Review of thermo-mechanical cracking and wear mechanisms in large caliber guns

Thermo-mechanical cracking has become a critical wear mechanism in modern large caliber guns, due to operational increases in severity of heating at the gun bore in order to increase gun performance. The chromium coating typically used in guns often cracks after one firing, as do tantalum coatings and ceramic liners being considered for future guns. Once small segments of coating are surrounded by cracks, rapid erosion failure is likely. In prior studies to address this problem, models have described the depth and spacing of thermal cracks that develop in bore coatings and identified the mechanism of final separation of a cracked segment of coating. The models have used finite difference calculations of near-bore temperatures and elevated temperature material properties to calculate the thermal expansion stresses which cause both the initial cracking and the final separation of a cracked segment of bore material. The current work will review prior and recent observed damage and the models developed to describe the damage, and then extend the models to a wider range of gun firing conditions and materials from recent tests. Included will be [i] recent firing damage in tantalum coated steel liners in 120 mm inner diameter guns, [ii] additional recent gun-combustion simulation of firing damage in a SiAlON bulk ceramic, and [iii] extension of thermo-mechanical and fracture models showing effects of transient cannon heating and different coating materials on the near-bore temperatures and stresses that control both initial crack formation and final segment separation.