Low energy, high current density ion implantation of materials at elevated temperatures for tribological applications

Low energy, high current density ion implantation of materials at elevated temperatures for tribological applications

ELSEVIER Surfaceand Coatings Technology83 (1996) 218-227 Low energy, high current density ion implantation of materials at elevated temperatures fo...

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ELSEVIER

Surfaceand Coatings Technology83 (1996) 218-227

Low energy, high current density ion implantation

of materials at elevated temperatures for tribological applications R. Wei

Hughes Research Laboratories,

RL57, 3011 Malibu

Canyon RONI, Malibu,

CA 90265-4799,

USA

Abstract Low energy, high current density ion implantation at elevated temperatureshas been shown to improve significantly the tribological propertiesof various materials.This paper summarizesthe resultspublishedpreviously in this researcharea and presentssomenew results.Comparisonsof this techniqueare madewith ion nitriding and high energyion implantation conducted under similar conditions (treatment temperature,treatment time and so on) OSIausteniticstainlesssteeland tool steelmaterials. The microstructural analysesand tribological evaluationspresentedhere show that all three techniquesgeneratealmostidentical microstructureson eachmetalstudied,but low energyion implantation producestreatedlayerswith highernitrogen concentrations and deeper diffusion, leading to higher wear resistance.A physical model is proposed to explore the mechanismsfor these advantageousphenomena.The analysissuggests that a high current densityis the primary mechanismresponsiblefor the formation of deepnitrogen-containinglayers.The ion energyis of secondaryimportance,aslong asit is sufliciently high to overcomecertain surfacebarrier potentials,to allow the removal of native oxide layers, to prevent surfaceoxidation and to allow the build-up of a high concentration of atomic nitrogen on the top of the treated surfaceto facilitate subsequentfast diffusion. Someapplications and limitations of this techniqueare also addressed.It seemsevident that low energy implantation (slightly higher than for ion nitriding, but much lower than for high energy ion implantation) at high current densities(much higher than thoseusedin both ion nitriding and high energy implantation) generatessuperior nitrogen-containinglayers on many materials, and hence the superiortribological performancecomparedto the other two techniques. Keywords: Low energyimplantation; Ion nitriding; Steels;Elevatedtemperatures;Mechanisms

1. Introduction

--It is well known from the theoretical calculations of Ziegler et al. [ 11 and from numerous experiments by manyresearchers including Picraux and Peercy [ 21 that, at low temperatures, high energy (about 100 keV) ion implantation (HEII) results in implanted layers about 0.1 pm thick for commonly used gaseous ion species such as nitrogen, implanted into most metals. However, for tribological applications involving wear, thicker nitrogen-containing surface layers are usually desirable. If the temperature can be tolerated, thicker layers can be attained by heating the metal parts. Thus Wei et al. [3,4], Williamson et al. [S] and Hutchings [6] have utilized high doseimplantation of N2 at elevated temper-

atures to take advantage of thermal diffusion and to obtain implanted layers up to 10 pm deep in stainless steels. Compared with HEII, low energy (about 1 keV) ion implantation (LEII) at room temperature generates

implanted layers only a few nanometers thick (for N, into steels by TRIM calculation) [l] but, when this is 0257-8972/96/$15.00 0 1996ElsevierScience S.A.All rightsreserved PPrlTn~~7.8477/_O~\A7R3i?-5~

done at an elevated temperature, significant diffusion of the near-surface-implanted species takes place and its effect dominates the effect of ion injection into the target. As a result, low energy ion processing can be made to produce microstructures and tribological performances of metals which are very similar to the results for high energy processing [ 71. Since it is a low voltage treatment, however, the facility demands on power supplies are alleviated dramatically (e.g. for insulation, X-ray shielding and safety). Therefore a cost-effective system which occupies much smaller space than a high energy system can be easily attained. LEII makes high current density treatment of metals possible, thus allowing a high dose to be delivered at a faster rate than in HEII. This is in agreement with the results showing that LEII generates much deeper implanted layers with much higher concentrations of the implanted species than HE11 does under similar conditions [ 7,8], one more advantage which is very appealing for industry. Since this process utilizes thermal diffusion and covers an ion energy range similar to that commonly

