Main Toughening Mechanisms Research Articles

Improved mechanical and tribological properties of (TiZrHfNbTa)C with the addition of silicon carbide whiskers

Dense (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C with up to 10 wt % SiC whisker were prepared by spark plasma sintering. The influence of SiCw on the microstructure development, mechanical and tribological properties has been investigated. Nanohardness of HEC and SiCw phases varied between 38 GPa and 40 GPa, and indentation modulus of elasticity was ∼605 GPa. The hardness of the composites increased from 22 GPa to 27 GPa and indentation fracture resistance from 3.55 MPa m1/2 to 4.59 MPa m1/2 with increasing SiCw content. The main toughening mechanisms were crack deflection, crack branching, and crack bridging. The system HEC +5 wt% of SiCw was found to possess the highest bending strength of 623 ± 25 MPa. The composites exhibited similar coefficients of frictions with around 0.3 and wear rates approximately 1.50 × 10−6 mm3/N⋅m at 5 N and 2.66 × 10−6 mm3/N⋅m at 25 N with positive influence of SiC phase on the wear mechanisms.

Steel–Steel Laminates Manufactured via Accumulative Roll Bonding

Accumulative roll bonding (ARB) is a repeated cladding process in which two or more sheets of material are joined together by rolling at temperatures below recrystallization. The present review is focused on ARB of high‐alloy steels, which, among other laminated metal composites (LMCs), deliver the highest mechanical properties. After a brief description of high‐strength steels, history, and state of the art of LMCs, the principal roll bonding mechanism is explained. Further, the methodology of ARB of steels and variable parameters (stacking, temperature, etc.) are discussed. Known examples of steel–steel laminates are summarized with respect to their rolling temperature and mechanical properties. Further, the main toughening mechanisms of steel‐based LMCs are listed. The most promising candidates of high‐alloy steel laminates are presented in more detail. The important deformation mechanisms of twinning‐ and transformation‐induced plasticity (TWIP and TRIP) high‐alloy steels are explained. Microstructural changes and layer bonding as well as mechanical properties and damage behavior of two‐ and four‐layered TRIP/TWIP steel laminates are illustrated, including some specific phenomena, such as deformation lenses. Finally, by summarizing the analyzed data on steel laminates, conclusions and outlook are formulated.

Tough and Ductile Architected Nacre‐Like Cementitious Composites

AbstractEnhancing fracture toughness and ductility of brittle materials such as concrete remains a challenge. Nature offers numerous mechanisms to enhance fracture toughness using purposeful designs of materials’ architecture. Natural nacre exhibits high fracture toughness by promoting inelastic deformation and hierarchical toughening mechanisms. Here, “nacre‐like‐separated” and “nacre‐like‐grooved” cementitious composites inspired by brick‐and‐mortar arrangement of mollusk shells are proposed. These nacre‐like composites are engineered by laser processing cement paste into individual tablets and grooved patterns (as intentional defects) and laminating them with limited amounts of suitable elastomeric (polyvinyl siloxane) interlayers. It is found that interlayer deformation, tortuous crack propagation guided by the defects, and crack bridging are the main toughening mechanisms in these composites that lead to rising resistance curves. The study hypothesizes tablet sliding as an additional toughening mechanism in “nacre‐like‐separated”, preventing tablet failure and leading to the postponed onset of bulk composite failure. These mechanisms significantly enhance both fracture toughness and ductility by 17.1 and 19 folds, compared to constituent hardened cement paste, respectively. By engineering laser‐induced defects into tabulated cementitious‐elastomeric material at meso‐scale, a class of tough and ductile cementitious composites is introduced, resulting in significantly high fracture toughness values (73.68 MPa.mm1/2), comparable to Ultra‐high‐performance‐concrete without sacrificing the strength.

