U.S. patent application number 16/149811 was filed with the patent office on 2019-06-06 for methods for improving adhesion of aluminum films.
The applicant listed for this patent is ALUMIPLATE, INC.. Invention is credited to Lucy Elizabeth Browning, William Charles Carlson, Jon Frederick Schulz, Gustavo Rolando Vallejo.
Application Number | 20190169763 16/149811 |
Document ID | / |
Family ID | 51391740 |
Filed Date | 2019-06-06 |
View All Diagrams
United States Patent
Application |
20190169763 |
Kind Code |
A1 |
Browning; Lucy Elizabeth ;
et al. |
June 6, 2019 |
METHODS FOR IMPROVING ADHESION OF ALUMINUM FILMS
Abstract
The described embodiments relate generally to aluminum films and
pretreatments for improving the adhesion of aluminum films on
substrate surfaces. Methods involve providing three-dimensional
adhesion surfaces on the substrate that promote adhesion to a
subsequently deposited aluminum film. The methods can avoid the use
of strike materials, such as nickel and copper, used in
conventional adhesion-promoting treatments. According to some
embodiments, methods involve providing an aluminum oxide adhesion
layer on the substrate prior to depositing aluminum. According to
some embodiments, methods involve providing a zincating layer on
the substrate prior to depositing aluminum. According some
embodiments, methods involve roughening the surface of the
substrate prior to depositing aluminum. Some embodiments involve a
combination of two or more substrate pretreatments. Described
methods can be used to provide more flexibility in subsequent
anodizing processes. In some embodiments, methods involve anodizing
the aluminum film and a portion of the substrate.
Inventors: |
Browning; Lucy Elizabeth;
(Cupertino, CA) ; Carlson; William Charles; (Coon
Rapids, MN) ; Schulz; Jon Frederick; (Coon Rapids,
MN) ; Vallejo; Gustavo Rolando; (Coon Rapids,
MN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ALUMIPLATE, INC. |
Coon Rapids |
MN |
US |
|
|
Family ID: |
51391740 |
Appl. No.: |
16/149811 |
Filed: |
October 2, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14768493 |
Aug 18, 2015 |
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PCT/US2014/016913 |
Feb 18, 2014 |
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16149811 |
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61766633 |
Feb 19, 2013 |
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61907323 |
Nov 21, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25D 5/48 20130101; C25D
3/44 20130101; C25D 5/10 20130101; C23C 28/322 20130101; C23C
18/1653 20130101; C23C 28/345 20130101; C25D 17/008 20130101; C23C
28/028 20130101; C25D 5/44 20130101; C23C 18/54 20130101; C25D
11/16 20130101; C25D 17/08 20130101; B32B 15/016 20130101; C23C
28/023 20130101; C25D 11/12 20130101; C25D 11/04 20130101 |
International
Class: |
C25D 5/10 20060101
C25D005/10; C23C 28/00 20060101 C23C028/00; C25D 11/16 20060101
C25D011/16; C23C 28/02 20060101 C23C028/02; C23C 18/54 20060101
C23C018/54; C25D 5/44 20060101 C25D005/44; C25D 11/12 20060101
C25D011/12; B32B 15/01 20060101 B32B015/01; C25D 11/04 20060101
C25D011/04; C25D 5/48 20060101 C25D005/48; C25D 3/44 20060101
C25D003/44; C25D 17/00 20060101 C25D017/00 |
Claims
1. A method of forming a protective coating on a surface of an
aluminum substrate, the method comprising: forming an
adhesion-promoting layer on a surface of the aluminum substrate,
the adhesion-promoting layer having a plurality of cavities having
side walls oriented substantially normal to the surface of the
aluminum substrate, wherein the adhesion-promoting layer is
chemically compatible with a subsequent anodizing process; and
depositing an aluminum layer on the adhesion-promoting layer, the
aluminum layer having a plurality of anchor portions disposed
within corresponding cavities of the adhesion-promoting layer,
wherein the anchor portions engage with the side walls of the
adhesion-promoting layer resisting a shearing force applied to the
aluminum layer securing the aluminum layer to the
adhesion-promoting layer.
2. The method of claim 1, further comprising: converting at least a
portion of the aluminum layer to an aluminum oxide layer using an
anodizing process.
3. The method of claim 2, wherein substantially the entire aluminum
layer is converted to the aluminum oxide layer.
4. The method of claim 3, further comprising: during the anodizing,
transcending a boundary between the adhesion-promoting layer and
the aluminum substrate converting a portion of the aluminum
substrate to a second aluminum oxide layer.
5. The method of claim 1, wherein the aluminum substrate makes up
at least a portion of an enclosure for an electronic device.
6. The method of claim 1, wherein the adhesion-promoting layer is
substantially free of copper or nickel.
7. The method of claim 1, wherein the aluminum oxide adhesion layer
has a thickness of about 3 micrometers or less.
