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High temperature wear performance of HVOF-sprayed Cr3C2-WC-NiCoCrMo and Cr3C2-NiCr hardmetal coatings
Accepted Manuscript Title: High temperature wear performance of HVOF-sprayed Cr3 C2 -WC-NiCoCrMo and Cr3 C2 -NiCr hardmetal coatings Authors: Wuxi Zhou, Kesong Zhou, Yuxi Li, Chunming Deng, Keli Zeng PII: DOI: Reference:
S0169-4332(17)31159-5 http://dx.doi.org/doi:10.1016/j.apsusc.2017.04.132 APSUSC 35813
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28-10-2016 13-4-2017 17-4-2017
Please cite this article as: Wuxi Zhou, Kesong Zhou, Yuxi Li, Chunming Deng, Keli Zeng, High temperature wear performance of HVOF-sprayed Cr3C2-WC-NiCoCrMo and Cr3C2-NiCr hardmetal coatings, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.04.132 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
High temperature wear performance of HVOF-sprayed Cr3C2-WC-NiCoCrMo and Cr3C2-NiCr hardmetal coatings Wuxi Zhou1,2*, Kesong Zhou1,3,4, Yuxi Li2, Chunming Deng3,4, Keli Zeng3 1. School of Materials Science and Engineering, Central South University, Changsha 410083, P.R.China; 2. Zigong Cemented Carbide Co.,Ltd. Zigong 643000, P.R.China; 3. Guangdong Institute of New Materials, National Engineering Laboratory for Modern Materials Surface Engineering Technology, Guangdong Provincial Key Lab for Modern Materials Surface Engineering Technology, Guangzhou 510650, P.R. China. Highlights
Cr3C2-WC-NiCoCrMo HVOF spray-grade powder was prepared by an agglomeration and sintering process.
Cr3C2-WC-NiCoCrMo coating was deposited by HVOF spraying process.
The Cr3C2-WC-NiCoCrMo coating exhibits superior wear resistance as compared with Cr3C2-NiCr coating at high temperatures (450oC, 550oC, 650oC).
Some of WC particles were pulled out and much pores were found in the worn surface of the Cr3C2-WCNiCoCrMo coating after wear testing at 650oC.
Abstract: A novel Cr3C2-WC-NiCoCrMo and commercial Cr3C2-NiCr thermal spray-grade powders with particle size of -45+15μm were prepared by an agglomeration and sintering process. Cr3C2WC-NiCoCrMo and Cr3C2-NiCr coatings were deposited by high velocity oxygen fuel (HVOF) spraying. The fundamental properties of both coatings were evaluated and friction wear test against Al2O3 counterbodies of both coatings at high temperatures (450 oC, 550oC, 650oC) were
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carried out ball-on-disk high temperature tribometer. All specimens were characterized by optical microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) and 3D non-contact surface mapping profiler. The results have shown that the Cr3C2-WC-NiCoCrMo coating exhibited lower porosity, higher micro-hardness compared to the Cr3C2-NiCr coating. The Cr3C2-WC-NiCoCrMo coating also exhibited better wear resistance and higher friction coefficient compared to the Cr3C2-NiCr coating when sliding against the Al2O3 counterpart. Wear rates of both coatings increased with raising temperature. Both coatings experienced abrasive wear; hard phase particles (WC and Cr 3C2) with different sizes, distributed in the matrix phase, will effectively improve the resistance against wear at high temperatures. Keywords: WC; Cr3C2; HVOF; wear; high temperature 1. Introduction Thermal sprayed carbide-based coatings are currently used for many applications in the surface treatment of components in various industries to improve the resistance against high temperature wear and corrosion [1-5]. These coatings commonly consist of carbide phases (WC, Cr3C2) embedded in a matrix phase (Co, Ni, CoCr, NiCr). In the range of thermal spray processes, high velocity oxygen-fuel (HVOF) spraying has many distinguished advantages for preparing carbide based coatings with high density, excellent bonding strength and low oxidation due to highest plume particles velocities and moderate particle temperatures [6-9]. The wear resistance of carbide-based coatings is mainly dependent on the hardness of the carbide particles. WC-based coatings are usually harder and more wear resistant than Cr3C2-based coatings due to the higher hardness of the WC particles embedded in the binder phase, but their operation temperature is
limited to less than 450oC [10-14]. Cr3C2-based coatings are utilized in corrosive and moderate wear environment at temperature up to 850oC [10, 15-16]. If wear is simultaneously involved in an aggressive corrosion process at an elevated temperature, the coating should be tailored to withstand wear and corrosion attack at the same time. Composite carbides (WC and Cr 3C2) based coatings are used to improve wear resistance compared to the WC or Cr3C2 based coatings at temperatures higher than 450oC. Previous studies have shown that HVOF sprayed WC20CrC7Ni coatings have excellent wear resistance compared with WC10Co4Cr coating and electrolytic hard chrome plating at different temperatures (25oC, 450oC, 750oC) [10,17]. Although the carbides are corrosion resistant in most environments, the corrosion resistance of the carbide based coating largely depends on the binder phase (Co, Ni, CoCr, NiCr). Furthermore, matrix materials consisting of metal alloys like NiCrBSi, FeCrAlY are usually having better corrosion resistance [18-19]. Therefore composite carbides and alloy binders are expected to achieve both excellent wear and corrosion resistant performance at elevated temperature. In this paper, a novel Cr3C2-based spray powder which consists of Cr3C2 and WC hard phase in a NiCoCrMo matrix phase was prepared by the agglomeration and sintering process. The fundamental properties of the Cr3C2-WC-NiCoCrMo and Cr3C2-NiCr coatings were investigated. Especially the wear resistances of Cr3C2-WC-NiCoCrMo and Cr3C2-NiCr coatings were evaluated at temperatures of 450oC, 550oC, 650oC. The data arising out from this work will be useful to explore the possibility of using the composite carbide based coating as a favorite material for high temperature wear application. 2. Experimental 2.1 Powder and coating preparation
Cr3C2-WC-NiCoCrMo (C:8.1; Co:2.0; Ni:9.0; Mo:3.0; W:37; Cr: balance; wt.% ) and commercially available Cr3C2-25(Ni20Cr) (wt.%) powders with a nominal particle size distribution of -45+15μm were prepared by attritor milling, spray drying, vacuum sintering, crushing, and sieving process in Zigong Tungsten Carbide Co., Ltd. (Zigong, China). The raw materials for the Cr3C2-WC-NiCoCrMo spray powder are Cr3C2, WC, Ni, Co, Cr and Mo powders. Plain carbon steel (C:0.42; Si:0.18; Mn:0.55; Ni:≤0.25; Cr:≤0.25; Fe: balance; wt.%) plates (55 mm×15 mm×5 mm) were used as substrate materials. Prior to HVOF spraying, the substrates were sand blasted with corundum grit (50-70 mesh). The coatings were deposited by Diamond Jet 2600 high velocity oxyfuel spraying equipment (Metco, Switzerland) manipulated with a robot (KUKA, Germany). The spraying parameters are shown in Table 1. 2.2 High temperature wear test Cr3C2-WC-NiCoCrMo and Cr3C2-NiCr coating samples (φ40 mm×5 mm) were ground to achieve a smooth surface with Ra of 0.18±0.02μm, tested by a profilometer (TR220 profilometer, China). Friction and wear tests were carried out on a ball-on-disk high temperature tribometer (Anton Paar CSM, Switzerland) at three different temperatures (450 oC, 550oC, 650oC). In all of the high-temperature wear tests, heating lasted about 1 hour and the system will automatically start when the sample is heated to the different temperatures. The wear test conditions are listed in Table 2. 2.3 Characterizations of feedstock powder and sprayed coating The apparent density and flow-ability of Cr3C2-WC-NiCoCrMo and Cr3C2-NiCr powders were characterized according to ASTM B213-03 and ASTM B212-99. The microstructure of the coatings
was investigated on cross sections by optical (OLYMPUS B×41M-LED, Japan) and scanning electron microscopy (SEM, ZEISS EVO-18, Germany) including energy dispersive spectroscopy (OXFORD XMaxN). Metallurgical microscope with image analysis software (OLYMPUS Stream, Janpan) was used to process the optical images (500×magnification) to evaluate the porosity in the coatings. Five readings were obtained and the average was reported. The microhardness of the coating was measured on polished cross section using a Vickers indenter (Future-tech Corp FM-700, Japan) with a load of 300g. In all hardness tests, dwell time was set to be 10 s. An average of 10 readings is reported. Test samples for microstructure evaluation were cut, hot mounted, ground and polished with Struers equipment (Struers, Denmark). Diamond disk and slurry (Struers, Denmark) were used to grind and polish the samples. The phase composition was determined by X-ray diffraction technique (D/MAX 2550, Japan). The used radiation was Ni-filtered Cu Kα (λ=0.1542nm). The element composition on the surface of both Cr3C2-based coatings after wear test at 450oC was analyzed by photoelectron spectroscopy (XPS) with Al Kα X-rays (ESCALAB 250Xi, ThermoFisher-VG Scientific, USA). XRD and XPS analysis were performed outside the wear track of both coatings. The morphology of the worn surfaces of the coatings after the wear test was characterized by SEM. The wear volume of the coatings was determined using a MicroXAM-100 3D non-contact surface mapping profiler (KLT-Tencor, USA). Four replicated tests were conducted for each wear test condition and the average value was given. The wear rate is calculated as: w=V/(FL), where V is the wear volume loss in mm3, L is the total sliding distance in m, and F is the normal load in N. 3. Results and discussion 3.1 Powders
