Towards new formulations for journal bearing alloys A.E. Bravo, H.A. Dura ´ n, V.H. Jacobo, A. Ortiz, R. Schouwenaars n Departamento de Materiales y Manufactura, Facultad de Ingenierı ´a, Edificio O. Avenida Universidad 3000, Universidad Nacional Auto ´noma de Me ´xico, D.F. 04510, Me ´xico article info Article history: Received 31 August 2012 Received in revised form 8 January 2013 Accepted 11 January 2013 Available online 23 January 2013 Keywords: Journal bearings Tribolayer Tribological compatibility Strength Strain hardening abstract For the last half century, the literature on journal bearing alloys has been limited, with some exceptions, to technological papers describing small improvements in existing alloys and textbook descriptions repeating in one way or another the few reviews available on this topic. Several of the comments in these reviews are now outdated, some contained incorrect statements and no evolution is visible in the formulation of commonly used journal bearing alloys. The absence of new solutions in this area imposes restrictions on the improvement of the efficiency of combustion engines. It will be seen that the classical terms used to describe the optimal properties for these alloys are mostly qualitative. Based on present-day knowledge on the wear mechanisms of ductile alloys, the paper shows that these criteria can be substituted with a smaller set of quantitative parameters, with yield strength and strain hardening capability as the most important ones. Others, like the concept of tribological compatibility remains as enigmatic as it was 50 years ago, although possible lines of exploration will be highlighted based on current understanding of friction and adhesion. To illustrate the former points, two sets of experimental results will be summarised in the text. The first set relates to an extensive study of classical Al–Sn alloys, illustrating that even in this system, improvement is still possible. The second one describes an attempt to use the concept of compatibility, as described by Rabinowicz, to define a new formulation for copper based triboalloys, in the form of the Cu–Mg–Sn system. These examples, together with the general principles derived from modern literature, indicate that there is no theoretical or practical reason why journal bearing alloys should be limited to the existing classes. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Journal bearings are a product with high added value: the cost of the materials used in their manufacturing is low compared to the price of the finished product, the main cost residing in the manufacturing process. In spite of a production of more than 10 9 parts per year [1], journal bearings and the alloys of which they are made are among the ones that have evolved least over the last fifty years among all the components in the automotive power train. The main literature resources in the field of journal bearing alloys are a review paper by Pratt from 1973 [2] and the periodically updated overviews by Kingsbury [3,4]. One of the possible reasons for the lack of innovation in this field is the set of specific properties to which the alloys have to comply. From the chemical viewpoint, they have to be corrosion resistant; traditionally, toxicity of was not considered critical, but Cd and As have been eliminated in commercial products and the use of Pb is under severe pressure. High thermal conductivity is desirable to transport the heat dissipated, a low expansion coefficient is advantageous and a high melting point is required to maintain reasonable strength at the operating temperature. Mechanical properties are described as follows. To allow higher peak pressures in the engine, the material must have a high resistance, but its hardness must be low enough to avoid damage to the shaft. Conformability refers to the capability of the bearing material to adapt its surface geometry to small deviations of the ideal geometry of the shaft-bearing assembly. Fatigue resistance is generally mentioned as an essential property. Embeddability is a very specific requirement which refers to the capability of trapping hard particles in the bearing surface, before they cause damage to the shaft. Considering that the hardness of the alloy must not be higher than the hardness of the shaft, it is not too difficult to find alloy systems which combine reasonable strength with sufficient conformability. However, most of such systems would lack tribological com- patibility. This term refers to the tendency to prevent adhesion between two alloys sliding under conditions of solid friction, as happens when partial breakdown of the lubricant film occurs. This concept was studied in-depth by Rabinowicz [5] who associated the friction coefficients between pairs of pure metals with the binary phase diagram for those metals. He found that if two metals were chemically incompatible, their friction Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/wear Wear 0043-1648/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2013.01.040 n Corresponding author. Tel.: þ52 55 56 22 80 57; fax: þ52 55 56 22 80 58. E-mail address: raf_schouwenaars@yahoo.com (R. Schouwenaars). Wear 302 (2013) 1528–1535