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Microstructure and properties of cold sprayed multimodal WC–17Co deposits
Int. Journal of Refractory Metals and Hard Materials 45 (2014) 196–203
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Microstructure and properties of cold sprayed multimodal WC–17Co deposits Hong-Tao Wang a,b, Xiao Chen a,b, Xiao-Bo Bai a,b, Gang-Chang Ji a,b,⁎, Zeng-Xiang Dong a,b, Deng-Liang Yi a a b
Jiujiang Key Laboratory of Green Remanufacturing, 332005 Jiujiang, China School of Mechanical & Materials Engineering, Jiujiang University, 332005 Jiujiang, China
a r t i c l e
i n f o
Article history: Received 17 December 2013 Accepted 19 April 2014 Available online 30 April 2014 Keywords: Multimodal WC–17Co WC size Cold spraying Microstructure Properties Abrasive wear
a b s t r a c t In the present paper, three multimodal WC–17Co deposits with different content of nano-sized WC (10%, 30% and 50%, in wt.%) were prepared by cold spraying process. The effect of micro/nano ratio of WC particles on the microstructure, phase composition and properties such as microhardness, fracture toughness and wear performance of cold-sprayed multimodal WC–12Co coating was investigated. The experimental results demonstrated that the cold-sprayed multimodal WC–17Co deposits exhibited dense microstructure with some stripes composing of Co and nano WC, and their phase structures were similar to that of feedstock. The microhardness and fracture toughness of cold sprayed multimodal WC–17Co deposits respectively increased and decreased with increasing the content of nano-sized WC. The obtained multimodal WC–17Co deposits had both a high hardness and high fracture toughness and exhibited a greatly enhanced wear resistance in comparison to the conventional high-velocity oxy-fuel (HVOF) sprayed WC–12Co coating. The abrasive wear behavior of cold sprayed multimodal WC–17Co deposits was mainly dominated by cutting or gouging related to the preferential wear of Co and the fracture and the peel-off of WC particle. © 2014 Elsevier Ltd. All rights reserved.
Introduction WC–Co cermet was commonly used as surface hardening and enhancing materials for improving the sliding, abrasive and erosion wear resistance of components. It was known that the mechanical properties of WC–Co cermet were related to several factors such as WC size and Co content [1–6]. Usually, on the one hand, it was considered that the fracture toughness of WC–Co cermets would increase with the increase of WC size and the Co content [2,4,6]. On the other hand, the hardness of these materials may be increased as the WC size and the Co content decreased [1,3,6]. The WC–Co cermet with fine WC particles was considered exhibiting better wear performance due to its higher hardness [5]. However, it is argued by Konyashin [7] that the higher hardness and the higher fracture toughness may be necessary to the application of WC–Co hard metals in mining operation. Therefore, it would be valuable to consolidate the fracture toughness and hardness of WC–Co cermets by a suitable selection of WC size and shape [2]. In order to improve the wear performance of components, it would encounter the hard and tough cermet which was commonly deposited on their surface. High Velocity Oxy-fuel Spray (HVOF) process was considered as the popular way to deposit WC–Co cermet coatings due to its lower temperature and short dwell time of particles within flame ⁎ Corresponding author at: School of Mechanical and Materials Engineering, Jiujiang University, Jiujiang, Jiangxi 332005, PR China. Tel./fax: + 86 792 8334039. E-mail addresses: [email protected], [email protected] (G.-C. Ji).
http://dx.doi.org/10.1016/j.ijrmhm.2014.04.018 0263-4368/© 2014 Elsevier Ltd. All rights reserved.
compared to air plasma spraying (APS), which would limit dissolving of WC [8–10]. It was reported that the dense and wear resistant HVOF WC–Co coatings can be deposited by using suitable powder [9] and in optimized spray condition [10,11]. In order to improve the enhancing effect of WC particles on hardness and the wear resistance of WC–Co materials, J.M. Guilemany et al. [12] have deposited WC–Co coating with nanometer and micrometer WC particles by HVOF process and found that this bimodal coating showed the improved abrasive and friction wear resistance. Skandan G et al. [13,14] researched the properties of HVOF sprayed multimodal WC–Co coatings with powders containing 30% nano WC and 70% micro WC and found that there was a significant improvement of wear performance for these coatings. However, it was reported that the decarburization, dissolution of WC and the formation of brittle η phases (CoxWyC) were also liable to occur during HVOF spraying nanostructured WC–Co [12,15,16] and multimodal WC–Co [13], which was detrimental to the fracture toughness of these thermal sprayed coatings. Moreover, because of the serious decarburization, the wear resistance of HVOF sprayed nanostructured WC–Co coatings was not superior compared with that of conventional ones [17–20]. Therefore, the detrimental effect of decarburization and dissolving of nano WC on the fracture toughness and abrasive wear performance of HVOF sprayed WC–Co deposits would blur the contribution of nano WC for improving wear performance of multimodal WC–Co coatings. Cold spray was a recently developed low temperature coating deposition process using powder feedstock. The coatings are formed though plastic deformation of the particles in completely solid state during
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impact with high velocity. Owing to its relative lower temperature of particles, the cold-spray process has a significant advancement to the spraying nanostructured WC–Co coatings [21–27]. Researches on the deposition of WC–Co particles using different types of powders demonstrated that the particles with small WC [25] and porous structure [27] may exhibit better deformation ability in cold spray and be liable to form dense coatings. The WC–Co coatings with high hardness and improved wear resistant can be deposited by cold spraying of the nanostructured WC–Co powders. However, considering the inherent decrement in fracture toughness with reduction of WC size [2,4] and the high cost for fabrication of nano WC powders [14], it was expected to deposit WC–Co coatings with multimodal WC in order to get high fracture toughness and better wear performance. It was well known that the microstructure of cold sprayed deposits was greatly influenced by the plastic deformation of incident particles and pre-deposited particles. For WC–Co cermets, WC size, Co content and particle density were the key structural factors influencing the deformation of particles and the microstructure of WC–Co deposits [25–27]. However, report about the effect of multimodal WC particles on microstructure of WC–Co deposits was very limited. The fracture toughness, microhardness and abrasive wear properties of cold sprayed multimodal WC–Co coating were not clear. In present study, the multimodal WC–17Co powders with different content of nano WC were fabricated by blending the agglomerated nano WC–17Co powders with micro WC–17Co powders in different proportion. The microstructure and phase constitution of cold sprayed multimodal WC–Co deposits were characterized. The microhardness, fracture toughness and abrasive wear performance of these deposits were experimentally investigated. The effect of WC size on the microstructure formation and properties of the deposits was discussed. Experimental materials and procedures Feedstock used in this experiment was commercially available multimodal WC–17Co powder produced by Ganzhou Zhangyuan Tungsten Advance Material Co., Ltd. These powders were fabricated by blending the agglomerated-sintered nano WC–17Co and micro WC–17Co powder. As shown in Table 1, three types of the powders contained about 10%, 30% and 50% nano WC–17Co were used in this study. In addition, the particles size and microhardness were also given in Table 1. The surface morphologies of three feedstocks and cross sectional microstructure of the micrometer and nanometer WC–17Co powder were shown in Fig. 1, respectively. It can be seen that all feedstock had global morphology, as shown in Fig. 1a–c. The micrometer WC–17Co particles were mainly composed of micro WC (seen in Fig. 1d) and the nano and near nano WC existed in nano WC–17Co particles (seen in Fig. 1e). The particle size distribution of feedstock was analyzed by a laser particle size analyzer (Malvern Mastersizer 2000, Malvern Instrument Ltd. UK). The tested result (as shown in Fig. 2) demonstrated that the actual particle size was about 4–25 μm and d(0.5) was about 13 μm, as shown in Table 1. Stainless steel with dimensions of 45 × 20 × 3 mm was used as a substrate for depositing coating and blasted with 20 mesh alumina before depositing. A cold spraying system (CS-2000) developed in Xi'an Jiaotong University was used for depositing coatings in the condition as shown in Table 2. A spray gun with a converging–diverging de Laval type nozzle of a throat
Fig. 1. Morphologies and cross sectional microstructure of multimodal WC–17Co powder. a–c: surface morphologies of powders A, B and C; d–e: cross sectional microstructure (d: micrometer WC–17Co; e: nanometer WC–17Co).
