Study On Stress Evolvement Of Plasma-sprayed Hydroxyapatite Coatings

The coating and substrate materials have some differences in thermo-physical properties and they undergo a series of rapid heating and cooling process,so the interfacial stress of the coating/substrate and internal stress are very large and they influence the coating properties and coating service life finally.In order to clarify the relationship between the stress and coating structure in the spraying process,this paper was devoted to the temperature and stress field in the spraying process by numerical simulation.Based on the simulation model,Coupling effect of maximum internal stress at interface and coating structure changes in different stress conditions were explored.Firstly,the temperature field in the spraying process was simulated.The temperature field simulation mainly includes the establishment of the model,the thermo-physical parameters,the heat input and the boundary conditions.The self-made heat flux meter can accurately measure the heat input under different process parameters,to heat dissipation mainly includes convection and radiation,by comparison with the experimental results,when heat convection coefficient is 6W/m2 K and coefficient of thermal radiation is 0.8W/(mK),the simulation of temperature field matched with the experimental value.Based on the simulation results of temperature field,the stress field is simulated.The coating microstructure was considered on the stress and the values were compared with the experimental result,found that joining the organization structure change,stress field simulation results are match with the actual stress in the coating field.Maximum stress and residual stress at the interface show that the spraying power is smaller,the distance is longer,and the temperature is smaller,the maximum stress and residual stress is smaller.The single thickness of the coating layer is thinner,the maximum stress at the interface of is smaller,and when the thickness of the coating is increased,the interface maximum stress first decreases and then increases steadily even.It is found that the predicted values of residual stress on the surface of the work piece are basically similar that of the X ray detection,and the greater thickness of the single layer coating is,the larger the maximum stress of the coating is.The effect of stress on the crack propagation was studied by using the extended finite element method(XFEM),it was found that if the compressive stress in the coating,crack was mainly the transverse crack,if the tensile stress exists,large cracks in the coating was mainly the longitudinal crack.Changing the experiment temperature of matrix was 300 ℃ to 500 ℃,coating crack propagation results were consistent with simulation results.The greater the residual stress in the coating,the more obvious the crack is.The larger the crack density is,the smaller the residual stress of the coating is.The crack propagation can release the residual stress in the coating to some extact.