Sintering of alumina ceramics reinforced with a bioactive glass of 3CaO.P₂O₅-SiO₂-MgO system

Sinterização da alumina reforçada com um vidro bioativo do sistema 3CaO.P₂O₅-SiO₂-MgO

Abstracts

Alumina-based ceramics, Al₂O₃, exhibit a combination of properties which favor its use as biomaterial, specifically as structural dental prosthesis. Its most important properties as biomaterial are its elevated hardness, chemical stability and biocompatibility. Usually, Al₂O₃ is processed by solid-state sintering at a temperature of about 1600 ^oC, but it is very difficult to eliminate the porosity due to its diffusional characteristics. The objective of this work was the development and characterization of sintered Al₂O₃ ceramics, densified with a transient liquid phase formed by a bioactive 3CaO.P₂O₅-SiO₂-MgO glass. Powder mixtures of 90 wt.% Al₂O₃ and 10 wt.% bioglass were milled, compacted and sintered at 1200 ^oC to 1450 ^oC. Comparatively, monolithic Al₂O₃ samples were sintered at 1600 ^oC/120 min. The sintered specimens were characterized by relative density, crystalline phases, microstructure and mechanical properties. The results indicate that the specimen sintered at 1450 ^oC/120 min present the best properties. Under this sintering condition, a relative density of 95% was reached, besides hardness higher than 9 GPa and fracture toughness of 6.2 MPa.m^1/2. XRD analysis indicate alumina (αAl₂O₃), whitlockite (3CaO.P₂O₅) and diopsite [3(Ca,Mg)O.P₂O₅], as crystalline phases. Comparatively, monolithic sintered Al₂O₃ samples presented 92% of relative density with 17.4 GPa and 3.8 MPa.m^1/2 of hardness and fracture toughness respectively.

Al₂O₃ ; bioactive glass ceramic; sintering; characterization; mechanical properties

Cerâmicas à base de alumina exibem combinações de propriedades as quais favorecem seu uso como biomaterial, com destaque para estruturas de prótese dentária. Entre as mais importantes propriedades para uso como biomaterial estão a dureza elevada, a estabilidade química e a biocompatibilidade. Normalmente, Al₂O₃é sinterizada no estado sólido em temperaturas superiores a 1600 ^oC; porem, devido às suas características difusionais, há grande dificuldade em eliminar completamente a porosidade. O objetivo deste trabalho foi o desenvolvimento e a caracterização de cerâmicas de Al₂O₃densificadas com uma fase liquida formada por um vidro bioativo do sistema 3CaO.P₂O₅-SiO₂-MgO. Misturas de pó com 90% em peso de Al₂O₃ e 10% em peso de vidro foram preparadas, compactadas e sinterizadas entre 1200 ^oC e 1450 ^oC. Comparativamente, amostras de Al₂O₃ monolíticas foram sinterizadas a 1600 ^oC/120 min. As amostras foram caracterizadas por densidade relativa, fases cristalinas, microestrutura e propriedades mecânicas. Os resultados indicaram que as amostras sinterizadas a 1450 ^oC/120 min apresentaram as melhores propriedades, com densidade relativa de 95% além de dureza de 9 GPa e tenacidade a fratura de 6,2 MPa.m^1/2. Análises de difração de raios X indicaram alumina (α-Al₂O₃), whitlockita (3CaO.P₂O₅) e diopsita [3(Ca,Mg)O.P₂O₅], como fases cristalinas após sinterização. Comparativamente, as amostras de Al₂O₃ apresentaram 92% de densidade relativa com 17,4 GPa e 3,8 MPa.m^1/2 de dureza e tenacidade a fratura, respectivamente.

