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Thin Solid Films

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Sapphire optical fiber with SiBCN coating prepared by chemical vapordeposition for high-temperature sensing applications

Shuang Chenb, Qiqi Zhanga, Xin’gang Luana,⁎, Rong Yua, Sam Zhangc, Laifei Chenga

a Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University, Xi'an, 710072, PR Chinab Changcheng Institute of Metrology and Measurement, Beijing, 100095, PR Chinac School of Materials and Energy, Southwest University, Chong Qing, 400715, PR China

A R T I C L E I N F O

Keywords:Sapphire optical fiberSilicon boron carbon nitrideCoatingThin filmsOxidation resistanceTensile strengthOptical properties

A B S T R A C T

To prevent the oxidation of BN-coated sapphire optical fiber (SOF) at 800 °C, a SiBCN ceramic coating was usedas an outer coating owing to its excellent high-temperature oxidation resistance. Compared with BN/SOF, thetensile strength of SiBCN/BN/SOF increases from 500 to 723 MPa at 1200 °C. The optical properties of doublecoating indicate that the addition of SiBCN does not diminish the total reflection of the BN coating. Furthermore,SiBCN-coated fibers indicate lower residual stress and performed better compared with other coated fibers interms of mechanical and optical properties. Hence, SiBCN is highly suitable as an outer coating of SOF.

1. Introduction

High-temperature structural materials are widely used in aerospace,automotive engines, nuclear power plants, and other fields. Thestress–strain relationship of materials in high-temperature environ-ments and temperature is important in industrial production processes[1]. Real-time and accurate measurements are crucial to the improve-ment of production safety and production efficiency. Although varioustypes of sensors are available, most of them cannot be used in high-temperature, high-pressure, and high-corrosion environments [2]. Op-tical fiber sensors can satisfy these environmental measurement re-quirements because of their unique advantages; therefore, a significantamount of research has been performed regarding these sensors. Owingto their high sensitivity, high precision, wide temperature range, andlong-distance measurement and control, fiber optic sensors are pro-mising for strain sensing and near-infrared energy transmission [4,5].

Although fiber optic sensors have demonstrated good results afterbeing systematically tested on domestically produced aviation equip-ment under various harsh conditions, their general dynamic response isstill slow and the measurement temperature is still not sufficiently high.Thus, a sapphire optical fiber (SOF) sensor cannot fully satisfy industryrequirements of the aerospace sector. As stress sensors, SOF sensors areexposed to continuous harsh environments, such as hot air flows andhigh temperatures [3]. This can damage the surface and interior of thefiber, resulting in light scattering and optical loss and consequently

affecting the long-term stability of the fiber sensor. Hence, the fibermust be protected to extend its working range and life [6].

As the fiber cladding material must withstand harsh environments,it must exhibit the following characteristics [7]: a) the refractive indexof the cladding material and the optical fiber should satisfy the re-quirements of total reflection, i.e., the refractive index of the claddingmaterial should be less than the refractive index of the fiber; b) thethermal expansion coefficients of the cladding material and optical fibershould match well, i.e., the residual stress between the core and thecladding material should be minimized; and c) the cladding materialshould have a higher melting point, good thermochemistry, mechanicalstability, and high temperature resistance.

In our previous study [8], we discovered that BN coating was ef-fective for SOFs. The tensile strength of the fiber increased when a 1-μm-thick BN coating was deposited on it. Besides, the BN coating/SOFcombination maintained a linear behavior even at 1200 °C, rendering itideal for strain sensors. The BN/SOF combination also demonstratedgood light transmission properties; hence, it can be used in opticalapplications. However, because BN begins to oxidize at 800 °C [9–11],an outer coating with excellent high-temperature and oxidation re-sistance for the BN/SOF needs to be prepared to protect the BN coatingfrom being oxidized. The outer material must resist erosion, wear, andoxidation. Suitable materials include platinum, tantalum, aluminumoxide, zirconium oxide, silicon carbide, silicon nitride, zirconium car-bide, silicon boron carbon nitride [12,13], and other high-temperature

https://doi.org/10.1016/j.tsf.2020.138242Received 24 October 2019; Received in revised form 16 July 2020; Accepted 16 July 2020

⁎ Corresponding author.E-mail address: xgluan@nwpu.edu.cn (X. Luan).

Thin Solid Films 709 (2020) 138242

Available online 17 July 20200040-6090/ © 2020 Elsevier B.V. All rights reserved.

