Eclet. Quim. 49 | e-1529, 2024 https://doi.org/10.26850/1678-4618.eq.v49.2024.e1529 ISSN 1678-4618 page 1/14 1Hyperion Materials & Technologies, Polígono Industrial Roca, Martorelles, Barcelona, Spain. +Corresponding author: Núria Cinca Luis, Phone: +34935717600, Email address: nuria.cinca@hyperionmt.com Review Article Last decade insights on cemented carbides: A review on alternative binders, new consolidation techniques and advanced characterization Daniela Andreina Sandoval Ravotti1 , Hortensia Melero Correas1 , Núria Cinca Luis1+ Abstract CONTENTS 1. Introduction 2. General insights on conventional knowledge of cemented carbides 2.1. Common constituents 2.2. Industrial processing steps 2.3. Properties characterization 3. Recent advances in cemented carbides 3.1. Alternative binders 3.2. New consolidation methods 3.2.1. Additive manufacturing technologies 3.2.2. Alternative sintering technologies 3.3. Advanced testing 4. Concluding remarks Author’s contribution Data availability statement Funding Acknowledgments References Cemented carbide alloys are well known powder metallurgically processed materials used for a wide range of tooling and components that require a good balance of hardness and fracture toughness, together with wear resistance. After 100 years of the first patent, research and development within this area continues to fulfil more demanding applications and adapt to new requirements. The last decade especially has witnessed important advances. In that sense, Co-free compositions are being studied due to the health issues that its use implies and its criticality in the supply chain. Secondly, the steps towards near-net-shape components by means of additive manufacturing technologies to avoid waste of powder and the technological advance of fast sintering processes are promising. Finally, new microstructural and mechanical characterization methods at micro and nanoscale provide helpful insights for a better understanding of these materials under performance. Article History Received November 03, 2023 Accepted April 25, 2024 Published July 04, 2024 Keywords 1. cemented carbides; 2. alternative binders; 3. consolidation routes; 4. micro mechanical; 5. tribological properties. Section Editor Mário Antônio Alves da Cunha Highlights More demanding wear and corrosion resistant applications incentive new developments in tooling industry. Novel binders, mainly those based on high entropy alloys, are promising for cobalt substitution. New powder metallurgy routes overcome limitations of conventional production methodologies. Micro and nano-scale testing characterization increases the understanding of failure mechanisms. https://doi.org/10.26850/1678-4618.eq.v49.2024.e1529 https://ror.org/039fsvq67 mailto:nuria.cinca@hyperionmt.com https://orcid.org/0000-0002-4743-475x https://orcid.org/0009-0004-2145-747x https://orcid.org/0000-0002-1622-8734 Review Article https://doi.org/10.26850/1678-4618.eq.v49.2024.e1529 Eclet. Quim. 49 | e-1529, 2024 ISSN 1678-4618 page 2/14 1. Introduction Cemented carbides (also referred to as hardmetals) are a group of composite materials that show outstanding mechanical properties such as hardness and fracture toughness and excellent wear resistance. Such combination of properties makes them suitable to produce component and tool materials capable of working under stringent requirements for high demanding applications. The microstructure of cemented carbides is heterogeneous. It is constituted by hard, brittle carbides bonded by a soft and tough metallic binder. The first ones are refractory carbides of transition metals (WC, TiC, TaC, Cr3C2 or Mo2C), and the last one is a metal from the iron group, more often cobalt (Co) or nickel (Ni) and their alloys (Exner, 1979). Refractory carbides of transition metals from the groups IV, V and VI show an interstitial structure that combines metallic, covalent, and ionic bonds, providing them with a high melting point. They also exhibit high hardness and strength with high thermal and chemical stability. The metallic binder phase is a ductile and softer phase that contributes to improve the toughness of cemented carbides (Upadhyaya, 1996). The unique combination of hardness and toughness given by its constitutive phases (in comparison with other hard materials), positions cemented carbides as the most versatile materials used mainly in the tooling, mining, and oil and gas industries (Hyperion Materials & Technologies, 2019). Figure 1 presents the different application sectors in which cemented carbides are used, according to their Co content and WC grain size, as well as hardness; it can be observed that by tailoring these parameters, cemented carbides can be applied in a broad range of applications. The development of cemented carbides started during World War I in Germany due to the need to replace diamond drawing dies with a less expensive material in the production of tungsten (W) filaments (Exner, 1979; Ortner et al., 2014). Several attempts were made without success, until 1923, when tungsten carbide (WC) with added Ni was sintered. With time, Ni was replaced by Co, obtaining a good ceramic-metal combination, suitable to produce fine W wires of good quality (Ettmayer et al., 2014; Ortner et al., 2014). The new material was commercialized in 1926 under the name “WIDIA” (from German terminology “wie Diamant” – meaning like diamond). With the 100th anniversary of its invention (Konyashin, 2023), it became evident that there are many opportunities to improve cemented carbides by tailoring their composition and microstructure through substitution and/or alloying of both carbide and binder phases, as well as by using novel processing and characterization methods. In the following review, we present current industrial knowledge around cemented carbides before focusing on the main developments evidenced during the last decade. Figure 1. Range of applications in which cemented carbides are used, defined by WC grain size, Cobalt content and hardness. Source: Adapted from Sandvik (s.d.) and Hyperion Materials & Technologies (2019). 