Mixed-Mode Fracture of Ceramics in Asymmetric Four-Point Bending: Effect of Crack-Face Grain Interlocking/Bridging

The mixed-mode fracture of a large-grain-size alumina ceramic and a soda-lime glass is investigated. These ceramics are tested using straight-through precracked or notched specimens. The straight-through precrack is introduced by the single-edge-precracked beam method. Precracked or notched specimens are subjected to combined mode I/II or pure mode II fracture, under asymmetric four-point bending, and pure mode I fracture, under symmetric four-point bending. A pure mode II fracture is never achieved in the precracked polycrystalline alumina by the crack-face friction inevitably induced by grain interlocking/bridging. The crack-face friction in sliding mode reduces the local mode II stress intensity factor in the crack-tip region and produces a sizable amount of mode I deformation. Accounting for the contribution of the crack-face friction to the crack-tip local stress intensity factors, K _I and K _II, in mixed-mode fracture tests, the experimental results of the K _I/ K _{I c} versus K _II/ K _{I c} envelope and the initial angle of noncoplanar crack extension are in good agreement with the theoretical predictions of the maximum hoop-stress theory.     

Measurement of Fracture Toughness of Plasma-Sprayed Al2O3 Coatings Using a Tapered Double Cantilever Beam Method

The fracture toughness of plasma-sprayed Al₂O₃ coatings in terms of the critical strain energy release rate G _Ic was measured using a tapered double cantilever beam (TDCB) approach. The fracture surfaces were examined using a scanning electron microscope (SEM). The measurement yielded the mean G _Ic values from 13 to 27 J/m² for the sprayed Al₂O₃ coatings at different spray distances. These values agree well with those obtained by the conventional double cantilever beam approach. The dependence of the observed G _Ic on spray distance is consistent with that for the lamellar bonding ratio. These results suggest that the TDCB test is a reliable approach for measuring the G _Ic of thermal-spray coatings without the crack-length measurement.     

Combined Mode I–Mode II Fracture of 12-mol%-Ceria-Doped Tetragonal Zirconia Polycrystalline Ceramic

The mode I, mode II, and combined mode Imode II fracture behavior of ceria-doped tetragonal zirconia polycrystalline (Ce-TZP) ceramic was studied. The single-edge-precracked-beam (SEPB) samples were fractured using the asymmetric four-point-bend geometry. The ratio of mode I to mode II loading was varied by varying the degree of asymmetry in the four-point-bend geometry. The minimum strain energy density theory best described the mixed-mode fracture behavior of Ce-TZP with the mode I fracture toughness, K _IC= 8.2 ± 0.6 MPa·m^1/2, and the mode II fracture toughness, K_IIC= 8.6± 1.3 MPa·m^1/2.     

Determining the Critical Strain Energy Release Rate of Plasma-Sprayed Coatings Using a Double-Cantilever-Beam Technique

A double-cantilever-beam (DCB) method for determining critical strain energy release rate (G_Ic) values from plasma-sprayed coatings is described in detail. This approach, involving acoustic emission (AE) methodology, yielded up to 25 results per specimen and was successful in providing cohesive G_Ic values for plasma-sprayed coatings of Al₂O₃-2.5 wt% TiO₂, Ni-20 wt% Al, as well as two ostensibly identical, 99.5% commercially pure Al₂O₃ coatings. Adhesive G _Ic data from Al₂O₃-40 wt% TiO₂ coatings was also obtained. Results showed a G _Ic dependence on crack length, and a number of possibilities, based on fractography, the AE response of the coatings during testing, and crack velocity measurements, are advanced to explain this occurrence. Differences in G _Ic values between coatings were found to correlate with differences in powder/coating properties. The DCB method was also used to investigate batch differences and the effect on toughness of sealing an alumina coating. Problems associated with this method of testing are addressed.     

Aqueous Colloidal Processing of ZTA Composites

Two different zirconia-alumina composites, ZTA-30 (70 wt% Al₂O₃+30 wt% ZrO₂) and ZTA-60 (40 wt% Al₂O₃+60 wt% ZrO₂), with potential for orthopedic applications, were processed in aqueous media and consolidated by slip casting (SC), hydrolysis-assisted solidification (HAS), and gelcasting (GC) from suspensions containing 50 vol% solids loading. For comparison purposes, the same ceramic compositions were also consolidated by die pressing of freeze-dried granules (FG). In the HAS process, 5 wt% of Al₂O₃ in the precursor mixture was replaced by equivalent amounts of AlN to promote the consolidation of the suspensions. Ceramics consolidated via GC exhibited higher green (three-point bend) strengths (∼17 MPa) than those consolidated by other techniques. Further, these ceramics also exhibited superior fracture toughness and flexural strength properties after sintering for 1 h at 1600°C in comparison with those consolidated by other techniques, including conventional die pressing (FG).     

