Chapter 6 Wear. Abrasive Wear (plus Erosive Wear). Encountered in % of cases. Surface Fatigue Wear. Encountered in % of cases.

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1 Chapter 6 Wear 1 Introduction Wear is the gradual removal of material obtained at contacting surfaces in relative motion. While friction results in important energy losses, wear is associated with increased maintenance costs and costly machine downtime. Wear phenomena are intimately linked to frictional processes. Recall that friction forces are generally the result of two main physical processes: shearing and ploughing. If solid surfaces in relative motion are not separated in some way, wear can be expected. Lubricants are used to separate contacting surfaces in relative motion and thus to reduce wear. Lubricants may completely separate the surfaces, as in fluid film lubrication or allow solid-solid contact only at a restricted number of locations (mitigated solid contact) as in boundary lubrication. The tribology of lubricants is discussed in another chapter. ere the focus is on wear resulting from direct soli-solid contact. Wear phenomena are heavily influenced by the fact that most engineering surfaces are rough (and hence surfaces come in contact at single asperities and the real area of contact is usually much smaller that the nominal contact area). Furthermore, wear behavior is also influenced by the presence of adsorbed species and/or surface layers. Many different wear mechanisms have been identified. A first classification of mechanisms is based on their relative importance in engineering practice. According to this, the following types of wear are often encountered: Adhesive Wear (plus Fretting Wear). Encountered in % of cases. Abrasive Wear (plus Erosive Wear). Encountered in % of cases Surface Fatigue Wear. Encountered in % of cases. Corrosive Wear. Encountered in 4-5 % of cases. Wear phenomena can also be classified based on the underlying physics. According to this, mechanistic-based classification, the following types of wear have been identified: Adhesion and Transfer. Wear takes place by gradual removal of adhered fragments of material picked up by the contacting surfaces during frictional interaction. 1

2 Corrosion Film Wear. Wear is associated with the removal of fragments of protective corrosion/passivating layers from the surface of the worn material. Cutting. Wear is the result of intermittent or continuous chip formation in the soft material due to cutting action by a harder tool. Plastic Deformation. Wear being associated with the removal of sheared layers resulting from excessive plastic deformation. Surface Jetting. Wear resulting from interfacial instabilities associated with localized softening at the contact interface. Surface Fracture. Wear resulting from breakage of atomic bonds in embrittled surface layers. Surface Fatigue. Wear resulting from subsurface cracking and fracture induced by cyclic loading. Surface Reactions. Wear associated with the removal of reaction products from the surface which in turn were produced by frictional flash heating. Melting. Wear associated with the melting transition. Electrochemical. Gradual wear associated with anodic reactions on the worn surface. Details about several of the above mentioned wear mechanisms are discussed in the sequel. owever, first we describe general aspects of wear processes, including the measurement of wear as well as some details about adhesive wear. 2 Measurement of Wear Wear rate measurements are routinely performed using standard or customized friction and wear testing equipment. The same configurations used for friction measurements are also used in wear research. While the occurrence of an initial running-in or breaking-in stage is often detected when measuring wear, experiments tend to focus on the also commonly encountered subsequent stage of the process: steady state wear. The amount of material worn in a tribological system is measured in relation to the duration and extent of contact. Specifically, a common measure of wear is the volume of material removed per unit sliding distance. Consider a tribological system where a volume V w of the softer component of the couple is removed by wear in the same amount of time it takes for the sliding distance to become L. The volume of worn material per unit sliding distance is then w = V w /L. The wear rate thus expressed has units of area and it is often useful to compare the observed values of wear rate against the other most important area 2

