Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR THE SURFACE TREATMENT OF STEEL
PRODUCTS BY THE ACTION OF A PLASMA
The present invention relate~ to a process for the
surface treatment of steel products of the type in which
said product is subjected to the action of a plasma
generated in a rarefied gaseous atmosphere.
It is known to employ such prccesses for cleaning metal
surfaces. In this case the atmosphere is formed by a rare
gas, most often argon. The material, negatively polarized,
attracts the gaseous ions of the plasma, and the ionic
bombardment has a cleaning effect by the removal of material
which results in a high reactivity of the surface with
respect to the atmosphere and an increase in the roughness.
It is also known to effect surface treatments with
plasma of the nitriding or cementation type. These
treatments are carried out on materials heated to several
hundred degrees Celsius.
Further, it is known to effect surface treatments of
metal materials by conventional chemical reactions, such as
oxidation, reduction, conversion treatment, etc., for the
purpose of imparting to the surface of these materials
particular properties such as for example the improvement of
the resistance to corrosion, surface hardening, improvement
in the adhesion of coverings, coatings or various protective
layers.
It has now been found that surface treatment processes
with a plasma could be employed instead of the chemical
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treatment processes for imparting to the treated surfaces
the aforementioned particular properties.
The present invention therefore provides a process for
the surface treatment with a plasma of the type indicated at
the beginning of this specification, said process being
characterized in that the product to be treated is
maintained at low temperature, and the product is subjected
to a surface treatment with a plasma at low temperature, at
a pressure of 1 to 10~ Pa.
Plasma at low temperature, or "cold" plasma, generally
designates a plasma obtained by a luminescent discharge in
an atmosphere at low pressure (lower than 103 Pa). The
discharge is obtained at a voltage of several hundred volts,
preferably a dc voltage and moreover preferably 400 to
800 V, this voltage possibly being in particular applied
between an anode and the negatively polarized metal product
which acts as the cathode. It is also possible to
superpose on the dc voltage a variable voltage at radio
frequency. The current is preferably lower than 10 mA/cm
The product to be treated is maintained "cold", i.e. its
temperature is lower than about 300C. In practice, the
temperature is generally maintained at around ambient
temperature lower than 100C. This may be achieved by the
use of a cathode which is cooled, for example by a
circulation of water. In the case of a treatment applied to
a cold metal sheet, the latter may be maintained at a
temperature of around ambient temperature simply by means of
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sufficiently brief treatment sequences in the treatment
enclosure, possibly completed by a cooling of the supports
of the sheet, this being more particularly adapted to the
treatment of a moving sheet. The required condition is that
the rise in temperature only brought about by the
treatment (in contrast to certain known processes in which a
treatment with a plasma is achieved on a product which is
intentionally heated and brought to temperatures of several
hundred C) does not deteriorate the characteristics of the
product.
Generally, the duration of the treatment is from one
second to 10 minutes.
In the known treatment processes in which the treated
material is heated either by specific heating means or by
the very action of the plasma, the specific action of the
plasma may be combined with chemical reactions in the
treated material, of the nitriding type, owing to the
relatively high temperature of this material. In some
case~, such a treatment may favour the formation of oxides.
Contrary to this, the process according to the invention
pexmits limiting the action of the treatment with a plasma
to a surface zone of the material and, depending on the
nature of the gas or gases in which the plasma is generated,
permits for example improving the anticorrosion aptitudes of
the treated material, or the adhesion to the surface of the
latter, by specifically acting on the causes which may
adversely affect the obtainment and the perennity of the~e
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characteristics.
Thus, in the case of the application of the process
according to the invention for improving the corrosion
resistance of a steel product, such as a sheet of steel,
the treatment is carried out by the action of a plasma at a
low temperature in an atmosphere comprising at least one
molecular gas selected from among oxygen, ozone, nitrogen,
hydrogen, air, carbon dioxide, carbon monoxide, nitrogen
oxides, water, gases of combustion or mixtures thereof with
a neutral gas, the product being maintained a low
temperature.
Under the effect of the electric field, the molecules of
the gas are dissociated, excited or ionized; in the electric
discharge thus created, a low energy plasma sweeps across
the surface of the material and the various gaseous species
react with the surface atoms in accordance with their
chemical affinity. By the combination of the chemical
effect of the gas (oxidizing or reducing for example) and
the sputtering effects, a large number of elements disappear
from the treated surface. After treatment, the surface is
generally passive with respect to the atmosphere, i.e. the
conventional pollution elements C, S, P, O...
One of the most interesting characteristics of a
treatment with a molecular plasma according to the invention
is that it practically does not change the surface roughness
of thP material, even on layers having a low melting point.
