This document summarizes the manufacturing process for a dental tooth crown made of 3 mol% yttria stabilized zirconia (3YSZ). The process involves powder synthesis, injection molding, debinding, and sintering. The 3YSZ powder is synthesized via solution precipitation and calcined to form a 98% tetragonal phase powder. A feedstock of 53 vol% 3YSZ powder, binders, and lubricants is made and injection molded into a tooth crown shape. Binder removal is done via water leaching and thermal debinding. Finally, the crown is sintered at high temperature to achieve 99.99% theoretical density and the desired mechanical properties for use as a dental crown.

1. Manufacturing of Dental Tooth
Crowns
By 906162675
Submitted to GLM, 12/14/14
MatSE 411 Comprehensive Report

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Abstract:
3 mol% yttria stabilized zirconia (3YSZ) is an excellent material selection for a dental tooth crown
due to high values of tensile strength (1 GPa), high fracture toughness5
(5.8 MPa·m1/2
), high hardness
(12.7 GPa5
), and low thermal expansion coefficient (10.5 ×10-6
/K). The tooth crown made of 3YSZ
can be manufactured with strict process control of powder synthesis, calcination, injection molding,
debinding, and sintering. The powder is synthesized by dissolving ZrOCl2·8H2O in excess nitric acid
and heating to 100°C to evaporate chlorine. Y2O3 powder is added to achieve a 3 mol% yttria
stabilized zirconia solution while mixing at a pH of 10 for 30 minutes. The 3YSZ powder is
precipitated by spray drying to 400°C at a pH of 10. To form 98% tetragonal phase, this powder is
calcined at 600 °C for 2 hours and milled using 3 mm alumina balls in an ethanol medium for 48
hours. The feedstock for injection molding is made by mixing 3.52 wt% PMMA nanopowder, 0.280
wt% stearic acid nanopowder, and 53 vol% 3YSZ powder using a roller blade-type bender mixer at
25°C and 30 rpm for 30 minutes, then adding 10.3 wt% PEG and mixing at 70 °C for an additional
hour. The feedstock is crushed to a size of 2 mm and injection molded using a plunger-type ram
extruder with a barrel temperature series of 130-150-170-180°C from the feed to the nozzle at a
holding pressure of 30 MPa and holding time of 3.0 s before the feedstock is injected into a stainless
steel mold at 60 °C and cooled for 3.0 seconds. The binder is removed by immersing in distilled
water for 6 hours at 55°C, drying at 50°C for 30 minutes, and heating to 450°C for 2.5 hours. Finally,
the part is solid state sintered by heating to 1100°C for 4 hours, then heating to 1300°C for 1 hour.
This results in the final product of a dental tooth crown that is 99.99% pure and 99.99% theoretical
density, leading to the desired mechanical properties.
Introduction:
Yttria stabilized zirconia (YSZ) is an important material with many applications including tooth
crowns. This polycrystalline YSZ is characterized by high hardness, toughness, and high oxygen
diffusivity.1
The room temperature stable phase of zirconia is monoclinic. The addition of yttria to
zirconia stabilizes the high-temperature tetragonal phase to room temperature, enabling better
mechanical properties and more diverse applications. Some other applications are the following: an
electrolyte for solid oxide fuel cells2
, optical diagnostics like skull implants to visualize brain
tumors1
, optically activated drug delivery1
, and thermal barrier coatings in jet and diesel engines1
.
YSZ for tooth crowns is desirable because it has a low thermal expansion coefficient, meaning it will
have low volume change with temperature fluctuations in the human mouth, is bioinert, and non-
reactive with water-base bodily fluids.
The proposed product is a 3 mol% yttrium oxide partially-stabilized zirconia (3YSZ) for dental tooth
crowns. The crown is a tooth-shaped cap that covers a tooth to protect it from decay, restore a broken
tooth, or for cosmetic reasons. The dimensions are 9.33 mm (width) by 7.84 mm (height) with a wall
thickness of 2.5 mm and a cavity 2 mm deep and 0.5 mm wide. An example of the proposed product
is shown in figure 1.
In order to obtain this high purity, dense bulk ceramic, the powder must have particular
characteristics. Since this is a dental application, the composition should be 99.99% pure 3 mol%
yttria stabilized zirconia (3 mol% Y2O3· ZrO2) to avoid impurities in the sample. A realistic particle
size is 60 nanometers and a grain size on the order of 0.15 μm. The microstructure should be 98%
metastable tetragonal phase.4
This tetragonal phase with a final density of 99.99% theoretical density,

