Effect of Mo and Y2O3 additions on the microstructure and properties of fine WC-Co cemented carbides fabricated by spark plasma sintering

Accepted Manuscript Effect of Mo and Y2O3 additions on the microstructure and properties of fine WC-Co cemented carbides fabricated by spark plasma sintering

Shengda Guo, Rui Bao, Jiangao Yang, Hao Chen, Jianhong Yi PII: DOI: Reference:

S0263-4368(17)30252-4 doi: 10.1016/j.ijrmhm.2017.07.010 RMHM 4480

To appear in:

International Journal of Refractory Metals and Hard Materials

Received date: Revised date: Accepted date:

24 April 2017 14 July 2017 18 July 2017

Please cite this article as: Shengda Guo, Rui Bao, Jiangao Yang, Hao Chen, Jianhong Yi , Effect of Mo and Y2O3 additions on the microstructure and properties of fine WCCo cemented carbides fabricated by spark plasma sintering, International Journal of Refractory Metals and Hard Materials (2017), doi: 10.1016/j.ijrmhm.2017.07.010

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ACCEPTED MANUSCRIPT Effect of Mo and Y2O3 additions on the microstructure and properties of fine WC - Co cemented carbides fabricated by spark plasma sintering Shengda Guo1,2, Rui Bao1, Jiangao Yang2, Hao Chen2, Jianhong Yi1*

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1. School of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China

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2. Institute of Engineering Research, Jiangxi University of Science and Technology, Ganzhou,

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Jiangxi 341000, China

Corresponding author: Jianhong Yi, E-mail address: [email protected]

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Present / permanent address: School of Material Science and Engineering, Kunming University of Science and Technology, Kunming City, Yunnan Province,

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China (Zip Code: 650093)

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Abstract: The effects of Mo and Y2O3 additions on the microstructure and properties

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of fine WC-6Co cemented carbides prepared by spark plasma sintering (SPS) were investigated. The microstructure, mechanical properties and corrosion behavior were analyzed by SEM, TEM, XPS, mechanical property tests and potentiodynamic

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polarization curve measurements. The results show that there are no significant differences in the microstructure and properties of alloys with 1.0 % Y2O3 addition.

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However, the addition of Mo was beneficial for refining the WC grains. The hardness of alloys increased until the maximun value, and then followed a decreasing trend with the increase of Mo content. In addition, the relative density of alloys decreased with the increase of Mo addition, which caused to an obvious decline in fracture toughness. With the additions of Mo and Y2O3, the corrosion resistance of alloys improved significantly both in acid and alkaline solutions (0.1 M HCl and 0.1 M NaOH solutions). The adding amount of Mo should be controlled within a certain 1

Corresponding author. Tel / fax: +86 87165916977 E-mail address: [email protected]

ACCEPTED MANUSCRIPT range and 1 wt. % of Mo has the most positive effect on the comprehensive properties of WC-6Co alloys. Key words: WC-6Co cemented carbides; Molybdenum; Yttrium oxide; Mechanical properties; Corrosion resistance

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1. Introduction Owing to the combination of excellent hardness, thermal stability, enhanced

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strength and good toughness [1, 2], cemented carbides have been widely used in

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various engineering applications such as mechanical processing tools, dies, general wear parts and so on [3-7]. The excellent mechanical properties of cemented carbides

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such as hardness, transverse rupture strength and fracture toughness are derived from a combination of the hard carbides and ductile binder matrix. Traditionally, WC is a

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common choice for hard carbide due to the high hardness and good wear resistance. Meanwhile, ductile Co is normally regarded as the binder phase for its excellent

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wetting, adhesion and adequate mechanical properties for the great strain without

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breaking [8, 9]. However, cemented carbides are increasingly employed in a variety of other industrial applications, such as fluid mixers, drilling tools and jet nozzles, with the development of industry.

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The different application areas mentioned above are performed in chemically aggressive environments which make the alloys susceptible to corrosion [10, 11].

