STUDY OF PARAMETERS AFFECTING FRACTURE OF TITANIUM NITRIDE FILMS (TiN) Marcos F. Odorczyk1,*, José D. Bressan1, Luiz C. Fontana1 1PGCEM, University of Santa Catarina State, Joinville-SC- 89219-710, Brasil * Corresponding author: Dem6mdo@joinville.udesc.br ABSTRACT: Present work examines theoretically the relationship between micro-cracks and failure criteria for fracture modes I and II in coating films. TiN films deposited by magnetron sputtering on substrates of steel or aluminum, with homogeneous (HMG) or functionally graded (FGM) structure were considered. The stress field in the film and substrate was generated from the Hertzian normal pressure and tangential force due to the friction from loading displacement at constant sliding velocity on the film surface. At the interface of the film and substrate, there are substantial differences among material properties. These differences, together with the film deposition conditions, can induce the formation of imperfections in the film condensation front, thus, becoming a region of potential micro-cracks formation. Stress-intensity factors at crack tip and a fracture criterion for combined rupture modes I and II in function of TiN film thickness, structure type (FGM x HMG), friction coefficient µ and substrate, were analyzed. A variance study by ANOVA has indicated that for both fracture failure mode I and II, the material factors, substrate and film structure, have higher significance than other factors analyzed. Film thickness is a significant factor, but not at the same level as material factors. Aluminium substrate with HMG coating has a higher risk level than steel substrate. FGM films reduce drastically the fracture failure risk mode I and II, when compared with HMG films,for both substrate. On FGM films, and HMG thin films over steel substrate, the mode II is more importante than the mode I, although the values are not high. For other cases, the mode I is more significative. The friction, altough is a important fator, has not the same significance level when compared with the others investigated. Key-words: FGM, micro-cracks, TiN film, fracture criterion, combined fracture mode I and II. 1. Introduction The use of coating films is based on the idea to combine materials with different properties, in order to obtain the benefit of both constituents. However, such differences lead to a non-continuous mechanical behavior at the interface film and substrate. To reduce this functional discontinuity, materials were developed whose properties vary gradually through the thickness direction and are called Functionally Gradade Materials (FGM). One consequence of this properties discontinuity is delamination of the film at the interface with the substrate due to load applications. This behavior depends on load factors, geometry, surface roughness and material of the film and substrate. The variation of the properties in films FGM can follow linear or exponential laws, making the transition smoother interface than the homogeneous films. Several works have been presented in this area. Shodja [1] presented a study with analytical solutions for the temperature field and stress of a tribological pair under thermomechanical loading. Odorczyk [2] presented a study with numerical solutions for the same case. The tribological pair study consists of a displacement normal force over the surface of a TiN film HMG or FGM deposited on a steel or aluminum substrate. The stress intensity factors KI and KII were analyzed in relation to the main normal and shear stresses at the interface. By analysis of variance, it was evaluated the significance of the factors friction, thickness, substrate material and film strucutre on a mixed KI/KIc and KII/KIIc criteria.
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