Description:4. DESCRIPTION

Technical Field of the Invention

The present invention generally relates to the field of advanced ceramic materials. More particularly, the invention focusing on methods to enhance the fracture toughness of ceramic components by uniformly dispersing alpha-alumina platelets coated with lanthanum phosphate throughout the ceramic matrix.

Background of the Invention

Alumina ceramics, widely recognized for their hardness, corrosion resistance, and high-temperature stability, are essential in high-performance applications such as aircraft exhaust components, gas turbine parts, and various high-stress industrial machinery. However, despite their advantages, alumina ceramics have a significant drawback: their susceptibility to sudden, catastrophic failure due to their inherently low fracture toughness. Typically, the fracture toughness of monolithic alumina is around 3 MPavm, which is substantially lower than required for many demanding applications. This low toughness results in the formation propagation of microcracks during use, which can rapidly propagate and lead to material failure without warning.

Numerous efforts have been made to enhance the fracture toughness of alumina ceramics. One of the traditional approaches includes reinforcing alumina matrices with alumina fibers. These alumina fiber-reinforced composites have shown increased toughness, achieving values up to 7 MPavm, which is a considerable improvement over the base material. However, this method introduces new challenges, such as the difficulty in molding the composites into complex shapes, increased porosity, and reduced strength (around 200 MPa) due to in the presence of fibers.

Another approach has involved the use of carbon nanotube (CNT) reinforcement, which has demonstrated fracture toughness in the range of 4.7 to 9.7 MPavm. Despite these promising results, the high cost of CNTs, the complexities of the manufacturing process which requires an inert gas environment, and the lack of scalability to large parts limit their practical application. Moreover, CNT-reinforced composites are prone to oxidation at high temperatures, where many of their intended applications are situated.

Given the limitations of current solutions, there is a critical need for a novel material that not only enhances the fracture toughness of alumina ceramics but also addresses the deficiencies of the existing methods. The ideal solution would be a composite material that can be formed into complex shapes, withstands high temperatures without degradation, and maintains or improves the inherent advantages of alumina ceramics such as corrosion and oxidation resistance.

The composite should provide a practical and economically viable solution that overcomes the barriers associated with fiber and nanotube reinforcements. It should allow for the large-scale production of components with complex geometries, suitable for advanced engineering applications requiring high material integrity under stress. Moreover, the enhancement in fracture toughness should not come at the expense of other critical material properties such as strength, wear resistance, and density.

This invention introduces a novel class of alumina composites reinforced with coated, single-crystalline a-alumina platelets, nanometric in their thickness, aiming to substantially increase the fracture toughness of the ceramic while maintaining its high-temperature capabilities and mechanical integrity. The a-alumina platelets are coated with lanthanum phosphate (LaPO4), providing an optimal interface with the alumina matrix, which facilitates effective energy dissipation and crack arrest mechanisms.

This new composite utilizes a-alumina platelets that are uniformly dispersed within the alumina matrix, which are key to its enhanced properties. The use of these platelets helps in arresting crack propagation, effectively increasing the fracture toughness of the composite. The coating process ensures adequate bonding between the platelets and the matrix, further enhancing the mechanical properties of the composite.

The application of LaPO4-coated a-alumina platelets of thickness in nanometric range, as reinforcement brings several advantages over existing alternatives. First, the innovative method of dispersing these platelets throughout the alumina matrix leads to a more homogeneous material that is better at resisting crack initiation and propagation. Second, the composite can be fabricated using standard ceramic processing techniques, which allows for the production of complex shapes that were previously challenging or impossible to achieve with fiber-reinforced or CNT-reinforced ceramics. Lastly, the material retains all the favorable characteristics of alumina ceramics, such as high hardness, lower density compared to metals, and resistance to high temperatures, while significantly improving upon the critical aspect of fracture toughness.

Brief Summary of the Invention

This invention introduces an advancement in the field of ceramic materials by developing a composite made of an alumina matrix reinforced with single-crystalline a-alumina platelets, of thickness in the nanometric range, that are coated with lanthanum phosphate (LaPO4). The primary goal of this innovation is to address and overcome the significant limitation of traditional alumina ceramics—specifically, their low fracture toughness, which restricts their application in environments where mechanical integrity is crucial. By significantly enhancing the fracture toughness, this composite material opens up new possibilities for the broader application of alumina ceramics.

