An investigation into which factors control the nanotribological behaviour of thin sputtered carbon films

Abstract

Ultra-thin (20–100 nm) films deposited on Si surfaces can improve their mechanical and tribological properties. As a stepping stone towards the optimization of such ultra-thin films, herein we report experimental nanoscratch and nanowear data on a-C films of thickness in the range 200–1000 nm on Si aiming to (1) understand the role of film thickness on the nanoscratch behaviour, (2) determine whether the same factors (substrate bias, H/E ratio, etc) are at play for thick films as for the thin films, (3) determine possible design rules for thinner films enabling their optimization for MEMS applications and (4) evaluate the use of the multi-pass (3-scan) procedure for clarifying the locus of failure. To a first approximation, the critical load for total film failure in the nanoscratch test is proportional to thickness provided the films are not too stressed. a-C films of 1 µm with very high H/E, deposited under high substrate bias, perform well at low load but very poorly in more highly loaded situations. Not only do they exhibit low critical loads but also failure involves extensive delamination outside of the scratch track. This is not observed on thinner films. A suitable strategy for optimizing wear resistance for thin films for MEMS applications is to aim to maximize H/E. For the 200 nm films studied here, the films with the highest H/E showed slightly improved scratch resistance.

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

“…The Si-a-C:H and especially a-C:H:W coatings have lower hardness and higher plasticity, a combination which has been associated with improved crack resistance in scratch testing in amorphous hydrogen-free carbon coatings [25][26][27][28]. Although the a-C:H:W coating is softer it has a higher threshold for cracking when tested with R = 5 and 25 m probes.…”

“…In nano-scratch testing it is commonly observed for a hard coating deposited on a hard and brittle substrate that higher hardness, H/E and H 3 /E 2 is correlated with lower scratch depths at low load but a lower L c2 . Reported examples of coating systems showing this behaviour include 1 m a-C on Si in nano-scratch testing with a R = 4 m probe [25] and ~0.8 m nc-TiN/a-SiN x on Si in nano-scratch testing with a R = 3 m probe [34]. When micro-scratch testing of hard nitride coatings on cemented carbide the on-load scratch depths do not vary significantly with coating properties but the same trend of higher hardness, H/E and H 3 /E 2 being connected with a lower L c2 is also commonly found [21,[35][36][37].…”

Advanced Nanomechanical Test Techniques

“…Beake and co-workers have reported nanomechanical and nanotribological (nano-scratch and nano-wear) characterisation of a wide range of thin films on Si, with thicknesses from 5 nm (ta-C) to 1500 nm (TiN) performing nanoindentation with Berkovich indenters combined with nano-scratch testing with spherical indenters [49,[54][55][56][57]. A key motivation for studying several of these was to understand the interplay between film thickness and interfacial toughness for MEMS and protective thin film applications.…”

“…Shi [57]. They found that on wear resistance grounds a suitable strategy for optimising wear resistance for MEMS applications was to maximise H/E.…”

“…Shi and co-workers performed sub-critical load nanowear testing to investigate the performance of 1 m a-C films that fail in single nano-scratches at ~200 mN [57]. The wear test loads were chosen so that the maximum stresses were either within the film (at 50 mN) or the substrate (at 150 mN) so that the additional stress due to the imposed stress field from the scratch reaches the interface for the higher load test.…”