## 3 Results and Discussion
### Analysis of Coating Performance Test
#### 3.2.4 Coating Thermal Shock Resistance Test  
As can be seen in Figure 7, the number of thermal shock cycles of the specimens decreases gradually with the increase of thermal shock temperature. Among them, the number of thermal shock cycles of the specimens reaches the maximum at 600 \({}^{\circ}\)C thermal  
\begin{table}
\begin{tabular}{c c c c} \hline
**Test item** & **Sample** & **Heating Rate** & **Shearing Strength (MPa)** \\ \hline Shear strength test of coating and substrate & 5 \({}^{\circ}\)C/min ceramic coating & 5 \({}^{\circ}\)C/min & 2.36 \\  & 2 \({}^{\circ}\)C/min ceramic coating & 2 \({}^{\circ}\)C/min & 4.13 \\ Shear strength of the coating itself & 5 \({}^{\circ}\)C/min ceramic coating & 5 \({}^{\circ}\)C/min & 0.39 \\  & 2 \({}^{\circ}\)C/min ceramic coating & 2 \({}^{\circ}\)C/min & 0.74 \\ \hline \end{tabular}
\end{table}
Table 5: Shear strength test results.  
shock temperature and the minimum at 800 \({}^{\circ}\)C thermal shock temperature. In addition, the composite ceramic coating with a heating rate of 2 \({}^{\circ}\)C/min showed superior thermal shock resistance compared to the coating with a heating rate of 5 \({}^{\circ}\)C/min. The composite coating with a 2 \({}^{\circ}\)C/min ramp rate had the highest number of thermal shock cycles under the 600 \({}^{\circ}\)C test condition, while the coating with a 5 \({}^{\circ}\)C/min ramp rate had the lowest number of thermal shock cycles under the 800 \({}^{\circ}\)C test condition. This suggests that a lower temperature increase rate may help to improve the thermal shock resistance of ceramic coatings.  
The thermal shock resistance of a coating is not only affected by the coefficient of thermal expansion of the substrate itself but is also closely related to the bond strength between the coating and the substrate. In general, the coefficient of thermal expansion of metallic substrates is usually larger than that of non-metallic oxides. Among them, the thermal expansion coefficients of Al\({}_{2}\)O\({}_{3}\) are relatively close to those of the metal matrix, while the thermal expansion coefficients of TiO\({}_{2}\) differ greatly from those of the metal matrix, and the surface of nano-TiO\({}_{2}\) contains unsaturated bonds and hydroxyl groups in different bonding states, which gives it high surface activity, thus promoting the chemical reaction of the ceramic coating during the curing process, and the new substances generated further enhance the bonding strength of the coating [58]. In addition, the coefficient of thermal expansion of nanocrystalline materials is usually more than one times higher than that of micrometer crystalline materials, so replacing micrometer particles with nanoceramic particles can effectively increase the coefficient of thermal expansion of the coatings and reduce the thermal stresses between the coatings and the substrate, thus improving the thermal shock resistance of the coatings [59].  
The experimental results show that the pore structure at the coating interface has an important effect on thermal shock resistance. Coatings with a small number of micropores at the interface show superior thermal shock resistance compared to coatings with large pores. This may be due to the fact that micropores can effectively relieve stress concentration and reduce crack extension during thermal shock, thus improving the thermal shock  
Figure 7: Experimental test results of ceramic coating against thermal shock.  
stability of the coating. However, coatings prepared at a 5 \({}^{\circ}\)C/min heating rate tended to form more large pores, which not only reduced the bonding strength of the ceramic coatings to the substrate, but also provided penetration channels for corrosive atmospheres and accelerated oxidization reactions at the interfaces, thus weakening the thermal shock resistance of the coatings. This phenomenon further suggests that reducing the heating rate can help optimize the densification of the coating and enhance its thermal shock resistance.