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Mechanical properties and features of erosion of cermets
Wear 250 (2001) 818–825
Mechanical properties and features of erosion of cermets Irina Hussainova∗ , Jakob Kubarsepp, Juri Pirso Department of Materials Technology, Tallinn Technical University, Ehitajate tee 5, Tallinn 19806, Estonia
Abstract The erosive wear resistance of cermets with different composition, structure and properties has been investigated. It has been shown that cermets erosive wear resistance cannot be estimated only by hardness, characterised by resistance to penetration. The differences in wear resistance between cermet materials with equal hardness level can be attributed to differences in their resistance to fracture. The present paper discusses some features of the material removal process during the particle–wall collision. Solid particle erosion tests on eight materials have been performed using silicon carbide and silica abrasive particles within a range of erodent size of 0.1–0.3 mm, impact angles from 30 to 90◦ and particle velocity from 30 to 80 m s−1 . In order to clarify the details of the impact, the process of interaction of solid particles with cermet targets was studied using a laser Doppler anemometer (LDA) measuring technique. Systematic studies of the influence of the impact variables on the collision process have been carried out. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Ceramic metal composites; Solid particle erosion; Particle–wall collision; Wear resistance
1. Introduction Widespread use of cermets as materials for working elements of various equipment and machine parts may be attributed to their unique combination of desirable properties such as high hardness, strength, stiffness and wear resistance. Because of lack of common information of different types and grades of cermets, it is of interest and importance to test the materials and to identify their wear behaviour under different conditions to choose the optimum metal-matrix composite. WC-based cobalt-bonded hard metals are most widely used because of their excellent wear resistance-strength combination. Shortage of tungsten and cobalt and their poor corrosion resistance in some corrosive mediums and elevated temperatures restricts application of these hard metals. For this reason, the so-called tungsten free hard metals have been developed and adopted in industry [1]. Nickel and iron or their alloys are used as binders in these metal-matrix composites. The most widely known among tungsten-free cermets are those based on TiC and Cr3 C2 cemented with Ni and Mo alloys. Steel-bonded cermets form a special group among hard metals. They are of interest because of their cheapness and possibility of heat treatment. For ductile materials the shape and kinetic energy of erodents are the most important factors determining the erosion rate [2]. For brittle materials, however, the erosion ∗ Corresponding author. Tel.: +372-620-3303; fax: +372-620-3196. E-mail address: [email protected] (I. Hussainova).
rate is determined by kinetic energy, particle size, hardness and toughness of erodents [3]. The erosion of WC–Co hardmetals and titanium carbide–base cermets is associated with a combination of ductile and brittle modes of erosion, although the brittle mode is dominant [4]. Recent work [3] has attempted to define regimes of erosion of hardmetals, ranging from “plasticity-dominated” to “fracture-dominated” behaviour. In the present paper, the erosion mechanism for such materials was suggested, although a methodology for defining such transitions was not outlined. In order to elucidate the modes and mechanism of particle erosion, the impact process has been studied using a one-dimensional laser Doppler anemometer (LDA) system. By using a special test facility supplied with a disk accelerator, several combinations of the particle–wall material and several particle collision velocities were investigated.
2. Test materials and experimental details The cobalt content of the WC–Co hardmetals tested was 8–15 wt.%. The content of TiC in TiC-based cermets investigated was 40–80 wt.%. Four grades of those materials contained binder with different composition and structure. All TiC-based cermets were sintered in vacuum. The average grain size of carbides was, depending on composition, 2.0–2.7 m, porosity 0.1–0.2 vol.%. In Cr3 C2 –Ni composites the content of metal binder was 15–30 wt.%. In Table 1 the tested materials and their properties have been given.
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Table 1 Composition and properties of cermets investigated Grade
Carbide content (wt.%)
Composition and structure of binder
Vickers hardness numbera (HV)
Density, ρ (kg m−3 )
Transverse rupture strength, RTZ (GPa)
Modulus of elasticity, E (GPa)
BK8 BK15 TZC40 TZC60 TH20A TH40A K31 KE3
92 85 40 60 80 60 85 70
Co Co FeCr9 Si1.5 FeCr7 Si1.5 Ni13 Mo7 Ni26 Mo14 Ni Ni
1350 1200 1150 1360 1378 1190 1410 980
14500 13900 6100 5800 5500 5770 6970 7190
2.3 2.35 1.83 2.0 1.08 1.32 0.9 1.4
650 560 300 380 400 380 340 320
a
WC WC TiC TiC TiC TiC Cr3 C2 Cr3 C2
Hardness measurements were made on cross-sectioned surfaces using a Vickers indenter and 10 kg load.
Table 2 Mechanical properties of abrasive particles Erodent
Hardness (HV) [7]
Density, ρ (kg m−3 )
Fracture toughness, Kc (MPa m1/2 ) [7]
SiC SiO2
3000 1100
3300 2150
3.52 1.6
All materials studied were produced at Institute of Materials Technology, TTU. To fabricate the composites, conventional P/M technology was used. The abrasive particles used in this work were silicon carbide, as used in most standard erosion tests, and silica, the most common naturally occurring erodent. The particles were sieved into the required size fractions before erosion testing. The erodents were of size 0.1–0.3 mm. The mechanical properties of abrasive particles are listed in Table 2. The SEM micrographs of particles are shown in Fig. 1. The silicon carbide was more angular, with sharper profiles (Fig. 1a), than the silica that was rather rounded (Fig. 1b). Studying of wear was performed in a centrifugal four-channel de-
Fig. 2. The scheme of equipment of solid particle erosion testing.
vice as shown in Fig. 2. The specimens with dimensions 20 mm × 12 mm × 5 mm were tested. The surface to be impacted was polished and then cleaned in acetone prior to impacting it with erodent. An accuracy of 0.1 mg could be obtained for the target mass loss measurements. Investigations of the steady state erosion rate were made as a function of the impact velocity and impact angle. Wear conditions were as given in Table 3. The number of test samples of each cermet grades was four. In order to clarify the details of the impact, the process of interaction of solid particles with cermet targets was studied using a LDA measuring technique [5]. Since the special one-dimensional LDA system measures velocities of the individual striking and rebounding particles, it also gives insight into the variation of particle velocities in an abrasive jet. This technique allows for direct measurement of the particle velocity without disturbing the particle flow. The rebound direction was determined as the point of the maximum particle concentration, obtained by the number of Doppler Table 3 Erosion test data
Fig. 1. SEM micrographs of the abrasive particles: (a) SiC; (b) SiO2 .
