Characterization of thermal sprayed hydroxyapatite powders and coatings

Calcium phosphate materials such as hydroxyapatite (HA) have biocompatible properties that can promote osteogenesis or new bone formation. Thermal spraying is an economical and effective process for coating the hydroxyapatite onto metal. It has been reported that plasma spraying changes the degree of crystallinity as well as the phase composition of the HA. This article reports the preparation and characterization of HA powders and coatings by two thermal spray processes (plasma and combustion flame) and suggests that the state of the starting powder adversely affects the coating characteristics. The raw HA powders are synthesized through a chemical reaction involving calcium hydroxide and orthophosphoric acid. Phase analysis using an X- ray diffractometer revealed that the synthesized powder consists of predominantly the HA phase. Calcined and crushed HA powders of various size ranges were fed into the plasma jet to produce HA coatings on metallic substrates. In addition, some HA powders were sprayed into distilled water by plasma spraying and combustion flame spraying to study powder melting characteristics. Other samples were plasma sprayed onto a solid rotating target to study atomization and impact behavior. The morphology of the rapidly solidified powders and thermal sprayed coatings were examined by scanning electron microscopy (SEM). An X- ray sedimentation particle size analyzer, laser diffraction particle size analyzer, and image analyzer performed the particle size analysis. Preliminary results indicate that particle cohesion, size range, and thermal treatment in the plasma affect the phase and structure of the as- sprayed coating, and some post- spray treatment may be necessary to produce a dense and adherent coating with the desired biocompatible properties.     

Plasma spraying of combustion flame spheroidized hydroxyapatite (HA) powders

Tailoring powder characteristics to suit the plasma spray process can alleviate difficulties associated with the preparation of hydroxyapatite (HA) coatings. Commercial HA feedstock normally exhibit an angular morphology and a wide particle size range that present difficulties in powder transport from the powder hopper to the plasma spray gun and in nonuniform melting of the powders in the plasma flame. Hence, combustion flame spheroidized hydroxyapatite (SHA) was used as the feedstock for plasma spraying. Spherical particles within a narrow particle size range are found to be more effective for the plasma spray processes. Results show coatings generated from spheroidized HA powders have unique surface and microstructure characteristics. Scanning electron microscope (SEM) observation of the coating surface revealed well-formed splats that spread and flatten into disc configurations with no disintegration, reflecting adequate melting of the HA in the plasma and subsequent deposition consistency. The surface topography is generally flat with good overlapping of subsequent spreading droplets. Porosity in the form of macropores is substantially reduced. The cross-section microstructure reveals a dense coating comprised of randomly stacked lamellae. The tensile bond strengths of the SHA coatings, phase composition, and characteristics of the coatings generated with different particle sizes (125 to 75 μm, 45 to 75 μm, 20 to 45 μm, and 5 to 20 μm) showed that a high bond strength of ∼16 MPa can be obtained with SHA in the size range from 20 to 45 μm. This can be improved further by a postspray treatment by hot isostatic pressing (HIP). However, larger particle size ranges exhibited higher degrees of crystallinity and relatively higher HA content among the various calcium phosphate phases found in the coatings.     

The evaluation of powder processing on microstructure and mechanical properties of hydroxyapatite (HA)/yttria stabilized zirconia (YSZ) composite coatings

In clinical applications, the mechanical failure of HA-coated titanium alloy implants suffered at the interface of the HA coating and titanium alloy substrate will be a potential weakness in prosthesis. Yttria stabilized zirconia (YSZ) reinforced HA coatings have been proven to enhance the mechanical properties of the HA coating significantly and reduce the formation of calcium oxide (CaO). In this paper, HA/YSZ (30 wt.% YSZ) composite coatings were sprayed by the plasma technique. The effects of the powder processing–mechanical ball milling method and spheroidization method on the microstructure and mechanical properties of the HA/YSZ composite coatings were evaluated. The experimental results showed that the spheroidized powders melted better than the ball milled powders during plasma spraying and formed higher mechanical property coatings (1.6326±0.08 MPa m^−0.5 of fracture toughness, 58.59±2.91 GPa of elastic modulus and 43.42±2.53 MPa of tensile bond strength). HA/YSZ solid solution formed during deposition on the substrate, which played a very important role in the mechanical properties of the HA/YSZ composite coatings. Tensile bond strength tests showed that the fracture mode was cohesive and that failure occurred at the interface of HA and unmelted YSZ particles. The molten state of YSZ had a great influence on the properties of the HA/YSZ composite coatings.     

