Investigation of fracture toughness of modified (KxNa1 − x)NbO3 lead-free piezoelectric ceramics

While most of the previous studies have focused on the processing and electrical properties of KNN-based ceramics, very little research has been carried out to evaluate their mechanical behavior. This work presents for the first time an examination of the fracture toughness, K_IC, of the most widely studied (K_xNa_1 ? x)NbO₃ (KNN)-based lead-free ceramics modified with lithium, tantalum and antimony. The samples were produced through the conventional mixed-oxide route and the K_IC values were measured using the single edge V-notched beam (SEVNB) method under four-point bending. The mean K_IC values were determined to be 0.48 ± 0.18 MPa m^1/2 for (K_0.48Na_0.48Li_0.04)NbO₃, 0.8 ± 0.18 MPa m^1/2 for (K_0.5Na_0.5)(Nb_0.9Ta_0.1)O₃, 0.86 ± 0.04 MPa m^1/2 for (K_0.48Na_0.48Li_0.04)(Nb_0.9Ta_0.1)O₃ and 1.06 ± 0.21 MPa m^1/2 for (K_0.48Na_0.48Li_0.04)(Nb_0.86Ta_0.1Sb_0.04)O₃ compositions. The microstructure, phase structure and dielectric constant values of the samples have been used to correlate the results of the K_IC values.     

High-temperature conductivity,stability and redox properties of Fe3−xAlxO4 spinel-type materials

Iron-based oxides are considered as promising consumable anode materials for high temperature pyroelectrolysis. Phase relationships, redox stability and electrical conductivity of Fe_3?xAl_xO₄ spinels were studied at 300–1773 K and p(O₂) from 10^?5 to 0.21 atm. Thermogravimetry/XRD analysis revealed metastability of the sintered ceramics at 300–1300 K. Low tolerance against oxidation leads to dimensional changes of ceramics upon thermal cycling. Activation energies of the total conductivity corresponded to the range of 16–26 kJ/mol at 1450–1773 K in Ar atmosphere. At 1573–1773 K and p(O₂) ranging from 10^?5 to 0.03 atm, the total conductivity of Fe_3?xAl_xO₄ is nearly independent of the oxygen partial pressure. The conductivity values of Fe_3?xAl_xO₄ (0.1 ≤ x ≤ 0.4) at 1773 K and p(O₂) ～10^?5 to 10^?4 atm were found to be only 1.1–1.5 times lower than for Fe₃O₄, showing high potential of moderate aluminium additions as a strategy for improvement of refractoriness for magnetite without significant deterioration of electronic transport.     

Effect of sintering temperature on the transparency and mechanical properties of lutetium aluminum garnet fabricated by spark plasma sintering

Transparent lutetium aluminum garnet (Lu₃Al₅O₁₂, LuAG) was fabricated by reactive spark plasma sintering. The effect of sintering temperature on the crystal phase, microstructure, transparency and mechanical properties of LuAG bodies was investigated. Fully dense and single-phase LuAG bodies were obtained at sintering temperatures 1573–1923 K. The average grain size increased from 0.18 to 0.52 μm with increasing sintering temperature from 1573 to 1773 K, and grain growth became significant at 1823 K. Transmittance showed a maximum value of 77.8% at 2000 nm at a sintering temperature of 1773 K after annealing at 1423 K in air for 43.2 ks. The Vickers hardness increased from 14.2 to 17.2 GPa with decreasing grain size from 7.45 to 0.23 μm.     

Influence of processing on fracture toughness of Si3N4 + graphene platelet composites

Silicon nitride + 1 wt% graphene platelet composites were prepared using various graphene platelets (GPL) and two processing routes; hot isostatic pressing (HIP) and gas pressure sintering (GPS). The influence of the processing route and graphene platelets’ addition on the fracture toughness has been investigated. The matrix of the composites prepared by GPS consists of Si₃N₄ grains with smaller diameter in comparison to the composites prepared by HIP. The indentation fracture toughness of the composites was in the range 6.1–9.9 MPa m^0.5, which is significantly higher compared to the monolithic silicon nitride 6.5 and 6.3 MPa m^0.5. The highest value of K_IC was 9.9 MPa m^0.5 in the case of composite reinforced by the smallest multilayer graphene nanosheets, prepared by HIP. The composites prepared by GPS exhibit lower fracture toughness, from 6.1 to 8.5 MPa m^0.5. The toughening mechanisms were similar in all composites in the form of crack deflection, crack branching and crack bridging.     

