The Evolution Mechanism Of Morphology And Microstructure For ZrO₂ Ceramic Powders Irradiated With A Laser Beam

During the high-speed flight of supersonic aircrafts,the surface temperature rise of the titanium alloy structural parts leads to the reduction of structural rigidity and stability.Therefore,the thermal protection of titanium alloy materials becomes more and more severe.The introduction of second phase-ceramic particles with very low thermal conductivity inside the metal can reduce the thermal conductivity of the overall composite layer and thus achieve the desired thermal insulation properties.The multi-scale gradient composite material prepared by the laser melt injection technique can realize metallurgical bonding between the second phase ceramic particles and the metal,at the same time,it can reduce the tendency of cracks,and can combine the important functions of“heat protection”and“bearing”,which has theoretical and practical significance.In this paper,Y₂O₃ partially stabilized ZrO₂ ceramics were used as the second phase particles.A multi-scale particle-reinforced gradient composite thermal barrier coating was prepared on the surface of titanium alloy by laser melt injection technology.The difficulties in the preparation of the composite material layer were studied.The changes of morphology and dispersion of ZrO₂ ceramic particles after laser irradiation were systematically studied,and the influence of these changes on the thermal conductivity of the composite layer was analyzed.Through systematic experiments,it was determined that the optimum injection position of ZrO₂ ceramic particles was 1.9 mm away from the center of the laser spot.The optimum process parameters of the ZrO_2p/Ti-6Al-4V composite material layer prepared by laser melt injection were obtained through orthogonal experiments.The influence of the substrate width and the melting degree of the zirconia ceramic particles on the cracking tendency of the composite material layer was investigated.The results show that the cracking tendency of the composite material layer increases with the increase of the substrate width and the zirconia melting degree.The high-speed imaging was used to observe the direct action of the laser on the zirconia ceramic particles,and the morphology of the zirconia ceramics was analyzed by SEM.When the laser energy density was less than 1 J/mm²,the zirconia ceramic particles did not melt,but crack due to the effect of thermal stress,producing irregular blocks and spherical particles smaller in size than the original particles.When the laser energy density is greater than 15 J/mm²,the zirconia ceramic particles are completely melted and a serious spheroidization phenomenon occurs.As the laser energy density increases,the spheroidized particle size becomes larger.After fully understanding the evolution of zirconia ceramic particles directly affected by the laser,further studies were conducted on whether the zirconia ceramic particles were exposed to the laser and the microstructure evolution rules during the laser melting process.The results showed that some of the ZrO₂ ceramic particles cracked due to thermal stress,and some of the particles absorbed the laser energy and the internal grains grew and the microstructure became denser.It was found that the internal grain growth of the ZrO₂ ceramic particles after laser irradiation promotes the penetration of the Ti melt into the zirconia ceramic particles and reduces the time for the complete dispersion of the zirconia ceramic particles.The change of the particle size of the zirconia ceramic particles by the laser has almost no effect on the overall thermal conductivity of the composite layer,and has a greater influence on the cracking tendency.When the particles are melted heavily,the cracking tendency of the composite layer is greatly increased.