Toughness Of Ceramics Research Articles

Fracture and Deformation Damage Accumulation in Tough Ceramics

Fracture and Deformation Damage Accumulation in Tough Ceramics

Comparison of three fracture toughness testing techniques using a dental glass and a dental ceramic

Objectives. Various methods aimed at determining the fracture toughness of ceramics in mode I ( K Ic ) have been described in the literature. The accuracy, scatter and the interexaminer reproducibility of K Ic depend strongly on the procedural approach, the test parameters used and the conditioning of the specimens. The purpose of the present study was to compare fracture toughness values obtained using two indentation methods as well as a newly established fracture mechanics test. Methods. The following methods for K Ic determination were applied: (1) indentation fracture (IF), (2) indentation strength (IS) and (3) the single-edge-V-notched-beam test (SEVNB). The materials tested were a low-fusing dental glass (Duceram LFC) and a feldspar-based porcelain (IPS classic). Data were compared by ANOVA and Tukey's multiple comparison test ( p≤0.05). Results. For both materials, K Ic coefficients of variation ranged between 10 and 14% for IF and 7 and 10% for IS. The IS technique demonstrated a load dependency for the IPS porcelain which was not observed when using the IF method. The SEVNB test provided consistent results with coefficients of variation between 1 and 3%. SEVNB toughness values for the IPS porcelain were in agreement with the IS technique. However, halfpenny shaped cracks were observed at the tip of the notch of all LFC specimens thus leading to underestimated K Ic values. Significance. The overall aim of this type of study is to select testing procedures that are as expedient and reliable as possible. This study has shown that all three methods agreed within 10%. However none of the procedures proved absolutely straightforward. Decision on which method to use should be based on a sound understanding of the conceptual limitations and technical difficulties inherent to each technique.

Model of Strength Degradation from Hertzian Contact Damage in Tough Ceramics

A model of strength degradation for ceramics subject to damage from contact with hard spheres is developed. Primary attention is focused on tough ceramics with heterogeneous microstructures which deform in a quasi‐plastic mode. Brief consideration is also given to ideally brittle ceramics which form classical ring cracks, as a comparative baseline. Strength vs indentation load data from two microstructurally controlled ceramics, silicon nitride and a micaceous glass‐ceramic, illustrate distinctive strength degradation responses: in fine‐grain (F) form, ideally brittle failure from ring cracks, with abrupt strength loss at the critical load for crack initiation followed by a slow falloff at increasing load; in coarse‐grain (C) form, failure from within the quasi‐plastic zone, with continuous strength loss beyond a load well above that for the onset of yield, and with even slower falloff. Failure in the latter materials occurs from contact‐induced microdamage flaws with two essential elements: an inner closed shear crack with frictional sliding faces (“shear fault”), which forms within the confining compression–shear contact field; an outer annular, kinked crack that initiates at the fault edges (“wing crack”), and that extends in tensile local mode. The critical fault–crack is modeled as a virtual crack, with the residual field from the inner fault stabilizing the net driving force on the outer wing crack during ensuing tensile loading. Finite element modeling is used to evaluate the nonlinear elastic–plastic contact fields, and to provide a relationship between residual shear fault stress and contact load. The model accounts for the essential qualitative and quantitative features of the strength–load data, with provision for catastrophic degradation at high fault densities and extreme loads by microcrack coalescence. The model also contains the ingredients for analysis of contact fatigue, via attrition of the frictional tractions on the residual fault.

Dispersibility of SiC Whiskers in Several Organic Solvents

As fundamental research to raise the strength and fracture toughness of ceramics, the dispersibility of SiC whiskers, W (A), and carbon-coated SiC whiskers, W (AC), in eight organic solvents was evaluated by measuring the zeta potential and sedimentation velocity. Ethanol and 2-propanol were selected from these organic solvents, and the dispersibility was studied in detail. The electric charge of W (A) was positive in ethanol and negative in 2-propanol. On the other hand, the charge of W (AC) was negative in both solvents. The charge of their whiskers was a minimum (negative) by the addition of 0.001-0.002g/l dispersant for W (A) and 0.0002-0.001g/l for W (AC). The dispersibility of W (AC) was worse than that of W (A) in ethanol. The dispersibility of both SiC whiskers was improved by the addition of the dispersant to ethanol and 2-propanol. It was estimated that the dispersibility of W (A) and W (AC) was considerably affected by steric hindrance of the polymer used as a dispersant.