3 measure encountered in friction studies, namely, the real area of contact of the friction couple A r which, for plastically deforming asperities, is simply given by A r = F n ere F n is the applied load and is the hardness of the softer component in the friction couple. The ratio of the wear in units of volume removed per unit sliding distance to the real interfacial area of contact is a meaningful dimensionless quantity useful in wear studies and is called the dimensionless wear coefficient, or simply the wear coefficient, K. From the above, this is defined as K = V w/l F n / = w A r A couple of useful physical interpretations can be given to the coefficient K. First, since, K is the ratio of two areas, the worn area A w and the real contact area A r. One can thus interpret K as the fraction of the real contact area A r removed by wear. Consider a tribological system consisting of a hard flat surface and a rough surface of a softer material with of identical asperities. Assume the surfaces make contact at N aspertities. ence, the wear coefficient represents the number n of those asperities were conditions are such the conditions that material is torn forming wear debris, i.e. K = A w A r = n N This interpretation, together with the commonly found low values of K found in practice indicates that while all asperity contacts contribute to friction only a very small fraction of contacts result in wear. Therefore, while friction is dominated by the contact events that occur without damage, wear is dominated by the (small) chance that a particular contact event results in rupture. Note also that the quantity (F n /)L has dimensions of volume. This represents the total volume of the plastically deformed zone underneath the surface of a worn area, V p following sliding by a distance L. ence K = V w V p Therefore, K also represents the proportion of the plastically deformed volume that is removed by the wear process. 3 Adhesive Wear An adhesive wear model assumes that wear is the result of adhesion between asperities followed by fracture. A first simple model of adhesive wear states that the volume of worn 3

4 material removed V w as a result of a tribological interaction is directly proportional to the load F n and also to the total sliding distance L. owever, since less wear is observed when the hardness of the softer member of the tribological couple increases, V w can also be regarded as inversely proportional to the hardness of the material being worn away. In symbols, the above is represented as V w = K F nl or expressed in terms of the worn volume per unit sliding distance w w = V w L = K F n where K is the previously introduced wear coefficient. This expression is sometimes called Archard s law. Although an equation of this form was first proposed by olm from his studies on electric contacts, Archard first obtained it using a simplified model of the contact interaction described below. Note that if the sliding distance is the result of sliding at constant velocity U, it is then given by L = Ut where t is the sliding time. If Archard s law is divided by the nominal (apparent) contact area A a, substitutes the sliding distance in terms of sliding velocity and time and solves for the time one gets t = d K 1 p m U where d = V w /A a is the worn depth and p m = F n /A a is the mean or nominal pressure. This is an indication of the life of a wearing component in terms of the admissible worn depth and the material and process parameter, K, p m and U. It is often the case, that measured worn volumes vary in direct proportion with the total sliding distance. In contrast, while worn volumes often vary in proportion with the applied load over certain load ranges, abrupt changes in wear rates (wear transitions) are observed at specific critical loads. Such changes are the result on the complex interplay between the softening and chemically reacting behaviors of the material induced by high flash temperatures. Abrupt increases in wear rates are commonly found at high loads and these are often associated with welding and seizure. owever, in some cases these high wear rates may revert to low values at even higher loads. Measured values of K are frequently small and range from 10 8 for incompatible metals rubbing against each other with good lubrication to 10 3 for clean unlubricated surfaces of like metals. The smallness of these values together with the original interpretation of K given above suggest a probabilistic interpretation of the wear coefficient, namely, it represents the fraction of the actual contact surface which is actually removed by the wear process. It could also represent the probability that any given individual friction contact event culminates with the breakage and removal of a wear particle. 4

5 3.1 Some Experimental data on Adhesive Wear Following is a short compilation of specific wear data obtained using a pin and disk system with a load of 0.4 kgf and a speed of 1.8 m/s (adapted from Archard s article in the Wear Control andbook). Wearing Surface Counter Surface w (mm 3 /m) (kg/mm 2 ) µ K Mild Steel Mild Steel Brass Tool Steel PTFE Tool Steel Stellite Tool Steel Ferritic SS Tool Steel Polyethylene Tool Steel WC WC Adhesive wear coefficient correlate well with friction coefficients, however, unlike friction coefficient values, wear rates vary over a very large range. Other experimental techniques useful in wear research include: Microscopic examination of wear debris. Metallographic examination of worn surfaces. Radioactive isotope methods. Besides the sharp differences in wear rates that exist between mild and severe wear, the wear debris produced in both cases also exhibits significant differences. Specifically, wear debris resulting from mild wear frequently consists of fine particles ( micrometer), many of them oxides. In contrast, the debris resulting from severe wear consists of much larger ( micrometer) plate-like particles consisting of an assembly of smaller particles stuck together. To gain insight into adhesive wear mechanisms it is useful to examine in some detail the results of selected wear experiments carried out in well characterized systems. The advantage of this approach is that the resulting information has been reproduced and the underlying processes are generally well understood. The systems examined below include: Wear of plain carbon steel rubbing against itself in air. Wear of leaded brass (59Cu-39Zn-2Pb) rubbing against stellite in air. Wear of brass and ferritic stainless steel against tool steel in air. 5