8. The method of claim 1, wherein forming the adhesion-promoting
layer comprises: forming an aluminum oxide adhesion layer on the
surface of the aluminum substrate, the aluminum oxide adhesion
layer having a plurality of pores defined by a plurality of
corresponding pore walls, wherein the plurality of pores have an
average pore size sufficiently large to allow anchoring portions of
the aluminum layer to form therein during the subsequent aluminum
layer depositing process.
9. The method of claim 8, wherein the aluminum oxide adhesion layer
has a thickness of about 3 micrometers or less.
10. The method of claim 1, wherein forming the adhesion-promoting
layer comprises: forming a zincate layer on the aluminum substrate,
the zincate layer having a crystalline structure having the
plurality of cavities.
11. The method of claim 10, wherein the zincate layer has a
thickness of less than about 0.5 micrometers.
12. The method of claim 1, further comprising: prior to forming the
adhesion-promoting layer, roughening the surface of the aluminum
substrate.
13. The method of claim 12, wherein the aluminum substrate is
comprised of an aluminum alloy.
14. The method of claim 1, wherein the aluminum substrate is
comprised of an aluminum alloy.
15. A method for forming an aluminum layer on a substrate, the
method comprising: forming an aluminum oxide adhesion layer on the
substrate, the aluminum oxide adhesion layer having a plurality of
pores defined by a plurality of corresponding pore walls; during
the forming, controlling an average pore size of the aluminum oxide
adhesion layer by simultaneously allowing growth of the pore walls
and dissolving the pore walls and dissolving that the average pore
size is sufficiently large to allow aluminum material to form
therein during a subsequent aluminum layer depositing process; and
depositing the aluminum layer on the aluminum oxide adhesion layer,
wherein during the depositing, anchoring portions of the aluminum
layer form within at least a portion of corresponding pores,
wherein the anchor portions engage with the pore walls resisting a
shearing force applied to the aluminum layer and securing the
aluminum layer to the aluminum oxide layer.
16. The method of claim 15, wherein the substrate is comprised of
an anodizable material, the method further comprising: converting
substantially the entire aluminum layer to an aluminum oxide layer
and converting a portion of the substrate to an oxide layer.
17. The method of claim 15, wherein the substrate is comprised of
an aluminum alloy.
18. The method of claim 16, wherein the substrate is comprised of
an aluminum alloy.
19. The method of claim 15, wherein the aluminum layer is comprise
of substantially pure aluminum.
20. The method of claim 15, wherein the aluminum substrate makes up
at least part of an enclosure for an electronic device.
21. (canceled)
22. (canceled)
23. (canceled)
24. (canceled)
25. (canceled)
26. (canceled)
27. (canceled)
28. (canceled)
29. (canceled)
30. (canceled)
Description
FIELD
[0001] This disclosure relates generally to aluminum films and
methods for depositing aluminum films. In particular, described are
various methods for improving adhesion of deposited aluminum
films.
BACKGROUND
[0002] Electroplating is a process widely used in industry to
provide a metal coating having a desirable physical quality on a
part. For example, electroplated coatings can provide abrasion and
wear resistance, corrosion protection and aesthetic qualities to
the surfaces of parts. Electroplated coating may also be used to
build up thickness on undersized parts.
[0003] Aluminum substrates, in particular, can be difficult to
plate since aluminum surfaces rapidly acquire an oxide layer when
exposed to air or water, and thus tend to inhibit good adhesion of
an electrodeposited film. In addition, since aluminum is one of the
more anodic metals, there is a tendency to form unsatisfactory
immersion deposits during exposure to a plating solution, which can
cause discontinuous plating or breakdown of the plating process.
Furthermore, if plating an aluminum film, plating methods usually
involve the plating of pure aluminum metal onto the substrate.
Although pure aluminum has an ordered microstructure and good
cosmetic properties, it is relatively soft and easily scratched.
Therefore, there are significant challenges to plating aluminum in
industrial applications where durability is a desirable
characteristic of a plated film.
SUMMARY
[0004] This paper describes various embodiments that relate to
aluminum films with improved adhesion.
[0005] According to one embodiment, a method for forming a
protective coating on a surface of an aluminum substrate is
described. The method includes forming an adhesion-promoting layer
on a surface of the aluminum substrate. The adhesion-promoting
layer has a number of cavities having side walls oriented
substantially normal to the surface of the aluminum substrate. The
adhesion-promoting layer is chemically compatible with a subsequent
anodizing process. The method also includes depositing an aluminum
layer on the adhesion-promoting layer. The aluminum layer has a
number of anchor portions disposed within corresponding cavities of
the adhesion-promoting layer. The anchor portions engage with the
side walls of the adhesion-promoting layer resisting a shearing
force applied to the aluminum layer securing the aluminum layer to
the adhesion-promoting layer.
