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Cu composite irradiated by high power laser
Ceramics International 45 (2019) 15272–15280
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Multi-synergy ablative effects of laminated SiO2–graphite/SiO2/Cu composite irradiated by high power laser
T
Wenzhi Lia,b, Lihong Gaoa,b,∗, Zhuang Maa,b, Fuchi Wanga,b, Taotao Wuc, Lijun Wangc, Hezhang Lia,b a
School of Materials Science and Engineering, Beijing Institute of Technology, Beijing, 100081, China National Key Laboratory of Science and Technology on Materials Under Shock and Impact, Beijing, 100081, China c State Key Laboratory of Laser Interaction with Matter, Northwest Institute of Nuclear Technology, Xi'an, Shanxi, 710024, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Multi-synergy effects Ablation resistance High-power laser
Faced with the challenge of ultra–high–temperature ablation problems, multi-synergy and efficient energy dissipation method is a feasible solution. A structure-modified silica-graphite composite with multiple copper layers was prepared by hot-pressing. The ablation behaviors and mechanisms were analyzed in detail. During the laser ablation, the designed composite exhibits excellent ablation resistance with a multi-synergy effects strategy. As ablation time increases, the ablation mechanisms change from reflection and horizontal thermal diffusion to reaction and horizontal thermal diffusion and vertical heat insulation. Moreover, the gap generated by the reaction volume contraction can further insulate against heat transfer in the vertical direction. The rate of increase of back-surface temperature can be further reduced. This multi-synergy effects strategy and designed structure accompanied with appropriate materials can greatly enhance the ablation resistance ability of the composite.
1. Introduction
improved by adding silica [14,15]. On one hand, the melt of silica reduces heat transfer and oxygen transport to the underlying materials. On the other hand, the chemical reaction among carbon fiber, the phenolic matrix, and silica can further reduce the temperature. However, compared with polymers, inorganic composites have higher ablation-resistant temperature and lower mass ablation rates. Therefore, they may have more widespread ablation resistant application potential, particularly for dense inorganic silica and carbon composites. Usually, the thermal ablation properties of materials are tested by oxyacetylene torch [16,17], arc jet [18], or plasma wind tunnel [19]. However, accurate control of the output power of torch and plasma arc is technically difficult. Meanwhile, the power density of these approaches is limited within 240–400W/cm2 [16,20,21]. As the speed of aircrafts increases, the surface heating power density will far beyond than that value. Besides, the experimental setup of plasma wind tunnel test method is extremely complicated with high cost. As a result, novel approaches with low cost, high feasibility and high power density are needed to test the ablation property of materials. Recently, high-power continuous wave (CW) laser ablation method has been considered as a new emerging tool to evaluate the ablation properties of materials, because both the output power and laser testing time are controllable and stable. More important is that the ablation response of the tested
Ablation is an endothermic and erosive process in which the thermophysical and thermochemical responses are very complicated, resulting in parts of the materials being removed by physical, chemical, and mechanical influences [1,2]. Ablation occurs on some thermal protective materials of ultra-high-temperature-affected surfaces, such as rocket nozzles [3], re-entry space vehicles [4,5], and other highenergy interacting surfaces [6,7]. High-energy interaction with matter will cause a significant temperature rise, leading to a serious ablation situation on the surface. Consequently, high-energy ablation resistance, especially high-temperature ablation resistance, is one of the most important properties to evaluate the usability of these high-temperature protective materials before they are applied in the ablation environment [8,9]. Because of the occurrence of carbothermal reaction between carbon and silica in an ultra-high-temperature environment [10], silica-reinforced resin composites have been widely researched in ablation resistance studies [11–13]. Silica is considered as a feasible additive to modify the ablation property of composites. In the past few decades, numerous investigations have clearly revealed that the ablation properties of phenolic composite such as mass ablation rates and strength can be further ∗
Corresponding author. School of Materials Science and Engineering, Beijing Institute of Technology, Beijing, 100081, China. E-mail address: [email protected] (L. Gao).
https://doi.org/10.1016/j.ceramint.2019.05.017 Received 19 March 2019; Received in revised form 3 May 2019; Accepted 3 May 2019 Available online 04 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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materials to the laser is quick and convenient, because the power density of laser can reach beyond 1 kW/cm2, which can achieve the heating rate at about 600 K/s [22,23]. Besides, due to the reliable, highly efficient, and low cost of laser technology, high-power CW laser has recently already been used to evaluate the ablation resistance of coatings and composites [24,25]. Moreover, the laser ablation behaviors differ from other ablation behaviors because of their different heating modes. For laser, the materials are heated by the photon–matter interaction, but for torch or plasma, the materials are directly heated by flame or arc. Therefore, evaluating ablation resistance by using highpower lasers can provide us with more knowledge about the usability of materials and help us develop protection against laser irradiation. In our previous work [26], a novel inorganic graphite/SiO2 composite prepared by a hot-pressing technique exhibited excellent ablation performance, being dependent on surface reflectivity under low laser power and high endothermic reaction enthalpy under a highpower laser test. However, because the initial reflection of the graphite/ SiO2 composite is very low, the initial rate of temperature increase of the graphite/SiO2 composite under laser ablation is extremely high. Once the temperature reaches the reaction point of carbothermal reactions and these reactions occur, there will be no other energy dissipation path, so that ablation of the graphite/SiO2 composite will become more serious. That means that the ablation resistance of the composite will be reduced sharply. Increasing laser power and irradiation time completely destroys the laser ablation resistance of the graphite/SiO2 composite. In this study, faced with the challenge of higher power energy ablation and the problems we have encountered in our previous studies, the idea of using multi-synergy protective effects is proposed. The strategies include enhancement of the initial reflectivity and thermal conductivity anisotropy. The structure of the graphite/SiO2 composite was modified by inserting multiple Cu foil layers into it and adding a SiO2 top layer (denoted as SGS–Cu hereafter). The optical, thermal conductivity, and ablation behaviors under laser irradiation were investigated. The special ablation mechanisms operating during ablation are also discussed.
