Two series of five diamond-like carbon (DLC) coatings were sputtered under nominally identical conditions, but to different film thicknesses of 20 nm and 105 nm. First, the hardness of each sample was determined by depth-sensing indentation. Hardness measurements were substrate-affected to some extent for all samples but especially so for the 20 nm coatings. Two types of scratch tests were performed in an attempt to isolate and characterize the top coatings. The first was a wear test, which consisted of moving the sample back and forth repeatedly under a small constant load. The residual damage was inconsistent, but appeared to be a function of the composite, or substrate-affected hardness. The second test was a single-pass scratch in which the normal load was rampled linearly. For all samples, the friction coefficient was approximately constant as a function of load. Furthermore, samples with the same top coats yielded similar friction coefficients, regardless of the coating thicknesses. Friction coefficient decreased with hydrogen content and to some extent, increased with hardness, as measured on the 105 nm samples. The friction coeffcient measured during a ramp-load scratch offers an alternative for characterizing ultra-thin films, then identation alone yields measurements that are significantly affected by the substrate.
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