Microstructure and mechanical properties of Al2O3–YSZ and Al2O3–YAG directionally solidified eutectic plates

Plates of Al₂O₃–YSZ and Al₂O₃–YAG eutectic composition with a thickness from 0.1 to 1 mm were prepared by directional solidification using a diode laser stack. The melt processed regions of plates exhibited colony microstructure consisting of finely dispersed phases. Due to the curved shape of the melted pool, the growth rate depends on the distance to the surface plate, decreasing from top to bottom. In this way, the microstructure characteristic length changes as a function of the distance to the plate surface. Vickers indentations and piezo-spectroscopy measurements were done on longitudinal and transverse cross-sections of the samples at different depths. From these measurements, we concluded that the Vickers hardness (H_V), indentation fracture toughness (K_IC) and residual stresses (σ_h) of the plates were mainly independent from the distance to the surface. The mean values that we obtained in the Al₂O₃–YSZ plates were H_V = 16 GPa, K_IC = 4.2 MPa m^1/2 and σ_h = −0.33 GPa, and in the Al₂O₃–YAG plates were H_V = 16 GPa, K_IC = 2.0 MPa m^1/2, and σ_h = −0.1 GPa. These values are similar to those found in directionally solidified eutectic rods.     

Densification and mechanical properties of TiC by SPS-effects of holding time,sintering temperature and pressure condition

The influence of sintering temperature, holding time and pressure condition on densification and mechanical properties of bulk titanium carbide (TiC) fabricated by SPS sintering has been systematically investigated. Experimental data demonstrated that relative density and Vickers hardness (H_V) increase with sintering temperature and holding time, but fracture toughness (K_IC) was not significantly influenced by sintering parameters. The H_V and relative density of samples consolidated by SPS technique at 1600 °C for 5 min under 50 MPa pressure (applied entire sintering cycle) reached 30.31 ± 2.23 GPa and 99.90%, respectively. H_V values of ～24–30 GPa and K_IC of ～3.7–5 MPa m^1/2 were obtained in all bulk samples with relative densities of 95.61–99.90% when fabricated under various conditions presented above, without abnormal grain growth. More pronounced effects of pressure condition on grain growth (promoted by grain-boundary diffusion) than on densification were observed. The relationship of fracture toughness and fracture mode is also discussed.     

Influence of experimental variables on fracture toughness determined on SEVNB in three points bending. Mullite a case study

The effect of testing variables on toughness of single edge “V” notched beams (4 mm × 6 mm × 50 mm, α = 0.6) of a fine grained mullite (d₅₀ = 0.7 ± 0.5 μm) in three points bending (span = 40 mm) is analysed. Mullite was selected as case material because it presents flat R-curve and subcritical crack propagation. Stable fracture was reached by using the CMOD as control variable (0.001 and 0.018 mm/min). Results for stable test and unstable displacement (0.05 mm/min) controlled tests are analysed. K_IC has been calculated from maximum loads, K_ICp, and from the total fracture energy determined in stable tests, K_ICγ. The fact that for materials with flat R-curve both K_IC values are coincident has been used as criterion for adequacy of the test. Stable fracture at high deformation rates is required to fulfil K_ICp = K_ICγ. Under such conditions, an intrinsic K_IC = 0.86 ± 0.06 MPa m^1/2, less than one half of those previously reported has been obtained.     

Room and high temperature flexural failure of spark plasma sintered boron carbide

Dense (95–98.6%) bulk boron carbide prepared by Spark Plasma Sintering (SPS) in Ar or N₂ atmospheres were subject to three-point flexural tests at room and at 1600 °C. Eight different consolidation conditions were used via SPS of commercially available B₄C powder. Resulting specimens had similar grain size not exceeding 4 µm and room-temperature bending strength (σ_25 °C) of 300–600 MPa, suggesting that difference in σ_25 °C is due to development of secondary phases in monolithic boron carbide ceramics during SPS processing. To explain such difference the composition of boron carbide and secondary phases observed by XRD and Raman spectroscopy. The variation in intensity of the Raman peak at 490 cm⁻¹ of boron carbide suggests modification of the boron carbide composition and a higher intensity correlates with a higher room-temperature bending strength (σ_25 °C) and Vickers hardness (HV). Secondary phases can modify the level of mechanical characteristics within some general trends that are not dependent on additives (with some exceptions) or technologies. Namely, HV increases, σ_25 °C decreases, and the ratio σ_1600 °C/σ_25 °C (σ_1600 °C – bending strength at 1600 °C) is lower when fracture toughness (K_IC) is higher. The ratio σ_1600 °C/σ_25 °C shows two regions of low and high K_IC delimited by K_IC=4.1 MPa m^0.5: in the low K_IC region, boron carbide specimens are produced in nitrogen.     

