Effect of atomic-scale microstructure on hardness, fracture and oxidation of metal-nitride coatings

Abstract

The main goal of this habilitation thesis is to identify and classify the types of interfaces and their role in hardening mechanisms in differently structured transition metal nitride coatings, by direct transmission electron microscopy (TEM) observations. The investigated coatings have been divided according to their structure into three groups: columnar coatings, multilayers and nanocomposites. The microstructural aspects of all three groups have been extensively studied by TEM in as-deposited and heat-treated state or after indentation in order to better visualize the mechanisms governing the coating behavior during plastic deformation. Also the combination of different chemical compositions and their influence on the coating’s microstructure has been analyzed. TEM studies down to the atomic scale, of a compositionally graded TiAlSiN coating allowed putting some light onto the mechanism of formation of the nanocomposite structure. The images taken in the Ti-rich part of the coating showed crystalline columns with neat interfaces, followed by a progressive appearance first of a crystalline and then of an amorphous boundary phase as the Al+Si concentration was increasing. At the top part of the coating the well-known nanocomposite structure consisting of crystalline grains surrounded by an amorphous matrix was observed. In addition it could be shown that the nanocomposite structure, exhibiting high hardness, can only be formed with two phases having sharp interfaces such as TiN/SiN. It was not possible to make a hardness enhanced nanocomposite out of AlN and SiN. This was due to local epitaxy at AlN/SiN interfaces, investigated on a models system of AlN/SiN multilayers. Indeed, it was found that 0.7 nm of SiN, corresponding to about two monolayers, grew crystalline on AlN favoring epitaxy. The nanocomposite structure is not the only way to achieve a hardness enhancement in these coatings: also columnar coatings can be hard,provided a sufficient density of dislocation barriers, not in form of column boundaries, is present. Two solutions to increase the density of dislocation barriers are presented: one consists in the introduction of compositionally graded multilayers, which distort the lattice but do not obstruct the columnar growth. The second solution is a phase modulated structure i.e. inside the columns zones of different phases are formed as it is the case for cubic and hexagonal NbN, which nevertheless has a columnar structure. In multilayered coatings three types of interfaces, influencing their properties were observed: completely epitaxial, such as in AlN 10 nm/SiN<0.7 nm layers or NbN/CrN at the NbN/CrN interfaces in the growth direction, semicoherent, with local epitaxy and noncoherent such as in TiN/SiN coatings. All of them were barriers for dislocation movement making thus these coatings harder that their reference columnar layouts. To better visualize and identify the hardening mechanisms based on dislocation blocking in metal nitride coatings, plastic deformation was deliberately induced by nanoindentation. Columnar and multilayered coatings have been extensively investigated by post-mortemTEM observations of indents cross-sections. In general, all coatings containing TiN, independently on their structure or layer thickness, deformed by shear sliding at grain boundaries. This mechanism was observed on two different scales: either entire pieces of multilayers or individual grains were vertically displaced over distances of several nanometers. It was particularly well visible for the NbN/TiN and the TiN/amorphous-SiN combinations. By microstructural observations it could be shown that the substrate governs the deformation in columnar TiN coatings. In the case of columnar TiN deposited on a soft Si substrate during indentation the columns underwent shear sliding at grain boundaries and were pushed into the substrate forming steps at the interface. If columnar TiN is grown on polycrystalline, hard WC-Co substrate, cracks at co lumnar boundaries are generated. Moreover, these columns, which grow on the soft Co matrix, are also pushed into it, similarly to the TiN/Si combination. Conversely, those columns, which grow on the hard WC grains are bent and internally fractured. The type of coating structure and thus the interfaces play an important role in oxidation resistance. It could be proved, by microstructural TEM investigations, that coatings with columnar grains, such as TiAlSiN and ZrAlN, are not oxidation resistant independently on their chemical composition. Indeed, the columnar boundaries serve as oxygen diffusion channels. Conversely, a coating with a nanocomposite structure, such as Al-rich TiAlSiN, has an abrupt interface with the oxide grown on its top and no penetration of oxygen into the structure is observed.