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Influence of the substrate hardness on the cavitation erosion resistance of TiN coating
Wear 263 (2007) 395–401
Influence of the substrate hardness on the cavitation erosion resistance of TiN coating Alicja Krella a,∗ , Andrzej Czyzniewski b a
b
The Szewalski Institute of Fluid-Flow Machinery, Polish Academy of Sciences, ul. Fiszera 14, 80-231 Gdansk, Poland Technical University of Koszalin, Department of Materials Science and Engineering, Raclawicka 15-17, 75-620 Koszalin, Poland Received 5 August 2006; received in revised form 14 December 2006; accepted 1 January 2007 Available online 23 May 2007
Abstract Nanocrystalline TiN coating was deposited by means of the cathodic arc method on stainless steels types X6CrNiTi18-10 and X39Cr13. Both steels were subjected to thermal treatment in order to obtain substrates of different hardness: 1.7 GPa, 2.8 GPa and 4.6 GPa. The TiN coating was 3.7 m thick. The TiN coating has strong (1 1 1) crystallographic orientation and fine crystalline structure of ␦-TiN phase. The TiN coating is characterized by high hardness (25.4 GPa) and good adhesion. The adhesion increases with the substrate hardness. The evaluation of TiN coating resistance to cavitation erosion is based on the investigation performed in a cavitation tunnel with a slot cavitator and tap water as a medium. The estimated cavitation resistance parameters of coating were the incubation period of damage and the total mass loss after the whole test. It has been confirmed that the incubation periods of the coated steels were from 2 to 4 times longer than that of the uncoated steels. The mass losses of the coated steels decrease approximately 2.5 times in comparison with the uncoated steels. The scanning microscope analysis indicates that the damage of TiN coating is mainly due to its delamination. The character of the coating and substrate damage in multiple locations indicates that the hard coating micro-particles torn off during the cavitation bubbles implosion hit against the coating and the revealed areas of substrate. As a result, the coating and especially the substrate of relatively low hardness beside cavitation erosion are subject to solid particle erosion with the hard torn off micro-particles of coating. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline TiN coating; Stainless steel; Cavitation erosion
1. Introduction TiN coatings are widely used for the protection of tools. These coatings generally provide high hardness, low coefficient of friction, good corrosion and oxidation wear resistance [1,2]. TiN coatings possess good impact resistance and cavitation resistance [1,2]. Cavitation phenomena usually occur in fluid systems where there are strong pressure fluctuations in fluids [3]. These fluctuations nucleate bubbles, which implode causing highenergy impact on solid surface. These implosions can fracture surface coating and can dislodging particles from the surface. Generally fine-grained materials with high Young modulus, high hardness, and smooth surface are best for resisting cavitation damage [4–7]. The mechanism of cavitation resistance of coated materials is more complicated due to the mismatch of mechan∗
Corresponding author. Tel.: +48 58 341 12 71; fax: +48 58 341 61 44. E-mail address: [email protected] (A. Krella).
0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2007.02.003
ical properties and the adhesion of the coating to the substrate [8]. This paper reports the results of a study of the cavitation resistance of 3.7 m thick TiN coatings on three hardnesses of stainless steel. Authors expect that higher substrate hardness causes better hard coating protection in cavitation erosion as a result of less plastic flow and therefore better adhesion. 2. Experimental details 2.1. Materials Stainless steel X39Cr13 and X6CrNiTi18-10 were used as the substrate materials. The chemical compositions of steels are presented in Table 1. The specimens made of the X6CrNiTi1810 steel were subjected to solutioning at 1050 ◦ C. The obtained hardness was 1.7 GPa. The specimens made of the X39Cr13 steel were subjected to quenching at 1050 ◦ C and tempering at 400 ◦ C and 600 ◦ C to obtain hardness of 4.6 GPa and 2.8 GPa, respec-
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Table 1 The chemical composition of steels Content of an element [%]
X39Cr13 X6CrNiTi18-10
C
Mn
Si
Ni
Ti
Cr
P
S
0.44 0.014
0.5 1.65
0.47 0.61
– 9.24
– 0.23
13.31 17.36
0.025 0.024
0.030 0.029
tively. All specimens (45 mm × 26 mm × 14 mm) were ground with a series of emery papers of 120, 320 and 600 grits, the final step was performed with diamond grinding paste to achieve the roughness of Ra ≤ 0.05 m. The uncoated specimens were marked 1.7, 2.8 and 4.6, where the number means the hardness of the specimen. 2.2. Deposition and investigation of the TiN coating The TiN coating was deposited by cathodic arc method (ARC) in a vacuum chamber equipped with arc sources, with a target 100 mm in diameter. The 99.9% pure titanium target, 99.995% pure argon and 99.995% pure nitrogen gases were applied. Substrates preparation for the process consisted of ultrasonic-aided cleaning with organic solvents and alkaline detergents. The typical process of coating deposition includes the following operations: pumping-off air from the chamber down to the pressure below 2 × 10−3 Pa; heating the substrates to the temperature of 350 ◦ C; cleaning of the substrates by argon and titanium ions; deposition of thin titanium interlayer (∼0.05 m thickness) in the argon atmosphere; deposition of TiN coating in the nitrogen atmosphere up to the thickness of approximately 3.7 m. Parameters applied to the deposition of TiN coating are presented in Table 2. The coated specimens were marked TiN-1.7, TiN-2.8 and TiN-4.6, where 1.7 means X6CrNiTi18-10 steel with hardness of 1.7 GPa, 2.8–X39Cr13 steel with hardness of 2.8 GPa, and 4.6–X39Cr13 steel with hardness of 4.6 GPa. Basic properties of TiN coating deposited on X39Cr13 and on X6CrNiTi18-10 steels are shown in Table 3. The phase composition of the coating measured on a DRON2 X-ray diffractometer using Co K␣ radiation showed that the TiN coating is consistent with phase of ␦-TiN, what was confirmed by strong (1 1 1) plane reflection (Fig. 1). The grains size was determined by means of the Scherrer method with reflex parameters (location and FWHM) using Gaussian analysis. The size of TiN crystallites was estimated to be approximately 16 nm. The coating morphology and thickness were examined with JEOL JSM 5500 LV scanning electron microscope (SEM). The microphotograph
