SPS hard metal alloy WC-8Ni-8Fe fabrication based on mechanochemical synthetic tungsten carbide powder

Journal Pre-proof SPS hard metal alloy WC-8Ni-8Fe fabrication based on mechanochemical synthetic tungsten carbide powder O.O. Shichalin, I.Yu Buravlev, A.S. Portnyagin, M.I. Dvornik, E.A. Mikhailenko, A.V. Golub, A.M. Zakharenko, A.E. Sukhorada, K.Yu Talskikh, A.A. Buravleva, A.N. Fedorets, V.O. Glavinskaya, A.D. Nomerovskiy, E.K. Papynov PII:

S0925-8388(19)33793-4

DOI:

https://doi.org/10.1016/j.jallcom.2019.152547

Reference:

JALCOM 152547

To appear in:

Journal of Alloys and Compounds

Received Date: 11 July 2019 Revised Date:

20 September 2019

Accepted Date: 2 October 2019

Please cite this article as: O.O. Shichalin, I.Y. Buravlev, A.S. Portnyagin, M.I. Dvornik, E.A. Mikhailenko, A.V. Golub, A.M. Zakharenko, A.E. Sukhorada, K.Y. Talskikh, A.A. Buravleva, A.N. Fedorets, V.O. Glavinskaya, A.D. Nomerovskiy, E.K. Papynov, SPS hard metal alloy WC-8Ni-8Fe fabrication based on mechanochemical synthetic tungsten carbide powder, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.152547. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.

SPS hard metal alloy WC-8Ni-8Fe fabrication based on mechanochemical synthetic tungsten carbide powder Shichalin O.O.1,2, Buravlev I.Yu.1,2, Portnyagin A.S. 1,2*, Dvornik M.I.3, Mikhailenko E.A.3, Golub A.V.1, Zakharenko A.M.2, Sukhorada A.E.2, Talskikh K.Yu.2, Buravleva A.A.2, Fedorets A.N.2, Glavinskaya V.O. 2, Nomerovskiy A.D. 2, Papynov E.K.1,2 1

Institute of Chemistry of Far Eastern Branch of Russian Academy of Sciences, Vladivostok, Russia 2

3

Far Eastern Federal University, Vladivostok, Russia

Institute of Material Science Far Eastern Branch of Russian Academy of Sciences, Khabarovsk, Russia

* Corresponding author: Portnyagin A.S. ([email protected]) Abstract. The paper presents fabrication of WC-Ni-Fe hard metal alloy with the density up to 99.9% from theoretical, hardness ~1303 HV, fracture toughness ~12.4 MPa m0.5, compressive strength ~1365 MPa and without open porosity. High quality of the materials is provided by the original synthesis route based on powder preparation and sintering. WC powder (particle size 0.1-14 µm) was obtained via mechanochemical synthesis using available precursors and polymethyl methacrylate to enhance the grinding. Rapid SPS consolidation was performed at 1200 °C and below achieving high density with the holding time being less than 18 min. Increased amount of NiFe (8 wt.% of each component) was used to enhance SPS heating due to deep wetting of the particles and, therefore, improved conductivity of the whole system. Earlier unknown data on densification dynamics and phase composition are presented for WC-8Ni-8Fe alloy under SPS sintering in the temperature range 1100-1200 °C. The results are proved by the means of XRD, SEM and EDX. Keywords: tungsten carbide; hard metal alloy; WC-Ni-Fe; mechanochemical synthesis; spark plasma sintering. 1

Introduction WC-based alloys including ones doped with Co are the most abundant materials for cutting,

pressing, drilling tools and etc. [1]. The common problem of these alloys is their high cost caused by expenses on preparation of the original WC and Co powders. Highly expensive conventional technologies of WC powder production and low rates of solid-phase reaction are the main reasons for that. WC powder is obtained by a number of methods [1,2], e.g., by tungsten and carbon interaction in hydrogen, methane, and in vacuum at temperatures of 1400-2000 °C or by heating tungsten-carbon mixture in the carbon tube at 2800 °C in microwave oven and etc. Solid-phase interaction in the reaction mixture is promoted usually by mechanochemical activation using hightemperature mechanochemical synthesis (HTMCS). Additionally, disperse powders of required 1