2. Experimental Commercial flake graphite (≈10 μm, AR, Forsman Scientific Co., Ltd., Beijing, China) and amorphous SiO2 powders (≈20 μm, AR, Forsman Scientific Co., Ltd., Beijing China) were used as raw materials. As done in our previous work [26], the mixed graphite and SiO2 with the mole ratio of 4:1 and single SiO2 were ball-mixed for 8 h in ethanol medium using ZrO2 balls, as shown in Fig. 1. Copper disks with a
diameter of 24 mm were cut from copper foil (Xing Rong Yuan Co., Ltd., Beijing, China). The thickness of the copper foil was 70 μm. During sample preparation in the graphite mold, three copper disks were inserted into the dried graphite/SiO2 mixture powder. Copper disk layers were used as a reinforcement layer to fill the pores during hot-pressing and increase the horizontal thermal conductivity of the prepared composite. SiO2 powder was then laid on top of the sample to form a SiO2 layer. Subsequently, the composites were hot-pressed in hotpressing furnace (R-C-ZKQY-07, Chen Rong Furnace Co., Ltd., Shanghai, China) at 1250 °C for 3 h in an Ar atmosphere under a uniaxial pressure of 20 MPa. The dimensions of the obtained laminated structure SGS–Cu composite were Φ25 mm × 2.5 mm. The compact thicknesses of SiO2 and the mixture layers were controlled as 2.3 mm and 200 μm, respectively. The surface morphology of SGS–Cu before and after laser ablation was observed by using three-dimensional super-depth digital microscopy (3D-SDDM, VHX-600, Keyence, Japan). The surface microstructure was observed by scanning electron microscopy (SEM, Hitachi S4800, Japan) and energy dispersive spectroscopy (EDS). Back-scattered electron (BSE) imaging method was used to present the distribution of some special elements. The phase was characterized by Xray diffraction (XRD, X'Pert PRO MPD, PANalytical, Inc., Holland) with Cu Kα radiation and the XRD patterns were analyzed by using JADE software (version 10.0, Adobe Systems Software Ireland, Ltd., USA). In addition, the optical property and thermal conductivity of SGS–Cu composites were characterized by an ultraviolet–visible–near-infrared (UV–VIS–NIR) spectrophotometer (Cray 5000, Australia) and a laser conductometer (LFA 447, NETZSCH, Germany), respectively. The composites were irradiated by using a commercial laser irradiation system, operating a 1070-nm-wavelength Nd:YAG continuous laser with a 1 cm2 uniform laser spot. The laser ablation behavior was investigated at a laser power density of 2000 W/cm2. The variations of back- and front-surface temperatures during laser irradiation were recorded by a K-type thermocouple and infrared thermometer (ThermoVision A40, FLIR System Inc., USA), respectively. To increase the accuracy of back-temperature measurement, the thermocouple was fixed onto the control point of a copper disk with the size of Φ25 mm × 0.5 mm. Then the copper disk was fitted closely to the back surface of the sample to measure the back-surface temperature during laser irradiation. Meanwhile, the front scattering light along the laser test was detected by a near-infrared (NIR) detector system (GD3561T, Zolix Instruments Co., Ltd., Beijing, China).
Fig. 1. Sketch of preparation process of laminated structure SGS–Cu composite. 15273
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Fig. 2. (a) XRD patterns and (b) SEM morphology of the as-calcined SGS–Cu composite surface.
3. Results and discussion
Cu-enriched layer (Fig. 3(b)), it can be seen that Cu exhibits a discontinuous distribution state, which proves that Cu was melted and diffused. In addition, Cu binds the particle together and this phenomenon has the positive effect in conductivity increase.
3.1. Phase and microstructure of the as-calcined SGS–Cu composite XRD patterns of the surface of SGS–Cu composites in Fig. 2(a) show that, after hot-pressing, the phase of the surface SiO2 maintains an amorphous state. No detectable crystalline peaks were found, which suggested that the selected preparation temperature did not reach the crystallizing point of SiO2 [27,28]. This phenomenon would facilitate the endothermic reaction of mixture graphite and SiO2, because the reaction activation energy of amorphous materials is higher than that of crystalline materials [29]. From Fig. 2(b), it can be observed that the particle size of the as-calcined SGS–Cu composite is obviously reduced (to just about 1–3 μm). The tiny particle size of the composite would increase the amount of interface and further promote the reaction between SiO2 and graphite under high temperature. Moreover, this dense surface structure of the composite would inhibit energy absorption by the pores or cracks. Meanwhile, by using the BSE image of the cross-section morphology, the distribution state of the inserted Cu layer was detected, as shown in Fig. 3. Visible contrast appears in the morphology and the highlighted region is confirmed as Cu by EDS. Even the calcination temperature of the composite is higher than the melting point of Cu. The thickness of the Cu layer in the sample was about 100 μm, which is similar to the thickness of raw Cu foil. Before Cu reached its melting point, the axial pressure during hot-pressing compressed the Cu foil in thickness direction. When the temperature rose to nearly the melting point of Cu, the compressed Cu layer began to melt and diffuse, then forming a diffused Cu layer, which was comparable in thickness to the original thickness of raw Cu foil. From the magnified morphology of the
3.2. Laser ablation behaviors of the SGS–Cu composite To evaluate the high-energy ablation resistance of the SGS-Cu composite, a high-power continuous laser was used as a heating source. The laser power density for the ablation test was chosen as 2000 W/ cm2. Fig. 4 shows the laser irradiation information after laser irradiation for 35 s. Compared with the laser ablation properties exhibited in our previous work on graphite/SiO2 composites [26], the laser ablation resistance of SGS–Cu was greatly enhanced, which contributed to the improved surface reflection. From Fig. 4(a), it can be found that, under such a high laser power density and long irradiation time, no obvious damage or material removal appeared on the surface; it just became whiter. During this long laser irradiation, the ablation area expanded to 16 mm by 16 mm, which is much larger than the laser spot. This expansion may be caused by the high thermal conductivity in the horizontal direction of the interior mixture materials, which promotes heat transition and ablation area expansion. The variations of optical reflection and thermal conductivity of the SGS–Cu composite will be discussed in the following section. XRD patterns in Fig. 4(b) show the very interesting result that the surface SiO2 phase structure still remains amorphous. This phenomenon may be because the time spent above the crystallization temperature is short, while the provided energy is insufficient to realize the SiO2 phase transition. From Fig. 4(c) and (d), it can be seen that the ablated surface illuminates a dense state with a few small pores. Based on the EDS test
Fig. 3. BSE cross-section morphology of the inserted Cu layer of the SGS–Cu composite under (a) low magnification and (b) high magnification. 15274
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Fig. 4. Sample information of SGS–Cu composite irradiated at 2000 W/cm2 for 35 s (a) Surface macro-morphology; (b) XRD pattern at irradiation area; (c) 3D-SDDM micrograph; (d) SEM surface morphology and EDS result.
