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.     

Ti (C,N)-based cermets sintered under high pressure

In this study, high pressure and high temperature sintering (HPHT) is adopted in the cermet fabrication process, and the microstructure and mechanical properties of cermets with TiC_0.5N_0.5–15WC–10Mo₂C–5TaC–10Ni–10Co (wt%) sintered under 5 GPa and different temperatures (900–1600 °C) using 6 × 14 MN cubic press are investigated. Results show that the densities of samples can reach up to 7.00 g/cm³. Vickers hardness and fracture toughness of the products are over 1727 HV30 and 7.2 MPa m^1/2 respectively. In addition, the sintering results are compared with the data that obtained from commercial samples which produced via conventional sintering technique. The conclusion is that high density and high hardness cermets can be obtained through HPHT sintering.     

Effect of B-S co-doping on large diamonds synthesis under high pressure and high temperature

Large single-crystal diamonds with n-type semiconductor were synthesized from S/B-S co-doping FeNiCo-C system under high pressure and high temperature (HPHT) in this paper. It was found that the slight variation of the additive S content had not made obvious change for the color of diamonds synthesized from FeNiCo-C system. The B-S co-doping samples became more transparent and yellow than the samples added alone by S. The analysis of X-ray photoelectron spectroscopy (XPS) spectra and Fourier transform infrared (FTIR) spectroscopy showed the presence of B and S in the obtained diamonds. The electrical properties of large diamond crystals were tested by Van der Pauw method with a four-point probe. The highest value of the hall mobility was 628.726 cm²/vs. And the lowest value of the resistivity was 9.33 × 10⁵ Ω·cm with boron additive of 0.8 wt.% and sulfur of 2 wt.% doping to diamond which was confirmed as n-type. This work indicated that B-S co-doping to synthesize diamond crystals was a trend to promote the electrical properties of large diamond crystals.     

Spark plasma sintering of a commercially available granulated zirconia powder: Comparison with hot-pressing

A commercially available granulated TZ3Y powder has been sintered by hot-pressing (HP). The “grain size/relative density” relationship, referred to here as the “sintering path”, has been established for a constant value of the heating rate (25 °C min^?1) and a constant value of the macroscopic applied pressure (100 MPa). It has then been compared to that obtained previously on the same powder but sintered by spark plasma sintering (SPS, heating rate of 50 °C min^?1, same applied macroscopic pressure). By coupling the analysis of a sintering law (derived from creep rate equations) and comparative observations of sintered samples using transmission electron microscopy, a hypothesis about the densification mechanism(s) involved in SPS and HP has been proposed. Slight differences in the densification mechanisms lead to scars in the microstructure that explain the higher total ionic conductivity measured, in the temperature range 300–550 °C, when SPS is used for sintering.     

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.     

Influence of consolidation process and sintering temperature on microstructure and mechanical properties of near nano- and nano-structured WC-Co cemented carbides

In this paper the influence of the consolidation process and sintering temperature on the properties of near nano- and nano-structured cemented carbides was researched. Samples were consolidated from a WC 9-Co mixture by two different powder metallurgy processes; conventional sintering in hydrogen and the sinter-HIP process. Two WC powders with different grain growth inhibitors were selected for the research. Both WC powders used were near nanoscaled and had a grain size of 150 nm and a specific surface area of 2.5 m²/g. Special emphasis was placed on microstructure and mechanical properties; hardness and fracture toughness of sintered samples. Consolidated samples are characterised by different microstructural and mechanical properties with respect to the sintering temperature, the consolidation process used and grain growth inhibitors in starting powders. Increasing sintering temperature leads to microstructure irregularities and inferior hardness, especially for samples sintered in hydrogen. The addition of Cr₃C₂ in the starting powder reduced a carbide grain growth during sintering, improved microstructural characteristics, increased Vickers hardness and fracture toughness. The relationship between hardness and fracture toughness is not linear. Palmqvist toughness does not change with regard to sintering temperature or the change of Vickers hardness.     

