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Bonding and wear behaviors of supersonic plasma sprayed Fe-based coatings on Al-Si alloy substrate
Surface & Coatings Technology 367 (2019) 288–301
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Bonding and wear behaviors of supersonic plasma sprayed Fe-based coatings on Al-Si alloy substrate ⁎
T
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Qiang Wanga,b, , Xing Ruia, Qing-Juan Wanga, , Yu Baic, Zhong-Ze Dua, Wen-Juan Niua, Wei Wanga, Kuai-She Wanga,b, Yuan Gaoa a
School of Metallurgical Engineering, Xi'an University of Architecture and Technology, Xi'an 710055, China National-Local Cooperative Research Centre for Functional Materials Processing Engineering, Xi'an University of Architecture and Technology, Xi'an 710055, China c School of Materials Science and Engineering, Xi'an Jiaotong University, Xi'an 710049, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Plasma spraying Iron-based coating Al-Si alloy Phase transformation Bond behavior Wear mechanism
The quality of plasma sprayed coatings is governed by both bond and wear properties during the service period, associated with a number of factors including substrate surface conditions, materials composition, phase transformation, and etc. In the present study, three different Fe-based powder materials including gray cast iron, high chrome steel and high chrome and nickle contained self-fluxing powders were plasma sprayed onto the AlSi alloy substrate. The microstructure, hardness, phase composition, bonding strength and wear properties of the deposited coatings were investigated. The XRD results revealed that severe oxidation of gray cast iron coatings occurred during the deposition, while no obvious oxidation and phase transformation were detected for chrome steel coatings after spraying. However, the transition of γ-(Fe, Ni) phase into α-(Fe,Cr) phase was confirmed in the self-fluxing coatings. In addition, the Cr and Ni alloying elements enhanced the diffusion between Fe and Al across the coating-substrate interface. Three types of Fe-based coatings showed improved wear resistance in comparison to Al-Si alloy substrate, in terms of coefficient of friction, mass and volume loss. The chrome steel coatings have better integrated performance of bond strength and wear properties. Distinct wear mechanisms were revealed for three types of coatings, i.e. a mixture of adhesive and abrasive wear for gray cast iron coatings, an oxidation dominant wear for chrome steel coatings, and a mixture of oxidation and fatigue wear for selffluxing coatings.
1. Introduction
cast iron liners with thermal sprayed wear resistant coatings. Several thermal spraying techniques have been used to produce wear resistant coatings on the aluminum engine block materials, including plasma spraying, wire arc spraying and high velocity oxygen fuel spraying [6]. Among them, plasma spraying has advantages of higher jet speed, less oxidation and higher deposition efficiency. Various types of coatings can be made, including metallic coatings [7], metal matrix composite (MMC) coatings [8], and ceramic coatings [9]. Owing to the low cost and good mechanical properties, Fe-based powders are the preferred materials for the industrial application of wear protection for various substrate materials [10–14]. During the service period, coatings are only functional when the interfacial bonding strength fulfills the requirement. Therefore, the successful application of wear-resistant coatings is governed by a sufficient bonding strength. The bonding of spraying formed coatings includes the adhesion between coatings and substrates and the cohesion between deposited splats within the coatings [15]. The adhesive bond
The application of lightweight materials has grown rapidly in the area of transportation including aircrafts, motors and high-speed trains, in order to save energy and achieve high performance [1]. Due to the low density, good thermal conductivity and machinability, aluminum alloys have been widely used as engine block materials instead of traditional cast iron materials in the passenger cars [2]. However, because of the poor wear resistance of aluminum alloy, low-cost cast iron liners are usually embedded in the aluminum engine cylinders to improve the wear resistance [3–4]. During the practical operation, the distinct thermal expansion coefficients between cast iron (~12 E-6/K) and aluminum alloys (~23 E-6/K) can result in the loosening of the embedded liners during cyclic heating and cooling processes, which leads to the failure of the engine [5]. In addition, the cast iron liners can further increase the engine weight by about 2 kg per set of four liners. In recent years, there has been a trend to substitute the implantation of
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Corresponding authors at: Xi'an University of Architecture and Technology, China. E-mail addresses: [email protected] (Q. Wang), [email protected] (Q.-J. Wang).
https://doi.org/10.1016/j.surfcoat.2019.04.003 Received 12 October 2018; Received in revised form 20 March 2019; Accepted 1 April 2019 Available online 03 April 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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Table 1 The compositions of original powder materials. Powder
P1 P2 P3
Chemical compositions (wt%) C
Si
B
Cr
Ni
Mo
Mn
S
P
O
Fe
3.0 1.81 1.83
0.54 0.87 0.75
– – 2.13
– 13.02 19.12
– – 10.83
– – 2.22
0.54 – –
≤0.12 – –
< 0.15 – –
0.25 0.02 0.06
Bal. Bal. Bal.
Fig. 1. The secondary electron SEM micrographs and particle size distribution of the three Fe-based powders. (a) and (a′) P1 powder, (b) and (b′) P2 powder, (c) and (c′) P3 powder.
strength, which is closely related to the microstructure, porosity and hardness of the deposited coatings, and these are influenced by the materials composition of original powders as well as the spraying parameters. In addition, the oxidation and phase transformation of the spraying powders could occur during the deposition process, and these factors affect both the adhesive and cohesive bond strength, and also the wear properties of final coatings [17].
strength primarily determines the quality of the coating while the cohesive bond strength indicates the tribological behaviors [16]. During plasma spraying, the successful deposition of the first layer is crucial for adhesive strength, which is influenced by a number of factors including spraying parameters, substrate surface conditions, and elemental reaction on the interface of coating and substrate materials. Subsequently, the continuous deposition of flying particles governs the cohesive 289
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Fig. 2. Backscatter SEM images of the cross section of Fe-based coatings on Al-Si alloy substrate. (a) C1, (b) C2, (c) C3, and (a′), (b′) and (c′) are the magnified images of coating areas.
and cast iron powder as a comparison material since the traditional cylinder liner is made of cast iron. The microstructure analysis, hardness, bonding and wear tests were performed. The correlation of materials composition, phase transformation, bonding and wear behaviors were discussed to provide an insight view when identifying the suitable coating materials for Al-Si alloy substrate.
Table 2 The porosity, oxide proportion and microhardness of three Fe-based coatings. Coating C1 C2 C3 Substrate
Porosity (vol%)
Oxide (vol%)
1.33 ± 0.03 1.20 ± 0.02 1.19 ± 0.04 –
25 ± 1 3.8 ± 0.2 4.9 ± 0.3 –
Hardness (HV0.1) 313 394 428 29
± ± ± ±
12 27 17 4
2. Experimental procedure The present study aims to investigate the effect of materials composition on the bonding and wear behaviors of plasma sprayed Fe-based coatings on Al-Si substrate. Three types of powder materials with distinct chemical compositions were used as the source powders, including high chrome steel powder which can improve the oxidation, wear and corrosion resistance of the coatings [18], and Fe-based high chrome and nickel alloyed self-fluxing powder which not only has good wear and corrosion resistance, but also has good fluidity and wettability [19–20],
The compositions of original powder materials measured by X Ray Fluorescence (XRF, Pioneer-S4) are listed in Table 1, including cast iron (P1), high chrome steel (P2) and high chrome and nickel self-fluxing (P3) powders, and the morphology (examined by scanning electron microscopy, JSM-6390A) and particle size distribution (measured by a two-laser diffraction particle size analyzer, OMEC LS800) are shown in Fig. 1. The P1 powder was produced by water-atomization (WA), and P2 and P3 powders were gas-atomized (GA), provided by Hunan 290
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Fig. 3. XRD patterns of three Fe-based powders and coatings. (a) P1 and C1, (b) P2 and C2, (c) P3 and C3.
