Study of oxide protective layers on stainless steel by AES, EDS and XPS

Transcription

1 Research Article Received: 23 July 2007 Revised: 15 November 2007 Accepted: 17 November 2007 Published online in Wiley Interscience: 20 February 2008 ( DOI /sia.2718 Study of oxide protective layers on stainless steel by AES, EDS and XPS Djordje Mandrino, Matjaž Godec, Matjaž Torkar and Monika Jenko A protective oxide layer was formed on the surface of stainless steel by standard industrial-type thermal treatment. Additional post-thermal treatment annealing and polishing followed by annealing were applied in an attempt to improve characteristics of the layers. Oxide layers formed on samples without and with different types of post-thermal treatment processing were analysed and compared. Several techniques were employed. SEMwasusedtoimagethesurface;AES,XPSandenergydispersive spectroscopy (EDS) were all used to probe the elemental composition of the oxide layers. AES was also used as the most straightforward method to obtain depth profiles. Low beam energy EDS was used in an attempt to obtain some fast averaged volume information from the layers. XPS was mainly used for determining chemical states on the surface of the sample but not for depth profiling (low sputtering rate due to large illuminated area). Analysed samples differed with regard to surface morphology of the oxide layers, oxide layer compositions and thicknesses. An attempt was made to relate these differences to differences in post-thermal treatment procedures. Copyright c 2008 John Wiley & Sons, Ltd. Keywords: AES; EDS; XPS; oxide layer; stainless steel Introduction Protective layers for use with stainless steels at high temperatures should exhibit adequate mechanical properties, should be resistant to chemical degradation by reaction with the environment, and should be easily fabricated. However, these are often conflicting requirements and materials used in practice are usually a compromise. To obtain the best functional and/or commercial results, protective layers with the most suitable mechanical properties are tested under environmental conditions very similar to those likely to be experienced in practice. The film developed should be adequately stoichiometric so that transport of metal or oxygen ions in the oxides lattice is slow. Cr 2 O 3 is reasonably protective in oxygen or air to about 900 C. At higher temperatures this oxide reacts further with oxygen to form CrO 3. [1] A continuous stoichiometry change may be preferred to an abrupt stoichiometry change, as in the case of the homogeneous layer, since it may improve adhesion of the protective layer to the substrate. [2] The aim of this study was to investigate the influence of post-thermal treatment processing of protective oxide layers produced by standard industrial-type procedure [3] on the AISI 321 stainless steel. Unprocessed and processed protective layers were analysed by SEM, AES, XPS and energy-dispersive spectroscopy (EDS). Morphologies, depth profiles and chemical information were obtained and compared. Experimental Protective oxide layers on the walls of stainless steel tubes of 10 mm diameter were formed during an actual technological process similar to the one for high temperature preparation of oxide layers in controlled atmosphere described in Ref. [3]. Stainless steel AISI 321 of nominal composition (also checked by EDS in another study [4] ) 68.3 at.% Fe, at.% Cr, at.% Ni, 0.4 at.% C, 2.0 at.% Si, and 2.0 at.% Mn was used. Additional post-thermal treatment processing of unpolished and polished surfaces was performed in tubular furnace in air at 1050 C. This temperature was achieved in an 8 min ramp, with samples kept at final temperature for 2 min and slowly cooled down after switching off the heaters. Commercial polishing paste was used for polishing of the samples. From these stainless steel surfaces with oxide protective layers platelet-like samples of approximately 5 10 mm 2 were cut out for AES, EDS and XPS analyses. Prepared samples were analysed by JEOL JSM 5610 SEM/EDS/WDS and by VG Microlab 310F SEM/AES/XPS. The former was operated at primary beam energies of 15 and 5 kev in EDS mode, corresponding to probing depths of the order of approximately and µm [5] in layers with predominantly Mn/Fe/Cr/Ni composition. In samples of layer thickness well below 1 µm and average layer density considerably decreased, due to oxide nature of the layer, lower primary beam energy may more likely derive considerable proportion of the compositional information from the layer only. Primary electron beam energies of 10 kev were used for AES depth profiling. Ar + wasusedof3kevenergyandorderof magnitude of 1 µa ion current over the area of 4 4mm 2. Rough estimate for sputtering rate at these parameters is approximately 1 nm/min. This is not inconsistent with some calibration measurements performed on metallic oxide type samples as well as with some reference data for sputtering rates forfeandcrandtheiroxides. [6] Also, sputtering rates differ somewhat between different samples since an effort was made to ensure them to be constant during individual depth profiles by keeping Ar pressure constant. However, they may have been much more severely influenced by the depth-dependent stoichiometry of the samples. Thus all information provided on depth or layer Correspondence to: Djordje Mandrino, Institute of Metals and Technology, Lepi Pot 11, 1000 Ljubljana, Slovenia. djordje.mandrino@imt.si Institute of Metals and Technology, Lepi Pot 11, 1000 Ljubljana, Slovenia 285 Surf. Interface Anal. 2008; 40: Copyright c 2008 John Wiley & Sons, Ltd.

