- Email: [email protected]
calcium phosphate composites produced by hot pressing
September
1997
Materials Letters 32 (1997) 229-233
ELSEVIER
Microstructure and mechanical properties of silicon carbide whisker/calcium phosphate composites produced by hot pressing K.Park a,*, T.
Vasilosa b
a Depanment of Materials Engineering, Chung-ju National Uniuersity, Chungju 380-702, South Korea b Department oj Chemicaland Nuclear Engineering, University of Massachusetts, Lowell, Massachusetts 01854, USA Received
13 January
1997; accepted
19 January
1997
Abstract The Sic whisker/calcium phosphate composites were fabricated by hot pressing at 1185°C and 7.6 MPa in vacuum. The calcium phosphate matrix of the composites was porous. The chemical reaction at the interface between the Sic whisker and matrix formed an amorphous SiO, layer. The SEM fractographs showed predominantly a transgranular failure. It is demonstrated that the Sic whisker/calcium phosphate composites would be good biomaterials, when a low mechanical strength is required. Keywords: SiC/Ca,(PO,),;
Biomaterials;
Mechanical
strength; Bone replacement;
1. Introduction The major requirements of acceptable bioceramics are a good biocompatibility, high mechanical strength, and ease of handling in an operating room environment. They must also be easily sterilized and fabricated and be available at a reasonable cost [I]. The calcium phosphate ceramics with molar Ca : P ratios of 1SO : 1 to 1.67 : 1 are widely accepted because of their good biocompatibility with bone tissues and good corrosion and abrasion resistances [2]. However, the ceramics are extremely brittle so that they fail in a brittle fashion with very little deformation to failure. This can be a major problem when they are utilized in weight-bearing areas of the human body. Recently, attempts were made to improve
* Corresponding
author. Fax: + 82-441-841-5380.
00167-577X/97/%17.00 Copyright PII SO167-577X(97)00041-4
SIC whisker; Hot pressing;
the mechanical properties by forcements into polymers or study, Sic whisker was added phate ceramics to produce the
Microstructure;
TEM
the addition of reinceramics [3]. In this into the calcium phoscomposites.
2. Experimental In order to fabricate the Sic whisker (5 vol%)/calcium phosphate composites, calcium phosphate powder and Sic whisker were mixed by a ball-milling for 30 min in methanol. The slurries were allowed to dry. The dried powders were then mixed again by a ball-milling for 5 min to remove all agglomerates. The dried powder mixtures were compacted by the hot pressing at 1185 t and 7.6 MPa for 30 min in vacuum to produce the composites. The composites were cooled to room temperature and then machined into rectangular bars (3 X 4 X 45
0 1997 Elsevier Science B.V. All rights reserved.
230
K. Park, T. Vasilosa/Materials Letters 32 (1997) 229-233
mm). The open porosity of the composites was determined in accordance with ASTM C373 [4]. The mechanical properties of the composites were measured at room temperature under a cross-head speed of 0.5 mm/min by four-point bending in accordance with ASTM D790 [5] using an Instron universal testing machine. The fracture surface of the failed composites was examined by scanning electron microscopy (SEM) using Hitachi 2400 and Hitachi 2460 scanning electron microscopes. Detailed microstructural information on the composites was obtained from transmission electron microscopy (TEM). TEM samples were sectioned with a low-speed diamond saw and mechanically ground to = 300 pm. Circular discs for the TEM samples, 3 mm in diameter, were core drilled from the ground sections. The samples were mechanically ground to = 120 pm, and then dimpled to = 30 pm. The samples were ion milled with 3 kV Ar+ ions at an incident angle of 12” until perforation occurred. The microstructure and chemistry of the composites were investigated by a bright-field image, selected area diffraction (SAD), and energy dispersive X-ray spectroscopy (EDS) using a Philips EM400T TEM operated at 120 kV and a Noran ultrathin window Micro-Z detector.
