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Slurry erosion characteristics of TiN coatings on α-Ti and plasma-nitrided Ti alloy substrates
Surface and Coatings Technology 122 (1999) 176–182 www.elsevier.nl/locate/surfcoat
Slurry erosion characteristics of TiN coatings on a-Ti and plasma-nitrided Ti alloy substrates J.P. Tu a,c, *, L.P. Zhu b, H.X. Zhao c a Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China b Zhejiang Technology School of Building Materials, Hangzhou 310014, People’s Republic of China c Department of Chemical Engineering, Hiroshima University, Kagamiyama 1-4-1, Higashi-Hiroshima 739-8527, Japan Received 4 January 1999; accepted in revised form 12 June 1999
Abstract The slurry erosion behavior of TiN coatings on a-Ti and plasma-nitrided Ti alloy substrates in aqueous silica slurry has been investigated using a jet-in-slit rig. Erosion tests were performed with slurry jet velocities between 6.4 and 15.2 m s−1. At the lower slurry velocity, pitting was caused by the slurry impact, and there was little volume loss for each of the coatings. The TiN coatings deposited on the a-Ti and nitrided Ti alloy substrate presented high slurry erosion resistance. With increasing slurry velocity, however, perforation and fragmentation of the hard TiN coatings occurred and the protection effect of the coating layer reduced with the erosion duration. The erosion rates for the coatings deposited on the nitrided Ti substrate were slightly lower than those for the coatings deposited on the a-Ti substrate under the same test conditions. After coating perforation, the surfaces of the substrates were eroded by crater formation and flaking on the center section of the erosion scars while cutting and plowing occurred at the outer regions of the erosion scars. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Plasma nitriding; Slurry erosion; a-Ti alloy; TiN coating
1. Introduction In recent years, increasing applications have been found for thin, hard, wear-resistant ceramic coatings in industry, particularly in cutting and forming tools [1– 4]. The most commonly used coating is titanium nitride ( TiN ), which possesses high hardness and good chemical and metallurgical stability [5]. Cutting and forming tools coated with such coatings usually exhibit improved service behavior and increased lifetime. The potential applications for this coating as erosion barriers include a variety of engineering components, such as blades and pipes. Many investigations [6–13] have focused on both the friction and the wear characteristics of TiN coatings deposited on various engineering materials, as well as the failure mechanisms of TiN films. Like most ceramic materials, TiN coatings exhibit brittle behavior; they sometimes have a great tendency to fracture and spall from the substrate during wear, especially on softer substrates. The nitriding of a steel substrate prior to the * Corresponding author. Fax: +86 571 7051358. E-mail address: [email protected] (J.P. Tu)
TiN coating deposition has a beneficial effect on reduced wear of TiN coatings under the same wear conditions [14]. Although physical, chemical and mechanical properties of TiN coatings are documented satisfactorily, relatively few investigations have been carried out to study their erosion behavior [15]. Slurry erosion occurs when surface material suffers from impingement of solid erodent particles suspended in a carrier liquid. Continued operation under particulate flow condition may cause severe erosion damage on blade or pipe surfaces and can be detrimental to their reliability. There are many factors which influence the erosion rate of materials, for example the flow regime and the kinetic energy of the impacting slurry. The difficulties in predicting erosion performance for coatings are more complex than for bulk materials. Levy [16 ] found that hertzian cone cracks occurred in the lower layer beneath the plastic indentation under particle impact conditions. As the radial cracks in the upper layer developed, they intersected and formed a network of chips that were removed from the surface by succeeding erodent particle impact. In brittle coating materials, erosion is the maximum at normal incidence and the
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principal mechanism for material removal is brittle fracture accompanied by a small amount of plastic deformation [17]. The failure of thin ceramic coatings was caused by intercrystalline or transcrystalline fracture, surface cracking and interfacial spalling. In the present work, the slurry erosion properties of TiN coated a-Ti and plasma-nitrided Ti alloy have been investigated. Based on the experimental data and results of microscopic studies, the effects of slurry velocity on erosion mechanisms of the TiN-coated specimens and the substrate nitriding on the erosion resistance of the TiN coatings are evaluated.
2. Experimental procedure 2.1. Preparation of nitrided substrate and TiN coatings The substrate material of the specimens (20 mm in diameter and 1.5 mm in thickness) used in the present work was a commercial a-Ti alloy (O: 0.10 wt.%, N: 0.03 wt.%, C: 0.05 wt.%, H: <0.015 wt.%, Fe: <0.15 wt.%, Si: <0.10 wt.%, Ti: bal.). Plasma nitriding was carried out in pure nitrogen with a constant gas flow of 20 l h−1. At the working pressure of 6.6×102 Pa, a glow discharge (with a current density of 8 mA cm−2) of the reaction gas was created by the d.c voltage (800 V ). The specimens were treated at a temperature of 900°C for 5 h and cooled slowly in the evacuated furnace. The nitrided layer of a-Ti alloy consisted of two different layers, i.e. a thin compound layer (about 3 mm) and a diffusion layer. The XRD data showed that the surface of the compound layer was composed mainly of the e phase ( Ti N ) and a small amount of the d phase 2 ( TiN ) after nitriding treatment. In the diffusion layer, small precipitates were observed after nitriding with subsequently slow furnace cooling. By measuring the characteristic N spectrum using wavelength-dispersive X-ray spectrometry ( WDS), the intensity of the nitrogen line decreased continuously with increasing distance from the surface, and the depth of nitrided layer was nearly 40 mm. As shown in Fig. 1, near the surface, a Vickers hardness value of about Hv 1220 (50 gf ) was observed. The microhardness decreased gradually with increasing distance from the surface. The TiN coatings were produced by cathodic arc ion plating. For the coating process, the a-Ti and nitrided Ti specimens were heated to 300°C in a vacuum deposition chamber at 1.33×10−3–1.33×10−4 Pa. The steered cathode arc plasma source with a titanium target was operated with an arc current between 90 and 100 A. After the titanium ion bombardment at −1000 V bias voltage, the Ti film was deposited with the substrate bias voltage at −210 V. Then N and Ar gases 2 (P =0.21 Pa) were fed into the vacuum deposition N2
Fig. 1. Microhardness as a function of the distance from the surface for a-Ti alloy plasma nitrided at 900°C for 5 h in nitrogen.
