Grain size effect on wear resistance of WC-Co cemented carbides under different tribological conditions

Grain size effect on wear resistance of WC-Co cemented carbides under different tribological conditions

Accepted Manuscript Title: Grain size effect on wear resistance of WC-Co cemented carbides under different tribological conditions Authors: Haibin Wan...

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Accepted Manuscript Title: Grain size effect on wear resistance of WC-Co cemented carbides under different tribological conditions Authors: Haibin Wang, Mark Gee, Qingfan Qiu, Hannah Zhang, Xuemei Liu, Hongbo Nie, Xiaoyan Song, Zuoren Nie PII: DOI: Reference:

S1005-0302(19)30236-1 https://doi.org/10.1016/j.jmst.2019.07.016 JMST 1650

To appear in: Received date: Revised date: Accepted date:

15 May 2019 10 June 2019 11 June 2019

Please cite this article as: Wang H, Gee M, Qiu Q, Zhang H, Liu X, Nie H, Song X, Nie Z, Grain size effect on wear resistance of WC-Co cemented carbides under different tribological conditions, Journal of Materials Science and amp; Technology (2019), https://doi.org/10.1016/j.jmst.2019.07.016 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.

Research Article

Grain size effect on wear resistance of WC-Co cemented carbides under different tribological conditions Haibin Wang

1, 2, *

[email protected], Mark Gee 2, Qingfan Qiu 1, Hannah Zhang 2, Xuemei

Liu1, Hongbo Nie 3, Xiaoyan Song 1, * [email protected] , Zuoren Nie 1 1

College of Materials Science and Engineering, Key Laboratory of Advanced Functional

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Materials, Education Ministry of China, Beijing University of Technology, Beijing 23, China

National Physical Laboratory, Hampton Road, Teddington, TW11 0LW, UK

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Xiamen Tungsten Co., Ltd., Xiamen 361009, China

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[Received 15 May 2019; Received in revised form 10 June 2019; Accepted 11 June 2019]

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* Corresponding authors.

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The grain-size dependence of wear resistance of WC-Co cemented carbides (with mean

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WC grain sizes of 2.2 μm, 1.6 μm, 0.8 μm and 0.4 μm, respectively) was investigated under different tribological conditions. The results showed that the grain size had opposite

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effects on wear resistance of the cemented carbides in dry sliding wear and microabrasion tests. In the former condition, with decrease of WC grain size hence the increase of hardness, plastic deformation, fracture, fragmentation and oxidation were all mitigated, leading to a drastic decrease in the wear rate. In the latter condition, pull-out of WC grains

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after Co removal dominated the wear, so that the hardness of cemented carbide was not a core factor. As a result, the wear resistance of the cemented carbide generally showed a

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decreasing trend with decrease of the grain size, except for a slight increase in the ultrafine-grained cemented carbide. Single-pass scratching of the cemented carbides under

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various loads indicated the same failure mechanism as that in the sliding wear tests. Furthermore, the reasons for severe surface oxidation of the coarse-grained cemented carbides were disclosed. Key words: Cemented carbide; Grain size; Fracture; Surface oxidation; Grain pull-out; Wear resistance

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1. Introduction Wear resistance of WC-Co cemented carbides has been a hot topic with widespread concern because of its importance in various industrial applications [1-3]. The wear performance of cemented carbides mainly depends on their inherent microstructural features and the testing conditions. Among these, WC grain size and Co content, which fundamentally determine the hardness and toughness of cemented carbides, are the most important influencing factors [4-8].

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However, a specific WC-Co grade may not always be the most competitive in various frictional conditions such as impact wear, fretting wear, sliding wear, abrasion or their combinations [9]. In a particular service environment, it is critical to get the best choice. In this regard, it is of great

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significance to evaluate the wear properties of cemented carbides with characteristic microstructures under different tribological conditions. The understanding obtained will enable engineers to select suitable WC-Co grades for specific applications.

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Generally, it is considered that coarse-grained cemented carbides have lower abrasive wear resistance than that of fine or medium grain size [5, 7, 10]. Some researchers even suggested that

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the wear resistance of cemented carbides (with the WC grain size of 0.8-3.6 μm) strongly

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decreased as the grain size and mean binder free path increased, regardless of the test

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conditions[2]. However, a recent study indicated that the wear performance of cemented carbides having nearly the same hardness showed different trends in the standard high -load (i.e.