compositions are obtained with the mechanical stimulation of thermal explosion being performed [3–5]. Application of Co as an alloying compound is done on a large scale to improve corrosion resistance and mechanical properties of WC alloys, however, it increases their production costs [6– 9]. More available elements with similar effects are Ni, Cr, Cu, Mo, Al, Si, Fe [10–14]. Ni-Fe mixture is very promising for alloying WC, because of low cost with higher corrosion resistance and mechanical properties obtained as compared to Co-containing alloys [15]. Good wetting ability of Ni and Fe within the WC powder provide efficient liquid-phase sintering at rather low temperature yielding in the dense low-porous alloy [15] that is characterized by nearly the same hardness as WC-Co system [16]. Also, WC-Ni-Fe constituents are mutually inert with no chemical interaction involved during sintering that allows preserving the shape and high viscosity of the alloy [1,16,17]. The most prospective are WC-Ni-Fe alloys with high content of alloying additive (to 20 wt.%) that are widely used in production of drilling tool [18]. Recently published papers [17,19–24] have shown WC of high density can be achieved using rapid consolidation of the powder with pulse current in accordance with spark plasma sintering (SPS) technology. During sintering, alloying compounds in the form of conducting metals can enhance heating of WC powder by electric current due to formation of additional percolation routes and spark discharges in the places of particle contacts [25]. High heating rates of the whole system and in the local regions as well as pressure load are the advantages of the SPS method as compared to conventional sintering routines as has been already proved in the works related to manufacturing of some dense alloys [19,25–28]. Thus, this works studies synthesis of WC-(8 wt.%)Ni-(8 wt.%)Fe by SPS of tungsten carbide powder obtained using high-temperature mechanochemical synthesis. Physico-chemical and mechanical characteristics of the prepared alloy are also investigated. Suggested method is focused on manufacturing of high-quality dense alloy by implementing modern technologies with soft technological regimes and using low-cost precursors instead of expensive counterparts. 2

Materials and Methods.

2.1 Materials. To prepare tungsten carbide (WC) we used tungsten (VI) oxide (99.9%, LLC “SZKhR”, Russia), magnesium metal (99.95%), carbon (99,6 %, “PM-15”), polymethyl methacrylate with molecular weight of 6⋅105. Chemically pure nickel (99.95%) and iron (99.95%) were used as alloying agents. 2.2 Mechanochemical synthesis of WC-8Ni-8Fe powder. Tungsten carbide was prepared according to scheme WO3 + 3Mg + C + PMMA→ WC + 3MgO as reported in [3,5]. 2

Mechanochemical activation and synthesis of the precursor powders were done on the vibration mill at the frequency of 750 min-1, the magnitude of 90 mm and using steel (steel 52100, US standard) grinding balls (Ø 12–14 mm). Ball size was chosen in a way to avoid their mutual blocking up in the mill. Ratio of powders to balls mass is 1:15. Steel balls occupy less than 40% of the mill’s volume, alternatively, the grinding efficiency drops drastically. The vibrational treatment was stopped after abrupt temperature increase in the reactor indicating synthesis end. IR laser pyrometer DT-9862 (Cem Tech, China) focused on the outer reactor wall was used to measure temperature changes. Obtained product was washed three times with hydrochloric acid (density 1.15 g cm-3) in hydrothermal conditions at 120 °C for 5 hours. To remove impurities, powders were washed with distilled water and with acetone. Obtained tungsten carbide was mixed with nickel (8 wt.%) and iron (8 wt.%) metal powders. The micture was homogenized on a planetary ball mill Pulverisette 7 (Fritsch, Germany) using cerium oxide grinding balls, rotation rate 600 rpm and for 5 cycles of 20 minutes each. 2.2 SPS consolidation of the WC-8Ni-8Fe. Spark plasma sintering was performed on a SPS-515S (Dr.Sinter*LABTM, Japan) device at temperatures 1000, 1100, 1150, and 1200 °C under pressure 57.3 MPa at heating rates of 50 and 87.5 °C min-1. To avoid melting followed by Ni-Fe compound concentrating on the surface, the maximal sintering temperature of 1200 °C was used. Sintering was performed according to the scheme: 8g of the powder was put into the graphite die (Ø 10.5 mm), pre-pressed (20 MPa), then the green body was placed into the sintering chamber, degassed till vacuum (10-5 bar) and sintered. SPS temperature was measured using optical pyrometer focused on the gap at the center of the outer die wall (5.5 mm deep). Sintered samples was held at maximal temperature for 5 minutes and cooled down to room temperature for 45 min. Heating was performed using direct pulse current (periodicity in the On/Off regime 12/2, packet duration 39.6 ms and pause 3.6 ms). Maximal current achieved was 600 A, voltage – 4 V. 2.3 Characterization methods. Particle size distribution of the powders was found on a laser particle analyzer Analysette-22 NanoTec/MicroTec/XT («Fritsch», Germany). XRD patterns of the powders and alloys were recorded on a diffractometer D8 Advance Bruker AXS (Germany) using CuKα-radiation source, Ni-filter, mean wavelength (λ) 1,5418 Å, angle range 10–80°, scanning step 0.02°, scanning rate — 5 ο min-1. Microstructural investigations were done on a scanning elecgtron microscope Hitachi TM 3000 and S3400N with the EDX add-on (Bruker, Germany). Nanosized sample for SEM was collected via gravimetric decantation of the powder suspended in water after ultrasonic treatment. The droplet was sampled from the upper part of the tube, then it was deposited on the support followed by drying at ambient conditions and gold spattering onto it. Packing density was found 3

