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erosive slurries
Wear 426–427 (2019) 612–619
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Elastomers and plastics for resisting erosion attack of abrasive/erosive slurries
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Yongsong Xie , Jiaren (Jimmy) Jiang, Md. Aminul Islam Mining Wear and Corrosion Laboratory, National Research Council Canada, 4250 Wesbrook Mall, Vancouver, BC V6T 1W5, Canada
A R T I C LE I N FO
A B S T R A C T
Keywords: Elastomers Polyethylene Slurry erosion Erosion tests
Elastomers and plastics are widely used as the materials for handling abrasive/erosive slurries because of their wear resistance, toughness, corrosion resistance and light weight. In this study, slurry jet erosion, Coriolis slurry scouring erosion and large particle slurry erosion are used to characterize the erosion resistance of selected elastomers and plastics, including two natural rubbers, a neoprene rubber, a polyurethane and three types of polyethylene, at different test conditions. The wear rates and wear modes of the tested materials are presented and the relationships between the erosion resistance of these materials and their mechanical properties are discussed. Suitable applications of these materials in slurry transport are also recommended.
1. Introduction Slurry transport is widely used for the transport of solid materials in various industrial operations because it has a number of handling advantages including low cost, minimum maintenance and small environmental impact [1]. In slurry transport, the concentrate of solid materials is mixed with water and then pumped over a distance through pipeline. The slurries are often abrasive or erosive because the solid materials in the slurries are hard and angular. For examples, mining slurry contains crushed ores and oil sands slurry contains silica sands. The mechanical interactions between the slurries and the component surfaces of slurry transport equipment result in wear damage of the equipment. To reduce the maintenance costs incurred for replacing and/or repairing worn components and minimize the losses in production during related equipment down-time, the most cost-effective wear-resistant materials should be applied [2–4]. Among commonly used wear-resistant materials for handling abrasive/erosive slurries, elastomers and plastics are increasingly widely used because of their wear resistance, corrosion resistance, toughness and light weight [5–7]. In order to use elastomers and plastics properly in different slurry transport systems, it is essential to understand their suitable application conditions. However, there is only scarce literature in this field. In the past twenty-two years, the Mining Wear and Corrosion research team at the National Research Council Canada (NRC) has characterized various elastomers and plastics for Canadian mining operations. In this paper, the erosion resistance and wear modes of several elastomers and plastics commonly used in slurry transport are presented and the
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relationships between the erosion resistance of these materials and their mechanical properties are discussed. Then, the suitable application conditions of these elastomers and plastics are recommended. 1.1. Elastomers commonly used for slurry transport Elastomer is a word commonly used to mean any rubber-like compound (natural or synthetic) which can be stretched repeatedly at room temperature to at least twice of its length but return quickly to its approximate original dimension when the applied stress is released. Because of their general similarity to rubbers, polyurethanes are also classified as elastomers. General advantages of elastomers over other wear protection options are in the areas of resilience, toughness, corrosion resistance, ease of fabrication, non-stick and self-lubricating qualities, noise/vibration damping capabilities and light weight. In wear situations, they rely mainly on their elastic properties to absorb the deformation induced by the abrasive or erosive medium elastically with minimal plastic deformation. However, they have clear limitations in the areas of tearing and gouging resistance, strength and ability to withstand elevated temperatures. Elastomers are often used as liners or hoses for slurry transport. Natural rubbers have excellent abrasion and erosion resistance when hydrocarbon and weathering resistance are not required. Neoprene rubbers are good materials where resistance to hydrocarbons and wear are needed [8]. The properties of polyurethanes vary depending on the base polymers. They can exhibit outstanding
Corresponding author. E-mail address: [email protected] (Y. Xie).
https://doi.org/10.1016/j.wear.2019.01.123 Received 31 August 2018; Received in revised form 9 January 2019; Accepted 20 January 2019 0043-1648/ Crown Copyright © 2019 Published by Elsevier B.V. All rights reserved.
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Table 1 Materials tested in this study. Material and Category Elastomers
Plastics
Steels
Natural rubber (40 Shore A) Hardened natural rubber (69 Shore A) Neoprene rubber (53 Share A) Polyurethane (90 Shore A) HDPE XPE UHMWPE Low alloy pipe steel AISI 1018 mild steel AR450 steel
Hardness (H)
Young's modulus (E)
0.62 MPa 1.5 MPa 0.61 MPa 5.6 MPa N/A 3.2 HV (35 MPa) 3.6 HV (39 MPa) 231 HB (2.6 GPa) 254 HB (2.9 GPa) 460 HB (5.2 GPa)
2.48 MPa 14.8 MPa 3.53 MPa 34.2 MPa N/A 958 MPa 791 MPa 200 GPa 200 GPa 200 GPa
varied up to 25 m/s with erodent concentration up to 20 vol% and slurry temperature up to 60 °C. By selecting suitable erodent and test parameters, erosions of many slurry handling components in different service conditions can be simulated. In NRC standard SJE testing procedure, AFS 50–70 silica sand, which is natural silica having a semi-rounded shape with size around 250 µm (Fig. 2(a)), is used as the erodent for testing most materials and the test duration is two hours. For most wear-resistant elastomers and plastics, the semi-rounded AFS 50–70 silica sand generates very small wear volume loss. Hence, to distinguish the wear resistance of these materials, angular 36 mesh SiC particles with size around 500 µm (Fig. 2(b)) are used as erodent and the test duration is extended to four hours. Fig. 3 presents the wear rates of some materials tested in SJE using semi-rounded AFS 50–70 silica sand as erodent. Fig. 4 gives the SJE results of several materials tested using 36 mesh SiC particles as erodent. Both tests were performed at jet velocity of 16 m/s and at room temperature (23 ± 1 °C). The wear volume losses of steel specimens were determined by the mass losses and densities of the specimens. For elastomers and plastics, the wear volume losses were measured directly using a laser profilometer. To minimize any potential effects from water absorption and swelling in the elastomeric and plastic specimens, the tested specimens were dried in a vacuum furnace for more than ten hours before measuring their wear volume losses. When using semi-rounded AFS 50–70 silica sand as erodent, the wear resistance of the tested elastomers and plastics were much better than the steels. When angular and larger SiC particles were used as the erodent, although the wear resistance of these elastomers and plastics were still better than the AR450 steel, all the tested elastomers and plastics displayed wear rates many times higher than those when using semi-rounded AFS 50–70 silica erodent. In contrast, for AR450 steel, when using the angular and larger SiC particles as the erodent, the increase in wear rate is much less than the elastomers and plastics. Fig. 5 shows the worn surfaces of four elastomers and plastics after the SJE tests using 36 mesh angular SiC erodent. On the worn surface of the neoprene rubber (Fig. 5(a)), the main feature is plastic flow along the slurry flow direction. On the worn surfaces of the polyurethane, the UHMWPE and the XPE (Fig. 5(b), (c) and (d)), the main features are cutting and ploughing grooves. On the worn surfaces of the UHMWPE and the XPE (Fig. 5(c) and (d)), some cutting debris are still visible.
