Study On The Formation Of Residual Stress In Thernal Spray Coatings And Its Controlling Factors

Due to the nature of the thermal spray process, residual stress formed in thick coating is a clarification of objective. Residual stress affects the microstructure and micro-hardness of the coating, result in micro-cracks in the coating and reduce the strength between coating and the substrate. In some cases it could lead to cracks, even pull out, causing the failure of work piece.The residual stress of coatings is not only related to the kind of materials, but also affected by the spraying method, spraying parameters and the size of work piece. The residual stress-state that evolves in a deposit is largely dependent on the thermal conditions under to the system has been subjected, and is a combination of quenching stresses which arise during deposition, and cooling stresses post-deposition. Therefore, study residual stress in theory and experiment has great value for get high-quality thermal spray coating and re-manufacturing engineering coating technology.The work of this paper analyzed the formation mechanism of thermal spray coating stress, elaborated the main residual stress test method which in the process the residual stress produces, introduced some test method principle and evaluated respective superiority and the limitation. And, it designed a special experimental device, used curvature to measure the residual stress and studied the effects of the change of the parameters of spraying on the residual stress in the coatings. The work also analyzed the coating microstructure by SEM and studied the transformation process of residual stress changes in the mechanism. By means of electrical measurement and curvature method, Young's modulus of two coatings were measured, providing a research method and data reference for the coating stress study.The results show that Young's modulus of 4Cr13 coatings is 53GPa. The Young's modulus of 65Mn coatings is 105 GPa. Young's modulus of the coating is less than the Young's modulus of original material. The residual stresses of the coatings sprayed at different temperatures are different when cooled to room temperature. For the coating with the transformation, it exist a critical temperature which leads to a zero coating's residual stress. Sprayed at higher temperatures than the critical, the coating was at the state of tensile stress, while sprayed at lower temperatures than the critical; the coating was at the state of compressive stress. The critical point of 4Cr13 coating is about 260℃and that of 65Mn coating is about 300℃. With the increase of spraying current, the tensile stress of coating increases. Similar to the influence of spraying current, the increase of spraying voltage also have the tensile stress of coating increased.