Wear in cylinder liners

269 zyxwvutsrq Wear, 91 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA (1983) 269 - 279 WEAR IN CYLINDER LINERS T. S. SUDARSHAN and S. B. BHADURI Department of Materials Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 (U.S.A.) (Received M arch 15, 1983) Over the years, investigators have extensively studied various aspects of wear in cylinder liners but a survey of the literature has revealed little information on the subject of high wear rates found in the upper portion of the liners. In order to provide a better understanding of the subject, an attempt is made in this paper to assess the primary factors responsible for the high wear rates. It is observed that, although several factors can operate at the same time in influencing the wear process, in general wear is controlled by adhesion with contributions later on from corrosive and abrasive processes. The metallurgy of the liner also plays an important role in determining wear rates. 1. Introduction Wear in a cylinder reaches a maximum at, or just below, the top of ring travel. Although researchers have addressed the general causes of wear in a cylinder, there is little information on attempts to understand the exact mechanism for the excessive wear observed in the topmost portion of the cylinder. The factors which determine the wear characteristics are many and it is possible that several factors work simultaneously. Ting and Mayer [l] have used theoretical calculations to predict the profiles of wear in various sections of the cylinder and have shown these to be in excellent correlation with in-service profiles. Typical profiles are shown in Fig. 1 [ 11. The‘ mechanisms which largely influence the wear in the cylinder can be broadly classified into two categories: (1) physical mechanisms that include adhesion, scuffing and abrasion and (2) chemical corrosion. Both of these mechanisms can operate independently or together, depending on the operating variables and the time the cylinder has been in operation. 0043-1643/83/$3.00 0 Elsevier Sequoia/Printed in The Netherlands in Dtstance from Top of Cylmder Wall, Dtstance from Top of Cylmder Wall, in Dwance from Top of Cylinder 00010 c ‘2 0.0005 @I ’ o Cl Dlstonce Wall, in from Top of Cyhnder Wall,in zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQ cylinder bore wear patterns Fig. 1. Comparison of measured (0, 0) and predicted ( -) on the major thrust side (from ref. 1): (a) after 5025 miles; (b) after 22485 miles; (c) after 26 053 miles;(d) after 47 306 miles. 2. Wear mechanisms zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFED 2.1. Phy sicd wear mechanisms Wear due to friction, in a cylinder, is almost fully confined to the piston ring track and is most pronounced during the process of “running in”. During this period, the piston rings have to form an effective seal against the passage of blow-by gases and also to smooth the surface between the cylinders and the rings so that they can slide freely against one another, without the asperities welding together, when the engine is under load. When surfaces slide against each other, contacts occur between asperities, resulting in the formation of junctions through local plastic deformation. These junctions will eventually rupture at their weakest points and may cause metal transfer between the two surfaces. The wear rate is influenced by the relative hardness of the two surfaces, the extent to which they are chemically clean (bonding and welding are possible) and their comF~ib~ity in forming a wear couple. In cylinder liners, adhesive wear occurs only in the very early stages of the operating life of the cylinder and is influenced by factors such as piston ring clearances, lubricating conditions and the nature of the materials in contact and their surface finish. 271 Another infrequent wear process in cylinder liners is scuffing, which takes place after a short run-in period. Scuffing occurs when two metallic surfaces are in sliding contact and when lubrication is inadequate. The inner surface of the liners can plastically deform to such an extent that work hardening can occur, resulting in the transformation to a hard material [2]. If the scuffed area is small it will heal with operating time but if the scuffed region is large the hard surface will suddenly spall off, giving rise to small hard particles and an increase in wear rate. A typical relationship between the applied load and wear rate is depicted in Fig. 2 [ 21. Abrasive wear occurs when a soft surface is ploughed by a harder one, causing a type of wear which is linked to the presence of asperities and hence to surface roughness. Particles can be generated through external or internal means. Externally, they can arise from [3] core sand, metal swarf (which may be left behind from the manufacturing process), dust (from the air intake required for combustion), fuel, lubricating oil and leakages from the cooling system, while internally they can be generated from corrosive and frictional products, besides abrasive ash formed during combustion of some WEAR RATE I White layers form on the surface in this area LOAD Fig. 2. Typical effect of applied load on wear rate (from ref. 2). heavy fuels. In cylinder liners, abrasive wear is often characterized by a series of regular grooves running vertically down the cylinder wall. The surface finish of the liner appears to be smoother than it was when the liner was first fitted into the cylinder. This surface is often characterized as a well-run-in surface. 