Sintering behavior and non-linear properties of ZnO varistors processed in microwave electric and magnetic fields at 2.45 GHz

A study of the densification behavior and grain growth mechanisms of ZnO-based varistors composed of 98 mol.% ZnO–2 mol.% (Bi₂O₃, Sb₂O₃, Co₃O₄, MnO₂) has been carried out. The pressed samples were sintered in microwave electric (E) and magnetic (H) fields using a single-mode cavity of 2.45 GHz. The effect of the sintering temperature (900–1200 °C), holding time (5–120 min) and sintering mode (E, H) on the microstructure and electrical properties of the sintered varistor samples were investigated. The grain growth kinetics was studied using the simplified phenomenological equation Gⁿ = kte^(?Q/RT). The grain growth exponent (n) and apparent activation energy (Q) values were estimated for both electric and magnetic heating modes and were found to be n = 3.06–3.27, Q = 206–214 kJ mol^?1, respectively. The lower value of n estimated in the E field was attributed to a volume diffusion mechanism, whereas the higher n value in the H field sintering was correlated mainly to a combined effect of volume and surface diffusion processes. Samples sintered in the H and E fields showed high final densities. Moreover, the ones sintered in the H field presented slightly higher density values and bigger grains for all sintering temperatures than E field heated ones. The optimal sintering conditions were achieved at 1100 °C for a 5 min soaking time for both H and E field processed samples, where respectively densities of 99.2 ± 0.5% theoretical density (TD) and 98.3 ± 0.5% TD along with grain size values of G = 7.2 ± 0.36 μm and G = 6.6 ± 0.33 μm were obtained. Regarding the electrical properties, breakdown voltage values as high as 500–570 V mm^?1 were obtained, together with high non-linear coefficients α = 29–39 and low leakage currents (J_l ≈ 5 × 10^?3 mA cm^?2), respectively, for E and H field sintered varistor samples. Moreover, samples sintered in an H field systematically exhibited higher breakdown voltage values compared to the ones sintered in the E field. This was attributed to an improved coupling between the H field and the present dopants within the ZnO matrix, this latter being mostly semiconductive, thus leading to an enhanced reactivity and improved properties of the electrostatic barrier.     

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.     

Effect of impingement angle and WC content on high temperature erosion behavior of SiC-WC composites

Hot pressed dense SiC-(0, 10, 30 or 50 wt%)WC composites were subjected to erosion against SiC particles at 800 °C. Effects of WC content and angle of impingement (30°, 60° or 90°) on the erosion performance of composites were evaluated. Erosion rate ranged from 2.1 × 10² mm³/kg to 7.7 × 10² mm³/kg with varying WC content or angle of impingement. The erosion rate of the composites increased with increasing the impingement angle from 30° to 90°, and decreased with WC content up to 30 wt%. Minimum and maximum erosion wear rates were obtained for SiC-30 wt% WC composites at 30° and for SiC-50 wt% WC composites at normal impact, respectively. Grain fracture and pull-out were observed as major mechanisms of material removal for the composites. Decreased angle of impingement led to reduced grain fracture and pull-out, and hence reduction in material removal. Owing to increased fracture toughness with incorporation of WC particles, the composites showed less fracture and removal of WC particles up to 30 wt% reinforcement.     

Near-nano and coarse-grain WC powders obtained by the self-propagating high-temperature synthesis and cemented carbides on their basis. Part I: Structure,composition and properties of WC powders

Near-nano WC powders with mean grain sizes of about 200 nm were prepared by the SHS method including the reduction of WO₃ by Mg in the presence of carbon and regulating additives. The chemical leaching and refinement of the SHS reaction products allowed one to obtain stoichiometric WC containing only traces of oxygen and magnesium. The thermal reduction of WO₃ and V₂O₅ by magnesium in the presence of carbon resulted in obtaining two carbide phases of WC and complex carbide (W,V)C with the fcc crystal lattice having a grain size of less than 300 nm. It was established that the tungsten oxide reduction by magnesium in the presence of carbon cannot be used to synthesize coarse-grain WC powders. Coarse-grained WC powders were obtained using the W + C mixture heated to high temperatures by a simultaneous exothermic reaction of interaction between magnesium perchlorate Mg(ClO₄) and magnesium. The coarse-grain WC powder synthesized in such a way is nearly stoichiometric and consists of sintered round-shaped agglomerates with the average grain size of up to 16 μm and containing only traces of magnesium and oxygen. The agglomerates comprise WC single-crystals of roughly 1 μm to 8 μm in size.     

