1、Relationship among Microstructure, Processing, and Applications,Ceramics,Relationship among Microstructure, Processing, and Application,The field of materials science and engineering is often defined by the interrelationship between four topicssynthesis and processing, structure and composition, pro
2、perties, and performance.,效能,合成/制备,性质,成分/结构,图 材料科学与工程学科四要素(左)与五要素(右)关系,（2）材料的发展和应用是系统工程 1989年Flemings提出四元体系 五要素模型,环境因素,组织结构,设计模拟,性质,效能(使用性能),成分,合成制备,To understand the behavior and properties of any material, it is essential to understand its structure. Structure can be considered on several levels,
3、all of which influence final behavior. At the finest level is the electron configuration, which affects properties such as color, electrical conductivity, and magnetic behavior.,能够对材料的颜色、电导性和磁性产生影响的电子形态是材料的最精细的水平。Finest:最精细的,The arrangement of electrons in an atom influences how it will bond to anot
4、her atom and this, in turn, impacts the crystal structure. The arrangement of the atoms or ions in the material also needs to be considered.Crystalline ceramics have a very regular atomic arrangement whereas in noncrystalline or amorphous ceramics (e.g., oxide glasses) there is no long-range order,
5、although locally we may identify similar polyhedra.,结晶陶瓷具有非常规则的原子排列，然而，这种长程有序性的排列在非晶体和无定形陶瓷中却不存在，尽管在局部我们可以开到相似的多面体结构,Polyhedron（多面体）的复数形式,Such materials often behave differently relative to their crystalline counterparts. Not only perfect lattices and ideal structure have to be considered but also t
6、he presence of structural defects that are unavoidable in all materials, even the amorphous ones.Examples of such defects include impurity atoms and dislocations.,Polycrystalline ceramics have a structure consisting of many grains. The size, shape, and orientation of the grains play a key role in ma
7、ny of the macroscopic properties of these materials, for example, mechanical strength.In most ceramics, more than one phase is present, with each phase having its own structure, composition, and properties.,Control of the type, size, distribution, and amount of these phases within the material provi
8、des a means to control properties.This microstructure of a ceramic is often a result of the way it was proceed. For example, hotpressed ceramics often have very few pores. This may not be the case in sintered materials.,The interrelationship among the structure, processing, and properties will be ev
9、ident throughout this text but are illustrated here by four examples.1. the strength of polycrystalline ceramics depends on the grain size through the Hall-Petch equation. In general, as the grain size decreases the strength increases. The grain size is determined by the size of the initial powder p
10、articles and the way in which they were consolidated.,The grain boundaries in a polycrystalline ceramic are also important. The strength then depends on whether or not the material is pure, contains a second phase or pores, or just contains glass at the grain boundaries. The relationship is not alwa
11、ys obvious for nano-ceramics.,2. transparent or translucent ceramics require that we limit the scattering of light by pores and second-phase particles. Reduction in porosity may be achieved by hot pressing to ensure a high-density product.This approach has been used to make transparent PLZT ceramics
12、 for electrooptical applications such as the flash-blindness goggles.,lead lanthanum zirconate titanate,3. Thermal conductivity of commercially available polycrystalline AlN is usually lower than that predicted by theory because of the presence of impurities, mainly oxygen, which scatter phonons. Ad
13、ding rear earth or alkaline metal oxides (such as Y2O3 and CaO, respectively) can reduce the oxygen content by acting as a getter. These oxides are mixed in with the AlN powder before it is shaped. The second phase, formed between the oxide additive and the oxide coating on the AlN grains, segregate
14、s to triple points.,4. Alumina ceramics are used as electrical insulators because of their high electrical resistivity and low dielectric constant. For most applications pure alumina is not used. Instead we blend the alumina with silicates to reduce the sintering temperature.These materials are know
15、n as debased aluminas and contain a glassy silicate phase between alumina grains. Debased aluminas are generally more conductive (lower resistivity) than pure aluminas, and are used in spark plugs.,Safety,When working with any material, safety considerations should be uppermost. There are several im
16、portant precautions to take when working with ceramics.,Toxicity of powders containing, for example, Pb and Cd or fluorides should be known. When shipping the material, the manufacturer supplies information on the hazards associated with their product. It is important to read this information and ke
17、ep it accessible. Some standard resources that provide information about the toxicity of powders and the “acceptable” exposure levels are given in the References.,Risk assessment,Small particles should not be inhaled. The effects have been well known, documented, and often ignored since the 1860s. P
