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.     

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.     

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.     

Combined effect of Ni and nano-Y2O3 addition on microstructure,mechanical and high temperature behavior of mechanically alloyed W-Mo

Nanostructured tungsten (W) based alloys with the nominal compositions of W₇₀Mo₃₀ (alloy A), W₅₀Mo₅₀ (alloy B), and 1.0 wt.% nano-Y₂O₃ dispersed W₇₉Ni₁₀Mo₁₀ (alloy C) (all in wt.%) have been synthesized by mechanical alloying followed by compaction at 0.50, 0.75 and 1 GPa pressure for 5 mins and conventional sintering at 1500 °C for 2 h in Ar atmosphere. The microstructure, evolution of phases and thermal behavior of milled powders and consolidated products has been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), High resolution transmission electron microscopy (TEM), Energy dispersive spectroscopy (EDS) and differential scanning calorimetry (DSC). Minimum crystallite size of 29.4 nm and maximum lattice strain and dislocation density of 0.51% and 18.93 (10¹⁶/m²) respectively has been achieved in alloy C at 20 h of milling. Addition of nano-Y₂O₃ reduces the activation energy for recrystallization of W based alloys. Alloy C compacted at 1 GPa pressure shows enhanced sintered density, hardness, compressive strength and elongation of 95.2%, 9.12 GPa, 1.51 GPa, 19.5% respectively as well as superior wear resistance and oxidation resistance (at 1000 °C) as compared to other W-Mo alloys.     

Mechanical behavior of diamond matrix composites with ceramic Ti3(Si,Ge)C2 bonding phase

Polycrystalline diamond, PCD, compacts are usually produced by high pressure–high temperature (HP–HT) sintering. This technique always introduces strong internal stresses into the compacts, which may result in self-fragmentation or graphitization of diamond. This may be prevented by a bonding phase and Ti₃(Si,Ge)C₂ was so investigated. This layered ceramic was produced by Self Propagating High Temperature Synthesis and the product milled. The Ti₃(Si,Ge)C₂ milled powder was mechanically mixed, in the range 10 to 30 wt.%, with 3–6 μm diamond powder (MDA, De Beers) and compacted into disks 15 mm in diameter and 5 mm high. These were sintered at a pressure of 8.0 GPa and temperature of 2235 K in a Bridgman-type high pressure apparatus. The amount of the bonding phase affected the mechanical properties: Vickers hardness from 20.0 to 60.0 GPa and Young's modulus from 200 to 500 GPa, with their highest values recorded for 10 wt.% Ti₃(Si,Ge)C₂. For this composite fracture toughness was 7.0 MPa m^1/2, tensile strength 402 MPa and friction coefficient 0.08. Scanning and transmission electron microscopy, X-ray and electron diffraction phase analysis were used to examine the composites.     

Mechanical behavior of Fe65.5Cr4Mo4Ga4P12C5B5.5 bulk metallic glass

Fe_65.5Cr₄Mo₄Ga₄P₁₂C₅B_5.5 bulk amorphous rectangular bars with a cross-section of 2×2 mm² and a length of 30 mm were produced by copper mold casting. The as-cast bars as well as annealed samples were investigated by compression and Vickers hardness tests. The fracture strength for the as-cast samples σ_f is 2.8 GPa and the fracture strain ε_f is 1.9%. Upon annealing at 715 K for 10 min, i.e. at a temperature below the calorimetric glass transition, the fracture strain drops to 1.6% and no plastic deformation is observed. The Vickers hardness HV for the as-cast samples is about 885, and increases to 902 upon annealing. The fracture behavior of this Fe-based bulk glassy alloy is significantly different in comparison with the well-studied Zr-, Cu- or Ti-based good glass-formers. The fracture is not propagating along a well-defined direction and the fractured surface looks irregular. Instead of veins, the glassy alloy develops a high number of microcracks.     

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.     