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200 N) ASTM B611 and low-load (i.e. 130 N) G6504 tests with the variation of WC grain size (0.2-4.5 μm) and Co content (3-24 wt%)[4]. Also, for a given WC-Co bulk hardness coarser-grained cemented carbides may be more wear-resistant than the fine-grained ones in microabrasion tests [11]. The distinct differences between coarse- and fine-grained WC-Co

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cemented carbides can be summarized as follows: coarse WC grains exhibit relatively large plastic deformation before fracturing; fine WC grains themselves are less likely to be cracked,

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but result in materials with lower fracture toughness hence higher possibility of intergranular fracture and pull-out of grains[4, 10, 12-14]. It is evident that the damage behaviors of WC grains in

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different scales are strongly related to the applied stress and the tribological conditions. To date, little attention has been given to compare the wear behaviors of WC-Co cemented

carbides with different grain sizes under a ultra-low load (<1 N) and normal higher loads. In such conditions, it is interesting to know whether there are any different grain size effects on the wear resistance of WC-Co cemented carbides. On basis of these considerations, in the present work a group of WC-Co cemented carbides with the mean WC grain size varying from 0.4-2.2 μm were prepared and their wear behaviors were investigated under different

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tribological conditions. The relationship between grain size of the prepared cemented carbides and the wear mode and load will be clarified. 2. Experimental 2.1. Materials preparation Commercially available WC, VC, Cr3C2 and Co powders were used as raw materials, four

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kinds of pre-treated WC-Co powders with different WC particle sizes were obtained by wet ball milling using ethyl alcohol as the milling medium. The preparation details of these powders are listed in Table 1. The powders were pressed and consolidated by sinter-HIP (hot

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isostatic pressure) (COD433R, PVA TePla, Germany) under the pressure of 6 MPa. The sintering temperature for all specimens was 1420 °C.

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2.2.Characterization of microstructure and mechanical properties

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The relative densities of as-prepared cemented carbides were measured with the Archimedes

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method. The hardness was evaluated by a Vickers hardness tester with a 30 kg load and 15 s dwell time according to the ISO-3878 standard. Cracks that originated from the Vickers

(1)

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KIC = 0.0028(HvP/L)1/2

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indentations were used to estimate the fracture toughness of the specimens as follows[15]:

where Hv is the indentation hardness, P is the load and L is the total crack length. The amount of magnetic Co of the prepared cemented carbides was measured by a magnetism apparatus

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(ACOMT-I cobalt, Chang Sha Xianyou in China). The apparatus has a high sensitivity up to 1 mg Co. The microstructure of the specimens was observed by scanning electron microscope

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(SEM, Zeiss Supra 55). Before observations, the specimens were polished with a final step of

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1μm diamond suspension for 30 min. 2.3. Evaluation of wear resistance The wear resistance of the cemented carbides was evaluated by a microabrasion tester (Plint PE66)[11] and dry sliding wear tribometer (CFT-1, Lanzhou Zhongke Kaihua of China), respectively. The details of the wear tests are given in Table 2. The wear volume loss of the specimens after microabrasion tests was measured based on the 3D morphology of the wear craters, which were measured using volume measurements made with a 3D optical micros cope 3

(Alicona InfiniteFocus G5). After dry sliding wear tests, the wear volume loss was calculated from the measured mean wear scar profiles at three different positions and the friction stroke length (5 mm). At least three repeated experiments were performed for each specimen under both wear conditions. Macro scratch tests were carried out using a high-load scratch tester (ST3001, Teer Coatings Ltd.) with a spherical diamond indenter. The tip radius is 200 µm. Single -pass scratch tests were done at 10, 20, 30, 40, 50, 60 and 70 N for each specimen at ambient conditions. The

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scratch width was measured by SEM observations.

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3. Results 3.1. Microstructure and mechanical properties

The SEM microstructures of the prepared cemented carbides are shown in Fig. 1. Using

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the linear intercept method, the mean WC grain sizes of the specimens S1, S2, S3 and S4 were

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estimated to be 2.2 μm, 1.6 μm, 0.8 μm and 0.4 μm, respectively. The WC grain size

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distribution of these specimens were shown in Fig. 2. The addition of VC and Cr3C2 in the initial powders effectively reduced the grain size of the prepared cemented carbides. Generally,

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VC and Cr3C2 can completely dissolve in the Co binder or partially precipitate at the WC/Co interfaces, which thereby inhibits the WC grain growth in the liquid-state sintering process.

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The detailed evidence can be found elsewhere in our previous work [16]. Due to a very low addition amount, VC and Cr 3C2 have little effect on wear resistance of the cemented carbide as compared to the WC grain size.