18、roper ventilation, improved cleanliness, and protective clothing have significantly reduced many of the industrical risks.Care should be taken when handling any powders (of both toxic and nontoxic materials).,High temperatures are used in much of ceramic processing. The effects of high temperatures
19、on the human body are obvious. What is not so obvious is how hot something actually is. Table 3.3 gives the color scale for temperature. From this tabulation you can see that an alumina tube at 400 will not show a change in color but it will still burn skin.,Table The Color Scale of Temperature,Orga
20、nics are used as solvents and binders during processing.Traditionally, organic materials played little role in ceramic processing. Now they are widely used in many forms of processing.Again, manufacturers will provide safety data sheets on any product they ship. This information is important and sho
21、uld always be read carefully.,As a rule, the material safety data sheets (MSDS) should be readily accessible for all the materials your are using; many states requires that they are kept in the laboratory.,化学品安全说明书,21,Composites,Introduction to CompositesProperties of Composite Materials,22,Introduc
22、tion to Composites,三大材料：Metalinorganic materialPolymerComposites取长补短协同作用产生原来单一材料没有本身所没有的新性能,23,Schematic illustration of composite constituents,24,composite,matrix,reinforcement,polymerceramicmetal,discontinuouscontinuous: fiber yarns,particlsflakewhiskerschopped fiber,Constitution of composite,Inte
23、rphase,dispersion strengthenparticle reinforced,25,Types of reinforced composites,26,27,KEVLAR纤维,无捻玻璃纤维,碳纤维,氧化铝纤维,碳化硅纤维,28,dispersion strengthened(弥散增强) :其粒子直径为0.10.01 m，体积分数为1%15%。particle reinforced(颗粒增强) :粒子直径为150 m，体积分数20%fiber reinforced(纤维增强) ：,strengthening mechenism,29,Properties of Composit
24、e Materials,Mechanics of Composite MaterialsFailure of CompositeAdvantage of Composite Materials,The mechanical properties of a material are those ones that involve a reaction to an applied load.The common properties considered are strength, ductility, hardness, impact resistance, and fracture tough
25、ness.,强度、延展性、硬度、冲击强度、断裂韧性,Mechanics of Composite Materials,Most structural materials are anisotropic, which means that their material properties vary with orientation. The variation in properties can be due to directionality in the microstructure (texture) from forming or cold working operation, the
26、 controlled alignment of fiber reinforcement and a variety of other causes. Mechanical properties are generally specific to product form such as sheet, plate, extrusion, casting, forging, and etc.,Additionally, it is common to see mechanical property listed by the directional grain structure of the
27、material. In products such as sheet and plate, the rolling direction is called the longitudinal direction, the width of the product is called the transverse direction, and the thickness is called the short transverse direction. longitudinal lnditju:dinl 轴向Transverse trnsv:s 横向,The mechanical propert
28、ies of a material are not constant and often change as a function of temperature, rate of loading , and other conditions. For example, temperatures below room temperature generally cause an increase in strength properties of metallic alloys; while ductility, fracture toughness, and elongation usuall
29、y decrease. Temperatures above room temperature usually cause a decrease in the strength properties of metallic alloys. Ductility may increase or decrease with increasing temperature depending on the same variables,It should be also be noted that there is often significant variability in the values
30、obtained when measuring mechanical properties. Seemingly identical test specimen from the same lot of materials will often produce considerable different results. Therefore, multiple tests are commonly conducted to determine mechanical properties and values reported can be an average value or calcul
31、ated statistical minimum value. Also, a range of values is sometimes reported in order to show variability.,loading,The application of a force to an object is known as loading. Materials can be subjected to many different loading scenarios and a materials performance is depend on the loading conditi
32、ons. There are five fundamental loading conditions:tension, compression, bending, shear, and torsion.scenarios:情况 情节,Tension is the type of loading in which the two sections of material on either side of a plane tend to be pulled apart or elongated. Compression is the reverse of tensile loading and
33、involves pressing the material together.Loading by bending involves applying a load in a manner that causes a material to curve and results in compressing the material on one side and stretching it on the other.,Shear involves applying a load parallel to a plane which caused the material on one side
