Micro-scale fracture toughness of textured alumina ceramics

Enhanced fracture resistance of textured alumina is ascribed to crack deflection along grain boundaries. In this work, we quantify and compare the micro-scale fracture toughness of textured alumina grains and grain boundaries by micro-bending tests. Notched micro-cantilevers were milled from single alumina textured grains (perpendicular to the [0001] direction) and across several textured grains (along the [0001] direction), using a focused ion beam technique. Bending tests were performed with a nanoindenter. A shape function for notched pentagonal-shaped cantilevers was developed using finite element analysis. The critical stress intensity factor at the notch tip was determined based on the measured fracture loads. The micro-scale fracture toughness of the textured alumina grain boundaries (2.3 ± 0.2 MPa m^1/2) was about 30% lower than that of the grains (3.3 ± 0.2 MPa m^1/2). These findings at the micro-scale are paramount for understanding the macroscopic fracture behaviour of textured alumina ceramics.     

Structure and electrical insulation characteristics of plasma-sprayed alumina coatings under pressure

In this work, alumina coatings were fabricated on 316LN austenitic stainless steel by a plasma spray technique. The pressure dependence of the surface electrical resistivity of alumina coatings was investigated in detail. A combination of scanning electron microscopy and X-ray diffraction was employed to understand the microstructure and properties of the as-sprayed alumina coatings. The coatings can endure high pressures under a practical working environment. The surface electrical resistivity of the alumina coatings decreases continuously with an increase in pressure to 250 MPa. Interestingly, the surface resistivity is still greater than 10⁷ Ω·mm for 250 MPa, demonstrating that the coatings have good electrical insulation properties and can be fully utilized in the magnet support of ITER.     

Fracture behaviours of in situ silica nanoparticle-filled epoxy at different temperatures

Fracture behaviours of nanosilica filled bisphenol-F epoxy resin were systematically investigated at ambient and higher temperatures (23 °C and 80 °C). Formed by a special sol-gel technique, the silica nanoparticles dispersed almost homogenously in the epoxy resin up to 15 vol.%. Stiffness, strength and toughness of epoxy are improved simultaneously. Moreover, enhancement on fracture toughness was much remarkable than that of stiffness. The fracture surfaces taken from different test conditions were observed for exploring the fracture mechanisms. A strong particle-matrix adhesion was found by fractography analysis. The radius of the local plastic deformation zone calculated by Irwin model was relative to the increment in fracture energy at both test temperatures. This result suggested that the local plastic deformation likely played a key role in toughening of epoxy.     

On the fracture toughness of polycrystalline alumina measured by SEPB method

Fracture toughness (K_IC) of a polycrystalline alumina was evaluated using a single edge precracked beam (SEPB) method. A Knoop indentation-induced microcrack was introduced into a bend bar specimen, and then a sharp pop-in precrack was developed by applying the bridge loading technique. The precrack length (a/W) was varied by changing the indentation load and/or the support groove width of the anvil. The precracked specimens were fractured by three-point bending under a cross-head speed of 0·5 mm/min at room temperature. K_IC values of a polycrystalline alumina were dependent on precrack length for a/W<0·35. The dependence was discussed in terms of residual stress around the indentation-induced crack and crack mouth opening displacement (CMOD).     

Devitrification behaviour of plasma-sprayed glass coatings

Glass and glass-ceramic coatings on ceramic tiles have been manufactured by plasma-spraying high-performance CAS (in wt%—SiO₂, 60%; Al₂O₃, 15%; CaO, 23%; others, traces) and CZS (in wt%—SiO₂, 50%; CaO, 31%; ZrO₂, 16.5%; Al₂O₃, 2%; others, traces) glass frits. The CZS system has a surface crystallization at about 1050 °C. Such behaviour would not easily allow to obtain a fully crystalline bulk glass-ceramic, but the defectiveness of the plasma-sprayed coating supplies many nucleation sites. Thus, it becomes completely crystalline and well sintered after a 850 °C for 30 min + 1050 °C for 15 min treatment. The CAS frit, designed not to produce significant crystallization, is well sintered after a 850 °C for 30 min + 950 °C for 30 min thermal treatment, but remains too brittle due to its glassy nature. A 1050 °C treatment allows a few pseudowollastonite crystals to form in a glassy matrix; their formation also hinders sintering. Thus, mechanical properties are inferior to heat-treated plasma-sprayed CZS.     

