LOCAL THERMAL CHARACTERIZATION OF INNER GUN BARREL REFRACTORY METALLIC COATINGS

Large caliber gun barrels are coated, on their internal surface, by hard and refractory metallic layers. The thermophysical properties of such coatings are generally very different of bulk materials ones, and liable to evolve following the thermal cyclings. Nevertheless, the knowledge of these properties is necessary for modeling the gun barrels behavior.
In the present work, a photothermal microanalysis method has been used to measure the local thermal diffusivity within such a coating. The periodic excitation is localized on a micron-scale spot and the temperature evolution in the thermally excited zone is monitored by photoreflexion on a spot with similar size. Local thermal properties are identified from thermal transfer function analysis, i.e. phase lag evolution versus frequency.
Results obtained on an aged chromium coating, on which several local diffusivity measurements have been undertaken along the depth, are presented. The diffusivity is higher near the surface than to the interior of the coating that has undergone less important heating during the gun lifetime. This behavior is compared to the microhardness evolution along the coating depth.