Reconstruction mechanisms of tantalum oxide coatings with low concentrations of silver for high temperature tribological applications

Abstract

Silver tantalate (AgTaO 3 ) coatings have been found to exhibit outstanding tribological properties at elevated temperatures. To understand the mechanisms involved in the tribological behavior of the Ag-Ta-O system, tantalum oxide coatings with a small content of silver were produced to investigate the metastable nature of this self-lubricating material. The coatings were produced by unbalanced magnetron sputtering, ball-on-disk wear tested at 750 C, and subsequently characterized by X-ray diffraction, Scanning Auger Nanoprobe, cross-sectional Scanning Electron Microscopy, and Transmission Electron Microscopy. Complementary molecular dynamic simulations were carried out to investigate changes in the chemical and structural properties at the interface due to sliding for films with varying silver content. Both the experimental characterization and the theoretical modeling showed that silver content affects friction and wear, through the role of silver in film reconstruction during sliding. The results suggest that the relative amount of silver may be used to tune film performance for a given application. V C 2014 AIP Publishing LLC. It is well established that high temperature working environments are still a major challenge for the tribology community. Oxidation of the commonly-used solid lubricants, such as MoS 2 and graphite, has hampered their use at elevated temperatures due to the degradation of material properties. 1 Solid lubricants that have been reported in the literature as well as used in industry when the temperature exceeds $300 C include noble metals, alkaline halides, Magn eli phases, and ternary oxides (also termed binary metal oxides).2 These systems usually exhibit friction coefficients in the range of 0.1-0.4, depending on the selected material, working temperature, and load. For more detailed information on the properties of these materials as high temperature solid lubricants, the reader is encouraged to refer to the following review articles. [3][4][5][6] Very recently, silver tantalate (AgTaO 3 ) was reported to be a promising lubricant material at elevated temperatures. 7,8 The measured coefficient of friction (CoF) at 750 C was found to be as low as 0.04-0.06 in two recent studies conducted by our group. Additionally, transmission electron microscopy (TEM) revealed that the AgTaO 3 phase produced clusters of silver surrounded by Ta 2 O 5 near the surface, where the applied stress was presumed to attain its maximum values. These features were reproduced in molecular dynamics (MD) simulations, 8,9 and results indicated that the lubricious nature of AgTaO 3 at high temperatures was enabled by the joint contributions of the hard Ta 2 O 5 phase and lubricious silver clusters in the shear-and temperature-induced surface layer. Theoretical and experimental cross-sectional images of the coatings in the wear track revealed that the presence of Ta 2 O 5 and silver increased dramatically closer to the interface whereas the remainder of the coating consisted primarily of AgTaO 3 with a very small amo...

(Ag,Cu)–Ta–O Ternaries As High-Temperature Solid-Lubricant Coatings

“…In our previous studies, [14][15][16]34 we proposed that the CoF and the wear rate are determined by the ability of the monovalent metal (in this case, Ag or Cu) to migrate to the surface and agglomerate into soft metallic clusters. In this section, we investigate the ability of this mechanism to explain the observed trends.…”

“…The MD and experimental observations of opposing friction and wear trends suggest that all three materials share similar sliding mechanisms. In our previous studies, − , we proposed that the CoF and the wear rate are determined by the ability of the monovalent metal (in this case, Ag or Cu) to migrate to the surface and agglomerate into soft metallic clusters. In this section, we investigate the ability of this mechanism to explain the observed trends.…”

“…These loads correspond to initial mean Hertzian contact stresses of 0.5, 0.6, 0.8, and 1.0 GPa, respectively. All test conditions were the same as those under which previous testing of AgTaO 3 was performed. − , Three sliding tests were performed at each temperature: the elemental compositions were determined for the material as-deposited and after tribotesting, inside and outside the wear track, using a Model 670xi Scanning Auger Nanoprobe (SAN). The composition of the as-deposited film was found to be 12 at.…”

Tribological behaviour of self-lubricating materials at high temperatures

“…AgTaO 3 has been reported to show a perovskite structure, with a layered microstructure expected to provide effective self-lubrication [152]. Recently, the tribological properties of tantalate-based PVD thin films have been studied by Stone, Gao et al [153][154][155]. In particular, Ag/AgTaO 3 coatings could be prepared by means of magnetron sputtering, with silver contents close to 20 at.-%.…”

Controllable Preparation and the Tribological Properties of Bimetal Oxide ZnxTiyOz

“…At high temperature, TMO x react with Ag to generate AgTM x O y . Due to the weak bond of Ag-O, AgTM x O y plays a good lubricating role at high temperature [14][15][16][17][18] .…”