ICMCTF2008 Session A1-2: Coatings to Resist High Temperature Oxidation and Wear

Thermal stability of functional coatings is of a key importance for their use at high temperatures in a number of application areas. Recently, novel quaternary Si-B-C-N coatings with an amorphous nanostructure, high hardness, low compressive stress, and tunable electrical and optical properties have been successfully prepared by magnetron sputtering in our laboratories [1-2]. Based on excellent results for the oxidation resistance of these coatings even above 1500°C in flowing air, a systematic investigation of their thermal stability in an inert atmosphere was carried out up to 1700°C using a symmetrical Setaram thermogravimetric system TAG 2400 and differential scanning calorimeter DSC Labsys 1600. The structure of the coatings was characterized by XRD. The Si-B-C-N coatings were deposited on Cu substrates by dc magnetron co sputtering of a single B₄C-Si target in a nitrogen-argon gas mixture at optimum process parameters. Subsequently, the substrate was diluted and fragments of the coatings were used as specimens for the thermal stability experiments. It was found that the deposited Si-B-C-N material is more stable at high temperatures in the inert atmosphere than the usually used substrates (e.g. SiC or BN). The amorphous structure of the coatings was thermally stable up to 1600°C without any DSC responses and the first crystallization of the Si₃N₄ phase was detected by XRD just at 1700°C.

[1] J. Vlcek, S. Potocky, J. Cizek, J. Houska, M. Kormunda, P. Zeman, V. Perina, J. Zemek, Y. Setsuhara, S. Konuma, J. Vac. Sci. Technol. A 23, 1513 (2005).

[2] J. Vlcek, S. Potocky, J. Houska, P. Zeman, V. Perina, Y. Setsuhara, Trans. Mater. Res. Soc. Jpn. 31, 447 (2006).

The demand for protective coatings to serve high temperature applications such as dry high speed machining or protection of special grades of automotive or aerospace alloys is ever growing. CrAlYN/CrN coatings utilising a superlattice structure with a typical bi-layer thickness of 4.2nm have been deposited on a light- weight Ti-45Al-8Nb alloys known as γ-TiAl. The surface pre-treatment was carried out by bombardment with Cr⁺ ions generated by High Power Impulse Magnetron Sputtering, (HIPIMS) discharge whereas the superlattice CrAlYN/CrN coating was deposited by Unbalanced Magnetron Sputtering (UBM).

Scanning Transmission Electron Microscopy, (STEM) revealed that the coating/substrate interface was extremely clean and sharp. Large areas of coating grown epitaxially were observed. STEM-Energy Dispersive Spectroscopy (EDS) profile analysis further showed that during the HIPIMS ion bombardment Cr had been implanted into the substrate to a depth of 5 nm.

Although the relatively soft γ -TiAl material, (HRC = 35) provides poor support to the hard coating, adhesion critical load values of as high as L_C = 37 N have been measured in scratch adhesion tests. Impact tests brought further evidence for the excellent coating- substrate bonding. The coating remained intact, no spalations and cracks were observed after 5.10⁶ impacts at normal load of 700 N.

CrAlYN/CrN showed potential for reliable protection of γ-TiAl alloys against wear and aggressive environmental attack. For coated γ-TiAl alloys, thermo gravimetric quasi-isothermal oxidation tests in air at 850°C after 1000 hours exposure showed four times smaller weight gain compared to the uncoated material. In sulphidation tests after 1000 hours exposure to aggressive H₂/H₂ S/H₂O atmosphere the CrAlYN/CrN protected γ-TiAl alloys showed reduced weigh gain by factor of four as compared to the uncoated substrate. High temperature pin-on-disc tests revealed that the friction coefficient is temperature dependent. However, unlike most of the nitride PVD coatings, which tend to increase their friction coefficient with temperature, CrAlYN/CrN reduces its friction coefficient from 0.56 at room temperature to 0.4 at 650°C, which demonstrates the excellent high temperature tribological behaviour of the coating.