Hot-pressing Sintering Technology Research Articles

High-Temperature Tribological Behavior of Fast-Hot-Pressed NiCr/Cr3C2-LaF3 Self-Lubrication Composite

This article details a method for preparing cermet matrix composites via Fast hot pressing (FHP) sintering technology and emphasizes their potential use in extremely high-temperature settings. The material primarily consists of NiCr alloy, Cr3C2, and LaF3. An in-depth investigation was conducted on the tribological properties of the specimen by conducting sliding tests against a Si3N4 ball at varying temperatures, including room temperature (RT), 400 °C, 600 °C, and 800 °C. Advanced techniques such as scanning electron microscopy, micro-XRD, and micro-Raman spectroscopy were employed to examine the friction surfaces formed under different frictional temperatures. The findings reveal a uniform composition and high density within the composites. It is noteworthy that as the LaF3 content increases, the hardness of the ceramic phase diminishes. Conversely, the hardness of the alloy phase augments with the addition of LaF3, provided that its content remains below 15 wt%. The composite material containing 15 wt% LaF3 demonstrates superior hardness values, with the ceramic phase reaching HV1412 and the alloy phase achieving HV384. Furthermore, the coefficient of friction of the composite material was evaluated. The coefficient of friction of the composite is between 0.74 and 0.4 and the wear rate is 4.46 × 10−6–5.72 × 10−5 mm3N−1m−1 from room temperature to 800 °C. The lubrication behavior at low temperature is mainly attributed to the lubricating effect of LaF3, and at high temperature it is due to the tribochemical reaction to form LaCrO3 with good lubricating properties, which plays a synergistic lubricating role with Cr2O3.

Novel fast preparation of MoAlB phase cermet with elevated temperature oxidation resistance

Owing to the small production scale, high energy consumption and low degree of the current sintering method, the facile and fast method to prepare tailored MoAlB phase cermets is urgent. Hence, the high purity, dense and large-size MoAlB phase cermet blocks are fabricated by a novel direct current (DC) fast hot pressing (FHP) sintering technology starting from Mo/Al/B powders with a molar ratio of 1:1.3:1. The phase composition and the microstructure of MoAlB phase cermets under different sintering conditions are characterized with X-ray diffraction and scanning electron microscopy. The results indicate that the as-prepared MoAlB samples are almost single-phase. MoAlB phase cermet has good oxidation resistance, forming a thin layer with Al2O3, B2O3 and MoO3 at 1000 °C.

The crack-healing and strength recovery mechanisms for the new developed Ti(C,N)-TiSi2-WC composite ceramic tool materials with crack-healing ability

Because the ceramic tool life will be reduced due to the crack growth, it is necessary to develop the new ceramic tools with crack-healing ability for improving the ceramic tool life. Ti(C,N)-TiSi2-WC composite ceramic tools were fabricated by vacuum hot pressing sintering technology. The Vickers hardness tester was used to prefabricate cracks with different lengths. The crack-healing abilities of materials were researched by air and vacuum heat treatments. The effects of different healing conditions on crack-healing behaviors were investigated based on the strength recovery and microstructure of crack morphology. The healing conditions included healing temperature, healing time, crack length and healing atmosphere. It could be found that the surface cracks were entirely healed after air heat treatment, but locally healed after vacuum heat treatment. After air heating at 800 °C for 90 min, the restoration ratio of flexural strength of the crack-healed samples returned to 109.02% compared to that of the smooth specimen. The strength of the crack-healed specimen would be fully recovered when the crack length was less than 400 μm. After vacuum heating at 1000 °C for 60 min, the recovery of the flexural strength of the crack-healed samples returned to 49.73%. X-ray diffraction and scanning electron microscope technology were used to analyze compositions and microstructure of crack-healed specimens. In the air atmosphere, the liquid filling of SiO2 and TiO2 and crack closure caused by WSi2 and MoSi2 were confirmed as the crack-healing mechanisms. SiO2 and TiO2 were produced by high temperature oxidation reaction of TiSi2 and WSi2. WiS2 and MoSi2 were new phases formed during sintering and heat treatment, and their formation could be explained by thermodynamically feasible reactions. In the vacuum atmosphere, local filling of the crack with silicide solid solutions such as WSi2, MoSi2 and NiSi was the main crack-healing mechanism.

