During EDM drilling process for TBCs with AEM, the phase transformation of 8YSZ and the generation of the conductive layer occur simultaneously. the continuous and stable generation of the conductive layer is the key to realize high-quality EDM machining for TBCs without cracks and delamination. Hence, the formation process of conductive layer with the phase transition of TBCs is analyzed in this section.
The discharge process of EDM can be divided into 4 stages: (a) the ionization and breakdown of dielectric liquid in discharge gap, and the formation of the plasma channel; (b) the pyrolysis of dielectric liquid, the thermal expansion of melted and vaporized electrode materials; (c) the throwing out of the electrode materials; (d) eionization of dielectric liquid between electrodes. Based on the above analysis results and the 4 stages of the discharge process, 8 stages of the conductive layer forming process on the TBCs surface are proposed which accompanied with the materials phase transition.
The schematic diagram of forming process of conductive layer is shown in Fig. 11. Figure 11(a) shows the preparation stage of the conductive layer during the formation of the plasma channel. Figure 11(b) illustrates the 3 stages of the generation of the conductive layer during the pyrolysis of dielectric liquid, the thermal expansion of melted and vaporized electrode materials. The 3 stages include: (1) The phase transition of 8YSZ at high temperature; (2) 8YSZ conducts electricity due to high temperature; (3) Brass and 8YSZ melting and vaporizing, and working oil undergoes pyrolysis. The analysis of 3 stages of the conductive layer formation is as follows:
(1)The phase transition of 8YSZ at high temperature When the temperature in the discharge crater is greater than 873K, 8YSZ in the crater starts to change from the monoclinic phase to the cubic phase then to the liquid phase, as the temperature continues to increase [42]. The temperature in the heat-affected zone cannot reach the melting point of 8YSZ, thus, 8YSZ in the heat-affected zone starts to change from the monoclinic phase or the tetragonal phase to the cubic phase.
(2)8YSZ conducts electricity due to high temperature When the temperature exceeds 1023K, 8YSZ can obtain higher conductivity [43]. The conductivity of 8YSZ increases with the temperature continues to increase. The temperature in the discharge crater and the heat-affected zone is extremely high, and the temperature is greater than 1023K in most areas due to the high-temperature heat generated by the plasma channel during the discharge process. Meanwhile, part of 8YSZ in the heat-affected zone transforms from the tetragonal phase to the cubic phase with fluorite structure. A mount of oxygen ions in 8YSZ can migrate directionally through oxygen vacancies, the conductivity of 8YSZ is improved significantly, and 8YSZ in the crater and the heat-affected zone change from the insulating status to the conductive status.
(3)Brass and 8YSZ melting and vaporizing, and working oil undergoes pyrolysis The instantaneous high-temperature heat generated by the plasma channel makes the working oil pyrolyze into pyrolytic carbon, brass and 8YSZ melt and boil until vaporize.
Fig. 11(c) shows the ejection stage of electrode materials during the formation of the conductive layer. It includes 4 stages: (4)The sputtering of melting and vaporization of brass and 8YSZ; (5)The deposition and adsorption stage of brass, 8YSZ and pyrolytic carbon; (6) Carbon penetrates into stable high-temperature phases of 8YSZ; (7)The re-deposition of the sputtered materials from discharge crater. The analysis of the stages about the generation of the conductive layer is as follows:
(4)The sputtering of melting and vaporization of brass and 8YSZ Due to the instantaneous high temperature, the working oil is vaporized and the electrode materials is melted or vaporized. The volume of materials expand outward due to vaporization, forming expanded bubbles. Because of the unequal pressure from inside and outside of expanding bubbles, some brass materials in the discharge crater of anode are melted and vaporized, and then they are sputtered, while some 8YSZ in the crater of cathode also be melted, vaporized and sputtered.
