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Experimental setup for testing and mapping of high temperature abrasion and oxidation synergy
Wear 267 (2009) 1798–1803
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Experimental setup for testing and mapping of high temperature abrasion and oxidation synergy Maksim Antonov ∗ , Irina Hussainova Department of Materials Engineering, Tallinn University of Technology, Ehitajate tee 5, 19086, Tallinn, Estonia
a r t i c l e
i n f o
Article history: Received 23 September 2008 Received in revised form 20 January 2009 Accepted 22 January 2009 Available online 14 February 2009 Keywords: High temperature Cermet Abrasion Oxidation Mapping
a b s t r a c t Performance and lifetime of engineering materials at high temperature are affected by degradation of a material under wear and corrosion to a great extent. To assess the material performance at high temperature, the most detrimental processes and their interactions should be known and understood for materials selection and design of new advanced materials. The present study introduces an experimental setup for testing and mapping of high temperature abrasion taking into consideration the process of oxidation. A new design of a test rig has been developed at Tallinn University of Technology to provide synergy study of wear and oxidation and to improve the effectiveness of control and monitoring the mechanisms of materials failure at room and high temperatures (up to 1000 ◦ C). Methodology for assessment and mapping of the effects of abrasion and corrosion on materials performance are presented along with some results obtained for high temperature abrasion of titanium carbide- and chromium carbide-based cermets as well as for steel. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Abrasive wear of materials under high temperatures is a very complex process that can be affected by many factors including prevailing mechanisms of materials degradation, corrosion–oxidation resistance and intensity of a process. The rate of corrosion is usually accelerating with temperature increase especially for metals and materials of high metallic phase content [1–3]. However, to a great extent the oxidation–corrosion process is controlled by a layer formed on the surface. The properties of such layer depend on temperature, surrounding environment and rate of its formation [4–6]. In the absence of corrosion, abrasion resistance of materials depends mostly on ability of surface to resist penetration, scratching, ploughing, etc. that in its turn can be estimated by the complex of mechanical properties such as hardness, strength, fracture toughness, etc. The energy or heat generated during the friction between materials is distributed between the solid body of a material, conterbody and environment. Materials of low thermal conductivity can hardly dissipate heat sufficiently; therefore there is a heat concentration at contact points. In some cases the temperatures equal or higher than a melting point of a material may be achieved at contacts. Such local increase in temperature influences the mechanical properties of a given material [7].
∗ Corresponding author. Tel.: +372 6203355; fax: +372 6203196. E-mail address: [email protected] (M. Antonov). 0043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2009.01.008
When material application involves abrasion at high temperatures, a combination of mechanical and chemical processes determines material durability. The mechanism of material loss in the presence of a surface layer is very complex and depends on a passive oxide film formation and removal and kinetics of corrosion under given conditions. In the context of the present study, the process of material degradation is the sum of the mechanical abrasion of a material surface and the subsequent deterioration of the mechanical damage and oxidation. When wear and corrosion acting concurrently, total material wastage may be estimated as the sum of four components: “pure” corrosion (in absence of wear), “pure” wear (in absence of corrosion), the effect of corrosion on wear and the effect of wear on corrosion [8–11]. The most unwanted are conditions when one of these processes is accelerated by the other one. However the conditions when (a) the oxide layer has better wear resistance than bare material or (b) the wear disabling the critical oxide film thickness growth and preventing its chipping and leading to the decreasing of material wastage are known and are the most desirable [9]. The fastening of the specimen and the scheme of specimenabrasive interaction during high temperature testing are critical tasks influencing the reproducibility of the real conditions. The present study aims in understanding of the response of material to the abrasive wear and development of the tester for investigation of synergy between high temperature abrasion and oxidation. Such kind of conditions is critical in energy applications involving transport of ash or other hard inclusions and high temperature gases [12–14].
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Several high temperature test schemes have been proposed in literature. For example, in [15,16] the apparatus using a particles jet attacking flat surface is applied to study the erosion resistance. The area subjected to wear and the intensity both depend on distribution of abrasive particles along the diameter of the jet. Using such setup, some part of the tested specimen is impacted by abrasive particles while the rest of the surface experiences corrosion. Area where specimen is in contact with holder undergoes corrosion of different mode (slot corrosion, for example) as compared to non-contacted areas. Usually it is one specimen test. In [14], the erosion inside the tubes has been investigated. Rate of abrasion is minimal on straight paths and pronounced in elbows. The tube itself is a test material at a large scale. The pin moving inside the abrasive bath has been studied in [17]. The pin of angular shape possesses sufficiently higher wear at the edges than at plain surfaces. The impact angle is varying along the circumference of cylindrical shaped pins. The rear part of pin is usually not subjected to action of abrasive particles. The fastening zone may be of temperature that is lower than test temperature due to the fact that the heat flow and the corrosion may be different in different areas. Testing of several specimens (materials under investigation as well as standard ones) simultaneously is preferred since it allows studying the effect of minor differences in mechanical or corrosion properties as well as controlling the test regime according to the behaviour of standard material with known wear rates. The transport of abrasive in real life may be accompanied by the long waiting period when the material undergoes mainly corrosion leading to growth of oxide layer. During the initiation of abrasive particles jet flow the material removal rate can differ from that of continuous wear-corrosion action and lead to unexpectedly short lifetime of components. This work was initiated to search for some suitable methods enabling the evaluation of the effect of various parameters on resistance of high temperature materials to wear and corrosion and to study interaction between these processes.
help of the pairs of disks with specially designed edges (2). The disks are hold together by tiny bolts which can be easily removed by wire-cutters in the case of excessive corrosion at high temperature. After tightening of the bolts there is a gap of minimum 0.1 mm between sample and disk to impede the formation of galvanic pairs and slot corrosion. Up to18 samples can be tested simultaneously in each chamber. The disks are fixed to the shaft (3) and rotated by the electrical motor (7) through reduction gear (8). The amount of abrasive can be varied (but its level should stay below the level of bearings (11) and the shaft) thus changing the duration and intensity of abrasive action. This design enables to maintain the temperature of the sample, specimen holder and abrasives at the same level during the test. The reduction gear enables to make testing at low frequency of rotation to study the interaction between oxidation and wear. Chamber temperature and motor frequency controllers (5 and 9) are used to provide the required test conditions. Coupling of the device (10) provides setting of the maximum torque transmitted through the shaft and disconnection of the reduction gear when opening of the door is necessary (15) to get the samples. The disks, chamber, shaft and three steel–steel sliding bearings (11) are made of 1.4746 (DIN X8CrTi25) heat resistant steel with good welding properties. Maximum service temperature is 1100 ◦ C. The heating elements (12) are distributed in the lower part while the thermal sensor (thermocouple, 13) is located in the upper part of the oven. The supporting area of sliding “bearings” is increased to diminish the wear. Design of the upper part is made to lead the abrasive away, i.e. to avoid the presence of abrasive in bearing zone. Pads (14) made of thermal insulation material with high compression strength are placed around the chambers (6) in order to prevent the damage of the interior of the oven due to vibration. The main features of the test rig are presented in Table 1.