T

resistant materials. Silicon-based ceramic materials, such as siliconcarbide, have high oxidation resistance, chemical stability, high tem-perature strength, and creep resistance at temperatures below 1400 °C.The fiber can be protected for a significant amount of time using anencapsulation coating. The SiBCN structure [14–16] comprises manycovalent bonds, such as SieN, SieC, BeN, and CeN. Its high-tem-perature performances are superior to those of SiC and Si3N4, includingthermal stability, oxidation resistance, and high-temperature creep re-sistance [17]. The oxidation product of SiBCN, which is a nano-composite ceramic containing silicon oxide and boron oxide, has astrong bonding strength at 1000 °C. In this study, we investigated theproperties of a SiBCN film as an outer coating for BN/SOF, including itsmicromorphology, residual stress, optical properties, and mechanicalproperties at different temperatures. Furthermore, the effects of dif-ferent coating types on the performance of SOFs were analyzed.

2. Coating design and simulation results

In our previous study, the residual thermal stress had a significanteffect on the film preparation [8]. We used a 1-μm-thick BN as the innercoating, and SiBCN and SiC as outer anti-oxidation protective coatingsfor analysis and comparison, respectively. In this study, COMSOL(Version 3.5a, Axisymmetric model, COMSOL, Inc.) was used to simu-late the residual stress distribution of rod-like materials from high-temperature cooling to room temperature using an axisymmetricmodel. The thermal stress of the composite material was calculated at950 °C. The main model ensured that the BN thickness was fixed at1 µm. The model was established by changing the thickness of the outerlayer. For the boundary conditions, the lower boundary was subject tothe constraints of the roller; whereas free constraints were used for theupper boundary. The model is shown in Fig. 1 and the physical prop-erties of each coating material are listed in Table 1.

Fig. 2 shows the residual stress distribution of the composite fromhigh temperature to room temperature. Fig. 2(a) shows the axial re-sidual stress distribution for the SiC/BN/SOF. As the thickness of theSiC coating increased and its tensile stress decreased, the compressivestresses of the SOF and BN coating gradually increased. Fig. 2(b) showsthe radial residual stress distribution for the SiC/BN/SOF. The radialcompressive stresses of the SOF, BN, and SiC coatings increased as thethickness of the SiC coating increased.

The axial stress of the SiBCN/BN/SOF is shown in Fig. 2(c). As thethickness of the SiBCN coating increased, the tensile stresses of the SOFand SiBCN coatings gradually decreased, whereas the compressive

stress of BN gradually increased. The radial stress of the SiBCN/BN/SOFis shown in Fig. 2(d). As the thickness of the SiBCN increased, the axialand radial stresses of the BN coating of the SOF changed from 3 μm tocompressive stress and gradually increased. Moreover, the radial com-pressive stress of the SiBCN coating gradually decreased.

Furthermore, the axial and radial residual stresses of the 1-µm-thickouter SiBCN and SiC coatings were compared. As shown in Fig. 3(a) and3(b), the SOF in the SiBCN/BN/SOF was less stressed in the axial andradial directions than that in the SiC/BN/SOF. Therefore, the SiBCNcoating is more suitable as an outer coating for the SOF.

3. Experimental procedure

3.1. Film deposition

We performed two chemical vapor deposition (CVD) processes todeposit SiBCN on the SOFs: (1) BN coating and SiBCN protectivecoatings were continuously deposited using one-step method; (2) a 1-μm-thick BN coating was deposited, and then a SiBCN or SiC protectivecoating was deposited using two-step method. The schematic of thereactor is shown in Fig. 4(a).

The SOFs provided by the Beijing 304 Research Institute have adiameter of 100 μm. The SOF are sonicated in acetone and ethanol for10 min, then taken out and dried at room temperature. A SOF waspassed through the holes of the graphite brackets fixed along the gra-phite mold as shown in Fig. 4(b) and the loaded mold suspended in theconstant temperature zone of the deposition furnace with carbon fiberattachments. Besides, the holes in the graphite mold itself allows gascirculation in the mold. Before deposition, the furnace is evacuated to0.1 Pa, then heated to 650 °C, and BCl3, NH3, H2 and Ar gasses areintroduced to deposit the BN coating. The specific deposition para-meters are shown in Table 2. After 150 min, the temperature is raised to950 °C, and SiCH3Cl3 (MTS), BCl3, NH3, H2 and Ar are introduced todeposit the SiBCN coating for 120 min with a pressure of 2 kPa andnMTS/(nNH3+nBCl3) ratio of 1. Finally, the samples are cooled naturallyin the reactor in vacuum. In addition, the SiC deposition process is al-most the same with that of SiBCN except for the longer heating time(180 min) and different deposition atmosphere. Hydrogen and argonare used as the carrier and diluent gas, respectively. SiCl3CH3 is broughtinto the deposition furnace by the bubbling method with a H2: MTSratio of α=10 and a pressure of 4 kPa. The specific SiC depositionparameters are also shown in Table 2.

