Volume 18 - Friction, Lubrication, and Wear Technology Part 16 ppt

Fig. 6 Variation in wear of carbon brake from taxi to RTO energies Each of the three energy ranges has its own wear mechanism. The taxi region exhibits the highest wear, and mechanical abrasion predominates. In the mid-energy (service energy) region, wear is lowest because a thin film protects the substrate. The third region occurs when temperatures are sufficiently high to produce oxidation of any debris or substrate (fiber or matrix). Oxidation usually occurs during overload and RTO conditions. PAN-base fibers wear differently than pitch-base fibers; these differences are associated with fiber processing as well as composite processing. A harder matrix will wear less than a softer one. Studies have shown that circumferentially oriented fibers have high wear and a high coefficient of friction. Wear Mechanism and Wear Debris Analysis. The worn surfaces of the carbon materials have bands of different reflectance when observed via macro examination. Debris material occurs in both bright and dull bands. The observed banding on the wear surface of the composite is related to variations in reflectance; these variations are thought to be related to differences in the character of the wear debris. The bright bands consist of a thin film of debris with a polished appearance that produces a high reflectance. The dull bands have a high number of fissures or shallow grooves and scratches in the longitudinal fiber bundles, both of which are indicative of fiber removal caused by abrasion. The debris material in the dull bands does not produce a high reflectance and appears less dense (more porous) than the bright bands. The debris in the dull bands is particulate and is comprised of fibers and matrix. This debris does not form a film, but fills in the original porosity, mainly in the matrix, and is found primarily under low-energy conditions. The debris in the bright band consists of a thin film (up to a few micrometers thick) that is smeared over fibers and matrix and also fills in the pores. The bright bands appear to be denser (less porous) than the dull bands. The wear debris film is grooved, whereas the fibers and matrix are not. This indicates that the wear debris film is protecting the fibers and matrix from abrasion. The thin film of wear debris is amorphous in character and is comprised of both fibers and matrix. High-energy braking conditions produce a high coverage of wear debris (film), which may result in a low coefficient of friction and low wear rate. More detailed information on wear debris analysis can be found in the article "Lubricant Analysis" in this Volume. Moisture Problems. Carbons and graphites have an affinity for moisture. These materials adsorb moisture; that is, water molecules are attracted to their exterior and interior surfaces. Consequently, the presence of moisture significantly reduces the coefficient of friction. When a brake is sitting for a period of several hours, the rubbing surfaces will adsorb moisture from the air. Thus, when a brake stop is initiated in this condition, the stopping power is significantly reduced because of the low coefficient. This is typically referred to as "morning sickness." As rubbing continues through the stop, the moisture evaporates and the coefficient returns to its normal dry value. Oxidation. Carbons and graphites are also subject to oxidation at elevated temperatures. Typically, the threshold of oxidation is considered to be 430 °C (805 °F). Technically there is oxidation at this temperature, but it is so low that it is considered to be negligible. Other than taxi conditions, the operating temperatures of a carbon brake will range between 500 and 1000 °C (930 and 1830 °F) during landings; under RTO energies the temperatures exceed 1300 °C (2370 °F) and oxidation becomes a significant factor. In order to keep the disks from deteriorating, an oxidation inhibitor is applied to the outer and inner diameters of each carbon brake disk. Inhibitor is not applied to the wear surfaces because it would alter the friction characteristics of the carbon. Friction Coefficient Variability. As stated previously, friction coefficients have a larger range over the operational spectrum of the aircraft when compared to the friction coefficients for steel brakes. This coefficient range has implications for brake control (antiskid) system design. In the carbon friction material, friction coefficients can vary by factors of 3 or more over the operation range of the brake. Therefore, the