nitridedsteel

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STAINLESS STEEL 2000Proceedings of an International

Current Status Seminar onThermochemical Surface Engineering

of Stainless Steel

Held in Osaka, JapanNovember 2000

Edited by Tom BellSchool of Metallurgy & MaterialsUniversity of Birmingham, UK

and

Katsuya AkamatsuKansai University, Japan

FOR THE INSTITUTE OF MATERIALSIN ASSOCIATION WITH

THE INTERNATIONAL FEDERATION FORHEAT TREATMENT AND SURFACE ENGINEERINGAND THE JAPAN SOCIETY FOR HEAT TREATMENT

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B0752First published in 2001 by

Maney Publishing forThe Institute of Materials1 Carlton House Terrace

London SW1Y 5DB

The Institute of Materials 2001All rights reserved

Maney Publishing is the trading name ofW. S. Maney & Son Ltd

Hudson RoadLeeds LS9 7DL

ISBN 1–902653–49–1

Typeset in the UK byDorwyn Ltd, Rowlands Castle, Hants

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177

Towards Quantifying the Composition of ExpandedAustenite

M.P. FEWELLa,*, P. GARLICKb, J.M. PRIESTa†, P.T. BURKEc,N. DYTLEWSKIc, K.E. PRINCEc, K.T. SHORTc, R.G. ELLIMANd,

H. TIMMERSd,e, T.D.M. WEIJERSd,e and B. GONGf

aPhysics and Electronics, University of New England, Armidale NSW 2351, AustraliabElectron Microscope Unit, University of New England, Armidale, NSW 2351, AustraliacAustralian Nuclear Science and Technology Organisation, Menai, NSW 2234, Australia

dDepartment of Electronic Materials Engineering, Australian National University, Canberra, ACT 0200,Australia

eDepartment of Nuclear Physics, Australian National University, Canberra, ACT 0200, AustraliafSurface Science and Technology, School of Chemistry, University of New South Wales, Sydney, NSW 2052,

Australia

This paper is dedicated to the memory of Trevor Richard Ophel, Professor of NuclearPhysics in the Australian National University, who died suddenly on 17th June 2000.

ABSTRACTSecondary-ion mass spectrometry (SIMS) is an attractive method for elemental depth profiling fornitrided stainless steel. However, if taken at face value, the results suggest migration of Mo and Cr fromthe nitrogen-rich layer. More plausibly, the results indicate the presence of significant matrix effects, andthis conclusion casts doubt on the interpretation of all the SIMS depth profiles, including that ofnitrogen. We explored this question by examining standard samples prepared by nitrogen-ion implanta-tion and comparing assays and depth profiles from SIMS, energy-dispersed X-ray microanalysis (EDX),heavy-ion elastic-recoil detection analysis (HI-ERDA) and X-ray photoelectron spectroscopy (XPS). Weconclude that there are negligible matrix effects in the SIMS sputter yields of nitrogen and carbon, butthat the yields of MoCs+ and CrCs+ ions are significantly affected by the concentration of nitrogen. Thus,the shape of the nitrogen and carbon depth profiles from SIMS can be taken at face value. HI-ERDAgives depth profiles of the same quality as SIMS, but is limited to a relatively thin nitrogen-containinglayer. Results from EDX, HI-ERDA and XPS show that the relative concentrations averaged over thetreated layer of the main metals in the alloy are the same as in the bulk material. Detailed examination ofthe peak positions and widths in XPS shows a clear chemical association between the nitrogen in thetreated layer and Cr and Mo, and probably also an N–Fe association, but none between nitrogen and Ni.These results support a model of the kinetics of nitriding in austenitic stainless steel.

1. INTRODUCTION

Stainless steel hardened by nitriding is potentially such a useful material, and plasma nitrid-ing gives such promise of a commercially practicable method of producing it, that much

*Corresponding author (e-mail address: [email protected]).†Present address: School of Chemistry, Physics and Earth Sciences, Flinders University, GPO Box 2100, Adelaide SA 5001,Australia.

Reprinted from SS2000: Thermochemical Processing of Stainless Steel, T.Bell and K.Aakamatsu eds, Maney, 2001


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Stainless Steel 2000

effort world wide is now being devoted to realising this potential.1–5 As in many otherfields, a significant issue for this research is the characterisation of treated samples, for thepurpose of comparing the efficacy of different treatment processes. Many diagnosticmethods are used; among them are techniques for determining the composition of thetreated layer. These range from methods familiar in metallurgy, such as glow-dischargeoptical spectroscopy, to the most sophisticated techniques developed in modern surfacescience. A feature of all methods is the need for calibration: whether one is measuring theyield of photons, electrons or ions, there is always the question of relating the observedyield to the concentration of the various species in the material under study. The workreported in this paper was prompted by just such a question of calibration, in this case inthe results of secondary-ion mass spectrometry (SIMS) on samples of nitrided AISI 316stainless steel.

Figure 1 shows a typical SIMS elemental depth profile from a nitrided sample. (Ex-perimental details are given in §§ 2.1 and 2.2.) There are many interesting features inthese data, not least the shape of the profiles for carbon and nitrogen. However, theprofiles for the four main alloy elements indicate that the interpretation may not bestraightforward. All four metals show a lower ion-current ratio IMCs+ in the nitrogen-containing layer compared with the bulk. This is to be expected from sputtering theory,which indicates that the ion-current ratio of a species M should be proportional to itsatomic concentration cM (atom %), and cM values for the metal elements are reduced bythe presence of the nitrogen. But closer inspection shows that the details do not matchthis explanation. Table 1 gives the ratio of ion-current ratios in the treated layer to thebulk for the four main alloy elements. A straightforward interpretation would explain thevalue for iron as reflecting a ∼25 at. % concentration of nitrogen in the treated layer.However, one would then expect the values to be the same for all the metals, and clearlythey are not. Either there has been migration of Cr and Mo from the treated layer, or theion-current ratios of these elements are influenced by the presence of nitrogen differentlyfrom those of Fe and Ni. Given that Fe is the main constituent of the alloy (∼65 at. %),there may even be evidence that INiCs+ is behaving in an opposite manner from ICrCs+and IMoCs+.

Influences of this sort are referred to as ‘matrix effects’; their presence clearly compli-cates the interpretation of depth profiles. One can no longer assume that, for example, ahalving of IMCs+ implies a halving of cM; rather there may be a change in the calibrationfactor relating these two. Of the elements profiled in Fig. 1, most interest perhaps lies innitrogen and carbon, but there is no reason why the profiles of these should be lessaffected by matrix effects than the profiles of the metal elements.

We have explored this problem by preparing samples of AISI 316 stainless steelcontaining known concentrations of nitrogen. We also examine the same or similarnitrided samples with a range of composition-determining techniques. Our results,though not absolutely conclusive, strongly suggest that the nitrogen profile in Fig. 1 canbe taken at face value, whereas the Mo and Cr profiles cannot. That is, matrix effects aresignificant for Mo and Cr, but at most minor for N. We argue that matrix effects are alsolikely to be negligible for carbon.


