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41 TRANSFER OF TECHNOLOGY FROM OTHER FIELDS TO THE EROSION OF GUN BARRELS The "Wear", referred to in this report, is a collective term for a number of phenomena capable of creating dimensional changes and includes mechanical wear (adhesion, plowing, abrasion), erosion by swiftly flowing gases, corrosion by the products of combustion, and surface cracking or deterioration due to thermal shock or thermal fatigue. Similar destructive phenomena limit the useful life of internal combustion engines, metal cutting tools and the dies used for forming metal in the casting, forging and extrusion processes. Discussion of wear problems in each of these fields follows, as does a summary of those corrective measures used in each area which might also be applicable to gun barrel use.
42 1. Internal Combustion Engine Technology While there are functional similarities between an engine piston and a projectile, a piston ring and a rotating band, or a cylinder barrel and a gun tube -- the magnitudes of pressure and temperature in the gun are far more extreme. This, plus the continuous presence of lubrication in the engine, makes direct comparison difficult. There are, notwithstanding, a few critical areas which may relate to the gun problem. Exhaust valves in four-cycle engines are exposed to high combustion chamber temperatures while closed C3000 degrees F., or more). At the moment the valve opens by lifting off its seat, combustion products at high temperature flow over the sealing edges of valve and seat at sonic velocity - circa 1600 feet per second in a typical case. This may be analogous to the flow of propellent gas around the origin of rifling prior to the time that engraving of the rotating band effects a seal. In the case of the engine, the combined actions of the hot gas flow and the mechanical hammering or pounding of the valve on the seat result in dimensional changes in both parts. Gas erosion, corrosion, oxidation and mechanical wear are all thought to play a part. Erosion of the valve itself has traditionally been the prime source of concern. The valve, as a result of its mushroom shape and the fact that it is constantly in motion is very difficult to cool effectively, and hence, operates at higher temperature than the seat. local erosion or "guttering" of the sealing rim of the valve can produce massive leakage of gases from the cylinder, ren- dering the engine unuseable in few operating hours. Steps taken by engine designers to minimize the problem include cooling of the valve, se^ction of valve materials with good physical properties and high corrosive resistance at elevated temperatures, and protective coatings for the sealing surface.
43 Cooling of the valve in engines of moderate specific output (horsepower developed per unit of piston displacement^ is limited to supplying ample coolant around the valve guide /in which the valve stem reciprocates^ and the valve seat (with which the sealing face of the valve makes intermittent contact) to remove heat from the valve through conduction. In higher output engines, the valve may be hollow and partially filled with sodium. In use, the sodium melts and sloshes to and fro in the valve to conduct heat from the head to the cooler stem portion. Valve materials are austenitic steel alloys of chrome, nickel and manganese. Typical are 214N /containing 21% chromium, 4% nickel and 9% manganese) and 2155N (21% chromium, 5% nickel, 5% manganese). Over the years a wide variety of protective coatings has been used on the sealing face of the valve but the accepted standard for severe applications is a Stellite facing, applied by "puddling", a form of welding. The valve seat, formed in a mass of relatively well-cooled metal, is less prone to catastrophic "guttering" or "burning" than the valve itself. In typical automotive use, for instance, the grey cast iron of the cylinder head has long been adequate as a seat material and the seat is simply machined into the iron. The wear or erosion of the seat typically appears as a gradual recession or sinking of the seat into the bulk material of the cylinder head. The geometry of the sealing face is maintained and negligible leakage occurs during the process. This recession continues at a slow pace until it causes the valve motion to be stopped by the valve driving linkage rather than by the seat - or, in other words, until the valve stands slightly open during the period when it should be closed. A high rate of leakage then occurs. As noted above, the rate of recession of the valve seat in automotive applications has long been very low. Recently, the trend toward "no lead" gasoline has changed the picture markedly.
