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.     

Weibull parameters obtained from dependence of fiber strength on fiber length and area

Weibull strength parameters of ceramic fibers can be inferred from variations in strength with fiber diameter or gauge length. The goal of this article is to provide a critical assessment of the efficacy of these methods. The issues are addressed using theorems in regression analysis and uncertainty propagation as well as Monte Carlo simulations. The results show that, when Weibull moduli are obtained from strength variations with fiber area, inordinately large sample sizes (>1000) are required to achieve reliable results. In contrast, Weibull moduli can be accurately estimated from the dependence of average fiber strength on gauge length for a modest sample size at each of two gauge lengths, provided the gauge length range is sufficiently large. The dependence of strengths of bundles containing many (ca. 500) fibers on gauge length yields yet more reliable results. The results are used to assess the fidelity of Weibull moduli obtained from these methods and provide guidance for preferred test methods.     

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.     

Effects of SiC content on the densification,microstructure, and mechanical properties of HfB2–SiC composites

To investigate the effects of SiC on microstructure, hardness, and fracture toughness, 0, 10, 20, and 30 vol% SiC were added to HfB₂ and sintered by SPS. Upon adding SiC to 30 vol%, relative density increased about 4%; but HfB₂ grain growth had a minimum at 20 vol% SiC. This may be due to grain boundary silicate glass, responsible for surface oxide wash out, enriched in SiO₂ with higher fraction of SiC. By SiO₂ enrichment, the glass viscosity increased and higher HfO₂ remained unsolved which subsequently lead to higher grain growth. Hardness has increased from about 13 to 15 GPa by SiC introduction with no sensible variation with SiC increase. Residual stress measurements by Rietveld method indicated high levels of tensile residual stresses in the HfB₂ Matrix. Despite the peak residual stress value at 20 vol% SiC, fracture toughness of this sample was the highest (6.43 MPa m^0.5) which implied that fracture toughness is mainly a grain size function. Tracking crack trajectory showed a mainly trans-granular fracture, but grain boundaries imposed a partial deflection on the crack pathway. SiC had a higher percentage in fracture surface images than the cross-section which implied a weak crack deflection.     

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.     

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.     

Ceramics Reliability: Statistical Analysis of Multiaxial Failure Using the Weibull Approach and the Multiaxial Elemental Strength Model

Methodology for designing reliable ceramic components requires a precise evaluation and correlation of strengths in different stress states. The present paper compares the merits of the Weibull approach and the multiaxial elemental strength model on an experimental case involving mixed-mode failure in the presence of bimodal flaw populations (surface and volume flaws). The experimental data were obtained using flexure specimens of Si₃N₄ tested at various spans, with the purpose of enhancing shearing effects. The analysis of data was refined by developing an advanced postprocessor program to finite-element codes for failure probability determination based upon the Barnett-Freudenthal approximation of the Weibull approach and the multiaxial elemental strength model. In a second step, the strengths of the specimens exhibiting failures from the two concurrent populations of flaws (intermediate span) were predicted using both approaches from data obtained with different span lengths (long and short spans). Comparison with experimental data showed that the multiaxial elemental strength model is an improvement over the Weibull approach. It also allowed the short-span bending test to be assessed. Finally, important implications for structural design with ceramics are discussed.     

Efficient multiscale strategy for toughening HfB2 ceramics: A heterogeneous ceramic–metal layered architecture

A multiscale structural design was innovatively adopted herein to increase the toughness of monolithic HfB₂ ceramics. SiC whiskers (SiC_w) and graphene oxide (GO) were used as fillers for the HfB₂ matrix, whereas a ductile W foil was introduced as an interlayer to synthesize laminated HfB₂-SiC_w-rGO/W ceramics. Monolithic HfB₂-SiC_p (particulate) and laminated HfB₂-SiC_p/W ceramics were prepared using the same routes and used as controls. Following tape casting and spark plasma sintering at 1800°C, the toughness of the prepared laminated HfB₂-SiC_w-rGO/W samples was increased to 14.2 ± 0.6 MPa·m^1/2, with minimal sacrifice in flexural strength (421 ± 16 MPa). Morphological analysis of the fracture surface revealed the synergistic effects of micro-toughening (including bridging and pullout of whiskers and rGO) and macro-toughening (including crack deflection, bifurcation, and delamination) mechanisms responsible for improving the fracture toughness of the laminated HfB₂-SiC_w-rGO/W composites.