The invention aims to develop a composite material with superior fracture toughness compared to monolithic alumina. This enhancement is achieved through the integration of a-alumina platelets of nanometric thickness, which are uniformly dispersed throughout the alumina matrix. A critical aspect of this innovation is the coating of these platelets with LaPO4, which improves the interface bonding between the platelets and the matrix, facilitating effective energy dissipation and crack arrest under stress. This uniform distribution is essential for maintaining consistent mechanical properties across the composite.

Furthermore, this composite material has been designed to be easily molded into complex shapes, which is a significant improvement over traditional fiber-reinforced or nanoparticle-reinforced ceramics. The ability to form complex components without losing material integrity allows this new composite to be used in a variety of high-demand applications. For example, in aerospace, components such as turbine blades, exhaust nozzles, and heat shields can benefit greatly from the material’s enhanced high-temperature capabilities and wear resistance. Additionally, its use in automotive and electronic industries is promising, particularly in high-stress parts where traditional ceramics could fail.

The development of this material not only improves the mechanical properties of alumina ceramics but also maintains their inherent advantages such as high hardness, corrosion resistance, and oxidation resistance. This makes the composite ideal for a wide range of technical applications that require materials to withstand harsh conditions while maintaining high performance and reliability. The novel features of this invention, including the method of incorporating nanometric a-alumina platelets and the coating process on the platelets, represent significant advancements in ceramic technology, offering practical and innovative solutions to longstanding material challenges.

Further objects, features, and advantages of the invention will be readily apparent from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings.

Brief Summary of the Drawings

The invention will be further understood from the following detailed description of a preferred embodiment taken in conjunction with an appended drawing, in which:

Fig. 1 (a-c) illustrates a visual representation of key elements involved in the fabrication process of the ceramic components, in accordance with an exemplary embodiment of the present invention;

Figure 2 illustrates the compacts of alumina composites with the inclusion of 4 wt% alumina platelets, in accordance with an exemplary embodiment of the present invention;

Figure 3 presents the fracture toughness (KIc) of alumina - a-Al2O3-platelet composites with varying weight percentages of nanometric a- Al2O3 platelets, in accordance with an exemplary embodiment of the present invention;

Figure 4 presents FESEM (Field Emission Scanning Electron Microscopy) images showcasing the microstructure of an Alumina composite consisting of alumina with 4 wt% alumina platelets, in accordance with an exemplary embodiment of the present invention.

Detailed Description of the Invention

It is to be understood that the present disclosure is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The present disclosure is capable of other embodiments and of being practiced or of being carried out in various ways. In addition, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.

According to an exemplary embodiment of the present invention, a method for preparing ceramic components comprising a-alumina platelets, of thickness in the range of a few hundred nanometers, is disclosed. This composite material consists of an alumina matrix reinforced with a-alumina platelets coated with lanthanum phosphate. These platelets are uniformly dispersed within the alumina matrix, enhancing its fracture toughness.

In accordance with an exemplary embodiment of the present invention, a method is provided for preparing ceramic components comprising a-alumina platelets. This method includes several sequential steps aimed at synthesizing a-alumina platelets, coating them with lanthanum phosphate, and preparing a suspension for casting ceramic components.

In accordance with an exemplary embodiment of the present invention, the synthesis of a-alumina platelets begins with the dissolution of aluminum nitrate in distilled water. Following this, urea is added to the solution, inducing a chemical reaction. The solution is then heated to boiling, resulting in foaming and proliferation. Subsequently, the yield is collected, milled for a predetermined duration, and calcined at 800°C in an oxidizing atmosphere to obtain a-alumina platelets with thickness in nanometric range with a hexagonal crystal structure.

In accordance with an exemplary embodiment of the present invention, once the a-alumina platelets are synthesized, they undergo a coating process with lanthanum phosphate to enhance their interface with alumina. This process involves soaking the platelets in a solution containing lanthanum phosphate hydrate dispersed in water. A dispersant is added to the solution, and the platelets are maintained at a predetermined temperature for a specified duration. Subsequently, the soaked and dried platelets are heated to 900°C to ensure the adherence of the coating.

In accordance with an exemplary embodiment of the present invention, the soaked platelets are then subjected to a heating process to ensure the adherence of the coating. This heating step is performed at a temperature of 900°C for a predetermined duration. Through this process, a robust interface is established between the a-alumina platelets and the lanthanum phosphate coating, enhancing the mechanical properties and durability of the resulting ceramic components.

