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2014 | Buch

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Mechanical Properties of Ceramics

verfasst von: Joshua Pelleg

Verlag: Springer International Publishing

Buchreihe : Solid Mechanics and Its Applications

Enthalten in: Springer Professional "Wirtschaft+Technik" , Springer Professional "Technik" , Springer Professional "Wirtschaft"

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Über dieses Buch

This book discusses the mechanical properties of ceramics and aims to provide both a solid background for undergraduate students, as well as serving as a text to bring practicing engineers up to date with the latest developments in this topic so they can use and apply these to their actual engineering work.

Generally, ceramics are made by moistening a mixture of clays, casting it into desired shapes and then firing it to a high temperature, a process known as 'vitrification'. The relatively late development of metallurgy was contingent on the availability of ceramics and the know-how to mold them into the appropriate forms. Because of the characteristics of ceramics, they offer great advantages over metals in specific applications in which hardness, wear resistance and chemical stability at high temperatures are essential. Clearly, modern ceramics manufacturing has come a long way from the early clay-processing fabrication method, and the last two decades have seen the development of sophisticated techniques to produce a large variety of ceramic material.

The chapters of this volume are ordered to help students with their laboratory experiments and guide their observations in parallel with lectures based on the current text. Thus, the first chapter is devoted to mechanical testing. A chapter of ductile and superplastic ceramic is added to emphasize their role in modern ceramics (chapter 2). These are followed by the theoretical basis of the subject. Various aspects of the mechanical properties are discussed in the following chapters, among them, strengthening mechanisms, time dependent and cyclic deformation of ceramics. Many practical illustrations are provided representing various observations encountered in actual ceramic-structures of particularly technical significance. A comprehensive list of references at the end of each chapter is included in this textbook to provide a broad basis for further studying the subject. The work also contains a unique chapter on a topic not discussed in other textbooks on ceramics concerning nanosized ceramics.

This work will also be useful as a reference for materials scientists, not only to those who specialize in ceramics.

Inhaltsverzeichnis

Frontmatter

Chapter 1. Mechanical Testing of Ceramics

Abstract

This chapter considers the most common mechanical testing methods which are usually expected to be performed by students entering the first time into a lab. Tensile test-related parameters are evaluated. Very popular tests of ceramics are the various hardness tests (for example Vickers hardness test), which is not only a cost saving test, but also requires shorter times, since no specific specimen preparation, except of a smooth (often polished) surface is required. On small size specimens, Knoop hardness test is the general approach to obtain hardness data. Another accepted method of evaluating the mechanical properties of a ceramic is by a bending (flexural) test. The tests can be performed by three or four point bending tests. Compression tests are more popular than tension tests, since they tend to close pores, cracks and other flaws resulting in higher test results than by those obtained by tension, which tends to open rather than close cracks and microcracks. Toughness is an important criterion in ceramic properties (mechanical) evaluation. Because of the brittle nature of ceramics, special instrumented Charpy Impact Test machines were developed, primarily to evaluate the dynamic toughness of such materials. Creep and Fatigue tests are not included in this chapter and they will be evaluated in separate chapters. Because of the large scatter in the experimental results, Weibull statistical distribution is applied to obtain a mean value of the experimental results.

Joshua Pelleg

Chapter 2. Ductile Ceramics

Abstract

Not all ceramics are brittle at room temperature. There are some ceramics which are ductile at ambient temperatures. Such ceramics, for example are single crystals MgO, SrTiO₃, etc. They undergo plastic deformation and by dislocation motion slip lines are observed on the deformed specimens. In pure MgO at room temperature, dislocations are very mobile at comparatively low stresses. Changing the microstructure, possibly by alloying, the mobility of dislocations may be reduced and an increase in strength may be achieved. As usually observed, material undergoing plastic deformation tend to strain harden, a feature observed also in ductile ceramics. Of the several factors influencing the strength properties of ductile ceramics, grain size is outstanding. Fine grained ceramics are desirable. Originally brittle ceramics show elongation at high temperatures which is a usual observation. There is a transition temperature from brittle to ductile behavior which depends on the ceramics. One of the common methods to determine the brittle to transition temperature is by impact testing, and for this purpose various sophisticated machines have been developed. An extraordinary phenomenon related to ductility is superplasticity, where very high values of strains can be achieved before fracture. Superplastic ceramics are oxide (zirconia) or non-oxide ceramics. Well-known superplastic ceramics are SiC and FeC. The common feature of superplastic materials is the requirement of very fine grains, namely, in the nanosize range.

