A.S. James

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.

TiN and CrN PVD coatings on electroless nickel-coated steel substrates

Abstract Plasma-assisted (PA) PVD ceramic coating such as TiN have so far achieved only very limited use on cheap low-alloy steels, owing to problems relating to both corrosion resistance and the need for load support from the underlying material. Here we report tests to asses the wear and corrosion performance of TiN and CrN PAPVD coatings on phosphorus-doped electroless nickel (ENiP)-coated steels. It is shown that this route offers a potentially cost-effective means of utilizing PAPVD ceramic films on lower grade steels. In particular, CrN/ENiP on AISI 304 stainless steel is shown to exhibit a promising combination of wet abrasion resistance with good corrosion properties.

Substrate surface finish effects in duplex coatings of PAPVD TiN and CrN with electroless nickel-phosphorus interlayers

The ability to deposit hard coating materials on to a wide range of substrate surface finishes and still maintain adequate structural and functional properties is a very desirable objective in the quest to optimise product economics. A better surface finish requires longer and more elaborate finishing processes which will result in substantial additional costs. In this investigation two different groups of substrate surface roughnesses were prepared for a number of appropriate testing procedures. The tests involved in this study were microhardness, surface profilometry, scratch adhesion and the wet sand rubber wheel abrasive test. Electrochemical corrosion testing was also carried out. The results reveal that the substrate roughness can be optimised to achieve the best compromise in terms of cost and performance. The addition of an electroless nickel-phosphorus (ENiP) interlayer, in particular, is beneficial in isolating the substrate from near-surface effects and in permitting the use of rougher surface finishes.

Thermal stability of partially-yttria-stabilized zirconia thermal barrier coatings deposited by r.f. plasma-assisted physical vapour deposition☆

Abstract The paper first summarizes the results of earlier work on the development of thermal barrier coatings produced by plasma-assisted physical vapour deposition (PAPVD). The results of burner rig trials on coatings produced by different processes are then discussed. All processes were based on electron beam PVD techniques. It was found that the most durable coating was deposited by use of two different techniques to form a coating with a thin dense layer adjacent to the substrate followed by a thicker, more open structure. In addition, the coatings were examined after failure and it was found that each different process had a separate failure mechanism.

Ionization in plasma-assisted physical vapour deposition systems

Abstract In this paper we discuss recent developments in plasma-assisted physical vapour deposition processes, extending on earlier work, and emphasizing the characteristics of the various enhanced ionization systems. The non-uniform natures of the ion and vapour distributions in these systems are discussed in the context of theoretical models and empirical studies covering these effects. The ionization efficiencies of the successful commercial systems are shown to be over 3.5%, although caution is necessary in determining the location of samples relative to sources which themselves enhance ionization, such as the unbalanced magnetron or arc source, as inhomogenous or “beamy” plasmas can be generated with consequential ionization variations. Nevertheless, with appropriate equipment design full-scale mass production of coated components can be achieved.

Thermionically assisted r.f. plasma assisted physical vapour deposition of stabilized zirconia thermal barrier coatings

Abstract In this work, yttria partially stabilezed zirconia has been deposited under a range of process conditions, by both electron beam physical vapour deposition (EBPVD) and radio-frequency plasma assisted (PA) PVD. The addition of thermionic assistance to the above techniques was also investigated. The effects of plasma assistance, thermionic assistance, pressure, surface finish and source-to-substrate distance on structure, phase, uniformity and adhesion are reported. PAPVD alone was found to improve adhesion and permit control of coating structure and densification over more conventional EBPVD coatings. The addition of thermionic emission for both PAPVD and EBPVD results in further improvements in adhesion and coating densification.

