C Composites Prepared by Ultrasonic Induction Heating Method

Rare Metal Materials and Engineering Volume 43, Issue 7, July 2014 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2014, 43(7): 1594-1599.

ARTICLE

Effect of Substrate Temperature on CaHPO4 Coating on HT-C/C Composites Prepared by Ultrasonic Induction Heating Method Xiong Xinbo,

Chu Cencen,

Zeng Xierong,

Zou Jizhao

Shenzhen Key Laboratory of Special Functional Material, Shenzhen University, Shenzhen 518060, China

Abstract: CaHPO4 coatings were deposited on H2O2-treated carbon/carbon composites (HT-C/C) substrates at different substrate temperatures by a ultrasonic induction heating method. The microstructures of these coatings were characterized by SEM, XRD, FTIR and EDS, and the adhesion of them to the HT-C/C substrates was evaluated by a scratch test. Meantime, the deposition kinetics of the CaHPO4 coatings was analyzed by measuring their weights using an analytical balance. The results show that the CaHPO4 coatings have a Ca/P ratio of 1.2±0.05, and some carbonate ions are found in their lattice. These coatings show no obvious changes in the morphologies, but their compactness decreases with increasing the temperature. The adhesive strength of the coatings to HT-C/C increases but their cohesive strength decreases with increasing substrate temperature. The induction heating deposition process of these CaHPO4 coatings is controlled by surface chemical reaction and the deposition activation energy is 46.7 kJ/mol. Key words: coating; carbon; CaHPO4; scratch test; induction heating deposition

Metal implants, such as titanium, CoCrMo alloy and stainless steel, have been applied in medical practice because of their excellent biocompatibility, high-strength/density ratios and corrosion resistance to biological environments. Nevertheless, the clinically used metal has a higher elastic modulus than 25 GPa of natural bone, which will lead to stress shielding and harmful effects in the vicinity of the implant area[1]. Therefore, how to get the elastic modulus of implant materials close to natural bone receives great attention. Carbon/carbon (C/C) composites potential as an implant has been highlighted[2-4]. They not only inherit the intrinsic biocompatibility of carbon materials, but also possess excellent mechanical properties, such as high toughness, high strength, fatigue proof and corrosion resistance. In particular their elastic modulus is in the range of 5~80 GPa, very close to that of cortical bone. Also, they have lots of micropores, which are beneficial to the growth of bone tissue. However, C/C composites are biologically inert and often release carbon debris into

the surrounding tissue due to the friction during surgical procedure. Thus, calcium phosphates could be coated onto C/C to provide bioactivity and reduce the release of carbon micro particles. Among calcium phosphates, the most commonly used calcium phosphate is hydroxyapatite (HA) because it is bioactive and similar in structure and composition to mineral fraction of bone[5]. However, its solubility in serum results in almost no resorption in vivo. On the other hand, β-tricalcium phosphate (β) shows higher solubility at physiological pH. But its resorption in vivo is too high, which results in poor bone remodeling capacity[6]. Brushite (CaHPO42H2O) is another calcium phosphate ceramic with higher solubility. But, brushite re-precipitates into poorly resorble HA, which will slow down its further dissolution. Recently, anhydrous calcium hydrogen phosphate (monetite or CaHPO4) has received much attention in the dental or orthopedic fields. Monetite not only shows good osteo-conductive and osteo-inductive properties but also re-

Received date: July 25, 2013 Foundation item: the National Natural Science Foundation of China (51172147, 50702034); the Shenzhen Science and Technology Research(JC201005280437A) Corresponding author: Xiong Xinbo, Ph. D., Professor, Shenzhen Key Laboratory of Special Functional Materials, Department of Materials, Science and Engineering, Shenzhen University, Shenzhen 518060, P. R. China, Tel: 0086-755-26536239, E-mail: [email protected] Copyright © 2014, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

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sorbablity in vivo. Particularly, after implantation, monetite could not re-precipitate into HA in vivo but it is in situ replaced by new bone. In addition, studies show that monetite can transform to HA without change in morphology by the topological transformation mechanism[7]. Thus, it is a useful route to prepare HA with special morphology via control over the morphology of monetite. Nevertheless, the use of monetite as a bone graft materials is limited to none- or low-load-bearing application due to its poor mechanical performance. Therefore, it is a logical choice to coat monetite onto bioinert implants with excellent mechanical properties, for example C/C composites in our study [8]. Many efforts have been made on the preparation of monetite coating via different methods, including plasma spraying [9], electrochemical deposition[10], electrophoretic deposition[6], chemical deposition method[8], induction heating deposition method (IHD)[11]. Among these methods, IHD introduced in our previous study is a convenient and effective technique to prepare coatings on conductive materials. However, the bonding strength of CaHPO4 coating on C/C prepared by induction heating was only 13 N of critical load, which need to be further enhanced [12]. Ultrasonic effects through liquid media are believed to induce strong mechanical, physical and chemical changes that can lead to the activation of chemical reactions. Hence, coupling of induction heating deposition with an ultrasound field should improve adhesion of coatings to C/C substrate. However， as far as we know, there was no investigation report about the kinetics and adhesive properties of CaHPO4 on C/C deposited by induction heating method to C/C. Thus, this study will focus on the composition, the structure, the morphology, the kinetic and adhesive property of CaHPO4 coatings on HT-C/C prepared by the ultrasonic induction heating method.

