Effect of nitrogen-incorporation on structure, properties and performance of magnetron sputtered CrB2

Transition metal (TM) boron nitrides are promising candidates for protective coatings with self-lubricating abilities as they can combine properties of TM diborides with the lubricity of hexagonal boron nitride (h-BN). Here, we report on Cr–B–N coatings prepared by unbalanced DC magnetron sputtering of a CrB2 target in argon/nitrogen atmosphere at 450 °C. By varying the nitrogen partial pressure (pN2) between 0 and 64% of the total pressure (pAr + pN2), the N-content in our coatings could be increased from 0 to 47 at.%. The results obtained from X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy show that for pN2 ≤ 11% a CrB2-based structure type develops, whereas with increasing pN2 the microstructure becomes then X-ray amorphous and finally CrN is detected as the sole crystalline constituent. With increasing pN2 from 0 to 11%, the hardness and indentation modulus rapidly decrease from 40.6 and 397 GPa for CrB2 to 13.4 and 108 GPa for CrB2.0N0.5. All coatings investigated yield only a moderate friction coefficients between 0.5 and 0.7. Based on detailed high-resolution TEM studies, we can conclude that the missing h-BN based lubricity is due to a lack of a significant long-range order.