Fig. 4. Backscatter SEM and EDX element mapping image of the cross section of C1 coating.
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Fig. 5. Backscatter SEM and EDX element mapping image of the cross section of C2 coating.
deposited coatings was examined by scanning electron microscopy (SEM, JSM-6390A), and the phase composition was examined using Xray diffraction (XRD, D8 ADVANCE A25). The porosity and oxide proportion of as-sprayed coatings were measured by image analysis based on 20 backscatter SEM images captured across the whole section of each coating, and the values were averaged [12]. Vickers hardness measurement was conducted on the polished cross-sectional surface, and the load and dwell time for each measurement were 100 g and 15 s, respectively. Six measurements were acquired, and values were averaged for each coating. According to ASTM C633 standard, the tensile test was carried out by the Instron M8801 electro-hydraulic servo machine, and three measurements were done for each coating. The morphology of fractured surface and chemical element changes were analyzed by SEM equipped with energy dispersive X-ray (Oxford Instrument SEM X-Max Extreme analytical system). Prior to the wear test, the coating was grinded with 600–2000 mesh sandpaper and polished, followed by ultrasonic cleaning for 15 min in an ethanol bath. The wear tests were performed at room temperature using a ball-on-disk type sliding wear apparatus (HT-1000). According to previous studies [8,22], AISI 52100 steel ball having a hardness of 62 HRC and composition of Fe-0.8C0.35Mn-0.25Si-1.5Cr (wt%) with a diameter of 6 mm was used as the counterpart during the wear test. The test load was 5 N, and the friction
Finepowd Materials co. Ltd. The P1 powder particles have an irregular shape with a size of 10 to 55 μm. The P2 powder consists of spherical morphology particles with size of 10 to 65 μm. Compared to the other two powders, the P3 powder has a smaller particle size of 5 to 25 μm. WA powders are generally characterized by high oxygen content, irregular appearance and inexpensiveness compared with those of GA powders. Ozdemir et al. [21] reported the anti-wear properties of plasma sprayed cast iron coatings with inexpensive WA powders were better than GA powders. Therefore, WA cast iron powders were used in the present study. In addition, the initial oxygen content of three original powder materials was measured by an oxygen-nitride‑hydrogen analyzer (LECO ONH836), as shown in Table 1. It is noted that there is only a small amount of oxygen in WA P1 powder. These powders were deposited on the Al-Si alloy (Al-12.87 wt% Si-0.6 wt% Fe) substrate by using a supersonic plasma spraying system (HEPJet). During the spraying process, argon was used as the main gas with a flow rate at 152 L·min−1, and hydrogen was used as the auxiliary gas at 3.3 L·min−1. The spraying voltage and current were set at 115 V and 385 A, respectively. The spraying distance from the nozzle exit to the substrate surface was 110 mm. Prior to spraying, the substrate was sand blasted with a roughness of Ra = 6.2 μm. The coatings were subsequently labelled as C1, C2 and C3 to donate the sequence of powders. The cross-sectional microstructure of the 292
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Fig. 6. Backscatter SEM and EDX element mapping image of the cross section of C3 coating.
3. Results and discussion
radius was 3 mm. The plate rotation speed was 224 r/min with a test time of 40 min for each sample. Before and after each test, the samples were ultrasonically cleaned in alcohol. The surface morphology of wear tracks was examined by SEM, and EDX was used to analyze the composition of specific points and areas on the worn surface.
3.1. Microstructure of the coatings Fig. 2a–c shows the cross-sectional microstructure of as-sprayed coatings. It can be seen that there is a well bonded interface between three kinds of Fe-based coatings and Al-Si alloy substrates. The magnified images of coating materials are shown in Fig. 2a′–c′, and all coatings shows a typical lamellar structure where the collapsed molten 293
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risk of oxidation, compared to P2 and P3 spherical powders. As shown in Fig. 2a′, the oxides were found to distribute mostly at the boundary of deposited splats. And considering the low content of oxygen in the initial P1 powder as listed in Table 1, such high oxide content in C1 was attributed to the oxidation occurred on the surface of powder particles in the hot gas stream before deposition [23]. It is also noted that some unmelted particles were found in C1 coating as well, which together with the oxide lead to an inhomogeneous microstructure. In contrast, C2 and C3 coatings have a much lower oxide content, due to the higher proportion of chrome element in the original powders. In addition, the lower oxide content of C3 coatings was also attributed to the existence of B and Si elements which had the function of deoxygenation during the solidification process of deposited splats [24]. Fig. 7. Bonding strength of plasma sprayed coatings on Al-Si substrate.
3.2. Phase composition of the powders and coatings
particles were flattened. The values of porosity, oxide content and microhardness of three deposited coatings are given in Table 2. It can be found that the porosity was in a similar level for three kinds of coatings, while the oxide proportion of gray cast iron coating (C1) is approximately six times that of the other two coatings. It is noted that irregular P1 powders have higher exposed surface during spraying and were more prone to get the
Fig. 3 shows the results of phase analysis of original powders and assprayed coatings. By comparing the XRD patterns before and after spraying, it can be seen that two types of oxides, including γ-FeO (wustite) and γ-Fe3O4 (magnetite), were identified in C1 (Fig. 3a) coating, while no oxide phases were detected in both C2 (Fig. 3b) and C3 (Fig. 3c) coatings under the present test conditions due to the small amount. This corresponds well with the results of oxide content
Fig. 8. SEM and EDX element mapping image of the coatings on the post-fractured substrate surface. (a) and (a′) C1, (b) and (b′) C2, (c) and (c′) C3. 294
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Fig. 9. The cross section and element diffusion on the interface of residual coating and substrate for (a) and (a′) C1, (b) and (b′) C2, (c) and (c′) C3.