diameter of 2 mm was adopted. The exit diameter and downstream length of nozzle were 6 mm and 100 mm, respectively. The nitrogen gas was used as accelerating and powder feed gas. The pressure was kept at 2.4 MPa for accelerating gas and 2.6 MPa for powder feed gas, respectively. The accelerating gas was preheated and the temperature of gas in the gun pre-chamber was kept at 750 ± 30 °C. The distance from the nozzle exit to the substrate surface was about 20 mm. The spray gun was kept stationary during deposition processes in order to prepare thick deposits for performance test. Microstructure of powders and deposits, and the morphology of worn surface of deposits were examined by SEM (Tescan Vega II LSU). The phase constitution of both feedstock and as-spayed deposits was characterized by XRD (Bruker D8 advance model) with Cu Kα radiation
Table 1 Powders used in cold spraying. Powder type
Proportion of nano to micro WC–17Co
Particle size (μm)
d(0.5) (μm)
Manufacture process
Microhardness (Hv0.1)
A B C
10/90 30/70 50/50
+4–25 +4–25 +4–25
13.465 13.148 13.077
Agglomerated and mixed Agglomerated and mixed Agglomerated and mixed
137 ± 25 197 ± 28 168 ± 36
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The elastic modulus was determined according to the following formula as established in [29]: 0 0 b =a ¼ ðb=aÞ−α ðHv =EÞ
ð2Þ
Where, a and b were the diagonal dimensions of Knoop indenter. a′ and b′ were the long and short diagonal dimensions measured on residual Knoop indentation impression. α was a constant equal to 0.45. Hv was the Vickers hardness. The abrasive wear resistance of the coating was evaluated using a pin-on-disk tester (Model ML-100, Zhangjiakou Chengxin Balance Testing Machineries Company, China) in the conditions as shown in Table 3. The pin sample was 4 mm in diameter and the coating was deposited on the end surface of the pin with a thickness about 500 μm. Before the tests, the coating surface was polished using an abrasive paper of 1000 grit in order to reduce the error caused by the different coating surface topography. During the tests, the pin specimen was loaded against a rotating disk covered with a SiC abrasive paper of 300 grit. The applied load was 10 N and the wear distance was 16 m. The rotation speed of the disk was 60 rpm and the radial feed rate of the pin was 4 mm/r. In order to ensure a fresh supply of abrasive particles, worn SiC abrasive paper was replaced after each test (total running distance: 16 m). The weight loss of the pin before and after wear was measured using an analytical balance with a precision of 0.0001 g. The weight loss of three samples was averaged for evaluating the abrasion resistance of the coating. Results and discussions Microstructure of cold sprayed multimodal deposits Fig. 2. Particle size distribution of feedstock (a, b and c corresponding to A, B and C feedstock in Table 1).
at 35 kV and 35 mA. Scanning speed and scanning step were 2°/min and 0.01°, respectively. The microhardness of original powder was tested using the HVS1000 Vickers Microhardness Tester with a load of 0.98 N and dwell time of 10 s. The microhardness of the deposits was tested under a load of 2.94 N and a dwell time of 20 s in all tests. The microhardness was evaluated using the average value of random ten point tests in the cross section of both powder particles and deposits sample. The plain strain fracture toughness, i.e. KIC, was determined based on an indentation method. A Vickers indenter was used on metallographically prepared cross-sections of the coatings with a 5 kgf load and dwell time of 20 s. The fracture toughness (KIC) was calculated by applying the following equation proposed by Niihara [28]: 2=5 −1=2
K IC ¼ 0:0193ðHνdÞðE=HvÞ
ð1Þ
a
Where KIC is the fracture toughness, Hv the Vickers hardness, E the elastic modulus, d the half-diagonal of the Vickers indentation and a is the indentation crack length. At least five indentations were made for each sample.
The microstructure of three cold sprayed multimodal WC–17Co deposits using powders A, B and C (as indicated in Table 1) were shown in Fig. 3. For convenience, three deposits were denoted as deposits A, B and C, respectively. It was found that three deposits presented dense microstructure with a little visible pore and theirs thickness varied between 0.5 and 2 mm. Compared with cross sectional microstructure of the feedstock as shown in Fig. 1d and e, it was recognized that some dense wavy stripes (indicated by arrows in Fig. 3b, e and h) that did not exist in feedstock appeared in the deposits. By close examination (in Fig. 3c, f and i), it was found that these wavy stripes were mainly composed of nano or near nano WC (bright particle in contrast) and Co (gray area in contrast) as indicated by arrows in Fig. 3f. Moreover, it can be seen that the WC particles within these stripes presented some orientation (as shown in Fig. 3c and f). This microstructure change indicated that there occur some rearrangement of WC and deformation of Co during deposition processes. Alkhimov et al. [30,31] reported that there existed a regional temperature rise and a high pulse-pressure after the shock wave leaving the contact area of impacting particles, which would lead to regional adiabatic shear instability and severe deformation of splats at its periphery in the form of ejections. Gao et al. [32] researched the deformation behavior of single WC–12Co particle and found that the manifest densification occurred near the impact interface in the form of slippage of small carbide particles.
Table 3 Abrasive wear parameters.
Table 2 Cold spraying parameters. Diameter of throat (mm)
2
Exit diameter of nozzle (mm) Pressure of propellant gas (MPa) Pressure of powder feed gas (MPa) Temperature of propellant gas in gun chamber (°C) Distance for exit of nozzle to substrate (mm)
6 2.4 2.6 750 ± 30 20
Wheel diameter (mm)
280
Rotation of wheel (r/min) Diameter of specimen (mm) Applied load (N) Radial feed rate of specimen (mm/r) Abrasive paper Duration of test (m)
60 4 10 4 300# SiC 16
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deformation of splats resulted from the sliding of small WC and plastic deformation of Co. Because the severe deformation was liable to happen on the periphery of splats close to impact interface, the dense stripes appeared in the area near impacting interface. Afterwards, the successive incident particles would strike the surface of pre-deposited WC–Co splats and form new dense stripes near the contact interface of up WC–Co splats and underlying WC–Co splats. In this case, the combined effects of hammering and deformability of particles would result in the formation of stripes. Owing to the better self-deformability of nano WC–17Co particle resulted from easier movement of nano WC, it would be believed that the dense stripes mainly formed through the deformation of nano WC–17Co particles on impact. In addition, the hammering effect of micro WC–17Co particles on impact may enhance the deformation of underlying nano WC–17Co splats in cold spraying due to the prominent peening effect of large WC as it happened in HVOF sprayed WC–Co [33]. By close examination on microstructure of deposits, it was revealed that the morphology of WC particles within the as-sprayed deposits exhibited unsmooth profile with an angular feature which is different from that existed in HVOF sprayed coating [12]. This unsmooth profile indicated that there was no surface melting of WC particle in cold spay as occurred in HVOF spraying. However, comparing with the shape of WC within original powder (as shown in Fig. 1d), it was revealed that the sharp angles of micro WC particles presented in original powder became blunt after deposition. The size of micro WC particles within as sprayed deposits seems slightly reduced. It was argued by Andrew [25] that, in cold spraying WC–Co, the high velocity impacting between the brittle WC particles would cause their fracture and splat breakup, and consequently leads to the formation of fine grains. Meanwhile, the scrubbing of WC particles during deposition would cause the blunting of shape angle. Therefore, the refinement of WC incurred by fracture of large WC on impact may be the reason for size reduction of WC particles within as-sprayed multimodal WC–Co coating. By comparing the microstructure of the three deposits, it was found that deposit A had the relative higher content of micro WC and deposit C had the relative higher content of nano WC. This was related to the different content of nano WC particles in three feedstocks. Moreover, it was recognized that deposit C exhibited the most dense microstructure compared with that of other deposits. As mentioned before, the nano WC–17Co particles would exhibit improved self-deformability due to the sliding of nano WC. Therefore, the deformation or densification of nano WC–17Co powders caused by impact and strike from other incident particles may be manifest compared to that of micro WC–17Co under in identical spray condition. As a result, deposit C showed a denser microstructure compared to that of other deposits.
Phase constitution of cold-sprayed multimodal deposits
Fig. 3. Microstructure of the cold-sprayed multimodal WC–17Co deposits. (a), (b), (c) for deposit A; (d), (e), (f) for deposit B and (g), (h), (i) for deposit C.
