Pulsed electric current sintered Fe3Al bonded WC composites

WC based composites with 5, 10 and 20 vol.% Fe₃Al binder were consolidated via pulsed electric current sintering (PECS) in the solid state for 4 min at 1200 °C under a pressure of 90 MPa. Microstructural analysis revealed a homogeneous Fe₃Al binder distribution, ultrafine WC grains and dispersed Al₂O₃ particle clusters. The WC-5 vol.% Fe₃Al composite combines an excellent Vickers hardness of 25.6 GPa with very high Young’s modulus of 693 GPa, a fracture toughness of 7.6 MPa m^1/2 and flexural strength of 1000 MPa. With increasing Fe₃Al binder content, the hardness and stiffness decreased linearly to 19.9 and 539 GPa, respectively with increasing binder content up to 20 vol.%, while the fracture toughness and flexural strength were hardly influenced by the binder content. Compared to WC–Co cemented carbides processed under exactly the same conditions, the WC–Fe₃Al composites exhibit a substantially higher hardness and Young’s modulus.     

Optimization of processing conditions and characterization investigations of ceramic injection molded and sintered ZrO2/WC composites

In this investigation, 3 mol% Y₂O₃ stabilized ZrO₂-based composites reinforced with 10 vol.%, 20 vol.% and 40 vol.% WC (named as 3Y-TZP/10WC, 3Y-TZP/20WC and 3Y-TZP/40WC) were fabricated by using injection molding and sintering. Mechanical properties of these composites varied due to WC addition and dwelling time. Density, strength and toughness decreased with shorter dwelling time and increasing WC content however a significant enhancement in fracture toughness was obtained by 3Y-TZP/20WC composite which had 9.2 MPa m^1/2 toughness. Severe unlubricated wear tests which were performed under 55 N normal load and 45 km sliding distance showed that 3Y-TZP/20WC composite had the lowest wear rate and wear volume values which are 2 × 10⁻⁸ mm³/(N m⁻¹) and 0.05 mm³, respectively.     

An evaluation of Al2O3-ZrO2 composites produced by coprecipitation method

The objective of this work is to produce Al₂O₃-ZrO₂ composite from nano-sized powders processed by coprecipitation method. Al₂O₃ and mixture of Al₂O₃ + 10 wt.% ZrO₂ precipitated successfully by chemical route from aluminum sulfate and zirconium sulfate were pressed under uniaxial compression of 170 MPa and sintered at 1600 °C for 1 h. SEM investigations revealed that, pure alumina sample has a microstructure with coarse grains which anisotropically grown up to 30-40 μm in size. In alumina-zirconia composite, the structure consists of very fine equiaxed grains of typically 2 μm in which zirconia precipitates were uniformly dispersed. By adding zirconia to alumina, hardness and indentation fracture toughness were increased from 11.6 GPa to 16.8 GPa and from 3.2 MPa m^1/2 to 4.9 MPa m^1/2, respectively. Improvement in fracture toughness was attributed to bridging effects of zirconia particles as well as transformation toughening.     

Densification and mechanical properties of fine-grained Al2O3-ZrO2 composites consolidated by spark plasma sintering

Alumina matrix composites containing 5 and 10 wt% of ZrO₂ were sintered under 100 MPa pressure by spark plasma sintering process. Alumina powder with an average particle size of 600 nm and yttria-stabilized zirconia with 16 at% of Y₂O₃ and with a particle size of 40 nm were used as starting materials. The influence of ZrO₂ content and sintering temperature on microstructures and mechanical properties of the composites were investigated. All samples could be fully densified at a temperature lower than 1400 °C. The microstructure analysis indicated that the alumina grains had no significant growth (alumina size controlled in submicron level 0.66-0.79 μm), indicating that the zirconia particles provided a hindering effect on the grain growth of alumina. Vickers hardness and fracture toughness of composites increased with increasing ZrO₂ content, and the samples containing 10 wt% of ZrO₂ had the highest Vickers hardness of 18 GPa (5 kg load) and fracture toughness of 5.1 MPa m^1/2.     

