Mechanical properties of ZrB2 ceramics determined by two laboratories

The mechanical properties for zirconium diboride (ZrB₂) were measured at two laboratories and compared. Two billets of ZrB₂ were prepared by hot-pressing commercial powder. The relative densities of the billets were >99% and with an average grain size of 5.9 ± 4.5 µm. Both laboratories prepared American Society for Testing and Materials (ASTM) C1161 B-bars for strength and ASTM C1421 bars with notch configuration A for fracture toughness. Specimens were machined by diamond grinding at the Army Research Laboratory (ARL) and electrical discharge machining (EDM) at Missouri S&T. Strength bars tested at Missouri S&T were polished to a .25 μm finish while the bars were tested as-ground at ARL. Strengths were 473 ± 79 MPa for the Missouri S&T bars and 438 ± 68 for the ARL bars while the fracture toughness values were 3.9 ± .7 MPa•m^1/2 for the Missouri S&T bars and 4.4 ± .6 MPa•m^1/2 for the ARL bars. Vickers hardness was measured by both laboratories over a range of indentation loads. The resulting hardness values were on the low end of previously reported values and were quite different from each other especially at indentation loads ≤20N. The study demonstrated that the properties of materials tested to ASTM standards at different laboratories can be compared directly. In addition, strength and fracture toughness were nearly identical for bars prepared by conventional diamond grinding or EDM.     

Revisiting the strength of micron-scale ceramic platelets

Individual platelets in nacre-like ceramics are able to locally withstand stress levels many times larger than the strength of larger specimens made of the same materials. This size effect, usually reported as being relative to the size of pre-existing defects, is described by considering both stress and energy failure conditions. We show that there is a characteristic length, defined by the material's Young's modulus, fracture energy, and strength, above which failure is governed by a stress criterion and below which energy considerations are dominant. Applying the coupled criterion to three-point bending of a single alumina platelet allows the identification of the platelet strength and fracture energy. The proposed approach also allows capturing the decrease in failure stress due to a surface defect accounting for its shape and size.     

Enhanced toughness of zirconia ceramics with graphene platelets consolidated by spark plasma sintering

Graphene platelets reinforced zirconia (GPLs/ZrO₂) composites were prepared by spark plasma sintering in the present work. The effects of GPLs content on the densification route, microstructure feather, mechanical properties, and aging behaviors of such composites were investigated. In spite of the impeding effect of GPLs, high relative density of 98% was achieved for the composites owing to the uniform dispersion of GPLs. The addition of GPLs contributed to enhanced fracture toughness of the composites; when the added content was 1.0 wt.%, its fracture toughness reached up to 8.6 MPa·m^1/2. Also, aging behavior of the GPLs/ZrO₂ composites was investigated at 134°C for 24 hours. The monolithic ZrO₂ ceramic and GPLs/ZrO₂ composites presented residual ratio of 55% and 72% in fracture toughness, respectively. Thus, the incorporation of GPLs inhibited phase transformation from tetragonal phase to monoclinic phase of zirconia.     

Artificial Intelligence-based determination of fracture toughness and bending strength of silicon nitride ceramics

Two mechanical properties, fracture toughness (K_IC) and bending strength (σ), of silicon nitride (Si₃N₄) ceramics were determined from their microstructural images via convolutional neural network (CNN) models. The Si₃N₄ samples used for database were fabricated using various kinds of sintering additives under different process conditions. In total, 330 data sets were prepared and used for building the CNN models for artificial intelligence-bassed determination of the two mechanical properties and testing the determination accuracy of the trained models. The determination coefficients (R²), which were used as accuracy indices, were approximately 0.85 for K_IC and 0.92 for σ. Although both the R² values were relatively high, the lower value for K_IC suggests that it is influenced more by what is little obtained from the microstructural information, such as grain-boundary characteristics. Furthermore, gradient-weighted class activation mapping, which can visualize which parts of the image the CNN models focus on, showed that the trained models determined the two mechanical properties based on correct recognition of the microstructural difference among the images.     

A statistical model for the failure of glass plates due to nickel sulfide inclusions

Nickel Sulfide (NiS) inclusions can provoke the rupture of thermally treated glass due to a phase transformation with volume increase that stresses the surrounding glass. Starting from a Pareto statistics for the population of inclusion sizes, from an assumed kinetics of the phase transformation, a micro-mechanically motivated model provides the statistical characterization of the probability of spontaneous failure of glass during lifetime. A distinction based upon the composition of NiS is used to discuss the effects of the heat soak test (HST), where glass remains at high temperature for a certain time to speed-up the phase transformation and destroy those elements with critical inclusions. Three functions à la Weibull for the probability of spontaneous rupture during lifetime are theoretically derived for the case of no HST, short HST, and long HST. In particular, the probability of collapse for long HSTs depends upon the holding time in the oven. An explanatory example shows the potentiality of the model for optimizing the HST parameters toward a target probability of failure, but experimental campaigns are needed for a proper calibration.     

