**1. Introduction**

In the 20th century, Laser surface alteration played a major role in enhancing the material surface properties. Among the number of ways to enhance the material properties, laser based surface alterations are used to enhance a better physical property in the machined surface and improved the component performance. The high power Neodymium Yttrium-Aluminum-Garnet (Nd: YAG) laser, carbon-dioxide (CO2) laser and excimer lasers are used to perform the laser surface treatment which is expensive, popular and operate at pulsed mode or continuous wave mode. These lasers are used to heat the near-surface area of the finished components for enhancing the properties. The laser surface modifications have the ability to control the amount of heat energy to work material with high directionality. The purpose of a surface hardening by laser is to improve the component wear properties. The laser surface hardening is defined as the heat energy from the laser beam that directly heated the component surface at a very short interval period without melting the work material. The heat input to the component surface is the reason for creating the tough and fine-grained structure in the hardened surface. The risk of crack forming is very low due to the self-quenching process. The laser surface melting (LSM) is heated to its melting point through a high power laser beam and rapidly solidified. The aim of LSM is to refine the surface microstructure, homogenization of composition, dissolution of precipitates. The LSM is also used to improve the corrosion resistance of steel and iron. The minimization of intergranular corrosion

is possible through LSM by avoiding the carbides formation during subsequent homogenization and sensitizing treatment. The laser surface alloying (LSA) is defined as the high heat energy used to melt the metal coating through laser and a portion of underlying substrate. This technique is used to form highly resistant gradient layers on the metal surface. The major benefit of this technique is sudden heating followed by cooling and the surface properties are improved. The laser cladding (LC) is a coating method that the surface melting and new material layer formation by addition of material are simultaneously processed in the substrate at the same time by using the laser power. The desired surface properties are achieved after solidification. The large component surface properties are easily increased by using LC. The complete metallurgical bond is necessary between the melting of substrate and forming of a new material layer at the interface. The laser surface texturing (LST) is defined as the process in which the change of material surface properties by modifying its texture and roughness. The laser beam is used to create the micro patterns on the surface by laser ablation. The micro patterns are created on the surface in various shapes such as dimples, grooves and free forms with precise dimension. This process is mostly used in biomedical applications.

The different types of laser have different abilities to perform the process on materials. All the lasers are producing the heat energy and the laser beam wavelength is majorly affecting the performance of materials. Generally, the total laser heat energy is supplied to work material in which can be divided into two ways such as the fraction of heat energy is observed by work material and remaining heat energy is reflected to the environment. This happens during the surface hardening by laser. The supply of heat energy to polished metal surface components is depending upon the heat absorbability of work material and wavelength of irradiation. Generally, the short wavelength has higher absorptivity. Hence, the Nd: YAG laser (λ = 1.064 μm) has produced the higher absorbing ability beam to work material than the CO2 laser (λ = 10.6 μm) for surface hardening of steel. In order to increase the CO2 laser absorbility (high wavelength) to work material, the coating or painting is required in the work material prior to the CO2 laser surface hardening. Therefore, the Nd: YAG laser surface hardening better than CO2 laser surface hardening because the Nd: YAG laser has short wavelength and produces a high absorbing rate to work material. The Nd: YAG laser produces heat energy to work material which is transferred through fiber cable whereas CO2 laser is impossible. The inert gases, helium, neon and argon are used to eliminate the atmospheric contamination. In order to reduce the wavelength of a laser, an excimer laser is developed with very short wavelength. This laser can be used to micromachining on medical parts. In this chapter, laser surface hardening, laser surface melting, laser surface alloying, laser surface cladding and laser surface texturing have been discussed to improve the microstructure, hardness and wear resistance of mechanical components.
