Yttrium α‐Sialon Ceramics by Hot Isostatic Pressing and Post‐Hot Isostatic Pressing

Dense α‐sialon materials were produced by hot isostatic pressing (HIP) and post‐hot isostatic pressing (post‐HIP) using compositions with the formula Y x (Si 12–4.5 x , Al 4.5 x )‐(O 1.5 x ,N 16–1.5 x ) with 0.1 ≤ x ≤ 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 β‐sialon ( x = 0.1) to large amounts of α‐sialon ( x = 0.4) and increasing amounts of mellilite and sialon polytypoids ( x = 0.8). Samples HIPed at 1600°C for 2 h contained unreacted α‐silicon nitride, while those HIPed at 1750°C for 1 h did not. This could be due to the fact that the time is to short to achieve equilibrium or that the high pressure (200 MPa) prohibits α‐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. Microstructrual analyses showed that the post‐HIPed samples had the highest α‐sialon content. A higher amount of α‐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.