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A (Hf-Ta-Zr-Nb-Ti)C high-entropy carbide was prepared by ball milling and a two-step Spark Plasma Sintering process, achieving a single-phase ceramic sample with a high relative density of 99.4 %. The wear resistance of the sample was measured by tribology and micro-scale mechanical behaviour was studied by nanoindentation on both the non-deformed and worn surfaces. Grains and the vicinity of grain boundaries exhibited high hardness values of 38.5 ± 0.5 GPa and 35.5 ± 1.0 GPa with similar Young’s moduli of 562 ± 11 GPa and 547 ± 16 GPa, respectively. The dominant wear mechanism was mechanical wear with limited grain pull-out and fracture, and with a localized and thin tribo-layer formation. The specific wear rate exhibited an increase with the increasing load from 2.53·10−6 mm3/Nm at 5 N to 9.03·10−6 mm3/Nm at 50 N. This was correlated to the decrease of nanohardness of the worn surfaces with increasing wear load, which is attributed to the increased number of microcracks.

Anisotropic slip activation of ZrB2 grains and deformation behaviour of constituents of ZrB2-SiC and ZrB2-B4C composites were investigated by nanoindentation. Samples were prepared by spark plasma sintering; crystallographic orientation of grains was determined by electron backscatter diffraction and deformation around indents in constituents was studied by scanning electron microscopy. Based on the Hertzian stress analysis of the measured strain bursts, so called pop-ins, the critical resolved shear stresses for slip system families of 10 1 ¯ 0 11 2 ¯ 0 , 10 1 ¯ 0 0001 and 10 1 ¯ 0 11 2 ¯ 3 were determined for ZrB2. Homogeneous dislocation nucleation was revealed with similar critical resolved shear stresses (∼35 GPa) for each slip system close to the theoretical value. Based on the analysis of the orientation dependent maximal resolved shear stress, anisotropic dislocation nucleation was inferred in ZrB2 grains with single and simultaneous activation of the investigated slip systems. Macromechanical properties of the composites were correlated to nanohardness and indentation modulus of grains and grain boundaries.

Bulk polycrystalline high-entropy carbides are a newly developed group of materials that increase the limited compositional space of ultra-high temperature ceramics, which can withstand extreme environments exceeding 2000 °C in oxidizing atmospheres. Since the deformability of grains plays an important role in macromechanical performance, in this work we studied the strength and slip behaviour of grains of a spark-plasma sintered (Hf-Ta-Zr-Nb)C high-entropy carbide in a specific orientation during micropillar compression. For comparison, identical measurements were carried out on the monocarbides HfC and TaC. It was revealed that (Hf-Ta-Zr-Nb)C had a significantly enhanced yield and failure strength compared to the corresponding base monocarbides, while maintaining a similar ductility to the least brittle monocarbide (TaC) during the operation of $${\boldsymbol{\{}}{\bf{110}}{\boldsymbol{\}}}{\boldsymbol{\langle }}{\bf{1}}\bar{{\bf{1}}}{\bf{0}}{\boldsymbol{\rangle }}$$ { 110 } 〈 1 1 ¯ 0 〉 slip systems. Additionally, it was concluded that the crystal orientation and stress conditions determine the operation of slip systems in mono- and high-entropy carbides at room temperature.

