U.S. patent application number 13/189739 was filed with the patent office on 2015-11-19 for magnesium mirrors and methods of manufacture thereof.
This patent application is currently assigned to RAYTHEON COMPANY. The applicant listed for this patent is John P. Schaefer, Clay E. Towery. Invention is credited to John P. Schaefer, Clay E. Towery.
Application Number | 20150331158 13/189739 |
Document ID | / |
Family ID | 47141715 |
Filed Date | 2015-11-19 |
United States Patent
Application |
20150331158 |
Kind Code |
A2 |
Schaefer; John P. ; et
al. |
November 19, 2015 |
MAGNESIUM MIRRORS AND METHODS OF MANUFACTURE THEREOF
Abstract
Low density mirrors for optical assemblies and methods of
manufacture thereof. In one example, a reflective mirror is formed
of a magnesium or magnesium alloy substrate that is single point
diamond turned to provide a reflective surface. The magnesium or
magnesium alloy substrate is compatible with thin-film finishing
processes and/or magnetorheological finishing which may be applied
to improve a surface finish of the mirror.
Inventors: |
Schaefer; John P.; (Plano,
TX) ; Towery; Clay E.; (Plano, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Schaefer; John P.
Towery; Clay E. |
Plano
Plano |
TX
TX |
US
US |
|
|
Assignee: |
RAYTHEON COMPANY
Waltham
MA
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20120287520 A1 |
November 15, 2012 |
|
|
Family ID: |
47141715 |
Appl. No.: |
13/189739 |
Filed: |
July 25, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61485939 |
May 13, 2011 |
|
|
|
Current U.S.
Class: |
29/527.3 ;
164/69.1; 29/527.6; 29/558; 359/838; 359/883; 427/162; 451/41;
82/1.11 |
Current CPC
Class: |
G02B 5/085 20130101;
B24B 13/0018 20130101; B24B 1/00 20130101; G02B 5/08 20130101 |
International
Class: |
G02B 5/08 20060101
G02B005/08; B24B 13/00 20060101 B24B013/00; B24B 1/00 20060101
B24B001/00; B05D 5/06 20060101 B05D005/06; B22D 25/00 20060101
B22D025/00 |
Claims
1. A method of manufacture of a reflective mirror comprising:
providing a magnesium substrate having a first surface; single
point diamond turning the first surface of the magnesium substrate
to produce a mirror surface.
2. The method of claim 1, further comprising thixotropically
molding a magnesium alloy to produce the magnesium substrate.
3. The method of claim 2, wherein thixotropically molding the
magnesium alloy includes forming at least one of weight-reducing
features and stress-relieving features on a second surface of the
substrate, the second surface being on a side of the substrate
opposite the first surface.
4. The method of claim 2, wherein thixotropically molding the
magnesium alloy includes thixotropically molding magnesium
AZ91-D.
5. The method of claim 1, further comprising casting one of
magnesium and a magnesium alloy to produce the magnesium
substrate.
6. The method of claim 1, further comprising finishing the first
surface of the magnesium substrate after the single point diamond
turning, the finishing including: depositing a thin-film finish
layer on the mirror surface; and polishing the thin-film finish
layer to provide a polished surface.
7. The method of claim 6, further comprising depositing a thin
reflective layer on the polished surface to produce a reflective
surface of the mirror.
8. The method of claim 1, further comprising finishing the mirror
surface of the magnesium substrate using a computer controlled
polishing process.
9. The method of claim 8, wherein finishing the mirror surface
includes magnetorheologically finishing the mirror surface.
10. The method of claim 1, wherein single point diamond turning the
first surface of the magnesium substrate includes using a free-form
single point diamond turning process.
11. An optical apparatus comprising: a mirror including a magnesium
substrate having a first surface that provides a reflective mirror
surface for the mirror, wherein the magnesium substrate is
thixotropically molded magnesium AZ91-D.
12. The optical apparatus of claim 11, wherein the magnesium
substrate has a second surface on a side of the magnesium substrate
opposite the first surface, the second surface being configured
with at least one of weight-reducing features and stress-relieving
features.
13. The optical apparatus of claim 11, wherein the mirror further
includes: a thin-film finish layer deposited over a first surface
of the magnesium substrate, the thin-film finish layer having on a
side thereof opposite to the magnesium substrate a second surface
that is polished to improve a surface finish thereof; and a
reflective layer disposed over the second surface of the thin-film
finish layer, the reflective layer forming the reflective mirror
surface of the mirror.
14. The optical apparatus of claim 13, wherein the surface finish
of the thin-film finish layer is less than approximately 20
Angstroms RMS.
15. A method of manufacture of a reflective mirror comprising:
providing a magnesium substrate formed of one of magnesium and a
magnesium alloy, the magnesium substrate having a first surface;
single point diamond turning the first surface to produce the
reflective mirror.
16. The method of claim 15, wherein providing the magnesium
substrate includes casting magnesium to produce the magnesium
substrate.
17. The method of claim 15, wherein providing the magnesium
substrate includes thixotropically molding magnesium AZ91-D to
produce the magnesium substrate.
18. The method of claim 17, further comprising: depositing a
thin-film finish layer on the first surface of the substrate; and
polishing the thin-film finish layer to provide a polished
surface.