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Towards Quantifying the Composition of Expanded Austenite

Fig. 1 Relative ion-current ratios IMCs+ = IMCs+/ICs+ as a function of sputter time and hencedepth for a sample of AISI-316 stainless steel nitrided in the UHV chamber for 3 h at 400°C in aN2 + 25% H2 gas mixture of total pressure 0.53 Pa without pretreatment. The vertical line showsthe thickness of the treated layer determined from a cross-sectional electron micrograph of the samesample (Fig. 4a). The broken line shows the carbon ion-current ratio in the bulk, to emphasise themanner in which carbon is piled up at the interface between the treated layer and the bulk.

Table 1 Ratio of ion-current ratios IMCs+ in the treated layer to those in the bulk for the main alloyelements (data from Fig. 1).

IMCs+ (treated layer)

Element IMCs+ (bulk)

Cr 0.625 ± 0.005Fe 0.748 ± 0.003Ni 0.816 ± 0.005Mo 0.338 ± 0.005

In addition, we note the strengths and weaknesses of each technique used in its applica-tion to nitrided stainless steel. This includes questions of the depth that can be profiled, thedepth and concentration resolution possible, mass discrimination, and the extent to whichthe composition of the sample may be altered by the technique. Not least, we also highlightthe information that can be obtained on aspects other than an assay of the sample.


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2. EXPERIMENTAL METHODS AND RESULTSWe examined a total of twelve samples of one grade of austenitic stainless steel, nitrided byexposure to a low-pressure rf plasma at elevated temperature, and several untreated andnitrogen-implanted samples as controls. Our experience in conducting nitriding in low-pressure rf plasmas6–10 gives us confidence in the reproducibility of the process. As far aspracticable, samples were examined by more than one technique. The following subsec-tions detail the nitriding process, the diagnostic techniques employed and the resultsobtained.

2.1. SAMPLES AND THE NITRIDING PROCESS

The material examined is AISI 316 austenitic stainless steel. Because its density is requiredfor the analysis of the results from several of the techniques reported herein, we determinedthis by direct measurements on an accurately machined cylinder of our stock material andalso by a method based on Archimedes’ principle. After correction for the buoyancy of air,the two methods agreed within uncertainty. The mean value of the density is 7950 ± 8 kgm–3.

The nitriding chambers, sample preparation and process conditions have been describedpreviously.6–10 In brief, discs of stainless steel are parted from a rod, surface ground andpolished, ultrasonically cleaned in ethanol, and loaded onto the stage of the nitridingchamber. All of our chambers are fabricated from stainless steel and pumped by tur-bomolecular pumps. In each, the chamber volume contains a single-turn inductivelycoupled antenna also fabricated of stainless steel. With an appropriate filling pressure, adiffuse rf glow discharge fills the chamber when rf power is applied to the antenna.

Samples may be treated either with or without a glow-discharge ‘pretreatment’ step. Ifwithout, the sample is heated to the working temperature in vacuum prior to filling thechamber with nitrogen or a nitrogen/hydrogen mixture and commencing the nitriding.Where a pretreatment is included, the chamber is filled with hydrogen or a hydrogen/argonmixture and an rf glow discharge is maintained as the sample is being heated. At the end ofthe pretreatment, the chamber is pumped out and filled with nitrogen for the nitridingstep. In all cases, the sample cools in vacuum following the treatment. The ranges oftreatment parameters used for samples studied herein are given in Table 2.

The samples were treated either in the hot-wall chamber at the Australian NuclearScience and Technology Organisation (ANSTO) or the UHV chamber at the Universityof New England (UNE).8,10 In the first, the whole volume is at the working temperature,which is monitored by both a thermocouple and an optical pyrometer. In the UHVchamber, the sample stage is heated and the temperature is measured by a thermocoupleembedded in the stage. This is calibrated prior the treatment to allow for the temperaturedrops across the vacuum gap between the sample and the stage.

All processes use flowing gas. Where a gas mixture is used, the percentages quoted inTable 2 refer to partial pressures. Both the UNE and ANSTO systems show differentialpumping, with hydrogen being pumped the most efficiently. The effect is not small: toobtain equal partial pressures of nitrogen and hydrogen in our chambers, the hydrogen


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Table 2 Nitriding conditions used for samples included in this study.

UHV chamber Hot-wall chamber

rf power and frequency 80 W at 13.56 MHz 250 W at 13.56 MHzsample bias 0 V –250 V

pretreatment (where applied):gas H2 Ar + 50% H2

total pressure 133 mPa 400 mPatemperature rising from ambient to nitriding temperaturepretreatment time 1.0–4.0 h 1.0–2.0 h

nitriding step:gas N2 or N2 + 25% H2 N2

total pressure 400–530 mPa 400–1000 mPatemperature 300°–400°C 350°–450°Ctreatment time 1.0–4.0 h 3.0–5.0 h

flow rate must be about three times that of the nitrogen. The actual values in each chamberwere determined prior to the treatments by noting the relationship between flow rate andpressure for each gas separately.

2.2. SECONDARY-ION MASS SPECTROMETRY

The present work extends a previous study of nitrided stainless steel by secondary-ion massspectrometry (SIMS),11 the principal finding of which is that the sputter rate (crater depthvs sputter time) is constant in nitrided AISI 316 stainless steel. As in the earlier work, weused the Cameca IMS 5F magnetic-sector mass spectrometer located at ANSTO, employ-ing a 10 keV Cs+ beam with the sample biased at +4.5 kV and counting caesium binaryions MCs+, where M is the species being monitored. Matrix effects in the yields of MCs+

ions are much less pronounced than those in the atomic ions M+,12 but are not necessarilyabsent.13 The caesium beam was rastered over a square 250 µm on a side and sputteredions were collected from circular area 60 µm in diameter centred on the raster pattern.During the determination of a depth profile, several elements were monitored by countingfor 1.0 s at each chosen value of mass, in repetitive sequence from the lowest mass to thehighest.

Figure 1 shows an example of such a depth profile. The quantity plotted is the ion-current ratio IMCs+: the measured current IMCs+ of the various MCs+ ions divided by themeasured Cs+-ion current. (Dividing by the Cs+-ion current eliminates effects of drifts inthe primary beam current and reduces matrix effects.)13,14 The sputter yields for the sevenelements shown encompass all the elements expected in nitrided stainless steel, except forthe ≤ 2.0 at. % of Si, which was not detectable. The depth scale shown at the top of thefigure was calibrated after the profiling by measuring the depth of the sputter crater with aTencor Alpha-Step stylus profilometer and relying on the previous observation of constantsputter rate in this material.11 The vertical line near the nitrogen profile shows thethickness of the treated layer as determined by electron microscopy (see §2.4 and Fig. 4a).