[0006] According to an additional embodiment, a method for forming
an aluminum layer on a substrate is described. The method includes
forming an aluminum oxide adhesion layer on the substrate. The
aluminum oxide adhesion layer has a number of pores defined by a
plurality of corresponding pore walls. The method also includes,
during the forming, controlling an average pore size of the
aluminum oxide adhesion layer by simultaneously allowing growth of
the pore walls and dissolving the pore walls such that the average
pore size is sufficiently large to allow aluminum material to form
therein during a subsequent aluminum layer depositing process. The
method also includes depositing the aluminum layer on the aluminum
oxide adhesion layer. During the depositing, anchoring portions of
the aluminum layer are formed within at least a portion of
corresponding pores. The anchor portions engage with the pore walls
resisting a shearing force applied to the aluminum layer securing
the aluminum layer to the aluminum oxide layer.
[0007] According to a further embodiment, a composite coating for
an aluminum substrate is described. The composite coating includes
a first aluminum oxide layer disposed on the aluminum substrate.
The first aluminum oxide layer has a first hardness. The composite
coating also includes a second aluminum oxide layer disposed on the
first aluminum oxide layer. The second aluminum oxide layer being
more optically transparent than the first aluminum oxide layer. The
first aluminum oxide layer is integrally bounded to the second
oxide layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] The described embodiments and the advantages thereof may
best be understood by reference to the following description taken
in conjunction with the accompanying drawings. These drawings in no
way limit any changes in form and detail that may be made to the
described embodiments by one skilled in the art without departing
from the spirit and scope of the described embodiments.
[0009] FIGS. 1A-1C show cross-section views of a part undergoing a
pretreatment involving an anodizing process to improve adhesion of
a deposited aluminum layer.
[0010] FIGS. 2A and 2B show cross-section scanning electron
microscope (SEM) views of a part that includes an aluminum oxide
adhesion layer formed using a phosphoric acid anodizing
process.
[0011] FIGS. 3A-3C show cross-section views of a part undergoing a
pretreatment involving a zincating process to improve adhesion of a
deposited aluminum layer.
[0012] FIGS. 4A-4C show cross-section views of a part undergoing a
pretreatment involving a surface roughening process to improve
adhesion of a deposited aluminum layer.
[0013] FIGS. 5A-5C shows cross-section views of a part undergoing
aluminum depositing and anodizing processes where a portion of
substrate is anodized.
[0014] FIG. 6 shows a flowchart indicating a high-level process
involving substrate pretreatment to improve adhesion of a deposited
aluminum layer.
[0015] FIG. 7 shows a plating rack assembly suitable for plating a
number of parts.
DETAILED DESCRIPTION
[0016] Representative applications of methods and apparatus
according to the present application are described in this section.
These examples are being provided solely to add context and aid in
the understanding of the described embodiments. It will thus be
apparent to one skilled in the art that the described embodiments
may be practiced without some or all of these specific details. In
other instances, well known process steps have not been described
in detail in order to avoid unnecessarily obscuring the described
embodiments. Other applications are possible, such that the
following examples should not be taken as limiting.
[0017] This application relates to aluminum films and providing
aluminum films on substrates. As used herein, the terms "film" and
"layer" are used interchangeably. Unless otherwise described, as
used herein, "aluminum" and "aluminum layer" can refer to any
suitable aluminum-containing material, including pure aluminum,
aluminum alloys or aluminum mixtures. As used herein, "pure" or
"nearly pure" aluminum generally refers to aluminum having a higher
percentage of aluminum metal compared to aluminum alloys or other
aluminum mixtures. The aluminum films are well suited for providing
both protective and attractive layers to consumer products. For
example, methods described herein can be used for providing
protective and cosmetically appealing coatings for enclosures and
casings for electronic devices.
[0018] Described herein are methods for improving adhesion of
deposited aluminum layers on a substrate. Methods described herein
can be used to improve the adhesion of an aluminum layer to a
substrate without the use of a strike layer. Methods involve
substrate pretreatments prior to depositing of an aluminum layer.
The pretreatments providing a three-dimensional surface having gaps
or cavities on the substrate that can act as anchoring regions for
securing the aluminum layer to the substrate. In some embodiments,
methods involve providing a thin aluminum oxide adhesion layer on
the substrate prior to depositing aluminum. In some embodiments,
methods involve providing a zincating layer on the substrate prior
to depositing aluminum. In some embodiments, methods involve
roughening the surface of the substrate prior to depositing
aluminum. Some embodiments involve a combination of two or more
substrate pretreatments.
[0019] These and other embodiments are discussed below with
reference to FIGS. 1-7. However, those skilled in the art will
readily appreciate that the detailed description given herein with
respect to these Figures is for explanatory purposes only and
should not be construed as limiting.
[0020] As described above, it can be difficult to deposit onto
aluminum substrates since aluminum substrates quickly acquire a
natural oxide layer when exposed to air or water. The natural oxide
layer can inhibit the adhesion of many metal materials, such as
aluminum, to the surface of the aluminum substrate. Conventional
methods for providing better adhesion include forming a thin layer
of copper or nickel plating, referred to as a strike or strike
layer. The strike layer generally has good adhesion to the aluminum
substrate and also to the subsequently deposited aluminum layer.