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As shown in Fig. 2, the grain size distribution of the ultrafine-grained sample is much narrower than that of the coarse-grained sample. A few WC grains in samples S1 and S2 have

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obviously larger size than the mean values. However, the abnormal WC grain growth accounted for only a very low fraction. Moreover, the size of the relatively larger WC grains in each sample shifts towards smaller values with the decrease of mean WC grain size. Therefore, the

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mean grain size is representative for evaluation of the grain-size effect on wear resistance of the cemented carbides. The measured relative density, amount of magnetic Co, hardness and fracture toughness of the cemented carbides are summarized in Table 3. The relative density values of the specimens with smaller WC grain sizes (S3 and S4) are slightly lower than that with larger grains (S1 and S2). The amounts of magnetic Co of all specimens are close to the nominal Co content (i.e. 8 wt%). With the decrease in WC grain size, the hardness of the prepared cemented carbides 4

increases considerably. Interestingly, the ultrafine-grained cemented carbide (S4) has the same mean fracture toughness as the submicron-grained specimen (S3) though they have significantly different hardness. The fracture toughness of the coarse- (S1) and medium coarse-grained (S2) cemented carbides were not obtained because cracks were not generated by indentations with a 30 kg load, indicating a higher fracture toughness than the submicron and ultrafine-grained cemented carbides. Besides, as shown in Table 3, the variation of hardness of the samples is very small (the highest error of hardness in sample S3 is only 1.46%). This

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indicates that the microstructure of these samples is homogeneous in terms of distributions of WC grain size and Co. Therefore, it is reasonable to believe that the grain size distribution in

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the cemented carbides has little effect on the mechanical properties and wear resistance. 3.2. Microabrasion and sliding wear properties

The microabrasion and sliding wear rates of the prepared cemented carbides with different

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WC grain sizes are shown in Fig. 3. The sliding wear rate of the sintered WC-Co cemented carbides increases monotonically with the increase of WC grain size. In contrast, the

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microabrasion wear rate of the specimens increases at first, then rapidly declines with the

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increase of grain size. Such a large difference is associated with the friction load, stress state of

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WC grains and the corresponding wear mechanism.

Fig. 4 shows the typical optical morphology and cross-sectional profiles of the craters

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caused on the sintered WC-Co cemented carbides after microabrasion tests. It is obvious that the crater depth of the submicron-grained cemented carbide, which has the largest wear loss, is much larger than those of the other specimens. The dynamic friction coefficient curves of the WC-Co cemented carbides as a function of

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time during sliding wear tests are shown in Fig. 5. The mean values at the steady friction stage (i.e. the time period of 10-30 min as marked in yellow background) are estimated to be 0.40,

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0.39, 0.38 and 0.45 with the decrease of WC grain size, respectively. It is clear that the ultrafine-grained cemented carbide has a higher friction coefficient than the others.

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The 3D surface topography and corresponding cross-sectional profiles of sliding wear

scars of the prepared WC-Co cemented carbides are shown in Fig. 6. Both the width and depth of the wear scars decreased with the WC grain size decreasing from 2.2 μm to 0.8 μm. However, as the WC grain size further decreased from 0.8 μm to 0.4 μm, the wear scar became wider and the depth decreased. This suggests that the ultrafine-grained cemented carbide has caused more severe damage on the Si 3N4 counterpart ball due to its extremely high hardness compared to the submicron-grained cemented carbide. Though the contact area between the

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ultrafine-grained specimen and counterpart ball increased, the penetration rate of the counterpart ball was significantly reduced. 3.3. Macro scratch tests The width of scratches on the sintered WC-Co cemented carbides is plotted as a function of scratch load, as shown in Fig. 7(a). Correspondingly, the compressive stresses generated beneath the indenter for each scratch can be calculated as follows:

 = 8P/πW2

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(2)

where  is the compressive stress on the specimens, P is the applied normal load and W is

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scratch width. The calculated compressive stress is also plotted as a function of scratch load, as shown in Fig. 7(b).

As shown in Fig. 7(a), the scratch width follows a linear increasing trend with the increase of load for a given specimen (except for the two data points of the coarse-grained cemented

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carbide at higher loads). At a fixed load, particularly above 40 N, the scratch width of the

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cemented carbide increases remarkably with the increase of WC grain size. As shown in Fig.

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7(b), the compressive stress generated on the specimens (except for the coarse-grained cemented carbide) is gradually increased with the rising of load and approaching to a constant

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value when the load is above 40 N. Interestingly, the limit compressive stress of the submicron and ultrafine-grained WC-Co cemented carbides are approximately equal to their hardness (see

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Table 3). In contrast, the limit values of the coarse- and medium coarse-grained specimens are slightly lower than their hardness. Moreover, the calculated compressive stress in the coarse-grained cemented carbide decreases significantly when the load exceeds 50 N. The main

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reason is that many coarse WC grains become crushed under the critical load, which will be presented and discussed later.

