*4.1.5. Laser ablation testing of C/C-ZrC composite*

temperature is not sufficient to lead to the sublimation of the carbon fibers, and they still remain

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

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

the original shapes. The ablated surface mainly shows the ablation of SiC matrix.

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

characterization results and discussion was proposed (**Figure 8**).

*4.1.4. Laser ablation mechanism*

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

Due to the higher melting temperature and high-temperature ablation resistance of ZrC compared with SiC, we produced a C/C-ZrC composite and tested the ablation resistance of the as-produced C/C-ZrC composite using an impulse laser beam. The ablation resistance of C/SiC composite was also tested in the same parameters for comparison.

**Figure 9** shows the linear ablation rates of C/C-ZrC composite and C/SiC composite versus the laser power densities. Both the linear ablation rates of the composites increase with increasing laser power densities. The linear ablation rates of C/C-ZrC composite are smaller than that of C/SiC composite. With increasing laser power densities, the difference of linear ablation rates between the two composites increases, which indicates that the ablation resistance of the C/C-ZrC composite is greatly improved at higher temperatures.

The much better ablation resistance of C/C-ZrC composite can be also illustrated by observing the morphologies of the ablated surface. As shown in **Figure 9**, the ablated surface of C/C-ZrC composite is covered by a white melting ZrO2 layer (determined by EDS) without obvious ablation pit, while the C/SiC composite is severely ablated with a deep ablation pit without any protecting layer on the ablated surface. ZrO2 owns a high melting point (2700 °C), low oxygen penetration rate, and excellent ablation resistance, which can act as a protecting layer for the C/C-ZrC composite, and thus, C/C-ZrC composite presents much better ablation resistance than the PIP C/SiC composite.

**Figure 9.** Linear ablation rates and macro-morphologies of (a) C/C-ZrC composite and (b) C/SiC composite after laser testing.


**Table 2.** Laser-testing parameters and weight changes of the composites before and after testing.

**Figure 10.** Surface morphologies of C/ZrC and C/Zr-Si-C composites after laser ablation testing.

Because of the excellent properties of carbon fiber-reinforced ZrC-based composites and their potential applications in hypersonic aerospace vehicles, Ultramet [26] developed a fast and economic reactive melt infiltration method to prepared C/ZrC and C/Zr-Si-C composites. Ablation resistance of the composites was tested using a continuous laser beam. Both C/ZrC and C/Zr-Si-C composites survived laser testing at 2871 °C and 2691 °C, respectively, under forced air flow at the Air Force Laser Hardened Materials Evaluation Laboratory. The testing parameters and weight changes of the composites before and after testing are shown

in **Table 2**. The composites show small weight change and excellent oxidation stability. Continuous protecting layers were formed on the ablated surface of C/ZrC and C/Zr-Si-C composites (**Figure 10**) and effectively protected the composites from severely ablation damage. Wang et al. [27] also prepared C/C-ZrC composite by reactive melt infiltration of liquid zirconium. They used a continuous wave CO2 laser (ROFIN DC080W, Germany) to test the ablation resistance of the composite. The ablation depth increased with the increase in laser power densities. The reactive melt-infiltrated C/C-ZrC composite presented much better ablation resistance than C/C composite.

### **4.2. Ultra-high-temperature ceramics and coatings**

**Figure 9.** Linear ablation rates and macro-morphologies of (a) C/C-ZrC composite and (b) C/SiC composite after laser

**Blackbody temp (°F)**

**Adjusted temp (°F)** **Weight**

**change/area (g/in2**

**)**

testing.

**Materials Test**

**length (s)**

**Heat flux (W/cm2**

**)**

C/ZrC 300 600 4858 5200 −0.029 C/Zr-Si-C 125 500 4556 4875 −0.051

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

**Table 2.** Laser-testing parameters and weight changes of the composites before and after testing.

**Figure 10.** Surface morphologies of C/ZrC and C/Zr-Si-C composites after laser ablation testing.

Because of the excellent properties of carbon fiber-reinforced ZrC-based composites and their potential applications in hypersonic aerospace vehicles, Ultramet [26] developed a fast and economic reactive melt infiltration method to prepared C/ZrC and C/Zr-Si-C composites. Ablation resistance of the composites was tested using a continuous laser beam. Both C/ZrC and C/Zr-Si-C composites survived laser testing at 2871 °C and 2691 °C, respectively, under forced air flow at the Air Force Laser Hardened Materials Evaluation Laboratory. The testing parameters and weight changes of the composites before and after testing are shown Owing to their high melting temperatures, good chemical stability and excellent oxidation, and ablation resistance at elevated temperatures, ultra-high-temperature ceramics have captured great attentions and have potential application in extreme aerothermal and nuclear power-generation environments. Laser heating, known as rapid heating and high power density, is a convenient technique to test the ablation resistance of ultra-high-temperature ceramics.