2. General insights on conventional knowledge of cemented carbides 2.1. Common constituents In cemented carbides, the most used ceramic phase is tungsten carbide (WC) (Upadhyaya, 2001), while the metallic binder is often cobalt (Co), followed by nickel (Ni) and iron (Fe) in order of importance (Prakash, 2008). These are still the most successful composite materials produced by powder metallurgy technologies. The binder is selected for its wettability on the carbide and interfacial adhesion characteristics. In this regard, Co shows outstanding wetting of WC, resulting in full densification during conventional liquid phase sintering methods, leading to higher values of toughness when comparing with other metallic binders (García et al., 2019; Lay and Missiaen, 2014). The formation of other undesirable phases, such as graphite or eta phase, occurs when the WC-Co system is exceeding or depleting in carbon content, respectively (García et al., 2019; Lay and Missiaen, 2014). WC particles exhibit a hexagonal close packed (HCP) crystal structure; WC crystals grown from liquid-metal solutions exhibit the shape shown in Fig. 2a, which is also the shape of WC grains in cemented carbides as seen in Fig. 2b and 2c. The shape adopted by WC in cemented carbides owes to the high polarity of the prismatic crystal planes of (1010) type due to the different spacing of the W and C planes of [1010] directions; therefore, there are two sets of equivalents (1010) planes, instead of six ones. Because of its non-centrosymmetric structure of WC, the microhardness is strongly anisotropic, i.e., the hardness of the basal plane (0001), the prismatic plane (1010) and intermediate orientations are different (Exner, 1979). https://doi.org/10.26850/1678-4618.eq.v49.2024.e1529 Review Article https://doi.org/10.26850/1678-4618.eq.v49.2024.e1529 Eclet. Quim. 49 | e-1529, 2024 ISSN 1678-4618 page 3/14 a) b) c) Figure 2. (a) Schematic representation of truncated WC grains shape in WC-Co composites; (b) a typical microstructure of WC-Co cemented carbides with microstructural parameters commonly determined for cemented carbides; (c) microstructure of cemented carbide with gamma phase. The light and dark phases correspond to the carbide and to the metallic binder, respectively. Source: Adapted from Lay and Missiaen (2014), Roa et al. (2015) and Sandoval Ravotti et al. (2019). TiC, ZrC, HfC, VC, NbC and TaC carbides are also used to produce cemented carbides. These carbides are face-centred cubic (FCC), melt congruently, and exhibit higher hardness than WC. When their content is higher than the solubility limit within the metallic binder, these elemental carbides precipitate, combining to form a mixed solid solution along with WC in the final part, which is commonly known as the cubic carbide phase or gamma (γ) phase (Fig. 2c); this phase is more brittle than WC. However, when their nominal content falls below the solubility limit, they behave as WC grain growth inhibitors (Exner, 1979). Cobalt is the most used material as a metallic binder in cemented carbides. Pure Co shows an allotropic transformation from an HCP structure up to around 415 °C to a cubic structure (FCC) at higher temperatures (Davis, 1995). The prevalence of one form or another may affect the mechanical properties of the composite material. In sintered WC-Co alloys, Co shows a cubic lattice because the stabilization of it by dissolved W and C cannot be transformed by annealing (Exner, 1979; Upadhyaya, 2001). Both the mechanical and tribological performance of cemented carbides are related to the chemical nature, amount, and size of carbide and binder phases (Chychko et al., 2022; Davis, 1995; Gurland, 1988). The common parameters used to characterize the microstructure of cemented carbides are the mean grain size of the carbide particles (dcarbide) and the binder content (wt.%). The proportion of the carbide phase is generally between 3-30 wt.% of the total weight of the composite and its grain size averages between 0.4 and 10 μm. Other important microstructural parameters are the contiguity of the carbide phase (Ccarbide) and the binder’s mean free path (λbinder) (Exner, 1979; Roebuck and Almond, 1988; Upadhyaya, 1998), which refer to the interface area fraction of WC carbides that is shared between them and to the mean size of the metallic phase, respectively (Fig. 2c). 2.2. Industrial processing steps Ammonium-para-tungstate (APT) is the usual starting raw material for standard WC powder production. After a hot reduction of the APT in hydrogen, pure tungsten powder is obtained, with controlled grain size. Tungsten and carbon are mixed in the right proportions, and this mixture is heated at high temperatures in hydrogen, forming tungsten carbide powder (Furberg et al., 2019). Different WC powder types and binders (Co, Ni) are the raw materials for the conventional manufacturing of cemented carbide, usually through a weigh-in, milling and then spray drying process. Once the powder is ready, the production of parts follows the traditional powder metallurgy routes, either compaction plus sintering or hot compaction. Finally, the parts are finished in one or several grinding/turning/polishing steps if needed (Raihanuzzaman et al., 2014). Figure 3 shows the workflow of the entire traditional manufacturing process, starting from the APT, passing through the production of the agglomerated powder, and then compacting the part and densifying it (typically with a liquid phase sintering process). Finally, the part can be finished to provide the required surface roughness. Figure 3. Traditional powder metallurgy process steps to produce parts. Source: Adapted from Hyperion Materials & Technologies (2019). 