Comparing hardness and wear data for tetrahedral amorphous carbon and hydrogenated amorphous carbon thin films

We compared nanoindentation and nanoscratch testing of 10 and 50 nm thick tetrahedral amorphous carbon (ta-C) and hydrogenated amorphous carbon (a-C:H). Raman spectroscopy shows the expected spectral features for the two carbon forms, however, luminescence from the ceramic substrate can alter the spectra. We find that hard ta-C films can blunt the diamond tip and hence use a tip area function re-calibration procedure after each indentation. This shows that, despite the correction, shallow depth hardness data is influenced by the tip geometry. Therefore, we also present a hardness slope ratio (sample/fused silica) protocol which is independent of tip geometry. The ta-C films are the hardest and the more wear resistant, i.e. for 50 nm thick ta-C films on Al2O3–TiC substrates; H (25 nm)=51 GPa and the wear rate under a 37 GPa contact pressure is 1.8×10−4 mm3/N m. Ramping load measurements of critical loads were complemented with scanning electron micrographs and energy dispersive X-ray (EDX) spot analysis of the wear regions. The comparison helped detect film delamination, particularly so for the thinner films. The ta-C films have higher critical loads than the a-C:H films. This finding contrasts with the large internal compressive stress usually associated with ta-C formation. In the present case, we believe that the filtered arc deposition at the floating potential provides sufficient energy for efficient atomic intermixing in the substrate but, appropriately, not enough to produce the large internal stress observed at higher energy. We note that the superior wear resistance of the ta-C films has more to do with their good adhesion to the substrate rather than their high hardness, as measured by the indenter tip. Finally, we find that a-C:H has better adhesion on silicon, whereas ta-C sticks better to the conducting ceramic substrate.