Crankshaft bearings absorb the forces created by the piston which are transmitted to the crankshaft via the connecting rods and ensure low friction, low noise rotation of the crankshaft. 

Crankshaft bearings are usually split friction or 'plain bearings'. A steel backing shell, generally 0.70 - 0.75mm thick is used to create a sturdy surface on which one or more anti-friction materials are added, resulting in an overall thickness of approximately 1.5mm.

There are multiple designs of crankshaft bearings:

• Two-component bearings: They comprise of a steel backing shell with a thin low-friction coating made of aluminium alloy.
• Three-component bearings: These also have a support layer between the steel backing and the low-friction coating made of a copper/tin alloy. Particles of dirt get embedded in this layer. A nickel diffusion barrier separates the low-friction coating from the support layer to prevent diffusion of the tin.
• Sputter bearings: Bearings such as these are used in turbocharged diesel and high performance engines. The coatings are applied using electromagnetic techniques which give them high density and high load tolerances. Sputter bearings are generally only installed on the thrust load side of the connecting rod and main bearings. Two or three component bearings are used on the unloaded side of the journals.
• Polymer bearings: These are used in engines with automatic start-stop capabilities, which generate higher combustion pressures and use low viscosity lubricating oils.  An extra resin layer is applied on top of a three-component bearing with hard-compound particles and solid lubricant particles embedded in the joins. These bearings demonstrate a low coefficient of friction and therefore minimise wear within the engine.

In the animation you can see a multi-layer journal bearing. This gets its name due to multiple layers, in this case a support layer (yellow), a nickel diffusion barrier and a low-friction coating (red).