Sprayed Hydroxyapatite Coatings Research Articles

Effect of vapor-flame treatment on plasma sprayed hydroxyapatite coatings.

A vapor-flame treatment was developed to modify the crystallinity of the as-sprayed hydroxyapatite (HA) coatings. The effects of the treatment on composition, structure, and properties of HA coatings were investigated. Results showed that the vapor-flame treatment is simple and efficient to adjust the crystallinity of as-sprayed HA coating. Its crystallinities can be raised from 53.5 to 98.7% in 3-7 min. The porosities of coatings increased with an increase in the vapor-flame treating time. The microhardness of coating decreased as a result of this treatment. It may be explained in terms of the extent of microcracks caused by recrystallization of amorphous HA and relaxation of stress of the coating. The porosity, bonding strength, and hardness of HA coatings treated for 7 min were 15.7%, 32.0 MPa (300 microm thickness), and 1.9 GPa, respectively.

Phase transformations in plasma sprayed hydroxyapatite coatings

In the present study, the kinetics of hydroxyapatite formation in different kaolin-natural phosphate mixtures was investigated using X-ray diffraction (XRD) and differential thermal analysis (DTA) measurements. The XRD patterns revealed the presence of both kaolinite and hydroxyapatite phases in all of the mixtures and full disappearing of kaolinite phase in the heat-treated samples. According to the DTA curve results, both the peak temperature formation of hydroxyapatite and the relative fraction-transformed increased as the heating rate increased and the amount of phosphate in the mixture decreased. The activation energy of hydroxyapatite phenomenon was estimated using Kissinger and Ligero analytical methods. The obtained values of the activation energy were in the range of ∼196.5 and 210.6 kJ/mol. The growth morphology parameters n and m were both found to be near to half unity indicating that the surface nucleation with plate-like growth was dominant in hydroxyapatite formation controlled by diffusion.

X-ray determination of crystalline hydroxyapatite to amorphous calcium-phosphate ratio in plasma sprayed coatings.

A method to measure the amorphous calcium phosphate (ACP) content of plasma sprayed hydroxyapatite (HA) coatings is presented. The areas of the crystalline peaks and the broad amorphous "hump" in an X-ray diffraction scan of a mixture of crystalline and amorphous HA can be fitted using conventional nonlinear least-squares methods. The weight fraction of the amorphous component in a mixture of unknown proportions is obtained from the measured hump fraction using a linear expression requiring a single calibration constant. This calibration constant may be determined by applying the same sample preparation, X-ray measurement, and least-squares fitting methods to a single standard of known (weighed) proportions as used to measure the unknown(s). Application of this method to a set of 12 samples yields an "index of crystallinity" for plasma sprayed HA coatings with an estimated standard deviation of 1 wt % ACP across the entire compositional range.

Laser treatment of plasma sprayed HA coatings

Laser treatment was conducted on plasma sprayed hydroxyapatite (HA) coatings using a Nd-YAG pulse laser. Various laser parameters were investigated. The results showed that the HA surface melted when an energy level of ≥2 J and a spot size of 2 mm was employed during continuous scanning. The melted structure displayed a multi-layered porous structure. The size range of these pores was about 30–150 μm. These pores have the propensity to promote osseo-induction. However, when the spot size was increased to 3 mm, the coating was found to display melted structure at energy levels of >6 J. XRD results for the laser treated coating showed increased crystallinity of the HA phase. Favourable amount of crystalline HA was found when the laser pulse energy <5 J and a laser spot size of 3 mm was employed.

Fretting wear behavior of thermal sprayed hydroxyapatite coating lubricated with bovine albumin

Fretting wear behavior of plasma sprayed hydroxyapatite (HA) coatings lubricated with bovine albumin solution has been investigated as a function of oscillatory cycle, normal load and slip amplitude. Wear mechanisms, wear volume and coefficient of friction of HA coating under lubricated fretting conditions were studied. Results indicated that the fretting wear mechanisms of plasma sprayed HA coatings under lubricated conditions were mainly delamination, pitting and abrasive wear. Bovine albumin can provide an effective lubrication during fretting. Results also showed that hot isostatic pressing (HIP) treatment could densify the coating structure, decrease the generation of fretting wear debris and improve the fretting wear resistance, therefore, it could be used as a post-treatment method for as-sprayed HA coating.

