This ratio effect of H/Ha on abrasive wear results from a change in the nature of the contact mechanics. At H/h a ratios between 0.6 and 0.8, the contact conditions give rise to extensive plastic deformation. At higher H/Ha ratios, the nature of the contact becomes essentially elastic(Ref 10, 11, 22). As a result, wear rates decrease unless material is removed by mechanisms other than cutting and plowing, for example, fragmentation As the hardness of the worn surface approaches that of the abrasive, plastic flow of the abrasive may occur, leading to a reduction in the cutting ability of the abrasive particle. In addition, it is possible to fracture the abrasive when the plastic zone in the abrasive particle reaches a critical size(Ref 10, 19). This effect for the abrasive particle is analogous to the transition from purely cutting to fragmentation in the abrasion of brittle materials The effect of H/Ha on wear behavior is also influenced by the size and compressive strength of the abrasive particles. Coarse abrasives are more likely to fracture than fine abrasives, partly due to a decrease in tensile strength with increasing particle size. In addition, fracture of the abrasive may regenerate sharp facets and abrasive fragments, which in turn increase wear rates. Loading conditions also have an effect, because increasing the contact stress between abrasive and wear material increases the probability of fracture of abrasive particles Contact Conditions. It is difficult to assess the effects of individual contact conditions on the wear interactions among abrasive and wear material, because their effect is synergistic in nature. Force, impact level, velocity and impingement angle combine to influence the wear rate of the material Increasing the contact stress between the abrasive and the wear surface results in greater indentation depths and an increased tendency for fracture and fragmentation in both brittle wear materials and abrasives(ref 10, 19) This generally leads to increased wear rates, although exceptions may occur if the abrasivity of particular minerals decreases with failure of the abrasive particle. The effect of nominal contact stress on the relative abrasion rating of a range of metallic materials is shown in Fig. 6 100 QT plate steels QT plate QT plate steels Cast martensitic Cast QT plate martensitic Cast Austenitic steels martensitic manganese Austenitic Austenitic Austenitic manganese Alloy cast irons steels manganese Crc hardfacing Alloy cast irons CrC hardfacing Alloy cast irons and irons and CrC hardfacing Crc hardfacing Abrasive type Gouging High stress Low stress Abrasive hardness 800-1100 HV 18002000HV 8001100Hv9001300HV Abrasive shape Angular Angular Angular Nominal stress 250 MPa 2.1 MPa 2.1 MPa 0. 6 MPa (305ps (305ps) (87ps) ig. 6 Effect of nominal contact stress on relative abrasion rating of metallic wear materials. QT, quenched and tempered. Source: ref5 The general effect of increasing nominal contact stress levels is to increase wear rates; however, the occurrence of significant impact during the abrasive/wear material contact may also accelerate the rate of material removal This acceleration results from the increased amount of kinetic energy dissipated during contact. This energy
This ratio effect of H/Ha on abrasive wear results from a change in the nature of the contact mechanics. At H/Ha ratios between 0.6 and 0.8, the contact conditions give rise to extensive plastic deformation. At higher H/Ha ratios, the nature of the contact becomes essentially elastic (Ref 10, 11, 22). As a result, wear rates decrease, unless material is removed by mechanisms other than cutting and plowing, for example, fragmentation. As the hardness of the worn surface approaches that of the abrasive, plastic flow of the abrasive may occur, leading to a reduction in the cutting ability of the abrasive particle. In addition, it is possible to fracture the abrasive when the plastic zone in the abrasive particle reaches a critical size (Ref 10, 19). This effect for the abrasive particle is analogous to the transition from purely cutting to fragmentation in the abrasion of brittle materials. The effect of H/Ha on wear behavior is also influenced by the size and compressive strength of the abrasive particles. Coarse abrasives are more likely to fracture than fine abrasives, partly due to a decrease in tensile strength with increasing particle size. In addition, fracture of the abrasive may regenerate sharp facets and produce loose abrasive fragments, which in turn increase wear rates. Loading conditions also have an effect, because increasing the contact stress between abrasive and wear material increases the probability of fracture of the abrasive particles. Contact Conditions. It is difficult to assess the effects of individual contact conditions on the wear interactions among abrasive and wear material, because their