(Components)Depending on the materials, applications, and require speed, various laser sources are use for cleaning. Table 7.1 presents the different types of laser systems used for cleaning along with the wavelengths and their typical usage.

1.7 Typical Contaminants Found in Industrial Components

Few contaminants are often bound very strongly to the surface, making their removal difficult. The adhesion of contaminant particles to a solid substrate depends on secondary bonds [17]. Secondary bonds are weak in comparison to primary bonds (ionic, covalent, and metallic). In dry surroundings the main forces of adhesion to solid surfaces are: (i) electrostatic, (ii) secondary van der Waals forces, and hydrogen bonding.

Electrostatic action is generate by electric charges, which form at the contact interface between two materials. Van der Waals forces are generate by dipole interaction between two molecules. A dipole can be induce on a molecule by the permanent or instantaneous dipole moment of another molecule. Thereby creating a force of attraction between the two molecules. Therefore, there exists a weak attractive force acting between all atoms and molecules at all times. Allowing molecules to stick together even when there are no attractive forces due to permanent charge separation in the molecules themselves. A hydrogen bond is formed by electrostatic attraction between a hydrogen atom bound to an electronegative atom and another electrically negative atom.

In general, contaminants adhere to the material surface by one or another of these secondary bonds.

The main force of adhesion of small particles, less than 50 μm diameter, on a dry surface are van der Waals forces. Electrostatic forces become important and predominate for particles larger than 50 μm diameter. Electrostatic forces also play an important role in attracting particles to the surface for adhesion. The total adhesion forces acting on a particle of only 1 μm diameter can exceed gravitational forces acting on that particle by a factor greater than 10⁶. In a humid atmosphere, other forces due to capillarity and surface tension will be active. The problems of cleaning can be consider as comparisons of the strength of adhesion between the dirt and the object being clean and the cohesion of the molecules of the object to one another.