3.2. Characterization

Tensile test was performed on an Instron-3345 electronic universaltesting machine loaded with aluminum rings. The gage lengths of thesamples were 50 and 160 mm at room temperature and 1200 °C, re-spectively. The structure of the prepared BN film was investigated viaRaman spectroscopy (Renishaw Raman, U.K.), which was equippedwith a HeeNe laser (I = 532 nm). The film thickness and morphologywere analyzed using scanning electron microcopy (S4700, Tokyo,Japan, 15 kV), and the chemical composition of the surface was de-termined using energy-dispersive spectrometry (EDS, Genesis, U. K,5 kV). The optical transmittance of the coated SOF was tested using anoptical fiber test pen (SG-A7R). The light source was a semiconductorFig. 1. Model of SOF composites.

Table 1Properties of various systems.

Constituent Young's modulus(GPa) E

Coefficient of ThermalExpansion (×10−6 /K) α

Poisson’ sRatio υ

SOF 344 5.3 0.27BN 58 2.3 0.16SiBCN 170 3.5 0.2SiC 450 5.3 0.24

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laser with a wavelength of 650 nm.

4. Result analysis

4.1. SiBCN/BN/SOF microstructure

Fig. 5(a) and (b) show the SiBCN coating prepared using two-stepmethod on the surface of a BN/SOF. A large part of the SOF surfacecracked and peeled off, and the coating was completely delaminatedfrom the SOF. Fig. 5(c) and (d) show the morphology of the continuousdeposition of the BN and SiBCN coatings on the SOF surface, using one-step method. The SOF surface was smooth and flat without cracks. Asshown in Fig. 5(d), the thickness of the BN coating was 0.75 μm, andthe thickness of the SiBCN coating was 1.5 μm.

As shown in Table 3, the surface of the SiBCN coating contained sixelements: B, C, N, O, Al, and Si, and no other impurity elements.Comparing the delamination and smooth areas in Fig. 5(a), the atomicpercentages of the elements in the smooth area reduced to varyingdegrees. The increase in Al and O indicates leakage on the SOF surface,signifying that part of the coating was spalled.

4.2. Mechanical properties

4.2.1. Bonding of SiBCN coating on BN/SOF compositesAs shown in Fig. 6(a), when the normal load reached approximately

42.95 N, the acoustic emission signal rapidly improved. The criticalload of the SiBCN coating on the BN/SOF composite was 42.95 N,which is similar to the binding force of the BN/sapphire system (50.1 N)[8]. As shown in Fig. 6(b), the critical load of the SiC coating on the BN/SOF composite was 25.6 N. This shows that the combination of BN/SOFand SiBCN coatings was better than that of BN/SOF and SiC coatings.

4.2.2. Room-temperature strengthAs shown in Table 4, SiBCN/BN/SOF had a higher tensile strength

and modulus of elasticity at room temperature, i.e., 1829 MPa and258 GPa, respectively, compared with those of the SOF. This indicatesthat the SiBCN coating can improve the room-temperature mechanicalproperties of the SOF, which are significantly better than those of theSiC/BN double coating.

As shown in Table 4, the SiBCN/BN/SOF composite has betterroom-temperature mechanical properties than those of SiC/BN/SOF butworse than those of BN/SOF. The residual stress of the SOF in theSiBCN/BN/SOF and BN/SOF composites was approximately equal, asshown in Fig. 3. Previously, we discovered that the thickness of the BNcoating in the BN/SOF composite increased from 0.5 to 1 μm, and thetensile strength at room temperature gradually increased [8]. The ac-tual thickness of the BN coating of the SiBCN/BN/SOF composite (usingone-step method) was 0.75 μm, indicating that the BN coating of lessthan 1 μm did not adequately repair the surface defects of the SOF,thereby resulting in a lower strength compared with the BN/SOFcomposite. Meanwhile, SiBCN did not exert a repairing effect on the BNsurface defects, and the residual stress of the fiber caused by the doublecoating decreased; hence, its room-temperature tensile strength wassuperior to that of the SOF. As shown in Fig. 3, the residual stress of theSiC/BN/SOF was much higher than those of the other two materials.The axial residual tensile stress of the SiC coating was approximately600 MPa, and the stress was greater than the strength of the SiC coating(500 MPa [18]), which is sufficient to crack the SiC matrix. Once theSiC surface is microcracked, the crack easily spreads when subjected totensile forces. As evident from the light leakage of the SiC/BN/SOFcomposite, SiC diminished the total reflection of the inner BN coatingand introduced defects into the BN coating. Therefore, its room-tem-perature tensile strength was much lower than those of the other twocoatings. As shown in Fig. 7(a), the SiBCN/BN/SOF maintained itslinear tensile properties at room temperature, which is critical for theapplication of SOFs to sensors. However, the stress–strain curve of theSiC/BN/SOF did not remain linear. Hence, the SiBCN coating is moresuitable as an outer coating of SOFs.