Preparation of glass-ceramic by low-pressure hot-pressing sintering of CaO-MgO-Al2O3-SiO2(-CaCl2) glass powder

CaCl2 can dissolve in the silicate glasses, and thus, using the CaCl2-bearing glass for the preparation of glass-ceramic could be a potential method for treating CaCl2-bearing wastes. In this work, two kinds of glass powders, CaO-MgO-Al2O3-SiO2 and CaO-MgO-Al2O3-SiO2-CaCl2, were sintered at 825–850 ℃ to prepare the glass-ceramic. Besides, a low-pressure (19.1kPa) hot-pressing sintering technology was applied to reduce the porosity and pore size of the glass-ceramic. The addition of CaCl2 (5 wt%) reduced the polymerization degree, glass transition temperature, crystallization peak temperature, and crystallization activation energy of the glass. Augite was the main crystalline phase of the glass-ceramic, and the addition of CaCl2 promoted the precipitation of anorthite. The glass-ceramic prepared from the Cl-containing glass always exhibited a higher bending strength than that obtained from the Cl-free glass. When sintered at 825 ℃ for 8 h, the glass-ceramic showed the highest bending strength of 171 MPa.

Effects of ambient humidity and sintering temperature on the tribological and antistatic properties of PEEK and CF/PEEK

Polyether ether ketone (PEEK) is one of the representatives of special engineering plastics. Due to the high molding temperature, the molding method of ordinary plastics and cavity abrasives are difficult to meet the requirements. In view of this, pure PEEK was prepared by vacuum hot pressing sintering technology at different sintering temperatures. The mechanical properties and microstructure characterization results showed that the pure PEEK prepared at 350 °C showed excellent friction and wear properties. Then PEEK and CF/PEEK composites were prepared at the optimum sintering temperature. The friction experiments of prepared materials were carried out using UMT-2. The effects of ambient humidity on the tribological properties, wear mechanism and antistatic properties of prepared materials were systematically studied. The surface analysis and properties of the materials were measured by 3D profiler, scanning electron microscope, friction electrostatic tester. The results showed that friction coefficient of PEEK and CF/PEEK composite changed slightly with increase of ambient humidity. The wear rate of PEEK decreased firstly and then increases, which reached the lowest of 3.09 × 10−5 mm3/Nm when the ambient humidity was 40%. The wear rate of CF/PEEK composite changed slightly, which was significantly lower than that of PEEK, and the main wear mechanism was adhesive wear. The surface friction static electricity of PEEK and CF/PEEK composites decreased with the increase of humidity.

Effect of layer thickness ratio on microstructure and mechanical properties of TiCN-HfN/TiCN-WC laminated ceramics

The TiCN-HfN/TiCN-WC(THTW) laminated ceramics with different layer thickness ratio were prepared by the vacuum hot-press sintering technology. The influence of the layer thickness ratio on the residual stress, microstructure, and mechanical properties of these laminated ceramics had been studied. The simulation results showed the compressive and tensile stress layers had a phenomenon of distribution alternately, and the residual compressive stress became smaller when the layer thickness ratio became larger. Cracks deflection and cracks bridging occurred during the crack propagation, and the simultaneous existence of the transgranular fracture and intergranular fracture were conducive to improving the comprehensive mechanical properties of these ceramics. With the increased of the layer thickness ratio, both the relative density and flexural strength initially increased and then decreased; the hardness of TiCN-HfN layer first decreased and then increased but the fracture toughness decreased continually. The fracture toughness and Vickers hardness of TiCN-WC layer increased constantly. When the layer thickness ratio was 0.8, the laminated ceramic obtained the best comprehensive mechanical properties: its surface hardness was 16.93 ± 0.30 GPa, its surface fracture toughness was 7.91 ± 0.12 MPa m1/2, its flexural strength was 1293.54 ± 10 MPa, and its fracture toughness was 8.25 ± 0.08 MPa m1/2.

High-Temperature Oxidation Behavior of TiB2-HfB2-Ni Cermet Material.