3. Results and discussion Fig. la and lb represent the bright-field images showing spherical pores (0.1 to 0.3 pm diameter) and both spherical (0.1 to 0.5 pm diameter) and cylindrical pores (0.1 pm diameter and 0.5 pm length), respectively, in the calcium phosphate matrix of the composites. The crack shown in the middle of Fig. lb was formed during TEM sample preparation. A high density of pores was observed within the matrix grains. The open porosity measured in the composites was 0.203. The porous matrix allows the ingrowth of bone tissue to achieve the full integration of the inert implant with living bone, resulting in an intimate bone contact [6]. However, the pores within the matrix deteriorate the mechanical properties of the composites because they act as a stress raiser. As the porosity increases, the strength of the composites decreases rapidly as expressed by the Ryskewitch equation [7]: cr= oOe-“‘, where u
Fig. 1. Bright-field images showing (a) spherical pores and (b) both spherical and cylindrical pores in the calcium phosphate matrix of the composites.
is the strength, o0 is the strength at zero porosity, c is a constant, and p is the porosity. Fig. 2a and 2b show the SAD pattern and EDS spectrum obtained from the matrix, respectively. The matrix was identified as Ca,(PO,), with a hexagonal structure (a = 0.935 nm and c = 0.682 nm). Fig. 3 shows a longitudinal view of SIC whisker in the calcium phosphate matrix. The whisker contained many twins. The interfacial layers (indicated by arrows) between the Sic whisker and matrix were formed by a chemical reaction at the interface during fabrication. No voids and cracks were observed within and around the interfacial layers. The thickness of the interfacial layers shown in Fig. 3 is in the range of 12 to 85 nm. The matrix had undergone an electron irradiation damage when observed in the
K. Park, T. Vasilosa /Materials Letters 32 (1997) 229-233
231
Fig. 4. SAD pattern from the SIC whisker.
._, 1..
__.l___._i_.__._._L___~~____~.
..----__-_.___F.._
t-
___.
1,
4
+..----.-..-~ I -It: __--__;_
+--...
\:
nova
!I ; I--
VFS
Fig. 2. (a) SAD pattern and (b) EDS spectrum calcium phosphate matrix.
ii
= 403s
5
the Sic fiber and interfacial layer, SAD patterns and EDS spectra were obtained. Fig. 4 shows a SAD pattern from the SIC whisker. This indicates that the whisker is a-Sic with a
ia3
obtained from the
electron microscope operated at 120 kV. The electron irradiation effect increased with increasing exposure times. There was no evidence for a phase transformation due to the electron irradiation. In order to identify the microstructure and chemistry of
(b)
I.--
I ..__
___.__~_.-
/
i
i
I__..___.~__._.-;--_____
..__.)
-_--_.--+-__ “‘__..._____,~
j
._,i__...~~~~~~~~~._~~~ ._..._
_ _j-_-_-_____-_,______-___-.+-_._---__ /ii
p-.._
..-._ -_.-&--_-
-...__+_______
t-
oeQ0
Fig. 3. Bright-field phosphate matrix.
image
of the Sic
whisker
in the calcium
MS=256
I
120
Fig. 5. (a) SAD pattern and (b) EDS spectrum obtained from the interfacial layer between the Sic whisker and calcium phosphate matrix.
K. Park, T. Vasilosa /Materials Letters 32 (1997) 229-233
232
Fig. 8. SEM fractograph composites.
from the Sic whisker/calcium
phosphate
Fig. 6. Bright-field image of the SIC whisker and calcium phosphate matrix in the composites.
hexagonal structure (a = 0.307 nm and c = 1.508 nm). The SAD pattern and EDS spectrum obtained from the interfacial reaction layer are shown in Fig. 5a and 5b, respectively. In Fig. 5b, the main elements are Si and 0 with other elements such as C, Ca and P. The other elements originated from the scattering of SIC whisker and calcium phosphate matrix, because the size of the electron probe was not small enough to illuminate only the inter-facial reaction layer. The interfacial layer was determined as amorphous SiO,. The interfacial SiO, layer may degrade the mechanical properties of the composites. A transverse view of Sic whisker in the calcium phosphate matrix of the composites is shown in Fig. 6. The interfacial SiO, layer is clearly shown.
