Throughout the 20th century, lead made a significant contribution to the development of the internal combustion engine. Initially this was in the form of Babbit-type materials (a mixture of tin, antimony, copper and lead) that were used in engine bearings. Later, lead played a more significant role in the development of ‘anti-knock’ compounds when added to gasoline fuels. In the former case, the softness of lead helped the lubrication in the bearings, whereas as an additive in the form of tetra-ethyl lead it improved the octane rating of the fuel.
However, 70 or so years later the link was made between the toxicity of airborne lead and the adverse health effects on children, so within a decade the presence of four-star leaded fuels in the West was largely eradicated. Once removed from the urban air, as part of ‘end of vehicle life’ legislation, the authorities in Europe have now removed it from virtually all areas of road-based automotive applications. So while lead-free bi-metal bearings are now common and working satisfactorily, for high-performance tri-metal units – especially in highly loaded diesel engines – another approach was needed. Cue the development of a new type of bearing overlayer.
Unfortunately lead has some very desirable properties when it comes to bearing materials. It is soft and has a comparatively low melting point, and these characteristics are essential in high-performance engines where peak bearing loading can be very high. But as well as these properties, lead has what is referred to as ‘lipophilic’ properties, in that it has an affinity for some types of oils, lowering the surface tension and making the bearing surface more ‘wettable’. All this – and of course its ability to absorb heat during the melting process – makes adhesive wear at the boundary with the crankshaft journal highly unlikely.
Combining lead with indium or copper (to add strength), however, made it possible to accept high peak loads; the loads would be reacted by the presence of the oil film, but if this broke down then any friction generated would melt the lead, taking heat away from the area, and the molten lead would be smeared to a position elsewhere in the bearing, giving up its heat back to the lubricant flowing past and re-solidifying. The result would be a redistribution of the load in the bearing, a situation often referred to as ‘bedding-in’. Under such conditions the presence of lead was ideal, for not only did it make the bearing shape eventually conform to that required, it also made seizure of the journal most unlikely, even under the most adverse conditions.
As a conclusion to a study for this lead replacement therefore, materials were evaluated for strength, melting point, thermal conductivity and adhesion to crank journal surfaces, so the tri-metal application using bismuth and silver was suggested. With a low melting point (271 C), pure bismuth was favoured as the top surface in contact with the journal, while beneath it and next to the base layer (a copper-tin based product) was the silver, chosen primarily for its high thermal conductivity.
Both layers were around 5 microns thick, giving a maximum thickness of the overlayer of 10 microns. At this thickness, optimum fatigue resistance was achieved with, at the same time, adequate ‘embeddability’ against the risk of dirt or foreign matter in the oil. Furthermore, by altering the crystal orientation of the bismuth and electroplating it into pyramid-type shapes, bearing surface wettability was improved, minimising the chances of seizure.
Including silver as a separate layer was in my opinion a masterstroke. As a distinct layer, and being easily picked up by ICP (inductively coupled plasma) oil analysis equipment, the presence of silver in the oil sample can indicate a certain degree of bearing wear long before the situation becomes critical.
It’s not often you get such warning signs of impending disaster in an engine.
Fig. 1 - The bismuth-silver overlayer
Written by John Coxon