BY EXOTHERMIC HEATING FIELD OF THE INVENTION

The present invention relates generally to the manufacturing of ceramic articles, and, more particularly, to processes for sintering ceramic formulations utilizing exothermic reactions to produce heat. BACKGROUND OF THE INVENTION

Metallic components are often susceptible to stress damage and corrosion when operated at high temperatures. As a result, research has been conducted on ceramic coatings for use on such metallic parts with the goal of improving high-temperature performance and component lifetimes. Nevertheless, the use of ceramic coatings is not itself without drawbacks. Ceramic coatings formed from powders must to be sintered after deposition to be suitably fused and densified. This sintering process needs to be performed at high temperature, which may itself act to degrade the underlying metallic component. At the same time, the high temperature sintering can cause undesirable reactions at the interface of the ceramic and the underlying metal, resulting in mechanical issues such as strain-induced defects and delamination.

For the foregoing reasons, there is a need for alternative ceramic coating techniques that reduce the temperature seen by the underlying component during sintering, while still producing ceramic coatings with acceptable properties. SUMMARY OF THE INVENTION

Embodiments of the present invention address the above-identified needs by providing methods of sintering a ceramic formulation at least in part utilizing the heat generated from an internal, exothermic chemical reaction. In the case of a ceramic coating on a substrate, these embodiments allow the ceramic coating to be effectively sintered at high temperature while the temperature of the underlying substrate is kept relatively low.

Aspects of the invention are directed to a method for manufacturing a ceramic article. Initially, a ceramic formulation is obtained comprising an oxidizable component. The ceramic formulation is then formed into a desired configuration. Subsequently, the ceramic formulation is heated to an initial temperature in a non-oxidizing ambient. Finally, the ceramic formulation is exposed to an oxidizing ambient so as to cause at least a portion of the oxidizable component to oxidize and release heat into the ceramic formulation. In one or more embodiments, the ceramic article may comprise a standalone object or a coating on a substrate. BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:

FIG. 1 shows a flow diagram of a process for forming a ceramic article, in accordance with an illustrative embodiment of the invention;

FIGS. 2A-2C show sectional views of intermediates formed during various stages of manufacturing a distinct ceramic object utilizing the FIG. 1 method;

FIGS. 3A-3C show sectional views of intermediates formed during various stages of manufacturing a ceramic coating on a substrate utilizing the FIG. 1 method;

FIG. 4 shows a diagrammatic representation of zirconia particles coated with zirconium carbide, in accordance with an illustrative embodiment of the invention; and

FIG. 5 shows a graph of temperature versus time while processing two YSZ ceramic formulations, one containing zirconium carbide and the other not containing zirconium carbide. DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described with reference to illustrative embodiments. For this reason, numerous modifications can be made to these embodiments and the results will still come within the scope of the invention. No limitations with respect to the specific embodiments described herein are intended or should be inferred.

FIG. 1 shows a flow diagram of a process 100 for forming a ceramic article, in accordance with an illustrative embodiment of the invention. As will be detailed below, the ceramic article may be a distinct ceramic object or, alternatively, may be a ceramic coating on an underlying substrate. Thus, as used herein, the term“article” is intended to be construed as encompassing both a standalone object as well as a constituent of a larger object.

In step 105 of the illustrative process 100, a ceramic formulation comprising an oxidizable component is obtained. The oxidizable component preferably comprises a material that undergoes an exothermic oxidation reaction when exposed to an oxidizer above a given temperature (hereinafter, the“required oxidation temperature”). If an yttria-stabilized zirconia (YSZ) ceramic is desired, the ceramic formulation may comprise, for example, zirconia and yttria mixed with zirconium carbide. Subsequently, in step 110, the ceramic formulation is formed into the desired configuration. When forming a distinct ceramic object, for example, the ceramic formulation may be extruded, pressed, or slip casted. Alternatively, when forming a ceramic coating on a substrate, the ceramic formulation may be deposited on top of the substrate by dip coating, painting, or spraying.

