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Evaluation of wear resistance of thin hard coatings by a new solid particle impact test
Wear 251 (2001) 861–867
Evaluation of wear resistance of thin hard coatings by a new solid particle impact test Y. Iwai a,∗ , T. Honda a , H. Yamada a , T. Matsubara b , M. Larsson c , S. Hogmark d a
Department of Mechanical Engineering, Fukui University, 3-9-1 Bunkyo, Fukui 910-8507, Japan b Macoho Co. Ltd., Ishido-cho, Nagaoka 940-2032, Niigata, Japan c Balzers Sandvik Coating AB, Box 42056, SE-126 12 Stockholm, Sweden d The Angstrom Laboratory, Uppsala University, SE-751 21 Uppsala, Sweden
Abstract For the evaluation of tribological properties of thin hard coatings, several experimental techniques are normally used, such as scratch testing and testing against abrasive, erosive, and sliding wear, and fretting. In this paper, we propose slurry jet, a new type of solid particle impact test, in order to quickly evaluate wear properties of thin, single layered or multilayered physical vapor deposited (PVD) coatings. By slurry jet 1 m alumina particles were impacted at high velocity perpendicular to thin PVD coatings of TiN, TiN/NbN, TiN/TaN, TiN/CrN, TiN/TiAlN deposited on a high speed steel (HSS) substrate material. The coatings proved to have much higher erosion resistance than the substrate material and, consequently, the wear rate increased significantly to the higher level of the HSS material when the coatings were penetrated. The maximum peak-to-valley roughness (Ry ) of the eroded coatings was of the order of 0.1–0.5 m, suggesting that the size of the detached particles was of the same order. The ranking of erosion resistance and correlation to the mechanical properties, such as hardness and critical normal load obtained by scratch testing are discussed. We conclude that the proposed evaluation test is fast and easy to accomplish. It generates reproducible results and is very sensitive to the quality of the coating. Thus, it can preferentially be used as a screening test when evaluating coatings and coated materials. In particular, the interface strength of multilayered coatings with very thin lamella can be assessed. For the coatings included in this study, the TiN/TiAlN proved to have the highest erosion resistance, whereas the TiN/CrN was the worst. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Slurry jet impact test; PVD coating; Evaluation; Wear resistance
1. Introduction Today, homogeneous and multilayered coatings are increasingly being used to improve the tribological properties of a wide variety of mechanical components such as tools for metal cutting and forming, and machine elements such as rolling or sliding bearings, seals, piston/cylinder systems, and valves [1,2]. As to the evaluation of tribological properties of thin hard coatings, several experimental techniques are being used, such as scratch testing; abrasive and erosive testing, sliding and fretting wear testing. A continuing development of versatile and reliable techniques for evaluation of coated components is important for the development and tribological assessment of new coating composites and their applications.
∗ Corresponding author. Tel.: +81-776-27-8544; fax: +81-776-27-8748. E-mail address: [email protected] (Y. Iwai).
Tests aimed at the intrinsic coating properties or at distinguishing between the properties of the coating and substrate material, respectively, should allow the wear rate of the coating to be determined independently of that of the substrate. Hutchings [3] has reviewed the application of solid particle erosion testing to coated samples. Erosion testing has proven to be a suitable tool for studies of crack initiation and propagation in thin hard coatings [4]. Conventional particle erosion tests such as gas-blast [5], centrifugal [6] and slurry erosion tests [7,8] have been used to study the erosion durability of coatings when impacted by relatively large solid particles of a diameter many times larger than the coating thickness. However, failure of thin hard coatings often occurs unevenly [6] and after a short test duration [7] due to this high ratio of particle diameter to coating thickness. It is also difficult with these techniques to focus the impingement of tiny solid particles on a small area of the target. If particle erosion is to be used as a means to test intrinsic coating properties, a test method which involves the impact of very
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small particles is needed to minimize the maximum depth to which the coating material is affected at each individual impact. In this paper, we propose a new type of slurry jet, i.e. a solid particle impact test which utilizes a focused slurry jet, in order to quickly evaluate wear properties of single layered and multilayered physical vapor deposited (PVD) coatings. The PVD coatings included in this study are TiN, TiN/NbN, TiN/TaN, TiN/CrN, TiN/TiAlN deposited on flat (20 mm ×40 mm ×2 mm) high speed steel (HSS) substrates. The ranking of erosion resistance and its correlation to the mechanical properties such as hardness and critical normal load obtained by scratch testing are discussed. From these results, we conclude that the proposed test can preferentially be used as a screening test when evaluating coated materials. 2. Experimental 2.1. Test apparatus and procedure Fig. 1 shows a schematic view of the test apparatus, which was developed by Matsubara at Macoho Co. Ltd. It consists of a specimen holder, a tank and a stirrer to mix solid particles in liquid, a nozzle to eject the test liquid, a regulator to adjust compressed air pressure and a solenoid valve connected with a timer to control on–off of liquid flow. The flowing stream of water containing solid particles sucked from the tank, is mixed with compressed air in the nozzle, and eventually a slurry jet is ejected at high velocity in atmosphere. The cross-section of the nozzle is square by 3 mm × 3 mm, which generates a rectangular scar on a flat specimen. The shape of a square was chosen because it has
Fig. 1. Schematic view of the slurry jet tester. The insert shows detail of the nozzle.
