Transfer layer evolution during friction in W-C:H coatings

The case study focuses on the development of nanocomposite W-C:H coatings with high hardness and simultaneously, with a lower coefficient of friction. We studied the W-C:H coating system with different hydrogenated carbon matrix content obtained during a hybrid PVD-PECVD process from hydrocarbons (mostly acetylene) added into sputtering atmosphere. The coatings were prepared using three different PVD techniques (DC magnetron sputtering, HiPIMS, HiTUS) and the contents of hydrogenated carbon were controlled via different additions of hydrocarbon gas. Since the friction behavior of the coatings studied herein is controlled by the formation of a transfer layer on the ball, while the focus of our earlier works was to study the basic relationships between hardness and coefficients of friction, the focus turned to the investigation of the evolution of the transfer layer itself.

It is important to note that that the humidity is a very important factor in friction in hydrogenated carbon (based) coatings. Several W-C:H coatings with different amounts of carbon and hydrogen were tested in humid air and in flowing nitrogen with reduced humidity. Up to now, conventional optical microscopy, SEM/EDS, SEM/FIB (Figure 1) and Raman spectroscopy were used to evaluate different aspects of the transfer layer formation. However, we  have introduced the information obtained with the 3D optical profiler, Plu neox, which provides further qualitative and quantitative information about the transfer layer within the whole contact area.

Figure 1. SEM (and light microscopy) micrographs of the transfer layers formed on the worn ball cap during friction with various W-C:H coatings and under different conditions

This approach is not absolutely new, already in 2003 Scharf and Singer [Tribol. Letters, vol. 14, No1 and No2] applied Raman combined with optical profilometry in 2D to evaluate thickness of the transfer layer in a-C:H nanocomposite coatings. To the best of our knowledge, we are not aware of any study where Confocal 3D optical profilometry is applied in nanocomposite W-C:H coatings and in such an extensive range.

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