International Journal of Refractory Metals and Hard Materials 96 (2021) 105497

Available online 5 February 20210263-4368/© 2021 Elsevier Ltd. All rights reserved.

Remarkable improvement in high temperature oxidation resistance of WC-12Co by the addition of CrSi2

Abdul Basyir , Hubby Izzuddin , Bambang Hermanto , Toto Sudiro *

Research Center for Physics, Indonesian Institute of Sciences, PUSPIPTEK Area, South Tangerang City, Banten Province 15314, Indonesia

A R T I C L E I N F O

Keywords: WC-12Co CrSi2

Spark plasma sintering Oxidation resistance Density Porosity Hardness

A B S T R A C T

WC-Co has excellent mechanical properties at a temperature below 700 ◦C, but above it, the mechanical properties of WC-Co decrease severely because of the oxidation effect. In the present study, the addition of metal silicide of CrSi2 was considered to enhance the oxidation resistance of cementite carbide. The WC-12Co compacts containing 0, 12.5, and 25 wt% CrSi2 were prepared using mechanical alloying and spark plasma sintering techniques at 1150 ◦C. According to the results of metallographic characterization, the sintered WC-12Co is composed mainly of WC and Co phases. The addition of CrSi2 leads to the formation of new phases as CrSi, CoSi, Co6W6C, and SiC depending on the compact composition. It is found that the WC-12Co compact with 12.5 wt% CrSi2 content exhibits a lower measured density and higher hardness compared to that of CrSi2-free compact. After the first cycle of exposure at 900 ◦C, the WC-12Co is severely oxidized, forming porous and crack oxide scales. On the contrary, the addition of 12.5 wt% CrSi2 shows the lowest mass gain and displays the most resistance to oxidation attributed to the formation of mainly SiO2 scales. Further increase in CrSi2 concentration, however, the alloy compact experiences breakaway oxidation after the fifth cycle of exposure. Accordingly, it can be proposed that the addition of 12.5 wt% CrSi2 is the most promising composition to allow the cemented carbide used at high temperatures and oxidizing atmospheres.

1. Introduction

It is well known that WC based cemented carbides have an excellent combination of mechanical properties such as high hardness, excellent toughness, and wear resistance [1–8]. So that it has been widely used in various industrial applications, such as hot rolling, mining, wear parts, machining, metal forming, cutting tools, and drilling equipment. For specific applications, however, like hot rolling steel or high-speed cut-ting and drilling, the cemented carbide components may be exposed to extreme conditions as complex corrosive atmospheres [2,9] and/or high temperatures of up to about 1000 ◦C or higher [1,3,5,7,10–12]. This can lead to accelerated oxidation and corrosion of WC-Co components, leading to the deterioration of the mechanical properties.

Several studies have investigated the oxidation behavior of WC based materials in the temperature range 300 ◦C-1100 ◦C [1–16]. Their results show that the oxidation resistance of cemented carbide is governed by several factors as cemented carbide composition, binder content, and atmospheric condition as oxygen concentration and pressure, tempera-ture exposure and time. The ultrafine cemented carbide is getting to

oxidize at a temperature of about 425 ◦C [6]. As the temperature in-creases, the oxidation rate also increases as well. The furious oxidation took place at a temperature higher than 700 ◦C [2,6]. It mostly leads to the formation of cracks and porous oxides, spallation of the oxide scale [5] and evaporation of WO3 [8,11,14]. The presence of aforesaid defects as cracks and pores in the oxide layer can allow the oxygen to diffuse inwardly more faster than the cations to diffuse outwardly [3,5,11], leading to the oxidation of WC-Co elements. The raise in Co content could enhance the oxidation resistance of cemented carbide due to the formation of more compact oxide layers of CoWO4 [3,9,11,12,15]. Nevertheless, at higher temperatures (> 700 ◦C), the oxidation of WC and the Co binder will become serious, resulting in a dramatic increase in mass gain and oxide thickness due to the formation of unprotective oxide scales [2]. Based on the aforementioned evidence, it is important to note that the oxidation resistance of WC-Co must be improved to restrain the mechanical properties deterioration.

Several studies were carried out to improve the properties of WC materials at high temperatures, for examples: the development of WC- FeAl [7] and WC/(Fe-Al-B) [17] composites, Ni/(WC-Co) coating [4]

* Corresponding author. E-mail address: toto011@lipi.go.id (T. Sudiro).

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International Journal of Refractory Metals and Hard Materials

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https://doi.org/10.1016/j.ijrmhm.2021.105497 Received 10 November 2020; Received in revised form 24 January 2021; Accepted 28 January 2021

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and considering the addition of an alloying element to cemented carbide like TiC, Mo2C, TiN, Ni [1]; TiC, (Ta, Nb)C [8]; and AlN [13]. However, a great attention is still needed to enhance the oxidation resistance of WC- Co based materials, mostly at a temperature higher than 700 ◦C and longer exposure times, and also to present a protective and adherence oxide scale.

Concurrently, metal silicides such as CrSi2, FeSi2, CoSi2, WSi2, NiSi2, MoSi2, etc. already known as a material possessing attractive features likes high melting points, a relatively low density, and excellent resis-tance to oxidation at high temperature due to the formation of a pro-tective SiO2 scale [18–20]. Hence, these materials become a potential candidate for advanced high temperature structural applications. In this research, we considered the addition of transition metal silicide of CrSi2 to WC-12Co to improve the oxidation resistance of tungsten carbide based materials. Probably, the addition of CrSi2 is advantageous in promoting the formation of SiO2 and/or Cr2O3 protective oxide layer on the compact surface, enhancing the oxidation resistance of WC-Co compact.

To obtain the desired microstructure and properties, mechanical alloying (MA) and spark plasma sintering (SPS) techniques have received much attention for specimen preparation. MA is a solid-state powder processing technique that involves repeated welding, frac-turing, and rewelding of powder particles in a high-energy ball mill. Thus, it allows the production of a homogeneous material with a controlled microstructure. Usually, process control agents were added during MA of metallic powders to avoid the oxidation of the powder elements and particle agglomeration [21–23]. Meanwhile, the SPS technique offers some advantages for powder consolidation as rapid solidification, microstructure control and is able to produce porous to compact material in a short sintering time. Thus, this technique is favorable to sinter the various kind of materials like metals, ceramics, cermets, intermetallic compounds and other materials [24,25].