brake torque can vary by a factor of 3 or more. Carbon processing and brake frame design must take this friction coefficient variation into account. Vibration. An aircraft wheel and brake is a system with multiple degrees of freedom that is subjected to high dynamic loads. This high dynamic loading is transient in nature and can be an exciter of vibration. In a carbon brake, the most critical vibration mode is known as whirl. This mode consists of an accordian-type action of the brake disks combined with an orbiting motion of the brake structure about the axle (Fig. 7). The torque output and friction coefficient of the brake, as functions of velocity and brake pressure, are significant parameters in determining whether or not whirl motion will occur and, if it does occur, the severity of the vibration. Severe whirl can result in damage to the carbon brake disks and other wheel/brake hardware. Fig. 7 Carbon brake vibration There are two basic approaches that can be taken to control whirl vibration. First, carbon friction coefficient characteristics are engineered in such a way that the brake coefficient, at a given velocity and brake pressure, remains low enough to minimize the potential for the whirl vibration. Second, the aircraft wheel/brake is designed to incorporate the required stiffnesses and damping characteristics. Adequate stiffnesses and damping, combined with favorable friction coefficient characteristics, will ensure the stability of the aircraft wheel/brake structure at given dynamic conditions. Testing Friction material testing is conducted on either direct-connected dynamometers or on landing wheel dynamometers. Typically, new friction materials are screened in subscale brakes on direct-connected dynamometers. Once a potential material has been selected, then full-scale brakes are tested on landing wheel dynamometers which use an aircraft tire and wheel. Full-scale brake testing is very expensive and therefore limited. During aircraft wheel and brake qualification testing, the full-scale brake is run through numerous tests before any aircraft testing is done. Test Requirements. Generally an aircraft brake must pass testing standards set up by the military or FAA. The military requirements are outlined in MIL-W-5013, and the FAA requirements are continued in TSO-C26. The Society of Automotive Engineers (SAE) also publishes an Aerospace Recommended Practice (ARP) for aircraft wheels and brakes (ARP 597). The airframe manufacturers also specify extensive supplemental qualification requirements that must be met before the brake can be qualified for service. Selected References • J.F. Archard, The Temperature of Rubbing Surfaces, Wear, Vol 2, 1958/59 • F.P. Bowden and J.E. Young, Proc. R. Soc. (London) A, Vol 208, 1951, p 444 • H.W. Chang, Correlation of Wear with Oxidation of Carbon-Carbon Composites, Internati onal Conference on Wear of Materials, American Society of Mechanical Engineers, 30 Mar to 1 Apr 1981 • T.S. Eyre and F. Wilson, Wear of Grey Cast Iron Under Unlubricated Sliding Conditions, ASME/ASLE International Lubrication Conference (New York), 9-12 Oct 1972 • D.B. Fischbach and D.R. Uptegrove, Oxidation Behavior of Some Carbon/Carbon Composites, 13th Biennial Conference on Carbon (Irvine, CA), 1977 • K. Gopinath, G.V.N. Rayudu, and R.G. Narayanamurthi, Friction and Wear of Sintered Iron, Wear, Vol 42, 1977, p 245-250 • B. Granoff, H.O. Pierson, and D.M. Schuster, Carbon-Felt, Carbon- Matrix Composites: Dependence of Thermal and Mechanical Properties on Fiber Volume Percent, J. Compos. Mater., Vol 7, Jan 1973 • T L. Ho, "Development and Evaluation of High- Energy Brake Materials," Ph.D. thesis, Rensselaer Polytechnic Institute, 1974 • J.M. Hutcheon and M.S.T. Price, The Dependence of the Properties of Graphite on Porosity, Proceedings of the Fourth Conference on Carbon, Pergamon Press, 1960 • W.V. K otlensky and P.L. Walker, Jr., Crystallographic and Physical Changes of Some Carbons Upon Oxidation and Heat Treatment, Proceedings of the Fourth Conference on Carbon, Pergamon Press, 1960 • I.V. Kragelskii, Friction and Wear, Butterworths, Washington, 1965, p 117 • J.K. Lancaster, Instabilities in the Frictional Behavior of Carbons and Graphites, Wear, Vol 34, 1975 • R.L. Lewis and R.E. Raymond, "Stopping Distance Analysis," Society of Automotive Engineers, Inc., Paper No. 730193, 1973 • F.F. Ling and E. Saibel, On Kinetic Friction Between Unlubricated Metallic Surfaces, Wear, Vol 1, 1957/58 • J. Molgaard and V.K. Srivastava, The Activation Energy of Oxidation in Wear, Wear, Vol 