Porosity Evaluation of Sintered Powder Compacts and Its Effect on Some Properties of Bioactive (Na

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91

Porosity Evaluation of Sintered Powder Compacts and Its

Effect on Some Properties of Bioactive (Na

2

O – CaO – SiO

2

– P

2

O

5

) Glass - Ceramic System

Waleed Asim Hanna

Dept. of Materials Eng., Univ. of Technology, Baghdad, Iraq [email protected]

Abstract. The aim of this research is the preparation and characterization of glass-ceramic that is able to mimic the structure of the natural bone, formed by cortical and cancellous bone and to study the effect of formed porosity on the prepared system. The method and materials described herein provide biocompatible materials, e.g. Na2O - CaO - SiO₂- P₂O₅ glasses prepared by melt – quench rout, ground, and sieved to obtain powder of specific size. The samples were characterized through morphological observation i.e. (SEM and OPM) to identify the porose formation also x- ray diffraction used to identify the major phases, i.e. sodium calcium silicate, calcium silicate and calcium phosphate. As well as porosity porosemetry test was used to reveal the volume & % of porosity and density measurements, some of mechanical tests also were done represented by hardness and fracture toughness.

Keywords: Porosity, Bioactive materials, Glass-Ceramic system.

1. Introduction

One of the great challenges of the 21^st Century is increasing life expectancy, while at the same time maintaining quality of life in an aging population ^[1]. Tissue engineering possesses new challenges in the area of biomaterials, specially for bone and cartilage tissue engineering applications. Bone is an open cell porous composite material made by osteoblast cells. Also it is composed of two kinds of tissue, cortical (compact / dense, placed on the exterior of the bone) and trabecular (concellous /porous, placed in the interior). Cortical bone is semi-brittle, viscous elastic and anisotropic material, and properties of these materials are influenced by porosity, mineralization level and the organization of

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the solid matrix. Knowing that cortical bone contains about 70% of several minerals, i.e. calcium, phosphorous sodium, magnesium and carbonate which are providing the body with structural strength and internal organ protection ^[2,3].

An ideal synthetic bone graft substitute should be a porous matrix with interconnecting porosity that promotes rapid bone in growth, and at the same time, it should possess a sufficient strength to prevent crushing under physiological load during osteointegration and healing^[4]. Bioactive ceramics have a variety of uses, including use as artificial vertebrae or bone defect supplementation material^[5].

Hench discovered in (1969) the so-called bioactive glasses, which can bond chemically to bone. The main characteristics of these bioactive glasses are the formation of a hydroxyapatite layer on their surface in contact with any aqueous solution. This layer is the equivalent in composition and structure to the mineral phase of bone ^[6]. Bioactive glasses and glass – ceramics have been widely studied during the past three decades after Hench et al. reported that some silicate glasses within the Na2O-CaO-P2O5-SiO2 system chemically bond with living bone ^[1].It has been found that reactions on bioactive glass surfaces led to the release of critical concentrations of soluble Si, Ca, P and Na ions, which induce favorable intracellular and extracellular responses leading to rapid bone formation ^[7].

For glass as a candidate tissue engineering material, porosity is desired for cellular growth and attachment to the implant materials surface. Recent studies have shown that bioactive glass forms a better bond with bone if it is porous. Irrespective of the size, it is important that the pores are interconnected for the flow of the fluids, migration of cell, etc. ^[8]. Although bioactive glasses are mechanically weak, but can partially crystallize when heated to high temperatures (> 950 ^oC) and that the mechanically strong crystalline phase can transform to a biodegradable, amorphous calcium phosphate at body temperature and in a biological environment ^[7]. Bioactive glass – ceramics bond strongly to bone and therefore have potential as important material. They have superior strength and toughness to the monolithic counterpart ^[9].

