Thermal stability of nanocomposite CrC/a-C:H thin films

AbstractThe thermal stability of low-friction Me–C/a-C:H coatings is important for their potential applications in the tool and automotive industry.Recently we showed that CrC x /a-C:H coatings prepared by unbalanced magnetron sputtering of a Cr target in Ar+CH 4 glow discharges exhibit ananocomposite structure where metastable fcc CrC nanocrystals are encapsulated by an a-C:H phase. Here, we present the structural evolution ofthese nanocomposite CrC/a-C:H coatings during annealing. High-temperature X-ray diffraction in vacuum and differential scanning calorimetry(DSC) combined with thermo-gravimetric analysis in Ar atmosphere indicate decomposition of the formed metastable fcc CrC phase andsubsequent formation of Cr 3 C 2 and Cr 7 C 3 and structural transformation of the a-C:H matrix phase towards higher sp 2 bonding contents attemperatures above 450 °C. Combined DSC and mass spectrometer analysis as well as elemental profiling after annealing in vacuum by elasticrecoil detection analysis relate this transformation to the loss of bonded hydrogen at temperatures above 200 °C.Due to these structural changes the coefficient of friction depends on the annealing temperature of the nanocomposite a-C:H coatings andshows a minimum of ∼0.13 for T=200 °C. The more complex tribochemical reactions, influenced by the hydrogen loss from the coating duringin-situ high temperatures ball-on disc tests, result in coefficient of friction values below 0.05 for Tb120 °C.© 2007 Elsevier B.V. All rights reserved.