Direct morphological comparison of vacuum plasma sprayed and detonation gun sprayed hydroxyapatite coatings for orthopaedic applications

Hydroxyapatite coatings on titanium substrates were produced using two thermal spray techniques—vacuum plasma spraying and detonation gun spraying. X-ray diffraction was used to compare crystallinity and residual stresses in the coatings. Porosity was measured using optical microscopy in conjunction with an image analysis system. Scanning electron microscopy and surface roughness measurements were used to characterise the surface morphologies of the coatings. The vacuum plasma sprayed coatings were found to have a lower residual stress, a higher crystallinity and a higher level of porosity than the detonation gun coatings. It is concluded that consideration needs to be given to the significance of such variations within the clinical context.

The effect of heat treatment on the morphology of D-Gun sprayed hydroxyapatite coatings

In this study, the morphology of the Hydroxyapatite (HA) coatings sprayed on Ti alloy samples by Detonation Gun Spray (D-Gun) and the effect of aging before and after heat treatment in physiological solution were observed. Cross-sectional porosity and percentages of amorphous and crystal phase were measured using optical, electron microscopy, and X-ray diffraction analysis. Differential Thermogravimetric Analysis (DTA) was performed to estimate the glass-crystalline phase transformation temperatures. Heat-treatment at 300, 500, 700, 800 and 1200 degrees C were carried out to confirm DTA results. As a final analysis, the aging effect using Ringer's solution for 1 week on heat-treated and non-heat-treated samples was measured. It was observed that, in D-Gun sprayed samples, the cross-sectional porosity stayed in the accepted 5% range as reported for other spraying techniques.(1-5) On the other hand, surface porosity measured using the water immersion method remained in the conventional porosity limit of 15% for non-heat-treated samples. Heat-treatment had a small influence on the porosity while the crystallinity increased considerably; in addition, aging had little effect on HA crystallinity for heat treated samples. This work showed that D-gun sprayed HA coatings had lower porosity and better integrity than other coatings, due to which we can expect better performance during in vivo applications.

The effect of heat treatment on the morphology of D-Gun sprayed hydroxyapatite coatings

In this study, the morphology of the Hydroxyapatite (HA) coatings sprayed on Ti alloy samples by Detonation Gun Spray (D-Gun) and the effect of aging before and after heat treatment in physiological solution were observed. Cross-sectional porosity and percentages of amorphous and crystal phase were measured using optical, electron microscopy, and X-ray diffraction analysis. Differential Thermogravimetric Analysis (DTA) was performed to estimate the glass-crystalline phase transformation temperatures. Heat-treatment at 300, 500, 700, 800 and 1200°C were carried out to confirm DTA results. As a final analysis, the aging effect using Ringer's solution for 1 week on heat-treated and non-heat-treated samples was measured. It was observed that, in D-Gun sprayed samples, the cross-sectional porosity stayed in the accepted 5% range as reported for other spraying techniques.1-5 On the other hand, surface porosity measured using the water immersion method remained in the conventional porosity limit of 15% for non-heat-treated samples. Heat-treatment had a small influence on the porosity while the crystallinity increased considerably; in addition, aging had little effect on HA crystallinity for heat treated samples. This work showed that D-gun sprayed HA coatings had lower porosity and better integrity than other coatings, due to which we can expect better performance during in vivo applications. © 1999 John Wiley & Sons, Inc. J Biomed Mater Res (Appl Biomater) 48: 861–868, 1999

Effects of Power Level on Characteristics of Vacuum Plasma Sprayed Hydroxyapatite Coating

In this study, hydroxyapatite coatings were obtained with a vacuum plasma spray system at different power levels that were achieved by altering the plasma current and voltage. The effects of spray power level on coating characteristics were investigated. X-ray diffraction was used to identify the crystallinities of as-sprayed coatings, Electron Probe Microanalysis was employed to detect the surface chemical composition of as-sprayed coatings and Scanning Electron Microscopy revealed the microstructure. The results indicated that spray power greatly affected the crystallinity, chemical composition, and microstructure of as-sprayed hydroxyapatite coatings, which were linked to the melting state of hydroxyapatite powder.

