Interface characterization of combination hard/solid lubricant coatings by specific methods

Abstract

Double or combination coatings and multilayered coatings were recently developed as a new generation of protective PVD coating for cutting tools. Some of these have already been applied in selected cases in modern machining techniques such as dry high-speed milling, turning, tapping and drilling, or when using a minimum quantity of lubricant (MQL). In this work we studied details of the combination of two coatings, one a solid lubricant (WC/C) coating sputtered onto a hard, wear- and temperature-resistant (TiAlN) coating, deposited onto high-speed steel and cemented carbide substrates. Using specific analytical methods, we focused our investigation on interface (interlayer) characterization between individual layers and the substrate. A detailed knowledge of the interlayer properties is the basis for understanding and interpretation of the macroscopic properties of WC/C–TiAlN combination coatings in a given tribological system. Depth profiles of Vickers microhardness, as determined on low-angle cross-sections of test samples, showed that the TiAlN layer had approximately 3000 HV 0.05, the W-rich interlayer approximately 2500 HV 0.05 and the WC/C solid lubricant approximately 1000 HV 0.05. High adhesion strength was proved by the REVETEST scratch test and by additional SEM+EDS analyses and depth profiles of the scratch tracks. Total delamination L c was observed at critical loads from 80 to 115 N. A low scratching coefficient μ* (0.05–0.20) was typical of thin (3 μm) and also of thick (10.8 μm) coatings. The width of the WC/C–TiAlN interlayer (0.15–0.20 μm) was measured from the SEM image of the kalotest ball crater and from fracture cross-sections. This narrow transient area was also detected and analysed by AES depth profile analysis and by EDS linescan and SEM microanalysis at low-angle cross-sections of the coated samples. All the results were correlated successfully with the declared basic macroscopic properties of the WC/C–TiAlN combination layer.