Fracture of LiF-MgF2 Precipitated Composites

The precipitation reaction in LiF-MgF₂ solid solutions was successfully used to toughen this composite. Incoherent, stable, rod-shaped MgF₂ precipitates were produced in the LiF matrix by quenching and aging treatments. They increased the fracture toughness of the system to >12 times that of undoped LiF single crystals. The maximum toughness was observed with 3 wt% MgF₂ after quenching and aging at 280°C for°300 min. The large toughness increase is attributed to pulling out of the incoherent MgF₂ rods and microcracking ahead of the primary crack.     

Electric-Field-Induced Crack Growth Behavior in PZT/Al2O3 Composites

Hard lead zirconate titanate (PZT) and PZT/Al₂O₃ composites were prepared and the alternating-electric-field-induced crack growth behavior of a precrack above the coercive field was evaluated via optical and scanning electron microscopy. The crack extension in the 1.0 vol% Al₂O₃ composite was significantly smaller than that in monolithic PZT and the 0.5 vol% Al₂O₃ composite. Secondary-phase Al₂O₃ dispersoids were found both at grain boundaries and within grains in the composites. A large number of dispersoids were observed at the grain boundaries in the 1.0 vol% Al₂O₃ composite. It appears that the Al₂O₃ dispersoids reinforce the grain boundaries of the PZT matrix as well as act as effective pins against microcrack propagation.     

Reinforcement of Hydroxyapatite Bioceramic by Addition of Ni3Al and Al2O3

The effects of Ni₃Al and Al₂O₃ additions on the mechanical properties of hydroxyapatite (HAp) were investigated. The addition of Ni₃Al particles increased the strength as well as the fracture toughness of HAp. However, the improvements in the properties were limited because of the formation of microcracks around the metal particles. The microcracks were formed because of the large difference in the coefficients of thermal expansion between HAp and Ni₃Al, and because of the relatively large size of Ni₃Al particles (∼20 µm). The addition of submicrometer Al₂O₃ powder was also effective in increasing the mechanical properties. The flexural strength and the fracture toughness were increased from about 100 MPa and 0.7 MPam^1/2, respectively, to 200 MPa and 1.5 MPam^1/2 by the addition of 20 vol% Al₂O₃. When Ni₃Al and Al₂O₃ were added together, the fracture toughness was further increased to 2.3 MPam^1/2. This increase in the fracture toughness was attributed to the synergistic effect of matrix strengthening and crack interactions with the metal particles.     

Effect of Alumina on the Strength, Fracture Toughness, and Crystal Structure of Fluorcanasite Glass-Ceramics

The influence of alumina content (0-15 wt% Al₂O₃) on the indentation strength, fracture toughness ( K _{I c} ), and crystal structure of fluorcanasite (Al₂O₃-CaO-F-K₂O-Na₂O-SiO₂) glass-ceramics was analyzed. Increasing the Al₂O₃ content from 0 wt% (CAN0) to 8 wt% (CAN8) caused the mean indentation strength and K _{I c} values to decrease from 213 ± 14 MPa and 2.7 ± 0.1 MPa·m^1/2, respectively, for the CAN0 glass-ceramic to 78 ± 16 MPa and 1.3 ± 0.2 MPa·m^1/2, respectively, for the CAN8 glass-ceramic. Increased Al₂O₃ concentrations (0-15 wt%) significantly affected the crystal size, crystal shape, aspect ratio, and crystal aggregation characteristics of the fluorcanasite glass-ceramics. The addition of greaterthan equal to8 wt% of Al₂O₃ to fluorcanasite glass caused a transformation from canasite to leucite.     

