**2. Characteristics of thermal oxidation on hot-pressed pure yttria ceramics**

#### **2.1 Introduction**

*Ceramic Materials - Synthesis, Characterization, Applications and Recycling*

**68**

**1.5 Summary**

**Figure 10.**

**Figure 9.**

*(b) ZrO2, (c) CY, (d) FY, and (e) HY [17].*

The plasma resistance and characteristics of the yttria ceramics were investigated in terms of calcination and three-step sintering. The crystal phase of the

*Coverage of the plasma-deposited yttria ceramics versus quartz, silicon, Al2O3, and ZrO2. (a) Etch depth and rate and (b) weight loss rate and relative weight loss rate. Deposition time of Si wafer (100) and quartz:* 

*Coverage of the etching profile for the topographies of the different samples by a plasma test: (a) Al2O3,* 

*10 min. ZrO2, Al2O3, and yttria (CY and HY [17]): 60 min. W. L. R. means the weight loss rate.*

Fundamental studies on the forming process that affects the sintering behavior of yttria ceramics and powder synthesis techniques to obtain full density have attracted considerable interest. In particular, the sintering temperature has direct effects on the porosity, density, microstructure or crystal phase, and grain boundary migration. Yttria is used widely as a liquid sintering additive in AlN, SiC, SiAlON, and ZrO2 [7, 12]. Yttria itself, however, is a sintering-limit material when heated and can induce abnormal grain growth during the sintering process [6].

Finely divided yttria, having a purity at least 99.9%, was pressed into compacts, sintered in a dry hydrogen atmosphere or a partial vacuum to 2150–2300°C, and refired in wet hydrogen at 1950–2300°C to redoxidize any yttria that had been reduced to yttrium during sintering [8]. The resulting polycrystalline yttria ceramics were examined to determine if there was any improvement in the intrinsic properties of yttria.

The previously studied yttria ceramics were divided into three groups. The first was developed from the control of yttria particles, such as 5 nm Y2O3 synthesized from a 50 to 120 nm Y2O3 powder and Y(OH)3 using a coprecipitation method [9, 24]. The second group was a transparent yttria ceramic used as an infrared window to examine the electrical properties of yttria ceramics with additives, Th, Nd, and Er [15, 28, 29]. The last group involved the development of unconventional sintering techniques, such as triaxial isostatic press, hot isostatic pressing, or vacuum sintering, to fabricate fulldensity polycrystalline yttria ceramics [13, 30].

Most studies reported that the synthesis of nano-yttria particles was dependent only on the process variables. On the other hand, derivation of a common denominator for the properties of yttria is more complex because of the sintering methods of yttria ceramics, sintering property, and grain boundary migration by dopants such as La, Sr, and Sc [25, 31].

Nevertheless, the purity of the starting material and packing behavior as a function of the particle size used in the granule preparation together with the particle surface area affects the sintering characteristics [13]. These are important factors in the synthesis of dense sintered bodies. When yttria powders are induced on the nanoscale and have high purity, the fine particles or submicron-sized

secondary particles agglomerate easily. Although deformation occurs in most agglomerated particles under applied stresses, strongly agglomerated secondary particles maintain their agglomerated shapes. Because the green compact containing secondary agglomerated particles produces discontinuous microstructures at the interface of the agglomerated zones to show a difference in heat transfer, its evasion is desirable.

From this point of view, a high-purity 99.9% Y2O3 powder as a starting material and a hot isostatic pressing method (hot-pressed), which are useful for obtaining a fully dense ceramic, were used to observe the sintering behavior as a function of temperature.

A foundation study was performed through comparisons between the microstructure of hot-pressed yttria ceramics as a function of temperature and that of the density, crystal phase, weight loss, Vickers hardness, the behavior of indentation, flexural strength, etc.

#### **2.2 Characteristics of the hot-pressed yttria ceramics**

**Figure 11** shows the X-ray diffraction (XRD) patterns of hot-pressed yttria ceramics as a function of temperature. The sintering temperature in hot isostatic pressing (FRET-18, Fuji Denfa, Japan) was increased from 1300 to 1800°C at 100°C intervals, where the heating rate, holding time, and Ar gas flow were 5°C/min, 8 h, and 3 kgf/cm2 , respectively.

