Zirconium carbide-modified polymer-matrix composites with improved reflectivity under high-energy laser ablation

Accepted Manuscript Zirconium carbide-modified polymer-matrix composites with improved reflectivity under high-energy laser ablation Chen Ma, Zhuang Ma, Lihong Gao, Taotao Wu, Fuchi Wang, Hatsuo Ishida PII:

S0272-8842(19)31462-2

DOI:

https://doi.org/10.1016/j.ceramint.2019.05.335

Reference:

CERI 21787

To appear in:

Ceramics International

Received Date: 13 March 2019 Revised Date:

29 May 2019

Accepted Date: 30 May 2019

Please cite this article as: C. Ma, Z. Ma, L. Gao, T. Wu, F. Wang, H. Ishida, Zirconium carbide-modified polymer-matrix composites with improved reflectivity under high-energy laser ablation, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.05.335. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Zirconium carbide-modified polymer-matrix composites with improved reflectivity under high-energy laser ablation Chen Ma1, 2, Zhuang Ma1, 2, Lihong Gao1, 2*, Taotao Wu3, Fuchi Wang1, 2, Hatsuo Ishida4 1

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School of Material Science and Engineering, Beijing Institute of Technology, Beijing, 100081, China 2 National Key Laboratory of Science and Technology on Material under Shock and Impact, Beijing 100081, China 3 Northwest Institute of Nuclear Technology, Xi’an 710024, China 4 Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106, USA *:Corresponding author (Lihong Gao: [email protected])

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Abstract:

High-energy continuous-wave (CW) lasers can cause severe damage to traditionally structured materials within several seconds because of the formation of an extremely high temperature field. To prevent laser damage, zirconium carbide

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(ZrC)-modified short-carbon-fiber-reinforced phenolic-resin matrix composites have been successfully prepared. The micro-morphologies and elemental distributions of

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the composites were investigated and phase detection was performed to study their laser-ablation behavior. The results reveal that the ZrC particles are oxidized rapidly,

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forming ZrO2, during laser ablation. Owing to binding of the short carbon fibers and sintering of neighboring ZrO2 particles, a compact ZrO2 layer was obtained. Owing to the formed ZrO2 layer, the reflectivity increased from 8.0% to 52.5% following irradiation at 500 W/cm2 for 180 s. This greatly mitigates the absorption of the laser energy. In addition, no distinct ablation pits were observed. This indicates that the composites exhibit excellent anti-laser ablation performance, and could be potentially 1

ACCEPTED MANUSCRIPT applied as protective shields for materials exposed to high-energy CW lasers. Keywords: Polymer-matrix composites; Laser-ablation behavior; High-energy

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continuous-wave laser; Reflectivity.

1. Introduction

Polymer-matrix composites are some of the most important structural materials

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because of their outstanding modulus–strength ratio, excellent corrosion resistance,

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and good formability[1-4]. With the extension of their application range, polymer-matrix composites are exposed to some extreme environments[5], including high-energy continuous-wave (CW) lasers[6].

With the rapid increase of laser power, high-energy CW lasers can destroy

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materials of a traditional structure in a period of several seconds because of the formation of a high-temperature field[7-10]. Polymer-matrix composites can also be destroyed. Owing to high-temperature fields and severe oxidation, such composites

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can easily be broken down[11, 12]. Therefore, to broaden their application range, it is

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critical to determine an effective method to improve the anti-laser performance of polymer-matrix composites. Anti-laser coatings which can protect materials through high reflectivity or

endothermic reactions have been prepared [13, 14]. For example, Jinpeng Zhu et al.[15] designed and prepared a novel ceramic coating to reduce laser-energy absorption. This coating exhibits good anti-laser performance and can provide protective effects without damage. Other researchers have also designed various kinds 2

ACCEPTED MANUSCRIPT of protective coatings. However, such coatings are only effective for periods of seconds or tens of seconds[6, 13, 16-18]. Once the coating has failed, the material requiring protection will be destroyed within a short period of time[13]. Therefore,

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these protective coatings have marked limitations and cannot provide long-term protection. In this case, designing a new polymer-matrix composite owning outstanding anti-laser performance could be a good solution.

