Toughening Behavior of a Two-Dimensional SiC/SiC Woven Composite at Ambient Temperature: II, Stress-Displacement Relationship in the Crack Process Zone

In this paper insight into the origin of the J_R -curve of a SiC/SiC woven composite was obtained by experimental characterization of the closure stress-crack opening displacement, sigma( u ), relationship in the process zone of the crack. This process zone included both a crack frontal zone and a crack wake damage zone so that quantitative estimates could be obtained of the magnitudes of toughening associated with these two separate zones. The research indicated that the closure stress-crack opening displacement curve has a positive slope in the crack frontal zone and a negative slope in the wake zone with a maximum stress capability on the order of 350 MPa. The toughness contributions from the crack wake and from the crack front were consistent with the J_R -curve results obtained in the previous paper. The stresses supported locally in the crack frontal zone were almost twice as large as those supported by tensile specimens even though this zone was considerably damaged by matrix cracks. This appears to be the result of stabilization of matrix cracks by arrest at fiber bundles. Application of a previously derived theoretical function, sigma_b( u ), solely based on crack bridging by continuous unidirectional fibers, suggested that the efficacy of bridging in the woven composite may in part be related to the woven fiber architecture. Such an architecture apparently induces greater sliding resistance of the SiC bundles against the surrounding SiC matrix.     

R-Curve Behavior in a Silicon Carbide Whisker/Alumina Matrix Composite

R -curve measurements were performed on a SiC whisker/Al₂O₃ matrix composite. A controlled flaw/strength technique was utilized to determine fracture resistance as a function of crack extension. Rising R -curve behavior with increasing crack extension was observed, confirming the operation of wake toughening effects on the crack growth resistance. Observations of crack/microstructure interactions revealed that bridging by intact whiskers in the crack wake was the mechanism responsible for the rising R -curve behavior.     

Processing and Characterization of SiC-Whisker-Reinforced Alumina-Matrix Composites

Brittle monolithic alumina can be reinforced with highstrength single-crystal SiC whiskers with the effect of increasing fracture toughness. In this study, well-mixed and nearly fully dense SiC whisker/alumina composites were fabricated by wet-blending the constituents and uniaxially hot-pressing the resulting powder. The alumina-matrix grain size depended on whisker volume fraction, whisker surface-oxygen content, and hot-pressing environment. Fracture toughness, measured by an indentation-fracture method, increased from 3.0 MPa·m^1/2 for the hot-pressed unreinforced alumina to 10.7 MPa·m^1/2 for the composite containing 25 vol% SiC whiskers. Fracture surfaces revealed evidence of toughening by the mechanisms of crack deflection, pullout, and crack bridging by the whiskers. The observed increase in fracture toughness of alumina due to the addition of SiC whiskers was correlated with existing models of toughening mechanisms.     

Influence of Cu2O and CuAlO2 Interphases on Crack Propagation at Cu/α-Al2O3 Interfaces

In-situ crack-propagation experiments, in conjunction with thermochemical experiments, have been used to examine the role of discontinuous interphases on the fracture behavior of solid-state diffusion-bonded Cu/α-Al₂O₃ couples. Clean, interphase-free interfaces exhibit crack extension by brittle decohesion at the crack tip at an initiation fracture energy of 125 J/m². Crack propagation is characterized by an increase in the fracture energy with increases in the crack length ( R -curve behavior). When interfacial chemical reaction products are present, the crack growth is altered, depending on the characteristics of the interphase. The presence of Cu₂O needles results in preferential debonding along the Cu₂O/Al₂O₃ interface. On the other hand, finer CuAlo₂ needles visibly impede crack propagation and result in a higher interface initiation fracture energy (}190 J/m²) than that of the interphase-free interface. The effects of the Cu₂O and CuAlo₂ phases on the fracture energy are discussed.     

Fabrication and Fracture Behavior of Novel SiC Ceramics Having Rodlike Grains

A new type of silicon carbide having rodlike grains has been fabricated by hot-pressing or hot-pressing followed by hot isostatic pressing using SiC whiskers as the raw powder. Preliminary relationships between sintering conditions and microstructure and fracture toughness of the processed ceramics were established. Ceramics with relative density greater than 98% were prepared at temperatures of 2050°C or greater. The fracture toughness of these ceramics was better than that of ordinary SiC, and its maximum value was 7.3 MPa · m^1/2. Grain pullout, grain bridging, and crack deflection were considered to be the main operative mechanisms which led to improved fractrue toughness.     