The room-temperature mechanical properties of the Sic whisker/calcium phosphate composites are shown in Fig. 7. The ultimate bending strength and strain to failure are 14.5 MPa and 0.14%, respectively. Many pores within the matrix degraded significantly the mechanical properties. Also, the interfacial SiO, layer between the matrix and whisker led to a slight decrease in the mechanical properties. Fig. 8 shows an SEM micrograph of fractured surface from the Sic whisker/calcium phosphate composites. The fracture faces revealed that the transgranular failure occurred predominantly. Also, the fractograph showed a little whisker pull out. The Sic reinforcement offered a resistance to crack extension by supporting load.
4. Conclusions
A // \ t t .f \
!
1 i
0.1
I
Strain (%) Fig. 7. Room-temperature whisker/calcium phosphate
mechanical composites.
properties
of the
Sic
A porous matrix in the composites was observed. The measured open porosity of the composites was 0.203. It was found that an amorphous SiO, layer was formed by a chemical reaction at the interface between the Sic whisker and matrix. The composites showed a low ultimate strength and failure strain mainly because of a high density of large size pores in the matrix. The SEM fractographs of the failed composites represented that fracture modes were dominantly transgranular. We believe that the Sic whisker/calcium phosphate composites would be good biomaterials for bone replacement, when a low mechanical strength is required.
K. Park, T. Vasilosa/Materials Letters 32 (1997) 229-233
References [l] C. yLavemia and J. M. Schoenung, Am. Gram. Sot. Bull. 70 (1991) 95. [2] K. de Groot, Biocompatibility of clinical implant materials, ed. D. F. Williams (CRC Press, Boca Raton, FL, 1981) p. 199. [3] C. Doyle, Handbook of bioactive ceramics, Vol. 2, eds. T. Yamamuro, L. L. Hench and J. Wilson (CRC Press, Boca Raton, FL, 1990).
[4] ASTM C373 (American Society Philadelphia, PA, 1993). 151 ASTM D790 (American Society Philadelphia, PA, 1993). 161 R.E. Holmes, R.W. Bucholz and Surg. 68 A (1986) 904. [7I L.L. Hench, J. Amer. Ceram. Sot.
233 for Testing
and Materials,
for Testing
and Materials,
V. Mooney,
J. Bone Joint
74. (1991) 1487.
1997
Materials Letters 32 (1997) 229-233
ELSEVIER
Microstructure and mechanical properties of silicon carbide whisker/calcium phosphate composites produced by hot pressing K.Park a,*, T.
Vasilosa b
a Depanment of Materials Engineering, Chung-ju National Uniuersity, Chungju 380-702, South Korea b Department oj Chemicaland Nuclear Engineering, University of Massachusetts, Lowell, Massachusetts 01854, USA Received
13 January
1997; accepted
19 January
1997
Abstract The Sic whisker/calcium phosphate composites were fabricated by hot pressing at 1185°C and 7.6 MPa in vacuum. The calcium phosphate matrix of the composites was porous. The chemical reaction at the interface between the Sic whisker and matrix formed an amorphous SiO, layer. The SEM fractographs showed predominantly a transgranular failure. It is demonstrated that the Sic whisker/calcium phosphate composites would be good biomaterials, when a low mechanical strength is required. Keywords: SiC/Ca,(PO,),;
Biomaterials;
Mechanical
strength; Bone replacement;
1. Introduction The major requirements of acceptable bioceramics are a good biocompatibility, high mechanical strength, and ease of handling in an operating room environment. They must also be easily sterilized and fabricated and be available at a reasonable cost [I]. The calcium phosphate ceramics with molar Ca : P ratios of 1SO : 1 to 1.67 : 1 are widely accepted because of their good biocompatibility with bone tissues and good corrosion and abrasion resistances [2]. However, the ceramics are extremely brittle so that they fail in a brittle fashion with very little deformation to failure. This can be a major problem when they are utilized in weight-bearing areas of the human body. Recently, attempts were made to improve
* Corresponding
author. Fax: + 82-441-841-5380.
00167-577X/97/%17.00 Copyright PII SO167-577X(97)00041-4
SIC whisker; Hot pressing;
the mechanical properties by forcements into polymers or study, Sic whisker was added phate ceramics to produce the
Microstructure;
TEM
the addition of reinceramics [3]. In this into the calcium phoscomposites.
