Freeze casting process for the generation of graded porosity in Al₂O₃ ceramics

Processo de freeze casting para a geração de porosidade graduada em cerâmicas de Al ₂ O ₃

Abstract

Alumina ceramic materials with graded functional porosity were prepared by the freeze casting method and characterized. The effects of processing route parameters on the pore formation were studied. Pore characteristics were investigated concerning morphology and distribution in the ceramic matrix. The mercury intrusion method was used to evaluate the porosity. Mechanical properties, as well as the fracture mode, were investigated by the uniaxial compression test. Scanning electron microscopy was used to analyze and correlate the created interface between the layers with the mechanical response. The results suggested that the porosity obtained in all samples was similar, even with varying parameters (different suspension solidification configurations). When evaluating the mechanical behavior, these distinct parameters showed drastic differences in compressive strength and failure mode. This was due to the formation of interfaces between the layers of different porosities, according to the solidification configuration. The interfaces modified the fracture mode, changing from a longitudinal-directed to an interface-directed fracture. The pore microstructure and morphology indicated highly connected pore channels.

Keywords: freeze casting; porosity; graded porosity

Resumo

Foram efetuadas a preparação e a caracterização de materiais cerâmicos de alumina com gradientes funcionais de porosidade utilizando o método de freeze casting. Foram estudados os efeitos dos parâmetros da rota de processamento na formação dos poros. As características dos poros foram analisadas quanto à morfologia e distribuição na matriz cerâmica. O método de porosimetria de mercúrio foi utilizado para avaliar a porosidade. As propriedades mecânicas, bem como o modo de fratura, foram investigadas em teste de compressão uniaxial. A microscopia eletrônica de varredura foi empregada para analisar a morfologia dos poros e correlacionar a interface formada entre as camadas com a resposta mecânica do material. Os resultados sugeriram que a porosidade obtida em todas as amostras foi semelhante, mesmo variando os parâmetros de processo (diferentes configurações de solidificação de suspensão). Ao avaliar o comportamento mecânico, esses diferentes parâmetros alteraram drasticamente a resistência à compressão e o modo de falha, devido à formação de interfaces entre as camadas de diferentes porosidades, de acordo com a configuração de solidificação. As interfaces modificaram a forma de fratura passando de uma fratura em direção longitudinal para uma na direção das interfaces. A microestrutura dos poros indicou que foram obtidos canais de poros altamente conectados.

Palavras-chave: freeze casting; porosidade; porosidade graduada

INTRODUCTION

Ceramic materials are usually used in situations where resistance to wear and hot environments are needed. However, these materials are not widely used for dynamic mechanical applications due to unpredictable failure during loading ¹. One of the reasons is the presence of pores in ceramic materials, impairing the strength. Even in non-structural applications, minimal mechanical strength is required, thus reducing the feasibility of the porous ceramic application, making porosity a critical factor ^{2), (3}. With the development of new technologies and with a growing need for porous materials with ceramic characteristics, a balance between porosity degree and properties has been studied ². A ceramic structure with a controlled degree of porosity can lead to the best combination of properties, especially when high resistance to wear, high temperatures, and corrosive environments are required ². For these applications, ceramic materials need to be processed with unconventional methods that generate designed and controlled microstructures, where pore distribution, size, and morphology can influence the desired properties. These applications range from filters, used in the metallurgy industry, and gas filters, used in industrial activities and combustion engines, to biomaterials, where it can be used as a bone growth substrate in hydroxyapatite ceramic prosthesis ^4)-(6. The processes for obtaining controlled porosity in ceramics basically depend on the desired pore morphology, distribution, and size. One of the simplest processes to obtain porous ceramics is by the partial sintering of ceramic powder compacts. This method can generate porosity values up to 60 vol% with pores homogeneously distributed through the matrix. To produce materials with higher than 60 vol% of pores, other processing routes are required ². Processes were developed where the porosity was induced and controlled, as the replica technique ^{7), (8}, sacrificial template ⁹, superficial depositions ¹⁰, direct foaming ², freeze casting ^{3), (6), (11)-(25}, and others. New materials were developed using those methods, with applications in filters ²⁶, support for catalysts ³, sensors ³, electrodes ³, biomechanical prostheses ²⁶, artificial bones ²⁶, and thermal insulators ²⁶.