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Short carbon fiber reinforced phenolic resin composite owns remarkable ablation

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resistance due to its good mechanical properties, heat resistance, and high char yield[19, 20]. Based on this composite, appropriate modification by particles introduction could effectively improve the anti-laser performance[21]. Zirconium carbide (ZrC) has been used to manufacture cutting tools because of its ultra-high

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melting point and hardness[22, 23]. In addition, it is also widely applied in the ablation-resistance field[24-26]. Dan Zhao et al.[27] prepared a three-dimensional (3D) C/ZrC composite and thoroughly investigated its flame-ablation behavior using

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oxyacetylene torch tests. C/ZrC exhibits good ablation-resistance performance, and

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benefits from the melting of ZrO2, which can seal the pores of the ablated composite. It is interesting that ZrO2 exhibits high reflectivity, which can help to reduce the absorption of laser energy[28]. If a ceramic layer with high reflectivity could be formed during laser ablation, such polymer-matrix composites can offer excellent anti-laser performance. However, it is unknown if such a concept could be effective during high-energy CW laser ablation, considering that laser ablation can result in much greater temperature fields and more severe heat-concentration issues than those 3

ACCEPTED MANUSCRIPT experienced during typical flame ablation[29, 30]. In this work, a ZrC-modified short-carbon-fiber-reinforced phenolic-resin matrix composite was prepared. The laser-ablation behavior of the composite was studied

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using a high-energy CW laser. The reflectivity and back-surface temperature of the composite were monitored in real-time to investigate the surface evolution of the composite during laser ablation. The micro-morphologies, elemental distributions,

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and detected phases of the composite were studied in detail, along with its specific

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physical and chemical responses. In addition, the mass ablation rate was determined under various test periods to evaluate the anti-laser performance of the composite.

2. Experimental

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2.1 Materials

Resol-type boron-modified phenolic resin (BPF) was purchased from Bengbu Tianyu High Temperature Resin Materials Co., Ltd (model: FB). Short carbon fibers

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were purchased from Yancheng Xiang Sheng Carbon Fiber Technology Co., Ltd.

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(model: T300, length: 9 mm). The ZrC was provided by Forsman Scientific (Beijing) Co., Ltd (10 µm, model: 4004003). 2.2

Preparation

of

the

ZrC-modified

short-carbon-fiber-reinforced

phenolic-resin composite The composite was prepared using a thermocompression method. The preparation process is shown in Figure 1 (a). The mass of the raw materials was determined according to the mass ratio of 8:3:9 (BPF: short carbon fiber: ZrC particles). First, 100 4

ACCEPTED MANUSCRIPT g of BPF was dissolved in 100 g of absolute ethyl alcohol to prepare the BPF solution. Then, 112.5 g of ZrC particles was added to the prepared BPF solution. The particles were uniformly dispersed under high-speed stirring (4000 r/min) for a period of 40

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min. In addition, 37.5 g of the short carbon fibers was quickly steeped in the aforementioned mixture, and then dried at 80 °C for 4 h within a convection oven. The dried prepregs were placed into a mold with a size of 200 mm × 200 mm prior to

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the thermocompression process. The thermocompression process was performed

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using the heating schedule shown in Figure 1 (b). The mold was maintained at 120 °C for 120 min under a pressure of 1 MPa, followed by 150 °C for 20 min at 1 MPa, 150 °C for 180 min at 10 MPa, and 180 °C for 120 min at 10 MPa. Subsequently, a ZrC-modified

short-carbon-fiber-reinforced

phenolic-resin

composite,

with

a

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thickness of 2.5 mm, was obtained. Samples, with sizes of 50 mm × 50 mm, were cut from the composite for the laser ablation experiments; these samples are denoted as BPF-ZF. In order to study the effect produced by the short carbon fiber and ZrC

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particles, Control experiment was designed. Sample containing only BPF and ZrC

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particles was prepared with a mass ratio of 8:9, which is denoted as BPF-Z. Sample containing only BPF and short carbon fiber was also prepared with a mass ratio of 8:3， which is denoted as BPF-F. The cross-section of the prepared composite (BPF-ZF) is shown in Figure 1 (c).