effect is synergistic in nature. Force, impact level, velocity, and impingement angle combine to influence the wear rate of the material. Increasing the contact stress between the abrasive and the wear surface results in greater indentation depths and an increased tendency for fracture and fragmentation in both brittle wear materials and abrasives (Ref 10, 19). This generally leads to increased wear rates, although exceptions may occur if the abrasivity of particular minerals decreases with failure of the abrasive particle. The effect of nominal contact stress on the relative abrasion rating of a range of metallic materials is shown in Fig. 6. Fig. 6 Effect of nominal contact stress on relative abrasion rating of metallic wear materials. QT, quenched and tempered. Source: Ref 5 The general effect of increasing nominal contact stress levels is to increase wear rates; however, the occurrence of significant impact during the abrasive/wear material contact may also accelerate the rate of material removal. This acceleration results from the increased amount of kinetic energy dissipated during contact. This energy
may arise either from the moving abrasive particles or from the moving wear surfaces, as in the case of impact crushers. For brittle materials, increased fragmentation occurs under impact conditions, whereas elastomers may only suffer an increase in cutting and tearing modes, associated with insufficient elastic recovery The nominal force level may also determine the level of constraint experienced by abrasive particles. Under low-stress abrasive conditions, the abrasive particles can be free to rotate as they move across the surface of the wear material and are less likely to indent and scratch the surface. The tendency for the abrasive particle to rotate also depends on the abrasive particle shape, with angular particles being more likely to slide rather than roll. Increasing the nominal force acts to constrain the abrasive particle in its orientation to the wear surface thereby increasing the wear rate The effect of velocity on wear behavior is associated with the dissipation of kinetic energy during abrasive/wear material contact. For a large number of abrasive wear environments, the velocity of abrasive particles is elatively low(<10 m/s)and therefore of little importance. However, in slurry pumps, that is, the transport of abrasive particles in slurry or pneumatic form, the effect of velocity is significant. Under these conditions, the wear rate is proportional to velocity (w av, where W is the wear rate, V is the velocity of the abrasive, and n is a constant for the abrasive/wear material synergism). The value of n falls in the range of 2 to 3, although the exact value for any particular condition is dependent on the properties of both the abrasive and wear material and on the angle of impingement(Ref 10, 22, 23). For softer abrasives, n tends to increase with decreasing abrasive particle size. For brittle materials and high impingement angles, the value of n tends toward the higher In erosion, a change in abrasive impact velocity can lead to a change in the dominant wear mechanism(Ref 24 ), resulting in a change in wear rate. Higher values for n are associated with increased cutting and fragmentation Figure 7 depicts the effect of erosive particle velocity on the wear of a range of materials Ceramics Rubbe Tool or Impact veloci Fig 7 Effect of impact velocity on erosive wear Source: Ref 12 The influence of impingement angle on wear depends on the properties of the wear material and is associated with changes to the dominant wear mechanism. At high impingement angles(60 to 90o), brittle materials typically experience elevated wear rates, resulting from increased fragmentation and spalling. Conversely, elastomers are more effective under these conditions, because much of the impact energy can be dissipated through elastic deformation At low impingement angles (10 to 30%), elastomers cut and tear more readily, leading to increases in wear rates Hard, brittle materials usually perform better under these conditions. For materials intermediate in their Thefileisdownloadedfromwww.bzfxw.com
may arise either from the moving abrasive particles or from the moving wear surfaces, as in the case of impact crushers. For brittle materials, increased fragmentation occurs under impact conditions, whereas elastomers may only suffer an increase in cutting and tearing modes, associated with insufficient elastic recovery. The nominal force level may also determine the level of constraint experienced by abrasive particles. Under low-stress abrasive conditions, the abrasive particles can be free to rotate as they move across the surface of the wear material and are less likely to indent and scratch the surface. The tendency for the abrasive particle to rotate also depends on the abrasive particle shape, with angular particles being