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The adhesion strength and residual stress of colloidal-sol gel derived

-Tricalcium-Phosphate/Fluoridated-Hydroxyapatite biphasic coatings

Kui Cheng a, Sam Zhang a,, Wenjian Weng b, Khiam Aik Khora,Shundong Miao b, Yongsheng Wang a

a School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singaporeb Department of Materials Science and Engineering, Zhejiang University, Hangzhou, Zhejiang, PR China 310027

Received 12 July 2006; received in revised form 15 November 2007; accepted 27 November 2007

Available online 4 December 2007

Abstract

-tricalcium phosphate (-TCP) powders are embedded in a fluoridated hydroxyapatite (FHA) matrix to form -TCP-FHA composites via

colloidal-sol gel method. This composite layer is deposited on top of a FHA layer to form a -TCP-FHA/FHA biphasic coating. The effect of the

nanosized powder on the residual stress is characterized through the X-ray diffraction peak shift. The powder incorporation increases the residual

stress, while a large amount of-TCP (Capowder/Casol ratio is higher than 1/2) results in less gel shrinkage that partially compensates the mismatch

of thermal expansion coefficient and thus the residual stress. Despite the elevated residual stress as more powders are embedded, the coating

adhesion strength remains virtually constant: around 430 mN500 mN in scanning scratch test.

2007 Elsevier B.V. All rights reserved.

Keywords: -TCP/FHA biphasic coatings; Colloidal-sol gel; Residual stress; Adhesion strength

1. Introduction

Hydroxyapatite (HA) is one of the best coating candidates on

metallic biomedical implants to improve the interaction

between the implant and host tissue. However, pure HA

coatings have relatively large solubility in vivo that leads to

long-term stability concerns [1,2]. Although incorporation of

fluorine into the HA structure to form fluoridated hydroxyapa-

tite (FHA) [35] imparts dissolution resistance, the cell

activities are adversely affected owing to the reduced Ca2+

and PO43

ion concentration at the vicinity of the implant,because these ions aid cell activities and osteointegration [68].

Aside from the biological and cell activity considerations, the

mechanical properties of the coatings especially cohesion

strength of the coating itself and the adhesion strength between

the coating and the implant also play critical roles in the

effectiveness and durability of the implant. It has been reported

that coating-substrate adhesion failure is one of the most

common modes of in vivo implantation failure [9]. Residual

stress in the coating has been identified as one of the main factors

adversely affecting adhesion strength [10]; in addition, residual

stresses also affect the dissolution behaviors of the coating [11].

In our recent work [12], a coating has been engineered to

have short term controlled dissolution and long term stability:

nanosized -tricalcium phosphate (-TCP) powders are

embedded in FHA to form a -TCP/FHA biphasic coating

through a colloidal-sol gel method, where -TCP serves as the

soluble phase to initiate good early implant

tissue interaction[13,14], while the FHA acts as the dissolution-resistant matrix.

This paper concentrates on the residual stresses of the coatings

and their influence on adhesion strength of this biphasic coating

as a function of-TCP incorporation.

2. Materials and procedures

2.1. Designed coating structure

As sketched in Fig. 1, the biphasic coatings have the

following structure in cross section: a dissolution resistant FHA

Thin Solid Films 516 (2008) 32513255

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E-mail address: [email protected] (S. Zhang).

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layer between the substrate and biphasic layer for long-term

stability and a composite layer of -TCP particles in FHA on

top of this layer for short term release of calcium and phosphate

ions.

2.2. Nanosized -TCP

The detailed preparation process of nanosized -TCP was

documented in [15]. In brief, calcium chloride (CaCl2, GR,Merck) and sodium dihydrogen phosphate (NaH2PO4, GR,

Merck) were dissolved in double deionized water to form CaCl2and NaH2PO4 solution respectively; polyethylene glycol (PEG,

GR, Merch) with an average molecular weight of 10,000 was

dissolved in the CaCl2 solution in designed molar ratio and held

in ice-water bath. The NaH2PO4 solution was then dripped in

the PEG-containing CaCl2 solution gradually to form pre-

cipitates. During the dripping process, the pH value of system

was maintained at over 10 with continuous addition of

ammonium hydroxide solution. The precipitates were rinsed

and lyophilized before calcination at 900 C for 2 h.

