Analysis of thermo-mechanical cracking in refractory coatings using variable pulse-duration laser pulse heating

Large caliber cannon firing represents a unique and demanding environment, characterized by a transient surface temperature spike of about 1500 K over about 2 ms. Design and evaluation of protective coatings that can withstand this intense thermal loading without thermo-mechanical failure and wear is an ongoing process, but live-fire tests to assess coating performance are difficult and expensive. Previous studies demonstrated that Cr and Ta coatings irradiated with a pulsed laser with a 5 ms pulse duration exhibited morphological features comparable to those observed in cannons, including recrystallization and grain growth of Cr, crack formation, a phase transition in the substrate steel, interface degradation, and chemical reactions in the steel [P.J. Cote, G. Kendall, M.E. Todaro, Laser pulse heating of gun bore coatings, Surf. Coat. Tech. 146–147 (2001) 65–69; P.J. Cote, M.E. Todaro, G. Kendall, M. Witherell, Gun bore erosion mechanisms revisited with laser pulse heating, Surf. Coat. Tech. 163–164 (2003) 478–483]. However, the laser was unable to induce the types of adhesive failures that accelerate wear and erosion in coatings that were subsequently observed to fail in this manner during live fire testing.