The fracture toughness of glassy carbons at elevated temperatures

The fracture toughness of three different glassy carbons heat treated at 1000, 2000 and 3000°C was measured at temperatures between 25 and 750°C. Young's elastic modulus varies inversely with measurement temperatures, whereas fracture toughness varies directly. Fracture characteristics of the glassy carbons are compared with those of float glass and graphites.     

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.     

Laser damage resistance of plasma-sprayed alumina and honeycomb skeleton/alumina composite ceramic coatings

Al₂O₃ and honeycomb skeleton-Al₂O₃ composite coatings on Titanium alloy (Ti–6Al–4V) were prepared by atmospheric plasma spraying. A laser ablation experiment on as-sprayed coatings was performed. In this paper, the laser damage resistance, microstructure, phase composition of Al₂O₃ coatings were examined. 3D Dimensional Confocal Microscopy, Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), and Energy Dispersive Spectrometry (EDS) characterized the laser damage morphology, microstructure, phase composition, and element analysis, respectively. The influence of the honeycomb skeleton on the laser ablation damage on as-sprayed coatings was investigated by a comparative analysis of the laser damage morphology with different laser ablation times and gas flow. The results show that the honeycomb skeleton raises thermal conductivity and thermal diffusivity. Moreover, a “tower”-like dendrite was generated during the laser irradiation of the composite coating. The honeycomb skeleton refined the structure, suppressed crack propagation, and reduced the influence of gas flow on cracks. Under the same experimental laser ablation parameters, the laser damage area of the honeycomb skeleton-Al₂O₃ composite coating was smaller than that of the Al₂O₃ coating. It was demonstrated that the laser damage resistance of the honeycomb skeleton-Al₂O₃ composite coating was superior to that of the Al₂O₃ coating.     

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.     

The wetting-to-nonwetting transition of CVD BN coatings deposited at different temperatures

Boron nitride (BN) coatings (thickness 20–40 μm) were prepared on graphite substrates by chemical vapor deposition, with precursors of BCl₃ and NH₃ (ratio of 1:4) and pressure of 500 ± 50 Pa. The influence of the deposition temperature (650°C–1250°C) on the wettability of BN coatings with deionized water was studied. The wetting angle rapidly increases at 1100°C–1250°C, and the wetting-to-nonwetting transition occurs. The crystal structure and surface morphology of the BN coatings were characterized by a stylus instrument, scanning electron microscopy, and transmission electron microscopy. Research shows that the contact angle or nonwettability increases with a higher degree of crystallinity and a lower surface roughness, which were both under the control of the deposition temperature since the pressure and gas flows were kept constant in this study. At a deposition temperature of 650°C–950°C, the increase in the degree of crystallinity dominates; at 950°C–1100°C, the increase in surface roughness takes over. At 1100°C–1250°C, the degree of crystallinity continues to increase, while the surface roughness decreases due to the advantage of nucleation and the breakage of large surface clusters into smaller clusters. This results in increases (650°C–950°C), then decreases (950°C–1100°C) and again fast increases (1100°C–1250°C) in the wetting angle between the BN coating and deionized water and finally in the wetting-to-nonwetting transition (1100°C–1250°C).     

Plane-strain fracture toughness of epoxies at different loading rates

The plane-strain fracture toughness of two common epoxy systems of different ductility were determined at different loading rates, according to ASTM E 399 for metallic materials. The ASTM standard was applicable, but underestimated slightly the specimen thickness required for K_Ic. K_Ic decreased with an increasing loading rate and with an increasing yield stress. The fracture surfaces were all very smooth as long as plane-strain conditions prevailed.     

Mixed-mode fracture model to quantify local toughness in nacre-like alumina

With the progress of ceramic engineering, tough technical ceramics such as nacre-like alumina now exhibit complex fracture patterns with high degrees of deflection, branching and bridging. If these mechanisms highly contribute to improving crack resistance, the crack geometries go beyond the hypotheses assumed in standard techniques to quantify toughness, making it difficult to understand the mechanisms responsible for crack propagation. We report a robust local solution based on a combination of analytical formulas and finite element analysis to accurately estimate the effective stress intensity factors at the tips of multiple deflected cracks. The influence of sample size on local R-curves is analysed and shown to be less important relative to standard methods. We use this method to evaluate the direct influence of zirconia nanoparticles in the nacre-like structure and highlight the role of these findings in the understanding of mechanical response and future optimisation of highly textured tough ceramics.     

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).     

Evaluation of fracture toughness of as plasma sprayed alumina–13 wt.% titania coatings by micro-indentation techniques

The fracture toughness of the as plasma sprayed alumina–13 wt.% titania (AT-13) coatings was evaluated using micro-indentation techniques. Indentations with smaller loads of 0.49 N show features such as partially melted particles to be extremely hard and brittle. However, the matrix made up of bi-modulus microstructure of splats was much tougher due to operative toughening mechanism for instance, crack bridging, which effectively retarded propagation of the cracks. Log plots of crack length (C) versus load (P) for loads varying from 1.96 to 9.8 N occurred with a slope of 0.65 ± 0.095 signifying sub-surface median or palmqvist type cracks to be rampant. The as sprayed microstructure made of pores; partially melted particles and splats were found to be anisotropic with regard to indentation toughness. Cracks were found to initiate and propagate easily in direction parallel to the coating growth directions than along the splat boundaries. Accordingly the bonding between the splats caused by plasma spray process could be deemed to be superior. The preferential directionality developed in the microstructure due to the imposed heat flow conditions essentially has weakened the microstructure.     

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.     

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.     

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.     

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.     

Absolute radiation tolerance of amorphous alumina coatings at room temperature

In this study structural and mechanical properties of a 1 μm thick Al₂O₃ coating, deposited on 316L stainless steel by Pulsed Laser Deposition (PLD), subjected to high energy ion irradiation were assessed. Mechanical properties of pristine and ion-modified specimens were investigated using the nanoindentation technique. A comprehensive characterization combining Transmission Electron Microscopy and Grazing-Incidence X-ray Diffraction provided deep insight into the structure of the tested material at the nano- and micro-scale. Variation in the local atomic ordering of the irradiated zone at different doses was investigated using a reduced distribution function analysis obtained from electron diffraction data. Findings from nanoindentation measurements revealed a slight reduction in hardness of all irradiated layers. At the same time TEM examination indicated that the irradiated layer remained amorphous over the whole dpa range. No evidence of crystallization, void formation or element segregation was observed up to the highest implanted dose. Reported mechanical and structural findings were critically compared with each other pointing to the conclusion that under given irradiation conditions, over the whole range of doses used, alumina coatings exhibit excellent radiation resistance. Obtained data strongly suggest that investigated material may be considered as a promising candidate for next-generation nuclear reactors, especially LFR-type, where high corrosion protection is one of the highest prerogatives to be met.     

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。     

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).