The use of MgO as a densification aid for alpha-SiC

The suitability of MgO as a densification aid for alpha-SiC has been invest igated. Samples of SiC containing additions of MgO, both alone and in combi nation with Al2O3 and Y2O3, have been hot pressed at temperatures between 1 500 and 1900 degrees C and pressureless sintered at temperatures up to 2000 degrees C. The MgO reacts with surface SiO2 from the SiC grains to form a liquid phase which promotes densification by particle rearrangement and sol ution-reprecipitation processes. With combined additions of MgO, Al2O3 and Y2O3, a eutectic in the system MgO-Al2O3-Y2O3 allows extensive liquid forma tion at similar to 1700 degrees C independent of the SiO2 content of the Si C powder, enabling efficient densification by hot-pressing. Volatilisation due to reactions between the MgO and the SiC or with the furnace environmen t, however, oppose densification by pressureless sintering, and must be com pensated for by the use of Mg-containing powder beds. (C) 1999 Elsevier Sci ence Ltd. All rights reserved.