Ablation Behavior of Plasma-Sprayed La1âxSrxTiO3+Î´ Coating

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Ablation behavior of plasma-sprayed La SrTiO coating irradiated by high-intensity continuous laser Jinpeng Zhu, Zhuang Ma, Yinjun Gao, Lihong Gao, Vladimir Pervak, Lijun Wang, Chenghua Wei, and Fuchi Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11034 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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ACS Applied Materials & Interfaces

Ablation behavior of plasma-sprayed La1-xSrxTiO3+δ coating irradiated by high-intensity continuous laser Jinpeng Zhua,b, Zhuang Maa,b,Yinjun Gaoc, Lihong Gaoa,b*, Vladimir Pervakd, Lijun Wangc,e, Chenghua Weic,e, Fuchi Wanga,b a

School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China, b National Key Laboratory of Science and Technology on Materials under Shock and Impact, China, c Northwest Institute of Nuclear Technology, Xi'an 710024, China, d Ludwig-Maximilians-Universität München, Faculty of Physics, Garching 85748, Germany, e State Key Laboratory of Laser Interaction with Matter, China, *Corresponding author, E-Mail: [email protected]

ABSTRACT: Laser protection for optical components, particularly those in high-power laser systems, has been a major concern. La1-xSrxTiO3+δ with its good optical and thermal properties can be potentially applied as a high-temperature optical protective coating or high-reflectivity material for optical components. However, the high-power laser ablation behavior of plasma-sprayed La1-xSrxTiO3+δ (x=0.1) coatings has rarely been investigated. Thus, in this study, laser irradiation experiments were performed to study the effect of high-intensity continuous laser on the ablation behavior of the La1-xSrxTiO3+δ coating. The results show that the La1-xSrxTiO3+δ coating undergoes three ablation stages during laser irradiation: coating oxidation, formation and growth of new structures (columnar and dendritic crystals), and mechanical failure. A finite-element simulation was also conducted to explore the mechanism of the ablation damage to the La1-xSrxTiO3+δ coating and provided a good understanding of the ablation behavior. The apparent ablation characteristics are attributed to the different temperature gradients determined by the reflectivity and thermal diffusivity of the La1-xSrxTiO3+δ coating material, which are critical factors for improving the anti-laser ablation property. Now, the stainless steel substrate deposited by it can effectively work as a protective shield layer against ablation by laser irradiation. KEYWORDS: La1-xSrxTiO3+δ coating, plasma spraying, laser ablation behavior, finite-element simulation, transient temperature field

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1. Introduction The perovskite La1-xSrxTiO3+δ (LST) has attracted considerable attention because of its excellent physical and chemical properties.1-5 It has potential applications in the areas of acoustics,6 electricity,7 and optics.8,9 At present, studies on LST materials have mainly focused on the high electrical conductivity, good magnetic properties, and high transmittance of these materials and some of these studies have made significant breakthroughs.10,11 For instance, Suzuki8 and Yun12 et al. reported that an LST thin film has a transmittance higher than 80% in the visible wavelength range. However, most of the studies have focused on the high transmittance of LST bulk materials and LST thin films instead of LST thick coatings. In our previous studies, simulation results showed that the theoretical reflectivity of LST materials can reach up to 99% theoretically, and we successfully prepared LST particles and bulk materials with high reflectivity (95%),9 which is comparable to that of metallic aluminum. Furthermore, plasma-sprayed LST thick coatings have a good mechanical properties.13 The deposited LST coating, which had a reflectivity of 85%, was prepared using an optimal set of plasma-spraying parameters and subsequent heat treatment.14 A LST thick coating may provide advantages over its thin film and bulk material forms and may thus extend its applications in a high-temperature or hot-corrosion environments. Compared to the metal materials used in optical fields, LST has better thermal stability. This property indicates that LST with its good optical property can be potentially applied as a high-temperature optical protective coating or high-reflectivity material for optical components, instead of metals. Optical coatings can be applied in many technology systems and optical instruments such as the LIDAR, ALADIN and the French LMJ instruments.15-17 Laser is widely used in many optical components, and damage to optical coatings may lead to the significant performance degradation and lifetime reduction of instruments.18-20 With the rapid development of laser technology, the laser intensity is continuously increasing, and laser-induced ablation has become increasingly important.21 Therefore, laser protection for optical components, particularly those in high-power laser systems, is a major concern, and optical coatings should have good anti-laser ablation 2