The anisotropic deformation behaviour of ZrB2 grains of basal and prismatic orientations in a polycrystalline ZrB2 sample was studied during in-situ micropillar compression at room temperature (RT), 350 °C and 500 °C. Micropillars were milled out of large grains by focused ion beam machining; the basal and prismatic orientations were previously identified using electron backscatter diffraction maps. The basal pillars showed brittle behaviour and cracking on the 10 1 ¯ 0 and 2 1 ¯ 1 ¯ 0 type prismatic planes that lead to their collapse. For this orientation, the yield/rupture stress ranged between 8 and 14 GPa and it exceeded 12 GPa even at 500 °C. In the prismatic orientation, considerable plasticity was observed, showing a temperature dependent yield stress from 6 to 8 GPa at room temperature to ~2GPa at 500 °C. The activated slip system family was identified as 10 1 ¯ 0 11 2 ¯ 3 type for all of the test temperatures (RT, 350 °C and 500 °C). Analysis of the Schmid-factors revealed that the plasticity of the prismatic micropillars is controlled by the temperature dependence of the critical resolved shear stress of the 10 1 ¯ 0 11 2 ¯ 3 slip systems. The brittleness of the basal micropillars was influenced by their defect structure (e.g. microcracks) and it was temperature independent due to the absence of slip activation.

In this study, the influence of alloying elements on the mechanical properties of iron borides FeB and Fe2B formed in Fe-B-X (X = C, Cr, Mn, V, W, Mn + V) alloys were evaluated using instrumented indentation measurement. The microstructural characterization of the alloys was performed by means of X-ray diffraction and scanning electron microscope equipped with an energy dispersive X-ray analyzer. The fraction of the phases present in the alloys was determined either by the lever rule or by image analysis. The hardest and stiffest FeB formed in Fe-B-X (X = C, Cr, Mn) alloys was observed in the Fe-B-Cr alloys, where indentation hardness of HIT = 26.9 &plusmn, 1.4 GPa and indentation modulus of EIT = 486 &plusmn, 22 GPa were determined. The highest hardness of Fe2B was determined in the presence of tungsten as an alloying element, HIT = 20.8 &plusmn, 0.9 GPa. The lowest indentation hardness is measured in manganese alloyed FeB and Fe2B. In both FeB and Fe2B, an indentation size effect was observed, showing a decrease of hardness with increasing indentation depth.

Support from the projects APVV-15-0469, APVV-15-0621, VEGA 2/0163/16, and VEGA 2/0082/17 is acknowledged. MJR and EGC acknowledge the support of EPSRC grant XMAT (EP/K008749/2).

The formation of TiC and Ti phases and their influence on their mechanical properties was studied in this work. Thin layers were deposited by DC magnetron sputtering at room temperature in ultrahigh vacuum from Ti and C targets. Cubic TiC phase (c-TiC) was formed from 58 to 86 at.% Ti content. First formation of hexagonal Ti (h-Ti) occurred from 86 at.% Ti content. The c-TiC disappears from 90 at.% Ti content. Films with 86 at.% Ti content the c-TiC structure can transform to h-Ti by sequential stacking faults. Dominance of c-TiC(111) texture with increasing Ti content was observed. The hardness of thin films agree with structural observations. The highest hardness value (∼26 GPa) showed the c-TiC thin film with 67 at% Ti content. The nanohardness values showed decreasing character with increasing Ti content over 70 at.%. The lowest values of nanohardness (∼10 GPa) was observed for thin films with only h-Ti phase.

Hardness anisotropy and deformation-induced dislocations in (Hf-Ta-Zr-Nb)C high-entropy carbide grains of low-index facets were investigated using nanoindentation and transmission electron microscopy (TEM). The (Hf-Ta-Zr-Nb)C sample was prepared by a two-step spark plasma sintering (SPS) and nanoindentation was performed on grains mapped previously by electron backscatter diffraction. To investigate the deformation zones under the indents, thin lamellas were extracted from grains of {001}, {101} and {111} orientations using a focused ion beam (FIB) technique and were studied by TEM. Nanoindentation revealed that grains of {100} orientation exhibit about 7% lower hardness (~35 GPa) compared to the {110} and {111} facets (~38 GPa) while there was no anisotropy found for the indentation modulus. TEM analysis of the lamellas revealed heavily deformed zones under the indents with dislocations of a / 2 1 1 ¯ 0 type Burgers vector. TEM-based trace analysis of dislocations revealed the activation of the 110 1 ¯ 11 and 110 1 ¯ 10 slip systems in grains indented on the {001} and {101} facets, respectively. The hardness anisotropy of (Hf-Ta-Zr-Nb)C was attributed to the different operating slip systems, similar to that was reported for the component monocarbides in the literature.

