Preparation And Failure Mechanism Of YSZ Coatings Deposited By Plasma Spray-Physical Vapor Deposition

With the development of engine,higher requirements have been put forward for the reliability and life of thermal barrier coatings(TBCs),which promotes the update of preparation technology of TBCs.At the moment,the two main typical technologies used for thermal barrier coatings deposition are plasma spraying(PS)and electron beam-physical vapor deposition(EB-PVD).The coatings prepared by the PS process have a typical layered microstructure with a large number of interfaces,micro-cracks and pores,which is beneficial to reduce the thermal conductivity and improve the thermal insulation performance of the coatings.However,the bonding strength of the coatings is relatively low,which is easy to induce the horizontal propagation of microcracks under the condition of thermal mechanical coupling,leading to the cracking and spalling of the coatings.PS equipment has the advantages of high deposition rate,wide range of coatings thickness control and relatively low cost,which is suitable for the preparation of thermal barrier coatings for large-size engine stationary components.The columnar coatings prepared by EB-PVD have a good bonding performance with the substrate.The inter columnar gap can alleviate the thermal mismatch stress in the process of high and low temperature,improve the strain tolerance,and thus improve the thermal shock resistance of the coatings.However,the low deposition rate of EB-PVD is not suitable for the preparation of thick TBCs since the uncontrollability of the vapor pressure of the gasification of multicomponent bonding layer material is likely to cause composition deviation in coatings.The complex process leads to high cost in facilities and manufacture craft.Plasma spray-physical vapor deposition(PS–PVD)combines the advantages of PS(high deposition rate and cost efficiency)and EB–PVD(the ability to produce columnar structured coatings).TBCs with the initially favorable columnar microstructure can be prepared by PS–PVD,which combines the high thermal insulating property of the PS coatings and the high strain tolerance of the EB–PVD coatings.Therefore,PS-PVD is expected to become a characteristic technology for the preparation of advanced engine TBCs.At present,nearly ten research institutions at home and abroad have carried out basic research on the preparation science and application of PS-PVD TBCs.The gasliquid-solid three-phase interaction of the PS-PVD plasma flame flow,the movement of the powder spraying medium in the high enthalpy plasma flow field,the coating deposition mechanism and the failure mechanism of the coating under high temperature simulated service condition are not yet available perfect.In this paper,YSZ TBCs were prepared by PS-PVD process.This work studied the influence of process parameters on the coatings microstructure and performance,investigated coatings deposition characteristics,and preliminarily explained the deposition mechanism.On this basis,thermal stability of the coatings under the condition of thermal mechanical coupling was investigated,and service performance and failure mechanism of the coatings at high operating temperatures were explored,which provided a scientific basis for the development and application of a new generation of high reliable and long service life thermal barrier coatings.The main conclusions obtained from this work are as follows:(1)The transport mechanism of YSZ powder in high enthalpy plasma flame is as follows:(1)50 mm from powder feeding port to muzzle: agglomerated powder was dispersed and partially melted.The 7.5 wt% Y₂O₃-ZrO₂ mixed powder with carrier gas into the spray gun was impacted,agitated and collided by gas flow.The soft agglomerated powder was dispersed into micron/submicron particles,which were pushed out of the gun muzzle by high temperature plasma and partially melted by heating;(2)Gun muzzle 500-600 mm: the powder was fully melted and gasified.Most of the solid and liquid phases were accelerated along the axis by the impact of plasma flow,and a small amount was dispersed to the edge of flame flow.The gas-liquid-solid three-phase flow away from the muzzle was characterized by shock wave oscillation,and the powder was fully melted and gasified under the action of high temperature;(3)At the muzzle 600-1400 mm,clusters were formed by the collision nucleation of gas phase monomer components in the steady laminar flame.(2)Using optimized spraying process parameters,the controlled deposition of 20-30 ?m single column diameter,metastable tetragonal phase and non-preferred orientation columnar coatings was realized on the surface of the pre-oxidized bond coat.In the non-line-of-sight deposition(shielding diameter is 45 mm)of static spray mode,the coatings perpendicular and back to the incident direction of plasma presented dense columnar structures,and the deposition thickness was 16% and 10% of the front deposition,respectively.(3)The deposition mechanism of PS-PVD columnar coatings was gas deposition.In the initial stage of vapor deposition of columnar coatings,nucleation dominated homogeneous and heterogeneous cluster deposition,accompanied by diffusion and mass transfer,the nuclei adsorbed atoms to form island structure,and gradually covered the substrate surface to form equiaxed denser layer.The adjacent island structures grew preferentially under the shadow effect,forming a columnar growth trend.In the middle and late stage of deposition,the surface diffusion enhanced and the nucleation was inhibited,which promoted the growth of columns,and the columns with low surface energy were dominant and continued to grow.The columns with fast growth preferentially adsorbed the gas phase components,while the columns with slow growth stopped growing.Finally,the columnar structure coatings were formed under the influence of the shadow effect enhanced with coatings growth.(4)The microstructure,phase and mechanical properties of PS-PVD coatings were changed by high temperature heat treatment.After thermal aging treatment,the feather-like structure of the columnar coatings disappeared,and the columns became smooth,adhered and bridged together.In the early stage of heat treatment,the grains of the coatings gradually transformed into equiaxed grains with increasing size,and the mechanical properties of the coatings such as hardness and elastic modulus were significantly improved.In the middle and late stage of heat treatment,t/m phase transformation occurred accompanied by 3-5% volume expansion,which lead to crack initiation and propagation,resulting in the decrease of mechanical properties and structural stability.Through process optimization,the solid particles of monoclinic phase in the gap between the columns were eliminated,which lead to the improvement of phase composition,thus increasing the strain tolerance and ensuring thermal stability of columnar coatings.(5)Based on the evolution of residual stress,the failure mechanism of PS-PVD coatings was explained.PS-PVD coatings can effectively release the residual stress generated in the cold and hot process through spot spallation of columns,which greatly improve thermal cycle life.After thermal shock cycle tests,the residual compressive stress of PS-PVD topcoat basically maintained a relatively stable value,TGO stress increased rapidly at first and then decreased gradually,and the coatings failure basically occured at the interface of topcoat/TGO/bond coat.After burner rig tests,the compressive stress of PS-PVD topcoat increased gradually with the increase of thermal cycles,TGO stress decreased first and then maintained a basically stable value,and the failure of the coatings occured not only at the interface,but also in the topcoat near the interface.After thermal shock cycles and burner rig tests,the crack initiation,propagation and penetration behavior were not exactly the same,which leads to significant different in failure mechanism of the coatings.