A method for measuring the high temperature emissivity of refractory metal surfaces

For high temperature applications, such as lighting components or heating elements, the thermal emissivity of refractory metals such as tungsten can be improved by altering the surface of the material. Laser structuring or porous sintered coatings have been shown to increase the emissivity, which leads to improvements in process stability and component lifetime. Measuring the emissivity is generally performed at room temperature by reflectometry as high temperature measurement is extremely difficult. This is a significant disadvantage for comparing surface modifications, as emissivity at application temperature might differ greatly. In this paper, a method is described with which samples are heated by a direct current flow and the emissivity is measured with an optical pyrometer by comparing temperature values with the actual surface temperature. The method allows a comparative measurement of different surface treatments at the application temperature. Degradation of the surface emissivity at the application temperature is also possible to observe. Measured values for tungsten with different surface modifications are shown and discussed.