**3.2. Chemical aspects**

material from the surface above the liquid-vapor interface. Assuming the thermal ablation as vaporization, the flow of material vaporized from the surface of a body at temperature *T* can be calculated according to the Hertz-Knudsen equation [13], leading to an ablation rate *ϑ* as

> *V B Bb*

é ù æ ö <sup>=</sup> - - ê ú ç ÷ ê ú ë û è ø (6)

and *ga* are the degeneracy of states for ions and atoms/

(7)

*mk T k T T*

 r

where *Tb* is the boiling temperature at pressure *p*0, *kB* is the Boltzmann constant, *β* is the back

Irradiated by the laser beams with sufficiently large intensity, a great amount of surface evaporation occurs as mentioned in the previous sections. Once the vaporization takes place, the interactions between the as-produced vapor and the incident laser beam become important in determining the overall effect of the laser irradiation on the substrate material. Interaction of the laser with the vapor can lead to the ionizing of the vapor. The highly ionized vapor is termed as plasma. In dynamic equilibrium, the degree of ionization *ε* in the vapor can be

( ) ( ) <sup>0</sup> 1 1 <sup>1</sup> 2

*m pL <sup>T</sup>*

382 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

p

 b

flux coefficient, and *LV* is the latent heat of vaporization of the material.

<sup>3</sup> <sup>2</sup> <sup>2</sup>

 p

2 2 exp <sup>1</sup>

e

e

is the ionization energy.

2

æ ö æ ö <sup>=</sup> ç ÷ - ç ÷ - è ø è ø

*iB i a g B g mk T E gN h k T*

with *ε* = *Ne*/*Ng* and *Ng* = *Ne* + *Na*. Here, *Ne* and *Na* are the number densities of electrons and

The vapor and the plasma can absorb and scatter the laser radiation, which changes the actual flux received by the substrate surface. Recoil from the vapor and plasma can also generate shock waves in the substrate material, which may cause plastic deformation and work hardening [19]. Expulsion of any remaining molten material as well as initiate shock waves can be further caused by the recoil as well. In this chapter, laser ablation method is used to characterize the ablation-resistance performance of materials. It is not reasonable to choose too large laser intensity. Therefore, we make very little consideration of plasma formation.

Irradiated by the laser beams with large power densities, the material surface is heated to a rather high temperature, and significant surface evaporation and sometimes plasma take place, which makes a positive pressure over the ablated surface. Surface temperature increment from 300 to 3500 K can lead to an enormous vapor pressure increase from 10 bar to almost 160 bar

J

[17],

*3.1.5. Plasma formation*

expressed by the Saha equation [18]:

atoms/molecules, respectively; *gi*

*3.1.6. Recondensation and resolidification*

molecules; and *Ei*

In addition to the physical aspects, the substrate materials and ablation products are believed to react with the ambient gas, and some new compounds may be formed during the laser ablation process. Thus, the laser ablation should also be discussed from the chemical aspects.

Normally, laser ablation is carried out in the air atmosphere. Irradiated by the laser beams, the substrate material is heated to a high temperature, and the phases in the materials can react with the oxygen in the air atmosphere. For carbon-based materials, such as graphite and C/C composite, the reaction of the materials with oxygen produces gas products, CO and CO2, which are liable to eject from the ablated surface. The substrate materials are severely ablated with a large decrease of external dimensions. However, for the refractory alloys and refractorybased ceramics, refractory oxides are produced due to the oxidation reaction. The refractory oxides have high melting points and low evaporation rates at high temperatures (see in **Table 1** [20]). Especially, the oxygen permeabilities in these oxides are very small. The substrate materials can be isolated from most of the oxygen, and thus, the substrate material can be prevented from the reaction of oxygen in the air atmosphere. Taking this into consideration, the former reaction-formed oxides provide protection for the substrate materials and reduce the damage of the laser ablation.


**Table 1.** Properties of some refractory oxides.

Irradiated by laser beams with very large intensity, the substrate materials may be heated to very high temperatures over the decomposed temperature of some phases. Taking carbon and SiC, for example, they sublimate once heated over 3827 and 2987 K, respectively [21]. The gas products eject from the ablation surface and may also result in a severe damage to the substrate materials. Besides, these gas products may also recondensate and form some thin films and nanoparticles at the rim and surrounding areas of the ablated region. Understanding these chemical reactions during laser ablation plays a great role to analyze the final morphologies of the ablated surface and study the ablation mechanism of the substrate materials. It should be noted that the above reactions are greatly affected by the temperatures during laser ablation, which depends on the laser parameters (pulse duration, energy, and wavelength), the substrate materials' properties, and the surrounding environment condition.