2.3. Properties characterization Cemented carbide products must accomplish some industrial quality checks to satisfy their demands in the application. These are related to the carbon content (through magnetic saturation measurements), carbide grain size (through coercivity values), metallographic examination, density, and hardness. Due to the heterogeneity of cemented carbides and the cost and complexity of field testing, the development of laboratory- scale testing becomes vital for characterization of microstructure, measurement of properties and response to service-like conditions. A wide variety of test methods according to specific standards for cemented carbides are available today (Table 1). Among them, hardness and fracture toughness prevail. Vickers hardness HV30 (load corresponding to 30 kg of weight), is the https://doi.org/10.26850/1678-4618.eq.v49.2024.e1529 Review Article https://doi.org/10.26850/1678-4618.eq.v49.2024.e1529 Eclet. Quim. 49 | e-1529, 2024 ISSN 1678-4618 page 4/14 most used measurement. Typical values for WC–Co cemented carbides range from about 700 up to 2200 HV30. Higher values are obtained for binderless and nano–grained grades of WC–Co (Shatov et al., 2014a). Finer carbide grain sizes and lower binder contents result in increased hardness, but decreased fracture toughness. The higher the hardness, the more resistance to plastic deformation, penetration and thus abrasion wear, whereas high fracture toughness implies better absorption of impact energy by deformation, therefore, delaying crack propagation. Hardness of cemented carbides was modelled as the sum of the in-situ hardness of the hard phase times the volume fraction of the contiguous carbide skeleton (VWCC) and the hardness of the remaining binder times the rest of the volume (Eq. 1) (Shatov et al., 2014a): HCC = HWCVWCC + HCo(1 − VWCC) (1) where CC denotes cemented carbides. On the other hand, the measurement of fracture toughness in cemented carbide can be performed by several methods, with the Palmqvist test being the most accepted one. This method gives a good approximation of toughness for brittle-like cemented carbides, with a good correlation with the Chevron-notched three- point bending method from 10 to 14 MPa m1/2. However, Palmqvist method becomes invalid when Chevron-notched three- point bending gives higher than 14 MPa m1/2 (Sheikh et al., 2015). A compilation of hardness-toughness relationship data has been recently carried out by Chychko et al. (2022). These authors have examined being plotted according to the microstructure, chemical composition, and processing. WC-Co data is very much dependent on particle size of WC and binder phase volume fraction, where the different grain growth inhibitors affect at a different extent the limitation of WC coarsening during conventional liquid phase sintering. The addition of cubic carbides, being present as complex solid solutions, increases the hardness but reduces the fracture toughness; high percentages of those result in cermet-like compositions. The substitution of cobalt by nickel or iron results in tendencies to high toughness/low hardness ratios and high hardness/low toughness ratios respectively. Finally, promising hardness-toughness ratios have been found by new sintering processes, but their industrialization is not yet ready. Table 1. Standards for testing properties in cemented carbides. Hardness ASTM B294-22 Standard Test Method for Hardness Testing of Cemented Carbides ISO 3878 Hardmetals – Vickers hardness test ISO 3738 Hardmetals – Rockwell hardness test ISO 22394 Hardmetals – Knoop hardness test Strength ASTM B406 Standard Test Method for Transverse Rupture Strength of Cemented Carbides ISO 3327 Hardmetals – Determination of transverse rupture strength Fracture toughness ISO 28079 Hardmetals – Palmqvist toughness test ASTM B 771 Standard Test Method for Short Rod Fracture Toughness of Cemented Carbides Stiffness ISO 3312 Sintered metal materials and hardmetals – Determination of Young modulus Wear resistance ASTM B611 Standard Test Method for Abrasive Wear Resistance of Cemented Carbides Note: Metallographic standards are not included here. 3. Recent advances in cemented carbides 3.1. Alternative binders Since their discovery in 1922 by Schröter, materials with a predominant phase of WC are known preferably as cemented carbide. However, there is a variety of compositions, including binder-free and those containing TiN, that fall into the ‘hardmetal’ classification. Another way to refer to cemented carbides is as ‘cermet,’ understood as a particulate composite consisting of ceramic particles bonded with metallic matrix (German, 2005). Co is the preferred metallic binder in cemented carbides, but provided that Co is considered carcinogenic, and both Co and W were classified by the European Union as Critical Raw Materials (Grilli et al., 2017; Grohol and Veeh, 2023) (materials that are economically and strategically important for the European economy and have high risk associated with their supply), efforts have been made in the past decade to substitute it either partially or fully as a metallic binder in cemented carbides. Although alternative binder systems have been explored since the 1980s, it is only recently that efforts have led to new materials, even more with the advances of computational systems that help tailor the properties for specific applications (Long et al., 2017; Nicolás et al., 2020). The most explored elements for substitution of Co are nickel (Ni) and iron (Fe) and their alloys, because they are the closest transition metals in the periodic table, and it is expected for them to have a similar affinity with carbon and tungsten as cobalt. For applications demanding acidic corrosion resistance, Ni binder is preferred (Prakash, 2014). However, the substitution of Co is not straightforward, and an understanding of the mechanisms for microstructure formation should be well understood. In this sense, Roulon et al. (2020) found that for C- rich alloys, WC-Ni cemented carbides show significant grain growth with a major increase of mean intercept length and a widening of distribution, with increasing the sintering time. In contrast, WC-Fe does not show any significant grain growth with a negligible change in the distribution of intercept length. Consequently, after 8 hours of isothermal sintering, the coarser microstructure was obtained for WC-Ni material followed by WC-Co and finally WC-Fe. In addition, in WC-Ni C-rich alloys, WC grains were more rounded, due to a more uniform precipitation of the solid on the rounded edges of the WC. The