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Author’s Accepted Manuscript

Effect of sintering temperature on mechanical properties of magnesia partially stabilized zirconiarefractory

Lan Jiang, Shuqiang Guo, Yuyang Bian, ManZhang, Weizhong Ding

PII: S0272-8842(16)30274-7DOI: http://dx.doi.org/10.1016/j.ceramint.2016.03.136Reference: CERI12507

To appear in: Ceramics International

Received date: 21 December 2015Revised date: 18 March 2016Accepted date: 18 March 2016

Cite this article as: Lan Jiang, Shuqiang Guo, Yuyang Bian, Man Zhang anWeizhong Ding, Effect of sintering temperature on mechanical properties o

magnesia partially stabilized zirconia refractory, Ceramics Internationahttp://dx.doi.org/10.1016/j.ceramint.2016.03.136

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Effect of sintering temperature on mechanical properties of magnesia partially stabilized zirconia

refractory

Lan Jiang, Shuqiang Guo,*, Yuyang Bian, Man Zhang, Weizhong Ding

State Key Laboratory of Advanced Special Steel & Shanghai Key Laboratory of Advanced

Ferrometallurgy & School of Materials Science and Engineering, Shanghai University, Shanghai

200072, China

∗Corresponding author. Tel.(Fax): +86 21 56338244. [email protected]

Abstract

The optimized sintering conditions for a 3.5wt% magnesia partially stabilized zirconia (Mg-PSZ)

refractory were proposed in our recent research. The influence of the sintering temperature on the

development of phase composition, microstructure, densification, thermal expansion and mechanical

strength were studied in detail by X-ray diffraction (XRD), scanning electron microscope (SEM),

He-pycnometer, high temperature dilatometry and three-point bending test. The samples sintered at

1670oC had the highest bend strength, the maximum densification, the lowest thermal expansion

coefficient (CTE), a homogeneous microstructure and a linear change in thermal expansion.

Keywords: A. Sintering; C. Mechanical properties; D. ZrO2; E. Refractories

1. Introduction

Vacuum induction melting (VIM) processing has been the primary melting method for

nickel-based superalloy because it does not impact melt chemistry and gives results in good

homogeneity of the melt [1]. But VIM is the only vacuum melting method which uses a refractory

crucible made of oxides such as Al2O3 and MgO [2]. Unfortunately, the refractory is a contamination

source of oxygen because of metal/refractory reactions leading to crucible disintegration. Oxygen is a


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harmful trace element that exists both in solid solution and oxide inclusions in the superalloy. Oxide

inclusions can act as crack initiation sites and propagation paths, so it can dramatically affect the

properties of the nickel-based superalloy [3]. Therefore, it is important to design an appropriate

refractory to melt this superalloy. The stability of the refractory can be described by the following

sequence:Y2O3＞CaO＞ZrO2＞Al2O3＞MgO [4]. It is apparent that Y2O3 is the most stable crucible

lining material. But Y2O3 is fairly expensive, which is a drawback [5]. CaO is not used because it is

very susceptible to humidity [6] and therefore not suitable for crucible lining in industrial furnaces.

MgO and Al2O3 crucible materials also cause a problem because of their unstability under high vacuum

melting. The dissociation of these refractories leads to oxygen pick-up in the melt, which could

eventually result in oxygen inclusion formation when the solubility limit is exceeded. Besides, the use

of MgO crucible accelerates the formation of high melting inclusions (MgAl 2O4) which deteriorate the

cleanliness of the alloy [7]. Compared with refractories such as magnesia and alumina, zirconia

ceramics show better chemical stability. Aneziris et al. [8] reported that the low porosity ZrO2 based

materials have been used in the near shape steel casting for high corrosion resistance. Hence, ZrO 2 is

the most suitable refractory for melting nickel-based superalloy.

