Fe–B–C composites produced using spark plasma sintering

Fe–B–C composites were produced using iron and boron carbide powders. The powders were mixed to produce various compositions, ranging from 1 vol.% Fe to 80.1 vol.% Fe. Spark plasma sintering (SPS) was used to densify the composite powder green compacts. The sintering temperatures used ranged from 900 °C for the composites with a high iron content to 2000 °C for those with a high boron carbide content. It was evident that during the sintering process the iron reacted with the boron carbide. XRD analysis showed the presence of FeB, Fe₂B, Fe₃C, Fe₃(B_0.6C_0.4), Fe₂₃(B,C)₆ and residual carbon as reaction products. The composites were found to have hardness values between 9.8 and 33.1 GPa with the higher hardness being associated with the higher boron carbide contents. The fracture toughness values determined were in the range of 2.8–5.3 MPa m^0.5. With increasing iron content from 1 to 5 vol.%, it is evident that the FeB formed begins to embrittle the material rather than increase the fracture toughness as a result of the high residual stresses between the B₄C and FeB phases.     

Sintering of tungsten powder with and without tungsten carbide additive by field assisted sintering technology

Tungsten powder (0.6–0.9 μm) was sintered by field assisted sintering technology (FAST) at various processing conditions. The sample sintered with in-situ hydrogen reduction pretreatment and pulsed electric current during heating showed the lowest amount of oxygen. The maximum relative density achieved was 98.5%, which is from the sample sintered at 2000 °C, 85 MPa for 30 min. However, the corresponding sintered grain size was 22.2 μm. To minimize grain growth, nano tungsten carbide powder (0.1–0.2 μm) was used as sintering additive. By mixing 5 and 10 vol.% WC with W powder, densification was enhanced and finer grain size was obtained. Relative density above 99% with grain size around 3 μm was achieved in W–10 vol.% WC sintered at 1700 °C, 85 MPa, for 5 min.     

High-frequency induction-heated consolidation of nanostructured WC and WC-Al hard materials and their mechanical properties

The rapid sintering of nanostructured WC-Al composites in a short time was investigated with a focus on the mechanical properties (hardness and fracture toughness) and consolidation using high-frequency induction heated sintering. This process allowed very quick densification to near theoretical density and prohibited grain growth in the nano-materials. The addition of Al to WC facilitated consolidation and improved fracture toughness. The hardness and fracture toughness of WC with 5 vol.% Al and WC with 10 vol.% Al composites were higher than those of monolithic WC.     

Processing,phase evaluation and mechanical properties of MoSi2 doped 4TaC–HfC based UHTCs consolidated by spark plasma sintering

In this research, binary 4TaC–HfC based composites were consolidated using carbide materials and addition of 0–15 vol.% MoSi₂ by means of spark plasma sintering at 2000 °C. The nearly full dense and monophase specimens were fabricated with a relative density value higher than 99%. Mechanical tests revealed values of 18–19 GPa and 4–4.3 MPa·m^1/2, for average Vickers hardness and fracture toughness of the composites, respectively. Analysis of linear shrinkage during densification revealed that MoSi₂ addition increased densification rate and decreased the time required to reach full density at 2000 °C. It is proposed that at the intermediate stage of sintering, mass transfer can be accelerated by formation of a silicide based liquid phase and viscous flow mechanisms. The formation of binary 4TaC–HfC solid solution phase enhanced the densification process at the final stage of sintering.     

Sub-micron binderless tungsten carbide sintering behavior under high pressure and high temperature

Pure tungsten carbide (WC) compacts of about 200 nm grain size were prepared by high pressure and high temperature (HPHT) method. The best property sample with high relative density (99.2%), high Vickers hardness (2925 kg·mm^− 2) and high fracture toughness (8.9 MPa·m^1/2) was obtained in the condition of 1500 °C temperature and 5 GPa pressure. By means of scanning electron microscopy (SEM) and transmission electron microscope (TEM) observations, a large number of twins and stacking faults appeared in sintered samples, and the grain size of sintered samples maintained in the initial range. The XRD patterns of bulk samples reveal that there is a phase transition from WC to W₂C with the increasing of temperature. Moreover, the effect of HPHT condition for sintering kinetics, microstructure evolutions, and mechanical properties of the sintered samples were also discussed.     