Hardness and toughness enhancement of CeO2 addition to ZTA ceramics through HIPping technique

The aim of this research is to investigate the combined effects of CeO₂ additions and hot-isostatic pressing sintering (HIPping) technique on the hardness and toughness of ZTA ceramics. Addition of CeO₂ to ZTA ceramics leads to formation of a secondary phase (CeAl₁₁O₁₈) which played a vital role in affecting the Vickers hardness and toughness. Microstructure investigations showed that HIPping had a significant role in the removal of pores, and consequently affected both hardness and toughness of the samples. The highest Vickers hardness (1838.3 HV) and toughness (8.92 MPa·√m) were obtained with the 5 wt.% CeO₂ additions that also had the highest bulk density (4.48 g/cm³) and the lowest percentage of porosity (0.37%).     

Deposition and characterization of superhard biphasic ruthenium boride films

Recently, the superhardness of rhenium diboride films was reported. In this study the first successful preparation and characterization of ruthenium boride films is presented. The morphology, topography, microstructure and hardness of films, prepared by pulsed laser deposition, were investigated. The films, which are 0.7 μm thick, have a dense grain texture, and are composed of two phases Ru₂B₃ (main phase, 65% volume fraction) and RuB₂ (35%). The RuB₂ phase does not show any preferred orientation, while Ru₂B₃ is textured preferentially along the (1 1 4) and (1 0 5) directions, with crystallite growth parallel within 1.9° of average mismatch. The composite Vickers microhardness of the film–substrate systems was measured, and the intrinsic hardness of the films was separated using an area law-of-mixtures approach. The obtained films were found to be superhard, the intrinsic film hardness value (49 GPa) being much higher than that for the RuB₂ bulk used as the target for film deposition and than that for the Ru₂B₃ bulk.     

A comparative ab initio study of the ‘ideal’ strength of single crystal α- and β-Si3N4

In this study, the ideal tensile and shear strengths of single crystal α- and β-Si₃N₄ were calculated using an ab initio density functional technique. The stress–strain curve of the silicon nitride polymorph was calculated from simulations of predefined strain deformation in various directions. In particular, the ideal strength calculated for an applied tensile γ₁₁ strain, in the [1 0 0] plane, was estimated to be ∼51 and 57 GPa, for α- and β-Si₃N₄, respectively. Using a reported empirical method an estimate was also made of the Vickers indentation hardness of the α- and β-Si₃N₄ single crystals, ∼23.0 and 20.4 GPa, respectively. Moreover, a hardness estimate has been reported for Si₃N₄ in the literature: ∼21.0 GPa.     

Preparation of dense TiN1 − X (X = 0–0.4) by pulsed electric current sintering: Densification and mechanical behavior

Bulks of TiN_1 − X in the range of X = 0–0.4 with high density were prepared by a new method which comprises the reaction between TiN with TiH₂ through pulsed electric current sintering. X-ray diffractograms revealed that single fcc phase with nitrogen vacancies was achieved after sintering; further observation by scanning electron microscopy showed homogeneous structure in all samples and larger grain size by increasing the amount of TiH₂. The maximum Vickers hardness measured on the samples was approximately 31 GPa at X = 0.3. An increase in hardness was observed in non-stoichiometric samples even its larger grain size. The grain size-indent diagonal ratio was calculated from 1.2 at X = 0 to 5.6 at X = 0.4. The addition of TiH₂ showed an improvement in both densification and hardness without significant degradation of fracture toughness. Based on these results, the mechanical properties of TiN_1 − X bulks can be controlled as a function of TiH₂/TiN ratios in order to be used for different applications.     

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.     

Synthesis and properties of nanostructured dense LaB6 cathodes by arc plasma and reactive spark plasma sintering

Nanostructured polycrystalline LaB₆ ceramics were prepared by the reactive spark plasma sintering method, using boron nanopowders and LaH₂ powders with a particle size of about 30 nm synthesized by hydrogen dc arc plasma. The reaction mechanism of sintering, crystal structure, microstructure, grain orientations and properties of the materials were investigated using differential scanning calorimetry, X-ray diffraction, Neutron powder diffraction, Raman spectroscopy, transmission electron microscopy and electron backscattered diffraction. It is shown that nanostructured dense LaB₆ with a fibrous texture can be fabricated by SPS at a pressure of 80 MPa and temperature of 1300 °C for 5 min. Compared with the coarse polycrystalline LaB₆ prepared by traditional methods, the nanostructured LaB₆ bulk possesses both higher mechanical and higher thermionic emission properties. The Vickers hardness was 22.3 GPa, the flexural strength was 271.2 MPa and the maximum emission current density was 56.81 A cm^?2 at a cathode temperature of 1600 °C.     

