**4. Results and discussion**

Conventional sintering experiments were carried out at temperature between 900 and 1500oC in air with 2 h dwell time. This sample also was sintered in a Netzsch – DIL 402C dilatometer at 15oC/min constant heating rate in air atmosphere. Based on these results, steps for the sintering were defined. The sintering process was performed in electric furnace (Model Lindberg) in the presence of MoSi2 heating elements in air atmosphere.

In addition to thermal analysis by dilatometry, sintered samples were further characterized by the apparent density taken the Archimedes method as reference, grain size measurements using an image analysis program, and the microstructure was analyzed by scanning electronic microscopy (SEM).

Figure 1 shows the linear shrinkage rate versus temperature during heating in dilatometer at15°C/min constant heating rate. Sintering shrinkage started at 1030oC and maximum shrinkage rate occurred at 1345oC. In figure 1, two different areas can be defined: the first area beginning between 900oC and 1000oC and until approximately 1080oC and refers to the temperature range before sample shrinkage beginning. As shrinkage is directly related to the densification of ceramic body during the sintering process, sintered samples did not begin the densification at temperatures lower than 1080oC having a rearrangement

Mechanisms of Microstructure Control in Conventional Sintering 409

900 1000 1100 1200 1300 1400 1500

Fig. 2. Density and grain size of Al2O3 compacts after sintering at various temperatures for 1

In the first sintering heating curve one hypothesis was assumed: the maximization of final density with minimum grain growth could be achieved by improving the narrowing of grain size distribution at a pre-densification sintering stage and producing the final densification at a maximum densification rate. To confirm this hypothesis, a temperature below the onset of the densification process was chosen. This effect can be observed in samples with the first step at 1050ºC. Samples produced by the first step at 1050ºC followed by a second step at 1500°C showed significantly smaller final grain sizes as shown in figure

(a) (b) Fig. 3. SEM micrographs of alumina samples after two-step sintering: a) T=1500ºC/2 h; and

Figure 4 shows micrographs of surface fracture of alumina compacts, one heated at 1050oC and cooled immediately upon reaching that temperature; and the other heated to the same

oC)

Temperature (

Density

Grain size

0

500

1000

Grain size (nm)

1500

2000

60

70

80

Relative Density (%)

hour.

3 (Chinelatto et al., 2010).

b) T=1050ºC/2 h and T=1500ºC/2 h.

90

100

process, coarsening of the particles and appearance of contact points among particles. The second area can be defined as the one where shrinkage occurs, from approximately 1080C to 1500oC. In this area, shrinkage rate reaches the maximum value at approximately 1350ºC.

The sintering temperature effect on the densification and grain growth of compacts sintered at temperature ranging between 900 and 1500oC for 1 hour is shown in figure 2. No significant densification was observed below 1030oC confirming the dilatometric results (figure 1). Densification was accelerated at the temperature between 1100oC and 1350oC, without, however, presenting great grain growth. At higher temperature, densification was minimal but the grain growth was fast. Final grain size of the nearly fully dense structure was higher than 1800 nm. While relative density increased from 95% to 99.2% with increase in temperature from 1300oC to 1500oC, the average grain size became coarser from 480 nm to 1800 nm; in other words, there was more than 250% increase in grain size.

Fig. 1. Linear shrinkage rate versus temperature during heating in dilatometer at 15°C/min constant heating rate.

It has reported that dispersed open pores can pin grain boundaries and hinder grainboundary migration in the second stage of sintering, for which the grain growth is suppressed (German, 1996). In contrast, a very sharp ascending of grain size is observable in the final sintering stage (relative density above 90% TD); however, there is remarkable increase in density. It has been confirmed that open pores referring to the intermediate stage of sintering collapse to form the closed ones after the final stage starts. Such a collapse results in a substantial decrease in pore pinning, which triggers the accelerated grain growth.

