Influence of the bias voltage on the structure and mechanical performance of nanoscale multilayer CrAlYN/CrN physical vapor deposition coatings

The effects of bias voltage on the microstructure and the related tribological properties of CrAlYN/CrN nanoscale multilayer superlattice coatings were investigated. The coatings were deposited at 450 degrees C substrate temperature by combined high power impulse magnetron sputtering (HIPIMS) and unbalanced magnetron sputtering techniques. The substrates were 304 stainless steel, M2 high speed steel for structural analysis and mechanical testing, as well as cemented carbide substrates end mills for dry high speed milling applications. Substrates were pretreated by HIPIMS etching. The bias voltage U-b was varied between -75 and -150 V. The chemical composition was determined by neutral mass spectroscopy. The microstructure was characterized by x-ray diffraction and cross sectional transmission electron microscopy. All coatings had a single phase B1 fcc structure. The chemical composition was not affected by the bias voltage. Local epitaxial or axiotaxial growth attributed to the HIPIMS etching pretreatment was observed on the large surface areas of the substrate crystals. This turned to columnar growth with {110} texture at low bias voltages U-b between -75 and -120 V, while at U-b=-150 V an equiaxed structure of large crystal sizes developed with {111} texture. At the same time the waviness of the superlattice significantly decreased. An increase in bias voltage resulted in a significant rise in both residual stress levels (from -3.3 to -9.5 GPa) and plastic hardness (from H-p=34-51 GPa), while the coating/substrate adhesion decreased from 61 to 45 N. The friction coefficient increased from 0.43 (at U-b=-75 V) to 0.55 (at U-B=-120 V), while the initial sliding wear rates decreased remarkably (2.6x10(-16) m(3) N-1 m(-1) at U-B=-75 V to 3.7x10(-17) m(3) N-1 m(-1) at U-b=-150 V). The life time of 8 mm ball-nosed cemented carbide end mills decreased from 39 min at U-b=-75 V to 19 min when U-b was raised to -150 V. These results highlight that the combination of HIPIMS substrate treatment and designed deposition parameters provides good opportunity to tailor coating structures with optimized properties.