**4. Laser ablation applications in ablation-resistance characterization of materials**

### **4.1. Carbon fiber-reinforced ceramic matrix composites**

Carbon fiber-reinforced ceramic matrix composites, combined with the excellent properties of carbon fiber and high-temperature ceramics, are one of the most promising candidate materials for high-temperature components due to their unique properties such as relative low density, low coefficient of thermal expansion, high specific strength/modulus, and excellent ablation resistance. In this section, we chose C/SiC composite as an example and comprehensively discussed the laser ablation behavior and mechanism. In addition, some recent work on laser ablation of C/C-ZrC composite in o*u*r group and by some other researchers was reviewed.

### *4.1.1. Production of C/SiC composite and the laser ablation test*

C/SiC composite for laser ablation testing was fabricated using a polymer infiltration and pyrolysis (PIP) method [22] by the Key Laboratory of Advanced Ceramic Fibers and Composites (National University of Defense Technology, Changsha, China). Three-dimensional braided carbon fiber preform was used as the reinforcement. Polycarbosilane (PCS) was chosen as the precursor of SiC matrix. Divinylbenzene (DVB) was used as solvent and cross-linking reagent for PCS. The preparation process of the composite contained three steps: (1) the carbon fiber preforms were immersed into the PCS/DVB solution and infiltrated with the PCS/DVB solution in vacuum at room temperature; (2) the preforms filled with PCS/DVB solution were cured at 150 °C; and (3) the cured preforms were pyrolyzed at 1200 °C to form the SiC matrix in an inert atmosphere. In order to densify the composites, the other several infiltration-curepyrolysis cycles were repeated. The as-produced C/SiC composite is composed of carbon and SiC (determined by XRD) and generally dense with two kinds of pores. One is small distributing in the intra-fiber bundles, and the other is larger locating in the interfiber bundles (**Figure 4**).

**Figure 4.** Cross-sectional microstructure (a and b) and XRD diffraction patterns (c) of C/SiC composite.

Ablation properties of the as-produced C/SiC composite were tested using a pulsed laser in the air. The laser ablation equipment is an Nd:YAG pulsed laser (wavelength 1.064 μm) with the following parameters: frequency 20 Hz and pulse width 1 ms. During the ablation testing, the C/SiC composite was located in a test chamber and was then vertically irradiated by the pulsed laser. The ablation depth of C/SiC composite was given by the thickness changes before and after the ablation test, which was measured by a microscope.

**Figure 5.** Linear ablation rates of C/SiC composite versus (a) laser power densities and (b) time.

### *4.1.2. Laser ablation resistance of C/SiC composite*

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

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

**4. Laser ablation applications in ablation-resistance characterization of**

Carbon fiber-reinforced ceramic matrix composites, combined with the excellent properties of carbon fiber and high-temperature ceramics, are one of the most promising candidate materials for high-temperature components due to their unique properties such as relative low density, low coefficient of thermal expansion, high specific strength/modulus, and excellent ablation resistance. In this section, we chose C/SiC composite as an example and comprehensively discussed the laser ablation behavior and mechanism. In addition, some recent work on laser ablation of C/C-ZrC composite in o*u*r group and by some other researchers was reviewed.

C/SiC composite for laser ablation testing was fabricated using a polymer infiltration and pyrolysis (PIP) method [22] by the Key Laboratory of Advanced Ceramic Fibers and Composites (National University of Defense Technology, Changsha, China). Three-dimensional braided carbon fiber preform was used as the reinforcement. Polycarbosilane (PCS) was chosen as the precursor of SiC matrix. Divinylbenzene (DVB) was used as solvent and cross-linking reagent for PCS. The preparation process of the composite contained three steps: (1) the carbon fiber preforms were immersed into the PCS/DVB solution and infiltrated with the PCS/DVB solution in vacuum at room temperature; (2) the preforms filled with PCS/DVB solution were cured at 150 °C; and (3) the cured preforms were pyrolyzed at 1200 °C to form the SiC matrix in an inert atmosphere. In order to densify the composites, the other several infiltration-curepyrolysis cycles were repeated. The as-produced C/SiC composite is composed of carbon and SiC (determined by XRD) and generally dense with two kinds of pores. One is small distributing in the intra-fiber bundles, and the other is larger locating in the interfiber bundles

materials' properties, and the surrounding environment condition.

**4.1. Carbon fiber-reinforced ceramic matrix composites**

*4.1.1. Production of C/SiC composite and the laser ablation test*

**materials**

(**Figure 4**).