From a thermodynamic point of view the stable crystal phase has lower free energy and higher density than the corresponding unstable amorphous phase with the same composition. Crystallization processes, related to the glass – ceramics production, assumed that would lead to

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some shrinkage of the products and to increasing the density ^[10]. Most ceramics are processed by sintering which consists of submitting the pieces to thermal treatment at sufficiently high temperatures. During sintering, various diffusion mechanisms occur, which lead to a densification of the porous piece: the porosity progressively decreases while the piece shrinks. However, it is difficult to obtain a perfectly dense state without any porosity. In many cases a low residual porosity is accepted for specific uses, a sufficient and often high porosity is required^[11].

Highly porous ceramics usually occur as a result of incompliant sintering but can be used in their own right in applications as diverse as bioimplants and leachable inserts ^[12]. The rapture of the pores (open or closed) their size, shape, localization (intergranular or intragranular) and the repartition spectrum must be known to accurately characterize the microstructure all these parameters having an influence on the properties

[11].

Generally, pores can be classified by their size and structure, i. e., as micropores those of width less than 2nm, as mesopores those ranging from (2-50) nm, and as macropores, those wider than 50 nm, with considering the periods of tests ^{[13, 14]}.

Our current research introduces a single phase system of (Na2O- CaO-P2O5-SiO2) bioactive glass that is crystallized to glass – ceramic with a certain porosity to avoid two main problems : first, the clinical methods such as implantation of transplantation poses well known drawback, such as lack of ability to self repair, limited vascularization of implants, limited number of donors, and possibility of rejection of transplanted tissues ^[1]. Second, the Bioactive ceramics specially bioglass type, have lower fracture toughness and higher young's modulus than human cortical bone, so that applications are limited to the replacement of bony parts under low loads and as bone fillers, especially when pores exist in the sintered powder compacts^[15].

2. Experimental Work 2.1 Materials Preparation

The glass of the so called 45s5 composition of [12 Na2O-28CaO- 10P2O5-50SiO2] (in wt %) was prepared by the melt quench technique.

The raw materials, including SiO2, sodium carbonate, calcium carbonate and phosphorus pentaoxides, were dry mixed and pressed. The melting of

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glass batch mixture was carried out in a 100 ml platinum crucible at 1500

oC for 2h. The glass was formed by casting the melt over a stainless steel plate (also copper plate is recommended), and then annealing for (1.5 – 3) h at (550 – 520) ôC, followed by slow cooling at room temperature, to remove the residual stresses. The melts were fritted by using porciliene hard pot, and the obtained frits broken milled and sieved to reveal powder of (< 53)µm [1, 8,10, 16]. To induce crystallization, the samples were subjected to an additional creaming heat treatment in two steps: (a) a nucleation step at a temperature which consisted of heating at 5 ôC /min to 670 ôC and heating for 1h ^[8],followed by, (b) a crystal growth step, which included heating at the same rate to different temperatures ranging from (0.7-0.9)Tm, i. e. ( 945, 1013, 1080, 1148) ôC for 1h, also the effect of soaking time for crystallization and the pressing pressure of the powder compact were studied at a range of (1 – 5)h, and (2 – 8) ton/cm², respectively.

The sintering process was accomplished by using a programmable electrical furnace, and at a rate of heating and cooling 5 ^oC /min.

2.2 Measurements

The pore size distribution (percent & volume) of the resulting samples was determined by Quanta Chrome Autoscan Mercury Porosimetry.

To identify the crystalline phases, the powder sintering compacts were analyzed by X – ray diffraction (XRD) (SHIMADZU – XRD – 6000), was used to investigate the samples, using Cu-k∞ radiation in the 2θ range of (200 – 600). The microstructure and surface porosity of resultant glass – ceramic were investigated with Scanning Electron Microscope (SEM) [Tescan VEGA series] and Optical Microscope (OPM) [XJL-101-Bel-company]. The examined samples with the SEM were coated with a thin film of gold after etching with 48% HF for (20) seconds. The crystallization was evaluated by Differential Thermal Analysis (DTA) at 10 ^oC /min using powder (< 53 µm).

