Investigation on machinability of the oxide layer in anodic oxidation of reaction-sintered silicon carbide by pure-water

Oxidation-assisted polishing has been developed as an efficient method for precision machining of reaction-sintered silicon carbide (RS-SiC), and characteristic of the oxide layer is the critical factor to obtain a fine surface property. Machinability of the oxide layer obtained in anodic oxidation of RS-SiC by the pure-water under high-frequency-square-wave potential is investigated by the ceria slurry polishing in this study. Quantitative analysis of surface quality of the oxidized RS-SiC sample is conducted by the scanning white light interferometer (SWLI) measurement. Along with increase of the oxidation time, surface quality of the oxidized RS-SiC is changing better at the beginning, and rapidly deteriorated in further oxidation process. Surface qualities of the RS-SiC sample before oxidation, after oxidation, after HF etching, and after abrasive polishing, are compared by the SWLI measurement. Surface roughness rms after anodic oxidation for 60min is 189.004nm. After removing the oxide by ceria slurry polishing, the rms can reach 3.688nm. Meanwhile, there is no visible scratch on the new revealed surface. Therefore, combination of anodic oxidation of RS-SiC by pure water and abrasive polishing of oxide layer by ceria slurry can be considered as an efficient method to machine RS-SiC.


Introduction
Oxidation-assisted polishing is a novel developed method for precision machining of reaction-sintered silicon carbide (RS-SiC), which is aim to promote the material removal rate (MRR) in figuring process and improve the surface quality in finishing process [1][2][3]. Relative to the ultraviolet Irradiation [4,5], thermal oxidation [6][7][8], or plasma oxidation [9,10], the anodic oxidation has the advantages of a high oxidation rate [11][12][13][14], which makes it a focus of research in the machining of RS-SiC.
Several electrolytes have been used to anodically oxidize RS-SiC, such as phosphoric acid (H 3 PO 4 ) [11], hydrogen peroxide (H 2 O 2 ) accompanied with hydrochloric acid (HCl) [13], and so on. However, fabrication of these electrolytes and post-process of these abandoned electrolytes have pollution to the environment [15]. Thus, anodic oxidation of RS-SiC by pure water only under high-frequency-squarewave potential is conducted, and machinability of oxide layer is investigated by ceria slurry polishing in this research. Quantitative analysis of surface quality of the oxidized RS-SiC is conducted by the scanning white light interferometer (SWLI) detection [16][17][18]. Meanwhile, surface qualities of RS-SiC before oxidation, after oxidation, after HF etching, and after abrasive polishing, are compared. By analysis of surface quality and validation of machinability of the oxide layer, the feasibility to machine RS-SiC through anodic oxidation by pure water with assistance of abrasive polishing are conducted.

Experimental design
The abrasive polishing system shown in figure 1 is used to validate machinability of the oxide in the anodic oxidation of RS-SiC. The abrasive is ceria (CeO 2 ) slurry. The oxidized sample is fixed on the stage, and surface of the oxidized sample contacts the ceria slurry. The polishing pad is immersed into the ceria slurry, and surface of the oxidized RS-SiC sample is abraded by the polishing pad. The load can be adjusted by adjusting the weights on the load plate. The oxidized RS-SiC sample, which was oxidized for 60min, is polished by the abrasive polishing system. Parameters in the abrasive polishing process are summarized in Table 1. Afterwards, the polished sample is detected by SEM observation to investigate the surface morphologies and by SWLI measurement to evaluate the surface qualities.

Results and discussions
For the purpose of quantitative analysis of the oxide morphologies, the initial surface obtained by the diamond lapping, the oxidized surface when the oxidation time is 10min, and that when the oxidation time is 60min are investigated by SWLI measurement, respectively. The results are summarized in the figure 2. Since hardness of RS-SiC is smaller than that of diamond, there are scratches on the original surface, as shown in figure 2(a). Surface roughness rms of the original RS-SiC sample in figure 2(a) is 2.814nm. After oxidation for 10min, the surface roughness rms is improved to 2.144nm, as shown in figure 2(b). The reason for this phenomenon is that the oxidation of RS-SiC is a volume expansion process. Therefore, the expanded oxides can fill the scratches which are generated from the diamond lapping of RS-SiC. Meanwhile, the oxidation time is short, so the projections and cracks generated by the volume expansion force are not obvious. However, when the oxidation time reaches 60min, the surface roughness rms is rapidly deteriorated to 189.004nm, as shown in figure 2(c). The reason for this phenomenon is that sizes of the projects and cracks are gradually increasing.  figure 3(b) indicate that there are some tiny scratches on the polished surface. We hypothesis there are two reasons for generation of these tiny scratches. Firstly, the initial RS-SiC surface has scratches which are generated by the diamond lapping process, and these scratches are filled and closed by the volume expansion of the oxides in the anodic oxidation process. However, during polishing process, these regions have a relatively higher micro-MRR; thus, the scratches are revealed. Nevertheless, sizes of them are reduced, as shown in the figure 4. Secondly, distribution of the oxide on the oxidized RS-SiC surface is asymmetric among different Si grains and different SiC grains, and there are plenty of cracks and projections on the oxidized surface, especially at the oxides generated from initial SiC grains, which are common characters in the anodic oxidation of RS-SiC. So polishing of the oxide layer is unavoidable to generate scratches, because the material removal is realized by the abrading between the oxide and the nano-scale ceria particles. Although tiny scratches are inevitable generated in combination of the anodic oxidation of RS-SiC and the abrasive polishing of the oxide layer, sizes and depths of these scratches are reduced relative to those on the initial RS-SiC surface obtained by diamond lapping in figure 2(a).

Conclusions
Machinability of the oxide layer obtained in anodic oxidation of RS-SiC by pure-water is investigated by ceria slurry polishing in this study. The following conclusions can be obtained.
(1) Quantitative analysis of the surface quality of the oxidized RS-SiC sample has been conducted by the SWLI measurement. Along with the increase of oxidation time, surface quality of the oxidized RS-SiC is changing better at the beginning, and rapidly deteriorated in the further oxidation process.
(2) Machinability of the oxide layer has been validated by the abrasive polishing of the oxidized RS-SiC. After the abrasive polishing, the obtained surface roughness rms is 3.688nm, which is better than the surface roughness rms 189.004nm of the oxidized RS-SiC sample. What's more, there is no visible scratch on the polished surface, which indicates that this anodic oxidation method is feasible.