ICMCTF2015 Session F4-1: Functional Oxide and Oxynitride Coatings

We report an investigation into the origin of the hydrophilic/hydrophobic switch from chromium oxide to chromium nitride thin films in contact with water. The presence of surface entities and bonds in the films that are responsible for the switch from hydrophilic to hydrophobic behaviour is probed with Raman spectroscopy, FTIR and X-ray photoelectron spectroscopy (XPS) including XPS satellite peaks. The role of the surface energy is further probed through the dispersive, polar and acid-base components of the films using the Owens-Wendt (OW) and the Van Oss-Chaudhury-Good (VOCG) methods. Kelvin probe investigations of the role of the surface entities on surface energy changes in the films are carried out through contact potential difference (CPD) measurements and evaluation of electron affinity changes. Electrochemical corrosion investigation in saline solution involving open circuit potential, Tafel plots and Potentiodynamic polarization indicates superior corrosion resistance by the chromium oxide phase compared with chromium nitride. Our investigations show the potential to use combined chromium oxide/nitride layers in aggressive environments for wettability gradient force generation applications. Potential application areas include Lab-on-a-chip, biotechnology, biomaterials and reduced gravity space experiments, when an appropriate plasma treatment is used to create combined chromium oxide/nitride layers.

During the last years the Al-Cr-O coatings are getting more important for cutting tool applications. Beside chemical vapour deposition (CVD) which requests deposition temperatures above 1000°C, physical vapour deposition (PVD) can be deployed for coating production at reduced temperature below 600°C. The most used PVD techniques are reactive arc evaporation and magnetron sputtering. Although, the arc evaporation method enables a stable coating process with a high degree of ionisation of the deposition flux, macro-particle incorporation into the growing film might reduce the coating quality and cutting performance. Utilization of magnetron sputtering improves coating quality with respect to the number of incorporated defects. However complicated and cost intensive regulation methods have to be used during magnetron sputtering in order to enable a stable production process for oxide coatings. Additionally, a low degree of ionization of the deposition flux might result in deteriorating wear resistance of the coating.

Recently, high power impulse magnetron sputtering (HIPIMS) was reported to be the novel technique which combines high degree of ionization of the deposition flux as well as absence of micro-particles. In this work we report on the synthesis and properties of Al-Cr-O wear protective coatings for cutting tool applications by using S3p™ in an industrial deposition plant. S3p™ technology enables scalability of the pulse power density and pulse length in a wide range and expands significantly the choice of the deposition parameters and process stability as compared to conventional HIPIMS.

Al-Cr-O coatings have been produced using Al_xCr_1-x targets, where x was varied from 0.5 to 1. The deposition process is discussed with respect to the special features for deposition of oxide layers. Advantages of S3p™ technology are emphasised. Crystal structure, morphology as well as hardness and elastic modulus of the coatings studied as a function of the coating composition.

Oxide-based thin film materials are widely used in optical and electronic applications due to their unique properties, e.g. transmittance, refractive index, electrical resistivity and chemical stability. The range of materials used includes transparent conducting oxides, dielectrics, diffusion barriers, and optical filter materials. With changes in stoichiometry or by alloying or doping the base materials, it is possible to tune the material properties to the desired level. The current work aims to explore structure and properties of molybdenum oxide thin films as various phases exist in the Mo-O system, potentially enabling to tune thin film properties.

Molybdenum oxide thin films were deposited by reactive dc magnetron sputtering from a molybdenum target on Si (100) and window glass. By varying the gas flows during deposition, the argon/oxygen ratio was changed from pure argon to pure oxygen. The total flow and the total pressure were kept constant at 40 sccm and at 0.5 Pa, respectively. The film growth rate decreased from 90 to 7 nm/min with increasing oxygen partial pressure. The optical appearance of the as-deposited films varied from metallic to transparent, which correlated well with the oxygen content changing from 35 up to 68 at.%. A detailed structural characterisation was done by X-ray diffraction and Raman spectroscopy. The dominating phases were MoO₂, different polymorphs of Mo₄O₁₁ and α–MoO₃, depending on the oxygen content in the film. Mechanical properties like hardness, Young’s modulus and residual stresses as well as electrical conductivity and optical properties like transmittance and reflectance also show a dependence on the oxygen content. In conclusion, structure and properties of the deposited molybdenum oxide thin films are tuneable by adjusting the argon/oxygen ratio in the background gas during deposition; these thin films are, hence, promising materials for a wide variety of optical and electronic applications.

