Recent progress in aerospace technology has brought the creation of reusable space vehicles, thereby increasing demand for thermal protection systems (TPS). Fibrous insulating materials coated with high-emissivity materials are used in modern reusable space vehicles as insulation for suppressing radiant heat transfer. To realize radiative insulation with a high-emissivity layer, low-thermal-conductivity materials with sufficient thickness have been required, and considerations such as reduced brittleness and ease of handling have generally been secondary. Porous MgAl2O4 ceramics with 1–5 μm pores and grain clusters exhibit high reflectance and low emissivity in the 1–5 μm wavelength range because of their diffuse reflectance based on Mie theory. These properties are the suppression of heat input through low emissivity, which results in a space-saving in the thermal insulation layer. Porous MgAl2O4 ceramic coatings were formed on Ti–6Al–4V surfaces by low-temperature flame spraying to mitigate brittleness and were investigated in vacuum under radiant heating using an infrared lamp system. The spraying changed the microstructural shape and denser. However, these pores and grains of 1–5 μm, corresponding to the wavelength of radiation, remained. Infrared heating tests in vacuum confirmed that the coating restricted radiation transfer only at 1 mm thickness. Consequently, materials that combine low emissivity with high-temperature stability are considered essential for enhancing the flexibility, reliability, and performance of thermal control systems in next-generation aerospace applications.
Akamine et al. (Thu,) studied this question.