chamber. The TiN coatings were deposited on the substrates at a substrate temperature of 400°C and with a coating thickness of 5 mm. Microstructural analysis and the surface and cross-section morphologies of the coatings were identified by X-ray diffraction and scanning electron microscopy (SEM ). The TiN coatings have a dense, fine structure and remain adherent to the substrate. The surface microhardnesses of the ceramic coatings deposited on a-Ti and a nitrided Ti substrate are about Hv 2065 and Hv 2250 (50 gf ), respectively. 2.2. Slurry erosion tests Slurry erosion tests were conducted at room temperature using the jet-in-slit rig, which has been described previously [18]. In this slurry erosion rig, four specimens are installed and immersed in the abrasive slurry. The solid particles are suspended uniformly by an upward flow of liquid. The water stream is jetted from the nozzle with an internal diameter of 1.8 mm which is fitted on the center of the test section, and the slurry is drawn up and mixed with the waterjet stream. The slurry then impinges on the surfaces of the test specimens and is exhausted through the slit between the specimen and guide plate. The specimens are mounted on the test section directly upper the nozzle with a distance of 2 mm to the outlet of the nozzle. Before conducting the erosion experiments, the relationship between the water flow rate and the slurry velocity was established. For all experiments, angular silica sand (109–177 mm, Hv 1280) was selected as the erodent and the liquid was distilled water (pH 6.95). The sand concentration in the slurry was controlled at 15 wt.%. The erosion tests were carried out at slurry impact velocities ranging from 6.4
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to 15.2 m s−1 and at a normal impact angle. Each of the erosion volumes was the product of the mean value of four profile areas. After the erosion experiments, the profile areas of the erosion scars were measured by surface profilometry and the eroded surfaces were observed using optical microscopy and scanning electron microscopy (SEM ).
Fig. 2 shows the volumetric loss on TiN-coated a-Ti specimens with erosion duration at various slurry velocities. It is seen from this figure that the experimental results present the tendency for the erosion volume to increase significantly as the slurry velocity increases. When the TiN-coated a-Ti specimens were subjected to the slurry impact at the lower slurry velocity, the volume losses, for which the results as a function of time for the TiN coatings were approximately linear, increased slightly with the test duration. At intermediate slurry velocity, where the successive slurry impact can result in perforation and fragmentation of the ceramic coatings, the variation of volume loss over the test duration can be reasonably fitted by three stages as indicated in this figure. In the initial slurry impact stage, the low linear erosion volume was maintained until the TiN coating had been perforated. As the test continued, once the TiN coating had been perforated, erosion occurred partially on the a-Ti substrate and the volumetric erosion represented the combined loss of the TiN coating and a-Ti substrate. A significant deviation from the low
erosion volume is observed in tests that extend beyond 40 min. At the transition stage, however, the coating layer had not been eroded through, and the volume losses of the coated specimens were affected very much by the coating layers present. The results also exhibit relatively small erosion volumes associated with a small radius of perforation of the TiN coating layers. The residual coating layer on the erosion spot can act as a protective layer to resist the erosion on the a-Ti substrate, though the effective protection provided by the TiN coating decreased with increasing erosion duration. When the coating layer on the erosion spot had eroded away, the a-Ti substrate was completely exposed to the slurry impact and the material removal then increased linearly with erosion duration. With further increasing slurry velocity, for the test specimens, an initial transient period during which the volume loss increased slowly with erosion duration was observed, followed by an accelerated removal period due to the highly damaged TiN coating. The variation of volume loss for TiN-coated plasmanitrided specimen with erosion duration is displayed in Fig. 3. At any given slurry velocity, the progress of the volume loss is nearly equal to that observed for the TiN-coated a-Ti alloy. The erosion resistances of the TiN coatings deposited on the nitrided Ti substrate were slightly higher than those of the coatings deposited on the a-Ti substrate under the same test conditions. After perforation of the coatings, the volume loss from the nitrided Ti substrate was lower than that from the a-Ti alloy. This is attributed to the fact that the compound layer shows a high erosion resistance. With increasing erosion duration, however, the volume loss of the nitrided Ti substrate increased gradually at a test interval
Fig. 2. Erosion volume loss versus time plots for the TiN-coated plasma-nitrided a-Ti alloy at various velocities.
Fig. 3. Erosion volume loss versus time plots for the TiN-coated plasma-nitrided Ti alloy at various slurry velocities.