Toughening and Strengthening of NiAl with Al2O3 by the Addition of ZrO2(3Y)

NiAl/10-mol%-ZrO₂(3Y) composites of almost full density have been fabricated via spark plasma sintering (SPS) for 10 min at 1300°C and 30 MPa. The former intermetallic compound, which contains a trace amount of Al₂O₃, has been prepared via self-propagating high-temperature synthesis. The composite microstructures are such that tetragonal ZrO₂ (∼0.2 μm) and Al₂O₃ (∼0.5 μm) particles are located at the grain boundaries of the NiAl (∼46 μm) matrix. Improved mechanical properties are obtained: the fracture toughness and bending strength are 8.8 MPa·m^1/2 and 1045 MPa, respectively, and high strength (>800 MPa) can be retained up to 800°C.     

Notch Sensitivity of Ceramic Composites with Rising Fracture Resistance

A theoretical framework is developed for the notched strength of ceramic composites that exhibit rising fracture resistance. It is based on established concepts of crack stability under stress-controlled loadings. On using a linear representation of the resistance curve (expressed in terms of an energy release rate), straightforward analytical solutions are obtained for the strength as well the amount of stable crack growth preceding fracture and the associated fracture resistance. Calculations are performed for several test configurations commonly used for material characterization, including single- and double-edge-notched tension, center-notched tension, and single-edge-notched bending. The results reveal salient trends in strength with notch length and specimen geometry. An assessment of the theory is made through comparison with experimental measurements on an all-oxide fiber composite. Transitions in the degree of notch sensitivity with notch length are identified and explored. The utility of the theoretical results both for rationalizing the trends in measured notched strength and for guiding experimental studies of notch sensitivity is demonstrated.     

Improved Estimation of Weibull Parameters with the Linear Regression Method

Monte Carlo simulations were used to search for the optimal probability estimator for estimating Weibull parameters with the linear regression method. Compared with commonly used probability estimators, the optimal one obtained gives a more accurate estimation of the Weibull modulus and the same estimation precision of the scale parameter. It is also concluded that the maximum likelihood method results in the highest precision, however, less conservative than the linear regression method.     

The breakdown of the Weibull behavior in dental zirconias

Here we attend to the controversy of the use of Weibull statistics for describing the strength distribution of dental brittle materials. Our approach is purely experimental by means of testing for the strength size effect, a requirement for Weibull materials. Zirconia materials of five important dental manufacturers were selected, each represented by two compositions, being one a 3 mol% Y₂O₃-stabilized zirconia, and the other being a “translucent” zirconia with 4 or 5 mol% stabilizer content. Specimens of increasing sizes were fractured, whether by using a biaxial flexure test in plates or a uniaxial bending test in beams, thereby sampling different ranges of effective surfaces and volumes. A systematic deviation from the Weibull behavior over the range of 1–40 mm² effective surface was demonstrated, regardless of manufacturer and Y₂O₃ content in the powder. Extensive testing using a wider range of specimen sizes narrowed down the threshold for the breakdown of the defect size distribution from the parent population to be located between 10 and 20 mm² effective surface. A comparable behavior was confirmed for the partly sintered white-bodies, with similar defect morphology to the fully sintered analogs, indicating a defect size distribution stemming from the pressing steps of manufacture. The defect shape related to open particle aggregate junctions, pointing to an association of their size distribution to that of the distribution of aggregate sizes in the source powders.     

Temperature dependent fracture toughness model for whisker-reinforced ceramic matrix composites

A temperature dependent fracture toughness model for whisker-reinforced ceramic matrix composites was developed in this study, which considers the effects of matrix fracture toughness, residual thermal stress, crack bridging, crack deflection, and their temperature dependence. Its predicted results were compared with the fracture toughness of six types of whisker-reinforced ceramic matrix composites at different temperatures, and good agreement between predicted results and experimental results is obtained. Furthermore, based on this model, we systematically analyzed the effects of the volume fraction and aspect ratio of whisker, Young's modulus of matrix and whisker, thermal expansion coefficient difference, stress-free temperature, the ratio between the fracture energy of matrix and that of interface, on their temperature dependent fracture toughness for the first time. Finally, insights and suggestions which could help to optimize and improve the composite fracture toughness at different temperatures are provided.     