the advantage over circle in the easy measurement of the central region of the scar. The jet velocity is regulated by the pressure of the compressed air, but could, unfortunately, not be measured. For the pressure of 0.5 MPa used in this experiment, the maximum velocity was estimated to over 100 m/s at the exit of the nozzle by the double-disc method [9]. However, the error of this method may be severe for small particles and particles of low density. The impingement angle of the jet relative to the test surface can be varied from 15 to 90◦ by tilting the specimen holder. In this test 90◦ impact was used. The test piece was mounted at 10 mm distance from the end of the nozzle. The test liquid was tap water containing angular alumina particles with a size distribution in the range of 0.5–2.5 m, with an average diameter of 1.0 m, as erodent. The alumina particles have a hardness ranged from 1800 to 2000 HV [10]. The erodent was added into tap water of the volume of 2 l in 3 wt.%, i.e. the concentration of the alumina particles was 3 wt.% in the tank, and the slurry was kept at room temperature. The wear loss of the coatings after the tests was too small to be resolved by weighing. Instead, the geometry of the eroded surface was measured with a stylus profilometer at three positions along the center-line of the square scar. In addition, the worn surfaces were studied by SEM and AFM to reveal the wear mechanisms. 2.2. Test materials 2.2.1. Substrate material As substrate material, a powder metallurgical (PM) HSS with the chemical composition (wt.%) of 1.3C, 4.2Cr, 5.0Mo, 6.4W, 3.1V and 8.5Co, was used. It was heat treated by austenitisation at 1180◦ C followed by tempering 3× 1 h at 560◦ C, resulting in a primary carbides volume fraction of 13% and a hardness of 9.2 GPa. All substrates were polished to mirror finish, corresponding to a center-line average roughness (Ra ) value of approximately 5 nm. Uncoated HSS was included in the erosion test as a reference material. 2.2.2. Coating materials The coating materials and their properties are listed in Table 1. The first four are experimental coatings, and the last one is commercially available. The TiN coating is a monolayer, while the others have a multilayered structure, i.e. they were obtained by alternately depositing two different compounds. The total thickness of the coatings was in the range of 2.5–4.4 m, as estimated from fractured cross-sections in the SEM. The individual layer thickness, see Table 1, was estimated similarly. All multilayered coatings with individual layers of 0.01/0.005 m thickness were found to be harder than the coarsely laminated TiN/TiAlN (21 GPa) and the homogeneous TiN (21 GPa), see Table 1. All the coatings show a higher surface roughness than the uncoated substrate (Ra : 60–250 nm).
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Table 1 Coating materials and their propertiesa Coating
Deposition technique
Coating structure
Coating thickness (m)
TiN TiN/TaN TiN/CrN TiN/NbN TiN/TiAlN
REB REB/RMS REB/RMS REB/RMS RAE
Homogenous Multilayered Multilayered Multilayered Multilayered
2.8 3.2 3.7 2.8 4.1
a
± ± ± ± ±
0.2 0.2 0.3 0.3 0.3
Individual layer thickness (m)
Hardness HV0.050 (GPa)
– 0.01/0.005 0.01/0.005 0.01/0.005 0.1/0.1
21 39 24 38 21
±1 ±4 ±1 ±2 ±1
Ra (nm) 100 110 170 160 220
± ± ± ± ±
30 50 80 40 60
Critical normal load (N) 50 53 36 40 40
± ± ± ± ±
10 7 4 5 20
REB: reactive electron beam evaporation; RMS: reactive dc magnetron sputtering; RAE: reactive arc evaporation.
A crucial property of coated components is the adhesion of the coating to the substrate. In the conventional scratch test, a Rockwell diamond stylus (tip radius 200 m) was traversing the coating while the normal load was continuously increased. All the coated samples were scratched with a loading rate of 10 N/min and a scratching speed of 1 mm/min. Four scratches were performed for each coating. The critical normal load (FN,C ), i.e. the load at the first coating failure, was determined by monitoring a sharp increase in the friction force and acoustic emission (AE), respectively, during the scratch process. All the coatings displayed relatively high critical normal loads, and ultimately failed by a mixture of adhesive and cohesive failures. The critical load was found to decrease in order TiN/TaN, TiN, TiN/NbN and TiN/TiAlN, TiN/CrN, see Table 1. At and above the critical load, TiN/CrN and TiN displayed small cohesive chippings at the rim of the scratch. In the case of TiN/NbN and TiN/TaN, the critical load corresponded to semicircular cohesive failures, which are typical for brittle materials. For loads higher than the critical load, TiN/NbN and TiN/TaN also displayed areas of exposed substrate.