The present work aims to investigate the synthesis of WC-12Co compacts with different content of CrSi2 (in wt%) using MA and SPS techniques. The effect of CrSi2 addition on the structure, density, porosity, hardness and high temperature oxidation resistance of WC- 12Co compacts is clarified and discussed.

2. Materials and methods

2.1. Material composition

This research used tungsten carbide–cobalt (WC-12Co) powder, Code: WOKA 3103 (Metco Oerlikon) consisting of balance W, 10.5–13.5 wt% Co, 5–5.8 wt% C and 0.2 max wt% Fe with a spherical shape and 11–45 μm in size as the starting material and chromium silicide CrSi2 powder, Code: T502860 (Japan New Metals Company Limited) con-sisting of balance Cr, 50.3–52.8 wt% Si, ≤ 0.35 wt% Fe, ≤ 0.10 wt% C and ≤ 1.0 wt% O with a particle size of 5–10 μm as alloying material to enhance the oxidation resistance of cemented carbide at high tempera-ture. In order to prepare the alloy compacts, WC-12Co powders with the addition of 0, 12.5, and 25 wt% CrSi2 were prepared by MA and SPS techniques.

2.2. Mechanical alloying (MA)

The powder composition as mentioned above was mechanically alloyed for 2 h using a shaker mill (PPF-UG, Ultimate Gravity) at an oscillation frequency of 700/min. The MA was carried out in the cy-lindrical steel vial with 125 ml in volume and balls mill to powder weight ratio of 4:1. In this study, 10 ml of n-hexane solution was added to the cylindrical steels vial as a control agent to inhibit the powder oxidation and agglomeration during MA. After the milling process ends, the mixing powder was dried at room temperature for 24 h.

2.3. Sintering process

The MA powder was then consolidated using a spark plasma sinter-ing technique (Dr. Sinter model SPS-625, Fuji Electronic Industrial Co. Ltd). First, the powder was packaged in a graphite die with 2 cm in diameter. It was loaded in the SPS machine under a compressive pres-sure of 40 MPa and evacuated for up to less than 6 Pa. The sample was then heated from room temperature to 600 ◦C according to the basic equipment setting. From 600 ◦C to 1000 ◦C and 1000 ◦C to 1150 ◦C, the heating rate was set to 50 ◦C/min and 10 ◦C/min, respectively. The sample was continuously heated at 1150 ◦C for 10 min to obtain a high sample density. The compressive pressure was kept constant during the sintering process. Hereafter, the sintered sample was cooled to room temperature at the same evacuated chamber.

2.4. Characterization

To perform the phase identification, microstructure and elemental analysis, density and hardness measurement, and also high temperature oxidation test, all surfaces of the samples before oxidation test were polished for up to mirror finish using a polish machine with MD Piano 300 mm in diameter (LaboPol-30 Struers). The phase analysis of the powders, sintered alloys before and after oxidation test was carried out by X-ray diffraction (XRD Rigaku Smartlab) with Cu Kα radiation at 40 kV and 30 mA, 10 deg./min in scan speed, 0.01 deg. in step width and 10o to 90o in scan range using D/teX Ultra 250 detector. The crystallite size (D) and lattice strain (ε) of the powders before and after MA were estimated using the following formula [26].

D =0.9 λβcosθ

(1)

ε =β

4tanθ(2)

Here θ is Bragg diffraction angle, β is FWHM of θ, and λ is X-ray wavelength (Cu = 1.541862 Å). The inductively coupled plasma optical emission spectroscopy (ICP-OES PlasmaQuant PQ 900 Elite Analy-tikjena) was used to measure the concentration of Fe and Ni elements that may be introduced into the system during milling derived from steel balls and vial. The morphology and elemental analysis of the sample were performed using a field emission scanning electron microscopy (FE-SEM JIB-4610F) at an accelerating voltage of 15 kV and energy dispersive X-ray spectroscopy (EDX Oxford Instruments XMaxN). The hardness and measured density of sintered alloys were determined using a micro-hardness tester (Leco LM100AT) and density tester (AND GF- 600) based on Archimedes principle, respectively. The theoretical den-sity of each composition was calculated using the rule of mixtures. A ratio between measured and theoretical densities indicates the relative density of an alloy compact. In this study, the sample indentation was conducted using 300 kgf for 13 s. Meanwhile, the high temperature oxidation test was carried out in a muffle furnace (PPF-1300) at 900 ◦C for 8 cycles. One cycle is equivalent to high temperature exposure at 900 ◦C for 20 h and cooling in the air for 4 h. The oxidation behaviors of the alloy compact at high temperatures were evaluated based on the mass gain or mass loss per unit area of the sample. For cross-sectional examination, the oxidized sample was embedded in epoxy resin, cross- sectionally cut and carefully polished using various grit of SiC paper for up to #5000. Based on the results of XRD and EDX analysis, the possible chemical reactions for the phase formation were also presented. The Gibbs free energy change for a reaction on this study was calculated using HSC Chemistry Software version 9.8.1.2 [27] and some thermo-chemical data were obtained from the literature. The results were then graphically represented as a function of temperatures to confirm the stability of the phases.

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3. Results and discussion

3.1. Starting materials and mechanically alloyed powders

Fig. 1 shows the X-ray diffraction patterns of WC-12Co and CrSi2 powders as the starting material, and (WC-12Co) – x wt% CrSi2 (x = 0, 12.5, and 25) powders after MA.

The results of XRD analysis confirm the strong reflection of a WC phase with low diffraction peak intensities of Co phase in the WC-12Co powder (Fig. 1a). For the chromium silicide CrSi2 powder (Fig. 1b), X- ray diffraction analysis observes the diffraction peaks of CrSi2 phase mainly with minor diffraction peaks of CrSi and SiO2 phases. This phase identification suggests the material not purely CrSi2.