Fig. 5. Cross-section morphologies of the SiO2 layer and graphite/SiO2 layer interface under (a) low magnification and (b) high magnification.
of area A in Fig. 4(d), the surface composition can be confirmed as SiO2. The generation of a few small pores may be the result of oxidized diffusion carbon during calcination. Besides, the absence of SiO2 phase transition is propitious to avoiding volume expansion from crystallization and swelling stress. According to the cross-sectional microstructure observed in Fig. 5(a), the thickness of the top SiO2 layer is uniform, being about 200–300 μm. In addition, the top SiO2 layer and lower graphite/SiO2 mixture layer exhibit a good bonding state, especially as viewed from the magnified morphology of Fig. 5(b). At this
moment (irradiation for 35 s), not only is the composition of the graphite/SiO2 layer unchanged but its structure is also stable. It can be concluded that, under 2000 W/cm2 laser irradiation for 35 s, the temperature does not reach the melting point of SiO2 or the reaction point of graphite with SiO2, so there is no damage appear. During this laser irradiation stage, optical and thermal physical processes are at work, including light reflection, heat transfer, and heat radiation multi-synergy effects, and we called this stage the thermophysical response stage. Under this condition, the SGS–Cu composite represents good
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Fig. 6. Sample information of SGS–Cu composite irradiated at 2000 W/cm2 for 43 s (a) Surface macro-morphology; (b) XRD pattern at irradiation area; (c) 3D-SDDM micrograph; (d) SEM surface morphology and EDS result.
ablation resistance, while the mass ablation rate is only 0.51 mg/s, which is much lower than that in our previous work (about 8.26–10.18 mg/s). This mass loss is caused by the oxidation of diffusion carbon. As time progresses, the deposited laser energy continuously increases the surface temperature. After laser irradiation at 2000 W/cm2 for 43 s, the surface temperature reaches the melting point of SiO2, and melt failure of the surface area occurs, leading to surface morphology change as shown in Fig. 6(a). Meanwhile, from the XRD patterns shown in Fig. 6(b), the bread-shaped diffraction peak is visible at a low angle, which proves that the main SiO2 phase still maintains an amorphous state. However, the SiO2 crystallization led to the emergence of a small peak at 22.019°, representing the cristobalite SiO2 phase in the XRD patterns. The reason for the absence of crystallization is that the time of the phase transition is still not long enough. More interesting is that the peaks of the reaction product SiC appear at 35.653°. According to the 3D-SDDM micrograph shown in Fig. 6(c), under the glassy SiO2 top layer, the brown interior SiC generation layer can be observed clearly. Besides, in the top layer, many tiny pores (of around dozens of micrometers in size) are formed. Because when the SiO2 layer melted, the temperature is over 1600 °C [30]. At this moment, the gas generated by the carbothermal reaction of graphite and SiO2 encountered the melted SiO2 layer and even formed through holes. From Fig. 6(d), the interior material under the SiO2 layer can be observed through those holes. The generated SiC can be detected by EDS through the holes. Some research has proved that the resulting carbon–silicon carbide (C–SiC) can withstand aerodynamic shear forces effectively under high-temperature erosive environments, thereby enhancing ablation resistance [31]. Consequently, when the SiC is exposed to air, the ablation resistance of the composite will be improved. The cross-section morphology and element line scanning results of the ablation area after 43 s are illuminated in Fig. 7. A gap between the top and interior layers apparently formed as a result of the volume contraction caused by the carbothermic reaction between graphite and SiO2 and the temperature change. According to the element line scanning starting from the top of the reaction layer, we can see that the thickness of the reacted SiC layer is about 60 μm based on the lack of
element O. Not only below the glassy SiO2 layer but also in the surrounding white SiO2 layer irradiated with the laser, the reaction layer was generated as long as the temperature reached the reaction temperature. However, the carbothermal reaction is accompanied by volume contraction. Once the reaction occurs, the gap would be subsequently generated. This phenomenon would block heat transfer from the top layer and greatly reduce the back-surface temperature of the sample. Because of the carbothermal reaction, the average mass ablation rate rose to 2.01 mg/s. Particularly, between the ablation time period ranging from 35 to 43 s, the mass ablation rate rose to 8.61 mg/s. At this stage, thermochemical response dominates the ablation process and the composite exhibits the multi-synergy protective effects of an endothermic reaction, horizontal thermal diffusion, and vertical heat insulation. We can predict that, as the laser irradiation time increases continuously, vaporization and penetration of the top SiO2 layer will occur subsequently, as seen in our previous work [26]. Then the ablation behavior will be similar to that observed in our previously research. 3.3. Optical reflection, thermal conductivity, and temperature evolution of the SGS–Cu composite After a long laser irradiation under 2000 W/cm2, the SGS–Cu composite exhibits unique laser ablation behaviors and excellent ablation-resistant properties. Optical reflection and thermal conductivity dominate the laser irradiation responses of the composite. These properties determine the temperature variation of the composite during laser irradiation and, in turn, influence the ablation behaviors. Therefore, investigating the relationship among optical reflection, thermal conductivity, and temperature variation contributes to obtaining a deep understanding of the ablation mechanisms. During laser irradiation, the forward-scattered light was measured by NIR detector. It should be noted that the NIR detector only receives the scattering signal in one specific direction reflected from the front surface. The intensity of forward-scattered light varies from one position to another and it also related to the incident laser power and coating surface state. Therefore, the data obtained from different
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Fig. 7. Element line scan of a cross section of SGS–Cu composite irradiated at 2000 W/cm2 for 43 s.