In accordance with an exemplary embodiment of the present invention, the preparation of suspensions for ceramic components begins with calculating the amount of alumina powder or powder mix with second phases required to achieve a free-flowing suspension with a solid loading of 50%. This calculation is crucial for ensuring the proper formulation of the suspension.

In accordance with an exemplary embodiment of the present invention, an organic premix solution is prepared by mixing monomer methacrylamide (MAM), cross-linker N, N'-methylenebisacrylamide (MBAM), and deionized water in a specified ratio. Additionally, DARVAN 821-A dispersant for alumina is added to the solution in an amount ranging from 1 to 2 wt%, depending on the solid loading of the suspension.

In accordance with an exemplary embodiment of the present invention, the suspension is then tumbled with alumina balls as mixing media for 24 hours at 15 rpm to achieve homogeneity. This tumbling process ensures the uniform distribution of the components within the suspension, leading to consistent properties in the resulting ceramic components.

In accordance with an exemplary embodiment of the present invention, the de-aired suspension is cast into suitable molds for the fabrication of ceramic components. This casting process is crucial for shaping the suspension into desired forms, enabling the production of complex-shaped components with enhanced mechanical properties.

In accordance with an exemplary embodiment of the present invention, the resulting ceramic components exhibit significantly enhanced fracture toughness compared to those prepared using conventional methods. This improvement in mechanical properties is attributed to the uniform distribution of a-alumina platelets with nanometric thickness,
within the ceramic matrix, which effectively impedes crack propagation and increases the overall durability of the components.

In accordance with an exemplary embodiment of the present invention, the fracture toughness of the resulting ceramic components is measured using the pb-notch method (ASTM C1421-10), providing quantitative data on the effectiveness of the a-alumina platelets in enhancing mechanical properties. This measurement serves as validation of the inventive concept and highlights the potential applications of the technology in various industrial sectors.

Referring to Fig. 1 illustrates the flow diagram (100) illustrating the method for preparing ceramic components comprising a-alumina platelets.

Start (102): The process begins.

Synthesizing a-Alumina Platelets (104): This step involves the chemical synthesis of a-alumina platelets. the synthesis of a-alumina platelets involves a systematic procedure. Initially, aluminum nitrate is dissolved in distilled water, creating a solution. Subsequently, urea is introduced into this solution, in which it dissolves. The solution is then heated to boiling, leading to foaming and proliferation, resulting in the formation of a-alumina platelets. These platelets are collected from the foam, and the next step involves milling them for a predetermined duration to refine their particle size. Following milling, the collected platelets undergo calcination at a temperature of 800°C in an oxidizing atmosphere. This calcination process is cleans up the surface of the platelets. The milled material consists of a-alumina platelets with a hexagonal crystal structure. They are of thickness in nanometric range. This synthesis method ensures the production of a-alumina platelets with the desired properties, setting the stage for their utilization in the fabrication of composites.

Coating with Lanthanum Phosphate (106): After obtaining the a-alumina platelets, the next step involves coating them with lanthanum phosphate. This coating process is executed through the following steps. The a-alumina platelets are soaked in a solution prepared by dispersing lanthanum phosphate hydrate in water. A dispersant is added to facilitate the uniform dispersion of lanthanum phosphate. The platelets are then maintained in this solution at a predetermined temperature for a specified duration to allow for thorough coating. Following the soaking process, the platelets are heated to a temperature of 900°C for a predetermined duration. This high-temperature treatment ensures the adherence of the lanthanum phosphate coating onto the surface of the platelets, enhancing their properties and functionality. These steps ensure the effective coating of a-alumina platelets with lanthanum phosphate, thereby improving their performance and suitability for use in ceramic applications.

Preparing Suspension for Ceramic Components (108): Once the platelets are coated, a suspension is prepared for casting ceramic components. The calculation of the precise amount of alumina powder or powder mix with second phases is necessary to achieve a free-flowing suspension with a solid loading of 50% by volume. Subsequently, the calculated amount of alumina powder or powder mix, along with the second phases, is added to an organic premix solution. This solution comprises monomer methacrylamide (MAM), cross-linker N, N'-methylenebisacrylamide (MBAM), and deionized water, blended in a specific ratio of 33:6:1 by weight to ensure proper dispersion and homogeneity within the suspension, DARVAN 821-A dispersant for alumina is introduced into the mixture. The amount of dispersant added varies depending on the solid loading of the suspension. The suspension undergoes tumbling with alumina balls as mixing media for a duration of 24 hours. This extended mixing period facilitates thorough blending and uniform distribution of the components, leading to homogeneity.

