Abstract The boundary lubrication of an aluminium–silicon alloy using zinc dialkyldithiophosphate (ZDDP) or ZDDP and an organic antiwear additive (OAW) has been investigated. Contact conditions on silicon grains within the aluminium alloy were replicated using a model silicon surface. On either substrate, ZDDP tribofilms did not form readily and possessed poor lubricating characteristics, but the addition of the organic additive improved film formation and wear performance. An increase in film thickness, surface coverage and mechanical properties of the ZDDP+OAW tribofilm was accredited with the wear improvement. The tribochemistry of worn aluminium–silicon and silicon surfaces was similar to ZDDP-lubricated ferrous substrates.