Hot-press Sintering Technique Research Articles

Effect of Cr3+ doping concentration on the microstructure, mechanical properties and optical properties of Cr3+: SrF2 transparent ceramics

Cr3+: SrF2 nanopowders were synthesized using a chemical co-precipitation method. The nanoparticles exhibited a structure consistent with the pure SrF2 phase. Cr3+: SrF2 transparent ceramics were fabricated by hot-pressed (HP) sintering technique. The transmittance of 1 at% Cr3+: SrF2 transparent ceramics reached up to 83 % at 1054 nm, which is superior to other SrF2 transparent ceramics with different Cr3+ doping concentration. Furthermore, the mechanical properties of Cr3+: SrF2 transparent ceramics improved with increasing Cr3+ doping levels. The 1 at% Cr3+: SrF2 transparent ceramics exhibited the best mechanical properties, with a microhardness of 2.573 GPa, fracture toughness of 0.70 MPa·m1/2, and compressive strength of 38.1 MPa. The luminescence spectra and fluorescence lifetimes of the SrF2 transparent ceramics as a function of Cr3+ doping concentration were also analyzed and discussed.

Influences of binder phase and post-HIP treatment on tribological behavior of WC-based composite

In this study, WC-12Co and WC-12CoCrMo cemented carbides were prepared by the hot press sintering technique and subsequently post-treated by applying the hot isostatic pressing (HIP) technique. The effects of the binder phase and post-HIP treatment on the microstructures and mechanical and tribological properties of the two cemented carbides were investigated for ambient temperature use. The hardness and wear resistance of WC-12CoCrMo were increased by 87 % and two orders of magnitude compared to WC-12Co, respectively. The post-HIP did not affect the wear mechanism of cemented carbide. The binder phase was the fundamental factor in changing the wear mechanism. Post-HIP is a more viable option to enhance mechanical properties for complex carbide systems, as it promotes solid-state chemical reactions and results in an increased precipitation of secondary phases.

Thermal Conductivity of AlSi12/Al2O3-Graded Composites Consolidated by Hot Pressing and Spark Plasma Sintering: Experimental Evaluation and Numerical Modeling

Functionally graded metal matrix composites have attracted the attention of various industries as materials with tailorable properties due to spatially varying composition of constituents. This research work was inspired by an application, such as automotive brake disks, which requires advanced materials with improved wear resistance on the outer surface as combined with effective heat flux dissipation of the graded system. To this end, graded AlSi12/Al2O3 composites (FGMs) with a stepwise gradient in the volume fraction of alumina reinforcement were produced by hot pressing and spark plasma sintering techniques. The thermal conductivities of the individual composite layers and the FGMs were evaluated experimentally and simulated numerically using 3D finite element (FE) models based on micro-computed X-ray tomography (micro-XCT) images of actual AlSi12/Al2O3 microstructures. The numerical models incorporated the effects of porosity of the fabricated AlSi12/Al2O3 composites, thermal resistance, and imperfect interfaces between the AlSi12 matrix and the alumina particles. The obtained experimental data and the results of the numerical models are in good agreement, the relative error being in the range of 4 to 6 pct for different compositions and FGM structure. The predictive capability of the proposed micro-XCT-based FE model suggests that this model can be applied to similar types of composites and different composition gradients.

Preparation and Performance Study of SiC-Reinforced Fe-Based Wear-Resistant Composite Grinding Media.

