Tetsu Takahashi, Koji Matsumaru and Kozo Ishizaki
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1833-122X) Volume 6 November 2010
Internal friction or sound wave damping capacity is a mechanical property of
a material. Generally, the damping capacity of a material is inversely proportional
to its Young’s modulus. It is difficult to produce a material with high
Young’s modulus and simultaneously high damping capacity, or high internal
friction coefficient. To clarify a mechanism to obtain a material with high
Young’s modulus and high internal friction coefficient, sintering mechanism
of porous alumina was studied to obtain different kinds of microstructure of
porous materials. Porous alumina ceramics were sintered by conventional method
and capsule-free hot isostatic press (HIP) sintering. A production method of
porous ceramics with high Young's moduli with high damping capacity is reported.
Porous Alumina, Internal Friction, Young’s Modulus, Neck Size, Specific
A mechanical vibration or sound wave that is excited by external kinetic energy is attenuated. This means the sound wave energy in a solid material is consumed within the material itself. This sound wave energy consuming phenomenon is called “Internal Friction”, whose capacity is damping capacity.
The damping capacity or the magnitude of internal friction is defined as how much sound wave energy is consumed or how fast sound wave attenuated. To indicate the magnitude of internal friction phenomenon, internal friction coefficient, Ǫ-1 is used. A material that possesses high internal friction coefficient consumes sound wave energy rapidly.
Generally, internal friction depends on Young‘s modulus. A material that possesses lower Young’s modulus pertains to a higher internal friction substance. On the other hand, the internal friction coefficient of porous materials is higher than dense materials of same substance [1-5]. This fact indicates that the macrostructure affects damping capacity.
Previously, we reported internal friction phenomenon of capsule-free Hot Isostatic Pressed (HIPed) porous alumina [6, 7], and revealed the internal friction phenomenon of porous alumina depends on porosity and specific surface area. In this paper, a method of producing high Young’s modulus with high internal friction coefficient using porous ceramics is reported by clarifying the relationship between principal properties (Young’s modulus and damping capacity) and sintering mechanisms.
High purity (>99.99%) α-alumina (mean particle size 0.5 μm, Sumitomo Chemical Co., Ltd., Advanced Alumina AA-04, Lot No. YD-1202) was used as a precursor. In this research, conventional sintering, capsule-free HIPing and Pulsed Electric Current Sintering (PECS) were employed to fabricate samples. Compaction process and conventional sintering and capsule-free HIPing schedules were the same as previous experiments . For PECS, 13 g alumina powder was loaded into a graphite die. The inner diameter of graphite die was 60 mm. The loaded powder was pressed by a graphite punch. The graphite die and punch, which contained the alumina was heated in PECS under uniaxial pressure of 10 MPa in vacuum. Sintering schedule is shown in Fig. 1. To avoid temperature overshoot, three heating rates, 77, 50 and 25 K/min, were used. Sintering temperature was 1100, 1200 or 1400 °C, and holding period at sintering temperature was 5 min. Temperature was measured by a pyrometer by focusing on the surface of the graphite die. The density of green body was calculated from dimension and their mass.
Porosity, p was measured by water dislacement method. The samples were soaked in boiling water for 3 h to fill up water in all open pores. After 3 h boiling, soaked samples and water were cooled to room temperature. The specific surface area, ss of green bodies and sintered samples was measured by means of gas adsorption (BET) method (Micromeritics FlowSorb 2300, Shimazu Co., Ltd.). The samples were machined into a rectangular parallelepiped with dimension of 0.7×6×40 mm by diamond grinding wheel (grain size, 30 and 40 μm and 10 and 20 μm) to measure the internal friction coefficient, Ǫ-1 and Young’s modulus, E, which were measured by resonant method (JE-RT, Nihon Techno-Plus Corp.). The value of Ǫ-1 was calculated by full width at half maximum. The values of Ǫ-1 and E were measured for 10 times and averaged.
Figure 1 PECS schedule. Sintering temperature was 1100, 1200 or 1300 °C. Uniaxial pressure of 10 MPa was applied during heating.
Figs. 2 (a) and (b) show porosity, p and the specific surface area, ss vs. sintering temperatures, T, respectively. Circles, squares and triangles represent conventionally sintered samples, capsule-free HIPed ones and PECSed ones, respectively. Open and double marks represent 1 h sintered samples and 50 h sintered ones, respectively, for conventionally sintered, capsule-free HIPed samples. The value of p and ss of green body are represented by dashed lines in Figs. 2 (a) and (b). The values of p and ss decrease with increasing T. In the case of 50 h sintered ones, conventionally sintered samples and capsule-free HIPed ones showed similar p at the same T, on the other hand the capsule-free HIPed ones showed lower ss than the conventionally sintered ones. The values of ss for samples sintered at temperatures over 1400 °C were below the measurement range (below 0.2 m2/g) due to low p. PECSed samples show lower value of p and ss than other samples at the same T.