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As mentioned in §1, sputtering theory12–14 can be summarised as predicting the in-crease in the ion-current ratios of Cr, Fe, Ni and Mo at the boundary between the treatedlayer and the bulk to be compensated for entirely by the reduction in the ion-current ratioof the nitrogen. This applies only in the absence of matrix effects, and is not the case in thedata shown in Fig. 1. Hence, to extract elemental concentrations from IMCs+ values, it isnecessary either to examine standard samples containing known amounts of nitrogen, or tofind another technique to determine the composition of each region of the nitridedsamples. Both approaches are explored in the following subsections.

2.3. STANDARDS FOR SECONDARY ION MASS SPECTROMETRY

To prepare standards for calibrating the SIMS measurements of nitrogen concentration, weimplanted samples of AISI 316 stainless steel with known fluences of nitrogen using a beamof 2.00 MeV N+ atomic ions from the 3 MV single-ended Van der Graaff accelerator atANSTO. The relatively high implantation energy was chosen to ensure that the region ofhigh nitrogen concentration is well away from the surface. During the implantation, thebeam current was kept below 0.5 µA to avoid elevated sample temperature and so minimisediffusion of the implanted nitrogen. The implanted area was 3.0 mm × 3.2 mm. This wasdefined by apertures upstream of the target chamber, with the beam defocused to more thanfill them. The area was determined by irradiating a polymer film in the target position, andmeasuring the extent of the damaged area using an optical microscope. The total implantedfluence was determined by beam-current integration, using the whole scattering chamber asan isolated Faraday cup. The fluence is expected to be accurate to ± 5%. Four samples wereprepared, with fluences ranging from 3.0 × 1015 to 3.0 × 1017 ions cm–2.

The resulting samples were analysed by SIMS in the usual manner; Fig. 2a shows anexample. As in Fig. 1, the depth scale was obtained by direct measurement of the depth ofthe sputter crater. The concentration cM of element M can be related to the ion-currentratio IMCs+ by

IMCs+ = cMSMCs+, (1)

where SMCs+ is the calibration factor. Values of cN and hence SNCs+ were extracted from thedata in Fig. 2a in the usual way;15 Fig. 3 shows the result. At nitrogen concentrations above 2at. %, the calibration factor is constant. Rather higher values of SNCs+ occur at very lowconcentrations, but this is of little consequence for the interpretation of the nitrogen profileof a nitrided sample. It must be noted that the highest concentration studied, 6.9 atom %, issomewhat below the 20–30 at. % expected in a nitrided sample, so that the possibility ofvariations in the calibration factor at very high values of cN is not ruled out. Nevertheless, theevidence of this work is that, for the nitrogen concentrations studied, the SIMS calibrationfactor for NCs+ from nitrided austenitic stainless steel is at most weakly dependent onnitrogen concentration for concentration values of interest.

The nitrogen-implanted samples give information only on the SIMS calibration factorfor nitrogen. To explore those for the other elements, we used other methods of elementalquantification, as detailed in the next three subsections.


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Fig. 2 (a) Relative ion-current ratios INCs+ = INCs+/ICs+ of NCs+ ions as a function of sputtertime and hence depth for a sample of AISI-316 stainless steel implanted with 1.00 × 1017 nitrogenatoms cm–2 and (b) calculated depth distribution for 2.00 MeV nitrogen ions in AISI 316 stainlesssteel using the computer code TRIM. (Panel (b) is discussed in §3.1.)

Fig. 3 SIMS calibration factors SNCs+ determined as described in the text as a function ofnitrogen concentration cN.

2.4. ENERGY-DISPERSED X-RAY MICROANALYSIS USING A SCANNING

ELECTRON MICROSCOPE

We have previously reported the use of scanning electron microscopy (SEM) of etchedsectioned samples as a means of measuring the thickness of the treated layer.11 Examplesare shown in Fig. 4. These were taken with the JEOL JSM-5800LV instrument at UNE.The samples were prepared by transverse sectioning, casting in epoxy resin, grinding andpolishing, etching for 10 s in Marble’s solution, and coating with carbon to minimisecharging under the electron beam.

The SEM is also equipped with an X-ray detector to monitor fluorescence from thebeam spot. We used this to perform chemical analysis in selected regions on severalsamples, a technique referred to as energy-dispersed X-ray microanalysis (EDX). The UNEsystem comprises an Oxford Instruments Si(Li) detector optimised for x rays from lightelements and viewing at 35° to the sample surface. We used a 20-keV electron beam. Thefluorescence yield was calibrated against a cobalt standard just prior to commencement of


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Fig. 4 Scanning electron micrographs of cross sections of (a) the same sample as in Fig. 1 and (b) asample nitrided in pure N2 in the hot-wall chamber for 3.0 h at 350°C following a 1.75 hpretreatment in H2/Ar. The light circular marks show locations at which X-ray microanalysis wasperformed.

the EDX analysis and every two hours thereafter. Initial tests showed the need to collecthigh-statistics X-ray spectra if the presence of nitrogen was to be detectable at all. Thisprecluded area or even line scans. Instead, X-ray spectra were recorded at a variety oflocations in the treated layer and the bulk.

Figure 5 shows observed spectra. The carbon peak comes mainly from the coating, andin addition from deposition under the electron beam during the collection of the spectrum.The nitrogen Kα line can be discerned by comparison of the treated-layer spectrum withthe spectrum from regions in the bulk, as shown in the inset; the line is too weak to providea useful measure of nitrogen concentration. The energy of the electron beam was too lowfor efficient formation of K-shell holes in Mo, but the Lα line from this element is clear, asis the Kα line from Si which, according to the specification of AISI 316, may be present atup to 2 atom %. Manganese may also be present at the same level; the line marked Mn inFig. 4 is a blend of the Mn Kα and Cr Kβ lines. Line intensities were converted to elementconcentrations using standard X-ray fluorescence yields.

The images in Fig. 4 were taken after the EDX analysis. They show circular beam burnsindicating the locations where X-ray spectra were collected. A total of nine locations in thebulk region of three samples were examined; all show the same composition withinuncertainties. In the treated layer, sixteen locations were examined, including the three inthe interface region shown in Fig. 4b, and again all showed the same composition. Further,there is negligible difference between the two regions. (Details are presented in §3.2.)Because nitrogen and oxygen could not be included in the assay of the treated layer, theassays for the heavier elements in the two regions are directly comparable. This means thatthe differences in the ratio of sputter yields shown in Table 1 reflects variations in SIMScalibration factors. That for the MoCs+ ion is particularly sensitive to the concentration ofnitrogen in the matrix.


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Fig. 5 X-ray fluorescence spectrum from locations in the treated layer. The spectrum shown is thesum of all spectra recorded from three samples, one of which is the same sample as in Figs 1 and 4a.The X-ray peaks are identified. The inset shows an expanded view of the low-energy part of thespectrum, compared with a summed spectrum from locations in the bulk shown as full points. Theonly difference between the two spectra is the occurrence of the nitrogen K line, as shown.

This conclusion assumes that the EDX calibration factors are not influenced by nitrogenconcentration. Support for this assumption was sought from other methods of quantitativeanalysis: heavy-ion elastic-recoil detection and x-ray photoelectron spectroscopy.

