Hot-press Sintering Research Articles

Densification mechanism and corrosion properties of h-BN/MgAl2O4 ceramics prepared by hot-pressed sintering

The dense h-BN/MgAl2O4 composites without additives were fabricated by hot-pressed sintering. The effect of sintering temperature and MgAl2O4 on the bulk density, mechanical properties and corrosion resistance were systematically investigated. The results showed that the collaboration of sintering temperature and MgAl2O4 dominated the densification behavior due to the enhanced pore filling effects and solid diffusion. The improved densification and induced microstructure development contributed to enhanced mechanical properties. Additionally, the residual MgAl2O4 is more conducive to forming plate-like complex oxide with molten steel after BN oxidation and B2O3 volatilization. Consequently, the plate-like complex oxide can effectively hinder direct contact between molten steel and the composite, which determines the corrosion resistance of the h-BN/MgAl2O4 composite. This study highlights the enormous potential of h-BN/MgAl2O4 composites for future use as side-sealing materials.

Tailoring the additional content of short carbon fiber in the reduced graphene oxide-Cu self-lubricating composites for enhanced mechanical and tribological performance

The reduced graphene oxide-Cu self-lubricating composites with short carbon fiber fillers were fabricated by powders wet-mixing combined with hot-pressed sintering process. The impacts of short carbon fiber content on microstructure, mechanical and tribological performance of reduced graphene oxide-Cu composites were characterized. The reduced graphene oxide/carbon fiber hybrid fillers were randomly distributed in the sintered bulk compacts. The hybrid filled composites achieved enhancement in mechanical and tribological properties. With increasing carbon fiber content, hardness and compressive yield strength of the prepared composites were increased, and both friction coefficient and wear rate showed a constant decrease trend under different loads. Owing to the synergy effect of reduced graphene oxide/carbon fiber hybrid lubricant fillers during sliding together with the greatly enhanced hardness and yield strength, up to 0.6 wt% carbon fiber incorporation resulted a lowest friction coefficient and decreased wear rate. Randomly distributed reduced graphene oxide/carbon fiber hybrid fillers provided the lubrication to reduce friction and wear for Cu matrix.

Microstructure evolution and mechanical properties of bioinspired interpenetrating Ti2AlNb/TiAl matrix composite with a crossed-lamellar structure

TiAl alloys with low density, high creep resistance and high temperature performance are considered as candidate materials to replace nickel-based superalloys in the range of 700∼800 °C. However, the intrinsic brittleness of TiAl alloys has always been the biggest bottleneck restricting their development. In this paper, a bioinspired interpenetrating Ti2AlNb/TiAl composite with crossed-lamellar structure was prepared by combining selective laser melting (SLM) and vacuum hot press sintering (HPS) under the condition of 1150 °C/1 h/45 MPa, to improve the strength and toughness of the composite. Meanwhile, the metallurgical defects and microstructure of Ti2AlNb reinforcement skeleton printed under different volume energy densities (VEDs) were investigated, as well as the evolution of the microstructure at the interface region of the composite was systematically studied. What's more, we studied the mechanical properties of the composite including nanoindentation test, room temperature tensile and bending tests. The results show that the VED is 88.89 J/mm3, an almost completely dense reinforcement skeleton (∼99.8 %) is obtained. The interface region can be divided into four different reaction layers, namely LⅠ, LⅡ, LⅢ and LⅣ, due to the diffusion of elements. LⅠ is mainly composed of Othick/thin lath-like phase and O short rod-like phase. LⅡ is mainly composed of B2/β phase, acicular α2 phase and nanoscale ω-Ti3NbAl2 phase. The LⅢ mainly consists of B2/β phase. The LⅣ is composed of α2 phase. The deformability of each phase in the composite: B2/β phase > O phase >γ phase >α2 phase >ω phase. The tensile strength and fracture toughness of bioinspired interpenetrating Ti2AlNb/TiAl matrix composite are increased by 24.0 % and 89.0 %, respectively, compared with TiAl alloy, which is mainly contributed to the strong interfacial bonding between matrix and reinforcement as well as the synergistic effect of Ti2AlNb reinforcement with high strength and toughness.

