Glassy phase influence on the optical finishing of sintered silicon carbide with Al2O3 and Y2O3

Developing low-roughness silicon carbide (SiC) mirror surfaces for aerospace applications is challenging due to their extreme hardness and brittle nature. In this work, liquid phase sintered silicon carbide (LPS-SiC) substrates were machined at different feed rates. The influence of the glassy phase on the final finishing process was evaluated. An innovative thermo-chemical etching was proposed to evince the glassy phase. X-ray diffraction (XRD) analysis revealed the change of crystal structure from beta-SiC to alpha-SiC in addition to the presence of YAP and YAG phases, constituents of the glassy phase. Vickers microhardness and scratch tests simulating the action of a single diamond grain on the surface of the substrates with and without the glassy phase showed that the hardness of the SiC grain and its fracture resistance exceeded the values obtained for the substrate with the glassy phase by 110% and 170% respectively. Reducing the feed rate decreased the damage to the surface of the substrate with the glassy phase by 25% and to the SiC matrix by 90%.