Thermal Damage, Cracking and Rapid Erosion of Cannon Bore Coatings

Abstract

Thermal damage observed at the bore of fired cannons has increased noticeably in the past decade, due to the use of higher combustion gas temperatures for improved cannon performance. Current authors and coworkers recently have described cannon firing damage and proposed new thermo-mechanical models to gain understanding of its causes, with emphasis on the severe damage that occurs in the steel beneath the chromium plating used to protect the cannon bore. Recent refinements in the models will be used here to characterize some additional damage observations in the area beneath the protective coating of fired cannons. Model results validated by microstructural observations give predictions of near-bore temperature and stress distributions and good agreement with observed depths of hydrogen cracking in the high strength steel substrate. Interest in damage and failure within a coating is also of concern for cannons, since coating failure leads to extremely rapid erosion of coating and substrate. The slip zone model of Evans and Hutchinson is adapted here to predict failure strength of cannon coatings based on observed crack spacing and microhardness of thermally damaged areas. Results are described for electroplated chromium coatings from fired cannons and for sputtered chromium and tantalum coatings with laser-heating damage to simulate firing. Coating mechanics analysis of fired and laser-heated samples provides an insitu measurement of coating failure strength, showing that sputtered chromium has more than twice the failure strength of electroplated chromium. An analysis of cyclic shear failure of a coating interface at an open crack shows a six-fold decrease in low cycle fatigue life compared to the life of a closed crack. Recommendations are given for preventing rapid coating failure and catastrophic erosion of fired cannon, with emphasis on methods to prevent deep, open cracks in coating and substrate.