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Material Science > Chapter 12 > Flashcards

Chapter 12 Flashcards

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1
Q

What is the purpose of experimentally determined phase diagrams for ceramic systems?

A

To understand the behavior of ceramic systems under different compositions and temperatures.

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2
Q

What is frequently the case for binary or two-component phase diagrams in ceramic systems?

A

The two components are compounds that share a common element, often oxygen.

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3
Q

What type of solid solution is the Al2O3–Cr2O3 solid solution?

A

A substitutional solid solution in which Al3+ substitutes for Cr3+ and vice versa.

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4
Q

What regions are present in the aluminum oxide–chromium oxide phase diagram?

A

Single liquid-phase and single solid-phase regions separated by a two-phase solid–liquid region.

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5
Q

How is the diffusion data for ionic solids often obtained?

A

From electrical conductivity measurements.

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6
Q

What is the compound formed by aluminum oxide and chromium oxide?

A

Spinel, with the chemical formula MgAl2O4.

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7
Q

What is the composition of spinel in terms of mol% Al2O3 and MgO?

A

50 mol% Al2O3 - 50 mol% MgO (or 72 wt% Al2O3 - 28 wt% MgO).

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8
Q

Why is spinel considered nonstoichiometric?

A

It is nonstoichiometric for compositions other than 50 mol% Al2O3 - 50 mol% MgO.

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9
Q

What is the reason for limited solubility of Al2O3 in MgO?

A

Differences in charge and radii of the Mg2+ and Al3+ ions.

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10
Q

Why is MgO virtually insoluble in Al2O3?

A

Due to the differences in charge and radii of the Mg2+ and Al3+ ions.

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11
Q

At what temperature does stoichiometric spinel melt congruently?

A

About 2100°C (3800°F).

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12
Q

What compound forms at about 31 wt% CaO in the zirconium oxide-calcium oxide system?

A

CaZrO3.

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13
Q

How many eutectics and eutectoid reactions are found in the zirconium oxide-calcium oxide system?

A

One eutectic and two eutectoid reactions.

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14
Q

What are the three different crystal structures of ZrO2 phases in the system?

A

Tetragonal, monoclinic, and cubic.

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15
Q

At what temperature does pure ZrO2 experience a tetragonal-to-monoclinic phase transformation?

A

About 1150°C (2102°F).

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16
Q

How is the problem of crack formation in zirconia overcome?

A

By ‘stabilizing’ the zirconia with 3-7 wt% CaO.

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17
Q

What is the term for a zirconia material with a calcia content within the range of 3-7 wt%?

A

Partially stabilized zirconia (PSZ).

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18
Q

Which other elements are used as stabilizing agents for zirconia?

A

Yttrium oxide (Y2O3) and magnesium oxide.

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19
Q

What is the significance of the silica-alumina system commercially?

A

It is important because the principal constituents of many ceramic refractories are silica and alumina.

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20
Q

What is the unit cell for cristobalite?

A

The unit cell for cristobalite is shown in Figure 12.10.

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21
Q

What compound exists as a narrow phase field in the silica-alumina phase diagram?

A

Mullite, 3Al2O3–2SiO2, exists as a narrow phase field in the phase diagram.

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22
Q

At what temperature does mullite melt incongruently?

A

Mullite melts incongruently at 1890°C (3435°F).

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23
Q

What are the prime constituents for refractory ceramic materials?

A

Silica and alumina are the prime constituents for refractory ceramic materials.

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24
Q

What was the principal drawback of ceramic materials prior to the Bronze Age?

A

Their principal drawback was a disposition to catastrophic fracture in a brittle manner with very little energy absorption.

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25
Q

What is the characteristic of most ceramic materials at room temperature?

A

They almost always fracture before any plastic deformation can occur in response to an applied tensile load.

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26
Q

What does the brittle fracture process in ceramics consist of?

A

The formation and propagation of cracks through the cross section of material in a direction perpendicular to the applied load.

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27
Q

What are stress raisers in ceramic materials?

A

Very small and omnipresent flaws in the material that serve as points at which the magnitude of an applied tensile stress is amplified.

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28
Q

How is the measure of a ceramic material’s ability to resist fracture specified?

A

In terms of fracture toughness.

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29
Q

What is the plane strain fracture toughness for ceramic materials?

A

It is defined as KIc and is typically smaller than for metals, often below 10 MPa 1 m (9 ksi 1 in.).

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30
Q

What is the phenomenon called when fracture of ceramic materials occurs by the slow propagation of cracks under static stresses?

A

Static fatigue or delayed fracture.