41, 1977 • N. Murdie, C.P. Ju, J. Don, and F.A. Fortunato, Microstructure of Worn Pitch/Resin/CVI C- C Composites, Carbon, Vol 29, 1991, p 335-342 • D. Pavelescu and M. Musat, Some Relations for Determining the Wear of Composite Brake Materials, Wear, Vol 27, 1974 • T.F.J. Quinn, A.R. Baig, C.A. Hogarth, and H. Muller, Transit ions in the Friction Coefficients, the Wear Rates, and the Compositions of the Wear Debris Produced in the Unlubricated Sliding of Chromium Steels, ASME/ASLE International Lubrication Conference (New York), 9-12 Oct 1972 • E, Rabinowicz, Friction and Wear of Materials, John Wiley & Sons, 1965 • D.M. Rowson, The Interfacial Surface Temperature of a Disk Brake, Wear, Vol 47, 1978 • L. Rozeanu, Friction Transients (Their Role in Friction Failures), Trans. ASLE, Vol 16, 1975, p 257- 266 • J.J. Santini and F. E. Kennedy, Jr., An Experimental Investigation of Surface Temperatures and Wear in Disk Brakes, Lubr. Eng., Aug 1975 • P. Stanek, N. Murdie, E.J. Hippo, and B. Howdyshell, The Effect of Fiber Orientation on Friction and Wear of C-C Composites (Extended Abstracts), Biannual Conference on Carbon (Santa Barbara, CA), 1991, p 378-379 • I.L. Stimson and R. Fisher, Design and Engineering of Carbon Brakes, Philos. Trans. R. Soc. (London) A, Vol 294, 1980 • E.M. Tatarzycki, Friction Characteristics of Some Graph ites and Carbon Composites Sliding Against Themselves, 13th Biennial Conference on Carbon, 1977 • A.K. Vijh, The Influence of Solid State Cohesion of Metals and Non- Metals on the Magnitude of Their Abrasive Wear Resistance, Wear, Vol 35, 1975 Wear of Jet Engine Components J.D. Schell and K.P. Taylor, General Electric Aircraft Engines Introduction A JET ENGINE is a sophisticated piece of machinery with many moving parts; the potential for wear problems exists whenever moving parts come into contact or unintended motion occurs between stationary contacting parts. As for any system, wear in jet engines can be controlled through proper design, material selection, and lubrication. A schematic cross section of a typical jet engine is shown in Fig. 1. The major engine subsystems consist of the fan, the high-pressure compressor (HPC), the combustor, the high- and low-pressure turbines (HPT and LPT), and the exhaust nozzle. The engine design contains one nonrotating system and two concentric rotating systems. The nonrotating (stator) system is made up of structural frames and casings. The low-pressure rotating system consists of the fan disk(s) and fan blades, the LPT disks and turbine blades, and a connecting shaft. The high-pressure rotating system consists of the HPC disks/spools and compressor blades, the HPT disks and turbine blades, and a connecting shaft. Fig. 1 Jet engine cross section showing important subsystems and potential areas of wear Operating environments vary widely between different sections of the engine and depend on where the engine is in its mission. Temperatures may vary from subzero to above 1095 °C (2000 °F), rotational speeds may climb to more than 15,000 rev/min, and contact loads may range from a few psi to local hertzian stresses well beyond 1720 MPa (250,000 psi) in rolling-element bearings. The relative motions of components may be unidirectional sliding of rotating parts on stators, oscillatory sliding varying from a few thousandths of an inch up to several tenths of an inch, or vibratory motion resulting in impact between components. Components also may be subjected to ingested particle impacts. The wide variety of operating conditions results in a wide variety of materials used to meet the design needs of the engine. Aluminum and titanium alloys, plastics, and resin-matrix graphite composites are frequently used in the fan and the engine nacelle. The HPC uses titanium alloys, nickel-base superalloys, such as Inconel 718, and steels, such as M152, 17-4PH, and A286. The combustor requires heat-resistant nickel or cobalt alloys, such as Hastelloy X or Haynes 188, and stainless steels for fuel tubing. The turbine sections rely on cobalt and nickel superalloys, such as Inconel X750, MAR-M- 509, René 77, René 80, René 125, and advanced directionally solidified and single-crystal alloys. Often the design demands on materials for jet engine components will not permit substitution of materials for wear purposes, so a number of surface coatings and treatments are employed for wear protection. The different operating environments and types of materials in each section of the engine result in a variety of wear types, including fretting, impact, adhesive, high-speed