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.

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

As one of the transition metal oxides, niobium pentoxide (Nb2O5) offers a broad variety of properties that make it a potentially useful and highly applicable material in many different areas. In comparison to many other transition metal oxides, Nb2O5 has received relatively little attention, which presents a significant opportunity for future investigations aimed at fundamentally understanding this material and finding new and interesting applications for it. In this article, a general overview of Nb2O5 is presented which focuses on its fundamental properties, synthesis methods and recent applications, along with a discussion on future research directions relevant to this material.

Thin films and nanostructures of niobium pentoxide: fundamental properties, synthesis methods and applications

Surface modification technologies

Thin films and nanostructures of niobium pentoxide : fundamental properties, synthesis methods and applications

This work gives a review on published results on the corrosion behavior of hard coatings deposited on steel substrates by mainly physical vapor deposition (PVD) or in some cases by plasma-assisted chemical vapor deposition (PACVD). Corrosion investigations of our own lab have been added at several sites to complete missing knowledge or to introduce new non-published data. A detailed discussion of thin film growth, microstructure and the kind of defects inherent in hard coatings has been given. Then the influence of coating thickness and some processing parameters on the corrosion resistance of coated steel is demonstrated. After this incipient part, the corrosion behavior of hard nitride coatings, multilayer coatings, alloyed or modified nitride coatings, insulating nitride, oxide or oxynitride coatings and duplex treated steel is being presented. Own investigations and published results show that single layered, conducting nitride coatings can protect steel to some extent only if very thick coatings (≥ 10 μm) are applied or if energetic deposition methods (e.g. ion-assisted deposition) are used. Distinct improvements of the corrosion behavior of hard coatings can be obtained by a proper multilayer structure deposited by deposition methods allowing energetic condensation, i.e. possessing a high ion-to-atom flux ratio. Alloying TiN by a sacrificial element, like magnesium, is also capable of significantly improving the corrosion resistance of low alloyed steel. Amazing improvements have also been reported for insulating coatings like AlN, oxynitride coatings or oxide coatings. However, the most significant improvement seems to be the application of duplex treatments. Plasma nitriding + PVD coating or PVD coating + atomic layer deposition (ALD) prolonged the time to red rust in neutral salt spray test for > 2 days up to one week, respectively. The only disadvantage of the duplex treatments is the higher cost. Recently, mapping by a confocal microscope of TiN coating surfaces before and after salt spray testing showed that open porosities in micro- and macro-scale growth defects mainly govern the corrosion behavior of hard coatings on steel. The coating microstructure – i.e. the submicron-scale morphology – seems to play only a secondary role in the corrosion behavior.

Corrosion protection with hard coatings on steel: Past approaches and current research efforts

PVD hard coatings can be a serious alternative to electrochemical deposited coatings in the field of corrosion protection, if they are dense and without pores. This study is part of a series of experiments with the aim to deposit thin films onto high-speed steel by magnetron sputtering which possess a high corrosion resistance. Two different methods were chosen: (1) deposition of single layers (CrN, NbN) and (2) deposition of multilayer films (Cr/CrN). Additionally the effect of intermediate plasma etching was studied for these coating systems. That means for single layer systems, that the film growth was stopped several times and the surface of the as-grown film was exposed to an ion bombardment. For multilayer systems the intermediate plasma etching was applied between the different layers. It is assumed that the plasma etching of the film leads to a denser coating, due to defect-induced renucleation and improved adatom mobility. The corrosion resistance of the coatings was studied by potentiodynamic corrosion tests in sodium chloride solution. Further film properties were investigated by SEM, XRD, Scratch testing, Rockwell indentation tests and microhardness measurements. The results reveal that intermediate etching improves the corrosion resistance of monolithic CrN films, whereas for the multilayered coatings no further improvement could be observed.

Improvement of the corrosion resistance of hard wear resistant coatings by intermediate plasma etching or multilayered structure

Corrosion behaviour of decorative and wear resistant coatings on steel deposited by reactive magnetron sputtering – Tests and improvements

Hard niobium nitride films were deposited onto high-speed steel (HSS) using reactive magnetron sputtering. Experiments were carried out by varying the nitrogen partial pressure. The porosity and corrosion behavior of the NbNx/HSS samples were studied by a copper decoration test and by potentiodynamic corrosion tests in 0.8 M NaCl solution (pH 7), respectively. The corrosion tests revealed that the NbN/HSS system has a very broad passivation range with a low current density. Further film properties were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), scratch testing and microhardness measurements. Good adhesion and very high plastic universal-hardness values in the range of 34 000–53 000 N/mm2 were found for the NbN thin films.

Deposition of NbN thin films onto high-speed steel using reactive magnetron sputtering for corrosion protective applications

(Ti,Mg)N coatings were deposited onto high speed steel (HSS) and glass substrates using reactive dc magnetron sputtering. Because of the addition of Mg to TiN the colour changed from golden to violet and metallic blue as measured by spectrophotometry. The chemical composition, morphology and structure of the coatings was investigated by energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The addition of Mg to TiN leads to a shift of the TiN peaks in the XRD patterns. Probably, this is due to the substitution of titanium atoms by the bigger magnesium atoms. The indentation hardness and the open-circuit-potential (in NaCl solution) of the (Ti,Mg)N coatings is decreasing with increasing Mg contents. The oxidation resistance of (Ti,Mg)N coatings could be drastically improved with increasing Mg content.

H. Kappl

Papers

Corrosion protection with hard coatings on steel: Past approaches and current research efforts

Improvement of the corrosion resistance of hard wear resistant coatings by intermediate plasma etching or multilayered structure

Corrosion behaviour of decorative and wear resistant coatings on steel deposited by reactive magnetron sputtering – Tests and improvements

Deposition of NbN thin films onto high-speed steel using reactive magnetron sputtering for corrosion protective applications

Some properties of (Ti,Mg)N thin films deposited by reactive dc magnetron sputtering