2.5 ELASTIC-RECOIL DETECTION WITH HEAVY IONS

Heavy-ion elastic-recoil detection analysis (HI-ERDA) is an ion-beam analysis techniquefor the compositional depth-profiling of materials. In this technique, incident ions pene-trate into a sample, interacting with atoms and much less frequently with nuclei in thesample. Some recoil ions from nuclear scattering are ejected through the sample surface,permitting their detection. Typically, a wide range of species with a wide range of energiesis ejected, so that particle identification is required. This is achieved either by time-of-flightmeasurements or by measurements of energy loss in a gas-ionisation detector. The nu-merous interactions of both the incident and the recoil ions with electrons in the samplecause loss of kinetic energy. Consequently, the energy with which the recoil ions emergefrom the surface is a function of the depth of the nuclear collision, with lower-energyrecoils originating from deeper inside the sample. By measuring the energy spectra of therecoil ions, stoichiometric information can be correlated with depth.

The HI-ERDA in this work was carried out at the Australian National University usingbeams of 200 MeV and 241 MeV gold ions from the 14UD Pelletron accelerator. Thebeam was incident at 22.5° to the sample surface. Recoil ions were detected with aposition-sensitive gas-ionisation detector that had been developed for the specific demandsof HI-ERDA with heavy-ion beams.16–18. The detector was centred at a recoil angle of 45°


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Fig. 6 Two dimensional plot of ion yield against energy loss ∆E and total energy E for elasticrecoils following 200-MeV 197Au bombardment of a sample treated in pure N2 in the UHVchamber for 3.0 h at a pressure of 400 mPa and a temperature of 300°C. The magnitude of the ionyield is indicated in gray-scale. Identified species are noted.

with a net detection solid angle of 3.50 ± 0.05 msr. Propane was passed through thedetector at a constant pressure of 7.0 kPa. The energy E of the recoil ions was measureddirectly using a grid electrode18 with an energy resolution of 1.6% for 28.6 MeV 16O ions.The anode of the detector is divided into three segments, allowing simultaneous energy-loss measurements for particles with different ranges in the gas. The first segment, adjacentto the entrance window, provides a measurement of the initial energy loss ∆E of ionsentering the detector. The middle segment is further subdivided in a saw-tooth pattern toprovide information on the lateral position of the ion, so allowing compensation for thekinematic variation of energy with recoil angle. Finally, the third segment of the anodegives a measurement of the residual energy Eres of the ions. It is also used to detect protons.

Combining the signals from the short ∆E section with those from the grid electrode in atwo-dimensional spectrum allows the identification of the atomic number of a recoil ion.This is shown in Fig. 6 for data from a nitrided stainless steel sample; all the elementsindicated can be readily identified and separated. The signal from Mn, which is nominallypresent at a level of ∼2 at. %, is obscured by those from the much more abundant elementsCr and Fe. Hydrogen is detected simultaneously but identified separately using the Eres

signal, and is therefore not shown in Fig. 6.A total of seven nitrided samples and one untreated sample were examined. For each,

energy spectra for each element were obtained by gating on appropriate areas in the ∆E–Eplots. Figure 7 shows these for N and C extracted from the data in Fig. 6 and followingcorrection for the kinematic energy spread over the detector acceptance angle. The nitro-gen energy spectrum clearly terminates below E ≈ 10 MeV, reflecting the interface between


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Fig. 7 Energy spectra of (a) carbon and (b) nitrogen recoils projected from the data shown in Fig.6. The smooth curves show simulations using RUMP.

the nitrogen-rich layer and the underlying bulk material. The stoichiometry of the sampleswas determined by fitting theoretical simulations to the energy spectra using the codeRUMP,19 as shown by the broken lines in Fig. 7. Fits such as those shown in Fig. 7 foreach element give the relative concentrations; results are compiled in §3.2. All samplesstudied give the same assay, within uncertainties.

All samples have a thin oxidised surface layer (∼1017 atoms cm–2). This is evident in Fig.7a, which is typical of all the samples. In this case, the surface layer contains ∼5 at. %carbon and ∼18 at. % oxygen. Underneath this layer, the oxygen content is much reduced,at ∼0.2 at. %. For carbon, the situation is more complex. Below the surface layer, theconcentration decreases to a constant level of ∼0.4 at. %. At the interface between thetreated and the bulk the carbon concentration piles up to a maximum of ∼0.8 at. % andthen decreases to a constant ∼0.4 at. % in the substrate material. The region of relativelyhigh carbon concentration is ∼1.5 × 1018 atoms cm–2 in thickness. The RUMP simulationshown as the full curve in Fig. 7b corresponds to a peak nitrogen concentration of 27.9 ±0.7 at. %. A detailed depth profile for nitrogen is presented in §3.1.

A feature of HI-ERDA is its sensitivity to hydrogen.20–1 This is of interest in theexamination of nitrided stainless steel because of the use of hydrogen in precleaning and asa gaseous additive during the nitriding step.9,22 Whether or not there is any retainedhydrogen in the treated layer is relevant to the question of how the hydrogen aids thenitriding process.

Protons recoiling from the sample were detected simultaneously with heavy ions usingthe transmission technique.20 The intensity of the detected hydrogen was related to thehydrogen concentration in the sample using tabulated stopping powers. The samplesexamined included some that had been exposed to hydrogen during the treatment. Allsamples, whether exposed to hydrogen or not and including the untreated control sample,showed the same [H]:[Fe] concentration ratio within uncertainty; the mean value is (1.3 ±0.7) × 10–3. It can thus be concluded that exposure to hydrogen during the treatment,


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whether in a pretreatment step or as part of the process gas in the nitriding step, does notlead to measurable retention of additional hydrogen in the treated layer.

2.6. X-RAY PHOTOELECTRON SPECTROSCOPY – COMPOSITIONS

X-ray photoelectron spectroscopy (XPS) is not primarily a technique for compositionalanalysis; its strength is the provision of information on the chemical states of atoms in asample, that is, on the measurement of binding energies of core-level electrons. However,the technique can also be used to give an indication of the surface composition of a sampleby measuring photoelectron peak areas, which are then normalised by standard sensitivityfactors and instrument transmission functions. Here, we look at both aspects of the XPSdata. XPS is very much a surface-analysis technique; facilities for sputter etching arecommonly provided, as with the instrument used in the present study, but it is usually notpracticable to penetrate more than several 100 nm below the as-presented surface. In thepresent analysis, both chemical identification and atomic quantification are restricted to asurface layer up to about 40 nm thick. This is sufficient to examine the treated layer belowthe oxygen- and carbon-rich region of surface contamination.