Characterization and mechanical testing of alumina-based nanocomposites reinforced with niobium and/or carbon nanotubes fabricated by spark plasma sintering

Alumina-based nanocomposites reinforced with niobium and/or carbon nanotubes (CNT) were fabricated by advanced powder processing techniques and consolidated by spark plasma sintering. Raman spectroscopy revealed that single-walled carbon nanotubes (SWCNT) begin to break down at sintering temperatures >1150 °C. Nuclear magnetic resonance showed that, although thermodynamically unlikely, no Al₄C₃ formed in the CNT-alumina nanocomposites, such that the nanocomposite can be considered as purely a physical mixture with no chemical bond formed between the nanotubes and ceramic matrix. In addition, in situ single-edge notched bend tests were conducted on niobium and/or CNT-reinforced alumina nanocomposites to assess their toughness. Despite the absence of subcritical crack growth, average fracture toughness values of 6.1 and 3.3 MPa m^1/2 were measured for 10 vol.% Nb and 10 vol.% Nb-5 vol.% SWCNT-alumina, respectively. Corresponding tests for the alumina nanocomposites containing 5 vol.% SWCNT, 10 vol.% SWCNT, 5 vol.% double-walled-CNT and 10 vol.% Nb yielded average fracture toughnesses of 3.0, 2.8, 3.3 and 4.0 MPa m^1/2, respectively. It appears that the reason for not observing improvement in fracture toughness of CNT-reinforced samples is because of either damage to CNTs or possibly non-optimal interfacial bonding between CNT-alumina.     

Effects of adding yttrium nitrate on the mechanical properties of hot-pressed AlN ceramics

Aluminum nitride (AlN) ceramics were prepared by hot-pressing with Y(NO₃)₃·6H₂O as sintering additive. The mechanical properties including flexural strength, Vickers’ hardness, and fracture toughness were studied. The relative density and mechanical property of the monolithic AlN were improved by adding Y(NO₃)₃·6H₂O, which decreased the porosity. At 2 wt% Y₂O₃, the AlN ceramic exhibited the highest strength of 383 MPa, the highest hardness of 15.39 GPa, and the highest fracture toughness of 3.1 MPa m^1/2. However, doping with more additive, the strength, hardness, and toughness of AlN ceramics decreased because of the weak interfacial bonding between AlN matrix and the yttrium aluminates phase.     

On the wide range of mechanical properties of ZTA and ATZ based dental ceramic composites by varying the Al2O3 and ZrO2 content

In this work the influence of pressureless sintering on the Vickers hardness and fracture toughness of ZrO₂ reinforced with Al₂O₃ particles (ATZ) and Al₂O₃ reinforced with ZrO₂ particles (ZTA) has been investigated. The ceramic composites were produced by means of uniaxial compacting at 50 MPa and the green compacts were heated to 1250 °C using a heating rate of 10 °C min⁻¹, then to 1500 °C at 6 °C min⁻¹ and maintained at this temperature during 2 h. After sintering, relative density over 94%, hardness values between 9.5 and 21.9 GPa, and fracture toughness as high as 3.6 MPa m^1/2 were obtained. The presence of TZ-3Y particles on the grain boundaries suggests that they inhibit notably the alumina grain growth. The grain sizes of pure Al₂O₃ and TZ-3Y as well as Al₂O₃ and TZ-3Y in the 20 wt% Al₂O₃+80 wt% TZ-3Y composite were 1.27 ± 0.51 μm, 0.57 ± 0.12 μm, 0.65 ± 0.19 μm and 0.41 ± 0.14 μm, respectively. The 20 wt% Al₂O₃ + 80 wt% ZrO₂ + 3 mol% Y₂O₃ (TZ-3Y) composite showed a hardness of 16.05 GPa and the maximum fracture toughness (7.44 MPa m^1/2) with an average grain size of 0.53 ± 0.17 μm. On the other side, the submicron grain size and residual porosity seem to be responsible for the high hardness and fracture toughness obtained. The reported values were higher than those obtained by other authors and are in concordance with international standards that could be suitable for dental applications.     

Mechanical properties and rapid consolidation of nanostructured WC and WC–Al2O3 composites by high-frequency induction-heated sintering

The short-term rapid sintering of nanostructured WC and WC–Al₂O₃ hard materials was fabricated using the high-frequency induction-heating sintering (HFIHS) process. The sintering behaviors, microstructure, and mechanical properties of the WC and WC–Al₂O₃ composites were investigated. The addition of Al₂O₃ to WC can facilitate sintering, and the grain size of WC decreases as the addition of Al₂O₃ is increased; furthermore, the hardness and fracture toughness of WC-15 vol% Al₂O₃ are greater than those of monolithic WC and Al₂O₃.     

