Synthesis and Preliminary Tests of Suspension Plasma Spraying of Fine Hydroxyapatite Powder

The synthetic hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂) is a very useful biomaterial for numerous applications in medicine, such as e.g., fine powder for suspension plasma spraying. The powder was synthesized using aqueous solution of ammonium phosphate (H₂(PO₄)NH₄) and calcium nitrate (Ca(NO₃) · 4H₂O) in the carefully controlled experiments. The synthesized fine powder was characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The powder was formulated into water and alcohol based suspension and used to carry out the initial tests of plasma spraying onto titanium substrate. The phase analysis of sprayed coating was made with the XRD.     

Recent Developments in Suspension Plasma Sprayed Titanium Oxide and Hydroxyapatite Coatings

The paper aims at reviewing of the recent studies related to the development of suspension plasma sprayed TiO₂ and Ca₅(PO₄)₃OH (hydroxyapatite, HA) coatings as well as their multilayer composites obtained onto stainless steel, titanium and aluminum substrates. The total thickness of the coatings was in the range 10 to 150 μm. The suspensions on the base of distilled water, ethanol and their mixtures were formulated with the use of fine commercial TiO₂ pigment crystallized as rutile and HA milled from commercial spray-dried powder or synthesized from calcium nitrate and ammonium phosphate in an optimized reaction. The powder was crystallized as hydroxyapatite. Pneumatic and peristaltic pump liquid feeders were applied. The injection of suspension to the plasma jet was studied carefully with the use of an atomizer injector or a continuous stream one. The injectors were placed outside or inside of the anode-nozzle of the SG-100 plasma torch. The stream of liquid was tested under angle right or slightly backwards with regard to the torch axis. The sprayed deposits were submitted to the phase analysis by the use of x-ray diffraction. The content of anatase and rutile was calculated in the titanium oxide deposits as well as the content of the decomposition phases in the hydroxyapatite ones. The micro-Raman spectroscopy was used to visualize the area of appearance of some phases. Scratch test enabled to characterize the adhesion of the deposits, their microhardness and friction coefficient. The electric properties including electron emission, impedance spectroscopy, and dielectric properties of some coatings were equally tested.     

Microstructure and properties of plasma-sprayed bio-coatings on a low-modulus titanium alloy from milled HA/Ti powders

New composite hydroxyapatite/titanium (HA/Ti) coatings were fabricated by plasma spraying on Ti–24Nb–4Zr–7.9Sn alloy from milled precursor powders. The microstructures, mechanical properties and apatite-induction abilities of the coatings were investigated, and the influences of the initial HA/Ti ratios on microstructure and properties were highlighted. XRD, SEM and TEM were used to analyze the microstructures of the coatings. The micro-hardness and elastic modulus were determined by indentation tests and the bond strength was determined by tensile tests. The apatite-induction ability of the coatings was evaluated in simulated body fluid (SBF) with ion concentrations similar to those of human blood plasma. The results showed that the microstructure, mechanical properties and apatite-induction ability were dependent on the HA/Ti ratios of the original powders. The mechanical properties increased, and the apatite-induction ability decreased with increasing Ti content. Various chemical reactions occurred during the preparation of the coatings, including the decomposition of HA, the reaction between HA and Ti and the oxidization of Ti, which resulted in the formation of new phases, such as CaTiO₃, Ca₃(PO₄)₂, TiP_x and titanium oxides in the different coatings. These new phases play an important role for the mechanical properties and the apatite-induction ability of the coatings.     