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Therefore, enhancing the corrosion resistance of the alloys gradually becomes an crucial issue since corrosion degrades the cemented carbides and thus shorten its lifetime [12]. As is known to all that for the pure samples of WC and Co, the stability of WC and Co exhibits an opposite pH dependence. That is, Co shows passivation at alkaline pH and continuously increasing dissolution rates towards the acidic domain, while WC is least stable in alkaline solution and becomes more stable with decreasing pH [10]. However, previous investigations have shown that the binder phase of WC-Co cemented carbides is not pure Co, but is a Co-W-C eutectic. In addition, A.J. Gant [13] et al indicated that in acidic solution, the WC/Co interface is the most

ACCEPTED MANUSCRIPT anodic based on the fact that there exists a thin layer of Co which has a lower tungsten content than the bulk cobalt. Thus, the corrosion behavior of WC-Co cemented carbides becomes more complicated due to the characteristics mentioned above. The chemical modifications of the alloys certainly had an effect on the electrochemical behavior. It has been recognized that several carbides such as Cr3C2,

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VC, TiC and TaC are introduced in WC-Co cemented carbides to enhance the properties and corrosion resistance [14-16]. An early study [15] indicated that the

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addition of NbC and VC effectively refines the grain size of WC by restraining the dissolution / precipitation processes of WC grains in Co binder phase. C. N. Machio

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[16] et al investigated the effect of VC addition on the corrosion behaviour of WC-11 wt. % Co cemented carbides in acids solutions. The results show that the introduction

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of VC in WC-Co improved the passivation resistance of the hard alloys in HCl solution, and at high VC contents in lower corrosion current densities than for the

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base alloy in both HCl and H2SO4 solutions.

It was found that the addition of Mo, Mo2C, Ni and rare earth is beneficial for

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increasing properties of WC-based cemented carbides [17-25]. A series of previous

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studies have demonstrated that hardness, abrasion resistance and corrosion resistance of WC-TiC-based alloys all increase significantly with the addition of Mo and Mo2C [19-22]. Furthermore, the corrosion resistance of WC-Co cemented carbides is

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improved by the introduction of Ni. However, it has been found that Ni added cemented carbides are inferior to WC-Co cemented carbides counterparts in

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mechanical properties [17, 18]. Meanwhile, the effect of nano-yttria addition on the microstructure and mechanical properties were also investigated [3, 23, 24]. The results show a significant improvement in the mechanical properties of WC-Co cemented carbides with addition of Y2O3. Although J. H. Potgieter [25] et al revealed that Ru addition is more effective in improving corrosion resistance of WC-Co cemented carbide in acid solution than a small quantity of VC addition, the influence of Y2O3 on the corrosion resistance of alloys was not investigated. Untill now, limited studies are available about the effects of Mo and Y2O3 on the corrosion behaviour of straight WC-Co cemented carbides in both acid and alkaline

ACCEPTED MANUSCRIPT solutions. Hence, the main purpose of this paper is to perform an systematic investigation on the effect of Mo and Y2O3 addition on the microstructure, mechanical properties and corrosion behaviour of WC-6Co cemented carbides. 2. Experimental

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2.1 Preparation The contents of the raw powders used in this work are listed in Table 1. Five sets

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of WC-6Co alloys with various Mo and Y2O3 content were prepared via three-roller grinder system combined with SPS. The mixtures were mechanically ball-milled with

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cemented carbide balls for 48 h in a three-roller grinder system using anhydrous alcohol as the liquid medium. The ball-to-powder ratio was 5:1 and the rotation speed

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was 100 r/min. Then the mixed powders were dried at 70 °C in vacuum for 4 h. Subsequently, the composite powders were put into a graphite die with diameter of 20

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mm and heated up to 1250 °C with a heating rate of 100 °C/min and holding time of 10 min under a sintering pressure of 50 MPa in vacuum.