Effect of initial particulate and sintering temperature on mechanical properties and microstructure of WC–ZrO2–VC ceramic composites

Owing to improving the mechanical properties of cemented carbides in high speed machining fields, a new composite tool material WC–ZrO₂–VC (WZV) is prepared from a mixture of yttria stabilized zirconia (YSZ) and micrometer VC particles by hot-press-sintering in nitrogenous atmosphere. Commercial WC, of which the initial particle sizes are 0.2 μm, 0.4 μm, 0.6 μm and 0.8 μm, is mixed with zirconia and VC powder in aqueous medium by following a ball mill process. The sintering behavior is investigated by isostatic pressing under different sintering temperature. The relative density and bending strength are measured by Archimedes methods and three-point bending mode, respectively. Hardness and fracture toughness are performed by Vickers indentation method. Microstructure of the composite is characterized by scanning electron microscopy (SEM). The correlations between initial particles, densification mechanism, sintering temperature, microstructure and mechanical properties are studied. Experimental results show that maximum densification 99.5% is achieved at 1650 °C and the initial particle size is 0.8 μm. When temperature is 1550 °C and particle size is 0.4 μm, the optimized bending strength (943 MPa) is obtained. The best hardness record is 19.2 GPa when sintering temperature is 1650 and particle size is 0.8 μm. The indention cracks propagate around the grain boundaries and the WC particles fracture, which is associated with particle and microcrack toughening mechanism.     

Synthesis,densification and characterization of EuB6

This paper presents the result of the investigation carried out on the synthesis and densification of Europium hexaboride. EuB₆ was synthesized by boron carbide reduction of Eu₂O₃. Effect of temperature on product quality was investigated. Pure EuB₆ powder was obtained at 1400 °C, in vacuum. Hot pressing of EuB₆ powder at 1750 °C and 32 MPa resulted in a density of only 86% of theoretical value. Addition of 10% TiSi₂ resulted in 96% of theoretical density at the same hot pressing conditions. The higher density achieved is attributed to liquid phase sintering due to low melting TiSi₂. Hardness and fracture toughness of the composite was measured as 18.49 GPa and 3.2 MPa m^1/2 respectively. Fractured surface of the composite reveals the mode of fracture to be transgranular. SEM and EDS analyses indicate the presence of TiSi₂ in EuB₆ matrix. Isothermal oxidation study revealed that EuB₆ + TiSi₂ composite has good oxidation resistance at 900 °C in air.     

Effect of Mo2C additions on the properties of SPS manufactured WC–TiC–Ni cemented carbides

The effect of spark plasma sintering (SPS) on the microstructure and mechanical properties of WC–Co and WC–Ni cemented carbides was studied, and compared to WC–Co produced by liquid phase sintering (LPS). There were finer WC grains with larger Co pools in the spark plasma sintered WC–Co, resulting in higher hardness and slightly lower fracture toughness than the liquid phase sintered WC–Co. The influence of the addition of 0.5–5 wt.%Mo₂C to WC-based cemented carbide containing 6.25 wt.%TiC and 9.3 wt.%Ni prepared by SPS was also studied. This addition improved the wettability between WC and Ni and lead to the improvements of microstructures, resulting in good combinations of hardness, fracture toughness and modulus of elasticity that were comparable to WC–Co based cemented carbides.     