R Wei/Swfaee and Coatings Techology

used in the ion-nitriding industry, research is being conducted to compare these two processes[8,9]. This paper presents an overview of LEII of metals (using nitrogen) at elevated temperatures for tribological applications. A summary of the results from several materials will be given, with the emphasis being on stainless steels, since a great deal of data is available on these metals. Similarities and differences in microstructures and tribological performance induced by HEII, ion nitriding and LEII will be discussed,but the emphasis will be on the low energy treatments. A physical model will be proposed and the mechanisms responsible for the formation of deep implanted layers, and hence good wear resistance, will be discussedin detail. Finally, we shall point out potential applications and limitations of this technology in industry, along with subjects for future research.

2. The effectsof low energy ion implantation In recent years the LEII of nitrogen has been studied by several groups worldwide [7,9,10], and some significant results have been obtained from austenitic stainless steels and tool steels. Some of the latest results are summarized as follows. 2.1. Austenitic stainless steels Austenitic stainless steels (SSs) contain a high chromium content and are commonly used engineering materials (mainly becauseof their corrosion resistance). They are rarely used for tribological applications such as wear, however, becauseof poor surface hardness and low loadbearing capacity. Great efforts have been made to improve their surface properties using two different approaches: the commonly used ion nitriding and ion implantation. Since austenitic SSs have generally been considered to be difficult candidates for conventional ion nitriding [ 111, to increase the nitriding depth, an intensified glow discharge has been employed [12] whereby the nitriding gas pressure is reduced and the current density is increased, consequently intensifying the surface bombardment by ions and energetic gas species.The observed result is a much higher diffusion rate of nitrogen. On the contrary, HEII, which was formerly done at a low current density to keep samples at low temperatures, has utilized high current densities to elevate the temperature for these materials. Here it has been found that ion bombardment at a high energy and a high current density during processing greatly enhances the diffusion of nitrogen, so as to increase the wear resistance [4,5]. Most recently, LEII has emerged which utilizes even higher current densities than those used in either HE11 or ion nitriding. This high current density treatment results in even further diffusion of

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nitrogen [ 7,131. The following paragraphs present some typical results. 2.1.I. Auger electron spectroscopyanalyses Typical Auger electron spectroscopy (AES) depth profiles of nitrogen-implanted and ion-nitrided AISI 304 SS samples at energies of 60, 0.7 and 0.4 keV, where the treatment time (30 min) and the temperature (400 “C) are kept constant for each energy level, are shown in Fig. 1. For comparison, an Auger profile of conventional ion implantation at 60 keV energy and at room temperature is presented. As can be seen,high temperature ion implantation at an energy of 60 keV, and a relatively high current density of 0.1 mA cmU2 results in a nitrogen-containing layer depth of about 1 pm, about ten times the projected ion range for low temperature implantation. When the energy is reduced to 0.7 keV, a much higher current density (2.5 mA cm-‘) can be used to maintain the same implantation temperature, resulting in a much higher dose. Consequently, not only does the nitrogen diffuse much faster, but also its concentration is higher, even in the deep regions. The nitrogen profile for a sample nitrided at 0.4 keV is also plotted. It shows that the nitrogen-containing layer is slightly deeper than for the HE11 sample, but with a lower nitrogen concentration. When compared with the profile for the LEII sample (0.7 keV), it exhibits both a lower concentration and a shallower diffusion. The reasons for this will be discussed later. 2.1.2. Microstmcture analyses Microstructural analyses have been conducted using X-ray diffraction (XRD), conversion electron Miissbauer spectroscopy (CEMS) and conversion X-ray Miissbauer spectroscopy (CXMS). The XRD and CEMS patterns are plotted in Figs. 2 and 3 respectively, corresponding to the same samples in Fig. 1. Qualitatively, all treatments at the same temperature induce a formation of the samedominant phase, YN(nitrogen in solid solution), which is called expanded austenite by Vardiman and co-workers [14-161 and is also observed in the AISI 240

