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2017 | Book

What Is Creep? Read chapter Read first chapter

Creep in Ceramics

Author: Joshua Pelleg

Publisher: Springer International Publishing

Book Series : Solid Mechanics and Its Applications

Part of: Springer Professional "Wirtschaft+Technik" , Springer Professional "Technik"

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About this book

This textbook is one of its kind, since there are no other books on Creep in Ceramics. The book consist of two parts. In part A general knowledge of creep in ceramics is considered, while part B specifies creep in technologically important ceramics, namely creep in oxide ceramics, carnides and nitrides. While covering all relevant information regarding raw materials and characterization of creep in ceramics, the book also summarizes most recent innovations and developments in this field as a result of extensive literature search.

Table of Contents

Frontmatter

Basics

Frontmatter
Chapter 1. What Is Creep?
Abstract
The concept of creep as presented originally by Andrade is discussed in this chapter. The conventional three stages of creep and the relevant equations are indicated. Creep rate as a function of the important parameters shown in the equation below summarizes the effects
$$ \dot{\varepsilon } = f\left( {\sigma ,t,T} \right) $$
of stress, time, and temperature. Creep at some stress and temperature is time dependent.
Joshua Pelleg
Chapter 2. General Mechanisms of Creep
Abstract
The general creep mechanism is discussed in this chapter, which is classified as: (i) dislocation slip; (ii) climb; (iii) grain-boundary sliding; and (iv) diffusion flow caused by vacancies. The relevant relations and illustrations are included. These provide the basic understanding of creep.
Joshua Pelleg
Chapter 3. Creep and Its Relation to Diffusion
Abstract
Creep occurs at some temperature and thus as being thermally activated is associated with diffusion. Almost every creep equation incorporates a diffusion coefficient in the relation. Creep can occur in grain boundaries also (Coble creep) and therefore lattice- or grain-boundary diffusion coefficients are indicated in the relations depending on the main creep involved in the process.
Joshua Pelleg
Chapter 4. Creep in Ceramics
Abstract
Creep rate in ceramics is smaller than in metals but experimental evidence indicates similar creep behavior between them. Metals and ceramics exhibit diffusion creep with n = 1 at low stresses and n ~ >3 at high stresses. Creep, both in metals and ceramics in the steady state, is diffusion controlled and the homologue temperature of T/T m , regarding the diffusion coefficient, also applies to both materials. One of the reasons for the smaller creep rate is related to the diffusion rate, which is smaller in ceramics than in metals at the same homologue temperature. The diffusion-controlled creep rate in ceramics has a higher activation energy, Q, than in metals causing a slower creep rate. There is an advantage of using single crystal due to the absence of grain boundaries. Further, one should choose
(i)
materials with high melting points;
 
(ii)
the use of strongly bonded ceramics; and
 
(iii)
alloying for dislocation pinning.
 
Joshua Pelleg
Chapter 5. Testing Methods for Creep
Abstract
The major techniques for collecting creep data are as follows:
(a)
tensile creep testing;
 
(b)
compressive creep testing;
 
(c)
flexural (bend) testing for creep; and
 
(d)
impression (hardness) creep testing.
 
