ICMCTF2014 Session F4: New Oxynitride and Oxide-based Coatings

Delafossites AMO₂ (A= Cu,Ag, Pd, Pt; M=Fe, Co, Ni, Cr, Al, Mn, etc) have interesting potential applications as transparent p-type conducting oxides, photocatalysts, luminescent materials, batteries and thermoelectric materials. Recently, high temperature superconductivity was also suggested for doped members of this type of oxide compounds. The delafossites can be described as natural nanolaminates with MO₂ layers separated by A planes. The chemical stability of these compounds is defined by the linear O-A-O bonds. Copper-based delafossites have been synthesized using solid-state reactions, hydrothermal synthesis methods, sol-gel methods and, in a few cases, reactive sputtering with high temperature post-annealing (≥700^oC). Very few studies, however, have previously been published on silver-based delafossite films deposited by sputtering. The main challenge is that the silver-based delafossites are easy to decompose at high temperatures (>400^oC) due to the less stable O-Ag-O bonds. Furthermore, the synthesis of Ag-based delafossites by reactive sputtering is usually restricted to a narrow parameter window.

In this work, we have deposited delafossite AgFeO₂ thin films with reactive magnetron sputtering using a combinatorial approach with gradient films. Rapid screening combined with XRD, XPS and XRF were used to determine the chemical composition and phase composition. The most interesting films were studied by TEM. Optical and electrical properties have also been investigated. Our results show that the AgFeO₂ successfully can be deposited without post-annealing using a pulsed Ag target. The process window is very narrow and strongly dependent on deposition temperature, sputtering power, pulse frequency, O₂ flow rate, working pressure, etc. An optimum temperature is 400 ^oC and a pulsing frequency of 150 kHz. The XRD and TEM showed that the AgFeO₂ films were textured with the (00l) planes perpendicular to the growth direction also on an amorphous SiO_x substrate. A very strong substrate dependence was observed with good quality films on Si and a poor crystallinity on glass. Furthermore, the films were p-type and transparent. Further post-annealing in air improved the electric properties.

α-Al₂O₃ made by PVD-processes has been a long sought goal in coating development for the cutting tool industry. Different approaches have been taken throughout the years, such as alloying with materials stabilizing the corundum phase, i.e. Cr, as reported in [1, 2, 3]. Having achieved α-(Cr, Al)₂O₃ phase from compound targets, there are still many problems to solve with respect to process stability and coating quality. The reactive arcing of mixed AlCr targets in oxygen creates a large amount of droplets as well as target poisoning [4], which leads to a reduced film quality and uneconomical use of target material, especially on an industrial scale. A recent discovery shows that Si can be added to the AlCr-material in order to stabilize the process, creating a more even erosion of the cathode [5]. However, the Si-content in the target need to be balanced carefully in the light of another study indicating that Si in the coating stabilizes the cubic phase instead of the desired corundum phase [6]. It remains to be found with which Si content the improved erosion of the targets may result in better film integrity, that is, fewer droplet and voids while retaining hardness, E-modulus, crystal structure, and improving the machining properties of the original AlCr oxide.

Thus, this work aims to study the influence of adding different amounts of Si to AlCr-target material with increasing Al/Cr ratio respect to erosion rate of the target as well as the resulting coating properties. SEM, XRD, TEM, and XPS among other techniques showed that some Si is incorporated into the coating when having 10 at% Si in the target with Al/Cr=1. The α-(Cr, Al)₂O₃ coating, free from silicon, change phase to possibly cubic-(Cr, Al)₂O₃ upon incorporating silicon. The hardness is also reduced when adding Si in the coating. The questions of where the silicon is incorporated in the coating is also further investigated in this work.

[1] Ramm, J., et al., Surface and Coatings Technology, 2007. 202(4–7): p. 876-883.

[2] Khatibi, A., et al. Acta Materialia, 2013. 61(13): p. 4811-4822.

[3] Edlmayr, V., et al., Thin Solid Films, 2013. 534(0): p. 373-379.

[4] Pohler, M., et al., Surface and Coatings Technology 2011. 206(6): p. 1454-1460.

[5] Paulitsch, J., et al., Vacuum, 2014. 100: p. 29-32

[6] Najafi, H., et al., Thin Solid Films, 2013. http://dx.doi.org/10.1016/j.tsf.2013.06.062.

Many investigations already indicated that alloying Al to ceramic-like nitride coatings, such as CrN or TiN, increase their mechanical and thermal properties. Especially the formation of dense alumina (Al₂O₃) outer layers during high temperature oxidations is highly acknowledged due to its chemical inertness and passivating character. However, synthesizing crystalline corundum-type alumina itself indicated to be challenging, especially when using deposition temperatures <700°C.