Microstructural analysis of wear micromechanisms of WC–6Co cutting tools during high speed dry machining

This original study investigates the damages of WC–6Co uncoated carbide tools during dry turning of AISI 1045 steel at mean and high speeds. The different wear micromechanisms are explained on the basis of different microstructural observations and analyses made by different techniques: (i) optical microscopy (OM) at macro-scale, (ii) scanning electron microscopy (SEM), with back-scattered electron imaging (BSE) at micro-scale, (iii) energy dispersive spectroscopy (EDS), X ray mapping with wavelength dispersive spectroscopy (WDS) for the chemical analyses and (iv) temperature evolution during machining. We noted that at conventional cutting speed Vc ≤ 250 m/min, normal cutting tool wear types (adhesion, abrasion and built up edge) are clearly observed. However, for cutting speed Vc > 250 m/min a severe wear is observed because the behavior of the WC–6Co grade completely changes due to a severe thermomechanical loading. Through all SEM micrographs, it is observed that this severe wear consists of several steps as: excessive deformation of WC–6Co bulk material and binder phase (Co), deformation and intragranular microcracking of WC, WC grain fragmentation and production of WC fragments in the tool/chip contact. Thus, the WC fragments accumulated at the tool/chip interface cause abrasion phenomena and pullout WC from tool surface. WC fragments contribute also to the microcutting and microploughing of chips, which lead to form a transferred layer at the tool rake face. Finally, based on the observations of the different wear micromechanisms, a scenario of WC–6Co damages is proposed through to a phenomenological model.     

Effects of ZrC and SiC addition on the microstructures and mechanical properties of binderless WC

ZrC-added WC ceramics and SiC-added WC–2 mol% ZrC ceramics were sintered at 1800 °C using a resistance-heated hot-pressing machine. Dense WC ceramics containing 0–1 mol% ZrC and WC–2 mol% ZrC ceramics containing 1–6 mol% SiC were obtained. The reaction products W₂C, ZrO₂ and ZrC-based solid solutions were formed in the ZrC-added WC ceramics during sintering. The relative amount of W₂C reached zero at 2 mol% ZrC, increased in the range of 2–6 mol% ZrC, and decreased again above 6 mol% ZrC. The average WC grain size decreased from 0.49 μm for the WC ceramic to 0.24 μm at 4 mol% ZrC. The SiC addition of 1–2 mol% to the WC–2 mol% ZrC ceramics caused abnormal growth of WC grains. The Vickers hardness of the ZrC-added WC ceramics decreased to 17 GPa at 2 mol% ZrC. The hardness of the SiC-added WC–2 mol% ZrC ceramics increased from 12.4 at 2 mol% SiC to 21.5 GPa at 6 mol% SiC. The fracture toughness of the ZrC-added WC ceramics decreased from 6.2 MPa m^0.5 for the WC ceramic to 5.2 MPa m^0.5 at 4 mol% added ZrC. The fracture toughness of the WC–2 mol% ZrC ceramics with 6 mol% SiC were relatively high at 6.7 MPa m^0.5. The addition of SiC to WC-based ceramics thus improved both hardness and fracture toughness.     

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.     

