The tribochemical mechanism of phosphorus-containing additives is important to understand, even if the ultimate aim is to replace them with something more environmentally benign. In a combinatorial experiment that explored a range of temperatures (RT-180°C), loads, two additives, and two substrate pairings, it was observed that zinc dialkyldithiophosphate (ZnDTP) reacted with steel surfaces to produce phosphate at room temperature, pyrophosphate at 130°C, and polyphosphate at 180°C. Metal-ceramic pairings did not seem to produce long-chain polyphosphates at 150°C. It was also observed that metal-metal pairings appear to benefit more from the presence of the additives than ceramic-metal pairings, which suggests that the role of the additives is largely to inhibit microwelding of the asperities across the sliding plane.

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