The importance of gradual transition is to minimize the residual stress when joining two dissimilar materials, such as metal/metal, metal/ceramic and etc. By applying functional gradient materials (FGM) for joining, interfacial coherence can be improved by continuous gradation in composition rather than having a sharp discontinuity. One technique to fabricate FGM is electrophoretic deposition (EPD) which allows forming a continuous compositional change by controlling the powder composition in the suspension from which particles are deposited on one of the electrodes. It involves a relatively high speed coating and sintering to achieve fully dense material.
In this study, two types of EPD which are continuous direct current (DC) EPD and pulsed DC EPD have been used to coat layers of Ni-Al2O3 FGM. Before deposition, suspension stability and optimal pH range for deposition were studied by measuring zeta potential and electrophoretic mobility.
To resolve problems involving differences in densities between Ni and Al2O3, NiO and Al2O3 powders were used to be electrophoretically deposited due to its high surface charge and relatively low density. Compositions of NiO and Al2O3 suspension were varied in gradients based on volume ratio using ethanol as a solvent.
Moreover, deposition mass with varying voltage and time was characterized by continuous DC EPD. After deposition, pressureless sintering was used to reduce NiO in Ar/H2 atmosphere for Ni-Al2O3 FGM. However, deposition using EPD with continuous DC voltage, has some problems such as controlling thickness, non-uniform deposits, deposits with low green density and bubble formation through electrolysis of solvent resulting in roughness.
To overcome these problems, pulsed DC EPD system was adopted. Moreover, the effect of an intermediate step during the sintering process and the number of layers were investigated to minimize crack problems.
And, pulsed EPD system was used to improve coating condition of each layer based on the square-wave pulse potential of 50% duty cycle. As a result, FGM layers from Ni to Al2O3 were obtained with smooth gradient of deposition. Fabricated Ni- Al2O3 FGM layer was characterized by various analysis techniques. X-ray diffraction (XRD) verified successful reduction of NiO into Ni. Moreover, these samples were characterized using back scattered electron microscopy (BSE) and its compositional gradients were verified using electron probe micro analyzer (EPMA). The mechanical properties of Ni-Al2O3 FGM coating hardness via indentation methods, tribology test and residual stresses with diffraction method were evaluated by measuring. Finally, Ni-Al2O3 FGM coating layers with a thickness of 20 ? 30㎛ were successfully fabricated by using pulsed DC EPD with no observable cracks. Moreover, controlling its sintering conditions through an intermediate step and varying the numbers of layers played important roles in reducing crack and thickness of the coating layer.