and oscillatory sliding, oxidational, ingested particle erosion, and abrasive wear. High-speed sliding wear occurs in rotating gas path seals. Impact wear can occur in loose part assemblies or blade midspan or tip shroud interlocks. Fretting wear is frequently seen in blade dovetails. Erosion occurs when dirt and sand particles are ingested with the air through the fan and compressor. Bearings and gears can experience rolling contact fatigue. High-temperature components can experience oxidational wear. This article will discuss some of the most significant of these wear problems in relation to specific jet engine components. Gas Path Seals A major area of wear in jet engines involves gas path sealing. Such seals include blade tip seals, labyrinth seals, and leaf and spline seals. Blade tip and labyrinth seal problems are concerned with clearances between rotating parts and their adjacent stators. Engine efficiency is significantly affected by the amount of gas leakage over blade tips or through labyrinth seals. In an ideal engine, the blade tips or labyrinth seal teeth would maintain minimum clearance with the adjacent stator surfaces at all points in the engine cycle. In practice, the rotor parts and stator parts experience differential growth rates because of thermal gradients in the engine and much larger mechanical growth of the rotor than the stator because of centrifugal forces. The results of these differences are depicted in Fig. 2. When the rotor and stator diameters are plotted against time after the throttle is applied for takeoff, they are seen to experience a period of interference (pinch point in Fig. 2) that causes wear. The issue can be further complicated by the stator going out of round (Fig. 3). Fig. 2 Rotor and stator growth rates as a function of time and engine throttle movements Fig. 3 Clearance change caused by rotor/stator eccentricity or maneuver deflections Both design and material approaches are employed in combating wear of blade tips. One design approach, known as active clearance control, applies heating or cooling to the stator to achieve a better match of the thermomechanical responses of the stator and rotor. A second design approach for the out-of-round condition involves local arc grinding to remove casing material in the areas where minimum radii would occur. A materials approach that has been used in recent years is called passive clearance control. The casing is made of an alloy with a low coefficient of expansion, such as IN909, to achieve a more favorable overall thermal transient response match of the rotor and stator diameters. The most commonly used materials approach involves the application of an abradable material to the stator. The abradable material wears preferentially in a limited arc when the stator is out of round or when the rotor moves off center. This results in local clearance increases during rotor/stator interferences instead of wearing the rotor and causing a 360° clearance increase. An alternative materials approach is to apply an abrasive to the rotor, which machines the stator material, thus achieving the same result. The materials used for the abradable stator seals or abrasive rotor coatings vary by location in the engine. Abradables can take several forms, including bonded elastomers, braze-attached sintered porous metals or honeycomb cells, or thermal spray coatings. Some of the more commonly used abradables are listed in Table 1. These materials are designed to wear in preference to the opposing blade tip or seal tooth. They rely on low densities created by included porosity or friable structures with weak bonding between constituent materials. Bill and Wisander (Ref 1) have provided a model for friable abradable seal materials. In practice, however, wear usually occurs on both surfaces, necessitating periodic overhaul. Table 1 Commonly used abradable seal materials Type of seal Material Phenolic/carbon microballons Aluminum 80/20 nickel-graphite Porous Teflon Aluminum-silicon/polyester Fan and booster seals Ni-Cr-Al/bentonite Nickel-graphites (75/25, 80/20, and 85/15) Nickel-aluminum Aluminum Aluminum bronze/nickel-graphite Ni-Cr-Al/nickel-graphite Ni-Cr-Al/bentonite Hastelloy X open-faced honeycomb High-pressure compressor seals FiberMetal Co-Ni-Cr-Al-Y High-pressure turbine seals Bradelloy (Hastelloy X honeycomb + braze/nickel-aluminum The abrasive materials approach has been used with success on rotating parts, allowing them to machine their own clearances and minimizing rotor wear. The most commonly