Microstructure and mechanical properties of bulk nanocrystalline Al88Mm5Ni5Fe2 alloy consolidated at high pressure

Bulk nanocrystalline Al₈₈Mm₅Ni₅Fe₂ alloys have been produced by consolidation of pulverised melt-spun ribbons at high pressure. Different production procedures were explored to improve the quality of compaction of the resulting bulk samples. Quality of compaction of samples pressed at room temperature is clearly improved by increasing applied pressure from 2 to 7.7 GPa. All hot compacted samples had good quality of compaction. Onset of crystallisation shifts to higher temperatures as the applied pressure increases. Nanocrystalline powder fails to be compacted at room temperature even at 7.7 GPa. Mechanical properties were studied in terms of Vickers' microhardness. Relationship between microhardness and microstructure of the bulk samples was studied in the frame of two different theoretical models.     

La-based bulk metallic glasses with critical diameter up to 30 mm

We report composition optimization, thermal and physical properties of new La-based bulk metallic glasses with high glass forming ability (GFA) based on a ternary La₆₂Al₁₄Cu₂₄ alloy. By refining the (Cu, Ag)/(Ni, Co) and La/(Cu, Ag) ratios in the La–Al–(Cu,Ag)–(Ni, Co) pseudo-quaternary alloy, the formation of 30 mm diameter of La₆₅Al₁₄(Cu_5/6Ag_1/6)₁₁(Ni_1/2Co_1/2)₁₀ bulk metallic glass (BMG) alloy is achieved using water quenching. The origin of the high GFA was investigated from the kinetic, structural and thermodynamic points of view, and was found to be due to the smaller difference in Gibbs free-energy between the amorphous and crystalline phases in the pseudo-quaternary alloy. These alloys exhibit low glass transition temperatures, below 430 K, and relatively wide supercooled liquid regions of 40–60 K. Mechanical tests on these alloys show a fracture strength of 650 GPa, Vicker’s hardness 200 kg mm⁻², Young’s modulus 35 GPa, shear modulus 13 GPa and Poisson ratio 0.356. The La-based BMGs are useful for both scientific and engineering applications.     

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.     

Preparation of Si3N4-TaC and Si3N4-ZrC composite ceramics and their mechanical properties

Si₃N₄-TaC and Si₃N₄-ZrC composite ceramics with sintering additives were consolidated in the sintering temperature range of 1500–1600 °C using a resistance-heated hot-pressing technique. The addition of 20–40 mol% carbide improved the sinterability of the ceramics. The ceramics were densely sintered under 0–40 mol% TaC or ZrC at 1500 °C, 0–80 mol% TaC at 1600 °C, and 0–60 mol% ZrC at 1600 °C. In ceramics sintered at 1500 °C, the proportion of α-Si₃N₄ was larger than that of β-SiAlON; α-Si₃N₄ transformed mostly to β-SiAlON at 1600 °C. Carbide addition was effective in inhibiting α-Si₃N₄-to-β-SiAlON phase transformation. Young's modulus for the dense Si₃N₄-TaC and Si₃N₄-ZrC ceramics increased with the carbide amount, and the hardness of dense Si₃N₄-ZrC and Si₃N₄-TaC ceramics increased from 14 GPa to 17 GPa with increasing α-Si₃N₄ content. Dense Si₃N₄-TaC and Si₃N₄-ZrC ceramics, with larger quantities of α-Si₃N₄ sintered at 1500 °C, exhibited high hardness; the fracture toughness of these ceramics decreased with increasing α-Si₃N₄ proportion. Both the hardness and fracture toughness of the dense Si₃N₄-TaC and Si₃N₄-ZrC ceramics were strongly related to the proportion of α-Si₃N₄ in the sintered body.     