mechanical properties, wear resistance and chemical resistance at moderate temperatures [9]. The capability of polyurethanes to be spray coated is a very significant attribute which increases the range of their application. 1.2. Plastics commonly used for slurry transport The use of plastics for wear protection is limited compared to elastomers. Polyethylene (PE), including high density polyethylene (HDPE), cross-linked polyethylene (XPE), high molecular weight polyethylene (HMWPE) and ultra-high molecular weight polyethylene (UHMWPE) are the most used wear-resistant plastics for slurry transport. They have good chemical resistance and their properties increase with molecular weight. Polyethylene have low friction surfaces with non-stick and self-lubricating properties to enhance slurry flow [10]. 2. Erosion resistance of commonly used elastomers and plastics 2.1. Materials Table 1 lists the seven elastomers and plastics studied in this paper, together with three steels for comparison. These elastomers and plastics are selected from materials commonly used for slurry transport. The hardness and Young's modulus values of all these materials are also listed in the table. The Young's modulus values of the elastomers and plastics were measured by the instrumented indentation (also called depth-sensing indentation) method according to ASTM E2546 standard [11] and those of the steels were from the manufacturers. The hardness values of the four elastomers were also measured by the instrumented indentation method, defined as the maximum indentation load divided by the projected area of the indent. For the plastics and steels, hardness values measured by Vickers or Brinell method are converted into the same unit of indentation load divided by projected area and are given in the brackets in Table 1. For the four elastomers, the commonly used Shore A hardness, which is a measure of the elasticity of an elastomer [12], is also given in the table. It should be noted that the properties of the materials belonging to the same material category vary from manufacturer to manufacturer. These selected materials are just some of the elastomers and plastics commonly used in slurry transport. They may not be the representative materials in their categories. 2.2. Erosion resistance assessed by slurry jet erosion
2.3. Erosion resistance assessed by Coriolis slurry scouring erosion
The NRC slurry jet erosion (SJE) test assesses material resistance to slurry attack at various impingement angles and has been proven to provide consistent and reproducible results [5,13]. In the SJE testing, as shown in Fig. 1, the slurry is circulated by a slurry pump from the slurry reservoir. The slurry exiting from the nozzle impacts the specimen surface and then returns to the reservoir. The specimen is clamped in a specimen holder and the holder can be adjusted to provide a selected slurry impingement angle between 10° and 90°. Slurry velocity can be
The Coriolis slurry erosion method was first introduced by Tuzson in 1984 [14] in an effort to develop a test method that simulated the motion of slurries and their interaction with container surfaces, such as slurry pumps or pipes. This method utilises the combination of centrifugal and Coriolis accelerations in a revolving rotor to pass slurry rapidly across a test surface such that the solid particles are forced against the surface, producing wear during their passage. A similar erosion test method has been used for ranking erosion resistant 613
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Fig. 1. Schematics of (a) NRC slurry jet erosion rig and (b) impingement angle.
Fig. 2. SEM images of erodent particles used in SJE tests. (a) AFS 50–70 silica sand and (b) 36 mesh SiC particles.
Fig. 3. Wear rates of selected materials in SJE at impingement angles of 20°, 45° and 90°. Semi-rounded AFS 50–70 silica sand was used as the erodent. The jet velocity was 16 m/s and the slurry temperature was 23 °C.
Fig. 4. Wear rates of selected materials in SJE at impingement angle of 20°. 36 mesh angular SiC particles were used as the erodent. The jet velocity was 16 m/ s and the slurry temperature was 23 °C.
materials of slurry pump components [15]. NRC has developed the Coriolis erosion tester further [16,17], allowing the use of flat plate specimens and accurate volume measurement of the eroded track. The Coriolis erosion method is particularly suitable for evaluating materials involved in handling slurries at locations where solid particles tend to impact a wear surface at a very low impingement angle. The data obtained from Coriolis erosion testing have correlated well with field trials [18,19]. As shown in Fig. 6, the Coriolis rig consists of a diametrically grooved rotor, which rotates at a preselected speed. The slurry is premixed before being fed into the centre of the rotor at a constant rate.
The considerable centrifugal force (Fcent) subsequently causes the slurry to be expelled along two slurry channels and the Coriolis force (Fcor) acts to force the slurry against the specimen surfaces. The impacts and the rebounds of the erodent particles on the specimen surfaces cause most of the particles “jump” along the specimen surface with very low impact angles (< 5 ° according to our modelling [16]). The other particles slide or roll along the specimen surfaces. The velocity with which the slurry traverses across the specimen surfaces increases with the increase in the distance from the centre of rotation. The main specifications of NRC Coriolis erosion tester are as follows: width of
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Fig. 5. SEM images of worn surfaces of the (a) neoprene rubber, (b) polyurethane, (c) UHMWPE and (d) XPE after SJE using 36 mesh SiC erodent. Slurry flow was from right to left.
Fig. 7. Wear rates of selected materials from Coriolis slurry erosion testing using semi-rounded AFS 50–70 silica sand as erodent at slurry temperatures of 23 °C and 45 °C.
