Hot-press Sintering Process Research Articles

Mechanical properties and first-principle calculation investigation of ZrHfNbTa series refractory high entropy alloys prepared by a combined process

ABSTRACT Refractory high-entropy alloys (RHEAs) are widely used in aerospace engine, gas turbine high-temperature components, capacitors and catalysis. The study presents a combined process for preparing ZrHfNbTa series RHEAs. HEAs powders were prepared from metal oxide by molten salt electro-deoxidation process, and the bulk HEAs were prepared by vacuum hot-pressing sintering process. The study aimed to explore the alloying mechanism of mixed metal oxide in molten salt electro-deoxidation process to optimise the properties. The hardness and wear resistance of the bulk alloy were tested. The test results show that the hardness of ZrHfNbTa and VZrHfNbTa bulk RHEAs prepared by this process is 1251 and 1603 HV. According to the friction surface characterisation of bulk alloy, VZrHfNbTa HEA has better wear resistance than ZrHfNbTa. Based on the first-principle calculation results, the doping of V element produces a strong hybridisation of electron orbitals in RHEAs, which can effectively improve the mechanical properties of RHEAs.

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.

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/

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.

Cr-Diamond/Cu Composites with High Thermal Conductivity Fabricated by Vacuum Hot Pressing.

Chromium-plated diamond/copper composite materials, with Cr layer thicknesses of 150 nm and 200 nm, were synthesized using a vacuum hot-press sintering process. Comparative analysis revealed that the thermal conductivity of the composite material with a Cr layer thickness of 150 nm increased by 266%, while that with a Cr layer thickness of 200 nm increased by 242%, relative to the diamond/copper composite materials without Cr plating. This indicates that the introduction of the Cr layer significantly enhanced the thermal conductivity of the composite material. The thermal properties of the composite material initially increased and subsequently decreased with rising sintering temperature. At a sintering temperature of 1050 °C and a diamond particle size of 210 μm, the thermal conductivity of the chromium-plated diamond/copper composite material reached a maximum value of 593.67 W∙m-1∙K-1. This high thermal conductivity is attributed to the formation of chromium carbide at the interface. Additionally, the surface of the diamond particles in contact with the carbide layer exhibited a continuous serrated morphology due to the interface reaction. This "pinning effect" at the interface strengthened the bonding between the diamond particles and the copper matrix, thereby enhancing the overall thermal conductivity of the composite material.

Investigation of mechanical characteristics of hot-pressed sintered tungsten based advanced shielding materials

This research aims to investigate the application of the hot pressing (HP) sintering process in the fabrication of W-B-Fe-Cr-C advanced shielding materials and analyze the relationship between their microstructure and mechanical properties. The findings reveal that the HP sintering process can effectively produce reactive sintered boride (RSB) materials with high density and engineering application size. The results indicate that the residual stress in RSB-HP improves its bending strength to some extent. Residual stress and granular structure are both related to phase aggregation during sintering. Microstructural analysis has unveiled the granular organizational characteristics and the pathways of crack propagation within the RSB-HP. These discoveries hold significant importance for comprehending the potential of the HP sintering process in the context of nuclear fusion shielding material preparation and provide a scientific foundation for further optimizing the sintering process and performance of RSB materials.

Enhancing mechanical properties and density of WC-Co with pressureless sintering via shell structure binder jetting additive manufacturing