The optical micrographs of the polished cross section of Cr3C2-NiCr and Cr3C2-WC-NiCoCrMo powders are shown in Fig.1. The Cr3C2-NiCr powder was nearly spherical with a certain degree of porosity. This is the typical structure obtained by agglomeration and sintering process. However, the Cr3C2-WC-NiCoCrMo powder exhibits dense microstructure and has higher apparent density (3.6 g/cm3) and better flowability (25 s/50 g) compared to the Cr3C2-NiCr powder (29 s/50 g). XRD patterns of both carbide-based powders are shown in Fig.2 (a). Cr3C2 and NiCr matrix phases were detected by X-ray analysis in the Cr3C2-NiCr powder. This result is consistent with the findings of previous studies [20-21]. WC, Cr3C2, Cr7C3 and Ni-based solid solution phases were detected in the Cr3C2-WC-NiCoCrMo powder. In a WC-20CrC-7Ni thermal spray feedstock powder, a small amount of nickel binder phase was also detected by XRD [22]. 3.2 Microstructure and properties of HVOF-sprayed coatings Optical and SEM micrographs of the polished cross section of the Cr3C2-NiCr and Cr3C2-WCNiCoCrMo coatings are shown in Fig.3 and Fig.4 respectively. The physical properties of both coatings are summarized in Table 3. These micrographs have shown that these coatings have low porosity, lamellar structure and defect-free interface with the substrate. Carbide particles (WC, Cr3C2) are surrounded by the matrix phases. That is the typical structure of carbide based coatings prepared by HVOF spraying due to the accumulation of the high speed molten or semi-molten particles on the substrate [23]. As the Table 3 shows, the porosity and micro-hardness of Cr3C2WC-NiCoCrMo coating are 1.1±0.1% and 1153±42HV0.3 respectively. The Cr3C2-WC-NiCoCrMo coating possesses relatively lower porosity and higher micro-hardness values compared to the Cr3C2-NiCr coating under the same spraying parameters. High-hardness WC particles embedded in the matrix phase determine the higher microhardness of the Cr3C2-WC-NiCoCrMo coating. The
NiCoCrMo binder phase possibly has sufficient wettability with WC grains compared to the NiCr phase with Cr3C2 grains, this may decrease the porosity of Cr3C2-WC-NiCoCrMo coating during the HVOF spraying process. As it can be seen from Fig.4, the gray area is rich in Cr, the bright area is rich in W, so the structure of Cr3C2-WC-NiCoCrMo powder could be mostly retained in the coating. Accordingly, WC, Cr3C2, Cr7C3 and Ni-based alloy were detected in Cr3C2-WC-NiCoCrMo coating. The detected phases in Cr3C2-WC-NiCoCrMo coating are also similar with those of the powder. So the cross section of the Cr3C2-WC-NiCoCrMo coating exhibits two obviously identifiable hard phases, including Cr3C2 and WC grains (Fig. 4). The XRD pattern of the Cr3C2-WC-NiCoCrMo coating also exhibits a broad peak round 44°probably belonging to an amorphous phase. Some similar results were reported for a WC20CrC7Ni coating due to the high cooling rate when molten particles were deposited on the substrate; WC, Cr3C2 and (W,Cr)2C carbide phases were also found in the WC20CrC7Ni coating; but in this study Cr3C2, WC, Cr7C3 carbide phases are found in the Cr3C2-WCNiCoCrMo coating [10]. 3.3 High temperature wear resistance of HVOF sprayed coatings The variation of friction coefficients (COF, computed over the entire sliding distance ) and wear rates of Cr3C2-NiCr and Cr3C2-WC-NiCoCrMo coatings at different temperatures (450oC, 550oC, 650oC) are shown in Fig. 5 and Table 4 respectively. It can be observed that the Cr 3C2-WCNiCoCrMo coating possesses relatively higher COF against Al2O3 counterbody as compared with the Cr3C2-NiCr coating. Composite hard phases (WC and Cr3C2) embedded in the matrix phase (NiCoCrMo) will significantly affect the friction coefficient at elevated temperatures compared
with the single hard phase (Cr3C2) due to the formation of different oxides films on the surface of the coatings [24, 25]. The COF of Cr3C2-WC-NiCoCrMo coating decreased from 0.574±0.038 to 0.481±0.041 while the temperature increased from 450oC to 650oC. However, the COF of the Cr3C2-NiCr coating did not follow this rule under these test conditions. The Cr 3C2-NiCr coating exhibits its highest COF at 450oC, but displays the lowest COF at 550oC. It also can be seen that the Cr3C2-WC-NiCoCrMo coating exhibits much smaller wear rates than the Cr3C2-NiCr coating when tested against the alumina counterbody at 450, 550 and 650 oC (Table. 4). The wear rate of the Cr3C2-NiCr coating is more than five times higher than that of the Cr3C2-WC-NiCoCrMo coating at 450oC. At 650oC, the wear rate of Cr3C2-NiCr coating is more than three times higher than that of the Cr3C2-WC-NiCoCrMo coating. The results of wear rates indicate that the Cr3C2-WC-NiCoCrMo coating has excellent wear resistance against the Al2O3 counterpart at high temperatures compared to the Cr3C2-NiCr coating. Wear rates of both coatings are increased with rising temperature. As shown in Fig.6, wear scars are obviously observed on the worn area of both coatings after wear testing. The wear rates of Cr3C2-NiCr coating at 550oC and 650oC are very similar. This corresponds to the slight changes of COF and indicates that the Cr 3C2NiCr coating exhibits stable performance at these temperatures. However the wear rate of Cr 3C2WC-NiCoCrMo coating increases nearly two times when the temperature is increased from 450 oC to 650oC, indicating that the microstructure and properties of the coating are prone to be affected by temperature compared to Cr3C2-NiCr coating. The XRD patterns of the surface of the Cr3C2-NiCr and Cr3C2-WC-NiCoCrMo coatings after wear testing at different temperatures (450oC, 550oC, 650oC) are shown in Fig.7. It can be found that the phases of the Cr3C2-NiCr coating after wear testing at high temperatures are carbide
phases and Ni-based alloy phase. Moreover, the diffraction peaks of Cr2O3 are detected after the wear testing, indicating that the Cr3C2-NiCr coating experienced oxidation on the surface of the coating during the wear process at high temperatures especially at 650 oC (Fig.7(a)). However, the main phases of the Cr3C2-WC-NiCoCrMo coating after wear testing at different temperatures are tungsten carbide, chromium carbides and Ni-based alloy phase. The diffraction peaks of WO3, Cr2O3 are also detected by XRD after wear testing. That demonstrates that embedded tungsten carbide particles in the coating experienced oxidation when the temperature rose (Fig.7(b)). The diffraction bands of the amorphous phase of both coatings become sharper and stronger with the temperature rising; indicating that the amorphous phase crystallized to secondary carbides like previous studies [20, 26-28] have shown. That improved the matrix phase ductility and increased the hardness of the carbide based coatings, and probably improved the wear resistance of carbide based coatings at high temperatures (450-650oC) [26-28]. SEM/BSD micrographs of the cross section of Cr3C2-NiCr coating and Cr3C2-WC-NiCoCrMo coating are shown in Fig.8. The precipitation of secondary carbides occurred during the wear process at 650oC. XPS spectra of the surface of the Cr3C2-NiCr and Cr3C2-WC-NiCoCrMo coatings after wear test at 450oC are shown in Fig.9. Oxygen peaks are relatively strong after wear test at 450oC, indicating the presence of surface oxides. The high-resolution spectra of Cr2p and Ni2p of the surface of the Cr3C2-NiCr coating are shown in Fig.10. The high-resolution spectra of Cr2p, Ni2p, Co2p and W4f of the surface of the Cr3C2-WCNiCoCrMo coating are shown in Fig.11. These are the characteristic peak of Cr 2O3, CrO3, Ni(OH)2, CoO and WO3 [29-35], indicating that the outermost layers of the Cr3C2-NiCr coating mainly contain Cr2O3, CrO3 and Ni(OH)2 and the outermost layers of the Cr3C2-WC-NiCoCrMo coating mainly contain Cr2O3, CrO3, Ni(OH)2, CoO and WO3 during the high temperature wear process.
To deeply understand the wear behavior, the worn area was examined by SEM and 3D noncontact surface mapping profiler. 3D non-contact surface mapping of the worn surface of both coatings at 450, 550, 650oC are shown in Fig.12, Fig.14 and Fig.16 respectively. SEM/EDS images of the worn scar of both coatings at 450, 550, 650oC are shown in Fig.13, Fig.15 and Fig.17 respectively. It is observed that Cr3C2-WC-NiCoCrMo coating has smaller wear scar width and depth than the Cr3C2-NiCr coating and both coatings show obvious signs of grooves. This is in accordance with the wear rates results (Table 4). Also it can be seen that numerous WC particles are still embedded in the matrix phase on the worn surface of the Cr 3C2-WC-NiCoCrMo coating at 450oC (Fig.13), while at 650oC just a small amount of WC particles and some pores are retained in the worn scar surface of the Cr3C2-WC-NiCoCrMo coating after sliding wear (Fig.17). Comparatively, the worn scars of the Cr3C2-NiCr coatings present similar morphologies with some pores at different temperatures. Tribo-films are obviously observed on the worn scar surface of both coatings after the sliding wear testing at 550, 650oC. According to the EDS analysis (Fig.13, Fig.15, Fig.17), major elements, Cr, W, Ni, Co, Mo, and O were found, this suggests that these tribo-films are composed of some oxides. SEM/EDS analysis of the surface of the Al2O3 counterpart balls for the wear test of the Cr3C2-NiCr coating and the Cr3C2-WC-NiCoCrMo coating at 450, 650oC are shown in Fig.18 and Fig.19 respectively. It can be observed that the tribo-films are also found on the surface of the Al2O3 counterparts at different temperatures. When the temperature increased, the tribo-films covered more the surface of the counterpart balls. These transfer tribofilms are composed of different oxides. This result is in accordance with the composition of the tribo-films which were found on the worn surface of the coatings. That indicates that a portion of oxidized layer on the surface of the coatings transfered to the surface of the Al 2O3 ball.
