A dense, fine‐grained, and toughened nonstoichiometry high entropy carbide (HEC) (MoNbTaTiW)C4.5 was synthesized via a two‐step way of carbothermal reduction‐solid solution using metal oxides and ...carbon. The effects of carbothermal reduction temperature, sintering temperature, and carbon content on the phase, microstructure, mechanical and thermal properties of (MoNbTaTiW)C4.5 were studied. The results showed dense and single phase (MoNbTaTiW)C4.5 were synthesized through reduction of metal oxides at 1600°C and sintered at and above 1950°C by spark plasma sintering. Reduced carbon content and higher solid solution temperature were beneficial to obtain dense HECs with better mechanical properties. With a carbon content of 90%, a reduction temperature of 1600°C, a holding time of 20 min, and a sintering temperature of 1950°C, the as‐synthesized (MoNbTaTiW)C4.5 showed a higher fracture toughness (5.2 MPa·m1/2). Besides, the (MoNbTaTiW)C4.5 showed a low thermal conductivity (5.9 W/m·K, at room temperature).
In this study, diamond/silicon carbide (SiC) composites were prepared by liquid Si infiltration. The effects of the infiltration temperature on the densification, diamond graphitization, and thermal ...conductivity of the composites were investigated. The effects of the diamond content on the thermal conductivity and bending strength were also explored. X‐ray diffraction and Raman spectroscopy detected diamond graphitization at an infiltration temperature of 1480°C. Almost no graphitized diamond was observed due to reactions between silicon and carbon to form SiC at infiltration temperatures higher than 1480°C. As the infiltration temperature increased, graphitized diamond was consumed, and the thermal conductivity of the composite rose. The thermal conductivity and bending strength of the composites depended on the diamond content, reaching values of 257 W/m K and 237 MPa, respectively, for the sample with a diamond content of 17.9 vol.%. The higher bending strength was mainly ascribed to the high diamond content and well‐bonded interface.
AlxCoCrFeNiTi (x = 0.1, 0.3, 0.6, 1) powders were prepared via mechanical alloying and were used as binders for SPS-produced Ti(C,N)-based cermets. The effects of AlxCoCrFeNiTi binder on phase ...composition, morphology, room-temperature mechanical properties, and oxidation resistance of cermets were studied. The research showed that cermets with AlxCoCrFeNiTi binders exhibited a more homogeneous core–rim structure than cermets with cobalt binders. The Vickers hardness and fracture toughness of cermets with AlxCoCrFeNiTi binders increased with the aluminum molar ratio due to the grain refinement and solid solution strengthening effect of carbonitrides. After static oxidation at 1000 °C, the mass gain of the cermets with AlxCoCrFeNiTi binders changed according to a quasi-parabolic law, and the lowest mass gain was obtained in the cermet with Al0.6CoCrFeNiTi binder. The oxidation kinetics curve of the benchmark cermet with cobalt followed a linear law. The oxidation product of Ti(C,N)-based cermet with cobalt was rich in TiO2, and the Ti(C,N)-based cermets with AlxCoCrFeNiTi binders were transformed into complex oxides, such as NiMoO4, NiWO4, FeMoO4, Fe3Ti3O9, and Ni3TiO7. The oxide layer on the cermet with Al0.6CoCrFeNiTi appeared to be dense and protective, which inhibited the diffusion of oxygen into the cermet and improved the oxidation resistance of the final product.
High-entropy carbide (NbTaTiV)C4 (HEC4), (MoNbTaTiV)C5 (HEC5), and (MoNbTaTiV)C5-SiC (HEC5S) multiphase ceramics were prepared by spark plasma sintering (SPS) at 1900 to 2100 °C, using metal carbide ...and silicon carbide (SiC) as raw materials. Their microstructure, and mechanical and tribological properties were investigated. The results showed that the (MoNbTaTiV)C5 synthesized at 1900–2100 °C had a face-centered cubic structure and density higher than 95.6%. The increase in sintering temperature was conducive to the promotion of densification, growth of grains, and diffusion of metal elements. The introduction of SiC helped to promote densification but weakened the strength of the grain boundaries. The average specific wear rates for HEC4 were within an order of magnitude of 10−5 mm3/N·m, and for HEC5 and HEC5S were within a range of 10−7 to 10−6 mm3/N·m. The wear mechanism of HEC4 was abrasion, while that of HEC5 and HEC5S was mainly oxidation wear.