The interior of the composite is very compact and no cracks can be observed. Based on the micro-morphology of the cross-section, it can be determined that the short carbon fibers and ZrC particles are uniformly dispersed within the BPF matrix. 5

ACCEPTED MANUSCRIPT Although the ZrC has a very high density compared with that of the BPF and short fibers, the composite prepared using the aforementioned method does not exhibit pronounced precipitation phenomena. Figure 1 (d) shows a high-magnification image

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of the cross-section; as shown, the ZrC particles are primarily distributed between the short carbon fibers. In addition, owing to the good wetting capability of the BPF

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solution, there is good amalgamation at the interface between the fibers and BPF.

Figure 1. (a) Schematic of the composite preparation process, (b) curing cycle used

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during thermocompression, (c) macro-morphology and micro-morphology of the

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BPF-ZF cross-section, (d) high-magnification image of the cross-section.

2.3 Characterization and testing A scanning electron microscope (SEM, HITACHI S4800, Japan) equipped with

an

energy-dispersive

spectroscope

(EDS)

was

used

to

observe

the

micro-morphologies of the laser-ablated region. X-ray diffraction (XRD, X’Pert PRO MPD, PANalytical Inc.) was used to detect the phase transitions of the composites 6

ACCEPTED MANUSCRIPT following laser ablation. An ultra-violet/ visible/ near infra-red (UV/VIS/NIR) spectrophotometer (CRAY 5000, Australia) was used to measure the reflectivity of the composites.

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The high-energy CW-laser ablation process was studied using a Nd:YAG continuous-fiber laser device. A CW laser with a 1070-nm wavelength was excited. The size of the laser spot was adjusted to 10 mm × 10 mm. A signal detector was

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placed in front of the sample to detect the intensity of the diffuse reflection. The

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sample was fixed by a point-contact fixture, which could eliminate the heat transmission between sample and fixture. The back-surface temperature was monitored using a thermalcouple that was fixed to the back surface of the sample. The laser parameters, including the laser-ablation time and laser-power density, were

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controlled using a computer.

3. Results and discussion

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3.1 Macro-morphologies of composites ablated by the high-energy CW laser

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When resin or resin-matrix composites are ablated using a high-energy CW laser, the resin continues to decompose into residual char because of the high temperature caused by the opto-thermal transformation[31]. This process rapidly releases many flammable gases, such as CO and H2[32], which results in the appearance of a visible flame. In addition, these gases are released from the interior of the material. Owing to severe laser ablation, the release rate of these gases is so high that a gas flow can form. This gas flow can peel off the particles located on the surface of the ablated region. 7

ACCEPTED MANUSCRIPT Therefore, resin-matrix composites that are reinforced with typical particles always form a distinct pit during laser ablation[33-35], which accelerates the failure of the composite.

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Figures 2 (a) and (b) reveal the macro-morphologies of the BPF-Z samples irradiated at 500 W/cm2 for 15 s and 30 s respectively. The size of the thermal affected area shows a slight increase from 14 mm to 16 mm. When the sample is

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irradiated for 30 s, an obvious crack running through the whole sample, indicating the

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failure of the composite. This could be caused by the shrinkage of the composite during cooling period after laser ablation. Figures 2 (c) and (d) show the macro-morphologies of the BPF-F samples irradiated at 500 W/cm2 for 15 s and 180 s respectively. Although no crack appears because of the reinforcement of carbon fiber,

oxidative damage.

