High damage tolerant Al2O3 composite ceramics constructed with short Al2O3 fibers

The catastrophic fracture characteristics of ceramic materials have become one of the most serious factors limiting their application in critical areas, as a result, it is urgent to overcome the brittleness and improve the damage tolerance of ceramic materials. Herein, a series of Al₂O₃ composite ceramics developed with short Al₂O₃ fibers and a compound interface phase composed of Al₂O₃ and h-BN powders, followed by investigating their fracture behaviors and damage tolerance. Results show that these composites present progressive fracture manners with the rising resistance curve (R-curve) behaviors, and the maximum crack growth toughness of the sample with 15% compound interface phase reaches above 10 MPa·m^1/2 (135% increase with respect to the reference alumina). Meanwhile, the composite ceramic exhibits an excellent ability to resist catastrophic failure with a large critical crack size (105.47 ± 19.11 μm) and high damage tolerance parameter (0.71 ± 0.06 m^1/2), which are close to 14.57 times and 5.92 times higher than those of the reference alumina. The superior performances are mainly attributed to the precise combination of compound interface phase for inducing crack and interlocking Al₂O₃ fibers for load capacity.     

Fracture toughness determination of Ti3Si(Al)C2 and Al2O3 using a single gradient notched beam (SGNB) method

In this work, we suggest a new and simple method named single gradient notched beam (SGNB) method for determining the fracture toughness of Ti₃Si(Al)C₂ and Al₂O₃ with four-point bending specimens. For the specimen with a gradient notch, a sharp natural crack will initiate and extends from the tip of the triangle under increasing load. Based on the straight through crack assumption or on the slice model, the stress intensity factor coefficient for this notched beam was derived. The fracture toughness can be calculated from the maximum load and the minimum of the stress intensity factor coefficient without knowing the crack length. To verify the feasibility and reliability of this suggested method, the SGNB method and two other conventional methods, e.g. the chevron notched beam (CNB) method and single edge notched beam (SENB) method, were performed to determine the fracture toughness of Ti₃Si(Al)C₂ and Al₂O₃. The measured fracture toughness values obtained from the SGNB method agreed well with those from conventional fracture toughness tests.     

Fracture toughness of zirconia from a shallow notch produced by ultra-short pulsed laser ablation

Fracture toughness of submicron grain size tetragonal zirconia polycrystals doped with 3 mol% yttria (3Y-TZP) is measured by the single edge V-notch beam (SEVNB) method from a shallow sharp notch produced by ultra-short pulsed femtolaser ablation (UPLA) on the surface of a bending bar. It is shown that the radius of the notch tip achieved is in the submicron range and the damaged volume in front of the notch tip is characterized by using focus ion beam milling and scanning electron microscopy. It consists of a narrow fully microcracked region less than ∼4 μm wide and ∼15 μm deep in front of the notch. If the extension of this region and the length of the notch are used in the determination of the fracture toughness (K_Ic) in the four bending test, the values obtained for submicron grain size 3Y-TZP are in agreement those obtained by using very sharp cracks. It is concluded that the SEVNB testing method with a sharp notch induced by UPLA may be used for K_Ic testing of submicron grain size ceramics.     

Fracture toughness improvement of epoxy resins with short carbon fibers

This study examined the thermal stability and fracture toughness of diglycidylether of bisphenol-A (DGEBA)/short carbon fiber (SCF) composites using several techniques. The thermal stability of the DGEBA/SCF composites was similar to that of neat epoxy resin. The fracture toughness of the composites was significantly improved relative to the neat resin. The SEM micrographs indicated that a relatively rough surface with shear deformation and tortuous cracks was formed, thereby preventing deformation and crack propagation and inducing higher fracture toughness in the DGEBA/SCF composites.     

Al2O歧化法制备微细Al2O3/Al复合粉体

真空条件下,以Al2O3和Al为原料,通过Al2O歧化法制备微细Al2O3/Al复合粉体.XRD和SEM分析表明:在反应温度为1200～1400℃时,随着温度的升高,粉体中氧化铝含量升高;冷凝温度约为550～750℃时,复合粉体中的氧化铝包括稳定晶型和不稳定晶型;冷凝温度约为1100～1300℃时,复合粉体中的氧化铝全部为稳定晶型;冷凝温度约为550～650℃时,复合粉体的平均粒径小于0.5μm;冷凝温度约为750℃时,铝熔化、微粒团聚;冷凝温度约为1100～1200℃时,铝形成铝珠,氧化铝为不规则状、平均粒径小于2μm;冷凝温度约为1300℃时,氧化铝为片状.因此,通过选取合适的反应温度、冷凝温度,可以控制Al2O3/Al复合粉体中氧化铝的含量、晶型和粒径.     

