Friction coefficient, microstructure and thermal stability of amorphous a-C coatings

Correlation between the microstructure, friction properties and ratio of sp2/sp3 carbon bonds in amorphous a-C coatings on AISI M2 steel was studied in this work. The coatings were formed using pulsed cathodic-arc vacuum deposition (CAVD), with negative bias voltage of 0.2–20 kV applied to the sample. The dose of assisting ion fluence was 3×1015–1017 ions cm−2. The best friction properties were found for amorphous a-C coatings formed with a negative bias voltage of 5–20 kV and ion assistance dose of 1017 ions ?m−2. Raman spectra with a diffused asymmetric maximum at 1545 ?m−1 and ID/IG ratio in the range 2.3–2.7 are characteristic for these coatings. Deterioration of the friction properties of a-C coatings occurs with an increase in annealing temperature from 300 to 600 °C. This is accompanied by a transformation of the amorphous carbon into submicron graphite crystallites.