Work of Fracture and Crack Healing in Glass

The work of fracture, γwoF, of a soda-lime-silica glass was determined as 5.5 J/m² in an inert atmosphere and 4.4 J/m² in air by using a short bar specimen with a chevron notch. The critical stress intensity factor K_IC calculated from γ_wof obtained in an inert atmosphere agreed with the value, 0.88 MN/m^3/2, determined by the indentation method. In unloading-loading cycles during stable crack growth, crack healing was observed both on the diagram of load vs load-point displacement and visually. Crack healing is more prevalent in an inert atmosphere, occurring only minimally in air. The energy for healing was measured as 0.65 J/m² in an inert atmosphere and ∼0.21 J/m² in air.     

Fracture Toughness Anisotropy and Toughening Mechanisms of a Hot-pressed Alumina Reinforced with Silicon Carbide Whiskers

The fracture toughness of a hot-pressed alumina and that of a hot-pressed alumina/SiC-whisker composite containing 33 vol% SiC whiskers were measured by four-point bending on single-edge precracked bend bars having sharp precracks created by "bridge indentation." Two batches of the composite were examined, one exhibiting a greater degree of whisker clustering than the other. The fracture toughness of the alumina was around 4 MN·m^-3/2 whereas that of the composite varied between 5 and 8 MN·m^-3/2 depending on microstructural uniformity and crack-propagation direction. Crack deflection in combination with a change in fracture from intergranular to transgranular fracture is proposed as an explanation of the superior fracture toughness of SiC-whisker-reinforced alumina as compared to unreinforced alumina. The composite exhibited a variation in fracture toughness with the crack-propagation direction in identical crack planes. This effect could with good accuracy be described in terms of crack deflection for the composite with uniform whisker distribution. However, in the material with whisker clustering the variation of the fracture toughness with crack-growth direction was greater and could not entirely be explained by crack deflection.     

Closure and Repropagation of Healed Cracks in Silicate Glass

Cracks in soda-lime-silica and vitreous silica glass close against a finite load at humidities between 0.01 % and 100%. The force associated with crack closure (0.15 J/m²) can be predicted by a model which involves hydrogen-bonded linkages of surface-adsorbed water molecules. The fracture energy to reopen healed cracks in vitreous silica is also in the range of hydrogen-bonding interactions. In the driest environments used, healed cracks in soda-lime-silica glass required 1.7 ± 0.2 J/m² to reopen. This bonding energy can be attributed to the formation of either cationic bridges or siloxane bonds between fracture surfaces. Since crack healing was observed to be independent of the number of cycles, a cationic bridging model is favored to explain healing effects at room temperature.     

Fracture Modes in MoSi2

The room-temperature fracture behavior of polycrystalline MoSi₂ was characterized using Vickers indentation fracture. Fracture analysis was aided by the optically active grain structure of MoSi₂ revealed under polarized light. Radial crack propagation from indentations was found to be predominantly transgranular. The approximate indentation fracture toughness of MoSi₂ was 3 MPa.m^1/2, while the measured hardness was 8.7 GPa. Fracture behavior is believed to be controlled by anisotropy and cleavage energy of the tetragonal MoSi₂ crystal structure.     

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.     

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.     

Crack-Tip Toughness Measurements on a Sintered Reaction-Bonded Si3N4

An evaluation of measurements of crack opening displacements (CODs) on a commercial sintered reaction-bonded silicon nitride (SRBSN) was performed. To determine the intrinsic fracture toughness of this material, a new evaluation method is presented, which takes into account not only the near tip CODs, but the CODs of the complete crack profile. The method is applied on through-thickness cracks in bend bars and contrasted to CODs of Vickers radial cracks. A crack-tip toughness of ∼1.7 MPa·m^1/2 is obtained.     

Subcritical Growth of Indentation Median Cracks in Soda-Lime-Silica Glass

The influence of load and dwell time on the subcritical growth of indentation cracks was investigated. The behavior of soda-lime-silica glass in both active and inert environments was considered. Vickers indentations were obtained.using loads ranging from 1.96 to 39.2 N and the dwell time.at maximum load was varied between 5 s and 30 min. Indented samples were broken and the fracture surfaces.observed using optical microscopy. Anomalous crack shapes were obtained in air and water for long dwell times, particularly at higher indentation loads. The cracks were markedly circular in shape and substantially deeper than conventional median/radial indentation cracks, evidencing a time-dependent growth. Comparison of the measured crack depth with a theoretical analysis demonstrated that the appearance of the "circular" cracks was related to the subcritical growth of median cracks.     

