ABSTRACT Two different oxide-based bioactive materials, namely tricalcium phosphate (TCP) and K2O... more ABSTRACT Two different oxide-based bioactive materials, namely tricalcium phosphate (TCP) and K2O–CaO–P2O5–SiO2 bioactive glass (“Bio-K”), were processed by the High Velocity Suspension Flame Spraying (HVSFS) technique. Single splats were deposited onto polished cold glass plates using a custom-built experimental set up. In addition, complete coatings were deposited onto pre-heated Ti plates.Deposited splats and coatings were examined by scanning and transmission electron microscope (SEM, TEM) techniques and their phase composition was investigated by micro-Raman spectroscopy. The 3-D profiles of single splats were also reconstructed through atomic force microscopy (AFM).Based on the experimental results, explanations of the deposition mechanisms were put forward, discussing the different heating and melting behaviour of particles and agglomerates. The critical particle sizes needed to obtain deposition of molten splats at impact, and their solidification behaviour onto cold vs. hot surfaces were studied as well. Most importantly, the splats can only be formed by molten droplets comprised within a given size range (“sprayability window”). Smaller droplets possess insufficient thermal and mechanical inertia to overcome the stagnation flow in front of the substrate; larger ones are mostly unmelted. This “sprayability window” depends on the deposition process itself and on the physical properties of sprayed materials.
ABSTRACT Bioactive glass coatings based on a novel SiO(2)-P(2)O(5)-CaO-K(2)O system ("Bi... more ABSTRACT Bioactive glass coatings based on a novel SiO(2)-P(2)O(5)-CaO-K(2)O system ("Bio-K") were deposited by the High-Velocity Suspension Flame Spraying (HVSFS) technique. Attrition-milled micrometric powder particles, dispersed in water + isopropanol, were sprayed onto Ti plates. The coatings remained entirely glassy, but the glass underwent some structural alterations during processing. After soaking in simulated body fluid (SBF), a rather uniform hydroxyapatite layer developed on the coatings, indicating that they may have the potential to faster osseointegration of prosthetic implants.
ABSTRACT In this study, hydroxyapatite (HAp) coatings were deposited on Ti plates by the high-vel... more ABSTRACT In this study, hydroxyapatite (HAp) coatings were deposited on Ti plates by the high-velocity suspension flame spraying (HVSFS) technique. The process characteristic, the microstructure and phase composition of the coatings are significantly influenced by the solvent and by the design of the combustion chamber (CC) of the HVSFS torch. Water-based suspensions always lead to fairly low surface temperatures (≈350 °C), deposition efficiencies <40%, and produce coatings with low amount of crystalline HAp, which tend to dissolve very rapidly in simulated body fluid (SBF) solutions. DEG-based suspensions, when sprayed with properly-designed CCs, produce deposition efficiencies of 45-55% and high surface temperatures (550-600 °C). In these coatings, the degree of crystallinity increases from the bottom layer to the top layer, probably because the increasingly large surface temperature can eventually favour re-crystallisation of individual lamellae during cooling. These coatings are much more stable in SBF solutions.