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high current density LEII produces deeper nitrogencontaining layers than do the other techniques. In general, if the temperature is slightly lower (at about 380 “C), the magnetic phaseof .s-Fe,N-likenitride occurs in addition to the YNphase.When the treatment temperature is further lowered (340 “C or below), the paramagneticphase of FezN-like nitride dominates.In contrast, at a slightly higher temperature(about 420 “C), chromium starts to migrate and CrN begins to precipitate. When the temperature is further increased to 450-500 “C, most of the expanded austenite converts to CrN and a Cr-depleted b.c.c. phase, a-(Fe, Ni). Increasingthe dose (or treatment duration) also induces a similar trend; at 400 “C for instance,&-Fe,N-likenitride usually remains in the implanted layer in addition to the dominant YNphaseif the doseis low (or the treatment time short), while a small amount of CrN emergesif the dose is high (or the treatment time long). Interested readers may refer to [3,5,7,13,19] for the detailed analyses. A more careful study conducted by Ozturk and Williamson [20] shows that there are two phasesin the TN layer formed at 400 “C: One is magnetic (YN(m)), formed on the foremost top of the expandedaustenitic layer with a thicknesslessthan 0.5 pm, and the other is paramagnetic (YN(p)), formed beneath the magnetic phasewhere the N concentration is lower. Higher levels of greater than 20 at.% N are required to stabilize the magnetic phase and high residual stressmay be important as well [13,20]. When the implantation dose is increased,the thicknessof the magnetic layer increases. This can be verified by sequential sputtering of the nitrogen-containing layer and subsequently using CEMS, which is most sensitiveto the top region (about 100nm from the surface of an implanted sample).The discussion above is valid regardless of the processing methods,including HEII, LEII and nitriding. However, the difference lies in the thickness and the nitrogen concentration of the expanded austenite layer and this can be seen easily from the above AES, XRD and CEMS-CXMS analyses. 2.1.3. Microhndness wensueeinents

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304 and 316 stainlesssteelsimplanted by plasmaimmersion ion implantation [ 17,181. Further quantitative analyses, however, show that the percentage of the bonded nitrogen is different, mainly depending on the processesand correlating well with the retained doses -calculated from Fig, 1. From Figs. 1-3, it is clear that

Vickers microhardnessmeasurementshave beenmade using an ultralight load tester [7], Selectedhardness data vs. the applied load are plotted in Fig. 4. As is known, the hardness at low loads characterizes the implanted layers,while at high loads the hardnesscorresponds to the substrate. As expected,the ion-nitrided sample,having the lowest concentration of nitrogen and a shallow layer, shows a relatively low hardnessat low loads and it approachesthe value for the untreated 304 SS quickly when the normal load is increased. The sampleimplanted at 60 keV showsa slight improvement for the light loads owing to its high concentration at the top surfacebut still cannot support heavy loads, since

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the nitrogen-enriched layer is thin. In contrast, the sample implanted at 1 keV for 125 min exhibits a much improved hardness, even to a deep region of the substrate, because of its thick expanded austenite layer and the high nitrogen concentration in the layer. This suggests that high current density LEII at elevated temperatures significantly improves the surface hardness for the austenitic steels. 2.1.4. Tribological evaluation An oscillating-pin-on-disc wear tester, shown to be suitable for the evaluation of surfaces and subsurfaces treated by ion implantation and ion nitriding [7,21], has been used to evaluate the wear resistance of the treated samples.Typical parameters are as follows: tungsten carbide pins with a curvature of 3.2 mm radius against the discs of 5 cm diameter; average sliding speed of 13 cm s-l; lubricant of 10% oleic acid in kerosene to obtain a mild sliding wear condition. Untreated and nitrogen-treated austenitic steels usually exhibit very different load-bearing capacities or critical loads [3,7,21], which define the transition of the wear mechanisms from mild wear to severe adhesive wear for all treatments. Below the critical load the wear rate of a nitrogen-treated sample is very low, of the order of 1O-3-1O-2 mg N-r km-l and the surface remains smooth, but above the critical load the wear rate of the sample is two to three orders of magnitude higher, about 2 mg N-l km-‘, and the surface roughness increases dramatically owing to the rupture of the surface. The critical load for both untreated 304 SS and untreated 310 SS is determined to be less than 0.1 N [3,7,21-j, the minimum load that can provide us with reliable results from this tester, while the critical load for both the nitrogen-treated steels is much higher, varying from a few newtons to over 100 N, depending on the treatment conditions. In the examples given in this paper, the critical loads for the ion-nitrided, 60 keV-implanted and 0.7 keV-implanted 304 SSare 8 N, 51 N and 93 N respectively. It is evident that a solution-strengthened thick layer with a high concentration of nitrogen is responsible

