Laser-induced layers peeling of sputtering coatings at 1064 nm wavelength

Large-scale layers peeling after the laser irradiation of dual ion beam sputtering coatings is discovered and a model is established to explain it. The laser damage morphologies relate to the laser fluence, showing thermomechanical coupling failure at low energy and coating layers separation at high energy. High-pressure gradients appear in the interaction between laser and coatings, resulting in large-scale layer separation. A two-step laser damage model including defect-induced damage process and ionized air wave damage process is proposed to explain the two phenomena at different energy. At relatively high energies (higher than 20 J/cm2), ionization of the air can be initiated, leading to a peeling off effect. The peeling effect is related to the thermomechanical properties of the coating materials.

Laser-induced damage parameters. In the experiment, the 1-on-1 laser damage performance test is carried out according to the standard ISO 21254 29 . The schematic diagram of laser damage test platform is shown in Fig. 1. The incident angle of laser to three samples is 0 degree. Sample I and II are tested on laser exit surface. Sample III is tested on laser incident surface. The pulse width of Nd: YAG laser is 12 ns at 1064 nm (1ω). The facula radius of the incident laser on the coatings is about 200 μm at 1/e 2 of the maximum intensity. In the laser damage experiment, there are 20 points irradiated by each energy step. The online CCD (charge coupled device) and offline optical microscope can be used to evaluate whether the test area is damaged.

