Chapter 12 Flashcards
What is the purpose of experimentally determined phase diagrams for ceramic systems?
To understand the behavior of ceramic systems under different compositions and temperatures.
What is frequently the case for binary or two-component phase diagrams in ceramic systems?
The two components are compounds that share a common element, often oxygen.
What type of solid solution is the Al2O3–Cr2O3 solid solution?
A substitutional solid solution in which Al3+ substitutes for Cr3+ and vice versa.
What regions are present in the aluminum oxide–chromium oxide phase diagram?
Single liquid-phase and single solid-phase regions separated by a two-phase solid–liquid region.
How is the diffusion data for ionic solids often obtained?
From electrical conductivity measurements.
What is the compound formed by aluminum oxide and chromium oxide?
Spinel, with the chemical formula MgAl2O4.
What is the composition of spinel in terms of mol% Al2O3 and MgO?
50 mol% Al2O3 - 50 mol% MgO (or 72 wt% Al2O3 - 28 wt% MgO).
Why is spinel considered nonstoichiometric?
It is nonstoichiometric for compositions other than 50 mol% Al2O3 - 50 mol% MgO.
What is the reason for limited solubility of Al2O3 in MgO?
Differences in charge and radii of the Mg2+ and Al3+ ions.
Why is MgO virtually insoluble in Al2O3?
Due to the differences in charge and radii of the Mg2+ and Al3+ ions.
At what temperature does stoichiometric spinel melt congruently?
About 2100°C (3800°F).
What compound forms at about 31 wt% CaO in the zirconium oxide-calcium oxide system?
CaZrO3.
How many eutectics and eutectoid reactions are found in the zirconium oxide-calcium oxide system?
One eutectic and two eutectoid reactions.
What are the three different crystal structures of ZrO2 phases in the system?
Tetragonal, monoclinic, and cubic.
At what temperature does pure ZrO2 experience a tetragonal-to-monoclinic phase transformation?
About 1150°C (2102°F).
How is the problem of crack formation in zirconia overcome?
By ‘stabilizing’ the zirconia with 3-7 wt% CaO.
What is the term for a zirconia material with a calcia content within the range of 3-7 wt%?
Partially stabilized zirconia (PSZ).
Which other elements are used as stabilizing agents for zirconia?
Yttrium oxide (Y2O3) and magnesium oxide.
What is the significance of the silica-alumina system commercially?
It is important because the principal constituents of many ceramic refractories are silica and alumina.
What is the unit cell for cristobalite?
The unit cell for cristobalite is shown in Figure 12.10.
What compound exists as a narrow phase field in the silica-alumina phase diagram?
Mullite, 3Al2O3–2SiO2, exists as a narrow phase field in the phase diagram.
At what temperature does mullite melt incongruently?
Mullite melts incongruently at 1890°C (3435°F).
What are the prime constituents for refractory ceramic materials?
Silica and alumina are the prime constituents for refractory ceramic materials.
What was the principal drawback of ceramic materials prior to the Bronze Age?
Their principal drawback was a disposition to catastrophic fracture in a brittle manner with very little energy absorption.
What is the characteristic of most ceramic materials at room temperature?
They almost always fracture before any plastic deformation can occur in response to an applied tensile load.
What does the brittle fracture process in ceramics consist of?
The formation and propagation of cracks through the cross section of material in a direction perpendicular to the applied load.
What are stress raisers in ceramic materials?
Very small and omnipresent flaws in the material that serve as points at which the magnitude of an applied tensile stress is amplified.
How is the measure of a ceramic material’s ability to resist fracture specified?
In terms of fracture toughness.
What is the plane strain fracture toughness for ceramic materials?
It is defined as KIc and is typically smaller than for metals, often below 10 MPa 1 m (9 ksi 1 in.).
What is the phenomenon called when fracture of ceramic materials occurs by the slow propagation of cracks under static stresses?
Static fatigue or delayed fracture.
What environmental conditions can affect the phenomenon of static fatigue in ceramic materials?
Moisture in the atmosphere.
