I have a long 8mm smooth steel rod of about 55cm long. This rods bend easily due to the length. If I replace them with 8mm solid carbon fiber rods, will the bending reduce? Will the bearings wear off the carbon fiber rod? I couldn't find too much information about this.
I agree that steel is the best material for rails but the info provided on carbon fiber is just wrong.
When making a carbon fiber part, whether a tube or any other shape, you lay the fabric in the orientation you need for the direction of the forces. If multi-directional strength and stiffness is needed then the fibers are laid in multiple directions. Nobody who knows what they are doing lays all the cf in one direction...
In general, carbon fiber is stronger than steel by both the strength to weight ratio and by volume. I say "in general" because parts need to be designed in the right way. A poorly designed carbon fiber part or a low quality steel can easily be weak.
There is a great video on YouTube showing a strength and stiffness comparison of a carbon fiber drive shaft vs the same part made in steel. The steel drive shaft bent and snapped with 3 times less force while the cf drive shaft had no visible bending until it snapped.
The main reason why steel is a better linear rail material is surface hardness. Linear rails have bearings traveling up and down continuously. The rails need to be hardened to prevent them wearing out too quickly. The surface hardness (how easy it is to scratch or penetrate) of cf is far lower than steel. There is no comparison.
Cf is great material for the frame but not the rails. For stiffer rails, I suggest buying some square THK rails. 15mm the rails will be plenty stiff enough for most 3d printers.