MULTILAYER NON-STICK COATING OF IMPROVED HARDNESS FOR ALUMINUM ARTICLES AND ARTICLES AND CULINARY UTENSILS INCORPORATING SUCH COATING

REVETEMENT MULTICOUCHE ANTIADHESIF DE DURETE AMELIOREE POUR ARTICLES EN ALUMINIUM; ARTICLES ET USTENSILES CULINAIRES POSSEDANT CE REVETEMENT

English Abstract

A multilayer non-stick coating of improved hardness
for aluminum articles includes an alumina-based hard
first layer obtained by anodic oxidation of the aluminum
article immersed in an alkaline solution. Micro-arcs are
applied to the article by means of a high current and a
high potential difference. A second layer based on
polytetrafluoroethylene and chemical agents constitutes a
primer for one or more polytetrafluoroethylene-based
finish layers.

Claims

6
CLAIMS
1. A multilayer non-stick coating of improved
hardness and increased resistance to scratching and wear
for aluminum articles including the following layers:
- a first layer comprising .alpha.-alumina and .gamma.-
alumina covered with a surface layer of mullite, such
first layer, that is provided with a hardness of greater
than 1500 Vickers, being obtained by anodic oxidation of
the aluminum article immersed in an alkaline solution
during application of micro-arcs to said article, said
mullite layer optionally being partially or totally
removed,
- a second layer comprising polytetrafluoro-
ethylene which constitutes a primer for
- one or more finished layers comprising
polytetrafluoroethylene.
2. The coating claimed in claim 1 wherein said
first layer has a thickness in the range of 5 microns to
100 microns.
3. The coating claimed in claim 1 or 2 wherein
said second layer and said finish layer(s) have a
thickness in the range of 5 microns to 50 microns.
4. An aluminum or aluminum alloy article having a
coating as claimed in any one of claims 1 to 3.
5. An aluminum or aluminum alloy culinary utensil
having a coating as claimed in any one of claims 1 to 3.
6. Method for manufacturing an aluminum article
coated with a multilayer non-stick coating, said coated
article having improved hardness and increased resistance
to scratching and wear, said method including the
following steps:

7
a) producing directly on an aluminum article a
porous first layer comprising .alpha.-alumina and .gamma.-alumina
covered with a surface layer of mullite, such porous
first layer, that is provided with a hardness of greater
than 1500 Vickers, being produced by anodic oxidation of
the aluminum article immersed in an alkaline solution
during application of micro-arcs,
b) applying on this porous first layer a second
layer comprising polytetrafluoroethylene which
constitutes a primer for one or more finished layers
comprising polytetrafluoroethylene,
c) applying thereto one or more finished layers
comprising polytetrafluoroethylene, and
d) sintering the coating thus obtained.
7. The method claimed in claim 6, wherein it
further comprises, prior to step b), a polishing step in
order to remove some or all of the mullite layer.
8. The method claimed in claim 6 or 7, further
including, after step d), a final step of polishing part
of the coating obtained.

Description

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1
Multilayer non-stick coating of improved hardness for
aluminum articles and articles and culinary utensils
incorporating such coating
The present invention concerns a multilayer non-
stick coating of improved hardness for aluminum or
aluminum alloy articles.
The invention is also directed to aluminum or
aluminum alloy articles and in particular culinary
utensils incorporating a coating in accordance with the
invention.
Coatings based on polytetrafluoroethylene (PTFE)
applied to the inside and/or outside surface of aluminum
culinary utensils have the disadvantage of being
sensitive to scratching and to wear.
To overcome this disadvantage, it has previously
been proposed to apply the PTFE-based coating on top of a
hard underlayer obtained by anodizing the aluminum or by
thermal plasma sputtering, for example.
However, these solutions have not yielded
satisfactory results.
The aim of the present invention is to provide a
non-stick coating for aluminum articles having increased
resistance to scratching and wear.
The invention consists in a multilayer non-stick
coating of improved hardness for aluminum articles
including an alumina-based hard first layer obtained by
anodic oxidation of the aluminum article immersed in an
alkaline solution during applications of micro-arcs to
said article by means of a high current and a high
potential difference and a second layer based on
polytetrafluoroethylene and chemical agents constituting
a primer for one or more polytetrafluoroethylene-based
finish layers.
The process for obtaining the first hard layer

