Study On The Mechanism For The Formation Of Surface Cracks In TIN Coating

With the development of science and technology, surface coating technology hasbeen increasingly applied in Engineering. Because of its good performance such aswear-resistance, heat-resistance and corrosion-resistance, TiN coatings are widelyapplied to the contact surface of cutters, bearings and gears, and it can effectivelyalleviate the friction of component surface, prolong their service time and meet theperformance requirement of modern equipments. However, when coating structures areoperated at a high temperature, and bear alternating loading under frictionaly slidingcontact, surface cracks will be induced. With the popagating of cracks, coating failurewill occur, so as to influence severely applications of coating components. Therefor, itis of great significance to study the forming mechanism of coating surface cracks.In order to reveal the mechanism for the formation of surface crack in the TiNcoatings, the works of below have been done in this paper:（1）Both scratch test and energy spectrum analysis are carried out on TiN coatingin this paper. Results of the scratch test show that arc-shaped cracks, which have anaverage distance of5.12μm, come into being on the surface of the scratch. The indenterdid not press into the basement in the scratch test process；Indentation test was carriedout on TiN coating. Results of the Indentation test show that the Hardness of TiNcoating is10.17GPa and the Fracture Toughness of TiN coating is3.30MPa·m1/2.（2）The three-dimension finite element method is applied to simulate the scratchprocess and the results indicat stress distributions in the coating and reveal the formingmechanism of surface crack. Through taking account of the stress distributions in thecoating, a predicative analysis is made of the causes possible for the generation ofsurface cracks in coatings.（3）The2D extending finite element method (XFEM) is applied to simulating thescratch process and the result shows that surface cracks and interface cracks induced bythe maximum tensile stress in the coating are modelⅠ. The sub-surface cracks, whichare caused by shear stress and tensile stress are compound one. The surface crack isformed in the following three ways. The first one is that at first, cracks emerge fromthe surface of a coating and then surface cracks are formed with the extension of crackstowards the interface. The second one is that surface cracks are directly formed on thesurface of a coating. The last one is that at first, cracks are formed on the sub-surfaceof a coating and then surface cracks come into being with the popagation of cracks towards the sub-surface. In the process of simulation, it is found that the surface andthe interface generate more than the sub-surface does. With the popagation and fusionof cracks, the coating will spall.（4）The3D extending finite element method (XFEM) is applied to simulating thescratch process and the result shows that surface cracks and interface cracks in thecoating that result from the maximum tensile stress are modelⅠ. The surface crack isformed in the following two ways. The one is that cracks occur from the coatinginterface and then surface cracks are formed with the popagation of interface crackstowards the surface. The other one is that surface cracks are directly formed on thesurface of a coating.These will provide basis for studying coating failure.