Velocity (m s−1 ) Impact angle (◦ ) Particle size (mm)
31, 46, 61, 80 30, 45, 60, 75, 90 0.1–0.3
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signals registered by the system in one time unit. This point was found by linear scanning along the direction parallel to the direction of particle outlet at some distance down from the plate in the most probable rebound region. The rebounding particle velocity at this point was determined by scanning on the maximum of measuring velocity. Velocity measurements were made during each erosion test. The accelerator (Fig. 2) was mounted on the base of a coordinate device and can be moved relative to measuring volume of LDA system. The cover of accelerator has got a rectangular opening, where the receiving unit — a truncated triangle prism — was mounted. On the top of the prism there is a regulating receiving hole and in the bottom there is a cover with outlet aperture to form a narrow bundle of particles. The plate to be investigated is fixed at a corresponding angle onto the bracket of the cover. Maximum measurable velocity is approximately 100 m s−1 . Scheme and additional information regarding the apparatus can be found in [5].
3. Results The erosion rate was determined as volume loss of the target sample per mass of erodent particles (mm3 kg−1 ) to facilitate the comparison of target materials with different densities. 3.1. Steady state erosion rates of the materials investigated Figs. 3 and 4 show the steady state erosion rates of the investigated materials impacting by silica and SiC particles travelling at velocity of 61 m s−1 under initial angle of 75◦ . In all tests, BK8 clearly outperforms the other materials. Remarkable differences between erosion rates could not be explained by differences in hardness; this parameter of cermets investigated differ to a little extent. The results (Fig. 5) confirm the inconclusive influence of hardness on erosion rate of cermets: the relationship between hardness and erosion rate differs substantially from the linear relationship. An
Fig. 3. Steady state erosion rates of the materials investigated. Abrasive: silica; velocity: 61 m s−1 ; angle: 75◦ .
Fig. 4. Steady state erosion rates of the materials investigated. Abrasive: SiC; velocity: 61 m s−1 ; angle: 75◦ .
increase of hardness does not always result in increase of wear resistance. D’Errico et al. [6] analysed the erosion behaviour of TiC–Ni-based cermets. In contrast to the present investigation, it was concluded that the hardness is the most important controlling factor under erosion by solid particle impact. Four cermet types were taken into consideration in that study, but in [1], 16 TiC–Ni-based alloys were investigated and it was shown that at the same level of hardness, the wear resistance of the various cermets differs up to 80%. Therefore, for erosive wear resistance evaluation hardness is only useful as a first approximation. It must be noted that the hardness values are based on low strain rate hardness measurements, but the hardness value actually relevant to the impact conditions occurring in erosion may be somewhat different. During the erosion process the fracture of material starts locally and in most cases, in the binder phase [1]. Carbide grains lose their protective binder and the eroded surface is almost entirely covered with the exposed carbides. If material hardness exceeds that of abrasive, the erodent particles hardly can cause plastic flow in hard target and selective nature of erosion prevails. The degree of elastic penetration and, therefore, energy transmitted to a surface depends on the elastic modulus and, if the latter is high, less elastic penetration occurs. Under these conditions, the impact of abrasive particles may cause a low-cycle fatigue failure of
Fig. 5. Erosion rate vs. Vickers hardness number of cermets.
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Fig. 6. Erosion rate vs. modulus of elasticity of cermets.
carbide matrix and carbide grains. Thus, the elastic modulus is an even more important parameter of wear resistance than hardness. Among materials investigated, WC-8% Co alloy has the highest modulus of elasticity. In Fig. 6, the erosion rate versus elastic modulus is plotted. If the hardness of an abrasive exceeds that of cermet, the following processes take place: penetration of abrasive into the material surface, microcutting or plowing, failure of large carbide grains, resulting in the detachment of small chips. Under these conditions there is no great difference between cermet grades. Since the erosion of brittle grains is primarily via a mechanism involving the initiation and propagation of microcracks one expects that the fracture toughness of the material will affect the erosion rate [7,8]. The low toughness of most hard constituents resulted in a loss of erosion resistance that explained the poor performance of brittle Cr3 C2 -based cermets (fracture toughness of about 3 MPa m1/2 ) and the possible benefit of the tougher WC-based ones (fracture toughness of about 15 MPa m1/2 ). In contrast to the brittle TiC and Cr3 C2 grains, tungsten carbide grains are tough enough to withstand contacts with high amounts of dissipated energy without microfracture [8]. The mechanical and wearing properties of each individual grain and the boundaries between adjacent grains are, therefore, important for the overall performance of the carbide materials. It seems that transverse rupture strength, RTZ , is an important parameter of materials that plays a role in the process of solid particle erosion. RTZ is generally accepted as a failure resistance characteristic of cermets [1,9]. In addition, there exists a good correlation between RTZ and impact strength that characterises the resistance to failure under impact loading [10]. As well, RTZ of WC–Co grades is about 2.3 GPa, which exceeds that of the other ones. The specific wear behaviour was shown by Cr3 C2 -based cermets. If the erodent is much harder than the target material, crack initiation is inevitable and crack propagation is the rate controlling factor. The grade with the content of brittle Cr3 C2 grains of 85 mass% has a poor wear resistance, that may be attributed to its low fracture toughness
Fig. 7. SEM micrograph of single crater on K31 cermet impacting by silica particle at impact angle of 45◦ (erodent velocity: 31 m s−1 ).