The results of the creep test are plotted as strain versus time to obtain a curve characterizing it. Often a creep power law relates creep strain to the applied stress. In ceramics a creep test at constant load and temperature is generally performed at prolonged times, because of the bond character in ceramics. Creep tests are often performed by compressive loading to eliminate growth of cavities and their opening and thus creep rate is slower compared to tension. In ceramics which are inherently brittle flexural tests are preferable since machining of test specimens is difficult and also because the tendency to break in the grips when test is performed by tension.
Joshua Pelleg
Chapter 6. Creep in Nanoceramics
Abstract
Modern interest in nanosized specimens is a result of their possessing outstanding strength, superior hardness, and good fatigue resistance. This chapter is unique since no other book discusses the mechanical properties—including creep—of nanoceramics. All the tests performed in macro-sized material are used to evaluate the properties of nanosized ceramics. Thus results were collected by tension tests, compressive test, flexural, and hardness tests. The unique properties of nanoceramics are discussed in this chapter.
Joshua Pelleg
Chapter 7. Creep Rupture
Abstract
Creep—a time-dependent deformation—occurs below its yield strength usually at elevated temperatures. Creep terminates in rupture if steps are not taken to bring it to a halt. The purpose of creep rupture tests is to determine the time-to-failure. For such tests higher stresses are applied until the specimen fractures. The objectives of the tests are to determine the minimum creep rate at stage II creep and to evaluate the time at which failure sets in. Such information is essential so that the proper ceramics will be selected to eliminate failure during service and to assess the time period of safe use during high-temperature applications. Creep rupture (failure) is the objective of this chapter.
Joshua Pelleg
Chapter 8. Superplasticity
Abstract
Some materials are capable of undergoing large tensile extension of the order of hundreds and even thousands of percent. These superplastic materials are strong and ductile at low temperatures and exhibit high plasticity at high temperatures. They deform without showing necking. The large plastic extension enables fabrication into intricate shapes by simple forming process which is quite important in the case of ceramics where usually machining is a difficult process. A much investigated ceramics is the Y-TZP which can show an extension over ~100%. Structural considerations are grain size, grain boundaries and cavitation. Fine-grained ceramics can be superplastic at elevated temperatures.
Joshua Pelleg
Chapter 9. Creep and Recovery
Abstract
The rate of decrease in deformation after load removal, following a long application during a creep experiment is discussed in this chapter. Constant temperature must be maintained in such experiments in order to eliminate the contributions of thermal expansion (contraction). Monolithic ceramics typically do not possess the elevated-temperature toughness required for safety-critical designs. For this reason, a considerable amount of effort has been devoted to the development of ceramics reinforced with continuous fibers. Most of the ceramics for various applications are usually in composite forms (and contain various additives in specified amounts), depending on their use for a definite purpose. A knowledge of creep-strain recovery behavior may be used to increase the lifetimes of components subjected to sustained- and cyclic-creep load.
Joshua Pelleg
Chapter 10. Empirical Relations
Abstract
Stress rupture and creep life are particularly important for preventing catastrophic failure. The knowledge on this important subject is based on empirical concepts. Even Andrade’s creep relations are based on empirical observations. Several empirical relations are considered in this chapter. These are the Larson-Miller parametric method, the Monkman–Grant relationship, the Sherby-Dorn parametric method, the Orr-Sherby-Dorn approach, and the Manson-Haferd parameter. Appropriate relations and illustration of these empirical methods to evaluate creep life are the subject of this chapter.
Joshua Pelleg
Chapter 11. Design for Creep Resistance
Abstract
In the absence of theoretical fundamentals for creep, one has to rely on experimental creep data in order to develop creep-resistant alloys and design components for service-lifetime evaluation. New designs must be based on extensive experimentation performed on a variety of materials, while applying all manner of possible work conditions while keeping in mind that creep consists of three stages.
Joshua Pelleg