3. Results and discussion 3.1. Erosion rates
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Fig. 4. Variation of steady-state erosion rates for the TiN coatings and uncoated a-Ti alloy with the slurry velocity.
owing to reduced hardness at the depth far from the surface. From Figs. 2 and 3 it can be seen that for the TiN coatings clear steady-state erosion rates are established during the periods of the linear volume losses of the coated specimens. As a comparison, the erosion data of uncoated a-Ti alloy were measured under the test conditions. Since the volume loss obtained for the nitrided substrate did not follow a linear relationship with the test duration, it was difficult to determine the erosion rate of the uncoated plasma-nitrided Ti alloy. Fig. 4 presents the influence of slurry velocity on the erosion rates of the TiN coatings and uncoated a-Ti alloy. It is seen from this figure that the experimental results present the tendency for the erosion rate to increase significantly as the slurry velocity increases. Under the same experimental conditions, the steady-state erosion rates for the TiN coatings deposited on a-Ti and nitrided Ti substrate were significantly lower than those for the uncoated a-Ti alloys. This is to be expected because the coating layers are harder than the underlying substrate materials. In many erosion studies, it was found that the erosion rate E is proportional to vn, where v is slurry velocity and n is a semi-empirical velocity exponent [19,20]. Fig. 5 presents the experimental results of the erosion rates for the TiN coating layer and uncoated a-Ti alloy related to slurry velocity in logarithmic plots. In this study, data for erosion rates of the uncoated a-Ti specimens could be fitted to a power law and the velocity exponent was 4.1. However, for the TiN coatings, there was no evidence that the velocity dependence followed a power law relationship. In the slurry velocity range 6.4–15.2 m s−1, for the TiN coatings deposited on a-Ti and nitrided Ti substrates, the lower and intermediate slurry velocities (in the range 6.4–9.6 m s−1) were associ-
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Fig. 5. Logarithm of steady-state erosion rate versus logarithm of velocity component for the TiN coatings and a-Ti alloy.
ated with higher exponents, and, subsequently, the velocity exponent decreased at higher slurry velocities above 9.6 m s−1. 3.2. Surface characteristics and erosion mechanisms The degree of coating surface deterioration exhibited, in terms of erosion mechanisms, is mainly dependent upon the operating conditions. After successive slurry impact at a low velocity, numerous pit formations were distributed over the coating surface on the erosion spot. In general, the pit number and size increased with test duration. However, pit-induced volume loss for each of the TiN coatings was small, as shown in Figs. 2 and 3. With increasing erosion duration, localized perforation initiated at sites where the pitting accumulated. After perforation of the TiN coating, the slurry impact caused fragmentation of the coating layer at the circumference of the perforated zone, resulting in the perforation expansion of the TiN coating in the lateral direction. The quantity of the coating fragmentation resulting from the slurry impact is strongly dependent upon both the slurry velocity and the erosion duration. Fig. 6 shows typical SEM micrographs of the eroded surfaces of the TiN-coated a-Ti alloys after exposure to erosion for 60 min at the lower and intermediate slurry velocities. Similar appearances were also observed on the eroded surface of the TiN-coated plasma-nitrided specimens. As seen in Fig. 6(a), at the lower slurry velocity, the eroded areas are very smooth and only pits are visible, indicating that the TiN coating layers deposited on the a-Ti substrate exhibit higher slurry erosion resistance under this condition. When the slurry velocity was increased to 9.6 m s−1 (see Fig. 6(b)), the erosion topog-
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Fig. 6. SEM micrographs of the eroded surfaces for the TiN-coated a-Ti specimens after exposure to slurry for 60 min at various velocities: (a) 6.4 m s−1; (b) 9.6 m s−1.
raphy showed perforation of the coating in the center of the erosion spot. The perforation and fragmentation of the ceramic coating accelerated as slurry velocity increased. It was found that the failure modes of the coatings at high slurry velocities were very similar to that of Fig. 6(b), indicating that the TiN coatings possess good adhesion to a-Ti and nitrided Ti substrates. Once the coating layers had been removed by the perforation and fragmentation processes, slurry erosion took place in the substrate and then the erosion mechanisms of the test specimens were changed. When the TiN coating layer on the erosion spot of the specimen was completely eroded out, the a-Ti or nitrided Ti substrate was thus exposed as the erosion surface. Fig. 7 reveals the eroded surfaces on the center section of the a-Ti and nitrided Ti substrate at a slurry velocity of 9.6 m s−1. For the softer a-Ti substrate, the eroded surface is characterized by craters and flake formation to some extent ( Fig. 7(a)). Because of solid solution hardening and precipitation hardening, the local plastic deformation on the eroded surface of the nitrided layer was greatly reduced. As seen in Fig. 7(b), the eroded surface of the nitrided Ti substrate exhibits
smaller craters and a small degree of flaking. The flake formation occurred when plastic deformation took place in the vicinity of the slurry impact and a strain-hardened layer formed on the alloy surface after multiple impact. As a critical strain is exceeded in the deformation volume beneath the alloy surface, the flake or the outer strainhardened layer is detached from the eroded surface by ductile fracture, thereby causing the erosive loss of the material. The profilometry measurements of the erosion induced on the TiN-coated a-Ti alloy at a slurry velocity of 9.6 m s−1 are shown in Fig. 8. The evolution of the erosion profile for the TiN-coated plasma-nitrided Ti alloy was quite similar. The depth and width of the erosion scar progressively increased with the test duration. During the initial erosion process, the surface profile for the specimens was relatively smooth owing to the protection of hard TiN coating layer. On further increasing the test duration, however, the slurry erosion penetrated through the coating layer reaching the substrate. The surface profile reveals the larger lateral expansion of the erosion spot after coating perforation. After exposure to slurry erosion for a long time, the size
Fig. 7. SEM micrographs of the eroded surfaces on the center section of the a-Ti and nitrided substrate after exposure for 60 min at a slurry velocity of 9.6 m s−1: (a) a-Ti; (b) nitrided Ti.
J.P. Tu et al. / Surface and Coatings Technology 122 (1999) 176–182
Fig. 8. Evolution of the surface profile of the TiN-coated a-Ti alloy with erosion duration at a slurry velocity of 9.6 m s−1
of perforation of the TiN coating approaches a constant value and remains at a radius of approximately 2.3 mm. Thereafter, an increasing part of the exposed surface is eroding against the substrate as the test is prolonged. The erosion scar expands now mainly in the depth direction. As a consequence, the erosion volume loss on
Fig. 9. Velocity profile for the liquid–particle flow in the test section of the jet-in-slit rig.