During industrial and laboratory processes involving material grinding, the grinding media endure prolonged high-collision and friction environments, resulting in substantial wear. Consequently, this study adopts the hot-pressing sintering technique in powder metallurgy to prepare SiC-reinforced Fe-based wear-resistant composite grinding media, aiming to increase wear performance. For this purpose, Fe with 10 wt% SiC powders were milled for the fabrication of the composite. Then, sintering was performed by hot press at 1100 °C in a furnace. Scanning electron microscopy (SEM) and X-ray diffraction were employed to investigate the microstructures and phase of SiC-reinforced Fe-based matrix composite. Subsequently, comparative performance evaluations of the newly developed grinding media and traditional chromium-based media were conducted in terms of wear rate and grinding efficiency. The wear resistance tests revealed that the SiC-reinforced composite media displayed significantly superior wear resistance across various abrasives compared to the chromium-containing alternatives. Specifically, the composite media achieved a wear rate reduction of 2.9 times against standard sand over 1 h, and 2.3 and 2.4 times against sandstone and iron slag, respectively. Moreover, extended grinding for 3 hours further enhanced these reductions to 3.1, 2.4, and 2.7 times, respectively. Additionally, efficiency assessments indicated that at a 1:1 material ratio, the composite media outperformed the chromium-containing media in grinding efficiency by 7.5%, 12.5%, and 10.3% for standard sand, sandstone, and iron slag, respectively. Further increasing the material ratio to 3:1 resulted in efficiency improvements of 7.4%, 17.5%, and 11.3%, correspondingly.

Effect of sintering temperature on microstructure and properties of Cu/graphite composites hybridized with MoAlB ceramic particles

Using a fast hot press sintering technique, Cu/graphite composites doped with MoAlB ceramic particles were synthesized. The impact of varying sintering temperatures on the composites' mechanical, tribological, and microstructure characteristics were studied. The Al element diffused from the MoAlB particles into the copper matrix and produced the MoB phase when the sintering temperature rose over 850 °C. The formation of MoB phase, which was well bound to the matrix, and the formation of Cu(Al) solid solution could improve the plastic deformation resistance of the composites. This, along with the lubrication supplied by graphite, synergistically improved the mechanical and frictional properties of Cu/graphite composites. The composites exhibited remarkable wear resistance (the wear rate was reduced to 3.36 × 10−5 mm3·N−1·m−1) and high compressive strength (266.25 MPa) when the sintering temperature was set to 900 °C. In order to create high-performance copper-based self-lubricating composites, this effort may offer a new research direction.

Electrical sliding friction wear behaviors and mechanisms of Cu–Sn matrix composites containing MoS2/graphite

Cu–Sn matrix composites were prepared by vacuum hot press sintering technique with the addition of MoS2, graphite and a mixture of MoS2/graphite, respectively. The friction wear behaviors and contact resistance of the three composites under different currents were investigated and compared. The friction film formed on the friction surface was analyzed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The wear mechanism and the evolution of friction film composition of the composites under different currents were revealed. The results showed that MoS2 and graphite had synergistic lubrication effects. The Cu–Sn-Graphite-MoS2 composite exhibited low wear rates and the lowest and most stable contact resistance. MoS2 could effectively promote the formation and integrity of the friction film and optimize the friction wear performance. With the increase of current, the graphite content in the friction film rose, which helped to reduce the oxidation and consumption of the friction film. The increase in current accelerated the shedding of lubricating particles and the softening of lubricating film, making the lubricating film denser and reducing the oxidation depth.

Thermal Conductivity and Sintering Mechanism of Aluminum/Diamond Composites Prepared by DC-Assisted Fast Hot-Pressing Sintering.

A novel DC-assisted fast hot-pressing (FHP) powder sintering technique was utilized to prepare Al/Diamond composites. Three series of orthogonal experiments were designed and conducted to explore the effects of sintering temperature, sintering pressure, and holding time on the thermal conductivity (TC) and sintering mechanism of an Al-50Diamond composite. Improper sintering temperatures dramatically degraded the TC, as relatively low temperatures (≤520 °C) led to the retention of a large number of pores, while higher temperatures (≥600 °C) caused unavoidable debonding cracks. Excessive pressure (≥100 MPa) induced lattice distortion and the accumulation of dislocations, whereas a prolonged holding time (≥20 min) would most likely cause the Al phase to aggregate into clusters due to surface tension. The optimal process parameters for the preparation of Al-50diamond composites by the FHP method were 560 °C-80 MPa-10 min, corresponding to a density and TC of 3.09 g cm-3 and 527.8 W m-1 K-1, respectively. Structural defects such as pores, dislocations, debonding cracks, and agglomerations within the composite strongly enhance the interfacial thermal resistance (ITR), thereby deteriorating TC performance. Considering the ITR of the binary solid-phase composite, the Hasselman-Johnson model can more accurately predict the TC of Al-50diamond composites for FHP technology under an optimal process with a 3.4% error rate (509.6 W m-1 K-1 to 527.8 W m-1 K-1). The theoretical thermal conductivity of the binary composites estimated by data modeling (Hasselman-Johnson Model, etc.) matches well with the actual thermal conductivity of the sintered samples using the FHP method.