used abrasive is plasma spray aluminum oxide on seal teeth or rotor lands. Figure 4 shows a sector from an HPC rotor with two sets of seal teeth coated with plasma spray aluminum oxide. The most common mating stator seal material for such applications is open honeycomb (Fig. 5). Commonly used abrasive coatings for clearance control in jet engines include: • Plasma spray aluminum oxide • Entrapment-plated cubic boron nitride (Borazon) • Entrapment-plated aluminum oxide The abrasive coatings approach is usually combined with honeycomb or an abradable seal to improve the overall wear system for both surfaces. Fig. 4 High- pressure compressor disk with seal teeth coated with plasma spray aluminum oxide between stages. Arrow indicates location of coated seal teeth. Fig. 5 Open-faced honeycomb seal showing cutting by seal teeth Blade Midspan Stiffeners and Tip Shrouds Some fan, HPC, and LPT rotating airfoils (blades) require the use of either a midspan stiffener or a Z-notch tip shroud (often called interlocks) to prevent mechanical flutter of the aerodynamically loaded blades. These must be designed so that the blades are sufficiently loose to allow easy assembly, but lock up into a solid stiffening ring as aerodynamic loads are imposed on the blades, causing them to untwist along the blade stacking axis. These two requirements result in a combination of impact and sliding as the interlocking contact surfaces engage and rotate into position to form the solid stiffening ring. The impact loads imposed on the contact surfaces can be on the order of 7 to 70 MPa (1000 to 10,000 psi) and can cause severe wear damage to most materials suitable for use as blades. Therefore, it is common practice to apply a wear material to the interlock contact surfaces. These wear treatments are usually coatings on the order of 0.13 to 0.25 mm (0.005 to 0.010 in.) thick or welded hardfacing deposits up to 2.5 mm (0.100 in.) thick. Much care must be taken in the design and assembly of alignment tolerances for interlocks to prevent excessive wear, chipping, or spallation of even the most successful wear treatments on the interlock contact surfaces. The materials used for fan and HPC blades with interlocks are usually titanium alloys, which have poor wear properties. Most fan blade and HPC interlocks use thermal sprayed WC-Co coatings or brazed-on WC-Co powder metallurgy wear pads to prevent excessive wear. The most widely used coating is Union Carbide's LW1N40, applied using a detonation gun (D-gun). Recent advances in thermal spray coatings have allowed the use of high-energy plasma spray WC-Co coatings, which hold promise for direct substitution, or high-velocity oxyfuel (HVOF) sprayed WC-Co coatings on titanium alloy interlocks. The WC-Co coatings are successful in the titanium alloy interlock applications because of the high wear resistance of the tungsten carbide, adequate fracture toughness because of the cobalt matrix, high adherence on the titanium alloy substrates, and a good match in coefficient of thermal expansion with the titanium alloy substrate materials. The typical range of temperatures for fan and HPC interlocks may vary from subzero to 95 °C (200 °F) in the fan and from about 40 to 260 °C (100 to 500 °F) in the HPC. Fortunately, WC-Co coatings appear to retain sufficient low- temperature ductility and high-temperature oxidation resistance over these temperature ranges. The formation of a wear glaze at the contact zones contributes to the good wear resistance of the WC-Co in these interlock applications. The LPT blade materials are typically nickel-base superalloys, such as René 77 or René 125, which usually possess fairly good sliding wear resistance. However, they have inadequate wear resistance in the combined impact and sliding wear environment of LPT blade interlock contact surfaces. Typical use temperatures for LPT interlocks are 540 to 925 °C (1000 to 1700 °F), so the oxidation properties of the alloys under the existing wear conditions also play a significant role in their wear resistance. Typical wear coating compositions applied by thermal spraying or weld buildup that are used for LPT blade interlocks include: • Tribaloy 800 (plasma sprayed, welded, HVOF) • Cost Metal 64 (welded) • Chromium-carbide/nickel-chromium (plasma sprayed, HVOF, D-gun) Most of these alloys are cobalt based for good wear resistance and benefit from the formation of cobalt oxide and/or spinel wear glaze films. Tribaloy 800 and Coast Metal 64 are the most commonly used LPT blade interlock coatings at GE Aircraft