Experimental results and analysis
Laser damage probability. Laser damage probability distribution of the Al 2 O 3 /SiO 2 coatings is shown in Fig. 2. It can be obtained that within 30 J/cm 2 energy, the probability of laser damage is low, and is about 40% around 70 J/cm 2 . The two-stage damage probability indicates that there are two different defects. One has a lower density but is prone to laser damage, and the other has a higher density but requires higher energy.
Laser damage morphology. Optical microscope (Leica) and optical profiler (Veeco) are used to characterize laser damage morphologies. Morphologies and damage pits depth of Al 2 O 3 /SiO 2 coatings are shown in Fig. 3(c) and (d) correspond to the depth distribution of (a) and (b), respectively. Figure 3(a) is single defectinduced damage, and (b) is multiple defect-induced damage. Obvious peeling off of coatings layer is observed, and no change in the color of the plasma ablation is observed. From the depth profile of Fig. 3(d), the damage depth is about 1.2 μm, which is close to the substrate. The defects of sample I is possible from interface of coatings and substrate. Field emission scanning electron microscopy (FE-SEM; Zeiss) is used to characterize the microscopic morphology of damage pits. Figure 4(a-c) show the damage morphologies of Al 2 O 3 /SiO 2 coatings at near damage threshold, medium energy, and high energy, respectively. Figure 4(d) and (e) are enlarged views of the central regions of (b) and (c), respectively. Laser damage near the threshold appears as thermal-mechanical coupling failure. The diameter of the damage pit is about 3 μm. The edge contour of the damage pit is clear and shows brittleness distortion, which indicates that the defects are far away from the coatings surface and the thermal effect is not obvious. The central thermal-mechanical coupling damage pit can still be observed at medium energy in Fig. 4(d), but the surrounding coatings are extensively damaged, which is manifested as peeling off. At high energy, the central damage pit is not deeper and only appears as more thermodynamic ablation. The area of ablation and damage of the surrounding peeling layers is larger. Ta 2 O 5 /SiO 2 and Nb 2 O 5 /SiO 2 coatings also show a similar phenomenon, that is, they only show thermal damage at low energy, and at high energy, in addition to thermal damage, the coatings show peeling off effect. The critical energy density of the three coatings is about 20 J/cm 2 . www.nature.com/scientificreports/ Figure 5 indicates that the size of the damage pit changes with the laser energy. Some damage pits of sample I and II are observed at relatively low energy, which are relatively small, especially sample I. Sample III is not damaged at relatively low energy, so no damage point was observed at relatively low energy. The damage pit size becomes significantly larger after energy above about 20 J/cm 2 , and with the increase of energy, the development of damage pit size approaches a linear increase. Under the same laser energy, sample I has the largest damage pit size, and sample III has the smallest size. This is related to the thermodynamic properties of the film composition of the samples, which will be explained in detail later.    30 , the shock wave is generated in the solid material, and the propagation speed is the speed of sound level. The propagation speed of shock waves in solid materials is much lower than the surface destruction speed. Therefore, the model we propose is to ionize air to generate plasma, and the speed of expansion and propagation in the air is in the same order of magnitude as the speed of destruction in the experiment. The possible formation of air laser supported detonation waves (LSD) is considered 31 . This happens when the free electron energy E can excite the neutral substance in the medium (mainly composed of O 2 and N 2 molecules) to ionize 32 . At the beginning of ionization, the maximum energy obtained by the electrons cannot be higher than the following value 33 : (1)   1), I is the laser light intensity, λ is the laser wavelength, The energy of air molecules (mainly N 2 and O 2 ) ionized by laser is 12 eV. Equation (1) can be used to calculate the laser power density required for ionizing air as 2.14GW/cm 2 at 1064 nm. According to the conversion formula ( F = 0.5 π/ ln 2Iτ ) of laser energy density and power density, the energy density of the laser can be obtained as 27.34 J/cm 2 . Laser damage will cause the temperature of the coating material around the defect to rise sharply and the absorption will increase 34,35 . The LSD wave front will absorb and reflect the laser 34,36-38 , making the initial electron avalanche ionization energy lower than 27.34 J/cm 2 , which is about 20 J/cm 2 in our experiment.
Thus, when the energy is low, the laser energy is lower than the ionization energy of the air, and no LSD wave in air can be generated. At this time, the laser and film defects interact with each other, and the defects absorb the laser energy, resulting in thermomechanical coupling damage, such as Fig. 4(a). Due to the deeper defects of the sample II compared to sample I and the strong layer binding force, the thermal effect is more obvious and the damage area is larger in the process of the sample II absorbing the thermal coupling effect of the defect. When the laser energy is greater than the ionization energy of air, LSD waves are generated in the air. Thus, the largescale emergence of peeling off of coatings is related to a propagation of LSD wave, which is similarly with ringpattern damage morphologies of the fused silica bulk material 39 . According to the experimental data in Fig. 5, the velocity of propagation of peeling off can be obtained as 21 km/s (laser energy: 70 J/cm 2 , maximum diameter: 500 μm), which is equivalent to the speed of a surface shock wave 23 . Multi-layer coatings deposited by dual ion beam sputtering usually possess high compressive residual stress 40 . The temperature of laser-induced plasma is higher than 10 4 K, and the pressure is higher than 1 GPa 41 . The laser-induced stress wave propagates horizontally and vertically in the coatings and reflects at the boundary of the coatings, thereby changing the residual stress field of the coatings. At the same time, when the stress wave propagates far away from the center of the laser spot, it attenuates exponentially, and gradually disperses. A stress field distribution similar to the shape of Airy Pattern is formed in the coatings 32 . At the same time, due to the high temperature gradient brought by the LSD wave, the samples are prone to peeling off. Thermodynamic parameters will affect the peeling size of the samples.
The separation of the coating layers originates from the changes in the local stress of different coating layers after the temperature rises, considering the case where the temperature has not reached the melting and vaporization of the coating layers. The change of coatings stress caused by temperature can be explained by the following formula 42 : Among them, α c , ν c , and E c are the thermal expansion coefficient, Poisson's ratio and Young modulus of different coating material. ΔT is the amount of change in temperature rise. Table 2 shows the mechanical parameters of SiO 2 , Al 2 O 3 , Ta 2 O 5 , and Nb 2 O 5 coating materials 43,44 . The stress change caused by the same temperature change in the Al 2 O 3 layer, Ta 2 O 5 , and Nb 2 O 5 are 63.59 times, 13.53 times, and 8.99 times that of SiO 2 layer, respectively. This explains that sample I which contains Al 2 O 3 /SiO 2 layers is more likely to occur peeling effect caused by temperature rise. Thus, the peeling off size of sample I is relatively larger.
Therefore, the laser-induced damage of dual ion beam sputtering coatings is mainly divided into two processes, as shown in the Fig. 6. The first step is the defect absorbing laser energy to induce damage, as can be seen in Fig. 6(a). In nanosecond laser damage realm, the distribution of defects is random, and the laser intensity is Gaussian, so the laser intensity of the defect location is random, and the size of the damage pit is also random. In the defect-induced damage process, damage morphology is also correlated with the thermomechanical parameters of the coatings, the type of defect, number of defects in the spot range, and the distribution depth of defect. In the interaction with the laser, the defect absorbs heat, and the local coatings is melted and gasified, resulting in initial damage to the film. Deeper defects require more layers to be destroyed, and the thermal coupling time is longer and the scale of the laser damage is larger.
The second step is the damage of ionized air waves, as shown in the Fig. 6(b), which only occurs at relatively high energies. Air is a wide band gap dielectric, which is basically transparent to the laser, and basically does not absorb laser energy. In the first step, the broken pieces and residual bonds become the precursor of the ionized air, providing the initial seed electrons. Electron avalanche occurs during laser irradiation, resulting in severe ionization of air and formation of plasma. Plasma almost completely absorbs laser energy. The heated gas expands to form a spherical shock wave in all directions, and the air is heated to dozens of eV. The ionization front expands, forming a temperature gradient, and then the film undergoes stress peeling off.

Conclusion
The phenomenon of peeling damage of the dual ion beam sputtering coatings was found and explained. A twostep laser damage model is proposed, including defect-induced damage process and ionized air wave damage process. At relatively high energies (higher than 20 J/cm 2 ), ionization of the air can be initiated, leading to a peeling off effect. The peeling effect is correlated with the thermomechanical properties of the coatings materials. For coatings with large stress differences, the peeling off effect is more serious. This article is helpful for the analysis of the damage process of the dual ion beam sputtering coatings, which can help improve the ability to resist laser damage from a process and design perspective. www.nature.com/scientificreports/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.