**3. Fundamentals of laser-material interactions**

Because of the unique energy sources characterized by spectral purity, spatial and temporal coherence, and high intensity, the laser beams have captured significant attention and widely been used in matrix-assisted laser desorption/ionization, laser surgery, micro-fabrication, pulsed laser deposition, etc. [1]. Recently, they were successfully used to evaluate the ablationresistant performance of materials, which provided us more knowledge about the usability of materials in the ablation environment and developing protection against laser irradiation. A great amount of work on laser ablation of polymer-based composites, ultra-high-temperature ceramics (UHTCs), ceramic-based composites, and ablation-resistant coatings has been reported. The linear and mass ablation rates of these materials were tested using the laser ablation method, and the laser ablation behavior and mechanism were investigated by

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

In this chapter, we firstly made a simple introduction of the ablation-resistance characterization methods for materials and compared the advantages and disadvantages of these methods with laser ablation method. To help understand the laser ablation mechanism of ultra-hightemperature materials, the fundamentals of laser-material interactions were discussed from the physical aspects and chemical aspects. The physical aspects mainly involved the absorption of laser radiation, heating and propagation, melting, vaporization, and solidification. The chemical aspects mainly involved the decomposition of the phases in materials and the reaction of the materials with the atmosphere. Finally, we presented some practical applications of laser ablation in ablation-resistance characterization of materials based on our research and some relevant literatures. The ablation rates of materials under different laser parameters were tested. The morphologies of the ablated surface were observed by electron microscopes, and

**2. Ablation-resistance characterization methods for materials**

Advanced aerospace structures and anti-ablation components, such as nose caps, sharp leading edges, and rocket engines for hypersonic aerospace vehicles, suffer from high heat fluxes and pressure, severe thermal shock, and perhaps high-speed erosion of ceramic particles in their working conditions [2]. The serving temperatures of these components may increase rapidly from the room temperature to over 2000 °C and last from several seconds to several hundreds of seconds. Materials with outstanding mechanical and ablation-resistant properties are required forthese structures and components. Refractory metals, carbon-based composites (graphite and C/C composite), ultra-high-temperature ceramics, and composites are potential candidates due to their extremely high melting points, high-temperature mechanical strength, and outstanding ablation resistance. Due to the special serving environments, it is necessary to validate the ablation properties of these materials. Ablation resistance has been one of the most important properties in evaluating the usability of these materials. Great efforts should be devoted to the investigation on the ablation-resistance characterization, microstructure evolution, and ablation mechanism of these materials before their practical applications in the

experiments and numerical simulation.

the ablation mechanism was discussed.

ablation environments.

Laser-material interactions are very important to understand laser ablation process of materials. Irradiated by the laser beams, the substrate materials absorb the irradiation energy. Absorption of radiation in the materials results in various effects such as heating, melting, vaporization, and plasma formation. The extent of these effects primarily depends on the characteristic of electromagnetic radiation and the thermophysical properties of the substrate materials [8]. The laser parameters include intensity, wavelength, angle of incidence, spatial and temporal coherence, illumination time, and polarization, whereas parameters of the substrate materials include absorption of the laser energy, thermal conductivity, specific heat, and density. They all should be taken into account in order to understand in details the effects of laser ablation processing on the substrate materials tested. In addition to these physical aspect effects, the substrate materials are believed to react with the atmosphere under the thermal impact. The phases in the substrate materials will be changed, and they even decompose and vaporize, which greatly affects the ablation resistance of the substrate material. Thus, the laser-material interactions are very complex, and only in some simple cases, the laser can be merely seen as a heat source. The laser-material interactions should be comprehensively considered from the physical aspects and chemical aspects.

### **3.1. Physical aspects**