(5)The deposition and adsorption of the brass, 8YSZ and pyrolytic carbon Fig. 12 shows schematic diagram of deposition of carbon and electrodes onto 8YSZ coating. The discharge plasma channel is composed of positively charged particles, negatively charged particles and neutral particles. The positively charged particles are Zr4+ decomposed from 8YSZ, Cu2+ and Zn2+ decomposed from brass, and positively charged Carbon ions. The negatively charged particles include electrons and negatively charged Carbon ions and O− 2. Neutral particles include vaporized 8YSZ, vaporized brass and pyrolytic carbon. The deposition processes of brass materials include physical deposition of molten and vaporized brass onto crater, sputtering deposition of molten and vaporized brass particles from the crater. The deposited materials exist in the form of Cu, Zn, ZnO and CuZn. The 8YSZ deposition includes physical deposition of 8YSZ onto crater, and sputtering deposition of molten and vaporized 8YSZ from discharge crater. The carbon deposition exists in the form of the pyrolytic carbon deposition, sputtering deposition of pyrolytic carbon, and the carbon micelle adsorption. The pyrolytic carbon deposition occures from the pyrolysis of kerosene at the crater and its vicinity. The sputtering deposition of products containing pyrolytic carbon are the sputtering deposition of carbon adhered to the molten brass, the vaporized brass, the molten 8YSZ and the vaporized 8YSZ inside and outside the plasma channel. Some pyrolytic carbon undergo the reduction reaction with ZrO2 at crater and its vicinity to generate ZrC, and then the generated products occur chemical adsorption.
(6) Carbon penetrates into stable high-temperature phases of 8YSZWhen the temperature in the plasma channel exceeds 1200K, carbon seeps into the cubic phase 8YSZ and the tetragonal phase 8YSZ transformed at high temperature, which inhibits the transformation of high temperature phase to monoclinic phase after the discharge.
(7)The re-deposition of the sputtered materials from discharge crater.
After a single-pulse discharge, the materials at the crater and its adjacent area are sputtered again owing to the bursting of bubbles around the pulse channel. The molten and vaporization brass at the crater of TE adheres to the carbon micelle and deposits on the machined surface of the workpiece. Meanwhile, some molten and vaporization 8YSZ at the crater of workpiece and its adjacent area adheres to the carbon micelle, sputters and deposits on the discharge spot on the surface of 8YSZ, and a small part of 8YSZ sputters and deposits on the TE surface. Other vaporization and molten ejected brass and 8YSZ are washed out from the discharge gap because of the high-speed flow field.
Fig. 11(d) shows the end stage of the conductive layer formation after the deionization of the dielectric liquid. The analysises are as follows:
(8)The end stage of the conductive layer formation After the discharge of 8YSZ, 8YSZ transforms from the liquid phase to the cubic phase or the tetragonal phase under the discharge spot, and the phases are maintained until the temperature drops to room temperature, then phase transformation of materials is complete. Meanwhile, the discharge crater and heat-affected zone are cooled rapidly due to the high-speed flow field in the discharge gap, and 8YSZ in this area rapidly recovers from the conductive status to the insulating status. Finally, C, Cu, Zn, ZrC, ZnO, CuZn and ZrO2 deposit on the surface and form a new conductive layer. Thus, the formation of the conductive layer is the result of the interaction or simultaneous action of various stages.
When the BC and the superalloy substrate are machined, the element composition of the conductive layer on the hole wall of 8YSZ coating changes significantly. The micro morphology and EDS analysis for the conductive layer on the hole wall machined by copper tube electrode with external and internal diameters of 0.9 mm and 0.3 mm respectively (the process parameters: peak current of 6 A, pulse duration of 8 μm, duty cycle of 0.5, and flushing pressure of 5 MPa) are carried out using SEM (Merlin Compact, Carl Zeiss AG, Germany). As shown in Fig. 13, the area of spectrum 1 is located on the inner wall of the hole, the spectrum 2 is located in the brittle fracture zone. Spectrum 1 and spectrum 2 show that the main elements on the surface are C, Ni, Cu, and Zr, at either the fracture area or the inner wall of the hole. Obviously, in the EDM process of metal, the debris produced from the wear of superalloy and TE is sputtered and adsorbed on the conductive layer. Furthermore, a secondary discharge occurs between the conductive layer on the hole wall of the ceramic coating and the tapered electrode surface formed due to the TE wear, which will also causes the debris to be sputtered and deposited on the surface of the conductive layer.