The orientation dependence of hardness and modulus of ZrB2 grains were determined on a polycrystalline specimen with low porosity using Berkovich nanoindentation at room temperature. Additionally, Cube corner indentation was carried out on differently oriented grains to generate visible slip patterns, which were used to analyze slip activation. The anisotropy ratio, which is the ratio of the hardest/stiffest to the softest/most compliant orientations, was higher for hardness (∼1.3) than for modulus (∼1.1). The orientation dependence of nanohardness was described using an analytical model based on the activation of multiple slip system families. Based on literature data and the present indentation results, only the { 10 1 ¯ 0 } 〈 11 2 ¯ 0 〉 , { 10 1 ¯ 0 } [ 0001 ] and { 10 1 ¯ 0 } 〈 11 2 ¯ 3 〉 type slip system families were considered in the model. Simulations showed that none of the three slip systems by itself could describe the orientation dependence of the hardness of ZrB2. Analysis of the model in combination with the results of Cube corner indentation revealed that the experimental results can be appropriately described by the simultaneous activation of the { 10 1 ¯ 0 } [ 0001 ] and { 10 1 ¯ 0 } 〈 11 2 ¯ 3 〉 slip systems. Further analysis of the model together with first principles calculations found in the literature were used to derive the hierarchy of slip activations for ZrB2 at room temperature.

Elastic constants of single carbon fibres were estimated by a novel nanoindentation based method and subsequently predicted by modified two- and three-phase Eshelby-Mori-Tanaka (EMT) micromechanical models which takes into account both crystalline, amorphous phases and microvoids of the fibre structure. This case study was carried out on a T-300 PAN-based carbon fibre reinforced SiC composite material at room temperature. Transversal and longitudinal cross-sections of individual fibres were indented by a sharp Berkovich tip up to the maximum depth of 150 nm. Assuming transverse isotropy of the fibres, their five elastic constants were deduced from the measured indentation moduli using numerical (Vlassak-Nix) and analytical (Delafargue-Ulm) models, and finite element method simulations. The elastic constants were estimated to c11 = 30.9, c12 = 9.7, c13 = 12.2, c33 = 237.3 and c44 = 10.9 GPa. The complete set of measured elastic constants were analyzed and then predicted by the proposed modified EMT models. The application of anisotropic nanoindentation coupled with the amorphous-crystalline structure model is unique to carbon fibre which revealed that an anisotropic ‘amorphous′ matrix should be assumed to achieve an appropriate fit with experimental results. This matrix was considered as a mixture of microvoids and aligned graphite planes, and their volume ratio was determined.

The orientation dependence of scratch resistance of zirconium diboride (ZrB2) grains was investigated during nanoscratch testing. The experimental material was produced by spark plasma sintering (SPS) and the crystal orientation of ZrB2 grains was determined by electron backscatter diffraction (EBSD) technique. Scratches were prepared on a previously mapped surface area by an instrumented indentation machine using Berkovich tip with constant loads of 50, 100 and 150 mN along lengths of 200 μm. The generated surface topography was studied by scanning electron microscopy (SEM), confocal and atomic force microscopy (AFM). Micro-plasticity was observed in the form of slip-lines and microcracking along the scratch tracks. The analysis of experimental data revealed significant anisotropy in residual scratch depth with the lowest resistance corresponding to the tilt angle of around Φ = 50°–60°. Anisotropic crack initiation was observed, which depends on the load and crystal orientation. The orientation dependence of scratch resistance showed a proportional tendency with hardness, yield and rupture stress anisotropies reported recently by the present authors.