What is the phenomenon that explains the variation in fracture strength for many specimens of a specific brittle ceramic material?
The dependence of fracture strength on the probability of the existence of a flaw that is capable of initiating a crack.
Why do brittle ceramics display much higher strengths in compression than in tension?
For compressive stresses, there is no stress amplification associated with any existent flaws.
How can the fracture strength of a brittle ceramic be enhanced dramatically?
By imposing residual compressive stresses at its surface, which may be accomplished by thermal tempering.
What is the purpose of statistical theories developed in conjunction with experimental data for brittle ceramic materials?
To determine the risk of fracture for a given material.
What does a failure analysis of ceramics focus on?
Determination of the location, type, and source of the fracture.
What is a fractographic study in ceramics analysis?
It involves examining the path of crack propagation and microscopic features of the fracture surface.
What equipment is used for conducting a fractographic study?
Simple and inexpensive equipment such as a magnifying glass, low-power stereo binocular optical microscope, and scanning electron microscope.
What is the critical velocity for crack acceleration in glass?
Approximately one-half of the speed of sound.
What happens when a crack reaches the critical velocity in ceramics?
It may branch or bifurcate, and this process may be successively repeated until a family of cracks is produced.
How does the rate of crack acceleration change with stress level in ceramics?
It increases with increasing stress level, and correspondingly, the degree of branching also increases.
What distinctive features are produced on the fracture surface during crack propagation in ceramics?
The interactions with the microstructure, stress, and elastic waves generate distinctive features on the fracture surface.
What happens to the crack surface when the crack reaches its critical velocity?
It begins to branch and changes propagation direction.
What are the two surface features formed when the crack surface changes on a microscopic scale?
Mist and hackle.
What is the mist region on the fracture surface of a brittle ceramic?
A faint annular region just outside the mirror.
What is the hackle region on the fracture surface of a brittle ceramic?
A region noted outside the mist region.
What is the origin region on the fracture surface of a brittle ceramic?
The point where the failure starts.
What is the hackle composed of?
A set of striations or lines that radiate away from the crack source in the direction of crack propagation.
What does the mirror radius (r_m) measure?
The acceleration rate of a newly formed crack.
What does the mirror radius indicate about fracture stress level?
As fracture stress level increases, the mirror radius decreases.
What are Wallner lines and what information do they provide?
Arc-shaped surface features that provide information regarding stress distributions and directions of crack propagation.
Why is a tensile test not usually used to ascertain the stress–strain behavior of brittle ceramics?
It is difficult to prepare and test specimens with the required geometry, difficult to grip brittle materials without fracturing them, and ceramics fail after only about 0.1% strain.
What test is most frequently used to ascertain the stress–strain behavior of brittle ceramics?
A transverse bending test using a three- or four-point loading technique.
What is the flexural strength also known as?
Modulus of rupture, fracture strength, or bend strength.
How is the flexural strength (s_fs) calculated for a specimen with a rectangular cross section?
s_fs = (3FfL) / (2bd^2)
What is the formula for flexural strength for a specimen with a circular cross-section?
sfs = FfL / (pR^3) where R is the specimen radius.
What factors does flexural strength depend on?
Flexural strength depends on specimen size and volume exposed to a tensile stress.
What is the magnitude of flexural strength for a specific ceramic material compared to its fracture strength from a tensile test?
Greater.
What is the reason for the greater flexural strength of a specific ceramic material compared to its fracture strength from a tensile test?
Only some volume fraction of a flexural specimen is subjected to tensile stresses.
What is the elastic stress-strain behavior for ceramic materials similar to?
Tensile test results for metals.
What is the range of moduli of elasticity for ceramic materials?
Between about 70 and 500 GPa (10^6 and 70^6 psi).
How does plastic deformation occur in crystalline ceramics?
By the motion of dislocations.
What is the predominant bonding type in ceramics?
Predominantly ionic.
Why is plastic deformation rarely measurable at room temperature in ceramics?
Due to restricted slip systems caused by electrostatic repulsion of like-charged ions.