Enhanced fracture toughness in nonstoichiometric magnesium aluminate spinel through controlled dissolution of second phase alumina

Dissolution of Al₂O₃ particles into a MgAl₂O₄ (spinel) matrix is accompanied by a volumetric expansion that is predicted to lead to a compressive stress field upon cooling, resulting in a promising microstructure for enhanced toughening of transparent spinel. This study explores the conditions to form such a microstructure by hot‐pressing powders of Al₂O₃ and spinel, at temperatures that promote dissolution of the Al₂O₃. Tough, particulate‐reinforced composites were formed under lower temperatures and shorter times, but single phase, cubic spinel was formed at 1700°C for 10 hours. The single phase spinel made in this way exhibited a toughness of 2.26 ± 0.17 MPa√m, significantly higher than the equivalent nonstoichiometric spinel made by traditional methods, 1.72 ± 0.06 MPa√m. X‐ray diffraction measurements revealed lattice parameter changes consistent with the dissolution of Al₂O₃ into spinel. Mechanics modeling reveals that toughening arises due to the volume expansion as Al₂O₃ dissolves into the spinel matrix.     

The fracture toughness of inorganic glasses

Measuring the fracture toughness (K_Ic) of glasses still remains a difficult task, raising experimental and theoretical problems as well. The available methods to estimate K_Ic are reviewed, with emphasis on their respective advantages and drawbacks. In view of our current understanding, this analysis gives precedence to the SEPB method. The ultimate glass strength, the critical flaw size, and the indentation load for the onset of crack initiation are discussed, in the light of the fundamentals of fracture mechanics and classical background regarding the mechanics of brittle materials. Analytical expressions were further proposed to predict the fracture energy and fracture toughness of glasses from different chemical systems from their nominal compositions. The theoretical values were compared with the experimental ones, as obtained by self‐consistent methods when available. The agreement observed in most cases suggests that measured K_Ic values correspond to the crack propagation regime (as opposed to the crack initiation threshold), and supports previous investigations in glasses and ceramics, which showed that a crack tip is nearly atomically sharp in these materials (but for metallic glasses). Some ideas to design tougher glasses are finally presented.     

Improved ageing-resistance and fracture toughness of zirconia-toughened alumina bioceramics via composition and microstructure design

Low-temperature degradation (LTD) is the main obstacle that hinders the application of zirconia-toughened alumina (ZTA) ceramics as artificial hip joints. In this study, a new type of ZTA bioceramic with LTD resistance, high fracture toughness, and superior wear resistance was prepared by the in-situ formation of plate-like crystals and co-stabilised tetragonal phase zirconia (t-ZrO₂). This new design is realised by synthesising nanocrystalline PrAlO₃ through a one-step solution combustion method and introducing it into a ZTA matrix to form praseodymium hexaaluminate (Pr_0.833Al_11.833O₁₉, PHA) plate-like crystals by solid-state sintering. PHA plays a key role in toughening and Y–Pr co-doped t-ZrO₂ slows down the low-temperature degradation of ZTA bioceramics. After hydrothermal ageing, the combination strategies have a positive influence on the flexural strength, fracture toughness, and Vickers hardness of the ceramics. This study provides a novel direction for ensuring long-term safety of bioceramics.     

An essential work of fracture study of the toughness of thermoset polyester coatings

The fracture toughness of a range of thermoset polyester paints with different cross-link densities has been studied, using the essential work of fracture (EWF) method. The glass transition temperature, T_g, of each of the materials was measured using differential scanning calorimetry, and found to lie between 8 and 46 °C. EWF tests were performed on the paint films at a range of temperatures around the measured glass transition temperature of each material. The essential work of fracture, w_e, at T_g was found to decrease with increasing cross-link density from around 20 kJ/m² at a cross-link density of 0.4 × 10⁻³ mol/cm³ to around 5 kJ/m² for cross-link densities of approximately 1 × 10⁻³ mol/cm³ or higher. A maximum in the essential work of fracture was observed at around T_g when w_e was plotted versus temperature, which could be attributed to the effect of an α-relaxation at a molecular level. The polyesters were found to be visco-elastic, and the applicability of the EWF test to the study of these visco-elastic thermoset materials is discussed.     

Decomposition voltage for the electrolysis of alumina at low temperatures

The apparent decomposition voltage for the electrolysis of alumina in an equimolar Na₃AlF₆-Li₃AlF₆ electrolyte was measured over a temperature range of 800 to 1000° C by the extrapolation of voltagecurrent plots to zero current. Temperature coefficients of –1.9 and –2.4 mV° C^–1 were determined for conditions of variable alumina activity (constant concentration) and unit activity (saturated), respectively. The overvoltage contribution to the temperature dependency was estimated to be about –1.6mV° C^–1 (versus a –0.6 mV° C^–1 dependency for the reversible decomposition voltage). Reduced alumina solubility at low temperatures also appeared to increase the overvoltage, but was of secondary importance.Based on work partially supported by the US Department of Energy (Contract DE-AC03-76CS40215: Energy Savings Through the Use of an Improved Reduction Cell Cathode).     

