Basic Researches On The Structural Integrity And Life Prediction Of Plasma-Sprayed Coating-Based Systems Aiming For Remanufacturing

The remanufacturing engineering technologies have been widely used with great success for the maintenance of modern constructions to extend the lives of components. The aim of remanufacturing engineering was to extend the service lives and improve the service performance of components by using the advanced surface technologies, such as thermal spraying, on the basis of the scientific evaluation of servicing conditions of components. The scientific evaluation of the structural integrity of remanufactured components and the life prediction of them should be included in the remanufacturing engineering.The aim of this paper was to provide some foundational researches on the structure integrity and life prediction of plasma-sprayed coating-based systems and to provide some important designing methodologies for these systems. These researches can provide some scientific insights for the development of remanufacturing engineering. Three different topics were involved, i.e., the thermo-mechanical integrity of the coating-based systems with residual stresses, the microstructural integrity of plasma-sprayed coatings relating to the processing parameters, and the durability of plasma-sprayed coatings under the applied loading conditions.The thermo-mechanical integrity of the coating-based systems with residual stresses was investigated systematically. The analytical models with closed-from solutions based on the force and moment balances were developed to predict the residual stresses within the multilayered coating-based systems and the systems with graded properties and compositions. The effects of material properties and thickness of the coating on the distributions of residual stresses within the coating-based system were studied. Several optimal designing methodologies were proposed to improve the performance of the coating-based systems on the basis of the knowledge on residual stress distributions. Additionally, the error analyses of different typical assumptions of residual stresses in single-layered coatings were performed and a correction factor was added to Stoney's equation to improve its prediction ability.The effects of plastic deformation, creep deformation of the coating and the geometrical discontinuities on the redistributions of residual stresses within the coating-based systems were investigated systematically. First, the effect of plastic deformation of the coating on the residual stresses within the coating-based system was studied. Result showed that, when the residual stress in the coating was of the same sign, the plastic zone thickness was proportional to the temperature difference. Second, the concept of stress relaxation ration was proposed to predict the residual stress redistributions in the coating-based system when the coating or the substrate was subjected to the creeping deformation. The stress relaxation ration can be characterized by the ration of the creep strain and the thermal misfit strain between the coating and substrate. Third, the analytical model on the basis of strength-of-material theory was developed to investigate the edge-effect on the residual stress redistributions within the coating-based systems. The closed-form solutions on the residual stresses within the coating and at the interface between the coating and the substrate were obtained. Fourth, an expression of the critical misfit strain, which was related to material properties and the dimensions of the coating and substrate, was proposed to determine the critical condition of the coating surface crack initiation. Fifth, the designing route for the interfacial delamination resistance of the coating-based systems was developed on the basis of the knowledge on interfacial stress distributions.The processing effects on the microstructures and mechanical properties of Ni-Cr-B-Si alloy coatings prepared by supersonic plasma spraying were investigated systematically. The processing-microstructure-properties maps were attempted to set up. Static analyses were performed on the variations of porosity, micro-hardness and Young's modulus of the coatings. The residual stresses at the surfaces of the coatings were also determined. Results showed that the experimental data of porosity variations, micro-hardness and Young's moduli of a coating followed the Weibull distributions. The scattering degree of the experimental data was related to the spraying parameters. With increasing the hydrogen gas flow rate, input power or decreasing the powder feed rate, the porosity level of the coating generally decreased. There was not a certain relationship between the porosity and the hardness as well as Young's modulus of the coating. This result indicated that the porosity along was not accurate for characterizing the effective mechanical properties of the coating. However, from the statistic trend, the micro-hardness as well as the Young's modulus of the coating decreased with increasing the porosity of the coating. Experimental results showed that the mean value of Young's modulus of the coating is proportional to the square root of the mean value of micro-hardness of it. The residual stress at the coating surface was generally increased with increasing the Young's modulus of the coating.The durability of supersonic plasma-sprayed CrC-NiCr cermet coatings was evaluated by the rolling contact fatigue tests under different tribological conditions of contact stress at room temperature. The coupling relationship between the contact stress and fatigue life of plasma-sprayed coating was obtained. At high contact stress, the fatigue lives of the coatings exhibited high scattering. Experimental results showed that it might be impossible to set up a certain relationship between the failure mode of coating and the magnitude of the contact stress. However, when the contact stress was low, the probabilities of the abrasion and spalling type failures were relatively high. At higher contact stresses, the coatings were prone to fail in interfacial delamination.The rolling contact performance of the supersonic plasma-sprayed Ni-Cr-B-Si coatings remelted by laser was also investigated. Results showed that spalling type failure was the main failure mode of remelted coatings. The formation of the spall was due to the joining of the cracks at the subsurface. A new processing model was proposed to illustrate the formation of the spalls on basis of the worn surface and cross-sectional observations.