For the mechanically alloyed powders, the main diffraction peaks are detected as WC phase (see Figs. 1c, d and e). A small diffraction peak of SiO2 which can be observed in the chromium silicide powder before milling (Fig. 1 b) is not found in the WC-12Co powder with CrSi2 addition after MA (see Figs. 1d and e). This could be related to its small fraction and SiO2 particle refinement. So the peak reflection becomes very weak. This evidence suggests that MA did not lead to the oxidation of milled powder. But, it results in XRD peak broadening due to the reduction in crystallite size and the increase in lattice strain of MA powders (see Table 1). As the CrSi2 concentration increases, the crys-tallite size of the powders after MA increases as well, in contrast to the lattice strain.

In order to further investigate the possibility of the presence of steel elements that may be introduced into the system during MA, the amount of Fe and Ni elements in the mechanically alloyed powders was measured using ICP-OES. The measurement result shows that the con-centration of Fe in the WC-12Co powders with 0, 12.5 and 25 wt% CrSi2 content are as follows 0.87, 0.53 and 0.54 wt%, respectively. While, the

Ni concentration in the aforesaid composition is 0.0030, 0.0026 and 0.0025 wt%, respectively. Compared to the initial Fe concentration in the WC-12Co powder before MA of about max. 0.2 wt%, there is an increase in the Fe content after MA to about 0.87 wt%. It tends to decrease with the addition of CrSi2 as well as Ni element. This may be due to the less abrasive of CrSi2 than WC-12Co. The obtained results also suggest that the use of steel balls and vial did not lead to significant contaminants.

3.2. Compact structure before oxidation test

3.2.1. Phase structure Fig. 2 shows the X-ray diffraction patterns of WC-12Co, (WC-12Co)

+ 12.5 CrSi2, and (WC-12Co) + 25 CrSi2 compacts fabricated at 1150 ◦C using a SPS technique.

It can be seen that the WC-12Co compact is composed of WC and Co phases. There is no difference in the phase composition between the sintered compact (Fig. 2a) and the starting powder material (Fig. 1a). Both phases are presents as individual phase. It appears that they did not

Fig. 1. XRD patterns of (a) WC-12Co, (b) CrSi2 powders as received and mechanically alloyed powders of WC-12Co containing (c) 0, (d) 12.5 and (e) 25 wt% CrSi2.

Table 1 Crystallite size and lattice strain of WC-12Co, CrSi2 and mechanically alloyed powders.

Starting Material Mechanical Alloying

WC- 12Co

CrSi2 WC- 12Co

(WC-12Co) + 12.5 CrSi2

(WC-12Co) + 25 CrSi2

Crystallite size (Å)

681.71 1366.10 162.56 181.14 221.14

Lattice strain (%)

0.12 0.07 0.52 0.47 0.38

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react to each other to form a new compound of WC-Co during SPS preparation.

In Figs. 2b and c the WC-12Co compacts with 12.5 and 25 wt% CrSi2 content are composed by WC and three other new phases as CrSi, CoSi and Co6W6C. Apart from the aforementioned phases, another phase found in the WC-Co compact with 25 wt% CrSi2 content is SiC phase. The disappearance of Co and CrSi2 peaks and the presence of CoSi and CrSi peaks as mentioned above in both 12.5 and 25 wt% CrSi2 compacts suggest that the Co is preferentially reacted with CrSi2 to form CoSi and CrSi (Eq. (3)).

Co+CrSi2→CrSi+CoSi ΔG1150 = − 12.445 kcal (3)

While the formation of Co6W6C is probably due to the local carbon deficiency caused by CO and CO2 formation [6] that may occur ac-cording to the following chemical reaction.

6 /5 WC+ 6 /5 Co+O2→1 /5Co6W6C+CO2 ΔG1150 = − 45.884 kcal (4)

12 /5 WC+ 12 /5 Co+O2→2 /5Co6W6C+ 2 CO ΔG1150 = − 15.756 kcal (5)

The thermochemical data of Co6W6C refer to Ref. [28]. As the Si concentration in the alloy compact increases, C from WC is potentially to react with Si from silicides compounds to form SiC as detected by X-ray diffraction analysis in Fig. 2c, as shown in the reaction below.

C+Si→SiC ΔG1150 = − 14.374 kcal (6)

The reaction between C from WC and the adsorbed oxygen in the SPS chamber as shown in Eqs. (4) and (5) are liable to occur due to the high affinity of C to oxygen. However, the SiC phase is not observed in the WC-12Co compact with 12.5 wt% CrSi2 content (see Fig. 2b). It should be related to its low fraction. Another interesting result found in this study is that the intensity of diffraction peaks depends on the alloy

composition. In the CrSi2-free compact, the reflection of Co6W6C is not observed. Nevertheless, as the CrSi2 content in the WC-12Co compact increases, the diffraction peak of Co6W6C peaks tends to increases (see Fig. 2c). It becomes clearly observed in the 25 wt% CrSi2 compact. This may be due to that the adsorbed oxygen in the mechanically alloyed powder is likely to increase with the increase of CrSi2 concentration, enhancing the local carbon deficiency and the formation of Co6W6C during sample sintering. In addition, the reaction between C from WC and Si from silicides phases leads to carbon deficiency too.

3.2.2. Microstructure and elemental analysis Fig. 3 shows the typical surface morphology of (WC-12Co), (WC-

12Co) + 12.5 CrSi2, and (WC-12Co) + 25 CrSi2 compacts prepared by a spark plasma sintering (SPS) technique at 1150 ◦C.

As shown in Fig. 3a, the surface of the WC-12Co compact mostly consists of two different areas: light-gray particles and dark-gray areas around the particles. Some small black regions in which pores or prob-ably oxide scales are randomly distributed in the alloy compact. In order to examine the chemical composition of each aforesaid area, an EDX analysis was carried out in the corresponding area of Fig. 3 with the results as listed in Table 2. According to the results, the light-gray area is composed mainly of 44.6 at% W and 53 at% C with a small content of Co and O, which refers to the WC phase. While the dark-gray area contains 36.9 at% W, 42.3 at% C and 17.6 at% Co with a small content of oxygen of about 3.2 at% which is suspected to be Co and WC phases. The Co phase presents at the WC grain boundaries and acts as a binder agent for WC sintering.

In Fig. 3b, the SEM microstructure analysis clearly distinct four different surface contrast in the WC-12Co compact with 12.5 wt% CrSi2 content, namely light-gray, gray and dim-gray areas with small black areas. The results of EDX analysis show the light-gray area composed of 41.7 at% W and 52.8 at% C with a small content of Co, Cr and O,

Fig. 2. XRD patterns of (WC-12Co) compacts with (a) 0, (b) 12.5 and (c) 25 wt% CrSi2 content prepared by a spark plasma sintering technique at 1150 ◦C.