6 Details about the observed wear mechanisms in these two systems are now described. In the first case, wear observations are performed on a plain carbon steel pin rubbing against a disk of the same material in air as a function of load under dynamic but otherwise steady-state conditions. When the applied normal load is relatively small, fine metal particles are removed from the rubbing surfaces and at least some of them are rapidly oxidized while dispersed in the gap between the contacting surfaces. The oxide particles then act as abrasive medium producing further wear. owever, the observed wear rates are relatively small, and this is called mild wear. As the load increases, larger metal particles are torn from the rubbing surfaces. The oxidation rate induced by flash temperature is not large enough to fully oxidize the wear particles and many particles are ejected from the gap and discarded as wear debris. Because the wear rates involved are larger, this is called severe wear. For the highest loads, the resulting high flash temperatures lead to rapid formation of oxides and a hard, tempered layer on the contact surfaces. Fine metal particles are formed, oxidized and removed from the gap between the rubbing surfaces as wear debris. Since the contacting surfaces harden rapidly, the resulting wear rate is relatively small. This is known as the high temperature regime of mild wear. When one uses instead a leaded brass pin and rubs it against a disk made of stellite in air the following scenario results. At the smallest loads (< 0.1N) and low temperatures. the contacting surfaces meet at fairly isolated asperity contact points. Small, discrete particles of brass are transferred to asperities on the stellite counterface and some oxidize rapidly. On further rubbing, the accumulated particles on counterface are smeared over and diffuse into the stellite surface and form a continuous thin layer of brass containing oxidized particles mixed with counterface material. Later on, material from the smeared layer at the counterface transfers back to the pin forming a microcomposite hard layer on the pin surface while small composite particles break away from the pin and the disk. forming loose wear debris. As a result of the above, the wear rate is relatively low (mild wear). As the applied load - and the resulting temperature - increase, the following distinct sequence of events is obtained. The real area of contact increases and electrical contact resistance at the interface reaches a minimum. Particles of brass are transferred to asperities at the counterface and at least some oxidize rapidly. The accumulation of discrete particles on the counterface results in aggregation into larger clusters on the surface of the disk. These clusters subsequently break away from the disk forming loose wear debris and the resulting wear rate is high (severe wear). At the highest loads, the contacting surfaces meet at over a much larger contact area. The resulting high flash temperatures lead to rapid surface oxidation. Particles of brass are transferred to asperities at the counterface and oxidize rapidly. The oxide particles are smeared over the contacting surfaces and effectively form an intervening layer between the rubbing surfaces, while small particles continuously break away from the pin and the disk. forming loose wear debris. The observed wear rate in this case is relatively low (high 6

