Adhesion and tribological properties of diamond films on various substrates
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I. INTRODUCTION
Diamond is the hardest material and the material with the largest thermal conductivity. Since the development of low pressure plasma-enhanced chemical vapor deposition (PECVD) methods for synthesizing diamond films, one of the research efforts has focused on developing diamond films as a hard coating on tool materials for tribological applications.1"6 The most important features of any coated object for such applications are the quality of adherence between the films and the substrate, and the tribological behavior with its counterpart. However, only limited reports have been published on the adhesion of the diamond films to the substrate,3'7 and no quantitative data have been provided. Regarding the tribological behavior of diamond films, steel or SiC instead of nonferrous materials has been employed as the friction partners,4"6 although it is known that steel is not a good candidate for the friction partner against a carbon material due to a strong mutual alloying tendency of the materials involved. This paper presents the results of the adhesion tests on diamond films deposited by the microwave PECVD method on various substrates. The friction behavior of the diamond-coated cemented carbide inserts against a brass (70% Cu-30% Zn) ring will be discussed, and the wear resistance will be evaluated by turning tests. Effects of coating conditions and surface pretreatment of the substrate on the adhesion strength, friction, and wear behaviors will also be discussed. II. EXPERIMENTAL
Diamond films were synthesized by a microwave (2.45 GHz) PECVD system. A mixture of hydrogen and methane was used as the reactants for deposition J. Mater. Res., Vol. 5, No. 11, Nov 1990
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under the following conditions: methane concentration, 1 vol. %; total gas flow rate, 100 cc/min; total pressure, 40 Torr; microwave power, 200-300 watts. The substrate temperatures were varied from 880 to 1050 °C, and were monitored by an IR pyrometer. The substrate materials, specimen designations, and the corresponding surface pretreatments before diamond deposition are shown in Table I. A detailed description of the indentation adhesion tests, which used a Rockwell hardness tester with a Brale diamond cone indenter, is given elsewhere.8 Since the Rockwell hardness tester employs discrete loads, only the minimum load at which the film cracks and the next lower available load were recorded as the range of crack initiation load, Pcr. The slope, dP/dX, of the indentation load versus the crack diameter curve is also used as a measure to differentiate the quality of adherence between the coating and the substrate. The coefficient of friction (/A) was tested using a block-on-ring tribotester, as shown in Fig. 1. The tests were performed by pressing the stationary diamondcoated specimen against a rotational ring made of brass (70% Cu-30% Zn). The tests were carried out in air without adding lubricants. The diamond-coated cemented carbide insert with tool geometry, ISO TPGN 110308, was fi
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