Fig. 7 shows the Coriolis erosion test results for selected materials. In this study, 10 wt% AFS 50–70 silica sand + water was used as abrasive slurry. Slurry feed rate was 60 ml/s and the slurry was not reused after exiting the rotor. When traversing across the specimen surfaces, slurry velocity ranged from 15.5 m/s to 26 m/s at 5000 rpm rotor speed. The slurry temperature was 23 °C. Neoprene rubber, polyurethane, UHMWPE, XPE and 1018 steel were also tested at slurry temperature of 45 °C. The test duration was varied (depending on the wear resistance of the materials) to obtain a minimum wear depth of 30 µm. Wear volume losses of the test specimens were measured using a laser profilometer. Same as SJE testing, tested elastomeric and plastic specimens were completely dried in a vacuum furnace before volume loss measurement. The wear rates are normalized with respect to the used erodent amount and the erosion scar length to obtain the unit of mm3/kg/mm (Fig. 7).
Fig. 6. Schematics of the rotor of NRC Coriolis slurry erosion tester, the forces acting on an erodent particle and the optical image of the wear scar generated on a carbon steel.
slurry channels 1 mm, erodent concentration in the slurry up to 40 vol %, slurry temperature up to 60 °C and the rotor speed up to 5000 rpm which gives slurry velocity up to 26 m/s when traversing across the specimen surfaces. 615
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Fig. 8. SEM images of worn surfaces of the (a) neoprene rubber, (b) polyurethane, (c) UHMWPE and (d) XPE after the Coriolis erosion tests. Slurry flow was from right to left. 4.44 kg AFS 50–70 silica sand erodent was used in each test.
Fig. 9. Schematics of the rotor of NRC large particle slurry erosion tester, (a) 3D view and (b) top view.
Fig. 11. Wear rates of selected materials from the centrifugal impact erosion testing using 2–4 mm angular silica particles as erodent at slurry temperature of 23 °C.
Fig. 10. Wear rates of selected materials from the Coriolis scouring erosion testing using 2–4 mm angular silica particles as erodent at slurry temperature of 23 °C.
At 45 °C, the wear resistance of elastomers (polyurethane and neoprene rubber) and plastics (XPE, UHMWPE) are 61 and 58 times better than 1018 steel, respectively. It is interesting to note that both elastomers and plastics displayed better wear resistance at higher temperature.
Average wear resistance of elastomers (polyurethane, neoprene rubber and hard natural rubber) and plastics (XPE, UHMWPE) used in this study are 13 and 32 times better than steels at 23 °C, respectively. 616
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slurry up to 40 vol%, impingement angle on the impact erosion specimen can be adjusted between 10° and 80°, slurry temperature up to 60 °C and the rotor speed up to 3500 rpm which gives slurry velocities up to 26 m/s. Figs. 10 and 11 give the wear rates of some materials in the Coriolis erosion testing and centrifugal impact erosion testing using 2–4 mm angular silica particles (Fig. 12) as erodent. In the testing, the erodent concentration in water was 10 wt% and the slurry feed rate was 100 ml/ s. The slurry was not re-used after exiting the rotor. The rotor speed was 1500 rpm in the centrifugal impact erosion testing, which gives slurry velocity of 11 m/s when impacting the impact erosion specimen surface, and was 3500 rpm in the Coriolis erosion testing, which gives slurry velocity from 16 m/s to 24 m/s when passing the Coriolis erosion specimen surface. The tests were performed at slurry temperature of 23 °C. The test durations depended on the wear resistance of the test materials and wear depths of more than 50 µm were generated. The tested elastomeric and plastic specimens were completely dried in a vacuum furnace before measuring their wear volume losses using laser profilometry. Figs. 13–15 show the worn surfaces of the neoprene rubber, the polyurethane and the UHMWPE after the Coriolis scouring erosion and centrifugal impact erosion at 20° impingement angle by the 2–4 mm angular silica particles. On the worn surfaces of the neoprene rubber (Fig. 13), the dominant damage mechanisms are plastic flow and tearing. On the worn surfaces of the polyurethane, the dominant damage mechanisms are cutting and ploughing. On the worn surfaces of the UHMWPE, the dominant damage mechanism is cutting and the cutting grooves are huge.
Fig. 12. Optical image of silica particles used in the large particle erosion tests.
Increase in wear resistance with temperature is more prominent with elastomers than plastics. An increase in wear resistance of around 3.7 and 1.4 times was observed for elastomers and plastics, respectively with an increase in slurry temperature from 23 °C to 45 °C. In contrast, metallic material (1018 steel) displayed similar wear resistance at the two slurry temperatures (23 °C and 45 °C). UHMWPE and XPE displayed excellent wear resistance in the slurry scouring erosion, which might be attributed to their low friction surfaces with non-stick and self-lubricating properties thus the interactions between the erodent particle and the wearing surfaces are minimal. Fig. 8 shows the worn surfaces of the neoprene rubber, the polyurethane, the UHMWPE and the XPE after the Coriolis erosion tests at slurry temperature of 23 °C. On the worn surface of the neoprene rubber (Fig. 8(a)), ripples appeared in the wear scar, which were the result of plastic flow. On the worn surfaces of the polyurethane, the UHMWPE and the XPE (Fig. 5(b), (c) and (d)), the main features are ploughing and cutting grooves.
3. Discussion The wear test results show that in most of these slurry erosion tests, the studied elastomers and plastics, especially the elastomers, displayed wear resistance much better than the steels. These laboratory wear test results are in agreement with field wear results. For example, high performance polyurethane internal pipe coatings have expanded the design life of slurry pipelines by up to a factor of 10 [9]. The excellent wear resistance of the elastomers and plastics can be mainly attributed to their excellent resilience to absorb deformation induced by the abrasive/erosive particles elastically with minimal plastic deformation. This can be explained by the “plasticity index”, ψ, which can be expressed as [20,21]
2.4. Erosion resistance assessed by large particle slurry erosion When using the NRC SJE tester and Coriolis slurry erosion tester, the erodent particles are smaller than 1 mm and 0.3 mm, respectively. To assess the resistance of materials to attacks by large particles in slurries, a large particle slurry erosion tester was developed at NRC for performing slurry impact erosion testing and slurry scouring erosion testing using erodent particles up to 5 mm [13]. As shown in Fig. 9, the large particle slurry erosion tester has a rotor bigger than that of the Coriolis erosion tester described in previous section. In addition to two scouring erosion specimens, two impact erosion specimens can be clamped at the two ends of the rotor. After exiting from the two slurry channels, the slurry impacts the two impact erosion specimens under the centrifugal force. The main specifications of the tester are as follows: width of slurry channels 10 mm, erodent concentration in the
ψ≈
E ×tan β H
where E is the Young's modulus of the wearing surface, H is the hardness of the wearing surface and β is the average slope of asperities on contacting surface which is equivalent to the average attack angle of
Fig. 13. SEM images of worn surfaces of the neoprene rubber after tests using 2–4 mm angular silica particles, (a) Coriolis scouring erosion and (b) impact erosion at 20° impingement angle. Slurry flow was from right to left. 617
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Fig. 14. SEM images of worn surfaces of the polyurethane after tests using 2–4 mm angular silica particles, (a) Coriolis scouring erosion and (b) impact erosion at 20° impingement angle. Slurry flow was from right to left.
