Creep of Hard Porcelain during Firing

Creep of Hard Porcelain during Firing
The stress exponent, activation energy, thermal expansion
behavior and compression strength of a porcelain body were
evaluated from temperature and stress data.
Suat Yilmaz and Z. Engin Erkmen
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The classical definition of
porcelain is a translucent,
impermeable ceramic with <5%
porosity and high sonorous
property. Porcelain has high
strength when compared with
other ceramics. Its white color
results from the high purity of
its components. Porcelain contains quartz and neddlelike mullite crystals dispersed in a viscous silicate glass.
Qu
ar t
In porcelain bodies, during
heat treatment, the major component kaolinite ((Al2Si2O5)(OH)4)
transforms
to
metakaolinite at 550–600°C,
which
is
a
dehydrated
metastable amorphous structure. Then, at 950–1000°C, the
latter transforms to a spinel
structure and amorphous silica.
Feldspar content (wt%)
At the same time, feldspar
grains react with silica and
1
kaolinite phases to form a vis- Fig. 1 Phase diagram of a porcelain body at a typical firing temperature of 1300°C.
cous liquid phase at 990°C. In
this stage, quartz grains begin to dissolve into the liquid phase. Above 1100°C, spinel transforms to primary
mullite and silica. Above 1200°C secondary mullite crystals nucleate and grow, following the reaction of clay
with feldspar relicts.
Aggregates of small equiaxed-shaped crystals that originate from clay (<0.5 µm) are called primary mullite,
whereas needle-shaped mullite crystals (>1 µm) grown from feldspar melt are called secondary mullite. During
heat treatment, primary mullite gradually crystallizes and transforms to secondary mullite. The dissolution of
quartz grains increases above 1250°C and forms a silica-rich solution of amorphous periphery around quartz
grains. When the temperature is increased, quartz and secondary mullite partly dissolve into the glassy melt,
whereas primary mullite remains stable at higher temperatures. Potash diffusion occurs simultaneously along
with these transformations.
Final phases that appear at 1300°C (which is a typical firing temperature) are mullite, silica and potassium
aluminum silicate glass (Fig. 1). Fine quartz grains (<20 µm) dissolve rapidly, whereas coarser grains dissolve
American Ceramic Society Bulletin, Vol. 86, No. 8
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Creep of Hard Porcelain during Firing
dL-rel (%)
dL-rel (662DIL)
Temperature (°C)
Fig. 2 Dilatometry results of porcelain sample.
completely at 1350°C. The threshold temperature is 1400°C. Therefore, a porcelain body above 1400°C
consists of mostly mullite and glass phases with little quartz. The presence of quartz frequently causes circumferential cracking around the grain when α ➝ β transformation occurs at ~573°C during cooling.1–3
Few studies have been performed about the high-temperature creep properties of porcelain. Chaudhuri3
has researched the short-term creep behavior of silica porcelains at 1075–1125°C. He has calculated the
viscous flow of the glassy phase and creep activation energy. Ponraj et al.4 have studied the creep properties
of silica and alumina porcelains. Other researchers also have determined that the glassy phase that exists in
porcelain increases creep rates at various temperatures. However, the vitrified phase retards creep by
increasing viscosity.4–9
The purpose of this research is to investigate residual deformation that occurs after creep of glassy melt
against swelling effect, which increases volume of the porcelain body during glaze firing between 1250 and
1400°C.
Sample Preparation
The raw materials were obtained from Germany (Meiβen Porzellan GmbH). The mineralogical composition of the batch was 30% quartz, 30% feldspar and 40% clay (Table 1). The specific surface area was measured as 11.39 m2/g using BET (Model Areameter 2, Ströhlein, Germany).
As-received porcelain paste was preshaped in the form of cylindrical pellets using hand-pressing. These
50 × 50 mm cylindrical samples were prepared according to DIN 51053 standard and were dried in ambient air for 24 h.