The binder jetting (BJT) additive manufacturing (AM) process is suitable for producing complex WC-Co cemented carbide parts. To achieve near-full-density in the final parts, the hot isostatic pressing (HIP) sintering process is typically employed to enhance densification. However, HIP sintering requires high equipment specifications and is relatively more costly compared to conventional pressureless sintering. To improve the density of WC-Co parts printed by BJT AM under the pressureless sintering environment, the shell structure printing method was used in this study. The shell structure printing only jets binder to the model's outer profile, creating a shell that encapsulates the central powder, mitigates binder pyrolysis and residual effects on particle densification, and enhances post-sintering density. In this work, the effect of shell thickness, binder type, and printing layer thickness on the density and mechanical properties of the WC-12Co parts under pressureless sintering conditions were investigated. The results show that the density and mechanical properties of the WC-12Co parts significantly improved with the decreasing shell thickness. Specifically, the WC-12Co printed using shell structure achieves a relative density of over 98% and a transverse rupture strength (TRS) of 1954 MPa after pressureless sintering, a result comparable to BJT printed samples treated with HIP sintering. Moreover, the results indicate using a binder with lower residual carbon content and smaller printing layer thickness has a more positive effect on improving density and mechanical strength when employing the shell structure printing method. Microstructure morphology and phase analysis results indicate that the shell structure printing method does not affect the growth of WC grains in the shell and center regions, nor does it influence its phase composition. The shell structure printing method offers a new approach to the manufacturing of near-full-dense WC-Co materials using the BJT process.

Effect of the TiB2 content on the mechanical and tribological properties of TiB2/AZ91 composites fabricated by vacuum hot-press sintering process incorporating vacuum ball milling

Magnesium alloys have poor strength and wear resistance, restricting their application greatly, while Mg matrix composites can improve these deficiencies. TiB2/AZ91 composites with high TiB2 content were prepared by vacuum hot-press sintering process incorporating vacuum ball milling. The effects of TiB2 content on the microstructure, mechanical properties and tribological properties of the composites were investigated in detail. The results suggested that the TiB2 particle exhibited good bonding with the Mg matrix. With the increasing TiB2 content, the ultimate tensile strength, yield strength and elongation of TiB2/AZ91 composites firstly increased and then decreased, while the wear rate and wear surface roughness presented an opposite trend, and the hardness and coefficient of friction gradually increased. The composites with 5 wt% TiB2 obtained the maximum tensile properties (UTS: 234 MPa YS: 91 MPa EL: 7.3 %). This improvement in mechanical properties was attributed to the uniform distribution of TiB2 particles and the well bonding at the interface. The strengthening mechanisms were found to be the load transfer strengthening and the thermal mismatch strengthening. Furthermore, the composites with 15 wt% TiB2 demonstrated the lowest wear rate of 1.06×10−6 mm3/Nm and lowest wear surface roughness of 2.971 μm. This wear resistance was enhanced because of the significant increasing in hardness. The dominant wear mechanisms observed in TiB2/AZ91 composites were abrasive wear and oxidation wear.

Synergistic reinforcement of Cu/Ti3SiC2/C laminated-like composites with the bimodal and highly oriented graphite flake and graphene nanoplatelets

This study describes the preparation of bimodal reinforced Cu/Ti3SiC2/C laminated-like composites using flake powder metallurgy and vacuum hot-press sintering process. The objective was to accurately modulate the formation of oriented reinforcement network in two-dimensional carbon materials. The powders used in the five designed composites were flake Cu powder (45–60 μm), Ti3SiC2 (1–5 μm), graphite flakes (GFs, 120 μm) and graphene nanoplatelets (GNPs, 15 μm). Among them, the proportions of Ti3SiC2 (10 wt %) and GFs (3 wt %) remained unchanged, GNPs accounted for 0 wt %, 0.5 wt %, 1.0 wt %, and 1.5 wt %, and Cu powder accounted for a decreasing proportion. Formation mechanisms of GFs and GNPs in oriented networks were quantitatively analyzed using the alignment model. Additionally, the related strengthening and failure mechanisms were investigated. According to the study, the bimodal laminated-like Cu/Ti3SiC2/C composites with 0.5 wt% GNPs added showed the best overall performance. Specifically, it exhibited a hardness of 125.7 HV, tensile strength of 276.3 MPa, thermal conductivity of 207.8 W m−1 K−1, and electrical conductivity of 23.6 MS/m. The network enhanced by the two-dimensional carbon material achieves efficient and orderly load transfer and dissipation mechanisms, optimizing stress distribution and enhancing fracture resistance, resulting in improved mechanical properties. In addition, the formation of the oriented reinforcement network optimizes the orientation arrangement of the reinforcing particles, which somewhat mitigates the scattering of thermoelectric carriers by structural defects in the composites, and the enlarged mean free-range improves the migration efficiency of electrons and phonons. Therefore, the erection of bimodal laminated-like structure achieves the goal of optimizing the properties of the composites.

