Efficient processing of reaction-sintered silicon carbide by anodically oxidation-assisted polishing

Abstract. Reaction-sintered silicon carbide (RS-SiC) is a promising optical material for the space telescope systems. Anodically oxidation-assisted polishing is a method to machine RS-SiC. The electrolyte used in this study is a mixture of hydrogen peroxide (H2O2) and hydrochloric acid (HCl), and the oxidation potential has two modes: constant potential and high-frequency-square-wave potential. Oxide morphologies are compared by scanning electron microscope/energy dispersive x-ray spectroscopy and scanning white-light interferometer. The results indicate that anodic oxidation under constant potential can not only obtain a relatively smooth surface but also be propitious to obtain high material removal rate. The oxidation depth in anodic oxidation under constant potential is calculated by comparing surface morphologies before and after hydrofluoric acid etching. The theoretical oxidation rate is 5.3  nm/s based on the linear Deal–Grove model. Polishing of the oxidized RS-SiC is conducted to validate the machinability of the oxide layer. The obtained surface roughness root-mean-square is around 4.5 nm. Thus, anodically oxidation-assisted polishing can be considered as an efficient method, which can fill the performance gap between the rough figuring and fine finishing of RS-SiC. It can improve the machining quality of RS-SiC parts and promote the application of RS-SiC products.


Introduction
Reaction-sintered silicon carbide (RS-SiC) is a promising material for making optical mirrors in space telescope systems due to its excellent physical, mechanical, and chemical properties. [1][2][3] With the increasing standard for ultrasmooth surface finishing and the demand for high material removal rate (MRR) in the machining of RS-SiC substrate, methods for processing RS-SiC have become the research focus in the fields of optics and ceramics. 4,5 However, RS-SiC is a known difficult-to-machine material because of its high hardness and chemical inertness. 3 These properties make it inefficient to process RS-SiC with traditional mechanical or chemical methods, such as diamond turning 6,7 or plasma chemical vaporization machining (PCVM). 8 Meanwhile, the RS-SiC substrate is mainly composed of SiC and Si. 9,10 Thus, the differences in physical hardness and chemical activity between the nonuniform SiC grains and Si grains make the homogeneous removal of RS-SiC almost impossible. 11 Therefore, a novel method for processing RS-SiC is urgently demanded to increase the MRR in the rough figuring process and to improve the surface quality in the fine finishing process.
In this research, a novel processing method, named anodically oxidation-assisted polishing, is proposed for the efficient machining of RS-SiC substrate. The method includes two important steps: generation of oxide layer on the RS-SiC surface by means of anodic oxidation and the validation of machinability of the oxidized RS-SiC sample by subsequently polishing the oxide using traditional ceria slurry abrasive. Anodic oxidation is a classical method for surface modification. 3,12 The electrolyte used in anodic oxidation of RS-SiC, which consists of hydrogen peroxide (H 2 O 2 ) and hydrochloric acid (HCl), has been proven as an effective oxidant to oxidize SiC specimen. 13,14 The anodic oxidation system is constructed to generate an oxide layer on RS-SiC, while a ceria slurry polishing system is built to verify the machinability of the oxide. The potential used in anodic oxidation of RS-SiC has two modes: constant potential and high-frequency-square-wave (HFSW) potential. The oxide morphologies obtained in the two oxidation modes are compared by the scanning electron microscope/ energy dispersive x-ray spectroscopy (SEM-EDX) observation and the scanning white-light interferometer (SWLI) detection. Afterward, the anodic oxidation depth of RS-SiC corresponding to a certain oxidation time in the constant potential mode is calculated by comparing the surface morphologies before and after the hydrofluoric acid (HF) etching. The oxidation rate is calculated based on the linear Deal-Grove model. Through tracing the surface qualities by the SWLI measurement after abrasive polishing for a certain time, the machinability of the oxide layer, which is obtained by anodic oxidation of RS-SiC, is validated. *Address all correspondence to: Xinmin Shen, E-mail: shenxmjfjlgdx2014@163 .com 2 Experimental Setup The system for anodic oxidation of RS-SiC in the constant potential mode is shown in Fig. 1(a) 3 and that for anodic oxidation of RS-SiC in the HFSW potential mode is shown in Fig. 1(b). The parameters in anodic oxidation under constant potential are summarized in Table 1 3 and those in anodic oxidation under HFSW potential are summarized in Table 2. Afterward, the oxidized RS-SiC sample is investigated by SEM-EDX observation and SWLI detection, and it is used to evaluate the two oxidation modes.
In order to make a fair comparison of the oxide morphologies obtained in the two oxidation modes, the equivalent oxidation potentials in both anodic oxidation modes are both set to 9.8 V. Meanwhile, the oxidation time for both cases is also set to be the same (2 min).     . Therefore, further investigation is needed to verify the detailed differences in the oxide morphologies between the two anodic oxidation modes.
To further investigate the differences of oxide morphologies between the two modes, SEM observation with high magnification has been conducted. The results are shown in Figs. 4 and 5. Under constant potential, there are protuberances and cracks on the oxidized RS-SiC sample, most of which are at the boundaries of the original SiC grains and Si grains, as shown in Fig. 4(b). The protuberances appear at the oxide generated from initial SiC grains, while surfaces of the oxide generated from initial Si grains are relatively smooth, as shown in Fig. 4(c). The fringe region of a certain SiC grain has a higher oxidation rate relative to the central area, as shown in Fig. 4(d). The reason for this phenomenon could be the asymmetric distribution of the oxidation rate during the oxidation process. 3 The oxidation of SiC/Si to SiO 2 is a volume expansion process, which generates the swelling pressure. The distribution of this pressure is determined by the distribution of the oxidation rate. Thus, in the thickness direction, the pressure ejects the oxide, resulting in the formation of protuberances; in the plane direction, the pressure presses the neighboring grains, causing the introduction of cracks in the weak areas. Meanwhile, there are Tu et al.: Efficient processing of reaction-sintered silicon carbide by anodically oxidation-assisted polishing many holes and ditches on the oxidized RS-SiC obtained in the HFSW potential mode, as shown in Fig. 5(a). The width of some ditches reaches 100 nm, as shown in Figs. 5(b) and 5(d), while the radius of some holes reaches 500 nm, as shown in Fig. 5(c). These holes and ditches on the oxidized RS-SiC sample are caused by the unsteady and desultory volume expansion force generated in the HFSW potential mode. Obviously, the ditches and holes on the oxidized RS-SiC obtained in the HFSW potential mode is difficult to remove relative to the protuberances and cracks on that generated in the constant potential mode. For the purpose of quantificational analysis of the oxidized surface, SWLI measurement is conducted to evaluate the oxidized samples, as shown in Fig. 6. The surface roughnesses, root mean square (rms), and roughness average (Ra) of the oxidized RS-SiC surface in the constant potential mode are 266.968 and 177.084 nm, while those under HFSW potential are 369.634 and 293.535 nm, respectively. The results indicate that the surface quality of the constant potential case is better than that of the HFSW potential case. Therefore, this study will focus on the anodic oxidation of RS-SiC in the constant potential mode.