Fig. 15. SEM images of worn surfaces of the UHMWPE after tests using 2–4 mm angular silica particles, (a) Coriolis scouring erosion and (b) impact erosion at 20° impingement angle. Slurry flow was from right to left.
abrasive/erosive particles. The higher the ψ, the more severe the plastic contact is. The reciprocal of the material parameter of the plasticity index, H/E, is proportional to the maximum elastic strain beyond which plastic contact deformation occurs. Therefore, the H/E ratio is a measure of the resistance of a material to plastic contact deformation. Fig. 16 gives the values of H/E ratio of the tested materials calculated from the H and E values given in Table 1. For the elastomers, which have much higher values of the H/E ratio than those of the steels, their surface deformation is much more likely to be elastic during the erosion tests, thus they displayed much better erosion resistance than the steels. For the plastics, although their values of the H/E ratio are lower than those of the elastomers, they are higher than the steels thus displayed better erosion resistance than the steels. Please note that, because the erosion resistance of elastomers and plastics is also determined by some other material properties (such as viscoelasticity), the measured erosion rates of the tested materials do not match the H/E results exactly. According to the images of erodent particles used in the wear tests (Figs. 2 and 12), the shapes of these erodent particles can be approximately regarded as spheres or polyhedrons with spherical tips at the corners. In the erosion tests, with the increase in the angularity of an erodent particle or the impact energy of the erodent particle (increased particle mass or particle velocity normal to the wearing surface, Vn) which results in increased penetration depth of the erodent particle, the attack angle β increases, as shown in Fig. 17. When using the small, semi-rounded AFS 50–70 silica sand as erodent, the deformations induced by the erodent particles on the wearing surfaces of the elastomer and plastic specimens were essentially elastic. Only some extremely sharp corners of the erodent particles caused plastic deformation. Therefore, the elastomers and plastics displayed very low wear rates which were many times lower than those of the steels (Figs. 3 and 7). At
Fig. 16. H/E values of the tested materials.
Fig. 17. Schematics of the shapes of erodent particles and the attack angles of the erosive particles.
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the service condition of slurry equipment for which the tested material will be used. The important test parameters include abrasive/ erosive particles (shape, size, hardness and density), slurry moving conditions (velocity and impingement angle) and temperature.
Table 2 Values of the H/E ratio of the neoprene rubber and polyurethane measured at two temperatures. Material
Temperature
H (MPa)
E (MPa)
H/E
Neoprene rubber
23 °C 60 °C 23 °C 60 °C
0.61 0.44 5.6 3.5
3.53 2.35 34.2 18.6
0.173 0.187 0.164 0.188
Polyurethane
Acknowledgement The authors would like to express their thanks to the members of the NRC/Industry Mining Wear Materials Research Consortium for their support of this work. They also wish to thank their NRC colleagues for their contributions for obtaining the test results.
the same SJE test conditions but changing the erodent particles from the AFS 50–70 silica sand to the larger and more angular 36 mesh SiC particles, the wear rates of the elastomers and plastics greatly increased (Figs. 3 and 4). Further increasing the sizes of the angular erodent particles to 2–4 mm, the wear rates of the elastomers and plastics further increased. At this test condition, in both the Coriolis scouring erosion and impact erosion, the wear rates of the UHMWPE were higher than those of 1018 steel and the wear rates of the polyurethane were close to those of 1018 steel (Figs. 10 and 11) because the plastic contact deformations on the surfaces of the UHMWPE and the polyurethane became significant. Since the UHMWPE and the polyurethane have much lower hardness than 1018 steel (Table 1) to bear the contact forces from the erodent particles, the erodent particles generated significant cutting and ploughing damages on the two materials. The results of the tests using the angular 2–4 mm erodent demonstrate that elastomers and plastics are not suitable to resist the attacks of large and angular particles. In the Coriolis erosion tests at different slurry temperatures, the tested elastomers and plastics displayed better wear resistance at slurry temperature of 45 °C than that at 23 °C, especially for the elastomers (Fig. 7). The same effect of slurry temperature on wear resistance of elastomers and plastics were also observed in our SJE and large particle slurry erosion tests, which are not presented in this paper, and by other researchers. In the air jet erosion tests conducted by Ashrafizadeh et al. [22], a polyurethane liner material displayed higher wear resistance at 60 °C than 22 °C. Among several possible reasons for the increased wear resistance at elevated temperature, one is the increased values of the H/ E ratio of these materials at elevated temperature. Table 2 gives the values of the H/E ratio of the neoprene rubber and the polyurethane at 23 °C and 60 °C, measured by the instrumented indentation method.