Processing of ZrO2 strengthened and toughened WC matrix composites based on ML

Binderless WC matrix cemented carbide was considered a novel material with the potential to replace WC-Co cemented carbide. However, current research was insufficient to address the trade-off between strength, toughness, and strength in binderless WC matrix cemented carbide. In this work, the machine learning was employed to estimate mechanical property and optimize the composition of WC matrix cemented carbide. WC-MgO-ZrO2 composites were successfully developed with uniform microstructure and excellent mechanical properties. The experimental results showed that the grain size of WC reached 1.05μm where the mass fraction of ZrO2 was 2%. The WC-MgO-2 wt%ZrO2 composite exhibited superior Vickers hardness, fracture toughness, and flexural fracture strength, which were 1851HV, 11.2 MPa·m1/2 and 1368 MPa, respectively. The fracture mode of WC-MgO-ZrO2 composites was mainly a mixture of transgranular fracture and intergranular fracture. The main toughening mechanisms were crack deflection, crack bridging, grain pull out and phase transformation. The comprehensive mechanical properties of WC-MgO-2 wt%ZrO2 composites rank at a superior level compared to similar binderless WC matrix composites reported in the current literature. Meanwhile, the wear behaviors of WC-MgO-ZrO2 composites were examined under three different loads. And the wear behavior and mechanism of WC MgO-2 wt% ZrO2 composite were revealed.

Super-tough polylactic acid blends via tunable dynamic vulcanization of biobased polyurethanes

Polylactic acid (PLA) is a biobased plastic with biodegradability, biocompatibility, and high-strength properties; however, its inherent brittleness limits its widespread application. Toughness can be improved by physical blending, chemical copolymerization, and reactive blending of PLA with flexible components. As highly engineered block polymers, polyurethanes (PUs) can improve the toughness of PLA through physical blending or in situ polymerization. However, current research on PU-toughened PLA is hindered by poor compatibility, toughening, and dependence on petrochemical resources. Therefore, to address these issues, we developed a novel biobased millable polyurethane (PO3G-MPU) containing unsaturated double bonds and used it to toughen PLA via peroxide-initiated dynamic vulcanization. The results showed that balancing toughness and strength is possible, with the notched impact strength and tensile strength of the prepared PLA/MPU blends reaching 59.01 kJ/m2 and 43.97 MPa, respectively. The massive shear yielding and plastic deformation of the PLA matrix induced by the crosslinked PO3G-MPU were responsible for the main toughening mechanism. These results provide a new perspective on research on PU-toughened PLA.

Repair and strengthening of existing structures using FRP composites externally-bonded with toughened epoxy adhesives

This literature review investigates the efficacy of toughened epoxy adhesives for structural repair and strengthening of existing structures with FRP composites. Highlighting the limitations of traditional epoxy adhesives, such as susceptibility to fatigue loading and impact actions, the study explores the potential of toughened formulations to mitigate fracture propagation issues. Through the incorporation of additional phases like rubber particles or nanomaterials, toughened adhesives offer increased fracture toughness and improved structural performance. However, this could also lead to trade-offs, including potential reductions in mechanical properties, glass transition temperature reduction and possible health and environmental concerns. This study presents an overview of the main toughening mechanisms occurring in modified epoxy resins. Two toughening techniques are described and discussed, namely the rubber-toughening and nanomaterial-toughening. The study further investigates the effects of the use of toughened adhesives in externally-bonded FRP strengthening/repair applications, involving both steel and concrete substrates. Eventually, the difference in bond-slip behavior of toughened and traditional adhesives is described, with reference to currently available literature studies on the topic. This review paper shed light on the promising advantages of using toughened epoxy adhesives in FRP applications, in terms of higher bond and strain capacity and enhanced bond transfer mechanisms between composite and substrate, if compared to traditional adhesives.

Effects of Boron Nitride Nanoplatelets on the Mechanical Properties of Spark Plasma Sintered Glass–Ceramics Composites

Boron nitride nanoplatelet (BNNPs) with superior mechanical properties can be a promising candidate as reinforcement to improve the toughness of glass–ceramics. Herein, glass–ceramics composites reinforced with well‐dispersed BNNPs are fabricated by spark plasma sintering. The addition of BNNP enhances the mechanical properties of glass–ceramics and inhibits the nucleation and growth of the diopside. The number of diopsides and the grain size gradually decrease with the increase of BNNP content. The mere addition of 1.0 wt% BNNPs can simultaneously increase the hardness and fracture toughness by 7.94% and 22.54%, respectively. Crack deflection, BNNP pull‐out, crack blocking, and crack bridging are the main toughening mechanisms, and the toughening model is established to optimize the design of BNNP‐reinforced composites.