Estimation of Thermal Shock Fracture Toughness of Si<sub>3</sub>N<sub>4</sub>

An infrared radiation heating method was proposed previously as a new technique for estimating thermal shock strength and thermal shock fracture toughness of ceramics using a thin-circular disk with or without an edge crack, heated by infrared rays at the center with a constant heat flux. In this paper, thermal shock fracture toughness was measured by the above method in four kinds of silicon nitride. Temperature distributions, thermal stresses and stress intensity factors for the specimens were also analyzed considering the temperature dependent thermal conductivity, thermal diffusivity and thermal expansion of the materials. The experimental results show that the highest value of thermal shock fracture toughness was observed in the material having the higher thermal conductivity, which was developed by growing the grains and decreasing the amount of the grain boundary phase.

A new approach for toughening of ceramics

Abstract A new approach for the toughening of ceramics was proposed and investigated, in which a piezoelectric secondary phase was incorporated into the ceramic matrix as toughening phase, and energy dissipation by the piezoelectric effect was suggested as a new toughening mechanism. In a BaTiO3 toughened Al2O3 system, fracture toughness was significantly enhanced, and KIC reached 5.1 MPa m 1 2 for a composition of 0.05BaTiO3/0.95Al2O3.

A review of issues in the fracture of interfacial ceramics and ceramic composites

This article reviews the fracture behavior of very fine grain ceramics and ceramics which contain fine grain inclusions. These are interfacial materials in the sense that interfaces make up a relatively large volume fraction of the total material, and may therefore significantly influence the properties. The theme is to examine established trends concerning fracture behavior and grain or inclusion size and then to evaluate the validity of extrapolating those trends to very small sizes. The first section examines possible toughening mechanisms in single phase ceramics with submicron grain sizes, including microcracking, crack tip plasticity due to grain boundary sliding and transformation toughening. There is little reason to expect that microcrack toughening or crack tip plasticity will occur in these very fine grain ceramics. The second section discusses the strength and fracture toughness of ceramics containing submicron inclusions of oxides or carbides. Possible reasons for enhancements in the fracture strength in these ‘nanocomposites’ are discussed.

Some remarks concerning scuffing of ceramics slid against steel

Scuffing of Cr 2C 3 ceramic coatings applied on steel surfaces, ground or polished, was investigated after sliding against a hard steel surface. Results in the form of diagrams for samples having different surface parameters are compared with one another and with those obtained by using the same apparatus for scuffing of steel/steel pairs. The contact loads were increased monotonously. Sliding velocity was constant at V h = 12 m s −1 until the scuffing caused the apparatus to come to a dead stop. The contact was lubricated with a gear lubricant Hipol EP-4 80W/90. The scuffing phenomena observed for steel/ceramic differ significantly from those observed for steel/steel pairs. The simple mechanism of scuffing observed for steel/steel contacts is replaced by a much more complicated process, beginning with a slow increase of traction forces, then transient scuffing phenomena occurred accompanied by dynamic incidents similar to stick-slip phenomena. Catastrophic destruction of the ceramic coatings, the result of the creation of inter-granular cracks when the Cr 2C 3 grains are pulled out from the surfaces with extraction of large aggregates of Cr 2C 3 grains as a result of the low fracture toughness of ceramics, was observed. Delamination of the ceramic coating from the substrate due to thermal stresses was also noticed.

Asymptotic analysis of plane-strain mode I steady-state crack growth in transformation toughening ceramics(II)

Based on a constitutive law which includes the shear components of transformation plasticity, the asymptotic solutions to near-tip fields of plane-strain mode I steadity propagating cracks in transformed ceramics are obtained for the case of linear isotropic hardening. The stress singularity, the distributions of stresses and velocities at the crack tip are determined for various material parameters. The factors influencing the near-tip fields are discussed in detail.

Comparative analysis of fracture toughness test methods for ceramics and crystals at room and lower temperatures

Flexural and indentation tests to estimate the fracture toughness of ceramics and crystals were compared. It was demonstrated that different methods can give reliable estimates in tests of homogeneous ceramics and quite ambiguous in tests of nonhomogeneous materials. Zirconia ceramics partially stabilized with magnesia were shown to exhibit a low-temperature inelastic-elastic transition in the temperature range of their fracture toughness variations.