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4. Conclusions Following conclusions can be drawn from current study:
• Elastomers have excellent erosion resistance because of their ex• • • •
cellent resilience to absorb the deformation induced by abrasive/ erosive particles elastically with minimal plastic deformation. Plastics have good erosion resistance because of their moderate resilience to absorb the deformation induced by abrasive/erosive particles elastically with small plastic deformation. Elastomers and plastics are suitable for resisting erosion attacks by small and rounded or semi-rounded particles. When attacked by large and angular particles, their erosion resistance decreases significantly. Elastomers and plastics have displayed increased erosion resistance at elevated temperature. This study demonstrates the importance of selecting a correct laboratory erosion test method and correct test parameters to simulate
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Wear journal homepage: www.elsevier.com/locate/wear
Elastomers and plastics for resisting erosion attack of abrasive/erosive slurries
T
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Yongsong Xie , Jiaren (Jimmy) Jiang, Md. Aminul Islam Mining Wear and Corrosion Laboratory, National Research Council Canada, 4250 Wesbrook Mall, Vancouver, BC V6T 1W5, Canada
A R T I C LE I N FO
A B S T R A C T
Keywords: Elastomers Polyethylene Slurry erosion Erosion tests
Elastomers and plastics are widely used as the materials for handling abrasive/erosive slurries because of their wear resistance, toughness, corrosion resistance and light weight. In this study, slurry jet erosion, Coriolis slurry scouring erosion and large particle slurry erosion are used to characterize the erosion resistance of selected elastomers and plastics, including two natural rubbers, a neoprene rubber, a polyurethane and three types of polyethylene, at different test conditions. The wear rates and wear modes of the tested materials are presented and the relationships between the erosion resistance of these materials and their mechanical properties are discussed. Suitable applications of these materials in slurry transport are also recommended.
1. Introduction Slurry transport is widely used for the transport of solid materials in various industrial operations because it has a number of handling advantages including low cost, minimum maintenance and small environmental impact [1]. In slurry transport, the concentrate of solid materials is mixed with water and then pumped over a distance through pipeline. The slurries are often abrasive or erosive because the solid materials in the slurries are hard and angular. For examples, mining slurry contains crushed ores and oil sands slurry contains silica sands. The mechanical interactions between the slurries and the component surfaces of slurry transport equipment result in wear damage of the equipment. To reduce the maintenance costs incurred for replacing and/or repairing worn components and minimize the losses in production during related equipment down-time, the most cost-effective wear-resistant materials should be applied [2–4]. Among commonly used wear-resistant materials for handling abrasive/erosive slurries, elastomers and plastics are increasingly widely used because of their wear resistance, corrosion resistance, toughness and light weight [5–7]. In order to use elastomers and plastics properly in different slurry transport systems, it is essential to understand their suitable application conditions. However, there is only scarce literature in this field. In the past twenty-two years, the Mining Wear and Corrosion research team at the National Research Council Canada (NRC) has characterized various elastomers and plastics for Canadian mining operations. In this paper, the erosion resistance and wear modes of several elastomers and plastics commonly used in slurry transport are presented and the
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relationships between the erosion resistance of these materials and their mechanical properties are discussed. Then, the suitable application conditions of these elastomers and plastics are recommended. 1.1. Elastomers commonly used for slurry transport Elastomer is a word commonly used to mean any rubber-like compound (natural or synthetic) which can be stretched repeatedly at room temperature to at least twice of its length but return quickly to its approximate original dimension when the applied stress is released. Because of their general similarity to rubbers, polyurethanes are also classified as elastomers. General advantages of elastomers over other wear protection options are in the areas of resilience, toughness, corrosion resistance, ease of fabrication, non-stick and self-lubricating qualities, noise/vibration damping capabilities and light weight. In wear situations, they rely mainly on their elastic properties to absorb the deformation induced by the abrasive or erosive medium elastically with minimal plastic deformation. However, they have clear limitations in the areas of tearing and gouging resistance, strength and ability to withstand elevated temperatures. Elastomers are often used as liners or hoses for slurry transport. Natural rubbers have excellent abrasion and erosion resistance when hydrocarbon and weathering resistance are not required. Neoprene rubbers are good materials where resistance to hydrocarbons and wear are needed [8]. The properties of polyurethanes vary depending on the base polymers. They can exhibit outstanding
Corresponding author. E-mail address: [email protected] (Y. Xie).
https://doi.org/10.1016/j.wear.2019.01.123 Received 31 August 2018; Received in revised form 9 January 2019; Accepted 20 January 2019 0043-1648/ Crown Copyright © 2019 Published by Elsevier B.V. All rights reserved.
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Table 1 Materials tested in this study. Material and Category Elastomers
Plastics
Steels
Natural rubber (40 Shore A) Hardened natural rubber (69 Shore A) Neoprene rubber (53 Share A) Polyurethane (90 Shore A) HDPE XPE UHMWPE Low alloy pipe steel AISI 1018 mild steel AR450 steel
Hardness (H)
Young's modulus (E)
0.62 MPa 1.5 MPa 0.61 MPa 5.6 MPa N/A 3.2 HV (35 MPa) 3.6 HV (39 MPa) 231 HB (2.6 GPa) 254 HB (2.9 GPa) 460 HB (5.2 GPa)
2.48 MPa 14.8 MPa 3.53 MPa 34.2 MPa N/A 958 MPa 791 MPa 200 GPa 200 GPa 200 GPa
varied up to 25 m/s with erodent concentration up to 20 vol% and slurry temperature up to 60 °C. By selecting suitable erodent and test parameters, erosions of many slurry handling components in different service conditions can be simulated. In NRC standard SJE testing procedure, AFS 50–70 silica sand, which is natural silica having a semi-rounded shape with size around 250 µm (Fig. 2(a)), is used as the erodent for testing most materials and the test duration is two hours. For most wear-resistant elastomers and plastics, the semi-rounded AFS 50–70 silica sand generates very small wear volume loss. Hence, to distinguish the wear resistance of these materials, angular 36 mesh SiC particles with size around 500 µm (Fig. 2(b)) are used as erodent and the test duration is extended to four hours. Fig. 3 presents the wear rates of some materials tested in SJE using semi-rounded AFS 50–70 silica sand as erodent. Fig. 4 gives the SJE results of several materials tested using 36 mesh SiC particles as erodent. Both tests were performed at jet velocity of 16 m/s and at room temperature (23 ± 1 °C). The wear volume losses of steel specimens were determined by the mass losses and densities of the specimens. For elastomers and plastics, the wear volume losses were measured directly using a laser profilometer. To minimize any potential effects from water absorption and swelling in the elastomeric and plastic specimens, the tested specimens were dried in a vacuum furnace for more than ten hours before measuring their wear volume losses. When using semi-rounded AFS 50–70 silica sand as erodent, the wear resistance of the tested elastomers and plastics were much better than the steels. When angular and larger SiC particles were used as the erodent, although the wear resistance of these elastomers and plastics were still better than the AR450 steel, all the tested elastomers and plastics displayed wear rates many times higher than those when using semi-rounded AFS 50–70 silica erodent. In contrast, for AR450 steel, when using the angular and larger SiC particles as the erodent, the increase in wear rate is much less than the elastomers and plastics. Fig. 5 shows the worn surfaces of four elastomers and plastics after the SJE tests using 36 mesh angular SiC erodent. On the worn surface of the neoprene rubber (Fig. 5(a)), the main feature is plastic flow along the slurry flow direction. On the worn surfaces of the polyurethane, the UHMWPE and the XPE (Fig. 5(b), (c) and (d)), the main features are cutting and ploughing grooves. On the worn surfaces of the UHMWPE and the XPE (Fig. 5(c) and (d)), some cutting debris are still visible.