The freeze casting process consists of freezing aqueous or nonaqueous ceramic slurries. During this process, the particles dispersed in the slurry are expelled from the newly formed solid, stacking between the ice crystals. The slurry is usually exposed to a surface at or below the freezing temperature of the solvent ¹. After complete solidification, the solvent is sublimed, producing green bodies. The pore structure produced by this method is similar to a network of unidirectional channels, where the formed pores are a replica of the frozen solvent ^3)-(14. The solvent is critical to the microstructure of ceramic materials produced by freeze casting and, by its direct influence, several solvents have already been studied such as water ³, camphene ²³, tert-butanol ^{16), (18), (27}, naphthalene camphor ¹⁷, and others. Certain additives may also interfere with the solvent, promoting morphology changes during solidification, like glycerol, ethylene glycol, propylene glycol, methanol, ethanol, dimethyl sulfoxide, and acrylamide ^{15), (16}. Several different morphologies may also be obtained depending on the solidification system employed. The solidification can occur in a single surface ³, in two surfaces (bottom and top ²⁵ or bottom and sides ²⁸), in a centrosymmetric orientation ¹⁹, or even by using nanometric magnetic particles in suspension, associated with an external magnetic field ²⁴. The freeze casting process is extremely attractive due to the easy control of porosity and high compressive strength. With this method, it is possible to reach compressive strength up to four times higher than other methods, due to its directional pore channels ^{10), (29}.

Controlling porosity also enables obtaining ceramics with nonhomogeneous structures and different properties in different regions of the same material. These types of materials are called functionally graded materials (FGM). According to their application, functionally graded materials can be labeled as graded, layer-graded, and layered materials ^{26), (30}. Functionally graded materials with different degrees of porosity have been applied mainly in the areas of thermal coating ³¹ and biomaterials ⁷. In ceramic biomaterials, bone tissue properties can be imitated with the use of graded porosity and materials such as hydroxyapatite, alumina, and zirconia ^{7), (32}. Hydroxyapatite with graded porosity using sequential freeze casting has been already studied in the literature. Sequential casting made it possible to achieve a bone-like structure ³³. This study focused on the evaluation of the relationships of the type of processing used with the mechanical strength of the material. Factors such as fracture mode and interface relationships were also discussed according to the type of processing. Sequential freeze casting was used to achieve different high porosity ratios in the same sample with significant strength. This study evaluated the freeze casting process to generate graded porosity in Al₂O₃ ceramics. Morphology, size, and distribution of pores, as well as the compressive strength and fracture mode of the produced samples, were evaluated.

EXPERIMENTAL PROCEDURE

For the study of the freeze casting process, alumina CT 3000 SG, Almatis, was used, which had the following specifications according to the supplier: 99.8% purity, average particle size of 0.5 μm, and 7.8 m²/g of specific surface area. To produce samples with different amounts of porosity, ceramics slurries were prepared with different amounts of alumina. These slurries were prepared with distilled water, using ammonium hydroxide to obtain a basic pH solution. To stabilize the ceramics suspension, Darvan 821a was used as a dispersant and Duramax TM B1022 was used as a binder to improve the green strength. The compositions of prepared ceramic suspensions are shown in Table I.

Table I
Compositions of Al₂O₃ suspensions prepared for freeze casting processes (dispersant and binder concentrations calculated according with Al₂O₃ mass).
Tabela I
Composições de suspensões de Al₂O₃ preparadas para os processos de freeze casting (concentrações de dispersante e ligante calculadas de acordo com a massa de Al₂O₃).

