Electrochemical Discharge-assisted Micro-grinding Device for Micro-components of Brittle and Hard Materials

Abstract

The invention provides an electrochemical discharge-assisted micro-grinding device for micro-components of brittle and hard materials. The device includes a micro-grinding tool, grinding fluid, a workpiece, an auxiliary electrode, a processing groove, and a pulsed DC power supply; the processing groove is filled with grinding fluid; the micro-grinding tool, the workpiece, and the auxiliary electrode are immersed in the grinding fluid; the micro-grinding tool is composed of a conductive grinding tool base, an electroplating layer, and insulated superabrasives. The micro-grinding tool is connected to the negative electrode of the pulsed DC power supply; the grinding fluid is composed of H.sub.2O.sub.2, Na.sub.2CO.sub.3, EDTA-Fe-Na, and deionized water; the workpiece material is brittle and hard; a large number of micro structures need to be produced on the surface of the workpiece.

Claims

1. An electrochemical discharge-assisted micro-grinding device for micro-components of brittle hard and difficult-to-conductive materials comprising: 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 insulated super abrasive (1-3); the micro-grinding tool (1) is connected to the negative electrode of the 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 brittle, hard and difficult-to-conductive materials; a 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 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; when the current of the pulsed DC power supply (6) flows through the loop, hydrogen bubbles (9) are generated around the micro-grinding tool (1) due to an electrochemical reaction; a gas film (7) is generated as hydrogen bubbles (9) converge and merge; under the action of a strong electric field, the gas film (7) is broken down and electrochemical discharge occurs accompanied by discharge spark (8); the discharge spark (8) directly ablates the adjacent workpiece (3) and generate a softened heat-affected layer (HAL) (10-1). namely, physical modification of the surface material of the workpiece (3); meanwhile, the discharge spark (8) increases the temperatures of the adjacent workpiece (3) and the grinding fluid (2), which triggers a chemical reaction between the workpiece (3) and the grinding fluid (2) to generate soft oxysalt (10-2), namely, chemical modification of the surface material of the workpiece (3); the mechanical properties of the modified layer (10) generated under the coupled effects of physical and chemical modification are reduced so it is ground away by the micro-grinding tool (1), which achieves efficiency, precision, and quality machining of the workpiece (3) of brittle, hard and difficult-to-conductive materials; and a number of micro structures (3-1) is produced on the surface of the workpiece (3), in conjunction with the movement of the workbench.

2. The electrochemical discharge-assisted micro-grinding device for micro-components of brittle hard and difficult-to-conductive materials according to claim 1 wherein the working portion of the micro-grinding tool (1) has a needle-, rod-, disc-, column-like, or spherical shape; the radial dimension of the working portion ranges from 0.02 mm to 1 mm; the immersion depth of micro-grinding tool (1) in the grinding fluid (2) ranges from 0.5 mm to 2 mm; the electroplating layer (1-2) covers the whole or part of the working surface of the micro-grinding tool (1), and its thickness ranges from 0.5 .Math.m to 100 .Math.m; the superabrasives (3-3) are diamond and cubic boron nitride (CBN), and their particle sizes range from 0.5 .Math.m to 50 .Math.m.

3. The electrochemical discharge-assisted micro-grinding device for micro-components of brittle hard and difficult-to-conductive materials according to claim 2, wherein the materials of the workpiece (3) include monocrystalline silicon, polycrystalline silicon, monocrystalline silicon carbide, polycrystalline silicon carbide, glass, and engineering ceramics.

4. The electrochemical discharge-assisted micro-grinding device for micro-components of brittle hard and difficult-to-conductive materials according to claim 3, wherein the block of inert conductive material of the auxiliary electrode (4) is bulk graphite.

5. The electrochemical discharge-assisted micro-grinding device for micro-components of brittle hard and difficult-to-conductive materials according to claim 4, wherein the inner space of the processing groove (5) accommodates the workpiece (3) and the auxiliary electrode (4) at the same time; the groove depth allows the immersion of the to-be-machined surface of the workpiece (3) in the grinding fluid (2) and is 2 mm or more above the to-be-machined surface.

Description

DESCRIPTION OF DRAWINGS

[0017] FIG. 1 is a schematic diagram of an electrochemical discharge-assisted micro-grinding device for micro-components of brittle hard and difficult-to-conductive materials, as stated in the present invention.

[0018] FIG. 2 is a partially enlarged view of the grinding head portion of the micro-grinding tool in FIG. 1.

[0019] FIG. 3 is a schematic diagram of the workpiece used in a specific example of the electrochemical discharge-assisted micro-grinding device for micro-components of brittle hard and difficult-to-conductive materials, as stated in the present invention.