The hot isostatic pressing (hot-pressed) yttria ceramic specimens were oxidized at 1200°C for 12 h. XRD revealed the characteristic peaks of cubic yttria (JCPDS 41-1105), as shown in **Figure 11(a)** and **(b)**, irrespective of the sintering temperature and oxidation reaction at 1200°C.

In previous studies using a conventional sintering method, the crystallite phase of yttria could not be described precisely as the yttria phase because the YO1.401 (JCPDS39-1064) and YO1.335 (JCPDS 39-10,654) phases were also detected in the XRD pattern [25, 31]. This was based on the possibility that lattice distortion due to cation invasion into the oxygen vacancies or the Y-O ions can escape the regular lattice [32]. On the other hand, yttria ceramics containing Sc, Gd, La, Yb, and Mg were accompanied by a change in translucency and grain boundary migration, but the crystal phase was not changed [5].

The change in the intensity ratio reversed the peak at 48 and 57° in the hightemperature region, 1800°C, as shown in **Figure 11(b)**. Therefore, it was necessary to further examine the fraction of the crystal lattice due to the decomposition and volatilization of the yttria component [19, 31]. This means that the yttria crystalline phase varied according to the synthesis conditions and sintering methods, but the Y2O3 crystal phase identified in hot-pressed yttria ceramics means that the sintering temperature and oxidation reaction had no effect [18, 33].

**Figure 12** shows the hot-pressed yttria ceramics sintered as a function of temperature. As the sintering temperature was increased, the color of the sintered body changed to black. This is dependent on the weight deviation and change in color due to an increase in oxygen defect concentration and Y:O ratio variation [12, 18]. This suggests that the black samples obtained from a traditional sintering method are due to the oxygen released in the lattice site during the sintering process.

In addition, the proposed hot-pressed yttria ceramics could be applied to marking samples with a crystalline structure and full density, because it has a larger effect on the color change than repeat-cycle sintering or conventional sintering. The oxidation reaction specimens were converted to white regardless of the sintering temperature, which suggests that oxygen diffusion affects the

**71**

sintering temperature.

**Figure 11.**

**Figure 12.**

at 1050–1250°C were >6 × 10<sup>−</sup><sup>6</sup>

*Plasma Resistance Evaluation and Characteristics of Yttria Ceramics Sintered by Using…*

*XRD patterns of yttria ceramic. (a) Hot-pressed yttria ceramics as a function of temperature and (b) tempered in an oxidation of the hot-pressed yttria ceramics at 1200°C in air for 8 h.*

oxygen vacancies in the lattice directly [21]. The 1500°C specimen was translucent and showed the considerable development of a glassy phase with increasing

*Colors of yttria samples sintered. (a)~(f) Hot-pressed yttria ceramics as a function of temperature and (a*′*)~(f*′*) tempered in an oxidation of the hot-pressed yttria ceramics at 1200°C in air for 8 h.*

The weight of the specimens after the oxidation of yttria ceramics showed a tendency to increase with increasing temperature in **Figure 13**. The self-diffusion coefficient of yttrium, Do, and activation energy, Q, for polycrystalline Y2O4 with

289 J/mole, respectively [31]. The Do and Q values in the interstitial mechanism

diffusion coefficient was expressed as D = Do exp(−Q/RT). O had 3.5 times the activation energy of Y, and the self-diffusion of oxygen ions was relatively higher than yttrium. In addition, the diffusion of oxygen anions means that they occur relatively quickly because the interstitial diffusion of yttrium cations was similar to the rate of grain boundary migration [5, 18, 21]. Therefore, the diffusion rate during the oxidation of the hot-pressed yttria ceramics was higher than that of Y atom, and the oxygen vacancy showed preferential filling. This suggests that the weight

