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An investigation of bond formation between alumina single crystals and nickel alloys 1959

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AN INVESTIGATION OF BOND FORMATION BETWEEN ALUMINA SINGLE CRYSTALS AND NICKEL ALLOYS by JOHN FRANK CLARKE A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the Department of MINING AND METALLURGY We accept t h i s t h e s i s as conforming t o the standard r e q u i r e d from candidates f o r the degree of MASTER OF APPLIED SCIENCE. Members of the Department of Mining and M e t a l l u r g y THE UNIVERSITY OF BRITISH COLUMBIA • August, 195? ABSTRACT An i n v e s t i g a t i o n was conducted on the mechanisms of bond formation between a l l o y s of n i c k e l and s i n g l e c r y s t a l s of alumina. N i c k e l - t i t a n i u m , nickel-chromium, and n i c k e l - z i r c o n i u m powder mixtures were cleaned w i t h p u r i f i e d hydrogen gas at 800°C and were i n d i v i d u a l l y melted under vacuum (10~5 m.m. of Hg) i n contact w i t h alumina. I n t e r f a c i a l energy measurements at 1500°C were made by the s e s s i l e - d r o p method. The bond surfaces were examined by X-ray flu o r e s c e n c e and X-ray d i f f r a c t i o n techniques. The bond formation i n a l l cases appeared t o i n v o l v e two basic mechanisms - metal s o l u t e segregation and i n t e r f a c i a l r e a c t i o n . By i n t e r f a c i a l measurements and X-ray fluorescence analyses, the s o l u t e atoms, t i t a n i u m and chromium, were shown to be s e l e c t i v e l y adsorbed at the metal-ceramic i n t e r f a c e . I n t e r - f a c i a l r e a c t i o n products were detected by X-ray d i f f r a c t i o n methods. The adsorbed t i t a n i u m reacted w i t h the alumina to produce an i n t e r f a c i a l l a y e r of alpha t i t a n i u m sesqui oxide ( a-TigOj). Adsorbed chromium s i m i l a r l y reacted w i t h the alumina to form an i n t e r f a c i a l compound. However, t h i s compound could not be i d e n t i f i e d . With n i c k e l - z i r c o n i u m a l l o y s , the v i o l e n c e of the r e a c t i o n between zirconium and alumina made experimental measurements impossible. In presenting t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree th a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference- and study. I f u r t h e r agree th a t permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood tha t copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission. Department of Mining and Metallurgy The U n i v e r s i t y of B r i t i s h Columbia, Vancouver 8, Canada. Date September 5 . 1959. ACKNOWLEDGEMENT The author i s indebted t o Mr. W.M. Armstrong f o r h i s s u p e r v i s i o n and encouragement, and to Mr. R. B u t t e r s f o r h i s t e c h n i c a l advice and a s s i s t a n c e . The i n v a l u a b l e a s s i s t a n c e given by Dr. R.M. Thompson i n i n t e r p r e t i n g X-ray data i s a l s o g r a t e f u l l y acknowledged. The work was financed by Research Grant 7510-32 provided by the Defence Research Board of Canada and a Scholar- ship provided by the U n i v e r s i t y of B r i t i s h Columbia. TABLE OF CONTENTS Pap;e I. INTRODUCTION 1 A. General Purpose and Scope 1 B. . Previous I n v e s t i g a t i o n s . . . . . . . . . . . 4 C. S p e c i f i c Aims of the Present I n v e s t i g a t i o n .... 10 I I \ EXPERIMENTAL 12 A, M a t e r i a l s . . . . . . . . . . . . . . . . . . . . 1 2 ' ' 1. Aluminum oxide . . . . . . . . . . . . . 12 2, . Metals 13 B. , Apparatus 15 1, Furnace . . . . . . . . . . . . . . . . . . '17 2, Vacuum system' . . . . . . . . 18 3, O p t i c a l system 19 C, P r e p a r a t i o n of M a t e r i a l s . . . . . . . . . . . 20 D. Experimental Procedure . . . . . . . . . . . . 24 1. S e s s i l e - d r o p experiments . . . . . . . . 24 2. X-ray fluorescence examinations , . 25 3. X-ray d i f f r a c t i o n i n v e s t i g a t i o n s . . . . 25 I I I . EXPERIMENTAL RESULTS AND "CALCULATIONS r " T . . ... . . . '27 A. Wetting R e s u l t s 27 B. X-ray Data . . . . . . . . ... . . . . . . . 52 1. X-ray fl u o r e s c e n c e analyses . . . . . . 32 2. Debye-Scherrer Powder P a t t e r n s , . . . . 35 C. Thermodynamic C a l c u l a t i o n s . . . . . . . . . . 37 (Continued ... Table of Contents (continued) Page IV. DISCUSSION AND CONCLUSIONS 39 A. D i s c u s s i o n of R e s u l t s 39 B. Conclusions 4-3 V. RECOMMENDATIONS FOR FURTHER WORK 46 VI. APPENDICES 47 V I I . BIBLIOGRAPHY 60 FIGURES No. Page 1. Surface t e n s i o n f o r c e s a c t i n g on a s e s s i l e drop . . . . 3 2. Graphs reproduced from the work by A l l e n and Kingery, and K u r k j i a n and Kingery 7 3. A phase diagram f o r the CrgO^ system 9 4. P i c t o r i a l r e p r e s e n t a t i o n of sapphire l a t t i c e . . . . . 13 3. The apparatus assembly of the i n d u c t i o n furnace, vacuum system and o p t i c a l system 13 6. Assembly drawing of the apparatus l£> 7. Molybdenum heating elements (dismantled) of the i n d u c t i o n furnace . . . . 17 8. (a) Rough-polished surface of sapphire rod; (b) F i n i s h e d surface of sapphire rod; (c) F i n i s h e d surface of s i n t e r e d oxide. M a g n i f i c a t i o n lOOOx ... . . 21 9. G r o s s - s e c t i o n a l view of a specimen showing shape of the compact to i n s u r e an advancing contact angle . . 22 10. a. Phase diagram f o r the nickel-chromium^^ 23 b. Phase diagram f o r ^ t h e n i c k e l - t i t a n i u m s y s t e m ^ . . . 23 11. Dimensions measured on photographic negatives of s e s s i l e drops. Two types are shown: f o r angles g r e a t e r than 90° on the l e f t , or l e s s than 90° on the r i g h t 23 12. S e s s i l e - d r o p photographs o f : : ( l ) Pure S h e r r i t t #0323 N i , (2) Ni+0.3% T i , (3) Ni+1.6% T i , and (4) Ni+9.3% T i . The m a g n i f i c a t i o n , lOx, has been reduced approximately 23 percent f o r r e p r o d u c t i o n . . 28 FIGURES (cont'd.) No. Page 13. S e s s i l e - d r o p photographs of: (1) Pure S h e r r i t t #C523 N i , (2) Ni+7% Cr, (5) Ni+10% Cr, and (4) Ni+198/o Cr. The m a g n i f i c a t i o n , lQx, has been reduced approximately 25 percent f o r reproduction . 28 14. Graph of i n t e r f a c i a l energy versus percent t i t a n i u m . . 30 15. Graph of i n t e r f a c i a l energy versus percent chromium . . 31 l b . X-ray fluorescence curves. 34 17. X-ray powder p a t t e r n No. 1438 of i n t e r f a c i a l m a t e r i a l i n N i . - T i experiments 51 18. A.S.T.M. Card No. 2-1359 f o r a-TigO^ 52 19. X-ray powder p a t t e r n s , Nos. 1489, 1485 and 1493. (Top - Pure AlgO^, Middle - A1 2 0^ and unknown, Bottom - S o l i d s o l u t i o n of C ^ Q j i n AI2Q3) . . . 55 20. A.S.T.M. Card No. 5-0712 f o r O r A l ^ j . . . . . . . . . 56 21. A.S.T.M. Card No. 6-0504 f o r C r 9 0 ^ 57 TABLES No. Page I. Analyses of M a t e r i a l s 15 I I . (a) N i - T i and Ni-Zr ( S h e r r i t t No. C 325 Ni used throughout) 29 (b) Ni-Cr A l l o y s ( S h e r r i t t No. C 525 Ni used throughout) , 29 I I I . . W e t t a b i l i t y Data from M o d i f i e d Procedures . . . . . 33 IV. X-ray Fluorescence A n a l y s i s 35 V. Drying Agents and Drying Powers 47 VI. Data from F i l m No. 1458 52 V I I . E x t i n c t i o n Rules3& f o r Space Group D j d R3c . . . 53 V I I I . C a l c u l a t e d and Observed Data f o r F i l m No. 14-58 . . 54 IX. Data from F i l m No. 1485 56 AN INVESTIGATION OF BOND FORMATION BETWEEN ALUMINA SINGLE CRYSTALS AND NICKEL ALLOYS I. INTRODUCTION A. General Purpose and Scope During the past decade, the development of metal- ceramic systems has been focused on the bonding of metals to r e f r a c t o r y m a t e r i a l s such as borides, carbides, n i t r i d e s , oxides, s i l i c a t e s , s i l i c i d e s , and ceramic mixtures. The important p r o p e r t i e s of many ceramic m a t e r i a l s i n c l u d e c o r r o s i o n and oxida- t i o n r e s i s t a n c e , high m e l t i n g p o i n t s , and r e l a t i v e l y high s t r e n g t h and creep r e s i s t a n c e at elevated temperatures. Compared to metals, however, they are very b r i t t l e and s e n s i t i v e t o thermal shock. The u l t i m a t e aim of i n v e s t i g a t i o n s i n the metal-ceramic f i e l d i s to produce combinations of metals and ceramics which are s u p e r i o r to e i t h e r the metal or ceramic alone. At present, ceramic m a t e r i a l s are commonly used i n conjunction w i t h metals as p r o t e c t - i v e coatings on metal p a r t s , as the r e f r a c t o r y components of s i n t e r e d powder mixtures known as "cermets", and as the f i n e l y dispersed phases of dispersion-hardened metals. The s e l e c t i o n of promising m a t e r i a l s appears to be based on bonding c h a r a c t e r i s t i c s between the ceramic and metal phases. Although the p r o p e r t i e s of the metal-ceramic combinations have been measured, there i s l i t t l e fundamental basis f o r the s e l e c t i o n of the components."'" Hence, a study of the nature of the metal-to-ceramic bonds i s necessary. - 2 - p The fundamental f a c t o r s to be considered i n an i n v e s t i g a t i o n of the mechanisms of bonding between metals and ceramics are: (1) chemical f a c t o r s i n c l u d i n g w e t t i n g phenomenon and chemical r e a c t i o n s , (2) mechanical f a c t o r s r e l a t e d to thermal expansion and bulk s t r e n g t h s , and (3) design f a c t o r s i n v o l v i n g s i z e , shape and use of the f i n a l product. I n t h i s paper, the author considers chemical f a c t o r s under two main headings, 3 surface i n t e r a c t i o n s and i n t e r f a c i a l r e a c t i o n s . Surface i n t e r - a c t i o n s are those a s s o c i a t e d w i t h surface t e n s i o n and w e t t i n g , and i n t e r f a c i a l r e a c t i o n s are those r e l a t e d to the formation of i n t e r f a c i a l s o l i d s o l u t i o n s and chemical compounds. P h y s i c a l w e t ting of the ceramic by the metal i s a major f a c t o r i n producing an e f f e c t i v e bond. Wetting can be s t u d i e d by c o n s i d e r i n g the i n t e r f a c i a l energy i n v o l v e d . I n g e n e r a l , a low i n t e r f a c i a l energy i s a s s o c i a t e d w i t h a strong metal-ceramic bond. I t should be noted, however, that other f a c t o r s such as i n t e r - f a c i a l s t r e s s e s a r i s i n g from heating or c o o l i n g o f t e n render a strong bond i n e f f e c t i v e . Because lower i n t e r f a c i a l t e nsions