Creep in Technologically Important Ceramics

Frontmatter
Chapter 12. Creep in Alumina (Al2O3)
Abstract
Creep in alumina and alumina composites are discussed in this chapter. Relevant relations for creep specifically related to this technologically important ceramic are presented. Both polycrystalline and single crystal experimental details are considered. Creep tests in tension and compression are noted. Steady state creep rate is indicated and the parameters Q (activation energy), p (grain size exponent), and n (stress exponent) were evaluated.
Joshua Pelleg
Chapter 13. Creep in MgO
Abstract
MgO and MgO composite (MgO·Al2O3) are discussed in this chapter. The value of the stress exponent determined as n = 3.3 suggests a dislocation model as the rate-controlling creep mechanism. In the absence of glide the dislocation motion is that of climb, which is the rate-controlling mechanism. Knowledge of the structure is of great importance for understanding the creep deformation mechanism in the power law range. It is revealed that the typical dislocation structure of creep-deformed MgO is qualitatively very similar to that of creep-deformed metals and that the grains are divided into well-defined subgrains. Creep in polycrystalline and single-crystal MgO are considered in this chapter and the experiments were performed by tensile, compressive and flexural tests. Creep rupture, superplasticity, and nano-MgO are important sections of this chapter.
Joshua Pelleg
Chapter 14. Creep in ZrO2 Zirconia
Abstract
ZrO2 is a very refractory ceramic with excellent chemical inertness, corrosion resistance up to high temperatures and low thermal conductivity; it is also electrically conductive above ~600 °C. Due to all these properties, ZrO2 has a very broad range of applications. Both polycrystalline and single-crystal zirconia were subject to creep tests. Alloying zirconia (composites) intends to enhance mechanical strength and to improve its physical properties. Zirconia has to be stabilized for technological applications. One of the most frequently used stabilizers is yttria. Stabilized zirconia might be partial or fully stabilized depending on the quantity of the stabilizer. Also in this ceramics superplasticity was observed, a section is devoted to nano-zirconia.
Joshua Pelleg
Chapter 15. Creep in Silicon Carbide (SiC)
Abstract
Silicon carbide (SiC) is a technologically important ceramic, due to its high hardness, optical properties, and thermal conductivity. The high strength of SiC is a consequence of the strong covalent bonds (similar to diamonds) providing resistance to high pressures. The properties of SiC, which are similar to those of diamonds, have opened the gem industry to this material for use as a possible diamond substitute. However, a very important application of SiC is in microelectromechanical systems (MEMS), such as in wide-band gap semiconductors and power semiconductors, due its inherent strength and durability. Creep in polycrystalline and single-crystal SiC is the subject of this chapter. SiC is reinforced with fibers among them SiC fibers. Creep rupture is evaluated in a section devoted to this subject. Superplasticity observed in SiC is also discussed.
Joshua Pelleg
Chapter 16. Creep in Boron Carbide (B4C)
Abstract
Boron carbide is an excellent choice for high-temperature applications because of its properties. These are high hardness, high elastic modulus high thermal conductivity at room temperature, low thermal expansion, electrical conductivity, and its very high melting point (2447 °C). Moreover, it has a large neutron-capture cross section (∼4000 barn), which makes B4C a possible candidate for use in nuclear reactor components. Despite all these properties, it is puzzling that investigations of creep are lacking in the case of B4C. A climb-glide power-law creep model is one concept regarding the mechanism. The density of dislocations and the presence of pileups support this creep model in B4C. It was also suggested that vacancy diffusion model is operating during B4C creep.
Joshua Pelleg
Chapter 17. Creep in Silicon Nitride (Si3N4)
Abstract
Si3N4 has a wide range of applications, such as in: automotive parts; mechanical bearings; high-temperature/thermal-shock-resistant ceramics; orthopedic solutions; metalworking tools; electronic insulators; and diffusion barriers in integrated circuits, to name just a few. The mechanical properties of Si3N4 play important roles in modern technology and industry. The evaluation of creep in Si3N4, is one of the subjects of this chapter performed by tensile and compressive creep tests in polycrystalline Si3N4. SiC-based composites were investigated by flexural and tensile loading of the specimens. A considerable discussion is devoted to cavitation in silicon nitride. Sections of superplasticity, nanosize silicon nitride and stress rupture are integral parts of this chapter. The final section deals with creep recovery in Si3N4.
Joshua Pelleg
Backmatter
Metadata
Title
Creep in Ceramics
Author
Joshua Pelleg
Copyright Year
2017
Electronic ISBN
978-3-319-50826-9
Print ISBN
978-3-319-50825-2
DOI
https://doi.org/10.1007/978-3-319-50826-9

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