Joshua Pelleg

Chapter 3. Imperfections (Defects) in Ceramics

Abstract

The periodic nature of crystalline materials can be interrupted by imperfections. The relevant imperfection determining the mechanical properties of ceramics are point defects, or dislocations, or both. The major point defects considered in the chapter are vacancies and interstitials, which are responsible for some observed phenomena via diffusional exchange with atoms in their vicinity. One such process relates to climb which is an essential process in creep phenomena. Edge dislocations are involved in the climb process which occurs by leaving the glide plane, either in the positive, or the negative direction. Point defect-atom exchange by diffusion is the basic mechanism. Although one can talk about point defect hardening, the important defects that determine the mechanical properties of materials are line defects, commonly known as dislocations (edge or screw character). Their presence in crystals is essential, because of the orders of difference between the theoretical and actual strength of materials. The presence of dislocations makes deformation easier by the application of smaller stress than would be required in their absence. Conservative motion of dislocations occurs by slip, whereas non-conservative motion is associated with climb. The strengthening of material is a consequence of retarding the motion of dislocations, either by their intersection, or by particles of a second phase or by grain boundaries. Closely associated with dislocations are partial dislocations which usually produce stacking faults when they form. Basically stacking faults are surface defects. The association of partial dislocations and stacking faults define the extended dislocation, which makes cross slip more difficult, thus strengthening the material against deformation. Various properties of dislocations are one of the subjects of this chapter.

Joshua Pelleg

Chapter 4. Deformation in Ceramics

Abstract

Deformation can be elastic or plastic. Understanding elastic deformation is very important in ceramics to eliminate instantaneous brittle fracture at some applied stress levels. The fracture stress is usually the same or very close to the elastic limit. Stresses have to be exercised with understanding of the limits of the specific ceramics and the level that it can endure before fracture. No dimensional changes in test pieces occur in elastic deformation. Plastic deformation of ductile ceramics at room temperature, and of low temperature brittle ceramics at elevated temperatures, produce slip marks due to the advance of dislocations. All the characteristic phenomena of plastic deformation are observed either at ambient temperature (of room temperature ductile materials) or at the elevated temperature (of brittle ceramics at low temperature) such as yielding, existence of resolved and critical shear stress and slip. Among the yield phenomena, serrated stress–strain curves and Lüders bands can be noted as existing features. Twinning deformation, -mechanical or annealing twins-, are also observed to operate under proper conditions. Among the many factors influencing the mechanical properties, special consideration should be given to the effect of grain size. Preferred orientation in polycrystalline ceramics are seldom observed in its natural state but it may be induced during processing or fabrication. It might be of interest sometimes to get anisotropy for specific purposes of interest to enhance some directional property.

Joshua Pelleg

Chapter 5. The Strength and Strengthening of Ceramics

Abstract

There are several mechanisms by which materials may be strengthened as listed: (i) Strain (or work) hardening in ductile ceramics, (ii) Solid-solution strengthening by pinning dislocations either by interstitial or substitutional atoms, (iii) Second-phase hardening, (iv) Transformation hardening, (v) Strengthening by grain boundaries. Strain hardening is a feature of ductile ceramics, but at high temperatures where brittle materials show plasticity, strain hardening does not necessarily occur. In brittle materials that show plasticity at elevated temperature, strain hardening depends on composition and conditions of the test. It is possible that, due to the recovery process, strain hardening will not be observed. Superplastic materials are characterized by high ductility and no strain hardening occurs. In particular in superplastic materials such as MgO, strain hardening is absent while in β-Si₃N₄ under compression it is not observed. However, it has often been observed that little or no hardening at all occurs. Either interstitial or substitutional atoms can pin dislocations and thus strengthen the material. Second phase particles not in solution can hinder dislocations in their motion with a consequent increase in strength in the ceramics. Ceramics such as those based on zirconia are likely to undergo phase transformation, in particular the yttria stabilized zirconia, which is associated with strengthening of the material. Clearly, grain boundaries are obstacles to dislocation motion and thus harden the ceramic.

Joshua Pelleg

Chapter 6. Time-Dependent Deformation: Creep

Abstract

Creep is time-dependent deformation under constant stress. It may occur at relatively moderate temperatures. Most ceramics are intended for use at high temperatures, where they are ductile and creep deformation might occur. For ceramics with low-temperature ductility, creep may occur at ~0.5 T_m or even at lower temperatures. Creep generally is a function of the stress applied, the time of load duration and the temperature. Many ceramics are characterized by a high melting point even above 2000 °C (MgO, Al₂O₃, SiC, etc.) which makes them natural candidates for high temperature applications without the risk of creep failure during their lifetimes. Single and polycrystals creep, but to eliminate grain boundary sliding single crystals are preferred in certain important applications, despite the cost factor involved. Although small grain size enhances most of the mechanical properties, for creep resistance large grained materials are preferred. Mechanisms of creep that can act individually or simultaneously (depending on conditions) are Nabarro-Herring Creep, Dislocation Creep and Climb, Climb-Controlled Creep, Thermally-Activated Glide via Cross-Slip and Coble Creep, involving Grain-Boundary Diffusion. Creep may terminate in rupture which has to be avoided by choosing the proper ceramics and the safe temperature use for the desired life time. The presence of flaws (cracks) in ceramics intended for high-temperature applications can be controlled by the manufacturing process, which should be reduced to a minimum. It is essential for design purposes to estimate the life time in service of a ceramics to avoid failure, which is evaluated by some parametric method. The most popular methods to predict life time are the Larson-Miller and Monkman–Grant methods.