Suspension	Al2O3 (wt%)	Dispersant (wt%)	Binder (wt%)	Al2O3 (vol%)
A25	25	2	5	21.25
A30	30	2	5	25.50
A35	35	2	5	29.75
A40	40	2	5	34.00

The experiments were designed to cover different possible configurations of pore production by the freeze casting process. There were 4 different processes: freeze casting (FC), graded freeze casting (GFC), freeze casting with core (FCC), and graded freeze casting with core (GFCC). These cores acted mainly as a possible reinforcement to the mechanical properties of the material. To obtain the cores used as reinforcements, suspension A40 was used as a powder precursor. These cores were produced by uniaxial pressing in a cylindrical mold, resulting in cores of 6 mm in diameter and 17 mm in height. To perform the freeze casting process, a freezing system was developed for the unidirectional growth of the ice crystals, as shown in Fig. 1. In this system, heat withdrawal from the suspension occurred at the base of the support by cooling it with a liquid nitrogen bath. The die used was cylindrical, with 21.11 mm in height and 16.3 mm in diameter. To obtain the same initial conditions for all samples, the ceramic slurries and freezing system were kept at room temperature until the starting of the process. For FC and FCC samples, suspension A40 was used, while for GFC and GFCC samples, suspensions A40, A35, A30, and A25 were used. For FCC and GFCC samples, solid cores were added. These configurations are shown in Fig. 2. For FC and FCC samples, suspension A40 was added to the mold, being completely solidified in the developed freeze casting system. For GFC and GFCC samples, suspension A40 was added first, and when the solidification was almost complete, suspension A35 was added; in a similar way, suspension A35 was followed by the suspension A30, and then suspension A30 was followed by the suspension A25. Each suspension took up approximately 25% of the volume of the mold used. For FCC and GFCC samples, which were cored, a hydrophobic protective layer of petroleum jelly (Vaseline) was applied on the cores so that they did not deteriorate during the process. After full solidification, the samples were lyophilized (Terroni, LD1500) and sintered. For the sintering process, the samples were initially heated from room temperature to 180 °C, at a heating rate of 10 °C/min, holding for 20 min. Then the samples were heated to 600 °C, with a heating rate of 1 °C/min, holding for 20 min to remove organic compounds. Afterward, heating from 600 to 1500 °C was performed at a heating rate of 10 °C/min, holding at 1500 °C for 2 h, so that densification occurred.

Figure 1:
Freeze casting system developed to prepare porous Al₂O₃ samples. Directional solidification occurred by withdrawing the heat from the ceramic suspension through the liquid nitrogen cooled metallic surface. After complete solidification of the suspension, the mold was removed from the surface and the sample was extracted.
Figura 1:
Sistema de freeze casting elaborado para a confecção das amostras porosas de Al₂O₃. A solidificação direcional ocorreu pela retirada de calor da suspensão cerâmica através da superfície metálica resfriada por banho de nitrogênio líquido. Depois da completa solidificação da suspensão, o molde foi retirado da superfície e a amostra extraída.

Figure 2:
Structural configurations for freeze casting samples according to the precursor suspensions: a) GFC (graded freeze casting): suspensions A40, A35, A30, and A25; b) FC (freeze casting): suspension A40; c) GFCC (graded freeze casting with core): suspensions A40, A35, A30, and A25 and dense core; d) FCC (freeze casting with core): suspension A40 and dense core.
Figura 2:
Configuração estrutural das amostras por freeze casting de acordo com as suspensões precursoras: a) GFC: suspensões A40, A35, A30 e A25; b) FC: suspensão A40; c) GFCC: suspensões A40, A35, A30 e A25 e posicionamento de núcleo denso; d) FCC: suspensão A40 e posicionamento de núcleo denso.