For building-up of WC–Co deposits, there required a successive deposition of WC–Co particles. The first layer deposit was formed by deformation of splats and stainless steel substrate. In this stage, the apparent
Fig. 4 shows the XRD patterns of three cold-sprayed multimodal WC–17Co deposits and their feedstocks. It was found that only the peaks of WC and Co phase existed in the pattern of as-sprayed deposits that was same to that of feedstock. This fact suggested that no phase transformation occurred in cold spraying of multimodal WC–17Co in our experiment condition. Nevertheless, some humped or broadened peaks appeared in the pattern of these deposits. It was known that peak broadening can be attributed to either a strain effect or refinement of particle size [25]. Li et al. [24] researched the effect of atomic level strain within cold sprayed nano WC–12Co coating by analyzing the microhardness variation before and after heat treatment at 1000 °C for 6 h and concluded that the effect of the atomic level strain on the broadening of XRD diffraction peaks was limited due to no obvious microhardness change. As for cold sprayed multimodal WC–17Co, buildingup of deposit would encounter the impact of WC–17Co particles on pre-deposited WC–17Co layer. The severe striking of high velocity incident particles could cause the fracture of brittle WC [25,27], which would lead to the refinement of WC particles. Therefore, it was reasonable
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the increment of FWHM (defined as Formula 3) of WC peaks in XRD patterns during deposits formation. Δ ¼ WWCd −WWCp
Fig. 4. XRD patterns of cold sprayed multimodal WC–17Co deposits and feedstock. (a) feedstock and (b) deposits.
to consider that the broadening of WC peaks in X-ray pattern was related to the grain refinement. In order to characterize the refinement of WC during deposition, the full width at half maximum (FWHM) of WC peaks was evaluated by quantitative analysis of the XRD patterns of the deposits. Fig. 5 shows
ð3Þ
Where, △ is the increment of FWHM of WC peaks during deposits formation; WWCd and WWCp are respectively the FWHM of strongest peaks of WC in deposits and corresponding feedstock. It was revealed that the FWHM of WC peaks in feedstock (Powders A, B, C) were 0.18, 0.18 and 0.17, respectively. However, the increment of FWHM of three WC–17Co deposits, A, B and C, were 0.32, 0.33 and 0.29, respectively. Therefore, it would be recognized that the broadening of WC peak in XRD pattern was obvious. It was well known that the preferential rebounding of large WC in HVOF sprayed WC–Co deposit would result in the reduction of WC content. For cold spraying multimodal WC–Co, the deposits were formed through deformation of the particles in completely solid state. Therefore, it was interesting to investigate the variation of WC content during deposit formation. Owing to no phase transformation in cold spraying of WC–Co, it would be reasonable to evaluate the change of WC content by comparison of W content in as-sprayed deposits and feedstock. The EDA analysis results (as shown in Table 4) indicated that the content of W in assprayed deposits was slightly reduced compared to feedstock. This phenomenon may demonstrate that there existed a little loss of WC during cold spraying of multimodal WC–17Co. However, there was no manifest difference of W content in three deposits. These results may imply that the micro and nano WC and Co possessed the almost same deposition efficiency in cold spraying which was different from that in HVOF spray WC–Co. It was known that the WC–Co particle in HVOF spray processes was presented in solid/liquid state. The rebound off of solid WC particle when impacting would result in the loss of WC. Nevertheless, for cold spraying of WC–Co, the deposits were formed though deformation of the particles in completely solid state. The preferential rebound off of WC was liable to take the Co binder along due to tightly bonding between Co and WC in agglomerate-sintered feedstock, which lead to the simultaneous reduction of both WC and Co. Consequently, there was no obvious variation of W content of during coating formation. Properties of cold sprayed multimodal deposits Considering that the WC–Co are mainly used to improving wear performance of components, the microhardness, fracture toughness and abrasive wear of multimodal WC–17Co deposits were experimentally investigated. Fig. 6 shows the microhardness of three multimodal WC–17Co deposits (A, B, C). It was revealed that the microhardness (Hv0.3) of these deposits increased from 1316 ± 80 Kgf/mm 2 to 1625 ± 115 Kgf/mm2 which was higher than that of corresponding powder (as shown in Table 1), and that of the cold sprayed nano WC– 17Co deposits (11.91 ± 0.35 GPa) reported by Andrew Siao Ming Ang [25] and HVOF sprayed multimodal WC–12Co (820 VHN) reported by Skandan et al. [13]. The higher hardness of cold sprayed multimodal WC–17Co deposits would be attributed to its high content of nano WC and high density. In addition, it was found that the deposits C exhibited the higher microhardness. This fact would be ascribed to the higher fraction of nano WC particles and dense microstructure of this deposits resulted from enhanced plastic deformation of cold sprayed splats [25,34]
Table 4 W content in deposits and original powder (wt.%).
Fig. 5. FWHM increment of WC peak in XRD patterns of cold sprayed multimodal WC–17Co deposits.
Wp Wc Wc/Wp
A
B
C
87.01 84.71 0.97
86.34 84.08 0.97
87.08 83.36 0.96
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Fig. 6. Microhardness of cold sprayed multimodal WC–17Co powder and its deposits.
201
Fig. 8. Fracture toughness of cold sprayed multimodal WC–17Co deposits.
The fracture toughness of the multimodal WC–17Co deposits was measured by the indentation methodology as described above. The typical indentation mark of the toughness measurements was shown in Fig. 7. It was seen that cracks arised from the tip of the indentation diagonal and propagated in the direction paralleled to coating surface. There was no visible crack in the direction perpendicular to coating surface. These experiment results showed that the deformation of the particles during cold spraying in this study is much less significant than HVOF. However, from Fig. 3, obvious flattening of the deposited particles was observed. Therefore, one can reasonably expect that the anisotropic performance was due to the lamella microstructure of the deposits. Fig. 8 shows the fracture toughness obtained by indenting on cross section of cold sprayed multimodal WC–17Co deposits. It was demonstrated that the indentation fracture toughness of three deposits decreased from 14.52 ± 3.39 MPa m1/2 to 10.95 ± 3.05 MPa m1/2 with increasing content of nano-sized WC, which was comparable to that of sintered WC–Co composites (9–20 MPa m1/2) reported in [35] and higher than that of HVOF spayed WC–17Co deposits (3–5 MPa m1/2) [36]. The higher fracture toughness of cold sprayed multimodal WC– 17Co deposits may partially be attributed to its dense microstructure and the absence of brittle phase (η phase) compared to that of HVOF sprayed WC–Co coatings [11,14,15]. The deflection in crack growth afforded by different size of WC and the increment of Co mean free path due to the existence of micro WC may also be liable to result in the higher fracture toughness for multimodal WC–17Co deposits. In
addition, it was found that the indentation fracture toughness of cold sprayed multimodal WC–17Co deposits slightly increased with the decrease of nano WC–17Co in feedstock, which was consistent with the influence of WC size on fracture toughness of sintered WC–Co composites. However, the decrement of fracture toughness with the increment of nano WC was not such obvious as expected. This fact can be ascribed to the absence of brittle phase in cold sprayed WC–Co deposits compared to HVOF sprayed WC–Co deposit. Based on the results as presented above, one can conclude that cold sprayed multimodal WC–17Co deposits exhibited both a high microhardness and relatively high fracture toughness. The differently sized WC particles play synergetic roles on enhancing simultaneously the mechanical properties of the WC–Co deposit. That is, small WC particles exert a more efficient reinforcing effect than large WC particles due to the large interfacial area and therefore the efficient stress transfer between the particles and the binder matrix [24,37]. Nevertheless, large WC particles toughen better the WC–Co coating due to the fact that lager WC particles hindered effectively the crack propagation of cracks [4,37]. It was demonstrated that the wear resistance of a WC–Co cermet was closely related to both its microhardness and fracture toughness [1,7]. WC–Co cermet can exhibit a high wear resistance only when the requirements for a high microhardness and a high fracture toughness are fulfilled [20,38]. In this study, three multimodal WC–Co deposits had relatively high microhardness and high fracture toughness, therefore, it may be expected to exhibit improved wear resistance. In this
Fig. 7. Typical image of the impression for fracture toughness evaluation.
Fig. 9. Weight loss of cold sprayed multimodal WC–17Co deposits.