Glass–alumina functionally graded coatings (FGCs) were produced via plasma spraying, a deposition... more Glass–alumina functionally graded coatings (FGCs) were produced via plasma spraying, a deposition technique for thick (>10–20μm) coatings production, which ensures high flexibility and good reliability. The samples were obtained by building a graded glass–alumina coating onto an alumina substrate; the coatings were designed as multi-layered systems, each layer having a mean composition slightly different from the neighbouring ones. Two different
Functionally graded materials (FGMs) are innovative composite materials characterized by a gradua... more Functionally graded materials (FGMs) are innovative composite materials characterized by a gradual spatial change in composition, microstructure and related properties. This work was focused on glass–alumina functionally graded materials, produced via percolation of molten glass into a sintered polycrystalline alumina substrate and via plasma spraying. The glass composition, belonging to the CaO–ZrO2–SiO2 system, was purposely designed in order to minimize
Glass-alumina functionally graded materials were obtained by percolation and alternatively by pla... more Glass-alumina functionally graded materials were obtained by percolation and alternatively by plasma spraying. The paper develops a reliable model to predict the functional gradient of the analysed systems. A finite element code, which was able to handle microstructural images, was employed to estimate the effective elastic properties along the gradient direction. The calculated values were compared with experimental data acquired
The viability of a high power diode laser source as effective post-deposition treatment technique... more The viability of a high power diode laser source as effective post-deposition treatment technique of functionally graded titania-HA coatings was checked. In particular, several laser treatments were performed on various coatings plasma-sprayed under different conditions to verify the presence of an operative window large enough for practical purposes and, subsequently, to identify the most promising settings of the laser parameters.
Among bioactive ceramics, hydroxyapatite (HAp) has been widely studied, especially as a coating o... more Among bioactive ceramics, hydroxyapatite (HAp) has been widely studied, especially as a coating onto metallic substrates. In clinical applications, coating delamination has been observed, close to the interface between coating and substrate. This is due to a mismatch in the thermal expansion coefficients of HAp and titanium/titanium alloy. In order to improve the adhesion, a proper bond coat may be
Thermal spray represents an advantageous technique for depositing large-area titanium dioxide coa... more Thermal spray represents an advantageous technique for depositing large-area titanium dioxide coatings that are of interest for both traditional wear-resistant coatings as well as functional applications such as photo-induced decontamination surfaces. Numerous past studies have examined the phase evolution and properties of TiO2 coatings using different thermal spray processes or parameters. In this paper, an integrated study of thermal sprayed
International Journal of Applied Ceramic Technology, 2009
ABSTRACT Titania–hydroxyapatite functionally graded coatings were deposited on titanium alloy sub... more ABSTRACT Titania–hydroxyapatite functionally graded coatings were deposited on titanium alloy substrates by plasma spraying. Because it was necessary to spray together the titania and the hydroxyapatite powders to obtain the graded system, the first target of the present study was to optimize the process parameters in order to obtain a high-quality coating. A 23 Design of Experiments was applied to define the optimal values of plasma torch power, hydrogen flux, and spraying distance. This defined set of parameters (38 kW, 5 SLPM, and 90 mm, respectively) was used to spray the most promising graded coating, which was characterized and postheat treated.
ABSTRACT In this work hydroxyapatite, beta-tricalcium phosphate and a new bioactive glass have be... more ABSTRACT In this work hydroxyapatite, beta-tricalcium phosphate and a new bioactive glass have been used to produce ternary composites for orthopedic applications. Thanks to the particular properties of the glass, whose formulation delays the devitrification processes at high temperature, sintering the composites at a relatively low temperature (800 degrees C) has been possible. In this way, two basic aims have been reached, since the glass preserved its amorphous nature and the reactions between the constituent phases were substantially reduced. Moreover, the ternary composites had a dense and uniform microstructure, which resulted in good mechanical properties (Vickers micro-hardness: 280 +/- 22HV; elastic modulus: 28.7 +/- 3.6 GPa). In vitro tests confirmed the apatite-forming ability of the composites soaked in a Simulated Body Fluid (SBF). Basing on the obtained results, the new ternary composites represent an intriguing alternative to conventional biomedical materials whenever a controlled bone-bonding rate is required. (C) 2013 Elsevier Ltd and Techna Group S.r.l. All tights reserved.
Glass–alumina functionally graded materials were obtained using two different methods: percolatio... more Glass–alumina functionally graded materials were obtained using two different methods: percolation, which was representative of natural transport based processes, and plasma spraying, which was representative of constructive processes. The specimens produced in this way were investigated to evaluate the effect of production techniques on the final microstructure and gradient, which, in turn, govern the properties and performances of the graded systems. Moreover, post-production heat treatments were performed in order to improve the reliability of the materials examined.