Enhancing the thermoelectric properties for hot-isostatic-pressed Bi2Te3 nano-powder using graphite nanoparticles

Bismuth telluride (Bi2Te3) is a promising thermoelectric material produced commercially. However, its poor electrical conductivity and low figure of merit, caused by grain boundaries and high thermal conductivity, limit its effectiveness in powder metallurgy production. Herein, effects of adding Graphite nanoparticles (GTNPs) to Bi2Te3 on thermoelectric properties were studied. Three ratios of GTNPs (0.2, 0.35, 0.5 wt%) were added to ball-milled Bi2Te3 nano-powder. The hot isostatic pressing (HIP) sintering technique was employed to prepare the pristine Bi2Te3 and the BT-xGTNPs samples for testing. The crystallographic measurements showed a reduction in the crystallinity of the BT-xGTNPs samples compared to the pure Bi2Te3, whereas the electron microscopy measurements showed smaller grain sizes. This was also confirmed with an increase in the samples’ relative density implying the formation of nano-sized grains. Full electrical, thermal, and thermoelectric measurements were performed and comprehensively discussed in this report for all samples in the temperature range from room temperature (RT) to 570 K. The measurements demonstrated an enhancement for x = 0.35 wt% GTNPs at 540 K up to 43% in the power factor and 51% in the ZT compared to pristine Bi2Te3, which was attributed to the optimum grain size, the lower grain boundaries, and better electrical and thermal conductivity aroused from the precise addition of GTNPs. The best electrical conductivity of ~ 8.2 × 104 S/m and lowest thermal conductivity of ~ 1 W/m·K for the Bi2Te3-containing 0.35 wt% GTNPs at RT even though the sample with 0.5 wt% attained the highest Seebeck coefficient of 154 µV/T at 540 K.

Effects of thermal, stress, and electric fields on microstructure and ion distribution of (Mg0.1Mn0.1Co0.1Ni0.2Ti0.1Cu0.1Zn0.2)Fe2.1O4 spinel ferrites

In this work, conventional sintering, hot-pressing sintering, and spark plasma sintering techniques were utilized to simulate the thermal field, thermal-stress field, and thermal-stress-electric field, respectively. The effects of these fields on the microstructure and cation distribution of the (Mg0.1Mn0.1Co0.1Ni0.2Ti0.1Cu0.1Zn0.2)Fe2.1O4 ferrite (HEF) material were comprehensively analyzed. The results indicate that in the temperature range of 1000–1100 °C, the thermal field provides the driving force for sintering, promotes grain growth (average particle size: 2→7 μm), and increases crystallinity degree. The stress field brings the particles into close contact with each other, facilitates diffusion, and delays grain growth (0.5→5 μm). The electric field reduces grain boundary energy, causes mass transfer, and makes the grains finer (0.5→0.7 μm) and more uniform. Mathematical model analysis reveals that the thermal field is beneficial to the random distribution of cations, which yields an inversion degree (γ) of about 2/3 and increases the configuration entropy of HEF. The stress field leads to lattice distortion, permits ions with larger radius to occupy tetrahedral positions, and changes the cell parameters and oxygen position parameter of HEF. Finally, the interaction between electric field and crystal charges increases the electrostatic field energy, which promotes the formation of positive spinel structure and maximizes the crystal field stability energy. X-ray photoelectron spectra, Raman spectra, and change in lattice constant confirm the rationality of the mathematical model, and can be used to establish the model of the crystal structure under field effects.

