A closed impeller has a base cover, which has a base interior surface, and a second cover. A vane, which has an inlet distal end and an outlet distal end, is on the base interior surface. The vane has low and high pressure side surfaces wherein they have a low or high pressure midpoint, respectively. A hard coating is on the low and high pressure side surfaces. The hard coating has a minimum low and high pressure coating thickness at their respective midpoints. The minimum low, as well as high, pressure coating thickness ranging between about 0.085 and about 0.8 of either one of the maximum outlet coating thickness at the outlet distal end or the maximum inlet coating thickness at the inlet distal end, respectively.