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the substrate is significantly increased. In the case of the eroded surface on the a-Ti substrate, it has been shown that the central section has a relatively low depth, and the maximum erosion depth occurs at a radius of 1.0– 1.1 mm from the center of the erosion spot. These characteristics were also seen in the cross-sectional profiles of the eroded a-Ti substrate surfaces at lower and higher slurry velocities. For the TiN-coated plasmanitrided specimen, within the nitrided layer, the difference between maximum erosion depth and the depth on the center of the erosion scar was relatively small. The radius where the maximum erosion depth occurred varied with the erosion duration. As compared with the TiN-coated a-Ti alloy, a smaller radius for the TiNcoated plasma-nitrided specimen was observed in the nitrided layer. Consequently, the erosion volume on the nitrided Ti substrate after coating perforation was reduced. Under the test conditions, the two-phase liquid– particle flow pattern along a given geometry is a complicated process and governed by the laws of hydrodynamics. Owing to jet spreading effects, the local impact angle differs all along the erosion spot, resulting in a distribution of impact angles for the normal impingement angle between the specimen surface and erosion jet. Fig. 9 shows a velocity profile for the liquid–particle flow in the test section of the jet-in-slit rig. The outer regions of the erosion scar are subjected to low angle impact while central regions are subjected to normal impact. Cutting and plowing processes by angular erodent were even more predominant in the outer erosion scar regions of a-Ti and nitrided Ti substrate, as seen in Fig. 10. The a-Ti substrate and the subsurface layer of the nitrided substrate exhibited ductile erosion behavior, namely maximum erosion occurred at low impact angle and minimum erosion at the normal angle [21]. This suggested the presence of a maximum erosion region, where the local impact angle was optimized. During slurry erosion at shallow angles, the hard precipi-
Fig. 10. SEM micrographs of the eroded surfaces at a distance of 1.0 mm from the center of the erosion scars: (a) a-Ti substrate, 9.6 m s−1, 60 min; (b) nitrided Ti substrate, 9.6 m s−1, 60 min.
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tates in the nitrided layer can act as a protective phase to resist the microcutting and microplowing action. The protective effect provided by the precipitates results in mild cutting and plowing appearance on the eroded surface (Fig. 10(b)).
4. Conclusions 1. The experimental results from the TiN-coated a-Ti and plasma-nitrided Ti alloy showed an increasing erosion volume loss as the slurry velocity increased. The erosion rates of the TiN coatings deposited on nitrided Ti substrate were slightly lower than those of the coatings deposited on a-Ti substrate under the same test conditions. 2. Only pitting was induced by the low-velocity slurry impact; the coated specimens presented high slurry erosion resistance due to the protection of hard ceramic coating layers at the low slurry velocities. As the slurry velocity increased, pitting was initially generated by the slurry impact. Nevertheless, perforation and fragmentation of the TiN coating occurred with the erosion duration and the protection of the hard TiN coating layer decreased gradually. 3. As compared with the a-Ti substrate, the nitrided Ti substrate exhibited small volume loss due to high hardness in the nitrided layer.
Acknowledgements This work was supported by a Grant-in-Aid administered by the Ministry of Education, Science, and Culture of Japan. Dr. J.P. Tu wishes to thank Prof. M.
Matsumura and the Japan Society for the Promotion of Science (JSPS) for fellowship.
References [1] H. Mohrbacher, B. Blanpain, J.P. Celis, J.R. Roos, L. Stals, M. Van Stappen, Wear 188 (1995) 130. [2] I.J. Smith, D. Gillibrand, J.S. Brooks, W.-D. Munz, S. Harvey, R. Goodwin, Surf. Coat. Technol. 90 (1997) 164. [3] W.D. Sproul, Thin Solid Films 126 (1985) 257. [4] H.K. Tonshoff, A. Mohlfeld, Surf. Coat. Technol. 93 (1997) 88. [5] L.E. Toth, Transition Metal Carbides and Nitrides, Academic Press, London, 1971. [6 ] H.-J. Spies, B. Larisch, K. Hock, E. Broszeit, Surf. Coat. Technol. 74 (1995) 178. [7] K.H. Habig, Tribol. Int. 22 (1989) 65. [8] J. Takadoum, H. Houmid Bennani, M. Allouard, Surf. Coat. Technol. 88 (1997) 232. [9] Z.P. Huang, Y. Sun, T. Bell, Wear 173 (1994) 13. [10] Y.Y. Guu, J.F. Lin, Surf. Coat. Technol. 85 (1997) 146. [11] S. Wilson, A.T. Alpas, Surf. Coat. Technol. 86 (1997) 75. [12] M. Bromark, M. Larsson, P. Hedenqvist, S. Hogmark, Surf. Coat. Technol. 90 (1997) 217. [13] E. Vancoille, J.P. Celis, J.R. Roos, Wear 165 (1993) 41. [14] Y. Sun, T. Bell, Wear 166 (1993) 119. [15] V.R. Parameswaran, J.-P. Immarigeon, D. Nagy, Surf. Coat. Technol. 52 (1992) 251. [16 ] A.V. Levy, in: J.E. Field, N.S. Corney ( Eds.), Proc. 6th Int. Conf. on Erosion by Liquid and Solid Impact, Newnham College, Cambridge, September (1983) 39-1. [17] J.E. Field, I.M. Hutchings, Proc. 3rd Conference on Mechanical Properties at High Rates of Strain, Oxford, Inst. Phys. Conf. Ser. No. 70 (1984). [18] J.P. Tu, J. Pan, H.X. Zhao, H. Fukunaga, Mater. Sci. Eng. A 263 (1999) 32. [19] C.T. Morrison, R.O. Scattergood, J.L. Routbort, Wear 111 (1986) 1. [20] I. Finnie, D.H. McFadden, Wear 48 (1978) 181. [21] W. Wu, K.C. Goretta, J.L. Routbort, Mater. Sci. Eng. A 151 (1992) 85.