Figure 2. Structural parameters of porous Al2O3. (a) Porosity, p against sintering temperature, T. Circles, squares and triangles represent conventionally sintered samples, capsule-free HIPed ones and PECSed ones, respectively. Open and double marks represent 1 h sintered samples and 50 h sintered ones, respectively. The dashed line represents porosity of green body (CIPed : 0.42, PECS : 0.54). (b) Specific surface area, ss after sintering at various temperatures, T. Dashed line represents initial ss (3.8 m2/g). The samples sintered at 1300, 1400 and 1500°C had lower ss than the limit of measurements, whose values are estimated as indicated by arrows.
Figure 3. Measured properties of sintered porous Al2O3. All marks have the same aforementioned meanings. (a) Young's modulus, E after sintering at various temperatures, T. (b) Internal friction coefficient, Ǫ-1 after sintering at various temperatures, T.
Figs. 3 (a) and (b) show Young's modulus, E, vs. internal friction coefficient, Ǫ-1 vs. sintering temperatures, T, respectively. All marks have the same aforementioned meanings. The values of E increased with increasing T, and 50 h sintered samples showed higher values of E than 1 h sintered ones. In the case of 50 h sintered samples, capsule-free HIPed ones showed slightly higher E than the conventionally sintered ones at the same sintering temperature. PECSed samples showed the highest value of E at the same T. The values of Ǫ-1 decreased with increasing T, and 50 h sintered samples showed the loweest Ǫ-1. In the case of 1 h sintered samples, capsule-free HIPed samples that were sintered in the range of 1100 to 1300 °C had lower Ǫ-1 than the conventionally sintered ones. PECSed samples showed lower value of Ǫ-1 than samples that were sintered conventionally or capsule-free HIPed for 1 h. However, PECSed samples showed higher value of Ǫ-1 than 50 h sintered samples at the same T.
Generally, Young's modulus and internal friction are closely related parameters. Materials with higher Young's modulus have a lower internal friction. Fig. 4 represents the relationship between E and Ǫ-1. All marks have the same aforementioned meanings. In the case of 1 h sintered samples, which Young's moduli were in the range of 50 to 150 GPa, capsule-free HIPed samples showed lower value of Ǫ-1 than conventionally sintered ones at the similar value of E. In the case of the both end ranges, i. e., E<50 and E>150, the samples have the similar values of E and Ǫ-1. The comparison of 1 h and 50 h sintered samples indicates that all 50 h sintered samples show the lower value of Ǫ-1 than 1 h sintered ones at the similar value of E. However, 50 h conventionally sintered samples and capsule-free HIPed ones did not show significant difference at similar value of E. The PECSed sample which Young’s modulus of 124 GPa showed higher value of Ǫ-1 than other samples at the similar value of E.
Figure 4. Relationship between Ǫ-1 and E. All marks have the same aforementioned meanings.
Normally, various kinds of diffusion occur during sintering process, and they are surface diffusion, evaporation-condensation, and/or bulk diffusion (including grain boundary diffusion, lattice diffusion etc). The geometrical characteristics of sintered porous ceramics strongly depend on the mass transport path. The relationship between geometry and mass transport of sintering has been discussed extensively [9-13]. The diffusion mechanisms can be classified broadly into two categories, with/without shrinkage. Grain boundary diffusion and lattice diffusion involve shrinkage, surface diffusion and evaporation-condensation do not involve shrinkage. The speed of diffusion depends on diffusion coefficients, a dominant controlling factor of these coefficients is temperature. Surface diffusion coefficient is the higher than lattice and grain boundary diffusion coefficient, and this diffusion become more effective at low sintering temperature. The activation energy of surface diffusion is lower than bulk diffusion, therefore, surface diffusion can be well progressed even at low temperature. Fig. 5 represents the relationship between ss and p. Samples sintered at the lower temperature (800 and 900°C for 50 h, p=0.411 and 0.413, respectively) showed small reduction of porosity from green state (800°C; -1.67 %, 900°C; -2.14 %), and showed large reduction of ss (800°C; -3.68 %, 900°C; -8.68 %). This means surface diffusion is more effective at low temperature, as a consequence of sintering progressing without significant shrinkage. In Fig. 4, these samples were show the lowest E region (800°C; 21.8 GPa, 900°C; 30.8 GPa), Ǫ-1 was decreased drastically with increasing E.