Failure criterion for brittle materials with U-notches: Unification of characteristic length-based and grain size-based criteria

Here, the limitations of characteristic length-based (L_chb) and grain size-based (G_b) criteria with two or three parameters were pointed out employing the apparent toughness tests of 12 different ceramics at a large span range of U-notch root radius (ρ) values. After comprehensively considering the potential influencing factors of stress intensity factor (K_c), ρ divided by critical notch tip radius (ρ_c) was proposed as the independent variable, and the data of 21 materials (covering ceramics, plastics, resins, rocks, and metals) was summarized and discussed to establish a simple and more applicable K_c prediction model. Results indicated that K_c/K_Ic was a power function of ρ/ρ_c with a power exponent n of 0.5 for ideal materials and less than 0.5 for actual materials. It was also found that ρ_c can be calculated simply by K_Ic²/(πσ₀²), where σ₀ represented the inherent strength. This semiempirical criterion succeeded in unifying the L_chb and G_b criteria without introducing more parameters to increase the prediction accuracy of the K_c at the U-notch root for brittle materials like ceramics.     

Advanced Structural Ceramics: A Round Robin

The strength of 1500 sintered alumina and reaction-bonded silicon nitride flexure specimens was measured by seven laboratories in four countries. Consistent results were obtained only with a well-defined standard test method. Older procedures were less consistent or gave erroneous results. A simple statistical procedure allowed a quick estimate of consistency, and outlier samples could be appraised for the likelihood of their occurrence.     

Orientation‐dependent mechanical and thermal properties of plasma‐sprayed ceramics

Due to droplet‐based assembly, microstructure anisotropy is expected in atmospheric plasma‐sprayed coatings (APS), with lamellar separations and interfaces having critical effects on properties. Quantitative determination of these anisotropic properties is difficult due to geometric test constraints. This has been overcome in the literature through a variety of indirect, local, or modeled evaluation, however direct measurement on like‐dimensioned coatings is not available. In this work, 25‐mm thick ceramic coating variants, deposited at two different feed rates, were obtained from industry and macroscopic mechanical and thermal properties were evaluated in both in‐plane and out‐of‐plane orientations using identical specimen geometries. As expected, and confirming select past work, coating anisotropy has a direct influence on measured properties. The response of each property is microstructure‐dependent, highlighting the specific interaction: for instance, the fracture toughness is 120% higher in the through‐thickness orientation versus in‐plane after thermal aging, while the thermal conductivity was 24% lower in the through‐thickness. The former benefits from the lamellar interfaces that provide obstacles to crack propagation while the latter sees these interfaces as efficient phonon scatters. The results provide insights for design through robust property measurements and into operational mechanisms in this class of highly defected ceramics.     

Mode I interlaminar fracture behavior of 2D woven ceramic matrix composites

Interlaminar fracture properties of melt-infiltrated woven SiC/SiC ceramic matrix composites were investigated using traditional and wedge-loaded double cantilever beam methods. The two methods produced comparable G_IC results for some specimens. The difference in boundary conditions between the two methods appeared to influence the crack propagation path. The DCB method, having free-end boundary condition, allowed more interaction between the crack and the composite microstructure than the wedge method did. The effect of fiber tow layout sequence had an effect on the interlaminar properties. Higher toughness was observed for the orientation where crack propagation occurs between planes with more transverse tows. Jump-arrest phenomenon was found to have higher significance on the rising R-curve behavior than fiber bridging.     

Fracture Strength of α- and β-SiAION Measured by Biaxial and Four-Point Bending

The fracture strength of an α- and β-SiAION was measured by biaxial and four-point bending. By utilizing Weibull statistics a close prediction could be made of the four-point values from the biaxial values. At a fracture strength of 460 MPa, fracture was initiated by linking of surface and subsurface flaws.     

Mechanical Strength and Microstructure of Zinc Oxide Varistor Ceramics

ZnO-based varistor ceramics were sintered under various conditions to optimize their mechanical strength. For highest strength, the optimum sintering temperature was 1070°C or below. At higher maximum temperature, the strength decreased because of grain coarsening and the increasingly inhomogeneous distribution of secondary phases thereby induced. Fracture typically started from holes associated with hollow or poorly compacted sprayed granules. All series contained the same type of critical flaws, but, depending on the sintering temperature, the fracture toughness changed, which led to different strengths. At sintering temperatures above 1050°C, the density started to decrease slightly because of swelling attributed to the pressure of gas entrapped in closed pores.