Corrosion Kinetics and Strength Degradation of Sintered δ-Silicon Carbide in Potassium Sulfate Melts

Pressureless sintered α-SiC ceramics containing carbon and boron as sintering aids and hot-pressed SiC containing aluminum nitride as a sintering aid were corroded in K₂SO₄ melts at 1080° to 1150°C. SiC ceramics were oxidized and dissolved into K₂SO₄ melts. Since the corrosion of SiC ceramics in K₂SO₄ melts proceeded autocatalytically, a reaction product such as K₂S_1.44 was suspected to promote the corrosion reaction. The corrosion resistance of SiC containing AlN in K₂SO₄ melt was superior to that of SiC containing boron and carbon. Apparent activation energies for the corrosion of SiC ceramics were 309 to 331 kJ mol^-1. The fracture strength of the specimens corroded by K₂SO₄ melt degraded to 40% to 70% of the original values up to a 20% weight loss and then was almost constant up to 45% weight loss.     

Fracture Stress as Related to Flaw and Fracture Mirror Sizes

Fractographically observed critical-flaw boundaries in strength-tested specimens of 2 polycrystalline ceramics were used in calculating critical stress-intensity factors ( K_IC )- Each ceramic exhibited a K_IC which had little or no dependence on critical-flaw characteristics and which agreed with the value obtained from independent determinations on artificially precracked specimens. Analyses both of fracture-mirror sizes and of water-enhanced subcritical crack growth data supported the evidence that K_1C is a material constant. The fracture-mirror analysis further indicated that the parameter A/K_IC , where A is the fracture-mirror constant, is a dimensionless, material-independent constant.     

Effect of Surface Flaw Size on Fracture Strength of Alumina Ceramics

Semielliptical surface flaws of different sizes were introduced into Al₂O₃ by Knoop microhardness indentation. The specimens were fractured by four-point bending and the profiles of the indentation flaws were determined by observing the fracture surfaces with a scanning electron microscope. The relation between the indentation flaw size and the fracture strength could be well explained by applying the fracture-mechanics analysis for semielliptical surface flaw in bending. The calculated values of the as-indented critical stress intensity factor, K_IC, were lower than previously reported presumably because of the influence of the residual stresses produced by the indenter.     

Controlled Surface Flaws in Hot-Pressed SiC

Controlled semi-elliptical surface flaws were produced in hot-pressed SiC by Knoop microhardness indentation. Flawed specimens were placed in 4-point bending in order to determine their critical stress intensity factor, K_IC, at both room and high temperatures. Room-temperature fracture and K_IC values after annealing were sensitive to the annealing environment; this behavior correlated with the active/passive nature of the oxidation process. Flaw healing was observed for annealing exposures in air. Room-temperature K_IC values increased with increasing annealing temperature. High-temperature K_ICvalues decreased with increasing temperature as a result of a decrease in the fracture surface energy.     

Mixed-Mode Fracture Toughness of Ceramic Materials

An experimental technique whereby pure mode I, mode II, and combined mode I-mode II fracture toughness values of ceramic materials can be determined using four-point bend specimens containing sharp, through-thickness precracks is discussed. In this method, notched and fatigue-precracked specimens of brittle solids are subjected to combined mode I-mode II and pure mode II fracture under asymmetric four-point bend loading and to pure mode I under symmetric bend loading. A detailed finite element analysis of the test specimen is performed to obtain stress intensity factor calibrations for a wide range of loading states. The effectiveness of this method to provide reproducible combined mode I-mode II fracture toughness values is demonstrated with experimental results obtained for a polycrystalline Al₂O₃. Multiaxial fracture mechanics of the Al₂O₃ ceramic in combined modes I, II, and III are also described in conjunction with the recent experimental study of Suresh and Tschegg (1987). While the mode II fracture toughness of the alumina ceramic is comparable to the mode I fracture toughness K _Ic, the mode III fracture initiation toughness is 2.3 times higher than K _Ic. The predictions of fracture toughness and crack path based on various mixed-mode fracture theories are critically examined in the context of experimental observations, and possible effects of fracture abrasion on the apparent mixed-mode fracture resistance are highlighted. The significance and implications of the experimental methods used in this study are evaluated in the light of available techniques for multiaxial fracture testing of brittle solids.     