3.4 Discussion Many factors affect the wear resistance of HVOF sprayed carbide based coatings [24, 36-39]. The preparation processes for both coatings for this wear test are the same. In this study, a comprehensive wear mechanism model of HVOF sprayed Cr3C2-based coatings is shown in Fig.20 to further understand the wear process at high temperatures. Due to the smooth surface of the coatings and the Al2O3 ball, for both coatings, single WC or Cr3C2 particles and also whole coating splats (which also contain hard phase particles) will be firstly cut out, removed and finally grooves and pores are formed on the surface of the coatings (Fig.13, Fig.15, Fig.17 and Fig.20). These pulled-out debris particles will act as abrasive particles leading to the severe three-body abrasive wear. What’s more, the wear process will be accelerated by forming oxides on the surface of the coatings and softening of the coatings at high temperature. During the high temperature wear process, oxide films were first formed on the surface of the Cr3C2-NiCr coating and the Cr3C2-WC-NiCoCrMo coatings. In this study, Cr2O3, CrO3 and Ni(OH)2 were formed on the surface of the Cr3C2-NiCr coating as proven by XPS (Fig.9(a), Fig.10), XRD (Fig.7(a)) and EDS analyses (Fig.13(b), Fig.15(b), Fig.17(b)). However, more oxides were formed on the surface of the Cr3C2-WC-NiCoCrMo coating during the wear process compared to the Cr3C2NiCr coating. Cr2O3, WO3, CrO3, Ni(OH)2, and CoO oxides were formed on the surface of the Cr3C2WC-NiCoCrMo coating, as further confirmed by XPS (Fig.9(b), Fig.11), XRD (Fig.7(b)) and EDS analyses (Fig.13(d), Fig.15(d), Fig.17(d)). Oxide like MoO may also form on the surface of the Al 2O3 ball (Fig.19) and on the worn tracks (Fig.13(d), Fig.15(d), Fig.17(d)) at different temperatures according to the EDS analyses. In the XRD patterns (Fig.7), the diffraction peaks of carbides and Nibased alloy phases are still clearly visible and as the most intense ones due to the low thickness of
the oxide films. The diffraction peaks of the oxides phases (Cr2O3, WO3) are clearly visible only at 650oC indicating that both coatings experience stronger oxidation at this temperature. At 450oC, the oxide based tribo-films were only found on the surface of the counterbody balls and cover a smaller fraction of the surface of the Al2O3 balls (Fig.18-Fig.19, Fig.20). With the temperature rising (550oC, 650oC), the oxides based tribo-films were found on the worn surface of both coatings and cover a lager fraction of the surface of the counterbody balls (Fig.13, Fig.15, Fig.17, Fig.18, Fig.19). During the high temperature wear process of both coatings, the contact between the hardmetal and the counterbody is not fully mediated by the oxide based tribo-films (Fig.18-Fig.19). The interposition of the oxides based tribo-films, a non-plastic medium, between the coatings surface and the Al2O3 balls prevents direct contact and decreases the friction coefficient, and the oxides based tribo-films will play an important role as lubricant to further reducing the COF of the coating [10-11, 25, 40]. The friction coefficient values produced by both of the tribo-pairs at 650oC are lower than those measured at 450oC due to the presence of tribo-films on the worn surface (Fig.13, Fig.17) and to the fact that the tribo-films cover a lager fraction of the surface of the Al2O3 counterbody (Fig.18-Fig.19). The wear rate value of the Cr3C2-NiCr coating at 550oC and 650oC is slightly higher than the wear rate value measured at 450oC. With the temperature increasing from 450oC to 650oC, the oxides layer will become thicker and more wear debris and tribo-films will be formed. The oxides based wear debris and tribo-films will act as abrasive particles leading to severe abrasive wear (Fig.20), which results in higher wear rate at 650oC. Many wear grooves, deep wear scars, pores and pulled-out particles are obviously observed on the worn surface of the Cr3C2-NiCr coating at different temperatures indicating that the Cr3C2-NiCr coating experiences abrasive wear. Similar
results are also reported in previous research: abrasion was an important wear mechanism of Cr3C2-NiCr coating when sliding at 800oC [40]. When the temperature is between 550oC and 650oC, the COF values and wear rates values of Cr3C2-NiCr coating are very close. Similar wear rates and COF values of Cr3C2-NiCr coatings at these temperatures indicate that the microstructure and the properties of the Cr3C2-NiCr coatings are stable in this range of temperature. That’s ascribed to the excellent oxidation resistance of this coatings at high temperatures (Fig.7(a)). Previous studies also reported that Cr3C2-NiCr coatings exhibited excellent oxidation resistance at 900oC [40-41]. However, due to the smaller size of the WC particles distributed in the coating, the wear process of Cr3C2-WC-NiCoCrMo coating at high temperatures is slightly changed compared to the Cr3C2-NiCr coating (Fig.20). As shown in Fig.18 and Fig.19, the oxide based tribo-films cover smaller fraction of the surface of the counterbody for the Cr3C2-WC-NiCoCrMo coating compared to that of the Cr3C2-NiCr coating. And some oxides such as WO3, which formed on the surface of the Cr3C2WC-NiCoCrMo coating, may act as solid lubricant [24]. So the Cr3C2-WC-NiCoCrMo coatings exhibit higher COF against the Al2O3 counterpart at different temperatures compared to the Cr3C2-NiCr coating. The friction coefficient of Cr3C2-WC-NiCoCrMo coating at 650oC is lower than that at 450oC, possibly because more WO3 with the structure of Magnéli phase was produced at 650oC to act as solid lubricant [24, 42]. That’s consistent with the previous studies that the COF of WC20CrC-7Ni coating decreased from 0.81±0.01 to 0.62±0.05 while the temperature increased from 400oC to 750oC and the COF of WC-10Co4Cr coating decreased from 0.64 to 0.41 while the temperature changed from 200oC to 550oC [10-11]. In addition, with the temperature increase, the area of contact between the coating surface and the Al2O3 ball become greater (Fig.19), that’s
leading to lowering of stress and thus reducing the friction coefficient [25]. That’s possible the other reason which reduce the friction coeffient of the Cr3C2-WC-NiCoCrMo coating at 650oC. The Cr3C2-WC-NiCoCrMo coating possesses higher hardness than the Cr3C2-NiCr coating and the tungsten carbide particles also have much higher hardness than chromium carbide particles. During the wear process, less binder phase will be cut due to the harder WC particles being distributed between the binder phase and chromium carbides in the coating as shown in Fig.20. It can be obviously seen that WC particles are retained in the worn surface after testing at different temperatures and the grooves on the worn surface are shorter compared to those on the Cr3C2NiCr coatings (Fig.13(b), Fig.15(b), Fig.17(b)). The morphology of the worn scar of the Cr3C2-WCNiCoCrMo coating indicates that the Cr3C2-WC-NiCoCrMo coating also experiences abrasive wear like the Cr3C2-NiCr coating. Additionally, according to the XRD patterns (Fig.7(b)) and SEM/EDS analysis (Fig.6, Fig.8) of the Cr3C2-WC-NiCoCrMo coatings, the microstructure of the Cr3C2-WCNiCoCrMo coating didn’t experienced catastrophic oxidation during this wear testing at 650oC. On top of that, tungsten carbide, chromium carbides and binder phase of the Cr3C2-WC-NiCoCrMo coating are retained after wear process at high temperatures (Fig.7(b), Fig.6). All this can significantly decrease the wear rate of the composite carbide-based coating. So the wear rate values of Cr3C2-WC-NiCoCrMo coatings are much lower compared to the Cr3C2-NiCr coatings at different high temperatures. However at 650oC, some of WC particles were pulled out and some pores were found in the worn surface of the Cr3C2-WC-NiCoCrMo coating (Fig.17(b) and Fig.20) due to the excessive oxidation (Fig.6, Fig.7(b)). These pulled out particles will act as abrasive particles and the oxide film will break down, that resulted in the highest wear rate at 650oC. Pure WC-Co or WC-CoCr coatings
were indeed reported to experience catastrophic oxidation at high temperature, and this will result in poor wear resistance [13, 43]; however, the Cr3C2-WC-NiCoCrMo coating still exhibited better wear resistance at 650oC in this study due to the WC and Cr3C2 particles embedded in the matrix of NiCoCrMo. Although the Cr3C2-WC-NiCoCrMo coating exhibits good results in high temperatures (450650oC) wear tests, the coating needs to be tested further for wear combined with corrosion conditions at high temperatures. 4. Conclusion In the present study, Cr3C2-WC-NiCoCrMo and Cr3C2-NiCr spray powders were prepared by an agglomeration and sintering process. The basic properties of the powders and the coatings were investigated. Especially the high temperature sliding wear resistance of both coatings was evaluated at 450 oC, 550 oC and 650 oC. The conclusions are summarized as follows: ● The Cr3C2-WC-NiCoCrMo spray powder exhibits higher apparent density (3.6 g/cm3) and
flow-ability (25 s/50g) compared to the Cr3C2-NiCr powder. ● The Cr3C2-WC-NiCoCrMo coating exhibits lower porosity (1.1±0.1%) and higher micro-
hardness (1153±42 HV0.3) compared to the Cr3C2-NiCr coating. Major phases of Cr3C2, WC, Cr7C3 are detected in both Cr3C2-WC-NiCoCrMo powder and coating. ● The Cr3C2-WC-NiCoCrMo coating produces relatively higher friction coefficients (COF)
compared to the Cr3C2-NiCr coating when tested against an Al2O3 counterbody at high temperatures (450oC, 550oC, 650oC) in this testing conditions.
● Cr3C2-WC-NiCoCrMo coatings show lower wear rates compared to the Cr 3C2-NiCr coatings at
this range of temperature (450oC-650oC). The wear rates of the Cr3C2-WC-NiCoCrMo coating increased by nearly two times when the temperature increased from 450 oC to 650oC, but the wear rates of the Cr3C2-NiCr coating are roughly the same, indicating that the microstructure and properties of the Cr3C2-WC-NiCoCrMo coating are prone to be affected at higher temperature (550oC-650oC) compared to the Cr3C2-NiCr coating. ● Both coatings experience abrasive wear during the high temperatures wear process. Oxides
like Cr2O3, WO3, CrO3, Ni(OH)2, CoO was initially formed on the surface of the surface of the Cr3C2WC-NiCoCrMo coating at 450oC. ● After wear testing at 650oC, although some of the WC particles are pulled out and some
pores are found in the worn surface of the Cr3C2-WC-NiCoCrMo coating, the Cr3C2-WC-NiCoCrMo coating nevertheless exhibits superior wear resistance compared to the Cr3C2-NiCr coating at this temperature. Acknowledgement We would like to acknowledge the financial support from Natural Science Foundation of Guangdong Province (2016A030312015), Guangdong Science and Technology Programming (No. 2013B050800031, No. 2014B050502008 and No. 2015B07070124). REFERENCE [1] K. Szymański, A. Hernas, G. Moskal, H. Myalska, Thermally sprayed coatings resistant to erosion and corrosion for power plant boilers- A review, Surf. Coat. Tech. 268 (2015) 153-164. [2] L. Pawlowski, The Science and Engineering of Thermal Spray Coatings: Second Edition, 2008
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Figures Fig.1 Optical micrographs of the polished cross section of (a) Cr3C2-NiCr powder and (b) Cr3C2-WCNiCoCrMo powder. Fig.2 XRD patterns of (a) Cr3C2-NiCr spraying powder and coating and (b) Cr3C2-WC-NiCoCrMo spraying powder and coating. Fig.3 (a) Optical and (b) SEM micrograph of the polished cross section of the Cr3C2-NiCr coating Fig. 4 (a) Optical and (b) SEM/EDS micrograph of the polished cross section of the Cr3C2-WCNiCoCrMo coating. Fig.5 Friction coefficients of Cr3C2-NiCr and Cr3C2-WC-NiCoCrMo coating at different temperatures. Fig.6 SEM/EDS analysis across the wear scar cross section of (a) Cr3C2-NiCr coating and (b) Cr3C2WC-NiCoCrMo coating after sliding wear tests at 650oC. Fig.7 XRD patterns of the surface of the (a) Cr3C2-NiCr coatings and (b) Cr3C2-WC-NiCoCrMo coatings after wear testing at different temperatures. Fig.8 SEM/BSD micrographs of the cross section of (a) Cr3C2-NiCr coating and (b) Cr3C2-WCNiCoCrMo coating after wear testing at 650oC. Fig.9 Full XPS spectra of the surface of the (a) Cr3C2-NiCr coating and (b) Cr3C2-WC-NiCoCrMo coating after wear testing at 450oC. Fig.10 XPS high-resolution spectrum of (a) Cr and (b) Ni element of the surface of the Cr3C2-NiCr coating after wear test at 450oC. Fig.11 XPS high-resolution spectrum of (a) Cr, (b) Ni, (c) Co, (d) W element of the surface of the Cr3C2-WC-NiCoCrMo coating after wear testing at 450oC. Fig.12 3D profiles of HVOF coatings after wear testing at 450oC. (a) Cr3C2-NiCr coating, (b) Cr3C2WC-NiCoCrMo coating. Fig.13 SEM/BSD micrographs of the wear scars of (a) , (b) Cr3C2-NiCr coating and (c), (d) Cr3C2-WCNiCoCrMo coating after wear testing at 450oC.
