 |
Description  |
|
|
TECHNICAL FIELD
This invention relates to hydrogenated boron-silicon alloy films having
unexpected properties permitting the bonding together of various metal and
ceramic substrates, the protecting of various metal and organic resin
substrates against attack by atmospheres containing dissociated oxygen,
molecular oxygen, water vapor and dissociated oxygen and nitrogen, and the
sealing and stabilizing of intermetallic compounds against structural
degradation.
BACKGROUND ART
Solid amorphous silicon-boron-hydrogen alloy films have heretofore been
utilized as electronic materials; see, "Fundamentals of Solar Cells",
chapter 11 section 2, Alan L. Fahrenbruch and Richard H. Bube, Academic
Press 1983. Most commonly they are used in the form of deposits, prepared
by radio frequency glow discharge decomposition of hydrides, in solid
state devices converting solar radiation to electrical energy. Preparation
and electrical current-voltage characteristics of semiconductor junctions
made of thin films of amorphous silicon doped with trace amounts of boron
are described by W. E. Spear, P. G. Le Comber, S. Kinmond and M. H.
Brodsky, "Amorphous Silicon p-n Junction", Applied Physics Letters, Vol.
28, No. 2, pg. 105, Jan. 1976. In this work, the fact that materials
prepared by decomposing the hydride gases always contain hydrogen, and the
role that this hydrogen plays in forming the amorphous structure
advantageous for electronic applications, were not considered. Hydrogen
content and bonding configurations and their influence on growth and
structure of amorphous silicon films prepared by decomposition of silane
have been examined by J. C. Knights, "Growth Morphology and Defects in
Plasma-Deposited a-SI: H Films", Journal of Non-Crystalline Solids, Vol.
35 & 36 (1980), pages 159-170.
The high efficiency of solar cells incorporating thin films of
hydrogen-containing amorphous silicon-boron has motivated scientific
interest in the solid state physical properties of compositions exceeding
in boron content the low concentrations utilized for semiconductors. For
example, C. C. Tsai in an article entitled "Characterization of Amorphous
Semiconducting Silicon-Boron Alloys Prepared by Plasma Decomposition"
published in Physical Reviews, Volume 19, page 2041, February 1979,
describes structure, optical absorption, electrical conductivity and
paramagnetic resonance of hydrogenated boron-silicon alloys, ranging in
composition from 0 to 100% boron. In this work, films deposited by glow
discharge decomposition of Si H.sub.4 +B.sub.2 H.sub.6 gas mixtures on a
variety of substrates, including glass, aluminum, crystalline silicon, and
saphire have been studied. The relative efficiency of incorporation of
boron from the gas into the film, compared to that of the silicon, is
about 0.65 + or - 0.15 at 270.degree. C. deposition temperature.
Therefore, amorphous hydrogenated silicon-boron alloys with any
composition in the binary system can be made by properly choosing the Si
H.sub.4 -B.sub.2 H.sub. 6 gas mixture. The hydrogen content in the films
has been found to range between 10 and 45 atomic percent depending on the
deposition parameters. In the film structure, hydrogen forms B-H and Si-H
bonds which represent the majority of the amorphous atomic lattice. The
structure and stability of the films depend in great measure on the
deposition temperature. Films deposited at 270.degree. C. are dense,
contain less than 1 atom percent of oxygen and are very stable to exposure
to ambient air. They loose their hydrogen when heated to about 350.degree.
C. to 550.degree. C. In contrast, films deposited at 25.degree. C. are
porous, easily oxidize upon exposure to air, and start loosing their
hydrogen near 350.degree. C.
In the current electronic art, also pyrolysis, or thermal decomposition, of
gaseous hydrides is practiced for preparation of amorphous
hydrogen-containing silicon and boron films. An example is the work of B.