Slurry erosion characteristics of TiN coatings on a-Ti and plasma-nitrided Ti alloy substrates J.P. Tu a,c, *, L.P. Zhu b, H.X. Zhao c a Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China b Zhejiang Technology School of Building Materials, Hangzhou 310014, People’s Republic of China c Department of Chemical Engineering, Hiroshima University, Kagamiyama 1-4-1, Higashi-Hiroshima 739-8527, Japan Received 4 January 1999; accepted in revised form 12 June 1999
Abstract The slurry erosion behavior of TiN coatings on a-Ti and plasma-nitrided Ti alloy substrates in aqueous silica slurry has been investigated using a jet-in-slit rig. Erosion tests were performed with slurry jet velocities between 6.4 and 15.2 m s−1. At the lower slurry velocity, pitting was caused by the slurry impact, and there was little volume loss for each of the coatings. The TiN coatings deposited on the a-Ti and nitrided Ti alloy substrate presented high slurry erosion resistance. With increasing slurry velocity, however, perforation and fragmentation of the hard TiN coatings occurred and the protection effect of the coating layer reduced with the erosion duration. The erosion rates for the coatings deposited on the nitrided Ti substrate were slightly lower than those for the coatings deposited on the a-Ti substrate under the same test conditions. After coating perforation, the surfaces of the substrates were eroded by crater formation and flaking on the center section of the erosion scars while cutting and plowing occurred at the outer regions of the erosion scars. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Plasma nitriding; Slurry erosion; a-Ti alloy; TiN coating
1. Introduction In recent years, increasing applications have been found for thin, hard, wear-resistant ceramic coatings in industry, particularly in cutting and forming tools [1– 4]. The most commonly used coating is titanium nitride ( TiN ), which possesses high hardness and good chemical and metallurgical stability [5]. Cutting and forming tools coated with such coatings usually exhibit improved service behavior and increased lifetime. The potential applications for this coating as erosion barriers include a variety of engineering components, such as blades and pipes. Many investigations [6–13] have focused on both the friction and the wear characteristics of TiN coatings deposited on various engineering materials, as well as the failure mechanisms of TiN films. Like most ceramic materials, TiN coatings exhibit brittle behavior; they sometimes have a great tendency to fracture and spall from the substrate during wear, especially on softer substrates. The nitriding of a steel substrate prior to the * Corresponding author. Fax: +86 571 7051358. E-mail address: [email protected] (J.P. Tu)
TiN coating deposition has a beneficial effect on reduced wear of TiN coatings under the same wear conditions [14]. Although physical, chemical and mechanical properties of TiN coatings are documented satisfactorily, relatively few investigations have been carried out to study their erosion behavior [15]. Slurry erosion occurs when surface material suffers from impingement of solid erodent particles suspended in a carrier liquid. Continued operation under particulate flow condition may cause severe erosion damage on blade or pipe surfaces and can be detrimental to their reliability. There are many factors which influence the erosion rate of materials, for example the flow regime and the kinetic energy of the impacting slurry. The difficulties in predicting erosion performance for coatings are more complex than for bulk materials. Levy [16 ] found that hertzian cone cracks occurred in the lower layer beneath the plastic indentation under particle impact conditions. As the radial cracks in the upper layer developed, they intersected and formed a network of chips that were removed from the surface by succeeding erodent particle impact. In brittle coating materials, erosion is the maximum at normal incidence and the
0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 30 1 - 1
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principal mechanism for material removal is brittle fracture accompanied by a small amount of plastic deformation [17]. The failure of thin ceramic coatings was caused by intercrystalline or transcrystalline fracture, surface cracking and interfacial spalling. In the present work, the slurry erosion properties of TiN coated a-Ti and plasma-nitrided Ti alloy have been investigated. Based on the experimental data and results of microscopic studies, the effects of slurry velocity on erosion mechanisms of the TiN-coated specimens and the substrate nitriding on the erosion resistance of the TiN coatings are evaluated.
2. Experimental procedure 2.1. Preparation of nitrided substrate and TiN coatings The substrate material of the specimens (20 mm in diameter and 1.5 mm in thickness) used in the present work was a commercial a-Ti alloy (O: 0.10 wt.%, N: 0.03 wt.%, C: 0.05 wt.%, H: <0.015 wt.%, Fe: <0.15 wt.%, Si: <0.10 wt.%, Ti: bal.). Plasma nitriding was carried out in pure nitrogen with a constant gas flow of 20 l h−1. At the working pressure of 6.6×102 Pa, a glow discharge (with a current density of 8 mA cm−2) of the reaction gas was created by the d.c voltage (800 V ). The specimens were treated at a temperature of 900°C for 5 h and cooled slowly in the evacuated furnace. The nitrided layer of a-Ti alloy consisted of two different layers, i.e. a thin compound layer (about 3 mm) and a diffusion layer. The XRD data showed that the surface of the compound layer was composed mainly of the e phase ( Ti N ) and a small amount of the d phase 2 ( TiN ) after nitriding treatment. In the diffusion layer, small precipitates were observed after nitriding with subsequently slow furnace cooling. By measuring the characteristic N spectrum using wavelength-dispersive X-ray spectrometry ( WDS), the intensity of the nitrogen line decreased continuously with increasing distance from the surface, and the depth of nitrided layer was nearly 40 mm. As shown in Fig. 1, near the surface, a Vickers hardness value of about Hv 1220 (50 gf ) was observed. The microhardness decreased gradually with increasing distance from the surface. The TiN coatings were produced by cathodic arc ion plating. For the coating process, the a-Ti and nitrided Ti specimens were heated to 300°C in a vacuum deposition chamber at 1.33×10−3–1.33×10−4 Pa. The steered cathode arc plasma source with a titanium target was operated with an arc current between 90 and 100 A. After the titanium ion bombardment at −1000 V bias voltage, the Ti film was deposited with the substrate bias voltage at −210 V. Then N and Ar gases 2 (P =0.21 Pa) were fed into the vacuum deposition N2
Fig. 1. Microhardness as a function of the distance from the surface for a-Ti alloy plasma nitrided at 900°C for 5 h in nitrogen.