The (VNbTaMoW)C
5
–SiC high-entropy ceramics were prepared by spark plasma sintering at 1900°C and 40 MPa. The effects of SiC content (0–30 wt.%) on the microstructure, mechanical properties, and ...tribological properties were examined. The results showed that the matrix phase (VNbTaMoW)C
5
exhibited a face-centered cubic structure, and the second phase (SiC) was uniformly distributed, inhibiting excessive grain growth. The relative density of (VNbTaMoW)C
5
– SiC composite ceramics decreased first and then dropped as SiC content increased. The fracture mode of (VNbTaMoW)C
5
–SiC composite ceramics changed from transgranular to mixed (transgranular fracture and intergranular) fracture with an increase in SiC content due to weak bonding between (VNbTaMoW)C
5
and SiC. The grains of the (VNbTaMoW)C
5
in multiphase ceramics were refined because of the grain growth-inhibiting effect of SiC. With the increase in SiC content, the hardness of (VNbTaMoW)C
5
–SiC multiphase ceramics increased, and the fracture toughness first increased and then decreased. The (VNbTaMoW)C
5
–20 wt.% SiC multiphase ceramics exhibited the best mechanical properties with Vickers' hardness and fracture toughness of 18.2 GPa and 5.7 MPa ∙ m
1/2
, respectively. Coupled with WC, (VNbTaMoW)C
5
–SiC multiphase ceramics exhibit good wear resistance with a specific wear rate of (5.7–8.1) ∙ 10
–8
mm
3
/N ∙ m.
The tribological behavior of self-mated Ti
3
SiC
2
is investigated from ambient temperature to 800 °C at a sliding speed of 0.01 m/s in air. The results show that at the temperatures lower than ...300 °C, friction coefficient and wear rates are as high as 0.95 and 10
−3
mm
3
/N m, respectively. With the temperature increasing to 600 °C, both the friction coefficient and wear rates show consecutive decrease. At 700 and 800 °C, friction coefficient and wear rates are 0.5 and 10
−6
mm
3
/N m, respectively. According to the wear mechanism, the tribological behavior of Ti
3
SiC
2
can be divided into three regimes: mechanical wear-dominated regime from ambient temperature to 300 °C characterized by pullout of grains; mixed wear regime (mechanical wear and oxidation wear) from 400 to 600 °C; and tribo-oxidation-dominated wear regime above 700 °C. The tribo-oxides on the worn surfaces involve oxides of Si and Ti. And, species transformation occurs to these two oxides with the increasing temperature. In the competition oxidation of elements Ti and Si, Si is preferably oxidized because of its high active position in the crystal structure. Additionally, plastic flow is another notable characteristic for the tribological behavior of self-mated Ti
3
SiC
2
.
The tribological behavior of self-mated Ti
3
SiC
2
in bath of alcohols are investigated at sliding velocity ranging from 0.005 to 0.3 m/s. The results show that the friction coefficient and wear ...rates of self-mated Ti
3
SiC
2
are reduced greatly under lubrication of alcohols compared to that under dry condition. In alcohols, smooth worn surfaces of Ti
3
SiC
2
can be obtained. The mechanical wear is inhibited and the oxidized Ti and Si species on the worn surface are TiO
2
and silica gel. The friction coefficients decrease with carbon numbers and sliding velocity in the n-alcohols. The friction coefficient decrease with sliding velocity in glycol and glycerol as well. The decrease is determined by the increase of the viscosity of alcohols. In glycerol, at 0.1 m/s and 5 N, the friction coefficient of self-mated Ti
3
SiC
2
is 0.08 which is the lowest in this paper. The lubrication regimes are calculated according to classical lubrication theory. At velocity from 0.005 to 0.3 m/s, in all of the alcohols except glycerol, the λ are below 1 which indicates that the lubrication regime is BL. While in glycerol, the lubrication regimes vary from BL to BL and EHL, then to EHL as the velocity increases.
