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

Stress-augmented thermal activation: Tribology feels the force

In stress-augmented thermal activation, the activation energy barrier that controls the rate of atomic and molecular processes is reduced by the application of stress, with the result that the rate of these processes increases exponentially with applied stress. This concept has particular relevance to Tribology, and since its development in the early twentieth century, it has been applied to develop important models of plastic flow, sliding friction, rheology, wear, and tribochemistry. This paper reviews the development of stress-augmented thermal activation and its application to all of these areas of Tribology. The strengths and limitations of the approach are then discussed and future directions considered. From the scientific point of view, the concept of stress-augmented thermal activation is important since it enables the development of models that describe macroscale tribological performance, such as friction coefficient or tribofilm formation, in terms of the structure and behaviour of individual atoms and molecules. This both helps us understand these processes at a fundamental level and also provides tools for the informed design of lubricants and surfaces

A Mass Balance Study of the West Antarctic Ice Sheet

The present state of the West Antarctic ice sheet (WAIS) is a prime concern of science, but its large size and remote location have limited the amount of reliable data that are available for mass balance calculations. The spatial pattern of mass balance for a 100-km2 portion of the WAIS is estimated by calculating the residual flux of ice through 1-km grid cells organized into a geographical information system (GIs). The input data used for this estimate include continent-scale compilations of ice thickness and snow accumulation rate measurements, and ground-based measurements of snow accumulation rate and ice velocity. The calculation was performed using different combinations of input data so that error sources could be identified. The largest sources of error were associated with the continent-scale compilations of accumulation rate and ice thickness. These errors are greatly reduced when using snow accumulation rates derived from ground-penetrating radar (GPR) surveys. The best results, which agree with two previous estimates, suggest that this area is nearly in balance. Results also indicate that the mass balance varies within this 100-km2 grid. In some portions of the grid, local variations in mass balance correspond with measured changes in ice velocity and snow accumulation rate. In other parts of the grid, the apparent spatial variability is attributed to errors in the ice thickness data. The results show that the demonstrated accuracy and spatial resolution of this high-resolution sampling approach are needed to understand the response of the entire West Antarctic ice sheet to recent changes in climate. However, the accuracy of the data compilations discussed above are examined using continuous, simultaneously recorded, ground-based measurements of ice-sheet surface topography, ice thickness, and snow accumulation rate that extend for hundreds of km beyond the grid at Byrd Station. Results from these analyses suggest that each of the compilations have larger errors than previously reported and therefore need to be improved before they are incorporated into estimates of WAIS mass balance

Performing Ethnographic Inquiry into America\u27s Black Youth Culture: The Impact of Hip Hop and Rap Music on Today\u27s Youth Society

This is an ethnographic inquiry into America\u27s black youth culture. It is an exploration of how the hip hop culture influences black youth culture. Drawing on the works of Rose (1994a, 1994b), Dimitriadis (2001), and Kitwana (2002), I explored impact of the hip hop culture in the areas of fashion, language and identity through the lens of young black males. To provide relevant theories for promoting higher academic achievement in African American males, I studied the works of Gay (2000), Ladson-Billings (2006, 2003, 2001, 1999), Weaver & Daspit (2003) and Watkins (2001). Sixth grade male students in a PreK-8 school, located in Savannah, Georgia were the focus of this study. Using group discussions and interviews, I documented the time spent with the students using hip hop and rap music. Each student\u27s view of hip hop and its significance allowed me to better understand their interest in and reverence of the rap and hip hop culture. Much of the literature on hip hop culture deals with music, fashion, language and identity and its impact on contemporary youth culture. A limited number of educational text explored the how hip hop culture can be used to engage African American youth in culturally relevant pedagogy. As a result of the drive for national standards in the United States today, school districts face the challenges of having all students performing at high academic levels. Superintendents, school officials and administrators, and teachers are frustrated by high stakes testing and its impact on meeting Adequately Yearly Progress (AYP). In a school district in which the academic achievement of all learners students is important, it is the intent of this study will be to promote the use of hip hop culture to engage all students in culturally relevant pedagogy

Diversifying Environmental Advocacy in Atlanta: A Case Study of Atlanta\u27s African American-Led Community-Based Groups Working Against Environmental Injustices

An assumption that marginalized communities are not interested in the environment is one barrier to diversifying environmental affairs. Barriers like this one have shaped the ways that these communities respond to both enduring and emerging injustices. It is vital to understand how marginalized communities address environmental injustices including both disparate exposure to environmental harm and exclusions from environmental decision-making. This case study investigates five of Atlanta’s African American-led community-based environmental groups that each work on water governance issues in predominantly Black watersheds in Atlanta: the Atlanta Watershed Learning Network, Environmental Community Action, the Proctor Creek Stewardship Council, the South River Watershed Alliance, and the West Atlanta Watershed Alliance. Through their continued efforts, these community-based-groups have worked to diversify mainstream environmentalism while destigmatizing myths associated with African American involvement in the urban environmental landscape

Effects of supplementing sources of zinc on digestibility parameters of beef steers

Zinc (Zn) has numerous functions and life sustaining processes depend on the presence of Zn within the body. Zinc sulfate is a common form of Zn supplemented in the beef industry. Inorganic sulfate based minerals have been associated with negative rumen effects, mainly, decreased rumen fermentation and protozoa numbers. Our studies concluded no difference in digestibility of three different forms of Zn (zinc glycinate, zinc sulfate, and zinc hydroxychloride). Although overall digestibility did not change, there were specific mineral concentration differences after steers consumed concentrate treatment for 10 days. These differences implied that the mineral fed was consumed and absorbed in post-treatment sample collection of rumen fluid, plasma, and liver. Overall liver Zn levels were within normal ranges and implies the zn fed was absorbed. Based on the results of this study bioavailability of, zinc glycinate, zinc sulfate, or zinc hydroxychloride were similar