Al2O3–Ni Composites with High Strength and Fracture Toughness

Al₂O₃–Ni composites were prepared by the reactive hot pressing of Al and NiO. The composites had a two-phase, interpenetrating microstructure and contained ∼35 vol% Ni. They exhibited an impressively high combination of strength and toughness at room temperature; the four-point bending strength was in excess of 600 MPa with a fracture toughness of more than 12 MPa·m^1/2. Examination of fracture surfaces showed that Ni ligaments underwent ductile deformation during fracture. SEM analysis revealed knife-edged Ni ligaments with a limited amount of debonding around their periphery (i.e., at the Ni–Al₂O₃ interface), indicating a strong Ni–Al₂O₃ bond.     

Processing and Properties of Intermetallic/Ceramic Composites with Interpenetrating Microstructure

Intermetallic/ceramic composites represent an interesting class of materials for high-temperature structural and functional applications. These materials can be prepared via high-energy milling of pure metals with Al₂O₃ as well as of aluminum with metal oxides. During subsequent compaction via pressureless sintering, the components react to form dense composites that consist of interpenetrating networks of the ceramic and intermetallic phases. Microstructural investigations, mechanical properties, and resistivity and wear resistance measurements of selected composites are presented. Improved fracture toughness and bending strength, with respect to monolithic Al₂O₃, have been achieved.     

Microstructure and Mechanical Properties of Porous Alumina Ceramics Fabricated by the Decomposition of Aluminum Hydroxide

The mechanical properties of Al₂O₃-based porous ceramics fabricated from pure Al₂O₃ powder and the mixtures with Al(OH)₃ were investigated. The fracture strength of the porous Al₂O₃ specimens sintered from the mixture was substantially higher than that of the pure Al₂O₃ sintered specimens because of strong grain bonding that resulted from the fine Al₂O₃ grains produced by the decomposition of Al(OH)₃. However, the elastic modulus of the porous Al₂O₃ specimens did not increase with the incorporation of Al(OH)₃, so that the strain to failure of the porous Al₂O₃ ceramics increased considerably, especially in the specimens with high porosity, because of the unique pore structures related to the large original Al(OH)₃ particles. Fracture toughness also increased with the addition of Al(OH)₃ in the specimens with higher porosity. However, fracture toughness did not improve in the specimens with lower porosity because of the fracture-mode transition from intergranular, at higher porosity, to transgranular, at lower porosity.     

Crack Bifurcation in Laminar Ceramic Composites

Crack bifurcation was observed in laminar ceramic composites when cracks entered thin Al₂O₃ layers sandwiched between thicker layers of Zr(12Ce)O₂. The Al₂O₃ layers contained a biaxial, residual, compressive stress of ∼2 GPa developed due to differential contraction upon cooling from the processing temperature. The Zr(12Ce)O₂ layers were nearly free of residual, tensile stresses because they were much thicker than the Al₂O₃ layers. The ceramic composites were fabricated by a green tape and codensification method. Different specimens were fabricated to examine the effect of the thickness of the Al₂O₃ layer on the bifurcation phenomena. Bar specimens were fractured in four-point bending. When the propagating crack encountered the Al₂O₃ layer, it bifurcated as it approached the Zr(12Ce)O₂/ Al₂O₃ interface. After the crack bifurcated, it continued to propagate close to the center line of the Al₂O₃ layer. Fracture of the laminate continued after the primary crack reinitiated to propagate through the next Zr(12Ce)O₂ layer, where it bifurcated again as it entered the next Al₂O₃ layer. If the loading was stopped during bifurcation, the specimen could be unloaded prior to complete fracture. Although the residual stresses were nearly identical in all Al₂O₃ layers, crack bifurcation was observed only when the layer thickness was greater than ∼70 μm.     