19. The method of claim 18, further comprising depositing a thin
reflective layer on the polished surface to produce a reflective
surface of the mirror.
20. The method of claim 15, further comprising plating the first
surface prior to single point diamond turning the first
surface.
21. The method of claim 15, further comprising finishing the first
surface using a computer controlled polishing process.
22. The method of claim 21, wherein finishing the first surface
includes magnetorheologically finishing the first surface.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority under 35 U.S.C.
.sctn.119(e) to co-pending U.S. Provisional Application No.
61/485,939 titled "MAGNESIUM MIRRORS AND METHODS OF MANUFACTURE
THEREOF" and filed on May 13, 2011, which is herein incorporated by
reference in its entirety.
BACKGROUND
[0002] High precision reflective mirrors are used in numerous
optical devices and applications. One existing technique for making
precision metal mirrors is to use a substrate of an aluminum alloy,
such as that commonly known in the art as aluminum 6061-T6, and to
carry out single point diamond turning (SPDT) of a surface on the
substrate, which then serves as the reflective surface. Aluminum
6061-T6 is primarily aluminum, with alloy elements of zinc,
chromium, iron, magnesium and silicon. The aluminum 6061-T6 alloy
is lightweight, is easily machined by SPDT, and has good long-term
stability. Accordingly, this alloy is commonly used to produce
reflective mirrors for optical devices.
SUMMARY OF INVENTION
[0003] Aspects and embodiments are directed to low density
reflective mirror fabrication processes that are compatible with
surface finishing techniques to provide broadband optical
performance in lightweight handheld devices. Various aspects and
embodiments include reflective mirrors formed of magnesium or
magnesium alloys, methods of manufacturing them, and optical
devices using them.
[0004] According to one embodiment, a method of manufacture of a
reflective mirror comprises providing a magnesium substrate having
a first surface, and single point diamond turning the first surface
of the magnesium substrate to produce a mirror surface.
[0005] In one example, the method further comprises thixotropically
molding a magnesium alloy to produce the magnesium substrate. The
act of thixotropically molding the magnesium alloy may include
forming weight-reducing and/or stress-relieving features on a
second surface of the substrate, the second surface being on a side
of the substrate opposite the first surface. Thixotropically
molding the magnesium alloy may include thixotropically molding
magnesium AZ91-D, for example. In another example, the method
further comprises casting one of magnesium and a magnesium alloy to
produce the magnesium substrate. The method may further comprise
finishing the first surface of the magnesium substrate after the
single point diamond turning, the finishing including depositing a
thin-film finish layer on the mirror surface, and polishing the
thin-film finish layer to provide a polished surface. In one
example, the finishing further includes depositing a thin
reflective layer on the polished surface to produce a reflective
surface of the mirror. In another example, the method further
comprises finishing the mirror surface of the magnesium substrate
using a computer controlled polishing process, such as
magnetorheological finishing for example. Single point diamond
turning the first surface of the magnesium substrate may include
using a free-form single point diamond turning process.
[0006] According to another embodiment, an optical apparatus
comprises a mirror including a magnesium substrate having a first
surface that provides a reflective mirror surface for the mirror,
wherein the magnesium substrate is thixotropically molded magnesium
AZ91-D.
[0007] In one example, the magnesium substrate has a second surface
on a side of the magnesium substrate opposite the first surface,
the second surface being configured with weight-reducing and/or
stress-relieving features. In another example, the mirror further
includes a thin-film finish layer deposited over a first surface of
the magnesium substrate, the thin-film finish layer having on a
side thereof opposite to the magnesium substrate a second surface
that is polished to improve a surface finish thereof, and a
reflective layer disposed over the second surface of the thin-film
finish layer, the reflective layer forming the reflective mirror
surface of the mirror. In one example, the surface finish of the
thin-film finish layer is less than approximately 20 Angstroms
RMS.
[0008] According to another embodiment, a method of manufacture of
a reflective mirror comprises providing a magnesium substrate
formed of one of magnesium and a magnesium alloy, the magnesium
substrate having a first surface, and single point diamond turning
the first surface to produce the reflective mirror.
[0009] Providing the magnesium substrate may include, for example,
casting magnesium to produce the magnesium substrate or
thixotropically molding a magnesium alloy (e.g., magnesium AZ91-D)
to produce the magnesium substrate. In one example, the method
further comprises depositing a thin-film finish layer on the first
surface of the substrate, and polishing the thin-film finish layer
to provide a polished surface. The method may further comprise
depositing a thin reflective layer on the polished surface to
produce a reflective surface of the mirror. In one example, the
method further comprises plating the first surface prior to single
point diamond turning the first surface. In another example, the
method further comprises finishing the first surface using a
computer controlled polishing process, such as magnetorheological
finishing, for example.
[0010] Still other aspects, embodiments, and advantages of these
exemplary aspects and embodiments, are discussed in detail below.