Tool steels are of interest because of their importance and wide applications in cutting and forming in industry. Byeli et al. [lo] implanted P6M5 steel at a low energy of 500 eV with nitrogen at elevated temperatures and found that the wear resistance of the implanted steel increased by a factor of 3. This paper presents some preliminary results from low energy nitrogen-implanted AISI M-2 and AISI H-11 steelsat elevated temperatures. Both gas and ion nitriding have also been performed under similar conditions; so the similarities and differences of these three processes can be compared. In addition, Wilbur et al. [22] conducted a detailed study on LEII of M-2 steel under different conditions using a block-on-ring tester. 2.2.1. AISI M-2 tool steel To study the effect of LEII on tool steels, some M-2 steel samples were nitrogen implanted, ion nitrided and gas nitrided at both 400 “C! and 500 “C. The ion implantation was conducted at an energy of 0.5 keV and a current density of 4 mA cm-‘. Both the ion nitriding and the gas nitriding were conducted using standard industrial procedures. In the ion nitriding, a mixture of gases(75 vol.% H2 + 25 vol.% N2) were back-tilled into a vacuum chamber to a pressure of 3.5 Torr. A glow discharge was generated by applying a voltage of about 600 V, and current densities of 1.6 mAcm-’ and 3.2 mA crnp2 were maintained for the 400 “C and 500 “C treatments respectively. Gas nitriding was conducted in a furnace filled with a mixture of NH3 and N2 gases, which resulted in 85% residual NH,. The detailed conditions can be found in [8]. The heating time used for implantation or nitriding (the time before the prescribed temperature was reached) was dependent on the process and was a few minutes for the ion-implanted samples and over 2 h for both ionnitrided and gas-nitrided samples. Once the prescribed temperature had been reached, the treatment time was kept the same for all the different processes. Shown in Fig. 5 are the AES nitrogen profiles for all these treated samples at 400 “C (Fig. 5(a)) and 500 “C (Fig. 5(b)). As can be seen,relative to ion nitriding, LEII results in a higher concentration of nitrogen on the top surfaces,up to a depth of 0.5 pm for the 400 “C run and 1 urn for the 500 “C run. Thereafter both profiles for the implanted and the ion-nitrided samples are almost identical. The higher concentration of nitrogen is attributed to the higher current density used in ion implantation. Compared with both ion nitriding and LEII, gas nitriding produces lower concentrations under both conditions. The recession of the nitrogen profile at each

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temperature is the result of the formation of a thick oxide layer, which has been recognized and emphasized by the ion-nitriding community [23]. Corresponding to Fig. 5(a), the XRD patterns for the 400 “C treatment are plotted in Fig. 6, which clearly shows the formation of a high fraction of E nitride in both ion-nitrided and ionimplanted samples (the patterns for 500 “C are similar) while, in the gas-nitrided samples, strong signals from oxides can be seen clearly. It seemsthat the bombardment by high energy ions in both ion nitriding and implantation is sufficient to prevent the surfaces from oxidizing. As a result, nitrogen diffuses at faster rates than for the gas nitriding, forming thick nitridecontaining layers. It should be pointed out here, however, that, even though both ion nitriding and LEII seem to produce similar nitride(s) and nitrogen profiles, the heating time for ion nitriding is much longer (120 min to reach 400 “C and 150 min to reach 500 “C). During this period, some nitrogen diffusion has already taken place even before reaching the prescribed temperatures. In contrast, the heating time for LEII at each temperature is less than 5 min and the nitrogen diffusion has barely started before the sample reaches the prescribed temperatures. Thus high current density LEII greatly increasesthe nitrogen concentrations and produces deep nitride-containing layers in a short time.

