DETAILED DESCRIPTION OF THE INVENTION Various transition metal oxides in combination with aluminum oxide form liquids upon equilibrium heating at temperatures in the range of about 1500.
degree.
C.
to about 2000.
degree.
C.
Transition metal oxides are defined, for the purpose of this invention, as oxides of the elements which have unfulfilled d-orbitals.
These elements are: Sc, Ac, Ti, V, Cr, Mn, Fe, Co, Ni, Os, Zr, Nb, Mo,Ru, Hf, Ta, W, Re, B and La.
In the sintering of SiC powder, the difficulty in obtaining a high density product is attributed to the exaggerated grain growth that takes place during the final stage of sintering and coarse grains having a grain size of 100 microns or more will thereby be formed in a substantial amount.
The same exaggerated grain growth was observed in sintered bodies manufactured from coarse SiC powders.
This exaggerated grain growth is known to retard densification and to degrade mechanical properties.
It is the object of the present invention to overcome the above mentioned difficulties inherent to the conventional processes and to provide a process for sintering SiC powders, whereby a high density sintered product is readily obtainable by simultaneous conversion of transition metal oxides to carbides and sintering which process leads to an increase in fracture toughness of over 80% over that of conventionally sintered bodies.
It has been discovered in the present invention that the conversion of transition metal oxides to carbides can be achieved by placing the samples in a sealed graphite crucible and surrounding the samples with powder having the composition of 30% to 60% by weight SiC, 38% to 60% by weight Al.
sub.
2 O.
sub.
3 and 0.
5% to 7% by weight carbon.
It was found that the level of carbon determines critically the extent of conversion of transition metal oxides to carbides.
For example, too high amount of carbon, normally above 7% by weight, allows formation of free carbon in the sintered silicon carbide bodies, whereas too little carbon (normally below 0
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