Properties of WC-Co Cemented Carbide Reinforced with Yttria-Stabilized Zirconia Nanoparticles

Abstract

To produce strong and wear-resistant tools for the rock drilling industry, the most commonly used metal matrix composites contain the reinforcing phase of cemented carbide. There are numerous research reports on attempts to improve the performance of WC-Co composites. The paper is a continuation of previously reported research on the SPS-processed WC–6 wt.%Co metal matrix composites with yttria-stabilized zirconia (YSZ) addition in amounts of 4 wt.% and 10 wt.%. The sintered specimens were polished and underwent the microindentation tests with a Vickers shape diamond tip. The following parameters were measured: stiffness S, the Poisson number ν, indentation creep C_IT, relaxation R_IT, indentation hardness H_IT, indentation Vickers hardness HV_IT, Martens hardness HM, reduced modulus E*, and indentation elastic modulus E_IT. The tests revealed hardness values of 16.2–17.0 GPa and indentation elastic moduli in the range of 607–670 GPa. Moreover, respective plastic and elastic parts of the indentation work W_plast and W_elast were determined. It was found that YSZ addition slightly reduced hardness and modulus, but all the three wear parameters, H/E, H³/E², and 1/(E²H), increased after addition of zirconia. Specifically, for 10 wt.% ZrO₂ H/E increased by 5%, H³/E² by 7%, while 1/(E²H) by 27% compared to 94WC–6Co composition.

1. Introduction

To produce strong and wear-resistant tools for the rock drilling industry, most commonly used metal matrix composites contain the reinforcing phase of cemented carbide. The matrices often contain several metal powders of different kinds with various melting temperatures, hardness, density, and particle sizes. Among the metals, Co, Fe, Ni, and Mn have melting temperatures ca. 1200–1500 °C, and they are usually used as binding phases [1]. The widely used carbide phase is the tungsten carbide of hexagonal structure, which is combined with cobalt (WC–Co), where the metal increases the wettability of the WC particles, improving the strength of their binding [2]. Even though WC is not the hardest one among carbides nor is cobalt the toughest metal, the WC-Co combination is widely recognized as the best system for hardmetals. Among the reasons, da Silva et al. [3] named the low dihedral angle, close to zero, of the WC-Co system, and the fact that Co is able to dissolve a significant amount of WC, while Co is not dissolved in WC. Due to this outstanding coupling, globally, around 54–72% of the produced tungsten is used in the form of WC monocarbide to manufacture cemented WC-Co carbides [4].

The WC-Co composite for cutting tools with a hard WC phase and tough Co binding phase is successfully fabricated by powder metallurgy techniques [5]. For instance, Matviichuk et al. [6] published results on WC–20 wt.%Co material fabricated using free liquid-phase sintering, while Sheremet and colleagues [7] reported research on cold isostatic pressing of WC-15 wt.%Co powder. Li with co-authors [8] investigated WC–13 wt.%Co composite with 2–4% Fe addition prepared by pressureless sintering, while Mariani with his team [9] produced and examined WC–12 wt.%Co, using binder jetting technique with subsequent densification by sintering at 1400 °C. However, it has been recognized that methods such as Field-Assisted Sintering Technique (FAST) or Spark Plasma Sintering (SPS), where electric current pulses heat the powder, are more efficient than other technologies due to the ability to reach higher heating rates [10]. Niu and team [11] demonstrated that superior properties could be obtained by the SPS process at a lower temperature, compared with conventional methods, and in a shorter time. Thus, in this study, WC-Co sintered using the SPS method was chosen as the material to be investigated in terms of improvement of its mechanical properties.

Common strategies to improve the mechanical performance of WC-Co composites are to refine the WC grain size, add grain-growth inhibiting metal carbides, e.g., TiC, VC, or Cr₃C₂, and alter the processing parameters via different sintering techniques [12]. Other researchers [13] reported results on sintering WC–Co composite with the addition of 316 L SS powder in order to enhance the bonding between phases. A successful attempt to strengthen the Co metal phase with CeO₂ was reported [14], demonstrating that the additive inhibited WC grain growth, thus improving the overall performance of the WC-Co composite. Also the sintering processes of WC-Co nanopowders were investigated with the addition of Cr₂(C,N), and the nitrogen-containing cemented carbide was obtained with a uniform microstructure and enhanced interfacial bonding [15].

