| Surface engineering technology is a key to the technology of green remanufacturingengineering and coatings of all kinds have been applied in every part of human social lives fortheir excellent properties, such as good corrosion and wear resistance. However, the differenceof performance parameters between coatings and substrates as well as the characteristics ofvarious preparation methods lead to the appearance of residual stress in both coatings andsubstrates. The existence of residual stress has an obvious effect on the superior properties ofthe coatings and it is also the main factor that causes the crack and break off of the coatings.Thus, the measurement of residual stresses in coatings is significant not only for theunderstanding of the origins of residual stress but also for the investigation of influences ofresidual stress on the coatings'properties.In our study, Fe314 alloy was cladded on the surface of 1045 steel by plasma claddingprocess and the nano-Al2O3/Ni composite coatings were plated on 1045 steel by electro-brushplating technique as well. Heat treatment of the corresponding coatings was conducted atdifferent temperatures. Scanning electronic microscope (SEM), X-ray Diffraction (XRD), X-raystress determinator and nano-indentation tester were used to investigate the morphologies, thephase structures, the residual stress and mechanical properties of the coatings, respectively. Themain part of this paper focus on the measurement of residual stress of the two coatings bynano-indentation and the discussion of the corresponding mechanics. The determination ofcharacteristic parameters and its affecting factors in this method were also investigated carefully.In addition finite element analysis (FEM) was used to simulating the indentation process of thecoatings as well. The true plastic properties and the distribution of stress of the coatings wereobtained during this process.The results showed that both the plasma cladding Fe314 coatings and the compositenano-Al2O3/Ni coatings belong to material that pile-up which indicated that the O&P methodwas no longer suitable to the calculation of characteristic parameters, such as hardness and elastic modulus. Then G&P method was adopted to detect the characteristic parameters of thecorresponding coatings and it proved that the degree of pile-up and residual stress of thecoatings had an combination effect on the calculation of characteristic parameters. Modelsestablished by S.Suresh and Yun-Hee Lee were used to measure the residual stresses of thecorresponding coatings at fixed depth and load, respectively combined with the application ofnano-indentation and the results were compared with the values obtained by X-ray stressdeterminator. It can be seen that the residual stress obtained by these methods showed the samedistribution while residual stress calculated by indentation model was much more accurate. Inaddition the numerical simulation found the true constitutive relations of the correspondingcoatings and showed the distribution of stress for different coatings at certain depth.The theoretical research of this dissertation demonstrated that the application ofinstrumented indentation technique in detecting residual stress of materials at small scale isavailable and has broad prospects for future development. |