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2.2 Characterization

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The alloys were wet ground using coarse diamond grinding disc with 180 μm grit to obtain a flat surface, and then by fine diamond grinding discs with of 400 μm and 600 μm. Subsequently, the flat surface was polished with a pan cloth with 7 μm

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followed by a 1μm diamond spray paste to obtain a mirror-like surface. The polished surfaces were etched by Murakami’s solution for SEM analysis. All the densities of

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the as-sintered alloy bulks were measured by the Archimedes method. The phase compositions of alloys were detected by X-ray diffraction (XRD) (PANalytical, Empyrean)with Cu Kα (λ=0.15406 nm) at a scanning step size of 0.026° in the scan range of 30-80°. Scanning electron microscopy (SEM) (FEI, MLA650F) equiped with an energy dispersive X-ray spectroscopy (EDS) was utilized to analyze the microstructures and the chemical composition of as- polarized alloys. A FEI Tecnai G2 TF30 S-Twin was used at 300 kV to analyse the microstructure and the chemical composition of the Co binder phase. The X-ray pholoelectron spectroscopy (XPS) measurements of alloys tested under different testing conditions were performed with

ACCEPTED MANUSCRIPT a PHI5000 Versaprobe-II apparatus. And the mean grain size of WC was measured by linear intercept method based on SEM images. At least five SEM images were randomly selected for measurement. On average, each SEM image contains about 200 WC grains. The hardness (kg•mm-2) was measured with Vikers hardness tester under a constant load of 30 kg. The reported hardness values of alloys were the mean value of

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five measurement results. The fracture toughness (Wk, which is expressed in MPa•m1/2) was calculated using the Palmqvist indentation method as follows.

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(1)

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Where P is the ratio of indent of load (N), li is the length of crack tip from the hardness indent (mm).

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Potentiodynamic polarization curve is employed to investigate the corrosion behaviour in HCl (PH=1) and NaOH (PH=13) solutions at room temperature. A

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conventional three-electrode cell equipment, consisting of a saturated calomel electrode (SCE) as the reference, a large platinum sheet as the counter electrode, and the alloys as working electrode, respectively, was used for the electrochemical test at

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room temperature. The electrochemical tests were carried out using an

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electrochemical working station (CHI660). 0.1 M HCl and 0.1 M NaOH aggressive solutions were selected as electrolytes in open air. Before the tests, all the alloys were

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immersed in the corresponding electrolyte for 30 min. The potentiodynamic polarization curve tests were performed for the five sets of hard alloys from -800 mV

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to +250 mV (SCE) at a scan rate of 5 mV·s-1 and the sensitivity was 10 mA/V. The corrosion potential (Ecorr) and corrosion current density (icorr) were determined by the General Purpose Electrochemical Software. 3. Results and discussion 3.1 Microstructure and Composition The morphologies of the WC-6Co composite powders, Mo powders, Y2O3 powders and WC-6Co-1Mo composite powders prepared by ball-milling method are shown in Fig. 1. It can be seen from Fig. 1 that the distribution of WC, Mo and Y2O3

ACCEPTED MANUSCRIPT particle size is narrow, which is beneficial for generating a uniform microstructure. Fig. 1(d) shows that the particle size of prepared WC-6Co-1Mo composite powders are also ultrafine and distributed uniformly. The XRD patterns of the as-prepared hard alloys with a variety of additions are shown in Fig. 2. By XRD, only WC phase and few weak fcc-Co binder phase are

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identified in each alloy due to the few content of Mo and Y2O3 additions [26]. Moreover, the magnified XRD patterns of Co binder phase were faintly observed in

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Fig. 2 (b). The five smooth curves in Fig. 2 (b) are the fitting results of XRD datas. According to the magnified XRD patterns in Fig. 2 (b), the (111) peaks of Co binder

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phase of alloy 3, alloy 4 and alloy 5 shift slightly toward the negative direction compared with that of alloy 1 and alloy 2, which indicates that the Co lattice expands

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with the addition of Mo. During the sintering process, Mo and C reacted to form Mo2C phase firstly before 900 ºC. Furthermore, Mo2C dissolved in Co binder phase

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[19]. Therefore, the negative shift of (111) peaks of Co binder phase may be due to the dissolution of Mo into the Co binder phase [11].

