Patent Application
20100075055
Alpha alumina (α-Al2O3, corundum) is one of the most widely utilized ceramic materials due to a favorable combination of such properties as high mechanical strength and hardness, good wear resistance, low electric conductivity, high refractoriness, and high corrosion resistance in a broad range of chemical environments. Applications of α-Al2O3 include abrasive materials, electric insulators, structural ceramics, vacuum tube envelopes, refractory bricks, liners, and sleeves used in metallurgical applications, kiln furnaces, etc., laboratory ware, catalytic supports, etc.
α-Al2O3 has been used in the form of coatings/films for several important applications. In thermal barrier coatings (TBC), the α-Al2O3 films act as diffusion and thermal barriers protecting underlying high-temperature alloys from damage in gas turbines and engines. α-Al2O3 wear-resistant coatings are applied on metals or cemented carbides to significantly prolong the lifetime of cutting tools. Very high purity alumina coatings can be used as electric insulators in electric/electronic applications. After doping with Cr, Ti, or rare-earth ions, films of α-Al2O3 can be used as planar optical waveguides in photonic devices.
Films and coatings of α-Al2O3 can be synthesized by several well-established methods, such as sol-gel, chemical vapor deposition (CVD), high-temperature oxidation of Al-containing alloys, PVD techniques, such as pulsed laser deposition, magnetron sputtering, and thermal spray. The later technique actually uses α-Al2O3 powders only as feedstock for spraying but due to the high temperature nature of the process, the coatings consist mostly of γ-Al2O3 phase with only small content of untransformed α-Al2O3 grains. All of the other methods require the use of high temperatures, in order to crystallize the α-Al2O3 phase. The synthesis temperatures vary by deposition method and are: 1,100-1,200° C. for sol-gel, 1,000-1,100° C. for CVD, 850-1,050° C. for pulsed laser deposition, and 1,200° C. for high-temperature oxidation. The very high synthesis temperatures lead to several detrimental effects, such as undesired oxidation/corrosion of the substrate metal (for example Inconel 718), formation of very large residual thermoelastic stresses between the coating and the substrate, which can result in cracking, peeling-off of the coatings, or diffusion of metals from the substrate into the coating. Besides, techniques such as CVD or PVD require expensive equipment, use corrosive gases, and thus are expensive and environmentally stressful. Deposition processes using lower temperatures of 280-560° C., such as rf magnetron sputtering, still necessitate using Cr2O3 template layer to promote formation of the α-Al2O3 phase.
A viable low-temperature, inexpensive, and environmentally benign alternative to the film deposition techniques described above is the hydrothermal method. Hydrothermal synthesis simultaneously deposits and crystallizes anhydrous coatings/films directly from aqueous solutions at low temperatures and under moderate pressures. This technology offers several advantages over conventional film deposition methods, such as one-step synthesis without high temperature calcination, unique chemical defect structure, excellent control of film microstructure, flexibility in substrate shape and size when compared to deposition techniques such as CVD or PVD, simplicity, and low cost. There is no need for expensive equipment (PVD), vacuum systems, or corrosive gases (CVD). The hydrothermal technique allows the direct deposition of crystalline films or coatings using simple aqueous solutions as precursors in simple autoclaves at low temperatures, greatly reducing or eliminating difficulties associated with thermal strain mismatch, film/substrate interdiffusion, films peel-off, and other deleterious effects that occur at high temperatures with other films/coatings deposition methods, particularly those requiring temperatures up to over 1,000° C. All these attributes make the hydrothermal process commercially appealing, particularly for α-Al2O3.
No α-Al2O3 films or coatings of any type have ever been synthesized by the hydrothermal method on any type of substrates (metallic, ceramic, or polymers).
The following presents a simplified summary of the invention in order to provide a basic understanding of some example aspects of the invention. This summary is not an extensive overview of the invention. Moreover, this summary is not intended to identify critical elements of the invention nor delineate the scope of the invention. The sole purpose of the summary is to present some concepts of the invention in simplified form as a prelude to the more detailed description that is presented later.
