U.S. patent application number 15/261699 was filed with the patent office on 2018-03-15 for interference colored titanium with protective oxide film.
The applicant listed for this patent is Apple Inc.. Invention is credited to James A. CURRAN, Herng-Jeng JOU, James A. WRIGHT.
Application Number | 20180073159 15/261699 |
Document ID | / |
Family ID | 61559208 |
Filed Date | 2018-03-15 |
United States Patent
Application |
20180073159 |
Kind Code |
A1 |
CURRAN; James A. ; et
al. |
March 15, 2018 |
INTERFERENCE COLORED TITANIUM WITH PROTECTIVE OXIDE FILM
Abstract
Coatings for titanium and titanium alloy substrates are
described. The coatings can include an aluminum oxide layer and, in
some cases, a thin titanium oxide layer. If the coating includes a
titanium oxide layer, the aluminum oxide layer can cover and
protect the titanium oxide layer from abrasion and scratching. In
some examples, the titanium oxide layer has a thickness sufficient
to provide a color by thin-film interference, which can be visible
through the overlying aluminum oxide layer. In some embodiments,
the aluminum oxide layer is colorized using an anodic dye, pigment
or metal colorant, which can combine with interference colors of
the titanium oxide layer.
Inventors: |
CURRAN; James A.; (Morgan
Hill, CA) ; JOU; Herng-Jeng; (San Jose, CA) ;
WRIGHT; James A.; (Los Gatos, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Apple Inc. |
Cupertino |
CA |
US |
|
|
Family ID: |
61559208 |
Appl. No.: |
15/261699 |
Filed: |
September 9, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25D 11/26 20130101;
C25D 11/243 20130101; C25D 11/246 20130101; C25D 11/16 20130101;
C25D 11/14 20130101 |
International
Class: |
C25D 11/04 20060101
C25D011/04; C25D 11/14 20060101 C25D011/14 |
Claims
1. A part, comprising: a substrate composed of titanium or a
titanium alloy; and a coating disposed on the substrate, the
coating including an aluminum oxide layer.
2. The part of claim 1, wherein the coating includes a titanium
oxide layer.
3. The part of claim 2, wherein the aluminum oxide layer is more
porous than the titanium oxide layer.
4. The part of claim 2, wherein the titanium oxide layer has a
thickness ranging from about 20 nm to about 150 nm.
5. The part of claim 2, wherein the titanium oxide layer imparts a
color to the coating via thin film interference coloring.
6. The part of claim 2, wherein the aluminum oxide layer has a
colorant infused within pores of the aluminum oxide layer.
7. The part of claim 1, wherein the aluminum oxide layer has a
colorant infused within pores of the aluminum oxide layer.
8. The part of claim 1, wherein the aluminum oxide layer has a
hardness of about 300 HV or greater.
9. The part of claim 1, wherein the coating includes a layer of
aluminum or aluminum alloy between the aluminum oxide layer and the
substrate.
10. The part of claim 1, wherein the aluminum oxide layer has a
thickness ranging from about 5 micrometers to about 20
micrometers.
11. A method of forming a coating on a substrate, the substrate
composed of titanium or a titanium alloy, the method comprising:
depositing a layer of aluminum or an aluminum alloy on the
substrate; and converting at least a portion of the layer of
aluminum or the aluminum alloy to an aluminum oxide layer.
12. The method of claim 11, wherein substantially all of the layer
of aluminum or aluminum alloy is converted to the aluminum oxide
layer.
13. The method of claim 12, wherein at least a portion of the
substrate is converted to a titanium oxide layer.
14. The method of claim 13, wherein the aluminum oxide layer is
more porous than the titanium oxide layer.
15. The method of claim 13, wherein the titanium oxide layer has a
thickness ranging from about 20 nm to about 150 nm.
16. The method of claim 11, further comprising infusing a colorant
within pores of the aluminum oxide layer.
17. An enclosure for an electronic device, the enclosure
comprising: a substrate composed of titanium or a titanium alloy,
the substrate having a coating corresponding to an outer surface of
the enclosure, the coating including an aluminum oxide layer.
18. The enclosure of claim 17, wherein the coating includes a
titanium oxide layer.
19. The enclosure of claim 18, wherein the titanium oxide layer has
a thickness ranging from about 20 nm to about 150 nm.
