Examination of mechanical properties and failure mechanisms of TiN and Ti−TiN multilayer coatings

Abstract

A number of studies have been carried out to establish mechanical properties of single and multilayer hard coatings. However, the mechanisms of deformation, cracking and delamination of coatings under ploughing and shear stress are not fully understood. A fractured cross-sectional specimen preparation technique through hardness indentation and scratch tests on hard coatings has been used in conjunction with high resolution SEM to observe deformation and fracture behaviour occurring as a result of these tests. TiN and Ti−TiN multilayer coatings were deposited on M2 high speed steel and silicon substrates using an unbalanced magnetron sputtering system. Hardness measurements and scratch tests were performed to monitor the mechanical properties. X-ray diffraction was used for phase identification. Coatings comprising fine columnar TiN behaved like closely congregated strong fibres: they were found to accommodate a large amount of ploughing and shear stress through densification and shear deformation. On increasing the load above a certain value, rupture of heavily deformed TiN initiated at defect locations and the cracks propagated and coalesced into macrocracks. When the applied load was increased to near the critical load, close packed columns separated from each other and detached from the substrate, resulting in total failure. For Ti−TiN multilayers, hardness and critical load are related to the different monolayer thickness of the Ti and TiN. The Ti layers dissipate most of the energy by means of shear deformation during the scratch test. At higher scratch loads, cracks occurred at Ti−TiN interfaces or at multilayer- substrate interfaces depending on the relative interface strengths. The influences of substrate hardness on the indentation crack pattern and scratch failure mechanism are also briefly covered in this paper.