Influence of electrodeposited crystallite size on interfacial adhesion strength of electroformed layers

Microelectroforming is an important method for fabricating metal devices in microelectromechanical systems. However, the metal devices are limited by poor interfacial adhesion strength between the electroformed coating and the substrate. To improve the adhesion strength, the authors´ research group presented a small current density deposition method. The purpose of this Letter is to investigate the mechanism of this method regarding the effect of the electrodeposited crystallite size. The mechanism is that a small current density can cause a big crystallite size which reduces the compressive stress. Therefore the adhesion strength is large under a small current density. To investigate this mechanism, different current densities were applied during the microelectroforming experiments. The crystallite size of the electroformed coating was measured by the X-ray diffraction method, and the adhesion strength was calculated by a scratch test. The experimental results indicate that within the range of the current density from 0.2 to 2 A/dm², the crystallite size decreases from 30.1 to 23.7 nm and the adhesion strength decreases from 14.89 to 8.12 J/m², respectively. The adhesion strength maintains a decreasing trend along with the decreasing crystallite size. Considering the effect of the crystallite size, a new mechanism of the small current density deposition method is presented in this Letter. This Letter may contribute to improving the adhesion strength of the electrodeposited layer.