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31
Q

What environmental conditions can affect the phenomenon of static fatigue in ceramic materials?

A

Moisture in the atmosphere.

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32
Q

What is the phenomenon that explains the variation in fracture strength for many specimens of a specific brittle ceramic material?

A

The dependence of fracture strength on the probability of the existence of a flaw that is capable of initiating a crack.

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33
Q

Why do brittle ceramics display much higher strengths in compression than in tension?

A

For compressive stresses, there is no stress amplification associated with any existent flaws.

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34
Q

How can the fracture strength of a brittle ceramic be enhanced dramatically?

A

By imposing residual compressive stresses at its surface, which may be accomplished by thermal tempering.

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35
Q

What is the purpose of statistical theories developed in conjunction with experimental data for brittle ceramic materials?

A

To determine the risk of fracture for a given material.

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36
Q

What does a failure analysis of ceramics focus on?

A

Determination of the location, type, and source of the fracture.

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37
Q

What is a fractographic study in ceramics analysis?

A

It involves examining the path of crack propagation and microscopic features of the fracture surface.

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38
Q

What equipment is used for conducting a fractographic study?

A

Simple and inexpensive equipment such as a magnifying glass, low-power stereo binocular optical microscope, and scanning electron microscope.

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39
Q

What is the critical velocity for crack acceleration in glass?

A

Approximately one-half of the speed of sound.

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40
Q

What happens when a crack reaches the critical velocity in ceramics?

A

It may branch or bifurcate, and this process may be successively repeated until a family of cracks is produced.

41
Q

How does the rate of crack acceleration change with stress level in ceramics?

A

It increases with increasing stress level, and correspondingly, the degree of branching also increases.

42
Q

What distinctive features are produced on the fracture surface during crack propagation in ceramics?

A

The interactions with the microstructure, stress, and elastic waves generate distinctive features on the fracture surface.

43
Q

What happens to the crack surface when the crack reaches its critical velocity?

A

It begins to branch and changes propagation direction.

44
Q

What are the two surface features formed when the crack surface changes on a microscopic scale?

A

Mist and hackle.

45
Q

What is the mist region on the fracture surface of a brittle ceramic?

A

A faint annular region just outside the mirror.

46
Q

What is the hackle region on the fracture surface of a brittle ceramic?

A

A region noted outside the mist region.

47
Q

What is the origin region on the fracture surface of a brittle ceramic?

A

The point where the failure starts.

48
Q

What is the hackle composed of?

A

A set of striations or lines that radiate away from the crack source in the direction of crack propagation.

49
Q

What does the mirror radius (r_m) measure?

A

The acceleration rate of a newly formed crack.

50
Q

What does the mirror radius indicate about fracture stress level?

A

As fracture stress level increases, the mirror radius decreases.

51
Q

What are Wallner lines and what information do they provide?

A

Arc-shaped surface features that provide information regarding stress distributions and directions of crack propagation.

52
Q

Why is a tensile test not usually used to ascertain the stress–strain behavior of brittle ceramics?

A

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.

53
Q

What test is most frequently used to ascertain the stress–strain behavior of brittle ceramics?

A

A transverse bending test using a three- or four-point loading technique.

54
Q

What is the flexural strength also known as?

A

Modulus of rupture, fracture strength, or bend strength.

55
Q

How is the flexural strength (s_fs) calculated for a specimen with a rectangular cross section?

A

s_fs = (3FfL) / (2bd^2)

56
Q

What is the formula for flexural strength for a specimen with a circular cross-section?

A

sfs = FfL / (pR^3) where R is the specimen radius.

57
Q

What factors does flexural strength depend on?

A

Flexural strength depends on specimen size and volume exposed to a tensile stress.

58
Q

What is the magnitude of flexural strength for a specific ceramic material compared to its fracture strength from a tensile test?

A

Greater.

59
Q

What is the reason for the greater flexural strength of a specific ceramic material compared to its fracture strength from a tensile test?

A

Only some volume fraction of a flexural specimen is subjected to tensile stresses.

60
Q

What is the elastic stress-strain behavior for ceramic materials similar to?

A

Tensile test results for metals.

61
Q

What is the range of moduli of elasticity for ceramic materials?

A

Between about 70 and 500 GPa (10^6 and 70^6 psi).

62
Q

How does plastic deformation occur in crystalline ceramics?

A

By the motion of dislocations.

63
Q

What is the predominant bonding type in ceramics?

A

Predominantly ionic.

64
Q

Why is plastic deformation rarely measurable at room temperature in ceramics?

A

Due to restricted slip systems caused by electrostatic repulsion of like-charged ions.