of the cross-section and the surface of TiN coating are shown on Fig. 2. Impurities visible on the coating surface are the microdroplets of titanium. These microdroplets always occur when the cathodic arc plasma method is used. The hardness and Young modulus were measured with a NanoHardness Tester (CSEM) using the method of Oliver and Pharr [9]. The maximum indentation depth of 300 nm was applied. The obtained hardnesses and Young modulus are presented in Table 3. A scratch tester Revetest® produced by CSEM was used to investigate the adhesion of coating. A diamond indenter with radius 0.2 mm was used and measurements were carried out at normal loading rate of 100 N/min, scratch speed of 10 mm/min and scratch length of 10 mm. At least three scratches were done. The first minor cracks for TiN-4.6, TiN-2.8 and TiN-1.7 occurred with a scratch made under the load of 29N, 20 N and 10N, respectively (Fig. 3). These loads correspond to the critical load LC1 defined by the occurrence of the first cohesive failure of the Table 3 Properties of TiN coating
Phase composition, structure Crystallographic orientation Mean crystallites size (nm0 Hardness (GPa) Young’s modulus (GPa) Coating thickness (m) Adhesion LC1 (N) LC2 (N) Roughness Ra (m)
TiN-4.6
TiN-2.8
TiN-1.7
␦-TiN (1 1 1) 16 ± 2 25.4 ± 2.5 295 ± 38 3.7 ± 0.1
␦-TiN (1 1 1) 16 ± 2 25.4 ± 2.5 295 ± 38 3.7 ± 0.1
␦-TiN (1 1 1) 16 ± 2 25.4 ± 2.5 295 ± 38 3.7 ± 0.1
29 ± 3 48 ± 3 0.31 ± 0.03
20 ± 3 36 ± 3 0.35 ± 0.03
10 ± 3 23 ± 3 0.35 ± 0.03
Table 2 Deposition parameters of TiN coating Pressure of residual gases Working pressure of argon Working pressure of nitrogen Arc current Substrate bias voltage Substrate temperature Target–substrate distance
2 × 10−3 Pa 1 Pa 1 Pa 80 A −100 V ∼350 ◦ C 150 mm Fig. 1. X-ray diffraction patterns of the TiN coating.
A. Krella, A. Czyzniewski / Wear 263 (2007) 395–401
Fig. 2. The cross-section and the surface of the TiN coating, SEM.
coating, that are the cracks caused by tensile stresses inside and on the edges of the scratch behind the sliding diamond cone. The critical loads LC2, at which the coating removal from inside of the scratch starts, were 48 N, 36 N and 23 N, respectively. The performed examinations show that the hardness of substrate has an influence on coating adhesion. Along with the increase of substrate hardness the increase of adhesion occurred. The stylus profilometer Hommel-Tester 2000 was used to measure roughness (Ra ) the substrate and TiN coating. 2.3. Procedure of cavitation erosion test The experimental tests were performed in a cavitation tunnel equipped with a system of barricades. The schematic of the cavitation chamber is shown in Fig. 4. Cavitation intensity is controlled by adjusting the slot witdh and the boost pomp speed. Flow conditions are defined by the p1 and p2 absolute pressures measured at the chamber inlet and outlet, respectively. Vertical positioning of the specimen is easily adjustable by means of spacer washer. Tap water is used as the working liquid. The cavities (cavitating vortices and bubbles) are generated by the pressure decrease in the slot between two semi-cylindrical barricades. The TiN coated and uncoated specimens were subjected to cavitation impingement in the cavitation chamber operated with inlet pressure p1 = 1000 kPa, outlet pressure p2 = 130 kPa, and
397
Fig. 4. Schematic view of cavitation chamber with a system of barricades. I: stationary barricade; II: moving counter-barricade.
the slot width – 5 mm. Tests were performed without spacer washer. In order to obtain the erosion curves, the mass loss was measured after each exposure interval. The specimens were cleaned, dried and weighed before the test and after each test interval. Mass losses of the tested specimens were measured using an analytical balance with the permissible error limit of the balance ±1.4 mg for the load up to 100 g. At the beginning of the cavitation test the measurements were conducted every 30 min of exposure (for the first 180 min of test) to estimate the incubation period, and then the duration of exposure intervals was gradually increased. The total cavitation test exposure was 600 min. After each particular exposure time the cavitation erosion damage was analysed with macroscopic sample surface observation and the microstructure was investigated using the scanning electron microscope. 3. Results The mass losses of TiN coated and uncoated specimens arising during cavitation tests are shown in the graphic form in Fig. 5. The incubation period of all uncoated specimens was identical, and lasted less than 90 min. The least mass loss occurred on X6CrNiTi18-10 austenitic steel (7 mg), while the highest mass loss occurred on X39Cr13 steel with hardness of
Fig. 3. TiN coating damage arising during scratch test.
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Fig. 5. Erosion curves of uncoated and TiN coated specimens.