G. Bagley, D. E. Aspnes, A. C. Adams and R. E. Benenson described in the
paper entitled "Optical Properties of LPCVD aB(H)" published in the
Journal of Non-Crystalline Solids, Vol. 35 & 36 (1980), page 441. The
paper reports the infra red and near ultra violet absorptions of films
deposited on single crystal silicon substrates by pyrolysis of diborane at
temperatures between 290.degree. C. and 400.degree. C. The films have
remained stable upon exposure to laboratory atmosphere for six months
without any degradation by oxidation.
Applicants are unaware of any other uses set forth in the prior art for
silicon-boron-hydrogen alloys. Such alloys have been studied, developed
and used exclusively for the purpose of utilizing their physical behavior
as electronic semiconductors. The only chemical aspects of these materials
that are recognized by the electronic art are the reactions occurring in
the process of their fabrication as solid thin films, by decomposition of
gaseous silicon and boron hydrides (silanes and boranes). Accordingly, the
prior art teaches the composition and pressure of the gases from which the
films are formed and the resulting film compositions, temperature and rate
of their nucleation and growth, atomic lattice structure and stability to
temperature and exposure to ambient air. The art also implies that the
silicon-boron-hydrogen films can be deposited on almost any and all known
solid substrates including metals, ceramics and organic resin materials.
However, the art is devoid of any teaching or consideration of the
chemical behavior of the films towards the substrate on which they have
been deposited and towards materials which come in contact with them from
the outside.
In the nonanalogous art of joining a specific material to itself or to
other materials, a structural transition interface is utilized between the
materials. This transition interface is created on atomic and crystal
lattice levels by direct chemical reactions between the two surfaces in
processes such as diffusion or fusion welding or bonding. In many
instances, for convenience of lower bonding process temperatures, an
intermediate filler material is interposed between the two surfaces to
interact with each one of them separately and thus bond them together.
Examples of this kind of state of the art processes are arc welding,
brazing, soldering, and organic adhesive bonding. All these chemically
generated bond interfaces create a region of transition in which the
interfaced composition and crystalline structures are forced to adjust to
each other. Particularly in the cases of soldering, brazing and adhesive
bonding the filler material is retained at the joint and constitutes an
additional adventitious material in the joined assembly. In general, the
chemical and physical characteristics of the transition region are quite
different from those of the bonded materials and pose problems and
disadvantages of diminished mechanical strength, sensitivity to thermal or
mechanical shock, or to chemical attack, undesirable electrical or thermal
conductivity, and the like.
Advantages of using boron and/or silicon as active reactants in creating
bonds between metals have been recognized by the current art, as shown,
for example, in U.S. Pat. Nos. 2,714,760; 2,868,639; 3,188,203; 3,530,568
and 3,678,570. These patents teach joining high temperature corrosion and
oxidation resistant iron, nickel and cobalt base alloys by means of
brazing compositions based on nickel-chromium, or nickel-cobalt-chromium,
to which boron, or boron and silicon, are added. Boron and/or silicon
lower the melting point of the brazes and, thus, the joint can be made at
temperatures low enough to avoid deteriorating the mechanical properties
of the bonded alloys. Further advantage of using boron and silicon in the
brazes is that although they act to generate a brazing liquid at a
conveniently low temperature, they also at the same time tend to diffuse
out of the bond into the metal and cause the liquid braze bond to
solidify. In this way, the remelt temperature of the braze is raised above
that of the original braze composition and the bonded assembly can be put
to use at desirably high temperatures. However, the use of boron and
silicon in this manner does not obviate the aforesaid difficulties
associated with prior art bonding techniques.
DISCLOSURE OF INVENTION
Briefly, in accordance with the invention, it has been discovered that
boron-silicon-hydrogen alloys, formed by conventional processes, have
unobvious chemical and structural properties which yield unexpected
results when applied as surface coatings or films to various metal,
ceramic, intermetallic and organic resin substrates.
