Research On Competing Failure Behavior And Life Prediction Of Plasma Spraying Coating

Drilling implement are the key parts in the equipment of drilling engineering and geotechnicalengineering. The common surface failure such as abrasive wear, adhesive wear, contact fatigue,and corrosion wear greatly affects the service reliability, production safety and workingefficiency of drilling implement. Thus, in order to improve the wear resistance and corrosionresistance, avoid failure as well as prolong the service life of drilling equipment, advancedsurface engineering technologies are often used to strengthen the surfaces of damageable parts ofdrilling implement. Supersonic plasma spraying Al₂O₃/40%wt. TiO₂composite ceramic coatingswith high density, toughness and bonding strength, possess excellent wear resistance, relativelygood corrosion resistance as well as resistance to contact fatigue properties, which is suitable forsurface strengthening of damageable parts of drilling implement.This dissertation used the orthogonal test to investigate the effects of spraying parameters suchas spraying distance, spraying current, spraying voltage and argon flow rate on the coatingquality to determine the optimal spraying parameters. Various means of microscopic analysiswere used to characterize the microstructure, chemical composition and mechanical properties ofthe composite ceramic coatings.In order to study the failure mechanism and life evolution laws of the composite ceramiccoatings under the real contact conditions, a contact fatigue/wear multi-function testing machinewith double roller was made. It can achieve the simulation of "rolling","rolling/slidingcoexistence" and "pure sliding" contact conditions by adjusting the slip ratio, and can be used asa mechanical experimental platform to evaluate the wear resistance and contact fatigueperformance of coatings.Based on the new type contact fatigue/wear multifunctional testing machine, the failurebehavior and life evolution laws of the composite ceramic coating were investigated underdifferent contact stresses. The finite element method was used to simulate the distribution of themaximum shear stress and orthogonal shear stress within the coating as well as in the interfacebetween the coating and substrate. The effects of coating microstructure and shear stressdistribution on the initiation and propagation process of contact fatigue cracks were alsodiscussed. Empirical mode decomposition method was used to obtain the waveform andspectrum of typical acoustic emission signals during the coating delamination failure process ofcontact fatigue. The P-N and P-S-N curves of contact fatigue life were established. Theseanalyses can realize the contact fatigue life prediction of coatings under a certain contact stressrange.The competing failure behavior and life evolution laws of the composite ceramic coating were researched under different rolling/sliding contact conditions. It was found that when the slidingcontact existed, the main failure modes of coatings are surface wear and interface delamination.The distributions of the maximum shear stress on the coating surface, within the coating as wellas in the interface between the coating and substrate were also investigated. The effectmechanisms of surface sliding wear, brittle fracture and thinning process on the interfacedelamination failure were discussed. Empirical mode decomposition method was used toseparate the waveform and spectrum of typical acoustic emission signals during the surface wearfailure process under the rolling/sliding coexistence condition. It was found that the competinglife of the coating obeys the Weibull distribution, and the life is more dispersed at higher slipratio. The competing life P-N curve of the coating was finally established.