Characterization of (Zr,Ti)B2-SiC composites obtained by hot press sintering of ZrB2-SiC-TiO2 powder mixtures

The ability to enhance mechanical and oxidation properties for severe environmental applications has led to substantial academic interest in multiphase ultra-high temperature ceramics (UHTC). The purpose of this work is to study the in-situ solid solution formation of (Zr,Ti)B2 from ZrB2 and TiO2 in a ZrB2-SiC composite using hot pressing reaction sintering. For this, a mixture of 10, 20, and 30 % vol% SiC with ZrB2 was mixed with 2.0 wt% TiO2. Hot pressing sintering was performed with a load of 20 MPa at a final temperature of 1850 °C/30 min in an argon atmosphere. The microstructures, crystalline phases, densities, mechanical properties, and oxidation resistance of the composites were examined and compared with ZrB2-SiC samples lacking TiO2. In samples where TiO2 was added, the matrix grain size slightly decreased, the fracture mode was mainly intergranular, and the SiC grain morphology changed the aspect ratio to be more equiaxed. The solid solution (Zr,Ti)B2 was produced, and it was demonstrated by EDS elemental map images and the XRD analysis that Ti atoms incorporate into the ZrB2 crystalline structure. The development of solid solutions showed no impact on relative densities or Vickers hardness. However, the solid solution formation favored an improvement in fracture toughness, probably owing to the smaller matrix grain size and intergranular fracture mode. Samples exhibiting (Zr,Ti)B2 formation presented lower oxidation resistance than undoped samples in the same oxidizing condition.

Effect of element N on microstructure and tribological properties of CoCrFeNiV alloy at severe temperature

In this study, CoCrFeNiV non-equiatomic high entropy alloys (HEAs) with varying N content were prepared using vacuum hot-press sintering. The effects of N content on the alloy's microstructure, mechanical properties, and tribological behavior at different temperatures were investigated. The results indicate that the N-added alloys primarily maintain an FCC phase. With increasing N content, the σ phase in the alloy gradually decreases, while the VN content increases, leading to grain refinement. This results in a 10.7 % increase in hardness, a 9 % increase in compression strength, and a 56 % increase in compression deformation. Below 400 °C, the primary wear mechanisms are adhesive wear, spalling wear, and slight oxidative wear, while above 600 °C, oxidative wear dominates. The addition of N reduced the wear rates of the alloy by 33.8 %, 8.1 %, and 31.3 % at RT, 200 °C, and 600 °C, respectively, and decreases the coefficient of friction at all temperatures. In summary, N addition improves the alloy's tribological performance at various temperatures.

Effect of Sintering Temperature on the Microstructure and Mechanical and Tribological Properties of Copper Matrix Composite for Brake Pads

Copper-based powder metallurgy materials are frequently utilized in fabricating brake pads for high-speed trains. The preparation process involves mixing, ball milling, pressing, and sintering. Among these steps, hot-pressed sintering stands out as a rapid and efficient method that significantly influences the properties and performance of the products. In this study, four samples (S700/S750/S800/S850) were prepared using hot-pressed sintering at various temperatures, as follows: 700 °C, 750 °C, 800 °C, and 850 °C. The mechanical and physical properties of the four samples were tested, and the microstructure and compositions were investigated using scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. The findings highlighted the close relationship between sintering temperature and the mechanical and physical properties of the samples, as it impacts the porosity and interfacial bonding of the particles. Notably, Sample S800 demonstrated superior mechanical and thermal conductivity. Furthermore, the coefficient of friction (COF), friction heat, and wear rate of the four samples were also tested under different braking speeds ranging from 150 km/h to 350 km/h. The results indicated that the COFs of the four samples remained relatively stable below 300 km/h but decreased notably above 300 km/h due to heat fading. Sample S800 displayed consistent and high COF under varied braking speeds and exhibited the lowest wear rate. The observed wear mechanisms included abrasive wear and oxidation wear. Additionally, the friction test results underscored the close correspondence of the COF curve of S800 with the standard of the Ministry of Railways of the People’s Republic of China.