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properties. Thus, the ablation behavior of coatings should be intensively investigated. Given that an LST coating has a complex composition and microstructure that comprises pores and interfaces, laser irradiation may have significant effects on the properties of this coating. However, studies on the high-power laser ablation behavior of LST coatings have rarely been reported. In this study, we conducted laser irradiation experiments to investigate the ablation behavior of the plasma-sprayed LST coating. Simulation results on the temperature field are also discussed to provide a better understanding of the mechanism of ablation damage. 2. Experimental details The La1-xSrxTiO3+δ (x=0.1) feedstock powder was prepared from La2O3, SrCO3, and TiO2 in the required stoichiometric ratio by the solid-state method at 1550 ℃ for 5 h.9 After sintering, the feedstock powders were obtained by sieving in the range of 40~80 µm. The flowability (52 s/50 g) and apparent density (1.2 g/cm3) of the powders were measured using a Hall flow meter. LST coatings with a thickness of 0.2 mm were deposited by the plasma spraying technique using a Praxair SG100. The plasma-spraying parameters are listed in Table 1. MCoAlY bond coatings with a thickness of 0.1 mm were deposited on the blasted surface of the substrate by high-velocity oxygen fuel spraying with a Praxair JP5000. A disk-shaped GH4169 high-temperature alloy (Ø24 mm×2 mm) was used as the substrate. The coating surface roughness (Ra 7.6 µm) was measured using a portable roughness tester. The phase of the LST coating was detected by X-ray diffraction (XRD) using an X’Pert PRO MPD. The lattice constants and volume of the LST coating were obtained using Jade 5.0, and the average crystal sizes were calculated using Scherrer’s equation on the diffraction peak with the highest intensity. The coating tensile bonding strength (28 MPa) was tested by a Multi-Specimen-Test Machine analysis system. The coating porosity (13%) was calculated by a Video Test-Master quantitative metallographic analysis system. The ablation morphology evolution was investigated by an S-4800 scanning electron microscope. A commercial laser irradiation system, operating a Nd:YAG continuous laser with 1070 nm wavelength in a 1 cm2 laser spot was utilized, 3



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as shown in Fig. 1. The thermal effect is studied by varying the laser power density from 500 W/cm2 to 1000 W/cm2 and the irradiation time from 3 s to 15 s. Table 1．．Air plasma spray parameters. Sample

LST

Primary gas Ar (scfh*) 120

Secondary gas He (scfh) 0

Current (A) 600

Carrier gas Ar (scfh) 10

Powder feed rate(g/min) 25

Spray distance (mm)

Gun speed (mm/s)

80

*scfh = standard cubic feet per hour

Fig. 1. Beam path schematic diagram of the laser ablation of the LST coating.

Given that only the back-surface temperature of the LST coating can be experimentally measured, whereas its front-surface temperature varying with the loaded laser cannot be detected, the transient temperature field during laser irradiation was simulated, to obtain an in-deep understanding of the mechanism of the ablation damage to the LST coating. Although some simulation models of the laser ablation of metals have been established based on one-dimensional formulations or two-dimensional finite-element models, investigations on the thermal phenomena in ceramic coatings with complex phase structures are still at an early stage.22, 23 In this paper, a three-dimensional finite element model (Fig. 2) is established. As functions of temperature, the materials’ physical properties used for the calculation are based on the experimental data listed in Table 2. Heat transfer is the primary process in the laser irradiation of a top-coated material. Thus, the Fourier heat equation can be used 4


500

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to describe the physical model. Considering that a phase transition may be induced by the high temperature during irradiation, such as melting or vaporization, it is reasonable to introduce the enthalpy of transition as a heat source in the transfer equation. Therefore, the equation can be written as

ρc

∂T = k∇2T + SH ∂t

where ρ is the density, c is the heat capacity, k is the heat transfer coefficient, T is the temperature, t is the time and SH is the enthalpy of phase transition. (If

there is no phase transition, it is zero.) Boundary conditions play a key role in the simulation. In our model, the heat flux on the front-surface of the coating should satisfy the equation

k

∂T = α I − εσ (T 4 − T04 ) ∂n

where α is the absorption coefficient, I is the power density, ε is the emissivity,

σ is the Stefan-Boltzmann constant, n is the normal direction of the front-surface, and T0 is the ambient temperature. The first item on the right side represents the absorption of the laser, and the second item represents the emission of thermal radiation, which results in a negative heat flux. The back surface of the coating should use a convection boundary complemented by thermal radiation when the temperature is high. Thus, the heat flux equation on the back-surface of the coating can be given as

k

∂T = h(T − T0 ) + εσ (T 4 − T04 ) ∂n

where h is the convective heat transfer coefficient. A finite-element method is used to solve the given three equations, and the temperature field as a function of both time and location can be obtained.