Spark plasma sintering of TiC particle-reinforced molybdenum composites

In order to improve the recrystallization resistance and the mechanical properties of molybdenum, TiC particle-reinforcement composites were sintered by SPS. Powders with TiC contents between 6 and 25 vol.% were prepared by high energy ball milling. All powders were sintered both at 1600 and 1800 °C, some of sintered composites were annealed in hydrogen for 10 h at 1100 up to 1500 °C. The powders and the composites were investigated by scanning electron microscopy and XRD. The microhardness and the density of composites were measured, and the densification behavior was investigated. It turns out that SPS produces Mo–TiC composites, with relative densities higher than 97%.The densification behavior and the microhardness of all bulk specimens depend on both the ball milling conditions of powder preparation and the TiC content. The highest microhardness was obtained in composites containing 25 vol.% TiC sintered from the strongest milled powders. The TiC particles prevent recrystallization and grain growth of molybdenum during sintering and also during annealing up to 10 h at 1300 °C. Interdiffusion between molybdenum and carbide particles leads to a solid solution transition zone consisting of (Ti_1 −x Mo_x)C_y carbide. This diffusion zone improves the bonding between molybdenum matrix and TiC particles. A new phase, the hexagonal Mo₂C carbide, was detected by XRD measurements after sintering. Obviously, this phase precipitates during cooling from sintering temperature, if (Ti_1 −x Mo_x)C_y or molybdenum, are supersaturated with carbon.     

Effect of medium temperature on the dispersion of WC/ZrO2/VC composite powders

This article proposed a novel method to disperse WC/ZrO₂/VC composite powders so as to attain a perfectly uniform suspension. Besides using conventional dispersing means such as adding dispersant (PEG, polyethylene glycol), mechanical stirring, ultrasonic vibration and ball milling, the temperature adjustment of dispersing-medium distilled water had also been employed. The agglomerating and dispersing mechanisms were analyzed by means of TEM observation of WC/ZrO₂/VC composite powders dispersed under five different temperatures, with the results showing that the most uniform dispersion was obtained under the temperature of 100 °C based on the criterion for conglomeration number per unit. The dispersed WC/ZrO₂/VC composite powders were dried and consequently sintered by hot-press sintering in nitrogen atmosphere at 1580 °C with pressure of 30 MPa. The testing results of mechanical properties such as relative density, hardness, bending strength and fracture toughness show that the optimal properties are obtained by using the WC/ZrO₂/VC composite powders dispersed under 100 °C. The surface crack morphologies of sintered samples are investigated and the results show that crack extended in a more tortuous path for the sample sintered from well-dispersed composite powders.     

Microstructure and mechanical properties of ZrB2–SiC composites prepared by gelcasting and pressureless sintering

ZrB₂–SiC ceramic composites were prepared through water-based gelcasting and pressureless sintering. Effects of the pressureless sintering temperature (1500–2000 °C), heating rate (5–15 °C/min) and soaking time (0.5–2 h) on the relative density, microstructure and mechanical properties of the ZrB₂–SiC composites were investigated in detail. A sintering temperature of 2000 °C, a heating rate of 5 °C/min and a soaking time of 2 h were found to be the optimal pressureless sintering procedure. The relative density, flexural strength and fracture toughness of the ZrB₂–SiC composite prepared under the optimum condition were 97.8%, 403.1 ± 27.8 MPa and 4.05 ± 0.42 MPa·m^1/2, respectively.     

Effects of sintering processes on the mechanical properties and microstructure of Ti(C,N)-based cermet cutting tool materials

In this study, two types of Ti(C_0.7,N_0.3)-based cermet cutting tool materials (Ti(C,N)–Mo–Ni–Co, named as TMNC, and Ti(C,N)–WC–Mo–Ni–Co–TaC–HfC, named as TWMNCTH) were fabricated by the hot pressed sintering process at different temperatures (from 1380 °C to 1500 °C) for different holding times (from 30 min to 60 min) in a vacuum atmosphere and at a compressive stress of 32 MPa. The polished surface and the fracture surface of the two types of cermets were observed by a scanning electron microscope (BSE/SEM) and energy dispersive spectrometry (EDS), and the relationships among sintering processes, mechanical properties and microstructure were discussed. The experimental results showed that the sintering temperature and holding time both had a great influence on the flexural strength and a small effect on the hardness and the fracture toughness of the two types of cermets. The two cermets both had the optimal comprehensive mechanical properties when they were sintered at 1400 °C for 30 min. The sintering temperature and holding time also had a great influence on the microstructure of the two cermets, and the grain sizes increased when the sintering temperature varied from 1400 °C to 1500 °C and the holding time varied from 30 min to 60 min. The properties and microstructure of the two cermets were also compared. The results indicated that the cermet TWMNCTH had a lower flexural strength, a similar value of fracture toughness, a higher hardness and a thicker rim in the microstructure.     

