Preparation And Microstructural Of Spark Plasma Sintered Thermal Barrier Coating On Titanium Alloy Surface

Titanium alloys are widely used in the aerospace field due to their low density and high specific strength,e.g.,compressors housing for aeroengine,leading edge of critical structures,etc.However,as the thrust-to-weight ratio increases,the temperature in an engine increase to values considerably above the service temperature of the titanium alloy.Therefore,the study of thermal barrier coatings with high strain tolerances and strong oxidation resistances is crucial to improving the service temperature of titanium alloys and the service time of TBCs on titanium alloys at high temperatures.Thermal barrier coatings（TBCs）are advanced technology that can breakthrough the extreme service temperatures in engines.The excellent oxidation resistance and thermal insulation properties of TBCs compensate for the poor oxidation resistance and deterioration of the mechanical properties of titanium alloys.However,TBCs on titanium alloys are easy to fail in long-term high-temperature environments.Therefore,how to increase the service time of the thermal barrier coating on a titanium alloy surface is an urgent problem to be solved.Effective microstructural control is a potential approach for improving the strain tolerance of TBCs.In this study,a bimodal structured TBCs（structures with different sizes and even cross-scale grains）with non-laminar and uniformly distributed nanoparticles were prepared on TC4 substrates by SPS.On this basis,the microstructure was designed and prepared on the surface of the BC using laser,and bimodal-structured dense vertically cracked TBCs were realized.The evolution mechanism of bimodal-structured DVC TBCs and ways to improve strain tolerance were discussed.The way of different interface microstructures induce segmentation cracks was explored.The spallation resistances,thermal insulation performances,and oxidation resistances of the coatings were evaluated.At the same time,the experimental phenomena were explored and verified by finite element analysis.In-depth understanding of the stress distribution and crack growth dynamic process of thermal barrier coatings with different structures.It not only provides a new preparation idea for bimodal structured DVC TBCs,but also lays a theoretical foundation and technical supplement for increasing the service temperature of titanium alloys and prolonging the service time of TBCs on titanium alloys at high temperatures.The main research content and conclusions are described as follows:（1）A bimodal structure was prepared on TC4 by SPS.The bimodal structure possessed dense particle contacts and porous aggregates of uniformly distributed nanoparticles.It was found that the SPS bimodal structure provides the coating with high strain tolerance,resistance to sintering,and low thermal conductivity,which prolongs the high temperature service time of the thermal barrier coating on titanium alloys.The TC and BC were well connected after isothermal oxidation at 800℃for100 h.The TBCs only shed 6%of their surface area at high temperatures and large-angle bending.Meanwhile,the bimodal-structured TBCs effectively improved the oxidation resistance of the TC4 substrate.The TC4 substrate with bimodal-structured TBCs only gained 0.51 times the mass gained by the bare TC4.（2）Diverse microstructures were engraved at the BC interface of the bimodal-structured TBCs by laser.The interface microstructure induced segmentation cracks.DVC could be formed at densely spaced areas of the microstructure.Based on the bimodal structure characteristic obtained by SPS,bimodal-structured DVC TBCs were realized,which provided the coating with higher strain tolerance and thermal insulation performance.Therefore,the comprehensive performance of TBCs on titanium alloys has been further improved.After 138 cycles of thermal shock at 800℃,the spallation area of TBCs with interface microstructure was smaller than that of the unengraved TBCs.The microstructure with the smallest spalling area was a 2-mm square,whose spalling area was only 34.55%of that of the unengraved TBCs.Meanwhile,the interface microstructure coating had better thermal insulation performance.The thermal insulation temperature of the unengraved coating reached 136℃.The denser structural spacing led to a better thermal insulation performance of the coating.The thermal insulation temperature of a hexagonal 2-mm coating reached 215℃.（3）Differences in stress distribution and crack dynamic expansion processes of diverse interfacial microstructured TBCs under thermal shock conditions were investigated using the finite element method.Meanwhile,the finite element method combined with fracture mechanics theory was used to analyze and verify the process of interfacial microstructure-induced segmentation cracks and the effect of segmentation cracks on coating stress distribution.It was found that the stress conditions for crack initiation in the unengraved TBCs could only be obtained after several thermal shock cycles,and crack initiation tends to occur at the peak position of the heating stage.The interface microstructure TBCs had the stress conditions for crack initiation in the early thermal shock cycle process,and cracking was easy to initiate at the peak or valley position during the heating and cooling process.Meanwhile,the horizontal crack was easier to propagate in unengraved TBCs,and the vertical crack was easier to propagate in interface microstructure TBCs.The interface microstructure TBCs had a largerσ₁₁ tensile stress at the initial heating,and the vertical crack initiation and propagation occurred rapidly at this stage.The above FEA results are consistent with the results of interface microstructures that easily induce segmentation cracks in the TC.