and weak bonding between the grains. In the case if hardness of abrasive (HVSiO2 = 1100) is less than hardness of target, K31 grade has enough good wear resistance that may be attributed to its high hardness and, therefore, high resistance to plastic penetration. SEM micrographs of impact craters are shown in Figs. 7 and 8. From the micrographs of the eroded surfaces [10,11] it is clear that silicon carbide erodent produced large-scale fracture of the surface of the cermets tested in this work, suggesting that lateral fracture was the main method of material removal. The particle impact generates a residual stress causing the subsurface lateral cracks and facilitates removal of the material ships. However, the surfaces of the cermets eroded by silica showed a different morphology [10,11], being smoothed and rounded. The abrasives impact the surface that results in slight displacement of the carbide grains in each collision and consequent gradual extrusion of the binder material. Small carbide grains lose their protecting binder and crumble off uncrushed. The large grains are fragmented by fatigue nature. The lateral cracking mechanism of erosion, operating with silicon carbide erodent, leads to much higher rates of erosion than the minor chipping mechanism which occurs if the erodent is silica. From this result it can be concluded that in quantifying the erosion resistance of materials for use in a particular environment, care must be taken to ensure that the test and service environments produce the same erosion mechanism. Comparative data obtained for materials eroded by silicon carbide particles, where the brittle fracture mechanisms dominates, may have little relevance to the comparative erosion resistance of the same materials when eroded with silica in an industrial environment if erosion then occurs by a completely different and less severe mechanism such as a low-cycle fatigue failure of the carbide skeleton and an extrusion of
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Fig. 9. Comparative evaluation of the relative performance of materials tested. Abrasive: silica; impact velocity: 31 m s−1 .
Fig. 10. Comparative evaluation of the relative performance of materials tested. Abrasive: silica; impact velocity: 80 m s−1 .
of collision, the higher the advantage of WC-based grades is over other grades. A decrease in the abrasive jet velocity results in diminishing erosion resistance difference between WC and TiC-based cermets. Fig. 8. SEM micrographs: (a) single crater on K31 cermet impacting by SiC particle at impact angle of 45◦ (erodent velocity: 31 m s−1 ); (b) cross section of eroded surface of K31 cermet.
the binder material. This means that the abrasive used in a test can have an extremely large effect on the erosion rate of alloys. A comparative evaluation of the relative performance of materials tested is given in Figs. 9–12. It shows that erosion resistance of the relatively harder alloy K31 compares unfavourably with that of other grades as using SiC abrasive particles. Decreasing of impact angle results in some diminishing of advantages of WC-based cermets over TiC-based ones (for almost equal hardness of alloys). The higher the angle
Fig. 11. Comparative evaluation of the relative performance of materials tested. Abrasive: SiC; impact velocity: 31 m s−1 .
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Fig. 13. A characteristic velocity distribution of silica particles. Fig. 12. Comparative evaluation of the relative performance of materials tested. Abrasive: SiC; impact velocity; 80 m s−1 .
The mechanism of erosion depends first on the testing conditions [1,12]. Independently on their hardness, at low velocity erodent can hardly cause a local destruction of carbide grains and damage may have a though nature across the binder material. Under these test conditions the selective erosion prevails when the failure starts in the binder as the weakest phase. Some fatigue cracks may develop. In the conditions of high velocity of abrasive particles a low-cycle fatigue failure of the carbide skeleton and large carbide grains and microcutting occur depending on the ratio of material and abrasive hardness. With the silica abrasives, at low velocity of particles (Fig. 9), two grades of TiC-based cermets show the similar wear rates; TZC60 alloy may be an attractive candidate material for the erosive wear problem provided the hardness of erodent is less than that of target at impact angle lower than 60◦ . Cr3 C2 shows the highest wear rate, especially at collision angle of 90◦ . At high particle velocity, the difference between different grades of cermets is large (Fig. 10). With the SiC abrasives, at low particle velocity (Fig. 11) the erosion resistance of WC–Co alloy is much higher than that of the other cermets. But at high particle velocity (Fig. 12), the advantage of WC–Co grade is not substantial any more. At those service conditions TiC–NiMo cermet may be an alternative to conventional tungsten carbides for use in erosive application.
and direction of particle velocity and schematic view of collision process is given in Fig. 14. The coefficient of velocity restitution describes the kinetic energy loss during the collision event and therefore knowledge of the restitution coefficient (K) gives a good indication of impact process. In theory, the restitution parameters should be proportional to the resulting erosion. This chapter mainly discusses effect of impact velocity and target material properties on the restitution coefficient K. The coefficient of velocity restitution (restitution ratio) is expressed by K=
v2 v1
(1)
where v1 and v2 are the particle velocities before and after impact, respectively. The restitution ratio (1) alone does not give sufficient information in regard to erosion [2]. Therefore, the restitution ratio was resolved into two components: the normal velocity restitution ratio Kn = v2n /v1n , where v1n = v1 sin α 1 is the normal component of particle velocity before impact (α 1 is the impact angle) and v2n = v2 sin α 2 is the same after impact (α 2 is the rebound angle), and the tangential velocity restitution ratio. In order to determine the effect of particle velocity on the restitution parameters, the materials investigated were
3.2. Investigation of particle–wall collision with LDA A one-dimensional LDA system [5] was used to investigate the particle–wall collision. The particle velocity was measured during the actual erosion experiments. Although the width of the velocity distribution is considerable, our interpretation uses the number-averaged mean velocity only. A characteristic velocity distribution as measured with the LDA system is shown in Fig. 13. Determination of the value
Fig. 14. Schematic view of the collision process and determination of the value and direction of particle velocity; SV: LDA sensitivity vector [2], N: number of particles crossing measuring volume per unit of time and registered with LDA, v1 and v2 : the particle velocity before and after impact.
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The kinetic energy lost by the particle during impact can be deduced from the measurements of impact and rebound velocity [2] W = 21 m(v12 − v22 ) = 21 mv12 (1 − K 2 ) In all cases, W increases with increasing impact velocity; thereby implying that the incident energy is more efficiently absorbed at larger particle velocity. Surface damages or material removal process during particle–wall collision is the result of materials’ response to the contact stresses and/or the penetration depth. For spherical indentor the Hertz formula for energy of elastic impact is expressed as We = 25 Fm he Fig. 15. Normal restitution coefficient vs. initial velocity of erodents (dotted lines: SiC abrasive and solid lines: silica abrasive). (䉬) Cermet K31; (䊉) cermet BK8; () cermet TH20A.