Reduced-temperature processing and consolidation of ultra-refractory Ta4HfC5

TaC, HfC, and WC powders were subjected to high-energy milling and hot pressing to produce Ta₄HfC₅, a composite of Ta₄HfC₅ + 30 vol.% WC, and a composite of Ta₄HfC₅ + 50 vol.% WC. Sub-micron powders were examined after four different milling intervals prior to hot pressing. XRD was used to verify proper phase formation. SEM, relative density, and hardness measurements were used to examine the resulting phases. Hot pressed compacts of Ta₄HfC₅ showed densification as high as 98.6% along with Vickers hardness values of 21.4 GPa. Similarly, Ta₄HfC₅ + 30 vol.% WC exhibited 99% densification with a Vickers hardness of 22.5 GPa. These levels of densification were achieved at 1500 °C, which is lower than any previously reported sintering temperature for Ta₄HfC₅. Microhardness values measured in this study were higher than those previously reported for Ta₄HfC₅. The WC additions to Ta₄HfC₅ were found to improve densification and increase microhardness.     

Preparation of polycrystalline cubic boron nitride compact by high-pressure infiltration using cemented carbide

Polycrystalline cubic boron nitride (PcBN) compacts, using the infiltrating method in situ by cemented carbide (WC–Co) substrate, were sintered under high temperature and high pressure (HPHT, 5.2 GPa, 1450 °C for 6 min). The microstructure morphology, phase composition and hardness of PcBN compacts were investigated by using scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). The experimental results show that the WC and Co from WC–Co substrate spread into cubic boron nitride (cBN) layer through melting permeability under HPHT. The binder phases of WC, MoCoB and Co₃W₃C realized the interface compound of PcBN compact, and the PcBN layer formed a dense concrete microstructure. Additionally the Vickers hardness of 29.3 GPa and cutting test were performed when sintered by using cBN grain size of 10–14 μm.     

Effect of sintering temperature on the structure and properties of polycrystalline cubic boron nitride prepared by SPS

The polycrystalline cubic boron nitride (PcBN) with Si₃N₄–AlN–Al₂O₃–Y₂O₃ ceramic system as binding agents was prepared by spark plasma sintering (SPS). The starting materials Si₃N₄, AlN, Al₂O₃, Y₂O₃, and cBN in the ratio of 22:14:10:4:50 were heated to a sintering temperature between 1250 °C and 1450 °C at a heating rate of 300 °C/min, with a holding time of 5 min in nitrogen atmosphere. The microstructure, phase constitution, microhardness and fracture toughness of the prepared PcBN were then studied. It was shown that the Si₃N₄–AlN–Al₂O₃–Y₂O₃–cBN polycrystalline materials were densified in a very short sintering time resulting in materials with relative densities of more than 95%. When the sintering temperature increased, the microhardness and fracture toughness of prepared PcBN were also increased. The microhardness of PcBN prepared at 1250–1450 °C was between 28.0 ± 0.5 GPa and 48.0 ± 0.9 GPa, and its fracture toughness K_IC was from 7.5 ± 0.2 MPa m^1/2 to 11.5 ± 0.3 MPa m^1/2. Microstructure study showed that the ceramic-binding agents bonded with cubic boron nitride particles firmly. Our work demonstrated that spark plasma sintering technology could become a novel method for the preparation of PcBN cutting materials.     

Corrosion synthesis of boron carbide with pore and hollow structure

In this study, corrosion synthesis of boron carbide particles with pore size ranging from hundreds of nanometers to several micrometers was reported. Firstly, the pristine boron carbide powders which contain free carbon have been synthesized at 350 °C in a steel autoclave. As the pristine boron carbide was refluxed by HClO₄ at 170 °C for 1–2 h, the boron carbide particles with macropores were produced. Similarly, the boron carbide nanocages can also be obtained. The corrosion of the embedded amorphous and/or low crystallinity carbon/boron carbide using HClO₄ was considered for the formation of boron carbide with macropores and hollow nanocages.     

