Characterization of carbon coatings on SiC monofilaments using Raman spectroscopy

The axial residual stresses in the carbon coatings deposited onto different silicon carbide monofilaments have been determined experimentally using Raman spectroscopy. The stress-dependent band shift for the carbon G-band at around 1600 cm–1, due to symmetric in-plane stretching mode of graphite, has been found to be –1.6 cm–1/GPa. Using this calibration, the axial residual stresses in carbon coatings can be estimated from measured band shifts between the broken end and middle of the monofilaments. It was found that the stresses in the coatings of all monofilaments were compressive and between –440 and –810 MPa. Modelling indicated that this was consistent with the coating stress arising from the difference in coefficients of thermal expansion of carbon and the underlying silicon carbide. The coating stress was measured as a function of distance from the broken monofilament end. It was found that the distance for the stress to build up varied greatly, from 40 lmin Ultra- SCS to 500 lm in SM1140+. This suggests there are significantly different shear stresses between the coatings and underlying silicon carbide in the different monofilaments.