This tendency corresponded to an approximate 35% reduction in weight due to gas evolution and volatilization in the precursor at temperatures up to 1250°C [10]. The density of the hot-pressed yttria ceramics of 1600°C was close to those

cm2

, and the density tended to decrease

/s and 82 kJ/mole, respectively [29]. The self-

/s and

a density of 99.9% at 1400–1700°C were reported to be 1.65 × 10<sup>−</sup><sup>2</sup>

cm2

increases with increasing weight of oxygen (see **Figure 14**).

of the theoretical density of yttria, 5.03 g/cm3

*DOI: http://dx.doi.org/10.5772/intechopen.81750*

*Plasma Resistance Evaluation and Characteristics of Yttria Ceramics Sintered by Using… DOI: http://dx.doi.org/10.5772/intechopen.81750*

#### **Figure 11.**

*Ceramic Materials - Synthesis, Characterization, Applications and Recycling*

**2.2 Characteristics of the hot-pressed yttria ceramics**

, respectively.

ture and oxidation reaction at 1200°C.

the crystal phase was not changed [5].

temperature and oxidation reaction had no effect [18, 33].

evasion is desirable.

flexural strength, etc.

temperature.

and 3 kgf/cm2

secondary particles agglomerate easily. Although deformation occurs in most agglomerated particles under applied stresses, strongly agglomerated secondary particles maintain their agglomerated shapes. Because the green compact containing secondary agglomerated particles produces discontinuous microstructures at the interface of the agglomerated zones to show a difference in heat transfer, its

From this point of view, a high-purity 99.9% Y2O3 powder as a starting material and a hot isostatic pressing method (hot-pressed), which are useful for obtaining a fully dense ceramic, were used to observe the sintering behavior as a function of

A foundation study was performed through comparisons between the microstructure of hot-pressed yttria ceramics as a function of temperature and that of the density, crystal phase, weight loss, Vickers hardness, the behavior of indentation,

**Figure 11** shows the X-ray diffraction (XRD) patterns of hot-pressed yttria ceramics as a function of temperature. The sintering temperature in hot isostatic pressing (FRET-18, Fuji Denfa, Japan) was increased from 1300 to 1800°C at 100°C intervals, where the heating rate, holding time, and Ar gas flow were 5°C/min, 8 h,

The hot isostatic pressing (hot-pressed) yttria ceramic specimens were oxidized at 1200°C for 12 h. XRD revealed the characteristic peaks of cubic yttria (JCPDS 41-1105), as shown in **Figure 11(a)** and **(b)**, irrespective of the sintering tempera-

In previous studies using a conventional sintering method, the crystallite phase

of yttria could not be described precisely as the yttria phase because the YO1.401 (JCPDS39-1064) and YO1.335 (JCPDS 39-10,654) phases were also detected in the XRD pattern [25, 31]. This was based on the possibility that lattice distortion due to cation invasion into the oxygen vacancies or the Y-O ions can escape the regular lattice [32]. On the other hand, yttria ceramics containing Sc, Gd, La, Yb, and Mg were accompanied by a change in translucency and grain boundary migration, but

The change in the intensity ratio reversed the peak at 48 and 57° in the hightemperature region, 1800°C, as shown in **Figure 11(b)**. Therefore, it was necessary to further examine the fraction of the crystal lattice due to the decomposition and volatilization of the yttria component [19, 31]. This means that the yttria crystalline phase varied according to the synthesis conditions and sintering methods, but the Y2O3 crystal phase identified in hot-pressed yttria ceramics means that the sintering

**Figure 12** shows the hot-pressed yttria ceramics sintered as a function of temperature. As the sintering temperature was increased, the color of the sintered body changed to black. This is dependent on the weight deviation and change in color due to an increase in oxygen defect concentration and Y:O ratio variation [12, 18]. This suggests that the black samples obtained from a traditional sintering method are due to the oxygen released in the lattice site during the sintering

In addition, the proposed hot-pressed yttria ceramics could be applied to marking samples with a crystalline structure and full density, because it has a larger effect on the color change than repeat-cycle sintering or conventional sintering. The oxidation reaction specimens were converted to white regardless of the sintering temperature, which suggests that oxygen diffusion affects the

**70**

process.

