ZDDPs adsorption

In XANES spectra of tribofilms, both the phosphorus and sulfur signals are very weak compared to ZDDP used alone. For all the concentrations (1%, 2%, 5%) of dispersant PIBS used, antiwear polyphosphate films were formed and unreacted ZDDP was not present in the film. These results imply that antiwear films are much thinner than either tribofilm generated by ZDDP alone or ZDDP with the detergent. This confirms that the PIBS dispersants compete for ZDDP adsorption on the surface (Yin, 1997b). 

Zinc dialkyldithiophosphates (ZDDPs), which act as antiwear additives in lubricating oils and were postulated to exist in various molecular forms (monomer, dimer or neutral form, and basic form), were studied by multi-edge (Zn K-, P K- and S K-) XAS for structural assessment [311]. Grazing incidence absorption spectroscopy measurements have provided evidence for breakdown of the ZDDP molecule following its adsorption on to a steel substrate surface [312]. XANES and CEMS were used to study the interaction of per-fluoropolyalkyl ether (PFPAE) additives with Fe-based alloys [313],... 

ZDDP is known to interact with most other additives employed in these formulations e.g., ZDDP is solubilized by soft-core and hard-core micelles and has been considered for reduced antiwear performance (Inoue, 1993 Kapsa et al., 1981 Rounds, 1981 Shiomi etal., 1986 Willermet 1995a and 1995b Yin et al., 1997). Based on the tribofilm formation (polyphosphate) and the presence (or absence) of unchanged ZDDP in the film, we can conclude that the additives compete with the adsorption of ZDDP on the surface (Varlot et al., 2000 Yin et al., 1997a and 1997b). 

A combination of ZDDP and hard-core RMs leads to a synergistic effect of metallic detergents on the degradation of ZDDP. These phenomena are observed in many tests and can be explained in terms of (a) the acid neutralization property of hard-core RMs that leads to the prevention of decomposition of ZDDP (in the valve train wear test and the thin film oxygen uptake test), (b) the competitive adsorption of detergents that reduce the effective concentration of ZDDP on the metal surface (in the four-ball test), (c) the formation of mixed films on the metal surface, formed through the decomposition of ZDDP in the presence of hard-core RM s (the coefficient of friction in the Falex wear test). 

Table 4.3. The effect of physical parameters such as adsorption, rubbing, temperature, concentration, load, surface roughness on the antiwear performance of ZDDPs...

Adsorption. Sulfur and phosphorus ratios change with rubbing time. Tribofilms accumulated on the surface become thicker with time. All co-additives cause reduction in ZDDP surface coverage under all conditions studied. A surface force apparatus (SFA), and atomic force microscopy were used to determine tribofilm thiclaiess, molecular structure and mechanical properties. For the neutral ZDDP, monomolecular layer thickness is 1 nm and for basic ZDDP it is 1.6 nm (Sutherland et al., 1993 Bames et al., 2001 Bee et al., 1999 Dacre and Bovington, 1983 Georges et al., 1998 Paddy et al., 1990 Wu and Dacre, 1997). 

The first step (eq. 4.2) is an adsorption process. ZDDP in solution is adsorbed on the rubbing surfaces. As time goes by, ZDDP is converted into LI-ZDDP (eq. 4.3) which in turn, will be adsorbed on the surface along with ZDDP (eq. 4.4). 

The antiwear mechanism of ZDDP in the presence of dispersants has been studied and it has been concluded that ZDDP forms an association complex with an amino group of a succinic type dispersant, and this complexation has been proved to be antagonistic to antiwear action (Gallopolous and Murphy, 1991 Rounds, 1986 Shiomi et al., 1986 Willermet et al. 1995b). The solubilization of ZDDP helps the adsorption of ZDDP on the surface, thus improving the antiwear performance of the additives (Forbes et al., 1970b). RMs would decrease the... 

Physical parameters By using Table 4.3. The effect of physical parameters such as adsorption, rubbing, temperature, concentration, load, surface roughness on antiwear performance of ZDDPs , which term is described by the physical parameter (a) the tribofilm accumulated on the surface becomes thicker with adsorption time (b) long-chain phosphates are formed on the topmost surface, but short-chain phosphates were present in the bulk (c) sulfur and phosphorus ratio changes with rubbing time and (d) when load increased the concentration of (S) and decreased that of (P) ... 