mechanical properties, wear resistance and chemical resistance at moderate temperatures [9]. The capability of polyurethanes to be spray coated is a very significant attribute which increases the range of their application. 1.2. Plastics commonly used for slurry transport The use of plastics for wear protection is limited compared to elastomers. Polyethylene (PE), including high density polyethylene (HDPE), cross-linked polyethylene (XPE), high molecular weight polyethylene (HMWPE) and ultra-high molecular weight polyethylene (UHMWPE) are the most used wear-resistant plastics for slurry transport. They have good chemical resistance and their properties increase with molecular weight. Polyethylene have low friction surfaces with non-stick and self-lubricating properties to enhance slurry flow [10]. 2. Erosion resistance of commonly used elastomers and plastics 2.1. Materials Table 1 lists the seven elastomers and plastics studied in this paper, together with three steels for comparison. These elastomers and plastics are selected from materials commonly used for slurry transport. The hardness and Young's modulus values of all these materials are also listed in the table. The Young's modulus values of the elastomers and plastics were measured by the instrumented indentation (also called depth-sensing indentation) method according to ASTM E2546 standard [11] and those of the steels were from the manufacturers. The hardness values of the four elastomers were also measured by the instrumented indentation method, defined as the maximum indentation load divided by the projected area of the indent. For the plastics and steels, hardness values measured by Vickers or Brinell method are converted into the same unit of indentation load divided by projected area and are given in the brackets in Table 1. For the four elastomers, the commonly used Shore A hardness, which is a measure of the elasticity of an elastomer [12], is also given in the table. It should be noted that the properties of the materials belonging to the same material category vary from manufacturer to manufacturer. These selected materials are just some of the elastomers and plastics commonly used in slurry transport. They may not be the representative materials in their categories. 2.2. Erosion resistance assessed by slurry jet erosion
2.3. Erosion resistance assessed by Coriolis slurry scouring erosion
The NRC slurry jet erosion (SJE) test assesses material resistance to slurry attack at various impingement angles and has been proven to provide consistent and reproducible results [5,13]. In the SJE testing, as shown in Fig. 1, the slurry is circulated by a slurry pump from the slurry reservoir. The slurry exiting from the nozzle impacts the specimen surface and then returns to the reservoir. The specimen is clamped in a specimen holder and the holder can be adjusted to provide a selected slurry impingement angle between 10° and 90°. Slurry velocity can be
The Coriolis slurry erosion method was first introduced by Tuzson in 1984 [14] in an effort to develop a test method that simulated the motion of slurries and their interaction with container surfaces, such as slurry pumps or pipes. This method utilises the combination of centrifugal and Coriolis accelerations in a revolving rotor to pass slurry rapidly across a test surface such that the solid particles are forced against the surface, producing wear during their passage. A similar erosion test method has been used for ranking erosion resistant 613
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Fig. 1. Schematics of (a) NRC slurry jet erosion rig and (b) impingement angle.
Fig. 2. SEM images of erodent particles used in SJE tests. (a) AFS 50–70 silica sand and (b) 36 mesh SiC particles.
Fig. 3. Wear rates of selected materials in SJE at impingement angles of 20°, 45° and 90°. Semi-rounded AFS 50–70 silica sand was used as the erodent. The jet velocity was 16 m/s and the slurry temperature was 23 °C.
Fig. 4. Wear rates of selected materials in SJE at impingement angle of 20°. 36 mesh angular SiC particles were used as the erodent. The jet velocity was 16 m/ s and the slurry temperature was 23 °C.
materials of slurry pump components [15]. NRC has developed the Coriolis erosion tester further [16,17], allowing the use of flat plate specimens and accurate volume measurement of the eroded track. The Coriolis erosion method is particularly suitable for evaluating materials involved in handling slurries at locations where solid particles tend to impact a wear surface at a very low impingement angle. The data obtained from Coriolis erosion testing have correlated well with field trials [18,19]. As shown in Fig. 6, the Coriolis rig consists of a diametrically grooved rotor, which rotates at a preselected speed. The slurry is premixed before being fed into the centre of the rotor at a constant rate.
The considerable centrifugal force (Fcent) subsequently causes the slurry to be expelled along two slurry channels and the Coriolis force (Fcor) acts to force the slurry against the specimen surfaces. The impacts and the rebounds of the erodent particles on the specimen surfaces cause most of the particles “jump” along the specimen surface with very low impact angles (< 5 ° according to our modelling [16]). The other particles slide or roll along the specimen surfaces. The velocity with which the slurry traverses across the specimen surfaces increases with the increase in the distance from the centre of rotation. The main specifications of NRC Coriolis erosion tester are as follows: width of
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Fig. 5. SEM images of worn surfaces of the (a) neoprene rubber, (b) polyurethane, (c) UHMWPE and (d) XPE after SJE using 36 mesh SiC erodent. Slurry flow was from right to left.
Fig. 7. Wear rates of selected materials from Coriolis slurry erosion testing using semi-rounded AFS 50–70 silica sand as erodent at slurry temperatures of 23 °C and 45 °C.