samples are not comparable. However, they can be used to evaluate the variation in optical reflection of a single tested sample during laser irradiation. Besides, the signal of forward-scattered light could reflect the laser irradiation time. The detailed optical reflection evolution of the SGS–Cu composite under laser irradiation is shown in Fig. 8. Fig. 8(a) and (b) present the surface reflectivity before and after laser irradiation for 35 s, respectively. After laser irradiation, the reflectivity at 1070 nm grew from 73% to 95% because of the oxidation of diffusion carbon and the appearance of pores. The forward-scattered light evolution during the whole laser irradiation is shown in Fig. 8(c). It can be seen that, once the sample surface is ablated by the laser, the intensity of forwardscattered light rises suddenly from 0.69716 to 0.89049 and then maintains a high value of about 0.90468. The signal intensities at starting point A and later point B reflect the surface reflection performance. We can see that the intensity at point B is a little stronger than that at point A, which means that the surface reflectivity of the sample ablated for 35 s is higher than that without laser ablation. The forwardscattered light results are in agreement with the reflection test and also consistent with the change of surface morphology. By comparing the surface morphologies before and after laser ablation shown in Figs. 2(b) and 4(d), it can be found that the surface becomes not that compact like before because of the oxidation of diffusion carbon during preparation. The reduction in the low-reflectivity carbon phase and the increase in the number of interfaces between SiO2 and pores further improve the reflection of the SGS–Cu during laser irradiation. After laser irradiation for 35 s, a reduction in the forward-scattered light intensity appears, as shown in Fig. 8(d). By magnifying the final stage of the intensity curve and making a tangential calculation, an
extreme drop can be observed at the final 1.5 s. This phenomenon is caused by the melting of the surface SiO2 layer. The melting leads to surface state change and reflectivity reduction, so the forward-scattered light reduces extremely. The front- and back-surface temperature curves of the central point can reveal the evolution of temperature. The related results are shown in Fig. 9. Because for the infrared thermometer, different temperature measurement ranges have different sensitive emission wavelengths. Therefore, during the temperature test, filters were added in front of the infrared thermometer lens to only collect the emissions with wavelength that corresponds to high temperature (around 600–2200 °C). Owing to the limitation of the starting test temperature of the infrared thermometer and to test hysteresis, until the measurement at 32 s, the front-surface temperature data are invalid. Therefore, a “disbelief data interval” appears in Fig. 9(a) at the beginning. After a big jump in the front-surface temperature, the data transition into the effective thermal data interval. In the effective thermal data interval, when the temperature grows to near 1600 °C, a sharp temperature rise appears. Before the surface layer melts and penetrates, the rate of the front-surface temperature increase varies over time. From the back-surface temperature, we can see that, during the 43 s laser irradiation, the rate of temperature increase trends to flatten. However, a sudden change occurs during the last period of laser irradiation, which is because the melting of the surface SiO2 layer increases the laser energy absorption. This sudden temperature change can be observed in more clearly in the front-surface temperature. The slope of the front-surface temperature curve is shown in Fig. 9(b), which can obviously reflect the rate of front-surface temperature increase. The rate of temperature increase changes along with
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Fig. 8. Surface reflectivity of the sample (a) at the beginning without laser ablation and (b) after laser ablation for 35 s; (c) forward-scattered light history during the whole laser irradiation duration; (d) tangent value of forward-scattered light at the final laser irradiation duration.
the laser irradiation time, which corresponds to the change of reflectivity and carbothermal reaction. For time below 41.5 s, no reaction occurs and the reflectivity remains at a high level, so at this stage only reflection and heat transfer exist. As the surface temperature increases continuously, the growth rate of temperature is reduced owing to the occurrence of endothermic reactions. Therefore, the deposited laser energy gets partly consumed by the reactions. During this stage, the temperature lies between that of the reaction point and the melting point. High reflection and high energy consumption characteristics were exhibited at the same time. It can be seen that the appearance of
the ablation reaction can relieve heat concentration in the composite. This stage only lasts for just about 3.2 s. Finally, a sharp temperature increase appeared, because the melt of the surface SiO2 led to a reflectivity decrease and the huge incident laser energy absorption caused a temperature rise. At this stage, even though the endothermic reaction occurs, it is not enough to offset the absorbed energy. Rapid failure of the composite begins to occur. Besides the optical property, thermal conductivity also plays an important role in the ablation resistance of the SGS–Cu composite. According to the thermal conductivity values in the horizontal and
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Fig. 9. (a) Front- and back-surface temperature curves at the central point; (b) slope of the front-surface temperature curve.
4. Conclusion
Table 1 Thermal conductivity of the SGS–Cu composite. Direction
Thermal conductivity (W/m·K)
Ratio
Horizontal Vertical
34.76 6.90
5.04
vertical directions listed in Table 1, we can find that the horizontal thermal conductivity is 34.76 W/m·K, which is nearly a factor of 5 higher than that in the vertical direction. Such large horizontal and low vertical thermal conductivity performance of the composite contributes to improving thermal diffusion in the horizontal direction and delay heat transfer in the vertical direction. The anisotropy of thermal conductivity is caused by the horizontal distribution of raw materials, as shown in Fig. 10, especially that of the flake graphite. Combined with the Cu layer, the designed structure indeed has improved the horizontal thermal conductivity of the composite and, in turn, promoted thermal uniformity under high-energy loading. The anisotropic nature of the thermal conductivity further increases the laser ablation resistance of the SGS–Cu composite. The ablation behaviors and mechanisms of SGS–Cu composites indicate that the composites with SGS–Cu-like laminated structure and endothermic reactions will have good laser ablation resistance under long-duration high-laser-power irradiation. These materials and structures have great application potential as high-power ablated surfaces to further enhance their ablation resistance.
Laminated structure-modified SGS–Cu composites were prepared by hot-pressing. Their laser ablation behaviors and mechanisms were investigated. Because of its structural design, the SGS–Cu composite exhibits high reflection and highly anisotropic thermal conductivity performance. Together with the high reaction enthalpy of the composite, they exhibited unique ablation resistance. Under 2000 W/cm2 laser irradiation, during the early stage of laser ablation, the improvement in surface reflectivity means that most of the incident laser energy gets reflected. The deposited laser energy can be homogenized by good thermal conductivity in the horizontal direction of the interior composite. Besides, the ratio of horizontal to vertical thermal conductivity reaches 5.04. Due to the anisotropy of thermal conductivity, the thermal conductivity in the vertical direction is low, being just 6.90 W/ m·K, so there is no sharp back-temperature rise. As irradiation time grows, when the temperature reaches the reaction point of the SiO2–graphite mixture, energy consumption by the reaction and the gap generated by the reaction volume contraction further provide ablation resistance. Furthermore, the melted SiO2 covering the reacted SiC layer could protect the SiC layer from oxidation and maintain its positive function. This multi-synergy effects strategy and designed structure accompanied with appropriate materials could enhance the ablation resistance of the composite greatly.
Acknowledgment The authors acknowledge the financial support of the National Natural Science Foundation of China (51772027).