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work, the wear resistances of three multimodal WC–17Co deposits were compared using the same two-body abrasive wear tester. Fig. 9 shows the abrasive wear weight loss of cold sprayed multimodal WC– 17Co deposits. For comparison, the wear weight loss of bulk 316 L stainless steel was also given in Fig. 9. It was found that the wear weight loss of three deposits decreased from 5.53 ± 0.11 mg to 2.67 ± 0.15 mg and the deposit C exhibited the lowest wear weight loss. However, the wear weight loss of all deposits was much less than that of stainless steel bulk (28.2 ± 3.11 mg) in identical wear condition. Moreover, it was revealed that abrasive wear resistance of these multimodal WC–17Co deposits were better than that of HVOF sprayed multimodal WC–12Co in identical wear condition as reported in our previous paper [39]. The better wear resistance of cold sprayed multimodal WC–17Co was ascribed to its both high hardness and high fracture toughness. In order to better understanding the characteristics of cold sprayed multimodal WC–17Co cermet deposits, the properties of cold sprayed and HVOF sprayed multimodal WC–17Co coatings including of microhardness, fracture toughness, wear resistances and deposition efficiency were compared. The three kinds of HVOF sprayed multimodal WC– 17Co deposits were fabricated with the following parameters: propane flow of 30 L/min, oxygen flow of 362 L/min and spray distance of 160 mm. The pressures of propane, oxygen and nitrogen were 0.4 MPa, 0.55 MPa and 0.6 MPa, respectively. The properties of HVOF sprayed multimodal WC–17Co deposits were evaluated under the same condition with that of cold sprayed multimodal WC–17Co deposits. The experimental results were shown in Table 5. It can be seen that microhardness of the cold sprayed multimodal WC–17Co deposits was much higher than that of HVOF sprayed ones (755–768 Kgf/mm2). This would be attributed to the disappearance of nano-sized WC due to high spraying temperature in HVOF deposition process. Moreover, It was demonstrated that the indentation fracture toughness of the cold sprayed multimodal WC–17Co deposits was higher than that of HVOF sprayed one (4.64–5.1 MPa m1/2), which can be attributed to the appearance of brittle phase (such as W2C etc.) in the HVOF WC–17Co deposits due to the high spraying temperature. From Table 5, it was also found that abrasive wear mass loss of the cold sprayed multimodal WC–17Co deposits was lower than that of HVOF ones (13.1–14.4 mg). The higher abrasive wear resistance of cold sprayed multimodal WC– Co deposits may be ascribed to its higher fracture toughness and higher microhardness. Form Table 5 it was found that the deposition efficiency of cold sprayed multimodal WC–17Co deposits varied from 5.88 to 12.7% when the content of nano WC particle in powder increased from 10% to 50%. However, it was demonstrated that the deposition efficiency of cold sprayed multimodal WC–17Co deposits was relatively lower when compared to that of HVOF sprayed ones (29.8–33.1%) which may be attributed to the low plastic deformation of WC particles in cold spray process. In order to characterize the wear behavior of cold sprayed multimodal WC–17Co deposits, the worn surface morphologies of specimen were examined by SEM. Fig. 10a shows the general view of wear track of deposit C after worn under the condition shown in Table 3. The obvious grooves presented on worn surface would confirm the occurrence of micro-cutting and micro-gouging caused by hard SiC abrasives. On the other hand, some plastic deformation can be observed although the
Fig. 10. Typical worn surface of cold sprayed multimodal WC–17Co deposits C.
cold sprayed WC–17Co coating had high microhardness. By close examination, it was also recognized that some WC particles were protruded from deposit surface (as indicated by arrows in Fig. 10b). This fact would indicate the preferential wear of Co binder around the WC. In addition, some cracks on surface of WC and pits can be found on WC particle, as indicated by arrows in Fig. 10c. Owing to the higher hardness of SiC, the gouging effect of abrasives would cause the fracture, loosing and pulling-off of WC. Therefore, it would be considered that the abrasive wear of multimodal WC–17Co deposits in the pin-on-disk test may be dominated by the cutting or gouging related to the preferential wear of Co and fracture and peel-off of WC particles. Conclusions The multimodal WC–17Co deposits were fabricated by cold spraying of the powder with different content of nano WC (10%, 30%, 50%). The microstructure of three deposits presented somewhat dense wavy stripes consisting of nano WC and Co. Some fracture of micro WC occurred during deposition. Microhardness, fracture toughness and abrasive wear of cold sprayed multimodal WC–17Co deposits were influenced by the content of nano WC–17Co powder in feedstocks. The deposit with 10% nano-sized WC exhibited the higher fracture toughness and deposit with 50% nano-sized WC had the highest abrasive wear resistance. The abrasive wear of multimodal WC–17Co deposits in the pin-on-disk test was dominated by the cutting or gouging related to the preferential wear of Co and the fracture and peel-off of WC particle.
Table 5 Properties comparison of cold sprayed and HVOF sprayed multimodal WC–17Co deposits. Deposition process
Coating type
Microhardness (Hv0.3)
Fracture toughness (MPa m1/2)
Wear mass loss (mg)
Deposition efficiency (%)
Cold spraying
A B C A B C
1316 1350 1625 755 760 768
14.52 13.15 10.95 5.1 4.64 4.67
5.5 3.8 2.7 13.1 14.4 14.3
5.88 10.8 12.7 32.4 29.8 33.1
HVOF
± ± ± ± ± ±
80 120 115 64 36 32
± ± ± ± ± ±
3.39 3.44 3.05 0.74 0.73 0.37
± ± ± ± ± ±
0.12 0.06 0.15 0.17 0.20 0.45
± ± ± ± ± ±
1.39 2.55 2.96 1.84 1.57 3.63
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Acknowledgments The present work is supported by the National Natural Science Foundation of China (Nos. 50961007, 51001056), the Natural Science Foundation of Jiangxi Province (Nos. 20114BAB206019, 20132BAB216014) and Science and Technology Project of Jiangxi Educational Bureau (Nos. GJJ12610, GJJ12622, GJJ13721, KJLD12096). References [1] Fang ZZ, Wang X, Ryu T, Hwang KS, Sohn HY. Synthesis, sintering, and mechanical properties of nanocrystalline cemented tungsten carbide—a review. Int J Refract Met Hard Mater 2009;27:288–99. [2] Bonache V, Salvador MD, Busquets D, Burguete P, Martínez E, Sapiña F, et al. Synthesis and processing of nanocrystalline tungsten carbide: towards cemented carbides with optimal mechanical properties. Int J Refract Met Hard Mater 2011;29:78–84. [3] O'Quigley DGF, Luyckx S, James MN. An empirical ranking of a wide range of WC–Co grades in terms of their abrasion resistance measured by the ASTM standard B61185 test. Int J Refract Met Hard Mater 1997;15:73–9. [4] Jia K, Fischer TE, Gallois B. Microstructure, hardness and toughness of nanostructured and conventional WC–Co composites. Nanostruct Mater 1998;10:815–91. [5] Scieska SF, Filipowicz K. An integrated testing method for cermet abrasion resistance and fracture toughness evaluation. Wear 1998;216:202–12. [6] Richter V, Ruthendorf MV. On hardness and toughness of ultrafine and nanocrystalline hard materials. Int J Refract Met Hard Mater 1999;17:141–52. [7] Konyashin I, Ries B, Lachmann F. Near-nano WC–Co hardmetals: will they substitute conventional coarse-grained mining grades. Int J Refract Met Hard Mater 2010;28:489–97. [8] Li CJ, Ohmori A, Harada Y. Effect of powder structure on the structure of thermally sprayed WC–Co coatings. J Mater Sci 1996;31:785–94. [9] Li CJ, Ohmori A, Tani K. Effect of WC particle size on the abrasive wear of thermally sprayed WC–Co coatings. Mater Manuf Process 1999;14:175–84. [10] Li CJ, Wang YY, Yang GJ, Ohmori A, Khor KA. Effect of solid carbide particle size on deposition behavior, microstructure and wear performance of HVOF cermet coatings. Mater Sci Technol 2004;20:1087–96. [11] De Villiers Lovelock HL, Richter PW, Benson JM, Young PM. Parameter study of HP/HVOF deposited WC–Co coatings. J Therm Spray Technol 1998;7:97–106. [12] Guilemany JM, Dosta S, Miguel JR. The enhancement of the properties of WC–Co HVOF coatings through the use of nanostructured and microstructured feedstock powders. Surf Coat Technol 2006;201:1180–90. [13] Skandan G, Yao R, Sadangi R, Kear BH, Qiao Y, Liu L, et al. Multimodal coatings: a new concept in thermal spraying. J Therm Spray Technol 2000;9:329–31. [14] Skandan G, Yao R, Kear BH, Qiao Y, Liu L, Fischer TE. Multimodal powders: a new class of feedstock material for thermal spraying of hard coatings. Scr Mater 2001;44:1699-02. [15] Yang QQ, Senda T, Ohmori A. Effect of carbide grain size on microstructure and sliding wear behavior of HVOF-sprayed WC–12%Co coatings. Wear 2003;254:23–34. [16] Li CJ, Ohmori A, Harada Y. Formation of an amorphous phase in thermally sprayed WC–Co coatings. J Therm Spray Technol 1996;5:69–73. [17] Stewart DA, Shipway PH, McCartney DG. Abrasion wear behavior of conventional and nanocomposite HVOF sprayed WC–Co coatings. Wear 1999;225–229:789–98.
