Effect of LZSA Glass-Ceramic Addition on Pressureless Sintered Alumina . Part II : Mechanical Behavior

This work aims to evaluate the influence of a Li2O-ZrO2-SiO2-Al2O3 (LZSA) glass-ceramic on the mechanical behavior of alumina. Composites were prepared from alumina with three different particle sizes and 7 to 21 vol% of an LZSA glass-ceramic composition (11.6Li2O-16.8ZrO2-68.2SiO2-3.4Al2O3,). Specimens were obtained by uniaxial pressing. The optimum sintering temperature and holding time were found to be different for each composite. Structural characterization (bulk density and crystalline phases); mechanical characterization (flexure strength, elastic modulus, fracture toughness, and fracture energy); and microstructural analyses were carried out. Fine-grained alumina-based composite containing 21 vol% of glass-ceramic (1470 oC and 3 h holding time, 2.0% porosity) showed a fracture toughness of 4.93 MPa·m0.5, an elastic modulus of 210 GPa, a fracture energy of 57 J·m-2, and a flexural strength of 170 MPa, in very good agreement with values reported by the literature. An increase of 37-177% in the fracture energy due to 21 vol% LZSA addition in the alumina was achieved for the range of grain size obtained in this work. Even though the final composition included a glassy component, the observed mechanical properties confirmed the effectiveness of the crystalline phases that were formed from LZSA glass-ceramic in reducing the propagation of cracks. The results showed that the addition of the LZSA glass-ceramic improved the mechanical properties of alumina.

Many studies have shown the influence of microstructure on the mechanical behavior of alumina, and most of them were focused on the effects of grain size [10][11][12][13][14][15][16][17] .A fine-grained microstructure 3,10,11 and narrow range of particle size distribution often result in an improvement of the mechanical behavior of alumina 10,11 .For instance, similar relationships between wear rates and grain size are observed in various wear modes, such as erosive wear, abrasive wear, cutting, and grinding 12 .Moreover, the mechanical strength of alumina may be improved when the microstructure shows fine grainsize and residual porosity less than 0.05% 15 .
Solid state sintering contributes to grain growth in alumina, because of the high temperatures and holding times applied.Thus, Liquid Phase Sintering (LPS) has emerged as a feasible alternative to obtain dense alumina with a refined microstructure and low porosity.The LPS application plays an important role in the processing of alumina, because the use of additives during this process allows the formation of a second phase that controls the grain growth phenomena.It seems that the erosive wear rate of LPS alumina is controlled by a combination of different features that may be related to grain size 14 .The types of glass that are most commonly used in LPS contain amorphous silica in their composition, which can degrade the mechanical behavior of the material 18 .Furthermore, the glassy phases are fragile and show low fracture toughness.
A reduction of the residual glassy phase can improve the mechanical behavior of alumina obtained by LPS.Thus, the use of a glass-such as a glass-ceramic-that encourages sintering during heating and also crystallizes in stable phases during the cooling cycle may be an alternative way to obtain highdensity alumina by LPS, while simultaneously producing a large amount of crystalline phase in the grain boundaries 19 .In addition, less grain growth and less residual glassy phase could be obtained.
The formed glass-ceramic must have a low coefficient of thermal expansion (CTE) in order to generate compressive residual stress at the interfaces with the alumina.This stress should strengthen the structure, hampering the stripping of  [20][21][22] , allowing the fabrication of materials with a low CTE (ranging from 5.1 to 5.3 × 10 -6 °C-1 , over the range of 25 to 325 °C) 20 .The high crystallinity, low porosity, and fine microstructure (crystal sizes of 1 µm) that are obtained [21][22] result in high wear resistance and flexural strength 23 .Moreover, LZSA shows surface crystallization and achieves high densification at lower temperatures: above 95% in the range of 630 to 770 °C22,24 .
Alumina-based composites have been studied in order to achieve high performance materials [4][5][6]8 , and it was found that the introduction of a second crystalline phase to improve the properties of alumina, such as fracture toughness, plays an important role. I fact, Montedo et al. 25 studied the effect of the LZSA (11.6Li 2 O-16.8ZrO 2 -68.2SiO 2 -3.4Al 2 O 3 ) glassceramic on the grain growth of alumina.The addition of 21 vol% LZSA to fine alumina (d 50 = 0.5 µm) enabled the lowering of the sintering temperature from 1600 to 1470 °C, and the holding time from 10 h to 40 min, for the same relative density.
Thus, Part II of this work aims to evaluate the effect on the mechanical behavior of alumina caused by the addition of an LZSA glass-ceramic.