eroded under standard conditions at different velocities from 5 to 50 m s−1 . The angle of impact was 75◦ . In the experiments with the erodent of irregular shape, the velocity and the particle size show a considerable width in their distribution (Fig. 13), leading to a comparable width in the restitution coefficient distribution. The average Kn was calculated using the number-averaged velocity: Kn = v2n /v1n . Initial impact velocity of abrasive particles v1 and the normal velocity restitution ratio Kn of cermets are plotted in Fig. 15. As it was emphasised above, for the case where the hardness of a target exceeds the hardness of the abrasive and at low velocity, impacting particles squeeze out the plastic binder, plunge hard carbide grains into soft substrate without any failure. Single impacts cause a little direct material loss, but multiple impingements will cause detachment of the extruded lips of binder material [4]. Carbide grains lose their protective binder forming thin subsurface transition layer with strength parameters differing from those of the bulk ones. Upwards of certain impact velocity a crack system develops. In all probability, some energy is accumulated by interphase (metal–carbide) boundaries up to the velocity when with increasing of erodent energy the tensile stresses in the target surface will initiate fatigue cracking. After the binder phase removal, worn areas of binder act as failure initiating concentrators. Small carbide grains crumble off uncrushed, whereas the main mechanism of large grains failure is chipping. Fig. 15 gives the manner in which the restitution coefficient decreases with increasing of the particle velocity (dotted lines were used in the case of SiC abrasive and solid lines were used in the case of silica abrasive). The coefficient of velocity restitution decreased with increasing of initial velocity of erodent and a loss of kinetic energy is much more appreciable when bombarding with hard SiC particles (Fig. 15).
where he is the total elastic deformation of the sphere and target in normal direction and Fm the maximum impact force.The kinetic energy of particle before the wall collision is W = 21 Fm hm where hm = he + hp is the maximum depth of indenter penetration into the material and hp the depth of impact crater in target surface. If Kn = v2n /v1n = (W e /W )0.5 , the magnitude of the elastic recovery he can be obtained as he = 1.25Kn2 hm
(2)
For the calculation of the indentation parameters it is assumed that the kinetic energy of the incoming particle is equal to the work done by the target on the indenting particle h 1 2 F (h) dh (3) mv1n = 2 0 where F(h) is the indentation force at a certain indentation depth h. Since the mass distribution could not be measured, the average mass was calculated using the mass-average particle size (number average mass), m = (4π/3)ρR 3 (R is mean radius of particle and ρ the material density). For non-spherical particles the tip radius r does not equal the mean particle radius R. The result is the introduction of a term (R/r) in following equations. The equation for the maximal indentation depth can be derived 1/2 2/5 5 R 2 hm = πρJ v1n R (4) 4 r where r is the tip radius of abrasive particles and J the apparent modulus, defined by J = (1 − ηm )/E m + (1 − ηa )/E a where Em , Ea and ηm , ηa are respectively the elastic modulus and Poisson’s ratio of the target material (m) and the indenting particle (a).
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To evaluate the magnitude of elastic recovery, Eq. (2) was used. In the case of interaction of SiC particles of average size 250 m with cermets he is he (K31) = 2 × 10−7 m, he (TH20A) = 2.7×10−7 m,
he (TZC60) = 4 × 10−7 m he (BK8) = 3.46×10−7 m
A depth of impact crater in target surface can be expressed as hp = hm − he . To get the erosion map it is necessary to learn the particle velocity of transition between wear regimes. Since elastic impact sites are not visible, the velocity at which the value of the indentation depth exceeds the value of elastic deformation he , represents the transition from elastic to plastic regime of impact. For cermet materials investigated this transition velocity is about 10 m s−1 .
825
grade over others is diminishing. At those service conditions TiC–NiMo cermet may be an alternative to conventional tungsten carbides for use in erosive application. The results indicate that the solid particle erosion of ceramic-metal composites is a complicated process controlled significantly by properties of each individual grain and the boundaries between adjacent grains. The restitution coefficients (and therefore kinetic energy loss) of materials tested are strongly influenced by the collision parameters. The velocity of transition from elastic to plastic regime for angular SiC particles is about 10 m s−1 for materials investigated.
Acknowledgements The authors would like to thank Dr. Krister Källström of KAMI Foundation, Sweden, for support.
4. Conclusions Erodent particles of different materials can cause different erosion rates and mechanisms. All materials show a significantly lower wear rate when abraded with silica than with SiC. This may be explained by the influence of hardness of the abrading particles compared to the test material. The harder silicon carbide particles cause plastic indentation and lateral cracking, whereas the softer silica particles cause minor chipping on a much finer scale than that of particle contact area. Main mechanism of wear is low-cycle fatigue. In the case of silica erodent, the erosion rate is higher for Cr3 C2 –Ni (KE3) and TiC-steel (TZC40) cermets with the lowest modulus of elasticity and hardness. For WC–Co (BK15) alloy with the same level of hardness, but highest (after BK8) modulus of elasticity and transverse rupture strength, the erosion rate is the lowest. In the case of SiC erodent, the erosion rate is higher for Cr3 C2 –Ni (K31) cermet with highest hardness and TiC-steel (TZC40) cermets with the lowest hardness. Cermets erosive wear resistance cannot be evaluated by single mechanical property like hardness. In the present study, the relative ranking of the materials investigated with respect to erosion rate is probably to a large extent explained by their different modulus of elasticity and fracture toughness values while the hardness seems to be of minor importance. The erosion resistance of WC–Co alloy (BK8) is much higher than that of the other cermets. But with the SiC abrasives, at high particle velocity, the advantage of WC–Co
References [1] H. Reshetnyak, J. Kübarsepp, Mechanical properties of hard metals and their erosive wear resistance, Wear 177 (1994) 185–193. [2] I. Hussainova, I. Kleis, Investigation of particle–wall impact process, Wear 233–235 (1999) 168–173. [3] A. Ball, Z. Feng, The erosion of four materials using seven erodents — toward an understanding, Wear 233–235 (1999) 674–684. [4] I. Hussainova, Some aspects of solid particle erosion of cermets, Tribol. Int. 34 (2001) 89–93. [5] I. Hussainova, J. Kübarsepp, I. Shcheglov, Investigation of impact of solid particles against hardmetal and cermet targets, Tribol. Int. 32 (1999) 337–344. [6] G. D’Errico, S. Bugliosi, D. Cuppini, E. Guglielmi, A study of cermets’ wear behaviour, Wear 203/204 (1997) 242–246. [7] R. Wellman, C. Allen, The effect of angle of impact and material properties on the erosion rates of ceramics, Wear 186/187 (1995) 117–122. [8] H. Engqvist, N. Axen, S. Hogmark, Tribological properties of a binderless carbide, Wear 232 (1999) 157–162. [9] G. Kreimer, Strength of Hardmetals, Metallurgia, Moscow, 1971, 274 pp. [10] J. Kübarsepp, H. Reshetnyak, Features of hardmetals wear resistance, NORDTRIB’98, in: Proceedings of the 8th International Conference on Tribology, Vol. 1, DTI Tribology Centre, Aarhus, Denmark, 1998, pp. 239−246. [11] J. Pirso, M. Viljus, J. Kubarsepp, Abrasive erosion of chromium carbide based cermets, NORDTRIB’2000, in: Proceedings of the 9th International Conference on Tribology, Vol. 3, Porvoo, Finland, 2000, pp. 959–967. [12] J. Larsen-Basse, Effect of composition, microstructure and service conditions on the wear of cemented carbides, J. Met. 11 (1983) 35–41.