Embodiments disclosed herein may be combined with other embodiments
in any manner consistent with at least one of the principles
disclosed herein, and references to "an embodiment," "some
embodiments," "an alternate embodiment," "various embodiments,"
"one embodiment" or the like are not necessarily mutually exclusive
and are intended to indicate that a particular feature, structure,
or characteristic described may be included in at least one
embodiment. The appearances of such terms herein are not
necessarily all referring to the same embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Various aspects of at least one embodiment are discussed
below with reference to the accompanying figures, which are not
intended to be drawn to scale. The figures are included to provide
illustration and a further understanding of the various aspects and
embodiments, and are incorporated in and constitute a part of this
specification, but are not intended as a definition of the limits
of the invention. Where technical features in the figures, detailed
description or any claim are followed by references signs, the
reference signs have been included for the sole purpose of
increasing the intelligibility of the figures and description. In
the figures, each identical or nearly identical component that is
illustrated in various figures is represented by a like numeral.
For purposes of clarity, not every component may be labeled in
every figure. In the figures:
[0012] FIG. 1 is a flow diagram illustrating one example of a
manufacturing process for a reflective mirror according to aspects
of the invention;
[0013] FIG. 2 is an image of a plurality of magnesium samples that
have been diamond point turned on a lathe according to aspects of
the invention;
[0014] FIG. 3A is a diagram of one example of a thixotropically
molded mirror substrate incorporating engineered structural
features on the non-reflective surface according to aspects of the
invention;
[0015] FIG. 3B is a diagram of a portion of FIG. 3A illustrating an
additional structural feature according to aspects of the
invention;
[0016] FIG. 4 is a diagrammatic fragmentary sectional view of one
example of a high precision magnesium mirror according to aspects
of the invention;
[0017] FIG. 5 is a graph illustrating test results for example
magnesium mirrors according to aspects of the invention to
demonstrate optical stability of the mirrors over time;
[0018] FIG. 6A is an image of a portion of an example
thixotropically molded magnesium AZ91-D mirror to demonstrate the
grain structure and achievable surface finish according to aspects
of the invention;
[0019] FIG. 6B is a fringe intensity image corresponding to FIG.
6A;
[0020] FIG. 6C is an enlarged image of the portion 310 identified
in FIG. 6A showing grain structure;
[0021] FIG. 6D is a fringe intensity image corresponding to FIG.
6C;
[0022] FIG. 7A is an image of a portion of an example
thixotropically molded magnesium AZ91-D mirror to demonstrate
surface finish of the reflective surface according to aspects of
the invention;
[0023] FIG. 7B is a fringe intensity image corresponding to FIG.
7A;
[0024] FIG. 8A is an image illustrating the surface finish of the
diamond point turned surface of an example magnesium mirror
according to aspects of the invention;
[0025] FIG. 8B is an image illustrating the surface finish of the
example mirror of FIG. 8A after application of a finish layer
according to aspects of the invention;
[0026] FIG. 8C is an image illustrates the surface finish of a
polished surface of the mirror of FIGS. 8A and 8B according to
aspects of the invention;
[0027] FIG. 9A is an image of a portion of an example of a
magnesium substrate illustrating the surface finish after SPDT
according to aspects of the invention;
[0028] FIG. 9B is an image of the surface of the example substrate
of FIG. 9A after application of a thin-film finishing process
according to aspects of the invention;
[0029] FIG. 9C is an image of the surface of the example substrate
of FIGS. 9A and 9B after magnetorheological finishing according to
aspects of the invention;
[0030] FIG. 10A is a fringe intensity image of the surface of one
example of a magnesium substrate after SPDT according to aspects of
the invention;
[0031] FIG. 10B is a corresponding fringe intensity image of
example substrate of FIG. 10A after magnetorheological finishing
according to aspects of the invention;
[0032] FIG. 10C is a fringe intensity image of the surface of
another example of a magnesium substrate after SPDT according to
aspects of the invention; and
[0033] FIG. 10D is a corresponding fringe intensity image of the
surface of the example substrate of FIG. 10C after
magnetorheological finishing according to aspects of the
invention.
DETAILED DESCRIPTION
[0034] Modern precision optical devices are configured to perform a
variety of different functions, including multi-wavelength imaging
(e.g., visible and infrared imaging) as well as laser ranging,
targeting and/or designation. Recently, there has been increased
interest in developing person-portable (e.g., handheld)
multi-function, multi-wavelength optical devices. In order for
these devices to both perform as intended/desired and to be easily
person-portable, they should be able to maintain optical alignment
(also referred to as "boresight") over a range of temperatures, be
lightweight (e.g., weigh only a few pounds), and be affordable in
volume. Conventional multi-aperture refractive optical assemblies
have been unable to affordably and reliably meet these goals. The
use of reflective mirrors in the optical assemblies resolves the
concern of maintaining boresight over temperature; however, in the
context of high-volume person-portable, precision optical devices,
conventional aluminum alloy mirrors have several disadvantages,
including weight and cost.
[0035] Accordingly, aspects and embodiments are directed to
reflective mirrors that are formed by carrying out a single point
diamond turning (SPDT) process on a low density substrate, such as
magnesium or a magnesium alloy for example, rather than an aluminum
alloy substrate. Magnesium is approximately 35% less dense than
aluminum 6061-T6 which is commonly used to form precision
reflective mirrors. In addition, embodiments of magnesium mirrors
may include engineered structural features that further reduce the
weight of the mirror, as discussed further below. Accordingly, the
use of magnesium mirrors may provide a significant weight advantage
for person-portable devices. Magnesium mirrors may also offer
significant cost advantages over aluminum mirrors, as discussed
further below. Examples discussed herein demonstrate that mirror
substrates formed of cast or molded magnesium or magnesium alloys
can be manufactured using SPDT to achieve broadband optical surface
quality. In addition, magnesium mirrors are demonstrated to be
compatible with surface finishing techniques to improve the surface
finish to beyond present SPDT capabilities, as discussed further
below.