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based on alumina is described, for example, in an article
by FEDOROV and others published in 1983 in the Russian
journal Physics and Chemistry of Materials Processing,
under a title translating as "Composition and structure
of the strength and surface layer of aluminum alloys
obtained by micro-arc oxidation" and in author's
certificates in the name MARKOV N 1200591 published in
1989, titled in translation "Method of application of
coatings to metals and alloys", and N 1713990 published
in 1992, titled in translation "Micro-arc anodization
process for metals and alloys".
The above layer comprises Cc alumina and 'y alumina
covered with a surface layer of porous mullite (silico-
alumina).
This hard layer is much harder and much more
resistant to wear than alumina layers obtained by
conventional anodization or by thermal plasma sputtering.
We have found that PTFE-based primer and finish
layers applied to the alumina-based hard layer adhere
very well to the latter and impart to the PTFE-based non-
stick coating excellent resistance to wear and to
scratching.
Other features and advantages of the invention will
become apparent in the following description.
In the accompanying drawings, which are provided by
way of non-limiting example:
Figure 1 is a diagrammatic representation of an
installation for applying a hard alumina layer to an
aluminum article.
Figure 2 is a sectional view of the hard non-stick
coating of the invention.
The application process for an aluminum article,
for example a culinary utensil, will be described first.
Figure 1 shows a container 1 containing an alkaline
solution 2 into which dip an anode 3 and a cathode 4.

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The anode 3 is an aluminum article whose surface is
to be oxidized.
The anode 3 and the cathode 4 are connected to a
generator 5 capable of applying a high potential
difference, for example in the range 500 volts to
1 000 volts, between the anode 3 and the cathode.
The generator 5 can also generate short pulses of
high current so as to form electrical micro-arcs for
oxidizing the surface of the aluminum article 3.
This oxidation forms a hard layer 6 of alumina on
the surface of the article (see figure 2).
The layer 6 is made up mainly of a alumina and a
low percentage of y alumina.
It is covered with a surface layer 7 of mullite
(silico-alumina) which is porous.
Depending on the period of application of the
micro-arcs, the thickness of the hard alumina layer 6 can
vary in the range 5 microns to 100 microns.
The alumina layer has a much higher hardness and a
much higher resistance to wear than coatings obtained by
conventional anodization or by thermal plasma sputtering.
The Vickers hardness of this layer is greater than 1 500
whereas that of convention ceramic layers obtained by
anodization is at most equal to 450.
In accordance with the invention, the following are
applied to the hard layer 6, 7 obtained in this way: a
second layer 8 based on polytetrafluoroethylene and
chemical agents to constitute a primer and then one or
two polytetrafluoroethylene-based finish layers 9, 10.
The layers 8, 9, 10 can have a total thickness in
the range 5 microns to 50 microns.
The table below gives examples of compositions of
the various layers 8, 9, 10 in the case of a non-stick
coating for culinary utensils.

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Component Layer 8 Layer 9 Layer 10
weight % weight % weight %
Silica sol precipitated to 10 - 30 0 0
30% dry extract in aqueous
SNOWTEX" C.30 solution
Polytetrafluoroethylene, 20 - 50 80 - 90 80 - 90
60% dzy extract, aqueous
dispersion. (* )
Perfluoroalkoxy, 50% dry 0 - 20 0 0
extract, aqueous dispersion
(PFA 6900 HOECHST)
Mica flakes coated with 0 0 - 3 0 - 3
Titanium dioxide
Iron oxide or carbon black 0 - 5 0 - 0.5 0
mineral pigments
Emulsion of spreading 0 - 15 10 - 20 10 - 20
agents, 15% dry extract
including approximately
5% - 10% acrylic copolymers
Polyamide-imide resin in 0 - 40 0 0
aqueous solution, 12% dry
extract
(*) Polytetrafluoroethylene dispersion is
preferably Daikin" DIK or D46
After application of layers 8, 9 and 10 the coating
obtained is sintered at a temperature in the range
400 C to 420 C for a period in the range 3 minutes to
minutes.
The PTFE-based layer 8, 9 and 10 are found to
adhere very well to the alumina-based hard layer 6, 7.
10 This result is explained by the porous nature of the
alumina layer 6, 7. Because of this, particles of PTFE
in the primer layer 8 can penetrate into the pores of the

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mullite layer 7 to assure excellent adhesion of the
layer 8.
Before applying the PTFE coating the surface can be
polished to remove some or all of the mullite and thereby
5 obtain a smoother surface.
The PTFE coating can be applied to the hard layer
either when polished as described above or not, as
previously, and polished after sintering it.
In all cases the same result is obtained: a coating
that is extremely resistant to scratching and having
the non-stick properties of a conventional
polytetrafluoroethylene coating, because the PTFE is
lodged in the pores present throughout the thickness of
the layer of A1203 and mullite.
The present invention is intended particularly for
non-stick coatings of culinary articles, but applies
equally to any article on which a slippery surface having
excellent hardness and resistance to wear is required
(for example the hotplate of an iron).

Representative Drawing