Kevin S. Fancey

Enhanced plasma nitriding at low pressures: A comparative study of d.c. and r.f. techniques☆

Abstract Increasing industrial interest in the use of enhanced low pressure ion plating techniques for surface coating applications led the authors to examine the applicability of discharge enhancement to the plasma nitriding process. The performance of various d.c. diode, triode and r.f. systems is discussed in terms of surface layer growth rate, hardness profiles and coverage uniformity. It is shown that a correlation of visual and optical emission spectroscopy analyses of practical discharges with predictive equations for plasma phenomena can be useful in explaining observed discharge effects, thus assisting development and optimization of the plasma nitriding process.

An investigation into dissociative mechanisms in nitrogenous glow discharges by optical emission spectroscopy

Abstract Nitrogen molecular dissociation is studied in DC diode and thermionic triode glow discharges of pure nitrogen and nitrogen-argon mixtures. Optical emission spectroscopy is used to determine the relative proportions of atomic and molecular nitrogen in the cathode sheath and plasma regions of these discharges operated at −2 kV cathode bias in the range 0.1–10 Pa. The results show that for nitrogen-argon diode discharges at 6.67 Pa total pressure, (i) dissociation rates within the sheath compare with those of pure nitrogen discharges operated at similar nitrogen pressure and the same mechanism (N2+N20 dissociative charge exchange) is believed to be primarily responsible; (ii) without argon, the proportion of atomic to molecular nitrogen in the plasma is very low compared with the sheath but increases when argon is introduced, the suggested cause being reduced quenching of electron energy by nitrogen gas, leading to increased electron impact dissociation. For nitrogen-argon thermionic triode discharges at 1.33 Pa total pressure, the proportion of atomic to molecular nitrogen in the sheath does not significantly vary with nitrogen concentration; this is thought to arise from increased incident ion energies in the sheath, compared with the higher pressure diode case, making ArN2 dissociative charge exchange collisions more prominent. The implications of these findings are discussed in relation to plasma nitriding techniques.

Some fundamental aspects of glow discharges in plasma-assisted processes☆

Abstract The applicability of some theoretical models for ionization-assisted processes, such as those of Davis and Vanderslice and Child-Langmuir, are discussed using data obtained by the authors and other researchers in the field. Information derived from argon discharges is used as a basis, and provides a convenient foundation from which to compare different system layouts, such as the direct current diode, and various triode systems. Detailed information is given on the estimation of the cathode fall distance L and the L /λ ratio, where λ is the mean free path for charge exchange collision. This allows the estimation of energy distributions for both ions and neutrals. Other important parameters are also discussed, such as ionization efficiency, as well as the effect of additional species within the discharge.

Prestressed Polymeric Composites Produced by Viscoelastically Strained Nylon 6,6 Fibre Reinforcement

A method for improving impact resistance is reported. Polymeric fibres are stretched under a load which is released prior to moulding them into a matrix. On solidification of the matrix, compressive stresses imparted by the viscoelastically strained fibres impede crack propagation. The prestressing principle is illustrated through a polariscope study of nylon 6,6 monofilament embedded in transparent resin. Previous work demonstrated the method through Charpy tests on nylon 6,6 fibre-polyester resin composites: prestressed samples absorbed 25% more impact energy than their control (unstressed) counterparts. Further impact studies are reported in the present work to evaluate probable service life. Batches are tested from 12 to 96 hours after moulding and (through heat treatment to accelerate ageing) at estimated ages of one to 10 years. They show no discernible change in impact performance with age.

Comparative tribological and adhesion studies of some titanium-based ceramic coatings

Abstract In this work different evaluation methods for thin hard coatings were studied in three laboratories. Different coatings and test equipment were used. An agreement on the procedure for testing and data appraisal was made. The results obtained in the different laboratories varied depending on the test procedure used. Two test methods, ball crater and scratch test, yielded smaller variations in the results, whereas the hardness measurements and pin-on-disc tests had larger variations, although in the former case these were within the standard deviations of the readings. It is clear from this work that tests performed in different laboratories cannot be used for comparison purposes, unless strict agreement is reached on the exact test procedure and on the basis of interpretation of the results. Even tests carried out under nominally identical conditions in one laboratory can in some cases give a spread in performance, since many variables, such as friction coefficient, wear rate and hardness, are not intrinsic materials properties and can statistically vary. It is thus important to devise standardized test methods, and in all cases to quote the standard deviations on data.