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which is 6.10 g/cm3
, leads to high crystallinity and low machinability. Also, this dense microstructure
leads to the following important mechanical properties: tensile strength of 1 GPa, fracture toughness5
of 5.8 MPa·m1/2
, and a hardness of 12.7 GPa5
. A low thermal expansion coefficient of 10.5 ×10-6
/K
decreases the stress associated with the inherent temperature fluctuations. Table 1 is a summary of
the required physical and chemical powder specifications for this application.
Powder Synthesis:
Multiple processing steps are needed to produce the powder specified above. The first is the solution
synthesis of the metal oxide powders. The precursors are ZrOCl2·8H2O (99% pure), Y2O3 (99.9%
pure), nitric acid (AR grade – 70%), and deionized water.2
The ZrOCl2·8H2O is dissolved in excess
nitric acid and heated at 100°C to burn off chloride2
. The absence of chloride in the solution is
confirmed by reacting some of the sample with silver nitrate solution. If chloride is still present in
solution, a white precipitate of AgCl will form. If there is no precipitate formation, then all of the
chloride has indeed burned off. This remaining chloride-free solution is diluted with DI water and
filtered to remove any undissolved solids.2
The concentration of Zr4+
in this solution is determined
using gravimetric analysis. A corresponding stoichiometric quantity of yttria necessary to get a 3
mol% yttria stabilized zirconia is then calculated and dissolved into the solution. The solution is then
stirred using a magnetic stir bar for 30 minutes to ensure a homogeneous composition. NH4OH is
dissolved in DI water and added as necessary to the precursor solution to adjust to pH 10. Next, the
solution is spray dried to 400°C to precipitate precursor and remove solvent3
. Again, the NH4OH
solution is used to maintain a pH of 10. The collected powder is calcined at 600 °C for 2 hours3
to
form 98% tetragonal phase. Finally, the calcined powder is milled for 48 hours in a polyethylene
container with 3 mm alumina balls (chosen so it doesn’t contaminate the powder) in an ethanol
medium to break up hard agglomerates and ensure uniform particle size.2
The synthesis process is
summarized in figure 2.
Powder Processing and Dispersion:
Multiple manufacturing steps are needed to produce a dental tooth crown by injection molding. This
is a desirable technique due to the complex shape needed for dental crowns with small dimensions.
First, the feedstock is made of a high solids loading of powder with a thermoplastic binder system
consisting of a major binder, minor binder, and a lubricant. The binder system needed is water-
soluble for ease of later binder burnout and one with a pseudoplastic rheology so the part will solidify
into the mold but the excess can be recycled. The precursor powder is the 99.99% pure 3YSZ powder
with the characteristics listed above and the binder system is 73 wt% polyethylene glycol (PEG) with
an average molecular weight of 3500, 25 wt% poly(methyl methacrylate) (PMMA), and 2 wt%
stearic acid as a lubricant and wetting agent.6
The feedstock does not require a pre-dispersed powder
system, so the pH and amount of dispersant is neglected. The iso-electric point (IEP) of dispersed
3YSZ powder is at a pH of 8.6.3
To make the feedstock, 3.52 wt% of PMMA and 0.280 wt% stearic
acid nanopowders are mixed with 53.0 vol% solids loading of YSZ powder using a roller blade-type
bender mixer at 25°C and 30 rpm for 30 minutes.6
The 10.3 wt% of PEG is added and temperature
is then increased to 70 °C and mixing continued for 1 hour.6
This ensures a homogeneous powder
distribution without voids between binder matrices or agglomeration.6
The viscosity of this feedstock