Next, in step 115, the result of step 110 is heated in a non-oxidizing ambient to an initial temperature above the required oxidation temperature for the oxidizable component of the ceramic formulation (e.g., 800-1,100°C), but below the temperature ultimately required to adequately sinter the ceramic formulation. When forming a coating, this initial temperature is also preferably below a temperature that might have adverse effects on the underlying substrate due to, for example, stress cracks, damage to the substrate microstructure, unwanted reactions between the ceramic coating layer and the substrate, and delamination. The non-oxidizing ambient may comprise, for example, argon, nitrogen, or a mixture of nitrogen and hydrogen, and may be conducted at atmospheric pressure or at a reduced pressure (e.g., 30-100 Torr). Because this heating occurs in a non-oxidizing ambient, the oxidizable component will undergo little or no oxidation during this step.

Lastly, in step 120, the ceramic formulation (configured as either a distinct object or a coating) is sintered. This sintering is effectuated by exposing the ceramic formulation to an oxidizing ambient (e.g., air or oxygen). This exposure causes the oxidizable component of the ceramic formulation to rapidly heat as a result of the exothermic nature of the oxidation reaction (e.g., zirconium carbide being converted to zirconia). Heat is thereby released into the ceramic formulation, causing it to be sintered. Advantageously, when forming a coating, the localized nature of the oxidation reaction causes the heat produced by the rapid oxidation reaction to be largely confined to the ceramic formulation. The underlying substrate therefore does not see most of the temperature rise associated with the sintering process.

To further illustrate aspects of the process 100, FIGS. 2A-2C show sectional views of intermediates formed during various stages of manufacturing a distinct ceramic object. FIG. 2A, for example, shows a ceramic formulation 200 (comprising an oxidizable component) formed into a desired shape, as would be a result of performing step 110. FIG. 2B, in turn, shows the exposure of the ceramic formulation 200 to an oxidizing ambient 205, as would occur while performing step 120. Finally, FIG. 2C shows a sintered distinct ceramic object 200' that would result from completing step 120.

FIG. 3A-3C, moreover, show sectional views of various intermediates formed when utilizing the process 100 to form a ceramic coating. FIG. 3A, for instance, shows a ceramic formulation 300 applied to the film stack comprising a substrate 305 and a bonding layer 310, as would occur after performing step 110. In one or more embodiments, for example, the substrate 305 may comprise a nickel alloy, while the bonding layer 310 may comprise aluminum. FIG. 3B, in turn, shows the resultant film stack exposed to an oxidizing ambient 315 during step 120. Lastly, FIG. 3C shows the film stack after being sintered in step 120 with a densified ceramic coating 300'.

Because the above-described discussion of the process 100 is somewhat generalized, additional aspects of the various steps are now presented. In one or more embodiments falling within the scope of the invention, for example, a ceramic formulation suitable for forming an YSZ ceramic may be prepared (step 110 of the process 100) by utilizing a novel wet chemical processing technique, namely, a sol-gel process.

The process depends on the coating of metal oxide powder particles with ultra-high surface area atomic or molecular scale metal carbides. Initially, zirconium particles with alkoxide functional groups, such as, but not limited to, zirconium-n-propoxide, zirconium isopropoxide, and zirconium-tetra-sec-butoxide, (each suitably diluted in ethanol or propanol), are mixed with yttrium acetate or yttrium nitrate, sucrose, and acetic acid. The resultant solution is then heated (to, e.g., about 300°C) to drive thermal decomposition. This process results in a zirconia/zirconium-carbide/yttria powder mixture with the zirconium carbide forming amorphous web-like coatings that at least partially surround the zirconia particles. Such coated particles are diagrammatically shown in FIG. 4 with zirconia particles 400 and zirconium carbide coatings 405. Configured in this manner, the zirconium carbide 405 is highly reactive, and is thereby well suited to rapidly oxidize when exposed to an oxidizer in step 120. Moreover, the zirconium carbide is uniformly distributed in the ceramic formulation, resulting in excellent temperature uniformity during sintering, which may minimize thermally-induced strain. Finally, heat production during the sintering occurs directly at the grain-boundary regions of the zirconia, exactly where it is needed for effective densification of the ceramic article.