Effect of nitrogen and secondary carbide on the microstructure and properties of (Ti0.93W0.07)C–Ni cermets

Solid-solution powders of (Ti_0.93W_0.07)C and (Ti_0.93W_0.07)(C_0.7N_0.3) were synthesized via high-energy ball milling and carbothermal reduction processes. After blending powders with Ni and other carbides, cermets were prepared by a blending and sintering process at 1510 °C for 1 h. We observed a typical core/rim structure consisting of solid solution phases. We also found that secondary carbide and nitrogen have a remarkable influence on the cermet microstructures. Further, with an increase in the Mo₂C content, the mechanical properties of these cermets were enhanced significantly; the hardness of carbide and carbonitride cermets increased from 9.3 and 12.7 to 12.9 and 13.9 GPa, respectively. All results are discussed in terms of the thermodynamic stability and dissolution behavior of the constituent carbides and carbonitride.     

Structural and mechanical properties of Cr–Al–O–N thin films grown by cathodic arc deposition

Coatings of (Cr_xAl_1?x)_δ(O_1?yN_y)_ξ with 0.33 ? x ? 0.96, 0 ? y ? 1 and 0.63 ? δ/ξ ? 1.30 were deposited using cathodic arc evaporation in N₂/O₂ reactive gas mixtures on 50 V negatively biased WC–10 wt.% Co substrates from different Cr and Al alloys with three different Cr/Al compositional ratios. For N₂ < 63% of the total gas, ternary (Cr,Al)₂O₃ films containing <1 at.% of N forms; as determined by elastic recoil detection analysis. Increasing the N₂ fraction to 75% and above results in formation of quaternary oxynitride films. Phase analyses of the films by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy show the predominance of cubic Cr–Al–N and cubic-(Cr,Al)₂O₃ solid solutions and secondary hexagonal α-(Cr,Al)₂O₃ solid solution. High Cr and Al contents result in films with higher roughness, while high N and O contents result in smoother surfaces. Nanoindentation hardness measurements showed that Al-rich oxide or nitride films have hardness values of 24–28 GPa, whereas the oxynitride films have a hardness of ～30 GPa, regardless of the Cr and Al contents. Metal cutting performance tests showed that the good wear properties are mainly correlated to the oxygen-rich coatings, regardless of the cubic or corundum fractions.     

Experimental and atomistic simulation based study of W based alloys synthesized by mechanical alloying

The present research work represents the synthesis of nanostructured W based alloys with the nominal compositions of W₉₀Mo₁₀ and W₈₀Ni₁₀Mo₁₀ (all in wt.%) by mechanical alloying and followed by conventional sintering at 1500 °C for 2 h in Ar atmosphere. The microstructure and evolution of phases during milling and consolidated products are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Energy dispersive spectroscopy (EDS). Crystallite size of 38.7 nm and 40 nm and lattice strain of 0.41% and 0.33% are achieved in W₉₀Mo₁₀ and W₈₀Ni₁₀Mo₁₀ alloy respectively at 20 h of milling. The lattice parameter of all the investigated alloys shows initial expansion at 10 h of milling and then contraction at 20 h of milling. W₈₀Ni₁₀Mo₁₀ shows maximum sintered density of 94.8% as compared to W₉₀Mo₁₀. The hardness as well as the compressive strength of W₈₀Ni₁₀Mo₁₀ alloy records maximum value of 8.57 GPa and 1.18 GPa, respectively. The minimum wear depth is attained in W₈₀Ni₁₀Mo₁₀ alloy to that of W₉₀Mo₁₀. Molecular dynamic simulation based study is also performed to reveal the mechanisms responsible for deformation. Atomistic simulation shows that addition of nickel lowers the flow stress and increases ductility of W-Mo alloy studied at nanoscale. Results of atomistic simulation based study correlates well with experimental analysis.