Wikis - An Investigation on Densification of Nanocrystalline 3Y … · significant densification...

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An Investigation on Densification of Nanocrystalline 3Y-TZP Using Master Sintering Curve M. Mazaheri a , F. Golestani-Fard b,* A.Simchi a,c a Department of Materials Science and Engineering, Sharif University of Technology, Tehran, Iran b Department of Materials and Metallurgical Engineering, Iran University of Science and Technology, Tehran, Iran c Institute for Nanoscience and Nanotechnology, Sharif University of Technology Tehran, Iran Abstract The densification of nanocrystalline ZrO 2 -3mol% Y 2 O 3 (3Y-TZP) powder compacts during isothermal and non isothermal sintering was studied. Master sintering curve was developed by non-isothermal sintering of the powder compacts in a sensitive dilatometer at different heating rates. Combined stage sintering model that relates the linear shrinkage rate at any given instant to the grain boundary and volume diffusion (GBD and VD) coefficients, was used. The sintering activation energy of 3Y-TZP nanoscaled aggregate (75 nm) was determined to be Q=485 kJ mol -1 . It is shown that the activation energy is reduced as the particle size decreases. Using the estimated Q-value, the master sintering curve was established and used for prediction of the densification behavior of nanocrystalline 3Y-TZP under different thermal histories. A good agreement between the results of the model and those obtained from isothermal sintering experiments was obtained. Keywords: Nano-sintering; 3Y-TZP; Master sintering curve; Activation energy Introduction Since the innovation of the transformation-toughening phenomenon in tetragonal zirconia ceramics, these materials have gained much interest for industrial applications [1]. Yttria stabilized zirconia ceramics display extremely high values of fracture toughness [2] that makes them suitable for a wide range of structural applications such as cutting tools, valve guides, extrusion dies, abrasive tools, etc. Recently, it has been shown that additional advantages including superplastic deformation and higher hardness are gained when an ultrafine-grained toughened ceramics are utilized [3]. The sintering of nanoparticle ceramics exhibits certain specific features. The high surface area of nanoparticles provides a high driving force for sintering. Therefore, the activation energy of sintering is reduced and low temperature sintering can be afforded [4]. Nevertheless, the ultrafine particles promote grain growth even at low temperatures. Since the grain size significantly contributes in the densification via grain boundary diffusion, grain coarsening retards the consolidation upon sintering and degrades the product properties. Hence, it is essential to select a suitable sintering cycle in order to achieve a high densification rate without profound microstructure coarsening [5]. From the earliest quantitative sintering studies over the past five decades, many models have been derived to relate the sintering rate to the particle characteristics, compactness and sintering atmosphere and temperature. Simplified geometries were considered for the sintering process to readily identify driving forces, mass transport paths, and geometric factors. Attempts made to extend the models for both spherical and non-spherical powders have been of a limited success. Recently, Su and Johnson [6] have proposed the concept of a master sintering curve (MSC) to characterize the sintering behavior of a given powder and green-body regardless of the heating profile. The MSC model enables to predict the densification behavior under arbitrary time- temperature excursions following a minimal set of preliminary experiments. The procedure has been successfully applied to several sintering systems such as ThO 2 [7], ZnO [8], BaTiO 3 [9], nickel and stainless steel [10] powders. In the present work, the isothermal and non-isothermal sintering response of nanocrystalline ZrO 2 -3mol% Y 2 O 3 (3Y-TZP) powder compact was studied. The activation energy of sintering was estimated and the master sintering curve was constructed. The validity of the MSC model was verified by the experimental data. Then, the contour map of sintering for fabrication of the toughened ceramic with a high density was established. Experimental ZrO 2 -3mol% Y 2 O 3 powder was supplied from Tosoh Co. (Tokyo, Japan). Fig. 1 shows a picture of the powder particles taken by high resolution scanning electron microscopy (FEM-SEM GEMINI, ZEISS, Germany). The average aggregate size is 75 nm and the particle shape is polygonal. The density and specific

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An Investigation on Densification of Nanocrystalline 3Y-TZP Using Master Sintering Curve

