（公財）日本板硝子材料工学助成会，41（2023）

付加製造技術（３D プリンタ）用セラミック原料粉末の高次設計～レーザ直接３D 造型を目指して～
豊橋技術科学大学

総合教育院

タンワイキアン

Design of Ceramics Powder for Additive Manufacturing Technology～ For Direct Laser 3D Printing ～直接レーザー焼結による付加製造のための静電統合造粒を使用したセラミック顆粒の製造に関する実現可能性を調査した。平均サイズが 140 nm のアルミナ粒子を出発原料として単分散な球状のアルミナ顆粒を作製した。この構造によって、流動性とハンドリング性が向上した。アルミナ顆粒を二酸化炭素レーザーで照射して、粉末の焼結および溶融を誘導した。レーザー出力や走査速度などのパラメーターを調査した結果、レーザー照射パラ
メーターを調整することにより、直接焼結または溶融を制御できることが明らかになった。
機能性添加材料をセラミック顆粒に組み込むなどのさらなる開発は、機能性複合セラミッ
クスの付加製造に向けて有益である。
The feasibility of ceramic granules fabricated using electrostatic integrated granulation for
additive manufacturing via direct laser sintering has been investigated. Monodisperse andspherical alumina (Al 2 O 3 ) granules were obtained using Al 2 O 3 particles with an average sizeof 140 nm, which exhibit an improved flowability and handling ability. The granules werethen irradiated with a CO 2 laser to induce sintering and melting of the powder. Parameterssuch as laser power and scanning speed were investigated. The results obtained indicate apossible control of direct sintering or melting by adjusting the laser irradiation parameters.
Further development such as incorporation of functional additives into ceramic granules
would be beneficial toward additive manufacturing of functional composite ceramics.
1．Introduction
1.1 Additive manufacturing
Ceramics have desirable proper ties but are dif ficult to mould using conventional
techniques 1 - 3 ). Additive manufacturing technology, including 3 D printing, offers a potential
solution by enabling the production of complex ceramic parts without the need for expensive
moulds 4 ). While polymer-based 3 D printing is already widely used, it is challenging to use
this method for ceramics due to their high melting point 4 ). Two methods are currently being
researched for ceramic 3 D printing: indirect manufacturing of a sintered body and direct
moulding using melted materials or laser irradiation of raw material powders. These
approaches offer new possibilities to produce ceramic materials with complex shapes and athigh speeds. However, the bottleneck of additive manufacturing is the development offunctional materials using this technique 5 ). A homogeneous distribution of powder is
indispensable in obtaining composite ceramics exhibiting desired properties via additive− 136 −NSG Found. Mat. Sci. Eng. Rep.

manufacturing. A continuous“layer-by-layer”additive manufacturing of multifunctional
ceramic composites is challenging due to the inability to precisely obtain a controlled
distribution of composite powder due to tendency of fine particles toward agglomeration 6 ).
One possible solution is to simultaneously apply laser sintering to a continuous feed of
granulated powder with homogenous distribution of nanosized par ticles. Therefore,
fundamental investigation and the feasibility with laser sintering of granulated ceramic
powder is carried out in this study.
1.2 Indirect laser sintering
There are various methods of indirect manufacturing of sintered bodies, such asstereolithography, indirect laser sintering, binder jet method, and thin film lamination
method. Stereolithography involves dispersing raw material ceramics in a photocurable
resin, irradiating it with an ultraviolet laser, hardening and forming a monolayer outline,
applying a photocurable resin to the surface, and performing the following steps. After that,a curing step is required. The formed green body is then sintered to produce the final
ceramic part. The advantages of this method include the ability to use a wide range of
ceramic materials, high production speeds, and low material waste. However, the resolutionof the final product is limited, and there may be some residual binder left in the sinteredbody, which can affect the mechanical properties. These indirect methods can produce
complex shapes without the need for expensive moulds, but they require carefulconsideration of the material properties and processing conditions to ensure the quality of
the final product 7 ).
1.3 Direct laser sintering
A typical direct moulding method is the direct laser sintering method. Laser irradiation on
an uniformly spread powder on a stage leads to sintering or melting forming layered sinteredar tifacts. The step is repeated forming a multilayer sintered ar tifact to obtain a threedimensional object. The main drawback is the large thermal shock resulting in cracks
formation. However, unlike indirect moulding, polymer binder is not used and therefore, nodegreasing step or post-sintering step is required. In order to improve the direct laser
sintering of ceramics with the feasibility to induce desired properties in ceramic composites,
this study investigated the fundamental of direct laser sintering of granulated alumina(Al 2 O 3 ) powder. Using ceramic granules with good flowability at a continuous feeder for
laser sintering process, this could enable a more efficient and lean additive manufacturing
process. Furthermore, it is envisaged that desired functional materials can be fabricated byincorporating additives into the composite granules during additive manufacturing.
1.4 Electrostatic integrated powder assembly
By using electrostatically integrating surface-charged modified particles, composite
particles and granules design can be achieved 6 , 8 ). The controlled assembly of particles in
nano- and submicron-sized forming composite particles enabled the fabrication of functional− 137 −