Thermal Characterization
A 6 × 6 mm porcelain sample was observed using hot-stage optical microscopy (Leitz, Germany). Sintering started at ~1220°C and
accelerated at 1300°C. After sintering was completed, new sample
dimensions were 5.7 × 5.8 mm, and the original form was preserved. However, when the temperature reached 1500°C, the sample corners softened and became ellipsoidal. At the same time,
swelling occurred on the sample surfaces. At 1580°C, sample
dimensions were 5.5 × 6.5 mm. Therefore, the softening point of
the porcelain body was ~1500°C, whereas its sintering temperature
was ~1220°C.
Thermal expansion behavior was studied using dilatometry
(Linseis) in ambient air up to 1000°C (Fig. 2). At higher tempera-
9302
Table 1 Chemical Composition of Porcelain Body
Component
SiO2
Al2O3
Fe2O3
TiO2
CaO
MgO
Na2O
K 2O
f. l.
Composition (wt%)
59.42
28.38
0.34
0.14
0.29
0.13
1.17
2.91
7.62
American Ceramic Society Bulletin, Vol. 86, No. 8
Creep of Hard Porcelain during Firing
tures, the experiment could not be evaluated because of slumping of the sample in the dilatometer.
Maximum expansion occurred at 516°C (~0.29% of maximum), and caving with ~1.20% deformation
occurred at 908°C. Above this temperature, collapse of the sample progressed rapidly, and caving reached
~5% at 1120°C.
Characterization of Creep Behavior
The general fast-firing regime of a porcelain plate was determined (Fig. 3). A heat treatment was applied
for ~0.5 h at ~1300°C during liquid-phase sintering during which the volume of the glassy phase increased.
At this temperature, the porcelain body was subjected to viscoelastic deformation by its own weight caused
by creep.
In this study, hand-pressed cylindrical samples were used directly in the creep experiment after they were
dried 24 h at room temperature. The temperature and stress-dependent creep behavior of the porcelain samples under constant temperature and pressure conditions (1250–1300–1350–1400°C at 0.05–0.1 MPa) were
studied using power-law creep (Model R.U.L/C.I.C Tester 42, Netzsch).4,10–13 For this purpose, ISO 1893
and EN 993-8 (DIN 51053) standards were followed. Creep in compression norms were applied at 1250°C
at 0.05 MPa, 1300°C at 0.05 MPa, 1350°C at 0.05 MPa, 1400°C at 0.05 MPa and 1300°C at 0.1 MPa.
Creep curves were obtained according to measured experimental data. Temperature-dependent creep
(Figs. 4(a) and (b)) and stress-dependent creep (Figs. 5(a) and (b)) were determined.
Determination of Creep Parameters
Power-law creep can be expressed using the Arrhenius rate equation:4,12,13
·
ξ= dε/dt = Aoσn exp(–Q/RT)
(1)
·
where ξ = dε/dt is the creep rate (h–1) (strain), Ao a constant, n the creep stress exponent, Q the activation
energy, R the gas constant and T the temperature (K).
The creep exponent and activation energy can be determined using Eq. (1) and simple algebra for variable stress with constant temperature (left side) and variable temperature with constant stress (right side).
Using the geometrical method,
·
ξ1 = Aoσ1n exp(–Q/RT1)
·
ξ2 = Aoσ2n exp(–Q/RT1)
· ·
ξ1/ξ2 =σ1n/σ2n
·
ξ3 = Aoσ1n exp(–Q/RT1)
·
ξ4 = Aoσ1n exp(–Q/RT2)
· ·
ξ3/ξ4 = exp{[–Q/R[(1/T2) – (1/T1)]}
(2)
(3)
Temperature (°C)
· ·
n = log(ξ1/ξ2)/log(σ1/σ2)
· ·
Q = [ln(ξ1/ξ2)(T1T2)R]/(T2 – T1)
Length of tunnel kiln (m)
Fig. 3 Typical fast-firing regime of porcelain.
