Various commercially available surface treatments are being explored for use on stainless steel components in mass spectrometer inlet systems. Type A-286 stainless steel coupons, approximately 12.5 mm in diameter and 3 mm thick, were passivated with one of five different surface treatments; an untreated coupon served as a control. The surface and near-surface microstructure and chemistry of the coupons was investigated using sputter depth profiling using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). All the surface treatments studied appeared to change the surface morphology dramatically, as evidenced by lack of tool marks on the treated samples in SEM images. In terms of the passivation treatment, Vendors AD appeared to have oxide layers that were very similar in thickness to each other (0.7-0.9 nm thick), as well as to the untreated samples (the untreated sample oxide layers appeared to be somewhat larger). Vendor E's silicon coating appears to be on the order of 200 nm thick.
Although the stainless steel surface has long been a subject of study [1] due to its technological relevance, the surface passivation of A-286 [2] has not been investigated as extensively as more common stainless steels [1]. A-286 (UNS 66286) is notable in its difference from the type 304 and 316 stainless steels more commonly used in vacuum systems, in that the concentration of nickel in A-286 is greater than the concentration of chromium and the nickel content is quite high at 23-25 atomic percent [3]. Another important difference is the presence of 2.2-2.7 atomic percent titanium. In previous work [2] with passivated A-286, titanium was found at the surface in concentrations exceeding its bulk composition. Otherwise though, the A-286 depth profile [2] appears qualitatively similar to those seen in 300 series stainless steels [2,[4][5][6][7][8][9], with an adventitious carbon layer that is gradually sputtered away and an oxide layer that is indicative of the passivated layer [2,[4][5][6][7][8][9]. Previously, the oxide layer seen in A-286 was 4 nm [2]. The oxide layers for untreated 300 series stainless steels ranged from 1 to 3.5 nm [4][5][6], while the oxide layer for specially treated 300 series stainless steels ranged from 25 nm to 2 microns thick [5][6][7][8]. Scanning electron microscopy (SEM) images from the 300 series stainless steels show a relatively smooth and flat untreated surface [4][5][6], except in reference [7], which shows a relatively rough surface for the untreated stainless steel. However, with passivation treatment, these surfaces become notably rougher [5][6][7] except for a flow polishing treatment in ref.[4].A-286 stainless steel coupons approximately 0.5" in diameter and 0.125" thick were passivated with five different surface treatments and an untreated coupon was left as a control. These surface treatments are being explored for use in mass spectrometer inlet systems, where it is very important to have inlet systems which do not change the composition of the analyte measured. These coupons were made to allow surface analysis of the surface treatments using well-known surface analysis techniques. Depth profiles using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were performed on these coupons to characterize the surface and near surface regions. Scanning electron microscope (SEM) images were collected using the AES instrument. All of the surface treatments studied here appear to change the surface morphology dramatically, as evidenced by lack of tool marks on the treated samples. In terms of the passivation treatment, Vendors A-D appeared to have oxide layers that were very similar in thickness to each other (0.7-0.9 nm thick) as well as the untreated samples (the untreated sample oxide layers appeared to be somewhat larger, see Table 2). Vendor E's silicon coating appears to be on the order of 200 nm thick.Morphologically, all of the surface treatments studied here appear to change the surface morphology dramatically, as evidenced by lack of tool mark...
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