In this work, the WC-Co carbide/1020 steel brazed joints which were prepared by Tungsten Inert Gas (TIG) process using different brazing currents have been studied. The peak temperature reached by increasing the brazing current caused remarkable inter-diffusion of Ni, Mn, Cu and Co elements between the braze and the base materials. The higher the brazing current was, the larger the diffusion extent of these elements became and the greater the WC rearrangement occured. As the diffusion rate of Ni towards the WC-Co was enhanced through increasing brazing current, a short range diffusion of Co element towards the braze was recorded. A Special Shear Test (SST) conducted at room temperature revealed the best bonding strength for the current of 80A. Besides, hardness (HIT) and elastic modulus (EIT) values of the interfaces were situated between those of the braze and the base materials.

In this overview, we summarize the results published to date concerning the small-scale mechanical testing of WC−Co cemented carbides and similar hardmetals, describing the clear trend in the research towards ever-smaller scales (currently at the nano-level). The load-size effect during micro/nanohardness testing of hardmetals and their constituents and the influence of the WC grain orientation on their deformation, hardness, indentation modulus, fracture toughness, and fatigue characteristics are discussed. The effect of the WC grain size/orientation, cobalt content, and testing environment on damage accumulation, wear mechanisms, and wear parameters are summarized. The deformation and fracture characteristics and mechanical properties, such as the yield and compression strength, of WC−Co composites and their individual WC grains at different orientations during micropillar compression tests are described. The mechanical and fracture properties of micro-cantilevers milled from WC−Co hardmetals, single WC grains, and cantilevers containing WC/WC boundaries with differently-oriented WC grains are discussed. The physical background of the deformation and damage mechanisms in cemented carbides at the micro/nano-levels is descri and potential directions for future research in this field are outlined.

The relationship between the structure, elemental composition, mechanical and tribological properties of TiC/amorphous carbon (TiC/a:C) nanocomposite thin films was investigated. TiC/a:C thin film of different compositions were sputtered by DC magnetron sputtering at room temperature. In order to prepare the thin films with various morphology only the sputtering power of Ti source was modified besides constant power of C source. The elemental composition of the deposited films and structural investigations confirmed the inverse changes of the a:C and titanium carbide (TiC) phases. The thickness of the amorphous carbon matrix decreased from 10 nm to 1–2 nm simultaneously with the increasing Ti content from 6 at% to 47 at%. The highest hardness (H) of ~26 GPa and modulus of elasticity (E) of ~220 GPa with friction coefficient of 0.268 was observed in case of the film prepared at ~38 at% Ti content which consisted of 4–10 nm width TiC columns separated by 2–3 nm thin a:C layers. The H3/E2 ratio was ~0.4 GPa that predicts high resistance to plastic deformation of the TiC based nanocomposites beside excellent wear-resistant properties (H/E=0.12).

Mechanical properties of a hard and stiff W-C coating on steel substrate have been investigated using nanoindentation combined with finite element modeling (FEM) and extended FEM (XFEM). The significant pile-up observed around the indents in steel substrate caused an overestimation of hardness and indentation modulus. A simple geometrical model, considering the additional contact surfaces due to pile-up, has been proposed to reduce this overestimation. The presence of W-C coating suppressed the pile-up in the steel substrate and a transition to sink-in behavior occurred. The FEM simulations adequately reproduced the surface topography of the indents in the substrate and coating/substrate systems as well. The maximum principal stresses of the indented W-C/steel coated system were tensile; they were always located in the coating and evolved in 3 stages. Cohesive cracking occurred during loading in the sink-in zone (stage III) when the ultimate tensile strength (σ max ) of the coating was reached. The obtained hardness (H c ), indentation modulus (E c ), yield stress (Y) and strength (σ max ) of the W-C coating were H c = 20 GPa, E c = 250 GPa, Y = 9.0 GPa and σ max = 9.35 GPa, respectively. XFEM resulted in fracture energy of the W-C coating of G = 38.1 J · m-2 and fracture toughness of K IC = 3.5 MPa · m0.5.

