Fracture origin Powder Processing Issues for High Quality Advanced Ceramics †

In this paper, the ef fect of powder processing conditions on the fracture strength of advanced ceramics is discussed. Manufacturing processes of silicon nitride ceramics and alumina ceramics are utilized to explain the relationship between powder processing conditions and major fracture origin in the ceramics. The preparation conditions of powder slurry are found affecting the structure and strength of powder granules, which can influence the properties of green compact and thus the quality of sintered ceramics. These phenomena can be investigated by using new characterization tools such as the liquid immersion method and the observation technique using thinned ceramics specimen under the transmission mode. Also, very small amount of coarse particles contained in the powder slurry is found starting the fracture in ceramics by using wet sieve analysis and the observation technique with thinned ceramic specimen. As a result, it is found that the major ceramics fracture is originated from the large pores and coarse particles in powder granules, green compact and sintered ceramics. They can be unintentionally introduced in the manufacturing steps of ceramics, such as powder slurry preparation, spray drying, and forming process of green compact. By making use of these characterization tools, ef fective processing conditions to eliminate the large pores and coarse particles can be identified for producing high quality advanced ceramics.


Introduction
Powder processing technology has been widely used in many industries.However, it needs a lot of know-how to make high quality and low cost products by making use of powder materials.For example, manufacturing process can have a significant influence on the properties of advanced ceramics.For a given production method, a minor change in the powder processing condition can cause a great difference in the ceramic quality.Fig. 1 shows a typical powder granule compaction process used in the ceramic manufacturing 1) to make sintered ceramics.It indi-cates that non-uniform structure, such as large pores or coarse particles in the green compact, can become fracture origin 2,3) , which affects the reliability of sintered ceramics.On the other hand, powder packing structure, such as particle orientation or apparent density distribution in the green compact, also influences the deformation of sintered body; thus leads to its fracture strength and near-net shaping.Therefore, the manufacturing process of green compact before firing is crucial for producing high quality advanced ceramics.In this paper, the manufacturing process of silicon nitride ceramics and alumina ceramics is used to elucidate the relationship between the powder processing conditions and the fracture origin in the ceramics.For examples, the effects of powder slurry preparation conditions and small amount of coarse particles in the powder slurr y on the properties of sintered ceramics are discussed.Detailed characterization of powder raw materials, powder slurry, powder granules, green body, and sintered body is found necessary to understand and control ceramic processing conditions and is essential to open the black box of ceramic manufacturing processes.