Estimation of Crack Closure Stresses for In Situ Toughened Silicon Nitride with 8 wt% Scandia

An 8-wt%-scandia silicon nitride with an elongated grain structure was fabricated. The material exhibited high fracture toughness (∼ 7 MPa · m^1/2) and a rising R -curve as measured by the indentation strength technique. The "toughening" exponent m was found to be m ∼ 0.1. The high fracture toughness and R -curve behavior was attributed mainly to bridging of the crack faces by the elongated grains. The crack closure (bridging) stress distribution in the wake region of the crack tip was estimated as afunction of crack size from the R -curve data, with an arbitrarily assumed distribution function.     

Silicon Carbide Platelet/Alumina Composites: III, Toughening Mechanisms

This is the last part of a series of papers on the processing and fracture behavior of SiC-platelet/Al₂O₃ composites. The objective of this paper was to identify the mechanisms involved in the toughening process. A hot-pressed composite with a SiC volume fraction of 0.3 was chosen as the model system for study. Based on microstructural observations, crack deflection and grain bridging were both identified as possible toughening mechanisms and were further investigated. A Modified two-dimensional crack deflection model is presented to account for the anisotropic microstructure in hot-pressed platelet composites, in which preferred platelet orientation was present. Relative toughness values were predicted for two crack propagation directions assuming crack deflection is the toughening mechanism. Fracture toughness measurements for specific crack directions were made using a bridge indentation technique. The correlation of experimental results with theoretical predictions is discussed. To distinguish the effect of grain bridging from crack deflection, an in situ observation of crack growth was conducted. The results showed no distinct rising T -curve behavior for cracks in the size range 80 to 500 μ m. Measurement of fracture surface roughness was also made and the implications are discussed. The results indicate that crack deflection is the dominant toughening mechanism in the SiC-platelet/ Al₂O₃ composites studied herein.     

Elevated-Temperature R-Curve Behavior of a Polycrystalline Alumina

The fracture behavior of a polycrystalline alumina was examined at temperatures ranging from ambient through 1400°C, using three-point bend bar test specimens. R -curves were determined at all temperatures studied, and when accompanied by renotching procedures, a wake removal technique, conclusive evidence was provided to support the existence of a following wake region in this monolithic ceramic material. The crack closure stresses identified in this region are responsible for all toughening with crack extension observed in this study. Room-temperature " K _IC" fracture toughness values of 4.5 MPa · m^1/2 for the chevron-notch specimen and 3.9 MPa · m^1/2 for the straight-notch configuration were obtained. The critical stress intensity factor of the renotched chevron-notch specimen compared very closely with that of the straight-notch specimen. These findings further confirm the toughening role of the microstructural features found in the following wake region. This paper considers, in detail, these observations in terms of the microstructure and its role in the toughening mechanism.     

R-Curve for a Lead Zirconate Titanate Ceramic Obtained from Tensile Strength Tests with Knoop-Damaged Specimens

A procedure was used that made it possible to determine the R -curve for piezoelectric ceramics from tensile strength tests conducted with Knoop-damaged specimens. The resulting crack-tip toughness K _I0 was 0.6 MPa·m^1/2, and the R -curve starting from this value increased to 1.4 MPa·m^1/2 within a 0.7 mm crack extension.     

Effects of α-Sic versus β-Sic Starting Powders on Microstructure and Fracture Toughness of Sic Sintered with Al2O3-Y2O3 Additives

Dense Sic ceramics were obtained by pressureless sintering of β-Sic and α-Sic powders as starting materials using Al₂O₃-Y₂O₃ additives. The resulting microstructure depended highly on the polytypes of the starting SiC powders. The microstructure of SiC obtained from α-SiC powder was composed of equiaxed grains, whereas SiC obtained from α-SiC powder was composed of a platelike grain structure resulting from the grain growth associated with the β→α phase transformation of SiC during sintering. The fracture toughness for the sintered SiC using α-SiC powder increased slightly from 4.4 to 5.7 MPa.m^1/2 with holding time, that is, increased grain size. In the case of the sintered SiC using β-SiC powder, fracture toughness increased significantly from 4.5 to 8.3 MPa.m^1/2 with holding time. This improved fracture toughness was attributed to crack bridging and crack deflection by the platelike grains.     

