On the mechanism of ZDDP antiwear film formation

Zinc dialkyldithiophosphate additives are used to control wear and inhibit oxidation in almost all engine oils as well as many other types of lubricant. They limit wear primarily by forming a thick, protective, phosphate glass-based tribofilm on rubbing surfaces. This film formation can occur at low temperatures and is relatively indifferent to the chemical nature of the substrate. There has been considerable debate as to what drives ZDDP tribofilm formation, why it occurs only on surfaces that experience sliding and whether film formation is controlled primarily by temperature, pressure, triboemission or some other factor. This paper describes a novel approach to the problem by studying the formation of ZDDP films in full film EHD conditions from two lubricants having very different EHD friction properties. This shows that ZDDP film formation does not require solid-solid rubbing contact but is driven simply by applied shear stress, in accord with a stress-promoted thermal activation model. The shear stress present in a high pressure contact can reduce the thermal activation energy for ZDDP by at least half, greatly increasing the reaction rate. This mechanism explains the origins of many practically important features of ZDDP films; their topography, their thickness and the conditions under which they form. The insights that this study provides should prove valuable both in optimising ZDDP structure and in modelling ZDDP antiwear behaviour. The findings also highlight the importance of mechanochemistry to the behaviour of lubricant additives in general

Alternate Degree Program Delivery Formats for Adult Students: What is Lost and What is Gained?

A growing number of adult learners are turning to accelerated degree programs to complete an initial or more advanced college degree. While various aspects of these types of degree programs have been discussed in the literature, relatively little is known about how the epistemological beliefs as well as the self-directed learning readiness of adults who are enrolled in accelerated degree programs change over time. A discussion of the findings of recent research about the relationship and the development of these phenomena by adult learners along with implications for improved adult education form the basis for this research roundtable

Correlates of Same-sex Behavior Disclosure to Health Care Providers Among Black MSM in the United States: Implications for HIV Prevention

Disclosure of same-sex behavior to health care providers (HCPs) by men who have sex with men (MSM) has been argued to be an important aspect of HIV prevention. However, Black MSM are less likely to disclose compared to white MSM. This analysis of data collected in the United States from 2006–2009 identified individual and social network characteristics of Black MSM (n = 226) that are associated with disclosure that may be leveraged to increase disclosure. Over two-thirds (68.1%) of the sample had ever disclosed to HCPs. Part-time employment (AOR = 0.32, 95% CI = 0.11–0.95), bisexual identity (AOR = 0.29, 95% CI = 0.12–0.70), and meeting criteria for alcohol use disorders (AOR = 0.32, 95% CI = 0.14–0.75) were negatively associated with disclosure. Disclosers were more likely to self-report being HIV-positive (AOR = 4.47, 95% CI = 1.54–12.98), having more frequent network socialization (AOR = 2.15, 95% CI = 1.24–3.73), and having a social network where all members knew the participant had sex with men (AOR = 4.94, 95% CI = 2.06–11.86). These associations were not moderated by self-reported HIV status. Future interventions to help MSM identify social network members to safely disclose their same-sex behavior may also help disclosure of same-sex behavior to HCPs among Black MSM

Mechanochemistry of phosphate esters confined between sliding iron surfaces

The molecular structure of lubricant additives controls not only their adsorption and dissociation behaviour at the nanoscale, but also their ability to reduce friction and wear at the macroscale. Here, we show using nonequilibrium molecular dynamics simulations with a reactive force field that tri(s-butyl)phosphate dissociates much faster than tri(n-butyl)phosphate when heated and compressed between sliding iron surfaces. For both molecules, dissociative chemisorption proceeds through cleavage of carbon−oxygen bonds. The dissociation rate increases exponentially with temperature and stress. When the rate−temperature−stress data are fitted with the Bell model, both molecules have similar activation energies and activation volumes and the higher reactivity of tri(s-butyl)phosphate is due to a larger pre-exponential factor. These observations are consistent with experiments using the antiwear additive zinc dialkyldithiophosphate. This study represents a crucial step towards the virtual screening of lubricant additives with different substituents to optimise tribological performance

Film thickness and friction of ZDDP tribofilms

Tribofilm formation by several zinc dialkyl- and diaryldithiophosphate (ZDDP) solutions in thin film rolling-sliding conditions has been investigated. A primary, a secondary alkyl and a mixed alkyl ZDDP show similar rates of film formation and generate films typically 150 nm thick. Another secondary ZDDP forms a tribofilm much faster and the film is partially lost after extended rubbing. An aryl ZDDP forms a tribofilm much more slowly. The films all have a pad-like structure, characterised by flat pad regions separated by deep valleys. Three different techniques have been used to analyse the thickness and morphology of the tribofilms: spacer layer imaging (SLIM), scanning white light interferometry (SWLI) of the gold-coated film and contact mode atomic force microscopy (AFM). The SLIM method measures considerably thicker films than the other two techniques, probably because of lack of full conformity of a glass disc loaded against the rough tribofilm. No evidence of a highly viscous layer on top of the solid tribofilm is seen. SWLI and contact mode AFM measure similar film thicknesses. The importance of coating the tribofilm with a reflective layer prior to using SWLI is confirmed. As noted in previous work, the formation of a ZDDP tribofilm is accompanied by a marked shift in the Stribeck friction curve towards higher entrainment speed. For a given ZDDP this shift is found to correlate with the measured tribofilm roughness, proving that it results from the influence of this roughness on fluid entrainment in the inlet