Moreover, the overall supply risk of Co is assessed at a high level [16], so that the European Union and the International Energy Agency have identified cobalt among the critical raw materials [17], making more favorable materials with less percentage of cobalt. Thus, Ju with collaborators [18] examined the effects of Co content on the microstructural features and related mechanical properties of the cemented carbides and found that SPS-processed WC–8 wt.% Co composition exhibited favorable synergic mechanical performance. Low cobalt WC-based hard materials have also been investigated, including 1 wt.% and less [19]. Thus, in this study, a reduced Co proportion of 6 wt.% was chosen.

Utilizing various additives, it is possible to obtain desired properties of WC-Co based compositions for specific applications. In particular, the addition of Cr₃C₂, VC, TaC, NbC, Nb, and Y₂O₃ was reported to promote densification and enhance the mechanical properties of sintered cemented carbide [20,21]. Moreover, zirconia, being itself an excellent material for engineering ceramic fabrication, additionally promotes densification of tungsten carbide powders [22], and it was proved feasible to be used as a reinforcement in WC–6Co composite [23].

The paper is a continuation of previously reported research on the SPS-processed WC–6 wt.%Co metal matrix composites with yttria-stabilized zirconia (YSZ) addition in amounts of 4 wt.% and 10 wt.% [24]. The YSZ additives were demonstrated to promote the formation of very thin layers of metallic phase binding the carbide grains, thus enhancing the strength of interfaces.

Despite extensive research on WC–Co cemented carbides, the combined influence of reduced cobalt content and ceramic nanoadditives on elastic–plastic balance and respective wear resistance remains insufficiently explored. The scientific novelty of this work lies in the systematic investigation of YSZ-modified WC–6Co composites produced by electroconsolidation, with emphasis on wear-related parameters rather than hardness or strength alone. This approach enables purpose-oriented tailoring of cemented carbides for drilling and cutting tools operating under severe abrasive and cyclic loading conditions.

2. Materials and Methods

2.1. Starting Powders

The composite samples for the investigations were fabricated out of the mixtures of powdered tungsten carbide (WC), cobalt (Co), and yttria-stabilized zirconia (YSZ) consisting of ZrO₂ with 3 wt.% of Y₂O₃. The tungsten carbide powder was delivered by Global Tungsten & Powders spol. s.r.o. (Bruntal, Czech Republic). Its average particle size was 2.0–8.0 μm, according to the specification. The cobalt and zirconia powders were the same as the ones described in [25]. Particle dimensions of cobalt were ca. 2–3 μm, while YSZ consisted of nanoparticles of 30–60 nm size that formed agglomerates of diameters reaching 30 and even 100 μm. However, under the mixing process, these agglomerates got broken, providing YSZ in the form of a nanoadditive.

The composition of refractory matrix 94WC-6Co was chosen due to the numerous engineering applications of this composite material [26]. Moreover, due to high level of cobalt supply risk [16] and ongoing works concerning the optimized recovery of Co and WC from a binary-phase WC-6Co [27], this composition was considered an optimal one.

The mixtures of basic material WC–6 wt.% Co were prepared for the comparative analysis with WC-Co-ZrO₂ composites, where YSZ was added in different proportions. Addition of 4 wt.% YSZ made up a mixture 90.24WC–5.76Co–4ZrO₂ and addition of 10 wt.% YSZ nanoparticles provided the composition of 84.6WC–5.4Co–10ZrO₂. The powders underwent dispersion in the distilled water with application of ultrasound, and then milled with the planetary micro mill Pulverisette 7 produced by Fritsch GmbH (Idar-Oberstein, Germany) for 20 min. The ball diameters were 1.5 mm, and acceleration was 640 m/s². The as-prepared mixtures were dried for 24 h at a temperature of 80 °C. Figure 1 demonstrates an example of the as-prepared mixture 84.6WC–5.4Co–10ZrO₂, where some agglomerates of size ca. 10 μm are seen along with numerous submicron particles. An example of EDS diagram confirming the elemental composition is shown in Figure 2.

