L. Major; R. Major; M. Kot; J. M. Lackner; B. Major
Wear 404-405 (2018) 82-91
The lifetime of protective coatings is usually limited due to the formation of defects in the form of cracks, resulting in the destruction of coatings. Understanding the initial stages of structural defects during the wearing process of coatings on the nano- and atomic scale allows for predictions of the lifetimes of fabricated coatings. The main goal of the presented work was to study the nanoscale wear mechanisms of advanced Zr/ZrxN multilayer coatings both ex situ and in situ. These coatings allow for reduced failures due to crack deflection characteristics and the ability for layers by layer coating remove. The analysis indicated thatto increase the quality of the coating’s adhesion, it was necessary to decrease the thickness of the metallic buffer layer. This decrease caused the formation of much finer, equiaxial grains, resulting in the stabilization of the isotropic residual stress of the total coating and the improved resistance to the formation of defects in the form of cracks. The adequate thickness of the buffer layer is a very important aspect in the explanation of the differences in the quality of coating adhesion; however, it is insufficient to explain differences in its wear resistance properties. The detailed microstructure study revealed that the nature of the tribofilm formed during the wear process might have a significant impact on the speed of its progression. The analysis showed that at higher phase ratios, namely, 1:4, with a larger amount of ceramic phase in the multilayer structure, a lower amount of the unfavourable hard tribofilm was produced. A similar behaviour was observed in the scratch experiments performed in situ of the scanning electron microscope. In the case of plastic dominated deformation, continuous chips were formed remaining within the scratch track. In the coating with ceramic dominated properties, the chips were discontinuous and moved away from the scratch track during the test. To have adequate coating adhesion to the substrate and appropriate wear resistance, it was necessary to limit the thickness of not only the first metallic buffer layer but also all the other metallic interlayers in the ceramic/metallic multilayer system. Limiting this thickness allowed for the stabilization of the residual stress at the substrate/coating interface and resistance to the formation of defects in the form of cracks, and it increased coating adhesion while minimizing the formation of unfavourable tribofilm, which accelerates the wear process.