[0036] It is to be appreciated that embodiments of the methods and
apparatuses discussed herein are not limited in application to the
details of construction and the arrangement of components set forth
in the following description or illustrated in the accompanying
drawings. The methods and apparatuses are capable of implementation
in other embodiments and of being practiced or of being carried out
in various ways. Examples of specific implementations are provided
herein for illustrative purposes only and are not intended to be
limiting. In particular, acts, elements and features discussed in
connection with any one or more embodiments are not intended to be
excluded from a similar role in any other embodiment.
[0037] Also, the phraseology and terminology used herein is for the
purpose of description and should not be regarded as limiting. Any
references to embodiments or elements or acts of the systems and
methods herein referred to in the singular may also embrace
embodiments including a plurality of these elements, and any
references in plural to any embodiment or element or act herein may
also embrace embodiments including only a single element. The use
herein of "including," "comprising," "having," "containing,"
"involving," and variations thereof is meant to encompass the items
listed thereafter and equivalents thereof as well as additional
items. References to "or" may be construed as inclusive so that any
terms described using "or" may indicate any of a single, more than
one, and all of the described terms. Any references to front and
back, left and right, top and bottom, upper and lower, and vertical
and horizontal are intended for convenience of description, not to
limit the present systems and methods or their components to any
one positional or spatial orientation.
[0038] Where the term "magnesium" is used as an adjective herein,
it is intended to cover both pure magnesium and magnesium alloys.
Magnesium alloys are those compounds having magnesium as the
primary or majority component.
[0039] Referring to FIG. 1 there is illustrated a flow diagram of
one example of a method of manufacturing a magnesium mirror
according to one embodiment. Embodiments and examples of the
manufacturing process are discussed below with continuing reference
to FIG. 1.
[0040] According to one embodiment, a mirror is formed from a
magnesium substrate having a surface which is machined and
optionally further processed to provide the reflective surface of
the mirror. In one embodiment, the surface of the substrate is
machined using precision technique known as SPDT step 110). As
known to those skilled in the art, SPDT is a process of mechanical
machining of precision elements using lathes equipped with natural
or synthetic diamond-tipped single-point cutting tools. The process
of diamond turning is widely used to manufacture high-quality
aspheric optical elements from metals (such as aluminum 6061-T6),
plastics, and other materials. It has now been demonstrated, as
disclosed herein, that magnesium and magnesium alloys may be
diamond point turned to achieve an optical quality surface. FIG. 2
illustrates an image of several different magnesium samples 210
that were diamond point turned on a lathe. The article 220 is
present in the image as a reference to demonstrate reflection by
the reflective surfaces of the magnesium mirror samples 210.
[0041] The surface finish quality (smoothness) of the reflective
surface of the mirror is measured as the averaged (RMS)
peak-to-valley (PV) of the microscopic features left by the SPDT
tool. Surface finish is generally measured using a
three-dimensional (3D) white light scanning interferometer. Diamond
point turning of aluminum 6061-T6 can typically achieve a surface
finish of approximately 80 .ANG. RMS, with the surface finish
quality being limited by defects or artifacts left after SPDT by
alloy elements such as zinc, chromium and iron. Examples discussed
below demonstrate that SPDT of magnesium substrates can achieve a
surface finish as good as or better than aluminum 6061-T6 using the
same or similar manufacturing processes.
[0042] Referring again to FIG. 1, in one embodiment, the SPDT
process 110 includes using a free-form SPDT process (step 140) to
achieve manufacturing of off-axis mirrors in a surface-normal, or
on-axis, position, thereby eliminating excess mirror bulk weight.
An off-axis optical device is one in which the optical axis of the
aperture is not coincident with the mechanical center of the
aperture, and the optical surface is therefore rotationally
non-symmetric. SPDT of rotationally non-symmetric surfaces may be
achieved using a slow tool servo device. In such a device, the
diamond turning lathe includes two linear axes (x- and z-axes) and
a spindle or rotary axis (c-axis). The diamond tool is mounted
along the z-axis of the lathe, and the optical device with the
rotationally non-symmetric surface is mounted on the c-axis. The
optical equation that defines the optical surface to be machined is
used to create a tool path that controls movement of the diamond
tool across the work surface of the optical device. The tool path
is encoded as a computer-readable file, and the SPDT machine is
controlled by the computer to execute the tool path to produce the
rotationally non-symmetric optical surface. Conventionally,
off-axis mirrors are designed to overcome deformations caused by
centrifugal forces. By using free-form SPDT, centrifugal forces are
greatly reduced and accordingly, the mirror design may be optimized
to minimize weight, for example by incorporating engineered
structural features as discussed below.
[0043] The magnesium substrate upon which the SPDT process is to be
performed may be produced, for example, by casting magnesium or a
magnesium alloy (step 120), machining magnesium or a magnesium
alloy (step 125), or by thixotropically molding a magnesium alloy
(step 130). Other methods by which the magnesium (or magnesium
alloy) substrate may be produced include forging, stamping and
hot-pressing. The magnesium substrate may be plated (step 115), for
example, electro-plated with magnesium, copper plated, or nickel
plated using an electroless process.