Some physical tests were also evaluated i. e. bulk and apparent densities they are revealed from the, Quanta Chrome Autoscan Mercury Porosimetry with the porosity results, and linear shrinkage calculated from equ. 1 ^[17]. Also some mechanical tests were evaluated like Hardness measured by Vickers method as shown in equ. 2 ^[18]and the fracture toughness measured by Vickers indentation method as shown in equ. 3 ^[19].

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L.Sh% = (D2-D1)/D1 (1) where:

D2 : The diameter before sintering D1 : The diameter after sintering

Hv= 1.854 F/d2 (2) Where:

P: The applied load (N)

d: The dent is made on the test surface (mm)

KIC= 0.0515 P/c^3/2 (3) where:

P: Theapplied load (N)

c : Radius of the surface crack, (mm) = a + L

a: Half-diagonal of Vickers indent (mm) L: Crack length

All measurements were performed at room temperature.

3. Results and Discussion 3.1 Thermal Analysis

Figure 1 shows the DTA curves of the annealed mother glass. The first endothermal peak at about 842 ^oC on the DTA curve is correlated with glass transition. The second exothermal peak at about 950 ^oC represents the crystallization processes i. e., the crystallization of the mainly eutectic components namely [Ca2P2O7], and wollastonit [CaSiO3], phases revealed after sintering of the glass powder compact were identified in crystallized specimens by (XRD).

3.2 The Pore Size Distribution

The effect of crystallization temperature, time of sintering (soaking time of crystallization ) (Fig. 2 &3), and pressing pressure of the powder compact in the range of (945 – 1148) ^oC, (1 – 5) h and (2 – 8) ton/cm² respectively, have been studied on the pores size and percentage that issued in the crystallized glass – ceramic from the sintered glass powder

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Fig. 1. DTA curve of the annealed mother glass at a heating range of 10⁰C/min.

End Exo.

DTA TG

200⁰C 150⁰C

100⁰C 50⁰C

450⁰C 400⁰C

350⁰C 300⁰C

700⁰C 650⁰C

600⁰C 550⁰C

950⁰C 900⁰C

850⁰C 800⁰C

2 1

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(A) (B)

Fig. 2. Effect of sintering temperature on porosity % size distribution of the crystallized, glass-ceramic.

(A) (B)

Fig. 3. Effect of sintering time on porosity % size distribution of the crystalized glass- ceramic.

(A) (B)

Fig. 4. Effect of Pressing Pressure on porosity % size distribution of the crystallized, glass- ceramic.

compact. As temperature is increased, porosity % and volume are decreased from (47.35% and 0.1613 cm³/g) to (35.7 % and 0.1179 cm³/g) respectively (Fig. 4). This may be due to appearance of vitreous phase Na2SiO7 as shown in the XRD pattern in Fig. 5 (b), and these results

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seem to agree with the SEM and OP results where porosity % and its size distribution seem less in appearance as shown in Fig. 6(a,b) and 7(a,b) respectively.

Fig. 5. X-ray diffraction patterns of crystallized glass powder compact at different.

conditions. Ca2P2O7 Ca3(Po4)2 CaSiO3 Na2Ca2SiO3 Na2SiO7

A) 945 °C

B) 1148°

C) 4 h

D) 5 h

E) 2 ton/cm²

F) 4 ton/cm²

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a- 945 ⁰C b- 1148 ⁰

c-4h d-5h

e- 2 ton/cm² f-4 ton/cm²

Fig. 6. SEM of sintered glass powder compact at different conditions and at magnification of Kx 2.5.

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a- 945 ⁰C b-1148 ⁰C

c- 4h d-5h

e- 2ton /cm² f-4ton/cm²

Fig. 7. Optical microscope of sintered glass powder compact at different conditions and at magnification of 200x.