Ef fect of Powder Processing Conditions on
Ceramic Properties

Powder slurr y preparation conditions
Slurr y preparation conditions can directly affect the powder granule properties made by spray drying and lead to non-uniform structure of green compact.Fig. 2 shows the fabrication process of silicon nitride ceramics by powder granule compaction 4) .Commercially available silicon nitride, alumina, and yttrium oxide powders were used as the starting materials.Average particle size of each powder measured by X-ray sedimentation was 0.44μm, 0.33μm, and 0.29 μ m, respectively.Silicon nitride powder (270g) was mixed with alumina (15g) and yttrium oxide (15g) by ball-milling with distilled and deionized water (155g) for 24 hours.Dispersant was not added because the pH of the slurr y moved to the basic region (up to 10.5) during mixing, at which silicon nitride could be deflocculated electrostatically due to the reaction of silicon nitride and water.After passing through a sieve (32μ m) to remove undesired large agglomerates or inclusions, the slurr y was divided into two groups; one was spray-dried in the as-dispersed state and the other in the flocculated state.For the flocculated one, the pH of the slurry was adjusted to 9.4 by slowly adding a HNO3-H2O solution with continuous stirring at room temperature.The slurr y was then dried to form powder granules using a spray dryer.Fig. 3 shows a micrograph of the granules prepared from a well-dispersed slurr y (pH=10.5, apparent viscosity: 300mPa・s) and that of flocculated slurr y (pH=9.4,apparent viscosity 6500mPa・s), respectively 4).They were observed by liquid immersion method 5) .Most of the granules prepared from welldispersed slurry had distorted spherical shapes and contain dimples, which were also clearly visible in the micrograph taken in the optical transmission mode, as shown in Fig. 3 (a).On the contrast, the granules prepared from flocculated slurry were essentially spherical and no dimples as shown in Fig. 3 (b).However, when observing the SEM micrographs of theses granules, some dimples shown in Fig. 3(a) were apparently absent, because they were placed on the specimen holder with the dimple down.It indicated that conventional characterization tools were insufficient to observe the granule structure as shown in Fig. 3.
The slurry dispersion state strongly influences the  granule structure 6) .Droplets injected into a spray dryer are dried with evaporation of water from the sur face.In case of dispersed slurr y, water flows from the interior to the surface of a droplet and the particles are carried along with the flow to the area near the surface, leaving behind an internal void, as drying proceeds.It is believed that a partial vacuum is formed in the internal void, leading to the collapse of the granules near the end of the drying cycle.As can be seen in Fig. 3 (a), the mechanism results in granule deformation and the formation of a dimple structure on almost all powder granules made from dispersed slurry.
On the other hand, droplets made from flocculated slurry are considered to be composed of agglomerates in which primary powder particles are touching each other.Because water can migrate and evaporate easily from the source without distributing the loosely packed structure in and between agglomerates, solid granules are easily formed.As a result, the granules tend to have a spherical shape without dimples as shown in Fig. 3 (b).Fig. 4. shows the Weibull distribution cur ves of the fracture strength of sintered bodies 4).Average strength of sintered body fabricated from flocculated slurry (717MPa) was obviously higher than that from well-dispersed slurr y (607MPa), although the calculated Weibull moduli were high for both of them.The fractographical analysis for the fractured specimen clearly indicated that large pore defects were responsible for fracture origins in the silicon nitride ceramics 4) .Such larger pore defects were developed in the ceramics originated from the structure and strength of powder granules.In this case, large pores developed starting at the centers of granules originated from dimples through powder packing during compaction and those at the boundaries caused by incomplete adhesion between granules affected the fracture strength of ceramics.In the experiment, large pore size was apparently larger in the thinned specimen of the sintered ceramics made by welldispersed slurr y.However, such difference cannot be observed by microstructure analysis alone even at higher magnification 7,8) .
Although the structure of powder granules is similar, the fracture strength of the sintered bodies may still be affected by the slurry preparation conditions 9) .Table 1 indicates the preparing conditions of the   three slurries with some difference in apparent viscosity 9) .Alumina powder (AL160-SG4, Showadenko, Japan, average par ticle size: 0.46μm) was mixed with 0.2, 0.5, 2.0mass% of polymer dispersant (Ammonium polyacrylate, CELUNA-D305, Chukyoyushi, Japan) and distilled and deionized water for 24h by ball milling.The solid concentration of the slurry was 35 vol%.The pH of slurry with 0.2 mass% dispersant was increased to 10 by adding dilute NH4OH solution, which also decreased its viscosity to 43mPa・s.The slurries were spray-dried for granulation.The granules were uni-axially pressed at 9.8 MPa, and then isostatically pressed at 176 MPa.The green compact were sintered at 1550 °C for 2hr in air.As indicated in Table 1, three kinds of slurries showed low apparent viscosity; and, the granules made by the slurries had distorted spherical shapes and contained dimples, which were also clearly visible in the micrograph taken under the optical transmission mode, as shown in Fig. 3 (a).Also, no big difference was observed in size distribution of granules made from the three slurry preparing conditions, and their average sizes were all about 60μm.Table 1 presents the density and fracture tough-ness of the specimens.The values are almost the same for these ceramics.Fig. 5 shows the strength distribution of the alumina ceramics 9) .However, a significant variation of strength associated with the slurry preparing condition was noted.The average strength is 486, 430 and 363MPa respectively for specimens made from the three slurr y preparing conditions.Fig. 6 shows a comparison of the compressive strength of the granules prepared from the three different slurries 9) .The strength of granules was measured with a micro compression test machine (MCTM-500, Shimadzu Co., Japan) and the compressive strength was calculated by applying the model proposed for the elastic deformation of spherical particle 10,11) .Fig. 7 shows the transmission optical micrograph of the thinned ceramics specimens 9) .The dark features are pores in the structures.Clearly, the present ceramics have defects at the center and boundaries of granules.They are developed from the irregularities of packing structure of powder particles in green compact.The pore structure was similar for the three specimens, except for the size of the pores associated with the slurry preparing condition.The relation between the pore number density versus the pore size of the sintered specimens made from the three slurries was measured 9) .The pore number density was defined as the number of pore per unit volume of specimen per unit size interval.The effective volume of specimens under the analysis was about 0.5 mm 3 .In this study, relatively large pores were subjected to the analysis.Pores were assumed to have a spherical shape and their sizes were represented by the equivalent diameter.
According to the fracture mechanics, the strength of ceramics σ, can be related to the size of fracture origin, c, by the following equation: Where KIC is the fracture toughness and Y is the   shape factor.As expected from the equation, fracture strength of ceramics should change depending on the size of fracture origin, provided that the fracture toughness and the shape factor are the same.Focusing on large pores, the size as shown in Fig. 7 (b) is 1.29 times larger than that in the specimen shown in Fig. 7 (a).Provided the shape factor is the same, the average strength of the sintered sample made from the slurr y of pH 10 is estimated 1.34 times higher than that made from the slurry of pH 8.1.This estimate is in good agreement with the measured strength as shown in Fig. 5.
The change of the large pore size can be ascribed to the difference in the granule strength as shown in Fig. 6.In compaction of harder granules, less deformation can occur at a certain compaction pressure and this leaves larger pores in compacts.The size of large pore increases with the granule strength, which was strongly affected by the amounts of dispersant in slurry as shown in Fig. 6.The granule strength is influenced by various factors, such as the powder packing structure as well as the amount and distribution of dispersant in granules 12) .In this study, the difference of the granule strength can be ascribed to the latter factor dominantly, since the granule structure was almost similar.It is considered that the amounts of dispersant added, 0.2, 0.5 and 2.0 mass %, are insufficient, a little excess and excess for covering the powder surfaces based on the relation between the amounts of dispersant and the slurry viscosity, respectively.With excessive dispersant, the non-adsorbed dispersant becomes free polymers in the slurry.In spray drying, the free polymers form solid bridges between powder  particles, increasing the granule strength considerably.Clearly, polymeric additives have critical effects on the cohesive force between particles and thus on the powder compaction process.

