Thick protective UHTC coatings for SiC-based structures: Process establishment

A new process to form thick and dense ultra-high-temperature ceramic (UHTC) composite coatings over SiC surfaces is described. Coatings of ZrB2/SiC/(ZrC) thicker than 100 mu m are formed by a reaction-bonded SiC (RBSC) approach based on Si infiltration into ZrB2/C preform coating. The residual Si, typically found in RBSC, can be eliminated efficiently to provide a coating material that performs at temperatures above 1500 degrees C. The process is performed at 1500 degrees C in Ar at ambient pressure. The interface between the in situ formed SiC and the Zr phases is very tight, as is the interface with the substrate. The ZrB2 particles used in this process are rearranged in their morphology and an additional new phase containing Zr-C is formed. The coatings exhibit excellent integrity, hardness, and bonding to the tested substrates. A preliminary oxidation study indicates good protection of substrates at 1500 degrees C under both passive and active oxidation conditions, provided that the coatings have sufficient thickness.