In accordance with various aspects, the present invention provides use of hydrothermal synthesis to prepare a variety of α-Al2O3 based coatings on several types of metals (316 stainless steel, 1018 carbon steel, Inconel 718, and Grade 5 Titanium) at low temperature around 400° C. without any template layers. The coatings are either 100% α-Al2O3 phase or consist of mixtures of various quantities of the α-Al2O3 phase and substrate metal-derived oxides. Their microstructures, i.e. grain size, coating thickness, or surface coverage, can be controlled in wide ranges by changing the synthesis conditions. The hydrothermal synthesis offers here several advantages, such as low synthesis temperature, which minimizes thermal stresses and interdiffusion, good control of the film microstructure and phase composition, uniform coverage on complex shapes, and possibility of coating metals, which are not resistant to high temperatures.
In accordance with one specific aspect, the present invention provides a process to deposit an Alpha Alumina (α-Al2O3) crystalline coating on a substrate surface, wherein the process includes hydrothermal synthesis of the α-Al2O3 crystalline coating.
The foregoing and other aspects of the present invention will become apparent to those skilled in the art to which the present invention relates upon reading the following description with reference to the accompanying drawings, in which:
Example embodiments that incorporate one or more aspects of the present invention are described and illustrated in the drawings. These illustrated examples are not intended to be a limitation on the present invention. For example, one or more aspects of the present invention can be utilized in other embodiments and even other types of devices. Moreover, certain terminology is used herein for convenience only and is not to be taken as a limitation on the present invention. Still further, in the drawings, the same reference numerals are employed for designating the same elements.
The hydrothermal syntheses of α-Al2O3 coatings in the present invention were performed in thoroughly cleaned and hermetically closed with modified Bridgman-type plug steel autoclaves (13″ Diameter×120″ Height, Autoclave Engineers, Erie, Pa.) equipped with two centrally positioned thermocouples, two PID temperature controllers, a pressure gauge, and a pressure relief system designed to vent excess pressure during synthesis and after the synthesis, as well as keeping pressure constant at a desired level (See
Coatings/films that contained either 100% α-Al2O3 phase or coatings/films consisting of various mixtures of α-Al2O3 phase with substrate-derived metal oxides, with various microstructures and levels of substrate coverage were synthesized in the present invention using the following procedure. First, appropriate weight of de-ionized water was added to HDPE containers or titanium liners. Then, desired weights of chemical additives, if any, were added to the containers, and the containers were stirred thoroughly in order to obtain homogeneous solutions. Then, appropriate weights of the precursor powder (Type A or Type B, see Table I for detailed descriptions) were added to each of the containers and stirred thoroughly to obtain uniform slurry. Finally, the seeds, if any, were added and content of the containers was stirred again for 1-2 minutes in order to disperse the seeds uniformly in the slurry.
If the slurry was prepared in a titanium liner, coupons of metals to be coated where subsequently placed in the bottom part of each liner, so they were completely covered by the precursor slurry. Alternately, if the slurry was prepared in separate HDPE container, the metal coupons were first placed in the bottom of empty titanium liners and then the precursor slurry was poured in, to obtain complete coverage of the coupons. The following metal coupons with sizes ½″×½″×0.125″ (all obtained from Metal Samples Company, Munford, Ala.) were used: Inconel 718 (chemical analysis Al=0.480%, Cr=18.320%, Mo=2.990%, S=0.0002%, B=0.004%, Cu=0.060%, Nb=5.190%, Si=0.080%, C=0.030%, Fe=18.020%, Ni=53.650%, Ta=0.010%, Co=0.100%, Mn=0.080%, P=0.008%, and Ti=0.980%), stainless steel 316 (Cr=16.793%, Mo=2.206%, S=0.001%, Cu=0.308%, N=0.041%, Si=0.225%, C=0.023%, Ni=10.025%, Mn=1.567%, P=0.029%, and Fe=balance), carbon steel 1018 (S=0.007%, Si=0.020%, C=0.160%, Mn=0.750%, P=0.010%, Al=0.050%, and Fe=balance), and titanium grade 5 (C=0.030%, Al=6.150%, N=0.020%, Y<50 ppm, 0=0.170%, Fe=0.150%, H=48 ppm, V=3.930%, and Ti=balance).