65
Q

Why is slip difficult in ceramics with highly covalent bonding?

A

Due to relatively strong covalent bonds, limited slip systems, and complex dislocation structures.

66
Q

How do noncrystalline ceramics deform?

A

By viscous flow, similar to the deformation of liquids.

67
Q

What is the characteristic property for viscous flow?

A

Viscosity, which is a measure of a noncrystalline material’s resistance to deformation.

68
Q

What happens to the viscosity of glasses as the temperature is raised?

A

It decreases as the magnitude of the bonding is diminished, facilitating the sliding motion or flow of the atoms or ions.

69
Q

What is the influence of porosity on the elastic properties and strength of ceramic materials?

A

Residual porosity has a deleterious influence on both the elastic properties and strength.

70
Q

How does the modulus of elasticity (E) change with volume fraction porosity (P) for some ceramic materials?

A

The modulus of elasticity (E) decreases with volume fraction porosity (P) according to the equation E = E0 (1 - 1.9P + 0.9P^2).

71
Q

What are the two reasons that porosity is deleterious to the flexural strength of ceramic materials?

A

Pores reduce the cross-sectional area across which a load is applied, and they act as stress concentrators.

72
Q

How does the flexural strength (sfs) change with volume fraction porosity (P) for ceramic materials?

A

The flexural strength (sfs) decreases exponentially with volume fraction porosity (P) according to the equation sfs = s0 exp(-nP).

73
Q

Why are accurate hardness measurements difficult to conduct for ceramic materials?

A

Ceramic materials are brittle and highly susceptible to cracking when indenters are forced into them.

74
Q

Why are spherical indenters not used for ceramic materials?

A

Because they produce severe cracking.

75
Q

What techniques are used to measure the hardness of ceramic materials?

A

Vickers and Knoop techniques using pyramidal indenters.

76
Q

Which technique is often preferred for measuring hardness of very brittle ceramic materials?

A

Knoop technique.

77
Q

What happens to the hardness of ceramics with increasing load?

A

It decreases but ultimately reaches a constant hardness plateau that is independent of load.

78
Q

What is the most desirable mechanical characteristic of ceramics?

A

Their hardness.

79
Q

What is the hardest known material group?

A

Ceramics.

80
Q

What is the influence of porosity on the flexural strength for aluminum oxide at room temperature?

A

The flexural strength decreases with increasing porosity.

81
Q

What are the Vickers hardness values for Diamond, Boron carbide, and Aluminum oxide?

A

130 GPa, 44.2 GPa, and 26.5 GPa respectively.

82
Q

What are the characteristics of interatomic bonding in ceramics?

A

It ranges from purely ionic to totally covalent.

83
Q

How is crystal structure determined in ceramics with predominantly ionic bonding?

A

By the charge magnitude on each ion and the radius of each type of ion.

84
Q

What are some of the simpler crystal structures described in terms of unit cells in ceramics?

A

Rock salt, Cesium chloride, Zinc blende, Fluorite, Perovskite.

85
Q

How is the theoretical density of a ceramic material computed?

A

Using Equation 12.1.

86
Q

How is the structure of silicates more conveniently represented?

A

In terms of interconnecting SiO4-4 tetrahedra.

87
Q

What are some examples of silicate ceramics?

A

Crystalline silica (SiO2), Layered silicates, Noncrystalline silica glasses.

88
Q

In what polymorphic forms may carbon exist?

A

Diamond, Graphite.

89
Q

What types of atomic point defects are possible in ceramics?

A

Interstitials and vacancies for each anion and cation type.

90
Q

Why do defects sometimes occur in pairs in ceramic materials?

A

To maintain charge neutrality.

91
Q

What is a stoichiometric ceramic?

A

A ceramic in which the ratio of cations to anions is exactly the same as predicted by the chemical formula.

92
Q

What are nonstoichiometric materials?

A

Materials possible in cases where one of the ions may exist in more than one ionic state.

93
Q

How does diffusion occur in ionic materials?

A

Normally occurs by a vacancy mechanism

94
Q

What are the general characteristics of ceramic phase diagrams?

A

Similar to those for metallic systems.

95
Q

What is the effect of microcracks on ceramic materials?

A

They result in amplification of applied tensile stresses and account for relatively low fracture strengths (flexural strengths).

96
Q

How is plastic deformation explained in crystalline ceramics?

A

It is a result of dislocation motion

97
Q

What is the mode of plastic deformation for noncrystalline materials?

A

It is by viscous flow

98
Q

What is the influence of residual porosity on ceramic bodies?

A

It is deleterious to both their moduli of elasticity and fracture strengths.

Material Science (3 decks)

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