To analyze the high-temperature oxidation behavior of TiB2-HfB2-Ni cermet material, TiB2-HfB2-Ni cermets were fabricated by hot-pressing sintering technology. The oxidation resistance and the thermal fracture of TiB2-HfB2-Ni cermet were investigated at 1100 °C for 1, 4, 7, and 10 h, respectively. Before oxidation, TiB2-HfB2-Ni cermet, consisting of TiB2, HfB2, and Ni, had the core-rim structure. The core was TiB2 grain and the rim was composed of Ni and solid solution (Ti, Hf)B2. After oxidation at 1100 °C, the oxides of the TiB2-HfB2-Ni cermet were mainly TiO2, HfO2, B2O3, and NiO, which the oxidation process abided by the parabolic law. With the oxidation time increasing from 1 h to 10 h, the oxidation degree of the TiB2-HfB2-Ni cermet increased, and the oxide layer became thicker. The oxide layer was thin and dense after oxidation at 1100 °C for 1 h. An obvious boundary was discovered between the transition layer and the substrate layer after oxidation at 1100 °C for 7 h. The thermal fracture occurred in the contact regions of different layers at 1100 °C for 10 h. TiB2-HfB2-Ni took place in oxidation at different levels from the outer to the inner, and the components of different oxide layers were certainly distinct.

Plastic deformation behavior of bending aluminum fiber porous material under compressive load

The 6061 aluminum alloy wires with diameters of 0.15 mm and 0.20 mm were cut into different structural sizes of 5 × 10 mm and 10 × 15 mm by a spring machine to bend the aluminum alloy fibers, one is 5 mm short side, 10 mm long side, and 1 mm arc radius, and the other is 10 mm short side, 15 mm long side, and 1 mm arc radius. Special cylindrical molds are used to prepare folding sheets with different porosities using vacuum hot pressing sintering technology. Bent aluminum fiber porous material. The gas displacement method was used to detect the pore size and pore size distribution, and the Plastic Deformation Behavior were analyzed by quasi-static uniaxial compression test. Research shows that the material presents a three-stage stress-strain curve, from the initial nonlinear elastic deformation stage to the pseudo-platform stage and the densification stage, the wire diameter and structure of the material The smaller the size, the more conducive to the densification of the silk skeleton, and the smaller the average pore size formed, the higher the compressive strength.

Erosion behavior and mechanism of Cu-B4C composites under arc discharge in different atmospheres

Ceramic particle-reinforced materials are an important part of high-performance contact materials because of the excellent performance in resistance to arc erosion. In particular, B4C is the ideal choice for the preparation of high-performance electrical contact materials because of its excellent physicochemical properties. In this paper, Cu-B4C composites were prepared by hot-press sintering technology to illustrate the arc erosion behavior of Cu-B4C composites in different atmospheres at high voltages. The erosion morphology and composition of Cu-B4C composites after erosion in air, carbon dioxide and sulfur hexafluoride atmosphere at 8kV were studied. The different erosion mechanisms of Cu-B4C composites in air, carbon dioxide and sulfur hexafluoride atmospheres were systematically discussed. The results showed that the Cu-B4C composites exhibited inhomogeneous erosion in all three atmospheres, and the erosion was mainly concentrated in the region around the B4C particles. In air, the Cu-B4C composites were most severely eroded, but showed better erosion resistance in carbon dioxide and sulfur hexafluoride. The experimental atmosphere decomposed and reacted with copper on the cathode surface at high temperatures, while B4C maintained a good structure after erosion.

Fabrication of Copper Matrix Composites Reinforced with Carbon Nanotubes Using an Innovational Self-Reduction Molecular-Level-Mixing Method.