Coarse particles in powder slurr y
The control of particle size distribution of powder is also important in slurr y preparation.Powder is processed using mechanical method such as ball milling.Very few numbers of coarse particles can affect the fracture strength of sintered ceramics; therefore, particle size control is very important to achieve high quality advanced ceramics.
A fundamental study was conducted to understand the effect of coarse particles on the fracture strength of ceramics.The specimens were prepared through the procedure as shown in Fig. 8 13) .Low soda alumina powder (AL-160SG-4, Showa Denko K.K., Ja-pan) was used as raw material.The nominal average particle size was 0.5μ m.The powder was placed in alumina pot mill (SSA-999, Nikkato, Japan, volume with 2kg of alumina balls (SSA-999, Nikkato; diameter 5mm) and 400g of aqueous solution (2 mass%) of dispersant of polyacrylic acid type (CE-RUNA D-305, Chukyo Yushi, Japan), and mixed for 24h to make a slurry with the solid content 50vol%.The slurry was passed through a mesh (2mm opening) to separate the balls.Weighed slurry was placed in a container and stirred continuously with a stirrer while small amount of coarse particles (0.01-0.1 mass%) was added.The coarse particles for addition were prepared from the unground raw material used in the production of the present fine alumina powder.The coarse particles were classified into three fractions by sieving before being used.Each fraction of coarse particles was added to individual slurry; therefore, three kinds of slurries with coarse particles were prepared.Each slurry was kept stirring for 2h after the coarse particles were added.Finally, each slurry was cast in gypsum molds (100 x 100 x 9mm) to prepare green compacts with coarse particles.After drying, the compact was heated at 1550°C for 2h in an electric furnace to sinter the model ceramics 13) .In the micrograph, at high magnification, plateletshape particles form aggregates of porous structure with the size of 10-20μm 13) .At lower magnification, these aggregates form the coarse aggregates in large scale.In this experiment, such coarse aggregates are referred to as coarse particles.Table 2 shows the measured densities of green compact and ceramics 13) .The densities were approximately the same for all compacts.The densities of all ceramics were again the same.Clearly, addition of a small amount of coarse particles has no effect on the densities of ceramics.Fig. 9 shows the Weibull plots and the fracture toughness for all specimens 13) .The specimen strength decreased with increasing size of coarse particles added.The Weibull moduli were similar and over 20 for all specimens.All ceramics basically have    the same fracture toughness.Fig. 10 shows SEM micrographs of representative fracture origins found in this study 13) .The specimen contains coarse particles of the size range 75-90 μm.The fracture origin was noted in the specimen of the lowest strength (370MPa) as seen in Fig. 10(a) and Fig. 10(b) shows that with the average strength of 406MPa.They were both coarse particles.Lower strength was observed for the specimen containing larger coarse particles.Similar results were obtained for all specimens examined in this study.Fig. 11 shows the IR photomicrographs 14) for the internal structures of ceramics containing the coarse particles of various sizes 13) .The size of coarse particles in the ceramic matrix increased with increas-ing size of coarse particles added.Again, the sizes of coarse particles are the same as those added in the preparation of specimens 13) .Actually, the estimated values of strength obtained by linear fracture mechanics, assuming that the fracture was always initiated at the coarse particles in the matrix, agreed ver y well with the measured strength 13) .It means that small amount of coarse particles governs the strength of ceramics.Therefore, carefully preventing the coarse particles is crucial to improve the strength of high quality advanced ceramics.

