Note: Descriptions are shown in the official language in which they were submitted.
RD-17884
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ABRASION-RESISTANT PLASTIC ARTICLES
AND METHOD FOR MAKING THEM
Background of the Invention
=
This invention relates generally to plastic
articles, and more particularly to polycarbonate articles
exhibiting improved abrasion resistance, together with
improved resistance to cracking under exposure to thermal
and mechanical stresses.
Engineering resins are well-known, commercially
available materials possessing physical and chemical proper-
ties which are useful in a wide variety of applications.
For example, polycarbonates, because of their excellent
breakage resistance, have replaced glass in many products,
such as automobile headlamps and stoplight lenses; safety
shields in windows, architectural glazing, and the like.
However, major defects exhibited by polycarbonates are their
very low scratch-resistance and their susceptibility to
ultraviolet light-induced degradation.
Methods for improving the scratch-resistance of
plastics such as polycarbonate have involved disposing an
inorganic protective layer on the surface of the polycar-
bonate. For example, in U.S. Patent 4,328,646, issued to
Kaganowicz, an abrasion-resistant article is formed by
subjecting a mixture of hardcoating precursors to a glow
discharge, and depositing the product directly on a plastic
substrate as a very thin film. However, inorganic hardcoat-
ings such as silicon dioxide (SiO2) deposited directly onto
plastics such as polycarbonate have performance problems
when the system is subjected to stresses produced by mechan-
ical or thermal effects. These problems are due to the
difference in property characteristics of inorganic and
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plastic materials. For example, the thermal expansion
coefficient for polycarbonate is about 7 x 10-5 m/m/C,
while the coefficient for Pyrex~ glass is 3 x 10-6
m/m/C. These differences result in tangential
stresses at the interface of the plastic and the
hardcoating which may in turn cause cracking of the
hardcoating as a stress relief mechanism, especially
when the article is subjected to various
heating/cooling cycles. In attempting to alleviate
this problem, Hall et al. and Geffcken et al. in U.S.
Patents 4,190,681 and 3,713,869, respectively, proposed
the use of an intermediate layer to improve adhesion
between the hard inorganic layer and the plastic layer.
There is a continuing interest in improving
methods for forming articles having still greater
abrasion resistance while also exhibiting improvements
in various other physical properties.
It is therefore an object of the present invention
to provide a method for forming plastic articles having
a high level of abrasion resistance, with improved
resistance to cracking under exposure to thermal and
mechanical stresses.
It is another object of the present invention
to provide an improved method for applying smooth,
hard, transparent layers over a thermoplastic
substrate.
It is yet another object of the present
invention to provide a thermoplastic article having
disposed thereover
... .
RD-17884
1 337484
a protective top layer characterized by uniform thickness,
high abrasion resistance, and freedom from pinholes and
microcracks.
Summary of the Invention
An improved method for forming an abrasion-resis-
tant plastic article has been discovered, comprising the
plasma-enhanced chemical vapor deposition of a coating
characterized by a gradual transition from a composition
consisting essentially of an interfacial material to a
composition consisting essentia~ly of an abrasion-resistant
material. The transition is achieved by gradually changing
the feed composition of the coating material precursors, as
described below.
Several different types of interfacial material
may be used, as further described below. Furthermore, many
different abrasion-resistant materials capable of being
applied by plasma-enhanced chemical vapor deposition (PECVD)
may be used. "Interfacial" is meant herein to describe a
material situated between the substrate and the abrasion-
resistant material and possessing some chemical character-
istics in common with each, as described in detail below.
Use of the method of the present invention for
plastic articles such as polycarbonates results in articles
having all of the typical attributes of polycarbonates, such
as high tensile and impact strength, while also exhibiting
excellent abrasion resistance. Furthermore, good adhesion
generally results between the polycarbonate substrate and
the coating formed thereon, which is also referred to herein
as a "gradational layer".
A further attribute of this method is that PECVD
may be carried out as disclosed herein at temperatures which
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~ 337484
are harmless to polycarbonate, in contrast to conventional
chemical vapor deposition (CVD) processes which require high
operating temperatures often damaging to polycarbonate
material. "Conven~ional" vapor deposition processes as used
herein denotes both CVD processes in which coating material
precursors react in the gas phase at elevated temperatures,
typically above 400C; and also denotes "physical vapor
deposition" processes in which preformed coating materials
are simply evaporated onto a substrate. These processes do
not involve the use of a plasma.
The scope of the present invention also includes
an article comprising a plastic substrate and an abrasion-
resistant gradational layer disposed thereon which consists
essentially of an interfacial material at a depth closest to
the surface of the substrate, the relative concentration of
the interfacial material gradually decreasing in a direction
perpendicular to the substrate surface and being replaced by
a corresponding concentration of an abrasion-resistant
material.
Detailed Description of the Invention
The article formed by the method of the present
invention may include any plastic material as a substrate.
Illustrative plastics include acrylic, polyester, polyethyl-
ene, polyimide, polyphenylene oxide, polycarbonate, poly-
amide, epoxy, phenolic, acrylonitrile-butadiene-styrene, and
acetal. Blends of these materials as well as blends with
other materials such as impact modifiers are also possible.
Furthermore, the substrates may contain various additives
such as UV absorbers, fillers, plasticizers, and the like.
Where transparency is required, the preferred
substrate is formed of polycarbonate or an acrylic resin
1 3 3 7 4 8 4 RD-17884
such as poly(methyl methacrylate). Polycarbonates are
especially preferred materials for transparent
substrates because of their excellent physical,
mechanical and chemical properties. In general, the
choice of substrate is ultimately determined by the end
use contemplated for the article.
Polycarbonates suitable for forming such a
substrate are well-known in the art and are described,
for example, in U.S. Patents 4,200,681 and 4,210,699.
Such polycarbonates generally comprise repeating units
of the formula
- R - 0 - C - 0 -
in which R is a divalent radical of a dihydric phenol,
e.g., a radical of 2,2-bis(4-hydroxyphenyl)-propane,
also known as bisphenol A,
Ho~H
Polycarbonates within the scope of the present
invention may be prepared by several well-known
methods. For example, preparation may be accomplished
by reacting a dihydric phenol with a carbonate
precursor. A wide variety of dihydric phenols, such as
bisphenol A, may be used in the
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~ 337484
present invention; many are disclosed in U.S. Patents
2,999,835; 3,082,365; 3,160,121; 3,334,154;
and U.S. Patent 4,190,681. Many carbonate_
precursors may be used; they are typically either a carbonyl
halide, a carbonate ester, or a haloformate. Exemplary
carbonate precursors are described in U.S. Patent 4,190,681.
