From the variations of friction coefficients (see Fig.1), it can be found that temperature and content both control the lubrication effectiveness of silver in Ag-BCF coatings when temperatures are below 500℃. The mechanism for silver reducing friction was consistently described to be the formation of an easy-to-shear silver layer . Its lubrication effectiveness could be enhanced by the increased ductility of metallic silver and by the accelerated Ag diffusion to surface under the action of temperature.
Fig.1.Friction coefficients of BCF and Ag-BCF coatings sliding against SiN4 balls at elevated temperature.
Therefore, the increase in temperature from 25℃to 200℃ significantly grows the lubrication effectiveness of Ag-BCF coatings. In particular, since a higher dosage of silver reasonably favors the generation and perfection of lubricative layer, the friction coefficients of 15Ag-BCF and 20Ag-BCF coatings drop more obviously. Compared with 200℃, higher temperature at 350℃, however, seems to be of no much more help to the lubrication effect for coatings with relatively high Ag content. This might be due to the increase of shear strength of 15Ag-BCF and 20Ag-BCF coatings at 350℃ associated with the enhanced local cold-welding of asperities, which impedes the relative movement of frictional pair to some extent [33]. Even so,20Ag-BCF coatings still possesses lower friction coefficient than 10Ag-BCF coating at 350℃. Its higher silver content more facilitates the formation of a continuous and smooth lubricative film, which can be confirmed by the microscopic morphologies of wom surface of 20Ag-BCF coating after sliding at 350℃ shown in Fig.2. This smooth film could alleviate local cold-welding and reduce friction.
Fig.2. SEM images of worn surfaces of (a)(d) BCF coating.(b)(e)10Ag-BCF coating and(c)(f) 20Ag-BCF coating after sliding at 350℃and 500℃.
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