Production and Mechanical Performance of Tantalum Strengthened Alumina–Zirconia Composites with Graphene Addition

Abstract

High density alumina–zirconia–tantalum ceramic–metal composites with the addition of 0.5 vol.% of graphene oxide were fabricated via a wet processing technique followed by spark plasma sintering. Scanning electron microscopy confirmed the even distribution of metal particles in the composite matrix. The thermal reduction of graphene oxide after consolidation at 1500 °C was proved using Raman spectroscopy. The engineered materials exhibit a fracture resistance of 16 MPa∙m^1/2, which is 30% greater than in the reference ZTA ceramic composites fabricated using the same technology. That increase in fracture toughness could be down to a synergetic interaction mechanism; more specifically, crack trapping, renucleation and blunting, and elongated tantalum particles bridging. In addition to the above-mentioned mechanisms, tetragonal monoclinic phase transformation in zirconia is also an additional source of increased crack resistance in the developed composites.

1. Introduction

Alumina (Al₂O₃) and zirconia (ZrO₂) have long been the subject of intensive research by developers of new materials [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. The capacity to manufacture these inorganic compounds from readily available and cost-effective starting materials has contributed to their widespread adoption, for example, as cutting tools for high-speed operations, dental and orthopedic implants, electrical and thermal insulation, wearable components, coatings, and more. Despite the exceptional chemical, thermal, mechanical, and tribological properties of these ceramics, it is important to remember their fragility. The fracture toughness values for alumina and zirconia typically fall into a range of 3.3–6 MPa·m^1/2 и 5–12 MPa·m^1/2, respectively. In recent years, vast numbers of studies have explored the benefits of combining aluminum and zirconia (ZTA) oxides. The purpose of creating these materials is to leverage the high hardness of alumina while simultaneously enhancing the toughness of the composite through the martensitic transformation in zirconia [15,16]. Furthermore, to avoid the self-induced transformation of ZrO₂, its concentration in the composite must be below the percolation limit, which was determined to be 16% by volume or 22% by weight [17]. However, the presence of defects in ceramic materials, which can occur during different stages of manufacturing and processing, can lead to catastrophic failure even when the stress intensity is below the strength of the material. Ceramic reinforcement is one of the ways to avoid brittle fracture. The toughening phases employed can be depicted as whiskers, filaments, flakes, and so on. These elements exert compressive forces that mitigate the stress concentration at the fracture end, thereby preventing the crack from propagating further. There are diverse methods for enhancing the fracture toughness, such as bending or pulling out, or plastic deformation of reinforcing phases. The extent to which the crack is closed is determined by the ratio of the stresses and deformations in the reinforcements, which are affected by the yield strength of the ligament, its diameter, orientation, volume ratio, and the essence of the relationship between the matrix and reinforced particles. In recent decades several investigations of zirconia–metal, alumina–metal or zirconia–alumina–metal composites have been conducted. Huang et al. investigated Al₂O₃–tantalum composite laminates fabricated by hot pressing [18]. The maximum value of flexural strength (1090 ± 340 MPa) of the studied composite was more than twice the bending strength of Al₂O₃ subjected to high-temperature pressing under identical circumstances. Recently, researchers investigated the high-temperature mechanical performance of alumina coarse-grained composites containing 11 or 21 vol. % of metals with exceptionally high melting points [19]. The compression testing conducted at temperatures ranging from 1300 °C to 1500 °C revealed that the studied composites were resistant to brittle fracture. Thompson et al. found that the average fracture toughness values of Al₂O₃-based nanocomposites strengthened with niobium (Nb) and/or carbon nanotubes were almost twice as high as those of alumina nanocomposites without metal phase [20]. The fracture toughness of hot-pressed zirconia–tantalum (ZrO₂/Ta) ceramic metal composites was evaluated in [21]. Due to crack bridging and plastic deformations of ductile ligaments, the high toughness value (16 ± 0.6 MPa m^1/2) of fabricated ceramic–metal composites was obtained. The fatigue performance of ZrO₂/Ta and ZrO₂/Ta ceramic–metal composites produced by spark plasma sintering was analyzed in [22]. It was reported that, in contrast to niobium, the presence of tantalum improves a material’s ability to withstand repeated stress and resistance and safely sustain defects within a zirconia matrix due to the numerous arrangements working at various dimensionalities. It also reduces topical strain because of the permanent deformation, which in turn causes internal crack resistance and also affects external factors. Bartolomé et al. studied the mechanical properties of ceramic–metal micro–nano-hybrid composites [23]. Adding 20 vol.% of metal ligaments to alumina–nano–zirconia ceramic allowed them to increase toughness from 6 to 12 MPa∙m^1/2 due to the bridging effect of metal inclusions. The ability to withstand defects safely and the crack resistance in comparison to the crack length of these composites were also evaluated [24]. It was stated that a synergistic toughening mechanism leads to enhanced damage tolerance and R-curve behavior through the combined effects of crack bridging by metal particles and stress-induced phase transformation of zirconia, resulting in a fracture toughness greater than the sum of individual contributions. Zirconia–alumina–tantalum micro-nanocomposite shows high values of crack resistance (16 MPa∙m^1/2) and strength (1300 MPa) due to the significant influence on the martensitic transformation of ZrO₂ and to the synchronous action of different strengthening mechanisms [25]. In addition to its outstanding mechanical properties, the studied micro-nanocomposite has low susceptibility to aging. However, it should be noted that the initial mixture of ceramic powders consisted of 37.4 wt.% alumina and 52.4 62.6 wt.% hybrid ZrO₂ nanopowders, which were obtained by the by CO₂ laser co-vaporization method. In the current investigation, Ta was chosen as a strengthening agent for manufacturing the ZTA ceramic composites. Metal phase exhibits a level of plasticity that is not present in ceramic materials. Consequently, in metal–ceramic composites, the energy required for irreversible strain can be harnessed when a crack spreading within the base material encounters a ductile ligament. A portion of the energy required for fracture spreading is absorbed through irreversible stress, while the rest of the energy is insufficient and may prevent it spreading further. Furthermore, deformed metal particulates can bridge cracks, thus redistributing stresses and preventing further spread. In addition, Al₂O₃, ZrO₂, and Ta have been employed in medical applications [26,27]. In vitro and in vivo biocompatibility examinations of ceramic–metal materials were performed. The studied composites had no deleterious effect on cell proliferation, extracellular matrix production, or on cell morphology, and exhibited biocompatibility and osteoconductivity [28]. The properties and behavior of zirconia–tantalum composites were evaluated in vitro and in vivo [29,30]. In vitro studies show that composites create a surface favorable for cell growth. In vivo observation demonstrates that there is no fibrous tissue or inflammatory response at the boundary between implants and newly formed bone indicates successful osseointegration. This means the bone directly bonds to the implant surface without any intervening soft tissue, which is a key characteristic of a stable and successful implant. However, in addition to the bio- and mechanical performance, the tribological performance of materials used in joint orthopedic devices must also be considered. Wear resistance is important because it can affect the following factors. Implant stability: low wear resistance can lead to loosening of the implant, which will require surgery to restore vision. Tissue reaction: wear and debris can cause adverse tissue reactions, such as inflammation and bone loss (osteolysis), and wear can contribute to metallosis, a condition involving the release of metal debris into surrounding tissues. Clinical longevity: ultimately, wear resistance significantly determines the implant’s long-term clinical performance and the need for future interventions.

