Analysis of residual stresses in diamond coatings deposited on cemented tungsten carbide substrates
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Leonid Lev and Michael Lukitsch R&D Center, General Motors Corporation, Warren, Michigan 48090
Ashok Kumara) Department of Mechanical Engineering, Nanomaterials and Nanomanufacturing Research Center, University of South Florida, Tampa, Florida 33620 (Received 28 March 2006; accepted 20 December 2006)
Residual stresses in diamond films grown on WC–Co substrates have been investigated by Raman spectroscopy, x-ray diffraction (XRD), and curvature methods. Microcrystalline diamond films were deposited at 650–700 °C in a conventional hydrogen–methane environment by the microwave plasma-enhanced chemical vapor deposition technique. The film thickness, measured from cross-sectional micrographs taken by scanning electron microscopy, changed from 1.5 to 16.5 m as the growth time increased from 1 to 12 h. The type and the magnitude of the total residual stress obtained from curvature and XRD measurements agreed very well in all of the samples and changed from tensile to compressive as film thickness increased. However, Raman spectroscopy results showed that all films exhibited compressive stress due to the domain size effect. Different methane fractions, varying from 1% to 3%, have been utilized for diamond growth, and the total residual stress increased as more methane was included.
I. INTRODUCTION
Diamond is considered to be an ideal coating material for cutting tools with its unique characteristics, such as high hardness and wear resistance, good thermal conductivity, low friction, and low coefficient of thermal expansion (CTE). However, poor adhesion between the diamond coating and the substrate resulting in coating delamination is the main obstacle for the wide application of diamond-coated tools. One of the reasons for poor adhesion is that high residual stresses inside the coating affects its microhardness, wear resistance, and, most importantly for diamond coating, the adhesion strength.1–5 Hence, to understand the origin of the residual stresses and their variation due to process parameters and coating thickness, it becomes very important to produce highly adherent diamond coatings with low residual stress. It is common to separate the total residual stress into a thermal stress and an intrinsic stress component. The thermal stress is due to different CTE of the coating and the a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0120 1012 J. Mater. Res., Vol. 22, No. 4, Apr 2007 http://journals.cambridge.org Downloaded: 29 Mar 2015
substrate during cooling down from high diamond deposition temperature, while the intrinsic stress is associated with the film growth, structural defects, and nondiamond materials at the grain boundaries.6 Residual stresses in thin films are typically measured by curvature measurements,7 x-ray diffraction (XRD)8 and Raman9 peak shift methods. The curvature method measures the sample curvature before and after deposition, and the total residual stress could be calculated by the aid of Stoney’s equation.10 Raman spectroscopy is a simple tech
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