However, pure zirconia (ZrO2) has three polymorphs depending on temperature: monoclinic (m)

up to1170oC, tetragonal (t) to 2370oC, and above this temperature, cubic (c) [9-10]. After sintering at

temperatures between 1500 and 1750oC, pure zirconia ceramics break into pieces at room temperature

[11]. Due to the destructive t→m phase transformation, pure zirconia is used quite rarely [12]. However,

the effect of this phase transformation can be eliminated by doping zirconia with appropriate amount of

oxides such as MgO and CeO2 [13]. In view of the tremendous technological applications, PSZ

sintering has received much attention from scientists and technologists. Mechanical properties can be


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enhanced by preparing PSZ ceramics that have high densities and small grain sizes after sintering [14].

Because of the different rates of pore closure, larger pores will develop into a porous microstructure

during densification. These pores can be removed only at a high sintering temperature and a long

sintering time [15]. Response surface methodology (RSM) was applied to determine the operational

conditions for the properties of PSZ. The sintering temperature and heating rate have different effects

on the densification and bending strength - sintering temperature is positive, heating rate is negative

[16]. Although Young’s modulus of the ZrO2-3mol% Y2O3 does not change with sintering temperature,

a slight decrease is observed in the hardness values above 1000oC, which is attributed to microstructure

coarsening [17].

The Mg-PSZ refractory has already become one of the most important materials because of its

corrosion resistant, excellent chemical and thermomechanical properties [18]. Nevertheless, full

exploitation of these advantages can only be realized if the final products have the required

specifications comprising desired phase, density, microstructure and the bend strength [19]. Based on

the equilibrium phase diagram MgO-ZrO2 [20], the transformation temperature of t→m is about

1240oC and the tetragonal and cubic solid solution becomes stable above 1400 oC. A feature of

technological importance is the cubic and tetragonal solid solution phase field [8]. The crystal structure

transformations involving volume changes exhibited thermal hysteresis, which may cause cracking

defects. The hysteresis was controlled by several microstructure and chemical factors, such as grain

size, tetragonal size, and the sintering process [21-22].

In this paper, the optimized sintering conditions for the 3.5wt% Mg-PSZ refractory are proposed.

The influence of the sintering temperature on the development of phase composition, microstructure,

densification, thermal expansion and mechanical strength were studied in detail.


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2. Experimental Procedure

Commercial magnesia partially stabilized zirconia (3.5wt% Mg-PSZ) powders (provided by

Sunshine Capital Ltd, China) were selected as the starting material with a grain size distribution of

0-10μm and d50=2.7μm. Solid discs (20 mm in diameter) and bars of 3.5wt% Mg-PSZ were attained by

slip casting. In order to form suspensions, the 3.5wt% Mg-PSZ powders were dispersed in deionized

water with an organic binder and dispersant, then milled for 4 h. The organic binder and dispersants

were Arabic gum and Triethanolamine respectively. The stable suspensions with 78wt% solids content

were slip cast in plaster mold to obtain the green body. The green samples were sintered at different

temperatures of 1600, 1620, 1650, 1670, 1690oC with a dwell time at temperature of 4 h.

The crystal structure of samples sintered at different temperatures was characterized by XRD

(D/MAX2200V PC). Morphology was examined using a scanning electron microscope (SEM, Hitachi

SU-1510). The amount of densification was obtained by the ratio of the densities of sintered samples

determined using a He-pycnometer (AccuPyc II 1340) to the theoretical density. Differential scanning

calorimetry analysis (DSC, Netzsch STA 449 F3) was employed to evaluate the phase transition

temperature of 3.5wt% Mg-PSZ powders in air from room temperature to 1350oC. The bend strength of

samples which were sintered at different temperatures was investigated using a three-point bending test

with a span length of 30mm on samples of 3mm thickness×4mm width×40 mm length. Five samples

were tested to obtain the average data.

Linear shrinkage (dL/L0) of the 3.5wt%Mg-PSZ powders was measured in air using a push-rod

dilatometer (Netzsch DIL 402C) from the room temperature to 1450°C. The size of the sample bars for

the dilatometer analysis was 8mm length×6 mm diameter.


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3. Results and discussion

3.1. Sintering behavior of the 3.5wt% Mg-PSZ refractory

The sintering of green samples after drying took place in the dilatometer, which recorded the

shrinkage value (dL/L0) and the rate of linear shrinkage (d(dL/L0) of green bodies

during heating. The dynamic sintering curve of 3.5wt% Mg-PSZ green body in air is presented in Fig.1.