Al₂O₃ ; vidro bioativo; sinterização; caracterização; propriedades mecânicas

INTRODUCTION

The development of technologies to produce novel bioceramic materials has been motivated by the growing demand of dental ceramics with improved properties in substitution of metal-based materials. The use of advanced ceramics as biomaterials started in the 1970's. Since then, they have been continually improved, specifically aiming applications in dentistry as dental restoration parts. A usual technique for obtaining dental ceramics is the infiltration of a porous matrix with a glass. However, due to varying stages of sintering and the infiltration process, the final product has a high cost and also the possibility of having defects formed during processing which may reduce the reliability. In Table I some of the mechanical properties of commercial ceramic dental materials are listed ⁽¹⁾.

Table I/ Tabela I
Hardness and fracture toughness of commercial ceramic dental materials ⁽¹⁾. Dureza e tenacidade à fratura de cerâmicas dentárias comerciais.

Material	Hardness (GPa)	Fracture toughness ( MPa.m1/2)
High Alumina Porcelain	4.3	2.0 - 2.9
Leucite Reinforced Glass	6.5	1.0 - 2.0
Lithium disilicate, Li2Si2O5	6.7	3.4
Glass Infiltration - In Ceram Spinel	10	2.7
Glass Infiltration - In Ceram Alumina	13	4.4 - 4.8
Glass Infiltration - In Ceram Alumina Zircon	11	6.8
Monolithic Alumina	16	3.8 - 4.5

Bioglass is a bioactive glass. Bioactivity is defined as the property to form tissue on the surface of biomaterial, establishing an interface able to supporting mechanical solicitations. Three classes of ceramic materials satisfy this criterion: the bioactive glasses and glass-ceramics, calcium-phosphate ceramics and its composites, and bio inert ceramics such as Al₂O₃ and ZrO₂-Y₂O₃. Glasses and sinter additions of Si₃N₄ and ZrO₂ have been studied ^{(2) , (3)}. Recently, two works have been published, investigating the liquid-phase sintering of ZrO₂ ^{(4) , (5)}. Some of these bioactive glasses can be partially crystallized by heat treatment. Studies on the effect of partial crystallization of glass-ceramics intended to improve the mechanical properties, while maintaining the bioactivity have been carried out ^{(6) , (7)}. Crystallization of the glass is extremely important for the improvement of the mechanical properties. It has been shown ^{(8) , (9)} that bioglasses with high crystalline fractions exhibit a decrease in their biological performance, because the formation of the HCA (Hydroxy Carbonate Apatite) layer is related to the amount of existing residual glassy phase, since its formation depends on the dissolution of calcium and silicon ions in the glassy phase ^{(10) , (11)}. On the other hand, the presence of crystalline phases, which exhibit a moderate/high dissolution rate, such as wollastonite (CaSiO₃) ⁽¹²⁾, combeite (Na₂Ca₂Si₃O₉) ⁽¹³⁾ and tricalcium phosphate (Ca₃(PO₄)₂) ⁽¹⁴⁾, may play a role similar to that of a glassy phase as source of calcium and silicon ions, thus maintaining the bioactivity of the material. As example of glass-ceramics that exhibit good fracture toughness and high bioactivity are the so-called glass-ceramic A-W Cerabon(r)⁽¹⁵⁾, and Bioverit ⁽¹⁶⁾.

Glass-ceramics of the 3CaO.P₂O₅-SiO₂-MgO system with different crystalline fractions have emerged as bone substitutes because of their interesting mechanical properties ^{(17) , (18)}, similar to those of natural bone tissue. In a previous work ⁽¹⁷⁾, it was shown that the phase transformations that occur in these materials during heat-treatment under different temperatures directly influence the microstructure and hence the mechanical properties. The effect of crystallization of bioglasses on the formation of HCA is still a controversial subject and for this glass system it has not been explored yet.

In this work, Al₂O₃-based ceramics containing a bioactive glass was densified at low temperature, and the properties are compared with monolithic-Al₂O₃ sintered at 1600 ^oC. The effects of the glass partial crystallization on the ceramic properties are investigated.