Why is slip difficult in ceramics with highly covalent bonding?
Due to relatively strong covalent bonds, limited slip systems, and complex dislocation structures.
How do noncrystalline ceramics deform?
By viscous flow, similar to the deformation of liquids.
What is the characteristic property for viscous flow?
Viscosity, which is a measure of a noncrystalline material’s resistance to deformation.
What happens to the viscosity of glasses as the temperature is raised?
It decreases as the magnitude of the bonding is diminished, facilitating the sliding motion or flow of the atoms or ions.
What is the influence of porosity on the elastic properties and strength of ceramic materials?
Residual porosity has a deleterious influence on both the elastic properties and strength.
How does the modulus of elasticity (E) change with volume fraction porosity (P) for some ceramic materials?
The modulus of elasticity (E) decreases with volume fraction porosity (P) according to the equation E = E0 (1 - 1.9P + 0.9P^2).
What are the two reasons that porosity is deleterious to the flexural strength of ceramic materials?
Pores reduce the cross-sectional area across which a load is applied, and they act as stress concentrators.
How does the flexural strength (sfs) change with volume fraction porosity (P) for ceramic materials?
The flexural strength (sfs) decreases exponentially with volume fraction porosity (P) according to the equation sfs = s0 exp(-nP).
Why are accurate hardness measurements difficult to conduct for ceramic materials?
Ceramic materials are brittle and highly susceptible to cracking when indenters are forced into them.
Why are spherical indenters not used for ceramic materials?
Because they produce severe cracking.
What techniques are used to measure the hardness of ceramic materials?
Vickers and Knoop techniques using pyramidal indenters.
Which technique is often preferred for measuring hardness of very brittle ceramic materials?
Knoop technique.
What happens to the hardness of ceramics with increasing load?
It decreases but ultimately reaches a constant hardness plateau that is independent of load.
What is the most desirable mechanical characteristic of ceramics?
Their hardness.
What is the hardest known material group?
Ceramics.
What is the influence of porosity on the flexural strength for aluminum oxide at room temperature?
The flexural strength decreases with increasing porosity.
What are the Vickers hardness values for Diamond, Boron carbide, and Aluminum oxide?
130 GPa, 44.2 GPa, and 26.5 GPa respectively.
What are the characteristics of interatomic bonding in ceramics?
It ranges from purely ionic to totally covalent.
How is crystal structure determined in ceramics with predominantly ionic bonding?
By the charge magnitude on each ion and the radius of each type of ion.
What are some of the simpler crystal structures described in terms of unit cells in ceramics?
Rock salt, Cesium chloride, Zinc blende, Fluorite, Perovskite.
How is the theoretical density of a ceramic material computed?
Using Equation 12.1.
How is the structure of silicates more conveniently represented?
In terms of interconnecting SiO4-4 tetrahedra.
What are some examples of silicate ceramics?
Crystalline silica (SiO2), Layered silicates, Noncrystalline silica glasses.
In what polymorphic forms may carbon exist?
Diamond, Graphite.
What types of atomic point defects are possible in ceramics?
Interstitials and vacancies for each anion and cation type.
Why do defects sometimes occur in pairs in ceramic materials?
To maintain charge neutrality.
What is a stoichiometric ceramic?
A ceramic in which the ratio of cations to anions is exactly the same as predicted by the chemical formula.
What are nonstoichiometric materials?
Materials possible in cases where one of the ions may exist in more than one ionic state.
How does diffusion occur in ionic materials?
Normally occurs by a vacancy mechanism
What are the general characteristics of ceramic phase diagrams?
Similar to those for metallic systems.
What is the effect of microcracks on ceramic materials?
They result in amplification of applied tensile stresses and account for relatively low fracture strengths (flexural strengths).
How is plastic deformation explained in crystalline ceramics?
It is a result of dislocation motion
What is the mode of plastic deformation for noncrystalline materials?
It is by viscous flow
What is the influence of residual porosity on ceramic bodies?
It is deleterious to both their moduli of elasticity and fracture strengths.
