ZDDPs chain structure

Table 2.6. Types of ZDDPs (primary, secondaiy) with R-group chain structure (long, medium, short) and examples of alcohols...

ZDDP type R-group chain structure example Chemical name Abbreviation... 

The tribofilms consist of amorphous, short-chain phosphates (ortho- and metaphosphates), with some evidence of sulfur incorporated into the phosphate chain structure. Tribofilm structure depends not only on the lubricant composition of the substrate material, but also on the severity of the rubbing contact. The ZDDP typically forms phosphates under antiwear conditions, whereas sulfides and sulfates are typically formed under severe or very severe conditions, li > 0.8 (Willermet et al., 1997). 

The case study 2. Evaluation of ZDDP (primary, secondary) with R-group chain structure (long, medium, short) in oil formulation was done during the field tests. The various aspects of valve train wear performance of lubricating oils through field testing in a passenger car vehicle were compared with laboratory data (Smolenski and Kabel, 1983). 

The current understanding of the structure of ZDDP-based tribofilms derives mainly from XANES spectroscopy (Armstrong et al., 1997 Bancroft et al., 1997 Ferrari et al., 1999a Fuller et al., 1995, 1998 and 2002 Martin et al., 2001 Yin et al., 1993 and 1997a), infrared spectroscopy (Harrison and Brown, 1991 Lindsay et al., 1993 Willermet et al., 1992 and 1995b), and a comprehensive multi-technique (AES/XPS/XANES) approached on the basis of the hard and soft acids and bases (HSAB) principle (Martin, 1999, Martin et al., 2001 ). Most prominent models involve a two-layer structure. The layer in contact with the metal surface is thought to be composed of short-chain polyphosphates, and a top layer, consists of long-chain zinc polyphosphate. 

A recent XANES experiment showed that the ZDDP tribofilm generally has a two-layer structure a short-chain phosphate layer covered by a thin, long-chain polyphosphate layer. XANES spectra analysis of tribofilms did not show a substantial difference between neutral and basic ZDDPs (Fuller et al., 1997 Martin, 1999 Martin at al., 2001 Varlot et al., 2000 Willermet et al., 1991 and 1992 Yamaguchi et al., 1993 and 1996). 

In order to identify the chemical nature of phosphorus and sulfur in complex matrices such as antiwear tribofilms, it is essential to compare the spectra of films with different model compounds in which the local chemical environments of phosphorus and sulfur are known. The high resolution of the technique allows characterization of the chemical nature of phosphorus and sulfur in the tribofilm. Investigators have shown that the chain length of polyphosphate is related to the length of alkyl groups in ZDDP. By comparison of the L-edge XANES spectra of the tribofilms with the spectra of model compounds with known structures, it has been possible to speciate the chemical nature of phosphorus and sulfur in the antiwear tribofilms. 

One of the main surface techniques used for the analysis of thin surface films formed by ZDDP is XPS. It is found that the films formed from ZDDP alone are composed of inorganic polymer materials which contain zinc, phosphorus, oxygen and sulphur [l,4,5,6,7j. Martin et al. [4] suggested a two layer structure of these films on the top surface the long chain polyphosphate (zinc phosphate) and in the bulk the short chain polyphosphate is a mixed Fe/Zn phosphate with a graded concentration, enriched in Fe at the base. This is supported also by the work of Fuller et al. [5] using the XANES technique, and mechanisms of conversion of long chain polyphosphates into short chain polyphosphates are... 

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