More particularly, the alloys permit the forming of metal to metal, ceramic
to ceramic and metal to ceramic joints, the protection of metallic and
organic resin substrates against atmospheres containing dissociated or
molecular oxygen and nitrogen, oxygen, water vapor and the sealing of the
surfaces of intermetallic compounds formed from elements selected from
groups III and V of the periodic table against structural degradation.
The bonding of metal and ceramic substrates is accomplished by forming the
solid boron-silicon-hydrogen alloy coating or film on the surfaces to be
joined. The film is heated at temperatures sufficient to expell hydrogen
from the film and to react the remaining activated boron-silicon bonds
with metal and ceramic substrates to form, respectively, liquid
metal-boron-silicon eutectic alloys and liquid borosilicate glasses which
chemically interact with the mated surfaces to generate the bond. The
chemical interaction might be limited to surface wetting and involve only
superficial layers of the material but in most cases the eutectic alloys
and the borosilicate glasses are at least partially and preferably
essentially fully absorbed by the substrates. To promote glass formation
in the reaction of the boron and silicon with the ceramic substrates, the
heating is done in an atmosphere containing oxygen, such as ambient air.
In the case of metal to ceramic bonding, the liquid metal-boron-silicon
eutectics are chemically active towards most ceramic surfaces,
particularly those containing oxygen. Upon completion of the heat
treatment, the liquid phase components and the substrate components
adjoining the mated surfaces have formed chemical bonds at the joint
interface and in many cases have at least partially and preferably
essentially fully exchanged places resulting in a high structural quality
joint. The metal substrates utilized are those which form with the
activated boron-silicon alloy a liquid ternary metal-boron-silicon
eutectic and which desirably have at least a limited solubility for boron
and silicon. The ceramic surfaces utilized are those which are able to
form in oxygen-containing atmospheres a chemical bond with liquid
borosilicate glasses and which desirably have at least a limited
solubility for boron and silicon oxides.
The chemical stability of the as-formed boron-silicon-hydrogen alloy is the
only property necessary for the sealing of intermetallic compounds and the
protection of metal and organic resin substrates against attack by
dissociated oxygen, molecular oxygen and water vapor. No further
treatments of the film are necessary to achieve these results.
Protection of metal and organic resin substrates against deterioration by
the effects of recombination of dissociated oxygen and nitrogen obtains
with the as-deposited boron-silicon-hydrogen coating as well as with
coatings which have been converted to boron-silicon oxides prior to
exposure to such atmospheres.
BEST MODE OF CARRYING OUT THE INVENTION
Hydrogen contributes two different chemical properties to
boron-silicon-hydrogen films both properties being important to achieving
the various utilities of the invention.
For the purposes of protecting and sealing various materials, hydrogen
confers to the films a chemical stability in the temperature range where
the films retain hydrogen in their composition. For joining and bonding
purposes, expulsion of hydrogen from the films enhances the heretofore
unrecognized chemical reactivity of the films because the boron and
silicon left behind find themselves in a state of unsaturated chemical
bonds and thus activated to find new chemical associations.
In that embodiment of the invention pertaining to the bonding or joining of
materials, the materials are put in contact with each other along surfaces
coated with a solid boron-silicon-hydrogen alloy film of the invention.
This joint is heated at temperatures sufficient to expell hydrogen from
the film and to react the remaining activated boron and silicon bonds with
metal including metal alloy surfaces and ceramic surfaces to form,
respectively, liquid eutectic alloys and liquid borosilicate glasses which
form a surface bond with, or are at least partially and preferably fully
absorbed into the bulk materials.
More particularly, this mechanism involves heating the solid film of the
invention to a temperature at which hydrogen bonds with boron and silicon
are ruptured and hydrogen is expelled from the film. Boron and silicon
then find themselves in a state of unsaturated chemical bonds and thus
activated to generate new chemical associations. In particular, the now
activated boron and silicon react at readily determinable temperatures to
form liquid phase eutectics with the important engineering metals and
alloys such as aluminum, copper, iron, nickel and cobalt. In similar
manner, the activated boron and silicon combined with oxygen in an
oxygen-containing atmosphere to form on ceramic substrates borosilicate
glasses which are fluid at temperatures considerably lower than the
melting points of most of the technologically important ceramics.