Fig.114 3D profiles of HVOF coatings after wear testing at 550oC. (a) Cr3C2-NiCr coating, (b) Cr3C2WC-NiCoCrMo coating. Fig.15 SEM micrographs of the wear scars of (a), (b) Cr3C2-NiCr coating and (c), (d) Cr3C2-WCNiCoCrMo coating after wear testing at 550oC. Fig.16 3D profiles of HVOF coatings after wear testing at 650oC. (a) Cr3C2-NiCr coating, (b) Cr3C2WC-NiCoCrMo coating. Fig.17 SEM/BSD micrographs of the wear scars of (a), (b) Cr3C2-NiCr coating and (c), (d) Cr3C2-WCNiCoCrMo coating after wear testing at 650oC. Fig.18 SEM/EDS micrographs of the surface of the Al2O3 counterpart balls for the Cr3C2-NiCr coating wear testing at (a) 450oC and (b) 650 oC. Fig.19 SEM/EDS micrographs of the surface of the Al2O3 counterpart balls for the Cr3C2-WCNiCoCrMo coating wear testing at (a) 450oC and (b) 650 oC. Fig.20 Schematic diagram of the wear mechanism of Cr3C2-NiCr and Cr3C2-WC-NiCoCrMo coatings.
Fig.1 Optical micrographs of the polished cross section of (a) Cr3C2-NiCr powder and (b) Cr3C2-WCNiCoCrMo powder
Fig.2 XRD patterns of (a) Cr3C2-NiCr spraying powder and coating and (b) Cr3C2-WC-NiCoCrMo spraying powder and coating (b)
Fig.3 (a) Optical and (b) SEM micrograph of the polished cross section of the Cr3C2-NiCr coating Composition(
(b)
wt.%) Cr: 60.6 Ni: 7.0 Composition( wt.%) W: 84.1 Cr: 1.7
Co: 3.8 W:7.5 Mo:2.3
Fig. 4 (a) Optical and (b) SEM/EDS micrograph of the polished cross section of the Cr3C2-WC-NiCoCrMo coating
Fig.5 Friction coefficients of Cr3C2-NiCr and Cr3C2-WC-NiCoCrMo coating at different temperatures.
(a)
(b)
13.2 wt. % O
3.0 wt. % O
14.3 wt. % O
3.5wt. % O
Fig.6 SEM/EDS analysis across the wear scar cross section of (a) Cr3C2-NiCr coating and (b) Cr3C2-WCNiCoCrMo coating after sliding wear tests at 650oC.
(a)
Fig.7 XRD patterns of the surface of the (a) Cr3C2-NiCr coatings and (b) Cr3C2-WC-NiCoCrMo coatings after wear testing at different temperature.
(a)
(b) Secondary carbides Secondary carbides
Fig.8 SEM/BSD micrographs of the cross section of (a) Cr3C2-NiCr coating and (b) Cr3C2-WC-NiCoCrMo coating after wear testing at 650oC.
(a)
(b)
Fig.9 Full XPS spectra of the surface of the (a) Cr3C2-NiCr coating and (b) Cr3C2-WC-NiCoCrMo coating after wear testing at 450oC.
(a)
(b)
Fig.10 XPS high-resolution spectrum of (a) Cr and (b) Ni element of the surface of the Cr3C2-NiCr coating after wear testing at 450oC.
(a)
(b)
(d)
(c)
Fig.11 XPS high-resolution spectrum of (a) Cr, (b) Ni, (c) Co, (d) W element of the surface of the Cr3C2-WCNiCoCrMo coating after wear testing at 450oC.
(a)
(b)
Fig.12 3D profiles of HVOF coatings after wear testing at 450oC. (a) Cr3C2-NiCr coating, (b) Cr3C2-WCNiCoCrMo coating
(a)
(b)
Pore Carbides
Composition( wt.%) C：16.6 Cr: 58.3
Grooves
Ni: 21.7 O: 3.4
(c)
(d)
Composition( wt.%) C：14.5
Carbides
Cr: 28.8 Ni: 6.2
Grooves
O: 6.3 Mo: 2.2
Fig.13 SEM/BSD micrographs of the wear scars of (a), (b) Cr3C2-NiCr coating and (c), (d) CrCo: 3C2-WC-NiCoCrMo 1.6 coating after wear testing at 450oC. W:40.4
(b)
(a)
Fig.14 3D profiles of HVOF coatings after wear test at 550oC. (a) Cr3C2-NiCr coating, (b) Cr3C2-WC-NiCoCrMo coating
(b)
(a)
Tribofilm (wt.%)
Carbides
C：9.7 Cr: 43.0
Grooves
Ni: 12.7 O: 34.6
(d) Carbides
(c)
Pore Carbides
Tribofilm (wt.%)
Carbides C：6.6 Cr: 23.0
Grooves
Ni: 5.8 O: 29.3 Mo: 2.2 Co: 1.9 Fig.15 SEM/BSD micrographs of the wear scars of (a), (b) Cr3C2-NiCr coating and (c), (d) Cr3C2-WC-NiCoCrMo
coating after wear testing at 550oC.
(a)
W: 31.2
(b)
Fig.16 3D profiles of HVOF coatings after wear testing at 650oC. (a) Cr3C2-NiCr coating, (b) Cr3C2-WCNiCoCrMo coating
(b) Tribofilm
(a)
(wt.%)
Carbides
C：5.0
Grooves
Cr: 54.9 Ni: 4.2
Pore O: 35.9
(c)
Grooves
(d)
Tribofilm (wt.%) C：5.6
Pore
Cr: 20.0
Carbides
Ni: 5.5 O: 34.5 Mo: 2.1
Fig.17 SEM/BSD micrographs of the wear scars of (a) , (b) Cr3C2-NiCr coating and (c), (d) Cr3C2-WCCo: 1.8 NiCoCrMo coating after wear testing at 650oC.
(a)
Tribofilm
(b)
W: 30.5 Tribofilm
(wt.%)
(wt.%)
C：5.2
C：14.0
Cr:37.0
Cr: 55.5
Ni: 16.8
Ni: 11.3
O: 37.2
O: 19.2
Al: 3.8
Fig.18 SEM/EDS analysis of the surface of the Al2O3 counterpart balls for the Cr3C2-NiCr coating wear testing at (a) 450oC and (b) 650 oC.
(a)
Tribofilm (wt.%)
(b)
C：5.2
Tribofilm (wt.%) C：22.8
Cr:23.4
Cr: 23.4
Ni: 4.6
Ni: 5.5
Co: 1.9
Co: 1.7
W:27.3
W: 21.3
Mo:1.3
Mo: 0.8
Fig.19 SEM/EDS analysis of the surface of the Al2O3 counterpart balls for the Cr3C2-WC-NiCoCrMo coating O: 32.6 at (a) 450oC and (b) 650 oC. wear testing
O: 24.5
Al: 3.6
Fig.20 Schematic diagram of the wear mechanism of Cr3C2-NiCr and Cr3C2-WC-NiCoCrMo coatings
Table Table 1 HVOF spraying parameters Powder
Cr3C2-NiCr ,Cr3C2-WC-NiCoCrMo
Gun
DJ2600
Hydrogen(L/min)
635
Oxygen(L/min)
280
Air(L/min)
345
Carrier gas flow rate ( Ar, L/min)
30
Powder feed rate (g/min)
40
Surface traverse velocity(m/min)
90
Spraying distance(mm)
230
Offset(mm)
3
Table 2 Wear test conditions Parameters
Values
Ball material
Al2O3
Diameter of Ball
6.00mm
Load
10.00N
Radius
5.00mm
Lin. Speed
0.25m/s
Sliding distance
600m
Acquisition rate
6.5HZ
Test temperature
450oC, 550oC,650oC
Table 3 Characteristics of coatings Coating
Hardness(HV0.3)
Porosity(%)
Cr3C2-NiCr
1029±94
1.4±0.1
Cr3C2-WC-NiCoCrMo
1153±42
1.1±0.1
Table 4 Sliding wear resistance of Cr3C2-NiCr and Cr3C2-WC-NiCoCrMo coatings
Sample
Temperature (oC)
Mean wear scar depth (μm)
Wear rate(10-6 mm3/Nm)
Friction coefficient
Cr3C2-NiCr
450
6.8±0.3
33.3±1.6
0.474±0.034
Cr3C2-NiCr
550
8.7±0.2
36.8±2.5
0.419±0.034
Cr3C2-NiCr
650
15.4±0.3
37.2±2.1
0.427±0.032
Cr3C2-WC-NiCoCrMo
450
2.1±0.2
6.3±0.5
0.574±0.038
Cr3C2-WC-NiCoCrMo
550
2.5±0.3
7.8±0.3
0.508±0.043
Cr3C2-WC-NiCoCrMo
650
5.7±0.2
11.5±0.3
0.481±0.041
S0169-4332(17)31159-5 http://dx.doi.org/doi:10.1016/j.apsusc.2017.04.132 APSUSC 35813
To appear in:
APSUSC
Received date: Revised date: Accepted date:
28-10-2016 13-4-2017 17-4-2017
Please cite this article as: Wuxi Zhou, Kesong Zhou, Yuxi Li, Chunming Deng, Keli Zeng, High temperature wear performance of HVOF-sprayed Cr3C2-WC-NiCoCrMo and Cr3C2-NiCr hardmetal coatings, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.04.132 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
High temperature wear performance of HVOF-sprayed Cr3C2-WC-NiCoCrMo and Cr3C2-NiCr hardmetal coatings Wuxi Zhou1,2*, Kesong Zhou1,3,4, Yuxi Li2, Chunming Deng3,4, Keli Zeng3 1. School of Materials Science and Engineering, Central South University, Changsha 410083, P.R.China; 2. Zigong Cemented Carbide Co.,Ltd. Zigong 643000, P.R.China; 3. Guangdong Institute of New Materials, National Engineering Laboratory for Modern Materials Surface Engineering Technology, Guangdong Provincial Key Lab for Modern Materials Surface Engineering Technology, Guangzhou 510650, P.R. China. Highlights
Cr3C2-WC-NiCoCrMo HVOF spray-grade powder was prepared by an agglomeration and sintering process.
