Study On Laser Shock Processing Technics And High Temperature Failure Mechanism Of CoNiCrAlY Bond Coat

With the rapid development of aero engines in recent years,traditional thermal barrier coatings?TBCs?have been unable to meet their needs.How to solve the premature failure of thermal barrier coatings is a key technical problem facing the development of high-performance aeroengines.The service life of thermal barrier coatings in high-temperature environment is mainly related to the high-temperature oxidation behavior of the transition layer-bond coat.There are many different methods to improve the high-temperature oxidation,the surface modification is one.Among various modification methods,laser shock processing can flexibly adjust the surface morphology and achieve grain refinement,introducing defects such as dislocations and twins.Therefore,in this paper,the CoNiCrAlY bond coat is used as a research carrier,and modified by laser shock processing,in order to improve the high temperature service performance of thermal barrier coatings.In this experiment,atmospheric plasma spraying?APS?and APS+laser shock processing were used to prepare CoNiCrAlY bond coat on the surface of GH4169 nickel-based superalloy,then depositting a ceramic coat 7⁸ wt.%Y₂O₃-ZrO₂?7-8YSZ?by electron beam physical vapor deposition?EB-PVD?to form the thermal barrier coatings.High temperature oxidation and thermal shock test were used to evaluate the high temperature service performance of thermal barrier coatings before and after laser shock processing.Using X-ray diffraction,laser Raman spectrometer,scanning electron microscope and other characterization,testing methods to focus on the micro-morphology,phase structure,hardness and the effect of stress distribution of thermally grown oxide?TGO?at ceramic coat/bond coat interface of thermal barrier coatings by laser shock processing.?1?The three-dimensional model of laser shock processing bond coat was established by using ANSYS finite element.The number of laser shock processing and the spot spacing were optimized by calculation,and three sets of better parameters were obtained:laser shock processing 1 time,spot spacing 4 mm;laser shock processing 2 times,spot spacing 4 mm;laser shock processing 1 time,spot spacing 5 mm.?2?The air plasma sprayed CoNiCrAlY bond coat was surface-modified using the optimized laser shock processing parameters obtained by simulation,and then 7-8 YSZ ceramic coat was deposited on the surface of it using EB-PVD to form a complete thermal barrier coatings.The results showed that after the laser shock processing of CoNiCrAlY bond coat,the diffraction peak of?-Ni phase shifts to a low angle,and the diffraction peak of?-NiAl phase is broadened.Laser shock processing reduces the porosity of the bond coat and increases the average cross-sectional hardness of thermal barrier coatings.When laser shock processing twice,and the spot spacing is 4 mm,the porosity of the bond coat is the smallest 2.2%;and the average hardness of the cross-section of thermal barrier coatings reaches the maximum 296.9 HV.?3?The high temperature oxidation behavior of thermal barrier coatings prepared by different laser shock processing technics was analyzed.The results showed that:defects such as cracks and holes appear in the TGO layer after 1000°C for 100 h without laser shock processing.Under the same conditions,laser shock processing results in continuous dense TGO layers.During the oxidation processing,the stresses at the ceramic coat/bond coat interface are all compressive stresses,and laser shock processing reduces the stresses.When laser shock processing twice and the spot spacing is 4 mm,the stress at the ceramic coat/bond coat interface is the lowest:68.5 MPa after oxidation at 900°C for 100 h,and 68.7 MPa at 1000°C for 100 h.?4?The thermal shock behavior of thermal barrier coatings prepared by different laser shock processing technics was analyzed.The results showed that laser shock processing slows down the consumption rate of Al element and reduces the stress caused by the continuous thickening of TGO at the interface between ceramic coat and bond coat.Among them,when the laser shock processing is carried out twice,and the spot spacing is 4 mm,the stress value of the interface between the ceramic coat and the bond coat is the smallest:15.8 MPa after 205 thermal shocks at1000°C.Moreover,at this time,the area of thermal barrier coatings thermal shock failure is the smallest,which is 4.2%.