Electrochemical Discharge-Enabled Micro-grinding process for Micro-Components of Silicon-based Materials

Abstract

This paper describes an invention involving an electrochemical discharge-enabled micro-grinding process for micro-components of silicon-based materials. The specific machining method is described below. A micro-grinding tool and an auxiliary electrode are respectively connected to the negative and positive electrodes of a pulsed DC power supply. When the current flows through the loop, an electrochemical hydrogen evolution reaction (HER) occurs at the micro-grinding tool in the grinding fluid, which generate multiple hydrogen bubbles. The bubbles coalesce into an insulating gas film and separate the micro-grinding tool from the grinding fluid; when the critical voltage is reached, the gas film is broken down and an electrochemical discharge occurs accompanied by discharge spark; under the action of the discharge spark, the surface material of the workpiece in the discharge-affected region is directly ablated to generate a heat-affected layer (HAL), namely, physical modification.

Claims

1. An electrochemical discharge-enabled micro-grinding process for micro-components of silicon-based materials, wherein the electrochemical discharge-enabled micro-grinding process for micro-components of silicon-based materials requires an electrochemical discharge-enabled micro-grinding device comprised of: a micro-grinding tool (1); grinding fluid (2); a workpiece (3); an auxiliary electrode (4); a processing groove (5) and a pulsed DC power supply (6), wherein the processing groove (5) is filled with grinding fluid (2); the micro-grinding tool (1), the workpiece (3) and the auxiliary electrode (4) are immersed in the grinding fluid (2); the micro-grinding tool (1) is composed of a conductive grinding tool base (1-1), a conductive electroplating layer (1-2) and insulating super abrasives (1-3); the micro-grinding tool (1) is connected to the negative electrode of a pulsed DC power supply (6) and serves as a cathode; the grinding fluid (2) is composed of H.sub.2O.sub.2, Na.sub.2CO.sub.3, EDTA-Fe—Na and deionized water and is electrically conductive; the workpiece (3) is made of silicon-based materials with brittle, hard, and difficult-to-conductive properties, a large number of micro structures (3-1) need to be produced on its surface; the workpiece (3) is located adjacent to the micro-grinding tool (1); the auxiliary electrode (4) is composed of a large block of inert conductive material, and connected to the positive electrode of the pulsed DC power supply (6) to serves as an anode; the size of the auxiliary electrode (4) is larger than that of the micro-grinding tool (1) by one to two orders of magnitude; and wherein the electrochemical discharge-enabled micro-grinding process comprises the following processing step: Step 1: formation of gas film (7) by electrochemical hydrogen evolution reaction (HER): when the current of the pulsed DC power supply (6) flows through the loop, an electrochemical HER occurs at the micro-grinding tool (1) in the grinding fluid (2); the H.sup.+ ions in the grinding fluid (2) obtain electrons from the cathode to generate hydrogen bubbles (9) that adhere to the outer circumference of the micro-grinding tool (1), the hydrogen bubbles (9) coalesce into an insulating gas film (7) and separate the micro-grinding tool (1) from the grinding fluid (2), thus, the HER is terminated; meanwhile, an electrochemical oxygen evolution reaction (OER) occurs at the auxiliary electrode (4), the OH.sup.− ions in the grinding fluid (2) loses electrons and generate oxygen bubbles, since the size of the auxiliary electrode (4) is larger than that of the micro-grinding tool (1) by one to two orders of magnitude, the auxiliary electrode (4) is unable to generate a gas film; Step 2: electrochemical discharge: under the action of the voltage of the pulsed DC power supply (6), a large number of charged particles will accumulate on both sides of the insulating gas film (7) and a strong local electric field is generated wherein free electrons with negative charges gather on the micro-grinding tool (1), and positive ions with positive charge gather in the grinding fluid (2); when the critical voltage is reached, the gas film (7) is broken down, and a discharge channel is generated between the micro-grinding tool (1) and the grinding fluid (2); the high-speed directional movement of charged particles in the discharge channel results in an electrochemical discharge accompanied by discharge spark (8); Step 3: modification of the workpiece (3) material: the discharge spark (8) directly ablates the adjacent workpiece (3), and the temperature of the workpiece (3) located in the discharge center can raised to the melting temperature or even gasification temperature that leading to the direct removal of such material; a heat-affected layer (HAL) (10-1) is generated from the material of the workpiece (3) located adjacent to the discharge center due to a sudden temperature change, this results in the ablation and softening of the workpiece (3), namely, physical modification of the surface material of the workpiece (3); meanwhile, the temperature rise of the workpiece (3) and the grinding fluid (2) due to thermal effect of discharge spark (8) causes a series of chemical reactions between the workpiece (3) of silicon-based materials and the grinding fluid (2), the chemical reactions includes that H.sub.2O.sub.2 in the grinding fluid (2) generate *OH under the catalysis of Fe.sup.2+ ions, than *OH react with silicon-based materials to generate SiO.sub.2, and SiO.sub.2 further reacts with OH.sup.− ions in the grinding fluid (2) to generate silicate (10-2); finally, the surface material of the workpiece (3) is transformed from hard and brittle silicon-based material to soft silicate (10-2), namely, chemical modification of the surface material of the workpiece (3); the HAL (10-1) produced by physical modification and the silicate (10-2) produced by chemical modification constitute the modified layer (10); Step 4: deionization and cooling: when the pulsed DC power supply (6) enters the pulse interval, the voltage difference between the micro-grinding tool (1) and the grinding fluid (2) rapidly drops to zero, the discharge spark (8) and the insulating gas film (7) disappear, and the contact between the grinding fluid (2) and the micro-grinding tool (1) is restored; the grinding fluid (2) flow cools the micro-grinding tool (1) and the workpiece (3) while discharge the ablation products; Step 5: grinding of the modified layer (10): the mechanical properties of the modified layer (10) generated under coupled effects of the physical and chemical modification enabled by electrochemical discharge are significantly reduced; the micro-grinding tool (1) is rotated at a high speed and continuously fed, than the modified layer (10) is rapidly ground away by the superabrasives (1-3) on the micro-grinding tool (1); this results in the high-quality, high-efficiency, and high-precision machining of the workpiece (3) of silicon-based materials with brittle, hard, and difficult-to-conductive properties; and Step 6: repeat the Steps 1 to 5, a number of micro structures (3-1) can be produced on the surface of the workpiece (3) as the micro-grinding tool (1) is fed along the preset machining path.

