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36 MECHANISMS OF GUN BARREL EROSION The erosion of the barrel of a gun is an unusually complex interdisciplinary problem because it is influenced by the design parameters of all elements of the gun system: those of the barrel, the projectile, and the propellant. In addition, it is influenced by the rate of firing. A gun barrel is subject to high temperatures and temperature gradients high pressures reactive gases thermal and mechanical shock cyclic stresses of thermal and mechanical origin high gas velocities high velocity sliding contact This gives rise to a large number of possible mechanisms for the loss of material generally referred to as gun barrel erosion. The problem is unusually difficult because the chief mechanism or combination of mechanisms undoubtedly changes when any of the elements of the gun system are changed, such as the size or range of the gun. The problem is further compounded by the fact that most of the erosion mechanisms will not act independently but will interact with each other. Possible gun barrel erosion mechanisms may be conveniently divided into the following three types: thermal, mechanical and chemical. Table 1 lists a number of examples of each type. Thermal Structural Change (including thermal softening) Loss of residual stress Surface melting Ablation
37 Mechanical Erosion (by gas flow or solid particles) Abrasion Attritious wear Surface fatigue Brittle microfracture Chemical Reaction with hot gases Oxidation Carburization or decarburization Diffusion alloying Surface temperatures in many applications will be sufficiently high to cause thermal softening and a loss of residual stress from a thin layer of metal at the surface. A thin surface layer may even transform to austenite and the soft aus- tenitic material may persist after firing if the cooling rate is sufficiently high. Melting and ablation probably do not occur with the original steel but could result following a chemical change. For example, there is evidence that some of the copper rotating band transfers to the rifling and then alloys with iron to form a material on the surface capable of melting. This action is sometimes prevented by the incor- poration of sheet lead in the propellent charge which acts as a decoppering agent. There is also evidence that iron reacts with CO in the combustion gas to form iron carbonyl (FeCO) which is volatile enough to ablate. This reaction is influenced by the CO/CO ratio which in turn is influenced by the presence of H S, SO , NH , ^ Â« Z w H , NO in the combustion gases. Â£t Hot gases move down the barrel of a gun at velocities that are sufficiently high to scour metal from the surface, particularly if the surface is in a thermally softened state. The entrainment of solid particles of combustion or of solid wear particles in the high velocity gas stream will augment the gas erosion process. The projectile is in solid sliding contact with the barrel as it moves forward and
this can lead to abrasive (large particle^ or attritious (small particle) wear if lubrication is not sufficient. The cyclic thermal and mechanical stresses that result each time a gun is fired can lead to the formation of microcracks in the surface which can join up to produce relatively large wear particles which them- selves then do further damage. This is apparently the situation when some gun barrels are given a thin (. 001-. 003) surface coating of hard chrome plate. Wear particles may also be generated by single-blow impact if a brittle surface layer results from a chemical reaction between the hot gases and the metal surface. These are further examples of interactions involving both mechanical and chemical mechanisms. Summerfield's study of gun damage resulting from the burning through of 19 aluminum cartridge cases is indicative of one aspect of the erosion problem. In addition to carbonyl formation, other chemical reactions that are pos- sible in a gun barrel are oxidation, carburization, nitridation and sulfurization. These may take place on the surface or at the tips of microcracks that are of thermal or mechanical origin. When such chemical action is present in combination with high surface stresses, stress corrosion cracking may occur. Because of the complexity of the problem and the fact that it involves so many mechanisms a universal solution does not appear likely. The best that can be hoped for is the possibility of identifying the principal type of erosion /thermal, mechanical or chemical) for a given gun system and then subsequently changing those items having the greatest influence on that type of difficulty. If temperature is demonstrated to be the nature of the problem then possible sources of improvement would involve improved additives, a cooler burning pro- pellant, spray internal cooling following firing or by use of a more refractory material in the form of an improved material, a sleeve or a coating applied at the breech end or throughout the length of the barrel.
39 If the difficulty is expected to be largely mechanical in nature this might be identified by comparing erosion results for a high-melting, non-transforming, strong material with the standard material, such as one of the refractory metals, even though the material cannot be used in practice. Having demonstrated the character of the difficulty it would then be possible to explore a number of alter- natives that would improve mechanical behavior of the barrel surface, such as use of sleeves or electrolytically or vapor phase deposited coatings. If chemical action is thought to be the source of the difficulty this might be demonstrated by introducing an ingredient into the propellant charge known to increase chemical action, and noting the relative change in the rate of erosion. Tests such as those just described should be run on the full scale proto- type system, since at the present time it does not appear possible to translate model studies into those for a full scale gun system. At the same time however, it would be useful to perform a series of model studies in order to screen new potentially useful additives, lubricants, or coating materials. These model studies should also provide an understanding as to the nature of the action involved with a given change. For example, the action (or actions) of additives are presently not known with sufficient certainty. The wax is thought to coat the barrel to prevent heat transfer while the inorganic ingredient (SiO , TiO , Talc) has been variously Â£* â- described as a heat absorber, a heat reflector or shield, a strengthening agent for the wax or a dispersing agent for the wax. One thing that seems clear is that the finer the degree of subdivision of the inorganic material the better. It is important that we have a clearer understanding of what an additive does that is useful so that we can estimate the degree of improvement possible and hence more quickly optimize the performance of additives. Model tests on a small bore gun system would appear to be of value for such purposes. A relatively small bore gun system should be adopted for use by the Army and Navy for long range model studies covering all three types of erosion. The purpose of this work would be to provide a better understanding of the fundamentals
40 of gun barrel erosion. This work should be carefully coordinated by an interservice gun barrel erosion committee with some non-government representation for breadth. The full scale tests that will always be essential to characterize and correct specific gun barrel erosion difficulties would constitute the short range portion of the program.