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Tribological characteristics of fine-grained dual-phase high-entropy carbide/boride ceramics were investigated using the ball-on-flat dry sliding method in air, applying linear reciprocation motion with SiC counterpart. Detailed fractographical analyses were used for the characterization of the deformation and damage mechanisms. The investigated system with composition (Ti 0.14 Zr 0.2 Nb 0.2 Hf 0.2 Ta 0.26)C + (Ti 0.38 Zr 0.18 Nb 0.22 Hf 0.115 Ta 0.105)B 2 showed density with value 8.72 g/cm3 and very high hardness of HV1 29.4 ± 2.0 GPa. The friction coefficient at the test with 5 N was 0.56 and during the test with 10 N and 25 N loads were 0.48 and 0.5, respectively. The effective wear rates at the loads of 5 N and 25 N were very similar with values of 7.93 × 10−7 mm3/Nm and 6.63 × 10−7 mm3/Nm. At load 50 N the effective wear rate is significantly higher with a value of 9.11 × 10−6 mm3/Nm. Detailed fractography revealed that the dominant wear mechanisms at loads of 5 N and 25 N were an oxidation-driven tribochemical reaction and tribo-layer formation in boride grains and mechanical wear in carbide grains and at the load of 50 N fracture of boride and carbide grains and formation of chemically complex tribolayers. [ABSTRACT FROM AUTHOR]

The fracture behaviour of grains and grain boundaries plays an important role in the design of novel ceramics with improved macromechanical deformability. This was investigated first time in a dual-phase high-entropy carbide/boride ceramic, using microcantilever bending, linear beam theory and micro/nanofractography. Microcantilevers were focused ion beam milled from the carefully polished sample surface with random locations containing both carbide and boride grains and grain boundaries in the beam volume. Scanning electron microscopy-based fractography analysis revealed that the bending strength of grain/phase boundaries varies in the range of 0.9–6.7 GPa, depending on the orientation and location of the boundaries in the beam. The fracture of carbide grains was found to be initiated mainly on submicron size volume defects with strength values of 4.5–9.5 GPa, while the fracture origin in most boride grains was identified as surface defects introduced by specimen preparation, which resulted in higher fracture strengths of 9.0–12.3 GPa. [ABSTRACT FROM AUTHOR]

Wear behavior of a fine‐grained dual‐phase high‐entropy carbide/boride ceramics was investigated using ball‐on‐flat dry sliding methods in air, applying rotational and linear reciprocation motion with SiC counterpart. The investigated system showed very high nanohardness of the carbide and boride grains with mean values of 37.4 ± 2.3 and 43.0 ± 2.9 GPa, respectively, with the microhardness of dual system HV1 29.4 ± 2.0 GPa. The stabilized friction coefficient values during the circular tests changing from.62 to.77. During the reciprocal test, the frictional coefficient values are very similar with an average value of.53. The specific wear rates during the circular motion were similar in the range from 4.65 × 10−7 to 1.68 × 10−7 mm3/N m. During the reciprocal test, the wear rates at 5 and 25 N were similar as in the case of circular motion, but at 50 N load, the wear rate increased significantly to the value of 9.11 × 10−6 mm3/N m. The dominant wear mechanisms in all cases were oxidation driven tribochemical reaction and tribolayer formation in boride grains and mechanical wear in carbide grains. During the linear reciprocation test, the loading mode created conditions resulted in relatively low coefficient of friction and very high specific wear rate. [ABSTRACT FROM AUTHOR]