Plasma sprayed hydroxyapatite coatings on titanium substrates Part 2: optimisation of coating properties

Heat treatment and the introduction of a Ti bond coat have been applied to hydroxyapatite (HA) coatings sprayed using different plasma powers and gas mixtures. Attempts were made in this way to achieve optimal coating properties for orthopaedic implants. In particular, the effects on the degree of crystallinity, the adhesion, the OH - ion content and the purity were evaluated. Heat treatment at 700°C for 1 h in air proved to be effective in increasing the crystallinity, regaining the OH - ion and removing other non-HA compounds, although it caused a significant decrease in the degree of adhesion (interfacial fracture toughness) for those specimens sprayed at high powers. This heat treatment was found to induce significant transformation of amorphous HA to the crystalline form, while not detrimentally changing the properties of the underlying Ti–6Al–4V substrates. Precoating with a 100 μm Ti layer increased the adhesion of the HA coatings on Ti–6Al–4V substrates, primarily by providing a rougher surface and promoting better mechanical interlocking. Changes in coating properties during immersion in biological fluids were also studied and were found to depend critically on the chemical composition of the fluids. Small precipitates formed on the coating surfaces when immersed in Ringers solution. These might account for the apparent drop in the degree of crystallinity when measured using X-ray diffraction. A significant drop in the interfacial adhesion was found for those coatings sprayed at high powers. This could be offset by prior precoating with a titanium bond coat and suitable heat treatment. In summary, the following processing sequence is suggested in order to achieve optimum coating properties: precoating the substrate with a layer of Ti (∼100 μm), spraying HA at a sufficiently high-power level (depending on particle size and gas mixture) and heat treatment at 700°C for 1 h in air.

Plasma sprayed hydroxyapatite coatings on titanium substrates Part 1: Mechanical properties and residual stress levels

Hydroxyapatite (HA) coatings have been sprayed on to substrates of Ti–6Al–4V, using a range of input power levels and plasma gas mixtures. Coatings have also been produced on substrates of mild steel and tungsten, in order to explore certain aspects of the mechanical behaviour of HA without the complication of yielding or creep in the substrate. Studies have been made of the phase constitution, porosity, degree of crystallinity, OH - ion content, microstructure and surface roughness of the HA coatings. The Young’s moduli in tension and in compression were evaluated by the cantilever beam bend test using a tungsten/HA composite beam. The flexural Young’s modulus was determined using a free-standing deposit under the same test. Adhesion was characterised using the single-edge notch-bend test; this is considered superior to the tensile bond strength test in common use. Measured interfacial fracture energies were of the order 1–10 J m -2. Stress levels were investigated using specimen curvature measurements in conjunction with a numerical process model. The quenching stress for HA was measured to be about 10–25 MPa and the residual stress level in HA coatings at room temperature are predicted to lie in the approximate range of 20–40 MPa (tensile). These residual stresses could be reduced in magnitude by maintaining the substrate at a low temperature (possibly below room temperature) during spraying and it may be worthwhile to explore this. Ideally, the HA coating should have low porosity, high cohesive strength, good adhesion to the substrate, a high degree of crystallinity and high chemical purity and phase stability. In practice, such combinations are rather difficult to achieve by just varying the spraying parameters.

Oxyapatite in hydroxyapatite coatings

Plasma sprayed hydroxyapatite coatings are used as bioactive surfaces for increasing the fixation of bone to the dental implants or orthopaedic prostheses. The variability in the performance of these coatings is partly attributed to the chemical phases that form at high temperatures in the plasma or fast cooling rates upon deposition. Hydroxyapatite can be accompanied by an amorphous phase constituent, tricalcium phosphate, tetracalcium phosphate, calcium oxide or the rarely mentioned oxyapatite. The high temperatures in the plasma produces a hydroxyl-depleted layer on the outside of the traversing hydroxyapatite particle. It is this dehydroxylated area which may form oxyapatite upon deposition. Higher cooling rates produce an amorphous phase, but a lower cooling rate will lead to the formation of oxyapatite. Examination of the coating at various depths with X-ray diffraction reveals a higher oxyapatite content in the underlying layers. The surface in contact with water vapour in air can be modified by the inclusion of hydroxyl ions to form oxyhydroxyapatite or hydroxyapatite. This presence and varying amounts of oxyapatite with coating thickness could influence the dissolution and mechanical performance of the coating for dental and orthopaedic prostheses.

