Investigation On The Mechanism And Methods Of Improving Interface Adhesion Between Micro Electroforming Layer And Metal Substrate

With the development of MEMS, metal micro devices attract more and more attention. There are methods to fabricate the metal micro devices, such as electrical discharge machining, laser processing and micro electroforming technology. Meanwhile, due to the advantages of high replication accuracy and mass production, the micro electroforming technology is more suitable to fabricate the metal micro devices. However, the metal micro devices fabricating by the micro electroforming technology are limited by low interface adhesion performance between micro electroforming layer and metal substrate. Due to the low interface adhesion performance, serious defects of the micro electroforming layer including desquamation and fracture may emerge. It severely restricts the application and development of the micro electroforming technology. This research is of great significance to improve the interface adhesion performance between the micro electroforming layer and the metal substrate.In order to investigate the quantitative relationship between crystallite size of the micro electroforming layer and interface adhesion energy, based on the energy balance criterion of fracture mechanics, surface stress model and Miedema model of experimental electron theory, the equation of the quantitative relationship between the crystallite size and the interface adhesion energy is established originally. Concerning this equation, the results show that the interface adhesion energy keeps an increasing trend and approaches to the stable value as the crystallite size increases. In order to prove the equation, micro electroforming experiments under different current densities were performed. The crystallite size of the micro electroforming layer was tested by X-ray Diffraction (XRD) method. The interface adhesion energy was characterized by scratch test. The experimental results show that, within the range of crystallite size, the interface adhesion energy keeps an increasing trend and approaches to the stable value. The theoretical and the experimental results show the same trend. The equation is verified by the experiments. In order to increase the crystallite size and to prove the interface adhesion energy, the equation provides theoretical foundation to select the experimental parameters including small current density and ultrasonic electroforming.The effect of current density on the interface adhesion energy is investigated. Based on the equation of the quantitative relationship between the crystallite size and the interface adhesion energy, the electroforming experiments was applied under different current densities. The crystallite size, the compressive stress and the real internal contact surface area of the micro electroforming layer were measured. The interface adhesion energy was characterized by the scratch test. The mechanisms of the small current density method improving the interface adhesion energy are investigated with the partial discharge theory and the cathodic polarization theory. On the one hand, the small current density improves the interface adhesion energy by increasing the crystallite size. On the other hand, the small current density increases the internal real contact surface area of the micro electroforming layer. This enhances the mechanical interlocking strength and consequently increases the interface adhesion energy. The small current density method can improve the interface adhesion energy.The effect of ultrasonic electroforming method on the interface adhesion energy is explored. To explore the effect of the ultrasonic electroforming method on the interface adhesion energy, electroforming experiments were carried out under ultrasonic agitation. The effects of the ultrasonic agitation on the electroforming process were investigated by polarization and alternating current impedance methods. The experimental results show that the ultrasonic electroforming method increases the crystallite size and the real internal contact surface area, and reduces the compressive stress. The interface adhesion energy keeps increasing with the increased ultrasonic power and gets the highest value at 200W. Then the interface adhesion energy decreases. The adhesion energy gets the higher value at 40kHz and keeps decreasing with the increased ultrasonic frequency. Based on the equation of the quantitative relationship between the crystallite size and the interface adhesion energy, mechanisms of the ultrasonic electroforming method improving the interface adhesion energy are explored. The mechanisms are as follows:due to the depolarization effect, the ultrasonic electroforming method increases the crystallite size and consequently improves the interface adhesion energy. Besides, the ultrasonic electroforming method enlarges the internal real contact surface area of the micro electroforming layer and enhances the interface adhesion energy. An optimum of improving the interface adhesion energy can be observed when the ultrasound parameters are 200W and 40kHz.This dissertation presents a new ultrasonic composite electrofroming method to improve the interface adhesion energy. Firstly, the effect of chemical etching method on the interface adhesion energy is explored. The chemical etching electroforming experiments were carried out. The experimental results show that the chemical etching method can enlarge the substrate surface roughness Ra and increase the real contact surface area of the micro electroforming layer. This enhances the mechanical interlocking strength and consequently increases the interface adhesion energy. Based on the chemical etching method, small current density method and ultrasonic electroforming method, the ultrasonic composite electroforming method improving the interface adhesion energy according to the equation of the quantitative relationship between the crystallite size and the interface adhesion energy is proposed and verified by experiments. The experimental results show that, comparing with the ordinary one, the ultrasonic composite electroforming method can enlarge the interface adhesion energy by 134%. In order to prove the effectiveness of the ultrasonic composite electrofroming method, metal micro pillar arrays structures were fabricated. The experimental results show that the ultrasonic composite electroforming method can improve the interface adhesion performance of the metal micro pillar arrays structures. This work contributes to enhancing the interface adhesion performance of the metal micro devices.