DEPOSIÇÃO DIRECTA DE DIAMANTE CVD EM COMPÓSITOS CERÂMICOS Si 3 N 4 /SiC

Abstract

An excellent film-substrate adhesion is a basic requirement for the production of thin films for tribological applications, where very high thermal-mechanical loading is attained. For CVD diamond coated materials, the adhesion is related to the thermal stresses originated by the mismatch between the thermal expansion coefficient of the diamond film (a~0.8x10-6K-1) and that of the conventional substrates WC-Co (a~5.5x10-6K-1). Silicon nitride is here presented as a viable option, because the respective value of a ~2.9x10-6K-1 is closer to the one of diamond. This ceramic is currently used as a cutting tool material and also as an active component of tribological systems without lubrication. In this work, Si3N4/ SiC (0-50%) ceramic composites were used for CVD diamond nucleation and growth experiments. The addition of SiC particles to Si3N4 matrix provides a better chemical affinity to diamond, being capable to promote a high adhesion throughout direct carbon- carbon bonding. The deposition was carried out in a commercial microwave reactor using gaseous mixtures of H2/CH4. Samples were pre-treated with distinct techniques: a) scratching with diamond dust; b) ultrasonic bath in diamond suspension; c) chemical etching with CF4 plasma; and d) hot chemical etching with acids. SEM analysis showed that the Si3N4/SiC composites achieved the highest nucleation density. Adhesion tests with Brale indentation proved the very good adhesion by the absence of film delamination under a load of 588N, as anticipated by RAMAN characterisation. In these experiments, the shift of the diamond peak (1.333 cm-1) occurred without appreciable deviation to the characteristic one of natural diamond (1.332 cm-1). It was also observed that the mechanical pre-treatments using diamond particles were more efficient than the chemical etching procedures in promoting nucleation and growth of diamond.