（公財）日本板硝子材料工学助成会，41（2023）

composites for various applications such as selective laser sintering 9 , 4 ), batteries 10 ), opticalproperties of aerosol-deposited composite films 11 ), improved mechanical properties of green
body 7 ) and ceramic composites 12 ) have been reported.
Recently, by using electrostatic integrated granulation of particles with similar size,
formation of composite granules with controlled microstructure have been reported. By
effectively incorporating functional additives such as carbon nanotubes and hexagonal boron
nitride into the ceramic granules with improved handling ability, a controlled property suchas electrical 13 ) and heat conduction 14 ) of the sintered ceramic artifacts have also been
demonstrated. Therefore, it is of interest to investigate the feasibility to obtain a controlledlaser sintering of ceramic granules obtained by electrostatic integrated granulation method.
The findings from this fundamental investigation can be a useful reference for future
development of functional ceramic composites by direct laser sintering process.
2．Experimental procedures
The Al 2 O 3 granules (average par ticle size 140 nm, Taimei Chemical Co., Ltd) wereprepared using electrostatic integrated granulation method. As Al 2 O 3 particles exhibit
positive surface change in aqueous suspension, they were used as received and polysodium
styrenesulfonate (PSS) (an average molecular weight of 70 000 , Sigma-Aldrich) was used aselectrolyte to induce a negative surface charge. Details for the electrostatic integrated
granulation process are reported elsewhere 13 , 15 ). The ratio of the positively and negatively
charged suspensions used was 1 : 1 and the mixed suspension was rotated in a glass vessel at
5 to 30 rpm for one week. After that, the granules obtained were dried in an oven at 80 oC for
24 h.
As for the laser irradiation, the granules were affixed onto an alumina plate and irradiatedwith a carbon dioxide laser (wavelength of 10 . 6 µm and a spot size of 0 . 2 mm). The granules
obtained were observed using a laser microscope (Olympus, OLS 4100 ) The microstructureof laser sintered artifacts obtained were observed using a field emission scanning electron
microscope (FE-SEM, Hitachi S- 4800 ).
3．Results and discussion
The Al 2 O 3 granules obtained after electrostatic
integrated granulation are shown in Fig. 1 .
Spherical monodisperse granules with an average
size of 500 µm, exhibiting good flowability andhandling ability compared to their powder state
was obtained. SEM image of a granule irradiatedwith a laser at 8 W and 1 mm/s is shown as Fig. 2 .
The image is taken in the direction of laser
irradiation from left to right. In the left region of
the granule, it can be observed that sintering did
not occur and as the laser gradually moved to the− 138 −

Fig. 1 : Optical miscrope image of the Al 2 O 3
granules obtained using electrostatic
integrated granulation process.