EXPERIMENTAL PROCEDURE

Processing

High-purity α-Al₂O₃, (A-1000, Almatis-USA), and a bioactive glass based on system 3CaO.P₂O₅-SiO₂-MgO ^{(19) , (20)} have been used as starting powders. A glass composition of 52.75 wt.% Ca₃(PO₄)₂-30wt.% SiO₂-17.25wt.% MgO was prepared from reagent-grade Ca(H₂PO₄)₂.H₂O (Synth), CaCO₃ (Synth), SiO₂ (Fluka) and MgO (Synth). The bioactivity of this composition was studied in samples heat-treated at 1100 ^oC ⁽²¹⁾.

The glass was prepared according to the conventional melting method in a platinum crucible at 1600 ºC/60 min. Batches of 100 g were obtained by mixing the raw materials in ethanol for 240 min, drying at 90 °C for 24 h and passing it through a sieve with openings of 32 µm for deagglomeration. Finally, the glass was cast into cylinders with 12 mm diameter in a stainless steel mould and annealed for 120 min at 700 ºC (30 ºC below the glass transition temperature, T_g, of this glass) and slowly cooled down to room temperature at a rate of 3 ºC/min.

A powder mixture containing 90 wt.% Al₂O₃ and 10 wt.% glass powder was prepared by planetary milling at 1000 rpm for 4 h, using ethanol as vehicle, subsequent drying at 100 ºC for 24 h and sieving. Cylindrical samples of 20 mm diameter were cold uniaxially pressed under 100 MPa. The samples were sintered in a MoSi₂ resistance furnace, at 1200 ºC, 1300 ºC and 1400 ºC for 60 min or at 1450 ºC for 120 min, at a heating and cooling rate of 10 ºC/min and 8 ºC/min, respectively. Similarly, samples of α-Al₂O₃ were uniaxial pressed and sintered at 1600 ^oC/120 min with heating and cooling rate of 10 ºC/min and 8 ºC/min, respectively.

Characterization

Starting powders and sintered samples were characterized by X-ray diffractometry (XRD), Shimadzu XRD6000, using Cu-k_α α radiation (λ=1.5418Å) in the 2θ range of 20 to 80°, with a step of 0.02° and 2 s of exposure time/degree. The crystalline phases were determined by comparison with the JCPDS files ⁽¹⁹⁾.

The non-isothermal crystallization kinetics was studied using differential scanning calorimetry DSC Netzsch-404. Glass powder was carried out in a platinum crucible at 10 °C/min from room temperature up to 1200 °C.

In addition, the thermal expansion coefficients (TEC) of the alumina and glass (ceramic) samples were determined by dilatometry, using an alumina rod dilatometer (Bahr Thermoanalyse GmbH 2000 Dil801L-1600 ºC, Germany). Samples with 8-10 mm thickness and 3 mm × 3 mm cross-section were used for dilatometry measurements. The thermal expansion coefficient of the bulk glass and Al₂O₃ was measured in air, using heating rate and cooling rate of 25 and 5 ºC/min, respectively.

The bulk density of the sintered samples was evaluated by the Archimedes method in distilled water. The relative density was calculated as the ratio between the apparent and respective theoretical density, calculated by the rule of mixture.

Starting glass powder and sintered samples were examined by scanning electron microscopy (SEM), in a Leo-1450VP microscope. The sintered samples were cut, their surfaces ground and polished with diamond paste, washed with acetone in an ultrasonic bath for 10 min, and dried at 100 ºC for 1 h. After this step, samples were thermally etched at 1200 ºC/15 min, with heating rate 30 ºC/min to reveal the grains microstructure.

Hardness and fracture toughness (K _Ic), were determined using a Vickers indentation method. In each sample, 10 indentations were performed and measured, under a load of 2000 gf for 30 s. The fracture toughness was calculated by measuring the relation between cracks length (c) and indentation length (a), using the relation valid for Palmqvist crack types, which present c/a relation < 3.5 ⁽²⁰⁾.