Cr3C2-WC-NiCoCrMo coating was deposited by HVOF spraying process.
The Cr3C2-WC-NiCoCrMo coating exhibits superior wear resistance as compared with Cr3C2-NiCr coating at high temperatures (450oC, 550oC, 650oC).
Some of WC particles were pulled out and much pores were found in the worn surface of the Cr3C2-WCNiCoCrMo coating after wear testing at 650oC.
Abstract: A novel Cr3C2-WC-NiCoCrMo and commercial Cr3C2-NiCr thermal spray-grade powders with particle size of -45+15μm were prepared by an agglomeration and sintering process. Cr3C2WC-NiCoCrMo and Cr3C2-NiCr coatings were deposited by high velocity oxygen fuel (HVOF) spraying. The fundamental properties of both coatings were evaluated and friction wear test against Al2O3 counterbodies of both coatings at high temperatures (450 oC, 550oC, 650oC) were
Corresponding author: Tel:+86 813 5516861 E-mail address: [email protected] *
carried out ball-on-disk high temperature tribometer. All specimens were characterized by optical microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) and 3D non-contact surface mapping profiler. The results have shown that the Cr3C2-WC-NiCoCrMo coating exhibited lower porosity, higher micro-hardness compared to the Cr3C2-NiCr coating. The Cr3C2-WC-NiCoCrMo coating also exhibited better wear resistance and higher friction coefficient compared to the Cr3C2-NiCr coating when sliding against the Al2O3 counterpart. Wear rates of both coatings increased with raising temperature. Both coatings experienced abrasive wear; hard phase particles (WC and Cr 3C2) with different sizes, distributed in the matrix phase, will effectively improve the resistance against wear at high temperatures. Keywords: WC; Cr3C2; HVOF; wear; high temperature 1. Introduction Thermal sprayed carbide-based coatings are currently used for many applications in the surface treatment of components in various industries to improve the resistance against high temperature wear and corrosion [1-5]. These coatings commonly consist of carbide phases (WC, Cr3C2) embedded in a matrix phase (Co, Ni, CoCr, NiCr). In the range of thermal spray processes, high velocity oxygen-fuel (HVOF) spraying has many distinguished advantages for preparing carbide based coatings with high density, excellent bonding strength and low oxidation due to highest plume particles velocities and moderate particle temperatures [6-9]. The wear resistance of carbide-based coatings is mainly dependent on the hardness of the carbide particles. WC-based coatings are usually harder and more wear resistant than Cr3C2-based coatings due to the higher hardness of the WC particles embedded in the binder phase, but their operation temperature is
limited to less than 450oC [10-14]. Cr3C2-based coatings are utilized in corrosive and moderate wear environment at temperature up to 850oC [10, 15-16]. If wear is simultaneously involved in an aggressive corrosion process at an elevated temperature, the coating should be tailored to withstand wear and corrosion attack at the same time. Composite carbides (WC and Cr 3C2) based coatings are used to improve wear resistance compared to the WC or Cr3C2 based coatings at temperatures higher than 450oC. Previous studies have shown that HVOF sprayed WC20CrC7Ni coatings have excellent wear resistance compared with WC10Co4Cr coating and electrolytic hard chrome plating at different temperatures (25oC, 450oC, 750oC) [10,17]. Although the carbides are corrosion resistant in most environments, the corrosion resistance of the carbide based coating largely depends on the binder phase (Co, Ni, CoCr, NiCr). Furthermore, matrix materials consisting of metal alloys like NiCrBSi, FeCrAlY are usually having better corrosion resistance [18-19]. Therefore composite carbides and alloy binders are expected to achieve both excellent wear and corrosion resistant performance at elevated temperature. In this paper, a novel Cr3C2-based spray powder which consists of Cr3C2 and WC hard phase in a NiCoCrMo matrix phase was prepared by the agglomeration and sintering process. The fundamental properties of the Cr3C2-WC-NiCoCrMo and Cr3C2-NiCr coatings were investigated. Especially the wear resistances of Cr3C2-WC-NiCoCrMo and Cr3C2-NiCr coatings were evaluated at temperatures of 450oC, 550oC, 650oC. The data arising out from this work will be useful to explore the possibility of using the composite carbide based coating as a favorite material for high temperature wear application. 2. Experimental 2.1 Powder and coating preparation
Cr3C2-WC-NiCoCrMo (C:8.1; Co:2.0; Ni:9.0; Mo:3.0; W:37; Cr: balance; wt.% ) and commercially available Cr3C2-25(Ni20Cr) (wt.%) powders with a nominal particle size distribution of -45+15μm were prepared by attritor milling, spray drying, vacuum sintering, crushing, and sieving process in Zigong Tungsten Carbide Co., Ltd. (Zigong, China). The raw materials for the Cr3C2-WC-NiCoCrMo spray powder are Cr3C2, WC, Ni, Co, Cr and Mo powders. Plain carbon steel (C:0.42; Si:0.18; Mn:0.55; Ni:≤0.25; Cr:≤0.25; Fe: balance; wt.%) plates (55 mm×15 mm×5 mm) were used as substrate materials. Prior to HVOF spraying, the substrates were sand blasted with corundum grit (50-70 mesh). The coatings were deposited by Diamond Jet 2600 high velocity oxyfuel spraying equipment (Metco, Switzerland) manipulated with a robot (KUKA, Germany). The spraying parameters are shown in Table 1. 2.2 High temperature wear test Cr3C2-WC-NiCoCrMo and Cr3C2-NiCr coating samples (φ40 mm×5 mm) were ground to achieve a smooth surface with Ra of 0.18±0.02μm, tested by a profilometer (TR220 profilometer, China). Friction and wear tests were carried out on a ball-on-disk high temperature tribometer (Anton Paar CSM, Switzerland) at three different temperatures (450 oC, 550oC, 650oC). In all of the high-temperature wear tests, heating lasted about 1 hour and the system will automatically start when the sample is heated to the different temperatures. The wear test conditions are listed in Table 2. 2.3 Characterizations of feedstock powder and sprayed coating The apparent density and flow-ability of Cr3C2-WC-NiCoCrMo and Cr3C2-NiCr powders were characterized according to ASTM B213-03 and ASTM B212-99. The microstructure of the coatings
was investigated on cross sections by optical (OLYMPUS B×41M-LED, Japan) and scanning electron microscopy (SEM, ZEISS EVO-18, Germany) including energy dispersive spectroscopy (OXFORD XMaxN). Metallurgical microscope with image analysis software (OLYMPUS Stream, Janpan) was used to process the optical images (500×magnification) to evaluate the porosity in the coatings. Five readings were obtained and the average was reported. The microhardness of the coating was measured on polished cross section using a Vickers indenter (Future-tech Corp FM-700, Japan) with a load of 300g. In all hardness tests, dwell time was set to be 10 s. An average of 10 readings is reported. Test samples for microstructure evaluation were cut, hot mounted, ground and polished with Struers equipment (Struers, Denmark). Diamond disk and slurry (Struers, Denmark) were used to grind and polish the samples. The phase composition was determined by X-ray diffraction technique (D/MAX 2550, Japan). The used radiation was Ni-filtered Cu Kα (λ=0.1542nm). The element composition on the surface of both Cr3C2-based coatings after wear test at 450oC was analyzed by photoelectron spectroscopy (XPS) with Al Kα X-rays (ESCALAB 250Xi, ThermoFisher-VG Scientific, USA). XRD and XPS analysis were performed outside the wear track of both coatings. The morphology of the worn surfaces of the coatings after the wear test was characterized by SEM. The wear volume of the coatings was determined using a MicroXAM-100 3D non-contact surface mapping profiler (KLT-Tencor, USA). Four replicated tests were conducted for each wear test condition and the average value was given. The wear rate is calculated as: w=V/(FL), where V is the wear volume loss in mm3, L is the total sliding distance in m, and F is the normal load in N. 3. Results and discussion 3.1 Powders