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Table 2．．Physical property parameters. Sample material

Heat capacity (J/g·°C)

LST MCrAlY GH4169 alloy

0.62 0.59 0.53

Density (g/cm3) 4.8 7.5 8.2

Thickness (mm) 0.2 0.1 2

25

Thermal conductivity (W/m·°C) 200 400 600 800 1000 (°C)

1.45 4.3 13.4

1.36 --15.9

1.3 6.4 18.3

1.28 --21.2

1.2 10.2 23.6

Fig. 2. Finite element model for the temperature-field simulation.

3. Results and discussion 3.1 Effect of laser ablation on the macrostructure of the LST coating The macroscopic morphological features of the laser-ablated coating surface are shown in Fig. 3. As shown in Fig. 3a, the square ablation area is consistent with the laser beam as well as the change in the coating color from gray to white with constant roughness. This result verifies the occurrence of oxidation in the irradiated area during laser ablation. This phenomenon was also observed in the post heat treatment on the LST coating in our previous work.13 When the irradiation time increases to 15 s, the oxidation area has extended to the entire surface as a result of lateral heat conduction, as shown in Fig. 3b. Furthermore, minor ablation-induced spalling can be observed at the center of the irradiated area. With an increase in the laser power density, the area of the oxidation expands, and the ablation behavior becomes significantly different, as shown in Fig. 3c and Fig. 3d. A further increase in the laser power density aggravates the ablation of the LST coating because it has absorbed higher energy within a very short time such that effective thermal diffusion is not possible, particularly at the laser beam center. Moreover, Fig. 3c shows that the ablation behavior has changed from oxidation to mechanical failure in the central 6


1.15 --30.4

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ablation area. The change in the color of the coating from white to black shows a sintered state with small cracks on the smooth ablation surface. The LST coating irradiated at 1000 W/cm2 for 5 s has been penetrated because of the thermal stress concentration effect, as shown in Fig. 3d.

Fig. 3. Macroscopic ablation morphology on the surface of the LST coating for different laser irradiation parameters.

3.2 Effect of laser ablation on the microstructure of the LST coating Changing the laser irradiation parameters induces a different ablation mechanism for the LST coating. Fig. 4 shows the reflectivity spectra and surface microstructure of the different areas of the LST coating irradiated at 500 W/cm2 for 10 s. The entire surface, which includes the two areas in Fig. 4a, retains the original structural characteristics with the certain porosity of the plasma-sprayed LST coating, as shown in Fig. 4b. For this set of irradiation parameters, the absorbed energy is mainly used for the temperature increase of the coating and as the activation energy for the oxygen diffusion in the LST lattice structure. On the basis of the reflectivity spectra, the oxidation induces a significant increase in reflectivity from 13% to 75% at 1070 nm, thereby significantly enhancing the laser irradiation resistance. This result suggests 7



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that an appropriate laser ablation process can help efficiently achieve the desired reflectivity, which may offer a novel technical idea in contrast to our previously reported long period of post heat treatment after deposition.14

Fig. 4. Reflectivity spectra and microscopic ablation behavior on the surface of the LST coating irradiated at 500 W/cm2 for 10 s.

As illustrated in Fig. 5a, when the laser ablation time is increased, the ablation behavior begins to enter a new stage, at which some uniformly distributed columnar crystals form in the central area of the LST coating because of the laser-sintering effect. This structure can also be found in some transition areas; however, the crystals are only at the early stage of grain growth, and the edge area demonstrates no apparent laser ablation behavior. In addition, the integrated effects of thermal stress and structural stress result in the formation and propagation of large cracks during the rapid heating and cooling of the coating, as shown in Fig. 5b and Fig. 5c. These cracks provide channels for heat diffusion. For this set of laser ablation parameters, the amount of reflected laser energy increases by improving the reflectivity, and the laser energy can also be consumed by overcoming the surface formation energy of columnar crystals.

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Fig. 5. Microscopic ablation behavior on the surface of the LST coating irradiated at 500 W/cm2 for 15 s: a-central area, b-transition area, c-edge area.