Fracture Behavior of SiC-Whisker-Reinforced Barium Aluminosilicate Glass-Ceramic Matrix Composites

Barium aluminosilicate (BAS) glass-ceramic composites reinforced with various volume percents (0, 10, 20, 30, 40 vol%) of SiC whiskers were fabricated by hot pressing. The microstructure, the whisker/matrix interface structure, the phase constitution, and the mechanical properties of the composites were systematically studied by means of SEM, TEM, and XRD techniques as well as by indentation crack microfracture and single-edge-notched-beam bend testing. It was demonstrated that the incorporation of SiC whiskers could significantly increase the flexural strength and fracture toughness of BAS glass-ceramic matrices. The addition of active Al₂O₃ to the BAS matrix reduced the amount of SiO₂ in the matrix, forming needlelike mullite, which further improved the mechanical properties.     

Phase Equilibria and Structural Species in MgF2-MgO, MgF2-CaO, and MgF2-Al2O3 Systems

Partial equilibrium phase diagrams for the systems MgF₂-MgO, MgF₂-CaO, and MgF₂-Al₂O₃ were determined by differential thermal analysis. Simple eutectics were observed at 8.5 mol% MgO and 1228°± 3°C in the MgF₂-MgO system, at 7.5 mol% CaO and 1208°± 3°C in the MgF₂-CaO system, and at 2.5 mol% Al₂O₃ and 1250°± 3°C in the MgF₂-Al₂O₃ system. On the basis of agreements between the activities calculated by the Clausius-Clapeyron equation and Temkin's model using the present data, the eutectic melt consists of Mg²⁺, F^-, and O^2- ions in the MgF₂-MgO system; Mg²⁺, Ca²⁺, F^-, and O^2- ions in the MgF₂-CaO system; and Mg²⁺, Al³⁺, F^-, and AlO_2¯ ions in the MgF₂-Al₂O₃ system. Well-defined long needles of MgO in the MgF₂-MgO system, less defined needles of CaO in the MgF₂-CaO system, and Al₂O₃ grains in the MgF₂-Al₂O₃ system were observed by optical microscopy.     

Precipitation in LiF-MgF2 Solid Solutions

Precipitation was studied for LiF single crystals containing up to 4 wt% MgF₂. Replica electron microscopy observations, combined with hardness and lattice parameter measurements, showed that, irrespective of the level of MgF₂, precipitation consistently occurs during quenching. Rapidly cooled crystals with lower MgF₂ concentrations (<1 wt%) develop large cruciform particles of a metastable phase with approximate composition MgF₂-6LiF. Lower cooling rates or higher MgF₂ contents caused transformation of these metastable particles to rod-shaped stable MgF₂ phase. This occurs via nucleation on the metastable crosses. The large misfit between the LiF matrix and the MgF₂ phase causes development of an incoherent interface and an associated disturbance of a propagating crack was observed near each MgF₂ particle.     

Phase Relations and Microstructural Development of Aluminum Nitride–Aluminum Nitride Polytypoid Composites in the Aluminum Nitride–Alumina–Yttria System

AlN–AlN polytypoid composite materials were prepared in situ using pressureless sintering of AlN–Al₂O₃ mixtures (3.7–16.6 mol% Al₂O₃) using Y₂O₃ (1.4–1.5 wt%) as a sintering additive. Materials fired at 1950°C consisted of elongated grains of AlN polytypoids embedded in equiaxed AlN grains. The Al₂O₃ content in the polytypoids varied systematically with the overall Al₂O₃ content, but equilibrium phase composition was not established because of slow nucleation rate and rapid grain growth of the polytypoid grains. The polytypoids, 24 H and 39 R , previously not reported, were identified using HRTEM. Solid solution of Y₂O₃ in the polytypoids was demonstrated, and Y₂O₃ was shown to influence the stability of the AlN polytypoids. The present phase observations were summarized in a phase diagram for a binary section in the ternary system AlN–Al₂O₃–Y₂O₃ parallel to the AlN–Al₂O₃ join. Fracture toughness estimated from indentation measurements gave no evidence for a strengthening mechanism due to the elongated polytypoids.     