chamber. The TiN coatings were deposited on the substrates at a substrate temperature of 400°C and with a coating thickness of 5 mm. Microstructural analysis and the surface and cross-section morphologies of the coatings were identified by X-ray diffraction and scanning electron microscopy (SEM ). The TiN coatings have a dense, fine structure and remain adherent to the substrate. The surface microhardnesses of the ceramic coatings deposited on a-Ti and a nitrided Ti substrate are about Hv 2065 and Hv 2250 (50 gf ), respectively. 2.2. Slurry erosion tests Slurry erosion tests were conducted at room temperature using the jet-in-slit rig, which has been described previously [18]. In this slurry erosion rig, four specimens are installed and immersed in the abrasive slurry. The solid particles are suspended uniformly by an upward flow of liquid. The water stream is jetted from the nozzle with an internal diameter of 1.8 mm which is fitted on the center of the test section, and the slurry is drawn up and mixed with the waterjet stream. The slurry then impinges on the surfaces of the test specimens and is exhausted through the slit between the specimen and guide plate. The specimens are mounted on the test section directly upper the nozzle with a distance of 2 mm to the outlet of the nozzle. Before conducting the erosion experiments, the relationship between the water flow rate and the slurry velocity was established. For all experiments, angular silica sand (109–177 mm, Hv 1280) was selected as the erodent and the liquid was distilled water (pH 6.95). The sand concentration in the slurry was controlled at 15 wt.%. The erosion tests were carried out at slurry impact velocities ranging from 6.4
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to 15.2 m s−1 and at a normal impact angle. Each of the erosion volumes was the product of the mean value of four profile areas. After the erosion experiments, the profile areas of the erosion scars were measured by surface profilometry and the eroded surfaces were observed using optical microscopy and scanning electron microscopy (SEM ).
Fig. 2 shows the volumetric loss on TiN-coated a-Ti specimens with erosion duration at various slurry velocities. It is seen from this figure that the experimental results present the tendency for the erosion volume to increase significantly as the slurry velocity increases. When the TiN-coated a-Ti specimens were subjected to the slurry impact at the lower slurry velocity, the volume losses, for which the results as a function of time for the TiN coatings were approximately linear, increased slightly with the test duration. At intermediate slurry velocity, where the successive slurry impact can result in perforation and fragmentation of the ceramic coatings, the variation of volume loss over the test duration can be reasonably fitted by three stages as indicated in this figure. In the initial slurry impact stage, the low linear erosion volume was maintained until the TiN coating had been perforated. As the test continued, once the TiN coating had been perforated, erosion occurred partially on the a-Ti substrate and the volumetric erosion represented the combined loss of the TiN coating and a-Ti substrate. A significant deviation from the low
erosion volume is observed in tests that extend beyond 40 min. At the transition stage, however, the coating layer had not been eroded through, and the volume losses of the coated specimens were affected very much by the coating layers present. The results also exhibit relatively small erosion volumes associated with a small radius of perforation of the TiN coating layers. The residual coating layer on the erosion spot can act as a protective layer to resist the erosion on the a-Ti substrate, though the effective protection provided by the TiN coating decreased with increasing erosion duration. When the coating layer on the erosion spot had eroded away, the a-Ti substrate was completely exposed to the slurry impact and the material removal then increased linearly with erosion duration. With further increasing slurry velocity, for the test specimens, an initial transient period during which the volume loss increased slowly with erosion duration was observed, followed by an accelerated removal period due to the highly damaged TiN coating. The variation of volume loss for TiN-coated plasmanitrided specimen with erosion duration is displayed in Fig. 3. At any given slurry velocity, the progress of the volume loss is nearly equal to that observed for the TiN-coated a-Ti alloy. The erosion resistances of the TiN coatings deposited on the nitrided Ti substrate were slightly higher than those of the coatings deposited on the a-Ti substrate under the same test conditions. After perforation of the coatings, the volume loss from the nitrided Ti substrate was lower than that from the a-Ti alloy. This is attributed to the fact that the compound layer shows a high erosion resistance. With increasing erosion duration, however, the volume loss of the nitrided Ti substrate increased gradually at a test interval
Fig. 2. Erosion volume loss versus time plots for the TiN-coated plasma-nitrided a-Ti alloy at various velocities.
Fig. 3. Erosion volume loss versus time plots for the TiN-coated plasma-nitrided Ti alloy at various slurry velocities.
3. Results and discussion 3.1. Erosion rates
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Fig. 4. Variation of steady-state erosion rates for the TiN coatings and uncoated a-Ti alloy with the slurry velocity.