In-situ 3D visualizations of microstructural evolution during hot-pressing sintering of 7055 alloy powders containing satellite particles

In this study, we developed an in-situ hot-pressing sintering (HPS) device that can be coupled to a laboratory X-ray microscope, offering laboratory-available observation of the morphology evolution. With the help of this device, in-situ three-dimensional (3D) visualizations of the microstructural evolution of 7055 aluminum alloys during the HPS process were conducted. The 3D results revealed that the two-dimensional (2D) methods usually underestimated sintering neck width and exhibited significant standard deviation in statistical analysis. Benefiting from the precise microstructure characterization of the in-situ 3D methods, the diffusion activation energy for the sintering of 7055 alloys was calculated, and the quantitative relationship between the sintering temperature and the sintering process was constructed. Moreover, it was experimentally found an accelerative effect of satellite particles on the sintering process, and its mechanisms were discussed. The satellite particles enhanced the curvature near the sintering neck and thus increased the sintering driving stress, promoting the densification process. These findings provide new insights for optimizing sintering processes.

Significantly enhanced transmittance and EO property in PMN-PT via optimized two-step pressure-free sintering

It is a great challenge to prepare Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT) ceramics of very high transparency with only oxygen atmosphere pressureless sintering process. To address this issue, this work proposed an optimal low-cost two-step oxygen atmosphere sintering process with a first step at high temperature (T1) for minutes, followed by a second step at lower temperature (T2) for hours without applied pressure. High transmittance of 70 % in the near-infrared wavelength was achieved by our novel two-step process, which far exceeds the performance of PMN–PT ceramics prepared by only oxygen atmosphere sintering process reported in the past and demonstrates comparable performance compared to the costly hot-pressing sintering process. Meanwhile, it shows a high quadratic electro-optic (EO) coefficient of 42.1 × 10−16 m2 V−2, which is meaningful for applications in EO devices. Furthermore, the effect of the temperature difference between the first and second sintering step on the microstructure, transmittance, domain structure, ferroelectric and dielectric property, and polarization switching were discussed. The enhanced transparency is mainly attributed to the higher relative density, and weaker photoluminescence effect with lesser light absorption loss at optimal T1-T2. The improved EO property is due to greater polarization response originating from the interaction of defect dipoles and nanodomains. Such a process should facilitate the cost-effective preparation of other transparent electro-optic materials for practical applications.

Tailoring the microstructure of Al/Ti laminated composite through hot press sintering process to achieve superior mechanical properties

A novel method based on hot press sintering was proposed to fabricate the Al/Ti laminated composite, and the effects of sintering parameters on microstructure, interfacial structure, and mechanical properties were thoughtful examined. The results showed that insufficient sintering caused obvious voids and cracks at Al/Ti interface. With the increase of temperature or holding time, a good metallurgical bonding without defects was achieved, and both the recrystallization fraction and grain size of α phase in Ti layer increased, accompanied by the transformation of α to β phase and growth of intermetallic phase. The microstructure variation of Al layer is not evident with changing the sintering parameters. Due to the relatively low formation energy, the nanoscale TiAl3 phase with massive stacking faults formed at Al/Ti interface, and lots of dislocations existed at Al layer near the interface. The voids and cracks formed at Al/Ti interface led to the premature failure of laminate. On the basis of the metallurgical bonding and small TiAl3 phase, the high fraction and texture intensity of α phase were the dominated strengthening factors. Moreover, the back stress generated at Al/Ti interface contributes to an extra strengthening. The superior mechanical properties of Al/Ti laminate with a tensile strength of 670.9 MPa and a fracture strain of 0.33 were obtained with the sintering temperature of 600 °C and holding time of 2 h.

