Leaded bronzes have been widely used as bulk material or
as coating of steel pieces for applications as bearings, shafts or
hydraulic pumps. Lead acts as an efficient solid lubricant in systems
functioning under boundary or dry conditions. For environmen-
tal and legal reasons, efforts are being made to remove lead from
bearing alloys. Numbers of materials are already proposed as alter-
native to leaded alloys, in particular metallic composites containing
self-lubricant particles like graphite, MoS2 or PTFE [1–4]. Increas-
ing the wear performances of alternative materials to leaded bronze
requires a scientific understanding of the relevant mechanisms, and
in particular of the relationship between materials microstructure
and tribological behavior. However the present understanding of
the mechanisms by which lead improves the tribological properties
of alloys is limited.
Lead-base alloys, like babbit alloy, are used as soft coating for
bearings. Upadhyaya et al. [5] compared the tribological properties
of lead-base babbit alloys produced by casting and thermal spraying
in relation with their microstructure. Due to the low melting point,
lead was found smearing on the counterpart at an early stage of
sliding process. This thin lubricating film efficiently protected the
sliding parts against wear. Samples produced by thermal spraying
showed a lower wear due to a smaller spacing between intermetal-
lic particles. Dispersed lead particles are also used as solid lubricant
in aluminum-based or copper-based alloys. Mohan et al. showedthe effect of lead content in a stir-cast Al-Pb alloy [6]. An increase
of the lead content led to a decrease of wear of up to 20 wt % Pb.
Higher Pb concentration increased the wear due to easier crack
propagation and to the removal of thick layer of lead. The effect
of lead content on Cu-Pb alloys was studied by Pathak et al. [7].
Increasing the lead concentration up to 40 wt % led to a decrease
in wear. The coefficient of friction also decreased up to 35 wt% Pb
before increasing at higher lead content.
Pandey and Prasad studied the effect of applied pressure and
sliding velocity on zinc-based and copper-based alloys [8]. The
highest wear rate of the copper-based alloy was attributed to its
microcracking tendency. The cracks were mostly present at low
sliding speed and did not form at higher velocity due to the higher
frictional heating.
Concerning leaded bronzes, the crucial role of third bodies in
dry sliding has been previously observed with similar tribologi-
cal conditions as in this study [9]. At low load, only small metal
oxide particles were generated. Under higher contact pressure,
lead-enriched larger flat debris were built up, decreasing the wear
coefficient and the friction. Microstructure played an important
role in the formation of the antifriction flat debris. Fewer but larger
lead inclusions allowed building up the beneficial flakes under
lower contact pressure.
In this study, the role of the alloy microstructure and of the
surface roughness on dry wear and friction of leaded bronze
is investigated with the aim to better identify critical mecha-
nisms. A CuSn10Pb10, with two dendritic microstructures and
a lead-free bronze, CuSn8, were studied. The sliding tracks and
the ball scars were analyzed with scanning electron microscopy
(SEM), energy dispersive electron microscopy (EDX), x-ray pho-
toelectron spectroscopy (XPS) and Auger electron spectroscopy
(AES).