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INTRODUCTION Confronted with problems associated with the continual upgrading of field guns, reducing the number of different items in supply and lowering overall costs, the Department of the Army has need for improved methods of reducing erosion and extending the wear life of long range cannon. The National Materials Advisory Board was asked to consider all facets of this need and to make recommendations. OBJECTIVES The objectives of this report are to make an assessment of the nature of the bore erosion problem; to identify fruitful areas of research; to assess the state of technology of materials and methods in areas which may become significant to gun tubes of improved performance; to suggest experimental techniques, devices, and instruments and methods of reducing data; to enhance the continuing coordina- tion of the activities of the three Services; if feasible, to concentrate on a large weapon such as the 8 inch M110E2 gun system in order to suggest helpful short- range solutions in addition to possible long-range solutions. DISCUSSION AND RECOMMENDATIONS While the problem of gun erosion has to some measure been alleviated, there is no implicit assurance that future gun systems will have acceptable wear life. This is particularly true if such systems reach for higher projectile veloci- ties at higher chamber pressure with new propellant formulations. Anticipating that this will surely happen, the need for a better understanding of the mechanism of gun erosion is apparent. It is evident that erosion of gun tubes is not a simple problem; cutting across the sciences of metallurgy, chemistry, physics, and mechanics. Any future effort should be constructed to recognize the multidisciplinary nature of the problem, both in planning and in direction. In the course of conducting this task the Committee was impressed by the number of separate and varied investigations and tests that have been performed.
It would probably be fruitful to formally collate and examine the existing data prior to planning further research. This effort might also suggest short-term investiga- tions aimed at quick fixes for immediate problems. The nature of the gun-erosion problem superficially would appear to be different among the three services. The Air Force is concerned with smaller cal- iber, high rate of fire, weapons. The Army and Navy have this problem plus larger caliber, high velocity guns and large bore, low performance systems. It is suggested that close liaison between the services would be beneficial. In fact, further effort should recognize that there is a commonality in the problems ex- perienced and the expertise acquired by the three services. Their active partici- pation in future, unified, efforts in this area would be most desirable. A long-term goal should be a better understanding of the erosion phenomena. It is doubtful that this can be achieved through purely analytical treatment. Rather, any predictive model would be to a large degree empirical. This implies that the various investigators have available valid test vehicles. The extrapolation of data from vented vessel firings, measuring wear of an orifice is open to questions. Much testing has been done in small bore (7. 62 to 20mm). It is not at all certain that there is a one-to-one correspondence with these results and those obtained by full-scale firings or that results of various investigators can be translated from one test device to another. A first course of action would be the establishment of a common test device(s). Probably two sizes should be chosen in order to obtain at least first-order estimates of scaling factors. Assuming a valid test vehicle is available, a parametric investigation of the erosion process should be conducted. The parameters of importance min- imally should include propellant chemistry, gun tube metallurgy, projectile ma- terials, and interior ballistic effects. While such a study could not be precisely definitive, because of the complexity of the problem, it could define the role of the leading variables.
3 The most recent and notable gain in the reduction of erosion has come through the use of additives (talc or TiO in wax) to the propelling charge. While m it is generally agreed that the additive is effective because it reduces the heat trans- fer to the gun tube, the mechanism by which this is accomplished cannot be de- scribed at this time. Also, the effectiveness of the additive is not always pre- dictable: cannot be scaled from one gun system to another. Driving towards higher and higher performance systems, one might soon be operating in regimes where the additive is not effective at all. This suggests a research program, in advance of that date, to elucidate the mechanism by which additives reduce erosion. The search for "better additives" without some well developed rationale can re- sult in "one at a time solutions" which must be continually reiterated. The role of the products of combustion of the propelling charge in gun tube erosion is not well understood. Some work on the effects of specific products and the CO/CO ratio have been performed. The need for higher force constant pro- m pellants, e.g., the nitramine compositions, will drastically alter the composition of the propellant products. This suggests the need to support an effort in this area. One can see that superior propellants (i. e., ballistically) might not be useful be- cause of increased gun tube erosion. Recent advances in polymers and reinforced polymer-fiber composites suggest that the concept of "plastic" rotating bands be reconsidered. There has been some success in demonstrating a reduction in erosion through the use of plastic bands. It is understood that the problem, to date, has been one of guar- anteeing the integrity of the band during engraving, travel down the tube and launching from the muzzle. The superior (and more importantly, the ability to tailor) mechanical properties of the newer polymers and composites is cer- tainly worthy of further investigation. The discussions of other fields of engineering endeavor presented in the sections on technology transfer (Page 41) provide a number of ideas which merit