Scaling of quasibrittle fracture: asymptotic analysis

Fracture of quasibrittle materials such as concrete, rock, ice, tough ceramics and various fibrous or particulate composites, exhibits complex size effects. An asymptotic theory of scaling governing these size effects is presented, while its extension to fractal cracks is left to a companion paper (1) which follows. The energy release from the structure is assumed to depend on its size , on the crack length, and on the material length governing the fracture process zone size. Based on the condition of energy balance during fracture propagation and the condition of stability limit under load control, the large-size and small-size asymptotic expansions of the size effect on the nominal strength of structure containing large cracks or notches are derived. It is shown that the form of the approximate size effect law previously deduced (2) by other arguments can be obtained from these expansions by asymptotic matching. This law represents a smooth transition from the case of no size effect, corresponding to plasticity, to the power law size effect of linear elastic fracture mechanics. The analysis is further extended to deduce the asymptotic expansion of the size effect for crack initiation in the boundary layer from a smooth surface of structure. Finally, a universal size effect law which approximately describes both failures at large cracks (or notches) and failures at crack initiation from a smooth surface is derived by matching the aforementioned three asymptotic expansions.

Differentiation between Pseudocubic and Cubic Phases in Y–ZrO2 Using Rietveld Analysis

The precise microstructural characterization of the yttriapartially stabilized zirconia (Y‐PSZ) system by means of X‐ray diffractometry (XRD) experiments faces a serious challenge: the discrimination beween the cubic, c, and pseudocubic, t', phases. We show how the confusion between the two phases goes beyond a simple qualitative problem, because the relative abundances for all the present polymorphs can be substantially modified. This is particularly true for the amount of transformable tetragonal phase, which is of paramount importance for the design of zirconia‐based tough ceramics. Similarly, we show that a conclusive discrimination between c and I' phases is possible on the basis of XRD experiments by a careful application of the Rietveld method and by emphasizing some subtle details around the angular domain close to the {200} planes. The procedure is applied to samples of Y‐PSZ (4.7 mol%).

Surface Topography and Mechanical Characteristics of Ceramics Roughened by Erosion

Surface topography and mechanical characteristics of ceramic substrates is very important for the thermal spraying for solid oxide fuel cells. Usually, surfaces are eroded by blasting before thermal spraying to in crease the adhesive strength of coatings. The erosion tests were performed for three kinds of commercial ceramics (Al2O3, machinable ceramics: MACOR, glass) with changing impingement velocity and time. The eroded surfaces were evaluated by average surface roughness (Ra), fractal dimension (F. D.) and wear volume based on Evans'theory. It was concluded that the relations between effective wear rate and eroded surface topography depend on the crack types, and fractal dimension increases with fracture toughness of ceramics.

Transformation criterion of phase transformation toughening ceramics with misoriented microcracks

Phase transformation criterion is the key to investigating the toughness of phase transformation ceramics. In this paper, the modified equivalent inclusion theory by the authors is employed to study the interaction between microcracking and transformation in ceramics. The transformation criterion is derived. The influence of microcracks and transformation particles on the critical transformation load is discussed.

Stress Analysis of Contact Deformation in Quasi‐Plastic Ceramics

A stress analysis is made of Hertzian contact deformation in relatively tough ceramics with heterogeneous microstructures, where the response is essentially quasi‐plastic rather than ideally elastic‐brittle. Contact data for two such heterogeneous ceramics, a micaceous glass‐ceramic with modest hardness and a silicon nitride with high hardness, are presented as illustrative cases. Data from a soft steel serve as a comparative baseline. Two distinctive aspects of the deformation response are explored: indentation stress‐strain nonlinearity; and size and shape of the damage zone. For the harder ceramics, the stress‐strain nonlinearity is less pronounced, and the quasi‐plastic zone is more tightly confined beneath the contact, than in traditional ductile metals. As in metals, the deformation process in the ceramic structures is essentially shear driven, but has its origin in microstructurally localized interfacial sliding faults rather than in dislocation slip. Finite element modeling (FEM) is used to compute the shear stress distributions beneath the spherical indenters for selected experimental loading conditions. The underlying basis of the FEM calculations is an elastic‐plastic constitutive relation based on a critical shear condition for yield, but incorporating a strain‐hardening characteristic to allow for local elastic constraints on the sliding shear faults. The FEM calculations are able to simulate the main features of the stress‐strain curves and the evolving deformation zone geometries. In addition, the calculated tensile stress distributions are able to account, at least in part, for the suppression of conventional brittle fracture tendencies in tougher ceramics.

