Smoothing of reaction sintered silicon carbide using plasma assisted polishing

Abstract

We proposed a novel polishing technique named plasma assisted polishing (PAP) for the finishing of some difficult-to-machine materials, such as diamond, silicon carbide, sapphire and so on. In this work, PAP was applied to the smoothing of reaction sintered silicon carbide (RS-SiC) surface. Experimental results indicated that a higher oxidation rate led to a smoother surface. The smoothing mechanism was discussed based on the relationship between oxidation rate of RS-SiC and polishing rate of oxide layer. Through increase in the oxidation rate and decrease in the polishing rate, a smooth surface with 11.42 nm p–v and 0.91 nm rms was obtained. The amount and depth of scratches also decreased after PAP using ceria abrasives compared with the surface polished by diamond abrasives.

Introduction

Reaction sintered SiC (RS-SiC) is manufactured by reaction sintering process just as it's named. In reaction sintering process, original SiC powder is firstly mixed with graphite powder and then reacts with molten silicon under a high temperature, through which newly sintered SiC can be obtained [1], [2]. RS-SiC has several excellent mechanical and chemical properties, such as a high hardness, a strong bending strength, a good thermal conductivity and so on. These properties enable RS-SiC to be applied in harsh environmental conditions. Nowadays, RS-SiC has been considered as one of the most promising ceramic material for glass lens molds and substrate of lightweight space telescope mirrors.

However, there are several reasons, such as its high hardness, chemical inertness and multiphase, make RS-SiC difficult to be precisely figured with high integrity surface. Since SiC is harder than most other materials except diamond, the available material for removing of RS-SiC is very limited. Also, due to its chemical inertness, RS-SiC is difficult to react with chemicals. That is to say it is inefficient to remove RS-SiC through traditional chemical methods such as etching or electrochemical machining. What's more, as it was once reported, α-SiC, β-SiC and residual silicon were the main compositions of RS-SiC [1], [2]. These different compositions are different in hardness and chemical activity. Thus, homogeneous polishing of RS-SiC is almost impossible, which make it difficult to obtain a flat and scratch-free smooth surface through chemical or mechanical methods.

Researches on the flattening of RS-SiC have been widely conducted. Yan et al. [3] and Zhang et al. [4] conducted diamond turning experiments on RS-SiC. A very high material removal rate was obtained in this method. However, much dislodgement of SiC grains was observed on the processed surface. Gao et al. [5] polished RS-SiC using diamond abrasives and discussed the generation mechanism of surface and subsurface damages. Novel polishing techniques were also applied to RS-SiC. Computer controlled optical surfacing was used for ultra-precision polishing of large aperture RS-SiC mirrors in which a smooth surface with rms roughness less than 1 nm was obtained [6]. Magnetorheological finishing of RS-SiC was conducted by Cheng et al. [7] in which an optical surface was obtained after 8 h smoothing. In all of the above mentioned processes, diamond abrasive was used as the machining media. Due to the high hardness of diamond, introduction of scratches was difficult to avoid. Till now, scratch-free finishing of RS-SiC is seldom reported.

Plasma assisted polishing (PAP) was proposed for the finishing of difficult-to-machine materials [8], [9], [10]. In PAP, oxidation by atmospheric water vapor plasma and polishing by soft abrasive were efficiently combined. The surface of workpiece was firstly oxidized by water vapor plasma, and then the oxidation products were removed by soft abrasive polishing. Since these processes were repeated, a smooth surface could be finally obtained. In our previous research, PAP was applied to single crystal SiC, in which an atomically smooth surface without scratches and subsurface damages was obtained [10].

In this work, PAP was applied to finishing of RS-SiC. The characteristic of processed surfaces was evaluated. The smoothing mechanism was also discussed and experimentally proved.