RESULTS AND DISCUSSION

Glass characterization

Fig. 1 presents the X-ray diffraction patterns of the starting materials used in this work. The Al₂O₃ powder consists of the highly crystalline α-Al₂O₃ phase, while the glass exhibits a typical amorphous pattern, indicating no crystalline phase in this material. Due to the importance of the characteristics of the bioglass used as sintering aid, the bioglass has been studied in greater detail.

Figure 1/Figura 1
X-ray diffraction patterns of the bioglass and Al₂O₃ powders. Difratogramas de raios X dos pós de Al ₂ O ₃ e biovidro.

The crystallization of this glass at 1100 ^oC was investigated by High Resolution X-ray Diffractometry, HRXRD, using a Hubber diffractometer with multiple axes ⁽²²⁾. The amount of the crystalline phases (crystallized volume fraction) contained in the glass-ceramic samples was determined according to the procedure used by Krimm and Tobolsky ⁽²³⁾. The percent crystallinity (IC) was calculated by the ratio of the crystalline area (AC) and the total area (AT = amorphous + crystalline), using the Origin software (OriginLab Corp., Northampton, MA) and the following equation:

This work confirmed the presence of crystalline phases in the material, identified as whitlockite (PDF #87-1582), a tricalcium-phosphate with magnesium in solid solution, Ca_2,589Mg_0,411(PO₄)_{2 ,} and is also known as the b-TCMP and diopside, CaMgSi₂O₆ (PDF #71-1067) ⁽¹⁹⁾.

The glass-ceramics studied in this work, independent on the temperature of the thermal treatment, showed whitlockite (PDF # 87-1582) as the major crystalline phase. In this phase Mg is partially substituted by Ca, forming a solid solution of [3(Ca,Mg)O.P₂O₅] as will be seen below in the characterization of sintered samples.

Fig. 2 shows SEM images of the bioglass particles used as starting material. The particles are smaller than 32 μm and of irregular shape because of the crushing and milling process used in its fabrication.

Figure 2/Figura 2
SEM micrographs of the bioglass particles obtained by planetary milling. Micrografias obtidas por microscopia eletrônica de varredura das partículas de biovidros obtidas por moagem em moinho planetário.

The results of the Differential Scanning Calorimetry (DSC) for a monolithic sample are presented in Fig. 3. They indicate that the heat treated glass exhibits a first endothermic peak at 717 °C, which is attributed to the glass transition temperature, T_g. Two exothermic peaks were also detected at T_p1 = 834 ºC and at T_p2 near 980 ºC.

Figure 3/Figura 3
Thermal analysis curves of the bioactive 3CaO.P₂O₅-SiO₂-MgO glass. Curvas de análise térmica do vidro bioativo 3CaO.P ₂ O ₅ -SiO ₂ -MgO.

Another exothermic reaction at 1046 ^oC was observed in the bulk sample, but of small intensity, suggesting a phase transformation. This phase transformation could be confirmed later by the analysis of the crystalline phases. At last during cooling of the glass, an exothermic peak at T_x ~ 1115 °C was observed. Previous studies suggest that the first peak corresponds to the formation of the tricalcium phosphate with partial substitution of Mg by Ca (whitlockite) of composition 3(Ca, Mg)O.P₂O₅ and the second corresponds to the precipitation of enstatite with partial substitution of Ca by Mg, (Mg,Ca)O.SiO₂ ⁽²²⁾. Daguano et al. ⁽²¹⁾ using phase analysis by X-ray diffraction, were able to identify whitlockite and diopside as crystalline phases at 1100 °C, representing near to 70 vol.% of the samples, and the remaining 30 vol.% are in the glassy state.

Characterization of the sintered samples

Fig. 4 shows the results of the relative density as a function of sintering temperature. An increase of relative density is observed as function of sintering temperature. Samples sintered at 1450 ^oC/120 min lead to 94.5% of theoretical density. Furthermore, comparatively, monolithic samples sintered 1600 ^oC/120 min present relative density near to 92%. The X-ray diffraction patterns of the sintered Al₂O₃-bioglass samples are shown in Fig. 5.