Yan et al. [28] prepared an ultra-high-temperature ceramic-based composite ZrB2/20SiC-Cu by spark plasma-sintering method and investigated the ablation behavior of the composite using a 20 MW/m2 high-intensity continuous laser beam. They characterized the phase and microstructure evolution of the composite during ablation with XRD and SEM. The results revealed that no macroscopic damage but only an ablated layer was observed after being ablated. The composite exhibited good ablation resistance against high-intensity continuous laser beam. Cu phase in the composite evaporated preferentially, and a ZrO2 layer with different forms such as closely packed nanoparticle and melting layer was generated at the laser spot center. Dendrites and columnar ZrO2 grains were found on the cross section morphologies of the composite after ablation, which further demonstrated the melting of ZrO2 layer during ablation. The liquid ZrO2 layer had capability of thermal insulation, which prevented the inner matrix from further ablation. Both the state transformation of Cu and generated ZrO2 dissipated a lot of energy, which was another important reason why ZrB2/20SiC-Cu composite had good ablation resistance in such ablation condition.

Due to the rapid achieving of ultra-high temperatures, Jayaseelan et al. [29] used a laser beam to heat and melt ultra-high-temperature ceramics. The laser beam is a 3 kW Nd:YAG laser with a 10 mm diameter collimated beam capable of delivering a heat flux of up to∼20 MW/m2 . Sample temperature was recorded by a pyrometer (Raytek 1MH) with a measurement range of 650 and 3000 °C. They investigated the microstructural evolution of spark plasma-sintered ZrB2, ZrB2/20 vol.% SiC (ZS20), and ZrC ultra-high-temperature ceramics (UHTCs) during laser heating. Laser heating at temperatures between 2000 and 3750 °C resulted in extensive bubble and crater formation on the surfaces of 10 mm diameter samples (**Figure 11**). Only crystalline zirconia with a wide range of morphologies including nodules, needles, nanofibers, and lamella was formed on the surface of ZrB2 and ZS20 samples laser heated in air up to 2700 °C (**Figure 11**). The surface of ZrC samples after laser heated in vacuum up to 3750 °C was characterized by dendritic and eutectic morphologies. Solidification cracks and trapped porosity were also observed on the samples' surface. A complex array of mechanisms involving solid, liquid, and vapor phases led to formation of these morphologies including melting, oxidation, volatilization, and liquid flow.

**Figure 11.** Macro-morphologies of laser-heated (a) ZrB2/20 vol.% SiC and (b) ZrB2 samples.

In addition to the laser ablation of ultra-high-temperature ceramics, the laser beams were also used to characterize the ablation resistance of ultra-high-temperature ceramic coatings. Liu et al. [30] prepared a two-layer SiC-ZrC coating on C/C composite by chemical vapor deposition method. They characterized the ablation resistance of the coating using a continuous CO2 laser beam and investigated the laser ablation behaviors and ablation mechanisms of the SiC-ZrCcoated C/C composites. The results indicated that the ablation depth and width increased with increasing the laser power. Laser ablation resistance of the composites was greatly improved by the SiC-ZrC coating. They found three ablation regions on the ablated coating surface and discussed the formation mechanism. At the ablation center, the ablation was thought to be dominated by the blast and sublimation processes. At the transitional zone, the main ablation mechanism was evaporation. At the ablation edge, the ablation was mainly controlled by the oxidation process. Li et al. [31] also employed the laser beam to characterize ablation resistance of ultra-high-temperature ceramic coatings. A TaC coating was prepared on C/C composite by chemical vapor deposition to improve ablation resistance of the composite at high temperatures. They tested the ablation resistance of TaC coating and investigated ablation mechanism.