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Microstructure and properties of cold sprayed multimodal WC–17Co deposits Hong-Tao Wang a,b, Xiao Chen a,b, Xiao-Bo Bai a,b, Gang-Chang Ji a,b,⁎, Zeng-Xiang Dong a,b, Deng-Liang Yi a a b
Jiujiang Key Laboratory of Green Remanufacturing, 332005 Jiujiang, China School of Mechanical & Materials Engineering, Jiujiang University, 332005 Jiujiang, China
a r t i c l e
i n f o
Article history: Received 17 December 2013 Accepted 19 April 2014 Available online 30 April 2014 Keywords: Multimodal WC–17Co WC size Cold spraying Microstructure Properties Abrasive wear
a b s t r a c t In the present paper, three multimodal WC–17Co deposits with different content of nano-sized WC (10%, 30% and 50%, in wt.%) were prepared by cold spraying process. The effect of micro/nano ratio of WC particles on the microstructure, phase composition and properties such as microhardness, fracture toughness and wear performance of cold-sprayed multimodal WC–12Co coating was investigated. The experimental results demonstrated that the cold-sprayed multimodal WC–17Co deposits exhibited dense microstructure with some stripes composing of Co and nano WC, and their phase structures were similar to that of feedstock. The microhardness and fracture toughness of cold sprayed multimodal WC–17Co deposits respectively increased and decreased with increasing the content of nano-sized WC. The obtained multimodal WC–17Co deposits had both a high hardness and high fracture toughness and exhibited a greatly enhanced wear resistance in comparison to the conventional high-velocity oxy-fuel (HVOF) sprayed WC–12Co coating. The abrasive wear behavior of cold sprayed multimodal WC–17Co deposits was mainly dominated by cutting or gouging related to the preferential wear of Co and the fracture and the peel-off of WC particle. © 2014 Elsevier Ltd. All rights reserved.
Introduction WC–Co cermet was commonly used as surface hardening and enhancing materials for improving the sliding, abrasive and erosion wear resistance of components. It was known that the mechanical properties of WC–Co cermet were related to several factors such as WC size and Co content [1–6]. Usually, on the one hand, it was considered that the fracture toughness of WC–Co cermets would increase with the increase of WC size and the Co content [2,4,6]. On the other hand, the hardness of these materials may be increased as the WC size and the Co content decreased [1,3,6]. The WC–Co cermet with fine WC particles was considered exhibiting better wear performance due to its higher hardness [5]. However, it is argued by Konyashin [7] that the higher hardness and the higher fracture toughness may be necessary to the application of WC–Co hard metals in mining operation. Therefore, it would be valuable to consolidate the fracture toughness and hardness of WC–Co cermets by a suitable selection of WC size and shape [2]. In order to improve the wear performance of components, it would encounter the hard and tough cermet which was commonly deposited on their surface. High Velocity Oxy-fuel Spray (HVOF) process was considered as the popular way to deposit WC–Co cermet coatings due to its lower temperature and short dwell time of particles within flame ⁎ Corresponding author at: School of Mechanical and Materials Engineering, Jiujiang University, Jiujiang, Jiangxi 332005, PR China. Tel./fax: + 86 792 8334039. E-mail addresses: [email protected], [email protected] (G.-C. Ji).
http://dx.doi.org/10.1016/j.ijrmhm.2014.04.018 0263-4368/© 2014 Elsevier Ltd. All rights reserved.
compared to air plasma spraying (APS), which would limit dissolving of WC [8–10]. It was reported that the dense and wear resistant HVOF WC–Co coatings can be deposited by using suitable powder [9] and in optimized spray condition [10,11]. In order to improve the enhancing effect of WC particles on hardness and the wear resistance of WC–Co materials, J.M. Guilemany et al. [12] have deposited WC–Co coating with nanometer and micrometer WC particles by HVOF process and found that this bimodal coating showed the improved abrasive and friction wear resistance. Skandan G et al. [13,14] researched the properties of HVOF sprayed multimodal WC–Co coatings with powders containing 30% nano WC and 70% micro WC and found that there was a significant improvement of wear performance for these coatings. However, it was reported that the decarburization, dissolution of WC and the formation of brittle η phases (CoxWyC) were also liable to occur during HVOF spraying nanostructured WC–Co [12,15,16] and multimodal WC–Co [13], which was detrimental to the fracture toughness of these thermal sprayed coatings. Moreover, because of the serious decarburization, the wear resistance of HVOF sprayed nanostructured WC–Co coatings was not superior compared with that of conventional ones [17–20]. Therefore, the detrimental effect of decarburization and dissolving of nano WC on the fracture toughness and abrasive wear performance of HVOF sprayed WC–Co deposits would blur the contribution of nano WC for improving wear performance of multimodal WC–Co coatings. Cold spray was a recently developed low temperature coating deposition process using powder feedstock. The coatings are formed though plastic deformation of the particles in completely solid state during
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impact with high velocity. Owing to its relative lower temperature of particles, the cold-spray process has a significant advancement to the spraying nanostructured WC–Co coatings [21–27]. Researches on the deposition of WC–Co particles using different types of powders demonstrated that the particles with small WC [25] and porous structure [27] may exhibit better deformation ability in cold spray and be liable to form dense coatings. The WC–Co coatings with high hardness and improved wear resistant can be deposited by cold spraying of the nanostructured WC–Co powders. However, considering the inherent decrement in fracture toughness with reduction of WC size [2,4] and the high cost for fabrication of nano WC powders [14], it was expected to deposit WC–Co coatings with multimodal WC in order to get high fracture toughness and better wear performance. It was well known that the microstructure of cold sprayed deposits was greatly influenced by the plastic deformation of incident particles and pre-deposited particles. For WC–Co cermets, WC size, Co content and particle density were the key structural factors influencing the deformation of particles and the microstructure of WC–Co deposits [25–27]. However, report about the effect of multimodal WC particles on microstructure of WC–Co deposits was very limited. The fracture toughness, microhardness and abrasive wear properties of cold sprayed multimodal WC–Co coating were not clear. In present study, the multimodal WC–17Co powders with different content of nano WC were fabricated by blending the agglomerated nano WC–17Co powders with micro WC–17Co powders in different proportion. The microstructure and phase constitution of cold sprayed multimodal WC–Co deposits were characterized. The microhardness, fracture toughness and abrasive wear performance of these deposits were experimentally investigated. The effect of WC size on the microstructure formation and properties of the deposits was discussed. Experimental materials and procedures Feedstock used in this experiment was commercially available multimodal WC–17Co powder produced by Ganzhou Zhangyuan Tungsten Advance Material Co., Ltd. These powders were fabricated by blending the agglomerated-sintered nano WC–17Co and micro WC–17Co powder. As shown in Table 1, three types of the powders contained about 10%, 30% and 50% nano WC–17Co were used in this study. In addition, the particles size and microhardness were also given in Table 1. The surface morphologies of three feedstocks and cross sectional microstructure of the micrometer and nanometer WC–17Co powder were shown in Fig. 1, respectively. It can be seen that all feedstock had global morphology, as shown in Fig. 1a–c. The micrometer WC–17Co particles were mainly composed of micro WC (seen in Fig. 1d) and the nano and near nano WC existed in nano WC–17Co particles (seen in Fig. 1e). The particle size distribution of feedstock was analyzed by a laser particle size analyzer (Malvern Mastersizer 2000, Malvern Instrument Ltd. UK). The tested result (as shown in Fig. 2) demonstrated that the actual particle size was about 4–25 μm and d(0.5) was about 13 μm, as shown in Table 1. Stainless steel with dimensions of 45 × 20 × 3 mm was used as a substrate for depositing coating and blasted with 20 mesh alumina before depositing. A cold spraying system (CS-2000) developed in Xi'an Jiaotong University was used for depositing coatings in the condition as shown in Table 2. A spray gun with a converging–diverging de Laval type nozzle of a throat
Fig. 1. Morphologies and cross sectional microstructure of multimodal WC–17Co powder. a–c: surface morphologies of powders A, B and C; d–e: cross sectional microstructure (d: micrometer WC–17Co; e: nanometer WC–17Co).
