Industrial methodology for the selection of hard PVD coatings: study of Cr(N)/DLC multilayer for high wear resistance and friction applications

Significance 

Following the ever increasing restrictions targeting sustainable manufacturing, industries are constantly generating novel material concepts with the purpose to substitute harmful additives in lubricants. Some published studies have already highlighted that low friction coatings such as diamond-like carbon (DLC) can significantly minimize the friction coefficients of dry and lubricated sliding contacts. Hydrogen free DLC coatings have been seen to provide low friction, high wear resistance and high thermal conductivity, however, their high hardness and high residual stress often lead to a weak adherence on steel substrates, which is problematic for many fields of high-technology industries. Fortunately, transitional metal elements such as titanium, chromium and tungsten can be used as intermediate layers and as dopants for DLC layers. Unfortunately, selection of the proper adhesion layer, optimized deposition parameters of coatings substrate preparation and multilayer structure are primary for obtaining high interface adhesion and still need to be thoroughly researched on.

Recently, scientists at Advanced Coatings and Construction Solutions – CRM Group in Belgium: Florin Duminica, Raphaëlle Belchi, Laure Libralesso and David Mercier searched for an acceptable compromise between mechanical/tribological properties and interfacial adhesion between DLC and the steel substrate, knowing that interlayers play an important role for mechanical properties of the global multilayer system. They elucidated on the link between the mechanical and tribological properties with the adherence in order to select the best DLC coating and under layer combination. Their work is currently published in the research journal, Surface & Coatings Technology.

The research method employed commenced with the investigation of microstructure by scanning electron microscopy. Next, Raman spectroscopy was employed to assess the chemical structure of the DLC layers. The researchers then proceeded to evaluate the mechanical and tribological properties by nanoindenter, scratch tester and Rockwell ‘C’ indentation test, ball-on-disk tribometer and surface profiler.

The authors found out the nano-hardness values of DLC layers to be about 21 to 23 giga Pascal for DLC layers deposited respectively on chromium (500 nm)/CrN (2500 nm) and about 16 to 18 giga Pascal’s for chromium (2500 nm) bases. In addition, a better adhesion of the DLC coating was observed with only chromium under layer. Lastly, different multi-criteria decision making approaches were used to classify the present DLC coatings.

In conclusion, CRM group scientists presented successfully the application of a multilayer coating graded composition in improving the global coating adhesion on stainless steel, and also increasing progressively the coating hardness. Collectively, a novel performance index based on a mix of mechanical and tribological properties was advanced. Altogether, the combination of a thick DLC layer with a chromium base seems to be a good candidate for steel substrate mechanical and wear protection.

Reference

F.-D. Duminica, R. Belchi, L. Libralesso, D. Mercier. Investigation of Cr(N)/DLC multilayer coatings elaborated by PVD for high wear resistance and low friction applications. Surface & Coatings Technology, volume 337 (2018) page 396–403.

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