20. The enclosure of claim 19, wherein the enclosure includes a
first portion that is interference colored and a second portion
that is not interference colored, wherein the titanium oxide layer
is part of the first portion.
Description
FIELD
[0001] The described embodiments relate to oxide coatings for
titanium and titanium alloys. The coatings can include a titanium
oxide film having a color produced by thin-film interference, and a
protective aluminum oxide film.
BACKGROUND
[0002] Titanium and its alloys are known for their high strength,
low density and corrosion resistance. For these reasons, titanium
and titanium alloys are utilized in a number of applications that
require a strong, lightweight and corrosion resistant metal.
Although titanium surfaces are regarded as inherently corrosion
resistant, they can readily oxidize, which makes them susceptible
to staining when exposed to certain environments. Furthermore, when
titanium is anodized, a thin barrier layer type film is formed,
which offers little abrasion protection. In particular, the
titanium oxide film it very thin and therefore susceptible to
uneven wearing, which can result in a cosmetically unappealing
titanium part. What are needed, therefore, are coatings for
titanium and titanium alloys that offer improved abrasion
resistance and that are cosmetically appealing.
SUMMARY
[0003] This paper describes various embodiments that relate to
titanium and titanium alloys. In particular embodiments, methods of
providing abrasion resistant and cosmetically appealing oxide
coatings on titanium substrates are described. In some cases, the
coatings are characterized as having a color produced by thin-film
interference effects.
[0004] According to one embodiment, a part is described. The part
includes a substrate composed of titanium or a titanium alloy. The
part also includes a coating disposed on the substrate. The coating
includes an aluminum oxide layer. The coating can also include a
titanium oxide layer.
[0005] According to another embodiment, a method of forming a
coating on a substrate is described. The substrate is composed of
titanium or a titanium alloy. The method includes depositing a
layer of aluminum or an aluminum alloy on the substrate. The method
also includes converting at least a portion of the layer of
aluminum or the aluminum alloy to an aluminum oxide layer. In some
cases, the method further includes converting a portion of the
substrate to a titanium oxide layer.
[0006] According to a further embodiment, an enclosure for an
electronic device is described. The enclosure includes a substrate
composed of titanium or a titanium alloy. The substrate has a
coating corresponding to an outer surface of the enclosure. The
coating includes an aluminum oxide layer. In some cases, the
coating also includes a titanium oxide layer.
[0007] These and other embodiments will be described in detail
below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] The disclosure will be readily understood by the following
detailed description in conjunction with the accompanying drawings,
wherein like reference numerals designate like structural
elements.
[0009] FIG. 1 shows perspective views of devices having metal
surfaces that can be treated with the coatings described
herein.
[0010] FIGS. 2A-2D show cross-section views of a part undergoing a
surface coating process, in accordance with some embodiments.
[0011] FIGS. 3A-3C show cross-section views of an interface portion
of the part of FIGS. 2A-2D, in accordance with some
embodiments.
[0012] FIG. 4 shows a chart showing a relationship between
anodizing voltage potential and interference color of a titanium
oxide layer, in accordance with some embodiments.
[0013] FIGS. 5A and 5B show a plan view and a partial cross-section
view of a part having a coating that includes a portion that
includes interference coloring by a titanium oxide layer.
[0014] FIG. 6 shows a flowchart indicating a process for forming a
protective oxide coating on a titanium substrate, in accordance
with some embodiments.
DETAILED DESCRIPTION
[0015] Reference will now be made in detail to representative
embodiments illustrated in the accompanying drawings. It should be
understood that the following descriptions are not intended to
limit the embodiments to one preferred embodiment. To the contrary,
it is intended to cover alternatives, modifications, and
equivalents as can be included within the spirit and scope of the
described embodiments as defined by the appended claims.
[0016] Titanium and its alloys can form strong, lightweight
structures, but offer limited options for surface finishing.
Described herein are coatings for titanium and titanium alloy
substrates that are durable, stain resistant and cosmetically
appealing. The coatings can include an aluminum oxide layer, and in
some embodiments, include a titanium oxide layer. The thicker and
more scratch resistant aluminum oxide layer is disposed over the
thinner titanium oxide layer so as to protect the titanium oxide
layer from abrasion and scratching. In some cases, the thickness of
the titanium oxide layer is tuned to cause thin-film interference
coloring effects, thereby imparting a color to the titanium part.