The term "polycarbonate" is meant herein to
additionally include polymer blends of polycarbonates with
various other materials such as polyesters and impact
modifiers.
The substrate may be shaped into a variety of
forms, depending on the end use contemplated for the arti-
cles. For example, a polycarbonate film substrate may be
formed by casting the molten polymer onto a flat open mold,
and then pressing the material to a uniform thickness.
After cooling, the film may then have a gradational layer
applied thereover, as further described below. Furthermore,
the substrate may be in the form of tubes, rods, or irregu-
lar shapes. When the article of the present invention is to
be used as a glazing material, a polycarbonate material may
be formed into flat or curved sheets by well-known methods,
e.g., extrusion, injection molding, or thermoforming.
As mentioned above, the gradational layer which is
applied over the surface of a substrate according to the
method of the present invention is formed substantially of
an interfacial material at a depth closest to the surface of
the substrate. For the purpose of clarification, the region
of the gradational layer containing at least about 99% by
weight interfacial material will sometimes be referred to
herein as the "interfacial sublayer", while the region above
the gradational sublayer containing more than about 99% by
weight of an abrasion-resistant material will sometimes be
referred to herein as the "abrasion-resistant sublayer".
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RD-17884
1 337484
Furthermore, the region between the interfacial sublayer and
the abrasion-resistant sublayer will sometimes be referred
to as the "gradational sublayer". The thickness of each
layer is determined by process conditions, e.g., changes
made in the gaseous reactant feed into the PECVD reactor.
Thus, the depth of each sublayer is predetermined by the
intended requirements for the particular abrasion-resistant
article. For example, the abrasion-resistant sublayer may
constitute a larger portion of the gradational layer when
greater abrasion resistance is required.
The composition of the plasma-applied interfacial
material of the present invention depends on the end use
contemplated for the article. Organosilicons are particu-
larly useful for forming the interfacial material, especial-
ly when the abrasion-resistant material is silicon dioxide.
"Organosilicon" as used herein is meant to embrace organic
compounds in which at least one silicon atom is bonded to at
least one carbon atom, and includes silicone materials, as
well as materials commonly referred to as silanes, silox-
anes, silazanes, and organosilicones. Many of the organo-
silicons suitable for the method and article of the present
invention are described in Organosilicon Compounds, C.
Eaborn, Butterworths Scientific Publications, 1960. Other
suitable organosilicon compounds are described in Organic
Polymer Chemistry, K. Saunders, Chapman and Hall Ltd., 1973.
Non-limiting examples of organosilicon composi-
tions useful for the present invention are compounds repre-
sented by the general formula
R nSiz~4-n)
wherein Rl represents a monovalent hydrocarbon radical or a
halogenated monovalent hydrocarbon radical, Z represents a
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1 337484
hydrolyzable group, and n may vary between O and 2. More
specifically, Z is preferably a member such as halogen,
alkoxy, acyloxy, or aryloxy. Such compounds are well-known
in the art and are described, for example, in S. Schroeter
et al.'s United States Patent Number
4,224,378.
Other exemplary organosilicons falling within the
scope of the present invention include silanols having the
formula
R2Si(OH)3
wherein R2 is selected from the group consisting of alkyl
radicals containing from about 1 to about 3 carbon atoms,
the vinyl radical, the 3,3,3-trifluoropropyl radical, the
gamma-glycidoxypropyl radical and the gamma-methacryloxy-
propyl radical, with at least about 70% by weight of thesilanol being CH3Si(OH)3. Such compounds are described in
U.S. Patent 4,242,381.
Preferred organosilicon compounds of the present
invention are hexamethyldisilazane, hexamethyldisiloxane,
vinyltrimethylsilane and octamethylcyclotetrasiloxane.
When abrasion-resistant materials other than those
containing silicon are to be employed, other classes of
interfacial materials, such as various organometallics,
would be appropriate. For example, titanium isopropoxide,
Ti4(03H7), could be a suitable interfacial material when
titanium dioxide is to be the abrasion-resistant material.
The interfacial material may alternatively com-
prise plasma-polymerized acrylic materials. For example, an
acrylic acid ester monomer or methacrylic acid ester monomer
may be vaporized and then plasma-polymerized to form a
polymeric coating which is deposited on the underlying
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1 3 3 7 4 8 4 RD-17884
coating surface. Many of these materials are generally
described in the Encyclopedia of Polymer Science and
Technology, Volume I, Interscience Publishers, John Wiley
and Sons, Inc., 1964, and in ChemistrY of Organic Film
Formers, by D. Solomon, John Wiley and Sons, Inc., 1967, as
well as reference cited in each of the texts. Other
exemplary acrylic materials are described in U.S. Patent
4,239,798 and in 4,242,383.
The interfacial material may alternatively be a
polyolefin. Nonlimiting examples of suitable polyolefins
include polyethylene, polypropylene, polyisoprene, and
copolymers of these types of materials. Further included
within the broad definition of polyolefin as used herein are
synthetic and natural elastomers, many of which are
described in the Encyclopedia of Polymer Science and
Technoloqy, Vol. 5, pp. 406-482 (1966). Many of these
materials can be deposited according to the present
invention by vaporizing and then plasma-polymerizing their
monomer precursors under the plasma conditions described
below.
Nonlimiting examples of compounds suitable for the
abrasion-resistant material include silicon dioxide, silicon
nitride, silicon oxynitride, silicon carbide, silicon
carbonitride, boron oxide, boron nitride, aluminum oxide,
aluminum nitride, titanium dioxide, tantalum oxide, iron
oxide, germanium oxide, and germanium carbide. Mixtures of
such materials are also possible. When the article is to be
used as a glazing material, a silicon dioxide top layer is
preferred because of its ease of plasma deposition, its
excellent transparency, and the relatively inexpensive cost
of its precursors. It should be understood that "precursor"
as used herein is meant to include either one precursor or
.~ . . ,
RD-17884
~ 337484
more than one precursor, depending on the particular materi-
als being used.
A primer may be applied to the surface of the
substrate prior to the application of the gradational layer.