Fabrication and magneto-optic property of infrared transparent gadolinium iron garnet (GdIG) ceramics by hot-press sintering process

A series of GdIG ceramics with good infrared transparency have been fabricated by a hot-press sintering process with vacuum/oxygen atmosphere. The effects of the powder synthesizing temperature and the ceramic sintering temperature on the phase structure and microstructure of the GdIG ceramics were examined and analyzed. The GdIG ceramic sample sintered at 1275 °C from the Gd2O3 and Fe2O3 powders pre-synthesized at 1000 °C demonstrated an exceptionally high infrared transmittance, it is ≥ 68% within the wavelength range of 1500–5000 nm (t = 0.5 mm). Furthermore, the magnetic and magneto-optic property of the GdIG ceramic samples were investigated and discussed. When a saturation external magnetic field of ∼50 mT acting on the GdIG ceramic sample, the specific Faraday rotation angles are about 104 deg./cm and 51 deg./cm at 1064 and 1550 nm wavelength respectively. It exhibits a good faraday effect of GdIG ceramics which is rarely reported until now.

High-Temperature Reliability of Nano-Ag Sintered Joints on Ag-Plated Cu Substrates

As an emerging lead-free packaging interconnect material, Nano-Ag paste exhibits superior high-temperature mechanical, electrical, and thermal properties. It can meet the stringent high-temperature and high-density packaging requirements of future high-power semiconductor devices and is garnering widespread attention. However, research on the high-temperature reliability of sintered joints remains rather limited. In this study, we fabricated high-strength hot-press sintered nano-Ag joints using a hot-press sintering process. These joints underwent aging at high temperatures of 200°C and 300°C, yielding insights into the variations in mechanical properties at different temperatures. The reasons behind joint failure were analyzed by investigating the evolution of joint microstructures and fracture surfaces. The results indicate that even after extended aging at 200°C, the joint strength can still be maintained at 176 MPa. However, after aging for over 100 hours at 300°C, the joint fails. The growth of Cu oxides was observed within the joint microstructure, which constitutes the primary cause of joint failure at 300°C.

Microstructure, mechanical and tribological properties of APC/Al laminated composites prepared by hydrothermal carbon adsorption on plates

Amorphous carbon (APC)/Al laminated composites were prepared by hydrothermal carbon adsorption on plates (HTCAP) and vacuum hot-press sintering process. The effect of the glucose solution concentration on the microstructure, mechanical properties and tribological properties was investigated. The results indicate that APC is uniformly distributed between layers and has good interfacial bonding with the matrix through the diffusion of Al. As the glucose solution concentration increases, the interlayer hardness, ultimate tensile strength and elongation first increase and then decrease, while the wear rate shows the opposite trend and the coefficient of friction (COF) gradually decreases. A maximum elongation of 16.4% and a minimum wear rate of 0.344×10−5 mm3/(N·m) are achieved at a glucose solution concentration of 0.3 mol/L. Adhesive wear is the dominant wear mechanism of APC/Al laminated composites. The improvement in wear resistance is attributed to the formation of uniform APC lubrication film on the wear surface.

Trace nano-Ti addition for graphene nanoplatelets/copper composites to simultaneously enhance strength-ductility and wear resistance

Graphene nanoplatelets (GNPs)/copper composites containing nano-Ti addition were prepared using the vacuum hot-pressing sintering process. The effect of Nano-Ti on microstructure and mechanical properties of GNPs/copper composite was investigated. Before the sintering of the composite powders, the massive nano-Ti particles decorated on the GNPs surface. The GNPs/Ti chemical reaction occurred during the sintering, and then formed abundant TiC particles. Hardness and strength of bulk GNPs/copper composites were enhanced evidently by nano-Ti addition, which increased with nano-Ti increasing. The nano-Ti addition achieved simultaneous strength-ductility enhancement of GNPs/copper composites. The hardness, tensile yield strength, ultimate tensile strength and tensile elongation of the composite with 3.0 wt% nano-Ti addition demonstrated respectively 128.2 %, 199.6 %, 125.9 %, and 30.6 % enhancement comparing with no nano-Ti added sample. The nano Ti addition can effectively improve the wear resistance of GNPs/copper composites. The relevant structural formation, strengthening-toughening and wear mechanisms of GNPs/copper composites with nano-Ti addition were discussed.