owing to reduced hardness at the depth far from the surface. From Figs. 2 and 3 it can be seen that for the TiN coatings clear steady-state erosion rates are established during the periods of the linear volume losses of the coated specimens. As a comparison, the erosion data of uncoated a-Ti alloy were measured under the test conditions. Since the volume loss obtained for the nitrided substrate did not follow a linear relationship with the test duration, it was difficult to determine the erosion rate of the uncoated plasma-nitrided Ti alloy. Fig. 4 presents the influence of slurry velocity on the erosion rates of the TiN coatings and uncoated a-Ti alloy. It is seen from this figure that the experimental results present the tendency for the erosion rate to increase significantly as the slurry velocity increases. Under the same experimental conditions, the steady-state erosion rates for the TiN coatings deposited on a-Ti and nitrided Ti substrate were significantly lower than those for the uncoated a-Ti alloys. This is to be expected because the coating layers are harder than the underlying substrate materials. In many erosion studies, it was found that the erosion rate E is proportional to vn, where v is slurry velocity and n is a semi-empirical velocity exponent [19,20]. Fig. 5 presents the experimental results of the erosion rates for the TiN coating layer and uncoated a-Ti alloy related to slurry velocity in logarithmic plots. In this study, data for erosion rates of the uncoated a-Ti specimens could be fitted to a power law and the velocity exponent was 4.1. However, for the TiN coatings, there was no evidence that the velocity dependence followed a power law relationship. In the slurry velocity range 6.4–15.2 m s−1, for the TiN coatings deposited on a-Ti and nitrided Ti substrates, the lower and intermediate slurry velocities (in the range 6.4–9.6 m s−1) were associ-
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Fig. 5. Logarithm of steady-state erosion rate versus logarithm of velocity component for the TiN coatings and a-Ti alloy.
ated with higher exponents, and, subsequently, the velocity exponent decreased at higher slurry velocities above 9.6 m s−1. 3.2. Surface characteristics and erosion mechanisms The degree of coating surface deterioration exhibited, in terms of erosion mechanisms, is mainly dependent upon the operating conditions. After successive slurry impact at a low velocity, numerous pit formations were distributed over the coating surface on the erosion spot. In general, the pit number and size increased with test duration. However, pit-induced volume loss for each of the TiN coatings was small, as shown in Figs. 2 and 3. With increasing erosion duration, localized perforation initiated at sites where the pitting accumulated. After perforation of the TiN coating, the slurry impact caused fragmentation of the coating layer at the circumference of the perforated zone, resulting in the perforation expansion of the TiN coating in the lateral direction. The quantity of the coating fragmentation resulting from the slurry impact is strongly dependent upon both the slurry velocity and the erosion duration. Fig. 6 shows typical SEM micrographs of the eroded surfaces of the TiN-coated a-Ti alloys after exposure to erosion for 60 min at the lower and intermediate slurry velocities. Similar appearances were also observed on the eroded surface of the TiN-coated plasma-nitrided specimens. As seen in Fig. 6(a), at the lower slurry velocity, the eroded areas are very smooth and only pits are visible, indicating that the TiN coating layers deposited on the a-Ti substrate exhibit higher slurry erosion resistance under this condition. When the slurry velocity was increased to 9.6 m s−1 (see Fig. 6(b)), the erosion topog-
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Fig. 6. SEM micrographs of the eroded surfaces for the TiN-coated a-Ti specimens after exposure to slurry for 60 min at various velocities: (a) 6.4 m s−1; (b) 9.6 m s−1.
raphy showed perforation of the coating in the center of the erosion spot. The perforation and fragmentation of the ceramic coating accelerated as slurry velocity increased. It was found that the failure modes of the coatings at high slurry velocities were very similar to that of Fig. 6(b), indicating that the TiN coatings possess good adhesion to a-Ti and nitrided Ti substrates. Once the coating layers had been removed by the perforation and fragmentation processes, slurry erosion took place in the substrate and then the erosion mechanisms of the test specimens were changed. When the TiN coating layer on the erosion spot of the specimen was completely eroded out, the a-Ti or nitrided Ti substrate was thus exposed as the erosion surface. Fig. 7 reveals the eroded surfaces on the center section of the a-Ti and nitrided Ti substrate at a slurry velocity of 9.6 m s−1. For the softer a-Ti substrate, the eroded surface is characterized by craters and flake formation to some extent ( Fig. 7(a)). Because of solid solution hardening and precipitation hardening, the local plastic deformation on the eroded surface of the nitrided layer was greatly reduced. As seen in Fig. 7(b), the eroded surface of the nitrided Ti substrate exhibits
smaller craters and a small degree of flaking. The flake formation occurred when plastic deformation took place in the vicinity of the slurry impact and a strain-hardened layer formed on the alloy surface after multiple impact. As a critical strain is exceeded in the deformation volume beneath the alloy surface, the flake or the outer strainhardened layer is detached from the eroded surface by ductile fracture, thereby causing the erosive loss of the material. The profilometry measurements of the erosion induced on the TiN-coated a-Ti alloy at a slurry velocity of 9.6 m s−1 are shown in Fig. 8. The evolution of the erosion profile for the TiN-coated plasma-nitrided Ti alloy was quite similar. The depth and width of the erosion scar progressively increased with the test duration. During the initial erosion process, the surface profile for the specimens was relatively smooth owing to the protection of hard TiN coating layer. On further increasing the test duration, however, the slurry erosion penetrated through the coating layer reaching the substrate. The surface profile reveals the larger lateral expansion of the erosion spot after coating perforation. After exposure to slurry erosion for a long time, the size
Fig. 7. SEM micrographs of the eroded surfaces on the center section of the a-Ti and nitrided substrate after exposure for 60 min at a slurry velocity of 9.6 m s−1: (a) a-Ti; (b) nitrided Ti.
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Fig. 8. Evolution of the surface profile of the TiN-coated a-Ti alloy with erosion duration at a slurry velocity of 9.6 m s−1
of perforation of the TiN coating approaches a constant value and remains at a radius of approximately 2.3 mm. Thereafter, an increasing part of the exposed surface is eroding against the substrate as the test is prolonged. The erosion scar expands now mainly in the depth direction. As a consequence, the erosion volume loss on
Fig. 9. Velocity profile for the liquid–particle flow in the test section of the jet-in-slit rig.
