PREPARATION OF CERAMIC COATINGS FROM PRE-CERAMIC PRECURSORS .2. SIC ON METAL SUBSTRATES

SiC-coatings derived from pyrolysis of polycarbosilane layers on stainless steel and mild steel substrates have been studied using a combination of scanning electron microscopy (SEM), energy dispersive X-ray analysis, X-ray photoelectron spectroscopy and ultramicrohardness techniques. The coatings on mild steel plates at firing temperatures of 700 degrees C are cracked but uncracked SiC coatings can be formed on stainless steel substrates at 700-800 degrees C. X-ray photoelectron studies show that these coatings are covered with a layer of SiO2, and contain graphitic carbon, while Rutherford backscattering studies indicate inhomogeneities in the coating layer due to mixing of substrate and coating components at the interface between the two. Ultramicrohardness results indicate the SiC/stainless steel coatings formed at 800 degrees C are softer than the equivalent SiC coatings formed on alumina substrates at 1100 degrees C. Above 800 degrees C, a possible combination of both thermal expansion mismatch and CrN formation, which causes the growth of chromium-rich nodules in the stainless steel, serve to disrupt and ultimately destroy the coherence of the SiC coatings. The use of sol-gel-derived SiO, coatings as a barrier does not prevent the destruction of the SiC coating by this mechanism,