A new type of high flexural strength micro-nano-composite ceramic tool material Ti(C,N)/WC/ZrO2 for integrated micro milling cutter

Ti(C,N) composites are widely applied in various advanced manufacturing industries. However, the intrinsic brittleness and relatively high metal content for Ti(C,N) composites significantly limit their further applications, especially in high-temperature and high-speed machining conditions. Herein, a new type of high flexural strength micro-nano-composite ceramic tool material Ti(C,N)/WC/ZrO2 for integrated micro milling cutter which will be used to mill difficult-to-machine material such as titanium alloy was proposed and developed successfully by hot press sintering technique under vacuum. The Ti(C,N) composites with low metal phase strengthened by nano-3Y-ZrO2 and micro-WC particles, the effect of compositions and sintering parameters on the mechanical properties and microstructure of the composite ceramic materials were systematically investigated. The results demonstrated that the Ti(C,N) composites with low metal phase containing 15 wt% nano-3Y-ZrO2 and 15 wt% micro-WC, sintered at 32 MPa and 1500 °C for 45 min had the optimal mechanical properties under vacuum and the flexural strength, Vicker's hardness, fracture toughness and relative density were 1501 ± 86 MPa, 19.97 ± 0.43GPa, 7.25 ± 0.76 MPa.m1/2 and 99.55%, respectively. Meanwhile, the superior mechanical properties of the Ti(C,N) composites with low metal binder phase could be achieved when sintered at higher temperatures and longer holding times compared to conventional cerments. Moreover, the toughening and strengthening mechanisms for Ti(C,N) composites with low metal phase are the synergistic effects of crack bridging, crack branching, crack deflection, pinning effect, particle pull-out, intragranular microstructure, transgranular and intergranular fracture. These toughening and strengthening mechanisms and the solid solution effects generated by the joint action of oxides and carbides could remarkably improve the properties for the Ti(C,N) composites. In this study, a novel insight on developing high-property Ti(C,N) composites was proposed, and it will make a significant contribution to material synthesis and design, application for cutting tools and composite ceramics investigations.

Advance processing of cluster-free CNT reinforced 6082 Al matrix nanocomposites: influence of mechanical milling and cryomilling

ABSTRACT In the present study, lightweight high-strength carbon nanotubes (CNTs) reinforced aluminium-based composite was fabricated through powder metallurgical processing. The effect of planetary milling and cryogenic assisted milling on the structure, morphology, mechanical properties, phase composition, and thermal stability of CNTs reinforced Al6082 Al nanocomposites powders was investigated using X-ray diffraction (XRD), scanning electron microscope (SEM)- Energy dispersive spectroscopy (EDS), transmission electron microscope (TEM), and indentation techniques. The CNTs in the Aluminium (AL) matrix were distributed uniformly after 50 h of milling. Further, the cryomilling of 50 h milled Aluminium matrix composites (AMC) decreases agglomeration even more and was successful in cleaving the nanotubes as well as minimising entanglements. The broadening of diffraction peaks suggests a reduction in the crystallite size (from ~36 to ~20 nm) and an increase in induced strain. Milled powders were consolidated through the hot-press and spark plasma sintering technique. The mechanical properties of these AMCs were evaluated through indentation for hardness (>250 HV) and compressive testing techniques. SPSed samples reported better properties in comparison to hot-pressed samples. It is discovered that post-CM of Mechanical Milling (MM) powder aids in overcoming issues related to the agglomeration of Al/CNT composites, and the goal to fabricate tangle-free CNT reinforced composite was achieved. Moreover, it showed enhanced mechanical properties with a maximum yield strength of 210 MPa and good synergy between strength and ductility.

