Recent years, titanium has been paid attention as a high vacuum material because of lightweight, non-magnetic material, a low Yong's modulus, and a small thermal expansion coefficient. The vacuum characteristics for some kinds of titanium materials, pure titanium of JIS grade 2, Ti-3Al-2.5V and Ti-0.35Fe-0.35O, have been investigated, and their outgassing rates are clarified to be lower than that for the standard materials. The mechano-chemical polishing was found to be effective to improve the outgassing rate for titanium. However, mechano-chemical polishing is not easily applied to complicated shaped chambers and components, because this process combines the wet polishing with the mechanical polishing. Other polishing processes were hardly evaluated so far. In this study, the chemical polishing for titanium materials was developed, and the surface condition and outgassing property for chemically polished titanium materials were discussed.
Unpolished basic metal Ti(BM), buff polished Ti(BP), mechano-chemically polished Ti(MCP) and chemically polished Ti(CP) pure titanium materials, JIS grade 2, and chemically polished stainless steel SUS(CP) were prepared as reference. Surface roughness was estimated by a stylus-type profilometer in a 1.25 mm range and an atomic force microscopy (AFM) in an 1 µm2 area. The surface oxide layer thickness, depth profiles were analyzed by an auger electron spectrometer (AES) with Ar etching gas, and transmitting electron microscope (TEM) images were observed. The outgassing rates were estimated with a modified orifice method, switching between two pumping paths (SPP), which detection limit of the outgassing rate is 7×10-13 Pams-1.
The surface roughness for TiCP is smaller than those for TiBM and TiBP samples, and is comparable to that for MCP in 1 µm2 of microscopic range. The surface oxide layer thickness for TiCP sample is 8 nm, which is smaller than those of the other polished samples. The thin oxide surface layer is considered to act as barrier for bulk gas, mainly hydrogen, and diffusion, resulting in low outgassing. The outgassing rates for TiCP correspond to 7×10-13 Pams-1 after the baking processes, which is two orders magnitude smaller than that of SUSCP and comparable to that of TiMCP. |