Dr. Dmitri Kopeliovich
Tin based overlays contain copper (3-6%) as a hardening component.
The alloys are developed to replace toxic lead based overlays.
Tin based overlays possess excellent corrosion resistance and cavitation resistance.
Overlay multilayer system of Sn6Cu alloy deposited on the bi-layer diffusion barrier Ni-SnNi allows to create a microhardness gradient across the overlay thickness due to varying content of copper (soft top layer and hard bottom layer of the overlay). Such multilayer bearings are capable to withstand alternating loads up to 11600-13000 psi (80-90 MPa).
Hardness of tin-copper overlays is 20-30 HV.
Tin-copper overlays are deposited by the electroplating methods.
Nickel diffusion barrier (nickel dam) is deposited between the intermediate layer and tin containing overlay in order to prevent migration of tin from the overlay into the intermediate material (copper).
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 migration into 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 (Watts nickel plating solutions or nickel sulfamate solutions) are used for the diffusion barrier deposition.