The sintered samples were characterized by measuring the porosity by mercury porosimetry (Micromeritics, Autopore IV), the mechanical strength by compression test (Shimadzu, AGI 10 kN) with 1 mm/min loading rate, and the pore morphology using a scanning electron microscopy (Shimadzu, SSX550). The estimated porosity was calculated considering the volume fraction of solids present in the suspension, the water expansion upon freezing, and the shrinkage of the solid phase during sintering. Water expansion during freezing was calculated using the volume variation of the water, considering the density at 25 °C (0.997048g/cm³ ³⁴) in relation to the ice density at 0 °C (0.9167 g/cm3 ³⁴); thus, 1 cm³ of water at 25 °C generated 1.0873 cm³ of volume of ice at 0 °C. The linear shrinkage of the solid phase during sintering was assumed to be 17%, which resulted in a volumetric shrinkage of 42.8%. The porosity of Al₂O₃ was assumed as 5%. The mathematical relationships used to calculate the estimated porosity are shown in Table II.

Table II
Mathematical relations used to calculate the estimated porosities for freeze casting processes.
Tabela II
Relações matemáticas utilizadas para calcular a porosidade estimada para o processo de freeze casting.

Sample	Mathematical relation
FC	Px = (100-Sol).1.0873. (1-0.428)
GFC	FRM
FCC	Pt = 0.7034.P40 + 0.2966.PC
GFCC	FRM

• P - estimated porosity (%); Sol - solid loading in suspension (%); x - suspension used (40, 35, 30, 25); Pt - estimated total porosity (%);
• P40, P35, P30, P25 - estimated porosity (%) of A40, A35, A30, A25, respectively; PC - Al₂O₃ core porosity (%).

RESULTS AND DISCUSSION

The results of porosity obtained by mercury intrusion porosimetry, the estimated porosity and the percentage errors related to these measurements are presented in Table III. For the sample FC, porosity was slightly lower than the estimated. This may have occurred due to a larger shrinkage during the sintering process than estimated by calculations. Moreover, the GFC sample, with layers formed by various suspensions, had higher porosity values than the FC sample, which used only suspension A40, as expected. For sample FC, the solid phase shrinkage led to a lower porosity value than the previously estimated. As is shown in the SEM images and the values of strength, an additional porosity was generated in the GFC sample in comparison with the estimated porosity. The presence of solid cores was supposed to decrease the porosity of the samples; however, the FCC sample had a higher porosity than the FC sample. This effect was also associated with the formation of an interface porosity between the core and the phase formed by suspension A40. All the samples that formed an interface, whether between layers or between the suspension and the core, had a higher porosity than the sample FC. These results suggested that porosity was formed at the interface, both between layers or between alumina phase and core, reducing adhesion at these interfaces. This result is demonstrated in SEM images and strength values later on.

Table III
Apparent porosity obtained by the mercury intrusion method, estimated porosity, and percentage error related to the measurements.
Tabela III
Porosidade aparente obtida via porosimetria de mercúrio, porosidade estimada e erro percentual relativo às medidas.

Sample	Estimated porosity (%)	Apparent porosity (%)	Relative error (%)
FC	54.7	53.3	-2.6
GFC	62.1	66.4	6.5
FCC	40.0	55.6	28.1
GFCC	45.7	58.8	22.2

The average pore sizes measured in all samples ranged from 10 to 25 mm. The mean pore size for FC, GFC, FCC, and GFCC samples were, respectively, 18.19, 23.25, 19.64, and 11.10 mm. The pore size distributions are shown in Fig. 3. In the FC sample, most pores ranged from 5 to 50 μm, and approximately 90% of the pore volume was between 10 and 40 μm. In turn, the GFC sample showed pores greater than 10 μm and smaller than 50 μm, where approximately 85% of the pores were smaller than 50 μm. Regarding the samples with a dense core, the FCC sample had most of its pores between 8 and 60 μm, where 90% of the pores were smaller than 60 μm. Finally, the GFCC sample had most pores ranging from 4 to 40 μm, with 90% of the pores between 4 and 38 μm. Thus, it was observed that the four compositions did not show a large variation in pore size distribution. Although the total porosity varied, the pore size distribution of the compositions studied had little influence from the process used.

Figure 3:
Cumulative pore size distribution curves of the samples FC, GFC, FCC, and GFCC.
Figura 3:
Curvas de distribuição cumulativa de tamanho de poros das amostras FC, GFC, FCC e GFCC.

