Novel Si-B-C-N materials thermally stable up to 2200°C

Three novel Si-C-B-N ceramic compositions, namely Si2.9B1.0 C14N2.9, Si3.9B1.0C11N3.2 and Si5.3B1.0C19N3.4 were synthesized using the polymer-to-ceramic transformation of the polyorganoborosilazanes [B(C2H4Si(Ph)NH)3]n, [B(C2H4Si(CH3)NH)2–(C2H4Si(CH3)N(SiH2Ph))]n, and [B-(C H Si(CH –N(SiH Ph))3]n,where Ph is phenyl (C6H5 at 1050°C in argon. The Si-B-C-N ceramics exhibited significant stability with respect to composition and mass change in the temperature range between 1000° and 2200°C, including isothermal annealing of the samples at the final temperature for 30 min in argon. The mass loss rate at 2200°C was as low as 1.4 wt%�h�1 for Si5.3B1.0C19N3.4, 1.7 wt%�h�1 for Si2.9B1.0C14N2.9 and 2.4 wt%�h�1 for Si3.9B1.0C11N3.2. The measured amount of mass loss rate was comparable to that of pure SiC materials. As crystalline phases, �-Si N and �-SiC were found exclusively in the samples annealed at 2200°C at 0.1 MPa in argon. For thermodynamic reasons, �-Si3N4 should have decomposed into the elements silicon and nitrogen at that particular temperature and gas pressure. However, the presence of �-Si3N4 in our materials indicated that carbon and boron kinetically stabilized the Si3N4-based composition