Energy action model of spark assisted chemical engraving (SACE) for improving surface quality of micro cavities in ZrO₂ ceramics

SACE achieves the removal of workpiece material by the physical and chemical process under the action of spark discharges. The discharging process is shown in figure 1. A tool electrode, a gas film, and an electrolyte film are the necessary components for generating spark discharges. Under the action of a processing voltage, the surfaces of the tool electrode and the auxiliary electrode undergo electrolysis to generate bubbles. As the bubbles are continuously generated and merged (figure 1(a)), a gas film is formed on the surface of the tool electrode, which separates the tool electrode from the electrolyte. Then, the negative charges move toward the surface of the tool electrode, and positive charges move toward the surface of the electrolyte film (figure 1(b)). When the charged voltage reaches a certain value, spark discharges occur, thus creating a high temperature and high pressure environment within the electrolyte (figure 1(c)). In the SACE process, the spark energy determines the high temperature and high pressure of the active region, which has an important influence on the processing effect. Additionally, in order to ensure that the high temperature and high pressure can act on the workpiece surface to achieve material removal, the workpiece must be enough close to the discharging position to reduce the loss of spark energy in the electrolyte. Thus, the transfer process of the spark energy was studied in terms of its effects on physical and chemical removal in the SACE process.

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The high temperature and the high pressure from spark discharges on the workpiece surface lead to the physical and chemical removal of the ZrO₂ material [10]. The spark energy transferal process is shown in figure 2. A spark with an initial energy Q_init passes through the gas film and the electrolyte film before it acts on the workpiece surface. The extremely high temperature of the spark heats and vaporizes part of the electrolyte as the loss of some energy Q_loss, which is positively correlated with the thickness h of the electrolyte film. The thickness h and the initial energy Q_init determine the spark energy Q that ultimately acts on the workpiece surface. This energy Q directly affects the effect of the physical and chemical removal of the workpiece material. Besides, considering that the spark also loses some energy in the form of light, radiation, etc, the energy Q can be expressed as:

$$\begin{equation}Q = {Q_{Init}} - {Q_{loss}}\left( h \right) - Q_{loss}^\prime{\rm{ }}\end{equation} \tag{ 1 }$$

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where Q is the spark energy that ultimately acts on the workpiece surface, Q_init is the initial spark initial energy, Q_loss(h) is the loss energy when the spark passes through the electrolyte, and $Q_{loss}^\prime$ is the loss energy from other forms.

In SACE, the electrical power of sparks can be derived from the voltage and current as follows:

$$\begin{equation}{Q_{init}} = \mathop \smallint \limits_0^{{t_{spark}}} U\left( t \right) \cdot I\left( t \right)dt\,\end{equation} \tag{ 2 }$$

where Q_init is the initial spark energy, t_spark is the duration of a single spark, I(t) is the discharge current, and U(t) is the discharge voltage. Obviously, the spark duration t_spark is an important factor affecting the initial spark energy Q_init. The value of t_spark is affected by the deionization of the discharging channel. If the discharging channel could have sufficient time to deionize (t_pulse-off≥ t_deionization), a continuous discharge could not occur, in which the pulse interval t_pulse-off represents the shortest interval between adjacent discharges, and t_deionization is the minimum time of the deionization. If the discharging channel could not have sufficient time to deionize (t_pulse-off < t_deionization), a continuous discharge even into an arc discharge with high brightness might occur, resulting in an increase of the spark duration t_spark which could generate excessive heat to damage the workpiece surface. Therefore, to avoid the occurrence of continuous discharge, it is necessary to constrain the spark energy to ensure that t_pulse-off≥ t_deionization.

The spark energy Q is positively correlated with the local temperature T_zone of the workpiece surface caused by a spark. According to equations (1) and (2), the relationship between T_zone and the processing parameters can be obtained as:

$$\begin{equation}{T_{zone}} \propto \mathop \smallint \limits_0^{{t_{spark}}} U\left( t \right) \cdot I\left( t \right)dt - \,{Q_{loss}}\left( {{d_{gap}} - {h_{gas\,film}}} \right)\,\end{equation} \tag{ 4 }$$

where d_gap is the machining gap between the tool electrode and the workpiece surface, h_{gas film} is the gas film thickness, and d_gap—h_{gas film} is equal to the thickness h of the electrolyte film. T_zone is the local temperature of the workpiece surface caused by a spark. t_spark is the spark duration affected by t_deionization and t_pulse-off. Equation (3) shows that T_zone can be adjusted by changing the processing voltage, pulse interval, and machining gap. If T_zone could not satisfy the condition of high temperature, workpiece material removal would not occur in the SACE process. This means that there is a critical temperature T_c of the material removal. In this study, the critical temperature of chemical removal is expressed by T_ccr, and the corresponding critical spark energy is expressed by Q_ccr. Similarly, the critical temperature of physical removal is expressed by T_cpr, and the corresponding critical spark energy is expressed by Q_cpr.

