J.P. van der Pal

On the nature of the coefficient of friction of diamond-like carbon films deposited on rubber

In this paper, the nature of the coefficient of friction (CoF) of diamond-like carbon (DLC)-protected rubbers is studied. The relative importance of the viscoelastic and adhesive contributions to the overall friction is evaluated experimentally by modifying the contact load and the adhesive strength between the surface and the counterpart. The results indicate that the increase of CoF during the tribotests under non-lubricated conditions is caused by the increase of the adhesive contribution to friction motivated by the growth of the contact area during the test. In the case of oil lubricating condition, the adhesive force is minimized and the CoF is observed to decrease during the tribotest. This is caused by the reduction of the viscoelastic contribution due to the variation of the shape of the contact area. The role of the microstructure of the DLC film on the efficiency of the oil lubrication is also discussed.

Performance of diamond-like carbon-protected rubber under cyclic friction. II. Influence of substrate viscoelasticity on the friction evolution

In this paper, the influence of the mechanical properties of rubber substrate on frictional behavior of DLC-protected rubber is studied by numerical methods. The viscoelastic contribution to the friction during a tribotest was simulated according to a “mattress” approach composed by Voigt or standard linear solid units. The latter approach demonstrated more accurate predictions. In both cases, the results show an increase of the contact depth, which is in agreement with the experimental observations. The simulations also show a progressive reduction of the viscoelastic contribution of the coefficient of friction (CoF) with the number of laps due to the elongation of the front part of the contact area. This prediction does not agree with the experimentally observed increase of CoF, and suggests a dominant contribution of the adhesive component to the friction under real conditions.

Flexible protective DLC films on rubber: fundamental concepts and applications

Dynamic rubber seals are major sources of friction of lubrication systems and bearings, which may take up to 70% of the total friction. The solution we present is to coat rubber with DLC thin films by which the coefficient of friction is reduced from above 1.5 to below 0.15. Coating rubber is very challenging because the film/coating must be flexible and strongly adhered to the surface. Here we present and discuss our novel approach by depositing flexible DLC films on various rubbers via self-segmentation. By making use of the substantial thermal mismatch between DLC film and rubber substrates a dense crack network forms in DLC films and contributes to flexibility. The size of film micro-segments can be tuned by varying the bias voltage of pulsed-DC plasma CVD, which governs the amplitude of the substrate temperature variation during deposition. An analytical model is developed to predict the crack spacing (equivalent to the size of film segments) and the result fits well to the measured data. The formation mechanism of crack network and its effect on the flexibility and friction of DLC film coated rubbers are scrutinized. This paper provides generic design rules for the deposition of flexible and ultra-low friction films on rubber seals and the approach can drastically reduce the energy consumption in bearings and lubrication systems.