Study On High Temperature Ablation And Rapid Thermal Shock Properties Of Ti-Al-C Max Phase Ceramics

Ternary layered machinable ceramic MAX phase combines the advantages of metals and ceramics.It has a highly wide application prospect because of its low hardness,machinability,high modulus,high strength,excellent damage tolerance,thermal shock resistance,high conductivity and high thermal conductivity.As one of the most representative systems of MAX phase ceramic materials,Ti-Al-C system,as a high-performance ceramic integrating structure and function,has been proved to be used in aerospace,nuclear energy,metallurgy and other high-temperature environments.However,considering the practical application environment,the research on the performance and mechanism of Ti-Al-C ceramics in high temperature and high-speed airflow ablation or rapid heating and cooling environment is quite rare.In this paper,the dense Ti-Al-C block material is synthesized by ultra-high temperature hot pressing sintering furnace,and then processed by electrical discharge machining.Using two new experimental platforms constructed in the laboratory,the ablation properties of Ti₃AlC₂and Ti₂AlC and the thermal shock resistance of Ti₃AlC₂were systematically characterized.The main results are as follows:1.After Ti₃AlC₂is ablated by nitrogen plasma flame,the results show that when the ablation temperature is 1500℃,Ti₃AlC₂on the surface will first decompose into Ti C and Al₂O₃,and finally react to form Al₂Ti O₅and Ti C with the extension of time.When the ablation temperature rises to 1700℃,the tic decomposed on the ablation surface is completely oxidized to Ti O₂,and the content of Al₂Ti O₅increases with the increase of temperature.As the ablation temperature is further increased,obvious ablation pits appear on the surface of the sample.2.Combined with the cross-section analysis of Ti₃AlC₂,the ablated layer can be divided into two structures.The ablated outer layer is mainly composed of Al₂O₃,Ti O₂and their reaction product Al₂Ti O₅,and the ablated inner layer is mainly composed of Ti C and a small amount of large particle Al₂O₃.When the ablation temperature is 1500℃,1600℃,1700℃and 1900℃,the linear ablation rates are-0.73,-1.86,-8.20 and 8.71μm/s respectively and the mass ablation rates are-0.095,-0.283,-0.043 and 2.103μmg/s respectively.3.The results of Ti₂AlC ablated by nitrogen plasma flame show that the main phases on the surface are Al₂Ti O₅and Ti O₂,and Al₂O₃is not detected.With the extension of ablation time,the structure of the oxide layer on the surface changes significantly,and the Al₂Ti O₅particles generated on the surface grow continuously.However,different from Ti₃AlC₂,when the ablation time is less than 180 s,the oxide layer is separated from the matrix.4.Ti₂AlC is ablated at 1600℃,combining with the cross-sectional analysis,the ablated outer layer is mainly a mixed layer composed of Al₂Ti O₅and Ti O₂.The ablated outer layer is mainly connected with the matrix through Ti C,and the peeling of the oxide layer is mainly due to the large difference between the thermal expansion coefficient and the matrix.When the ablation time is 60 s,90 s,120 s and 180 s,the linear ablation rates are 1.50,2.50,3.56 and 3.09μm/s respectively,and the mass ablation rates are-0.951,-0.623,-0.595 and-0.473μmg/s respectively.5.When Ti₃AlC₂ceramics are quenched in water at 270,950 and 1250°C,the residual bending strength is 160 MPa,91 MPa and 27 MPa respectively,and the corresponding strength loss rates are 70.2%,82.9%and 94.9%respectively.The obvious decrease of strength is attributed to the lack of dense Al₂O₃film on the surface,the chemical reaction between water and matrix and the large transient tensile stress on the surface during quenching.Experiments show that the maximum thermal shock performance of Ti₃AlC₂ceramics is at least 1250°C in water.6.When Ti₃AlC₂ceramics are quenched in air,a dense oxide layer is attached to the quenched surface,which acts as a thermal barrier and effectively protects the matrix from large thermal shock.Therefore,the residual strength does not decrease significantly.The results show that the thermal shock temperature of Ti₃AlC₂ceramics in air is at least 1400°C.