Graphene (G) has the ability to reduce friction, making it a potential solid lubricant. It can also decrease the force of friction operating on the surfaces in contact at the micro- and nanolevels [31]. Recent articles confirm that the presence of graphene in ceramic-based composites can enhance their wear performance [32,33,34,35,36,37,38,39,40]. Su et al. showed that incorporation of 0.2 wt% graphene nanoplatelets led to a significant reduction in the wear rate of tungsten carbide-ZrO₂-Al₂O₃ ceramics [41]. Llorente et al. reported that cubic yttria-stabilized ZrO₂ materials containing graphene nanoplatelets (GNPs) provide outstanding triboresistance and, in particular, an excellent enhancement of the durability of more than two orders of magnitude versus composites without GNP [42]. Sun et al. [43] fabricated different ceramic matrix/graphene composites and studied their tribology properties. An unprecedentedly weak (0.06) frictional force between the tribosurfaces was recorded because of the synergistic effect between the mechanical reliability and self-lubricating performance. Cygan et al. [44] investigated the synergistic effect of mechanical performance and tribofilm formation on the wear performance of the alumina-based materials toughened with multilayered graphene and graphene oxide. The conducted analysis showed that formation of tribofilm on the rubbing surfaces was sufficient for an effective reduction in wear. Duntu et al. [45] investigated the tribological behavior of nanocomposites with an alumina matrix strengthened with ZrO₂, G and a nanosized, hollow cylindrical form of carbon. Unlike the monolithic Al₂O₃, the produced alumina matrix composites showed improved wear resistance (~93%) due to the generation of the solid-surface thin layer formed on the rubbed surfaces. Based on the above factors, to provide steps for further investigation—namely the evaluation of the tribological performance of studied ZTA-Ta ceramic–metal composites—graphene oxide (GO) was also incorporated into this composite. Therefore, the primary goal of this research is to analyze the mechanical capabilities of sintered ZTA-20 vol.%Ta ceramic metal composites and, at the same time, an assessment of the effect of the presence of 0.5 vol.% of graphene oxide on the mechanical performance of the fabricated materials.

