Mechanism of Mechanical Strength Increase of Soda–Lime Glass by Aging

Two models have been proposed to explain the mechanical strength increase of abraded or indented soda–lime glasses upon aging, namely, crack tip blunting and the release of residual tensile stress near the crack tip. To clarify the mechanism, the time dependence of the strengthening of an abraded soda–lime glass was investigated. Effects of aging media, such as moist air, distilled water, 1 N HCI and 1 N NaOH solutions, as well as the abrasion flaw depth, were determined. The strength increase rate in water of abraded soda–lime glass was compared with those of borosilicate and high-silica glasses. The effect of stressing during aging was also investigated. It was found that the rate of strength increase was faster with decreasing abrasion flaw depth and with decreasing chemical durability. For a given flaw depth, an acidic solution produced the fastest strengthening. The strengthening rate was found to accelerate because of the "coaxing'effect of stressing during aging. From these observations, it was concluded that the strengthening rates relate to the diffusion process and chemical reactions, especially the alkali–hydrogen (or hydronium) ion-exchange reaction, near the crack tip. The role of the residual tensile stress appears to be similar to that of the applied tensile stress, helping the diffusion process near the crack tip. The observed strength increase of soda–lime glass by aging was thus attributed to the effective blunting of the crack tip geometry by the glass–water reaction.     

Time-Dependent Strength of Siliconized Silicon Carbide under Stress at 1000° and 1100°C

The time-dependent strength of a fine-grained siliconized silicon carbide under stress at 1000° and 1100°C was investigated. Both macroscopic stress redistribution and localized flaw blunting were found to contribute to the strengthening of siliconized silicon carbide in bending tests. Strengthening through macroscopic stress redistribution involved nonlinear creep behavior which decreased the maximum outer fiber stress in the bending beam. Localized flaw blunting processes were determined to be operative in this material through artificial flaw tests using a prestress to prevent flaw healing. The sharp artificial cracks were blunted during static load tests by localized deformation processes at the crack tip.     

Improved Fracture Behavior of Alumina Microstructural Composites with Highly Textured Compressive Layers

Alumina‐based microstructural composites combining equiaxed and textured layers were fabricated to examine how cracks propagate and the mechanical properties are affected as a function of the residual stress and volume fraction of texture in a multilayer structure. By combining equiaxed and highly textured alumina layers of varying thermal expansion, the embedded textured layers were placed under compressive residual stresses as high as ?670 MPa. Composites with a near constant maximum failure stress of up to 300 MPa were shown to be almost independent of the initial defect size as result of the compressive residual stress in the textured layers. An apparent fracture toughness of up to 10.1 MPa·m^1/2 was obtained for composites with an equiaxed to textured volume ratio of 7.4:1. The high compressive stress in the textured layers arrested cracks, whereas the weak bonding parallel to the basal surfaces of the textured alumina grains caused cracks to deflect within the textured layers. The coupling of these two mechanisms resulted in crack arrest and a maximum work of fracture of ~1200 J/m² or almost 50 times higher than equiaxed alumina. We believe that embedding textured layers having compressive stresses below the surface of multilayer composites represent an important strategy for designing flaw‐tolerant materials with pronounced crack growth resistance and a high work of fracture.     

Control of Stress Concentration in Surface‐Mounted Multilayer Ceramic Capacitor Subjected to Bending

Failure of surface‐mounted multilayer ceramic capacitor (MLCC) results mainly from bending of the printed circuit board during handling and applications. Cracking of MLCC normally initiates at the junction of the bottom end of termination and the dielectric ceramics and it would cut through dielectric ceramics and electrodes to result in the capacitance loss and failure of MLCC. The purpose of this study was to mitigate the stress concentration and to shift the stress concentration location toward the side end of MLCC to minimize the number of electrodes cut by cracking. To achieve this, effects of the stiffness and the thickness of the solder and the size of MLCC on the location of crack initiation and the fracture load were examined. Nanoindentation was performed to obtain the mechanical properties of the constituents of the system. Finite element analyses were conducted to simulate the stress field in the surface‐mounted MLCC subjected to bending and the results were compared to the observed crack initiation locations and the fracture load. The outcome would provide guidelines in mounting and designing of MLCCs to enhance its reliability and lifetime.     

