M. Samandi

Microstructure, corrosion and tribological behaviour of plasma immersion ion-implanted austenitic stainless steel

The surface modification of AISI 316 stainless steel by plasma immersion ion implantation (PI3) has been investigated over a range of treatment temperatures. Below 250°C the results are similar to those obtained by conventional ion beam implantation of nitrogen, but the depth of nitrogen penetration increases dramatically with temperature. Up to 450 °C a nitrogen-expanded austenite phase is formed which is shown to have improved corrosion performance over the untreated material. At 520 °C chromium nitride is precipated and the expanded austenite transforms to martensite, leading to a reduction in corrosion resistance. Pin-on-disc testing indicates improved wear resistance at all temperatures, with reduction in the wear volume by factors of several hundred at high loads. This can be attributed to the formation of an oxide layer which prevents the initiation of severe metallic wear.

Nitriding of Austenitic Stainless-Steel by Plasma Immersion Ion-Implantation

Plasma immersion ion implantation (PI3™), in which the diffusion of nitrogen from a low pressure r.f. plasma is combined with the implantation of nitrogen ions at energies up to 45 kV, is an effective means of nitriding austenitic stainless steel. At temperatures up to 450 °C, tribological properties can be improved without loss of corrosion resistance. In common with other nitriding processes in this temperature range, a supersaturated f.c.c. phase is formed, sometimes described as expanded austenite, which is maintained to very high nitrogen concentrations. At higher temperatures, chromium nitride is precipitated and the expanded austenite decomposes, leading to a reduction in corrosion resistance. Glancing-angle X-ray diffraction (XRD) of PI3-treated AISI 316 stainless steel at temperatures between 350 and 450 °C suggests that a highly homogeneous layer of expanded austenite is produced. The expansion increases with increasing process time, but decomposition of the supersaturated phase occurs after several hours of treatment if the temperature is too close to 450 °C. For a fixed process time, the expansion appears to be greatest at the lower temperatures (350 °C), although it can also be influenced by other processing parameters such as plasma density. Microstructural examination by cross-sectional transmission electron microscopy (TEM) has challenged the identification of the supersaturated phase as expanded austenite and reveals the complexity of the modified layer not seen by glancing-angle XRD. Most striking is the formation of a thick (2–3 μm) amorphous zone which may contain nanocrystalline precipitates of CrN and α-ferrite. A highly defective layer (up to 2 μm thick) of expanded austenite has been observed to underlie the amorphous zone where nitrogen diffusion is facilitated by the high defect density. Only partial reconciliation of the TEM results with the XRD observations has been possible to date.

Wear resistance of plasma immersion ion implanted Ti6Al4V

Abstract The plasma immersion ion implantation (PI 3 tm ) process has been employed in the treatment of the Ti6Al4V alloy in order to improve its notoriously poor tribological properties. In particular, this study was undertaken with a view to its potential application for the surface engineering of orthopaedic implants. PI 3 has been developed over recent years at the Australian Nuclear Science and Technology Organisation (ANSTO). The hybrid nature of this technique combines elements of both ion implantation and plasma nitriding, and has been shown to produce components with unique surface properties and optimum performance characteristics. A detailed study of the PI 3 process on the Ti6Al4V alloy has been undertaken. Treatment was carried out in a pure nitrogen atmosphere at temperatures of 350, 450 and 550 °C. In each case, specimens were treated for 5 h, with a high voltage pulse (typically 40 kV) applied directly to the workpiece. Wear resistance of the treated samples was assessed using a standard CSEM pin-on-disc wear machine, with a single crystal ruby ball as the contact tip. Glancing angle X-ray diffraction (GAXRD) was employed to determine the phases present in the surface modified layer. These findings were then compared to those achieved from parallel work with conventionally ion implanted and low temperature plasma nitrided samples. It was established that a high treatment temperature of 550 °C was necessary for substantial improvements in the properties of the Ti6Al4V material. Under these conditions the PI 3 technique promoted significant increases in Knoop hardness, and wear resistance an order of magnitude greater than conventional ion implantation. Wear rates were typically reduced by four orders of magnitude compared to those of the untreated Ti6Al4V. This is thought to be associated with the increased mobility of nitrogen in α -Ti at these temperatures, producing a deeper, hardened case. The presence of TiN was observed in the microstructure of PI 3 Ti6Al4V samples at all temperatures in the range.