2.3. FHA precursor

Calcium nitrate tetrahydrate (Ca(NO3)24H2O, GR, Merck)

was dissolved in ethanol (GR, Merck) to form the 2 mol/L Ca-

precursor; ethanol was gradually poured into phosphor

pentoxide (P2O5, GR, Merck) to form 2 mol/L solution which

was refluxed for 24 h to become the P-precursor. Hexafluor-

ophosphoric acid (HPF6, GR, Merck) in F/Ca ratio of 1/5 was

added drop-wise into the P precursor. After that, the Ca

precursor was added into the mixture in Ca/P ratio of 1.67. The

mixed solutions were further refluxed for 24 h to be as-refluxed

sols. The as-refluxed sol is able to produce almost pure

fluorapatite (FA) coating [16]. The nanosized -TCP powderswere added into the as-refluxed sols slowly in design Capowder/

Casol ratios (1/8, 1/4, 1/2 and 3/4), and ultrasonically dispersed

for 20 min to be the colloidal-sols.

2.4. Biphasic coatings

Titanium alloy (Ti6Al4V) substrates were polished down

to #1200 grade SiC paper. The substrates were rinsed in double

distilled water and ultrasonically washed in acetone for 10 min

and dried.

For the preparation of the FHA bottom layer, the substrates

were first dipped in the as-refluxed sols and withdrawn at a

speed of 8 cm/min, followed by immediate 150 C oven drying

for 15 min and firing at 600 C for another 15 min. Then, the -

TCP/FHA composite layers were deposited following the samedippingdryingfiring process with -TCP powder dispersed

colloidal sols. Coatings with thickness 12 m resulted. As a

control, FHA coating was also prepared with the as-refluxed sol

following the same procedure and repeated five times to achieve

a thickness of about 1.5 m.

2.5. Coating characterization

The coatings were characterized by X-ray Diffractometry

(XRD, RIGAKU, D-Max, RA, using Cu K1 radiation with a

2/min scanning speed, 50 ms dwelling time and 0.02 step) for

phase identification. X-ray Photoelectron Spectroscopy (XPS,

AXIS Kratos Ultra) was used to determine the surface chemicalcomposition with monochromatic Al K X-ray source

(1486.7 eV, 1 eV per step for survey scan and 0.1 eV per step

for narrow scan). In order to avoid any non-stoichiometric loss

of F and Ca, the coatings are characterized directly without any

sputter etching. The atomic ratio of F/(CaTCP+ CaFHA) was

calculated based on the relative sensitivity factors (given by the

manufacturer) and the narrow scan areas of F and Ca. The

surface morphology of the coatings was observed using

Fig. 1. Sketch of the designed coating structure.

Fig. 2. XRD patterns of the as-prepared powders and coatings.

Fig. 3. Typical XPS result of the coatings.

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Scanning Electron Microscopy (SEM, JEOL, JSM5600LV,

operating at 20 kV) after gold sputtering.

The residual stress was evaluated through XRD peak shift

[17]:

r E ctgh

2 1 m sinu D2h 1

Where, E and are the Young's modulus and Poisson's

ratio of the FHA matrix, is a half of the diffraction angle. isthe angle between the normal and the preferred direction of

residual stress, thus is actually 90 as the biaxial residual

stresses is parallel to the coating surface. 2 is the shift of the

chosen diffraction peaks in comparison with the standard value.

In this study, the E and values, as well as the standard peak

positions of FA were used for the FHA matrix, since its F

content is very close to pure FA [16]. The peaks located at

around 25.863 (ascribe to (0 0 2) peak of FA [18]) and 31.026

(ascribe to (0 2 10) peak of -TCP [19]) were chosen to

investigate the residual stress level of the FHA matrix and -

TCP powder respectively, due to their relatively strong intensity

and less intervention from other peaks. The middle point at the

full width at half maximum of the fitted peak was regarded as

the exact peak position.

The adhesion strength of the coatings was evaluated by

Scanning Scratch Tester (Shimadzu, SST-101) with a tip size of

15-m in diameter and scanning amplitude of 50 m. The mean

value of the load at total coating peeling-off of the scratches wasused as a measurement of the adhesion strength between the

coating and the substrate. The measured output voltage

indicates the lateral friction force encountered by the scratching

tip on its displacement over the coating surface.