Importance of Powder Characterization Tools
As already explained, small amount of large pores    and coarse particles has large effect on the material properties.Therefore, coarse particles in raw powder materials or in powder slurry must be carefully controlled.This is critical to obtain not only higher quality of advanced ceramics but also other kind of materials such as toner and polishing materials.However, conventional characterization tools are insuf ficient to detect such small amount of nonuniform components.For example, Fig. 12 shows the particle size distribution of ground silicon nitride powder by wet ball milling 15) .It was measured by Xray sedimentation method 16,17) .The obtained particle size distribution indicates the ground powder has no particles coarser than 45μm.And, Fig. 13 shows the relationships between average particle size of ground powder and ball-milling time 15) .It shows that average particle size decreases with the milling time, and media ball size has no effect in this case.However, the situation was quite different when using wet sieve to examine the 45 μm oversize mass fraction of the ground powder with ball-milling time 15) , as seen in Fig. 14.Different from Fig. 12, it shows that coarse particles larger than 45μm are apparently contained in the ground powder.Media ball size has effect on the change of its mass fraction in the ground powder with milling time.The 10mm sized media ball is the most effective to grind coarse particles.It means the wet sieve analysis would be a reliable method to measure even just a few particles in the ground powder.From Fig. 14, we can identify the processing conditions to effectively grind very small amount of coarse particles.As a result, the method to measure coarse particles in advanced ceramic powder was developed 18, 19)   , and it has been already filed as an international standard in advanced ceramics (ISO/TC206).By relating the coarse particle information of raw powder or powder slurry to that of sintered ceramics, we can easily understand how small amount of coarse particles lower the reliability of sintered ceramics thorough its manufacturing process.Characterization of large pores in powder granules, green compact and sintered ceramics is also very important for producing high quality advanced ceramics 20) .Conventional characterization tools such as SEM or mercury porosimetry are not sufficient to evaluate the small amount of large pores in granules, green compact and sintered ceramics.Liquid immersion method 5) is a very powerful tool to observe the large pores in granules and green compact.And, thinned specimen obser vation is also ver y effective for de-      tecting large pores in sintered ceramics.Effectively using these tools, we can investigate what those processing conditions in the ceramic manufacturing create large pores in sintered ceramics causing product reliability problems.So far, on the microstructure and sintered strength of ceramics, the following issues were clarified using these characterization tools: seasonal variation of microstructure and sintered strength of dry-pressed alumina [21][22][23] , effects of spray drying conditions [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] , dewaxing procedures [25][26][27] and granule compaction procedures [28][29][30] on the fracture origin of sintered ceramics. O course, these tools are also suitable for other manufacturing processes, such as injection molding process 31) , to improve the properties of their ceramic parts.

Conclusions
In this paper, the effect of powder processing conditions on the fracture strength of advanced ceramics was clarified.Manufacturing processes of silicon nitride ceramics and alumina ceramics were utilized to explain the relationship between ceramics processing conditions and its major fracture origin.Powder slurry preparing conditions were found creating the large pores in powder granules, green compact and sintered ceramics.These phenomena can be investigated by using new characterization tools such as the liquid immersion method and the obser vation technique using thinned ceramics specimen under the transmission mode.Also, very small amount of coarse particles contained in the powder slurr y is found weakening the strength of ceramics by using wet sieve analysis and the observation technique with thinned ceramic specimen.As a result, it is believed that major fracture origin is caused by the large pores and coarse particles, which are generated in the manufacturing steps of ceramics, such as powder slurr y preparation, spray dr ying, and forming process of green compact.By making use of these characterization tools, effective processing conditions to eliminate the large pores and coarse particles can be identified for producing high quality advanced ceramics.

Fig. 3
Fig. 3 Internal structure of granules observed by liquid immersion method (a) prepared from well-dispersed slurry; (b) prepared from flocculated slurry.

Fig. 4
Fig. 4 Weibull distributions of the fracture strength measured for sintered specimens.

Fig. 4
Fig. 4 Weibull distributions of the fracture strength measured for sintered specimens.

Fig. 5
Fig. 5 Strength distribution of the alumina ceramics.

Fig. 5 Fig. 6 Fig. 7 Fig. 7
Fig. 5 Strength distribution of the alumina ceramics.Fig. 6 Compressive strength distribution of alumina granules associated with different slurry preparing conditions.Fig. 6 Compressive strength distribution of alumina granules associated with different slu preparing conditions.

Fig. 12
Fig. 12 Particle size distribution of the powder ground with 5mm media balls.

Fig. 13
Fig. 13 Relationships between average particle size of ground powder and milling time.

Fig. 12
Fig. 12 Particle size distribution of the powder ground with 5mm media balls.

Fig. 13
Fig.13 Relationships between average particle size of ground powder and milling time.

Fig. 14
Fig. 14 Change of 45Pm oversize mass fraction of ground powder with milling time.

Fig. 14
Fig. 14 Change of 45μm oversize mass fraction of ground powder with milling time.

Table 1
Slurry preparing conditions and properties of the sintered ceramics

Table 1
Slurry preparing conditions and properties of the sintered ceramics

Table 2
Densities of green bodies and sintered bodies

Table 2
Densities of green bodies and sintered bodies