*XRD patterns of yttria ceramic. (a) Hot-pressed yttria ceramics as a function of temperature and (b) tempered in an oxidation of the hot-pressed yttria ceramics at 1200°C in air for 8 h.*

#### **Figure 12.**

*Colors of yttria samples sintered. (a)~(f) Hot-pressed yttria ceramics as a function of temperature and (a*′*)~(f*′*) tempered in an oxidation of the hot-pressed yttria ceramics at 1200°C in air for 8 h.*

oxygen vacancies in the lattice directly [21]. The 1500°C specimen was translucent and showed the considerable development of a glassy phase with increasing sintering temperature.

The weight of the specimens after the oxidation of yttria ceramics showed a tendency to increase with increasing temperature in **Figure 13**. The self-diffusion coefficient of yttrium, Do, and activation energy, Q, for polycrystalline Y2O4 with a density of 99.9% at 1400–1700°C were reported to be 1.65 × 10<sup>−</sup><sup>2</sup> cm2 /s and 289 J/mole, respectively [31]. The Do and Q values in the interstitial mechanism at 1050–1250°C were >6 × 10<sup>−</sup><sup>6</sup> cm2 /s and 82 kJ/mole, respectively [29]. The selfdiffusion coefficient was expressed as D = Do exp(−Q/RT). O had 3.5 times the activation energy of Y, and the self-diffusion of oxygen ions was relatively higher than yttrium. In addition, the diffusion of oxygen anions means that they occur relatively quickly because the interstitial diffusion of yttrium cations was similar to the rate of grain boundary migration [5, 18, 21]. Therefore, the diffusion rate during the oxidation of the hot-pressed yttria ceramics was higher than that of Y atom, and the oxygen vacancy showed preferential filling. This suggests that the weight increases with increasing weight of oxygen (see **Figure 14**).

This tendency corresponded to an approximate 35% reduction in weight due to gas evolution and volatilization in the precursor at temperatures up to 1250°C [10]. The density of the hot-pressed yttria ceramics of 1600°C was close to those of the theoretical density of yttria, 5.03 g/cm3 , and the density tended to decrease

**Figure 13.**

*Behavior of weight of hot-pressed yttria ceramics after oxidation at 1200°C for 12 h in air.*

**Figure 14.** *Bulk and full density of hot-pressed yttria ceramics.*

with increasing temperature. **Figure 15** presents SEM images of the microstructure according to the sintering temperature.

The initial 0.5 μm uniform crystal grains changed to a glassy phase, in which the grain size was unclear depending on the sintering temperature, as shown in **Figure 15(a)–(d)**. **Figure 15(e)** and **(f )** presents the grain shape of the pentagonal or hexagonal of 15 μm grain size in yttria ceramics hot pressed at 1700°C. Grain growth tended to show a continuous increase with increasing temperature, which has a direct effect on the porosity, density, and crystal phase (see **Figure 15(a′)–(f′)**). On the other hand, the microstructure of hot-pressed yttria ceramics had an obscurity grain boundary like a glassy phase after the sintering process, which was illustrated by the change in density and weight from **Figure 15(e′)** and **(f )**. The Young's modulus of 120–170 GPa measured by the pulse echo method using an oscilloscope with X and Y modulation in **Figure 16** is believed to be dependent on the yttria ceramic microstructure. **Figure 17** shows the behavior of Vickers indentation of the specimen sintered as a function of temperature.

The indentation shape of the 1400°C specimen showed slightly ragged edges and interior surface, whereas in the 1500°C specimen, the origin, mirror, and mist-like noncrystalline fracture surface were distinguished clearly. The relationship between the grain size and crystal phase was examined by measuring the shape and length of the indentation according to the sintering temperature. This tendency increased with increasing indentation crack length when the crystal

**73**

**Figure 16.**

**Figure 15.**

*oxidized at 1200°C for 8 h in air.*

*Plasma Resistance Evaluation and Characteristics of Yttria Ceramics Sintered by Using…*

grains of the MgAl2O4 ceramics were grown from 0.4 to 24 μm and were attributed to the presence of local cracks acting as grain spalling [26]. **Figure 18** shows the KIC and Vickers hardness measured by applying a load of 500 g according to JIS R1610 method. KIC was calculated to be 1.2–1.9 MPa. The displacement point

