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? Types of Materials
Materials may be grouped in several ways. Scientists often classify materials by their state: solid, liquid, or gas. They also separate them into organic (once living) and inorganic (never living) materials.
For industrial purposes, materials are divided into engineering materials or nonengineering materials. Engineering materials are those used in manufacture and become parts of products.
Nonengineering materials are the chemicals, fuels, lubricants, and other materials used in the manufacturing process, which do not become part of the product.
Engineering materials may be further subdivided into: ①Metal ②Ceramics ③Composite ④Polymers, etc.
Metals and Metal Alloys
Metals are elements that generally have good electrical and thermal conductivity. Many metals have high strength, high stiffness, and have good ductility.
Some metals, such as iron, cobalt and nickel, are magnetic. At low temperatures, some metals and intermetallic compounds become superconductors.
What is the difference between an alloy and a pure metal? Pure metals are elements which come from a particular area of the periodic table. Examples of pure metals include copper in electrical wires and aluminum in cooking foil and beverage cans.
Alloys contain more than one metallic element. Their properties can be changed by changing the elements present in the alloy. Examples of metal alloys include stainless steel which is an alloy of iron, nickel, and chromium; and gold jewelry which usually contains an alloy of gold and nickel.
Why are metals and alloys used? Many metals and alloys have high densities and are used in applications which require a high mass-to-volume ratio.
Some metal alloys, such as those based on aluminum, have low densities and are used in aerospace applications for fuel economy. Many alloys also have high fracture toughness, which means they can withstand impact and are durable.
What are some important properties of metals?
Density is defined as a material’s mass divided by its volume. Most metals have relatively high densities, especially compared to polymers. Materials with high densities often contain atoms with high atomic numbers, such as gold or lead. However, some metals such as aluminum or magnesium have low densities, and are used in applications that require other metallic properties but also require low weight.
Fracture toughness can be described as a material’s ability to avoid fracture, especially when a flaw is introduced. Metals can generally contain nicks and dents without weakening very much, and are impact resistant. A football player counts on this when he trusts that his facemask won’t shatter.
Plastic deformation is the ability of bend or deform before breaking. As engineers, we usually design materials so that they don’t deform under normal conditions. You don’t want your car to lean to the east after a strong west wind.
However, sometimes we can take advantage of plastic deformation. The crumple zones in a car absorb energy by undergoing plastic deformation before they break.
The atomic bonding of metals also affects their properties. In metals, the outer valence electrons are shared among all atoms, and are free to travel everywhere. Since electrons conduct heat and electricity, metals make good cooking pans and electrical wires. It is impossible to see through metals, since these valence electrons absorb any photons of light which reach the metal. No photons pass through.
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Alloys are compounds consisting of more than one metal. Adding other metals can affect the density, strength, fracture toughness, plastic deformation, electrical conductivity and environmental degradation.
For example, adding a small amount of iron to aluminum will make it stronger. Also, adding some chromium to steel will slow the rusting process, but will make it more brittle.
? Ceramics and Glasses A ceramic is often broadly defined as any inorganic nonmetallic material． By this definition, ceramic materials would also include glasses; however, many materials scientists add the stipulation that “ceramic” must also be crystalline.
A glass is an inorganic nonmetallic material that does not have a crystalline structure. Such materials are said to be amorphous.
Properties of Ceramics and Glasses
Some of the useful properties of ceramics and glasses include high melting temperature, low density, high strength, stiffness, hardness, wear resistance, and corrosion resistance.
Many ceramics are good electrical and thermal insulators. Some ceramics have special properties: some ceramics are magnetic materials; some are piezoelectric materials; and a few special ceramics are superconductors at very low temperatures. Ceramics and glasses have one major drawback: they are brittle.
Ceramics are not typically formed from the melt. This is because most ceramics will crack extensively (i.e. form a powder) upon cooling from the liquid state.
Hence, all the simple and efficient manufacturing techniques used for glass production such as casting and blowing, which involve the molten state, cannot be used for the production of crystalline ceramics. Instead, “sintering” or “firing” is the process typically used.
In sintering, ceramic powders are processed into compacted shapes and then heated to temperatures just below the melting point. At such temperatures, the powders react internally to remove porosity and fully dense articles can be obtained.
An optical fiber contains three layers: a core made of highly pure glass with a high refractive index for the light to travel, a middle layer of glass with a lower refractive index known as the cladding which protects the core glass from scratches and other surface imperfections, and an out polymer jacket to protect the fiber from damage.
In order for the core glass to have a higher refractive index than the cladding, the core glass is doped with a small, controlled amount of an impurity, or dopant, which causes light to travel slower, but does not absorb the light.