Experimental
Five compositions were prepared from an LZSA glassceramic composition (Tecnofrita, Brazil) and three grades of alumina (99.8 wt% of Al 2 O 3 , Almatis, USA); the chemical composition, particle size, and specific surface area of these materials were presented in the previous work 25 .The experimental design used two factors: the particle size of alumina and the glass-ceramic content varied on two levels (−1 and +1).A full factorial design 2² was established with three central points.The variation ranges of the factors are shown in Table 1.The description A i is related to the particle size of alumina, where A F is the fine alumina, A M is the medium alumina, and A C is the coarse alumina.The number next to this description refers to the glass-ceramic content.Compositions were wet-mixed (with 0.1 wt% sodium tripolyphosphate as dispersant, 1.0 wt% carboxymethylcellulose as plastifier, and 1.5 wt% polyvinyl alcohol as binder) and dried in a spray-dryer (LabMaq do Brasil Ltda LM MSD 1.0, Brazil) to obtain powders (8 wt% water).The powders were formed by uniaxial pressing (Gabbrielli GT 0785, Italy) at 128 MPa specific pressure with a green density ranging from 1.92 to 2.61 g•cm -3 , (i.e., 50.9 to 65.4% of theoretical density) depending on the LZSA content, and dried at 110 ± 5 ºC.The sintering temperature was measured by an optical dilatometer (Expert System Solutions S.R.L Misura HSM ODHT 1400, Italy); and the temperature cycle included a 1 ºC•min -1 heating rate, 90 min holding times at 1100 and 1300 ºC, and a 1600 ºC maximum temperature).Sintering temperatures and holding times are shown in Table 1.The compacted alumina and the composites were sintered in an electrical kiln (Fortelab ME 1700/10, Brazil).Controlled cooling (10 ºC•min -1 cooling rate, 30 min holding time at 760 ºC, 10 ºC•min -1 cooling rate up to room temperature) was carried out in order to form crystalline phases in the glass-ceramic.Porosities were calculated from d rel ; the values of which were published by Montedo et al. 25 .The flexural strength (FS) of the sintered samples was determined based on ASTM 1161-02 using a mechanical testing machine (EMIC DL10000, Brazil).The elastic modulus was determined in a transitory vibration analyzer (ATCP Engenharia Física Sonelastic, Brazil).Fracture toughness measurements (K IC ) were performed using the notch method (Single Edged Notched Beam, SENB), which consists of making a notch in the specimen by means of a diamond cutting disk (0.8 mm thickness, 0.5 mm pitch).The depth of the notch was equal to 40% of the total thickness of the specimen.Then the specimens were submitted to flexure in a mechanical testing machine (EMIC DL10000, Brazil).The value of K IC was calculated by the Griffith equation: (1)   where σ is the rupture stress, Y is the calibration factor and / is the depth of the notch (or the natural flaw).
The calibration factor for this type of notch is given by Eq. 2, where b is the width of the specimen. (2) From the data of K IC and FS measurements, the natural flaw size, a, may be calculated from Griffith equation.
According to the theory of fracture mechanics, one can obtain the fracture energy (γ) of the material from Eq. 3, where E is the elastic modulus.
(3) Five specimens of each condition were used for measurements of mechanical properties.
The residual stress resulting from the interaction between alumina and LZSA was obtained by X-ray diffractometry (Shimadzu XRD-6000, Japan; radiation CuKα, 0.02º step) by means of the evaluation of the displacement in the 2θ angle of the highest α-alumina peak (Bragg's law, 57.47º (116), JCPDS card number 42-1468).The microstructure of the sintered samples was evaluated by scanning electron microscopy (SEM, Zeiss EVO MA10, Germany).Fractured samples were used to assess the interaction between the alumina and the glass-ceramic.Specimens were etched in 2 vol% HF for 25 s and coated with a thin Au film.This chemical etch was carried out to eliminate the glass-ceramic existing on the surface of the samples and thereby allow the visualization of the grain morphology and particles.