2. Description of the tester
Materials of some metals content exhibit the weight gain during the corrosion in gaseous environment. However, wear usually results in weight loss. The oxidation rate of a material KO is measured firstly at a separate test in order to account the weight gain due to corrosion during the following test for measuring the wear rate WAO at the same temperature. The equation for the calculation of the wear rate at high temperature for the designed tester is (see
2.1. Design of the tester Fig. 1 is a schematic of the abrasive-oxidation wear testing setup. Heated inside the electrical oven (4), the test is run in two chambers filled with abrasive (6). The samples (1) are fastened with the
2.2. Methodology of the high temperature wear rate calculation for the designed tester
Fig. 1. (a) Scheme of the high temperature multi sample, two chamber abrasion tester; (b) photo of the main unit taken out of the oven. Explanations—see the text. Sizes are given in millimetres.
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Table 1 Main design features and working parameters of tester. Feature/parameter
Description Available
Tested
Size of one chamber (L × W × H), mm Diameter of disk/specimens centre location, mm Size of specimen (L × W × H), mm Area of specimen subjected to wear, cm2 Area of specimen that is not subjected to wear, cm2 Frequency of disk rotation, s−1 Speed of specimen movement, m s−1 Number of specimens tested in one chamber Number of chambers with abrasive Abrasive, properties
125 × 120 × 120 105/77 25 × 15 × 5 4.5 4.8 0.01–0.40 0.004–0.1 3 × 6 = 18 2 Loose abrasive 0–3 mm
Temperature, ◦ C Duration of test, h Environment
20–1000 Limited by wear of bearings and disks Air
also Figs. 1 and 2): WAO =
W − (AOO KO tAO ) AAO tAO
(1)
where KO =
WO AO tO
(2)
In Eqs. (1) and (2) AO —area of the sample exposed to oxidation, 9.3 cm2 ; AOO —area of the sample protected against wear, 4.5 cm2 ; AAO —area of the sample subjected to wear (not protected), 4.8 cm2 ; KO —oxidation rate, mg cm−2 h−1 ; tO —duration of the oxidation test, h; tAO —duration of the wear test, h; WO —materials wastage during the oxidation test, mg; W—materials wastage during the wear test, mg; WAO —wear rate corrected for the designed tester, mg cm−2 h−1 . The wear and oxidation rates have positive sign if the weight of specimen is decreased during the test. The negative sign of the wear or corrosion rate indicates that the specimen has gained the weight during the test. 2.3. Estimation of the specific wear rate and mean pressure of abrasive The units in which the wear and corrosion rates are expressed (mg cm−2 h−1 ) throughout this paper are more characteristic to corrosion measurements and less to monitoring of wear. This is done in order to have the corrosion and wear rates expressed in the same units and to facilitate the use of given equations (Eqs. (1), (2), (5) and (6)). However, it is of interest to convert the wear results into the specific wear rate that is given in mm3 N−1 m−1 . The equation for the conversion is presented in the following chapter with the explanation of other characteristic parameters of the designed tester. The curvature of the abrasive bed was measured after the test at 0.01 and 0.05 m s−1 for SiC and SiO2 abrasives (Fig. 3). The angle ˇ that characterizes the duration of the specimens centre immersion to the abrasive, shown by positions A and C, was determined
Fig. 2. Location of specimen’s surfaces subjected to oxidation only (AOO ) and abrasion plus oxidation (AAO ) during wear test.
0.04, 0.21 0.01, 0.05 3×6 2 SiO2 , 0.2–0.4 mm, rounded, A = 2150 kg m−3 , 700 g per test SiC, 1.0–2.0 mm, angular, A = 3300 kg m−3 , 700 g per test 20, 400, 700, 900 5h
with the help of AutoCAD 2004 program as 98◦ ± 3◦ for the range of conditions tested. During disk rotation the abrasive is shifted due to the friction forces between disk and abrasive particles. The curvature shown in Fig. 3 represents the equilibrium of interplay of the friction forces, gravity force, rolling and sliding downhill, interlocking of abrasive, etc. Visual observations with the use of single painted abrasive grains has shown that the abrasive bed could be treated as stationary and the speed of the specimen relative to the abrasive bed could be determined as the angular velocity of the disks multiplied by the radial distance to the specimens centre. However, each abrasive grain in contact with rotating disk is forced to limited rotation and migration due to interlocking with surrounding grains. To calculate the specific wear rate through WAO the assessment of the mean pressure of abrasive particles acting normal to the specimen’s surface subjected to wear (AAO ) is necessary. The following assumptions should be made: 1. The mean pressure is calculated through the hydrostatic pressure. 2. The centre of sample is used for specifying its position. 3. The depth of the immersion into the abrasive h is determined in the middle of the duration of immersion (position B in Fig. 3, when ˇ = 49◦ ).
Fig. 3. Scheme of the test chamber showing specimen movement within the abrasive. Sizes are given in millimetres.