While zirconium carbide is used in the above-described ceramic formulation as the oxidizable component, yttrium carbide may also be utilized. Yttrium alkoxide precursors such as yttrium isopropoxide and yttrium acetate allow similar wet chemical processing to produce yttrium carbide coatings on yttria particles. So formed, these modified yttria particles may be added to a zirconia/zirconium-carbide/yttria powder mixture so that the yttrium carbide may also react exothermically when exposed to an oxidizing ambient, and thereby also produce heat for sintering.

Additionally and optionally, yttrium nitrate may be used as a source of yttrium in a ceramic formulation for the formation of YSZ ceramic articles by the process 100. Yttrium nitrate is an oxidizer, which during thermal decomposition may also contribute to higher temperatures in the ceramic formulation during step 120. In one or more alternative embodiments, moreover, a ceramic formulation may be produced by a direct blending process, wherein zirconium carbide is mixed with yttria-stabilized zirconia. Finely nano-grained zirconium carbide may be formed by thermally decomposing zirconium nanopowders with alkoxide functional groups at high temperature (e.g., about 1200°C). In such a process, a finely mixed, stoichiometric blend of zirconia and carbon may be formed initially, which reacts to form zirconium carbide at the elevated temperature. Alternatively, zirconium carbide powders may simply be purchased from commercial suppliers, such as PlasmaChem GmbH (Berlin, Germany) and Sigma-Aldrich Co. LLC (St. Louis, MO, USA). Once obtained, the zirconium carbide can then be mixed with yttria-stabilized zirconia powder using ball milling and ultrasonification in an alcohol solvent to produce the desired ceramic formulation. Suitable yttria-stabilized zirconia may be purchased from Tosoh USA, Inc. (Grove City, OH, USA) with differing yttria contents (i.e., different levels of stabilization). Moreover, Tosoh offers formulations comprising small concentrations of aluminum oxide, which has been demonstrated to improve bonding between YSZ coatings and metallic substrates without affecting ceramic coating performance.

Importantly, while YSZ ceramic formulations are described in the several proceeding paragraphs, other ceramic formulations comprising oxidizable components would also fall within the scope of the invention. More specifically, refractory metal carbides such as, but not limited to, tantalum carbide, titanium carbide, hafnium carbide, niobium carbide and molybdenum carbide, may be incorporated into other ceramic formulations to allow processing in accordance with the illustrative process 100 set forth above. In even other embodiments, moreover, powdered refractory metals or Group IIIB elements (e.g., scandium, yttrium, and lanthanum) may also act as effective oxidizing components. As used herein, the term“refractory metal” includes the following chemical elements: niobium, molybdenum, tantalum, tungsten, rhenium, titanium, vanadium, chromium, zirconium, hafnium, ruthenium, osmium and iridium.

Once so formed, the ceramic formulation may, in one or more non-limiting embodiments, be formed into a distinct ceramic object in step 110 of the process 100 by, for example, extrusion or pressing. This processing produces a“green” object ready for further processing in accordance with the remaining processing steps, namely, steps 115 and 120. If, instead, a ceramic coating on an already existing substrate is desired, step 110 may instead be performed by coating techniques such as, but not limited to, dip coating, painting, and spraying. Additional methods such as, for example, tape casting and additive forming methods (e.g., robocasting and laminated object manufacturing) may be utilized to form both distinct ceramic object as well as ceramic coatings. These several methodologies for configuring the ceramic formulation into the desired configuration (i.e., as a distinct ceramic object or as a ceramic coating) will already be familiar to one having ordinary skill in the relevant ceramic arts. Additional information is also provided in readily available publications, including, for example, M.N. Rahaman, Ceramic Processing and Sintering, CRC Press, 2003, and R.W. Rice, Ceramic Fabrication Technology, CRC Press, 2002, both of which are hereby incorporated by reference herein.

