Mechanical properties of hard chromium tungsten nitride coatings

Abstract

Due to its excellent oxidation and wear resistance, CrN is a prominent material for protective coatings. Still, its comparatively low hardness prevents CrN from being widely used in industrial applications. Thin films of Cr1−xWxNy with 0≤x≤1 were deposited on silicon, glass and steel substrates by reactive r.f. magnetron sputtering. The B1-NaCl phase is the only phase detected. In tungsten-rich films the compressive residual stress increases linearly with increasing tungsten content, but films with a high chromium content show tensile stresses. The morphology, examined by cross-section scanning electron microscopy, changed with increasing tungsten content from a columnar to a fine-grained microstructure. Hardness values, measured by nanoindentation, show a steep increase in hardness with only small additions of tungsten (hardness increases by 85% at a tungsten content of 10%). The maximum hardness value was measured for WN0.7 (30GPa). The ternary Cr1−xWxNy films with 0.1≤x≤0.8 and 0.8≤y≤1 were significantly harder (29–24GPa) compared to CrN (13GPa). In scratch tests with progressively increasing load, the films prove to have good adhesion and wear resistance, although some cracking is observed in films with tensile residual stress. In multi-pass scratch tests at low load, brittle flank wear is observed in films with moderate to high tungsten contents, while the softer CrN film exhibit cracks in the wear track.