2. The electrochemical discharge-enabled micro-grinding process for micro-components of silicon-based materials according to claim 1, wherein the mass fraction range of each component of the grinding fluid (2) is described as 50-70% deionized water, 16-26% Na.sub.2CO.sub.3, 8-16% H.sub.2O.sub.2, and 5-9% EDTA-Fe—Na.

3. The electrochemical discharge-enabled micro-grinding process for micro-components of silicon-based materials according to claim 2, wherein controlling the electrical parameters of the pulsed DC power supply (6) can achieve localized and efficient electrochemical discharge-enabled modification of the workpiece (3), the process is able to realize the brittle-ductile removal transition of hard and brittle materials when greater values of grinding parameters are used than the single forms of micro-grinding, namely, the maximum undeformed chip thickness of the brittle-ductile removal transition is increased; the pulsed DC power supply (6) has a peak voltage range of 20 V to 100 V, a pulse width of 0.1 μs to 1 ms, and a duty cycle of 0.1 to 0.8.

4. The electrochemical discharge-enabled micro-grinding process for micro-components of silicon-based materials according to claim 3 wherein the processing speed of the micro-grinding tool (1) is greater than 10,000 r/min, its grinding depth ranges from 0.2 μm to 25 μm, and its feeding speed ranges from 0.1 mm/min to 10 mm/min.

5. The electrochemical discharge-enabled micro-grinding process for micro-components of silicon-based materials according to claim 4, wherein the material of the processing groove (5) is non-conductive and alkali-proof, and can resist the erosion of the grinding fluid (2).

Description

DESCRIPTION OF DRAWINGS

[0018] FIG. 1A schematic diagram of the processing principle of the present invention.

[0019] FIG. 2A schematic diagram of the device required by the present invention.

[0020] FIG. 3A partial enlarged view of the grinding head portion of the micro-grinding tool in FIG. 2.

[0021] FIG. 4A schematic diagram of the workpiece processed by the present invention.

