What are placed between metal joints?

There are two options: plastic washers or a grease of some sort. The purpose of this kind of barrier is to reduce friction and wear and tare from a metal-on-metal contact. Plastic washers are useful because they are usually made of high density polyethylene or polycarbonate - both extremely slippery substances. however, this type of washer has bean surpassed by some of the modern heavy-duty greases, which don't break and last for a long time. Washers are still used for "clean applications" where grease would be considered a "contaminant" and in instances where the metal pieces would interfere with each other's movements if they were allowed to touch.