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the ablation depth reaches to 0.95 mm after irradiated for 180 s, indicating a severe

The BPF-ZF sample exhibits totally different behavior. Figures 2 (e), (f), (g), and

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(h) reveal the macro-morphologies of the BPF-ZF samples irradiated at 500 W/cm2

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for 15 s, 30 s, 60 s, and 180 s, respectively. It is noteworthy that the surface of the ablated region gradually becomes compact and develops a white color. The specific reason for this will be explained in the section that follows. When the BPF-ZF sample is ablated for 15 s, the size of the resultant ablated region is 14 mm. With the ablation time is extended to 60 s, the size of the ablated region increases to 23 mm, which indicates that thermal diffusion occurs when the laser-ablation period is extended. However, no failure phenomena can be observed. Instead, the white-colored surface 8

ACCEPTED MANUSCRIPT of the ablated region becomes more compact. In addition, no distinct ablation pits are formed. These observations indicate that the composite offers excellent anti-ablation performance. Remarkably, the composite exhibits a relatively stable state during the

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extended period of laser ablation. Even when the ablation time is extended to 180 s (Figure 2 (h)), the ablated surface remains compact and no distinct cracks or pits can be observed. Resin-matrix composites that can withstand high-energy CW-laser

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ablation for such long periods of time are rarely encountered.

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Fig. 2 Macro-morphologies of different samples irradiated at 500 W/cm2 for various

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periods: (a) BPF-Z, 15 s, (b) BPF-Z, 30 s, (c) BPF-F, 15 s, (d) BPF-F, 180 s, (e) BPF-ZF, 15 s, (f) BPF-ZF, 30 s, (g) BPF-ZF, 60 s, (h) BPF-ZF, 180 s.

3.2 Micro-morphologies and element analysis of composites ablated by the high-energy CW laser To achieve the aforementioned excellent anti-laser ablation performance, the formation of the white-colored material is considered to be key. Figure 3 (a) shows 9

ACCEPTED MANUSCRIPT the micro-morphology of the edge of the white-colored region of the BPF-ZF sample following its irradiation at 500 W/cm2 for 180 s. This region is formed by the accumulation of particles, as shown in Figure 3 (b). The EDS results indicate that

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these particles consist of O and Zr. Based on the atomic ratio of O (64%) and Zr (36%), the particles could be ZrO2. The XRD pattern of the white-colored particles, shown in Figure 2 (h), also indicates that the main phase of the white-colored particles

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oxidation of ZrC and the formation of ZrO2.

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is ZrO2. Hence, it can be confirmed that the white-colored region is formed by the

Although the macro-morphologies of the center and edge of the white-colored region are almost identical, it can be observed that the micro-morphology of the central region is more compact (Figure 3 (d)). In addition, distinct sintering necks can

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be observed in the high-magnification image shown in Figure 3 (e). These are caused by the high temperature of the central region, which is due to the heat concentration[15]. The sintering of the ZrO2 particles leads to a more stable structure,

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and thus the composite also remains stable during the extended ablation process. It is

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interesting to note that no carbon fibers remain on the surface of the white-colored region. However, some distinct dents can be observed (Figure 3 (f)). This is likely to be due to the oxidation of the carbon fibers. Therefore, the surface of the central region has a compact, white-colored surface with no black-colored carbon fibers present.

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Fig. 3 Micro-morphologies of the white-colored region that was irradiated at 500

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W/cm2 for 180 s: (a) edge of the white-colored region, (b) morphology and EDS results of the white-colored particles, (c) XRD pattern obtained for the white-colored particles, (d) center of the white-colored region, (e) morphology of the sintered

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material, (f) morphology of dents.

Figure 4 (a) reveals the cross-section of the BPF-ZF sample that was irradiated at

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500 W/cm2 for 180 s. Carbon fibers can be observed to exist under the ZrO2 layer, indicating that the carbon fiber that exists within the composite is protected because

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of the formation of the ZrO2 layer. Based on the above analysis, the ZrO2 particles on the surface of the composite were sintered during laser ablation. However, no sintering necks can be observed between neighboring ZrO2 particles existing within the ZrO2 layer (Figure 4 (b)). This could be because the temperature of the interior of the ZrO2 layer is lower than that of its surface. Figures 4 (c), (d), and (e) show the distributions of C, Zr, and O within the area shown in Figure 4 (a), respectively. It is 11

ACCEPTED MANUSCRIPT noted that Zr is concentrated at the top surface of the ablated region because of the accumulation of ZrO2. In addition, C almost disappears from the top surface because of oxidation. However, most of carbon fiber is remained in the region under the ZrO2