Effect of NiO additive on microstructure,mechanical behavior and electrical properties of 0.2PZN–0.8PZT ceramics

A NiO-added Pb((Zn_1/3Nb_2/3)_0.20(Zr_0.50Ti_0.50)_0.80)O₃ system is prepared and investigated. The results reveal that Ni doping induces a phase transformation from the morphotropic phase boundary to the tetragonal phase side. Above the solubility limit of 0.3 wt% in NiO form, excess Ni ions segregate at the grain boundaries and triple junctions, which facilitate the formation of a liquid phase with excess PbO and lead to remarkable grain growth. The mechanical behavior (Vickers hardness (H_v) and fracture toughness (K_IC)) can be tailored by controlling the content of additive; this is accompanied by a transition in the fracture mode changed from transgranular without NiO additive to intergranular with 1.0 wt% NiO additive. Moreover, the NiO addition weakens the dielectric relaxor behavior and improves the piezoelectric properties simultaneously. The 0.2PZN–0.8PZT with 0.5 wt% NiO addition shows good transduction coefficient (d₃₃·g₃₃ = 10,050 × 10^?15 m²/N) and large fracture toughness (K_IC = 1.35 MPa m^1/2).     

Slow crack growth and hydrothermal aging stability of an alumina-toughened zirconia composite made from La2O3-doped 2Y-TZP

A very tough zirconia matrix is interesting to fabricate alumina-toughened zirconia (ATZ) and composites generally processed from 3Y-TZP do not exhibit very high toughness. The strategy of lowering the yttria content to increase toughness however is normally associated with an increased hydrothermal aging susceptibility. In this work, a 0.4 mol% La₂O₃ doped 2Y-TZP matrix was investigated to realize a 20 wt.% alumina toughened zirconia composite with a substantially high aging resistance. The higher transformation toughening in the composite shifted the V-K_I towards higher K_I values, while preserving the slope of the curve, resulting in a threshold K_I0 of 4.0 MPa m^1/2 and fracture toughness (K_IC) of 7.1 MPa m^1/2. These composites can offer a better compromise between aging and crack resistance than traditional 3Y-TZPs and plain ATZ composites without La₂O₃ doping.     

Fabrication of transparent SiO2 glass by pressureless sintering and spark plasma sintering

Transparent SiO₂ bodies were prepared by pressureless sintering (PLS) and spark plasma sintering (SPS). The effects of sintering and annealing temperature on the transmittance of the SiO₂ bodies were investigated. The SiO₂ bodies sintered by SPS and PLS at 1073–1573 K were amorphous. With increasing the sintering temperature to 1673 K, the SiO₂ bodies sintered by PLS were crystallized while those sintered by SPS were still amorphous. The relative density of the SiO₂ bodies sintered by SPS was 98.5% at 1373 K and 100% at 1573 K, whereas that sintered by PLS was 92.6% at 1373 K and 98.9% at 1573 K. The transmittance was 91.0% and 81.5% at a wavelength (λ) of 2 μm for the SiO₂ sintered bodies by SPS and PLS, respectively. In the ultraviolet range, the transmittance of the SiO₂ bodies sintered by SPS at 1573 K was about 40% at λ = 200 nm and increased to 75% after annealing at 1073 K for 1 h, which was about three times of the transmittance of the SiO₂ bodies sintered by PLS (24.8%).     

