Hardness and adhesion of hard coatings on cutting tools
Hard coatings are used to increase the wear and temperature resistance 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. All these coatings must have good adhesion to the substrate and high hardness in order to ensure high wear resistance. This application report demonstrates the methods for measurements of adhesion, hardness and elastic modulus of hard coatings for cutting tools.
Introduction to hard coatings
Hard coatings are routinely used to reduce the wear in many types of applications. Coated tools (Figure 2) are used for cutting, punching, forming, tapping, thread forming or stamping. Hard coatings are also used for injection molding, die casting, precision parts or automotive components. Their main advantage is the reduction of production costs due to increase of lifetime of the tools and improved product quality. In addition, hard coatings with low friction in automotive and forming industry contribute to energy savings and longer service intervals.
Hard coatings are deposited mainly by Physical Vapor Deposition (PVD) or Chemical Vapor Deposition (CVD) methods or their variants (plasma assisted CVD, plasma enhanced CVD, HiPiMS, etc.). Their thickness varies typically in the range of ~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. High hardness, high wear resistance and good adhesion are crucial properties of these coatings and therefore have to be measured during coating development and manufacturing.
Mechanical properties of these coatings have traditionally been characterized using nanoindentation testers and high-load scratch testers. However, for many coatings it is now recommended to use lower scratch loads in order to concentrate the maximum shear stresses in coating-substrate interface. In such test conditions the failure of the coating-substrate system (determined as critical load) corresponds to adhesive failure and not to damage of the substrate due to large deformation.
Anton Paar Micro Combi Tester is a recommended instrument for adhesion and mechanical characterization of these coatings. It can apply loads from ~10 mN up to 30 N and is also capable of measurements of hardness and elastic modulus.
- N. Schwarzer, Q.-H. Duong, N. Bierwisch, G. Favaro, M. Fuchs, P. Kempe, B. Widrig, J. Ramm, Optimization of the Scratch Test for specific coating designs, Surf. Coat. Technol. 206 (2011) 1327–1335. doi.org/10.1016/j.surfcoat.2011.08.051.
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