Mechanical properties and features of erosion of cermets Irina Hussainova∗ , Jakob Kubarsepp, Juri Pirso Department of Materials Technology, Tallinn Technical University, Ehitajate tee 5, Tallinn 19806, Estonia
Abstract The erosive wear resistance of cermets with different composition, structure and properties has been investigated. It has been shown that cermets erosive wear resistance cannot be estimated only by hardness, characterised by resistance to penetration. The differences in wear resistance between cermet materials with equal hardness level can be attributed to differences in their resistance to fracture. The present paper discusses some features of the material removal process during the particle–wall collision. Solid particle erosion tests on eight materials have been performed using silicon carbide and silica abrasive particles within a range of erodent size of 0.1–0.3 mm, impact angles from 30 to 90◦ and particle velocity from 30 to 80 m s−1 . In order to clarify the details of the impact, the process of interaction of solid particles with cermet targets was studied using a laser Doppler anemometer (LDA) measuring technique. Systematic studies of the influence of the impact variables on the collision process have been carried out. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Ceramic metal composites; Solid particle erosion; Particle–wall collision; Wear resistance
1. Introduction Widespread use of cermets as materials for working elements of various equipment and machine parts may be attributed to their unique combination of desirable properties such as high hardness, strength, stiffness and wear resistance. Because of lack of common information of different types and grades of cermets, it is of interest and importance to test the materials and to identify their wear behaviour under different conditions to choose the optimum metal-matrix composite. WC-based cobalt-bonded hard metals are most widely used because of their excellent wear resistance-strength combination. Shortage of tungsten and cobalt and their poor corrosion resistance in some corrosive mediums and elevated temperatures restricts application of these hard metals. For this reason, the so-called tungsten free hard metals have been developed and adopted in industry [1]. Nickel and iron or their alloys are used as binders in these metal-matrix composites. The most widely known among tungsten-free cermets are those based on TiC and Cr3 C2 cemented with Ni and Mo alloys. Steel-bonded cermets form a special group among hard metals. They are of interest because of their cheapness and possibility of heat treatment. For ductile materials the shape and kinetic energy of erodents are the most important factors determining the erosion rate [2]. For brittle materials, however, the erosion ∗ Corresponding author. Tel.: +372-620-3303; fax: +372-620-3196. E-mail address: [email protected] (I. Hussainova).
rate is determined by kinetic energy, particle size, hardness and toughness of erodents [3]. The erosion of WC–Co hardmetals and titanium carbide–base cermets is associated with a combination of ductile and brittle modes of erosion, although the brittle mode is dominant [4]. Recent work [3] has attempted to define regimes of erosion of hardmetals, ranging from “plasticity-dominated” to “fracture-dominated” behaviour. In the present paper, the erosion mechanism for such materials was suggested, although a methodology for defining such transitions was not outlined. In order to elucidate the modes and mechanism of particle erosion, the impact process has been studied using a one-dimensional laser Doppler anemometer (LDA) system. By using a special test facility supplied with a disk accelerator, several combinations of the particle–wall material and several particle collision velocities were investigated.
2. Test materials and experimental details The cobalt content of the WC–Co hardmetals tested was 8–15 wt.%. The content of TiC in TiC-based cermets investigated was 40–80 wt.%. Four grades of those materials contained binder with different composition and structure. All TiC-based cermets were sintered in vacuum. The average grain size of carbides was, depending on composition, 2.0–2.7 m, porosity 0.1–0.2 vol.%. In Cr3 C2 –Ni composites the content of metal binder was 15–30 wt.%. In Table 1 the tested materials and their properties have been given.
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Table 1 Composition and properties of cermets investigated Grade
Carbide content (wt.%)
Composition and structure of binder
Vickers hardness numbera (HV)
Density, ρ (kg m−3 )
Transverse rupture strength, RTZ (GPa)
Modulus of elasticity, E (GPa)
BK8 BK15 TZC40 TZC60 TH20A TH40A K31 KE3
92 85 40 60 80 60 85 70
Co Co FeCr9 Si1.5 FeCr7 Si1.5 Ni13 Mo7 Ni26 Mo14 Ni Ni
1350 1200 1150 1360 1378 1190 1410 980
14500 13900 6100 5800 5500 5770 6970 7190
2.3 2.35 1.83 2.0 1.08 1.32 0.9 1.4
650 560 300 380 400 380 340 320
a
WC WC TiC TiC TiC TiC Cr3 C2 Cr3 C2
Hardness measurements were made on cross-sectioned surfaces using a Vickers indenter and 10 kg load.