Fig. 10. Mixture layer fractured microstructure of the as-calcined SGS-Cu composite under (a) low magnification and (b) high magnification. 15279
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Multi-synergy ablative effects of laminated SiO2–graphite/SiO2/Cu composite irradiated by high power laser
T
Wenzhi Lia,b, Lihong Gaoa,b,∗, Zhuang Maa,b, Fuchi Wanga,b, Taotao Wuc, Lijun Wangc, Hezhang Lia,b a
School of Materials Science and Engineering, Beijing Institute of Technology, Beijing, 100081, China National Key Laboratory of Science and Technology on Materials Under Shock and Impact, Beijing, 100081, China c State Key Laboratory of Laser Interaction with Matter, Northwest Institute of Nuclear Technology, Xi'an, Shanxi, 710024, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Multi-synergy effects Ablation resistance High-power laser
Faced with the challenge of ultra–high–temperature ablation problems, multi-synergy and efficient energy dissipation method is a feasible solution. A structure-modified silica-graphite composite with multiple copper layers was prepared by hot-pressing. The ablation behaviors and mechanisms were analyzed in detail. During the laser ablation, the designed composite exhibits excellent ablation resistance with a multi-synergy effects strategy. As ablation time increases, the ablation mechanisms change from reflection and horizontal thermal diffusion to reaction and horizontal thermal diffusion and vertical heat insulation. Moreover, the gap generated by the reaction volume contraction can further insulate against heat transfer in the vertical direction. The rate of increase of back-surface temperature can be further reduced. This multi-synergy effects strategy and designed structure accompanied with appropriate materials can greatly enhance the ablation resistance ability of the composite.
1. Introduction
improved by adding silica [14,15]. On one hand, the melt of silica reduces heat transfer and oxygen transport to the underlying materials. On the other hand, the chemical reaction among carbon fiber, the phenolic matrix, and silica can further reduce the temperature. However, compared with polymers, inorganic composites have higher ablation-resistant temperature and lower mass ablation rates. Therefore, they may have more widespread ablation resistant application potential, particularly for dense inorganic silica and carbon composites. Usually, the thermal ablation properties of materials are tested by oxyacetylene torch [16,17], arc jet [18], or plasma wind tunnel [19]. However, accurate control of the output power of torch and plasma arc is technically difficult. Meanwhile, the power density of these approaches is limited within 240–400W/cm2 [16,20,21]. As the speed of aircrafts increases, the surface heating power density will far beyond than that value. Besides, the experimental setup of plasma wind tunnel test method is extremely complicated with high cost. As a result, novel approaches with low cost, high feasibility and high power density are needed to test the ablation property of materials. Recently, high-power continuous wave (CW) laser ablation method has been considered as a new emerging tool to evaluate the ablation properties of materials, because both the output power and laser testing time are controllable and stable. More important is that the ablation response of the tested
Ablation is an endothermic and erosive process in which the thermophysical and thermochemical responses are very complicated, resulting in parts of the materials being removed by physical, chemical, and mechanical influences [1,2]. Ablation occurs on some thermal protective materials of ultra-high-temperature-affected surfaces, such as rocket nozzles [3], re-entry space vehicles [4,5], and other highenergy interacting surfaces [6,7]. High-energy interaction with matter will cause a significant temperature rise, leading to a serious ablation situation on the surface. Consequently, high-energy ablation resistance, especially high-temperature ablation resistance, is one of the most important properties to evaluate the usability of these high-temperature protective materials before they are applied in the ablation environment [8,9]. Because of the occurrence of carbothermal reaction between carbon and silica in an ultra-high-temperature environment [10], silica-reinforced resin composites have been widely researched in ablation resistance studies [11–13]. Silica is considered as a feasible additive to modify the ablation property of composites. In the past few decades, numerous investigations have clearly revealed that the ablation properties of phenolic composite such as mass ablation rates and strength can be further ∗
Corresponding author. School of Materials Science and Engineering, Beijing Institute of Technology, Beijing, 100081, China. E-mail address: [email protected] (L. Gao).
https://doi.org/10.1016/j.ceramint.2019.05.017 Received 19 March 2019; Received in revised form 3 May 2019; Accepted 3 May 2019 Available online 04 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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materials to the laser is quick and convenient, because the power density of laser can reach beyond 1 kW/cm2, which can achieve the heating rate at about 600 K/s [22,23]. Besides, due to the reliable, highly efficient, and low cost of laser technology, high-power CW laser has recently already been used to evaluate the ablation resistance of coatings and composites [24,25]. Moreover, the laser ablation behaviors differ from other ablation behaviors because of their different heating modes. For laser, the materials are heated by the photon–matter interaction, but for torch or plasma, the materials are directly heated by flame or arc. Therefore, evaluating ablation resistance by using highpower lasers can provide us with more knowledge about the usability of materials and help us develop protection against laser irradiation. In our previous work [26], a novel inorganic graphite/SiO2 composite prepared by a hot-pressing technique exhibited excellent ablation performance, being dependent on surface reflectivity under low laser power and high endothermic reaction enthalpy under a highpower laser test. However, because the initial reflection of the graphite/ SiO2 composite is very low, the initial rate of temperature increase of the graphite/SiO2 composite under laser ablation is extremely high. Once the temperature reaches the reaction point of carbothermal reactions and these reactions occur, there will be no other energy dissipation path, so that ablation of the graphite/SiO2 composite will become more serious. That means that the ablation resistance of the composite will be reduced sharply. Increasing laser power and irradiation time completely destroys the laser ablation resistance of the graphite/SiO2 composite. In this study, faced with the challenge of higher power energy ablation and the problems we have encountered in our previous studies, the idea of using multi-synergy protective effects is proposed. The strategies include enhancement of the initial reflectivity and thermal conductivity anisotropy. The structure of the graphite/SiO2 composite was modified by inserting multiple Cu foil layers into it and adding a SiO2 top layer (denoted as SGS–Cu hereafter). The optical, thermal conductivity, and ablation behaviors under laser irradiation were investigated. The special ablation mechanisms operating during ablation are also discussed.