Comparison of ZrB2-SiC ceramics with Yb2O3 additive prepared by hot pressing and spark plasma sintering

This work compared phase composition, microstructures and mechanical properties of ZrB₂—20 vol.% SiC—5 vol.%Yb₂O₃ prepared by hot pressing (HP) and spark plasma sintering (SPS) in vacuum. Despite the same raw material composition, the two densification techniques led to the appearance of different secondary phases. The HP ceramics had coarsened microstructure, predominantly intergranular fracture characteristic and high fracture toughness. The SPS ceramics exhibited refined microstructure, transgranular fracture model and high bending strength.     

Y2O3- and NbC-doped ultrafine WC–10Co alloys by low pressure sintering

The nanocomposite WC–10Co powders were prepared through planetary ball milling method. The effects of minor Y₂O₃ and NbC additions on structure, hardness and fracture toughness of ultrafine WC–10Co alloys were investigated using X-ray diffraction, optical microscope, scanning electron microscope and mechanical properties tests. The results show that minor NbC additions refine the WC grains and increase the hardness of the base alloys. The additions of Y₂O₃ decrease the volume fraction of Co₃W₃C phase in ultrafine WC–10Co alloys after low pressure sintering, and thus increase the fracture toughness of the base alloys from 6.2 MN m^−3/2 to 9.8 MN m^−3/2.     

Fabrication of full density near-nanostructured cemented carbides by combination of VC/Cr3C2 addition and consolidation by SPS and HIP technologies

The aim of present work is to study the effect of VC and/or Cr₃C₂ in densification, microstructural development and mechanical behavior of nanocrystalline WC-12wt.%Co powders when they are sintered by spark plasma sintering (SPS) and hot isostatic pressing (HIP). The results were compared to those corresponding to conventional sintering in vacuum. The density, microstructure, X-ray diffraction, hardness and fracture toughness of the sintered materials were evaluated. Materials prepared by SPS exhibits full densification at lower temperature (1100 °C) and a shorter stay time (5 min), allowing the grain growth control. However, the effect of the inhibitors during SPS process is considerably lower than in conventional sintering. Materials prepared by HIP at 1100 °C and 30 min present full densification and a better control of microstructure in the presence of VC. The added amount of VC allows obtaining homogeneous microstructures with an average grain size of 120 nm. The hardness and fracture toughness values obtained were about 2100 HV₃₀ and close to 10 MPa m^1/2, respectively.     

Effects of sintering additives on microstructure and mechanical properties of TiB2-WC ceramic-metal composite tool materials

TiB₂-WC ceramic-metal composite tool materials were fabricated using Co, Ni and (Ni, Mo) as sintering additives by vacuum hot-pressing technique. The microstructure and mechanical properties of the composite were investigated. The composite was analyzed by the observations of scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive spectrometry (EDS). The microstructure of TiB₂-WC ceramic-metal composites consisted of the fine WC grains and uniform TiB₂ grains. The brittle phase of Ni₃B₄ and a few pores were found in TiB₂-WC-Ni ceramic-metal composite. A lot of pores and brittle phases such as W₂CoB₂ and Co₂B were formed in TiB₂-WC-Co ceramic-metal composite. The liquid phase of Co was consumed by the reaction which led to the formation of the pores and the coarse grains of TiB₂. The pores, brittle phases and coarse grains of TiB₂ were harmful to the improvement of the mechanical properties of the composite. The sintering additive of (Ni, Mo) had a significant effect on the density and the mechanical properties of TiB₂-WC ceramic-metal composite. The formation of intermetallic compound of MoNi₄ inhibited the consumption of liquid phase of (Ni, Mo). The liquid phase of (Ni, Mo) not only inhibited the formation of the pores and the coarse grains of TiB₂ but also strengthened the interface energy between WC and TiB₂ grains. The grain size was fine and the average relative density of TiB₂-WC-(Ni, Mo) ceramic-metal composite reached 99.1%. The flexural strength, fracture toughness and Vickers hardness of TiB₂-WC-(Ni, Mo) ceramic-metal composite were 1307.0 ± 121.4 MPa, 8.19 ± 0.29 MPa m^1/2 and 22.71 ± 0.82 GPa, respectively.     