[0020] FIG. 4 is a schematic diagram of the processing principle of the electrochemical discharge-assisted micro-grinding device for micro-components of brittle hard and difficult-to-conductive materials, as stated in the present invention.

[0021] Description of reference numerals in the drawings: 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. Oxysalt.

DETAILED EMBODIMENT

[0022] The device of the present invention is further described by specific examples in conjunction with the accompanying drawings below.

[0023] An electrochemical discharge-assisted micro-grinding device for micro-components of brittle hard and difficult-to-conductive materials. The structure of the device is shown in FIGS. 1 and 2 and includes six parts: 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 constituents and structure of each part are presented as follows.

[0024] The tungsten material micro-rod is used as the grinding tool base (1-1) of the micro-grinding tool (1), the micro-rod has a slender shank (length: 30 mm; diameter: Φ3 mm) and a minute tool bit (length: 8 mm; diameter: Φ0.45 mm). Between the shank and the tool bit, a transitional cone (length: 10 mm) with gradually reducing diameters is used to improve the structural strength of the tool bit. At the end of the tool bit, electroplating technology is employed to coat the diamond abrasives (1-3) with a particle size of 1,000 mesh in an electroplated nickel layer (1-2) with a length of around 4 mm and a thickness of 0.025 mm on single side. The tool bit coated with the abrasive grain layer is used as the grinding head, i.e. the working portion of the micro-grinding tool (1).

[0025] The mass fractions of each component in the grinding fluid (2) are set according to the chemical reaction conditions of monocrystalline SiC, including 68 wt% deionized water, 16 wt% Na.sub.2CO.sub.3, 10% H.sub.2O.sub.2, and 6% EDTA-Fe-Na. The Na.sub.2CO.sub.3 powder is not deliquescent; the 30 wt% H.sub.2O.sub.2 reagent is selected; during preparation of the grinding fluid (2), the Na.sub.2CO.sub.3 powder is added into the deionized water, stirred well, and used as the base solution. Then the H.sub.2O.sub.2 is injected as the oxidant into the base solution. The EDTA-Fe-Na powders are added as a catalyst and stirred until they are completely dissolved. The preparation of the grinding fluid (2) is thus completed. The 1 kg grinding fluid (2) is prepared for future use according to the requirements of the device.

[0026] The monocrystalline SiC workpiece (3) is a single block with a size of 15 mm × 10 mm × 2 mm. It is featured by high hardness, high brittleness, and a high melting point, and difficult-to-conduct electricity; the auxiliary electrode (4) is a bulk of graphite with a size of 40 mm × 20 mm × 5 mm, which has good electrochemical stability and is resistant to the corrosion of alkaline working fluid; the processing groove (5) is a PTFE plastic groove with an internal size of 50 mm × 50 mm × 30 mm and a wall thickness of 5 mm. Four through-holes (diameter: Φ0.8 mm) are drilled at the bottom of the groove for connection and fixation. The PTFE plastic has great resistance to strong alkali and so it can resist the chemical erosion of grinding fluid (2). The pulsed DC power supply (6) of the transistor type is employed, which has good pulse waveform and low cost; the adjustable range of its output voltage is 0 V - 100 V, and the maximum peak current is 30 A.

[0027] The relative position of each part of the device is as follows: the grinding fluid (2) with a depth of 22 mm is injected into the processing groove (5); the micro-grinding tool (1), the monocrystalline SiC workpiece (3), and the auxiliary electrode (4) are placed in the processing groove (5). The immersion depth of the grinding head portion of the micro-grinding tool (1) in the grinding fluid (2) is 1.5 mm; the monocrystalline SiC workpiece (3) and the auxiliary electrode (4) are completely immersed in the grinding fluid (2), the upper surface of the monocrystalline SiC workpiece (3) is adjacent to the grinding head of the micro-grinding tool (1); the micro-grinding tool (1) is connected to the negative electrode of the pulsed DC power supply (6) and serves as an electrochemical cathode; the auxiliary electrode (4) is connected to the positive electrode and serves as an electrochemical anode.

[0028] As shown in FIG. 3, this example aims to show the production of eight micro structures (3-1) (0.5 mm wide and 20 .Math.m deep) of flow channel on the surface of the monocrystalline SiC workpiece (3), using the aforesaid device. Since the precise feeding and rotation of external sources are required by the device during processing, the micro-grinding tool (1) in the device is clamped on the spindle of the machine tool used for micro-machining, and a special clamp is used to locate and to clamp the monocrystalline SiC workpiece (3); bolts are used to fix the clamp together with the processing groove (5) on the workbench of the machine tool. The high-speed rotation of the micro-grinding tool (1) and the precise feeding between the micro-grinding tool (1) and the monocrystalline SiC workpiece (3) are achieved using the precision machine tool.