Fig. 6 shows the laser ablation behavior of the LST coating irradiated at a higher laser power density. As shown in Fig. 6a, the uniformly distributed columnar crystals grow into brittle dendritic crystals presenting a mutual crisscross shape in the central area. The formation of dendritic crystals is also related to the temperature gradient (G) and grain growth rate (R). As the G-R ratio decreases, the laser-sintered structure tends to dendritic crystal transformation during the laser irradiation. As a result of the increase in the length-diameter ratio as the grains grow, stress concentration can easily occur and induce crack formation and propagation. These cracks significantly reduce the mechanical properties of the LST coating, especially for the bonding strength. For this set of laser ablation parameters, the heat-affected area expands, and a larger temperature gradient leads to a different grain growth rate during the thermal diffusion from the center to the edge, so that two distinct ablation behaviors occur in the transition area, as shown in Fig. 6b: dendritic crystal formation and columnar crystal formation. However, no ablation behavior occurs in the edge area, where the original structural characteristics of the plasma-sprayed LST coating are retained, as shown in Fig. 6c.

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Fig. 6. Microscopic ablation behavior on the surface of the LST coating irradiated at 1000 W/cm2 for 3 s: a-central area, b-transition area, c-edge area.

The intensified ablation behavior in the LST coating irradiated by a high laser power density is closely related to the relevant optical behavior. Some literatures including our previous work have reported that oxygen vacancies are easily formed and result in δ˂0 in LST material system when the LST material is prepared at a low oxygen partial pressure or in a high-temperature environment (e.g., plasma spraying or high laser power density irradiation, shown in Fig. 7), and they have a significant impact on its microstructure and optical performance.14, 24-26. Specifically, the effect of oxygen vacancies formed by high laser power density irradiation on the optical reflectivity of the LST coating could be explained in two aspects. From the view of the microstructure, the low reflectivity of the LST (δ˂0) coating is attributed to oxygen depletion in the LST crystal lattice because the oxygen vacancies can act as absorption centers during the propagation of the laser within the crystal, and laser phonon scattering easily occurs in crystal structural defects.27 In addition, the distortion of the titanium-oxygen octahedra induced by the oxygen vacancies also contributes to the reduction of the reflectivity. The oxygen vacancy drives the Ti valence state from Ti4+ to Ti3+ by giving additional electrons, and the defect reaction equation is shown in Fig. 7, which also plays an important role in determining the optical reflectivity of the LST coating. 10


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From the energy band theory, the optical reflectivity of the LST coating is changed by the Ti3+ and oxygen vacancy, in essence, which leads to the generation of subbands or impurity levels below the conduction band. The distances between these states of subbands are less than the bandgap of the crystal without Ti3+ and oxygen vacancies, which indicates that the subbands can behave as trap centers inside the bandgap of the ideal case.28 This phenomenon is consistent with the reduction in the optical transparency of La-doped SrTiO3 thin films because of the oxygen vacancies.29 Therefore, some laser photons are absorbed rather than reflected (Fig. 7). Then, more laser energy is transformed to heat energy to intensify the ablation behavior of the LST coating, which is consistent with the change in microstructure in the irradiated areas.

Fig. 7. Schematic diagram of oxygen vacancy/Ti3+ formation and the corresponding energy level in the LST coating.

3.3 Phase structure analysis of the central laser ablated area The XRD patterns in Fig. 8 suggest that the phase structure in the central laser ablated area irradiated by the laser at 500 W/cm2 for 10 s is similar to that of the original LST coating. They both have the same main-phase orthorhombic structure (SrLa8Ti9O31) and a relatively small amount of cubic phase (SrTiO3, SrLa8Ti9O31 → SrTiO3 + La2O3, La2O3 with high saturation vapor pressure could easily be volatilized 11



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during plasma spraying.30), indicating that the LST coating with good thermal stability undergoes no other phase transformation at a low laser power density. This result is consistent with the microstructure analysis result. However, some relative diffraction intensities of the XRD patterns after laser irradiation at 1000 W/cm2 for 3 s become significantly stronger than those of the XRD patterns of the original LST coating. As a result, the irradiated LST coating exhibits a pronounced preferred orientation of a (401) crystal plane, and sufficient kinetic energy exists to initiate grain growth and grain boundary migration for this set of laser parameters. Furthermore, the nucleation of the columnar and dendritic crystals in the laser ablated areas is related to the growth extent of the grains with a preferred orientation that is proportional to the absorbed laser energy of the LST coating. This is mainly determined by reflectivity and laser irradiated parameter, especially for the laser power density. This result is in good agreement with the microstructure analysis and the following temperature field simulation of the laser-ablated LST coatings result. The results of the XRD pattern analysis are shown in Table 3. There is a slight shift to a high diffraction angle among the 2θ values corresponding to the (401) diffraction peaks upon increasing the laser power density, which indicates that lattice distortion has occurred, especially for the LST coating irradiated by a high laser power density. This phenomenon can be attributed to the incorporation of oxygen into the crystal lattice of the LST coating irradiated at 500 W/cm2 for 10 s, which improves the reflectivity due to the decrease of lattice defects. On the contrary, the reflectivity has decreased for the LST coating irradiated at 1000 W/cm2 for 3 s because of the generation of many oxygen defects with the sharp temperature rise, which is consistent with some literatures that reported at high temperature oxygen can be released from materials with a perovskite structure.31, 32 Normally, vacancy defects could lead to lattice contraction,33,