Poly-lactic acid self-reinforcing composites with improved toughness utilising solid-state drawn oriented tape

With the increasing white pollution problem and the enhancement of people’s awareness of environmental protection, the bio-based and biodegradable polymers have received unprecedented attention. Poly-lactic acid (PLA) is considered as one of the most potential alternatives to petroleum-based polymer materials. However, Due to its inherent brittleness and the difficulties of post-disposal and recycling caused by traditional processing methods, PLA has not been fully accepted by the market. To overcome the issues, self-reinforcing PLA composites (SRPLA) were prepared with different PLA grades as the matrix and the reinforcement respectively. High toughness SRPLA were prepared by hot pressing using PLA3052 casting films as the matrix and PLA6202 oriented tapes fabricated by solid state drawing as the reinforcement. In addition, the toughening mechanism was analyzed. Mechanical tests showed that the strength, modulus and elongation at break were increased by 124.3%, 114% and 145.2% respectively compared with pure PLA hot pressed film. Particularly, the tensile toughness of SRPLA is more than 200% that of pure PLA. The interface debonding and tape pullout were the main toughening mechanisms.

Betholletia excelsa Fruit: Unveiling Toughening Mechanisms and Biomimetic Potential for Advanced Materials.

Dry fruits and nutshells are biological capsules of outstanding toughness and strength with biomimetic potential to boost fiber-reinforced composites and protective structures. The strategies behind the Betholletia excelsa fruit mechanical performance were investigated with C-ring and compression tests. This last test was monitored with shearography and simulated with a finite element model. Microtomography and digital and scanning electron microscopy evaluated crack development. The fruit geometry, the preferential orientation of fibers involved in foam-like sclereid cells, promoted anisotropic properties but efficient energy dissipating mechanisms in different directions. For instance, the mesocarp cut parallel to its latitudinal section sustained higher forces (26.0 ± 2.8 kN) and showed higher deformation and slower crack propagation. The main toughening mechanisms are fiber deflection and fiber bridging and pullout, observed when fiber bundles are orthogonal to the crack path. Additionally, the debonding of fiber bundles oriented parallel to the crack path and intercellular cracks through sclereid and fiber cells created a tortuous path.

EPOXY-SILICA NANOCOMPOSITE: CREEP RESISTANCE AND TOUGHENING MECHANISMS

In this article, the effect of modified nanosilica (m-nanosilica) on the mechanical and creep performance of epoxy resin were investigated. At the same time, the toughnening mechanisms of epoxy resin with m-nanosilica were also considered. The results show that the crack pinning and crack deflection mechanism contributes only a small part to nanocomposite, this due to the size of the nanosilica particles is such smaller than crack-opening displacement. The FE-SEM surface fracture shows evidence of the debonding of the nanoparticles from the polymer matrix and the expansion of the holes around the nanoparticles as well as the existence of the plastic deformation zone. Plastic deformation and plastic void growth are the main toughening mechanism in nanocomposite material.

Tough and antibacterial poly(l-lactic acid) composites prepared via blending with the bifunctional macromolecular ionomer

In order to expand the application of PLLA in the packaging field, improving its toughness and antibacterial activity has been widely concerned. However, seldom researches can simultaneously efficiently improve the toughness and antibacterial activity of PLLA by adding one kind of additions. To address above problems, the bifunctional branched poly(butylene adipate) ionomer additive (b-PBAUi) was synthesized. For b-PBAUi, its branched structure not only increased the plasticizing effect of additive, but also acted as reaction sites to introduce more antibacterial ionic salt. Due to the special structure of b-PBAUi, PLLA/b-PBAUi blends achieved excellent toughness and antibacterial efficiency. The elongation of blend reached 125 % even by adding 5 wt% b-PBAUi, which was 10 times higher than that of PLLA. From the analysis of phase morphology, it could be found that the microvoids promoting tensile yielding was the main tensile toughening mechanism for PLLA/b-PBAUi blends. In addition, the antibacterial activity of PLLA was significantly improved by adding b-PBAUi. For PLLA/b-PBAUi10 and PLLA/b-PBAUi15, the antibacterial efficiency against E. coli and S. aureus bacteria exceeded 99.0 %. By comprehensive consideration, the optimal blend ratio was achieved by PLLA/b-PBAUi10 due to its excellent toughness and antibacterial efficiency.

