Propriétés physiques et chimiques des couches minces de ZrN, Zr-Al-N et Zr-Cr-N déposées par pulvérisation magnétron réactive
Because of their outstanding mechanical and chemical properties, transition metal nitrides are widely accepted materials in the protective coatings industry. This work aims at elucidating the origin of their exceptional properties. The study concentrates on zirconium nitride based thin films, since ZrN is a promising coating material especially in a corrosive and high temperature environment. Thin films of the binary Zr-N compounds, and of the ternary Zr-Al-N and Zr-Cr-N, material systems are deposited by reactive RF or DC magnetron sputtering. A comprehensive characterization of the films is performed in order to shed light on the interrelated dependencies of the material properties. This analysis includes X-ray diffraction, electron probe microanalysis, spectroscopic ellipsometry, optical reflectivity and transmission, imaging techniques like scanning and transmission electron microscopy and nanoindentation. The chemical composition of the thin films of ZrNy lies between y = 0.81 and y = 1.35. Indeed, the X-ray diffraction patterns and the optical and electric properties show the existence of two phases of different structures. A structure cfc-ZrN is formed in a broad range of composition of y = 0.81 to y = 1.30 and an insulating orthorhombic phase Zr3N4 for 1.30 < y ≤ 1.35. The stress-free lattice parameter increases with increasing y. The analyses of the structural and morphological properties show the existence of only the fcc NaCl type of structure (solid solutions) for Zr1-xAlxN with 0 ≤ x ≤ 0.43 and for Zr1-xAlxN with 0 ≤ x ≤ 0.48. Columnar morphology does not change with the increase in the chromium or aluminium contents in ZrN. The stress-free lattice parameter decreases linearly as function of x. The hardness values gradually increases from 21 GPa up to 28 GPa as the Al content increase from x = 0 to x = 0.43. In contrast, the hardness and Young's modulus in Zr1-xAlxN remains unchanged for every Cr content in the films. The difference in hardness and Young's modulus, between Zr1-xAlxN and Zr1-xAlxN films is interpreted in terms of local bond strengthening and conduction band states occupancy. In addition, all the ternary ZrAlN and ZrCrN thin films exhibit lower compressive stress values. The residual stress generally remains too low to influence the mechanical properties. The optical and electrical properties of ZrNy and Zr1-xAlxN thin films were studied and interpreted as a function of the chemical composition. When the ratio y = N/Zr or x = Al/(Al+Zr) increases, the optical properties continuously change from metallic to semiconducting behavior, even insulator. The model of Drude-Lorentz is used to interpret the optical measurements in order to separate the contributions due to free carriers and interband transitions. The effects of the variation of the chemical composition on the optical properties can be interpreted as a function of the type of defect (vacancies or interstitiels nitrogen in ZrNy and substitution of zirconium by aluminium or chromium in Zr1-xMxN) which exists in the crystalline structure. The exact nature of the defects is confirmed by the analyses of X-ray diffraction. The study of the kinetics of the growth of oxide layers in the system Zr-Al-N showed that the resistance to oxidation of ZrN thin films is improved by the addition of aluminium.