In the next example, some H-11 steel samples were nitrogen implanted at an energy of 600 eV and a current density of 4 mA cm-’ for 30 min at a temperature of 400 “C. The Vickers microhardness data obtained from the untreated and implanted samples are plotted as a function of the applied normal load in Fig. 7. A significant increase in hardness can be observed for the implanted sample. Even at the highest load, 10 N, the hardness is about 40% higher than for the untreated

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accuracy of these assumptions, we can refine our model. In accordance with this approach, the following discussions will first examine the mechanisms for ion nitriding and then explore the mechanisms for LEII. 3.1. The yoles of ion energy and current density in ion nitding

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H-11 steel. The hardness increase is mainly due to the formation of s-Fe,N, as indicated in Fig. 8. Compared with the untreated sample, a significant amount of E phase appears in addition to the original carbide (Fe, Cr),C,. When the implantation temperature is increased to 680 “C, diffusion of chromium takes place, forming CrN, and severe oxidation also occurs.

3. On the nitriding mechanismsdue to low energy ion bombardment Regarding the physical model, ion nitriding has been studied for decades and several mechanisms have been proposed. Hudis [24] suggested that high energy ion bombardment is mainly responsible for the high nitrogen diffusion rates that cannot be explained by the conventional diffusion theories. Strack [25] and later Brokman and Tuler [26] proposed that ion bombardment generates vacancies on the metal surface, and they diffuse into the metal together with the ions in the form of vacancy-ion pairs. Since the vacancies have a low activation energy, nitrogen diffuses more rapidly than it does in a metal without ion bombardment as in gas nitriding. The observed phenomenon that the diffusion coefficient is proportional to the ion current density seemsto support this ionic theory. However, Tibbetts [27] had demonstrated that neutral nitrogen atoms alone are responsible for ion nitriding. Szasz et al. [28] later developed the Tibbetts model and suggested that thermal chemical reaction is the main mechanism for ion nitriding, in which a dissociative adsorption reaction of nitrogen, assisted by the addition of hydrogen, N,+ N,(ad)-+2N(ad), takes place and results in the subsequent diffusion of the nitrogen atoms. Since the effects of LEII resemble those of ion nitriding, some of these mechanisms for ion nitriding are naturally assumed to apply to LEII. Then, as more detailed experimental study reveals limitations in the

The ionic model with vacancy generation mentioned above is widely accepted in ion nitriding and it assumes that the intense bombardment of a material being nitrided by high energy species(ions and activated neutrals) is the driving force for fast nitrogen diffusion compared with conventional diffusion. Hence, an increase in either ion energy or current density (or both of them) will result in further enhanced diffusion. First, let us consider the theoretical effect of ion energy. When a glow discharge is generated in ion nitriding, a Child-Langmuir sheath of thickness L forms between the plasma and the cathode surface (the part to be nitrided), where L is proportional to P’3/4/j1/2with I/ the potential difference between the plasma and the cathode and j the current density. In the sheath, the mean free path for charge-exchange collisions is ?,= l/no, where n is the gas number density and 0 is the collision cross-section for ion-neutral charge exchange. The ratio L/A characterizes energy loss due to charge exchange collisions. A high ratio implies that the ions entering the cathode sheath region will be subjected to many collisions, losing their energy to the neutral gas before they reach the cathode surface. The typical processing pressure for conventional glow discharge nitriding is l-10 Torr, and this gives a high L/2 ratio of about lo-15 [29,30]. Consequently, if the part is biased at 500 V, only less than 0.001% of the high energy species bombard the surface with the maximum energy, 500 eV, and the mean energy is less than 50 eV. This mean energy may be too low to sputter the surface oxide layer and to facilitate nitrogen diffusion, as is discussedin the next section. On the contrary, if now the ratio is reduced to unity (L//1.= l), 37% of the high energy particles will reach the surface with the maximum energy. This is in fact realized in the low pressure triode-plasma systems where the processing pressure is on the order of a few tens of millitorrs and a thermionic filament is used to enhance the discharge. As the mean ion energy increases, the bombardment of the part being nitrided owing to high energy speciesis intensified. As a result, the vacancy production increases and hence nitrogen diffuses into deeper regions [ 12,29-311. Now let us consider the effect of current density. From the same ion bombardment theory, a higher current density is also favorable for fast diffusion owing to the higher concentration of vacancies produced. More specifically, in ion nitriding, a high ratio of ion current density to total particle flux, defined as the “ionization effi-