The presence of the core was also harmful to the compressive strength, decreasing its value drastically. A comparison was established between the compressive strength obtained in this study and in the other studies ^{16), (18), (35), (36} that used the freeze casting method as the processing route. This comparison is shown in Table IV. It was observed that the compressive strength for FC, GFC, FCC, and FCC samples were closer to the lower limits of compressive strength from other studies and far from the upper limits. However, some considerations regarding methodologies should be highlighted to explain this behavior. The samples produced in ¹⁸ were made by freeze-gelcasting with tert-butanol as a solvent, with 58%±3% porosity. Li and Li ¹⁶ also used tert-butanol as a solvent, with 65%-82% porosity. Hautecoeur et al. ³⁵ used water as a solvent with different cooling rates, which changed the pore morphology, directly impacting the compressive strength. In ³⁶, camphene was used as a solvent and the porosity of the samples ranged from 59% to 82%.

Table IV
Results of compressive strength and standard deviation for alumina samples formed by freeze casting.
Tabela IV
Resultados de resistência à compressão e desvio padrão das amostras de alumina conformadas por freeze casting.

Sample	Compressive strength (MPa)
FC	24.8±6.3
GFC	12.2±4.4
FCC	8.4±1.3
GFCC	5.0±1.8
18	54±14
16	11-50
35	6-111
36	11-95

It was also observed that the presence of a graded porosity induced by different suspensions decreased the mechanical strength and the way the fracture occurred, as it can be observed in Fig. 4, which presents the fracture surfaces of the samples studied. It was observed the similarity of the fracture mode in the FC/FCC and GFC/GFCC pairs. In the FC/FCC pair, the fracture occurred in the longitudinal direction of the sample, parallel to the compressive stress. During load application, pieces of the material were released from the samples. For the GFC/GFCC pair, the fracture occurred at the interfaces between the layers, and in the case of the GFCC, the fracture also occurred at the interface with the core. In these samples, during loading, the layers of each suspension composition separated. This behavior can be observed by analyzing the stress-strain curves of the samples presented in Fig. 5. For the sample FC, no stress plateau was observed, and the failure occurred at the single maximum stress. Regarding the sample FCC, the failure at the interface between the core and the matrix generated plateaus of stress before the failure at the maximum stress, however, the total failure occurred in a similar way to sample FC. In GFC and GFCC samples, several plateaus were observed due to the failures of the interfaces between the layers, and in the case of GFCC also between the layers and the core. The same behavior in the stress-strain curve was obtained by Kim et al. ²⁷, who stated that the presence of small peaks during the test could be explained by the crack deflection. Once the crack reached the interface of a pore, it deflected in the direction of the interface, resulting in a gradual failure.

Figure 4:
Images of fractured specimens after compression test of the samples FC (a), GFC (b), GFCC (c), and FCC (d).
Figura 4:
Imagens de espécimes fraturados no ensaio de compressão das amostras FC (a), GFC (b), GFCC (c) e FCC (d).

Figure 5:
Stress versus strain curves of FC, GFC, FCC, and GFCC samples.
Figura 5:
Curvas tensão x deformação das amostras FC, GFC, FCC e GFCC.

Therefore, the modes of fracture presented by the samples were directly influenced by the microstructure induced by the processing in each type of sample. The morphology and pore structure of the FC sample are shown in Fig. 6. Ice crystals grew in the longitudinal direction, forming long plates of alumina. Roughness formation was observed on the alumina plate wall, following the transverse direction. This morphology is characteristic of freeze casting processes, which is similar to the morphology obtained with the two front freezing method ²⁰. The major difference between the single freezing front used in this study and the double freezing front used in ²⁰ was the alignment of the lamellae. The use of only one solidification front apparently generated an unstable freezing direction during the process. Thus, the lamellae generated exhibited some degree of randomness in the growth direction. The use of additives in the suspensions seems to have no significant influence on the structure, resulting in a usual freeze casting microstructure obtained with water. This characteristic was observed after comparing with the results obtained by Lebreton et al. ²², who used PVA and glycerol in their suspensions. Analyzing the pore shape, there was an intense connection between the pores, forming well-defined channels in the freezing direction, as shown in Fig. 6c. These pore channels spread throughout the material in an oriented direction, which resulted in a considerable mechanical strength as presented in ³⁵.

