|Title:||Surface generation and damage mechanism in ultra-precision grinding of brittle materials|
|Advisors:||To, Sandy (ISE)|
|Subject:||Grinding and polishing|
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Industrial and Systems Engineering|
|Pages:||xxiii, 210 pages : color illustrations|
|Abstract:||To ensure the surface finish and form accuracy simultaneously, ultra-precision grinding has been widely employed in the machining of hard and brittle materials, such as engineering composites, carbides, optical glasses, and cermet materials. However, there are still great challenges in achieving an optical mirror surface with good surface integrity. Indeed, the combined effects of material properties, processing parameters and wheel wear on the surface generation and damage mechanism still need further investigation in ultra-precision diamond grinding of brittle materials. Besides, as a potential candidate for machining functional surface, ultra-precision grinding with a sharp edge wheel has been developed, but its wide application still need more efforts and exploration. In this thesis, the theoretical and experimental study on the surface damage mechanism and surface generation of typical engineering ceramics (WC/Co and RB-SiC/Si) under ultra-precision grinding with a sharp edge wheel is divided into four parts. In the first part, Vickers-indentation and single point diamond scratch tests are firstly utilized to investigate the damage mechanism induced by the interaction between the abrasive grit and workpiece materials. Moreover, to analyze the surface damage mechanism in high spindle speed grinding (HSSG), a novel plunge grinding experiment is conducted at the creep feed condition, and the typical surface characteristics between WC/Co and RB-SiC/Si are analyzed and compared. The second part is dedicated to investigate the grinding induced surface damage mechanism and the surface generation in relation to the machining parameters, spindle vibration and material removal rate. New grinding induced damage mechanisms, such as amorphization of SiC, preferred phase growth and the impact of C segregation, are identified. In addition, the non-uniform surface finish of a specific workpiece is explored experimentally and theoretically.|
The effects of materials microstructure on the surface damage mechanism and surface generation is then studied in the third part, focusing on the effect of binder addition. Even though Co and Si can improve the density and toughness of bulk materials, the different mechanical properties between the composition phases and the existence of phase boundaries both contributed to the non-uniform material removal rate and resulted in the formation of reliefs, edge chipping and grain dislodgement. Moreover, the phase transformation induced volume change of Si and the extrusion of Co under the dynamic pressure of the diamond grits lead to the generation of projections on the machined surface. No obvious oxidation of Co and Si occurred for WC/Co and RB-SiC/Si under high spindle speed grinding (HSSG) with minimum quantity lubrication (MQL). In the fourth part, the wear mechanism of the diamond wheel is studied, and its impact on the surface generation is analyzed. The wheel wear mechanism involves rapid loss of the sharp edge, grit splintering, flattening, and oxidation. Two appropriate dressing methods are proposed to obtain a sharp edge on the diamond wheel. With the well prepared wheels, two types of functional surfaces are machined by ultra-precision grinding. The results showed that with the proposed grinding protocol, the form accuracy and surface finish could reach 0.28 μm (PV), 9 nm (Ra) for the Φ15 mm TiC based hemisphere couples and 0.64 μm (PV), 6 nm (Ra) for the Φ20 mm 'Water-drop' surface on binderless tungsten carbide. The originality and significance of the present research is shown in the following three aspects: (i) new grinding induced surface damage mechanism is identified, including the impact of C segregation, preferred phase growth, etc., so the present research contributes to the understanding of the machining induced surface damage mechanism; (ii) the effects of materials microstructure and wheel wear on the surface characteristics of typical engineering carbides in ultra-precision grinding with a sharp edge wheel are analyzed, and comprehensive knowledge of the surface generation in ultra-precision grinding of brittle materials is achieved; (iii) this study provides clear comprehension on the technique to machine functional surfaces by wheel normal grinding, and it promotes the development of the grinding technology of hard and brittle materials.
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