increase the p r o b a b i l i t y of forming an e f f e c t i v e bond, a method of d e t e c t i n g changes i n t h i s q uantity i s e s s e n t i a l . The s e s s i l e - d r o p 4 5 6 7 method »•".»' can be employed very e f f e c t i v e l y f o r measurements of t h i s type at h i g h temperatures. The forces a c t i n g at the i n t e r f a c e of a s e s s i l e drop of l i q u i d metal on a smooth ceramic 8 surface are r e l a t e d according to the equation: y y - y cos 0 (1) SL SY LV - 3 - VAPOUR Figur e 1. Surface t e n s i o n f o r c e s a c t i n g on a s e s s i l e drop. where Vg^ i s the i n t e r f a c i a l energy, Vgv i s t n e surface energy of the ceramic, y^y i s the surface energy of the l i q u i d metal and 9 i s the angle of contact or wetting angle (see F i g u r e 1 ) , For many of the metals and ceramics of i n t e r e s t f o r p r a c t i c a l uses, very l i t t l e data i s a v a i l a b l e on the i n d i v i d u a l surface, energies. T h i s has l i m i t e d the choice of systems f o r the measure- ment of v a r i a t i o n s i n i n t e r f a c i a l energy. The measure of the a t t r a c t i o n between two d i f f e r e n t m a t e r i a l s i s the work of adhesion. T h i s can be c a l c u l a t e d d i r e c t l y from experimental data by the f o l l o w i n g equation: W a d = ( yLV ' y S v ) " y L S = y L E ( l + C 0 S * { 2 ) where Waa i s the work of adhesion. Thus, the surface t e n s i o n of the metal i s of major importance. A v a i l a b l e s u r f a c e t e n s i o n values i n d i c a t e that surface t e n s i o n s of l i q u i d metals are approximately twice those of the s o l i d oxides. - 4 - The important i n t e r f a c i a l r e a c t i o n s are the formation of s o l i d s o l u t i o n s and the production of d e f i n i t e chemical compounds at or near the metal-ceramic i n t e r f a c e . These r e a c t i o n s are o f t e n e s s e n t i a l to the formation of an e f f e c t i v e bond i n conjunction w i t h surface i n t e r a c t i o n s . ^ M o d i f i c a t i o n s of the m a t e r i a l present at the i n t e r f a c e change the bonding f o r c e s . The chemical r e a c t i o n s which modify the i n t e r f a c e depend on the thermodynamics of the system.10'"'""'" Un f o r t u n a t e l y , thermodynamic data i s u s u a l l y incomplete or non-existent f o r the systems and elevated temperatures of i n t e r e s t . A study of i n t e r f a c i a l compounds must o f t e n be accomplished by experimental methods only. S i m i l a r l y , s o l i d s o l u t i o n s formed from the metal phase at the i n t e r f a c e u s u a l l y cannot be d e f i n i t e l y p r e d i c t e d from thermo- 3 dynamic data alone. The choice of the metal-rceramic systems f o r the i n v e s t i g a t i o n of bond mechanisms i s , t h e r e f o r e , l i m i t e d p r i m a r i l y by l a c k of thermodynamic data at elevated temperatures f o r both the ceramic and metal components. Recently, however, thermo- dynamic i n v e s t i g a t i o n s have been completed on the aluminum oxide- 7 12 13 14 n i c k e l a l l o y systems.'' ' ' Consequently, these m a t e r i a l s were chosen f o r an i n v e s t i g a t i o n i n t o the mechanisms of bond formation. B, Previous I n v e s t i g a t i o n s . P r i o r work i n d i c a t e s that a l l o y s of n i c k e l , chromium, t i t a n i u m and zirconium bond to aluminum oxide. I n many cases, the aluminum oxide was used i n the powdered or s i n t e r e d form without regard f o r some important experimental v a r i a b l e s . Many i n v e s t i g a t o r s were concerned only w i t h production of a strong bond f o r engineering a p p l i c a t i o n s and not w i t h the mechanisms of bond formation. Only the fundamental work w i l l be c i t e d here. The s e s s i l e - d r o p technique was used e x t e n s i v e l y by Kingery and co-workers 7,12,13,15,16 i n a thorough study of surface i n t e r a c t i o n s between s i n t e r e d alumina and va r i o u s metals and a l l o y s . Surface energies and i n t e r f a c i a l energies were measured i n the systems. Measurements i n d i c a t e d that the more e l e c t r o p o s i t i v e components of a l l o y s , which were s e l e c t i v e l y adsorbed at the l i q u i d m e t a l - s o l i d ceramic i n t e r f a c e , decreased the i n t e r f a c i a l energy. In a narrow concentration range, the s e l e c t i v e a d s o r p t i o n caused a marked decrease i n the i n t e r f a c i a l energy. The c r i t i c a l concen- t r a t i o n s were b e l i e v e d to be approximately equivalent to the a c t i v i t i e s at which a monolayer of the more e l e c t r o p o s i t i v e atoms was present at the i n t e r f a c e . The excess . i n t e r f a c i a l c o n c e n t r a t i o n can be c a l c u l a t e d o from Gibbs' equation at constant temperature and pressure, -dv = Ii'dU! + l 2 d u 2 (3) where T± and r 2 are the excess i n t e r f a c i a l c o n c e n t r a t i o n s of the two components, u i and u 2 are the chemical p o t e n t i a l s of the components of the system, and y i s the surface t e n s i o n . For d i l u t e s o l u t i o n s , the chemical p o t e n t i a l of the major component, U]_, i s constant and the chemical p o t e n t i a l of the s o l u t e , u 2 , i s given by, ,u 2 = constant + R T Jjx c 2 (4) - 6 - The change i n the chemical p o t e n t i a l of the s o l v e n t , chi^ , i s approximately zero. Thus, combining equation (J) and (4) r e s u l t s equation: -dy = r 2RT d i n C 2 (5) where R i s the gas constant, T i s the absolute temperature and C 2 i s the c o n c e n t r a t i o n of the s o l u t e . This equation can be a p p l i e d to nickel-chromium, 1^ and f i i c k e l - t i t a n i u m 1 2 '"^ a l l o y s melted on alumina supports. F i g u r e 2 i l l u s t r a t e s some of r e s u l t s t o which the above theory has been a p p l i e d . Most N i - T i experiments were done by A l l e n 12 and Kingery, and the Ni-Cr experiments were done by K u r k j i a n and K i n g e r y . 1 ^ Measurements were made at 1475°C. using s i n t e r e d alumina as the ceramic supports. The surface energy of s o l i d alumina used f o r c a l c u l a t i o n of i n t e r f a c i a l energies was known to a good approximation 1^ to be 905 ergs per cm.^ at l850°C. (assuming a temperature c o e f f i c i e n t of -0.1 ergs per cm. 2 p e r °c ) , The surface energy c a l c u l a t e d f o r pure n i c k e l i n a vacuum was 1725 ergs per cm.2 a t 1475°C., This compares favourably w i t h other 1 1 ft recent v a l u e s . ' Since the surface energies of t i t a n i u m and chromium are s i m i l a r to the surface energy of n i c k e l , small a d d i t i o n s of t i t a n i u m and chromium to n i c k e l do not a p p r e c i a b l y a f f e c t the surface energy of the n i c k e l . The temperature c o e f f - i c i e n t s of surface energies of l i q u i d metals are, i n g e n e r a l , very s m a l l 1 ? and the temperature c o e f f i c i e n t s of i n t e r f a c i a l energies f o r n i c k e l and n i c k e l a l l o y s are about -1.0 ergs per cm.2 per °C. 1 2 Reactions between the metals and oxides were - 7 T 0 0 0 1 0 0 1 0 1 1 0 WEIGHT PERCENT ADDITION 1 1 1 i I iOOO a u •» - c I S O O w V ^ / • SLOPE FOR PACKING OF T I T A N I U M / A T O M S IN (III) PLANE Of TiO J m K \ v s . / 1 • • 1 0 0 0 tt w - \ \ \ X w w u 2 soo c X - • • a o • N I - T I N I - T I I N C O N C L X 9 S 0 I H C O roo S U P E R I O R W A J P A L O T H t i l K U K K J U N T H I S S T U D Y \ \ 0 0 1 1 O O O I 0 . 0 1 0 . 1 1 . 0 10 TIT»mUM C 0 » C E I I T « * T I 0 « , W«T. % Figure 2. Graphs reproduced from the work by A l l e n and Kingery, and K u r k j i a n and Kingery. - 8 - 9 thoroughly i n v e s t i g a t e d by Economos and Kingery. S i n t e r e d ceramics were used i n t h e i r i n v e s t i g a t i o n as closed r e a c t i o n chambers under vacuum. Metals were heated to temperatures greater than 1400°C and the r e s u l t i n g r e a c t i o n products were m i c r o s c o p i c a l l y observed. Although n i c k e l forms a NiO.A^Oj s p i n e l when heated i n contact w i t h alumina i n an o x i d i z i n g atmos- phere,^ 0 Economos and Kingery found no r e a c t i o n at the n i c k e l - alumina' i n t e r f a c e under a vacuum. Zirconium showed considerable c o r r o s i o n of the oxide, p e n e t r a t i o n along g r a i n boundaries and •black d i s c o l o r a t i o n of the g r a i n s at l800°C but no new phase was formed at the i n t e r f a c e . S i m i l a r r e s u l t s were obtained w i t h t i t a n i u m at l800°C but no appreciable attack of the alumina by t i t a n i u m was n o t i c e d below l600°C o According to c a l c u l a t i o n s , t i t a n i u m should react e x t e n s i v e l y w i t h a l l oxides except ThGg and BeO. 14 Baxter and Roberts do report an i n t e r f a c e a l t e r a t i o n when n i c k e l - t i t a n i u m a l l o y s were heated to 1490°C i n contact w i t h s i n t e r e d alumina. The alumina surface was c o l o r e d from brown to blue-black which was presumably due to the formation of TiO]_<>25 ~ l i g h t brown, TiO-|_ 44_]_ ^ ~ d a r k brown, and TiO]^ 73.1, £,5 - blue- black. But, the formation of these lower oxides of t i t a n i u m adversely a f f e c t e d the bond st r e n g t h . Considerable a t t e n t i o n has been g i v e n to the alumina- chromium system. Changes i n the composition of the oxide phase 2 3 r e s u l t i n the formation of a C ^ O J - A ^ O J s o l i d s o l u t i o n . ' ̂ This combination of a few percent by weight of C^O^ i n the A ^ O j (see Figure 3) surface appears to be an e s s e n t i a l step i n the - 9 - 2300 Al 20 3 2573 - 247 3 - 2373 1 2273 0.2' 04 06 0 8 MOLE .FRACTION C r 2 0 3 F i g u r e 3° A phase diagram f o r the CrgO^-AlgO^ system 21 formation of a strong bond. The f r e e energy change on formation of the s o l i d s o l u t i o n i s of the corr e c t order of magnitude to reduce the i n t e r f a c i a l energy the req u i r e d amount to promote we t t i n g . Microscopic examination of the i n t e r f a c e shows no evidence of intermediate phases or d i f f u s e boundary regions,, I n e f f e c t , the chromium i s bonded to the CrgO^-AlgOj s o l i d s o l u t i o n r a t h e r than the AlgOj ceramic. However, there i s no fundamental reason why chromium should have a g r e a t e r a f f i n i t y f o r the CrgO-^-AlgO-j s o l i d s o l u t i o n than f o r AlgO^. A somewhat d i f f e r e n t approach to the study of the 22 nature of the bond