[0044] Cast magnesium substrates exhibit some porosity which limits
the surface finish achievable with SPDT. However, as discussed
below, according to one embodiment magnesium substrates have been
demonstrated to be compatible with certain surface finishing
processes which can improve the surface finish of the magnesium
substrate to optical quality for many applications. Thixotropic
fluids shear when the material flows, but thicken when standing.
For magnesium alloys, thixotropic molding uses a machine similar to
injection molding. In one example of a single step thixomolding
process, room temperature magnesium alloy chips (with a chip size
of approximately 4 mm) are fed into a heated barrel (maintained
under an argon atmosphere to prevent oxidation of the magnesium
chips), where they are heated into a semi-solid state and shearing
force is applied to generate a globular slurry. The slurry may then
be injected into a die for molding, similar injection molding of
plastics.
[0045] Magnesium alloys have been found to be well suited to
thixotropic molding for mirror substrates. Some examples of
suitable magnesium alloys for mirror substrates include magnesium
AZ91-D and AM60B. Magnesium AZ91-D is a high-purity alloy
comprising approximately 90% magnesium, 9% aluminum, and trace
amounts of zinc, silicon and iron (less than 0.005% iron).
Magnesium AZ91-D has excellent corrosion resistance and is widely
available and relatively inexpensive. Table 1 below contains
example physical properties of magnesium AZ91-D.
TABLE-US-00001 TABLE 1 Density 1830 kg/m.sup.3 (at 20.degree. C.)
Solidus temperature 470.degree. C. Liquidus temperature 595.degree.
C. Kinematic viscosity 1.0 .times. 10.sup.-6 m.sup.2/s (at
590.degree. C.) Specific heat 1014 J/kgK (at 20.degree. C.) Thermal
conductivity 72 W/mK (at 20.degree. C.) Latent heat 3.73 .times.
10.sup.5 J/kg
[0046] According to one embodiment, magnesium AZ91-D is well suited
to thixomolding for mirror substrates due to its ability to become
amorphous, resulting in well-refined grain structure that leads to
the excellent surface finish quality achievable with the alloy, as
demonstrated in the examples discussed below. In addition, since
during thixomolding the alloy is mixed under high temperature and
pressure (for example, temperatures of approximately 560-630
degrees Celsius and an injection pressure of approximately 500-1200
kgf/cm2), the resulting substrate is very stable and dense, and
lacks the porosity present in cast magnesium substrates. This also
contributes to the ability to obtain surface finishes of less than
80 .ANG. RMS with thixotropically molded magnesium substrates.
Furthermore, thixomolding is a well-developed, inexpensive process,
allowing the magnesium substrates to be produced far more cost
effectively (particularly in volume) than comparable aluminum
6061-T6 substrates. For example, the cost of a thixotropically
molded magnesium mirror substrate may be more than an order of
magnitude less than a comparable aluminum 6061-T6 mirror
substrate.
[0047] It has further been found, as disclosed herein, that wear on
the diamond cutting tool is significantly reduced for magnesium
substrates, including substrates formed of thixotropically molded
magnesium AZ91-D alloy, compared with the tool wear from processing
substrates formed of the aluminum 6061-T6 alloy. As discussed
further below, SPDT was performed on 15 thixotropically molded
magnesium substrates, after which there was found to be no
measurable tool variation. By contrast, tool variation typically
would be measurable after processing 15 aluminum 6061-T6
substrates, and the tool cutting path would need to be modified to
account for the tool variation. Tool wear is a significant cost
factor, particularly for high-volume devices. A significant factor
contributing to tool wear from aluminum 6061-T6 alloy substrates is
the presence of substantial amounts of iron in the alloy which
reacts with the diamond tip of the cutting tool, causing chemical
wear. By contrast, several magnesium alloys, including magnesium
AZ91-D and other magnesium AZ91 alloys, contain only a trace amount
of iron (no more than 0.005% for magnesium AZ91-D and magnesium
AZ91-E, another high-purity alloy with excellent corrosion
resistance) or even no iron at all, and therefore chemical wear on
the diamond tip is greatly reduced. In addition, magnesium alloys
such as the AZ91 series of alloys are softer, more ductile and less
dense (approximately 35%) than the aluminum 6061-T6 alloy,
resulting in decreased mechanical wear on the cutting tool.
Although purer aluminum alloys (containing less iron) are
available, aluminum 6061-T6 has been demonstrated to be very stable
over temperature and time, and is therefore presently the most
popular alloy used for precision optical mirrors. Thus, the reduced
tool wear of magnesium alloys relative to aluminum 6061-T6 is a
significant advantage. Reduced tool wear may result in reduced
set-up time and/or labor costs associated with monitoring and/or
modifying the tool during the fabrication process, as well as
reduced cost per device for tool replacement.