The overlying aluminum oxide layer can protect the integrity of the
titanium oxide layer, thereby preserving the interference coloring.
Compared to colors provided by some conventional oxide coloring
techniques, interference colors of the titanium oxide layer are
stable when exposed to ultraviolet (UV) light.
[0017] The coating can be formed using a number of techniques. In
some embodiments, this involves applying an aluminum layer on the
titanium substrate, then anodizing the aluminum layer. In some
cases, the entire aluminum layer is converted to a corresponding
aluminum oxide layer, and a portion of the titanium substrate is
also converted to a titanium oxide layer. In some applications, the
anodizing conditions are chosen so as to yield an optically clear,
porous oxide film (for example, a "Type II" anodic oxide as defined
by Mil-A-8625 specifications). This can permit optimal viewing of
the underlying interference colored titanium oxide layer. The
anodizing process can also be chosen so as to provide a hard
aluminum oxide layer that protects the underlying titanium oxide
layer from scratching and abrasion.
[0018] The substrates and coatings described herein are well suited
for providing cosmetically appealing consumer products. For
example, the methods described herein can be used to form durable
and cosmetically appealing finishes for housing of computers,
portable electronic devices, wearable electronic devices, and
electronic device accessories, such as those manufactured by Apple
Inc., based in Cupertino, Calif. As used herein, the terms oxide,
oxide coating, oxide film, oxide layer, etc. can be used
interchangeably and can refer to suitable oxide material, unless
otherwise specified.
[0019] These and other embodiments are discussed below with
reference to FIGS. 1-6. 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] FIG. 1 shows consumer products than can include titanium or
titanium alloy substrates described herein. FIG. 1 includes
portable phone 102, tablet computer 104, smart watch 106 and
portable computer 108, each of which can include enclosures that
are made of metal or have metal sections. These metal or metal
sections can be composed of titanium or titanium alloys in order to
provide high strength and stiffness. Titanium has a high specific
strength, thus the enclosures made of titanium and its alloys can
be more lightweight with the same strength compared to enclosures
made of other metals. Titanium surfaces also present good hardness,
which generally translates to good scratch and abrasion resistance
in everyday use. For example, Ti-6Al-4V titanium alloy has a
hardness of about 350 HV (per Vickers hardness test), which is in
itself comparable to the surface hardness of a typical anodized
aluminum surface (about 300-400 HV as measured at 50 gram
load).
[0021] In spite of these advantages, surfaces of titanium can be
less abrasion resistant and scratch resistant compared to, for
example, anodized aluminum surfaces. Conventional anodizing of
titanium does not offer the thick, clear and hard oxides, which are
useful in the protection of cosmetic aluminum surfaces--but instead
forms thin, interference colored barrier layers, which offer little
abrasion protection and can remain vulnerable to chemical staining.
Even slight changes in the thickness of these films (e.g., about 10
nm) can result in significant changes in the interference coloring,
and hence obvious changes in surface appearance after even light
abrasion.
[0022] Another problem associated with titanium surfaces relates to
corrosion. Whilst titanium and its alloys are noted for excellent
corrosion resistance, their surfaces readily oxidize. Thus,
titanium surfaces can be prone to staining and tarnishing under
exposure to certain environments. Although generally limited to
surface of the titanium or titanium alloy, and often only to a
limited area of the surface, these stains can significantly and
permanently change the local visual appearance of the surface.
Again, the extremely thin oxide films formed by conventional
anodizing of titanium do not provide significant protection against
this effect, and can even exacerbate it due to the greater optical
contrast that varying oxide film thicknesses and chemistries of
distinct colors can give.
[0023] The coatings described herein can be applied to surfaces of
titanium or titanium alloys in order to improve their scratch
resistance and retention of good cosmetic quality. Thus, the
coatings are well suited for coating titanium or titanium alloy
portions of consumer products such as enclosures of devices 102,
104, 106 and 108, which are subject to abrasion and scratching
during normal use. In some cases, the coatings are applied to
exterior portions of the enclosures such that the coatings are
visible and tactilely accessible to users of devices 102, 104, 106
and 108.
[0024] FIGS. 2A-2D show cross-section views of part 200 undergoing
a surface coating process in accordance with some embodiments. Part
200 can correspond to a portion of an enclosure, such as enclosures
for devices 102, 104, 106 and 108. FIG. 2A shows substrate 202,
with surface 204 corresponding to a surface undergoing treatment.