*Young's modulus of yttria ceramic hot-pressed as a function of temperature.*

*SEM images of yttria sintered by hot pressing as a function of temperature. (a)~(f) Hot-pressed and (a*′*)~(f*′*)* 

*DOI: http://dx.doi.org/10.5772/intechopen.81750*

*Plasma Resistance Evaluation and Characteristics of Yttria Ceramics Sintered by Using… DOI: http://dx.doi.org/10.5772/intechopen.81750*

#### **Figure 15.**

*Ceramic Materials - Synthesis, Characterization, Applications and Recycling*

*Behavior of weight of hot-pressed yttria ceramics after oxidation at 1200°C for 12 h in air.*

with increasing temperature. **Figure 15** presents SEM images of the microstructure

The indentation shape of the 1400°C specimen showed slightly ragged edges and interior surface, whereas in the 1500°C specimen, the origin, mirror, and mist-like noncrystalline fracture surface were distinguished clearly. The relationship between the grain size and crystal phase was examined by measuring the shape and length of the indentation according to the sintering temperature. This tendency increased with increasing indentation crack length when the crystal

The initial 0.5 μm uniform crystal grains changed to a glassy phase, in which the grain size was unclear depending on the sintering temperature, as shown in **Figure 15(a)–(d)**. **Figure 15(e)** and **(f )** presents the grain shape of the pentagonal or hexagonal of 15 μm grain size in yttria ceramics hot pressed at 1700°C. Grain growth tended to show a continuous increase with increasing temperature, which has a direct effect on the porosity, density, and crystal phase (see **Figure 15(a′)–(f′)**). On the other hand, the microstructure of hot-pressed yttria ceramics had an obscurity grain boundary like a glassy phase after the sintering process, which was illustrated by the change in density and weight from **Figure 15(e′)** and **(f )**. The Young's modulus of 120–170 GPa measured by the pulse echo method using an oscilloscope with X and Y modulation in **Figure 16** is believed to be dependent on the yttria ceramic microstructure. **Figure 17** shows the behavior of Vickers indentation of the specimen sintered as a function of

according to the sintering temperature.

*Bulk and full density of hot-pressed yttria ceramics.*

**72**

temperature.

**Figure 13.**

**Figure 14.**

*SEM images of yttria sintered by hot pressing as a function of temperature. (a)~(f) Hot-pressed and (a*′*)~(f*′*) oxidized at 1200°C for 8 h in air.*

**Figure 16.** *Young's modulus of yttria ceramic hot-pressed as a function of temperature.*

grains of the MgAl2O4 ceramics were grown from 0.4 to 24 μm and were attributed to the presence of local cracks acting as grain spalling [26]. **Figure 18** shows the KIC and Vickers hardness measured by applying a load of 500 g according to JIS R1610 method. KIC was calculated to be 1.2–1.9 MPa. The displacement point

#### **Figure 17.**

*Indentation images of hot-pressed yttria ceramics; (a)~(f) hot-pressed yttria ceramics as a function of temperature and (a*′*)~(f*′*) tempered in an oxidation of the hot-pressed yttria ceramics at 1200°C in air for 8 h.*

of KIC is dependent on the crystal phase according to the sintering temperature. The Vickers hardness of hot-pressed yttria ceramics was 28 GPa, indicating similar properties to those of the conventional polycrystalline yttria ceramics in **Figure 18(b)** [24].