[0048] A further advantage of using a thixotropically molded
magnesium alloy is that the die used in the molding process may be
configured to impart any of numerous shapes and features to the
magnesium substrate. For example, referring to FIG. 3A there is
illustrated an example of the back side (i.e., the non-reflective
surface) of a magnesium mirror 310 which incorporates engineered
structural features, such as support struts 320 and grooves or
recesses 330. The back side of the mirror 310 may be designed with
a multitude of thin support webs to provide the mirror with high
stiffness and low weight without additional machining costs because
these features may be easily thixotropically molded. In addition,
the mirror 310 may incorporate features to improve manufacturing
reliability. For example, FIG. 3B illustrates a portion of the
mirror 310 including isolation cuts 340 to decrease SPDT mounting
stress in the mirror substrate. Mounting features may also be
incorporated into the mirror structure. These and other features
may be easily and inexpensively molded into the magnesium substrate
during the thixomolding process. By contrast, at present machining
such features into an aluminum mirror is difficult and/or
prohibitively expensive. In one example, a mirror 310 including the
illustrated structural features formed of thixotropically molded
magnesium AZ91-D and having a 3 inch by 3 inch (75 mm by 75 mm)
aperture has a weight of approximately 1.47 ounces (42 grams).
Thus, due to the lower density of magnesium relative to aluminum
and the ability to include weight-decreasing features, embodiments
of the magnesium mirrors may be three to four times lighter than
comparable mirrors formed of aluminum 6061-T6, while also being
more affordable to produce in volume and having excellent optical
properties such as surface finish quality. Lighter mirrors not only
reduce the weight of the optical assemblies in which they are used
directly, but also allow for lighter gimbals, torquers, angle
resolvers and other devices that move the optical assemblies, which
may significantly reduce the overall weight of systems.
[0049] An important consideration for mirrors used in precision
optical devices is optical stability over time and temperature. As
discussed below, experimental data has been obtained demonstrating
that magnesium mirrors can become optically stable over time with
proper thermal conditioning, and also that surface finishes equal
to or better than those achievable with SPDT aluminum alloys can be
obtained. These results are unexpected given that Magnesium is
typically considered to be optically unstable. The commonly-used
aluminum 6061-T6 alloy contains magnesium as an alloy element. This
magnesium alloy element in aluminum 6061-T6 frequently causes
defects in the surface of aluminum 6061-T6 mirrors due to oxidation
and other reactions, particularly if the mirror substrate is
exposed to a humid environment. This known concern regarding the
presence of magnesium in the aluminum 6061-T6 alloy, together with
the known highly reactive nature of magnesium, suggests that
attempts to form optical components from magnesium would be
unsuccessful due to an expectation that the optic would not be
stable and that acceptable surface finish quality would not be
achievable.
[0050] As discussed above, according to certain embodiments,
magnesium mirrors formed using SPDT are compatible with surface
finishing techniques to improve the surface smoothness (step 150).
The examples presented below demonstrate that magnesium substrates
can be produced using SPDT to have a surface finish quality of at
least between approximately 58 .ANG. and 80 .ANG.. Finishes of this
quality provide adequately low scatter for many applications,
particularly those in which the reflected radiation of interest has
a relatively long wavelength, for example, greater than
approximately 3 microns. For shorter wavelengths, for example
applications using visible light, the surface finish may need to be
improved in order to achieve sufficiently low scatter. In addition
as discussed above, some cast magnesium substrates may not have
sufficiently good surface finish after SPDT, at least for some
applications, and therefore it may be desirable to improve the
surface finish. Accordingly, in some embodiments a finishing
process (step 150) may be applied after SPDT to improve the surface
finish of the mirror.
[0051] One process for producing an aluminum mirror having a
surface finish quality that is improved over the 80-90 .ANG. RMS
typically achievable with SPDT is described in U.S. Pat. No.
6,921,177, which is herein incorporated by reference in its
entirety. This process includes forming a thin-film finish layer
over the surface of the mirror substrate using thin-film techniques
and polishing the surface of the finish layer. A thin reflective
layer is then formed on the polished surface of the finish layer.
Optionally, a thin overcoat may be applied over the reflective
layer to protect the reflective layer and/or increase the
reflectance within a selected waveband.
[0052] In certain examples, embodiments of the finishing process
described in U.S. Pat. No. 6,921,177 are applied to the diamond
turned magnesium mirror to improve the surface finish. Referring to
FIG. 4 there is illustrated a diagrammatic fragmentary sectional
view of one example of a high precision magnesium mirror 410. The
mirror 410 includes a substrate 420 formed of cast or
thixotropically molded magnesium or magnesium alloy (for example,
magnesium AZ91-D) as discussed above. The substrate 420 has a
surface 430 that is processed using SPDT as discussed above. After
the SPDT process (step 110) is complete, a finish layer 440 is
deposited on the surface 430 (step 160) using thin-film vapor
deposition, for example. As discussed in U.S. Pat. No. 6,921,177,
the finish layer 440 may comprise any suitable material that can be
polished, including for example, a nickel-chromium alloy or
amorphous silicon. The finish layer 440 may have a thickness of
approximately 5000 .ANG.. Since the finish layer 440 is thin, its
upper surface 450 will initially conform at least to some degree to
the SPDT surface 430. Accordingly, in step 170, the surface 450 is
polished. A thin reflective layer 460 is then formed on the
polished surface 450 (step 180) using thin-film vapor deposition
techniques, for example. As discussed in U.S. Pat. No. 6,921,177,
the thin reflective layer may comprise any suitable reflective
material, for example, silver, gold or aluminum. The reflective
layer may have a thickness of approximately 2000 to 5000 .ANG.. The
reflective layer 460 provides a high precision reflective surface
470 which can reflect radiation 480. Since the reflective layer 460
is a thin film layer, its surface will conform, at least
substantially, to the polished surface 450.