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representing the area of WC phases. The gray area consists of 10.0 W, 24.2 C, 16.9 Co, 24.7 Cr, and 24.1 Si (in at%) which is suspected to be Co6W6C and CrSi phases as detected by X-ray diffraction analysis (see Fig. 2 b). While the dim-gray area contains 1.3 at% W, 9.0 at% C, 44.6 at % Co, 0.5 at% Cr and 44.7 at% Si. The atomic ratio of Co and Si is closed to 1:1. Therefore, this dim-gray area corresponds to CoSi phase as detected by XRD analysis in Fig. 2b. All phase is randomly distributed in the alloy compact. Similarly in the WC-12Co alloy compact, the small black areas seem to be pores or oxide precipitates.

With the increase of CrSi2 content in the WC-12Co compact into 25 wt%, its addition significantly alters the microstructure of WC-12Co compact. The BSE-SEM characterization indicates that the fraction of light-gray area decreases. While the fraction of gray area appears to increase. An EDX measurement performed in the areas in Fig. 3c shows that the chemical composition of light-gray area and gray 1 area in the

25 wt% CrSi2 compact is almost similar to light-gray and gray areas of 12.5 wt% CrSi2 compact (see Table 2), revealing the areas of WC phase, and Co6W6C and CrSi phases, respectively. A considerable difference in chemical composition is found in the gray 2 and 3 areas. An EDX analysis in area 2 indicates that it consists of 32.7 W, 41.5 C, 5.5 Co, 5.9 Cr, 11.6 Si and 2.7 O (in at%). Probably, this area is composed of WC and SiC phases. The other gray area contains 19.4 W, 31.2 C, 9.2 Co, 10.6 Cr, 26.1 Si and 3.6 O (in at%) that seems to be the areas of WC, CrSi and CoSi phases. In addition, the results also indicate that a localized black area consisting mainly of Si and O with an atomic ratio near to 1:2 which refers to SiO2 is also formed. Based on the results as presented above, it is worth mentioning that the addition of 12.5 wt% CrSi2 to WC-12Co promotes the formation of CrSi, CoSi and Co6W6C phases. Further in-crease in CrSi2 concentration also leads to the SiC formation and oxidation of Si element to form SiO2 in the alloy compact. The fraction of

Fig. 3. FE-SEM Compo images of (WC-12Co) compacts with (a) 0, (b) 12.5 CrSi2 and (c) 25 CrSi2 content prepared by a spark plasma sintering technique at 1150 ◦C.

Table 2 The result of EDX analysis in the corresponding points of WC-12Co compacts with 0, 12.5 and 25 wt% CrSi2 content prepared by SPS at 1150 ◦C.

Samples Areas Atomic Percentage (at%) Predicted Phases

W C Co Cr Si O

WC-12Co Light-gray 44.6 53.0 0.9 – – 1.4 WC Dark-gray 36.9 42.3 17.6 – – 3.2 Co + WC

(WC-12Co) + 12.5 CrSi2 Light-gray 41.7 52.8 1.8 1.0 0.0 2.7 WC Gray 10.0 24.2 16.9 24.7 24.1 – CrSi + Co6W6C Dim-gray 1.3 9.0 44.6 0.5 44.7 – CoSi

(WC-12Co) + 25 CrSi2 Light-gray 41.5 51.3 1.1 0.9 – 5.2 WC Gray 1 9.8 20.3 11.7 29.0 24.5 4.7 CrSi + Co6W6C Gray 2 32.7 41.5 5.5 5.9 11.6 2.7 WC + SiC Gray 3 19.4 31.2 9.2 10.6 26.1 3.6 WC + CrSi + CoSi Black 0.3 6.7 0.2 1.1 32.5 59.4 SiO2

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WC in the alloy compact is likely to decrease with the increase of CrSi2 concentration. It may affect the density, hardness and oxidation resis-tance of (WC-12Co) + CrSi2 compacts.

3.3. High temperature oxidation resistance of (WC-12Co) + CrSi2

3.3.1. Oxidation behavior Fig. 4 shows the mass change of WC-12Co compact with different

content of CrSi2 as a function of cyclic oxidation time exposed at 900 ◦C. From Fig. 4a, it can be clearly seen that the CrSi2 content in the WC-

12Co remarkably affects the resistances of the sample against oxidation at 900 ◦C. The WC-12Co compact has a very high mass gain of about 3.79 mg/mm2 compared to the CrSi2-added compacts just after the first cycle of exposure attributed to accelerated oxidation, resulting in the formation of voluminous unprotective oxide scales. This result infers that the WC-12Co compact at 900 ◦C has poor oxidation resistance. While the other two samples exhibit very low mass gain at this stage (see Fig. 4a). So it is noteworthy that the addition of CrSi2 significantly im-proves the oxidation resistance of WC-12Co compact. Since the CrSi2- free compact is prone to oxidation just after 20 h of exposure, we then stopped its cyclic oxidation test. Only the WC-12Co compacts with 12.5 and 25 wt% CrSi2 were subjected to further oxidation test.

To investigate in more detail the effect of 12.5 and 25 wt% CrSi2 addition on the oxidation resistance of WC-12Co compact at 900 ◦C for 8 cycles, we magnified the mass change curve for the two aforesaid compacts as shown in Fig. 4b. The most striking results are found in the WC-12Co compact containing 12.5 wt% CrSi2 that the mass change of the alloy compact is very small and almost steady. Although, after the fourth cycle there is a very small downward trend. But, in general the sample exhibits excellent resistance against high temperature oxidation for up to eighth cycles than WC-12Co compact. As the CrSi2 content in the compact alloys was increased to 25 wt%, however, initially the alloy compact shows a slight increase in mass gain at the first cycle of expo-sure and then gradually decreases for up to the fifth cycle (0.023 mg/ mm2). Nevertheless, after the fifth cycle, the mass gain suddenly in-creases again reaching about 0.23 mg/mm2 due to breakaway oxidation. Hereafter, the mass gain of 25 wt% CrSi2 compact is slightly increasing for up to eighth cycles of about 0.24 mg/mm2 as shown in Fig. 4b.