Fig. 10. The results of (a) coefficient of friction and (b) mass and volume loss of three coatings and Al-Si substrate materials.
mainly γ-(Fe,Ni) austenitic with small amount of α-(Fe,Cr), as shown in Fig. 3c. After spraying, the intensity of diffraction peaks of α-(Fe,Cr) phase increased dramatically, while it was reduced for γ-(Fe,Ni) phase. This confirmed that the transformation of γ-(Fe,Ni) austenite into α(Fe,Cr) occurred. Due to the low melting point of the P3 powder material, it can be fully melted at extremely high temperature of the plasma core, and the rapid cooling during the solidification of molten
presented in Table 2. These two types of wustite and magnetite oxides can play the role of solid lubricant during wear conditions rather than the trivalent oxide Fe2O3 (hematite) which is abrasive [25]. As seen in Fig. 3b, no phase transformation of high chrome powder (P2) occurred during the deposition, attributed to the Cr element which can stabilize the α-Fe phase [26]. Before spraying phases of self-fluxing powder (P3) consisted of 295
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the cross section of Fe-based coatings. In case of C1 coatings, as shown in Fig. 4, it can be seen that the carbon element which mainly exists in the form of cementite distributed uniformly in the Fe matrix [29]. The increased hardness of C1 coatings was attributed to the consolidated microstructure and also the precipitation of cementite during the rapid solidification. As shown in Fig. 5, the uniform distribution of chromium alloying element played the role of solid solution strengthening effect, which contributed to the enhanced hardness [30]. As seen from Fig. 6, a certain amount of nickel exists in the C3 coating, which also had the effect of solid solution strengthening on the austenite matrix [31]. Therefore, the combined alloying strengthening of chromium and nickel is responsible the improved hardness of sprayed self-fluxing coatings. Due to the distinct microstructure and phase compositions, the direct comparison of hardness cannot reflect the bonding and wear properties of these different coatings as discussed in the following parts.
Fig. 11. The relationships between hardness with coefficient of friction in steady periods and volume loss of substrate and coating materials.
3.3.2. Bond strength Fig. 7 shows the results of bond strength, and it can be seen that C1 coatings have the lowest values among three types of plasma sprayed coatings. As discussed above, compared with the other two coatings, C1 coatings had much higher oxide content which resulted in the formation of higher amount of weak oxide-substrate bonding areas at the coating-substrate interface [32]. Therefore, the lower oxide content of C2 and C3 coatings was beneficial for the enhanced bond strength. The topography and elemental distribution of post-fractured substrate surfaces are shown in Fig. 8. For all of the three coatings most of exposed fracture area was the aluminum substrate, and the amount of residual coating materials which randomly distributed on the fractured surface was measured to be ~10% of substrate surface area, representing the typical adhesive failure. To further interpret the correlation between bonding behavior and fracture mechanism, EDX line scanning analysis across the interface between residual coating and substrate materials was conducted, and the results are given in Fig. 9. In general, the bonding mechanism between thermal sprayed coatings and substrates includes mechanical bond resulting from the interlocking of splats and substrate surface, and
splats, i.e. the rapid quenching process of the molten particles, resulting in the phase transformation of the C3 coating [27]. Therefore, from the above discussion of phase transformation of these three coating materials before and after spraying, the chrome steel powder was least affected by the spraying process, i.e. less oxidation and no phase transformation.
3.3. Mechanical properties 3.3.1. Microhardness The hardness of spraying formed coating materials can reflect the integrated properties of deposited splats, which rely on the microstructure, chemical and phase compositions [28]. The microhardness values of deposited coatings are summarized in Table 2. It can be seen that three types of sprayed coatings have much higher hardness than AlSi alloy substrate. In comparison, the hardness of traditional cast iron cylinder liner material is about 220 HV [27]. Figs. 4–6 show the backscatter SEM and corresponding EDX element mapping images from
Fig. 12. The morphology of worn surface of substrate (a) secondary and (b) backscatter SEM images, EDX analysis result of (c) point G. 296
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Fig. 13. The morphology of worn surface of C1 coating (a) secondary and (b) backscatter SEM images, EDX analysis results of (c) point A and (d) point B.
roughness, and the large contact stress under the small contact area causes a faster wear speed at this stage. As seen in Fig. 10a, in the running-in period COF of Al-Si alloy substrate and three coatings peaks at around 0.9, 0.6, 1.2 and 0.8, respectively. The average stable COF of substrate and three coatings is about 0.83, 0.49, 0.62 and 0.65, respectively. The lower COF of C1 coating may be attributed to the flake graphite which has the lubricating effect during sliding process, while C2 and C3 have similar values. Fig. 10b shows the results of mass and volume loss of both substrate and coating materials. Because of different densities, especially between the substrate (Al-based) and the coatings (Fe-based), the wear performance is evaluated by calculating the volume loss based on the mass loss and the density (considering the porosity of sprayed coatings). It can be seen that the mass and volume loss of Fe-based coating materials is significantly lower than those of AlSi alloy substrate, indicating an improved wear resistance. Among three types of coatings, C1 has the greatest mass and volume loss, which is twice as much as that of C3, while C2 has the lowest mass and volume loss. The reasons for such difference will be discussed with the following results of worn surface analysis. Although it is known that hardness of commercially pure metals influences its wear resistance that higher hardness implies a higher wear resistance, the increase of wear resistance depends on the way how the material is being hardened (alloying, heat treatment or workhardening) [43–44]. In particular, thermal sprayed coatings represent a complex form of wear processes due to their lamellar structure. As shown in Fig. 11, it revealed different dependence of COF and volume loss on hardness for three types of coatings. For instance, COF of three coatings increased with the increase of hardness, while C2 coating with
metallurgical bond resulting from the elemental diffusion of coating and substrate materials [33–36]. In the present experiment, the high temperature molten Fe-based particles impacted upon the Al-Si alloy substrate material which has relatively lower melting point, and the mutual diffusion of Fe and Al could occur across the coating-substrate interface. Such diffusion can promote the bonding strength between the coating and the substrate [37–39]. In the diffusion couple of Fe and Al, the diffusion coefficient of Al is much higher than Fe [40]. The slope of Al content curve at the coating-substrate interface was calculated to characterize the degree of element diffusion, i.e. 0.46, 0.59, and 0.75 for C1, C2 and C3 coatings, respectively, indicating that Al diffusion was prompted in chrome steel and self-fluxing steel coatings. This can be explained that on the one hand the existence of Cr, Ni elements in coating materials increased the instability of substrate-coating interface, and on the other hand the distortion of stress field induced by the diffusion of Cr, Ni elements into Al substrate resulted in the formation of vacancies and reduced the activation energy for diffusion of Al to the coating side [41–42]. Therefore, the promoted interfacial diffusion was beneficial to increase the bonding strength of C2 and C3 coatings, as shown in Fig. 7.
3.3.3. Wear properties The results of coefficient of friction (COF) and wear rate of coatings and substrate materials are shown in Fig. 10. The friction curves can be divided into two parts, including an initial running-in period of increasing COF and a plateau of constant COF which indicates the steady state of friction regime. The running-in stage mainly occurs in the initial movement stage of the friction pair which has a certain surface 297
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Fig. 14. The morphology of worn surface of C2 coating (a) secondary and (b) backscatter SEM images, EDX analysis results of (c) point C and (d) point D.
