Investigation Of The Mechanical Properties And Fracture Behavior Of Plasma-sprayed 8YSZ Thermal Barrier Coatings

Thermal barrier coatings(TBCs)have been widely employed in the hot-section of gas turbine to protect the superalloy components against high temperature and oxidation,and thereby to prolong the lifetime of components and improve engine efficiencies.Howerver,TBCs are inevitably subjected to mechanical loads,thermal or residual stresses and high-temperature sintering in service,which strongly affect their mechanical properties,trigger the damage evolution and even result in the delamination of ceramic coatings.In this paper,8wt.%yttria-stabilized zirconia thermal barrier coatings were prepared by air plasma-sprayed(APS)technology and then subjected to thermal exposure at a temperature of 1100? in a normal atmosphere for various times.Scanning electron microscope(SEM)equipped with an energy dispersive spectrometer(EDS)was adpoted to analyze the microstructural evolution of the thermal barrier coatings and the growth process of thermally grown oxide(TGO).And next,an instrumented indentation testing system with appropriate settings for indentation load and load control was used to measure Young's modulus,hardness,fracture toughness and residual stress in cross sections of 8YSZ coatings with different heat treatment time.Subsequently,the evolution of mechanical properties of APS 8YSZ coatings during the process of heat treatment was discussed.In addition,acoustic emission(AE)technique was adopted to monitor the damage evolution of 8YSZ coatings during instrumented indentation testing,and then the failure mechanism was investigated by cluster analysis and wavelet transform methods.The effect of high-temperature heat treatment on coating damage evolution was explained as well.The experimental results showed that TGO growth followed a parabolic law with a growth rate constant of 0.13236 ?m2/h and reached a thickness of 6.85?m after 350 h.EDS analysis confirmed that,the TGO was eventually composed of an Al2O3 layer and mixed oxide clusters.These mixed oxides,typically of(Cr,Al)2O3,(Co,Ni)(Cr,Al)2O4 and NiO,abbreviated as CSN,which can compromise the durability of TBCs.Image analysis showed that the average porosity in 8YSZ coatings showed a reduction trend with the increase of heat treatment time as a result of activated diffusion-controlled sintering.These variations in microstructure can affect the stress distribution and mechanical properties in the 8YSZ coatings.The indentation testing results showed that the variation of Young's modulus and hardness with the increase of heat treatment time can be divided into two regimes:a rapid initial increase,followed by a more gradual increase.On the contrary,fracture toughness of 8YSZ coatings decreased from the initial value of 1.48 to 1.05 MPa·m1/2 after 350 h.And the corresponding residual stress in top coat varied from the deposition stress of-29.9 MPa to a maximum of-56.0 MPa within 100 h,and then decreased to-15.0 MPa after 350 h.The results of cluster analysis showed that there are three classes AE signals associated with distinct failure modes for the 8YSZ coatings under Vickers indentation.Based on wavelet transform,these three classes can be clearly distinguished from their dominant frequency bands,which were concentrated on levels A5(0-156.25 kHz),D5(156.25-312.5 KHz)and D4(312.5-625 kHz),respectively.Thus,failure mechanism of 8YSZ coatings under Vickers indentation can be clarified by the distribution of different failure modes in indentation depth.The results indicated that,as indentation load increases,the 8YSZ coatings can accommodate the indenter by elastic or little plastic deformation,microcracks propagation and then disbonding at the splat boundaries.By comparing the features of AE ring-down counts for samples with different heat treatment time,it can be inferred that high-temperature heat treatment can accelerate the degradation of APS 8 YSZ TBCs.