It can be seen that there was no shrinkage below 1000oC, but dramatically declined between 1000oC

and 1240oC. The rapid shrinkage is caused by the formation of grain boundaries between the particles

and a reduction in porosity. The maximum rate of shrinkage around 1240 oC was related to the phase

transformation from monoclinic to tetragonal according to the equilibrium phase diagram MgO-ZrO2

[20]. A large volume change is associated with this transformation, which was believed to be the reason

for the catastrophic failure of zirconia ceramic [23]. It was noteworthy that the rate of shrinkage

increased at 1426oC, which may be caused by the phase transformation from tetragonal to cubic

according to the phase diagram of MgO-ZrO2. Thus, it was important to control sintering condition

around this temperature. With increasing temperature, the rate of linear shrinkage gradually decreased.

The DSC curve of 3.5wt% Mg-PSZ green samples is shown in Fig.2. Two endothermic peaks

occurred at 330oC and 1240oC respectively. Because decomposition temperature of Arabic gum and

Triethanolamine are about 330oC, the initial endothermic peak could be attributed to the decomposition

of organics which were introduced during the slip casting. At this temperature, the effect of organics

can be eliminated by prolonging soaking time during sintering. The other endothermic peak at 1240 oC

corresponded to the phase transformation, which was consistent with the phase diagram and sintering

curve.


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Based on the equilibrium phase diagram MgO-ZrO2 [20], the dynamic sintering curve and DSC, a

controlled sintering process for 3.5wt% Mg-PSZ samples was determined as shown in Fig.3. From

room temperature to 330oC, a low heating rate of 1.5oC min-1 and a holding time of 60 min were used

to decompose the organic binder. Subsequently, the sintering temperature was increased to 1000 oC

using a heating rate of 3oC min-1. From 1000oC to 1240oC, the initial sintering of the 3.5wt% Mg-PSZ

refractory occurred and a lower heating rate of 1.5 oC min-1was used. A holding time of 120 min was

used at 1240oC to allow the Mg-PSZ phase to transform from monoclinic to tetragonal. From 1240oC

to 1426oC, a lower heating rate of 1.5oC min-1was used, and a hold time of 120 min at 1426oC due to

the phase transformation from tetragonal to cubic. Samples were further sintered at various final

sintering temperatures from 1600oC to 1700oC using a heating rate of 1.5oC min-1. The holding time at

the final temperature remained 240 min to allow grain growth and solidification. The optimal final

sintering temperature was determined by the thermomechanical properties and microstructure of

samples sintering at different temperatures.

3.2 Phase evolution

Polyphase ZrO2 is generally observed when the MgO content is below 13mol% [24]. Fig.4 shows

XRD patterns of 3.5wt% Mg-PSZ refractory sintered at different temperatures. The constituent phases

in the sintered samples were the tetragonal and cubic phases, with small amounts of monoclinic phase.

The (101)t and (111)c peaks overlap each other because they have nearly the same lattice parameters for

the tetragonal and cubic phases [13]. By XRD spectrum, with increasing sintering temperature, the

tetragonal and cubic phases were found to increase. When the sintering temperature are above 1600oC,

the tetragonal and cubic phases become stable on the basis of the equilibrium phase diagram

MgO-ZrO2 [20].


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The relative amount of the monoclinic phase of zirconia is of considerable importance in attempts

to understand the mechanical properties of zirconia-based ceramics [25]. Therefore, it is necessary to

determine the amount of monoclinic phase. The amount of monoclinic phase Xm was calculated in

accordance to Eq. (1) as previously described [24]:

Xm = (Im(̅ ) Im())/(It(0)+c() Im(̅ ) Im()) (1)

Where Im(̅ ), Im()and It(0)+c() represent the integrated intensities of the corresponding

peaks. The results of the amount of monoclinic phase Xm sintered at different final temperatures are

shown in Fig.5. It can be seen that the amount of m-phase approached nearly 80% at 1600oC. However,

the zirconia materials with high amounts of monoclinic phase (above 50%) made the mechanical

strengths and corrosion resistance worse [8]. Though high amounts of monoclinic phase are bad for

mechanical properties, a certain amount of monoclinic in zirconia materials is necessary for good

thermal shock performance. Aneziris et al. [26] reported that the zirconia based materials with an

amount of 13% m-phase can achieve a good thermal shock performance. At 1670oC, the amount of

m-phase was about 15%, which is similar value as the report, thus having a minimum value for good

thermal shock performance. When the sintering temperature reached 1690oC, the amount of monoclinic

phase would be predicted to be less than 10%.