Evaluation on the microstructure and corrosion properties of in situ prepared FeCrNiCo-boride cermet by hot press sintering

Traditional studies on boride cermet have rarely considered the effects of the synergistic interactions among their multiple components on their microstructure and properties. In this study, FeCrNiCo-boride cermet is prepared for the first time via hot-press sintering. The microstructure evolution and corrosion resistance of the cermet are investigated for sintering temperatures in the range of 1000–1300 °C. The results show that the phase comprises Mo2FeB2, MoB2, and FCC phases (including that of FeCrNiCo or various intermetallic solid solutions formed by FeCrNiCo decomposition). Owing to its low Gibbs free energy, Mo2FeB2 occurs in the hard phase. Cr atoms dissolve into the crystal lattice and stabilise the Mo2FeB2 crystals. A certain amount of MoB2 is also present in the hard phase. The bond phase is formed by alloying elements and solid solutions. The cermet sintered at 1100 °C exhibits the least structural defects, highest cermet content, and best corrosion resistance; the porosity is 9.94 % and phase ratio of cermet is 48.9 %. The cermet phase has the smallest grain size of 5.96 μm. The equilibrium potential (Ecorr) is −0.52 V, corrosion current density (Icorr) is 2.07 × 10−6 A/cm2, and open-circuit potential (OCP) in a 3.5 % NaCl solution is −0.34 V.

Effect of multi-directional compression on microstructure and mechanical properties of Tip/WE43 composite

Titanium (Ti) particle-reinforced Mg-4Y–2Nd-1Gd-0.5Zr (WE43) magnesium (Mg) matrix composite (Tip/WE43) was prepared via vacuum hot-pressing sintering. The multi-directional compression (MDC) treatment significantly enhanced the material's strength, and the MDC 6 wt% Tip/WE43 composite exhibits exceptional mechanical properties, with a yield strength (YS) of 255 MPa, ultimate tensile strength (UTS) of 316 MPa, and elongation (EL) of 8.9 %. The presence of Ti particles within the composite hinders the basal slip of the Mg matrix during plastic deformation, facilitating the early onset of tensile twinning. The Ti particles contribute to grain refinement, inhibit slip deformation, and promote the occurrence of additional twins within the grains. The second phase, Mg41Nd5, is distributed along the powder boundaries, with dislocation tangles observed within the grains. Twin boundary strengthening is identified as the primary factor for the increased YS in the MDC 6Tip/WE43 composite. Additionally, the MDC 6Tip/WE43 composite demonstrates superior thermal conductivity compared to the MDC WE43 alloy, which is attributed to its less pronounced basal texture.

Fine-grained BCZT piezoelectric ceramics by combining high-energy mechanochemical synthesis and hot-press sintering

Different stoichiometries of lead-free BaZr0.2Ti0.8O3–Ba0.7Ca0.3TiO3 (BCZT) prepared by mechanosynthesis and sintered by either conventional sintering (CS) or hot pressing (HP) techniques were studied to establish the dependence of piezoelectric and dielectric properties on sintering parameters and microstructure. All synthesized stoichiometries showed a pseudocubic perovskite phase with homogeneously distributed A- and B-cations in the structure. The BCZT retained the pseudocubic symmetry after sintering and an average grain size <1.8 µm was obtained in all cases. HP sintering hindered the secondary phase segregation observed in the CS ceramics and increased the relative density. Piezoelectric coefficients (d33) ranging from 5.1 to 21 pC/N and from 10.0 to 88.0 pC/N were obtained for CS and HP ceramics, respectively, despite the pseudocubic symmetry and the fine grain size. The higher d33 values for the HP ceramics are a consequence of the higher density, better chemical homogeneity and lower sintering temperature and time required for the mechanosynthesized BCZT powders with high sintering activity.