evaluation for gun barrel use and are recommended to be investigated. In order of ascending difficulty of application, these recommendations are: 1. Insulating and/or Lubricating Sprays or Washes Mold coatings, or "die slicks," as discussed in the casting and forging technology section, would be simple to introduce to the origin of rifling area as a spray between rounds. Their effectiveness in extending die life under conditions approaching those met in gun barrels warrants investiga- tion of both proprietary compounds and experimental compounds expressly developed for gun use. 2. Botating Bands It is understood that rotating bands are normally made of gilding metal or copper. Copper, by itself, is not noted for its bearing and wear properties. Substituting a copper-lead bearing alloy for the copper might be helpful, particularly since the lead is dispersed throughout the bulk material and is always present for its anti-wear function - even after undergoing engraving. This would clearly entail experimentation with the compounding of the mixture to provide enough lead for "lubrication" and to provide adequate physical properties for rotation. The dispersion of the lead within the copper might offer better resistance to "fouling" than would the alternative of lead plating the exterior of a copper rotating band. Similarly, the use of aluminum bearing alloys in place of copper merits consideration. Mr. Edward Smith, Delco Maraine Di- vision of General Motors, Dayton, Ohio (phone no. 513-445-4465), is an excellent source of information on bearings alloys and mix- tures of both the aluminum and copper-lead varieties. The sintering process also provides an opportunity for tailoring a "semi-bearing" material specifically for rotating band use. Typical might be a porous iron sintering, impregnated with an appropriate lubricating material. The behavior (friction, heat generation by deformation, and heat transfer to the barrel) of rotating bands as they are en- graved in the forcing cone is amendable to direct measurement
27 and to treatment by well-established extrusion theory. Calorimetry is recommended to establish the heat partition between the shell and the barrel. The coefficient of friction can be calculated by measur- ing the force required to push shells having rotating bands of two widths. The heat generated by redundant work (in transforming the circular cross-section of the band into the serrated cross- section) can be estimated from relationships developed for metal extrusion. 3. Protective Surface Coatings As was observed at Watervliet, the arsenal is already deep into the use of hard platings for wear reduction. Chromium-plating and chromium-plating on cobalt have been evaluated and experi- ments with nickel coatings containing hard particles (like Elnasil) are underway. Tungsten carbide was dismissed as being too brittle and for having poor adhesion. It is suggested that alloys of tungsten carbide (as used in the Wankel engine) be reconsidered for local application in gun tubes at the origin of rifling. A very thin deposit, plasma sprayed, might require no finishing operations, have minimal internal stress, and offer the best opportunity for good adhesion. The rapid growth of expertise in this field, as stimulated by the Wankel effort, has resulted in new techniques for application and bonding. Mr. Frank Longo, of Metco, is a good source of detail informa- tion. Further toward this end, cutting tool experience has shown thin (0. 0002 inch) overlays of titanium carbide or aluminum oxide to greatly enhance the wear resistance of tungsten carbide. It is also suggested that attention be given to the use of a thin stellite coating at the origin of rifling. Application of this material is an everyday commercial procedure in the manufac- ture of engine valves. It can be oxy-acetylene sprayed or, for thin coatings, plasma sprayed. Again, Metco is a good source of practical information on these processes. Another protective coating which might merit investigation is a proprietary DuPont product called "Triballoy." A cobalt- nickel based alloy, it is reportedly very wear-resistant at elevated temperatures and has good hot-hardness capability. Plasma spray is used for application. Dr. Donald Ferriss,
6 Chief Metallurgist, Triballoy Products of DuPont (302-453-2631), is a source of information on this material and its application. A key question on the use of any of these hard coatings is whether they would be effective in the gun tube in sprayed layers thin enough to be used without any finishing operations such as grind- ing or honing. 4. Wear-Resistant Inserts As has been noted, the use of wear resistant inserts is common in the engine field - cylinder liners, valve seats, etc., and in cutting tools. It is suggested that consideration be given to the use of a permanently installed wear-resistant insert at the origin of rifling. This would permit the continued use of the relatively soft bulk material needed in the tube for fatigue re- sistance, but provide a harder (at working temperature) mate- rial where it is needed for erosion resistance. Anchoring the insert firmly enough to resist the rotating force imposed by the projectile on the rifling might be handled by furnace brazing the insert into the gun tube during heat treat- ment of the tube. It is common practice in gear manufactur- ing to simultaneously furnace braze a pre-carburized insert into a gear and harden it while heat treating the bulk of the gear. Experimental inserts might be anchored in finished tubes by electron beam welding. Use of an insert opens up a whole new area for material selec- tion . . . including through-hardened steels, carburized and hardened (for compressive stress at surface) steel, die steels, high speed tool steels and others of good properties at high temperatures. Nitriding becomes feasible. 5. Cooling The drastic decrease in cutting tool life with increased temper- atures prompts investigation into means for reducing gun barrel temperatures. Cooling of the wear surfaces (i.e. the bore) might be accom- plished by a jet of expanding gas directed at the origin of rifling between rounds. Or, some cooling might be achieved by appropriate selection of a spray or wash which would also supply the insulating and/or lubricating function of Recom- mendation #1.