3. Test results and discussion Fig. 2 shows the depth profiles of the erosion crater of the TiN coating, measured after various test duration. During the first 21 min the wear of the coating was gradual and slow, generating a smooth surface, where after the coating was penetrated and severe damage of the substrate commenced.
Fig. 2. Surface profiles along the center-line of the square erosion scar of the TiN coating after various test duration.
At each test interval the distance between the original and the worn surfaces at the deepest position was measured and designated as the wear depth. The TiN coating proved to have much higher erosion resistance than the HSS material as can be seen from the wear depth variation with test duration in Fig. 3. For the TiN coating the wear depth increased linearly at a moderate slope until the coating is penetrated after about 21 min when it increased significantly. The slope of the initial part of the wear curve of the coating was calculated by means of the least-squares method, and defined as the coating wear rate in this study. The presumptive time of coating penetration tp is estimated as the time when the extrapolated initial wear curve reaches a wear depth corresponding to the measured coating thickness. On the other hand, the wear depth curve drawn from high slope points means the wear curve of substrate material beneath the coatings. The time at which both wear curves of coating and substrate intersect is designated as the experimental penetration time te of the coating, see the illustration in Fig. 3. For TiN, the slope of the high wear rate curve-section was almost identical to that of the uncoated substrate. In addition, tp and te were found to be almost the same (20.7 and 21.1 min, respectively). In order to confirm the reproducibility of the wear rate of the TiN coating, additional four 15 min tests were carried
Fig. 3. Maximum crater depth of the substrate (HSS) and the TiN coating as a function of test duration.
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Fig. 7. Wear rates for all coatings.
Fig. 4. Reproducibility of the wear rate for the TiN coating.
Fig. 5. Wear depth vs. time for all coatings.
Fig. 6. Relation between the experimental time for coating penetration te and the presumptive time for coating penetration tp .
Fig. 8. Representative SEM photographs of the worn surface of eroded substrate material (HSS) (a), TiN (b) and TiN/CrN (c)
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out, and the corresponding wear rates are plotted in Fig. 4. The mean-value and standard deviation of the five tests were 0.136 and 0.0065 m/min, respectively. Their relative error, i.e. the value of S.D./mean-value, was 4.8%. Consequently, our test was found to be highly reproducible. There are strong deviations in wear rate among the investigated coatings, as seen in Fig. 5. However, the general shape of the wear curves is the same. In addition, it was seen that te completely agrees with tp for all coatings, see Fig. 6, which means that our test is able to determine the durability and wear rate of thin hard coatings independently of the substrate. This is the great benefit of using our equipment. Therefore, it is possible to compare the wear resistance between various coatings accurately. The wear rate was found to decrease in the order TiN/CrN, TiN/TaN, TiN/NbN, TiN/TiAlN, TiN, see Fig. 7.
Fig. 11. Wear rate vs. hardness for all coatings.
3.1. Observation of worn surfaces Fig. 8 shows representative SEM photographs of the worn surface of substrate (a), TiN (b) and TiN/CrN (c). A characteristic pattern of protruding carbides is seen in the HSS substrate indicating a very mild, selective wear mechanism, primarily attacking the softer steel matrix. The coatings were worn gradually, and eventually the substrate was exposed and eroded to show the surface characteristics as the worn surface of the substrate specimen, see Fig. 8b and c. For TiN, a smooth surface was produced and the substrate slightly appeared on the whole worn surface, whereas for TiN/CrN a very smooth worn surface was produced compared to that of TiN, but the initial exposure of the substrate was more local. Cutting traces and cracks, which usually are characteristic of surfaces exposed to solid particle erosion [11], were neither observed for the substrate nor the coatings. It is concluded that the material is removed in the form of very small fragments due to repeated attacks from the very small eroding particles. The micro-topography of the worn surfaces was quantified by AFM, see Fig. 9. The original surface of TiN had a regular
Fig. 10. Equilibrium maximum peak-to-valley roughness (Ry ) for all coatings.
variation in roughness, produced during the coating process, see Fig. 9a. The higher parts of the original surface were preferentially worn down to form a uniform and smooth surface, see Fig. 9b. All tested coatings showed a smooth worn surface similar to that of TiN, cf. Fig. 9c–f. The evolution of equilibrium maximum peak-to-valley roughness (Ry ) of the worn surface of the coatings is shown in Fig. 10. For all coatings, the roughness was rapidly reduced at the beginning of the test and reached a constant-value, of the order of 0.1–0.5 m, after less than 5 min. The homogeneous TiN and the TiN/TiAlN multilayered coating with 0.1 m lamella thickness, showed a higher Ry compared to the three multilayers with 0.01/0.005 m lamella (TiN/NbN, TiN/TaN and TiN/CrN). These results suggest that the size of the detached debris is of the order of 0.1–0.5 m. The wear rate of the coatings does not simply correlate to the coating hardness, see Fig. 11. The homogeneous TiN and the multilayered TiN/TiAlN with relatively thick lamella
Fig. 12. Wear rate vs. critical normal load for all coatings.