Microstructural dependence of fracture energy and toughness of ceramics and ceramic composites versus that of their tensile strengths at 22 °C

The microstructural dependence of fracture energy and toughness of ceramics and ceramic particulate, platelet, and whisker composites is compared with the corresponding dependence of their tensile (flexure) strengths at 22 °C. These comparisons show that fracture energy and toughness often do not have the same porosity, or grainor particle-size dependence as strength. This is attributed to the scale of the cracks for measuring fracture energy or toughness often being too large in comparison to the cracks controlling strength. The large cracks reflect crack-microstructure interaction phenomena such as crack-wake bridging and R-curve effects that are not, or are much less, involved in the control of propagation of most strength-controlling cracks. Thus fracture mechanics must account for the scale of the cracks used in measuring fracture mechanics parameters relative to the scale of the cracks controlling the strength behaviour that is to be explained or predicted.

Biaxial flexural strength and indentation fracture toughness of three new dental core ceramics

The traditional gold and porcelain fused to metal crowns have been challenged by the esthetic all-ceramic crown materials. Only previous experience with poor mechanical properties, lack of standardized tooth preparation, and processing challenges have prevented universal acceptance of all-ceramic crowns. However, stronger and tougher ceramics and unique processing methods for ceramics have been developed in the past 20 years. In this study, three new ceramic crown core materials were tested to compare their biaxial flexural strength and indentation fracture toughness. Ten specimens of Empress, In-Ceram, and Procera AllCeram ceramics were prepared according to their manufacturers' recommendations. The results revealed significant differences in flexural strength for the three materials ( p ≤ 0.05). The average flexural strengths of AllCeram, In Ceram, and Empress ceramics were 687 MPa, 352 MPa, and 134 MPa respectively. There was no statistically significant difference between the fracture toughness of Procera ( 4.48 MPa · m 1 2 ) and In-Ceram ceramics ( 4.49 MPa · m 1 2 ); however, both ceramics had significantly higher fracture toughness ( p < 0.005) than Empress ceramic ( 1.74 MPa · m 1 2 ).

Orthorhombic zirconia phase in ZrO 2-MgAl 2O 4 composite material

Zirconia is one of the major components of modern ceramic materials. Those last two decades, a very large number of studies were devoted to zirconia based ceramics. Two classes of materials can be distinguished: Zirconia Toughened Ceramics (ZTC) and zirconia based alloys in the form of Partially Stabilized Zirconia (PSZ) or Tetragonal Zirconia Polycrystals (TZP). In the field of ZTC, zirconia toughened alumina (ZTA) is certainly the most important material. Magnesium aluminum spinel (MgAl{sub 2}O{sub 4}) has similar elastic properties as alumina and therefore can as well be toughened by zirconia adding. Among very few previous papers reporting on zirconia-spinel composites, N. Claussen and col. have published results on mechanical properties of such materials. Those products could yet exhibit specific microstructural and mechanical behavior, because of the magnesium readiness to form solid solutions as well with zirconia as with alumina. Until now very few studies have been devoted to microscopic analysis of such materials. The aim of this work is to investigate by transmission electron microscopy the presence of orthorhombic zirconia grains in spinel zirconia composites.

Zero-Brittleness Size-Effect Method for One-Size Fracture Test of Concrete

This paper proposes a new and less laborious version of the size-effect method for measuring the fracture energy or fracture toughness of concrete as well as other nonlinear fracture characteristics, such as the effective length of fracture process zone or critical crack-tip opening displacement. The size-effect method, based on the size-effect law, is the simplest to carry out because only the maximum loads of specimens need to be measured. No measurements of postpeak deflection and unloading stiffness, nor observations of crack-tip location, are needed; the testing machine need not be very stiff, and there is no need for closed-loop displacement control. The simplicity of the method makes it suitable not only for the laboratory but also for field quality control. In the original version of the size-effect method, notched specimens of different sizes are tested. The proposed new version further simplifies testing by allowing the use of notched specimens of only one size. The idea is to supplement the one-size notched-beam tests with an evaluation of the maximum load value for specimens with a zero-brittleness number. There are two types of methods that will achieve this. In one type of method, zero-brittleness data are obtained by using plastic limit analysis based on the modulus of rupture or compression strength of concrete to calculate the limiting nominal strength for zero specimen size. In the second type of method, zero-brittleness data are obtained by testing the maximum loads of notchless specimens of the same size. The former type leads to simpler calculations but has the drawback that the material strength to calculate the maximum load for zero size depends on specimen geometry. Both types of the zero-brittleness version of the size-effect method are validated by previously reported test data. The proposed method should also be applicable to other quasibrittle materials such as rock, ice, tough ceramics, and brittle composites.

An energy method to evaluate toughening in ceramics

An energy method to evaluate toughening in ceramics