Structure Control,Properties Assessment and Durability Improvement of Air Plasma Sprayed Thermal Barrier Coatings

Author:Zou Zhong Hua

Supervisor:xiao ping zhao xiao feng


Degree Year:2018





Air plasma sprayed(APS)thermal barrier coatings(TBCs)are widely used in aero-engine and land-based gas turbine to reduce the temperature of the superalloy component and improve the operation efficiency.Typical TBC system consists of four layers:ceramic top coat,metallic bond coat,a thermally grown oxide layer formed at top coat/bond coat interface during high temperature service and the superalloy substrate.The drving force of the failure cracks comes from the thermal stresses due to the thermal mismatch between the ceramic and metal,and the stresses due to interface oxidation.Both the changes in microstructure of top coats and oxidation of bond coats can reduce the driving force of crack initiation and propagation.To reduce the failure drving force,new ceramic top coats were studied by the formation of the vertical cracks and using a novel powder in APS processing.In addition,the changes in oxidation behavior by modification of the bond coat structure can also reduce the failure driving force.The research content and conclusions is described as follows:(1)Formation of the vertical cracks during thermal test reduced the residual stresses in the top coat,then enhanced the lifetime.The microstructure and the stress state of the TBC were adjusted by controlling the APS parameters.To determine the residual stresses in the top coat accurately,Cr3+piezospectroscopy sensor within the alumina impurity in the top coats was used.The results suggested that drving force to generate the vertical cracks in the top coat is the tensile stresses resulting from coating processing and M-T phase transformation.The measured value and the theoretical estimation is on the same level,suggesting the stress measurement by Cr3+piezospectroscopy sensor is a practical way to APS-TBCs.(2)The zirconia powder with novel porosity was used to deposite thermal barrier coatings.Compared with the TBCs sprayed by commercial powders,the lifetime of the new TBCs increased by 2-3 times.The higher strain tolerance to reduce the thermal stresses plays the key role to the enhanced durability.To test the strain tolerance,the combination of the Raman spectroscopy with the indentation technique was developed.The TBCs sprayed by the novel powder showed a higher strain and damage tolerance.The TBCs showed a low in-plane Young’s modulus by three-point bending test.It also suggests that the comparison results of the strain tolerance by Raman spectroscopy and indentation is an effective way.(3)The interface cracking behavior in APS-TBCs with a double-layered bond coat was investigated,which can establish a theoretical basis for the lifetime improvement.An inner bond coat was firstly sprayed on the substrate by high velocity oxy-fuel(HVOF),then the outer bond coat was deposited by the HVOF or APS.Significant internal oxidation occurred in the two types of outer bond coats during high temperature exposure.In the APS outer bond coat,the internal oxides formed along the splat boundaries.The laminate structure of the internal oxides supressed the supply of Al from the inner bond coat to the outer layer,then amount of spinel formed within the outer bond coat and around the interface,finally to promote the coating failure.The internal oxides also formed in the HVOF outer bond coat,while the scattered distribution did not affect the diffuson path of Al from inner bond coat toward the interface.The internal oxidation resulted the top coat/TGO(thermally grown oxide)interface roughening due to the non-uniform distribution of internal oxides close to the interface.The latter is the direct cause to the interface delamination.