Microhardness and surface roughness of silicon carbide by chemical vapour deposition

Abstract

Chemical vapour deposition (CVD) is one of the most attractive techniques for producing highly pure and dense materials, because sintering is not required and films are obtained directly from the raw vapours or gases in the CVD process. Silicon carbide synthesized by CVD is the candidate with the most potential as a high temperature structural material because of its excellent high temperature oxidation, corrosion, erosion and=or abrasion resistance, its high degree of elastic modulus and strength, and its high thermal shock resistance [1, 2]. Thus, it is used as diffusion barrier coatings for susceptors in semiconductor industries [3], oxidation resistance coatings for reusable space vehicles [4], coatings for continuous filament [5], and matrices for fibre reinforced composites [6]. For application of high temperature structural materials, it is required to investigate factors that affect the mechanical properties such as hardness and roughness. In this letter, the effect of preferred orientation and microstructure on the microhardness of CVD SiC is reported. Surface roughness was also investigated along with preferred orientation and microstructure to determine the dominant factors influencing roughness. The deposition experiments were carried out in a hot-wall horizontal reactor. Methyltrichlorosilane (MTS, CH3SiCl3) was used as a precursor for depositing SiC onto isotropic graphite. The flow rates of MTS and H2 were 100 and 1000 sccm, respectively. MTS vapour was carried from a bubbler maintained at 0 8C in an ice bath. The flow rate of MTS was adjusted by controlling the flow rate of H2 carrier gas. All depositions were performed in the temperature range 1000–1400 8C with a total pressure of 1.3 kPa. The crystal structure and preferred orientation were analysed by X-ray diffraction (XRD). The microstructure of the as-grown films was observed by scanning electron microscopy (SEM). Chemical compositions of the as-grown films were analysed using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) after argon sputtering. In microhardness testing, the Vickers indenter was loaded on the CVD SiC surface, and the major diagonal of the resulting indentation was measured using a microhardness tester. The test load was 100 gf and the indentation time was 15 s. The thickness of all SiC layers for hardness testing was 250 im to exclude the effect of substrate. Surface roughness was measured by a surface roughness tester, and the thickness of SiC was about 50 im. Fig. 1 shows the XRD results for the SiC films deposited at various deposition parameters. Below a reactor temperature of 1100 8C, the SiC films were mainly amorphous. However, the SiC films formed above 1100 8C had a strong preferred orientation, with only â-SiC being present. In a relatively low temperature region from 1100 to 1200 8C, the â-SiC films exhibited (1 1 1) preferred orientation. However, in a higher temperature above 1300 8C, the âSiC films had two different strong preferred orientations, (1 1 1) and (2 2 0), depending on the deposition rate resulting from depletion of reactant gas [7]. At a higher substrate temperature and a lower deposition rate, preferred orientation took place in the most densely populated atomic planes or the lowest surface energy crystal facets parallel to the substrate. At the higher substrate temperature and a higher deposition rate, molecules attached to the substrate have the preferred orientation that places the less densely populated atomic planes or the higher