The EDS spectrum in Figure 2 confirms the presence of all key elements of the system, tungsten (W), cobalt (Co), zirconium (Zr), oxygen (O) and carbon (C). The main peak in the spectrum is represented by W line (ca. 1.775 keV), which correlates with the highest percentage by mass, 68.2 wt.% of tungsten. The peaks of cobalt and zirconium are not high, but clearly identified, which indicates the successful introduction of alloying additives and bonding. According to the data of semi-quantitative analysis, the mixture consisted of high content of tungsten and carbon, which corresponded to the tungsten carbide phase. The content of oxygen (5.9 wt.%) and zirconium (8.2 wt.%) is in a stoichiometric ratio close to zirconium dioxide ZrO₂. Low values of statistical error with At.% Error below 0.6 for all elements indicated high reliability of the obtained results and sufficient spectrum acquisition time with more than 141,000 pulses for tungsten. Distribution of elements is shown in Figure 3.

The mixture shown in Figure 3a consisted of particles of different shapes and sizes. The main element of matrix was tungsten represented by red color in Figure 3b. It exhibited large conglomerates and individual irregularly shaped particles with tungsten (W), indicating the tungsten carbide (WC) dominating phase. Carbon (C) distribution is shown in green color in Figure 3c. A uniform background of carbon confirms the stoichiometry of the carbide phase and the presence of adsorbed carbon compounds on the surface. Cobalt (Co) being added as a binder was distributed unevenly, as it is seen in Figure 3d. Some individual inclusions and zones of cobalt localization are observed on the surface of the WC particles, which is a typical feature after mechanical mixing of the powders. Zirconia additive is represented by Zr and O maps in Figure 3e,f. The distributions correlate with each other, which confirms the presence of zirconia particles. The identified ZrO₂ particles had a smaller fraction compared to WC and were distributed mainly in the interstitial space. The scale bar of 10 μm indicates that most of the aggregates are between 2 and 8 μm in size. The mixture demonstrates sufficient homogenization suitable for subsequent liquid-phase or solid-phase sintering [28].

2.2. Sample Preparation

The as-prepared powders were then sintered by the electroconsolidation method to fabricate cylindrical specimens of 25 mm diameter and height of 5 mm. The sintering process described in detail elsewhere [29] represented a modified spark plasma method (SPS) with direct application of alternating electrical current. The initial results concerning the effect of various parameters on the final properties of the sintered composites published earlier [30] provided solid ground for the choice of the experimental conditions. In the present research, the sintering temperature was T_s = 1350 °C and the holding time was 3 min. This combination of relatively low sintering temperature and short holding time has been proven experimentally to provide sufficient densification of the material. The relative density above 98% can be reached, and at the same time, the excessive grain growth can be prevented. The heating rate of 500 °C/min was ensured by the electric current of 5000 A at voltage 5 V applied directly to the powders through the graphite molds. The uniaxial mechanical pressure was 30 MPa while the vacuum in the chamber was kept at 6 Pa. Proper control of the process parameters helped to avoid the grain recrystallization and to obtain equilibrium, ensuring smaller grain size and high density. These features are extremely important when fabricating enhanced composite materials.

The sintered specimens underwent a grinding procedure with diamond disks 1A1-200 × 20 × 51 D213 (K50) G. Then the surfaces were polished for 3 h and 20 min using the single wheel automatic polishing machine ATM Saphir 550 (ATM Qness GmbH, Mammelzen, Germany) dedicated for the metallographic sample preparation. Platinum and silver disks and diamond suspension were applied to obtain a mirror surface of the samples with Ra below 0.1 μm, according to ISO 4287 standard [31].

