Friction and thickness of hard coatings on cutting tools

Hard coatings are used to increase the wear and frictional properties of cutting tools. The coatings are deposited using Physical Vapor Deposition (PVD) or Chemical Vapor Deposition (CVD) with typical thickness of several micrometers. Among the most important coatings belong TiAlN, CrN, TiN, DLC, ZrN, Al2O3 and TiCN. Many of these coatings must have good frictional properties to ensure good wear resistance and long life time of the tool. This application report demonstrates the results of tribological characterization and thickness measurements of hard coatings for cutting tools.

Tribology of hard coatings Hard coatings are used for cutting tools and other applications that require surface protection. Their main advantage is the reduction of production costs due to increase of lifetime of the tools and improved product quality. Among the most important domains belong machining tools (Figure 2) that are used for cutting, punching, milling, tapping, thread forming or stamping. Hard coatings are also used for injection molding, die casting, precision parts or automotive components. The coatings are generally deposited in vacuum by Physical Vapor Deposition (PVD) or Chemical Vapor Deposition (CVD) methods or their variants. The thickness of hard coatings varies between ~0.5 µm to ~10 µm. The most commonly used coatings are TiN, TiCN, TiAlN, CrN, ZrN, AlTiCrN, Al2O3 and their combinations. Basic coatings are usually single-layered, advanced coatings are composed of several layers in order to achieve the best working performance (adhesion, hardness, wear resistance). Since almost all hard coated components are in some type of sliding contact, their tribological and wear properties are very important. Low friction leads not only to energy savings but also to increased life time due to lower thermal load. High contact temperature due to high coefficient of friction can result in coating delamination and decreased machinability of the heated material. Hard coatings in sliding applications must therefore have sufficiently low coefficient of friction in order to avoid such damage or malfunctioning. The coefficient of friction and wear behavior can be characterized either using real life test such as drilling test[1], which is timeconsuming, or by a pin-on-disk tribometer[2,3]. The pin-on-disk method is used mainly in the initial phases of development of the coating because it is simple, fast and allows controlling of many contact parameters (counter body material, sliding speed, load, lubrication, duration, type of movement, etc.). The results of pin-on-disk test help in selecting perspective coating which will then be used for more extensive and more time consuming specific tests.

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