Aluminum-silicon carbide coatings by plasma spraying

An aluminum base composite (Al-SiC) powder has been developed for producing plasma sprayed coatings on Al and other metallic substrates. The composite powders were prepared by mechanical alloying of 6061 Al alloy with SiC particles. The concentration of SiC was varied between 20 and 75 vol%, and the size of the reinforcement was varied from 8 to 37 μm in the Al-50 vol% SiC composites. The 44 to 140 μm composite powders were sprayed using an axial feed plasma torch. Adhesion strength of the coatings to their substrates were found to decrease with increasing SiC content and with decreasing SiC particle sizes. The increase in the SiC content and decrease in particle size improved the erosive wear resistance of the coatings. The abrasive wear resistance was found to improve with the increase in SiC particle size and with the SiC content in the composite coatings.     

Phase evolution of hydroxapatite coatings suspension plasma sprayed using variable parameters in simulated body fluid

Fine, hydroxyapatite (HA) powder, synthesized using calcium nitrate and diammonium nitrate was formulated with water and alcohol to obtain a suspension used to plasma spray coatings onto titanium substrates. The deposition process was optimized using statistical design of 2ⁿ experiments with two variables: spray distance and electric power input to arc plasma. The sprayed coatings were soaked in simulated body fluid (SBF) for the periods of 3, 7, 14, 28, and 60 days at controlled temperature of 37 °C. The reference intensity ratio (RIR) method basing onto X-ray diffraction (XRD) data was used to determine quantitatively the phase composition of as-sprayed and soaked deposits. Electron probe microanalysis (EPMA) enabled to make the profiles of calcium to phosphorus atomic ratio along a line though the coatings' cross sections. Raman spectroscopy of selected samples enabled to localize the different crystal phases in sprayed coatings. The coatings included some porosity and have two characteristic zones: (i) dense zone corresponding to the lamellas, observed usually in thermally sprayed coatings; (ii) sintered zone containing fine hydroxyapatite grains which correspond to the fine solids from the suspension which were agglomerated in the cold regions of plasma jet and sintered on the substrate. The soaking in SBF homogenizes the morphology of coatings. The sintered zone disappears and the pores get filled by the reprecipitated calcium phosphates.     

Next Generation Thermal Barrier Coatings for the Gas Turbine Industry

The aim of this study is to develop the next generation of production ready air plasma sprayed thermal barrier coating with a low conductivity and long lifetime. A number of coating architectures were produced using commercially available plasma spray guns. Modifications were made to powder chemistry, including high purity powders, dysprosia stabilized zirconia powders, and powders containing porosity formers. Agglomerated & sintered and homogenized oven spheroidized powder morphologies were used to attain beneficial microstructures. Dual layer coatings were produced using the two powders. Laser flash technique was used to evaluate the thermal conductivity of the coating systems from room temperature to 1200 °C. Tests were performed on as-sprayed samples and samples were heat treated for 100 h at 1150 °C. Thermal conductivity results were correlated to the coating microstructure using image analysis of porosity and cracks. The results show the influence of beneficial porosity on reducing the thermal conductivity of the produced coatings.     

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.     

Effect of steam treatment during plasma spraying on the microstructure of hydroxyapatite splats and coatings

