Although, the FSP treatment increased the friction during sliding contact, it showed much lower wear in both the flat and the ball counterface. Fig. 6a and b shows the amount of wear in the 52100 steel ball counterface and the 4140 flat specimens respectively. The heat treated flat produced much more wear on the counterface ball compared to the baseline flat. This can be attributed to higher hardness of the heat treated material. The FSP treated flat however produced the least amount of wear on the counterface ball in spite of having the highest surface hardness of the three. This may be the result of microstructural refinement of the near surface layer by FSP. Both thermal heat treatment and the FSP treatment reduced wear in the flat specimens as shown in Fig. 6b, again with FSP treated surface showing the lowest amount of wear. Optical micrographs of the wear track on the three flat specimens are shown in Fig. 7. Both the baseline material and the heat treated specimen showed identical wear mechanism. There is evidence of abrasion, possible sliding fatigue wear and formation of transfer layer in patches (Fig. 7a and b). For the FSP treated flat, wear occurred primarily by abrasion and some limited amount of transfer layer patches (Fig. 7c). Compared to the other two flats, the extent of surface damage and wear was much reduced.