Influence of Surface Stress on indentation Cracking

A simple, two-dimensional fracture mechanics analysis was used to determine the influence of nonuniform residual surface stresses on the formation of radial indentation cracks. The indentation behavior depends on the depth of the compressive stresses, such that the apparent fracture toughness passes through a maximum with increasing indentation load. The analysis was used to estimate the surface stress from indentation data for a zirconia-toughened ceramic and was compared to previous X-ray diffraction measurements of this stress. The comparison gives only fair agreement; the sources of possible error are discussed. Such surface stresses also influence the accuracy of K _{I C} measurements when an indentation crack length technique is used; surface preparation is a critical factor in the measurement. Finally, the K _{I C} values obtained from indentation crack sizes were compared with those obtained by the double-cantilever-beam technique.     

Effects of Residual Stress and Loading Geometry on Single-Edge Precracked-Beam (SEPB) Fracture-Toughness Test Results

The single-edge precracked-beam (SEPB) specimen test, using the "bridge indentation" method, is one of the standard test methods in the new ASTM standard for the determination of the fracture toughness of ceramics. In support of this effort, the effect of several test parameters on the fracture-toughness ( K _{I C} ) values was established. The effect of the residual stress, because of the indent and the alignment in three- and four-point bending, on the K _{I C} values was determined using aluminum nitride specimens. The results show that indent loads should be kept below 10 kgf, and the crack-plane angle should be <5°, to prevent artificially high K _{I C} values.     

Error Propagation Biases in the Calculation of Indentation Fracture Toughness for Ceramics

Indentation fracture toughness models generally share the derived parameter Pc ^−3/2, where P is the indentation load and c the measured crack length. Biases, inherent to error propagation through this nonlinear transformation ( c to c ^3/2), can be introduced into calculated values for K _{I C} , depending upon the amount of averaging of crack length data performed prior to the transformation. This work utilizes Monte Carlo simulations to evaluate the bias in K _{I C} calculated using both mean and linear regression methods. Significant positive biases were demonstrated when using mean-based calculations where coefficients of variation (cv) in c exceeded 10%. Regression methods produced significantly less bias. With cv < 10% or when c is averaged per load, both methods produce essentially unbiased estimates for K _{I C} .     

Effect of Crystallinity on Strength and Fracture Toughness of Li2O-Al2O3-CaO-SiO2 Glass-Ceramics

The influence of crystal volume fraction on fracture toughness ( K _{I C} ) and indentation strength was analyzed for Li₂O-Al₂O₃-CaO-SiO₂ (LACS) and LACS glass-ceramics containing 0.58 mmol% AgNO₃ (LACS-0.58Ag) or 0.78 mmol% AgNO₃ (LACS-0.78Ag). The mean flexure strength, indentation strength, and K_{I C} values of the LACS-0.78Ag groups increased with volume fraction of crystallinity. To achieve the greatest strength and K_{I C} in LACS-Ag specimens, a high volume fraction of crystallinity (95%) had to be produced. However, the relationship between volume fraction of crystal phase and translucency had to be analyzed to determine the influence of crystallization on the potential esthetic results that are essential for dental applications. Addition of AgNO₃ to LACS glass produced a change from surface crystallization to bulk crystallization.     

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.     

A Novel Technique for Producing Fine Metal Fibers for Enhancing Mechanical Properties of Glass-Matrix Composites

Micrometer-sized refractory metal fibers of Nb were produced by an unconventional method of mechanically reducing arc-cast ingots of Nb-Cu in which the original microstructure contains ductile Nb precipitates. During mechanical reduction the precipitates were elongated into fibers, which were extracted by chemically dissolving the Cu matrix. Composite specimens of Nb fibers in a glass matrix were fabricated by dry-blending a mixture of fibers and powdered glass, then hot-pressing into disks. Fracture toughness, K _{I C} , was measured using microin-dentation and three-point bending methods. The fracture toughness increased with increasing volume fraction of fibers with the greatest improvement noted at 0.10≤ v_f ≤0.20. The values of K _{I C} increased as the interfiber spacing and the fiber diameter decreased. Both the effective fracture surface energy and the fracture strength were enhanced by increasing v_f . The small fibers embedded in the brittle matrix showed virtually no evidence of microcracking; Nb fibers and glass matrices also demonstrated good interfacial adhesion. Fractographs suggested that the fibers were very effective in resisting crack propagation. Experimental observations were explained using the concept of crack-front bowing and fiber bridging.     