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is 20 Pa·s at a shear rate of 5000/s at 140°C.6
The addition of these binders create shear-thinning
pseudoplastic rheological behavior of the feedstock for further manufacturing. Table 2 is a summary
of the formulation for the feedstock.6
Forming:
The feedstock is now particulates on the order of 2 mm and is injection molded into an injection
molding machine for forming. The injection molding is done with a plunger-type ram extruder. A
barrel temperature series of 130-150-170-180°C from the feed to the nozzle is used.4
The holding
pressure is 30 MPa and the holding time is 3.0 s before the feedstock is injected into a stainless steel
mold at 60 °C and cools for 3.0 seconds. 4
The manufacturing process is summarized in figure 2.
Binder Burn-out and Densification:
Binder removal, or debinding, is required to remove the binding additives from the green part prior
to sintering. A two-step process including water leaching and thermal debinding is sufficient for this
product. Since PEG is water soluble, water leaching is first used to extract from the green part by
immersing in distilled water for 6 hours in air at 55°C.6
The green part is then removed from the
water and dried at 50 °C for 30 minutes in air to evaporate water from pores.6
The second step is
thermal debinding to extract the PMMA and stearic acid. The part is heated to 450°C at a rate of
1°C/minute for 2.5 hours6
in air at which point the part is binder-free and ready for the final stage of
manufacturing: sintering.
The sintering mechanism for this product is solid state sintering, in which grain boundary and bulk
diffusion are the primary densification mechanisms. To obtain 99.5% density, sinter the part at
1100°C at 5°C/minute for 4 hours in air.7
However, since 99.99% theoretical density of the product
is required for this medical application, further densification is needed. The remaining pores are
located at the grain corners and can be further sintered at 1300°C at 5°C/minute for 1 hour7
to increase
the densification. Another problem can be residual chlorine ions in the system from the synthesis of
the powder, which can lead to decomposition of the 3YSZ. This issue can be eradicated by ensuring
the react some of the ZrOCl2·8H2O dissolved in excess nitric acid with silver nitrate solution. The
solution will react with chlorine ions and will form a white precipitate if chlorine is still present. So,
by checking this while heating the ZrOCl2·8H2O dissolved in excess nitric acid continuously at 100°,
this should not be a problem. The total shrinkage of the final part due to pore removal is estimated
to be 19% from the injection modeled green part.6
This results in a dental tooth crown that is tooth-
shaped, 99.99% pure 3 mol% yttria stabilized zirconia (3 mol% Y2O3· ZrO2), and 99.99% theoretical
density with a grain size of 0.15μm. All the steps of the manufacturing process, including binder
burnout and sintering, are summarized in figure 2.
Summary:
A dental tooth crown made of 3YSZ is made following specific manufacturing parameters for the
powder synthesis, injection molding, debinding, and sintering processes to ensure excellent
repeatability and mechanical properties for implantation into the human body. The precursor powder
is ZrOCl2·8H2O dissolved in excess nitric acid. Chlorine is evaporated by heating to 100°C. The
chloride-free solution is diluted with deionized water and filtered to remove undissolved solids.
Gravimetric analysis is performed to determine the Zr4+
concentration, so a corresponding
stoichiometric quantity of Y2O3 can be added to get 3 mol% yttria stabilized zirconia. Diluted

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NH4OH is added to adjust the pH to 10 while stirring the solution with a magnetic stir bar for 30
minutes for uniform composition. To remove the solvent and precipitate the precursor, the YSZ
solution is spray dried to 400°C while maintaining a pH of 10. The powder is then calcined at 600
°C for 2 hours to form 98% tetragonal phase and milled for 48 hours in a polyethylene container with
3 mm alumina balls in an ethanol medium to break up agglomerates and ensure uniform particle size.
3.52 wt% PMMA nanopowder, 0.280 wt% stearic acid nanopowder, and 53.0 vol% 3YSZ powder
are mixed using a roller blade-type bender mixer at 25°C and 30 rpm for 30 minutes. Then, 10.3 wt%
PEG is added and mixer at 70 °C and for 1 hour before crushing this feedstock into 2 mm size. The
feedstock is injection molded with a plunger-type ram extruder using a barrel temperature series of
130-150-170-180°C from the feed to the nozzle at a holding pressure of 30 MPa for a holding time
of 3.0 s before the feedstock is injected into a stainless steel mold at 60 °C and cooled for 3.0 seconds.
Then a two-part binder removal process occurs by: immersing the part in distilled water for 6 hours
at 55°C to extract the PEG, removing the part and drying at 50°C for 30 minutes to evaporate the
water from the pores, and finally removing the PMMA and stearic acid by heating the part to 450°C
for 2.5 hours. Finally, the part is solid state sintered by heating to 1100°C for 4 hours, then heating
to 1300°C for 1 hour to remove pores and obtain the necessary high density. This results in the final
product of a dental tooth crown that is 99.99% pure and 99.99% theoretical density.
References:
1. E. Farshihaghro, "Pyrolysis of Yttria Stabilized Zirconia and its Characterization"; M. S.
Thesis, University of California, Riverside, CA, 2013. [thesis]
2. K. Prabhakaran, M.O. Beigh, J. Lakra, N.M. Gokhale, and S.C. Sharma, “Characteristics of
8 mol% Yttria Stabilized Zirconia Powder Prepared by Spray Drying Process,” J. Mater.
Process. Technol., 189 [1-3] 178-181 (2007).
3. R. Nagel, "Dental Crown: Benefits and Problems," Article published online: 2010;
http://www.curetoothdecay.com/Dentistry/dental_crown.htm.
4. C.A.M. Volpato, L.G.D. Garbelotto, M.C. Fredel, and F. Bondioli, “Application of Zirconia
in Dentistry: Biological, Mechanical and Optical Considerations”; Chapter 17 in Advances
in Ceramics – Electric and Magnetic Ceramics, Bioceramics, Ceramics and Enivronment;
Edited by Prof. C. Sikalidis. InTech, Rijeka, Croatia, 2011. [chapter in edited online book]
5. M. Ghatee, M. H. Shariat, and J. T. S. Irvine, “Investigation of Electrical and Mechanical
Properties of 3YSZ/8YSZ Composite Electrolytes,” Solid State Ionics, 181 [1] 57-62
(2009).
6. J. Rajabi and H. Zakaria, “Fabrication of Miniature Parts Using Nano-sized Powders and an
Environmentally Friendly Binder through Micro Powder Injection Molding,” Microsyst
Technol., 7 1-6 (2014).
7. W. H. Rhodes, “Agglomerate and Particle Size Effects on Sintering Yttria-Stabilized
Zirconia,” J. Am. Ceram. Soc., 64 [1] 19–22 (1981). doi: 10.1111/j.1151-
2916.1981.tb09552.x