34

which is

consistent with the variation trends of the lattice constants and volume with the increasing laser power density. Moreover, the average crystal size has a significant increase in the LST coating irradiated at 1000 W/cm2 for 3 s, which is because of the greater amount of laser energy absorbed as the source of grain growth kinetic energy 12


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leading to the clearly preferred grain orientation. The formation of dendritic crystals with a porous microstructure also enhances the laser absorptivity during irradiation. Table 3. Results of XRD pattern analysis. Sample

LST

2θ (401)

FWHM (401)

Crystal size

(°)

(°)

(Å)

Lattice constants (Å)

a

b

Lattice volume

c

(Å3)

46.361

0.252

374

7.8173

5.5411

57.2218

2481.79

LST (500 W/cm2, 10 s) 46.372

0.240

396

7.8220

5.5490

57.2401

2484.47

LST (1000 W/cm2, 3 s) 46.448

0.189

520

7.8143

5.5341

57.1703

2472.33

Fig. 8. XRD patterns of the laser-ablated area on the surface of the LST coating.

3.4 Simulated temperature field of the laser-ablated LST coatings The temperature field was simulated using a finite element method for the LST coating laser-irradiated at 500 W/cm2 and 1000 W/cm2 to intensively explore the cause of the laser ablation behavior. First, the precision of the simulated model was optimized by comparing the measured and calculated back-surface temperatures of the LST coating irradiated at 1000 W/cm2. The curves for the measured temperatures at 3 s and 5 s laser irradiation times converge at the rising stage (Fig. 9), and thus the experimental tests have good stability. The simulated calculation temperature curve 13



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for the parameter set of a 1000 W/cm2 laser power density and 5 s laser irradiation time are in good agreement with the experimentally measured temperature curves. This result validates that the optimal simulation model is accurate and reliable, as it considers the changes in the physical property functions of temperature during laser ablation. The temperature field analysis can thus provide a good route for studying the laser ablation behavior of LST coatings.

Fig. 9. Back-surface temperature as a function of time in the central area irradiated at 1000 W/cm2.

According to the front-surface isothermal temperature diagram in Fig. 10, the external normal of the isothermal line is the direction of temperature gradient, which presents a radial distribution and depends on the value and density of the isothermal line. This temperature field provides a realistic representation of the ablation characteristic that the maximum temperature is reached in the central laser-ablated area with the length of square heat-affected zone from 12 mm (1000 W/cm2, 3 s) to 14 mm (1000 W/cm2, 5 s); this characteristic agrees well with the real ablation behavior depicted in Fig. 3. Clearly, the high laser power density can lead to a large energy deposition in the irradiated area, in which a large temperature gradient is easily formed because of not enough time for the absorbed laser energy diffusion. It is precisely because of the different temperature gradients determined by the laser power density that there is the formation of new microstructures. Therefore, as for the LST coating, as the temperature gradient reaches a critical level (500 W/cm2, 15 s), the 14


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columnar crystals gradually form and grow up. With further increasing temperature gradient (1000 W, 3 s), the crystal microstructure changes from columnar to dendritic. Under laser ablation at 1000 W/cm2 for 5 s, the temperature of the penetrated area of the LST coating soared to approximately 1700 °C, which is higher than the melting point of the LST material (1650 °C). This result illustrates that vaporization also occurs along with coating peeling during the laser ablation.