The initial heating of the ceramic formulation in a non-oxidizing ambient (step 115 of the process 100) and the subsequent exposure of the ceramic formulation to an oxidizing ambient (step 120) can, in one or more embodiments of the invention, be performed in a controlled atmosphere furnace with heating provided by radiant heating, microwave energy, or induction. Such furnaces are capable of controlling the temperature of a sample inside the furnace as well as the type and flow rate of the ambient gas. In the case of microwave heating, susceptors placed near the sample may be heated wirelessly by microwave energy. Suitable furnaces are readily available from several commercial vendors. For example, Sentro Tech (Strongville, OH, USA) makes radiant-heating-based controlled atmosphere furnaces capable of precisely heating a sample to 1100°C and 1600°C in both an inert (e.g., argon or nitrogen) and an oxidizing atmosphere (e.g., air or oxygen). At the same time, microwave furnaces are available from, for example, Col-Int Tech (Columbia, SC, USA). Finally, induction-based furnaces are available from, as just one example, Inductotherm Corp. (Rancocas, NJ, USA).

Thermodynamic calculations can estimate the temperature rise in a ceramic formulation containing zirconia and zirconium carbide as the result of the oxidation of zirconium carbide when starting at an initially elevated temperature (“the initial temperature”; T ₀ ). For these calculations, the initial temperature is assumed to be 1027°C (1300 K), but this value is largely arbitrary and should not be construed as limiting the scope of the invention.

Two reactions may occur when oxidizing zirconium carbide (ZrC) to form zirconia (ZrO ₂ ): Reaction 1: ZrC + O ₂ → ZrO ₂ + C; and

Reaction 2: ZrC + 2O ₂ → ZrO ₂ + CO ₂ . The enthalpies of formation (' H _f ) of the molecules in these reactions are listed in Table I (where the data is taken from I. Barin, Thermochemical Data of Pure Substances, 3 ^rd Edition, Wiley, 1995 (hereby incorporated by reference herein)). Based on these values, the heat released during Reaction 1 is about -894 kilojoules per mole of zirconium carbide (kJ/mol) or -7.26 kilojoules per gram of zirconium carbide (kJ/g). For Reaction 2, the heat released is about -1,289 kJ/mol or -12.49 kJ/g.

Accordingly, both reactions are highly exothermic. Moreover, both reactions show a net consumption of gas molecules, suggesting a localized reduction in gas pressure, which may have the effect of drawing more oxidizing gas (e.g., air or oxygen) into the reaction. This dynamic is advantageous when compared to a reaction that produces a net surplus of gas molecules. A net surplus may act to expel gas and/or particulates from the material.

The heat energy available to a ceramic formulation from the Reactions, in turn, allows the temperature rise of the ceramic formulation to be estimated utilizing the heat equation: ' T = Q/(m*C _p ), where ' T is the temperature change, Q is the heat energy, m is the mass of the material being heated, and C _p is the specific heat of the material being heated. Tables 1 and 2 tabulate the estimated maximum temperatures for Reactions 1 and 2, respectively, for ceramic formulations with differing starting concentrations of zirconium carbide. In these calculations, it is assumed that the heat energy is heating only zirconia, and that the mass of that zirconia includes the additional mass created by converting all of the zirconium carbide into zirconia. In these calculations, the specific heat for the zirconia was also obtained from Barin (referenced above).

From Tables II and III, it is apparent that the oxidation reactions may act to significantly increase the temperature of the ceramic formulation. Nevertheless, the above-described thermodynamic calculations are somewhat idealized, and the actual temperature rise and the maximum temperature achieved in a ceramic formulation may be lower than that calculated due to radiative, convective, and conductive heat losses. Therefore the faster the reaction occurs, the higher the local heat rise will be. High surface area zirconium carbide (e.g., nanopowder zirconium carbide) may achieve higher temperatures as a result of kinetically faster reactions. Moreover, while the maximum temperatures may only be achieved locally, such temperatures, even attained only briefly, may almost instantaneously fuse the zirconia particles.

Notably, moreover, Reactions 1 and 2 results in a net volume increase because of zirconium carbide being converted to zirconia, with an estimated volume expansion of about 34%. Advantageously, this volume increase may somewhat offset the simultaneous decrease in the volume of zirconia as a result of densification, estimated to be about 50%. As a result, aspects of the invention may also act to control density and relieve residual stress associated with sintering ceramic articles.