The tribological behavior of self-mated Ti
3
SiC
2
in low viscosity fluids, that is, water and ethanol, is investigated. The results show that both water and ethanol effectively reduce the friction ...and wear of self-mated Ti
3
SiC
2
. The friction coefficient and total wear rates of Ti
3
SiC
2
tribo-pair are 0.82 and 2.17 × 10
−2
mm
3
/N m under dry condition, 0.41 and 6.50 × 10
−4
mm
3
/N m in water and 0.16 and 2.22 × 10
−4
mm
3
/N m in ethanol. As sliding distance is prolonged from 0.5 to 22.9 km, the total wear rates of Ti
3
SiC
2
in ethanol can be further reduced. Mechanical wear of Ti
3
SiC
2
is partly inhibited in water and greatly inhibited in ethanol. Surface polishing of Ti
3
SiC
2
pin and tribochemistry are the two distinct characteristics of self-mated Ti
3
SiC
2
in ethanol. The surface film in ethanol is composed of TiO
2
and SiO. Tribo-products in ethanol, that is, silica gel and titanium dioxide, are found. Polycondensation of ethanol by the catalysis of TiO
2
and SiO
2−
x
gives birth to paraffins and olefins.
•Single-phased high-entropy ceramics of (VNbTaMoW)C5 could be prepared at 1600 °C.•Element distribution homogeneity and microstructure could be improved by increasing the SPS temperature.•Metal ...oxides were formed at grain boundaries at the different sintering temperature.
High-entropy ceramics (HEC) with a fixed composition of (VNbTaMoW)C5 were prepared by spark plasma sintering (SPS) from 1500 °C to 2200 °C. XRD, TEM, HRTEM, SAED and EDX were used to investigate effects of the sintering temperatures on compositional homogeneity, constituent phases and microstructure of the HECs. The results showed that single-phase HEC formed at a temperature as low as 1600 °C while ultimate elemental distribution homogeneity could be obtained at 2200 °C. Elemental distribution homogenization was accompanied by microstructural coarsening and oxide impurities aggregating at grain boundaries as temperature increased. SPS at 1900 °C for 12 min could yield uniform HECs (VNbTaMoW)C5 with Vickers hardness, nanohardness, fracture toughness and Young’s modulus reaching 19.6 GPa, 29.7 GPa, 5.4 MPa m1/2 and 551 GPa, respectively. The resultant HECs showed excellent wear resistance when coupled with WC at room temperature.
ZrB2-20%volSiC composites were hot pressed with ZrB2 and SiC powders and the effect of temperature and friction couples (WC or Al2O3) on the tribological properties of these composites was ...investigated. The results show that the coefficient of friction (CoF) of ZrB2–SiC composites coupled with WC or Al2O3 was almost constant with respect to temperature (0.5–0.6). Meanwhile, their wear rates (WRs) were of the order of 10−4 mm3 N−1 m−1. Either discontinuous (with WC) or continuous (with Al2O3) tribo-layers were found to have been formed on the worn surfaces of the composites and these tribo-layers did not exhibit any lubricious or anti-wear effects. At temperatures of 25–400 °C, the wear mechanism of the composites coupled with WC and Al2O3 included abrasion, tribo-oxidation, and adhesion. At higher temperatures (600–800 °C), the wear mechanism of both tribo-pairs included tribo-oxidation and adhesion.
•There are three main findings in our paper.•CoF of ZrB2–SiC composites coupled with WC or Al2O3 almost remain unchanged from 200 °C to 800 °C.•Tribo-layers are formed on the worn surface of the composites but they have no lubrication or anti-wear effect.•At elevated temperatures, the wear mechanism of both tribo-pairs are tribo-oxidation and adhesion.
