Investigation Of Thermal Shocking Life And CMAS Corrosion Of Nano 8YSZ Thermal Barrier Coatings

This paper prepared nanostructured coatings by atmospheric plasma spraying(APS)method using spry-dried nano 8YSZ powders.The thermal shock lives of nanostructured 8YSZ coatings prepared by different grain size of powders and the corrosion resiatence by CMAS glass were investigated.The different grain size of nano 8YSZ powders were got by heat treatment in different temperature.The average particle size,the internal morphorlogy and its influence on the melting behavior in the sprayings process were investigated;the main physical properties changes of strength,expansion coefficient,density,elastic modulus and hardness of the nanostructured coatings were tested,in order to analyze the influence on thermal shock lifetime changes of coatings;in addition,nano and conventional 8YSZ coatings were prepared by APS,respectively.The corrosion mechanism and failure process on 8YSZ powders and coatings by CMAS glass melt were analyzed by XRD,SEM and TEM,and the corrosion resistance of nanostructured coatings and traditional coatings were compared.The results showed that the higher of the treatment temperature on the agglomerated nano 8YSZ powder,the greater of the average grain size(from 18 nm increased to 55 nm at 1200?),the tammann temperature of grain growth was about 900?,grain growth was rapider when the temperature higher than 900?.At the same time,the average particle size of the powders decreased with powder densificated,especially at high temperature(up 1000?).In the process of plasma spraying,the surface part of 8YSZ powders melted,while the internal part still maintained a nanostructure,the melting degree increased with the average size of powders reducted and powders densitificated.The properties of coatings changed with the powders' changed,significant increases in density,bonding strength,elastic modulus and hardness were observed about the coatings prepared by powders calcined above 900?.However,the abnormal phenomenon occurred with the coating prepared by 1200?,which might be the increase of micro cracks in the coating as the degree of melting increased,and was been proved by nano indentation method.The nanostructured coatings had a bimodal distribution,i.e.,the columnar crystals,equiax crystals from melt-recrystallization(melted zone),and the nanozone zone reserved from nonmelted part of nano powders,and there were large number of pores and cracks in nanostructured coatings,too.The service life of the coatings is inversely proportional to the elastic modulus,due to the failure of the coatings was mainly by the internal stress which produced by the different thermal expansion coefficient between the coatings and substrates,so the higher the elastic modulus,the shorter thermal shoking lifes of the coatings.On CMAS corrosion,nano 8YSZ coating showed similar reaction to the reports on traditional 8YSZ coating,chemical damages and physical damages contributed to the failure of coatings,and the physical destruction contributed more.But compared with the conventional coatings,nanostructured coatings were more prone to be damaged by the more phase transition(from T phase to M phase),and more porosity and micro cracks in nano coatings,which provides a channel for the CMAS melt.So it was easy for corrosion damage and the CMAS glass corrosion on nanostructured coatings should be paid more attention.