ABSTRACT Titanium plates were coated by high-velocity suspension flame spraying (HVSFS) technique... more ABSTRACT Titanium plates were coated by high-velocity suspension flame spraying (HVSFS) technique using a novel bioactive glass composition based on the K2O–CaO–P2O5–SiO2 composition (“Bio-K”). On half of the samples, an atmospheric plasma sprayed (APS) TiO2 bond coat was preliminarily deposited; suspensions of attrition-milled micron-sized glass powders, dispersed in a water + isopropanol mixture, were then sprayed onto both bare and bond-coated plates using five different process parameter sets.The microstructure of the coatings is independent of the presence of the bond coat but is strongly influenced by the deposition parameters. If the latter result in surface temperatures larger than the glass transition temperature of the Bio-K composition, large-scale viscous flow allows the expansion of the air entrained in the porosities, developing large rounded pores. When this phenomenon is avoided, denser layers are obtained. In tensile adhesion tests, porous layers fail cohesively at low loads, whereas adhesive/cohesive failure occurs in denser layers. In this latter case, the adhesion strength is significantly improved by the bond coat, reaching maximum values of 17 MPa.When immersed in simulated body fluid (SBF), the coating surface is rapidly converted into a silica gel because of ion leaching. A hydroxyapatite layer starts precipitating on top of it after 3 days and grows into a uniform film (of ≈ 10 μm thickness) after 2 weeks.
Functionally Graded Materials (FGMs) are heterogeneous systems whose composition and microstructu... more Functionally Graded Materials (FGMs) are heterogeneous systems whose composition and microstructure vary in space, thus creating a resulting gradient in properties and functions which can be tailored to the prescribed thermo-mechanical loading conditions [1]. Moreover, FGMs allow to fully exploit the peculiar properties of the ingredient materials, whilst the smooth change in composition and microstructure minimizes the stresses which can
ABSTRACT Two different oxide-based bioactive materials, namely tricalcium phosphate (TCP) and K2O... more ABSTRACT Two different oxide-based bioactive materials, namely tricalcium phosphate (TCP) and K2O–CaO–P2O5–SiO2 bioactive glass (“Bio-K”), were processed by the High Velocity Suspension Flame Spraying (HVSFS) technique. Single splats were deposited onto polished cold glass plates using a custom-built experimental set up. In addition, complete coatings were deposited onto pre-heated Ti plates.Deposited splats and coatings were examined by scanning and transmission electron microscope (SEM, TEM) techniques and their phase composition was investigated by micro-Raman spectroscopy. The 3-D profiles of single splats were also reconstructed through atomic force microscopy (AFM).Based on the experimental results, explanations of the deposition mechanisms were put forward, discussing the different heating and melting behaviour of particles and agglomerates. The critical particle sizes needed to obtain deposition of molten splats at impact, and their solidification behaviour onto cold vs. hot surfaces were studied as well. Most importantly, the splats can only be formed by molten droplets comprised within a given size range (“sprayability window”). Smaller droplets possess insufficient thermal and mechanical inertia to overcome the stagnation flow in front of the substrate; larger ones are mostly unmelted. This “sprayability window” depends on the deposition process itself and on the physical properties of sprayed materials.
ABSTRACT Bioactive glass coatings based on a novel SiO(2)-P(2)O(5)-CaO-K(2)O system ("Bi... more ABSTRACT Bioactive glass coatings based on a novel SiO(2)-P(2)O(5)-CaO-K(2)O system ("Bio-K") were deposited by the High-Velocity Suspension Flame Spraying (HVSFS) technique. Attrition-milled micrometric powder particles, dispersed in water + isopropanol, were sprayed onto Ti plates. The coatings remained entirely glassy, but the glass underwent some structural alterations during processing. After soaking in simulated body fluid (SBF), a rather uniform hydroxyapatite layer developed on the coatings, indicating that they may have the potential to faster osseointegration of prosthetic implants.