2.2. Chemical wear mechanisms The phenomenon of corrosive wear in a cylinder occurs as a result of low or high operating temperatures. Chemical attack arises from the reactive products (air, humidity, oxygen) which are contained in the atmosphere or those that are formed with the lubricant by oxidation (formation of organic 272 acids). Most of the reactive products are obtained during combustion, giving rise to corrosive derivatives. The mechanism of corrosive wear [4] in the cylinder occurs in two stages. (1) Rapid formation of the protective coherent film on the surface takes place first. These films can be metallic oxide, organic salt, sulfides or chlorides. (2) Removal of the protective film by wear and re-exposure of the surface then takes place so that corrosive attack can continue. The film thickness determines the wear characteristics in most cases. Usually films of corrosion products are harder than the surfaces they come from until they reach a critical thickness. At this point, they are removed by spalling and the process of film formation is repeated (Fig. 3 [4]). However, sometimes the films formed by corrosion are of low shear strength (metallic chlorides, sulfides, phosphates). These serve as good boundary lubricants and hence we have a low rate of wear. In this case, the thicker the corrosive film, the less is the possibility of metal-to-metal contact and subsequent adhesive wear. By contrast, this corrosive film can rupture easily, thus promoting corrosive wear. A compromise therefore exists between corrosive and adhesive wear. The above two wear rates can be plotted in terms of surface reactivity as shown in Fig. 4 [ 41. This figure clearly shows that, for a particular surface reactivity, an optimum balance can be obtained between the two wear rates. / (a) TIME ) (b) Fig. 3. Dependence of corrosive wear on the formation and spalling of protective surface films (from ref. 4): (a) formation of a protective reaction product; (b) no protective film. Corrosive wear can also be accentuated by allowing excess quantities of condensed water to enter the cylinder, when the scavenge air temperatures are low (humid climates). Water in the cylinder can wash down the oil films from the walls or contribute to corrosion-rusting by combining with the sulfuric acid derived from the fuel. Low concentrations of water give little cause for concern, but higher concentrations of water result in a slowing down of the combustion rate. As a result of this fuel droplets which are still burning strike the cylinder surface, thereby raising the wall temperature and affecting lubrication [Ei]. This leads to inefficient combustion which promotes hard deposit formation, thereby promoting the wear process. 213 zyxwvutsrq The major amount of corrosive wear in engines is caused by sulfur from the fuels and the additives from the lubricants. A typical relationship between the fuel sulfur content and wear rate is shown in Fig. 5 [4]. Adhesive Wear -. AdditivrlSurfacc Fig. 4. Optimization ref. 4). Reactivity - ” 0 1 2 3 FUEL SULFUR PER CENT WEIGHT of wear rate between adhesive and corrosive wear mechanisms (from Fig. 5. Influence of sulfur contents on the wear rate (from ref. 4). 3. The wear process in cylinders When an engine is operated for the first time at low loads, after assembly, the friction in the cylinders has a maximum value owing to the process of running in. During this period, the piston ring serves as an effective seal for the passage of blow-by gases and delivers maximum power efficiency. Since the gas pressure is acting on the ring, it adds to the ring bearing pressure and pushes it into closer contact with the cylinder wall, thereby squeezing the oil film and causing conditions of boundary lubrication. A typical profile of the variation in oil film thickness between the top and bottom centers is shown in Fig. 6 [6]. The area of contact between the two surfaces is therefore limited to the contacting peaks of surface roughness. When sliding occurs, frictional work is dispersed as heat at these contacts. Since only a small volume of peaks are in contact, the temperatures at these peaks are high and limited only by the melting point of the materials. Consequently, welding occurs at these peaks and these welds are sheared during sliding. Since cylinder liners are generally honed, lubricating oil settles in the valleys and crevices between the asperities and is held there by attractive forces. Therefore, until smoothing of the surface is complete, the oil in the crevices can maintain conditions of boundary lubrication and.minimize friction [6]. 