The key solution of damage-free process is to achieve the energy leading to the dominantly chemical removal on machined surfaces by adjusting the processing parameters. Physical removal can occur only if Q> Q_cpr. Chemical removal can occur only if Q> Q_ccr. If Q_cpr> Q_ccr and Q_cpr> Q > Q_ccr can be realized by adjusting Q_loss(h) to control Q according to equation (1), the damage-free process can be realized due to the lack of physical removal. For determining the feasibility of chemical removal dominated damage-free process, it is necessary to compare Q_cpr and Q_ccr experimentally.

Figure 3 shows the experimental system consisting of a tool electrode, an auxiliary electrode, a ZrO₂ workpiece, a liquidometer, a force sensor, a tilt table, a Z-axis spindle, an XY-axis worktable, and a CNC system. The auxiliary electrode uses a graphite block with dimensions of 40 × 40 × 5 mm to increase resistance to electrolytic corrosion. The liquidometer was used to read the level of the electrolyte with a resolution of 0.1 mm. The force sensor was used to measure the force on the workpiece surface with a resolution of 0.5 g. The tilt table was used to adjust the workpiece tilt for leveling the workpiece surface. The positioning accuracy of the XY-axis and the Z-axis was ±1 μm and ±0.5 μm respectively. The experimental parameters are shown in table 1. Before the experiments, the following preparatory steps were performed [25]: the surface of the ZrO₂ workpiece was leveled, the end of the tool electrode was flattened, the immersion depth of the tool electrode was adjusted, and the initial machining gap was set.

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Table 1. Experimental parameters.

Parameter	Initial value
Diameter of tool electrode (mm)	Φ0.5
Material of tool electrode	Copper
Material of workpiece	ZrO2 ceramic
Pulse duration (μs)	3.2
Pulse interval (μs)	4.8
Output voltage (${\text{V}}$)	90–150
Immersion depth (mm)	2
Electrolyte concentration (wt%)	5
Electrolyte type	NaOH solution

Under the conditions given in table 1 with a processing time of 30 s, experiments were performed by using different initial machining gaps and processing voltages. Figure 4 shows the experimental results observed with an optical microscope. As shown in the figure, for a machining gap of ≤15 μm and a voltage of 150 V, the machined surface showed obvious signs of thermal spalling, indicating the dominance of physical removal over chemical removal. Meanwhile, for a gap of ≥20 μm regardless of the voltage of 120 V or 150 V, the machined surface was smooth and shallow, indicating only chemical removal without physical removal, namely Q_cpr> Q> Q_ccr. For a voltage of 120 V and a gap of ≤15 μm, the workpiece surface showed the combined effect of physical and chemical removal. For a voltage of 120 V and a gap of ≥20 μm, there were no obvious machining marks. For the first result of 150 V, a large number of microcracks are visible, indicating that serious surface damage was caused by the larger spark energy, although the material removal rate was higher.

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As shown in figure 4, when the initial machining gap was increased from 15 μm to 20 μm under a voltage of 120 V, the processing effects changed abruptly. Furthermore, the experiments were performed using initial machining gaps of 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, and 22 μm, respectively. As shown in figure 5, the abrupt change occurred at a machining gap of 20 μm. As we know, a tool electrode thermally expands due to spark discharges in the SACE process [18]. In the experiment with an initial machining gap of 20 μm, the force sensor showed that the tool electrode after thermal expansion was in contact with the workpiece surface, resulting in the obvious material removal. However, for an initial machining gap of 21 μm, when the tool electrode after thermal expansion was not in contact with the workpiece surface, the removed amount of workpiece material was extremely small. Even if the initial spark energy was increased or the processing time was extended to dozens of minutes, obvious material removal did not occur due to the fact that the tool electrode was not in contact with the workpiece surface during the SACE process. In this paper, this phenomenon is named the 'contact effect'. The contact effect can explain the SACE sensitivity to the initial machining gap. This indicates that a necessary condition for the effective removal of ZrO₂ ceramic material during the SACE process is to ensure direct contact between the tool electrode and the workpiece.