44 Commercial gasolines have long contained tetraethyl lead as an octane improver and engines were developed on that basis. Recent emphasis on exhaust emission reduction has necessitated use of catalytic converters in the exhaust systems. The lead compounds were found to poison the catalysts, hence it was decreed that fuel must be made available which was free of lead compounds. It was soon noted that engines using the lead-free fuel suffered early failure from valve seat recession. The nature of the recession was similar to that found with leaded fuel, but the rate of recession was far greater - perhaps a factor of 10- 15 times greater. The mechanism by which the tetraethyl lead protects the valve seat is not fully clear, although best opinion is that lead oxides serve as an anti-welding agent to reduce metal transfer when the valve contacts the seat. Whatever the mechanism, steps taken to regain the valve seat life when using the no-lead fuels have concentrated on material changes in the seat. Harden- ing of the integrally-machined iron seats (induction hardening) has been helpful in engines of moderate specific output. For high-output or heavy-duty applications, it has been necessary to use valve seats of superior materials inserted into the cast iron cylinder heads. Typical of such insert materials are high chromium and nickel irons (20% chromium). In some cases the sealing faces of the seats are stellite-coated, by welding or oxy-acetylene spraying. Piston rings in an engine are somewhat analogous to the rotating bands of projectiles in that they must seal the expanding gases resulting from combustion of fuel (propellant) while sliding with the piston /projectile) along the cylinder 'gun bore). On the other hand, they are not subjected to the plastic flow suffered by a rotating band and are provided with a reasonably good supply of lubricant on each stroke. Sliding, as they do, at mean velocities of up to 4000 feet per minute while reversing direction at up to 20, 000 times per minute, sealing pressures ranging
45 up to 2000 psi at sealing surface temperatures of 600-800 degrees F., modern piston rings are remarkable in resisting wear to the extent that they do. Again, the materials of the sliding surfaces are critical. The cylinder walls are almost universally of gray cast iron at a hardness of 200-240 Brinell with an initial surface finish of 15 to 20 microinches RMS. There are exceptions. The piston-type aircraft engines used steel sleeves held in aluminum housings, with the steel often nitrided or chromium-plated. Small air-cooled engines sometimes use aluminum cylinders with chromium-plated bore surfaces. The Chevrolet Vega automobile engine block is cast of high-silicon aluminum alloy and the piston rings run on the aluminum surface - which has been electro-etched to make the sili- con particles protrude microscopically above the aluminum. Cast iron, however, is the norm. Piston rings are usually cast of iron or, more recently, formed and sintered of iron powder. Again, protective coatings are common. Tin plating is used to provide anti-scuffing protection, particularly during the run-in period. Chromium-plating is common for heavy-duty applications. Similarly, molybdenum disulphide coatings are effective in extreme cases. Wankel engine apex seal development provides an additional case where technology might be transferred to help solve the gun barrel erosion problem. Here sealing between the apexes of the triangular rotor (piston) and the "bore" of the trochoidal rotor housing (cylinder) is accomplished by a single seal head in contact with the housing surface by spring. Normal piston ring technology is not applicable. Wear conditions are severe and have not yielded to numerous attempts at analytical solutions. Edisonian research has evolved several workable combinations of seal and trochoid housing surface. One, which has had some commercial success (Toyo Kogyo Mazda auto- mobile) uses a hard chromium-plated trochoid housing and seals made of a highly