3.3.2. Appearance of oxidized alloy compact Fig. 5 shows the macroscopic appearances of the WC-12Co compacts

with and without CrSi2 addition after exposure at 900 ◦C for the first and eighth cycles in air.

Depending on the sample composition, WC-12Co based compacts with different content of CrSi2 demonstrates a significant difference in macroscopic appearance after the first cycle of exposure (see Figs. 5a, b and c). The CrSi2-free compact was completely oxidized to form bulky oxide scales with an irregular shape (Fig. 5a). This is consistent with that of the oxidation kinetic curve as shown in Fig. 4a. This evidence is also confirming the reason why the mass gain of this compact alloy is

significantly high since the initial oxidation test. On the contrary, the other two sintered compacts containing 12.5 and 25 wt% CrSi2 do not seem to be affected by oxidation attack at this stage. Apparently from Figs. 5b and c, all surfaces of the aforesaid samples are greenish and brownish, respectively due to the formation of oxide scales on the compact surface.

After 8 cycles of exposure, a noticeable difference is found in the 12.5 and 25 wt% CrSi2 compacts. The macroscopic appearance of 12.5 CrSi2 compact (Fig. 5d) is almost similar to that after the first cycle of the test (see Fig. 5b). No significant change on the macroscopic appearance of this alloy compact suggests that the WC-12Co compact with 12.5 wt% CrSi2 content has the best oxidation resistance at 900 ◦C as aforemen-tioned among the other two samples. Meanwhile, for WC-12Co compact with 25 wt% CrSi2 content, with the increase of cyclic oxidation time, the compact becomes susceptible to oxidation. After 8 cycles of expo-sure, the alloy compact appears to form thick oxide scales. In addition, the macro cracks can also be visually observed in Fig. 5e. Based on this sign, it is reasonable to presume that the dramatic increase in the mass gain after the fifth cycle of exposure is related to the transformation of a protective to the unprotective oxide layer, namely breakaway oxidation. The presence of defects in the oxide layer becomes the transport part for oxygen to diffuse inwardly or cations to diffuse outwardly leading to oxidation of compact elements or fresh surface of the alloy compact. This significantly increases the sample mass gain.

3.3.3. Phase composition Fig. 6 shows X-ray diffraction patterns of WC-12Co, (WC-12Co) +

12.5 wt% CrSi2, and (WC-12Co) + 25 wt% CrSi2 samples after cyclic oxidation test at 900 ◦C.

According to the results of X-ray diffraction analysis, the major constituent of formed oxide scales of WC-12Co compact after the first cycles of exposure at 900 ◦C is composed by CoWO4, Co3O4, CoO, WO2 and WO3 phases, which is almost similar to the oxidation products of WC-10Co at 450–800 ◦C [2,6] and WC-Co containing TiC and (Ta,Nb)C at 600–800 ◦C [8]. The difference lies in the identification of CoO and WO2 in this study. No diffraction peak of WC and Co can be observed in Fig. 6a. This is due to the fact that the bulk material is completely oxidized, forming voluminous oxide scales as mentioned before.

For (WC-12Co) + 12.5 wt% CrSi2 compact, XRD analysis confirms that the scales formed on the surface of this alloy compact after exposure at 900 ◦C for 8 cycles is significantly different from that of formed on the CrSi2-free compact after exposure for 1 cycle. It consists primarily of SiO2 and CoCr2O4 with a minor phase of Cr2SiO4 (see Fig. 6b). The diffraction peak of WC compound from the alloy compact can still be observed. This is an indication that the formed oxide scale on WC-12Co compact with 12.5 wt% CrSi2 content is relatively thin. As a result, an X- ray could penetrate and determine the structure of compact alloy beneath the formed oxide scale.

On the contrary, for (WC-12Co) + 25 wt% CrSi2 after oxidation at 900 ◦C for 8 cycles the alloy compact forms oxide scale consisting of

Fig. 4. (a) Mass change of (WC-12Co) compacts with 0, 12.5 and 25 wt% CrSi2 content vs. the cyclic oxidation time at 900 ◦C in air and (b) magnified image of (WC- 12Co) curves with 12.5 and 25 wt% CrSi2 content.

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CoWO4, SiO2, Cr2WO6, and CoCr2O4 (see Fig. 6c). No reflection of the alloy compact can be observed. It seems that thick oxide scales are formed on the sample surface and thereupon X-ray could not penetrate the alloy compact.

3.3.4. Microstructure and elemental analysis Fig. 7 shows the cross-sectional morphologies of WC-12Co compact

after oxidation test at 900 ◦C for 1 cycle, and WC-12Co compacts with 12.5 and 25 wt% CrSi2 content oxidized at 900 ◦C for 8 cycles.

It is apparent in Fig. 7a that after the first cycle of exposure at 900 ◦C, the WC-12Co compact forms a typical unprotective oxide layer. Pores and cracks can be clearly observed in the oxide layer. According to the results of BSE SEM cross-sectional observation and EDX analysis (Table 3) in the corresponding area of Fig. 7a, it can be seen that the oxide scale formed can be divided into 4 layers (marked by a red dashed line). The outer layer with a columnar-like structure is composed mainly of 16.5 Co, 18.4 W and 65.1 O (in at%) which refers to the composition of CoWO4 phase. In the second layer, EDX analysis confirms that the oxide scale contains 19.2 Co, 20.6 W and 60.2 O (in at%). It is noticeable that the Co and W content in the second layer are slightly higher. While O concentration is lower compared to that of in the first layer. Comparing to the results of XRD analysis, the second layer of the oxide scale seems to be composed of cobalt oxides of Co3O4, CoO, and tungsten dioxide WO2. Further getting inside in the third layer, and EDX analysis at Points 3 and 4 indicates that the oxide layer is composed mainly by 36.2 at% W and 63.3 at% O, with a small content of Co, and also 42.6 at % W, 2.1 at% Co and 55.3 at% O, respectively that is suspected to be intermediate products of tungsten oxides, mainly WO2 scale as detected by XRD analysis (Fig. 6a). In the fourth layer, EDX analysis at Points 5 and 6 reveals that the oxide layer is consisting mainly of W and O (see Table 3) with an atomic ratio of approximately 1 and 3. This is closed to the ratio of the WO3 phase. Based on the results as presented above, it

can be inferred that the first, second, third and fourth layers are composed mainly by CoWO4; Co-oxides and WO2; WO2; and WO3 scales, respectively.