Preparation of graphene film reinforced CoCrFeNiMn high-entropy alloy matrix composites with strength-plasticity synergy via flake powder metallurgy method

Inspired by the design principle of pearl structure, a bottom-up flake powder self-assembly arrangement strategy, flake powder metallurgy, is used to prepare graphene films (GFs) reinforced CoCrFeNiMn high-entropy alloy (HEA) matrix composites with a pearl laminated structure. Flaky HEA powder was prepared by ball milling method and homogeneously mixed with Ni plated GFs. Vacuum hot-press sintering (VHPS) technique was carried out to solidify the mixed powders to obtain composites with uniform distribution of GFs(Ni) and flaky HEA. The results show that the bottom-up preparation strategy can effectively fabricate bionic laminated HEA matrix composites, and the composites have a distinct pearly laminated structure. The tensile strength of the composites with 5 vol% GFs(Ni) content reached 834.04 MPa, and the elongation reached 26.58 %. The compressive strength in parallel and perpendicular laminar directions reached 2069.66 MPa and 2418.45 MPa at 50 % strain, respectively. The laminated GFs(Ni)/HEA matrix composites possessed excellent strength and maintained good plasticity. In this study, the strengthening and toughening mechanism of the laminated GFs(Ni)/HEA matrix composites is discussed in detail, and the results show that the laminated structure and GFs(Ni) are favorable for the hardening and strengthening of the HEA matrix.

Preparation and tribological properties of NiCr–Mo–Ag–O/Mo–V–Ag–O bilayer film at RT-1000 °C

The primary objective of this research is to investigate the structure and tribological properties of NiCr–Mo–Ag–O/Mo–V–Ag–O bilayer films in a wide temperature range (RT-1000 °C). The bilayer films were prepared by pulsed laser deposition (PLD) technology, so as to provide the theoretical basis and experimental guidance for the study of the synergistic lubricating effect of Mo and V bimetallic lubricating oxides. The NiCr–Mo–Ag–O composite target was fabricated by the hot-press sintering technique. Subsequently, a NiCr–Mo–Ag–O/Mo–V–Ag–O bilayer film was deposited on Inconel 718 by pulsed laser deposition (PLD). The tribological properties of the film were evaluated by HRT-1000 high-temperature ball-on-disc configuration at RT-1000 °C. The obtained results reveal that the NiCr–Mo–Ag–O target is marked by a compact structure and homogeneous composition. Furthermore, the composite film exhibits a smooth surface and dense structure, comprised of polycrystalline and amorphous phases. The measured hardness and elastic modulus of the film approximated 4.9 GPa and 153.5 GPa, respectively. Additionally, the lowest friction coefficient of the film is about 0.28 at 800 °C. Upon reaching this temperature, the cooperative lubrication resulting from NiMoO4, Ag2Mo2O7, and AgVO3 contributed effectively towards reducing the film's friction coefficient.

Reinforcing potential of MWCNTs on mechanical and machining performance of hot-pressed ZTA-MgO ceramic cutting inserts

A new generation cutting insert was manufactured by multi-walled carbon nanotube (MWCNT) reinforcement (0–0.25 weight percentage (wt%)) into zirconia toughened alumina (ZTA)- magnesium oxide (MgO) ceramic matrix using a powder metallurgy route. The hot-press sintering technique was used to compact the uniformly mixed ZTA-MgO-MWCNT powders, sintered at 1500 °C followed by morphological, mechanical and functional characterizations. The relative densities of the samples were between 98 % and 99 %. The mechanical properties of the matrix were improved up to 0.15 wt% MWCNTs. The highest microhardness was ~2366 HV0.5 for 0.15 wt% due to grain refinement by synergistic pinning between spinel (MgAl2O4) and MWCNTs. Similarly, the highest indentation fracture toughness was ~7.33 MPa.m1/2 for 0.15 wt% MWCNT content due to micromechanical interlocking mechanism from a combined effect of MWCNT size variation and inter-particle distance between MWCNTs featuring proper crack bridging and crack deflection. To function as a cutting insert, the sintered samples up to 0.15 wt% was selected and shaped according to SNGN120408 cutting tool geometry. For minimum cutting force with high surface roughness, the optimized parameters were observed as a cutting speed of 300 m/min, feed rate of 0.12 mm/rev and 0.2 mm depth of cut using the design of experiment technique while machining AISI 4340 steels. Furthermore, regression models were developed to correlate the machinability with machining conditions. The comparative study suggested that the significant improvement of cutting force (~26.4 %), surface roughness (~55.3 %) and tool life (~29.4 %) was achieved for ZTA-MgO-0.15 wt% MWCNT ceramic tool with respect to ZTA-MgO inserts depending upon self-lubrication phenomenon alongside with the retention of proper tool geometry for a longer tool life ~44 min. The morphological and Raman spectra analysis of the worn region justifies the self-lubricating phenomenon at the interface by breaking and rolling of MWCNTs.