Measurement of Rising R-Curve Behavior in Toughened Silicon Nitride by Stable Crack Propagation in Bending

A simple procedure for measuring the R -curve properties of ceramics by a stable fracture test in three-point bending is described. As a typical case, data are displayed for a Si₃N₄ material toughened by the presence of acicular grains in situ grown during the sintering process. The fracture mechanics specimen was a single-edge double-notched beam (SEDNB), whose notch was sharpened to a radius of <10 μm in order to reduce the amount of elastic energy stored at its root prior to crack extension. Furthermore, a stabilizer, specially designed for the bending geometry, was used to control crack stability. During stable extension, the crack could be easily arrested at selected locations of the load-displacement curve, the load quickly released, and the stable crack extension directly measured by the die-penetration technique. The crack resistance, K _R, of the material was calculated from the measured crack extent and the onset load value before unloading. This method enabled us to precisely monitor the critical load value at which the load-displacement curve deviated from linear behavior, as well as crack extensions from a few tens of micrometers to about 1 mm. As an application of this method, the fracture resistance of a Si₃N₄ material with rising R -curve behavior was measured and found to increase from about 5.5 to 9.0 MPaμm^1/2 within a 0.8-mm extension.     

Fracture Resistance of SiC-Fiber-Reinforced Si3N4 Composites at Ambient and Elevated Temperatures

The crack growth behavior in unidirectional SiC-fiber-rein-forced Si₃N₄-matrix composites fabricated in our laboratories was investigated as a function of fiber volume fraction and temperature. Both the stress-intensity factor and an energy approach were adopted in the characterization of the crack growth behavior. Crack resistance increased with crack extension ( R -curve or T -curve) as a result of bridging effects associated with the intact fibers. Large-scale bridging was observed, and was considered in the determination of the R -curves. Temperature and fiber volume fraction affected the crack propagation behavior. At room temperature a single crack was initiated at the notch tip; it then branched and delaminated upon further loading. In contrast, at 1200°C, little crack branching was observed. Increasing fiber volume fraction increased the degree of crack branching. Temperature and fiber volume fraction also affected the R -curve behavior. Raising the temperature to 1200°C did not significantly degrade the room-temperature R -curve effect. Increasing the fiber volume fraction from 14% to 29% substantially enhanced the toughening effect and the R -curve behavior.     

Strength and Toughness of Slip-Cast Fused-Silica Composites

The effects of fiber composition, size, and surface treatment on the mechanical behavior of slip-cast fused-silica composites were investigated. The ambient and 1000°C stiffness and strength, fracture toughness, G _R -curve behavior, and fiber-matrix interface bond strength were determined. Quantitative fractography and scanning electron microscopy were used to ascertain the mechanisms of toughening and strengthening. Composites with weak interface bonding exhibited good strength retention and rising G _R -curve behavior. The fracture resistance was improved primarily through crack deflection.     

Development of SiC-Whisker-Reinforced Lithium Aluminosilicate Matrix Composites by a Mixed Colloidal Processing Route

Based on the surface properties of aqueous silica, boehmite, and SiC-whisker dispersions, SiC-whisker-reinforced lithium aluminosilicate (LAS) matrix composites were fabricated by a mixed colloidal processing route. The composites were characterized by a uniform spatial distribution of SiC whiskers throughout the matrix. The fracture toughness increased from 1.3 MPa.m^1/2 for the LAS specimen to 5.0 MPa.m^1/2 for the hot-pressed composite (950°C and 20 MPa for 20 min) containing 20 wt% SiC whisker. The increase in fracture toughness appears to result mainly from crack deflection and crack bridging by whiskers with some additional toughenings from load transfer and whisker pullout.     

Si3N4/SiC-Whisker Composites without Sintering Aids: II, Fracture Behavior

The fracture behavior of an Si₃N₄/SiC-whisker composite fabricated without sintering aids is investigated using a double approach based on the examination of R -curve behavior and a statistical analysis of crack propagation. In the composite with 20 vol% whisker, a 30% increase in toughness over the matrix value can be attributed to crack-tip phenomena. Strong interfacial bonding prevents any contribution to toughening by mechanisms operating in the wake region of the crack. Based on experimental observations of microfracture in both SiC whiskers and Si₃N₄ grains, toughening caused by crack-tip phenomena is quantitatively discussed in terms of fracture energy and whisker-distribution parameters.     