Orientation relationships of unidirectionally aligned GdAlO3/Al2O3 eutectic fibers

Unidirectionally solidified rare-earth activated GdAlO₃(GAP)/Al₂O₃ eutectic crystal with well-aligned fibrous structure exhibits excellent light guiding property and can be used as a scintillator plate for high-resolution X-ray imaging. In this paper, the microstructures and orientation relationships of the GAP/Al₂O₃ eutectic fibers were investigated. The regular GAP single crystal fibers with a hexagonally close-packed arrangement grew straight in the same direction along the solidification direction, and were embedded in a c-axis oriented Al₂O₃ single crystal matrix. The majority of GAP fibers had the orientation relationships of [0 1 0]GAP//[0 0 0 1]Al₂O₃ to the growth direction and $(100)\text{GAP}//(11\overline{2}0){\text{Al}}_2{\text{O}}_3$ to the interface plane, while slight misorientation angle of both [0 1 0]GAP axis and (1 0 0)GAP plane were observed. In the GAP/Al₂O₃ interface boundary, the lattice misfit between the two phases was relieved by insertion of extra half-planes on the Al₂O₃ side.     

Effects of post-sintering annealing on the microstructure and toughness of hot-pressed SiCf/SiC composites with Al2O3-Y2O3 additions

This study examined the effects of post-sintering heat treatment on enhancing the toughness of SiC_f/SiC composites. Commercially available Tyranno^® SiC fabrics with contiguous dual ‘PyC (inner)-SiC (outer)’ coatings deposited on the SiC fibers were infiltrated with a SiC + 10 wt% Al₂O₃-Y₂O₃ slurry by electrophoretic deposition. SiC green tapes were stacked between the slurry-infiltrated fabrics to control the matrix volume fraction. Densification of approximately 94% ρ_theo was achieved by hot pressing at 1750 °C, 20 MPa for 2 h in an Ar atmosphere. Sintered composites were then subjected to isothermal annealing treatment at 1100, 1250, 1350, and 1750 °C for 5 h in Ar. The correlation between the flexural behavior and microstructure was explained in terms of the in situ-toughened matrix, phase evolution in the sintering additive, role of dual interphases and observed fracture mechanisms. Extensive fractography analysis revealed interfacial debonding at the hybrid interfaces and matrix cracking as the key fracture modes, which were responsible for the toughening behavior in the annealed SiC_f/SiC composites.     

压痕法测量ZrO2/Al2O3陶瓷断裂韧性的研究

使用压痕法研究ZrO2/Al2O3陶瓷的断裂韧性,通过实验分析放大倍数和载荷两个因素对试样M31和M34测量结果的影响.放大倍数低于600倍时,很难测量出实际裂纹尺寸,而在6000倍时测得了比较准确的裂纹尺寸.随着载荷的增加,样品M31断裂韧性对比误差逐渐降低,样品M34误差呈波浪式变化.选择最适宜的载荷,可得到最接近实际情况的KIC值;本文通过大量实验数据证实,样品M31最适宜的载荷范围在8～12 kg,M34的载荷范围在6～8 kg.裂纹的扩展形式包括沿相界断裂,沿晶界断裂和潜藏断裂.沿相界断裂消耗能量较低,对陶瓷材料强度和韧性贡献较小;沿晶界断裂消耗能量较高,对陶瓷材料强度和韧性贡献较大.     

Formation of an Al2O3-coated catalyst with a metallic core by anodic oxidation of aluminium

The formation of a novel type of coated catalyst with definite texture is described. The oxidic coating is formed by anodic oxidation of aluminium and a metallic core is preserved. The textural properties for two separately produced catalysts were determined. To obtain this result, high-pressure Hg-porosimetry, N₂ adsorption (BET), light-optical microscopy and electron microscopy (SEM, TEM) were applied. On the basis of the hexagonal cell model, proposed by Keller, Hunter and Robinson the specific surface areas and the specific pore volumes were calculated. The comparison between calculated and measured values correlates well and shows, that the formation of Al₂O₃-coated catalysts by anodic oxidation of aluminium is reproducible.     