3. Result and discussion

3.1. Crystalline phase and morphology

In Fig. 2, the XRD patterns show the as-prepared powders

are pure -TCP, no impurity phase is observed. All the coatings

contain apatite phase. For those derived from colloidal sols, the-TCP phase appears when the Capowder/Casol ratio (R) reaches

1/4, and the peak intensity becomes increasingly higher with the

increasing R. Non-detection of -TCP phase at R =1/8 is

probably due to the detecting limit of XRD.

Fig. 3 shows the result of a typical XPS survey scan of a

coating. Ca, P, O and F are detected. The inset indicates the

typical narrow scan result of F1s peak: the binding energy of

F (684.6 eV) is in good agreement with F in FA [20]. Based on

the narrow scan results, the F/(CaTCP+ CaFHA) molar ratios are

calculated and tabulated in Table 1: the F/CaFHA ratio of the as-

refluxed sol derived FHA coating is around 0.19, which is quite

Table 1

The F content and -TCP content of the coatings

Coatings

annotation

FHA R =1/8 R =1/4 R =1/2 R =3/4

F/Ca

molar ratio

0.190.01 0.180.01 0.170.00 0.130.00 0.100.01

CaTCP/CaFHAmolar ratio

0 0.03 0.12 0.47 0.92

Fig. 4. SEM micrographs of the as-prepared powders and coatings a: as-prepared powder; b: FHA coating; c: coating derived from colloidal-sol with R =1/4; d: coatingderived from colloidal-sol with R =3/4.

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close to pure FA. Since all the colloidal-sols are prepared based

on exact the same as-refluxed sol, the matrix of those colloidal

sol derived coatings is supposed to have the same F/CaFHAratios. Based on this and the calculated F/(CaTCP+ CaFHA) ratio,

the relative content of-TCP phase in term of CaTCP/CaFHA is

obtained and tabulated in Table 1. Obviously, CaTCP/CaFHA

ratio increases with increasing R of colloidal sols. These resultsindicate that the -TCP amount in the coatings is tailorable

through variation of the amount of powders in the sols.

The size of the powders and morphology of coatings are

shown in Fig. 4. The average size of the as-prepared -TCP

powders is around several hundred nanometers (Fig. 4a). The

FHA coating has a rather smooth surface (Fig. 4b). With

introduction of-TCP powder (R =1/4), the coating becomes

rougher but crack-free (Fig. 4c); increasing amount of the

powders leads to even rougher surface (Fig. 4d, R =3/4), but

still crack free. Comparing with the original particle size

(Fig. 4a), the size of the protruding powders is larger, indicating

a slight agglomeration of powders.

3.2. Residual stress and adhesion strength

Compared to the XRD standard pattern (FA, 15-876), the

(002) peak of FHA phase of all the coatings experiences

certain shift: FHA has the smallest shift, the biphasic coatings

have more and more shift as R increases. Based on equation 1,

where E and of FA is 103.6 GPa (calculated from bulk

modulus) and 0.344 respectively [21], the specific residual

stresses were calculated and plotted in Fig. 5. The residual

stresses are tensile and increase linearly from 79.0 MPa

for FHA to 286.6 MPa at R =1/2, and decrease to 274.9 MPa

at R =3/4. During the calculation, the possible interference ofthe (1 0 10) peak of-TCP is ignored, because its intensity is

only about 1/5 of the strongest peak at around 31.026,

which is so low that it could be totally submerged in the

background.

In solgel derived coatings, the residual stresses come from

the following two aspects: 1) thermal stress as a result of

coefficient of thermal expansion (CTE) mismatch between the

coating and the substrate; 2) shrinkage stress as a result of

evaporation and decomposition of organic residuals in the gel.

The thermal stress can be estimated by [22]:

r Da Dt E

1 m2

Where, is the Coefficient of Thermal Expansion (CTE)

difference between the coating and the substrate, t is the

difference between the heat-treatment and ambient tempera-

tures, E and are the Young's modulus and Poisson's ratio of

the coating respectively. Calculated from this equation, the

residual stress of FHA coating in this work is around 50.0 MPa,

lower than that calculated from XRD, indicating gel shrinkage

also plays a role [23].