An innovational self-reduction molecular-level-mixing method was proposed as a simplified manufacturing technique for the production of carbon nanotube copper matrix composites (CNT/Cu). Copper matrix composites reinforced with varying amounts of (0.1, 0.3, 0.5 and 0.7 wt%) carbon nanotubes were fabricated by using this method combined with hot-pressing sintering technology. The surface structure and elemental distribution during the preparation of CNT/Cu mixing powder were investigated. The microstructure and comprehensive properties of the CNT/Cu composites were examined by metallography, mechanical and electrical conductivity tests. The results revealed that the CNT/Cu could be produced by a high temperature reaction at 900 degrees under vacuum, during which the carbon atoms in the carbon nanotubes reduced the divalent copper on the surface to zero-valent copper monomers. The decrease in the ratio of D and G peaks on the Raman spectra indicated that the defective spots on the carbon nanotubes were wrapped and covered by the copper atoms after a self-reduction reaction. The prepared CNT/Cu powders were uniformly embedded in the grain boundaries of the copper matrix materials and effectively hindered the tensile fracture. The overall characteristics of the CNT/Cu composites steadily increased with increasing CNT until the maximum at 0.7 wt%. The performance was achieved with a hardness of 86.1 HV, an electrical conductivity of 81.8% IACS, and tensile strength of 227.5 MPa.

High-performance Ti(C,N)-based cermet with ZrO2 whiskers and Al0.3CoCrFeNi high-entropy alloy

In this study, a high–performance Ti(C,N)-based cermet was prepared by vacuum hot-press sintering technology using Ti(C,N) as the hard phase material, WC, TaC and Mo2C as the second-phase carbides, Al0.3CoCrFeNi high-entropy alloy (HEA) as the metal binding phase and ZrO2 whiskers as the reinforcement phase. The effects of whiskers and the HEA on the room-temperature and high-temperature properties of cermets were studied, including oxidation resistance, hardness, bending strength, fracture toughness and frictional wear properties. The research showed that the high-temperature properties of cermets were improved by ZrO2 whiskers and HEAs compared to those of conventional cermets with Ni/Co as the binding phase. The mechanisms for the weakening of cermets at high temperatures were grain boundary softening and material oxidation. Moreover, the high-temperature strengthening mechanism was attributed to the strength of the grain boundaries in the cermet, high–temperature hardness and oxidation resistance of the HEA binding phase, as well as the strengthening of the ZrO2 whiskers at high temperatures. The results proved that the wear mechanism of cermet materials was adhesive wear at high temperatures.

Microstructure, mechanical properties and high temperature tribological behavior of NiAl-based composites

NiAl-based composites were fabricated in-situ using vacuum hot-press sintering technology. The Ti3AlC2, TiC and Ti3Al were formed during the sintering process, and the intensity of peaks grow higher with the addition of Al, Ti and C powder contents, respectively. The microhardness of the composites was significantly improved by adding these powders and gradually increased with the incremental content of Al, Ti and C powders. Meanwhile, the tribological properties of the composites at high temperature were also improved. The worn surface formed during the tribo-oxidation reactions and produced the glaze film in the high-temperature friction process, which could improve the composites' tribological properties. The TiO2 and Al2O3 phases were observed on the worn surface after the friction test, which could act as a high-temperature lubricant and improve the composites’ tribological properties.

Study on corrosion behavior of (CuZnMnNi)100−xSnx high-entropy brass alloy in 5 wt% NaCl solution

(CuZnMnNi)100−xSnx (x = 0, 4, at%) high-entropy brass alloys (HEBAs) were prepared via mechanical alloying and hot-press sintering technology, the corrosion behavior of the HEBAs in 5 wt% NaCl solution have been systematically studied. The results show that (CuZnMnNi)96Sn4 has more BCC phase than CuZnMnNi, and the BCC phase distributes continuously along the phase boundaries of FCC phase. In 5 wt% NaCl solution, corrosion occurs first at the grain and phase boundaries, and BCC phase exhibits worse corrosion resistance than FCC phase. The continuous distribution of the BCC phase promotes the corrosion to extend into the alloy, accelerating the HEBA corrosion. The corrosion product of the HEBA has a two-layer structure. The outer layer is relatively dense, mainly composed of Cu2O and ZnO, and the inner layer has a loose structure and more cracks, which is an amorphous oxide layer rich in Ni, Cu and Zn elements.