Therefore, for metal and metal alloy surfaces, the solid
boron-silicon-hydrogen alloy film is first converted to a solid
boron-silicon alloy film which reacts with the substrate to form a liquid
eutectic. A ceramic suface coated with the boron-silicon-hydrogen film is
likewise converted to the boron-silicon alloy film which reacts with
oxygen at temperatures of formation of liquid borosilicate glasses.
An optimum bond is ensured when the components of the substrates that are
to be joined diffuse across the bond and merge the two structures
together. It is well understood by the art that chemical reactions and
diffusion are enhanced in a liquid medium, in this case liquid eutectics
and liquid glasses. The most desirable diffusive reaction occurs for full
dissolution of the liquid eutectic and liquid glass in the substrates. The
mechanism of dissolution is that of absorbing the dissolving species into
the substrate atomic lattice and is connected with enhanced atomic
movement where the liquid phase components and the substrate components
exchange places in the crystal lattice. This mechanism results in a direct
bond between the atoms of the two substrates while the liquid components
are absorbed by dissolution.
The structural quality of joints made by films of the invention is promoted
by the fact that conventional processing of film deposition by chemical
absorption from a gas allows for easy control of the deposit thickness
and, thereby, of the amount of the reactive material which generates the
joint. In this way, only the minimum amount necessary to achieve the
desired reactions is used. Increasing the film thickness above this amount
necessitates either higher temperatures or longer times to promote full
dissolution of the liquid components into the substrates. Less than full
dissolution that is retaining increasing thicknesses of the liquid
components between the substrates after completion of processing,
increasingly weakens the joint. What is an actual thickness of remaining
material excessive for the desired joint is a function of the nature of
the joint materials and readily determinable by one of ordinary skill in
the art.
The invention produces a true integration of the joined materials on the
atomic crystalline structure level in the case of metals which are capable
of dissolving in solid solution elemental boron and silicon and in the
case of ceramics which are capable of dissolving in their structure boron
and silicon oxides. The invention is not so limited, however, and has
useful applicability to metals and ceramics which have a limited
solubility for boron and silicon. For metals, intermetallic boride and
silicide compounds are formed by reactions with the liquid phase at the
joint interface. In most cases, these compounds tend to remain at the
joint location because they are thermodynamically stable and affected very
little by the diffusive matter transport mechanism. Similarly, a
borosilicate glass bond residue is retained at the joint with ceramics
which have limited tendency of dissolving boron and silicon oxides in
their structure. The influence of these segregations on the structural
properties of the joints is restricted, however, because of the small
amount of the boron and silicon participating in the bonding process.
The affinity of boron and silicon towards both metals and oxygen is also
advantageous for joining metals to ceramics. Here, the liquid
metal-boron-silicon eutectics are chemically active towards most ceramic
substrates, particularly those containing oxygen, and easily wet and bond
to the ceramics.
Another novel chemical use of the boron-silicon-hydrogen films of the
invention is for the sealing, that is preserving, the chemical bonds and
atomic lattice of surfaces of intermetallic compounds such as gallium
arsenide, selected from the compounds formed between elements belonging to
groups III and V of the periodic table.
In the process of deposition by decomposition of a borane-silane gas
mixture, the deposited solid boron-silicon hydrogenated alloy film bonds
with the external, unsaturated chemical valencies at the substrate
surface. The film does not disrupt or break the bonds of the underlying
atomic lattice layers of the bulk material. At the deposition temperature,
which is below the hydrogen evolution temperature of approximately
350.degree. C., reaction of the film with the diffusion into the bulk
material is essentially precluded.