Preparation and Microstructure of Multi-Wall Carbon Nanotubes-Toughened Al2O3 Composite

With multi-wall carbon nanotubes (MWNTs) as reinforcement, a 12 vol% MWNTs/alumina (Al₂O₃) ceramic composite was obtained by hot pressing. A fracture toughness of 5.55±0.26 MPa·m^1/2, 1.8 times that of pure Al₂O₃ ceramics, was achieved. Experimental results showed that the enveloping of carbon nanotubes (CNTs) with sodium dodecyl sulfate (SDS) is effective in changing the hydrophobicity of CNTs to hydrophilicity and improving the dispersion of CNTs in aqueous solution. Enveloped with SDS, CNTs can be homogeneously mixed with Al₂O₃ at a microscopic level by heterocoagulation. This mixing method can obviously improve the chemical compatibility between CNTs and Al₂O₃, which is important for enhancement of interfacial strength between them.     

Fracture Toughness of Al2O3-Particle-Dispersed Y2O3-Partially Stabilized Zirconia

The fracture toughness of 3 mol% Y₂O₃-ZrO₂ (3Y-PSZ) composites containing 10–30 vol% Al₂O₃ with different particle sizes was investigated. It was found that Al₂O₃ dispersion of up to 30 vol% increased the fracture toughness by 17% to 30%, and the toughness increase was more remarkable in the composite dispersed with Al₂O₃ particles of larger sizes. By combining the effects of the dispersion toughening and phase transformation toughening, the toughness change in the present materials was theoretically predicted, which was in good agreement with the experimental data.     

Crack Deflection in Ceramic/Ceramic Laminates with Strong Interfaces

Crack deflection in electrophoretically deposited Al₂O₃/ TZ-3Y (3 mol% Y₂O₃-stabilized tetragonal ZrO₂) lamellar composites with strong interfaces is described. The fracture behavior of, and crack paths in, these materials were evaluated using indentation and four-point bend tests. The effects of residual and induced stresses on crack deflection are considered.     

Transformation Plasticity and Toughening in CeO2-Partially-Stabilized Zirconia–Alumina (Ce-TZP/Al2O3) Composites Doped with MnO

Microstructure, phase stability, and mechanical properties of CeO₂-partially-stabilized zirconia (12 mol% Ce-TZP) containing 10 wt% Al₂O₃ and 1.5 wt% MnO were studied in relation to the base Ce-TZP and the Ce-TZP/Al₂O₃ composite without MnO. The MnO reacted with both CeO₂ and Al₂O₃ to form a new phase of approximate composition CeMnAl₁₁O₁₉. The reacted phase had a magnetoplumbite structure and formed elongated, needlelike crystals. The MnO-doped Ce-TZP/Al₂O₃ composites sintered at an optimum temperature of 1550°C exhibited high strength (650 MPa in four-point bending) and rising crack-growth-resistance behavior, with fracture toughness increasing from 7.6 to 10.3 MPa.In¹² in compact tension tests. These improved mechanical properties were associated with relatively high tetragonal-to-monoclinic transformation temperature ( M _s=−42°C) at small grain size (2.5 μm), significant transformation plasticity in mechanical tests (bending, uniaxial tension, and uniaxial compression) and transformation zones at crack tips in compact tension specimens. The transformation yield stress, zone size, and fracture toughness were sensitive to the sintering temperature varied in the range 1500° to 1600°C. Analysis of the transformation zones using Raman microprobe spectroscopy and calculation of zone shielding for the observed zones indicated that a large fraction of the fracture toughness (∼70%) was derived from transformation toughening.     

Fabrication of an Al2O3/YAG/ZrO2 Ternary Eutectic by Combustion Synthesis Melt Casting Under Ultra-High Gravity

This work presents a novel method for preparing an Al₂O₃/YAG/ZrO₂ ternary eutectic whereby combustion synthesis melt casting has been combined with the ultra-high gravity (UHG) technique. The fabricated product had a relative density of 99.3% of the theoretical one. Phase composition and microstructure analyses indicated that the application of UHG resulted in a metal-free ceramic microstructure with no porosity or microcracks. The microstructure comprises ZrO₂ rods dispersed in Al₂O₃. The product had 17.82 GPa Vickers hardness and 5.51 MPa·m^1/2 fracture toughness.     

Effects of Zirconium Doping on Grain-Boundary Bonding in Alumina-Silicon Carbide Composites

In this work, 800 ppm of Zr⁴⁺ dopants were added to Al₂O₃-5 vol% SiC particle composite. Zr⁴⁺ doping led to a weak Al₂O₃ grain-boundary bonding so that the fracture mode changed from transgranular in undoped composite to intergranular in Zr⁴⁺-doped composite. The fracture mode change increased the fracture toughness of the composite. Transmission electron microscopy and energy-dispersive spectroscopy examinations revealed that the weak grain-boundary bonding in the doped composite was caused by the segregation of Zr⁴⁺ and Si⁴⁺ ions at the Al₂O₃ grain boundary.     