Separation of Phases by Spinodal Decomposition in the Systems Al2O3−Cr2O3 and Al2O3−Cr2O3−Fe2O3

Separation of phases was investigated in the hexagonal (rhombohedral) systems Al₂O₃−Cr₂O₃ and Al₂O₃−Cr₂O₃−Fe₂O₃. The binary system shows a miscibility gap with a T_c of 950°C; the miscibility gap for the ternary system was determined for a constant Cr₂O₃ content of 16.6 mol%. Dark field transmission electron microscopy of solid solutions annealed within the miscibility gap showed dark and light lamellas ∼50 to 200 Å thick. X-ray diffraction results for the solid solutions in the ternary system indicated that, in the early stages of annealing, broadening occurred only on (hkl) reflections where l≠0. There was no major change in the X-ray diffraction patterns of the annealed solid solutions in the binary system. Electron diffraction results indicated, however, that phase separation in both systems proceeded in the [001] direction. Solid solutions in the binary system separated very slowly; the separation could be enhanced hydrothermally. The mechanism of the separation of phases in both systems is spinodal and proceeds as follows: solid solution→intermediate modulated phase→equilibrium phases.     

Mechanical properties of Al2O3 and Al2O3/Al with Gyroid structure obtained by stereolithographic additive manufacturing and melt infiltration

To obtain both plasticity and toughness of the material at the same time, various manufacturing techniques of ceramic-metal composites and structures have been studied. In this work, a bio-inspired Al₂O₃ ceramic scaffold with Gyroid structure was designed and prepared by stereolithographic (SL) additive manufacturing, then the Al₂O₃/Al ceramic-metal hybrid structure was prepared by infiltrating molten Al into the Al₂O₃ ceramic structure. The performances of the Al₂O₃ ceramic scaffold and the Al₂O₃/Al ceramic-metal hybrid structure were compared and analyzed by a quasi-static compression experiment. The quasi-static compressive strength of the pristine Al₂O₃ scaffold was 14.36 MPa, while that of the Al₂O₃/Al ceramic-metal hybrid structure was up to 89.06 MPa. Moreover, the plasticity of the Al₂O₃/Al ceramic-metal hybrid structure was much higher than that of the Al₂O₃ scaffold. During compression, the Al₂O₃/Al ceramic-metal hybrid structure had excellent energy absorption, reaching up to 2569.16 KJ/m³, 15 times that of the Al₂O₃ scaffold. Therefore, this method can obtain materials with excellent ductility and toughness.     

Fracture toughness of an epoxy system

The fracture toughness of epoxy used in the bulk and adhesive form was measured by a previously developed technique. The uniform double cantilever-beam specimen, which was described earlier, was modified to a tapered beam, which simplified the experimental procedure and calculations for obtaining toughness measurements. by varying the ratio of hardener to resin and post-cure temperature on a single epoxy system (DER 332-TEPA), it was found that the toughness of the epoxy used in either bulk or bond form varied by a factor of approximately five. A particular combination of composition and post-curing temperature generally yielded higher toughness in the bulk than in the bond form. This was not always the case, however. At high post-cure temperatures, where the bonds were very tough, their toughness exceeded that of the bulk material. Hence, it does not appear possible to predict joint toughness from bulk toughness measurements. The toughness of joints was found to be a single-valued function of tensile modulus. For the bulk material, on the other hand, the toughness obtained on the epoxy having a specific modulus depended on the combination of composition and post-cure temperature. Joint toughness for any combination of composition and post-cure temperature depended only on the cracking rate. If the epoxy was the type that caused cracks to jump rapidly, the epoxy was tough and vice versa. For a particular epoxy system, toughness was increased by driving the crack at an increasing rate.     

Air plasma-sprayed Y2O3 coatings for Al2O3/Al2O3 ceramic matrix composites

Al₂O₃/Al₂O₃ ceramic matrix composites (CMC) are candidate materials for hot-gas leading components of gas turbines. Since Al₂O₃/Al₂O₃ CMC are prone to hot-corrosion in combustion environments, the development of environmental barrier coatings (EBC) is mandatory. Owing to its favorable chemical stability and thermal properties, Y₂O₃ is considered a candidate EBC material for Al₂O₃/Al₂O₃ CMC. Up to 1 mm thick Y₂O₃ coatings were deposited by means of air plasma spraying (APS) on Al₂O₃/Al₂O₃ CMC with a reaction-bonded Al₂O₃ bond-coat (RBAO). APS Y₂O₃ coatings exhibit a good adherence in the as-deposited state as well as upon isothermal annealing up to 1400 °C. Moreover, furnace cyclic testing performed at 1200 °C revealed an excellent durability. This is explained by the formation of a continuous, approximately 1 μm thick reaction zone at the APS Y₂O₃/RBAO interface. The reaction zone between Y₂O₃ and Al₂O₃ comprises three layers of thermodynamically stable yttrium-aluminates exhibiting strong bonding, respectively.     