Fabrication and mechanical properties of ultrafine grained WC-10Co-0.45Cr3C2-0.25VC alloys

The ultrafine grained WC-10Co-0.45Cr₃C₂-0.25VC alloys were fabricated through planetary ball milling and low pressure sintering. The effects of the cobalt particle size, milling speed and sintering temperature on the microstructure, hardness and fracture toughness of the ultrafine grained alloys were investigated using optical microscopy, scanning electron microscopy and mechanical testing. The results showed that the mechanical properties of the low pressure-sintered alloys substantially depend on the milling speed and sintering temperature. At the same time, the hardness and fracture toughness of the samples can be increased from 1703 MPa and 8.90 MN m^−3/2 to 1789 MPa and 11.21 MN m^−3/2, respectively, when the cobalt particle size is reduced from 17 μm to 1.4 μm.     

Investigations on synthesis of ZrB2 and development of new composites with HfB2 and TiSi2

This paper presents the results of experimental investigations carried out on the synthesis of pure ZrB₂ by boron carbide reduction of ZrO₂ and densification with the addition of HfB₂ and TiSi₂. Process parameters and charge composition were optimized to obtain pure ZrB₂ powder. Monolithic ZrB₂ was hot pressed to full density and characterized. Effects of HfB₂ and TiSi₂ addition on densification and properties of ZrB₂ composites were studied. Four compositions namely monolithic ZrB₂, ZrB₂ + 10% TiSi₂, ZrB₂ + 10% TiSi₂ + 10% HfB₂ and ZrB₂ + 10% TiSi₂ + 20% HfB₂ were prepared by hot pressing. Near theoretical density (99.8%) was obtained in the case of monolithic ZrB₂ by hot pressing at 1850 °C and 35 MPa. Addition of 10 wt.% TiSi₂ resulted in an equally high density of 98.9% at a lower temperature (1650 °C) and pressure (20 MPa). Similar densities were obtained for ZrB₂ + HfB₂ mixtures also with TiSi₂ under similar conditions. The hardness of monolithic ZrB₂ was measured as 23.95 GPa which decreased to 19.45 GPa on addition of 10% TiSi₂. With the addition of 10% HfB₂ to this composition, the hardness increased to 23.08 GPa, close to that of monolithic ZrB₂. Increase of HfB₂ content to 20% did not change the hardness value. Fracture toughness of monolithic sample was measured as 3.31 MPa m^1/2, which increased to 6.36 MPa m^1/2 on addition of 10% TiSi₂. With 10% HfB₂ addition the value of K_IC was measured as 6.44 MPa m^1/2, which further improved to 6.59 MPa m^1/2 with higher addition of HfB₂ (20%). Fracture surface of the dense bodies was examined by scanning electron microscope. Intergranular fracture was found to be a predominant mode in all the samples. Crack propagation in composites has shown considerable deflection indicating high fracture toughness. An oxidation study of ZrB₂ composites was carried out at 900 °C in air for 64 h. Specific weight gain vs time plot was obtained and the oxidized surface was examined by XRD and SEM. ZrB₂ composites have shown a much better resistance to oxidation as compared to monolithic ZrB₂. A protective glassy layer was seen on the oxidized surfaces of the composites.     

Low temperature sintering of ZrC-SiC composite

High-energy ball milling and spark plasma sintering were adopted to prepare ZrC-SiC composite. Zirconium carbide, silicon, and graphite powders were used as raw materials. ZrC-30 vol.%SiC was sintered to a relative density of >96.1% at 1800 °C. The composite showed a fine microstructure. The fracture strength reached up to 523.4 MPa, Vickers’ hardness 18.8 GPa, fracture toughness 4.0 MPa m^1/2, and elastic modulus 390.5 GPa.     

