In this dissertation, an electrical discharge coating uses an isolated sleeve to form a closed space between the end surface of electrode and the work to deposit a thin film onto a substrate. The discharge occurs at the location where the two surfaces are closest and the dielectric fluid ionizes at this location to create a path for the discharge. Hence, this closed space is heated to extremely high temperature, so that a small portion of the work surface is suddenly melted with the particles in the dielectric fluid and then coated to increase its coating speed and quality. The electrode material is made of brass, the work material SKD11, and the dielectric fluid is kerosene with the WC powder concentration of 50g/L. The pulse-on and pulse-off times are 25 and 500£gs, respectively. The effects of supply voltage, electrical discharge coating time, electrical discharge gap, and powder added cycle on the coating characteristics are investigated. According to the experimental results, the electrical discharge with isolated sleeve can achieve a complete coating layer onto the surface of work. The coating thickness increases with increasing electrical discharge coating time and gap as the supply voltage is larger than the threshold voltage of electrical discharge. At the supply voltage below 250V and the added powder cycle less than 10, the coating thickness increases with increasing supply voltage and cycle. The quality of coating layer is better at the low gap distance and the high supply voltage. The hardness of coating layer is about HV 1687 which is approximately 5 times of substrate hardness using micro-hardness test. The electrical discharge without isolated sleeve cannot achieve the above-mentioned advantages.

¡@¡@An electrical discharge coating method, which an isolated sleeve is used to form a closed space between the end surface of electrode and the workpiece so that the particles, the ions, and the pressure during the discharge process are concentrated on this space, is employed to increase the coating speed and the quality of the coating. The electrode for the cathode is made of brass, and the workpiece for the anode SKD11. They are immersed in kerosene containing W/C powder with the concentration of 50g/L. The time for the pulse-on is 25£gs and that for the pulse-off times 500£gs. The growing thickness and the quality of coating layer are investigated under the supply voltage of 50-400V, discharge coating time of 34-284s, and the gap distance of 50-300£gm. ¡@¡@Experimental results show that when the supply voltage is larger than 100V, the gap distance less than 150£gm and discharge coating time 142s, the area covered by the coating can achieve more than 90 percent of the total surface area. Moreover, the coating thickness increases with increasing gap distance and discharge coating time, but as the gap distance is larger than 150£gm, the coating thickness decreases with increasing gap distance. ¡@¡@For a special case, under the supply voltage of 200V, the gap distance of 100£gm and the discharge coating time of 142s, the coating thickness can achieve about 17£gm with little pore in the coating layer. The hardness of the coating layer can almost achieve the level of W/C hardness.

This paper aims to create a hard modification layer of WC/Co/Fe on the surface of SKD11 work steel by using a new closed-type method of surface electrical discharge coating with the self-made tool electrode and CNC electrical discharge machine. The tool electrode is the composition of a semi-sintered powder compacted electrode and a cooper rod. The sintered powder compacted electrode making process is first to mix the WC/Co and Fe powders uniformly at 8:1, 4:1, 2:1 and 1:1 in weight ratio. Continually, it will form the cylinder of 8mm in diameter and 18mm in length approximately by compacting in different pressure (50~200 MPa) and sintering temperature (300~900 ¢J). The EDM condition is 1~12A discharge current, 25~500£gs pulse time, and 50% duty factor. The electrical discharge machining is proceeding in kerosene with tool electrode as cathode and workpiece as anode. The result is as following. It can be concluded that the best condition to fabricate the sintered powder compacted electrode is 1:1 in weight ratio between WC/Co and Fe powders; with 200Mpa compacting pressure and sintering temperature at 900¢J, which results in lowest electrical resistivity. Under such condition, the area covered ratio can reach 100% at best EDM condition, which is pulse time £non = 50 £gs, rest time £noff = 50 £gs, 8A in current and 1.5 min in machining time. The surface hardness of workpiece increases with machining time. The surface hardness dramatically increases to Hv1500 as machining time over 1.5 min. The hardness of modification layer equals to the WC/Co particle itself which brings to the best wear ability. Moreover, the hardness of under surface in between 48 £gm is much higher than it of the SKD11 work steel. The hardness in between 30 £gm can reach up to Hv1200 in particular. The hardness of surface modification layer increases linearly with machining time. However, the limit of surface modification layer is about 30 £gm, and the needed time is below 5 min.