However, use of a strike layer can have has several disadvantages.
For example, a strike layer can make the part more susceptible to
galvanic corrosion during a plating process. In particular, if
external coating layers are scratched exposing the strike layer
next to the plated aluminum layer (and possibly the aluminum
substrate), the exposure of dissimilar materials can create a
galvanic cell on the part. This can increase the risk of corrosion
in the plated and anodized part later on.
[0021] There can also be manufacturing challenges to using a strike
layer. In some manufacturing processes, an entire aluminum layer is
converted to aluminum oxide using an anodizing process. During the
anodizing process, the strike layer can be exposed in localized
areas creating a varied current density distribution across the
part. Locally thinner areas of the aluminum layer can become
anodized through sooner, resulting in an anodized layer having a
varying thickness across the part. Furthermore, materials from the
strike layer can contaminate the anodizing bath and create defects
in the resultant aluminum oxide. To avoid exposing the strike layer
during these processes, a buffer layer of plated aluminum can be
positioned between the substrate and the aluminum oxide layer.
However, the buffer layer can add thickness to the overall aluminum
and aluminum oxide stack.
[0022] To avoid reaching the strike layer, a buffer layer of
aluminum can be left between the strike layer and the remainder of
the aluminum layer. Since the thickness of the aluminum layer can
be variable across a part (due to variations in current density),
the thickness of the buffer layer is generally dictated by the
minimum thickness across the part. One of the disadvantages of
using a buffer layer is that it can add an undesired extra
thickness to the aluminum and aluminum oxide stack. In addition, if
during an anodizing process the aluminum oxide layer grows too
close to or beyond the thickness of the aluminum layer, the
anodizing solution can contact and react with the strike layer. The
reaction products can contaminate the anodizing solution and cause
defects in the resultant aluminum oxide layer. For at least these
reasons, it can be beneficial in certain applications to avoid the
use of a strike layer. However, it can be difficult to plate
aluminum directly onto substrates since aluminum generally does not
adhere well to substrates during electroplating, especially when
plating pure or nearly pure aluminum. In addition, if the substrate
is also made of aluminum, the aluminum substrate has a strong
affinity to form a natural oxide layer on its surface making it
difficult to plate thereon.
[0023] In order to improve the adhesion of an aluminum layer on a
substrate, methods described herein involve pretreating the
substrate prior to an aluminum deposition process. The
pretreatments avoid the use of a strike layer and, therefore, do
not include some of the downsides of using a strike layer. The
pretreatments involve creating a three-dimensional
adhesion-promoting surface on the substrate. When an aluminum layer
is deposited on the adhesion-promoting surface, portions of the
aluminum can become deposited within the gaps or cavities of the
three-dimensional adhesion-promoting surface. These portions of the
aluminum layer can anchor the aluminum layer to the substrate
surface and provide better adhesion of the aluminum layer to the
substrate. The adhesion-promoting layer can be substantially free
of non-aluminum metal agents, such as agents containing copper
and/or nickel, and therefore chemically compatible with a
subsequent anodizing process. In some embodiments, creating the
adhesion-promoting surface involves forming an aluminum oxide
adhesion layer on the substrate surface, which is described below
with reference to FIGS. 1A-1C and 2A-2B. In other embodiments,
creating the adhesion-promoting surface involves forming a
zincating layer on the substrate surface, which is described below
with reference to FIGS. 3A-3C. In other embodiments, creating the
adhesion-promoting surface involves roughening the substrate
surface, which is described below with reference to FIGS. 4A-4C. It
should be understood that these embodiments are presented as
suitable examples and are not meant to limit the types and scope of
possible methods of providing adhesion-promoting surfaces.
[0024] One way of forming an adhesion-promoting surface on a
substrate is by forming a thin aluminum oxide layer that has
adhesion-promoting properties on the substrate. FIGS. 1A-1C show
cross-section views of part 100 undergoing an aluminum deposition
process involving formation of an aluminum oxide adhesion layer in
accordance with some embodiments. At FIG. 1A, a thin portion of
aluminum substrate 102 is converted to aluminum oxide adhesion
layer 104. Prior to forming aluminum oxide adhesion layer 104, the
top surface of aluminum substrate 102 can be treated with any
suitable finishing technique. For example, aluminum substrate 102
can be polished to have mirror shine. In other embodiments,
aluminum substrate 102 is textured to have a textured or roughened
surface. Since forming aluminum oxide adhesion layer 104 is a
conversion process whereby a portion of aluminum substrate 102 is
consumed, aluminum oxide adhesion layer 104 is integral to and well
adhered to aluminum substrate 120. Aluminum oxide adhesion layer
104 should be thick enough to create a good anchoring surface and
thin enough so that the surface of substrate 102 remains conductive
for a subsequent electroplating process. In some embodiments,
aluminum oxide adhesion layer 104 has a thickness of less than
about 5 micrometers. In some embodiment, aluminum oxide adhesion
layer 104 has a thickness of about 3 micrometers or less.
