Development of Ni-based lubricating moulds for defects-free polymeric micro/nano fabrication

Electroformed nickel (Ni) moulds are widely used in micro hot embossing/injection moulding and nanoimprinting for the production of polymeric parts. However, Ni moulds still have issues such as low hardness, high surface energy, low wear resistance against polymer materials, and short lifespan. Therefore, the aim of this project is to develop a Ni-based lubricating mould by incorporating lubricating nanoparticles into the Ni matrix via one-pot electroforming. The fundamental aspects of co-deposition, including the dispersion of lubricating nanoparticles, the co-deposition of nanoparticles with Ni into micro/nano structures, and the resulting mechanical and tribological performance of Ni nanocomposite moulds, are comprehensively investigated. This thesis starts with a literature review in Chapter 1 to discuss the recent research findings of the Ni-based nanocomposites incorporated with lubricating nanoparticles. In Chapter 2, Ni-WS2 nanocomposite mould materials were developed with greatly improved hardness and tribological performance. This was achieved through the assistance of surfactant mixtures to avoid particle aggregation. In Chapter 3, cobalt (Co) was added as a second phase to enhance the hardness and wear resistance of the Ni-Co alloy composite moulds, with the incorporation of WS2/MoS2 nanoparticles and PTFE microparticles. The surface properties and tribological performance of resulting alloy composite moulds were thoroughly investigated. In Chapter 4, intermittent ultrasonication was introduced to assist the long-term dispersion of the WS2/MoS2 nanosheets within the electrolyte solutions. The effects of ultrasonic power/treatment time and surfactant concentration on the surface properties of the nanosheets (surface charge, zeta potential and polydispersity index) were studied. Four-inch Ni-based nanocomposite moulds were developed for the first time under the assistance of intermittent ultrasonication and surfactant mixtures. In Chapter 5, PTFE micro/nanoparticles were introduced into the Ni matrix to investigate the effect of particle size on the properties of the resulting composite moulds. PTFE nanoparticles were found to alter the Ni mould surface from hydrophilic to hydrophobic without roughening the mould surface. In Chapter 6, a 4-inch micro structured Ni-PTFE nanocomposite mould was developed for micro injection moulding. Its surface energy dynamics before and after over 100-cycle injection moulding, tribological performance including friction, wear resistance and adhesion, were explored. The biocompatibility of using such mould was studied. The PMMA and COC chips produced from this Ni-PTFE mould were detected regarding the surface morphology and structural integrity. In Chapter 7, PTFE nanoparticles were co-deposited with Ni atoms into anodic aluminium oxide (AAO) templates to form nano structured Ni-PTFE mould. This mould was used for thermal/UV nanoimprinting to validate its lubrication performance in demoulding intricate nano structures. In Chapter 8, the surface wettability and tribological performance of Ni-WS2 mould and Ni-PTFE mould were compared by measuring the polymer melt contact angle (CA) and performing pin-on-disk tests using various polymer pins. Ni-PTFE showed higher CA across all polymer melts and maintained its wear resistance against polymers under higher applied loads, while Ni-WS2 lost its lubrication properties in this situation. In Chapter 9, the overall conclusions and future perspectives were stated. Overall, this project offers valuable insights into the dispersion mechanism of lubricating nanoparticles within the electrolyte solutions and their subsequent co-deposition mechanism into the composite mould. Meanwhile, this project reveals the lubrication mechanism of such nanocomposite moulds for the first time. Understanding this lubrication mechanism holds significant promise for the development of nanocomposite moulds towards defect-free polymeric micro/nano fabrication.