Linear ablation rates of C/SiC composite tested with different laser power densities are shown in **Figure 5(a)**. It is indicated that the linear ablation rates of the composites increase with increasing laser power densities. During the ablation process, the laser energy is absorbed by the composite. Along the heat penetration depth and conduction width, the conduction laser energy decreases progressively from their input value, which in turn affects the corresponding temperature distribution. The larger the testing laser power density is, the higher the temperature of the composite is heated to be and the greater the heat penetration depth is. Therefore, the linear ablation rates increase with increasing of the laser power densities.

Ablation resistance of C/SiC composite was also tested for different time periods. **Figure 5(b)** shows the linear ablation rates of C/SiC composites tested for different time periods with laser power density of 1000 W/cm2 . It is indicated that the linear ablation rates of C/SiC composites decrease with an increase of ablation time.

**Figure 6.** Ablated surface morphologies of C/SiC composite at 1000 W/cm2 for 20 s (a and b) three ablated regions on the ablated surface, (c) macro-morphologies of the ablation center, (d and e) large magnification morphologies of ablation center, and (f) large magnification of the marked area in (e).

### *4.1.3. Microstructure morphologies of the ablated composite [23]*

Ablation resistance of C/SiC composite was also tested for different time periods. **Figure 5(b)** shows the linear ablation rates of C/SiC composites tested for different time periods with laser

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

. It is indicated that the linear ablation rates of C/SiC composites

for 20 s (a and b) three ablated regions on

power density of 1000 W/cm2

decrease with an increase of ablation time.

**Figure 6.** Ablated surface morphologies of C/SiC composite at 1000 W/cm2

tion center, and (f) large magnification of the marked area in (e).

the ablated surface, (c) macro-morphologies of the ablation center, (d and e) large magnification morphologies of abla-

**Figure 6** shows the surface morphologies of the C/SiC composite after laser ablation testing at 1000 W/cm2 for 20 s. Three ablated regions can be identified on the ablated surface of the composite according to the difference of the morphologies: region I ablation center with a deep pit, region II transitional zone with a lot of spheric particles, and region III ablation edge covered by a white glassy layer (**Figure 6(a** and **b**)). Detailed scanning electron microscope (SEM) characterization of region I (**Figure 6(c** and **d)**) shows that the ablation center region exhibits a needle-like structure with taper-ended carbon fibers standing on the ablated surface filled with some nanostructure sheet. In order to determine the composition of the above three ablated regions and the nanostructure sheet, EDS analysis was carried out. The results indicate that the nanostructure sheet is nanocarbon sheet composed of pure carbon. The spheric particles in the transitional region are composed of silicon and carbon owning the atom proportion of 54.47:45.53, which are SiC particles. The white layer covered on the ablated surface of region III ablation edge is SiO2 composed of silicon and oxygen.

During the laser ablation testing, the center region of the composite was instantly heated to a very high temperature, which was thought to be approximately higher than 3500 °C due to the large amount of ablated carbon fibers observed in the ablated center region. At such a high temperature, SiC matrix in the composite decomposes and sublimates, which forms a hot mixture of gases and vapors. Carbon fibers in the composite also get to its sublimation temperature to form a carbon vapor. The decomposition and sublimation of SiC and carbon fibers can be described as the following reactions:

$$\text{SiC(s)} \rightarrow \text{Si(g)} + \text{C(g)}\tag{8}$$

$$\text{SiC(s)} \rightarrow \text{SiC(g)}\tag{9}$$

$$\mathbf{C}(\mathbf{s}) \to \mathbf{C}(\mathbf{g})\tag{10}$$

Because the ablation testing was performed in the air, carbon fibers and SiC matrix are believed to react with the oxygen in the atmosphere and form the products of CO, CO2, SiO, and SiO2 due to the reaction with oxygen. The possible reactions that take place in the oxidation of carbon fibers and SiC matrix are as follows:

$$\rm{SiC(s) + O\_2(g) \to SiO\_2(g) + CO(g)}\tag{11}$$

$$\rm{SiC(s) + O\_2(g) \to SiO(g) + CO(g)}\tag{12}$$

$$\rm{C}(s) + \rm{O}\_{2}(g) \rightarrow \rm{CO}\_{2}(g) \tag{13}$$

$$\rm{C}(s) + \rm{O}\_{2}(g) \rightarrow \rm{CO}(g) \tag{14}$$

Among the products of the above reactions, SiO, CO, and CO2 are gases. SiO2 immediately gasifies due to its low boiling temperature, 2230 °C [24]. These gas products result in a positive pressure atmosphere on the ablation surface. Oxygen in the atmosphere on the ablation surface can be instantly exhausted, and the oxygen in the outside atmosphere hardly diffuses into the atmosphere on the ablation surface because of the positive pressure kept by the gasses and vapors produced according to Eqs. 8–14. Therefore, the oxidation of SiC and carbon fibers just takes place at the very beginning and is made no consideration in the subsequent analysis.