[0022] 1. Micro-grinding tool; 1-1. Grinding tool base; 1-2. Electroplating layer; 1-3. Superabrasive; 2. Grinding fluid; 3. Workpiece; 3-1. Micro structure; 4. Auxiliary electrode; 5. Processing groove; 6. Pulsed DC power supply; 7. Gas film; 8. Discharge spark; 9. Hydrogen bubble; 10. Modified layer; 10-1. HAL; 10-2. Silicate.

DETAILED EMBODIMENT

[0023] The technical solution of the present invention is further described by specific examples in conjunction with the accompanying drawings below.

[0024] A method for electrochemical discharge-enabled micro-grinding process for silicon-based materials. The device required by the process are shown in FIGS. 2 and 3, including the micro-grinding tool (1), the grinding fluid (2), the monocrystalline silicon carbide (SiC) workpiece (3), the auxiliary electrode (4), the processing groove (5), and the pulsed DC power supply (6). The micro-grinding tool (1) and the auxiliary electrode (4) are respectively connected to the negative and positive electrodes of the pulsed DC power supply (6) and are immersed in the processing groove (5) containing the grinding fluid (2) to serve as the electrochemical cathode and anode, respectively. The monocrystalline SiC workpiece (3) is located adjacent to the micro-grinding tool (1); the micro-grinding tool (1) consists of the tungsten material grinding tool base (1-1), the electroplated nickel layer (1-2) and diamond abrasives (1-3). The size of the auxiliary electrode (4) is 20 times that of the micro-grinding tool (1).

[0025] As shown in FIG. 4, this example aims to show the production of eight micro structures (3-1) of flow channel on the surface of the monocrystalline SiC workpiece (3), using the process of the present invention.

[0026] Before machining, the grinding fluid (2) was prepared according to the chemical reaction conditions of the monocrystalline SiC material of the workpiece (3); 68 wt % deionized water and 16 wt % Na.sub.2CO.sub.3 were used as the base fluid of the grinding fluid (2), 10 wt % H.sub.2O.sub.2 was added as the oxidant, and 6 wt % EDTA-Fe—Na was added as the catalyst; the mixed grinding fluid (2) doesn't react with monocrystalline SiC at the room temperature but reacts with it at a high temperature to produce silicate (10-2), and the reaction rate increases significantly as the temperature rise.

[0027] Based on the aforesaid device and the grinding fluid (2) at a specific ratio, the technical solution of the present invention is implemented by following steps of processing.

[0028] Step 1. Formation of gas film (7) by electrochemical HER: set the pulse width of the pulsed DC power supply (6) to 60 μs, the duty cycle to 0.2, and the peak voltage to 40 V; when the current of the pulsed DC power supply (6) flows through the loop, the negative potential from the negative electrode of the pulsed DC power supply (6) loaded on the micro-grinding tool (1) cause the H.sup.+ ions in the grinding fluid (2) to obtain electrons from the surface of the micro-grinding tool (1) and to generate H.sub.2. This represents the hydrogen bubbles (9) are generated by electrochemical HER and adhere to the surface of the micro-grinding tool (1). Than the multiple hydrogen bubbles (9) are gradually converge and merge to form an insulting gas film (7), and the insulating gas film (7) separate the micro-grinding tool (1) from the grinding fluid (2), thus, the HER is terminated; meanwhile, the positive potential from the positive electrode of the pulsed DC power supply (6) loaded on the auxiliary electrode (4) cause the OH.sup.− ions in the grinding fluid (2) to lose electrons and to generate O.sub.2. This represents the oxygen bubbles are generated by electrochemical OER, since the size of the auxiliary electrode (4) is 20 times that of the micro-grinding tool (1) and the generation rate of oxygen bubbles is only half of the hydrogen bubbles (9), the auxiliary electrode (4) is unable to form a gas film.