2 D. Mandrino et al. up to 5% for high concentration components and deteriorates considerably towards the detectability limit ( 1at.%). Results and Discussion In Fig. 1 SEM images of unmodified, unpolished additionally annealed, and polished additionally annealed samples are shown. Distinct micron-sized crystallites can only be found on the surface of the sample additionally annealed after thermal processing. These somewhat resemble Cr 2 O 3 crystallites in the size range of µm that can be formed on a stainless steel 316L of an equally high chromium content as AISI 321 by electrochemical anodic roughening; [7] however, they are an order of magnitude larger. Also in some previous studies, protective oxide layers Figure 1. SEM images of unmodified (a), unpolished additionally annealed (b) and polished additionally annealed (c) samples. 286 thickness should be regarded as estimative, at best, based on roughly 1 nm/min sputtering rate estimate. For XPS measurements Mg K α radiation at ev with anode voltage emission current = 12.5kV 16 ma = 200 W power was used. Sputtering parameters for pre-measurement cleaning were no different from AES depth profiling apart from larger sputtering area. VG Microlab 310F acquisitioned data were processed by the Avantage v3.41 software produced and supplied by the manufacturer of the instrument and by the CasaXPS commercially available software for processing of the XPS and AES spectra. Average accuracy of atomic composition determination from EDS and AES is approximately 10% for both techniques as roughly estimated from data scatter in unprocessed spectra. It may improve Figure 2. AES depth profiles of unmodified (a), unpolished additionally annealed (b) and polished additionally annealed (c) samples. Copyright c 2008 John Wiley & Sons, Ltd. Surf. Interface Anal. 2008; 40:

3 Study of oxide protective layers growth in air produced grain-like morphology with average linear dimension of the grain close to the thickness of the layer (e.g. Ref. [3]). All the above suggest that this particular oxide layer is much thicker than the other two, probably due to the accelerated growth of the oxide onto much thinner oxide layer already formed by the primary thermal treatment. The absence of large crystallites on the surface of the polished and additionally annealed sample suggests that polishing effectively removed most of the primary oxide layer but a new one was formed by the additional annealing. AES depth profiles of unpolished additionally annealed and polished additionally annealed samples are shown in Fig. 2. Absolute values for depth scale estimates should be taken with some caution, however they are mutually comparable: it is probable that effective thickness of the oxide layer in Fig. 2(c) is some 20% lower than that of the layer in Fig. 2(a) as suggested by measurements. Unpolished additionally annealed sample shows highest effective thickness of approximately 2000 nm, several times more than the other two (Fig. 2(b)), corroborating what was suggested by SEM images. Chromium and oxygen are the components of the oxide layers with the highest concentrations in all three types of samples. Cr concentrations in the oxide layers are slightly increased with respect to the substrate concentrations. At temperatures above 950 C diffusion of iron through the Cr 2 O 3 layer is enhanced, and the Fe 3 O 4 starts to form throughout the Cr 2 O 3 layer. [8] This mixture of two oxide phases with delayed formation of iron oxide may explain why chromium oxide is the predominantcomponentof the oxide layer. Itis not, however, quite consistent with O/(Fe + Cr) atomic ratios in Fig. 3(a), (b) and (c). For Figure 3. O/(Fe + Cr) and Mn/(Fe + Cr + Ni) ratios of unmodified (a) and (d), unpolished additionally annealed (b) and (e) and polished additionally annealed (c) and (f) samples. 287 Surf. Interface Anal. 2008; 40: Copyright c 2008 John Wiley & Sons, Ltd.