Within this study we present results on selected alloying materials, obtained from first principle calculations, which not only influence the crystallographic appearance of Al-Cr-O coatings but also enhance the target evaporation characteristic during cathodic arc evaporation, hence the process itself. Furthermore architectural optimized Al-O-based multilayer coatings were deposited using low and high ionizing deposition techniques, like conventional direct current magnetron sputtering or cathodic arc evaporation, respectively, to indicate the possibility of stabilizing preferential crystallographic configurations by tailoring the bi-layer thickness.

Metal oxynitride thin films, a novel class of mixed anionic solid structures, have gained an intense interest during recent years related to the unique interplay of oxygen (O) and nitrogen (N) with metals in the material structure. In this regard, we studied the structure and electronic properties of Cr-rich oxynitrides (Cr/Al~1) showing a cubic (B1) structure in the nitride regime, O/(O+N) < 0.6. The obtained films in this growth regime exhibited a well-developed columnar structure and high nanohardness values ranging from 30 to 33 GPa. Our results show the incorporation of oxygen into the films without any oxide region segregation results in the development of a strong (002) crystallographic preferred orientation and a decrease of the cubic lattice parameter, which allows to suggest the formation of a substitutional AlCr(O_xN_1-x) solid solution. Material system electronically behaves like nitrides; however, the higher charge transfer between metals and oxygen, as compared to less electronegative nitrogen, favors the formation of an iono-covalent solid structure. In the range of oxygen content from 0.6 < O/(O+N) < 0.97, transition regime, coatings with diffuse columnar structure and high values of metal vacancies were formed. However, the cubic lattice survives despite the large proportion of oxygen. According to the structural and electronic properties of corresponding layers, we assign this region to the formation of metastable monoxides with B1 structure, which are stabilized by the presence of a high amount of vacancies. The nanohardness decreased to about 21-25 GPa as a consequence of the formation of vacancies and changes in chemical bonding states as well as in the coating microstructure. Coatings in the oxide regime with nitrogen contents close to or even below the detection limit of chemical composition analysis, O/(O+N)>0.97, consisted of a solid solution of (Al,Cr)₂(O_0.97,N_0.03)₃ with corundum lattice and finer columnar structure. It was verified that the incorporation of even small amounts of N in the corundum lattice results in the development of hard coatings with proper wear resistance and less brittle nature.

Oxynitrides of transition metals are an important scientific and technological topic. Some of them exhibit tunable properties according to the O/N ratio, but not all the metals can form them, and their properties are dependent on the growing conditions, if prepared as thin films. The authors found a lack of knowledge in the literature on the production of CrON by medium frequency (MF) magnetron sputtering, so this is a report on the synthesis and characterization of MF sputtered CrON thin films on silicon and M2 high speed steel substrates. Temperature control was employed, together with substrate biasing to influence the coatings microstructure. X-ray diffraction (XRD) and glow discharge optical emission spectroscopy (GDOS) analysis point to a stoichiometry gap between an oxygen doped CrN-like film and a nitrogen doped CrOx one. Frequency and duty cycle variation of the DC pulses on the chromium target shifted the oxygen flow threshold between the two compositions. Polarization tests for corrosion test, stress measurements and instrumented micro and nanoindentation were employed to fully characterize the coatings produced.

Microstructural characterization and basic properties description of atmospheric plasma spraying oxide coatings modified by nano- and submicro-crystalinne different types of oxides will be presented in this article. In first part the short characterization of nano- and submicro-crystalinne oxides powders will by showed, especially theirs characterization form grain size and phases composition point of view. Analyzed powders were as follow: RE₂O₃ (RE=Sm, Eu, La), ZrO₂, 3YSZ, Y₂O₃, CeO, α-Al2O3 and γ-Al2O3. Those powders will by described by STEM, XRD and laser diffractions methods. In second part of investigations microstructural characterization of APS coatings of 8YSZ, Al₂O₃ and Al₂O₃ x 13 TiO₂ with addition of nano- and submicro-crystalinne oxides powders will be showed. The morphology of the coatings will be described, their hardness and results of erosion tests in different conditions. The value of K_Ic parameters will be estimated as well.

Financial support of Structural Funds in the Operational Program - Innovative Economy (IE OP) financed from the European Regional Development Fund - Project No POIG.0101.02-00-015/09 is gratefully acknowledge.