Because the refractive index of the core glass is greater than that of the cladding, light traveling in the core glass will remain in the core glass due to total internal reflection as long as the light strikes the core/cladding interface at an angle greater than the critical angle.
The total internal reflection phenomenon, as well as the high purity of the core glass, enables light to travel long distances with little loss of intensity.。
? Composites
Composites are formed from two or more types of materials. Examples include polymer/ceramic and metal/ceramic composites. Composites are used because overall properties of the composites are superior to those of the individual components.
For example: polymer/ceramic composites have a greater modulus than the polymer component, but aren’t as brittle as ceramics.
Two types of composites are: fiber-reinforced composites and particle-reinforced composites.
Fiber-reinforced Composites
Reinforcing fibers can be made of metals, ceramics, glasses, or polymers that have been turned into graphite and known as carbon fibers. Fibers increase the modulus of the matrix material.
The strong covalent bonds along the fiber’s length give them a very high modulus in this direction because to break or extend the fiber the bonds must also be broken or moved.
Fibers are difficult to process into composites, making fiber-reinforced composites relatively expensive. Fiber-reinforced composites are used in some of the most advanced, and therefore most expensive sports equipment, such as a time-trial racing bicycle frame which consists of carbon fibers in a thermoset polymer matrix.
Body parts of race cars and some automobiles are composites made of glass fibers (or fiberglass) in a thermoset matrix.
Fibers have a very high modulus along their axis, but have a low modulus perpendicular to their axis. Fiber composite manufacturers often rotate layers of fibers to avoid directional variations in the modulus.
Particle-reinforced composites
Particles used for reinforcing include ceramics and glasses such as small mineral particles, metal particles such as aluminum, and amorphous materials, including polymers and carbon black.
Particles are used to increase the modulus of the matrix, to decrease the permeability of the matrix, to decrease the ductility of the matrix. An example of particle-reinforced composites is an automobile tire which has carbon black particles in a matrix of polyisobutylene elastomeric polymer.
Polymers
A polymer has a repeating structure, usually based on a carbon backbone. The repeating structure results in large chainlike molecules. Polymers are useful because they are lightweight, corrosion resistant, easy to process at low temperatures and generally inexpensive.
Some important characteristics of polymers include their size (or molecular weight), softening and melting points, crystallinity, and structure. The mechanical properties of polymers generally include low strength and high toughness. Their strength is often improved using reinforced composite structures.
Important Characteristics of Polymers
Size. Single polymer molecules typically have molecular weights between 10,000 and 1,000,000g/mol—that can be more than 2,000 repeating units depending on the polymer structure!
The mechanical properties of a polymer are significantly affected by the molecular weight, with better engineering properties at higher molecular weights.
Thermal transitions. The softening point (glass transition temperature) and the melting point of a polymer will determine which it will be suitable for applications. These temperatures usually determine the upper limit for which a polymer can be used.
For example, many industrially important polymers have glass transition temperatures near the boiling point of water (100℃, 212℉), and they are most useful for room temperature applications. Some specially engineered polymers can withstand temperatures as high as 300℃(572℉).
Crystallinity. Polymers can be crystalline or amorphous, but they usually have a combination of crystalline and amorphous structures (semi-crystalline).
Interchain interactions. The polymer chains can be free to slide past one another (thermo-plastic) or they can be connected to each other with crosslinks (thermoset or elastomer). Thermo-plastics can be reformed and recycled, while thermosets and elastomers are not reworkable.
Intrachain structure. The chemical structure of the chains also has a tremendous effect on the properties. Depending on the structure the polymer may be hydrophilic or hydrophobic (likes or hates water), stiff or flexible, crystalline or amorphous, reactive or unreactive.