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proved to give a lower wear rate than expected form of their hardness-values. Nordin et al. [12] demonstrated a higher resistance for multilayered TiN/TaN coatings when eroded with SiC particles (20–30 m in size) that they believe is a result of the superior toughness gained from the large number of interfaces. Some proportion of the impact energy is consumed by crack deflection and correspondingly less energy is used in the creation of wear fragments, as compared to homogeneous coatings. However, in the present investigation, homogeneous TiN and the coating with thick lamella (TiN/TiAlN) showed a higher wear resistance than the thin lamella multilayered coatings. The reason for these unexpected results is probably found in the very small scale of the wear fragments. Any crack involved in the formation of the wear fragments in this very mild erosion is smaller, or of the same order of magnitude as the inter distance between the thin lamella. Thus, on this small scale, the interfaces rather play the role of defects than improving the toughness of the coatings through crack deflection. On the other hand, with the more homogeneous coatings, there is a chance for cracks to grow to larger depths through successive impacts, before any particle is removed. Consequently, these fragments are larger, explaining the higher roughness of the corresponding wear surfaces, but they also appear a lot scarcer. There is no correlation between the erosion rate of the coatings and the critical normal load in scratch testing, see Fig. 12. The simple explanation is found in the very small scale by which the erosion occurs. There is virtually no influence from the substrate material.
4. Conclusions By using slurry jet, a new type of solid particle impact test, fast and highly reproducible evaluation of wear properties of thin coatings can be made. The following conclusions can be drawn from this investigation: • Linear wear is obtained for both coating and substrate material, and the penetration through the coating into the substrate is signified by a sharp increase in slope of the wear versus time curve. • The test is fast and easy to accomplish, and the wear occurs on such a small scale that the erosion resistance of coating and substrate materials is assessed independently.
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These advantages of using our equipment are a great benefit compared to other erosion tests. • It is believed that this test can preferentially be used to evaluate the interlamella strength of multilayered coatings with thin lamella. • For the coatings included in this study, homogeneous TiN and a multilayered coating with coarse lamellae (TiN/TiAlN) proved to have a higher erosion resistance than multilayers with thin lamellae (TiN/NbN, TiN/TaN, TiN/CrN). Acknowledgements This study was supported by a Grant-in-Aid administered by the Ministry of Education of Japan (no. 11650149), and The Swedish Research Counsil for Engineering Sciences (TFR). The authors are grateful to Mr. Tetsuo Suehiro for his experimental help, and also to Mr. Ulrik Beste for operating a scanning electron microscope. References [1] K. Holmberg, A. Matthews, Coating Tribology, Tribology Series 28, Elsevier, Amsterdam, 1994. [2] S. Hogmark, S. Jacobson, M. Larssson, Design and evaluation of tribological coatings, Wear 246 (2000) 20–33. [3] I.M. Hutchings, Abrasive and erosive wear tests for thin coatings: a unified approach, Tribol. Int. 31 (1–3) (1998) 5–15. [4] M. Bromark, M. Larsson, P. Hedenqvist, M. Olsson, S. Hogmark, E. Bergman, PVD coatings for tool applications: tribological evaluation, Surf. Eng. 10 (3) (1994) 205–214. [5] P.H. Shipway, I.M. Hutchings, Measurement of coating durability by solid particle erosion, Surf. Coat. Technol. 71 (1995) 1–8. [6] M. Bromark, M. Larsson, P. Hedenqvist, S. Hogmark, Determination of coating erosion resistance using the mass-loss technique, in: Proceedings of the 6th Nordic Symposium on Tribology, Nordtrib’94, 1994, pp. 207–213. [7] R.J.K. Wood, Y. Puget, K.R. Trethewey, K. Stokes, The performance of marine coatings and pipe materials under fluid-borne sand erosion, Wear 219 (1998) 46–59. [8] Y. Iwai, K. Numbu, Slurry wear properties of pump lining materials, Wear 210 (1997) 211–219. [9] A.W. Ruff, L.K. Ives, Measurement of solid particle velocity in erosion wear, Wear 35 (1975) 195–199. [10] I.M. Hutchings, Tribology: friction and wear of engineering materials, Edward Arnold, London, 1992, pp. 137–139. [11] I.M. Hutchings, Tribology: Friction and Wear of Engineering Materials, Edward Arnold, London, 1992, pp. 171–197. [12] M. Nordin, M. Larsson, S. Hogmark, Wear resistance of multilayered PVD TiN/TaN on HSS, Surf. Coat. Technol. 120/121 (1999) 528–534.
