Amorphorization and recrystallization during plasma spraying of hydroxyapatite

Biomaterids 16 (1995) 829-632 fQ 1995 Elsevier Science Limited

Printed in Great Britain. All rights reserved 0142-9612/95/$10.00

Amorphorization and recrystallization during plasma spraying of hydroxyapatite Weidong Tong, Jiyong Chen and Xingdong Zhang institute of Materials

Science and Technology, Sichuan University,

Chengdu 610064, People’s Republic of China

X-ray diffraction was used to characterize the dependence of mean crystallite size and crystallinity on the thickness of coatings. A fall in mean crystallite size but a rise in crystallinity with increased thickness was observed. The reason might be due to the differences in cooling rate of partially molten particles of hydroxyapatite. The thicker the coating, the longer the cooling time. The longer cooling time was beneficial to the occurrence of recrystallization. It was found that the critical thickness of recrystallization (rJ was about 20,um. Keywords: Plasma spraying,

hydroxyapatite

coatings, amorphorization,

recrystallization

Received 17 August 1994; accepted 18 October 1994

hydroxyapatite (HA)-titanium (Ti) implants show excellent mechanical strength and good bioactivity, and can bond directly to bone after implantation’, ‘. Plasma spraying is one of the methods to obtain ceramic coatings on metal substrates. During spraying HA powder is melted at a very high temperature. The temperature of the plasma flame is far beyond the melting point of HA (1550°C)~. Therefore, change in component phases and crystallinity is unavoidable. The possible calcium phosphates in HA coatings are: crystalline HA, calcium oxide (CaO), amorphous phase c(- and /I-tricalcium phosphate (TCP) (a, P-Ca,(PO,),) and tetracalcium phosphate (TP, Ca40 (PO,),). HA almost does not degrade in vivo, and the other components have different solubilities4. The stability of the coatiq is related directly to its crystallinity (percentage of crystalline HA in the coating)5,6. The crystallinity of ihe coating is initially dependent on the conversion to an amorphous state and the recrystallization process during plasma spraying. In this paper, coatings of different thicknesses were used to study the dependence of mean crystallite size and crystallinity on ilmkness in order to characterize the recrystallization and amorphous conversion (amorphorization) process. The critical thickness for recrystallization (r,) was determined. The effect of r, on the cohesion of coatings is discussed.

ASTM B600-74’. HA powder was sintered at 1256” C before spraying. Coatings were plasma sprayed onto titanium substrates (MRTCO MN plasma spray system + AR-2000 Thermal Spray Robot, Metco, Westbay, NY, USA). The thickness of the coatings was controlled by varying the spraying parameters. X-ray diffraction (XRD) was employed to detect the structure of the coatings. Crystallinity was calculated by comparing the 211 peak area on XRD patterns of the coating with that of the HA powder. The mean crystallite size was determined by using the well-known Scherrer formula for the direction [211].

Plasma-sprayed

MATERIALS

to

RESULTS

AND DISCUSSION

The XRD pattern shows that the powder for spraying was pure HA (Figure 2). Figure 2 shows a typical XRD pattern for an HA coating, consisting of a series of sharp peaks, the diffusion background and some additional peaks. The diffusion background represents the amorphous phase and sharp peaks represent the crystalline HA. Curves A and B in Figure 3 show respectively the dependences of crystallinity (SC) and mean crystallite size (R) on thickness of coatings. It can be seen that R decreased with thickness, whereas SC increased with thickness. Curves A and B are almost parallel to the vertical axis at a thickness of 20 pm, above which recrystallization of the molten particles began. It can be deduced herein that the critical thickness for recystallization (r,) was about 20 pm. In order to analyse the recrystallization, we divided the HA coating into discrete layers each having uniform temperature and the same thickness (Figure 4). The heat input during spraying occurred at the front surface and heat loss occurred mainly at the rear surface of the

AND METHODS

Titanium substrates (12 x 8 x 2mm3) were polished, sandblasted with Sic and cleaned before use according

Correspondence to Dr Jiyong Chen. 829

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28 Figure 2 X-ray diffraction pattern of as-received coating consisting of a series of sharp peaks, diffusion background and additional peaks (arrows). Sharp peaks are representatives of crystalline hydroxyapatite, the diffusion background is the amorphous phase produced by fast cooling.

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29 Figure 1 X-ray diffraction pattern of starting showing pure and well-crystallized hydroxyapatite.

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substrate, causing the heat flux to pass through the coating perpendicularly. A steep temperature gradient would be built up in the coating due to the low thermal conductivity of the ceramic and the resistance to heat conduction caused by the pores in the coating’. The HA particle was partially molten on reaching the substrateg*JO. Each particle was composed of a molten portion and an unmelted core. The molten portion was either recrystallized or converted to the amorphous phase depending on the cooling rate of the particle. Therefore, the crystalline phase of HA in the coating was composed of the unmelted cores of the starting powder and the recrystallized HA crystals. The crystallinity of the HA coating was dependent on the total amount of unmelted cores and newly formed (recrystallized) HA crystals; the mean crystallite size of the HA coating was determined by the number and crystallite size of both the unmelted cores and the newly formed HA crystals. The crystallite size of the unmelted cores was several thousand BngstrGms’l, about the same as that for the starting powder. The crystallite size of the newly formed HA crystals must be far smaller than that of the unmelted cores, as fast cooling and rapid solidification prevented the growth of newly formed HA crystals”. Zaat13 deduced that the cooling time (fk) of a molten lamella (the flattened drop) obeyed the following formula:

0 -- SC . __ R

90 - f:

powder,

A

3

/--

Z 3 Nz

/-

0 Kr)G=l .,v 30/

X w

-

---

-1

&

10 i--i

I i 20

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Thickness

of coatings

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(urn)

Figure 3 The dependence of crystallinity (SC) and mean crystallite size of the coatings on thickness. Curve A shows that crystallinity increases with thickness, whereas mean crystallite size decreases with thickness (in curve B). Curves A and B are almost parallel to the vertical axis at a that the critical thickness of about 20pm, indicating thickness of recrystallization is about 20pm.

N

N 1 N12

Coating 2 1

Substrate Figure 4 Heat flux model in coatings during plasma spraying. The input of heat occurs at the front surface and heat loss occurs mainly at the rear surface of substrate, causing the heat flux to pass through the coating perpendicularly (arrow).

Amorphorization

and recrystallization

of HA in plasma spraying:

x is the position of the solidification front in the lamella; p is higher if the substrate material conducts heat more efficiently; and up is higher if the material of the lamella transports heat more easily. Here p and a,, are the parameters related only to the material. In this case, p and up are independent of x, and hence tk is proportional to x2. Coatings near the substrate (x is small) will cool more rapidly due to greater heat direct contact with metal. conduction through Therefore, recrystallization is limited and conversion of the molten portion to the amorphous phase is the main process occurring. We have pointed out before that HA particles reaching the substrate were partially molten and comprised a molten portion and an unmelted core. The molten portion ‘of the particle near the substrate solidified rapidly as amorphous material as its t* was rather short, and recrystallized HA only dominated a minor fraction of crystalline HA. This poor recrystallization process led to low crystallinity for very thin coatings. In this case, the mean crystallite size was determined mainly by the unmelted cores, with the crystallite size about the same as that of the starting powder. This was in agreement with the result shown in Figure 3, that S, was low but R was very large for very thin coatings (~3’0 pm). As the thickness increased, x for the particles became larger. The increase in x was beneficial to recrystallization as th is proportional to .?. Therefore more and more tiny HA particles were formed, resulting in a rise in crystallinity (Figure 3). It is known that the higher the cooling rate the smaller the crystallite size of the newly formed crystals as rapid cooling prevented the growth of crystals. So the crystallite size of these newly formed crystals should increase with thickness as tk increases with thickness, representing a decreased cooling raie. However, it can be concluded from Figure 3 that even at a thickness of about 380pm the crystallite size of the newly formed crystals only reached 50 nm, indicating the nanostructure of recrystallized HA. This size was far smaller than the crystallite size of HA powder. The mean crystallite size of the coating was determined by the number and crystallite size of both the unmelted cores and newly formed HA crystals. The number of unmelted cores detected by XRD can be assumed to be unchanged over the thickness of 50pm (for the depth detected by XRD was in the range of 40 to !jOpm in our experiments). As the thickness increased, more and more tiny HA crystals (crystallite size less than 5Onm) were formed in the coating, and thus the mean crystallite size R decreased with the increased thickness of coatings. Increasing the thicrkness of the coatings can raise crystallinity, but two limitations exist for this approach. One is that an increase in thickness resulted in an increase in iniernal stress14 and a decrease in bond strength to substrate15; the other is that the newly formed crystallites during plasma spraying were nanocrystals and would be more subject to dissolution than HA ceramics because of the difference in crystallite size. Both aspects have an adverse effect on the stability of HA coatings and as a result limit the application of this approach. It was reported that heat treatment could reduce greatly the dissolution of HA

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coatings16’17. An increase in both the crystallinity and crystallite size of the coatings can be obtained after hence raising the stability of the heat treatment”, coating. Zaat13 studied the relationship between the cohesion of sprayed layers and the contact temperature (Tcont). It was said that the higher the Tcont, the better the cohesion. Tcont for the layers was low below r,, as they were cooled down rapidly by the substrate, so cohesion must be poorer below r, than above it. In mechanical experiments the fracture site of the coating was usually located at the place where poorest cohesion existed. Mechanical experiments in vitro and in vivo had shown that the fracture mode was at the metal-ceramic interface for as-received coatings1gs20. So adhesion at the interface was poorest for asreceived coatings, and hence many efforts have been devoted to increasing the interface strength. As the metal-ceramic interface was enhanced to be stronger than the coating itself, fracture first occurred in the ceramic coating very near the interface”. This showed that cohesion adjacent to the interface was poorer than at other parts of the coating. Our analysis explains how cohesion below r,, where it was adjacent to the interface, can be poorer than that above r,. The experimental results” proved our analysis to be correct. Appropriately increasing the contact temperature below r, may be a useful way to raise the adhesion of the coating.

CONCLUSIONS HA particles were partially melted in the plasma flame to contain molten portions and unmelted cores. The mean crystallite size decreased but crystallinity increased with increasing thickness. The mean crystallite size of recrystallized HA would be less than 50 nm. The thickness of recrystallization (rc) was about 20pm (in different experiments, it might fluctuate slightly), above which recrystallization began.

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