Among the various utilities for this type of surface sealant is its use as
an intermediate protective layer when it is necessary to provide on the
intermetallic compound surface a layer of another material without
interfering with the atomic lattice of the compound. At temperature below
formation of liquid phases but above that of hydrogen evolution, the
deposited hydrogenated film converts to the solid boron-silicon alloy film
which is still effective in protecting the underlying intermetallic
surface.
The chemical stability of the as-formed film is the only property necessary
for this embodiment of the invention and no further treatment is necessary
to achieve this result.
A further novel chemical use of the hydrogenated films of the invention is
the protection of metallic and organic resin substrates against harmful
attack by atmospheres containing dissociated oxygen, molecular oxygen,
water vapor and dissociated oxygen and nitrogen.
Suitable metallic substrates are those containing iron, nickel, cobalt,
chromium and aluminum, for example, nickel or iron base high temperature
alloys such as Inconel 617 and MA 956, and titanium. Suitable organic
resin substrates are those that do not deteriorate during film formation,
for example, kapton.
For applications for protection against attack by dissociated oxygen in,
for example, the upper earth atmosphere, molecular oxygen and water vapor,
the films are used as deposited, needing no further thermochemical
treatment to accomplish their function. Their as-formed chemical stability
is the only property necessary for these embodiments of the invention.
For protection of metallic substrates against deterioration by the effects
of recombination of dissociated oxygen and nitrogen species generated, for
example, at the surface of a body reentering earth atmosphere, the films
are converted to boron-silicon oxides by the atmospheric oxygen under heat
generated by friction at the surface against the atmosphere. The
boron-silicon oxygen compound layer so formed is also protective against
oxidation of the substrate for a limited amount of time during reentry.
For applications where it is necessary to first form an oxide layer on the
surface of the reentry body or shield to insure it has desirable heat
emissivity during reentry, it is convenient to convert the as-deposited
boron-silicon-hydrogen film to boron-silicon oxides by heat treatment in
air, for example at 1000.degree. C. for one hour.
The boron-silicon-hydrogen films of the invention are formed by
conventional techniques well understood by the art. Illustrative of such
techniques are the methods used by C. C. Tsai, cited previously, and U.S.
Pat. No. 4,064,521. These processes involve subjecting to a glow discharge
a mixture of silicon and boron hydride gases (silane and borane) carried
in an inert gas carrier such as argon at a pressure which ranges between
0.1 and 10 torrs. Electrons emitted from the electrodes in the discharge
ionize and dissociate the hydride molecules which are attracted to the
substrate to form the silicon-boron-hydrogen deposit.
The preferred method of the film deposition for the practice of the
invention is by thermal decomposition of the hydride gases. It is chosen
over the glow discharge, or plasma, activated processes generally utilized
by the electronic technology because it is less dependent on the geometry
of the substrate. The glow discharge activation is produced by electrons
generated from gases ionized in an electric field created between two
electrodes, one of which is, most commonly, the substrate itself.
Therefore, the best results in terms of uniform coverage of the substrate
surface are secured with flat surfaces directly exposed to the discharge.
Thermal activation, on the other hand, is effected over all the external
substrate surfaces uniformly heated at the process temperature.
In the preferred method of the invention, the silicon-boron-hydrogen films
are formed by adsorption on chemically active surface sites of
hydrogen-silicon and hydrogen-boron complex molecules generated by thermal
decomposition, at a pressure of 1 atmosphere, of a mixture of silane
(SiH.sub.4), and diborane (B.sub.2 H.sub.6) gases contained in a hydrogen
carrier. At the temperature of the process, which is maintained between
200.degree. C. and 350.degree. C., the adsorbed species are mobile and
diffuse along the surface such that the coverage is independent of the
substrate geometry. Since the coating is formed by chemisorption, it
becomes bonded only to the atomic, or molecular, layers which physically
constitute the surface, and, at the low deposition temperature, it is
prevented from forming compounds with, or diffusing into, the substrate.
