Characterization of the boundary films formed in sliding on steel surfaces at high temperatures.

Abstract

Boundary lubrication is now recognized as existing in almost every kind of sliding and rolling component. Particularly, in early operation of those components, initial formation of a boundary film is crucial for preventing unexpected, catastrophic failure of lubricated surfaces. Characterization of boundary films and their formation is essential to the development of better friction and wear models and improved lubricants. This thesis describes a study of boundary films formed in sliding of steel surfaces with practical lubricants over a range of temperature. Thickness and refractive index of films were measured by ellipsometry during step loading tests, and composition was obtained from XPS. A mechanism of film formation and loss consistent with these measurements is presented. As temperature increases, chemical reactivity increases film formation rate, while film removal rate increases due to decreased durability of the film. There is a balance between these two competing mechanisms and this balance is reflected in the boundary film thickness. The films formed in plain mineral oil are composed of a layer of organo-iron compound (OIC), on the bottom of which lies a layer of iron oxide and metallic iron compound. The organo-iron compound is soft and weak; at high temperatures the OIC layer remains thinner and disappears at lower loads than at room temperature. ZDP-induced films also consist of two layers. At high temperatures, the OIC layer is thicker than at room temperature. The organo-iron compound contains Zn, S and P, which improve durability of the films over that of plain mineral oil. Higher concentration of ZDP exhibits faster film formation rate. However, 1% ZDP provides the most durable film. The films formed in X-1P effectively reduce friction and prevent scuffing. They are composed of a mixture of iron oxide, iron fluoride and organometallic compound, all of which are almost uniformly distributed throughout the thickness of the film. Iron fluoride, which forms only by sliding, appears to influence film performance more than other film components. The film formed in polyphenyl ether is as durable as that formed in X-1P, but above 250$\sp\circ$C, it becomes ineffective.