Considering the results of sintering experiments (figures 1 and 2) and to suppress the accelerated grain growth at the final sintering stage, two different sintering heating curves were applied to produce densification of Al2O3 compacts. These experiments were carried out using 15oC/min heating rate.

process, coarsening of the particles and appearance of contact points among particles. The second area can be defined as the one where shrinkage occurs, from approximately 1080C to 1500oC. In this area, shrinkage rate reaches the maximum value at

The sintering temperature effect on the densification and grain growth of compacts sintered at temperature ranging between 900 and 1500oC for 1 hour is shown in figure 2. No significant densification was observed below 1030oC confirming the dilatometric results (figure 1). Densification was accelerated at the temperature between 1100oC and 1350oC, without, however, presenting great grain growth. At higher temperature, densification was minimal but the grain growth was fast. Final grain size of the nearly fully dense structure was higher than 1800 nm. While relative density increased from 95% to 99.2% with increase in temperature from 1300oC to 1500oC, the average grain size became coarser from 480 nm to

200 400 600 800 1000 1200 1400

alumina **T=1345o**

**T=1030o C**

**C**

**T( o C)**

Fig. 1. Linear shrinkage rate versus temperature during heating in dilatometer at 15°C/min

It has reported that dispersed open pores can pin grain boundaries and hinder grainboundary migration in the second stage of sintering, for which the grain growth is suppressed (German, 1996). In contrast, a very sharp ascending of grain size is observable in the final sintering stage (relative density above 90% TD); however, there is remarkable increase in density. It has been confirmed that open pores referring to the intermediate stage of sintering collapse to form the closed ones after the final stage starts. Such a collapse results in a substantial decrease in pore pinning, which triggers the accelerated grain

Considering the results of sintering experiments (figures 1 and 2) and to suppress the accelerated grain growth at the final sintering stage, two different sintering heating curves were applied to produce densification of Al2O3 compacts. These experiments were carried

1800 nm; in other words, there was more than 250% increase in grain size.


constant heating rate.

out using 15oC/min heating rate.

growth.




**dL/Lodt(%/min)**


0,0

approximately 1350ºC.

Fig. 2. Density and grain size of Al2O3 compacts after sintering at various temperatures for 1 hour.

In the first sintering heating curve one hypothesis was assumed: the maximization of final density with minimum grain growth could be achieved by improving the narrowing of grain size distribution at a pre-densification sintering stage and producing the final densification at a maximum densification rate. To confirm this hypothesis, a temperature below the onset of the densification process was chosen. This effect can be observed in samples with the first step at 1050ºC. Samples produced by the first step at 1050ºC followed by a second step at 1500°C showed significantly smaller final grain sizes as shown in figure 3 (Chinelatto et al., 2010).

Fig. 3. SEM micrographs of alumina samples after two-step sintering: a) T=1500ºC/2 h; and b) T=1050ºC/2 h and T=1500ºC/2 h.

Figure 4 shows micrographs of surface fracture of alumina compacts, one heated at 1050oC and cooled immediately upon reaching that temperature; and the other heated to the same

Mechanisms of Microstructure Control in Conventional Sintering 411

(%TD)

3456789

time (h)

Mean Grain Size (nm)

> 1050<sup>o</sup> C

mean grain size (nm)

**Sintering procedure** Relative Density

TSS1 - T1=1000oC/3h and T2=1350oC/3h 93.8 797.4 TSS2 - T1=1000oC/6h and T2=1350oC/3h 94.2 763.5 TSS3 - T1=1000oC/9h and T2=1350oC/3h 94.6 683.5 TSS4 - T1=1050oC/3h and T2=1350oC/3h 93.9 717.1 TSS5- T1=1050oC/6h and T2=1350oC/3h 94.1 685.1 TSS6 - T1=1050oC/9h and T2=1350oC/3h 94.2 659.3

Table 2. **Sintering procedure and results of relative density (%TD) and mean grain size of** 

mean grain size (nm)

relative densiity% TD

(a) (b) Fig. 5. Relative density and mean grain size of alumina compacts sintered versus holding

According to Lin and Dejonghe (Lin & DeJonghe, 1997a, 1997b), with the steps at low temperature, the onset of densification is delayed due to the elimination of the finest particles (and smallest pores associated with them) during the first step. The local densification associated with the finest particles in the conventional sintering is significantly reduced in compacts subjected to the first step. Thus, removal of the finest particles due to the first step reduces the differential densification and formation of densest regions in the early sintering stages. This fact causes reduction in density fluctuations in the compact and

The other two-step sintering is based on works of Chen and Wang (Chen &Wang, 2000), in which samples are first heated to a higher temperature to achieve intermediate density, and then cooled down and kept at lower temperature until they are dense. A pre-requisite for successful densification during the second step of sintering is that pores become subcritical