Evaporative ion plating: process mechanisms and optimization

The basic discharge mechanisms pertinent to ion plating process are reviewed and some recent findings are highlighted, relating in particular to thermionically enhanced ion plating with an evaporative vapor source. By enhancing the discharge, the cathode sheath can approach collisionless conditions, which allows most of the energy to be transported to the substrate by ions. The energy distribution is also improved, this being significant if a critical energy E* is thought to be required in order to achieve high-quality coatings. It is suggested that E* is about 40 to 70 eV, and that higher bombarding energies may not necessarily improve coating quality, this being alternatively achievable by increasing the ionization efficiency. The role of metal vapors is outlined with reference to evidence from enhanced discharges that clusters of metal atoms exist in the vapor phase and that most of the ion current is transported to the substrate by the metal species. Further practical aspects are discussed such as the spatial distribution of bombardment intensity, which is shown to decrease exponentially with distance from the thermionic emitter used for discharge enhancement. It is suggested that positioning the thermionic emitter in close proximity to the vapor source will provide a more consistent ratio between energetic bombardment effects and vapor arrival rates throughout the deposition volume. >

Ion plating processes: Design criteria and system optimization☆

Abstract Important factors to be considered in the design and optimization of ionization-assisted physical vapour deposition systems include (i) non-uniform plasma effects caused by ionization enhancement, (ii) permissible mixing of different sized substrates and (iii) coating thickness uniformity. We have shown that plasma intensification with an electron emitter can impart non-uniform effects which are primarily related to the mean free path for electron impact ionization. Secondly, the influence of cathode fall distance on factor (ii) is highlighted by comparing sputter weight loss measurements with theoretical predictions. Finally, coating uniformity in terms of front-to-back thickness ratios is shown to follow a hyperbolic function while the absolute thickness can be expressed as an inverse power function of the source-to-substrate distance.

Process effects in ion plating

Abstract By the use of Optical Emission Spectroscopy and cathode sheath thickness measurements in ion plating discharges, we have (i) identified the influence of ionisation intensification sources on plasma spatial distribution, (ii) shown that nitrogen dissociative charge exchange in the sheath may be significant under certain conditions, and (iii) found evidence for the possible presence of metal clusters in the discharge. The implications of these findings are discussed.

Metal clusters in ion plating

Conventional ion plating theory assumes that the metal vapor arrives at the substrate in monatomic form. Our results suggest this is not the case, and that a large proportion of the metal arrives as clusters. This is based on measurements of the cathode fall distance before and during titanium deposition in a thermionically supported argon glow discharge. The results have been used to evaluate the charge‐to‐mass ratio of the metal species in accordance with the Child–Langmuir equation. This predicts that some titanium is present as atomic clusters which contain at least tens of atoms per unit charge. Also we show that at least 90% of the ion current arriving at the substrate is carried by the metallic species.

Investigation into the feasibility of viscoelastically generated pre-stress in polymeric matrix composites

Abstract A method of producing compressive stresses within a composite material, to improve resistance to crack propagation, is described. This involves stretching polymeric fibres under a load that is released prior to moulding the fibres into a matrix material. On solidification of the matrix, continuing viscoelastic recovery of the strained fibres imparts compressive stresses to the surrounding material. To evaluate the technique, batches of samples, comprising nylon 6,6 fibres (3–4% fill) in a polyester resin matrix, were produced for Charpy impact testing. Each batch consisted of five test and five control samples, the latter containing the fibres in unstressed condition. Of the 15 batches tested, test samples from 11 batches absorbed, on average, 25% more impact energy than their control counterparts. The remaining four batches showed little difference between test and control samples, and this may be attributed to inadequate fibre–matrix bonding. Batches were tested up to 3500 h after moulding, and there was no discernible deterioration in impact performance with age.