TiO2 nanofiber-derived in-situ Al2O3 particles reinforced TiAl matrix composites

In this study, Al2O3/TiAl composites were synthesized via powder metallurgy by incorporating TiO2 nanoparticles and nanofibers as oxygen sources into Ti-45Al-8Nb pre-alloy powders, followed by vacuum hot-pressing sintering to form in-situ Al2O3 particles as reinforcements. The addition of TiO2 nanofibers results in a better grain refinement effect and a more uniform distribution of Al2O3 particles within the composites. High-temperature tensile testing revealed that the composites prepared using TiO2 nanofibers exhibited slightly higher strengths and significantly improved ductility compared to those synthesized with TiO2 nanoparticles. This work not only introduces a novel additive for fabricating high-performance in-situ Al2O3/TiAl composites but also demonstrates a unique application of TiO2 nanofibers.

One-step ultrafast Joule heating synthesis of EuB6 ceramics and their electronic transport and magnetic properties

The synthesis of high-performance rare-earth borides (e.g., RB2, RB4, R2B5, RB6, RB12, and RB66) typically necessitates sintering at high temperatures and high pressures for prolonged durations. Traditional methods, such as hot-pressing sintering and spark plasma sintering, are marked by high production costs and low efficiency. Here, using EuB6 as an example, we demonstrate the successful synthesis of rare-earth boride ceramics utilizing the one-step ultrafast joule heating technology. Using this method, we optimized the sintering temperature and duration and synthesized EuB6 ceramics with superior electronic and magnetic properties at 1600 °C in just 5 min. The EuB6 ceramics prepared under optimized conditions show distinct paramagnetic to ferromagnetic phase transition, significant negative magnetoresistance, and enhanced magnetization. The combination of the one-step ultrafast Joule heating technology and boron thermal reduction method offers a simple and fast synthesis approach for fabricating polycrystalline EuB6 and may be extended to prepare other rare-earth and transition-metal borides.

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.

Microstructure, Mechanical and Tribological Properties of Cu40Zn-Ti3AlC2 Composites by Powder Metallurgy

The exploration of unleaded free-cutting Cu40Zn brass with excellent mechanical and tribological properties has always drawn the attention of researchers. Due to its attractive properties combining metals and ceramics, Ti3AlC2 was added to Cu40Zn brass using high-energy milling and hot-pressing sintering. The effects of Ti3AlC2 on the microstructure, mechanical and tribological properties of Cu40Zn-Ti3AlC2 composites were studied. The results showed that Ti3AlC2 could suppress the formation of ZnO by adsorbing oxygen impurity and promote the formation of the β phase by releasing the β-forming element Al to the substrate. The hardness and wear resistance of Cu40Zn-Ti3AlC2 composites increased with increasing Ti3AlC2 content from 0 to 5 wt.%. The proper Ti3AlC2 additive was beneficial to both the strength and plasticity of the composites. The underlying mechanisms were discussed.

Hierarchical modification of bimodal grain structure in Al/Ti laminated composites for extraordinary strength-ductility synergy

Al/Ti laminates with altering Al grain sizes was fabricated via hot press sintering. Fine Al powders results in low sintering density and obvious cracks at Al/Ti interface. Large Al powders greatly increased the grain size, grain aspect ratio, LAGBs fraction, and recrystallization fraction of the Al layers. The texture heterogeneity is also significant, with rolling texture in Ti layer and random texture in Al layer. Ti5Si3 phase precipitated at Al/Ti interface, and it gradually partitioned Ti atoms from TiAl3 and hindered the formation of TiAl3. Moreover, numerous stacking faults, dislocation loops, dislocation pinning, and dislocation tangles were observed at Al/Ti interface, resulting in an increased back stress. Large Al grains contributes the highest bending strength of 734.8 MPa, tensile strength of 753.2 MPa, and fracture strain of 71 %. The effect of grain size on work hardening was attributed to the fraction of LAGBs, dislocation storage capacity and additional HDI strengthening.