[0053] According to another embodiment, the surface figure of a
magnesium mirror may be improved beyond present diamond point
turning capabilities by applying magnetorheological finishing (step
190) to the mirror surface after the SPDT process. In other
examples, computer controlled polishing (CCP), such as
magnetorheological finishing for example, may be applied directly
to a bare or plated magnesium substrate (following step 115 in FIG.
1). Magnetorheological finishing (MRF) is a computer-controlled
precision surface finishing process. As discussed above, the
surface finish of the mirror after the SPDT process may be measured
interferometrically. MRF uses the interferometer data to
characterize a removal map of the optical surface that allows the
surface to be selectively machined to reduce peak-to-valley
variation. The MRF process uses an interferometrically controlled
magnetorheological (MR) finishing slurry (a suspension of
micrometer-sized magnetic particles composed of carbonyl iron in a
carrier field) as a polishing tool. A thin ribbon of the MR slurry
is drawn onto a rotating wheel. An electromagnet below the wheel
causes the MR slurry to stiffen in milliseconds. The MR slurry
returns to its original viscosity as it leaves the electromagnetic
field of the electromagnet. Shear stress caused by pressing the
optical surface against the MR slurry creates polishing pressure
over the optical surface. A computer-controlled algorithm generates
the interferometrically characterized removal map and calculates
the dwell time and position of the MR slurry to accomplish
deterministic removal of selected portions of the substrate surface
to polish the surface and "smooth" the surface figure of the
finished substrate. MRF or other CCP methods may be applied to the
magnesium substrate alone or in combination with the thin-film
finishing process discussed above.
EXAMPLES
[0054] The function and advantages of these and other embodiments
will be more fully understood from the following examples. The
examples are intended to be illustrative in nature and are not to
be considered as limiting the scope of the systems and methods
discussed herein. In each example discussed below, SPDT was carried
out on spherical magnesium substrates using a using a Precision 350
SPDT lathe produced by Precitech (of Keene, N.H.). Several mirrors
were diamond point turned, thermal conditioned (as discussed in
Example 1 below), and finished. Data and test results for these
mirrors are provided in the examples below. The images of the
processed substrates discussed below were taken using an
interferometer produced by Zygo Corporation (Middlefield, Conn.).
The mirror surfaces were of a spherical shape to facilitate
interferometric testing, and to minimize measurement errors that
could skew the results of the long term stability testing discussed
in Example 1 below.
Example 1
[0055] As discussed above, long-term optical stability is an
important criterion for precision optical mirrors. Accordingly,
accelerated long term stability testing was performed on the
example magnesium mirrors to determine their optical stability. 15
thixotropically molded magnesium AZ91-D mirrors were diamond point
turned and divided into three groups. Twelve of these mirrors (four
from each of the three groups) were then conditioned using three
different conditioning cycles/processes, one applied to each group
of mirrors. The twelve mirrors were thermal cycled from 225.degree.
F. to -30.degree. F. and re-tested interferometrically after each
of the ten cycles. Referring to FIG. 5, which illustrates a graph
of the test results for the mirrors, the conditioning cycles
included a hot/cold test (data points 510), a soak test (data
points 520), and a cryogenic test (data points 530). Each test was
performed over the ten temperature cycles, as specified in
MIL-STD-810, revision "G" promulgated by the Institute of
Environmental Sciences and Technology (IEST).
[0056] The target specification for the mirror is given in terms of
RMS surface figure (deviation between an actual optic and its ideal
surface) with a target maximum being 0.030, corresponding to an RMS
wavefront error of approximately .lamda./33. As illustrated in FIG.
5, the data demonstrates that magnesium AZ91-D mirrors may be
optically stable over time.
Example 2
[0057] This example demonstrates the disclosed approaches of
applying SPDT to a thixotropically molded magnesium substrate made
of magnesium AZ91-D.
[0058] Referring to FIGS. 6A and 6B there are illustrated images of
a microscopic view of the surface of an example mirror after SPDT.
FIG. 6A illustrates a magnified image (approximately 20.times.
magnification) of a portion of the reflective surface of the
mirror. The surface finish was measured to be approximately 90
.ANG.. A surface finish of approximately 80-90 .ANG. RMS is typical
for aluminum 6061-T6 substrates. Thus, this example demonstrates
that a surface finish quality at least equal to that of the
aluminum 6061-T6 alloy is achievable with the thixotropically
molded magnesium AZ91-D alloy. The turning marks 620 present in
FIGS. 6A and 6B are SPDT process-related, and were created because
this example mirror was cut with "air only," no coolant, and thus
small "friction induced" tooling sleeks are present due to the lack
of coolant lubrication.
[0059] FIG. 6C is a further magnified image (approximately
100.times. magnification) of the portion of FIG. 6A within boundary
610. FIG. 6D is a corresponding image illustrating the peak and
valleys in the surface finish. As can be seen with reference to
FIG. 6C, there is a small amount of grain structure; however, this
example demonstrates that surface finishes approaching 30 .ANG. RMS
are achievable with the magnesium AZ91-D alloy.