ABSTRACT In this study, hydroxyapatite (HAp) coatings were deposited on Ti plates by the high-vel... more ABSTRACT In this study, hydroxyapatite (HAp) coatings were deposited on Ti plates by the high-velocity suspension flame spraying (HVSFS) technique. The process characteristic, the microstructure and phase composition of the coatings are significantly influenced by the solvent and by the design of the combustion chamber (CC) of the HVSFS torch. Water-based suspensions always lead to fairly low surface temperatures (≈350 °C), deposition efficiencies <40%, and produce coatings with low amount of crystalline HAp, which tend to dissolve very rapidly in simulated body fluid (SBF) solutions. DEG-based suspensions, when sprayed with properly-designed CCs, produce deposition efficiencies of 45-55% and high surface temperatures (550-600 °C). In these coatings, the degree of crystallinity increases from the bottom layer to the top layer, probably because the increasingly large surface temperature can eventually favour re-crystallisation of individual lamellae during cooling. These coatings are much more stable in SBF solutions.
Glass–alumina functionally graded coatings (FGCs) were produced via plasma spraying, a deposition... more Glass–alumina functionally graded coatings (FGCs) were produced via plasma spraying, a deposition technique for thick (>10–20μm) coatings production, which ensures high flexibility and good reliability. The samples were obtained by building a graded glass–alumina coating onto an alumina substrate; the coatings were designed as multi-layered systems, each layer having a mean composition slightly different from the neighbouring ones. Two different
Functionally graded materials (FGMs) are innovative composite materials characterized by a gradua... more Functionally graded materials (FGMs) are innovative composite materials characterized by a gradual spatial change in composition, microstructure and related properties. This work was focused on glass–alumina functionally graded materials, produced via percolation of molten glass into a sintered polycrystalline alumina substrate and via plasma spraying. The glass composition, belonging to the CaO–ZrO2–SiO2 system, was purposely designed in order to minimize
Glass-alumina functionally graded materials were obtained by percolation and alternatively by pla... more Glass-alumina functionally graded materials were obtained by percolation and alternatively by plasma spraying. The paper develops a reliable model to predict the functional gradient of the analysed systems. A finite element code, which was able to handle microstructural images, was employed to estimate the effective elastic properties along the gradient direction. The calculated values were compared with experimental data acquired
The viability of a high power diode laser source as effective post-deposition treatment technique... more The viability of a high power diode laser source as effective post-deposition treatment technique of functionally graded titania-HA coatings was checked. In particular, several laser treatments were performed on various coatings plasma-sprayed under different conditions to verify the presence of an operative window large enough for practical purposes and, subsequently, to identify the most promising settings of the laser parameters.
Among bioactive ceramics, hydroxyapatite (HAp) has been widely studied, especially as a coating o... more Among bioactive ceramics, hydroxyapatite (HAp) has been widely studied, especially as a coating onto metallic substrates. In clinical applications, coating delamination has been observed, close to the interface between coating and substrate. This is due to a mismatch in the thermal expansion coefficients of HAp and titanium/titanium alloy. In order to improve the adhesion, a proper bond coat may be
Thermal spray represents an advantageous technique for depositing large-area titanium dioxide coa... more Thermal spray represents an advantageous technique for depositing large-area titanium dioxide coatings that are of interest for both traditional wear-resistant coatings as well as functional applications such as photo-induced decontamination surfaces. Numerous past studies have examined the phase evolution and properties of TiO2 coatings using different thermal spray processes or parameters. In this paper, an integrated study of thermal sprayed
International Journal of Applied Ceramic Technology, 2009
ABSTRACT Titania–hydroxyapatite functionally graded coatings were deposited on titanium alloy sub... more ABSTRACT Titania–hydroxyapatite functionally graded coatings were deposited on titanium alloy substrates by plasma spraying. Because it was necessary to spray together the titania and the hydroxyapatite powders to obtain the graded system, the first target of the present study was to optimize the process parameters in order to obtain a high-quality coating. A 23 Design of Experiments was applied to define the optimal values of plasma torch power, hydrogen flux, and spraying distance. This defined set of parameters (38 kW, 5 SLPM, and 90 mm, respectively) was used to spray the most promising graded coating, which was characterized and postheat treated.