Effect of load on the tribological properties of Si3N4-based composite with N-GQDs

Purpose The purpose of this paper is to improve the mechanical and tribological properties of Si3N4 ceramics and to make the application of Si3N4 ceramics as tribological materials more extensive. Design/methodology/approach Si3N4-based composite ceramics (SN-2L) containing nitrogen-doped graphene quantum dots (N-GQDs) were prepared by hot press sintering process through adding 2 Wt.% nanolignin as precursor to the Si3N4 matrix, and the dry friction and wear behaviors of Si3N4-based composite against TC4 disc were performed at the different loads by using pin-on-disc tester. Findings The friction coefficients and wear rates of SN-2L composite against TC4 were significantly lower than those of the single-phase Si3N4 against TC4 at the load range from 15 to 45 N. At higher load of 45 N, SN-2L/TC4 pair presented the lowest friction coefficient of 0.25, and the wear rates of the pins and discs were as low as 1.76 × 10−6 and 2.59 × 10−4mm3/N·m. The low friction and wear behavior could be attributed to the detachment of N-GQDs from the ceramic matrix to the worn surface at the load of 30 N or higher, and then an effective lubricating film containing N-GQDs, SiO2, TiO2 and Al2SiO5 formed in the worn surface. While, at the same test condition, the friction coefficient of the single-phase Si3N4 against TC4 was at a range from 0.45 to 0.58. The spalling and cracking morphology formed on the worn surface of single-phase Si3N4, and the wear mechanism was mainly dominated by adhesive and abrasive wear. Originality/value Overall, a high-performance green ceramic composite was prepared, and the composite had a good potential for application in engineering tribology fields (such as aerospace bearings). Peer review The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-05-2024-0161/

Arc erosion behaviour of Cu–diamond composites at different load voltages in air atmosphere

ABSTRACT To investigate the arc erosion performance of Cu–diamond composites, Cu–5vol.-% diamond composites were fabricated using the hot-pressing sintering method. Compared to pure copper, Cu–5vol.-% diamond composites exhibited improvements in Brinell hardness of 60.1 HB. The relative density and electrical conductivity of Cu–5vol.-% diamond composites were 97.54% and 83.29% IACS (International Annealing Copper Standard), respectively, with no significant decrease compared to pure copper. The thermal conductivity was 423.48 W·(m·K)−1, showing a slight enhancement. The effect of different load voltages on the arc erosion behaviour of Cu–5vol.-% diamond composites was investigated. Experimental results indicated that as the voltage rose from 3 to 7 kV, the breakdown strength decreased from 7.164 × 106 V m–1 to 2.535 × 106 V m–1, while the arc duration increased from 21.752 × 10−3 s to 28.864 × 10−3 s, and the breakdown current rose from 10.8 to 27.2 A. Additionally, the presence of diamond particles enhanced the viscosity of the molten pool, thus the splashing phenomenon of copper liquid during arc discharge was not obvious, indicating that the material loss was small. Few cracks and holes were detected. The Cu–5 vol.-% diamond composites possess good resistance to arc erosion.

Insight into Grain Refinement Mechanisms of WC Cemented Carbide with Al0.5CoCrFeNiTi0.5 Binder.

High-entropy alloys (HEA) as a kind of new binder for cemented carbide have garnered significant attention. In this work, WC/(17~25 wt.%)Al0.5CoCrFeNiTi0.5 cemented carbides were prepared by hot pressing sintering (HPS), and the reactions between WC powder and Al0.5CoCrFeNiTi0.5 powder during hot pressing sintering were elucidated. It found that different from traditional Co binder, the Al0.5CoCrFeNiTi0.5 binder effectively inhibited WC grain growth. During HPS, the decomposed W and C atoms from WC diffused into the Al0.5CoCrFeNiTi0.5 binder, reacted with the elements in the binder, and then formed the M(Co, Fe, Ni)3W3C phase. The back-diffusion of W and C atoms to WC grains was restricted by the Al0.5CoCrFeNiTi0.5 alloy and inhibited them from re-precipitating onto the large undissolved WC grains. As a result, the average size of WC grains in the cemented carbides was less than 200 nm. This work bright new insight into the grain refinement mechanisms of WC cemented carbide with HEA binder and provide a guidance for designing performance-stable WC/HEA cemented carbide and promoting their application.