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4. Discussion

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The wear resistance of the prepared WC-Co cemented carbides shows a different changing

trend with the WC grain size in microabrasion and sliding wear tests. However, the results of scratch tests at various loads are consistent with those of sliding wear tests. In this section, the wear behaviors of the cemented carbides under different tribological conditions are discussed, based on which the grain size effects on wear resistance of these cemented carbides are disclosed. 4.1. Microabrasion behavior 6

Fig. 8 shows the worn surfaces of the WC-Co cemented carbides after microabrasion tests. The low-magnification images (Fig. 8(a), (c), (e) and (g)) show that the depth of the grooves in the submicron- and ultrafine-grained cemented carbides is much smaller than those in the coarse-grained cemented carbides. This result is consistent with the variation of their hardness, that is, the cemented carbide with higher hardness has increased resistance against the penetration of abrasives. However, the cemented carbides with finer grain sizes are less

indicating that hardness is not the dominant factor in this condition.

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wear-resistant as compared with the coarse-grained counterpart in the microabrasion test, In the coarse-grained (2.2 μm) specimen, large amounts of Co have been removed (Fig. 8(b)), followed by slight fracture at the edges of WC grains, as shown in Fig. 8(b).

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Interestingly, the typical deformation characteristic, i.e. the occurrence of slip bands within individual WC grains [12, 17], is not observed. This implies that the applied load (0.2N) is not sufficient to give rise to large plastic deformation of the coarse WC grains.

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In the medium coarse-grained (1.6 μm) specimen, besides the preferential removal of Co

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binder, the fracture and fragmentation of WC grains become slightly more severe (Fig. 8(d))

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compared to the coarse-grained cemented carbide. This can be attributed to the decreased dislocation slip plasticity of the WC grains with smaller sizes and the thinner Co binder.

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In the submicron grained (0.8 μm) specimen, Co removal and fracture of WC grains played an equivalently important role in the wear process. As shown in Fig. 8(f), the pull-out of

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submicron WC grains after Co removal has led to a relatively high irregularity of the worn surface. This can be explained in terms of the lower fracture toughness and decreased binding area between submicron WC grains and the Co binder.

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In the ultrafine-grained (0.4 μm) WC-Co cemented carbide, the depth of the grooves is further decreased (see Fig. 8(e) and (g)), whereas less WC fragments are observed (Fig. 8(h)).

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The preferential removal of Co binder followed by pull-out of WC grains should be the dominant wear mechanism of the ultrafine-grained cemented carbides. Actually, the worn microstructures only exhibited very limited wear characteristics of the specimen. Some

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information has been lost because of the overall pull-out of WC grains. However, from the higher wear rate of the ultrafine-grained cemented carbide, it can be assured that the pull-out of ultrafine WC grains after Co removal is faster than that of the coarse-grained specimen. Generally, in the low-load (0.2 N) microabrasion test, the dominant wear mechanism of the cemented carbides is preferential removal of Co followed by WC grain pull-out. Besides, fracture of WC grains also leads to the wear of the cemented carbides. With the decrease of WC grain size, the thickness of Co binder decreases and the hardness of cemented carbide 7

increases. As a result, the penetration of abrasive into the Co layer is hindered and the Co binder is not easily removed. However, once Co is removed, the adjacent WC grains with smaller sizes are more likely to be detached because of less binding areas between WC and Co. Therefore, there is a competition of wear mechanisms between Co removal and WC grain pull-out for cemented carbides with different WC grain sizes. In the coarse-grained cemented carbide, the Co binder is easily removed; however, it is more difficult to cause the pull-out of coarse WC grains. Moreover, fracture and fragmentation

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of the coarse WC grains are less at the low load, which also reduces their removal rate. These two aspects contribute to the high wear resistance of the coarse-grained cemented carbide in the microabrasion condition. In contrast, though the submicron-grained specimen has a higher

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hardness and a good fracture toughness which are beneficial to its wear resistance, these points are weak as compared with the overall loss of WC grains after Co removal. Therefo re, the submicron cemented carbides have the highest wear rate among the specimens for tests. The

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slight decrease in wear rate of the ultrafine-grained specimen is mainly attributed to its thin Co

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layer and extremely high hardness, which reduce the damage of abrasive penetration to the Co

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binder. This also explains that in the ultrafine- and nano-grained cemented carbides, the interfacial bonding between carbides and metallic binder plays a more important role in the

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wear resistance rather than the hardness [14]. The above finding provides a reference for understanding the different wear resistance of these samples in high-load sliding wear tests (see

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Fig. 3).