To demonstrate the ability to produce a significant temperature rise in an actual ceramic formulation utilizing aspects of the invention, two pellets were initially heated to 1000°C in an argon ambient and then exposed to atmospheric-pressure air. Both pellets were 25.4 millimeters in diameter, weighed five grams, and were dry pressed using 11,000 pounds per square inch with a hydraulic press. The first pellet consisted of Tosoh TZ-3Y powder, an yttria-stabilized zirconia with nano-sized zirconia crystallites, containing 3 mol% yttria. The second pellet, in contrast, consisted of 75 weight% Tosoh TZ-3Y blended with 25 weight% zirconium-carbide nanopowder obtained from PlasmaChem. The pellets were initially heated using microwave heating with silicon carbide susceptors placed next to the pellets. Temperatures of the pellets were measured by optical pyrometry.

FIG. 5 shows a graph of pellet temperature versus time with the introduction of air occuring when each of the pellets reached 1000°C. The gas was not pre-heated before introduction, and in both cases, the introduction of the gas resulted in an initial small temperature dip (approximately 5°C). After this small dip, however, the first zirconia pellet (without zirconium carbide) continued to heat at a similar heating rate for the following two minutes until the microwave power was shut off. The second pellet (with zirconium carbide), in contrast, experienced a sudden rapid temperature rise to over 1050 °C, followed by a second prolonged temperature rise to nearly 1200 °C. This two stage exothermic heating, at constant microwave power, is likely to be due to an initial surface reaction of the zirconium carbide followed by the subsequent reaction of zirconium carbide deeper within the second pellet.

After processing, the first pellet was only lightly bonded, with material easily scratched off of the surface. In contrast, the second pellet, having experienced a more developed sintering process through the oxidation-induced temperature rise, was more fully sintered, with notably higher hardness and strength.

Accordingly, the process 100, and, more generally, processes in accordance with aspects of the invention, provide several advantages. When forming a ceramic coating on a metal substrate, for example, the ceramic coating formulation can be sintered without having to raise the temperature of the underlying substrate much above the initial temperature. Accordingly, much higher sintering temperatures can be utilized, giving greater control of coating densities. Depending on the process conditions, localized temperature increases in the ceramic coating can range from hundreds to thousands of degrees, enough to effectively densify almost all ceramic materials. The profile and duration of these localized temperature increases can ultimately be controlled by factors such as: the mass of the ceramic formulation, the mass fraction of the oxidizing component, the distribution of the oxidizing component, as well as the flow rate and composition of the oxidizing gas ambient. Where the oxidizing component is very well distributed in the ceramic formulation, as would be the case in, for example, a ceramic formulation comprising zirconium-carbide-coated zirconia formed by sol-gel processing, temperature uniformity during oxidation-induced sintering should be exceedingly uniform and ideally placed at the grain boundaries of the zirconia, where fusing is desired.

As discussed above, moreover, in those embodiments wherein YSZ articles are formed with ceramic formulations containing zirconium carbide, the volume expansion of the zirconium carbide converting to zirconia (e.g., about 34%) can help to offset the volume reduction of the ceramic formulation due to densification (e.g., about 50%). Here again, strain may be advantageously reduced.

In closing, it should again be emphasized that the above-described embodiments of the invention are intended to be illustrative only. Other embodiments can use different types and arrangements of articles, as well as differing processing steps, for implementing the described functionality. These numerous alternative embodiments within the scope of the appended claims will be apparent to one skilled in the art.

Moreover, all the features disclosed herein may be replaced by alternative features serving the same, equivalent, or similar purposes, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

Any element in a claim that does not explicitly state“means for” performing a specified function or“step for” performing a specified function is not to be interpreted as a“means for” or “step for” clause as specified in 35 U.S.C. §112, ¶6. In particular, the use of“step of” in the claims herein is not intended to invoke the provisions of 35 U.S.C. §112, ¶6.