Processing and characterization of sintered reaction bonded Si3N4 ceramics

The present contribution reports the influence of nitridation and sintering conditions on the densification, microstructure, mechanical and thermal conductivity properties of sintered reaction bonded Si₃N₄ (SRBSN) mixed with 3.5% Y₂O₃-1.5% MgO. The nitridation of samples was carried out at 1450 and 1500 °C for different time schedules (2.5, 8 and 16 h) in order to increase β Si₃N₄ phase and subsequently sintering was performed at various temperatures (1850, 1900 and 1950 °C) for 10 h to enhance densification and properties of SRBSN ceramics. It was observed that the density of the samples slightly decreased and β Si₃N₄ phase significantly increased to 87% with increasing nitridation temperature and time. The density of gas pressure sintered (GPS) samples increased with increasing sintering temperature, almost full density was measured for all the samples at the respective sintering temperature (except those samples which were given nitridation at 1500 °C for 16 h). The microstructure of SRBSN samples were characterized by bimodal microstructure with equiaxed and rod like elongated grains and average grain size of SRBSN samples varied between 1.62 and 2.43 μm and aspect ratio of grains varied from 3.78 to 6.88 with varying the sintering temperature. Depending on the sintering density and microstructure, the SRBSN samples exhibited hardness (16.69 to 19.47 GPa), fracture toughness (7.02 to 9.20 MPa·m^1/2) and thermal conductivity (77.32 to 98.52 W/m·K). The coarsening of grain size and aspect ratio negatively affected hardness and fracture toughness, on the contrary the thermal conductivity increased. Among all samples, the SRBSN (which was subjected to nitridation at 1500 °C for 16 h; GPS at 1950 °C for 10 h) measured with good combination of hardness: 17.32 GPa, fracture toughness: 8.36 MPa·m^1/2and thermal conductivity: 98.52 W/m·K.     

High temperature mechanical properties of WC–Co hard metals

Understanding of the load situation and consequently the lifetime of cutting tools made of WC–Co hard metal requires quantitative data for thermo-mechanical properties. For the elevated temperatures present in application, these data are currently rather rare. The present work does discuss elastic material properties up to 1100 °C and compressive yield strength up to 900 °C, both as a function of Co content. The fracture toughness was determined as a function of the WC grain size and Co content up to 800 °C. Young's modulus and yield strength decrease with increasing temperature. A significant rise in fracture toughness was observed at 800 °C with increasing Co content and decreasing WC grain size. A possible reason for this increase is an increase in the plastic zone size at elevated temperatures.     

Microstructure of the cBN/WC6Co composite produced by the pulse plasma sintering (PPS) method

The cBN/WC6Co composite with the relative density of 99.8% and hardness of 2130 HV5 was produced by sintering at a temperature of 1150 °C under a pressure of 100 MPa for 5 min. The composite was sintered using electric pulses generated periodically by discharging a capacitor battery. The constituent phases of the composite, as identified by the NBED method, were the cBN, WC, and Co phases. The HR STEM observations have shown that the interfaces between the individual phases are continuous and no pores or precipitates of other phases can be seen there. Thanks to the specific heating realized by electric pulses, the composite is heated during each current pulse to a temperature of 1950 °C at a rate of 10⁵ °C/s. As a result of these quick changes of the temperature, transient thermal compressive stresses of about 3 GPa are induced in the composite, which results in the grains of the WC composite matrix being refined and defected.     

Reaction spark plasma sintering of niobium diboride

Niobium diboride (NbB₂) is synthesized and consolidated by the spark plasma sintering technique. Elemental reactants such as niobium (Nb) and boron (B) were subjected to two stage heat treatment, initially at 1200 °C for synthesis and followed by densification at the temperatures in the range of 1700 °C to 1900 °C. High dense NbB₂ (~ 97.7%ρ_th) is obtained at 1900 °C after 15 min holding period. Load application during heat treatment stage is found to improve the sinterability of the niobium diboride compacts. Hardness, elastic modulus and indentation fracture toughness of the high dense NbB₂ are measured as 20.25 GPa, 539 GPa and 4 MPa m^1/2 respectively.