Treated and untreated samples were examined using the ESCALAB 200i-XL X-rayphotoelectron spectrometer at the University of New South Wales. This instrument uses a200 W double-focused monochromated Al Kα X-ray source. The X-rays are incident on thesample surface at an angle of 58° to the sample normal. The area analysed is approximatelyrectangular with dimensions 0.8 mm × 0.5 mm. Photoelectrons emitted normal to thesample surface are detected with six electron channel multipliers in parallel. Photoelectronspectra were acquired at a pass energy of 20 eV. Under these conditions, the resolution of theelectron spectrometer is 0.55 eV for Ag 3d5/2 photoelectrons. Sputter etching of the samplesurface was carried out with a 5.0 keV Ar+ beam. When activated, the ion beam is incidenton the sample at an angle of 50° to the surface. The sputter rate was not measured butprevious experience indicates a sputter rate of order 0.2 nm µC–1, where the charge refers tothe integrated beam current: the total charge delivered as Ar+ ions to the surface.

Three samples were examined, two nitrided and one untreated. Spectra were taken fromthe as-presented surfaces and following many successive episodes of sputtering up to a totaldelivered Ar+ charge of 300 µC. Figure 8 shows a photoelectron spectrum obtained from anitrided sample at a location in the region of surface oxidation, which a depth profile showswas not penetrated after about 120 µC of delivered Ar+.

A comparison of depth profiles from treated and untreated samples shows a thickeroxidised surface layer in the untreated sample. The treated samples have a region of veryhigh nitrogen concentration immediately below the oxygen-rich surface layer. The nitro-gen concentration peaks at 32.6% after 18 µC of delivered Ar+ (Fig. 8) and then settlesdown to a steady value of ∼20% deeper into the sample. The depth profile of Cr is verysimilar to that of N, showing the same pattern of surface enrichment. Molybdenum is alsosurface-enriched: its surface concentration after 18 µC of delivered Ar+ is 4.9%, more thantwice the value deep into the treated sample. On the other hand, iron and nickel are bothsurface-deficient but, unlike Fe, Ni shows a peak in its concentration profile below the


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Fig. 8 X-ray photoelectron spectrum (Al Kα X-rays) from a sample treated in the hot-wallchamber with a 2 h pretreatment followed by nitriding at 600 mPa and 400°C for 5.0 h. The mainpeaks are identified; the others are, at low binding energy, 3s, 3p and Mo 4p photoelectrons, andelsewhere various Auger-electron lines. The sample was sputter etched for an integrated Ar+ beamcurrent of 18 µC before recording the spectrum. The sputter rate is expected to be of order 0.2 nmµC–1.

layer rich in N and Cr. For the untreated sample, Mo, Fe and Ni all show surfacedeficiency, due mainly to the high concentration of oxygen, but Cr is slightly surface-enriched. The maximum Cr concentration appears ∼ 18 µC of delivered Ar+. These resultsare compared with those of the other techniques in §3.2.

2.7. X-RAY PHOTOELECTRON SPECTROSCOPY – CHEMICAL BINDING

The previous subsection indicates the ability of XPS to give concentration profiles withhigh depth resolution in the very near-surface region. The second capability of XPS is thedetermination of the chemical relationship between nitrogen and the four metals through adetailed examination of the binding energies of the Cr 2p3/2, Mo 3d5/2, Fe 2p3/2 and Ni2p3/2 photoelectron lines. Figure 9 shows regions of the photoelectron spectra relevant tothe lines mentioned for near-surface and deeper layers of a treated and an untreatedsample. For chromium, the binding energies of 2p3/2 electrons in Cr(0) and Cr(III) are574.2 eV and 576.6 eV respectively.23 Hence the most prominent component in thespectrum from the surface region of the untreated sample (Fig. 9a(i )) is probably due toCr2O3. For the treated sample, the Cr 2p3/2 peak starts at 575.3 eV on the very surface(not shown in Fig. 9) and its width is clearly larger than that of the Cr(0) peak shown inFig. 9a(iv). After 18 µC of etching (trace (ii )), the peak has shifted to 574.9eV. Thisindicates the presence of CrN.23–6 The only other likely candidate is Cr2N, but the Cr2p3/2-electron binding energy for this species is even higher.24 Deep in the sample (trace(iii )), the peak position has shifted again, this time to 574.2 eV, the same value as Cr(0),but the peak width remains larger than that from Cr(0) deep in the untreated sample (trace(iv)). This is attributed to ion-beam-induced reduction of the nitride.


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Fig. 9 Expanded sections of X-ray photoelectron spectra in the regions of the (a) Cr 2p3/2, (b) Mo3d doublet, (c) Ni 2p3/2 and (d) Fe 2p3/2 peaks taken from (i ) an untreated sample without sputteretching (Mo 3d and Fe 2p3/2) or after 18 µC of etching (Cr 2p3/2 and Ni 2p3/2), (ii ) a treatedsample after 18 µC of etching, (iii ) a treated sample after 210 µC of etching and (iv) an untreatedsample after 210 µC of etching. Electron binding energies of some features are indicated.

Molybdenum behaves similarly to chromium. The binding energy of Mo 3d5/2

photoelectrons (the member of the doublet at lower binding energy) is: Mo(0) 227.6 eV,Mo(II) 228.3 eV, Mo(IV) 229.3 eV.23 Values for nitrides of Mo are not known to us. Theuntreated sample shows Mo(0) peaks both at the surface (Fig. 9b(i )) and at depth (Fig.9b(iv)), with a FWHM of 0.7 eV. The 3d5/2 peak in the spectrum from the surface (trace(i )) has a tail extending to higher binding energy; this is attributed to Mo(VI) oxide. Forthe treated sample after 18 µC of etching (trace (ii )), the 3d5/2-photoelectron peak is at228.6 eV with a FWHM of 1.2 eV. This could be due to either the mixture of Mo(II) andMo(IV) or to nitrides of Mo. However, the value of the FWHM is not large enough for thefirst explanation, in view of the binding energy difference between these two oxidationstates. This leaves nitrides of Mo as the most likely explanation of the Mo 3d5/2 spectrumin trace (ii ). The association of a region enriched in Mo with the presence of N supportsthe suggestion of Mo nitrides. Also, the layer of surface oxidation on the treated sample isthinner and the surface concentration of Mo higher compared with the untreated sample,so Mo(0) would be seen were the atoms not bonded to N. Deep into the treated sample(trace (iii )), the peak has shifted down to 228.0 eV. This is again due to ion-bombardment-induced reduction of the Mo nitrides.


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The Ni 2p3/2 photoelectron peaks (Fig. 9c) are not affected by the treatment. Thistogether with the surface depletion of the element indicates that nickel is not bonded tonitrogen. The situation concerning iron, shown in Fig. 9d, is not so clear. Figure 9d(i ),taken from the untreated sample without etching, shows binding energies of 707.0 eV and710.5 eV for 2p3/2 photoelectrons from Fe(0) and Fe oxide respectively, the latter beingmost likely Fe(III). A comparison of the near-surface spectrum from the treated sample(Fig. 9d(ii )) with both spectra from the untreated sample(traces (i ) and (iv)) shows a peakshift of 0.20 eV. It is not clear whether this is due to nitrides of Fe, although the value ofthe shift is of the right magnitude.23,25–6 Further evidence for nitrides of Fe comes fromthe widths of the 2p3/2-photoelectron peaks. The peak in trace (ii ) is clearly broader thanthat in trace (iv). Even deep in the treated sample (trace (iii )), the peak is 0.19 eV widerthan at the corresponding position in the untreated sample (trace (iv)), although the 0.2-eV peak shift apparent near the surface (trace (ii )) has disappeared. Depth nprofilingshows that the N concentration in the treated sample is the highest after 18 µC of etching,the same amount of etching as in Fig. 9d(ii ). This suggests some chemical relationshipbetween Fe and N. Taken together, the data indicate a significant possibility of ironnitrides in a very thin surface layer of the treated sample, probably no more than 5 nmthick. Such a layer has been identified by transmission electron microscopy.27 This sugges-tion does not contradict the results of X-ray diffraction because such a thin layer of ironnitride would not be detectable by XRD.