Mode I Fracture Toughness of a Small-Grained Silicon Nitride: Orientation, Temperature, and Crack Length Effects

The Mode I fracture toughness ( K _{I C} ) of a small-grained Si₃N₄ was determined as a function of hot-pressing orientation, temperature, testing atmosphere, and crack length using the single-edge precracked beam method. The diameter of the Si₃N₄ grains was <0.4 µm, with aspect ratios of 2–8. K _{I C} at 25°C was 6.6 ± 0.2 and 5.9 ± 0.1 MPa·m^1/2 for the T–S and T–L orientations, respectively. This difference was attributed to the amount of elongated grains in the plane of crack growth. For both orientations, a continual decrease in K _IC was observed through 1200°C, to ∼4.1 MPa·m^1/2, before increasing rapidly to 7.5–8 MPa·m^1/2 at 1300°C. The decrease in K _IC through 1200°C was a result of grain-boundary glassy phase softening. At 1300°C, reorientation of elongated grains in the direction of the applied load was suggested to explain the large increase in K _IC. Crack healing was observed in specimens annealed in air. No R -curve behavior was observed for crack lengths as short as 300 µm at either 25° or 1000°C.     

Effects of an Electric Field on the Fracture Toughness of Poled Lead Zirconate Titanate Ceramics

Compact tension tests and indentation-fracture tests have been conducted to study the effects of an applied electric field on the fracture toughness ( K _{I C} ) of poled commercial lead zirconate titanate (PZT) ceramics. The experimental results show that an applied electric field, either parallel or antiparallel to the poling direction, considerably reduces the K _{I C} value of the PZT ceramics. The reduction in K _{I C} for a negative field is larger than that for a positive field of the same strength. The failure mode in the PZT ceramics is basically transgranular, insensitive to the applied electric field.     

Stoichiometry Effects on the Fracture of TiO2−x

Fracture characteristics of TiO_{2− x} were studied as a function of Stoichiometry. With increasing x , the fracture toughness K _{I e} and the fracture surface energy γ _f decrease and the amount of transgranular cleavage increases, corresponding to the increased concentration of planar defects within the grains. Increasing nonstoichiometry also shifted the ( K _I- V ) diagram to lower K _I values, commensurate with the K_{I e} decrease. Water accelerated stress corrosion by promoting intergranular failure during subcritical crack growth.     

Fracture Toughness of Dental Porcelains Evaluated by IF, SCF, and SEPB Methods

Fracture toughness of six dental porcelains with leucite content ranging from 0 to 22 vol% was evaluated by indentation fracture (IF), surface crack in flexure (SCF), and single edge pre-cracked beam (SEPB) methods. The results of the IF method were similar to those of the SCF method for all the porcelains investigated. The results of the SEPB were similar to those of the other two methods only for the glassy porcelains, but for leucite-based porcelains this method resulted in higher values of K _{I c} . Based on microstructure, fractographic analysis, and an additional single edge V-notched beam test, it was concluded that the pre-crack size influences the value of K _{I c} for porcelains reinforced by leucite. For design and failure analysis purposes, the K _{I c} determined by SCF method should be preferred, since fracture of dental restorations usually starts from small surface cracks.     

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.     

Advances in the Grinding Efficiency of Sintered Alumina Abrasives

The study relates the grinding power of different grades of sintered alumina abrasives to their microstructures and to basic mechanical properties in comparison with conventionally fused electrocorundum and with an electrofused alumina/ zirconia eutectic. Contrary to the traditional approach of the Battelle test, the fracture toughness K _{I c} of individual grains is measured by a quantitative indentation analysis. Compared with fused corundum, sintered alumina grits exhibit an increased toughness and grinding efficiency, but the further increase of K _{I c} in the eutectic does not improve the grinding performance. The key parameter for grinding is the inherent hardness of the abrasive. The elimination of flaws by a new approach results in a strong increase in the grinding power of sintered alumina abrasives.     

Hot Isostatic Pressing of Cubic Boron Nitride–Tungsten Carbide/Cobalt (cBN–WC/Co) Composites: Effect of cBN Particle Size and Some Processing Parameters on their Microstructure and Properties

Cubic boron nitride (cBN)-cemented carbide composites have gained attraction over the last few years because of their potential uses as wear parts. The densification behavior of cBN—hard metal composites by glass encapsulation hot isostatic pressing—has been investigated. Composites with different cBN grades (from 0/0.5 to 6/12 μm particle sizes) and cBN content (up to 50 vol%) were selected for this study. Near-full densification was obtained at temperatures between 1100° and 1200°C, and pressures between 150 and 200 MPa, respectively, while no phase transformation of cBN into the low-hardness hexagonal form has been detected by X-ray diffraction. The addition of cBN to the hard metal base material led to an increase of hardness, a significant increase of fracture toughness ( K _{I C} measured by Vickers indentation), and a moderate decrease of mechanical strength (determined by three-point bending).     