Therefore, the deposition by low temperature chemisorption from a gas
medium accomplishes two purposes important for the embodiments of the
invention: uniformity of coverage independent of the complexity of the
surface geometry and sealing of the substrate surface with a film of a
highly reactive composition.
The deposition is carried out from a flowing stream of the gas fed at
atmospheric pressure into a reaction chamber containing the substrate
heated to the process temperature which is kept between 200.degree. C. and
350.degree. C. Below 200.degree. C. the process is slow and produces non
uniform powdery deposits, above 350.degree. C. the composition of the film
becomes unstable due to loss of hydrogen. The films best suited for the
purposes of the invention are prepared at about 250.degree. C. Prior to
initiating the deposition, the reaction chamber is evacuated, heated under
vacuum to the process temperature and then purged with pure nitrogen for
one hour. After purging, the reactive gas is admitted and kept flowing at
the desired rate for the duration of the treatment. The temperature
usually rises spontaneously at the beginning for a short time due to the
exothermic nature of the adsorption of the first layer of the coating.
After that the temperature falls down agains and remains stable while the
film's thickness increases to its final level. After the film has attained
its full thickness, the reactive gas mixture is replaced with nitrogen,
the chamber is cooled down under the flow of nitrogen to ambient
temperature and the coated substrate is taken out. Due to chemical
stability at low temperatures, the storage of as coated articles under
ambient conditions does not pose aging and deterioration problems.
The composition of the gas feed for the film deposition and the duration of
the deposition depend on the embodiment of the invention for which the
film has been prepared. The gas feed is made up of two components: a
mixture of about 2% silane (SiH.sub.4) and about 98% hydrogen and a
mixture of about 1% diborane (B.sub.2 H.sub.6) and about 99% of hydrogen.
These two components are mixed together prior to being admitted to the
reaction chamber by feeding them each at a different rate through a mixing
flowmeter. For all the above invention embodiments, except for ceramic to
ceramic bonding, the optimum flow measured in cubic centimeters per minute
has been found to be 15 cc of borane-hydrogen mixture and 385 cc of
silane-hydrogen mixture. Conditions for bonding ceramics which require
formation of borosilicate glasses are 105 cc of borane-hydrogen and 210 cc
of silane-hydrogen. The time of treatment for all five embodiments of the
invention is the same, about 20 minutes. It produces a film between 1.0
and 1.5 micrometers thick. This thickness has been found satisfactory in
all cases.
In preparation for film deposition, the surfaces are cleaned with
appropriate commercial solvents or soap solutions to remove impurities and
grease. Metals are additionally pickled and smut is removed by such
procedures as normally applied in industrial practice prior to any coating
process.
The most efficient bonding of metals obtains when the bond interface is
liquid during the process. Although solid state diffusion bonds can be
made using the films of the invention, they might require excessively high
temperatures and long times. The invention is accordingly best suited for
those cases where at least one of the materials being joined is able to
react with the film to produce a low-melting phase, i.e., melting at a
temperature low enough not to impair the properties of the materials
during the joining operation. The invention, therefore, is most
advantageous when applied to metals containing in their compositions such
elements as gold, silver, copper, iron, nickel, or cobalt, all of which
react with either or both boron and silicon to generate liquid eutectic
phases at temperatures not higher than 1200.degree. C. These metals can be
joined to themselves, to each other, to other metals and to oxide ceramics
by depositing on them the film of the invention, putting them in contact
with the surface to which they are to be joined under a pressure of a few
pounds per square inch and heating the assembly to a temperature between
10.degree. C. and 50.degree. C. above the respective eutectic temperature
in an inert atmosphere, or vacuum, to prevent oxidation. The eutectic
temperatures of interest are easily found in scientific and technical
literature. The time of heating is chosen according to the structure
expected for the joint. The best procedure is to determine by experiment
the structure best suited for each case. In general, the increase of time
at joining temperature will increase the diffusion effects producing
dissolution of boron and silicon away from the joint interface and grain
growth across the bond. The bond between ceramics using conversion of the
boron-silicon-hydrogen films into a borosilicate glass is made by heating
the assembled joint in air at illustrative temperatures of about
1200.degree. C. to 1300.degree. C.
