Characteristics of vitrified binders for Cubic Boron Nitride grinding wheels.
| Autor: | Chao-Hua Peng, 彭昭華 |
|---|---|
| Rok vydání: | 2015 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 103 In this study, the glass-ceramic binding agent of cubic boron nitride (CBN) grinding wheel were developed and investigated. The Diopside (CaMgSi2O6) was selected to be used in glass-ceramic systems. This type of ceramic binders have high dressibility, high rigidity, high wear resistance, high heat resistance and other characteristics. Glass ceramics have high density and can be sintered at low-temperature. Furthermore, the glass-ceramic phase structure and crystallization degree can be controlled by post-treatment. Because of these reasons, this study uses different heat treatment methods to control the phase composition ratio, to improve the physical properties in order to increase the internal structure of the glass network, as a result TiO2 was added to obtain high strength glass ceramic binder. This experiment was designed to use glass compositions with Ca-Mg-Si-B (CMSB) as the main components, and TiO2 as the modifier, in order to form ceramic binding agent. When more than 30% of B2O3 was added, we can effectively increase the CBN abrasive wetting property. Mg, Ca, Ba and other alkaline earth (AE) elements, possess larger ionic radius, higher electronic valence, higher bonding energy, and therefore oxide bonding is more stable than those in the alkali metal group. The covalent/ionic bonding of AE elements can increase structural strength, enhance physical properties of the glass and therefore decrease thermal expansion coefficient of glass. Therefore in this experimental we use Ca: Mg: Si = 1: 1: 2 ratio of ingredients to formulate the binder, and add 2 ~ 8 mol% of TiO2 into the final glass melt in order to produce Ti4+ bonds inside the glass to increase network density and to increase physical strength of the glass. FT-IR were also employed to investigate the glass, and TiO2 addition was found to generate [TiO4] and [TiO6] bonding in the glass, which increases network density, and physical strength of the glass phase. TiO2 can also work as a nucleation agent in the experiment and therefore we perform heat treatments to make diopside crystal phase precipitate with different heat treatment parameters to control the ratio of glass-ceramic phases in CMSB glass. Heat treatment at 760oC for 5 hours and at 800oC for 2 hours of the CMSB4T specimens produce surface hardness of up to 572 HV and 592 HV, and bending strength of 97.4 Mpa and 98.5 Mpa. Analyzing the crystal phase using Rieveld method indicates 18% and 44% of formation of the diopside phase. Although longer holding time and higher annealing temperature generate more diopside crystal phase, no significant increase in hardness and bending strength, indicating an appropriate arrangement of the glass network and the amount of crystal phase provides optimum hardness and bending strength of the binding agent. As to toughness of the CMSB4T glass ceramics, the material treated at 760oC, 2hrs with fewer diopside phase (6%) exhibit fracture toughness of about 0.073 MPa√m; on the other hand, the specimen treated at 760oC, 10 hrs with a crystallinity of 82.7% show a the fracture toughness of 0.177 MPa√m, implying that fracture toughness of the glass-ceramics increases with the amount of the diopside phase. The current results show that a designed glass-ceramic system based upon alkaline-earth element provides better toughness, strength and thermal expansion than those with alkaline elements, which may work as a better CBN binding agent in grinding wheel applications. |
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