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Effect of spraying power on the microstructure and mechanical properties of supersonic plasma-sprayed Ni-based alloy coatings
Applied Surface Science 254 (2008) 6318–6326
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of spraying power on the microstructure and mechanical properties of supersonic plasma-sprayed Ni-based alloy coatings X.C. Zhang a,b,*, B.S. Xu c, S.T. Tu b, F.Z. Xuan b, H.D. Wang c, Y.X. Wu a a
Shanghai Key Laboratory of Materials Laser Processing and Modification, Shanghai Jiao Tong University, Shanghai, 200030, China School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai, 200237, China c National Key Laboratory for Remanufacturing, Beijing, 100072, China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 January 2008 Received in revised form 13 March 2008 Accepted 14 March 2008 Available online 30 March 2008
The aim of this paper was to investigate the microstructure and mechanical properties of the supersonic plasma-sprayed Ni–Cr–B–Si–C coatings prepared at different spraying powers. The microstructure, phase composition, porosity, Young’s modulus, micro-hardness, and residual stresses of the coatings were investigated and determined. The variations of the porosity, Young’s modulus and micro-hardness of the coatings were evaluated by using statistical method. Results showed that the variations of porosity, Young’s modulus and micro-hardness of the coatings followed the Weibull distributions. With increasing the porosity, the micro-hardness and Young’s modulus of the coating decreased. The mean value of the Young’s modulus of the coating calculated from Weibull plot was almost proportional to the square root of the micro-hardness of the coating. With increasing the power, Young’s modulus of the coating increased, which, in turn, resulted in the increment of the residual stress at the coating surface. ß 2008 Published by Elsevier B.V.
Keywords: Plasma spraying Spraying power Porosity Mechanical properties Statistical analysis
1. Introduction Plasma spraying (PS) is a versatile method which can be used to deposit powders as dense, adherent and homogeneous coatings with low porosity. During PS, the coating material in the form of powder is heated to a molten state. The heated material is propelled in the form of particles or droplets in a plasma jet at high velocities to impact the substrate, where they flatten and solidify rapidly forming a stacking of lamellae with micro-defects, such as pores, micro-cracks, unmelted particles, etc. Some of these defects are open and connected, while some of them are closed and isolated. Due to the presence of these micro-defects, the plasmasprayed coating has a unique microstructure and behaves differently from a comparable monolithic material [1,2]. The properties, such as porosity, hardness, elastic modulus and residual stress are of great interest for plasma-sprayed coating. During PS, the pores and micro-cracks can be generated from different sources, such as the entrapped gases, the incomplete filling in of the rapidly solidifying splats [3], and the shrinking of the splats during rapid solidification [4,5], etc. If no distinction is made of the nature of pores and the micro-cracks, the porosity in plasma-sprayed coatings can verify from less than 2% to more than
* Corresponding author at: School of Mechanical and Power Engineering, Meinlong Road 130, Xuhui District, Shanghai, 200237, China. Tel.: +86 21 64253425. E-mail address: [email protected] (X.C. Zhang). 0169-4332/$ – see front matter ß 2008 Published by Elsevier B.V. doi:10.1016/j.apsusc.2008.03.148
20%, depending on the type of powders and the deposition parameters used [6]. The methods such as mercury intrusion porosimetry (MIP) and image analysis (IA) are often used to evaluate the porosity level in the coating. MIP is a quantitative porosity measurement method and does not guarantee determination of true porosity value due to the presence of closed pores in the coating. IA is somewhat subjective, but it is suitable for a relative comparison among different coatings [7]. Coating porosity is determined by IA technique due to the principle of gray value analysis. Usually, the surface of the defects, such as pore, inhomogeneous phase and inter-lamellar cracks, had ‘‘spots’’ darker than other area in the coating [8–11]. Young’s modulus and micro-hardness are two intrinsically physical properties of the plasma-sprayed coating. Young’s modulus of the coating is dependent upon the bulk material as well as its micro- or macro-structures of the coating. The measured Young’s modulus of the coating is generally lower than the bulk coating material from which the coating is made due to the presence of the micro-defects in the coating [12,13]. The microhardness of coatings is strongly influenced by the distribution and amount of defects existed in the microstructure as well as by the selection of the testers. Depth sensing indentation (DSI), also known as nanoindentation, is often used to determine the microhardness and Young’s modulus of the coating through the use of continuous depth recording to measure the penetration depth of the indenter, together with load and time [14]. However, when DSI is used to determine the mechanical properties of sprayed
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coatings, the applied load is relatively low during testing. The micro-defects on the indentation impression, the unmelted particles and the voids in the coating make the scattering of the measured data. The residual stress is also an important mechanical property of the sprayed coating. Different analytical models and experimental methods have been developed to determine the residual stresses in coatings. Among the experimental methods, Xray diffraction (XRD) is commonly used to determine the residual stress at the coating surface. The basic principals of this method for evaluating the residual stress can be seen elsewhere [15,16]. The microstructure and mechanical properties of the plasmasprayed coatings are dependent on the processing parameters used for spraying. The spraying power is an important parameter that affects the quality of the coating, since it can influence the temperature and velocity of the particles at the moment of impinging the substrate. Gao et al. [17] investigated the effect of spraying power on the velocity of the powder particles and microhardness of the plasma-sprayed coating. Results showed that the velocity of the particles increased with increasing the power. The similar results can be seen elsewhere [2,18]. In the present paper, the effect of spraying power on the microstructure and mechanical properties of the supersonic plasma-sprayed Ni–Cr–B–Si–C coatings was investigated. The variations of the porosity, Young’s modulus and micro-hardness of the coatings were statistically evaluated. The relationship between the porosity and the micro-hardness as well as Young’s modulus of the coating was determined. The effect of spraying power on the residual stress at the coating surface was also investigated. These topics discussed should provide some important insights on establishing the thermal spray process–structure– property correlations. 2. Experimental procedures 2.1. Spraying parameters and coating material For conventional air PS guns, the gas velocities of 300–800 m/s and velocities of the particles in the range of 130–220 m/s were obtained with 5–8 mm diameter nozzles and 50–75 l/min total gas flow rates [19]. Additionally, the disordered plasma jet generated by conventional plasma gun has a ‘‘boundary effect’’. As the distance from the nozzle exit increases, the enthalpy decreases. Consequently, conventional air PS has limited coating quality with low bond strength and high porosity. A high-efficiency PS system with a hypersonic PS gun was used to prepare Ni-based alloy coatings. The advanced system developed by national key laboratory for remanufacturing, China, was composed of plasma torch, power feeder, gas supply, water cooling circulator, control unit with PC interface and power supply unit. The key of this system was a novelly supersonic PS gun, as shown in Fig. 1. Using the hypersonic PS gun, the velocities of the plasma jet and particles of 2400 m/s and 500 m/s can be achieved at the normal distance of 100 mm from the nozzle exist [20]. Near the nozzle exist, the temperature of the gas flow of
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Table 1 Nominal composition of the sprayed powder Element
Cr
Si
B
C
Fe
Ni
wt.%
18.09
4.84
3.6
0.6
15.63
54.27
Fig. 2. SEM photograph of the Ni–Cr–B–Si–C powder.
around 10,000 8C can be reached [21]. Compared with the conventional air PS, high-efficiency hypersonic PS improved greatly the coatings quality but at low cost [20]. The substrate material used was a commercial middle carbon steel, which was cut to yield 50 mm 10 mm 6 mm specimens. Prior to spraying, one face of the steel substrate was cleaned in acetone solution and preheated to 100–200 8C, and then sandblasted by using corundum powder. For sandblasting and spraying, the substrates were fixed by the corresponding fixtures. In order to achieve good temperature control of the substrate and coating, rectangular specimens were disposed in the holes on a stainless steel cylindrical holder (SCH), 200 mm in diameter and 4 mm in thickness. The rotation of the SCH was 120 rpm during the spraying. The plasma torch, whose axis was orthogonal to that of the SCH, was translated parallel to the SCH at a constant velocity of 12 mm/s. The spray distance was kept to be 100 mm. A commercially available Ni–Cr–B–Si–C powder (Ni60 AA) was used. The nominal composition of the powder can be seen in Table 1. The powder was prepared by Beijing general research institute of mining & metallurgy, China, and was characterized by near-perfect spherical particles, as shown by the scanning electron microscopy (SEM) photograph in Fig. 2. The particle diameter
Fig. 1. Schematic of plasma spray torch with a radial powder injection system.
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far apart so that the indention behavior was not affected by the adjacent indentations. Since the load was relatively low and the indent was relatively small, the tester was calibrated to avoid the indentation size effects. The residual stresses at individual coating surfaces were also determined by using XRD with sin2 C method. For the stress calculation, the Poisson’s ratio of the coating was assumed to be 0.25. However, for each coating, the elastic modulus was taken as the mean value of Young’s modulus that was obtained from Weibull plot. 2.3. Porosity analyses
Fig. 3. Probability density function of particle diameter of the initial Ni–Cr–B–Si–C powder.
ranges between 20 mm and 120 mm, as shown in Fig. 3. The average particle diameter determined by using the IAM is about 63.5 mm with the relative standard deviation of 8%. A series of coatings were prepared at different spraying powers. Details of the spraying parameters were listed in Table 2.
An IAM was developed to evaluate coating porosity. The procedure of the proposed IA method to determine the porosity of the coating can be divided into four steps, i.e., gray level transformation, fuzzy enhancement of image, binary segment of image, and removing the impurities and identification of pores, inter-lamellar voids and intralamellar cracks. The detailed introduction of the IAM can be seen in Refs. [22,23]. To get a representative porosity value, a series of SEM images with a magnification 1000 were obtained for individual specimens. Twenty digital micrographs were randomly taken from each coating cross-section using a blend of 70% BSE and 30% SE images on SEM. The high magnification images allowed a closer view of the pore structure as well as the crack distribution.
2.2. Characterization of coatings 2.4. Weibull distribution The microstructures and morphologies of coatings were observed by using a Philips Quant 200 SEM (FEI, the Netherlands) equipped with an energy dispersive spectrometer (EDS, EDAX Inc., USA) apparatus. Prior to observations, the coating samples were mounted in the epoxy resin. The cross-sectional surface for SEM observation was wet-ground with 400, 600, 800 and 1200 grit SiC paper. The samples were ultrasonically cleaned in water and then in acetone between each grit paper. A mixture of secondary electron (SE) and back scattered electron (BSE) modes was used to obtain digital SEM images with the best possible resolution and contrast to investigate the different types of microstructural defects in greater detail. Crystal structures of the coatings were determined by means of X-ray diffraction (Bruker AXS, Germany) with Cu Ka (l = 1.54A˚) radiation, step 0.028 on a D8-Advanced apparatus. Young’s modulus and micro-hardness were measured at the polished coating surface. DSI measurements were carried out with a Micro-Materials Nano-test 600 indentation tester ((NanoTest, MML, UK) by using a pyramidal indenter at room temperature (with a maximum load 15 mN and a loading rate of 0.3 mN s1). For each specimen, 20 indentations were randomly produced on the coating surface to get representative values of Young’s modulus and micro-hardness. Indentions were spaced sufficiently Table 2 Spraying parameters used for coating process
It was well known that the measured data of porosity, Young’s modulus and micro-hardness showed high scattering [22]. Hence, statistical analyses of variations of porosity, Young’s modulus and micro-hardness of each coating were made on the bases of Weibull plot [24]. The Weibull function in two-parameter form was given as, a N F ðNÞ ¼ 1 exp N0
(1)
where F(N) is the cumulative density function of probability, N is the experimental data, N0 is the characteristic value below which 63.2% of the data lie, and a is Weibull modulus which reflects the data scattering within the distribution. The detailed introduction on the Weibull distribution can be seen elsewhere [25]. 3. Results and discussion 3.1. Porosity variations The cross-sectional views of the coatings prepared under different spraying powers are shown in Fig. 4. For the coatings
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Fig. 4. Cross-sectional views of the coatings S1–S5, deposited at different spraying powers, (a) 43.2 kW, (b) 46.8 kW, (c) 47.6 kW, (d) 50.4 kW, and (e) 57.0 kW.
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Fig. 6. Variations of characteristic values and mean values of porosity of coatings as functions of spraying power.
and can be explained as follows. First, at low spraying powers, the powder particles are poorly melted. When they impacted on the substrate or the already formed splats, they are not able to spread out completely to form splats and therefore, could not conform to
Fig. 5. Weibull plots for porosity data of the coatings prepared at different spraying powers, (a) cumulative probability and (b) probability function.
prepared at the powers of 43.2 kW and 46.8 kW, the splats poorly flatten and spread out. There are numerous pores existing at the boundaries of overlapping splats. Moreover, the bonding strength between the adjacent splats is low since the cracks are visible at the interface. In addition, some unmelted particles in which the diverse phases can be seen are existed in the coating, as indicated by the arrows in Fig. 4(a and b). This result indicates that, the low spraying power cannot provide enough energy to melt the particles. With increasing the spraying power, the coating microstructure becomes denser. When the power is 57 kW, the lamellar structure is not visible and the inter-lamellar pores and cracks are almost not visible in the coating either, instead it resembled the microstructure of a bulk material, as seen Fig. 4(e). Only a few pores with small dimensions are formed at the boundaries between splats. Fig. 5 shows Weibull plots for porosity variations in the coatings prepared at different spraying powers. Fig. 5(a) shows the cumulative probability against the porosity and Fig. 5(b) shows the probability function against the porosity. It can be seen that the porosity data are heavily scattered and have wide ranges of values for the coating prepared under lower spraying power. This result indicates that, when the spraying power is low, the microstructure of the coating shows high inhomogeneity. Numerous unmelted and partially melted particles exist in the coating, resulting in the high scattering of the porosity data. Characteristic values (at cumulative probability of 63.2%) and mean values (at cumulative probability of 50.0%) with 95% reliability limit of the porosity of the coatings under different spraying powers are shown in Fig. 6. It can be seen that when the power is lower than 50 kW, the porosity of the coating decreases rapidly with increasing the spraying power. When the power is higher than 50 kW, the coating porosity is not very sensitive to the spraying power. This phenomenon is expected
Fig. 7. Microstructure of the unmelted particle in the coating prepared at the spraying power of 43.2 kW, (a) diverse phases and (b) EDS analyses on different phases.
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Fig. 8. X-ray diffraction patterns of the coatings prepared at different powers.
the surface. In such a case, the inter-lamellar pores and cracks will be formed due to the solidification of the spats. Moreover, when the power is relatively low, the unmelted and partially melted particles are existed in the coating. During the cooling process, the micro-cracks and pores are formed near the boundary of the unmelted particle, since the material mismatch between the unmelted particles and the around splats, as shown in Fig. 4(a). Second, when the power is high, the fully melted Ni with low melting temperature is considered to be well distributed and infiltrated in the spat boundary. When the power is sufficiently high, most of the powder particles have been melted and the flowability of splats is good. In such a case, when the power is further increased, the coating porosity is not very sensitive to the magnitude of the spraying power. 3.2. Phase composition Fig. 7 shows the detail of the microstructure containing an unmelted particle in the coating prepared under the spraying power of 43.2 kW. It is clear that the micro-cracks and pores are formed near the boundary of the unmelted particle. The diverse precipitates with a darker field can also been found in the particle, as illustrate by the arrow in Fig. 7(a). Analysis using EDS indicates higher Cr content of the diverse phases, as shown in Fig. 7(b). Given the high amounts of B and C in the powder, and taking into account earlier researches [26–28], these phases may be Cr carbides and/or borides. The formation of the unmelted precipitates is promoted by the low spraying temperature and a low residence time of particles during spraying [26]. X-ray diffraction patterns of the coatings prepared at different spraying powers are shown in Fig. 8. It can be revealed that the crystalline phases of plasma-sprayed Ni–Cr–B–Si–C coating consist mainly of g-Ni solid solution, Carbide (Fe,Cr)7C3, CrB type chromium boride, and nickel boride Ni3B. The peak located at about 44.48 (2u) corresponding to (Fe,Cr)7C3 is observed in all coatings. With increasing the spraying power, the intensity of the sharp peak at 44.48 (2u) decreases. This phenomenon can be attributed to the following reason. It is well known that the structure of the coating is mainly determined by the cooling rate of the particles onto the substrate or formed coating [29]. The amount of (Fe,Cr)7C3 is indicative of the presence of unmelted and/or recrystallized particles in the coating [7]. When the spraying power is low (e.g., 43.2 kW), numerous particles are partially melted and the
Fig. 9. Weibull plots for (a) micro-hardness, HIT, and (b) Young’s modulus, Ec, data for the coatings prepared at different spraying powers.
temperature of the particles is low, which leads to the high cooling rate of the particles. In such case, the degree of the crystallinity is low since the degree of melting of particles is high [30]. 3.3. Mechanical properties Weibull plots for the data of micro-hardness, HIT, and elastic modulus, Ec, for coatings prepared at different spraying powers are shown in Fig. 9. It can be seen that the micro-hardness and Young’s modulus of the coatings might follow Weibull distribution. From Fig. 9(a), it can be seen that when the power is extremely low (e.g., 43.2 kW), the micro-hardness data have wide ranges of values. This result also indicates that, when the power used is low, the microstructure of the coating shows high inhomogeneity. Numerous unmelted and partially melted particles exist in the coating, resulting in the high scattering of the micro-hardness data. With increasing the power used in the spraying, the scattering of microhardness data decreases. The similar result has been obtained by Chwa et al. [31]. Fig. 10(a) shows the variation of the characteristic value and the mean value with 95% reliability limit of micro-hardness of the coating as a function of the spraying power. The upper bound and low bound of each value are also included. It can be seen that when the power is less than 47.6 kW, with increasing the power, the characteristic value and the mean value of micro-hardness of the coating increase. The micro-hardness of the coating is almost not influenced by the spraying power when the power is high than 47.6 kW. For Young’s modulus against the spraying power, as shown in Fig. 10(b), the similar trend can be found. The mechanical properties of the coating are influenced by the microstructure of
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Fig. 12. Measured Young’s modulus and the corresponding micro-hardness with a square-root form.
Fig. 10. Variations of (a) micro-hardness and (b) Young’s modulus of the coating against the spraying power.
the coatings. Generally, with increasing the spraying power, the micro-hardness and Young’s modulus of the coating increase, due to the decrement of the micro-defects in the coating. When the power is sufficiently high, the micro-defects are almost eliminated. In such a case, the mechanical properties of the coating are almost not influenced by spraying power. The similar result can be seen elsewhere [32].
micro-crack) porous materials has always been difficult, but attractive. The relationship between the mean value of Young’s modulus as well as the mean value of micro-hardness of the Ni–Cr– B–Si–C coating and the mean value of overall porosity of the coating can be seen in Fig. 11. Young’s modulus and microhardness of the coating generally decrease with increasing the porosity. Young’s modulus and micro-hardness are two essential parameters of structural materials, and the relationship between them is of keen interest to material scientists. From statistical trend, Young’s modulus is usually considered to be an increasing function of micro-hardness [33]. Bao et al. [34] developed an analytical model to describe the relationship between the reduced modulus and micro-hardness for solid materials on the basis of the conventional DSI method of Oliver & Pharr. It was found that the two properties could be related through a material parameter that is defined as the recovery resistance Rs. This parameter represented the energy dissipation during indentation. Based on indentation measurements, the relationship was given as [34]: Er ¼ D
pffiffiffiffiffiffiffiffiffi HRs
(2)
3.4. Relationship between porosity and mechanical properties The research on the relationship between the mechanical properties and the porosity of the two phase (solid-pore or solid-
Fig. 11. Variations of mean values of micro-hardness and Young’s modulus of the coating against the mean value of the porosity.
where Er is the reduced modulus, D is a constant depending on the indenter used.
Fig. 13. Relationship between the mean value of Young’s modulus and square root of the mean value of the micro-hardness.
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4. Conclusions The microstructure, porosity and mechanical properties of the supersonic plasma-sprayed Ni–Cr–B–Si–C coatings prepared at different spraying powers were experimentally investigated. Result showed that the measured data of porosity, Young’s modulus and micro-hardness of the coatings followed Weibull distribution. When the spraying power was low, there were numerous unmelted and partially melted particles existing in the coating, resulting in high scattering of the measured data of porosity, Young’s modulus and micro-hardness. With increasing the spraying power, the mean value and characteristic value of the porosity calculated from Weibull plot decreased, while the mean and characteristic values of Young’s modulus as well as the microhardness increased. With increasing the porosity, the microhardness and Young’s modulus of the coating decreased gradually. From the statistical trend, Young’s modulus can be considered to be an increasing function of micro-hardness. The mean value of the elastic modulus of the coating was almost proportional to the square root of the mean value of the micro-hardness of the coating. Moreover, with increasing elastic modulus of the coating, the residual stress at the coating surface generally increased. Acknowledgements The authors are grateful for the supports provided by National Development Scheme of Key Fundamental Research (Nation ‘‘863’’ Project) of China (2007AA04Z408). Two of the authors (X.C. Zhang and F.Z. Xuan) are also grateful for the support by National Natural Science Foundation of China (50505012) and Fok Ying Tung Education Foundation (101054).
Fig. 14. Measured residual stress at the coating surface as a function of (a) spraying power and (b) mean value of Young’s modulus of the coating.
For each coated specimen investigated in this paper, total 20 indentations were randomly produced on the coating surface to get representative values of Young’s modulus and hardness. Young’s modulus and the corresponding hardness with a square-root form are shown in Fig. 12. It can be concluded that, from the statistical point of view, with increasing Young’s modulus, the hardness of the coating increases. Fig. 13 shows the relationship between the mean value of Young’s modulus and the square root of the mean value of the corresponding hardness of the coatings prepared under different powers. The linear relationship between elastic modulus and the square root of the micro-hardness of the coating can be fitted. This experimental result agrees well with the analytical modeling result from Bao et al. [34]. 3.5. Residual stress The variation of the residual stress at the coating surface as a function of the spraying power is shown in Fig. 14(a). For all coatings, the residual stress is tensile. The residual stress almost increases with increasing the spraying power, especially when the power is less than 47.6 kW. The similar result was obtained by Vaidya et al. [2]. The magnitude of residual stress within the coating is mainly related to Young’s modulus of the coating. The residual stress variation as a function of the mean value of Young’s modulus is shown in Fig. 14(b). It can be observed that the coatings with higher tensile residual stresses generally exhibit higher elastic moduli. The similar results can be seen elsewhere [35,36].
References [1] R. McPherson, Surf. Coat. Technol. 39–40 (1989) 173. [2] A. Vaidya, T. Streibl, L. Li, S. Sampath, O. Kovarik, R. Greenlaw, Mater. Sci. Eng. A403 (2005) 191. [3] Z. Wang, A. Kulkarni, S. Deshpande, T. Nakamura, H. Herman, Acta Mater. 51 (2003) 5319. [4] P. Bengtsson, T. Johannesson, J. Therm. Spray Technol. 4 (1995) 245. [5] S. Kuroda, T.W. Clyne, Thin Solid Films 200 (1991) 49. [6] S.C. Gill, Residual stresses in plasma sprayed deposites, Ph.D thesis, University of Cambridge, Cambridge England 1991. [7] L.C. Erickson, Wear and microstructural integrity of ceramic plasma sprayed coatings. Ph.D. Dissertation, The University of British Columbia, 1998. [8] S. Deshpande, A. Kulkarni, S. Sampath, H. Herman, Surf. Coat. Technol. 187 (2004) 6. [9] L. Cunha, M. Andritschky, K. Pischow, K. Pischow, Z. Wang, Thin Solid Films 355– 356 (1999) 465. [10] Z.H. Tong, F.M. Jiang, C.X. Ding, Q.R. Yin, J. Inorg. Mater. 7 (1992) 345. [11] D.B. Fowler, W. Riggs, J.C. Russ, Adv. Mater. Process 138 (1990) 41. [12] R. McPherson, B.V. Shafer, Thin Solid Films 97 (1982) 201. [13] C.J. Li, A. Ohmori, J. Therm. Spray Technol. 11 (2002) 365. [14] H.M. Pollock, Nanoindentation, in Friction, Lubrication and Wear Technology, ASM Handbook 18 (1992) 419–429. [15] A.J. Perry, J.A. Sue, P.J. Martin, Surf. Coat. Technol. 81 (1996) 17. [16] I.C. Noyan, J.B. Cohen, Residual Stresses-Measurement by Diffraction and Interpretation, Springer-Verlag, New York, 1987, p. 117. [17] Y. Gao, X.L. Xu, Z.J. Yan, G. Xin, Surf. Coat. Technol. 154 (2002) 189. [18] J.F. Li, H.L. Liao, C.X. Ding, C. Coddet, J. Mater. Process. Technol. 160 (2005) 34. [19] J.F. Coudert, M.P. Planche, O. Betoule, M. Vardelle, P. Fauchais, in: Proceedings of the 1993 National Thermal Spray Conference, Anaheim, CA, June 7–11, 1993, ASM International, Metals Park, OH, 1993, pp. 19–24. [20] S. Zhu, B.S. Xu, J.K. Yao, Mater. Sci. Forum 457–459 (2005) 3981. [21] S. Zhu, B.S. Xu, Key Eng. Mater. 280/283 (2005) 1203. [22] X.C. Zhang, B.S. Xu, F.Z. Xuan, H.D. Wang, Y.X. Wu, S.T. Tu., Statistical analyses of porosity variations in plasma-sprayed Ni-based coatings, J. Alloys Compd., in press, doi:10.1016/j.jallcom.2007.12.083. [23] X.C. Zhang, B.S. Xu, Y.X. Wu, F.Z. Xuan, S.T. Tu, Appl. Surf. Sci. 254 (2008) 3879. [24] Abernethy Robert B, J.E. Breneman, C.H. Medlin, G.L. Reinman, Weibull Analysis Handbook, US Air Force AFWAL-TR-83-2079, Prepared for the Aero Propulsion and Power Laboratory, Wright-Patterson AFB, Ohio, 1983. [25] C.K. Lin, C.C. Bernet, J. Mater. Sci. 30 (1995) 111. [26] J.M. Miguel, J.M. Guilemany, S. Vizcaino, Tribol. Int. 36 (2003) 181.
6326
X.C. Zhang et al. / Applied Surface Science 254 (2008) 6318–6326
[27] C. Navas, R. Vijande, J.M. Cuetos, M.R. Ferna´ndez, J. de Damborenea, Surf. Coat. Technol. 201 (2006) 776. [28] D.W. Zhang, T.C. Lei, J.G. Zhang, J.H. Ouyang, Surf. Coat. Technol. 115 (1999) 176. [29] M.P. Planche, H. Liao, B. Normand, C. Coddet, Surf. Coat. Technol. 200 (2005) 2465. [30] G.Y. Kim, H.S. Choi, C.M. Han, S.H. Uhm, C.H. Lee, Surf. Coat. Technol. 195 (2005) 107.
[31] S.O. Chwa, D. Klein, F.L. Toma, G. Bertrand, H. Liao, C. Coddet, A. Ohmori, Surf. Coat. Technol. 194 (2005) 215. [32] Y.C. Tsui, C. Doyle, T.W. Clyne, Biomaterials 19 (1998) 2015. [33] S.L.A. Prasad, M.M. Mayuram, R. Krishnamurthy, Mater. Lett. 41 (1999) 234. [34] Y.W. Bao, W. Wang, Y.C. Zhou, Acta Mater. 52 (2004) 5397. [35] J. Matejicek, S. Sampath, D. Gilmore, R. Neiser, Acta Mater. 51 (2003) 873. [36] C.A. Johnson, J.A. Ruud, R. Bruce, D. Wortman, Surf. Coat. Technol. 108–109 (1998) 80.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of spraying power on the microstructure and mechanical properties of supersonic plasma-sprayed Ni-based alloy coatings X.C. Zhang a,b,*, B.S. Xu c, S.T. Tu b, F.Z. Xuan b, H.D. Wang c, Y.X. Wu a a
Shanghai Key Laboratory of Materials Laser Processing and Modification, Shanghai Jiao Tong University, Shanghai, 200030, China School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai, 200237, China c National Key Laboratory for Remanufacturing, Beijing, 100072, China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 January 2008 Received in revised form 13 March 2008 Accepted 14 March 2008 Available online 30 March 2008
The aim of this paper was to investigate the microstructure and mechanical properties of the supersonic plasma-sprayed Ni–Cr–B–Si–C coatings prepared at different spraying powers. The microstructure, phase composition, porosity, Young’s modulus, micro-hardness, and residual stresses of the coatings were investigated and determined. The variations of the porosity, Young’s modulus and micro-hardness of the coatings were evaluated by using statistical method. Results showed that the variations of porosity, Young’s modulus and micro-hardness of the coatings followed the Weibull distributions. With increasing the porosity, the micro-hardness and Young’s modulus of the coating decreased. The mean value of the Young’s modulus of the coating calculated from Weibull plot was almost proportional to the square root of the micro-hardness of the coating. With increasing the power, Young’s modulus of the coating increased, which, in turn, resulted in the increment of the residual stress at the coating surface. ß 2008 Published by Elsevier B.V.
Keywords: Plasma spraying Spraying power Porosity Mechanical properties Statistical analysis
1. Introduction Plasma spraying (PS) is a versatile method which can be used to deposit powders as dense, adherent and homogeneous coatings with low porosity. During PS, the coating material in the form of powder is heated to a molten state. The heated material is propelled in the form of particles or droplets in a plasma jet at high velocities to impact the substrate, where they flatten and solidify rapidly forming a stacking of lamellae with micro-defects, such as pores, micro-cracks, unmelted particles, etc. Some of these defects are open and connected, while some of them are closed and isolated. Due to the presence of these micro-defects, the plasmasprayed coating has a unique microstructure and behaves differently from a comparable monolithic material [1,2]. The properties, such as porosity, hardness, elastic modulus and residual stress are of great interest for plasma-sprayed coating. During PS, the pores and micro-cracks can be generated from different sources, such as the entrapped gases, the incomplete filling in of the rapidly solidifying splats [3], and the shrinking of the splats during rapid solidification [4,5], etc. If no distinction is made of the nature of pores and the micro-cracks, the porosity in plasma-sprayed coatings can verify from less than 2% to more than
* Corresponding author at: School of Mechanical and Power Engineering, Meinlong Road 130, Xuhui District, Shanghai, 200237, China. Tel.: +86 21 64253425. E-mail address: [email protected] (X.C. Zhang). 0169-4332/$ – see front matter ß 2008 Published by Elsevier B.V. doi:10.1016/j.apsusc.2008.03.148
20%, depending on the type of powders and the deposition parameters used [6]. The methods such as mercury intrusion porosimetry (MIP) and image analysis (IA) are often used to evaluate the porosity level in the coating. MIP is a quantitative porosity measurement method and does not guarantee determination of true porosity value due to the presence of closed pores in the coating. IA is somewhat subjective, but it is suitable for a relative comparison among different coatings [7]. Coating porosity is determined by IA technique due to the principle of gray value analysis. Usually, the surface of the defects, such as pore, inhomogeneous phase and inter-lamellar cracks, had ‘‘spots’’ darker than other area in the coating [8–11]. Young’s modulus and micro-hardness are two intrinsically physical properties of the plasma-sprayed coating. Young’s modulus of the coating is dependent upon the bulk material as well as its micro- or macro-structures of the coating. The measured Young’s modulus of the coating is generally lower than the bulk coating material from which the coating is made due to the presence of the micro-defects in the coating [12,13]. The microhardness of coatings is strongly influenced by the distribution and amount of defects existed in the microstructure as well as by the selection of the testers. Depth sensing indentation (DSI), also known as nanoindentation, is often used to determine the microhardness and Young’s modulus of the coating through the use of continuous depth recording to measure the penetration depth of the indenter, together with load and time [14]. However, when DSI is used to determine the mechanical properties of sprayed
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coatings, the applied load is relatively low during testing. The micro-defects on the indentation impression, the unmelted particles and the voids in the coating make the scattering of the measured data. The residual stress is also an important mechanical property of the sprayed coating. Different analytical models and experimental methods have been developed to determine the residual stresses in coatings. Among the experimental methods, Xray diffraction (XRD) is commonly used to determine the residual stress at the coating surface. The basic principals of this method for evaluating the residual stress can be seen elsewhere [15,16]. The microstructure and mechanical properties of the plasmasprayed coatings are dependent on the processing parameters used for spraying. The spraying power is an important parameter that affects the quality of the coating, since it can influence the temperature and velocity of the particles at the moment of impinging the substrate. Gao et al. [17] investigated the effect of spraying power on the velocity of the powder particles and microhardness of the plasma-sprayed coating. Results showed that the velocity of the particles increased with increasing the power. The similar results can be seen elsewhere [2,18]. In the present paper, the effect of spraying power on the microstructure and mechanical properties of the supersonic plasma-sprayed Ni–Cr–B–Si–C coatings was investigated. The variations of the porosity, Young’s modulus and micro-hardness of the coatings were statistically evaluated. The relationship between the porosity and the micro-hardness as well as Young’s modulus of the coating was determined. The effect of spraying power on the residual stress at the coating surface was also investigated. These topics discussed should provide some important insights on establishing the thermal spray process–structure– property correlations. 2. Experimental procedures 2.1. Spraying parameters and coating material For conventional air PS guns, the gas velocities of 300–800 m/s and velocities of the particles in the range of 130–220 m/s were obtained with 5–8 mm diameter nozzles and 50–75 l/min total gas flow rates [19]. Additionally, the disordered plasma jet generated by conventional plasma gun has a ‘‘boundary effect’’. As the distance from the nozzle exit increases, the enthalpy decreases. Consequently, conventional air PS has limited coating quality with low bond strength and high porosity. A high-efficiency PS system with a hypersonic PS gun was used to prepare Ni-based alloy coatings. The advanced system developed by national key laboratory for remanufacturing, China, was composed of plasma torch, power feeder, gas supply, water cooling circulator, control unit with PC interface and power supply unit. The key of this system was a novelly supersonic PS gun, as shown in Fig. 1. Using the hypersonic PS gun, the velocities of the plasma jet and particles of 2400 m/s and 500 m/s can be achieved at the normal distance of 100 mm from the nozzle exist [20]. Near the nozzle exist, the temperature of the gas flow of
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Table 1 Nominal composition of the sprayed powder Element
Cr
Si
B
C
Fe
Ni
wt.%
18.09
4.84
3.6
0.6
15.63
54.27
Fig. 2. SEM photograph of the Ni–Cr–B–Si–C powder.
around 10,000 8C can be reached [21]. Compared with the conventional air PS, high-efficiency hypersonic PS improved greatly the coatings quality but at low cost [20]. The substrate material used was a commercial middle carbon steel, which was cut to yield 50 mm 10 mm 6 mm specimens. Prior to spraying, one face of the steel substrate was cleaned in acetone solution and preheated to 100–200 8C, and then sandblasted by using corundum powder. For sandblasting and spraying, the substrates were fixed by the corresponding fixtures. In order to achieve good temperature control of the substrate and coating, rectangular specimens were disposed in the holes on a stainless steel cylindrical holder (SCH), 200 mm in diameter and 4 mm in thickness. The rotation of the SCH was 120 rpm during the spraying. The plasma torch, whose axis was orthogonal to that of the SCH, was translated parallel to the SCH at a constant velocity of 12 mm/s. The spray distance was kept to be 100 mm. A commercially available Ni–Cr–B–Si–C powder (Ni60 AA) was used. The nominal composition of the powder can be seen in Table 1. The powder was prepared by Beijing general research institute of mining & metallurgy, China, and was characterized by near-perfect spherical particles, as shown by the scanning electron microscopy (SEM) photograph in Fig. 2. The particle diameter
Fig. 1. Schematic of plasma spray torch with a radial powder injection system.
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far apart so that the indention behavior was not affected by the adjacent indentations. Since the load was relatively low and the indent was relatively small, the tester was calibrated to avoid the indentation size effects. The residual stresses at individual coating surfaces were also determined by using XRD with sin2 C method. For the stress calculation, the Poisson’s ratio of the coating was assumed to be 0.25. However, for each coating, the elastic modulus was taken as the mean value of Young’s modulus that was obtained from Weibull plot. 2.3. Porosity analyses
Fig. 3. Probability density function of particle diameter of the initial Ni–Cr–B–Si–C powder.
ranges between 20 mm and 120 mm, as shown in Fig. 3. The average particle diameter determined by using the IAM is about 63.5 mm with the relative standard deviation of 8%. A series of coatings were prepared at different spraying powers. Details of the spraying parameters were listed in Table 2.
An IAM was developed to evaluate coating porosity. The procedure of the proposed IA method to determine the porosity of the coating can be divided into four steps, i.e., gray level transformation, fuzzy enhancement of image, binary segment of image, and removing the impurities and identification of pores, inter-lamellar voids and intralamellar cracks. The detailed introduction of the IAM can be seen in Refs. [22,23]. To get a representative porosity value, a series of SEM images with a magnification 1000 were obtained for individual specimens. Twenty digital micrographs were randomly taken from each coating cross-section using a blend of 70% BSE and 30% SE images on SEM. The high magnification images allowed a closer view of the pore structure as well as the crack distribution.
2.2. Characterization of coatings 2.4. Weibull distribution The microstructures and morphologies of coatings were observed by using a Philips Quant 200 SEM (FEI, the Netherlands) equipped with an energy dispersive spectrometer (EDS, EDAX Inc., USA) apparatus. Prior to observations, the coating samples were mounted in the epoxy resin. The cross-sectional surface for SEM observation was wet-ground with 400, 600, 800 and 1200 grit SiC paper. The samples were ultrasonically cleaned in water and then in acetone between each grit paper. A mixture of secondary electron (SE) and back scattered electron (BSE) modes was used to obtain digital SEM images with the best possible resolution and contrast to investigate the different types of microstructural defects in greater detail. Crystal structures of the coatings were determined by means of X-ray diffraction (Bruker AXS, Germany) with Cu Ka (l = 1.54A˚) radiation, step 0.028 on a D8-Advanced apparatus. Young’s modulus and micro-hardness were measured at the polished coating surface. DSI measurements were carried out with a Micro-Materials Nano-test 600 indentation tester ((NanoTest, MML, UK) by using a pyramidal indenter at room temperature (with a maximum load 15 mN and a loading rate of 0.3 mN s1). For each specimen, 20 indentations were randomly produced on the coating surface to get representative values of Young’s modulus and micro-hardness. Indentions were spaced sufficiently Table 2 Spraying parameters used for coating process
It was well known that the measured data of porosity, Young’s modulus and micro-hardness showed high scattering [22]. Hence, statistical analyses of variations of porosity, Young’s modulus and micro-hardness of each coating were made on the bases of Weibull plot [24]. The Weibull function in two-parameter form was given as, a N F ðNÞ ¼ 1 exp N0
(1)
where F(N) is the cumulative density function of probability, N is the experimental data, N0 is the characteristic value below which 63.2% of the data lie, and a is Weibull modulus which reflects the data scattering within the distribution. The detailed introduction on the Weibull distribution can be seen elsewhere [25]. 3. Results and discussion 3.1. Porosity variations The cross-sectional views of the coatings prepared under different spraying powers are shown in Fig. 4. For the coatings
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Fig. 4. Cross-sectional views of the coatings S1–S5, deposited at different spraying powers, (a) 43.2 kW, (b) 46.8 kW, (c) 47.6 kW, (d) 50.4 kW, and (e) 57.0 kW.
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Fig. 6. Variations of characteristic values and mean values of porosity of coatings as functions of spraying power.
and can be explained as follows. First, at low spraying powers, the powder particles are poorly melted. When they impacted on the substrate or the already formed splats, they are not able to spread out completely to form splats and therefore, could not conform to
Fig. 5. Weibull plots for porosity data of the coatings prepared at different spraying powers, (a) cumulative probability and (b) probability function.
prepared at the powers of 43.2 kW and 46.8 kW, the splats poorly flatten and spread out. There are numerous pores existing at the boundaries of overlapping splats. Moreover, the bonding strength between the adjacent splats is low since the cracks are visible at the interface. In addition, some unmelted particles in which the diverse phases can be seen are existed in the coating, as indicated by the arrows in Fig. 4(a and b). This result indicates that, the low spraying power cannot provide enough energy to melt the particles. With increasing the spraying power, the coating microstructure becomes denser. When the power is 57 kW, the lamellar structure is not visible and the inter-lamellar pores and cracks are almost not visible in the coating either, instead it resembled the microstructure of a bulk material, as seen Fig. 4(e). Only a few pores with small dimensions are formed at the boundaries between splats. Fig. 5 shows Weibull plots for porosity variations in the coatings prepared at different spraying powers. Fig. 5(a) shows the cumulative probability against the porosity and Fig. 5(b) shows the probability function against the porosity. It can be seen that the porosity data are heavily scattered and have wide ranges of values for the coating prepared under lower spraying power. This result indicates that, when the spraying power is low, the microstructure of the coating shows high inhomogeneity. Numerous unmelted and partially melted particles exist in the coating, resulting in the high scattering of the porosity data. Characteristic values (at cumulative probability of 63.2%) and mean values (at cumulative probability of 50.0%) with 95% reliability limit of the porosity of the coatings under different spraying powers are shown in Fig. 6. It can be seen that when the power is lower than 50 kW, the porosity of the coating decreases rapidly with increasing the spraying power. When the power is higher than 50 kW, the coating porosity is not very sensitive to the spraying power. This phenomenon is expected
Fig. 7. Microstructure of the unmelted particle in the coating prepared at the spraying power of 43.2 kW, (a) diverse phases and (b) EDS analyses on different phases.
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Fig. 8. X-ray diffraction patterns of the coatings prepared at different powers.
the surface. In such a case, the inter-lamellar pores and cracks will be formed due to the solidification of the spats. Moreover, when the power is relatively low, the unmelted and partially melted particles are existed in the coating. During the cooling process, the micro-cracks and pores are formed near the boundary of the unmelted particle, since the material mismatch between the unmelted particles and the around splats, as shown in Fig. 4(a). Second, when the power is high, the fully melted Ni with low melting temperature is considered to be well distributed and infiltrated in the spat boundary. When the power is sufficiently high, most of the powder particles have been melted and the flowability of splats is good. In such a case, when the power is further increased, the coating porosity is not very sensitive to the magnitude of the spraying power. 3.2. Phase composition Fig. 7 shows the detail of the microstructure containing an unmelted particle in the coating prepared under the spraying power of 43.2 kW. It is clear that the micro-cracks and pores are formed near the boundary of the unmelted particle. The diverse precipitates with a darker field can also been found in the particle, as illustrate by the arrow in Fig. 7(a). Analysis using EDS indicates higher Cr content of the diverse phases, as shown in Fig. 7(b). Given the high amounts of B and C in the powder, and taking into account earlier researches [26–28], these phases may be Cr carbides and/or borides. The formation of the unmelted precipitates is promoted by the low spraying temperature and a low residence time of particles during spraying [26]. X-ray diffraction patterns of the coatings prepared at different spraying powers are shown in Fig. 8. It can be revealed that the crystalline phases of plasma-sprayed Ni–Cr–B–Si–C coating consist mainly of g-Ni solid solution, Carbide (Fe,Cr)7C3, CrB type chromium boride, and nickel boride Ni3B. The peak located at about 44.48 (2u) corresponding to (Fe,Cr)7C3 is observed in all coatings. With increasing the spraying power, the intensity of the sharp peak at 44.48 (2u) decreases. This phenomenon can be attributed to the following reason. It is well known that the structure of the coating is mainly determined by the cooling rate of the particles onto the substrate or formed coating [29]. The amount of (Fe,Cr)7C3 is indicative of the presence of unmelted and/or recrystallized particles in the coating [7]. When the spraying power is low (e.g., 43.2 kW), numerous particles are partially melted and the
Fig. 9. Weibull plots for (a) micro-hardness, HIT, and (b) Young’s modulus, Ec, data for the coatings prepared at different spraying powers.
temperature of the particles is low, which leads to the high cooling rate of the particles. In such case, the degree of the crystallinity is low since the degree of melting of particles is high [30]. 3.3. Mechanical properties Weibull plots for the data of micro-hardness, HIT, and elastic modulus, Ec, for coatings prepared at different spraying powers are shown in Fig. 9. It can be seen that the micro-hardness and Young’s modulus of the coatings might follow Weibull distribution. From Fig. 9(a), it can be seen that when the power is extremely low (e.g., 43.2 kW), the micro-hardness data have wide ranges of values. This result also indicates that, when the power used is low, the microstructure of the coating shows high inhomogeneity. Numerous unmelted and partially melted particles exist in the coating, resulting in the high scattering of the micro-hardness data. With increasing the power used in the spraying, the scattering of microhardness data decreases. The similar result has been obtained by Chwa et al. [31]. Fig. 10(a) shows the variation of the characteristic value and the mean value with 95% reliability limit of micro-hardness of the coating as a function of the spraying power. The upper bound and low bound of each value are also included. It can be seen that when the power is less than 47.6 kW, with increasing the power, the characteristic value and the mean value of micro-hardness of the coating increase. The micro-hardness of the coating is almost not influenced by the spraying power when the power is high than 47.6 kW. For Young’s modulus against the spraying power, as shown in Fig. 10(b), the similar trend can be found. The mechanical properties of the coating are influenced by the microstructure of
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Fig. 12. Measured Young’s modulus and the corresponding micro-hardness with a square-root form.
Fig. 10. Variations of (a) micro-hardness and (b) Young’s modulus of the coating against the spraying power.
the coatings. Generally, with increasing the spraying power, the micro-hardness and Young’s modulus of the coating increase, due to the decrement of the micro-defects in the coating. When the power is sufficiently high, the micro-defects are almost eliminated. In such a case, the mechanical properties of the coating are almost not influenced by spraying power. The similar result can be seen elsewhere [32].
micro-crack) porous materials has always been difficult, but attractive. The relationship between the mean value of Young’s modulus as well as the mean value of micro-hardness of the Ni–Cr– B–Si–C coating and the mean value of overall porosity of the coating can be seen in Fig. 11. Young’s modulus and microhardness of the coating generally decrease with increasing the porosity. Young’s modulus and micro-hardness are two essential parameters of structural materials, and the relationship between them is of keen interest to material scientists. From statistical trend, Young’s modulus is usually considered to be an increasing function of micro-hardness [33]. Bao et al. [34] developed an analytical model to describe the relationship between the reduced modulus and micro-hardness for solid materials on the basis of the conventional DSI method of Oliver & Pharr. It was found that the two properties could be related through a material parameter that is defined as the recovery resistance Rs. This parameter represented the energy dissipation during indentation. Based on indentation measurements, the relationship was given as [34]: Er ¼ D
pffiffiffiffiffiffiffiffiffi HRs
(2)
3.4. Relationship between porosity and mechanical properties The research on the relationship between the mechanical properties and the porosity of the two phase (solid-pore or solid-
Fig. 11. Variations of mean values of micro-hardness and Young’s modulus of the coating against the mean value of the porosity.
where Er is the reduced modulus, D is a constant depending on the indenter used.
Fig. 13. Relationship between the mean value of Young’s modulus and square root of the mean value of the micro-hardness.
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4. Conclusions The microstructure, porosity and mechanical properties of the supersonic plasma-sprayed Ni–Cr–B–Si–C coatings prepared at different spraying powers were experimentally investigated. Result showed that the measured data of porosity, Young’s modulus and micro-hardness of the coatings followed Weibull distribution. When the spraying power was low, there were numerous unmelted and partially melted particles existing in the coating, resulting in high scattering of the measured data of porosity, Young’s modulus and micro-hardness. With increasing the spraying power, the mean value and characteristic value of the porosity calculated from Weibull plot decreased, while the mean and characteristic values of Young’s modulus as well as the microhardness increased. With increasing the porosity, the microhardness and Young’s modulus of the coating decreased gradually. From the statistical trend, Young’s modulus can be considered to be an increasing function of micro-hardness. The mean value of the elastic modulus of the coating was almost proportional to the square root of the mean value of the micro-hardness of the coating. Moreover, with increasing elastic modulus of the coating, the residual stress at the coating surface generally increased. Acknowledgements The authors are grateful for the supports provided by National Development Scheme of Key Fundamental Research (Nation ‘‘863’’ Project) of China (2007AA04Z408). Two of the authors (X.C. Zhang and F.Z. Xuan) are also grateful for the support by National Natural Science Foundation of China (50505012) and Fok Ying Tung Education Foundation (101054).
Fig. 14. Measured residual stress at the coating surface as a function of (a) spraying power and (b) mean value of Young’s modulus of the coating.
For each coated specimen investigated in this paper, total 20 indentations were randomly produced on the coating surface to get representative values of Young’s modulus and hardness. Young’s modulus and the corresponding hardness with a square-root form are shown in Fig. 12. It can be concluded that, from the statistical point of view, with increasing Young’s modulus, the hardness of the coating increases. Fig. 13 shows the relationship between the mean value of Young’s modulus and the square root of the mean value of the corresponding hardness of the coatings prepared under different powers. The linear relationship between elastic modulus and the square root of the micro-hardness of the coating can be fitted. This experimental result agrees well with the analytical modeling result from Bao et al. [34]. 3.5. Residual stress The variation of the residual stress at the coating surface as a function of the spraying power is shown in Fig. 14(a). For all coatings, the residual stress is tensile. The residual stress almost increases with increasing the spraying power, especially when the power is less than 47.6 kW. The similar result was obtained by Vaidya et al. [2]. The magnitude of residual stress within the coating is mainly related to Young’s modulus of the coating. The residual stress variation as a function of the mean value of Young’s modulus is shown in Fig. 14(b). It can be observed that the coatings with higher tensile residual stresses generally exhibit higher elastic moduli. The similar results can be seen elsewhere [35,36].
References [1] R. McPherson, Surf. Coat. Technol. 39–40 (1989) 173. [2] A. Vaidya, T. Streibl, L. Li, S. Sampath, O. Kovarik, R. Greenlaw, Mater. Sci. Eng. A403 (2005) 191. [3] Z. Wang, A. Kulkarni, S. Deshpande, T. Nakamura, H. Herman, Acta Mater. 51 (2003) 5319. [4] P. Bengtsson, T. Johannesson, J. Therm. Spray Technol. 4 (1995) 245. [5] S. Kuroda, T.W. Clyne, Thin Solid Films 200 (1991) 49. [6] S.C. Gill, Residual stresses in plasma sprayed deposites, Ph.D thesis, University of Cambridge, Cambridge England 1991. [7] L.C. Erickson, Wear and microstructural integrity of ceramic plasma sprayed coatings. Ph.D. Dissertation, The University of British Columbia, 1998. [8] S. Deshpande, A. Kulkarni, S. Sampath, H. Herman, Surf. Coat. Technol. 187 (2004) 6. [9] L. Cunha, M. Andritschky, K. Pischow, K. Pischow, Z. Wang, Thin Solid Films 355– 356 (1999) 465. [10] Z.H. Tong, F.M. Jiang, C.X. Ding, Q.R. Yin, J. Inorg. Mater. 7 (1992) 345. [11] D.B. Fowler, W. Riggs, J.C. Russ, Adv. Mater. Process 138 (1990) 41. [12] R. McPherson, B.V. Shafer, Thin Solid Films 97 (1982) 201. [13] C.J. Li, A. Ohmori, J. Therm. Spray Technol. 11 (2002) 365. [14] H.M. Pollock, Nanoindentation, in Friction, Lubrication and Wear Technology, ASM Handbook 18 (1992) 419–429. [15] A.J. Perry, J.A. Sue, P.J. Martin, Surf. Coat. Technol. 81 (1996) 17. [16] I.C. Noyan, J.B. Cohen, Residual Stresses-Measurement by Diffraction and Interpretation, Springer-Verlag, New York, 1987, p. 117. [17] Y. Gao, X.L. Xu, Z.J. Yan, G. Xin, Surf. Coat. Technol. 154 (2002) 189. [18] J.F. Li, H.L. Liao, C.X. Ding, C. Coddet, J. Mater. Process. Technol. 160 (2005) 34. [19] J.F. Coudert, M.P. Planche, O. Betoule, M. Vardelle, P. Fauchais, in: Proceedings of the 1993 National Thermal Spray Conference, Anaheim, CA, June 7–11, 1993, ASM International, Metals Park, OH, 1993, pp. 19–24. [20] S. Zhu, B.S. Xu, J.K. Yao, Mater. Sci. Forum 457–459 (2005) 3981. [21] S. Zhu, B.S. Xu, Key Eng. Mater. 280/283 (2005) 1203. [22] X.C. Zhang, B.S. Xu, F.Z. Xuan, H.D. Wang, Y.X. Wu, S.T. Tu., Statistical analyses of porosity variations in plasma-sprayed Ni-based coatings, J. Alloys Compd., in press, doi:10.1016/j.jallcom.2007.12.083. [23] X.C. Zhang, B.S. Xu, Y.X. Wu, F.Z. Xuan, S.T. Tu, Appl. Surf. Sci. 254 (2008) 3879. [24] Abernethy Robert B, J.E. Breneman, C.H. Medlin, G.L. Reinman, Weibull Analysis Handbook, US Air Force AFWAL-TR-83-2079, Prepared for the Aero Propulsion and Power Laboratory, Wright-Patterson AFB, Ohio, 1983. [25] C.K. Lin, C.C. Bernet, J. Mater. Sci. 30 (1995) 111. [26] J.M. Miguel, J.M. Guilemany, S. Vizcaino, Tribol. Int. 36 (2003) 181.
6326
X.C. Zhang et al. / Applied Surface Science 254 (2008) 6318–6326
[27] C. Navas, R. Vijande, J.M. Cuetos, M.R. Ferna´ndez, J. de Damborenea, Surf. Coat. Technol. 201 (2006) 776. [28] D.W. Zhang, T.C. Lei, J.G. Zhang, J.H. Ouyang, Surf. Coat. Technol. 115 (1999) 176. [29] M.P. Planche, H. Liao, B. Normand, C. Coddet, Surf. Coat. Technol. 200 (2005) 2465. [30] G.Y. Kim, H.S. Choi, C.M. Han, S.H. Uhm, C.H. Lee, Surf. Coat. Technol. 195 (2005) 107.
[31] S.O. Chwa, D. Klein, F.L. Toma, G. Bertrand, H. Liao, C. Coddet, A. Ohmori, Surf. Coat. Technol. 194 (2005) 215. [32] Y.C. Tsui, C. Doyle, T.W. Clyne, Biomaterials 19 (1998) 2015. [33] S.L.A. Prasad, M.M. Mayuram, R. Krishnamurthy, Mater. Lett. 41 (1999) 234. [34] Y.W. Bao, W. Wang, Y.C. Zhou, Acta Mater. 52 (2004) 5397. [35] J. Matejicek, S. Sampath, D. Gilmore, R. Neiser, Acta Mater. 51 (2003) 873. [36] C.A. Johnson, J.A. Ruud, R. Bruce, D. Wortman, Surf. Coat. Technol. 108–109 (1998) 80.