Figure 6:
SEM micrographs of sample FC showing: a) alumina plates; b) roughness on the wall of the ceramic plate formed by the difference in solidification rate in different directions; and c) pore channels.
Figura 6:
Micrografias de MEV da amostra FC mostrando: a) lamelas de alumina; b) rugosidade na parede da lamela cerâmica formada pela diferença na velocidade de solidificação em direções diferentes; e c) canais de poros.

The morphology and pore structure of the GFC sample are shown in Fig. 7. The microstructure had the same type of lamella growth observed in the sample FC, forming long plates of alumina with a certain roughness on the walls. Comparing the layers, it was observed that the precursor suspension with lower solid content resulted in greater spacing between layers. This phenomenon occurred because there was an increase in the proportion of water in relation to the amount of solids; in this way, the ice sheets formed during freezing were thicker. This behavior was most easily observed when comparing the layers with the highest (Fig. 7a) and lowest (Fig. 7d) solid contents. The sequential form in which the suspensions were added during the freeze casting generated a porosity gradient starting from a denser base to a more porous surface. The pores presented in this configuration were similar to those obtained in the FC sample, being unidirectional channels in the freezing direction throughout the sample. However, the presence of more than one suspension generated interfaces between the layers, which can be shown in Fig. 8. The interfaces in the GFC samples were formed during the mold filling process and the solid-state of the previous layer favored the formation of the interface. During the freezing process, as the solidification of the previous layer progressed, the interaction between the liquids of the suspensions weakened, thus forming well-defined flat interfaces. This behavior is shown in Fig. 8c, between layers A30 and A25. However, when there was better interaction between suspensions, the interface became less pronounced and noticeable, as observed between layers A40/A35 and A35/A30 (Figs. 8a and 8b, respectively). The presence of these interfaces was the main reason for the fracture mode of the GFC sample, as observed in the stress-strain curve shown in Fig. 5. This type of fracture is quite interesting because the crack deflection mechanism changes the failure from a fragile to a non-catastrophic one ³⁷. This crack deflection mechanism occurred due to the presence of weak interfaces ^{38), (39}, and it was also present in FCC and GFCC samples.

Figure 7:
SEM micrographs of each layer in sample GFC: a) A40; b) A35; c) A30; and d) A25.
Figura 7:
Micrografias de MEV de cada camada da amostra GFC: a) A40; b) A35; c) A30; e d) A25.

Figure 8:
SEM micrographs of the interfaces in the sample GFC between layers: a) A40 and A35; b) A35 and A30; and c) A30 and A25.
Figura 8:
Micrografias de MEV das interfaces da amostra GFC entre as camadas: a) A40 e A35; b) A35 e A30; e c) A30 e A25.

The FCC sample presented a microstructure similar to that observed in the sample FC; the only difference was the presence of an extra interface between the core and the matrix. The FCC sample exhibited long alumina plates with a typical roughness, with unidirectional pore channels aligned to the freezing direction. These channels were observed throughout the sample, except in the region surrounding the solid core. This morphology is shown in Fig. 9. In this sample, the phenomenon of solid core swelling occurred by the penetration of water from the suspension used for freeze casting. This phenomenon was visualized by comparing the surface morphology of the unaffected core with that affected by water, as shown in Fig. 10. In this way, the dense core was compromised, generating a scaly microstructure in the direction transverse to freezing. Despite this failure in the core, no influence was observed on the matrix structure near it. This behavior possibly occurred because the presence of the core acted as a substrate for matrix solidification before the desired directional solidification. This early solidification would eventually alter the orientation of the ice crystals locally. The presence of this defect may have been the main factor for decreased mechanical strength and increased porosity in the cored samples.