use of alternative binders was first compiled in the review by García et al. (2019). Apart from exposing Ni-based and Fe-based binders as they are the closest elements to Co in the periodic table for its potential substitution, they exposed the use of: (i) precipitated reinforce binders for increasing hardness, wear resistance, bending strength, (ii) aluminide intermetallics for their high oxidation and corrosion resistance, (iii) High Entropy Alloys (HEA) for additionally improving hot hardness and fracture toughness, and (iv) ruthenium for the increase of hardness of the cemented carbide without losing the corresponding amount of toughness. https://doi.org/10.26850/1678-4618.eq.v49.2024.e1529 Review Article https://doi.org/10.26850/1678-4618.eq.v49.2024.e1529 Eclet. Quim. 49 | e-1529, 2024 ISSN 1678-4618 page 5/14 To better assess their feasibility, thermodynamic interfacial energy models between WC and liquid binder phase were proposed by Warren (1980), because this parameter is responsible for the resulting microstructure (Lay and Missiaen, 2014). The solid-liquid interfacial energy is composed by two contributing factors: the chemical contribution and the structure contribution. The last one is constant (0.31 J/m2), as it considers only the melting point of WC grains. On the other hand, the chemical contribution is given by the binder metal, which changes as the composition of metallic alloys changes. Long et al. (2017) evaluated the chemical contribution of WC-50 (Co-Ni-Al) interfacial energy at various Ni3Al contents. The simulation showed that the chemical contribution increases from a minimum value of 0.215 J/m2 at the composition of WC- 50Co, up to 1.003 J/m2 at the composition of WC-25Co-25Ni3Al. Based on this information, phase diagrams can be calculated to determine the two-phase region in which a cemented carbide composed by WC and a metallic binder are the stable phases, the carbon (C) content, and the sintering temperature. Fe- and Ni-based binders have been explored using thermodynamic simulations made with Thermo-Calc Software. Fe-Cr-Al and Fe-Ni-Cr binders showed good densification and homogeneity in agreement with simulations (Nicolás-Morillas et al., 2020; 2023). Comparable or even improved combination of hardness, toughness and corrosion resistance to acidic media compared to Co binders was obtained (Nicolás-Morillas et al., 2024). Thermodynamic calculations can also help to explain variations in the grain size of the binder, which is known to be very large in comparison to the carbide phase (Mingard et al., 2011). In the case of WC-50(Co-Ni-Al), alloys containing 0, 11.4 and 15.2 wt.% Ni3Al, the grain size of the binder phase decreases as the amount of Ni3Al increases due to the elevated driving forces for nucleation that contribute to the nuclei formation during solidification, resulting in a larger number of grains with smaller sizes (Long et al., 2017). In addition, the interfacial energy between WC particles and liquid Co-Ni-Al binder becomes higher with addition of Ni3Al, becoming an important obstacle for grain growth of the binder phase. Another reported feature was the precipitation of the cuboidal γ’ phase within the Co-Ni-Al binder, also observed in Co- Al-W superalloys (Suzuki et al., 2015), which can contribute to the superior mechanical properties of the cemented carbide at elevated temperatures. Correspondingly, Konyashin et al. (2014; 2015) have shown that reinforced metallic binder with nanoprecipitates dramatically improves wear resistance by avoiding the detachment of the WC grains during wearing. Besides mechanical properties, corrosion resistance and behaviour can be tailored by modifying the metallic binder. A methodical study of the corrosion behaviour of three groups of cemented carbides (WC-20 wt.% Ni, WC-20 wt.% Ni15Cr and WC-20 wt.% Ni11Cr6Mo), showed that solubility of W in Ni alloys is higher than in Co, greatly affected by the gross carbon content in the material (solubility decreases by increasing the carbon content) (Steinlechner et al., 2022). For cemented carbides with low carbon i.e., high degree of alloying, WC-Ni(Cr,W) and WC-Ni(Cr, Mo, W) showed superior corrosion resistance in pH ranging from 1.5 to 7, as compared to WC-Ni(W), WC-Co(Cr, W) and WC-CoNiCr(W). More recently, high entropy alloys (HEA) are the focus of study as substitutes for Co binder alloys. HEA are complex metallic alloys with at least five components in which any of them is principal in the composition. The advantage of using HEA is that they increase the hardness (also at elevated temperatures), fracture toughness, wear, oxidation, and corrosion resistance of cemented carbides (Straumal and Konyashin, 2023). HEAs allow the decrease of the sintering temperature and suppress the WC grain growth during sintering (Zhou et al., 2016). Refinement of WC grains is especially expected in Cr-containing compositions due to its known inhibition effect (Straumal and Konyashin, 2023). However, a two-phase material is difficult to obtain because of segregation and reaction during sintering, allowing the formation of numerous carbide phases (Mueller- Grunz et al., 2019). Despite difficulties in production of cemented carbides with HEA as binder, the improvement of properties has been demonstrated. Zhou et al. (2016) found an improved corrosion resistance of cemented carbides with 10 and 20 wt.% of HEA in a 0.1 M sulphuric acid solution, compared to a 10 wt.% Co material. Additionally, they found that the corrosion resistance depends only on the chemical nature of the binder and not on the binder content. Luo et al. (2021) found a decrease in the oxidation rate by increasing the aluminium content in the HEA binder. Hardness and toughness have been studied more extensively. For several compositions, Zhou et al. (2018) found the optimal sintering temperature of 1,400°C, to obtain the highest hardness for 10 and 20 wt.% HEA cemented carbides. Moreover, when the HEA content increased to 20 wt.