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4. The packing density of abrasive is 0.6 that is typical for almost spherical particles of this size. Although abrasive particles vibration influences the packing density, it typically can increase or decrease the density by not more than 0.05 [18–20]. The mean pressure of abrasive may be expressed as following: p = hA KP g
(3)
where p—mean pressure of abrasive, N m−2 ; h—immersion depth of the sample during wear; A —bulk density of the abrasive, kg m−3 ; KP —packing density of abrasive bed (0 ≤ KP ≤ 1); g—standard gravitational acceleration. For the conditions described in Table 1, the mean abrasive pressure can be calculated as following: pSiO2 = 0.02 · 2150 · 0.6 · 9.8 = 253 N m−2 Fig. 4. Schematic representation of the abrasive–corrosive wastage modes.
pSiC = 0.02 · 3300 · 0.6 · 9.8 = 388 N m−2 The specific wear rate is then expressed in the following form: k=
WAO 2778 pv(ˇ/360)M
(4)
where k—specific wear rate, mm3 N−1 m−1 ; WAO —wear rate, mg cm−2 h−1 ; v—speed of specimen movement, m s−1 ; ˇ—the duration of the specimens centre immersion to the abrasive, degrees; M —density of specimen, kg m−3 . The correction factor taking into account the conversion of units is equal to 2778. For example, the calculated k the for the AISI 316 stainless steel abraded at speed of 0.05 m s−1 by a sand particles at room temperature (WAO is 10.4 × 10−3 mg cm−2 h−1 —see Fig. 5) is: kAISI 316 =
10.4 × 10−3 2778 = 1 × 10−3 mm3 N−1 m−1 253 · 0.05(98/360)7950
The range of the specific wear rates of AISI 316 stainless steel in dry conditions by loosen abrasive at room temperature varies from 0.25 × 10−3 mm3 N−1 m−1 (pin on top of the lath with loosen abrasive [21]) and up to 19 × 10−3 mm3 N−1 m−1 (ASTM G65 rubber wheel tests [22]). This indicates that the wear rate determined by the presented tester is comparable with the results obtained using other schemes of testers and that the efficiency of material removal through the action of abrasive particles is comparably low. 3. Estimation of abrasive action effect on material wastage For the description of the effect of the abrasive action on material wastage at high temperature three possible modes are proposed (Fig. 4 and Eq. (5)): • Corrosion dominated mode is characterized by (ABR = 0, Eq. (5)) ineffective material loss due to abrasion or removal of the oxide film from the surface of the sample. • Corrosion–abrasion mode with prevailing corrosion (ABR is minimal) is characterized by slight material degradation due to abrasion and high rate of oxidation so that material wastage still has a negative sign. • The positive sign of material wastage rate points to a strong abrasive action (ABR is high). ABR = WAO − KO , mg cm−2 h−1
(5)
To assess the effect of abrasion on the wear rate of materials at a high temperature it is convenient to compare it with the corrosion
rate (Eq. (6)). Synergy =
ABR |KO |
(6)
4. Test conditions and results The set of corrosion and abrasion–corrosion tests were performed at room, elevated and high temperature as indicated in Table 1. Materials tested in the present study were TiC–NiMo cermets produced at Tallinn University of Technology by means of a conventional powder metallurgy routine described elsewhere [23]. Materials of different binder metal composition and contents were chosen as the test materials. Cr3 C2 –Ni cermet and conventional AISI 316 stainless steel were tested for comparison. The description of the materials properties and their performance (effect of binder content, binder composition, abrasive, sliding velocity, characteristic wastage mechanisms, etc.) is planned to be published in more details as a separate paper. The choice of the test temperatures was influenced by the oxidation kinetic of binder and ceramic phases in air environment. Below 400 ◦ C the oxidation of metal is hardly measured. At 700 ◦ C the oxidation of the metal phase is at sufficiently high level and ceramic phase could have only some minor oxidation. At 900 ◦ C there is dramatic oxidation of metal and moderate of ceramic phase. Each sample was step by step tested in all conditions that would facilitate the assessment of materials performance under various conditions. The sequence of operations at every temperature selected for testing included: the grinding of the specimen, evaluation of oxidation rate during 15 min, oxidation during 45 min, oxidation during 4 h, regrinding, testing at low velocity, regrinding, testing at high velocity. The oxidation tests of different duration were done to see the kinetic of oxidation process. The mass gain (oxidation) rate based on 5 h (the same duration as that of the wear test) was used for calculation of WAO , ABR and Synergy. Before the wear test, the specimen had always fresh (not oxidized) surface. During the regrinding the layer of 0.015–0.025 mm was removed from each face. The weight of the specimens before and after the test was measured with the help of A and D GR-202 electronic scales with 0.01 mg resolution and automatic recording by computer (RS-232C type connection). Mapping is a powerful tool for material selection, optimisation of working conditions and understanding of materials response under wide ranges of conditions. Estimation of the materials durability and reliability give the ideas for new materials design [24].
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Fig. 5. Temperature dependant mapping of the material degradation under conditions of oxidation and high temperature abrasion. Abrasive—sand; velocity—0.05 m s−1 . Values of W are given in ×10−3 mg; values for KO , WAO are given in ×10−3 mg cm−2 h−1 .
The map presented in Fig. 5 displays some selected results in order to explain the parameters (ABR, Synergy) introduced in the present paper. The map shown in Fig. 5 indicates the areas of high and medium synergy. W is the difference between samples weight measured before and after testing. Wear wastage is positive at low temperature and becomes negative at high temperatures of 700 ◦ C and 900 ◦ C. This fact evidences that sample gains the weight during wear testing at high temperature. That may be explained by the effect of corrosion. However, SEM imaging of the materials surface has revealed the events of material removal that are characteristic features of abrasive wear even at low velocities and low pressure of abrasive (Fig. 6a). On the other hand, the gain in weight is possible due to the embedding of the abrasive dust into the surface facilitated by sufficient softening of metal phase at high temperature, Moreover, it should be noticed that the sample parts protected by fixing clampers may contribute to mass gain of oxidising materials that influence the results sufficiently. WAO is a corrected wear rate. It takes into account the existence of two areas with different mechanisms of degradation (AOO and AAO ). The wear rate of the chromium carbide based cermets is lower
than that of titanium carbide based ones at high temperatures. The wear of AISI 316 stainless steel is comparable to that of Cr3 C2 -based cermets at 400 and 700 ◦ C but is much higher at 900 ◦ C. The corrosion rate (KO ) of Cr3 C2 based cermets at room and even at 400 ◦ C is very low. It is higher for materials with high binder content. Corrosion rate of TiC-based cermet and stainless steel is comparably higher. Increase in corrosion rate of these materials is dramatic at high temperature. Oxide layer on the surface of TiC–NiMo cermet is cracked (Fig. 6b) and had a low adhesion to the substrate. To assess the effectiveness of the abrasive action as compared to corrosion, the Synergy value has been calculated. For materials tested, the Synergy was not lower than 1.4 demonstrating that the abrasive particles are able to remove the corrosion products from the surface quite efficiently. Wear rates for TiC–NiMo cermet and AISI 316 stainless steel at 900 ◦ C are both very high. However, the Synergy value of the steel is extremely high at this temperature indicating that the abrasion is the leading process acting much more effectively than corrosion compared to other conditions tested. The highest Synergy values have been obtained for Cr3 C2 –Ni cermets at 400 ◦ C. These
Fig. 6. SEM micrograph of 70TiC–24Ni 6Mo wt% after wear test at 700 ◦ C in sand at 0.05 m s−1 (a) and after 5 h of oxidation at 700 ◦ C (b). The cracking of the surface layer is shown by the arrows.