Interfacial Fracture Toughness of a Plasma Sprayed Hydroxyapatite Coating Used for Orthopedic Implants

AbstractThis study introduces a new method for evaluating the adhesion strength of a coating on a substrate. The interfacial fracture toughness, Γi is used to assess the work per unit area required to separate an interface. Γi is measured for the as-received specimens of hydroxyapatite plasma sprayed on Ti-6A1-4V substrate. Calculation of the interfacial fracture toughness requires that the elastic modulus of the coating to be known. The Young's modulus of the plasma sprayed hydroxyapatite is assessed using a bend test. The effect of aqueous environment on the interfacial fracture toughness is also investigated.

Acoustic emission analysis of thermal sprayed hydroxyapatite coatings examined under four point bend loading

Acoustic emission (AE) has been used to analyse hydroxyapatite (HA) coatings on Ti substrates subjected to four point bend testing. The coatings were produced using air plasma spraying, vacuum plasma spraying, or a continuous detonation system. Quantitative comparison of the AE results was made possible by the use of a D parameter, the value of which gives an indication of the relative AE count rate for a particular coating. Results showed that a high positive D parameter was associated with both thicker coatings and those coatings found to have the highest residual stresses. Optical and scanning electron microscopy observations of crack formation and distribution in the coatings correlate with the AE results indicating that the D parameter may be used to predict the performance of an HA coating subjected to high loading.

Mechanical and chemical consequences of the residual stresses in plasma sprayed hydroxyapatite coatings

The residual stresses in thick hydroxyapatite coatings, deposited by plasma spraying, have been determined experimentally using Raman piezo-spectroscopy. The stress dependence of the centre position of the 980cm −1 Raman band, owing to the symmetric stretching of the phosphate ion, PO 4 3−, has been established and found to be 2.47cm −1 GPa −1. Using this calibration, the residual stresses in hydroxyapatite coatings deposited onto Ti-6AI-4V substrates in air have been found to be 100 MPa (tensile), whereas those deposited in a vacuum have been found to be 60 MPa (compressive). Although desirable from a mechanical point of view, it is shown that coatings under residual compression are thermodynamically more stable and, hence, the dissolution of the ionic species, necessary in the exchange between bone and hydroxyapatite coating, can be impeded. It is calculated that for the coatings under examination the stresses have an effect comparable with almost an order of magnitude change of the [OH −] concentration. The analysis explains the dissolution behaviour of hydroxyapatite coatings subject to cyclic stress reported previously.

Influence of powder characteristics on plasma sprayed hydroxyapatite coatings

Phase transformations, particle breakdown, and partial decomposition occur in hydroxyapatite (HA) powder feedstock during plasma spraying. The biological responses of the coatings consequently change from the bioactive nature of the starting material to a less biocompatible one. This paper investigates the influence of powder characteristics on the phase composition and microstructure of plasma sprayed HA coatings. The raw HA was prepared by chemically reacting calcium hydroxide with orthophosphoric acid. Subsequently, HA was either calcined and crushed, flame spheroidized, or spray dried. These three types of HA powders were plasma sprayed on steel substrates to form coatings. A previous study showed that the calcined HA powder suffered from particle breakdown in the plasma. The plasma sprayed HA powders contained other calcium phosphate phases (amorphous and crystalline) apart from hydroxyapatite. The flow properties and stability of spheroidized HA were better than calcined HA and spraydried HA. Standard metallographic preparation of the cross sections of the coatings revealed different microstructural features among the coatings. The HA coatings prepared from calcined HA were highly porous and lacking in intimate lamellar contact. The spheroidized HA powders produced the coating with the lowest porosity. Characterization of the powders and coatings was carried out using x-ray diffraction (XRD), scanning electron microscopy (SEM), and optical microscopy.