%, the fracture toughness increased. Dong et al. (2020) found that both Vickers hardness and fracture toughness increased with increasing HEA content in WC-Co materials. On the other hand, hot hardness was improved in WC-HEA, as studied by Hering et al. (2023). In the work of R. Chen et al. (2022) they obtained increased density of WC-CoxFeNiCrCu (x= 1, 1.5, 2, 2.5) by increasing the sintering temperature from 1,300°C up to 1450°C, with the corresponding improvement of hardness. Increasing the temperature from 1,400°C to 1,450°C promoted WC grain growth, which translated into an increase in toughness. Comparison of hardness and fracture toughness ranges obtained for different cemented carbides with HEA as binders, with WC-10 wt.% Co, is shown in Fig. 4. Figure 4. Vickers hardness vs. Fracture Toughness maps for cemented carbides with 10 wt.% Co, compared to cemented carbides with HEA as binders. Source: Elaborated by the authors using the data from (García et al., 2019; R. Cheng et al., 2022; Dong et al., 2020; Hering et al., 2023; Zhou et al., 2016; 2018). 3.2. New consolidation methods Traditional powder metallurgy considers two main groups of forming technologies: cold compaction and sintering (or two- step consolidation routes) and hot compaction (or single-step consolidation routes). Figure 5 presents the main ones for each subdivision. https://doi.org/10.26850/1678-4618.eq.v49.2024.e1529 Review Article https://doi.org/10.26850/1678-4618.eq.v49.2024.e1529 Eclet. Quim. 49 | e-1529, 2024 ISSN 1678-4618 page 6/14 Figure 5. Main consolidation routes used for cemented carbides. As many of these technologies have been well known for decades, the research conducted in past years has been mainly focused on two topics: - Additive manufacturing (AM) technologies, as opposed to conventional subtractive technologies; - Alternative sintering technologies, as opposed to conventional, time-consuming, sintering technologies, or alternative applications for well-known sintering technologies, mainly Spark Plasma Sintering (SPS). This section will cover these two subjects. However, additionally to those, some research groups have made attempts to bring novelties on traditional technologies: micro-powder injection moulding for micro components (Heng et al., 2014; Fayyaz et al., 2014; 2018), casting processes using complex moulds (Kim et al., 2022), or different film deposition technologies (Hu et al., 2020; Xu and Huang, 2022a; 2022b; Zhao et al., 2020) are examples of it. 3.2.1. Additive manufacturing technologies The European Powder Metallurgy Association (EPMA) defines additive manufacturing as “The process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies” (Booklet, 2019). Conventional methodologies usually go through forming a blank that will be either green-machined, turned or ground to obtain the desired dimensions. However, these conventional approaches generate important waste and successive manufacturing operations, increasing the production cost of complex geometries. Due to these limitations of conventional technologies, additive manufacturing provides several advantages: the agility, or capability of acting quickly without manufacturing extra tooling, the flexibility to print single parts or short series without time loss (changeover time), the creation of new complex and optimized geometries, and an important contribution to sustainability avoiding unnecessary waste (Javaid et al., 2021). There are many different additive manufacturing technologies and several ways to group them (Carreño-Morelli et al., 2020; Yang et al., 2020). A division that works well for cemented carbides is between one-step technologies (also called Melting Processes) and two-step technologies (or Shaping- Debinding-Sintering Processes) (Fig. 6) (C. Chen et al., 2023). Figure 6. Metal Additive Manufacturing Technologies. Selective Melting Processes include technologies such as Selective Laser Melting (SLM) or Electron Beam Melting (EBM) that build parts by directly melting the powder (Padmakumar, 2020). In these processes, the melting of the metallic binder is achieved in a very short time, and successive heating-cooling cycles are performed. This usually leads to many defects appearing within the structure: porosity, thermal cracks, grain growth and undesired phases, among others. Some research groups have tried alternative approaches such as higher amounts of Co, optimization of the process parameters or modification of the starting powder, but generally, these studies have not led to mechanical properties comparable to traditional powder metallurgy routes (Padmakumar, 2020; Yang et al., 2020). Shaping-Debinding-Sintering (SDS) processes are usually more suitable to process hardmetals, as the shaping/forming process is done at low temperatures and does not induce modifications within the material. The following sintering process also contributes to homogenize, remove potential defects, and fully densify the structures (Miyanaji et al., 2018). Among the different SDS processes, two have proven to give positive results for cemented carbides (WC-Co): Binder Jetting and Extrusion-Based Additive Manufacturing (see Fig. 7). https://doi.org/10.26850/1678-4618.eq.v49.2024.e1529 Review Article https://doi.org/10.26850/1678-4618.eq.v49.2024.e1529 Eclet. Quim. 49 | e-1529, 2024 ISSN 1678-4618 page 7/14 Figure 7. (a) Schematics for binder jetting technology (FFF process, Schematic of the binder-jet 3D printing machine with the printing, 2023); (b) extrusion-based AM (pellets extrusion process, 2023). Source: Adapted from Gupta and Taulik (2021) and Mostafaei et al. (2018). Binder Jetting is based on selectively bonding powder particles and layers together with a binder. After that, the printed parts are cured to improve the green strength and are further subjected to thermal debinding and sintering (Elliott, 2020). Studies with Co contents equal to or higher than 12wt% showed microstructures fully densified after conventional sintering processes, with medium hardnesses and acceptable wear resistances (Cramer et al., 2020; Enneti and Prough, 2019; Wolfe et al., 2023a; 2023b). The feedstock powder characteristics play a crucial role in the process. The possibility of using available commercial powders other than the typical