4.6 GPa (11 mg). The SEM images of damaged specimens after the whole test are shown in Fig. 6. The surface of uncoated X6CrNiTi18-10 steel under cyclic action of cavitation pulses underwent undulation; originated slip bands agglomerates and big twins (Fig. 6a). The microcracks have been initiated at intrusions in slip bands (Fig. 6a) like in fatigue [10]. Microstructure of uncoated X39Cr13 (both hardness) steel under action of cavitation has been disclosed (Fig. 6 b, c). In case of X39Cr13 steel with hardness of 2.8 GPa, degradation has proceeded along martensite lathings causing material groove (Fig. 6b), therefore the shape of martensite lathings are not so clear as that arisen on X39Cr13 steel with hardness of 4.6 GPa (Fig. 6c). On X39Cr13 steel with hardness of 2.8 GPa some pits/microtunels (arrows) are also observed in Fig. 6b. In case of X39Cr13 steel with hardness of 4.6 GPa, because of high hardness the deformation is restrained in martensite lathings. Material is crumbled along streak lines (Fig. 6c). These lines are combinated with water flow and construction of cavitation tunnel. The best cavitation erosion resistance of uncoated austenitic steel is probably linked with absorption some degradation energy on plastic deformation (the surface undulation and the origin of some slip bands – Fig. 6a) and phase transformation due to cyclic impact during cavitation test (Fig. 7). The X-ray diffraction pattern of the X6CrNiTi18-10 steel after the cavitation test (Fig. 7) shows the increase of diffraction peak of Fe-␣ (1 1 1) and additionally appearance of the Fe-␣ (2 0 0), Fe-␣ (2 1 1) and Fe-␣ (2 2 0) reflections and the weakness of Fe-␥ reflections. The temporary arrest of mass loss that occurred between 90 and 120 min of erosion was most likely correlated with the phase transformation. The phase transformation, in turn, caused changes in the mechanical and physical properties on the specimen surface. Deposition of TiN coating on specimens caused the lengthening of incubation period and the decrease of mass loss for all tested coated specimens in comparison to uncoated specimens after the whole cavitation test (Fig. 5). The incubation period amounted <180 min, <240 min and <360 min, respectively, for TiN-2.8, TiN-1.7 and TiN-4.6. The highest elongation of incubation period was observed for TiN-4.6 system, which has the
Fig. 6. SEM images of cavitation damages arisen on (a) uncoated X6CrNiTi1810 austenitic steel with hardness of 1.7 GPa, (b) uncoated X39Cr13 steel with hardness of 2.8 GPa and (c) uncoated X39Cr13 steel with hardness of 4.6 GPa.
highest substrate hardness. Also the highest decrease in mass loss occurred for TiN-4.6 system. This reduction amounted 7 mg (from 11 mg for uncoated steel to 4 mg for TiN coated steel). In case of TiN-2.8 system the decrease of mass loss amounted 4 mg (from 9 mg for uncoated steel to 5 mg for TiN coated steel) whereas in case of TiN-1.7 system – 3 mg (from 7 mg for uncoated steel to 4 mg for TiN coated steel). The arrest of mass loss observed in TiN-1.7 was probably caused by phase transformation (Fig. 8) like in uncoated X6CrNiTi18-10 steel (Fig. 7). The SEM images of coated specimens damages after the whole test are shown in Fig. 9. TiN coated X6CrNiTi18-10 steel
A. Krella, A. Czyzniewski / Wear 263 (2007) 395–401
Fig. 7. X-ray diffraction patterns of the X6CrNiTi18-10 steel before and after cavitation test.
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Fig. 8. X-ray diffraction patterns of the TiN coated X6CrNiTi18-10 steel before and after cavitation test.
Fig. 9. SEM images of cavitation damages of TiN coated X6CrNiTi18-10 steel with hardness of 1.7 GPa (a, b), TiN coated X39Cr13 steel with hardness of 2.8 GPa (c, d) and TiN coated X39Cr13 steel with hardness of 4.6 GPa (e, f).
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(TiN-1.7) underwent deformation (Fig. 9a), which was probably related to the substrate undulation, most of TiN microdroplets were removed from the coating. The microcracks were initiated mostly on the ripples (Fig. 9b) probably due to arising tensile stresses that might have exceeded the coating toughness. The cracks developed along the ripples as a result of the micro-cracks merging. Afterwards, they radiated to further ripples through concave traces of post-microdroplets that are the discontinuous spots and reduce coating durability. It shows that coating defects affect the acceleration of the microcracks growth. The undulation of the coating causes also the delamination process. Delamination was observed on the peripheral side of the indentations (Fig. 9b). At some areas the TiN coating was detached; there none of the TiN coating particles remained (Fig. 9a). This could be evidence that adhesion was not high enough and the coating was possibly removed by delamination. On the uncovered surfaces an erosion of the substrate can be observed. SEM images of degraded TiN coating deposited on X39Cr13 steel with hardness of 2.8 GPa (TiN-2.8) are given in Fig. 9 c, d. In the low eroded places the TiN coating underwent undulating (Fig. 9c), but the degree of undulation is lower then that arose at TiN-1.7 (Fig. 9a). Most of TiN microdroplets were removed from the coating. At intensively eroded area the TiN coating was removed, but some particle of TiN coating remained (Fig. 9d). This could be evidence that adhesion was quite high in this dynamic loading condition. On the uncovered surfaces an erosion of the substrate was observed. The larger magnification analysis (Fig. 9d) implies that the TiN-2.8 system was degraded in a brittle manner and probably microparticles removed from hard coating during implosion hit against both the coating and exposed areas of substrate. An image of eroded TiN coating deposited on X39Cr13 with hardness of 4.6 GPa reveals lack of surface undulation (Fig. 9e). Nearly all the TiN microdroplets were removed from the coating. At intensively eroded areas the TiN coating was detached; there none of the TiN coating particles remained. This might suggest that the TiN coating was removed by delamination. On the uncovered surfaces an erosion of the substrate was observed. The larger magnification analysis (Fig. 9f) discloses that cavitation pulses hitting the surface stab the TiN coating and stick some coating particles into the substrate despite the substrate hardness. 4. Discussion Cavitation erosion resistance of conventional materials depends on: tensile strength, strain energy, resilience, ductility, hardness, fatigue resistance and fine structure [3–5,11,12]. Increase of strength and hardness tend to increase the incubation period and the cavitation erosion resistance providing that brittleness does not increase. Increase of brittleness causes increase of failure. Erosion resistance of coated steel depends on coating and substrate properties and also on coating adhesion [8]. Mann and Arya [13] have noticed that hardness of coatings plays a significant role in the lengthening of the incubation period. Presented in this paper results of cavitation tests have also conducted that the TiN coating deposition lengthened the incubation
