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

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. 1). 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. 1. 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.

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