Article published on juin 2008Tantalum
Thin but protective films
To protect equipment in corrosive environments, CEA is working to optimise a cold spray process for tantalum deposition. Objective: The goal is to obtain thin, dense and perfectly leaktight films.
With its anticorrosion properties
and extreme strength, tantalum
shows promise for protecting
industrial equipment in adverse
environments. To obtain thin films of this
metal that are dense and leaktight enough
to resist external stress, researchers at
CEA's Military Applications Division (DAM)
are using the deposition technique known as
"cold spraying". This method, developed 20
years ago, has been improved to meet industry
requirements and specifically adapted to
tantalum deposition.
The basic principle is to mix the metal particles
(in powder form) with a supersonic jet
of gas, giving the particles enough energy so
that they undergo plastic deformation upon
impact with the substrate and adhere to
it. At CEA, the researchers modified spray
parameters in order to reduce the number
of passes needed to obtain a deposit, while
at the same time guaranteeing high density.
"The nozzle, where the mixing occurs, is the
system's key component", explains Thierry
Laguionie, engineer-researcher at DAM. "Its
design determines the output velocity of
the gas, and as a result, that of the particles.
We've developed a nozzle that has a
geometry and an internal surface specially
designed to obtain a stable jet of tantalum,
with a velocity of 500 metres per second."
The result is twofold: the robust new components
not only deposit films with very
similar characteristics to massive tantalum
(density greater than 98% of the metal's
theoretical density); they also make industrialisation
possible.
The cold spray process is flexible. It can be
used to obtain a dense, leaktight metal film,
to change deposition thickness (between a
few dozen microns and several millimetres),
even during the process, and to adapt the
spray composition according to the protection
needed. It is ideal for complex or hardto-
reach surfaces, since deposition can be
obtained by internal spraying in cylinders
with diameters of 250 mm or larger.
Possible applications reach across several
sectors, from the chemical and pharmaceutical
industries to the storage of corrosive
waste.