period. The highest prolongation occurred for the TiN coating deposited on substrate with hardness of 4.6 GPa, which had the highest adhesion among investigated substrates (Table 3). This substrate exhibits as well the highest hardness. It has been noted (Table 3) that with increase of the substrate hardness the adhesion increases as well. However, there has not been found straight correlation between substrate hardness or/and adhesion and the incubation period. For X39Cr13 substrates, which were subjected to different thermal treatment to obtain different hardness, correlation between coating adhesion and the incubation period was noted. The coated X6CrNiTi18-10 steel, which has the lowest substrate hardness (1.7 GPa), exhibited longer incubation period than the substrate with hardness of 2.8 GPa (Fig. 5). Authors attributed this elongation of the incubation period to the phase transformation of X6CrNiTi18-10 steel (Fig. 7). The authors of [4,6,14] also ascribed good cavitation resistance of austenitic steels to the phase transformation that occurred during cavitation. The phase transformation needs energy and typically it occurs during heating. During cavitation bubbles collapse and induce microjets, which may produce intense local heating [15]. The material surface subjected to cavitation may experience the temperature of 103 K magnitude, pressure shock wave of 103 MPa and heating and rates approximately 1010 K/s [15,16]. The energy delivered from collapsing bubbles must be either absorbed or dissipated by the solid material or reflected as shock waves in liquid. Some of this energy is absorbed by changes in dislocation structure [4,17], some of it used by the phase transformation Fe␥ → Fe␣ + carbide in metastable austenitic steels [6,14,18]. Changes in dislocation structure of austenitic steel start with the beginning cavitation test [4,14,17]. It was noticed in [17] that after 5 min of exposure dense dislocation network, small dislocation cells, dislocation loops and slip bands crossed by cutting bands developed due to very high degradation energy. The change in phase and dislocation structure causes changes in material properties and changes the cavitation erosion resistance of material. Degradation process of the TiN coating was determined by substrate behaviour. The TiN coating underwent plastic deformation in case of substrates with hardness of 2.8 and 1.7 GPa. The degree of the coating deformation was correlated to the substrate undulation. The authors of [19,20] noticed that the substrate deformation occurs faster than the deformation of the coating. This could lead to coating delamination. The coating delamination was observed in Fig. 9. When the delamination occurs, the substrate does not aid the coating with carrying the loads. This could lead to crumble the TiN coating away. Time needed to start the delamination and crumbling the TiN coating is correlated with wear resistance of the coated steel. As long as the system was able to absorb the degradation energy the split of the TiN coating did not occur and the incubation period lasted. Presented results show (Fig. 5) that the longest incubation period occurred for TiN-4.6 system, which had the hardest substrate. Later on, the TiN coating was crushed in brittle mode (Fig. 9). Hard crushed TiN coating hammers itself into the substrate, especially if the substrate is relatively soft in comparison with the coating hardness (Fig. 9). Moreover, hard micro-particles removed from the TiN coating hit again coated
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steel causing acceleration of the delamination process and consequently cavitation erosion damages. Also, Mann and Arya [13] noticed that hard-coated steels are failed in brittle mode. Nevertheless, mass losses of the TiN coated steels are clearly less than uncoated steels and the decrease of the mass losses is correlated with the substrate hardness/adhesion. Degradation energy needed to cause the coating delamination and the removal TiN coating particles is linked with adhesion and consequently with erosion resistance of the system. With increase of the adhesion/substrate hardness the decrease of the mass loss occurred as well. 5. Conclusion • Deposition TiN coating on steel substrates lengthened the incubation period and lessened the mass loss of the coated steel. The incubation periods of the coated steels were from 2 to 4 times longer than that of the uncoated steels. The mass losses of the coated steels decrease approximately 2.5 times in comparison with the uncoated steels. • In case of X6CrNiTi 18–10 steel degradation energy delivered to material from imploding cavitation bubbles is partially absorbed by phase transformation Fe␥ → Fe␣ causing the change of the material properties and the increase of the resistance to cavitation loss of uncoated and coated steels. • The TiN coating deposited on X39Cr13 steel with hardness of 4.6 GPa enhances the cavitation erosion resistance the most among all tested coated steels, in comparison to uncoated steels. Acknowledgements Results presented in this paper were funded by budget resources allocated to scientific research in 2005–2007 as a research project no. 4 T07B 015 28. References [1] S. M¨unsterer, K. Kohlhof, Cavitation protection by low temperature TiCN coatings, Surf. Coat. Technol. 74–75 (1995) 642–647.