214 Lubrmmt film profile 0 1 2 3 L 5 6 7 l/, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLK ABRASIVE PARlICLESlwe~ght 1 BDC zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Fig. 6. Typical profile of variation in dil film thickness between top dead center and bottom dead center (BDC) (from ref. 6). @‘DC) Fig. 7. Influence of abrasive particles in lubricating oil on cylinder wear (from ref. 4). zyxwvutsrqponm As the load on the engine increases, the peaks of the surfaces in contact are gradually worn down and the valleys of the surfaces are filled up with wear debris until the surface finish is smooth [ 7,8]. The surface temperature of the cylinder at the top is around 250 “ C and the gas temperature in the combustion chamber is around 1700 “ C [9]. With increasing load on the engine, a small amount of gas leakage begins to occur. This is because of the greater heat expansion of the liner at the top than at the bottom. Gas leakage past the piston rings is normal up to a certain extent, but under certain conditions the leakage becomes excessive. The leakage determines the vertical and radial pressures of the rings on the cylinder wall and grooves, and these in turn influence the amount of wear. With every stroke, the high temperatures on the cylinder wall and gas leakage together cause a breakdown of the oil film between the cylinder wall and the piston ring and hence mew-to-meal contact occurs. This is accentua~d by the moment stopping of the piston as it reverses direction before starting on the return stroke. Over a period of time, the contact between metal and metal leads to the wear of the piston rings and the edges of the rings are gradually rounded. The gas leakage in such cases has been found to increase by about 30% [9]. Theoretical calculations, used to predict the friction and film thickness when the piston ring was rounded, showed that the film thickness increased proportionally to the third root of the radius of rounding, while friction varied inversely with the radius. However, some experiments have shown that piston rings with rounded edges offered a frictional resistance about 35% greater than rings with sharp edges when the cylinder was operated with pressure [9]. Apart from loss of power efficiency and greater frictional resistance, excessive gas leakage may disturb the lubricating film and lead to gas erosion. Gas erosion occurs when the materials in the boundary layer of the wearing surfaces are heated above their melting points and also swept by 275 leakage gases at high velocity. Surface melting cannot be produced by the heat in the gases alone but it may be produced in combination with the frictional heat generated at the points of contact between the rings and the cylinder wall. The molten metal at the contact spots can then be tom loose and carried away by the gases. In cases where the fuel is of a high sulfur content, the sulfur forms sulfur dioxide during combustion. This is converted to sulfur trioxide in the presence of water vapour from the combustion process and the excess intake of air. Sulfur trioxide can then combine with moisture to form a dilute acid. The surfaces of the cylinder are then subjected to a rapid corrosive attack and the effect of the attack decreases with the formation of a coherent film on the surface. When the film is thin the wear is at a minimum but when it reaches a critical thickness it is removed by spalling and the surfaces are reexposed so that corrosive attack can continue. Sulfur trioxide formation is possible only in areas where the combustion gases have been sufficiently cooled and where the reaction is catalysed by the presence of metallic oxide deposits on the chamber walls. Since the metallic oxide deposits are covered with an oil film, sulfur trioxide formation is minimized. During normal combustion these products are vaporized and discharged through the exhaust system. In order to prevent sulfur trioxide and acid formation, additives are added to lubricants so as to form a protective layer between the cylinder and the sulfur dioxide. Typical additives used are barium, magnesium and calcium phenates and/or sulfonates. These react with the sulfur dioxide to form salts which have high fusion temperatures and do not adhere to the metal surfaces, thereby generating particles which lead to abrasion. Abrasive particles can also be present from the air drawn for combustion and the generation of ash formed during the combustion of certain fuels. Unless filtration is effective or the lubricating oil changed frequently, abrasive wear cannot be minimized. A typical graph showing the effect of abrasive particles on cast iron cylinder wear with cast iron rings is shown in Fig. 7 [ 41. 3.1. