Study On The Failure Mechanism Of TiN Coating Under Frictional Contact Based On The Scratching Experiment

At present, the surface coating technology has become an important means to improve the fatigue resistance and the abrasion resistance of material. Many parts such as cutting tools, gears and bearings that improve contact performance by surface coating technology. Due to the inevitable defects and the elastic parameter discontinuity between the coating substrate in the coating manufacturing process, the coating structure is easy to crack under the contact stress, The coating will flake off with the crack propagation that causes the failure of the parts at last. In order to meet the reliability requirements of the coating structure in engineering application, it is necessary to study the failure mechanism of the coating structure under the contacting friction.In this paper, titanium nitride coating is made by Plasma assisted chemical vapor deposition technology. Its thickness is 10 μm and its base is high-speed steel. The hardness of the coating is 2000HV-4000 HV which is measured by the micro hardness tester. The elastic modulus and fracture toughness of the coatings is respectively 590 Gpa and 3.30 MPa21/ 2m which are measured by the nano indentation meter. The method of scratch is essentially belong to the friction contact problem. The result which is observated and analysed for the coating scratch surface by SEM show that the coating surface has arc cracks and the crack spacing is 5.1 microns. The friction coefficient of the coating surface is about 0.25.The finite element software is used to simulate the sliding process of the indenter in the coating surface. It can get the stress nephogram and the stress change curve at the stage of the indentation and sliding. It can analyse the stress distribution characteristics of the coating. According to the distribution orderliness, it reveals the cause of the crack initiation and the location of the crack.According to the extended finite element theory, the ABAQUS is used to simulate the initiation and propagation of the coating crack during the sliding process of the indenter in the coating surface. The crack produced firstly at the coating interface and then expanded to the surface along the thickness direction at the stage of the indentation. The coating surface and interface appeared a lot of cracks, and there were a few cracks in the subsurface at the stage of the sliding. The cracks of the coating surface and interface are mainly the I type of crack caused by the maximum tensile stress. The simulating result of the indentation reciprocating sliding in the coating surface showed that coating surface cracks under stress will expand to the coating interface. And the coating interface cracks will extend to the coating surface. This will finally form the through cracks which can lead to the spalling of the coating.The above work provides certain basis for engineering application of coating and optimized design of coating structure.