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To achieve a damage-free process by inducting the dominantly chemical removal, the spark energy Q acting on the workpiece surface needs to satisfy Q_cpr> Q> Q_ccr. In this study, this was achieved by adjusting the electric parameters, the electrolyte thickness, and the initial machining gap. For the damage-free process under the contact effect, the processing voltage was adjusted so as to change of the value of Q to investigate the effect on the material removal.

Figure 6 shows the experimental results obtained by white light interferometer (WLI) and scanning electron microscope (SEM). At the processing voltage of 150 V, the material removal starts from the edge of the tool electrode end, and then gradually extends inward until the workpiece material under the tool electrode is removed completely. The processed surface from the dominantly physical removal is very rough as the energy Q is greater than the critical energy Q_cpr of physical removal. At a processing voltage of 90 V, the initial pattern of material removal is similar to that at 150 V since the dominantly physical removal due to the very narrow machining gap making Q> Q_cpr. With increasing processing time, the processed surface becomes increasingly smooth and fine, indicating the effect of chemical removal. In this case, the damage-free process was realized by achieving Q_cpr> Q> Q_ccr, that is, dominantly chemical removal. In fact, in the case the discharge constraint effect appeared in the SACE process due to the small spark energy at a voltage of 90 V [23]. The discharge constraint causes the discharge area to be concentrated at the edge of the tool electrode end, leading to the unremoved areas in the center of the end face of the tool electrode. Obviously, the discharge constraint has an important influence on the energy distribution and processing results.

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To explain the experimental results shown in figure 6, models of the SACE process under the contact effect were established for ZrO₂ ceramics. Figures 7 and 8 show the process models without the discharge constraint (for example, 150 V in figure 6) and with the discharge constraint (for example, 90 V in figure 6), respectively.

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As shown in figure 7, when the spark energy is too large to constrain the discharges onto the edge of tool electrode end, the discharges occur across the whole of the surface of the tool electrode including the end and its side wall under the electrolyte. In the initial SACE process under the condition of tight direct contact, there is no electrolyte between the end face of the tool electrode and the surface of the workpiece, as a result of no discharges under the end face (figure 7(a)). When discharges occur at the boundary of the tool electrode end, the electrolyte film h passed by the sparks is very thin, which means that the energy Q can be sufficiently large to satisfy Q > Q_cpr due to a low energy loss Q_loss(h). The dominantly physical removal mainly occurs in the form of thermal spalling, resulting in terrible surface quality (figure 7(b)). With the increase of the amount of removed workpiece material, the electrolyte flows into the gap to cause discharges to occur at the gap. In this way, the workpiece material below the tool electrode end can be continuously removed (figure 7(c)). When the workpiece material in contact with the tool electrode is completely removed, the material removal process is stopped regardless of the physical or chemical removal since the contact effect cannot occur (figure 7(d)). Therefore, if the discharge energy is large to Q > Q_cpr as no discharge constraint, it usually results in a poor surface quality such as the processing results at a voltage of 150 V shown in figure 6.

As shown in figure 8, when the spark energy is small enough to bring the discharge constraint, the discharges are mainly concentrated at the edge of the tool electrode end. In the initial stage of the SACE process, the energy Q can still be large enough to satisfy Q > Q_cpr due to a low energy loss Q_loss(h) resulting from the very thin electrolyte film h passed by the sparks (figure 8(a)). In this case, similar to the process shown in figure 7(b), the material removal is achieved by the combination of thermal spalling and chemical removal (figure 8(b)), however the proportion of physical removal under the lower energy condition is relatively smaller than that in figure 7(b). The material removal causes the electrolyte film thickness h and the loss energy Q_loss(h) to increase simultaneously, meaning that the energy Q is decreased, thus reducing the physical removal effect. Until the energy Q decreases so as to satisfy Q_cpr > Q > Q_ccr, although the thermal spalling is stopped, the chemical removal continues to occur, thus improving the surface quality (figure 8(c)). As the material continues to be removed, the thickness of the electrolyte film continues to increase, thus decreasing the energy Q. Until Q < Q_ccr, the chemical removal is also stopped and the workpiece material is no longer removed (figure 8(d)). The effect of the discharge constraint can improve the surface quality, however leads some unremoved material under the end face of the tool electrode.