4.2. Sliding wear behavior

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The typical sliding wear characteristics and EDS analysis results of the sintered cemented carbides with the mean WC grain sizes of 2.2 μm, 0.8 μm and 0.4 μm are shown in Figs. 9 -11,

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respectively. As shown in Fig. 9(a), most WC particles at the surface of the coarse-grained specimen are cracked, and many fragments are formed accordingly (see Fig. 9(b)). The EDS

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analysis result indicates that these fragments consist of both WC and Co. Moreover, severe oxidation has occurred in local regions (i.e. the dark areas in Fig. 9(a)), as evidenced by the EDS maps. The magnified image of the dark areas shows that the W and Co containing oxides have been crushed into ultrafine and nanoscale particles during the repeated sliding friction, eventually forming the oxide scales with a scaly structure. Similar worn microstructure has been reported in a previous work[18]. In another area, the rod-like oxides (Fig. 9(d)) can be observed, which might well be WO 2.72[19]. Though this oxide does not account for a large area

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fraction in the wear scar, it clearly indicates that the coarse-grained cemented carbide has experienced such an oxidation stage during the dry sliding wear process. Meanwhile, the fracture of a WC grain (the composition was confirmed by EDS analysis which was not shown here) occurring in early stage was also observed, as shown in Fig. 9(e). This feature may result from a subsequent damage on the slip bands formed by dislocation glide in coarse WC grains[12]. In the WC planes without active slip systems, irregular fracture and fragmentation are more easily to take place, as shown in the area marked by a circle in Fig. 9(a).

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In the medium coarse-grained cemented carbide, irregular fracture and subsequent fragmentation of WC grains, like those described in the coarse-grained specimen, are also observed. However, a decreased area fraction of the worn surface is covered with W and Co

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containing oxides.

The worn microstructure of the submicron-grained cemented carbide also shows the presence of surface oxidation, as indicated in Fig. 10(a) and (c) and corresponding EDS maps.

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However, the oxidation degree is obviously decreased compared to that in coarse-grained

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specimens. Moreover, from a magnified image of the wear scar (Fig. 10(b)), it is found that lots

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of WC grains have a step-shaped surface morphology (as marked by the arrows), which is thought to form as a result of plastic deformation of WC grains, stress concentrations

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accumulated by dislocation pile-up and eventual fractures. The fractures are mostly confined to the surface areas of the submicron WC grains and complete breakage is rarely observed.

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In the ultrafine-grained cemented carbide, there are no significant changes in the microstructure compared to that without wear testing (Fig. 1(d)). Only slight scratches can be observed on the surface of several WC grains, as indicated by the arrows in Fig. 11(b).

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Moreover, the EDS analysis result of the whole imaged area of Fig. 11(a) shows that oxidation did not occur in this specimen during the sliding wear process. A point EDS scanning at the

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dark region of Fig. 11(b) confirms the existence of the Co binder phase. In fact, the maximum wear depth of the ultrafine-grained WC-Co sample is only 2.484 μm (see Fig. 6(d)), which may only result from plastic deformation of the loading area rather than loss of materials. This is

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distinctly different from that in microabrasion tests during which the abrasives could result in the removal of Co and subsequently the pull-out of WC grains in large amounts. It is found that whether there are abrasives also plays a crucial role in the measured wear resistance of cemented carbides. 4.3. Microstructure evolution of scratches under different loads

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In the dry sliding wear tests, the coarse-grained and ultrafine-grained WC-Co cemented carbides exhibit significantly different wear behaviors at the load of 70 N. Scratch tests under a wide range of loads further confirm that the cemented carbides with smaller WC grain sizes are much more wear-resistant in dry sliding wear conditions (see Fig. 7). The typical worn microstructures of the coarse-grained and ultrafine-grained WC-Co samples after scratching tests are shown in Fig. 12. The fracture of coarse WC grains can occur after single-pass scratch at a load of 10 N [20],

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as indicated by the arrows in Fig. 12(a). With the load increases, more WC grains are cracked. At 40 N, fragmentation of WC grains arises, as marked by the arrows in Fig. 12(c). The Co layers also become thinner due to large plastic deformation of the specimen. At 70 N, almost

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the entire surface of the coarse-grained specimen is covered with WC-Co fragments. It is interesting that the abnormal decrease of the calculated compressive stress on the coarse-grained WC-Co sample starts from the load of 50 N, which is in accordance with the

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formation of many WC-Co fragments. This indicates that the drastic increase of the scratch

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width under higher loads might well be caused by the severe fragmentation of coarse WC

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grains.