[0025] Aluminum oxide adhesion layer 104 can be formed using an
anodizing process that includes the use of an acidic electrolytic
solution. In some embodiments, the electrolytic solution includes
phosphoric acid, oxalic acid, or a combination of phosphoric acid
and oxalic acid. The phosphoric and/or oxalic acid can promote the
formation of pores 105 having a larger average diameter compared to
the average diameter of standard anodic pores. In addition,
aluminum oxide adhesion layer 104 generally has lower pore density
compared to standard anodized aluminum oxide layers. It is believed
that the phosphoric and/or oxalic acid tends to dissolve portions
of the pore walls of pores 105 as pores 105 are being grown,
thereby creating the larger diameter pores 105 and lower pore
density. That is, anodizing in phosphoric acid and/or oxalic acid
conditions allow for simultaneous growth and dissolving of the pore
walls. In some embodiments, pores 105 have a diameter of about 100
nm or greater. In some embodiments, the acidic electrolytic
solution contains chromic acid and/or sulfuric acid. In some
embodiments, the acidic electrolytic solution contains a mixture of
two or more of phosphoric acid, oxalic acid, chromic acid, and
sulfuric acid. In some embodiments, aluminum oxide adhesion layer
104 is exposed to an inert atmosphere prior to a subsequent
aluminum plating process in order to activate aluminum oxide layer
104 and promote better adhesion with the plated aluminum. The inert
atmosphere can include exposing part 100 to a non-oxidative
atmosphere such as a nitrogen or argon environment.
[0026] At FIG. 1B, aluminum layer 106 is deposited on aluminum
oxide adhesion layer 104. Aluminum layer 106 can be deposited using
any suitable process including a plating process, such as an
electroplating process. In some embodiments, aluminum layer 106 is
deposited on aluminum oxide adhesion layer 104 very soon after
formation of aluminum oxide adhesion layer 104 to avoid exposure of
aluminum oxide adhesion layer 104 to moisture or air. In some
embodiments, aluminum oxide adhesion layer 104 is placed in a
moisture free atmosphere, such as a nitrogen or argon environment.
As shown, pores 105 are sufficiently large such that anchoring
portions 107 of aluminum layer 106 can form within pores 105. Pore
walls 105 are substantially normal to the top surface of substrate
102 such that when a shearing force 109 is applied to aluminum
layer 106, anchoring portions 107 can engage with pore walls 105
and secure aluminum layer 106 to aluminum oxide adhesion layer 104.
The adhesion strength of aluminum layer 106 to aluminum oxide
adhesion layer 104 can be measured by a standard pull test. In some
embodiments, the adhesion of aluminum layer 106 to aluminum oxide
adhesion layer 104, as measured by pull testing, ranges from about
70 MPa and about 85 MPa.
[0027] At FIG. 1C, a portion of aluminum layer 106 is optionally
converted to aluminum oxide layer 108. As shown, aluminum oxide
layer 108 has pores 112 that are generally smaller in diameter
compared to pores 105 of aluminum oxide adhesion layer 104.
Aluminum oxide layer 108 can be formed using any suitable process,
including conventional anodizing processes. Note that part 100 does
not include a strike layer, thereby eliminating the occurrence of
defects caused by anodizing interfering materials in a strike
layer. That is, there is no chance of reaching a defect-causing
material during an anodizing process. Thus, aluminum layer 106 can
be thinner than a corresponding aluminum layer using a strike
layer, thereby reducing the stack thickness of aluminum oxide
adhesion layer 104, aluminum layer 106, and aluminum oxide layer
108. In some embodiments, substantially all of aluminum layer 106
is converted to an aluminum oxide layer. In some embodiments,
substantially all of aluminum layer 106 and a portion of aluminum
substrate 102 are converted to an aluminum oxide layer. These
embodiments are achievable because there is no intervening strike
layer. That is, aluminum oxide adhesion layer 104 is compatible
with an anodizing process.
[0028] FIGS. 2A and 2B shows cross-section SEM (scanning electron
microscope) images of an actual aluminum part 200 illustrating the
features described above with reference to FIGS. 1A-1C. FIG. 2B is
an inset showing a close-up view of a portion of the SEM image of
FIG. 2A. Part 200 includes aluminum substrate 202, aluminum oxide
adhesion layer 204, aluminum layer 206, and aluminum oxide layer
208. Aluminum oxide adhesion layer 204 was formed using an
anodizing solution containing phosphoric acid. As clearly shown in
the inset FIG. 2B, portions of aluminum layer 206 deposit within
the pores of aluminum oxide adhesion layer 204, mechanically
interlocking aluminum layer 206 and aluminum oxide adhesion layer
204 together. The pores of aluminum oxide adhesion layer 204 are
substantially normal to the top surface of substrate 202 such that
when a shearing force is applied to aluminum layer 206, the
deposited portions of aluminum layer 206 can engage with the pore
walls of the pores and secure aluminum layer 206 to aluminum oxide
adhesion layer 204.