[0029] Step 2. Electrochemical discharge: under the action of the voltage of the pulsed DC power supply (6), a large number of charged particles will accumulate on both sides of the insulating gas film (7) and a local strong electric field is generated wherein free electrons with negative charges gather on the micro-grinding tool (1), and Na.sup.+, Fe.sup.2+, and H.sup.+ ions gather in the grinding fluid (2); when the critical voltage is reached, the micro-grinding tool (1) starts to emit field electrons, and the field electrons (accelerated by the electric field) bombard the hydrogen molecules in the insulating gas film (7), ionize them and release more electrons. This process develops like an avalanche, during which the insulating gas film (7) is quickly broken down and a discharge channel is generated between the micro-grinding tool (1) and the grinding fluid (2); meanwhile, the micro-grinding tool (1) continuously emits electrons to transfer to the grinding fluid (2), the Na.sup.+, Fe.sup.2+, H.sup.+ ions (accelerated by the electric field) in the grinding fluid (2) also bombard the surface of the micro-grinding tool (1); the high-speed directional movement of the charged particles in the discharge channel leads to an electrochemical discharge accompanied by discharge spark (8).

[0030] Step 3. Modification of material of the monocrystalline SiC workpiece (3): when the charged particles in the discharge spark (8) collide with each other or hit the micro-grinding tool (1) and the grinding fluid (2) at a high speed, their kinetic energy is rapidly converted into thermal energy and a high-energy local heat source is generated instantaneously, namely, the thermal effect of the discharge spark (8). Under the action of the thermal effect of the discharge spark (8), the adjacent monocrystalline SiC workpiece (3) is directly ablated; the materials of the SiC workpiece (3) located in the discharge center absorb the heat flux with maximum density. The temperature of a small amount of SiC materials can be raised to the gasification temperature, then a sublimation occurs and such material are eliminated under the action of the explosive force and local thermal shock force generated by the discharge spark (8). The temperature of the monocrystalline SiC workpiece (3) located adjacent to the discharge center rises rapidly under the action of the thermal effect of the discharge spark (8). Due to an extremely short action time of the discharge spark (8), the heat flux absorbed by the material adjacent to the discharge center has no time to diffuse to the surrounding substrate. This results in a sudden temperature change followed by the generation of a HAL (10-1). The numerous thermal cracks and lattice distortion caused by the ablation of discharge sparks (8) in the HAL (10-1) reduces the mechanical properties of the HAL (10-1); thereby, the ablation and softening of the monocrystalline SiC workpiece (3) are achieved, which represent the physical modification of the surface material of the monocrystalline SiC workpiece (3); physical softening is conducive to the rapid elimination of the modified layer (10) when the diamond abrasives (1-3) on the micro-grinding tool (1) subsequently scratch the monocrystalline SiC workpiece (3), that improving the processing efficiency. Due to the thermal effect of discharge sparks (8), the electroplated nickel layer (1-2) on the working surface of the micro-grinding tool (1) is slightly ablated, it ensures that the diamond abrasives (1-3) on the micro-grinding tool (1) have sufficient height of protrusion and achieve self-sharpening. In other words, the micro-grinding tool (1) can always maintain a sharp state in grinding and have a more stable performance Meanwhile, the thermal energy generated by the thermal effect of discharge sparks (8) acts on the monocrystalline SiC workpiece (3) as well as on the grinding fluid (2) to increases their temperature. The increase in temperature causes a series of reactions between the SiC workpiece (3) and the grinding fluid (2). The chemical reactions includes H.sub.2O.sub.2 in the grinding fluid (2) under the catalysis of Fe.sup.2+ can produce *OH with strong oxidizing properties. The solid-liquid phase oxidation reaction of monocrystalline SiC and *OH occurs to breaks the Si—C bond and combines it into a Si—O bond to generate SiO.sub.2 which have greater bond energy. The SiO.sub.2 further reacts with OH.sup.− in the grinding fluid (2) to generate silicate (10-2). The specific chemical reaction formulas are presented as below:

H.sub.2O.sub.2+catalyst.fwdarw.*OH;

SiC+*OH+O.sub.2.fwdarw.SiO.sub.2+CO.sub.2+H.sub.2O;

SiO.sub.2+OH.sup.−.fwdarw.SiO.sub.3.sup.2−+H.sub.2O;

The instantaneous high-energy thermal field of discharge sparks (8) causes a rapid chemical reaction between the grinding fluid (2) and monocrystalline SiC workpiece (3). Finally, the surface material of hard and brittle monocrystalline SiC workpiece (3) is converted into the soft silicate (10-2), which represents the chemical modification of the surface material of the monocrystalline SiC workpiece (3); the efficient chemical softening further improves the modified layer (10) machining efficiency of subsequent processing by the micro-grinding tool (1). Moreover, using high temperature generated by electrochemical discharge for softening can make the modification area and the products of the monocrystalline SiC workpiece (3) adjustable and controllable, which can improving the dimensional accuracy of the micro structures (3-1) of flow channel.