高铝砖高温弯曲应力-应变关系

采用研制的高温弯曲应力-应变测试仪研究了Ⅰ等高铝砖DL-80、Ⅱ等高铝砖GL-75和Ⅲ等高铝砖GL-55在不同温度下的力学性能,包括应力-应变曲线、弹性模量、抗折强度和断裂时的最大变形量。结果表明:高铝砖在不同温度下的应力-应变关系可以分为弹性阶段、塑性阶段和粘滞流动阶段;在低、中温范围内,高铝砖的弹性模量和抗折强度随温度的上升而增加,到达某一转折温度后,随温度的上升而明显下降;3种高铝砖高温力学性能从高到低的排列顺序为:Ⅱ等>Ⅰ等>Ⅲ等。     

Simulation of spall fracture of uranium at different temperatures in the region of polymorphic phase transitions

This paper presents the results of numerical simulation of Zaretskii’s experiments on loading of natural uranium in the phase-transition region at temperatures of 27–862°C. Simulation of these experiments is of interest because of the observed features of spall fracture of uranium in the phase-transition region. Spall fracture and compaction was simulated using the DGC-L model of the dynamics of growth and compaction in a liquid medium, which takes into account the effect of strength properties, pressure, surface tension, viscosity, and inertial forces on the growth and collapse of pores. Calculations were carried out according to the UP program— a Lagrangian method for calculating deformation problems of continuum mechanics in a onedimensional approximation.     

The fracture toughness of thin polymeric films

A fracture test using the single edge notch (SEN) geometry was developed for thin polymeric films. The SEN specimens were videotaped while they were stressed to fracture within an Instron tensile testing machine operating at a constant crosshead speed. Critical stress intensity factors for fracture instability and for the initiation of crack growth, along with the corresponding fracture energies, were obtained using a residual stress analysis. The method is applicable to brittle films and films of moderate ductility. Critical stress intensity factors ranging from 0.12 to 4.03 MPa√m were determined for thin films under plane stress conditions. Results of the test are presented for poly(di-n-hexylsilane), Kapton polyimide, polystyrene, and Trycite biaxially oriented polystyrene film. Validation of the test and interpretation of the results are also discussed.     

Fracture toughness variability of foamed alumina cements

Preexisting micropores and microcracks within solid cement paste cause a variation in both the cell wall modulus of rupture and fracture toughness of foamed cements. Here, we determine for the variability of cell wall modulus of rupture by assuming that it follows a Weibull statistical distribution. Consequently, the variation of fracture toughness of foamed cements depends on the Weibull modulus of solid cement paste. A series of measurements on the Mode I fracture toughness of foamed alumina cements with various relative densities were conducted. Results indicate that the Weibull moduli of solid cement paste for foamed alumina cements are within the range of 7 to 10 except for the foamed alumina cement with a relative density of 0.167.     

Analysis of instrumented scratch hardness and fracture toughness properties of laser surface alloyed tribological coatings

The mechanical characteristics of ceramic matrix composite (CMC) coatings are widely different from the same materials in bulk form or the individual constituents and are very important to be assessed to carry out application oriented studies on CMC coatings with novel compositions. In the present work, a composite coating of TiB₂, TiN and SiC is fabricated in-situ through a combination of high temperature chemical reaction and laser surface alloying. The formation of the surface layer is due to the laser-assisted chemical reaction followed by laser melting. A mixture of TiO₂, SiO₂, hBN and graphite in stoichiometric proportions is used as the precursor for the chemical reaction. The presence of all the reaction products in the CMC coatings developed is confirmed by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). A thorough evaluation of various mechanical properties achieved more insight into the CMC coatings developed. Hardness and fracture toughness of the coatings are measured with a scratch tester. The property evaluations are performed in a similar way for two more coatings fabricated with precursor mixtures containing more than a stoichiometric amount of SiC and hBN respectively. For comparison, a number of composites fabricated through various other routes are characterized afresh with the same set of techniques. Coatings formed with SiC in precursor show higher values of scratch hardness (14.37 GPa), microhardness (24.37 GPa) and fracture toughness (6.63 MPa-m^1/2).     

Characterization of high strength mortars with nano alumina at elevated temperatures

In this study, the effect of elevated temperatures on chemical composition, microstructure and mechanical properties of high strength mortars with nano alumina was investigated. Mortars with 1, 2 and 3% nano alumina as cement replacement were prepared and then exposed to 100 °C, 200 °C, 300 °C, 400 °C, 600 °C, 800 °C and 1000 °C. XRD, DSC and SEM tests were carried out to identify chemical composition and microstructure changes in the cement matrix after being exposed to elevated temperatures. Residual compressive strength, relative elastic modulus and gas permeability coefficient of samples were also obtained. A brittleness index was defined to monitor changes in brittleness of samples after being exposed to elevated temperatures. Nano alumina enhanced compressive strength of samples up to 16% and improved residual compressive strength. An increase in the relative elastic modulus, higher energy absorption and lower permeability were also observed when 1% nano alumina was added.