Table 2 Mechanical properties of abrasive particles Erodent
Hardness (HV) [7]
Density, ρ (kg m−3 )
Fracture toughness, Kc (MPa m1/2 ) [7]
SiC SiO2
3000 1100
3300 2150
3.52 1.6
All materials studied were produced at Institute of Materials Technology, TTU. To fabricate the composites, conventional P/M technology was used. The abrasive particles used in this work were silicon carbide, as used in most standard erosion tests, and silica, the most common naturally occurring erodent. The particles were sieved into the required size fractions before erosion testing. The erodents were of size 0.1–0.3 mm. The mechanical properties of abrasive particles are listed in Table 2. The SEM micrographs of particles are shown in Fig. 1. The silicon carbide was more angular, with sharper profiles (Fig. 1a), than the silica that was rather rounded (Fig. 1b). Studying of wear was performed in a centrifugal four-channel de-
Fig. 2. The scheme of equipment of solid particle erosion testing.
vice as shown in Fig. 2. The specimens with dimensions 20 mm × 12 mm × 5 mm were tested. The surface to be impacted was polished and then cleaned in acetone prior to impacting it with erodent. An accuracy of 0.1 mg could be obtained for the target mass loss measurements. Investigations of the steady state erosion rate were made as a function of the impact velocity and impact angle. Wear conditions were as given in Table 3. The number of test samples of each cermet grades was four. In order to clarify the details of the impact, the process of interaction of solid particles with cermet targets was studied using a LDA measuring technique [5]. Since the special one-dimensional LDA system measures velocities of the individual striking and rebounding particles, it also gives insight into the variation of particle velocities in an abrasive jet. This technique allows for direct measurement of the particle velocity without disturbing the particle flow. The rebound direction was determined as the point of the maximum particle concentration, obtained by the number of Doppler Table 3 Erosion test data
Fig. 1. SEM micrographs of the abrasive particles: (a) SiC; (b) SiO2 .
Velocity (m s−1 ) Impact angle (◦ ) Particle size (mm)
31, 46, 61, 80 30, 45, 60, 75, 90 0.1–0.3
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signals registered by the system in one time unit. This point was found by linear scanning along the direction parallel to the direction of particle outlet at some distance down from the plate in the most probable rebound region. The rebounding particle velocity at this point was determined by scanning on the maximum of measuring velocity. Velocity measurements were made during each erosion test. The accelerator (Fig. 2) was mounted on the base of a coordinate device and can be moved relative to measuring volume of LDA system. The cover of accelerator has got a rectangular opening, where the receiving unit — a truncated triangle prism — was mounted. On the top of the prism there is a regulating receiving hole and in the bottom there is a cover with outlet aperture to form a narrow bundle of particles. The plate to be investigated is fixed at a corresponding angle onto the bracket of the cover. Maximum measurable velocity is approximately 100 m s−1 . Scheme and additional information regarding the apparatus can be found in [5].
3. Results The erosion rate was determined as volume loss of the target sample per mass of erodent particles (mm3 kg−1 ) to facilitate the comparison of target materials with different densities. 3.1. Steady state erosion rates of the materials investigated Figs. 3 and 4 show the steady state erosion rates of the investigated materials impacting by silica and SiC particles travelling at velocity of 61 m s−1 under initial angle of 75◦ . In all tests, BK8 clearly outperforms the other materials. Remarkable differences between erosion rates could not be explained by differences in hardness; this parameter of cermets investigated differ to a little extent. The results (Fig. 5) confirm the inconclusive influence of hardness on erosion rate of cermets: the relationship between hardness and erosion rate differs substantially from the linear relationship. An
Fig. 3. Steady state erosion rates of the materials investigated. Abrasive: silica; velocity: 61 m s−1 ; angle: 75◦ .
Fig. 4. Steady state erosion rates of the materials investigated. Abrasive: SiC; velocity: 61 m s−1 ; angle: 75◦ .
increase of hardness does not always result in increase of wear resistance. D’Errico et al. [6] analysed the erosion behaviour of TiC–Ni-based cermets. In contrast to the present investigation, it was concluded that the hardness is the most important controlling factor under erosion by solid particle impact. Four cermet types were taken into consideration in that study, but in [1], 16 TiC–Ni-based alloys were investigated and it was shown that at the same level of hardness, the wear resistance of the various cermets differs up to 80%. Therefore, for erosive wear resistance evaluation hardness is only useful as a first approximation. It must be noted that the hardness values are based on low strain rate hardness measurements, but the hardness value actually relevant to the impact conditions occurring in erosion may be somewhat different. During the erosion process the fracture of material starts locally and in most cases, in the binder phase [1]. Carbide grains lose their protective binder and the eroded surface is almost entirely covered with the exposed carbides. If material hardness exceeds that of abrasive, the erodent particles hardly can cause plastic flow in hard target and selective nature of erosion prevails. The degree of elastic penetration and, therefore, energy transmitted to a surface depends on the elastic modulus and, if the latter is high, less elastic penetration occurs. Under these conditions, the impact of abrasive particles may cause a low-cycle fatigue failure of
Fig. 5. Erosion rate vs. Vickers hardness number of cermets.
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Fig. 6. Erosion rate vs. modulus of elasticity of cermets.
carbide matrix and carbide grains. Thus, the elastic modulus is an even more important parameter of wear resistance than hardness. Among materials investigated, WC-8% Co alloy has the highest modulus of elasticity. In Fig. 6, the erosion rate versus elastic modulus is plotted. If the hardness of an abrasive exceeds that of cermet, the following processes take place: penetration of abrasive into the material surface, microcutting or plowing, failure of large carbide grains, resulting in the detachment of small chips. Under these conditions there is no great difference between cermet grades. Since the erosion of brittle grains is primarily via a mechanism involving the initiation and propagation of microcracks one expects that the fracture toughness of the material will affect the erosion rate [7,8]. The low toughness of most hard constituents resulted in a loss of erosion resistance that explained the poor performance of brittle Cr3 C2 -based cermets (fracture toughness of about 3 MPa m1/2 ) and the possible benefit of the tougher WC-based ones (fracture toughness of about 15 MPa m1/2 ). In contrast to the brittle TiC and Cr3 C2 grains, tungsten carbide grains are tough enough to withstand contacts with high amounts of dissipated energy without microfracture [8]. The mechanical and wearing properties of each individual grain and the boundaries between adjacent grains are, therefore, important for the overall performance of the carbide materials. It seems that transverse rupture strength, RTZ , is an important parameter of materials that plays a role in the process of solid particle erosion. RTZ is generally accepted as a failure resistance characteristic of cermets [1,9]. In addition, there exists a good correlation between RTZ and impact strength that characterises the resistance to failure under impact loading [10]. As well, RTZ of WC–Co grades is about 2.3 GPa, which exceeds that of the other ones. The specific wear behaviour was shown by Cr3 C2 -based cermets. If the erodent is much harder than the target material, crack initiation is inevitable and crack propagation is the rate controlling factor. The grade with the content of brittle Cr3 C2 grains of 85 mass% has a poor wear resistance, that may be attributed to its low fracture toughness
Fig. 7. SEM micrograph of single crater on K31 cermet impacting by silica particle at impact angle of 45◦ (erodent velocity: 31 m s−1 ).