NSG Found. Mat. Sci. Eng. Rep.

right region, temperature increase during acontinuous laser led to sintering or melting,which can be observed at the surface of the
granule. As the diameter of the laser used issmaller than the diameter of the granule, only apartial region of the granule was irradiated andunder went a sintering or melting process.
Cracks were also observed on the surface oflaser ir radiated granules which could becaused by the thermal shock during laser
sintering process 4 ). Although it is difficult toobtain a complete and uniform sintering

Fig. 2 : SEM image an Al 2 O 3 granule after laser
irradiation at 8 W with a scan speed of
1 mm/s.

process of the entire granule, this study provides the fundamental obser vations on thefeasibility for direct laser sintering of Al 2 O 3 granules and the morphological transformation
which could be beneficial for future development and study.
Different laser irradiation parameters were used for the laser sintering process of the
Al 2 O 3 granules and SEM images showing the surface morphologies obtained are shown in
Fig. 3 . From the SEM images, molten state was observed when laser irradiated at 4 and6 . 4 W with a scan speed of 1 to 10 mm/s as shown in Fig. 3 (a)-(c) and (f)-(g), respectively. At
higher laser irradiation power of 8 W, molten state was observed for all the scan speed from 1
to 20 mm/s as shown in Fig. 3 (k)-(o).
At a low scan speed and low laser power, suf ficient heat generation for melting was
obtained. However, upon irradiation at a lower laser irradiation power of 4 W with a high scan
speed of 15 to 20 mm/s, sintering or melting was not obtained due to insufficient heat
conversion and generation as shown in Fig. 3 (d) and (e). A higher magnification of the
microstructure obtained after laser irradiation at 4 W with a scan speed of 20 mm/s is shown

Fig. 3 : Surface morphologies SEM showing the microstructure obtained at the surface of Al 2 O 3 granules
using different laser sintering parameters.

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（公財）日本板硝子材料工学助成会，41（2023）

in Fig. 4 . It can be clearly seen that particulate
Al 2 O 3 par ticles that match the size of thestarting materials used.
It is noteworthy that a sintered morphology
was obtained upon laser irradiation with laser
power of 6.4W and a scan speed of 15-20mm/
s as shown in Fig. 3 (i) and (j). Higher
magnification SEM images of the sinteredmicrostructure obtained at 6 . 4 W with scan
speed if 15 and 20 mm/s are shown as Fig. 5(a) and (b), respectively. At a scan speed of

Fig. 4 : High magnification SEM image of the
microstructure obtained after laser
irradiation at 4 W with a scan speed of
20 mm/s.

15 mm/s, a distinguished grain boundary canbe obser ved. At a higher scan speed of
20 mm/s, a rather low degree of sintering was obtained. The morphologies observed arecomparable with those of sintered ceramic powder obtained using powder metallurgy
technique.
The findings from this study indicates the feasibility to obtain a controlled direct laser
sintering or melting of Al 2 O 3 granules formed using electrostatic integrated granulation
method. However, as the size of the granules obtained is larger than the diamter of the laser
used, further investigation and control in the formation of granule size is necessary and will
be carried out. Using granules with average size that is equivalent or smaller than laser
irradiation spot size, it is envisaged that a more uniform selective laser sintering or melting
can be obtained.

Fig. 5 : High magnification SEM image of the microstructure obtained after laser irradiation at 6 . 4 W witha scan speed of (a) 15 and (b) 20 mm/s.

4．Conclusions
In this study, a fundamental investigation on the feasibility of direct laser sintering of Al 2 O 3
granules obtained using electrostatic integrated granulation process. Using the Al 2 O 3
granules with improved flowability and handlingability, different degrees of sintering or
melting was obtained by altering the laser irradiation power and scan speed. This indicates apotential to use ceramic granules for additive manufacturing via a continuous feeder.
Nevertheless, further investigation is still required to obtain a desired size of granules and− 140 −NSG Found. Mat. Sci. Eng. Rep.

incorporation of functional additives toward a possible fabrication of functional composite
materials with desired properties using additive manufacturing.
5．Acknowledgment
This study was supported by Nippon Sheet Glass Foundation and the author would like toexpress his utmost gratitude for the research funding.
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