Unit 1 Engineering Materials (Ⅰ)

1.1 Introduction

For industrial purposes, materials can be divided into engineering materials and non-engineering materials. Engineering materials are those used in manufacturing and will become parts of products through definite processing. In generally, engineering materials may be further subdivided into metals, ceramics, composites and polymers.

Metals. Common metals are gold, silver, copper, iron, nickel, aluminum, magnesium and titanium, etc. Among these metals, gold and silver (also platinum) are precious metals; iron and nickel are magnetic materials (they are subject to the action of magnetic force); aluminum, magnesium and titanium are commonly called light metals. Of course, metal materials are seldom used in their pure states but in alloy states. Alloys contain more than one metallic element. Their properties can be modified by changing the element contents present in them. Examples of alloys include stainless steels which are alloys of Fe, Ni and Cr; and brass which is an alloy of Cu and Zn. In addition, metal materials can also be broadly divided into two groups, i.e. ferrous metals and nonferrous metals. Ferrous metals often refer to iron alloys (iron and steel materials) and nonferrous metals include all other metallic materials.

Ceramics. It seems that there hasn’t been an exact and complete definition about advanced ceramics so far, but from a viewpoint of modern engineering and technology, advanced ceramics (differentiating from traditional household ceramics) may be defined as the new engineering materials composed of some special kinds of metallic oxides (e.g. alumina or corundum and zirconia) or carbides or nitrides of metallic and nonmetallic elements (e.g. tungsten carbide, silicon carbide, boron nitride, silicon nitride, etc.).[1] They have some unique properties such as high-temperature strength; hardness; inertness to chemicals, food, and environment; resistance to wear and corrosion; and low electrical and thermal conductivity. So they are widely used in turbine, automobile, aerospace components, heat exchangers, semiconductors and cutting tools.

Polymers. The word polymer was first used in 1866. In essence, they are organic macromolecular compounds. And in 1909, the word plastics was employed as a synonym for “polymers” . Plastic is from a Greek word plastikos, meaning “able to be molded and shaped”. Plastics are one of numerous polymeric materials and have extremely large molecules. Because of their many unique and diverse properties, polymers have increasingly replaced metallic components in applications such as automobiles, civilian and military air craft, sporting goods, toys, appliances, and office equipment.

Composite materials. Among the major developments in materials in recent years are composite materials. In fact, composites are now one of the most important classes of engineered materials, because they offer several outstanding properties as compared with conventional materials. A composite material is a combination of two or more chemically distinct and insoluble phases; its properties and structural performance are superior to those of the constituents acting independently.[2]

Nanomaterials refer to those materials, at least one of whose three dimensions is at the nano-scale(1-100 nm) and hence we may have nano-powders, nano-fibers and nano-films.[3] They were first investigated in the early 1980s. However, we can not classified them in nature as distinct from other four common engineering materials, i.e. metals, ceramics, composite and macromolecular materials, because various nano-materials developed so far are all composed of one or combination of the above four materials.[4] Nevertheless, when the sizes of some common materials are reduced to the nano-scale, they will possess some special properties superior to traditional and commercially available materials. These properties can include strength, hardness, ductility, wear resistance and corrosion resistance suitable for structural (load-bearing) and nonstructural applications, in combination with unique electrical, magnetic, and optical properties. For example, nano-powders have very large specific surface area, up to hundreds even thousands of square meters per gram, making them become highly active adsorbents and catalysts with wide application prospect in organic synthesis and environmental protection.[5]