ABSTRACT In this work hydroxyapatite, beta-tricalcium phosphate and a new bioactive glass have be... more ABSTRACT In this work hydroxyapatite, beta-tricalcium phosphate and a new bioactive glass have been used to produce ternary composites for orthopedic applications. Thanks to the particular properties of the glass, whose formulation delays the devitrification processes at high temperature, sintering the composites at a relatively low temperature (800 degrees C) has been possible. In this way, two basic aims have been reached, since the glass preserved its amorphous nature and the reactions between the constituent phases were substantially reduced. Moreover, the ternary composites had a dense and uniform microstructure, which resulted in good mechanical properties (Vickers micro-hardness: 280 +/- 22HV; elastic modulus: 28.7 +/- 3.6 GPa). In vitro tests confirmed the apatite-forming ability of the composites soaked in a Simulated Body Fluid (SBF). Basing on the obtained results, the new ternary composites represent an intriguing alternative to conventional biomedical materials whenever a controlled bone-bonding rate is required. (C) 2013 Elsevier Ltd and Techna Group S.r.l. All tights reserved.
Glass–alumina functionally graded materials were obtained using two different methods: percolatio... more Glass–alumina functionally graded materials were obtained using two different methods: percolation, which was representative of natural transport based processes, and plasma spraying, which was representative of constructive processes. The specimens produced in this way were investigated to evaluate the effect of production techniques on the final microstructure and gradient, which, in turn, govern the properties and performances of the graded systems. Moreover, post-production heat treatments were performed in order to improve the reliability of the materials examined.
ABSTRACT Titanium plates were coated by high-velocity suspension flame spraying (HVSFS) technique... more ABSTRACT Titanium plates were coated by high-velocity suspension flame spraying (HVSFS) technique using a novel bioactive glass composition based on the K2O–CaO–P2O5–SiO2 composition (“Bio-K”). On half of the samples, an atmospheric plasma sprayed (APS) TiO2 bond coat was preliminarily deposited; suspensions of attrition-milled micron-sized glass powders, dispersed in a water + isopropanol mixture, were then sprayed onto both bare and bond-coated plates using five different process parameter sets.The microstructure of the coatings is independent of the presence of the bond coat but is strongly influenced by the deposition parameters. If the latter result in surface temperatures larger than the glass transition temperature of the Bio-K composition, large-scale viscous flow allows the expansion of the air entrained in the porosities, developing large rounded pores. When this phenomenon is avoided, denser layers are obtained. In tensile adhesion tests, porous layers fail cohesively at low loads, whereas adhesive/cohesive failure occurs in denser layers. In this latter case, the adhesion strength is significantly improved by the bond coat, reaching maximum values of 17 MPa.When immersed in simulated body fluid (SBF), the coating surface is rapidly converted into a silica gel because of ion leaching. A hydroxyapatite layer starts precipitating on top of it after 3 days and grows into a uniform film (of ≈ 10 μm thickness) after 2 weeks.
Functionally Graded Materials (FGMs) are heterogeneous systems whose composition and microstructu... more Functionally Graded Materials (FGMs) are heterogeneous systems whose composition and microstructure vary in space, thus creating a resulting gradient in properties and functions which can be tailored to the prescribed thermo-mechanical loading conditions [1]. Moreover, FGMs allow to fully exploit the peculiar properties of the ingredient materials, whilst the smooth change in composition and microstructure minimizes the stresses which can
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