diameter of 2 mm was adopted. The exit diameter and downstream length of nozzle were 6 mm and 100 mm, respectively. The nitrogen gas was used as accelerating and powder feed gas. The pressure was kept at 2.4 MPa for accelerating gas and 2.6 MPa for powder feed gas, respectively. The accelerating gas was preheated and the temperature of gas in the gun pre-chamber was kept at 750 ± 30 °C. The distance from the nozzle exit to the substrate surface was about 20 mm. The spray gun was kept stationary during deposition processes in order to prepare thick deposits for performance test. Microstructure of powders and deposits, and the morphology of worn surface of deposits were examined by SEM (Tescan Vega II LSU). The phase constitution of both feedstock and as-spayed deposits was characterized by XRD (Bruker D8 advance model) with Cu Kα radiation
Table 1 Powders used in cold spraying. Powder type
Proportion of nano to micro WC–17Co
Particle size (μm)
d(0.5) (μm)
Manufacture process
Microhardness (Hv0.1)
A B C
10/90 30/70 50/50
+4–25 +4–25 +4–25
13.465 13.148 13.077
Agglomerated and mixed Agglomerated and mixed Agglomerated and mixed
137 ± 25 197 ± 28 168 ± 36
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The elastic modulus was determined according to the following formula as established in [29]: 0 0 b =a ¼ ðb=aÞ−α ðHv =EÞ
ð2Þ
Where, a and b were the diagonal dimensions of Knoop indenter. a′ and b′ were the long and short diagonal dimensions measured on residual Knoop indentation impression. α was a constant equal to 0.45. Hv was the Vickers hardness. The abrasive wear resistance of the coating was evaluated using a pin-on-disk tester (Model ML-100, Zhangjiakou Chengxin Balance Testing Machineries Company, China) in the conditions as shown in Table 3. The pin sample was 4 mm in diameter and the coating was deposited on the end surface of the pin with a thickness about 500 μm. Before the tests, the coating surface was polished using an abrasive paper of 1000 grit in order to reduce the error caused by the different coating surface topography. During the tests, the pin specimen was loaded against a rotating disk covered with a SiC abrasive paper of 300 grit. The applied load was 10 N and the wear distance was 16 m. The rotation speed of the disk was 60 rpm and the radial feed rate of the pin was 4 mm/r. In order to ensure a fresh supply of abrasive particles, worn SiC abrasive paper was replaced after each test (total running distance: 16 m). The weight loss of the pin before and after wear was measured using an analytical balance with a precision of 0.0001 g. The weight loss of three samples was averaged for evaluating the abrasion resistance of the coating. Results and discussions Microstructure of cold sprayed multimodal deposits Fig. 2. Particle size distribution of feedstock (a, b and c corresponding to A, B and C feedstock in Table 1).
at 35 kV and 35 mA. Scanning speed and scanning step were 2°/min and 0.01°, respectively. The microhardness of original powder was tested using the HVS1000 Vickers Microhardness Tester with a load of 0.98 N and dwell time of 10 s. The microhardness of the deposits was tested under a load of 2.94 N and a dwell time of 20 s in all tests. The microhardness was evaluated using the average value of random ten point tests in the cross section of both powder particles and deposits sample. The plain strain fracture toughness, i.e. KIC, was determined based on an indentation method. A Vickers indenter was used on metallographically prepared cross-sections of the coatings with a 5 kgf load and dwell time of 20 s. The fracture toughness (KIC) was calculated by applying the following equation proposed by Niihara [28]: 2=5 −1=2
K IC ¼ 0:0193ðHνdÞðE=HvÞ
ð1Þ
a
Where KIC is the fracture toughness, Hv the Vickers hardness, E the elastic modulus, d the half-diagonal of the Vickers indentation and a is the indentation crack length. At least five indentations were made for each sample.
The microstructure of three cold sprayed multimodal WC–17Co deposits using powders A, B and C (as indicated in Table 1) were shown in Fig. 3. For convenience, three deposits were denoted as deposits A, B and C, respectively. It was found that three deposits presented dense microstructure with a little visible pore and theirs thickness varied between 0.5 and 2 mm. Compared with cross sectional microstructure of the feedstock as shown in Fig. 1d and e, it was recognized that some dense wavy stripes (indicated by arrows in Fig. 3b, e and h) that did not exist in feedstock appeared in the deposits. By close examination (in Fig. 3c, f and i), it was found that these wavy stripes were mainly composed of nano or near nano WC (bright particle in contrast) and Co (gray area in contrast) as indicated by arrows in Fig. 3f. Moreover, it can be seen that the WC particles within these stripes presented some orientation (as shown in Fig. 3c and f). This microstructure change indicated that there occur some rearrangement of WC and deformation of Co during deposition processes. Alkhimov et al. [30,31] reported that there existed a regional temperature rise and a high pulse-pressure after the shock wave leaving the contact area of impacting particles, which would lead to regional adiabatic shear instability and severe deformation of splats at its periphery in the form of ejections. Gao et al. [32] researched the deformation behavior of single WC–12Co particle and found that the manifest densification occurred near the impact interface in the form of slippage of small carbide particles.
Table 3 Abrasive wear parameters.
Table 2 Cold spraying parameters. Diameter of throat (mm)
2
Exit diameter of nozzle (mm) Pressure of propellant gas (MPa) Pressure of powder feed gas (MPa) Temperature of propellant gas in gun chamber (°C) Distance for exit of nozzle to substrate (mm)
6 2.4 2.6 750 ± 30 20
Wheel diameter (mm)
280
Rotation of wheel (r/min) Diameter of specimen (mm) Applied load (N) Radial feed rate of specimen (mm/r) Abrasive paper Duration of test (m)
60 4 10 4 300# SiC 16
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deformation of splats resulted from the sliding of small WC and plastic deformation of Co. Because the severe deformation was liable to happen on the periphery of splats close to impact interface, the dense stripes appeared in the area near impacting interface. Afterwards, the successive incident particles would strike the surface of pre-deposited WC–Co splats and form new dense stripes near the contact interface of up WC–Co splats and underlying WC–Co splats. In this case, the combined effects of hammering and deformability of particles would result in the formation of stripes. Owing to the better self-deformability of nano WC–17Co particle resulted from easier movement of nano WC, it would be believed that the dense stripes mainly formed through the deformation of nano WC–17Co particles on impact. In addition, the hammering effect of micro WC–17Co particles on impact may enhance the deformation of underlying nano WC–17Co splats in cold spraying due to the prominent peening effect of large WC as it happened in HVOF sprayed WC–Co [33]. By close examination on microstructure of deposits, it was revealed that the morphology of WC particles within the as-sprayed deposits exhibited unsmooth profile with an angular feature which is different from that existed in HVOF sprayed coating [12]. This unsmooth profile indicated that there was no surface melting of WC particle in cold spay as occurred in HVOF spraying. However, comparing with the shape of WC within original powder (as shown in Fig. 1d), it was revealed that the sharp angles of micro WC particles presented in original powder became blunt after deposition. The size of micro WC particles within as sprayed deposits seems slightly reduced. It was argued by Andrew [25] that, in cold spraying WC–Co, the high velocity impacting between the brittle WC particles would cause their fracture and splat breakup, and consequently leads to the formation of fine grains. Meanwhile, the scrubbing of WC particles during deposition would cause the blunting of shape angle. Therefore, the refinement of WC incurred by fracture of large WC on impact may be the reason for size reduction of WC particles within as-sprayed multimodal WC–Co coating. By comparing the microstructure of the three deposits, it was found that deposit A had the relative higher content of micro WC and deposit C had the relative higher content of nano WC. This was related to the different content of nano WC particles in three feedstocks. Moreover, it was recognized that deposit C exhibited the most dense microstructure compared with that of other deposits. As mentioned before, the nano WC–17Co particles would exhibit improved self-deformability due to the sliding of nano WC. Therefore, the deformation or densification of nano WC–17Co powders caused by impact and strike from other incident particles may be manifest compared to that of micro WC–17Co under in identical spray condition. As a result, deposit C showed a denser microstructure compared to that of other deposits.
Phase constitution of cold-sprayed multimodal deposits
Fig. 3. Microstructure of the cold-sprayed multimodal WC–17Co deposits. (a), (b), (c) for deposit A; (d), (e), (f) for deposit B and (g), (h), (i) for deposit C.