The primer may be applied by conventional methods well-known
in the art, e.g., spraying, roll coating, curtain coating,
dip coating, brushing, and other art-recognized techniques.
This layer tends to increase the adhesion of the interfacial
material to the substrate surface, while also acting as an
incorporation site for one or more ultraviolet light (UV)
absorbers. The primer material generally exhibits some com-
positional differences from the interfacial material.
Various well-known materials may be used to form
the primer, with the proviso that they be chemically compat-
ible with both the substrate and the gradational layer
material. Among the suitable primer materials for poly-
carbonate substrates are thermoplastic and thermoset acrylic
polymers, as described in the Devins et al- patent
mentioned above.
Preferred acrylic materials for the primer are
those which are ultraviolet light-curable. These materials
are typically applied to the substrate as a monomer. An
exemplary composition of this type comprises:
(A) at least one polyfunctional acrylate monomer
represented by the general formula
(H2C=IC-C-O)n~R3
wherein n is an integer having a value of from l to 4, and
R is selected from the group consisting of aliphatic
hydrocarbon groups, an aliphatic hydrocarbon group
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RD-17884
1 337484
containing at least one ether linkage, and a substituted
aliphatic hydrocarbon group containing at least one ether
linkage; and R' is selected from hydrogen or lower alkyl
radicals;
(B) colloidal silica,
(C) at least one acryloxy functional silane of
the formula
R6 o
I ll 5 4
(H2C=C -C-O-R )x-Si-(OR )4-x
wherein R4 is a monovalent hydrocarbon radical, R5 is a
divalent hydrocarbon radical, R6 is selected from the group
consisting of hydrogen atoms and monovalent hydrocarbon
radicals, x is an integer of from 1 to 4 inclusive; and
(D) a photoinitiator.
After the composition is applied to the substrate,
it is exposed to ultraviolet light for a period of time
sufficient to polymerize and crosslink the polyfunctional
acrylate monomers, thereby forming a cured coating. Ketones
are useful as initiators when curing the acrylic composi-
tions in inert atmospheres such as nitrogen, while blends of
at least one ketone and at least one amine are useful as
initiators when curing in oxygen-containing atmospheres, as
described in R. Chung's United States Patent Number
~,478,876.
Many of the organosilicon materials discussed
above are also suitable as primers (not applied by PECVD),
and in that instance, they may often have colloidal silica
dispersed therein, which increases the hardness of the
material. Dispersions of colloidal silica in organosilicon
materials are well-known in the art, and are described, for
example, in United States Patent Numbers 3,986,997: .
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1 37484
4,027,073; 4,239,798; 4,294,685 and 4,436,851.
Typically, the colloidal silica is dispersed in an aqueous
solution of the organosilicon. For example, these compounds
may comprise a dispersion of colloidal silica in a lower
aliphatic (e.g., less than about 6 carbon atoms) alco-
hol-water solution of the partial condensate of a silanol.
When used, colloidal silica should comprise about
5% to about 40% by weight of the total nonvolatile weight of
the primer. Furthermore, aqueous colloidal silica disper-
sions used in the present invention generally have a parti-
cle size in the range of about 5 to about 150 nanometers in
diameter. An especially preferred particle size range is
from about 5 to about 20 nanometers in diameter.
An especially preferred colloidal silica-contain-
ing organosilicon material for use as the primer is de-
scribed in B. Ashby et al.'s United States Patent
4,374,674, and comprises: ~
(a) a dispersion of colloidal silica in a solution
of the partial condensate of a silanol having the formula
RSi(OH)3 or R2Si(OH)2 ,wherein R is selected from the group
consisting of alkyl groups-having about 1 to 3 carbon atoms
and aryl groups having about 6 to 20 carbon atoms, wherein
at least 70 weight percent of the silanol is CH3Si(OH)3 or
(CH3)2Si(OH)2 in a mixture of an aliphatic alcohol and
water,
said dispersion containing from 10 to 50 percent
by weight of solids, said solids consisting essentially of
10 to 70 percent by weight of the colloidal silica and 30 to
90 percent by weight of the partial condensate, and
(b) an effective amount of an ultraviolet light
absorbing agent comprising a compound having the formula
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1 3 3 7 4 8 4 RD-17884
Y 2
~X~
Z OQ
wherein:
X is
11
C=O, or C=CI-C-OW
CN
Y is H or OH;
Z is H, OH, OQ or OW, where at least one Z is OH if Y
is H;
Q is -CH2(CH2)nSi(R )X(ORl)y; and
W is CmH2m+l;
where x = O, 1 or 2, y = 1, 2 or 3, x + y = 3, and
is an alkyl or alkanoyl group having about 1 to 6 carbon
atoms, R is an alkyl group having from about 1 to 6 carbon
atoms, n = O, 1 or 2 and m = 1 to 18. The composition
forming this material typically contains sufficient acid to
provide a pH in the range of about 3.0 to 7Ø The Ashby et
al. patent also describes methods of applying and curing
these coatings.
Exemplary condensates of R2Si(OH)2-type silanols,
and compositions formed therefrom, are disclosed in U.S.
Patent 4,159,206.
Another preferred organosilicon material for use
as the primer comprises a water/aliphatic alcohol dispersion
of ammonium hydroxide-stabilized colloidal silica and a
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1 }37484
RD-17884
partial condensate derived from organotrialkoxy silanes.
Such a material is described by B. Anthony in U.S. Patent
4,624,870 and is preferably used at an alkaline pH, i.e., a
pH of at least about 7.1.
Exemplary W absorbers which may be incorporated
into the primer are those of the hydroxy benzophenone and
benzotriazole type, although other W absorbers might also
be used. When a W -curable primer material is employed,
latent W absorbers which do not interfere with the curing
reaction should be used. These types of UV absorbers are
known in the art and are described, for example, in U.S.
Patents 4,372,835, issued to Chung et al, 4,478,876 and
4,486,504, each issued to Chung; and 4,455,205, issued to
Olson et al. An example of a latent W absorber is
resorcinol monobenzoate.
The amount of W absorber depends in part on the
particular composition of the acrylic, and in part on
whether a W absorber is also present in the substrate
- material itself. Those skilled in the art will be able to
determine an appropriate quantity without undue
experimentation.
Other additives which may be incorporated into the
primer include anti-oxidants, fillers, reinforcing agents,
wetting agents, and the like.