Engines. Tribaloy 800, applied by thermal spraying or tungsten inert gas (TIG) welding, provides excellent wear resistance and oxidation resistance to about 840 °C (1550 °F). Above this temperature. TIG-welded Coast Metal 64 provides better wear and oxidation resistance than Tribaloy 800. In general, wear coating performance for LPT blade interlocks correlates to the chromium content and use temperature, with better performance at elevated temperatures for coatings with higher chromium contents and better performance at lower temperatures for coatings with lower chromium contents. Mainshaft Bearings The materials traditionally used for gas turbine mainshaft bearings are 52100 and M50 steels. More recently, powder metallurgy (P/M) bearings and case-carburized M50NiL steel, a modified M50, have been introduced as race materials. Several factors have contributed to this recent trend. Newer gas turbine mainshafts operate at higher speeds. This has pushed the bearing DN values (bore diameter in millimeters times shaft revolutions per minute) well past 2 million, which increases race hoop stresses and the hertzian contact stresses between rolling elements and the races. The higher hoop stresses can cause fracture of the 52100 or M50-type races because they are through-hardened materials, typically in the 50 to 60 HRC range, and thus have low fracture toughness. At high DN values, this can become a fracture reliability problem for a statistically significant number of these bearings. The higher hertzian stresses, approaching 2400 MPa (350,000 psi) for a 2.5 million DN mainshaft bearing outer race, also can cause significant reductions in rolling contact fatigue life. This is undesirable, because changing a mainshaft bearing requires costly disassembly of the engine. Bearing races made from the new P/M alloys and forged low-carbon alloys with carburized surfaces do not have these shortcomings. These materials are designed for high DN use and require special manufacturing processes. GE Aircraft Engines has concentrated on a variation of M50 steel with reduced carbon and increased nickel to improve fracture toughness. The race is then carburized to produce a fine dispersion of carbides for high hardness. Compressive residual stresses are frozen in to the raceway surfaces to improve rolling contact fatigue, while the low hardness (<50 HRC) rare core material remains tough to deal with high hoop stresses. This M50NiL material with its finely dispersed carbides, as well as the fine-grained (to improve fracture toughness) P/M race materials, can suffer from a low tolerance to wear. Wear can occur at the ball cage guide lands under marginal lubrication conditions even for "normal" bearing cleanliness operation. The rolling-element cage shoulders are silver plated, which provides solid lubrication and low friction to prevent wear when direct metal-to-metal cage skidding occurs on the cage guide land of the race. This works quite effectively for 52100 and M50 steels. However, the P/M and M50NiL steels sometimes experience rapid wear under similar operating parameters. Research by Budinski (Ref 2) has shown that the size, distribution, type, and volume fraction of carbides in tool steels can significantly alter their abrasive wear resistance, with coarser carbide grains having better resistance than finer carbide grains. Thus, it has been suggested that the coarser carbide stringers in M50 or 52100 forged bearing races can adequately resist the initiation of abrasive wear, while the very fine; evenly dispersed carbides in M50NiL or P/M bearing races cannot. The abrasive particles found in the bearings originate in the oil supply system. Sump castings, abrasively cut tubes, and grit-blast-cleaned parts in the bearing lubrication system all likely contain very fine alumina or silicon carbide contaminant particles. Most of these are removed during cleaning prior to assembly or by in-line filtering, but some of the finer particle (<50 m) get through to the bearings even under the most stringent clean-room assembly conditions. Once inside a bearing, contaminant particles can become embedded in the silver plating on the ball cage shoulders, where they protrude, causing abrasive wear to initiate during transient cage shoulder/race guide land contacts. The abrasive particles soon become "capped" with race transfer material, and adhesive wear ensues. The high differential sliding speeds between the orbiting cage and race cause frictional heating, local oxidation, and carburization by oil coking of the thin metallic transfer