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the substrate is significantly increased. In the case of the eroded surface on the a-Ti substrate, it has been shown that the central section has a relatively low depth, and the maximum erosion depth occurs at a radius of 1.0– 1.1 mm from the center of the erosion spot. These characteristics were also seen in the cross-sectional profiles of the eroded a-Ti substrate surfaces at lower and higher slurry velocities. For the TiN-coated plasmanitrided specimen, within the nitrided layer, the difference between maximum erosion depth and the depth on the center of the erosion scar was relatively small. The radius where the maximum erosion depth occurred varied with the erosion duration. As compared with the TiN-coated a-Ti alloy, a smaller radius for the TiNcoated plasma-nitrided specimen was observed in the nitrided layer. Consequently, the erosion volume on the nitrided Ti substrate after coating perforation was reduced. Under the test conditions, the two-phase liquid– particle flow pattern along a given geometry is a complicated process and governed by the laws of hydrodynamics. Owing to jet spreading effects, the local impact angle differs all along the erosion spot, resulting in a distribution of impact angles for the normal impingement angle between the specimen surface and erosion jet. Fig. 9 shows a velocity profile for the liquid–particle flow in the test section of the jet-in-slit rig. The outer regions of the erosion scar are subjected to low angle impact while central regions are subjected to normal impact. Cutting and plowing processes by angular erodent were even more predominant in the outer erosion scar regions of a-Ti and nitrided Ti substrate, as seen in Fig. 10. The a-Ti substrate and the subsurface layer of the nitrided substrate exhibited ductile erosion behavior, namely maximum erosion occurred at low impact angle and minimum erosion at the normal angle [21]. This suggested the presence of a maximum erosion region, where the local impact angle was optimized. During slurry erosion at shallow angles, the hard precipi-
Fig. 10. SEM micrographs of the eroded surfaces at a distance of 1.0 mm from the center of the erosion scars: (a) a-Ti substrate, 9.6 m s−1, 60 min; (b) nitrided Ti substrate, 9.6 m s−1, 60 min.
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tates in the nitrided layer can act as a protective phase to resist the microcutting and microplowing action. The protective effect provided by the precipitates results in mild cutting and plowing appearance on the eroded surface (Fig. 10(b)).
4. Conclusions 1. The experimental results from the TiN-coated a-Ti and plasma-nitrided Ti alloy showed an increasing erosion volume loss as the slurry velocity increased. The erosion rates of the TiN coatings deposited on nitrided Ti substrate were slightly lower than those of the coatings deposited on a-Ti substrate under the same test conditions. 2. Only pitting was induced by the low-velocity slurry impact; the coated specimens presented high slurry erosion resistance due to the protection of hard ceramic coating layers at the low slurry velocities. As the slurry velocity increased, pitting was initially generated by the slurry impact. Nevertheless, perforation and fragmentation of the TiN coating occurred with the erosion duration and the protection of the hard TiN coating layer decreased gradually. 3. As compared with the a-Ti substrate, the nitrided Ti substrate exhibited small volume loss due to high hardness in the nitrided layer.
Acknowledgements This work was supported by a Grant-in-Aid administered by the Ministry of Education, Science, and Culture of Japan. Dr. J.P. Tu wishes to thank Prof. M.
Matsumura and the Japan Society for the Promotion of Science (JSPS) for fellowship.
References [1] H. Mohrbacher, B. Blanpain, J.P. Celis, J.R. Roos, L. Stals, M. Van Stappen, Wear 188 (1995) 130. [2] I.J. Smith, D. Gillibrand, J.S. Brooks, W.-D. Munz, S. Harvey, R. Goodwin, Surf. Coat. Technol. 90 (1997) 164. [3] W.D. Sproul, Thin Solid Films 126 (1985) 257. [4] H.K. Tonshoff, A. Mohlfeld, Surf. Coat. Technol. 93 (1997) 88. [5] L.E. Toth, Transition Metal Carbides and Nitrides, Academic Press, London, 1971. [6 ] H.-J. Spies, B. Larisch, K. Hock, E. Broszeit, Surf. Coat. Technol. 74 (1995) 178. [7] K.H. Habig, Tribol. Int. 22 (1989) 65. [8] J. Takadoum, H. Houmid Bennani, M. Allouard, Surf. Coat. Technol. 88 (1997) 232. [9] Z.P. Huang, Y. Sun, T. Bell, Wear 173 (1994) 13. [10] Y.Y. Guu, J.F. Lin, Surf. Coat. Technol. 85 (1997) 146. [11] S. Wilson, A.T. Alpas, Surf. Coat. Technol. 86 (1997) 75. [12] M. Bromark, M. Larsson, P. Hedenqvist, S. Hogmark, Surf. Coat. Technol. 90 (1997) 217. [13] E. Vancoille, J.P. Celis, J.R. Roos, Wear 165 (1993) 41. [14] Y. Sun, T. Bell, Wear 166 (1993) 119. [15] V.R. Parameswaran, J.-P. Immarigeon, D. Nagy, Surf. Coat. Technol. 52 (1992) 251. [16 ] A.V. Levy, in: J.E. Field, N.S. Corney ( Eds.), Proc. 6th Int. Conf. on Erosion by Liquid and Solid Impact, Newnham College, Cambridge, September (1983) 39-1. [17] J.E. Field, I.M. Hutchings, Proc. 3rd Conference on Mechanical Properties at High Rates of Strain, Oxford, Inst. Phys. Conf. Ser. No. 70 (1984). [18] J.P. Tu, J. Pan, H.X. Zhao, H. Fukunaga, Mater. Sci. Eng. A 263 (1999) 32. [19] C.T. Morrison, R.O. Scattergood, J.L. Routbort, Wear 111 (1986) 1. [20] I. Finnie, D.H. McFadden, Wear 48 (1978) 181. [21] W. Wu, K.C. Goretta, J.L. Routbort, Mater. Sci. Eng. A 151 (1992) 85.