Mechanical properties of nano-grain SiO2 glass prepared by spark plasma sintering

Silicon carbide (SiC) layers were deposited on silica (SiO₂) glass powder by rotary chemical vapor deposition (RCVD) to form SiO₂ glass (core)/SiC (shell) powder; this powder was consolidated by spark plasma sintering (SPS). SiO₂ glass powder with a particle size of 250 nm was coated with 5–10-nm-thick SiC layers. The resultant SiO₂ glass (core)/SiC (shell) powder was consolidated to form a nano-grain SiO₂ glass composite at a relative density above 90% by SPS in the sintering temperature range of 1573–1823 K. The Vickers hardness and fracture toughness of the SiO₂ glass composite at 1723 K were found to be 14.2 GPa and 5.4 MPa m^1/2, respectively.     

Tribo-mechanical characterization of spark plasma sintered chopped carbon fibre reinforced silicon carbide composites

Short carbon fibre (C_f) reinforced silicon carbide (SiC) composites with 7.5 wt% alumina (Al₂O₃) as sintering additive were fabricated using spark plasma sintering (SPS). Three different C_f concentrations i.e. 10, 20 and 30 wt% were used to fabricate the composites. With increasing C_f content from 0 to 20 wt%, micro-hardness of the composites decreased ~28% and fracture toughness (K_IC) increased significantly. The short C_f in the matrix facilitated enhanced fracture energy dissipation by the processes of crack deflection and bridging at C_f/SiC interface, fibre debonding and pullout. Thus, 20 wt% C_f/SiC composite showed >40% higher K_IC over monolithic SiC (K_IC≈4.51 MPa m^0.5). Tribological tests in dry condition against Al₂O₃ ball showed slight improvement in wear resistance but significantly reduced friction coefficient (COF, μ) with increasing C_f content in the composites. The composite containing 30 wt% C_f showed the lowest COF.     

Round robin on indentation fracture resistance of silicon carbide ceramics by using a powerful optical microscope

Reproducibility of indentation fracture resistance, K_IFR of silicon carbides sintered with B and C was evaluated by a round robin with ten laboratories. When the crack length was measured with an optical microscope at a low magnification of ～100×, K_IFR varied widely from 3.43 to 4.20 MPa m^1/2, whereas those obtained by a powerful microscopy with both an objective lens of 40× and a traveling stage exhibited a consistent value of 3.20±0.12 MPa m^1/2. The wide scatter of K_IFR for the former measurements was attributed mainly to the variation in misreading of the crack length. It was revealed that the high resolving power of the objective lens of 40× enabled to find exact crack tips easily, which resulted in the good matching of K_IFR between laboratories for the latter case. It was suggested that the observation of indentations with powerful optics was effective for improving the reproducibility of the IF method.     

Processing and mechanical properties of Si3N4 composites employing polymer-derived SiAlOC as sintering aid

Silicon nitride ceramics have been densified with polymer-derived SiAlOC sintering aid. Dense samples were prepared at relatively mild temperatures (1600 °C) from blends with 30 wt.% of pyrolysed SiAlOC additives. Decreasing the SiAlOC aid content to 15 wt.% resulted in porous Si₃N₄ samples (～85% rel. density). The properties of dense samples were influenced by the remaining SiAlOC glass (HV = 15.5 GPa, K_IC = 4 MPa m^1/2). Increasing the sintering temperature to 1780 °C for 5 min significantly changed the phase composition and properties of the composites. The major phase was O′-sialon in the sintered samples. Additional annealing of the samples at 1530 °C for 16 h further decreased the amount of the residual glassy phase and consequently affected the mechanical properties. The Vickers hardness of dense samples was 18.5 GPa and the fracture resistance ranged between 4.0 and 4.5 MPa m^1/2. The compressive creep test (1400 °C/100 MPa/24 h) of the SNA30-A sample sintered at 1600 °C for 30 min without an additional crystallisation step showed a promising low creep rate of 8.6 × 10^?8 s^?1. Further improvement of creep resistance is expected for the crystallised samples.     

A mullite/SiC oxidation protective coating for carbon/carbon composites

To protect carbon/carbon (C/C) composites against oxidation, a mullite coating was prepared on SiC precoated C/C composites by a hydrothermal electrophoretic deposition process. The phase composition, microstructure and oxidation resistance of the prepared mullite/SiC coatings were investigated. Results show that hydrothermal electrophoretic deposition is an effective route to achieve crack-free mullite coatings. The mullite/SiC coating displays excellent oxidation resistance and can protect C/C composites from oxidation at 1773 K for 322 h with a weight loss rate of only 4.89 × 10^?4 g/cm² h. The failure of the multi-layer coatings is considered to be caused by the volatilization of silicate glass layer, the formation of microholes and microcracks on the coating surface and the formation of penetrative holes between the SiC bonding layer and the C/C matrix at 1773 K. The corresponding high temperature oxidation activation energy of the coated C/C composites at 1573–1773 K is calculated to be 111.11 kJ/mol.     