材料的類型
材料可以按多種方法分類?？茖W(xué)家常根據(jù)狀態(tài)將材料分為：固體、液體或氣體。他們也把材料分為有機材料(曾經(jīng)有生命的)和無機材料(從未有生命的)。就工業(yè)效用而言，材料被分為工程材料和非工程材料。那些用于加工制造并成為產(chǎn)品組成部分的就是工程材料。非工程材料則是化學(xué)品、燃料、潤滑劑以及其它用于加工制造過程但不成為產(chǎn)品組成部分的材料。工程材料還能進一步細分為：①金屬材料②陶瓷材料③復(fù)合材料 ④聚合材料，等等。
金屬和金屬合金
金屬就是通常具有良好導(dǎo)電性和導(dǎo)熱性的元素。許多金屬具有高強度、高硬度以及良好的延展性。某些金屬能被磁化，例如鐵、鈷和鎳。在極低的溫度下，某些金屬和金屬化合物能轉(zhuǎn)變成超導(dǎo)體。
合金與純金屬的區(qū)別是什么？純金屬是在元素周期表中占據(jù)特定位置的元素。例如電線中的銅和制造烹飪箔及飲料罐的鋁。
合金包含不止一種金屬元素。合金的性質(zhì)能通過改變其中存在的元素而改變。金屬合金的例子有：不銹鋼是一種鐵、鎳、鉻的合金，以及金飾品通常含有金鎳合金。為什么要使用金屬和合金？許多金屬和合金具有高密度，因此被用在需要較高質(zhì)量體積比的場合。
某些金屬合金，例如鋁基合金，其密度低，可用于航空航天以節(jié)約燃料。許多合金還具有高斷裂韌性，這意味著它們能經(jīng)得起沖擊并且是耐用的。
金屬有哪些重要特性？
密度定義為材料的質(zhì)量與其體積之比。大多數(shù)金屬密度相對較高，尤其是和聚合物相比較而言。
高密度材料通常由較大原子序數(shù)原子構(gòu)成，例如金和鉛。然而，諸如鋁和鎂之類的一些金屬則具有低密度，并被用于既需要金屬特性又要求重量輕的場合。
斷裂韌性可以描述為材料防止斷裂特別是出現(xiàn)缺陷時不斷裂的能力。金屬一般能在有缺口和凹痕的情況下不顯著削弱，并且能抵抗沖擊。橄欖球運動員據(jù)此相信他的面罩不會裂成碎片。
塑性變形就是在斷裂前彎曲或變形的能力。作為工程師，設(shè)計時通常要使材料在正常條件下不變形。沒有人愿意一陣強烈的西風過后自己的汽車向東傾斜。然而，有時我們也能利用塑性變形。汽車上壓皺的區(qū)域在它們斷裂前通過經(jīng)歷塑性變形來吸收能量。
金屬的原子連結(jié)對它們的特性也有影響。在金屬內(nèi)部，原子的外層階電子由所有原子共享并能到處自由移動。由于電子能導(dǎo)熱和導(dǎo)電，所以用金屬可以制造好的烹飪鍋和電線。
因為這些階電子吸收到達金屬的光子，所以透過金屬不可能看得見。沒有光子能通過金屬
合金是由一種以上金屬組成的混合物。加一些其它金屬能影響密度、強度、斷裂韌性、塑性變形、導(dǎo)電性以及環(huán)境侵蝕。
例如，往鋁里加少量鐵可使其更強。同樣，在鋼里加一些鉻能減緩它的生銹過程，但也將使它更脆。
? 陶瓷和玻璃
陶瓷通常被概括地定義為無機的非金屬材料。照此定義，陶瓷材料也應(yīng)包括玻璃；然而許多材料科學(xué)家添加了“陶瓷”必須同時是晶體物組成的約定。
玻璃是沒有晶體狀結(jié)構(gòu)的無機非金屬材料。這種材料被稱為非結(jié)晶質(zhì)材料。
陶瓷和玻璃的特性高熔點、低密度、高強度、高剛度、高硬度、高耐磨性和抗腐蝕性是陶瓷和玻璃的一些有用特性。許多陶瓷都是電和熱的良絕緣體。某些陶瓷還具有一些特殊性能：有些是磁性材料，有些是壓電材料，還有些特殊陶瓷在極低溫度下是超導(dǎo)體。陶瓷和玻璃都有一個主要的缺點：它們?nèi)菀灼扑椤?
