Ceramic composites: TiB₂-TiC-SiC

Abstract

In the ternary system TiB₂-TiC-SiC, the different two-phase composites, TiC-TiB₂, SiC-TiB₂ and SiC-TiC exhibit remarkable mechanical properties in regard with the single phase ceramics. The evolution of those properties, i.e. modulus of rupture σ_f, fracture toughnessK _1c, critical flaw sizea _c, hardnessHv, coefficient of thermal expansion α and electrical resistivity ρ, over the complete ternary diagram was investigated.

A methodology of research using optimal design was used to minimize the number of composites to be elaborated. In this study, 16 samples were sufficient to empirically determine a provisional mathematical model for each property. A model, then, enables the plot of isoresponse curves in the ternary diagram. The samples were hot pressed and the optimal hot-pressing cycles were determined using densification rates against temperature curves. The concordance between computed and experimental values is excellent, e.g. a sample containing 20 mol % of TiB₂, 55 mol % of TiC and 25 mol % of SiC has σ_fexp = 1080 M Pa, σ_fcomp=1070 MPa;K _1cexp=6.7 MPa m^1/2,K _1ccomp=6 M Pa m^1/2;Hv _exp=1 6.6 G Pa,Hv _comp=17.3 GPa; and ρ_exp=57.4 μΩ cm, ρ_comp=55 μΩm cm.

Although titanium diboride does not react with silicon carbide, a strong interface bond is developed between titanium diboride and titanium carbide, and between titanium carbide and silicon carbide. This explains the bend strength evolution in the ternary system, and more particularly the fact that, in the area σ_f > 1000 MPa andK _1c > 6 MPam^1/2, to high SiC contents correspond to low TiB₂ contents and conversely. The relevant microstructures will be discussed.