The major problems with plasma sprayed hydroxyapatite (HA) coatings for hard tissue replacement are severe HA decomposition and insufficient mechanical properties of the coatings. Loss of crystalline HA after the high-temperature spraying is due mainly to the loss of OH⁻ in terms of water. The current study used steam to treat HA droplets and coatings during both in-flight and flattening stages during plasma spraying. The microstructure of the HA coatings and splats was characterized using scanning electron microscope, Raman spectroscopy, Fourier transform IR spectroscopy, and x-ray diffraction. Results showed that a significant increase in crystallinity of the HA coating was achieved through the steam treatment (e.g., from 58 to 79%). In addition, the effects were dependent on particle sizes of the HA feedstock, more increase in crystallinity of the coatings made from smaller powders was revealed. The Raman spectroscopy analyses on the individual splats and coatings indicate that the mechanism involves entrapping of water molecules by the individual HA droplets upon their impingement. It further suggests that the HA decomposition has already taken place before the impingement of the droplets on precoating or substrate. The improvement in crystallinity and phases, for example, from tricalcium phosphate and amorphous calcium phosphate to HA, was achieved by reversing the HA decomposition through providing extra OH⁻. Furthermore, the steam treatment during the spraying also accounts for remarkably increased adhesion strength from 9.09 to 23.13 MPa. The in vitro testing through immersing the HA coatings in simulated body fluid gives further evidence that the economic and simple steam treatment is promising in improving HA coating structure. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Hydroxyapatite coating on titanium substrate by electrophoretic deposition method: Effects of titanium dioxide inner layer on adhesion strength and hydroxyapatite decomposition

Nanosized hydroxyapatite (HA) powders were prepared by a chemical precipitation method and electrophoretically deposited on Ti6Al4V substrates. The powders were calcined before the deposition process in order to obtain crack-free coating surfaces. As an inner layer between Ti6Al4V substrate and HA coating, nanosized titanium dioxide (TiO₂) powders were deposited, using different coating voltages, in order to connect substrate and HA tightly. Moreover, this layer is considered to be acting as a diffusion barrier, reducing the HA decomposition due to ion migration from the metal substrate into the HA. After the sintering stage, adhesion strengths of coatings were measured by shear testing, phase changes were studied by X-ray diffraction, and coating morphology was analyzed through scanning electron microscopy observations. Results showed that usage of the TiO₂ inner layer prevented HA decomposition. Furthermore, decreasing the voltage used in TiO₂ deposition resulted in crack-free surfaces and increased adhesion strength of the overall coating.     

微束等离子喷涂制备羟基磷灰石涂层

采用微束等离子喷涂系统制备羟基磷灰石涂层,通过扫描电子显微分析(SEM)和X射线衍射分析(XRD)对涂层形貌、相组成和结晶度进行了研究.结果表明,随着喷涂电流和离子气流量的增加,羟基磷灰石粒子的熔化和撞击后的铺展更充分,在70～130 mm范围内随着喷涂距离的增加粒子熔化程度增加.适当的喷涂工艺条件下微束等离子喷涂制备羟基磷灰石涂层的结晶度可以相当或高于传统大气等离子喷涂制备的涂层,有利于涂层在体液环境中稳定性的提高.     

Microstructure and composition analysis in plasma sprayed coatings of Al2O3/ZrSiO4 mixtures

Plasma spraying of Al₂O₃/ZrSiO₄ was performed using spray dried and plasma spheroidised powder feedstock. The mixtures were sprayed using different spray stand-off distances and plasma power levels. X-Ray diffraction (XRD) was used to characterise the phase composition and scanning electron microscopy (SEM) examined the morphology of the sprayed surface and polished cross-sections. The results showed that the plasma spray process parameters played an important role in the final outcome of microstructures of the coatings. The coatings produced with spheroidised powders displayed a much denser structure than those produced with the spray-dried powders. The phase composition analysis showed the presence of amorphous phases in addition to crystalline alumina, zircon and tetragonal (t) zirconia (ZrO₂). Transmission electron microscopy (TEM) showed that amorphous phases and t-ZrO₂ crystals with particle size 100–200 nm could coexist within a single splat due to the relatively low local cooling rate.     

Spheroidization of Nd-Fe-B Powders by RF Induction Plasma Processing

以不规则形的钕铁硼粉为原料,使用射频等离子体球化处理工艺,制备球形钕铁硼粉.研究了原料的加料速率和粉末粒度对粉末球化率的影响.通过扫描电子显微镜观察对比了等离子球化处理前后粉末及截面形貌,采用X射线衍射方法测试分析了球化过程中氧化物的生成.检测了球化前后粉末的松装密度及其粒度分布.结果表明:不规则形状的钕铁硼粉经等离子球化处理后其球化率可达到100％,松装密度由2.778 g/cm3提高到3.785 g/cm3,粉末流动性由43.3 s/50 g提高到27.5 s/50 g.该粉末适用于凝胶注膜成型及注射成型.     

