Microstructure And Properties Of ZrO_2p/Ti-6Al-4V Functionally Graded Thermal Barrier Composite Layers Produced By Laser Melt Injection

With the development of high Mach aircrafts,the surface temperature rise of structural parts caused by aerodynamic heating has been more and more serious.Using thermal protection structure to protect the surface of structural parts is a effective way to improve aircrafts’performance.However,the conventional thermal protection structures mainly depend on the low thermal conductivity of the coating materials for thermal protection.They are heavy and have low adhesion strength to substrate.Moreover,they only have thermal protection function,can’t bear and spall easily,which reduce the reliability of thermal protection structures.By introducing appropriate ceramic particles into the inner of metal matrix and utilizing the interfacial resistance between metal and ceramic,the metal matrix composite（MMC）layer would possess low thermal conductivity.Ceramic particle has a sound metallurgical bonding to the metal matrix,which would avoid coating spallation and improve bearing property.The new thermal barrier structure which based on interfacial resistance effect has not only thermal protection,but can undertake the load,which exhibit great advantage in academic study and practical application.By injecting the ZrO₂ ceramic particles directly into a laser melt pool,the multi-scale particulate reinforced titanium matrix composite layer was fabricated successfully.Base on the study of laser melt injection（LMI）process characteristics,the phase composition and microstructure of MMC layer was investigated.The formation mechanism of MMC layer was interpreted.The thermal conduction behavior between the interface of metal and ceramic was analyzed.The thermal conductivity of MMC layer was measured and compared with the model calculated data.A appropriate parameter window of LMI was acquired after a series of experiments.The influence of experimental parameters on the particle distribution and volume fraction was studied.It was found that the relative position between laser and powder stream is the key element to affect the quality of MMC layer.High particle volume fraction,crack-free and porosity-free MMC layer was obtained when the powder stream was injected away1.8².0 mm from the center of laser beam.The phase composition and microstructure of MMC layer was analyzed by using XRD and scanning electron microscope（SEM）and transmission electron microscope（TEM）.The results show that owing to the ZrO₂ particle disintegration behavior in the molten titanium pool and non-equilibrium solidification at high cooling rates,the different regions of MMC layer have different morphologies.Based on the analysis of formation mechanism of MMC layer,a novel method was applied to produce ultra-fine particle reinforced composite layer for thermal barrier function.After the investigation of thermal conduction behavior of Ti-ZrO₂ interface,the thermal conductivity of MMC layer was measured by laser flash method.The lowest thermal conductivity of specimens is 3.4 W/mK,which is closed to ZrO₂ bulk.The thermal insulation mechanism can be described as follows.Because of the low thermal conductivity of ZrO₂ and the interfacial thermal resistance,heat transfer between particles and surrounding matrix is difficult to reach the thermal equilibrium rapidly and a large temperature gradient exhibit from particle to surrounding matrix.Under the driving of temperature-pressure,there is heat flux component pointing to the particles and the heat flow is impended in the depth direction.