Friction and wear testing of multilayer coatings on carbide substrates for dry machining applications

The principle aim of this paper is to investigate three wear-protective coatings with multilayer structures, which are frequently used in the cutting tool industry and to assess their frictional behaviour under the test conditions equivalent to those for the cutting of medium carbon steel. A modified pin-on-disc tester was used to conduct experiments in which both the friction coefficient and the linear wear of the tribo-pair were recorded vs. sliding distance. The volumetric wear rate was proposed as a parameter for quantitative comparison of the wear resistance of the tribo-pairs tested. It was found that the principle stage of the specimen wear takes place during the first 200-240 s of sliding independently of the applied speed; obviously, after this stage the intensity of wear decreases drastically. The maximum values of specific wear rate observed at the lowest sliding speeds applied decrease significantly with the elevation of the sliding speed for all tribo-pairs, especially for coated specimens. The use of coatings with an Al2O3 intermediate layer at the lower sliding speed of approximately 0.5 m/s deteriorates the wear resistance of the coated specimens, but its protective role becomes more important at the high speed conditions in comparison to an uncoated carbide specimen. The maximum wear resistance was revealed in the case of a carbon steel-TiC/TiN coating tribo-pair for sliding speeds ranging from 0.5 to 3.0 m/min. It is concluded that the proposed methodology for experimental friction and wear quantification provides the new knowledge for multilayer coating structures on the cutting tool wear resistance and can be helpful in the optimum selection of coated tools for the dry cutting of steels. (C) 2002 Elsevier Science B.V. All rights reserved.