In recent years, several new solid lubricant and modern lubrication concepts have been developed to achieve better lubricity and longer wear life in demanding tribological applications. Most of the traditional solid lubricants were prepared in the form of metal, ceramic and polymer-matrix composites. They have been used successfully in various engineering applications. Recent progress in thin-film deposition technologies has led to the synthesis of new generations of adaptative, self-lubricating coatings with composite or multilayered architectures, by using duplex/multiplex surface treatments. These modern self-lubricating coatings progressively make their way into the commercial marketplace and meet the ever-increasing performance demands of more severe applications. The present paper reviews our recent understanding of the lubrication mechanisms of both traditional and new solid lubricants, with particular emphasis on solid lubricant methods and practices.

Historical developments and new trends in tribological and solid lubricant coatings.

http://www.123seminarsonly.com/Seminar-Reports/2013-04/80827267-Coatings-tribology-contact.pdf

Coatings tribology—contact mechanisms and surface design

There is increasing scientific and commercial interest in the development of nanostructured coatings, particularly those based on low-miscibility ‘ceramic–ceramic’ or ‘ceramic–metal’ crystalline/amorphous nanocomposite phase mixtures deposited by plasma-assisted PVD or CVD. In laboratory mechanical testing, extreme values of hardness (which may be in excess of 70 GPa) are often found for such films, similar to those claimed for intrinsically hard materials such as c-BN and diamond. High hardness is, however, often accompanied by an associated high elastic modulus, which although desirable in principle for cutting tool materials and/or coatings, may in practice limit coating durability, on low-strength, low-modulus substrates (e.g. low-alloy steels and the light alloys) and in many wear applications other than metal cutting. In this paper, we discuss the benefits of using the ratio of hardness to elastic modulus ( H / E ) as an indicator of coating durability since this parameter essentially describes the elastic strain to failure capability (and resilience ) of a candidate material. Furthermore, we consider the likely need for tribological coatings to accommodate some degree of substrate deformation; in this respect film toughness , i.e. ‘engineering toughness’ in the sense of an ability to absorb deformation energy (both elastic and plastic) needs to be considered. The concept of predominantly metallic films with a nanograined and/or glassy microstructure (containing little or no high-modulus ceramic constituents) is introduced, through which we point to the importance of retaining ‘sufficient’ coating hardness, whilst reducing coating elastic moduli to more closely match those of candidate substrate materials. With regard to the implications of H / E for practical tribological coating applications, we propose that closer matching of the coating/substrate interfacial elastic properties and thus an improved ability for the coating to accommodate substrate strain, where necessary, is often a more important factor in wear resistance than is extremely high hardness.

Design criteria for wear-resistant nanostructured and glassy-metal coatings

Since the inception of electroless coating by Brenner & Riddell in 1946, it has been the subject of research interest and, in the past two decades, emphasis has shifted to the studies of its properties and applications. The co-deposition of paniculate matter or substance within the growing film has led to a new generation of electroless composite coatings, many of which possess excellent wear and corrosion resistance. This valuable process can coat not only electrically conductive materials including graphite but also fabrics, insulators like plastics, rubber etc. The low coating rates with these can provide better reflectivity of plated surfaces and many more applications. Coatings can be tailored for desired properties by selecting the composition of the coating alloy/composite/metallic to suit specific requirements. The market for these coatings is expanding fast as the potential applications are on the rise. In the present article, an attempt has been made to review different electroless alloy/composite coatings with respect to bath types and their composition, properties and applications. Different characterisation studies have been conducted on various electroless nickel-based coatings with emphasis on wear and corrosion properties.

Electroless alloy/composite coatings: A review

In recent years, great strides have been made in the formulation of solid lubricant coatings for a wide range of industrial applications These coatings are now available in nano-structured and/or -composite forms to provide better performance and durability even under very severe sliding conditions By coupling these coatings with smart surface engineering strategies (such as micro-texturing and/or -patterning), researchers have achieved higher levels of performance and durability in demanding tribological applications Some of these advanced coatings are now commercially available and can meet the ever-increasing performance and durability requirements of severe tribological applications This paper will provide a historical overview of recent developments in solid lubricant coatings and will expand on the lubrication mechanisms of both traditional and new solid lubricants Special emphasis will be placed on modern practices that are aimed at enhancing the properties of these coatings and expanding their uses in practical applications

Solid Lubricant Coatings: Recent Developments and Future Trends

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.

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

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.

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

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.

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

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.

Ionization in plasma-assisted physical vapour deposition systems

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

A.S. James

Papers

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

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

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

Ionization in plasma-assisted physical vapour deposition systems

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