Using oxygen as alloying element in transition metal aluminum nitride coatings has recently attracted significant attention [1-4]. In this talk, it is shown that oxygen incorporation on nitrogen lattice sites leads to metal vacancy formation in the transition metal oxynitride, irrespective of whether the deposition was done by cathodic arc or high power pulsed magnetron sputtering [3,4]. Similar vacancy formation has been observed in the Al-Si-N system by metal doping [5]. Additionally, the impact of oxygen addition and metal vacancies on elastic properties will be discussed based on comparative ab initio and experimental data. Furthermore, initial experimental data on the composition dependence of the thermal stability will be presented.

[1]: Stueber et al., Thin Solid Films 519 (2011) 4025.

[2]: Khatibi et al., Acta Mater. 60 (2012) 6494.

[3]: Shaha et al., Appl. Phys. Let. 103 (2013) 221905.

[4]: Hans et al., J. Appl. Phys. 116 (2014) 093515.

[5]: Pignedoli et al., Appl. Phys. Lett. 96 (2010) 071908.

The paper presents results of research of duplex oxide layers obtained on the Ti-13Nb13-Zr alloy. The d ouble layers were prepared in two steps by two different methods. The f irst method used the gas oxidation process to achieve thin oxide layer on which nanotubes were produced by an electrochemical method . In the second method, based on the fully electrochemical production of oxide layers, on the amorphous titanium layer the nanotube layer was formed . The gas oxidation process was conducted at air atmosphere, using three different temperatures of 700, 900 and 1100°C (1292, 1652, 2012°F ) during 5h . The electrochemical oxidation was achieved in the 1M H₃PO₄ ( orthophosphoric acid ) and 1 M H₃PO₄ with the addition of HF ( hydrofluoric acid ) over 0.5 h, at constant current values of 20V and 40V . The SEM ( Scanning Electron Microscope ), EDX (Energy Dispersive X - ray Spectroscopy), chemical analysis, AFM (Atomic Force Microscopy ), Raman Spectroscopy, and potentiokinetic corrosion tests at different pH studies were used to examine the oxide layers.

The obtained results show the possibility of producing duplex layers on titanium alloy Ti- 13Nb - 13Zr. Electrochemical formation of nanotubular layer as a second one, is possible only to a specific thickness of the first layer. The different colonies of nanotubes are observed. The obtained results indicate significant decrease of the corrosion resistance of titanium alloy Ti13 -Nb- 13Zr following the appearance of duplex oxide layers on the surface of titanium alloy.

Oxide and nitride films of various low-electronegativity metals were prepared by dc reactive magnetron sputtering in an Ar+O₂ or Ar+N₂ gas mixture. Hydrophobic (the water droplet contact angle (WDCA) and surface free energy) and mechanical (the hardness H, effective Young’s modulus E^* and elastic recovery W_e) properties were investigated in detail. The hydrophobicity of the sputtered films was measured by the sessile drop method with polar and non-polar testing liquids. These tests demonstrated that the surfaces of all sputtered nitride and oxide films of low-electronegativity metals (unlike those of most metals) are of non-polar nature. The van Oss-Good-Chaudhury approach, based on the Lifshitz-van der Waals/acid-base theory, was selected for the calculation of the film surface free energy. It was found that: (1) All studied nitride and oxide films of low-electronegativity metals are hydrophobic with WDCA’s ranging from 94^o to 105^o, i.e. the obtained WDCA’s are comparable with those achieved for fluoropolymers, (2) The nitrides of low-electronegativity metals exhibit greater WDCA’s than the corresponding oxides, which is explained by the difference in the electronic structure of the oxygen and nitrogen anions and a different number of bonds of these anions with water molecules . (3) The hardness H of the nitride and oxide films of low-electronegativity metals ranges from 8 GPa to 18 GPa, and (4) The hydrophobic properties of nitride and oxide films of low-electronegativity metals do not depend on the electronic structure of the metal cations. The main result of our investigation is the finding that the low value of electronegativity of metal atoms in the film is of key importance for its hydrophobicity.