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ciency”, is desirablesinceit increasesthe diffusivity and reactivity of the ionic species [32] and the diffusion coefficientis linearly proportional to the current density [26]. Therefore an increase in either the mean ion energy or the ion current density (or both) will result in further nitrogen diffusion. It seemsthat we could use this ionic bombardment theory to explain the mechanismof LEII since now we have an even higher energy and current density, and hence a higher vacancy production, implying a faster diffusion. However, an early test [24] and a recent experiment [ 131 both contradict the vacancy-enhanced diffusion concept. In the first test, a mixture of argon and nitrogen was used and the results compared with thosefor the previousmixture of hydrogen and nitrogen, all at the samenitrogen partial pressure.Argon should generatea much higher vacancy production and hence faster nitrogen diffusion; however, the Ar mixture resultedin a shallowernitrided layer than the H, mixture for the same treatment time and temperature. In the second test, an argon-nitrogen mixture was used and the resultscomparedwith thosefor pure nitrogen. Again, the mixed gases produced a shallower nitrided layer, even though a much higher vacancy concentration was produced. It does seem that we need a new physical model that fits the experimental results and overcomes the discrepancies.Basedon the latest researchon LEII, the following model is proposed and its mechanismsfor fast diffusion are discussed,particularly for LEII. In this model, both the ion energy and the current density are considered but it does not require considering the sequentialprocessesof bombardment of the surfaceby high energyspecies,vacancyproduction and subsequent vacancy-ion pair diffusion. 3.2. The roles of ion energy and current density in low energy ion implantation

In an LEII system,an ion source is commonly used to produce ions and the part to be treated is located in a low pressurevessel( 10-6-10-4 Torr) which communicates with the source. Owing to the low pressure,ion charge exchangecollisions are minimized. The ratio L/,J approacheszero and the ionization efficiencyis closeto unity. Now the ions are mainly responsiblefor nitriding, and the high energy neutral ions consist of only a small fraction comparedwith those in high pressureion nitriding. When the ions extracted from the source have passedthrough the “free space”, they collide with the surfacewith their full energy,typically 1 keV. Although this energy is much lower than for conventional HEII, it is much higher than the mean energy used in both conventional and low pressureglow dischargenitriding. Upon impact, the nitrogen ions (over 70% of them in the form of N2+) having sufficient energy to overcome surface barriers transfer their kinetic energy to the

substrate, resulting in heating of the substrate. The molecular nitrogen ion is dissociatedto atomic nitrogen and subsequentlybecomesimplanted into the surface. Even though the implanted depth is shallow (a few nanometers for 1 keV Nz ions in steels),a high concentration of nitrogen atoms (estimated at about 5 x 1O22atoms crne3 assuming a current density of 1 mA crnw2N2 flux) is embeddedand maintained in this region, thus facilitating the subsequentnitrogen diffusion at a fast rate when the part is heated to elevated temperaturesowing to kinetic energy transfer. This model states that the ion energy is important only if it allows the ions to overcomesurface barriers and be implanted into the surface to form an atomic nitrogen layer, but a higher energymay not be necessary. This is based on the observations that the implanted depth is much shallower, even if a high energy is used (0.1 ym for 100keV N, implantation), than the diffusion zone (typically greater than 1 pm); hence the nitrogen atoms in thesedeep regions have no “memory” of their energy history and their transport is solely determined by thermal diffusion. In contrast with the ion energy, the ion current density is far more important becausea high ion flux will provide this implanted region with a high concentration of nitrogen atoms and hencesustain the subsequent fast diffusion, as predicted from the traditional diffusion theory. Using this model, we can now explain that pure nitrogen produced a thicker nitrided layer than the mixture of argon and nitrogen becauseof the higher concentration of nitrogen atoms [ 131. This mechanismdoes not discount the ion bombardment theory in ion nitriding; however, it makes a bridge to the second theory in ion nitriding, in which the dissociative adsorption reaction, N2 +N,(ad)+ 2N(ad), is the driving force for the subsequentdiffusion of nitrogen into steel substrates. In the model proposed in this paper, the nitrogen atoms are obtained through the ion implantation and the spontaneousdissociation of molecular nitrogen ions as the ion energy becomesmuch higher. To verify the mechanismdiscussedabove,we needed to investigatethe effectof ion energy and current density on the subsequentfast nitrogen diffusion. In doing so, two tests were conducted. In the first, three samplesof AISI 304 SS were ion bombarded, all at 420 “C for 60 min, but at three energy levels: 40 eV, 120eV and 400 eV. The corresponding current densitieswere about 5 mA cmw2, 2 mA cm-* and 1 mA crnM2 respectively. After the treatments, the samples were studied using XRD and then they were cross-sectioned,etched and examined using scanning electron microscopy (SEM). The XRD data show that the nitrogen ion bombardment at all the three energylevelsproducessimilar microstructures with the dominant yN phase in addition to some CrN precipitation, as observedand discussedearlier in this paper. More importantly, the SEM study reveals