Preferred orientation of plasma sprayed hydroxyapatite coatings

The long-term stability of hydroxyapatite (HA) coatings has been under investigation for a long time. The evaluation of crystallinity is important since a relationship exists between the degree of crystallinity and the degradation by in vitro cellular reaction. However, the choice of a representative peak to calculate the crystallinity is the subject of some controversy with the (2 1 1) peak and the (0 0 2) peak being the most commonly analysed peaks. Thus the determination of the degree of crystallinity may be biased due to preferred orientation (texture) in the coatings. Therefore, the dependence of texture on thickness during plasma spraying was investigated by the calculation of a texture coefficient (TC). Experiment results showed that the TC value of the (0 0 2) and (0 0 4) peaks increased with thickness and that the TC value in the (2 1 1) peak decreased. This was caused by a high temperature gradient during spraying and also the growth direction for a hexagonal system. It was observed that appropriately controlled temperature increases during annealing did not bring about notable texture to the recrystallized crystallites. However, if the temperature gradient was high during annealing, notable (0 0 2) texture can exist. The effect of particle size on the texture was also investigated.

Vacuum plasma sprayed hydroxyapatite coatings on titanium alloy substrates: Surface characterization and observation of dissolution processes using atomic force microscopy

Hydroxyapatite (HA), Ca5(PO4)3OH, is a biologically important material that occurs naturally in bone. Synthetic HA is thus a natural choice as a surface coating for materials that are used in implant surgery due to its inherent biocompatibility. HA coatings can be applied by several techniques, one of which is vacuum plasma spraying (VPS). This method is used commercially to apply HA to a titanium alloy substrate. A complete characterization of the initial, as-sprayed surface state was carried out using scanning electron microscopy (SEM), x-ray diffraction (XRD), and also atomic force microscopy (AFM). XRD measurements reveal the coating to be semicrystalline and SEM observations showed it to be irregular in appearance but with flatter regions interspersed. Large scale AFM scans (≳20 μm) of the as-sprayed coatings were difficult to obtain as the tip would frequently snag on higher outcrops, sometimes snapping the cantilever in the process. To prevent this from occurring, the scan range was reduced and images were only obtained within the flatter regions observed previously with SEM. From these data, the roughness values characterizing the flatter surfaces were estimated. Further, ex situ images of the coatings after they had been aged for periods of up to 28 days in a solution simulating the physiological environment within the body (buffered Ringer’s solution) were obtained. Changes in the topography were observed that may have a bearing on processes such as the cellular response of the surrounding tissue to the implanted material. The structural changes in the surface were again characterized by estimating the surface roughness values. Conclusions are drawn regarding the formation of the coating from plasma spraying and also the importance of the dissolution process in promoting bony ingrowth and rapid fixation of the implant.

Water vapour-treated hydroxyapatite coatings after plasma spraying and their characteristics

A novel way to enhance the ability of hydroxyapatite (HA) coatings in resisting degradation was revealed. The as-received plasma sprayed HA coatings were kept in water vapour at 125 °C, with a pressure of 0.15 MPa for 6 h; most of the amorphous phase in the coating was converted into crystalline HA and enhanced the crystallinity significantly. Meanwhile, the α-tricalcium phosphate, tetracalcium phosphate and CaO which decomposed from HA during plasma spraying were also transformed into crystalline HA. The dissolution experiment in distilled water at room temperature showed that the post-water vapour-treated coatings were more stable than post-heat-treated ones. The average interfacial tensile bond strength between HA and substrate before and after water vapour treatment was 45.0 and 39.1 MPa, respectively.

Crystallite orientation and anisotropic strains in thermally sprayed hydroxyapatite coatings

Thermally sprayed hydroxyapatite powders and coatings have been studied using X-ray powder diffraction and scanning electron microscopy. Preferred orientation of the crystallites has been found on a number of coated plates using X-ray powder diffraction, and full profile fitting of the patterns from the extracted powders indicates anisotropically thermally strained crystallites. This is a result of rapid crystal regrowth and associated thermal gradients on deposition. Electron micrographs of some coatings show a consistent picture of columnar crystallites orientated perpendicular to the substrate surface.