layers. Thus, a very hard abrasive transfer layer results and the wear process accelerates. These deposits also increase friction. Therefore, when cage-to-cage encounters occur, a more severe rebound force results from the skidding contact, generally increasing the cage orbiting and number of skid contacts and producing further wear. Combustor and Nozzle Assemblies These engine components are subjected to a variety of severe wear environments. Combustor hardware includes fuel nozzles, swirlers, and cowl damping wires, which experience relatively high temperatures (540 °C, or 1000 °F, and up) during operation. Exhaust nozzle assemblies are characterized by many parts, such as pins, bushings, links, and overlapping flaps, which aid in motion of the nozzle to control engine thrust. Some of these nozzle parts are directly in the hot gas stream and experience temperatures up to 815 °C (1500 °F); others are bathed in bypass cooling air and remain relatively cool (approximately 315 °C, or 600 °F). Combustor and nozzle assemblies experience large amounts of vibration from turbulent air flows both inside and outside the engine. This vibrational/impact wear can cause significant material removal as well as high-cycle fatigue of some components. The combined effects can cause liberation of hardware; in the case of the combustor, this will in turn cause severe damage to downstream components, such as turbine nozzles and blades. The majority of wear problems in both of these assemblies is cause by vibration and impact. Because of the elevated temperatures in the combustor, oxidational wear occurs as scales are formed and subsequent chipped off by impact. Contact pressures between parts are nominally low, but can be aggravated by high-frequency impacts, which may locally yield the materials. Oscillatory sliding (galling) wear sometimes occurs on exhaust nozzle flaps as they are actuated during mission cycles over several hundred accumulated flight hours. The design of these components addresses temperature and fatigue concerns. Both combustor and exhaust nozzle hardware are made from heat-resistant superalloy sheet materials, such as Hastelloy X, René 41, or Haynes 25. Because these materials vibrate in the turbulent hot gas stream, high-cycle fatigue life at elevated temperature is important. The cooler sections of the exhaust nozzle sometimes use high-temperature titanium alloys, such as Ti-6Al-2Sn-4Zr-2Mo, to reduce engine weight and maintain mechanical properties at elevated temperature. Many pins and bushings are manufactured from steel alloys, such as 17-4PH and A286. Coatings can be applied to problem areas on specific components, but they must withstand the application temperatures and not degrade the mechanical properties of the base alloy to unacceptable levels. Therefore, specification of the material and/or coating can be a complicated process. In general, cobalt-base alloys, such as Haynes 25 and Haynes 188, tend to perform best at temperatures above 540 °C (1000 °F) in both sliding and impact wear. Some nickel-base alloys, such as René 41, also possess good wear resistance at high temperatures. Effective coatings for wear problems in these temperature regimes include Tribaloy 800 and chromium carbide/nickel chromium. Tribaloy 800 derives its good elevated-temperature wear resistance from a hard Laves phase in a cobalt-base matrix. This matrix produces a cobalt oxide, which provides lubricity to the interface. Chromium carbide/nickel chromium derives its good performance chiefly from the hard carbide phase and the formation of favorable oxide wear glazes. Cooler titanium components in the exhaust nozzle generally have poor wear resistance and almost always require coatings for mating parts in relative motion. Here, the coating of choice is generally WC-Co, which again derives its wear resistance from the hard carbide phase. Oxidation of the carbide limits use of this coating to temperature regimes below 480 °C (900 °F). Because of the aggressive nature of the carbide, both mating surfaces should be coated. The steels used in the actuation systems for the nozzle flaps are usually ion nitrided to develop a hard case layer on the surface (hardness of up to 72 HRC can be achieved). For a particularly severe wear environment, ion nitriding may not provide sufficient wear protection, and chromium carbide or tungsten carbide coatings may be required. Dovetails [...]