2.3. Materials Characterization

Scanning Electron Microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDS) analyses were performed using Axia Chemi SEM (Thermo-Fisher Scientific, Eindhoven, The Netherlands). The system employs the secondary and backscatter electron signals that allow for acceleration of the EDS analysis with reduced noise.

The specimens underwent the microindentation tests using a universal device, Micro Combi Tester (MCT, Anton Paar GmbH, Ostfildern, Germany), with a Vickers shape diamond tip. The tests were performed under maximal load of 1 N with loading/unloading rates of 1 N/min and a holding time under 1 N load of 30 s. The parameters corresponding with ISO 14577-1:2015 [32], including stiffness S, the Poisson number ν, indentation creep C_IT, and relaxation parameter R_IT, were calculated accordingly. The values of indentation hardness H_IT, indentation Vickers hardness HV_IT, reduced modulus E*, and indentation elastic modulus E_IT, were determined using Oliver & Pharr method [33]. Martens hardness HM, related to plastic and elastic deformation, was determined by the applied loading force. HM is defined as the quotient of loading force F and indenter area As(h) at depth h [32]. For the Vickers indenter, HM was calculated from the following equation:

$$HM={\displaystyle \frac{F}{26.43 h^2}}$$

(1)

Moreover, respective plastic and elastic parts of the indentation work W_plast and W_elast were determined. The measurement was repeated 10 times, so each parameter was processed statistically, determining arithmetic mean values, standard deviations, and range from minimal to maximal values.

Since the investigated composites with nanoadditives were inhomogenous, surface mapping was performed to assess mechanical properties of the matrix and inclusions of the particulate reinforcement ZrO₂. For that purpose, Hysitron TS 77 Select nanoindenter was used (Bruker, Billerica, MA, USA). The measurement was performed in testing mode, In Situ SPM Imaging with Berkovich indenter and projected contact area A = 24.5 h_c² [34]. This mode allowed for topography imaging and the collection of nanomechanical characteristics.

3. Results and Discussion

SEM images in Figure 4 illustrate the microstructures of the analyzed composites, and

Table 1. Phase composition of the tested specimens, wt.%.

Phase	94WC–6Co	90.24WC–5.76Co–4ZrO2	86.60WC–5.4Co–10ZrO2
WC	97.97	94.68	91.39
C	0.77	1.60	3.25
Co3W3C	1.26	1.39	1.35
ZrO2	–	2.33	4.01

shows the results of their phase compositions.

Comparing images in Figure 4a,b, important differences can be noted between WC-Co and WC-Co-ZrO₂ structures, respectively. On the one hand, the carbide grains became larger after addition of zirconia. On the other hand, the metal binder between grains appears to be thinner, which probably contributed to the improvement of mechanical properties of composites with zirconia reinforcement, described below. And importantly, microstructure in Figure 4c exhibits smaller grains than that in Figure 4b, but still thinner cobalt binder layers than those in Figure 4a. Thus, further improvement of the mechanical properties can be expected with increase in zirconia proportion from 4 wt.% up to 10 wt.%.

Phase compositions of the specimens without zirconia, shown in Table 1, consisted of the hexagonal tungsten carbide, hexagonal form of carbon (graphite), and the cubic phase of Co₃W₃C compound. In turn, the specimens with zirconia reinforcement consisted of two main phases of structural WC and Co₃W₃C. The carbon phase was represented by amorphous C, and the zirconia additive formed hexagonal ZrO₂.

The diagrams F–h of application of the test force and its removal for the three tested materials are presented in Figure 5. Significant shift in the peaks is seen for both composites with YSZ compared to the WC-Co basic composition. In turn, the curves for zirconia-reinforced composites appeared very close to one another. It corresponds with the differences between the calculated parameters collected in

Table 2. Results of hardness from microindentation measurement.