Detection and Analysis of Crack Propagation in PZT Multilayer Ceramics

The flaw propagation in Lead zirconate titanate (PZT) multilayer ceramics under mechanical load was examined using impedance spectroscopy and three‐point bending studies. Initial flaws were generated by applying a positive sinusoidal electric field to the specimens. The cracks were sequentially propagated and after the release of the external mechanical load, impedance spectroscopy was conducted. The shift in the resonance frequencies and the subresonance height of the impedance spectroscopy were used as a measure of flaw extension. A functional dependence of the resonance frequency and the phase shift on the crack length was found. The crack propagation was studied on flaws starting at the positive and negative electrode, respectively. The maximum fracture strength, as well as the crack path, depends on the electrode potential. The variation in the fracture strength was caused by different observed fracture mode: interface cracking, matrix‐cracking, or a combination of both. The morphology of the fracture surfaces was ascribed to a textured microstructure, which is created by the sample processing, for example, by the poling process. A modified poling procedure with a lower poling temperature was analyzed, which yielded a reduction of the anisotropy of the electrode strength. Impedance spectroscopy was found to be a reliable measurement tool for automated flaw detection in PZT multilayer ceramics.     

Controlled relative humidity storage for high toughness and strength of binderless green pellets

An equation was developed to predict fracture toughness of green powder compacts. The model combines crack tip toughness predicted by Kendall's model with crack tip shielding due to bridging of moisture meniscuses across the crack. The model predicts that crack tip shielding due to moisture should be dominant. Fracture tests on ceria green pellets verified that storing pellets at a high relative humidity (98% RH) for an extended period of time led to fracture strength more than double those stored at lower RH. However, at lower RH there is no significant increase in fracture strength with increased RH as predicted by the model. The lower strength at low RH is due to insufficient capillary and surface forces but may also be related to the lack of sufficient adsorbed moisture to form bridging meniscuses. The high green strengths achieved by storing pellets at a high RH suggest a method of strengthening green parts without adding binder.     

Finite Element Analysis and Design of Thermal–Mechanical Stresses in Multilayer Ceramic Capacitors

A three‐dimensional finite element model describing the thermal–mechanical stress distribution in multilayer ceramic capacitors (MLCCs) during termination firing, soldering, and bending tests is presented. Numerical results indicate that the thermal residual stresses originating from the soldering process are approximately one‐fifth to half of the magnitude of the flexural stresses at the crack occurrence during the board flex test. The peak tensile stress from numerical simulations correlates with the crack initiation site observed in situ in board flex tests. The effects of inner electrode number, solder wicking height, lateral margin length, and the thickness of nickel in the termination component on mechanical failure during the board flex test are also investigated. Numerical results demonstrate that the maximum tensile stress could be effectively relieved by increasing the length of the lateral margin. In addition, a judicious combination of the solder wicking height and nickel termination thickness can further diminish the peak tensile stress during the board flex test. Finally, better design criteria are also developed by modifying the geometric parameters of MLCCs using Taguchi orthogonal arrays to decrease the peak tensile stresses that occur during board flex tests.     

Rising Fracture Toughness from the Bending Strength of Indented Alumina Beams

The analytical function of crack extension to a fractional power is used to represent the fracture resistance of a vitreous-bonded 96% alumina ceramic. A varying flaw size, controlled by Vickers indentation loading between 3 and 300 N, was placed on the prospective tensile surfaces of four-point bend specimens, previously polished and annealed. The lengths of surface cracks were measured by optical microscopy. Straight lines were fitted to the logarithmic functions of observed bending strength versus indentation load in two series of experiments: (I) including the residual stress due to indentation and (II) having the residual stress annealed out at an elevated temperature. Within the precision of measurement these lines have the same slope, being about 32% less than the -1/3 slope which a fracture toughness independent of crack extension would indicate. Considering the criteria for crack extension and specimen failure, the fracture mechanics equations were solved for the conditions of the two series of experiments. Approximately the same values of fracture toughness, rising as a function of indentation flaw size, were obtained from both series of experiments.     