Results and Discussion
Part I of this work demonstrated the effect of an LZSA glass-ceramic on the grain growth of alumina 25 , since the grain size is one of the most important microstructural features that must be controlled in order to obtain high performance alumina.Microstructural control by means of dopants and processing techniques can improve the mechanical properties of alumina-wear for example 16 .Thus, suppression of grain growth plays a crucial role.
In this Part II, LZSA glass-ceramic was added to alumina to cause suppression of grain growth; however, crystalline phases were formed during heating in the range of 640-820 °C19 .
Because the composites were sintered at higher temperatures (> 1450 °C), those crystalline phases were dissolved into the glassy phase after the melting of the LZSA.Nevertheless, after sintering, the controlled cooling of composites allowed the crystallization of LZSA.In fact, Figure 1 shows that β-spodumene ss (Li 0-6 Al 0-6 Si 2-4 O 6 , JCPDS No. 21-503, and LiAlSi 3 O 8 , JCPDS No. 15-27), zirconium oxide (ZrO 2 , JCPDS No. 13-307), and quartz (SiO 2 , JCPDS No. 5-490) were formed during the controlled cooling of composite A F 21.Those crystalline phases caused the formation of compressive stress at the alumina/glass-ceramic interface as confirmed by the displacement of the main alumina peak in the XRD patterns 19 .
Figure 2 shows the elastic modulus (E) of the alumina ceramics and composites that were investigated in this study, as a function of the LZSA content.One may observe that E shows an inversely proportional behavior in relation to porosity.The maximum value in the E-LZSA content plot was obtained at 7 vol% LZSA (94.8% Al 2 O 3 purity) regardless of the amount of alumina used (porosity of 5.1 and 4.1% for A F 7 and A C 7, respectively).However, at higher LZSA contents E diminishes probably because of the poor distribution of LZSA into the bulk of alumina.Terheci 26 obtained higher E values for pure alumina compositions (386 GPa, 1600 ºC/30 min holding time), while Munro 27 obtained 416 ±30 GPa for 99.5% purity Al 2 O 3 sintered at 1700 ºC and showing 5-µm grain size and 2.0% porosity; the value of E decreased for longer sintering holding times.For the types of alumina used in this study, alumina particle size had no significant effect on the E values for the LZSA compositions that were investigated.Figure 3 shows K IC results for the alumina ceramics and composites.The LZSA addition increased K IC regardless of the alumina particle size used in this work.The K IC values were found to be 4.24, 4.93, 3.9, and 4.6 MPa•m 0.5 for A F 7, A F 21, A C 7, and A C 21, respectively.Taking into account the obtained standard deviation, one can say that there is no difference between these K IC values, although one can observe a tendency of K IC increasing for higher values of LZSA content and lower particle sizes.Lube et al. 28 obtained 3.8 MPa•m 0.5 , while Marques 29 cited K IC values ranging from 3.85 to 3.95 MPa•m 0.5 for sintered alumina (relative density of 99.5%).Tuan et al. 30 found 5.0 MPa•m 0.5 for sintered alumina (1.7% porosity, 13.3-µm grain size), and Wu et al. 18 found 3.6 MPa•m 0.5 for pure alumina and 4.8 MPa.m 0.5 for LPS alumina.Therefore, even though composites A F 7, A F 21, A C 7, and A C 21 had glass in their composition, they showed K IC values larger than those for pure alumina reported for the literature, confirming the effectiveness of the crystalline phases formed from LZSA glass-ceramic to reduce the crack propagation.Fracture energy (γ) was calculated from the data of K IC and E, using Eq. 3. Figure 4 shows the effect of LZSA content on the fracture energy of alumina.Fracture energy increased with increasing LZSA content for the investigated composites.This effect may be attributed to the alumina/LZSA interaction and the obtained microstructures, in particular, the increase of roughness.However, it is important to emphasize that pure alumina (A F , A M , and A C ) were sintered at the same temperature as the composites, i.e. 1600 °C, and because of this, the obtained relative densities were much lower than that of dense alumina.Nevertheless, fracture energy data of alumina with very similar grain sizes and porosities in relation to the investigated composites were obtained from National Institute of Standards and Technology -NIST 31 , as shown in Table 2. Table 2 shows that the fracture energy ranges from 18.0 to 36.5 J•m -2 for alumina similar to that used in this work.Taking into account that the grain size obtained in this work ranged from ~1 to 12 µm, one may say that the addition of LZSA glass-ceramic increased the fracture energy of alumina up to 50 J•m -2 (21 vol% LZSA addition), which represents an increase of 37-177%.
Figure 5 shows images (photographies) of composites A   and A C 21 could be used for some applications where high impact resistance is required in addition to wear resistance.Composites A F 7 and A C 7 showed higher flexural strength (FS) values than alumina A F , A M , and A C , as shown in Figure 6.Sathinyakumar and Gnanam 33 obtained 119 MPa for pure alumina sintered at 1400 ºC, while Goswami and Das 34 obtained 288 MPa for LPS alumina, just slightly higher than the highest value found in this study (273 MPa for composite A F 7).With the increase of LZSA content, the FS value decreased, probably due to the increase of the natural defect size (a), obtained from Eq. 1, of the composites (Figure 7).By comparison, Figures 6 and 7 show that FS is higher for lower values of natural defect size.Figure 7 shows that a increased with the increase of alumina particle size and LZSA content.However, these effects tend to be insignificant at higher LZSA contents (~ 21 vol%).Alumina particle size seems to be the main factor that determined the sizes of the natural defects in the samples with up to 7 vol% LZSA.On the other hand, the addition of 21 vol% LZSA (composites A F 21 and A C 21) increased the natural defect size for all investigated alumina.It is possible that the residual vitreous phase in these composites has increased the natural defect size, as reported by De Noni et al. 35 .toward the left side of the XRD patterns; the higher the LZSA content, the higher the displacement.This displacement demonstrates the existence of compressive residual stresses that were caused by the lower coefficient of thermal expansion of LZSA (5.2 × 10 -6 °C-1 ) 20 in comparison to alumina (8.1 × 10 -6 °C-1 ).The other composites also showed the same behavior.Figure 8 also shows that the peak displacement was higher for composite A F 21 than for composite A C 21, possibly due to the greater specific surface area of the former; and consequently, the greater interface region between the alumina and the LZSA.Thus, the interaction between fine alumina (A F ) and the LZSA may help to explain the mechanical behavior of the investigated composites.As mentioned before, LZSA glass-ceramic was added to alumina in order to generate residual stress and improve the mechanical properties.Figure 8 presents the XRD patterns of composites A F , A F 7, and A F 21. Peak displacement occurred Figure 9 shows SEM observations of fractured specimens of the alumina ceramics and composites.LZSA is homogeneously dispersed in all the specimens, as detailed in Figure 10.It seems as if the microstructure of composites A M 15 and A C 21 are quite similar, which could explain the similarity in their mechanical properties.
The LZSA addition also changed the morphology of the alumina, as shown in Figure 11 for the etched specimens.The morphology is influenced by the chemical composition of the intergranular phase of materials obtained by LPS and can significantly change the mechanical properties of alumina [36][37][38] .As shown in the SEM observations, the composites have   elongated grains.Elongated grains can act as reinforcement material in the microstructure and indeed contribute to the increased fracture toughness 36,38 .Composite A F 21 achieved a high relative density at a lower temperature (1470 °C); and because of this, alumina grains did not experience as much growth.In fact, sintering at lower temperatures in the presence of the liquid phase may cause a suppression of grain growth 38 .Although composites A F 7 and A C 7 show the presence of elongated grains, one can also see coarse, equiaxed grains.