Microstructures and mechanical properties of functionally graded TiCN–TaC ceramics prepared by a novel layer processing strategy

Functionally graded TiCN–TaC ceramics (FGTTCs) were fabricated using a novel layer processing method based on a vacuum hot-press sintering technology. Microstructural investigations revealed a visibly layered structure for the FGTTCs with relatively flat boundaries between the neighboring layers; additionally, the layer thickness was facilely controlled. With an increase in the sintering temperature, the hardness and flexural strength of the surface and middle layers of the FGTTCs initially increased, and then decreased. The fracture toughness of the surface layer did not undergo significant changes after sintering at various temperatures, except at 1500 °C. The FGTTC sintered at 1350 °C contained uniform fine grains and simultaneously exhibited transgranular and intergranular fracture modes. Further, it presented excellent comprehensive mechanical properties, i.e., surface layer hardness = 20.28 ± 0.18 GPa, flexural strength = 1553.76 ± 22 MPa, surface layer fracture toughness = 7.29 ± 0.24 MPa m1/2. Under the same sintering conditions, our FGTTCs presented superior mechanical properties against homogeneous TiCN–TaC ceramics (HTTCs), achieving a considerably higher flexural strength (1553.76 ± 22 vs 953.35 ± 24 MPa).

Characterization of Mo-6Ta alloy targets and its magnetron sputtering deposited thin film

In this study, pure Mo and Mo-6Ta alloy targets were produced by a manufacturing method, which is called die pressing with a combination of cold isostatic pressing and hot-pressing sintering technology, and molybdenum thin films were deposited on corning glass substrates using a DC magnetron sputtering system. The effect of alloying with tantalum on the phase structure, grain size, orientation, thin film surface morphologies and electrochemical properties has been investigated by a variety of techniques such as X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) with electron back-scattered diffraction (EBSD), atomic force microscopy (AFM), four-probe resistivity tester, and electrochemical analyzer. The observation of the Mo targets reveals that this manufacturing technology with alloying of tantalum can reduce the target's grain size to lower than 50 μm with random grain orientation. The microstructure of the deposited Mo thin films indicates that the alloyed films have the higher deposition velocity with a dense fine grain structure and smoother surface. The sheet resistivity of the alloyed Mo film increases continuously with the increasing of the sputtering time, and eventually becomes higher than that of the pure Mo film. In electrochemical tests, both of the Mo films show capacitive reactance, and the capacitive arc radius of the alloyed Mo films are obviously larger than that of pure Mo films. The result of alloying with tantalum shows an increase in the corrosion potential of the Mo film from −0.5 V to −0.2 V, indicating improvement of the corrosion resistance of Mo films.

Tribological Behavior of VN-MoS2/Ag Composites over a Wide Temperature Range

As a type of thermal structure ceramic, vanadium nitride (VN) ceramics are widely used in harsh conditions, such as high temperatures, variable temperatures, or oxidizing atmospheres. However, high friction coefficients and wear rates under wide temperature ranges limit the application of VN in moving parts. In this article, VN-based composites with different contents of MoS2/Ag lubricants are prepared by vacuum hot-press sintering technology, and the microstructure and tribological behavior of VN-MoS2/Ag composites over a wide temperature range are studied. The results show that sintered VN-based composites are composed of the VN phase and corresponding MoS2/Ag lubricants, where MoS2 and Ag are uniformly distributed in the VN matrix. The tribological behavior of VN-based composites confirms that the addition of MoS2 lubricant significantly improves the friction coefficient of the VN ceramic at room temperature and 300 °C, whereas the MoS2/Ag lubricants obviously decrease the friction coefficient of the VN ceramic at room temperature to 700 °C; in particular, the friction coefficient of the VN-MoS2/Ag composites at 700 °C is lower than 0.17. Furthermore, the synergistic effect between MoS2 and Ag lubricants is the key factor for the excellent friction coefficient and wear performance of VN-based composites over a wide temperature range.