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the relationship between the ion energy and the nitrided layer thickness. Shown in Fig. 9 are the SEM photographs for the samples nitrogen bombarded at 40 eV (Fig. 9(a)), 120 eV (Fig. 9(b)) and 400 eV (Fig. 9(c)). Measured from Fig. 9, the nitrided layer thickness is plotted in Fig. 10 against the ion energy. It is clear that, at the lowest energy, the nitrided layer depth is thin because the energy is low, even though the sample is bombarded with the highest ion flux. When the energy I

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Fig. 10. The effect of ion energy on the nitrided layer growth.

Fig. 9. Scanning electron micrographs of AISI 304 SS after nitrogenion bombardment at (a) 40 eV, (b) 120eV and (c) 400 eV.

is increased, the nitrided layer depth increases, despite the fact that the ion flux decreases. Apparently, ion bombardment at a low energy, 40 eV for instance, neither sufficiently overcomes surface barriers nor results in efficient dissociation of nitrogen molecules, thus remaining incapable of the subsequent fast diffusion. The issues related to the surface barriers will be addressed in the next section. In the second test, nine samples of AISI 304 SS were implanted at three energy levels (400, 700 and 1000 eV) and three current densities (1, 2 and 3 mA cm-‘) at the same temperature (400 “C) and for the same implantation time (60 min). A special fixture which could be either heated or cooled when necessary was used to maintain the sample temperature to within a few degrees Celsius. Preliminary results appear to verify the proposed model; nitrogen diffusion depends on the ion energy but even more strongly on the current density. These results confirm the data discussed in Section 2.1 of this paper (see Fig. l), and earlier data in [7,9,13]. The reference works show that the implanted layer thickness slightly increases with increasing energy up to about 1-2 keV and then decreasesas the energy increasesfurther, owing to the low current density that has to be used to obtain the same treatment temperature. The quantitative relationship between the ion current density and the diffusion coefficient will be published elsewhere. It should be emphasized here that the model proposed in this paper does not predict new mechanisms for diffusion but only indicates that high current density implantation at certain energy levels overcomes surface barriers, builds up a high concentration of nitrogen atoms and facilitates the subsequent diffusion. As far as the diffusion of nitrogen in steels is concerned, Williamson et al. [ 13,191investigated the nitrogen diffusion data published in a number of papers and proposed the following diffusion model. In austenitic SSs, for instance, a high flux of nitrogen will occupy all the Cr sites quickly and form strong bonds of YN in the near surface, thus allowing the incoming nitrogen to diffuse

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through this layer at a much higher rate, just as if it were diffusing in pure y-Fe, to reach to the front of the layer where new Cr sites are available.In contrast, in a low current density treatment, there is not sufficient nitrogen to bond all the Cr; therefore the incoming nitrogen will have to diffuse through the nitrogencontaining layer in which plenty of Cr sitesare available, thus slowing the nitrogen diffusion. 3.3. Swface oxidation during nitriding and its removal by ion bombardment

It is well known that the oxide layer commonly formed on the top of a metal prohibits nitrogen diffusion; thereforepre-cleaningis often necessarybefore nitriding, as well as for low pressure glow discharge nitriding where the mean ion energy is much higher than for the conventionalion nitriding [23,29]. In addition, an oxide layer can actually grow on a part during nitriding if the ion energy is not sufficiently high. This oxide growth is mainly due to the outgassing of water vapor absorbed by the part to be treated and the vacuum chamber before it is pumped down. When nitriding begins, the temperaturesfor both the part and the chamberincrease, hence resulting in an outgassing of the water vapor. If the partial pressureof the HZ0 is high in an ion-nitriding system,ionization of H,O can occur and this will cause severeoxidation of the part in addition to its “native” oxide. Shown in Fig. 11 is the history of the partial pressure of HZ0 in a typical vacuum chamber during nitriding, measuredusing a residual gas analyzer (RGA). It can be seenthat, as the systemis being heatedup, the partial pressure of H,O increasesdramatically, and within a tirne it reaches the peak value. Then it decays /

0

I

5

1

I

I

IO HEATING TIME (MIN.)