The deformation and fracture behaviour of constituents of a WC-Co hardmetal were investigated by microcantilever bending technique. The compositions of FIB fabricated microcantilevers were: I) single grains of WC, II) WC grains of different orientations and III) the mixture of WC grains and Co phase. The crystallographic orientation of WC grains and the fracture surface of beams were studied by EBSD and SEM analyses, respectively. It was revealed that the elastic deformation depends mainly on the composition of the beams and the orientation of the WC grains. The Young's modulus of WC grains showed an orientation dependence with decreasing values from the basal (E~800 GPa) towards the prismatic orientations (E~500 GPa), which is in agreement with the theoretical predictions. The deformation behaviour of WC grains exhibited plasticity before their fracture with an average fracture strength of σ = 12.3 ± 3.8 GPa. It was found that the effect of dislocations and nanometre-sized defects (e.g. pores) plays an important role in the bending test of WC grains. Most of the WC/WC boundaries showed brittle failure with an average fracture strength of σ = 4.1 ± 2.5 GPa. It was concluded that the majority of the boundaries in the WC-Co composite are high energy WC/WC boundaries and their fracture strength is generally much lower than that of the WC grains.

The influence of the crystal orientation of WC grains on their nanoscratch resistance in WC–Co system was investigated. Instrumented nanoindentation with Berkovich tip was used for nanoscratch measurement. Electron backscatter diffraction (EBSD), atomic force microscopy (AFM) and scanning electron microscopy (SEM) investigations were performed to determine the orientation of WC grains and to study the surface morphology after the scratch test. The scratch resistance of the WC grains exhibits significant angle dependence with higher resistance of grains close to the basal orientation in comparison to the grains close to prismatic orientation.

The hardness, reduced modulus and indentation fatigue of the constituents of WC–Co cemented carbides have been investigated at a nanolevel using instrumented indentation in the range of applied loads from 0.25 mN to 50 mN. The influence of indentation load, microstructure, heat treatment and crystallographic orientation of WC grains/crystals has been studied. During random indentation a wide range of hardness values are connected with “mix-mode” indents, when under the indent, in its plastic/process zone not only one phase is present. Heat treatment has only a slight effect on the hardness of the constituent phases. An evident load–size effect was found during the indentation of WC grains but this was absent in the case of the Co binder. Significant influence of the crystallographic orientation of WC crystals on the hardness, indentation modulus and indentation fatigue has been found with enhanced properties in the basal plane.

Bulk equiatomic (Hf-Ta-Zr-Ti)C and (Hf-Ta-Zr-Nb)C high entropy Ultra-High Temperature Ceramic (UHTC) carbide compositions were fabricated by ball milling and Spark Plasma Sintering (SPS). It was found that the lattice parameter mismatch of the component monocarbides is a key factor for predicting single phase solid solution formation. The processing route was further optimised for the (Hf-Ta-Zr-Nb)C composition to produce a high purity, single phase, homogeneous, bulk high entropy material (99% density); revealing a vast new compositional space for the exploration of new UHTCs. One sample was observed to chemically decompose; indicating the presence of a miscibility gap. While this suggests the system is not thermodynamically stable to room temperature, it does reveal further potential for the development of new in situ formed UHTC nanocomposites. The optimised material was subjected to nanoindentation testing and directly compared to the constituent mono/binary carbides, revealing a significantly enhanced hardness (36.1 ± 1.6 GPa,) compared to the hardest monocarbide (HfC, 31.5 ± 1.3 GPa) and the binary (Hf-Ta)C (32.9 ± 1.8 GPa).