Fracture Resistance and Stable Crack-Growth Behavior of 8-mol%-Yttria-Stabilized Zirconia

The crack-growth behavior of a yttria-stabilized zirconia ceramic (8 mol% of cubic-phase yttria) was studied at room temperature. Double-cantilever-beam specimens were loaded with pure bending moments in a specially designed loading fixture inside an environmental scanning electron microscope. Crack-growth data were obtained from truly sharp (arrested) cracks, bypassing interpretation problems that involve crack initiation from a machined notch. The crack-growth study was conducted over a range of applied energy-release rates that allowed crack arrest on one hand and fast fracture on the other. Three energy-release-rate values were relevant: initiation of crack growth (3.5 J/m²), crack arrest (2.8 J/m²), and fast fracture (8.0 J/m²). At the macroscopic scale, subcritical crack growth occurred as a continuous process. In situ observations revealed that, at the microscopic scale, crack growth occurred in small jumps. The fracture mode for stable crack growth was identified to be transgranular.     

R-Curve Behavior of Si3N4–BN Composites

R -curve behavior of Si₃N₄–BN composites and monolithic Si₃N₄ for comparison was investigated. Si₃N₄–BN composites showed a slowly rising R -curve behavior in contrast with a steep R -curve of monolithic Si₃N₄. BN platelets in the composites seem to decrease the crack bridging effects of rod-shaped Si₃N₄ grains for small cracks, but enhanced the toughness for long cracks as they increased the crack bridging scale. Therefore, fracture toughness of the composites was relatively low for the small cracks, but it increased significantly to ∼8 MPa·m^1/2 when the crack grew longer than 700 μm, becoming even higher than that of the monolithic Si₃N₄.     

Fracture and Fatigue Behavior at Ambient and Elevated Temperatures of Alumina Bonded with Copper/Niobium/Copper Interlayers

Interfacial fracture toughness and cyclic fatigue-crack growth properties of joints made from 99.5% pure alumina partially transient liquid-phase bonded using copper/niobium/copper interlayers have been investigated at both room and elevated temperatures, and assessed in terms of interfacial chemistry and microstructure. The mean interfacial fracture toughness, G _c, was found to decrease from 39 to 21 J/m² as temperature was raised from 25° to 1000°C, with failure primarily at the alumina/niobium interfaces. At room temperature, cyclic fatigue-crack propagation occurred both at the niobium/alumina interface and in the alumina adjacent to the interface, with the fatigue threshold, Δ G _TH, ranging from 20 to 30 J/m²; the higher threshold values in that range resulted from a predominantly near-interfacial (alumina) crack path. During both fracture and fatigue failure, residual copper at the interface deformed and remained adhered to both sides of the fracture surface, acting as a ductile second phase, while separation of the niobium/alumina interface appeared relatively brittle in both cases. The observed fracture and fatigue behavior is considered in terms of the respective roles of the presence of ductile copper regions at the interface which provide toughening, extrinsic toughening due to grain bridging during crack propagation in the alumina, and the relative crack propagation resistance of each crack path, including the effects of segregation at the interfaces found by Auger spectroscopy.     

Toughening of Silicon Nitride Matrix Composites by the Addition of Both Silicon Carbide Whiskers and Silicon Carbide Particles

Si₃N₄ matrix composites reinforced by SiC whiskers, SiC particles, or both were fabricated using the hot-pressing technique. The mechanical properties of the composites containing various amounts of these SiC reinforcing materials and different sizes of SiC particles were investigated. Fracture toughness of the composites was significantly improved by introducing SiC whiskers and particles together, compared with that obtained by adding SiC whiskers or SiC particles alone. On increasing the size of the added SiC particles, the fracture toughness of the composites reinforced by both whiskers and particles was increased. Their fracture toughness also showed a strong dependence on the amount of SiC particles (average size 40 μm) and was a maximum at the particle content of 10 vol%. The maximum fracture toughness of these composites was 10.5 MPa·m^1/2 and the flexural strength was 550 MPa after addition of 20 vol% of SiC whiskers and 10 vol% of SiC particles having an average particle size of 40 μm. These mechanical properties were almost constant from room temperature to temperatures around 1000°C. Fracture surface observations revealed that the reinforcing mechanisms acting in these composites were crack deflection and crack branching by SiC particles and pullout of SiC whiskers.