The subject program is part of a continuing effort to identify distinct mechanisms that contribute to gun barrel wear and erosion. The chemical effect of a carburizing atmosphere in a gun tube was the main topic for investigation in this program. Experiments were conducted in the Shock Tube Gun (STG), a ballistic compressor, designed and developed by Calspan Corporation. This facility can compress mixtures of pure gases to simulate propellant gas flow condtions and cycle time experienced in large caliber guns. Test were conducted with a mixture of argon and nitrogen to delineate the threshold of simple melting erosion which served as a chemical inert baseline. Progressive substitution of carbon monoxide for nitrogen in the mix quantified the carburizing effect as a function of CO concentration. Subsequent tests were conducted with a gas mixture containing carbon monoxide, argon and helium to measure the carburizing effect as a function of cycle time. The basic carburizing effect was observed to be a shift in the erosion threshold to less severe convective heating conditions in response to surface chemistry. The magnitude of the shift appeared to be directly proportional to heating cycle time.

Experiments to determine the individual contributors to gun barrel erosion in an oxidizing atmosphere were conducted in the Shock Tube Gun (STG), a ballistic compressor, designed and developed by Calspan Corporation. This facility can compress mixtures of pure gases and can generate flow conditions and cycle times similar to those experienced in large caliber guns. Tests were conducted with a mixture of argon and nitrogen to define the threshold erosion for an inert baseline. Small concentration of oxygen were substituted for nitrogen in the mix to quantify the oxidation effect. Subsequently, tests were conducted with various gas mixtures containing carbon dioxide, which dissociates to produce oxygen when compressed to high temperatures. The basic oxidation effect was observed to be nearly linear increase in erosion with oxygen concentration. A corresponding shift in the erosion threshold to less severe convective heating conditions was observed in response to surface chemistry. A similar effect was observed for carbon dioxide mixture where erosion was correlated in terms of the equilibrium concentration of oxygen at peak pressure. Surface cracking was observed and found to be most severe near the erosion threshold where the oxide layer was thickest. A white layer was observed in tests with oxygen in the absence of carbon dioxide.

Metallurgical analysis was conducted on 7.62mm machine gun barrels, in the chromium-plated and unplated conditions, to describe the erosion process. The chronological analysis involved the examination of severly eroded gun barrels as well as those subjected to test firings of 1 to 3000 rounds. Inherent defects in the form of cracks were noted in the chromium plate prior to firing. These cracks were extended to the chromium-steel interface as early as the first round; thus the underlying steel was exposed to the reactive environment. Continued firing resulted in the propagation of these cracks into the steel substrate followed by crack-branching. Branching proceeded until linkup was achieved, which resulted in the removal of chromium plate steel fragments. The factors considered to be responsible for crack extension include gaseous and liquid metal reactions. The type of erosion found in unplated steel gun barrels was in contrast with that of the plated barrels in that substrate-cracking was delayed and land wear occurred much earlier in the firing sequence.

An update is provided for the previously-presented gun barrel thermochemical erosion modeling code. This code addresses wall degradations due to transformations, chemical reactions, and cracking. As a predictive tool, it provides analysis and design information that is either unattainable or expensive by experiment. Single- or multiple-shot comparisons can be made of either the same gun wall material for different rounds, or different gun wall materials for the same round., this complex computer analysis is based on rigorous scientific thermochemical erosion considerations that have been validated in the reentry nosetip and rocket nozzle/chamber community over the last forty years. A gun system example is used to illustrate the five module analyses for chromium and gun steel wall materials for the same round. The first two modules include the somewhat modified standard gun community XNOVAKTC interior ballistics and BLAKE nonideal gas thermochemical equilibrium codes. The last three modules, significantly modified for gun barrels, include the standard rocket community two-dimensional kinetics!mass addition boundary layer (TDK/MABL), gas-wall thermochemistry TDK/CBT), and wall material ablation conduction erosion (MACE) codes. These five analyses provide thermochemical ablation, conduction, and erosion profiles for each material as a function of time, travel, and rounds. For the gun system example, at two axial positions, with cold and hot firing rates. predictions of rounds required to achieve 0.040-inch wall loss are made for cracked and uncracked 0.005-inch chromium plated A723 steel and A723 steel alone. Thermochemical erosion increases by a factor of about 2.0 from cracked chromium plated A723 steel to A723 steel alone. For a given%wall, thermochemical erosion decreases by a factor of 1.4 from the hot to the cold firing rates.