The gradational layer may be applied on the
substrate or "over" the substrate, i.e., onto the surface of
a primer coating disposed on the substrate. The application
is accomplished by PECVD, which in general is a method of
applying films from a gaseous discharge to a substrate. For
example, the Kirk-Othmer EncycloPedia of Chemical Technolo-
gy, Volume 10, discusses the plasma deposition of inorganic
materials. Furthermore, details regarding the plasma
, . , ~
~ ~e
1 337484 RD-17884
deposition of inorganic thin films are given in Thin Film
Processes, Ed. by Vossen and Kern, Academic Press, 1978.
Exemplary plasma deposition methods are also described in
U.S. Patents 4,096,315, 4,137,365, 4,361,595, and 4,396,641.
While all of the above-listed references generally describe
plasma deposition, the process disclosed herein must be
carried out according to the various operating parameters
discussed below in order to obtain an article having excel-
lent abrasion resistance, optical properties, and adhesion
between its layers.
The following general statement regarding the
operation of PECVD for the present invention applies to both
the deposition of the interfacial material and the abra-
sion-resistant material. When a discharge is produced at
low pressure in the film-forming reactants, the reactants
become ionized, forming a plasma. A portion of the material
is in the form of ions, electrons, and atomic free radicals
generated in the plasma prior to formation of the film over
or upon the substrate. Most of the reactive species consist
of the atomic free radicals. Although the inventors do not
wish to be bound by a specific theory, it is thought that at
the higher plasma frequencies, e.g., 13.56 MHz, and at the
typical gas pressures employed, e.g., 1 Torr, most of the
film formation on or over the substrate occurs when the free
radical species diffuse out of the plasma to the deposition
surface. Thus, free radicals react on or over the primed or
unprimed substrate to form the desired layer. A distinct
advantage of PECVD over conventional chemical vapor deposi-
tion processes lies in the fact that the applied electric
field ~nh~nces free radical formation, thereby permitting
the use of deposition temperatures which are low enough to
prevent damage to substrates such as polycarbonates, i.e.,
temperatures less than about 130C. Furthermore, when used
1 3 3 7 4 8 4 RD-17884
under the process conditions disclosed herein, PECVD can be
carried out with a much higher percentage of free radicals
than is possible with conventional CVD.
One PECVD system suitable for the process dis-
closed herein is designated as Model 2411 and sold by
Placm~Therm~ Inc. However, in order to achieve the ex-
cellent results obtained by the present invention, use of
this or any other PECVD apparatus must fall within the
processing and compositional parameters disclosed herein.
In applying the gradational layer by PECVD, the
primed or unprimed substrate is placed in a reactor chamber
in which an electric field can be generated. The reactor
chamber must be capable of being substantially evacuated,
i.e., to a pressure of less than or equal to about 1.0
millitorr.
The method of generating and applying the electric
field is not critical to this process. For example, the
field may be generated by inductive coupling systems, as
described, for example, by J. Vossen in Glow Discharge
Phenomena in Plasma Etching and Plasma Deposition, J.
Electrochemical Society, February 1979, pp. 319-324.
A capacitively coupled system may also be used to
generate an electric field, and is preferred for use in the
present invention. By this technique, which is generally
described in the Vossen article referred to above, two
electrodes are situated within the reaction chamber, and the
plasma is formed therebetween. Each electrode may be a
plate of a material that is a good electrical conductor,
e.g., aluminum. The electrodes preferably each have a
planar face parallel to the other electrode.
In preferred embodiments of the present process
wherein the capacitively coupled system is utilized, the
electrodes are horizontally arranged, i.e., an upper
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electrode is affixed in the upper region of the reactor
chamber with a planar surface facing a planar surface of a
lower electrode affixed in the lower region of the vacuum
chamber. The spacing between the electrodes depends on the
S desired strength of the applied electric field, as well as
the size of the article being coated. Those skilled in the
vapor deposition art appreciate the interrelationship of
these processing variables and are therefore able to make
adjustments for a particular use of this invention without
undue experimentation. In preferred embodiments, the
substrate is positioned on the surface of the lower elec-
trode which faces the upper electrode, such that the sub-
strate surface to be coated is parallel to the facing
surface of the upper electrode. Alternatively, the elec-
trodes might be arranged vertically or along other geometricplanes within the chamber as long as a plasma can be gener-
ated therebetween.
Film-forming materials must be in vapor or gaseous
form for the PECVD process. Vapor reactants, such as
acrylic, olefinic, or organosilicone monomers, are vaporized
from the liquid form prior to entry into the reactor cham-
ber. A preferred technique when sufficient vapor pressures
are difficult to obtain is to introduce a mist of the liquid
into the plasma region.
In preferred embodiments, the liquid material may
be degassed by cooling it and then subjecting it to a
vacuum. Depending on its particular boiling point, the
liquid is then heated to ambient temperature or higher in
order to provide sufficient positive vapor pressure to flow
through a channeling system such as that described below.
Alternatively, a carrier gas such as helium can be blown
through the liquid to obtain a diluted vapor mixture of
desired composition.
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1 337484 7 4
Gaseous reactants such as silane or nitrous oxide
are suitable in natural form for reaction in the plasma,
alone or with a carrier gas to insure proper metering into
the reactor chamber. Sometimes, e.g., in the case of
nitrous oxide, the reactants may be stored in liquid form
beforehand.
The reactor chamber is evacuated prior to entry of
the gaseous reactants. Cham~er pressures as required for
the process of the present invention range from about 50
millitorrs to about 10 Torrs, with a preferred pressure
being in the range of about 0.3 Torr to about l.0 Torr.
The gaseous reactants which form the composition
of the gradational layer may be supplied from an external
source through a series of inlet pipes into the reactor
chamber. The technical particularities of channeling the
various gases into the reactor chamber are well-known in the
art and need not be described in detail here. For example,
each gas inlet may be connected to a central feed line which
carries the gases into the reactor chamber. In preferred
embodiments, gaseous reactants for the abrasion-resistant
composition are mixed with a carrier gas such as helium to
improve the flow of the reactants into the chamber. The
flow of carrier and reactant gases into the reactor may be
governed by mass flow controller valves which are` well-known
in the art and which serve to both measure the flow of gases
and to control such flow. Furthermore, the carrier gas,
when used, may be premixed with the gaseous reactants or may
be fed into the central feed line by a separate inlet. For
example, when silane (SiH4) is used as a reactant for
forming silicon dioxide, it may be premixed with helium in a
SiH4/He volume ratio ranging from about 2:98 to 20:80.
