Materials used for leaf spring bushings.
Rubber
Rubber is excellent at absorbing vibration and is the most common bushing type for light-duty vehicles. Rubber bushings will have an inner steel sleeve to prevent wear between the attaching bolts and the rubber and they may be designed with a steel outer sleeve although, some are designed with no outer sleeve. There are many styles of rubber bushing and certain suspension designs with call for specific styles of rubber leaf spring bushing. Rubber bushings have no maintenance, and they will naturally wear over time.
Brass
Brass is a softer metal combination of zinc and copper. Brass bushings are softer than the hardened steel spring pins that are used in suspensions, and they are designed to wear before the spring pins. Brass is a dissimilar metal to steel which makes galling less likely and prevents binding between the leaf spring bushing and spring pin. Brass bushings will use greaseable smooth spring pins, and they will require grease to be added as part of a periodic maintenance schedule.
Bi-Metal
Brass inside and steel on the outside, a bi-metal bushing features a steel outer section, so they are less likely to be damaged when they are pressed into leaf springs. Bi-metal bushings will accept a hardened steel smooth pin, and they will be greaseable. They will require lubrication periodically just as the full brass bushings will.
Steel
Steel is typically used for threaded type bushings. Threaded spring pins and threaded type bushings are used on heavy duty applications where the suspension will be put under high side loads. The coarse threads hold the spring centered on the pin; this prevents the leaf springs from shifting side to side on tight turns and putting wear on spring hangers or shackles. Threaded pins and bushing will require even stricter maintenance and greasing as there is more surface area contact between the bushing and the pin and more chance for wear.
Nylon
Nylon bushings are commonly used in light trailer leaf springs. Nylon is durable, and the material has self-lubricating properties. Nylon leaf spring bushings are inexpensive and can easily be replaced when they are worn out. Nylon spring bushings can normally be installed by hand or with a hammer and will not require a press.
Polyurethane
Polyurethane is used as a replacement for rubber when extreme temperatures or chemical exposure may shorten the life of rubber bushing and make them less practical. Polyurethane bushings are typically more durable than their rubber equivalents, but they are stiffer, so more vibration may be transferred to the chassis of the vehicle.