Effect of the machining method on the catalycity and emissivity of ZrB2 and ZrB2-HfB2-based ceramics

The emissivity and the catalytic efficiency related to atomic oxygen recombination were investigated experimentally in the range 1000-2000 K for ZrB2 and ZrB2-HfB2-based ceramics. In order to evaluate the effect of the machining method, two series of samples, one prepared by electrical discharge machining and the other machined by diamond-loaded tools, were tested. High emissivity (about 0.7 at 1700 K) and low recombination coefficients (on average 0.08 at 1800 K) were found for all the materials. The experimental data showed an effect of the surface machining on the catalytic behavior only on the ZrB2-based composite; conversely, small variations were found in the recombination coefficients of ZrB2-HfB2-based samples for the different machining processes. The surface finish affected the emissivity at lower temperatures in both compositions, with the effect becoming negligible at temperatures above 1500 K.