7 temperature mild wear). When the same pin and disk experiment is carried out using a stainless steel pin rubbing against a tool steel disk one finds that Stainless steel exhibits mild wear for loads below 30 N and there is a transition to severe wear mode for larger loads. When the pin and disk experiments are performed in a high temperature environment, a somewhat different behavior is observed. Specifically, for the case of a brass pin at low sliding speeds one finds that the imposed high temperatures produce some softening of the material while the low flash temperatures resulting from the low sliding speed lead to minor softening and oxidation. Small discrete particles of brass are transferred to asperities at the counterface and oxidize. The brass particles are smeared over the counterface forming a continuous thin layer of microcomposite material consisting of a brass matrix containing oxidized particles and tool steel. The material from the layer continuously transfers back to the pin forming a corresponding microcomposite hard layer on the pin surface. while small composite particles break away from the pin and the disk. forming loose wear debris. The resulting wear rate is low (mild wear). For the same system, as the sliding speed increases the increased flash temperatures lead to further material softening and oxidation, while the prevailing high temperatures produce additional softening of the material. Small discrete particles of brass are transferred to asperities at the counterface and accumulate there resulting in larger aggregates on the surface of the disk. Such aggregates break away periodically from the disk forming loose wear debris and the resulting wear rate is high (severe wear). If beside increasing the sliding speed, one also increases the temperature of the pin and disk system, one observes additional softening and significant oxidation (due to both the high flash temperatures as well as the experiment temperature). Small particles of brass are transferred to asperities at the counterface, oxidize rapidly while they accumulate and smear over the counterface forming a continuous thin and microcomposite layer containing large amounts of oxidized particles and counterface material that make it relatively hard. From time to time, small composite particles break away from the pin and the disk surfaces forming loose wear debris. As a result, the wear rate is relatively low (mild wear). Many studies similar to those described above have been performed and a couple of generic observations are the following. Wear occurs in both materials of a tribological system regardless of the differences in hardness of the materials involved. owever, in most systems, the softer material wears out at a faster rate, as expected. Transitions from mild wear to severe wear are the result of the competing processes of surface oxidation and surface softening. At high oxidation rates, intervening oxide layers form readily reducing both friction and wear. When the softening rate predominates large material removal rates result. 3.2 Adhesive Wear Modeling In the adhesion theory of friction, asperities of the contacting surfaces come in contact against each other briefly forming an adhesive junction. Assuming, correctly, that plastic behavior 7

8 prevails at the asperities, the friction coefficient becomes µ = F t F n = τ where τ is the shear strength of the junction and the hardness of the softer material. The adhesive junctions formed during frictional processes play a key role in the development of the adhesive wear mechanism and in the production of wear debris. The following wear model was first proposed by Archard. Consider two asperities in a tribological couple. The asperities first approach each other, then establish adhesive contact and finally move past each other. As a result of the interaction, a wear particle may be created. To simplify, consider asperities of hemispherical shape and radius a and assume the volume of the wear particle δv w is approximately given by δv w = 2 3 πa3 Moreover, let the normal load δf n supported by the (plastically deforming) asperity be given by δf n = πa 2 where is the hardness of the softer component in the contact couple. Finally, the sliding distance for this asperity interaction is δl = 2a. Only a small fraction of contact interactions result in worn particles, therefore, let the proportion of interactions resulting in wear be κ so that the volume of material removed per unit sliding distance δw is given by δw = κ δv w δl = κπa2 3 The total amount of wear (per unit sliding distance) w is the sum over all interactions, while the total load is the sum of all the loads supported by individual asperities. The wear rate is then given by w = κ F n 3 = K F n where K = κ/3 is the wear coefficient. Archard s model yields an expression for the wear rate as a function solely of the macroscopic quantities F n and. It is well known that other factors significantly influence adhesive wear rates, the most important include Material Compatibility. Crystal Structure. Microstructure. These effects are accounted for only implicitly in Archard s model though the wear coefficient K. 8