has been taken by Pincus, He studied the metal-ceramic i n t e r a c t i o n s between molybdenum and aluminum oxide - 10 - w i t h the a i d of taper s e c t i o n s and X-ray p a t t e r n s . The r e a c t i o n s of Mo and M0O3 w i t h pure alumina s i n g l e c r y s t a l s and s i n t e r e d mixtures were i n v e s t i g a t e d . X-ray d i f f r a c t i o n powder pa t t e r n s i n d i c a t e d that a new phase, A^fMoO^^, was formed by an i n t e r - f a c i a l r e a c t i o n i f a c o n t r o l l e d degree of metal o x i d a t i o n was allowed, ' I t can be seen from the above survey t h a t , although va r i o u s methods have been used to study the nature of metal alumina bonds, l i t t l e can be sai d about them that i s fundamental or c o n c l u s i v e . S i m i l a r l y , general r u l e s which would apply to a l l metal-ceramic systems have not been developed. Each system must, t h e r e f o r e , be stu d i e d s e p a r a t e l y i n order t o evaluate the mechanism involved i n bonding, C„ S p e c i f i c Aims of the Present I n v e s t i g a t i o n The o r i g i n a l purpose of t h i s i n v e s t i g a t i o n was to study bond formation between s i n g l e c r y s t a l s of AlgO^ (sapphire) and, various metals and a l l o y s , A c o n s i d e r a t i o n of s u b s t i t u t i o n a l d i f f u s i o n of the metal i o n i n t o the aluminum oxide l a t t i c e and of re d u c t i o n of the aluminum oxide by r e a c t i v e metals was to be of prime importance. L i k e l y elements f o r s u b s t i t u t i o n a l d i f f u s i o n would be those which have: (1) favorable s i z e s f o r d i f f u s i o n , (2) s i m i l a i M2Q3 oxides, and (3) oxides which are s t a b l e above the mel t i n g point of the metal. Elements which meet a l l three c o n d i t i o n s are: manganese, i r o n , chromium, vanadium, molybdenum, g a l l i u m , t i t a n i u m and bismuth. The r e a c t i v e metals commonly used to promote bonding are: t i t a n i u m , zirconium, tantalum and colum- bium. - 11 - A l l of the above p o s s i b i l i t i e s were to be i n v e s t i g a t e d by measurements of contact angle of a s e s s i l e drop of l i q u i d metal on the sapphire. The examinations of the bond were t o be made by me t a l l o g r a p h i c , c r y s t a l l o g r a p h i c and t r a c e r techniques. However, p r e l i m i n a r y experiments i n d i c a t e d the p o s s i b i l i t y of numerous experimental d i f f i c u l t i e s . The f i e l d of i n v e s t i g a t i o n was, t h e r e f o r e , narrowed i n order t o a l l o w a d e t a i l e d study of the systems used. Titanium and chromium of the f i r s t group 0f elements mentioned p r e v i o u s l y , and, zirconium and t i t a n i u m of the second group were chosen f o r the study surface i n t e r a c t i o n s and i n t e r - f a c i a l r e a c t i o n s . I t was hoped t h a t the use of s i n g l e - c r y s t a l , sapphire plaques of c o n t r o l l e d c r y s t a l l o g r a p h i c o r i e n t a t i o n would lead t o a more fundamental understanding o f the e f f e c t o f i n t e r - f a c i a l energy on bond formation. For the same reason, the Debye- Scherrer X-ray d i f f r a c t i o n analyses and X-ray fluorescence analyses were planned f o r i d e n t i f i c a t i o n of r e a c t i o n products at the metal-ceramic i n t e r f a c e . With t h i s approach, the chemical f a c t o r s i n f l u e n c i n g the bond formation should be detected. - 12 - I I . EXPERIMENTAL A. M a t e r i a l s 1. Aluminum Oxide The s i n g l e c r y s t a l plaques of aluminum oxide used throughout the i n v e s t i g a t i o n were high p u r i t y s y n t h e t i c sapphire (99.99?° AI2O3) s u p p l i e d by Linde A i r Products Co., L t d . This m a t e r i a l i s a v a i l a b l e i n the form of windows, rods, t u b i n g and s p e c i a l shapes. The rods used i n t h i s work were of c o n t r o l l e d 23 o r i e n t a t i o n s p e c i f i e d w i t h i n ±5 degrees. The c r y s t a l s t r u c t u r e of sapphire i s based on a hexa- 24 gonal c l o s e s t packing of atoms. The oxide ions are very n e a r l y i n a hexagonal c l o s e s t packing w i t h the aluminum ions i n the octahedral i n t e r s t i c e s of the oxygen framework. Because the number of aluminum ions i s d i f f e r e n t than the number of oxygen, one - t h i r d of the octahedral s i t e s are vacant. The arrangement of f i l l e d and empty i n t e r s t i c e s , however, i s ordered (see F i g u r e 4 ) , Small amounts of impurity elements i n the octahedral i n t e r - s t i c e s produce c h a r a c t e r i s t i c c o l o r s , A sapphire-blue c o l o r i s produced by 0,001 to 0.1/* t i t a n i u m , 2 ' ' a ruby red by l e s s than 8 mole percent of 0 ^ 0 3 i n s o l i d s o l u t i o n w i t h AI2O3, and a l i g h t green by more than 8 mole percent C ^ O j , D For the experiments, sapphire rods, 0,35 inches i n diamet e r fwere used to make the ceramic supporting plaques. These rods were suppli e d w i t h the rod a x i s at 60±5 degrees to the c r y s t a l l o g r a p h i c c - a x i s . C u t t i n g the rods p e r p e n d i c u l a r t o the rod a x i s produced a working surface approximately c o i n c i d e n t to 0 = oxygen atoms • = aluminum atoms o = vacant interstices f i g u r e 4. P i c t o r i a l r e p r e s e n t a t i o n of sapphire l a t t i c e . 2 4 the O o i 2 ^ plane.. The d i r e c t i o n p e r p e n d i c u l a r t o t h i s plane i s the p r e f e r r e d growth d i r e c t i o n . The b a s a l plane i s the c l o s e s t packed plane f o r sapphire. 2. Metals. A l l the metals used i n t h i s work were obtained i n the powder form. N i c k e l , the major c o n s t i t u e n t i n the a l l o y s used, was s u p p l i e d by S h e r r i t t Gordon Mines L t d , and Mond N i c k e l Co., L t d . Chromium powder coated w i t h n i c k e l was a l s o s u p p l i e d by S h e r r i t t Gordon Mines L t d . The t i t a n i u m hydride and zirconium hydride powders used to make n i c k e l - t i t a n i u m and n i c k e l - z irconium a l l o y s were obtained from Metal Hydrides Incorporated. The chemical analyses of these m a t e r i a l s 'are g i v e n i n Table I . TABLE I Analyses of M a t e r i a l s A n a l y s i s (Maximum percent) M a t e r i a l Ni T i Cr Zr Co Cu Ee S C H 2 N 2 Ca Mg A l S i Other Titanium hydride 95 0.1 3.5 0.2 0.05 0.05 0.1 0.1 Zirconium hydride • - 0.1 95.5 0.15 0.01 0.15 2.1 0.7 0.05 0.03 0.15 0.03 N i c k e l coat- ed chromium 7 ?.6 19.0 0.57 0.003 0.018 0.01 S h e r r i t t N i JC 525 B a l . 0.08 0.029 0.022 0.016 0.01 C005 S h e r r i t t high ..purity Ni B a l . 0.008 0.009 .004 0.006 .08 Mond carhonyl Ni B a l . 0.005 0.008 0.007 0.002 0.091 <.003 I H -15 - B. Apparatus The apparatus was designed w i t h the f o l l o w i n g o b j e c t i v e s (1) to produce temperatures as high as 1800 C by i n d u c t i o n heating and vacuums as high as 5 x 10"^ m.m. of Eg, (2) to permit the i n t r o d u c t i o n of reducing atmospheres at pressures l e s s than atmospheric pressure, and (3) t o a l l o w accurate measurements on s e s s i l e drops at temperatures greater than the me l t i n g p o i n t s of the a l l o y s used. The equipment i s shown i n F i g u r e s 5 and 6. F i g u r e 5. The apparatus assembly of the i n d u c t i o n furnace,vacuum system and o p t i c a l system. OPTICAL SYSTEM FURNACE ASSEMBLY (1) Oround glass or pbotojjraphlc plate. (2) Vertical adjustment screw. (5) Horizontal adjustment track. (k) Focussing screw.. - (5) Adjustable bellows. (6) Ocular lens. (7) Objective lens, abutter and Iris diapbn (8) Vertical adjustment screw. (9) Waterrcooled optical f l a t . (10) Water-cooled brass f i t t i n g . (11) Magnetic abutter. (12) Vycor tube. (15) Induction c o l l . (lh) Heating element, radiation shield and specimen. (15) Thermocouple gauge. (16) Ionization gauge. {17} Oaa inlet control. (18) . Viewing window. (19) Brass fittings. (20) Optical pyrometer. (21) Light source interchangeable with pyrometer. ON Fi g u r e 6. Assembly drawing of the apparatus, -17 - 2. Furnace. The heating elements of the furnace were constructed from 0.005 inch molybdenum sheet (see Figure 7). The susceptor of the i n d u c t i o n furnace. c o n s i s t e d of a "closed l o o p " i n the form of a c y l i n d e r 0.75 inches i n diameter and 5.5 inches long, open at both ends. A r a d i a t i o n s h i e l d surrounded the susceptor to prevent excessive heat l o s s from the core of the heating element. T h i s s h i e l d c o n s i s t e d of an "open l o o p " c y l i n d e r 1.5 inches i n diameter and 5 inches long, a l s o open at both ends. The susceptor and r a d i a t i o n s h i e l d assembly was supported i n s i d e the i n d u c t i o n c o i l by a molybdenum-rod framework. This rod was separated from the vycor tubing c o n t a i n i n g the heating elements by s i n t e r e d alumina i n s u l a t o r s . The furnace-tube assembly c o n s i s t e d of a vycor (fused s i l i c a ) tube, 2£ inches i n diameter and 18 inches i n lengt h . Rubber 0-ring s e a l s and s i l i c o n e high vacuum grease were used i n the vycor-to-brass j o i n t s at the end o f the tube. - 18 - At the camera end of the furnace, a water-cooled o p t i c a l f l a t was used to protect the camera lenses from heat r a d i a t i o n . Because of the high vapour pressures of metals at operating temperatures, 2 7 a magnetic s h u t t e r was placed i n s i d e the furnace to protect the o p t i c a l f l a t from metal vapours. At the other end of the furnace, a viewing window was b u i l t i n t o the elbow l e a d i n g to the vacuum system. Outside t h i s window, a l i g h t source and an o p t i c a l pyrometer were placed such that e i t h e r could be p o s i t i o n e d on the c e n t r e - l i n e of the furnace. Through a water-cooled, copper i n d u c t i o n c o i l ( i i n c h O.D. tubi n g , 21 turns) around the outside of the vycor tube, power was supplied to the heating element by a high-frequency i n d u c t i o n generator. This u n i t , L e p e l model T-10-.3, produces power from 0 to 23..5 K.V.A. at 400,000 c y c l e s per second. The power could be adjusted p r e c i s e l y by var y i n g e i t h e r the p l a t e current or the g r i d current i n the tube of the generator. The r e s u l t i n g temperature i n the furnace was measured to ±5°C by a Hartmann and Braun pyrometer, model T0-10-e. E m i s s i v i t y c o r r e c t - ions were not necessary because of the design of the susceptor. Power input t o the furnace was l i m i t e d by the lo s s , of strength of the vycor tube at elevated temperatures. The tube was, t h e r e f o r e , a i r - c o o l e d by means of a 10 inch diameter f a n . 