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Process engineering of thick dielectric films by Chemically Bonded Composite Sol-Gel Kim, Hyungkeun

Abstract

This research explores new processing methods to decrease residual stress in ceramic films on metallic substrates, and thus to prevent large-scale cracking in the films. The system of particular concern is alumina-based ceramic coating on aluminum alloy, wherein coefficient of thermal expansion (CTE) of the ceramic is about 3x smaller than that of the alloy. The specific goal was to achieve relatively thick (~0.2mm) and substantially low density of cracks in dielectric films of alumina-based ceramic on aluminum alloy AA5052, by Chemically Bonded Composite Sol-Gel (CB-CSG) process. The principal strategies undertaken in the materials process engineering involved: (i) multi-layer film deposition - to introduce the intermediate steps of stress relaxation; (ii) composite sol-gel slurries with bi-modal particle size distribution - to decrease the overall process temperature, and to decrease film strain during thermal treatment as well as to increase the density and stability of the slurries; (iii) chemical bonding of the film through phosphating - to further decrease the process temperature to the level of 300°C; and (iv) introduction of organic-phase (citric acid) derived bond coats at the interface between the AA5052 substrate and the ceramic coating -to achieve residual stress relaxation through viscoelastic deformation of the bond coat. The coatings were processed through spray deposition of consecutive -40 µm thick layers, heat-treated at 300°C for 10 min after each deposition. Two size fractions of alumina powders (average size of 0.5 µm of "fine" and 3 µm of "coarse") were used in formulation of the Composite Sol-Gel (CSG) slurry, and the fine/coarse particle content was optimized based on slurry viscosity and stability, as well as properties of the final coating. The coatings were characterized for microstructure, residual stresses and dielectric strength, as a function of the process parameters. The most important finding of this work is that it is possible to deposit thick ceramic films on aluminum alloy substrates, if all four processing strategies listed above are implemented simultaneously. In particular, the citric acid - derived organo-ceramic bond coats seem to play an important role in relaxing residual stresses resulting from differential thermal contraction and expansion. It is concluded that the viscoelastically deforming organo-ceramic bond coat helps to relax residual stresses in the coating layers due to differential thermal contraction/expansion, and thus allows deposition of films of up to 200 µm thickness. Dielectric strength of the CB-CSG alumina coated AA5052 aluminum reached a maximum of 15±1 kV/mm for the first layer, and subsequently decreased to 10.5±1 kV/mm. It is believed that this decrease in the dielectric strength after the first layer is caused by increased density of cracks in the coating, as evidenced by decrease of the residual stress in the coatings.

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