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layer, which demonstrates the good oxidation resistance of the formed ZrO2 layer. To further study the morphology and composition of the interface between the ZrO2 layer and composite, the ZrO2 layer was removed manually; the

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micro-morphology of the exposed interface is shown in Figure 4 (f). The interface is

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composed of carbon fibers and particles. Based on the XRD patterns obtained for the exposed interface (Figure 4 (g)), some unoxidized ZrC particles also exist in this layer, as well as ZrO2 particles. This means that compared with that of the composite surface, oxidation is greatly mitigated. At this exposed interface, it is interesting to

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observe that a severe oxidation phenomenon has occurred on the surface of the carbon fiber (Figure 4 (h)). This morphology further indicates that the disappearance of the

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carbon fibers from the surface of the composite is due to oxidation[36].

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ACCEPTED MANUSCRIPT Fig. 4 Morphologies and detected phases of the BPF-ZF sample that was irradiated at 500 W/cm2 for 180 s: (a) cross-section of the BPF-ZF sample following laser ablation, (b) particles within the cross-section, (c) elemental distribution of C, (d) elemental

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distribution of Zr, (e) elemental distribution of O, (f) surface morphology of the exposed interface following removal of the ZrO2 layer, (g) XRD patterns obtained for

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3.3 Laser-ablation mechanism of BPF-ZF

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the exposed interface, (h) oxidation morphology of carbon fiber.

The micro-morphologies and phase evolution of the composite were observed during laser ablation, and the interface between the ZrO2 layer and composite was studied. The results of the above analysis can be used to determine the specific

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laser-ablation behavior of the composite (Figure 5). When the BPF-ZF is ablated using a high-energy CW laser, the BPF continues to decompose into residual char and the pyrolysis depth gradually reaches the interior of the composite. Owing to the

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existence of oxygen, the ZrC particles are oxidized, forming ZrO2 particles. Therefore,

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the ablated region is white colored, as shown in Figure 2 (h). During the initial ablation period, the formed ZrO2 particles are fixed on the surface of the composite, which is caused by the binding of the carbon fibers and residual char. Thus, no distinct ablation pit can be observed within the central region. As the laser ablation proceeds, the carbon fibers and residual char at the surface of the composite were completely oxidized. Meanwhile, with the deposition of heat, the temperature at the surface of the composite becomes so high that sintering occurs 13

ACCEPTED MANUSCRIPT between neighboring ZrO2 particles, and a compact and stable ZrO2 layer forms. Owing to the oxidation of the carbon fibers and residual char, a perfect, white-colored coating (ZrO2 layer) can be observed. This coating can prevent oxygen from entering

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the composite, and demonstrates excellent oxidation resistance performance. Therefore, although the thickness of the residual char continues to increase because of the heat transmission and resin pyrolysis, the entire composite remains stable during

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the extended period of laser ablation because of the protection provided by the formed

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ZrO2 layer.

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Fig. 5 Schematic of the laser-ablation process

3.4 Improvement in reflectivity after laser ablation Owing to the formation of the compact ZrO2 layer, the reflectivity of the ablated region markedly improves, as shown in Figure 6. In the wavelength of the Nd:YAG continuous fiber laser device (1070 nm), the reflectivity of BPF-ZF is only 8.0% prior 14

ACCEPTED MANUSCRIPT to laser ablation. When the sample is ablated at 500 W/cm2 for 15 s, the reflectivity increases to 30.7%. In addition, the reflectivity increases as the ablation period is extended. When the ablation period is 180 s, the reflectivity reaches 52.5%, which is

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6.6 times greater than the reflectivity of the original composite. This indicates that the extended ablation period not only fails to destroy the composite but improves the reflectivity of the ablated region. The improved reflectivity can effectively mitigate

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the strong absorption during laser ablation. This suggests that the laser-ablation

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resistance gradually improves.

Fig. 6 Reflectivity of BPF-ZF sample irradiated at 500 W/cm2 for various periods of time.