Joshua Pelleg

Chapter 7. Cyclic Stress: Fatigue

Abstract

Components in engineering applications operating under cyclic loads, commonly known as fatigue, may become unstable and cause catastrophic failure to occur unexpectedly because of structural instability. It is generally thought that over 80 % of all service failures are associated with fatigue. Therefore, operation of machines or their components under cyclic loads are of prime concern. To overcome the difficulty in predicting fatigue failure—because of a large spread of statistical results—it is essential to use many test specimens to reach a meaningful average value below which the probability for fatigue fracture is quite low. Fatigue-resistance evaluation is done by plotting applied stress against the number of cycles, usually referred to as the S–N (curve) relation. In some cases a horizontal line is observed in the plot known as the “knee” representing the endurance limit. At this level of stress or below it, ceramics have the ability to endure a large number of stress-cycles without failure. A favored location of failure initiation is the surface; therefore good surface finish (often by polishing) is recommended which significantly improves fatigue resistance. Introducing compressive stress by any of the following methods, namely, laser treatments, sand blasting or shot peening improve greatly the fatigue resistance. Regardless of the origin of stress when cycling is applied, fatigue damage may result. Thus stress cycling associated with temperature changes is of great concern because it can induce fatigue damage known as thermal fatigue with premature failure in components operating at elevated temperatures. Design to overcome fatigue failure and to increase resistance to cyclic deformation is essential. Environmental effects, among them corrosion, are important in design considerations. Corrosive environments may accelerate the growth of fatigue cracks, which initiate at the surface and, therefore, reduce overall fatigue performance.

Joshua Pelleg

Chapter 8. Fracture

Abstract

In ceramics it is essential to consider all kinds of fractures that a material might experience during its service life time as a consequence of deformation. Fracture propensity is critical in ceramics which does not show elongation (plasticity) because failure can set in at deformations which basically are elastic (brittle ceramics). It is important to understand the theories of fracture, and relate them to the theoretical strength of materials. Among the important theories one can mention Griffith’s theory on fracture, Orowan’s fracture theory, and the dislocation theory of brittle fracture including the Stroh model of fracture. One of the most important parameters regarding fracture is toughness. Fracture toughness is the property that describes the ability of a material containing a crack to resist fracture and is one of the most important properties of any material for design applications. Related to fracture toughness is the term R-curve, which refers to fracture toughness that increases as a crack grows. Prediction of the effect of existing flaws in ceramics on fracture strength is the R-curve. Fracture toughness is an indicator for failure in ceramics and the R-curve expresses ceramic crack resistance. Another way to characterize a ceramic is by the energy absorption concept which is related to its fracture toughness. The J-integral as a fracture criterion is used to express the energy absorbed during crack extension. Fracture may occur in ceramics under static load, time dependent and cyclic deformation. Toughness can be improved by changing the course of crack, by crack tip shielding, crack bridging and crack healing. In ceramics undergoing transformation, transformation-toughening can improve the toughness.

Joshua Pelleg

Chapter 9. Mechanical Properties of Nanoscale Ceramics

Abstract

The mechanism of deformation in nanosize ceramics occur either by dislocation motion or by grain boundary sliding depending on the size of the grains. In nanoceramics of grain size above ~100 nm the main deformation mechanism is by dislocation motion. At ultra-fine nano grain sizes below ~100 nm in the range <50 nm, the deformation mechanism is by grain boundary sliding. Dislocations cannot be accommodated conveniently in such nanosize materials and are prevented from motion and interactions. At levels in the hundreds of nanosized grains, a probable partial-dislocation mechanism may occur concurrently with other deformation mechanisms such as grain boundary sliding. For grain boundary sliding atomic mobility is essential, which results in a metal-like plasticity in nanoscale ceramics. One is interested in the behavior of nanoceramics under applied loads; therefore the various responses effecting static mechanical properties (tension–compression, hardness, etc.) time-dependent deformation (creep) and cyclic (fatigue) deformation are relevant. Making ceramics superplastic requires producing ultra-fine grains in the lower nanosize level, preferentially below 50 nm or even less. Various sophisticated techniques have been developed over the past decade or so, such that certain nanoceramics can now be produced with some measure of superplasticity. Superplastic materials may be thinned down, usually in a uniform manner, before breaking, without neck formation. The actual deformation mechanism is still under debate and may be material-dependent as well. Despite the various views on the exact mechanism responsible for the observed nano-behavior, it is clear from the experiments that nanoceramics may exhibit increased strength (hardness, for example), improved toughness, improved ductility and high resistance to fatigue. All these improved properties serve as safeguards against unexpected or premature fracture in service.

Joshua Pelleg

Backmatter

Titel: Mechanical Properties of Ceramics
verfasst von: Joshua Pelleg
Verlag: Springer International Publishing
Electronic ISBN: 978-3-319-04492-7
Print ISBN: 978-3-319-04491-0
DOI: https://doi.org/10.1007/978-3-319-04492-7

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