Chen and Wang (Chen &Wang, 2000) have explained that to achieve densification without grain growth, grain-boundary diffusion needs to remain active, while the grain-boundary

93,94 93,96 93,98 94,00 94,02 94,04 94,06

**alumina samples.** 

93,8 93,9 94,0 94,1 94,2 94,3 94,4 94,5 94,6 94,7

relative density (% TD)

3456789

1000o C

time (h)

promotes a more homogeneous final microstructure.

time at: (a) 1000oC and (b) 1050oC.

and unstable against shrinkage.

temperature and kept at such temperature for 2 hours. The heat treatment made finest particles to disappear and slightly coarsened the coarsest particles, decreasing the specific surface area and slightly increasing the mean grain size as indicated in Table 1. De Jonghe et al. (Lin & DeJonghe, 1997a, 1997b) suggested that during the first step, coarsening of the microstructure by surface diffusion, vapor transport, or some combination of these mechanisms produces a more uniform microstructure by an Ostwald ripening process. The evolution to a more homogeneous microstructure can be expected from the trend of porous system to evolve towards a quasi-steady state structure. Such steady-state structural distributions are generally significantly narrower than that usually produced in a powder compact (Chu et al., 1991).

Fig. 4. SEM fracture surfaces of Al2O3 compacts: (a) heated at 1050oC and cooled immediately e (b) heated at 1050oC for 2 hours.


#### Table 1. **Superficial area and mean grain size of particles.**

Other heating curves were developed applying sintering curves coherent with the temperature ranges in which the two processes, i.e., narrowing grain size distribution and final densification, were expected to occur. The following conditions were defined for the sintering heating curves: the first step for alumina was at 1050°C and 1000oC and the second step was at the maximum sintering temperature of 1350°C. Table 2 describes the sintering conditions and findings regarding density and average grain size of samples produced in the two-step sintering experiments.

Changes in the relative density and mean grain size with the holding time obtained are shown in Figure 5. Increased holding time results in increase of relative density and decrease in mean grain size.


temperature and kept at such temperature for 2 hours. The heat treatment made finest particles to disappear and slightly coarsened the coarsest particles, decreasing the specific surface area and slightly increasing the mean grain size as indicated in Table 1. De Jonghe et al. (Lin & DeJonghe, 1997a, 1997b) suggested that during the first step, coarsening of the microstructure by surface diffusion, vapor transport, or some combination of these mechanisms produces a more uniform microstructure by an Ostwald ripening process. The evolution to a more homogeneous microstructure can be expected from the trend of porous system to evolve towards a quasi-steady state structure. Such steady-state structural distributions are generally significantly narrower than that usually produced in a powder

(a) (b)

1050oC 1050oC/2 hours

Fig. 4. SEM fracture surfaces of Al2O3 compacts: (a) heated at 1050oC and cooled

Superficial area (m2/g) 12.3 10.5

Mean grain size (nm) 119 ± 33 133 ± 28

Other heating curves were developed applying sintering curves coherent with the temperature ranges in which the two processes, i.e., narrowing grain size distribution and final densification, were expected to occur. The following conditions were defined for the sintering heating curves: the first step for alumina was at 1050°C and 1000oC and the second step was at the maximum sintering temperature of 1350°C. Table 2 describes the sintering conditions and findings regarding density and average grain size of samples produced in

Changes in the relative density and mean grain size with the holding time obtained are shown in Figure 5. Increased holding time results in increase of relative density and

immediately e (b) heated at 1050oC for 2 hours.

the two-step sintering experiments.

decrease in mean grain size.

Table 1. **Superficial area and mean grain size of particles.** 

compact (Chu et al., 1991).


Table 2. **Sintering procedure and results of relative density (%TD) and mean grain size of alumina samples.** 

Fig. 5. Relative density and mean grain size of alumina compacts sintered versus holding time at: (a) 1000oC and (b) 1050oC.