I

15

I

20

slowly, Typically, evenif a vacuumvesselfor ion nitriding is pumped down to a base pressure in the 10-6-10-5 Torr range, the peak Hz0 pressurewill still get into the millitorr range, and this will induce the formation of an oxide layer even before the nitriding starts. Furthermore, if the ion energy is not sufficiently high, the oxide layer, as a surface barrier, cannot be removed.Consequently,it inhibits the nitriding process. To investigatethe oxide growth during nitriding and its removal by low energy ion sputtering, the same samplesbombarded at 40, 120 and 400 eV, mentioned above,wereexaminedusing secondary-ionmassspectroscopy. Figure 12 shows the depth profiles of the oxygen in these samples.It is evident that an oxide layer, in addition to the native oxide layer, does grow on the top of the sample when a low energy (40 eV) is used in nitriding owing to the outgassingof water vapor. As the ion energy increases,the thickness of the oxide layer decreasesand eventually vanisheswhen an energy of 400 eV or higher is used.Therefore,considering Figs. 9, 10 and 12, we can conclude that implantation at a relatively high energy effectively removes the native oxide layer, preventsthe surfacefrom oxidation during nitriding and enhancesnitrogen diffusion. 4. Conclusions

High current density LEII at elevated temperatures shows significant improvement in forming thick nitrogen-containinglayers with high concentrations of nitrogen;hencethe tribological performanceis superior than both HE11 and ion nitriding, with the microstructures in the treated layers of various steelsbeing essentially identical for all thesethree different processes.Basedon the experimentalresults, a physical model is proposed. It is apparent that the ion energy is important only to the extent that it must be sufficiently high to overcome surfacebarriers, to sputter-removenative oxide, to prevent oxidization during treatments and to build up an

40 60 DEPTH (nm)

Fig. 11. Typical outgassing history of water vapor in a vacuum system

Fig. 12. The effect of ion energy on the oside growth

due to its heating.

removalduring nitrogen-ion bombardment.

and oxide

R. WeijSwface and Coatings Technology83 (1996) 218-227

implanted layer of nitrogen atoms on the top of a material for subsequent fast diffusion. An even higher energy may not be necessary or even advisable, since a lower current density would have to be used to avoid overheating of the part being treated. On the contrary, the high current density is crucial for the formation of thick layers with high nitrogen concentrations. Compared with the ion-nitriding process where multiple parts with complicated geometry can be treated simultaneously and quite uniformly, the major disadvantage of LEII is that currently it is a line-of-sight process, since an ion beam is commonly used for exploratory study. However, it may not be too difficult to overcome this limitation, for instance by modifying the technique of plasma ion implantation (in which at present a pulsed high voltage and a low average current are required) to a pulsed low voltage and a high current system. Other techniques are also being studied in order to achieve large-scale, low cost processing. Moreover, even though LEII generates thicker nitrogen-containing layers than ion nitriding does for the same treatment time, so far only layers a few to tens of micrometers thick have been studied. For some tribological applications, nitrided layers submillimeter thick may still be needed and more work has to be done for such applications. In addition to nitrogen, future research may also include carbon and boron and be aimed at achieving results similar to those obtained from nitrogen. In time, given its advantages, low energy, high current ion implantation may be used on all the steels and titanium and aluminium alloys currently being ion nitrided to achieve faster processing and superior tribological performance.

Acknowledgment The author is very grateful to Dr. P.J. Wilbur, Dr. D.L. Williamson, Dr. J.J. Vajo, Dr. R.E. Doty and Dr. J.N. Matossian for their contributory discussions and for performing some of the implantation work, tribological tests and microstructural analyses.

Referen’ces [l]

J.F. Ziegler, J. Biersack and U. Littmark, The Stopping and Range ofIons in Sol&, Vol. 1, Pergamon, Oxford, 1985.

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