2

Fig. 10. Front-surface temperature field distribution with isothermal lines irradiated at 500 W/cm2 and 1000 W/cm

To further investigate the effect of temperature on the formation of columnar and dendritic crystals, Fig 11 shows the temperature and temperature gradient as a function of the radius in the front-surface irradiated area under different irradiation parameters. The temperature gradient was calculated by the slopes of the temperature versus distance along the radial direction. As seen from the figure, there is a higher temperature in the formation region of the dendritic crystal compared with that of the columnar crystal, which is consistent with the great temperature gradient in the range from the center to edge of the coating irradiated by a high laser power density. This is because the higher temperature and great temperature gradient can effectively contribute to the crystal transformation from columnar to dendritic as the driving force. This is due to the high heat diffusion at the edge of the irradiated area, where 15



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there is a larger temperature gradient than in the central area, especially for the high laser power density (1000 W/cm2, 5 s). In addition, the temperature gradient is mainly decided by the laser power density, which leads to the formation of different crystal microstructures. Therefore, the nucleation of the columnar and dendritic crystals is closely

related

to

and

proportional

to

the

absorbed

laser

energy/temperature/temperature gradient determined by the laser power density in the irradiated area.

Fig. 11. Temperature and corresponding temperature gradient as a function of the radius in the front-surface irradiated area under different irradiation parameters.

As shown in Fig. 12, the front-surface temperature and relative reflectivity change as functions of time in the central area irradiated at 1000 W/cm2 for 5 s. The trend of the front-surface temperature can be divided into three stages according to the curve slope: the rate of temperature increases follows a fast-slow-fast rule. At the first fast heating (FH, ≈ 600 °C/s) stage, the LST coating absorbs considerable laser energy because of its low reflectivity. Subsequently, the rate of the temperature increase decelerates, as a consequence of the reflectivity improvement (RI, ≈ 300 °C/s) of the LST coating. This phenomenon is consistent with the analysis of the ablation behavior. However, as the laser ablation behavior worsens, reflectivity deterioration (RD, ≈ 420 °C/s) occurs. In this third stage, only the formation and growth of columnar 16


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crystals are the main modes of energy consumption. After the laser irradiation, the front-surface temperature of the LST coating sharply drops.

Fig. 12. Front-surface temperature and relative reflectivity change as functions of time in the central area irradiated at 1000 W/cm2 for 5 s.

Fig. 13 shows the temperature in the central area as a function of the thickness direction and back-surface temperature field under 1000 W/cm2 irradiation. As expected, the temperature rises with an increase in the laser irradiation time, and the calculated temperature in the edge area is lower than that in the central area irradiated by the square laser beam. This result, which is consistent with the experimental observations in Fig. 3, is attributed to the geometric effect associated with the thermal transfer. Furthermore, a large temperature gradient exists in the top coating, particularly with an approximately 700 °C temperature drop when the LST coating is irradiated for 5 s, which shows a better thermal resistance than the bond coating because of its low thermal conductivity. There is also a significant gap at approximately 600 °C in the front-surface temperature of the LST coating irradiated between 3 s and 5 s because 3 s later, the reflectivity continues to deteriorate. The central temperatures at the interface between the bond coating and the substrate, that is, the maximum temperatures the substrate has suffered, do not exceed the melting point of the stainless steel substrate. Since the reflectivity of the LST coating is mainly related to the laser irradiated parameters35-37, which contributes to the different microstructure and oxygen vacancies content, and the reflectivity strongly influences anti-laser ablation property, this result indicates that the LST coating with an 17



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appropriate thickness can work as a protective shield layer against ablation by lasers, particularly a continuous laser at moderate laser irradiated parameters.

Fig. 13. Temperature in the central area as a function of thickness direction and the back-surface temperature field under 1000 w/cm2 irradiation: (a) t=3 s, (b) t=5 s.

4. Conclusion The laser ablation behavior of the plasma-sprayed LST coating has been investigated through experiments and simulation. The effect of laser ablation on the microstructure reveals three laser ablation stages: coating oxidation, formation and growth of new structures (columnar and dendritic crystals), and mechanical failure. The temperature field during the laser irradiation has been simulated accurately using a finite element analysis method to provide a good understanding of the ablation behavior. The simulated temperature field demonstrates physical and qualitative agreement with the experiments and indicates that the apparent ablation characteristics are attributed to the different temperature gradients. During the laser irradiation, the rate of temperature increase is inversely proportional to the reflectivity of the LST coating. According to the experimental and simulation results, the reflectivity and thermal diffusivity of the LST coating material are the critical factors for improving the anti-laser ablation property in the central area. Acknowledgement The authors acknowledge the financial support of the National Natural Science Foundation of China (51302013). 18


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Ablation Behavior of Plasma-Sprayed La1âxSrxTiO3+Î´ Coating

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Ablation Behavior of Plasma-Sprayed La1âxSrxTiO3+Î´ Coating