## 3 Results and Discussion
Mechanical properties are very crucial for the practical application of high-power laser coatings. Coating/substrate adhesion determines the lifetime and hardness decides the reliability of the coating. Figure 3 illustrates the adhesion and hardness of the HRI coating with different PI concentrations. Each sample was tested at three different places, and the results displayed in Figure 3 are the average value. Coating/substrate adhesion of three samples almost stays the same around 1.7 MPa, indicating no obvious effect of PI molecule on the adhesion. Meanwhile, coating hardness exhibits prominent variation along with the increasing of PI concentration. An increase followed by a decrease is observed in the coating hardness results, which may be ascribed to the role switch of the PI molecule. Increasing PI at the beginning provides more free radicals that help to facilitate the curing of prepolymer, but further increasing of PI concentration might separate the prepolymer and prevent it from cross-linking, which means the PI molecule actually acts partly as photoinitiator and partly as isolator. It acts more like isolator than photoinitiator and incomplete curing of coating takes place if its concentration increases. Figure 3: Adhesion and hardness of high refractive index (HRI) coatings with different PI concentrations. Figure 2: N\({}_{2}\) adsorption–desorption results of HRI coatings with different photoinitiator (PI) concentrations. LIDT is the acknowledged standard to estimate the laser-damage resistance, which is an important characteristic for the coatings being applied in the high-power laser system, and such a test has been performed on the HRI coatings with different PI concentrations. As the HRI coating only suffers 1\(\omega\) and 2\(\omega\) laser, its 1\(\omega\) and 2\(\omega\) LIDT has been verified using R-on-1 mode. Figure 4 depicts the testing results of different HRI coatings, in which the LIDT of 1\(\omega\) is estimated to be around 48 J/cm\({}^{2}\) (5 ns equiv.) and that of 2\(\omega\) is about 32 J/cm\({}^{2}\) (5 ns equiv.). No obvious variation trend can be observed for the LIDT with the increasing of PI concentration, indicating that the addition of UV PI does not deteriorate the 1\(\omega\) and 2\(\omega\) laser-damage resistance of coating. In the high-power laser system, some other key characteristics that influence the transmission quality of the laser beam also need to be taken into account, especially for the roughness, such as the R\({}_{\text{q}}\) (high-frequency spatial part) and PSD (middle-frequency spatial part) [25; 26; 27]. The R\({}_{\text{q}}\) (0.01-0.12 mm) results of HRI coating are illustrated in Figure 5a, in which a gradual but slight increase can be observed while the PI concentration increases. Figure 5b shows the PSD2 (0.12-2.5 mm) results of HRI coating with different PI concentrations. Nearly the same trend as the R\({}_{\text{q}}\) results is noticed, but the variation is a little larger than that of R\({}_{\text{q}}\). This means that the addition of the PI molecule gradually degrades the R\({}_{\text{q}}\) and PSD2 of HRI coating and deteriorates the transmission quality of high-power laser beam further. This is assumed to be attributed to the behavior of the PI molecule mentioned above. With the increasing of the PI molecule, more and more micropores are occupied inside the coating, resulting in coating structure deformation (as demonstrated in Figure 6) and unexpected degradation of R\({}_{\text{q}}\) and PSD2. Figure 4: 1\(\omega\) and 2\(\omega\) laser-induced damage threshold (LIDT) of HRI coatings with different PI concentrations. Figure 5: (**a**) Surface roughness and (**b**) power spectral density (PSD)2 of HRI coatings with different PI concentrations. Figure 7 demonstrates the transmission spectra of the consolidated coating system with different PI concentrations in HRI coating.