3.3 Densification and Microstructure

The densification of zirconia specimens sintered at different final temperature is shown in Fig.6.

As can be seen from the result, the sintering temperature had a significant effect on the densification.

With rising sintering temperature, the densification initially increased and then decreased, reaching a

maximum value at 1670oC. When temperature reached 1690oC, because of grains growth, some pores

were trapped by grains, resulting in a slight reduction of densification [27]. The micrographs of the


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samples sintered at different temperatures are presented in Fig.7. The green body contained a large

amount of pores, and there were only point contact between particles. At 1450oC, a microstructure with

clear grain boundaries was obtained. But many pores were also found on the surface (Fig.7a). With

increasing temperature, the pores decreased gradually (Fig.7b). From Fig.7c, it shows the grain grew

further and the particles contacted with each other more and more close. It is clearly observed the

agglomerations of the fine particles. It means that the decrease in the numbers of pores results in the

increase of densification with increased sintering temperature. At the same time, larger grain size

implies a reduction in the grain boundaries. However, the microphotograph revealed an exaggerated

grain growth, with grain sizes becoming as coarse as 60μm when the sintering temperature was 1690oC

(Fig.7e). This behavior was in good agreement with densification. Higher temperatures promote

exaggerated grain growth rather than further densification. This indicated that the combined effects of

particle size and the numbers of pores are all equally critical factors for optimal densification. Besides,

the increased grain size may result in enhanced crack formation [28]. When the sintering temperature

reached 1670oC, the size of grain was only about 40μm and the structure uniformity was not found to

have exaggerated grain growth (Fig.7d). The similar micrographs of 3.5wt% Mg-PSZ samples sintered

at 1670oC were found.

3.4 Thermal and Mechanical properties

Thermal stress resistance can be used to predict the thermal shock behavior of material. The

thermal shock parameter R was calculated using the equations [29]:

R = (1 − )/Eα (2)

where is the bend strength, E is Young’s modulus, is the thermal expansion coefficient, and is

Poisson’s ratio. Considering the parameter R, it is clear that better thermal shock behavior can be


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achieved in materials with high strength and Poisson’s ratio, and with low values of thermal expansion

coefficient and Young’s modulus. Besides, the change of linear thermal expansion also affects thermal

shock behavior. In spite of the higher values of the thermal expansion, more thermal expansion led to

better thermal shock behavior of the samples.

The thermal expansion curves of 3.5wt% Mg-PSZ refractory during heating and cooling, which

were sintered at different temperatures, are shown in Fig.8. With increased sintering temperature, the

linear shrinkage at the temperature around 1240oC and the linear thermal expansion at the temperature

around 1400oC were both reduced. It was considered that the t→m transformation temperature during

heating and cooling was directly related to the thermal stabilities of the tetragonal and monoclinic

phase [30]. Sample sintered at 1600oC exhibited a remarkable thermal hysteresis. The t→m phase

transformation took place at around 800oC on cooling, which was much lower than the m→t

transformation on heating, causing the hysteresis loops and the associated linear change. The thermal

expansions trended to a linear change with increasing sintering temperatures. The thermal hysteresis

area of curves has been decreased for the samples when sintering temperatures are higher than 1650 oC.

It means that the tetragonal and cubic phase of Mg-PSZ materials could be stabilized at room

temperature.