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materials are almost inert at this temperature and abrasion is only the mechanism of degradation. The Synergy value proposed in this study represents the effect of abrasive action on the wear rate compared to the corrosion rate of material. To obtain the value of abrasive wear at high temperature in the absence of corrosion and the effect of abrasive action on corrosion rate are also of a great interest and importance. Abrasive wear can be studied during the test within the protective media. 5. Conclusions • The device for testing of as much as 36 specimens simultaneously in abrasive conditions at room and high temperatures (up to 1000 ◦ C) is designed and fabricated. • The methodology for calculation of the wear rate for the given design is presented. • The performance results enable us to arrange studied materials in the row of decrease of resistance to wear at 900 ◦ C as following: 90Cr3 C2 –10Ni-60Cr3 C2 –40Ni-70TiC–24Ni6Mo-AISI 316. The effect of abrasion on the wear rate of stainless steel at this temperature compared with the corrosion rate was the highest. Acknowledgements The authors wish to thank D.Eng. J. Pirso for help with samples manufacturing. Paroc Eesti AS is appreciated for supplying high temperature insulation slab. Estonian Science Foundation under grants nos. G6660, MT and SF T062 is acknowledged for financial support of this research. References [1] L.L. Shreir, R.A. Jarman, G.T. Burstein (Eds.), Corrosion, 3rd edition, Elsevier, 1994. [2] J.R. Blachere, F.S. Pettit, High Temperature Corrosion of Ceramics, William Andrew Publishing/Noyes, 1989. [3] D.J. DeRenzo, Corrosion Resistant Materials Handbook, William Andrew Publishing/Noyes, 1985. [4] B.D. Craig, Fundamental Aspects of Corrosion Films in Corrosion Science, Plenum Press, New York, 1990.
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[5] C. Brundle, C. Evans, S. Wilson, Encyclopedia of Materials Characterization— Surfaces, Interfaces Thin Films, Elsevier, 1992. [6] G.W. Stachowiak (Ed.), Wear. Materials, Mechanisms and Practice, John Wiley & Sons, 2005. [7] G.W. Stachowiak, A.W. Batchelor, Engineering Tribology, Second edition, Butterworth-Heinemann, 2001. [8] M.M. Stack, N. Pungwiwat, Erosion–corrosion mapping of Fe in aqueous slurries: some views on a new rationale for defining the erosion–corrosion interaction, Wear 256 (2004) 565–576. [9] S.W. Watson, F.J. Friedersdorf, B.W. Madsen, S.D. Cramer, Method of measuring wear-corrosion synergism, Wear 181–183 (1995) 476–484. [10] J.O. Bello, R.J.K. Wood, J.A. Wharton, Synergistic effects of micro-abrasion– corrosion of UNS S30403, S31603 and S32760 stainless steels, Wear 263 (2007) 149–159. [11] ASTM standard G119-93 (1998): standard guide for determining synergism between wear and corrosion. [12] B.-Q. Wang, K. Luer, The relative erosivity of limestone, dolomite and coal samples from an operating boiler, Wear 215 (1998) I80–190. [13] D. Jianxin, D. Zeliang, Y. Dongling, Erosion wear mechanisms of coal–water– slurry (CWS) ceramic nozzles in industry boilers, Materials Science and Engineering 417 (2006) 1–7. [14] T. Deng, J. Lib, A.R. Chaudhry, M. Patel, I. Hutchings, M.S.A. Bradley, Comparison between weight loss of bends in a pneumatic conveyor and erosion rate obtained in a centrifugal erosion tester for the same materials, Wear 258 (2005) 402–411. [15] M. Suckling, C. Allen, Critical variables in high temperature erosive wear, Wear 203–204 (1997) 528–536. [16] A.L. Ham, J.A. Yeomans, J.F. Watts, Elevated temperature solid particle erosion of silicon carbide continuous fibre reinforced calcium aluminosilicate glassceramic matrix composite, Wear 203–204 (1997) 387–392. [17] P. Hou, S. MacAdamb, Y. Niuc, J. Stringerd, High temperature degradation by erosion–corrosion in bubbling fluidized bed combustors, Materials Research 7 (2004) 71–80. [18] A.B. Yu, J. Bridgwater, A. Burbidge, On the modelling of the packing of fine particles, Powder Technology 92 (1997) 185–194. [19] X.Z. An, R.Y. Yang, R.P. Zou, A.B. Yu, Effect of vibration condition and interparticle frictions on the packing of uniform spheres, Powder Technology 188 (2008) 102–109. [20] X. Jia, M. Gan, R.A. Williams, D. Rhodes, Validation of a digital packing algorithm in predicting powder packing densities, Powder Technology 174 (2007) 10–13. [21] M.S. Bingley, S. Schnee, A study of the mechanisms of abrasive wear for ductile metals under wet and dry three-body conditions, Wear 258 (2005) 50–61. [22] K.G. Budinski, Resistance to particle abrasion of selected plastics, Wear 203–204 (1997) 302–309. [23] I. Hussainova, Effect of microstructure on the erosive wear of titanium carbidebased cermets, Wear 255 (2003) 121–128. [24] M.M. Stack, Bridging the gap between tribology and corrosion: from wear maps to Pourbaix diagrams, International Materials Reviews 50 (2005) 1–17.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Experimental setup for testing and mapping of high temperature abrasion and oxidation synergy Maksim Antonov ∗ , Irina Hussainova Department of Materials Engineering, Tallinn University of Technology, Ehitajate tee 5, 19086, Tallinn, Estonia
a r t i c l e
i n f o
Article history: Received 23 September 2008 Received in revised form 20 January 2009 Accepted 22 January 2009 Available online 14 February 2009 Keywords: High temperature Cermet Abrasion Oxidation Mapping
a b s t r a c t Performance and lifetime of engineering materials at high temperature are affected by degradation of a material under wear and corrosion to a great extent. To assess the material performance at high temperature, the most detrimental processes and their interactions should be known and understood for materials selection and design of new advanced materials. The present study introduces an experimental setup for testing and mapping of high temperature abrasion taking into consideration the process of oxidation. A new design of a test rig has been developed at Tallinn University of Technology to provide synergy study of wear and oxidation and to improve the effectiveness of control and monitoring the mechanisms of materials failure at room and high temperatures (up to 1000 ◦ C). Methodology for assessment and mapping of the effects of abrasion and corrosion on materials performance are presented along with some results obtained for high temperature abrasion of titanium carbide- and chromium carbide-based cermets as well as for steel. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Abrasive wear of materials under high temperatures is a very complex process that can be affected by many factors including prevailing mechanisms of materials degradation, corrosion–oxidation resistance and intensity of a process. The rate of corrosion is usually accelerating with temperature increase especially for metals and materials of high metallic phase content [1–3]. However, to a great extent the oxidation–corrosion process is controlled by a layer formed on the surface. The properties of such layer depend on temperature, surrounding environment and rate of its formation [4–6]. In the absence of corrosion, abrasion resistance of materials depends mostly on ability of surface to resist penetration, scratching, ploughing, etc. that in its turn can be estimated by the complex of mechanical properties such as hardness, strength, fracture toughness, etc. The energy or heat generated during the friction between materials is distributed between the solid body of a material, conterbody and environment. Materials of low thermal conductivity can hardly dissipate heat sufficiently; therefore there is a heat concentration at contact points. In some cases the temperatures equal or higher than a melting point of a material may be achieved at contacts. Such local increase in temperature influences the mechanical properties of a given material [7].