2. Vacuum system. The pumping system was designed to produce a high vacuum or a c o n t r o l l e d low vacuum. A mechanical fore pump and a two-stage o i l d i f f u s i o n pump produced vacuums i n the range of 10" 4 to 10 m.m. Hg. With only the for e pump o p e r a t i n g , a reducing atmosphere could he continuously f l u s h e d through the system to maintain a vacuum of 0.5 m.m. of Hg. The high vacuums were measured w i t h an N.R.C. i o n i z a t i o n gauge, type 507, and the low vacuums were measured by an N.R.C. thermocouple gauge, type 501. P u r i f i e d reducing atmospheres were obtained by c o n t i n u - ously f l u s h i n g dry hydrogen through the system. The hydrogen was supp l i e d by Canada L i q u i d A i r Co., L t d . i n 2000 p . s . i . tanks. The gas was p u r i f i e d at a c o n t r o l l e d r a t e of l e s s than one cubic foot per minute. The p u r i f i c a t i o n and d r y i n g t r a i n c o n s i s t e d of a hydrogen "deoxo" c a r t r i d g e i n s e r i e s w i t h columns of anhydrous calcium sulphate, s i l i c a g e l , and phosphorous pentoxide. The "deoxo" c a r t r i d g e converted the oxygen i m p u r i t y to water. The s i l i c a g e l and anhydrous calcium sulphate reduced the water con- c e n t r a t i o n i n the hydrogen to 0.005 m i l l i g r a m s per l i t e r of gas and the phosphorous pentoxide reduced the c o n c e n t r a t i o n .further to l e s s than 0.0002 m i l l i g r a m s per l i t e r . (See Appendix I) 3. O p t i c a l system. At temperatures of greater than 1100°C, obj e c t s emit enough l i g h t to produce an o u t l i n e image on a photographic f i l m . Sharp images of the incandescent s e s s i l e drops i n t h i s i n v e s t i - g a t i o n could be r e a d i l y obtained at 1500°C i f a camera were c o r r e c t l y designed f o r t;his purpose, A camera was constructed to give a t e n - f o l d m a g n i f i c a - t i o n . This was accomplished by the combination of a two-component, o b j e c t i v e lens ( f o c a l l e n g t h of 11 inches) and a s i n g l e component, oc u l a r lens ( f o c a l l e n g t h of 42 m i l l i m e t e r s ) . The f i x e d , - 20 - o b j e c t i v e lens was equipped w i t h an ad j u s t a b l e s h u t t e r and i r i s diaphragm. The o c u l a r lens was separated from the o b j e c t - i v e lens by a 10 to 15 inch a d j u s t a b l e bellows, and from the ground g l a s s p l a t e by a 42 to 48 inch a d j u s t a b l e bellows. The camera was p o s i t i o n e d so that the o p t i c a l a x i s was'in the plane of the ceramic surface and p a r a l l e l t o a x i s of the furnace by means of v e r t i c a l adjustment screws and h o r i z o n t a l adjustment t r a c k s . These were securely f i x e d to the camera frame. , Because of the le n g t h of the camera, s e n s i t i v t y to v i b r a t i o n s was high and a very sturdy framework was necessary. Focusing adjustments were made by means of a screw- thread adjustment on the o c u l a r lens holder which moved p a r a l l e l t o the o p t i c a l a x i s of the camera. Before the specimen was heated, p r e l i m i n a r y f o c u s i n g was accomplished w i t h the a i d of a l i g h t source at the f a r end of the furnace. The l i g h t source produced a s i l h o u e t t e of the specimen on the ground g l a s s . C. P r e p a r a t i o n of M a t e r i a l s . The s i n g l e - c r y s t a l aluminum oxide was obtained as sapphire rods. D i s c s , approximately 1/8 inch t h i c k , were cut from the rod per p e n d i c u l a r to rod a x i s w i t h a F e l k e r "Di-Met" diamond saw. These d i s c s were mounted i n l u c i t e and were p o l i s h e d w i t h diamond powder suspended i n kerosene. Rough p o l i s h i n g was done w i t h 325-micron diamond powder i n kerosene on p l a t e g l a s s (see Figure 8a). Fin e p o l i s h i n g was completed w i t h the a i d of a v i b r a t o r y oQ p o l i s h e r , The bottom of the p o l i s h i n g bowl was covered w i t h a - 21 - heavily-etched copper p l a t e . A mixture of diamond paste micron) and kerosene was used as the p o l i s h i n g compound. A f t e r 48 t o 72 Figure 8. (a) Rough-polished surface of sapphire rod (b) F i n i s h e d surface of sapphire rod (c) F i n i s h e d surface of s i n t e r e d aluminum oxide. M a g n i f i c a t i o n lOOOx hours of continuous p o l i s h i n g , a s u i t a b l e surface was obtained (see F i g u r e 8b). The d i s c s were then >roken away from the l u c i t e mounting. The p o l i s h e d plaques were soaked i n concentrated n i t r i c a c i d and washed w i t h e t h y l a l c o h o l and acetone. Care was taken i n handling the plaques to avoid contamination of the p o l i s h e d surface. Metal tweezers were used whenever p o s s i b l e . The same precautions were necessary i n the handling of the metals because minute amounts of contaminants o f t e n a f f e c t surface energy measurements g r e a t l y . Metal powders, mixed i n the des i r e d p r o p o r t i o n s , were - 22 - compacted i n a c y l i n d r i c a l d i e , £ in c h i n diameter, under a pressure of 40 tons per square i n c h . The compacting d i e was machined to form a compact of a shape r e q u i r e d t o i n s u r e an advancing contact angle when the a l l o y melted (see F i g u r e 9). The die c a v i t y was designed to hold 0.5 t o 0.7 grams of powder. Clean metal t o o l s were used throughout to avoid contamination of the compact. POWDER COMPACT Fig u r e 9. C r o s s - s e c t i o n a l view of a specimen showing shape of the compact to insure an advancing contact angle. For contact angle measurements on the n i c k e l - t i t a n i u m and hickel-chromium.alloys,compositions were used w i t h i n the s o l i d - s o l u b i l i t y l i m i t s . The s o l i d - s o l u b i l i t y ranges are 0 to 10.8 percent t i t a n i u m i n the n i c k e l - t i t a n i u m system (see F i g u r e 10a) and 0 t o 47 percent chromium i n the nickel-chromium system (see F i g u r e 1.0b). The compositions of n i c k e l - z i r c o n i u m , which were i n v e s t i g a t e d , were s i m i l a r to those of n i c k e l - t i t a n i u m . - 23 - °C Atomic Percentage Chromium °F 10 20 30 40 50 60 70 80 90 1900 1700 1500 1300 1100 900 700 1 1 1 r i i i -* ^ 1800* " / a + L L + L - s / / — / / / ~ ~-~L • — — 43* P / _ ATI C , /T^ r N s \ c i / a + p \ \ r 1 ~ 1 1 - 3000 - 2600 - 2200 /eoo 7400 A» 10 20 30 40 50 60 70 80 90 Cr Weight Percentage Chromium F i g u r e 10b. Phase diagram f o r the n i c k e l - chromium system.29 °C Atomic Percentage Titanium °F Weight Percentage Titanium F i g u r e 10a. Phase diagram f o r the n i c k e l - t i t a n i u m system. ' - 24 - D. Experimental Procedure. 1„ S e s s i l e - d r o p experiments. The metal was placed on the p o l i s h e d sapphire i n s i d e the susceptor. With the a i d of the l i g h t source, the specimen was l e v e l e d . The furnace was assembled and pumped to a f o r e pump vacuum of 5 x 10"^ m.m. of Hg. P u r i f i e d hydrogen was then f l u s h e d through the furnace at a rat e s u f f i c i e n t to maintain a vacuum of 0.5 mm. of Hg. The temperature was slowly r a i s e d to approximately 800°C„ Near t h i s temperature, the hydrogen gas i o n i z e d and hydrogen-ion c l e a n i n g of the specimen took place. A f t e r a heating pe r i o d of 10 minutes, the power was turned o f f and the system was pumped down to a vacuum of 10"-̂  m.m. Hg. The power was again slowly increased u n t i l the temperature reached 1500°C as measured by the o p t i c a l pyrometer. The specimen was maintained at t h i s temperature f o r 20 minutes to insure e q u i l i b r i u m c o n d i t i o n s . ^ ' ^ During the h o l d i n g time, the image of the s e s s i l e drop was care- f u l l y focused on the ground g l a s s . At the end of the heating p e r i o d , the ground g l a s s was replaced by a Kodak "M" p l a t e , the fo r e pump and f a n were turned o f f to reduce v i b r a t i o n s , and the photographic p l a t e was exposed. The power' was then decreased to zero and the furnace was allowed to c o o l s l o w l y t o room tempera- t u r e . The s e s s i l e - d r o p method of c a l c u l a t i n g surface tensions and contact angles i n v o l v e s the measurement of the dimensions of a s t a t i o n a r y l i q u i d drop on a smooth f l a t s u r f a c e . From the measurements i n d i c a t e d i n Figu r e 11, s u f f i c i e n t l y accurate calculations^"*" can be made by a method described l a t e r . - 25 2X ~" 2 X X ; 2T1- — Fig u r e 11. Dimensions measured on photographic negatives of s e s s i l e drops. Two types are shown: f o r angles greater than 90° on the l e f t . o r l e s s than 90° on the r i g h t . For measurements on the photographic p l a t e s , the p l a t e s were mounted on the i l l u m i n a t e d window of a standard viewing box. Measurements were made w i t h d i v i d e r s and m i c r o c a l i p e r s to the nearest 0.001 in c h . 2. X-ray fluorescence examinations. In the n i c k e l - t i t a n i u m and nickel-chromium experiments, some specimens were s e l e c t e d f o r examination by X-ray methods. A f t e r s eparation of the metal and sapphire, X-ray fluorescence analyses were made on the mating surfaces of the metal drop and the sapphire plaque. The i n t e r f a c i a l surfaces were analyzed f o r n i c k e l , t i t a n i u m , and chromium t o i n v e s t i g a t e s e l e c t i v e adsorp- t i o n at the i n t e r f a c e . A tungsten-target X-ray tube(40 K.V., 20 m.a.) and a l i t h i u m f l u o r i d e a n a l y z i n g c r y s t a l were used. The r e s u l t s were recorded i n g r a p h i c a l form by a s c i n t i l l a t i o n counter. 