The phase and grain boundary characteristics of recently developed fine-grained dual-phase high-entropy (Ti-Zr-Nb-Hf-Ta)C/(Ti-Zr-Nb-Hf-Ta)B 2 was investigated throughout all the accessible length scales using scanning electron microscopy (SEM), aberration-corrected scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS). The system exhibits relatively homogeneous grain size distribution where the average size is approximately 0.97 µm, with chemical composition (Ti 0.14 Zr 0.2 Nb 0.2 Hf 0.2 Ta 0.26)C + (Ti 0.38 Zr 0.18 Nb 0.22 Hf 0. 115 Ta 0.105)B 2. SEM analyses revealed no micro-crack formation and second – phase segregation at the boundaries or micro-pores at the triple – points. Investigation down to the sub-nanometer scale revealed that the phase and grain boundaries were typically clean and sharp with an indistinct 1 – 1.5 nm thin gradient of metallic elements at boride/boride and carbide/carbide interfaces. The sharp phase and grain boundaries do exhibit elemental enrichment from a trace amount of Fe being incorporated in interstitial positions of carbide and boride grains locally at boride/carbide boundaries or are present in boride and carbide grains in the form of continuous thin layer at boride/boride and carbide/carbide interfaces with the probably origin from starting powders. [ABSTRACT FROM AUTHOR]

Fine-grained (Ti-Zr-Nb-Ta-Hf)C/(Ti-Zr-Nb-Ta-Hf)B 2 dual-phase high-entropy boride/carbide ceramic (HEC/HEB) was prepared from powders synthesized via a boro-carbothermal reduction and sintered by spark plasma sintering. The developed ceramic has a very high density and relatively homogeneous chemical composition of HEC and HEB grains with a size of HEC grains from 0.1 μm to 1.5 μm and HEB grains from 0.1 μm to 5 μm. The nanohardness of the HEC and HEB grains are very high with values of 37.4 ± 2.3 GPa and 43.3 ± 2.9 GPa, respectively. The nanohardness in the vicinity of grain boundaries is 30.0 ± 5.2 GPa. The Young's modulus of HEC with a mean value of 536.5 ± 34.2 GPa is significantly lower in comparison to Young's modulus for HEB grain with a mean value of 766 ± 45.7 GPa. Vickers hardness HV1 of the developed HEC/HEB ceramic is very high with a value of 29.4 ± 2.0 GPa which is the highest between the up-to-now reported dual-phase high-entropy ceramics. The developed system shows good indentation fracture resistance with a value of 3.9 ± 0.62 MPa m1/2. The most significant toughening mechanism is crack branching in larger HEC grains with sizes from 1.0 μm to 1.5 μm. [ABSTRACT FROM AUTHOR]

A novel dual-phase (Ti-Zr-Nb-Hf-Ta)B 2 + (Ti-Zr-Nb-Hf-Ta)C high-entropy, compositionally complex composite was synthesized by reactive spark plasma sintering using commercial ZrB 2 , NbB 2 , HfB 2 , TaB 2 and TiC powders as starting materials. The composite with high density, consists predominantly of boride (∼39.7 vol%) and carbide (∼56.4 vol%) phases and with residual amount of (Hf-Zr)O 2. The microstructure is homogenous and relatively fine due to the reaction and solid solution coupling effect with average grain sizes of 3.4 μm and 2.2 μm of the boride and carbide phases, respectively. Large-area SEM analyses confirmed no microcracks or micropore formation and no presence of large grains or clusters. Investigation from micro to nano and atomic level, focused on local chemical composition and spatial distribution of constituent elements, revealed that Ti preferentially dissolves into the boride and all other elements to the carbide phase creating a (Ti 0.82 Zr 0.04 Nb 0.08 Hf 0.03 Ta 0.03)B 2 + (Ti 0.49 Zr 0.12 Nb 0.13 Hf 0.11 Ta 0.15)C system. The analysis of grain and phase boundary interfaces revealed a continuous, <1.5 nm wide, segregation of impurity atoms of Fe and Co. Continuous (>20 nm) and atomically flat basal interfacial termination planes (001) decorated with monoatomic Zr enriched layer in the boride phase were observed. Triple points at grain and phase boundaries exhibit metal-rich composition due to impurities from the starting raw powders. [ABSTRACT FROM AUTHOR]