In contrast, with rare gases, the erosion is greater and may
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lead to a very high reactivity with respect to the
contaminants of the atmosphere.
In the case of the application of the process according
to the invention for improving the adhesion to the surfaces
thus treated, the gaseous atmosphere preferably comprises at
least one gas selected from among hydrogen, nitrogen,
chlorinated compounds and rare gases.
Preferably, the gaseous atmosphere does not include
oxygenated compounds.
10By the application of this process, the inventors were
able to find a suppression of segregated elements on the
~ face which are h~ l to ~ adhesion i such as aluminium,
lead, calcium and magnesium oxides, silicon, manganese...
The surface treatments most generally employed up to the
present time for improving the adhesion to the surface of
ferrous products are carried out under wet conditions by
putting the surfaces to be treated in contact with acid or
alkaline chemical reagents. However, such treatments have
several drawbacks:
20most of the reagents employed are corrosive and their
use presents problems of safety and pollution,
the solution of the compounds to be eliminated from the
surface is not always selective and one is unable to avoid a
surface solution of the metal matrix resulting in a
modification in the state of the surface and a higher
reactivity with respect to the atmosphere.
The process according to the invention permits avoiding
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these drawbacks and is advantageously substituted for
chemical treatments in the preparation of products such as
metal sheets, in particular galvanized sheets intended to be
8ubsequently subjected to phosphating or chromating
treatments, to be coated with lacquers, to be assembled by a
hot pressing with a sheet of polymer, for example for
manufacturing "sandwich" sheets, or to be assembled by
adhesion.
The results obtained in the improvement of adhesion by
means of the process according to the invention can be
explained by the cleaning effect and the passivation
resulting from the combination of the mechanical action of
the ions of the plasma on the treated surface with the
chemical action of the gas, for example reduction by the
hydrogen or formation of volatile compounds, eliminated by
the pumping employed for maintaining the required low
pressure, with chlorinated gases.
As illustrative examples of the application of the
process for improving adhesion on the surface of the treated
products, characteristics and results of different tests are
mentioned hereinafter.
Example 1: Surface treatment carried out on bare mild
steel sheets.
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The treatment was carried out at a dc voltage of 400 V
and a current of 200 mA, the distance between the anode andthe product (cathode) being 4 mm, the test specimen having a
dimension of 70 x 120 mm.
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Several tests were carried out with different gases:
a) treatment for 5 minutes under an N2 ~ H2 plasma
b) treatment for 5 minutes under an N2 ~ 2 plasma
c) treatment for 5 minutes under an N2 ~ H2 plasma
followed by a treatment for 5 minutes under an N2 ~ 2
plasma.
It was found, from a subsequent analysis of the surfaces
by luminescent discharge spectrometry ~LDS), that all the
treatments eliminate the contaminants of the extreme surface
such as sulphur, phosphorus, aluminium and boron. The
calcium only disappears with a reducing treatment under
an N2 ~ H2 p~asma.
Sheet specimens treated in this way were then
phosphated.
The tests for phosphating by means of a trication bath
effected without a prior alkaline degreasing gave very good
results for the treatments N 2 ~ H2 and N2 ~ H2 followed by
N2 ~ 2 : the phosphating is fine, homogeneous and in the
form of small cubic blocks.
On the other hand, after an N 2 ~ 2 treatment alone, the
crystals are blunted, irregular and zones are non-
phosphated, which confirms the advantage of a treatment with
a plasma in a gaseous atmosphere which does not include
oxygenated compounds.
Furthermore, adhesion tests were carried out on
specimens of bare mild steel sheets.
The reference specimen is simply degreased with
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chlorotene. The specimens treated in accordance with the
invention had undergone a treatment with an N 2 ~ H2 plasma
for 4 min. at a voltage 400 V and a current 200 mA.
The adhesion was effected with a bicomponent epoxy
5 adhesive which polymerizes at ambient temperature (sold by
the firm CI13A GEIGY under the reference AW134).
The adhered assemblies were then aged by exposure for 48
hours in a hot and humid atmosphere (65C with 1009
relative humidity).
The results obtained by the 3 point bending test ~French
standards NFT 76143 and NFT 30010) are indicated in the
following table where Fmax is the maximum fracture force.
Non-aged Aged
Fmax Fmax
Reference 67 43
N2 ~ H2 Treatment 81 55
There is observed for the adhesions of the treated
products an increase in the maximum fracture force of 20% in
the non-aged state and 28% in the aged state.
Example 2: Surface treatment carried out on sheets
coated with zinc
(Galvanized IFS steel sheets of the"Monogal type"treated on
the zinc-coated side)
The treatment was carried out at 400 V and 200 mA, the
specimen having the same dimensions as before and the gases
employed being respectively N2 ~ H2 and N2 ~ 2 .