M. Mazaheria, F. Golestani-Fardb,* A.Simchia,c

aDepartment of Materials Science and Engineering, Sharif University of Technology, Tehran, Iran

bDepartment of Materials and Metallurgical Engineering, Iran University of Science and Technology, Tehran, Iran

cInstitute for Nanoscience and Nanotechnology, Sharif University of Technology Tehran, Iran Abstract The densification of nanocrystalline ZrO2-3mol% Y2O3 (3Y-TZP) powder compacts during isothermal and non isothermal sintering was studied. Master sintering curve was developed by non-isothermal sintering of the powder compacts in a sensitive dilatometer at different heating rates. Combined stage sintering model that relates the linear shrinkage rate at any given instant to the grain boundary and volume diffusion (GBD and VD) coefficients, was used. The sintering activation energy of 3Y-TZP nanoscaled aggregate (75 nm) was determined to be Q=485 kJ mol-1. It is shown that the activation energy is reduced as the particle size decreases. Using the estimated Q-value, the master sintering curve was established and used for prediction of the densification behavior of nanocrystalline 3Y-TZP under different thermal histories. A good agreement between the results of the model and those obtained from isothermal sintering experiments was obtained. Keywords: Nano-sintering; 3Y-TZP; Master sintering curve; Activation energy Introduction Since the innovation of the transformation-toughening phenomenon in tetragonal zirconia ceramics, these materials have gained much interest for industrial applications [1]. Yttria stabilized zirconia ceramics display extremely high values of fracture toughness [2] that makes them suitable for a wide range of structural applications such as cutting tools, valve guides, extrusion dies, abrasive tools, etc. Recently, it has been shown that additional advantages including superplastic deformation and higher hardness are gained when an ultrafine-grained toughened ceramics are utilized [3]. The sintering of nanoparticle ceramics exhibits certain specific features. The high surface area of nanoparticles provides a high driving force for sintering. Therefore, the activation energy of sintering is reduced and low temperature sintering can be afforded [4]. Nevertheless, the ultrafine particles promote grain growth even at low temperatures. Since the grain size significantly

contributes in the densification via grain boundary diffusion, grain coarsening retards the consolidation upon sintering and degrades the product properties. Hence, it is essential to select a suitable sintering cycle in order to achieve a high densification rate without profound microstructure coarsening [5]. From the earliest quantitative sintering studies over the past five decades, many models have been derived to relate the sintering rate to the particle characteristics, compactness and sintering atmosphere and temperature. Simplified geometries were considered for the sintering process to readily identify driving forces, mass transport paths, and geometric factors. Attempts made to extend the models for both spherical and non-spherical powders have been of a limited success. Recently, Su and Johnson [6] have proposed the concept of a master sintering curve (MSC) to characterize the sintering behavior of a given powder and green-body regardless of the heating profile. The MSC model enables to predict the densification behavior under arbitrary time-temperature excursions following a minimal set of preliminary experiments. The procedure has been successfully applied to several sintering systems such as ThO2 [7], ZnO [8], BaTiO3 [9], nickel and stainless steel [10] powders. In the present work, the isothermal and non-isothermal sintering response of nanocrystalline ZrO2-3mol% Y2O3 (3Y-TZP) powder compact was studied. The activation energy of sintering was estimated and the master sintering curve was constructed. The validity of the MSC model was verified by the experimental data. Then, the contour map of sintering for fabrication of the toughened ceramic with a high density was established. Experimental ZrO2-3mol% Y2O3 powder was supplied from Tosoh Co. (Tokyo, Japan). Fig. 1 shows a picture of the powder particles taken by high resolution scanning electron microscopy (FEM-SEM GEMINI, ZEISS, Germany). The average aggregate size is 75 nm and the particle shape is polygonal. The density and specific

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surface area of the powder, as reported by the supplier, is 6.2 g cm-3 and 16±3 m2 g-1, respectively. The powder was compacted in a cylindrical die at 150 MPa to produce green compacts with 12.7 mm diameter and 3 mm height. The green density was 43% of the pore free density. Isothermal sintering was performed in a laboratory furnace at temperature ranging from 1250˚C to 1400 ˚C for various times up to 8 hr. In order to remove the pressing binder, 30 min soaking at 500 ˚C was applied. The heating ramp to the sintering temperature was 5 ˚C min-1 . After cooling down to the ambient temperature with a rate of 10 ˚C min-1, the sintered density was measured by the water displacement (Archimedes) method.

