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Letter Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Hard, Flexible, and Transparent Nanolayered SiNx/BN Periodical Coatings Oleksiy V. Penkov,†,‡ Mahdi Khadem,†,‡ and Dae-Eun Kim*,†,‡ †

Center for Nano-Wear, Yonsei University, Yonsei-ro 50, Seoul 03722 Korea Department of Mechanical Engineering, Yonsei University, Yonsei-ro 50, Seoul 03722 Korea

‡

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S Supporting Information *

ABSTRACT: In this study, SiNx/BN periodical nanolayered coatings (PNCs) are developed. PNCs were deposited at the room temperature on plastic and glass substrates. They demonstrate the excellent mechanical durability of inorganic materials and optical transparency and flexibility of organic ones. The 150 nm thick PNC shows optical transparency, sapphire-like hardness, high wear protection, and flexibility. Such a coating with a superior combination of optical and mechanical properties has not been reported previously.

KEYWORDS: flexible hard coatings, transparent coatings, wear resistance, reactive sputtering, periodical nanolayered coating he rapid growth of the wearable and flexible electronics industry has spurred the need to develop novel materials with excellent mechanical and optical properties.1 In particular, materials with high scratch resistance and elasticity/flexibility are desired, but it is challenging to develop a material with such a combination of properties.2 For example, organic coatings typically demonstrated high flexibility but low hardness and scratch resistance.3,4 However, inorganic films are hard and abrasion resistant. Besides, they are often brittle and inelastic.5 Thus, still, there is a need to develop flexible hard coatings with high optical transparency. The formal criteria for “flexible hard coatings” were given by Musil.6 According to these criteria, a coating is considered flexible and hard if it is hard, tough, and crack resistant simultaneously. Such coatings should exhibit high hardness H and relatively low effective Young’s modulus E* so that the ratio H/E* > 0.1. Elastic recovery should be higher than 60%. The coating should have a compressive macro-stress (σ) of 0.15.10,12 Since these coatings are deposited by magnetron sputtering at room temperature, they can be deposited on plastic substrates. However, these coatings are not transparent. On the other hand, hard transparent films such as Al−Si−N show high hardness (around 30 GPa) and transparency but minimal elasticity. Moreover, they require high deposition temperatures, and hence cannot be deposited on plastic substrates.5 Recently, polycrystalline cubic silicon nitride has been reported.13 This material exhibits a hardness of >40 GPa and high transparency. However, this material requires high synthesis temperature and pressure and hence can be manufactured only in bulk. Thus, despite the significant improvement in the development of coatings, a single coating exhibiting hardness, transparency, and flexibility has not been developed yet. Received: December 18, 2018 Accepted: February 22, 2019 Published: February 22, 2019 A

DOI: 10.1021/acsami.8b22091 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Structure and mechanical properties of the PNCs. (A) The cross-sectional TEM image of the three-stack PNCs where the SiNx layer thickness was 1.2 nm and the thickness of the BN layers was 1.1, 0.7, and 0.3 nm. It was deposited at a N2:Ar ratio of 0.2 and bias voltage of 35 V. (B) Indentation curves for the PNCs and Gorilla 4, tempered, and soda-lime glasses. (C) Hardness and H/E ratios of the PNCs and commercial glasses. (D) Hardness and H/E ratios of the single-layered coatings and PNCs (N2:Ar ratio of 0.2 and bias voltage of 35 V). (E) Effect of the N2:Ar ratio on the hardness and optical transmission of the PNCs (bias voltage of 35 V).

In this study, we developed hard, transparent, and flexible periodical nanolayered coatings (PNCs). The properties of the PNCs developed in this study and previously reported coatings have been compared in Table S1. The PNCs were deposited layer-by-layer by reactive magnetron sputtering. A mixture of argon and nitrogen was used as the working gas for sputtering. The deposition setup consisted of a vertical cylindrical vacuum chamber with magnetrons mounted on the top cap. Silicon and boron nitride targets were installed on the magnetrons. A substrate holder was mounted below the magnetrons and was transferred from one magnetron to another by a robotic arm to deposit alternate layers of SiNx and BN (Figure S1). Glass, polyethylene terephthalate (PET), and acryl substrates were used in this work. The thickness of PET and acryl was 0.2 and 2 mm, respectively. The substrate temperature during the process did not exceed 55 °C. No sign of melting or degradation of PET and acryl substrates was observed after the deposition process. Preliminary studies showed that SiNx layers were subjected to gradual oxidation under contact with the ambient atmosphere. Oxidation caused reduction of hardness and increase of transparency of coatings. The periodical structure was covered with a 5 nm thick BN layer to avoid oxidation. Also, it was found that SiNx provided better adhesion to glass substrates in comparison with BN. Thus, the 5 nm thick SiNx adhesion layer was coated on the substrate before the multilayer structure. The total thickness of each coating was 150 nm, and the number of SiNx/BN-pairs varied from 70 to 180 depending on