Fig. 2. Residual stress distribution at different coating thicknesses: (a) axial distribution; (b) radial residual stress of SiC/BN/SOF; (c) axial distributions; and (d)radial residual stress of SiBCN/BN/SOF.

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One of the most well-known methods for detecting the residualstress of carbon-containing materials is Raman spectroscopy [19].Compared with the Raman spectrum of graphene in Fig. 8(a) [20], theG peak (1580 cm−1) of SiBCN coating in Fig. 8(b) was relatively broadand not sharp, showing that SiBCN was in fact amorphous. The G*(2278 cm−1) and D + D* (3045 cm−1) peaks slightly shifted, which isrelated to the D + G combination mode of the carbon peak and causedby disorder [16]. The G* peak shifted by 167 cm−1, whereas theD + D* peak shifted by 95 cm−1. The crystallization degree of C is

Fig. 3. Residual stress of structural units at 1 μm in SiBCN/BN/SOF, SiC/BN/SOF, and BN/SOF composites: (a) axial residual stress and (b) radial residual stress.

Fig. 4. Schematics of (a) the reactor for CVD SiC and (b) of the graphite mold.

Table 2Processing conditions of CVD process.

T ( °C) H2 (ml/min) BCl3 (ml/min) NH3 (ml/min) Ar (ml/min)

BN 650 50 10 30 50SiBCN 950 250 4 16 50SiC 1100 200 0 0 250

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related to the heat treatment temperature of SiBCN.

4.2.3. High-temperature strengthAs shown in Table 4, compared with the high-temperature me-

chanical properties of SOFs, the strength of the SiBCN/BN/SOF in-creased to 723 MPa at 1200 °C, and the modulus increased to 141 GPa,which is significantly higher than the values of the high-temperaturemechanical properties of the outer SiC coating. Compared with the BN/

SOF, the tensile strength of the SiBCN/BN/SOF increased from 512 [8]to 723 MPa at 1200 °C. As shown in Fig. 9(a), the SOF coated withSiBCN can maintain linear tensile properties at high temperaturescompared with the SiC coating. As shown in Fig. 9b), the SOF wassubjected to compressive stress in both the radial and axial directions,

Fig. 5. Surface morphology and cross-sectional morphology of SiBCN coating on SOF: (a) (b) two-step method and (c) (d) one-step method.

Table 3EDS analysis of the surface of a SiBCN coating using different preparationprocesses.

At (%) B C N O Al Si

Two-step Smooth 44 10 27 1 0 18method Delamination 27 7 3 43 19 1

One-step method 34 12 37 1 3 13

Fig. 6. Acoustic emission curve and surface morphology of sapphire double coating: (a) SiBCN/BN/SOF and (b) SiC/BN/SOF.

Table 4Tensile test results of SiBCN/BN/SOF and SiC/BN/SOF at room temperature(RT) and 1200 °C.

Elastic modulus (GPa) Tensile strength (MPa)RT 1200 °C RT 1200 °C

SOF 244 ± 98 125 ± 9 1425 ± 250 658 ± 89BN/SOF (BN = 1 μm) 478 ± 13 123 ± 7 5269 ± 304 512 ± 24SiBCN/BN/SOF

(BN = 0.75 μm)258 ± 16 141 ± 10 1829 ± 228 723 ± 67

SiC/BN/SOF(BN = 1 μm)

127 ± 25 90 ± 12 1022 ± 157 434 ± 76

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and the coating did not undergo debonding and cracking. Therefore,the SiBCN coating is more suitable as an outer coating of SOFs than SiCcoating at high temperatures. As shown in Fig. 10, the fiber sig-nificantly peeled, whereas the interface of the fracture site of thesample blurred and teared, indicating that the strength of the fibersignificantly improved.