Fig. 1: FEM-SEM micrograph of ZrO2-3 mol% Y2O3 particles. A TMA 801 sinter-dilatometer (NETZCH, Germany) was used to study the non-isothermal sintering behavior of the powder compacts at various heating rates. Small pieces (2 mm diameter) were carefully cut from the green compacts for testing. The heating and cooling cycles were similar to the isothermal sintering, but different heating rates of 2, 5 and 20 ˚C min-1 were utilized. The axial shrinkage of the specimens was measured with an accuracy of ± 0.1 µm. The relative density of the sintered specimen (s) was calculated using the following equation:

gsTTLdL

ρα

ρ3

00 )(/1

1

−+−= (1)

where 0/ LdL is instantaneous liner shrinkage obtained

by the dilatometer test, 0L is the initial length of the

specimen, s is the green density, T0 is the room temperature, and is the coefficient of thermal expansion. Results and Discussion Theory of Master Sintering Curve The master sintering curve is derived from the densification rate equation of combined stage sintering model proposed by Hansen et al. [11]. Sintering is traditionally viewed in terms of three distinct stages (initial, intermediate and final) and most modeling efforts have focused on one of these stages. Hansen et al. [11] proposed a generalized model with a sintering equation of shrinkage rate quantifying sintering as a continuous process from the beginning to the end. The

microstructure is characterized by two separate parameters representing geometry and scale (average grain size). The model relates the linear shrinkage rate of a compact at any given instant to the grain boundary and volume diffusion coefficients, the surface tension and certain aspects of the instantaneous microstructure of the compact as below:

Γ+ΓΩ=− 43 G

D

G

D

kTLdT

dL bbvv δγ (2)

where γ is the surface energy, Ω is the atomic

volume, k is the Boltzmann constant, T is the absolute temperature, G is the mean grain size,

vD is the

coefficient for volume diffusion, and bD is the

coefficient for grain boundary diffusion. bΓ and

vΓ are

represent all microstructural scaling parameters for grain boundary and volume diffusion, respectively. Eq. (2) can be rearranged and integrated to obtain two separate functions of ))(,( tTtΘ and )(ρΦ as follows:

ρρρ

ργ

ρρ

ρ

dG

D

k n

ΓΩ≡Φ

0)(3

))(()(

0

(3)

and

dtRT

Q

TtTt

t

)exp(1

))(,(0

−≡Θ (4)

)(ρΦ is related to the microstructural evolution and

))(,( tTtΘ is a function of the thermal history. The

power n represents the dominant mechanism, i.e. volume diffusion (n=3) and grain boundary diffusion (n=4), of the sintering response. The relationship between density () and -function is defined as the master sintering curve. If the sintering activation energy (Q) of a given powder compact is known, MSC can be constructed by using -function (Eq. 4). Consequently, the contour map of sintering as a function of thermal history can be established. In order to estimate the activation energy, a trial and error procedure based on the data of non-isothermal sintering is commonly used. Non-isothermal sintering Figure 2 shows strain of 3Y-TZP compact during non-isothermal sintering at three different heating rates (HRs) of 2, 5 and 20 ˚C min-1.

700 800 900 1000 1100 1200 1300 1400-28

-24

-20

-16

-12

-8

-4

0

20 oC min-1

5 oC min-1

2 oCmin-1

Temperature, οC

ε, %

(a)

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Fig. 2: Effect of heating rate on the strain of 3Y-TZP compact as a function of temperature. The comparative data of the sintering response is summarized in Table 1. Apparently, increasing the heating rate increased the temperatures at which sintering starts (0.5% shrinkage) and proceeds. For instance, 0.5% shrinkage occurred at 952 and 980 °C when HRs increased from 2 and 20 ˚C min-1. All compacts exhibited dramatic shrinkage rate at temperatures beyond 1050 °C. The maximum shrinkage rate was found to be dependent on HR; it increased from 1211 to 1277 °C as the hearing rate increases from 2 to 20 ˚C min-1. Nevertheless, the analysis of the amount of shrinkage at the temperature of maximum shrinkage rate (Table 1) indicates that at an approximate density of 0.72, the maximum densification strain rate occurred for each heating rates. Similar results were reported by Long [12] for sintered Al2O3 compacts. This would suggest that sintering kinetics dominated the densification process up to a relative density of ca. 0.75, after which coarsening kinetics dominated [12]. Table 1: Dilatometry data for 3Y-TZP powder compact sintered at different heating rates

Heating rate [˚C min-1]

T0.5 [˚C]

Tmax

[˚C] max [%]

f

[%]

2 952 1211 14.3 23.6

5 961 1255 16.6 21.6

20 980 1288 17 18.7 T0.5: Temperature at 0.5% shrinkage Tmax: Temperature at maximum shrinkage rate max: Amount of shrinkage at Tmax

f: Amount of shrinkage at 1300 °C

Fig. 3 shows the effect of heating rate on the sintered density as a function of temperature. The density-temperature curves exhibit the familiar sigmoidal shape and generally are shifted to higher temperatures with increasing the heating rate. It can be noted that the achieved sintered densities at any temperature showed a modest but systematic dependence on the heating rate.

