A.D Wilson

Characteristics of a plasma electrolytic nitrocarburising treatment for stainless steels

Abstract In this work, we investigate the feasibility of a plasma electrolytic nitrocarburising (PEN/C) treatment applied to AISI 316 stainless steel using a modified aqueous solution of urea as the treatment electrolyte. The substrate samples were connected cathodically to a high-current DC power supply and biased with a negative voltage in the range 220–260 V. The treatment time was typically in the range of 30–60 s. Investigations of the characteristics of the treated component show that the friction coefficient against a WC–Co ball counterface can be slightly reduced, and that the wear rate decreases by several decades. The microstructure of the treated layers depends strongly on the electrical parameters (e.g. the applied voltage, which controls the treatment temperature) and can be adjusted from a single expanded austenite phase (γ N ) to multi-phase combinations, including mixtures of Fe(Fe,Cr) 2 O 4 , γ N and (Cr,Fe)N x . The corrosion properties of the treated layers are closely correlated to the microstructure and composition and can be significantly improved, particularly if a thin, but dense, magnetite-based iron–chromium oxide layer is produced at the surface. For improvement of the tribological properties of substrates treated at low temperature, a diamond-like-carbon coating was deposited on the PEN/C pre-treated substrate using a plasma-immersion ion-assisted deposition (PIAD) process. Such duplex treatments show great potential for surface modification of stainless steels for applications in aggressive corrosive-wear environments.

A comparative study of the influence of plasma treatments, PVD coatings and ion implantation on the tribological performance of Ti–6Al–4V ☆

Abstract The mechanical and tribological properties of the titanium alloy Ti–6Al–4V have been characterized by low-frequency reciprocating wear, rotating–bending fatigue and Knoop microhardness tests and surface roughness (Ra) measurements before and after various treatments. These treatments included ion-beam nitrogen implantation, plasma-assisted physical vapour deposition (PAPVD) of TiN and plasma diffusion of nitrogen and carbon. Using a commercial thermionic triode plasma-assisted PVD system, hot-filament-supported glow discharges were produced with different compositions of nitrogen, carbon and hydrogen. The plasma-diffusion treatments all increased the load support provided by the substrate but reduced the bulk fatigue stength considerably. The PAPVD TiN coating increased surface microhardness but provided limited load support. Nitrogen-ion implantation increased low-load surface microhardnesses and improved the fatigue resistance without improving the overall load support provided by the substrate. It was found that the elevated temperatures and increased process times in plasma-diffusion treatments increased the surface roughness significantly, although this roughening effect could be reduced by depositing a thin titanium layer by PAPVD, prior to the plasma treatment.

A comparison of the wear and fatigue properties of plasma-assisted physical vapour deposition TiN, CrN and duplex coatings on Ti-6Al-4V

Abstract The study sets out to establish a comparison between duplex systems of plasma nitriding followed by plasma-assisted physical vapour deposition (PAPVD) of TiN deposited on Ti-6Al-4V, compared with PAPVD of TiN and CrN alone. The fatigue resistance has also been examined since conventional surface modifications can often impair fatigue resistance. A rubber-wheel-type abrasion tester and pin-on-disc sliding wear tester were used to examine the wear resistance of the coatings and the load-bearing capacity of the substrate respectively. A Wohler-type rotating tester was used to study the fatigue properties. Using smooth rotating-bending fatigue specimens tested in air at 5700 rev min-1 for 106 cycles or until failure it was found that TiN, CrN and the duplex coating did not impair fatigue resistance but actually improved the S-N curves and increased the endurance limit. From the wear results it was found that, although TiN and CrN do improve the wear resistance of Ti-6Al-4V significantly, it is the duplex coating that has much the greater load-bearing capacity on the titanium substrate and gives a significant improvement on PAPVD TiN or CrN coatings in sliding and abrasive wear conditions.

Duplex surface treatments combining plasma electrolytic nitrocarburising and plasma-immersion ion-assisted deposition