Fabrication and properties of monolayer graphene reinforced Al2O3/TiN ceramic tool material

Al₂O₃/TiN/graphene ceramic tool materials were prepared by spark plasma sintering technology and the strengthening and toughening mechanisms were studied. The influence of monolayer graphene content on the mechanical properties and microstructure of the composite material were analyzed and the strengthening and toughening mechanisms were researched. The results showed that with an addition of .5 vol.% graphene the mechanical properties of the material reached the best. The bending strength, hardness, and fracture toughness were 624 MPa, 23.24 GPa, and 6.53 MPa·m^1/2, respectively. Graphene existed in the forms of few-layer and multilayer. The toughening mechanism of few-layer graphene was mainly graphene breaking, and that of multilayer graphene included graphene breaking and pulling-out. Graphene could contribute to the uniform growth of grains due to the excellent electrical conductivity and the high thermal conductivity. The addition of nano-TiN introduced many endocrystalline structures and graphene promoted this phenomenon. Micro-TiN grains made the crack extension show a combination of transgranular fracture, intergranular fracture, crack bridging, and crack deflection, while graphene introduced weak grain interfaces and made the crack appear more branches. The layered graphene made the material fracture change from two-dimension to three-dimension.     

Ytterbia–neodymia–costabilized TZP—Breaking the limits of strength–toughness correlations for zirconia?

TZP ceramics were manufactured by hot pressing of pyrogenic zirconia nanopowder which was costabilized by 1 mol% ytterbia and 2 mol% neodymia (1Yb–2Nd–TZP) via the nitrate route. The evolution of microstructure, phase composition and mechanical properties with variation of sintering temperature from 1200 °C to 1400 °C was investigated. 1Yb–2Nd–TZP consists of a bimodal microstructure of small very transformable tetragonal grains and large cubic grains. At intermediate sintering temperature the materials combine a bending strength of 1250 MPa with a fracture resistance >13 MPa √m. The high threshold stress intensity of 7 MPa √m indicates high resistance to subcritical crack growth. An increase in fracture resistance before the crack tip induced by compressive residual stress shifts the strength–toughness correlations to higher values than previously considered possible.     

Effect of abrasive grinding on the strength of Y-TZP

The residual stress and flaw size in yttria-stabilized zirconia (Y-TZP) before and after abrasive grinding are examined in the present study. As long as the distribution of flaw size remains the same after grinding, the strength of zirconia increases with increasing residual stress. For the grinding conditions employed, a compressive residual stress as high as 1 GPa is introduced on the surface. Such stress is large enough to induce lattice distortion. The strength of Y-TZP is consequently increased by 190 MPa. The compressive residual stress can be partly removed by subsequent polishing or annealing treatment.     

Improved toughness and electromagnetic shielding-effectiveness for graphite-doped SiC ceramics with a net-like structure

The graphite-doped SiC ceramics with net-like structure was fabricated via tape casting and pressureless sintering. The ceramics exhibited a step-like fracture mode, which could be attributed to the net-like structure composed of long columnar SiC grains, layered graphite, and the three-modal pore distribution. The formation of warped epitaxial graphene and large size graphite could be attributed to the pyrolysis of organics in the tape casting system. In the net-like structure, the SiC grains provide the high strength, whereas the layered graphite and three-modal pores were used to deflect the cracks and release the stress at the tip, following the crack-tip-shielding mechanism. The sample with a net-like structure exhibited a combination of a variety of extrinsic toughening mechanisms, such as crack deflection, crack bridging, crack branching and delamination, pull-out, and rupture of layered graphite, which led to improved fracture toughness of 7?MPa?m^1/2, flexural strength of 400?MPa, and (work of fracture) WOF of 3.3?kJ?m^?2. When increasing the graphite content, the electrical conductivity of the graphite-doped SiC ceramics significantly increased from 7.15?×?10^?4 to 216 S/m. The high shielding effectiveness of 34.1?dB was due to the multi-absorption on the various surfaces during the multi-reflection by the net-like structure.     

