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Aluminum-silicon carbide composite coatings by plasma spraying Ghosh, Kunal


The use of aluminum in the automobile engines and other critical parts require a superior surface property of the same. This has led to the development of plasma sprayable surface coatings for the components. To impart the maximum bonding strength, along with hardness to the coatings an aluminum based composite (Al-SiC) was found to be the most suitable. The presence of a hard ceramic second phase within a soft metallic matrix greatly improves the wear resistance of the composite material. The powders for spraying were prepared by mechanical agglomeration of 6061 Al alloy (particle size between 40 and 60 um) with fine SiC particles ( « 8 um) by using high energy vibratory mills. The concentration of SiC was varied from 20-75 vol%, the balance being the matrix A l alloy. The size of the reinforcement was varied from 8 to 37 um in the Al-50vol%SiC composite coatings. A Process Control Agent (PCA) was used to modify the morphology of the powders during the process of mechanical alloying. Mechanical alloying produced composite powders in a size range between 40 and 120 u.m with the SiC phase uniformly dispersed within the matrix. The powders used for spraying were fractionated between the size range of 44 and 149 um by sieving. The powders were sprayed using two types of axial feed plasma torches. Coatings were sprayed on mild steel coupons, rods and thin foils of A l , Ni, plain carbon steel and stainless steel, which were used for conducting tests to assess the physical properties of the coatings. The cross sections of the coatings sprayed on the coupons were observed under an SEM and optical microscope. The hardness, porosity and SiC distribution of the coatings were assessed on these cross sections. The coatings were tested for different physical and mechanical properties like adhesion and wear strength. Adhesion was tested on the mild steel rods using the standard ASTM C633 pull tests but the results were mostly inconclusive. Adhesion strength on the foils was also measured by peel tests which is a modification of the ASTM D-3167 tests. The coatings showed high adhesion strength compared to the other commercially available coatings reported in a recent work [40]. Adhesion strength was found to decrease with the increase in the SiC content and decrease in SiC particle sizes. Erosive wear of the coatings was assessed using a dry erosion test which is a modification of the ASTM G76-83 test. The increase in the SiC content and decrease in the reinforcing particle size improved the wear resistance of the coatings. The abrasive wear resistance was found to improve with the increase in SiC particle size and also with the SiC content in the composite powders (or coatings).

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