Abstract This paper discusses the feasibility of improving the tribological performance of AISI 304 stainless steel by applying a duplex surface treatment which combines the methods of plasma electrolytic nitrocarburising (PEN/C) and plasma-immersion ion-assisted deposition (PIAD) of a diamond-like carbon (DLC) coating. The effects of different regimes of plasma electrolytic diffusion treatment on the surface microstructural, mechanical and tribological characteristics of the stainless steel substrate are studied. The major advantages of the PEN/C technique are shown to be extremely high growth rate of the N/C diffusion layer (typically 3–6 μm/min) and the near-ambient (bulk) treatment temperature; this combination of characteristics far exceeds the capabilities of most conventional methods of diffusion treatment. Diffusion hardened layers 15–60 μm thick were produced after 5–10 min treatment, consisting predominantly of expanded austenite (γ N,C ) with a Knoop microhardness of 800–1400 HK 10 , providing significant improvement in the substrate load bearing capacity for subsequent deposition of a PIAD DLC coating. Reciprocating-sliding friction and wear tests against a WC–Co sintered-carbide or SAE 52100 chromium-steel ball counterface have shown that the application of a duplex treatment promotes a change in wear mechanism of the friction pair from relatively severe adhesion/abrasion to mild asperity deformation and ‘polishing’. This results in simultaneous reduction of the volumetric wear rate from ∼10 −4 to ∼10 −7 mm 3 /Nm and of the friction coefficient from (0.59–0.63) and (0.68–0.74) to (0.09–0.13) and (0.24–0.32) for WC–Co and chromium–steel counterfaces, respectively. It is also shown that the ball-on-plate impact wear resistance of the PIAD DLC coating can be considerably increased by application of a nitrocarburised load-supporting sublayer.

Deposition of duplex Al2O3/DLC coatings on Al alloys for tribological applications using a combined micro-arc oxidation and plasma-immersion ion implantation technique

Abstract Micro-arc discharge oxidation (MDO) is a cost-effective plasma electrolytic process which can be used to improve the wear resistance of aluminium alloy parts by creating a hard thick alumina coating on the component surface. However, for sliding wear applications, such alumina coatings often exhibit relatively high friction coefficients against many counterface materials. Therefore, a duplex treatment, combining a load-supporting MDO alumina layer with a low friction diamond-like carbon (DLC) coating, produced by a modified plasma-immersion ion implantation (PI3) process, has been investigated. PI3 provides a flexible method of implanting ions into complex-shaped parts using a low temperature, low voltage plasma onto which high voltage pulses are superimposed. It can also be used to enhance the adhesion and growth characteristics of films formed under plasma conditions. In this work, a weakly-ionized, hot-filament supported low-voltage argon–acetylene plasma (with C2H2/Ar ratios from 1.0 to 0.15) was used, in combination with a low-frequency dc pulse voltage PI3 system (in this case 100 μs, 5 kV pulses at 850 Hz) to deposit a low friction DLC top layer onto MDO-treated Al alloy coupons. Microhardness measurements and pin-on-disc sliding wear tests were performed to evaluate the mechanical and tribological properties. Ball-on-plate impact tests were also carried out to assess coating layer adhesion/cohesion. Scanning electron microscopy (SEM) was used to observe coating morphology, and to examine wear scars from pin-on-disc tests and crater scars from impact tests. The work demonstrates that a hard and uniform DLC coating, with good adhesion and a low coefficient of friction, can be successfully deposited on top of an alumina intermediate layer, which provides excellent load support; such that the coating can withstand much higher contact stresses than would normally be the case with aluminium-alloy substrate materials. The C2H2/Ar ratio significantly influences the interfacial adhesion between the DLC and alumina layers, but has no significant effect on coating hardness. It is suggested that the C2H2/Ar ratio should be selected in the range of 0.25–0.35 to obtain a hard a-C:H carbon film with low-hydrogen-content and excellent adhesion. The investigations indicate that a duplex combination of micro-arc oxidation and PI3 represents a promising technique for surface modification of Al-alloys for tribological applications in which high contact loads are anticipated.

A comparative study of the cyclic thermal oxidation of PVD nickel aluminide coatings

Abstract Static components used in the hot sections of gas turbines increasingly have a thermal barrier coating (TBC) of partially yttria-stabilized zirconia (PYSZ) routinely applied to improve their high temperature properties. Developments are also in progress to make TBCs better suited for use on rotating components such as blades. To help protect against high temperature oxidation of such components, an intermediate bond coat is applied, typically of an MCrAlY-type metal alloy. An alternative bond coat material is nickel-aluminum intermetallic alloy. Various processing routes have been studied for both bond coat and TBC deposition. A potentially attractive processing route is to deposit both bond coat and TBC by a physical vapor deposition (PVD) method; this would have the advantage of permitting sequential deposition in the same coating cycle. Whilst much research has been carried out on PVD MCrAlY and PYSZ coatings, relatively little work has been carried out on PVD NiAl, which represents a simpler intermetallic alloy with less critical composition control requirements. Thus, we have investigated NiAl deposition by three ion-assisted PVD coating routes: arc, electron-beam, and sputter ion plating. Coatings were deposited on a nickel-based alloy (Inconel 600) and an AISI 304 stainless steel. The differences in microstructure and phase composition from each deposition method are reported, together with data on the cyclic oxidation performance. The influence of process parameters on coating characteristics and degradation mechanisms is discussed.