In contrast, there are no significant changes in scratched surfaces of the ultrafine -grained

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WC-Co sample expect for several shallow scratches when the load increases from 10 N to 40 N, as shown in Fig. 12(b) and (d). At 50 N, the scratched surface shows the occurrence of

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fragments in local areas as marked by the arrows in Fig. 12(f). At 70 N, some WC-Co materials in the vicinity of severe plastic deformation zones are detached from the sample, as shown in Fig. 12(h). Moreover, cracks can also be observed in certain ultrafine WC grains (as marked by

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the arrows in Fig. 12(h)). The differences between scratch test and the sliding wear test are mainly the counterpart materials and the generated stress fields due to different tip radius. It is

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reasonable that the diamond indenter with a higher hardness than the Si 3N4 ball could cause more severe plastic deformation and fractures in the ultrafine-grained cemented carbide in

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scratch test compared to that in sliding wear test under the same load. 4.4. Grain size effects on wear resistance of cemented carbides Based on the above results, it is found that the wear resistance of cemented carbides exhibits differently the dependence on WC grain size under different tribological conditions. The reasons can be explained as below. In conditions without abrasives such as sliding wear and scratch tests, the contacting area between a specimen and the mating Si 3N4 ball or diamond

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indenter is large enough to cover a large number of grains. In this case, the stress state generated beneath the surface of the material varies greatly with the grain size. The coarse-grained cemented carbide with lower hardness encounters more severe plastic deformation compared to the fine-grained material, which leads to ease of fracture and fragmentation of coarse WC grains. With the decrease of grain size, the hardness of the cemented carbide increases, and the friction-induced plastic deformation is reduced. Consequently, the degree of stress concentration and fracture occurred in the ultrafine WC

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grains decrease significantly, which contribute to high wear resistance under such tribological conditions.

Besides, an interesting phenomenon is that the surface oxidation of the samples becomes

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less intensive with the decrease of WC grain size during sliding wear tests. There are no signs that oxidation occurred in the ultrafine-grained WC-Co specimen. The reason of oxidizing is the significant rise in temperature of contact areas because of frictional heat. The temperature

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rise ΔT can be roughly estimated by the following equation [21, 22]:

(3)

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ΔT = fν(πHP)1/2/[4J(K1+K2)]

where f is the friction coefficient, P is the applied load, v (m/s) is the sliding velocity, K1 and K2

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(W/(m K)) are thermal conductivities of the two contacting materials, J is Joule’s constant

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(J=1) and H (N/m2) is the measured hardness of the samples. For the coarse-, medium coarse- and submicron-grained WC-Co cemented carbides, a

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large number of WC-Co fragments existed between the counterpart ball and the samples. Thus, the temperature increase mainly acted on the fragments rather than the subsurface material. The thermal conductivities (K1 and K2) should be those of WC-Co fragments and the Si 3N4 ball. For

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the ultrafine-grained WC-Co specimen, the friction process did not generate fragments. Thus, the two contacting materials are WC-Co (~135 W/(m K)[23]) and Si3N4 ball (26.5-82.5 W/(m K) ), respectively. Using the above equation and the present experimental data, the calculated

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[24]

value of temperature rise at the surface of the ultrafine-grained WC-Co material is in the range of 88-118 °C. However, the thermal conductivity of loosely stacked WC-Co fragments may be

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decrease by 1-2 orders of magnitude, according to similar experiences in metal powders [25]. Assuming that the thermal conductivity of WC-Co fragments is only 1/10 of the bulk material, the calculated temperature rise for those cemented carbides with severe WC fragmentation is in the range of 198-477 °C. Obviously, this temperature is sufficient to cause the oxidation of WC-Co fragments during the sliding wear process. With the increase in hardness of cemented carbides, less fragments were generated at the surface, and therefore the friction-induced temperature rise and surface oxidation could be decreased. 11

In microabrasion tests, the abrasives could act on individual WC grains and Co layers. At the load of 0.2 N, even the coarse WC grains did not encounter severe fractures. However, the Co binder could be easily removed due to the penetration, embedment and cutting of abrasives. The hard WC particles without sufficient support of the Co binder were more likely to be pulled out because of frictional forces. As compared with those with smaller sizes, the coarser WC particles embedded into the Co binder more deeply, thus having increased critical stress for their detachment. This explains why the microabrasion resistance of the cemented carbides

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is reduced with the decrease of WC grain size from 2.2 μm to 0.8 μm. The slight increase in wear resistance of the ultrafine-grained cemented carbide is attributed to its extremely high hardness and thin Co layer, which can retard the penetration of abrasives. The above

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mechanisms dominated the wear of the cemented carbides under the microabrasion condition. It is expected that the grain-size dependence of wear resistance of the sintered WC-Co

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cemented carbides may vary with the size of abrasives and the pH values of slurries.

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5. Conclusions

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The wear resistance of the prepared WC-Co cemented carbides with varying grain sizes

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was investigated under different tribological conditions. The following conclusions can be drawn.

