Dr. Dmitri Kopeliovich
Lead based alloys are still widely used as overlay materials mainly in copper based tri-metal bearings.
However in many applications the alloys containing toxic lead are replaced with new developed lead-free materials.
Lead based overlay materials have excellent soft anti-friction properties (compatibility, conformability, embedability) but relatively low fatigue strength limiting their applications in highly loaded engines (eg. direct fuel injection diesel engines). In addition to this pure lead has low corrosion resistance in acidic oils therefore conventional lead based overlay materials contain tin (not less than 10%), which inhibits corrosion attack of the alloy.
In order to impart the alloys higher fatigue strength lead-tin compositions are alloyed by copper (2-8%), indium (9-12.5%) or dispersed alumina particles (0.5-2%). Pb10Sn2Cu and Pb10Sn3Cu compositions are the most popular Pb-based overlay alloys.
Hardness of lead based overlay materials is in the range 8-20 HV depending on the content of the hardening component.
Fatigue strength of the lead based overlays is about 7250-10150 psi (50-70 MPa).
Lead based overlays compositions
|Lead (Pb), %||Tin (Sn), %||Copper (Cu), %||Indium (In), %||Alumina (Al2O3), %|
Lead based overlays are deposited by Electroplating methods. The same methods are used for the deposition of thin cosmetic tin flash.
Fluoborate bath formulations:
(Organic brighteners/additives - as recommended by the supplier)
|Tin alloy electroplating in fluoborate solutions|
|Tin||Lead||Copper||Fluoboric acid||Boric acid|
|Tin flash (100Sn)||5||37||-||-||-||-||26||200||4||30|
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.