Download Applications of Highly Reflective Silver Coatings for Car

Applications of Highly Reflective Silver Coatings
for Car Headlamp Reflectors
Torsten Schmauder, Stephan Küper
Leybold Optics GmbH, Alzenau
Keywords:
Headlamps, Reflectivity, Device Efficiency,…
1 Introduction
Modern car headlamps pose high requirements to their device efficiency. LED
light sources still fall short in luminous flux compared to HID lights and they often
have one or more optical elements to shape the beam. As a consequence, the
reflectivity of the (reflective) optical elements becomes one of the key factors for
the overall performance and efficiency of the device.
Further, conventional systems with halogen or HID light sources test the thermal
limits of the materials used in their design. Here, reflectivity is a key critical factor
for the thermal design of the components. Finally, with energy conservation
becoming more and more important, poor device efficiency becomes
unacceptable for environmental reasons.
Still, most of today’s head lamp reflectors are coated conventionally with
Aluminum and deliver a reflectivity between 85 and 90 % on a manufacturing
quality level. Silver, on the contrary, offers a reflectivity up to 97 % and can
therefore contribute to the improvement of the device efficiency.
One of the major challenges for applying Silver as the reflective material is its
corrosion stability. Even though Silver is a noble metal and Aluminum is
chemically very reactive, Aluminum can be passivated and protected to excellent
quality levels by mature and established manufacturing processes.
Finally, the cost of manufacturing Silver coated parts is important, as Silver
targets come with the price tag of a noble metal. However, a complete cost per
piece calculation reveals that the costs of Silver coated parts may be
overestimated if viewed in relation with the total cost of a headlamp and with the
performance increase achieved.
The paper discusses the advantages and challenges of reflectors which are
vacuum coated with Silver instead of Aluminum.
2 Optical Properties of Silver and Aluminum
2.1 Comparison of Spectra
Silver and Aluminum exhibit quite different reflection spectra. Silver rises to a
level of > 98 % for all wavelengths above 600 nm. In the visible range from 400
to 700 nm the integral reflectivity is 96.9 %. Aluminum, on the other hand has a
(theoretical) integral reflectivity of 92.5 % in the visible range and rises to app.
97.5 % in the infrared. In day to day production, however, the reflectivity of
Aluminum mirrors is frequently lower (sometimes below 85 % in the visible
range) due to the application of a corrosion protective siloxane top coat. The
calculations below are based on the ideal reflectivities of Aluminum plus Siloxane
top coat and Silver. The differences between Silver and Aluminum coated
reflectors calculated in this paper therefore represent a conservative estimation
for the achievable improvements.
100
99
98
Reflectivity [%]
97
96
95
94
93
92
Ag
91
90
200
Al + Siloxan TC
400
600
800
1000
1200
1400
1600
1800
2000
2200
2400
2600
Wavelength [nm]
Figure 1: Optical reflection spectra of mirrors coated with Aluminum with Siloxane top coat and
Silver respectively. Silver has a significantly higher reflectivity in the visible and near IR.
2.2 Device Efficiency
The device efficiency of car lights describes the percentage of the light emitted
by the source which actually leaves the device in the intended way and is
available for illumination. It is determined exclusively by the optical design of the
device and can be calculated from the optical data and reflection spectra of the
individual elements. Losses are due to absorption on reflective optical elements
(mirrors) and due to reflection on refractive elements (lenses, transparent
covers). For the illumination performance we consider the visible range from 400
to 700 nm for the calculations. In case of a single reflective optical element the
integral reflectivity is at the same time the device efficiency. In case of 2 reflective
elements (like in example 1) the device efficiency is the product of the two
integral reflectivities of both reflective elements. In all head lights there is also a
front lens, mostly made of Polycarbonate, which finally reflects another 10 % of
the light into the lamp body. As an example, in a head light device like in figure 1,
about 14.3 % of the initial light are lost only due to the reflectors, if they are
coated with Aluminum. After the front lens little more than 75 % of the initial light
is available for road illumination. If the mirrors were coated with Silver, the
reflective losses could be reduced to 6.2 %, which would lift the overall efficiency
after the front lens to 84.4 %. In total, the light output from the Silver coated
device will be 11 % higher than from the Aluminum coated device. The reflection
on the front lens could also be reduced drastically by interference anti reflex
coatings, which however are not topic of this paper.
Al Mirror 2
PC Front Lens
7,4 %
85,7 %
75,7 %
93,6 %
Al Mirror 1
100 %
7,4 %
10 %
Figure 2: Optical Losses in a head light utilizing two Aluminum reflectors for beam shaping. Due
to the reflectors alone, 14.3 % of the light is lost. At the PC front lens additional 10 % are reflected
within the device. Only 75.7 % of the light from the light source leave the device in the intended
way and are available for illumination
.
