X.L. Bui

Microstructure and tribological behavior of tungsten-containing diamondlike carbon coated rubbers

Tungsten-containing diamondlike carbon (W-DLC) coatings have been deposited on FKM (fluorocarbon), ACM (acrylate), and HNBR (hydrogenated nitrile butadiene) rubbers via unbalanced magnetron reactive sputtering from a WC target in C2H2/Ar plasma. The surface morphology and fracture cross sections of coated rubbers have been scrutinized by high resolution scanning electron microscopy (SEM). The random crack networks formed due to the large difference in the coefficients of thermal expansion break down the W-DLC coatings into segments of a couple of hundred micrometers in size, facilitating good flexibility if the interfacial adhesion between the coating and a rubber substrate is strong enough. The size and density of growth defects in the W-DLC coatings strongly depend on the surface roughness of the rubber sheets. The tribological behavior of uncoated and coated rubbers has been investigated with ball-on-disk tribotest under dry sliding condition against a 6 mm 100Cr6 ball. Uncoated rubbers exhibited a ver...

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.

Modification of rubber surface with DLC thin films for low friction and self lubrication

Thin films of hydrogenated diamond-like carbon (DLC) have been deposited on hydrogenated nitrile butadiene rubber (HNBR) via magnetron-enhanced plasma chemical vapor deposition (ME-PCVD). Pre-deposition plasma treatment of HNBR substrate is proven to be crucial for the improvement of film performance due to enhanced interfacial adhesion. Moreover, enhancement of concurrent ion impingement via magnetron sputtering of graphite in poisoning condition raises further the adhesion and hardness of the films. The columnar structure and the crack network developed during deposition enhance the flexibility of DLC thin films and exhibit strain tolerance up to 5%. After unloading from 50% strain stretch, thin DLC films of similar to 300nm thickness still adhere very well on the rubber substrates and no spallation or delamination is observed. The thin DLC film on 400V plasma treated HNBR rubber exhibits very low coefficient of friction of 0.175 under dry sliding against 06mm steel ball (compared to > 1 of uncoated HNBR rubber). After tribotests even at high normal load of 3N, almost no damage can tie seen on the films. Such tribological property is even better than that of 1 mu m thick DLC or Me-DLC coated rubbers.