Experimental design of plasma spraying and laser treatment of hydroxyapatite coatings

Plasma spray process of hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HA) followed by laser treatment of obtained coatings were optimized by an advanced statistical planning of experiments. The full factorial design of 2⁴ experiments was used to find effects of four principal parameters, i.e. electric power, plasma forming gas composition, carrier gas flow rate and laser power density onto microstructure of hydroxyapatite (HA) coatings and powders and depth of laser melted zone. The SAS and Statgraphics commercial softwares have been applied to obtain the mathematical model of influence of process parameters onto experimental responses. The chosen responses were the fraction of HA crystal phase and two phases of its decomposition α-tricalcium phosphate (Ca₃(PO₄)₂, α-TCP), tetracalcium phosphate (Ca₄P₂O₉, TTCP) and, on the other hand, the depth of laser melted zone in the coating. The two most important factors influencing these responses are electric power supplied to torch, laser power density. Laser power density is very important for the depth of laser melted zone. The crystal phase content of powders and coatings was determined using X-ray diffraction (XRD) quantitative analysis. The morphologies of coatings surfaces, cross-sections were characterized using scanning electron microscope (SEM).     

Anti-corrosion microarc oxidation coatings on SiCP/AZ31 magnesium matrix composite

The corrosion-resistant ceramic coatings up to 80 μm thick were fabricated on SiC_P/AZ31 magnesium matrix composite by microarc oxidation (MAO) technique in Na₃PO₄ + KOH + NaF solution. The microstructure, composition and phase constituent of ceramic coatings were analyzed by SEM and XRD, and the electrochemical corrosion behaviour of coatings was evaluated by the electrochemical polarization method. The thicker coating is compact and displays a good adhesion to the composite substrate. The ceramic coatings consist of MgO, Mg₂SiO₄, MgF₂, Mg₃(PO₄)₂, furthermore, a few residual SiC phases were also found in the coatings by means of SEM observation and EDX analysis. Most of SiC reinforced particles in the oxidized composite substrate have transformed into the oxides under microarc discharge sintering, but a few residual SiC reinforcements in the MAO coatings have not disrupted the continuity of coatings. So the corrosion resistance of the SiC_P/AZ31 composite is greatly improved by MAO surface treatment, however, the corrosion resistance of coated composite also depends on the coating thickness.     

The Influence of the Substrate on the Deposition of Cold-Sprayed Titanium: An Experimental and Numerical Study

The deposition of cold-sprayed titanium on various substrates is studied in this work. A rather coarse powder of titanium (−70 + 45 μm) was sprayed under uniform spraying conditions using a cold spray system onto five different substrates: two aluminum-based alloys (AISI 1050-H16 and AISI 2017-T4), copper, stainless steel AISI 304L, and Ti-6Al-4V. All the spraying experiments were carried out using alternatively nitrogen (N₂) or helium (He) as the process gas. Thick coatings were formed on the various substrates, with the exception of the AISI 2017 substrate. When N₂ was used as the process gas, only a few particles remained adhering to the AISI 2017. The thick pre-existing superficial oxide layer on AISI 2017, which was detected by Electron MicroProbe Analysis (EPMA), appeared to prevent adhesion of cold-sprayed titanium particles. The interaction of the sprayed particles with the various substrates was also studied by means of numerical simulations to better understand the adhesion mechanisms. The microstructure and the characteristics of the coatings were investigated. Deposition efficiency and coating density were found both to be strongly improved by spraying helium as the process gas.     

The surfactant addition effect in the elaboration of electrodepositated NiP-SiC composite coatings

In this work, NiP-SiC composite coatings were prepared by electrodeposition from a plating bath containing NiSO₄·6H₂O; NiCl₂·6H₂O; H₃PO₃; H₃PO₄; Na₂SO₄ and SiC particles (average diameter of 600 nm) in suspension. Anionic surfactant (SDS — sodium dodecyl sulfate, CH₃(CH₂)₁₁SO₄Na) or cationic surfactant (CTAHS — cetyltrimethylammonium hidrogensulfate, C₁₉H₄₃NO₄S) were also added to this suspension. The number of incorporated particles by area unit (α_np/A) was evaluated by image analysis of micrographs obtained by scanning electron microscopy (SEM). It was observed that organic additive addition influences the incorporated SiC particles rate on the metallic matrix. Addition of organic additives such as SDS or CTAHS modifies the characteristic of suspensions. Depending on particle size organic additive addition can change the number of incorporated particles in deposits during growth of the matrix. Decrease of the incorporated particle amount with the increase of particle size was verified. The selective incorporation process is associated to the presence of organic additive and it was not dependent on surfactant charge.     

