Mechanical and physical properties of C and C-Cr sputter coatings measured at the nano-scale

X-ray photoelectron spectroscopy and nano-indentation were used to investigate the chemical/structural composition and the nano-scale mechanical/tribological properties of a group of sputtered C coatings doped with zero, low and high amounts of Cr. At zero and low Cr concentration, the coatings were composed of sp ² and sp ³ hybridised C-C bonds and at high Cr concentration the coating reverted to a Cr:C ratio of 3:2, suggesting a Cr ₃C ₂ composition. The diffusion of adsorbed atmospheric O/H ₂O from the surface of the coating into the bulk was reduced by the inclusion of Cr which implies a more dense surface structure. An increasing concentration of Cr produced a reduction of hardness, an increase of reduced modulus (with a high amount of Cr), an increase of wear rate and an increase of the (initial) coefficient of friction. A comparison was made between the nano-scale results in this work with those previously obtained from similar coatings, but using micro-hardness and macro-scale pin-on-disc techniques. In this comparison nano and micro-hardnesses followed similar trends, reducing with increasing Cr concentration; however, micro-hardness gave significantly higher absolute values and showed a greater difference between samples. Comparing wear rates for the two scales, trends were again similar at low load pin-on-disc measurements with wear rate increasing with increasing Cr concentration at, but no such agreement was found at higher load. The results for coefficient of friction were similar between the two scales of test.