Hot-pressing of zirconium diboride-molybdenum disiliside mixtures

Powder mixtures of zirconium diboride and molybdenum disiliside were hot-pressed at the temperatures ranging from 1500° to 2200°C in graphite dies under the pressure of 200kg/cm2 in order to improve densifying and oxidation resistance properties. Pressure was applied through out the experiment and shrinkage of the compact was measured by a dilatometric method during hot-pressing at the temperatures below 2000°C.Final density was measured and density change of the compact during hot-pressing was calculated. Microstrctures were also observed with microscope and electron micro-probe. The compacts were heated in air at the temperatures ranging from 860° to 1600°C and weight changes were measured to estimate the oxidation resistance of the compacts.Densification of zirconium boride was strongly accelerated by addition of small amount of molybdenum siliside. No other compound except zirconium boride and molybdenum siliside was detected in the compacts by X-ray diffraction. Wetting of molybdenum siliside to zirconium boride was considered to be good by observation of the microstructures.When the densification data were processed with Murray's equation for hot-pressing ceramics based on plastic flow mechanism, fairly good linearities were observed in the relations between log (1-ρ) and t expect early period. From these relations, viscosities of the compacts were able to be calculated and it was found that the viscosity decreased with increasing content of molybdenum siliside. The densification data were also processed with the rate equations based on stress enhanced diffusion mechanism. It was found that there were two steps in the densification process. First step of the densification was considered to be a deforming process of the particles at the contact area between them. This step corresponds with the early period in Murray's relations. Although second step which corresponds with the linear period in Murray's relations was not decided to be either plastic flow or stress enhanced diffusion mechanism, this step was considered to be a diffusion process. For the compacts containing molybdenum siliside, these steps were not able to be distinguished clearly.The compacts showed good oxidation resistance with increasing content of molybdenum siliside.