the early stage of dry sliding wear tests, the pre-existing FeO oxides can transform to Fe3O4 and then to Fe2O3 as revealed in the previous study [13], and this phenomenon was also responsible for increasing COF values in the running-in period. Once a Fe2O3 layer was formed, a steady state regime was obtained with a reduced COF of 0.49 that was accompanied by the adhesion of the oxide debris generated during sliding. However, the abrasive nature of trivalent Fe2O3 oxide was responsible for the higher mass and volume loss of C1 coating than C2 and C3 coatings, as shown in Fig. 10b. In case of C2 coating, there are no obvious grooves on worn surfaces as shown in a secondary SEM image (Fig. 14a), while pores and spalling are presented. The corresponding backscatter SEM image (Fig. 14b) reveals a large number of adhesives appeared on the worn surface. EDX analysis of point C (Fig. 14c) revealed that the composition in the dark gray phase were mainly oxides. The composition of point D (Fig. 14d) is close to that of C2 coating. From the evidence revealed by XRD results in Fig. 3b where no oxides were detected in as-sprayed coatings, this indicates that oxidation occurred on the surface of C2 coating during the wear tests. The initial stage of oxide formation corresponded to the higher COF in running-in period, and the progression of oxide formation was beneficial to reduce the values of COF and the wear rate. The evidence of peeling pits was possibly due to the porosity in coating microstructure where the formed oxide layer has week bonding on the surface and been scrubbed off during the wear test. Therefore, the wear mechanism of C2 coating is mainly oxidation wear. The secondary SEM image of typical worn surface of C3 coating is shown in Fig. 15a, and it demonstrates a more sever wear damages in comparison with C2 coating. There are no wear grooves, while obvious
the moderate hardness had the lowest wear rate. To further reveal the wear mechanism of sprayed coatings and substrate, the surface morphology and chemical compositional changes after wear tests were examined. The worn morphology and elemental analysis results of the substrate surface are presented in Fig. 12. From the secondary electron SEM image of the worn surface of Fig. 12a, it can be clearly seen that there are a lot of grooves and adhesives on the worn surface. The backscattered image in Fig. 12b shows that the worn surfaces are all covered by Al-oxide, as revealed by EDX result of point G in Fig. 12c. It is mainly due to the large difference in hardness between the substrate and steel ball, resulting in excess wear damage on the substrate surface. Therefore, the wear mechanism of substrate is mainly adhesive wear. Fig. 13 shows the results of surface morphology and element analysis of C1 coating. As shown by the secondary SEM image in Fig. 13a, there are a lot of debris sitting on the worn surface, and some shallow discontinuous grooves are visible as well, indicating a mixture of adhesive and abrasive wear. Fig. 13b shows the corresponding backscatter SEM image where debris show a dark gray color in comparison with light gray matrix. It is noticed that furrows mostly occurred at regions of debris. To reveal the chemical composition of such debris and residual matrix, EDX point analysis results of point A and B are given in Fig. 13c and d, respectively. There is about 20 wt% of oxygen in the dark gray phase (Point A), which corresponds to iron oxide phases. It is also noticed that the carbon content in residual coating matrix is lower than that of P1 original powder material, indicating the loss of graphite during the spraying process [15]. As revealed by XRD results in Fig. 3a, FeO and Fe3O4 oxide phases existed in as-sprayed C1 coatings. During 298
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Fig. 15. The morphology of worn surface of C3 coating (a) secondary and (b) backscatter SEM images, EDX analysis results of (c) point E and (d) point F.
wear track in comparison to that of C2 and C3 coatings. During sliding, the interfacial oxides with high hardness and some unmelted particles were also fallen off, and the scratch of these debris resulted in grooves on the worn surface [46]. In addition, the higher porosity of C1 coating was also responsible for its greater mass and volume loss. In cases of C2 and C3 coatings, the existence of alloying elements, including Cr, Ni and Mo, resulted in the formation of a compacted tribo-oxide layer on worn surfaces under the periodic friction [47]. The relatively higher mass and volume loss of C3 than C2 was attributed to the microcracks and spalling, which were generally formed near the pores and intersplats regions with poor cohesion bond.
spalling and cracks can be found, indicating the existence of fatigue wear. The corresponding backscatter SEM image in Fig. 15b reveals that a large amount of adhesive debris occupied most of the wear track area. EDX analysis gives the composition for adhesive debris (Point E) and residual matrix (Point F), which corresponds to oxides and C3 coating, respectively. Therefore, the wear mechanism of C3 coating consists of oxidation and fatigue wear. The surface morphologies of the counter-body (steel ball) in contact with the substrate and the coatings after wear tests are also shown in Fig. 16. It can be seen that the amount of adhesives varies on the surface of balls, especially for the ones which are in contact with the substrate (Fig. 16a) and the C1 coating (Fig. 16b) showed relatively large amount of adhesives. The EDX analysis results of area A in Fig. 16a revealed the adhesives are mainly Al-oxides, which corresponds to the wear product on substrate surface. For areas of B, C and D regions in sprayed coatings, the corresponding EDX analysis revealed that the main compositions are mainly Fe-oxides. Therefore, the adhesives were mainly derived from the wear products produced on the surface of the substrate and coatings during the wear process. According to the above observation and compositional analysis of the worn surface of three Fe-based coatings, the wear mechanism depends both on the phase composition of as-sprayed coatings and the compositional changes during the wear process. For instance, the formation of wear surface oxides was significantly different. The gray cast iron coating (C1) contained certain amount of graphite, which detached from the coating during sliding and covered the worn surface, resulting in less generation of oxides [45]. Therefore, in contrary to the amount of oxides in as-sprayed coatings, there was less amount of oxide on the
4. Conclusions In the present work, the bonding and wear behaviors of plasma sprayed iron-based coatings on Al-Si alloy substrate was investigated by analyzing the microstructure, phase transformation and mechanical properties of the coating. The following main conclusions can be drawn from this work: 1. The three iron-based coatings prepared by plasma spraying exhibited typical lamellar structures, and the coatings were well bonded to the substrate. The coatings presented a similar level of porosity, but the oxide content of gray cast iron coatings was significantly higher than the other two coatings. 2. The XRD results revealed that after spraying two types of oxides, FeO and Fe3O4, formed in gray cast iron coating, while no oxide was detected in chrome steel and self-fluxing coatings. No phase
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Fig. 16. The secondary SEM images of counter-body (ball) worn surface and EDX analysis results: (a) counter-body with substrate contact, (b) counter-body with C1 coating contact, (c) counter-body with C2 coating contact, (d) counter-body with C3 coating contact.
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transformation occurred during the deposition of chrome steel coatings. However, the transformation of γ-(Fe, Ni) to α-(Fe,Cr) phase occurred owing to the rapid cooling during the deposition of self-fluxing coatings. 3. The gray cast iron coating has lower bond strength compared to the other two coatings, due to the higher oxide content. EDX line scanning analysis confirmed the element diffusion across the coating-substrate interface, and the Cr and Ni alloying elements enhanced the diffusion between Fe and Al which was beneficial to improve the bonding strength of chrome steel and self-fluxing coatings. 4. Three types of Fe-based coatings showed improved wear resistance in comparison to Al-Si alloy substrate, in terms of coefficient of friction, mass and volume loss. Among them, chrome steel coatings have better integrated performance with moderate coefficient of friction and much lower mass and volume loss. Distinct wear mechanisms were revealed for these coatings, i.e. a mixture of adhesive and abrasive wear for gray cast iron coatings, an oxidation dominant wear for chrome steel coatings, and a mixture of oxidation and fatigue wear for self-fluxing coatings.
[18]
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[26] [27]
Acknowledgements The authors are grateful to financial support from Natural Science Foundation of China (51801143), Natural Science Foundation of Shaanxi Province (2017JZ012), Education Department of Shaanxi Province (18JK0445). Thanks for the supersonic plasma spraying experiment done by Xi'an Jiaotong University.