Mechanical properties and rapid sintering of nanostructured WC and WC–TiAl3 hard materials by the pulsed current activated heating

Tungsten carbides are primarily used as cutting tools and abrasive materials in the form of composites with a binder metal, such as Co or Ni. However, these binder phases have inferior chemical characteristics compared to the carbide phase and the high cost of Ni or Co. Therefore, low corrosion resistance of the WC–Ni and WC–Co cermets has generated interest in recent years for alternative binder phases. In this study, TiAl₃ was used as a novel binder and consolidated by the pulsed current activated sintering (PCAS) method. Highly dense WC–TiAl₃ with a relative density of up to 99% was obtained within 2 min by PCAS under a pressure of 80 MPa. The method was found to enable not only the rapid densification but also the inhibition of grain growth preserving the nano-scale microstructure. The average grain sizes of the sintered WC and WC–TiAl₃ were lower than 100 nm. The addition of TiAl₃ to WC enhanced the toughness without great decrease of hardness due to crack deflection and decrease of grain size.     

Microstructure,grain size distribution and grain shape in WC–Co alloys sintered at different carbon activities

The properties of cemented carbides strongly depend on the WC grain size and it is thus crucial to control coarsening of WC during processing. The aim of this work was to study the effect of sintering at different carbon activities on the final microstructure, as well as the coarsening behavior of the WC grains, including the size distribution and the shape of WC grains. These aspects were investigated for five WC–Co alloys sintered at 1410 °C for 1 h at different carbon activities in the liquid, in the range from the graphite equilibrium (carbon activity of 1) to the eta (M₆C) phase equilibrium (carbon activity of 0.33). The grain size distribution was experimentally evaluated for the different alloys using EBSD (electron backscatter diffraction). In addition, the shape of the WC grains was evaluated for the different alloys. It was found that the average WC grain size increased and the grain size distribution became slightly wider with increasing carbon activity. Comparing the two three-phase (WC–Co–eta and WC–Co–graphite) alloys a shape change of the WC grains was observed with larger grains having more planar surfaces and more triangular shape for the WC–Co–graphite alloy. It was indicated that in alloys with a relatively low volume fraction of the binder phase the WC grain shape is significantly affected by impingements. Moreover, after 1 h of sintering the WC grains are at a non-equilibrium state with regards to grain morphology.     

Effect of sintering temperature on the structure and magnetic properties of SmCo5/Fe nanocomposite magnets prepared by spark plasma sintering

Highly dense SmCo₅/Fe nanocomposite bulk magnets were prepared by spark plasma sintering of magnetic field-milled SmCo₅/Fe nanocrsytalline powders. The sintering experiments were conducted with varying temperatures of 973–1123 K. The resultant bulk materials had densities of 85–98% and mean grain sizes of 17–30 nm. The SEM analysis showed that the bulk samples prepared at higher sintering temperature exhibited dense and uniform microstructure. The XRD studies in complement with energy dispersive X-ray analysis revealed that the bulk magnets sintered at or above 1073 K exhibited Sm(Co,Fe)₅ as main phase, along with other secondary phases such as Sm₂(Co,Fe)₁₇ and α-Fe(Co). A single-phase behavior with high remanence ratios (0.67–0.77) for the nanocomposite magnets was demonstrated by the magnetic measurements. In the present study, the sintering temperature of 1073 K was found to be optimum in achieving relatively high coercivity (8.2 kOe), magnetization (97.5 emu/g) and energy product (278.7 kJ/m³) for the SmCo₅/Fe nanocomposite bulk magnets.     

Stainless steel bonded NbC matrix cermets using a submicron NbC starting powder

The microstructure and mechanical properties of 316 L and 430 L stainless steel bonded NbC cermets were assessed. NbC starting powder mixtures with 15 and 30 vol% steel binder were pressureless vacuum sintered for 1 h at 1420 °C. The liquid forming temperature and shrinkage behaviour of the green powder compacts were investigated by differential scanning calorimetry and dilatometry. Microstructural and compositional analysis were conducted by electron probe microanalysis (EPMA) and XRD to investigate the effect of the steel binder on NbC grain growth and Cr-rich carbide precipitation. Rapid NbC grain growth was observed and the average NbC grain size decreased with increasing binder content. The residual Cr-rich carbide located at NbC grain boundaries can be eliminated by the addition of carbide forming metal precursors such as TiH₂ or by a thermal annealing process of the sintered NbC cermets at 1200 °C. The hardness and fracture toughness of the NbC-steel cermets was influenced by the steel binder type and content. A maximum hardness of 13.6 GPa was measured for the NbC-15 vol% 430 L cermet, combined with a modest fracture toughness of 7.3 MPa m^1/2.     