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(1) In the microabrasion tests, the wear rate of the cemented carbide generally increases with the decrease of WC grain size, except the ultrafine-grained cemented carbide which has extremely high hardness and good fracture toughness. The pull-out of WC grains after Co removal dominates the wear of the cemented carbide rather than hardness.

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(2) In the sliding wear tests, the wear rate of the cemented carbide remarkably decreases with the decrease of WC grain size due to mitigation of multiple risks caused by plastic

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deformation, fracture, fragmentation and oxidation of grains. Hardness plays an essential role in the wear resistance in this case.

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(3) The results of scratch tests are in accordance with those obtained under sliding wear

condition. Moreover, it is indicated that the fracture of coarse WC grains can occur at a load of only 10N after a single-pass scratch. (4) The severe surface oxidation of the coarse-grained cemented carbide during dry sliding wear process can be attributed to the formation of WC-Co fragments which block the transfer of friction-induced heat and cause a significant increase in surface temperature.

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Acknowledgements This work was supported financially by the National Natural Science Foundation of China (Nos. 51601004, 51631002, 51425101 and 51621003), the China Scholarship Council (201806545002) and the Program of Top Disciplines Construction in Beijing (No. PXM2019_014204_500031). The authors would also like to thank Ken Mingard, Linda Orkney, John Nunn and Eric Bennett for their assistance with experimental aspects.

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References [1] J. Heinrichs, H. Mikado, A. Kawakami, U. Wiklund, S. Kawamura, S. Jacobson, Wear 420–421 (2019) 96–107.

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[2] F. Bosio, E. Bassini, C.G.O. Salazar, D. Ugues, D. Peila, Wear 394–395 (2018) 203–216. [3] J. Pirso, S. Letunovitš, M. Viljus, Wear 257 (2004) 257–265.

[4] Konyashin, B. Ries, Int. J. Refract. Met. Hard Mater. 46 (2014) 12–19.

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[5] D. Jianxin, Z. Hui, W. Ze, L. Yunsong, Z. Jun, Int. J. Refract. Met. Hard Mater. 31 (2012) 196–204.

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[6] D. Tkalich, A. Kane, A. Saai, V.A. Yastrebov, M. Hokka, V.T. Kuokkala, M. Bengtsson,

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Wear 387 (2017) 106–117.

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[7] H. Saito, A. Iwabuchi, T. Shimizu, Wear 261 (2006) 126–132. [8] M. Chandrashekar, K.V.S. Prasad, Mater. Today Proc. 5 (2018) 7678–7684.

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[9] M. Antonov, R. Veinthal, D.L. Yung, D. Katušin, I. Hussainova, Wear 332–333 (2015)

[10] K. Bonny, P. DeBaets, J. Vleugels, S. Huang, O. Van der Biest, B. Lauwers, Wear 267 (2009) 1642–1652.

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[11] A.J. Gant, M.G. Gee, A.T. May, Wear 256 (2004) 954–962. [12] X. Liu, J. Zhang, C. Hou, H. Wang, X. Song, Z. Nie, Mater. Des. 150 (2018) 154–164.

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[13] M. Gee, K. Mingard, J. Nunn, B. Roebuck, A. Gant, Int. J. Refract. Met. Hard Mater. 62 (2017) 192–201.

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[14] I. Konyashin, B. Ries, D. Hlawatschek, Y. Zhuk, A. Mazilkin, B. Straumal, F. Dorn, D. Park, Int. J. Refract. Met. Hard Mater. 49 (2015) 203–211.

[15] D.K. Shetty, I.G. Wright, P.N. Mincer, A.H. Clauer, J. Mater. Sci. 20 (1985) 1873–1882. [16] X.W. Liu, X.Y. Song, H.B. Wang, X.M. Liu, F.W. Tang, H. Lu, Acta Mater. 149 (2018) 164–178. [17] J. Heinrichs, M. Olsson, K. Yvell, S. Jacobson, Int. J. Refract. Met. Hard Mater. 77 (2018) 141–151.

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[18] K. Bonny, P. De Baets, Y. Perez, J. Vleugels, B. Lauwers, Wear 268 (2010) 1504–1517. [19] X. Huang, Z. Zhang, H. Li, H. Wang, T. Ma, J. Energy Chem. 29 (2019) 58–64. [20] M. Gee, K. Mingard, J. Nunn, B. Roebuck, A. Gant, Int. J. Refract. Met. Hard Mater. 62 (2017) 192–201. [21] Y. Liu, A. Erdemir, E.I. Meletis, Surf. Coat. Technol. 82 (1996) 48–56. [22] N. Vashishtha, S.G. Sapate, P. Bagde, A.B. Rathod, Tribol. Int. 118 (2018) 381–399.

