Jiaming Feng

Effect of atmosphere on phase transformation in plasma‐sprayed hydroxyapatite coatings during heat treatment

The purpose of this study was to evaluate the effect of atmosphere on the phase transformation in hydroxyapatite (HA) coatings during heat treatment by varying the atmosphere in the furnace pipe. Heat treatment always increased the crystallinity of HA coatings regardless of the kind of atmosphere. Water molecules in atmosphere further promoted HA recrystallization during heat treatment. In a dry atmosphere, tricalcium phosphate (TCP) and tetracalcium phosphate (TTCP) were more stable than HA, so heat treatment could not convert them into HA. However, in a humid atmosphere, heat treatment would transform TCP and TTCP into HA by hydrolytic reactions.

Dissolution and mineralization behaviors of HA coatings.

The dissolution and mineralization behavior of HA coatings are two of the main factors governing the bioactivity of coatings. After different post treatment operators, the plasma-sprayed HA coatings have different characteristics, including different chemical composition, crystallinity, crystallite size and dissolution behavior. In this study, HA coatings were characterized by X-ray diffraction, scanning electron microscope, and Fourier transform infrared spectra before and after immersion in simulated body fluid (SBF). When immersed in SBF, both dissolution and precipitation occurred at the same time, but the kinetics of dissolution was quite different from that of precipitation. The former was dominated by ion exchange, while the latter was controlled by the ion concentration product and the solubility of the particles. Therefore, the dissolution behaviors of phosphate ions partly depended on the dissolution behaviors of calcium ions. With the increase of ions concentration in solution by dissolution, more nucleation sites appeared on the surface of coatings. Crystalline grains gradually grew up on the nucleation sites and developed into biomineral layers. The biomineral layers were the results of the precipitation of the ions in the solution; and the carbonates partially substituted phosphates to form bone-like apatite. The different dissolution characters resulted in quite different morphology of the biomineral layers: the coatings with low solubility induced biomineral layers of large grains; on the contrary, the biomineral layers of network structure were observed on the more soluble coatings.

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 degrees 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 alpha-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.

Effect of particle size on molten states of starting powder and degradation of the relevant plasma-sprayed hydroxyapatite coatings

Crystallinity of hydroxyapatite (HA) coatings is an important parameter to evaluate their stability. Variation of the size distribution of the starting powder is one way to alter crystallinity of coatings. The fundamental reason might be the variation of molten states of HA powders with different particle sizes. In the experiments, HA particles sized between 1 and 180 microns were divided into six groups by sieving. It was observed that the trend of crystallinity of coatings on particle size is not linear but fluctuates. The fluctuation of crystallinity was caused by the alteration of molten states of HA powders with different size distributions. It is concluded that the molten state of starting powder also fluctuated with particle size but the trend was different from that of crystallinity. Coatings sprayed with different particle sizes were immersed in deionized water for 1 month. After immersion, severe degradation and break-up were observed on the surface of coatings with the highest crystallinity, which were sprayed with large sized HA powders. It may be the high porosities in these coatings that cause the severe degradation. This shows that high crystallinity is not necessarily related to high stability of coatings and microstructure is of great importance when stability of coatings is considered.

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.

Studies on diffusion maximum in x‐ray diffraction patterns of plasma‐sprayed hydroxyapatite coatings

Study of an amorphous phase in plasma-sprayed hydroxyapatite (HA) coatings is important owing to its unique characteristics and nonnegligible amount of the amorphous phase compared to crystalline HA. However, little is known about the component parts of an amorphous phase. It is known that amorphous phase usually appears as the diffusion maximum (Dmax) in X-ray diffraction (XRD) patterns. Analyzing Dmax, including the position (Pmax) and area of Dmax, we can indicate the component parts of an amorphous phase and their transitions. In this study, the variation of Dmax in XRD patterns of the coatings during plasma spraying, in postheating, and in dissolving in vitro was studied with the aid of XRD. It was found that component parts of the amorphous phase in the coating varied with increasing thickness, consisting of two part represented by Dmax1, located between 29.4 and 29.8 degrees (2 theta), and Dmax2, located between 31.0 and 31.4 degrees (2 theta). It was concluded that Dmax3, located between 32.0 and 32.4 degrees (2 theta), should be referred to as nanocrystals of HA. In addition, the particle size of the starting powder may affect the component parts of the amorphous phase in the coating in addition to thickness. With vacuum heating (650 degrees C) and water vapor treatment at a low temperature (125 degrees C) in a saturated vaporic atmosphere, transition of the amorphous components was not as efficient as that at 490 degrees C with water vapor. The reason might be that the amorphous-to-crystalline HA conversion is dependent on both temperature and water vapor pressure. It was found that amorphous components were transformed completely into crystalline HA after heating at 490 degrees C with a partial water vapor pressure of 0.01 MPa for 2 h. It was concluded that the unstable amorphous components (Dmax1, Dmax2) converted into more stable nanocrystals of HA (Dmax3). Degradation in vitro showed that Dmax3 was more stable than Dmax1 and Dmax2. It was concluded that nucleation of apatite in vitro should be attributed to nanocrystals of HA (Dmax3) except for the amorphous components. It is recommended that the optimal phasic contents of the plasma-sprayed HA coating be mainly composed of crystalline HA and nanocrystals of HA (Dmax3) in terms of the stability and biocompatibility of the coating.