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Figures, Tables, and Flow Sheet:
Figure 1. An example of how a crown fits onto a tooth.3
Table 1. Outline of necessary powder specifications for 3YSZ.
Powder Specifications of Physical Characteristics for
3 mol % Yttria-Stabilized Zirconia (3YSZ)
Physical
60 nm particle size
0.15 μm grain size
98% tetragonal phase
99.99% theoretical density
6.10 g/cm3
theoretical density
pH = 8.6 isoelectric point
1 GPa tensile strength
5.8 MPa·m1/2
fracture toughness
12.7 GPa hardness
10.5 ×10-6
/K thermal expansion coefficient
Chemical 99.99% pure 3 mol % Y2O3· ZrO2
Table 2. Formulation of feedstock of 3YSZ. 6
Formulation of Feedstock of 3 mol % Yttria-Stabilized Zirconia (3YSZ)
Material
Average
Molecular
Weight
Purpose Volume % Weight %
3 mol % YSZ powder Ceramic Powder 53.0 85.9
PEG 3,500 Binder (major) 34.6 10.3
PMMA 3,400 Binder (minor) 11.3 3.52
Stearic acid Lubricant/Wetting Agent 1.10 0.280

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Manufacturing Process of 3 mol% Yttria Stabilized Zirconia
Dissolve ZrOCl2·8H2O (99% pure) in stoichiometric excess nitric acid (AR grade – 70%)
Heat precursor solution to 100°C to evaporate chlorine
React small sample of solution with silver nitrate solution to determine absence of chlorine
If white (AgCl) precipitate forms, chlorine is still present. If solution stays clear, solution is chlorine-free.
Dilute chlorine-free solution with deionized water and filter solution to remove undissolved solids
Determine Zr4+
concentration in solution using gravimetric analysis
Dissolve corresponding stoichiometric quantity of Y2O3
(99.9% pure) to get 3 mol% yttria stabilized zirconia
Dissolve NH4OH
in DI water
Adjust pH to 10
using NH4OH
Stir solution using magnetic stir bar for 30 minutes to
ensure uniform composition of solution
Precipitate precursor by spray drying YSZ solution to 400°C to remove solvent
Maintain pH at
10 using NH4OH
Calcine collected powder at 600 °C for 2 hours to form 98% tetragonal phase
Mill powder 48 hours in polyethylene container with 3 mm alumina balls in ethanol medium to break up hard
agglomerates for uniform particle size
Mix 3.52 wt% PMMA with an average molecular weight of 3,400
nanopowder, 0.280 wt% stearic acid nanopowder, and 53 vol% 3YSZ powder
using roller blade-type bender mixer at 25°C and 30 rpm for 30 minutes
Mix in 10.3 wt% PEG
with an average molecular
weight of 3,500 at
increased temperature of
70 °C and for 1 hour
Crush feedstock into 2 mm size
granules
Injection mold feedstock with a plunger-type ram extruder using a barrel temperature series of 130-150-170-
180°C from the feed to the nozzle at a holding pressure of 30 MPa for a holding time of 3.0 s before the
feedstock is injected into a stainless steel mold at 60 °C and cool for 3.0 seconds
Immerse green part in distilled water for 6 hours in air at 55°C to extract PEG from green part
Remove part from water and dry at 50 °C for 30 min in air to evaporate water from the pores
Heat part to 450°C at a rate of 1°C/minute for 2.5 hours in air to remove PMMA and stearic acid
Sinter binder-free part at 1100°C at 5°C/minute for 4 hours in air, then heat to1300°C 5°C/minute for 1 hour to
remove pores and obtain high density
Final product: dental tooth crown that is tooth-shaped, 99.99% pure 3 mol% yttria stabilized zirconia (3 mol%
Y2O3• ZrO2), and 99.99% theoretical density with a grain size of 0.15μm
Figure 2. Flow chart for the manufacturing process of 3 mol% Yttria Stabilized Zirconia.