Fabrication of Al/Al-TiB2 laminate composites via hot press sintering process: An insight into the mechanical properties and fracture behavior

The Al/Al-TiB2 laminated composites with different numbers of layers was fabricated by hot press sintering method. The results show that Al/Al-TiB2 interface is well bonded without obvious micropores, cracks, or other defects, while the stacking faults and dislocation loops are observed at the interface. The average grain sizes of Al and Al-TiB2 layers are 45.17 and 5.51 μm in the composite with 3 layers, while they greatly increase to 80.07 and 6.51 μm in the composite with 5 layers. The hardness increases gradually along Al layer, Al/Al-TiB2 interface, and Al-TiB2 layer. TiB2 particles mainly contributes to the enhancement of bending strength, and the maximum strength of 110.0 MPa is achieved in the composite with 3 layers due to the fine grain size, small particle spacing, and more geometrically necessary dislocations in Al-TiB2 layer. During bending deformation, Al layer exhibits ductile fracture with shear lip, slip band, fatigue striation and dimple, while Al-TiB2 layer shows typical brittle fracture with micro-voids and cleavage. The stress concentration and crack always appear in Al-TiB2 layer, while Al layer can transfer strain and inhibit strain localization. An increasing ductility of the laminated composites is achieved due to crack deflection, interfacial delamination, and strain transferring.

Study on the Thermophysical Properties of 80% 10B Enrichment of B4C.

In this paper, a specific type of Boron Carbide (B4C) with a high enrichment of 80 ± 0.3 at% 10B was prepared as an absorbing material for control rods in nuclear reactors. The enrichment of 10B was achieved using a chemical exchange method, followed by obtaining boron carbide powder through a carbothermal reduction method. Finally, B4C with a high enrichment of 68.3~74.2% theoretical density was obtained using a hot-pressed sintering process. This study focused on investigating the basic out-of-pile thermophysical properties of the high enrichment B4C compared to natural B4C reference pellets under non-irradiated conditions. These properties included the thermal expansion coefficient, thermal conductivity, emissivity, elastic limit, elastic modulus, and Poisson's ratio. The research results indicate that the enriched B4C pellet exhibits good thermal stability and meets the technical requirements for mechanical capability. It was observed that porosity plays a significant role in determining the out-of-pile mechanical capability of B4C, with higher porosity samples having a lower thermal conductivity, elastic-plastic limit, and elastic modulus. In short, all the technical indexes studied meet the requirements of nuclear-grade Boron Carbide pellets for Pressurized Water Reactors.

Strengthening and toughening mechanisms of tailored Al/Al-TiB2 laminated composites fabricated by hot press sintering process

Strengthening and toughening mechanisms of tailored Al/Al-TiB2 laminated composites fabricated by hot press sintering process

Microstructure and bending behavior of Ti/Al laminated composites reinforced with high-entropy alloy particles

Ti/Al laminated composites embedded with Al0.5CoCrFeNi high entropy alloy particles were prepared by vacuum hot press sintering process at temperatures of 660 °C and 730 °C. The microstructure of the material was initially examined using XRD, SEM, EDS, EBSD, and TEM techniques. The results of microstructural characterisation show that Al3Ti is the only newly generated phase in the materials prepared at two temperatures. The Al3Ti generated at the temperature of 660 °C is concentrated at the Ti/Al interface position. The Al3Ti generated at a temperature of 730 °C is dispersed in the Al matrix, leading to the formation of numerous Al3Ti/Al interfaces. The residual HEA particles were observed at the sites of oxide aggregation, potentially enhancing the connectivity of the weak interfaces. Following that, the flexural strength and fracture toughness of the composites were evaluated. The results indicate that the materials prepared at a temperature of 730 °C exhibit superior properties. The flexural strength perpendicular to the layer direction reaches 515.89 MPa, while the flexural strength parallel to the layer direction reaches 487.1 MPa. These findings demonstrate clear anisotropy in the flexural performance. Additionally, the material exhibits excellent resistance to crack extension, with a fracture toughness of 32.67 MPa√m.