[0029] Another way forming an adhesion-promoting surface on a
substrate is by forming a zincating layer on the substrate. FIGS.
3A-3C show cross-section views of part 300 undergoing an aluminum
deposition process involving formation of a zincating layer in
accordance with some embodiments. At FIG. 3A, zincating layer 304
is deposited onto aluminum substrate 302. Zincating layer 304 is
generally a thin conductive crystalline layer that can be formed by
exposing aluminum substrate 302 to a zincate solution. The zincate
solution contains tetrahydroxozincating ion (Zn(OH).sub.4.sup.2),
which can remove a natural oxide layer that forms on aluminum
substrate 302. Once formed, the zincating layer 304 can prevent
re-oxidation of aluminum substrate 302. In some embodiments, a
cyanide multi-metal based zincatc solution is used. Zincating layer
304 is generally chemically computable with a subsequent anodizing
process. In some embodiments, zincating layer 304 is very thin, in
some cases less than about 0.5 micrometers thick. In some
embodiments, zincating layer 304 is exposed to an inert atmosphere
prior to a subsequent aluminum plating process in order to promote
better adhesion with the plated aluminum.
[0030] At FIG. 3B, aluminum layer 306 is deposited on zincating
layer 304. As shown by the inset view, zincating layer 304 has a
three-dimensional crystalline structure that includes cavities
surrounded by walls 305. When aluminum layer 306 is deposited onto
zincating layer 304, anchoring portions 307 become deposited within
the cavities of zincating layer 304. Walls 305 can be substantially
normal to the top surface of substrate 302 such that when a
shearing force 309 is applied to aluminum layer 306, anchoring
portions 307 can engage with walls 305 and secure aluminum layer
306 to substrate 302. In some embodiments, the adhesion of aluminum
layer 306 to zincating layer 304, as measured by pull testing,
ranges from about 30 MPa and about 65 MPa.
[0031] At FIG. 3C, a portion of aluminum layer 306 is optionally
converted to aluminum oxide layer 308, which has anodic pores 312
formed therein. Aluminum oxide layer 308 can be formed using any
suitable process, including a conventional anodizing process.
Zincating layer 304 allows for the elimination of a strike layer
within part 300, thereby eliminating the occurrence of defects
caused by anodizing interfering materials in a strike layer. Thus,
in some embodiments, substantially all of aluminum layer 306 is
converted to an aluminum oxide layer. In some embodiments,
substantially all of aluminum layer 306 and a portion of aluminum
substrate 302 are converted to an aluminum oxide layer.
[0032] An additional way of forming an adhesion-promoting surface
on a substrate is by creating a textured or roughened surface on
the substrate. FIGS. 4A-4C show a cross-section view of part 400
undergoing an aluminum deposition process involving substrate
surface roughening in accordance with some embodiments. At FIG. 4A,
top surface of aluminum substrate 402 is textured to have a rough
surface 404 having a series of peaks and valleys. Any suitable
surface texturing or roughing process can be used. In some
embodiments, a blasting procedure is used, whereby a blasting media
is impinged on top surface of aluminum substrate 402. In some
embodiments, a laser texturing procedure is used, whereby a
continuous or pulsed laser is scanned across the top surface of
aluminum substrate 402 creating random or organized patterns of
pits. In some embodiments, an acid etching procedure is used,
whereby an acidic solution etches and creates a roughened top
surface of aluminum substrate 402. In some embodiments, aluminum
substrate 402 is exposed to an inert atmosphere prior to a
subsequent aluminum plating process in order to promote better
adhesion with the plated aluminum.
[0033] At FIG. 4B, aluminum layer 406 is deposited on aluminum
substrate 402. As shown, anchoring portions 407 are formed within
the valleys of roughened surface 404. Walls 405 of the valleys of
roughened surface 404 are substantially normal to the top surface
of substrate 402 such that when a shearing force 1309 is applied to
aluminum layer 406, anchoring portions 407 engage walls 405
securing aluminum layer 406 to substrate 402. Note that each of the
valleys of rough surface 404 are generally larger in width compared
to the width of each pore 105 of aluminum oxide adhesion layer 104
described above with reference to FIGS. 1A-1C. In some embodiments,
the adhesion of aluminum layer 306 to substrate 402, as measured by
pull testing, is about 29 MPa.
[0034] At FIG. 4C, a portion of aluminum layer 406 is optionally
converted to aluminum oxide layer 408, which has pores 412 formed
therein. Aluminum oxide layer 408 can be formed using any suitable
process, including a conventional anodizing process. In some
embodiments, substantially all of aluminum layer 406 is converted
to an aluminum oxide layer. Roughened surface allows for the
elimination of a strike layer within part 400, thereby eliminating
the occurrence of defects caused by anodizing interfering materials
in a strike layer. Thus, in some embodiments, substantially all of
aluminum layer 406 is converted to an aluminum oxide layer. In some
embodiments, substantially all of aluminum layer 406 and a portion
of aluminum substrate 402 are converted to an aluminum oxide
layer.