Compressive strength, hardness and fracture toughness of Al2O3 whiskers reinforced ZTA and ATZ nanocomposites: Weibull analysis

The aim of this investigation was to study the variability in compressive strength, fracture toughness and microhardness applying the well-known Weibull statistics and to be able to provide a wide spectrum of mechanical properties in Al₂O₃ whisker reinforced alumina toughened zirconia (ATZ) and zirconia toughened alumina (ZTA) nanocomposites for possible dental applications. Uniaxial compression tests at room temperature of samples 6.35 ± 0.03 mm in diameter and 12.50 ± 0.63 mm in length and Vickers hardness measurements on polished surfaces were carried out. The indentation fracture toughness (K_IC) was derived from the average crack length. Weibull analysis was performed on the data. The ATZ2 (18.0 wt.% Al₂O₃ + 2.0 wt.%_(w) + 80.0 wt.% ZrO₂ (TZ-3Y)) nanocomposite reported the highest average compressive load of 1200 MPa, the highest value of characteristic strength, σ_o, of 1340 MPa with Weibull modulus of 3.25 and relatively high fracture toughness (4.7 ± 0.7 MPa m^1/2), suggesting that with the wide range of mechanical properties obtained in our work, different dental applications could be offered without lead to premature failure.     

Effects of LaB6 addition on the microstructure and mechanical properties of ultrafine grained WC-10Co alloys

The ultrafine grain WC-10Co alloys were prepared by the planetary ball milling method and low pressure sintering. The effects of LaB₆ addition on the microstructure and mechanical properties of the based alloys were investigated by X-ray diffraction, scanning electron microscope and mechanical property testing. It has been shown that the grain growth and regularization of WC particles occur simultaneous with the addition of LaB₆. Adding suitable amount of LaB₆ improves the density, hardness and fracture toughness of the ultrafine WC-10Co alloys, and decreases the volume fraction of Co₃W₃C phase in ultrafine WC-10Co alloys.     

Microstructure and mechanical properties of ZrB2–Nb composite

The high relative density of the ZrB₂-based composite toughened by 25 vol.%Nb (ZN) was hot-pressed at reduced temperatures with low pressure of 30 MPa. Compared with the toughness of 2.3–3.5 MPa m^1/2 and strength of 350 MPa of the monolithic ZrB₂, the toughness and strength of the ZN composite were improved to 6.7 MPa m^1/2 and 773 MPa, respectively, due to the addition of ductile Nb. The toughening mechanisms are crack deflection and branching as well as stress relaxation near the crack tip. Furthermore, the densification mechanism was analyzed and discussed. The results here pointed to a potential method for improving fracture toughness and strength of ZrB₂-based ceramics.     

Synthesis, microstructure and mechanical properties of reaction-infiltrated TiB2-SiC-Si composites

TiB₂-C preforms formed with different compositions and processing parameters were reactively infiltrated by Si melts at 1450 °C to fabricate TiB₂-SiC-Si composites. Phase constituent and microstructure of these composites were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The resulting composites are generally composed of TiB₂ and reaction-formed β-SiC major phases, together with a quantity of residual Si. Unreacted carbon is detected in the samples with a starting composition of 2TiB₂ + 1C formed at higher pressure and in all of the ones at the composition of 1TiB₂ + 1C. The distribution of these phases is fairly homogeneous in microstructure. TiB₂-SiC-Si composites show good mechanical properties, with representative values of 19.9 GPa in hardness, 395 GPa in elastic modulus, 3.5 MPa m^1/2 in fracture toughness and 604 MPa in bending strength. The primary toughening and strengthening mechanism is attributed to the crack deflection of TiB₂ particles.     

Synthesis and consolidation of TiN/TiB2 ceramic composites via reactive spark plasma sintering

The simultaneous synthesis and densification of TiN/TiB₂ ceramic composites via reactive spark plasma sintering (RSPS) was investigated. Different component ratios (TiH₂/BN (TiN, B)) and heating rates (112.5-300 °C/min) were used to initiate the chemical reaction for TiN/TiB₂ synthesis. The omit RSPS process was revealed to have three stages, which are described separately. The relationships between the RSPS conditions, the microstructure and the properties of sintered ceramic composites were established. A Vickers hardness of 16-25 GPa and a fracture toughness of 4-6.5 MPa m^1/2 were measured for various compositions. Sintered ceramic composites containing 36 wt% TiB₂ with the highest relative density of 97.4 ± 0.4% and an average grain size of 150-550 nm have been obtained.