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28ADAPTIVE COATINGS FOR HIGH TEMPERATURE APPLICATIONS
S. M. Aouadi
Department of Materials Science and Engineering, The University of North Texas,
Denton, TX 76203, USA e-mail: samir.aouadi@unt.edu
DOI: 10.24411/9999-0014A-2019-10011
Abstract. This talk will provide an overview of the latest research developments in the design and exploration of ceramics with high temperature adaptive behavior. The adaptive behavior, triggered by thermal or thermo-mechanical stimulus, may be used to create smart surfaces such that change their chemistry and structure depending, to achieve a desired functionality. The initial focus of the talk will be on understanding the major mechanisms that lead to a reduction in friction and/or wear in high temperature lubricious materials. Adaptive mechanisms will be discussed and will include metal diffusion and formation of lubricant phases at worn surfaces, thermally- and mechanically-induced phase transitions in hexagonal solids, contact surface tribo-chemical evolutions to form phases with low melting point, and formation of easy to shear solid oxides. The second focus of this talk will be on self-healing ceramics and how thermal stimulus may trigger the migration and/or formation of a new phase to heal cracks that may have initiated as a result of repeated mechanical stimuli. Specific examples will be provided for applications that pertain to thermal barrier coatings. Changes in the structural and chemical properties of these materials as a function of temperature will be correlated to their performance using a range of experimental tools in addition to simulations based on ab initio and molecular dynamics calculations. This review also includes a discussion of the industrial applications of these materials as well as of potential design improvements and other anticipated future developments.
The quickly emerging need to finish surfaces of additive manufactured parts in addition to parts produced using more traditional techniques requires the use of materials with adaptive surfaces that automatically adjust their properties in response to external stimuli via interaction with both the ambient environment and contact with the parts. The new design requires a surface that satisfies a specific protective functionality that will enable the main part to last longer. Most protective surface finishes require the use of ceramics in pure or composite form that may be produced using a variety of techniques that include but are not limited to physical vapor deposition, chemical vapor deposition, laser cladding, plasma spray, and cold spray. Functional ceramics are prone to cracking as a result of external mechanical stimuli, which are further exacerbated by thermal stimuli. Once cracks have formed within a ceramic, the integrity of the protective surface is significantly compromised. A potential solution would be to resort to the self-healing/surface reconstruction concept in the design of next-generation protective surfaces, which would significantly increase the lifetime and reliability of materials and would drastically reduce replacement costs.
Surface reconstruction (self-organization) during sliding contact has received less attention so far but has the potential to create self-healing and self-lubricating materials that are crucial for environmentally-friendly tribological applications. Friction and wear are usually viewed as irreversible processes that lead to energy dissipation (friction) and material deterioration (wear). These adverse effects can be mitigated using solid lubricants that are able to self-organize on sliding surfaces to minimize friction and/or wear. The formation of an optimum solid lubricant at the interface between sliding surfaces is crucial to the efficiency and lifetime of applications,
XV International Symposium on Self-Propagating High-Temperature Synthesis
especially when operating in harsh environments. For example, ternary oxides that contain a noble metal were recently shown to shear easily at elevated temperatures and to exhibit extremely low friction coefficients (< 0.2) when tested at temperatures that exceed T > 500°C [1]. These oxides were also successfully incorporated into the design of an adaptive coating, whereby multiple lubricious phases and a hard phase are combined to form a composite material that reduces both friction and wear over a broad temperature range [2]. The most promising high-temperature solid lubricant that has been reported in the literature is silver tantalate (AgTaO3) [3]. In these studies, the measured coefficient of friction (COF) was found to vary with load and values as low as 0.04 were reported when tested at 750°C using a 1 N load. More recently, the author investigated surface reconstruction mechanisms that result from thermal and mechanical stimuli during high temperature sliding for Nb-based surfaces (Figs. 1 and 2) [4] and for self-healing thermal barrier coatings (Fig. 3) [5].
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Revolutions Révolutions Revolutions
Fig. 1. Preparation of the bulk niobium oxide sample by (a) pressing and (b) sintering the pellet. The sample was further tested for the tribological performance (c). Tribology test of Nb2O5 with and without the presence of Ag at (d) 25°C, (e) 400°C, and (f) 600°C. Results indicate a reduction in the coefficient of friction in case of silver presence at 600°C (f).
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Fig. 2. (a) Cracks formed during the indentation test on the Nb2O5/Ag2O/Nb2O5 sample; (b) after annealing the cracks became invisible; Cross-sectional micrograph of (c) region 1 and (d) region 2; EDS analysis of cross section 2 for (e) Si, (f) Nb, (g) Ag, and (h) O, indicating a larger content of silver and oxygen at the crack site.
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S.M. Aouadi
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Fig. 3. Cross-sectional SEM micrographs of: (a) laser-processed YSZ coating, (b) annealed YSZ coating, (c) laser-processed YSZ-AhO3-TiC coating, and (d) annealed YSZ-AhO3-TiC coating. Processing Power = 800 W.
The research was supported by the Army Research Laboratory.
1. S.M. Aouadi, H. Gao, A. Martini, T.W. Scharf, C. Muratore, Lubricious oxide coatings for extreme temperature applications: a review, Surf. Coat. Technol., 2014, vol. 257, pp. 266277.
2. S.M. Aouadi, D.P. Singh, D. Stone, K. Polychronopoulou, F. Nahif, C. Rebholz, C. Muratore, A.A. Voevodin, Adaptive VN/Ag nanocomposite coatings with lubricious behavior from 25 to 1000°C, Acta Mater, 2010, vol. 58, pp. 5326-5331.
3. A. Martini, T. Scharf, S.M. Aouadi, Load-dependent high temperature tribological properties of silver tantalate coatings, Surf. Coat. Technol., 2014, vol. 244, pp. 37-44.
4. A. Shirani, J. Gu, B. Wei, J. Lee, S.M. Aouadi, D. Berman, Tribologically enhanced self-healing of niobium oxide surfaces, Surf. Coat. Technol., 2019, vol. 364, pp. 273-278.
5. J.J. Gu, S.S. Joshi, Y.-S. Ho, B.W. Wei, T.Y. Hung, Y.Y. Liu, N.B. Dahotre, S.M. Aouadi, Tunable self-healing YSZ-AhO3-TiC laser processed thermal barrier coatings, Surf. Coat. Technol. (in press).