High energy milling on tungsten powders

Nanocrystalline tungsten powders were produced by high energy mechanical milling, using both tungsten carbide (WC) and tungsten (W) balls as grinding media. X-ray diffraction study indicated that the lattice parameter of tungsten decreased (from 3.162 to 3.149 Å) with increasing milling time from 0 to 15 h. Considerable decrease in particle size was observed in both W and WC grinding media after 15 h of milling duration. Rietveld analysis of the X-ray data along the Williamson-Hall plots revealed that the crystallite size also decreased with increasing milling time. Chemical analyses showed that the total amount of cobalt and carbon in the milled samples were higher in WC grinding media, as compared to W grinding media. The sintered density increased from 80% to 98% from as received to milled tungsten powders, when sintered at 1790 °C. The mechanical properties of as sintered alloys were evaluated and were found to be strongly influenced by the milling time and grinding media.     

Two-step hot pressing of bimodal micron/nano-ZrB2 ceramic with improved mechanical properties and thermal shock resistance

The densification and grain growth behaviors for micron- and nano-sized ZrB₂ particles were investigated. The densification on-set temperature (T_d-micron) and grain growth on-set temperature (T_g-micron) for micron-sized ZrB₂ particles were about 1500 °C and 1800 °C, respectively. And the densification on-set temperature (T_d-nano) and grain growth on-set temperature (T_g-nano) for nano-sized ZrB₂ particles were about 1300 °C and 1500 °C, respectively. A bimodal micron/nano-ZrB₂ ceramic was therefore prepared using a novel two-step hot pressing. A high relative density of 99.2%, an improved flexural strength of 580.2 ± 35.8 MPa and an improved fracture toughness of 7.2 ± 0.4 MPa·m^1/2 were obtained. The measured critical thermal shock temperature difference (ΔT_c) for this bimodal micron/nano-ZrB₂ ceramic was as high as 433 °C.     

Microstructure and tribological performance of NbC-Ni cermets modified by VC and Mo2C

The current study reports on the influence of the addition of 5–15 vol% VC or/and Mo₂C carbide on the microstructure and mechanical properties of nickel bonded NbC cermets, which are compared to cobalt bonded NbC cermets. The NbC, Ni and secondary carbides powder mixtures were liquid phase sintered for 1 h at 1420 °C in vacuum. The fully densified cermets are composed of a cubic NbC grains matrix and an evenly distributed fcc Ni binder. NbC grain growth was significantly inhibited and a homogeneous NbC grain size distribution was obtained in the cermets with VC/Mo₂C additions. The mechanical properties of the NbC-Ni matrix cermets are strongly dependent on the carbide and Ni binder content and are directly compared to their NbC-Co equivalents. The liquid phase sintered NbC-12 vol% Ni cermet had a modest Vickers hardness (HV₃₀) of 1077 ± 22 kg/mm² and an indentation toughness of 9.1 ± 0.5 MPa·m^1/2. With the addition of 10–15 vol% VC, the hardness increased to 1359 ± 15 kg/mm², whereas the toughness increased to 11.3 ± 0.1 MPa·m^1/2. Addition of 5 and 10 vol% Mo₂C into a NbC-12 vol% Ni mixtures generated the same values in HV₃₀ and K_IC when compared to VC additions. A maximum flexural strength of 1899 ± 77 MPa was obtained in the cermet with 20 vol% Ni binder and 4 vol% VC + 4 vol% Mo₂C addition, exhibiting a high fracture toughness of 15.0 ± 0.5 MPa·m^1/2, but associated with a loss in hardness due to the high Ni content. The dry sliding wear behaviour was established at room temperature and 400 °C from 0.1 to 10 m/s.