1．Description:
The selection abrasive of vitrified diamond grinding wheels directly affects the grinding effect of wheels. So, abrasive selection is one of the key factors to study the vitrified diamond wheel. This experiment studies the micromorphology, antioxidant performance of different kinds of diamond abrasives and grinding performance of the diamond wheel. It has certain reference value for developing high-performance vitrified diamond grinding wheels.
Vitrified bond diamond grinding wheel not only features sharp cutting, small grinding force, high production efficiency, long service life, easy dressing and sharpening, high grinding accuracy but also has the features of low temperature and high durability of the workpiece when grinding. Therefore, many scholars at home and abroad have been committed to the development of vitrified bond diamond grinding wheel in recent years.
2.Experiment
The main parameters of experimental diamond abrasives are shown in Table 1.
Main parameters of experimental diamond abrasives
| Abrasive type | Grain size | Manufacturer | Item |
| Crushed diamond | 120/140 | Changsha Research Institute of Mining and Metallurgy Superabrasives Co., Ltd. | RVD |
| Raw material diamond | 120/140 | Changsha Research Institute of Mining and Metallurgy Superabrasives Co., Ltd. | K-RVD |
| Ti-coating diamond | 120/140 | Beijing Polaris Superabrasives Co., Ltd. | Ti-RVD |
Observed the surface morphologies of different kinds of abrasive under the JM S-5600LV
scanning electron microscope of Japan. Measured the antioxidant performance of different kinds of abrasives on the comprehensive thermal analyzer of Leitz 200 of Germany, and the heating rate was 10℃/ min, and the atmosphere was air. Mixed different kinds of abrasives with a certain amount of vitrified bond to form cylinder specs of φ25mm×10mm under 50MPa pressure. The formula of the sample is shown in Table 2. Sintered the sample at 750℃ for 1 hour under the protection of nitrogen. Installed the diamond grinding wheel that is sintered on a self-made grinder, and done the wear resistance test.
The workpiece was SiC grinding wheel made of Zhengzhou White Dove Abrasives Co., Ltd., The hardness of grinding wheel was K grade and grain size was 80/100. The speed of the workpiece was 16m/s, the loading pressure on the grinding wheel was 19.6N. Measured the weightlessness of diamond wheel and SiC grinding wheel after grinding for 10 minutes and calculated the abrasive ratio.
Table 2 Weight percentage(wt%) of the grinding wheels with different kinds of abrasives
| No. | Bond | RVD(raw, material diamond) | RVD(crushed diamond) | RVD-Ti | Auxiliary abrasives | Clay | Dextrin |
| 1 | 30 | 50 | – – | – – | 16 | 2 | 2 |
| 2 | 30 | – – | 50 | – – | 16 | 2 | 2 |
| 3 | 30 | – – | – – | 50 | 16 | 2 | 2 |
3.Experiment results and discussions:
3.1 Microstructure analysis of diamond abrasive
The main diamond abrasives used for vitrified bond diamond grinding wheel in China market are RVD diamond at present. The popular RVD diamond for a vitrified bond diamond grinding wheel is divided into raw material, crushed material, and Ti-coated material according to the different surface conditions. 3 kinds of micromorphology of the diamond are shown in Fig. 1.
1-B is the morphology of raw material diamond. It can be seen from the figure that diamond grains remain in the state of synthesis, and the sharp angle is good, the crystal shape is irregular. There are twins and symbiosis of crystals, and many defects on the crystal surface.
Fig.1-A is a crushed diamond, which is made up of larger diamond grains and crushed into smaller diameters. The shape of the grains has also changed. In the process of crushing, diamond crushed from the defects, so the surface detects of crushed materials is less. At the same time, the loss of abrasive sharp angle is more and the sharpness is worse.
Fig.1-C is Ti-coated diamond. The surface of the Ti layer of the Ti-coated diamond is compact. Usually, it is good for improving the oxidation resistance of diamond and improve the bonding strength of the diamond and vitrified bond.
3.2 Analysis of the antioxidant performance of different diamond abrasives
Fig.2, Fig.3, and Fig.4 are thermal analysis figures of 3 kinds of abrasives in air.
Crushed diamond, raw material diamond, and Ti-coated diamond.
It can be seen from the figure that the weightlessness of Ti-coated diamond is the smallest before ℃, and there is almost no weightlessness before 685℃, and the antioxidant performance is the best. There is basically no weightlessness of crushed diamond before 650℃. But the DSC curve of raw material diamond also began to have inflection point at 400℃, and the corresponding TG curve also began to weight lost at 400℃.
This is because the surface of raw material diamond is rougher than crushed diamond, the surface area is large, and the contact area with oxygen is larger, so the weightlessness is faster and the antioxidant performance is poor. At the same time, there are many crystal defects on the surface and inside of the raw material diamond, which also affects the antioxidant performance of the abrasive.
The DTG curve of Ti-coated diamond at 490℃ shows an obvious exothermic peak, and the corresponding TG curve shows a slight weight gain. 2 reactions may occur at this time. Ti on the abrasive surface is oxidized, or diamond is oxidized. This is because Ti is oxidized in air. From the thermodynamic point of view, any change of free energy is related to temperature according to the thermodynamic law.
Two reactions may occur at this time. Ti on the abrasive surface is oxidized, or diamond is oxidized. This is because Ti is oxidized in air. From the thermodynamic point of view, according to the thermodynamic law, any change of free energy is related to temperature.
Formula(1)is less than the formula(2). So, in the same system, Formula (1) reacts first, this is to say, Ti is oxidized first. In Fig.4, there is slight weight gain at 490℃, because Ti is oxidized in air. At the same time, there is an inflection point of DSC at 685℃. The sample has no weightlessness before this temperature. It indicates that the antioxidant of Ti coating is the best, but the antioxidant performance improves little compared with the crushed diamond.
3.3 The effect of diamond abrasives type on the durability of grinding wheels
The grinding ratio of 1#, 2#, 3# grinding wheel is 79.0, 42.6 and 17.3 respectively. From the grain shape, the grain shapes of crushed diamond and Ti-coated diamond are regular, and the defects of the surface are less. The surface of raw material diamond is rough and has more defects, irregular shape, larger surface area and bigger mechanical holding force of the bond. Therefore, the raw material diamond is difficult to falling off and has high wear resistance in the process of grinding.
Fig.5 and Fig.6 are SEM of the fracture surface of 2# and 3# grinding wheels respectively. The figures show that there are many large air holes at the interface between the Ti-coated diamond and vitrified bond. The sample was sintered under the protection of N2, but there are a lot of alkali metal oxides in the vitrified bond, so a lot of free oxygen exist in the bond. According to the related research, the free oxidation chemical potential is high and the activity is high. So the chemical reaction occurs with the abrasive surface element Ti. There are large pores at the interface and it reduces the bonding strength of the abrasive and the bond.
4.Conclusions:
The micromorphology of RVD diamond abrasives, crushed diamond, raw material, and Ti-coated diamond shows that the crystal shape of raw material is the most irregular and has more defects. Thermal analysis shows that Ti-coated diamond features the best antioxidant among 3 kinds of materials, and no oxidation reaction occurs before 685℃ in air. Wear resistance tests show that the bonding strength between raw material and vitrified bond is the highest, and the wear resistance is the best. It most suitable as abrasive of vitrified diamond grinding wheels