Surface Engineering

Four different tool steel materials, P20, H13, M2 and D2, were nitrocarburised at 570°C in a fluidised bed furnace. The reactive diffusion of nitrogen and carbon into the various substrate microstructures is compared and related to the different alloy carbide distributions. The effect of carbon bearing gas (carbon dioxide, natural gas) on carbon absorption is reported, as well as its influence on compound layer growth and porosity. Partial reduction of Fe₃O₄ at the surface resulted in the formation of a complex, ϵ-nitride containing oxide layer. In H13, carbon was deeply absorbed throughout the entire diffusion zone, affecting the growth of grain boundary cementite, nitrogen diffusivity and the sharpness of the compound layer: diffusion zone interface. When natural gas was used, carbon became highly concentrated in the compound layer, while surface decarburisation occurred with carbon dioxide. These microstructural effects are discussed in relation to hardness profiles, and compound layer hardness and ductility. The surfaces were characterised using glow discharge optical emission spectroscopy, optical and scanning electron microscopy and X-ray diffraction.

Nitrocarburised H13 disks were tested in dry, sliding wear against a stationary ruby ball (pin). Three different 4 h nitrocarburising treatments were compared, using N₂/NH₃/CO₂, N₂/NH₃/natural gas and N₂/NH₃ gas mixtures, resulting in compound layers of varying thickness, hardness, porosity and oxide morphology. During mild, oxidative wear, with the formation of abrasive wear debris, the most brittle and oxidised surfaces performed poorly. Polishing to a bright, reflective finish greatly reduced wear. However, the N₂/NH₃/CO₂ sample also frequently maintained a ‘very mild’ wear regime, owing to the formation of a protective film between the wear surfaces, and resulting in a lowering of the friction coefficient. This treated surface was porous and covered in a complex layer of coarse oxide+ϵ-carbonitride. Nitrocarburised samples and wear tracks were characterised by optical microscopy, SEM, atomic force microscopy and stylus profilometry.

Langmuir–Blodgett (LB) nanolayers of alkyl hydroxamic and phosphonic acids on glass, iron and copper surfaces were studied in the presence of different, corrosion relevant micro-organisms. The quality of monomolecular and multimolecular LB layers was characterised by dynamic contact angle measurement, atomic force microscopy and the sum frequency vibration technique. The inhibition efficiency of microbial adhesion and biofilm formation, which was analysed by the surface visualising technique, epifluorescence microscopy and microbiological methods, increased with layer thickness. There were some differences in the blocking activity of the two amphiphiles (octadecylphosphonic acid C18P and octadecanoyl hydroxamic acid C18N) applied in LB films. Their repellent activity did not differ in cooling water but, in the presence of Acidithiobacillus ferrooxidans, the C18N nanolayer inhibited the biofilm formation much better.

Laser processed Si coatings with thickness in the range 100–150 μm were produced on AISI type 316L stainless steel substrate using a continuous wave CO₂ laser. The experiments were done at a laser power between 1·5 kW and 1 kW, process speeds in the range 33·6–8·3 mm s⁻¹, with a beam diameter of 1·6 mm. The microstructure of the laser processed coating consists of columnar dendrites and fine cellular structures. High temperature phases such as δ-ferrite and γ-austenite were found on the resolidified microstructure. The microanalysis technique demonstrated that the laser clad zone possessed a high concentration of Si. The crack free laser clad zone exhibited high hardness in the range 850–900 HV.

Aluminium is a very important material used in the semiconductor industry as an interconnection layer. The most popular method of depositing aluminium films is the physical vapour deposition (PVD) process. The objective of the present study is to improve aluminium film uniformity by optimising the process conditions to use a thicker target and meet the demand for more uniform films on wafer surfaces. Aluminium films were deposited by a sputtering process on 200 mm wafers in PVD chambers. Experiments were carried out to find the process factors that affect film uniformity. The results revealed that film uniformity was affected significantly by wafer to target spacing and process pressure. Film characterisation was then performed by measuring grain size and film surface roughness, and performing package level electron migration, ball shear and bond pull tests. A higher process pressure resulted in more uniform films with bigger grains, lower surface roughness, longer reliability lives and higher ball shear values. With optimised process conditions, the aluminium layer uniformity can be maintained at <1·5% throughout the whole target life (∼1000 kWh).

Composite films consisting of cobalt hydroxide and weak cationic polyelectrolytes, such as poly(allylamine hydrochloride) and chitosan, were obtained by cathodic electrodeposition. The proposed method is based on the electrosynthesis of cobalt hydroxide from the solutions of CoCl₂ and electrophoretic deposition of the polymers. Cathodic deposits were obtained by the galvanostatic method on Pt and Ni substrates. The deposition yield was studied at different concentrations of the polymers and CoCl₂ in solutions. The deposits were studied by thermogravimetric analysis, X-ray diffraction analysis and atomic force microscopy. The deposition mechanism is discussed. The results obtained pave the way for the fabrication of other organic–inorganic composites.