Figure 4/Figura 4
Relative density as a function of sintering temperature. Densidade relativa como função da temperatura de sinterização.

Figure 5/Figura 5
X-ray diffraction patterns of the Al₂O₃-glass composite ceramic sintered at 1200 ºC/60 min and 1450 ºC/120 min. Difratogramas de raios X do compósito Al ₂ O ₃ -vitrocerâmico, sinterizado a 1200 ºC/60 min e 1450 ºC/120 min.

The peaks of the α-Al₂O₃ phase remain unaltered, independent on the sintering temperature, and show the same positions and relative intensities as the starting powder, see Fig. 2. Besides α-Al₂O₃, the phases whitlockite [3CaO.P₂O₅] and diopsite [3(Ca,Mg)O.P₂O₅], can be observed in all sintered samples. This observation is consistent with the results of the thermal analysis of the glass, Fig. 3, which indicates the crystallization of these phases. The microstructures of the sintered samples are shown in Fig. 6.

Figure 6/Figura 6
SEM micrographs of the sintered samples at temperatures of (a) 1200 ºC; (b) 1300 ºC; (c) 1400 ºC and (d) 1450 ºC. Micrografias em MEV de amostras sinterizadas a: (a) 1200 ºC; (b) 1300 ºC; (c) 1400 ºC e (d) 1450 ºC.

With increasing sintering temperature, the pore size decreases, especially when sintered at 1400 ºC or 1450 ºC. The pore sizes decrease from approximately 15 μm when sintered at 1200 ºC/60 min to less than 8 μm when sintered at 1450 ºC/120 min. These observations are consistent with the results of the final relative densities shown in Fig. 4, indicating a significant increase at relative density in the samples when sintered at 1400 ºC or 1450 ºC. These results are attributed to a decrease of the viscosity of the reminiscent glassy phase at temperatures above 1400 ºC, enhancing the liquid phase sintering mechanism ⁽²⁴⁾.

Mechanical properties

The hardness and toughness of samples sintered at different temperatures are shown in Table II.

Table II/Tabela II
Hardness and fracture toughness of the samples as function of sintering conditions. Dureza e tenacidade a fratura em função das condições de sinterização.

Samples	Sintering conditions	Relative density (%TD)	Vickers hardness HV2000gF (GPa)	Fracture toughness (MPa.m1/2)
Al2O3-glass ceramic composite (90:10)	1200 ºC -1 h	77.9 ± 3.8	1.2 ±0.3	0.4 ± 0.3
1300 ºC -1 h	80.9 ± 3.2	1.4 ±0.2	2.3 ± 0.2
1400 ºC -1 h	92.0 ± 4.1	8.0 ± 0.5	5.7 ± 0.3
1450 ºC -2 h	94.5 ± 4.2	9.0±0.5	5.9 ± 0.4
Monolithic Al2O3	1600 ºC -2 h	92.1 ± 3.7	17.4 ± 0.4	3.8 ± 0.5

Both hardness and fracture toughness of the samples increase significantly when sintered at temperatures higher than 1400 ºC. This improvement is directly related to the increase of the relative density. For sintering temperatures higher than 1400 ºC, relative density higher than 90% and the hardness exceeds 8 GPa, while the fracture toughness ranges in the order of 6 MPa.m^1/2. In comparison, conventionally solid-state sintered Al₂O₃ at 1600 ºC exhibit hardness near to 17 GPa, relative density of 92% and fracture toughness 3.8 MPa.m^1/2. In applications of ceramic materials as structural components in prosthesis, lower hardness and high fracture toughness are important because they permit the preparation and restoration of parts with complex geometry, which must be machined to high quality finishing. Besides, high fracture toughness materials have also improved strength and reliability. A brief comparison with the materials properties presented in Table I permits to state that the Al₂O₃-bioglass material has good fracture toughness and similar hardness.