Investigation into microstructural properties of fluorapatite Nd-YAG laser clad coatings with PVA and WG binders

Biomedical implants are generally coated with a thin layer of hydroxyapatite (HA) using a plasma spraying or pulsed laser deposition method. However, the bonding strength between the coating layer and the substrate is relatively low. Moreover, the high temperature of the deposition process causes a structural instability of the HA, and therefore degrades its bioactivity and reduces the service life of the coating layer following implantation. Accordingly, the present study replaces HA with fluorapatite (FA) as the coating material, and examines the effects of two different binders, namely polyvinyl alcohol (PVA) and water glass (WG), on the properties of FA coatings deposited on Ti-6Al-4V substrates using an Nd-YAG laser cladding process. Scanning electron microscopy (SEM) observations reveal that the FA coating has a coral- and dendrite-like structure when mixed with the PVA binder, but a pure dendrite-like structure when mixed with the WG binder. In addition, the Ca/P value of the WG-based FA coating is significantly higher than that of the PVA-based coating. Fluorapatite, Al₂O₃, CaTiO₃ and Ca₃(PO₄)₂ phases are observed in both coating layers. However, the XRD analysis results indicate that the microstructure of the WG sample is dominated by Ca₃(PO₄)₂ phase while that of the PVA sample is dominated by FA. After soaking in SBF, it was observed that the specimen produced with PVA binder or less laser output power possessed a better apatite induction ability.     

Fabrication of calcium phosphate/chitosan coatings on AZ91D magnesium alloy with a novel method

Four calcium phosphate/chitosan composite films were fabricated on the surface of micro-arc oxidized (MAO)-AZ91D alloy through electrophoretic deposition (EDP) followed by a conversion process of the coatings in a phosphate buffer solution (PBS). In the EPD process, nano hydroxyapatite (n-HA, Ca₁₀(PO₄) ₆(OH)₂) and Ca(OH)₂ in the layers were obtained from a n-HA/ethanol suspension and a n-HA/chitosan-acetic acid aqueous solution, respectively. After immersion into PBS, brushite (DCPD, CaHPO₄·2H₂O) and new HA were introduced into the deposited layers. The percentage of Ca(OH)₂ in the deposited layers played an important role in developing the new phase in the conversion layers. When the percentages of Ca(OH)₂ in the deposited layers were 32 wt. % and 54 wt. %, the main phase of the conversion layers was DCPD with a little HA. However, when the percentages of Ca(OH)₂ were 64 wt. % and 100 wt. %, the main phase of the conversion layers became HA with a little DCPD. The calcium phosphate/chitosan coatings with more homogeneous bioactive layers and better adhesion strength on MAO-AZ91D alloy substrate were obtained from the electrolyte whose volume percentages of the n-HA/chitosan-acetic acid aqueous solution being 60% and 80%.So, EPD combined with a conversion process into PBS could be a promising method for the preparation of new calcium phosphate/chitosan coatings.     

Composition and magnetic characteristics of plasma electrolytic oxide coatings on titanium