Calculation of Oxidation Rate
By HF etching of the oxidized RS-SiC samples, the oxide product silica (SiO 2 ) of RS-SiC can be removed Therefore, by comparing surface morphologies of the oxidized RS-SiC samples before and after HF etching, the oxidation depth can be obtained. Experiments were carried out in the constant potential mode with the parameters in Table 1. The measurement results of oxidation depth after a certain oxidation time are shown in Fig. 7.
Taking the SWLI measurement result in Fig. 7(a), for example, the oxide SiO 2 in the oxidized area is etched by the HF, which makes a "step" at the boundary region between the oxidized area and the nonoxidized area. By calculating the average height difference of the step, the oxidation depth can be obtained. The calculated anodic oxidation depths are 25, 38, 47, 58, 68, 88, 102, 120, 135, 145, 156, and 163 nm, respectively, and the corresponding oxidation times are 5, 10, 20, 30, 60, 120, 180, 300, 600, 900, 1200, and 1800 s. For the purpose of calculating the oxidation rate in the constant potential mode according to the classical Deal-Grove model, 3,15 the relationships between the anodic oxidation depth and the corresponding oxidation time are summarized in Fig. 8. It can be observed that the increasing rate of the oxidation depth reduces when the anodic oxidation time increases. This is because the continuously widened oxide layer counteracts the further occurrence of the oxidation process.
Through data fitting (isolated singularities are filtered) based on the Deal-Grove model, 15 the relationship between the oxidation depth x (nm) and the oxidation time t (s) for anodic oxidation of RS-SiC by H 2 O 2 and HCl under the constant potential can be obtained, as shown in Eq. (2). Therefore, in the simultaneous anodically oxidation-assisted polishing, in which the anodic oxidation of RS-SiC and the abrasive polishing of oxide layer are carried out at the same time, the theoretical oxidation rate is 5.3 nm∕s. In other words, the MRR in this machining process can reach 5.3 nm∕s (0.318 μm∕ min). This MRR is similar to the MRRs obtained from the other rough figuring methods for machining of RS-SiC, and is much larger than those obtained from those fine finishing techniques. Meanwhile, the oxidation depth after oxidation of RS-SiC for 30 min can reach 0.163 μm. Thus, nonsimultaneous anodically oxidation-assisted polishing, in which the abrasive polishing of oxide layer is carried out after the anodic oxidation of RS-SiC, is also a feasible method for machining RS-SiC samples.

