Composition and structure-property relationships of chromium-diboride/molybdenum-disulphide PVD nanocomposite hard coatings deposited by pulsed magnetron sputtering

The composition and structure-property relationships of physical vapour deposited coatings containing mixtures of CrB2 and MoS2 are reported. The coatings were produced by pulsed magnetron sputtering of loosely-packed powder targets formed from a blend of chromium and boron powders, alloyed with 12.8, 18.9 and 24.0 atom percent MoS2. Results of coating characterisation (by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Auger electron spectroscopy and nanoindentation measurement of hardness and elastic modulus) revealed that increasing amounts of MoS2 produced the following effects: frustration of crystallisation and phase separation; a decrease in average grain sizes (from similar to 5.5 to similar to 4.3 nm) and a decrease in coating hardness (from similar to 15 to similar to 10 GPa). Scratch testing also showed that the load-bearing capability of coatings was altered; coatings possessing an intermediate concentration of MoS2 exhibited the best behaviour with no failure observed in mechanical testing, due to an optimal nanocomposite structure. The corrosion resistance (investigated by potentiodynamic polarisation tests) however tended to improve as more MoS2 was introduced. An investigation of the effects of generating an amorphous structure by adding Ti and C into Cr-B-MoS2 coatings revealed improved corrosion behaviour, which significantly exceeded that of uncoated stainless steel and CrB2-coated samples.