Smaller titanium liners were placed inside large 12″ Diameter titanium liners, which were then closed with lids, placed in a special steel holder (up to 5 containers per holder), and put into the autoclave as described earlier. Detailed concentrations and types of used precursors, seeds, chemical additives, and dopants are summarized in Table II.
The subsequent hydrothermal treatments in accordance with aspects of the present invention were accomplished in either single-ramp or in dual-ramp regime (see
Phase compositions of metal coupons, both as-received (i.e. untreated) and after hydrothermal treatment with deposited coatings, were characterized by X-ray diffraction using Advanced Diffraction System X1 diffractometer (XRD, Scintag Inc.) using Cu Kα radiation, in the 20 range between 20-70° with a 0.05° step size and 1.0 s count time. The chemical identity of the materials was determined by comparing the experimental XRD patterns to standards compiled by the Joint Committee on Powder Diffraction and Standards (JCPDS), i.e. card #10-0173 for α-Al2O3 (corundum), #03-0066 for γ-AlOOH (boehmite), #33-0664 for Fe2O3 (hematite), #19-0629 for Fe3O4 (magnetite), and #80-1012 for Na1.97Al1.82Ti6.15O16.
The microstructures of the metal coupons before and after hydrothermal treatment were examined using both optical microscope (Vanox, Olympus, Tokyo, Japan) under 50-500× magnifications and scanning electron microscope (SEM, Model S-4500, Hitachi, Japan) at 5 kV accelerating voltage. Prior to the SEM examination, the materials were attached to aluminum holders using a conductive carbon tape and subsequently sputtered with thin conductive layers of palladium. Chemical compositions of various regions of the coatings formed on metal coupons during the hydrothermal treatment were determined using Noran X-ray energy-dispersive spectrometry (XEDS) detector attached to the SEM. During the XEDS measurements, 20 kV accelerating voltage was used and the data was accumulated over 60-180 seconds.
Morphology of α-Al2O3 coatings, surface roughness, and residual thermoelastic stresses present in the α-Al2O3 coatings were measured using laser scanning confocal microscope Olympus FV1000, connected to a high-resolution spectrometer, which allows stress measurements from wavelength shift of the 694 nm fluorescence line of Cr3+ ions present in the α-Al2O3 lattice.
Typical properties of the α-Al2O3-based coatings in accordance with various aspects of the present invention, which were deposited on all types of metal substrates using the hydrothermal method, are summarized in Tables II A-D. It is apparent that the properties of the films, such as phase composition, microstructure, grain size and shape, and film thickness are strong functions of all synthesis parameters, such as temperatures/durations of the hydrothermal treatment, precursor composition, type of the metal substrate, etc. Thus, the selection of appropriate precursors, seeds, dopants (if any), and chemical additives for the hydrothermal synthesis of α-Al2O3 coatings is part of the process to obtain product coatings with desired properties as per the requirements of the user.
The α-Al2O3-based coatings in accordance with various aspects of the present invention cover uniformly all types of metal substrates. The uncoated (as-received) metal substrate coupons were covered with machining groves/scratches (see
The surface adhesion of the films in accordance with aspects of the present invention was good. No cracks or peeled-off layers were typically observed. This can be attributed to such factors as low deposition temperature thus low residual stresses (see
A wide variety of microstructures of the α-Al2O3-based coatings were obtained in accordance with several aspects of the present invention, as shown in Tables II A-D. Coatings consisting of equiaxed α-Al2O3 crystals of various sizes (See
The coatings in accordance with the various aspects of the present invention were either single-phase α-Al2O3, like these shown in
Chemical composition of selected coatings in the present invention suggests the presence of substrate metal-derived atoms even in the α-Al2O3 grains. XEDS analysis suggests the presence of Fe, Cr, Ni and/or Ti in α-Al2O3 grains deposited on steel, Inconel or titanium substrates (see
At the low film deposition temperatures, the diffusion coefficients are insufficient to dope the α-Al2O3 phase via the bulk diffusion mechanism. Thus the most likely mechanism is incorporation of the ions dissolved from the substrate in the growing film. This in-situ doping is unique for the hydrothermal deposition conditions of the films in the present invention and may result in unique film properties. Again, this effect can be controlled by changing the deposition conditions and either high-purity α-Al2O3 coatings or doped α-Al2O3 coatings can be synthesized hydrothermally by the methodology described in the present invention.