Investigation of Polishing Property
In order to further investigate the machinability of the oxide layer obtained in anodic oxidation of RS-SiC by the mixture of H 2 O 2 and HCl under the constant potential, the oxidized sample is polished with the ceria slurry polishing system, as shown in Fig. 9. 3 The reason for applying the ceria slurry polishing method is that it is easier to obtain high MRR and ultrasmooth surface simultaneously in the polishing of silica (SiO 2 ) by CeO 2 in the chemical mechanical polishing. 16 The oxidized RS-SiC sample is fixed on the stage, while the oxidized surface contacts the ceria slurry. The polishing pad is immersed into the slurry, and the oxidized RS-SiC surface is abraded by the polishing pad. The load is adjusted by changing weights on the load plate. The polishing parameters are summarized in Table 3. The anodic oxidation parameters for the preparation of the oxidized RS-SiC samples are the same as those parameters in Table 1. The anodic oxidation time is 10 min.  During the polishing process, the RS-SiC sample is detected by the SWLI measurement after polishing for a certain time. The obtained data corresponding to an oxidation time from 20 to 240 min at the interval of 20 min is exhibited from Figs. 10(a)-10(l). The relationship between the surface roughnesses (rms and Ra) and the polishing time is summarized in Fig. 11.
The oxidized RS-SiC surface is very rough, which can be judged from the data in Fig. 6(a). The entire polishing process can be divided into three periods: the removal of protuberances (the first 60 min), the removal of cracks (from 60 to 140 min), and overpolishing (the remaining time). During the period of removing protuberances, the surface roughness rms is rapidly improved to 39.453 nm, because the component of the oxide protuberance is soft SiO 2 , which can be easily removed. During the period of removing cracks, the surface roughness rms is improved from 39.453 to 5.002 nm, since most of the cracks penetrate into the inside of the oxide and the MRR in this period is relatively low. During the period of overpolishing, the surface roughness rms stays around 4.5 nm with little undulation, because the oxide has already been removed and the MRR for polishing hard RS-SiC with soft ceria particles is extremely low.
Anodically oxidation-assisted polishing of RS-SiC can obtain a smooth surface with roughness rms around 4.5 nm. Although the surface property is not as good as that obtained with the fine finishing methods, it is obviously better than those obtained with the rough figuring methods, such as PCVM and diamond turning. Moreover, from the calculation of oxidation rate, it can be found that the MRR in simultaneous anodically oxidation-assisted polishing of RS-SiC can reach 0.318 μm∕ min, which is close to those in the rough figuring methods and is much larger than those in the fine finishing methods. Therefore, anodically oxidation-assisted polishing can be considered as an efficient processing method, which can fill the performance gap between the rough figuring and fine finishing of RS-SiC.

Conclusions
Anodically oxidation-assisted polishing is conducted in this research for efficient machining of RS-SiC. The electrolyte used in anodic oxidation of RS-SiC is a mixture of H 2 O 2 and HCl, and the applied potential has two modes: the constant potential and HFSW potential. The oxide morphologies in the two oxidation modes are obtained using SEM-EDX observation and SWLI detection. The comparison results indicate that anodic oxidation under constant potential can not only obtain a smooth surface but also be propitious to achieve high MRR with simultaneous anodically oxidation-assisted polishing. By measuring the anodic oxidation depth with different oxidation times, a theoretical oxidation    rate based on the linear Deal-Grove model is obtained, which shows that the MRR in simultaneous anodically oxidationassisted polishing of RS-SiC can reach 0.318 μm∕ min. By ceria slurry polishing of the oxidized RS-SiC, the machinability of the oxide is investigated using the SWLI measurement, and the obtained surface roughness rms is around 4.5 nm.
Based on the results in this research, anodically oxidationassisted polishing can be considered as an efficient processing method, which can fill the performance gap between the rough figuring and fine finishing of RS-SiC. Also, it will improve the processing quality of RS-SiC parts and promote the application of RS-SiC products in the optical and ceramic fields. Biographies for the other authors are not available. Fig. 11 The relationship between surface roughnesses (rms and Ra) with the corresponding polishing time.