Experimental investigation on electrical discharge diamond grinding of RB-SiC ceramics

Journal of the European Ceramic Society

Luo

et al. 2019

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Thermal-assistant is considered potentially as an effective approach to improve machinability of hard and brittle materials. Understanding the material removal and friction behaviour influenced by the purposely introduced heat is crucial to obtain high quality machined surface. This paper aims to reveal material removal and friction behaviours of RB-SiC ceramics scratched by a Vickers indenter at elevated temperatures. Material removal mode, scratching hardness, critical depth of ductile-brittle transition, scratching force and friction were discussed under different penetration depths. Size effect of scratching hardness was used to assess the plastic deformation at elevated temperature. A modified model was established to predict the critical depth at elevated temperatures by taking into account of the changes of mechanical properties. The results revealed that the material deformation and adhesive behaviour enhanced the material removal in ductile regime and the coefficient of friction at elevated temperatures.

Section: Introductionmentioning

confidence: 99%

Material removal mode and friction behaviour of RB-SiC ceramics during scratching at elevated temperatures

Journal of the European Ceramic Society

Luo

et al. 2019

Self Cite

“…Reaction-bonded silicon carbide (RB-SiC), as one of the most prosperous material for substrate of lightweight space optical mirrors [1,2], is very difficult to machine because of its high hardness and brittleness. In recent years, some thermal-involved hybrid processes, such as laser assisted machining [3][4][5], ultrasonic assisted electrical discharge machining [6,7], ED milling and mechanical grinding compound process [8][9][10] and electrical discharge diamond grinding [11], have been attempted to improve the machinability of SiC ceramics. However, the thermal effect in these processes has a significant influence on the mechanical property of the machined surface material, which will change the material removal mode in the hybrid machining process, especially in the hybrid process combined with a mechanical action such as milling, turning and grinding.…”

Section: Introductionmentioning

confidence: 99%

Characterization of hardness, elastic modulus and fracture toughness of RB-SiC ceramics at elevated temperature by Vickers test

Materials Science and Engineering: A

Luo

et al. 2019

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In this paper, mechanical properties of RB-SiC ceramics, such as hardness, elastic modulus and fracture toughness, are characterized through indentation technique using a Vickers indenter at elevated temperatures ranging from room temperature to 1200 °C realized by laser heating.. The indentation size effect, load-displacement curves and relationship between crack length and applied load are studied in order to determine hardness, elastic modulus and fracture toughness accurately. The results show that the Meyer's index and Vickers hardness decrease with the increase temperature. It indicates that the permanent plastic deformation of RB-SiC ceramics is mainly responsible for the indentation size effect and the reduction of hardness at elevated temperature. Both material softening and plastic deformation will contribute to the indentation creep at elevated temperature as shown in the load-displacement curves. The elastic modulus decreases with the increase of temperature due to increase of contact depth as a result of less elastic recovery. In the indentation test for calculating fracture toughness, only radial-median cracks are identified by the relationship between crack length and applied load at all temperatures, although the fracture mode observed at the indent corner changes from transgranular at room temperature to intergranular at elevated temperature. As more energy is consumed by intergranular facture and cracking-healing takes place due to oxidation, only short crack length appears in the indentation test which implies an increase of fracture toughness with the increase of temperature. However, this tendency has an exception at the highest temperature of 1200℃. This is because the free Si softening in RB-SiC specimen fails to resist crack propagation at extremely high temperature. Consequently, the crack length increases again which leads to the increase of the calculating fracture toughness at the highest temperature. These variations of hardness, elastic modulus and fracture toughness with temperatures will account for the possible change of material removal regimes occurred in some thermal-involved hybrid machining of RB-SiC ceramics.

“…After the EDDG process, to investigate the subsurface damage at different dischargeenergy levels, a TEM specimen was prepared. In a previous study [5], the machined surface of an RB-SiC sample subjected to EDDG was characterized by an EDM zone and a grinding zone. A cross-sectional TEM specimen was prepared with a boundary between the two zones to investigate the effects of EDM and diamond grinding on the subsurface damage, as shown in Fig.…”

Section: Specimen Preparation For Tem Detectionmentioning

confidence: 99%

“…Additionally, efficient grinding of large-scale RB-SiC mirrors faces the problems of serious clogging and wear of the metal-bonded grinding wheel [3,4]. To solve these problems, a hybrid process called electrical discharge diamond grinding (EDDG) has been employed for grinding RB-SiC by introducing electrical discharge machining (EDM) [5]. Owing to the effects of the discharge-induced erosion on the metal bonds of the grinding wheel and the workpiece, this process has the advantages of reduced grinding force [6], improved material removal rate [7,8], and effective declogging of the wheel surface [9].…”

Section: Introductionmentioning

confidence: 99%

Surface and subsurface damage of reaction-bonded silicon carbide induced by electrical discharge diamond grinding

International Journal of Machine Tools and Manufacture

et al. 2020

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Reaction-bonded silicon carbide (RB-SiC) ceramic, one of the best candidates for large optical mirrors, is difficult to machine because of its high hardness and brittleness. A hybrid process called electrical discharge diamond grinding (EDDG) exhibits potential for improving the machinability of RB-SiC by combining electrical discharge machining (EDM) and diamond grinding. However, this hybrid process leads to damages that differ from those in conventional processes owing to the simultaneous actions of EDM and diamond grinding. In the present study, surface and subsurface damages induced by the interactions between EDM and diamond grinding during the EDDG of RB-SiC were examined. The effect of the discharge energy was considered.The surface and subsurface topographies and microstructures were characterized via scanning electron microscopy, Raman spectroscopy, and transmission electron microscopy. The EDM and grinding zones exhibited distinctive surface topographies and different dominant material removal mechanisms. An increase in the discharge energy facilitated ductile removal of the material and decomposition of SiC. Thus, a thinner subsurface damage layer was obtained compared with that in the less-thermally affected zone. The decomposed C and material migration tended to increase with the discharge energy. Owing to the interactions between EDM and diamond grinding, the subsurface was a mixture of amorphous/crystalline C, polycrystalline/nanocrystalline