the thickness of the individual layers. The nominal thickness of the layers was controlled by the exposition time and was maintained in the range of 0.3−1.2 nm. The nominal thickness of a coating is the product of its exposition time and deposition rate. The deposition rate was initially calibrated by measuring the thickness of the relatively thick single-layer coating. A negative 10 kHz dc pulse bias voltage was applied between the substrate holder and the magnetron anodes during the deposition. The use of pulse bias during the deposition increased the deposition rate and improved the quality of the coatings because of the effective illumination of the charging substrate.14,15 Figure 1 shows the structure and mechanical properties of the PNCs. Cross-sectional high-resolution transmission electron microscopy (HRTM) analysis was carried out to examine the layered structure of the PNCs (Figure 1A). The PNCs consisted of three stacks with different layer thicknesses. The nominal thickness of the SiNx layers was 1.2 nm, while the thicknesses of the BN layers were 1.2, 0.7, and 0.3 nm. The stacks consisted of 9, 12, and 40 SiNx/BN pairs. The bright stripes in the HRTEM image represent the BN layers, while the darker stripes represent the SiNx layers. The brightness of the BN layers decreased with a decrease in their thickness. As a result, the periodical structure in the top stack could be hardly seen (Figure S2). The real thickness of the layers was lower than their nominal thickness, especially in the top stack. This can be attributed to the intermixing of the components. The crystal structure of the PNCs was assessed using their microdiffraction images. The microdiffraction image of the B


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Figure 2. Optical and mechanical performance of the PNCs. (A) Transmission spectra of the PNCs on the glass. The inset shows the corresponding glass substrate (left) and the PNCs (right). The PNC had the nominal SiNx/BN thicknesses 1.2/0.3 nm. It was deposited at a N2:Ar ratio of 0.2 and bias voltage of 35 V. (B) Transmittance of the single-layered and multilayer coatings with different SiNx:BN ratios (N2:Ar ratio of 0.2 and bias voltage of 35 V). Optical images and 2D profile of the wear tracks for the bare PET substrate (C) and the PNC-coated PET substrate (D). The PNC had a SiNx/BN thickness of 1.2/0.7 nm and a total number of pairs of 150. It was deposited at a N2:Ar ratio of 0.2 and bias voltage of 35 V. (E) Flexibility of the PNC-coated PET substrate.

especially the bias voltage. Initially, the hardness of the coatings increased as the bias voltage was increased from 10 to 35 V. The maximum hardness (35 GPa) was obtained at the bias voltage of 35 V. A further increase in the bias voltage resulted in a decrease in the hardness of the PNCs (27 GPa at 40 V) (Figure S5). The coatings exhibited compressive residual stress, and the stress level was also affected by the bias voltage. When the voltage increased from 10 to 40 V, the residual stress changed from −0.35 to −0.92 GPa (Figure S6). Despite the fact that the presence of compressive stress is typical for sputtered hard materials, these values were low compare to other nanolayered periodical coatings.11,12 Another important factor affecting the properties of the PNCs was the gas atmosphere. It was found that the N2:Ar ratio was crucial for both the hardness and transparency of the PNCs (Figure 1E). An increase in the nitrogen content increased the transmittance and reduced the hardness of the PNCs. The best combination of the hardness and transmittance was achieved at the gas ratio of about 0.4. At this gas ratio, the PNCs showed a hardness of >32 GPa and high transparency. The transmission spectra of the PNCs on the glass substrate in the visible light are shown in Figure 2.