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Fig. 3 shows the SEM images in back scattering mode of the alloys with

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different kinds of additions. The lighter phase corresponds to WC grains, which can be easily observed as they are distinctly lighter than the darker cobalt binder phase. There are no significant differences in the microstructure morphologies of alloy 1,

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alloy 2 and alloy 3. Additionally, abnormal grain growth which is a very vexing problem in cemented carbides was not observed. The mean grain size of WC in the

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alloy 1 is 0.95 μm. It decreased to 0.91 μm when 1.0 wt. % Y2O3 was added into the alloy 2, as shown in Fig. 3 (a) and (b). This can be explained as follows. Firstly, Y2O3 exhibits good ability in inhibiting both the continuous and discontinuous WC grain growth at the solid phase sintering stage [24]. Secondly, the Y2O3 is located in the grain

boundary

of

the

WC/Co

grains,

which

can

slightly

inhibit

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dissolution-reprecipitation of WC by separating the WC and Co phases [3]. Although the ability of Y2O3 to inhibit the grain growth can still remain to a considerable extent during the sintering process, it becomes ineffective in controlling the preferential grain growth at the liquid phase sintering stage [24].

ACCEPTED MANUSCRIPT After adding 1.0 wt. % Mo, the mean grain size of the alloy is 0.88 μm. Moreover, the mean grain size of WC reduced to 0.81μm when the content of Mo was 2.0 %, and decreased to 0.73 μm for alloy 5. It is concluded that the mean grain size of WC decreases with the increase of Mo addition. Previous works [27, 28] have reported that there is a temperature gradient inside

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the sample and die depended on the design of the die and conductivity of the materials of the SPS process, rating between 30 ºC and 300 ºC. Thus, WC/Co can be sintered in

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solid state or at least with a minimum time close to the liquid phase temperatures due to the rapid heating and high pressure in SPS [29-31]. During sintering process, Mo

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and C reacted to form Mo2C phase firstly before 900 ºC. And then, Mo2C dissolved in Co binder phase preferentially, which can restrain the dissolution-reprecipitation of

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WC in Co binder phase. Therefore, the WC grains were refined with the inhibition of

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WC growth. 3.2 Density and mechanical properties

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The variation of relative density and mechanical properties of as-prepared alloys

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are listed in Table 2. It should be noted that the relative density of the alloy 2 is 99.11 %, which is almost the same as that of alloy 1 (99.12 %). It is indicated that the addition of small amount of nano-grained Y2O3 has almost no effect on the relative

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density. This is attributed to the outstanding coupling between WC and Co represented by low dihedral angle WC-Co system, which is reported to be zero,

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dissolution of significant amount of WC by Co and formation of a ternary eutectic reaction at 1275 ºC [3]. It is crucial to notice that the relative density of alloy 3 was 98.96% when 1.0 wt. % Mo was added, and the relative density decreased to 95.43 % for alloy 5. The main densification mechanisms for cemented carbides are the carbide particle rearrangement, strengthening of the diffusion and viscous flow of the binder [6, 32]. During the sintering process, the viscous flow of the Co binder phase plays a major role in the densification behavior of the alloy. However, the Mo2C generated during the sintering process preferentially dissolves in Co binder phase, which causes a

ACCEPTED MANUSCRIPT negative influence on the flow ability of the liquid phase [33]. Therefore, the pores in the alloys can not be filled by the liquid phase during the sintering process, which leads to a decline in relative density [32, 34]. The evolutions of the Vickers hardness and fracture toughness as a function of different additions are also listed in Table 2. The hardness of alloy 1 is 2060 kg·mm-2, which is about 19.2 % higher that that in the literature [18]. This could be explained

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from two following aspects. The first was that the WC grain size of alloy 1 is 0.95 μm,

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which is much smaller than that in the literature. According to the Hall-Petch relationship [35], the hardness of alloy increases with the decrease of the WC grain

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size. The second was that the raw materials used in this experiment is finer, and the distribution of WC, Mo and Y2O3 particle size is more homogeneous than that in the

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literature. It is apparent that finer powders are quite effective in generating finer and uniform microstructure, which leads to a decrease in Co pooling and volume of the

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residual porosity. Therefore, the hardness of WC-6Co is improved. The hardness of alloy 2 (2100 kg·mm-2) is close to that of alloy 1 due to the few content of Y2O3

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addition.