Determining the Toughness of Ceramics from Vickers Indentations Using the Crack-Opening Displacements: An Experimental Study

Recently, a method for evaluating the fracture toughness of ceramics has been proposed by Fett based on the computed crack-opening displacements of cracks emanating from Vickers hardness indentations. To verify this method, experiments have been conducted to determine the toughness of a commercial silicon carbide ceramic, Hexoloy SA, by measuring the crack-opening profiles of such Vickers indentation cracks. Although the obtained toughness value of K _o= 2.3 MPa·m^1/2 is within 10% of that measured using conventional fracture toughness testing, the computed crack-opening profiles corresponding to this toughness display poor agreement with those measured experimentally, raising concerns about the suitability of this method for determining the toughness of ceramics. The effects of subsurface cracking and cracking during loading are considered as possible causes of such discrepancies, with the former based on direct observations of lateral subsurface cracks below the indents.     

Mechanical Properties and Failure Analysis of Alumina-Glass Dental Composites

Strength measurements and fractography were used to investigate the failure of alumina-glass dental composites containing 75 vol% alumina and 25 vol% glass. Alumina compacts were prepared by slip casting and sintering at 1100°C for 2 h. Dense composites were made by infiltrating partially sintered alumina with glass at 1150°C for 8 h. Young's modulus and the hardness of the composites were 270 GPa and 12 GPa, respectively. The mean strength (460 MPa) and fracture toughness (4.0 MPa·m^1/2) of the composites were insensitive to the glass thermal expansion coefficient (α_glass= 5.9 × 10⁻⁶ to 7.8 × 10⁻⁶°C⁻¹). Typical flaws were pores and cracklike voids formed by poor particle packing and differential sintering near agglomerates of alumina in the composite. Crack deflection and crack bridging were observed in indentation cracks. Fracture toughness was single-valued because the alumina particle size was small (∼3 μm). Alumina-glass composites are promising new ceramics for dental crown and bridge applications, because their strength and fracture toughness are ∼2 times greater than those of current dental ceramics.     

Measurement of the Fracture Toughness of Ceramic Materials Using a Miniaturized Disk-Bend Test

The fracture toughness of several ceramic materials has been measured using a miniaturized disk-bend test apparatus and methodology based on small disk-shaped samples 3 mm in diameter. The method involves the Vickers indentation of specimens ranging in thickness from 300 to 700 μm, and testing them in a ring-on-ring bending mode. New experiments on a glass-ceramic (GC) and Si₃N₄ have been performed to demonstrate the validity of the technique, supplementing the original work on ZnS. The fracture resistances of these materials increase with increasing crack length ( R -curve behavior). The data are analyzed using a specific model for the relationship between fracture resistance and crack length; this model enables the R -curve behavior to be treated analytically, and the fracture resistance at "infinite" crack length to be evaluated using a straightforward graphical procedure. The resulting values of the fracture toughness for ZnS, GC, and Si₃N₄ are 0.74 ± 0.02, 2.18 ± 0.09, and 4.97 ± 0.07 MPa-m^1/2, respectively, which are all in very good agreement with values obtained from conventional fracture toughness tests on large specimens. The results verify the utility of the miniaturized diskbend method for measuring the fracture toughness of brittle materials.     

Sintering Behavior of Gehlenite, Part II. Microstructure and Mechanical Properties

A self-toughened gehlenite (2CaO·Al₂O₃·SiO₂ or "C₂AS") ceramic with randomly distributed platelet grains was prepared by the organic steric entrapment (PVA) route. The gehlenite ceramic had a density of 2.698–2.875 g/cm³, corresponding to a relative density of 90%–96%. The platelet gehlenite grains had an average thickness of 3.6±0.8 μm and a width of 12.9±3.7 μm, respectively, with an average aspect ratio of 3.6. The three-point bending strength, fracture toughness, and Young's modulus attained were 142.1±12.1 MPa, 2.32±0.12 MPa·m^1/2, and 108±6.8 GPa, respectively. Fractography as well as Vickers indentation crack propagation profiles showed that crack deflection, crack blunting, and pinning effects due to the randomly distributed platelet grains were considered to be responsible for the good mechanical properties of the gehlenite ceramic.     

Mechanical Properties of Polycrystalline Translucent Cubic Boron Nitride as Characterized by the Vickers Indentation Method

Mechanical properties of polycrystalline translucent cubic boron nitride (cBN) were characterized by Vickers indentation measurement. The calculated hardness decreased from 54 to 49 GPa as the load increased to 39 N, and then remained constant for values above this load. According to the relationship between crack length and applied indentation load, the formation of the median/radial type of cracks seems to take place at an applied load above 29 N. Assuming that the ratio of hardness and Young's modulus is constant in the polycrystalline cBN, the fracture toughness, K_IC , of cBN was estimated to be 5.0 ± 0.5 MPa·m^1/2.     