The Vickers hardness of the hot-pressed yttria ceramics had high hardness because the particle size was smaller due to the Hall-Petch effect, but the hardness behavior tended to decrease with increasing particle size [27, 34]. The correlation between KIC and the hardness in hot-pressed yttria ceramics was affected by the

#### **Figure 18.**

*Hardness and KIC of hot-pressed yttria ceramic. (a) KIC and (b) hardness, 500 g loads, giving typical Vickers indent diagonals. The vertical bars are the standard deviations.*

**75**

**Table 3.**

**Figure 19.**

*Silicon carbide*

*Silicon nitride*

*Alumina oxide*

*Plasma Resistance Evaluation and Characteristics of Yttria Ceramics Sintered by Using…*

grain size and the presence or absence of pores. The mean bending strength of five samples for each specimen was determined according to the JIS R1601 method. **Figure 19** shows the bending strength of the specimen. The specimens before heat treatment showed high strength in the high-temperature region, and those after heat treatment showed relatively high strength in the low-temperature region. The bending strength of the hot-pressed yttria ceramics was approximately 140 MPa,

Zirconia (3 mol% Y2O3) 7.0–12.0 800–1500

Hot-pressed 4.8–6.0 230–825 Sintered 14.8 96–520

Hot-pressed 4.1–6.0 700–1000 Reaction bonded 3.6 250–345 Sintered 5.3 414–650

99.9% pure 4.2–5.9 282–551 96% 3.85–3.95 358 Hot-pressed yttria 1.0–1.9 20–140 Glass ceramics (Pyroceram) 1.6–2.1 123–370 Fused silica 0.79 104 Borosilicate glass (Pyrex) 0.77 69 Soda-lime glass 0.75 69 Polyethylene terephthalate (PET) 5.0 59.3 Polypropylene (PP) 3.0–4.5 31.0–37.2 *Source: ASTM Handbooks, Vol. 1 and 19, Engineered Materials Handbook, Vol. 2 and 4, and Advances Materials &* 

**\_\_**

*m***) Strength (MPa)**

*DOI: http://dx.doi.org/10.5772/intechopen.81750*

and it was similar to that of fused silica in **Table 3**.

*Processes, Vol. 137, No. 6 ASM International Materials Park, OH.*

*Fracture toughness and strength of various materials.*

**Materials Fracture toughness (MPa√**

*Flexural strength of hot-pressed yttria ceramics.*

*Plasma Resistance Evaluation and Characteristics of Yttria Ceramics Sintered by Using… DOI: http://dx.doi.org/10.5772/intechopen.81750*

**Figure 19.** *Flexural strength of hot-pressed yttria ceramics.*

*Ceramic Materials - Synthesis, Characterization, Applications and Recycling*

of KIC is dependent on the crystal phase according to the sintering temperature. The Vickers hardness of hot-pressed yttria ceramics was 28 GPa, indicating similar properties to those of the conventional polycrystalline yttria ceramics in

*Indentation images of hot-pressed yttria ceramics; (a)~(f) hot-pressed yttria ceramics as a function of temperature and (a*′*)~(f*′*) tempered in an oxidation of the hot-pressed yttria ceramics at 1200°C in air for 8 h.*

The Vickers hardness of the hot-pressed yttria ceramics had high hardness because the particle size was smaller due to the Hall-Petch effect, but the hardness behavior tended to decrease with increasing particle size [27, 34]. The correlation between KIC and the hardness in hot-pressed yttria ceramics was affected by the

*Hardness and KIC of hot-pressed yttria ceramic. (a) KIC and (b) hardness, 500 g loads, giving typical Vickers* 

*indent diagonals. The vertical bars are the standard deviations.*

**74**

**Figure 18.**

**Figure 18(b)** [24].

**Figure 17.**

grain size and the presence or absence of pores. The mean bending strength of five samples for each specimen was determined according to the JIS R1601 method. **Figure 19** shows the bending strength of the specimen. The specimens before heat treatment showed high strength in the high-temperature region, and those after heat treatment showed relatively high strength in the low-temperature region. The bending strength of the hot-pressed yttria ceramics was approximately 140 MPa, and it was similar to that of fused silica in **Table 3**.


*Source: ASTM Handbooks, Vol. 1 and 19, Engineered Materials Handbook, Vol. 2 and 4, and Advances Materials & Processes, Vol. 137, No. 6 ASM International Materials Park, OH.*

#### **Table 3.**

*Fracture toughness and strength of various materials.*