using hydrostatic weighing on a balance AdventurerTM OHAUS Corporation (USA). Fracture toughness was calculated according to ISO 28079: 2009. Vickers’ hardness was measured in accordance with ISO 6507–1:2018 using hardness testing instrument HVS-50 at the load of HV30. The mean grain diameters of WC in alloys were determined by averaging of individual intercept lengths using parallel straight test lines applied randomly to the structure (ASTM E112 - 13). Bending strength limit was measured according to ISO 3327–2009 using specifically designed samples on a tensile machine Autograph AG-X plus 50 (Shimadzu, Japan), load rate 0.5 mm min-1. Phase composition and microstructure of the obtained alloys were identified from the polished cross-section region. 3

Results and Discussion. To obtain nanosized fraction in the WC powder in the process of ball milling, we have

added polymethyl methacrylate (PMMA) to the mixture of starting materials (WO3, Mg, C). PMMA acts as a high-molecular compound decomposing under high-energy ball milling, which promotes the metal grinding. Vibrational treatment of polymer-metal mixture leads to PMMA decomposition into volatile components. The products of decomposition enter the surface cracks of metal particles and start to polymerize there yielding in high-molecular compounds that tend to increase in volume within the crack. The outflow of the polymer product is suppressed by adhesion to the solid surface that leads to growth of strains in the micro-crack area and promotes advance of crack front inside the particle. Kinetics of crack growth is governed by rate of mechanochemical processes and concentration of polymer degradation products formed under ball milling conditions [5]. Retention time and synthesis temperature increase with PMMA mass added to the milled mixture (Table 1), because additional grinding of initial mixture occurs and required energy conditions are to be met. Temperature of milling chamber at the moment of reaction grows linearly with the mass of PMMA in the mixture. Table 1. Composition of the dry mix and parameters of WC nanopowders milling

PMMA content with respect to WO3+Mg+C, %

Retention time, s

Temperature of the outer wall of the milling chamber, °C

1 2 3

255 328 346 426

130 134 144 156

According to XRD, peak intensity of W2C is inversely proportional to the concentration of PMMA in the dry mix (Fig. 1). Optimal PMMA concentration corresponding to absence of W2C is 4

3 wt.% from the total amount of WO3+3Mg+C mixture that evidences all residual carbon is involved into reaction. Further increase of PMMA content is not reasonable, because there occurs unreacted carbon on the inner walls of milling chamber after the end of synthesis. Unreacted carbon after treatment with HCl floats on the surface of the solution and requires further mechanical separation, i.e. it may require introduction of one additional technological step of WC powder purification from impurities.