Figure 5. Relationship between ss and p. All marks have the same aforementioned meanings. The samples had the lower porosity (less than 0.1) showed lower ss than the limit of measurements, whose values are estimated as indicated by arrows.
Nanko and Ishizaki investigated about the sintering mechanism of capsule-free HIPing and revealed that the high gas pressure enhances surface self diffusivity . The enhanced surface self-diffusion provides porous material that has larger surface area reduction with larger grown necks which connect two particles. The enhanced surface self-diffusion should change the relation between porosity and specific surface area. In Fig. 5, 50 h capsule-free HIPed samples showed lower value of ss than conventionally sintered ones at the similar value of p. This indicates inter-particle distance shrinks at the similar magnitude for samples sintered by capsule-free HIPing or conventional sintering, but surface area decreases more for samples sintered by capsule-free HIPing than those sintered conventionally. The cause of this phenomenon is enhanced surface self-diffusion.
All the 50 h sintered samples showed lower value of Ǫ-1 than 1 h sintered ones at similar E, but no significant difference between capsule-free HIPing and conventional sintering was observed. The driving force of diffusion is reducing free energy. Under long sintering period at the same temperature, atoms sufficiently diffuse and the free energy of the system decreases. Under a condition with the free energy of a system close to a minimum point, diffusion progresses slow. Surface self-diffusion is enhanced under capsule-free HIP [14-16], this means the free energy reaches its equilibrium value faster than conventional sintering. The reason why 50 h sintered samples did not show significant difference between capsule-free HIPing and conventional sintering is that the sintering progresses sufficiently.
Young's modulus of porous material has been estimated by porosity, and some empirical equation had been proposed. In recent years, Young's modulus dependence on neck size is reported as follows: Green and coworkers reported a dramatic increase in the Young’s modulus of alumina that were sintered at low temperature (800 to 1000°C) with no porosity change and no densification [17,18]. Rice et al. discussed on domination of Young’s modulus of porous materials using neck size between particles, and they concluded the higher Young’s modulus of porous materials produced by the lager neck size [19,20]. Takata et al. reported on Young’s modulus on capsule-free HIPed porous copper, and indicated the Young’s modulus was increased by capsule-free HIPing . Kinemuchi et al. revealed that the capsule-free HIPed porous alumina showed higher fracture strength (averaged value: 120 GPa, porosity: 0.392) than conventionally sintered ones (averaged value: 95 GPa, porosity: 0.399), and they indicated this increment of fracture strength was caused by well grown neck produced by high gas pressure . According to the literature, higher Young's modulus is produced by large neck size, and the large neck size produces enhanced surface diffusion. However, experimental results indicate larger necks reduce the value of Ǫ-1. Previously, we reported both porosity and specific surface area affect the value of Ǫ-1 . For this reason, the value of Ǫ-1 can be changed even in the same value of E, as shown in Fig. 4.
In the case of PECS, heating rate is higher than conventional sintering and capsule-free HIPing (PECS : 77 K/h [max.], conventional and HIP : 6.7 K/h). Additionally, uniaxial pressure (10 MPa) is applied during sintering. Higher heating rate reduce the effects of surface diffusion, because the period of lower temperature region is short. The uniaxial pressing assists densification because of particle re-arrangement. Therefore, the PECSed samples have lower p and higher ss. Comparison between PECS and normal conductive sintering (conventional and HIPing) showed high heating rate can promote higher value of E and Ǫ-1.
The relationship between Young’s modulus and internal friction phenomenon of porous alumina is reported. To elaborate porous ceramics with high Young's moduli with high damping capacity, both larger neck size and lager specific surface area are required. Ideally, lower surface diffusion effect and higher volume diffusion effect can be produced larger neck with larger specific surface area, and as a consequence, the porous material which possesses higher Young's moduli with higher internal friction can be produced. Also pressure assist and high heating rate are effective to produce porous material with high Young’s modulus and high internal friction coefficient. Because, sintering progresses quickly.
The authors wish to express their gratitude to the Japanese government for partially supporting this work through the 21st Century Centers of Excellence (COE) Program and Promotion of Independent Research Environment for Young researchers of the Ministry of Education, Culture, Sports, Science and Technology. The authors also wish to express their gratitude to Sumitomo Chemical Co. Ltd (Japan) for supplying the powders.
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Tetsu Takahashi, Koji Matsumaru and Kozo Ishizaki
Nagaoka University of Technology
1603-1 Kamitomioka Nagaoka Niigata JAPAN
E-mail: [email protected]
This paper was also published in print form in "Advances in Technology
of Materials and Materials Processing", 10 (2008) 71-76.