∗ Corresponding author. Tel.: +372 6203355; fax: +372 6203196. E-mail address: [email protected] (M. Antonov). 0043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2009.01.008
When material application involves abrasion at high temperatures, a combination of mechanical and chemical processes determines material durability. The mechanism of material loss in the presence of a surface layer is very complex and depends on a passive oxide film formation and removal and kinetics of corrosion under given conditions. In the context of the present study, the process of material degradation is the sum of the mechanical abrasion of a material surface and the subsequent deterioration of the mechanical damage and oxidation. When wear and corrosion acting concurrently, total material wastage may be estimated as the sum of four components: “pure” corrosion (in absence of wear), “pure” wear (in absence of corrosion), the effect of corrosion on wear and the effect of wear on corrosion [8–11]. The most unwanted are conditions when one of these processes is accelerated by the other one. However the conditions when (a) the oxide layer has better wear resistance than bare material or (b) the wear disabling the critical oxide film thickness growth and preventing its chipping and leading to the decreasing of material wastage are known and are the most desirable [9]. The fastening of the specimen and the scheme of specimenabrasive interaction during high temperature testing are critical tasks influencing the reproducibility of the real conditions. The present study aims in understanding of the response of material to the abrasive wear and development of the tester for investigation of synergy between high temperature abrasion and oxidation. Such kind of conditions is critical in energy applications involving transport of ash or other hard inclusions and high temperature gases [12–14].
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Several high temperature test schemes have been proposed in literature. For example, in [15,16] the apparatus using a particles jet attacking flat surface is applied to study the erosion resistance. The area subjected to wear and the intensity both depend on distribution of abrasive particles along the diameter of the jet. Using such setup, some part of the tested specimen is impacted by abrasive particles while the rest of the surface experiences corrosion. Area where specimen is in contact with holder undergoes corrosion of different mode (slot corrosion, for example) as compared to non-contacted areas. Usually it is one specimen test. In [14], the erosion inside the tubes has been investigated. Rate of abrasion is minimal on straight paths and pronounced in elbows. The tube itself is a test material at a large scale. The pin moving inside the abrasive bath has been studied in [17]. The pin of angular shape possesses sufficiently higher wear at the edges than at plain surfaces. The impact angle is varying along the circumference of cylindrical shaped pins. The rear part of pin is usually not subjected to action of abrasive particles. The fastening zone may be of temperature that is lower than test temperature due to the fact that the heat flow and the corrosion may be different in different areas. Testing of several specimens (materials under investigation as well as standard ones) simultaneously is preferred since it allows studying the effect of minor differences in mechanical or corrosion properties as well as controlling the test regime according to the behaviour of standard material with known wear rates. The transport of abrasive in real life may be accompanied by the long waiting period when the material undergoes mainly corrosion leading to growth of oxide layer. During the initiation of abrasive particles jet flow the material removal rate can differ from that of continuous wear-corrosion action and lead to unexpectedly short lifetime of components. This work was initiated to search for some suitable methods enabling the evaluation of the effect of various parameters on resistance of high temperature materials to wear and corrosion and to study interaction between these processes.
help of the pairs of disks with specially designed edges (2). The disks are hold together by tiny bolts which can be easily removed by wire-cutters in the case of excessive corrosion at high temperature. After tightening of the bolts there is a gap of minimum 0.1 mm between sample and disk to impede the formation of galvanic pairs and slot corrosion. Up to18 samples can be tested simultaneously in each chamber. The disks are fixed to the shaft (3) and rotated by the electrical motor (7) through reduction gear (8). The amount of abrasive can be varied (but its level should stay below the level of bearings (11) and the shaft) thus changing the duration and intensity of abrasive action. This design enables to maintain the temperature of the sample, specimen holder and abrasives at the same level during the test. The reduction gear enables to make testing at low frequency of rotation to study the interaction between oxidation and wear. Chamber temperature and motor frequency controllers (5 and 9) are used to provide the required test conditions. Coupling of the device (10) provides setting of the maximum torque transmitted through the shaft and disconnection of the reduction gear when opening of the door is necessary (15) to get the samples. The disks, chamber, shaft and three steel–steel sliding bearings (11) are made of 1.4746 (DIN X8CrTi25) heat resistant steel with good welding properties. Maximum service temperature is 1100 ◦ C. The heating elements (12) are distributed in the lower part while the thermal sensor (thermocouple, 13) is located in the upper part of the oven. The supporting area of sliding “bearings” is increased to diminish the wear. Design of the upper part is made to lead the abrasive away, i.e. to avoid the presence of abrasive in bearing zone. Pads (14) made of thermal insulation material with high compression strength are placed around the chambers (6) in order to prevent the damage of the interior of the oven due to vibration. The main features of the test rig are presented in Table 1.