Figure 1. Phase composition of the powders obtained at various content of PMMA in the dry mix: ● — WC; ○ — W2C Reaction retention time increases with PMMA content in the mixture due to damping the mechanical energy by PMMA and products of its degradation. As a result, to create necessary energy conditions the additional amount of energy should be provided, which is achieved at longer milling times. After milling, magnesium oxide is formed as the main by-product of the reaction, removal of which requires multi-stage treatment in various solutions and even at hydrothermal conditions (120 °C, ~2 bar). Fig. 2 shows particle morphology, their agglomerates before and after treatment in HCl.

5

Figure 2. Surface morphology of WC powder obtained using 3 wt.% PMMA added to the dry mix. a – before HCl treatment; b – after HCl treatment. Surface of large particles in the prepared powder before HCl treatment is formed from the agglomerates of smaller (<1 µm) particles (Fig. 2a). Treatment of the powder with hydrochloric acid removes the residual magnesium oxide. Dissolution of magnesia changes particle morphology and provides additional chemical dispersing the whole powder fraction. Deagglomeration of sintered mass liberates particles sized 150-300 nm with pronounced morphological signs of melting. Particle melting is caused by high-temperature conditions in the local regions during mechanochemical activation. SEM images show powder particles either smaller than 0.5 µm or sized from 1 to 5 µm that indicates rather wide particle size distribution. Particle size distribution of the WC-8Ni-8Fe powder is bimodal with 5% of the particles being sized below 1 µm (Fig. 3, Area B). Such powder fraction consisting of submicrone- and micron-sized (Fig. 3, Area A) particles is rather advantageous for packing and sintering, because empty gaps among the larger particles are easily filled with the smaller ones, thus, increasing the density of the final consolidated material and reducing required sintering temperature. Obtained size distribution is a particular feature of high-temperature mechanochemical synthesis, which involves 6

high-energy milling to produce small particles combined with high temperature leading to particle aggregation [5]. SEM images reveal the particles of WC are characterized by fragmental shape with melt edges, particle size corresponds to the one found by laser scattering (Fig. 3). It can be also seen that the largest particles are the agglomerates of the smaller ones. Additionally, both submicrone and microne-sized particles are present on SEM images proving the results of laser granulometry

Figure 3. Particle size distribution of the tungsten carbide (Qvol – volume content of particles; Qmass – mass content of particles; d – particle’ size) and SEM images of its powder. Consolidation kinetics of WC-8Ni-8Fe powder was studied using densification vs. time curves at SPS heating to 1200 °C and at constant pressure of 57.3 MPa. Dilatometry curves (Fig. 4) show that all the samples tend to consolidate the same way at the beginning. At low temperatures (to 600 °C), the slight packing occurs as a result of mechanical load that promotes particle rearrangement and destruction (region A). Intense densification and sintering begin as 700 °C on the 7th minute of heating (region B) and enter the active sintering phase at 850-900 °C (region C). Active densification in that region is apparently related to plastic flow and creeping of the sintered powder. When the heating is stopped, the densification becomes slow. Increase of the sintering temperature from 1000 to 1150 °C leads to significant sample’s densification. Further temperature increase from 1150 to 1200 °C has a slight effect on compaction.

7

Figure 4. Densification of WC-8Ni-8Fe powder at various SPS temperatures: (Ppress = 57.3 MPa): 1 — 1000 °C; 2 — 1100 °C; 3 — 1150 °C; 4 — 1200 °C. Hypothetically, under given SPS conditions the formation of Ni3C (3Ni(s) + C(s) → Ni3C(s)) and metastable W2C (2WC(s) + 3Ni(s) → W2C(s) + Ni3C(s)) is possible and may give rise to η-phase (W2C+Ni→Ni3W3C). Formation of η-phase arising from lack of carbon is not advisable because it leads to dramatic decrease of mechanical properties [29]. Gibbs free energy change (∆G) of these compounds is known to reduce with the increase of temperature [17]. Our results show phase composition of all samples sintered at different temperatures (1000-1200 °C) remains practically the same (Fig. 5). Absence of possible “negative” phases (Ni3C, W2C or Ni3W3C) is particularly important. XRF analysis proves the preservation of the phase composition revealed by XRD (Table 2) and indicates higher temperature has no effect on the composition and on alloying compounds content in the solid solution either (within the error of the method).