ones used for additive manufacturing has been explored by Berger et al. (2023), finding that they can be appropriate for use in binder jetting as well. On the other hand, extrusion-based AM technologies use as feedstock either pellets or filament manufactured with high polymer content (Altiparmak et al., 2022). The principle of the technology is like commercial home printers: extruding the material (pellets or filament) through a screw and a nozzle to build the part layer-upon-layer. After the printing, the parts need to go through a debinding step (usually solvent for hardmetals) to remove one of the polymers used, then through a thermal debinding plus sintering step to eliminate the backbone binder and fully densify the parts. These technologies have proven to obtain very good microstructures for 10% Co, although they exhibit some macroporosities due to printing limitations (Lengauer et al., 2018; Yang et al., 2020). An alternative to this extrusion-based AM has been developed by the Fraunhofer IKTS: Thermoplastic 3D printing (T3DP) builds the part using a precise deposition of small droplets of molten thermoplastic hard-metal-containing suspensions. After debinding and sintering, full densities were achieved, no defects were found and homogeneous microstructures were obtained (Scheithauer et al., 2017; Pötschke et al., 2017). 3.2.2. Alternative sintering technologies Conventional sintering cycles are very well suited to cemented carbides and can be designed to optimize the densification of different grades. However, they also present several drawbacks: - Time, as they tend to be long cycles, and that makes the process inefficient for short series; - High energy consumption, leading to an important carbon footprint of these processes; - Difficulty in sintering some alternative binders using long conventional cycles, as they might react with each other. All those reasons lead to alternative sintering technologies being evaluated (Guillon et al., 2023). The commercial alternative technology mostly used in the past years for hardmetals is Field-assisted Sintering Technique/Spark Plasma Sintering (FAST/SPS). This technology, which has been in the market for decades, is a low-voltage, current-activated, pressure-assisted sintering process (Fig. 8). This combination of heating and pressure gives high heating rates and short cycle times (5–20 min), making this process very suitable for materials with low sintering activity or high reactivity (Eriksson et al., 2013; Guillon et al., 2014). Figure 8. Schematics of the SPS process. Source: Adapted from Bram et al. (2022). Due to these characteristics, many studies have been conducted using FAST/SPS to sinter binderless powders (Tang et al., 2017), nanopowders (Chuvil’deev et al., 2017) or composites of tungsten carbide with other ceramics (W. Chen et al., 2014; Sribalaji et al., 2017; Wang et al., 2018), leading to interesting results. In recent years, some research has been dedicated to exploring additional sintering mechanisms that are not present in conventional sintering. Although the physics behind those sintering mechanisms has been shown, it is too soon to implement these processes in the industry (Guillon et al., 2014; 2023). An example of these technologies is flash sintering (applying an electric field to a green body while it is heated in a conventional furnace) (Wachowicz et al., 2023). 3.3. Advanced testing Cemented carbides face such a wide array of stress- temperature-environment combinations that only a few standard parameters, as exposed in previous section 2.3, cannot describe the performance in all different applications. In terms of macroscale testing of mechanical properties, mechanical strength and abrasion wear resistance are standardized. Strength, typically https://doi.org/10.26850/1678-4618.eq.v49.2024.e1529 Review Article https://doi.org/10.26850/1678-4618.eq.v49.2024.e1529 Eclet. Quim. 49 | e-1529, 2024 ISSN 1678-4618 page 8/14 measured by bending as transverse rupture (TRS), is limited by size, shape, and distribution of flaws (pore, crack, or inclusion), and by the stress distribution in the structure, i.e., the larger the body or test piece, the weaker it is since a large part has a greater chance of having a large flaw (Zak Fang et al., 2014). The abrasion resistance measured through ASTM B611 is known to be correlated with hardness following an expression of the form V = A exp(−BH), where V is the wear volume, H is the sample hardness and A and B are constants (Gee et al., 2007). Although the abrasion resistance is the only wear mode standardized for cemented carbides, it is not the only one that can be found. Other wear modes experienced by these materials are adhesive wear, fatigue, erosion, and cavitation coming from sliding, rolling, impact, fretting or slurry contacts (Table 2) (Beste et al., 2001; Bonny et al., 2010; Gee et al., 2007; Lavigne et al., 2022; Perez Delgado et al., 2011). Typical features that result from these wear mechanisms can be binder extrusion and removal, accumulation of plastic strain in WC grains, cracking of individual WC grains and between them, and even WC grain pullout. A compilated literature about the influence of microstructure and composition around these topics is very scarce given the many different testing parameters that can be considered in each wear test method. Figure 9 (left side) illustrates some of the morphologic features that can be observed during those tests. The situation becomes more complex when these mechanical requirements need to accommodate different ranges of temperatures and surrounding media. Therefore, hot hardness, oxidation, creep, and corrosion, as well as wear-corrosion synergy effects have been tested (Boukantar et al., 2021; Fathipour et al. 2024; Gant et al., 2004; Guo et al. 2018; Lebedev et al. 2023; Maier et al. 2021; Tang et al., 2023). All these properties are generally evaluated according to the microstructural parameters and binder nature. Table 2. General standards used to test wear in cemented carbides. Standard ref Test ASTM G65 Standard Test Method for Measuring Abrasion Using the Dry