Energy transfer and color tunable up-conversion luminescence in PrF3 and YbF3 co-doped SrF2 transparent ceramics

Near-infrared pumped up-conversion luminescent PrF3 and YbF3 co-doped SrF2 ceramics were fabricated by the hot-pressing sintering technique, and the pellets were transparent with a primrose yellow appearance. The influence of YbF3 co-doping levels on the optical qualities and microstructures of ceramics was investigated. We identified the mechanisms to realize up-conversion luminescence in the visible range from 3P0,1 levels (Pr3+) under near-infrared wavelength pumping sources other than the near ultraviolet to blue region photons. Yb3+ ions inducing a broad and intense absorption band at the 900–1050 nm region, and the absorption coefficient and cross-section at 980 nm are 7.57 cm−1 and 4.19 × 10−21 cm2 in the ceramic co-doped with 15 wt% YbF3, respectively. The intense up-conversion luminescence bands at 480, 520, 540, and 600 nm under 980 nm excitation were attributed to 3P0→3H4, 3P1→3H5, 3P0→3H5 and 1D2→3H4 transitions of Pr3+ ions, respectively.

Trapping Magnetic Field in Bulk Iron-based Superconductor Sintered under High Pressure

We fabricated two bulk superconductors of (Ba,Na)Fe2As2 using a cubic press and hot press sintering technique. For bulk #1 sintered by cubic press, despite its small dimensions, with diameter 4.1 mm and height 3.64 mm, a trapped field value of ~9.97 kG has been achieved on its surface, which is only 230 G lower than the value reported in a bulk with a diameter of 10 mm before. Critical current density (J c) is estimated to be 81.2 kA cm−2 at 4.2 K under self-field, which is larger than the value reported before. It suggests that the improvement of J c achieved by high pressure could make up for the negative influence from the smaller dimensions and achieve a large trapped field. In the second larger bulk #2 sintered by hot pressing technique, with diameter 9.45 mm and height 4.58 mm, the value of J c is ~ 33.9 kA cm−2 and the trapped field is ~8.81 kG on the surface. The expected better texturing has not been observed. Possible origins for inferior properties of bulk #2 are discussed.

Self-lubricating Ti3SiC2 strengthening Cu composites with electrical conductivity and wear resistance trade off

Introduction of hard particles into the soft copper is an effective method to strengthen copper and improve its wear resistance, while it normally sacrifices the electrical conductivity. The type and content of the addition phase have great influence on the electrical conductivity and wear resistance trade off. Herein, we introduced self-lubricating and conductive Ti3SiC2 particles into the soft copper, and self-lubricating Ti3SiC2/Cu composites were fabricated by vacuum hot pressing sintering technique. It was found that dense composites with homogeneous Ti3SiC2 distribution was obtained when addition content of Ti3SiC2 was relatively low, less than 20 vol% Ti3SiC2 additions, while the composite became porous after larger content, especially over 40 vol%. Introduction of Ti3SiC2 significantly enhanced the wear resistance without obviously sacrificing of electrical conductivity. The composites with 20 vol% Ti3SiC2 content had the lowest wear rate of 3.135 × 10−5 mm3 N−1 m−1 with conductive 63% IACS. Self-lubricating film was detected on the friction surface, which sharply reduced the composite’s wear rates. At the same time, hardness of the composite was obviously increased by 98% compared with copper, up to 121 HV, which was beneficial to the improvement of wear resistance. Introduction of Ti3SiC2 has been demonstrated to obviously strengthen copper with electrical conductivity and wear resistance trade off.