more likely to slide rather than roll. Increasing the nominal force acts to constrain the abrasive particle in its orientation to the wear surface, thereby increasing the wear rate. The effect of velocity on wear behavior is associated with the dissipation of kinetic energy during abrasive/wear material contact. For a large number of abrasive wear environments, the velocity of abrasive particles is relatively low (<10 m/s) and therefore of little importance. However, in slurry pumps, that is, the transport of abrasive particles in slurry or pneumatic form, the effect of velocity is significant. Under these conditions, the wear rate is proportional to velocity (W α V n , where W is the wear rate, V is the velocity of the abrasive, and n is a constant for the abrasive/wear material synergism). The value of n falls in the range of 2 to 3, although the exact value for any particular condition is dependent on the properties of both the abrasive and wear material and on the angle of impingement (Ref 10, 22, 23). For softer abrasives, n tends to increase with decreasing abrasive particle size. For brittle materials and high impingement angles, the value of n tends toward the higher extreme. In erosion, a change in abrasive impact velocity can lead to a change in the dominant wear mechanism (Ref 24), resulting in a change in wear rate. Higher values for n are associated with increased cutting and fragmentation. Figure 7 depicts the effect of erosive particle velocity on the wear of a range of materials. Fig. 7 Effect of impact velocity on erosive wear. Source: Ref 12 The influence of impingement angle on wear depends on the properties of the wear material and is associated with changes to the dominant wear mechanism. At high impingement angles (60 to 90°), brittle materials typically experience elevated wear rates, resulting from increased fragmentation and spalling. Conversely, elastomers are more effective under these conditions, because much of the impact energy can be dissipated through elastic deformation. At low impingement angles (10 to 30°), elastomers cut and tear more readily, leading to increases in wear rates. Hard, brittle materials usually perform better under these conditions. For materials intermediate in their The file is downloaded from www.bzfxw.com
mechanical properties, for example, some metals, the effect of impingement angle depends on the ductility of the material(Ref 24). The effect of impingement angle on the erosive wear behavior of a number of materials is shown schematically in Fig 8 White cast iron Tool or bearing steel Mild steel Rubber 90° Fig 8 Effect of impingement angle on erosive wear Source: Ref 12 Properties of the Wear Material. The properties of the wearing material that influence wear behavior are grouped into the following categories: mechanical properties, microstructure effects, and other properties (corrosion resistance, friction, thermal effects). Because abrasive wear is primarily a mechanical process particularly in the absence of corrosive environments, mechanical properties are of major importance, whereas the role of microstructure depends on the severity of the wear environment. The following discussion is patterned after the approach of mutton(Ref 10)and Moore (Ref 11) Mechanical Properties. The resistance to indentation(hardness) is an important variable in determining abrasion resistance. Laboratory wear tests indicate that the abrasive wear resistance for particular material types increases with increasing hardness(Fig. 9). However, large differences in abrasion resistance can also occur at similar hardness levels. These differences arise from variations in the plastic flow characteristics of the various materials, which in turn influence the predominant wear mechanism(Ref 7). The general relationships between abrasion resistance and hardness for plowing and fragmentation are similar to those shown in Fig. 10 for pure metals and ceramics
mechanical properties, for example, some metals, the effect of impingement angle depends on the ductility of the material (Ref 24). The effect of impingement angle on the erosive wear behavior of a number of materials is shown schematically in Fig. 8. Fig. 8 Effect of impingement angle on erosive wear. Source: Ref 12 Properties of the Wear Material. The properties of the wearing material that influence wear behavior are grouped into the following categories: mechanical properties, microstructure effects, and other properties (corrosion resistance, friction, thermal effects). Because abrasive wear is primarily a mechanical process, particularly in the absence of corrosive environments, mechanical properties are of major importance, whereas the role of microstructure depends on the severity of the wear environment. The following discussion is patterned after the approach of Mutton (Ref 10) and Moore (Ref 11). Mechanical Properties. The resistance to indentation (hardness) is an important variable in determining abrasion resistance. Laboratory wear tests indicate that the abrasive wear resistance for particular material types increases with increasing hardness (Fig. 9). However, large differences in abrasion resistance can also occur at similar hardness levels. These differences arise from variations in the plastic flow characteristics of the various materials, which in turn influence the predominant wear mechanism (Ref 7). The general relationships between abrasion resistance and hardness for plowing and fragmentation are similar to those shown in Fig. 10 for pure metals and ceramics