A silica coating was deposited on the nickel substrate by the sol–gel process, and the structure and composition were evaluated. The sols were made from several acid catalysts, including HNO₃–HCl, CH₃COOH, HCl, a base catalyst (NH₄OH, NH₃) and two step catalysts such as CH₃COOH–NH₃. To produce adherent silica coatings on nickel, various additives such as surfactants, viscous additives and drying control chemical additives were introduced to the system, and appropriate drying and firing cycles were applied after dipping in solution. The structure and uniformity of the coatings were examined by SEM. Coating compositions were studied by glow discharge optical spectroscopy and EDAX microanalysis. The results indicate that a uniform coating of silica can be produced on a nickel substrate by optimising the composition of the sol and the process variables.

Pulsed nitrided plasma of Mo₃Si intermetallic alloys with different Nb concentrations were treated in a nitrided atmosphere of 50%H₂–50%N₂ at pressure 533 Pa and temperature 950°for 8 h. The pulsed nitrided plasma was studied by optical emission spectroscopy (OES). Strong intensities of N₂, N₂⁺and NH excited states were observed in the OES in the negative glow of H₂–N₂ pulsed plasma. Several alloys with different additions of Nb solid solution were treated. Nitrided specimens were analsed by SEM and X-ray diffraction; microhardness measurements were also evaluated. After the nitrided treatment, the alloy with more than 10 at.-%Nb increased its surface hardness by 16·6%, presumably by the formation of NbN.

TiCN coatings were deposited by magnetron sputter ion plating onto high speed AISI M2 steel. The physical vapour deposition TiCN coatings were produced with various partial pressures of nitrogen and methane, at a constant substrate bias of 0, 50 or 100 V and constant deposition temperature of 300, 350, 400 or 450°C, respectively. Next, the substrate bias was varied for constant ratios of partial pressures and constant temperature. The amount of methane flow affects the coating composition as well as its lattice parameters and residual stresses. These properties are also affected by the substrate bias. The joint effect of increasing the substrate bias, the presence of interstitial carbon and the formation of CN–Ti bonds could probably be responsible for strong fcc lattice distortion and consequently high residual stresses of the TiC_xN_y coatings. The deposition temperature affects the lattice parameters and the residual stresses only to a small extent. The substrate bias of 0 to −100 V yields known strong effects on the lattice parameters and residual stresses, because this deposition parameter is adjusted more or less independently of the other deposition parameters. Electrochemical analysis confirms the effects of different deposition parameters. Coating–substrate systems produced at low bias exhibit better corrosion resistance compared with those manufactured at −100 V. Coating embrittlement from hydrogen incorporation, the formation of CN–Ti bonds and large numbers of defects from high bias obviously favour corrosion attack, and high values of compressive residual stresses are not able to prevent early electrolyte attack on the substrate.

Investigations into nickel electrodeposits on dc magnetron sputtered copper substrates are reported. Thin layers of copper were deposited by dc magnetron sputtering on mild steel substrates. Onto these sputtered copper surface, layers of nickel were electrodeposited using a Watt's bath. The coatings were vacuum annealed at 200°C for 120 min. The growth of the deposit is discussed in terms of structural and microstructural analysis by X-ray diffraction (XRD), SEM and atomic force microscopy (AFM). The orientation along (111), (200) and (220) was observed for the vacuum annealed Cu_3·8Ni alloy deposits. Uniform and pinhole free morphology was observed from SEM. AFM images reveal that these coatings have a granular morphology. The corrosion behaviour of these samples in a 3·5 wt-%NaCl solution was examined. A decrease in I_corr and high charge transfer resistance indicated the improved corrosion resistant behaviour of the nickel sample plated over a sputtered copper base.

Electroless deposition of nickel based composites produces outstanding tribiological behaviour. These composite coatings are formed by addition to the electroless nickel solution of the material to be codeposited, in powder form, and by maintaining it in suspension during the deposition process so that it is incorporated into the deposit. The most commonly used hard dispersed compounds are SiC, diamond powder, alumina, TiC, BN, chromium carbide or WC. Recently, electroless nickel containing PTFE as a composite material has been used because it is uniform, highly adherent, hard wearing, dry lubricating, non-galling, has a lower coefficient of friction and good corrosion resistance properties. The present paper studies the effect of PTFE in an electroless Ni–acid bath on the rate of deposition, incorporation of PTFE and phosphorous content in the deposit, wear and corrosion resistance.