Anisotropic dislocation nucleation of differently oriented zirconium diboride (ZrB 2 ) grains in a polycrystalline ZrB 2 -based ceramic composite was investigated during nanoindentation with a Berkovich tip. Based on the Hertzian stress analysis of the measured strain bursts, the critical resolved shear stresses for slip systems of 10 1 ¯ 0 11 2 ¯ 0 , 10 1 ¯ 0 0001 and 10 1 ¯ 0 11 2 ¯ 3 types were determined. Homogeneous dislocation nucleation was revealed with near theoretical critical resolved shear stress (~35 GPa) for each slip system. The analysis of the orientation dependence of the calculated maximal resolved shear stress values inferred anisotropic dislocation nucleation in ZrB 2 grains with single and simultaneous activation of the investigated slip systems. [ABSTRACT FROM AUTHOR]

The series of polyurethane – carbon nanofiber (PU/CNF) elastomers was prepared and characterized. Polyurethane elastomeric matrix was made from polybutadiene-based macrodiol of MW ca. 2000, 1,6-diisocyanatohexane, and butane-1,4-diol. Carbon nanofibers (up to 2.25 wt.%) were dispersed in the mixture of monomers, i.e., before polyurethane formation. The influence of the nanofiber concentration on functional properties was studied by the series of microscopic, thermal, mechanical and electrical techniques of analysis. The best functional properties has PU/CNF composite containing 0.75 wt.% of CNF. [ABSTRACT FROM AUTHOR]

Indentation fatigue of WC grains on basal and prismatic planes in WC-Co cemented carbide has been investigated using instrumented indentation at nano level and applied loads from 5mN to 50mN. The influence of indentation load and crystallographic orientation of WC grains has been studied. Scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and atomic force microscopy (AFM) have been used for the characterization of microstructure, orientation of WC grains, topography of indents and slip lines in WC grains. Significant influence of the crystallographic orientation of WC grains on the indentation fatigue has been found. The indentation depth increased with the increasing load more intensively during the fatigue test of the prismatic planes than the basal planes. This behavior is probably connected with the different slip and dislocation mechanisms during the indentation of basal and prismatic planes. [ABSTRACT FROM AUTHOR]

Abstract: Indentation fatigue of WC–Co systems with different microstructure parameters was investigated using instrumented indentation. A Berkovich indenter was used at maximum loads from 25mN to 200mN, then unloaded to 50% of the maximum load and repeatedly re-loaded to 100 cycles. FIB/TEM observations have been used for characterization of the deformation and damage mechanisms in the zone beneath the indenter to study the influence of microstructure parameters on fatigue and fatigue mechanisms. Deformation induced fcc–hcp phase transformation of Co binder was the main mechanisms in the system with higher volume fraction of binder and slip in WC grains and fracture of WC/WC bridges in the system with lower Co content and higher contiguity. These mechanisms result in different fatigue behavior of the systems. [Copyright &y& Elsevier]

Abstract: Silicon nitride+1wt% graphene platelet composites were prepared using various graphene platelets (GPL) and two processing routes; hot isostatic pressing (HIP) and gas pressure sintering (GPS). The influence of the processing route and graphene platelets’ addition on the fracture toughness has been investigated. The matrix of the composites prepared by GPS consists of Si3N4 grains with smaller diameter in comparison to the composites prepared by HIP. The indentation fracture toughness of the composites was in the range 6.1–9.9MPam0.5, which is significantly higher compared to the monolithic silicon nitride 6.5 and 6.3MPam0.5. The highest value of K IC was 9.9MPam0.5 in the case of composite reinforced by the smallest multilayer graphene nanosheets, prepared by HIP. The composites prepared by GPS exhibit lower fracture toughness, from 6.1 to 8.5MPam0.5. The toughening mechanisms were similar in all composites in the form of crack deflection, crack branching and crack bridging. [Copyright &y& Elsevier]

Silicon nitride+1wt.% graphene platelet composites were prepared using various graphene platelets with the aim to improve the fracture toughness of Si3N4. The fracture toughness was significantly higher for all composites in comparison to the monolithic Si3N4, with the highest value of 9.92MPam0.5. The main toughening mechanisms originated from the presence of graphene platelets, and the increase in the fracture toughness values was attributable to crack deflection, crack branching and crack bridging. [Copyright &y& Elsevier]