Synthesis of nanocrystalline hydroxyapatite by using precipitation method

In this investigation, hydroxyapatite powder has been synthesized from the calcium nitrate hydrated and di-ammonium hydrogen phosphate solution by precipitation method and heat treatment of hydroxyapatite powders. In order to study the structural evolution, the Fourier transform infrared spectroscopy (FTIR), the X-ray diffraction (XRD) and simultaneous thermal analysis (STA) were used. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to estimate the particle size of the powder and observe the morphology and agglomeration state of the powder. Results show that hydroxyapatite nanocrystalline can successfully be produced by precipitation technique from raw materials. Hydroxyapatite grain gradually increased in size when temperature increased from 100 to 1200 °C, and the hydroxyapatite hexagonal-dipyramidal phase was not transformed to the other calcium phosphates phases up to 1200 °C.     

Functionally graded carbon nanotubes/hydroxyapatite composite coating by laser cladding

Functionally graded carbon nanotubes/hydroxyapatite (CNTs/HA) composite coatings have been fabricated by laser cladding technique using CNTs/HA composite powders. As the feedstock for laser deposition, CNTs/HA composite powders were prepared by ball-milling different weight ratios (1%, 3% and 5%) of CNTs with HA powders. CNTs/HA composite coatings were fabricated with CNTs/HA composite powders and functionally graded coating was fabricated by sequentially depositing different CNTs/HA composite coatings on pure titanium. The phase composition, microstructure, micro-hardness, bonding strength and in vitro cellular responses of the composite coatings and the functionally graded composite coating were studied. The results show that the crystallinity of CNTs/HA composite coatings increased with increasing amount of CNTs in the powder mixture. The CNTs were dispersed homogeneously in the coatings to form an interconnected web and the cylinder graphic structure of CNTs was not changed after laser irradiation. Compared with pure HA coating, the maximum increase of the micro-hardness of CNTs/HA composite coatings was 46.8% and the micro-hardness of the functionally graded coating increased gradually through the thickness of this coating. Furthermore, the bonding strength of the functionally graded coating was nearly twice higher than that of pure HA coating. The in vitro cellular biocompatibility tests reveal that the functionally graded composite coating has comparable in vitro bioactivity with pure HA coating.     

Advanced Microstructural Study of Suspension Plasma Sprayed Hydroxyapatite Coatings

Fine, home-synthesized, hydroxyapatite powder was formulated with water and alcohol to obtain a suspension used to plasma spray coatings onto a titanium substrate. The deposition process was optimized using statistical design of 2 ⁿ experiments with two variables: spray distance and electric power input to plasma. X-ray diffraction (XRD) was used to determine quantitatively the phase composition of obtained deposits. Raman microscopy and electron probe microanalysis (EPMA) enabled localization of the phases in different positions of the coating cross sections. Transmission electron microscopic (TEM) study associated with energy-dispersive x-ray spectroscopy (EDS) enabled visualization and analysis of a two-zone microstructure. One zone contained crystals of hydroxyapatite, tetracalcium phosphate, and a phase rich in calcium oxide. This zone included lamellas, usually observed in thermally sprayed coatings. The other zone contained fine hydroxyapatite grains that correspond to nanometric and submicrometric solids from the suspension that were agglomerated and sintered in the cold regions of plasma jet and on the substrate.     

Protection of titanium metal by nanohydroxyapatite coating with zirconia and alumina second phases

In this study hydroxyapatite (HA)/zirconia/alumina composite coatings on titanium metal was carried out using Sol-Gel dip coating and calcination process. Hydroxyapatite-Alumina-Zirconia sol, coated samples in three processes by changing final sol stirring time, aging time, calcination temperature of synthesized powder and prepared coating and rate of coating. Some parts of prepared sol were also synthesized and became powder in all three processes. Scanning electron microscopy was used to estimate the particle size of the surface and for morphological analysis. The functional group and crystallization characteristics of the powders were analyzed using (FTIR) and X-Ray diffraction (XRD). Results show that the morphology of HA-Alumina-Zirconia coatings is more homogenous in the second process with 2 hours final sol stirring time, 20 hours aging time under stirring at 60, 675°C calcination temperature for coating and 850°C for powder and 60mm/min rate of dip coating.     

