Laser pulse heating of gun bore coatings

The authors have developed laser pulse heating (LPH) as a tool for studying erosion processes at gun bore surfaces and for evaluating candidate coatings for bore protection. During firing, the bore coating and steel substrate experience thermal shock from rapid heating and cooling. Thermal pulse durations are in the millisecond range. Despite the short thermal pulse durations, severe coating and substrate degradation processes can occur; these include melting, gas–metal reactions, metallurgical transformations, interface reactions, transformational stresses, and thermal stresses. The degradation processes lead to coating cracking and spallation, which, in turn, allow erosion of the vulnerable underlying steel. Gas–metal interactions may also enhance fracture propagation in the steel. LPH is particularly convenient for performing multiple tests on coupon specimens for early evaluations of candidate bore coatings. This allows initial screening of candidate coatings without the difficulty and expense of testing in an actual gun or in a vented combustor. LPH was applied to gun steel that was electroplated with high-contractile chromium, which is the conventional coating for gun bores. LPH was also applied to low-contractile electroplated chromium and bare steel to provide baseline information. In the present LPH experiments, samples were repeatedly pulse heated to simulate the effects of multiple gun firings. The samples were then sectioned through the pulse-heated areas and examined to assess the resulting degradation. The results validate the approach. LPH reproduces the many features of thermal damage from actual gun firing: recrystallization and grain growth in the chromium, development of major cracks in the chromium, a heat-affected zone in the substrate steel, interface degradation, white layer formation (carburization and/or nitriding), and gray layer formation (rapid oxidation) in the steel.