According to Lin and Dejonghe (Lin & DeJonghe, 1997a, 1997b), with the steps at low temperature, the onset of densification is delayed due to the elimination of the finest particles (and smallest pores associated with them) during the first step. The local densification associated with the finest particles in the conventional sintering is significantly reduced in compacts subjected to the first step. Thus, removal of the finest particles due to the first step reduces the differential densification and formation of densest regions in the early sintering stages. This fact causes reduction in density fluctuations in the compact and promotes a more homogeneous final microstructure.

The other two-step sintering is based on works of Chen and Wang (Chen &Wang, 2000), in which samples are first heated to a higher temperature to achieve intermediate density, and then cooled down and kept at lower temperature until they are dense. A pre-requisite for successful densification during the second step of sintering is that pores become subcritical and unstable against shrinkage.

Chen and Wang (Chen &Wang, 2000) have explained that to achieve densification without grain growth, grain-boundary diffusion needs to remain active, while the grain-boundary

Mechanisms of Microstructure Control in Conventional Sintering 413

To choose the temperature for the second step T2, it is necessary to choose a temperature in which volume diffusion or grain boundary diffusion operate while the grain boundary movement is restricted (Mazaheri et al., 2008). Therefore, the second step temperatures were 1260oC and 1300oC. Sintering conditions and results of density relative and mean grain size

**(%TD)** 

**Mean grain size (nm)** 

**Sintering procedure Relative density** 

TSS7 - T1=1400oC and T2=1260oC/3h 91.0 518.8

TSS8 - T1=1400oC and T2= 1260oC/6h 92.1 579.2

TSS9 - T1=1400oC and T2= 1260oC/9h 93.1 647.8

TSS10 - T1=1400oC and T2= 1300oC/3h 95.9 668.5

TSS11 - T1=1400oC and T2= 1300oC/6h 96.5 692.5

TSS-12 - T1=1400oC and T2= 1300oC/9h 96.6 718.7 Table 3. Sintering procedure and results of relative density (%TD) and mean grain size of

According to the results of the second step of TSS10, TSS11 and TSS12, holding the samples at 1300oC resulted in accentuate densification. Diffusive mechanisms that seem to be time dependent are therefore active at this stage. Grain-boundary diffusion and volumetric diffusion are possibly responsible for the shrinkage of the samples. On the other hand, TSS7, TSS8 and TSS9 do not lead to a dense structure, showing the inactivity of the grainboundary diffusion at 1260oC. Considering all these facts, one can infer that 1300oC is the minimum temperature after which the grain boundary diffusion mechanism dominates.

Due to the relatively low temperature of the second stage (1260oC), densification stops before reaching a fully dense sample. A similar trend has also been reported for the twostep sintering behavior of Y2O3 (Wang and Chen, 2006) and ZnO (Mazaheri et. al., 2008)

Fig. 7. SEM micrograph of alumina sintered at 1400oC.

are presented in table 3.

alumina samples.

migration must to be suppressed. A mechanism to inhibit grain-boundary movement is a triple-point (junction) drag. Consequently, to prevent accelerated grain growth, it is essential to decrease grain-boundary mobility. The grain growth entails a competition between grain-boundary mobility and junction mobility. Once the latter becomes less at low temperatures in which junctions are rather motionless, the mentioned drag would occur. Therefore, the grain growth is prohibited. Network mobility follows the grain-boundary mobility at high temperatures. At low temperatures, junction mobility dominates. Below the temperature at which the two rates become equal, junction mobility is essentially reduced despite the considerable grain-boundary diffusion.

Figure 2 shows that grain growth is most intense at temperatures above 1400oC. Since samples conditions after the first stage affect the second stage of sintering, grain growth resulting from heating in the first stage must be avoided. Thus, the temperature chosen for the first stage of sintering was 1400 °C.

Figure 6 shows the behavior of relative density (%TD) versus temperature for sintering at constant heating rate of alumina. Density of alumina when temperature reaches 1400°C is 81% TD. The relative density during sintering was determined from green density (dv) and measured shrinkage (ΔL/Lo), using the approximate Eq. (1), assuming that deformation is isotropic and all axial strain is devoted to specimen's densification (Ray, 1985).

$$d\_i = \frac{d\_v}{(1 + \frac{\Delta L}{L\_o})^3} \tag{1}$$

Fig. 6. Variation of relative density (% TD) versus temperature for alumina sintered at 15°C/min until the temperature of 1500 °C.