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[2] L.Z. Yu, The corrosion resistance and wear resistance of thick TiN coatings deposited by arc ion plating, Surf. Coat. Technol. 145 (2000) 80– 87. [3] R.T. Knapp, J.W. Daily, F.G. Hammit, Cavitation, McGraw-Hill, New York, 1970. [4] I.N. Bogachev, R.I. Mints, Cavitation erosion and means for its prevention, Israel Program for Scientific Transactions, Jerusalem, 1966. [5] Z. Reymann, K. Steller, Ocena odporno´sci materiał´ow na działanie kawitacji przepływowej (Assessment of materials resistance to flow cavitation – in Polish). IMP PAN Rep. 67–68, Gdansk, 1975. [6] Y. Zhang, Z. Wang, Y. Cui, The cavitation behavior of metastable Cr–Mn–Ni steel, Wear 240 (2000) 231–234. [7] F.J. Heymann, On the time dependence of the rate of erosion due to impingement or cavitation, ASTM 408, Baltimore, 1967. [8] Y. Iwai, T. Honda, H. Yamada, T. Matsumura, M. Larson, S. Hogmark, Evaluation of wear resistance of thin hard coatings by a new solid particle impact test, Wear 251 (2001) 861–867. [9] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (1992) 1564–1583. [10] J. Schijve, Fatigue of structures and materials in the 20th century and the state of the art, Int. J. Fatigue 25 (2003) 679–702. [11] F.G. Hammit, Cavitation and Multiphase Flow Phenomena, McGraw-Hill Inc., 1980. [12] R.H. Richman, W.P. McNaughton, Correlation of cavitation erosion behavior with mechanical properties of metals, Wear 140 (1990) 63–82. [13] B.S. Mann, V. Arya, An experimental study to correlate jest impingement erosion resistance and properties of metallic materials and coatings, Wear 253 (2002) 650–661. [14] C.J. Heathcock, B.E. Protheroe, A. Ball, Cavitation erosion of stainless steels, Wear 81 (1982) 311–327. [15] K.S. Suslick, Y. Didenko, M.M. Fang, T. Hyeon, K.J. Kolbeck, W.B. McNamara III, M.M. Mdleleni, M. Wong, Acoustic cavitation and its chemical consequences, Phil. Trans. Roy. Soc. A (1999) 1–21. [16] B. Lernuwat, K. Sugiyma, Y. Matsumoto, Modeling of thermal behaviour inside a bubble. Presented at CAV 2001, in: Four International Symposium on Cavitation, California Institute of Technology, Pasadena CA, USA, June 20–23, 2001. [17] A. Krella, Influence of cavitation intensity on X6CrNiTi 18-10 stainless steel performance in the incubation, Wear 258 (2005) 1723–1731. [18] C.Z. Wu, Y.J. Chen, T.S. Shih, Phase transformation in austempered ductile iron by microjets impact, Mater. Charact. 48 (2002) 43–54. [19] A.A. Voevodin, R. Bantle, A. Matthews, Dynamic impact wear of TiCx Ny and Ti-DLC composite coatings, Wear 185 (1995) 151–157. [20] T.D. Raju, K. Nakasa, M. Kato, Relation between delamination of thin films and backward deviation of load–displacement curves under repeating nanoindentation, Acta Materialia 51 (2003) 457–467.
Influence of the substrate hardness on the cavitation erosion resistance of TiN coating Alicja Krella a,∗ , Andrzej Czyzniewski b a
b
The Szewalski Institute of Fluid-Flow Machinery, Polish Academy of Sciences, ul. Fiszera 14, 80-231 Gdansk, Poland Technical University of Koszalin, Department of Materials Science and Engineering, Raclawicka 15-17, 75-620 Koszalin, Poland Received 5 August 2006; received in revised form 14 December 2006; accepted 1 January 2007 Available online 23 May 2007
Abstract Nanocrystalline TiN coating was deposited by means of the cathodic arc method on stainless steels types X6CrNiTi18-10 and X39Cr13. Both steels were subjected to thermal treatment in order to obtain substrates of different hardness: 1.7 GPa, 2.8 GPa and 4.6 GPa. The TiN coating was 3.7 m thick. The TiN coating has strong (1 1 1) crystallographic orientation and fine crystalline structure of ␦-TiN phase. The TiN coating is characterized by high hardness (25.4 GPa) and good adhesion. The adhesion increases with the substrate hardness. The evaluation of TiN coating resistance to cavitation erosion is based on the investigation performed in a cavitation tunnel with a slot cavitator and tap water as a medium. The estimated cavitation resistance parameters of coating were the incubation period of damage and the total mass loss after the whole test. It has been confirmed that the incubation periods of the coated steels were from 2 to 4 times longer than that of the uncoated steels. The mass losses of the coated steels decrease approximately 2.5 times in comparison with the uncoated steels. The scanning microscope analysis indicates that the damage of TiN coating is mainly due to its delamination. The character of the coating and substrate damage in multiple locations indicates that the hard coating micro-particles torn off during the cavitation bubbles implosion hit against the coating and the revealed areas of substrate. As a result, the coating and especially the substrate of relatively low hardness beside cavitation erosion are subject to solid particle erosion with the hard torn off micro-particles of coating. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline TiN coating; Stainless steel; Cavitation erosion
1. Introduction TiN coatings are widely used for the protection of tools. These coatings generally provide high hardness, low coefficient of friction, good corrosion and oxidation wear resistance [1,2]. TiN coatings possess good impact resistance and cavitation resistance [1,2]. Cavitation phenomena usually occur in fluid systems where there are strong pressure fluctuations in fluids [3]. These fluctuations nucleate bubbles, which implode causing highenergy impact on solid surface. These implosions can fracture surface coating and can dislodging particles from the surface. Generally fine-grained materials with high Young modulus, high hardness, and smooth surface are best for resisting cavitation damage [4–7]. The mechanism of cavitation resistance of coated materials is more complicated due to the mismatch of mechan∗
Corresponding author. Tel.: +48 58 341 12 71; fax: +48 58 341 61 44. E-mail address: [email protected] (A. Krella).
