Anti-wear film

Reactive FFs can only be applied to a few specific cases for which they have been developed, such as the hydrocarbon systems discussed in the first part of this section. For other systems, describing tribochemical reactions requires the use of quantum chemical methods. In recent studies, such methods have been applied to investigate the behavior of zinc phosphates (ZPs) in response to high pressures. ZPs form the basis of anti-wear films derived from zinc dialkyldithiophosphates (ZDDPs), which are additives that have... 

One key experimental observation regarding the ZP films is that the films found on the tops of asperities are stiffer and exhibit chemical spectra indicative of longer phosphate chain lengths than films found in the valleys between asperities. These observations that differences in the conditions at the two distinct locations alter the elastic and chemical properties of the films. One of the key differences between the tops of asperities and the valleys is the pressure experienced by the zinc phosphates. Since the highest pressures, and greatest potential for wear, are achieved at the tops of the asperities, determining the response of ZPs to these pressures may aid in developing a clear picture of how the anti-wear films work. 

Overall, this work highlights how quantum chemical methods can be used to study tribochemical reactions within chemically complex lubricant systems. The results shed light on processes that are responsible for the conversion of loosely connected ZP molecules derived from anti-wear additives into stiff, highly connected anti-wear films, which is consistent with experiments. Additionally, the results explain why these films inhibit wear of hard surfaces, such as iron, yet do not protect soft surface such as aluminum. The simulations also explained a large number of other experimental observations pertaining to ZDDP anti-wear films and additives.103 Perhaps most importantly, the simulations demonstrate the importance of cross-linking within the films, which may aid in the development of new anti-wear additives. 

Considering these results, the main difference between the antiwear action of the ZDDP soft-core RMs and the hard-core RMs is clear. In the case of ZDDP soft-core RMs, the anti wear film formation requires that chemical reactions occur between the additive and the metallic surfaces. In the case of hard-core RMs, the mineral material (CaC03) is directly introduced to the sliding contact and undergoes small physicochemical changes during the film build-up. Consequently no chemical reaction with the substrate surfaces is required. 

MoDTC+ZDDP. A synergistic effect in reducing friction and wear has been attributed to anti-wear films containing primarily MoS2 and polyphosphates. The counter ion to the phosphates is zinc, rather than iron. The XANES spectra indicate that sulfur and phosphorus form MoS2 and polyphosphate chains (Kasrai et ah, 1998). 

Figure 4.3 Reprinted from Wear, Vol. 202, Z. Yin, M. Kasrai, M. Fuller, G.M. Bancroft, K. Fyfe and K.H. Tan, Application of soft X-ray absorption spectroscopy in chemical characterization of anti wear films generated by ZDDP the effect of physical parameters, Part I, pp. 172-191. Copyright 1997, with permission from Elsevier.

Kasrai, M., Cutler, J.N., Gore, K., Canning, G.W., Bancroft, G.M. and Tan, K.H., The Chemistry of Anti-Wear Films generated by the Combination of ZDDP and MODTC Examined by X-Ray Absorption Spectroscopy, World Tribology Congress, London, 8-12 September, 1997. (STLE Preprint No. 97-WTC-9). 

It follows from this that the ZDDPs which are most efficient at anti-wear film formation will also be likely to suffer depletion due to thermal effects. The thermal degradation of ZDDPs in service has often been confirmed by P NMR and IR studies [45] and is not really important unless decomposition proceeds to the stage where significant reduction in phosphorus and sulphur levels as insoluble or volatile products occurs. Indeed, according to [6], the early stage decomposition products are ... 

The principal problem, which is compounded by the above, is that the most cost-effective source of anti-wear film formation and the most effective peroxide decomposing agent has, since the 1940s, been ZDDPs which contain both sulphur and phosphorus as the active elements. 

Despite the large number of studies on the tribo-chemistry of ZnDTP reported in the literature, several points regarding the antiwear action of ZnDTP are stUl unclear and under discussion. Two open questions concern the kinetics of the trihofilm formation and the relative roles of tribochemical and thermal processes. Film formation in the presence of Zn DTP also occurs at lower apparent temperatures due to flash temperatures induced by the metal-metal contact itself [39,41]. The dependence of the ZnDTP decomposition rate on the temperature has been also described in the Uterature and considered one of the most important features in the anti-wear film formation mechanism [10]. 

