Solution Combustion Synthesis of Calcium …
177
1.1 Hydroxyapatite (HAp)
In the last several years, with the growth in medical science and advancements
in surgical techniques, there is an increasing demand for artificial bone implants
and synthetic hydroxyapatite has commanded extensive attention as a material for
medical implants. Among the above described bioactive ceramics, from the viewpoint of biocompatibility, hydroxyapatite (HAp) is the most appropriate choice for
hard tissue replacement implants. It directly binds to the bone and shows excellent
biocompatibility with hard tissues and also with skin and muscle tissues and results
in better osseointegration. However, it has inferior mechanical properties and hence
it is applied in the form of a coating on metallic components such as titanium and its
alloys, which bear the loads. HAp crystallizes in hexagonal structure and belongs to
the apatite family wherein the compounds in this family possess a similar hexagonal
structure but have different compositions. Calcium apatites have the general formula
of Ca 5 (PO 4 ) 3 X (X = Halide ion or OH group). The important calcium apatites
are HAp, fluorapatite (FA) (Ca 5 [PO 4 ] 3 F) and chlorapatite (CA) (Ca 5 [PO 4 ] 3 Cl). The
apatite structure can be substituted by ions such as Sr
2+ , Pb
2+ , Cd
2+ , Mn
2+ , etc., at
Ca position which results in changes in the properties.
Hydroxyapatite coatings are applied by the atmospheric plasma spray (APS)
method. However, due to the high processing plasma spray temperatures, careful
control of plasma processing parameters is paramount to prevent the thermal decomposition of HAp into other soluble calcium phosphates. While dense as well as porous
HAp blocks have found application as implant material in the field of dentistry and
orthopedics, HAp powder and granules have been used for bone grafting and augmentation [10]. Hydroxyapatite is described as a non-stoichiometric, calcium-deficient
carbonated apatite and is represented by the chemical formula Ca 10 (PO 4 ) 6 (OH) 2 ,
wherein the molar ratio Ca/P is 1.67 [10]. Some of the important properties of hydroxyapatite are as follows: low thermal conductivity (0.013 W/cm.K), higher decomposition temperature (> 1000 °C), higher melting point (1614 °C), lower fracture
toughness (0.7–1.2 MPam
1/2 ), lower Vickers hardness (3–7 GPa for dense HAp) and
lower Young’s modulus (35–130 GPa), besides higher biocompatibility, bioactivity,
cellular compatibility and osteoconduction.
The different methods employed for the development of hydroxyapatite coatings include physical vapor deposition, chemical vapor deposition, sol-gel, electrophoretic deposition, plasma spray technique, etc., [5]. Among these methods,
plasma spraying is the only method that has been approved by the Food and Drug
Administration (FDA), USA [11]. The required key HAp coating properties include
stable phase composition, phase purity, sufficient crystallinity (> 45%), suitable
coating thickness, adequate porosity and roughness, high adhesive and cohesive
strengths and devoid of residual coating stresses to avoid delamination of coatings
during in vivo conditions. A thin (< 50 μm) HAp layer exhibited better adhesion
and the porosity of plasma-sprayed HAp coatings was in the range of 3–20%. High
porosity facilitates ingrowth of bone cells into the bioceramic coatings. However,
very high porosity results in decreased tribological properties.
177
1.1 Hydroxyapatite (HAp)
In the last several years, with the growth in medical science and advancements
in surgical techniques, there is an increasing demand for artificial bone implants
and synthetic hydroxyapatite has commanded extensive attention as a material for
medical implants. Among the above described bioactive ceramics, from the viewpoint of biocompatibility, hydroxyapatite (HAp) is the most appropriate choice for
hard tissue replacement implants. It directly binds to the bone and shows excellent
biocompatibility with hard tissues and also with skin and muscle tissues and results
in better osseointegration. However, it has inferior mechanical properties and hence
it is applied in the form of a coating on metallic components such as titanium and its
alloys, which bear the loads. HAp crystallizes in hexagonal structure and belongs to
the apatite family wherein the compounds in this family possess a similar hexagonal
structure but have different compositions. Calcium apatites have the general formula
of Ca 5 (PO 4 ) 3 X (X = Halide ion or OH group). The important calcium apatites
are HAp, fluorapatite (FA) (Ca 5 [PO 4 ] 3 F) and chlorapatite (CA) (Ca 5 [PO 4 ] 3 Cl). The
apatite structure can be substituted by ions such as Sr
2+ , Pb
2+ , Cd
2+ , Mn
2+ , etc., at
Ca position which results in changes in the properties.
Hydroxyapatite coatings are applied by the atmospheric plasma spray (APS)
method. However, due to the high processing plasma spray temperatures, careful
control of plasma processing parameters is paramount to prevent the thermal decomposition of HAp into other soluble calcium phosphates. While dense as well as porous
HAp blocks have found application as implant material in the field of dentistry and
orthopedics, HAp powder and granules have been used for bone grafting and augmentation [10]. Hydroxyapatite is described as a non-stoichiometric, calcium-deficient
carbonated apatite and is represented by the chemical formula Ca 10 (PO 4 ) 6 (OH) 2 ,
wherein the molar ratio Ca/P is 1.67 [10]. Some of the important properties of hydroxyapatite are as follows: low thermal conductivity (0.013 W/cm.K), higher decomposition temperature (> 1000 °C), higher melting point (1614 °C), lower fracture
toughness (0.7–1.2 MPam
1/2 ), lower Vickers hardness (3–7 GPa for dense HAp) and
lower Young’s modulus (35–130 GPa), besides higher biocompatibility, bioactivity,
cellular compatibility and osteoconduction.
The different methods employed for the development of hydroxyapatite coatings include physical vapor deposition, chemical vapor deposition, sol-gel, electrophoretic deposition, plasma spray technique, etc., [5]. Among these methods,
plasma spraying is the only method that has been approved by the Food and Drug
Administration (FDA), USA [11]. The required key HAp coating properties include
stable phase composition, phase purity, sufficient crystallinity (> 45%), suitable
coating thickness, adequate porosity and roughness, high adhesive and cohesive
strengths and devoid of residual coating stresses to avoid delamination of coatings
during in vivo conditions. A thin (< 50 μm) HAp layer exhibited better adhesion
and the porosity of plasma-sprayed HAp coatings was in the range of 3–20%. High
porosity facilitates ingrowth of bone cells into the bioceramic coatings. However,
very high porosity results in decreased tribological properties.