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From Table 2, it is know that the hardness of alloy 3 is improved by up to 5.6 % with the addition of 1.0 % Mo than that of alloy 1. It could be seen in Fig. 3 that the WC grain size of alloy 3 is smaller than that of alloy 1, which leads to the improved

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with the addition of Mo according to the Hall-Petch relationship. Furthermore, the Co binder phase is enhanced due to the dissolution of Mo2C during the sintering process,

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which would have a positive influence on the hardness of WC-Co cemented carbides [22]. However, the hardness decreases sharply with increasing the content of Mo equaled to or more than 2.0 %. The hardness and density of sintered alloys have the important relations. That is, hardness decreased with the decrease of density [19]. According to Table 2, the relative density reduced with the increase of Mo addition, which leads to a decrease of hardness. Thus, the highest hardness is obtained in alloy 3 with 1.0 % Mo addition. It could be seen from Table 2 that there are no significant differences in the fracture toughness of alloy 1, alloy 2 and alloy 3, since the uncertainties associated

ACCEPTED MANUSCRIPT with the Palmqvist test are generally about ± 1.5 MN·m-3/2 for calculations of toughness from the formula (1). However, the fracture toughness decreases sharply with increasing the content of Mo equaled to or more than 2.0 %. This could be explained from two following aspects. The first was that the finer WC grain size results in the shorter mean free path of Co binder phase, which directly contributes to

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the lower toughness [36]. The second was that the decrease of relative density with the increase of Mo content, which leads to lower fracture toughness [18] since pores

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act as crack initiation and crack propagation sites.

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3.3 Corrosion behavior

The SEM images in secondary electrons mode of alloy 1 and alloy 5 after the

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potentiodynamic polarization tests in the HCl solution are shown in Fig. 4. The surface of alloy 1 shows the exposed WC grains islets and netting crack figures,

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which might be caused by the stress derived from the generation of new phases during polarization and drying processes. The EDS analysis result of the selected spectrum is

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shown in Fig. 4(b). It could be seen that the Co content is low (about 2.08 %), which

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is due to the dissolution of Co binder phase during the potentiodynamic polarization test. However, only negligible distinctions could be observed on the morphologies of alloy 5 after the electrochemical measurements, as shown in Fig. 4(c). The corroded

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surface is reasonably complete, indicating a slight corrosion state. The EDS analysis result of the selected spectrum shows that the Co content is about 4.60 %, indicating

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that the Co binder phase has not been dissolved in the aggressive solution greatly when 4.0 % Mo was added. The SEM morphologies of alloy 1 and alloy 5 after the potentiodynamic polarization tests in the NaOH solution are shown in Fig. 5. There are no significant differences in the morphologies of the two alloys. The pits on the surface are left by the dissolution of WC during the potentiodynamic polarization tests. However, the pits on the surface of alloy 1 are much larger than that of alloy 5. Besides, the W content of the selected spectrum in Fig. 5(a) is about 47.05 %, which is 21.82 % lower than that of alloy 5 (about 60.14 %), indicating that the WC grains of alloy 1 are

ACCEPTED MANUSCRIPT dissolved more fully in the NaOH solution. Fig. 6 shows the XPS spectras for alloy 5 after the potentiodynamic polarization tests in HCl and NaOH conditions. As for Co 2p tested in HCl, the result indicated that the peaks associated with Co were weaker than that in NaOH, which is owing to the dissolution of Co binder phase. Contrarily, the Co peaks in NaOH was much

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higher due to the good corrosion resistance of Co binder phase. The generated Co(OH)2 and Co3O4 were distributed on the surface of alloy 5. Fig. 6(b) shows the

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different intensities of Mo peaks depending on the testing conditions. It could be seen that the Mo was mostly dissolved into NaOH solution. However, the MoO3 was

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generated on the surface when tested in HCl solution.

