Internal Stress Measurement on CVD Diamond Coatings by X-ray Diffraction and Raman Spectroscopy
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INTERNAL STRESS MEASUREMENT ON CVD DIAMOND COATINGS BY X-RAY DIFFRACTION AND RAMAN SPECTROSCOPY K. Van Acker, H. Mohrbacher, B. Blanpain, P. Van Houtte, and J.P. Celis Departement Metaalkunde en Toegepaste Materiaalkunde (MTM), Katholieke Universiteit Leuven, de Croylaan 2, B-3001 Leuven, Belgium
ABSTRACT Internal stresses are measured in CVD diamond coatings deposited on a variety of substrate materials by low incident beam angle X-ray diffraction and by micro-Raman spectroscopy. The X-ray diffraction technique gives an integral stress value over a comparably large coating area whereas the Raman technique results in localized stress information with a lateral resolution of down to lpm. The stress values obtained from both methods compare well for diamond coatings of 6 pm thickness whereas a difference is observed for thicker coatings.
1. INTRODUCTION Residual stresses are generally present at room temperature in diamond surface coatings grown on a substrate material by chemical vapor deposition techniques. The stress field is basically induced by the growth of crystallites in the film during the deposition process as well as during the cooling down stage after the deposition [1]. The magnitude of the stress field depends mainly on the nucleation density of diamond crystallites on the substrate and the final grain size as well as on the mismatch in the coefficients of thermal expansion and the stiffness between diamond and the substrate material. A quantitative prediction of the stress field is difficult to make because of the complicated interrelation between all involved parameters. From a qualitative estimation over sign and relative size of the stress field it is expected that the magnitude of the residual stresses in the diamond film is highest when a substrate material with a comparably large coefficient of thermal expansion, high stiffness, and a high yield point is used. Experimental results by Johnston et al. [1] for diamond coatings on alumina and silicon substrates indicate the validity of this simplistic model. The presence of such high stresses in a thin diamond coating is expected to have a large influence on the mechanical stability of the coating. High shear tractions parallel to the coatingsubstrate interface can lead to adhesional failure and spalling. The beneficial aspect of a compressive stress field in the coating is seen in the retardation or avoidance of surface crack propagation under the action of applied loads. Stress free films are desired for the production of free standing diamond windows. In the present work, residual stresses in diamond coatings deposited on tungsten carbide, silicon nitride, and silicon substrates have been analyzed by a low incident beam angle X-ray diffraction method [2, 3] and by micro-Raman spectroscopy. The results obtained from both methods are compared.
2. EXPERIMENTAL Diamond coatings of 6 pm thickness were grown from a 1% CH 4/H2 atmosphere on two different Si3N 4 substrates (Cerasiv SL100 and SL200) and on one WC-Co substrate (Krupp THM K10/20) by hot f
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