S. Bee, A. Tonck, J.M. Georges, R.C. Coy, J.C. Bell, G.W. Roper, Relationship between mechanical properties and structures of zinc didiiophosphate anti-wear films, Proc. Roy. Soc. London A 455 (1999) 4181 203. 

J.C. Bell, K.M. Delargy, AM. Seeney, The removal of substrate material through zinc dithiophosphate anti-wear films, in D. Dowson, et al. (Eds.), Proceedings of the 18th Leeds/Lyon Symposium on Wear Particles, Elsevier, Amsterdam, 1992, pp. 387-396. 

Pidduck, A.J., Smith, G.C. Scanning probe microscopy of automotive anti-wear films. Wear 212, 254-264 (1997)... 

Figure 4.2 (a) TEM image recorded onto an anti-wear film particle and corresponding electron diffraction pattern, (b) X-ray emission spectrum (EDXS) analysis. The antiwear film is composed of iron and elements present in the ZDDP additive (O, P, S, Zn). (c) and (d) Fe and Zn radial distribution functions (RDFs) uncorrected from phase shift, extracted from EXAFS experiments on ZDDP antiwear film particles. The electron diffraction pattern and the presence of one major peak in each RDF corresponding to oxygen first-neighbours shell point out the amorphous structure of the antiwear film, (e) Schematic structure of the ZDDP tribo-film according to the multitechnique approaches [8]... 

Figure 4.16 Evolution of the wear scar width (wear) and electrical contact resistance as a function of the number of cycles for tests carried out in the presence of various lubricants. The nonsignificant wear rate recorded in the case of Sr and Ca octanoate dispersions is attributed to the immediate build-up-of an anti wear film (increase in the ECR in the first cycles)...

G C Smith, J C Bell, Multi-technique surface analytical studies of automotive anti-wear films . Applied Surf. Science, 144-145, pp 222-227, 1999... 

Tribological tests have been performed and are supported by post-test examination using X-ray Photoelectron Spectroscopy (XPS), and Environmental Scanning Electron Microscope (ESEM) with Energy Dispersive X-ray analysis (EDX). Preliminary results show that fully formulated lubricant oils are effective in the reduction of wear and friction for ferrous-based systems but not for aluminium systems. Initial XPS and EDX data suggests that a relatively thick anti-wear film is formed on ferrous materials but is thin (and therefore unstable) for steel on aluminium systems. In completely non-ferrous systems the wear film is absent. In this paper the fundamental aspects of the film formation as well as the practical aspects of the results will be discussed. 

Lubricant technologies for ferrous materials are well documented [2-9]. In particular, the study of anti-wear additives to reduce the amount of wear of the material surface has been investigated. Studies have shown the formation of a relatively thick, stable anti-wear film when lubrieated in formulations containing anti-wear P-based additives [2, 3]. The anti-wear film formed at the material surface, as a result of decomposition of anti-wear additives (typically zinc dialkyl dithiophosphate. 

The results from this paper confirm the formation of a relatively thick anti-wear film on the steel plate. This is evident from post-test analysis conducted by EDX (which has a penetration depth of several microns) where elements such as S, P, Zn,... 

XPS results show the formation of phosphates, sulphides and oxides on the surface. These observations are similar to those made by Tonck et al. [5], whose studied anti-wear film formation from simple ZDDP systems. In his models, the ferrous substrates were covered with a thin sulphide/oxide layer. This in turn was covered with a thicker phosphate layer. In this paper the model of the film formation (determined from XPS depth profiling), as shown in Figure 6a, shows similar sulphides, oxides and phosphates. 

No evidence of anti-wear film formation has been found for totally non-ferrous systems (Al-Si alloy pins on Al-Si alloy plates) from EDX and XPS analysis. This, however, is not the case for systems containing iron (cast-iron pins on Al-Si alloy plates), where traces of phosphates and sulphides are detected from XPS. EDX analysis of these systems does not however detect the P and S, which suggests that the anti-wear film formed was thinner, and hence less stable compared to films formed on ferrous systems (Figure 6b). 

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