In some embodiments, substrate 202 is composed of titanium or a
titanium alloy. In some cases, substrate 202 is composed of a
Ti-6Al-4V titanium alloy. In some cases, substrate 202 is composed
of a custom titanium alloy having alloying elements in
predetermined amounts for providing certain benefits, such as high
strength, corrosion resistance or cosmetic quality. In other
embodiments, substrate 202 is composed of zirconium, zirconium
alloys, tantalum, tantalum alloys, hafnium, hafnium alloys,
niobium, or niobium alloys. In some cases, substrate 202 includes
titanium and one or more of zirconium, tantalum, hafnium and
niobium.
[0025] At FIG. 2B, substrate 202 has optionally undergone one or
more surface treatment processes. For example, surface 204 can be
planarized to be substantially flat, or can be machined to have
curved surface profile. In some embodiments, surface 204 is
polished. Additionally or alternatively, surface 204 can undergo a
texturing process, such as one or more of a blasting and chemical
etching process. In some embodiments, a very thin metal
intermediate layer (e.g., about 2-6 nanometers of silver) is
deposited on surface 204 prior to a subsequent step of depositing
an aluminum layer in order to enhance the adhesion of the aluminum
layer to substrate 202. Since metals such as silver are inert with
respect to an anodizing process and the intermediate layer would
nominally be very thin, such an intermediate metal layer may be
beneficial in some cases.
[0026] At FIG. 2C, aluminum layer 206 is deposited on surface 204
of substrate 202. In some embodiments aluminum layer 206 is
composed of an aluminum alloy, such as a 6000 or 7000 series alloy.
In some embodiments, aluminum layer 206 is composed of
substantially pure aluminum, which generally results in a more
transparent and colorless aluminum oxide layer compared to aluminum
alloys. Aluminum layer 206 can be deposited using any suitable
process For example, aluminum layer 206 can be deposited using a
physical vapor deposition (PVD), hot dip aluminizing, cladding
and/or electroplating processes. If a hot dip aluminizing process
is used, it may be necessary to remove some of the aluminum
material using, for example, a chemical etching process, in order
to achieve an appropriate thickness 214. If a conformal deposition
technique is used, the geometry of surface 208 of aluminum layer
206 can correspond to the geometry of surface 204 of substrate 202.
For example, a conformal deposition process can result in flat,
curved, polished and/or textured surface 208 of aluminum layer 206
to match corresponding flat, curved, polished and/or textured
surface 204 of substrate 202.
[0027] The thickness 209 of aluminum layer 206 can vary depending
on particular applications. In some embodiments, aluminum layer 206
is deposited to a thickness 209 that ranges from about 5
micrometers to about 10 micrometers. In some applications, after
aluminum layer 206 is deposited, surface 208 undergoes one or more
finishing processes, such as one or more polishing, buffing,
abrasive blasting, and chemical etching operations, in order to
form a textured, matte, smoothed or polished surface 208. However,
in some cases such a surface finishing process is not
necessary.
[0028] It should be noted that surfaces titanium and titanium
substrates do not traditionally need a metal coating since titanium
and its alloys are generally known for their hardness and corrosion
resistance. Thus, adding aluminum layer 206 on a titanium substrate
202 is not considered in conventional applications. This is
understandable, given that aluminum can be softer and less
corrosion-resistant than titanium and its alloys, and therefore
offers no obvious benefits in conventional applications.
[0029] At 2D, at least a portion of aluminum layer 206 is converted
to aluminum oxide layer 210. In some embodiments, an anodizing
process is used, whereby at least a portion of aluminum layer 206
is electrolytically oxidized to a corresponding aluminum oxide.
Since anodizing is a conversion process, surface 212 of aluminum
oxide layer 210 replaces surface 208 of aluminum layer 206. Surface
212 of aluminum oxide layer 210 generally has a geometry
corresponding to that of surface 208 of aluminum layer 206. That
is, a smooth and polished surface 208 of aluminum layer 206 can
result in a smooth and polished surface 212 of aluminum oxide layer
210. Likewise, a textured and matte surface 208 of aluminum layer
206 can result in a textured and matte surface 212 of aluminum
oxide layer 210.
