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The effect of a silicate-based plasma anodization treatment on the corrosion and stress corrosion cracking behaviour of a cast AM50 magnesium alloy was studied. Electrochemical tests revealed the beneficial effect of the plasma electrolytic oxidation (PEO) in improving the corrosion resistance of the alloy. Although the coating had provided an improved resistance to stress corrosion cracking in this test environment at a nominal strain rate of 10 −6 s −1 , it could not completely eliminate the SCC susceptibility of the alloy. Cracking of the coating under conditions of straining was found to be the reason for SCC of PEO-coated alloy.

An attempt was made to understand the effect of silicate based plasma electrolytic oxidation (PEO) coating on the stress corrosion cracking (SCC) behaviour of an AZ61 wrought magnesium alloy. The SCC behaviour of untreated and PEO coated specimens was assessed using slow strain rate tensile tests at two different nominal strain rates, viz. 1 × 10−6 s−1 and 1 × 10−7 s−1, in ASTM D1384 test solution at ambient conditions. The PEO coating was found to improve the general corrosion resistance to a significant extent; however, the improvement in the resistance to stress corrosion cracking was only marginal.

Ferritic alloys with different contents of carbon, aluminum and chromium are nitrided by plasma immersion ion implantation at 300 °C for 3 h. The thermal stability of the nitrided surface layers is evaluated by annealing the specimens in vacuum and in air both at 400 °C. The composition and the structure of the surface layers is studied by means of GDOES-spectroscopy and X-ray diffraction. In comparison to the nitrided reference specimens the nitrogen concentration decreases after heat treatment at 400 °C in air as well as under vacuum condition. The possibly main reasons are outward (degassing) and inward diffusion of nitrogen at elevated temperature. The e-nitride layer of a Fe–Cr alloy with high chromium content transforms to a stable Fe–Cr–N solid solution.

The X6CrNiTi1810 austenitic stainless steel samples nitrided by plasma immersion ion implantation at 300 °C/3 h are studied by slow positron implantation and conversion electrons Mossbauer spectroscopies completed with X-ray diffraction (XRD) measurements. The surface layers consist of γ N -Fe expanded austenite and e-Fe 2 N nitride. The changes in hyperfine parameters and relative representation of both phases during the interrupted long-time annealing at 150 °C are discussed from the point of view of nitrogen migration and changes in defect structure.

Nitride formation was studied in various steels with different alloying elements. Results of PI 3 nitriding, r.f. nitriding with self bias and DC voltage at 300 and 400°C were compared. It was found that nitriding of ferritic steels depends strongly on the formation of surface nitride layers. The habit of the nitrides and the supersaturation of nitrogen is strongly influenced by the HV pulsing during PI 3 and thus also the treatment depth and layer properties.

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.

The influence of different alloying elements and their concentration on the nitriding behaviour of steel by plasma immersion ion implantation (PI 3 ) was examined. Treatments were performed at 300°C using different high voltage (HV) repetition rates of 100, 200 and 300 Hz. The steels were alloyed either with 0.09, 0.45 and 0.9 wt.% C, 0.11, 0.3 and 0.93 wt.% Al or 0.93, 3.6 and 13.2 wt.% Cr. The nitride formation for the various alloys and parameters was studied using X-ray diffraction (XRD) and metallographic cross-sections in scanning electron microscopy (SEM), showed a nitride formation that is strongly influenced by the material composition. Additional information is obtained by glow discharge optical emission spectroscopy (GDOS) depth profiles and instrumented microhardness measurements. The effect of the HV repetition rate is not very clear, as on the one hand the implanted dose is increased, but on the other nitride formation seemed to be disturbed by higher repetition rates. The results will be discussed under the aspect of improving the nitriding efficiency of PI 3 .

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.

Plasma immersion ion implantation is an effective surface treatment for stainless steels. The influence of treatment parameters (temperature, plasma density and pressure) on the sliding wear resistance are studied here. At moderate temperatures, nitrogen remains in solid solution without forming nitrides. This increases the surface hardness and the wear resistance without affecting the passivation of the steel. This may allow the use of such steels in applications where their poor wear resistance would normally prohibit their use.

The structure and phase composition of the modified surface of two different stainless steels (austenitic steel X6CrNiTi1810 and austenitic–ferritic steel X2CrNiMoN2253) after 3 h plasma immersion ion implantation of nitrogen at 400°C were compared using XRD, SEM, TEM and Mossbauer spectroscopy. Both steels show a surface layer with very similar composition. The major phase found is expanded austenite. Very fine precipitates in the near surface region are most likely decomposition products (ferrite/martensite and CrN) of the metastable expanded austenite. The stability of the expanded austenite in the austenitic–ferritic steel is lower and the amount of decomposition products is higher under the same treatment conditions.

Nitriding of steels by Plasma Immersion Ion Implantation (PI 3 ) allows access to a large process parameter space. Although the parameters associated with the high-energy ion bombardment (implantation energy, high-voltage pulse length and frequency, ion current density and time-averaged dose rate) are important, the treatment temperature and plasma parameters such as ion density, excited neutral density and plasma potential also play a vital role. Previous investigations have been hampered by the use of the high-energy ion bombardment to heat the workpiece. In this paper, we present the results of a study in which the treatment temperature and the ion bombardment were decoupled by radiatively heating. The effect of varying high-voltage pulse length, repetition rate, total implanted dose, plasma density and potential on the nitrogen uptake during PI 3 treatment depends strongly on whether nitrides are formed in the surface (e.g. Ck45 mild steel) or nitrogen is incorporated in solid solution (e.g. X6CrNiTi 1810 austenitic stainless steel). In the first case, nitride formation can be suppressed by increasing the high-voltage pulse frequency or can be enhanced by treating at a high pressure or plasma potential. In the second case, the thickness of the modified layer can be increased by increasing the ion current density or time-averaged dose rate. In both cases, nitrogen uptake by direct thermochemical absorption from the plasma is significant.