Microstructure and secular instability of the (Ti1 − x,Alx)N films prepared by ion-beam-assisted-deposition

Abstract(Ti1 − x,Alx)N films were prepared by ion beam assisted deposition (IBAD). The films were synthesized by depositing titanium and aluminum metal individual vapor under simultaneous bombardment with nitrogen ions in the energy range of 0.2–20 keV with the (Ti1 − x,Alx)/N transport ratio in the range of 0.5–2.0. The films were formed onto Si(111) wafers at room temperature. Structural characterization of the films was performed with x-ray diffraction and selected area electron diffraction. The crystalline structure of the (Ti0.64,Al0.36)N and (Ti0.33,Al0.67)N films were found to be a metastable single-phase B1-NaCl structure. The (Ti0.29,Al0.71)N films revealed a two-phase mixture consisting of NaCl and würtzite structural phases. The AlN solubility limit into TiN, which approximately equal with x value, calculated by using electron theory was about x = 0.65, which shows good agreement to the experimental results. Phase separation after half a year of aging at room temperature in air was observed on the (Ti0.33,Al0.67) films whose AlN content is close to the solubility limits.

The influence of ion energy on the nitriding behaviour of austenitic stainless steel

Abstract Recent work on austenitic stainless steels has indicated that low energy, high current density nitrogen implantation can result in nitrided layers several micrometers in depth and of considerable hardness. This work was initiated to examine the effects of ion energy during ion implantation at elevated temperatures. Three nitriding methods were considered: plasma immersion ion implantation (PI 3 ), radio frequency (r.f.) plasma nitriding and ion beam nitriding. The structure and nitrogen profile of austenitic stainless steel were examined after the different treatments by a range of analytical techniques including glancing angle X-ray diffraction (GAXD), ultra microhardness indentations (UMIS), glow discharge optical spectroscopy (GDOS) and nuclear reaction analysis (NRA). It was found that similar surface structures can be formed by PI 3 treatments at high energies and ion beam nitriding at low energies. The resultant microstructures, as determined by GAXD, consist primarily of an expanded austenite layer. It appears that the adherent oxide film present on stainless steel must be either removed by sputtering, at low ion energies, or passed through by implantation, at high energies. Subsequent diffusion at elevated temperatures allows the formation of a nitrided layer several micrometers thick in both cases.

Advanced surface treatments by plasma ion implantation

Abstract Plasma ion implantation techniques have emerged recently as non-line-of-sight alternatives to ion beams for surface modifications in the energy range 10–100 keV. Experimental work performed over the last 7 years is reviewed, pointing out the progress that has been made in both the physical understanding of the process and the technological developments necessary for its implementation. Improvements in the surface properties of a wide range of alloys have been obtained by the implantation of nitrogen while field test results for industrial tools and components from a diverse range of applications have been positive. Although the process has been extended to the implantation of other gaseous and metal ions, it is in the combination of the technique with other surface treatments such as ion-assisted deposition and ion nitriding that the most interesting developments have been taking place. For deposition, it has been demonstrated that, for certain coating/substrate systems, increasing the ion energy can result in improved film properties. Treatment at elevated temperatures, particularly in steels, has shown that a substantial diffusion zone can be produced supporting the implanted layer. Some of the results of these advanced surface treatments involving high energy ion bombardment from a plasma are presented and the advantages and limitations of the process are discussed.