proprietary carbon compound containing aluminum particles. Pulse-plating processes (Novachrome, for example) are common. Another (NSU Bo 80 automobile) is based upon an Elnasil-coated trochoid housing with apex seals of an iron compound akin to piston ring material (Goetze- werke's IKA). Elnasil is a plating process which provides a matrix of nickel containing particles of silicon carbide. For high-output or heavy-duty applications (marine applications, for example), best results have been obtained with a coating of tungsten carbide on the trochoid housing and seals of tool steel. The "tungsten carbide" is a mixture of tungsten carbide, nickel, cobalt and alumina applied by spraying with a plasma gun. This material is proprietary and is made by Metco, of Westbury, Long Island, New York... as is the plasma gun. A typical application uses Metco 439 alloy, plasma-sprayed to a thickness of not more than 0. 015 inch to minimize internal stress. This is ground to a thickness of 0. 005 to 0. 010 and honed to the desired surface finish. Remarkable progress has been made in the last 12-18 months in evolving tungsten carbide coating of superior adhesion characteristics and minimum crack- ing propensity. Engine bearings are designed for use with ample quantities of oil as a lubricant. They must, however, cope with transient or emergency situations where they operate for brief periods in the regime of boundary lubrication or even under conditions of dry friction. Toward this end, multi-material bearings have been devised and used to good effect. During World War II the use of silver-lead-indium bearings was common in aircraft engines. The silver was plated onto a steel shell or backing to a thickness of . 020 to 0. 030 inch, providing good heat conducting and fatigue resistance. A 0.001 to 0.003 inch layer of lead plated over the silver offered
47 resistance to scoring and provided some degree of embeddability for small particles. A very thin layer of indium protected the lead from the corrosive action of the lubricating oil and its content of combustion products. While cost and availability have led to the demise of the silver-lead-indium bearing for most applications, thin plated layers of silver are currently used as a solid lubricant in critical operations. For example, the side faces of roller-bearing connecting rods used in two-cycle engines are so plated (about 0.001 inch thick), as are the bearing cages or retainers used in such applications. The silver makes the difference between satisfactory operation and failure. A current substitute for the silver-lead-indium bearing is the copper-lead bearing. Here the bearing is a mixture - not an alloy - of copper and lead with the lead dispersed throughout the copper. A typical mix might contain one-third lead, two-thirds copper, with a trace of tin or silver to increase the hardness. Bearings of this type have excellent wear characteristics under extreme conditions as the lead forms a low-friction layer on the surface which is self-replenishing. Such mixtures can be cast or sintered. In casting, the resulting structure is a matrix of dendritic copper with the lead mechanically filling the interstices. For sintered material, a porous copper matrix is first formed by sintering and then impregnated with lead. Aluminum has also moved into the bearing field, generally alloyed with cadmium and often containing tin, nickel, or silicon in the form of deliberate "inclusions." Typical are Federal Mogul's AT6 material and G. M. 's Moraine 410. They can be either cast or wrought. Traditionally, engine wear is determined by disassembling the engine after running and measuring the individual parts with conventional measuring tools - micrometers, dial bore gages, etc.
48 Engine cylinders present a problem in that permanent distortion of the bore ("out-of-round") is frequently greater than actual wear, thus making measurement of bore wear impossible. During World War II, Mr. S. A. McKee of the National Bureau of Standards devised a method for measuring wear without regard to 20 distortion. The McKee technique involves indenting the wear surface, prior to starting the test, with a diamond penetrator akin to an oversize Knoop hardness penetrator. By observing the reduction in length of the pyramidal indentation as wear proceeds, one can easily compute the depth of wear from the known geometry of the penetra- tor. The geometry of the Knoop indentor is such that the change in the pyramidal length is approximately 30 times the radial depth of the layer. Special tools for applying the penetrator to the surfaces of cylinder bores and periscopic optical devises for reading the dimensions of the indentations were devised and marketed during World War II and were widely used in engine labora- tories at that time. The same basic technique has since been used in laboratory work on the nature of the wear process, using simply a Knoop diamond and a measuring micro- scope to measure very small amounts of wear in bench tests. An application of this technique to the wear of gun barrels is questionable as the scuffing, smearing and other plastic deformation involved will tend to fill the indentation with debris and obscure any measurement of real wear.