Face-centered cubic Martensite and pure metals and annealed retained austenite steels body-centered cubic Austenitic"a High Cr-Mo white cast iron Martensitic Heat treated 0Q0g=o3g>m Increasing carbon content Cold-worked metals Ceramics recipitation hardening Bulk hardness Fig 9 Abrasion resistance versus hardness for various material types in high-stress pin abrasion tests (silicon carbide abrasive). Source: Ref 6 0.7 0.6 0.5 0.4 Hardness 0.3 E0.2 a0.1 200 400 800 Hardness and Eσ Fig 10 Influence of hardness and Eloy on dominant wear mechanism. Source: Ref5 In the absence of fragmentation, plowing and cutting involve plastic flow of the wear material either in front of or to the sides of the indenting abrasive particles. The plastic flow behavior of the material can be characterized by the ratio Eloy where E is the elastic modulus and oy is the flow stress of the non-work-hardened material Thefileisdownloadedfromwww.bzfxw.com
Fig. 9 Abrasion resistance versus hardness for various material types in high-stress pin abrasion tests (silicon carbide abrasive). Source: Ref 6 Fig. 10 Influence of hardness and E/σy on dominant wear mechanism. Source: Ref 5 In the absence of fragmentation, plowing and cutting involve plastic flow of the wear material either in front of or to the sides of the indenting abrasive particles. The plastic flow behavior of the material can be characterized by the ratio E/σy where E is the elastic modulus and σy is the flow stress of the non-work-hardened material. The file is downloaded from www.bzfxw.com
Decreasing Elo, favors a cutting mechanism, in which a high proportion of material is removed as microchips Increasing the hardness of the wear material has a similar effect, as shown in Fig. 10 Although Fig. 10 indicates that changing Elo, can significantly affect the dominant wear mechanism, its impact on wear resistance is not as clearly established. For a constant value of E, a decrease in o, favors plowing. This should lead to lower material wear rates. However, decreasing oy also decreases hardness, which favors greater depths of indentation and a reduction in the wear resistance of the material. For materials of equivalent hardness, the general trend is for wear resistance to increase with higher E/o, values(Ref 11) The wear material properties discussed thus far (i.e, hardness, elastic modulus, and flow strength) relate to material behavior at relatively low strains. For example, the plastic strain produced by indentation hardness testing of metals is typically between 8 and 10%. During the abrasive wear process, the extent of plastic strain experienced by a worn surface may approach these values. For these cases, the behavior of the wear material at high plastic strains is important, and here, high values of the work-hardening coefficient and ductility are favorable Tensile strength and ductility are also important mechanical property parameters in a wear environment especially for polymers, which undergo elongations-to-fracture approaching 500%. Laboratory wear studies rupture,which is defined as the product of the rupture stress and elongation-to-fracture ortional to the work of indicate that the wear resistance of both rigid and ductile polymers is roughly prop The abrasion resistance of polymers also increases with increasing indentation resistance, although this relationship is not as well defined for polymers as it is for metals and ceramics. This is due in part to the viscoelastic nature of polymer deformation behavior. In addition, the absolute hardness levels for polymers are luch lower than those for metals and ceramics For elastomeric materials such as natural rubbers and polyurethanes, the deformation behavior during abrasive wear may be entirely elastic, in which case resilience or hysteresis loss is important. High resilience favors increased abrasion resistance, while increasing hysteresis losses under high impact levels result in material breakdown, exacerbated by heat buildup For materials that undergo wear by fragmentation, abrasion resistance varies with fracture toughness. However, the contribution from microcracking and fragmentation depends on the severity of the wear environment, in particular the applied load and abrasive particle