Nanoscale chemistry and mechanical properties of tribofilms on Al–Si alloy (A383): interaction of ZDDP, calcium detergent and molybdenum friction modifier

Abstract

The interaction of a friction modifier and a calcium phenate detergent additive, with zinc dialkyl dithiophosphates (ZDDPs) in the formation of antiwear films on A383, has been studied using synchrotron radiation and nanoindentation techniques. X-ray absorption near edge structure (XANES) spectroscopy has shown that films prepared from oils containing both ZDDP and detergent, and ZDDP and molybdenum dithiocarbamate (MoDTC), are chemically similar to, but thicker than those made from oils containing only ZDDP. In addition, wear was greatly reduced in the presence of the detergent which was correlated with the basicity and the presence of the friction modifier. The phosphorus K and L edge XANES spectra show that the tribofilms are polyphosphate glasses of similar nature to those found on steel, but characterised by a shorter chain length. The sulphur K edge shows a MoS 2 like film and under certain conditions, the presence of a sulphate species is detected. High resolution topographic images and mechanical properties were determined by atomic force microscopy and imaging nanoindentation. The films formed in the presence of the detergent exhibited similar mechanical responses independent of the conditions tested. The indentation modulus of the films on the Al matrix always appear much softer than the films formed on the Si grains whether or not the lubricant contains only ZDDP, or both ZDDP and MoDTC.

Tribological performance of an Al—Si alloy lubricated in the boundary regime with zinc dialkyldithiophosphate and molybdenum dithiocarbamate additives

“…This suggests that the ZDDP additive cannot form a stable and durable tribofilm on Al-Si alloys. The MoDTC at higher concentration, (oil B1), together with the ZDDP provided the highest wear protection in the Al-Si/CI tribocouple, in agreement with other published research [7]. After 10 minutes the wear coefficient was the lowest of the three oils.…”

“…In both cases the Mo intensity after 3 h was greatest and for oil B1, consistent with the PO 2 results a steady state was reached after 1 h where Mo was maximum. shown in a study by Pereira et al [7] using the XANES surface technique. The current study confirms this and shows that the MoS 2 formation is not dependent on the substrate properties but mainly on the MoDTC concentration in the oil and the rubbing process.…”

“…MoDTC is known as a good friction modifier for steel tribocouples lubricated in the boundary regime due to its decomposition to form MoS 2 sheets in the interface. In the case of Al-Si alloy surfaces, XANES [7] has shown that MoS 2 , ZnS and sulphate form on the Al-Si plate wear scar when fully formulated oil is used for lubrication. The detailed friction and wear performance as a function of time, additive concentration and testing conditions remains to be understood.…”

Lubrication of aluminium–silicon surfaces with ZDDP and detergents

“…37 At each operating temperature studied (60, 100 and 150uC), the combination of detergent and ZDDP afforded superior wear results than the use of solitary ZDDP. 37 In addition, the higher total base number detergent reduced the wear in the contact by a further amount. 37 A similar benefit was witnessed in terms of the friction coefficient; the addition of detergent to ZDDP reduced the frictional response in the tribosystem compared to ZDDP alone.…”

“…37 Compared to ZDDP alone, detergent and ZDDP increased the amount of generated polyphosphate; these chains ranged from short to medium in length. 37 At each operating temperature studied (60, 100 and 150uC), the combination of detergent and ZDDP afforded superior wear results than the use of solitary ZDDP. 37 In addition, the higher total base number detergent reduced the wear in the contact by a further amount.…”