Sintering and oxidation resistance of compositions in the SiC-La sub 2 O sub 3 -Al sub 2 O sub 3 -Cr sub 2 O sub 3 system. I. Pressureless sintering of compositions in the SiC-La sub 2 O sub 3 -Al sub 2 O sub 3 -Cr sub 2 O sub 3 system at low temperatures

SiC is known to be a material with high-temperature strength and oxidation resistance. However, the covalent bonding of SiC results in a brittle nature and in poor sintering at temperatures lower than 2000C under normal presure. Dense ceramic bodies without using HIP can be obtained only at about 2100-2200C and with sintering aids. The groups of oxides which could be probable candidates for SiC sintering additives were theoretically defined by Negita on the base of thermodynamics of reactions of oxides with SiC. Sintering of materials, which were prepared from beta -SiC and LaCrO sub 3 with Al sub 2 O sub 3 , beta -SiC and LaAlO sub 3 , beta -SiC and LaAlO sub 3 with Cr sub 2 O sub 3 and Al sub 2 O sub 3 via heating at 1700 deg C in graphite powder, was studied. Significant densification of material was achieved in SiC-LaCrO sub 3 -Al sub 2 O sub 3 composition due to a formation of liquid phase. The formation of liquid phase is considered to be promoted by a reaction of chromium oxide with starting components. SiC-LaCrO sub 3 -Al sub 2 O sub 3 compositions were sintered to approx90% of theoretical density at 1700 deg C. The material had a structure comprising a matrix made up of SiC grain with tiny LaAlO sub 3 crystals and 2-4 mu m inclusions of chromium carbide. As result, the material showed improved fracture toughness of 5 MPa m exp 1/2 compared to that of SiC.