Structural changes of doped ta-C coatings at elevated temperature

Doped hydrogen-free amorphous carbon coatings, ta-C, ta-C:B, ta-C:Si, a-C:Fe and a-C:Mo, deposited from laser-assisted vacuum arc evaporation, were annealed in air up to 600 °C and in vacuum up to 800 °C. Subsequently they were studied regarding mechanical properties and structure, using nanoindentation, Raman spectroscopy, light microscopy, EDS and wafer curvature method. It was found that the coatings with high ratio of sp3 bonded carbon, high hardness and amorphous structure were stable up to 800 °C in vacuum, with a small increase in hardness (+5%–15%) above 400 °C, while compressive stresses were relieved significantly. In air, undoped ta-C was fully oxidized first, followed by ta-C:B and ta-C:Si, which improved temperature stability best. Metal dopands led to a decrease of sp3 in the as deposited coating, with structural change and hardness loss above 400 °C in vacuum. While a-C:Fe formed clear graphite peaks in the Raman spectrum, a-C:Mo showed only small changes in the amorphous structure. In air, a-C:Fe fully decomposed with remaining iron oxides at 500 °C whereas a-C:Mo formed a MoO3 overlayer at 600 °C. Failure mechanism at 500 °C in air was studied in detail for ta-C:Si. It was found that failure occurred from both degradation of the chromium interlayer as well as from oxidation of the topmost coating, both initiated at growth defects found in the respective layer.