3.3 Surface Coatings and Liners
Although coatings have been used to protect barrels since World War II, there has been renewed, active research in this area over the last decade [13, 16, 18, 22, 34, 52–54]. Rather than the development of new coating materials, recent work has mostly been directed at understanding the mechanisms of coating failure, performance assessment of known potential coatings, and proposed new coating application techniques. Conroy and coworkers [34] have proposed several criteria for a successful coating:
• The coating should not react with the propellant gases.
• The coating should help insulate the base material from the heat load, distribute the heating, and be resistant to thermal erosion.
• The coating must be resistant to mechanical wear from projectile passage.
• The coating must adhere well to the base material.
• The coating must have a coefficient of thermal expansion similar to that of the base material to prevent thermal stress cracking.
• The coating material and application method must be cost effective.
According to Conroy, these myriad requirements may explain the paucity of new coatings and application techniques. Electrodeposited chromium remains the most popular barrel coating in fielded guns, despite being originally developed over sixty years ago. Other coating and liner materials that are still being actively pursued as alternatives include ceramics, and refractory metals such as molybdenum, niobium, tantalum, rhenium and tungsten.
The most common commercial technique for chromium coating is aqueous electrodeposition [54], where chromium is initially deposited as chromium hydride. During deposition and the subsequent heat treatment to outgas hydrogen, residual stress causes microcracks to form in the coating [13]. Usually the cracks do not penetrate through the entire coating thickness, however, and a crack-free sublayer exists near the base material. Refinements to the process have lead to the development of low contractile (LC) chromium coatings. LC chromium coatings exhibit fewer cracks and higher strength, at the expense of reduced hardness [1, 13]. Mawella [54] reports that recent studies on pulsed electrodeposition have demonstrated that reduced cracking or crack-free coatings are possible. A number of other experimental coating methods are also cited. Physical vapour deposition, via magnetron sputtering or the use of an RF plasma discharge, can reportedly produce crack-free coatings and deposit a range of refractory metals which cannot be electrodeposited. Chemical vapour deposition, where a volatile vapour containing the coating material decomposes on the bore surface, is noted as producing highly uniform coatings. Conventional chemical vapour deposition requires high temperatures (over 1100 K) for decomposition, thus triggering phase changes in the gun steel. Mawella proposes metal-organic chemical vapour deposition (MOCVD) as more amenable to gun barrel applications, which requires temperatures of 700 K or lower. He cites firing trials where barrels coated with chromium using MOCVD showed improved erosion resistance, compared to those coated with electrodeposited chromium. A more thorough description of these and other possible coating processes is contained in Reference [55].
Through numerical modelling and vented vessel tests, Sopok has assessed the compatibility of different refractory metal coatings and propellant types [18]. Bare gun steel, and chromium, tantalum, molybdenum rhenium and niobium coatings were subjected to oxidizing, carburizing, and intermediate propellant gas environments. Erosivity was gauged by the threshold surface temperature at which erosive processes (melting, phase transformation, and reactions) initiated. In an oxidizing propellant gas environment, rhenium and niobium had the lowest threshold (corresponding to most erosion), chromium and tantalum had the highest threshold, while the thresholds for gun steel and molybdenum were intermediate. In a carburizing environment, tantalum had the highest threshold temperature, followed by similar thresholds for chromium, molybdenum, rhenium and niobium, with gun steel performing worst. Chromium was the only material not to show a variation in threshold temperature between the different environments, which may explain its popularity as a coating material. For the other materials, the significant difference in threshold temperatures between the propellant gas environments highlights the need to match propelling charge to coating type. The chemical mechanisms responsible for the variations are discussed at length by Sopok in the paper.
The high melting point, low reactivity and high hardness of coating materials render them resistant to direct thermal, chemical and mechanical erosion. The melting point of chromium (Table 1), for example, is much higher than typical bore surface temperatures [52]. Coated barrels still erode, however, and once erosion is initiated they may erode at a faster rate than uncoated barrels [54]. Much attention has recently been given to understanding the erosion process for coated barrels.
