Recently, the internationally renowned journal ACS Applied Materials & Interfaces in the field of materials science published the latest research results from our institute, which improved the reactive magnetron sputtering and coating deposition process using pulsed DC power supply, successfully preparing SiNx/BN periodic nano-layered coatings (PNCs) with high hardness, high transparency, and good flexibility. The first author of the article is Wu Pengyuan, a ZJUI 2022 doctoral student, and the corresponding author is Assoc Prof. Oleksiy Penkov of ZJUI. Other authors include Yu Tianxiang, a ZJUI 2022 master's student, Wang Yusi, a 2021 doctoral student from the School of Optical Science and Engineering, Zhejiang University, Shen Boyang, a ZJUI 2021 doctoral student, Paul C. Uzoma, a postdoctoral researcher at ZJUI, and Prof. Shen Weidong from the School of Optical Science and Engineering, Zhejiang University. 

Building on this research, Wu Pengyuan participated in the 6th International Conference on Applied Surface Science (ICASS 2024) and the Chinese Materials Conference 2022-2023, where he was honored with the Best Academic Presentation Award. "The excellent research platforms and relaxed, enjoyable atmosphere of ZJUI and Oleksiy Penkov’s research group have greatly influenced me, allowing me to focus on learning and research," Wu Pengyuan shared with us. 

Research Background

Durable transparent protective coatings are crucial for the development and application of new electronic devices. Only coatings that combine glass-like hardness, polymer-like flexibility, and high transparency can meet the demands of wearable and flexible electronic products. However, enhancing the hardness, flexibility, and transparency of coatings remains a challenge. Traditional inorganic coatings, while offering high hardness and wear resistance, as well as maintaining good optical properties, are typically more brittle. Organic coatings, on the other hand, excel in flexibility and feature self-cleaning, antibacterial, and high light-transmitting properties, but they tend to have lower hardness. These challenges highlight the urgent need for innovative solutions in the field of protective coatings. 

Research Highlights

The use of magnetron sputtering technology to fabricate periodic nano-coatings (PNCs) effectively combines the properties of different materials, allowing for precise control over coating performance. The research team led by Prof. Oleksiy Penkov, utilizing a self-built advanced magnetron sputtering system, investigated the effects of sputtering atmosphere and pulse DC power parameters on the composition and performance of SiNx coatings, ultimately determining the optimal deposition conditions for SiNx. Building upon this, the study further refined the modulation periods and thickness ratio of SiNx and BN, resulting in a SiNx/BN periodic nano-layered coating with good mechanical and optical properties. 

▲ An advanced magnetron sputtering system independently developed by Prof. Oleksiy Penkov's research group 

By employing pulse DC power supplies, the adverse effects of target poisoning and arc phenomena during reactive magnetron sputtering can be mitigated, stabilizing the reaction process and promoting thorough reactions between target atoms and the working gas. Moreover, there is currently limited research on how the operating parameters of pulse DC power—specifically pulse frequency (F) and duty cycle (D)—affect SiNx-based coatings. This study systematically addresses this issue, revealing that pulse parameters significantly influence the composition, hardness, and elastic modulus of SiNx coatings. When a continuous (non-pulsed) power supply is used, substantial amounts of elemental silicon and SiO₂ are present in the coatings, which severely impair their mechanical and optical properties. As the pulse frequency increases, the impurity content within the coatings gradually decreases. At a frequency of 40 kHz and a duty cycle of 32%, the coatings exhibit almost no elemental silicon or oxide impurities. At this point, the coatings demonstrate optimal mechanical properties and high optical performance, achieving a hardness of 25 GPa and optical absorption rates of 0.80% in the visible region and 0.32% in the near-infrared region. 

(A) Mechanical properties of SiNx coatings at different pulse duty cycles when the pulse frequency is 40 kHz. 

(B) Mechanical properties of SiNx coatings at different pulse frequencies when the duty cycle is 32%. 

(C) and (D) Transmittance and optical absorption of the coatings under different pulsing conditions. 

After determining the optimal parameters for preparing SiNx coatings, the research team combined SiNx with BN to fabricate SiNx/BN periodic nano-layered coatings. They explored how the modulation periods and the thickness ratios of each layer affect the overall performance of the coatings. The results indicate that when the SiNx layer has a moderate thickness (approximately 2.4 nm) and the BN layer is relatively thin (less than 0.8 nm), the strengthening effect of the periodic nano-layered coating is most pronounced. Under these conditions, the coating achieves a hardness of up to 32.4 GPa, an elastic modulus of 212.7 GPa, and a hardness-to-elastic modulus ratio (H/E) of 0.153. Moreover, the coatings exhibit good wear resistance and bending durability. Wear tests show that the wear rate of the coatings is only 3.05×10⁻⁹ mm³·N⁻¹·mm⁻¹. When the coatings are deposited on PET surfaces and subjected to repeated rubbing 300 times under a load of 0.5 N using a steel brush, no obvious scratches appear on the coating surface; after 1,000 cycles of inward bending, the coatings show no signs of cracking or peeling. Additionally, the average optical absorption of the coatings in the visible and near-infrared regions is only 0.4%. By adjusting the thickness ratios of each layer, the refractive index of the coatings can be tuned within a certain range (1.72–1.92 at 600 nm). 

(A) Indentation curves of SiNx/BN periodic nano-layered coatings (PNC), Corning Gorilla Glass, PET with PNC deposition, and bare PET. 

(B) Surface wear conditions of PET and PNC-deposited PET after 300 cycles of steel wool abrasion. 

(C) Structural design diagram of the PNC. 

(D) Transmittance and optical constants curves of the PNC. 

(E) PNC-deposited PET still exhibits good flexibility. 

Conclusion and Outlook

The unique mechanical and optical properties of the SiNx/BN periodic nano-layered coatings (PNCs) make them highly promising for applications in flexible electronic devices, wearable technology, and vehicle-mounted autonomous driving radars. Additionally, their high hardness, good flexibility, and tunable refractive index suggest that these coatings have potential applications in enhancing anti-reflective optical films, self-cleaning coatings, and related fields. 

Article Link：https://doi.org/10.1021/acsami.4c10220