Fracture Toughness of TiB2 and B4C Using the Single-Edge Precracked Beam, Indentation Strength, Chevron Notched Beam, and Indentation Strength Methods

The mode I fracture toughness ( K _Ic) of boron carbide (B₄C) and titanium diboride (TiB₂) was determined using four competing techniques. The indentation strength (IS), chevron notched beam (CNB), and indentation fracture (IF) methods are common techniques that were compared to the recently standardized single-edge precrack beam (SEPB) method. The SEPB method was more difficult to apply, but it represents the most rigorous method for K _Ic determination, because it uses few assumptions and requires a direct measurement of crack length. The IS method was an expeditious and economical alternative when low indentation loads were used. CNB K _Ic values were virtually rate-independent when displacement rates less than or equal to 0.5 mm/min were used. The IF method was the least satisfactory technique, because of high variability in K _c values and because of the low differentiation between the two materials studied.     

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.     

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.     

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.     

Phase Changes in a Leucite-Containing Dental Porcelain Frit

The high average thermal expansion required for thermal compatibility of dental porcelains with their substrate alloys is supplied by the mineral leucite (K₂O Al₂O₃ 4SiO₂). Since leucite is not an equilibrium phase at the composition of component No. 1 of the Weinstein et al. patent, phase changes in this leucitecontaining frit could be responsible for the observed thermally induced changes in dental porcelain expansion behavior. This study sought to explore such phase changes.     

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} .     

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.     

VAMAS Round Robin on Fracture Toughness of Silicon Nitride

Eight laboratories in Germany, Japan, U.K., and U.S. participated in the VAMAS round robin. The fracture toughness of silicon nitride at room temperature and at 1200^deg;C was measured by three methods: the single-edge V-notched beam (SEVNB), single-edge precracked beam (SEPB), and chevron notched beam (CNB). The obtained values show hardly any crosshead speed dependence, irrespective of test temperature and atmosphere. Results may have been influenced by a small amount of slow crack growth, but distinct R -curve behavior could not be detected within the scope of the tests. The values at 1200^deg;C in N₂ can be measured by the SEVNB and SEPB methods with small scatters. The oxidation of silicon nitride, caused by heating in air, increases the SEVNB and SEPB values. The CNB values are free from the effects of test temperature and atmosphere, but they show a large scatter between laboratories. However, the chevron V-notched beam (CVNB) method, which is an improved CNB method, shows values with a small scatter, irrespective of the measurement conditions. The SEVNB and SEPB measurements in N₂ and the CVNB measurement under any conditions are recommended for the measurement of high-temperature fracture toughness.     

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.     

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.     

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.     

Short-Crack Fracture Toughness of Silicon Carbide

The fracture toughness of four different silicon carbides was measured using single-edge precracked beam (SEPB) and indentation/strength techniques. Two were development grades with similar microstructures and chemistries, and yet exhibited different fracture modes. The grade that exhibited a predominantly intergranular fracture had an SEPB fracture toughness (6.4 MPa√m) 88% higher than the one that showed primarily a transgranular fracture (3.4 MPa√m). The higher fracture toughness was associated with a modest increase in average strength (25%), although there was a significant increase in the Weibull modulus (11–32). Fracture toughness at short crack lengths was assessed by an indentation method that used fracture strengths, crack lengths at fracture, and a new method of estimating the constant δ that characterizes the residual driving force of the plastic zones based on the stable growth of the indentation cracks from the initial ( c ₀) to the instability ( c ^*) lengths. The results showed a rising crack-growth-resistance behavior for the grade exhibiting intergranular fracture, while the grade showing transgranular fracture had a flat crack-growth resistance. Tests on two commercial grades of silicon carbide showed similar behaviors associated with the respective fracture modes.     

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.     

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.     

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.     

Mechanical Behavior of a Borosilicate Glass Under Aqueous Corrosion

In France, fission products are being vitrified for a possible final geological disposal. Under disposal conditions, corrosion of the glass by groundwater as well as stress corrosion because of stresses occurring at surface flaws cannot be excluded. Within this framework, the mechanical behavior of the French simulated nuclear waste glass SON68 was studied by Vickers indentation and fracture experiments in air and in a corrosive solution. The glass was corroded at 90°C in a solution enriched with Si, B, and Na. The results showed that the glass corrosion enhances the cracks propagation relative to experiments in air. The indentation fracture toughness ( K _{I C} ) obtained using a four-point bending test showed that the K _{I C} of the glass decreased with increasing corrosion time.     

Mechanical Properties of Dental Porcelain with Different Leucite Particle Sizes

This research aims to investigate the effect of leucite particle size on the mechanical properties of dental porcelain with a similar leucite content and chemical composition. Leucite powders of different particle sizes were synthesized by a hydrothermal method and a high-temperature fusing-crystallization method, respectively. Dental porcelains with different average leucite particle sizes (i.e., 0.5±0.2, 1.2±0.3, and 5±2 μm) were prepared by sintering the mixture of different leucite powders and a low temperature frit. The crystalline phase, crystalline content, relative density, hardness, flexural strength, and fracture toughness of the porcelains were measured by X-ray diffraction (XRD), quantitative XRD analysis, the Archimedes method, a Vickers microhardness tester, a universal testing machine, and a single-edge precracked beam method, respectively. The microcrack density and the distribution of leucite particles were also quantitatively assessed from micrographs. The results showed that the leucite particle size did not have a significant effect on the average of the measured flexural strength, fracture toughness, and hardness of dental porcelains. However, because of a existence of the large number of microcracks, the relative density and the Weibull modulus of the sample groups with an average leucite particle size of 5 μm were statistically lower.     

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.     

High-Toughness Silicon Carbide as Armor

Silicon carbide, with single-edge precracked beam (SEPB) toughness greater than 7 MPa·m^1/2, was made by hot-pressing using Al–B–C (ABC) or Al–Y₂O₃ (YAG) as additives. The hardness of SiC processed with a liquid phase was always less than SiC densified without a liquid phase despite having a similar or finer grain size. With increasing Al content, the ABC system changed from trans- to intergranular fracture with a drop in hardness and a two- to threefold increase in SEPB toughness. Strength and Weibull modulus for materials processed with a liquid phase were higher than those of solid-state densified SiC. Ballistic testing, however, did not show any improvement over SiC densified with B and C additives. Depth of penetration was controlled by hardness of the SiC-based materials, while V ₅₀ values for 14.5 mm WC–Co cored projectiles were in the range of 720–750 m/s for all materials tested.     

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.