Although a carrier gas is not critical to the present
invention, its use improves the uniformity of plasma density
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~ 337484 RD-17884
and gas pressure within the reactor chamber. Furthermore,
use of the carrier gas tends to prevent gas phase particu-
lation of the plasma-formed coating material, and also
improves film quality in terms of transparency (when de-
sired) and abrasion resistance.
Formation of the gradational layer is facilitated
by the use of separate inlet pipes for reactants forming the
interfacial material and for reactants forming the abra-
sion-resistant material. The flow of each of these gases is
also controlled by the mass flow controller valves described
above. Since the interfacial material is often formed from
reactants which are liquids at room temperature, the materi-
al is advantageously stored in a supply vessel located in an
oven to allow for the vaporization of the material prior to
entry into the reactor chamber.
When using the capacitively coupled system, the
gaseous reactants entering the reactor chamber from the
central feed valve are passed between the upper and lower
electrodes and over the substrate to be coated. The quality
of the gradational coating on or over the substrate or
primer depends greatly on both the flow rate of the reac-
tants and the flow dynamics, i.e., laminar characteristics,
as described below. For example, excessive flow rates would
force the active, film-forming reactants past the zone above
the deposition surface before they react to form the coating
on the surface. Conversely, if the flow rate is too small,
the film-forming reactants will quickly be depleted and
thereby lead to nonuniformities in film thickness. The flow
rate of interfacial material reactants may range from about
5 sccm to about 250 sccm, with about 20 sccm to about 100
sccm being preferred. For coating surfaces larger than
about 10 square feet, which might require reactor chambers
larger than the PlasmaTherm reactor described below, higher
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RD-17884
~ 337484
flow rates may be desirable, e.g., up to about 2000 sccm.
As further described below, the interfacial material reac-
tants may be passed into the reactor chamber with a carrier
gas.
The individual flow rates of reactants forming the
abrasion-resistant material range from about 500 sccm to
about 10,000 sccm for each reactant when a carrier gas is
used, and from about 5 sccm to about 2000 sccm without a
carrier gas. For example, a silicon dioxide coating may
advantageously be formed by flowing silane at a rate of
about 10 sccm to about 100 sccm and nitrous oxide at a rate
of about 300 sccm to about 5000 sccm into the reactor along
with a carrier gas flowing at a constant value in the range
between about 500 sccm and 5000 sccm. As in the case of the
interfacial material precursor flow rates, higher abrasion-
resistant material precursor flow rates may be desirable for
coating surfaces larger than about 10 square feet. For
example, in forming silicon dioxide, silane flow rates up to
about 250 sccm, nitrous oxide flow rates up to about 8000
sccm, and an increase in carrier gas flow proportional to
the increase in silane flow might be used. Those of ordi-
nary skill in the art will be able to easily select a proper
flow rate for a particular substrate and coating material if
the teachings herein are followed.
While gas flow, gas pressure, and plasma power may
be varied within the ranges described above in order to suit
the requirements of a particular end use, it may be desir-
able in some embodiments to maintain these three parameters
as fixed values during formation of the gradational layer in
order to maintain a steady plasma. Preferred embodiments
also call for each gaseous reactant passing into the reactor
chamber to be mixed with a carrier gas which flows at a
constant rate throughout plasma deposition.
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~ 337484
~ RD-17884
Preferred embodiments also call for the total gas
flow into the reactor chamber to be a constant amount during
the deposition of the gradational coating. This should not
be viewed as an absolute requirement for good quality
coatings, but as a more efficient means for process control.
Laminar flow of the gaseous reactants relative to
the deposition surface is of great importance to the present
invention because it enhances the uniformity of the coating
in terms of thickness and properties such as hardness,
clarity, and, for the interfacial material, adhesive and
thermal expansion compensation capabilities.
"Laminar flow" as used herein is defined as smooth
and steady flow, i.e., a substantially streamlined flow of
gaseous reactants relative to the substrate and character-
ized by the absence of turbulent flow of reactant molecules.
This type of gas flow is described, for example, in Fluid
Mechanics, by F. White, McGraw-Hill Book Company, 1979,
p. 305 et seq. As described in the White text, laminar flow
- may be generally characterized by a Reynolds value of
between about 1 and 1000. In preferred embodiments of this
invention, a particularly preferred Reynolds value is about
2.5. Those skilled in the art understand that small areas
of turbulence may be present, but do not significantly
affect the properties of the deposited coating. Further-
more, as pointed out above, the mass flow of each gas may be
regulated by adjustment means to control the laminar flow
characteristics of the gaseous reactants.
In preferred embodiments, the coating surface is
heated to a temperature between about 100C and 130C during
plasma deposition, 100C being the most preferred tempera-
ture. The heating can be accomplished by a variety of
well-known methods. For example, the resistively-heated
,; ~ .
~ , ~ .
1 3 3 7 4 8 4 RD-17884
lower electrode upon which the substrate rests serves to
provide heat to the coating surface through the substrate.
In some embodiments of this invention, coating surface
temperatures of 100C or higher increase the deposition rate
of the abrasion-resistant material onto the underlying
surface. Furthermore, the elevated temperature may also
result in greater abrasion resistance. It should also be
understood that deposition onto a coating surface maintained
at between about room temperature and 100C is also within
the scope of this process.
In preferred embodiments of this invention, the
substrate surface may be cleaned by washing with an alcohol
solvent such as isopropanol prior to application of the next
layer. This step removes dirt, contaminants, and additives
such as wetting agents from the surface. The primer surface
may also be washed in this manner.
After being washed, the substrate is vacuum-desic-
cated by well-known methods to remove any water on or in the
surface region which would interfere with the adhesion of
the subsequently-deposited layers. The desiccation treat-
ment may also be used on the primer surface after it has
been applied to the substrate. Desiccation temperatures
range from about ambient temperature to about 120C, with
the preferred range being about 80C to about 90C. Desic-
cation duration ranges from about 2 hours to about 16 hours,with longer times within this range compensating for lower
temperatures, and vice versa.