Modeling Slow Crack Growth Behavior of Glass Strengthened by a Subcritical Tensile Stress Using Surface Stress Relaxation

Glasses exhibit slow crack growth under stress intensities below the fracture toughness in the presence of water vapor or liquid water. It has been observed by several authors that when an oxide glass with a large crack is held under a subcritical stress intensity (where no slow crack growth occurs) in room‐temperature water vapor or liquid water, upon reloading to a higher stress intensity, a finite restart time is observed prior to measurable crack extension. This phenomenon of apparent strengthening, or crack arrest, has been attributed to concepts such as corrosive dissolution of the crack tip, crack tip blunting, or water diffusion, and subsequent swelling of the material around the crack tip. Recently, a newly observed surface stress relaxation process that is aided by molecular water diffusion was used to improve the mechanical strength of glass fibers and to explain the subsurface compressive stress peak observed in ion‐exchange strengthened glasses. The same process is employed here to explain these delayed slow crack growth data. A simple mathematical model has been developed utilizing water‐assisted surface stress relaxation and fracture mechanics. Predictions of restart times using the model agreed well with published experimental data, indicating that surface stress relaxation is responsible for the anomalous delayed slow crack growth behavior.     

Temperature-dependent fracture strength and the effect of oxidation for ZrB2-SiC ceramics

Using the stress distribution of the body containing a spherical inclusion, the stress intensity factor at the tip of the annular flaw emanating from the inclusion is formulized. Since the thermal expansion coefficient of matrix and inclusion is not matched, the residual stress is also taken into account. Introducing into the proposed temperature-dependent fracture surface energy or fracture toughness, the temperature-dependent fracture strength for ZrB₂-SiC is obtained. The influence of oxidation on the fracture strength is also discussed and the analysis reveals that the oxidation has significant effect on the fracture strength under some circumstances. The calculated results are compared with the experimental data and they have very good consistency.     

Critical crack-healing condition for SiC whisker reinforced alumina under stress

Alumina reinforced by SiC whisker, called here “alumina(w)” was developed with the objective of improving fracture toughness and crack-healing ability. The composites were crack-healed at 1200 °C for 8 h in air under elevated static and cyclic stresses. The bending strength at 1200 °C of the crack-healed composites were investigated. The threshold static stress during crack-healing of alumina(w) has been determined to be 250 MPa, and the threshold cyclic stress was found to be 300 MPa. Considering that the crack growth is time-dependent, the threshold stress of every condition during crack-healing of alumina(w) was found to be 250 MPa. The results showed that the threshold stress intensity factor during crack-healing was 3.8 MPa m^1/2. The same experiment conditions were applied to specimens cracked and annealed at 1300 °C for 1 h in Ar, to remove the tensile residual stress at a tip of the crack. Thus, the threshold stress intensity factor during crack-healing was found to be 3.2 MPa m^1/2 for the specimens crack-healed with annealing. The threshold stress intensity factor during crack-healing of alumina(w) was chosen to be 3.2 MPa m^1/2 to facilitate comparison with the values of the threshold stress intensity factor during crack-healing. The residual stress was slightly larger than the intrinsic value.     

Fracture Origin and Strength in Advanced Pressureless-Sintered Alumina

Advanced raw materials and shaping approaches enable the production of pressureless-sintered alumina parts where, in bending, the average maximum stress at the fracture origin is as high as 800 MPa. In individual specimens that fracture at lower stresses (450–600 MPa), failure often originates at volume flaws, as known for hot-pressed alumina with a similar strength. Also, transgranular and intergranular fracture modes along the crack path are the same as those observed in hot-pressed alumina. If the size and the frequency of volume flaws are reduced, fracture initiates at smaller defects in the ground surfaces and bodies with a bending strength of >800 MPa are produced without hot pressing. The grain-size dependence of grinding-induced surface damage contributes to a grain-size effect for the strength.     