The value of free corrosion potential (Ecorr) represents a thermodynamic stability

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of the tested alloys under a given electrochemical corrosion condition [37]. The potentiodynamic polarization curves of alloys in acid solution are presented in Fig.

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7(a). The electrochemical corrosion data of investigated alloys are listed in Table 2. It is observed that the Ecorr value of the alloy 2 increased with Y2O3 addition. This

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positive shift of potentials is due to the increase of solution content of W in Co binder

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phase caused by the addition of Y2O3, which decrease the dissolution of the alloy [38]. According to Fig. 7(a) and Table 2, alloy 3 shows the most positive Ecorr value of -307 mV, while alloy 5 possesses the most negative one (-371 mV). The corrosion

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properties of WC-Co cemented carbides in acidic solution are mainly affected by the corrosion resistance of the Co binder phase and WC/Co interface. Fig. 8 shows a

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TEM field image of alloy 5 and the EDS linecan measurement of W, Co and Mo. The WC grains appear bright, whereas the Co binder phase appears dark, as shown in Fig. 8(a). The EDS spectrum shows that the dark binder phase predominantly consists of Co. Moreover, it should be noted that the distribution of Mo element is consistent with that of Co. And the content of Mo increase with the decrease of W, indicating that Mo can preferentially dissolve in the Co binder phase and inhibit the dissolution of W. The results mentioned above show that the enhancement of the corrosion resistance of alloy 3 probably primarily ascribe to the strengthening of Co binder phase caused by the dissolution of Mo, which would lead to the improvement of the

ACCEPTED MANUSCRIPT corrosion resistance of Co binder phase by stabilizing the fcc crystal structure [11, 39, 40]. Moreover, it is worthwhile to note that the addition as high as 2 % may be harmful to the corrosion resistance. The relative density of alloys decreases significantly with increasing the content of Mo equaled to or more than 2.0 %, which causes an increase of contact area between the alloy and the aggressive solution.

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The corrosion current density (icorr) implies the corrosion rate in terms of dynamics. According to Table 2, the alloy 3 shows the lowest icorr value (3.38

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μA/cm2), while the alloy 1 shows the maximum value. This is to say that the corrosion rate of WC-Co alloys in 0.1 M HCl solution decreases with the addition of Mo and

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Y2O3. In addition, the corrosion rate of alloy 3 is the minimum. This could be explained from two following aspects. The first was that Mo may suppress the pitting

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corrosion and enhance the passivation behavior of alloys [19], which would reduce the corrosion rate of alloys. The second was that Mo exhibits higher corrosion

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resistance to acidic solution than Co binder phase due to the much lower electronic conductivity of the Mo-containing passivation film [41]. With the increase of Mo

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content, the icorr of alloy 4 and alloy 5 are greater than that of alloy 3, which is due to

grain refinement.

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the decrease of relative density and the increase of WC/Co interfaces caused by the

According to Fig. 7(b) ant Table 2, the Ecorr value increases continuously from

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-1032 mV for alloy 1 to -572 mV for alloy 2, then to -502 mV for alloy 3 in 0.1 M NaOH solution. Moreover, the Ecorr value of alloy 4 and alloy 5 is -441 mV and -504

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mV, respectively. Meanwhile, the icorr value decreases from 12.80 μA/cm2 for alloy 1 to 5.15 μA/cm2 for alloy 2, then to 0.01 μA/cm2 for alloy 3 obviously. However, the icorr value of alloy 4 and alloy 5 is 4.84 μA/cm2 and 4.09 μA/cm2, respectively. This result indicates that the corrosion resistance of cemented carbide in alkaline solution could be enhanced by adding Y2O3 and Mo. The corrosion resistance of the alloy 3 is the highest when the amount of Mo is 1.0 %. The enhancement of corrosion resistance can be explained from three following aspects. The first was the decrease of WC grain size with the addition of Y2O3 and Mo, which facilitates the improvement of corrosion resistance owing to the increase of the number of the WC/Co interfaces [10]. The

ACCEPTED MANUSCRIPT second was the formation of Mo-containing passivation film on the surface of alloys during corrosion process, which enhances the corrosion resistance of alloys in alkaline solution [42, 43]. The third was that the various competing reactions between passivation and dissolution reactions [37], which inhibits the conduction of electrons through the passivation film.