Use of Crack Branching Data for Measuring Near-Surface Residual Stresses in Tempered Glass

An analytical procedure based on fracture mechanics is used to obtain the amount of residual stress in glass from measurements on the fracture surface. The technique utilizes the measurement of microcrack branching distances, known as the mirror — mist boundary, which occur at a critical crack branching stress intensity (K _m ) value. This procedurre shows that σ _A r _m ^1/2 Y _F (θ) =σ _R r ^1/2 _m +Ψ₀, where σ _A is the applied stress, r _m is the microcrack branching radius, σ _R is the residual stress, Y _F ( θ ) is the crack-border correction factor, and Ψ₀ is a material constant based on K _m . Thus, the equation is that of a straight line with the slope equal to the magnitude of the residual stress. Data for tempered glass from the literature are used to demonstrate the applicability of the technique.     

Fracture Toughness of Chalcogenide Glasses of the System Se-Ge-As

Measurements of the critical stress-intensity factor, K₁₀ are reported for eight glasses of the chalcogenide system Se-Ge-As. The K₁₀ values are between 5.5 and 9.4 N.mm-^3/2. The dependence on glass composition can be understood qualitatively according to different short-range order units of the Se-Ge-As glasses. A relation between fracture toughness, K₁₀, microhardness, L₂VH, and Young's modulus, E, may not be excluded for these glasses .     

Crack Branching in Transformation-Toughened Zirconia

Crack propagation and branching were investigated in calcia partially stabilized zirconia aged at 1300°C for various times. The crack-branching radii were measured and used to calculate the apparent stress intensity factors at crack branching ( K₈ ) which increase with increasing aging time. The K_B/K_IC ratios increased with aging time with a maximum of 5.3. This maximum ratio is much greater than the ratios previously observed for ceramics.     

Mixed-Mode Fracture from Controlled Surface Flaws in Hot-Pressed Si3N4

The room-temperature mixed-mode fracture of commercial hot-pressed Si₃N₄ was examined using controlled surface flaws in 4-point bending, oriented at various angles 6 with respect to the outer fiber tensile stress direction. Catastrophic fracture paths were non-coplanar with the initial flaw plane, and the stress intensity factor ratio K_I/K_IC was < 1 for fracture in modes II and III. A non-coplanar maximum strain-energy release rate fracture criterion best described mixed-mode fracture.     

Fracture-Surface Energy and Fracture Toughness of (U,Pu)C and (U,Pu(C,O)

The fracture surface energy, γ _f , and the fracture toughness, K_1c , were measured for the advanced fast-breeder reactor fuels (U_0.80Pu_0.20)C and (U_0.82Pu_0.18) C_0.9O_0.1, using the method of Hertzian indentation with steel indentors. Elastic behavior was observed with transgranular cracks yielding K_1c , values of ∼ 1.5 MN·m^−3/2. The surface energy of the oxicarbide was smaller than that of the carbide. Annealing of the (U,Pu) (C,O) led to a completely different behavior. Permanent "quasi-plastic" deformation was observed, in addition to Hertzian ring cracks, due to pronounced grain-boundary weakening and grain separation. The plausible cause, i. e. precipitation of He formed by decaying Pu into grain-boundary bubbles, indicates that the in-pile behavior of a nuclear fuel after some burn-up does not necessarily correspond to the behavior observed in laboratory experiments, even in the absence of restructuring due to thermal gradients that exist during operation.     

Fracture Properties of Polycrystalline Lithia-Stabilized β"- Alumina

The critical stress-intensity factor, K_1C , and the fracture strength, σ _f , have been investigated on both hot-pressed and sintered lithia-stabilized β "-alumina. The hot-pressed material possessed a strong preferred orientation with many of the basal planes aligned perpendicular to the direction of hot-pressing. Both K_1C and σ _f were found to be orientation-dependent. Two regimes of fracture were identified. In fine-grained material (<120 μm), the strength was slightly dependent on grain size.
For larger grain sizes, the strength decreased rapidly with increasing grain size and the fracture mode was almost entirely transgranular. The K_1C values for sintered β "-alumina were in the same range as those obtained on the hot-pressed material.     

The Activation Effect of K2SO4 on the Hydration of Gypsum Anhydrite, CaSO4(II)

The effect of K₂SO₄ activator on the hydration of chemical anhydrite obtained from burned FGD-gypsum has been studied by different experimental techniques. Results obtained show that the degree of hydration increases when the K₂SO₄ concentrations increase from 0.5 to 3.3 wt%. Their heat evolution rate and maximum value also increase with the increase of K₂SO₄ concentration. The highest values were obtained when the hydration degree was about 50%. Also a correlation between the hydration degree and the total heat evolved was obtained. The X-ray and SEM/EDX studies have shown that K₂SO₄ is adsorbed at the surface of CaSO₄ even within 5 min of hydration and a syngenite—a double salt K₂SO₄·CaSO₄·H₂O is formed (even in the presence of 1.0 wt% K₂SO₄). Also, important changes in the morphology of the dihydrate crystal are detected. Finally, in the presence of 1.0 wt% K₂SO₄ (water/anhydrite ( W / A ) ratio=0.33), it was found that the resonance frequency, modulus of elasticity, compressive strength, and tensile strength increase with the degree of hydration whereas the total porosity decreases.     

