What are 'Sputtered' Bearings?
In company with the small end bearing there’s no more hard working bearing surface than that found in the big end of high performance gasoline and diesel race engines.
In a good example of a reverse technology flow - series production to racing - bearings originally developed to cope with extreme cylinder pressures found in the latest generation of high speed passenger car and commercial diesel engines have found their way into Formula One and sport car engines.
In the mid 90’s, vehicle manufacturers found that the bearing materials available to them that used bi and tri metal construction processes were unable to provide the durability required for full engine life when the cylinder pressures exceeded 160bar. Enter ‘sputter’ bearing technology.
Sputter materials and processes for engine bearings were first developed by Glyco, part of the Federal-Mogul engine components group. The sputtering process produces a material that combines the high wear-resistance properties of an aluminum-tin sliding layer with the extremely high-load withstanding capacity of a cast copper-lead-bearing metal layer.
Sputter overlays are deposited by Physical Vapor Deposition (PVD) method, utilising argon ions for bombarding a cathodically connected target, made of the final bearing coating material, normally Al20Sn or Al40Sn.
The sputter process starts with the base bearing shells being loaded into a vacuum chamber which is then evacuated before the introduction of argon gas. The gas is then heated until it enters a plasma state causing positively charged argon ions to form.
Then follows an etching and activation stage during which stage the bearings become the cathodes and the argon ions bombard the bearing substrate to both clean it and provide good adhesion for bearing surface material.
In the next step the current is reversed and the bearing becomes the anode and a nickel or nickel chrome (Ni or NiCr) diffusion layer is deposited on to the cathode target. The argon ions bombard the target knocking the atoms of nickel (nickel and chromium) out from the target. The atoms moving off the target meet the bearing – now the anode - substrate surface and stick to it, creating a diffusion layer of 0.00004”-0.00008” (1-2 µm) in thickness.
Finally, the working surface overlay is deposited. At this stage the cathode is a target made of an aluminum-tin alloy, Al20Sn or Al40Sn. Atoms of the target are knocked out by the high energy ions and are deposited on the substrate surface forming the finished AlSn overlay.
The sputtering process provides an extremely homogeneous distribution of tin within an aluminum matrix. Hardness of aluminum-tin sputter material is about 90 HV, which is three times higher than hardness of aluminum-tin alloy prepared by conventional methods (casting). Cast copper based bearings or high strength aluminum based bearings are commonly plated by sputter overlays.
Load carrying capacity of sputter bearings is highest of all bearing materials, being in the region of 100-120 MPa (14500-17400 psi).
There are however some disadvantages with sputter bearings. High production costs caused by the slow deposition process and low soft anti-friction properties due to poor material compatibility, conformability, embeddability – the ability of a bearing to allow small particles to embed themselves into the soft bearing surface, thereby cause no further bearing damage – have typically confined the use of sputtered bearings to premium and high performance niche markets.
However, a significant cost reduction can be achieved by a combination of a sputter bearing shell in the high load conrod position, with a common tri-metal bearing shell being used in the less loaded cap position.
The physical vapor deposition process produces an exceptionally uniform alloy matrix with superior wear resistance, capable of withstanding the very high loads found in very high speed gasoline engines and the current generation of racing diesels such as the Audi V10 and Peugeot Le Mans series engines.
Written by David Wood.