Fracture Toughness of Chemically Vapor-Deposited Diamond

The fracture toughness of chemically vapor-deposited diamond is estimated by a Vickers indentation method. Freestanding diamond films of 400-μm thickness are produced with plasma-enhanced chemical vapor deposition and highly polished for indentation testing. Indentation testing was performed with a microhardness tester using a load range of 5 to 8 N. The average fracture toughness is estimated as 5.3 ± 1.3 MPA · m^1/2.     

Quantitative Characterization of Indentation Crack Path in a Cubic Zirconia-10 vol% Alumina Composite

Stereological measurements were performed to characterize the indentation crack path in a cubic zirconia-10 vol% alumina (c-ZrO₂-10 vol% Al₂O₃) composite. Cracks were generated using Vickers indentation, and the crack propagation behavior was characterized as a function of the indentation loading/unloading rates. Cracks that were produced by Vickers indentation formed at higher crack velocities as the loading/unloading rates increased. The amount of contact between the crack and the Al₂O₃ particles increased as the indentation rate decreased. The total number of crack-particle interactions per unit crack length also increased as the indentation rate decreased, because of an increase in the number of particles that were fractured per unit crack length, whereas the number of particles that were debonded remained relatively constant as the indentation rate changed. These results suggest that residual thermal mismatch stresses have predominant control of the crack path at lower crack velocities (low indentation loading/unloading rate), whereas elastic mismatch stresses predominate at higher crack velocities (high indentation loading/unloading rate).     

Effect of Indenter Geometry on Controlled-Surface-Flaw Fracture Toughness

Fracture toughness values obtained using both Knoop and Vickers-indentation-produced controlled surface flaws were compared as a function of indentation load for a well-characterized glass-ceramic material. At the same indentation load, Knoop cracks were larger than Vickers. As-indented K_c values calculated from fracture mechanics expressions for surface flaws were higher for Knoop flaws than Vickers, but both types gave low K_c values due to indentation residual stress effects. Analysis suggested that theoretical formalisms for indentation residual stress effects based on fracture mechanics solutions for a center-loaded penny crack in an infinite medium should apply to both indentation types. K_c values calculated using the residual stress approach were identical for Knoop and Vickers controlled surface flaws when a "calibration" value for a constant term in the expression for K_c was used for both indentation types.     

Microhardness and Fracture Toughness Anisotropy in Cubic Zirconium Oxide Single Crystals

Vickers and Knoop indentation tests have been used to study the fracture and deformation characteristics of 9.4-mol%-Y₂O₃-stabilized ZrO₂ single crystals. K_c is anisotropic, with values of 1.9 and 1.1 MPa·m^1/2 for radial cracks propagating along (100) and (110), respectively. The toughness for these two orientations was also determined using the single-edge notched-beam geometry, and yielded values of 1.9 and 1.5 MPa·m^1/2.     

Zirconia/Alumina Functionally Gradiented Composites by Electrophoretic Deposition Techniques

An electrophoretic deposition and sintering route was used to prepare YSZ/Al₂O₃ composites with a compositional gradient. The YSZ content was continuously decreased from the YSZ-rich surface to the Al₂O₃-rich surface, Microstructural and Vickers hardness (16–24 GPa) evidence tracked the compositional development, and the indentation fracture toughness was found to vary across the section (10–3 MPa·m^1/2).     

Hardness and Fracture Toughness of Pressureless-Sintered Boron Carbide (B4C)

This is a companion work to our previous study on the pressureless sintering of boron carbide (B₄C). The Vickers hardness and indentation fracture toughness of B₄C compacts were measured after various sintering heat treatments. Increases in hardness and decreases in indentation fracture toughness as the grain size decreased in sintered B₄C were attributed to the effects of more rapid strain hardening associated with dislocation pileups at grain boundaries.     

Sodium Diffusion in SiO2 Glass

Diffusion of the radioactive tracer ²²Na in a commercial SiO₂ glass was investigated from 170° to 1000°C. The temperature dependence curve had discontinuities at about 573° and 250°C. The resulting Arrhenius equations are D = 3.44 × 10² exp(-21.1 kcal/RT) cm²/sec between 1000° and 573°C, D = 0.398 exp(-25.8 kcal/RT) cm²/sec between 573° and 250°C, and D = 2.13 exp(-28.3 kcal/RT) cm²/sec between 250° and 170°C. The two anomalies are discussed in terms of "quartz-like" and "cristobalite-like" precrystalline elements in the structure of the glass. Comparison of the Na diffusion in SiO, glass with that in soda-silica and soda-lime-silica glasses shows that SiO₂ glass occupies a boundary position with respect to these systems. A possible diffusion mechanism is discussed.