2.3 Heat Management
All light that is lost due to absorption is converted to heat in the system. For the
heat management, the range between 350 and 2500 nm needs to be considered
and covers most of the light that is emitted by a halogen or HID lamp. The heat
load onto a reflector is due to radiation and thermal convection. The part that is
due to radiation can be influenced by the reflective properties of the optical
elements. Because the absolute percentage of absorption is small (and still the
reflector materials are often stressed to their thermal limits), the relative
difference between Aluminum and Silver coated mirrors is large. The integral
reflectivity of Silver over the full range from 350 to 2500 nm is twice as high as for
Aluminum. Therefore, as an example, in a headlight device like in figure 1, the
heat load due to radiation on each mirror could be cut in half, if the mirrors were
coated with Silver instead of Aluminum.
3 Corrosion Protection
3.1 Silver vs. Aluminum
Aluminum reflectors are generally coated with a siloxane based top coat which is
deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD). This
mature technology provides an excellent protection against the current corrosion
tests (damp heat, sodium hydroxide, salt spray, climatic change etc.). The first
thing that comes to mind, if the protection of Silver layers is addressed, is to
simply copy the PECVD technology from Aluminum to Silver. The results of this
approach, however, are disappointing. Silver parts that are protected with a
siloxane based top coat do not reach anywhere near the performance of
Aluminum coated parts in accelerated climatic tests.
3.2 New Approaches
Therefore, new approaches have to be found to provide corrosion protection for
Silver. In the literature several ways have been explored, a large part of the work
being performed for solar concentrating mirrors. Progress was made with
several µm thick oxide layers and underlying layers from various metals including
Copper, Chromium and Nickel. Especially for the automotive industry however,
the costs of the layer stack always have to be kept in mind. In this study coating
stacks with underlying and covering metal layers like Copper, Aluminum or
Chromium have been tested besides the initial approach with PECVD siloxane
top coats. The different samples were evaluated by damp heat, damp UV and
climatic change tests. It could be shown that thin barrier layers on top of the
reflectors in combination with underlying protective and adhesion layers can
boost the climatic performance of Silver mirrors to promising levels.
The testing methods for Aluminum mirrors are as mature as the Aluminum
technology itself. It may be useful to reevaluate, if the same tests that are used
for Aluminum mirrors are also adequate to test Silver mirrors. Besides new
coating processes also new testing methods may be necessary to accompany
the development of better optical devices.
Figure 3: Progress in corrosion protection of Silver mirrors. From left to right: Silver with siloxane
PECVD topcoat, Silver with Al base coat plus PECVD top coat, Silver with copper base coat plus
PECVD top coat , Silver with metal top coat and Silver with metal base- and top coat. All samples
were submitted to 1 week climate change test (65 °C , 85 % r.H., -40 °C)
4 Cost Comparison
4.1 Silver vs. Aluminum
The automotive industry is very cost sensitive. Silver as a representative of the
noble metals clearly meets severe prejudices against the extra cost of a Silver
vs. an Aluminum mirror. Since the reflective layers are very thin however, many
reflectors can be made from an (admittedly expensive) Silver target. Further, the
remainder of the target as well as the material deposited on the shields can be
recycled. A true cost of ownership calculation shows that a reflector sized
20 X 20 cm2 can be coated with Aluminum for app. 17 € Cents, whereas the
same part with Silver is coated for app. 65 € Cents. The additional coating costs
need to be put into perspective with
a) the costs that can be saved because of the better optical performance
- lower power light sources
- lower thermal grade reflector material
- lower energy consumption during use and/or
b) the costs that would have to be spent to reach the higher performance in
terms of lighting.
Compared to the overall costs of an advanced automotive headlight the extra
costs of Silver coating are small.
5 Conclusions
Coating of reflectors in car lights with Silver offers the advantage of increased
device efficiency and lower thermal load. Considering the fact that LED
headlights still fall short of their Xenon or Halogen competitors, every possible
percent of light that LEDs produce should be delivered to the road instead of
being converted to heat inside the head light. Further, expensive materials that
are necessary just to tolerate the heat load produced by the light sources, could
be avoided. In a headlamp device with two reflective elements for beam shaping
Silver coated reflector surfaces increase the overall device efficiency by 11 % (or
more, if the Aluminum coating is of poor quality). At the same time the heat load
to the reflectors due to absorption is reduced by 50 %.
Silver competes with Aluminum as a cheap, mature coating material. In order to
utilize Silver in the automotive industry, traditional testing and performance
evaluation may have to be reevaluated against today´s requirements. As a result
further (co-)development of the manufacturing methods may be necessary until a
mature technology like Aluminum coating will be achieved. However, the benefits
in performance will be well worth the efforts.
References:
1. C.E. Kennedy, K. Terwillger. “Optical durability of candidate solar reflectors”,
Journal of Solar Energy Engineering., Vol. 127, p. 262, 2005
2. Jesse D. Wolfe, Norman L. Thomas, U.S. Patent #6,078,425, “Durable Silver
coating for mirrors“, June 20, 2000
3. J. Wolfe, D. Sanders, “Optical & Environmental Performance of Durable Silver
Mirror Coatings Fabricated at LLNL“, Mirror technology days 2004, Huntsville,
AL, 17-19 Aug 2004