[0035] In some embodiments, one or more of the above-described
pretreatment techniques can be used in combination. For example, an
aluminum substrate can be treated with a surface roughening
process, followed by formation of an aluminum oxide adhesion layer,
and followed by deposition of an aluminum layer. In another
embodiment, an aluminum substrate is treated with a surface
roughening process, followed by formation of a zincating layer, and
followed by deposition of an aluminum layer. In some embodiments,
combining multiple pretreatment techniques can improve the adhesion
of an aluminum layer to a substrate.
[0036] As described above, one of the advantages of the absence of
a strike layer is that more portions of the aluminum layer, and
possibly the substrate itself, can be converted to aluminum oxide
without creating strike layer material defects. This allows for
more flexibility during a subsequent anodizing process and the more
possible variations in forming aluminum oxide layers on a
substrate. FIGS. 5A-5C shows cross-section views of part 500
undergoing aluminum depositing and anodizing processes where a
portion of substrate is anodized.
[0037] At FIG. 5A, adhesion-promoting surface 504 is formed on
aluminum substrate 502. Adhesion-promoting surface 504 is any
suitable surface that has a three-dimensional quality that allows
for formation of anchoring portions during a subsequently aluminum
depositing process. As described above, adhesion-promoting surface
504 can correspond to a surface of an aluminum oxide adhesion
layer, a surface of a zincating layer, or a roughed surface of
substrate 502. Substrate 502 can be made of any suitable anodizable
metal or metal alloy.
[0038] At FIG. 5B, aluminum layer 506 is deposited on
adhesion-promoting layer. Aluminum layer 506 can have the same or
different composition as substrate 502. In one embodiment, aluminum
layer 506 and substrate 502 are both made of the substantially the
same aluminum alloy. In some embodiments, aluminum layer 506 is
made of substantially pure aluminum and substrate 502 is made of an
aluminum alloy. Embodiments where aluminum layer 506 is
substantially pure aluminum may be preferable in applications where
it is desirable to have an optically brighter top layer for part
500. Substrate 502 can be made of aluminum alloy since aluminum
alloy is generally harder than pure aluminum and can provide good
structural support for part 500.
[0039] At FIG. 5C, substantially all of aluminum layer 506 is
converted to first oxide layer 508 and a portion of substrate 502
is converted to second oxide layer 510, forming a composite coating
for part 500. Since first oxide layer 508 and second oxide layer
510 are formed during a single anodizing process, first oxide layer
508 can be integrally bonded with second oxide layer 510. First
oxide layer 508 and second oxide layer 510 are separated by
interface 514. Pores 512 formed during the anodizing process can be
formed within first oxide layer 508, transcend interface 514 and
continue to within second oxide layer 510. In embodiments where
aluminum layer 506 and substrate 502 have different compositions,
first oxide layer 508 and second oxide layer 510 can have different
physical qualities and/or appearances. For example, an aluminum
oxide layer resulting from conversion of a pure aluminum layer can
be more optically transparent or translucent compared to an
aluminum oxide layer resulting from conversion of an aluminum alloy
layer. That is, aluminum oxide obtained from aluminum alloys can
appear more yellow and have more of a hazy or matt quality. First
oxide layer 508 and second oxide layer 510 can also have different
hardness qualities. In one embodiment, second oxide layer 508 is
harder than first oxide layer 510.
[0040] FIG. 6 shows flowchart 600 indicating a high-level process
involving substrate pretreatment and aluminum depositing, in
accordance with described embodiments. At 602, a surface of a
substrate is pretreated forming an adherence-promoting surface. The
substrate can be made of an anodizable material such as aluminum or
alloys thereof. The pretreatments can include providing a
three-dimensional surface that has gaps or cavities. Examples of
pretreatments include one or more of forming an aluminum oxide
adherence layer, forming a zincating layer and providing a roughen
substrate surface. In some embodiments, the adhesion-promoting
layer has a thickness of less than about 3 micrometers. At 604, the
adherence-promoting surface is optionally activated by exposing the
adherence-promoting surface to an inert atmosphere. Suitable inert
atmospheres can include exposure to nitrogen and/or argon gas.
[0041] At 606, an aluminum layer is deposited onto the
adherence-promoting surface of the substrate. In some embodiments,
the aluminum layer is deposited using a plating process, such as an
electroplating process. The aluminum layer can have substantially
the same or different composition as the substrate. In one
embodiment, the substrate is made of an aluminum alloy and the
aluminum layer is made of substantially pure aluminum. The aluminum
layer can be deposited to any suitable thickness. In some
embodiments, the aluminum layer is deposited to a thickness ranging
from about 1 micrometer and about 10 micrometers. In some
embodiments, the aluminum layer is deposited to a thickness ranging
from about 2 micrometers and about 4 micrometers.