微束等离子喷涂制备羟基磷灰石涂层微观结构的特征研究

目的 通过与大气等离子喷涂和超音速火焰啧涂的对比,研究微束等离子喷涂制备的羟基磷灰石涂层的微观组织特点.方法 以高结晶度的羟基磷灰石粉末为原料,采用三种不同的喷涂方法(微束等离子喷涂、大气等离子喷涂和超音速火焰喷涂),在Ti-6Al-4V基体上制备羟基磷灰石(HA)涂层.利用冷场发射扫描电子显微镜和X射线衍射仪,对三种涂层的形貌、相组成和择优取向进行分析.结果 与大气等离子喷涂及超音速火焰喷涂制备的涂层相比,应用该设备制备的羟基磷灰石涂层表面平整致密,无大量的气孔存在;涂层截面呈典型的层状结构,在近三分之一表面处观察到柱状晶;涂层中仅有少量的非晶相及分解相,结晶度高达90％以上.这些特征均有利于羟基磷灰石涂层在体液环境中的稳定性.结论 比较三种喷涂方法,采用微束等离子喷涂制备的羟基磷灰石涂层致密,结晶度高,杂相少,且存在择优取向的柱状晶.     

Mullite and Mullite/ZrO2-7wt.%Y2O3 Powders for Thermal Spraying of Environmental Barrier Coatings

Mullite and mullite/ZrO₂-7wt.%Y₂O₃ coatings could be thought among the main protective layers for environment barrier coatings (EBCs) to protect Si-based substrates in future gas turbine engines. Considering that feedstock of the compound powder is not commercially available, two powder processing routes Spray Drying (SD) and Flame Spheroidization (FS) were implemented for both types of powders. For each method the particle size, the morphology, and microstructure of the powder particles was determined. In addition, the effect of the heat treatment on the powder crystallinity and microstructure of FS powders was also investigated. To evaluate their suitability as feedstock materials, the powders were plasma sprayed and their in-flight particle characteristics monitored for coatings production. The powder morphology was correlated to the in-flight particle characteristics and splat morphology to gain insight about into the influence of powder characteristics on the coating formation.     

HVOF-Sprayed Nano TiO2-HA Coatings Exhibiting Enhanced Biocompatibility

Biomedical thermal spray coatings produced via high-velocity oxy-fuel (HVOF) from nanostructured titania (n-TiO₂) and 10 wt.% hydroxyapatite (HA) (n-TiO₂-10wt.%HA) powders have been engineered as possible future alternatives to HA coatings deposited via air plasma spray (APS). This approach was chosen due to (i) the stability of TiO₂ in the human body (i.e., no dissolution) and (ii) bond strength values on Ti-6Al-4V substrates more than two times higher than those of APS HA coatings. To explore the bioperformance of these novel materials and coatings, human mesenchymal stem cells (hMSCs) were cultured from 1 to 21 days on the surface of HVOF-sprayed n-TiO₂ and n-TiO₂-10 wt.%HA coatings. APS HA coatings and uncoated Ti-6Al-4V substrates were employed as controls. The profiles of the hMSCs were evaluated for (i) cellular proliferation, (ii) biochemical analysis of alkaline phosphatase (ALP) activity, (iii) cytoskeleton organization (fluorescent/confocal microscopy), and (iv) cell/substrate interaction via scanning electron microscopy (SEM). The biochemical analysis indicated that the hMSCs cultured on n-TiO₂-10 wt.%HA coatings exhibited superior levels of bioactivity than hMSCs cultured on APS HA and pure n-TiO₂ coatings. The cytoskeleton organization demonstrated a higher degree of cellular proliferation on the HVOF-sprayed n-TiO₂-10wt.%HA coatings when compared to the control coatings. These results are considered promising for engineering improved performance in the next generation of thermally sprayed biomedical coatings.