For building-up of WC–Co deposits, there required a successive deposition of WC–Co particles. The first layer deposit was formed by deformation of splats and stainless steel substrate. In this stage, the apparent
Fig. 4 shows the XRD patterns of three cold-sprayed multimodal WC–17Co deposits and their feedstocks. It was found that only the peaks of WC and Co phase existed in the pattern of as-sprayed deposits that was same to that of feedstock. This fact suggested that no phase transformation occurred in cold spraying of multimodal WC–17Co in our experiment condition. Nevertheless, some humped or broadened peaks appeared in the pattern of these deposits. It was known that peak broadening can be attributed to either a strain effect or refinement of particle size [25]. Li et al. [24] researched the effect of atomic level strain within cold sprayed nano WC–12Co coating by analyzing the microhardness variation before and after heat treatment at 1000 °C for 6 h and concluded that the effect of the atomic level strain on the broadening of XRD diffraction peaks was limited due to no obvious microhardness change. As for cold sprayed multimodal WC–17Co, buildingup of deposit would encounter the impact of WC–17Co particles on pre-deposited WC–17Co layer. The severe striking of high velocity incident particles could cause the fracture of brittle WC [25,27], which would lead to the refinement of WC particles. Therefore, it was reasonable
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the increment of FWHM (defined as Formula 3) of WC peaks in XRD patterns during deposits formation. Δ ¼ WWCd −WWCp
Fig. 4. XRD patterns of cold sprayed multimodal WC–17Co deposits and feedstock. (a) feedstock and (b) deposits.
to consider that the broadening of WC peaks in X-ray pattern was related to the grain refinement. In order to characterize the refinement of WC during deposition, the full width at half maximum (FWHM) of WC peaks was evaluated by quantitative analysis of the XRD patterns of the deposits. Fig. 5 shows
ð3Þ
Where, △ is the increment of FWHM of WC peaks during deposits formation; WWCd and WWCp are respectively the FWHM of strongest peaks of WC in deposits and corresponding feedstock. It was revealed that the FWHM of WC peaks in feedstock (Powders A, B, C) were 0.18, 0.18 and 0.17, respectively. However, the increment of FWHM of three WC–17Co deposits, A, B and C, were 0.32, 0.33 and 0.29, respectively. Therefore, it would be recognized that the broadening of WC peak in XRD pattern was obvious. It was well known that the preferential rebounding of large WC in HVOF sprayed WC–Co deposit would result in the reduction of WC content. For cold spraying multimodal WC–Co, the deposits were formed through deformation of the particles in completely solid state. Therefore, it was interesting to investigate the variation of WC content during deposit formation. Owing to no phase transformation in cold spraying of WC–Co, it would be reasonable to evaluate the change of WC content by comparison of W content in as-sprayed deposits and feedstock. The EDA analysis results (as shown in Table 4) indicated that the content of W in assprayed deposits was slightly reduced compared to feedstock. This phenomenon may demonstrate that there existed a little loss of WC during cold spraying of multimodal WC–17Co. However, there was no manifest difference of W content in three deposits. These results may imply that the micro and nano WC and Co possessed the almost same deposition efficiency in cold spraying which was different from that in HVOF spray WC–Co. It was known that the WC–Co particle in HVOF spray processes was presented in solid/liquid state. The rebound off of solid WC particle when impacting would result in the loss of WC. Nevertheless, for cold spraying of WC–Co, the deposits were formed though deformation of the particles in completely solid state. The preferential rebound off of WC was liable to take the Co binder along due to tightly bonding between Co and WC in agglomerate-sintered feedstock, which lead to the simultaneous reduction of both WC and Co. Consequently, there was no obvious variation of W content of during coating formation. Properties of cold sprayed multimodal deposits Considering that the WC–Co are mainly used to improving wear performance of components, the microhardness, fracture toughness and abrasive wear of multimodal WC–17Co deposits were experimentally investigated. Fig. 6 shows the microhardness of three multimodal WC–17Co deposits (A, B, C). It was revealed that the microhardness (Hv0.3) of these deposits increased from 1316 ± 80 Kgf/mm 2 to 1625 ± 115 Kgf/mm2 which was higher than that of corresponding powder (as shown in Table 1), and that of the cold sprayed nano WC– 17Co deposits (11.91 ± 0.35 GPa) reported by Andrew Siao Ming Ang [25] and HVOF sprayed multimodal WC–12Co (820 VHN) reported by Skandan et al. [13]. The higher hardness of cold sprayed multimodal WC–17Co deposits would be attributed to its high content of nano WC and high density. In addition, it was found that the deposits C exhibited the higher microhardness. This fact would be ascribed to the higher fraction of nano WC particles and dense microstructure of this deposits resulted from enhanced plastic deformation of cold sprayed splats [25,34]
Table 4 W content in deposits and original powder (wt.%).
Fig. 5. FWHM increment of WC peak in XRD patterns of cold sprayed multimodal WC–17Co deposits.
Wp Wc Wc/Wp
A
B
C
87.01 84.71 0.97
86.34 84.08 0.97
87.08 83.36 0.96
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Fig. 6. Microhardness of cold sprayed multimodal WC–17Co powder and its deposits.
201
Fig. 8. Fracture toughness of cold sprayed multimodal WC–17Co deposits.
The fracture toughness of the multimodal WC–17Co deposits was measured by the indentation methodology as described above. The typical indentation mark of the toughness measurements was shown in Fig. 7. It was seen that cracks arised from the tip of the indentation diagonal and propagated in the direction paralleled to coating surface. There was no visible crack in the direction perpendicular to coating surface. These experiment results showed that the deformation of the particles during cold spraying in this study is much less significant than HVOF. However, from Fig. 3, obvious flattening of the deposited particles was observed. Therefore, one can reasonably expect that the anisotropic performance was due to the lamella microstructure of the deposits. Fig. 8 shows the fracture toughness obtained by indenting on cross section of cold sprayed multimodal WC–17Co deposits. It was demonstrated that the indentation fracture toughness of three deposits decreased from 14.52 ± 3.39 MPa m1/2 to 10.95 ± 3.05 MPa m1/2 with increasing content of nano-sized WC, which was comparable to that of sintered WC–Co composites (9–20 MPa m1/2) reported in [35] and higher than that of HVOF spayed WC–17Co deposits (3–5 MPa m1/2) [36]. The higher fracture toughness of cold sprayed multimodal WC– 17Co deposits may partially be attributed to its dense microstructure and the absence of brittle phase (η phase) compared to that of HVOF sprayed WC–Co coatings [11,14,15]. The deflection in crack growth afforded by different size of WC and the increment of Co mean free path due to the existence of micro WC may also be liable to result in the higher fracture toughness for multimodal WC–17Co deposits. In
addition, it was found that the indentation fracture toughness of cold sprayed multimodal WC–17Co deposits slightly increased with the decrease of nano WC–17Co in feedstock, which was consistent with the influence of WC size on fracture toughness of sintered WC–Co composites. However, the decrement of fracture toughness with the increment of nano WC was not such obvious as expected. This fact can be ascribed to the absence of brittle phase in cold sprayed WC–Co deposits compared to HVOF sprayed WC–Co deposit. Based on the results as presented above, one can conclude that cold sprayed multimodal WC–17Co deposits exhibited both a high microhardness and relatively high fracture toughness. The differently sized WC particles play synergetic roles on enhancing simultaneously the mechanical properties of the WC–Co deposit. That is, small WC particles exert a more efficient reinforcing effect than large WC particles due to the large interfacial area and therefore the efficient stress transfer between the particles and the binder matrix [24,37]. Nevertheless, large WC particles toughen better the WC–Co coating due to the fact that lager WC particles hindered effectively the crack propagation of cracks [4,37]. It was demonstrated that the wear resistance of a WC–Co cermet was closely related to both its microhardness and fracture toughness [1,7]. WC–Co cermet can exhibit a high wear resistance only when the requirements for a high microhardness and a high fracture toughness are fulfilled [20,38]. In this study, three multimodal WC–Co deposits had relatively high microhardness and high fracture toughness, therefore, it may be expected to exhibit improved wear resistance. In this
Fig. 7. Typical image of the impression for fracture toughness evaluation.
Fig. 9. Weight loss of cold sprayed multimodal WC–17Co deposits.
