Flexspline　is　the　most　vulnerable　component　of　harmonic　drives,　and　its　wear　is　likely　to　induce　failure.　Therefore,　this　study　investigated　the　tribological　properties　of　three　typical　flexspline　materials　with　lubricating　grease.　The　evolution　of　two　oxide　forms　(amorphous　FeOOH　and　crystalline　Fe2O3)　on　a　frictional　interface　was　proposed　as　the　wear　mechanism,　and　this　mechanism　was　confirmed　using　co-analyses　of　X-ray　photoelectron　spectroscopy,　transmission　electron　microscopy,　atomic　force　microscopy,　and　scanning　electron　microscopy.　Amorphous　FeOOH　generated　during　sliding　was　observed　to　enhance　the　oxide　layer,　resulting　in　compact　structure　and　improved　toughness　that　reduced　friction　at　the　interface,　whereas　with　insufficient　amorphous　FeOOH,　the　frictional　crystalline　Fe2O3　tended　to　strip　under　shear　force,　destabilizing　the　oxide　layer.　The　formation　ratio　of　FeOOH/Fe2O3　during　sliding　regulated　the　oxide　film　characteristics　and　the　tribological　properties　of　the　materials.　Crystalline　Fe2O3　partly　evolved　from　amorphous　FeOOH　is　mainly　dependent　on　heat　generated　during　sliding.　The　grease　adsorption　properties　of　tribo-pairs　significantly　affected　the　temperature　of　the　frictional　contact　interfaces.　The　characteristics　of　this　wear　mechanism　can　be　leveraged　to　reduce　wear　in　future　drive　designs.