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Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Bonding and wear behaviors of supersonic plasma sprayed Fe-based coatings on Al-Si alloy substrate ⁎
T
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Qiang Wanga,b, , Xing Ruia, Qing-Juan Wanga, , Yu Baic, Zhong-Ze Dua, Wen-Juan Niua, Wei Wanga, Kuai-She Wanga,b, Yuan Gaoa a
School of Metallurgical Engineering, Xi'an University of Architecture and Technology, Xi'an 710055, China National-Local Cooperative Research Centre for Functional Materials Processing Engineering, Xi'an University of Architecture and Technology, Xi'an 710055, China c School of Materials Science and Engineering, Xi'an Jiaotong University, Xi'an 710049, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Plasma spraying Iron-based coating Al-Si alloy Phase transformation Bond behavior Wear mechanism
The quality of plasma sprayed coatings is governed by both bond and wear properties during the service period, associated with a number of factors including substrate surface conditions, materials composition, phase transformation, and etc. In the present study, three different Fe-based powder materials including gray cast iron, high chrome steel and high chrome and nickle contained self-fluxing powders were plasma sprayed onto the AlSi alloy substrate. The microstructure, hardness, phase composition, bonding strength and wear properties of the deposited coatings were investigated. The XRD results revealed that severe oxidation of gray cast iron coatings occurred during the deposition, while no obvious oxidation and phase transformation were detected for chrome steel coatings after spraying. However, the transition of γ-(Fe, Ni) phase into α-(Fe,Cr) phase was confirmed in the self-fluxing coatings. In addition, the Cr and Ni alloying elements enhanced the diffusion between Fe and Al across the coating-substrate interface. Three types of Fe-based coatings showed improved wear resistance in comparison to Al-Si alloy substrate, in terms of coefficient of friction, mass and volume loss. The chrome steel coatings have better integrated performance of bond strength and wear properties. Distinct wear mechanisms were revealed for three types of coatings, i.e. a mixture of adhesive and abrasive wear for gray cast iron coatings, an oxidation dominant wear for chrome steel coatings, and a mixture of oxidation and fatigue wear for selffluxing coatings.
1. Introduction
cast iron liners with thermal sprayed wear resistant coatings. Several thermal spraying techniques have been used to produce wear resistant coatings on the aluminum engine block materials, including plasma spraying, wire arc spraying and high velocity oxygen fuel spraying [6]. Among them, plasma spraying has advantages of higher jet speed, less oxidation and higher deposition efficiency. Various types of coatings can be made, including metallic coatings [7], metal matrix composite (MMC) coatings [8], and ceramic coatings [9]. Owing to the low cost and good mechanical properties, Fe-based powders are the preferred materials for the industrial application of wear protection for various substrate materials [10–14]. During the service period, coatings are only functional when the interfacial bonding strength fulfills the requirement. Therefore, the successful application of wear-resistant coatings is governed by a sufficient bonding strength. The bonding of spraying formed coatings includes the adhesion between coatings and substrates and the cohesion between deposited splats within the coatings [15]. The adhesive bond
The application of lightweight materials has grown rapidly in the area of transportation including aircrafts, motors and high-speed trains, in order to save energy and achieve high performance [1]. Due to the low density, good thermal conductivity and machinability, aluminum alloys have been widely used as engine block materials instead of traditional cast iron materials in the passenger cars [2]. However, because of the poor wear resistance of aluminum alloy, low-cost cast iron liners are usually embedded in the aluminum engine cylinders to improve the wear resistance [3–4]. During the practical operation, the distinct thermal expansion coefficients between cast iron (~12 E-6/K) and aluminum alloys (~23 E-6/K) can result in the loosening of the embedded liners during cyclic heating and cooling processes, which leads to the failure of the engine [5]. In addition, the cast iron liners can further increase the engine weight by about 2 kg per set of four liners. In recent years, there has been a trend to substitute the implantation of
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Corresponding authors at: Xi'an University of Architecture and Technology, China. E-mail addresses: [email protected] (Q. Wang), [email protected] (Q.-J. Wang).
https://doi.org/10.1016/j.surfcoat.2019.04.003 Received 12 October 2018; Received in revised form 20 March 2019; Accepted 1 April 2019 Available online 03 April 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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Table 1 The compositions of original powder materials. Powder
P1 P2 P3
Chemical compositions (wt%) C
Si
B
Cr
Ni
Mo
Mn
S
P
O
Fe
3.0 1.81 1.83
0.54 0.87 0.75
– – 2.13
– 13.02 19.12
– – 10.83
– – 2.22
0.54 – –
≤0.12 – –
< 0.15 – –
0.25 0.02 0.06
Bal. Bal. Bal.
Fig. 1. The secondary electron SEM micrographs and particle size distribution of the three Fe-based powders. (a) and (a′) P1 powder, (b) and (b′) P2 powder, (c) and (c′) P3 powder.
strength, which is closely related to the microstructure, porosity and hardness of the deposited coatings, and these are influenced by the materials composition of original powders as well as the spraying parameters. In addition, the oxidation and phase transformation of the spraying powders could occur during the deposition process, and these factors affect both the adhesive and cohesive bond strength, and also the wear properties of final coatings [17].
strength primarily determines the quality of the coating while the cohesive bond strength indicates the tribological behaviors [16]. During plasma spraying, the successful deposition of the first layer is crucial for adhesive strength, which is influenced by a number of factors including spraying parameters, substrate surface conditions, and elemental reaction on the interface of coating and substrate materials. Subsequently, the continuous deposition of flying particles governs the cohesive 289
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Fig. 2. Backscatter SEM images of the cross section of Fe-based coatings on Al-Si alloy substrate. (a) C1, (b) C2, (c) C3, and (a′), (b′) and (c′) are the magnified images of coating areas.
and cast iron powder as a comparison material since the traditional cylinder liner is made of cast iron. The microstructure analysis, hardness, bonding and wear tests were performed. The correlation of materials composition, phase transformation, bonding and wear behaviors were discussed to provide an insight view when identifying the suitable coating materials for Al-Si alloy substrate.
Table 2 The porosity, oxide proportion and microhardness of three Fe-based coatings. Coating C1 C2 C3 Substrate
Porosity (vol%)
Oxide (vol%)
1.33 ± 0.03 1.20 ± 0.02 1.19 ± 0.04 –
25 ± 1 3.8 ± 0.2 4.9 ± 0.3 –
Hardness (HV0.1) 313 394 428 29
± ± ± ±
12 27 17 4
2. Experimental procedure The present study aims to investigate the effect of materials composition on the bonding and wear behaviors of plasma sprayed Fe-based coatings on Al-Si substrate. Three types of powder materials with distinct chemical compositions were used as the source powders, including high chrome steel powder which can improve the oxidation, wear and corrosion resistance of the coatings [18], and Fe-based high chrome and nickel alloyed self-fluxing powder which not only has good wear and corrosion resistance, but also has good fluidity and wettability [19–20],
The compositions of original powder materials measured by X Ray Fluorescence (XRF, Pioneer-S4) are listed in Table 1, including cast iron (P1), high chrome steel (P2) and high chrome and nickel self-fluxing (P3) powders, and the morphology (examined by scanning electron microscopy, JSM-6390A) and particle size distribution (measured by a two-laser diffraction particle size analyzer, OMEC LS800) are shown in Fig. 1. The P1 powder was produced by water-atomization (WA), and P2 and P3 powders were gas-atomized (GA), provided by Hunan 290
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Fig. 3. XRD patterns of three Fe-based powders and coatings. (a) P1 and C1, (b) P2 and C2, (c) P3 and C3.
