ICMCTF2011 Session F5: New Oxynitride Coatings

PVD coatings for high performance applications such as protective coatings for metal cutting require outstanding properties including high hardness, chemical inertness and high temperature stability. In the last decade the synthesis of hard Al-O and related protective thin films attracted large scientific interest and such coatings were successfully introduced in industrial production processes.

We present a combinatorial approach for the synthesis of ternary Al-Cr-O and quaternary Al-Cr-O-N solid solution strengthened thin films, grown in α-phase structure, by reactive r.f. magnetron sputtering in argon-oxygen-nitrogen atmospheres. The deposition experiments were carried out with a laboratory scale Leybold Z 550 PVD machine by sputtering from a segmented Al-Cr target at non-equilibrium conditions. The substrate temperature during deposition was adjusted to 500°C and the r.f. substrate bias was varied from zero up to -300 V. The metastable thin films were characterized by determining the Vickers micro hardness, the residual stress, the chemical composition by EPMA, the structure by XRD and the microstructure by SEM and TEM.

This work reports on the correlation between deposition parameters, chemical composition, crystal structure, mechanical properties and cutting performance of Al-Cr-O-N coatings produced by reactive cathodic arc evaporation. The coatings have been synthesized using Al_xCr_1-x alloyed targets where x varies from 0 to 0.85. The reactive gas mixture was adjusted in a way to obtain coatings with composition varying from AlCrN to (Al,Cr)₂O₃. The chemical composition of the coatings was investigated by means of Rutherford backscattering spectroscopy (RBS). The evolution of the crystallographic structure as a function of chemical composition was studied by X-ray diffraction (XRD). Based on the results of RBS and XRD analysis we propose formation of the oxide coatings with cubic crystal structure if the ratio of oxygen atoms to metal atoms during the deposition is close to one. The oxide coatings with the cubic structure exhibit the highest deposition rate, the highest hardness as well as significantly improved tool life as compared to (Al,Cr)N and (Al,Cr)₂O₃ coatings.

In this work we focus on the influence of oxygen content on the properties of cathodic arc-deposited AlCr(O_xN_1-x) coatings. The samples were prepared in a mixture of O₂ and N₂ at 550°C using lateral rotating arc cathodes (LARC) technology and a pulsed bias voltage. The obtained coatings were characterized by various techniques including XRD, SIMS, SEM, TEM, pin-on-disk tests and nanoindentation.

The diffraction results show that the samples can be classified in three groups with respect to their oxygen content, x. For the first group of samples with x ≤ 0.6, single-phase films with fcc lattice are obtained despite the large proportion of oxygen. In the second group, composite coatings consisting of a mixture of α-(Al,Cr)₂O₃ with corundum structure and (Al,Cr)(O,N) with cubic structure are achieved within a range of oxygen from 0.6 < x ≤ 0.95, as confirmed by XRD and TEM. The third group is formed by coatings with x > 0.95, where a well-crystalline α-(Al,Cr)₂O₃ corundum phase is observed. In all three groups, coatings heat-treated for 1h at 1000°C in argon environment show enhanced crystallinity and kept their structure except for those close to the threshold of the first group (x ≈ 0.6), where a huge difference between as-deposited and heat-treated states was observed. Our results indicate that these single-phase oxynitrides are metastable and after heat treatment convert to a composite from equal volume fractions of nitride and oxide. This behaviour was attributed to the presence of a sufficient quantity of oxygen in the coatings to enable local formation of oxide lattice during annealing.

The characteristics of the AlCr(O_xN_1-x) arc-PVD coatings are compared to both chemically equivalent and silicon-doped pulsed DC magnetron sputtered layers, and friction and wear for the different series were studied at both ambient and high temperatures. The low wear rates observed for the oxynitride coatings suggest their suitability for turning and milling applications, which will be demonstrated by cutting test examples.

Deposition of oxynitride hard coatings was realized with an arc-PVD process by use of an oxygen-nitrogen gaseous mixture. For a stabilized and reproducible deposition process without micro arcs, an adapted coating hardware and optimized coating procedure were used. To determine the optimal process design, several parameters of the arc-PVD-process were varied: the material system (Zr-O-N; Cr-Al-O-N), the coating structure (gradient, multilayer, block), the pre- and post-treatment and the process condition (current, pressure, hf-pulsed or constant bias …). The results of optimization the coating process are oxynitride coating systems with improved coating properties and a higher thermal stability.

Applicability of the developed coating systems are demonstrated in several investigations in machin-ing heavy machinable materials. In various analyses an increase in tool life of oxynitride coated tools at higher cutting speed could be proved. This shows the applicability of the developed oxynitride coating systems to improve the efficiency of cutting processes.

Zirconium oxide (ZrO₂) exhibits excellent optical properties such as high refractive index, large band gap, low optical loss, high transparency in the visible and near-infrared regions, and high dielectric constant. However, ZrO₂ is UV-light sensitive. In addition, interfacial reactions during thin-film growth suppress the effective dielectric constant and degrade the optical performances. The present work was performed on ZrO_xN_y thin films to effectively alter the electronic structure by the method of nitridation, which well-known to suppress the interfacial reactions. ZrOxNy thin films were produced by magnetron-sputter deposition under the reactive pressure of nitrogen and oxygen. The effect of nitrogen/oxygen flow rate on the structure and optical properties of ZrO_xN_y thin films was investigated and compared with that of ZrO₂. The optical measurements of ZrO₂ films show a very high optical transmission with a band gap of 5 eV. The optical absorption measurements on ZrOxNy thin films grown at various reactive nitrogen pressures indicate a progressive shift from insulating to semiconductor behavior. The corresponding changes in the profiles of index of refraction were also remarkably distinct. The results indicate that tailoring the electronic structure and optical constants of ZrO_xN_y thin films to meet the requirements of visible-light functionality can be achieved by carefully controlling the reactive pressure. The results will be presented and discussed in detail.

The aim of this work is to develop zirconium oxynitride coatings by RF magnetron sputtering on silicon substrates. The film properties were analyzed as a function of oxygen partial pressure in two different inert gas atmospheres namely argon and helium. At low oxygen partial pressure, Zr₂ON₂ and m-ZrO₂ phases are present as observed from the structural characterization done by X-ray diffraction. The surface morphology was investigated by AFM. The proportion of metalloid content rises with increase in oxygen partial pressure which was determined by FE-SEM/EDS. The thickness of the film was measured by surface profiler and varies inversely with oxygen partial pressure. The films deposited are hydrophobic and the contact angle was measured by contact angle measuring system. Higher surface roughness and contact angle values are observed at low oxygen partial pressure. The surface energy of films was calculated by two methods: Owens–Wendt’s geometric mean and Wu’s harmonic mean approach. The elevated surface energy values were observed with increase in oxygen partial pressure. The stress measurements of the deposited films were done by sin²ψ X-ray diffraction method which depends on the variation of Zr₂ON₂ and m-ZrO₂ phases.