... particle separators to eliminate very large grit sizes, but finer grit sometimes enters the compressors This particle ingestion is severe enough to cause erosion of titanium-, nickel-, and steel-base materials to various degrees Erosion occurs by two distinct mechanisms: high-impingement angle and low-impingement angle High-impingementangle erosion is characteristic on the front stages of the high-pressure... underwing-mounted jet engines has brought with it the problem of the ingestion of materials left on airport runways Sand and dirt are used during winter months to improve traction on snow-covered runways This grit is easily ingested by the engine when the snow melts and evaporates Because the current "hub -and- spoke" airline system produces many takeoff/landing cycles in a short time, compressor blades and. .. phases in composites on the wear process require further investigation References 1 R.C Bill and D.W Wisander, Friction and Wear of Several Compressor Gas-Path Seal Materials, NASA TP-1128, National Aeronautic and Space Administration, 1978 2 K.G Budinski, Surface Engineering for Wear Resistance, Prentice-Hall, 1988 Wear of Pumps William D Marscher, Dresser Pump Division, Dresser Industries Introduction... industrial types, including single -and multistage centrifugal, single-stage axial flow, reciprocating, and positive-displacement progressive cavity pumps, such as screw pumps Rubbing Wear Centrifugal and Axial Flow Pumps Rubbing wear in centrifugal and axial flow pumps occurs in the following components, illustrated by the multistage pump in Fig 1: • • • • Impeller neck ring or wear ring annular seals, which... Ductile Metals, Wear, Vol 19, 1972, p 8 1-9 0 23 M.C Roco and G.R Addie, Analytical Model and Experimental Studies on Slurry Flow and Erosion Flow and Erosion in Pump Casings, Proceedings of the Slurry Transfer Association, March 1981 24 W O'Keefe, Pumps, Valves, and Piping, Power Mag., March 1992, p 1 9-3 0 25 A.P Smith, Unexplained Wear in Large Centrifugal Pumps, Chem Eng., Aug 1979, p 15 3-1 55 26 A Prang,... important Unfortunately, copper-nickel and copper-nickel-indium alloys and MoS2 can oxidize above about 315 °C (600 °F), while the organic binder breaks down and loses strength at such temperatures, resulting in higher friction coefficients, increased shear forces, and wear Improved dovetail coatings for the 315 to 540 °C (600 to 1000 °F) range need to be developed, as new high-temperature titanium alloys... evenly suspended (Ref 21) This requirement is particularly strong if the slurry particles are larger than roughly 65 m The areas of a centrifugal pump most prone to abrasive wear are the impellers and the casing interior, where flow-path velocities are high One approach to decreasing erosive wear of these areas and to maintaining the pump is the use of hard, wear- resistant, replaceable liners Elastomeric... Institution of Mechanical Engineers, 1986 16 S Collier, Know Your Triplex Mud Pump, Parts 1-6 , World Oil Mag., Jan-June 1982 17 J Miller, "Reciprocating Pumps for Slurry Service," ASLE Preprint 84-AM-6A-1, American Society of Lubrication Engineers, 1984 18 J Campbell et al., Bearings for Reciprocating Machinery: A Review of the Present State of Theoretical, Experimental, and Service Knowledge, Proceedings... contact stress and high-frequency motion admits a large amount of mechanical work into the metal surface Temperatures for dovetails range from subzero (fan) to close to 650 °C (1200 °F) (rear compressor) The combination of these factors creates wear problems for critical rotating hardware Fan and front-stage compressor blades and disks are manufactured from titanium alloys; middle- and rear-stag compressor... section of crosshead-type reciprocating compressor Source: Dresser-Rand The cylinder, which is the gas-handling part of the reciprocating compressor, contains the piston, which is the gas displacer Motion is transmitted to the piston by the connecting rod in the trunk-type compressor, and by the piston rod in the crosshead type The cylinder includes the valves that control the inlet and outlet gas flow . bronze/nickel-graphite Ni-Cr-Al/nickel-graphite Ni-Cr-Al/bentonite Hastelloy X open-faced honeycomb High-pressure compressor seals FiberMetal Co-Ni-Cr-Al-Y High-pressure turbine seals. nickel-graphite Porous Teflon Aluminum-silicon/polyester Fan and booster seals Ni-Cr-Al/bentonite Nickel-graphites (75/25, 80/20, and 85/15) Nickel-aluminum Aluminum Aluminum bronze/nickel-graphite. G.V.N. Rayudu, and R.G. Narayanamurthi, Friction and Wear of Sintered Iron, Wear, Vol 42, 1977, p 24 5-2 50 • B. Granoff, H.O. Pierson, and D.M. Schuster, Carbon-Felt, Carbon- Matrix Composites: