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Microstructure and properties of Fe-based amorphous metallic coating produced by high velocity axial plasma spraying
Journal of Alloys and Compounds 484 (2009) 300–307
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Microstructure and properties of Fe-based amorphous metallic coating produced by high velocity axial plasma spraying X.Q. Liu a , Y.G. Zheng b , X.C. Chang a , W.L. Hou a , J.Q. Wang a,∗ , Z. Tang c , A. Burgess c a b c
Shenyang National Laboratory for Materials Science, Institute of Metal Research, CAS, Shenyang, 110016, PR China State Key Laboratory for Corrosion and Protection, Institute of Metal Research, CAS, Shenyang 110016, PR China Northwest Mettech Corp., North Vancouver, BC, Canada
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Article history: Received 12 February 2009 Received in revised form 10 April 2009 Accepted 16 April 2009 Available online 23 April 2009 Keywords: Amorphous metallic coating High velocity axial plasma spraying Micro-hardness Corrosion resistance Erosion–corrosion resistance
a b s t r a c t An FeCrMoMnWBCSi amorphous metallic coating with thickness of about 1 mm and porosity less than 0.1% was deposited onto a mild steel by a high velocity axial plasma spraying. The microstructure evolution of the coating was characterized with SEM, XRD, and the coating micro-hardness, erosion–corrosion and corrosion behavior were tested. It was found that the formation of amorphous phase in the coating is sensitive to plasma spray parameters, and powder size. The coating exhibited a good combination of high micro-hardness and excellent erosion–corrosion resistance even in a serious media. This amorphous metallic coating could be applied as a good alternative material in erosion and corrosion environments. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Corrosion, wear and fracture are considered as major problems in engineering environments, especially for the material surface. Not only enormous economy lost, but also great injuries are caused by these serious material failures. Nowadays great attentions have been paid on the surface engineering field for exploring highperformance coatings deposited onto different bulk materials to alleviate the deteriorations. As a new kind of potential application material, amorphous alloys exhibit high strength and hardness, superior wear and corrosion resistance [1–5]. Many attempts have been made to fabricate amorphous metallic coatings by using different deposition methods, e.g. high velocity oxygen fuel (HVOF) [6–10], plasma spraying [11–14], kinetic spraying [15–17], electrospark deposition [18], laser cladding [19]. Among these techniques, plasma spraying has attracted much more attention due to the advantages of low cost and simplicity. In typical plasma spraying, powder feedstocks are fed to a spray gun, heated to a molten state, accelerated, and impacted on a substrate, and then spread laterally and rapidly solidified. Many overlapping and connecting lamellar splats formed finally build up coatings. The powders are fed radially during traditional thermal spraying, particle segregation of the feedstock occurs inevitably at the point of injecting into the plasma stream and on the deposit
surface, resulting in non-uniform particle temperature and velocity. Thus the coating produced usually presents an inhomogeneous structure and a low deposition efficiency [20]. To avoid the above problems, an axial plasma spraying system has recently been developed (branded Axial IIITM ) [21]. The uniqueness of this design is tri-electrode plasma torch with central axial powder injection. During spraying three plasma streams are converged at the injection point of the feedstock. The plasma/powder flow is then accelerated through a specially designed nozzle. The coatings produced by this process present superior characteristic with a lower porosity, less oxidation and higher deposition efficiency than any other plasma spraying system [20]. Fe-based amorphous metallic coatings (mostly partially amorphous structure) fabricated by thermal spraying are important materials for industrial applications based on their merits of low cost, high hardness, good abrasive wear resistance and corrosion resistance. It has indicated that the properties of the coatings are much sensitive to the amorphous content and microstructure evolution of the coatings. The aim of this paper is to prepare a highperformance Fe-based amorphous metallic coating by the axial plasma spraying process. Accordingly, the correlation of the coating properties with the coating structural characteristics has been presented. 2. Experimental
∗ Corresponding author. Tel.: +86 24 2397 1902; fax: +86 24 2397 1215. E-mail address: [email protected] (J.Q. Wang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.04.086
An Fe54.2 Cr18.3 Mo13.7 Mn2.0 W6.0 B3.3 C1.1 Si1.4 (wt.%) master alloy was prepared by induction-melting of high-purity elemental constituents (Fe: 99.9%, Cr: 99.9%, Mo: 98.5%, Mn: 99.7%, W: 99.9%, FeB: 99% (20.06 wt.% boron), C: 99.9%, Si: 99.9%) in an
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Fig. 1. SEM micrographs of the as-atomized Fe54.2 Cr18.3 Mo13.7 Mn2.0 W6.0 B3.3 C1.1 Si1.4 powders with particle sizes of 15–45 m. (a): low magnification, (b): high magnification.
Table 1 Axial III plasma spray parameters. Run Spray distance (mm) Current (A) Powder size (m) Feed rate (g/min)
A1 175 220 45–53 85
was calculated by measuring the area weight loss of samples per unit of time. The detailed information of test apparatus was described elsewhere [22]. A2 A3 175 125 220 230 15–45 15–45 Ar:N2 :H2 = 73:10:17
A4 150 230 15–45
A5 175 230 15–45
A6 150 190 15–45
yttria-stabilized zirconia crucible under an argon atmosphere. The powders were produced by high-pure Ar gas atomization at a dynamic pressure of 7 MPa after heating up to about 1600 K using a close-coupled annulus nozzle with a melt delivery inner diameter of 3 mm. The atomized powders were sieved according to conventional sieve analysis and divided into different size ranges. The as-atomized powders with particle size range of 15–45 m as well as 45–53 m ones for comparison were used for the thermal spraying. The plasma spraying system used in our experiments (Axial III Series 600, Northwest Mettech Corp., North Vancouver, BC, Canada) contains three-electrodes and uses axial powder injection. Axial powder injection ensures that virtually all of the powder injected passes through the hottest part of the plasma jet, thus facilitating the uniform melting of the powder. Deposition was carried out onto sandblasted mild carbon steel substrates mounted on a turntable rotating at 400 rpm. The selected spraying parameters were listed in Table 1. The oxygen content in the powders and detached coatings were analyzed by a Nitrogen/Oxygen determinator (Leco TC-436, St. Joseph, MI). The microstructure and elemental characterization of the powders and coatings were examined by scanning electron microscopy (SEM) (JMS-6301) incorporated with energy dispersive spectroscopy (EDS). X-ray diffraction (XRD) analysis of the powders and coatings was undertaken on an X-ray diffractometry (Rigaku D/max 2400) with Cu K␣ radiation. The percentage of porosity in the coatings was evaluated by using image analysis on optical microscopy (MEF-4). The thermal stabilities of the melt-spun ribbons and coatings were examined in differential scanning calorimeter (PerkinElmer DSC-7) in a continuous heating mode at a rate of 20 K/min. For comparison, the melt-spun Fe54.2 Cr18.3 Mo13.7 Mn2.0 W6.0 B3.3 C1.1 Si1.4 ribbon with ∼3 mm width and ∼150 m thickness and the electroplated chromium sample with ∼10 m thickness were prepared. The micro-hardness of the coating was measured on the cross section of the coatings using a hardness tester (MVK-H3) with indentation load of 100 g for duration of 10 s. During the hardness testing closed porosities and micro-cracks within the coating were avoided to ensure that the measured values were a true hardness of the coating. The values given are the average of 10 measurements. The corrosion behavior of the coatings were evaluated by electrochemical measurement on a Potentiostat/Galvanostat (EG & G Princeton Applied Research Model 273). Prior to electrochemical measurements, the samples were ground to 1200grit SiC papers and degreased in acetone, washed in distilled water and dried in air. A standard cell with three-electrode using a platinum counter electrode and a saturated calomel electrode (SCE) reference electrode was employed for the electrochemical measurements. As the open-circuit potential became almost steady, the samples with 1 cm2 exposed to the electrolyte were scanned at potential sweep rate of 0.33 mV/s in 1 M hydrochloric aqueous solutions open to air at 298 K after sample immersing for several minutes. Erosion–corrosion tests were conducted using a modified rotating disk rig with six specimen holders on its edge. The test media used for the rotating disk rig was tap water with silica sand (10 kg/m3 , 200–300 mesh) and the effects of corrosion on erosion were investigated through adding 1 wt.% NaCl. The linear rotating speed of specimens was 15 m/s holding for 4 h. The samples were ultrasonic cleaned with acetone and dried prior to and behind the experiments. The erosion–corrosion rate
3. Results and discussion 3.1. Characterization of gas-atomized powders SEM observations of the powders are shown in Fig. 1. It is clear that the powder particles are spherical or near-spherical with a mean diameter of 33 m. Some large particles are being capped with small ones. The solidification rates are different for particles with different diameters. During atomization, particles with various sizes would collide with each other in the gas turbulence. Thus, some small particles with a higher solidification rate would easily adhere to the molten surfaces of large particles, forming such an attached particle morphology [23]. From Fig. 1(b), it is also noted that the surface of larger particles is ragged with marked shrinkages. This is associated with the occurrence of a non-planar solidification in the larger particles with lower solidification rate. XRD patterns of the powders and ribbons are shown in Fig. 2. There exhibits only a broad halo of fully amorphous phase in the ribbon samples. For the powders, it shows a broad halo and some crystalline peaks due to Fe2 C, Cr2 B and M23 C6 , reflecting that the cooling rate of gas-atomization is apparently lower than that of melt-spinning. From DSC measurements, glass transition and onset crystallization temperatures of the powders are determined to be 597 ◦ C and 632 ◦ C, respectively.
Fig. 2. XRD patterns of the as-spun ribbon and the atomized powders of Fe-based alloy.
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Fig. 3. SEM images of the Fe-based coatings prepared at different conditions (a) the cross-section of coating A1, (b) the cross-section of coating A2, (c) the cross-section of coating A5, (d) the plane view of coating A1, (e) the plane view of coating A2, (f) the plane view of coating A5.
3.2. Microstructure of the coatings The microstructure of plasma spraying coatings depends not only on the feeding powders, but also the spraying process parameters. Fig. 3 shows the cross-section and plane view SEM images of as-sprayed Fe-based alloy splats and resulting coatings under different spraying conditions. The detailed spray parameters (A1–A6) are listed in Table 1. It can be seen from Fig. 3(a) and (b) that the coatings sprayed by powders with different sizes have a uniform structure with the presence of some pores and the partially melted particles. For the as-sprayed coating with the coarser powders (A1), the coating is more porous with more unmelted particles, compared to that of using the finer powders (A2). The porosity was 2.36% for A1 and 0.95% for A2, respectively, as measured by area fraction measurement performed on optical microscopy. As for the porosities there are several possible types of pores in the plasma spraying coating, depending upon their positions [24]: (1) within the unmelted powder particles that presumably fractured
on impact with the substrate or the pre-deposited coating; (2) near the unmelted or partially melted powder; (3) at the junction of lamellar splats; (4) within the matrix of the coating. From Fig. 3(a) and (b), the former two effects seem to be prevailing in the two coatings. It is noted that some big pores have obviously occurred in the vicinity of large unmelted particles. The reason is that during thermal spraying, the heat transfer in the larger powder particles was inhomogeneous from the molten surface to the core of particle, and the large unmelted or partially melted powder particles prevented good flattening and splashing behaviors of the fully melted droplets when they impacted on the substrate. The torch current is a critical factor in plasma spraying which relates to the change of coating morphology. Fig. 3(b) and (c) show the characteristics of the as-sprayed coating with different torch current. Under a given set of spraying conditions (A2 and A5), the porosity and the number of unmelted or partially melted particles decreased sharply as the spraying current increased from 220 A to 230 A, which can be seen from Fig. 3(e) and (f). Correspondingly,
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Fig. 4. (a) Comparison of XRD patterns of the coatings A1, A2 and A5. (b) Elemental distribution spectrum along the depth of the coating A5.
the porosity measured decreased from 0.95% to 0.1%, much less than typical values of plasma spraying coating, above 1% [25]. The spraying current parameter that determines the variance in coating morphology is considered to be the reflection of melting state of the in-flight particle. In the case of fully melted particle of Fig. 3(f), the in-pressure of the impacting molten liquid induces the mass flow and the resulting well-flattened splat is produced by rapid cooling. For a splat like the one presented in Fig. 3(d), the unmelted solid and the solidified parts coexist in a single splat. Therefore, it could be deduced that the higher spraying current enhanced the in-flight particle melting state (decrease in the unmelted particle number density), resulting in the decrease of pore forming frequencies and splat boundaries. Phase identification of these Fe-based coatings was performed as shown in Fig. 4(a). There are some crystalline phases which tend to overlap in a diffuse halo peak. As the powder size decreases, the number and the intensity of crystalline peaks reduce. These crystalline phases are mainly retained from these unmelted or partially molten particles that embedded in the coatings [13]. The XRD results indicate that the smaller the powder particle size is, the more the amorphous phase fraction is. Fig. 4(b) illustrates the distributions of elemental constituents along the depth of the A5 coating. All of the elemental concentrations are almost homogenous, suggesting such a coating is very dense and uniform. Fig. 5 shows the cross-sectional microstructures of Fe-based coatings deposited under different spray distances. The coating thickness can be used as a qualitative indicator for the deposition efficiency. Under the same feed rate and spray passes, the coating
Fig. 5. SEM backscattered images of typical region from the cross-section of the coatings sprayed at different stand-off distances (a) coating A3, 125 mm, (b) coating A4, 150 mm, (c) coating A5, 175 mm.
thickness increases as the spraying distance is increased. For A3 coating sprayed at the shortest distance of 125 mm, there exhibit apparent defective microstructures, such as pores and splat boundaries, as seen in Fig. 5(a). As the stand-off distance increases from 150 mm to 175 mm, the build up density appears to increase. In thermal spraying, the velocity and temperature of the powder particles prior to impact on the substrate or pre-deposited coating play major roles in controlling coating characteristics. An increase in particle
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Fig. 6. Comparison of XRD patterns of the coatings A3, A4 and A5.
velocity at impact causes an increase in the kinetic energy of the particle, and a rise in particle temperature brings about a decrease in material viscosity. Both result in diminished splat thickness and make splashing easier [26]. The variation of process parameters can be expected to influence the particle velocity and temperature. As the stand-off distance increases, the resident time of particles in the high temperature plasma stream becomes longer, which renders a much more homogeneous melting condition, but the particle velocity reduces to some extent, especially when the distance is too far. This will enhance the rate of heat extraction between the particles and substrate through effectively decreasing the number of partially melted or unmelted powders on impact, resulting in a denser coating with less pores and splat boundaries. An XRD analysis carried out on these coatings is shown in Fig. 6. All of the coatings show partial crystalline phase coexisted with the amorphous phase. This result indicates that the change of spraying distance has slight influence on the phase constituent of Fe-based metallic coating. 3.3. Amorphous phase content of the coatings From the coating microstructures shown in Figs. 3 and 5, obviously, the A5 coating, with the thickness up to 1 mm, presents the best dense characteristics with little porosity. Compared with the feedstock, the crystallinity in the A5 coating is alleviated (see Figs. 2 and 4). For plasma spraying, individual particle deposition is primarily dependent on the particle melting state at the moment of impact. Partially melted or fully melted particles can be deposited to form splats through the rapid flattening and solidification processes [27]. Thus, the feedstock undergoes remelting and subsequent rapid solidification in the course of coating formation, resulting in the increment of amorphous phase in the A5 coating. However, from the XRD results in Fig. 6, there still exhibit a few crystalline phases in these coatings. The occurrence of crystallization in the as-sprayed coatings is mainly related to the intrinsic features of materials and spraying process. The GFA of the present iron-based alloy system is limited. To obtain a fully amorphous coating structure, it requires a much higher cooling rate during particles flattening and piling up on the substrate. In fact, in thermal spraying, coatings are deposited in a layer-by-layer manner, the self-annealing or adiabatic recalescence in the deposited layers occurs inevitably, though it is sometimes beneficial to stress relief and bonding improvement [28]. Thus, a marked temperature gradient has developed in the thicker layers, resulting in a variation of the coating structure with thickness and the occurrence of crystallization reaction.
Fig. 7. DSC traces of coating A1, A5 and ribbon heated at a rate of 20 K/min, Fe-based ribbon used for reference for comparison.
The oxidation is proposed to be a disadvantageous factor to affect the formation of amorphous structure. XRD results indicate that oxides do not appear apparently in the coatings. Further, we have measured the oxygen contents in the feedstock powders and assprayed coatings by a Nitrogen/Oxygen determinator. The oxygen mass contents of the A1–A5 coatings are in the range of 0.12–0.19%, whereas 0.046% in the powders. The present coating oxygen contents are much lower than a typical value of coatings fabricated by the traditional air plasma spray, which often above 1% [29]. This result reflects that the use of inert gas shroud in Axial III plasma spraying process is an effective way to protect coating from oxidation, also excluding the operative effect of oxidation on coating formation. Fig. 7 shows the DSC traces for the A1 and A5 coatings and the as-spun ribbon. There is an obvious exothermic reaction in the ribbon. However, the area of exothermic reaction becomes weaker in the coatings, which is due to the partial crystalline phases precipitated in the coatings. Compared with the ribbons which can be considered as fully amorphous structure, the amorphous phase volume fraction contained in the coatings can be evaluated by the following equation: Vf = Hcoating /Hribbon , where Hcoating and Hribbon are the crystallization enthalpy of the coating and ribbon samples, respectively. From which, we calculated the amorphous phase contents of the coatings where the A5 coating has the highest amorphous phase content which is about 72.9% and the A1 coating is the least one. This reflects that the particle melting behavior is closely related to the amorphous phase content of coatings. The more sufficient the powder melts, the more amorphous phase content forms in the coating. A good particle melting state can cause the splash thickness decreased, resulting in a higher cooling rate [30]. However, as for the unmelted particles, there exhibit a lower amorphous phase content and a lower cooling rate due to the existence of crystalline phases. To examine the change in amorphous phase contents with coating thickness, the XRD analyses of the coating A5 at the top and the bottom surfaces were performed, as shown in Fig. 8(a) and (b). It is certain that the amorphous phase content in the coating decreases with the increase of the thickness, i.e., the coating surface near substrate holds the higher amorphous phase content than that of the top surface. To confirm this phenomenon, we have prepared the coating A6 with the thickness of only about 50 m. The corresponding XRD result and SEM observation of the A6 coating are shown in Figs 8(c) and 9, respectively. Apparently, an almost completely amorphous structure is obtained in such a thinner coating.
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Fig. 8. XRD patterns of coatings scanned at different surfaces (a) the top surface of coating A5, (b) the bottom surface of coating A5, (c) the top surface of coating A6.
As for the variation of the amorphous phase content with the coating depth, it is understandable on the basis of the thermal spraying characteristics. During the first layer splat formation, the temperature of the substrate is relatively lower and the cooling rate will be maximized (typically about 106 K/s). For the coating with only one pass, the self-annealing and crystallization would be inhibited. As the first layer solidified on the substrate, the thermal conductivity of the sample becomes lower that that of the substrate. This renders the coating forming a temperature gradient along the coating depth [31]. Generally, amorphous alloys display a lower heat conductivity [13,32,33], thus, the deposited thicker coating leads to the less amorphous phase content. 3.4. Corrosion resistance of the coatings Fig. 10 shows potentiodynamic polarization curves of the coatings in 1 M HCl aqueous solution, herein, the ribbon, substrate and electroplated Cr also presented for comparison. All the coatings and the ribbon passivate spontaneously with lower passive current densities in the order of magnitude less than 10−4 A/cm2 . They are much lower than those of the electroplate Cr and the substrate, in which dissolutions of active metals occur. A wide passive region without transpassive behavior is determined to be about 1.2 V in the coatings, indicating that these coatings hold an excellent ability to resist localized corrosion. From Fig. 10, it is tempting that the amor-
Fig. 9. A cross-sectional SEM image of the coating A6 with one-pass spray.
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Fig. 10. Potentiodynamic polarization curves of the coatings A5, A4, A3 and A2, in comparison with the amorphous ribbon, the substrate and the electroplate Cr samples in 1 M HCl aqueous solution.
phous coatings present superior corrosion resistance performance, as compared with the electroplate Cr, the mild steel and the previous reported Ni- and Fe-based amorphous metal coatings [34], Further, we can also note that the corrosion behavior of the coatings varies with spray parameters. The further spraying distance, the higher plasma current and the finer powders in thermal spray are beneficial to the corrosion resistance of the coatings. In essence, these factors can lead to a lower porosity, a lower crystalline content as well as a lower oxidation that are directly related to the coating corrosion resistance. 3.5. Micro-hardness and erosion–corrosion performance Fig. 11 illustrates the variations of erosion–corrosion rate and micro-hardness in the A5 coating, substrate and nonmagnetic steel sheet. The A5 coating exhibits a very high micro-hardness of about 1081 HV, which is much higher than that of the low carbon steel, nonmagnetic steel sheet and other Fe-based amorphous coating reported earlier [34]. The erosion–corrosion resistances of these samples are measured in the media of 1% NaCl solution with 1% silicon sand. The flow velocity is 15 m/s and the duration time is 4 h. The erosion–corrosion rate of the coating A5 is about 0.19 mg/cm2 h, which is much lower than those of the carbon steel substrate and
Fig. 11. Comparison of erosion–corrosion rate and micro-hardness of the coating A5, low carbon steel and shipbuilding steel sheet.
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Fig. 12. SEM images of the samples after erosion–corrosion test in 1% NaCl + 1% silica sand at the flow rate of 15 m/s (a) low carbon steel (b) shipbuilding steel sheet and (c) coating A5.
nonmagnetic shipbuilding steel sheet. The actual value of coating should be a little higher than the measured one since the silica sand can be contained into the pores of the coating during the test [35]. It is found that as the material micro-hardness increases, the erosion–corrosion rate decreases. The surface morphologies of these samples after the erosion–corrosion tests are presented in Fig. 12. No marked destruction caused by silica sand even in the position of pores can be determined in the coating (see Fig. 12(c)), whereas some pronounced long scratch scars induced by sand ploughing and some pitting corrosion products are displayed in the low carbon steel and shipbuilding steel sheet (Fig. 12(a) and (b)). Considering the high erosion–corrosion resistance is the reflections of high micro-hardness, high strength and excellent corrosion resistance, hence, the iron-based amorphous alloy coating can be developed as an excellent alternative material used in many aggressive fields where some components and equipments are exposed in the severe erosion and corrosion environments, such as the pump, valve and hull for shipbuilding and hydraulic sets.
taining more unmelted particles. The plasma current has a positive effect on the amorphous phase content and decreasing porosity level. In addition, the deposition efficiency, amorphous phase content and structure of the coating are found to be sensitive to the spray distance. (3) The coating exhibits very high micro-hardness and excellent erosion–corrosion resistance even in such a serious media, which suggests that these amorphous alloy coatings can be used as a good alternative material in erosion and corrosion environments, especially for the application in the areas of marine and hydraulic.
4. Conclusions
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An Fe-based amorphous metallic coating, with a high corrosion resistance and micro-hardness, has been prepared by Axial III plasma thermal spray, and the influences of spraying parameters on microstructures, amorphous phase content as well as corrosion resistance of the coatings were evaluated. The main conclusions are as follows:
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(1) A thick uniform coating of about 1 mm with very low porosity of about 0.1% and low oxidation can be prepared by the unique Axial III plasma spray technique. As the coating thickness increases, the amorphous phase content decreases. This is related to the occurrence of crystallization and the low thermal conductivity of the coating. (2) The appropriate powder size for Axial III plasma spraying is 15–45 m. The increase of powder size makes the coating con-
Acknowledgements This work was supported by the National Natural Science Foundation of China (grant No. 50323009). Thanks are due to Dr. Z.M. Wang and Z. Liu for their assistances in experiment and helpful discussion.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Microstructure and properties of Fe-based amorphous metallic coating produced by high velocity axial plasma spraying X.Q. Liu a , Y.G. Zheng b , X.C. Chang a , W.L. Hou a , J.Q. Wang a,∗ , Z. Tang c , A. Burgess c a b c
Shenyang National Laboratory for Materials Science, Institute of Metal Research, CAS, Shenyang, 110016, PR China State Key Laboratory for Corrosion and Protection, Institute of Metal Research, CAS, Shenyang 110016, PR China Northwest Mettech Corp., North Vancouver, BC, Canada
a r t i c l e
i n f o
Article history: Received 12 February 2009 Received in revised form 10 April 2009 Accepted 16 April 2009 Available online 23 April 2009 Keywords: Amorphous metallic coating High velocity axial plasma spraying Micro-hardness Corrosion resistance Erosion–corrosion resistance
a b s t r a c t An FeCrMoMnWBCSi amorphous metallic coating with thickness of about 1 mm and porosity less than 0.1% was deposited onto a mild steel by a high velocity axial plasma spraying. The microstructure evolution of the coating was characterized with SEM, XRD, and the coating micro-hardness, erosion–corrosion and corrosion behavior were tested. It was found that the formation of amorphous phase in the coating is sensitive to plasma spray parameters, and powder size. The coating exhibited a good combination of high micro-hardness and excellent erosion–corrosion resistance even in a serious media. This amorphous metallic coating could be applied as a good alternative material in erosion and corrosion environments. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Corrosion, wear and fracture are considered as major problems in engineering environments, especially for the material surface. Not only enormous economy lost, but also great injuries are caused by these serious material failures. Nowadays great attentions have been paid on the surface engineering field for exploring highperformance coatings deposited onto different bulk materials to alleviate the deteriorations. As a new kind of potential application material, amorphous alloys exhibit high strength and hardness, superior wear and corrosion resistance [1–5]. Many attempts have been made to fabricate amorphous metallic coatings by using different deposition methods, e.g. high velocity oxygen fuel (HVOF) [6–10], plasma spraying [11–14], kinetic spraying [15–17], electrospark deposition [18], laser cladding [19]. Among these techniques, plasma spraying has attracted much more attention due to the advantages of low cost and simplicity. In typical plasma spraying, powder feedstocks are fed to a spray gun, heated to a molten state, accelerated, and impacted on a substrate, and then spread laterally and rapidly solidified. Many overlapping and connecting lamellar splats formed finally build up coatings. The powders are fed radially during traditional thermal spraying, particle segregation of the feedstock occurs inevitably at the point of injecting into the plasma stream and on the deposit
surface, resulting in non-uniform particle temperature and velocity. Thus the coating produced usually presents an inhomogeneous structure and a low deposition efficiency [20]. To avoid the above problems, an axial plasma spraying system has recently been developed (branded Axial IIITM ) [21]. The uniqueness of this design is tri-electrode plasma torch with central axial powder injection. During spraying three plasma streams are converged at the injection point of the feedstock. The plasma/powder flow is then accelerated through a specially designed nozzle. The coatings produced by this process present superior characteristic with a lower porosity, less oxidation and higher deposition efficiency than any other plasma spraying system [20]. Fe-based amorphous metallic coatings (mostly partially amorphous structure) fabricated by thermal spraying are important materials for industrial applications based on their merits of low cost, high hardness, good abrasive wear resistance and corrosion resistance. It has indicated that the properties of the coatings are much sensitive to the amorphous content and microstructure evolution of the coatings. The aim of this paper is to prepare a highperformance Fe-based amorphous metallic coating by the axial plasma spraying process. Accordingly, the correlation of the coating properties with the coating structural characteristics has been presented. 2. Experimental
∗ Corresponding author. Tel.: +86 24 2397 1902; fax: +86 24 2397 1215. E-mail address: [email protected] (J.Q. Wang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.04.086
An Fe54.2 Cr18.3 Mo13.7 Mn2.0 W6.0 B3.3 C1.1 Si1.4 (wt.%) master alloy was prepared by induction-melting of high-purity elemental constituents (Fe: 99.9%, Cr: 99.9%, Mo: 98.5%, Mn: 99.7%, W: 99.9%, FeB: 99% (20.06 wt.% boron), C: 99.9%, Si: 99.9%) in an
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Fig. 1. SEM micrographs of the as-atomized Fe54.2 Cr18.3 Mo13.7 Mn2.0 W6.0 B3.3 C1.1 Si1.4 powders with particle sizes of 15–45 m. (a): low magnification, (b): high magnification.
Table 1 Axial III plasma spray parameters. Run Spray distance (mm) Current (A) Powder size (m) Feed rate (g/min)
A1 175 220 45–53 85
was calculated by measuring the area weight loss of samples per unit of time. The detailed information of test apparatus was described elsewhere [22]. A2 A3 175 125 220 230 15–45 15–45 Ar:N2 :H2 = 73:10:17
A4 150 230 15–45
A5 175 230 15–45
A6 150 190 15–45
yttria-stabilized zirconia crucible under an argon atmosphere. The powders were produced by high-pure Ar gas atomization at a dynamic pressure of 7 MPa after heating up to about 1600 K using a close-coupled annulus nozzle with a melt delivery inner diameter of 3 mm. The atomized powders were sieved according to conventional sieve analysis and divided into different size ranges. The as-atomized powders with particle size range of 15–45 m as well as 45–53 m ones for comparison were used for the thermal spraying. The plasma spraying system used in our experiments (Axial III Series 600, Northwest Mettech Corp., North Vancouver, BC, Canada) contains three-electrodes and uses axial powder injection. Axial powder injection ensures that virtually all of the powder injected passes through the hottest part of the plasma jet, thus facilitating the uniform melting of the powder. Deposition was carried out onto sandblasted mild carbon steel substrates mounted on a turntable rotating at 400 rpm. The selected spraying parameters were listed in Table 1. The oxygen content in the powders and detached coatings were analyzed by a Nitrogen/Oxygen determinator (Leco TC-436, St. Joseph, MI). The microstructure and elemental characterization of the powders and coatings were examined by scanning electron microscopy (SEM) (JMS-6301) incorporated with energy dispersive spectroscopy (EDS). X-ray diffraction (XRD) analysis of the powders and coatings was undertaken on an X-ray diffractometry (Rigaku D/max 2400) with Cu K␣ radiation. The percentage of porosity in the coatings was evaluated by using image analysis on optical microscopy (MEF-4). The thermal stabilities of the melt-spun ribbons and coatings were examined in differential scanning calorimeter (PerkinElmer DSC-7) in a continuous heating mode at a rate of 20 K/min. For comparison, the melt-spun Fe54.2 Cr18.3 Mo13.7 Mn2.0 W6.0 B3.3 C1.1 Si1.4 ribbon with ∼3 mm width and ∼150 m thickness and the electroplated chromium sample with ∼10 m thickness were prepared. The micro-hardness of the coating was measured on the cross section of the coatings using a hardness tester (MVK-H3) with indentation load of 100 g for duration of 10 s. During the hardness testing closed porosities and micro-cracks within the coating were avoided to ensure that the measured values were a true hardness of the coating. The values given are the average of 10 measurements. The corrosion behavior of the coatings were evaluated by electrochemical measurement on a Potentiostat/Galvanostat (EG & G Princeton Applied Research Model 273). Prior to electrochemical measurements, the samples were ground to 1200grit SiC papers and degreased in acetone, washed in distilled water and dried in air. A standard cell with three-electrode using a platinum counter electrode and a saturated calomel electrode (SCE) reference electrode was employed for the electrochemical measurements. As the open-circuit potential became almost steady, the samples with 1 cm2 exposed to the electrolyte were scanned at potential sweep rate of 0.33 mV/s in 1 M hydrochloric aqueous solutions open to air at 298 K after sample immersing for several minutes. Erosion–corrosion tests were conducted using a modified rotating disk rig with six specimen holders on its edge. The test media used for the rotating disk rig was tap water with silica sand (10 kg/m3 , 200–300 mesh) and the effects of corrosion on erosion were investigated through adding 1 wt.% NaCl. The linear rotating speed of specimens was 15 m/s holding for 4 h. The samples were ultrasonic cleaned with acetone and dried prior to and behind the experiments. The erosion–corrosion rate
3. Results and discussion 3.1. Characterization of gas-atomized powders SEM observations of the powders are shown in Fig. 1. It is clear that the powder particles are spherical or near-spherical with a mean diameter of 33 m. Some large particles are being capped with small ones. The solidification rates are different for particles with different diameters. During atomization, particles with various sizes would collide with each other in the gas turbulence. Thus, some small particles with a higher solidification rate would easily adhere to the molten surfaces of large particles, forming such an attached particle morphology [23]. From Fig. 1(b), it is also noted that the surface of larger particles is ragged with marked shrinkages. This is associated with the occurrence of a non-planar solidification in the larger particles with lower solidification rate. XRD patterns of the powders and ribbons are shown in Fig. 2. There exhibits only a broad halo of fully amorphous phase in the ribbon samples. For the powders, it shows a broad halo and some crystalline peaks due to Fe2 C, Cr2 B and M23 C6 , reflecting that the cooling rate of gas-atomization is apparently lower than that of melt-spinning. From DSC measurements, glass transition and onset crystallization temperatures of the powders are determined to be 597 ◦ C and 632 ◦ C, respectively.
Fig. 2. XRD patterns of the as-spun ribbon and the atomized powders of Fe-based alloy.
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Fig. 3. SEM images of the Fe-based coatings prepared at different conditions (a) the cross-section of coating A1, (b) the cross-section of coating A2, (c) the cross-section of coating A5, (d) the plane view of coating A1, (e) the plane view of coating A2, (f) the plane view of coating A5.
3.2. Microstructure of the coatings The microstructure of plasma spraying coatings depends not only on the feeding powders, but also the spraying process parameters. Fig. 3 shows the cross-section and plane view SEM images of as-sprayed Fe-based alloy splats and resulting coatings under different spraying conditions. The detailed spray parameters (A1–A6) are listed in Table 1. It can be seen from Fig. 3(a) and (b) that the coatings sprayed by powders with different sizes have a uniform structure with the presence of some pores and the partially melted particles. For the as-sprayed coating with the coarser powders (A1), the coating is more porous with more unmelted particles, compared to that of using the finer powders (A2). The porosity was 2.36% for A1 and 0.95% for A2, respectively, as measured by area fraction measurement performed on optical microscopy. As for the porosities there are several possible types of pores in the plasma spraying coating, depending upon their positions [24]: (1) within the unmelted powder particles that presumably fractured
on impact with the substrate or the pre-deposited coating; (2) near the unmelted or partially melted powder; (3) at the junction of lamellar splats; (4) within the matrix of the coating. From Fig. 3(a) and (b), the former two effects seem to be prevailing in the two coatings. It is noted that some big pores have obviously occurred in the vicinity of large unmelted particles. The reason is that during thermal spraying, the heat transfer in the larger powder particles was inhomogeneous from the molten surface to the core of particle, and the large unmelted or partially melted powder particles prevented good flattening and splashing behaviors of the fully melted droplets when they impacted on the substrate. The torch current is a critical factor in plasma spraying which relates to the change of coating morphology. Fig. 3(b) and (c) show the characteristics of the as-sprayed coating with different torch current. Under a given set of spraying conditions (A2 and A5), the porosity and the number of unmelted or partially melted particles decreased sharply as the spraying current increased from 220 A to 230 A, which can be seen from Fig. 3(e) and (f). Correspondingly,
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Fig. 4. (a) Comparison of XRD patterns of the coatings A1, A2 and A5. (b) Elemental distribution spectrum along the depth of the coating A5.
the porosity measured decreased from 0.95% to 0.1%, much less than typical values of plasma spraying coating, above 1% [25]. The spraying current parameter that determines the variance in coating morphology is considered to be the reflection of melting state of the in-flight particle. In the case of fully melted particle of Fig. 3(f), the in-pressure of the impacting molten liquid induces the mass flow and the resulting well-flattened splat is produced by rapid cooling. For a splat like the one presented in Fig. 3(d), the unmelted solid and the solidified parts coexist in a single splat. Therefore, it could be deduced that the higher spraying current enhanced the in-flight particle melting state (decrease in the unmelted particle number density), resulting in the decrease of pore forming frequencies and splat boundaries. Phase identification of these Fe-based coatings was performed as shown in Fig. 4(a). There are some crystalline phases which tend to overlap in a diffuse halo peak. As the powder size decreases, the number and the intensity of crystalline peaks reduce. These crystalline phases are mainly retained from these unmelted or partially molten particles that embedded in the coatings [13]. The XRD results indicate that the smaller the powder particle size is, the more the amorphous phase fraction is. Fig. 4(b) illustrates the distributions of elemental constituents along the depth of the A5 coating. All of the elemental concentrations are almost homogenous, suggesting such a coating is very dense and uniform. Fig. 5 shows the cross-sectional microstructures of Fe-based coatings deposited under different spray distances. The coating thickness can be used as a qualitative indicator for the deposition efficiency. Under the same feed rate and spray passes, the coating
Fig. 5. SEM backscattered images of typical region from the cross-section of the coatings sprayed at different stand-off distances (a) coating A3, 125 mm, (b) coating A4, 150 mm, (c) coating A5, 175 mm.
thickness increases as the spraying distance is increased. For A3 coating sprayed at the shortest distance of 125 mm, there exhibit apparent defective microstructures, such as pores and splat boundaries, as seen in Fig. 5(a). As the stand-off distance increases from 150 mm to 175 mm, the build up density appears to increase. In thermal spraying, the velocity and temperature of the powder particles prior to impact on the substrate or pre-deposited coating play major roles in controlling coating characteristics. An increase in particle
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Fig. 6. Comparison of XRD patterns of the coatings A3, A4 and A5.
velocity at impact causes an increase in the kinetic energy of the particle, and a rise in particle temperature brings about a decrease in material viscosity. Both result in diminished splat thickness and make splashing easier [26]. The variation of process parameters can be expected to influence the particle velocity and temperature. As the stand-off distance increases, the resident time of particles in the high temperature plasma stream becomes longer, which renders a much more homogeneous melting condition, but the particle velocity reduces to some extent, especially when the distance is too far. This will enhance the rate of heat extraction between the particles and substrate through effectively decreasing the number of partially melted or unmelted powders on impact, resulting in a denser coating with less pores and splat boundaries. An XRD analysis carried out on these coatings is shown in Fig. 6. All of the coatings show partial crystalline phase coexisted with the amorphous phase. This result indicates that the change of spraying distance has slight influence on the phase constituent of Fe-based metallic coating. 3.3. Amorphous phase content of the coatings From the coating microstructures shown in Figs. 3 and 5, obviously, the A5 coating, with the thickness up to 1 mm, presents the best dense characteristics with little porosity. Compared with the feedstock, the crystallinity in the A5 coating is alleviated (see Figs. 2 and 4). For plasma spraying, individual particle deposition is primarily dependent on the particle melting state at the moment of impact. Partially melted or fully melted particles can be deposited to form splats through the rapid flattening and solidification processes [27]. Thus, the feedstock undergoes remelting and subsequent rapid solidification in the course of coating formation, resulting in the increment of amorphous phase in the A5 coating. However, from the XRD results in Fig. 6, there still exhibit a few crystalline phases in these coatings. The occurrence of crystallization in the as-sprayed coatings is mainly related to the intrinsic features of materials and spraying process. The GFA of the present iron-based alloy system is limited. To obtain a fully amorphous coating structure, it requires a much higher cooling rate during particles flattening and piling up on the substrate. In fact, in thermal spraying, coatings are deposited in a layer-by-layer manner, the self-annealing or adiabatic recalescence in the deposited layers occurs inevitably, though it is sometimes beneficial to stress relief and bonding improvement [28]. Thus, a marked temperature gradient has developed in the thicker layers, resulting in a variation of the coating structure with thickness and the occurrence of crystallization reaction.
Fig. 7. DSC traces of coating A1, A5 and ribbon heated at a rate of 20 K/min, Fe-based ribbon used for reference for comparison.
The oxidation is proposed to be a disadvantageous factor to affect the formation of amorphous structure. XRD results indicate that oxides do not appear apparently in the coatings. Further, we have measured the oxygen contents in the feedstock powders and assprayed coatings by a Nitrogen/Oxygen determinator. The oxygen mass contents of the A1–A5 coatings are in the range of 0.12–0.19%, whereas 0.046% in the powders. The present coating oxygen contents are much lower than a typical value of coatings fabricated by the traditional air plasma spray, which often above 1% [29]. This result reflects that the use of inert gas shroud in Axial III plasma spraying process is an effective way to protect coating from oxidation, also excluding the operative effect of oxidation on coating formation. Fig. 7 shows the DSC traces for the A1 and A5 coatings and the as-spun ribbon. There is an obvious exothermic reaction in the ribbon. However, the area of exothermic reaction becomes weaker in the coatings, which is due to the partial crystalline phases precipitated in the coatings. Compared with the ribbons which can be considered as fully amorphous structure, the amorphous phase volume fraction contained in the coatings can be evaluated by the following equation: Vf = Hcoating /Hribbon , where Hcoating and Hribbon are the crystallization enthalpy of the coating and ribbon samples, respectively. From which, we calculated the amorphous phase contents of the coatings where the A5 coating has the highest amorphous phase content which is about 72.9% and the A1 coating is the least one. This reflects that the particle melting behavior is closely related to the amorphous phase content of coatings. The more sufficient the powder melts, the more amorphous phase content forms in the coating. A good particle melting state can cause the splash thickness decreased, resulting in a higher cooling rate [30]. However, as for the unmelted particles, there exhibit a lower amorphous phase content and a lower cooling rate due to the existence of crystalline phases. To examine the change in amorphous phase contents with coating thickness, the XRD analyses of the coating A5 at the top and the bottom surfaces were performed, as shown in Fig. 8(a) and (b). It is certain that the amorphous phase content in the coating decreases with the increase of the thickness, i.e., the coating surface near substrate holds the higher amorphous phase content than that of the top surface. To confirm this phenomenon, we have prepared the coating A6 with the thickness of only about 50 m. The corresponding XRD result and SEM observation of the A6 coating are shown in Figs 8(c) and 9, respectively. Apparently, an almost completely amorphous structure is obtained in such a thinner coating.
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Fig. 8. XRD patterns of coatings scanned at different surfaces (a) the top surface of coating A5, (b) the bottom surface of coating A5, (c) the top surface of coating A6.
As for the variation of the amorphous phase content with the coating depth, it is understandable on the basis of the thermal spraying characteristics. During the first layer splat formation, the temperature of the substrate is relatively lower and the cooling rate will be maximized (typically about 106 K/s). For the coating with only one pass, the self-annealing and crystallization would be inhibited. As the first layer solidified on the substrate, the thermal conductivity of the sample becomes lower that that of the substrate. This renders the coating forming a temperature gradient along the coating depth [31]. Generally, amorphous alloys display a lower heat conductivity [13,32,33], thus, the deposited thicker coating leads to the less amorphous phase content. 3.4. Corrosion resistance of the coatings Fig. 10 shows potentiodynamic polarization curves of the coatings in 1 M HCl aqueous solution, herein, the ribbon, substrate and electroplated Cr also presented for comparison. All the coatings and the ribbon passivate spontaneously with lower passive current densities in the order of magnitude less than 10−4 A/cm2 . They are much lower than those of the electroplate Cr and the substrate, in which dissolutions of active metals occur. A wide passive region without transpassive behavior is determined to be about 1.2 V in the coatings, indicating that these coatings hold an excellent ability to resist localized corrosion. From Fig. 10, it is tempting that the amor-
Fig. 9. A cross-sectional SEM image of the coating A6 with one-pass spray.
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Fig. 10. Potentiodynamic polarization curves of the coatings A5, A4, A3 and A2, in comparison with the amorphous ribbon, the substrate and the electroplate Cr samples in 1 M HCl aqueous solution.
phous coatings present superior corrosion resistance performance, as compared with the electroplate Cr, the mild steel and the previous reported Ni- and Fe-based amorphous metal coatings [34], Further, we can also note that the corrosion behavior of the coatings varies with spray parameters. The further spraying distance, the higher plasma current and the finer powders in thermal spray are beneficial to the corrosion resistance of the coatings. In essence, these factors can lead to a lower porosity, a lower crystalline content as well as a lower oxidation that are directly related to the coating corrosion resistance. 3.5. Micro-hardness and erosion–corrosion performance Fig. 11 illustrates the variations of erosion–corrosion rate and micro-hardness in the A5 coating, substrate and nonmagnetic steel sheet. The A5 coating exhibits a very high micro-hardness of about 1081 HV, which is much higher than that of the low carbon steel, nonmagnetic steel sheet and other Fe-based amorphous coating reported earlier [34]. The erosion–corrosion resistances of these samples are measured in the media of 1% NaCl solution with 1% silicon sand. The flow velocity is 15 m/s and the duration time is 4 h. The erosion–corrosion rate of the coating A5 is about 0.19 mg/cm2 h, which is much lower than those of the carbon steel substrate and
Fig. 11. Comparison of erosion–corrosion rate and micro-hardness of the coating A5, low carbon steel and shipbuilding steel sheet.
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Fig. 12. SEM images of the samples after erosion–corrosion test in 1% NaCl + 1% silica sand at the flow rate of 15 m/s (a) low carbon steel (b) shipbuilding steel sheet and (c) coating A5.
nonmagnetic shipbuilding steel sheet. The actual value of coating should be a little higher than the measured one since the silica sand can be contained into the pores of the coating during the test [35]. It is found that as the material micro-hardness increases, the erosion–corrosion rate decreases. The surface morphologies of these samples after the erosion–corrosion tests are presented in Fig. 12. No marked destruction caused by silica sand even in the position of pores can be determined in the coating (see Fig. 12(c)), whereas some pronounced long scratch scars induced by sand ploughing and some pitting corrosion products are displayed in the low carbon steel and shipbuilding steel sheet (Fig. 12(a) and (b)). Considering the high erosion–corrosion resistance is the reflections of high micro-hardness, high strength and excellent corrosion resistance, hence, the iron-based amorphous alloy coating can be developed as an excellent alternative material used in many aggressive fields where some components and equipments are exposed in the severe erosion and corrosion environments, such as the pump, valve and hull for shipbuilding and hydraulic sets.
taining more unmelted particles. The plasma current has a positive effect on the amorphous phase content and decreasing porosity level. In addition, the deposition efficiency, amorphous phase content and structure of the coating are found to be sensitive to the spray distance. (3) The coating exhibits very high micro-hardness and excellent erosion–corrosion resistance even in such a serious media, which suggests that these amorphous alloy coatings can be used as a good alternative material in erosion and corrosion environments, especially for the application in the areas of marine and hydraulic.
4. Conclusions
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An Fe-based amorphous metallic coating, with a high corrosion resistance and micro-hardness, has been prepared by Axial III plasma thermal spray, and the influences of spraying parameters on microstructures, amorphous phase content as well as corrosion resistance of the coatings were evaluated. The main conclusions are as follows:
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Acknowledgements This work was supported by the National Natural Science Foundation of China (grant No. 50323009). Thanks are due to Dr. Z.M. Wang and Z. Liu for their assistances in experiment and helpful discussion.
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