49 2. Cutting Tool Technology High temperature (1000 F. and higher) and pressures (100, 000 psi and higher) exist on the face of a metal cutting tool. In addition, a clean freshly genera- ted surface passes across the face of the tool at high speed. These conditions result in interaction between chip and tool which in turn results in rapid wear. Review of this technology should be useful in providing clues to solutions that might prove successful in reducing gun barrel erosion. To withstand such extreme conditions, cutting tools must be refractory enough to retain their shape under high operating temperatures, hard enough to resist the erosive action of the chip and ductile enough to avoid chipping when microwelds which form on the tool face are broken. Material characteristics that are useful in prolonging tool life should be useful in preventing gun barrel erosion. High speed steels that contain substantial amounts of tungsten or molybdenum to increase the hot hardness of the tool are far superior to ordinary alloy steels and such materials might be useful as inserts at the breech end of a gun. Where increased abrasion resistance is called for increased amounts of chromium and vanadium are added to provide hard complex carbides. Increased amounts of cobalt are employed to increase the hot hardness of a tool steel. Cemented tungsten carbides and hot pressed ceramics are still more refractory materials but suffer from an increased tendency to chip. If such materials were to be used in a gun barrel they would have to be in the form of spray coatings. The lack of ductility of the coating would then be largely offset by that of the underlying steel. Recently, the crater resistance of tungsten carbide tools has been sub- stantially increased by the vapor phase deposition of small amounts (0. 0002 inch) of titanium carbide or aluminum oxide on the surfaces of tungsten carbide tools. These materials are more stable than tungsten carbide when operating in contact with the thin layer of austenite that forms on the surface of a chip. Although titanium carbide and aluminum oxide are too brittle to be used in bulk, they have sufficient ductility when used as a very thin overlay. These surface coatings
50 diffuse into the tool face ahead of the wear zone and hence are effective in decreas- ing the rate of wear to depths that are far greater than the thickness of the layer initially applied. If tungsten carbide coatings are used in gun barrels it should be useful to employ thin coatings of titanium carbide or aluminum oxide to the tungsten carbide to further increase wear resistance. A CVD coating of titanium carbide has been found effective also in reducing erosion of steel compressor blading for helicopter gas turbines, even though the fatigue resistance of the steel blading was reduced. Although only 0. 0002" is used as an overlay on tungsten carbide tools, thicknesses of up to 0. 002" have been applied on steel compressor blading. Its very good adherence and erosion resis- tance makes it attractive as a gun barrel materials concept. There is some question regarding the minimum temperature at which TiC can be deposited. Most of the available data indicate 1000-1100°C is necessary. These temperatures would make the process impractical for coating gun barrels. However, there is other proprietary information that TiC coating processes of about 500-600°C were used to coat some of the compressor blading materials. Research should be conducted and aimed at applying TiC at these lower temperatures. The strong tendency for clean metals to adhere when operating in sliding contact is frequently reduced by use of cutting fluids which contaminate the sliding surfaces and hence prevent adhesion and galling. Some of the extreme pressure additives used in cutting fluids might prove useful if a spray coating were to be used at the breech end of a gun between firings or if it would be pos- sible to coat the outside surfaces of shells with suitable boundary or solid lubri- cants. The rate of tool wear is proportional to about the 20th power of the tem- perature for a high speed steel tool and to about the 10th power of the temperature for a carbide tool. This suggests that strong cooling should be beneficial in metal cutting operations and in fact this is usually the case provided the tool remains buried in the cut (as in a turning operation). However, when a cutting edge cuts
51 intermittently (as in a milling operation) use of a coolant makes things worse if the tool material is tungsten carbide. This is due to the formation of thermal fatigue cracks that result from the thermal stresses that arise as the tool is alternately heated and cooled. High speed steel tools that cut intermittently are helped by strong cooling since this class of tool materials is ductile enough to avoid cracks due to thermal shock and thermally induced fatigue. This ex- perience suggests that the life of a gun barrel might be extended by use of mist cooling between firings if a high speed tool steel liner were used but probably not if a sprayed layer of tungsten carbide were to be used.
