Atom consist of protons and neutrons located in the nucleus. Electrons moves in the orbits. Protons are a positive charge, electrons negatively and neutrons are neutral. Atoms are electrically neutral because the number of protons and electrons is equal.
However, atoms may gain or lose electrons in order to increase their stability and the resulting charged entity is called an ion. Atoms of different elements have different atomic structure because they consist of different numbers of electrons and protons.
Electrons located on the last outermost atomic shell are called valence electrons. The number and energy levels of valence electrons deciding about the properties of the atoms.
Metals have between 1 – 3 valence electrons, non -metals between 5 – 8.
The characteristic of valence electrons depend on which bonds can occur between atoms.
There are 3 types of bonds:
- Ionic bonds (metal atom bond to non-metallic atom)
- Covalent bond (non-metallic atom bond to non-metallic atom)
- Metallic bond (metallic atom bond to metallic atom)
Classification of material and their structure linked to properties
Solid metals have a crystal structure. The 3 basic metallic structure:
- Hexagonal Close Packed Structure
- Face Centred Cubic Structure
- Body Centred Cubic Structure
- Metallic bond
The formation of metallic bond involves the transformation of atoms of the same metal or atoms of different metals into a set of cations and electrons moving freely between them.
In the solid state the nodes of the metal crystal lattice are occupied by cations making only oscillatory movements around the node, while delocalized electrons move freely within the entire crystal, like particles of a substances in a gaseous state. For this reason, we called that electron sea of a metallic bond. The cations that make up atomic cores stay in their position due to electrostatic attractions of electrons.
Ionic bond involves the transition of one or more valence electrons from the atoms of the electropositive element to the atoms of electronegative element. The atom giving away electrons becomes the cation, and the atom receiving electrons becomes the anion. The resulting ions attract electrostatic forces, forming a bond -ion network. These bounds occur when a metallic atom bond to non-metal atom.
Ions bonds are strong bonds
In molten metals ions are randomly distributed -this structure is called amorphous. When the molten metal is cooled down it begins to form a lattice structure. A lattice begins to form around the impurity which is called nucleation afterwards the Lettice begins to develop shaping a tree structure which we called dendrite.
When the molten metal is cooled down it begins to form a lattice structure.
A lattice begins to form around the impurity which is called nucleation afterwards the Lettice begins to develop shaping a tree structure which we called dendrite.
The properties of metal are associated with existing metallic bonds:
Good thermal and electrical conductivity can be justified by the mobility of electrons belonging to electron sea.
The metallic shine result from the fact that under the influence of visible light electrons on the crystal surface vibrate at the frequency of the incident radiation. The reflected rays have the same frequency as the incident rays, which is perceive as the characteristic shine of metal.
Plasticity, ductility, malleability is explained by the lack of privileged directions in crystal so network can be mowed and cause metal crack.
Metals have high boiling and melting points due to the fact that the giant structure of a metal have strong metallic bonding and lots of energy need to be use to surmount metallic bonds in boiling and melting process.
Explain relation between metals properties and their structure.
To show relationship between the properties of metals and theirs structure I use example of cold rolling method. Cold rolling is a process of reducing the thickness or changing the cross-sectional area of work piece by compressive forces. Cold rolling process increase strength and hardness of metal by deform and flatted grains in the metal, but decrease toughness and ductility.
Ceramics material are made of compounds of metallic and non-metallic elements. Contain mix of ionic and covalent bonds. Sometimes they are ionic -in this case structure of crystal is thought because they have electrically charged ions instead of atoms, sometimes pure covalent. They do not have metallic properties although many of them like metals have a crystalline structure. Ceramic materials are resistant to high temperatures and their melting points significantly exceeds the melting points of metals. Ceramic materials are composed of at least two elements and sometimes more and their crystal structure is more complex than that of metals. Ceramic materials are a good electrical insulator and have a good chemical and corrosion resistance. Ceramic materials are also very fragile
Ceramic materials are divided into three main groups:
- Traditional clay-based ceramics
- Advanced ceramics based on oxides, carbides, nitrides
In ceramic materials in which ionic bonds predominate, there is a balance between electrically positively charged cations – metal ions giving their valence electrons and negatively charged anions – non-metallic ions. When the anions are in contact with the surrounding cations, the ceramic materials are characterized by a stable structure.