The orientation dependence of hardness and nanoindentation-induced deformation mechanisms of differently orientated tungsten carbide (WC) grains in WC–Co hardmetal were studied. Electron backscatter diffraction, atomic force microscopy and scanning electron microscopy investigations were performed to determine the grain orientation, and to study the surface morphology and the resulting deformation fields around the indents. The hardness of the differently orientated WC grains showed significant angle dependence from the basal towards the prismatic directions, but there was only a slight change in hardness between the two types of prismatic orientations ( ( 1 0 1 ¯ 0 ) and ( 2 1 ¯ 1 ¯ 0 ) ). Sink-in and pile-up effects, together with highly deformed regions and dislocation steps, were revealed around the imprints in the case of basal and prismatic orientations, respectively. A theoretical model is proposed in which the critical force for slip activation is determined as a function of orientation, based on the possible slip systems of WC. The predictions of the present model concerning the measured hardness values and the deformation field around the indents together with the sink-in effect are in good agreement with the experimental results.

Room-temperature deformation and fracture behaviors under microcompression of single crystal β-silicon nitride (Si3N4) micropillars were investigated. Pillars were fabricated by focused ion beam (FIB) in large Si3N4 grains, located close to the basal and prismatic orientations, selected by electron backscatter diffraction (EBSD). The micromechanical test and the damage characterization were performed by nanoindentation and scanning electron microscopy (SEM), respectively. The elastic, plastic, and fracture properties, such as Young's modulus, yield stress, and rupture stress are considerably influenced by the pillar orientation. The activation of the {101¯0}[0001] type slip system was identified in case of basal oriented micropillars.

The stress–strain relationships characterizing plastic deformation of aluminum are described over a wide range of testing temperatures by applying both a widely used phenomenological relationship and a well-known dislocation-based model. It is shown that over the whole range of testing temperatures the trapping of mobile dislocations and the annihilation of forest dislocations are controlled by the same thermally-activated dislocation motion, thereby leading to a simplified model which uses only two parameters to describe the multiplication and the annihilation rates of dislocations. The temperature dependence of these two microscopic quantities is explained. Furthermore, correlations between the characteristics of macroscopic and microscopic descriptions were established over a wide range of testing temperatures for pure Al.

The plastic behavior of face-centered cubic metals was investigated over a wide range of strain and testing temperature. The experimental stress-strain data were described using both macroscopic and microscopic, well-established relationships. The characteristics of these descriptions are discussed and compared with each other. The analysis of the characteristics leads to a definition of the low and high temperature deformation regions, where the kinetics of both the dislocation-multiplication and the dislocation-annihilation (recovery) are different. For pure aluminum, it is shown that the boundary between these two regions occurs at a homologous temperature of the order of ≈ 0.5T m where T m is the absolute melting temperature. From this analysis, correlations are also drawn between the macroscopic parameters describing the stress-strain relationship and the fundamental characteristics of the microscopic processes both at room temperature and elevated temperatures.

The room temperature macroscopic and microscopic plastic behavior of four face-centered cubic metals (Al, Au, Cu and Ni) is investigated experimentally over a wide strain range, and theoretical modeling is used to simulate the established major micromechanisms describing the evolution of mobile and forest dislocations during plastic flow. It is shown that forest dislocations develop primarily due to interaction between mobile dislocations, while the contribution from forest–mobile interactions is only minor. The trapping of mobile dislocations and the annihilation of forest dislocations are both controlled by the same thermally activated dislocation motion. These observations permit a simplification of the theoretical model that leads to an analytical relationship for the evolution of the total dislocation density as a function of strain. From this analysis, correlations are drawn between the macroscopic parameters describing the stress–strain relationship and the fundamental characteristics of the microscopic processes.

The plastic behavior of four pure face-centered-cubic (fcc) metals (Al, Au, Cu and Ni) was studied at room temperature up to a strain value of ∼8 achieved by severe plastic deformation. It is shown that at high strains the saturation flow stress and the total dislocation density can be scaled with the melting point, and the deformation process can be regarded as steady-state flow leading to a uniform description of the plastic behavior of these fcc metals at all temperatures. The results suggest that the plastic flow at room temperature of fcc metals processed by severe deformation is primarily attributable to diffusion-controlled and thermally activated processes due to the high concentration of deformation-induced vacancies.