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Figure 5 shows that as time of sintering increase from (1) h to (5) h porosity % and volume are decreased from (47.35% and 0.722 cm³/g) to (10% and 0.0438 cm³/g) respectively, and after an hour of sintering the porosity % and volume starts to unincrease and reach 33.4% and o.528 cm³/g respectively. This may be due to atoms bond to each other forming small necks during sintering. This kind of pores may be seen during necking and before densification reaches its optimum amount. Further decrease in porosity % and volume, i.e. after 5 h of sintering may be related to good densification, this may be due to the grain boundary mechanism of sintering which happens at the end of sintering at which atoms move on grain boundary, trying to fill up the residual holes (pores), and hence increasing the densification and decreasing the porosity % and its volume.

When pressing pressure of powder compact increased from (2 to 8 ton/cm²), the porosity % is increased from 28.8% to 59.8% at (2 and 4) ton/cm² and then decreased to 31% at 8 ton/cm² but still higher than the use of 2 ton/cm². The volume of formed pores decreased from (0.2038 to 0.0971) cm³/g with a pressing pressure increase from (2 – 8) ton/cm², as shown in Fig. 6. An increase in porosity % and volume may be related to low permeability between particles at higher pressures which causes gas blowing during the sintering process leading to formation of holes aided as big pores, which are interconnected with inducing high percent of pores. While as pressing pressure exceeds (4 ton/cm²) the amount and volume of porosity decrease and that may be due to the appearance of a vitreous phase, Na2Ca2SiO3 as shown in the XRD pattern in Fig. 5(f) and these results seem to coincide with SEM & OPM results shown in Fig. 6 (c) and 7(c) respectively.

3.3 X-Ray Diffraction Analysis (XRD)

The XRD patterns of the sintered glass powder compact in the system (Na2O-CaO-SiO2-P2O5) depicted in Fig. 5 shows the presence of Ca2P2O7, Ca3(PO4)2, CaSiO3, Na2SiO7,and Na2Ca2SiO3 phases and at different crystallization conditions of temperature, time, and pressing pressure. The patterns reveal the crystallization of a glass – ceramic system from a mother glass with certain porosity affected by the alteration between the calcium phosphate phases and the formation of vitreous phases that affect the densification process.

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3.4 SEM and OPM Observation

Figures 6 and 7 show the effect of temperature, time, and pressing pressure on structural behavior of sintered glass – ceramic system [Na2O – CaO – SiO2 – P2O5]. The SEM pictures in Fig. 6 show how the main Phase of bone mineralization Ca2P2O5 with a less amount of Ca3(PO4)2

phase, starting to precipitate mainly from the surface of the original glass particles, exhibiting a rise – grain from. The apatite grains become larger and their size increase with temperature rise. This confirms that each Ca2P2O7 grain consists of many smaller crystallites, which are grown radically from the center of each grain. This agrees with X – ray patterns in Fig. 7 were the Ca2P2O7 one of the two major phases, i.e. Ca2P2O7 and CaSiO3. Also the SEM revealed that the formation of porosity and its changes in % and volume at different conditions which agree with the pictures of (% and volume) OPM that show the topography porosity distribution.

3.5 Physical Properties

a) Linear Shrinkage

An incremental increase in linear shrinkage had resultant and reaches its maximum amount (10.7%) at sintering temperature of (1148⁰C) as shown in Fig. 8. Evaporation of binder, moisture and then densification explain the increase in shrinkage, also the formation of other phases which crystallize from the residual glass i.e. Na2SiO7 or transformation of Ca2P2O7 to other phase of calcium phosphate i.e., Ca3(PO4)2 as shown in XRD pattern in Fig. 5(b), which exhibits a volume change in the sintered sample, at high temperatures.

Linear shrinkage increases as time of crystallization increases till it reaches 4 h of crystallization, then a decrease is seen after 4h of crystallization and as shown in Fig. 9. Generally, an increase in shrinkage may be due to volume change of sample due to change in relative amount of constituent phases, i.e. higher crystallization of Ca2P2O7 and start the formation of vitreous phases, while a decrease in shrinkage may be related to decrease in amount of residual glass and further densification of the crystallized glass – ceramic.