Evaluation of wear resistance of thin hard coatings by a new solid particle impact test Y. Iwai a,∗ , T. Honda a , H. Yamada a , T. Matsubara b , M. Larsson c , S. Hogmark d a
Department of Mechanical Engineering, Fukui University, 3-9-1 Bunkyo, Fukui 910-8507, Japan b Macoho Co. Ltd., Ishido-cho, Nagaoka 940-2032, Niigata, Japan c Balzers Sandvik Coating AB, Box 42056, SE-126 12 Stockholm, Sweden d The Angstrom Laboratory, Uppsala University, SE-751 21 Uppsala, Sweden
Abstract For the evaluation of tribological properties of thin hard coatings, several experimental techniques are normally used, such as scratch testing and testing against abrasive, erosive, and sliding wear, and fretting. In this paper, we propose slurry jet, a new type of solid particle impact test, in order to quickly evaluate wear properties of thin, single layered or multilayered physical vapor deposited (PVD) coatings. By slurry jet 1 m alumina particles were impacted at high velocity perpendicular to thin PVD coatings of TiN, TiN/NbN, TiN/TaN, TiN/CrN, TiN/TiAlN deposited on a high speed steel (HSS) substrate material. The coatings proved to have much higher erosion resistance than the substrate material and, consequently, the wear rate increased significantly to the higher level of the HSS material when the coatings were penetrated. The maximum peak-to-valley roughness (Ry ) of the eroded coatings was of the order of 0.1–0.5 m, suggesting that the size of the detached particles was of the same order. The ranking of erosion resistance and correlation to the mechanical properties, such as hardness and critical normal load obtained by scratch testing are discussed. We conclude that the proposed evaluation test is fast and easy to accomplish. It generates reproducible results and is very sensitive to the quality of the coating. Thus, it can preferentially be used as a screening test when evaluating coatings and coated materials. In particular, the interface strength of multilayered coatings with very thin lamella can be assessed. For the coatings included in this study, the TiN/TiAlN proved to have the highest erosion resistance, whereas the TiN/CrN was the worst. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Slurry jet impact test; PVD coating; Evaluation; Wear resistance
1. Introduction Today, homogeneous and multilayered coatings are increasingly being used to improve the tribological properties of a wide variety of mechanical components such as tools for metal cutting and forming, and machine elements such as rolling or sliding bearings, seals, piston/cylinder systems, and valves [1,2]. As to the evaluation of tribological properties of thin hard coatings, several experimental techniques are being used, such as scratch testing; abrasive and erosive testing, sliding and fretting wear testing. A continuing development of versatile and reliable techniques for evaluation of coated components is important for the development and tribological assessment of new coating composites and their applications.
∗ Corresponding author. Tel.: +81-776-27-8544; fax: +81-776-27-8748. E-mail address: [email protected] (Y. Iwai).
Tests aimed at the intrinsic coating properties or at distinguishing between the properties of the coating and substrate material, respectively, should allow the wear rate of the coating to be determined independently of that of the substrate. Hutchings [3] has reviewed the application of solid particle erosion testing to coated samples. Erosion testing has proven to be a suitable tool for studies of crack initiation and propagation in thin hard coatings [4]. Conventional particle erosion tests such as gas-blast [5], centrifugal [6] and slurry erosion tests [7,8] have been used to study the erosion durability of coatings when impacted by relatively large solid particles of a diameter many times larger than the coating thickness. However, failure of thin hard coatings often occurs unevenly [6] and after a short test duration [7] due to this high ratio of particle diameter to coating thickness. It is also difficult with these techniques to focus the impingement of tiny solid particles on a small area of the target. If particle erosion is to be used as a means to test intrinsic coating properties, a test method which involves the impact of very
0043-1648/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 ( 0 1 ) 0 0 7 4 3 - 8
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Y. Iwai et al. / Wear 251 (2001) 861–867
small particles is needed to minimize the maximum depth to which the coating material is affected at each individual impact. In this paper, we propose a new type of slurry jet, i.e. a solid particle impact test which utilizes a focused slurry jet, in order to quickly evaluate wear properties of single layered and multilayered physical vapor deposited (PVD) coatings. The PVD coatings included in this study are TiN, TiN/NbN, TiN/TaN, TiN/CrN, TiN/TiAlN deposited on flat (20 mm ×40 mm ×2 mm) high speed steel (HSS) substrates. The ranking of erosion resistance and its correlation to the mechanical properties such as hardness and critical normal load obtained by scratch testing are discussed. From these results, we conclude that the proposed test can preferentially be used as a screening test when evaluating coated materials. 2. Experimental 2.1. Test apparatus and procedure Fig. 1 shows a schematic view of the test apparatus, which was developed by Matsubara at Macoho Co. Ltd. It consists of a specimen holder, a tank and a stirrer to mix solid particles in liquid, a nozzle to eject the test liquid, a regulator to adjust compressed air pressure and a solenoid valve connected with a timer to control on–off of liquid flow. The flowing stream of water containing solid particles sucked from the tank, is mixed with compressed air in the nozzle, and eventually a slurry jet is ejected at high velocity in atmosphere. The cross-section of the nozzle is square by 3 mm × 3 mm, which generates a rectangular scar on a flat specimen. The shape of a square was chosen because it has
Fig. 1. Schematic view of the slurry jet tester. The insert shows detail of the nozzle.