and weak bonding between the grains. In the case if hardness of abrasive (HVSiO2 = 1100) is less than hardness of target, K31 grade has enough good wear resistance that may be attributed to its high hardness and, therefore, high resistance to plastic penetration. SEM micrographs of impact craters are shown in Figs. 7 and 8. From the micrographs of the eroded surfaces [10,11] it is clear that silicon carbide erodent produced large-scale fracture of the surface of the cermets tested in this work, suggesting that lateral fracture was the main method of material removal. The particle impact generates a residual stress causing the subsurface lateral cracks and facilitates removal of the material ships. However, the surfaces of the cermets eroded by silica showed a different morphology [10,11], being smoothed and rounded. The abrasives impact the surface that results in slight displacement of the carbide grains in each collision and consequent gradual extrusion of the binder material. Small carbide grains lose their protecting binder and crumble off uncrushed. The large grains are fragmented by fatigue nature. The lateral cracking mechanism of erosion, operating with silicon carbide erodent, leads to much higher rates of erosion than the minor chipping mechanism which occurs if the erodent is silica. From this result it can be concluded that in quantifying the erosion resistance of materials for use in a particular environment, care must be taken to ensure that the test and service environments produce the same erosion mechanism. Comparative data obtained for materials eroded by silicon carbide particles, where the brittle fracture mechanisms dominates, may have little relevance to the comparative erosion resistance of the same materials when eroded with silica in an industrial environment if erosion then occurs by a completely different and less severe mechanism such as a low-cycle fatigue failure of the carbide skeleton and an extrusion of
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Fig. 9. Comparative evaluation of the relative performance of materials tested. Abrasive: silica; impact velocity: 31 m s−1 .
Fig. 10. Comparative evaluation of the relative performance of materials tested. Abrasive: silica; impact velocity: 80 m s−1 .
of collision, the higher the advantage of WC-based grades is over other grades. A decrease in the abrasive jet velocity results in diminishing erosion resistance difference between WC and TiC-based cermets. Fig. 8. SEM micrographs: (a) single crater on K31 cermet impacting by SiC particle at impact angle of 45◦ (erodent velocity: 31 m s−1 ); (b) cross section of eroded surface of K31 cermet.
the binder material. This means that the abrasive used in a test can have an extremely large effect on the erosion rate of alloys. A comparative evaluation of the relative performance of materials tested is given in Figs. 9–12. It shows that erosion resistance of the relatively harder alloy K31 compares unfavourably with that of other grades as using SiC abrasive particles. Decreasing of impact angle results in some diminishing of advantages of WC-based cermets over TiC-based ones (for almost equal hardness of alloys). The higher the angle
Fig. 11. Comparative evaluation of the relative performance of materials tested. Abrasive: SiC; impact velocity: 31 m s−1 .
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Fig. 13. A characteristic velocity distribution of silica particles. Fig. 12. Comparative evaluation of the relative performance of materials tested. Abrasive: SiC; impact velocity; 80 m s−1 .
The mechanism of erosion depends first on the testing conditions [1,12]. Independently on their hardness, at low velocity erodent can hardly cause a local destruction of carbide grains and damage may have a though nature across the binder material. Under these test conditions the selective erosion prevails when the failure starts in the binder as the weakest phase. Some fatigue cracks may develop. In the conditions of high velocity of abrasive particles a low-cycle fatigue failure of the carbide skeleton and large carbide grains and microcutting occur depending on the ratio of material and abrasive hardness. With the silica abrasives, at low velocity of particles (Fig. 9), two grades of TiC-based cermets show the similar wear rates; TZC60 alloy may be an attractive candidate material for the erosive wear problem provided the hardness of erodent is less than that of target at impact angle lower than 60◦ . Cr3 C2 shows the highest wear rate, especially at collision angle of 90◦ . At high particle velocity, the difference between different grades of cermets is large (Fig. 10). With the SiC abrasives, at low particle velocity (Fig. 11) the erosion resistance of WC–Co alloy is much higher than that of the other cermets. But at high particle velocity (Fig. 12), the advantage of WC–Co grade is not substantial any more. At those service conditions TiC–NiMo cermet may be an alternative to conventional tungsten carbides for use in erosive application.
and direction of particle velocity and schematic view of collision process is given in Fig. 14. The coefficient of velocity restitution describes the kinetic energy loss during the collision event and therefore knowledge of the restitution coefficient (K) gives a good indication of impact process. In theory, the restitution parameters should be proportional to the resulting erosion. This chapter mainly discusses effect of impact velocity and target material properties on the restitution coefficient K. The coefficient of velocity restitution (restitution ratio) is expressed by K=
v2 v1
(1)
where v1 and v2 are the particle velocities before and after impact, respectively. The restitution ratio (1) alone does not give sufficient information in regard to erosion [2]. Therefore, the restitution ratio was resolved into two components: the normal velocity restitution ratio Kn = v2n /v1n , where v1n = v1 sin α 1 is the normal component of particle velocity before impact (α 1 is the impact angle) and v2n = v2 sin α 2 is the same after impact (α 2 is the rebound angle), and the tangential velocity restitution ratio. In order to determine the effect of particle velocity on the restitution parameters, the materials investigated were
3.2. Investigation of particle–wall collision with LDA A one-dimensional LDA system [5] was used to investigate the particle–wall collision. The particle velocity was measured during the actual erosion experiments. Although the width of the velocity distribution is considerable, our interpretation uses the number-averaged mean velocity only. A characteristic velocity distribution as measured with the LDA system is shown in Fig. 13. Determination of the value
Fig. 14. Schematic view of the collision process and determination of the value and direction of particle velocity; SV: LDA sensitivity vector [2], N: number of particles crossing measuring volume per unit of time and registered with LDA, v1 and v2 : the particle velocity before and after impact.