The optical micrographs of the polished cross section of Cr3C2-NiCr and Cr3C2-WC-NiCoCrMo powders are shown in Fig.1. The Cr3C2-NiCr powder was nearly spherical with a certain degree of porosity. This is the typical structure obtained by agglomeration and sintering process. However, the Cr3C2-WC-NiCoCrMo powder exhibits dense microstructure and has higher apparent density (3.6 g/cm3) and better flowability (25 s/50 g) compared to the Cr3C2-NiCr powder (29 s/50 g). XRD patterns of both carbide-based powders are shown in Fig.2 (a). Cr3C2 and NiCr matrix phases were detected by X-ray analysis in the Cr3C2-NiCr powder. This result is consistent with the findings of previous studies [20-21]. WC, Cr3C2, Cr7C3 and Ni-based solid solution phases were detected in the Cr3C2-WC-NiCoCrMo powder. In a WC-20CrC-7Ni thermal spray feedstock powder, a small amount of nickel binder phase was also detected by XRD [22]. 3.2 Microstructure and properties of HVOF-sprayed coatings Optical and SEM micrographs of the polished cross section of the Cr3C2-NiCr and Cr3C2-WCNiCoCrMo coatings are shown in Fig.3 and Fig.4 respectively. The physical properties of both coatings are summarized in Table 3. These micrographs have shown that these coatings have low porosity, lamellar structure and defect-free interface with the substrate. Carbide particles (WC, Cr3C2) are surrounded by the matrix phases. That is the typical structure of carbide based coatings prepared by HVOF spraying due to the accumulation of the high speed molten or semi-molten particles on the substrate [23]. As the Table 3 shows, the porosity and micro-hardness of Cr3C2WC-NiCoCrMo coating are 1.1±0.1% and 1153±42HV0.3 respectively. The Cr3C2-WC-NiCoCrMo coating possesses relatively lower porosity and higher micro-hardness values compared to the Cr3C2-NiCr coating under the same spraying parameters. High-hardness WC particles embedded in the matrix phase determine the higher microhardness of the Cr3C2-WC-NiCoCrMo coating. The
NiCoCrMo binder phase possibly has sufficient wettability with WC grains compared to the NiCr phase with Cr3C2 grains, this may decrease the porosity of Cr3C2-WC-NiCoCrMo coating during the HVOF spraying process. As it can be seen from Fig.4, the gray area is rich in Cr, the bright area is rich in W, so the structure of Cr3C2-WC-NiCoCrMo powder could be mostly retained in the coating. Accordingly, WC, Cr3C2, Cr7C3 and Ni-based alloy were detected in Cr3C2-WC-NiCoCrMo coating. The detected phases in Cr3C2-WC-NiCoCrMo coating are also similar with those of the powder. So the cross section of the Cr3C2-WC-NiCoCrMo coating exhibits two obviously identifiable hard phases, including Cr3C2 and WC grains (Fig. 4). The XRD pattern of the Cr3C2-WC-NiCoCrMo coating also exhibits a broad peak round 44°probably belonging to an amorphous phase. Some similar results were reported for a WC20CrC7Ni coating due to the high cooling rate when molten particles were deposited on the substrate; WC, Cr3C2 and (W,Cr)2C carbide phases were also found in the WC20CrC7Ni coating; but in this study Cr3C2, WC, Cr7C3 carbide phases are found in the Cr3C2-WCNiCoCrMo coating [10]. 3.3 High temperature wear resistance of HVOF sprayed coatings The variation of friction coefficients (COF, computed over the entire sliding distance ) and wear rates of Cr3C2-NiCr and Cr3C2-WC-NiCoCrMo coatings at different temperatures (450oC, 550oC, 650oC) are shown in Fig. 5 and Table 4 respectively. It can be observed that the Cr 3C2-WCNiCoCrMo coating possesses relatively higher COF against Al2O3 counterbody as compared with the Cr3C2-NiCr coating. Composite hard phases (WC and Cr3C2) embedded in the matrix phase (NiCoCrMo) will significantly affect the friction coefficient at elevated temperatures compared
with the single hard phase (Cr3C2) due to the formation of different oxides films on the surface of the coatings [24, 25]. The COF of Cr3C2-WC-NiCoCrMo coating decreased from 0.574±0.038 to 0.481±0.041 while the temperature increased from 450oC to 650oC. However, the COF of the Cr3C2-NiCr coating did not follow this rule under these test conditions. The Cr 3C2-NiCr coating exhibits its highest COF at 450oC, but displays the lowest COF at 550oC. It also can be seen that the Cr3C2-WC-NiCoCrMo coating exhibits much smaller wear rates than the Cr3C2-NiCr coating when tested against the alumina counterbody at 450, 550 and 650 oC (Table. 4). The wear rate of the Cr3C2-NiCr coating is more than five times higher than that of the Cr3C2-WC-NiCoCrMo coating at 450oC. At 650oC, the wear rate of Cr3C2-NiCr coating is more than three times higher than that of the Cr3C2-WC-NiCoCrMo coating. The results of wear rates indicate that the Cr3C2-WC-NiCoCrMo coating has excellent wear resistance against the Al2O3 counterpart at high temperatures compared to the Cr3C2-NiCr coating. Wear rates of both coatings are increased with rising temperature. As shown in Fig.6, wear scars are obviously observed on the worn area of both coatings after wear testing. The wear rates of Cr3C2-NiCr coating at 550oC and 650oC are very similar. This corresponds to the slight changes of COF and indicates that the Cr 3C2NiCr coating exhibits stable performance at these temperatures. However the wear rate of Cr 3C2WC-NiCoCrMo coating increases nearly two times when the temperature is increased from 450 oC to 650oC, indicating that the microstructure and properties of the coating are prone to be affected by temperature compared to Cr3C2-NiCr coating. The XRD patterns of the surface of the Cr3C2-NiCr and Cr3C2-WC-NiCoCrMo coatings after wear testing at different temperatures (450oC, 550oC, 650oC) are shown in Fig.7. It can be found that the phases of the Cr3C2-NiCr coating after wear testing at high temperatures are carbide
phases and Ni-based alloy phase. Moreover, the diffraction peaks of Cr2O3 are detected after the wear testing, indicating that the Cr3C2-NiCr coating experienced oxidation on the surface of the coating during the wear process at high temperatures especially at 650 oC (Fig.7(a)). However, the main phases of the Cr3C2-WC-NiCoCrMo coating after wear testing at different temperatures are tungsten carbide, chromium carbides and Ni-based alloy phase. The diffraction peaks of WO3, Cr2O3 are also detected by XRD after wear testing. That demonstrates that embedded tungsten carbide particles in the coating experienced oxidation when the temperature rose (Fig.7(b)). The diffraction bands of the amorphous phase of both coatings become sharper and stronger with the temperature rising; indicating that the amorphous phase crystallized to secondary carbides like previous studies [20, 26-28] have shown. That improved the matrix phase ductility and increased the hardness of the carbide based coatings, and probably improved the wear resistance of carbide based coatings at high temperatures (450-650oC) [26-28]. SEM/BSD micrographs of the cross section of Cr3C2-NiCr coating and Cr3C2-WC-NiCoCrMo coating are shown in Fig.8. The precipitation of secondary carbides occurred during the wear process at 650oC. XPS spectra of the surface of the Cr3C2-NiCr and Cr3C2-WC-NiCoCrMo coatings after wear test at 450oC are shown in Fig.9. Oxygen peaks are relatively strong after wear test at 450oC, indicating the presence of surface oxides. The high-resolution spectra of Cr2p and Ni2p of the surface of the Cr3C2-NiCr coating are shown in Fig.10. The high-resolution spectra of Cr2p, Ni2p, Co2p and W4f of the surface of the Cr3C2-WCNiCoCrMo coating are shown in Fig.11. These are the characteristic peak of Cr 2O3, CrO3, Ni(OH)2, CoO and WO3 [29-35], indicating that the outermost layers of the Cr3C2-NiCr coating mainly contain Cr2O3, CrO3 and Ni(OH)2 and the outermost layers of the Cr3C2-WC-NiCoCrMo coating mainly contain Cr2O3, CrO3, Ni(OH)2, CoO and WO3 during the high temperature wear process.
To deeply understand the wear behavior, the worn area was examined by SEM and 3D noncontact surface mapping profiler. 3D non-contact surface mapping of the worn surface of both coatings at 450, 550, 650oC are shown in Fig.12, Fig.14 and Fig.16 respectively. SEM/EDS images of the worn scar of both coatings at 450, 550, 650oC are shown in Fig.13, Fig.15 and Fig.17 respectively. It is observed that Cr3C2-WC-NiCoCrMo coating has smaller wear scar width and depth than the Cr3C2-NiCr coating and both coatings show obvious signs of grooves. This is in accordance with the wear rates results (Table 4). Also it can be seen that numerous WC particles are still embedded in the matrix phase on the worn surface of the Cr 3C2-WC-NiCoCrMo coating at 450oC (Fig.13), while at 650oC just a small amount of WC particles and some pores are retained in the worn scar surface of the Cr3C2-WC-NiCoCrMo coating after sliding wear (Fig.17). Comparatively, the worn scars of the Cr3C2-NiCr coatings present similar morphologies with some pores at different temperatures. Tribo-films are obviously observed on the worn scar surface of both coatings after the sliding wear testing at 550, 650oC. According to the EDS analysis (Fig.13, Fig.15, Fig.17), major elements, Cr, W, Ni, Co, Mo, and O were found, this suggests that these tribo-films are composed of some oxides. SEM/EDS analysis of the surface of the Al2O3 counterpart balls for the wear test of the Cr3C2-NiCr coating and the Cr3C2-WC-NiCoCrMo coating at 450, 650oC are shown in Fig.18 and Fig.19 respectively. It can be observed that the tribo-films are also found on the surface of the Al2O3 counterparts at different temperatures. When the temperature increased, the tribo-films covered more the surface of the counterpart balls. These transfer tribofilms are composed of different oxides. This result is in accordance with the composition of the tribo-films which were found on the worn surface of the coatings. That indicates that a portion of oxidized layer on the surface of the coatings transfered to the surface of the Al 2O3 ball.