Improvement of the hydroxyapatite mechanical properties by direct microwave sintering in single mode cavity

The main purpose of this study consists in investigating the direct microwave sintering of hydroxyapatite (HA) in a single mode cavity. Firstly, stoichiometric HA powders were synthesized by a coprecipitation method from diammonium phosphate and calcium nitrate solutions and shaped by slip-casting. Then, using the one-step microwave process, dense pellets with fine microstructures were successfully obtained in short sintering timespan. A parametric study permitted to determine the influence of powder grain size, sintering temperature and dwell time on the sintered samples microstructures. The Young's modulus (E) and hardness (H) were measured by nanoindentation and the values discussed according to the microstructure. Finally, the resulting mechanical properties determined on the microwave sintered samples (E = 148.5 GPa, H = 9.6 GPa, σ_compression = 531.3 MPa and K_IC = 1.12 MPa m^1/2) are significantly higher than those usually reported in the literature, whatever the sintering process, and could allow the use of HA for structural applications.     

Spark plasma sintering of Al2O3–Ni nanocomposites using Ni nanoparticles produced by rotary chemical vapour deposition

Al₂O₃–Ni nanocomposites were fabricated by spark plasma sintering (SPS) using Ni nanoparticle produced by rotary chemical vapour deposition. Carbon-free Ni nanoparticles were prepared by reacting NiCp₂ with O₂ to form NiO and then reducing to Ni by H₂ for 30 min at 823 K. The highest Ni content and grain size were 7.8 wt.% and 47.7 nm, respectively, using a NiCp₂ supply rate (R_s) of 1.67 × 10⁻⁶ kg s⁻¹. At a sintering temperature (T_SPS) of 1573 K, the hardness of Al₂O₃–3.8 wt.% Ni was 20.5 GPa, around 1 GPa higher than that of monolithic Al₂O₃ sintered at the same temperature. The tensile strength of Al₂O₃–4.6 wt.% Ni was 170 MPa, 60 MPa higher than that of Al₂O₃ sintered at 1573 K.     

Spark plasma sintering of TiN–TiB2 composites

TiN–TiB₂ composites were fabricated by spark plasma sintering at 1773–2573 K. Effects of TiN and TiB₂ content on relative density, microstructure, and mechanical properties were investigated. Above 2373 K, TiN–TiB₂ composites exhibited relative densities over 95%. A high density of 99.7% was obtained at 2573 K with 20–30 vol% TiB₂. Shrinkage of the TiN–70 vol% TiB₂ composite was the highest at 1573–2473 K. For the TiN–70 vol% TiB₂ composite prepared at 1973–2373 K, TiN grains were small, while at 2573 K, TiB₂ became a continuous matrix, in which irregular-shaped TiN dispersed. hBN was formed in the TiN–TiB₂ composite containing 50–60 vol% TiB₂ above 2373 K. The maximum Vickers hardness and fracture toughness obtained for the TiN–80 vol% TiB₂ composite sintered at 2473 K was 26.3 GPa and 4.5 MPa m^1/2, respectively.     

Pressureless sintered SiC matrix toughened by in situ synthesized TiB2: Process conditions and fracture toughness

This paper reports the fabrication of SiC toughened by in situ synthesized TiB₂ based on pressure-less sintering technique using TiO₂, B₄C, C and SiC as starting materials. The process conditions were investigated in detail, including the pre-sintering temperatures, carbon contents, differently sized TiO₂ powders, TiB₂ volume contents, final sintering temperature and time. These conditions were found to have great influence on the TiB₂ toughened SiC in terms of relative density, TiB₂ particle size and fracture toughness. Homogeneous dispersion of in situ synthesized TiB₂ secondary phase was confirmed to enhance the K_IC of the SiC matrix. The K_IC of SiC toughened by in situ synthesized TiB₂ (15 vol%) reaches 6.3 MPa m^1/2, which is among the highest values reported so far on TiB₂ reinforced SiC composites based on the pressure-less sintering technique using TiO₂ as Ti source.