Understanding and formalization of the fretting-wear behavior of a cobalt-based alloy at high temperature
Introduction
The changes in the wear behavior of metallic alloys when the temperature varies is a crucial aspect in many industrial components such as the turbine and the compressor components in aircraft engines. The blade/disk contact is subjected to cyclic thermal loading combined with cyclic sliding (fretting) which can significantly damage the interface (Fig. 1). Many works can be found in the literature dealing with this industrial issue. For example, Mary et al. studied the wear behavior at different temperatures of a Cu–Ni–In plasma coating/Ti17 contact representative of the blade/disk contact in high pressure compressors [1]. Viat et al. [2] studied the high temperature wear behavior of a cobalt-based alloy/ceramic tribocouple representative of the blade/disk contact of turbines. These studies deeply examined the influence of temperature on wear but did not try to model such behavior. Hence, the understanding and the formalization of the temperature effect during fretting-wear is needed. In the present paper, the wear behavior of a cobalt-based alloy, widely used for aeronautical components [[2], [3], [4]], is studied in a more fundamental way (simplified contact configuration and inert counterbody) in order to deepen knowledge of the dependence of the Co-alloy wear mechanism with temperature.
The wear rate of cobalt-based alloys highly depends on temperature [1,5]. Above a certain transition temperature, wear changes from severe to mild. In the mild wear domain, a lubricant tribolayer is naturally created in the interface leading to a drastic reduction of the wear volume and the friction coefficient [6,7]. The so-called “glaze layer” is formed of oxidized and compacted/sintered wear debris generated during the transient regime [8,9]. According to Jiang et al. [9], the compaction of triboparticles is enhanced by elevated temperatures and small grain sizes. Kato et al. [10] called “tribo-sintering” the capacity of debris to sinter during sliding. They showed that mild wear is promoted by enhanced diffusion properties of the oxide debris particles. Hence, they demonstrated that the formation of the glaze layer can be related to a sintering process of the debris particles. Showing the significant influence of diffusion to form the protective glaze layer structure, they suggested that the wear volume is proportional to the sliding distance until the glaze layer is formed (). Viat et al. [11] confirmed that the formation of glaze layer on Haynes 25 is mainly controlled by cobalt oxides which presents the highest auto-diffusion coefficient among the main alloying elements (Co, Cr, Ni).
In the literature, it is shown that the glaze layer is mainly nanocristalline [12,13] with the presence of some amorphous zones near the fretted interface. Nanocrystalline materials present high strength and as high hardness, such as the glaze layer [14,15], explaining the use of nanocristalline structure for tribological applications since they better resist to wear than conventional alloys [16,17]. In addition, Viat et al. [15] proposed that the tribological properties of the glaze layer are mainly due to the perfect ductile behavior observed at high temperature whereas a brittle behavior is observed for low temperature.
At low temperature, when the agglomeration and compaction of the wear debris are not promoted, the wear rate and the friction coefficient are high. Recently, it was shown that the wear evolution in the severe wear domain is controlled by a synergistic action of the oxidation of the surface and the abrasion of the oxidized surface [18]. The oxidized layer is continuously ejected out of the interface leading to high abrasive wear rates. An extended energetic wear model was also proposed in order to capture the transition from severe to mild wear [19], when the tribological conditions are sufficient to generate the compaction and sintering of the powdered debris layer.
The objective of this paper is to investigate in a fundamental, rather than industrial, way the high temperature wear mechanisms of cobalt-based alloys subjected to fretting. First, an overview of the different wear processes occurring over a large range of temperatures (25°C - 600°C) is presented. Then, the investigation focuses on the high temperatures at which the formation of the glaze layer leads to an unworn regime. A microstructural, chemical and mechanical description of the glaze layer is proposed in order to completely describe this protective structure. It is then proposed to understand the protective tribolayer formation by considering a sintering formalism as previously intuited by Kato and co-authors [10,20]. The effectiveness of the glaze layer is also discussed by considering two hypotheses; one based on the mechanical behavior of the fretted interface and the other based on the sintering formalism. Finally, the paper extends an “effective” friction energy wear approach taking into account the tribo-oxidation and sintering processes activated at the fretted interface. The proposed wear model is then able to predict the wear volume for a cobalt-based alloy/alumina contact irrespective of the operating temperature.
Section snippets
Materials
The specimens used in this study are a cobalt-based alloy (HS25) and a ceramic (alumina). Cobalt-based alloys are extensively used in the industry since they present good mechanical properties and corrosion resistance at high temperature [2,3]. Chromium, nickel, tungsten and other alloying elements, presented in Table 1, are present in solid-solution or in the form of carbides [18]. The mechanical properties of the Co-based alloy are given in Table 2. The main alloying element is chromium which
Effect of temperature on wear
As previously observed on this tribosystem [18,19,21], the temperature has a great effect on the wear mechanism. Fig. 4 a) shows the evolution of wear volume and the friction coefficient as functions of the operated temperature, while Fig. 4b) and c) respectively display the related wear kinetics and the wear scar associated to each domain (reference temperatures). Three domains can be distinguished:
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Domain I (T ≤ T1): this domain is characterized by a continuous increase in the wear volume up
Formalization of the tribo-sintering process
It is now proposed to formalize the glaze layer formation through a tribo-sintering concept as previously performed by Kato and Komai [10]. According to the authors, the wear volume at high temperature is directly dependent on the tribolayer formation (since there is no additional wear when the glaze layer is effective) and is inversely proportional to the sintering rate S of the triboparticles:Where V is the wear volume and LGL the sliding distance until the glaze layer is formed.
Quantitative wear formulation
The formation of the glaze layer is now well explained by performing several microscopic and nanoscopic observations and by formalizing its creation through a tribo-sintering concept. The capacity of the glaze layer to withstand wear is probably due to its capacity to continuously re-incorporate the created wear debris thanks to tribo-sintering. The last section of this paper exposes a formalization of the wear evolution at high temperature.
Conclusion
The paper focused on the wear mechanism of a cobalt-based alloy, enriched with chromium, subjected to fretting at high temperature. The wear volume is very low at high temperature due to the formation of a protective layer (glaze layer) at the interface which is able to resist fretting wear. The main points of the present paper are summarized:
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It appeared that the glaze layer is a thin (less than 1 μm-thick) cobalt oxide layer at the top of the interface. This layer was suggested to be formed by
CRediT authorship contribution statement
Alixe Dreano: Methodology, Conceptualization, Investigation, Writing - original draft, Writing - review & editing. Siegfried Fouvry: Methodology, Conceptualization, Writing - original draft, Writing - review & editing, Supervision, Funding acquisition. Gaylord Guillonneau: Methodology, Writing - review & editing, Supervision, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
A part of this work was supported by the LABEX MANUTECH-SISE (ANR-10- LABX-0075) of Université de Lyon, within the program “Investissements d'Avenir” (ANR-11-IDEX-0007) operated by the French National Research Agency (ANR) and by l'EQUIPEX MANUTECH-USD (ANR-10-EQPX-36-01).
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