Figure 10 shows decreases in the linear shrinkage of sintered powder compact with increasing the pressing pressure, this may be attributed to the decrease in distance between powder particles, so, the resultant shrinkage will be smaller at higher pressures.

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Fig. 8. Effect of sintering Temperature on L.Sh% of crystallized Glass-ceramic.

Fig. 9. Effect of sintering Time on L.Sh % of crystallized Glass-ceramic.

Fig. 10. Effect of Pressing Pressure on L .Sh % of crystallized Glass-ceramic.

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b) Apparent and Bulk Densities

Apparent and bulk densities relatively increase with temperature rise but they are decreased at 1080 ^oC, as shown in Fig. 11. These densities are affected by chemical composition (since a multiphase is formed in the glass matrix, which exhibits different densities). This exhibits a low apparent density at the earliest stages of sintering process especially at nucleation step. The apparent density increases due to change in particle shape of Ca2P2O7 that become like a rice grain (i.e.

become rounded) see the SEM picture in Fig. 6 (a, b). While the decrease in these densities at 1080 ^oC may be due to the phase changes and the material passes through verification step while densification goes on.

Later increase in these densities may be attributed to the higher crystallization of Ca2P2O7 and CaSiO3 and formation of new phases Ca2(PO4)3 and Na2SiO7 associated with grain growth of the crystallized structure as shown in Fig. 5 (b) and 6 (b) respectively.

Apparent and bulk densities have varied with time of crystallization. They are decreased after (2 and 4) h of crystallization but they increase after three and five hours of crystallization; specially the apparent density which reaches its maximum amount (4.8583 g/cm³) after 3 h of crystallization (Fig. 12). Their decrease may be due to densification process and the volume change, while new phases are formed or the earliest one becomes more crystallized but their magnitude increases after 5 h of crystallization perhaps due to decrease in amount of residual glass in the glass matrix with prolonged time of crystallization associated with grain growth in the crystal structure as revealed from SEM picture in Fig. 6 (d).

The apparent and bulk densities are varying with pressing pressure they reach their maximum amount when 4 ton /cm² is used while minimum amount were at use of 6 ton /cm² and as shown in Fig. 13.

Decrease in distance between particles makes sintering much easier, so fast densification is expected with pressure increase, but further increase may cause gases capturing, and then blowing, this may affect sintering aid, chemical composition of phases in the structure, which in turn affect the behavior of apparent and bulk densities. At higher pressures, even if gas blowing happens, the refilling of the holes will be easy, perhaps because of easy grain boundary atoms migration due to the movement of atoms for a short distance.

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Fig. 11. Effect of sintering temperature on bulk &apparent densities of crystallized Glass - ceramic.

Fig. 12. Effect of sintering time on bulk &apparent densities of crystallized Glass -ceramic.

Fig. 13. Effect of pressing pressure on bulk &apparent densities of crystallized Glass -ceramic.

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3.6 Mechanical Properties

a) Hardness

Maximum hardness has been revealed at (945 ^oC) then an incremental decrease was seen as temperature of crystallization increase for sintered glass powder compact, and as shown in Fig. 14, decrease in hardness may be attributed to grain growth at high temperatures in multi–

phase materials which comprise phases with different thermal expansion may exhibit cracking near to the phase boundaries, also cracking behavior that may be associated with surface machining ^[20]. Also weaknesses of low melting point of reinforcing phase Na2Ca2Si3O as shown in Fig. 5 (b) will cause the hardness to be decreased.

Hardness decreases as time of crystallization increases, but after 5 h of crystallization hardness increases again, as shown in Fig. 15. Also hardness decreases as pressing pressure increases as shown in Fig. 16.

Fig. 14. Effect of Sintering temperature on hardness of crystallized Glass -ceramic.