4 D. Mandrino et al. the proposed mixture these ratios should be between 1.33 and 1.50, while their top values inside the oxide layers are somewhat over 3. This may be due to very inhomogeneous nature of thinner layers but in case of the thicker layer also some additional oxygen may be adsorbed in the spaces between the crystallites. It may also be the case, that some of the oxide formed has already passed from Cr 2 O 3 into CrO 3 due to temperatures involved during thermal treatment and further processing. [1] Manganese concentrations also increase compared to the substrate as shown in Fig. 3(d), (e) and (f). Concentration increase is highest towards the top of the oxide layer. While this is not so pronounced at thinner layers it can be very well observed in Fig. 3(e) where at the half depth Mn concentration already drops to its substrate value. There is also a noticeable peak in silicon concentration in thinner films at the oxide layer substrate interface which does not exist in depth profiles of unpolished additionally annealed samples. This is an additional argument that polished additionally annealed sample is very close to a repeated formation of oxide layer after removing the oxide layer formed by the primary thermal treatment. In case of further growth of the oxide layer (additional annealing without polishing thick oxide layer) Si concentration peak most probably disappears due to accelerated diffusion processes. Even without benefit of the profiling data, extreme thickness of the oxide film in the unpolished additionally annealed sample compared to the other two samples can be perceived and even roughly estimated from the surface morphology. In Fig. 1(c) it is shown to be grainy with grains in the range of µm; 2 µmis also a rough estimate of the probing depth in Fe/Cr/Mn oxides for 15 kevelectronbeamusedateds. [5] Asseenfromtheprofilingdata this value also very closely matches the effective thickness of the layer (Fig. 2(b)). Lower energy, 5 kev, was also employed for thinner oxide layers; however, due to the near overlap of the Cr and O peaks quantification of the results is problematic. To check the validity of this layer averaged composition approach, average compositions of the unpolished additionally annealed protective layer derived from the EDS measurements at 15 kev and from the averaged first 2000 nm of the depth profiles in Fig. 2(b) are compared in Table 1. Rather close match in average concentration values can be observed for all elements but for Fe and O, which suggests that estimated probing depth is slightly underestimated, possibly since closer to the surface oxygen content of the protective layer is higher than in usual Fe/Cr/Mn oxides thus decreasing average density of the layer. XPS measurements of protective layers were also performed; due to XPS depth profiling being rather time consuming, single level measurement was performed on each sample after standard cleaning by intense sputtering estimated to remove approximately 10 nm from the top of each sample. Some of the more interesting results obtained by the XPS are shown in Fig. 4. XPS high resolution spectra around Cr 2p 3/2 and Mn 2p 3/2 peaks of unmodified (Fig. 4(a) and (d)) samples with peaks at and ev that can be attributed to Cr 2 O 3 and MnO, respectively [9,10] suggest the presence of one type of chromium and manganese oxide only. XPS high resolution spectra around Cr 2p 3/2 and Mn 2p 3/2 peaks of unpolished additionally annealed (Fig. 4(b) and (e)) samples Table 1. Average compositions of the unpolished additionally annealed protective layer derived from the EDS measurements and from the averaged first 2000 nm of the depth profiles in Fig. 2(b) Element C EDS (at.%) < C AES > (at.%) C Cr Fe Mn Ni O Si Figure 4. XPS high resolution spectra around Cr 2p 3/2 and Mn 2p 3/2 peaks of unmodified (a) and (d), unpolished additionally annealed (b) and (e) and polished additionally annealed (c) and (f) samples. Copyright c 2008 John Wiley & Sons, Ltd. Surf. Interface Anal. 2008; 40:

5 Study of oxide protective layers show peaks with components: 576.9, and 641.1, ev. In case of Cr 2p 3/2 additional component of ev may be ascribed to metallic Cr [9,10] especially so in view of layer s grain-like structure which may allow small contribution to the signal to derive directly from the substrate in spite of layer s thickness. Metallic Cr may also appear due to Cr 2 O 3 reduction by Ar + sputtering via preferential removal of oxygen atoms; however, XPS was performed on samples that were all sputter cleaned and metallic Cr was not detected on all of them (Fig. 4(a)). It is also interesting to note that Cr 2p 3/2 peaks from all three samples can be quite successfully interpreted without using additional component at approximately 579 ev corresponding to CrO 3, [9,10] though this oxide is hinted at by stoichiometric considerations. Mn 2p 3/2 peaks however suggest two different manganese oxides. According to binding energy reference data [9,10] most likely candidates for binding energy values of and ev are MnO 2 and MnO. XPS high resolution spectra around polished additionally annealed (Fig. 4(c) and (f)) sample also show multicomponent peaks: 576.9, and 640.7, ev. In case of Cr 2p 3/2 additional component of ev may be again ascribed to metallic Cr. [9,10] Mn 2p 3/2 peaks can be again ascribed to two different manganese oxides, MnO 2 and MnO. [9,10] Precise binding energy values may in this case indicate slight non-stoichiometricity of the MnO 2 y,mno 1 x type, especially since Cr 2p 3/2 binding energy values are within 0.1 ev of corresponding values for unpolished additionally annealed sample and polished additionally annealed sample. Conclusion First conclusions about the oxide layers prepared by thermal treatment solely and subjected to post-thermal treatment processing could be drawn from SEM images of their surfaces on the basis of comparison with some similar oxide layer studies. Some educated guesses as to thicknesses and main metallic components of the oxide layers could be made. Also, hints about influence of postthermal treatment processing could be found in SEM images of the sample s surface. All these conclusions were corroborated by AES depth profiling that provided detailed quantitative data of all three types of oxide layers and virtually demonstrated that while additional annealing of an unpolished protective layer produces nearly an order of magnitude thicker oxide layer through continued oxide layer growth, additional annealing of the polished protective layer very nearly recreates the original layer since the type of polishing as used in this study removes most of the original oxide layer. Potentially important differences between the original oxide layer and the recreated one are discerned by XPS: an additional manganese oxide appears compared to original layer and small metallic chromium component appears in the spectrum. At this point it is yet unclear whether this is due to the inhomogeneous nature of the oxide layer. It was shown that while EDS can be valuable for obtaining first qualitative results, quantification may still be unreliable for thin layers, especially with prolonged layer substrate interface. In future, it is planned to measure the corrosion properties of the post-thermal treatment processed oxide layers to check for actual improvement and to test several polishing techniques. References [1] Stott FH.Mater. Sci. Technol. 1989; 5(8):734. [2] Grant WK, Loomis C, Moore JJ, Olson DL, Mishra B, Perry AJ. Surf. Coat. Technol. 1996; 788: 86. [3] Mandrino Dj, Lamut M, Godec M, Torkar M, Jenko M. Surf. Interface Anal. 2007; 39: 438. [4] Mandrino Dj, Lamut M, Godec M, Torkar M, Jenko M. Metallurgy (in press). [5] ScottVD, LoveG, ReedSBJ. Quantitative Electron-Probe Microanalysis. Ellis Horwood: New York, [6] Jagielski J, Khanna AS, Kucinski J, Mishra DS, Racolta P, Sioshansi P, Tobin E, Thereska J, Uglov V, Vilaithong T, Viviente J, Yang SZ, Zalar A. Appl. Surf. Sci. 2000; 156: 47. [7] Stefanov P, Stoychev D, Stoycheva M, Marinova Ts. Mater. Chem. Phys. 2000; 65: 212. [8] Evans HE, Lobb RC. Corros. Sci. 1984; 24: 209. [9] Chastain J (ed.) Handbook of X-ray Photoelectron Spectroscopy. Physical Electronics: Eden Prairie, [10] Wagner CD, Naumkin AV, Kraut-Vass A, Allison JW, Powell CJ, Rumble JR (eds.) NIST X-ray Photoelectron Spectroscopy Database Available at: Surf. Interface Anal. 2008; 40: Copyright c 2008 John Wiley & Sons, Ltd.