2. Description of the tester
Materials of some metals content exhibit the weight gain during the corrosion in gaseous environment. However, wear usually results in weight loss. The oxidation rate of a material KO is measured firstly at a separate test in order to account the weight gain due to corrosion during the following test for measuring the wear rate WAO at the same temperature. The equation for the calculation of the wear rate at high temperature for the designed tester is (see
2.1. Design of the tester Fig. 1 is a schematic of the abrasive-oxidation wear testing setup. Heated inside the electrical oven (4), the test is run in two chambers filled with abrasive (6). The samples (1) are fastened with the
2.2. Methodology of the high temperature wear rate calculation for the designed tester
Fig. 1. (a) Scheme of the high temperature multi sample, two chamber abrasion tester; (b) photo of the main unit taken out of the oven. Explanations—see the text. Sizes are given in millimetres.
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Table 1 Main design features and working parameters of tester. Feature/parameter
Description Available
Tested
Size of one chamber (L × W × H), mm Diameter of disk/specimens centre location, mm Size of specimen (L × W × H), mm Area of specimen subjected to wear, cm2 Area of specimen that is not subjected to wear, cm2 Frequency of disk rotation, s−1 Speed of specimen movement, m s−1 Number of specimens tested in one chamber Number of chambers with abrasive Abrasive, properties
125 × 120 × 120 105/77 25 × 15 × 5 4.5 4.8 0.01–0.40 0.004–0.1 3 × 6 = 18 2 Loose abrasive 0–3 mm
Temperature, ◦ C Duration of test, h Environment
20–1000 Limited by wear of bearings and disks Air
also Figs. 1 and 2): WAO =
W − (AOO KO tAO ) AAO tAO
(1)
where KO =
WO AO tO
(2)
In Eqs. (1) and (2) AO —area of the sample exposed to oxidation, 9.3 cm2 ; AOO —area of the sample protected against wear, 4.5 cm2 ; AAO —area of the sample subjected to wear (not protected), 4.8 cm2 ; KO —oxidation rate, mg cm−2 h−1 ; tO —duration of the oxidation test, h; tAO —duration of the wear test, h; WO —materials wastage during the oxidation test, mg; W—materials wastage during the wear test, mg; WAO —wear rate corrected for the designed tester, mg cm−2 h−1 . The wear and oxidation rates have positive sign if the weight of specimen is decreased during the test. The negative sign of the wear or corrosion rate indicates that the specimen has gained the weight during the test. 2.3. Estimation of the specific wear rate and mean pressure of abrasive The units in which the wear and corrosion rates are expressed (mg cm−2 h−1 ) throughout this paper are more characteristic to corrosion measurements and less to monitoring of wear. This is done in order to have the corrosion and wear rates expressed in the same units and to facilitate the use of given equations (Eqs. (1), (2), (5) and (6)). However, it is of interest to convert the wear results into the specific wear rate that is given in mm3 N−1 m−1 . The equation for the conversion is presented in the following chapter with the explanation of other characteristic parameters of the designed tester. The curvature of the abrasive bed was measured after the test at 0.01 and 0.05 m s−1 for SiC and SiO2 abrasives (Fig. 3). The angle ˇ that characterizes the duration of the specimens centre immersion to the abrasive, shown by positions A and C, was determined
Fig. 2. Location of specimen’s surfaces subjected to oxidation only (AOO ) and abrasion plus oxidation (AAO ) during wear test.
0.04, 0.21 0.01, 0.05 3×6 2 SiO2 , 0.2–0.4 mm, rounded, A = 2150 kg m−3 , 700 g per test SiC, 1.0–2.0 mm, angular, A = 3300 kg m−3 , 700 g per test 20, 400, 700, 900 5h
with the help of AutoCAD 2004 program as 98◦ ± 3◦ for the range of conditions tested. During disk rotation the abrasive is shifted due to the friction forces between disk and abrasive particles. The curvature shown in Fig. 3 represents the equilibrium of interplay of the friction forces, gravity force, rolling and sliding downhill, interlocking of abrasive, etc. Visual observations with the use of single painted abrasive grains has shown that the abrasive bed could be treated as stationary and the speed of the specimen relative to the abrasive bed could be determined as the angular velocity of the disks multiplied by the radial distance to the specimens centre. However, each abrasive grain in contact with rotating disk is forced to limited rotation and migration due to interlocking with surrounding grains. To calculate the specific wear rate through WAO the assessment of the mean pressure of abrasive particles acting normal to the specimen’s surface subjected to wear (AAO ) is necessary. The following assumptions should be made: 1. The mean pressure is calculated through the hydrostatic pressure. 2. The centre of sample is used for specifying its position. 3. The depth of the immersion into the abrasive h is determined in the middle of the duration of immersion (position B in Fig. 3, when ˇ = 49◦ ).
Fig. 3. Scheme of the test chamber showing specimen movement within the abrasive. Sizes are given in millimetres.