The effect of the addition of carbon nanofibers (CNFs) on the microstructure, fracture/mechanical and electrical properties of the CNF/zirconia composite has been investigated. The microstructure of both ZrO2 and ZrO2–CNF composites consists of a very low grain sized matrix (approximately 160nm) with relatively well dispersed carbon nanofibers in the composite. The mechanical properties slightly decreased after the addition of CNFs to the ZrO2 but the electrical resistivity decreased significantly, exhibiting approximately 0.1Ωcm. [Copyright &y& Elsevier]

Abstract: Sintering and hot pressing have been used for preparation of monolithic ZrO2 and zirconia–carbon nanotube (CNT) composite. The effect of the processing route and the CNT''s addition on the microstructure, fracture/mechanical and electrical properties of the zirconia has been investigated at room temperature. The microstructure of the sintered and hot-pressed monolithic ZrO2 consists of a submicron-sized grains. The matrix of the ZrO2–CNT composite consists of a grains with even smaller size (approximately 140nm) with relatively well dispersed carbon nanotubes. The hardness and the indentation toughness of the sintered monolithic zirconia are 1297kg/mm2 and 8.01MPam0.5 and of the hot-pressed monolithic zirconia 1397kg/mm2 and 6.24 MPam0.5, respectively. The addition of the CNT''s decreased the hardness and indentation toughness to 830kg/mm2 and 5.6MPam0.5 however the electrical resistivity decreased significantly in comparison to the monolithic zirconia to the value of 0.1Ωcm. [Copyright &y& Elsevier]

The indentation load-size effect was investigated during hardness testing of high entropy carbides with different hardness, applying indentation loads from 50 mN to 10 N. The experimental systems were recently developed (Hf-Ta-Zr-Nb-Ti)C and (Mo–Nb–Ta–V–W)C high-entropy carbides, prepared by ball milling and a spark plasma sintering, with single-phase and high relative density of 99.4% and 99.0%, respectively. The load dependence of hardness was analysed using the traditional Meyer's law, the proportional specimen resistance model and the modified proportional specimen resistance model. The best correlation (R 2 > 0.99) between the measured values and the used models was achieved using the modified proportional specimen resistance model. This resulted in the so-called load-independent hardness of (Hf-Ta-Zr-Nb-Ti)C and (Mo–Nb–Ta–V–W)C systems 21.96 and 14.81 GPa in the case of analyses for 50 mN–10 N load range and 25.42 GPa and 14.13 GPa in the case of 50 mN–1 N load range, respectively. The different deformation and damage mechanisms detected as a potential reason for the origin of the load-size effect were microcracks at grain boundaries during the micro/macro-indentation and plastic deformation/nanocrack formation at the indents during nano/micro-indentation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: Silicon nitride+1wt% graphene platelet composites were prepared using various graphene platelets (GPLs) as filler. The influence of the addition of GPLs on the microstructure development and on the fracture toughness of Si3N4 +GPLs composites was investigated. The GPLs with thickness from 5nm to 50nm are relatively homogeneously distributed in the matrix of all composites, however overlapping/bundle formation of GPLs was found, containing 2–4 platelets as well. The single GPLs and overlapped GPLs are located at the boundaries of Si3N4, and hinder the grain growth and change the shape of the grains. The fracture toughness was significantly higher for all composites in comparison to the monolithic Si3N4 with the highest value of 9.9MPam0.5 for the composite containing the GPLs with smallest dimension. The main toughening mechanisms originated from the presence of graphene platelets, and responsible for the increase in the fracture toughness values are crack deflection, crack branching and crack bridging. [Copyright &y& Elsevier]

