Annex IX: Co-operative program on friction reduction and lifetime control by advanced coatings via characterization, modeling and simulation

Overview of Scope:
The objective is to integrate modeling and coating testing to guide the development of next generation of advanced coatings for energy efficiency and durability in engines.

Annex Participants:
Finland: Led by Dr. Kenneth Holmberg, VTT, Finland (Chair)
Australia: Led by Dr. Gwidon Stachowiak, Curtin University, Australia
Israel: Led by Dr. Izhak Etsion, Technion, Israel
China: Led by Dr. Junyan Zhang, State Key Lab of Solid Lubrication, Lanzhou, China
UK: Led by Dr. Mark Gee, National Physical Laboratory, UK

Activities and Accomplishments:
The work was carried out in three work packages focusing on three scale levels in a sliding contact with coated surfaces:
WP1: Integrated surface and microstructural modelling in a DLC coated tribological contact: study covering topographical and microstructural material modelling on macro and micro level.
WP2: Optimization of thin film coated surfaces in tribological sliding contacts: a study focusing on asperity level micro scale modelling.
WP3: Phase transformation (sp3 to sp2) in DLC coatings caused by friction: a study on nano scale by molecular dynamic modelling.

Diamond-like carbon (DLC) coatings were chosen to be studied because of their excellent low friction and low wear properties and great potential for extensive use in transportation. In WP1 a very detailed DLC (diamond-like carbon) coated surface material characterization at the microstructural level was carried out (Fig. 1a). The surface roughness was characterized by determination of topographical parameters and orientations by 3D profilometry (Fig. 1b) and variance orientation transformation on several fractal scales from 30 – 840 μm (Fig. 2a).



Fig. 1. (a) Focused ion beam cross-section of a DLC coating with three layers from top: the DLC layer, the CrCx gradient layer, the Cr buffer layer and the substrate. (b) A 3D topography image by chromatic confocal surface profilometer of and average roughness DLC coated surface.

The surfaces studied had three levels of roughness (Ra 0.004, 0.012 and 0.1 μm) and three orientations (0º, 45º, 90º) in relation to the grinding grooving marks. The topographical effects on surface failure by fracture was studied by scratch test (Fig. 2b) and five failure mechanisms were identified, two on macro level and three on micro level (Fig. 3a).



Fig. 2. (a) Rose Hurst plots showing topographical heights and orientations on 360 μm fractal scale of an average roughness DLC coated surface. (b) Scratch test load, friction, acoustic emission and residual deformation measurements of an average roughness DLC coated surface.

The topographical effects on friction and wear were measured both for rotational and reciprocal movements (Fig. 3b). New interesting surface strengthening, surface weakening, frictional as well as wear effects of topographical orientation were identified, analyzed and reported in a journal paper. The multiscale analysis brought new insight to the basic contact mechanisms that control the friction and wear behavior and offers a platform for computational modelling based coated surface optimization with regard to reliability and effective lifetime.



Fig. 3. (a) Scratch test image from above after tip sliding contact showing surface failure pattern with angular cracks and delamination of an average roughness DLC coated surface. (b) Coefficient of friction for DLC vs DLC coated surfaces in rotational pin-on-disc and reciprocating pin-on-plate tribotesting with three different roughness levels and three orientations.

In WP2 a universal model for the load-displacement relation in an elastic coated spherical contact was developed. These contact conditions correlates with the conditions found on asperity tips of coated surfaces. The model provides a universal expression for the effective modulus of elasticity that is based only on mechanical properties of the coating and the substrate.

In WP3 both the growth mechanisms (Fig. 4a) of hydrogenated DLC coatings and the interactions and degradation behavior in DLC a-C/a-C self-mated contacts (Fig. 4b) were modelled and simulated on atomistic level by molecular dynamic simulation technique. These nano level studies gives an explanation of the transformation of molecular structure from a diamond-like crystalline to a more graphite-like amorphous sliding interface, which is one of the key elements giving the DLC surfaces extremely low friction in automotive and other applications.



Fig. 4. Molecular dynamic simulation images of (a) growth of a hydrogenated DLC film and (b) molecular dynamics of a self-mated DLC a-C/a-C contact (blue is sp3, green is sp2 and red is sp1).

2019 Activities and Accomplishments:
The efficiency and durability of engines can be improved by the use of thin surface coatings on sliding components. Diamond-like carbon (DLC) coatings are of the most promising and versatile coatings for low friction and low wear performance. Integrated computational materials engineering (ICME) techniques were used to explore the optimal tribological performance from a coating. Integrated multiscale simulation models were developed to link tribological contacts from nanometer events to micrometer results. This multi-scale optimization of material properties and performance is not possible to reach by empirical testing alone. At microscale, the aim is a universal model for a frictionless elastic-plastic contact with optimum coating thickness. The influence of phase transformation on friction in DLC coatings was also explored.

Topographical fractal measurements of DLC coated steel samples of three roughness levels were carried out. A computer model was built based on the surface characteristics. The model integrates microstructural and topographical features of the surface. New models had been developed to include DLC coating and martensitic stainless steel substrate as well as the bond and gradient layers between the DLC coating and substrate. Computer simulations were carried out where microscale features were linked to surface durability and wear properties.

The computer simulations showed the mechanism how coatings failure during sliding against flat rigid surfaces (Fig. 5). Based on the modelling results it was possible to predict the performance of the coatings in relation with surface roughness and angle of sliding.

The integrated multi-scale model can now be used for optimal coating system design for low friction and strong protection against surface fracture and wear. The optimal coating combinations can be defined to specific loading conditions.

Fig. 5. Coating cracking failure during sliding against flat rigid surface was simulated with the digital material model developed

Significance and Impact
Recent studies show that more than 30% of all energy used in transportation originates from tribological contacts and 25% of that could be saved by implementing advanced tribological technologies. Properly tailored and optimized surface coatings are one of the promising ways to make this possible. New coatings properly designed will have significant impact on energy consumption and emissions reduction worldwide. Digital material modelling enables faster coating development and selection as well as definition of the optimal parameters, such as hardness, elastic modulus etc.

Current and Future Focus
Annex IX (model based coatings) will continue developing the current multiscale computation models including surface topography and material microstructure to provide insights on how coatings breakdown and how the lifetime of the coating can be extended. In 2019 digital material models for lubricated contacts were developed and merged with earlier developed models. This work will continue in 2020.