[0042] At 608, at least a portion of the aluminum layer of the
aluminum layer and the substrate is optionally converted to an
oxide layer. In some embodiments, an anodizing process is used to
form the oxide layer. In some embodiments, only a portion of the
aluminum layer is converted to an aluminum oxide layer. The absence
of a strike layer makes it possible to allow the anodizing process
to convert a relatively larger percentage of the aluminum layer
without concern as to strike layer material related defects. Thus,
in some embodiments, substantially the entire aluminum layer,
including portions proximate the substrate, is converted to
aluminum oxide. In some embodiments, substantially the entire
aluminum layer is converted to an aluminum oxide layer and a
portion of the substrate is converted to an oxide layer. Anodizing
process conditions can be chosen such that the aluminum oxide layer
is durable and cosmetically appealing. In general, an aluminum
oxide layer converted from a substantially pure aluminum layer can
have a relatively transparent or translucent visual quality
compared to an aluminum oxide layer converted from an aluminum
alloy.
[0043] In a production environment, a number of parts can be
plating in a single plating bath. The parts can be situated on a
rack assembly, such as rack assembly 700 shown in FIG. 7. Rack
assembly 700 is configured to support parts 702a-7021 during a
plating process and, in some embodiments, during processes prior to
or subsequent to a plating process. For example, rack assembly 700
can be used to support parts 702a-7021 during forming of an
adhesion-promoting surface, during exposure to an inert atmosphere,
during a plating process and/or during a post-plating anodizing
process. This way, parts 702a-7021 can be transferred together as a
unit from process station to process station without removing parts
702a-7021 from rack assembly 700.
[0044] Rack assembly 700 can be placed within a plating bath during
a plating process with bottom portion 711 oriented toward a bottom
of the plating cell and top portion 713 oriented toward a top of
the plating cell. Rack assembly 700 includes rack frame 704,
drainage bars 706, and cut outs 710. Parts 702a-7021 can be
positioned within cut outs 710 such that each of parts 702a-7021 is
separated a distance 712 from an edge of rack frame 704. In
addition, outward surfaces of parts 702a-7021 and outward surfaces
of rack frame 704 are along the same plane. Distance 712 should be
small enough such that, during a plating process, parts 702a-7021
and rack frame 704 approximate a single flat surface. The proximity
of parts 702a-7021 to rack frame 704 and the positioning of parts
702a-7021 along the same plane as rack frame 704 can promote even
current density and plating along edges, corners, and flat surfaces
of parts 702a-7021. In some embodiments, drainage bars 706 are
added to rack assembly 700. Drainage bars 706 are connected with
and extend outward from rack frame 704 along a different plane as
parts 702a-7021 and rack frame 704. Drainage bars 706 can be
positioned at an angle relative to rack frame 704 to promote good
drainage of chemicals during the plating process. Drainage bars 706
can include connector portions 708 that connect with and fix parts
702a-7021 to drainage bars 706. In some embodiments, connector
portions 708 are secured to parts 702a-7021 using fasteners such as
screws. It should be noted that rack assembly 700 illustrates a
particular embodiment and that the shape and arrangement of rack
frame 704, drainage bars 706 and parts 702a-7021 can vary in other
embodiments.
[0045] It should be noted that in processing aluminum alloy
substrates for coating with a high purity aluminum, the rack
material should be chemically compatible with various processing
steps that may be employed. In some embodiments, the adhesion
improvement processing (e.g., phosphoric anodizing) requires that
substantially all surfaces presented for processing uniformly
evolve a tenacious dielectric oxide layer for the process to
proceed correctly. A subsequent processing step (inert atmosphere
activation) can also benefit from having only aluminum surfaces
exposed. Bare titanium can work successfully for adhesion but
potentially cause cosmetic defects. Use of aluminum coated titanium
racks avoids these defects.
[0046] Racks made entirely of an aluminum alloy may be successfully
employed for the adhesion improvement step, the inert activation
step and also for any cosmetic finish anodization after the high
purity aluminum coating process, without changing the rack. In some
embodiments, a titanium rack may also be employed for all these
processing steps if it is first coated with aluminum. The utility
of the titanium rack is that it will not be substantially degraded
by normal post processing cleaning and restoration treatments. A
rack made entirely of aluminum could potentially be consumed and
destroyed by some number of complete processing cycles.
[0047] The foregoing description, for purposes of explanation, used
specific nomenclature to provide a thorough understanding of the
described embodiments. However, it will be apparent to one skilled
in the art that the specific details are not required in order to
practice the described embodiments. Thus, the foregoing
descriptions of specific embodiments are presented for purposes of
illustration and description. They are not intended to be
exhaustive or to limit the described embodiments to the precise
forms disclosed. It will be apparent to one of ordinary skill in
the art that many modifications and variations are possible in view
of the above teachings.
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