Research On Residual Stresses Of Coatings By Nanoindentation Technology

Drilling implement is indispensable equipment in engineering geological exploration. Thesecurity and service life of drilling implement are seriously affected by common surface failuresuch as wear, corrosion and fatigue. Advanced surface engineering technologies are often used toremanufature the drilling implement to improve the wear resistance, corrosion resistance andfatigue resistance, which is an effective way to ensure the service safety. Laser cladding coatingsand supersonic plasma spraying coatings having excellent wear resistance, corrosion resistanceand fatigue resistance are suitable for remanufaturing of drilling implement. However, theresidual stresses produced in the preparation process have important influence on the mechanicalproperties and service persistence of remanufatured coatings. Therefore, the study on the residualstresses of remanufatured coatings has important significance to improve the service life andreveal the failure mechanism.In this dissertation, nanoindentation tests were performed on the single crystal copper and1045steel after applied to different stresses by a new designed stress-applying device. Theeffects of residual stresses on the nanoindentation parameters such as loading and unloadingcurves, pile-up deformation around indents, and contact area were researched. The resultsshowed that compared with stress-free state, tensile stress will increase the indentation depthwhen the load is fixed, and will decrease the indentation load when the depth is fixed. No matterwhich indentation mode, the residual tensile stress will reduce the elastic recovery whenunloading, the pile-up height and contact area. While the influence of residual compressive stresson the indentation parameters shows the opposite law.The real contact area is an important characteristic parameter for the determination ofresidual stresses, but the traditional O&P model ignores the pile-up area in the calculation of thereal contact area, and thus serious errors will be introduced. Aiming at this problem, thisdissertation considered the real contact area as the sum of pile-up area and the triangle areacalculated by O&P model. The new proposed method is suitable for the area calculation ofmaterials that pile up, which well corrects the error of O&P model.Based on the new designed stress-applying device, the accuracy and applicability of theSuresh models (fixed load model and fixed depth model) and the Lee model were compared andanalyzed in the calculation of residual stresses. The Suresh models and Lee model were not suitable for soft materials. For hard materials, the residual stresses calculated by the fixed loadSuresh model were too large; the tensile stresses determined by the Lee model had big errors.The stresses calculated by the fixed depth Suresh model were the most accurate.Based on the fixed depth Suresh model, the surface and cross-section residual stresses of Fe90laser cladding coating and FeCrBSi supersonic plasma spraying coating were studied andverified by traditional X-ray diffraction method. The results showed that nanoindentation methodcan accurately measure the surface and cross-section residual stresses of the Fe90laser claddingcoating with dense structure, as well as the surface residual stress of FeCrBSi coating with lesspores. However, for the cross-section of supersonic plasma spraying coating with relatively morepores, the calculated stress value may have certain errors.