Thermal plasma CVD of SiC under reduced pressure

Abstract

The main purpose of this study is to develop a novel coating process by thermal plasma chemical vapor deposition (TPCVD) under reduced pressure around 3.5×104Pa. We began with the calculation of temperature and flow fields in hybrid Ar plasmas which were operated under reduced pressure and local thermodynamical equilibrium (LTE) conditions in order to investigate its effectiveness for TPCVD. The derived results suggested that the reduced pressure operation causes two main effects; (1) heating efficiency of the exit gas is raised to about 73% because of the reduction of radiation energy loss, and (2) plasma velocity is drastically accelerated to the order of 100 m/s at the torch exit. Moreover, the excellent stability and velocity controllability of hybrid plasma were also shown. Based on these modeling works, high rate and large area coating of SiC was tried at 3.5×104Pa by injecting SiCl4 and CH4 as reactants into Ar plasmas. During deposition, plasma tail flame revealed two distinct zones, that is, a central high intensity luminous zone and a relatively low intensity greenful one. Dense grossy SiC films with grain size of 3 to 7 nm and Vickers hardness over 3000 kgf/mm2 were deposited successfully at the maximum deposition rate of 70 nm/s on a graphite substrate in the latter zone at the deposition temperature less than 1470 K. Particularly noteworthy is that the dense film depositing area spread over 150 mm in diameter. In conclusion, these theoretical and experimental investigations have confirmed the possibility of high rate and large area ceramics deposition by TPCVD using a hybrid plasma system.