CsxWO3 films with excellent infrared shielding ability and high visible light transmittance prepared via magnetron sputtering

Caesium tungsten bronze materials are widely used in various applications, such as architectural glass, medicine, insulation coating and textile fibre, owing to their excellent mechanical, optical and thermal properties. In this study, a caesium tungsten bronze (CsxWO3, x = 0.3–0.32) target with a relative density of 99.63 % and uniform microstructure was successfully prepared via hot-pressing sintering. CsxWO3 films were deposited on a quartz glass substrate via magnetron sputtering, and the mechanisms and effects of the substrate temperature and oxygen flow rate on the phase structure, microstructure and optical properties of CsxWO3 films were investigated. When the substrate temperature increased, the crystallinity and oxygen vacancy concentration of the CsxWO3 film improved. Consequently, the visible light transmittance of the film decreased, whereas the infrared blocking rate considerably increased, reaching up to 93.877 %. With the increase in the oxygen flow rate, the visible light transmittance exhibited an increasing trend, reaching a maximum value of 84.319 %, whereas the infrared blocking rate remained relatively stable. We successfully fabricated CsxWO3 films possessing a visible light transmittance of 66 % and infrared blocking rate of 94.97 % under the conditions of a substrate temperature of 300 °C and an oxygen flow rate of 2.5 sccm. Our study offers a method for the large-scale production of CsxWO3 films, which are excellent infrared shielding materials, rendering a substantial contribution to global energy conservation and emission reduction.

Influence of grain size and grain edge phase on the flexural strength of aluminum nitride ceramics with Y2O3 sintering additive

The necessity for efficient heat dissipation compels AlN packaging materials towards a thinner profile. In response to the ensuing concerns regarding the reliability of strength, the effects of the grain edge phases and grain size on flexural strength are investigated in this study. Different preparation processes, including (hot-pressed sintering, pressureless sintering, and annealing) are employed to prepare AlN ceramics with different grain edge phases and grain sizes. After classifying the morphology of the grain edge phases and fitting the Hall-Petch relationship, it is found that the grain edge phases had a more significant influence on the flexural strength. Depending on the morphology, the negative effects can be divided into three degrees. While the finer grain size proves advantageous for the flexural strength. Consequently, in the non-additive system, the flexural strength of AlN ceramic is 422 MPa, with an average grain size of 8.47 μm. For the additive-doped system, the maximum flexural strength can reach 532 MPa, which can be attributed to the limited average grain size resulting from the hot-pressed sintering process, approximately 1.32 μm. These results could potentially be employed in the microstructural design of high-strength AlN ceramics.

Preparation and tribological properties of PAI/PTFE@Mg-Al LDH polymer-based solid self-lubricating materials

PAI/PTFE composites with 1 %, 2 %, 3 %, 4 % and 5 % of Magnesium aluminum hydrotalcite (Mg-Al LDH) particles were prepared by DC electric field-assisted hot-pressing sintering. The friction and wear properties of the composites were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), water contact angle and UMT-5 friction and wear tests. The effect of the amount of Mg-Al LDH particles added on the physical properties and friction and wear properties of the composites were analyzed. Experimental results showed that the friction and wear properties of PAI/PTFE composite films can be improved by adding appropriate Mg-Al LDH particles. Among them, The PAI/PTFE polymer based solid lubrication containing 3 % Mg-Al LDH has the best friction coefficient and wear amount, the average friction coefficient is 0.0962 and the wear extent is 1.47×10−5 g. The improved tribological properties of the composite films are mainly due to the excellent mechanical properties of Mg-Al LDH and the good lubrication properties of PTFE.