Cross-sectional transmission electron microscopy characterisation of plasma immersion ion implanted austenitic stainless steel

Abstract Cross-sectional transmission electron microscopy (XTEM), selected area diffraction (SAD) and nano-beam diffraction (NBD) techniques were used to investigate the surface microstructure of 316 stainless steel, implanted with high doses of nitrogen ions at 150, 250, 350, 450 and 520 °C using plasma immersion ion implantation. It has been found that the treatment temperature has a strong influence on the evolution of the microstructure. An amorphous layer of about 1 μm thick with a heavily stressed substrate underneath was observed on the 150 °C implanted sample. Both the 250 and 350 °C implanted samples showed a thin nanocrystalline sublayer at the outermost surface and an amorphous sublayer between the nanocrystalline sublayer and the substrate. A thick amorphous layer up to 3 μ thick was formed on the 450 °C implanted sample whereas at 520 °C, cellular precipitation of CrN and α-ferrite dominated the system. It is suggested that a solid state chemical reaction and the poor mobility of the reactant atoms are the key factors for the solid state amorphisation by nitrogen ion implantation into austenite.

Plasma nitriding of microalloyed steel

Abstract 3icroalloyed or high strength low alloy (HSLA) steels are carbon-manganese steels containing small amounts of Nb, V or Ti. The excellent mechanical properties of these alloys, particularly high yield strength, usually obviate the need for expensive quench and tempering operations. Furthermore, the presence of a significant amount of nitride-forming elements in some microalloyed steels has generated interest in the applicability of these alloys as a new generation of nitriding steels. In this paper, a study of the plasma nitriding behaviour of a commercially available microalloyed steel MAXIMATM is reported. A comparison is made with a traditional quenched and tempered nitriding steel (En19), plasma nitrided under similar conditions. Optical and scanning electron microscopy in conjunction with microhardness measurements and X-ray diffraction were utilized to characterize the nitrided surfaces. The observed differences in the thickness and structure of the compound layer and the diffusion zone are discussed in terms of chemical composition and microstructure of these steels.

Surface modification of austenitic stainless steel by titanium ion implantation

Abstract The wear properties of AISI 316 austenitic stainless steel implanted with Ti were investigated for ion doses in the range (2.3−5.4) × 10 16 ions cm −2 and average ion energies of 60 and 90 keV. The implanted layer was examined by Rutherford backscattering, from which the retained doses were determined, and glow discharge optical emission spectroscopy. Following implantation, the surface microhardness was observed to increase with the greatest change occurring at higher ion energy. Pin-on-disc wear tests and associated friction measurements were also performed under both dry and lubricated conditions using applied loads of 2 N and 10 N. In the absence of lubrication, breakthrough of the implanted layer occurred after a short sliding time; only for a dose of 5.1 × 10 16 ions cm −2 implanted at an average energy of 90 keV was the onset of breakthrough appreciably delayed. In contrast, the results of tests with lubrication showed a more gradual variation, with the extent of wear decreasing with implant dose at both 2 N and 10 N loads. Finally, the influence of Ti implantation on possible wear mechanisms is discussed in the light of information provided by several surface characterization techniques.

Microstructure and tribological behaviour of plasma immersion ion implanted tool steels

Abstract Plasma Immersion Ion Implantation (PI 3 ) has emerged as an alternative non-line-of-sight technique for implanting nitrogen ions into ferrous materials. In this work, the effect of PI 3 on the surface hardness and microstructure of two shock resistant tool steels has been studied. The influence of nitrogen implantation temperatures on the microstructure was investigated using optical metallography and glancing angle X-ray diffraction. Chemical depth profilling by Auger electron spectroscopy and glow discharge spectroscopy demonstrated that nitrogen profiles typical of conventional implantation and high temperature nitriding can be obtained depending on the process temperature. On the basis of these findings, it has been established that PI 3 treatment of shock-resistant steels can, in a single process, produce a duplex structure consisting of an extremely hard implanted surface which is supported by a deep nitrided case. The tribological behaviour was also investigated with the aid of a pin-on-disc tribometer, computer-assisted profilometry, optical and scanning electron microscopy. The wear mechanisms and coefficient of friction of the two steels were studied as a function of applied load, sliding distance and implantation temperature. Significant improvement in wear resistance was obtained especially for high temperature implantation of the higher alloy steel.