Effect of water vapor pressure and temperature on the amorphous‐to‐crystalline HA conversion during heat treatment of HA coatings

X-ray diffraction was used to characterize the increment of crystallinity of HA coatings after heat treatment. Coatings were heated over the temperature (T) interval of 300 degrees-460 degrees C with a partial water vapor pressure of 0.01 MPa and 0.001 MPa. Heat treatment also was done in air, as a contrast. It was found that the ratio (n) of the increment of crystallinity to the crystallinity of the as-received HA coatings was more significant for the coatings heated in atmosphere with water vapor than for those heated in air. This ratio also increased with water vapor pressure. The logarithm of the ratio increased linearly with 1/T, indicating that the ratio is exponential to T. The reason might be that recrystallization of the amorphous phase is a diffusion controlled process; the nucleation rate and growth velocity of the crystallites are in proportion to the diffusion coefficient, which is exponential to the temperature (T). Incorporation of water vapor in the atmosphere during heat treatment may decrease the activation energy for diffusion, which helps raise the diffusion coefficient of the atoms. Thus recrystallization of the amorphous phase can be accelerated.

Phase transitions of hydroxyapatite coatings during post-heat treatment and their performances under ultrasonic tests.

Highly or completely crystalline hydroxyapatite (HA) coatings can be obtained by post-heat treatment. We have developed a high-temperature (490 degrees C) and a low-temperature (125 degrees C) heat treatment to improve the crystallinity of HA coatings. Both methods transform entirely the amorphous phase into crystalline HA. However, the microstructure of the coating is dependent on the post-heating method. Nanocrystalline HA is about half of the component of the low-temperature heated coating while highly crystalline HA dominates the high-temperature heated coating, as detected by X-ray diffraction. The effects of both methods on the disintegration of the coatings were tested by ultrasonic treatment. The high-temperature heated coatings exhibited poor integrity while the low-temperature heated coatings exhibited better integrity, possibly due to their different microstructure. SEM revealed that the coatings disintegrated via different mechanisms: the high-temperature heated coatings failed via crack initiation and propagation while the low-temperature heated coatings failed via pit formation and subsequent widening.

Effect of Post-Treatment on Dissolution and Biomineralization on Surface of ha Coatings in Simulated Body Fluid (SBF)

The HA coatings were heated separately in vacuum, air and water vapor. The dissolution of the HA coatings was investigated by immersion in Tris buffer and SBF The dissolubility of HA coatings in the solutions decreased in this order: as-received, heated in vacuum, in air and in water vapor. The nucleation of bone-like apatite on the surfaces of HA coatings after immersing a period of 11 days in SBF was observed by SEM. The microenvironment with a sufficient supersaturation of Ca and P ions was crucial for the nucleation and growth of apatite in SBF. The dissolution of amorphous phase in coatings played an important part in establishing the supersaturation of Ca and P ions.

Further study on post-treatment of plasma sprayed hydroxyapatite coatings with water vapour [dental/hip prostheses application]

It is important to transform the amorphous and the additional phases, such as /spl alpha/,/spl beta/-Ca/sub 3/(PO/sub 4/)/sub 2/ (/spl alpha/,/spl beta/-TCP) and Ca/sub 4/O(PO/sub 4/)/sub 2/ (TP), into HA and thus increase crystallinity of the coatings in terms of long-term stability. Post-treatment is an effective way to raise crystallinity of the coating, which includes water vapour treatment with water vapour pressure of 0.15 MPa and post-heat-treatment in different atmosphere. It was found that TCP and TP disappeared almost completely after water vapour treatment but the amorphous to crystalline conversion was still incomplete. Post-heat-treatment at 490/spl deg/C in air with water vapour can obtain a completely crystalline HA coating after 2 hr treatment, which is superior to both water vapour treatment and post-heat-treatment in air without water vapour in terms of phase transition. Amorphous to crystalline conversion is significantly promoted when temperature and water vapour pressure increased whereas increase in water pressure or temperature alone is not efficient in this conversion. Mean crystallite size after both water vapour and post-heat-treatment is also compared.