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.     

Controlled Surface Flaws in Hot-Pressed Si3N4

Surface flaws of controlled size and shape were produced in high-strength hot-pressed Si₃N₄ with a Knoop microhardness indenter. Fracture was initiated at a single suitably oriented flaw on the tensile surface of a 4-point-bend specimen, with attendant reduction in the measured magnitude and scatter of the fracture strength. The stress required to propagate the controlled flaw was used to calculate the critical stress-intensity factor, K _IC, from standard fracture-mechanics formulas for semielliptical surface flaws in bending. After the bend specimen had been annealed, the room-temperature K _IC values for HS-130 Si₃N₄ increased to a level consistent with values obtained from conventional fracture-mechanics tests. It was postulated that annealing reduces the residual stresses produced by the microhardness indentation. The presence of residual stresses may account for the low K _IC, values. Elevated-temperature K_IC values for HS-130 Si₃N₄ were consistent with double-torsion data. Controlled flaws in HS-130 Si₃N₄ exhibited slow crack growth at high temperatures.     

Effect of Subcritical Crack Growth on Fracture Toughness and Work-of-Fracture Tests Using Chevron-Notched Specimens

A correlation between the plane strain stress intensity factor K_I , load, and crack extension has been analyzed for constant displacement and constant loading rate experiments, using chevron-notched, four-point-bend specimens. It is assumed that at the beginning of the experiment the chevron triangle tip is not ideally sharp. As loading continues, the crack initially moves with velocity v_t at K_I equal to a threshold value K_t . Maximum crack velocity is reached at K_I= K_IC , the fracture toughness. Depending on the type of material tested, a specific displacement or loading rate must be used to correlate the maximum load with K_Ic . An error in K_IC calculation is estimated if different displacement rates are applied. Repeated loading-unloading work-of-fracture (WOF) experiments generate values related to the resistance of the material to fracture initiation, K_t , only when the crack length approaches 100% of the specimen width. Values related to material's fracture toughness, K_IC are not generated in WOF tests.     

Transformation Toughening in ZrO2-Containing Ceramics

Transformation toughening in ZrO₂-containing ceramics is discussed. Specifically, microstuctures of the three distinct types of ZrO₂-toughened ceramics are presented, after which microstructural evolution in MgO-partially-stabilized ZrO₂ Mg-PSZ) is reviewed. The mechanical properties of such transformation-toughened ceramics are dominated by "resistance-curve" ( R -curve) behavior, wherein the crack resistance increases during the course of crack propagation. Ceramics subject to R -curve behavior require a more detailed failure criterion than those subject to the usual linear elastic fracture mechanics criterion involving a critical stress intensity factor, K_IC.R -curve-controlled fracture in ceramics provides a degree of very desirable flaw insensitivity, but can lead to counterintuitive relationships concerning strength, toughness, and initial flaw size. Examples of R curves of Mg-PSZ with different thermal histories are given.     

Temperature Dependence of KIC for High-Pressure Hot-Pressed Si3N4 Without Additives

Temperature dependence of K_IC values for Si₃N₄ bodies sintered at high pressures without additives was studied from room temperature to 1400°C. Little change in K_K was found in this range.     

Equilibrium Compressibilities and Density Fluctuations in K2O–SiO2 Glasses

The density fluctuations contributing to light scattering in a glass are governed by the flctive temperature of the glass and the equilibrium compressibility of the melt. Using ultrasonic velocity data for K₂O–SiO₂ melts, these compressibilities were evaluated, and the magnitude of the density fluctuations were calculated. In this system, the mean–square amplitude of the fluctuations reaches a minimum value (about half that of pure SiO₂) for a composition of ∼20 mol% K₂O. By extrapolating the equilibrium compressibilities to zero K₂O content, the density fluctuations can be calculated for pure SiO₂ glass; this calculation agrees well with the result obtained from light–scattering measurements.     

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.     

The Effect of Stoichiometry on Mechanical Properties of Boron Carbide

The mechanical properties of chemically vapor-deposited boron carbides (B₄C) with varied B/C ratios were investigated as a function of composition. The maximum hardness, H, and fracture toughness, K_1c, were observed at an almost stoichiometric composition. For nonstoichiometric B₄C (B/C>4), H and K_1c decreased with increasing B content, suggesting that excess B diminishes the bond strength in the B₄C structure. The decrease in H and K_1C at B /C <4 was attributed to free C in the microstructure.