2. Experimental Commercial flake graphite (≈10 μm, AR, Forsman Scientific Co., Ltd., Beijing, China) and amorphous SiO2 powders (≈20 μm, AR, Forsman Scientific Co., Ltd., Beijing China) were used as raw materials. As done in our previous work [26], the mixed graphite and SiO2 with the mole ratio of 4:1 and single SiO2 were ball-mixed for 8 h in ethanol medium using ZrO2 balls, as shown in Fig. 1. Copper disks with a
diameter of 24 mm were cut from copper foil (Xing Rong Yuan Co., Ltd., Beijing, China). The thickness of the copper foil was 70 μm. During sample preparation in the graphite mold, three copper disks were inserted into the dried graphite/SiO2 mixture powder. Copper disk layers were used as a reinforcement layer to fill the pores during hot-pressing and increase the horizontal thermal conductivity of the prepared composite. SiO2 powder was then laid on top of the sample to form a SiO2 layer. Subsequently, the composites were hot-pressed in hotpressing furnace (R-C-ZKQY-07, Chen Rong Furnace Co., Ltd., Shanghai, China) at 1250 °C for 3 h in an Ar atmosphere under a uniaxial pressure of 20 MPa. The dimensions of the obtained laminated structure SGS–Cu composite were Φ25 mm × 2.5 mm. The compact thicknesses of SiO2 and the mixture layers were controlled as 2.3 mm and 200 μm, respectively. The surface morphology of SGS–Cu before and after laser ablation was observed by using three-dimensional super-depth digital microscopy (3D-SDDM, VHX-600, Keyence, Japan). The surface microstructure was observed by scanning electron microscopy (SEM, Hitachi S4800, Japan) and energy dispersive spectroscopy (EDS). Back-scattered electron (BSE) imaging method was used to present the distribution of some special elements. The phase was characterized by Xray diffraction (XRD, X'Pert PRO MPD, PANalytical, Inc., Holland) with Cu Kα radiation and the XRD patterns were analyzed by using JADE software (version 10.0, Adobe Systems Software Ireland, Ltd., USA). In addition, the optical property and thermal conductivity of SGS–Cu composites were characterized by an ultraviolet–visible–near-infrared (UV–VIS–NIR) spectrophotometer (Cray 5000, Australia) and a laser conductometer (LFA 447, NETZSCH, Germany), respectively. The composites were irradiated by using a commercial laser irradiation system, operating a 1070-nm-wavelength Nd:YAG continuous laser with a 1 cm2 uniform laser spot. The laser ablation behavior was investigated at a laser power density of 2000 W/cm2. The variations of back- and front-surface temperatures during laser irradiation were recorded by a K-type thermocouple and infrared thermometer (ThermoVision A40, FLIR System Inc., USA), respectively. To increase the accuracy of back-temperature measurement, the thermocouple was fixed onto the control point of a copper disk with the size of Φ25 mm × 0.5 mm. Then the copper disk was fitted closely to the back surface of the sample to measure the back-surface temperature during laser irradiation. Meanwhile, the front scattering light along the laser test was detected by a near-infrared (NIR) detector system (GD3561T, Zolix Instruments Co., Ltd., Beijing, China).
Fig. 1. Sketch of preparation process of laminated structure SGS–Cu composite. 15273
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Fig. 2. (a) XRD patterns and (b) SEM morphology of the as-calcined SGS–Cu composite surface.
3. Results and discussion
Cu-enriched layer (Fig. 3(b)), it can be seen that Cu exhibits a discontinuous distribution state, which proves that Cu was melted and diffused. In addition, Cu binds the particle together and this phenomenon has the positive effect in conductivity increase.
3.1. Phase and microstructure of the as-calcined SGS–Cu composite XRD patterns of the surface of SGS–Cu composites in Fig. 2(a) show that, after hot-pressing, the phase of the surface SiO2 maintains an amorphous state. No detectable crystalline peaks were found, which suggested that the selected preparation temperature did not reach the crystallizing point of SiO2 [27,28]. This phenomenon would facilitate the endothermic reaction of mixture graphite and SiO2, because the reaction activation energy of amorphous materials is higher than that of crystalline materials [29]. From Fig. 2(b), it can be observed that the particle size of the as-calcined SGS–Cu composite is obviously reduced (to just about 1–3 μm). The tiny particle size of the composite would increase the amount of interface and further promote the reaction between SiO2 and graphite under high temperature. Moreover, this dense surface structure of the composite would inhibit energy absorption by the pores or cracks. Meanwhile, by using the BSE image of the cross-section morphology, the distribution state of the inserted Cu layer was detected, as shown in Fig. 3. Visible contrast appears in the morphology and the highlighted region is confirmed as Cu by EDS. Even the calcination temperature of the composite is higher than the melting point of Cu. The thickness of the Cu layer in the sample was about 100 μm, which is similar to the thickness of raw Cu foil. Before Cu reached its melting point, the axial pressure during hot-pressing compressed the Cu foil in thickness direction. When the temperature rose to nearly the melting point of Cu, the compressed Cu layer began to melt and diffuse, then forming a diffused Cu layer, which was comparable in thickness to the original thickness of raw Cu foil. From the magnified morphology of the
3.2. Laser ablation behaviors of the SGS–Cu composite To evaluate the high-energy ablation resistance of the SGS-Cu composite, a high-power continuous laser was used as a heating source. The laser power density for the ablation test was chosen as 2000 W/ cm2. Fig. 4 shows the laser irradiation information after laser irradiation for 35 s. Compared with the laser ablation properties exhibited in our previous work on graphite/SiO2 composites [26], the laser ablation resistance of SGS–Cu was greatly enhanced, which contributed to the improved surface reflection. From Fig. 4(a), it can be found that, under such a high laser power density and long irradiation time, no obvious damage or material removal appeared on the surface; it just became whiter. During this long laser irradiation, the ablation area expanded to 16 mm by 16 mm, which is much larger than the laser spot. This expansion may be caused by the high thermal conductivity in the horizontal direction of the interior mixture materials, which promotes heat transition and ablation area expansion. The variations of optical reflection and thermal conductivity of the SGS–Cu composite will be discussed in the following section. XRD patterns in Fig. 4(b) show the very interesting result that the surface SiO2 phase structure still remains amorphous. This phenomenon may be because the time spent above the crystallization temperature is short, while the provided energy is insufficient to realize the SiO2 phase transition. From Fig. 4(c) and (d), it can be seen that the ablated surface illuminates a dense state with a few small pores. Based on the EDS test
Fig. 3. BSE cross-section morphology of the inserted Cu layer of the SGS–Cu composite under (a) low magnification and (b) high magnification. 15274
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Fig. 4. Sample information of SGS–Cu composite irradiated at 2000 W/cm2 for 35 s (a) Surface macro-morphology; (b) XRD pattern at irradiation area; (c) 3D-SDDM micrograph; (d) SEM surface morphology and EDS result.