陶瓷一般不是由熔化形成的。因為大多數(shù)陶瓷在從液態(tài)冷卻時將會完全破碎(即形成粉末)。 因此，所有用于玻璃生產(chǎn)的簡單有效的—諸如澆鑄和吹制這些涉及熔化的技術(shù)都不能用于由晶體物組成的陶瓷的生產(chǎn)。作為替代，一般采用“燒結(jié)”或“焙燒”工藝。
在燒結(jié)過程中，陶瓷粉末先擠壓成型然后加熱到略低于熔點溫度。在這樣的溫度下，粉末內(nèi)部起反應(yīng)去除孔隙并得到十分致密的物品。
光導(dǎo)纖維有三層：核心由高折射指數(shù)高純光傳輸玻璃制成，中間層為低折射指數(shù)玻璃，是保護核心玻璃表面不被擦傷和完整性不被破壞的所謂覆層，外層是聚合物護套，用于保護光導(dǎo)纖維不受損。 為了使核心玻璃有比覆層大的折射指數(shù)，在其中摻入微小的、可控數(shù)量的能減緩光速而不會吸收光線的雜質(zhì)或攙雜劑。
由于核心玻璃的折射指數(shù)比覆層大，只要在全內(nèi)反射過程中光線照射核心/覆層分界面的角度比臨界角大，在核心玻璃中傳送的光線將仍保留在核心玻璃中。全內(nèi)反射現(xiàn)象與核心玻璃的高純度一樣，使光線幾乎無強度損耗傳遞長距離成為可能
復(fù)合材料
復(fù)合材料由兩種或更多材料構(gòu)成。例子有聚合物/陶瓷和金屬/陶瓷復(fù)合材料。之所以使用復(fù)合材料是因為其全面性能優(yōu)于組成部分單獨的性能。
例如：聚合物/陶瓷復(fù)合材料具有比聚合物成分更大的模量，但又不像陶瓷那樣易碎。
復(fù)合材料有兩種：纖維加強型復(fù)合材料和微粒加強型復(fù)合材料。
纖維加強型復(fù)合材料
加強纖維可以是金屬、陶瓷、玻璃或是已變成石墨的被稱為碳纖維的聚合物。纖維能加強基材的模量。沿著纖維長度有很強結(jié)合力的共價結(jié)合在這個方向上給予復(fù)合材料很高的模量，因為要損壞或拉伸纖維就必須破壞或移除這種結(jié)合。把纖維放入復(fù)合材料較困難，這使得制造纖維加強型復(fù)合材料相對昂貴。纖維加強型復(fù)合材料用于某些最先進也是最昂貴的運動設(shè)備，例如計時賽競賽用自行車骨架就是用含碳纖維的熱固塑料基材制成的。
競賽用汽車和某些機動車的車體部件是由含玻璃纖維(或玻璃絲)的熱固塑料基材制成的。
纖維在沿著其軸向有很高的模量，但垂直于其軸向的模量卻較低。纖維復(fù)合材料的制造者往往旋轉(zhuǎn)纖維層以防模量產(chǎn)生方向變化。
微粒加強型復(fù)合材料
用于加強的微粒包含了陶瓷和玻璃之類的礦物微粒，鋁之類的金屬微粒以及包括聚合物和碳黑的非結(jié)晶質(zhì)微粒。 微粒用于增加基材的模量、減少基材的滲透性和延展性。微粒加強型復(fù)合材料的一個例子是機動車胎，它就是在聚異丁烯人造橡膠聚合物基材中加入了碳黑微粒。
聚合材料
聚合物具有一般是基于碳鏈的重復(fù)結(jié)構(gòu)。這種重復(fù)結(jié)構(gòu)產(chǎn)生鏈狀大分子。由于重量輕、耐腐蝕、容易在較低溫度下加工并且通常較便宜，聚合物是很有用的。聚合材料具有一些重要特性，包括尺寸(或分子量)、軟化及熔化點、結(jié)晶度和結(jié)構(gòu)。聚合材料的機械性能一般表現(xiàn)為低強度和高韌性。它們的強度通?？刹捎眉訌姀?fù)合結(jié)構(gòu)來改善。
聚合材料的重要特性
尺寸：單個聚合物分子一般分子量為10,000到1,000,000g/mol之間，具體取決于聚合物的結(jié)構(gòu)—這可以比2,000個重復(fù)單元還多聚合物的分子量極大地影響其機械性能，分子量越大，工程性能也越好。
熱轉(zhuǎn)換性：聚合物的軟化點(玻璃狀轉(zhuǎn)化溫度)和熔化點決定了它是否適合應(yīng)用。這些溫度通常決定聚合物能否使用的上限。
例如，許多工業(yè)上的重要聚合物其玻璃狀轉(zhuǎn)化溫度接近水的沸點(100℃, 212℉)，它們被廣泛用于室溫下。而某些特別制造的聚合物能經(jīng)受住高達300℃(572℉)的溫度。
結(jié)晶度：聚合物可以是晶體狀的或非結(jié)晶質(zhì)的，但它們通常是晶體狀和非結(jié)晶質(zhì)結(jié)構(gòu)的結(jié)合物(半晶體)。
原子鏈間的相互作用：聚合物的原子鏈可以自由地彼此滑動(熱可塑性)或通過交鍵互相連接(熱固性或彈性)。熱可塑性材料可以重新形成和循環(huán)使用，而熱固性與彈性材料則是不能再使用的。
鏈內(nèi)結(jié)構(gòu)：原子鏈的化學(xué)結(jié)構(gòu)對性能也有很大影響。根據(jù)各自的結(jié)構(gòu)不同，聚合物可以是親水的或憎水的(喜歡或討厭水)、硬的或軟的、晶體狀的或非結(jié)晶質(zhì)的、易起反應(yīng)的或不易起反應(yīng)的。