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
U.S.: Led by Dr. Stephen Hsu, George Washington University, USA
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
Germany: Led by Dr. Carsten Gachot, Saarland University, Germany
Hong Kong: Led by Dr. Lawrence Li, City University of Hong Kong, Hong Kong

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).



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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).


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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.


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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.


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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).

Significance and Impact
The combination of modeling and testing will yield important insights into the material behavior. The study is its initial stages and theoretical analysis of data is to be followed in 2015. Undoubtedly, this will lead to more durable coatings.