[0030] Thickness 214 of aluminum oxide layer 210 will depend on
particular applications. In some applications, aluminum oxide layer
210 is grown to a thickness 214 that ranges from about 8
micrometers and 30 micrometers. In some cases, thickness 214
ranging from about 5 micrometers to about 20 micrometers is found
to be thin enough to provide an aluminum oxide layer 210 with good
visible clarity (i.e., little cloudiness), yet thick enough to
provide good abrasion resistance. The hardness of aluminum oxide
layer 210 can vary depending, in part, on the anodizing conditions.
In some embodiments, a "Type II anodizing" (as defined by
Mil-A-8625) process is used to form aluminum oxide layer 210 having
a hardness of about 300 HV or greater.
[0031] In some embodiments, only a portion of aluminum layer 206 is
converted to aluminum oxide layer 210. In other cases,
substantially all of aluminum layer 206 is converted to aluminum
oxide layer 210. In some cases, all of aluminum layer 206 is
converted to aluminum oxide layer 210, and also a portion of
substrate 202 is converted to a corresponding oxide layer. These
embodiments are described below with reference to FIGS. 3A-3C,
showing cross-section views of interface portion 216 of part
200.
[0032] FIG. 3A shows interface portion 216 of part 200 after an
anodizing process has converted a portion of aluminum layer 206 to
aluminum oxide layer 210. In some cases, the anodizing forms pores
302 within aluminum oxide layer 210. The size of pores 302 will
depend, in part, on the conditions of the anodizing process, as
will the hardness and cosmetics of aluminum oxide layer 210. If a
Type II anodizing process is used, which involves anodizing in a
sulfuric acid electrolyte, pores 302 can have diameters ranging
from about 10 nanometers (nm) and about 50 nm. In one embodiment,
the Type II anodizing is performed using a voltage of about 15 V,
resulting in pores 302 having a diameter of about 30 nm. A Type II
anodizing process can result in aluminum oxide layer 210 being
substantially colorless and transparent, which allows the remainder
of aluminum layer 206 to be visible through aluminum oxide layer
210. Thus, any surface texturing applied to aluminum layer 206
(e.g., at FIG. 2C described above) may be visible to a user of part
200. For example, a polished and shiny surface of aluminum layer
206 would be visible through aluminum oxide layer 210. Likewise, a
textured or matte surface of aluminum layer 206 would be visible
through aluminum oxide layer 210.
[0033] As described above, in some cases a very thin intermediate
metal layer (not shown), such as a 2-6 nanometer thick layer of
silver, is positioned between aluminum oxide layer 210 and titanium
oxide layer 304. It is preferable for the intermediate layer to be
composed of a metal that is inert to the anodizing process (i.e.,
does not chemically react with and contaminate the anodizing bath).
The anodizing process can then terminate at the intermediate layer,
making a top surface of the intermediate layer visible. In some
cases, the intermediate layer is composed of a highly reflective
metal, such as silver, thereby providing a shiny and cosmetically
appealing underlying surface that is visible through aluminum oxide
layer 210.
[0034] FIG. 3B shows part 200 after the anodizing process is
allowed to proceed until substantially all of aluminum layer 206 is
converted to aluminum oxide layer 210. When all of aluminum layer
206 is consumed by oxidation, substrate 202 starts to anodize at
the terminuses of pores 302. If substrate 202 is composed of
titanium or a titanium alloy, a corresponding titanium oxide layer
304 forms between aluminum oxide layer 210 and substrate 202. A
voltage limit can control the extent of substrate 202 oxidation. In
one example, when a Type II anodizing of aluminum layer 206 is
conducted under galvanostatic control with an applied current
density of about 1.5 Amps/dm.sup.2, then the potential will start
to rise (from an approximate value of 15 V) upon complete
conversion of aluminum layer 206 to aluminum oxide layer 210. From
this point onwards, the titanium substrate 202 can be oxidized to
titanium oxide layer 304, which is a pore-free barrier layer type
film, having a thickness proportional to the applied voltage.
[0035] At FIG. 3C, the anodizing process is allowed to proceed
until titanium oxide layer 304 achieves a desired thickness 306.
Unlike aluminum oxide layer 210, titanium oxide layer 304 is a
non-porous, barrier layer type film and nominally reaches up to
tens or hundreds of nanometers in thickness 306. Anodizing of
titanium is a self-limiting process, based on the anodizing voltage
used, with higher voltages generally resulting in titanium oxide
layers having greater thicknesses. In some embodiments, thickness
306 of titanium oxide layer 304 is chosen to provide a
predetermined color to part 200 by thin film interference effects.