PNCs (Figure S3) showed a spread halo, which indicated that the layers had an amorphous structure. The diameter of the halo was close to the first coordination rings of Si3N4 and cBN.16 The mechanical properties of the PNCs were evaluated by high-precision ultranano indentation.17,18 The typical indentation curves of the PNCs and commercial glasses are shown in Figure 1B. The PNCs showed the lowest penetration depth and the largest elastic recovery (>80%). The hardness of the PNCs was approximately 3 times higher than that of commercial Gorilla 4 glass (Figure 1C). The hardness of the single-layer BN and SiNx coatings was 2.6 and 24.9 GPa, respectively (Figure 1D). Since the PNCs consisted of pairs of hard and soft layers, their mechanical properties depended on the ratio of the thicknesses of the SiNx and BN layers (in a pair) (Figure S4). An increase in the SiNx content resulted in an increase in both the hardness and H/E ratio of the PNCs. Moreover, the hardness and H/E ratio of the multilayer structures were much higher than those of the individual materials in pairs. Apart from the layer thickness, the mechanical properties of the coatings were also sensitive to the deposition conditions, C


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Figure 3. Chemical composition of the PNCs. The PNC had a SiNx/BN thickness of 1.2/0.7 nm and a total number of pairs of 150. It was deposited at a N2:Ar ratio of 0.2 and bias voltage of 35 V. (A) Fragment of the Raman spectra. N1s, B1s, and Si2p XPS peaks (B)−(D).

Because of the relatively low thickness of the PNCs and high H/E ratio of the coatings, the PNC-coated PET substrate maintained its flexibility (Figure 2E). The adhesion of the coatings and their degree of flexibility were further evaluated using a bending tester (made in-house). The specimens were bent forward and backward, i.e., the twisted area was subjected to both the tensile and compressive stresses. After the tests, the bent area was observed using a three-dimensional (3D) laser confocal microscope to examine the physical damage. The PNCs did not show any cracking or delamination even after 1000 cycles (Figure S7). The chemical structure of the PNCs was evaluated using Xray photoelectron spectroscopy (XPS) and Raman spectroscopy (Figure 3). Figure 3A shows the Raman spectra of the PNCs. The peak at ∼1100 nm−1 corresponded to boron nitride (BN).20 The PNCs exhibited XPS peaks associated with the N1s, B1s, and Si2p orbitals (Figure 3B−D). The nitrogen peak (Figure 3B) had two components corresponding to silicon nitride and boron nitride. The boron peak (Figure 3C) could also be deconvoluted into two components. The more intense peak corresponded to the B−N bond. The less intense peak at 190.2 eV corresponded to the B−Si bond. The silicon peak (Figure 3D) consisted of four components. The most intense peak at 101.6 eV corresponded to the Si−N

Coatings exhibited an average reflectivity of about 8% in the visible range (Figure S7). Since BN is transparent and SiNx is semitransparent, the transmittance of the PNCs also depended on the thickness of the individual layers. An increase in the BN content increased the transparency (Figure 2B) and reduced the hardness (Figure 1C) of the PNCs. A series of tribological tests were performed to evaluate the protective properties of the PNCs.19 A hard silicon nitride ball was slid against the bare and PNC-coated PET substrates. Parts C and B of Figure 2 show the wear tracks of the bare and PNC-coated PET substrates, respectively, after 1000 sliding cycles. In the case of the bare PET substrate, plastic deformation was observed along with the formation of deep scars (Figure 2C). Wear was not detected on the counter surface after the wear test and only a small number of wear particles were accumulated on it (Figure S9A). In the case of the PNC-coated PET substrate, no significant wear of the coating was observed. Moreover, the formation of a protrusion along the wear track was found in this case (Figures 2D and S8). The surface of the silicon nitride ball was found to be scratched along the sliding direction (Figure S9B). Thus, the formation of protrusion was attributed to the wear of the ball and the transfer of the ball material to the surface of PNC. D


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the nanoindenter. The indentation depth was maintained at 20% of the coating thickness to avoid the effect of the substrate on the hardness measurements. Substrate curvature was measured using a 3D stylus profilometer (Dektak, Bruker). Residual stress was calculated on the basis of the substrate curvature using the Stoney’s equation.21 The transmittance was measured using an ultraviolet−visible spectrophotometer (V-650; JASCO) over the visible light wavelength range (400−700 nm). The beam spot size was 100 μm, and the scan speed was 1000 nm/min. Optical properties were measured for PNCs coated on glass and having a thickness of ∼150 nm. Reciprocating sliding tests were performed using a tribotester made in-house. A normal load of 50 mN was applied to a 1.6 mm silicon nitride ball. The sliding stroke was 2 mm, and the number of sliding cycles was 1000. Each specimen was tested three times with a fresh ball. The bending test for PNCs coated on PET was performed using a custom-built bending tester (Figure S10). The coatings were bent to 180° for 1000 cycles at a rate of 0.5 Hz. The bending radius was set to 5 mm. The surface of the coatings after the bending tests and wear tracks was characterized using a high-resolution 3D laser microscope (Keyence VK-X210).