0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2007.02.003
ical properties and the adhesion of the coating to the substrate [8]. This paper reports the results of a study of the cavitation resistance of 3.7 m thick TiN coatings on three hardnesses of stainless steel. Authors expect that higher substrate hardness causes better hard coating protection in cavitation erosion as a result of less plastic flow and therefore better adhesion. 2. Experimental details 2.1. Materials Stainless steel X39Cr13 and X6CrNiTi18-10 were used as the substrate materials. The chemical compositions of steels are presented in Table 1. The specimens made of the X6CrNiTi1810 steel were subjected to solutioning at 1050 ◦ C. The obtained hardness was 1.7 GPa. The specimens made of the X39Cr13 steel were subjected to quenching at 1050 ◦ C and tempering at 400 ◦ C and 600 ◦ C to obtain hardness of 4.6 GPa and 2.8 GPa, respec-
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Table 1 The chemical composition of steels Content of an element [%]
X39Cr13 X6CrNiTi18-10
C
Mn
Si
Ni
Ti
Cr
P
S
0.44 0.014
0.5 1.65
0.47 0.61
– 9.24
– 0.23
13.31 17.36
0.025 0.024
0.030 0.029
tively. All specimens (45 mm × 26 mm × 14 mm) were ground with a series of emery papers of 120, 320 and 600 grits, the final step was performed with diamond grinding paste to achieve the roughness of Ra ≤ 0.05 m. The uncoated specimens were marked 1.7, 2.8 and 4.6, where the number means the hardness of the specimen. 2.2. Deposition and investigation of the TiN coating The TiN coating was deposited by cathodic arc method (ARC) in a vacuum chamber equipped with arc sources, with a target 100 mm in diameter. The 99.9% pure titanium target, 99.995% pure argon and 99.995% pure nitrogen gases were applied. Substrates preparation for the process consisted of ultrasonic-aided cleaning with organic solvents and alkaline detergents. The typical process of coating deposition includes the following operations: pumping-off air from the chamber down to the pressure below 2 × 10−3 Pa; heating the substrates to the temperature of 350 ◦ C; cleaning of the substrates by argon and titanium ions; deposition of thin titanium interlayer (∼0.05 m thickness) in the argon atmosphere; deposition of TiN coating in the nitrogen atmosphere up to the thickness of approximately 3.7 m. Parameters applied to the deposition of TiN coating are presented in Table 2. The coated specimens were marked TiN-1.7, TiN-2.8 and TiN-4.6, where 1.7 means X6CrNiTi18-10 steel with hardness of 1.7 GPa, 2.8–X39Cr13 steel with hardness of 2.8 GPa, and 4.6–X39Cr13 steel with hardness of 4.6 GPa. Basic properties of TiN coating deposited on X39Cr13 and on X6CrNiTi18-10 steels are shown in Table 3. The phase composition of the coating measured on a DRON2 X-ray diffractometer using Co K␣ radiation showed that the TiN coating is consistent with phase of ␦-TiN, what was confirmed by strong (1 1 1) plane reflection (Fig. 1). The grains size was determined by means of the Scherrer method with reflex parameters (location and FWHM) using Gaussian analysis. The size of TiN crystallites was estimated to be approximately 16 nm. The coating morphology and thickness were examined with JEOL JSM 5500 LV scanning electron microscope (SEM). The microphotograph
of the cross-section and the surface of TiN coating are shown on Fig. 2. Impurities visible on the coating surface are the microdroplets of titanium. These microdroplets always occur when the cathodic arc plasma method is used. The hardness and Young modulus were measured with a NanoHardness Tester (CSEM) using the method of Oliver and Pharr [9]. The maximum indentation depth of 300 nm was applied. The obtained hardnesses and Young modulus are presented in Table 3. A scratch tester Revetest® produced by CSEM was used to investigate the adhesion of coating. A diamond indenter with radius 0.2 mm was used and measurements were carried out at normal loading rate of 100 N/min, scratch speed of 10 mm/min and scratch length of 10 mm. At least three scratches were done. The first minor cracks for TiN-4.6, TiN-2.8 and TiN-1.7 occurred with a scratch made under the load of 29N, 20 N and 10N, respectively (Fig. 3). These loads correspond to the critical load LC1 defined by the occurrence of the first cohesive failure of the Table 3 Properties of TiN coating
Phase composition, structure Crystallographic orientation Mean crystallites size (nm0 Hardness (GPa) Young’s modulus (GPa) Coating thickness (m) Adhesion LC1 (N) LC2 (N) Roughness Ra (m)
TiN-4.6
TiN-2.8
TiN-1.7
␦-TiN (1 1 1) 16 ± 2 25.4 ± 2.5 295 ± 38 3.7 ± 0.1
␦-TiN (1 1 1) 16 ± 2 25.4 ± 2.5 295 ± 38 3.7 ± 0.1
␦-TiN (1 1 1) 16 ± 2 25.4 ± 2.5 295 ± 38 3.7 ± 0.1
29 ± 3 48 ± 3 0.31 ± 0.03
20 ± 3 36 ± 3 0.35 ± 0.03
10 ± 3 23 ± 3 0.35 ± 0.03
Table 2 Deposition parameters of TiN coating Pressure of residual gases Working pressure of argon Working pressure of nitrogen Arc current Substrate bias voltage Substrate temperature Target–substrate distance
2 × 10−3 Pa 1 Pa 1 Pa 80 A −100 V ∼350 ◦ C 150 mm Fig. 1. X-ray diffraction patterns of the TiN coating.
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Fig. 2. The cross-section and the surface of the TiN coating, SEM.
coating, that are the cracks caused by tensile stresses inside and on the edges of the scratch behind the sliding diamond cone. The critical loads LC2, at which the coating removal from inside of the scratch starts, were 48 N, 36 N and 23 N, respectively. The performed examinations show that the hardness of substrate has an influence on coating adhesion. Along with the increase of substrate hardness the increase of adhesion occurred. The stylus profilometer Hommel-Tester 2000 was used to measure roughness (Ra ) the substrate and TiN coating. 2.3. Procedure of cavitation erosion test The experimental tests were performed in a cavitation tunnel equipped with a system of barricades. The schematic of the cavitation chamber is shown in Fig. 4. Cavitation intensity is controlled by adjusting the slot witdh and the boost pomp speed. Flow conditions are defined by the p1 and p2 absolute pressures measured at the chamber inlet and outlet, respectively. Vertical positioning of the specimen is easily adjustable by means of spacer washer. Tap water is used as the working liquid. The cavities (cavitating vortices and bubbles) are generated by the pressure decrease in the slot between two semi-cylindrical barricades. The TiN coated and uncoated specimens were subjected to cavitation impingement in the cavitation chamber operated with inlet pressure p1 = 1000 kPa, outlet pressure p2 = 130 kPa, and
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Fig. 4. Schematic view of cavitation chamber with a system of barricades. I: stationary barricade; II: moving counter-barricade.
the slot width – 5 mm. Tests were performed without spacer washer. In order to obtain the erosion curves, the mass loss was measured after each exposure interval. The specimens were cleaned, dried and weighed before the test and after each test interval. Mass losses of the tested specimens were measured using an analytical balance with the permissible error limit of the balance ±1.4 mg for the load up to 100 g. At the beginning of the cavitation test the measurements were conducted every 30 min of exposure (for the first 180 min of test) to estimate the incubation period, and then the duration of exposure intervals was gradually increased. The total cavitation test exposure was 600 min. After each particular exposure time the cavitation erosion damage was analysed with macroscopic sample surface observation and the microstructure was investigated using the scanning electron microscope. 3. Results The mass losses of TiN coated and uncoated specimens arising during cavitation tests are shown in the graphic form in Fig. 5. The incubation period of all uncoated specimens was identical, and lasted less than 90 min. The least mass loss occurred on X6CrNiTi18-10 austenitic steel (7 mg), while the highest mass loss occurred on X39Cr13 steel with hardness of
Fig. 3. TiN coating damage arising during scratch test.
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Fig. 5. Erosion curves of uncoated and TiN coated specimens.
4.6 GPa (11 mg). The SEM images of damaged specimens after the whole test are shown in Fig. 6. The surface of uncoated X6CrNiTi18-10 steel under cyclic action of cavitation pulses underwent undulation; originated slip bands agglomerates and big twins (Fig. 6a). The microcracks have been initiated at intrusions in slip bands (Fig. 6a) like in fatigue [10]. Microstructure of uncoated X39Cr13 (both hardness) steel under action of cavitation has been disclosed (Fig. 6 b, c). In case of X39Cr13 steel with hardness of 2.8 GPa, degradation has proceeded along martensite lathings causing material groove (Fig. 6b), therefore the shape of martensite lathings are not so clear as that arisen on X39Cr13 steel with hardness of 4.6 GPa (Fig. 6c). On X39Cr13 steel with hardness of 2.8 GPa some pits/microtunels (arrows) are also observed in Fig. 6b. In case of X39Cr13 steel with hardness of 4.6 GPa, because of high hardness the deformation is restrained in martensite lathings. Material is crumbled along streak lines (Fig. 6c). These lines are combinated with water flow and construction of cavitation tunnel. The best cavitation erosion resistance of uncoated austenitic steel is probably linked with absorption some degradation energy on plastic deformation (the surface undulation and the origin of some slip bands – Fig. 6a) and phase transformation due to cyclic impact during cavitation test (Fig. 7). The X-ray diffraction pattern of the X6CrNiTi18-10 steel after the cavitation test (Fig. 7) shows the increase of diffraction peak of Fe-␣ (1 1 1) and additionally appearance of the Fe-␣ (2 0 0), Fe-␣ (2 1 1) and Fe-␣ (2 2 0) reflections and the weakness of Fe-␥ reflections. The temporary arrest of mass loss that occurred between 90 and 120 min of erosion was most likely correlated with the phase transformation. The phase transformation, in turn, caused changes in the mechanical and physical properties on the specimen surface. Deposition of TiN coating on specimens caused the lengthening of incubation period and the decrease of mass loss for all tested coated specimens in comparison to uncoated specimens after the whole cavitation test (Fig. 5). The incubation period amounted <180 min, <240 min and <360 min, respectively, for TiN-2.8, TiN-1.7 and TiN-4.6. The highest elongation of incubation period was observed for TiN-4.6 system, which has the
Fig. 6. SEM images of cavitation damages arisen on (a) uncoated X6CrNiTi1810 austenitic steel with hardness of 1.7 GPa, (b) uncoated X39Cr13 steel with hardness of 2.8 GPa and (c) uncoated X39Cr13 steel with hardness of 4.6 GPa.
highest substrate hardness. Also the highest decrease in mass loss occurred for TiN-4.6 system. This reduction amounted 7 mg (from 11 mg for uncoated steel to 4 mg for TiN coated steel). In case of TiN-2.8 system the decrease of mass loss amounted 4 mg (from 9 mg for uncoated steel to 5 mg for TiN coated steel) whereas in case of TiN-1.7 system – 3 mg (from 7 mg for uncoated steel to 4 mg for TiN coated steel). The arrest of mass loss observed in TiN-1.7 was probably caused by phase transformation (Fig. 8) like in uncoated X6CrNiTi18-10 steel (Fig. 7). The SEM images of coated specimens damages after the whole test are shown in Fig. 9. TiN coated X6CrNiTi18-10 steel
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Fig. 7. X-ray diffraction patterns of the X6CrNiTi18-10 steel before and after cavitation test.
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Fig. 8. X-ray diffraction patterns of the TiN coated X6CrNiTi18-10 steel before and after cavitation test.
Fig. 9. SEM images of cavitation damages of TiN coated X6CrNiTi18-10 steel with hardness of 1.7 GPa (a, b), TiN coated X39Cr13 steel with hardness of 2.8 GPa (c, d) and TiN coated X39Cr13 steel with hardness of 4.6 GPa (e, f).