Ablation of the composite is greatly influenced by the temperatures functioning on the ablated areas. Irradiated by the laser, the center area of C/SiC composite was heated to the highest temperature, and it was severely ablated. Because the carbon fibers are thermally more stable than the SiC matrix, carbon fibers with taper ends protruded on the ablation surface without the SiC matrix as shown in **Figure 6(c)**. Inside the composite under the ablated center region, the heat absorbed from the laser penetrates into the inner of the body composite along the direction of the laser beam. It is known from Section 3 that the penetrated heat decreases with the increasing of the penetration distance. Thus, the temperature at this region decreases with the increase of the heat penetration depth. The ablation of carbon fibers is gradually alleviated due to the temperature decrease and finally avoided along the heat penetration direction. Therefore, carbon fibers on the ablated surface in the center region present a needled-like structure with taper ends. Though the ablation of carbon fibers is alleviative and avoidable along the heat penetration direction, the temperature at this region is high enough to lead to the SiC decomposition. Therefore, lots of grooves without SiC matrix are formed among the carbon fibers which are lower than the carbon fibers' taper ends. With further increase of the depth, the temperature in the grooves decreases. At a certain depth, the temperature reaches a critical value, and the decomposition of SiC matrix stops. The SiC decomposition according to Eq. 8 and carbon fiber sublimation according to Eq. 10 produce a carbon-rich atmosphere on the ablation surface. In the grooves among carbon fibers, the SiC matrix can decompose while the temperature is not high enough to sublimate the carbon fibers. Some nanocarbon sheet is deposited at the bottom region of the grooves (**Figure 6(c** and **d)**), which is quite similar to the preparation of carbon nanotubes by laser ablation method [25].

Different from the ablated center region, the conducted heat and the corresponding heat penetration depth at region II transitional zone of the ablation surface are much smaller. It is not high enough to lead to the decomposition, vaporization, and sublimation of SiC matrix. The mixed gases of C, Si, and SiC escaped from the ablation center region can be cooled down in this area with relatively lower temperature. SiC grains re-nucleate and grow to spherical particles (see **Figure 6(b)**). Though the temperature at the transitional zone is lower than the ablation center and cannot lead to the decomposition, vaporization, and sublimation of SiC matrix, it is still believed to be high enough to volatilize the SiO2 phase. Thus, no SiO2 was found at the transitional zone.

C s O g CO g ( ) + ® 2 2 ( ) ( ) (13)

C s O g CO g ( ) + ® <sup>2</sup> ( ) ( ) (14)

Among the products of the above reactions, SiO, CO, and CO2 are gases. SiO2 immediately gasifies due to its low boiling temperature, 2230 °C [24]. These gas products result in a positive pressure atmosphere on the ablation surface. Oxygen in the atmosphere on the ablation surface can be instantly exhausted, and the oxygen in the outside atmosphere hardly diffuses into the atmosphere on the ablation surface because of the positive pressure kept by the gasses and vapors produced according to Eqs. 8–14. Therefore, the oxidation of SiC and carbon fibers just takes place at the very beginning and is made no consideration in the subse-

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

Ablation of the composite is greatly influenced by the temperatures functioning on the ablated areas. Irradiated by the laser, the center area of C/SiC composite was heated to the highest temperature, and it was severely ablated. Because the carbon fibers are thermally more stable than the SiC matrix, carbon fibers with taper ends protruded on the ablation surface without the SiC matrix as shown in **Figure 6(c)**. Inside the composite under the ablated center region, the heat absorbed from the laser penetrates into the inner of the body composite along the direction of the laser beam. It is known from Section 3 that the penetrated heat decreases with the increasing of the penetration distance. Thus, the temperature at this region decreases with the increase of the heat penetration depth. The ablation of carbon fibers is gradually alleviated due to the temperature decrease and finally avoided along the heat penetration direction. Therefore, carbon fibers on the ablated surface in the center region present a needled-like structure with taper ends. Though the ablation of carbon fibers is alleviative and avoidable along the heat penetration direction, the temperature at this region is high enough to lead to the SiC decomposition. Therefore, lots of grooves without SiC matrix are formed among the carbon fibers which are lower than the carbon fibers' taper ends. With further increase of the depth, the temperature in the grooves decreases. At a certain depth, the temperature reaches a critical value, and the decomposition of SiC matrix stops. The SiC decomposition according to Eq. 8 and carbon fiber sublimation according to Eq. 10 produce a carbon-rich atmosphere on the ablation surface. In the grooves among carbon fibers, the SiC matrix can decompose while the temperature is not high enough to sublimate the carbon fibers. Some nanocarbon sheet is deposited at the bottom region of the grooves (**Figure 6(c** and **d)**), which is quite similar

to the preparation of carbon nanotubes by laser ablation method [25].