SEM micrograph of alumina when it reaches 1400oC in the first step of sintering is showed in Figure 7. The mean grain size of alumina in this condition is about 330 nm.

migration must to be suppressed. A mechanism to inhibit grain-boundary movement is a triple-point (junction) drag. Consequently, to prevent accelerated grain growth, it is essential to decrease grain-boundary mobility. The grain growth entails a competition between grain-boundary mobility and junction mobility. Once the latter becomes less at low temperatures in which junctions are rather motionless, the mentioned drag would occur. Therefore, the grain growth is prohibited. Network mobility follows the grain-boundary mobility at high temperatures. At low temperatures, junction mobility dominates. Below the temperature at which the two rates become equal, junction mobility is essentially reduced

Figure 2 shows that grain growth is most intense at temperatures above 1400oC. Since samples conditions after the first stage affect the second stage of sintering, grain growth resulting from heating in the first stage must be avoided. Thus, the temperature chosen for

Figure 6 shows the behavior of relative density (%TD) versus temperature for sintering at constant heating rate of alumina. Density of alumina when temperature reaches 1400°C is 81% TD. The relative density during sintering was determined from green density (dv) and measured shrinkage (ΔL/Lo), using the approximate Eq. (1), assuming that deformation is

> <sup>3</sup> (1 ) *<sup>v</sup> <sup>i</sup>*

200 400 600 800 1000 1200 1400

temperature (

SEM micrograph of alumina when it reaches 1400oC in the first step of sintering is showed

Fig. 6. Variation of relative density (% TD) versus temperature for alumina sintered at

in Figure 7. The mean grain size of alumina in this condition is about 330 nm.

oC)

*<sup>d</sup> <sup>d</sup> <sup>L</sup>*

<sup>=</sup> <sup>Δ</sup> +

*o*

(1)

81%TD

*L*

isotropic and all axial strain is devoted to specimen's densification (Ray, 1985).

despite the considerable grain-boundary diffusion.

the first stage of sintering was 1400 °C.

15°C/min until the temperature of 1500 °C.

relative density (% TD)

Fig. 7. SEM micrograph of alumina sintered at 1400oC.

To choose the temperature for the second step T2, it is necessary to choose a temperature in which volume diffusion or grain boundary diffusion operate while the grain boundary movement is restricted (Mazaheri et al., 2008). Therefore, the second step temperatures were 1260oC and 1300oC. Sintering conditions and results of density relative and mean grain size are presented in table 3.


Table 3. Sintering procedure and results of relative density (%TD) and mean grain size of alumina samples.

According to the results of the second step of TSS10, TSS11 and TSS12, holding the samples at 1300oC resulted in accentuate densification. Diffusive mechanisms that seem to be time dependent are therefore active at this stage. Grain-boundary diffusion and volumetric diffusion are possibly responsible for the shrinkage of the samples. On the other hand, TSS7, TSS8 and TSS9 do not lead to a dense structure, showing the inactivity of the grainboundary diffusion at 1260oC. Considering all these facts, one can infer that 1300oC is the minimum temperature after which the grain boundary diffusion mechanism dominates.

Due to the relatively low temperature of the second stage (1260oC), densification stops before reaching a fully dense sample. A similar trend has also been reported for the twostep sintering behavior of Y2O3 (Wang and Chen, 2006) and ZnO (Mazaheri et. al., 2008)

Mechanisms of Microstructure Control in Conventional Sintering 415

(a) (b)

(c)

1210<sup>o</sup> C

> 1440o C

200 400 600 800 1000 1200 1400

Temperature (<sup>o</sup>

Fig. 10. Linear shrinkage rate versus temperature during heating in dilatometer at 15°C/min

C)

Fig. 9. SEM micrographs (a) TSS3; (b) TSS12 and (c) CS.


constant heating rate for alumina-zirconia compacts.

alumina-zirconia




dL/LodT



0,00

0,01

confirming that the reason for exhaustion in the second stage of densification is attributed to low temperature which retards grain-boundary diffusion as the sintering mechanism.

It can be seen in figure 8 that density variation results in increased grain size of the sample, showing that this condition is not yet the ideal to control the grain size in two-step sintering.

Fig. 8. Grain size/relative density trajectory obtained by two-step sintering T1=1400oC and T2=1300oC.