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4. Conclusions The effect of Mo and Y2O3 addition on the microstructure and corrosion

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resistance of fine WC-6Co cemented carbides prepared via SPS were investigated in this work. The investigation could be summarized as follows.

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(1) There are no significant differences in the mean grain size of WC and the mechanical properties of alloys with 1.0 % Y2O3 addition. The corrosion resistance of

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alloys increased when 1.0 % Y2O3 was added. However, the grain size of WC decreases with the increase of Mo addition due to the inhibiting effect on the

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dissolution-reprecipitation process. In addition, the relative density and fracture toughness of alloys decreases with the increase of Mo content.

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(2) The hardness of alloys and corrosion resistance both in acid and alkaline

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solutions increase to a certain value, and then follows a decreasing trend with the increase of Mo content due to the combined action of microstructure, the enhancement of Co binder phase and relative density.

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(3) The adding amount of Mo should be controlled within a certain range and 1 wt. % of Mo has the most positive effect on the microstructure and comprehensive

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properties of WC-6Co alloys. Suggestions for Further Work (1) The effect of Mo and rare earth on the WC/Co interface needs more investigations, including (a) orientation relationships of WC, Co binder, (W,Mo)C and other second phases, (b) the effect of additions on the characteristics of the thin Co layer. (2) Untill now, most investigations on the wear resistance and corrosion resistance of cemented carbides are performed separately. However, the interaction of abrasion and corrosion has a great influence on the lifetime of alloys in service.

ACCEPTED MANUSCRIPT Therefore, the frictional behaviour under corrosion conditions and the corrosion behaviour under wear conditions would be investigated using a device that combines sliding wear and electrochemical corrosion. Acknowledgements The work was financially supported by the Natural Science Foundation of (2015FB127),

Natural

Science

Foundation

of

Jiangxi

Province

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Yunnan

(20151BBE50002), Project of Education Department of Jiangxi Province (GJJ150648)

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and Analysis Foundation of Kunming University of Science and Technology

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(2016P20151130003).

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ACCEPTED MANUSCRIPT SPS. Int. J. Refract. Met. Hard Meter. 62(2017)155-160 [19] N. Lin, C. H. Wu, Y. H. He, D. F. Zhang. Effect of Mo and Co additions on the microstructure and properties of WC-TiC-Ni cemented carbides. Int. J. Refract. Met. Hard Meter. 30(2012)107-113 [20] Z. Guo, J. Xiong, M. Yang, X. Song, C. Jiang. Effect of Mo2C on the microstructure and properties of WC–TiC–Ni cemented carbide. Int. J. Refract.

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[22] Z. Lin, J. Xiong, Z. Guo, W. Zhou, W. Wan, L. Yang. Effect of Mo 2C addition on the microstructure and fracture behavior of (W, Ti)C-based cemented carbides.

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Effects of Y2O3 addition on microstructures and mechanical properties of WC–Co functionally graded cemented carbides. Int. J. Refract. Met. Hard Meter. 50(2015)53-58

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carbide platelet-containing cemented carbide with yttrium containing dispersed phase. Trans. Nonferrous Met. Soc. China. 18(2008)104-108

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[25] J. H. Potgieter, N. Thanjekwayo, P. Olubambi, N. Maledi, S. S. Potgieter-Vermaak. Influence of Ru additions on the corrosion behaviour of

