Tribological Behavior Of DLC Films Coated Nitrile Rubber By Closed-field Unbalanced Magnetron Sputtering

Diamond-like carbon(DLC) films are an optimal solution as protective coating for many applications, due to the combination of relatively high hardness, chemical inertness, low friction coefficient and low wear rate. Their chemical composition,which is mainly composed by C and H, suggests a good compatibility with rubber materials. Rubber seals are incorporated in ball bearings widely used in many technical fields, like aerospace or automotive industries, in order to prevent the leakage of lubricant and the entrance of dirt. However, under operation dynamic rubber seals suffer from severe wear and cause high friction, leading to an ultimate failure of the bearing. Therefore, a protective coating with optimal tribological performance is of much interest in the respects of energy saving and environment protection.In this paper, four excitation coils are used to make up the closed field unbalanced magnetron sputtering system(CFUBMS), and the DLC coatings are deposited on the nitrile rubber by CFUBMS. The discharge characteristics of pulsed bias power under different parameters by CFUBMS are studied; with two graphite target, a five-factors and four-levels orthogonal experiment by CFUBMS was made;the three-pattern friction and wear testing and SEM-microstructure testing was used to analysis average friction coefficient variations, the factors poor and the DLC coatings microstructure.Pulse bias power discharge characteristics show that: in the CFUBMS(GD),with the increase of the excitation coil current, the deposition pressure and the substrate pulsed bias voltage, the substrate current increases; with the increase with C2H2/Ar flow rate, the substrate current firstly increases, then decreases; in the CFUBMS(DC), With the increase of the excitation coil current and graphite target currents, the substrate current increases firstly, then remains the same; with the increase of deposition pressure and the C2H2/Ar flow rate, the substrate current increases rapidly at first, then decreases, which can be explained by the protection of the bias voltage. The curve of the substrate current and C2H2/Ar flow rate turn appears a turning point on both method, which suggested that the process parameters of C2H2/Ar flow rate makes a big difference on the deposited DLCcoatings.Friction and wear result of the orthogonal experiment samples suggested that the friction factor of the DLC coatings increase respectively with the increase of friction force and friction loading rate. Specifically, when The load increases, the friction head pressed into deeper and The Friction is mainly caused by Hysteresis friction; while friction rate increases, The Friction is mainly caused by viscous friction. Based on the trend and range chart analysis, the best process parameters are as follows: graphite target current 1.0A, deposition pressure 2Pa, C2H2:Ar = 1: 3,pulse bias-200 V, 60% duty factor. And according to the degree of influence on the DLC coating, the decreasing order are as follows: C2H2/Ar flow ratio, graphite target currents, bias duty factor, pulsed bias and deposition pressure.DLC coatings microstructure analysis suggested that the surface of the DLC coatings generally consisted of a mesh structure(related to sp3 bonds) and cluster-like structure(related to sp2 bond).When the DLC coatings is thicker and the sp2 bond content is higher, the friction factor will be lower.