A study of neon–nitrogen interactions in d.c. glow discharges by optical emission spectroscopy

Abstract Neon–nitrogen d.c. diode glow discharges have been investigated through the use of spatially resolved optical emission spectroscopy. This technique has enabled the discharges to be sampled from inside the cathode sheath and plasma regions. All discharges were operated at –2 kV cathode bias and 6 Pa total pressure. By evaluating spectral line intensity ratios (SLIRs) that incorporate ion species, the principal findings are as follows: first, the cathode current density at low nitrogen partial pressures is boosted by Penning ionisation of nitrogen by neon; evidence of this mechanism is provided by maxima in nitrogen ion-based SLIRs and minima in neon ion-based SLIRs occurring at low nitrogen concentrations. Second, although the optimum nitrogen partial pressure for Penning ionisation appears to be approximately 5%, the maxima in cathode current density and N 2 + /N 2 0 SLIRs occur at 10–15%; we speculate that a dissociative Penning ionisation mechanism is predominant at 5% but conventional (non-dissociative) Penning ionisation, with possible contributions from other mechanisms (such as electron impact ionisation of nitrogen) become significant at 10–15% nitrogen concentration.

Investigation into nitrogen–inert gas interactions in d.c. diode glow discharges

Abstract Neon–nitrogen and argon–nitrogen d.c. diode discharges have been investigated using spatially resolved optical emission spectroscopy (OES) to enable sampling inside the cathode sheath and plasma regions. The discharges were operated at −2 kV cathode bias and at 6.0 Pa total pressure with cathode current density and sheath thickness measurements evaluated for all gas mixtures. The results show that a greater proportion of ions reaching the cathode are nitrogen when neon is used as the primary gas instead of argon. Penning ionisation of nitrogen by neon metastables is the suggested mechanism and the effect is maximised at low nitrogen partial pressures. For 10–50% nitrogen in neon, the current density is higher than that achieved from neon or nitrogen alone or the equivalent argon-based mixtures. Moreover, information from cathode sheath thickness studies suggest that the dominant ionic species reaching the cathode is N + rather than N 2 + or Ne + at low nitrogen partial pressures. From OES studies, the production of nitrogen atomic species is attributed to N 2 + –N 2 0 dissociative charge exchange collisions in the sheath and since this mechanism depends on the availability of N 2 + , the effect is significant in argon-based mixtures only at high nitrogen partial pressures. The implications for plasma processing are also discussed.

Ion plating discharges: evidence of cluster formation during metal evaporation

Previous work has demonstrated the possible presence of metal clusters in ion plating discharges that utilise evaporative sources. In the present study, cathode sheath thickness measurements under ion plating conditions and mass spectroscopy studies (up to 2500 a.m.u.) of gas evaporation have provided further evidence of metal clusters in the vapour phase. The view that these clusters nucleate and grow by vapour cooling, through metal atoms losing energy by collisions with gas atoms, is supported by the following observations: (i) for evaporation at a fixed gas pressure, the abundance of clusters seems to be enhanced if argon is used instead of neon, indicating the greater cooling effect from a gas with larger atomic mass; (ii) cluster detection rate appears to be increased with increasing gas pressure, and this can be attributed to higher collision frequencies leading to enhanced cooling.

Structure and tribological properties of thin vacuum arc coatings on polysulfone

Abstract Thin coatings of the pure metals Ti, Zr, Cu, and multi-layer metal-nitride coatings Ti/TiN and Ti/ZrN of 0.2–1 μm thickness were deposited onto polysulfone S2010 substrate using a multi-cathode vacuum arc plasma gun connected to a magnetized straight plasma duct. A few Ti coatings were also deposited with the aid of transverse current injection (TCI). The structure and composition of the coatings were studied by means of X-ray diffraction, Auger electron spectroscopy and scanning electron microscopy (SEM) analysis. Scratch tests were used for evaluation of the adhesive strength between the substrate and the coating, and reciprocating wear tests against a steel ball were used to study the friction and wear rates of the coated samples. The wear tracks were examined by SEM. The nitride coatings possessed a nanocrystalline structure or a mixture of amorphous and nanocrystalline structures. The TiN and the ZrN coatings were stoichiometric. The adhesive strength was found to be dependent on the pre-coating treatment and the nature of the metallic bond layer. The Ti/ZrN coating exhibited improved tribological properties as compared to uncoated polysulfone. For this coating a negligible wear rate and a coefficient of friction of 0.1 against a steel ball (lower than the uncoated substrate) was obtained. In addition, TCI significantly reduced the wear rate of the Ti films by a factor of 5 to 6.5.