8

Figure 5. Phase composition of the original WC-8Ni-8Fe powder and SPS alloys based on it (● – WC; ■ – Ni/Fe). Table 2. XRF analysis of the original WC-8Ni-8Fe SPS alloys based on it. Sample, # 1 2 3 4

Sintering temperature, °C 1000 1100 1150 1200

W, wt.%

Fe, wt. %

Ni, wt. %

Total, wt.%

84.1±2,4 % 84.1±2,3 % 84.1±2,2 % 84.5±2,3 %

8.0±0,2 % 7.9±0,2 % 7.7±0,3 % 7.5±0,2 %

7.9±1,7 % 8.0±1,9 % 8.2±1,8 % 8.0±1,8 %

100.0 100.0 100.0 100.0

Microstructure of the WC-8Ni-8Fe alloys consists of WC grains (Fig. 6). SEM images reveals the influence of sintering temperature on WC particle packing density (light regions) and on distribution of Ni-Fe (dark regions).

9

Figure 6. Morphology of the WC-8Ni-8Fe alloy surface cross-section depending on the SPS temperature (a — 1000 °C; b — 1100 °C; c — 1150 °C; d — 1200 °C), prepared at constant pressure 57.3 MPa, heating rate 50 °C min-1 and holding time of 5 min. Sample sintered at 1000 °C is characterized by non-uniform distribution of Ni-Fe in the bulk of WC. This temperature is not enough to spread the alloying compound uniformly, thus, leading to higher concentration of Ni-Fe in the bulk of the alloy as well as reducing the density and hardness of the SPS-sintered material (Table 3). With the increase of sintering temperature, Ni-Fe binding agent is spread more uniformly (Fig. 6, 7a, b). Surface of the samples sintered at 1150 and 1200 °C is quite different in terms of Ni-Fe distribution. Alloy sintered at 1200 °C is characterized by slightly more dense WC packing proved by higher mechanical properties (Table 3).

10

Figure 7. Distribution of W, Ni and Fe in the samples obtained at (a) 1000 °C and (b) 1200 °C In the temperature range 1100-1200 °C, liquid phase sintering is the main densification route. The closest WC particle packing is found for the sample sintered at 1200 °C, which is characterized by the most uniformly distributed Ni-Fe binding agent (Fig. 6d). Samples’ porosity evaluated from the density values falls from 13% down to 0% with the sintering temperature increasing from 1000 °C to 1150 °C. Raising the temperature further has no effect on density. Thus, even at 1150 °C the theoretical density of the material is achieved and does change at higher temperatures that is proved by sintering dynamics (Fig. 4). At higher temperatures Ni-Fe binder melts and flows out of the die. Table 3. Physico-mechanical characteristics of WC-8Ni-8Fe SPS samples depending on sintering temperature

11

Sample, #

Sintering temperat ure, °C

Density, % from theoretical

Porosity, %

Mean grain diameter, µm

Hardness, HV

Fracture toughness, MPa m0.5

Bending strength, MPa

1

1000

87,7±0,1

13,0

1,25

354±5

impossible to measure*

—

2

1100

94,6±0,1

6,1

1,32

1131±5

8,0

1097±15

3

1150

99,9±0,1

0

1,38

1276±5

11,9

1325±15

4

1200

99,9±0,1

0

1,40

1303±5

12,4

1365±15

*Sample obtained at 1000 °C do not observe fractures during measurements **Table provides results obtained from three measurements Increasing the sintering temperature from 1000 to 1200 °C monotonously enhance hardness, fracture toughness, and bending strength of the alloy. Temperature plays the key to form physicomechanical characteristics in the obtained alloys. In the high temperature range, at 1150-1200 °C, the considered properties change slightly within a range of 5% caused by grain rearrangement, which was observed on SEM images. Properties of WC alloys sintered with different alloying compounds are given in Table 4. Presented data shows increase of sintering temperature by 200 °C leads to mean grain growth from 1.25 to 1.40 µm. Such insignificant growth is caused by merging and dissolving small grain of WC with their number being decreased at higher sintering temperatures. Alloy sintered at 1200 °C (Table 3) is characterized by higher hardness compared to medium-grained counterparts containing nearly the same amount of alloying compound [30,31]. Lower sintering temperature allows preserving not only grains, but also crystallites constituting them, thus, improving hardness and fracture toughness. Obtained hard metal alloy in terms of hardness and toughness is closer to submicron WC-Co alloys containing the same amount of alloying compound [14,15]. Table 4. The composition and mechanical characteristics of sintered samples and its analogues Binder composition and content

