My first attempt at an extruder had stainless steel bearings and a stainless steel drive shaft, more by accident than anything else. I wondered at the time how the bearings would last compared to the recommended brass ones. Obviously stainless steel is harder than brass, but brass should have less friction, so less wear.
The first drive shaft got retired because the bearing lands were off centre. At the end of its life the bearings and lands were still in good condition.
I ran it from 10/07 to 01/08 but up to that point all I had made was lots of HDPE test shapes.
I replaced it with a plain steel shaft that I bought from BitFromBytes. That had the big advantage of being solder-able.
It worked well for a long time but eventually the bottom land on the shaft wore down so much that the pump halves closed together when using undersized filament (2.7mm).
The bottom land has worn down from 3mm to 2mm. The bearings show a little wear but still have some life left in them. This is after running 5lbs of ABS through the extruder.
In the last few weeks I replaced the shaft with a new zinc steel one and switched to brass bearings in an ABS extruder with HDPE filament guide. That worked well until I noticed the pump halves closed together again. When I opened it up I found that the brass bearings had rotated in the ABS, but they had also worn down a lot, considering the short time I had used them.
The drive shaft lands are still fine though.
So it would appear that the best combination is stainless steel bearings and a stainless steel shaft, but I would have to find another way of attaching the nut.
Most metal bearings I have recovered from old equipment are bronze. I don't know how that compares to brass but it seems to be the thing to use.
Maybe it is time to look at ball bearings and an offset shaft like Ian Adkins' design.