3.4 Discussion Many factors affect the wear resistance of HVOF sprayed carbide based coatings [24, 36-39]. The preparation processes for both coatings for this wear test are the same. In this study, a comprehensive wear mechanism model of HVOF sprayed Cr3C2-based coatings is shown in Fig.20 to further understand the wear process at high temperatures. Due to the smooth surface of the coatings and the Al2O3 ball, for both coatings, single WC or Cr3C2 particles and also whole coating splats (which also contain hard phase particles) will be firstly cut out, removed and finally grooves and pores are formed on the surface of the coatings (Fig.13, Fig.15, Fig.17 and Fig.20). These pulled-out debris particles will act as abrasive particles leading to the severe three-body abrasive wear. What’s more, the wear process will be accelerated by forming oxides on the surface of the coatings and softening of the coatings at high temperature. During the high temperature wear process, oxide films were first formed on the surface of the Cr3C2-NiCr coating and the Cr3C2-WC-NiCoCrMo coatings. In this study, Cr2O3, CrO3 and Ni(OH)2 were formed on the surface of the Cr3C2-NiCr coating as proven by XPS (Fig.9(a), Fig.10), XRD (Fig.7(a)) and EDS analyses (Fig.13(b), Fig.15(b), Fig.17(b)). However, more oxides were formed on the surface of the Cr3C2-WC-NiCoCrMo coating during the wear process compared to the Cr3C2NiCr coating. Cr2O3, WO3, CrO3, Ni(OH)2, and CoO oxides were formed on the surface of the Cr3C2WC-NiCoCrMo coating, as further confirmed by XPS (Fig.9(b), Fig.11), XRD (Fig.7(b)) and EDS analyses (Fig.13(d), Fig.15(d), Fig.17(d)). Oxide like MoO may also form on the surface of the Al 2O3 ball (Fig.19) and on the worn tracks (Fig.13(d), Fig.15(d), Fig.17(d)) at different temperatures according to the EDS analyses. In the XRD patterns (Fig.7), the diffraction peaks of carbides and Nibased alloy phases are still clearly visible and as the most intense ones due to the low thickness of
the oxide films. The diffraction peaks of the oxides phases (Cr2O3, WO3) are clearly visible only at 650oC indicating that both coatings experience stronger oxidation at this temperature. At 450oC, the oxide based tribo-films were only found on the surface of the counterbody balls and cover a smaller fraction of the surface of the Al2O3 balls (Fig.18-Fig.19, Fig.20). With the temperature rising (550oC, 650oC), the oxides based tribo-films were found on the worn surface of both coatings and cover a lager fraction of the surface of the counterbody balls (Fig.13, Fig.15, Fig.17, Fig.18, Fig.19). During the high temperature wear process of both coatings, the contact between the hardmetal and the counterbody is not fully mediated by the oxide based tribo-films (Fig.18-Fig.19). The interposition of the oxides based tribo-films, a non-plastic medium, between the coatings surface and the Al2O3 balls prevents direct contact and decreases the friction coefficient, and the oxides based tribo-films will play an important role as lubricant to further reducing the COF of the coating [10-11, 25, 40]. The friction coefficient values produced by both of the tribo-pairs at 650oC are lower than those measured at 450oC due to the presence of tribo-films on the worn surface (Fig.13, Fig.17) and to the fact that the tribo-films cover a lager fraction of the surface of the Al2O3 counterbody (Fig.18-Fig.19). The wear rate value of the Cr3C2-NiCr coating at 550oC and 650oC is slightly higher than the wear rate value measured at 450oC. With the temperature increasing from 450oC to 650oC, the oxides layer will become thicker and more wear debris and tribo-films will be formed. The oxides based wear debris and tribo-films will act as abrasive particles leading to severe abrasive wear (Fig.20), which results in higher wear rate at 650oC. Many wear grooves, deep wear scars, pores and pulled-out particles are obviously observed on the worn surface of the Cr3C2-NiCr coating at different temperatures indicating that the Cr3C2-NiCr coating experiences abrasive wear. Similar
results are also reported in previous research: abrasion was an important wear mechanism of Cr3C2-NiCr coating when sliding at 800oC [40]. When the temperature is between 550oC and 650oC, the COF values and wear rates values of Cr3C2-NiCr coating are very close. Similar wear rates and COF values of Cr3C2-NiCr coatings at these temperatures indicate that the microstructure and the properties of the Cr3C2-NiCr coatings are stable in this range of temperature. That’s ascribed to the excellent oxidation resistance of this coatings at high temperatures (Fig.7(a)). Previous studies also reported that Cr3C2-NiCr coatings exhibited excellent oxidation resistance at 900oC [40-41]. However, due to the smaller size of the WC particles distributed in the coating, the wear process of Cr3C2-WC-NiCoCrMo coating at high temperatures is slightly changed compared to the Cr3C2-NiCr coating (Fig.20). As shown in Fig.18 and Fig.19, the oxide based tribo-films cover smaller fraction of the surface of the counterbody for the Cr3C2-WC-NiCoCrMo coating compared to that of the Cr3C2-NiCr coating. And some oxides such as WO3, which formed on the surface of the Cr3C2WC-NiCoCrMo coating, may act as solid lubricant [24]. So the Cr3C2-WC-NiCoCrMo coatings exhibit higher COF against the Al2O3 counterpart at different temperatures compared to the Cr3C2-NiCr coating. The friction coefficient of Cr3C2-WC-NiCoCrMo coating at 650oC is lower than that at 450oC, possibly because more WO3 with the structure of Magnéli phase was produced at 650oC to act as solid lubricant [24, 42]. That’s consistent with the previous studies that the COF of WC20CrC-7Ni coating decreased from 0.81±0.01 to 0.62±0.05 while the temperature increased from 400oC to 750oC and the COF of WC-10Co4Cr coating decreased from 0.64 to 0.41 while the temperature changed from 200oC to 550oC [10-11]. In addition, with the temperature increase, the area of contact between the coating surface and the Al2O3 ball become greater (Fig.19), that’s
leading to lowering of stress and thus reducing the friction coefficient [25]. That’s possible the other reason which reduce the friction coeffient of the Cr3C2-WC-NiCoCrMo coating at 650oC. The Cr3C2-WC-NiCoCrMo coating possesses higher hardness than the Cr3C2-NiCr coating and the tungsten carbide particles also have much higher hardness than chromium carbide particles. During the wear process, less binder phase will be cut due to the harder WC particles being distributed between the binder phase and chromium carbides in the coating as shown in Fig.20. It can be obviously seen that WC particles are retained in the worn surface after testing at different temperatures and the grooves on the worn surface are shorter compared to those on the Cr3C2NiCr coatings (Fig.13(b), Fig.15(b), Fig.17(b)). The morphology of the worn scar of the Cr3C2-WCNiCoCrMo coating indicates that the Cr3C2-WC-NiCoCrMo coating also experiences abrasive wear like the Cr3C2-NiCr coating. Additionally, according to the XRD patterns (Fig.7(b)) and SEM/EDS analysis (Fig.6, Fig.8) of the Cr3C2-WC-NiCoCrMo coatings, the microstructure of the Cr3C2-WCNiCoCrMo coating didn’t experienced catastrophic oxidation during this wear testing at 650oC. On top of that, tungsten carbide, chromium carbides and binder phase of the Cr3C2-WC-NiCoCrMo coating are retained after wear process at high temperatures (Fig.7(b), Fig.6). All this can significantly decrease the wear rate of the composite carbide-based coating. So the wear rate values of Cr3C2-WC-NiCoCrMo coatings are much lower compared to the Cr3C2-NiCr coatings at different high temperatures. However at 650oC, some of WC particles were pulled out and some pores were found in the worn surface of the Cr3C2-WC-NiCoCrMo coating (Fig.17(b) and Fig.20) due to the excessive oxidation (Fig.6, Fig.7(b)). These pulled out particles will act as abrasive particles and the oxide film will break down, that resulted in the highest wear rate at 650oC. Pure WC-Co or WC-CoCr coatings
were indeed reported to experience catastrophic oxidation at high temperature, and this will result in poor wear resistance [13, 43]; however, the Cr3C2-WC-NiCoCrMo coating still exhibited better wear resistance at 650oC in this study due to the WC and Cr3C2 particles embedded in the matrix of NiCoCrMo. Although the Cr3C2-WC-NiCoCrMo coating exhibits good results in high temperatures (450650oC) wear tests, the coating needs to be tested further for wear combined with corrosion conditions at high temperatures. 4. Conclusion In the present study, Cr3C2-WC-NiCoCrMo and Cr3C2-NiCr spray powders were prepared by an agglomeration and sintering process. The basic properties of the powders and the coatings were investigated. Especially the high temperature sliding wear resistance of both coatings was evaluated at 450 oC, 550 oC and 650 oC. The conclusions are summarized as follows: ● The Cr3C2-WC-NiCoCrMo spray powder exhibits higher apparent density (3.6 g/cm3) and
flow-ability (25 s/50g) compared to the Cr3C2-NiCr powder. ● The Cr3C2-WC-NiCoCrMo coating exhibits lower porosity (1.1±0.1%) and higher micro-
hardness (1153±42 HV0.3) compared to the Cr3C2-NiCr coating. Major phases of Cr3C2, WC, Cr7C3 are detected in both Cr3C2-WC-NiCoCrMo powder and coating. ● The Cr3C2-WC-NiCoCrMo coating produces relatively higher friction coefficients (COF)
compared to the Cr3C2-NiCr coating when tested against an Al2O3 counterbody at high temperatures (450oC, 550oC, 650oC) in this testing conditions.
● Cr3C2-WC-NiCoCrMo coatings show lower wear rates compared to the Cr 3C2-NiCr coatings at
this range of temperature (450oC-650oC). The wear rates of the Cr3C2-WC-NiCoCrMo coating increased by nearly two times when the temperature increased from 450 oC to 650oC, but the wear rates of the Cr3C2-NiCr coating are roughly the same, indicating that the microstructure and properties of the Cr3C2-WC-NiCoCrMo coating are prone to be affected at higher temperature (550oC-650oC) compared to the Cr3C2-NiCr coating. ● Both coatings experience abrasive wear during the high temperatures wear process. Oxides
like Cr2O3, WO3, CrO3, Ni(OH)2, CoO was initially formed on the surface of the surface of the Cr3C2WC-NiCoCrMo coating at 450oC. ● After wear testing at 650oC, although some of the WC particles are pulled out and some
pores are found in the worn surface of the Cr3C2-WC-NiCoCrMo coating, the Cr3C2-WC-NiCoCrMo coating nevertheless exhibits superior wear resistance compared to the Cr3C2-NiCr coating at this temperature. Acknowledgement We would like to acknowledge the financial support from Natural Science Foundation of Guangdong Province (2016A030312015), Guangdong Science and Technology Programming (No. 2013B050800031, No. 2014B050502008 and No. 2015B07070124). REFERENCE [1] K. Szymański, A. Hernas, G. Moskal, H. Myalska, Thermally sprayed coatings resistant to erosion and corrosion for power plant boilers- A review, Surf. Coat. Tech. 268 (2015) 153-164. [2] L. Pawlowski, The Science and Engineering of Thermal Spray Coatings: Second Edition, 2008
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Figures Fig.1 Optical micrographs of the polished cross section of (a) Cr3C2-NiCr powder and (b) Cr3C2-WCNiCoCrMo powder. Fig.2 XRD patterns of (a) Cr3C2-NiCr spraying powder and coating and (b) Cr3C2-WC-NiCoCrMo spraying powder and coating. Fig.3 (a) Optical and (b) SEM micrograph of the polished cross section of the Cr3C2-NiCr coating Fig. 4 (a) Optical and (b) SEM/EDS micrograph of the polished cross section of the Cr3C2-WCNiCoCrMo coating. Fig.5 Friction coefficients of Cr3C2-NiCr and Cr3C2-WC-NiCoCrMo coating at different temperatures. Fig.6 SEM/EDS analysis across the wear scar cross section of (a) Cr3C2-NiCr coating and (b) Cr3C2WC-NiCoCrMo coating after sliding wear tests at 650oC. Fig.7 XRD patterns of the surface of the (a) Cr3C2-NiCr coatings and (b) Cr3C2-WC-NiCoCrMo coatings after wear testing at different temperatures. Fig.8 SEM/BSD micrographs of the cross section of (a) Cr3C2-NiCr coating and (b) Cr3C2-WCNiCoCrMo coating after wear testing at 650oC. Fig.9 Full XPS spectra of the surface of the (a) Cr3C2-NiCr coating and (b) Cr3C2-WC-NiCoCrMo coating after wear testing at 450oC. Fig.10 XPS high-resolution spectrum of (a) Cr and (b) Ni element of the surface of the Cr3C2-NiCr coating after wear test at 450oC. Fig.11 XPS high-resolution spectrum of (a) Cr, (b) Ni, (c) Co, (d) W element of the surface of the Cr3C2-WC-NiCoCrMo coating after wear testing at 450oC. Fig.12 3D profiles of HVOF coatings after wear testing at 450oC. (a) Cr3C2-NiCr coating, (b) Cr3C2WC-NiCoCrMo coating. Fig.13 SEM/BSD micrographs of the wear scars of (a) , (b) Cr3C2-NiCr coating and (c), (d) Cr3C2-WCNiCoCrMo coating after wear testing at 450oC.