3. DISCUSSION

3.1 MATRIX AND OTHER EFFECTS IN SIMS

The great attraction of SIMS lies in its visually striking results: data such as those in Fig. 1invite immediate and straightforward interpretation. Also, the data are relatively simple tocollect; depth resolution is good, being limited by the depth of penetration of the Cs+ ions;and depths of several µm can be profiled in a practicable amount of machine time.However, the question of matrix effects intervenes, and this prompted the other studiesreported here. As noted in §1, the data in Table 1 suggest at face value that some Mo and alesser amount of Cr migrated out of the treated layer during the treatment. However, EDXcontradicts this, showing the same ratios [Cr]:[Fe]:[Ni]:[Mo] in the treated layer as in thebulk. As discussed in the next subsection, HI-ERDA and XPS support the EDX result.Hence we have evidence for a strong SIMS matrix effect for Mo and a lesser one for Cr.

On the other hand, the analysis of implanted samples (§2.3) suggests a negligible matrixeffect for N, at least at high concentration. Another way of looking for matrix effects is tocompare depth profiles from different techniques. Figure 10 shows nitrogen depth profilesfrom the same sample determined by SIMS and HI-ERDA. The degree of similarity is veryclose, providing good evidence of the absence of SIMS matrix effects in the case of nitrogen.

Internal evidence in Fig. 1 suggests a negligible matrix effect also for carbon. The brokenline on the carbon profile in Fig. 1 shows the concentration in the bulk. The area abovethis line in the interface region equals the ‘missing’ area below the line in the treated layer.


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Fig. 10 Nitrogen depth profiles from the same sample as in Figs 6 and 7 according to (a) SIMSand (b) HI-ERDA. The HI-ERDA depth profile was obtained by dividing the measured energyspectrum (Fig. 7b) by a calculated spectrum based on a constant nitrogen content of 27.9 atom %.The filled points show the level of agreement with a simulation using RUMP.

This is only circumstantial evidence, but it nevertheless points strongly to a constant SIMSsensitivity factor; for otherwise it would require a finely balanced variation to produce theobserved equality in areas. All carbon depth profiles examined show similar features, and allshow the equality of areas.

A comparison of Figs 2a and 2b indicates another aspect of the SIMS results. Figure 2bshows the results of a calculation of the range distribution of 2.00 MeV nitrogen ions inAISI 316 stainless steel, carried out using the code TRIM.28 The calculated depth of thepeak of the profile agrees well with the measured value in fig. 2a, but the shape does not:TRIM indicates a asymmetric distribution with a tail toward the surface; the SIMS datashow a tail in the opposite direction. Thermal diffusion of the implanted nitrogen might bea possible explanation for this, despite the efforts taken to keep the substrate temperaturedown during implantation. It is, however, hard to see how diffusion could convert the peakshape in Fig. 2b to that in Fig. 2a. Alternatively, there may be roughening of the floor ofthe sputter crater, so that the value of INCs+ reflects the nitrogen distribution at a range ofdepths. This effect can be significant in some materials,15 and usually increases with thedepth of the crater. However, in previous work on nitrided stainless steel, many the sputtercraters were examined with a stylus profilometer;11 all showed reasonably flat floors.

A third explanation for the shape of the depth profile in Fig. 2a may lie in impact-drivenrearrangement of the elemental distribution caused by the Cs+ beam, in particular byknocking nitrogen atoms further into the sample. A TRIM calculation gives the range of5.5 keV Cs+ ions in stainless steel as ∼50 nm. Hence, the material just beneath the floor ofthe sputter crater will be homogenised to about that depth. This represents more than justan effective instrumental resolution profile to be convoluted with the actual depth profile;it provides a source of nitrogen, perhaps only slowly diluted, for implanting further intothe sample. Whatever the explanation of the difference in shape between Figs 2a and 2b,the fact of the difference must be borne in mind when interpreting a SIMS depth profile.


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3.2 COMPARISON OF THE TECHNIQUES

Like SIMS, EDX microanalysis produces straightforwardly interpretable results, but unlikeSIMS, it seems to be relatively unaffected by variations in calibration factors. However,EDX has several features limiting its application. First, it requires the preparation of a crosssection of the sample, which involves substantial machining and polishing. Perhaps thismight alter the composition of the surface layer under observation, although the degree ofconsistency with the other methods suggests not for the case of stainless steel. Secondly, thedepth resolution is limited to the size of the beam spot. Thus, it is not applicable to sampleswith thin nitrided layers, and finely resolved depth profiles cannot be obtained. Thirdly,because of interference with L lines from Fe and the decrease of X-ray yield with Z,assaying for nitrogen is very difficult and oxygen is impossible. The last limitation is, ofcourse, applicable mainly to iron and steel samples.

HI-ERDA is also little affected by composition-dependent variations in its calibrationfactors. The use of energetic very heavy ions, such as 200 MeV gold ions, allows thesimultaneous detection of all elements in the periodic table with comparable sensitivity. Lightelements (Z <14) are fully separated from their neighbours in the periodic table, unlike withRutherford backscattering. HI-ERDA also can profile across the whole treated layer withreasonable depth resolution.29 The total depth probed is determined not only by the energyand atomic number of the incident species, but also by the mass and atomic number of therecoils, since these must escape from the sample to be detected. The straggling of speciesemerging from depth coupled with the energy resolution of the detector limits the range ofenergies over which particle identification is possible. The limitation is readily apparent inFig. 6: as the recoil energy E falls, a point is reached where the signatures of the differentspecies merge. In the present case, the depth accessible for Cr, Fe and Ni is limited to ∼0.5µm because they are so close in Z, and the intensity of these groups completely obliteratesMn. For nitrogen, the profiling depth is mainly dependent on the range of gold nuclei instainless steel. Gold ions with a bombarding energy of 241 MeV can access layers up to ∼1µm thick. This is somewhat of a restriction in examining nitrided stainless steel; samples withthicker layers can be assayed but not profiled over the whole layer. In contrast, the onlylimitation on the thickness of the layer that can be profiled by SIMS is the length of time thatone has access to the machine, although the effects of roughening of the crater floor mayimpose limitations on the composition resolution at large depths.