Figure 9:
SEM micrographs of FCC sample: a) matrix obtained from suspension A40; and b) interface between the A40 matrix and the dense alumina core.
Figura 9:
Micrografias de MEV da amostra FCC: a) matriz obtida da suspensão A40; e b) interface entre a matriz A40 e o núcleo denso de alumina.

Figure 10:
SEM micrographs of the surfaces of sintered cores: a) affected by swelling due to the penetration of the water used in the suspension of a FCC sample; and b) not affected after freeze casting of GFCC sample.
Figura 10:
Micrografias de MEV das superfícies dos insertos (núcleos) sinterizados: a) afetada pelo inchamento devido à penetração da água utilizada na suspensão da amostra FCC; e b) não afetada pelo processo de freeze casting da amostra GFCC.

The GFCC sample exhibited the same layered structure that was observed in the GFC sample, as shown in Fig. 11. The sample showed unidirectional pore channels aligned in the freezing direction, interrupted at each new layer added. Interface regions are marked in Fig. 11. In the microstructure of the GFCC sample, it was observed that the interfaces between layers behaved in the same way as in the GFC sample, being highly dependent on the layer previously added. During processing, the closer to fully solidified the precursor layer became, the more apparent was the interface. As for the relationship between core and matrix, the interface was very apparent due to the large density difference between the phases. However, even apparent, a cohesion between core and matrix was observed. In this type of configuration, the phenomenon of swelling was not observed. The presence of a further group of interfaces between layers and core modified the fracture mode when compared to the GFC sample. In the GFCC sample, besides layer delamination, delamination between layers and core also occurred.

Figure 11:
SEM micrographs of GFCC sample showing the interfaces between: a) layers A40, A35 and dense alumina core (N); and b) layers A35, A30 and dense alumina core (N). Interface regions are marked in the images.
Figura 11:
Micrografias de MEV da amostra GFCC mostrando interfaces entre: a) camadas A40, A35 e núcleo denso de alumina (N); e b) camadas A35, A30 e núcleo denso de alumina (N). As interfaces estão indicadas nas imagens.

CONCLUSIONS

Samples produced by the freeze casting method with different suspension solidification configurations had the same pore morphology, and their mean pore sizes were very close to each other. All samples showed the presence of long lamellae of alumina in the direction of freezing, and roughness in the transverse direction. The behavior of pore distribution was similar despite the different variations of the freeze casting method. The samples GFC (graded freeze casting) and GFCC (graded freeze casting with core) showed disruption of their structures due to the interfaces induced by the use of various suspensions during processing. These interfaces were defined mainly during the changing of suspensions in the freeze casting process and according to the degree of solidification of the previous layer. Basically, the higher the degree of solidification of the previous layer, the more apparent and detrimental the interface was. The pore gradient induced by the sequential addition of suspensions reduced the mechanical strength of the material. It also changed the direction that the crack propagated in the material, from a fracture previously parallel to the applied stress to a perpendicular one. The fracture that previously occurred by ejecting pieces of material changed to delamination between the layers. In the samples with a dense core, many presented the phenomenon of solid core swelling during the freeze casting. This phenomenon occurred due to failures in the protective layer of Vaseline applied to the surface of the cores. Because of this failure, the mechanical strength of the samples with a dense core, FCC (freeze casting with core) and GFCC (graded freeze casting with core), was much lower than that of the samples without a dense core. The swelling phenomenon, as well as the presence of the Vaseline film, may have influenced the total porosity of the samples with a dense core, increasing the porosity.

ACKNOWLEDGMENTS

The authors thank CAPES and the Ponta Grossa State University for the grants provided to develop this research.

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Publication Dates

• Publication in this collection
  13 Dec 2019
• Date of issue
  Jan-Mar 2020

History

• Received
  15 Mar 2019
• Reviewed
  03 Aug 2019
• Reviewed
  13 Sept 2019
• Accepted
  16 Sept 2019