Sintering temperatur e, °C

Porosity, %

7,5% Fe+7,5%Ni 7,5% Fe+7,5%Ni 15%Co 15%Co 8%Fe+8%Ni+4%Co 16,3%Co 9,2%Co 15%Co 11.6% Fe+3%Ni+0.4%Co

1350 1400 1350 1400 1380 – – 1350 1300

– 0,32 0,24 – 0,05 – – 0,03 0,03

Main grain diameter , µm – – – – 1,33 1,83 1,73 0,42 0,32

Hardness, HV (HRA)

Fracture toughness, MPa m0.5

Bending strength, MPa

Refe rence

1463 1488 1417 1416 850 1075 1300 (90,92) (90,83)

12,3 15,1 10,2 9,5 18,4 17,0 12,6 12,33 16,2

– – – – 2800 3554 3912 2861 2568

[15] [15] [15] [15] [30] [31] [31] [14] [14]

Hardness of the alloy obtained at maximal temperature is higher than that of mediumgrained alloys with nearly the same amount of binding agent. Obtained WC-8Ni-8Fe alloy is as 12

good as commercial tungsten-cobalt alloys in terms of hardness and fracture toughness [32]. Fracture toughness (12.4 MPa m0.5) and hardness (1303 HV) of the obtained alloy is slightly different in comparison with the respective values of the tungsten-cobalt medium-grained alloys (12.5 MPa m0.5, 1300 HV, respectively) containing the same amount of binding agent (6, 10, and 15 wt.% of Co), however, these properties are in close limits. This can be attributed to smaller grain size and increased hardness of the binding agent.

4

Conclusion The paper studied the way to obtain WC-Ni-Fe alloy with the increased content of Ni/Fe

binding agent (8 wt.% of each component) using spark plasma sintering. WC powder was prepared via mechanochemical synthesis involving reduction of tungsten oxide with magnesium in the presence of PMMA. Adding 3 wt.% of PMMA into the dry mix yields in W2C-free WC powder. Treatment of the obtained powder with HCl solution removes the impurities and liberates nanosized (150-300 nm) melt particles from the sintered agglomerates due to magnesium oxide dissolution. It is shown that SPS allows fabricating WC-8Ni-8Fe alloy from synthetic WC powder at temperatures 1000, 1100, 11500 and 1200 °C and at short times of heating and holding (less than 18 min), without formation of troublesome Ni3C, W2C or Ni3W3C phases in the product’s composition. Physico-chemical characterization has shown hardness, fracture toughness, and bending strength of WC-8Ni-8Fe alloy increase with the sintering temperature rising from 1000 to 1200 °C. Additionally, the porosity falls from 13% down to 0%, therefore, density reaches 100% from theoretical. Optimal sintering temperature was found to be 1200 °C, because it yields in the most uniform distribution of binder Ni-Fe phase within the compound and alloy with the highest physicochemical characteristics is achieved: hardness ∼1303 HV, fracture toughness ∼12.4 MPa m0.5 and bending strength ∼1365 MPa. Suggested approach is promising for technological implementation, because of high efficiency of SPS and mechanochemical methods and due to available precursors and cheap binders (Ni and Fe) are used. Obtained products can serve as a cheap alternative to the known solid carbides.

Acknowledgements The work was financially supported by the Institute of Chemistry FEB RAS State Order (0265-2019-0001). The equipment of CUC “Far Eastern center of structural investigation” was used in the work. Additionally, the use of Shimadzu equipment was financially supported by the Genzo Shimadzu scholarship. 13

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SPS-derived WC-Ni-Fe alloy with high binder phase content to improve compaction; Mechanochemical synthesis was used to obtain powders from cheap, available precursors; Original high-temperature mechanochemical method yielded in optimal powder fractions; SPS regime was optimized using densification dynamics based on dilatometry curves; Obtained alloy shows the same mechanical properties as the best WC-Co alloys.