Bamboo as a substitute for plastic: Underlying mechanisms of flexible deformation and flexural toughness of bamboo at multiple scales

To develop bamboo-based products by weaving, winding and molding for promoting "bamboo as a substitute for plastic", it is urgent to fully understand the underlying mechanisms of flexible deformation and flexural toughness of bamboo at multiple scales. In this study, we quantified the micro deformation and strain of bamboo cells at different curvatures by in situ high-resolution field emission scanning electron microscopy (FE-SEM) and X-ray micro-computed tomography (Micro-CT) to analyze its flexible deformation at cellular and nano scales. The toughening mechanism embodied in fracture morphology was quantitatively calculated using mathematical models at macro and cell-wall scales. Results showed that bamboo possessed both excellent flexible deformation and flexural toughness in the longitudinal direction with the variation of fiber contents in the radial direction of bamboo culm. Flexible deformation of bamboo was achieved through synergistic cellular deformation between tensile sclerenchyma fibers (SFs) and compressible parenchyma cells (PCs) adapted to external stress distribution. SFs with greater tensile strain (0.027) bore the load even after the PCs broke (0.019), while the PCs in the compression layer acted as a buffer through plastic deformation. Meanwhile, the cell-wall sublayer wound by microfibrils at different angles, demonstrated significantly different Young's modulus and fracture modes verified by a mathematical model. Multiwalled fiber pullout and crack deflection were the main toughening mechanisms in bamboo at macro- and cellular scales. Multiwalled fibers pullout energy contributed the most to fracture energy dissipation, accounting for 46.0∼77.5%, whereas the weak intercellular interface effectively promoted crack deflection, reduced the local stress and delayed bamboo failure.

Preparation and characterization of porcupine-fish-spine-inspired AZ91/SiC composites with radial lamellar structures

There is a growing demand for lightweight, damage-tolerant, and wear-resistant structural materials, and it is difficult to produce such materials with conventional methods. In this work, we prepared AZ91/SiC composites with a porcupine-fish-spine-inspired structure through radial freeze casting and pressureless infiltration, and investigated the effect of the thermal conductivity of the freezing mold (copper, brass, and stainless steel molds) on the structure and mechanical properties of the composite prepared. Owing to a higher axial-radial temperature gradient ratio, scaffolds prepared with the steel mold had a stronger orientation of ceramic layers, and the corresponding composites had higher flexural strength and fracture toughness compared with those prepared using the copper mold. The main toughening mechanisms were crack deflection and uncracked-ligament bridging. In particular, a radially frozen composite had a lower dry wear rate (by 20%) at cross-sectional surfaces than a conventional axially frozen composite. This paper presents an economical and scalable method for the preparation of high-performance lightweight metal matrix composites.

Microstructure and mechanical properties of WC-TiC0.4 composites doped with ZrO2 or 3Y-ZrO2

The ZrO2 or 3Y-ZrO2 incorporated WC-TiC0.4 composites were consolidated employing spark plasma sintering (SPS) technology at different temperatures. The impacts of ZrO2 and 3Y-ZrO2 as toughening agents on the densification, phase composition, microstructure and mechanical properties of the WC-based composites were assessed. The results revealed that the WC-5 wt% TiC0.4–9 wt% ZrO2 (W5Ti9Zr) composite required a lower sintering temperature of 1700 °C for nearly full densification than WC-5 wt% TiC0.4–9 wt% 3Y-ZrO2 (W5Ti9Zr-3Y) composite. The diffraction peaks of monoclinic ZrO2 (m-ZrO2) existed only in the W5Ti9Zr composite. The evenly dispersed ZrO2 and 3Y-ZrO2 achieved the relatively homogeneous and fine microstructure in W5Ti9Zr and W5Ti9Zr-3Y composites. Moreover, the semi-coherent interfaces of (W1-x, Tix)Cn layer with WC, (W1-x, Tix)Cn layer with m-ZrO2, and (W1-x, Tix)Cn with m-ZrO2 brought about a rise in the fracture toughness of W5Ti9Zr composite. The W5Ti9Zr composite yielded the better comprehensive mechanical properties with a combination of hardness (20.8 GPa) and fracture toughness (9.9 MPa·m1/2) as compared to those of W5Ti9Zr-3Y composite. Furthermore, the main toughening mechanisms of W5Ti9Zr and W5Ti9Zr-3Y composites included grain refinement of WC matrix, a mixed transgranular and intergranular fracture, stress-induced transformation of t-ZrO2 to m-ZrO2, and crack deflection.