As I mentioned above ceramic materials consist a mix of covalent and ionic bonds (I have described Ionic bonds in metals section).
Covalent bond is formed by creating a common pair of electrons (known as bonding pairs) by two or more atoms, each atom providing the same number of electrons. Covalent bond occurs in non-metal atoms with equally electronegativity value.
Above picture shows covalent bond.
Ceramic materials are weak conductors of electric current and due to the lack of free electrons which are missing in both ceramic materials with ionic bonds and covalent bonds. That is why it is a good electric insulator and is often used in electrical and electronics industry. (capacitors, fuses)
Ceramic materials are also brittle because the atoms cannot shift and this is because they have ionic bonds, alternately positive and negative ions. If we try to shift the row of atoms, this will shift the negative ions towards negative ions and positive ions towards positive ions. For the same reason, ceramic materials are hard, resistant to plastic deformation or cracking in compression, but not very resistant to stretching and bending. The fragility also hampers thermal treatment. Ceramic materials are resistant to chemical action because when they are ionically or covalently bonded, they rarely react with another atom.
Chemical substances with high molecular weight, which consist of repeated units called monomers. Mer is the simplest, repeated pattern of polymer molecules, which consists of a long chain of monomers. Hence the name Polymer:
- Poly means many
- Mono means one
- Mer means unit.
- Mer is formed when a double carbon bond is broken by catalysis or heat.
Amorphous polymers have an amorphous form. They take on a form similar to spaghetti in a thong. They form disordered swirled structures. Amorphous polymers soften gradually on heating and become liquid in a certain temperature range.
Crystalline polymers have a regular linear structure of the chain form an ordered-crystalline structure. They are formed under the influence of thermal movements and intermolecular forces of chains. Crystalline polymers are characterized by regular arrangement of atoms, molecules or ions. Therefore, they have a strictly defined melting point at which viscosity and density change rapidly.
Above picture shows of crystalline form of Polymer.
Liquid crystalline polymers
Liquid crystalline polymers – this group includes polymers capable of generating liquid crystalline phases. The structure of these polymers resembles the shape of a rod.
Polymers have four types of chains:
- Linear – they are polymers in which the chains are straight and have no branches.
Figure a shows linear polymer`s chain.
- Branched – they have polymers in which the main chains are branched.
Figure b shows branched polymer`s chains.
- Cross linked – They have polymers in which there are two parallel main chains connected by side chains.
Figure c shows cross linked polymer`s chain.
- Network – they are polymers forming a spatial continuous network.
Relationship between polymers properties and structure:
Polymers are also divided into 3 groups due to their behaviour when heated:
Thermoplastic – at high temperature they are plastic while at room temperature and below they are solid. These materials can be repeatedly melted and reworked. Thermoplastics can be processed many times; however, each heating and cooling cycle deteriorates their properties. The properties of thermoplastics result from their internal structure – their long chains are connected by intermolecular bonds that are becoming sensitive when they are heated. That’s why they deform easily and when they cool down, they return to a solid state. Due to the ease and quick technological process, thermoplastics are used for the production of dishes, bottles, toys, furniture and car bodies.
These are polymers which, once formed, cannot be melted again. When subjected to high temperature again, they do not melt but decompose. This is due to their internal structure. The polymer chains in thermosetting materials have the form cross linked into network structure. Cross linking bond connections are formed during a chemical reaction. That is why thermosetting material are hard, inflexible and have high resistance to stretching and compression. Because the cross-linked process eliminates the risk of melting thermosets under the influence of heat, thermosetting materials are often used in electronics.