3. X-ray d i f f r a c t i o n i n v e s t i g a t i o n s . F o l l o w i n g the X-ray fluorescence analyses, the i n t e r - - 26 - f a c i a l l a y e r s on the sapphire were cleaned and prepared f o r i d e n t i f i c a t i o n by the X-ray powder method. The powder patterns were obtained by exposures to copper r a d i a t i o n (40 K.V, , 15 m.a.) f o r 2 to 4 hours. These patterns were indexed and compared w i t h p a t t e r n s of standard m a t e r i a l s recorded i n the A.S.T.M. card index f o r powder p a t t e r n s . I I I . EXPERIMENTAL RESULTS AND CALCULATIONS A. Wetting R e s u l t s . To c a l c u l a t e i n t e r f a c i a l energy data, p r e c i s e measure- ments of surface energies and contact angles are r e q u i r e d . The s e s s i l e - d r o p dimensions, X, Z, X"*" , and (see Figu r e 11), were used to c a l c u l a t e contact angles and surface energies by the methods o u t l i n e d i n Appendix I I . The v a r i a t i o n s of contact angle f o r the n i c k e l - t i t a n i u m and nickel-chromium systems are i l l u s t r a t e d i n F i gures 12 and 13 and l i s t e d i n Table I I . The average l i q u i d surface energy value f o r these a l l o y s was l84j? ergs per cm.2 c a l c u - l a t e d from drop dimensions. By s u b s t i t u t i n g the appropriate values of contact angle and the values of surface energy, V L V = p • ? 1 7 1845 ergs per cm.* and "Kgy = 940 ergs per cm. , i n t o equation (1), the r e s p e c t i v e i n t e r f a c i a l energies can be c a l c u l a t e d . These i n t e r f a c i a l energies have been l i s t e d i n Table I I and p l o t t e d i n Figures 14 and 15. Although a number of nickel-chromium a l l o y s were used, none of the experiments i n v o l v i n g zirconium were e n t i r e l y success- f u l . The a l l o y s e i t h e r erupted a f t e r melting; or reacted v i o l e n t l y w i t h the sapphire to form a rough s c a l e on the metal drop. P r e c i s e measurements on the r e s u l t i n g photographs were i n a c c u r a t e , o r impossible. Therefore, only one contact angle was measured (see Table I l a ) and no i n t e r f a c i a l energies were c a l c u l a t e d . Considerable d i f f e r e n c e s between the above data and previous values f o r n i c k e l - t i t a n i u m and nickel-chromium a l l o y s on - 28 - F i g u r e 12. S e s s i l e - d r o p photographs of: (1) Pure S h e r r i t t #C525 N i , (2) Ni + 0.3% T i (3) Ni + 1.6% T i , and (4) Ni + 9.3% T i . The m a g n i f i c a t i o n , lOx, has been reduced approximately 25 percent f o r reproduction. F i g u r e 13. o e s s i l e - d r o p photographs of: (1) Pure S h e r r i t t #0325 N i , (2) Ni + 7% Or, (3) Ni + 10* Cr, and (4) Ni + 19% Cr. The m a g n i f i c a t i o n , lOx, has been reduced approximately 25 percent f o r reproduction. - 29 - TABLE I I Experimental W e t t a b i l i t y Data. (a) N l - T l and Ni-Zr ( S h e r r i t t N0.C525 Ni used throughout) A l l o y 0 (deg.) y S L ( e r g s cm.2) Pure Ni 108.0 (av.) 1510 Ni + 0.3% T i 101.6 1311 Ni + 1.0 91A 1178 Ni + 1.2 95.1 1104 Ni +1.40 92.4 1017 Ni + 1.42 85.7 800 Ni + 1.44 105.7 1439 Ni + 1.6 90.0 940 Ni + .1.8 85»3 789 ' Ni + 1 „ 9 87.6 863 Ni + 2.1 85.1 782 Ni + 3.4 90.0 940 Ni + 3.7 82.9 713 Ni + 4.1 89.9 938 Ni + 4.8 87.9 874 Ni + 7.1 87.5 860 Ni + 8.4 83.6 735 Ni + 8.5 85.6 798 Ni + 9.3 8 0.I 623 Ni + 10.4% Zr 75 (approx, (b) Ni-Cr A l l o y s ( S h e r r i t t N0.C525 Ni and S h e r r i t t Ni coated Cr used). A l l o y 0 (deg.) y S L ( e r g s cm.2) Pure Ni 108.0 (av.) 1510 Ni + 2% Cr 101.1 1295 Ni + 2 107 .7 150I Ni + 4 106.4 1461 Ni + 6 105 .9 1446 Ni + 7 100.7 1 2 8 3 - Ni + 8 96.2 1139 Ni + 8 97.4 1178 Ni + 9 99.5 1117 Ni + 9 89.1 911 Ni + 10 86.7 834 Ni + 10 87.3 853 Ni + 11 8 8 . 3 885 Ni + 11 89.O 908 Ni + 12 84.9 776 Ni + 14 9 0 . 0 940 Ni + 14 82.3 725 Ni + 14 84.8. . 773 Ni + 19 82.3 725 Ni + 19 82.1 690 Ni + 19 84.1 750 o o o o 0.2 -I L I I I I I I .3 .4 .5 .6 .7 J8 .9 1 PERCENT TITANIUM J I I I C 5 6 7 8 9 Figure 14. Graph of interfacial energy versus percent titanium. PERCENT CHROMIUM Figure 15. Graph of Interfacial Energy versus Percent Chromium - 32 - s i n t e r e d alumina supports are apparent. The i n t e r f a c i a l energies of the pure n i c k e l and l o w - a l l o y samples appear^to "be too l o w 1 2 , 1 3 12 and the h i g h - a l l o y i n t e r f a c i a l energies appear too high. P o s s i b l e causes of these d i f f e r e n c e s are: (1) surface c o n d i t i o n of the ceramic support, (2) s i z e of the metal drop, (3) o r i e n t a t i o n of 13 the ceramic support, and (4) p u r i t y of the metals. Experiments were, t h e r e f o r e , designed to i n v e s t i g a t e these p o s s i b i l i t i e s . The m o d i f i c a t i o n s and the r e s u l t i n g changes have been l i s t e d i n Table I I I . The three types of ceramic surfaces used f o r i n v e s t i g a t i n g surface and o r i e n t a t i o n e f f e c t s are shown i n F i g u r e 8 . Analyses f o r the d i f f e r e n t grades of n i c k e l used f o r the m o d i f i c a t i o n of metal p u r i t y are l i s t e d i n Table I . The surface energy c a l c u l a t e d f o r the S h e r r i t t high- p p u r i t y n i c k e l i s 1595 ergs per cm, and the corresponding value f o r the Mond carbonyl n i c k e l i s 1725 ergs per cm. 2 The d i f f e r e n c e between these values and the value f o r S h e r r i t t N0.C525 n i c k e l (1845 ergs per cm.2) i s probably due to the d i f f e r e n t concentra- t i o n s of s u r f a c e - a c t i v e i m p u r i t i e s such as carbon and sulphur i n the metal, B. X-ray Data, 1, X-ray fluorescence analyses. Metal and ceramic specimens were s e l e c t e d f o r analyses from the Ni + 2.1% T i and Ni + 10% Cr experiments. The r e s u l t i n g curves, which have been reproduced i n F i g u r e 1 6 , show that t i t a n i u m and chromium concentrations are higher at the ceramic surface than i n the metal drop compared to the r e s p e c t i v e n i c k e l TABLE I I I W e t t a b i l i t y Data from M o d i f i e d Procedures Metal Approximate! metal weight - Ceramic support 0 (dego) y S L ergs/cm 2 V a r i a b l e #C325 Ni 0 .5 gms. Fin e - p o l i s h e d sapphire d i s c 108.0 1510 None #C525 Ni 0 . 5 gms. Rough-polished sapphire disc 111.6 1619 Surface c o n d i t i o n #C525 Ni 0 .75 gms. Fi n e - p o l i s h e d sapphire d i s c 112.4 1650 Drop s i z e #C525 Ni 0 . 5 gms. Si n t e r e d alumina pdr.(600mesh) 119.4 1846 O r i e n t a t i o n and surface c o n d i t i o n Ni + 9.3% T i 0 . 5 gms. Fi n e - p o l i s h e d sapphire d i s c 80.1 623 None Ni + 10% T i 0 , 5 gms. Sin t e r e d alumina pdr „ ( 6 0 0 mesh) 7 9 o l 591 O r i e n t a t i o n and ; surface c o n d i t i o n : Hi g h " p u r i t y Ni 0„5 gms. Fin e - p o l i s h e d sapphire d i s c 113 .7 1581 Metal p u r i t y Carbonyl Ni 0 . 5 gms. Fi n e - p o l i s h e d sapphire d i s c 104.1 1360 Metal p u r i t y I 20 Values for Ni.Ti .and Cr F i g u r e / i G . X-ray fluorescence curves.--v -35 - concentrations. Although t h i s method i s i n h e r e n t l y inaccurate f o r absolute values i f the samples are not standardized, the r e l a t i v e values f o r any one X-ray specimen are s i g n i f i c a n t . A f t e r c o r r e c t i n g the curves f o r s e n s i t i v i t y and s c a l e f a c t o r s of the counter, r a t i o s of the metals i n the drop and on the sapphire can be obtained. These approximate r a t i o s are shown i n Table I"V. U n f o r t u n a t e l y , t h i s method gives the r e l a t i v e amounts of the elements but no i n d i c a t i o n of the form i n which the elements e x i s t . TABLE IV X-ray Fluorescence A n a l y s i s A l l o y Composition Composition R a t i o a f t e r Test In Metal Dron On Sapphire Surfaces Ni + 2.1 T i Ni + 10% Cr N i = 42 T i 1 Ni = _ Cr 1 Ni = 0.1 T i 1 Ni = 0.2 Cr 1 2. Debye-Scherrer Powder P a t t e r n s . During the heating c y c l e i n the n i c k e l - t i t a n i u m and n i c k e l - chromium experiments, c o l o r e d i n t e r f a c i a l l a y e r s formed between the a l l o y s and sapphire. N i c k e l - t i t a n i u m a l l o y s produced a blue-black f i l m on the p o r t i o n of the sapphire surface covered by the metal drop. V a r i a t i o n s i n temperature about the m e l t i n g point of the a l l o y caused t h i s blue-black f i l m t o f l a k e o f f the sapphire. N i c k e l - -36 - chromium a l l o y s , on the other hand, produced a pale green m a t e r i a l at the i n t e r f a c e . This m a t e r i a l was not a f i l m but appeared to be a d i f f u s i o n l a y e r which was p h y s i c a l l y inseparable from the under- l y i n g sapphire. The blue-black f l a k e s from the n i c k e l - t i t a n i u m experiments were cleaned and powdered i n preparation f o r X-ray d i f f r a c t i o n i n a Debye-Scherrer camera. P r e l i m i n a r y c a l c u l a t i o n s on the r e s u l t - i ng powder patterns i n d i c a t e d that the i n t e r f a c i a l f i l m was the r e a c t i o n product, alpha t i t a n i u m sesqui oxide (a-TigC^). Because the recorded data f o r t h i s m a t e r i a l i n the A.S.T.M. card index was incomplete, f u r t h e r c a l c u l a t i o n s were necessary to p o s i t i v e l y i d e n t i f y the m a t e r i a l as being a-TigGj. The X-ray powder patterns were subsequently indexed by the use of the e x t i n c t i o n r u l e s f o r a-TigOj and the Bunn chart f o r hexagonal m a t e r i a l s . The d e t a i l s of these c a l c u l a t i o n s have been included i n Appendix I I I . The c a l c u l a t e d and observed r e s u l t s were i n c l o s e enough agreement to prove that the m a t e r i a l was a-TigO^,. The p e r t i n e n t data, which was t a b u l a t e d and submitted to the J o i n t Committee on Chemical A n a l y s i s by Powder D i f f r a c t i o n Methods, was accepted f o r p u b l i c a t i o n i n the card index. A technique f o r the s e p a r a t i o n of the green, i n t e r f a c i a l l a y e r formed i n the nickel-chromium experiments from the under- l y i n g sapphire was not discovered. Powder samples of the combined m a t e r i a l and a sample of pure sapphire were, t h e r e f o r e , prepared f o r X - r a y . d i f f r a c t i o n . Because the green i n t e r f a c i a l m a t e r i a l was p r e v i o u s l y reported , p ' to be a s o l i d s o l u t i o n of CrgOj i n A l g O j , a known s o l i d s o l u t i o n of t h i s type was X-rayed i n a s i m i l a r - 37 - manner f o r use as a reference. The d e t a i l s of the r e s u l t i n g powder patterns are presented i n Appendix IV. I f the i n t e r f a c i a l l a y e r were a s o l i d s o l u t i o n of C ^ O j i n AI2O3;. the powder p a t t e r n of the l a y e r would be s i m i l a r t o the p a t t e r n of the major components. This was not the case. F i l m s showed strong AI2Q3 l i n e s along w i t h a very weak p a t t e r n of an unknown m a t e r i a l . Although t h i s weak p a t t e r n could not be i d e n t i f i e d , i t was concluded that the i n t e r f a c i a l l a y e r c o n s i s t e d of a compound or compounds and p o s s i b l y a s o l i d s o l u t i o n . The d i f f r a c t i o n p