Mechanical performance and microstructure of 3Y‐TZP/Al2O3/GNPs medical ceramic surgical scalpel material prepared through SPS–HF dual sintering method

AbstractIn this study, 3 mol% yttria‐stabilized tetragonal zirconia polycrystal (3Y‐TZP)/Al2O3/graphene nanoplatelets (GNPs) medical ceramic materials for manufacturing surgical scalpels were sintered in vacuum in an SPS–625HF furnace. The mechanical performances and microstructures of the composites were investigated, and the influence mechanisms of the sintering temperature and amount of added GNPs were studied. During the sintering process at 1400°C and 30 MPa for 5 min, the added GNPs enhanced the mechanical properties of the 3Y‐TZP/Al2O3 composites. The results showed that the composite with .1 wt.% GNPs had 6.4% (910 ± 11 MPa) higher flexural strength than 3Y‐TZP/Al2O3. The composite with .4 wt.% GNPs had 38.7% (12.95 ± .22 MPa m1/2) greater fracture toughness than 3Y‐TZP/Al2O3. The main toughening mechanisms of 3Y‐TZP/Al2O3/GNPs were crack bridging, crack deflection, crack branching, GNPs bridging, transgranular fracture structures, and phase transformation of t‐ZrO1.95. The two‐stage densification displacement curve appeared at the optimal sintering temperature of the materials, and the 3Y‐TZP/Al2O3/GNPs composites with a two‐stage densification displacement curve had excellent mechanical properties. The added GNPs can inhibit the grain growth during the sintering process, thereby refining the zirconia grains. With the increase in GNPs content, the grain size and flexural strength of the composites decreased gradually. However, higher content of GNPs was beneficial to improve the relative density and thermal diffusivity of 3Y‐TZP/Al2O3/GNPs composite material.

Investigating inter/intralayer interface-triggered toughening mechanisms of three-dimensional printed polylactic acid using double-notch four-point-bending method

Three-dimensional (3D) printing technology is widely adopted in smart manufacturing, automotive, and aerospace fields for manufacturing complex geometric structures. Understanding the inter/intralayer interface-triggered toughening mechanisms of the 3D printed polylactic acid (PLA) materials produced by material extrusion is crucial. The fracture performance of the printed specimens in different fracture directions is evaluated with the single-edge-notch three-point-bending (SEN-3PB) test. Based on the double-notch four-point-bending (DN-4PB) method, the deformation mechanism around the crack tip and the crack propagation in the damage zone are systematically analyzed. It is clarified that the geometrical arrangement between the pre-crack and inter/intralayer and the interface bonding strength play decisive roles during the fracture process. The crack deflection, crack branching, and plastic deformation of the filaments are the main toughening mechanisms which endow 3D printed materials with better fracture performance than injection molded ones. However, the crack deflection and crack blunting could be inhibited if the intralayer interfacial bonding strength is excessively high. The results provide a good insight into the toughening mechanisms provoked by the inter/interlayer interface and could guide the design of the 3D printed specimens with optimized fracture performance.

A New Strategy to Improve the Toughness of Epoxy Thermosets-By Introducing Poly(ether nitrile ketone)s Containing Phthalazinone Structures.

As high brittleness limits the application of all epoxy resins (EP), here, it can be modified by high-performance thermoplastic poly(ether nitrile ketone) containing phthalazinone structures (PPENK). Therefore, the influence of different PPENK contents on the mechanical, thermal, and low-temperature properties of EP was comprehensively investigated in this paper. The binary blend of PPENK/EP exhibited excellent properties due to homogeneous mixing and good interaction. The presence of PPENK significantly improved the mechanical properties of EP, showing 131.0%, 14.2%, and 10.0% increases in impact, tensile, and flexural strength, respectively. Morphological studies revealed that the crack deflection and bridging in PPENK were the main toughening mechanism in the blend systems. In addition, the PPENK/EP blends showed excellent thermal and low-temperature properties (-183 °C). The glass transition temperatures of the PPENK/EP blends were enhanced by approximately 50 °C. The 15 phr of the PPENK/EP blends had a low-temperature flexural strength of up to 230 MPa, which was 46.5% higher than EP. Furthermore, all blends exhibited better thermal stability.