2. Experimental In order to fabricate the Sic whisker (5 vol%)/calcium phosphate composites, calcium phosphate powder and Sic whisker were mixed by a ball-milling for 30 min in methanol. The slurries were allowed to dry. The dried powders were then mixed again by a ball-milling for 5 min to remove all agglomerates. The dried powder mixtures were compacted by the hot pressing at 1185 t and 7.6 MPa for 30 min in vacuum to produce the composites. The composites were cooled to room temperature and then machined into rectangular bars (3 X 4 X 45
0 1997 Elsevier Science B.V. All rights reserved.
230
K. Park, T. Vasilosa/Materials Letters 32 (1997) 229-233
mm). The open porosity of the composites was determined in accordance with ASTM C373 [4]. The mechanical properties of the composites were measured at room temperature under a cross-head speed of 0.5 mm/min by four-point bending in accordance with ASTM D790 [5] using an Instron universal testing machine. The fracture surface of the failed composites was examined by scanning electron microscopy (SEM) using Hitachi 2400 and Hitachi 2460 scanning electron microscopes. Detailed microstructural information on the composites was obtained from transmission electron microscopy (TEM). TEM samples were sectioned with a low-speed diamond saw and mechanically ground to = 300 pm. Circular discs for the TEM samples, 3 mm in diameter, were core drilled from the ground sections. The samples were mechanically ground to = 120 pm, and then dimpled to = 30 pm. The samples were ion milled with 3 kV Ar+ ions at an incident angle of 12” until perforation occurred. The microstructure and chemistry of the composites were investigated by a bright-field image, selected area diffraction (SAD), and energy dispersive X-ray spectroscopy (EDS) using a Philips EM400T TEM operated at 120 kV and a Noran ultrathin window Micro-Z detector.
3. Results and discussion Fig. la and lb represent the bright-field images showing spherical pores (0.1 to 0.3 pm diameter) and both spherical (0.1 to 0.5 pm diameter) and cylindrical pores (0.1 pm diameter and 0.5 pm length), respectively, in the calcium phosphate matrix of the composites. The crack shown in the middle of Fig. lb was formed during TEM sample preparation. A high density of pores was observed within the matrix grains. The open porosity measured in the composites was 0.203. The porous matrix allows the ingrowth of bone tissue to achieve the full integration of the inert implant with living bone, resulting in an intimate bone contact [6]. However, the pores within the matrix deteriorate the mechanical properties of the composites because they act as a stress raiser. As the porosity increases, the strength of the composites decreases rapidly as expressed by the Ryskewitch equation [7]: cr= oOe-“‘, where u
Fig. 1. Bright-field images showing (a) spherical pores and (b) both spherical and cylindrical pores in the calcium phosphate matrix of the composites.
is the strength, o0 is the strength at zero porosity, c is a constant, and p is the porosity. Fig. 2a and 2b show the SAD pattern and EDS spectrum obtained from the matrix, respectively. The matrix was identified as Ca,(PO,), with a hexagonal structure (a = 0.935 nm and c = 0.682 nm). Fig. 3 shows a longitudinal view of SIC whisker in the calcium phosphate matrix. The whisker contained many twins. The interfacial layers (indicated by arrows) between the Sic whisker and matrix were formed by a chemical reaction at the interface during fabrication. No voids and cracks were observed within and around the interfacial layers. The thickness of the interfacial layers shown in Fig. 3 is in the range of 12 to 85 nm. The matrix had undergone an electron irradiation damage when observed in the
K. Park, T. Vasilosa /Materials Letters 32 (1997) 229-233
231
Fig. 4. SAD pattern from the SIC whisker.
._, 1..
__.l___._i_.__._._L___~~____~.
..----__-_.___F.._
t-
___.
1,
4
+..----.-..-~ I -It: __--__;_
+--...
\:
nova
!I ; I--
VFS
Fig. 2. (a) SAD pattern and (b) EDS spectrum calcium phosphate matrix.
ii
= 403s
5
the Sic fiber and interfacial layer, SAD patterns and EDS spectra were obtained. Fig. 4 shows a SAD pattern from the SIC whisker. This indicates that the whisker is a-Sic with a
ia3
obtained from the
electron microscope operated at 120 kV. The electron irradiation effect increased with increasing exposure times. There was no evidence for a phase transformation due to the electron irradiation. In order to identify the microstructure and chemistry of
(b)
I.--
I ..__
___.__~_.-
/
i
i
I__..___.~__._.-;--_____
..__.)