Fig. 4. Backscatter SEM and EDX element mapping image of the cross section of C1 coating.
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Fig. 5. Backscatter SEM and EDX element mapping image of the cross section of C2 coating.
deposited coatings was examined by scanning electron microscopy (SEM, JSM-6390A), and the phase composition was examined using Xray diffraction (XRD, D8 ADVANCE A25). The porosity and oxide proportion of as-sprayed coatings were measured by image analysis based on 20 backscatter SEM images captured across the whole section of each coating, and the values were averaged [12]. Vickers hardness measurement was conducted on the polished cross-sectional surface, and the load and dwell time for each measurement were 100 g and 15 s, respectively. Six measurements were acquired, and values were averaged for each coating. According to ASTM C633 standard, the tensile test was carried out by the Instron M8801 electro-hydraulic servo machine, and three measurements were done for each coating. The morphology of fractured surface and chemical element changes were analyzed by SEM equipped with energy dispersive X-ray (Oxford Instrument SEM X-Max Extreme analytical system). Prior to the wear test, the coating was grinded with 600–2000 mesh sandpaper and polished, followed by ultrasonic cleaning for 15 min in an ethanol bath. The wear tests were performed at room temperature using a ball-on-disk type sliding wear apparatus (HT-1000). According to previous studies [8,22], AISI 52100 steel ball having a hardness of 62 HRC and composition of Fe-0.8C0.35Mn-0.25Si-1.5Cr (wt%) with a diameter of 6 mm was used as the counterpart during the wear test. The test load was 5 N, and the friction
Finepowd Materials co. Ltd. The P1 powder particles have an irregular shape with a size of 10 to 55 μm. The P2 powder consists of spherical morphology particles with size of 10 to 65 μm. Compared to the other two powders, the P3 powder has a smaller particle size of 5 to 25 μm. WA powders are generally characterized by high oxygen content, irregular appearance and inexpensiveness compared with those of GA powders. Ozdemir et al. [21] reported the anti-wear properties of plasma sprayed cast iron coatings with inexpensive WA powders were better than GA powders. Therefore, WA cast iron powders were used in the present study. In addition, the initial oxygen content of three original powder materials was measured by an oxygen-nitride‑hydrogen analyzer (LECO ONH836), as shown in Table 1. It is noted that there is only a small amount of oxygen in WA P1 powder. These powders were deposited on the Al-Si alloy (Al-12.87 wt% Si-0.6 wt% Fe) substrate by using a supersonic plasma spraying system (HEPJet). During the spraying process, argon was used as the main gas with a flow rate at 152 L·min−1, and hydrogen was used as the auxiliary gas at 3.3 L·min−1. The spraying voltage and current were set at 115 V and 385 A, respectively. The spraying distance from the nozzle exit to the substrate surface was 110 mm. Prior to spraying, the substrate was sand blasted with a roughness of Ra = 6.2 μm. The coatings were subsequently labelled as C1, C2 and C3 to donate the sequence of powders. The cross-sectional microstructure of the 292
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Fig. 6. Backscatter SEM and EDX element mapping image of the cross section of C3 coating.
3. Results and discussion
radius was 3 mm. The plate rotation speed was 224 r/min with a test time of 40 min for each sample. Before and after each test, the samples were ultrasonically cleaned in alcohol. The surface morphology of wear tracks was examined by SEM, and EDX was used to analyze the composition of specific points and areas on the worn surface.
3.1. Microstructure of the coatings Fig. 2a–c shows the cross-sectional microstructure of as-sprayed coatings. It can be seen that there is a well bonded interface between three kinds of Fe-based coatings and Al-Si alloy substrates. The magnified images of coating materials are shown in Fig. 2a′–c′, and all coatings shows a typical lamellar structure where the collapsed molten 293
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risk of oxidation, compared to P2 and P3 spherical powders. As shown in Fig. 2a′, the oxides were found to distribute mostly at the boundary of deposited splats. And considering the low content of oxygen in the initial P1 powder as listed in Table 1, such high oxide content in C1 was attributed to the oxidation occurred on the surface of powder particles in the hot gas stream before deposition [23]. It is also noted that some unmelted particles were found in C1 coating as well, which together with the oxide lead to an inhomogeneous microstructure. In contrast, C2 and C3 coatings have a much lower oxide content, due to the higher proportion of chrome element in the original powders. In addition, the lower oxide content of C3 coatings was also attributed to the existence of B and Si elements which had the function of deoxygenation during the solidification process of deposited splats [24]. Fig. 7. Bonding strength of plasma sprayed coatings on Al-Si substrate.
3.2. Phase composition of the powders and coatings
particles were flattened. The values of porosity, oxide content and microhardness of three deposited coatings are given in Table 2. It can be found that the porosity was in a similar level for three kinds of coatings, while the oxide proportion of gray cast iron coating (C1) is approximately six times that of the other two coatings. It is noted that irregular P1 powders have higher exposed surface during spraying and were more prone to get the
Fig. 3 shows the results of phase analysis of original powders and assprayed coatings. By comparing the XRD patterns before and after spraying, it can be seen that two types of oxides, including γ-FeO (wustite) and γ-Fe3O4 (magnetite), were identified in C1 (Fig. 3a) coating, while no oxide phases were detected in both C2 (Fig. 3b) and C3 (Fig. 3c) coatings under the present test conditions due to the small amount. This corresponds well with the results of oxide content
Fig. 8. SEM and EDX element mapping image of the coatings on the post-fractured substrate surface. (a) and (a′) C1, (b) and (b′) C2, (c) and (c′) C3. 294
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Fig. 9. The cross section and element diffusion on the interface of residual coating and substrate for (a) and (a′) C1, (b) and (b′) C2, (c) and (c′) C3.