Parameter		Specimen Composition
		94WC–6Co	90.24WC–5.76Co–4ZrO2	86.60WC–5.4Co–10ZrO2
HIT [MPa]	Mean	17,033.594	16,920.033	16,241.339
	Std Dev	3007.783	440.302	1291.676
	Min	15,720.130	15,440.521	13,952.075
	Max	25,866.066	16,547.338	17,127.111
HVIT [Vickers]	Mean	1692.679	1512.066	1483.515
	Std Dev	283.892	41.558	101.916
	Min	1483.759	1457.368	1316.880
	Max	2441.393	1561.836	1573.190
HM [MPa]	Mean	13,042.417	12,042.519	11,947.111
	Std Dev	2002.895	389.368	815.055
	Min	11,623.484	11,664.136	10,657.615
	Max	18,446.531	12,944.982	12,992.006

,

Table 3. Results of modulus, stiffness, creep, and relaxation from microindentation measurement.

Parameter		Specimen Composition
		94WC–6Co	90.24WC–5.76Co–4ZrO2	86.60WC–5.4Co–10ZrO2
EIT [MPa]	Mean	669,580.875	649,884.813	606,737.938
	Std Dev	97,791.117	102,174.414	35,051.184
	Min	587,612.313	564,093.250	543,561.500
	Max	922,189.375	920,274.688	664,891.563
E* [MPa]	Mean	696,279.875	682,938.038	637,596.688
	Std Dev	102,764.938	102,764.938	36,833.961
	Min	617,499.250	592,784.000	571,208.000
	Max	969,093.500	967,081.375	698,709.125
S [N/μm]	Mean	3.894	3.644	3.685
	Std Dev	0.314	0.145	0.095
	Min	3.618	3.468	3.480
	Max	4.666	3.871	3.710
CIT [%]	Mean	2.072	1.799	1.781
	Std Dev	0.278	0.176	0.201
	Min	1.685	1.589	1.497
	Max	2.614	2.117	2.184
RIT [%]	Mean	–0.058	–0.064	–0.055
	Std Dev	0.036	0.085	0.052
	Min	–0.127	–0.202	–0.130
	Max	–0.017	0.078	0.032

and

Table 4. Results of elastic and plastic works from microindentation measurement.

Parameter		Specimen Composition
		94WC–6Co	90.24WC–5.76Co–4ZrO2	86.60WC–5.4Co–10ZrO2
Welast [μJ]	Mean	0.163	0.167	0.171
	Std Dev	0.009	0.008	0.005
	Min	0.144	0.157	0.151
	Max	0.176	0.186	0.193
Wplast [μJ]	Mean	0.462	0.421	0.410
	Std Dev	0.018	0.017	0.016
	Min	0.433	0.424	0.419
	Max	0.493	0.418	0.411
Wtotal [μJ]	Mean	0.625	0.588	0.581
	Std Dev	0.021	0.126	0.023
	Min	0.596	0.568	0.670
	Max	0.664	0.594	0.585

.

Table 2 presents the values for hardness H_IT, HV_IT, and HM. It should be noted that the maximal hardness, measured by all three methods, exhibited the composition WC–6 wt.%Co with no zirconia additive. In particular, H_IT, HV_IT, and HM were 17,933.594 MPa, 1692.679, and 13,042.417 MPa, respectively. Notably, the Vickers hardness of the recently reported novel ultrafine WC–15Co cemented carbides was lower, 1579 kg/mm², i.e., 15,484 MPa, and it was increased up to 1652 kg/mm², i.e., 16,201 MPa, after increase of carbon content up to 6.16 wt.% [35]. Decrease in the hardness after zirconia was added could be expected, since ZrO₂ itself exhibited lower hardness than WC. Moreover, to some extent, reduction in hardness and modulus could be attributed to the appearance of the nanoscale pores around the agglomerated zirconia particles, as indicated in [36,37].

An important observation is that a significant drop of standard deviation of all three hardness methods took place after YSZ addition. The smallest standard deviation exhibited hardness of the composite with 4 wt.% of zirconia, which proved a more homogeneous structure and better distribution of reinforcement phases in the matrix. Compared to that of WC–Co ceramic, indentation hardness H_IT of WC–Co–4ZrO₂ composite decreased by less than 1%, while its standard deviation decreased by 85%. For the composition WC–Co–10ZrO₂ this difference is not so dramatic. Thus, taking into account the overlapping of the areas covered by standard deviation, decrease in the hardness can be considered negligible. It is seen in the diagram of H_IT(mean) ± s presented in Figure 6.