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4. The packing density of abrasive is 0.6 that is typical for almost spherical particles of this size. Although abrasive particles vibration influences the packing density, it typically can increase or decrease the density by not more than 0.05 [18–20]. The mean pressure of abrasive may be expressed as following: p = hA KP g
(3)
where p—mean pressure of abrasive, N m−2 ; h—immersion depth of the sample during wear; A —bulk density of the abrasive, kg m−3 ; KP —packing density of abrasive bed (0 ≤ KP ≤ 1); g—standard gravitational acceleration. For the conditions described in Table 1, the mean abrasive pressure can be calculated as following: pSiO2 = 0.02 · 2150 · 0.6 · 9.8 = 253 N m−2 Fig. 4. Schematic representation of the abrasive–corrosive wastage modes.
pSiC = 0.02 · 3300 · 0.6 · 9.8 = 388 N m−2 The specific wear rate is then expressed in the following form: k=
WAO 2778 pv(ˇ/360)M
(4)
where k—specific wear rate, mm3 N−1 m−1 ; WAO —wear rate, mg cm−2 h−1 ; v—speed of specimen movement, m s−1 ; ˇ—the duration of the specimens centre immersion to the abrasive, degrees; M —density of specimen, kg m−3 . The correction factor taking into account the conversion of units is equal to 2778. For example, the calculated k the for the AISI 316 stainless steel abraded at speed of 0.05 m s−1 by a sand particles at room temperature (WAO is 10.4 × 10−3 mg cm−2 h−1 —see Fig. 5) is: kAISI 316 =
10.4 × 10−3 2778 = 1 × 10−3 mm3 N−1 m−1 253 · 0.05(98/360)7950
The range of the specific wear rates of AISI 316 stainless steel in dry conditions by loosen abrasive at room temperature varies from 0.25 × 10−3 mm3 N−1 m−1 (pin on top of the lath with loosen abrasive [21]) and up to 19 × 10−3 mm3 N−1 m−1 (ASTM G65 rubber wheel tests [22]). This indicates that the wear rate determined by the presented tester is comparable with the results obtained using other schemes of testers and that the efficiency of material removal through the action of abrasive particles is comparably low. 3. Estimation of abrasive action effect on material wastage For the description of the effect of the abrasive action on material wastage at high temperature three possible modes are proposed (Fig. 4 and Eq. (5)): • Corrosion dominated mode is characterized by (ABR = 0, Eq. (5)) ineffective material loss due to abrasion or removal of the oxide film from the surface of the sample. • Corrosion–abrasion mode with prevailing corrosion (ABR is minimal) is characterized by slight material degradation due to abrasion and high rate of oxidation so that material wastage still has a negative sign. • The positive sign of material wastage rate points to a strong abrasive action (ABR is high). ABR = WAO − KO , mg cm−2 h−1
(5)
To assess the effect of abrasion on the wear rate of materials at a high temperature it is convenient to compare it with the corrosion
rate (Eq. (6)). Synergy =
ABR |KO |
(6)
4. Test conditions and results The set of corrosion and abrasion–corrosion tests were performed at room, elevated and high temperature as indicated in Table 1. Materials tested in the present study were TiC–NiMo cermets produced at Tallinn University of Technology by means of a conventional powder metallurgy routine described elsewhere [23]. Materials of different binder metal composition and contents were chosen as the test materials. Cr3 C2 –Ni cermet and conventional AISI 316 stainless steel were tested for comparison. The description of the materials properties and their performance (effect of binder content, binder composition, abrasive, sliding velocity, characteristic wastage mechanisms, etc.) is planned to be published in more details as a separate paper. The choice of the test temperatures was influenced by the oxidation kinetic of binder and ceramic phases in air environment. Below 400 ◦ C the oxidation of metal is hardly measured. At 700 ◦ C the oxidation of the metal phase is at sufficiently high level and ceramic phase could have only some minor oxidation. At 900 ◦ C there is dramatic oxidation of metal and moderate of ceramic phase. Each sample was step by step tested in all conditions that would facilitate the assessment of materials performance under various conditions. The sequence of operations at every temperature selected for testing included: the grinding of the specimen, evaluation of oxidation rate during 15 min, oxidation during 45 min, oxidation during 4 h, regrinding, testing at low velocity, regrinding, testing at high velocity. The oxidation tests of different duration were done to see the kinetic of oxidation process. The mass gain (oxidation) rate based on 5 h (the same duration as that of the wear test) was used for calculation of WAO , ABR and Synergy. Before the wear test, the specimen had always fresh (not oxidized) surface. During the regrinding the layer of 0.015–0.025 mm was removed from each face. The weight of the specimens before and after the test was measured with the help of A and D GR-202 electronic scales with 0.01 mg resolution and automatic recording by computer (RS-232C type connection). Mapping is a powerful tool for material selection, optimisation of working conditions and understanding of materials response under wide ranges of conditions. Estimation of the materials durability and reliability give the ideas for new materials design [24].
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Fig. 5. Temperature dependant mapping of the material degradation under conditions of oxidation and high temperature abrasion. Abrasive—sand; velocity—0.05 m s−1 . Values of W are given in ×10−3 mg; values for KO , WAO are given in ×10−3 mg cm−2 h−1 .
The map presented in Fig. 5 displays some selected results in order to explain the parameters (ABR, Synergy) introduced in the present paper. The map shown in Fig. 5 indicates the areas of high and medium synergy. W is the difference between samples weight measured before and after testing. Wear wastage is positive at low temperature and becomes negative at high temperatures of 700 ◦ C and 900 ◦ C. This fact evidences that sample gains the weight during wear testing at high temperature. That may be explained by the effect of corrosion. However, SEM imaging of the materials surface has revealed the events of material removal that are characteristic features of abrasive wear even at low velocities and low pressure of abrasive (Fig. 6a). On the other hand, the gain in weight is possible due to the embedding of the abrasive dust into the surface facilitated by sufficient softening of metal phase at high temperature, Moreover, it should be noticed that the sample parts protected by fixing clampers may contribute to mass gain of oxidising materials that influence the results sufficiently. WAO is a corrected wear rate. It takes into account the existence of two areas with different mechanisms of degradation (AOO and AAO ). The wear rate of the chromium carbide based cermets is lower
than that of titanium carbide based ones at high temperatures. The wear of AISI 316 stainless steel is comparable to that of Cr3 C2 -based cermets at 400 and 700 ◦ C but is much higher at 900 ◦ C. The corrosion rate (KO ) of Cr3 C2 based cermets at room and even at 400 ◦ C is very low. It is higher for materials with high binder content. Corrosion rate of TiC-based cermet and stainless steel is comparably higher. Increase in corrosion rate of these materials is dramatic at high temperature. Oxide layer on the surface of TiC–NiMo cermet is cracked (Fig. 6b) and had a low adhesion to the substrate. To assess the effectiveness of the abrasive action as compared to corrosion, the Synergy value has been calculated. For materials tested, the Synergy was not lower than 1.4 demonstrating that the abrasive particles are able to remove the corrosion products from the surface quite efficiently. Wear rates for TiC–NiMo cermet and AISI 316 stainless steel at 900 ◦ C are both very high. However, the Synergy value of the steel is extremely high at this temperature indicating that the abrasion is the leading process acting much more effectively than corrosion compared to other conditions tested. The highest Synergy values have been obtained for Cr3 C2 –Ni cermets at 400 ◦ C. These
Fig. 6. SEM micrograph of 70TiC–24Ni 6Mo wt% after wear test at 700 ◦ C in sand at 0.05 m s−1 (a) and after 5 h of oxidation at 700 ◦ C (b). The cracking of the surface layer is shown by the arrows.
