CERAMIC PHASE DIAGRAMS These diagrams are especially useful in assessing the high temperature performance of ceramic materials.
Phase diagrams have been experimentally determined for a large number of ceramic systems. For binary or two-component phase diagrams, it is frequently the case that the two components are compounds that share a common element, often oxygen.
The Al2O3 – Cr2O3 System consists of single liquid and single solid phase regions separated by a two-phase solid–liquid region having the shape of a blade. A substitutionalsolid solutiononein which Al3+substitutes for Cr3+and vice versa.
The Al2O3 – Cr2O3 System Fig. 12.24
The MgO–Al2O3 System There exists an intermediate phase, or better, a compound called spinel, which has the chemical formula MgAl2O4 (or MgO– Al2O3 ). SPINEL Spinel is a distinct compound 50 mol% Al2O3–50 mol% MgO
The MgO–Al2O3 System Fig. 12.25
The ZrO2–CaO System The horizontal axis extends to only about 31 wt% CaO (50 mol% CaO), at which composition the compound CaZrO3 forms. one eutectic and two eutectoid reactions are found for this system.
It may also be observed that ZrO2 phases having three different crystal structures exist in this system—namely, tetragonal, monoclinic, and cubic. A relatively large volume change accompanies this transformation, resulting in the formation of cracks
This problem is overcome by “stabilizing” the zirconia. adding between about 3 and 7 wt% CaO. Partially stabilized zirconia, or PSZ Yttrium oxide (Y2O3 ) and magnesium oxide are also used as stabilizing agents.
The ZrO2–CaO System Fig. 12.26
The SiO2–Al2O3 System Commercially, the silica–alumina system is an important one since the principal constituents of many ceramic refractories are these two materials.
cristobalite polymorphic form of silica that is stable mullite 3Al2O3–2SiO2 a rare silicate mineral
The SiO2–Al2O3 System Fig. 12.27
Mechanical Properties limited in applicability by their mechanical properties. inferior to those of metals. The principal drawback is a disposition to catastrophic fracture in a brittle manner with very little energy absorption.
BRITTLE FRACTURE OF CERAMICS At room temperature, virtually all ceramics are brittle.
Microcracks Its presence results in amplification of applied tensile stresses and accounts for relatively low fracture strengths (flexural strength).
At room temperature, both crystalline and noncrystalline ceramics almost always fracture before any plastic deformation can occur in response to an applied tensile load. Crack growth in crystalline ceramics may be either transgranularor intergranular
The measured fracture strengths of ceramic materials are substantially lower than predicted by theory from interatomic bonding forces. The measure of a ceramic material’s ability to resist fracture when a crack is present is specified in terms of fracture toughness.
very small and omnipresent flaws in the material stress raisers may be minute surface or interior cracks (microcracks), internal pores, and grain corners, which are virtually impossible to eliminate or control.
static fatigue, or delayed fracture fracture of ceramic materials will occur by the slow propagation of cracks, when stresses are static in nature this type of fracture is especially sensitive to environmental conditions, specifically when moisture is present in the atmosphere.
FRACTOGRAPHY OF CERAMICS A fractographic study involves examining the path of crack propagation as well as microscopic features of the fracture surface.
during propagation, a crack accelerates until a critical velocity is achieved; for glass, this critical value is approximately one-half of the speed of sound. Upon reaching this critical velocity, a crack may branch, a process that may be successively repeated until a family of cracks is produced.
mirror region - The crack surface that formed during the initial acceleration stage of propagation mist region- a faint annular region just outside the mirror hackle region- a set of striations or lines that radiate away from the crack source in the direction of crack propagation
STRESS-STRAIN BEHAVIOR The stress–strain behaviorof brittle ceramics is not usually ascertained by a tensile test.
First, it is difficult to prepare and test specimens having the required geometry.
Second, it is difficult to grip brittle materials without fracturing them;
Third, ceramics fail after only about 0.1% strain, which necessitates that tensile specimens be perfectly aligned to avoid the presence of bending stresses, which are not easily calculated.
Flexural Strength The stress at fracture using flexure test.
Elastic Behavior a linear relationship exists between stress and strain.
MECHANISMS OF PLASTIC DEFORMATION Any plastic deformation of crystalline ceramics is a result of dislocation motion; the brittleness of these materials is explained, by the limited number of slip systems.
Crystalline Ceramics plastic deformation occurs, as with metals, by the motion of dislocations. One reason for the hardness and brittleness of these materials is the difficulty of slip (or dislocation motion).
Non-crystalline Ceramics These materials deform by viscous flow, the same manner in which liquids deform; the rate of deformation is proportional to the applied stress.
Viscosity a measure of a noncrystalline material’s resistance to deformation.
For viscous flow in a liquid that originates from shear stresses imposed by two flat and parallel plates, the viscosity is the ratio of the applied shear stress and the change in velocity dvwith distance dyin a direction perpendicular to and away from the plates
The units for viscosity are: Poises (P): 1P = 1 dyne-s/cm2 and Pascal-seconds (Pa-s): 1 Pa-s = N-s/m2 Conversion from one system of units to the other is according to: 10P = 1Pa-s
MISCELLANEOUS MECHANICAL CONSIDERATIONS
Influence of Porosity As will be discussed in chapter 13, the precursor material is in the form of a powder. Subjected to compaction for formation of desired shape, pores will exist between the powder particles. During heat treatment, much of this porosity will be eliminated; however, it is often the case that this pore elimination process is incomplete and some residual porosity will remain.
Any residual porosity will have a deleterious influence on both the elastic properties andstrength. For example, it has been observed for some ceramic materials that the magnitude of the modulus of elasticity E decreases with volume fraction porosity.
The influence of porosity on the modulus of elasticity for aluminum oxide at room temperature.
The influence of porosity on the flexural strength for aluminum oxide at room temperature.
Hardness One beneficial mechanical property of ceramics is their hardness, which is often utilized when an abrasive or grinding action is required; in fact, the hardest known materials are ceramics.
A listing of a number of different ceramic materials according to Knoop hardness is contained in Table 12.6. Only ceramics having Knoophardnesses of about 1000 or greater are utilized for their abrasive characteristics.
Creep Often ceramic materials experience creep deformation as a result of exposure to stresses (usually compressive) at elevated temperatures.
In general, the time–deformation creep behavior of ceramics is similar to that of metals; however, creep occurs at higher temperatures in ceramics. High-temperature compressive creep tests are conducted on ceramic materials to ascertain creep deformation as a function of temperature and stress level.