Of techniques employed here, HI-ERDA has the highest sensitivity for hydrogen. WithSIMS, there is the difficulty of resolving HCs+ from the very intense Cs+ group, and yieldsof H+ and H– are low and likely to suffer from significant matrix effects. None of the othertechniques has any sensitivity to hydrogen at all.

XPS can provide high-resolution depth profiles, but practical considerations limit this tothe outermost few 100s of nm. As discussed in the next paragraph, calibration factors seemto be less reliable for XPS than for EDX. On the other hand, XPS provides uniqueinformation on the chemical state of the atoms in the sample.

Table 3 collates assays of bulk material from the techniques that provide this informa-tion and compares this with the specifications of AISI 316. (The official specifications are


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Table 3 Official composition of AISI 316 as specified in wt %, with a conversion to at. %, comparedwith results from locations in the bulk material obtained with techniques used in this work (balance: Fe).

Specification* EDX HI-ERDA† XPS (at. %)†‡

Element (wt %) (at. %) (at. %) (at. %) 18 µC 210 µC

H 0.08 ± 0.02C <0.08 <0.37 0.40 ± 0.10 2.9 2.7O 48.6 3.9Si <1.00 <1.98 0.40 ± 0.05 0.50 ± 0.10Cr 16.0–18.0 17.0–19.3 17.10 ± 0.14 17.3 ± 1.0 14.2 16.7Mn <2.00 <2.03 1.41 ± 0.06Ni 10.0–14.0 9.4–13.4 9.56 ± 0.19 9.3 ± 1.0 4.3 10.0Mo 2.0–3.0 1.1–1.8 2.39 ± 0.14 1.30 ± 0.13 1.1 3.1

*In addition, P < 0.045 wt %, S < 0.030 wt %.†The HI-ERDA and XPS values were measured on an untreated sample.‡The uncertainty in the values is ± 0.2 percentage points. The µC values refer to the delivered charge from the Ar+ sputteringbeam; the sputter rate is of order 0.2 nm µC–1.

given as wt %; these were converted to at. % by taking into account the extremes of all theranges.) By and large, the EDX, HI-ERDA and deeper (210 µC) XPS results agree withintheir uncertainties except for Mo. As noted in §2.6, Mn was not resolved from Cr and Fein the HI-ERDA, and so its contribution is included in those elements. The near-surface(18 µC) XPS results shows the composition of the oxidised region at the surface. Elevatedconcentrations of carbon and oxygen in this region were also seen with HI-ERDA, asshown for carbon in Fig. 7a, although the values were different (5 at.% for C and 18 at.%for O). This probably reflects different oxidation responses of the two samples concerned.

The situation with Mo is interesting: only HI-ERDA give a result in agreement with thespecifications of AISI 316. This may possibly indicate calibration problems in EDX andXPS for this element. In the EDX analysis, the concentration of Mo is derived from theareas of the unresolved L-line multiplet, whereas well resolved K lines are used for the otherelements. In XPS, the Mo results rely on the analysis of the third-shell photoelectrons,whereas all the other elements use electrons from the second shell. Perhaps the discrepan-cies are to be found in these distinctions.

Table 4 presents assays of the treated layer. The EDX results are must be rescaled beforecomparison with the others, because of the insensitivity to N. As presented, they aredirectly comparable with the assays of the bulk. It is evident that the relative proportions ofthe main alloying elements are the same in the treated layer as in the bulk. The differencesin the C, N and O concentrations between HI-ERDA and the 210-µC XPS results maysimply reflect real differences in the two samples examined. The differences in Cr, Ni andMo are more difficult to rationalise. XPS suggests that there is some fractionation of themetals in the outermost regions of the treated layer; perhaps this can account for theobservations. However, XPS depth profiling also relies on sputtering, and so experiencesmany of the problems of SIMS: roughening of the crater floor, preferential sputtering andsputter-driven implantation. Whatever the cause, comparison of the results in Tables 3 and


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Table 4 Compositions measured in the treated layer (at. %, balance: Fe).

XPS†

Element EDX HI-ERDA 18 µC 210 µC

H 0.08 ± 0.02C 0.80 ± 0.05* 4.5 2.7N 27.9 ± 1.2* 32.6 19.6O 0.16 ± 0.02 15.8 2.1Si 0.41 ± 0.10 0.39 ± 0.05Cr 16.82 ± 0.17 12.4 ± 0.7 25.2 10.7Mn 1.26 ± 0.09Ni 9.76 ± 0.33 6.7 ± 0.7 2.5 14.4Mo 2.40 ± 0.22 0.90 ± 0.10 4.9 1.8

*Peak values.†The uncertainty in the XPS values is ± 0.2 percentage points. The µC values refer to thedelivered charge from the Ar+ sputtering beam; the sputter rate is of order 0.2 nm µC–1.

4 gives the clear impression of greater scatter in the XPS results, especially when comparedwith claimed uncertainties, than in the results from other methods.

3.3 ELEMENTAL DEPTH PROFILE FOR NITRIDED AISI 316 STAINLESS STEEL

We have taken SIMS depth profiles on dozens of samples; they all show the same features:reduced ion-current ratios for Mo and Cr, carbon piled up at the interface between thetreated layer and the bulk and a characteristic shape to the nitrogen profile. On theevidence presented here the variations observed for Mo and Cr do not correspond tovarying compositions of these species: the ratios [Cr]:[Fe]:[Mo] are the same in the treatedlayer as in the bulk. However, the shapes of the carbon and nitrogen profiles can be takenseriously. To set these on an absolute scale, they must be calibrated at some point on theprofile. For nitrogen, this can be done by analysis of implanted standard samples, by HI-ERDA or some other technique. In fact, for the sample shown in Figs 1 and 4a, it provedmost convenient to use some old results from proton backscattering.

Proton backscattering (pBS) is essentially the same as Rutherford backscattering, but weused bombarding energies well above the Coulomb barrier because of the thickness of thenitrogen-rich layer. Also for this reason, we choose protons rather than the often-used He+

ions. Details of the method and representative spectra have been publishedelsewhere.7,22,30 The spectra were analysed with the computer program RUMP19 andchecked with the computer program SIMNRA.31 The observed spectra can be reproducedusing no more than four layers with constant nitrogen concentration in each layer. Thisrather crude representation of the depth profile is the best that can be obtained from thedata. However, the nitrogen concentration at the surface is well determined; the valueobtained is 22.0 ± 0.5 at. % and this provides a means of calibrating the SIMS sputterprofile.


196


Fig. 11 Elemental depth profile for the same sample as in Figs 1 and 4a constructed as described inthe text. Uncertainties are shown at selected locations on the depth profiles; in some cases these aresmaller than the symbols.

The result is shown in Fig. 11, together with the results for the other elements. Theprofile for carbon is from SIMS, normalised to the concentration in the bulk determinedby HI-ERDA (Table 3). The constant hydrogen profile is also from HI-ERDA. Theconcentrations of Si, Cr and Ni in the bulk material are taken as the means of the valuesfrom HI-ERDA in Table 3 and EDX in Tables 3 and 4. The value for Mn comes fromEDX alone and, in view of the doubts about the EDX results for Mo, the Mo value is fromHI-ERDA alone. The concentrations of all these elements in the treated layer are deter-mined by simply scaling to allow for the carbon and nitrogen profiles.