Sand/Rubber Wheel Apparatus ASTM G133 Standard Test Method for Linearly Reciprocating Ball-on-Flat Sliding Wear ASTM G99 Standard Test Method for Wear Testing with a Pin-on-Disk Apparatus ASTM G32 Standard Test Method for Cavitation Erosion Using Vibratory Apparatus ASTM-G76 Standard Test Method for Conducting Erosion Tests by Solid Particle Impingement Using Gas Jets Figure 9. Some morphological features from cemented carbides tested in different conditions; left (standard) and right (advanced) characterization techniques. Source: Elaborated by the authors using data from Cinca et al. (2019), Ortiz-Membrado et al. (2021), Sandoval Ravotti (2019), Hyperion Materials & Technologies (2019). Due to the cost and time-consuming efforts to provide complete characterization, and to facilitate new materials development, machine learning strategies have also been introduced in this sector (Guan et al., 2022). However, their use is not yet of widespread knowledge. Interestingly, many of the material developments have been linked and helped by emerging advanced testing and characterization methods that appear due to the necessity in materials science to further explore connections between microstructure and bonding with final properties. Hence, in recent years, the use of conventional techniques has been complemented with the use of more sophisticated ones working at the micro and nanoscale. The understanding of the material performance at small scales is important because the mechanical failure of any bulk material is triggered by the formation, extension, or local accumulation of initial small defects (Naughton-Duszová et al., 2019). Specifically for cemented carbides, classical works focused on the intergranular and intragranular fracture of WC grains, the formation and rupture of Co ligaments and the decohesion of the Co-WC interface, even on crack propagation and plastic deformation of the binder. https://doi.org/10.26850/1678-4618.eq.v49.2024.e1529 Review Article https://doi.org/10.26850/1678-4618.eq.v49.2024.e1529 Eclet. Quim. 49 | e-1529, 2024 ISSN 1678-4618 page 9/14 In recent years, 3D visualization techniques, such as serial sectioning, Focused Ion Beam /Field Emission Scanning Electron Microscopy (FIB/FESEM) Tomography, X-ray tomography, Transmission Electron Microscopy (TEM) based tomography and atom probe tomography are becoming helpful in providing images of the internal structures, specifically on the spatial distribution of phases, real feature shapes and sizes, and feature connectivity. The FIB/FESEM tomography has been used by some scientists to reveal the complexity of the WC carbide skeleton and binder ligaments in cemented carbides to study crack propagation (Jiménez-Piqué et al., 2017; Tarragó et al., 2015). Nanoindentation is the most appropriate technique to determine mechanical properties such as hardness and elastic modulus for the constitutive phases. However, intrinsic response of constitutive phases is usually not considered in the overall behavior of the composite material. For example, it has been established that hardness of WC is anisotropic (Naughton- Duszová et al., 2013; Roebuck et al., 2012), with hardness values for WC basal planes significantly higher than for the prismatic orientation. Focused ion-beam (FIB) has also been used to investigate the morphological features below local phase nanoindentations (Walbrühl et al., 2018). Electron Backscattered Diffraction (EBSD), Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) have been used to evaluate the dependence of hardness and deformation mechanisms on orientation for WC grains in coarse WC-Co materials (Csanádi et al., 2015). Furthermore, binder is a Co-W-C alloy in which the amount of the alloying elements plays an important role on the intrinsic hardness and deformation mechanisms. On the other hand, disposition of the surrounding carbide particles, i.e., constraint degree, affects its flow stress in the composite (Roa et al., 2016). All these measurements have been lately complemented by the statistical analysis provided in massive nanoindentation studies on the small scale, reporting hardness values for Co-based binder of around 8 GPa constrained by the WC grains. Around 30 GPa and 22 GPa were measured for the basal and prismatic WC planes, respectively (Roa et al., 2015). Walbrühl et al. (2018) studied thoroughly the hardness of an alternative Ni-based binder in cemented carbides (85Ni15Fe alloy). In their study, they assessed the effect of the WC particles surrounding the indentation made in the metallic binder. According to their results, the measured hardness of the metallic binder first should be corrected to neglect the pile-up effect found in Ni alloys. Moreover, the measured hardness increases with the proximity of WC particles to the indentation, obtaining the maximum value when the indentation touches a carbide particle. After correction, intrinsic hardness of the metallic binder approximates its value, to that of a bulk alloy with similar composition. Such finding is critical as input for FEM simulations. Regarding small scale, many studies have found estimated data in agreement with experimental data by assuming an effect of the length scale, such as a Hall-Petch relationship in intrinsic hardness of constitutive phases, regardless the constrain factors (Shatov et al., 2014b; Xu and Ågren, 2004). Besides nanoindentation, uniaxial compression of micropillars gives further insights on the local response of cemented carbides. Samples for uniaxial compression of micropillars are micron-sized pillars prepared by FIB milling. Micropillars are afterwards deformed with a nanoindenter equipped with a flat punch. The failure initiation at WC/Co and WC/WC has been investigated according to the specimen size through in-situ uniaxial compression and FESEM microscopy. Different mechanical responses depend on the ratio of the WC grain size and micropillar diameter (Csanádi et al., 2014; Sandoval Ravotti et al., 2018; 2019). Material deformation and fracture have also been examined by micro-beam bending through free-standing micro- cantilevers, where also the specimen size is of importance (Csanádi et al., 2020; Elizalde et al., 2018; Klünsner et al., 2011; Ortiz-Membrado et al., 2021; Trueba et al., 2014). Load vs. displacement curves have been reported with the analysis of fractographic behaviours. The technique has contributed to evaluating the role of individual microstructural features in cemented carbides (single WC crystals and single Co ligaments as well as WC/WC and WC/Co boundaries) (Csanádi et al., 2020; Trueba et al., 2014). An average mechanical strength of 6.32 GPa for ultrafine specimens with 8.2 wt.