2. Materials and Methods

2.1. Raw Powders

The powders that are readily available in the market were utilized as the starting materials: (1) graphite powder (d₅₀ = 3 mm, Plasmotherm, Moscow, Russia); (2) t-ZrO₂ (d₅₀ = 0.26 µm, 3Y-TZP, 3 mol% Y₂O₃; TZ-3YS-E, Tosoh Corp., Tokyo, Japan); (3) alumina (d₅₀ = 0.30 µm, α-Al₂O₃, A16SG, Alcoa, New York, NY, USA); (4) tantalum (d₅₀ = 58 µm, Alfa Aesar, Karslruhe, Germany).

2.2. Materials Processing and Sintering

GO was synthesized by oxidizing graphite powder based on the modified Hummers’ method [46,47]. Tantalum flakes with d₅₀ = 42 µm were obtained after attrition milling the initial metal powder in a teflon jar for 4 h in isopropyl medium. To produce a ceramic matrix incorporating 20 vol.% Ta lamellar particles, the powders were initially mixed in distilled water medium and then subjected to spark plasma sintering (SPS) at 1500 °C (100 °C/min) for 3 min with uniaxial pressures of 80 MPa in vacuum environment. Detailed information on ceramic and metal slurry processing, as well as sintering parameters, can be found elsewhere [48]. The as-sintered samples with a 20 mm and ±3 mm of diameter and height, respectively, were fabricated. After consolidation, GO was thermally restored in situ. Hereafter, the resulting carbon material will be referred to as reduced graphene oxide (rGO).

2.3. X-Ray Diffraction (XRD) and Raman Characterization

Analysis of the phases of studied materials and the presence of m- and t-zirconia phases on the samples’ polished and fractured surfaces was performed using an Empyrean diffractometer (PANalytical, Almelo, The Netherlands) with a radiation source Cu–Kα (λ = 1.5405981 Å) operated with an intensity of 30 mA at 40 kV. The amount of m-ZrO₂ and its volume fraction were estimated based on the work of Garvie and Nicholson [49] as well as calculations by Toraya et al. [50], respectively. The presence of a graphic structure in the composition before and after sintering was determined via a Raman spectroscopy analyzer (DXRTM2, Thermo Fisher Scientific, Waltham, MA, USA) using a laser with a wavelength of 532 nm and a power of 8.0 mW.

2.4. Microstructural and Mechanical Characterization

The densities of the SPSed specimens were measured by the Archimedes hydrostatic weighing procedure. The microstructure of the metalographically ground and polished samples was analyzed using scanning electron microscopy (SEM; LYRA3, Tescan, Brno, Czech Republic) and a Focused Ion Beam Scanning Electron Microscope (FIB-SEM; AURIGA 60 CrossBeam Workstation, Carl Zeiss Microscopy, Jena, Germany). Energy-dispersive spectroscopy (EDS) was performed using X-Act (Oxford Instruments, Abingdon, UK) detector. The hardness (Hv) was determined by the Vickers method. The measurements were performed using a hardness indenter (Qness, Salzburg, Austria). The hardness testing conditions per measurement (10 imprints) were 294 N loading force with 10 s loading time. To determine the mean value, the following equation was used:

$$\mathrm{Hv}={\displaystyle \frac{0.1891\mathrm{P}}{{\mathrm{d}}^2}}$$

where P is load (N) and d is the average length of two diagonals (mm). The values of the Vickers fracture toughness (KC) were calculated using the approach described in [51]. The flexural strength (σf) was calculated through a biaxial bending test [52]. This scheme test is recognized as a reliable and preferred method for studying fragile specimens, since the highest stress occurs in the middle of the specimens, while defects on the edges of the prismatic samples used in the three-point bending test can act as stress concentration sites and may affect the test results. The tested sample was mounted on three steel balls (∅ 3 mm) arranged in a circle with a diameter of 10 mm. To determine flexural strength, 12 SPSed disks were broken using a 5 kN universal testing machine AutoGraph AG-X (Shimadzu Corp., Kyoto, Japan). The testing scheme and method used to calculate strength have been explained in a previous article [53].

3. Results and Discussion

SEM images and X-ray diffractions graphs of raw metallic powder before (Figure 1A) and after (Figure 1B) attrition milling was analyzed.

For XRD examinations, a small volume of the Ta-acetone suspension was carefully applied to the silicon (510) single-crystal wafer within a specially produced support device and the suspension was dried by evaporating the acetone. The location and area under the peaks were compared with the reference (PDF 004-0788) pattern. During milling, metal particles deformed between the grinding zirconia balls, the impeller shaft and the walls of the teflon jar. Therefore, visible texturing made the (200) reflection parallel to the specimen surface and broadening occurred as a result of deformation and fundamentally altered the crystal lattice distortion. As a result, the shape of most of the initial Ta powder particles changed to a lamellar one. The distinctive X-ray patterns and Raman spectra of ZTA-Ta-GO mixture and consolidated materials are displayed in Figure 2.

According to the data provided by the ICDD, the locations and strengths of the peaks observed in the analyzed samples were consistent with the expected peaks for monoclinic (24-1165) and tetragonal (83-0113) ZrO₂, aluminum oxide (81-1667), and Ta (004-0788). The data obtained only showed the presence of phases of zirconium dioxide, aluminum oxide, and tantalum, which confirms the absence of the formation of any secondary phases or contamination during processing. It is also necessary to note the complete transformation of monoclinic zirconia in the initial powder mixture into tetragonal zirconia after consolidation. The Raman spectra of ZTA-20Ta-GO powder mixture and sintered ZTA-20Ta-GO composites are shown in Figure 2B. The spectrum in Figure 2B demonstrates the wide G peak and the slight second-order area features of sp¹-, sp²-, and sp³-hybridized C-C bonds in graphene [54] and the D band (∼1350 cm⁻¹) exhibit structural defects such as lattice distortion in ZTA-20Ta-graphene oxide powder [55]. The shift for the SPSed samples (Figure 2B) shows that graphene oxide was reduced in situ (rGO) after consolidation. This is supported by the lower-intensity proportion between D- and G-bands (I_D/I_G). The analysis revealed that the proportion of bands in the powder mixture was 1.02, whereas in the compact samples, the I_D/I_G ratio decreased to 0.35, suggesting a reduced defect level in the rGOs in the heat-treated samples. Moreover, a distinct and well-defined bivariate symmetrical shift was observed at approximately 2700 cm⁻¹. The I_2D/I_G ratio in sintered composites was observed to be 0.68, compared to the initial powder mixture, which had a ratio of 0.14. This also demonstrates the recovery of the graphene structure following the sintering process. Consequently, the findings demonstrate the thermal transformation of graphene oxide, including the reduction of extensive regions of sp², during the sintering at 1500 °C.

Particles of various shapes (whiskers, fibers, flakes, etc.) can be used to harden brittle materials. The phenomenon of the bridge effect arises when ductile inclusions either absorb the strain energy during fracture (1, 9), detach from the sintered body and elongate (2, 8), or relieve the stress at the crack tip without breaking (3, 7), as illustrated in Figure 3. Furthermore, the phenomenon of crack deflection takes place when the top of the fracture encounters a particle, causing the crack to alter its trajectory of expansion (4). Conversely, these inclusions contribute to the blunting of fracture top (5) or the dissipation of stress through plastic strain in the areas where cracks are growing (6).