[0031] Step 4. Deionization and cooling: When the pulsed DC power supply (6) enters the pulse interval, the voltage difference between the micro-abrasive tool (1) and the grinding fluid (2) sharply drops to zero; the discharge channel gradually shrinks and collapses, and the discharge spark (8) disappears after the support from the electric field is lost. Under the action of the buoyancy of the grinding fluid (2) and the impact force of discharge sparks (8), the insulating gas film (7) is shattered and loses its integrity. The contact between the grinding fluid (2) and the micro-grinding tool (1) is restored, the grinding fluid (2) flow cools the micro-grinding tool (1) and the monocrystalline SiC workpiece (3) while discharge the products of ablation.

[0032] Step 5. Grinding of the modified layer (10): the surface layer material of the monocrystalline SiC workpiece (3) located in the discharge-affected region is transformed into a modified layer (10) with significantly reduced mechanical properties under the action of coupled effects of physical and chemical modification. When the micro-grinding tool (1) performs micro-grinding at a rotational speed of 60,000 r/min, a feeding rate of 500 μm/min, and a grinding depth of 5 μm, the modified layer (10) is rapidly ground away under the action of mechanical shear stress of the diamond abrasives (1-3) on the micro-grinding tool (1). This enables the performance of the micro-grinding tool (1) to be more concentrated in the grinding of a small amount of hard and brittle monocrystalline SiC base materials and weakens the mechanical force between the diamond abrasives (1-3) and the monocrystalline SiC workpiece (3). Meanwhile, the shape retention of the micro-grinding tool (1) and the shape accuracy of the micro structures (3-1) of flow channel are improved; compared with the single forms of micro-grinding, the critical depth of cuts for the brittle-ductile removal transition of the micro-grinding tool (1) increases at the same grinding speed, thus, it is easier to achieve the removal in ductile domain when grinding monocrystalline SiC workpiece (3). Doing so may reduce or even eliminate processing damages to the surface/subsurface and processing defects such as chipping, breakage and cracking. Meanwhile, the elimination of the modified layer (10) by the ultra-fine diamond abrasives (1-3) also enables the micro structures (3-1) of flow channel to have nano-scale surface roughness after processing.

[0033] Step 6. Repeat steps 1 to 5, the micro-grinding tool (1) is fed along the preset machining path. Under the electric supply of the pulsed DC power supply (6), the micro-grinding tool (1) continuously performs efficient localized modification on the to-be-processed area of the monocrystalline SiC workpiece (3); meanwhile, the generated modified layer (10) is quickly ground away, and a large number of micro structures (3-1) of flow channel are produced on the surface of the monocrystalline SiC workpiece (3); due to a significant decrease in the mechanical properties of the modified monocrystalline SiC workpiece (3), the mechanical force between the micro-grinding tool (1) and the monocrystalline SiC workpiece (3) is greatly reduced. This slows the speed of the wear, breakage and shedding of the diamond abrasives (1-3), improves the shape retention of the micro-grinding tool (1) and prolongs its service life.

[0034] According to the aforesaid processing process, the process of the present invention realizes the high-precision, high-efficiency and low-damage machining of the micro structures (3-1) of flow channel on the surface of the monocrystalline SiC workpiece (3); the micro-grinding tool (1) has a long service life and can be self-sharpened; adjusting the ratio of the grinding fluid (2) and the other aforesaid processing parameters is applicable to machining of all kinds of hard and brittle silicon-based materials that are difficult-to-conduct electricity and have high-melting point. It effectively resolves the defects of existing methods for processing micro components of hard and brittle silicon-based materials that are difficult-to-conduct electricity and have high-melting point.

[0035] The specific description of the foregoing embodiment is only used to elaborate the present invention explicitly, not to limit the scope of the present invention. Any equivalent replacement and modification and so on made within the scope of the present invention are in the protection scope of the invention.