Synthesis–structure–property relations for Cr–B–N coatings sputter deposited reactively from a Cr–B target with 20 at% B

Abstract

Cr–B–N films were synthesized by unbalanced magnetron sputtering from a sintered Cr–B target with 20 at% B in an Ar–N 2 discharge at varying N 2 partial pressures ( p N 2 ) of up to 64% of the total pressure ( p Ar + p N 2 = 0.4 Pa). Coating composition and microstructure were investigated by X-ray diffraction, X-ray photoelectron spectroscopy and wavelength-dispersive electron-probe microanalysis and correlated with mechanical and tribological properties measured by microindentation and dry sliding ball-on-disk tests. For low nitrogen partial pressures ( p N 2 ⩽ 22 % ), the XRD patterns are composed of broad overlapping peaks with low intensity. These films have hardness values of ∼15 GPa and indentation moduli of ∼150 GPa. Increasing p N 2 from 22% to 28% results in an increase of the N content from ∼38 to 50 at% where the films meet the quasi-binary CrN–BN composition. Thereby, an increase of the hardness from ∼15 to 32 GPa is obtained. A further increase in p N 2 up to 64% results in minor changes of the chemical composition, micro and bonding structure as well as mechanical properties. While in ball-on-disk testing early failure was observed for coatings grown at p N 2 ⩽ 22 % , higher nitrogen contents in the discharge yielded friction coefficients of ∼0.43 independent of chemical composition, microstructure and mechanical properties.