Toughening of SiC with Ti3SiC2 Particles

Composites in the SiC–TiC–Ti₃SiC₂ system were synthesized using reactive hot pressing at 1600°C. The results indicate that addition of Ti₃SiC₂ to SiC leads to improved fracture toughness. In addition, high microhardness can be retained if TiC is added to the material. The best combination of properties obtained in this study is K _{I c} =8.3 MPa·m^1/2 and H _v=17.6 GPa. The composition can be tailored in situ using the decomposition of Ti₃SiC₂. Ti₃SiC₂ decomposed rapidly at temperatures above 1800°C, but the decomposition could be conducted in a controlled manner at 1750°C. This can be used for synthesis of fully dense composites with improved properties by first consolidating to full density a softer Ti₃SiC₂-rich initial composition, and then using controlled decomposition of Ti₃SiC₂ to achieve the desired combination of microhardness and fracture toughness.     

Method to Estimate Crack-Tip Toughness from Bending Tests on Prenotched Bars

A strain-gauge procedure that enables determination of the crack-tip toughness ( K _I0) from bending-strength tests is described. The procedure is applied to coarse-grained alumina and yields an average K _I0 value of 2.51 MPa·m^1/2, with a standard deviation of 0.16 MPa·m^1/2.     

Indentation Fracture Response and Damage Resistance of Al2O3-ZrO2 Composites Strengthened by Transformation-Induced Residual Stresses

Indentation fracture behavior of three-layer Al₂O₃-ZrO₂ composites with substantial compressive residual stresses was compared with the behaviors of monolithic Al₂O₃ and Al₂O₃-ZrO₂ ceramics without intentionally introduced residual stresses. The indentation cracks were smaller in the three-layer specimens relative to the monolithic specimens in agreement with the predictions of indentation fracture mechanics theory. Indentation and strength testing were used to show that a residual compressive stress of approximately 500 MPa exists in the outer layers of the three-layer composites. The three-layer specimens showed excellent damage resistance in that the strength differential between the three-layer and monolithic indented specimens was maintained at indentation loads up to 1000 N, the maximum indentation load used in the experiments.     

A New Low-Brittleness Glass in the Soda-Lime-Silica Glass Family

In the soda-lime-silica glass family, the effect of each constituent of the composition on the brittleness was first investigated. Vickers indentation was employed to estimate the brittleness (ratio of harness ( H ) to fracture toughness ( K _c)) by measuring the C / a ratios (where C and a are the characteristic crack and indentation diagonal lengths, respectively). It was observed that a higher silica content and a lower lime content helped to lower the brittleness. Substitution of potash and magnesia for soda and calcia, respectively, was effective in lowering the brittleness. From these results, a higher molar volume was found to be a key factor for reducing the brittleness. A new low-brittleness glass was then developed with a brittleness as low as 5.1 µm^-1/2 as compared with the brittleness of 7.1 µm^-1/2 for commercial soda-lime-silica glass. The crack initiation load ( P *), measured by the Vickers indentation method, for this new low-brittleness glass was almost 10 times as high as P * of commercial soda-lime-silica glass. The new glass shows lower hardness and higher fracture toughness than the commercial soda-lime-silica glass.     

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.     

The Study on Carbon Nanofiber (CNF)-Dispersed B4C Composites

Carbon nanofiber (CNF)-dispersed B₄C composites have been synthesized and consolidated directly from mixtures of elemental raw powders by pulsed electric current pressure sintering (1800°C/10 min/30 MPa). A 15 vol% CNF/B₄C composite with ∼99% of dense homogeneous microstructures (∼0.40 μm grains) revealed excellent mechanical properties at room temperature and high temperatures: a high bending strength (σ_b) of ∼710 MPa, a Vickers hardness ( H _v) of ∼36 GPa, a fracture toughness ( K _{I C} ) of ∼7.9 MPa m^1/2, and high-temperature σ_b of 590 MPa at 1600°C in N₂. Interfaces between the CNF and the B₄C matrix were investigated using high-resolution transmission electron microscopy, EDS, and electron energy-loss spectroscopy.     

Microstructure and Mechanical Properties of Silicon Nitride Ceramics with Controlled Porosity

Porous silicon nitride ceramic with a porosity from 0–0.3 was fabricated by partial hot-pressing of a powder mixture of α-Si₃N₄ and 5 wt% Yb₂O₃ as sintering additive. Irrespective of the porosity, the samples exhibited almost the same microstructural features including grain size, grain aspect ratio, and pore size. Porosity dependences of Young's modulus, flexural strength, and fracture toughness ( K _{I C} ) were investigated. All these properties decreased with increasing porosity. However, because of the fibrous microstructure, the decreases of flexural strength and fracture toughness were moderate compared with the much greater decrease of Young's modulus. Thus, the strain tolerance (fracture strength/Young's modulus) increased with increasing porosity. The critical energy release rate also increased slightly with an increasing volume fraction of porosity to 0.166 and remained at the same level with that of the dense sample when the porosity was 0.233. They decreased as porosity increased further.