ZrN based Nanostructured Hard Coatings : Structure-Property Relationship

Abstract: Ever since the hard coatings have been introduced, there has been a constant push for better mechanical properties, which motivates for deeper understanding of the microstructure-mechanical properties correlation. The aim of this thesis is to extend the knowledge on how microstructural variation influences the deformation, fracture and wear behavior of ZrN based nanostructured coatings.Few microns thick, monolithic Zr-Si-N and multilayered Zr-Al-N coatings were deposited by reactive arc deposition and unbalanced reactive magnetron sputtering techniques respectively. The microstructures of the coatings were studied using xray diffraction, transmission electron microscopy and scanning electron microscopy. Indentation induced plastic deformation and fracture behavior was visualized by extracting the lamellae under the indent using focused ion beam milling technique combined with transmission electron microscopy. Wear behavior of the coatings were characterized by reciprocating sliding wear test following microscopic observations of the wear track.Monolithic Zr-Si-N coating shows a systematic variation of microstructure, hardness and fracture resistance as a function of Si content. Si forms a substitutional solid solution in the cubic ZrN lattice up to 1.8 at. % exhibiting a fine columnar structure. Further Si additions result in precipitation of an amorphous SiNX phase in the form of a nanocomposite structure (nc ZrN- a SiNX) that is fully developed at 6.3 at. % Si. Dislocation based homogeneous deformation is the dominating plastic deformation mode in the columnar structure, while grain boundary sliding mediated plastic deformation causing localized heterogeneous shear bands dominates in the nanocomposite structure.Indentation induced cracking shows the higher fracture resistance for columnar structure compared to the nanocomposite coatings. Crack branching and deflection were observed to be the key toughening mechanisms operating in the columnar structured coating. Reciprocating wear tests on these coatings show a bi-layer wear mode dominated by tribo-oxidation. Nanocomposite coatings offer superior resistance to both static and tribo-oxidation, resulting in higher wear resistance even though they are soft and brittle.Monolithic and multilayers of Zr0.63Al0.37N coatings were grown at a deposition temperature of 700 °C. Monolithic Zr0.63Al0.37N coating shows a chemically segregated nanostructure consisting of wurtzite-AlN and cubic-ZrN rich domains with incoherent interfaces. When the same composition is sandwiched between ZrN nanolaminates, Zr0.63Al0.37N shows a layer thickness dependent structure, which results in systematic variation of hardness and fracture resistance of the coatings. Maximum hardness is achieved when the Zr0.63Al0.37N layer shows semicoherent wurtzite-AlN rich domains. While the maximum toughness is achieved when AlN- rich domains are pseudomorphically stabilized into cubic phase. Stress induced transformation of metastable cubic-AlN to thermodynamically stable wurtzite-AlN was suggested to be the likely toughening mechanism.