Theoretical residual stress

Table III shows the coefficient of thermal expansion and Young modulus of the Al₂O₃ and glass-ceramic composite phases. Previous studies ^{(19) , (23)} found that these ceramics present, after heat treatment at 1100 °C, Young modulus 130 GPa with porosity 5% and 70% crystalline phase.

Table III/Tabela III
Coefficient of thermal expansion and Young's modulus of the Al2O3 and glass. Coeficiente de expansão térmica e modulo de elasticidade da Al2O3 e do vidro.

Material	Coefficient of thermal expansion (CTE) α(25 ºC-800 ºC)	Young modulus (GPa)
3CaO.P2O5-SiO2glass	10.2 x 10-6/ºC	130
Al2O3	7.5 x 10-6/ºC	450

The calculation of the average thermal residual stress, generated during cooling of the sintered samples, is based on the homogeneous distribution of the second phase in the ceramic matrix, and it is directly related to the thermal expansion coefficient difference between Al₂O₃-matrix and the glassy (intergranular) phase ^{(25) - (27)}. This average thermal residual stress in both phases can also be calculated as a function of the volume fraction of sintering additive, following the approach using equation B and C ⁽²⁵⁾.

Here, σ_g and σ_m are residual stresses in the system (glass and substrate matrix, respectively). E_m and E_g indicate the Young modulus of the matrix and glass, respectively, and α_m and α_gindicate the average thermal expansion coefficient of the composite, matrix and glassy phase, respectively. The average thermal expansion coefficient for each composition can be calculated using equation D:

where C_g and C_m are, respectively, the fraction of glass and matrix. Based on the above calculation, one finds that, on average, when α_m > α_g and σ_g < 0, the grain boundary will be in compression and the matrix will be in tension ^{(25) , (26)}. Residual stress in multiphase composites develops due to the mismatch of the E-modulus and the thermal expansion coefficient in the constituent phases. Due to the lower TEC of the glass α_b than that of the matrix α_m, residual tensile stresses are developed in the Al₂O₃-glass ceramic matrix during cooling. The residual stress in the multiphase composites is developed because of the mismatch in the E-modulus and the thermal expansion coefficient among the constituent phases. Fig. 7 shows the theoretical residual stress of the composite and indicates a tensile stress in order of 500 MPa in the glass-ceramic phase, which promotes the crack deflection and the formation of grains bridging.

Figure 7/Figura 7
Theoretical residual stress as function of sintering temperature. Tensão residual teórica como função da temperatura de sinterização.

CONCLUSIONS

This work shows that it is possible to sinter alumina ceramics by liquid phase sintering, using bioglass as additive, at lower temperatures as compared with conventional solid-state sintered alumina. Adding 10 wt.% of bioglass as sintering aid to Al₂O₃, a relative final density of 95% could be achieved at temperatures as low as 1450 ºC, with a hardness of ~ 9G Pa and a fracture toughness of 6.5 MPa.m^1/2. Furthermore, a partial crystallization of the intergranular glassy phase into 3(Ca,Mg)O.P₂O₅, whitlockite, and (Mg,Ca)O.SiO₂, enstatite, has been observed and that inhibited the full densification of these ceramics. The comparison with monolithic α-Al₂O₃ sintered at 1600 ^oC/120 min indicates an increasing of 2.6% in relative density and 54% in fracture toughness, while the hardness was reduced to ~ 48% relative to monolithic Al₂O₃.

ACKNOWLEDGMENTS

The authors would like to thank the Brazilian research funding agency FAPESP (Fundação de Amparo à Pesquisa do Estado de S. Paulo) for financial support (Grants 04/04386-1 and 06/50510-1).

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Publication Dates

• Publication in this collection
  Apr-Jun 2015

History

• Received
  06 Nov 2014
• Accepted
  06 Mar 2015