A study of the response of the nitriding steel, 34CrAlNi7, to two different nitriding techniques revealed remarkable differences. The techniques used were pulsed plasma nitriding and plasma immersion ion implantation over a range of temperatures from 300 to 500°C. The surface produced by the pulsed plasma nitriding consisted of a nitride layer whose thickness varied with temperature, with an underlying diffusion zone. Plasma immersion ion implantation treatment, however, formed nitrides only at the lowest temperature, with most of the nitrogen in solid solution. Although quite different surface structures resulted from the two treatments, similar improvements in wear performance were obtained.

Plasma immersion ion implantation (PI 3 ) is a new hybrid technology using elements of ion implantation as well as plasma nitriding. In addition, and due to the high energetic ion bombardment, thermal diffusion can be used to obtain thicker layers than in conventional ion implantation. Furthermore, the process can be used in an ion beam enchanced deposition (IBED) mode to produce various kinds of coatings. This enables the treatment to be adapted to the material structure and state and also to the intended service conditions. Furthermore, economic aspects make PI 3 an interesting technique for surface modification of a wide range of materials. An overview of the worldwide activities in the treatment of components and the achieved improvements will be given.

In this paper we present our preliminary results of the plasma immersion ion implantation (PI3) treatment of boronised and laser surface treated layers. The substrate treatment was followed by the PI3 treatment. The structural changes in the surface layers of pretreated substrates due to PI3 treatments at various temperatures will also be discussed. In addition, the property changes characterised by microhardness and wear tests are presented.

A thin nitride layer containing high amounts of oxygen is formed on the surface of A199.5 treated by Plasma Immersion Ion Implantation (PIII) at temperatures up to 500°C. Below this layer, nitrogen is found in a diffusion zone several microns thick but does not form nitrides. The thickness of the diffusion zone is dependent on the treatment temperature. Microhardness measurements reveal a decrease in hardness due to a loss of work-hardening. In spite of this annealing, both the wear and corrosion resistance are improved.

It has been shown previously in the literature that plasma immersion ion implantation (PHI) can increase the wear resistance of austenitic stainless steel without losing its corrosion resistance. In this work, the effect of PHI treatment on the microstructure and the properties of an austenitic (X6CrNiTi1810, AISI 321) and a duplex austenitic-ferritic (X2CrNiMoN2253, AISI 318) stainless steel has been studied and the results compared. Three different treatment temperatures and treatment times were used. The microstructures were studied by optical metallography and glancing angle X-ray diffraction (XRD). The formation of expanded austenite was observed in both steels up to treatment temperatures of 400 °C. The ferrite in the duplex austenitic-ferritic steel was also transformed to expanded austenite. At 500 °C, a surface layer consisting of CrN was formed on the duplex austenitic-ferritic steel whereas the modified layer on the austenitic steel was still expanded austenite with a small amount of CrN precipitation. Elemental depth profiling by sputtered neutral mass spectrometry (SNMS) revealed a similar treatment depth for both materials up to 400 °C, which was a function of treatment temperature and time. A pin on disc tribometer was used to determine the tribological behaviour. A change in the wear behaviour was observed and the wear depth decreased relative to untreated material. This was due to an increase in the surface hardness and a decrease in the coefficient of friction. The decrease in wear depth correlated with the thickness of the modified layer. The best results were found with the duplex austenitic-ferritic steel at a treatment temperature of 500 °C and can be attributed to the formation of a CrN layer. Corrosion tests have shown that good corrosion resistance was preserved up to 400 °C for both materials with only a small decrease being observed. This is due to nitrogen remaining in solid solution without CrN-precipitation. At a treatment temperature of 500 °C, the corrosion resistance decreased dramatically, especially for the duplex austenitic-ferritic steel where a layer of CrN was formed. These results show the capability of PIII treatment to increase the wear resistance of these stainless steels without losing their good corrosion performance. This may allow the use of such steels in applications where the poor wear resistance of the untreated material would normally prohibit their use. In comparison to the austenitic steel, the duplex austenitic-ferritic steel performed better after PIII treatment. For an optimum surface treatment, it is necessary to consider the substrate material as well as the treatment parameters.

Most low alloy steels are tempered at relatively low temperatures, limiting the opportunities for improving their wear resistance and surface hardness by traditional heat treatment processes. In this work, we use plasma immersion ion implantation (PI 3 ) to apply a hybrid ion implantation/thermochemical diffusion treatment to ball bearing steel 100Cr6 (1% C, 1.5% Cr). By a judicious choice of implantation voltage, temperature and time, it is possible to combine the PI 3 treatment with the tempering stage of heat treatment, without compromising the hardness of the alloy. Up to 200 °C, the bulk hardness (750 HV tempered) of the material is maintained, and nitrogen implantation increases the surface hardness due to the precipitation of fine Fe 2 N. At 300 °C, where enhanced nitrogen and carbon diffusion can occur, the bulk hardness decreases, although in many applications this can be compensated by the increase in surface hardness to around 900 HV0.1 due to Fe 3 N and Fe 4 N formation. Wear and corrosion tests reveal significant improvements in comparison with untreated steel.