As already noted, surface microcracks are present in chromium coatings from the time of manufacture. ´e pressure and thermal cycling of firing causes the microcracks to grow deeper until reaching the substrate material, and also propagate laterally to combine and form a network [54]. The result is fragmented but contiguous coating elements still attached to the substrate, described by Cote and Rickard [13] as a series of separate, isolated islands or plates of chromium. The dimensions of these plates are of the same order as the coating depth. Conroy and coworkers contrast these microcracks with their theory of macroscopic cracks caused by stresses at the coating-substrate interface [34]. These stresses are generated by direct loading from the barrel internal pressure, and the difference in thermal expansion of coating and substrate at the interface itself. They formulate an analytical treatment to calculate the spacing of such macroscopic cracks and, subject to a number of assumptions, find that tantalum should show less cracking (a greater spacing between cracks) than chromium. It is also determined that neither chrome nor tantalum should fail by debonding from the gun steel; instead the analysis indicates that cracking and plastic strain are the most likely results of interfacial thermomechanical stress.
Once cracks in the coating have reached the substrate, the exposed gun steel begins to erode. Jets of hot combustion gases wash through the crack, recirculate, react with the substrate, and cause pitting via thermal and chemical erosion. It has been discovered that, at the interface, oxides of refractory metal coatings may seed cracking in the substrate [22]. Specifically, Conroy and coworkers calculated that tantalum engenders more rapid pit growth in the substrate compared to chromium [34].
Numerical modelling of a 20 mm gun by Heiser and coworkers [53] showed that chromium coatings lower bore surface temperature because they conduct heat to the substrate faster. Thus the temperature at the coating-gun steel interface is higher than it would have been at the identical depth for a steel-only barrel. The high temperature at the interface encourages thermochemical erosion to traverse laterally under the coating, from the initial crack site, attacking the substrate material [16]. Eventually the coating is undermined, and susceptible to removal by mechanical processes. The small plates of coating may simply lift out due to complete separation from the steel, or be removed by engagement with the projectile or spallation [52] driven by choked high pressure gas [34] (see Section 2.3). Underwood and coworkers have experimentally observed that deep, open cracks are the preferred site of plate loss [56]. However, Sopok notes that erosion in coated cannon barrels always correlates with interface degradation and substrate exposure, regardless of whether or not this actually occurs at the deepest crack sites [22]. Hordijk and Leurs have additionally observed that once erosion of a coated barrel begins, after further firings the number of exposed spots tends to stay constant, while the damaged area per spot increases [40]. While the described process is generally agreed to be the prime cause of erosion for high temperature propellants, Cote suggests that fatigue fracture of the coating due to sliding forces may be more significant for cooler propellants [13].
Methods to prevent or reduce the undermining process have been suggested. Conroy and coworkers suggest that, aer firing, storage conditions may induce oxidation of the newly exposed substrate gun steel [34]. Corrosion control through post-firing treatment of coated barrels is thus advocated as a possibility of extending barrel life. Also suggested is pre-nitriding of the steel bore before coating, to increase hardness and reduce chemical erosion at the interface once the coating is penetrated by cracks. Likewise, reducing the carbon content of the steel near the interface may decrease its susceptibility to hydrogen cracking aer the coating is breached [56]. Alternatively, a tough cobalt interlayer located between the coating and substrate may prevent cracks penetrating through to the gun steel, and has been successfully trialed in the past [1, 13]. Underwood and coworkers also suggest that interlayers may aid in preventing the exposure of the gun steel to chemical attack, as well as decrease the transfer of shear stress from coating to substrate [56].
As alternatives to refractory metals, ceramic liners have been identified as a promising technology due to very good wear and thermal resistance. The propensity of ceramics to fracture due to susceptibility to stress concentration and flaws, however, must be addressed before widespread practical use is possible [1, 57]. Grujicic and coworkers present structural reliability studies of segmented and monolithic ceramic liners using finite element analysis, and for their 25 mm barrel test case find a failure probability of once per 400 single shots [57, 58]. The primary cause of failure was identified as cracking of the ceramic liner near the barrel ends, as a result of stress due to axial thermal expansion of the steel jacket. The use of segmented liners was found to reduce failure probability by as much as 18%, by relieving tensile stress in the ceramic. Functionally graded ceramic-tometal barrel liners provide an alternative means to avoid the abrupt mismatch of thermal expansion between a ceramic and metal interface. The response of candidate functionally graded liner materials to thermal shock, conductivity, and wear tests, are reported in an initial study by Huang and coworkers [59]. As an alternative to using ceramics as liners, Kohnken describes the use of composite reinforced ceramics for the construction of entire small-calibre barrels [60]. The concept is to use a carbon fibre/resin composite as an outer wrap, to reinforce and compress a zirconia-ceramic tube from the outside.
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