34、 of the plane to want to slide across the material on the other side of the plane. Torsion is the application of a force that causes twisting in a material.,If a material is subjected to a constant force, it is called static loading. If the loading of the material is not constant but instead fluctua
35、tes, it is called dynamic or cyclic loading. The way a material is loaded greatly affects its mechanical properties and largely determines how, or if, a component will fail; and whether it will show warning signs before failure actually occurs.,Stress,The term stress (S ) is used to express the load
36、ing in terms of force applied to a certain cross-sectional area of an object. From the perspective of loading, stress is the applied force or system of forces that tends to deform a body. From the perstective of what is happening within a material, stress is the internal distribution of forces withi
37、n a body that balance and react to the loads applied to it. The stress distribution may or may not be uniform, depending on the nature of the loading condition.,For example, a bar loaded in pure tension will essentially have a uniform tensile stress distribution. However, a bar loaded in bending wil
38、l have a stress distribution that changes withDistance perpendicular to the normal axis.,垂直的,Strain,Strain is the response of a system to an applied stress. When a material is loaded with a force, it produces a stress, which then causes a material to deform. Engineering strain is defined as the amou
39、nt of deformation in the direction of the applied force divided by the initial length of the material. The results in a unitless number, although it is often left in the unsimplified form, such as inches per inch or meters per meter.,工程应变可定义为：所施加力方向上的材料的改变量与材料原始长度的比值,41,(a) brittle materials 脆性材料(b)
40、 ductile materials 延性材料(c) elastic materials 弹性材料,stress-strain curves,For example, the strain in a bar that is being stretched in tension is the amount of elongation or change in length divided by its original length. As in the case of stress, the strain distribution may or may not be uniform in a
41、complex structural element, depending on the nature of the loading condition.,If the stress is small, the material may only strain a small amount and the material will return to its original size after the stress is released. This is called elastic deformation, because of liking elastic, it returns
42、to its unstressed state. Elastic deformation only occurs in a material when stresses are lower than a critical stress called the yield strength. If a material is loaded beyond it elastic limit, the material will remain in a deformed condition after the load is removed. This is called plastic deforma
43、tion.,Tensile properties,Tensile properties indicate how the material will react to forces being applied in tension. A tensile test is a fundamental mechanical test where a carefully prepared specimen is loaded in a very controlled manner while measuring the applied load and the elongation of the sp
44、ecimen over some distance. Tensile tests are used to determine the modulus of elasticity, elastic limit, elongation, proportional limit, reduction in area, tensile strength, yield point, yield strength and other tensile properties.,拉伸试验是一种基本的力学测试，它是对所制备好的样品施加一种可以控制的负荷，来测量所施加的负荷和在一段距离内样品的拉长。,Hardness
45、,Hardness is the resistance of a material to localized deformation. The term can apply to deformation from indentation, scratching, cutting or bending. In metals, ceramics and most polymers, the deformation considered is plastic deformation of the surface. For elastomers and some polymers, hardness
46、is defined at the resistance to deformation of the surface.,The lack of a fundamental definition indicates that hardness is not be a basic property of a material, but rather a composite one with contributions from the yield strength, work hardening, true tensile strength, modulus, and other factors.
47、Hardness measurements are widely used for the quality control of materials because they are quick and considered to be nondestructive tests when the marks or indentations produced by the test are in low stress areas.,Toughness,The ablity of a metal to deform plastically and to absorb energy in the p
48、rocess before fracture is termed toughness. The emphasis of this definition should be placed on the ablity to absorb energy before fracture. Recall that ductility is a measure of how much something deforms plastically before fracture, but just because a material is ductile does not make it tough.,Th
49、e key to toughness is a good combination of strength and ductility. A material with high strength and high ductility will have more toughness than a material with low strength and high ductility. Therefore, one way to measure toughness is by calculating the area under the stress strain curve from a tensile test. This value is simply called “material toughness” and it has units of energy per volume. Material toughness equates to a slow absorption of energy by the material.,
