Phase formation and microstructure evolution of reactively r.f. magnetron sputtered Cr-Zr oxynitride thin films

摘要

Thin films in the system Cr-Zr-O-N were deposited by reactive r.f. magnetron sputtering at a substrate temperature of 500 °C in various argon-oxygen-nitrogen plasmas. The depositions were realized by an experimental combinatorial approach: five substrates were placed linearly in front of a circular segmented target consisting of a metallic Cr and Zr half plate. Consequently, the coated samples exhibit different elemental compositions (with respect to their position in relation to the target half plates, i.e. from Cr-rich to Zr-rich) and constitutions. Additionally to the impact of the Cr/Zr concentration ratio (mainly determined by the sample position in relation to the target) the influence of reactive gases, i.e. oxygen and nitrogen, on phase formation, microstructure evolution, hardness and Young'smoduluswas investigated. For this purpose, the O_2 and N_2 gas flowswere varied systematically,while the total gas pressurewas kept constant for all depositions. The coatingswere characterized by electron probe micro analysis (EPMA), X-ray diffraction (XRD) and microindentation. In dependence of the sample position and the reactive gas flow ratio coatings with various phases were grown: i) coatings with a high Cr/(Cr + Zr) concentration ratio (i.e. grown at the Cr-rich side of the target) exhibited a single-phase solid solution corundum structure with up to 1.9 at.% nitrogen incorporation, (Cr,Zr)_2(O,N)_3. ii) coatings with similar Cr and Zr concentration (i.e. grown at the center positions of the target) exhibited a single-phase solid solution cubic ZrO_2 structure with approximately 2 at.% nitrogen incorporation, c-(Zr,Cr)(O,N)_2. iii) coatings with a higher Zr concentration (i.e. grown at the Zr-rich side of the target) grew in mixed-phase monoclinic and tetragonal structure, incorporating up to 2.3 at.% nitrogen, m + t-(Zr,Cr)(O,N)_2. These coating structures developed over a wide range of oxygen and nitrogen gas flow settings and substrate positions. It indicates that oxygen determines the microstructure evolution under the chosen experimental conditions, and only little nitrogen contents can be incorporated into these oxide structures. For high nitrogen gas flows (i.e. N_2/(N_2 + O_2) >0.75) this situation changed significantly, and only coatings with facecentered- cubic CrN structures were grown. These contained up to 11.4 at.% oxygen and were clearly understoichiometric in the non-metal sites. All deposited coatings exhibited hardness values between 15 GPa and 27 GPa, indicating their suitability as protective coatings.