Fig.114 3D profiles of HVOF coatings after wear testing at 550oC. (a) Cr3C2-NiCr coating, (b) Cr3C2WC-NiCoCrMo coating. Fig.15 SEM micrographs of the wear scars of (a), (b) Cr3C2-NiCr coating and (c), (d) Cr3C2-WCNiCoCrMo coating after wear testing at 550oC. Fig.16 3D profiles of HVOF coatings after wear testing at 650oC. (a) Cr3C2-NiCr coating, (b) Cr3C2WC-NiCoCrMo coating. Fig.17 SEM/BSD micrographs of the wear scars of (a), (b) Cr3C2-NiCr coating and (c), (d) Cr3C2-WCNiCoCrMo coating after wear testing at 650oC. Fig.18 SEM/EDS micrographs of the surface of the Al2O3 counterpart balls for the Cr3C2-NiCr coating wear testing at (a) 450oC and (b) 650 oC. Fig.19 SEM/EDS micrographs of the surface of the Al2O3 counterpart balls for the Cr3C2-WCNiCoCrMo coating wear testing at (a) 450oC and (b) 650 oC. Fig.20 Schematic diagram of the wear mechanism of Cr3C2-NiCr and Cr3C2-WC-NiCoCrMo coatings.
Fig.1 Optical micrographs of the polished cross section of (a) Cr3C2-NiCr powder and (b) Cr3C2-WCNiCoCrMo powder
Fig.2 XRD patterns of (a) Cr3C2-NiCr spraying powder and coating and (b) Cr3C2-WC-NiCoCrMo spraying powder and coating (b)
Fig.3 (a) Optical and (b) SEM micrograph of the polished cross section of the Cr3C2-NiCr coating Composition(
(b)
wt.%) Cr: 60.6 Ni: 7.0 Composition( wt.%) W: 84.1 Cr: 1.7
Co: 3.8 W:7.5 Mo:2.3
Fig. 4 (a) Optical and (b) SEM/EDS micrograph of the polished cross section of the Cr3C2-WC-NiCoCrMo coating
Fig.5 Friction coefficients of Cr3C2-NiCr and Cr3C2-WC-NiCoCrMo coating at different temperatures.
(a)
(b)
13.2 wt. % O
3.0 wt. % O
14.3 wt. % O
3.5wt. % O
Fig.6 SEM/EDS analysis across the wear scar cross section of (a) Cr3C2-NiCr coating and (b) Cr3C2-WCNiCoCrMo coating after sliding wear tests at 650oC.
(a)
Fig.7 XRD patterns of the surface of the (a) Cr3C2-NiCr coatings and (b) Cr3C2-WC-NiCoCrMo coatings after wear testing at different temperature.
(a)
(b) Secondary carbides Secondary carbides
Fig.8 SEM/BSD micrographs of the cross section of (a) Cr3C2-NiCr coating and (b) Cr3C2-WC-NiCoCrMo coating after wear testing at 650oC.
(a)
(b)
Fig.9 Full XPS spectra of the surface of the (a) Cr3C2-NiCr coating and (b) Cr3C2-WC-NiCoCrMo coating after wear testing at 450oC.
(a)
(b)
Fig.10 XPS high-resolution spectrum of (a) Cr and (b) Ni element of the surface of the Cr3C2-NiCr coating after wear testing at 450oC.
(a)
(b)
(d)
(c)
Fig.11 XPS high-resolution spectrum of (a) Cr, (b) Ni, (c) Co, (d) W element of the surface of the Cr3C2-WCNiCoCrMo coating after wear testing at 450oC.
(a)
(b)
Fig.12 3D profiles of HVOF coatings after wear testing at 450oC. (a) Cr3C2-NiCr coating, (b) Cr3C2-WCNiCoCrMo coating
(a)
(b)
Pore Carbides
Composition( wt.%) C：16.6 Cr: 58.3
Grooves
Ni: 21.7 O: 3.4
(c)
(d)
Composition( wt.%) C：14.5
Carbides
Cr: 28.8 Ni: 6.2
Grooves
O: 6.3 Mo: 2.2
Fig.13 SEM/BSD micrographs of the wear scars of (a), (b) Cr3C2-NiCr coating and (c), (d) CrCo: 3C2-WC-NiCoCrMo 1.6 coating after wear testing at 450oC. W:40.4
(b)
(a)
Fig.14 3D profiles of HVOF coatings after wear test at 550oC. (a) Cr3C2-NiCr coating, (b) Cr3C2-WC-NiCoCrMo coating
(b)
(a)
Tribofilm (wt.%)
Carbides
C：9.7 Cr: 43.0
Grooves
Ni: 12.7 O: 34.6
(d) Carbides
(c)
Pore Carbides
Tribofilm (wt.%)
Carbides C：6.6 Cr: 23.0
Grooves
Ni: 5.8 O: 29.3 Mo: 2.2 Co: 1.9 Fig.15 SEM/BSD micrographs of the wear scars of (a), (b) Cr3C2-NiCr coating and (c), (d) Cr3C2-WC-NiCoCrMo
coating after wear testing at 550oC.
(a)
W: 31.2
(b)
Fig.16 3D profiles of HVOF coatings after wear testing at 650oC. (a) Cr3C2-NiCr coating, (b) Cr3C2-WCNiCoCrMo coating
(b) Tribofilm
(a)
(wt.%)
Carbides
C：5.0
Grooves
Cr: 54.9 Ni: 4.2
Pore O: 35.9
(c)
Grooves
(d)
Tribofilm (wt.%) C：5.6
Pore
Cr: 20.0
Carbides
Ni: 5.5 O: 34.5 Mo: 2.1
Fig.17 SEM/BSD micrographs of the wear scars of (a) , (b) Cr3C2-NiCr coating and (c), (d) Cr3C2-WCCo: 1.8 NiCoCrMo coating after wear testing at 650oC.
(a)
Tribofilm
(b)
W: 30.5 Tribofilm
(wt.%)
(wt.%)
C：5.2
C：14.0
Cr:37.0
Cr: 55.5
Ni: 16.8
Ni: 11.3
O: 37.2
O: 19.2
Al: 3.8
Fig.18 SEM/EDS analysis of the surface of the Al2O3 counterpart balls for the Cr3C2-NiCr coating wear testing at (a) 450oC and (b) 650 oC.
(a)
Tribofilm (wt.%)
(b)
C：5.2
Tribofilm (wt.%) C：22.8
Cr:23.4
Cr: 23.4
Ni: 4.6
Ni: 5.5
Co: 1.9
Co: 1.7
W:27.3
W: 21.3
Mo:1.3
Mo: 0.8
Fig.19 SEM/EDS analysis of the surface of the Al2O3 counterpart balls for the Cr3C2-WC-NiCoCrMo coating O: 32.6 at (a) 450oC and (b) 650 oC. wear testing
O: 24.5
Al: 3.6
Fig.20 Schematic diagram of the wear mechanism of Cr3C2-NiCr and Cr3C2-WC-NiCoCrMo coatings
Table Table 1 HVOF spraying parameters Powder
Cr3C2-NiCr ,Cr3C2-WC-NiCoCrMo
Gun
DJ2600
Hydrogen(L/min)
635
Oxygen(L/min)
280
Air(L/min)
345
Carrier gas flow rate ( Ar, L/min)
30
Powder feed rate (g/min)
40
Surface traverse velocity(m/min)
90
Spraying distance(mm)
230
Offset(mm)
3
Table 2 Wear test conditions Parameters
Values
Ball material
Al2O3
Diameter of Ball
6.00mm
Load
10.00N
Radius
5.00mm
Lin. Speed
0.25m/s
Sliding distance
600m
Acquisition rate
6.5HZ
Test temperature
450oC, 550oC,650oC
Table 3 Characteristics of coatings Coating
Hardness(HV0.3)
Porosity(%)
Cr3C2-NiCr
1029±94
1.4±0.1
Cr3C2-WC-NiCoCrMo
1153±42
1.1±0.1
Table 4 Sliding wear resistance of Cr3C2-NiCr and Cr3C2-WC-NiCoCrMo coatings
Sample
Temperature (oC)
Mean wear scar depth (μm)
Wear rate(10-6 mm3/Nm)
Friction coefficient
Cr3C2-NiCr
450
6.8±0.3
33.3±1.6
0.474±0.034
Cr3C2-NiCr
550
8.7±0.2
36.8±2.5
0.419±0.034
Cr3C2-NiCr
650
15.4±0.3
37.2±2.1
0.427±0.032
Cr3C2-WC-NiCoCrMo
450
2.1±0.2
6.3±0.5
0.574±0.038
Cr3C2-WC-NiCoCrMo
550
2.5±0.3
7.8±0.3
0.508±0.043
Cr3C2-WC-NiCoCrMo
650
5.7±0.2
11.5±0.3
0.481±0.041