Toughness Enhancement in Hard Single-Crystal Transition-Metal Nitrides : V-Mo-N and V-W-N Alloys
Abstract: Transition-metal nitrides are known for their high hardness, good wear resistance, high-temperature stability, and chemical inertness. Because of these properties, they are extensively used in many industrial applications, notably as protective wear, erosion, and scratch resistant coatings, which are often subjected to high thermo-mechanical stresses. While high hardness is essential, most applications also require high ductility, to avoid brittle failure due to cracking. However, transitionmetal nitrides, as most ceramics, generally exhibit low ductility and hence poor toughness.Improving toughness, the combination of hardness and ductility, of ceramic materials requires suppression of crack initiation and/or propagation, both of which depend on the microstructure, electronic structure, and bonding nature of the coating material. This, however, is an extremely challenging task that requires a fundamental understanding of the mechanical behavior of materials. Theoretical studies, for example, ab initio calculations and simulations are therefore useful in the design of “unbreakable” materials by providing information about the electronic origins of hardness and ductility. Recent density functional theory calculations predicted that alloying can increase toughness in a certain family of transition-metal nitrides such as V-Mo-N and V-W-N alloys. Toughness enhancement in these alloys arises from a near optimal filling of the metallic d-t2g states, due to their high valence electron concentrations, leading to an orbital overlap which favors ductility during shearing.This thesis focuses on the growth and characterization of V1-xMoxNy (0 ≤ x ≤ 0.7, 0.55 ≤ y ≤ 1.03) and V1-xWxNy (0 ≤ x ≤ 0.83, 0.75 ≤ y ≤ 1.13) cubic alloy thin films. I show that alloying VN with WN increases the alloy hardness and reduces the elastic modulus, an indication of enhanced toughness. I investigated the growth, nanostructure, and atomic ordering of as-deposited V1-xWxNy(001)/MgO(001) thin films. In addition, I studied the growth, structural and mechanical properties, and electronic structure of V1-xMoxNy(001)/MgO(001) and V0.5Mo0.5Ny(111)/Al2O3(0001) thin films. I demonstrate that these alloys exhibit not only higher hardness than the parent binary compound, VN, but also dramatically increased ductility. V0.5Mo0.5N hardness is more than 25% higher than that of VN. Using nanoindentation I show that while VN and TiN reference samples undergo severe cracking typical of brittle ceramics, V0.5Mo0.5N films do not crack. Instead, they exhibit material pile-up around nanoindents, characteristic of plastic flow in ductile materials. Furthermore, the wear resistance of V0.5Mo0.5N is significantly higher than that of VN. I also show, for the first time, anion-vacancyinduced toughening of single-crystal V0.5Mo0.5Ny/MgO(001) films. Nanoindentation hardness of these alloys increases with the introduction of N-vacancies, while the elastic modulus remains essentially constant. In addition, typical scanning electron micrographs of nanoindents show no cracks, which demonstrate that N-vacancies lead to toughness enhancement in these alloys. Valence band x-ray photoelectron spectroscopy analyses show that vacancy-induced toughening is due to a higher electron density of d-t2g(Metal) – d-t2g(Metal) orbitals with increasing N-vacancy concentration, and essentially equally dense p(N) – d-eg(Metal) first neighbor bonds.Overall, I demonstrate that it is possible to design and deposit hard and ductile transition-metal nitride coatings. My research results thus provide a pathway toward the development of new tough materials.
CLICK HERE TO DOWNLOAD THE WHOLE DISSERTATION. (in PDF format)