The CVD of Ceramic Protective Coatings on SiC Monofilaments
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THE CVD OF CERAMIC PROTECTIVE COATINGS ON SIC MONOFILAMENTS K. L. CHOY AND B. DERBY Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, UK
ABSTRACT The TiB 2 , TiC and TiN coatings were studied as potential protective coatings for SiC fibre reinforced titanium alloys. These coatings were deposited by a CVD technique on resistively heated SiC monofilament fibres at reduced pressure in a cold wall reactor. The deposition conditions that are required to coat each of these materials were determined. The morphology, microstructure, phase composition and nature of the coatings were examined. The comparison and evaluation of the effectiveness and potential of these as protective coatings were carried out by incorporating the coated fibre into Ti-6A1-4V using the diffusion bonding method at 1100'C under 10 MPa for 1 hour. INTRODUCTION The development of Ti-based composites is hindered largely due to the interfacial reaction between SiC and Ti matrices at elevated temperatures which leads to the formation of a brittle reaction zone consisting of titanium carbides and titanium silicides, which weakens the mechanical properties of the composites [1,2]. This has prompted research into the development of chemically inert protective coatings onto SiC fibres to inhibit the fibre/matrix interaction. The prospective coatings of interest in this study are TiB2 , TiC and TiN. These coatings are all chemically inert. EXPERIMENTAL The SiC fibres used in this study were BP Sigma SiC monofilaments (-100 gm diameter with tungsten core). The schematic diagram and general operation of the CVD apparatus used for the deposition has been described elsewhere [3]. A SiC monofilament was drawn into a cold wall reactor. Subsequently it was heated to the deposition temperature. Then a gaseous mixture of the reactants was fed into the reactor, these underwent reactions and deposited the coatings onto the SiC substrate. The reactants and deposition reaction used for the deposition of TiB2 , TiC and TiN, were as follows: TiCL1 (g) + 2BC13(g) + 5 H2(g) TiC14(g) + CH4(g) 2TiCl4(g) +
TiB 2 (s) + 10HCl (g) H--!--c-,TiC (s) + 4HCl (g) N2(g) + 4 H2(g) ---- '---2TiN(s) + 8HCl (g) -
The coating process variables chosen were substrate temperature, deposition pressure and input gas ratio. The experimental parameters were systematically varied to investigate the effect of these parameters on the surface morphology, Mat. Res. Soc. Symp. Proc. Vol. 250. @1992 Materials Research Society
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deposition rates and microstructures of the coatings. Table I summarizes the chemical vapour deposition conditions for different types of coatings.The coated fibres were characterized by optical microscope, SEM, TEM, X-ray diffraction and chemical analysis. Table 1. The chemical vapour deposition conditions for different types of coatings Coating Temperature Pressure Flow rate(cm 3 min-1) 0 ( c) kPa TiCI 4 BC13 CH4 N2 H2 TiB 2 TiC TiN
1050-1450 1000-1350 950-1050
6-23 8-25 10-25
24-216 30-60 30-60
72-192
-
30-180
-
300
0-450
1200-400 200-6
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