Fig. 7 shows the Coriolis erosion test results for selected materials. In this study, 10 wt% AFS 50–70 silica sand + water was used as abrasive slurry. Slurry feed rate was 60 ml/s and the slurry was not reused after exiting the rotor. When traversing across the specimen surfaces, slurry velocity ranged from 15.5 m/s to 26 m/s at 5000 rpm rotor speed. The slurry temperature was 23 °C. Neoprene rubber, polyurethane, UHMWPE, XPE and 1018 steel were also tested at slurry temperature of 45 °C. The test duration was varied (depending on the wear resistance of the materials) to obtain a minimum wear depth of 30 µm. Wear volume losses of the test specimens were measured using a laser profilometer. Same as SJE testing, tested elastomeric and plastic specimens were completely dried in a vacuum furnace before volume loss measurement. The wear rates are normalized with respect to the used erodent amount and the erosion scar length to obtain the unit of mm3/kg/mm (Fig. 7).
Fig. 6. Schematics of the rotor of NRC Coriolis slurry erosion tester, the forces acting on an erodent particle and the optical image of the wear scar generated on a carbon steel.
slurry channels 1 mm, erodent concentration in the slurry up to 40 vol %, slurry temperature up to 60 °C and the rotor speed up to 5000 rpm which gives slurry velocity up to 26 m/s when traversing across the specimen surfaces. 615
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Fig. 8. SEM images of worn surfaces of the (a) neoprene rubber, (b) polyurethane, (c) UHMWPE and (d) XPE after the Coriolis erosion tests. Slurry flow was from right to left. 4.44 kg AFS 50–70 silica sand erodent was used in each test.
Fig. 9. Schematics of the rotor of NRC large particle slurry erosion tester, (a) 3D view and (b) top view.
Fig. 11. Wear rates of selected materials from the centrifugal impact erosion testing using 2–4 mm angular silica particles as erodent at slurry temperature of 23 °C.
Fig. 10. Wear rates of selected materials from the Coriolis scouring erosion testing using 2–4 mm angular silica particles as erodent at slurry temperature of 23 °C.
At 45 °C, the wear resistance of elastomers (polyurethane and neoprene rubber) and plastics (XPE, UHMWPE) are 61 and 58 times better than 1018 steel, respectively. It is interesting to note that both elastomers and plastics displayed better wear resistance at higher temperature.
Average wear resistance of elastomers (polyurethane, neoprene rubber and hard natural rubber) and plastics (XPE, UHMWPE) used in this study are 13 and 32 times better than steels at 23 °C, respectively. 616
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slurry up to 40 vol%, impingement angle on the impact erosion specimen can be adjusted between 10° and 80°, slurry temperature up to 60 °C and the rotor speed up to 3500 rpm which gives slurry velocities up to 26 m/s. Figs. 10 and 11 give the wear rates of some materials in the Coriolis erosion testing and centrifugal impact erosion testing using 2–4 mm angular silica particles (Fig. 12) as erodent. In the testing, the erodent concentration in water was 10 wt% and the slurry feed rate was 100 ml/ s. The slurry was not re-used after exiting the rotor. The rotor speed was 1500 rpm in the centrifugal impact erosion testing, which gives slurry velocity of 11 m/s when impacting the impact erosion specimen surface, and was 3500 rpm in the Coriolis erosion testing, which gives slurry velocity from 16 m/s to 24 m/s when passing the Coriolis erosion specimen surface. The tests were performed at slurry temperature of 23 °C. The test durations depended on the wear resistance of the test materials and wear depths of more than 50 µm were generated. The tested elastomeric and plastic specimens were completely dried in a vacuum furnace before measuring their wear volume losses using laser profilometry. Figs. 13–15 show the worn surfaces of the neoprene rubber, the polyurethane and the UHMWPE after the Coriolis scouring erosion and centrifugal impact erosion at 20° impingement angle by the 2–4 mm angular silica particles. On the worn surfaces of the neoprene rubber (Fig. 13), the dominant damage mechanisms are plastic flow and tearing. On the worn surfaces of the polyurethane, the dominant damage mechanisms are cutting and ploughing. On the worn surfaces of the UHMWPE, the dominant damage mechanism is cutting and the cutting grooves are huge.
Fig. 12. Optical image of silica particles used in the large particle erosion tests.
Increase in wear resistance with temperature is more prominent with elastomers than plastics. An increase in wear resistance of around 3.7 and 1.4 times was observed for elastomers and plastics, respectively with an increase in slurry temperature from 23 °C to 45 °C. In contrast, metallic material (1018 steel) displayed similar wear resistance at the two slurry temperatures (23 °C and 45 °C). UHMWPE and XPE displayed excellent wear resistance in the slurry scouring erosion, which might be attributed to their low friction surfaces with non-stick and self-lubricating properties thus the interactions between the erodent particle and the wearing surfaces are minimal. Fig. 8 shows the worn surfaces of the neoprene rubber, the polyurethane, the UHMWPE and the XPE after the Coriolis erosion tests at slurry temperature of 23 °C. On the worn surface of the neoprene rubber (Fig. 8(a)), ripples appeared in the wear scar, which were the result of plastic flow. On the worn surfaces of the polyurethane, the UHMWPE and the XPE (Fig. 5(b), (c) and (d)), the main features are ploughing and cutting grooves.
3. Discussion The wear test results show that in most of these slurry erosion tests, the studied elastomers and plastics, especially the elastomers, displayed wear resistance much better than the steels. These laboratory wear test results are in agreement with field wear results. For example, high performance polyurethane internal pipe coatings have expanded the design life of slurry pipelines by up to a factor of 10 [9]. The excellent wear resistance of the elastomers and plastics can be mainly attributed to their excellent resilience to absorb deformation induced by the abrasive/erosive particles elastically with minimal plastic deformation. This can be explained by the “plasticity index”, ψ, which can be expressed as [20,21]
2.4. Erosion resistance assessed by large particle slurry erosion When using the NRC SJE tester and Coriolis slurry erosion tester, the erodent particles are smaller than 1 mm and 0.3 mm, respectively. To assess the resistance of materials to attacks by large particles in slurries, a large particle slurry erosion tester was developed at NRC for performing slurry impact erosion testing and slurry scouring erosion testing using erodent particles up to 5 mm [13]. As shown in Fig. 9, the large particle slurry erosion tester has a rotor bigger than that of the Coriolis erosion tester described in previous section. In addition to two scouring erosion specimens, two impact erosion specimens can be clamped at the two ends of the rotor. After exiting from the two slurry channels, the slurry impacts the two impact erosion specimens under the centrifugal force. The main specifications of the tester are as follows: width of slurry channels 10 mm, erodent concentration in the
ψ≈
E ×tan β H
where E is the Young's modulus of the wearing surface, H is the hardness of the wearing surface and β is the average slope of asperities on contacting surface which is equivalent to the average attack angle of
Fig. 13. SEM images of worn surfaces of the neoprene rubber after tests using 2–4 mm angular silica particles, (a) Coriolis scouring erosion and (b) impact erosion at 20° impingement angle. Slurry flow was from right to left. 617
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Fig. 14. SEM images of worn surfaces of the polyurethane after tests using 2–4 mm angular silica particles, (a) Coriolis scouring erosion and (b) impact erosion at 20° impingement angle. Slurry flow was from right to left.