American Ceramic Society Bulletin, Vol. 86, No. 8
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Creep of Hard Porcelain during Firing
Substitution of creep data into Eqs. (2) and (3) shows
that n = 0.42 and Q = 44.374 cal/(mol·K), which are similar to data found in the literature.
These results show that the predicted sagging of porcelain following creep at 1250, 1300 and 1350°C terminate
and reverse to a slight expansion at ~1400°C. This
behavior can be attributed to the increase in the rate of
crystallization of secondary mullite at 1400°C, as reported in the literature.2,6,13,14
Table 2 Cold Compressive Strength of Porcelain
Samples
Sintering
temperature (°C)
Compressive
strength (MPa)
1300
1350
1400
140
160
95
Measurement of Mechanical Behavior
A liquid penetration test was performed to determine the porosity of the porcelain bodies fired at
1300–1400°C. As expected, porosity levels were low, and zero porosity could be claimed.
After the creep experiments were completed, the fast-fired cylindrical samples were subjected to compression tests using cold isostatic pressing (AIP3-12-60C) at ambient temperature. A significant decrease in
compression strength occurred at 1400°C although similar results were measured after sintering at 1300 and
1350°C (Table 2). The needle-shaped secondary mullite crystals might have caused microcrack formation
because of stress accumulation along the crystal tips in the glassy matrix when cooling occurred.
Microstructural Characterization
The porcelain sample heat-treated at 1350°C for 0.5 h was examined using scanning electron microscopy
(SEM). Round-shaped quartz crystals were dispersed in an amorphous silica phase, and secondary mullite
needles were present in the microstructure (Figs. 6(a) and (b)). A secondary mullite phase formed following the reaction of clay relicts with feldspar relicts above 1200°C.
Moreover, primary mullite gradually transformed to secondary mullite by crystallization. Circular cracks
appeared along the edges of quartz grains. These were caused by residual stresses that originated during volume shrinkage of α ➝ β transformation of silicate phase at ~572°C.
Application
The study of creep behavior during firing of vitrified ceramics such as porcelain is important for industry
and computer simulation of products. The experimentally determined creep parameters can be used as input
data in ANSYS,12,15 which is a CAD/CAE program that provides a model for predicting stress and strain distributions in porcelain bodies. ■
Acknowledgments
We are grateful to TUBITAK–DFG (Turkish and German Research Foundations) for their financial support, Istanbul University Research Funds and Freiberg Technical University, Institute for Ceramics, where
most of this work has been evaluated.
About the Authors
Suat Yilmaz is a faculty member in the Dept. of Metallurgy and Materials Engineering, Faculty of
Engineering, Istanbul University, Istanbul, Turkey. He is a graduate of the Dept. of Metallurgical
Engineering, Istanbul Technical University. He completed his M.S. and Ph.D. studies at the same university. Z. Engin Erkmen is a faculty member in the Dept. of Metallurgy and Materials Engineering, Faculty of
Engineering, Marmara University, Istanbul, Turkey. He is a graduate of the Dept. of Metallurgiacal
Engineering. He completed his M.S. and Ph.D. studies at the University of Michigan and in the University
of Florida, respectively.
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Creep of Hard Porcelain during Firing
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American Ceramic Society Bulletin, Vol. 86, No. 8
(a)
Strain, ξ
dL/lo (⫻10-1)
(b)
Time, t (h)
Time (h)
Fig. 4 (a) Creep test results at various temperatures. (b) Temperature-dependent creep behavior of porcelains.
(a)
Strain, ξ
dL/lo (⫻10-1)
(b)
Time (h)
Time, t (h)
Fig. 5 (a) Creep test results at various applied stresses. (b) Stress-dependent creep behavior of porcelains.
(a)
(b)
Fig. 6 SEM micrographs of porcelain body: (a) lower and (b) higher magnification.