Research On Microstructure And Performance Of Thermal Barrier Coatings Modified By Laser Biomimetic Coupling Technology

Thermal barrier coatings（TBCs）applied for aero-engine blades are usually used as the advanced high-temperature protective coatings,which can significantly reduce the surface temperature of turbine blades,greatly extend the service life of the blades,and improve the thrust and efficiency of the engine.Therefore,TBCs,high-temperature structural materials and high efficiency air film cooling technology are regarded as the three key technologies for advanced aero-engine turbine blades.During the frequent taking off,endurance,landing cycles of the aircraft,the aero-engine blades will be affected by the interaction effects of high-temperature and high-speed gas,high stress,alternating load,foreign object impact,corrosive medium and other factors.The TBCs are prone to thermal fatigue spalling,high temperature oxidation,erosion and hot corrosion,which ultimately lead to the premature failure.Among them,the thermal fatigue spalling is the main form of the TBCs failure,and it is also a bottleneck problem in service of thermal barrier coating prepared by atmospheric plasma spraying（APS）.Therefore,the improvement in the thermal fatigue resistance of zirconia-based thermal barrier coatings prepared by plasma spraying has become the primary mission for extending the service life of aero-engine blades.Based on the principle of coupling crack-arrest and anti-fatigue function of biological structure in nature,the biomimetic coupling model for the zirconia-based thermal barrier coating prepared by APS has been designed to improve the thermal fatigue resistance.Meanwhile,the biomimetic coupling structure is prepared by laser surface modification technology on the TBCs.The optimization of laser processing parameters and post-heat treatment technology were used to effectively improve the preparation technology of biomimetic coupling TBCs.The influences of morphology and material coupling elements on the performances of the biomimetic coupling TBCs were investigated,and the failure mechanisms of thermal fatigue resistance,solid particle erosion resistance and hot corrosion resistance of laser biomimetic coupling modified TBCs were revealed.On the basis,laser alloying technology was also used for further improving the thermal shock resistance of the biomimetic coupling TBCs.The main findings are as follows:（1）The morphology coupling element has an obvious influence on the performances of the biomimetic coupling TBCs.The biomimetic coupling TBCs with dotted units has the best bond strength and thermal shock resistance,and the biomimetic coupling TBCs with grid units has the best thermal insulation performance and solid particle erosion resistance.The optimization of unit distance can further improve the thermal shock resistance of the biomimetic coupling TBCs.The thermal shock resistance of the dotted biomimetic coupling TBCs with unit distance of 3 mm is the best,and the thermal cycle lifetime is 2.5 times that of the untreated sample.The columnar crystal microstructure and the segmented cracks of the biomimetic unit,which provides the coating with high strain tolerance capability and can release the thermal stress during the thermal cycle.Meanwhile,the crack propagation driving force is reduced,and the thermal crack propagation resistance is enhanced,and finally the thermal shock resistance of the biomimetic coupling TBCs is improved.（2）The performance increment ratio of the biomimetic coupling TBCs is different based on different ceramic coating base material.When the ceramic coating material is the same,the biomimetic coupling TBCs have better bond strength,thermal shock resistance and hot corrosion resistance than the untreated TBCs.After the laser biomimetic coupling modification,the bond strength increment ratio is CYSZ coating（16%）>7YSZ coating（11%）,thermal insulation performance decrement ratio is CYSZ coating（15%）>7YSZ coating（12%）,thermal shock resistance increment ratio is 7YSZ coating（150%）>CYSZ coating（26%）,hot corrosion resistance increment ratio is CYSZ coating（13.8%）>7YSZ coating（8.5%）.（3）The biomimetic units fabricated by laser alloying technology show different microstructures and materials with the matrix of ceramic coating,which is beneficial to further improving the thermal shock resistance effect of the biomimetic coupling TBCs.The thermal shock resistance of the biomimetic coupling TBCs with different mass fractions of TiAl₃ is significantly better than that of the untreated TBCs.The TiAl₃ mass fractions sort the10%TiAl₃>15%TiAl₃>5%TiAl₃>20%TiAl₃>25%TiAl₃ by the improvement degree of the thermal shock resistance of the biomimetic coupling TBCs.Among them,the thermal shock resistance of the biomimetic coupling TBCs with 10%TiAl₃ is 3.1 times that of the untreated TBCs.In the later stage of thermal shock test,the TiAl₃ in the biomimetic units oxidizes at high temperature to realize the crack self-healing,then the connection of vertical cracks and horizontal cracks is postponed,the spalling of the coating is delayed,and thus the thermal shock resistance of the biomimetic coupling TBCs is further improved.（4）The erosion failure process of the biomimetic coupling TBCs mainly includes two stages:during the early erosion period,the untreated zone with porous structure is more susceptible to the impact of erosive particles,resulting in the fracture and fragmentation of individual splats.During the later erosion period,the biomimetic unit with dense microstructure and high hardness gradually protrudes,which is is obviously resistant to the erosion of solid particles.The erosion failure mode of the biomimetic coupling TBCs was dominated by brittle and some plastic erosion.（5）The reaction between the molten salts and zirconia stabilizers（Y₂O₃ and CeO₂）leads to the formation of YVO₄ in the 7YSZ TBCs,the generation of YVO₄ and CeVO₄ and the mineralization of CeO₂ in the CYSZ TBCs.The depletion of stabilizers causes the detrimental phase transformation of t-ZrO₂ to m-ZrO₂.Finally,the stresses resulting from phase transformation and the generation of hot corrosion products,and the bond coat oxidation cause the failure of 7YSZ and CYSZ TBCs.The minimal surface roughness and the dense microstructure of biomimetic units significantly contribute to improving the hot corrosion resistance of the biomimetic coupling TBCs.