-_--_.--+-__ “‘__..._____,~
j
._,i__...~~~~~~~~~._~~~ ._..._
_ _j-_-_-_____-_,______-___-.+-_._---__ /ii
p-.._
..-._ -_.-&--_-
-...__+_______
t-
oeQ0
Fig. 3. Bright-field phosphate matrix.
image
of the Sic
whisker
in the calcium
MS=256
I
120
Fig. 5. (a) SAD pattern and (b) EDS spectrum obtained from the interfacial layer between the Sic whisker and calcium phosphate matrix.
K. Park, T. Vasilosa /Materials Letters 32 (1997) 229-233
232
Fig. 8. SEM fractograph composites.
from the Sic whisker/calcium
phosphate
Fig. 6. Bright-field image of the SIC whisker and calcium phosphate matrix in the composites.
hexagonal structure (a = 0.307 nm and c = 1.508 nm). The SAD pattern and EDS spectrum obtained from the interfacial reaction layer are shown in Fig. 5a and 5b, respectively. In Fig. 5b, the main elements are Si and 0 with other elements such as C, Ca and P. The other elements originated from the scattering of SIC whisker and calcium phosphate matrix, because the size of the electron probe was not small enough to illuminate only the inter-facial reaction layer. The interfacial layer was determined as amorphous SiO,. The interfacial SiO, layer may degrade the mechanical properties of the composites. A transverse view of Sic whisker in the calcium phosphate matrix of the composites is shown in Fig. 6. The interfacial SiO, layer is clearly shown.
The room-temperature mechanical properties of the Sic whisker/calcium phosphate composites are shown in Fig. 7. The ultimate bending strength and strain to failure are 14.5 MPa and 0.14%, respectively. Many pores within the matrix degraded significantly the mechanical properties. Also, the interfacial SiO, layer between the matrix and whisker led to a slight decrease in the mechanical properties. Fig. 8 shows an SEM micrograph of fractured surface from the Sic whisker/calcium phosphate composites. The fracture faces revealed that the transgranular failure occurred predominantly. Also, the fractograph showed a little whisker pull out. The Sic reinforcement offered a resistance to crack extension by supporting load.
4. Conclusions
A // \ t t .f \
!
1 i
0.1
I
Strain (%) Fig. 7. Room-temperature whisker/calcium phosphate
mechanical composites.
properties
of the
Sic
A porous matrix in the composites was observed. The measured open porosity of the composites was 0.203. It was found that an amorphous SiO, layer was formed by a chemical reaction at the interface between the Sic whisker and matrix. The composites showed a low ultimate strength and failure strain mainly because of a high density of large size pores in the matrix. The SEM fractographs of the failed composites represented that fracture modes were dominantly transgranular. We believe that the Sic whisker/calcium phosphate composites would be good biomaterials for bone replacement, when a low mechanical strength is required.
K. Park, T. Vasilosa/Materials Letters 32 (1997) 229-233
References [l] C. yLavemia and J. M. Schoenung, Am. Gram. Sot. Bull. 70 (1991) 95. [2] K. de Groot, Biocompatibility of clinical implant materials, ed. D. F. Williams (CRC Press, Boca Raton, FL, 1981) p. 199. [3] C. Doyle, Handbook of bioactive ceramics, Vol. 2, eds. T. Yamamuro, L. L. Hench and J. Wilson (CRC Press, Boca Raton, FL, 1990).
[4] ASTM C373 (American Society Philadelphia, PA, 1993). 151 ASTM D790 (American Society Philadelphia, PA, 1993). 161 R.E. Holmes, R.W. Bucholz and Surg. 68 A (1986) 904. [7I L.L. Hench, J. Amer. Ceram. Sot.
233 for Testing
and Materials,
for Testing
and Materials,
V. Mooney,
J. Bone Joint
74. (1991) 1487.