The reduction in hardness observed after YSZ addition can be generally attributed to the lower intrinsic hardness of zirconia compared to WC, as well as to nanoscale porosity localized near zirconia agglomerates. More importantly, the elastic indentation work and wear-related indices should be used to assess the ability of the composite to elastically accommodate contact stresses. Such an elastic–plastic balance is particularly beneficial for rock drilling tools subjected to cyclic impact and abrasive wear, where crack deflection and interfacial energy dissipation determine the service life.

In the case of the values of modulus shown in Table 3, they were higher than the recently reported ones, namely, 466.0–516.5 GPa for the commercial WC–Co cemented carbide [38]. Both reduced E* and indentation E_IT moduli after addition of 4 wt.% zirconia exhibited decrease below 3% compared to WC–Co, while standard deviation remained almost the same. However, addition of 10 wt.% reduced indentation elastic modulus E_IT by ca. 10% compared to that of WC–6 wt.%Co composition, while the respective standard deviation reduced by ca. 45%. Thus, in terms of zirconia effect on the hardness and modulus, some reduction can be noted, but it was accompanied by significant homogenization throughout the material’s volume and it did not drop below the typical values for the commercial WC–Co.

The results generally correlate with widely known physical properties of tungsten carbide, which consists of hexagonal layers of W atoms separated by C layers one-half of the interstices, so that a six-fold trigonal prismatic structures emerge. As a result, individual phases and the crystallographic orientation have an effect on the hardness of WC and its indentation modulus [39]. For instance, variation in hardness from 8 GPa in the metal matrix to 25 GPa in WC η phase was reported along with modulus values different for the WC prismatic planes (700–900 GPa) and for WC basal plane (450–550 GPa) [40]. The modulus above 600 GPa obtained in this study is closer to that of prismatic plane phase.

The highest values of stiffness S and creep C_IT were also exhibited by the material without zirconia additive. At the same time, the creep values of both composites with zirconia added are almost the same, 1.799% and 1.781%, which corresponds with the diagrams of indentation in Figure 5. Along with the decrease in the indentation work W_total, zirconia additive contributed to the increase in the values of its elastic part W_elast, presented in Table 4.

Unlike in the case of hardness, the indentation work W_total exhibited the largest standard deviation for the composite with 4 wt.% YSZ. Nevertheless, reliable results were obtained for the plastic part of work W_plast with comparable standard deviations between 0.16 and 0.18 μJ. In that case, mean values of W_plast significantly decreased when zirconia was added to WC-Co composite and further decreased when amount of YSZ additive reached 10 wt.%.

Considering high dispersion of the measured values, it was found necessary to make the hardness and modulus mapping to check the structural homogeneity of the samples. Figure 7 shows the examples of the SPM mapping of tested composites after nanoindentation. The presence of micron-sized softer inclusions and nanoscale inclusions of extremely high hardness appeared to be the decisive factor influencing dispersion of microhardness results.

The differences in hardness demonstrated that the maximal values of 22.45 GPa belonged to the WC–Co material, while maximal values of the other two composites with zirconia were almost the same, 20.35 GPa and 20.45 GPa. Similarly, maximal modulus of 476 GPa was found in the map of WC–Co, while it was 466 GPa for 4 wt.% of zirconia and 453 GPa for 10 wt.% ZrO₂. However, the specimen with no zirconia had many areas of lower modulus, 153 GPa, compared to the respective values of 189 GPa and 230 GPa. It can be seen that the WC–Co matrix had higher hardness than that of the zirconia additive.

It is also noteworthy that the tested composite materials exhibited high homogeneity. All maps of hardness revealed several single submicron-sized points of significantly higher hardness corresponding with the particulate reinforcement. This feature indicates that the applied sintering technology allowed for retaining some nanoscale structures in the composites. The presence of hard inclusions in the soft matrix is always beneficial for the composite, and the submicron dimensions of the particles increase the interfacial effect. It can be expected that the fracture energy will be dissipated by debonding on interfacial and particulate levels, which is crucial for the resistance of the composite to wear and catastrophic failure [41].