Air-brazed Al2O3 joint with a novel bismuth glass

Polycrystalline alumina ceramics were air-brazed using bismuth glass with a chemical composition of 50Bi₂O₃–40B₂O₃–10ZnO (mol.%) at a relatively low temperature. ZnAl₂O₄ particles were formed insitu in the joints and the particles grew rapidly with an increase in joining temperature and holding time increasing. In addition, penetration of the alumina substrate by the glass became increasingly serious at higher temperatures and holding durations. The mechanical properties of the joints were investigated and the maximum shear strength was determined to be 50 MPa when brazed at 650 °C for 0 min.     

In-Situ Determination of bridging stresses in Al2O3/Al2O3-platelet composites by fluorescence spectroscopy

Bridging stresses arising from interlocking and frictional effects in the crack wake have been quantitatively evaluated in an Al₂O₃/Al₂O₃-platelet ceramic, using in-situ microprobe fluorescence spectroscopy. Crack opening displacement (COD) profile has also been quantitatively measured in the scanning electron microscope (SEM), in order to substantiate the reliability of the piezo-spectroscopic measurements of microscopic bridging stresses. Mapping the crack wake (at critical condition for crack propagation) with a laser probe of 2 μm spatial resolution led to determine a discrete map of closure stresses over a crack extension of about 800 μm. Relatively high bridging stress values ≈350 MPa were revealed due to platelet interlocking in a near-tip bridging zone <100 μm, whereas frictional sites of lower stress magnitude <100 MPa were monitored in the crack profile farther away from the crack tip. The availability of microscopic fracture parameters like as the bridging stress distribution and the near-tip COD profile enables to quantitatively explain the rising R-curve behavior of the Al₂O₃/Al₂O₃-platelet material. Bridging stress distribution, COD profile and R-curve data are discussed in comparison with those collected in previous studies on equiaxed Al₂O₃ and toughened Si₃N₄. The present study supports the notion that crack bridging is by far the most important toughening mechanism in non-transforming ceramics.     

Structural investigation of Al2O3 coatings by PECVD with a high deposition rate

The deposition of alumina on thermal barrier coatings can effectively avoid hot corrosion and increase durability. Al₂O₃ coatings were prepared on an yttria-stabilized zirconia (YSZ) substrate by plasma chemical vapor deposition (CVD). The effects of microwave power (P_M) and total pressure (P_tot) on the crystalline phase and microstructure of Al₂O₃ coatings were investigated, and the effect of the mechanism on the deposition rate was also analyzed. The α-Al₂O₃ coatings with a needle-like microstructure were formed at a higher P_M and P_tot, whereas the γ-phase coatings exhibited a cauliflower-like microstructure at a lower P_M and P_tot. A maximum deposition rate (R_dep) of 58 μmh⁻¹ was obtained, which is significantly higher than those of conventional CVD methods.     

Refractory alumina cement based on Al2O3 with an aluminochromophosphate binder

Conclusions The possibility of producing refractory cements based on industrial and fired alumina, and white and normal electrocorundum, using ACPB has been studied. It has been established that the use of these fillers in conjunction with ACPB makes it possible to make refractory materials which can be used in equipment with a high thermal load. At 1600°C, it is reasonable to use a cement based on white electrocorundum and industrial alumina with a specific surface of 8850 cm²/g.Translated from Ogneupory, No. 1, pp. 52–55, January, 1980.     

Optical absorption and selective thermal emission in directionally solidified Al2O3-Er3Al5O12 and Al2O3-Er3Al5O12-ZrO2 eutectics

Al₂O₃-Er₃Al₅O₁₂ and Al₂O₃-Er₃Al₅O₁₂-ZrO₂ eutectic ceramic rods were directionally solidified using the laser floating zone technique at several growth rates. Binary eutectic microstructure consisted in a three-dimensional interpenetrated network of the eutectic phases whereas the ternary eutectic showed a geometrical microstructure at low growth rates and a nanofibrillar pattern at high rates. The microstructure size was strongly dependent on the growth rate, decreasing when the processing rate increased. The optical absorption was measured in the samples at room temperature and Judd–Ofelt analysis was used to model the optical absorption of the Er³⁺ ions. Thermal emission of the eutectic rods was studied at temperatures up to 1600 °C. An intense narrow emission band at 1.55 μm matching with the sensitive region of the GaSb photoconverter was obtained. The intensity of the selective emission band is larger for the binary eutectic than for the ternary compound and increases as the microstructural size decreases.