The results of studies of the composition and magnetic characteristics of coatings formed in different time periods on titanium in suspension electrolytes of compositions Na₃PO₄ + Na₂B₄O₇ + Na₂WO₄ + Fe₂(C₂O₄)₃ (PBWFe, рH 7.8) and Na₂НPO₄ + Na₂B₄O₇ + Na₂WO₄ + Fe₂(C₂O₄)₃ (P*BWFe, рН 7.4), as well as the characteristics of coatings fabricated in filtrates of both electrolytes, are presented. PBWFe and P*BWFe electrolytes contain colloid particles of iron hydroxides. Crystallites are contained in pores of PBWFe and P*BWFe coatings. The averaged Fe/Ti atomic ratio for crystallites in pores of PBWFe coatings formed in different periods of time is equal to ~2.6, while in pores of P*BWFe coatings it is equal to ~1.1. The samples with PBWFe coatings manifest ferromagnetic properties, while samples with P*BWFe coatings and those obtained in filtrates of both electrolytes are paramagnetics. It was shown that, for PBWFe and P*BWFe coatings formed in different time periods, the composition, thickness, and presence and composition of crystallites in pores, as well as magnetic and other controlled parameters, are fairly reproducible.     

A study on microstructure and properties of calcium phosphate coatings processed using ion beam assisted deposition on heated substrates

In this study a set of thin Hydroxyapatite (HA) [Ca₁₀(PO₄)₆(OH)₂] coatings was deposited on heated silicon and titanium substrates using Ion Beam Assisted Deposition (IBAD). The effects of substrate temperature and processing parameters on the microstructural properties and composition of the coatings are being studied. Analytical techniques include transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) with an energy dispersive X-ray spectroscopy (EDS), as well as scanning electron microscopy (SEM) with EDX, X-ray diffraction (XRD), and Fourier Transform Infrared Spectroscopy (FTIR). The current results indicate that as substrate temperature increases the Ca/P ratio of the coatings both on titanium and silicon substrates increases. The crystallinity of the coatings and the number of calcium phosphate compounds within the coating including HA also increases. STEM-EDS revealed an atomically diffused intermediate layer at the interface between the coating and substrate. XRD results along with TEM selected area diffraction (SAD) revealed that the coatings are composed of HA, other calcium phosphate, and calcium oxide compounds.     

Structural, Mechanical and Erosion Properties of Yttrium Oxide Coatings by Axial Suspension Plasma Spraying for Electronics Applications

Yttrium oxide (Y₂O₃) coatings have been prepared by axial suspension plasma spraying with fine powders. It is clarified that the coatings have high hardness, low porosity, high erosion resistance against CF₄ -containing plasma and retention of smooth eroded surface. This suggests that the axial suspension plasma spraying of Y₂O₃ is applicable to fabricating equipment for electronic devices, such as dry etching. Surface morphologies of the slurry coatings with splats are similar to conventional plasma-sprayed Y₂O₃ coatings, identified from microstructural analysis. Dense coating structures with no lamellar boundaries have been seen, which is apparently different from the conventional coatings. It has also been found that crystal structure of the suspension coatings mainly composed of metastable monoclinic phase, whereas the powders and the conventional plasma spray coatings have stable cubic phase. Mechanism of coating formation by plasma spraying with fine powder slurries is discussed based on the results.     

Electrical Properties of Atmospheric Plasma-Sprayed La10(SiO4)6O3 Electrolyte Coatings

In this present work, La₁₀(SiO₄)₆O₃ as a promising electrolyte candidate for intermediate temperature solid oxide fuel cells (IT-SOFCs) has been synthesized and its electrical property was investigated as a function of temperature. In order to improve the density and oxide ion conductivity of La₁₀(SiO₄)₆O₃, the feedstock powder was prepared by sintering the oxide mixture powders at proper sintering temperatures and times. The hexagonal apatite-type ceramic coatings with a typical composition of La₁₀(SiO₄)₆O₃ were deposited by atmospheric plasma spraying (APS) with different hydrogen flow rates. With increasing hydrogen flow rate oxide ion conductivity successively decreases. The highest ionic conductivity of the dense composite electrolyte coatings reaches a value of 2.4 mS/cm at 900 °C in air, which is comparable to other apatite-type lanthanum silicate (ATLS) conductors.     