[24] F. Hu, L. Zhao, Z.P. Xie, J. Ceram. Sci. Technol. 7 (2016) 423–428.

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[23] H. Wang, T. Webb, J.W. Bitler, Int. J. Refract. Met. Hard Mater. 49 (2015) 170–177.

[25] M.J. Powell-Palm, J. Beuth, L.E. Ehrlich, J.A. Malen, L.C. Wei, C. Montgomery, Addit.

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Manuf. 21 (2018) 201–208.

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Figure list:

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Fig. 1. Microstructures of the cemented carbides prepared with WC-Co powders of different

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characteristics: (a) S1; (b) S2; (c) S3; (d) S4.

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Fig. 2. WC particle size distribution of the prepared cemented carbides.

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Fig. 3. Wear rate of the cemented carbides with different WC grain sizes.

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Fig. 4. Typical optical images of craters caused on the sintered samples with (a) 2.2 μm, (b) 1.6

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μm, (c) 0.8 μm and (d) 0.4 μm after microabrasion tests, and (e) the cross-sectional profiles of

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the craters.

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CC

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Fig. 5. Friction coefficient of the sintered WC-Co cemented carbides during sliding wear tests.

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Fig. 6. 3D surface topographies obtained from vertical scanning white-light interference

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microscopy and corresponding cross-sectional profiles of sliding wear scars of the sintered

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CC

WC-Co cemented carbides with (a) 2.2 μm, (b) 1.6 μm, (c) 0.8 μm and (d) 0.4 μm.

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Fig. 7. Variation of scratch width (a) and calculated compressive stress (b) with the increase of

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applied load on the sintered WC-Co cemented carbides.

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Fig. 8. Microstructures of the sintered WC-Co cemented carbides after microabrasion test

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under the load of 0.2 N: (a, b) 2.2 μm; (c, d) 1.6 μm; (e, f) 0.8 μm; (g, h) 0.4 μm.

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Fig. 9. Typical wear characteristics and elemental analyses of the coarse-grained cemented carbide after sliding wear test under the load of 70 N: (a) low-magnification image; (b) mixed WC and Co fragments; (c) powdery oxides containing W and Co; (d) rod-like W oxides; (e) fracture of WC grain. EDS maps show the distributions of O, Co and W in (a). The area marked with circle in (a) shows irregular fracture and fragmentation of WC grains. 23

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Fig. 10. Typical wear characteristics EDS maps of the submicron-grained cemented carbide

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after sliding wear test under the load of 70 N: (a) low-magnification image; (b) irregular

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fracture of WC; (c) oxide scales. The EDS maps show the distributions of O, Co and W in (a).

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Fig. 11. Worn microstructures of the ultrafine-grained cemented carbide after sliding wear test

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at 70 N: (a) low-magnification image; (b) high-magnification image; (c) EDS spectrum

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obtained by scanning the entire imaged area of (a); (d) EDS spectrum obtained at the point marked with ―+‖ in (b).

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Fig. 12. Typical microstructures of scratches on the coarse-grained cemented carbides with loads of (a) 10 N, (c) 40 N, (e) 50 N and (g) 70 N and ultrafine-grained with loads of (b) 10 N, (d) 40 N, (f) 50 N and (h) 70 N.

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Table list:

Table 1 Preparation details of the WC-Co powders. Powder

Particle size of raw powders (μm)

Composition

Milling

WC

Co

VC

Cr3C2

S1

WC-8Co

3

10

-

-

S2

WC-8Co

3

30

-

-

3

30

0.5

0.5

0.8

30

0.5

0.5VC-0.5Cr3C2 WC-8Co0.5VC-0.5Cr3C2

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S4

WC-8Co-

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S3

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time (h)

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(wt%)

0.5

Table 2 Experimental details of the wear tests. Microabrasion

Dry sliding wear

Normal load (N)

0.2

70

Wear counterpart

Stainless steel ball, Φ25

Si3N4 ball, Φ5

Wear time (min)

30

30

Wear distance (m)

220

150

SiC (4 μm) slurry with a concentration of 0.2 g/ml and flow rate of 44 ml/h



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Abrasive flow rate

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Parameter

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Table 3 Properties of the prepared cemented carbides. Fracture

hardness

toughness (MPa

(HV30)

m1/2)

7.87

1489 ± 10



99.8

7.68

1549 ± 9



S3

97.7

7.34

1715 ± 25

11.3 ± 0.7

S4

98.5

7.41

2154 ± 27

11.3 ± 0.3

Amount of

No.

(%)

magnetic Co (wt%)

S1

99.5

S2

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Relative density

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Vickers

Specimen

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