The residual stresses of two selected α-Al2O3-based coatings in accordance with various aspects of the present invention on Inconel 718 and stainless steel 316L were measured and it was found that the in-plane stress distribution was very uniform and narrow and the residual thermal stresses averaged around 1.8-2.0 GPa (see
One factor in the hydrothermal deposition process of the α-Al2O3 coatings in accordance with an aspect of the present invention was the type of the substrate.
Interactions between the substrate and the deposited α-Al2O3 films include reactive substrate, inert substrate and a combination of both. The reactive substrate releases metal ions into the surrounding solution. The ions can subsequently form metal oxide coating, which will be mixed with the α-Al2O3 crystals producing α-Al2O3-based composite coatings (
Aluminum tri-hydroxide (trihydrate) powders (gibbsite or hydrargillite, chemical formula Al(OH)3) or aluminum oxide-hydroxide powders (boehmite, chemical formula γ-AlOOH) can be used as precursor powders in hydrothermal synthesis of α-Al2O3 coatings in the present invention. During the course of this work, several precursors were tested, however the best results, which provided the highest chemical purity and most consistent and reproducible morphological features of the α-Al2O3 powders, were obtained using the following precursors: Type A and Type B, which both are various types of Al(OH)3. Available typical properties of the precursor powders are summarized in Table I.
Seeds can be advantageously used to control the size, composition and rate of crystallization of oxides under hydrothermal conditions See for example U.S. application Publication No. 2007/0280877. The α-Al2O3 seeds, mixed with the precursor powder were found to be effective modifiers of microstructure of the synthesized α-Al2O3 coatings in the present invention. Seeds having a wide range of median particle sizes between 100 nm and 40 μm can be used. The seeds could be hydrothermally synthesized α-Al2O3 powders, either milled or as-synthesized (aggregated), or suitable commercially available α-Al2O3 powders. The relationship between the α-Al2O3 seeds used as starting materials and the final α-Al2O3 hydrothermal products is a complex function of seed quantity (weight/volume fraction of seeds with respect to the precursor powder), particle size, aggregation level, and type of seeds, as well as type of precursor, conditions of hydrothermal synthesis of the α-Al2O3 coatings, and method of mixing the seeds with the precursor. This complex relationship has to be established experimentally in each case. Nevertheless, some general observations were made in this work. The smaller the α-Al2O3 seeds, the finer the microstructures of the hydrothermally synthesized α-Al2O3 coatings of the present invention (see Table II).
During the hydrothermal synthesis, the conversion of the precursor into α-Al2O3 can be complete or limited. Several factors, such as lower temperature, shorter synthesis time, conditions of the temperature ramp(s) in hydrothermal treatment, etc. (see for example U.S. application Publication No. 2007/0280877) can be used to make unique composite α-Al2O3/γ-AlOOH coatings. In at least one aspect of the present invention, the α-Al2O3/γ-AlOOH coatings could be deposited on titanium, Inconel 718, and 316L stainless steel (Examples 3, 7, and 13). Presence of boehmite can result in unique properties of the α-Al2O3-based coatings, moreover the boehmite phase could be converted into transition aluminas upon subsequent heat treatment in air, resulting in composite α-Al2O3/transition alumina coatings.
α-Al2O3 phase can be doped with a variety of elements during the hydrothermal synthesis, such as Mn or Cr. The doping additives can be selected for specific applications and/or creating unique defect structures. Preferred sources of doping additives can be water-soluble salts of the doping elements, but they can also be derived from dissolved components of metal substrates during the hydrothermal synthesis. It is presumed that any type of salts can be used, providing that they do not introduce unwanted impurities, which could change properties of the α-Al2O3. Use of CrCl3 or KMnO4 in order to introduce doping elements of Cr and Mn in concentrations of 0.01%, and 0.05%, respectively, is not expected to introduce any modifications to the coatings microstructure (see for example U.S. application Publication No. 2007/0280877). In some cases, however, doping can be used to modify properties of the α-Al2O3 (chemical composition, microstructure, etc.).