a t t e r n of the unknown m a t e r i a l d i d not correspond e x a c t l y t o any of the recorded patterns f o r the oxides of chromium. C. Thermodynamic C a l c u l a t i o n s . Assuming that the n i c k e l - t i t a n i u m and nickel-chromium a l l o y s are i d e a l s o l u t i o n s (the a c t i v i t y c o e f f i c i e n t s , are u n i t y and the heats of mixing are zero.1,, the G-ibbs adsorption equation can be a p p l i e d to these a l l o y s . ^ The excess surface c o n c e n t r a t i o n of the s o l u t e , T, i s given by: r = -d'y (6) RT d i n c where y i s the surface energy, R i s the gas constant, T i s the absolute temperature and, c i s the bulk c o n c e n t r a t i o n of the s o l u t e . Because small a d d i t i o n s of t i t a n i u m ( l e s s than 10% T i ) and chromium ( l e s s than 19°/> Cr) have no measurable e f f e c t on the l i q u i d surface energy, the excess concentrations of t i t a n i u m and chromium at the l i q u i d - v a p o u r i n t e r f a c e are zero. S i m i l a r a d d i t i o n s do show a marked e f f e c t on the s o l i d - l i q u i d i n t e r f a c i a l energy i n the composition ranges of 1 to 2 percent t i t a n i u m and - 38 - 6 to 12 percent chromium (see Figures 14 and 15). A p p l i c a t i o n of equation (6) i n these ranges i n d i c a t e s excess s o l u t e concentrations of 24.5 x 1 0 1 4 T i atoms per cm.2 and 40.7 x 1 0 1 4 Cr atoms per cm.2 at the i n t e r f a c e (see Appendix V) . These surface excess values r e f e r t o the number of s o l u t e atoms at the i n t e r f a c e above the number i n the l i q u i d a l l o y . The extent to which the excess t i t a n i u m and chromium at the i n t e r f a c e react w i t h the sapphire to form oxides of these metals can be p r e d i c t e d from thermodynamic data."^'"'"^ Although the f r e e energy f o r the r e a c t i o n , AI2O5 + 2 T i — • TigOj + 2 A l , i s p o s i t i v e , the e q u i l i b r i u m constant i s s u f f i c i e n t l y l a r g e that a n o t i c e a b l e r e a c t i o n can be p r e d i c t e d (see Appendix V) . The f r e e energy f o r the r e a c t i o n , AI2O3 + 2 C r — ^ G r ^ O j , + 2 A l , i s s i m i l a r l y p o s i t i v e , but the very low value of the e q u i l i b r i u m constant i n d i c a t e s only a very s l i g h t r e a c t i o n (see Appendix V). - 39 - IV. DISCUSSION AND CONCLUSIONS A. D i s c u s s i o n of R e s u l t s . The surface energy of l i q u i d n i c k e l i s not a f f e c t e d g r e a t l y by a d d i t i o n s of t i t a n i u m and chromium 1 2' 1^ but i t i s markedly changed by the presence of minor s u r f a c e - a c t i v e i m p u r i - ties." 1"^ The surface energies of the three grades of n i c k e l used i n the work v a r i e d as much as 15 percent. The v a r i a t i o n s , however, could not be r e l a t e d to a s p e c i f i c impurity on the b a s i s of the experimental values. On the other hand, the i n t e r f a c i a l energy between n i c k e l a l l o y s and alumina i s a f f e c t e d by the surface c o n d i t i o n of the alumina, the c r y s t a l l o g r a p h i c o r i e n t a t i o n of the alumina c r y s t a l s , and the composition of the metal drop. Because these e f f e c t s are a d d i t i v e , the e f f e c t of a l l o y a d d i t i o n s to n i c k e l was measured by m a i n t a i n i n g constant surface c o n d i t i o n s and c r y s t a l o r i e n t a t i o n . Experimental values showed a decrease i n i n t e r f a c i a l energy w i t h i n c r e a s i n g s o l u t e c o n c e n t r a t i o n . Because the s c a t t e r of the r e s u l t s was so great, e s p e c i a l l y i n the higher a l l o y compositions, d i f f i c u l t i e s i n e s t a b l i s h i n g the curves of best f i t were encountered (see Figures 14 and 15). T h i s s c a t t e r was b e l i e v e d to be caused by inhomogeneous mixing of the metal powders i n the compact. Regardless of the s c a t t e r , however, the i n t e r f a c i a l energies show a marked decrease w i t h i n a very narrow composition range. The decrease i n i n t e r f a c i a l energy w i t h i n c r e a s i n g bulk c o n c e n t r a t i o n of the s o l u t e i n the n i c k e l a l l o y s has been - 40 - a t t r i b u t e d to the s e l e c t i v e a d s o r p t i o n of the s o l u t e atoms at the i n t e r f a c e ' • ? , / . The r a p i d decrease i n i n t e r f a c i a l energy i. above a c r i t i c a l bulk c o n c e n t r a t i o n i s caused by the a d s o r p t i o n of the s o l u t e i n a complete monolayer at the i n t e r f a c e . The constant slope i n a narrow range above the c r i t i c a l c o n c e n t r a t i o n i n d i c a t e s that the i n t e r f a c i a l s o l u t e c o n c e n t r a t i o n remains constant (at the monolayer concentration) throughout the range. At higher concentrations, the i n t e r f a c i a l energy reaches a n e a r l y constant value which presumably corresponds to the m u l t i l a y e r a d s o r p t i o n of s o l u t e atoms. The t h e o r e t i c a l c o n c e n t r a t i o n of t i t a n i u m atoms i n a close-packed plane i s 12 x 1 0 1 4 atoms per cm.2 ^ A p p l i c a t i o n of the G-ibbs' adsorption equation t o the experimental curve f o r the n i c k e l - t i t a n i u m a l l o y s of t h i s i n v e s t i g a t i o n i n d i c a t e s a t i t a n i u m excess c o n c e n t r a t i o n of 24.5 x 1 0 1 4 atoms per cm.2 at the i n t e r - f ace. Although t h i s c oncentration i s p h y s i c a l l y impossible, i t does show that at l e a s t a close-packed monolayer i s completed at 1.1 percent t i t a n i u m (the c o n c e n t r a t i o n at which the constant slope b e g i n s ) . S i m i l a r l y , the experimental value f o r the excess c o n c e n t r a t i o n of chromium i n the constant-slope range (40.7 x 1 0 1 4 Cr atoms per cm.2) i n d i c a t e s that at l e a s t a monolayer of chromium (13 x 1 0 1 4 atoms per cm.2)-'-^ j _ s completed at a p p r o x i - mately 6 percent chromium. Because the experimental excess concentrations disagree w i t h the monolayer concentrations, the t h e o r e t i c a l slopes were c a l c u l a t e d from the Gibbs' equation f o r monolayers of t i t a n i u m and chromium. These slopes have been drawn on the graphs.(Figures 14 and 15) as dotted l i n e s . - 41 - The n o t i c e a b l e d i f f e r e n c e s i n the experimental and t h e o r e t i c a l slopes f o r both t i t a n i u m and chromium could be due to er r o r s i n the experimental values or due t o assumptions r e q u i r e d to apply the theory. The d i f f e r e n c e s could be p a r t i a l l y caused by the s c a t t e r of experimental p o i n t s r e s u l t i n g i n i n c o r r e c t p o s i t i o n i n g of the curve on the graph. Regardless of the disagreement between the experimental and t h e o r e t i c a l s l o p e s , the concentrations at which i n t e r f a c i a l energy decreases r a p i d l y w i t h i n c r e a s i n g s o l u t e c o n c e n t r a t i o n are approximately 1.1 percent t i t a n i u m and 6 percent chromium. A l l X-ray analyses were made on samples c o n t a i n i n g higher s o l u t e concentrations. Thus, s i g n i f i c a n t amounts of adso r p t i o n would be expected i n these samples. The s e l e c t i v e a dsorption of both t i t a n i u m and chromium at the metal-sapphire i n t e r f a c e was d e f i n i t e l y detected i n the X-ray fluorescence analyses. Although p r e c i s e q u a n t i t a t i v e analyses of only a few atom l a y e r s are impossible by t h i s method, the r e s u l t s d i d i n d i c a t e a much higher r a t i o of t i t a n i u m or chromium to n i c k e l on the surface of the sapphire than i n the o r i g i n a l a l l o y . From thermodynamic c a l c u l a t i o n s , a l i m i t e d r e a c t i o n between the excess t i t a n i u m at the i n t e r f a c e and the un d e r l y i n g sapphire i s p r e d i c t e d . By X-ray d i f f r a c t i o n , a r e a c t i o n product was shown to be a-TigO^. The oxygen atoms f o r the r e a c t i o n l i e i n the ^1012) planes of the A l 2 0 j l a t t i c e . The co n c e n t r a t i o n of oxygen atoms i n t h i s plane i s o n e - t h i r t e e n t h the con c e n t r a t i o n - 42 - i n the basal plane (15.5 x lO^- 4 oxygen atoms per cm,2),''''' However, t h i r t e e n (1012") planes are at l e a s t p a r t i a l l y exposed to the surface. The e f f e c t i v e c o n c e n t r a t i o n of oxygen atoms at the surface of a (l012) plane i s , t h e r e f o r e , approximately the same as the con c e n t r a t i o n i n the basal plane. To s a t i s f y the t i t a n i u m - oxygen r a t i o f o r the formation of a - T i 2 0 3 , approximately 10 x 10^-4 t i t a n i u m atoms would tend to concentrate at the i n t e r f a c e . T h i s i s l e s s than a close-packed monolayer. F u r t h e r r e a c t i o n of the t i t a n i u m and alumina would i n v o l v e the d i f f u s i o n of t i t a n i u m atoms from the i n t e r f a c e i n t o the alumina l a t t i c e and the d i f f u s i o n of fr e e d aluminum atoms from the alumina i n t o the metal. The r e a c t i o n would continue u n t i l the a c t i v i t y of the t i t a n i u m i n the metal drop was reduced to a value at which a monolayer of t i t a n i u m would not form, or u n t i l the a c t i v i t y of the aluminum i n the metal drop was high enough f o r a s i g n i f i c a n t b ack-reaction. For bulk concen- t r a t i o n s w e l l above the c r i t i c a l c o n c e n t r a t i o n required f o r the formation of a monolayer, r e a c t i o n l a y e r s s e v e r a l atom l a y e r s i n thick n e s s would be expected t o form. This was the case. The i n t e r f a c i a l l a y e r s of a-TigOj were of v i s i b l e t h i c k n e s s e s . The r e a c t i o n product,o-Ti20^>and the sapphire plaques, C1-AI2O3, have s i m i l a r c r y s t a l l o g r a p h i c p r o p e r t i e s . Both m a t e r i a l s are i n the rhombohedral d i v i s i o n of the hexagonal system and both are i n the R3c 'space group. The u n i t a x i s length and a x i a l angle f o r a-TigOj are a 0 = 5.42 S. and a= 36°32» and f o r a _ A l 2 0 ^ are a Q = 5,12 £ and a = 5 5 ° 1 7 T . Thus, the s t r u c t u r e of a-TigO^ i s compatible w i t h both the sapphire and the adsorbed t i t a n i u m . The adsorbed chromium atoms would s i m i l a r l y be expected - 4} - to produce a s i g n i f i c a n t amount of r e a c t i o n product. Although thermodynamic c a l c u l a t i o n s do not p r e d i c t any