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.     

Bulk WC-Al2O3 composites prepared by spark plasma sintering

WC and WC-Al₂O₃ materials without metallic binder addition were densified by spark plasma sintering in the range of 1800-1900 °C. The densification behavior, phase constitution, microstructure and mechanical properties of pure WC and WC-Al₂O₃ composite were investigated. The addition of Al₂O₃ facilitates sintering and increases the fracture toughness of the composites to a certain extent. An interesting phenomenon is found that a proper content of Al₂O₃ additive helps to limit the formation of W₂C phase in sintered WC materials. The pure WC specimen possesses a hardness (HV₁₀) of 25.71 GPa, fracture toughness of 4.54 MPa·m^1/2, and transverse fracture strength of 862 MPa, while those of WC-6.8 vol.% Al₂O₃ composites are 24.48 GPa, 6.01 MPa·m^1/2, and 1245 MPa respectively. The higher fracture toughness and transverse fracture strength of WC-6.8 vol.% Al₂O₃ are thought to result from the reduction of W₂C phase, the crack-bridging by Al₂O₃ particles and the local change in fracture mode from intergranular to transgranular.     

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.     

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.     

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.     

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.     

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.     

The effect of particle size,sintering temperature and sintering time on the properties of Al–Al2O3 composites,made by powder metallurgy

Al₂O₃ is a major reinforcement in aluminum-based composites, which have been developing rapidly in recent years. The aim of this paper is to investigate the effect of alumina particle size, sintering temperature and sintering time on the properties of Al–Al₂O₃ composite. The average particle size of alumina were 3, 12 and 48 μm. Sintering temperature and time were in the range of 500–600 °C for 30–90 min. A correlation is established between the microstructure and mechanical properties. The investigated properties include density, hardness, microstructure, yield strength, compressive strength and elongation to fracture. It has been concluded that as the particle size of alumina is reduced, the density is increased followed by a fall in density. In addition, at low particle size, the hardness and yield strength and compressive strength and elongation to fracture were higher, compared to coarse particles size of alumina. The variations in properties of Al–Al₂O₃ composite are dependent on both sintering temperature and time. Prolonged sintering times had an adverse effect on the strength of the composite.     

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.     

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.     

Investigations on synthesis of HfB2 and development of a new composite with TiSi2

This paper presents the results of investigation carried out on synthesis and densification of monolithic HfB₂ and the effect of TiSi₂ as sinter additive. Pure phase HfB₂ was prepared by boron carbide reduction of HfO₂ and hot pressed to full density with the addition of TiSi₂. Isothermal oxidation study of this composite was carried out at 850 °C up to 64 h. Formation of HfB₂ was seen at 1200 °C but pure HfB₂ was formed at a much higher temperature of 1875 °C in vacuum. Hot pressing of HfB₂ at 1850 °C and 35 MPa pressure gave a compact of 80% TD. Addition of TiSi₂ helped in achieving a much higher density at a lower temperature of 1600 °C and a pressure of 20 MPa. A fully dense composite of HfB₂ and TiSi₂ was obtained with 15% TiSi₂. Hardness and fracture toughness of this composite were 27.4 ± 1.9 GPa and 6.6 ± 0.2 MPa m^1/2, respectively. Considerable deflection was observed in the crack propagation in composites. Oxidation studies indicated the formation of HfO₂, SiO₂, TiO₂ and HfSiO₄ with some glassy phase and the composite with 15% TiSi₂ was seen to be completely covered with a protective glassy layer.     

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.     

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 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.     

Selective single pulse femtosecond laser removal of alumina (Al2O3) from a bilayered Al2O3/TiAlN/steel coating

We studied surface modification of a double layer protective coating on steel induced by single fs laser pulse irradiation in ambient air. The outer alumina (Al₂O₃) layer, which protects against aggressive environments, was 1.7 μm thick and the titanium aluminum nitride (TiAlN) layer in contact with the steel surface had a thickness of 1.9 μm. The pulses (λ = 775 nm, τ = 200 fs) were generated by a Ti:sapphire laser source. The pulse energy was varied from 0.32 μJ to 50 μJ, corresponding to an incident laser fluence of 0.11 J cm^− 2 to 16.47 J cm^− 2. The surface damage threshold was found to be 0.20 J cm^− 2 and the alumina layer removal was initiated at 0.56 J cm^− 2. This selective ablation of alumina was possible in a wide range of fluences, up to the maximum applied, without ablating the TiAlN layer beneath.     

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.     

Preparation and characterization of high-toughness ZrB2/Mo composites by hot-pressing process

ZrB₂/Mo composites with various volume fractions of Mo (0–10%) were prepared by a hot-pressing process at 1950 °C. The addition of Mo both facilitated the densification and improved the mechanical properties of the ZrB₂/Mo composites. Compared with the pure ZrB₂ ceramics, the fracture toughness of the ZrB₂/Mo (10%) composite increased from 4.52 to 7.98 MPa m^1/2 and its bending strength increased from 424 to 450 MPa. The enhancement in mechanical properties of the ZrB₂/Mo composites could be attributed to the reduced grains and the MoB phase formed in the reaction between ZrB₂ and Mo.     

A new TiB2 + CrSi2 composite – Densification,characterization and oxidation studies

A new composite of TiB₂ with CrSi₂ has been prepared with excellent oxidation resistance. Dense composite pellets were fabricated by hot pressing of powder mixtures. Microstructural characterization was carried out by XRD and SEM with EDAX. Mechanical and physical properties were evaluated. Extensive oxidation studies were also carried out. A near theoretical density (99.9% TD) was obtained with a small addition of 2.5 wt.% CrSi₂ by hot pressing at 1700 °C under a pressure of 28 MPa for 1 h. The microstructure of the composite revealed three distinct phases, (a) dark grey matrix of TiB₂, (b) black phase – rich in Si and (c) white phase – Cr laden TiB₂. Hardness and fracture toughness were measured as 29 ± 2 GPa and 5.97 ± 0.61 MPa m^1/2, respectively. Crack branching, deflection and bridging mechanisms were responsible for the higher fracture toughness. With increase in CrSi₂ content, density, hardness and fracture toughness values of the composite decreased. Thermo gravimetric studies revealed the start of oxidation of the composite at 600 °C in O₂ atmosphere. Isothermal oxidation of these composites showed better oxidation resistance by formation of a protective oxide layer. TiO₂, Cr₂O₃ and SiO₂ phases were identified on the oxidized surface. Effects of CrSi₂ content, temperature and duration of oxidation on the oxide layer formation are reported. Activation energy of the composite was calculated as ∼110 kJ/mol using Arrhenius equation. Diffusion controlled mechanism of oxidation was observed in all the composites.     

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.