Effects of applied voltage on the microstructure and properties of hydroxyapatite bioceramic coatings formed on Ti7Cu5Sn titanium alloy by micro-arc oxidation

The effects of the applied voltage on the morphology, composition and corrosion behaviour of Ti7Cu5Sn coated were investigated. At applied voltages lower than 250?V, the composite coatings consist of anatase-TiO₂, rutile-TiO₂, DCPD(CaHPO₄·2H₂O) and a small amount of amorphous calcium phosphate phase. When the applied voltage is increased, the ceramic coatings transform from DCPD (CaHPO₄·2H₂O) to HA (Ca₁₀(PO₄)₆(OH)₂, 300?V), and new phases of Ca₂P₂O₇, CaTiO₃ and TCP(Ca₃(PO₄)₂) form at 350?V. The passive current densities at body potential are one order of magnitude lower than that of the uncoated sample, indicating better corrosion resistance. The MAO film is a tri-layer system: a compact inner layer, a mesosphere porous oxide layer, and an outer layer.     

Characterization and corrosion behavior of ceramic coating on magnesium by micro-arc oxidation

In this study, the commercial pure magnesium was coated in different aqueous solutions of Na₂SiO₃ and Na₃PO₄ by the micro-arc oxidation method (MAO). Coating thickness, phase composition, surface and cross sectional morphology and corrosion resistance of coatings were analyzed by eddy current method, X-ray diffraction (XRD), scanning electron microscope (SEM) and tafel extrapolation method, respectively. The average thickness of the coatings ranged from 52 to 74 μm for sodium silicate solution and from 64 to 88 μm for sodium phosphate solution. The dominant phases on the coatings were detected as spinal Mg₂SiO₄ (Forsterite) and MgO (Periclase) for sodium silicate solution and Mg₃(PO₄)₂ (Farringtonite) and MgO (Periclase) for sodium phosphate solution. SEM images reveal that the coating is composed of two layers as of a porous outer layer and a dense inner layer. The corrosion results show the coating consisting Mg₂SiO₄ is more resistant to corrosion than that containing Mg₃(PO₄)₂.     

Effect of Spraying Powders Size on the Microstructure,Bonding Strength,and Microhardness of MoSi2 Coating Prepared by Air Plasma Spraying

Molybdenum disilicide (MoSi₂) coatings were deposited on carbon steel by air plasma spraying technology with different feedstock powder sizes (i.e., powder A: ?15 + 2.5 μm, powder B: ?30 + 15 μm, powder C: ?54 + 30 μm, powder D: ?74 + 54 μm and powder E: ?106 + 74 μm). Phase composition and microstructure of coatings were characterized by x-ray diffraction (XRD) and scanning electron microscope. The bonding strength and microhardness of coatings were also evaluated. The XRD results show that there exists mutual transformation between T-MoSi₂ and H-MoSi₂ phase and part of Mo-rich phases are formed because of oxidization during the spraying process. With the increase of spraying powders size, the content of Mo-rich phases (Mo or Mo₅Si₃) and molybdenum oxide (MoO₃) in coatings decreases, and that of disilicide-rich phase (MoSi₂) in coatings increases. The oxidation degree of MoSi₂ particle gradually decreases during the spraying process with the increase of spraying powders size. The MoSi₂ is the main phase of the as-sprayed coatings when the spraying powders size is beyond 30 μm. With the increase of spraying powders size, the porosity of the as-sprayed coating increases, and the bonding strength of the coating gradually decreases. The hardness of coatings first increases and then decreases with the increase of spraying powders size.     

电压对AZ31镁合金微弧氧化涂层微观结构及腐蚀性能的影响

为研究过程参数对镁合金微弧氧化涂层的微观结构及耐腐蚀性能的影响,在AZ31镁合金基体上,采用不同电压,在电解液磷酸三钠(Na3PO4)中制备微弧氧化涂层.采用扫描电子显微镜(SEM)及光学显微镜,分析膜层腐蚀前后的微观组织结构;通过X射线衍射仪(XRD)分析涂层样品腐蚀前后的相组成.采用动电位极化曲线和电化学阻抗谱(EIS)测试对涂层的耐腐蚀性能进行评价.结果表明:AZ31镁合金微弧氧化涂层主要由Mg3(PO4)2,MgO,Mg和少量MgAl2O4组成,腐蚀产物由Mg(OH)2,quintinite和Ca10(PO4)6 (OH)2组成.在电压为325 V,频率3 000 Hz,氧化时间为5 min下制备的微弧氧化涂层具有最致密均匀的微观形貌和最小的腐蚀电流密度,因此表现出最强的耐腐蚀性能.     