In general, thin film interference coloring occurs when light is
reflected by boundaries of a thin film of a transparent or
partially transparent material. To illustrate, optical pathways for
incident light ray 308 are illustrated in FIG. 3C. Incident light
ray 308 enters and passes through aluminum oxide layer 210. A first
portion of light ray 308 reflects off interface surface 310 between
aluminum oxide layer 210 and titanium oxide layer 304, and is
reflected as light ray 314. A second portion of light ray 308
travels through aluminum oxide layer 210 and is reflects off
interface 312 (which can also be referred to as the surface of
underlying substrate 202) between titanium oxide layer 304 and
substrate 202, and is reflected as light ray 316.
[0036] The different optical paths of reflected light rays 314 and
316 causes constructive and destructive interference between
reflected light rays 314 and 316, with the degree of constructive
and destructive interference depending upon differences in their
phases. The difference in the phases, in turn, depends on the
degree of thickness 306 of titanium oxide layer 304. In this way,
the light interference can impart a perceived color to titanium
oxide layer 304, and thereby provided a colored coating to part
200. Thus, the term "interference color" can refer to a color
imparted to a thin film by thin film interference effects,
including first order and second order interference. The resulting
color can depend not only on the thickness of the thin film, but
also the material of the thin film. Thus, for example, a titanium
oxide thin film may be characterized as having a different range of
colors compared to an aluminum oxide thin film. Other factors that
may affect a perceived color can include the color of the
underlying substrate 202, which will depend on the composition of
metal(s) within the substrate 202.
[0037] Note that differences in the indices of refraction of
aluminum oxide layer 210 and titanium oxide layer 304 can also
cause some amount of diffraction of incident light 308. However,
the thin film interference coloration provided by titanium oxide
layer 304 is still apparent and observable through aluminum oxide
layer 210 as long as aluminum oxide layer 210 and titanium oxide
layer 304 are sufficiently transparent. In addition, variations of
the thickness of aluminum oxide layer 210 may not be cosmetically
apparent where aluminum oxide layer 210 is nominally transparent to
visible light. This allows for greater tolerance with regard to
thickness variations of aluminum oxide layer 210.
[0038] As described above, the interference coloration provided by
titanium oxide layer 304 depends on thickness 306, which, in turn,
can be tuned by controlling the applied potential used during the
anodizing process. For example, in a particular embodiment, a 30-40
V potential results in a 50-60 nm thick titanium oxide layer 304,
resulting in imparting a blue interference color to part 200.
Examples of other anodizing voltage potentials used to form
different interference colors are described below with reference to
FIG. 4. In some applications, thickness 304 ranges between about 20
nm to about 150 nm.
[0039] It should be noted that the interference coloring provided
by titanium oxide layer 304 is colorfast in that the color cannot
be washed away or faded, for example, by exposure to ultraviolet
(UV) light, as long as the interfaces 310 and 312 of titanium oxide
layer 304 remain substantially intact. This is in contrast to oxide
films colorized by dyes or pigments, which can in some cases be
susceptible to fading by UV light or by exposure to certain
chemicals.
[0040] As noted above, titanium oxide layer 304 is nominally very
thin and, by itself, can be prone to abrasion and scratching.
Aluminum oxide layer 210 addresses this issue by covering and
protecting titanium oxide layer 304 from exposure to forces that
could otherwise abrade or scratch titanium oxide layer 304. The
hardness of aluminum oxide layer 304 will depend on a number of
factors, such as the anodizing process parameters and the type of
aluminum alloy anodized. As described above, in some embodiments,
aluminum oxide layer 210 can have a hardness of about 300 HV or
higher, which can provide substantial protection for may consumer
product applications. Furthermore, if aluminum oxide layer 210 does
become partially eroded, the integrity of titanium oxide layer 304
can be preserved, and therefore may not significantly change the
visual appearance of part 200. That is, interface surfaces 310 and
312, which dominates the optical reflections for thin film
interference coloring, and hence the visual appearance of part 200,
can remain undamaged since they can be micrometers below the
surface of aluminum oxide layer 210. In these ways, aluminum oxide
layer 210 preserves the aesthetics of titanium surface finishes,
such as intense and colorfast interference coloring of titanium
oxide layer 304.