Therefore, in order to realize high surface quality when using SACE process on ZrO₂ ceramics, it is necessary to achieve the discharge constraint by adjusting the processing parameters. According to equation (3), the spark energy Q can be adjusted by changing the pulse interval and processing voltage. In this research, the discharge constraint can be realized under the conditions of a processing voltage of 90 V, a pulse duration of 3.2 μs, and a pulse interval of 6.4 μs. It is well known that the surface quality is determined by the final stage of the SACE process on ZrO₂ ceramics. In the final stage, only if the spark energy Q can satisfy Q_cpr > Q > Q_ccr, microstructures of ZrO₂ ceramics with high surface quality can be machined by the chemical removal effect for removing a certain thickness of workpiece material.

By controlling the processing time of 300 s to ensure that the chemical removal depth is greater than the depth of the surface roughness layer caused by the physical removal before, a circular ring micro cavity with high surface quality (see figure 9) was achieved by the chemical removal dominated damage-free process. In this process, the discharges are constrained to the edge of the tool electrode end. In this method, the circular ring micro cavity conforming to the edge shape of the tool electrode end was machined with high quality surface, a depth of 50 μm, and a width of 139 μm. This experimental result verifies the process model shown in figure 8.

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The die-sinking process can machine a ring channel with a consistent shape and a smaller size compared with the tool electrode end. This process can make full use of the effect of chemical removal to improve the machining quality. However, this process is subject to the scope of the chemical removal. In this research, the limited machining depth was about 50 μm. Additionally, the shape of the machined microstructure depends on the shape of the tool electrode end. Therefore, complex microstructures can potentially be machined by changing the shape of the tool electrode end. Thus, this approach has promise for application in micro manufacturing fields such as micro molds, microfluidic channels, and so on.

For the purpose of realizing complex micro cavities with a damage-free surface in ZrO₂ ceramics using the SACE method, a layer-by-layer scanning process was experimented according to a designed path. Our previous experiments found that the feed strategy of the tool electrode is very important during the scanning process. If the feed rate of each layer in the depth direction is too small, it is likely that the tool electrode and the workpiece will not contact, resulting in an extremely low processing efficiency according to the contact effect (section 3.2). If the feed rate is too high, the contact force between the tool electrode and the workpiece is too large, which means that the tool electrode is prone to bending during the scanning process. Figure 10 shows the case in which a large contact force causes processing errors or even failure.

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Micro-drilling experiments using the SACE process showed that maintaining a lower contact force between the tool electrode and the workpiece can improve the machining speed and consistency [17]. In order to obtain a small contact force, a tool electrode with a spring structure shown in figure 11 was designed to reduce the tool stiffness and thereby ensure a smaller contact force under the same compression amount. A guide tube was used to ensure the movement stability of the tool electrode. Thus, the bending problem of the tool electrode during the scanning process was avoided. The spring structure (figures 11(c) and (d)) effectively reduced the tool stiffness from 2 g μm to 0.004 g μm, as measured by the force sensor (figure 3) in the experimental system. The low stiffness not only ensured a micro contact force of 0.4 g of the tool electrode at a compression of 100 μm, but also ensured the stability of the scanning process after each layer feed of several hundred microns. Using this low-stiffness tool electrode with a rotation speed of 200 rpm in the scanning process, a square micro cavity (figure 12) was machined on the ZrO₂ ceramic coating fabricated by a ceramic spraying process. As shown in figure 12, compared to the roughness of the original surface (9.83 μm Sa), the machined surface has a lower roughness (3.94 μm Sa). Additionally, compared with that of figure 10, the accuracy of the machined shape of figure 12 is obviously superior.

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Unlike the die-sinking process, the scanning process has a wider range of applications for the machining of complex 3D microstructures. By reducing the contact force between the tool electrode and the workpiece and by adopting the discharge constraint strategy, the processing quality was effectively improved. This approach is suitable for application in the fabrication of micro devices made of ZrO2 ceramics. These micro devices have superior mechanical properties than those made from Si-based materials in micro-electromechanical systems (MEMS).