The LDS analysis of the surfaces thus treated showed
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that the treatment carried out under N - H2 permits the
rapid elimination of the extreme surface pollutants such as
P and S, the elimination of Ca, Al and Mg requiring a longer
period of about 5 minutes. Similar results were observed
with a treatment with an N2 ~ 2 plasma, but a substantially
double treatment time is required.
The analysis of the surface of the sheets thus treated
and then phosphated show that the treatment with an N2 ~ H
plasma results in a homogeneous and fine phosphating without
a prior alkaline degreasing.
After treatment under N2 ~ 2 ~ the phosphating is
slightly homogeneous.
Adhesion tests were carried out on these galvanized
sheets under the same conditions as for the previously-
mentioned bare sheets.
The results of these tests are shown in the followingtable where dmax is the maximum deformation before fracture
at the interface of the adhered assembly in the standard 3
point bending test.
Non-aged Aged
dmax dmax
Reference 0.29 0.5
N 2- 2 treatment for 5 min. 0.32 0.42
N ~ - H2 treatment for 4 min. 0.32 0.68
N 2- H2 treatment for 9 min. 0.45 1.08
These results confirm the increase in the adhesion after
treatment with an N2 -H2 plasma with respect to the merely
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degreased sheet (reference). This improvement is all the
better because the plasma cleaning is complete, which
requires a sufficient treating time depending on the initial
substrate.
The maximum deformation is increased by 55% after 9 min.
of treatment in the non-aged state and by 116~ in the aged
state.
Treatment with a gas of the N - O type has been found
to be but little effective.
Example 3: Surface treatment carried out on qalvanized
steel sheets (of the tvPe currentlv desiqnated bv the name
'Galvadur")
Four treatments were carried out on specimens of the
same product having dimensions 70 x 120 mm.:
T1: sheets subjected to an alkaline cleaning and then
to chromating, these specimens acting as comparative
references.
T2: sheet treated with an N2 ~ 2 plasma having 20%
oxygen for 5 min. at 400 V and 200 mA, then chromated.
T3: sheet treated with an N2 - H2 plasma having 10%
hydrogen for 4 min. at 400 V and 200 mA, then chromated.
T4: sheet treated with an N2 ~ 2 plasma for 5 min.,
then for 4 min. with an N2 ~ H2 plasma at 400 V and 200
mA, then chromated.
The sheets thus treated were then all covered with a
lacquer of polyester type in two coats: 5 ~um thickness of
the first coat and 15 ~um thickness of the finishing coat.
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The adhesion of the assembly thus formed is tested by
means of the 3 point bending test before and after aging.
The adhesive employed for the 3 point bending test is a
bicomponent epoxydic adhesive polymerized for 60 min. at
80C.
The results of this test are indicated in the following
table.
Non-aged state Aged state
TREATMENT Fmax (N) Fmax (N)
T1 134.9 77
T2 138.2 119.2
T3 129.9 109.1
T4 147.6 107.6
All the treatments result, before aging, in maximum
fracture forces (Fmax) which are close to one another.
On the other hand, it was observed that, for the
treatments T3 and T4 (with an N2 + H2 plasma), the fracture
occurs exclusively at the interface between the first coat
and the finishing coat, whereas for the treatments T1 and T2
(alkaline cleaning with an N2 + 2 plasma), certain fracture
zones are observed at the metal sheet - lacquer interface.
Thus it can be observed that, in the case of a treatment
with an N + H plasma, the adhesion at the metal sheet
lac~uer interface is very good.
After accelerated aging, the maximum fracture force
(Fmax) decreases relative to the non-aged state, by only
about 20% for the sheets which had undergone a treatment
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with a plasma, as against 50% for those which had undergone
the conventional chromating cycle including an alkaline
cleaning.
Fracture zones at the lacquer - metal sheet interface
are then observed.
Microscopic observation and X-ray analysis show that,
for a sheet which had undergone alkaline cleaning (T1), the
delamination starts in the oxidized zones.
On the other hand, with the treatment with an N2 + H2
plasma (T3), no presence of oxides is noted, which may
explain the small decrease in the Fmax relative to the non-
aged state.
These different tests show that the treatment according
to the invention generally permits improving the adhesion on
the surface of the sheets thus treated relative to the
conventional treatments employing an alkaline cleaning in
the case of phosphating or subsequent chromating, or
relative to the simple cleanings and degreasing employed in
the case of the adhesion. Even in the case where this
relative improvement is less distinct, this process for
surface treatment with a plasma permits replacing treatments
with corrosive chemical products and thereby eliminating the
risks related to the use of such products.
The process according to the invention is also
applicable, as already stressed, for improving the corrosion
resistance of the products thus treated, in particular
stainless steel sheets.
