Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
This tester provides an ideal measurement of the relative abrasion response (SAR number) of any material in any slurry, as related to the instantaneous rate of mass-loss of a specimen at a specific time on the cumulative abrasion-corrosion time curve, converted to volume or thickness loss rate. A schematic of the apparatus is given in Figure 4.
Future updates to the system allow for the incorporation of a galvanostat/potentiostat for electrochemical measurements simultaneously, as in the case of sliding contacts. Different abrasive particles can be used, whereas the wear damage in mainly evaluated by weight loss measurements.
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The repeatability of results Once tribocorrosion experiments are being done, and the wear damage after an experiment is measured, the next important industrial issue is the long term wear prediction, especially under lubricating conditions (including water lubrication). Long term testing is often required to evaluate or predict the ‘wear life’, and a statistically sufficient number of repeats needs to be done to predict the behavior with sufficient confidence. Figure 8 illustrates the importance of confidence intervals in the prediction of long term wear : for the same mechanism, the confidence bands using 4 measurements are widely diverging, whereas those using 15 measurements are narrow enough to allow extrapolation to longer test duration.
Figure 6. (left) Miller abrasivity tester and (right) schematic of tester.
Example: of Ttribocorrosion under slurry-abrasion conditions: Materials undergoing a combination of slurry abrasion and corrosion are common in a lot of industrial systems, where abrasive products have to be transported or manipulated. Water treatment plants, ore leaching baths, clay and brick manufacturing, dredging systems... they all have in common that equipement and materials have to resist the combined effects of abrasion and corrosion. The ASTM G75 protocol and the calculation of the Slurry Abrasivity Response (SAR) number of a material provide a systematic, standardized and repeatable way to classify materials or to determine the severity of an abrasive slurry in terms of the wear it causes. Figure 7 shows the relative wear resistance of 3 materials undergoing slurry abrasion in a G75 test. The evolution of wear can be observed, and repeatable results are obtained thanks to the design of the equipment and consistent methodology.
Figure 8. statistical analysis with confidence levels for wear data.
This challenge – of obtaining a statistically relevant number of repeats on the lab scale – is particularly important when introducing new materials or lubricants into industry. Conventionally, these materials are often tested with accelerated methods but the long term wear behavior under realistic conditions can only be evaluated with long term wear tests.
LUBE MAGAZINE NO.136 DECEMBER 2016 35
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