A coating thickness uniformity model for physical vapour deposition systems—further validity tests

Abstract A model has been developed which considers the ratio between the coating thicknesses on sample faces towards and away from the vapour source. The model demonstrates that this ratio is an explicit hyperbolic cotangent function of ( s 2l ), where s is the source-to-substrate distance and l is a mean free path related to the distance which a vapour particle will travel before becoming thermalized. Previous work has shown the model to be valid under gas evaporation and simple diode ion-plating conditions, though investigations were generally restricted to deposition by electron beam (EB) evaporation. In this work we demonstrate the validity of the model for TiN coatings produced by reactive ion plating with discharge enhancement and for metallic coatings deposited by resistive evaporation, magnetron sputtering and cathodic arc evaporation. The results indicate how thickness uniformity can be influenced by the type of source used and we suggest that this is primarily due to the initial energy of the vapour flux and the effects of source geometry. In particular, changes in the vapour emission characteristics of EB bun sources are shown to have a significant effect.

Use of PVD deposited TiN coating in retarding high temperature sulphidation

Abstract TiN coated Inconel 600 and Nimonic PE11 alloys were exposed to an atmosphere comprising a high sulphur potential (pS 2 ∼ 10 −1 Pa) and a low oxygen potential (pO 2 ∼ 10 −18 Pa) at 750 °C for periods up to 72 h. The sulphidation kinetics, determined by a discontinuous gravimetric method, demonstrated that the TiN coating greatly enhanced the sulphidation resistance of the substrates, particularly during the early stages of exposure. For TiN coated Inconel 600 the post-exposure analysis by scanning electron microscopy, energy-dispersive X-ray analysis, X-ray diffraction and glancing angle X-ray diffraction, showed the formation of an outer layer containing Ni 3 S 2 on the surface of the TiN coating while an inner layer consisting of Cr 2 S 3 developed at the coating/substrate interface. The double layered scale formed on uncoated Inconel 600 consisted of Ni 3 S 2 and Cr 2 S 3 . For the TiN coated Nimonic PE11, sulphide nodules consisting of three layers — (Fe,Ni) 9 S 8 (outmost) Cr 2 S 3 MoS 2 (innermost) were observed to develop and the TiN coating was sandwiched between (Fe,Ni) 9 S 8 and Cr 2 S 3 . The portion of the TiN coating which was enveloped by those sulphide nodules became unstable after long-term exposure and subsequently dissociated thereby causing the loss of environmental protection. Moreover, the coating was damaged mechanically by the growth of the sulphide nodules. The scale formed on the uncoated Nimonic PE11 showed the formation of a similar structure in a similar sequence.

An investigation into the use of a simple model for thickness uniformity on horizontal surfaces to describe thickness variations on vertical substrates

Abstract The present authors have previously demonstrated a model for the determination of the thickness uniformity on flat, horizontal plates which is easy to use because it only requires a small amount of measured data (a minimum of two known thickness and their location in the chamber). The model is based on an empirically observed inverse power law relationship between coating thickness and source-to-substrate distance and has been shown to be applicable both to surfaces of large substrates (i.e. facing both towards and away from the vapour source) and for a variety of deposition processes, including electron beam and sputtering based techniques. In the current paper we report further investigations into the model and its extension to incorporate coating thickness distributions on vertical surfaces, although at present shielding effects are not included in the model. The substrates used in this current work were mounted with the side facing the source being totally in its line of sight. The inclusion of thickness prediction on vertical as well as horizontal surfaces, however, enables the model to start to be of use in the determination of thicknesses on real components.

A COMPARISON OF THE PROPERTIES OF HARD CARBON FILMS PRODUCED BY DIRECT GAS DEPOSITION AND PLASMA-ASSISTED EVAPORATION

Abstract The mechanical and optical properties of carbon films grown using direct gas deposition from a fast atom beam source are compared with those of films grown by evaporation of graphite in butane and argon d.c. or low frequency r.f. plasmas. The characteristics are found to be highly dependent on the substrate material (glass or tool steel) and are also related to process conditions, such as bias voltage and hydrogen to carbon ratio in the plasma.