Since the sample was completely destroyed after the first cycle of exposure, oxygen can diffuse freely from multiple directions. Based on the results of XRD, microstructure and elemental analysis as presented above, there are some possible reactions for the formation of CoWO4 in the first layer as:

1. Simultaneously reaction between WC, Co and O:

1 /3 WC+ 1 /3 Co+O2→1 /3 CoWO4 +1 /3 CO2 ΔG900 = − 81.074 kcal

(7)

2 /5 WC+ 2 /5 Co+O2→2 /5 CoWO4 +2 /5 CO ΔG900 = − 80.032 kcal

(8)

2. The reaction between diffused W and Co with Co-oxides and W-ox-ides, respectively:

3 /4 W+ 1 /4Co3O4 +O2→3 /4 CoWO4 ΔG900 = − 89.921 kcal (9)

2 /3 W+ 2 /3 CoO+O2→2 /3 CoWO4 ΔG900 = − 80.588 kcal (10)

Co+WO2 +O2→CoWO4 ΔG900 = − 66.754 kcal (11)

2 Co+ 2 WO3 +O2→2 CoWO4 ΔG900 = − 54.189 kcal (12)

3. or solid state reaction between cobalt oxides and tungsten dioxide:

Co3O4 + 3 WO2 +O2→3 CoWO4 ΔG900 = − 88.745 kcal (13)

Fig. 5. Macroscopic features of (WC-12Co) compacts with (a) 0, (b) 12.5 and (c) 25 wt% CrSi2 content after first cycle of exposure and (WC-12Co) compacts with (d) 12.5 and (e) 25 wt% CrSi2 content after eighth cycle of exposure at 900 ◦C in air.

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2 CoO+ 2 WO2 +O2→2 CoWO4 ΔG900 = − 61.138 kcal (14)

While, the formation of Co-oxides and W-oxides scales may take place according to the reaction below:

3 /2 Co+O2→1 /2Co3O4 ΔG900 = − 55.759 kcal (15)

2 Co+O2→2CoO ΔG900 = − 72.370 kcal (16)

1 /2 WC+O2→1 /2 WO2 +1 /2 CO2 ΔG900 = − 88.234 kcal (17)

2 /3 WC+O2→2 /3 WO2 +2 /3 CO ΔG900 = − 88.884 kcal (18)

2 /5 WC+O2→2 /5 WO3 +2 /5 CO2 ΔG900 = − 86.451 kcal (19)

1 /2 WC+O2→1 /2 WO3 +1 /2 CO ΔG900 = − 86.493 kcal (20)

2WO2 +O2→2WO3 ΔG900 = − 79.319 kcal (21)

Apart from the oxidation of Co element (Eqs. (15), (16)), the for-mation of CoO beneath the first layer may occur due to the dissociation of Co3O4 into CoO and O2 [16]. It was then reacted with WO2 at the scale interface to form CoWO4 as shown in Eq. (14). On the other hand, the oxidation of WC compound can lead to the formation of an intermediate oxidation product of tungsten dioxide WO2 (Eqs. (17), (18)) in the third layer and tungsten trioxide WO3 (Eqs. (19), (20)) in the fourth layer. Further reaction of WO2 with O2 also results in the establishment of WO3 [8] which takes place according to the chemical reaction as shown in Eq. (21). The growth of WO3 scale seems to be inward growing controlled by oxygen inward diffusion [7,11].

In this study, the opening porosity and cracks formation in the oxide

scale are predominantly caused by the formation of volatile gases of CO and CO2 during WC oxidation [7,8] compared to the WOx gas formation because WOx gas is formed at a higher temperature of about 1000 ◦C [11,14]. In addition, cracks within the oxide scales are very closely related to the compressive internal stress coming from the growth oxide layer [1,8] and thermal stress [4] due to the coefficient of thermal expansion (CTE) difference during rapid heating and cooling along with cyclic oxidation test. This evidence cannot inhibit the inward diffusion of oxygen, leading to severe oxidation of WC-Co compact at 900 ◦C. Since the mass gain is significantly high, the formation of oxide scales is overweighing than the mass loss caused by the formation of volatile gas species as CO and CO2.

A surprising result is found in the cross-sectional image of WC-12Co with 12.5 wt% CrSi2 after oxidized in air at 900 ◦C for 8 cycles (Fig. 7b), the compact forms a considerable thin oxide layer of about 10–15 um in thickness. This slow growth oxide layer strongly suggests that the alloy compact exhibits a high resistance against oxidation at 900 ◦C as noticed earlier. To examine the scale structures formed on this alloy compact, the metallography observation was carried out at higher magnification as given in Fig. 7d. It is visible that the oxide scales formed can be divided into two parts as marked by a red dashed line. The outer layer consists of dark-gray particles surrounded by the dark area. Secondly, beneath the oxide layer, dark areas and gray particles can also be observed. According to the results of EDX analysis, the dark-gray par-ticle of Point 1 is composed of 15 at% Co, 30.6 at% Cr, 2.2 at% Si, 52.1 at % O with a small concentration of W. From the atomic ratio of Co, Cr and O, it should be CoCr2O4. While the dark areas (Points 2 and 3) consist mainly of Si and O (see Table 3) with an atomic ratio of about 1:2 which corresponds to SiO2 phase. In the inner layer, however, an EDX analysis indicates that the element composition and concentration of dark area in the outer layer and beneath the oxide scale is different. Beneath the

Fig. 6. XRD patterns of (WC-12Co) compacts with (a) 0, (b) 12.5 and (c) 25 wt% CrSi2 content after oxidation at 900 ◦C for (a) first, (b) and (c) eighth cycles.

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oxide layer at Point 4, it is composed mainly of 18.8 at% Cr, 14.4 at% Si and 62.6 at% O, probably a mixture of Cr-oxide and Si-Oxide in the form of Cr2SiO4, as confirmed by XRD analysis. Other interesting results are found here that the gray particle (Point 5) which is a typical morphology

of WC particle (light-gray in color) is composed of 5.9 at% W, 0.3 at% Co, 6.1 at% Cr, 25.7 at% Si and 62.0 at% O. It seems that SiO2 scale is covering the WC particles beneath the oxide layer.