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work, the wear resistances of three multimodal WC–17Co deposits were compared using the same two-body abrasive wear tester. Fig. 9 shows the abrasive wear weight loss of cold sprayed multimodal WC– 17Co deposits. For comparison, the wear weight loss of bulk 316 L stainless steel was also given in Fig. 9. It was found that the wear weight loss of three deposits decreased from 5.53 ± 0.11 mg to 2.67 ± 0.15 mg and the deposit C exhibited the lowest wear weight loss. However, the wear weight loss of all deposits was much less than that of stainless steel bulk (28.2 ± 3.11 mg) in identical wear condition. Moreover, it was revealed that abrasive wear resistance of these multimodal WC–17Co deposits were better than that of HVOF sprayed multimodal WC–12Co in identical wear condition as reported in our previous paper [39]. The better wear resistance of cold sprayed multimodal WC–17Co was ascribed to its both high hardness and high fracture toughness. In order to better understanding the characteristics of cold sprayed multimodal WC–17Co cermet deposits, the properties of cold sprayed and HVOF sprayed multimodal WC–17Co coatings including of microhardness, fracture toughness, wear resistances and deposition efficiency were compared. The three kinds of HVOF sprayed multimodal WC– 17Co deposits were fabricated with the following parameters: propane flow of 30 L/min, oxygen flow of 362 L/min and spray distance of 160 mm. The pressures of propane, oxygen and nitrogen were 0.4 MPa, 0.55 MPa and 0.6 MPa, respectively. The properties of HVOF sprayed multimodal WC–17Co deposits were evaluated under the same condition with that of cold sprayed multimodal WC–17Co deposits. The experimental results were shown in Table 5. It can be seen that microhardness of the cold sprayed multimodal WC–17Co deposits was much higher than that of HVOF sprayed ones (755–768 Kgf/mm2). This would be attributed to the disappearance of nano-sized WC due to high spraying temperature in HVOF deposition process. Moreover, It was demonstrated that the indentation fracture toughness of the cold sprayed multimodal WC–17Co deposits was higher than that of HVOF sprayed one (4.64–5.1 MPa m1/2), which can be attributed to the appearance of brittle phase (such as W2C etc.) in the HVOF WC–17Co deposits due to the high spraying temperature. From Table 5, it was also found that abrasive wear mass loss of the cold sprayed multimodal WC–17Co deposits was lower than that of HVOF ones (13.1–14.4 mg). The higher abrasive wear resistance of cold sprayed multimodal WC– Co deposits may be ascribed to its higher fracture toughness and higher microhardness. Form Table 5 it was found that the deposition efficiency of cold sprayed multimodal WC–17Co deposits varied from 5.88 to 12.7% when the content of nano WC particle in powder increased from 10% to 50%. However, it was demonstrated that the deposition efficiency of cold sprayed multimodal WC–17Co deposits was relatively lower when compared to that of HVOF sprayed ones (29.8–33.1%) which may be attributed to the low plastic deformation of WC particles in cold spray process. In order to characterize the wear behavior of cold sprayed multimodal WC–17Co deposits, the worn surface morphologies of specimen were examined by SEM. Fig. 10a shows the general view of wear track of deposit C after worn under the condition shown in Table 3. The obvious grooves presented on worn surface would confirm the occurrence of micro-cutting and micro-gouging caused by hard SiC abrasives. On the other hand, some plastic deformation can be observed although the
Fig. 10. Typical worn surface of cold sprayed multimodal WC–17Co deposits C.
cold sprayed WC–17Co coating had high microhardness. By close examination, it was also recognized that some WC particles were protruded from deposit surface (as indicated by arrows in Fig. 10b). This fact would indicate the preferential wear of Co binder around the WC. In addition, some cracks on surface of WC and pits can be found on WC particle, as indicated by arrows in Fig. 10c. Owing to the higher hardness of SiC, the gouging effect of abrasives would cause the fracture, loosing and pulling-off of WC. Therefore, it would be considered that the abrasive wear of multimodal WC–17Co deposits in the pin-on-disk test may be dominated by the cutting or gouging related to the preferential wear of Co and fracture and peel-off of WC particles. Conclusions The multimodal WC–17Co deposits were fabricated by cold spraying of the powder with different content of nano WC (10%, 30%, 50%). The microstructure of three deposits presented somewhat dense wavy stripes consisting of nano WC and Co. Some fracture of micro WC occurred during deposition. Microhardness, fracture toughness and abrasive wear of cold sprayed multimodal WC–17Co deposits were influenced by the content of nano WC–17Co powder in feedstocks. The deposit with 10% nano-sized WC exhibited the higher fracture toughness and deposit with 50% nano-sized WC had the highest abrasive wear resistance. The abrasive wear of multimodal WC–17Co deposits in the pin-on-disk test was dominated by the cutting or gouging related to the preferential wear of Co and the fracture and peel-off of WC particle.
Table 5 Properties comparison of cold sprayed and HVOF sprayed multimodal WC–17Co deposits. Deposition process
Coating type
Microhardness (Hv0.3)
Fracture toughness (MPa m1/2)
Wear mass loss (mg)
Deposition efficiency (%)
Cold spraying
A B C A B C
1316 1350 1625 755 760 768
14.52 13.15 10.95 5.1 4.64 4.67
5.5 3.8 2.7 13.1 14.4 14.3
5.88 10.8 12.7 32.4 29.8 33.1
HVOF
± ± ± ± ± ±
80 120 115 64 36 32
± ± ± ± ± ±
3.39 3.44 3.05 0.74 0.73 0.37
± ± ± ± ± ±
0.12 0.06 0.15 0.17 0.20 0.45
± ± ± ± ± ±
1.39 2.55 2.96 1.84 1.57 3.63
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Acknowledgments The present work is supported by the National Natural Science Foundation of China (Nos. 50961007, 51001056), the Natural Science Foundation of Jiangxi Province (Nos. 20114BAB206019, 20132BAB216014) and Science and Technology Project of Jiangxi Educational Bureau (Nos. GJJ12610, GJJ12622, GJJ13721, KJLD12096). References [1] Fang ZZ, Wang X, Ryu T, Hwang KS, Sohn HY. Synthesis, sintering, and mechanical properties of nanocrystalline cemented tungsten carbide—a review. Int J Refract Met Hard Mater 2009;27:288–99. [2] Bonache V, Salvador MD, Busquets D, Burguete P, Martínez E, Sapiña F, et al. Synthesis and processing of nanocrystalline tungsten carbide: towards cemented carbides with optimal mechanical properties. Int J Refract Met Hard Mater 2011;29:78–84. [3] O'Quigley DGF, Luyckx S, James MN. An empirical ranking of a wide range of WC–Co grades in terms of their abrasion resistance measured by the ASTM standard B61185 test. Int J Refract Met Hard Mater 1997;15:73–9. [4] Jia K, Fischer TE, Gallois B. Microstructure, hardness and toughness of nanostructured and conventional WC–Co composites. Nanostruct Mater 1998;10:815–91. [5] Scieska SF, Filipowicz K. An integrated testing method for cermet abrasion resistance and fracture toughness evaluation. Wear 1998;216:202–12. [6] Richter V, Ruthendorf MV. On hardness and toughness of ultrafine and nanocrystalline hard materials. Int J Refract Met Hard Mater 1999;17:141–52. [7] Konyashin I, Ries B, Lachmann F. Near-nano WC–Co hardmetals: will they substitute conventional coarse-grained mining grades. Int J Refract Met Hard Mater 2010;28:489–97. [8] Li CJ, Ohmori A, Harada Y. Effect of powder structure on the structure of thermally sprayed WC–Co coatings. J Mater Sci 1996;31:785–94. [9] Li CJ, Ohmori A, Tani K. Effect of WC particle size on the abrasive wear of thermally sprayed WC–Co coatings. Mater Manuf Process 1999;14:175–84. [10] Li CJ, Wang YY, Yang GJ, Ohmori A, Khor KA. Effect of solid carbide particle size on deposition behavior, microstructure and wear performance of HVOF cermet coatings. Mater Sci Technol 2004;20:1087–96. [11] De Villiers Lovelock HL, Richter PW, Benson JM, Young PM. Parameter study of HP/HVOF deposited WC–Co coatings. J Therm Spray Technol 1998;7:97–106. [12] Guilemany JM, Dosta S, Miguel JR. The enhancement of the properties of WC–Co HVOF coatings through the use of nanostructured and microstructured feedstock powders. Surf Coat Technol 2006;201:1180–90. [13] Skandan G, Yao R, Sadangi R, Kear BH, Qiao Y, Liu L, et al. Multimodal coatings: a new concept in thermal spraying. J Therm Spray Technol 2000;9:329–31. [14] Skandan G, Yao R, Kear BH, Qiao Y, Liu L, Fischer TE. Multimodal powders: a new class of feedstock material for thermal spraying of hard coatings. Scr Mater 2001;44:1699-02. [15] Yang QQ, Senda T, Ohmori A. Effect of carbide grain size on microstructure and sliding wear behavior of HVOF-sprayed WC–12%Co coatings. Wear 2003;254:23–34. [16] Li CJ, Ohmori A, Harada Y. Formation of an amorphous phase in thermally sprayed WC–Co coatings. J Therm Spray Technol 1996;5:69–73. [17] Stewart DA, Shipway PH, McCartney DG. Abrasion wear behavior of conventional and nanocomposite HVOF sprayed WC–Co coatings. Wear 1999;225–229:789–98.
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