Elastomers are synthetic or natural polymers that have the ability to be reversibly deformed under the action of mechanical forces while continuity of structure is hold. They have long chains braided and heavily cross-linked and coiled. Under the influence of force, the chains uncoil. Due to the possibility of performing segmental movements, polymer chains may be deformed under the influence of external stresses (e.g. straighten), which causes the dimensions of the materials made of them to change. Therefore, Elastomers have the ability to change in a wide range of their dimensions when it is subjected to tensile, shear or compressive stress and then return to previous dimensions. One of the most popular elastomers is rubber.
Composites are materials created from at least two components with different properties in such a way that it has better properties than can be obtained in each component separately. Composites with the simplest structure are those that are composed of two phases. The deeper layer which is the reinforcement and the outer phase called the Matrix.
Composite materials can be divided by shape of reinforcement into:
We divide composite materials into three main groups:
- Structural composite.
- Particle reinforced composite.
- Fibre reinforced composite.
The role of matrix in the composite material is gives the shape, holds the reinforcements together and protects reinforcement from chemical and environmental attack and ensures compressive strength.
In composite materials, the reinforcing phase consists of molecules or fibres. The reinforcement is responsible for improving the mechanical properties and strengthening the material.
The structure of the composite is heterogeneous, in cross-section we can distinguish layers that we call phases. The fundamental feature of the structure of the composite is the even distribution of all constituent substances throughout the volume, which means that all phases run in parallel throughout the entire body of material. Properties of composite material depend to their internal structure, the arrangement of their molecules relative to each other, how close they are.
The presentation of the properties of composite materials is not easy because they depend on the properties of the materials that make up the composite, and we have many of them. The basic feature of most composites is the durability of the structure as well as the resistance of the upper phase to mechanical damage. There are composite materials that endure high temperatures, others can carry large weight.
Relationship between composites properties and structure. As I mentioned earlier, the properties of composite materials depend on the arrangement, direction and proximity of the molecules and fibre in the composite material. I will explain this using the graphics attached below. For the composite to be strong, the load must be oriented towards the fibre. In the direction of the number (1), the fibres are arranged in the direction of the arrow, the load is in the same direction so the composite is strong. In (2) and (3) direction, the composite material is weak due to the fact that there are no fibres and the load falls on the Matrix.
Rule of Mixture
The relationship of composite materials can be determined using the rule of mixtures. We can use it to specify properties such as e.g. density or young`s module. This rule determines the percentage number of composite material properties that will come from matrix and reinforcement.
Explain the following types of degradation found in metals and non-metals, use image and diagrams and give examples of when and how each degradation can occur:
This is one of the types of material degradation occurring under the influence of load on a material depending on time and temperature. Material degradation takes the form of deformation. This deformation is caused by the fact that atomic bonds under the action of load and temperature are deformed, they shift in the direction of the load. These damages are durable, they cannot be reversed. This situation occurs in metals and plastics or composites. Creep in various materials depends on the temperature. In most metals, it will be a higher temperature than in polymers. Higher temperature promotes this degradation. This is due to the fact that the materials under the influence of higher temperature become more ductile thus more sensitive to deformation. An example of a creep is the deformation of metal parts of machines (shafts or rotors, parts of lifts) that work at high temperatures and are subjected to high loads and forces.
There are three stages of creep:
- Primary Creep
- Secondary Creep
- Final Creep
It is a process of material degradation consisting in changing the structural properties of a material caused by repeated deformation. The material decreases strength and finally cracks. The degradation process in this case is gradual, at the beginning there are small micro-cracks, which then grow and combine to finally bring to separate the material. This can also be defined as the number of load cycles that the material can transfer. The most common phenomenon of fatigue occurs where we are dealing with cyclical loads, i.e. in industry:
- machines (cutting tools)
- cars (engine components)
- aviation (turbine elements and wings)