Fig. 5. Cross-section morphologies of the SiO2 layer and graphite/SiO2 layer interface under (a) low magnification and (b) high magnification.
of area A in Fig. 4(d), the surface composition can be confirmed as SiO2. The generation of a few small pores may be the result of oxidized diffusion carbon during calcination. Besides, the absence of SiO2 phase transition is propitious to avoiding volume expansion from crystallization and swelling stress. According to the cross-sectional microstructure observed in Fig. 5(a), the thickness of the top SiO2 layer is uniform, being about 200–300 μm. In addition, the top SiO2 layer and lower graphite/SiO2 mixture layer exhibit a good bonding state, especially as viewed from the magnified morphology of Fig. 5(b). At this
moment (irradiation for 35 s), not only is the composition of the graphite/SiO2 layer unchanged but its structure is also stable. It can be concluded that, under 2000 W/cm2 laser irradiation for 35 s, the temperature does not reach the melting point of SiO2 or the reaction point of graphite with SiO2, so there is no damage appear. During this laser irradiation stage, optical and thermal physical processes are at work, including light reflection, heat transfer, and heat radiation multi-synergy effects, and we called this stage the thermophysical response stage. Under this condition, the SGS–Cu composite represents good
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Fig. 6. Sample information of SGS–Cu composite irradiated at 2000 W/cm2 for 43 s (a) Surface macro-morphology; (b) XRD pattern at irradiation area; (c) 3D-SDDM micrograph; (d) SEM surface morphology and EDS result.
ablation resistance, while the mass ablation rate is only 0.51 mg/s, which is much lower than that in our previous work (about 8.26–10.18 mg/s). This mass loss is caused by the oxidation of diffusion carbon. As time progresses, the deposited laser energy continuously increases the surface temperature. After laser irradiation at 2000 W/cm2 for 43 s, the surface temperature reaches the melting point of SiO2, and melt failure of the surface area occurs, leading to surface morphology change as shown in Fig. 6(a). Meanwhile, from the XRD patterns shown in Fig. 6(b), the bread-shaped diffraction peak is visible at a low angle, which proves that the main SiO2 phase still maintains an amorphous state. However, the SiO2 crystallization led to the emergence of a small peak at 22.019°, representing the cristobalite SiO2 phase in the XRD patterns. The reason for the absence of crystallization is that the time of the phase transition is still not long enough. More interesting is that the peaks of the reaction product SiC appear at 35.653°. According to the 3D-SDDM micrograph shown in Fig. 6(c), under the glassy SiO2 top layer, the brown interior SiC generation layer can be observed clearly. Besides, in the top layer, many tiny pores (of around dozens of micrometers in size) are formed. Because when the SiO2 layer melted, the temperature is over 1600 °C [30]. At this moment, the gas generated by the carbothermal reaction of graphite and SiO2 encountered the melted SiO2 layer and even formed through holes. From Fig. 6(d), the interior material under the SiO2 layer can be observed through those holes. The generated SiC can be detected by EDS through the holes. Some research has proved that the resulting carbon–silicon carbide (C–SiC) can withstand aerodynamic shear forces effectively under high-temperature erosive environments, thereby enhancing ablation resistance [31]. Consequently, when the SiC is exposed to air, the ablation resistance of the composite will be improved. The cross-section morphology and element line scanning results of the ablation area after 43 s are illuminated in Fig. 7. A gap between the top and interior layers apparently formed as a result of the volume contraction caused by the carbothermic reaction between graphite and SiO2 and the temperature change. According to the element line scanning starting from the top of the reaction layer, we can see that the thickness of the reacted SiC layer is about 60 μm based on the lack of
element O. Not only below the glassy SiO2 layer but also in the surrounding white SiO2 layer irradiated with the laser, the reaction layer was generated as long as the temperature reached the reaction temperature. However, the carbothermal reaction is accompanied by volume contraction. Once the reaction occurs, the gap would be subsequently generated. This phenomenon would block heat transfer from the top layer and greatly reduce the back-surface temperature of the sample. Because of the carbothermal reaction, the average mass ablation rate rose to 2.01 mg/s. Particularly, between the ablation time period ranging from 35 to 43 s, the mass ablation rate rose to 8.61 mg/s. At this stage, thermochemical response dominates the ablation process and the composite exhibits the multi-synergy protective effects of an endothermic reaction, horizontal thermal diffusion, and vertical heat insulation. We can predict that, as the laser irradiation time increases continuously, vaporization and penetration of the top SiO2 layer will occur subsequently, as seen in our previous work [26]. Then the ablation behavior will be similar to that observed in our previously research. 3.3. Optical reflection, thermal conductivity, and temperature evolution of the SGS–Cu composite After a long laser irradiation under 2000 W/cm2, the SGS–Cu composite exhibits unique laser ablation behaviors and excellent ablation-resistant properties. Optical reflection and thermal conductivity dominate the laser irradiation responses of the composite. These properties determine the temperature variation of the composite during laser irradiation and, in turn, influence the ablation behaviors. Therefore, investigating the relationship among optical reflection, thermal conductivity, and temperature variation contributes to obtaining a deep understanding of the ablation mechanisms. During laser irradiation, the forward-scattered light was measured by NIR detector. It should be noted that the NIR detector only receives the scattering signal in one specific direction reflected from the front surface. The intensity of forward-scattered light varies from one position to another and it also related to the incident laser power and coating surface state. Therefore, the data obtained from different
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Fig. 7. Element line scan of a cross section of SGS–Cu composite irradiated at 2000 W/cm2 for 43 s.