The surface of the substrate can often be etched
after placement in the reaction chamber. Etching tech-
niques, which in general are well-known in the art, may also
be used to treat the primer surface to create free radical
species thereof which will later bond with the free radical
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~ 3 3 7 4 8 4 RD-17884
species of the plasma-applied gradational material, thereby
improving adhesion between these layers.
As the reactants enter the reaction chamber after
the coating surface is treated as described above, an
electric field is generated under preselected frequency and
power conditions to ionize the gas mix, thereby forming a
plasma. Methods of generating an electric field between
electrodes are well-known in the art and therefore do not
require an exhaustive description here. A dc field, or an
ac field from 50 Hz to about 10 GHz, may be used. Power
values range from between about 10 watts to 5000 watts. A
particularly suitable electrical field-generating means for
this process is the use of a high frequency power supply to
initiate and sustain the plasma. When such a power supply
is used, a preferred operating frequency is 13.56 MHz, as
described, for example, in R. Kubacki's U.S. Patent
4,096,315. The particular frequency and power values
utilized will depend in part on the particular deposition
requirement for the coating material. For example, when
organosilicone monomers are reacting in the plasma, lower
frequencies and higher electrical power values within the
above-described ranges increase the polymerization rate and
deposition rate of the material, especially when lower
chamber pressures within the above-mentioned range are also
employed.
An additional refinement, well-known in the art,
which offers the potential for beneficially modifying the
plasma (e.g., by increasing the ionization and providing
improved spatial control of the plasma), uses separate
magnetic fields in conjunction with the electric field. An
example of such magnetic enhancement is "ECR" (electron
cyclotron resonance) microwave plasma technique.
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t 337484 RD-17884
As mentioned above, the gradational layer may be
formed by initially feeding plasma-polymerizable interfacial
material precursors into the reactor, and then gradually
changing the feed composition to abrasion-resistant material
precursors. The change in feed composition may be accom-
plished by manually adjusting the flow of each gas into the
central feed line. Those skilled in the art appreciate that
such gas flow adjustment can also be accomplished automati-
cally by various means. Each adjustment is made according
to a prescribed time and flow rate regimen based on data
obtained from the mass flow controller valves.
In preferred embodiments, the PECVD deposition of
the gradational layer occurs in three stages: a first stage
in which only the interfacial material precursor is fed into
the reactor and plasma-polymerized; a second stage in which
the interfacial precursor flow is gradually reduced while
the flow of abrasion-resistant material precursors is
initiated and gradually increased; and a third stage in
which only the abrasion-resistant material precursors are
fed into the reactor and deposited. A carrier gas is often
used during each stage of the deposition. The length of
each stage is determined by the desired thickness of each
sublayer. The resulting gradational layer has a sublayer of
interfacial material closest to the substrate, a sublayer of
abrasion-resistant material farthest from the substrate, and
a gradational sublayer therebetween which is characterized
by a gradual transition from the interfacial material to the
abrasion-resistant material. The change in feed composition
may be effected linearly or nonlinearly, e.g., exponential-
ly, from the interfacial precursor flow to the abrasion-re-
sistant precursor flow. The examples which follow further
describe this technique.
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1 337484 RD-17884
It is also within the scope of this invention that
the initial gas composition flowing into the reactor contain
a small amount of abrasion-resistant precursor material,
e.g., about 0.1% of the total gas flow, the remainder of the
composition comprising interfacial material precursors. The
abrasion-resistant precursor composition is then gradually
increased in proportion to the gradual decrease in the
interfacial material precursor composition, resulting in a
gradational layer characterized by a gradual transition from
a composition consisting essentially of an interfacial
material to a composition consisting essentially of an
abrasion-resistant material.
The overriding consideration for feed composition
adjustment is, of course, the desired characteristics of the
deposited gradational layer. For example, an article
requiring an especially high level of abrasion-resistance
but not likely to be subjected to rapid thermal changes or
wide temperature variations may be formed by decreasing the
time period of interfacial material precursor flow and
increasing the time period of abrasion-resistant material
precursor flow. The resulting article would thus have a
thicker abrasion-resistant layer than an article formed
according to the regimen exemplified above.
Conversely, an article likely to be subjected to
wide temperature variations may be formed by decreasing the
time period of abrasion-resistant precursor flow and in-
creasing the time period of interfacial material precursor
flow to produce an article having more of its depth as
interfacial material.
The thickness of the gradational layer is in part
determined by the contemplated end use of the article, and
generally may range from about 0.01 micron to about 5.0
microns. Similarly, the thickness of each zone or sublayer
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1 337484 RD-17884
- when distinctly present - depends on the end use of the
article.
After passing over the coating surface, the
carrier gas and any gas reactants or products which have not
been deposited on the substrate surface may be directed out
of the chamber through an exit valve and then to a gas
pumping and exhaust system. Means for expelling these
excess materials from the chamber are well-known in the art.
When the electrodes are circular and flat as described
above, the exhaust manifold can be located in the center of
the lower electrode. Furthermore, after the application of
the gradational layer, residual gases may be removed from
the reactor chamber by pumping means.
Embodiments of the present invention result in the
formation of articles having a high degree of hardness and
an abrasion resistance. Furthermore, when the process is
utilized to form a transparent glazing material, the re-
sulting articles are very smooth and free from microcracks.
Moreover, the gradational layer is capable of accommodating
large differences in thermal expansion when the article is
subjected to heating/cooling cycles.
Examples
The following examples are provided to more fully
describe the present invention. It is intended that these
examp~es be considered as illustrative of the invention,
rather than limiting what is otherwise disclosed and claimed
herein.
A brief description of the tests utilized in some
or all of the following examples will now be given:
Abrasion resistance was measured by a combination
of two ASTM test methods. The Taber Abrasion Test, ASTM
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1 3 3 7 4 8 4 RD-17884
D1044, was used with a 1,000 gram total weight load evenly
distributed on the two wear wheels. 300 and/or 1,000 cycles
were used, as indicated below. The second test method was
ASTM D1003, which uses a Gardner Hazemeter, Model UX 10. In
this method, the percentage of light scattering is measured
before and after the specimen is taber-abraded. The lower
the value, the better the abrasion resistance and hardness.
Optical transparency was measured on a UV-Visible
Spectrometer, Model 330, manufactured by the Perkin Elmer
Corporation.