Fig. 10. The results of (a) coefficient of friction and (b) mass and volume loss of three coatings and Al-Si substrate materials.
mainly γ-(Fe,Ni) austenitic with small amount of α-(Fe,Cr), as shown in Fig. 3c. After spraying, the intensity of diffraction peaks of α-(Fe,Cr) phase increased dramatically, while it was reduced for γ-(Fe,Ni) phase. This confirmed that the transformation of γ-(Fe,Ni) austenite into α(Fe,Cr) occurred. Due to the low melting point of the P3 powder material, it can be fully melted at extremely high temperature of the plasma core, and the rapid cooling during the solidification of molten
presented in Table 2. These two types of wustite and magnetite oxides can play the role of solid lubricant during wear conditions rather than the trivalent oxide Fe2O3 (hematite) which is abrasive [25]. As seen in Fig. 3b, no phase transformation of high chrome powder (P2) occurred during the deposition, attributed to the Cr element which can stabilize the α-Fe phase [26]. Before spraying phases of self-fluxing powder (P3) consisted of 295
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the cross section of Fe-based coatings. In case of C1 coatings, as shown in Fig. 4, it can be seen that the carbon element which mainly exists in the form of cementite distributed uniformly in the Fe matrix [29]. The increased hardness of C1 coatings was attributed to the consolidated microstructure and also the precipitation of cementite during the rapid solidification. As shown in Fig. 5, the uniform distribution of chromium alloying element played the role of solid solution strengthening effect, which contributed to the enhanced hardness [30]. As seen from Fig. 6, a certain amount of nickel exists in the C3 coating, which also had the effect of solid solution strengthening on the austenite matrix [31]. Therefore, the combined alloying strengthening of chromium and nickel is responsible the improved hardness of sprayed self-fluxing coatings. Due to the distinct microstructure and phase compositions, the direct comparison of hardness cannot reflect the bonding and wear properties of these different coatings as discussed in the following parts.
Fig. 11. The relationships between hardness with coefficient of friction in steady periods and volume loss of substrate and coating materials.
3.3.2. Bond strength Fig. 7 shows the results of bond strength, and it can be seen that C1 coatings have the lowest values among three types of plasma sprayed coatings. As discussed above, compared with the other two coatings, C1 coatings had much higher oxide content which resulted in the formation of higher amount of weak oxide-substrate bonding areas at the coating-substrate interface [32]. Therefore, the lower oxide content of C2 and C3 coatings was beneficial for the enhanced bond strength. The topography and elemental distribution of post-fractured substrate surfaces are shown in Fig. 8. For all of the three coatings most of exposed fracture area was the aluminum substrate, and the amount of residual coating materials which randomly distributed on the fractured surface was measured to be ~10% of substrate surface area, representing the typical adhesive failure. To further interpret the correlation between bonding behavior and fracture mechanism, EDX line scanning analysis across the interface between residual coating and substrate materials was conducted, and the results are given in Fig. 9. In general, the bonding mechanism between thermal sprayed coatings and substrates includes mechanical bond resulting from the interlocking of splats and substrate surface, and
splats, i.e. the rapid quenching process of the molten particles, resulting in the phase transformation of the C3 coating [27]. Therefore, from the above discussion of phase transformation of these three coating materials before and after spraying, the chrome steel powder was least affected by the spraying process, i.e. less oxidation and no phase transformation.
3.3. Mechanical properties 3.3.1. Microhardness The hardness of spraying formed coating materials can reflect the integrated properties of deposited splats, which rely on the microstructure, chemical and phase compositions [28]. The microhardness values of deposited coatings are summarized in Table 2. It can be seen that three types of sprayed coatings have much higher hardness than AlSi alloy substrate. In comparison, the hardness of traditional cast iron cylinder liner material is about 220 HV [27]. Figs. 4–6 show the backscatter SEM and corresponding EDX element mapping images from
Fig. 12. The morphology of worn surface of substrate (a) secondary and (b) backscatter SEM images, EDX analysis result of (c) point G. 296
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Fig. 13. The morphology of worn surface of C1 coating (a) secondary and (b) backscatter SEM images, EDX analysis results of (c) point A and (d) point B.
roughness, and the large contact stress under the small contact area causes a faster wear speed at this stage. As seen in Fig. 10a, in the running-in period COF of Al-Si alloy substrate and three coatings peaks at around 0.9, 0.6, 1.2 and 0.8, respectively. The average stable COF of substrate and three coatings is about 0.83, 0.49, 0.62 and 0.65, respectively. The lower COF of C1 coating may be attributed to the flake graphite which has the lubricating effect during sliding process, while C2 and C3 have similar values. Fig. 10b shows the results of mass and volume loss of both substrate and coating materials. Because of different densities, especially between the substrate (Al-based) and the coatings (Fe-based), the wear performance is evaluated by calculating the volume loss based on the mass loss and the density (considering the porosity of sprayed coatings). It can be seen that the mass and volume loss of Fe-based coating materials is significantly lower than those of AlSi alloy substrate, indicating an improved wear resistance. Among three types of coatings, C1 has the greatest mass and volume loss, which is twice as much as that of C3, while C2 has the lowest mass and volume loss. The reasons for such difference will be discussed with the following results of worn surface analysis. Although it is known that hardness of commercially pure metals influences its wear resistance that higher hardness implies a higher wear resistance, the increase of wear resistance depends on the way how the material is being hardened (alloying, heat treatment or workhardening) [43–44]. In particular, thermal sprayed coatings represent a complex form of wear processes due to their lamellar structure. As shown in Fig. 11, it revealed different dependence of COF and volume loss on hardness for three types of coatings. For instance, COF of three coatings increased with the increase of hardness, while C2 coating with
metallurgical bond resulting from the elemental diffusion of coating and substrate materials [33–36]. In the present experiment, the high temperature molten Fe-based particles impacted upon the Al-Si alloy substrate material which has relatively lower melting point, and the mutual diffusion of Fe and Al could occur across the coating-substrate interface. Such diffusion can promote the bonding strength between the coating and the substrate [37–39]. In the diffusion couple of Fe and Al, the diffusion coefficient of Al is much higher than Fe [40]. The slope of Al content curve at the coating-substrate interface was calculated to characterize the degree of element diffusion, i.e. 0.46, 0.59, and 0.75 for C1, C2 and C3 coatings, respectively, indicating that Al diffusion was prompted in chrome steel and self-fluxing steel coatings. This can be explained that on the one hand the existence of Cr, Ni elements in coating materials increased the instability of substrate-coating interface, and on the other hand the distortion of stress field induced by the diffusion of Cr, Ni elements into Al substrate resulted in the formation of vacancies and reduced the activation energy for diffusion of Al to the coating side [41–42]. Therefore, the promoted interfacial diffusion was beneficial to increase the bonding strength of C2 and C3 coatings, as shown in Fig. 7.
3.3.3. Wear properties The results of coefficient of friction (COF) and wear rate of coatings and substrate materials are shown in Fig. 10. The friction curves can be divided into two parts, including an initial running-in period of increasing COF and a plateau of constant COF which indicates the steady state of friction regime. The running-in stage mainly occurs in the initial movement stage of the friction pair which has a certain surface 297
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Fig. 14. The morphology of worn surface of C2 coating (a) secondary and (b) backscatter SEM images, EDX analysis results of (c) point C and (d) point D.
