A fundamental surface study of phosphorus based antiwear films on iron

Abstract

The interaction of PH3 with Fe(lOO) at different temperatures has been studied with LEED, TPD and ellipsometry. At 160 K, PH3 dissociates and phosphorus uptake saturates at 0.5 monolayers in a chemisorbed c(2x2) overlayer. From 300 to 445 K, phosphorus saturates in very-thin (20 A or less), structurally disordered, optically absorbing films with optical constants (n - ik) of about (3.5 -12.8). This is interpreted as the dissolution of phosphorus, by place exchange, into the subsurface region. From 495 to 640 K, films do not reach limiting thickness but grow with a temperature-dependent, near linear rate over several hundred Angstroms. These films are also structurally disordered and have optical constants that typically vary from (3.32 - i2.85) to (3.37 - i2.66) depending on pressure, temperature and film thickness. These results are interpreted as showing the initiation and growth of iron phosphide where small differences in optical constants reflect compositional inhomogeneity.

The kinetics of the oxidation of Fe(lOO) and the affect of preadsorbed phosphorus has also been studied. Oxygen uptake on the clean surface has been followed with ellipsometry and LEED, and results are in broad agreement with previous studies. The range of uptake curves has been extended, and for first time the uptake kinetics have been correlated with LEED. Disagreement between the uptake kinetics and previous work may be explained by small compositional variations in the oxide. A saturated c(2x2)P overlayer is found to promote oxide growth.

Adsorption of PH3 on Fe(llO) has been investigated with LEED and TPD. At 160 K, PH3 dissociates and phosphorus uptake saturates, at 0.25 monolayers, in a poorly ordered structure which on annealing forms a p(lx4) structure which can be interpreted as a surface reconstruction. At 540 K, a p(2x2) overlayer or a complex, unidentified structure precedes formation of a well-ordered p(lx4) structure. The appearance of disorder at high PH3 may be associated with uptake beyond 0.25 monolayers. Several reconstructions are detected at high temperatures (800 to 990 K).

Then structural environment of elements in the antiwear films of ZDDP and similar oil additives has been characterised with EXAFS. Phosphorus is located in a phosphate glass structure and is insensitive to the type of additive. Zinc is intimately mixed into the glass matrix as a modifier cation. Structure about sulphur depends on the type of additive. Two sulphur environments can be distinguished, in some cases together in the same film: as an iron sulphide and as a glass forming sulphate.

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ORCID for Brian E. Hayden: orcid.org/0000-0002-7762-1812

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