To examine the effect of-TCP on residual stress, the CTEvalues are considered first: the CTE of FA is 1010 6/C [24],

Ti6Al4 V substrate is 9.19.810 6/C [25] and -TCP is

14.810 6/C [26]. Thus, incorporation of-TCP in FHA will

increase the mean CTE, and its difference is directly proportional

to the amount of-TCP. That explainsthe linear increase in Fig. 5.

However, with the increase of the amount-TCP, the volume

fraction of the powder in the gel increases. That leads to less and

less shrinkage of the gel, therefore, a decreasing contribution to

Fig. 5. Effect of-TCP powder amount on the residual stress of the coatings.

Fig. 6. Typical scanning scratch test result of the coatings.

Fig. 7. Effect of-TCP powder amount on the adhesion strength of the coatings.

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residual stress from the evaporation and shrinkage. As a result,

the increase in residual stress should taper off when R is large.

The slight dip observed atR =3/4 in Fig. 5 may also come from

the interference of the (1 0 10) peak of -TCP, which may

become not negligible with increasing content of -TCP: the

position of (1 0 10) peak (25.802), which may be ignored at low

-TCP concentrations, but whose influence will becomenoteworthy at higher concentrations, is lower than that of the

stress-free FHA (002) peak (25.863), while becomes even

higher when FHA is under relatively high tensile stress. With the

increase of-TCP amount in the coatings, the position of FHA

(002) peak will be affected. Ignoring that results in the reduction

of calculated residual stress.

The trend of residual stress variation is discernable through

the stress state of-TCP powders in the coating. The diffraction

peaks of the in-coating powder register a very small shift in

diffraction pattern indicating that the in-coating powders are also

stressed, though slightly. Although the exact values can not be

calculated due to lack of standard E and values of-TCP, thequantity Ectg/2(1+) in Equation 1 is actually constant for the

given peaks, thus the extent of peak shift is directly proportional

to the resultant residual stress. The peak shifts ((0 2 10) peak) of

the in-coating -TCP are calculated based on the XRD patterns

(Fig. 2) and plotted in Fig. 5. Since the coatings have the same

powder-matrix interface (-TCP-FHA), the structural mismatch

stress will have few effects on this variation. Thus, the peak shift

tendency in Fig. 5 indicates the trend of the overallresidual stress

in the coatings: with increasing amount of powder, the effect of

its addition on the residual stress tapers off. This corroborates the

results for the residual stress of the FHA matrix.

In Fig. 6, a typical scratch curve was recorded.With increaseof

the load, the relative output voltage fluctuates with an overalltrend of increasing, and jumps up abruptly to a steady value. The

fluctuation at the beginning indicates continuous damage inflicted

to the coating as the load increases; finally, the coating peels off

from the substrate resulting in the abrupt jump; the load where the

abrupt jump occurs thus indicates the adhesion strength

between the coating and the substrate. Note that before the total

peeling off, no other abrupt changes are observed, this indicates

the bonding between the upper composite layer and the bottom

FHA layer is perfect, no delamination of the layer has happened.

The coating adhesion strength is plotted against -TCP

content in Fig. 7. All the coatings show comparable adhesion

strength although they believed to have different residual stresses(in Fig. 5). As studied in our previous work[27], in FHA coatings,

there are active chemical reactions at the interface, which result in

the formation of nanometer thick titanium oxide layer. This

interfacial layer should be responsible to relieve residual stresses.

4. Conclusion

The residual stresses and the adhesion strengths of-TCP-

FHA/FHA biphasic coatings are characterized using XRD and

Scratch Test. It was found that:

1. The biphasic -TCP-FHA/FHA coatings prepared through

colloidal-sol gel method demonstrate crack-free surface

morphology and good adhesion strength to biomedical-

grade Ti-alloy substrate;

2. The incorporation of nanosized -TCP powders results in two

opposite effects:a nearly linear increasein residual stressin the

coatings, and a reduction of residual stress due to less volume

shrinkage of the matrix; the net increase in residual stress

vanishes after the molar ratio of Capowder/Casol reaches 1/2.3. The incorporation of nanosized -TCP powder brings about

no obvious detrimental effect on adhesion between the coating

and the substrate despite the increased residual stresses.

Acknowledgement

The authors would like to thank the Agency for Science

Technology and Research, Singapore (AStar project 032 101

0005) for the financial support.

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