Abstract: Tribological behaviur of carbon fibrous phases (nanofibers and nanotubes) containing composites with Si3N4, ZrO2 and Al2O3 matrices was studied by pin-on-disk technique in conditions of dry sliding. Coefficients of friction and wear rates were measured, wear damage mechanisms were observed and identified. The resulting tribological behaviur was related to microstructure and mechanical properties of respective materials. Electrical conductivity was measured in wide range of frequencies by two-point method and effect of volume fraction and distribution of CNTs and CNFs on percolation threshold was evaluated. Both coefficient of friction and electrical resistivity decreased with increasing amount of carbon phases, in both cases the nanofibers were more efficient than the nanotubes. The wear resistance in most cases decreased but for Si3N4–CNT composite a certain optimum (∼5wt.% CNT) was found. [Copyright &y& Elsevier]

The friction and wear behavior of ZrO2 +1.07wt.% carbon nanofiber composite and monolithic ZrO2 has been investigated using the ball-on-disk technique with alumina balls as friction partners in an unlubricated condition at room temperature. The coefficient of friction for the composite was found to be significantly lower compared to that for monolithic zirconia with relatively low wear rates for both materials. The main wear mechanism in the nanocomposite was abrasion accompanied with a pull-out of the carbon nanofibers which act in the interface as a sort of lubricating media. [Copyright &y& Elsevier]

Anisotropic slip activation of ZrB 2 grains and deformation behaviour of constituents of ZrB 2 -SiC and ZrB 2 -B 4 C 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 ZrB 2. 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 ZrB 2 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. [ABSTRACT FROM AUTHOR]

Abstract The wear resistance and tribological characteristics of spark plasma sintered ZrB 2 + B 4 C, ZrB 2 + SiC and ZrB 2 + ZrC composites were investigated under dry sliding conditions at applied loads of 5 N and 50 N in air. The microstructure, deformation and damage characteristics were studied using scanning electron microscopy, confocal electron microscopy, a focused ion beam and atomic force microscopy. The friction coefficient values were very similar for all composites with values ranging from 0.63 to 0.72 and with the lowest value recorded for the ZrB 2 + SiC composite at a 5 N applied load. The ZrB 2 + ZrC composite was the most wear resistant, with wear rates at a 5 N load of 6.15 × 10−6 mm3/(N·m) and at a 50 N of 7.3 × 10−6 mm3/(N·m). None, or a very limited number of grain pull-outs and/or lateral fractures of grains were found during the wear tests. At the 5 N load, abrasive grooves connected with deformations and Hertzian crack formations were the main wear mechanisms in all systems, with limited crack formations in the ZrB 2 + ZrC composite. Tribofilm formations connected with debris origin, oxidation and tribochemical reactions were dominant in all composites, with similar chemical compositions but different sizes and thicknesses at the 50 N load. Highlights • To enhance the oxidation resistance, COF, WR and KIC different additives (SiC, B 4 C and ZrC) have been introduced to ZrB 2. • Detailed understanding of the tribological characteristics of Zirconium Diboride composites. • Description and understanding of tribochemical reactions. [ABSTRACT FROM AUTHOR]

Deformation, damage and wear characteristics of spark plasma sintered VC, NbC and ZrC refractory carbides have been investigated using nanoindentation, tribology and micro/macro − indentation tests. Fractography, using SEM, AFM and confocal microscopy, was used for the characterization of deformation and damage mechanisms. Considerable indentation load − size effect was found in all systems with hardness values from 30 to 36 GPa to 13–17 GPa corresponding to the applied loads of 1 mN and 100 N, respectively. During nanoindentation, characteristic stress-drops were observed on hardness-displacement profiles of NbC and ZrC at depth region of 15–30 nm while this was not typical in VC. The highest coefficient of friction was measured for NbC with a value of 0.45 and the lowest for ZrC with an average value of 0.3. The wear rate of the NbC, and VC was similar, approximately 3 × 10 −6 mm 3 /Nm only the wear rate of ZrC was larger, approximately 2 × 10 −5 mm 3 /Nm. [ABSTRACT FROM AUTHOR]