High-Strength and High-Fracture-Toughness Ceramics in the Al2O3/LaAl11O18 Systems

Control of microstructure in the Al₂O₃/LaAl₁₁O₁₈ system was performed. Elongated alumina grains were formed by doping with small addition of silica, and 20 vol% lanthana- luminate was formed in situ by the reaction of LaAlO₃- A1₂O₃ in an alumina matrix. Strengths of over 600 MPa and a high fracture toughness (6 MPa.m^1/2) were achieved in the material with both elongated A1₂O₃ grains and LaAl₁₁O₁₈ platelets. Generally antagonistic properties such as strength and fracture toughness have been made compatible in the same ceramic system.     

Electroconductive Al2O3–NbN Composites

Electroconductive Al₂O₃–NbN ceramic composites were prepared by hot pressing. Dense sintered bodies of ball-milled Al₂O₃–NbN composite powders were obtained at 1550°C and 30 MPa for 1 h under a nitrogen atmosphere. The bending strength and fracture toughness of the composites were enhanced by incorporating niobium nitride (NbN) particles into the Al₂O₃ matrix. The electrical resistivity of the composites decreased with increasing amount of NbN phase. For a 25 vol% NbN–Al₂O₃ composite, the values of bending strength, fracture toughness, Vickers hardness, and electrical resistivity were 444.2 MPa, 4.59 MPa·m^1/2, 16.62 GPa, and 1.72 × 10⁻²Ω·cm, respectively, making the composite suitable for electrical discharge machining.     

Properties of (Y)ZrO2–Al2O3 and (Y)ZrO2–Al2O3–(Ti or Si)C Composites

The effect of Al₂O₃ and (Ti or Si)C additions on various properties of a (Y)TZP (yttria-stabilized tetragonal zirconia polycrystal)–Al₂O₃–(Ti or Si)C ternary composite ceramic were investigated for developing a zirconia-based ceramic stronger than SiC at high temperatures. Adding Al₂O₃ to (Y)TZP improved transverse rupture strength and hardness but decreased fracture toughness. This binary composite ceramic revealed a rapid loss of strength with increasing temperature. Adding TiC to the binary ceramic suppressed the decrease in strength at temperatures above 1573 K. The residual tensile stress induced by the differential thermal expansion between ZrO₂ and TiC therefore must have inhibited the t - → m -ZrO₂ martensitic transformation. It was concluded that a continuous skeleton of TiC prevented grain-boundary sliding between ZrO₂ and Al₂O₃. In contrast, for the ternary material containing β-SiC in place of TiC, the strength decreased substantially with increasing temperature because of incomplete formation of the SiC skeleton.     

Mechanical Behavior of Alumina–Silicon Carbide „Nanocomposites”

The fracture behavior of Al₂O₃ containing 5 vol% 0.15μm SiC particles was investigated using indentation techniques. A significant increase in strength was achieved by the addition of SiC particles to the base Al₂O₃. Specifically, the strength increased from 560 MPa for Al₂O₃ to 760 MPa for the composite samples (average values for unindented hotpressed bars tested in four-point bending). After annealing for 2 h at 1300°C, the average strength of the composite samples increased to about 1000 MPa. Toughness was estimated using indentation-strength data. While there was a slight increase in toughness, the increase was not sufficient to account for the increase in the unindented strength on SiC particle addition. It is suggested that the observed strengthening and apparent toughening were due to a machining-induced compressive surface stress.     

Chemical Vapor Deposition for Silicon Cladding on Advanced Ceramics

Polycrystalline Si was used to clad several advanced ceramic materials such as SiC, Si₃N₄, sapphire, Al₂O₃, pyrolytic BN, and Si by a chemical vapor deposition (CVD) process. The thickness of Si cladding ranged from 0.025 to 3.0 mm. CVD Si adhered quite well to all the above materials except Al₂O₃, where the Si cladding was highly stressed and cracked or delaminated. A detailed material characterization of Si-clad SiC samples showed that Si adherence to SiC does not depend much on the substrate surface preparation; that the thermal cycling and polishing of the samples do not cause delamination; and that, in four-point bend tests, the Si–SiC bond remains intact, with the failure occurring in the Si.