Tribological study of novel metal-doped carbon-based coatings with enhanced thermal stability

Low energy consumption and reduced emission of greenhouse gases are two main challenges which trigger competitive growth of future automobiles. National Research Council, Washington DC (2011) on an assessment of fuel economy of light-duty vehicles reports that 21% of total fuel energy is spent to move the car and 17% to overcome frictional losses in engine parts. An attempt to curb these losses has already been commercially appreciated through the use of advanced surface engineering techniques on mating engine parts and new engine lubricants. In addition, friction effects wear and sometimes corrosion in these parts which recommends a robust system which would improve the tribological behaviour of surface engineered parts. Thus low friction and high temperature wear resistant coatings are in high demand for use on engine components. Diamond-like carbon (DLC) coatings are extensively used for this purpose due to their excellent tribological properties in presence of commercial lubricants. However, DLC coatings degrade at high temperature and pressure conditions leading to significant increase in friction and wear rate even in the presence of lubricant. This is a major concern of automobile manufacturers as the degradation is further aggravated by poor coating-substrate adhesion, finally delaminating the DLC coating from the base material. Hence to develop a thermally stable tribological coating with good adhesion quality for engine components, both the transitional metals Mo and W are simultaneously introduced in a carbon-based coating (Mo–W–C) for the first time utilising the benefits of smart material combination and High Power Impulse Magnetron Sputtering (HIPIMS).

This research includes development of Mo–W–C coating and investigation of thermal stability and tribological properties at ambient and high temperatures. The as-deposited Mo–W–C coating contains nanocrystalline almost X-ray amorphous structure and show dense microstructure, good adhesion with substrate (Lc~80 N) and high hardness (~17 GPa). During boundary lubricated sliding (commercially available engine oil without friction modifier as lubricant) at ambient temperature, Mo–W–C coating outperforms commercially available state-of-the-art DLC coatings by providing significantly low friction (µ~0.03 – 0.05) and excellent wear resistance (no measurable wear). When lubricated sliding tests are carried out at 200°C, Mo–W–C coating provides low friction similar to ambient temperature, whereas degradation of DLC coating properties fails to maintain low friction coefficient.

A range of surface analyses techniques reveal "in-situ" formation of solid lubricants (WS2 and MoS2) at the tribo-contacts due to tribochemically reactive wear mechanism at ambient and high temperature. Mo−W−C coating reacts with EP additives present in the engine oil during sliding to form WS2 and MoS2. This mechanism is believed to be the key-factor for low friction properties of Mo−W−C coating and presence of graphitic carbon particles further benefits the friction behaviour. It is observed that low friction is achieved mostly due to formation of WS2 at ambient temperature, whereas formation of both WS2 and MoS2 significantly decreases the friction of Mo–W–C coating at high temperature. This further indicates importance of combined Mo and W doping over single-metal doping into carbon-based coatings.

Isothermal oxidation tests indicate that Mo–W–C coating preserves it's as-deposited graphitic nature up to 500°C, whereas local delamination of DLC coating leads to substrate exposure and loss of its diamond-like structure at the same temperature. Further, thermo-gravimetric tests confirm excellent thermal stability of Mo–W–C compared to DLC. Mo–W–C coating resists oxidation up to ~800°C and no coating delamination is observed due to retained coating integrity and its strong adhesion with substrate. On the other hand, state-of-the-art DLC coating starts to delaminate beyond ~380°C.

The test results confirm that Mo–W–C coating sustains high working temperature and simultaneously maintains improved tribological properties during boundary lubricated condition at ambient and high temperature. Thus Mo–W–C coating is a suitable candidate for low friction and high temperature wear resistant applications compared to commercially available state-of-the-art DLC coatings.