[0041] In addition, because the overlying aluminum oxide layer 210
is porous, it has the capacity to absorb colorants, which may be
sealed in as per any suitable coloring techniques, such as infusing
one or more of dye, pigment and metal within pores 302. This
colorizing of aluminum oxide layer 210 can be used to adjust the
color of the inference colorized titanium oxide layer 304. That is,
the color imparted by a colorant within pores 302 can combine with
the color imparted by interference coloring of titanium oxide layer
304 to accomplish a predetermined color to part 200, which may not
be achievable through interference coloring or coloring of aluminum
oxide layer 210 alone. For example, a UV-stable red organic dye may
be infused within pores 302 of aluminum oxide layer 210 to achieve
red colors, which may not be achievable by interference coloring
alone. As another example, a UV-stable gray or black dye may be
used to change the lightness of an interference color of titanium
oxide layer 304. It should be noted, however, that colorizing of
aluminum oxide layer 210 is optional and may not be used in some
applications.
[0042] FIG. 4 shows a chart indicating a relationship between
anodizing voltage potential and interference color of a titanium
oxide layer, as measured by a* and b* color opponent dimension
values in accordance with CIE 1976 color space model. In general,
a* values represent amounts of green and red/magenta, and b* values
represent amounts of blue and yellow of a sample. Negative a*
values indicate a green color while positive a* values indicate a
red or magenta color. Negative b* values indicate a blue color and
positive b* values indicate a yellow color. In the chart of FIG. 4,
b* values are represented on the horizontal axis and a* values are
represented on the vertical axis.
[0043] As described above, the thickness of a titanium oxide layer
is directly dependent upon the voltage used during the anodizing
process. Thus, the interference color of the titanium oxide layer
directly correlates with the applied anodizing voltage. The chart
of FIG. 4 indicates that the interference color of a titanium oxide
layer will cycle through hues between yellow, blue, green and red
depending on the applied voltage used during the anodizing process.
For example, a 10-volt potential (corresponding to a titanium oxide
thickness of about 15-20 nm) used during anodizing of titanium
results in a brown color, and a 40-volt potential (corresponding to
a titanium oxide thickness of about 50 nm) results in a light blue
color. In some embodiments where a neutral is preferred, a titanium
oxide thickness of about 50-60 nm may be preferred since the
resulting light blue color is very slight and the blue color can
approximate a metallic appearance of the underlying substrate.
However, the titanium oxide layer can have any suitable thickness
and hue. According to some embodiments, the thickness of the
titanium oxide layer is between about 20 nm and about 150 nm.
[0044] FIGS. 5A and 5B show a plan view of part 500, which includes
a coating 502 having interference coloring, in accordance with some
embodiments. FIG. 5B shows a partial cross-section A-A of part 500.
Part 500 can correspond, for example, to an enclosure, or part of
an enclosure, for an electronic device. Coating 502 corresponds to
an oxide material formed over substrate 512, which can be composed
of a titanium or titanium alloy.
[0045] Coating 502 includes first portion 504, which is
characterized as having a color by interference coloring, and
second portion 506, which does not have interference coloring. In
particular, second portion 506 includes aluminum oxide layer 510,
which can be a porous anodic film, such as aluminum oxide layer 210
described above with reference to FIGS. 2A-2D and 3A-3C. Aluminum
oxide layer 210 can be transparent, or partially transparent, such
that substrate 512 is viewable through aluminum oxide layer 210.
First portion 504 includes aluminum oxide layer 210 as well as
titanium oxide layer 514. Titanium oxide layer 514 is sufficiently
transparent and has a thickness suitable for providing a color to
first portion 504 via interference coloring. Thus, the length and
shape of titanium oxide layer 514 defines boundary 508 of first
portion 504. In some embodiments, boundary 508 has a shape in
accordance with a logo, writing or other cosmetic or identifying
feature.
[0046] Coating 502 can be formed using the aluminum layer
deposition and anodizing processes described above with reference
to FIGS. 2A-2D, with the addition of a masking process. For
example, after an aluminum layer is deposited, a first anodizing
process can be used to form aluminum oxide layer 510 of both first
portion 504 and second portion 506. After substantially the entire
aluminum layer is converted to aluminum oxide layer 510, second
portion 506 can be masked. Then, first portion 504 is exposed to a
second anodizing process, whereby a portion of substrate 512 is
converted to titanium oxide layer 514. Note that in some
embodiments, it may be desirable to colorize aluminum oxide layer
510 (e.g., using dye, pigment or metal) in one or both of first
portion 504 and second portion 506.
