Characterization and properties of controlled nucleation thermochemical deposition (CNTD)-silicon carbide

Abstract

The present investigation was undertaken to characterize the microstructure of controlled nucleation thermochemical deposition (CNTD)-SiC material and to evaluate the room-temperature and high-temperature bend strength and oxidation resistance. Utilizing the CNTD process, ultrafine grained (0.01 to 0.1μm) SiC was deposited on W wires (0.5 mm diameter by 20cm long) as substrates. The deposited SiC rods had superior surface smoothness and were without any macrocolumnar growth commonly found in conventional CVD material. At both room and high temperature (1200 to 1380° C), the CNTD-SiC exhibited a bend strength of ~ 200 000 psi (1380 MPa), several times higher than that of hot-pressed, sintered, or CVD SiC. The excellent retention of strength at high temperature was attributed to the high purity and fine grain size of the SiC deposit and the apparent absence of grain growth at elevated temperatures. The rates of weight change for CNTD-SiC during oxidation were lower than for NC-203 (hot-pressed SiC), higher than for GE's CVD-SiC and CVD-Si3N4 but considerably below those for HS-130 (hot-pressed Si3N4). The high-purity fully dense and stable grain size CNTD-SiC material shows potential for high-temperature structural applications, however, problem areas might include scaling the process to make larger parts, deposition on removable substrates, and the possible residual tensile stress.