Anti-wear multilayer coatings based on chromium nitride for wood machining tools
Research highlights
▶This article presents investigation results of multilayer CrN/CrCN coatings deposited using cathodic arc evaporation method. There is no information in the available literature about multilayer CrN/CrCN coatings or their comprehensive investigations. ▶ The results of mechanical and tribological tests on substrates of HS 6-5-2 steel are presented. ▶ Results of semi-industrial operating investigations on wood are significant part of the presented study. The comparison of wear of the planer knife without the protective coating and with different types of new CrN/CrCN coating: i.e. wear area of the cutting edge, average worn edge displacement, rake face wear and nose width are shown. The roughness of the machined wood indicates that modified by the coating planer knife improve the wood surface.
Introduction
From the point of view of its mechanical and chemical properties, timber should be treated as heterogeneous material. In order to improve the quality of the processed wood and wood-derived elements, it is necessary to employ tools which have been appropriately prepared and to elaborate such surface processing technologies of the applied tools which will improve significantly their life and reliability. Increased resistance to abrasion and mechanical strokes is of paramount importance in the case of tools used in timber industry.
Both fibrous structure and viscoelastic properties of wood as well as high values of friction coefficients and use of high cutting and feed speeds place considerable requirements regarding the adhesion of the coating to the tool. Coatings of tools employed in wood processing must be characterised by numerous, frequently mutually excluding, characters, such as: considerable hardness, good adhesion to the base and small roughness. Other important traits include: low friction coefficient and wear rate.
At present, the following materials are used to manufacture wood processing tools: tool steel, high-speed steel, sintered carbides and polycrystalline diamond. The tools manufactured from high-speed steel covered with hard antiwear coatings can provide a valuable alternative for expensive tools made of sintered carbides or sintered polycrystalline diamond [1].
The dominant wear process in wood machining is abrasion, hence one can often observe erosion of the tool material and blunting of milling edges limiting possibilities of its application. This explains numerous current investigations concerned with the prolongation of tool life thanks to the introduction of proper coatings.
Due to their properties [2], [3], [4], [5], [6], [7], chromium nitride coatings are employed to cover tools used for metal, wood and wood-based material processing [8], [9], [10], [11]. Coating architecture modification [12] makes it possible to improve its useful properties. The application of the multilayer coating structure reduces the increase of column structure, blocking cracks in the interlayer zone. In addition, it may also reduces stresses forming in the coating. The coating adhesion to its substrate can also be enhanced by the introduction of transitional layers which decrease stress between the coating and the substrate [13].
There is no information in the available literature about multilayer CrN/CrCN coatings or their comprehensive investigations.
This article presents investigation results of multilayer CrN/CrCN coatings deposited using cathodic arc evaporation method. It also discusses measurement results of hardness of coatings deposited onto hardening and tempering substrates of HS 6-5-2 steel as well as results of tribological experiments performed in a ball-on-disc system. Results of industrial operating investigations are significant part of the presented study.
Section snippets
Experimental details
CrN/CrCN coatings were deposited in the TINA 900 M system employing cathodic arc evaporation from a chromium cathode of 99.99 at.% purity and of 100 mm diameter. Pressure and gas (argon, nitrogen and acetylene) flows were controlled using a Baraton type capacity gauge and MKS Company flowmeters.
Coatings were deposited onto high-speed steel (HS 6-5-2) substrates, polished mechanically to the roughness of Ra ∼ 0.02 μm and then ultrasonically washed in alkaline bath. The substrates prepared in the
General characteristics of coatings
Experiments were conducted on multilayer CrN/CrCN coatings approximately 3 μm thick. Each multilayer coating consisted of 7 CrN/CrCN modules described in Table 3. The CrN layer composition determined by the EDX method was characterised by the ratio of chromium to nitrogen of 53–47 at.%. The CrCN layer, apart from a similar ratio of chromium to nitrogen, also contained about 10 at.% carbon. The choice of the CrCN layer composition was the result of its very good properties [7] as well as lower
Conclusions
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Multilayer CrN/CrCN coatings deposited using the cathodic arc evaporation exhibited very good antiwear properties.
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Together with the loading increase, the coating friction coefficient against beechwood decreased. The friction coefficient for wet wood was two times lower than for dry wood.
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The wear of coatings with the thickness ratio of the layer CrN and CrCN in module 1:1 and 2:1 was similar but higher adhesion of the coatings with 2:1 thickness ratio indicates their potential application. This
Acknowledgement
Publication part-financed by the European Union within the European Regional Development Fund, 2007–2013.
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