On the other hand, comparing the two-step sintering with conventional sintering, it is observed that the two-step sintering is efficient to control the grain growth. Figure 9 shows the micrographs of alumina sintered at 1500oC for 2 hours and sintered at TSS3 and TSS12 conditions.

The heating curve control, through using steps of sintering, associated with control of grain size by addition of nanometric zirconia inclusions is also control the microstrucuture in conventional sintering.

Figure 10 shows the linear shrinkage rate as function of temperature for alumina-5%vol zirconia at 15oC/min constant heating rate and 1500oC. The presence of zirconia particles increases the maximum densification rate temperature; for alumina this temperature is 1350oC (figure 1) and for alumina-zirconia the temperature is increased for 1440oC. The temperature at the beginning of shrinkage process is also altered from 1030 to 1210oC with the addition of zirconia particles. Zirconia inclusions hinder the movement of grain boundary, reducing the densification rate and grain growth (Hori et al., 1985; Liu et al., 1998; Stearns & Harmer, 1996). In the figure 11 (a) and (b), that shows the micrographs of samples of alumina-zirconia and alumina, respectively, sintered at 1500oC for 2h, the influence of zirconia on microstructure evolution is noted through observing the grain growth behavior. The addition of nanometric zirconia is very efficient to promote a controlled grain growth. The inhibitive trend is due to the pinning effect which is associated with locations of small zirconia particles at grain boundaries or triple junctions of alumina.

confirming that the reason for exhaustion in the second stage of densification is attributed to low temperature which retards grain-boundary diffusion as the sintering mechanism.

It can be seen in figure 8 that density variation results in increased grain size of the sample, showing that this condition is not yet the ideal to control the grain size in two-step sintering.

> =1300o C

C and T2

TSS10

95,6 95,8 96,0 96,2 96,4 96,6 96,8 97,0 97,2

TSS11

TSS12

relative density (% TD)

Fig. 8. Grain size/relative density trajectory obtained by two-step sintering T1=1400oC and

On the other hand, comparing the two-step sintering with conventional sintering, it is observed that the two-step sintering is efficient to control the grain growth. Figure 9 shows the micrographs of alumina sintered at 1500oC for 2 hours and sintered at TSS3 and TSS12

The heating curve control, through using steps of sintering, associated with control of grain size by addition of nanometric zirconia inclusions is also control the microstrucuture in

Figure 10 shows the linear shrinkage rate as function of temperature for alumina-5%vol zirconia at 15oC/min constant heating rate and 1500oC. The presence of zirconia particles increases the maximum densification rate temperature; for alumina this temperature is 1350oC (figure 1) and for alumina-zirconia the temperature is increased for 1440oC. The temperature at the beginning of shrinkage process is also altered from 1030 to 1210oC with the addition of zirconia particles. Zirconia inclusions hinder the movement of grain boundary, reducing the densification rate and grain growth (Hori et al., 1985; Liu et al., 1998; Stearns & Harmer, 1996). In the figure 11 (a) and (b), that shows the micrographs of samples of alumina-zirconia and alumina, respectively, sintered at 1500oC for 2h, the influence of zirconia on microstructure evolution is noted through observing the grain growth behavior. The addition of nanometric zirconia is very efficient to promote a controlled grain growth. The inhibitive trend is due to the pinning effect which is associated with locations of small

zirconia particles at grain boundaries or triple junctions of alumina.

660

680

mean grain size (nm)

T2=1300oC.

conditions.

conventional sintering.

700

720

T1 =1400<sup>o</sup>

Fig. 9. SEM micrographs (a) TSS3; (b) TSS12 and (c) CS.

Fig. 10. Linear shrinkage rate versus temperature during heating in dilatometer at 15°C/min constant heating rate for alumina-zirconia compacts.

Mechanisms of Microstructure Control in Conventional Sintering 417

respectively. These micrographs confirmed that the two-step sintering used have been efficient to the sintering process control. It can be observed that the sample conventionally sintered presents larger grain size. Finally, it was observed that the step of sintering with

(a) (b)

(a) (b)

(c) Fig. 13. SEM image of the alumina-zirconia sintered under conditions: (a) CS; (b) TSS13;

Fig. 12. SEM micrographs of sintered samples: (a) T1=1460oC and cooled and (b) TSS13

addition of inclusions is also efficient in grain growth.

condition.