To make the joints of the invention, it is not necessary to coat both
surfaces which are to be bonded together with the boron-silicon-hydrogen
film. It is sufficient to deposit the film only on one surface. Joining
different materials, it is advantageous to put the film on the material
which has the better affinity to react with the film and generate the
bond-forming phases such as the liquid eutectic. However, from the point
of view of protecting the surfaces from deteriorating by exposure to
ambient air prior to making the joint, it is desirable to have both of the
surfaces coated because the film is an efficient protection against
oxidation, even by humid atmosphere. Several examples are given to
illustrate the preparation and characteristics of the hydrogenated
boron-silicon alloy films of the invention.
EXAMPLE 1
Boron-silicon-hydrogen film was used to bond together two pieces of mild
steel (SISI-SAE type 1020). The surfaces were ground flat on a fine emery
paper and etched by immersion for 30 seconds in an acid solution composed
of 17 volume percent hydrofluoric acid, 44 to 55% concentrated, 33 volume
percent nitric acid, 70% concentrated, and 50 volume percent deionized
water. After etching, the samples were rinsed in running water and the
oxide smut produced on the surface by the etching was removed by immersion
for 150 seconds in a solution composed of 7.5 volume percent nitric acid,
70% concentrated, 48 volume percent sulfuric acid, 90% minimum
concentrated, and 44.5 volume percent sulfuric acid, 90% minimum
concentrated, and 44.5 volume percent deionized water. The etched and
cleaned samples were rinsed in running water, drained and allowed to dry.
They were then placed in a reaction chamber which was evacuated, heated
under vacuum to a temperature of 250.degree. C. and then purged with pure
nitrogen for one hour. The flow of nitrogen was then replaced by a flow of
a mixture of two gases, hydrogen containing 2% silane (SiH.sub.4) and
hydrogen containing 1% diborane (B.sub.2 H.sub.6). Their flow was adjusted
to 15 cubic centimeters per minute of the borane-hydrogen gas and 385
cubic centimeters per minute of the silane-hydrogen gas, giving a total
gas mixture flow through the chamber of 400 cubic centimeters per minute.
This flow was maintained for 26 minutes after which time it was replaced
by a flow of pure nitrogen. The heat source was then shut off and the
samples were allowed to cool down to room temperature under the pure
nitrogen flow. The coated samples were assembled by mating the coated
surfaces under a pressure of 10 pounds per square inch and heating at a
temperature of 1200.degree. C. in vacuum for 1 hour. The bond generated by
this process between the two steel samples was entirely absorbed in the
material by steel crystals grown across the joint.
EXAMPLE 2
Two pieces of Inconel 617 alloy (22.63% Cr, 12.33% Co, 9.38% Mo, 1.155 Al,
0.76% Fe, 0.27% Ti, 0.15% Si, balance Ni) were bonded together using
boron-silicon-hydrogen film coatings deposited on the mating surfaces. The
procedure and process parameters applied for film deposition and bonding
treatment were identical to those described in Example 1. Also in this
case, the bond had been entirely absorbed by Inconel crystals grown across
the joint.
EXAMPLE 3
Two plates of fused quartz glass (pure SiO.sub.2) were bonded together
using boron-silicon-hydrogen film coatings deposited on the mating
surfaces. The procedure and processing time and temperature applied for
film deposition were identical with those described in Example 1 except
cleaning of the surfaces to be coated was confined to a wash with organic
solvent and the flow of the coating gases through the deposition chamber
was adjusted to 105 cubic centimeters per minute of the borane-hydrogen
gas and 210 cubic centimeters per minute of silane-hydrogen gas, to a
total flow of 315 cubic centimeters per minute of the gas mixture. The
coated samples were assembled as described in Example 1 and heated at a
temperature of 1250.degree. C. in an air furnace for 1 hour. The bond
generated between the quartz surfaces was completely absorbed in the
material. No segregations or voids were found at the joint.