9 4 Other Forms of Wear Material can be removed from a surface in a tribological couple by mechanisms other than adhesive/sliding wear. Particularly important are abrasive wear mechanisms involving either hard asperities or third bodies, erosive mechanisms involving high velocity swarms of particles and wear assisted by chemical reactions. 4.1 Abrasive Wear Abrasive wear takes place when hard asperities or third phase particles rub under load against a relatively softer surface. If the wear process involves only two materials it is known as two-body abrasive wear. If extraneous abrasive particles are used one has three-body wear. In general, wear rates for two body wear are higher than for three body since loose abrasive particles tend to roll over the soft surface while abrading asperities firmly attached to the abrading surface do not. Depending on the attack angle and the interfacial shear strength three modes of abrasive wear are usually encountered in ductile metals: Ploughing: Ridges form along the sides of wear track. Wedging: A short wedge forms in front of the abrading asperity. Cutting: A long ribbon-like chip forms in front of the abrading asperity. In all cases wear particles form when ridges, wedges or chips fracture and detach from the underlying surface, perhaps by a process of delamination. The Archard wear equation derived for adhesive wear situations has also been found useful in the representation of abrasive wear. Consider for instance the case of an abrasive surface consisiting of conical asperities of included angle α that ploughs through the surface of a softer material. The volume of material removed from the ploughed groove per unit sliding distance is w = V w L = cotα K ( π )F n = K F n abr where cot α is the average value of cotα for all asperities. Therefore, the abrasive wear coefficient K abr depends on the geometry of the abrading asperity and typically ranges between 10 6 to In cutting wear, material removal proceeds as in machining by forming a chip. Specifically, for cutting by a single, hard wedge asperity of width b perpendicular to the direction of motion and rake angle α under orthogonal cutting conditions the chip thickness ζ has been given by Merchant as ζ = cos(φ α) f sinφ 9

10 where φ is the shear plane angle formed by the thin, primary deformation zone, extending from the tip of the asperity to the free surface of the workpiece and the direction of motion and f is the undeformed chip thickness (called the feed in machining application). While the feed is a process parameter set by the machinist, the feed in this case can be expected to be directly proportional to the applied (normal) load and inversely proportional to the hardness of the material, i.e. f = K F n The volume of material removed as a chip by the wedge over a cutting time t is simply given by V w = Ufbt = Lfb = K cut F n L Moreover, the thickness of the resulting worn particles should be of the same order as the chip thickness ζ. 4.2 Wear by Brittle Fracture Brittle solids often fail under abrasive wear by subsurface lateral fracture crack propagation. The volume of worn material per unit sliding distance as a result of N asperities rubbing on a surface of modulus E, hardness and fracture toughness K c under load F n has been found to be given by where c is a material constant w = V w L 9/8 (E/)Fn = cn Kc 1/2 5/8 4.3 Fatigue Wear Subsurface cracks may also form in more ductile materials by fatigue processes associated with cyclic loading. While little wear may be observed prior to fatigue events extensive pitting may be observed once fatigue sets in. Consider again a tribological system formed by a large number of microscopic contacts. Each individual asperity experiences multiple contact interactions with asperities of the counterface. When fatigue failure takes place at a particular asperity contact, the volume of material removed can be expected to be proportional to a 3 where a is the contact radius. Moreover, the sliding distance over a particular contact is 2a. An expression for the worn volume per unit sliding distance for the system can then be obtained by adding the contributions from all contacting asperities where fatigue has occurred, i.e. w = V w L = K F n fatiguea r = K fatigue 10

11 Since K can be interpreted as the proportion of contacts that contribute to wear debris by failing, the number of stress cycles to failure N f is simply N f = 1 K fatigue Fatigue wear may also occur in rolling contacts even if they function under a fluid film lubrication regime. The reason for this are the large cyclical stresses transmitted through the lubricating film. The resistance of bearings against rolling contact fatigue is estimated statistically through their lives. Specifically, the Weibull distribution with density function (for t > 0) given by f(t) = β α ( t α )β 1 exp( ( t α )β ) where β and α are the parameters of the distribution is used as a foundation of the Weibull life model. The life (in millions of revolutions) for 90 percent of the bearings in a given population is given by L 10 = (C/F n ) 3 where C is the bearing s basic load capacity. The above equation is adjusted by the use of multiplicative factors to incorporate variations in geometry, material properties and lubrication conditions. 4.4 Impact Wear Impact wear is the gradual wastage of material due to repeated impact by particulate streams (erosive wear) or by continued hammering with a hard object (percussive wear). In solid particle erosion, the kinetic energy of impacting particles is transferred to the worn surface where is transformed into work of plastic deformation. Worn particles may then by removed by ploughing, wedging or cutting (ductile materials) or by fracture (brittle materials). By equating the kinetic energy of impinging particles to the work of plastic deformation and recognizing that only a fraction β of impacts results in worn particles the following expression has been obtained for the erosion ratio E (mass of worn particles divided by mass of erosive particles) due to a total mass m of erosive particles impinging the surface with velocity U, E = β ρu2 2 where ρ is the density of the worn material. Note that is again similar to Archard s equation but with the load F n replaced by ρu 2. Wear coefficients for erosive wear range between 10 5 and