3.5 Evolution of the back-surface temperature and mass ablation rate The intensity of the diffuse reflection is used to monitor the change in the 15

ACCEPTED MANUSCRIPT reflectivity of the composite in real time. Figure 7 (a) shows the intensity of the diffuse reflection and the back-surface temperature evolution of the BPF-ZF sample that was ablated by a laser at 500 W/cm2 for 180 s. The reflectivity increases for a

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period of about 110 s, and then remains relatively stable over the following 70 s. This suggests that a stable ZrO2 layer is formed following an ablation period of 110 s. In addition, the back-surface temperature rises rapidly during an initial period of 19 s,

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and reaches 1065 °C. Then, as the laser-ablation process continues, the back-surface

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temperature gradually decreases. When the composite is ablated for 110 s, the back-surface temperature remains almost stable at around 830 °C. The increase or decrease in the back-surface temperature depends on the rate that the laser energy transforms into heat (denoted as H1) and the rate that the heat in the

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ablated region transfers to its surroundings (denoted as H2), which is demonstrated in the schematic shown in Figure 7 (b). During the initial period of ablation, the absorptivity of the composite is so high (92%) that H1>>H2; therefore, the

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back-surface temperature increases rapidly. As the process continues, the ablated

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region forms a ZrO2 layer. Owing to the increased reflectivity, H1 gradually decreases. When H1 is lower than H2, the back-surface temperature starts to decrease, which corresponds to the ablation period between 19.1 s and 110 s. Owing to the reduction of the back-surface temperature, the temperature difference between the ablated region and its surrounding regions gradually declines; consequently, there is a decrease in H2. With the compaction of the newly formed ZrO2 layer, the values of the reflectivity of the ablated region and H1 remain stable. Finally, when the value of H2 16

ACCEPTED MANUSCRIPT decreases to that of H1, the laser-ablation process reaches dynamic equilibrium, and the back-surface temperature will remain relatively stable as the ablation process continues.

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Owing to the increase in the reflectivity, the mass ablation rate exhibits a declining trend during laser ablation (Figure 7 (c)). Over the ablation period of 0 s to 10 s, the average mass ablation rate is 16.5 mg/s. As the ablation period is extended

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from 10–20 s to 20–30 s, the average mass ablation decreases to 10.6 mg/s and 7.9

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mg/s, respectively. Remarkably, when the ablated region gradually stabilizes, the average mass ablation rate over the ablation period of 30 s to 180 s becomes as low as 1.1 mg/s. This low mass ablation rate indicates that the composite can further withstand laser ablation for significant period of time. Polymer-matrix composites

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that can remain in a good state following ablation by high-energy CW lasers for such long periods are rarely encountered, which indicates that the BPF-ZF composite offers

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excellent anti-laser ablation performance.

Fig. 7 (a) Back-surface temperature and intensity of the diffuse reflection evolution, (b) schematic of heat deposition and heat diffusion, (c) mass ablation rate as a 17

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4. Conclusions

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A ZrC-modified short-carbon-fiber-reinforced phenolic-resin composite was prepared via compression molding. Its laser-ablation behavior was studied using a high-energy CW laser. During the initial laser-ablation period, the BPF decomposed

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into residual char, and the ZrC particles were gradually oxidized, forming ZrO2. With

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the binding of the carbon fiber and residual char, the formed ZrO2 particles were fixed on the surface of the ablated region. As the laser-ablation process proceeded, sintering occurred between neighboring ZrO2 particles, and a compact ZrO2 layer was formed. The elemental distribution and micro-morphologies of the cross-section demonstrated

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that the compact ZrO2 layer possesses excellent oxidation resistance performance. This can prevent oxygen from causing damage to the carbon fibers and residual char located within the composite. In addition, the formed ZrO2 layer possesses high

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reflectivity. Following ablation for 180 s, the reflectivity of the ablated region

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increased from 8.0% to 52.5%. This can greatly mitigate the absorption of laser energy. Owing to the oxidation resistance performance of the composite and its improved reflectivity, the composite can withstand high-energy CW laser ablation for over 180 s. Such performance is rarely observed. Such composites could be applied in fields that involve exposure to high-energy CW lasers.

Acknowledgements 18

ACCEPTED MANUSCRIPT This work was supported by the National Natural Science Foundation of China (No. 51772027).

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