Adhesion was measured by the scribed adhesion
test, ASTM Test D3359, in which a 0.75 inch (1.9 cm) square
of the material is cross-hatched into 2.0 mm squares. A 3M
Company No. 610 adhesive tape is then pressed onto the
surface of the grid pattern and removed with a swift, even
pull. The amount of material remaining on the sample is
indicative of the adherence characteristics of the coating
to an underlying surface.
Two types of temperature cycling tests were
carried out. Test A was very severe because of the large
temperature excursion and very abrupt rate of temperature
change (dT/dt). The thermal cycle profile consisted of two
stages: first, a cool-down to -35C (dT/dt = 22C/min);
hold for 90 minutes; heat up to 50C (dT/dt = 17C/min);
hold for 90 minutes; cool down to 20C (dT/dt = 26C/min);
hold for 15 minutes, and then examination of the sample; and
second, cool down to -35C (dT/dt = 22C/min); hold for 90
minutes, heat up to 100C (dT/dt = 15C/min); hold for 90
minutes; cool down to 20C (dT/dt = 30C/min); hold for 15
minutes, and then examination of the sample.
Test B consisted of ten cycles of the following
thermal cycle: cool down -3SC; hold for 120 minutes, heat
up to 85C, hold for 120 minutes; cool down to -35C, etc.
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-
i 337484 RD-17884
In this case, the rate of temperature change was substan-
tially lower than in Test A, and was held constant through-
out the entire ten cycles at dT/dt = 1.41C/min.
Strain to microcracking measurements were made on
4" x ~" x ~" samples. For this purpose the samples were
placed in a three point bending jig attached to an Instron
device. The latter bent the samples and recorded the sample
deflection at the time microcracking was visually observed.
The strain to microcracking, E, was then calculated from =
4 dlT/L , where dl is the deflection at microcracking, L the
length of the sample (4"), and T, its thickness (~").
Impact strength was determined by the use of a
Gardner Heavy-Duty Variable Impact Tester.
The thickness of the coating applied to the
substrate by plasma deposition was controlled and determined
by process conditions and processing time, as described
above. Once the reactant gas mix flow rate, substrate
temperature, frequency, and pressure have been set, thick-
ness can be determined within about +5% by simply timing the
duration of the process.
The thickness uniformity of an applied coating is
assessed from the interference color produced by the coat-
ing; such a method is suitable for coatings having thick-
nesses of about 0.3 microns. For greater coating thick-
nesses (from about 0.4 micron-10 microns), a profilometer
(Sloan Dektak II) provides a determination of coating
uniformity. Small, thin silicon wafers are positioned at
strategic locations prior to deposition of the coating and
then removed afterwards, exposing the steps used for measur-
ing thickness.
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- 1 337484 RD-l7884
Example 1
Using the processing conditions shown in Table 1,
a gradational layer was deposited on bisphenol A-based
polycarbonate (samples 1 and 2). The abrasion-resistant
material was SiO2 and the interfacial material was plas-
ma-polymerized vinyltrimethylsilane (VTMS).
Table 1
(Samples 1 and 2)
Pre-Process Conditions
Sublayer calculated (Pressure, Time of
Composition Thicknessflow rates, power) Gas Feed
SiO2 0.7~m 1000 mTorr 20 min
1200 sccm SiH4 (He)
800 sccm N20
50 watts
Gradational 0.6~m 1000 mTorr 20 min
(Composition varied
linearly)
50 watts
20 VTMS* 1000 A 1000 mTorr 20 min
40 sccm VTMS
1960 sccm He
50 watts
*Vinyltrimethylsilane
The linear variation of composition for the
gradational sublayer occurred as follows:
-29-
~ }37484 RD-17884
Table 2
Time of
Gas FeedSiHq Flow N20 Flow VTMS Flow He Flow
(minutes)(sccm) (sccm) (sccm) (sccm)
0 0 40 1960
22.5 150 100 35 1680
300 200 30 1440
27.5 450 300 25 1200
600 400 20 960
32.5 750 500 15 720
900 600 10 480
37.5 1050 700 5 240
1200 800 0 0
8ased on measurements on the gradational coatings
deposited onto silicon (Si) wafers at strategic locations in
the reactor, it was concluded that the gradational coatings
had good thickness uniformity. The coatings were also
completely transparent, as demonstrated by their appearance,
and also from absorption measurements made in the visible
region (400-800 nm). In the W region, below 400 nm, the
measured absorption paralleled that of the polycarbonate.
Taber abrasion measurements carried out on sample
1 showed an improvement as compared to bare polycarbonate.
Thus, the increase in haze after 300 cycles was 20-40%,
which is essentially that of polycarbonate after 50 cycles.
The effect of temperature cycling of sample 2,
using Test B as described above, is shown in Table 3 and
compared with data obtained for control sample 3, a struc-
ture made by depositing SiO2 onto polycarbonate dip-coated
with an organosilicone primer comprising a dispersion of
colloidal silica in~a solution of the partial condensate of
an alkyl silanol which further included a W screening
agent. The thickness of the organosilicone material after
heat-curing was about 5.0 microns. The gradational
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t 337484 RD-17884
structure shows a distinct improvement in the temperature
cycling performance.
Table 3
Temperature Cycling, Test B
Thickness (~m)
Sample VTMS Gradational SiO2
No. Primer* Sublayer Sublayer Sublayer Result
2 - 0.1 0.6 0.7 No
micro-
cracks
3 5 --- --- 4.0 Micro-
cracked
*Organosilicone
Example 2
Using the processing conditions shown in Table 4,
a gradational layer formed from the same precursors as in
Example 1 was deposited on bisphenol A-based polycarbonate
(sample 4). The gradational layer was also applied on
polycarbonate material which had first been dip-coated with
an organosilicone material comprising a dispersion of
colloidal silica in a solution of the partial condensate of
an alkyl silanol which further included a UV screening agent
(sample 5). The thickness of the organosilicone material
after heat-curing was abGut 5.0 microns.