1050 1100 1150 1200 1250 1300 1350

0.5

0.6

0.7

0.8

0.9

1.0

5 oC min-1

20 oC min-1

Fra

ctio

nal d

ensi

ty

Temperature, oC

2 oC min-1

Fig. 3: Density of 3Y-TZP compacts as a function of temperature during non-isothermal sintering at different heating rates. Isothermal sintering The density of fired specimens at temperatures ranging from 1250 to 1400 °C at various holding times is shown in Fig. 4. The results indicate that significant densification (more than 55% for heating up to 1400 °C) occurred during heat-up. For instance, the density increased from 0.42 to 0.72 during heating up to 1250 °C. A greater fraction of densification took place upon heating at higher temperatures, e.g., high density of 0.95 was obtained at 1350 °C. This is in agreement with the data of non-isothermal sintering which showed the importance of heating rate on the shrinkage. In fact, if significant densification occurs during heating, changes in the heating segment would influence microstructural development (grain coarsening) in the nanocrystalline ceramic. Hence, the sintering kinetics is affected.

0.0 0.5 1.0 1.5 2.0 2.50.65

0.70

0.75

0.80

0.85

0.90

0.95

1.001400 oC

1300 oC

1250 oC

1350 oC

Fra

ctio

nal d

ensi

ty

Holding time, Hr

Fig. 4: Fractional density of 3Y-TZP compacts sintered at different temperatures as a function of holding time. Heating rate of 5 ˚C min-1 was applied. Master sintering curve One of the essential data for obtaining MSC is the activation energy (Q). In the present work, various Q-values in the range of 300 to 800 kJ mol-1 are taken into account and the s- diagram for a given Q-value was constructed according to Eq. (4). The resulting curves for different heating rates had to be converged; otherwise, new activation energy was chosen and the calculation was repeated. Fig. 5 shows the mean residual squares of error for the various values of activation energy. The minimum error has been obtained at Q=485 kJ mole-1.

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200 300 400 500 600 700 800 900

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

M

ean

resi

dual

squ

aers

of e

rror

Sintering activation energy, kJ mole-1

485 kJ mole-1

Fig. 5: Mean residual squares of error for the various values of activation energy. Shojai et al. [13] have reported the activation energy of 735 kJ mole-1 for 3Y-TZP with an average particle size of 790 nm. Maca et al. [4] have shown that the activation energy decreases from 550 to 237 kJ mol-1

when utilizing 9 nm particles instead of 152 nm. Therefore, the result of the present work is in reasonable agreement with the reported values by others; the sintering activation energy of 3Y-TZP ceramic is reduced with decreasing the particles size (Fig. 6). In 3Y-TZP nanoceramic, significantly enhanced volume diffusion (volume diffusion activation energy of 3Y-TZP is 423 kJ mol-1 [16]) is observed over the intermediate and final stage of sintering.

Fig. 6: Effect of 3Y-TZP particle size on the activation energy of sintering. Fig. 7 shows the master sintering curve for 3Y-TZP powder compact. The obtained curve illustrates the general trend of slow density increase at the onset of sintering and steep increase in the intermediate density range. In order to show the validity of the model, the isothermal sintering cycles described in the previous section were modeled by the master sintering curve. Both densification during heat-up and holding were considered.

Fig. 7: Master sintering curve for nanocrystalline 3Y-TZP powder compact. The results are compared with MSC in Fig. 8. A convincing agreement is evidently seen. Therefore, the constructed MSC is a characteristic measure of the sinterability of the nanocrystalline 3Y-TZP powder over a wide range of density. Fig. 9 shows the iso-density contour map in time-temperature diagram constructed by the model at heating rate of 5 ˚C min-1. One can use this diagram as a helpful tool to design the proper heating profile to reach the desired density.

Fig. 8: Validation of master sintering curve for nanocrystalline 3Y-TZP powder compact. The experimental values for twelve isothermal sintering cycles are shown.