Nanostructured Si,Mg, CO<Subscript>3</Subscript><Superscript>2−</Superscript> Substituted Hydroxyapatite Coatings Deposited by Liquid Precursor Plasma Spraying: Synthesis and Characterization

In this study, a novel liquid precursor plasma spraying (LPPS) process was used to deposit Si, Mg, CO₃ ²⁻ substituted hydroxyapatite (HA) coatings (alone and cosubstituted) onto Ti-6Al-4V substrates. Salts of silicon, magnesium, and carbonate elements were directly added into the HA liquid precursor for subsequent plasma spraying. The phase composition, structure, and morphology of all HA coatings were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Fourier transform infrared (FTIR) spectroscopy. The results indicated that the trace elements were successfully incorporated into the HA structure and nanostructured coatings were obtained for all doped HA formulations. The incorporation of trace elements into the HA structure reduced its crystallinity, especially when silicon, magnesium and carbonate ions entered simultaneously into the HA structure. FTIR spectra showed that the Si-HA and Mg-HA coatings had decreased intensities in both the O-H and P-O bands and that the CO₃ ²⁻-HA coating was mainly a B-type carbonate-substituted HA. The results showed that the LPPS process is an effective and simple method to synthesize trace element substituted biomimetic HA coatings with nanostructure.     

Highly Segmented Thermal Barrier Coatings Deposited by Suspension Plasma Spray: Effects of Spray Process on Microstructure

Effects of the ceramic powder size used for suspension as well as several processing parameters in suspension plasma spraying of YSZ were investigated experimentally, aiming to fabricate highly segmented microstructures for thermal barrier coating (TBC) applications. Particle image velocimetry (PIV) was used to observe the atomization process and the velocity distribution of atomized droplets and ceramic particles travelling toward the substrates. The tested parameters included the secondary plasma gas (He versus H₂), suspension injection flow rate, and substrate surface roughness. Results indicated that a plasma jet with a relatively higher content of He or H₂ as the secondary plasma gas was critical to produce highly segmented YSZ TBCs with a crack density up to ~12 cracks/mm. The optimized suspension flow rate played an important role to realize coatings with a reduced porosity level and improved adhesion. An increased powder size and higher operation power level were beneficial for the formation of highly segmented coatings onto substrates with a wider range of surface roughness.     

The effects of nano-SiO2 additive on the zinc phosphating of carbon steel

In this paper, nano-SiO₂ was used as an accelerator for improving the microstructure and the corrosion resistance of phosphate coating on carbon steel. The chemical composition and microstructure of the coatings were analyzed by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). The effects of nano-SiO₂ on weight, roughness and corrosion resistance of the phosphate coatings were also investigated. Results show that the compositions of phosphate coating were Zn₃(PO₄)₂·4H₂O (hopeite), and Zn₂Fe(PO₄)₂·4H₂O (phosphophylite). The phosphate coatings became denser due to the addition of nano-SiO₂ which reduced the size of the crystal clusters. The average weight of phosphate coatings approximately linearly increased with the nano-SiO₂ content in the bath from 0 to 4 g/L, and the phosphate coatings formed in bath containing 2 g/L nano-SiO₂ showed the highest corrosion resistance in 5 wt.% sodium chloride solution at ambient temperature. Nano-SiO₂ would be widely utilized as a phosphating additive to replace the traditional nitrite, due to its less pollutant and its better quality of the coating.