more than a very l i m i t e d r e a c t i o n between chromium and alumina, the r e a c t i o n would proceed to approximately the same point as the titanium-alumina r e a c t i o n i f the r e a c t i o n product, aluminum, were again removed from the r e a c t i n g l a y e r . Assuming that the chromium-alumina r e a c t i o n i s one of simple replacement of aluminum i n the alumina l a t t i c e by chromium, the oxide product would form i n a s t r u c t u r e s i m i l a r to alumina. The chromium oxide s t r u c t u r e which c l o s e l y resembles the alumina s t r u c t u r e i s that of CrgO^ (see F i g u r e 21, Appendix I V ) . The CrgO^ would a l s o be compatible w i t h both the alumina l a t t i c e and the adsorbed metal at the i n t e r f a c e . However, the chromium- alumina r e a c t i o n l a y e r was not p o s i t i v e l y i d e n t i f i e d . The r e a c t i o n of n i c k e l - z i r c o n i u m a l l o y s w i t h alumina appeared to be v i o l e n t and an oxide s c a l e formed on the metal drop. Zirconium probably segregated at the i n t e r f a c e and reacted w i t h the alumina as i n the cases of t i t a n i u m and chromium. The oxide product formed by the r e a c t i o n , however, must have been incompatible w i t h the alumina l a t t i c e to such an extent to cause i t to s p a l l o f f the surface of the alumina and gather on the surface of the l i q u i d metal. B. Conclusions. Bond formation between s i n g l e c r y s t a l s of alumina and n i c k e l a l l o y s i n v o l v e s two b a s i c mechanisms - the formation of an excess s o l u t e c o n c e n t r a t i o n at the metal-ceramic i n t e r f a c e , and a r e a c t i o n between these excess atoms and the alumina. The two - 44 - mechanisms were studied w i t h two d i f f e r e n t approaches - measure- ment of i n t e r f a c i a l energies between the metal and ceramic by the s e s s i l e - d r o p method, and a n a l y s i s of the bonded i n t e r f a c e s by X-ray techniques. The decrease i n the metal-alumina i n t e r f a c i a l energy w i t h i n c r e a s i n g concentrations of t i t a n i u m and chromium i n n i c k e l i s a r e s u l t of the s e l e c t i v e adsorption of t i t a n i u m and chromium atoms at the i n t e r f a c e . The most s i g n i f i c a n t decrease occurs when the i n t e r f a c e i s saturated w i t h adsorbed atoms. S a t u r a t i o n occurs at approximately 1.1 percent, t i t a n i u m and 6 percent chromium i n n i c k e l . At higher concentrations of t i t a n i u m and chromium, the i n t e r f a c i a l energy remains approximately constant w i t h a l l o y composition. This i s a t t r i b u t e d t o three-dimensional s a t u r a t i o n of the i n t e r f a c e . X-ray fluorescence analyses of the alumina surfaces and n i c k e l a l l o y s i n d i c a t e d s e m i - q u a n t i t a t i v e l y that t i t a n i u m and chromium segregate at the metal-ceramic i n t e r f a c e . However, the form i n which the adsorbed atoms e x i s t at the i n t e r f a c e cannot be determined by t h i s method. X-ray d i f f r a c t i o n data showed c o n c l u s i v e l y that the adsorbed t i t a n i u m and chromium atoms reacted w i t h the alumina at the i n t e r f a c e to form new compounds. For n i c k e l - t i t a n i u m a l l o y s on alumina, the r e a c t i o n product, a-TigO^, was p o s i t i v e l y i d e n t i f i e d . This compound i s c r y s t a l l o g r a p h i c a l l y compatible w i t h the alumina l a t t i c e and the adsorbed t i t a n i u m atoms at the i n t e r f a c e . From a s i m i l a r study of nickel-chromium on alumina, - 45 - one would expect the formation of C^O^. The r e a c t i o n product, however, could not be i d e n t i f i e d but i t was concluded that compound formation d i d occur at the i n t e r f a c e . With the assumption that the r e a c t i o n s of both t i t a n i u m and chromium w i t h alumina occur by simple replacement of the aluminum, the r e l a t i v e l y t h i c k e x p e r i - mental r e a c t i o n l a y e r s on the alumina surface can be p r e d i c t e d from adsorption c a l c u l a t i o n s . A l i m i t e d study of n i c k e l - z i r c o n i u m a l l o y s on alumina i n d i c a t e d a v i o l e n t r e a c t i o n between the zirconium and alumina. Q u a n t i t a t i v e measurements of i n t e r f a c i a l energies and i d e n t i f i c a t i o n of the r e a c t i o n products were impossible. - 46 - V. RECOMMENDATIONS FOR FURTHER WORK In t h i s i n v e s t i g a t i o n * the mechanisms of bond forma- t i o n were thoroughly i n v e s t i g a t e d f o r only two metal-ceramic systems. As mentioned p r e v i o u s l y , the o r i g i n a l purpose of t h i s i n v e s t i g a t i o n was to study the bond between s i n g l e c r y s t a l s of sapphire and a number of n i c k e l a l l o y s . Because of experimental d i f f i c u l t i e s , a d d i t i o n s of manganese, i r o n , vanadium, molybdenum, g a l l i u m , bismuth, tantalum and columbium were not attempted. With these d i f f i c u l t i e s now e l i m i n a t e d , i t would be i n t e r e s t i n g to complete the study t o e s t a b l i s h the bond mechanisms which are common to a l l s i m i l a r systems. I t appears that X-ray techniques are most promising f o r the development of general r u l e s f o r the s e l e c t i o n of metal-ceramic combinations w i t h optimum p r o p e r t i e s . - 47- APPENDIX I Drying Agents The common dr y i n g agents and t h e i r r e s p e c t i v e d r y i n g powers are l i s t e d i n Table V. S i l i c a g e l , anhydrous CaSC>4 and ?2 05 were used i n t h i s work t o e x t r a c t water from hydrogen gas. TABLE V Drying Agents and Drying Powers M a t e r i a l MgH2Q/liter of gas C U S O 4 (granular) 2.8 C a C l 2 (anhydrous) . 1.5 GaCl2 (fused) 1.25, ZnCl2 (anhydrous) 0.9 B a ( 0 1 0 4 ) 2 (anhydrous) 0.8 NaO'H ( s t i c k s ) 0.35 C a C l 2 (dehyd.) 0.20 GaBr2 0.15 B 2 O 3 0.032 MgC104•3H2° 0.03 S i l i c a g e l 0.014 KOH ( s t i c k s ) 0.008 MgO 0.008 AlpOj 0.005 CaS04 (anhydrous) 0.005 H 2 S O 4 (cone.) 0.005 CaO 0.003 M g ( 0 1 0 4 ) 2 (anhydrous) 0.002 BaO 0.0007 p 2 o 5 r. 0.0002 - 48 - APPENDIX I I C a l c u l a t i o n s on Sessile-Drop Measurements. C a l c u l a t i o n s of surface t e n s i o n s and contact angles can be made from s e s s i l e - d r o p dimensions u s i n g the Bashforth and 4 Adams t a b l e s . These t a b l e s have been derived from the mathematics of the e q u i l i b r i u m f o r c e s of surface t e n s i o n tending t o form a s p h e r i c a l surface and of g r a v i t y tending to f l a t t e n the drop. The methods of c a l c u l a t i o n are presented i n d e t a i l by E l l e f s o n and 5 6 Taylor. ' Sample c a l c u l a t i o n s f o r both methods used i n t h i s i n v e s t i g a t i o n are given below: Example I . For obtuse contact angles^ Consider, the measurements (:see F i g u r e 11) on a s e s s i l e drop of S h e r r i t t #C525 N i . X = . 3 2 0 8 cm. X 1 = .3109 cm. 2 = .2951 cm. z l = . 3 6 0 3 cm. Therefore X = 1 .0871 Z Reference i s now made t o the Bashforth and Adams' t a b l e which r e l a t e s values of 3 and X. These 3 -values are r e l a t e d t o surface Z t e n s i o n by the f o l l o w i n g equation: ; p = g b 2 (7) ' • y IN . where g = the a c c e l e r a t i o n due t o g r a v i t y 5 = the d e n s i t y of the l i q u i d b = the radius of curvature o f the apex of the drop y - the surface t e n s i o n of the l i q u i d - 49 - By i n t e r p o l a t i o n , the f o l l o w i n g value i s obtained: 0 = 0.4723 By using t h i s 3-value i n reference t o a second Table r e l a t i n g to X and Z at 0 - 90 degrees, i t i s p o s s i b l e t o o b t a i n the b b f o l l o w i n g values by i n t e r p o l a t i o n : X = 0.93604, t h e r e f o r e b T = 0.3427 cm. b x and Z = 0.86147, therefore b z = 0.3425- cm. b z The mean value of the radius of curvature of the apex of the drop i s , t h e r e f o r e b = 0.3426 cm. The surface t e n s i o n can then be c a l c u l a t e d from Equation (7): v = 1848 dynes per cm.2 where the density of molten nickel"'"? i s 7.6 gms, per cm.^ and the a c c e l e r a t i o n due to g r a v i t y i s 980.7 cm. per s e c . 2 . To o b t a i n the contact angle, the reverse.procedure i s used: ]_ X = 0.34-26 Z = 0.9074 b b R e f e r r i n g to the second Table again, the angles given by the X 1 and Z 1 values at p = 0.4734 can be obtained by i n t e r p o l a t i o n : b b 0 X •= 106.2° 0 Z = 105.2° The mean angle, 105.7 , i s equal to 0, the contact angle. A c a l c u l a t e d value of the i n t e r f a c i a l energy can then be obtained from Equation ( 1 ) , i f the surface energy of the s o l i d ceramic i s known. Example I I . For acute contact angles. -50- 6 Assuming that the drop i s small enough that the drop shape i s a t r u e s p h e r i c a l segment, the contact angle i s given by (see Fig u r e 11): 9 = 2 t a n _ 1 Z 1 ' (8) XT Consider the experimental values f o r Ni + 1?% Cr on sapphire: X 1 = 0.317 cms. Z 1 = 0.276 cms. From Equation (8), t h e r e f o r e , 0 = 82.1° - 51 - APPENDIX I I I D e t a i l e d C a l c u l a t i o n s on a-Ti203 Powder P a t t e r n s F i g u r e 17 i s a photographic p r i n t of a t y p i c a l X-ray powder p a t t e r n produced from a sample of i n t e r f a c i a l m a t e r i a l from the n i c k e l - t i t a n i u m experiments. Table VI shows the Figu r e 17. X-ray powder p a t t e r n No. 14-58 of i n t e r f a c i a l m a t e r i a l i n N i - T i experiments. p r e l i m i n a r y data obtained from the p a t t e r n . The t a b u l a t e d d- valuefland l i n e i n t e n s i t i e s compare to those recorded i n the A.S.T.M. X-ray Card Index f o r alpha t i t a n i u m sesqui oxide (see Figure 18). However, the data on t h i s card i s incomplete. For p o s i t i v e i d e n t i f i c a t i o n of the i n t e r f a c i a l m a t e r i a l , f u r t h e r c a l c u l a t i o n s on the observed data were necessary. These c a l c u - l a t i o n s are o u t l i n e d below: From Figure 18, the rhombohedral i n d i c e s f o r a-Ti203 are: a 0 = 5.42 £ and a = 56°32T -52- TABLE VI Data from F i l m No.1458 L i n e No. I / I l dI(obs'd.) .Line No. I / I l dI\(obs'd.) 1 50 3.728 17 5 .980 2 60 2.687 18 5 .973 "3 60 2.564 19 20 .915 4 40 2.232 20 20 .901 5 40 1.853 21 20 .895 6 100 1.694 22 5 .863 7 2 1.634 23 5 .859 8 .40 1.504 24 3 .836 9 40 1.482 25 10 .816 10 20 1.298 26 5 .795 n 10 1.284 27 5 .793 12 8 1.240 28 3 .781 13 5 1.216 29 3 .771 14 30 1.159 15 30 1.121 16 30 1 .059 376B d 2-1367 1.68 0 . 