Indentation Studies on Y2O3-Stabilized ZrO2: II, Toughness Determination from Stable Growth of Indentation-Induced Cracks

Stable indentation cracks were grown in four-point bend tests to study the fracture toughness of two Y₂O₃-stabilized ZrO₂ ceramics containing 3 and 4 mol% Y₂O₃. By combining microscopic in situ stable crack growth observations at discrete stresses with crack profile measurements, the dependence of toughness on crack extension was determined from crack extension plots, which graphically separate the crack driving residual stress intensity and applied stress intensity factors. Both materials exhibit steeply rising R -curves, with a plateau toughness of 4.5 and 3.1 Mpa·m^1/2 for the 3- and 4-mol% materials, respectively. The magnitude of the plateau toughness reflects the fraction of tetragonal grains contributing to transformation toughening.     

Lateral Cracks and Microstructural Effects in the Indentation Fracture of Yttria

The fracture properties of three polycrystalline Y₂O₃ materials: one fully cubic phase, one containing an Al₂O₃ grain-boundary phase, and one containing hexagonal phase, were examined by indentation over a wide range of contact loads. The two former microstructures displayed tendencies at large indentation loads to radial crack lengths shorter than those extrapolated from the ideal response at low loads. The deviations correlated with the development of lateral cracks at the larger contacts, rather than with any observable change in the interaction between the cracks and the microstructure. After taking the lateral crack influence into account, the toughness of all three materials was estimated to be constant over the range of crack lengths studied, in contrast to the phenomena observed in similar grain size noncubic materials and inferred from earlier fractographic studies. The toughness of the partially hexagonal material was estimated to be 50% greater than the cubic materials. The general phenomenon of partitioning energy into lateral cracks at the expense of radial cracks at large indentation loads has been characterized by a lateral crack development parameter, L_D , which varies from 0 to 1 as lateral cracks progressively develop and remove material.     

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.     

R Curves and Crack-Stability Map: Application to Ce-TZP/Al2O3

The concept of a crack-stability map is developed by considering the interaction between the crack-driving force and the rising crack-growth resistance of a toughened ceramic. The map plots normalized transition crack length as function of the ratio of the crack-initiation fracture toughness and the plateau toughness to delineate regimes of stable and unstable crack growth. The plot is used to analyze R curves and fracture stresses of a transformation-toughened Ce-TZP/Al₂O₃. It is shown that the fracture stress and the small scatter measured for this ceramic are consistent with its R- curve behavior, which enables stable growth of surface cracks from flaws (pores and second-phase particles), leading to a flaw-insensitive ceramic.     

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.     

Novel Fabrication Route to Al2O3–TiN Nanocomposites Via Spark Plasma Sintering

A novel method for the preparation of Al₂O₃–TiN nanocomposites was developed. A mixture of TiO₂, AlN, and Ti powder was used as the starting material to synthesize the Al₂O₃–TiN nanocomposite under 60 MPa at 1400°C for 6 min using spark plasma sintering. X-ray diffractometry, scanning electron microscopy, and transmission electron microscopy were used for detailed microstructural analysis. Dense (up to 99%) nanostructured Al₂O₃–TiN composites were successfully fabricated, the average grain size being less than 400 nm. The fracture toughness ( K _{I C} ) and bending strength (σ_b) of the nanostructured Al₂O₃–TiN composites reached 4.22±0.20 MPa·m^1/2 and 746±28 MPa, respectively.     

Fabrication and Mechanical Properties of Al2O3-WC-Co Composites by Vacuum Hot Pressing

Al₂O₃-WC-Co composites were fabricated by vacuum hot-pressing mixtures of Al₂O₃, WC, and cobalt powders. The phases formed with WC additions of up to 40 wt% were α-Al₂O₃, WC, Co₃W₃C, and small amounts of f-Co (face-centered cubic cobalt) and carbon (graphite); no cobalt or carbon phases formed at >40 wt% WC. A more-uniformly distributed and connected WC matrix formed as the WC content increased. The 10Al₂O₃-80WC-10Co (in wt%) composite exhibited high bending strength (1250 MPa), fracture toughness (9 MPam^1/2), and hardness (20.6 ± 0.5 GPa) simultaneously. The high bending strength was mainly attributed to fewer fracture origins due to the uniformly distributed and connected WC matrix together with a lower porosity. Increased fracture toughness was caused mainly by crack deflection and crack bridging in a uniformly connected WC matrix. High hardness resulted from finer WC metallic compounds and Co₃W₃C precipitation in almost all ranges.     

