The effect of surface mechanical attrition treatment on low temperature plasma nitriding of an austenitic stainless steel
Highlights
► The formation of a nanocrystalline surface layer improves the nitrogen diffusivity. ► It yields a 50% thicker nitrided layer. ► This layer is 5 times harder than the 316L substrate. ► This result is obtained by adding a polishing step between SMAT and nitriding.
Introduction
Austenitic stainless steel AISI 316 — ASTM F138 is a typical medical grade material that is used in many industrial and biomedical applications such as orthopaedic implants, due to its excellent corrosion resistance and biocompatibility. However, its hardness and wear resistance are relatively poor [1]. Many attempts have been made in order to harden its surface [2]. For example, at low temperature nitriding [3], the transformation of austenite into expanded austenite (γN, or S-phase) increases the surface hardness while keeping a reasonable corrosion resistance [4].
Another process for surface hardening is the Surface Mechanical Attrition Treatment (SMAT) [5]. It generates a nanocrystalline surface layer by severe plastic deformation. This enhances several mechanical properties such as yield and ultimate strengths, but it also decreases the ductility [6]. SMAT has already been combined with other processes such as co-rolling [7], gas nitriding [8], [9] or low-temperature plasma nitriding on AISI 321 steel [10]. It has been shown that SMAT combined with nitriding can enhance surface hardness and corrosion resistance [8], [9], [10]. The SMAT increases dislocation density and grain boundary fraction near the surface, thereby providing fast diffusion pathways for the nitrogen atoms into the material. Improved nitrogen diffusivity due to smaller grain size was already observed in AISI 304 steel [11]. Conversely, Cemin et al. [12] studied the influence of another mechanical attrition process, ball milling, on low temperature plasma nitriding of AISI 316 steel, and they demonstrated that ball milling oxidises the metal surface, which blocks the nitrogen flux into the bulk material. Several studies have been performed on similar mechanical surface treatments and materials [1], [2], [10], however no research has been carried out on the duplex treatment SMAT/nitriding of medical grade austenitic stainless steels.
In this work, a medical grade AISI 316 — ASTM F138 stainless steel is first SMATed and then plasma nitrided. The idea is that SMAT will improve the subsequent nitrogen diffusion, so that a thicker nitrided layer is formed, which would enhance several mechanical properties. Based on the thermal stability TTT diagram of γN [13], plasma nitriding is carried out for 20 h at 425 °C [2]. However, even if the nanostructure generated by SMAT is known to remain stable for at least 10 min at 600 °C [14], no information is available for longer dwell times. Thus, some of the SMATed samples are annealed at 425 °C for various durations between 5 and 20 h to study the stability of the nanostructure.
The effect of an intermediary polishing step is also investigated. If any oxides would be present, as in [12], this step would remove them. The resulting nitrogen layers obtained with and without polishing are then compared to each other using different techniques (as explained below), as well as to an un-SMATed nitrided sample. Finally, the results are discussed and analysed.
Section snippets
Material and surface treatments
Coupon samples 6 mm thick were cut from 25 mm diameter bars. Their chemical composition is given in Table 1. Several SMATed samples were annealed (A) in an air furnace at 425 °C (see Table 2). The other samples were subjected to different combinations of SMAT (S), intermediate polishing (P), and nitriding (N), always in that order (see also Table 2). In each case two samples were used. During the SMAT, spherical shot is set in motion by a high frequency (20 kHz) ultrasonic generator. Random shot
Thermal stability of the nanostructure generated by SMAT
In order to establish the thermal stability of the nanostructure during the nitriding, the grain sizes of different annealed samples (SS, SA5 and SA20) were determined at 2 μm below the surface from multiple SEM and EBSD observations. Fig. 1 shows a close-up inside the layer affected by SMAT (which extends to beyond 200 μm), of a typical example of one of these microstructures (SS). The average grain sizes are summarised in Table 3. It can be observed that the different annealing treatments
Conclusion
SMAT and nitriding were combined into a duplex process in order to increase the thickness of the nitrided layer, thereby improving the hardness profile near the sample surfaces of a medical-grade austenitic stainless steel AISI 316 — ASTM F138. First, SEM observations coupled with EBSD were carried out in order to establish whether the superficial nanostructure induced by SMAT remained stable under subsequent nitriding conditions. It was demonstrated that a nanocrystalline layer composed of
Acknowledgements
The authors gratefully acknowledge financial support from the Regional Council of Champagne-Ardenne (France) through the NANOSURF project as well as from the European FEDER programme.
They also acknowledge the technical assistance of Dr P. Trimby and L. Aschehoug of the Australian Microscopy & Microanalysis Research Facility at the Australian Centre for Microscopy and Microanalysis, University of Sydney.
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