JOURNAL OF MATERIALS SCIENCE 33 (1998) 173 — 182 Effect of methane concentration on physical properties of diamond-coated cemented carbide tool inserts obtained by hot-filament chemical vapour deposition M. A. TAHER, W. F. SCHMIDT, H. A. NASEEM, W. D. BROWN, A. P. MALSHE Materials and Manufacturing Research Laboratory, Mechanical Engineering Department— High Density Electronics Center HiDEC, University of Arkansas, Fayetteville, AR 72701, USA S. NASRAZADANI, Department of Materials Science, Iofahan Institute of Technology, Iofahan, Iran Diamond-coated tools can greatly improve the productivity of machining highly abrasive materials such as high silicon—aluminium alloys used in the automotive industry. Cemented- carbide diamond-coated tool inserts have not become an off-the-shelf product owing to several difficulties including insufficient adhesion of diamond to the substrate and questionable reproducibilty in their machining performance in the manufacturing. In order to overcome these difficulties, a better understanding of the effects of the chemical vapour deposition (CVD) conditions such as methane concentration, reactor pressure and substrate temperature is important. In this work, cemented tungsten carbide tool inserts with 6 wt% Co (WC — Co) were coated with diamond films deposited at five different methane concentrations (1—9 vol %). Here we present preliminary results of the effect of methane concentration variation on the following physical properties of the diamond coating: surface morphology; crystal structure; chemical quality; surface roughness; residual stress. The results indicate that the best physical properties of diamond-coated tool inserts using hot-filament CVD are achieved with diamond coatings deposited at methane concentrations ranging from 1 to 3%. 1. Introduction Diamond is a unique material for tooling applications because of its high Vickers hardness (10000 HV), high compressive strength (greater than 110 GPa) and high thermal shock resistance (greater than 1000 ¹ (K)), together with a low dynamic coefficient of friction (0.03) [1]. Owing to recent advances, it is possible to apply diamond coatings to cemented carbide and silicon nitride tool inserts [2 — 14]. Unlike diamond high-pressure obtained by high-temperature physical vapour deposition, chemical vapour deposition (CVD) thin-film coating technology allows the deposition of diamond films on complex geometries in a relatively cost-effective manner and, hence, there is increasing interest in its application for cutting inserts. Numerous studies examining the effects of the CVD conditions on diamond film properties deposited on silicon [15 — 18] and thin pure tungsten substrates [19, 20] have been reported. No systematic studies were found in the published literature that address the effects of deposition conditions on diamond coatings deposited on state-of-the-art cemented carbide tools. Such an investigative study would provide the optimal set of conditions that results in a good-quality coated tool insert. Film quality is a subjective term and depends on the type of application. In the case of tool inserts used for machining, the required diamond coating should have a desired graphitic content with well-faceted crystals, good adhesion with the substrate, uniform coverage over flank and rake regions, and uniform grain size. All have to be achieved at an acceptable growth rate. In practice, however, to obtain specific results, compromises have to be made in the choice of deposition parameters through an optimization process. All deposition parameters such as the reactor pres- sure, the gas flow rates, the substrate surface temper- ature and the hydrocarbon gas percentage are interde- pendent. Therefore, for the reactor used in this work, an optimization process was undertaken to achieve an optimal thermal configuration at a fixed pressure, gas flow rate and methane concentration. Details of the thermal optimization process have been described elsewhere [21]. In this paper, the focus of the work is to examine several diamond coatings on WC—Co tool inserts 0022—2461  1998 Chapman & Hall 173