Based on the results as presented above, the formation of SiO2 scale on this alloy compact occurs attributed to the preferential oxidation of Si from CrSi and CoSi according to the following chemical reaction:

CrSi+O2→SiO2 +Cr ΔG900 = − 153.425 kcal (22)

CoSi+O2→SiO2 +Co ΔG900 = − 146.457 kcal (23)

Whereas the CoCr2O4 scale can be formed through the following oxidation reaction [29]:

2 Co+O2 + 2 Cr2O3→2 CoCr2O4 ΔG900 = − 83.830 kcal (24)

Cr2O3 +CoO→CoCr2O4 ΔG900 = − 5.730 kcal (25)

As can be seen in Fig. 7d, the Cr2SiO4 scale is formed beneath the oxides layer of CoCr2O4 and SiO2. Its formation is related to the Cr2O3 reduction by metallic Cr beneath the oxide layer. The reduction product of CrO is subsequently reacted with SiO2 to form Cr2SiO4 [30].

Based on the results of cross-sectional analysis as shown in Figs. 7 b and d, it is important to note that the CoCr2O4 scale formed on the compact surface is not continuous. Therefore, it is reasonable to believe that the high oxidation resistance protection of WC-12Co compact containing 12.5 wt% CrSi2 is provided mainly by SiO2 scale formation. The presence of this oxide scale appears to play a significant role in retarding the oxidation of WC compound and the formation of volatile COx species. Meanwhile, a very small downward trend in mass gain after fourth cycle seems to be caused by the spallation of CoCr2O4 oxide particles. The difference in coefficient of thermal expansion between CoCr2O4 (7.3 × 10− 6/K [31]) and SiO2 (5.5 × 10− 7/K [32]) may lead to

Fig. 7. FE-SEM Compo images of (WC-12Co) compacts with (a) 0, (b) 12.5 CrSi2, (c) 25 CrSi2 content and (d) magnified image of Fig. 7 (b) after oxidation at 900 ◦C.

Table 3 The result of EDX analysis in the corresponding points of WC-12Co compacts with 0, 12.5 and 25 wt% CrSi2 content after oxidation at 900 ◦C.

Samples Points Atomic Percentage (at%) Predicted Phases

W Co Cr Si O

WC-12Co 1 18.4 16.5 – – 65.1 CoWO4

2 20.6 19.2 – – 60.2 Co-oxides +WO2

3 36.2 0.5 – – 63.3 WO2

4 42.6 2.1 – – 55.3 5 28.3 – – – 71.7 WO3

6 22.2 0.4 – – 77.4

(WC- 12Co) +12.5 CrSi2

1 0.1 15.0 30.6 2.2 52.1 CoCr2O4

2 0.5 0.1 4.5 31.5 63.4 SiO2

3 0.6 0.5 4.3 29.3 65.3 4 4.0 0.2 18.8 14.4 62.6 Cr2SiO4

5 5.9 0.3 6.1 25.7 62.0 SiO2

(WC- 12Co) +25 CrSi2

1 9.7 2.7 11.4 12.9 63.2 Complex mixture of Cr, W, Co and Si oxides

2 13.1 9.3 5.1 11.5 61.1 3 8.6 7.6 11.3 22.4 50.2 4 16.8 22.0 2.4 5.3 53.6 5 5.4 1.8 6.2 16.9 69.7 6 8.5 4.3 8.0 8.3 70.9 7 16.8 25.0 3.0 12.9 42.3 8 5.9 0.2 28.6 5.9 63.0 Cr2O3 + Cr2WO6

9 21.4 0.3 1.0 1.1 76.2 WO3

10 26.1 0.2 0.2 5.2 68.3 WO3

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CoCr2O4 particles spallation during cyclic oxidation test. Conversely, the results of cross-sectional observation of WC-12Co

compact with 25 wt% CrSi2 content reveals that the formed oxide scales on this alloy compact seem to consist of four layers with an interface marked by a red dashed line as shown in Fig. 7c. In the first layer, typically thick and porous oxide scales with a thickness of about 100 μm are formed. The formation of a crack in the oxide layer can also be clearly observed. Moreover, it was visible that the second, third and fourth oxide layers are relatively thinner and denser than the first layer. According to the results of EDX analysis carried out in the corresponding areas of Points 1, 2, 3, 4 and 5 with the result as listed in Table 3, the first layer is collected by a complex mixture of Cr, W, Co and Si oxides. These complex structures are suspected to be composed of CrWO4, Cr2WO6, SiO2 and CoCr2O4 as detected in the X-ray diffraction analysis (see Fig. 6c). In the second layer, EDX elemental analysis in Points 6 and 7 demonstrate that the oxide scales have a composition almost the same as the first layer. Some elements have a slightly higher concentration and vice versa. So it can be concluded that mostly, the first and second layers are composed of the same oxide scales. However, the second layer has a denser microstructure than the first layer. This may be due to the evaporation of volatile species during the oxidation test. The pores then grow inwardly with the increase of exposure times. On the other hand, the third layer at Point 8 is consisting of 5.9 at% W, 0.2 at% Co, 28.6 at% Cr, 5.9 at% Si and 63.0 at% O that seems to be Cr2O3 and Cr2WO6 scales. While the fourth layer at Points 9 and 10 is composed mainly of W and O with a small content of Co, Cr and Si (see Table 3). The atomic ratio of W and O is closed to the ratio of WO3 phase.

The formation of CoWO4, SiO2 and CoCr2O4 on this alloy compact can proceed according to the reaction as explained earlier. In addition, the oxidation of Co6W6C and SiC may occur according to the following reaction.

1 /13Co6W6C+O2→6 /13 CoWO4 +1 /13 CO2 ΔG900 = − 90.545 kcal (26)

2 /25Co6W6C+O2→12 /25 CoWO4 +2 /25 CO ΔG900 = − 90.716 kcal (27)

1 /2 SiC+O2→1 /2 SiO2 +1 /2 CO2 ΔG900 = − 123.562 kcal (28)

2 /3 SiC+O2→2 /3 SiO2 +2 /3 CO ΔG900 = − 135.988 kcal (29)

While the Cr2WO6 formation seems to follow the reaction below [33,34]. Here, the equation was presented in 1 mol of oxygen.