Enhancing the Interface Behavior on Polycarbonate/Elastomeric Blends: Morphological, Structural, and Thermal Characterization.

A systematic study was performed to provide better understanding of the effect of elastomeric materials on the behavior of polycarbonate blends (PC). Thus, blends of PC with different amounts of elastomers, such as copolyether ester elastomer (COPE), acrylonitrile-butadiene-styrene (ABS), maleic anhydride-grafted ABS (ABS-g-MA), and styrene-ethylene-butylene-styrene (SEBS-g-MA) were prepared in a co-rotating twin-screw extruder. The materials were characterized by an electronic microscopy (SEM), an infrared spectroscopy (FTIR), and thermal (DSC) and thermo-mechanical (DMA) techniques. The incorporation of elastomeric phases was observed by changes in the FTIR band's intensity, whereas a new shoulder of the ester band of COPE at 1728 cm-1 indicates the occurrence of a transesterification reaction. Unmodified and modified ABS (5% and 10%) did not affect the glass transition temperature (Tg) of PC, while 1% SEBS-g-MA slightly increased this value. PC/10% COPE showed that a decrease in Tg of 25 °C has a result of better compatibilization between both phases, which is visible via SEM. SEM analysis identified three main toughening mechanisms, depending on the type of elastomer. Unlike any other study, this work deepens the knowledge, in a comparative way, to understand the elastomeric effect at the interface and consequently, on the mechanical behavior of PC systems.

Refractory high-entropy alloys: A focused review of preparation methods and properties

• We reviewed the preparation methods of RHEAs, including metallurgical smelting, powder metallurgy, magnetron sputtering and additive manufacturing technology. • The microstructure and phase transformation process of RHEAs were analyzed. • The mechanical properties and main strengthening and toughening mechanisms of RHEAs, such as solid solution strengthening, precipitation strengthening and the transformation-induced plastic (TRIP) were discussed. • The properties of RHEAs, including high strength, oxidation resistance, corrosion resistance and wear resistance were reviewed. • We particularly focus on the generation of the ideal room temperature toughness of brittle BCC-structure RHEAs to achieve the combination of high strength and high toughness as engineering materials. In recent years, high-entropy alloys (HEAs) have become prominent metallic materials due to their unique design strategies and excellent mechanical properties. The HEAs-inherent high-entropy, lattice-distortion, sluggish-diffusion, and cocktail effects make HEAs maintain high strength, oxidation resistance, corrosion resistance, wear resistance, and other excellent comprehensive properties, showing stronger competitiveness relative to traditional alloys. Refractory high-entropy alloys (RHEAs) are considered as a new kind of high-temperature materials with great application prospects due to their excellent mechanical properties and have the potential to replace nickel-based superalloy as the next generation of high-temperature materials. We reviewed the research status and preparation methods of RHEAs in recent years, including the metallurgical smelting, powder metallurgy, magnetron sputtering, and additive manufacturing technologies. The microstructure and phase-transformation process of RHEAs were analyzed. The mechanical properties and main strengthening and toughening mechanisms of RHEAs, such as solid-solution strengthening, precipitation strengthening, and the transformation-induced plasticity (TRIP), were discussed, and the deformation mechanism of RHEAs was revealed. The properties of RHEAs, including high strength, oxidation resistance, corrosion and wear resistance were reviewed. RHEAs will meet the huge market demand in the engineering materials field, but there are still many challenges, such as the trade-off between high strength and high ductility, structural design, and performance optimization of RHEAs with brittle BCC structures. We believe that this combination of knowledge may shape the future of RHEAs and break through the mutually exclusive conundrum of high strength and high toughness for RHEAs.

Toughening Effect of Graphene Oxide in WC-Co Cemented Carbide

To study the effect of graphene oxide (GO) addition on the microstructure and mechanical properties of WC-Co cemented carbides, sintered bulks were fabricated and characterized. The study shows that the GO added in cemented carbide have insignificant effect on the relative density and the crystalline size, therefore, the higher hardness is maintained. Moreover, the fracture toughness has increased by 29.1%. Bridging is found to be the main toughening mechanism in WC-Co-GO cemented carbide. GO can be expected to be used to prepare cemented carbide with both high hardness and high fracture toughness.