Page 18 - Tribology in Machine Design

Introduction to the concept of tribodesign 5

consisting of a material that is much softer and weaker than both the
substrate material and the material of the mating surface. Such a layer can
be utilized without incurring too great a risk of structural failure of the
relatively weak material of the protective layer considered here. In the case
of conformal surfaces this may be explained by a very shallow penetration
of the protective layer by surface asperities. In fact, the depth of penetration
is comparable to the size of the micro-contacts formed by the contacting
asperities. This is a characteristic feature of the nature of contact between
conformal surfaces. Unless the material of the protective layer is exceed-
ingly soft, and the layer very thick indeed, the contact areas, and thus the
depth of penetration, will never become quite as large as those on
counterformal rubbing surfaces.
Other factors to be considered are the strengthening and stiffening effects
exerted on the protective layer by the substrate. It is true that the soft
material of the protective layer would be structurally weak if it were to be
used in bulk. But with the protective layer thin enough, the support by the
comparatively strong substrate material, particularly when bonding to the
substrate is firm, will considerably strengthen the layer. The thinner the
protective layer, the greater is the stiffening effect exerted by the substrate.
But the stiffening effect sets a lower bound to the thickness of the layer. For
the layer to be really protective its thickness should not be reduced to
anywhere near the depth of penetration. The reason is that the stiffening
effect would become so pronounced that the contact pressures would, more
or less, approach those of the comparatively hard substrate material. Other
requirements, like the ability to accommodate misalignment or deform-
ations of at least one of the two rubbing bodies under loading, and also the
need for embedding abrasive particles that may be trapped between the two
rubbing surfaces, set the permissible lower bound to thicknesses much
higher than the depth of penetration. In fact, in many cases, as in heavily
loaded bearings of high-speed internal combustion engines, a compromise
has to be struck between the various requirements, including the fatigue
endurance of the protective layer. The situation on solid protective layers
formed on counterformal rubbing surfaces, such as gear teeth, is quite
different, in that there is a much greater depth of penetration down to which
the detrimental effects of the constriction of the flow of force are still
perceptible. The reason lies in the fact that the size of the Hertzian contact
area is much greater than that of the tiny micro-contact areas on conformal
surfaces. Thus, if they are to be durable, protective layers on counterformal
surfaces cannot be thin, as is possible on conformal surfaces. Moreover, the
material of the protective layer on a counterformal surface should be at
least as strong in bulk, or preferably even stronger, as that of the substrate.
These two requirements are indeed satisfied by the protective layers
obtained on gear teeth through such surface treatments as carburizing. It is
admitted that thin, and even soft, layers are sometimes used on counter-
formal surfaces, such as copper deposits on gear teeth; but these are meant
only for running-in and not for durability.
Liquids or gases form protective layers which are synonymous with full