Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Patent
1994-07-22
1996-06-25
Nelson, Peter A.
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
75229, 75230, 75246, 428548, 428567, 428569, 149 30, 149 32, 149 48, B22F 508
Patent
active
055296009
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a material for friction components which operate in lubricated tribological systems, and in particular, but not exclusively, to the manufacture of synchronization rings for use in manual gearboxes.
BACKGROUND OF THE INVENTION
The development of materials for gearboxes is subject to many demands, some of them mutually contradictory. On the one hand, the gears must be effectively lubricated, i.e. the coefficient of friction between them must be as low as possible, while on the other hand, the synchronizing rings must have a high coefficient of friction which remains constant independently, in particular, of the temperature, the speed and the pressure.
One suggestion is to cover the active surface of the synchronizing rings with a suitable material such as molybdenum. This method is expensive.
Another technique is aimed at preventing an oil layer from forming, or causing the oil film to break by creating geometric irregularities by machining grooves or the like or by means of finer heterogeneities by using a non-homogeneous material, in particular a relatively soft matrix containing harder particles. Nevertheless, these friction materials have until now given results which vary according to the conditions under which they are used.
The studies carried out have lead to the conclusion that these materials could provide good results, at a relatively low cost, if certain conditions are fulfilled.
SUMMARY OF THE INVENTION
The aim of the present invention is therefore to provide a friction material that enables a high coefficient of friction to be achieved, with little dependence on the conditions of use, and with which it is possible to obtain components in a suitable way at a low cost.
To achieve this, the invention provides a material designed for making friction components in lubricated media, this material having the particular characteristic that it comprises different regions, between 60 and 500, advantageously from 60 to 100, microns in size, and at least two substances with different hardnesses and different coefficients of friction, the harder substance being the one with the higher coefficient of friction and the one which occupies a volume between 1/3 and 4/5 of the total volume.
The remaining volume of the material is occupied by the softer substance and by the porosity resulting from the method of manufacture.
It has been shown that if the proportion of the harder substance is less than 1/3 of the total volume the desired result is not achieved. If the proportion of the harder substance is increased in the manufacture of the material, a sintered compression technique becomes the only practical possibility and it is very difficult or very expensive to prevent the formation of a considerable amount of porosity. In practice it is therefore very difficult to exceed the limit of 4/5 of the total volume for the regions of the harder substance. Advantageously, the material according to the invention has the form of grains of hard material joined together by a matrix which fills most of the intergranular space, the rest of this space constituting porosity.
It is clear that the wear of the material of the invention causes a micro-relief to appear on its surface and that according to the dimensions specified for the respective regions, this micro-relief is sufficient to cause the oil film to break, thereby leading to a high coefficient of friction.
The hard material is chosen from among those which retain their surface hardness, have a high coefficient of friction and have a surface which is "passivated" by reaction in the tribological system mentioned above.
A passivatable surface is taken to mean a surface on which a continuous, impermeable oxide layer is formed in the medium in question. This layer constitutes a barrier between the material and its environment.
When the material is to be used in the presence of a lubricant containing an additive, the hard material is chosen preferably from those materials which retain their coefficient of friction in the presence of the
REFERENCES:
patent: 4233073 (1980-11-01), Takemura
patent: 4552590 (1985-11-01), Nakata et al.
patent: 5080713 (1992-01-01), Ishibashi et al.
Belair Pascal
Fernandez Antonio R.
Gras Jean R.
Greaves John N.
Nelson Peter A.
Sintermetal S.A.
Striker Michael J.
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