Elaboration
and mechanical properties of diamond coatings obtained by flame process
LPMM-ERMES, CNRS-UMR 7554
2, avenue de la forêt de Haye 54516 Vandoeuvre France
LEPMI-ENSEEG Domaine universitaire, BP75 38402 Saint Martin d'Hères cedex
Abstract
The flame process, one of the various
methods allowing the diamonds deposition, offers two advantages : it is, on the
one hand, relatively easy to realise (a welding torch is needed, and the
deposition can be done in ambient air) ; the crystals growth speed is, on the
other hand, is more than hundred times faster than the one observed with the
classical low pressure CVD processes.
This method, initially devised by Hirose in 1988, presents however many
problems which are still not yet solved. It leads to “local” coatings, not
bigger than 1 cm2 : the covering of large surfaces consequently
needs a motion of the torch across the sample surface, that induces a coating
exposure to the flame periphery which is colder, as well as a modification of
the gases mixture (increase of the oxygen amount). That finally results in a
crystals graphitization. To avoid these phenomena, the present processes
involve an argon shield or are carried out in a chamber.
An
other crucial problem is the coating/substrate adhesion : the local surface
temperature is very high, but decreases during the cooling ; important stresses
appear then between the substrate and the crystals, making thus the interface
brittle.
The
density of the nucleation sites, the crystals size and orientation on the
surface and the growth direction are hardly controlled as they greatly depend
on the hot gases speed, the amount of the various chemical species in the gases
(C2H2/O2 ratio), and on the combustion
instability due to the injection nozzle shape ; these parameters variations
lead to temporal and local modifications of the flame and finally induce
irregularities in the crystals growth.
The
mechanical properties of the thus obtained coatings are linked to the residual
stresses in the crystals ; these lattest can be observed when using scanning
Raman spectroscopy.
In
spite of these drawbacks, the flame process remains an attractive method for
covering cutting tools like inserts.
The tribological behaviour of these coatings is
quite different from the one of those obtained by CVD as the dangling bonds are
here mostly saturated by oxygen (instead of hydrogen termination for the CVD
process).
This paper presents a review dealing with the technological and
scientific knowledge as well as the evolution of this process during the ten
past years.
Keywords : diamond coatings ; flame process ; mechanical properties ; cutting
tools
Daniel PAULMIER
LPMM-ERMES, CNRS-UMR 7554
2, avenue de la forêt de Haye 54516 Vandoeuvre
France
Email : Daniel.Paulmier@ensem.inpl-nancy.fr
Fax : 00-33-(0)3-83-59-56-17
Phone : 00-33-(0)3-83-59-56-16