-31-
~ 33~484 RD-17884
Table 4
(Samples 4 and 5)
Pre- Process Conditions
Sublayercalculated (Pressure, Time of
5 Composition Thicknessflow rates, power)Gas Feed
SiO2 2 ~m 1000 mTorr 40 min
2500 sccm SiH4 (He)
1650 sccm N20
50 watts
10Gradational1.2 ~m 1000 mTorr 40 min
(Composition varied
linearly)
50 watts
VTMS* 50 A 1000 mTorr 2 min
100 sccm VTMS
2400 sccm He
50 watts
*Vinyltrimethylsilane
The linear variation of composition for the
gradational sublayer occurred as follows:
Table 5
Time of
Gas Feed SiHg Flow N20 Flow VTMS Flow He Flow
(sccm) (sccm) (sccm) (sccm)
252 min. 0 0 100 2400
12 min. 625 408 75 1800
22 min. 1250 816 50 1200
32 min. 1875 1235 25 600
42 min. 2500 1625 0 0
As in Example 1, the coatings were very uniform.
Furthermore, their optical characteristics were excellent.
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1 337484 RD-17884
Supporting evidence of a gradational structure was
obtained from Secondary Ion Mass Spectrometry measurements.
The device used was a second generation Cameca IMS-3F ion
microscope. These measurements were carried out on a
gradational coating deposited on a silicon wafer simul-
taneously with the deposition made on samples 4 and 5. The
gradational portion of the structure had a gradual decrease
in the carbon content from the polycarbonate surface to the
SiO2 layer, where it was essentially zero.
Taber measurements showed that sample 4 possessed
even greater abrasion resistance than sample 1. The in-
crease in haze after 300 cycles was only 8%.
The scribed adhesion test showed 80% removal of
the SiO2 from the gradational layer. However, since the
tape sticks more strongly to an SiO2 surface than to a
typical organosilicone surface, the adhesion is considered
to be much better than indicated by the test.
The effect of temperature cycling on sample 5,
using Test A as described above, iæ shown in Table 6 and
compared with data obtained on structures (samples 6 and 7)
made by depositing SiO2 on polycarbonate precoated with the
same organosilicone. Again, the gradational structure
outperformed the non-gradational structures, even when the
SiO2 sublayer was ~uite thin (sample 7).
-33-
~ 337484 RD-17884
Table 6
Temperature Cycling - Test A
Thickness (~m) Result
Grada-
VTMS tional SiO2 1st 2nd
Sample Sub- Sub- Sub- Examin- Examin-
No. Primer* layer layer layer ation ation
0.05 1.2 2.0 O.K. A few
micro-
cracks at
edges
6 5 ---- --- 2.0 O.K. Micro-
cracked
7 5 ---- --- 0.3 O.K. Micro-
cracked
*Organosilicone
Table 7 below demonstrates that the gradational
layer does not affect the properties of the underlying
substrate, such as impact strength. Sample 5, as mentioned
above, was gradationally coated according to the present
invention. Control sample 7(a) was uncoated bisphenol
A-based polycarbonate. Control sample 7(b) was the same
polycarbonate material dip-coated with the organosilicone
used for sample 5.
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1 337484 RD-17884
Table 7
Impact Strength
Thickness (~m)
Impact
5 Sample VTMS Gradational SiO2 Strength
No. Primer* Sublayer Sublayer Sublayer (in./lb.)
5** 5 0.05 1.2 2.0 '320
7(a) - ____ ___ ___ '320
7(b) 5 ---- -__ ___ '320
*Organosilicone
**After thermal cycling
Upon impact, cracking of the coating occurred in
the immediate vicinity of the impact area, but no cracking
or delamination occurred elsewhere.
Example 3
Using the processing conditions and compositions
shown in Table 8, a gradational coating was deposited on
bisphenol A-based polycarbonate (sample 8), and on the same
polycarbonate dip-coated with an organosilicone material
comprising a dispersion of colloidal silica in a solution of
the partial condensate of an alkyl silanol which further
included a UV screening agent (sample 9). The thickness of
this organosilicone material after heat-curing was about 5.0
microns.
Samples 10 and 11 were controls. Sample 10 was
formed by PECVD of SiO2 directly onto polycarbonate. Sample
11 was formed by PECVD of SiO2 onto polycarbonate which had
been dip-coated with an organosilicone of the type described
in Example 2.
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- i337484 RD-17884
Table 8
(Samples 8 and 9)
Pre- Process Conditions
Sublayer calculated (Pressure, Time of
5 Composition Thickness flow rates, power) Gas Feed
SiO2 2.5 ~m 1000 mTorr 48 min
2500 sccm SiH4 (He)
1625 sccm N20
50 watts
10Gradational1 ~m 1000 mTorr 40 min
(Composition varied
exponentially)
50 watts
VTMS* 50 A 1000 mTorr 2 min
175 sccm VTMS
- 2500 He
50 watts
*Vinyltrimethylsilane
The variation in composition for the gradational
sublayer was approximately exponential and occurred as
follows:
Table 9
Time of
Gas Feed SiHq Flow -2 Flow VTMS Flow He Flow
252 min. 0 0175 2400
22 min. 625 408 130 1800
38 min. 1250 816 88 1200
40 min. 1875 1235 44 600
42 min. 2500 1625 0 0
As in the previous examples, the coatings were
very uniform, and their optical characteristics were ex-
cellent.
-36-
- ~ 337484 RD-17884
Results of strain-to-microcracking ( E ) tests
obtained on samples 8 and 9 are shown in Table 10 and
compared with data obtained for control samples 10 and 11.
Table 10
Strain-To-Microcracking Results
Thickness (~m) Strain to
micro-
Sample VTMS Gradational SiO2 cracking E
No. Primer* Sublayer Sublayer Sublayer (in/in x 102)
8 - 0.05 1.0 2.5 0.45
9 5 0.05 1.0 2.5 0.47
- ---- --- 2.0 0.33
11 5 ---- --- 2.0 0.35
*Organosilicone
Higher E values represent better strain-to-micro-
cracking performance. Thus, it is readily apparent from
this data that the gradational coating outperforms the other
SiO2-containing structures, demonstrating that a gradational
coating indeed reduces the tensile stresses present at the
interface with the polycarbonate.
The desirable qualities achievable through a
PECVD-applied gradational coating may also be confirmed by
calculations of the mechanical stresses in such coatings as
compared to a non-gradational coating. For example, at a
given deposition temperature, the resulting stresses in the
coating and the substrate (after the structure had returned
to room temperature) can be calculated by assuming known
thermal expansion values and Young's modulus values for each
material.
Modifications and variations of the present
invention are possible in light of the above teachings. It
i 337484 RD-17884
should therefore be understood that changes may be made in
the particular embodiments of the invention described which
are within the full intended scope of the invention as
defined by the appended claims.
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