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(TiN-1.7) underwent deformation (Fig. 9a), which was probably related to the substrate undulation, most of TiN microdroplets were removed from the coating. The microcracks were initiated mostly on the ripples (Fig. 9b) probably due to arising tensile stresses that might have exceeded the coating toughness. The cracks developed along the ripples as a result of the micro-cracks merging. Afterwards, they radiated to further ripples through concave traces of post-microdroplets that are the discontinuous spots and reduce coating durability. It shows that coating defects affect the acceleration of the microcracks growth. The undulation of the coating causes also the delamination process. Delamination was observed on the peripheral side of the indentations (Fig. 9b). At some areas the TiN coating was detached; there none of the TiN coating particles remained (Fig. 9a). This could be evidence that adhesion was not high enough and the coating was possibly removed by delamination. On the uncovered surfaces an erosion of the substrate can be observed. SEM images of degraded TiN coating deposited on X39Cr13 steel with hardness of 2.8 GPa (TiN-2.8) are given in Fig. 9 c, d. In the low eroded places the TiN coating underwent undulating (Fig. 9c), but the degree of undulation is lower then that arose at TiN-1.7 (Fig. 9a). Most of TiN microdroplets were removed from the coating. At intensively eroded area the TiN coating was removed, but some particle of TiN coating remained (Fig. 9d). This could be evidence that adhesion was quite high in this dynamic loading condition. On the uncovered surfaces an erosion of the substrate was observed. The larger magnification analysis (Fig. 9d) implies that the TiN-2.8 system was degraded in a brittle manner and probably microparticles removed from hard coating during implosion hit against both the coating and exposed areas of substrate. An image of eroded TiN coating deposited on X39Cr13 with hardness of 4.6 GPa reveals lack of surface undulation (Fig. 9e). Nearly all the TiN microdroplets were removed from the coating. At intensively eroded areas the TiN coating was detached; there none of the TiN coating particles remained. This might suggest that the TiN coating was removed by delamination. On the uncovered surfaces an erosion of the substrate was observed. The larger magnification analysis (Fig. 9f) discloses that cavitation pulses hitting the surface stab the TiN coating and stick some coating particles into the substrate despite the substrate hardness. 4. Discussion Cavitation erosion resistance of conventional materials depends on: tensile strength, strain energy, resilience, ductility, hardness, fatigue resistance and fine structure [3–5,11,12]. Increase of strength and hardness tend to increase the incubation period and the cavitation erosion resistance providing that brittleness does not increase. Increase of brittleness causes increase of failure. Erosion resistance of coated steel depends on coating and substrate properties and also on coating adhesion [8]. Mann and Arya [13] have noticed that hardness of coatings plays a significant role in the lengthening of the incubation period. Presented in this paper results of cavitation tests have also conducted that the TiN coating deposition lengthened the incubation
period. The highest prolongation occurred for the TiN coating deposited on substrate with hardness of 4.6 GPa, which had the highest adhesion among investigated substrates (Table 3). This substrate exhibits as well the highest hardness. It has been noted (Table 3) that with increase of the substrate hardness the adhesion increases as well. However, there has not been found straight correlation between substrate hardness or/and adhesion and the incubation period. For X39Cr13 substrates, which were subjected to different thermal treatment to obtain different hardness, correlation between coating adhesion and the incubation period was noted. The coated X6CrNiTi18-10 steel, which has the lowest substrate hardness (1.7 GPa), exhibited longer incubation period than the substrate with hardness of 2.8 GPa (Fig. 5). Authors attributed this elongation of the incubation period to the phase transformation of X6CrNiTi18-10 steel (Fig. 7). The authors of [4,6,14] also ascribed good cavitation resistance of austenitic steels to the phase transformation that occurred during cavitation. The phase transformation needs energy and typically it occurs during heating. During cavitation bubbles collapse and induce microjets, which may produce intense local heating [15]. The material surface subjected to cavitation may experience the temperature of 103 K magnitude, pressure shock wave of 103 MPa and heating and rates approximately 1010 K/s [15,16]. The energy delivered from collapsing bubbles must be either absorbed or dissipated by the solid material or reflected as shock waves in liquid. Some of this energy is absorbed by changes in dislocation structure [4,17], some of it used by the phase transformation Fe␥ → Fe␣ + carbide in metastable austenitic steels [6,14,18]. Changes in dislocation structure of austenitic steel start with the beginning cavitation test [4,14,17]. It was noticed in [17] that after 5 min of exposure dense dislocation network, small dislocation cells, dislocation loops and slip bands crossed by cutting bands developed due to very high degradation energy. The change in phase and dislocation structure causes changes in material properties and changes the cavitation erosion resistance of material. Degradation process of the TiN coating was determined by substrate behaviour. The TiN coating underwent plastic deformation in case of substrates with hardness of 2.8 and 1.7 GPa. The degree of the coating deformation was correlated to the substrate undulation. The authors of [19,20] noticed that the substrate deformation occurs faster than the deformation of the coating. This could lead to coating delamination. The coating delamination was observed in Fig. 9. When the delamination occurs, the substrate does not aid the coating with carrying the loads. This could lead to crumble the TiN coating away. Time needed to start the delamination and crumbling the TiN coating is correlated with wear resistance of the coated steel. As long as the system was able to absorb the degradation energy the split of the TiN coating did not occur and the incubation period lasted. Presented results show (Fig. 5) that the longest incubation period occurred for TiN-4.6 system, which had the hardest substrate. Later on, the TiN coating was crushed in brittle mode (Fig. 9). Hard crushed TiN coating hammers itself into the substrate, especially if the substrate is relatively soft in comparison with the coating hardness (Fig. 9). Moreover, hard micro-particles removed from the TiN coating hit again coated
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steel causing acceleration of the delamination process and consequently cavitation erosion damages. Also, Mann and Arya [13] noticed that hard-coated steels are failed in brittle mode. Nevertheless, mass losses of the TiN coated steels are clearly less than uncoated steels and the decrease of the mass losses is correlated with the substrate hardness/adhesion. Degradation energy needed to cause the coating delamination and the removal TiN coating particles is linked with adhesion and consequently with erosion resistance of the system. With increase of the adhesion/substrate hardness the decrease of the mass loss occurred as well. 5. Conclusion • Deposition TiN coating on steel substrates lengthened the incubation period and lessened the mass loss of the coated steel. The incubation periods of the coated steels were from 2 to 4 times longer than that of the uncoated steels. The mass losses of the coated steels decrease approximately 2.5 times in comparison with the uncoated steels. • In case of X6CrNiTi 18–10 steel degradation energy delivered to material from imploding cavitation bubbles is partially absorbed by phase transformation Fe␥ → Fe␣ causing the change of the material properties and the increase of the resistance to cavitation loss of uncoated and coated steels. • The TiN coating deposited on X39Cr13 steel with hardness of 4.6 GPa enhances the cavitation erosion resistance the most among all tested coated steels, in comparison to uncoated steels. Acknowledgements Results presented in this paper were funded by budget resources allocated to scientific research in 2005–2007 as a research project no. 4 T07B 015 28. References [1] S. M¨unsterer, K. Kohlhof, Cavitation protection by low temperature TiCN coatings, Surf. Coat. Technol. 74–75 (1995) 642–647.
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