Example 3
[0060] This example further demonstrates the disclosed approaches
of applying SPDT to a thixotropically molded magnesium substrate
made of magnesium AZ91-D. In this example, a coolant (odorless
mineral spirits) was used to provide lubrication and eliminate the
"sleeking" (e.g., turning marks 620) present in Example 2. In
addition, the mirror substrate used in this example was formed with
engineered structural features on the back surface, as discussed
above.
[0061] Referring to FIGS. 7A and 7B there are illustrated images of
a portion of the reflective surface of the example thixotropically
molded magnesium mirror substrate. In this example, a surface
finish of approximately 58 .ANG. RMS was achieved. This level of
surface finish quality is 20 to 30% better than typically achieved
for aluminum 6061-T6 substrates and may provide sufficiently low
scatter for many applications, reducing or removing the need to
perform additional finishing processes.
Example 4
[0062] As discussed above, selected ones of the diamond turned
mirror substrates were finished using the processed discussed
herein. This example demonstrates the compatibility of a mirror
substrate made of thixotropically molded magnesium AZ91-D with an
embodiment of the finishing process described in U.S. Pat. No.
6,921,177. The finish layer applied over the diamond turned surface
was made of silicon and was approximately 12,000 .ANG. thick. The
finish layer was polished, as discussed above, to form the mirror
surface.
[0063] FIGS. 8A-8C illustrate images of a portion of the example
mirror formed as discussed above. Experimental data demonstrates
the compatibility of the thixotropically molded magnesium AZ91-D
substrate with the finishing process discussed above.
[0064] FIG. 8A illustrates the surface finish of the diamond point
turned surface 430 of the example mirror prior to the finishing
steps 160-180. The surface has a surface finish of approximately 80
.ANG. RMS.
[0065] FIG. 8B illustrates the surface finish of the surface 450 of
the example mirror after application of the finish layer and prior
to the polishing step 170. The surface still has a surface finish
of approximately 80 .ANG. RMS, and no adverse material or process
interactions are present.
[0066] FIG. 8C illustrates the surface finish after the polishing
step 180. The surface of the mirror now has a surface finish of
approximately 13 .ANG. RMS, demonstrating that magnesium substrate
is compatible with the surface finishing techniques discussed in
U.S. Pat. No. 6,921,177 to achieve exceptional surface finish
quality.
Example 5
[0067] The disclosed approaches have been further demonstrated by
applying magnetorheological finishing to the example magnesium
mirror substrates after application of the thin-film finishing
process discussed above. In this example, after SPDT, a finish
layer of silicon, approximately 12,000 .ANG. thick, was applied, as
discussed above in Example 4. The finish layer was pre-polished
using a sub-micron diamond slurry, and then magnetorheological
finishing was applied to the polished surface to improve the
surface figure and finish. Experimental data, as illustrated in
FIGS. 9A-9C and 10A-10d demonstrated the compatibility of the
magnesium substrate with the MRF process.
[0068] FIG. 9A illustrates the surface finish of the diamond point
turned surface of the example mirror. The surface has a surface
finish of approximately 80 .ANG. RMS, similar to the examples
discussed above. FIG. 9B illustrates the surface finish of the
example mirror after the thin-film deposition and pre-polishing
steps 160 and 170 discussed above. The surface has a surface finish
of approximately 20 .ANG. RMS. FIG. 9C illustrates the surface
finish after the MRF process is also applied to the surface. The
MRF process improves the surface finish of the mirror to
approximately 10 .ANG. RMS. Thus, MRF and/or thin-film finishing
processes may be applied to the magnesium mirror substrates to
improve the surface finish to below 20 .ANG. RMS and a surface
figure of less than .lamda./20.
[0069] FIGS. 10A-10D illustrate further experimental data for two
of the example mirrors.
[0070] FIG. 10A is an image of the surface of the first example
magnesium substrate after the SPDT process and after application of
the thin-film finish layer discussed above. FIG. 10B is a
corresponding image of the surface of the same example magnesium
substrate after the application of MRF. In this example, the MRF
process improved the surface figure to approximately
.lamda./100.
[0071] FIG. 10C is an image of the surface of another example
magnesium substrate after the SPDT process and after application of
the thin-film finish layer discussed above. FIG. 10D is a
corresponding image of the surface of the same example magnesium
substrate after the application of MRF. In this example, the MRF
process improved the surface figure to approximately
.lamda./80.
[0072] These examples demonstrate that magnesium substrates can be
processed using SPDT to achieve optical quality mirror surfaces.
Surface finishing processes, including MRF, can be applied to
improve the surface finish and/or surface figure to beyond what is
presently achievable with SPDT. The magnesium substrates have been
demonstrated to be optically stable over time, and can be
cost-effectively designed to optimize weight, making them suitable
for precision person-portable optical devices.
[0073] Having described above several aspects of at least one
embodiment, it is to be appreciated various alterations,
modifications, and improvements will readily occur to those skilled
in the art. Such alterations, modifications, and improvements are
intended to be part of this disclosure and are intended to be
within the scope of the invention. Accordingly, the foregoing
description and drawings are by way of example only, and the scope
of the invention should be determined from proper construction of
the appended claims, and their equivalents.
* * * * *