2 /3 W+ 2 /3Cr2O3 +O2→2 /3Cr2WO6 ΔG900 = − 100.119 kcal (30)

Cr2O3 +WO3→Cr2WO6 ΔG900 = − 3.624 kcal (31)

It can be seen that the reaction between Co6W6C with O2 also en-hances the formation of CoWO4 scale too. This supports the results of XRD analysis which shows that the CoWO4 reflections are more pro-nounced (see Fig. 6c) in the WC-12Co with 25 wt% CrSi2 addition compared to CrSi2-free compact. Moreover, the reaction as shown in Eqs. (26)–(29) also results in the formation of COx gases. Accordingly, it can be deduced that the formation of porous oxide scales in the first layer should be related to the formation of volatile COx species during cyclic oxidation test. This phenomenon is also believed to cause a slight decrease in mass gain for up to fifth cycles as shown in Fig. 4. Besides the aforementioned, a decrease in mass gain could also be caused by oxide scale spallation during cyclic oxidation test. Whereas, as previously explained, cracks in the oxide layer can be formed due to several factors such as the formation of volatile species [7,8], internal compressive stress [1,8] and thermal stress [4] arose from high heating and cooling. Its existence is allowing the oxygen to diffuse inwardly and freely. It will oxidize the new surface beneath the oxide layer, thereby causing the formation of new oxide scales and noticeable mass gain of the sample in the sixth cycle of exposure. Concurrently, the formation of Cr-based protective oxide scales as Cr2O3 or Cr2WO6 plays a role as a barrier for oxygen inward diffusion. The Cr2WO6 layer also prevents the further

oxidation of Cr2O3 to form volatile species CrO3 [34]. Thus, the mass gain caused by oxidation from the sixth cycles to eighth cycles is rela-tively small.

Additionally, in order to investigate the stability of the phases, the Gibbs free energy changes of Eqs. (3)–(31) is graphically represented as a function of temperatures as shown in Fig. 8.

The results show that at the temperature range 0 to 1150 ◦C, all re-actions have negative free energy changes. This means that thermody-namically all reactions can proceed spontaneously from the left to the right at given temperatures. A more negative free energy changes of a reaction, the more potential for the reaction to take place.

3.4. Mechanical properties

The theoretical, measured and relative densities, porosity and Vickers hardness of WC-12Co compacts with and without CrSi2 addition are listed in Table 4.

It can be seen that the theoretical and measured densities of WC- 12Co compacts decrease with the increase of CrSi2 content. The CrSi2- free compact has a lower relative density (95.94%) compared to the WC- 12Co compact with 12.5 wt% CrSi2 addition (96.31%). While 25 wt% CrSi2 addition exhibits the lowest relative density (87.95%) than WC- 12Co compacts with 0 and 12.5 wt% CrSi2 addition. The results also show that the porosity value of the alloy compact is inversely propor-tional to the relative density. This strongly suggests that the (WC-12Co) + 12.5% CrSi2 is denser compared to CrSi2-free and 25 wt% CrSi2 compacts.

The micro-Vickers hardness of WC-12Co compact is about 1589.28 HV. The addition of 12.5 wt% CrSi2 reaches the maximum hardness value of about 1737.45 HV. Further increase in CrSi2 content leads to decreasing the hardness of WC-12Co compact to about 1157.11 HV. This could be related to the relative density and porosity of the alloy com-pacts. A higher relative density associated with a lower porosity of the alloy compact results in a higher sample hardness. In addition, the previous study was also reported that the hardness of WC composite can be affected by several factors such as particle size of WC, binder phase quantity and binder hardness [17]. So based on the results as presented above, we presume that the increase in the hardness of WC-12Co compact with 12.5 wt% CrSi2 addition is also due to the fact that Co binder was already reacted, forming CoSi and Co6W6C. These phases seem to be harder than Co. While the decrease in the hardness of 25 wt% CrSi2 alloy compact is also attributed to a decrease in WC fraction in the alloy compact as clearly shown in the SEM images of Fig. 3. Based on the results obtained it can be concluded that the addition of 12.5 wt% CrSi2 offers a potential beneficial effect in increasing the hardness and lowering the density of WC-12Co compact.

4. Conclusions

In the present work, spark plasma sintered of WC-12Co containing 0, 12.5 and 25 wt% CrSi2 content is successfully synthesized. The effect of CrSi2 addition on structure, density, porosity, hardness and high tem-perature cyclic oxidation resistance of WC-12Co compact are clarified and intensively discussed. The results obtained can be summarized as follow:

1. The WC-12Co compact is composed mainly of WC and Co phases. While, the addition of CrSi2 leads to the formation of CrSi, CoSi, Co6W6C and SiC phases depending on the alloy constituent.

2. At 900 ◦C, WC-12Co compact is prone to oxidation. After the first cycle of exposure, the compact alloy is completely oxidized, forming bulky unprotective oxide scales.

3. The addition of 12.5 wt% CrSi2 leads to a lower measured density and a higher hardness compared to that of WC-12Co compact. In addition, the sample exhibits excellent resistance against oxidation at 900 ◦C for 8 cycles due to the formation of mainly SiO2 scale.

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4. However, further increase in CrSi2 concentration of about 25 wt%, its addition results in lower measured density and hardness. Moreover, the alloy compact also experiences breakaway oxidation after the fifth cycle of exposure, leading to the formation of a thick, cracks and porous oxide layer in the outer layer consisting of CoWO4, Cr2WO6, SiO2 and CoCr2O4 scales.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to thanks Research Center for Physics- Indonesian Institute of Sciences (LIPI) for providing research facilities and financial support. We also would like to thank Surip Kartolo for his valuable help during SPS preparation and Slamet Sumardi, MT. for ICP- OES measurement at Research Unit for Mineral Technology-LIPI.

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Table 4 Theoretical, measured and relative densities, porosity and Vickers hardness of (WC-12Co) + CrSi2 compacts.

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Measured density (g/ cm3)

Relative density (%)

Porosity (%)

Vickers Hardness (Hv)

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