The thermal expansion coefficient α at temperature T was calculated as [31]:

α = (L/0)/ dT (3)

where L0 is the length of a sample at room temperature. The value of linear shrinkage (dL/L 0) of

samples was measured using a dilatometer. The curves of thermal expansion coefficient (CTE) are

shown for the different sintered samples in the Fig.9. The lower value of CTE should improve thermal

shock behavior of the samples. Though the curve of CTE was the lowest when samples were sintered at


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1600oC, the thermal expansion curves as shown in Fig.8 exhibited a remarkable thermal hysteresis,

which would result in poor thermal shock behavior. The thermal expansion coefficients increased with

increasing sintering temperature. This phenomenon was consistent with the amount of m-phase in

samples as discussed earlier. The value of CTE of monoclinic phase was lower than cubic and

tetragonal phases [32]. The amount of monoclinic phase decreased with increasing sintering

temperature, which lead to an increase in the CTE value. As can be seen from Figs.8 and Figs.9, when

the samples were sintered at 1670oC, the amount of m-phase was about 15%. This composition offers a

low thermal shock coefficient accompanied by an acceptable thermal hysteresis. With increased

sintering temperature, the decrease of the amount of m-phase result in an increase of the thermal

expansion coefficient. This is a reason why zirconia based material should contain a certain amount of

monoclinic phase.

The effect of sintering temperature on bend strength was investigated by studying densification

and grain size [33]. Fig.10 shows the bend strength of Mg-PSZ refractory sintered at different

temperatures which was tested at room temperature. The strength-change behavior was observed to be

similar to the strength-sintering temperature behavior. With increasing sintering temperature, ceramic

particles connected closer and the structure of the sintered compacts became stronger, which led to

higher strength. The highest bend strength of 634 MPa was obtained with 3.5wt% Mg-PSZ sintered at

1670oC. As can be seen from Figs.6 and Figs.7, at temperature higher than 1670oC, the grain size of the

3.5wt% Mg-PSZ refractory becomes larger and the densification of the 3.5wt% Mg-PSZ refractory

decreases gradually. It must be mentioned that higher temperatures cause a lower densification and

larger grains, which results in decreasing mechanical strength of the 3.5wt% Mg-PSZ refractory.

Therefore, 1670o

C was the most suitable temperature for sintering Mg-PSZ refractory because of the


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high mechanical properties and lower CTE.

4. Conclusions

In this work, 3.5wt% Mg-PSZ refractories were prepared by slip casing using commercially

produced MgO stabilized zirconia powders and then sintered under a controlled sintering process. The

optimized sintering conditions were determined using MgO-ZrO2 phase diagram, high temperature

dilatometry and differential scanning calorimetry analysis. A sintering temperature of 1670oC proved to

be the optimum final sintering temperature. The samples sintered at 1670oC had the highest bend

strength of 634 MPa, the maximum densification and the lower value of CTE. Therefore, by selecting

this sintering temperature, it is possible to obtain optimized mechanical properties of Mg-PSZ for

potential applications in melting superalloy.

Acknowledgements

This study was financially supported by the ShangHai University and the Joint Funds of the National

Natural Science Foundation of China (U1560202).

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Fig.1.Temperature dependence of shrinkage (dL/L0) and shrinkage rate (d(dL/L0)/dT) of the 3.5wt%

Mg-PSZ sample in the course of heating(5 oC /min).

Fig.2.Differential scanning calorimetry of 3.5wt% Mg-PSZ green body.


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Fig.3. Heating pattern for sintering the 3.5wt% Mg-PSZ samples.

Fig.4.X-ray diffraction patterns of 3.5wt% Mg-PSZ powders sintered at different temperatures.

Fig.5. Amount of M-phase of 3.5wt% Mg-PSZ powders sintered at different temperatures.

Fig.6.Densification of 3.5wt% Mg-PSZ powers sintered at different temperatures.


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Fig.7.SEM micrographs of 3.5wt% Mg-PSZ powers sintered at different temperatures: (a) 1450oC, (b)

1600oC, (c) 1650oC, (d) 1670oC, (e) 1690oC.

Fig.8.Thermal expansion curve of 3.5wt% Mg-PSZ samples sintered at different temperatures.


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Fig.9. The CTE curve of 3.5wt% Mg-PSZ samples sintered at different temperatures.

Fig.10.The bend strength of 3.5wt% Mg-PSZ with sintering at different temperatures under at room

temperature.