In recent years vanadium-doped hard coatings have become available as possible candidates for self-lubrication at high temperatures. Their low coefficient of friction has mainly been attributed to the formation of the V2O5 phase at temperature of about 500°C. However, the formation of vanadium oxides must be controlled by the out-diffusion of vanadium in order to achieve the combination of a low coefficient of friction and good mechanical properties for the protective coatings. In this study a nanolayer CrN/(Cr,V)N hard coating was investigated to see whether it could better control the vanadium out-diffusion. It is based on the idea that the top chromium oxide layer acts as a diffusion barrier to the vanadium ions during the oxidation at elevated temperatures ()500 °C) due to the immiscibility between the vanadium oxides and the chromium oxides. Special attention was given to the oxidation process that takes place at the growth defects.
COBISS.SI-ID: 28907559
The CrN/CrVN coatings were extensively analyzed from the tribological point of view, while for reference we also analyzed the standard monolayer coatings CrN and CrVN. Numerous factors can influence the coating tribological properties. In contrast to most publications which typically use standard lab conditions we put an emphasis on the aspect of the surrounding atmosphere. We performed tribological tests in air (high, medium and low humidity), nitrogen and oxygen environment. To fully understand the processes during tribological testing, the wear mechanisms were investigated for each hard coating separately. We found important differences between results of tribological tests carried out in ordinary air, oxygen (main point is absence of moisture) and nitrogen (main point is absence of oxidation). The lowest wear volume was measured at the above mentioned nanolayer coatings.
COBISS.SI-ID: 28214055
We studied the oxidation resistance of CrVN single-layer, CrN/(Cr,V)N nanolayer and CrN/(Cr,V)N/VN nanolayer hard coatings with respect to the Cr:V atomic ratio. All the coatings were deposited by DC magnetron sputtering in an industrial deposition system. Using different target configurations we were able to prepare a series of samples with various Cr:V ratios. The following target arrangements were used: a) two sources with triangle-like, segmental Cr/V targets for depositing a single-layer CrVN coating with different Cr:V atomic ratios; b) two triangle-like, segmental Cr/V targets and two chromium targets for depositing the nanolayer CrN/(Cr,V)N coating; and c) two triangle-like, segmental Cr/V targets, one chromium and one vanadium target for depositing the nanolayer CrN/(Cr,V)N/VN coating. The analysis of the oxidation mechanisms for all three types of coatings showed that the formation of a thin chromium oxide layer slows down the diffusion of vanadium towards the surface. However, at the locations where the growth defects were present the vanadium kept diffusing at high rates and formed V2O5 dendritic structures at the surface.
COBISS.SI-ID: 30083879
Defects are one of the major obstacles in application of hard coatings. The corrosion resistance on these spots is severely reduced and the possibility of delamination is increased. We have analysed defect concentration by using statistical approach and by studying morphology, microstructure and composition of individual defects. We analyzed the composition on the spot of the defect and microstructural properties and found out that there are three types of defects: (i) pieces of foreign material, covered by the growing coating – they are a consequence of dust and delamination from chamber walls, (ii) microdroplets, covered by the growing coating with a porous microstructure, (iii) depressions, formed after delamination of the defects, Studies further revealed that defects originate either from inclusions in the substrate (e.g. sulphur inclusions found in different tool steel) or from deposition process itself. Both origins are important in all PVD techniques although process conditions normally dominate. In the deposition process, different stages, i.e. pump-down, heating, deposition and cool-down, cause formation of defects.
COBISS.SI-ID: 27288615
In the chapter »Hard Coatings on Cutting Tools and Surface Finish« of the book »Comprehensive Materials Finishing« (Publisher Elsevier, Oxford) an outline of the effect of hard coatings on surface finish in machining applications is presented. Main hard coatings designs were introduced and the required properties from a hard coating were summarized. It was shown that cutting performance of cutting tools and surface finish can be significantly improved by deposition of a hard coating on cutting tool materials. The relation of hard coatings with tool wear, cutting temperature, chip formation, surface roughness, and residual stress was discussed in detail.
COBISS.SI-ID: 29793063