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.

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.

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.

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.

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.

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.

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.

Screws and bolts are usually made of steel. Where great resistance to weather or corrosion is required, like in very small screws or medical implants, materials such as stainless steel, brass, titanium, bronze, silicon bronze or monel may be used.

Galvanic corrosion of dissimilar metals can be prevented (using aluminum screws for double-glazing tracks for example) by a careful choice of material. Some types of plastic, such as nylon or polytetrafluoroethylene (PTFE), can be threaded and used for fastenings requiring moderate strength and great resistance to corrosion or for the purpose of electrical insulation.

Often a surface coating is used to protect the fastener from corrosion (e.g. bright zinc plating for steel screws), to impart a decorative finish (e.g.japanning) or otherwise alter the surface properties of the base material.

Selection criteria of the screw materials include: size, required strength, resistance to corrosion, joint material, cost and temperature.