samples are not comparable. However, they can be used to evaluate the variation in optical reflection of a single tested sample during laser irradiation. Besides, the signal of forward-scattered light could reflect the laser irradiation time. The detailed optical reflection evolution of the SGS–Cu composite under laser irradiation is shown in Fig. 8. Fig. 8(a) and (b) present the surface reflectivity before and after laser irradiation for 35 s, respectively. After laser irradiation, the reflectivity at 1070 nm grew from 73% to 95% because of the oxidation of diffusion carbon and the appearance of pores. The forward-scattered light evolution during the whole laser irradiation is shown in Fig. 8(c). It can be seen that, once the sample surface is ablated by the laser, the intensity of forwardscattered light rises suddenly from 0.69716 to 0.89049 and then maintains a high value of about 0.90468. The signal intensities at starting point A and later point B reflect the surface reflection performance. We can see that the intensity at point B is a little stronger than that at point A, which means that the surface reflectivity of the sample ablated for 35 s is higher than that without laser ablation. The forwardscattered light results are in agreement with the reflection test and also consistent with the change of surface morphology. By comparing the surface morphologies before and after laser ablation shown in Figs. 2(b) and 4(d), it can be found that the surface becomes not that compact like before because of the oxidation of diffusion carbon during preparation. The reduction in the low-reflectivity carbon phase and the increase in the number of interfaces between SiO2 and pores further improve the reflection of the SGS–Cu during laser irradiation. After laser irradiation for 35 s, a reduction in the forward-scattered light intensity appears, as shown in Fig. 8(d). By magnifying the final stage of the intensity curve and making a tangential calculation, an
extreme drop can be observed at the final 1.5 s. This phenomenon is caused by the melting of the surface SiO2 layer. The melting leads to surface state change and reflectivity reduction, so the forward-scattered light reduces extremely. The front- and back-surface temperature curves of the central point can reveal the evolution of temperature. The related results are shown in Fig. 9. Because for the infrared thermometer, different temperature measurement ranges have different sensitive emission wavelengths. Therefore, during the temperature test, filters were added in front of the infrared thermometer lens to only collect the emissions with wavelength that corresponds to high temperature (around 600–2200 °C). Owing to the limitation of the starting test temperature of the infrared thermometer and to test hysteresis, until the measurement at 32 s, the front-surface temperature data are invalid. Therefore, a “disbelief data interval” appears in Fig. 9(a) at the beginning. After a big jump in the front-surface temperature, the data transition into the effective thermal data interval. In the effective thermal data interval, when the temperature grows to near 1600 °C, a sharp temperature rise appears. Before the surface layer melts and penetrates, the rate of the front-surface temperature increase varies over time. From the back-surface temperature, we can see that, during the 43 s laser irradiation, the rate of temperature increase trends to flatten. However, a sudden change occurs during the last period of laser irradiation, which is because the melting of the surface SiO2 layer increases the laser energy absorption. This sudden temperature change can be observed in more clearly in the front-surface temperature. The slope of the front-surface temperature curve is shown in Fig. 9(b), which can obviously reflect the rate of front-surface temperature increase. The rate of temperature increase changes along with
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Fig. 8. Surface reflectivity of the sample (a) at the beginning without laser ablation and (b) after laser ablation for 35 s; (c) forward-scattered light history during the whole laser irradiation duration; (d) tangent value of forward-scattered light at the final laser irradiation duration.
the laser irradiation time, which corresponds to the change of reflectivity and carbothermal reaction. For time below 41.5 s, no reaction occurs and the reflectivity remains at a high level, so at this stage only reflection and heat transfer exist. As the surface temperature increases continuously, the growth rate of temperature is reduced owing to the occurrence of endothermic reactions. Therefore, the deposited laser energy gets partly consumed by the reactions. During this stage, the temperature lies between that of the reaction point and the melting point. High reflection and high energy consumption characteristics were exhibited at the same time. It can be seen that the appearance of
the ablation reaction can relieve heat concentration in the composite. This stage only lasts for just about 3.2 s. Finally, a sharp temperature increase appeared, because the melt of the surface SiO2 led to a reflectivity decrease and the huge incident laser energy absorption caused a temperature rise. At this stage, even though the endothermic reaction occurs, it is not enough to offset the absorbed energy. Rapid failure of the composite begins to occur. Besides the optical property, thermal conductivity also plays an important role in the ablation resistance of the SGS–Cu composite. According to the thermal conductivity values in the horizontal and
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Fig. 9. (a) Front- and back-surface temperature curves at the central point; (b) slope of the front-surface temperature curve.
4. Conclusion
Table 1 Thermal conductivity of the SGS–Cu composite. Direction
Thermal conductivity (W/m·K)
Ratio
Horizontal Vertical
34.76 6.90
5.04
vertical directions listed in Table 1, we can find that the horizontal thermal conductivity is 34.76 W/m·K, which is nearly a factor of 5 higher than that in the vertical direction. Such large horizontal and low vertical thermal conductivity performance of the composite contributes to improving thermal diffusion in the horizontal direction and delay heat transfer in the vertical direction. The anisotropy of thermal conductivity is caused by the horizontal distribution of raw materials, as shown in Fig. 10, especially that of the flake graphite. Combined with the Cu layer, the designed structure indeed has improved the horizontal thermal conductivity of the composite and, in turn, promoted thermal uniformity under high-energy loading. The anisotropic nature of the thermal conductivity further increases the laser ablation resistance of the SGS–Cu composite. The ablation behaviors and mechanisms of SGS–Cu composites indicate that the composites with SGS–Cu-like laminated structure and endothermic reactions will have good laser ablation resistance under long-duration high-laser-power irradiation. These materials and structures have great application potential as high-power ablated surfaces to further enhance their ablation resistance.
Laminated structure-modified SGS–Cu composites were prepared by hot-pressing. Their laser ablation behaviors and mechanisms were investigated. Because of its structural design, the SGS–Cu composite exhibits high reflection and highly anisotropic thermal conductivity performance. Together with the high reaction enthalpy of the composite, they exhibited unique ablation resistance. Under 2000 W/cm2 laser irradiation, during the early stage of laser ablation, the improvement in surface reflectivity means that most of the incident laser energy gets reflected. The deposited laser energy can be homogenized by good thermal conductivity in the horizontal direction of the interior composite. Besides, the ratio of horizontal to vertical thermal conductivity reaches 5.04. Due to the anisotropy of thermal conductivity, the thermal conductivity in the vertical direction is low, being just 6.90 W/ m·K, so there is no sharp back-temperature rise. As irradiation time grows, when the temperature reaches the reaction point of the SiO2–graphite mixture, energy consumption by the reaction and the gap generated by the reaction volume contraction further provide ablation resistance. Furthermore, the melted SiO2 covering the reacted SiC layer could protect the SiC layer from oxidation and maintain its positive function. This multi-synergy effects strategy and designed structure accompanied with appropriate materials could enhance the ablation resistance of the composite greatly.
Acknowledgment The authors acknowledge the financial support of the National Natural Science Foundation of China (51772027).
Fig. 10. Mixture layer fractured microstructure of the as-calcined SGS-Cu composite under (a) low magnification and (b) high magnification. 15279
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