4.3. Optical properties

Transmittance detection was performed on an SOF coated withvarious coatings using an SG visual fault locator. The SOF was placed inthe optical interface, and the semiconductor laser emitted an opticalsignal of wavelength 650 nm and pulse frequency 2–3 Hz, which wasexpanded by the beam expander and projected onto the SOF. As shownin Fig. 11(a), the original SOF had poor transmittance, and the laser

could not smoothly pass through the surface of the fiber. However, asshown in Fig. 11(b), when the BN/SOF was tested, an apparent red dotwas observed at the other end of the indicator. This indicates that theindicator can emit a stable red laser, and BN can be used as a totalreflection coating to protect the SOF from light leakages. As shown inFig. 11(c), when the SiC/BN/SOF was tested, a red laser appeared in theentire length of the SOF, and SiC diminished the total reflection of theinner BN coating. As shown in Fig. 11(d), when the SiBCN/BN/SOF wastested, a distinct red dot was still observed at the other end of the in-dicator, indicating that the indicator can emit a stable red laser. Thisshows that SiBCN is highly compatible with BN, does not damage theintegrity of the BN coating, and better protects the SOF. Therefore, BNand SiBCN are suitable as inner coatings of SOFs.

The numerical aperture is calculated using NA = −n n12

22 , where

n1 is the refractive index of the fiber core (n1 of sapphire fiber [8] is

Fig. 7. Tensile stress–strain curves of SiBCN/BN/SOF and SiC/BN/SOF and fracture morphology of SiBCN/BN/SOF at room temperature.

Fig. 8. Raman spectra of (a) grapheme/SiC [20] and (b) SiBCN/BN/SOF composites.

Fig. 9. Tensile stress–strain curves of SiBCN/BN/SOF and SiC/BN/SOF and residual stress distribution of SiBCN/BN/SOF at 1200 °C.

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1.75), n2 is the refractive index of the cladding, and the optical wave-length is 650 nm. In a previous study, the refractive index of the BNcladding [8] was discovered to be 1.73 after testing; therefore, NA ofthe BN cladding was approximately 0.26, which was higher than that ofthe general multimode fiber. The representative optical fiber can bettercapture the light rays, thereby improving the coupling efficiency of thelight and light source, as well as increasing the optical power enteringthe optical fiber, thereby reflecting the high information transmissioncapacity of the optical fiber [21].

5. Conclusion

The residual stresses of SOF in SiBCN/BN/SOF were minimal in theaxial and radial directions in comparison with those of SiC/BN/SOF.The mechanical properties of the SiBCN/BN/SOF were better thanthose of SiC/BN/SOF at room or higher temperatures, which is

consistent with the calculation. Furthermore, the tensile strength of theSiBCN/BN/SOF was higher than that of BN/SOF at high temperatures.The bonding of SiBCN coating to BN/SOF was 42.95 N, which is sig-nificantly higher than that of SiC coating to the BN/SOF (25.6 N). Forthe outer SiC coating, a red laser appeared in the entire length of theSOF. It was also observed that SiC diminished the total reflection of theBN inner coating. Consistent with the calculation, SiBCN demonstratedgood compatibility with BN, did not destroy the integrity of the BNcoating, and better protects the SOF. Hence, SiBCN is a more suitableouter coating for SOFs.

CRediT authorship contribution statement

Shuang Chen: Resources, Funding acquisition, Validation. QiqiZhang: Supervision, Funding acquisition, Writing - review & editing.Xin’gang Luan: Supervision, Funding acquisition, Writing - review &

Fig. 10. Fracture morphology at 1200 °C of SiBCN/BN/SOF composites: (a) fracture and (b) high-magnification morphology of (a).

Fig. 11. Transmittance analysis: (a) SOF, (b) BN/SOF, (c) SiC/BN/SOF, and (d) SiBCN/BN/SOF.

S. Chen, et al. Thin Solid Films 709 (2020) 138242

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editing. Rong Yu: Methodology, Data curation, Formal analysis. SamZhang: Supervision, Funding acquisition, Writing - review & editing.Laifei Cheng: Project administration, Methodology.

Declaration of Competing Interest

The authors declare that they have no known competing financialinterests or personal relationships that could have appeared to influ-ence the work reported in this paper.

Acknowledgements

This study was supported by the National Basic Research Program ofChina(973 Program), the Open Fund of the State Key Laboratory onIntegrated Optoelectronics, China(No. IOSKL2018KF05), and theFundamental Research Funds for Central Universities, China (No. SWU118105). The authors acknowledge the Key Laboratory of SpecialtyFiber Optics and Optical Access Networks, Shanghai University of Chinafor providing the refractive index data.

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