G.A. Collins

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.

The nature of expanded austenite

Abstract This paper attempts to reduce some of the confusion that exists over the nature of the nitrogen-rich layer produced by nitriding austenitic stainless steel at temperatures below 500°C. Cross-sectional transmission electron microscopy shows that the modified layer is dominated by a cubic phase with considerable expansion of the austenite lattice. In some cases, a thin (

Nitrogen and carbon expanded austenite produced by PI3

Abstract Expanded austenite can be formed either by nitrogen or carbon plasma immersion ion implantation (PI3 ™) from a nitrogen or methane plasma at elevated temperatures. The structure and properties of nitrogen and carbon expanded austenite layers produced on austenitic stainless steel X5CrNi189 are compared. A new structural model of expanded austenite based on a defect rich face centred cubic (fcc) lattice is proposed. Although the structure of the two expanded austenite layers is similar, there is a remarkable difference in the uptake of nitrogen or carbon, despite the use of similar treatment conditions. The modified surfaces have different hardness, corrosion and wear properties.

A comparative study of beam ion implantation, plasma ion implantation and nitriding of AISI 304 stainless steel

Abstract This paper presents the results of a comparative study using beam ion implantation (BII), plasma ion implantation (PII), ion nitriding and gas nitriding of AISI 304 stainless steel. We have demonstrated that under controlled conditions (the same treatment times of 30 and 60 min, and the same treatment temperature of 400 °C), the microstructures produced by all four techniques are similar, being mainly the formation of nitrogen in solid solution (γ N phase). However, the concentrations of nitrogen and the detectable depths of the nitrogen-enriched layers are significantly different, depending on the process. Both BII and PII produce thick nitrogen-enriched layers (greater than 1 μm) at high concentrations (20–30 at.%) compared with either ion nitriding or gas nitriding (layers less than 1 μm thick with low nitrogen concentrations). As a result, the load-bearing capacity after either BII or PII is much greater than after either ion or gas nitriding. It has also been found that high current density implantation is crucial for the formation of the thick N-enriched layers.

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.

Plasma immersion ion implantation of steels

Abstract Plasma immersoin ion implantation (PI3) is a new technique with certain advantages over conventional ion implantation. We have developed an implanter based on an inductively coupled r.f. glow discharge plasma. Ion densities of 1010 cm−3 are obtained with filling pressures of about 10−3 mbar. Ions are accelerated from the plasma by high voltage pulses (typically – 45 kV) applied directly to the work-piece. In this paper we report on the application of PI3 to nitrogen implantation in steels. Dramatic increases in microhardness and wear resistance have been observed for a number of steels, ranging from 0.3 wt.% C mild steel to austenitic stainless steels. Despite the relatively implantation energy, the modified layer can be greater than 1 μm in thickness. The nitrogen concentration profile can be controlled by the implantation temperature and dose. Glancing-angle X-ray diffraction has been used to determine the structural changes that occur in the surface layer.

Nitriding at low temperature

Abstract This paper reports advances in the use of low-pressure rf plasmas for nitriding with an emphasis on treatments at temperatures of 250–450°C; i.e. well below those used by more conventional methods. The treatment of austenitic stainless steel AISI-316 was chosen to represent the efficacy of such plasmas for nitriding over a wide temperature range, producing thicker nitrogen-rich layers at low temperature than more conventional methods in the same process time. This is due to a lower activation energy. Application of high-voltage pulses to the workpiece (plasma-immersion ion implantation, PI3) increases the thickness of the nitrogen-rich layer but does not significantly alter the activation energy. Other aspects of the process investigated include the role of hydrogen, various regimes of plasma-based cleaning, process gas purity and the variation of workpiece bias, from zero up to the 10s of kV characteristic of PI3.

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.

Development of a plasma immersion ion implanter for the surface treatment of metal components

Abstract Plasma immersion ion implantation (PI3) has emerged as a viable alternative to conventional ion implantation for specific applications, such as the implantation of non-planar components and in hybrid treatments such as high energy, ion-assisted deposition and energetic ion nitriding. It is particularly suitable for the treatment of metal components where improvement in surface hardness and wear resistance is required. This paper describes the development of a PI3TM system, aimed at treating metal components up to a few hundred square centimetres in surface area with a hybrid implantation/diffusion process. Heating is provided by high energy ion bombardment. Complete process control has been implemented using industry standard equipment familiar to the process engineering community where possible. The implanter is described and its operating capabilities and reliability in a process environment are discussed.

Characterisation of duplex layer structures produced by simultaneous implantation of nitrogen and carbon into austenitic stainless steel X5CrNi189

Abstract Plasma immersion ion implantation has been used for simultaneous implantation of carbon and nitrogen into austenitic stainless steel X5CrNi189 at 400°C. Duplex layer structures are formed with a nitrogen-rich layer close to the surface and a carbon-rich layer at greater depth. The influence of different gas compositions on the structure and phase composition of the layers is examined and compared with results from pure nitrogen and methane. The results provide further information about ‘expanded austenite’ (S-phase) which is formed during the low temperature nitriding of austenitic stainless steels.