the advantage over circle in the easy measurement of the central region of the scar. The jet velocity is regulated by the pressure of the compressed air, but could, unfortunately, not be measured. For the pressure of 0.5 MPa used in this experiment, the maximum velocity was estimated to over 100 m/s at the exit of the nozzle by the double-disc method [9]. However, the error of this method may be severe for small particles and particles of low density. The impingement angle of the jet relative to the test surface can be varied from 15 to 90◦ by tilting the specimen holder. In this test 90◦ impact was used. The test piece was mounted at 10 mm distance from the end of the nozzle. The test liquid was tap water containing angular alumina particles with a size distribution in the range of 0.5–2.5 m, with an average diameter of 1.0 m, as erodent. The alumina particles have a hardness ranged from 1800 to 2000 HV [10]. The erodent was added into tap water of the volume of 2 l in 3 wt.%, i.e. the concentration of the alumina particles was 3 wt.% in the tank, and the slurry was kept at room temperature. The wear loss of the coatings after the tests was too small to be resolved by weighing. Instead, the geometry of the eroded surface was measured with a stylus profilometer at three positions along the center-line of the square scar. In addition, the worn surfaces were studied by SEM and AFM to reveal the wear mechanisms. 2.2. Test materials 2.2.1. Substrate material As substrate material, a powder metallurgical (PM) HSS with the chemical composition (wt.%) of 1.3C, 4.2Cr, 5.0Mo, 6.4W, 3.1V and 8.5Co, was used. It was heat treated by austenitisation at 1180◦ C followed by tempering 3× 1 h at 560◦ C, resulting in a primary carbides volume fraction of 13% and a hardness of 9.2 GPa. All substrates were polished to mirror finish, corresponding to a center-line average roughness (Ra ) value of approximately 5 nm. Uncoated HSS was included in the erosion test as a reference material. 2.2.2. Coating materials The coating materials and their properties are listed in Table 1. The first four are experimental coatings, and the last one is commercially available. The TiN coating is a monolayer, while the others have a multilayered structure, i.e. they were obtained by alternately depositing two different compounds. The total thickness of the coatings was in the range of 2.5–4.4 m, as estimated from fractured cross-sections in the SEM. The individual layer thickness, see Table 1, was estimated similarly. All multilayered coatings with individual layers of 0.01/0.005 m thickness were found to be harder than the coarsely laminated TiN/TiAlN (21 GPa) and the homogeneous TiN (21 GPa), see Table 1. All the coatings show a higher surface roughness than the uncoated substrate (Ra : 60–250 nm).
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Table 1 Coating materials and their propertiesa Coating
Deposition technique
Coating structure
Coating thickness (m)
TiN TiN/TaN TiN/CrN TiN/NbN TiN/TiAlN
REB REB/RMS REB/RMS REB/RMS RAE
Homogenous Multilayered Multilayered Multilayered Multilayered
2.8 3.2 3.7 2.8 4.1
a
± ± ± ± ±
0.2 0.2 0.3 0.3 0.3
Individual layer thickness (m)
Hardness HV0.050 (GPa)
– 0.01/0.005 0.01/0.005 0.01/0.005 0.1/0.1
21 39 24 38 21
±1 ±4 ±1 ±2 ±1
Ra (nm) 100 110 170 160 220
± ± ± ± ±
30 50 80 40 60
Critical normal load (N) 50 53 36 40 40
± ± ± ± ±
10 7 4 5 20
REB: reactive electron beam evaporation; RMS: reactive dc magnetron sputtering; RAE: reactive arc evaporation.
A crucial property of coated components is the adhesion of the coating to the substrate. In the conventional scratch test, a Rockwell diamond stylus (tip radius 200 m) was traversing the coating while the normal load was continuously increased. All the coated samples were scratched with a loading rate of 10 N/min and a scratching speed of 1 mm/min. Four scratches were performed for each coating. The critical normal load (FN,C ), i.e. the load at the first coating failure, was determined by monitoring a sharp increase in the friction force and acoustic emission (AE), respectively, during the scratch process. All the coatings displayed relatively high critical normal loads, and ultimately failed by a mixture of adhesive and cohesive failures. The critical load was found to decrease in order TiN/TaN, TiN, TiN/NbN and TiN/TiAlN, TiN/CrN, see Table 1. At and above the critical load, TiN/CrN and TiN displayed small cohesive chippings at the rim of the scratch. In the case of TiN/NbN and TiN/TaN, the critical load corresponded to semicircular cohesive failures, which are typical for brittle materials. For loads higher than the critical load, TiN/NbN and TiN/TaN also displayed areas of exposed substrate.