The scanning electron microscopy images of the metallographic and fractured surfaces of the sintered specimens subjected to the flexural test are presented in Figure 4. The dark and light phases pertain to ZTA matrix and tantalum, respectively (Figure 4A). The metal particles are evenly distributed throughout the ceramic matrix.

Examination of the fracture areas revealed that the failure process involved either full or incomplete plastic strain and the debonding of Ta inclusions. The composite mechanical characteristics are defined by a multifaceted equilibrium of linkage at different levels, and they are largely influenced by the adhesion between the ductile particles and ceramic components. If the interface is tremendous, the high level of strain it experiences will result in brittle failure of the reinforcing particles, thereby preventing a considerable gain in crack resistance. Conversely, if the bonding is poor, the ductile inclusions will simply detach from the matrix, also preventing an improvement in fracture toughness. Thus, the boundary between ceramic and ductile particles must be capable of resisting cleavage and acting as a crack arrestor, particularly when the tantalum ligaments are big or have poor ductility. The sintered composite exhibits a variety of fracture patterns in tantalum particles of diverse shapes. On the other hand, these are fragments of metal that have been shattered due to plastic deformation, either fully or partially, as a result of stretching. In such instances, the rupture of metallic lamellae takes the form of a ductile dimpled fracture, characterized by the presence of rounded depressions and protrusions on the fractured surface (Figure 4B). Conversely, there are tantalum inclusions that have detached from the ceramic, as well as those that have been fractured predominantly by cleavage (Figure 4C). Therefore, it can be inferred that an enhancement in KC will occur when the interface surfaces show moderate levels of separation, allowing the ductile ligaments to deform plastically. The tantalum inclusions within the ceramic base demonstrate elastic properties until a fracture occurs. When this happens, the particle’s surface undergoes a change in shape, but the particle itself remains unbroken, and is still attached to the surrounding material. Even after the particle is surrounded by a crack, the metal stretches due to plastic deformation, resulting in deformation. The intense plastic deformation of ductile ligaments prior to fracture ultimately results in a substantial increase in crack resistance.Figure 5 shows a scanning electron microscopy image of the fracture surface of zirconia–alumina–tantalum/rGO composite.

As can be seen in Figure 5, a very thin layer of graphene flake seems to be strongly anchored within the ceramic matrix and a large interface area between the graphene and ceramic matrix is created during the consolidation process. The development of the crack propagation within the microstructure of the specimens after the indentation has been investigated using the FIB-SEM microscope (Figure 6). These findings indicate the occurrence of plastic deformation and the formation of cracks in the Ta ligaments. Crack-tip blunting and renucleation mechanisms are also visible (Figure 6, yellow arrows). The crack can be halted at the metal particle in such a way that it must re-form in other places.

The main role in increasing fracture resistance in the samples under consideration is played by the crack bridging and irreversible strain of the ductile ligaments. The fracture can be halted at the Ta phase, such that it must reintegrate on the other side. The results of density measurements showed that a level of compaction close to the theoretical level (99.8%) was achievable. In other publications [56,57,58,59,60], it has been reported that a small quantity (0.1 to 0.5 percent by volume) of graphene may improve the mechanical characteristics of the specimens. The composites with and without graphene addition exhibited the following average values of strength 826 ± 29 MPa and 841 ± 22 MPa, respectively. No significant increase in Hv was found in the studied samples to which graphene had been added. The values of the specimens with (8.8 ± 0.3 GPa) and without (8.7 ± 0.4 GPa) rGO were virtually identical. This can be attributed to the fact that graphene, being a malleable substance, does not enhance hardness.