End (110): The process concludes here.

Fig. 2 (a-c) illustrates a visual representation of key elements involved in the fabrication process of the ceramic components described in the invention.

Fig. 2a illustrates the raw material used in the synthesis process. The alumina powder, with an approximate size of 600 nm, serves as the base material for creating the ceramic composite.

Fig. 2b shows a-Alumina platelets are the primary reinforcement material incorporated into the ceramic composite. Their distinctive shape and composition contribute to enhancing the mechanical properties, particularly the fracture toughness, of the final ceramic components.

Fig. 2c a platelet with a coating of lanthanum phosphate (LaPO4). The coating serves to improve the interface between the platelets and the alumina matrix, enhancing the bonding and overall performance of the ceramic composite. The LaPO4 coating is a key aspect of the invention, aimed at further enhancing the mechanical properties of the ceramic components.

Fig. 3 illustrates the compacts of alumina composites with the inclusion of 4 wt% alumina platelets. The bars shown in the figure are cut to a size of 25x2.0x2.5 mm specifically for measuring mechanical properties. These bars serve as specimens for conducting mechanical tests to evaluate the performance of the ceramic composite material. The inclusion of nanometric alumina platelets in the composite is a key aspect of the invention, aimed at improving fracture toughness and other mechanical properties compared to monolithic alumina.

Fig. 4 illustrates a graph where the x-axis represents the weight percentages of a-alumina platelets incorporated into the composite material, while the y-axis represents the fracture toughness (KIc) of the resulting alumina - a-alumina platelet composites. The curve depicted in the graph represents a fit to an exponential function, showcasing the relationship between the weight percentage of a-Al2O3 platelets and the resulting fracture toughness of the composite material.

Figure 5 presents FESEM (Field Emission Scanning Electron Microscopy) images showcasing the microstructure of an Alumina composite consisting of alumina with 4 wt% alumina platelets. These platelets are introduced into the composite as reinforcement materials. The images reveal a uniform distribution of the platelets throughout the alumina matrix. Additionally, the platelets are coated with LaPO4, as indicated, which serves to enhance their interface with the alumina matrix. This uniform distribution and the presence of the LaPO4 coating are critical factors contributing to the improved mechanical properties of the composite material, particularly its fracture toughness.

The experimental results from the study involving the alumina composites reinforced with coated single-crystalline a-alumina platelets demonstrate significant enhancements in mechanical properties, particularly fracture toughness. The core findings from the experiments are summarized as follows:

1. Fracture Toughness Improvement: The introduction of a-alumina platelets into the alumina matrix has shown a marked increase in fracture toughness. The composites containing 4 wt% of these platelets achieved a fracture toughness of 5.4 MPavm, which is a substantial improvement over the approximately 3 MPavm observed in monolithic alumina. This enhancement represents a nearly 80% increase in toughness, significantly extending the potential applications of alumina in environments where mechanical durability and resistance to crack propagation are critical.

2. Optimal Platelet Concentration: The experimental results indicated that adding 4 wt % of a-alumina platelets to the alumina matrix provides the best balance between toughness and other mechanical properties. This concentration allows for effective crack deflection and energy dissipation, leading to increased toughness without adversely affecting the material's structural integrity and manufacturability.

3. Microstructural Analysis: Field Emission Scanning Electron Microscopy (FESEM) images of the composites revealed a uniform distribution of the LaPO4-coated a-alumina platelets within the alumina matrix. This uniform dispersion is crucial for achieving consistent mechanical properties throughout the composite material. The images also confirmed the successful coating of platelets with LaPO4, which enhances their bonding with the matrix and contributes to the overall mechanical performance of the composites.

4. Manufacturing Process and Consistency: The ceramic composites were fabricated using a combination of rapid prototyping and gelcasting method, which allowed for precise control over the shape and dimensions of the resulting components. The process involved creating a uniform slurry of the alumina and coated platelets, which was then cast into molds. This method proved effective for producing complex-shaped parts with consistent properties, demonstrating the scalability and practicality of the manufacturing process for industrial applications.