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materials are almost inert at this temperature and abrasion is only the mechanism of degradation. The Synergy value proposed in this study represents the effect of abrasive action on the wear rate compared to the corrosion rate of material. To obtain the value of abrasive wear at high temperature in the absence of corrosion and the effect of abrasive action on corrosion rate are also of a great interest and importance. Abrasive wear can be studied during the test within the protective media. 5. Conclusions • The device for testing of as much as 36 specimens simultaneously in abrasive conditions at room and high temperatures (up to 1000 ◦ C) is designed and fabricated. • The methodology for calculation of the wear rate for the given design is presented. • The performance results enable us to arrange studied materials in the row of decrease of resistance to wear at 900 ◦ C as following: 90Cr3 C2 –10Ni-60Cr3 C2 –40Ni-70TiC–24Ni6Mo-AISI 316. The effect of abrasion on the wear rate of stainless steel at this temperature compared with the corrosion rate was the highest. Acknowledgements The authors wish to thank D.Eng. J. Pirso for help with samples manufacturing. Paroc Eesti AS is appreciated for supplying high temperature insulation slab. Estonian Science Foundation under grants nos. G6660, MT and SF T062 is acknowledged for financial support of this research. References [1] L.L. Shreir, R.A. Jarman, G.T. Burstein (Eds.), Corrosion, 3rd edition, Elsevier, 1994. [2] J.R. Blachere, F.S. Pettit, High Temperature Corrosion of Ceramics, William Andrew Publishing/Noyes, 1989. [3] D.J. DeRenzo, Corrosion Resistant Materials Handbook, William Andrew Publishing/Noyes, 1985. [4] B.D. Craig, Fundamental Aspects of Corrosion Films in Corrosion Science, Plenum Press, New York, 1990.
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[5] C. Brundle, C. Evans, S. Wilson, Encyclopedia of Materials Characterization— Surfaces, Interfaces Thin Films, Elsevier, 1992. [6] G.W. Stachowiak (Ed.), Wear. Materials, Mechanisms and Practice, John Wiley & Sons, 2005. [7] G.W. Stachowiak, A.W. Batchelor, Engineering Tribology, Second edition, Butterworth-Heinemann, 2001. [8] M.M. Stack, N. Pungwiwat, Erosion–corrosion mapping of Fe in aqueous slurries: some views on a new rationale for defining the erosion–corrosion interaction, Wear 256 (2004) 565–576. [9] S.W. Watson, F.J. Friedersdorf, B.W. Madsen, S.D. Cramer, Method of measuring wear-corrosion synergism, Wear 181–183 (1995) 476–484. [10] J.O. Bello, R.J.K. Wood, J.A. Wharton, Synergistic effects of micro-abrasion– corrosion of UNS S30403, S31603 and S32760 stainless steels, Wear 263 (2007) 149–159. [11] ASTM standard G119-93 (1998): standard guide for determining synergism between wear and corrosion. [12] B.-Q. Wang, K. Luer, The relative erosivity of limestone, dolomite and coal samples from an operating boiler, Wear 215 (1998) I80–190. [13] D. Jianxin, D. Zeliang, Y. Dongling, Erosion wear mechanisms of coal–water– slurry (CWS) ceramic nozzles in industry boilers, Materials Science and Engineering 417 (2006) 1–7. [14] T. Deng, J. Lib, A.R. Chaudhry, M. Patel, I. Hutchings, M.S.A. Bradley, Comparison between weight loss of bends in a pneumatic conveyor and erosion rate obtained in a centrifugal erosion tester for the same materials, Wear 258 (2005) 402–411. [15] M. Suckling, C. Allen, Critical variables in high temperature erosive wear, Wear 203–204 (1997) 528–536. [16] A.L. Ham, J.A. Yeomans, J.F. Watts, Elevated temperature solid particle erosion of silicon carbide continuous fibre reinforced calcium aluminosilicate glassceramic matrix composite, Wear 203–204 (1997) 387–392. [17] P. Hou, S. MacAdamb, Y. Niuc, J. Stringerd, High temperature degradation by erosion–corrosion in bubbling fluidized bed combustors, Materials Research 7 (2004) 71–80. [18] A.B. Yu, J. Bridgwater, A. Burbidge, On the modelling of the packing of fine particles, Powder Technology 92 (1997) 185–194. [19] X.Z. An, R.Y. Yang, R.P. Zou, A.B. Yu, Effect of vibration condition and interparticle frictions on the packing of uniform spheres, Powder Technology 188 (2008) 102–109. [20] X. Jia, M. Gan, R.A. Williams, D. Rhodes, Validation of a digital packing algorithm in predicting powder packing densities, Powder Technology 174 (2007) 10–13. [21] M.S. Bingley, S. Schnee, A study of the mechanisms of abrasive wear for ductile metals under wet and dry three-body conditions, Wear 258 (2005) 50–61. [22] K.G. Budinski, Resistance to particle abrasion of selected plastics, Wear 203–204 (1997) 302–309. [23] I. Hussainova, Effect of microstructure on the erosive wear of titanium carbidebased cermets, Wear 255 (2003) 121–128. [24] M.M. Stack, Bridging the gap between tribology and corrosion: from wear maps to Pourbaix diagrams, International Materials Reviews 50 (2005) 1–17.