High temperature, oxidation and wear resistant coatings

Abstract

Highly sophisticated production processes such as dry cutting or press hardening constitute tremendous mechanical and thermal stresses, as well as sever wear for the tool materials applied. Especially, the surfaces of the tools require a significant protection, where physical vapor deposition (PVD, e.g., sputtering or arc evaporation), allows for the preparation of well-defined ceramic-like coatings to protect the tool materials and hence increase their service life times. Based on their excellent mechanical properties and thermal stabilities, Ti-Al-N coatings are well established for various industrial applications. However, increasing industrial demands for even more wear and thermally resistant protections require further developments, especially for applications in the high-temperature regime. Therefore, this thesis focused on the further improvement of Ti-Al-N based coatings with respect to oxidation and wear resistance. A major problem during machining and forming processes, next to abrasion and diffusion driven wear, is also adhesive wear, which basically is an atomic-scale mechanism. Therefore, density functional theory based ab initio studies are used for a detailed description of the interaction between Al and Fe based transfer materials (basic components most worked materials) and various nitride coatings. Ternary TiN-based coatings, like Ti(0.50)Al(0.50)N and Ti(0.90)Si(0.10)N exhibit lowest tendencies for adhesion of Al and Fe combined with the highest barriers against Al and Fe implantation. While there are several studies on the influence of various deposition conditions (e.g., deposition temperature, bias potential, discharge current) on coating properties in general, and also on Ti-Al-N, there are no data available on the impact of residual oxygen (from the deposition process) on their growth morphology, structure and mechanical properties. By varying the oxygen impurity content within the Ti-Al targets in combination with a variation in base pressure during preparation of Ti-Al-N, the oxygen impurity content could be systematically modi ed between 0.3 and 1.32 at.%. Within the deposition conditions used, the lowest value was always defi ned by the target oxygen impurity content. Increasing oxygen content promotes the formation of an open porous columnar growth-morphology with 111-orientation. Whereas the purest coatings exhibit hardnesses of 35 GPa, only 25 GPa and 20 GPa are obtained for the coatings containing 1.10 and 1.32 at.% oxygen. Consequently, the development of high quality Ti-Al-N thin fi lms requires base pressures below 10--E-6 mbar (at the desired deposition temperature) in combination with high quality targets. The further development of Ti-Al-N focused on two different strategies, the improvement by alloying with a reactive element (yttrium) and the improvement by developing Ti-Al-N/Mo-Si-B multilayers. Reactive elements, such as yttrium, are known to promote the formation of well-adherent and dense protective oxide scales, where yttrium-oxides also act as effective plugs at the high-diffusion pathways, like grain and phase boundaries within the oxide scale. Mo-Si-B materials are known (mostly from bulk materials science) to allow for excellent thermal stability and outstanding oxidation resistance through the formation of well adherent and dense protective borosilica oxide scales. The addition of only ~2 at.% Y to the metal sublattice of Ti-Al-N - hence, the formation of a solid solution Ti(1-x-0.02)Al(x)Y(0.02)N - leads to significantly improved thermal stability and oxidation resistance. Special attention needs to be taken on the Al content, because yttrium decreases the maximum solubility of Al within the face centred cubic Ti-Al-N lattice. The development of Ti-Al-N/Mo-Si-B multilayers required a detailed investigation of Mo-rich Mo-Si-B thin films, as almost no data were available on PVD prepared materials. In conclusion, excellent mechanical properties (hardnesses of -20 GPa) combined with outstanding thermal stability and oxidation resistance require Si contents above ~20 at.%, B/Si ratios of - 0.25, combined with Mo-contents of 50-60 at.%. Best results were obtained for Mo(0.58)Si(0.28)B(0.14), which only showed an ~500 nm thin oxide scale after 60 min exposure to air at 1300 °C. The developed multilayers, Ti(0.57)Al(0.43)N/Mo(0.54)Si(0.30)B(0.16) and Ti(0.57)Al(0.43)N/Mo(0.68)Si(0.12)B(0.20), exhibit excellent mechanical properties (hardnesses of 32-34 GPa and indentation moduli of 385-405 GPa) and oxidation resistance, where even after 1000 min exposure to air at 900 °C an only ~1.0 µm thin oxide scale is formed. Furthermore the formation of lubricious oxide phases (based on Mo(n)O(3n-1) Magnéli phases) is envisioned to be responsible for the excellent wear-performance of the higher Mo-containing Ti(0.57)Al(0.43)N/Mo(0.68)Si(0.12)B(0.20) multilayer, exhibiting wear rates below 10E-6 mm³/Nm after 500 m sliding against Al2O3 at RT, 500, and 700 °C. In summary, the development of high-quality protective coatings requires optimized deposition conditions, optimized alloys and knowledge-based designed architectural arrangements.