The phase characterization of the polished surfaces of SPSed materials demonstrated that the tetragonal zirconia phase was the most prevalent. The results proved that the volume fractions of t- and m-ZrO₂ on the polished surface in sintered composites were 97% and 3%, respectively. However, on the surface of a broken material, the volume fraction of the tetragonal phase decreased to a value of 80%, while the monoclinic phase increased to a value of 20%. This can be attributed to the formation of an oxide film of tantalum oxide (Ta₂O₅) which covered the initial metal powder particles that came into in contact with air. The Ta₂O₅-ZrO₂ phase diagram shows the solubility of pentavalent oxide in zirconium dioxide (approximately 2 mol.% Ta₂O₅) for compositions containing a high amount of zirconium dioxide at temperatures above 1400 °C. We presume that upon consolidation, the layer that formed in the metal particles was incorporated into the solid solution. The enhanced transformability can be related to the alloying effect on the tetragonality, i.e., the cell parameter ratio c/a, of stabilized t-ZrO₂ [61]. The amount of tantalum oxide that enters the solid solution inside the matrix is an important factor. Kim et al. reported that solid solutions of t-ZrO₂ in the ZrO₂-Y₂O₃-Ta₂O₅ system, where Ta₂O₅ is added as a secondary phase and with c/a axial ratio smaller than 1.0203, have fracture toughness values within the range of 5–14 MPa∙m^1/2 [62]. Therefore, it is very important to maintain such a concentration of Ta₂O₅ in order not to exceed this value and create conditions to support the material’s ability to achieve t-m transformation. The tetragonality (c/a) of the samples was determined as the coefficient between the parameters of the crystal lattice. The results obtained are shown in

Table 1. Lattice parameters and tetragonality of studied materials.

Materials	c	a	c/a
Matrix	5.1735	5.0997	1.014
ZTA-Ta	5.1827	5.0865	1.0189
ZTA-Ta-rGO	5.1827	5.0865	1.0189

.

Thus, the presence of tantalum in the ceramic matrix increases the tetragonality values and, accordingly, increases the material’s predisposition to undergo tetragonal–monoclinic transformation, while at the same time this value does not exceed the established limit (1.0203). Utilizing the same technique as in previous studies [22,25,47], the molar percentage was calculated using the elemental weight percentages obtained by energy dispersion spectroscopy and the atomic masses of the elements. The proportion of tantala was determined to be less than 2 mol.%, and no other phases were observed. The occurrence of Ta₂O₅ enhances the tetragonal deformation of the cubic structure, which significantly increases the ability of zirconium dioxide to undergo transformation. Therefore, t→m phase transformation of zirconia in ceramic–metal composites is an additional mechanism for enhancing fracture toughness. The present findings indicate that the mutually reinforcing effect between crack bridging and the irreversible strain of the ductile tantalum inclusions and the tetragonal-to-monoclinic phase transformation of ZrO₂ induces an improvement in the fracture toughness of ZTA-Ta material of up to 16.1 ± 0.3 MPa∙m^1/2 compared to value (8.6 ± 0.3 MPa∙m^1/2) obtained in previously studied ZTA ceramic composites [51]. The application of the Wilcoxon test to the biaxial flexure test results showed no differences between the samples (p-value > 0.05). When comparing fracture toughness results, statistically significant differences between specimens (p-value < 0.05) with and without rGO were revealed.

4. Conclusions

For the first time, high-density ZTA-Ta-GO material using a combination of wet processing and field-assisted sintering technology was successfully fabricated. The examination of the microscopic structure revealed a consistent dispersion of tantalum particles throughout the ceramic framework. The results of the Raman analysis confirmed that the selected consolidation temperature (1500 °C) was sufficient for thermal reduction of graphene oxide. The presence of ductile metal particles of tantalum (20 vol.%) allowed us to increase the crack resistance of sintered composites to 15.5 MPa∙m^1/2. In addition to crack deflection and interfacial debonding of metal ligaments, a significant role in crack growth resistance in sintered composites is played by crack bridging and plastic deformation of elongated tantalum ligaments. It should also be noted that the presence of an oxide layer on the surface of the initial tantalum particles is a strong stabilizer, which, in turn, promotes the martensitic transformation in zirconium dioxide and also helps to increase the crack resistance of the sintered composites.

Combined use of metal phase and graphene oxide (0.5 vol.%) only slightly increases the crack resistance (16.1 MPa∙m^1/2) of sintered composites. The hardness values of specimens with and without GO are almost the same, since graphene oxide is a soft material and therefore does not significantly affect hardness. Based on studies of the mechanical behavior of developed ceramic metal materials with the addition of small quantities of graphene oxide, it can be concluded that in future, these materials could be excellent candidates for use in medicine. However, it is worth noting that, despite the outstanding mechanical properties, this is not enough. Tribological studies must be carried out to assess the impact of graphene on the wearability of the composite being fabricated. These studies will be described in our next article. Only after careful analysis of tribological test data will a decision be made regarding the need for in vitro and in vivo studies.