(c) TSS14.

Fig. 11. SEM images of sample sintered at 1500oC for 2 h: (a) alumina and (b) aluminazirconia.

The heating curve control, combined with the presence of nanoparticles inclusions can further optimize the microstructure control. Table 4 shows the sintering procedure and results of relative density (%TD) and mean grain size for alumina-zirconia samples. Results for TSS13 and TSS14 conditions show that the two-step sintering promoted reduction of the mean grain size compared to the conventional sintering (CS1) (Manosso et al., 2010).


Table 4. Sintering procedure and results of relative density (%TD) and mean grain size of alumina-zirconia samples.

The microstructure of the sample heated at 1460oC and cooled immediately and the sample sintered under TSS13 conditions are showed in Figure 12. It can be noticed an initial densification and 77%DT relative density for this sample. When the sample was heated at 1460oC and cooled down to 1350oC (TSS13 condition) the sample could be densified without grain growth (see table 2). It means that, 77% DT reached density in the first step at high temperature for this sample can be considered the critical density. In spite of the smaller grain size presented by TSS1 condition, its relative density was lower than densities of TSS14 and CS conditions. It suggests that the time of soaking in the second step can still be prolonged. Many studies (Tarjat & Trajat, 2009; Mazaheri et al., 2008) have been demonstrated that long times in the second steps allowed the total densification without grain growth.

In the TSS14 condition, pre-densification sintering stage at 1300oC for 2 hours was effective in grain growth control and final densification. Figure 13 (a), (b) and (c) presents the microstructure of alumina-zirconia sintered under CS1, TSS13 and TSS14 conditions,

(a) (b)

The heating curve control, combined with the presence of nanoparticles inclusions can further optimize the microstructure control. Table 4 shows the sintering procedure and results of relative density (%TD) and mean grain size for alumina-zirconia samples. Results for TSS13 and TSS14 conditions show that the two-step sintering promoted reduction of the

CS – T=1500oC/2h 99.0 550

**(%TD)** 

**Mean grain size (nm)** 

Fig. 11. SEM images of sample sintered at 1500oC for 2 h: (a) alumina and (b) alumina-

mean grain size compared to the conventional sintering (CS1) (Manosso et al., 2010).

TSS13 - T1=1460oC/h and T2=1350oC/3h 97.8 330

TSS14 - T1=1300oC/2h and T2=1460oC/2h 99.7 410

the second steps allowed the total densification without grain growth.

Table 4. Sintering procedure and results of relative density (%TD) and mean grain size of

The microstructure of the sample heated at 1460oC and cooled immediately and the sample sintered under TSS13 conditions are showed in Figure 12. It can be noticed an initial densification and 77%DT relative density for this sample. When the sample was heated at 1460oC and cooled down to 1350oC (TSS13 condition) the sample could be densified without grain growth (see table 2). It means that, 77% DT reached density in the first step at high temperature for this sample can be considered the critical density. In spite of the smaller grain size presented by TSS1 condition, its relative density was lower than densities of TSS14 and CS conditions. It suggests that the time of soaking in the second step can still be prolonged. Many studies (Tarjat & Trajat, 2009; Mazaheri et al., 2008) have been demonstrated that long times in

In the TSS14 condition, pre-densification sintering stage at 1300oC for 2 hours was effective in grain growth control and final densification. Figure 13 (a), (b) and (c) presents the microstructure of alumina-zirconia sintered under CS1, TSS13 and TSS14 conditions,

**Sintering procedure Relative density** 

zirconia.

alumina-zirconia samples.

respectively. These micrographs confirmed that the two-step sintering used have been efficient to the sintering process control. It can be observed that the sample conventionally sintered presents larger grain size. Finally, it was observed that the step of sintering with addition of inclusions is also efficient in grain growth.

Fig. 12. SEM micrographs of sintered samples: (a) T1=1460oC and cooled and (b) TSS13 condition.

Fig. 13. SEM image of the alumina-zirconia sintered under conditions: (a) CS; (b) TSS13; (c) TSS14.

Mechanisms of Microstructure Control in Conventional Sintering 419

Chen, P. L. & Chen, I. W. (1996). Sintering of Fine Oxide Powders: I, Microstructural

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