[0047] FIG. 6 shows flowchart 600 indicating a process for forming
a coating on a titanium substrate, in accordance with some
embodiments. At 602, a surface of the titanium substrate is
optionally finished using, for example, one or more of a buffing,
polishing, abrasive blasting and chemical etching processes. The
titanium substrate can be composed of a titanium alloy, such as a
Ti-6Al-4V titanium alloy, or may be composed of substantially pure
titanium.
[0048] At 604, an aluminum layer is deposited onto the surface of
the titanium substrate. The aluminum layer can be composed of an
aluminum alloy or substantially pure aluminum. The aluminum layer
can be deposited using any suitable technique. In some cases, the
aluminum layer is deposited using one or more physical vapor
deposition (PVD), hot dip aluminizing, cladding and electroplating
processes. At 606, a surface of the aluminum layer is optionally
finished using, for example, one or more of a buffing, polishing,
abrasive blasting and chemical etching processes.
[0049] At 608, at least a portion of the aluminum layer is
converted to an aluminum oxide layer. In some embodiments, an
anodizing process is used, whereby the resulting aluminum oxide
layer will have pores. In some applications, it is preferable that
the aluminum oxide layer be mostly transparent to visible light and
also has a hardness of at least about 300 HV. In a particular
embodiment, a Type II anodizing process is used, whereby the
anodizing process is performed in an electrolytic bath having
sulfuric acid, resulting in a substantially transparent,
cosmetically appealing aluminum oxide layer. In some embodiments,
substantially the entire aluminum layer is converted to a
corresponding aluminum oxide layer. Complete conversion of the
aluminum layer will be apparent by a rise in potential during the
anodizing process if conducted under current control, or by a fall
in current if the process is conducted under voltage control.
[0050] At 610, a portion of the titanium substrate is optionally
converted to a titanium oxide layer. These embodiments involve
anodizing through the entire aluminum layer, and continuing the
anodizing process until a portion of the titanium substrate is
converted to a corresponding titanium oxide layer. In some
embodiments, the part remains within the same electrolyte as used
to anodize the aluminum layer. In other embodiments, a different
electrolyte is used to anodize the titanium layer versus the
aluminum layer. The titanium oxide layer is a barrier type film
that is non-porous. That is, the aluminum oxide layer formed by
anodizing will have more pores than the titanium oxide layer since
the titanium oxide layer has substantially no pores formed by the
anodizing process. In some cases, the anodizing voltage is chosen
based on a desired interference coloring of the titanium oxide
layer. In some embodiments, the titanium oxide layer has a
thickness ranging from about 20 nm and about 150 nm.
[0051] At 612, a colorant is optionally deposited within the
aluminum oxide layer. The colorant can include, for example, a dye,
pigment or metal material that is infused within pores of the
aluminum oxide layer. A color of the colorant can combine with the
interference coloring provided by the titanium oxide layer,
resulting in a final color that may not be achievable using
standard coloring techniques. At 614, pores of the aluminum oxide
are optionally sealed in order to improve the corrosion resistance
of the coating and/or to seal in any colorant within the pores. Any
suitable sealing process can be used, including suitable
hydrothermal aqueous-based anodic film sealing techniques.
[0052] It should be noted that whilst the preceding description
focuses on titanium and its alloys as a substrate metal, the
processes described herein may also be applied to any of a number
of other suitable metals and their alloys that yield similar thin
nonporous oxides under certain anodizing conditions. Such metals
can include one or more of zirconium, tantalum, hafnium, niobium
and alloys thereof. In some cases, the substrate includes titanium
and one or more of zirconium, tantalum, hafnium and niobium. In
addition, chromium oxide or other metal oxides based on at least
one of rhenium, molybdenum, cobalt, or tantalum may provide a wide
range of interference colors. Thus, the term "interference colors"
can refer to a range of colors, which are formed because of the
thin film interference effect. The interference colors can include
first order, or second order interference colors such as yellow,
orange, pink, purple, blue, or green hues.
[0053] The foregoing description, for purposes of explanation, uses
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 the specific embodiments described herein are
presented for purposes of illustration and description. They are
not intended to be exhaustive or to limit the 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.
* * * * *