Effects of Ho3+ concentration on the fabrication and properties of Ho: Y2O3-MgO nanocomposite for Mid-infrared laser applications

Infrared transparent Ho: Y2O3-MgO nanocomposite ceramics with a volume ratio of 50:50 (RE2O3: MgO) were prepared by combining sol-gel powder synthesis and hot-pressing sintering techniques. In order to obtain Ho: Y2O3-MgO nanocomposite ceramics with fine grain size, dense microstructure and homogeneous phase domains, the effect of sintering temperature and Ho3+ doping concentration were studied. Transmittance and SEM measurement revealed that the grain size of 3 at.% Ho: Y2O3-MgO ceramic sintered at 1250 °C is 141 nm, and the transmission is up to 85.2% at 5 μm. The detailed spectroscopic investigation of x at.% Ho: Y2O3-MgO (x = 1, 3, 5, 7, 9, 15) ceramics was performed. The nanocomposites exhibited photoluminescence properties similar to that of Ho: Y2O3 crystals and ceramics. In addition, the thermal conductivity of 3 at.% Ho: Y2O3-MgO ceramic is 13.04 W/m·K, which is superior to that of Ho:Y2O3 ceramics. The high transmission, excellent thermal conductivity, and outstanding optical characteristics indicated that Ho: Y2O3-MgO ceramics is a promising material for efficient infrared solid-state laser.

The microstructure characteristics and self-lubrication mechanism of the NiAl matrix composites prepared by vacuum hot-pressing sintering

NiAl–Ti3AlC2 and NiAl–Mo2TiAlC2 composites were fabricated in-situ using a vacuum hot-pressing sintering technique. The microstructure and high temperature tribological performance of the composites were studied. When compared with NiAl–Ti3AlC2 composite, the NiAl–Mo2TiAlC2 composite contained the Mo2TiAlC2 precipitated phases and some nano-precipitated phases of Mo2TiAlC2 distributed along the grain boundaries. Meanwhile, the high temperature tribological property of NiAl–Mo2TiAlC2 composite was more superior than that of NiAl–Ti3AlC2 composite. The worn surface of NiAl–Mo2TiAlC2 composite formed a dense and continuous lubricating layer of MoO3 when subjected to the friction test at 800 °C, which could effectively improve the overall tribological properties of the composites.

Phase evolution and thermal stability of novel high-entropy (Mo0.2Nb0.2Ta0.2V0.2W0.2)Si2 ceramics

Owing to the high melting points and high-temperature stability, transition-metal disilicides are potential components for aerospace, automotive, and industrial engineering applications. However, unwanted oxidation known as PEST oxidation severely limits their application owing to the formation of volatile transition metal oxides, especially in the temperature range of 500–1000 °C. To overcome this problem, a new class of high-entropy disilicides, (Mo0.2Nb0.2Ta0.2V0.2W0.2)Si2, was selected by first-principles calculations and then successfully fabricated using a hot-pressing sintering technique. Furthermore, the phase evolution, thermal expansion behavior, thermal conductivity, and oxidation behavior were systematically investigated. Compared with MoSi2, (Mo0.2Nb0.2Ta0.2V0.2W0.2)Si2 possessed a lower thermal conductivity (10.9–14.7 W·m−1·K−1) at 25–1000 °C, higher thermal expansion coefficients (8.6 ± 1.3–6 K–1) at 50–1200 °C, and especially an excellent thermal stability at 500–1000 °C owing to slow diffusion and selective oxidation. This work provides a strong foundation for the synthesis and application of high-entropy disilicides.