Chemical parameters What is meant by chemical parameters in oil formulation Which of the following is a chemical parameter adsorption, detergent, dispersant, concentration, ZDDP, or surface roughness ... 

The lubrication system is extremely complex. The mechanism of lubrication is partly dictated by the nature of interactions between the lubricant and the solid surface. Additives blended into lubricating oil formulations either adsorb onto the sliding surfaces, eg., fatty alcohols, fatty amines, amides, phosphoric acid esters (friction modifiers), or react with the surface, eg., ZDDP, MoDTC, MoDDP organic phosphates (extreme pressure). Some interactions affecting the surfaces of metals include adsorption, chemisorption, and tribochemical reactions-these form new compounds on the surface and lubrication by reaction products (Bhushan and Gupta, 1991 Briscoe et al., 1973 Briscoe and Evens, 1982 Heinicke, 1984 Hsu and Klaus, 1978 and 1979 Klaus and Tewksbury, 1987 Lansdown, 1990 Liston, 1993 McFadden et al., 1998 Studt, 1989). 

It is deduced that competitive adsorption between ZDDP and MoDTC onto the surface occurred during the run, because Mo concentration on the surface with ZDDP + MoDTC oil was lower than that obtained with the oil containing MoDTC alone. The adsorption of the additive on a surface results in formation of a film composed of the decomposition compounds or reaction compounds of the additives with the metal surface. The decline in the Zn/Mo atomic concentration ratio in the film composition with test time indicates that the decomposition products of ZDDP, which had a high coefficient of friction, were preferentially formed and subsequently, Mo compounds were formed. Similarly, an increase in the concentration of MoS2 with time was also observed. A necessary condition for the production of an effective surface film for reducing friction is the previous formation of the compounds derived from ZDDP. 

The more interesting results obtained were for the use of both additives together, all of which showed a further increase in load-carrying capacity, so that any interaction in these tests was beneficial. The greatest improvement was approximately 39 kg increase in Initial Seizure Load compared with the solution of ZDDP alone. Curiously, 1% of molybdenum disulphide gave only about 18 kg improvement over the ZDDP solution. There was virtually no increase in weld load compared with the ZDDP, and this again suggests that the concentrations of molybdenum disulphide were too low to be very effective. Thorp explained these results on the basis that molybdenum disulphide cannot compete with the base oil for adsorption on the steel surfaces, but can adsorb on top of an adsorbed ZDDP film, but there is no real proof of this explanation. 

Equations of this type can describe both the physical dipole/dipole interactions and the subsequent desorption from the film. Since ka and k have different temperature coefficients, increasing temperature can lead to either increased, decreased or unchanged surface coverage. Provided that a critical minimum surface is maintained, wear and friction can be controlled. But once 0 falls below this critical value, believed to be approx 0.5, friction and wear will rise. The adsorption of dilinoleic acid [4], a series of organic sulphur compounds [5] and a ZDDP (zinc dialkyldithiophosphate) [6] has been described in these terms. 

The model for film formation described above suggests that interactions between basestock and film-forming additives, which retard their adsorption, will have a detrimental influence on wear and friction. For instance, highly napthenic basestocks are good solvents for polar species whilst paraffinic basestocks are relatively poor solvents for polar species and will therefore enhance film-forming activity. Clear evidence of this effect has been shown for molybdenum chemistries [7] and for ZDDPs [8, 9]. 

The mechanisms of film formation previously described involve both physical and chemical processes. It follows that factors favourable to film formation can influence friction and wear. Coefficients of friction were shown to vary from 0.04 for a reactive metal such as zinc, lubricated with 1% lauric acid, to 0.55 for an inert metal, silver, with the same lubricant [10], These factors include strong dipole interactions or strong hydrogen bonding which aid physical adsorption and the ease of chemical reaction from this adsorbed layer. Both interactions favour the formation of low-shear-strength films and similar influences have been reported by many workers including wear for mixtures of dilinoleic and linoleic acids [11], and for ZDDPs [6]. 

A high degree of lateral interaction forms a cohesive adsorbed layer which results in improved adsorption. Although this class of molecules is used in lubricants to reduce friction, they may also serve to reduce wear and protect against scuffing, particularly at low temperatures where ZDDP protective film formation rates are low. 

---