5. Comparative Analysis: The study also compared the performance of these composites with other alumina-based materials reported in the literature. The alumina composites reinforced with LaPO4-coated a-alumina platelets showed simultaneously the possibility of fabrication into complex shapes while possessing superior fracture toughness compared to those reinforced with other types of fibers or particulates, underlining the effectiveness of the platelet reinforcement strategy.

6. Temperature Stability: The experimental setups included high-temperature treatments to evaluate the stability of the composites under operational conditions likely to be encountered in applications like aerospace and automotive components. The composites maintained their integrity and mechanical properties at elevated temperatures, confirming their suitability for high-temperature applications.

Overall, the experimental results validate the effectiveness of incorporating LaPO4-coated a-alumina platelets into alumina matrices to significantly enhance fracture toughness. This advancement opens up new possibilities for using alumina ceramics in applications that require materials to withstand high mechanical stresses without failure, thereby extending the material's applicability in more demanding engineering fields.
, Claims:5. CLAIMS
I/We Claim:
1. A method (100) for preparing ceramic components comprising a-alumina platelets, comprising:
a. Synthesizing a-alumina platelets (104) by:
dissolving aluminum nitrate in distilled water;
adding urea to the solution obtained in step (a);
heating the solution to boiling, foaming, and proliferation;
collecting the resulting yield;
milling the collected yield for a predetermined duration;
calcining the milled yield at a temperature of 800°C in an oxidizing atmosphere to obtain a-alumina platelets of nanometric thickness with a hexagonal crystal structure;
b. Coating the a-alumina platelets obtained in step 1 with lanthanum phosphate (106) by:
soaking the platelets in a solution prepared by dispersing lanthanum phosphate hydrate in water, adding a dispersant, and maintaining at a predetermined temperature for a predetermined duration;
heating the soaked platelets to a temperature of 900°C for a predetermined duration to ensure adherence of the coating;
c. Preparing a suspension for ceramic components (108) comprising:
calculating the amount of alumina powder or powder mix with second phases required to generate a free-flowing suspension with a solid loading of 50 vol%;
adding the calculated amount of alumina powder or powder mix with second phases to an organic premix solution comprising monomer methacrylamide (MAM), cross-linker N, N'-methylenebisacrylamide (MBAM), and deionized water in a ratio of 33:6:1 by weight;
adding DARVAN 821-A dispersant for alumina in an amount ranging from 1 to 2 wt% dependent on the solid loading of the suspension;
tumbling the suspension with alumina balls as mixing media for 24 hours to achieve homogeneity; and
de-airing the suspension in a vacuum desiccator to eliminate any trapped air before casting.

2. The method (100) as claimed in claim 1, wherein the ceramic components are prepared using rapid prototyping and net shaping techniques.

3. The method (100) as claimed in claim 1, wherein the resulting ceramic components exhibit enhanced mechanical properties compared to components prepared using conventional methods.

4. The method (100) as claimed in claim 1, wherein the fracture toughness of the resulting ceramic components is measured using the pb-notch method (ASTM C1421-10).

5. The method (100) as claimed in claim 1, wherein the a-alumina platelets are synthesized with a hexagonal crystal structure as confirmed by X-ray diffraction (XRD).

6. The method (100) as claimed in claim 1, wherein the a-alumina platelets are present in the ceramic components in an amount ranging from 1 to 5 wt% based on the total weight of the components.

7. The method (100) as claimed in claim 1, wherein the ceramic components exhibit an enhancement in fracture toughness compared to components prepared using monolithic alumina.

8. The method (100) as claimed in claim 1, wherein the fracture toughness of the resulting ceramic components is measured using the pb-notch method (ASTM C1421-10).

9. The method (100) as claimed in claim 1, wherein the a-alumina platelets are coated with lanthanum phosphate to enhance their interface with alumina.

10. The method (100) as claimed in claim 1, wherein the organic premix solution further comprises a dispersant for alumina.

11. The method (100) as claimed in claim 1, wherein the suspension for ceramic components has a solid loading in the range 40 to 60%.

12. The method (100) as claimed in claim 1, wherein the ceramic components are prepared using rapid prototyping and net shaping techniques