Classification of materials: Metals, Polymers, Ceramics, Composite materials, Biomaterials

Classification of materials

 Like many other things, materials are classified  in groups, so that  our brain can handle the complexity. One can classify them  based on many criteria, for example crystal structure (arrangement of atoms and bonds between them), or properties, or use. Metals, Ceramics, Polymers, Composites, Semiconductors, and Biomaterials constitute the main classes of present engineering materials. Metals:  These materials are characterized by  high thermal and electrical conductivity; strong yet deformable under applied mechanical loads; opaque to light (shiny if polished). These characteristics are due to  valence electrons that are detached from atoms, and spread in an  electron sea  that  glues  the ions together, i.e. atoms are bound together by metallic bonds and weaker van der Waalls forces. Pure metals are not good enough for many applications, especially structural applications. Thus metals are used in alloy form  i.e. a metal mixed with another  metal to improve the desired qualities. E.g.: aluminum, steel, brass, gold. Ceramics:  These are inorganic compounds, and usually  made either of oxides, carbides, nitrides, or  silicates of  metals. Ceramics  are  typically partly crystalline and partly amorphous. Atoms (ions often) in ceramic materials behave mostly like either positive or negative ions, and are bound by very strong  Coulomb forces between them. These materials are characterized by very high strength under compression, low ductility; usually insulators to heat  and electricity. Examples: glass, porcelain, many minerals. 

Metals

These materials are characterized by  high thermal and electrical conductivity; strong yet deformable under applied mechanical loads; opaque to light (shiny if polished). These characteristics are due to  valence electrons that are detached from atoms, and spread in an  electron sea  that  glues  the ions together, i.e. atoms are bound together by metallic bonds and weaker van der Waalls forces. Pure metals are not good enough for many applications, especially structural applications. Thus metals are used in alloy form  i.e. a metal mixed with another  metal to improve the desired qualities. E.g.: aluminum, steel, brass, gold.


Polymers

Polymers in the form  of thermo-plastics (nylon, polyethylene, polyvinyl chloride, rubber, etc.) consist of molecules that have covalent bonding within each molecule and van der Waals forces between them. Polymers in the form  of  thermo-sets (e.g., epoxy, phenolics, etc.) consist of a network of covalent bonds. They are based on H, C and other non-metallic elements. Polymers are amorphous, except for a minority of thermoplastics. Due to the kind of bonding, polymers are typically electrical and thermal insulators. However, conducting polymers  can be obtained by doping, and conducting polymer-matrix  composites  can be obtained  by the use of conducting fillers. They decompose at moderate temperatures (100 –  400 C), and are lightweight. Other properties vary greatly. 

Ceramics

 These are inorganic compounds, and usually  made either of oxides, carbides, nitrides, or  silicates of  metals. Ceramics  are  typically partly crystalline and partly amorphous. Atoms (ions often) in ceramic materials behave mostly like either positive or negative ions, and are bound by very strong  Coulomb forces between them. These materials are characterized by very high strength under compression, low ductility; usually insulators to heat  and electricity. Examples: glass, porcelain, many minerals. 


Composite  materials

Composite materials are multiphase materials obtained  by artificial combination of different materials  to attain  properties that the individual components cannot attain. An example is a  lightweight brake disc obtained by embedding SiC particles in Al-alloy  matrix. Another  example is reinforced cement concrete, a structural composite obtained by combining cement (the matrix, i.e., the binder, obtained by a reaction known as hydration, between cement and water), sand (fine aggregate), gravel (coarse aggregate), and, thick steel  fibers. However, there are some  natural composites available in nature, for example –  wood. In general, composites are classified according to their matrix materials.  The main classes of composites are metal-matrix, polymer-matrix, and ceramic-matrix. Semiconductors:  Semiconductors are covalent in nature. Their atomic structure is characterized by the highest occupied energy band (the valence band, where the valence electrons reside energetically) full such that the energy gap between the top of the valence band and the bottom  of the empty  energy band (the conduction band) is small enough for some  fraction of the valence electrons to be excited from  the valence band to the conduction band by thermal, optical, or  other forms  of energy. Their electrical properties depend extremely strongly on minute proportions of contaminants. They are usually doped in order to enhance electrical conductivity. They are used in the form of single crystals without dislocations  because grain boundaries and dislocations would degrade electrical behavior. They are opaque  to visible light but transparent to the infrared.  Examples: silicon  (Si), germanium (Ge),  and  gallium  arsenide (GaAs, a compound semiconductor). 


Biomaterials

These are any type material that can be used  for replacement of damaged or diseased human body parts. Primary requirement of these materials is that they must be biocompatible with body tissues, and must not produce toxic substances. Other important material factors are: ability to  support forces; low friction, wear, density, and cost; reproducibility. Typical  applications involve heart  valves, hip joints, dental implants, intraocular lenses. Examples: Stainless steel, Co-28Cr-6Mo,  Ti-6Al-4V, ultra high molecular weight  poly-ethelene,  high purity dense Al-oxide, etc. 


 

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