the early stage of dry sliding wear tests, the pre-existing FeO oxides can transform to Fe3O4 and then to Fe2O3 as revealed in the previous study [13], and this phenomenon was also responsible for increasing COF values in the running-in period. Once a Fe2O3 layer was formed, a steady state regime was obtained with a reduced COF of 0.49 that was accompanied by the adhesion of the oxide debris generated during sliding. However, the abrasive nature of trivalent Fe2O3 oxide was responsible for the higher mass and volume loss of C1 coating than C2 and C3 coatings, as shown in Fig. 10b. In case of C2 coating, there are no obvious grooves on worn surfaces as shown in a secondary SEM image (Fig. 14a), while pores and spalling are presented. The corresponding backscatter SEM image (Fig. 14b) reveals a large number of adhesives appeared on the worn surface. EDX analysis of point C (Fig. 14c) revealed that the composition in the dark gray phase were mainly oxides. The composition of point D (Fig. 14d) is close to that of C2 coating. From the evidence revealed by XRD results in Fig. 3b where no oxides were detected in as-sprayed coatings, this indicates that oxidation occurred on the surface of C2 coating during the wear tests. The initial stage of oxide formation corresponded to the higher COF in running-in period, and the progression of oxide formation was beneficial to reduce the values of COF and the wear rate. The evidence of peeling pits was possibly due to the porosity in coating microstructure where the formed oxide layer has week bonding on the surface and been scrubbed off during the wear test. Therefore, the wear mechanism of C2 coating is mainly oxidation wear. The secondary SEM image of typical worn surface of C3 coating is shown in Fig. 15a, and it demonstrates a more sever wear damages in comparison with C2 coating. There are no wear grooves, while obvious
the moderate hardness had the lowest wear rate. To further reveal the wear mechanism of sprayed coatings and substrate, the surface morphology and chemical compositional changes after wear tests were examined. The worn morphology and elemental analysis results of the substrate surface are presented in Fig. 12. From the secondary electron SEM image of the worn surface of Fig. 12a, it can be clearly seen that there are a lot of grooves and adhesives on the worn surface. The backscattered image in Fig. 12b shows that the worn surfaces are all covered by Al-oxide, as revealed by EDX result of point G in Fig. 12c. It is mainly due to the large difference in hardness between the substrate and steel ball, resulting in excess wear damage on the substrate surface. Therefore, the wear mechanism of substrate is mainly adhesive wear. Fig. 13 shows the results of surface morphology and element analysis of C1 coating. As shown by the secondary SEM image in Fig. 13a, there are a lot of debris sitting on the worn surface, and some shallow discontinuous grooves are visible as well, indicating a mixture of adhesive and abrasive wear. Fig. 13b shows the corresponding backscatter SEM image where debris show a dark gray color in comparison with light gray matrix. It is noticed that furrows mostly occurred at regions of debris. To reveal the chemical composition of such debris and residual matrix, EDX point analysis results of point A and B are given in Fig. 13c and d, respectively. There is about 20 wt% of oxygen in the dark gray phase (Point A), which corresponds to iron oxide phases. It is also noticed that the carbon content in residual coating matrix is lower than that of P1 original powder material, indicating the loss of graphite during the spraying process [15]. As revealed by XRD results in Fig. 3a, FeO and Fe3O4 oxide phases existed in as-sprayed C1 coatings. During 298
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Fig. 15. The morphology of worn surface of C3 coating (a) secondary and (b) backscatter SEM images, EDX analysis results of (c) point E and (d) point F.
wear track in comparison to that of C2 and C3 coatings. During sliding, the interfacial oxides with high hardness and some unmelted particles were also fallen off, and the scratch of these debris resulted in grooves on the worn surface [46]. In addition, the higher porosity of C1 coating was also responsible for its greater mass and volume loss. In cases of C2 and C3 coatings, the existence of alloying elements, including Cr, Ni and Mo, resulted in the formation of a compacted tribo-oxide layer on worn surfaces under the periodic friction [47]. The relatively higher mass and volume loss of C3 than C2 was attributed to the microcracks and spalling, which were generally formed near the pores and intersplats regions with poor cohesion bond.
spalling and cracks can be found, indicating the existence of fatigue wear. The corresponding backscatter SEM image in Fig. 15b reveals that a large amount of adhesive debris occupied most of the wear track area. EDX analysis gives the composition for adhesive debris (Point E) and residual matrix (Point F), which corresponds to oxides and C3 coating, respectively. Therefore, the wear mechanism of C3 coating consists of oxidation and fatigue wear. The surface morphologies of the counter-body (steel ball) in contact with the substrate and the coatings after wear tests are also shown in Fig. 16. It can be seen that the amount of adhesives varies on the surface of balls, especially for the ones which are in contact with the substrate (Fig. 16a) and the C1 coating (Fig. 16b) showed relatively large amount of adhesives. The EDX analysis results of area A in Fig. 16a revealed the adhesives are mainly Al-oxides, which corresponds to the wear product on substrate surface. For areas of B, C and D regions in sprayed coatings, the corresponding EDX analysis revealed that the main compositions are mainly Fe-oxides. Therefore, the adhesives were mainly derived from the wear products produced on the surface of the substrate and coatings during the wear process. According to the above observation and compositional analysis of the worn surface of three Fe-based coatings, the wear mechanism depends both on the phase composition of as-sprayed coatings and the compositional changes during the wear process. For instance, the formation of wear surface oxides was significantly different. The gray cast iron coating (C1) contained certain amount of graphite, which detached from the coating during sliding and covered the worn surface, resulting in less generation of oxides [45]. Therefore, in contrary to the amount of oxides in as-sprayed coatings, there was less amount of oxide on the
4. Conclusions In the present work, the bonding and wear behaviors of plasma sprayed iron-based coatings on Al-Si alloy substrate was investigated by analyzing the microstructure, phase transformation and mechanical properties of the coating. The following main conclusions can be drawn from this work: 1. The three iron-based coatings prepared by plasma spraying exhibited typical lamellar structures, and the coatings were well bonded to the substrate. The coatings presented a similar level of porosity, but the oxide content of gray cast iron coatings was significantly higher than the other two coatings. 2. The XRD results revealed that after spraying two types of oxides, FeO and Fe3O4, formed in gray cast iron coating, while no oxide was detected in chrome steel and self-fluxing coatings. No phase
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Fig. 16. The secondary SEM images of counter-body (ball) worn surface and EDX analysis results: (a) counter-body with substrate contact, (b) counter-body with C1 coating contact, (c) counter-body with C2 coating contact, (d) counter-body with C3 coating contact.
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transformation occurred during the deposition of chrome steel coatings. However, the transformation of γ-(Fe, Ni) to α-(Fe,Cr) phase occurred owing to the rapid cooling during the deposition of self-fluxing coatings. 3. The gray cast iron coating has lower bond strength compared to the other two coatings, due to the higher oxide content. EDX line scanning analysis confirmed the element diffusion across the coating-substrate interface, and the Cr and Ni alloying elements enhanced the diffusion between Fe and Al which was beneficial to improve the bonding strength of chrome steel and self-fluxing coatings. 4. Three types of Fe-based coatings showed improved wear resistance in comparison to Al-Si alloy substrate, in terms of coefficient of friction, mass and volume loss. Among them, chrome steel coatings have better integrated performance with moderate coefficient of friction and much lower mass and volume loss. Distinct wear mechanisms were revealed for these coatings, i.e. a mixture of adhesive and abrasive wear for gray cast iron coatings, an oxidation dominant wear for chrome steel coatings, and a mixture of oxidation and fatigue wear for self-fluxing coatings.
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Acknowledgements The authors are grateful to financial support from Natural Science Foundation of China (51801143), Natural Science Foundation of Shaanxi Province (2017JZ012), Education Department of Shaanxi Province (18JK0445). Thanks for the supersonic plasma spraying experiment done by Xi'an Jiaotong University.
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