Mechanical Properties of Al2O3-NiAl Composites

The mechanical properties of the Al₂O₃-NiAl system are investigated in the present study. Specimens containing 0 to 100 vol% NiAl in Al₂O₃ were prepared by hot pressing. Both the strength and toughness of the Al₂O₃-NiAl composites are higher than the values predicted by the rule of mixtures. The grain growth of Al₂O₃ and NiAl in the composites is constrained by each component. The increase in strength is thus partly attributed to microstructural refinement. The toughness enhancement is contributed by a combination of crack deflection and crack bridging.     

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.     

Relationship Between Microstructures and Material Properties of Novel Fibrous Al2O3–(m-ZrO2)/t-ZrO2 Composites

Novel fibrous Al₂O₃–(m-ZrO₂)/t-ZrO₂ (m, monoclinic; t, tetragonal) composites having a core/shell structure were fabricated by multi-extrusion, and their microstructures and material properties were investigated depending on the number of extrusions. The composites acquired a homogeneously fine fibrous structure as the number of extrusions increased. The bending strength and fracture toughness increased remarkably as the number of extrusions increased. In the fracture surface of the second passed composite, an Al₂O₃–(m-ZrO₂) core region appeared, flat type, although some local regions existed with an intergranular fracture. However, the fracture mode of the t-ZrO₂ region was of intergranular type having a sharp and rough surface. In the composite made by the fifth passed extrusion, the fracture strength and toughness values were high at about 665 MPa and 9.6 MPa·m^1/2, respectively. The main fracture mode was a typical intergranular mode having a rough fracture surface, and the main multi-toughening was because of mechanisms such as crack bridging, microcracking, and phase transformation.     

Interdiffusion in SiC–AlN and AlN–Al2OC Systems

AlN, Al₂OC, and the 2 H form of SiC are isostructural. Both SiC–AlN and AlN–Al₂OC form homogeneous solid solutions above 2000° and 1950°C, respectively. The kinetics of phase separation in the two systems, however, are quite different. Interdiffusion in both SiC–AlN and AlN-Al₂OC systems was examined in the solid-solution regime in an attempt to elucidate differences in the kinetics of phase separation that occur in the two systems when annealed at lower temperatures. Diffusion couples of (SiC)_0.3(AlN)_0.7/(SiC)_0.7(AlN)_0.3 and (AlN)_0.7(Al₂OC)_0.3/(AlN)_0.3(Al₂OC)_0.7 were fabricated by hot pressing and were annealed at high temperatures by encapsulating them in sealed SiC crucibles to suppress loss due to evaporation. Interdiffusion coefficients in (SiC)_0.3-(AlN)_0.7/(SiC)_0.7(AlN)_0.3 diffusion couples were measured at 2373, 2473, and 2573 K, and the corresponding activation energy was determined to be 632 kJ/mol. (AlN)_0.7(Al₂OC)_0.3/ (AlN)_0.3(Al₂OC)_0.7 samples were annealed at 2273 K. The interdiffusion coefficient measured in the AlN–Al₂OC system was much larger than that in the SiC–AlN system.     

Fabrication of Elongated α-SiAlON via a Reaction-Bonding Process

A reaction-bonding process, which offers low sintering shrinkage and is a low-cost process, was applied to fabricate Y–α-SiAlON ceramics. The green compacts composed of Si, Y₂O₃, Al₂O₃, and AlN were nitrided and subsequently postsintered. Dense single-phase Y–α-SiAlON with elongated grain morphology could be achieved in the specimen postsintered at 1900°C. The material exhibited high hardness (1850 HV10) and high fracture toughness (5.1 MPa·m^1/2).