The following reactions take place under hydrothermal conditions to make α-Al2O3 powders from alumina hydrates (see
Al(OH)3→γ-AlOOH(boehmite)+H2O (1)
2AlOOH→α-Al2O3(corundum)+H2O (2)
Reaction (1) can occur above ≈100° C. practically independently of the water vapor pressure. Reaction (2) can occur above ≈350° C., but up to ≈450° C. only at pressures not exceeding ≈15 MPa (≈2,200 psi), because of the presence of AlOOH (diaspore)-stability region, which extends from 270° C. to 450° C. and from ≈15 MPa to over 100 MPa. In addition to raw materials and reactor design, very specific time, temperature and pressure “ramps” are required to produce α-Al2O3 of the desired characteristics. Due to constraints imposed by the strength of the autoclave, conducting synthesis above 450° C. at high pressure does not seem to be practical. Therefore, at α-Al2O3 synthesis temperatures below 450° C. (practical range is 380-430° C.), the pressure is reduced to or below ≈15 MPa (≈2,200 psi). In order to achieve this objective, water vapor pressure is released simultaneously with increasing temperature in the autoclave.
Effects of various possible conditions of the hydrothermal synthesis of α-Al2O3 powders have been studied in great detail and described elsewhere. It is believed that growth mechanisms and relationships found in that previous study are of general nature and can be also applied to the hydrothermal synthesis of α-Al2O3 coatings/films provided in accordance with aspects of the present invention. This relates not only to the use of dopants but also acidic synthesis conditions (for example use of diluted H2SO4), etc.
The present invention provides many aspects. Some example aspects are as follows, however it is to be appreciated that the present invention need not be so limited to the following examples.
A process to deposit an Alpha Alumina (α-Al2O3) crystalline coating on a substrate surface, wherein the process includes hydrothermal synthesis of the α-Al2O3 crystalline coating. The process may include utilizing an elevated temperature within the range of about 380° C.-430° C. for a time duration of within the range of about 1-10 days. The process may include utilizing a precursor having approximately 65% Al2O3, approximately 0.1-0.35% Na2O of which a maximum of 0.17% is soluble, approximately 0.007-0.01% Fe2O3, approximately 0.001-0.005% SiO2, 0.02-0.05% Free Moisture, a Specific Gravity of approximately 2.42 (g/cm3), and a Refractive Index of approximately 1.57. The process may provide the coating as an essentially pure α-Al2O3 crystalline coating. The substrate may be a metal in the process. The process may include some amount of dissolution of the metal substrate during the hydrothermal process and the process may provide the coating to include metal oxide. The metal oxide may be within the range of below 90%.
Dopants may be dissolved from the substrate during the hydrothermal process and incorporated into the coating. The process may include utilizing a precursor that includes dopants that are dissolved from the precursor during the hydrothermal process and incorporated into the coating. The process may provide the coating to contain boehmite. The boehmite may be within the range of below 90%. At least one of grain size, thickness and porosity may be controlled during the hydrothermal process by controlling at least one of temperature cycle, seeds and precursor selection. The process may include the use of acidic media to control submicron/nano particle size. The process may include the use of acidic aluminum salts to control submicron/nano particle size. At least one morphology of being equiaxed, elongated or platelets may be controlled during the hydrothermal process. The coating may include α-Al2O3 particles within the range of approximately 2 nm-1000 microns. The coating may include α-Al2O3 particles within the range of approximately 10 nm-40 microns.
The substrate may be one of a metal, ceramic and plastic. The substrate may be at least one of porous and fibrous. The substrate may be particulate. The substrate may be nano sized. The provided coating may have a strong resistance against release from the substrate. The provided coating may be porous.
Of course, the process provides an α-Al2O3 coating. An apparatus is used to deposit the α-Al2O3 coating and is used in conjunction with the process. The apparatus may include an autoclave and a heat exchanger. The apparatus may provide varied coatings in a single hydrothermal heating cycle by using separate liners.
The invention has been described with reference to the example embodiments described above. Modifications and alterations will occur to others upon a reading and understanding of this specification. Example embodiments incorporating one or more aspects of the invention are intended to include all such modifications and alterations insofar as they come within the scope of the appended claims
The present application claims benefit of priority from U.S. Provisional Patent Application No. 61/094,137, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61094137 | Sep 2008 | US |