12 An expression identical to Archard s law can be obtained by recognizing that the term F n L represents a sliding energy E s. Consider n identical particles of mass m impacting a surface with the same velocity v. Their individual kinetic energy is 1 2 mv2 Of the total kinetic energy of the particles only a fraction β results in material removal. Therefore the impacting energy resulting in wear is given by E i = βe k = β 1 2 mv2 n. Therefore, using this impact energy instead of the sliding energy into Archard s equation yields V w = K impact E k The wear coefficient in this case can then be interpreted as the fraction of all impact events that results in material removal. Liquid impingement erosion may result when liquid droplet sprays hit a surface at high speeds due to the high pressures developed when the kinetic energy of the fluid particle is transferred to the surface. Erosion can also result from imploding bubbles near the surface since implosion results in a fluid microjet directed at high speed against the surface. When a surface is subjected to repetitive hard body impact, multiple wear mechanisms may become simultaneously active. The wear volume is estimated per impact cycle. Specifically for the impact of a slug with mass m of duration t i and peak force F 0, onto a surface moving tangentially with speed u, the worn volume is given b y V w = k ut if 0 S 2π where S = πmu/µf 0 t i is the slip factor and µ is the friction coefficient between the slug and the surface. 4.5 Wear associated with Chemical Reactions Tribological systems embedded in a corrosive environment often experience chemical attack. Also, the localized heating produced by flash temperatures stimulates chemical reactions which may also result in wear Fretting Wear, the wastage of materials subjected to oscillatory motions of small amplitude is often associated with chemical reactions. As asperities on one surface slide past the surface of the counterface, the latter may be rapidly oxidized. The oxide layer may be intermittent or continuous and may stay in place or be removed by subsequent sliding. Depending of the fate of the oxide layer, the surface clearance may increase leading to sloppiness or undesired seizure may occur. A model of corrosive wear can be developed by considering that the contact interaction takes places in a reactive environment that leads to the formation of a tribofilm on the wearing surface. Wear takes place once the film reaches a critical thickness λ that becomes easily removed by rubbing. At a single contacting asperity covered with a tribofilm of critical thickness λ the worn volume is πa 2 λ. Since the sliding distance is 2a the volume removed 12

13 per unit sliding distance during a single asperity interaction is 1 πaλ. The result due to n 2 such identical asperity interactions is w = V w L = λ 2a nπa2 = λ 2a A F n r = K ch because A r = F n /. Note that K ch is directly proportional to the tribofilm thickness. ence, increasing the chemical aggressiveness of the environment and/or the temperature should increase the wear rate since both effects will make the film grow faster. 5 Examination of Wear Debris Insight about wear mechanisms may be obtained by careful examination of wear debris. Microscopic (optical and electron), Coulter analysis and Ferrography have all been used. Particular interest exist in the deployment of wear debris examination methods for on-line condition monitoring of tribological systems since abrupt changes in wear debris characteristics are indicative of changes in wear mechanisms. 6 Wear Mechanism Maps The various wear regimes and transitions observed in practice can often be usefully summarized in the form wear mechanism maps. A map is usually a two dimensional graph of normalized applied pressure against normalized relative velocity. Depending on the application, other combinations of coordinates may be selected. Regions within the map indicate process regions where a specific wear mechanism is dominant. Many different types of wear mechanism maps are now available for a variety of tribological systems. These are useful since one can identify at a glance specific values of tribosystem parameters that are most likely to yield desired results. 13