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The kinetic energy lost by the particle during impact can be deduced from the measurements of impact and rebound velocity [2] W = 21 m(v12 − v22 ) = 21 mv12 (1 − K 2 ) In all cases, W increases with increasing impact velocity; thereby implying that the incident energy is more efficiently absorbed at larger particle velocity. Surface damages or material removal process during particle–wall collision is the result of materials’ response to the contact stresses and/or the penetration depth. For spherical indentor the Hertz formula for energy of elastic impact is expressed as We = 25 Fm he Fig. 15. Normal restitution coefficient vs. initial velocity of erodents (dotted lines: SiC abrasive and solid lines: silica abrasive). (䉬) Cermet K31; (䊉) cermet BK8; () cermet TH20A.
eroded under standard conditions at different velocities from 5 to 50 m s−1 . The angle of impact was 75◦ . In the experiments with the erodent of irregular shape, the velocity and the particle size show a considerable width in their distribution (Fig. 13), leading to a comparable width in the restitution coefficient distribution. The average Kn was calculated using the number-averaged velocity: Kn = v2n /v1n . Initial impact velocity of abrasive particles v1 and the normal velocity restitution ratio Kn of cermets are plotted in Fig. 15. As it was emphasised above, for the case where the hardness of a target exceeds the hardness of the abrasive and at low velocity, impacting particles squeeze out the plastic binder, plunge hard carbide grains into soft substrate without any failure. Single impacts cause a little direct material loss, but multiple impingements will cause detachment of the extruded lips of binder material [4]. Carbide grains lose their protective binder forming thin subsurface transition layer with strength parameters differing from those of the bulk ones. Upwards of certain impact velocity a crack system develops. In all probability, some energy is accumulated by interphase (metal–carbide) boundaries up to the velocity when with increasing of erodent energy the tensile stresses in the target surface will initiate fatigue cracking. After the binder phase removal, worn areas of binder act as failure initiating concentrators. Small carbide grains crumble off uncrushed, whereas the main mechanism of large grains failure is chipping. Fig. 15 gives the manner in which the restitution coefficient decreases with increasing of the particle velocity (dotted lines were used in the case of SiC abrasive and solid lines were used in the case of silica abrasive). The coefficient of velocity restitution decreased with increasing of initial velocity of erodent and a loss of kinetic energy is much more appreciable when bombarding with hard SiC particles (Fig. 15).
where he is the total elastic deformation of the sphere and target in normal direction and Fm the maximum impact force.The kinetic energy of particle before the wall collision is W = 21 Fm hm where hm = he + hp is the maximum depth of indenter penetration into the material and hp the depth of impact crater in target surface. If Kn = v2n /v1n = (W e /W )0.5 , the magnitude of the elastic recovery he can be obtained as he = 1.25Kn2 hm
(2)
For the calculation of the indentation parameters it is assumed that the kinetic energy of the incoming particle is equal to the work done by the target on the indenting particle h 1 2 F (h) dh (3) mv1n = 2 0 where F(h) is the indentation force at a certain indentation depth h. Since the mass distribution could not be measured, the average mass was calculated using the mass-average particle size (number average mass), m = (4π/3)ρR 3 (R is mean radius of particle and ρ the material density). For non-spherical particles the tip radius r does not equal the mean particle radius R. The result is the introduction of a term (R/r) in following equations. The equation for the maximal indentation depth can be derived 1/2 2/5 5 R 2 hm = πρJ v1n R (4) 4 r where r is the tip radius of abrasive particles and J the apparent modulus, defined by J = (1 − ηm )/E m + (1 − ηa )/E a where Em , Ea and ηm , ηa are respectively the elastic modulus and Poisson’s ratio of the target material (m) and the indenting particle (a).
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To evaluate the magnitude of elastic recovery, Eq. (2) was used. In the case of interaction of SiC particles of average size 250 m with cermets he is he (K31) = 2 × 10−7 m, he (TH20A) = 2.7×10−7 m,
he (TZC60) = 4 × 10−7 m he (BK8) = 3.46×10−7 m
A depth of impact crater in target surface can be expressed as hp = hm − he . To get the erosion map it is necessary to learn the particle velocity of transition between wear regimes. Since elastic impact sites are not visible, the velocity at which the value of the indentation depth exceeds the value of elastic deformation he , represents the transition from elastic to plastic regime of impact. For cermet materials investigated this transition velocity is about 10 m s−1 .
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grade over others is diminishing. At those service conditions TiC–NiMo cermet may be an alternative to conventional tungsten carbides for use in erosive application. The results indicate that the solid particle erosion of ceramic-metal composites is a complicated process controlled significantly by properties of each individual grain and the boundaries between adjacent grains. The restitution coefficients (and therefore kinetic energy loss) of materials tested are strongly influenced by the collision parameters. The velocity of transition from elastic to plastic regime for angular SiC particles is about 10 m s−1 for materials investigated.
Acknowledgements The authors would like to thank Dr. Krister Källström of KAMI Foundation, Sweden, for support.
4. Conclusions Erodent particles of different materials can cause different erosion rates and mechanisms. All materials show a significantly lower wear rate when abraded with silica than with SiC. This may be explained by the influence of hardness of the abrading particles compared to the test material. The harder silicon carbide particles cause plastic indentation and lateral cracking, whereas the softer silica particles cause minor chipping on a much finer scale than that of particle contact area. Main mechanism of wear is low-cycle fatigue. In the case of silica erodent, the erosion rate is higher for Cr3 C2 –Ni (KE3) and TiC-steel (TZC40) cermets with the lowest modulus of elasticity and hardness. For WC–Co (BK15) alloy with the same level of hardness, but highest (after BK8) modulus of elasticity and transverse rupture strength, the erosion rate is the lowest. In the case of SiC erodent, the erosion rate is higher for Cr3 C2 –Ni (K31) cermet with highest hardness and TiC-steel (TZC40) cermets with the lowest hardness. Cermets erosive wear resistance cannot be evaluated by single mechanical property like hardness. In the present study, the relative ranking of the materials investigated with respect to erosion rate is probably to a large extent explained by their different modulus of elasticity and fracture toughness values while the hardness seems to be of minor importance. The erosion resistance of WC–Co alloy (BK8) is much higher than that of the other cermets. But with the SiC abrasives, at high particle velocity, the advantage of WC–Co
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