Isotropic graphite is a kind of new carbon material formed by isostatic pressing forming and has performances of high purity, high strength and high volume density, which is widely used in fields of edm, nuclear industry, electronic industry and moulds, etc, and has broad application prospect. With its good physical and chemical properties, isotropic graphite material gradually become a kind of important material in mold production .At present, domestic studies of isotropic graphite processing are more concentrated in high speed machining technologies. The surface roughness of graphite after high speed milling can not reach the quality requirement of optical mold. Therefore, grinding and polishing of isotropic graphite materials is necessary.
Through the study of the application range, processing status and material properties of isotropic graphite materials, the cutting force when processing graphite is low, and it is easy to grind and polish. However, due to the porous and non-homogeneous nature of the material itself, the grinding and polishing method is difficult to achieve the same surface roughness as metal materials. In this chapter, the isotropic graphite is polished by diamond abrasive paste and silicon carbide sandpaper to analyze the advantages and disadvantages of the two methods.
Grinding test of diamond abrasive paste
Grinding is a polishing method in which the convex and concave surfaces are gradually flattened and smoothed by the continuous rolling of the abrasive during the grinding process. The abrasive paste is a soft abrasive made of abrasive and paste-like binder. The binder of the abrasive paste is oil-soluble and water-soluble. According to the type of abrasive, the abrasive paste can be roughly divided into four types, in which the abrasive mesh number is from F600~F8000, and the diamond abrasive paste is from W40~W0.25, which can complete the rough research to the ultra-fine research.
To achieve the desired isotropic graphite surface finish, a water-soluble diamond abrasive paste is required. Therefore, the diamond paste was selected for testing. The particle size of the paste was three kinds of W5, W1.5, and W0.5, and the diamond abrasive grains were 5, 1.5, and 0.5 μm, respectively.
The isotropic graphite R8650 workpiece is clamped on the main shaft by a clamp. The spindle rotates at 100 rpm. The polished wool wheel with diamond abrasive paste is reciprocated from the edge of the workpiece to the center with a certain pressure. The pressure and speed are kept as constant as possible, and W5, W1.5, W0.5 diamond polishing paste is used in sequence. Each particle size abrasive paste use time is 30min. In order to improve the grinding precision, when the polishing time of the above-mentioned particle size polishing paste is reached, the workpiece is not removed, so as to avoid the error caused by repeated installation, and only the surface of the workpiece is cleaned with deionized water and alcohol to avoid the influence of the coarse-grained abrasive paste on the next grinding process.
The grinding test mainly measures material removal rate and surface quality. The material removal rate measures the removal rate of the grinding method, and the surface quality measures the grinding effect of the grinding method on the material. Since the workpiece remains clamped during the test, the material removal rate of the workpiece is not measured, which also increases the grinding speed. The surface roughness Ra and surface topography of the workpiece are mainly measured to characterize the surface quality. After the test is completed, the surface of the graphite is clearly scratched, and it is difficult to provide a constant pressure by the hand-held method, so that the diamond abrasive grains are embedded in the surface of the graphite workpiece to cause scratches. However, the surface finish of the workpiece has increased. The surface roughness measured by the ZYGO New Viewer 7100 white light interferometer can reach Ra 0.06 μm, which is greatly improved compared with the surface roughness Ra after CNC milling.
The diamond paste test achieved certain grinding results, which made the surface roughness of isotropic graphite drop to Ra 0.06μm, but the three repeated test surfaces had deep scratches and did not achieve the desired grinding effect. The reasons for the analysis are as follows:
(1) The pressure of the polishing wheel cannot be kept constant due to manual grinding in the test.
(2) The hardness of diamond abrasive grains is much larger than that of isotropic graphite, and the abrasive grains are too sharp.
(3) The isotropic graphite material has pores, and the abrasive grains are easily embedded.
Polishing test of silicon carbide sandpaper
The isotropic graphite material has pores, and the effect of grinding the isotropic graphite using the fine abrasive diamond paste of free abrasive grains is not satisfactory, and it is difficult to avoid scratches. Sandpaper belongs to coated abrasives and is manufactured by electrostatic sand-grinding process. The abrasive grains are arranged in order to some extent, and there is certain flexibility in the polishing process. There are many kinds of abrasive grains and a wide range of applications. Therefore, polishing experiments were conducted using sandpaper in which the abrasive grains were arranged in an orderly manner.
Sandpaper is a kind of coated abrasive. It consists of abrasive particles, binder and backing. The abrasive grains are evenly arranged on the sandpaper by electrostatic sanding. The abrasive grains mainly include synthetic diamond, alumina, silicon carbide and zirconia. . The hardness of the abrasive particles is characterized by Knoop hardness, which is tested with a small force using a special shape of the indenter to measure the hardness of the indentation diagonal. After grinding the isotropic graphite with diamond abrasive paste, the surface roughness is also greatly reduced, but the surface of the graphite workpiece is scratched, partly because the diamond abrasive grains are too sharp and do not match the graphite hardness, so the sandpaper The polishing test uses common silicon carbide abrasive grains, which are more economical while meeting the requirements of use.
According to the number of meshes, 400#, 600#, 1000#, 1200#, 1500#, 2000# are commonly used. In order to achieve the purpose of mirror polishing, it is also necessary to use sandpaper with a mesh number of 3000#, 5000#, and 7000#.