Fig. 15. Effect of Sintering time on hardness of crystallized Glass -ceramic.

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Fig. 16. Effect of Pressing Pressure on hardness of crystallized Glass -ceramic.

b) Fracture Toughness

Maximum fracture toughness was 3.5MPa.m^1/2 for the crystallized glass-ceramic at 1013 ^oC and shown in Fig. 17. It is believed that surface roughness, crack tilting and twisting during propagation, caused by thermal expansion mismatch and elastic modulus mismatch stresses, play an important role in toughening the glass-ceramic produced in this study i.e., (toughening by crack deflection) ^[21].

Fig. 17. Effect of Sintering Temperature on fracture toughness of crystallized class-ceramic.

Fracture toughness showed an increase during the first 2h and reached its maximum amount 4MPa.m^1/2 of crystallization, then an obvious decrease happens after 4h of crystallization, later toughness increased, and as shown in Fig. 18. It seems that as time of sintering increase fracture toughness is vice proportional with porosity, i.e.

toughness increases as porosity decreases. But sometimes when porosity is high that will reveal an increase in fracture toughness when crack bridging becomes much easier with higher amount of porosity; also crack

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blunting caused by line tension, will cause impeding of crack propagation^[21].

Maximum Fracture toughness was found to be 3.5 MPa.m^1/2 when 4 ton/ cm² is used as shown in Fig. 19. Perhaps a suitable inter-particles distance provided a good bridging during the sintering process, also micro cracks interaction with residual strain field around line tension create process zone ahead of crack tip causing increase in fracture toughness ^[21]. But further increase in pressing pressure decreases the fracture toughness. It is believed, that decrease in mechanical properties under study in this research, i.e. hardness and fracture toughness with increasing pressure of sintered glass powder compact may be due to decrease in permeability between glass powder particles, which in turn may affect the formation and diffusion of phases. Gas interruption between particles may cause blowing and forming large voids which accumulate to form large cracks that may degrade the mechanical properties.

Fig. 18. Effect of Time of sintering on fracture toughness of crystallized Glass -ceramic.

Fig. 19. Effect of Pressing Pressure on fracture toughness of crystallized Glass -ceramic.

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4. Conclusion

The glass ceramic with weight percentage of Na2O 12%, CaO 28%, SiO2 50%, P2O5 10%, exhibited a maximum porosity of (47.35% and 0.1613 cm³/g) at 945^oC and minimum one after one hour of crystallization (10% and 0.0438 cm³/g). An hour of crystallization revealed a minimum linear shrinkage (2%) while the use of 2 ton /cm² revealed a maximum one (12.2%). The crystallization at 1148°C resulted in a higher apparent density i.e. (6.7832g/cm³), and maximum bulk density occurred when 4 ton/cm³ is used (3.9878 g/cm³). The greatest hardness was achieved when 2 ton /cm³ are used and one hour of crystallization at 945^OC while a greater toughness i.e. (4 MPa.m^1/2) is achieved after two hours of crystallization. X-ray radiography follow up is recommended for specimens with maximum and minimum % of porosity after implantation either in vivo or in vitro to evaluate its affect in fastening of biological bonding.

5. Recommendation for Future Research

Study the effect of some ceramic additives on pores formation in the system [Na2O - CaO – SiO2 – P2O5] and how does that affect the studied properties of the system. Also a biological study in vivo will reveal a certain result of the benefit of the porosity and how does its % and volume affect the stimulation of bone growth.

Acknowledgement

The author would like to thank Univ. of Philadelphia – Faculty of Engineering Dept. of Mechanical Eng. - Jordan for hosting this research was performed. Also many thanks goes to center of nanotechnology – Univ. of Technology Baghdad – Iraq and to Eng. Saad M. Elia – Univ. of Technology -Dept. of Materials Eng., Baghdad-Iraq and Eng. Fatma al- widyan- Univ. of Philadelphia- Dept. of Mechanical Eng. -Jordan for the moral support during the period of the present work.

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