Introduction The development of infrared imaging detection technology for new generation of ultra-high-speed vehicles has put forward harsh requirements for infrared windows. For ultra-low altitude (e.g., sea level) to perform ultra-high-speed flight mission, the surface of its infrared window is usually accompanied by ultra-high density heat flow. In the high-speed flight environment, the force and thermal environment introduced by the aerodynamic effect on the surface of infrared window materials is complicated, and the related engineering experiment testing costs are extremely high. Finite element simulation can predict the phenomena of window warming and thermal stress failure caused by aerodynamic effect to a certain extent, which can provide important information in engineering or applications. In this paper, two conventional commonly used infrared window materials (i.e., hexagonal single-crystal sapphire and cubic ZnS) with a low thermal conductivity were selected to compare with cubic diamond infrared window materials with a high thermal conductivity. The numerical simulations of the window temperatures, thermal stresses and their distributions were carried out under the action of ultra-high-density heat flow, and the results were compared with and without the cooling medium. This work could provide a theoretical reference for the design, selection and application of infrared windows of the new-generation ultra-high-speed vehicles.Materials and method In this work, numerical simulations (Finite Element Analysis, FEA) of stagnation temperatures and thermal stresses were carried out on some infrared materials (i.e., sapphire, ZnS and diamond) with a size of 50 mm×2 mm for window application when facing ultra-high heat flux (3-8 MW/m2) for 5 s at an angle range of 10°-90° with and without constant temperature circulating coolant. The service performance of the infrared materials as well as the property requirements of windows in environments with ultra-high heat flux, were analyzed. A contrast of the infrared materials between sapphire or ZnS and diamond was investigated.Results and discussion The results show that the conventional infrared materials cannot survive as a result of high temperature or thermal stress due to the poor thermal and mechanical properties. For sapphire window, the stagnation temperature is 378 ℃, a value that already exceeds its limiting operating temperature for infrared imaging (about 350 ℃) with a small angle (10°) under 3 MW/m2 for 5 s, possibly resulting in a failure to image properly. For ZnS window, the temperature reaches 600 ℃, a temperature at which the material oxidizes and fails to image applications. Their stagnation temperatures both continue to increase as the angle and heat flux increase. When the heat flux continues to increase (i.e., 5 MW/m2 for 5 s), the stagnation temperatures of sapphire and ZnS reach 500 ℃ and 1 000 ℃ or more, respectively, even with a small angle (10°). At the maximum heat flux (i.e., 8 WM/m2), the stagnation temperatures can even exceed the melting point of the material. Meanwhile, in terms of thermal stress, only sapphire can keep the structure from failing under 3 MW/m2 at 10°. With the addition of coolant, conventional infrared materials (i.e., sapphire and ZnS) may fare worse. The stagnation temperature almost does not change, and the thermal stresses rather increases. For diamond as the infrared window, a failure risk sharply decreases due to the ultrahigh thermal conductivity, low thermal stress and more even stress distribution. Although diamond is equally difficult to apply under large angle working conditions (i.e., 45°-90°) without coolant (stagnation temperature reaches 750 ℃,which is above the oxidation temperature of diamond.), the situation improves dramatically with the addition of coolant. Diamond for benefiting from the cooling system can even survive under a high thermal flux of 8 MW/m2 at ≤45°. This study can provide a reference for the design, material selection, and application of infrared windows for a new generation of hypersonic aircraft.Conclusions Conventional infrared materials such as sapphire and ZnS could hardly achieve the requirements of the working conditions operating at any angles (i.e., 10°-90°) under ultrahigh heat flux (i.e., 3-8 MW/m2) for 5 s due to the failure risk caused by excessively high temperatures or structural failure from high thermal stresses. However, diamond had a better performance. Diamond could only be used as an infrared optical window at a small angle (i.e., 10°) without coolant, but could be used at a heat flux of 3-5 MW/m2 and any angle, as well as at 8 MW/m2 and 45° and below with coolant. The surface temperature of diamond was higher than the oxidation temperature, and the high stresses with a possibility of destruction under 8 MW/m2 at 90° occurred. In addition, diamond showed a lower failure risk and some advantage of its outstanding properties rather than sapphire and ZnS under cooling conditions