EXAMPLE 4
Two plates of sintered aluminum oxide (Al.sub.2 O.sub.3) were bonded
together using boron-silicon-hydrogen film coatings deposited on the
mating surfaces. The procedure and process parameters applied for film
deposition and bonding treatment were identical to those described in
Example 3. Also in this case, the bond had been absorbed in the material.
Only traces of borosilicate glass bond remained at the joint interface in
the form of dispersed isolated inclusions.
EXAMPLE 5
Two pieces of sintered silicon carbide (SiC) coated with
boron-silicon-hydrogen film were joined together with a 0.003 inch thick
aluminum metal foil interposed between the mating surfaces. The mating
surfaces of the silicon carbide pieces were coated with the film using
procedure and process parameters described in Example 1. The bond between
the coated silicon carbide surfaces and the aluminum sandwiched between
them was formed by heating the assembly at 700.degree. C. for one hour in
vacuum under a pressure of 10 pounds per square inch. The bond was uniform
and free of structural defects such as voids and foreign material
inclusions. Under a shear load, the bond ruptured within the silicon
carbide.
EXAMPLE 6
Structural alloys for thermal protection systems of spacecraft reentering
earth atmosphere require on the surface a high emissivity oxide layer with
a low catalytic activity to the recombination of dissociated species, such
as atomic oxygen and nitrogen, present in the boundary layer during
reentry. Several samples of alloy MA 956 (19.30% Cr, 4.28% Al, 0.49% Y,
0.39% Ti, 0.28% Ni, 0.20% O, balance Fe) were lightly grit blasted with
120 mesh alumina and oxidized for 2 hours at 2000.degree. F. in static
air. The oxidized surfaces were then coated with boron-silicon-hydrogen
film using the procedure described in Example 1. The coated specimens were
again oxidized in air and then exposed in an arc-heated wind tunnel to
repeated cyclic tests of 0.5 hour each, under simulated reentry conditions
at surface temperatures ranging from 1500.degree. to 2300.degree. F.
Catalytic activity of the sample surfaces was assessed by comparing the
response of coated and uncoated specimens for up to 5.5 hours of total
exposure to test temperature during cycling. While the surface catalysis
ratio, expressed as the ratio of net aerothermal heating rate and the
catalytic wall heating rate, for uncoated MA 956 alloy surface was close
to 1, that of the coated one was only about 0.5.
EXAMPLE 7
Development of field effect transistor devices using gallium arsenide
semiconductor materials is seriously hindered by difficulty in securing a
surface passivation layer which would not interfere with free movement of
electrical charges at the interface between the dielectric and the
semiconductor. Hydrogenated boron-silicon film was used as a transition
layer between GaAs and a silicon oxide passivation to eliminate charge
injection into the dielectric. Samples of thin wafers of single crystal
GaAs were coated with the film using the procedure described in Example 1.
About 0.25 micrometer thick layer of silicon oxide was deposited on the
top of the film by standard microelectronic art chemical vapor deposition
methods. The nature of the interface between the passivation film and the
GaAs surface was investigated by slow-sweep 10-KHz capacitance-voltage
plots. Most of the best quality oxide dielectrics available in the current
state of the art display a clockwise hysteresis in the C-V curves which is
attributed to charge injection into the oxide. The hysteresis, and,
therefore, the charge injection it indicates, were eliminated in the
samples with the silicon oxide dielectric deposited on the top of the
hydrogenated boron-silicon film.
* * * * *
|
|
 |
|
 |
Description |
|
