Mechanism of wear at extreme load and boundary conditions with ashless anti-wear additives: Analysis of wear surfaces and wear debris

Ashless antiwear additives have begun to find applications in automotive and heavy industry applications as sludge formation is reduced leading to improved efficiency. In this study, six different ashless antiwear agents, including an ashless neutral dialkyl dithiophosphate (DDP-1), an acidic dialkyl dithiophosphate (DDP-2), an alkylated triphenyl phosphorothionate (A-TPPT), a butylated triphenyl phosphorothionate (B-TPPT), a triphenyl phosphorothionate (H-TPPT), and an amine phosphate (AP-1) were studied. Conditions of tribotesting were chosen to simulate harsh boundary lubrication where extreme loads are applied and the anti-wear additives are stressed. These test conditions can potentially separate out high performing additives from ones that provide marginal protection. The mechanism of wear in the best-case scenario with acidic dialkyl dithiophosphate was associated with the formation of well-developed tribofilms even under the harsh conditions. On the other hand the neutral dialkyl dithiophosphate exhibited severe abrasive wear and premature failure. The amine phosphate, while it has a functioning tribofilm, also exhibited weak adhesion of the tribofilm resulting in stick slip behavior and extensive polishing wear. All the phosphorothionates exhibited poor wear outcomes with the alkylated triphenyl phosphorothionate being the best and the protonated triphenyl phosphorothionate being the worst. The mechanism of wear in the phosphorothionates was associated with polishing wear in A-TPPT and B-TPPT where wear debris trapped between contacting surfaces removes the tribofilms that form on the surface and abrasive wear in the case of H-TPPT. Transmission electron microscopy of the harvested wear debris indicated the formation of amorphous debris with embedded nanocrystalline particles that were identified as Fe3O4 in most of the cases. The wear protection afforded by the anti-wear additive can be correlated with the number of the nanocrystalline oxide particles embedded in the debris with the ones with smallest number of oxide particles exhibiting the best wear performance.