Dr. Dmitri Kopeliovich
Most engine bearings have layered structure, which provides optimum combination of the bearing material properties: fatigue strength (load capacity), seizure resistance (compatibility), wear resistance, conformability, embedability, corrosion resistance, cavitation resistance.
According to the number of the main functional bearing layers the bearings may be mono-metal (solid), bi-metal, tri-metal or multi-layer.
Most engine bearings are either bi-metal or tri-metal.
Typical engine bearings structures are presented in the figure below.
Intermediate layer is made of a bearing material with moderate or low anti-friction properties (conformability, compatibility, embedability) but high fatigue strength providing durability of the bearing working under alternating loads generated by the internal combustion engine.
A certain level of surface (anti-friction) properties is necessary for preventing catastrophic failure of the bearing in emergency conditions of boundary lubrication, oil starvation, significant misalignments or shaft deflections. Intermediate layer is referred to tri-metal and multi-layer bearings.
Intermediate layers are made mainly of Copper alloys such as CuPbSn (leaded bronzes), CuAl (aluminum bronze), CuBiSn. Copper based intermediate materials, CuSnNi, CuZn are applied on the steel back surface by either casting or sintering process.
Some Aluminum alloys such as Al-6Sn, Al-11Si, Al-4Si are also used as intermediate layer materials. Aluminum based bearing materials are bonded to the steel back by the cold rolling boding method.
Thickness of intermediate layers is usually within the range 0.008”-0.020” (0.2-0.5 mm).
Overlays are referred to tri-metal bearings. An overlay is applied to the surface of the bearing intermediate lining after final machining operation (boring or broaching).
Overlay properties become crucial particularly under the conditions of boundary lubrication at low rotation speeds of the engine (engine start and shutdown).
Low thickness of overlays is necessary for achieving required level of fatigue strength is dependent on the overlay thickness (the thicker the overlay the lower its fatigue limit).
Types of engine bearing overlays:
Diffusion of tin into copper causes formation of brittle Cu-Sn intermetallic compounds (Cu3Sn, Cu6Sn5), which decrease the adhesion strength of the overlay to the intermediate layer. In addition to this decrease of the tin content in the overlay due to the migration into the intermediate material deteriorates the Corrosion resistance of the lead-based overlay alloy.
Thickness of nickel diffusion barrier is about 0.00004”-0.00006” (1-1.5 µm).
Nickel electroplating methods are used for the diffusion barrier deposition. If the overlay is coated by Physical vapor Deposition (sputter bearings) the nickel barrier is also applied by the vapor deposition method.
Presence of tin particles on the surface of an aluminum-tin alloy bonded to a steel strip weakens the adhesion strength between the materials.
A bonding layer of pure aluminum between the AlSn alloy and steel allows to achieve strong adhesion.
Pure aluminum is commonly bonded to the aluminum-tin alloy and then the aluminum strip is bonded to steel back. The thickness of the aluminum bonding layer in a bearing is about 0.001”-0.002” (25-50 µm).
Another type of bonding layer is a nickel layer deposited on the steel surface prior to bonding with aluminum-tin alloy.
No overlays are deposited on the surface of the material. Non-plated materials combine fair surface (anti-friction) properties with good fatigue strength.
Most aluminum-tin bearing alloys (Al20Sn1Cu, Al12Sn3Si1Cu) do not require overlays.
Aluminum-tin alloys are bonded to the steel back by the cold rolling boding method.
Thickness of non-plated bearing alloys in is usually within the range 0.008”-0.016” (0.2-0.4 mm).