size For metals and ceramics, the general form of the relationship between wear resistance, hardness, and fracture toughness(Kc)is illustrated in Fig. 11(Ref 25). For low-fracture-toughness materials (14 MPa m, or 13 ksi vin ) wear resistance increases with increasing fracture toughness, despite a marked decrease in hardness The increase in the wear resistance of the material results from a reduction in the fragmentation contribution te the total wear rate For materials with high fracture toughness, wear occurs by cutting and plowing only. Hence, the wear rate of the material is controlled by the indentation resistance. In this case wear resistance decreases with decreasing hardness Fragment Cutting/plowing Increasing applied load, abrasive size, hardness and angularity ear resistance Hardness Fracture toughness(K) Fig 1l Schematic relationship between wear resistance, hardness, and fracture toughness. Source: Ref 6 The transition from fracture(fragmentation) to plastic deformation(cutting and plowing) is dependent on the critical groove size (pcrit)and the contact stress. These parameters, in turn, also depend on the ratio of fracture
Decreasing E/σy favors a cutting mechanism, in which a high proportion of material is removed as microchips. Increasing the hardness of the wear material has a similar effect, as shown in Fig. 10. Although Fig. 10 indicates that changing E/σy can significantly affect the dominant wear mechanism, its impact on wear resistance is not as clearly established. For a constant value of E, a decrease in σy favors plowing. This should lead to lower material wear rates. However, decreasing σy also decreases hardness, which favors greater depths of indentation and a reduction in the wear resistance of the material. For materials of equivalent hardness, the general trend is for wear resistance to increase with higher E/σy values (Ref 11). The wear material properties discussed thus far (i.e., hardness, elastic modulus, and flow strength) relate to material behavior at relatively low strains. For example, the plastic strain produced by indentation hardness testing of metals is typically between 8 and 10%. During the abrasive wear process, the extent of plastic strain experienced by a worn surface may approach these values. For these cases, the behavior of the wear material at high plastic strains is important, and here, high values of the work-hardening coefficient and ductility are favorable. Tensile strength and ductility are also important mechanical property parameters in a wear environment, especially for polymers, which undergo elongations-to-fracture approaching 500%. Laboratory wear studies indicate that the wear resistance of both rigid and ductile polymers is roughly proportional to the work of rupture, which is defined as the product of the rupture stress and elongation-to-fracture. The abrasion resistance of polymers also increases with increasing indentation resistance, although this relationship is not as well defined for polymers as it is for metals and ceramics. This is due in part to the viscoelastic nature of polymer deformation behavior. In addition, the absolute hardness levels for polymers are much lower than those for metals and ceramics. For elastomeric materials such as natural rubbers and polyurethanes, the deformation behavior during abrasive wear may be entirely elastic, in which case resilience or hysteresis loss is important. High resilience favors increased abrasion resistance, while increasing hysteresis losses under high impact levels result in material breakdown, exacerbated by heat buildup. For materials that undergo wear by fragmentation, abrasion resistance varies with fracture toughness. However, the contribution from microcracking and fragmentation depends on the severity of the wear environment, in particular the applied load and abrasive particle size. For metals and ceramics, the general form of the relationship between wear resistance, hardness, and fracture toughness (Kc) is illustrated in Fig. 11 (Ref 25). For low-fracture-toughness materials (>14 MPa m , or 13 ksi in ), wear resistance increases with increasing fracture toughness, despite a marked decrease in hardness. The increase in the wear resistance of the material results from a reduction in the fragmentation contribution to the total wear rate. For materials with high fracture toughness, wear occurs by cutting and plowing only. Hence, the wear rate of the material is controlled by the indentation resistance. In this case, wear resistance decreases with decreasing hardness. Fig. 11 Schematic relationship between wear resistance, hardness, and fracture toughness. Source: Ref 6 The transition from fracture (fragmentation) to plastic deformation (cutting and plowing) is dependent on the critical groove size (pcrit) and the contact stress. These parameters, in turn, also depend on the ratio of fracture