Fig. 15. SEM images of worn surfaces of the UHMWPE after tests using 2–4 mm angular silica particles, (a) Coriolis scouring erosion and (b) impact erosion at 20° impingement angle. Slurry flow was from right to left.
abrasive/erosive particles. The higher the ψ, the more severe the plastic contact is. The reciprocal of the material parameter of the plasticity index, H/E, is proportional to the maximum elastic strain beyond which plastic contact deformation occurs. Therefore, the H/E ratio is a measure of the resistance of a material to plastic contact deformation. Fig. 16 gives the values of H/E ratio of the tested materials calculated from the H and E values given in Table 1. For the elastomers, which have much higher values of the H/E ratio than those of the steels, their surface deformation is much more likely to be elastic during the erosion tests, thus they displayed much better erosion resistance than the steels. For the plastics, although their values of the H/E ratio are lower than those of the elastomers, they are higher than the steels thus displayed better erosion resistance than the steels. Please note that, because the erosion resistance of elastomers and plastics is also determined by some other material properties (such as viscoelasticity), the measured erosion rates of the tested materials do not match the H/E results exactly. According to the images of erodent particles used in the wear tests (Figs. 2 and 12), the shapes of these erodent particles can be approximately regarded as spheres or polyhedrons with spherical tips at the corners. In the erosion tests, with the increase in the angularity of an erodent particle or the impact energy of the erodent particle (increased particle mass or particle velocity normal to the wearing surface, Vn) which results in increased penetration depth of the erodent particle, the attack angle β increases, as shown in Fig. 17. When using the small, semi-rounded AFS 50–70 silica sand as erodent, the deformations induced by the erodent particles on the wearing surfaces of the elastomer and plastic specimens were essentially elastic. Only some extremely sharp corners of the erodent particles caused plastic deformation. Therefore, the elastomers and plastics displayed very low wear rates which were many times lower than those of the steels (Figs. 3 and 7). At
Fig. 16. H/E values of the tested materials.
Fig. 17. Schematics of the shapes of erodent particles and the attack angles of the erosive particles.
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the service condition of slurry equipment for which the tested material will be used. The important test parameters include abrasive/ erosive particles (shape, size, hardness and density), slurry moving conditions (velocity and impingement angle) and temperature.
Table 2 Values of the H/E ratio of the neoprene rubber and polyurethane measured at two temperatures. Material
Temperature
H (MPa)
E (MPa)
H/E
Neoprene rubber
23 °C 60 °C 23 °C 60 °C
0.61 0.44 5.6 3.5
3.53 2.35 34.2 18.6
0.173 0.187 0.164 0.188
Polyurethane
Acknowledgement The authors would like to express their thanks to the members of the NRC/Industry Mining Wear Materials Research Consortium for their support of this work. They also wish to thank their NRC colleagues for their contributions for obtaining the test results.
the same SJE test conditions but changing the erodent particles from the AFS 50–70 silica sand to the larger and more angular 36 mesh SiC particles, the wear rates of the elastomers and plastics greatly increased (Figs. 3 and 4). Further increasing the sizes of the angular erodent particles to 2–4 mm, the wear rates of the elastomers and plastics further increased. At this test condition, in both the Coriolis scouring erosion and impact erosion, the wear rates of the UHMWPE were higher than those of 1018 steel and the wear rates of the polyurethane were close to those of 1018 steel (Figs. 10 and 11) because the plastic contact deformations on the surfaces of the UHMWPE and the polyurethane became significant. Since the UHMWPE and the polyurethane have much lower hardness than 1018 steel (Table 1) to bear the contact forces from the erodent particles, the erodent particles generated significant cutting and ploughing damages on the two materials. The results of the tests using the angular 2–4 mm erodent demonstrate that elastomers and plastics are not suitable to resist the attacks of large and angular particles. In the Coriolis erosion tests at different slurry temperatures, the tested elastomers and plastics displayed better wear resistance at slurry temperature of 45 °C than that at 23 °C, especially for the elastomers (Fig. 7). The same effect of slurry temperature on wear resistance of elastomers and plastics were also observed in our SJE and large particle slurry erosion tests, which are not presented in this paper, and by other researchers. In the air jet erosion tests conducted by Ashrafizadeh et al. [22], a polyurethane liner material displayed higher wear resistance at 60 °C than 22 °C. Among several possible reasons for the increased wear resistance at elevated temperature, one is the increased values of the H/ E ratio of these materials at elevated temperature. Table 2 gives the values of the H/E ratio of the neoprene rubber and the polyurethane at 23 °C and 60 °C, measured by the instrumented indentation method.
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4. Conclusions Following conclusions can be drawn from current study:
• Elastomers have excellent erosion resistance because of their ex• • • •
cellent resilience to absorb the deformation induced by abrasive/ erosive particles elastically with minimal plastic deformation. Plastics have good erosion resistance because of their moderate resilience to absorb the deformation induced by abrasive/erosive particles elastically with small plastic deformation. Elastomers and plastics are suitable for resisting erosion attacks by small and rounded or semi-rounded particles. When attacked by large and angular particles, their erosion resistance decreases significantly. Elastomers and plastics have displayed increased erosion resistance at elevated temperature. This study demonstrates the importance of selecting a correct laboratory erosion test method and correct test parameters to simulate
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