52 3. Casting, Forging and Extrusion Technologies Industrial practices used in casting, forging and extrusion of various metallic materials that might provide information relevant to the gun tube erosion problem have been reviewed. (References 1-5). A typical recent contribution is given in reference 21. Practices used for permanent mold and die castings, forging, including high energy rate forging (HERF), and extrusion incorporate a mold or die coating in nearly all cases. Such coatings are of two general types - insulating and lub- ricating - with some coatings performing both functions. A number of types of mold coatings are used, depending on the specific practice. Sodium silicate plus kaolin, fireclay, metal oxides, diatomaceous earth, whiting (chalk), soapstone, mica and talc are examples of insulating types of coatings. Graphite, soot, oil- containing materials, greases and other carbonaceous materials, and molybdenum disulfide are types of coatings used for lubrication purposes. Molten glass, such as used in the Ugine-Sejournet process for extrusion, is used both as a lubricant and an insulator. In all of these metal forming and shaping processes, the selection of mold or die material and of the coating system have been developed to give good quality formed or shaped parts along with acceptable mold or die life. The mold or die life can vary considerably.. .from as little as only a few pushes on certain extrusion dies, to more than a quarter of a million parts produced in permanent mold cast dies, for example. For dies and molds that run hot, more highly alloyed tool steels are the usual mold or die materials. Generally the hotter the temperature reached by the mold or die, the shorter its life because of greatly increased abrasive wear, cracking and heat checking, and erosion. In spite of the wide diversification of types of mold or die materials, coatings, and types of metals processed, a feature common to all of these pro- cesses is that the coating system employed acts to prevent metal-to-metal contact
53 between the casting or work piece and the mold or die. When the coating systems fail and metal-to-metal contact occurs, mold or die life can be exceedingly short. The conditions of gun tube erosion appear to produce a more severe and complex operating environment than most, if not all, of the industrial metal casting, forging and extrusion practices. However, there are several items from the technology of these many and varied metal forming and shaping operations that might be applied to the gun barrel erosion problem. One of these is a development of a coating system that will provide the most effective barrier between the pro- jectile and gun tube to avoid metal-to-metal contact. Another is the selection of materials with higher hot strength, and abrasive resistance in the form of either inserts or possible surface coatings, for use in the most critical areas of erosion in the gun barrels. Another possibility that is not generally a feature of the ma- terials and coating systems used for casting, forging and extrusion but that could be an important aid in improving life of gun barrels would be the use of gun tube materials or surface coatings that would provide improved resistance to the chemical corrosion and oxidation aspects of the gun tube erosion mechanism.
54 , 22, 23, 24, 25, 26 4. Rocket Nozzles In the unrealized hope that sufficient parallels exist between the condi- tions existing in rocket nozzles and in gun tubes, this field was surveyed. There are two separate problems - Liquid Fuels and Solid Fuels. With liquid fuel, ero- sion does not seem to be a problem although some corrosion may be involved. Cooled metal throats are often used in which tubes form the throat and the fuel acts as a coolant. Steel, Nb, coated Mb, and Be have been used. SiC and C in- serts are also used. Ablative throats which involve controlled erosion are used (in the Apollo system for example). Materials with high specific heat and heat of vaporization are desirable. Phenolics reinforced with carbon or Quartz fibers and Poco graphite which recedes slowly are two of the materials employed. Solid fuel rockets often use carbon or graphite inserts which are resistant to erosion. Here it is said that the stoichiometry and ignition conditions are critical. Presumably this is to maintain the carbon in a reducing atmosphere. All of this, while not completely unrelated to gun tubes, does not suggest useful solutions.
55 5. Gas Turbine Engines The burner and turbine in gas turbines are subject to oxidation and selec- tive chemical attack (sulfidation) as a result of high gas flow rates at elevated temperatures. This problem is kept under control by the use of inherently oxida- tion-resistant materials (nickel or cobalt base alloys containing chromium), the use of (usually) aluminide coatings, and designs to incorporate cooling of the metal. Important differences from gun barrels are in the sizes of the parts, and the easy justification for the use of expensive alloys. A cursory survey of the technical status of the industry indicates that past improvements have been made largely on an empirical basis. An under- standing of the basic phenomena, such as oxidation or sulfidation under the existing conditions, is just beginning to be obtained. Operating conditions are sufficiently different that any direct transfer of technology so as to reduce gun barrel erosion seems unlikely. However, it is recommended that basic theoretical understanding now being obtained by the gas turbine industry be monitored for possible applica- tion to the gun erosion problem. Technology transfer, if it eventuates, would be most likely from industrial gas turbines, which operate in industrial atmospheres and use fuels containing contaminents such as sulfur.