3. Test results and discussion Fig. 2 shows the depth profiles of the erosion crater of the TiN coating, measured after various test duration. During the first 21 min the wear of the coating was gradual and slow, generating a smooth surface, where after the coating was penetrated and severe damage of the substrate commenced.
Fig. 2. Surface profiles along the center-line of the square erosion scar of the TiN coating after various test duration.
At each test interval the distance between the original and the worn surfaces at the deepest position was measured and designated as the wear depth. The TiN coating proved to have much higher erosion resistance than the HSS material as can be seen from the wear depth variation with test duration in Fig. 3. For the TiN coating the wear depth increased linearly at a moderate slope until the coating is penetrated after about 21 min when it increased significantly. The slope of the initial part of the wear curve of the coating was calculated by means of the least-squares method, and defined as the coating wear rate in this study. The presumptive time of coating penetration tp is estimated as the time when the extrapolated initial wear curve reaches a wear depth corresponding to the measured coating thickness. On the other hand, the wear depth curve drawn from high slope points means the wear curve of substrate material beneath the coatings. The time at which both wear curves of coating and substrate intersect is designated as the experimental penetration time te of the coating, see the illustration in Fig. 3. For TiN, the slope of the high wear rate curve-section was almost identical to that of the uncoated substrate. In addition, tp and te were found to be almost the same (20.7 and 21.1 min, respectively). In order to confirm the reproducibility of the wear rate of the TiN coating, additional four 15 min tests were carried
Fig. 3. Maximum crater depth of the substrate (HSS) and the TiN coating as a function of test duration.
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Fig. 7. Wear rates for all coatings.
Fig. 4. Reproducibility of the wear rate for the TiN coating.
Fig. 5. Wear depth vs. time for all coatings.
Fig. 6. Relation between the experimental time for coating penetration te and the presumptive time for coating penetration tp .
Fig. 8. Representative SEM photographs of the worn surface of eroded substrate material (HSS) (a), TiN (b) and TiN/CrN (c)
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out, and the corresponding wear rates are plotted in Fig. 4. The mean-value and standard deviation of the five tests were 0.136 and 0.0065 m/min, respectively. Their relative error, i.e. the value of S.D./mean-value, was 4.8%. Consequently, our test was found to be highly reproducible. There are strong deviations in wear rate among the investigated coatings, as seen in Fig. 5. However, the general shape of the wear curves is the same. In addition, it was seen that te completely agrees with tp for all coatings, see Fig. 6, which means that our test is able to determine the durability and wear rate of thin hard coatings independently of the substrate. This is the great benefit of using our equipment. Therefore, it is possible to compare the wear resistance between various coatings accurately. The wear rate was found to decrease in the order TiN/CrN, TiN/TaN, TiN/NbN, TiN/TiAlN, TiN, see Fig. 7.
Fig. 11. Wear rate vs. hardness for all coatings.
3.1. Observation of worn surfaces Fig. 8 shows representative SEM photographs of the worn surface of substrate (a), TiN (b) and TiN/CrN (c). A characteristic pattern of protruding carbides is seen in the HSS substrate indicating a very mild, selective wear mechanism, primarily attacking the softer steel matrix. The coatings were worn gradually, and eventually the substrate was exposed and eroded to show the surface characteristics as the worn surface of the substrate specimen, see Fig. 8b and c. For TiN, a smooth surface was produced and the substrate slightly appeared on the whole worn surface, whereas for TiN/CrN a very smooth worn surface was produced compared to that of TiN, but the initial exposure of the substrate was more local. Cutting traces and cracks, which usually are characteristic of surfaces exposed to solid particle erosion [11], were neither observed for the substrate nor the coatings. It is concluded that the material is removed in the form of very small fragments due to repeated attacks from the very small eroding particles. The micro-topography of the worn surfaces was quantified by AFM, see Fig. 9. The original surface of TiN had a regular
Fig. 10. Equilibrium maximum peak-to-valley roughness (Ry ) for all coatings.
variation in roughness, produced during the coating process, see Fig. 9a. The higher parts of the original surface were preferentially worn down to form a uniform and smooth surface, see Fig. 9b. All tested coatings showed a smooth worn surface similar to that of TiN, cf. Fig. 9c–f. The evolution of equilibrium maximum peak-to-valley roughness (Ry ) of the worn surface of the coatings is shown in Fig. 10. For all coatings, the roughness was rapidly reduced at the beginning of the test and reached a constant-value, of the order of 0.1–0.5 m, after less than 5 min. The homogeneous TiN and the TiN/TiAlN multilayered coating with 0.1 m lamella thickness, showed a higher Ry compared to the three multilayers with 0.01/0.005 m lamella (TiN/NbN, TiN/TaN and TiN/CrN). These results suggest that the size of the detached debris is of the order of 0.1–0.5 m. The wear rate of the coatings does not simply correlate to the coating hardness, see Fig. 11. The homogeneous TiN and the multilayered TiN/TiAlN with relatively thick lamella
Fig. 12. Wear rate vs. critical normal load for all coatings.
