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S. T. Aruna and M. Shilpa
1.2 Atmospheric Plasma Spraying of Hydroxyapatite
The deposition of HAp coatings by APS is a matured well-researched technique to
coat metallic hip endoprosthetic and dental root implants. The advantages of plasmasprayed HAp coatings are as follows: macro-micro porosity, enhanced bio-adhesion,
nanostructured surface topography, it provides osseointegration with adsorbed osteostimulating biological agents due to the chemical composition resembling that of the
inorganic component of natural bone. Some of the drawbacks of the plasma spray
technique are the thermal decomposition of the feedstock during spraying, lineof-sight process, difficulty to control pore sizes and porosity, and the inability to
deposit coatings with lower thickness. A prudent design of plasma spray conditions
and stringent quality management is required to develop HAp coatings with strong
adhesion and provide bone regeneration over a long time. It has been established that
a HAp coating must meet the minimum requirements described by the U.S. FDA
and the ISO [12–16]. According to ISO 13779-2, some of the important desirable
properties of plasma-sprayed hydroxyapatite coatings are as follows: hydroxyapatite
content > 50%; Non-apatite CaP (%): > 50; Crystallinity > 45%; adhesion strength
> 15 MPa; and Ca/P ratio 1.67–1.76 [17].
Plasma spraying is the most popular thermal spray technique [18]. In plasma
spray technique, a plasma flame is produced by using argon as the primary gas and
nitrogen/hydrogen is used as the secondary gas by striking an arc between the cathode
and the anode. The electrical energy ionizes the gas and produces plasma. The plasma
flame is characterized by relatively high temperatures (15,000 °C) and low velocity.
The ceramic powder feed is fed into the plasma flame by using a carrier gas and the
particles get melted in the plasma plume, traverses with high speed and impinges on
the substrate forming a coating. The schematic of a plasma spray set-up is shown in
Fig. 2.
Many of the methods employed for the synthesis of hydroxyapatite powder
does not yield plasma sprayable grade flowable powders. Generally, HAp fine
powders are prepared using methods such as coprecipitation, hydrothermal synthesis,
solvothermal method, sol-gel method, hydrothermal method and solution combustion method. To obtain flowable powders, the powders are subjected to spray drying
or fusing and drying. Thus obtained powders possess the required flowability for
plasma spraying. Figure 3 shows the steps involved in the fabrication of the HAp
coating starting from the fine powders to a coating on real component.
The review article by Mehdi et al. provides various methodologies employed in the
synthesis of nanosized HAp powders [19]. The review by Surmenev et al. elaborates
on the numerous techniques employed for enhancing the surface compatibility of
implants with bone [5]. Generally, the hydroxyapatite fine powder is synthesized
by solid-state, coprecipitation, sol-gel, flux, hydrothermal and solution combustion
methods. The obtained powders generally do not possess the required flowability and
hence are subjected to spray drying, agglomeration, crushing and fusing, etc. Thus it
is an overarching challenge to prepare flowable hydroxyapatite powder suitable for
plasma spraying without any agglomeration or spray drying process.
S. T. Aruna and M. Shilpa
1.2 Atmospheric Plasma Spraying of Hydroxyapatite
The deposition of HAp coatings by APS is a matured well-researched technique to
coat metallic hip endoprosthetic and dental root implants. The advantages of plasmasprayed HAp coatings are as follows: macro-micro porosity, enhanced bio-adhesion,
nanostructured surface topography, it provides osseointegration with adsorbed osteostimulating biological agents due to the chemical composition resembling that of the
inorganic component of natural bone. Some of the drawbacks of the plasma spray
technique are the thermal decomposition of the feedstock during spraying, lineof-sight process, difficulty to control pore sizes and porosity, and the inability to
deposit coatings with lower thickness. A prudent design of plasma spray conditions
and stringent quality management is required to develop HAp coatings with strong
adhesion and provide bone regeneration over a long time. It has been established that
a HAp coating must meet the minimum requirements described by the U.S. FDA
and the ISO [12–16]. According to ISO 13779-2, some of the important desirable
properties of plasma-sprayed hydroxyapatite coatings are as follows: hydroxyapatite
content > 50%; Non-apatite CaP (%): > 50; Crystallinity > 45%; adhesion strength
> 15 MPa; and Ca/P ratio 1.67–1.76 [17].
Plasma spraying is the most popular thermal spray technique [18]. In plasma
spray technique, a plasma flame is produced by using argon as the primary gas and
nitrogen/hydrogen is used as the secondary gas by striking an arc between the cathode
and the anode. The electrical energy ionizes the gas and produces plasma. The plasma
flame is characterized by relatively high temperatures (15,000 °C) and low velocity.
The ceramic powder feed is fed into the plasma flame by using a carrier gas and the
particles get melted in the plasma plume, traverses with high speed and impinges on
the substrate forming a coating. The schematic of a plasma spray set-up is shown in
Fig. 2.
Many of the methods employed for the synthesis of hydroxyapatite powder
does not yield plasma sprayable grade flowable powders. Generally, HAp fine
powders are prepared using methods such as coprecipitation, hydrothermal synthesis,
solvothermal method, sol-gel method, hydrothermal method and solution combustion method. To obtain flowable powders, the powders are subjected to spray drying
or fusing and drying. Thus obtained powders possess the required flowability for
plasma spraying. Figure 3 shows the steps involved in the fabrication of the HAp
coating starting from the fine powders to a coating on real component.
The review article by Mehdi et al. provides various methodologies employed in the
synthesis of nanosized HAp powders [19]. The review by Surmenev et al. elaborates
on the numerous techniques employed for enhancing the surface compatibility of
implants with bone [5]. Generally, the hydroxyapatite fine powder is synthesized
by solid-state, coprecipitation, sol-gel, flux, hydrothermal and solution combustion
methods. The obtained powders generally do not possess the required flowability and
hence are subjected to spray drying, agglomeration, crushing and fusing, etc. Thus it
is an overarching challenge to prepare flowable hydroxyapatite powder suitable for
plasma spraying without any agglomeration or spray drying process.