In Situ Transformed Carbon Fibers/Al2O3 Nanocomposites Using Cao–MgO–SiO2 Sintering Agent

In Situ transformed carbon fibers/Al2O3 ceramic matrix nanocomposites with Cao–MgO–SiO2 sintering agent were prepared by hot-pressed sintering technology in vacuum. In the sintering process, pre-oxidized polyacrylonitrile fibers (below named as pre-oxidized PAN fibers) were used as the precursors of In Situ transformed carbon fibers. The micro/nanostructure of composites and interface between In Situ transformed carbon fibers and matrix were investigated, as well as the properties of composites. The results showed that the composites could be sintered well at a relatively low temperature of 1650 °C. During the sintering, the precursors, pre-oxidized PAN fibers, were In Situ transformed into carbon fibers, and the In Situ transformed carbon fibers had the graphitelike structure along the fiber axial direction. The carbon atoms arrangement in the surface layer of the fiber was more orderly than the core. A typical diffraction peak of carbon fiber at 26°, which corresponded to the (002) crystal plane, was observed, and the inter-planar spacing was approximately 0.34 nm. The CaO–MgO–SiO2 sintering agent formed MgAl2O4 and CaAl2Si2O8 phases in the interface between In Situ transformed carbon fibers and matrix, therefore improving the interface bonding, and thereby modifying the mechanical properties of the composites.

Dynamic Mechanical Properties of Ti–Al3Ti–Al Laminated Composites: Experimental and Numerical Investigation

The Ti–Al3Ti–Al laminated composites with different Al contents were prepared by vacuum hot pressing sintering technology. The effects of Al content on the dynamic mechanical properties of the composites were studied using the combination of Split Hopkinson Pressure Bar experiment and finite element analysis. The results showed that different Al content changes the fracture mode of the composites. The laminated composites without Al have higher brittleness and lower fracture strain. The Ti–Al3Ti–Al laminated composites containing 10–15%Al have better dynamic mechanical properties than those without Al, but the subsequent increase of Al content is not conducive to the improvement of strength. However, when the Al content in the specimen reaches 30%, the dynamic mechanical properties of the composites decrease, multi-crack phenomenon and relatively large strain occur, and the Al extruded from the layers fills the crack.

Strengthening and toughening mechanism of (W,Ti,Ta)C based cermet with the addition of c-BN@Al2O3

Core-shell particles are considered to be one of the primary strengthening and toughening methods of cermet. In this study, a (W,Ti,Ta)C based cermet was prepared by vacuum hot pressing sintering technology with the addition of c-BN@Al2O3 (Al2O3 coated c-BN). Localized high residual stress around the core-shell particles causes the strengthening and toughening mechanism at short distances behind the crack tip, such as crack deflection, secondary cracks, crack branching, and grain pull-out, even at a small volume fraction of 10 vol%. Compared with cermet without Al2O3 and c-BN, the flexural strength and fracture toughness of the cermet with 10 vol% c-BN@Al2O3 are increased by 35.62% and 28.45%, reaching 1462 ± 49 MPa and 13.68 ± 0.51 MPa m1/2, respectively.

Improved tribological performance of nickel based high temperature lubricating composites with addition of metallic oxides

NiCr–Mo–Bi2O3/TiO2 composites were prepared by vacuum hot-press sintering technology. The oxides of Bi4Ti3O12, Cr2O3 and Ti2O3 were formed by the reaction of Cr, Bi2O3 and TiO2 during the sintering process. The intensity of Bi4Ti3O12, Cr2O3 and Ti2O3 diffraction peaks became higher with the incremental content of Bi2O3 and TiO2. Mo element was mainly distributed in the matrix, which had the effect of solid solution strengthen in the composites. The microhardness and yield strength improved with addition of Mo. With Bi2O3 and TiO2 adding, the microhardness obviously increased. The microhardness of NiCr–10Mo–30Bi2O3/TiO2 composite was the highest of 491.6 HV. Nevertheless, the yield strength and ultimate strength significantly decreased. It may be caused by cracks in the interface between metallic oxides and matrix, where stress concentration would occur. The NiCr–10Mo–20Bi2O3/TiO2 composite had the lowest wear rate of 2.1 × 10−5 mm3/Nm, which was probably due to the oxidation layer was very dense and was very hard to shed in the process of friction. The composites after adding of Bi2O3 and TiO2 presented the excellent tribology property due to the fact that the Bi4Ti3O12 formed on the worn surfaces which had been proved to be an outstanding lubricant at high temperature.