Abstract
The application of diamond like carbon (DLC) coatings in tribological applications, the range of deposition methods employed and techniques for characterising the structure and properties of the films produced are reviewed. Thus far, DLC coatings have found broad industrial application, particularly in optical and electronic areas. In tribological applications, DLC coatings are used successfully as coatings for ball bearings where they decrease the friction coefficient between the ball and race; in shaving applications where they increase the life of razor blades in wet shaving applications; and increasingly in automotive applications such as racing engines and standard production vehicles. The structure and mechanical properties of DLC coatings are dependent on the deposition method and the incorporation of additional elements such as nitrogen, hydrogen, silicon and metal dopants. These additional elements control the hardness of the resultant film, the level of residual stress and the tribological properties. As DLC films increasingly become adopted for use in industry, knowledge of the factors that control their properties, and thus the ultimate performance of coated components in practical tribological applications, becomes increasingly important. The current state of tribological research on DLC is reviewed.
Highlights
diamond-like carbon (DLC) is a generic term that is commonly used to describe a range of different types of amorphous carbon films
Two factors have limited the further exploitation of DLC, the first of these is that the coating thickness is limited by the build up of residual stress as film thicknesses increase which can lead to delamination failure and the second factor is that at relatively low temperatures (250 ̊C) the properties begin to degrade as DLC converts to graphite
The arc spot formed on the cathode is of a small diameter (1 to 10 μm), and carries an extremely high current density (106 to 108 A cm-2)[26] which produces the plasma, and releases micrometer-sized particulates from the cathode which can result in rough surfaces, in turn leading to film failure due to cracks initiated at grain boundaries. The formation of these droplets is an unwanted by-product of the explosive emission process that removes the evaporant species from the cathode surface, and to counteract this the plasma can be passed through a magnetic filter, a method known as filtered cathodic vacuum arc (FCVA).[35]
Summary
DLC is a generic term that is commonly used to describe a range of different types of amorphous carbon films These films include hydrogen-free diamond-like carbon, a-C, hydrogenated DLC, a-C:H, tetrahedral amorphous carbon, ta-C, hydrogenated tetrahedral amorphous carbon, ta-C:H, and those containing dopants of either silicon or metal such as Si-DLC and Me-DLC respectively. The coatings can possess high hardness, low coefficients of friction against materials such as steel, and they are generally chemically inert. These desirable tribological properties arise as the properties of the film can be manipulated to give either diamond-like or graphite-like properties by controlling the deposition process. In addition to the excellent mechanical properties, DLC coatings can be smooth, pinhole and defect free and provide a good diffusion barrier to moisture and gases.[1]
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