Different from the ablated center region, the conducted heat and the corresponding heat penetration depth at region II transitional zone of the ablation surface are much smaller. It is not high enough to lead to the decomposition, vaporization, and sublimation of SiC matrix. The mixed gases of C, Si, and SiC escaped from the ablation center region can be cooled down in this area with relatively lower temperature. SiC grains re-nucleate and grow to spherical particles (see **Figure 6(b)**). Though the temperature at the transitional zone is lower than the ablation center and cannot lead to the decomposition, vaporization, and sublimation of SiC

quent analysis.

**Figure 7.** Surface morphologies of C/SiC composite ablated at 150 W/cm2 for 20 s.

Compared with the ablated center region and the transitional zone, the heat at region III ablation edge is only conducted form the ablation center, and the temperature is the lowest. The oxygen in the outside atmosphere can continuously diffuse into the atmosphere over the composite surface of this area, and the C/SiC composite is oxidized. Due to the lowest temperature, SiO2 produced from the oxidation of SiC matrix cannot be volatilized. The composite at this region is covered by a white glassy SiO2 layer (**Figure 6(a)**). The electrical conductivity of SiO2 is not very well. Thus, the layer shows a white fuzzy pattern under the scanning electron microscope.

**Figure 7** shows the surface morphologies of the C/SiC composite ablated at 150 W/cm2 for 20 s. As can be seen, most carbon fibers remain their original shape without obvious ablation damage, which is owing to the low temperature heated by laser beam with low laser power density. Nevertheless, some nanocarbon sheets were found on the ablated surface and the SiC matrix among the carbon fibers decomposed (the same situation on the surface of the composite ablated at 1000 W/cm2 ), which indicates that the temperature on the composite surface ablated at 150 W/cm2 is high enough to induce the decomposition of SiC matrix. However, that temperature is not sufficient to lead to the sublimation of the carbon fibers, and they still remain the original shapes. The ablated surface mainly shows the ablation of SiC matrix.

### *4.1.4. Laser ablation mechanism*

Laser ablation of composite materials is very complicated and influenced with the laser power density and properties of phases in the composite substrate. In order to further understand the laser ablation processes of the C/SiC composite, an ablation model based on the previous characterization results and discussion was proposed (**Figure 8**).

As aforementioned, the temperature at the ablated center during laser ablation is the highest. The composite is heated to a very high temperature over 3500 °C. The SiC matrix reaches its decomposition and sublimation temperatures to form a hot mixture of gasses and vapors, and the carbon fibers get to its sublimation temperature to form a carbon vapor, which results in a positive carbon-rich atmosphere on the ablated surface. The carbon fibers are thermally more stable than the SiC matrix and carbon fibers with taper ends protrude on the ablation surface without the SiC matrix. Nanocarbon sheet is formed in the grooves among the protuberant

**Figure 8.** Schematic of the laser ablation processes.

carbon fibers where the temperature is high to decompose the SiC matrix but is not high enough to sublimate carbon fibers. The laser ablation of C/SiC composite in this center area is dominated by the decomposition and simulation process.

With the proceeding of the laser ablation, the positive gases escape from the surface of the ablated center due to the positive atmosphere produced by the decomposition, sublimation, and oxidation in the center region. Some gases deposit at the transitional zone (region II) where the temperature is relatively low and not high enough to lead to the decomposition of SiC phase. SiC grains re-nucleate and grow to spherical particles at the transitional zone, and the region becomes protuberant on the ablated surface.

At the ablation edge (region III), the heat is only conducted form the ablation center, and the temperature is the lowest among the three ablated regions. The temperature in this region cannot lead to the decomposition and sublimation of the phases. The C/SiC composite in this area is only oxidized and is covered by a white glassy SiO2 layer. The oxidation products of C/ SiC composite include CO, CO2, SiO, and SiO2. CO, CO2, and SiO are gaseous and escape from the ablated surface. SiO2 formed during the oxidation is in the liquid state at such a temperature, which can flow on to the carbon fibers and protects the C/SiC composite from further oxidation damage. The laser ablation of C/SiC composite in this region is dominated by the oxidation reaction of the composite with the atmosphere.