Effect of Residual Stresses on the Fracture of Ground Ceramics

Grinding induces resiual stresses and cracks at and near the surfaces of ceramic workpieces. The residual stresses in several ground ceramics are measured using the curvature method. Four-point bend tests are conducted to measure the fracture strength of the ground specimens. A fracture mechanics analysis includes the measured residual stress to calculate the grinding-induced surface crack size. It is shown that the residual stresses sometimes have a significant effect on the strength-controlling flaw size.     

Crack-healing behavior and static fatigue strength of Si3N4/SiC ceramics held under stress at temperature (800, 900, 1000 °C)

Si₃N₄/SiC composite ceramics were sintered and subjected to three-point bending. A semi-elliptical surface crack of 100 μm in surface length was introduced on each specimen. The pre-crack was healed under constant bending stress of 210 MPa at 800, 900 and 1000 °C. Applied stress of 210 MPa is ∼70% of the bending strength of pre-cracked specimen. Bending strength and static fatigue strength of crack-healed specimens were systematically investigated at each crack-healing temperature. The bending strength of crack healed specimen showed almost the same value as smooth specimen. Thus, Si₃N₄/SiC composite ceramics could heal a crack even under constant bending stress of 210 MPa at 800, 900 and 1000 °C. Moreover, crack-healed zone had quite high static fatigue limit at each crack-healing temperature. These conclusions indicate that Si₃N₄/SiC composite ceramics has an ability to heal a crack under service condition, i.e. high temperature and applied stress.     

Deducing the Fatigue Crack Growth Rates of Natural Flaws in Silicon Nitride Ceramics: Role of R‐Curves

Fatigue failure is a concern when high‐strength, high‐toughness silicon nitride ceramics are used in mechanical components and the growth of natural flaws will determine the usable upper bound strength. In this study a fracture resistance curve (R‐curve) model is incorporated into an established method for deducing natural flaw growth rates from a combination of strength and fatigue life data for smooth specimens. Experimental data for a commercial silicon nitride, SL200, were examined. When compared with results deduced using a constant fracture toughness model, the new method gives more physically realistic growth rate results. Specifically, by incorporating the R‐curve the deduced fatigue threshold is equal to the reported intrinsic toughness for crack propagation of 2.2 MPa√m, whereas the constant fracture toughness model gives a physically unrealistic threshold value. Furthermore, much better agreement is achieved with the growth rates measured using macroscopic compact‐tension specimens. Overall, it is concluded that the R‐curve effect should not be ignored when deducing the fatigue crack growth rates of natural flaws in high‐toughness silicon nitride ceramics.     

Microstructure and mechanical properties of 3D-printed nano-silica reinforced alumina cores

Ceramic cores are an important component in the preparation of hollow turbine blades for aero-engines. Compared with traditional hot injection technology, 3D printing technology overcomes the disadvantages of a long production cycle and the difficulty in producing highly complex ceramic cores. The ceramic cores of hollow turbine blades require a high bending strength at high temperatures, and nano-mineralizers greatly improve their strength. In this study, nano-silica-reinforced alumina-based ceramic cores were prepared, and the effects of nanopowder content on the microstructure and properties of the ceramic cores were investigated. Alumina-based ceramic cores contained with nano-silica were prepared using the vat photopolymerization 3D printing technique and sintered at 1500 °C. The results showed that the linear shrinkage of ceramic cores first increased and then decreased as the nano-silica powder content increased, and the bending strength showed the same trend. The fracture mode changed from intergranular to transgranular. The open porosity and bulk density fluctuated slightly. The weight loss rate was approximately 20%. When the nano-silica content was 3%, the bending strength reached a maximum of 46.2 MPa and 26.1 MPa at 25 °C and 1500 °C, respectively. The precipitation of the glass phase, change in the fracture mode of the material, pinning crack of nanoparticles, and reduction of fracture energy due to the interlocking of cracks, were the main reasons for material strengthening. The successful preparation of 3D printed nano-silica reinforced alumina-based ceramic cores is expected to promote the preparation of high-performance ceramic cores with complex structures of hollow turbine blades.