Conclusions
The influence of LZSA (Li 2 O-ZrO 2 -SiO 2 -Al 2 O 3 ) glassceramic on the mechanical behavior of alumina was investigated.Liquid phase sintering promoted higher densification than the use of pure alumina.A composite containing fine grain alumina and 21 vol% of glass-ceramic sintered at 1470 ºC (3 h holding time) showed lower porosity (2%) than pure alumina sintered at 1600 °C (5%).The initial particle size of alumina had little influence on the mechanical properties; however, the glass-ceramic addition caused a significant effect on the mechanical properties.Elongated grains of alumina were observed in the composites as a result of glass-ceramic addition, which caused strengthening of the structure and improved the fracture toughness.Fine-grained alumina-based composite containing 21 vol% of glass-ceramic (1470 ºC and 3 h holding time, 2% porosity) showed a fracture toughness of 4.93 MPa•m 0.5 , elastic modulus of 210 GPa, fracture energy of 57 J•m -2 , and flexural strength of 170 MPa, in very good agreement with values reported in the literature.Thus, the introduction of a liquid phase in the sintering of alumina, which crystallizes during controlled cooling, i.e. the use of a glass-ceramic, allowed us to modify significantly the microstructure of alumina, and consequently, the mechanical properties.The results showed that the addition of the LZSA glass-ceramic improved the mechanical properties of alumina.

Figure 2 .
Figure 2. Elastic modulus (E) and porosity in function of the LZSA content of the alumina ceramics and LZSA/alumina composites.

Figure 3 .
Figure 3. Fracture toughness in function of the LZSA content of the alumina ceramics and LZSA/alumina composites.
F 7, A F 21, A C 7, and A C 21.The textures of the composites containing 7 and 21 vol% of LZSA are quite different from each other.One can see that the 21-vol% based-composites (A F 21 and A C 21) are much rougher than the 7-vol% basedcomposites (A F 7 and A C 7).The rougher the material, the greater the contact between particles; consequently, more

Figure 5 .
Figure 5. Images (photographies) showing the texture of composites A F 7, A F 21, A C 7 and A C 21.

Figure 6 .
Figure 6.Flexural strength in function of LZSA content of the alumina ceramics and LZSA/alumina composites.

Figure 7 .
Figure 7. Calculated natural defect size in function of LZSA content of the alumina ceramics and LZSA/alumina composites.

Figure 8 .
Figure 8. Partial XRD patterns of the alumina A F and composites A F 7, A F 21 and A C 21.

Figure 10 .
Figure 10.Detail of the fractured composite A C 21: alumina and ○ LZSA glass-ceramic.

Table 1 .
Experimental design with the combinations among each factor.

Table 2 .
Fracture energy data from NIST.