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

Abstract

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 tools, MEMS and oil and gas sectors. The results from several nanomechanical and tribological test techniques -nanoindentation, nano-scratch and nanofretting (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 m 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 m spherical probe the stress field extends further towards the sublayers 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.3

Elevated temperature repetitive micro-scratch testing of AlCrN, TiAlN and AlTiN PVD coatings

“…Coatings with very high H/E can behave poorly due to their limited ductility, with reduced critical load. For hard coatings deposited on silicon there is more extensive delamination at failure [16,17,20,29], or chipping for harder DLC coatings on hardened steel [35]. Consistent with this general trend the smallest critical load was found for the hardest coating, TiAlN.…”

Nano-scratch testing of (Ti,Fe)N x thin films on silicon

“…For metal-doped carbon coatings on steel substrates improved scratch toughness has usually been associated with a significant decrease in coating hardness compared to undoped coatings. The highest scratch 18 toughness was for carbon films with H ~10-12 GPa [47][48][49]. Similarly, Zhang and co-workers reported improvements in scratch toughness that were at the cost of hardness in metal-doped nc-MenN/a-SiNx coating systems [17][18][19][20]63].…”

Investigations on Mechanical and Tribological Behavior of dcMS/HPPMS CrN and (Cr,Al)N Hard Coatings Using Nanoscratch Technique

“…Furthermore, due to its flexibility in terms of different indenter types and the wide range of loads (nN to mN), the nanoscratch method could be utilized to make comprehensive investigations on the tribological behavior of substrate/coating systems. However, correlating the results of tribological nanoscratch analyses with the mechanical properties of the coatings is usually challenging . Due to the complexity of PVD coatings, for example, in terms of microstructure or architecture, a precise evaluation of their deformation during a nanoscratch test would require more considerations than the individual mechanical properties, such as universal hardness HU or indentation modulus E IT only.…”