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1100 1150 1200 12500

10

20

30

40

50

0.95

0.9

0.85

0.8

0.75

0.7

0.65

T

ime,

ks

Temperature, oC

0.6

Fig. 9: Iso-density counters for nanocrystalline 3Y-TZP powder compacts in time-temperature sintering diagram. The heating rate is taken as 5 ˚C min-1. Conclusions In this work, the master sintering curve has been applied successfully to nanocrystalline 3Y-TZP powder compact. The ability to predict and control the sintering of nanocrystalline powder was demonstrated by isothermal sintering of the compacts at different heating cycles. The sintering activation energy was determined to be 485 kJ mol-1, which is significantly lower than that of conventional (submicron) 3Y-TZP powder compacts. This difference is attributed to a scaling effect, i.e. the influence of particle size (surface area) on the sintering driving force. A direct relationship between the particle size and sintering activation energy was noticed. The MSC model was used to predict the densification of nanocrystalline 3Y-TZP powder compact under arbitrary time-temperature excursion. Consistent results with the isothermal sintering experiment were obtained. References 1. R.C. Garvie, R.H.J. Hannink, R.T. Pascoe, Ceramic Steel. Nature 258 (1975), 703-704. 2. B.A. Cottom, M.J. Mayo, Fracture Toughness of Nanocrystalline ZrO2 – 3mol% Y2O3 Determined by Vickers Indentation. Scripta Mat. 34 (1996), 809-814. 3. F. Wakai, S. Sakaguchi, Y. Matsuno, Superplasticity of Yttria-Stabilized Tetragonal Zro2 Polycrystals. Adv. Ceram. Mat. 1 (1986), 259-263. 4. K. Maca, M. Trunec, P. Dobask, Bulk Zirconia Nanoceramics Prepared by Cold Isostatic Pressing and Pressureless Sintering. Review Advanced Material Science 10 (2005), 84-88. 5. X.-H. Wang, P.-L. Chen, I.-W. Chen, Two-Step Sintering of Ceramics with Constant Grain-Size, I. Y2O3. J. Am. Ceram. Soc. 89 (2006), 431-437. 6. H. Su, D.L. Johnson, Master Sintering Curve: A Practical Approach to Sintering. J. Am. Ceram. Soc. 79 (1996), 3211-3217. 7. T.R.G. Kutty, K.B. Khan, P.V. Hegde, J. Banerjee, A.K. Sengupta, S. Majumdar, H.S. Kamath,

Development of a Master Sintering Curve for ThO2. J. Nuclear Mat. 327 (2004), 211-219. 8. K.G. Ewsuk, D.T. Ellerby, B. DiAntonio, Analysis of Nanocrystalline and Microcrystalline ZnO Sintering Using Master Sintering Curve. . J. Am. Ceram. Soc. 89 (2006), 2003-2009. 9. M.V. Nicolic, V.P. Pavlovic, V.B. Pavlovic, N. Labus, B. Stajanovic, Application of the Master Sintering Curve Theory to Non-Isothermal Sintering of BaTiO3 Ceramics. Mat. Sci. Forum 494 (2005), 717-722. 10. D.C. Blaine, S.J. Park, P. Suri, R.M. German, Application of Work-of-Sintering Concepts in Powder Metals. Metal. and Mat. Trans. 37A (2006), 2827-2832. 11. J. D. Hansen, R. P. Rusian, M. H. Teng, D. L. Johnson, Combined Stage Sintering Model. J. Am. Ceram. Soc. 75 (1992), 1129-1135. 12. F.F. Long, Approach to Reliable Powder Processing. Ceram. Trans. 1 (1989), 1069-1083. 13. F. Shojai, T. A. Mantyla, Effect of Sintering Temperature and Holding Time on the Properties of 3Y-ZrO2 microfiltration members. J. of Mat. Sci. 36 (2001), 1-10. 14. P. Duran, M. Villegas, F. Capel, C. Moure, Low-Temperature Sintering and Microstructure Development of nanocrystalline Y-TZP Powder. J. Eur. Ceram. Soc. 16 (1996), 945-952. 15. M.-H. Teng, Y.-C. Lai, Y.-T. Chen, A Computer Program of Master Sintering Curve Model to Accurately Predict Sintering Results. Western Pacific Earth Sci. 2 (2002) 171-180. 16. R. M. German, Sintering Theory and Practice, MPIF, Princeton, NJ, 1996, 534.