Combined surface characterization and tribological (friction and wear) studies of CVD diamond films
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Joel W. Ager, III Materials Sciences Division, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720
Gabor A. Somorjai Department of Chemistry, University of California, Berkeley, California 94720 (Received 3 December 1992; accepted 8 June 1993)
The tribological properties of polycrystalline chemically vapor deposited (CVD) diamond films grown on silicon substrates and containing varying amounts of amorphous carbon impurities were investigated. Films were characterized by secondary electron microscopy (SEM) and atomic force microscopy (AFM) for surface morphology and roughness and by spatially resolved Raman spectroscopy for amorphous carbon (a-C) content. Friction measurements were performed with a Rockwell C hemispherical diamond tip in ultrahigh vacuum (UHV) and in ambient air. In vacuum, the friction coefficient rises monotonically from 0.6 in a region with substantial a-C to 0.85 in a region with pure diamond. Under ambient conditions, the friction coefficient is substantially lower than that in vacuum and deceases slightly (from ~0.19 to ~0.16) with the decreasing a-C content. Under both vacuum and ambient conditions, the friction coefficient was observed to be independent of load over the range of 0.1-0.5 N. The friction values are discussed in terms of adhesion between the diamond tip and the film. Qualitative scratch hardness measurements were performed in UHV by measuring the minimum load at which plastic deformation occurs for a single traversal of the tip. Scratch hardness is found to increase with increasing diamond content of the films. The wear mechanism of the pure diamond regions was evaluated by examining wear tracks with SEM and AFM. The wear tracks showed evidence of spalling, buckling, and grain pull-out indicative of a cohesive mode of failure (failure at grain boundaries). A decrease in surface roughness in the wear tracks indicates asperity wear. Adhesive failure at the Si substrate interface or of a phase transformation of the diamond film was not observed in this load regime.
I. INTRODUCTION Within the past ten years there has been a flurry of activity in the field of diamond film growth. Following the discovery of high growth rates of diamond from a gas phase source of carbon,1 research has expanded to address methods of preparation, models of growth, and characterization of film properties. Several reviews have been published recently summarizing much of this work.2"^ Research in the field of diamond films has quickly led to the development of new technologies based upon the production of very thin films of an extremely hard material. One application of diamond films is as abrasion resistant coatings in extreme wear environments.5 A more complete understanding of the tribological properties of diamond films is necessary for successful applications in this regard, as indicated in earlier adhesion and wear studies. 6 ^ J. Mater. Res., Vol. 8, No. 10, Oct 1993 http://journals.cambridge.org
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The friction, wear, and adhesion of natural
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