Stainless steel, Co-Cr and Ni-Cr alloys have played an important role in many industrial sectors to combat environmental degradation. However, low hardness and poor wear properties have impeded their tribological and tribochemical applications. Conventional thermochemical treatments can be used to significantly harden these passive alloys but at the expense of their corrosion resistance due to precipitation induced depletion of Cr in the matrix. Research in 1980s led to the discovery of a new expanded austenite phase, i.e. so called S-phase with combined improvement in wear and corrosion resistance. Recent research has revealed that S-phase can be formed not only in stainless steels but also in Co-Cr alloys and Ni-Cr alloys. It is the purpose of this paper to critically review the S-phase surface engineering of stainless steels, Co-Cr alloys and Ni-Cr alloys. Particular attention will be paid to the structure, formation conditions, supersaturation, hardening mechanisms and metastability of...

S-phase surface engineering of Fe-Cr, Co-Cr and Ni-Cr alloys

The use of elevated target temperatures near 400 °C during high flux ion implantation of N2+ at energies ranging from 60 keV to 0.4 keV leads to a metastable, f.c.c. high nitrogen solid solution phase induced in austenitic (f.c.c.) Cr-containing stainless steels. This phase has not been produced in an f.c.c. Ni-Fe alloy containing no Cr. Penetration depths of the N are significantly larger than expected on the basis of known diffusion coefficients of N in Cr-containing stainless steels or in pure f.c.c.-Fe. X-ray diffraction data suggest unusual differences in N penetration and concentration depending on the f.c.c. grain orientation. Consideration is given to possible residual stresses induced by N expansion of the lattice and to the anisotropic elastic constants for austenitic stainless steels. The amount of N in interstitial solid solution approaches 40 at.% under the lower energy, higher flux conditions, but only in the (200) crystallographic planes parallel to the surface. The N-expanded f.c.c. phase with the highest amount of N is found by conversion electron Mossbauer spectroscopy to be magnetic in AISI 304 and 310 stainless steels. A comparison of our results with those from related methods such as conventional plasma ion nitriding and pulsed plasma ion implantation is made to demonstrate that the observed metastable solid solution phase is often produced by other methods provided that appropriate temperatures and processing times are used. The similar penetration depths of N in the f.c.c. Cr-containing stainless steels for the various methods are consistent with thermal diffusion and metastable solubility that are enhanced by the Cr. The metastability is associated with the low Cr mobility below about 450 °C and the strong N-Cr bond. Evidence for vacancy-enhanced diffusion is not found and we see at present no advantage to using ion energies higher than about 2 keV at these elevated temperatures for producing surfaces with optimized tribological behavior based on the extremely high strength, metastable, N solid solution phase.

Metastable phase formation and enhanced diffusion in f.c.c. alloys under high dose, high flux nitrogen implantation at high and low ion energies

A series of plasma-nitriding experiments has been conducted on AISI 304L austenitic stainless steel at temperatures ranging from 400 to 600 °C using a pulsed d.c. plasma with various pulse duration/repetition ratios in an N2H2 gas mixture. The structure and composition of the plasma-nitrided surface layer were analysed by means of X-ray diffraction (XRD), scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), optical microscopy and microhardness testing. The corrosion behaviour of the S phase was also investigated. The maximum Knoop hardness after plasma nitriding is about 1400 HK 0.01 and the maximum thickness of the compound layer is 34 μm. XRD patterns show that the surface layer consists of the S phase only. The corrosion performance was tested in 0.05 M Na2SO4 solution at pH 3.3 and in neutral 3.5% NaCl solution at ambient temperature. Potentiostatic and potentiodynamic experiments yielded slightly higher passive corrosion currents for plasma-nitrided 304L. Pitting corrosion in neutral electrolytes containing chloride ions was observed only for untreated 304L. The passive layers formed on both types of samples were similar in constitution and thickness as determined from XPS sputter profiles.

Structure and properties of plasma-nitrided stainless steel

To avoid the drop in corrosion resistance of stainless steels in conventional nitriding (precipitation of CrN), low-temperature techniques like ion implantation, plasma immersion ion implantation (PIII, PI3) and low-temperature plasma nitriding were developed. In this investigation, four stainless-steel grades (ferritic: X6Cr17, austenitic–ferritic: X2CrNiMoN22.5.3, austenitic: X8CrNiTi18.10 and X5CrNi18.10) were plasma-nitrided between 250 and 500°C. Nitrogen-enriched layers with a high nitrogen content were produced, leading to a significant increase in surface hardness. X-ray diffraction indicated that CrN did not precipitate if treatment temperatures did not exceed 400°C. ‘Expanded austenite’ formed in the austenitic and duplex steels and E-nitride (Fe2N1−x) in the ferritic steel. The optically visible structure of the nitrided cases is comparable with that of the PIII layers, with higher charging densities being possible in the plasma nitriding. Also, in comparison to conventional ion implantation, large charges and parts with complicated shapes can be treated.

Plasma nitriding of stainless steels at low temperatures

Metals, in addition to ceramics and polymers, are important class of materials considered for replacement of non-functional parts in the body. Stainless steel 316, titanium and titanium alloys, Co-Cr, and nitinol shape memory alloys are the most frequently used metallic materials. These alloys are prone to corrosion in various extents. This review briefly discusses the important biomaterials, their properties, and the physiological environment to which these materials are exposed. Corrosion performance of currently used metallic materials has been assessed and threat to the biocompatibility from corrosion products/metal ions is discussed. The possible preventive measures to improve corrosion resistance by surface modification and to increase the bioactivity of the metallic surfaces have also been discussed. Importance of the formation of oxide layers on the metal surface, another aspect of corrosion process, has been correlated with the host response. The gap areas and future direction of research are also outlined in the paper.

Corrosion degradation and prevention by surface modification of biometallic materials

AISI316 stainless steel has been plasma nitrided at 570°C over a range of processing conditions, and the resultant corrosion properties have been investigated by a potentiodynamic polarization technique. X-ray diffraction and transmission electron microscopy studies have been used to map the process parameters under which a duplex surface compound layer of γ' phase and austenite is formed. This surface compound layer has better corrosion resistance than a plasma nitrided stainless steel surface, where the normal hardened layer consists of austenite' and chromium nitride precipitates. It has been found that the improvement in corrosion resistance is related to the presence of the γ' nitride. Furthermore; low temperature plasma nitriding at 400° C produces a nitrided layer which has a corrosion resistance equivalent to that of the original material.

T. Bell

Papers

Structure and Corrosion Resistance of Plasma Nitrided Stainless Steel