There are many manufacturers of sandpaper. The Warrior brand 991A series sandpaper produced by STARCKE of Germany has good wear resistance and full abrasive specifications (80#~7000#), which are widely used and recognized worldwide. And the sandpaper adopts high-voltage electrostatic sand-implanting technology, so that the silicon carbide abrasive can be aligned to ensure the best sharpness of the abrasive; the adhesive improves the wear resistance and softness of the sandpaper, making the sandpaper water-resistant and not easy to curl. The Warrior brand sandpaper has the characteristics of wet and dry: in the process of dry grinding, the heat dissipation is fast, the grinding amount is small, and the tear resistance is improved, thereby improving the surface quality of the workpiece; in the process of water grinding, the sandpaper backing is resistant to water and toughness. The abrasive particles will not fall off. The sandpaper selected for the test has 600#, 1000#, 1500#, 2000#, 3000#, 5000#, 7000#, a total of seven models.
When polishing an isotropic graphite workpiece with sandpaper, refer to the preparation process of the metallographic sample, try to make the polishing process flow and standardize as much as possible, to reduce the error caused by manual polishing, and obtain a graphite workpiece with good polishing consistency, which is conducive to summarizing the test rule. . When polishing, the graphite workpiece is fixed on the optical test platform by using a clamp, and the sandpaper is held by the thumb of both hands, and the other fingers are pressed on the sandpaper to uniformly distribute the applied pressure on the entire polishing surface to ensure that the polished surface of the sample is flat without Deformation. When polishing, the grinding should be carried out in one direction, grinding when moving forward, lifting the sandpaper away from the workpiece during the return stroke, and the same sandpaper is ground in the same direction. Polishing starts from 600# sandpaper, and replaces the fine sandpaper one by one. When replacing a sandpaper, the sample should be rotated 90° to eliminate the influence of uneven finger pressure and achieve a better polished surface. Using the clamp to clamp the workpiece, respectively, using silicon carbide sandpaper along the polishing direction in the figure. When polishing, dry grinding is used, graphite is an easy-to-remove material, and the grinding heat is not high when the sandpaper is polished, and the porous nature of the graphite makes the dry grinding more convenient for measuring the removal rate of the material. Each type of sandpaper was polished for 5 minutes. During the polishing process, the powder on the surface of the graphite is periodically cleaned to prevent the powder from rolling on the surface of the workpiece to increase surface scratches. The scratch removal on the surface of the sample was observed at any time during the polishing process. The use of silicon carbide sandpaper to polish isotropic graphite is more efficient, especially in coarse-grained (600#~2000#) sandpaper, when using fine-grained (3000#, 5000#, 7000#) sandpaper, increase the polishing time to10 minutes.
This test measures two test data: mass removal rate MRR and surface roughness Ra. The MRR can reflect the polishing speed and the sharpness of the abrasive grain; the surface roughness Ra is an index for verifying whether the sandpaper polishing can achieve the test purpose. The mass removal rate MRR is equal to the mass of the workpiece before polishing minus the mass of the workpiece after polishing, divided by the processing time. Before using the electronic balance to weigh, the surface of the workpiece must be cleaned. Because the porous material of graphite absorbs a certain amount of water, it is not washed with water. Only the blower is used to blow off the surface debris and other debris. Each workpiece is weighed five times and averaged to improve measurement accuracy.
The surface roughness Ra was measured using a ZYGO New Viewer 7100 white light interferometer, and five point measurements were selected in different areas of the workpiece surface.
The material removal efficiency of R8650 graphite material is higher in silicon carbide sandpaper. With the decrease of silicon carbide abrasive grains, the mass removal rate MRR is greatly reduced. When the type of silicon carbide sandpaper is in the range of 600#~3000#, the quality of R8650 The removal rate MRR decreased rapidly. The average mass removal rate MRR of the three tests decreased rapidly from 49.17 mg/min to 9.35 mg/min. When the type of silicon carbide sandpaper was in the range of 5000#~7000#, the average material removal rate MRR of the three tests has small differences as 1.67 mg / min and 1 mg / min. The isotropic graphite material is powder impregnated and then graphitized by isostatic pressing. The bulk density is not high (R8610 is 1.85g/mm3), the particle size is finer (R8650 is 7μm), and the bonding force between the particles is low. The material is easily removed, so the silicon carbide sandpaper has a high material removal efficiency for graphite. In the coarse abrasive grain (600#~3000#) stage, the silicon carbide sandpaper polishing graphite material has high efficiency, and the sandpaper abrasive grain size of 3000# is equivalent to the particle size of R8650, about 7μm, and the graphite powder can only block the sandpaper to a certain extent; In the stage of fine abrasive grains (5000#~7000#), the size of the abrasive grains is smaller than the particle size of R8650. The graphite powder is very easy to block the sandpaper, and the abrasive grains cannot be protruded, so that the material removal rate is lowered.
In summary, the silicon carbide sandpaper polishing test achieves a better polishing effect, although manual polishing, but by controlling the influence of human factors in manual polishing, such as the uneven pressure of the sandpaper contact surface and the number of grinding times per minute, etc. The surface roughness of the isotropic graphite decreased to about Ra 25 nm, indicating that the silicon carbide sandpaper is suitable for polishing isotropic graphite. There are three reasons for the better polishing effect of silicon carbide sandpaper:
(1) The sandpaper is coated with abrasives, and the abrasive grains are arranged in an orderly manner, which is more suitable for polishing graphite materials with micropores on the surface.
(2) The hardness of the silicon carbide abrasive grains is small, and it is suitable for polishing isotropic graphite having a low hardness.
(3) The sandpaper model is complete, from 600#~7000#.
Author: ZHANG Wenwu