Development of DLC coating architectures for demanding functional surface applications through nano- and micro-mechanical testing

DLC coatings can combine high hardness with low friction. However, they are often deposited with high levels of intrinsic stress and display low adhesion strength resulting in poor performance in demanding applications. A highly topical challenge is to develop advanced DLC coatings capable of withstanding more demanding applications in the automotive, cutting tool, MEMS and oil and gas sectors. The results from several nanomechanical and tribological test techniques - nanoindentation, nano-scratch and nano-fretting (nano-wear) - can be used together to aid the design of DLC coating architectures for enhanced durability in specific applications. In this study the behaviour of multilayered DLC coatings (Cr/W-C:H/a-C:H, Cr/W-C:H/Si-a-C:H) was compared to that of CrN/a-C:H:W (WC/C). We have previously reported that in nano-wear tests the coating with the highest hardness and H/E displayed greater wear resistance [T.W. Liskiewicz et al., Surf. Coat. Technol. 237 (2013) 212]. By employing nano- and micro-scale tribological testing with probes of differing sharpness it is possible to change the sensitivity of the test to probe the response of the coating top layer or the entire multilayer coating-substrate system. In the nano-scratch tests using a spherical indenter with a 5 μm end radius the maximum stresses are located well within the top layer of the multilayer coatings and consequently the mechanical properties of this top layer dominate the nano-tribological behaviour. In the micro-scratch using a 25 μm spherical probe the stress field extends further towards the sub-layers and steel substrate and consequently the behaviour is completely different. Under these conditions the coating with the lowest hardness and H/E showed improved performance with higher critical loads for cracking and total coating failure. High resolution SEM imaging has been used to investigate this further. A simple contact model strongly suggests that cracking and failure events occur on the harder coatings when the maximum von Mises stress was located close to the interfaces in the multilayer systems.