[0029] The spindle speed of the machine tools used for micro-machining is set to 60,000 r/min, the feeding rate to 500 .Math.m/min, and the grinding depth to 5 .Math.m; the pulse width of the pulsed DC power supply (6) is set to 60 .Math.s, the duty cycle to 0.2, and the peak voltage to 40 V. The operation process of each part of the device during machining is as follows:

[0030] When the current provided by the pulsed DC power supply (6) flows through the loop, an electrochemical reaction occurs to the portion of the micro-grinding tool (1) immersed in the grinding fluid (2). An insulating gas film (7) is generated around the micro-grinding tool (1), as multiple hydrogen bubbles (9) converge and merge. Since the auxiliary electrode (4) is much larger than the micro-grinding tool (1), oxygen bubbles are constantly produced by the electrochemical reaction in the grinding fluid (2); under the action of the voltage provided by the pulsed DC power supply (6), the gas film (7) is broken down by the micro-grinding tool (1), the electrochemical discharge occurs accompanied by the discharge spark (8); since the discharge spark (8) has a local, instantaneous, and strong thermal effect, during electrochemical discharge, the micro-grinding tool (1) can directly ablate the adjacent monocrystalline SiC workpiece (3) leading to the generation of the softened HAL (10-1), namely, physical modification of the surface material of the monocrystalline SiC workpiece (3); meanwhile, The micro-abrasive tool (1) also increases the temperature of the monocrystalline SiC workpiece (3) and of the grinding fluid (2) adjacent to its discharge area. The increase in temperature causes a chemical reaction between the monocrystalline SiC workpiece (3) and the grinding fluid (2). During the reaction, soft silicate (10-2) is produced, namely, chemical modification of surface material of the monocrystalline SiC workpiece (3); during electrochemical discharge, the electroplated nickel layer (1-2) on the working surface of the micro-grinding tool (1) is slightly ablated, and the micro-grinding tool (1) can achieve self-sharpening to ensure a sharp state suitable for grinding and a stable performance. When the pulsed DC power supply (6) enters the pulse interval, the electrochemical discharge on the micro-grinding tool (1) stops. The ruptured gas film (7) causes the grinding fluid (2) to resume contact with the micro-grinding tool (1). The grinding fluid (2) flow cools the micro-grinding tool (1) and the monocrystalline SiC workpiece (3) while discharge the product of ablation. The electrochemical discharge generated by the micro-grinding tool (1) doesn’t rely on the material of the workpiece (3), and the electrochemical discharge is used mainly to trigger the modification of the workpiece (3) material, rather than the erosion of the material. Therefore, the device is highly adaptable to the processing of the monocrystalline SiC, that is difficult-to-conduct electricity and has a high gasification temperature, and of other hard and brittle materials with similar characteristics.

[0031] The mechanical properties such as hardness, elastic modulus, and tensile strength of the modified layer (10) of the monocrystalline SiC workpiece (3) generated under the coupled effects of physical and chemical modification are significantly reduced. Accordingly, the modified layer (10) has better cutting performance; when the micro-grinding tool (1) is rotated at a high speed and fed precisely, the modified layer (10) of the monocrystalline SiC workpiece (3) can be quickly removed by the diamond abrasive (1-3) on the micro-grinding tool (1) that scratch the layer. This enables the performance of the micro-grinding tool (1) to be more concentrated in removal of small amounts of hard and brittle monocrystalline SiC base materials, while weakening the mechanical force between the diamond abrasive (1-3) and the monocrystalline SiC workpiece (3). It slows down the wear evolution speed of the micro-grinding tool (1). 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. As only a small amount of the unmodified monocrystalline SiC workpiece (3) materials are removed by the micro-grinding tool (1), 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). This can reduce or even eliminate processing damage 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 the flow channel to have nano-scale surface roughness after processing. The micro-grinding tool (1) repeats the aforesaid process of electrochemical discharge-assisted modification and grinding. Based on feeding of the workbench in the preset machining path, micro structures (3-1) of the flow channel are finally produced on the surface of monocrystalline SiC workpiece (3).

[0032] According to the foregoing structure and its working process, the device realizes high-efficiency, high-precision, and low-damage machining of the micro structures (3-1) of the flow channel on the surface of a monocrystalline SiC workpiece (3). Regulating parameters of the micro-grinding tool (1) and the other aforesaid structures is applicable to machining of all kinds of hard and brittle materials that have high melting points and difficult-to-conduct electricity. It effectively resolves defects of the existing methods for processing micro-components of hard and brittle materials that have high melting points and are difficult-to-conduct electricity.

[0033] The specific description of the foregoing embodiment is only intended to elaborate the present invention explicitly, not to limit the scope of the present invention. Any equivalent replacement and modification etc. made within the scope of the present invention are in the protection scope of the invention.