The authors have studied the tribological behaviour of hureaulite coat
ing previously modified by annealing. Hureaulite is a manganese phosph
ate used as an anti-friction coating on steel parts in relative motion
. This coating possesses no intrinsic lubricating characteristics and
must therefore be used in combination with a lubricant. It is the resp
onse of the tribosystem 'coating-lubricant' which is discussed. Using
a friction test to determine the seizure resistance, it is shown that
the modifications of the coating (dehydration, new phases and new comp
ounds forming) lead to a great improvement in the resistance to seizur
e whose value is 5 to 10 times higher according to the annealing tempe
rature. The micrographs show that these modifications generate cracks
through the crystals; these cracks are perpendicular to the surface an
d do not lead to scaling phenomenon. They act like lubricant traps and
so delay the seizure. Annealing temperatures ranging from 100 to 700
degrees C and treatment duration from 1 to 100 min are studied. The po
ssibility of reducing temperature while reasonably increasing time is
shown so the heat treatment does not affect the steel substrate and co
uld be industrialised. (C) 1998 Elsevier Science S.A. All rights reser
ved.