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WC–Co cemented carbide alloys in sulphuric acid. Int. J. Refract. Met. Hard Meter. 29(2011)478-487 [26] R. M. Raihanuzzaman, S. T. Han, R. Ghomashchi, H. S. Kim, S. J. Hong. Conventional sintering of WC with nano-sized Co binder: Characterization and mechanical behavior. Int. J. Refract. Met. Hard Meter. 53(2015)2-6 [27] U. Anselmi-Tamburinia, S. Gennarib, J.E. Garaya, Z.A. Munir. A fundamental investigations on the spark plasma sintering/synthesis process II. Modeling of current and temperature distributions. Mater Sci Eng A. 394(2005):139–48 [28] M. Eriksson, M. Radwan, Z. Shen. Spark plasma sintering of WC, cemented

ACCEPTED MANUSCRIPT carbide and functional graded materials. Int. J. Refract. Met. Hard Meter. 36(2013)31-37 [29] N. Al-Aqeeli, K. Mohammad, T. Laoui, N. Saheb. VC and Cr3C2 doped WC-based

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sintering. Ceram. Int. 37(2011):1139-1142

[31] C.B. Wei, X.Y. Song, J. Fu, X.M. Liu, Y. Gao, H.B. Wang, S.X. Zhao.

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Microstructure and properties of ultrafine cemented carbides—Differences in spark plasma sintering and sinter-HIP. Mater. Sci. Eng. A. 552(2012)427-433

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ACCEPTED MANUSCRIPT corrosion behavior of WC–Co based hardmetals in alkaline solutions. Int. J. Refract. Met. Hard Meter. 27(2009)806-812 [40] S. Sutthiruangwong, G. Mori. Corrosion properties of Co-based cemented carbides in acidic solutions. Int. J. Refract. Met. Hard Meter. 21(2003)135-145 [41] W.J. Tomlinson, C.R. Linzell. Anodic polarization and corrosion of cemented carbides with cobalt and nickel binders. J Mater Sci. 23(1988)914–918

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[42] W.A. Badawy, F.M. Al-Khara. Corrosion and passivation behaviors of molybdenum in aqueous solutions of different pH. Electrochim. Acta.

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wide pH range. J. Power. Sources. 202(2012)11-17

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SEM images of the raw materials. (a) WC-6Co composite powder, (b) Mo powder, (c) Y2O3 powder and (d) WC-6Co-1Mo composite powder

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The XRD patterns of alloys. (a) as-sintered alloys and (b) the negative shift of Co (111) peak of alloys with Mo addition BSE-SEM images of alloys etched by Murakami’s solution. (a) WC-6Co, (b)

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The XRD patterns of alloys. (a) as-sintered alloys and (b) the negative shift of

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BSE-SEM images of alloys etched by Murakami’s solution. (a) WC-6Co, (b)

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The SEM morphologies and corresponding EDS results of alloys after

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XPS spectras of Co 2p and Mo 3d for alloy 5 under different testing

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Fig. 7 NaOH

Potentiodynamic polarization curves of alloys in (a) 0.1 M HCl, (b) 0.1 M

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TEM image of alloy 5 for EDS-linescan-measurement. (a) TEM field image

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WC-6Co composite powder

Y2O3

Mo

Alloy 1

100

0

0

Alloy 2

99

1

0

Alloy 3

99

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1

Alloy 4

98

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Alloy 5

96

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The nominal composition of alloys (wt. %)

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icorr (μA/cm2)

Hardness

Wk

density (%)

(kg•mm-2)

(MPa·m1/2)

HCl

NaOH

HCl

NaOH

Alloy 1

99.12

2060

8.56

-368

-1032

62.99

12.80

Alloy 2

99.11

2100

8.42

-343

-572

4.76

5.15

Alloy 3

98.96

2175

9.01

-307

-502

3.38

0.01

Alloy 4

97.74

1930

7.75

-326

-441

6.04

4.84

Alloy 5

95.43

1618

5.90

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8.08

4.09

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ACCEPTED MANUSCRIPT Highlights 1. The addition of 1.0 % Mo or Y2O3 can improve the corrosion resistance of WC-6Co cemented carbides both in 0.1 M HCl, and 0.1 M NaOH solutions. 2. The content of Mo should be controlled within a certain range and 1.0 % of Mo has

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3. The Co binder phase is strengthened owing to the dissolution of Mo.