Characterization of the kinetic and mechanistic differences between free-surface and bulk grain growth in WC-Co material
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I. INTRODUCTION
ADVANCES in chemical vapor deposition (CVD) diamond synthesis have led to the development of deposition techniques that are characterized by high deposition rates and large, uniform deposition areas. These techniques have made economical synthesis of diamond films possible and have motivated researchers to explore the use of CVD diamond films for a host of applications. However, several issues that prevent CVD diamond films from achieving their full potential remain unresolved. Paramount among these issues is the poor adhesion of diamond films to their typical WC-Co substrates during direct deposition. The adhesion of CVD diamond films to WC-Co is limited by several factors, including the thermal-expansion mismatch between the film and the WC-Co substrate, the low nucleation density of diamond on WC-Co, and the relatively high solubility of carbon in cobalt. Since traditional CVD diamond synthesis takes place at elevated temperatures (between 700 ⬚C and 900 ⬚C), the thermal-expansion mismatch between the diamond and base material can result in very high residual stresses when the coated material is cooled from the processing temperature to room temperature. These residual compressive stresses can cause spontaneous delamination of the film or may reduce the critical stress for delamination during use. Since CVD diamond growth on WC-Co typically occurs by a Volmer–Weber nucleation and growth mechanism,[1] a low nucleation density can result in the formation of large, internucleic voids at the substrate/diamond interface. These voids can provide crack nucleation sites for interfacial crack propagation and cause delamination of the film. In addition, cobalt is a catalyst for the formation of graphite under typical conditions for CVD of diamond films. J.M. OLSON, formerly Research Engineer with Saint-Gobain Industrial Ceramics, Northborough, MA, is Staff Process Engineer, Fairchild Semiconductor, South Portland, ME 04106. M.M. MAKHLOUF, Associate Professor of Mechanical Engineering, is with Worcester Polytechnic Institute, Worcester, MA 01609. Manuscript submitted January 13, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
Therefore, if cobalt is present at the diamond/WC-Co interface, it can catalyze the formation of a layer of graphite that can obstruct chemical bonding between the film and the substrate. Moreover, cobalt can transform the diamond film that has already formed into graphite. At the elevated temperatures of CVD diamond growth, the transformed graphite is driven into solid solution with cobalt, thus dissolving the diamond film from the interface and decreasing adhesion strength. Therefore, in order to maximize the adhesion strength of diamond coatings to WC-Co substrates, the WCCo surface must be carefully tailored. This may be accomplished by optimizing three key characteristics of the surface: (1) its chemical composition, (2) its microstructure, and (3) the relative amounts of the phases at the surface. The chemical composition of the surface must be optimized to ensure that deleter
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