% Co content and 5.46 Gpa for those with 12 wt.% Co content were found (Klünsner et al., 2011). Fracture strength values as high as 20-25 Gpa for WC/WC boundaries in a 6.5 wt.% Co coarse cemented carbide were found (Elizalde et al., 2018). Also, in a coarse specimen but with 16.7 vol.% Co, most of the WC/WC boundaries exhibited brittle failure with an average fracture strength of 4.1 ± 2.5 GPa (Csanádi et al., 2020). Fracture toughness of single WC grains has been also assessed as 5.6 ± 0.8 MPa⋅m1/2 by means of the micro-beam bending technique (Ortiz-Membrado et al., 2021). Additionally, the use of spherical indenters for monotonic and cyclic Hertzian indentation loading has attracted attention with the aim to evaluate plastic deformation features beneath the indenter as well as residual strength. In the last years, monotonic loading has been used to evaluate the damage tolerance of cemented carbides, i.e., deformation prevailing over fracture as damage mode. This allows the optimization of the microstructural design to improve reliability by mapping the transition from brittle to quasi-plastic damage (Góez et al., 2012; Mahani et al., 2024). On the other hand, the use of cyclic loading is useful to evaluate fatigue sensitivity. The use of macro, micro and nano-scale impact tests have been useful to test WC-Co specimens with thin coatings where the ratio of coating thickness t to the indenter radius R (t/R), has been explored. Deformation and failure mechanisms depend on applied load and indenter sharpness (Beake, 2022). In nano-impact test for coated specimens, due to force limitations, very sharp probes are used, which can break eventually if testing tough materials. Micro- impact tests (repetitive loads at 1-3N) have been used to avoid probe damage. This test is severe/accelerated even at low forces, and it is an “in situ” test where the impact damage is followed cycle by cycle. Deformation and cracking of uncoated cemented carbides has been found to be dependent on the H/E and H3/E2 ratios (H being the hardness and E the elastic modulus), known as the elastic strain to break and the resistance to plastic deformation, respectively (Cinca et al., 2019). Wear at the micro and nanoscale also has been examined in the last decade by means of scratch testing. Diamond indenters with different radii have been used while applying different loads and sliding speeds. The friction coefficient is systematically recorded, and the damage mechanisms examined. Intergranular and intragranular WC cracking as well as at WC-Co interfaces have been found (Gee et al., 2017; Gee and Nimishakavi, 2011). As it can be defined as an abrasion wear mode, it has been also found that smaller WC grain sizes lead to better wear resistance. The same methodology as in a scratch test but with different stylus material has been used to evaluate adhesive wear, concluding that submicrometric surface irregularities have more influence than carbide composition on metal transfer (Olsson and Cinca, 2024). https://doi.org/10.26850/1678-4618.eq.v49.2024.e1529 Review Article https://doi.org/10.26850/1678-4618.eq.v49.2024.e1529 Eclet. Quim. 49 | e-1529, 2024 ISSN 1678-4618 page 10/14 Finally, nanoindentation, scratch and contact damage tests in aggressive media show evidence of corrosion effects (by removal of the metallic binder) in the mechanical integrity of cemented carbides (Gant et al., 2013; Zheng et al., 2019; 2022a; 2022b). Figure 9 (right side) illustrates features of tested cemented carbides by means of those micromechanical characterization methods above described. 4. Concluding remarks Within these 100 years of existence for cemented carbides, progress on new formulations and processing methods has been made. However, the last decade has witnessed an increased challenge given the necessity of substituting Co, not only due to its carcinogenic effects, but to its consideration as a critical raw material by the European Union. With the necessity to assist more demanding applications, academic and industrial experts have extensively worked to overcome these potential challenges. This has resulted in increased publications where High Entropy Alloys (HEAs) have been explored due to their high hardness at elevated temperatures in addition to high wear and corrosion resistance. Moreover, the boom in additive manufacturing technologies and the application of Spark Plasma Sintering or Field-assisted Sintering Technique (FAST/SPS) to sinter binderless powders, nanopowders or composites of tungsten carbide with other ceramics has opened new possibilities. Finally, new, and advanced characterization techniques such as nanoindentation, compression of micropillars and micro- beam bending through free-standing micro-cantilevers, are helping to provide insights into the mechanical properties of cemented carbides and their constitutive phases. Authors’ contributions Conceptualization: Sandoval Ravotti, D. S.; Melero, H.; Cinca, N.; Data collection: Not applicable; Formal Analysis: Not applicable; Funding acquisition: Not applicable; Investigation: Not applicable; Methodology: Not applicable; Project administration: Not applicable; Resources: Not applicable; Software: Not applicable; Supervision: Cinca, N.; Validation: Not applicable; Visualization: Not applicable; Writing – original draft: Sandoval Ravotti, D. S.; Melero, H.; Cinca, N.; Writing – review & editing: Sandoval Ravotti, D. S.; Melero, H.; Cinca, N. Data availability statement Data sharing is not applicable. Funding Not applicable. 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