High-temperature softening of nickel-based carbon nanotube composite coatings for the fabrication of nickel-based nanoimprint molds by thermal imprinting

Abstract

The development of nickel mold materials that softened at high temperatures was investigated, and their use in the fabrication of replica nickel molds by thermal nanoimprinting was demonstrated. Ni-based carbon nanotube (CNT) composite coatings were formed by ultrasonic assisted electroplating using a horn sonicator. 1.6 g/L CNTs, which were 9.5 nm in diameter and 1.5 μm in average length, were added to a nickel sulfamate plating bath. The Vickers hardness of the Ni-based CNT composite coatings was over 500 HV at room temperature and under 50 HV at high temperatures over the range of 400 to 600 °C. After heat treatment at 500 °C, the grain size of the Ni-based CNT coatings became larger than that of normal Ni coatings. Ni-based CNT composite coatings had random crystals, instead of the columnar crystals in pure nickel coatings. It is thought that the softening of the Ni-based CNT composite was based on containing CNTs in grain boundaries, as well as a crystal structure transformation. To fabricate the Ni-based replica mold, a Ni-based CNT composite coating was directly imprinted at 3 MPa and 500 °C; higher than the softening temperature of the coatings. 5 μm square dot patterns from a mother silicon mold was successfully replicated by thermal nanoimprinting on the Ni-based CNT composite coating. The fabrication method of replica nickel molds was based on the high-temperature softening property of Ni-based CNT composite coatings.