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Factors which influence metallurgy of cy linder liners Cylinder liners have to withstand high mechanical stresses, heat and corrosion during service and it is these operating conditions which play a major role in determining the metallurgy of the liners. In order to achieve minimum wear rates the parameters which need to be controlled are (1) surface topography, (2) hardness of the materials in contact and their microstructures and (3) lubrication. The surface topography of the liner plays a significant role in influencing the wear process. It is advantageous to have the inner liner surface composed of peaks (pla~aux) separated by a cross-hatched pattern of grooves so as to prevent continuous smearing of the surface and to provide small quantities of the lubricant during operation (marginal lubrication). The surface should be free from manufacturing debris, as this can fill up the valleys or can be embedded between the ring and the peaks, giving rise to 276 regions of high contact loading. The optimum surface can therefore be produced by exercising careful control of the honing operation. Plastic replica studies of the surface indicate that wide or deep cross-hatch grooves, folded and torn metal, low cross-hatch angle and lack of plateau area are all factors which are responsible for the increased wear rates observed in liners [lo]. Hesling [ lo] suggests that all the above factors can be remedied by the selection of proper honing speeds, optimum pressure control for removal of material and the use of coolants which have suitable viscosities. Recent developments include the use of silicon carbide powder as a slurry which results in the embedding of these particles on the surface after honing, thereby giving rise to local hard points which help in improving wear resistance [lo]. The hardness of the materials in contact also plays a significant role in determining the wear rates of the liner. The difference in hardness between the liner and the ring should be minimized and investigations [4] have shown that any deviation from this minimum difference in hardness results in a significant increase in wear rate. A typical profile of this phenomenon is shown in Fig. 8 [ 81. /ooo 900 i?.900 .F ii ‘0° 600 5op5 I I 1-x I I I -so -25 I I I I I I I zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCB I I I zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA I I :0 +.50 +2s l75 Difference in hardness of the moving and the zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONML stotionory port The movmgport horder + thon the stotlbnorypwt Fig. 8. Dependence (from ref. 8). L zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONM * The movingportsofie~ thou stotionoly port of wear rate on the difference in hardness of the parts in contact The hardness of the material represents only an average index of the microstructural constituents of the material. Microstructure can be modified either by adding certain alloying elements which are responsible for the precipitation of hard phases in the matrix or by the use of suitable processing techniques. Alloying elements which are often used include phosphorus, titanium, chromium and molybdenum. These elements produce phases such as_ phosphides, carbides or nitrides which are distributed throughout the matrix. The individual hardness of these phases results in their protruding from the matrix on a fully run-in surface, thereby minimizing wear rates. Researchers at Fiat [ll] have also shown that the method of casting influences the wear rates of liners and extensive trials of different processing techniques are necessary before an optimum process can be chosen. The pro- 277 zyxwvutsrq cessing methods are also responsible for the phases produced and the graphite morphology in a liner. Lubrication in a liner is enhanced by the presence of soft dispersed phases such as graphite which are readily available on the wear face, thereby reducing metal-to-metal contact and preventing adhesive wear during the very early stages of the running-in process [ 121. The morphology of graphite (flake, nodular or compacted) determines its effectiveness as a lubricant and can affect the wear process in two ways. (1) Graphite can produce a substantial interruption of the metallic structure and can, for mechanical reasons, crumble and increase wear. (2) Graphite can also contribute to wear resistance because under pressure it can adhere to the base metal to form a graphite film. The adhesion of a lubricant to this graphite film is much greater than to bare metal surfaces and this increases the lubricating action of the oil. Hoegh [9] and Eyre [13] have both shown that optimum wear rates are observed in grey cast iron when the ASTM flake graphite size is 4 - 6 and when it is randomly distributed throughout the matrix. A typical profile of the wear rate for flake graphite iron is shown in Fig. 9 [4]. Undercooled graphitic structures or zyxwvutsrqpo 6 f i 6 5 L z 2 3 % 2 z I ; z 0 0 LINER 0.6 0.6 1.0 02 0.4 ROUGHNESS Ra I JJmI 1.2 Fig. 9. Influence of liner roughness on abrasive wear (from ref. 4): -, against cast iron ring; - - -, cast iron liner against chromium-plated ring. cast iron liner nodular graphite show poor wear characteristics because of the large spacings between the graphite and hence are not preferred. Nodular irons also form hard white products on the surface during running in and this can give rise to abrasive particles and increased wear rates. 4. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Discussionand conclusions From the review of literature and an examination of the sequence of events involved in the wear phenomena of cylinders, it is clear that the excessive wear observed in the top portion of the cylinders is primarily influenced by adhesion and is accentuated with time by corrosive and abrasive processes. When metal-to-metal contact occurs during the early stages of running in 278 the surfaces are clean. The high temperatures generated by friction accompanied by the pressure exerted by the piston rings (from the blow-by gases) are sufficient to cause plastic flow and microwelding of the contacting asperities. Microwelding can be accompanied by diffusion of the material near the interface when the temperatures are high or formation of junctions because of adhesion when the temperatures are low. The sliding motion of the rings can break these microwelds. However, since surfaces are initially clean, the microwelds can be stronger than the asperities in contact (generally cast iron cylinders-cast iron rings are used in engines) and in this situation the breaks occur in the asperities. As an oxide film begins to form on the surfaces of the asperities in contact, during the operation of the engine, the interface welds become weaker than the asperities and hence shearing occurs at the interface. When conditions of boundary lubrication exist between the cylinder wall and the ring at the top portion of the cylinder, increases in temperature at the asperities cause the molecules of the lubricant to be desorbed. Under such conditions metal-to-metal contact cannot be prevented and the coefficient of friction and wear increase rapidly. A typical representation of this phenomenon is shown in Fig. 9 [4]. Scanning electron microscopy studies on a grey iron cylinder liner [ 141 which showed very low wear indicated that the wear phenomenon was largely influenced by friction in combination with corrosion. As load, or operation time, of the engine is increased, wear due to friction is replaced by corrosion and is characterized by the presence of phosphide and/or carbide phases standing out of the matrix. The total volume of these phases should be restricted to a maximum of 15%; the presence of a higher volume results in cracking during cooling and the formation of shrinkage pores during the manufacturing process [ 131. It has also been observed that phosphide particles were preferentially attacked, while carbides stand out proud of the surface [14]. It is therefore advantageous to control the amount of phosphorus in the liner and low wear rates have been observed with phosphorus contents below 0.2576, beyond which the disadvantage of increased brittleness results [5]. Silicon contents in excess of 1% have also been found to increase the tendency to corrosive wear but no clear reason is apparent for this behavior. The continuous efforts to decrease wear rates have also led to various developments in the metallurgical characteristics of liners. The presence of the carbide phase and its resistance to wear have promoted the idea of impregnating the liner surface with silicon carbide during the honing operation so that the particles, instead of being removed by the motion of the rings, are instead embedded into the material, thereby decreasing the wear rates. This has, however, caused some concern about the wear rates of piston rings and plasma-coated or molybdenum-sprayed rings have been used to overcome this problem [ 121. Hardened liner surfaces are also being produced by nitriding (Sursulf, Tufftride or Melonizing) and by chromium plating, and these all improve the abrasive wear rates of liners. Although some of them do not confer a long-term benefit, substantial improvements have been observed 279 during the early stages of running in, particularly where the graphite is either limited in quantity or inadequate in form. It is seen from this review that the mechanism which governs wear in the top portion of the cylinder liner is primarily adhesion due to friction. Under more or less uniform operating conditions wear at the top portion of the cylinder occurs by a three-step process which involves adhesion, corrosion and abrasion. The metallurgy of the liner and the compatibility of the materials in contact need to be carefully evaluated in order to achieve minimum wear rates. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGF References zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 1 L. L. Ting and J. E. Mayer, J. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJ L&r. Tec~no~, 96 (April 1974) 258 - 266. 2 M. J. Neale and T. S. Eyre, Paper C7/82, 1982, pp. 55 - 64 (Institution of Mechanical Engineers, London). 3 Automob. Eng., (September 1953) 373 - 378. 4 A. Schilling, Automobile Engine Lubrication, Scientific Publications, 1972. 5 D. W. Golothan, 2’mns. Inst. M ar. Eng., 90 (1978) 137 - 163. 6 A. D. Sarkar, W ear of Metals, Pergamon, Oxford, 1976. 7 A. V. Sreenath, Tribal. Zni., (April 1976) 55 - 62. 8 A. V. Sreenath and N. Raman, W ear, 38 (1976) 271- 289. 9 C. Hoegh, Cy linder W ear in Diesel Engines, Chemical Publishing Co., 1949, 10 D. M. Hesling, Lubr. Eng., (October 1963) 414 - 422. 11 L. Bruni and P. Iguera, Automobile Engineering Symp., 1978. Paper 20. 12 R. A. Day, Znd. Lubr. Tribal., (April 1982) 44 - 49. 13 T. S. Eyre, M icrostruct. Sk., 7 (1979) 275 - 286. 14 T. S. Eyre and J. Nadel, Tribal. Znt., (October 1978) 267 - 271.