From Table 4 it is seen that some reduction in hardness and modulus caused by zirconia additive was accompanied by the increase in elastic part of indentation work. This trend could contribute to the improvement of the parameters related to the wear resistance. The ratio of elastic strain to failure H/E and index of resistance to plastic deformation H³/E² are the two key indexes of mechanical properties of a material [42], and index of tolerance to abrasion damage 1/(E²H) is used for determining the resistance of materials to abrasive wear [43]. These parameters calculated for the tested composites after indentation are presented in

Table 5. Experimental parameters related to the wear resistance.

Parameter	Specimen Composition
	94WC–6Co	90.24WC–5.76Co–4ZrO2	86.60WC–5.4Co–10ZrO2
H/E	0.0254	0.0260	0.0268
H3/E2, MPa	11.02	11.47	11.84
1/(E2H), MPa−3	1.31 × 10−16	1.40 × 10−16	1.67 × 10−16

.

The results are comparable to the values recently reported for high entropy alloy coatings AlCoCrFeNiSi/WC, where H/E indexes were between 0.0116 and 0.0343, and specifically for the coating denoted H20, it was 0.0281 [44]. Also electrodeposited Ni–SiC nanocomposite coatings exhibited H/E ratios between 0.013 and 0.026 for various compositions [45].

It is seen in Table 5 that all the three wear parameters increased after addition of zirconia. Specifically, for 10 wt.% ZrO₂ H/E increased by 5%, H³/E² by 7%, while 1/(E²H) by 27% compared to 94WC–6Co composition. It is closely connected with the fact that some decrease in modulus is accompanied by much smaller, almost negligible decrease in hardness. From the perspective of abovementioned indexes, it can be assumed that the composite with 10 wt.% of zirconia is the optimal one, even though its hardness was not the highest one among the tested materials. This assumption seems to be confirmed by the results of the plastic part of work W_plast which appeared to be the smallest for this composite.

Though the H/E ratio may overestimate the wear resistance [46], it was demonstrated that optimized indexes H/E and H³/E² could be more effective than achievement of extremely high hardness [47]. Anyway, from the indexes shown in Table 5, some improvement in the performance after zirconia addition can be expected. Thus, in future research, it is planned to perform tribological tests to check the trend indicated by the obtained data.

4. Conclusions

The presented results demonstrated that it was possible to produce metal matrix composites WC-Co with zirconia additive obtaining different combinations of the plastic and elastic characteristics. Thus, it will be possible to adjust the composites with required properties dependent on the planned applications.

Addition of YSZ in amounts of 4 wt.% and 10 wt.% resulted with slight decrease in the hardness of WC-6Co composites and their modules. Correspondingly, the elastic part of the work W_elast slightly increased. Presence of small, nanoscale inclusions of hardness and modulus significantly higher than that of the matrix, appeared to be favorable from the perspective of the composite performance.

It is important to note that the indexes related to wear and plastic deformation resistances, H/E, H³/E², and 1/(E²H), appeared several percent higher in the case of composites with 4 wt.% zirconia and further improved when YSZ content was increased up to 10 wt.%. From the perspective of application in tools for drilling hard and abrasive rocks, the composite 84.60WC–5.4Co–10ZrO₂ appeared to be the most promising one, though requiring further investigations. It is planned to perform some tribological and wear tests to check the validity of the obtained indexes.

5. Patents

Patent of Republic of Kazakhstan: Ratov, B.T.; Mechnik, V.A.; Hevorkian, E.S.; Bondarenko, N.A.; Rucki, M.; Seidaliev, A. A.; Kuvanov, E.U.; Kalzhanova, A.B.; Morozov, D.; Samociuk, W. Diamond-reinforced composite material. No. 37698, Submission no. 2024/0909.1, 01.11.2024. Published 19.12.2025, Bulletin No. 51.