YTTRIUM ALPHA-SIALON CERAMICS BY HOT ISOSTATIC PRESSING AND POST-HOT ISOSTATIC PRESSING

Dense alpha-sialon materials were produced by hot isostatic pressing (HIP) and post-hot isostatic pressing (post-HIP) using compositions with the formula Yx(Si12-4.5x,Al4.5x)-(O1.5x,N16-1.5x) with 0.1 less-than-or-equal-to x less-than-or-equal-to 0.9 and with the same compositions with extra additions of yttria and aluminum nitride. X-ray diffraction analyses show how the phase content changes from large amounts of beta-sialon (x = 0.1) to large amounts of alpha-sialon (x = 0.4) and increasing amounts of mellilite and sialon polytypoids (x = 0.8). Samples HIPed at 1600-degrees-C for 2 h contained unreacted alpha-silicon nitride, while those HIPed at 1750-degrees-C for 1 h did not. This could be due to the fact that the time is too short to achieve equilibrium or that the high pressure (200 MPa) prohibits alpha-sialon formation. Sintering at atmospheric pressure leads to open porosity for all compositions except those with excess yttria. Therefore, only samples with excess yttria were post-HIPed. Microstructural analyses showed that the post-HIPed samples had the highest alpha-sialon content. A higher amount of alpha-sialon and subsequently a lower amount of intergranular phase were detected at x = 0.3 and x = 0.4 in the post-HIPed samples in comparison to the HIPed. The hardness (HV10) and fracture toughness (K(IC)) did not differ significantly between HIPed and post-HIPed materials but vary with different x values due to different phase contents. Measurements of cell parameters for all compositions show a continuous increase with increasing x value which is enhanced by high pressure at high x values.