Composite Sol-Gel Ceramic Coatings

Abstract

Apart from microelectronics where sub-micron coatings are sufficient, the conventional sol get coating method has never fully reached its potential. One of the reasons is that the conventional sol-gel films tend to crack when a certain thickness is reached. This is primarily because sol get films shrink significantly during firing and this problem is reduced in thinner layers. The large strain of the conventional sol-gel films upon heat treatment and densification can be prevented by dispersing in the sol get matrix particulates of calcined ceramics. The composite sol-gel coatings (CSG) are produced by dispersing calcined ceramics in the sol and hydrolising the mixture into a composite gel. The resulting slurry can be applied onto a substrate and heat treated to full density by methods similar to the conventional sol gel. The preliminary results indicate, that high density composite coatings can be obtained for variety of matrix/filler combinations, at relatively low heat treatment temperature of 500 - 1000C. For the first time therefore the possibility opened up for deposition of protective ceramic coating in a relatively fast and economic non-line-of-sight process. The CSG technique offers the primary advantages of the classical sol get, i.e. ease of application, low - temperature sinterablity and good adhesion due to formation of chemical bonds with the substrate. Simultaneously, CSG avoids the biggest disadvantage of the classical sol - gel: large strains upon densification , leading to large stresses and cracking of layers thicker than few microns.