As can be seen from Fig. 11, the nitrogen concentration is not uniform across the treatedlayer. It is highest at the surface, falling gradually to about half of the surface value at theinterface with the bulk. After this point, the nitrogen concentration decays exponentiallyinto the bulk. This non-error-function behaviour has been widely observed with a varietyof profiling techniques.1,25,32–48 It has been described with a trapping model, in whichnitrogen is trapped in the treated layer, presumably by chemical binding with Cr.36 Thetreated layer is then viewed as a region of occupied trap sites through which furthernitrogen can diffuse quickly to the interface. That is, the nitrogen diffusion coefficientvaries with nitrogen concentration.33,36 Our observation of a chemical association betweennitrogen and Cr, Mo and probably Fe supports this view. Recent work using 14N and 15Nas successive nitriding agents46 has required revision of the model to include the possibilityof nitrogen escape from shallow traps.49 This could find justification in the range ofbinding energies observed in our XPS work, with nitrogen being at most only weaklybound to Fe.

The displacement of carbon by the incoming nitrogen is a ubiquitous feature of ourdepth profiles. This behaviour has been observed in nitriding of stainless steel by plasma-immersion ion implantation,34 in a pulsed dc glow35 and by ion implantation with anitrogen beam.41


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4. CONCLUSIONSThis work seeks to establish a method of placing an absolute concentration scale on SIMSelemental depth profiles of nitrided stainless steel. The task was separated into two parts:establishing the reliability of relative measurements of concentration (the shape of thedepth profile) and then obtaining a reliable absolute concentration at one point in theprofile. From a combination of the use of standard samples prepared by nitrogen-ionimplantation and comparisons of assays and depth profiles from SIMS, EDX, HI-ERDAand XPS, we conclude that there are negligible matrix effects in the SIMS sputter yields ofnitrogen and carbon, but that the yields of MoCs+ and CrCs+ ions are significantly affectedby the concentration of nitrogen, with the matrix effect being most marked for Mo. Thus,the shape of the nitrogen and carbon depth profiles from SIMS can be taken at face value.HI-ERDA gives depth profiles of the same quality as SIMS, but is limited to a relativelythin nitrogen-containing layer.

For the metals in the AISI 316 alloy, results from EDX, HI-ERDA and XPS show thattheir relative concentrations averaged over the treated layer are the same as in the bulkmaterial. Hence, the depth profile of these elements, and also Si, can be obtained by simplyscaling the values in the bulk to allow for the dilution caused by the presence of nitrogenand, to a much lesser extent, carbon. The SIMS carbon profile is placed on an absolutescale by using the concentration in the bulk measured by HI-ERDA. For the nitrogenprofile, its absolute value could be determined with ion-implanted standards, by HI-ERDAif the treated layer is thin enough or, as in the present work, by using the surface nitrogenconcentration determined from proton backscattering.

Detailed examination of the peak positions and widths in XPS leads to the conclusionthat there is a clear chemical association between the nitrogen in the treated layer and Crand Mo, and none between nitrogen and Ni. The case of iron is less certain, but the weightof evidence supports an N–Fe association with quite low binding energy. These resultssupport the model of the kinetics of nitriding in austenitic stainless steel, which views thenitrogen as residing in traps shallow enough for atom exchange with incoming nitrogen.

ACKNOWLEDGEMENTSThe authors gladly acknowledge the contributions of the late Professor T.R. Ophel to thiswork. Had he lived to see the drafting of this paper, he would have been a co-author. Thisresearch was supported by the Australian Institute of Nuclear Science and Engineering, theAustralian Research Council and by an Australian Postgraduate Research Award.

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35. A. Leyland, D.B. Lewis, P.R. Stevenson and A. Matthews, ‘Low Temperature Plasma DiffusionTreatment of Stainless Steels for Improved Wear Resistance’, Surf. Coat. Technol., 1993, 62,608–617.

36. D.L. Williamson, O. Ozturk, R. Wei and P.J. Wilbur, ‘Metastable Phase Formation andEnhanced Diffusion in F.c.c. Alloys under High Dose, High Flux Nitrogen Implantation atHigh and Low Ion Energies’, Surf. Coat. Technol., 1994, 65, 15–23.

37. F. Bodart, Th. Briglia, C. Quaeyhaegens, J. D’Haen and L.M. Stals, ‘Investigation of NitridePhases in Stainless Steels by Nuclear Reaction Analysis and X-Ray Diffraction’, Surf. Coat.Technol., 1994, 65, 137–141.

38. O. Ozturk and D.L. Williamson, ‘Phase and Composition Depth Distribution Analyses of LowEnergy, High Flux N Implanted Stainless Steel’, J. Appl. Phys., 1995, 77, 3839–3850.

39. R. Wei: ‘Low Energy, High Current Density Ion Implantation of Materials at Elevated Tem-peratures for Tribological Applications’, Surf. Coat. Technol., 1996, 83, 218–227.

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41. C. Blawert, A. Weisheit, B.L. Mordike and F.M. Knoop, ‘Plasma Immersion Ion Implantationof Stainless Steel: Austenitic Stainless Steel in Comparison to Austenitic–Ferritic StainlessSteel’, Surf. Coat. Technol., 1996, 85, 15–27.

42. S. Leigh, M. Samandi, G.A. Collins, K.T. Short, P. Martin and L. Wielunski, ‘The Influence ofIon Energy on the Nitriding Behaviour of Austenitic Stainless Steel’, Surf. Coat. Technol., 1996,85, 37–43.

43. K. Marchev, C.V. Cooper, J.T. Blucher and B.C. Giessen, ‘Conditions for the Formation of aMartensitic Single-Phase Compound Layer in Ion-Nitrided 316L Austenitic Stainless Steel’,Surf. Coat. Technol., 1998, 99, 225–228.

44. S. Mandl, R. Gunzel, E. Richter and W. Moller, ‘Nitriding of Austenitic Stainless Steels usingPlasma Immersion Ion Implantation’, Surf. Coat. Technol., 1998, 100–101, 372–376.


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46. S. Parascandola, R. Gunzel, R. Grotzschel, E. Richter and W. Moller, ‘Analysis of DeuteriumInduced Nuclear Reactions Giving Criteria for the Formation Process of Expanded Austenite’,Nucl. Instr. Meth., 1998, B136–138, 1281–1285.

47. E. Menthe and K.-T. Rie, ‘Further Investigation of the Structure and Properties of AusteniticStainless Steel after Plasma Nitriding’, Surf. Coat. Technol., 1999, 116–119, 199–204.

48. B. Larisch, U. Brusky and H.-J. Spies, ‘Plasma Nitriding of Stainless Steels at Low Tempera-tures’, Surf. Coat. Technol., 1999, 116–119, 205–211.

49. D.L. Williamson, private communication.

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