Surface nanostructures on stainless steels
Stainless steels (FeCr-based alloys) are very favorable materials for many demanding application environments. In addition to their excellent mechanical properties, they possess an intrinsic corrosion resistance due to a protective oxide film that forms on the surface. Stainless steel materials are also considered highly versatile, as their applications range from home appliances and construction materials to refined high technology products such as solar thermal collectors and biomedical implants.
The functional properties of stainless steels result from processes that take place on their surfaces at molecular level (Fig. 1). For instance, the protective (passive) oxide layer grows by reaction between oxygen from the environment and various elements from the alloy. This layer is also self-repairing, since it forms spontaneously under the right conditions. Moreover, many important interactions with other phases or materials, e.g. during fouling or coating of stainless steel, are largely controlled by the morphology and chemical composition of the surface oxides. Therefore, better knowledge of the formation and modification of surface nanostructures on stainless steel is essential for the development of advanced alloys and novel surface treatments.
Figure 1. Scanning tunneling microscope image of protective oxide growth on ferritic stainless steel.
Surface Science Laboratory is a member of FIMECC Ltd. – Finnish Metals and Engineering Competence Cluster – Demanding Applications (DEMAPP) program, which aims to increase and deepen the cooperation between companies, universities, and research institutes in the area of top quality research. The program focuses on research and development of advanced materials for demanding application environments, including low-nickel stainless alloys for automotive exhaust systems, solar thermal collectors, fuel cells, and catalyst supports. Moreover, we are collaborating with the Institute of Biomedical Technology at University of Tampere in order to extend the applicability of stainless steels within the biomedical industry. Synchrotron radiation-based research of these materials is carried out at MAX-lab (Lund University, Sweden) in collaboration with the Institute of Physics at University of Tartu (Estonia).
Our role in the DEMAPP program is to provide experimental facilities for obtaining quantitative information concerning the local atomic structure of the alloy surfaces, adsorbate induced segregation effects, and chemical state of alloy constituents upon high temperature treatments and simulated corrosion tests of stainless steel materials. The surface analyses are performed primarily by electron spectroscopic methods utilizing both conventional and synchrotron radiation sources [Appl. Surf. Sci. 2011]. The technological objectives of controlling the surface-mediated processes include improving the durability and self-healing properties of the protective oxide layer, and optimizing the surface composition resulting from industrial surface finishing processes (Fig. 2).
Figure 2. Surface analysis of the passive layer on EN 1.4509 stainless steel (after pickling and skin pass rolling) by electron spectroscopy.
In addition to resolving performance issues related to surface segregation and oxidation phenomena, our long-term scientific goal is to develop novel functional materials by modifying metal alloy surfaces at the nanometer scale. Recently, we have demonstrated that bonding of organofunctional silanes on stainless steel [Surf. Interface Anal. 2010] is strongly influenced by surface hydroxylation resulting from water adsorption [Surf. Sci. 2009]. This knowledge has been applied to facilitate metal-polymer adhesion [Appl. Surf. Sci. 2011] and synthesis of biofunctional thin films [NanoBio-Zürich 2010] on stainless steel materials.
[1] H. Ali-Löytty, P. Jussila, M. Hirsimäki, and M. Valden, Influence of CrN surface compound on the initial stages of high temperature oxidation of ferritic stainless steel, Applied Surface Science 257, 7783-7791 (2011). [Abstract]
[2] P. Jussila, H. Ali-Löytty, K. Lahtonen, M. Hirsimäki, and M. Valden, Effect of surface hydroxyl concentration on the bonding and morphology of aminopropylsilane thin films on austenitic stainless steel, Surface and Interface Analysis 42, 157-164 (2010). [Abstract]
[3] P. Jussila, H. Ali-Löytty, K. Lahtonen, M. Hirsimäki, and M. Valden, Inhibition of initial surface oxidation by strongly bound hydroxyl species and Cr segregation: H2O and O2 adsorption on Fe-17Cr, Surface Science 603, 3005-3010 (2009). [Abstract]
[4] M. Honkanen, M. Hoikkanen, M. Vippola, J. Vuorinen, P. Jussila, H. Ali-Löytty, M. Lampimäki, M. Valden, and T. Lepistö, Characterization of silane layers on modified stainless steel surfaces and related stainless steel-plastic hybrids, Applied Surface Science 257, 9335-9346 (2011). [Abstract]
[5] L. Kanninen, N. Jokinen, K. Lahtonen, P. Jussila, H. Ali-Löytty, M. Hirsimäki, J. Leppiniemi, V. Hytönen, M. Kulomaa, N. Ahola, K. Paakinaho, M. Kellomäki, and M. Valden, Surface science analysis and surface modification methods for biomaterials research, NanoBio-Zürich, Zürich, Switzerland, August 24-27, 2010. [Abstract]