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proved to give a lower wear rate than expected form of their hardness-values. Nordin et al. [12] demonstrated a higher resistance for multilayered TiN/TaN coatings when eroded with SiC particles (20–30 m in size) that they believe is a result of the superior toughness gained from the large number of interfaces. Some proportion of the impact energy is consumed by crack deflection and correspondingly less energy is used in the creation of wear fragments, as compared to homogeneous coatings. However, in the present investigation, homogeneous TiN and the coating with thick lamella (TiN/TiAlN) showed a higher wear resistance than the thin lamella multilayered coatings. The reason for these unexpected results is probably found in the very small scale of the wear fragments. Any crack involved in the formation of the wear fragments in this very mild erosion is smaller, or of the same order of magnitude as the inter distance between the thin lamella. Thus, on this small scale, the interfaces rather play the role of defects than improving the toughness of the coatings through crack deflection. On the other hand, with the more homogeneous coatings, there is a chance for cracks to grow to larger depths through successive impacts, before any particle is removed. Consequently, these fragments are larger, explaining the higher roughness of the corresponding wear surfaces, but they also appear a lot scarcer. There is no correlation between the erosion rate of the coatings and the critical normal load in scratch testing, see Fig. 12. The simple explanation is found in the very small scale by which the erosion occurs. There is virtually no influence from the substrate material.
4. Conclusions By using slurry jet, a new type of solid particle impact test, fast and highly reproducible evaluation of wear properties of thin coatings can be made. The following conclusions can be drawn from this investigation: • Linear wear is obtained for both coating and substrate material, and the penetration through the coating into the substrate is signified by a sharp increase in slope of the wear versus time curve. • The test is fast and easy to accomplish, and the wear occurs on such a small scale that the erosion resistance of coating and substrate materials is assessed independently.
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These advantages of using our equipment are a great benefit compared to other erosion tests. • It is believed that this test can preferentially be used to evaluate the interlamella strength of multilayered coatings with thin lamella. • For the coatings included in this study, homogeneous TiN and a multilayered coating with coarse lamellae (TiN/TiAlN) proved to have a higher erosion resistance than multilayers with thin lamellae (TiN/NbN, TiN/TaN, TiN/CrN). Acknowledgements This study was supported by a Grant-in-Aid administered by the Ministry of Education of Japan (no. 11650149), and The Swedish Research Counsil for Engineering Sciences (TFR). The authors are grateful to Mr. Tetsuo Suehiro for his experimental help, and also to Mr. Ulrik Beste for operating a scanning electron microscope. References [1] K. Holmberg, A. Matthews, Coating Tribology, Tribology Series 28, Elsevier, Amsterdam, 1994. [2] S. Hogmark, S. Jacobson, M. Larssson, Design and evaluation of tribological coatings, Wear 246 (2000) 20–33. [3] I.M. Hutchings, Abrasive and erosive wear tests for thin coatings: a unified approach, Tribol. Int. 31 (1–3) (1998) 5–15. [4] M. Bromark, M. Larsson, P. Hedenqvist, M. Olsson, S. Hogmark, E. Bergman, PVD coatings for tool applications: tribological evaluation, Surf. Eng. 10 (3) (1994) 205–214. [5] P.H. Shipway, I.M. Hutchings, Measurement of coating durability by solid particle erosion, Surf. Coat. Technol. 71 (1995) 1–8. [6] M. Bromark, M. Larsson, P. Hedenqvist, S. Hogmark, Determination of coating erosion resistance using the mass-loss technique, in: Proceedings of the 6th Nordic Symposium on Tribology, Nordtrib’94, 1994, pp. 207–213. [7] R.J.K. Wood, Y. Puget, K.R. Trethewey, K. Stokes, The performance of marine coatings and pipe materials under fluid-borne sand erosion, Wear 219 (1998) 46–59. [8] Y. Iwai, K. Numbu, Slurry wear properties of pump lining materials, Wear 210 (1997) 211–219. [9] A.W. Ruff, L.K. Ives, Measurement of solid particle velocity in erosion wear, Wear 35 (1975) 195–199. [10] I.M. Hutchings, Tribology: friction and wear of engineering materials, Edward Arnold, London, 1992, pp. 137–139. [11] I.M. Hutchings, Tribology: Friction and Wear of Engineering Materials, Edward Arnold, London, 1992, pp. 171–197. [12] M. Nordin, M. Larsson, S. Hogmark, Wear resistance of multilayered PVD TiN/TaN on HSS, Surf. Coat. Technol. 120/121 (1999) 528–534.