9 0 1.B5 2. fa a - T i 2 0 3 Ifli 2 - l » 9 100 100 70 50 ALPHA T ITANIUM SESOUI OXIDE Rad. A 0 . 7 0 9 Filter D U . • Cut off Coll I/T-i d corr. ib«.T Ret F . H A L L A , Z . *IJOF<G. CHEM. 1 8 4 , 123 ( 1 9 2 9 ) Sy». HE)iAr,onAu(flr.C/M0. Div ) S.G. D§D R3c U 5 .42 b, c. A C a 56032* fi y Z 2 Ret WV49 to ngjt f Y S ign J V D 4 . 6 mp 1900 Color VELVET BLACK Ret C . C . , HCP WY48 GIVES . = 5 . 3 7 , a = 5 6 ° ' d A 2 . 68 2 . £-2 2.21 1.85 1.69 1.68 1.62 1.49 1.47 1.37 1.23 1.16 1.12 1.06 0 . 9 B F i g u r e 18. A.S.T.M. Card No. 2-1359 f o r a - T i 2 0 3 . - 53 - These rhombohedral values can he expressed i n terms of hexagonal values by the use of the f o l l o w i n g equations 3 S i n * = — = , 2 2)3 + ( c / a ) 2 ' (9) (10) a 0 - l/3a2 + c 2 The equivalent hexagonal u n i t c e l l dimensions are: a = 5.14 A and c = 13. !6l A Therefore, c/a = 2.648 and l o g c/a = 0.4229. From the Bunn chart f o r hexagonal m a t e r i a l s , a l l the p o s s i b l e low-index planes f o r ct-Ti203 are obtained. Only the planes w i t h i n d i c e s that obey the e x t i n c t i o n r u l e s f o r a-Ti,,^ (see Table VII) can d i f f r a c t X-rays. These i n d i c e s can then be used to o b t a i n calcu- TABLE V I I E x t i n c t i o n R u l e s ^ f o r Space Group R^c Plane Required (n=integer) i h k i i h + i + £ must equal 3n hOhi* h + i + Jl = 3n and £ = 2n htiZKl h + i + t = 3n 0002 Sum 6 ! l a t e d d-values f o r the r e s p e c t i v e planes from the f o l l o w i n g 35 (11) formula: 1 = d 2 ? a J These c a l c u l a t e d d-values should --.gree w i t h the observed d-values 1 = 4 h 2 + k 2 + X 2 + £ 2 — CT obtained from the powder p a t t e r n i f the specimen i s a - T i 2 0 3 (see Table V I I I ) ; . - 5 k - TABLE V I I I C a l c u l a t e d and Observed Data f o r F i l m No. 1458 dA(obs'd.) I / I l Rhomb. Index dA(calc.!d.) 3.728 50 110 3.725 2.687 60 211 2. 700 2.564 60 101 2.569 2.232 40 210 2.236 1.853 40 220 1.862 1.694 100 321 1.700 1.634 2 211 1.633 1.504 40 310 1.507 1.482 40 2TT 1.483 1.298 20 433 20I 432 1.302 1 .304 1.284 10 1.285 1.240 8 411 1.241 1.216 5 301 1.215 1.159 30 442 321 1.161 l .160 1.121 30 410 420 1.124 1 .118 1.059 30 532 400 1.058 1 .057 .980 5 531 411 .979 .978 .973 5 312 .971 .915 20 541 .914 .901 20 633 .901 .895 20 510 .893 .863 5 332 .861 .859 5 654 301 .856 .856 .836 3 655 402 .836 .835 .816 10 422 .816 .795 5 664 .795 .793 5 323 .794 .781 3 501 .778 .771 3 531 .772 The hexagonal i n d i c e s (hk£) used i n the above c a l c u l a t i o n s can be corresponding rhombohedral i n d i c e s (HKL). by the f o l l o w i n g formulae:^ 4 H = 1/3 (21V + k + i) (12) K = 1/3 (-h + k + 2) (13) L = 1/3(-h - 2k + X) (14) The rhombohedral i n d i c e s have been l i s t e d i n Table V I I I . Thus, the powder p a t t e r n has been completely indexed and the m a t e r i a l has been i d e n t i f i e d as ajTjigQ^. APPENDIX IV. X-ray D i f f r a c t i o n Data from Ni-Cr Experiments. -55- Photographic p r i n t s of X-ray powder patterns produced by samples of sapphire, sapphire mixed w i t h the i n t e r f a c i a l l a y e r of the Ni-Cr experiments, and a s o l i d s o l u t i o n of GrgO^ i n AI2O3 are shown i n F i g u r e 19. a l l — F i g u r e 19. X-ray powder p a t t e r n s , Nos. 1489, 1485 and 1493, (Top - Pure AI2O3, Middle - AI2O3 and unknown, Bottom - S o l i d s o l u t i o n of Cr203 i n AI2O3). The observed X-ray data f o r the mixture of the unknown m a t e r i a l and sapphire are l i s t e d i n Table IX. The reported d- values f o r AI2O3 given i n the A.S.T.M, Card Index are shown i n Figure 20. The d-values i n Table IX, which do not appear i n Figure 20, are undoubtedly due to the presence of the i n t e r f a c i a l l a y e r i n the specimen. These d-values have been s t a r r e d {&) i n - 56 - TABLE IX Data from F i l m No. 1485 Li n e No. I / I l dA(obs8d.) L i n e No. I / I l dA(obs'd. ) 1 20 4.244 4 25 30 1.187 2 20 3.871 A 26 30 1.145 3 80 3.474 27 30 1.122 4 50 * 3.342 & 28 30 1.096 5 20 2.801 i t 29 30 1.076 6 10 2.607 & 30 50 1.043 7 100 2.545 . 31 10 1.016 8 80 2.376 32 40 .998 9 . 10 2.311 & 33 10 .980 10 100 2.078 34 20 .935 n 10 1.953 & 35 10 .917 12 10 1.926 4 36 40 .908 13 ' 10 1.814 £ 37 30 .899 14 10 1.766 £ 38 20 .881 i X5 8o 1.737 39 10 .869 16 100 1.597 40 30 .858 17 20 1.541 41 20 .850 18 40 1.507 42 10 .846 £ 19 80 1.400 43 50 .830 .814 20 90 1.372 44 10 21 .. 10 1.335 & 45 30 .807 22 10 1.272 46 20 .799 A 23 10 1.254 & 47 20 .797 & 24 40 1.236 48 20 .793 & s-rrn? d 2 . 0 9 2 . 5 5 1 . 6 0 3 . 4 7 9 O - A L 2 0 3 * I/I, 100 92 81 74 ALPHA ALUMINUM Ox i r e CORUNDUM R«d.Cu x 1.5405 Duu Cut of! I/I, dcorr.»b*.? ReL SBANSOU AND F U ' A T , IBS CIRCULAR 53' S y i . HEXAGONAL U 4.758 b, S.C. D 3 n - R3c SAMPLE FROM MALLINCKRODT CHEW. WORKS. S P E C T . ANAL. I <0.w K . N A . S I I <0.0ll C A , C U , F E , M G , P O ; <0.00u S . C R . L I . M N . N I . X-RAY PATTERM AT 26°C REPLACES 1-1243, 1-1296, 2-1227. 2-1373. 3-1033 2.379 2 . ICS 2 . C S S 1.740 l . f O l 1.54C 1.510 1.239 1.1B98 l . K O l 1.1470 1.1382 1.12! 1.09BB 1.0B31 d A 1.0791 l . C 4 2 f i 1.0175 0.5976 .981? .9345 .9178 .907C .9052 .8991 L.0.14 41C F i g u r e 20. A.S.T.M. Card No.5-0712 f o r a-Al 203 - 57 - Table IX. The same l i n e s are missing from the GrgO^-AlgO^ s o l i d s o l u t i o n p a t t e r n except f o r l i n e No.4. Hence, the e x t r a l i n e s are not due to the presence of a CrgC^-A^Qj s o l u t i o n . \ A compound w i t h a d e f i n i t e c r y s t a l s t r u c t u r e must form at the i n t e r f a c e . From previous experience w i t h the N i - T i a l l o y s , the expected compound would be C^O^ because i t has s i m i l a r c r y s t a l l o - graphic p r o p e r t i e s . However, the e x t r a l i n e s do not correspond to l i n e s of C^Oj (see F i g u r e 21). Other m a t e r i a l s , e s p e c i a l l y the 6-0504 d 2.67 2.46 1.67 3.633 C R a 0 3 * I/It 100 96 90 74 CHROMIUM ( H I ) O X I D E Rid. Cu * 1.5405 Filter DU. Cut off ColL I/Ii d COTT. *ba.T RetS.ANGON ET A L . , NBS C I R C U L A R 539 V O L . S(195!I Sy*.HEK AGONAL S.G. DS„ - R3c a, 4.954 b, c. 13.584 A C a P V Z 6 Ret I B I D . l a nwfi Sign 2V Dx5.23 mp Color Ret SAMPLE TROU JOHNSON, MATTHEY AND CO. L T D . S P E C T . A N A L . ' < 0 . 0 0 l % C A , Mat * . 0 . 0 0 0 l % Si Cu. X-RAY PATTERN AT 26°C. F E J O S STRUCTURE T Y P E . R E P L A C E S 1-1294. 2-1362. 3-1124. 4-0765 d A I/I. hkl d A I/Ii hkl 3.633 74 012 0.9370 12 410 2.666 100 104 .8957 14 1.3.10 2.480 96 110 .8883 7 3.0.12 2.264 12 006 .8658 23 416 2. 176 38 113 .8425 8 4.0.10 2.048 9 202 .8331 11 1.0.16 1.8156 39 024 .8263 9 330 1.672 90 116 .7977 15 3.2.10 1.579 13 122 1.465 25 214 1.4314 40 300 1.2961 20 1.0.10 1.2398 17 220 X.2101 7 306 1.1731 14 128,312 1.1486 10 0.2.10 1.1239 10 134 1.0874 17 226 1.0422 16 2.1.10 0.9462 13 324 F i g u r e 21. A.S.T.M. Card No. 6-0^04 f o r C ^ O j oxides of chromium, were s i m i l a r l y i n v e s t i g a t e d but a l l the e x t r a l i n e s could not be i d e n t i f i e d w i t h any one m a t e r i a l . This could be due to the low i n t e n s i t y of the l i n e s and the r e s u l t i n g e r r o r s i n c a l c u l a t i o n s of d-values or to the presence of more than one compound i n the unknown m a t e r i a l . - 58 - APPENDIX V Thermodynamic C a l c u l a t i o n s . (a) A p p l i c a t i o n of Gibbs* adsorption equation. From F i g u r e 14, At, 1.3% T i , y S L = 1060 ergs per cm.2 and at 2.0% T i , r S L = 800 ergs per cm. 2 Therefore, d Xn c = 0.433 and d = -260 ergs per cm.2 or -621 x l O - ^ gram-calories per cm.2. Applying the Gibbs' equation at T = 1773°K, T = -(-621 x I0~Q) moles per cm,2 (2)(1773)(0.433) = (4.05 x 10" 9j(6.06 x 10 2^) T i atoms per cm.' 14 ? = 24.5 x 10 T i atoms per cm. From F i g u r e 15 . At 7.4% Cr, y S L = 1240 ergs cm.2 and at 10.0% Cr, y g ^ = 940 ergs per cm. 2 Therefore, d £n c = 0.301 and dyg^ = -300 ergs per cm. or -717 x 10~8 gram-calories per cm.2 A p p l y i n g the Gibbs' equation at T = 1773°5, ' ' r = -(-717 x 1Q-8) moles per cm.2 2(1773)(0.301) = (6.71 x 10 - 9)(6.06 x 10 2^) Cr atoms per = 40.7 x 10 Cr atoms per cm. cm, - 59 - (b) Reaction C a l c u l a t i o n s . For the T i - A l 2 0 3 r e a c t i o n at 1500°C. 2 A I 2 O 3 + 4 T i =2. T i 2 0 3 + 4 A l 3 3 3 3 ^ F r x n = (-163) - (-176) k c a l per mole O 2 = 13,000 c a l per mole 0 2 i n K = - ̂ F = -13,000 = -3.67 RT 2(1773) Therefore, the e q u i l i b r i u m constant, K = 2.5 x 10" For the Cr-AlgO^ r e a c t i o n at 1500°C. 2 A1 20* + 4 Cr = 2 Cr 203 + 4 A l 3 3 3 3 A F r x n = (-103) - (-176) k c a l per mole 0 2 = 73,000 c a l per mole 0 2 J-n K = - A F = -73,000 » -20.6 RT 2(1773) Therefore, the e q u i l i b r i u m constant, K = 1 x 10~ 9 APPENDIX VI Evaluation of Experimental Errors in Sessile-drop Calculations, rSL = ySV " yLV C 0 S 9 (1) Error in temperature = ±5°C Therefore, resulting experimental errors are: y 3 V = ±0.5 ergs/cm.2 y = ±5.0 ergs/cm.2 LV and, y = ±5.5 ergs/cm.2 SL (2) Error in sessile-drop measurements = ±.00254 cm. Resulting experimental errors are: y = ± 2 ergs/cm.2 J-iV 9 = ±0.1° 31 Error factor for drop size-' = 10 Therefore, y = ±20 ergs/cm.2 LV 0 = ±1°C and, y = ±40 ergs/cm. (3) Total resulting experimental error in y = ±45.5 ergs/cm.2 SL - 6o - BIBLIOGRAPHY 1. Van Houten, G.R.; Ceramic B u l l e t i n , 18 {1939) 301. 2. Kingery, W.D. ; J . Am. Ceram. S o c , 3_6 (1933) 362. 3. W i l l i a m s , L.S. and Murray, P.; M e t a l l u r g i a , 49_ (1954) 210. 4. Bashforth, F. and Adams. S.C.; "An Attempt to Test the Theories of C a p i l l a r i t y " , Cambridge U n i v e r s i t y Press (1883). 3. E l l e f s o n , B.S. and T a y l o r , N.W.; J.Am.Ceram.Soc,, 21,(1938) 193. 6. E l l e f s o n , B.S. and T a y l o r , N.W.; J.Am.Ceram.Soc., 21 (1938) 205. 7. Kingery, W.D. and Humenik, M. ; J . Phys. Chem., 5_1 (1953) 359. 8. Adam, N.K.; "The Physics and Chemistry of Surfaces", Oxford U n i v e r s i t y P r e s s , London (1941). 9. Economos, G. and Kingery, W.D. ; J . Am. Ceram. S o c , 3Ji (1953) 403. 10. 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Trans., 19J. (1953) 643. 32. R u s s e l l , A.S.; Alcoa Research L a b o r a t o r i e s T e c h n i c a l Paper No. 10, (1956). 33. B e s t u l , A.B. ; J . Am. Ceram. S o c , 42 (1959) 236, 34. Bewer, J.H.; Bureau of Standards, J . Research, 2 (1934). 35. C u l l i t y , B.D.; "X-ray D i f f r a c t i o n " , Adison Wesley (1956). 36. Donnay, J.D.H. and Harker, D.; N a t u r a l i s t e Canadien, 67 (1940) 33.

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