bond. The other less intense peaks corresponded to the Si−B, Si−O, and Si−Si bonds. Assuming the presence of all combination of bonds, it may be presumed that the periodical structure consisted of alternated SiNx and BN layers. The SiNx layers mostly consisted of silicon nitride with a small amount of silicon oxide and pure silicon. Besides, SiBn formed on the interface between SiNx and BN layers. Presence of pure silicon explained the reason for the degradation of the mechanical properties of coatings with the top Si-based layer mentioned above. As for silicon oxide, the source of oxygen could not be identified clearly. Since high-purity gases were used during the deposition, it could be assumed that oxygen entered the system from the surroundings during the deposition or analysis process. However, further investigation is required to confirm the source of oxygen. Since both the SiNx and BN layers contained nitrogen, the direct evaluation of the Si/N ratio was not possible. Thus, it was measured for a single SiNx coating deposited under the same conditions. It was found that the SiNx layer consisted of ∼55 at. % silicon, 42 at. % nitrogen, and 3 at. % oxygen. The measured Si:N ratio (55:42) was significantly larger than the stoichiometric ratio of silicon nitride (43:57). In summary, the SiNx/BN PNCs developed in this study showed excellent optical and mechanical properties. The use of magnetron sputtering allowed low-temperature deposition of the coatings, which is suitable for a wide range of polymeric materials. The periodical nanolayered combination of two different materials provided the coatings a hardness of up to 35 GPa and optical transmittance of >85% (on glass). The desired balance between the hardness and transparency could be achieved by varying the thickness of the individual layers or by adjusting the gas atmosphere during the deposition.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b22091. Comparison chart of protective coatings, images showing the schematic of the deposition system, TEM images of PNC, micro-diffraction for PNC, mechanical properties of PNCs, hardness of PNCs as a function of bias voltage, evaluation of residual stress in PNCs, reflectivity of PNCs, wear resistance of PNCs, wear of a counter surface, and the bending tester and surface images (PDF)

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MATERIALS AND METHODS

Coatings Deposition. The PNCs and single-layer coatings were deposited by reactive magnetron sputtering. A mixture of Ar and N2 gases was used. The coatings were deposited on glass and PET substrates. Si and BN targets with a diameter of 100 mm were used. The base pressure before the deposition was 2.5 × 10−4 Pa. The substrates were precleaned using a 30 W RF plasma for 15 min in an Ar atmosphere. A gas mixture with the desired N2:Ar ratio was prepared using high-precision flow controllers. The working pressure was measured by a Baratron gauge and was controlled by an automatic throttle valve. The working pressure during the deposition was 0.6 Pa. A negative pulse bias with a frequency of 10 kHz was applied between the substrate holder and the magnetron anodes during the deposition. The thickness of the individual layers in the coating was controlled by varying the exposure time. The deposition rates were about 0.11 and 0.025 for the SiNx and BN layers, respectively. The substrate temperature during the deposition was monitored using a thermo-resistive sensor attached to the substrate holder. Structure and Property Evaluation. The nanostructure of the PNCs was examined by a HRTEM (JEOL JEM-ARM200F). The cross sections for the HRTEM analysis were prepared using a focused ion beam (FIB, JEOL JIB-4601F). The chemical structure of the PNCs was assessed by Raman spectroscopy (Thermo Scientific DXR2) and XPS (Thermo Scientific Mono). Nanoindentation was performed using a high-precision hardness tester (Anton Paar, UNHT).17,18 A Berkovich pyramid indenter having a rounded end with a diameter of 20 nm was used. Each indentation test was carried out at 30 points, and the results were treated statistically. Before the indentation tests, the device was calibrated using a standard reference sample (fused silica). The indentation hardness of the coatings was calculated using the OliverPharr method. These measurements were carried out automatically by

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Dae-Eun Kim: 0000-0002-6095-5138 Author Contributions

The manuscript was written through the contributions of all authors. All the authors have given approval to the final version of the manuscript. All the authors contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2010-0018289) and the Brain Korea 21 Plus Project in 2019.

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ABBREVIATIONS PNC, periodical nanocomposite coating; HRTEM, highresolution transmission electron microscopy; XPS, X-ray photoelectron spectroscopy; HRTEM, high-resolution transmission electron microscopy

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REFERENCES

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