1

Experiment

Induction heating deposition (IHD) was performed using SP-15 high frequency induction power with a frequency of 15 kHz made in Shenzhen SuanPin Power Co. Lit. The experimental setup for induction heat deposition process was described elsewhere [13]. IHD process was performed in the ultrasonic bath with the power 150 W and 20 kHZ. Additionally, a calibrated T-type thermocouple was in contact with the surface of C/C sample for measuring the substrate temperature. The mother solution used in this study was prepared by dissolving given amounts of reagent-grade chemicals of 0.08 mol/L Ca(NO3)2 and 0.048 mol/L NH4H2PO4 into distilled water. C/C composites were prepared by chemical vapor infiltration (CVI) processing in Northwest Polytechnical University in China. The density and Shore scleroscope hardness of them are average 1.72 g/cm3 and 361 MPa, respectively. Cylindrical C/C samples with a diameter of 8 mm

and a length of 10 mm were cut from the block. Prior to the coating operation, each sample was polished with No.600 and No.1000 abrasive paper, rinsed with distilled water, then cleaned ultrasonically in acetone, and dried in a desiccator. After that, samples were pretreated in high pressure steam in a 50 mL autoclave with 40 mL 2 mol/L H2O2 solution at 433 K. After removal from H2O2 solution, these cylinders were rinsed ultrasonically with deionized water and dried in air. The deposition experiments were carried out at the C/C substrate temperatures of 343, 353, 363 to 373 K in deposition time ranging from 5 to 60 min. The crystalline structure, the morphologies and the compositions of the coated samples were characterized by X-ray diffraction (XRD) using a D8 Advance X-ray diffractometer (CuKα radiation), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis with a s-3400N (Japan) microscope. The functional groups of HA coatings were quantitatively identified by Fourier transform infrared (FTIR). The FTIR spectra were recorded in the 400~4000 cm-1 range, resolution 4 cm-1, using Perkin Eimer instruments Spectrum One Spectrometer and KBr pellet technology. The content of monetite coating was measured by an analytical balance with the precision of 10-4 g. The adhesion strength of the HA coatings deposited on C/C substrates was determined by the scratch test method using an s-3400 N scratch tester fitted with a Rochwell C 0.2 mm-diamond stylus with a preload of 1 N. The load speed, maximum load and scratch speed were 20 N/min, 2.5 mm/min and 40 N respectively, for the C/C samples with monetite coating whose thickness was 100 µm. The scratch trance of the HA coating was observed by a Stereomicroscop (STM).

2

Results and Discussion

Fig.1a shows a typical SEM image of coating cross section. It can be seen that the coating is linked tightly to C/C substrate. The corresponding EDS spectrum of the as-achieved CaHPO4 coatings given in Fig.1b confirms that the coating consists of calcium phosphate, and its Ca/P atomic ratio is 1.2±0.05, larger than a theoretic ratio of CaHPO4[14]. Fig.2 presents the XRD patterns of the CaHPO4 coatings deposited at different substrate temperatures. It can be seen that all the peaks are in good agreement with the reference data JCPDS 70-0360(CaHPO4, monetite) and there are no obvious difference among these XRD patterns[15]. Fig.3 shows a typical FTIR spectrum of the as-deposited products at different substrate temperatures. The peaks at 560, 887, 989, 1068, and 1126 cm-1 are attributed to the characteristic bands of PO43-. The bands at 1126, 1064 and 989 cm-1 are assigned to the P-O stretching modes, and the P-O(H) stretching mode appears at 887 cm-1. The O-P-O(H)

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a

Intensity/a.u.

b

P O Ca C 0 1

Fig.1

Ca

2 3 4 Energy/keV

5

6

SEM image of cross-section (a) and EDS spectrum (b) of

(022)

(120)

C/C Other peaks are attributed to CaPO4

(121)

(112)

373 K

(001)

Intensity/a.u.

(200)

coatings on HT-C/C

363 K 353 K 343 K

10

Fig.2

20

30 2θ/(º)

40

50

XRD patterns of achieved coatings on HT-C/C at different substrate temperatures

1570

Transmittance/%

60

989

50 1645

3428

560 1383

30 20

Fig.3

2840 2360

40

887

1126 1086

3600

2400 1200 Wavenumber/cm-1

Typical FTIR spectrum of the as-deposited CaHPO4 coatings

bending modes at 560 cm-1 and strong peak at 1383 cm-1 are assigned to P-O-H scissoring vibration modes. The broad O-H stretching bands induced by hydrogen bonds are observed clearly at 2840 and 2360 cm-1, indicating a relatively strong hydrogen bonds in the CaHPO4 structure. The broad peaks at 3428 and 1645 cm-1 are assigned to the O-H stretching and H-O-H bending of residual free water, respectively [16]. The FTIR result confirms the functional groups of the as-deposited CaHPO4 coatings. Additionally, a peak at 1570 cm-1 attributed to the v3b mode of A-type CO32- could be observed[17], which indicates that some carbonate functional groups are incorporated into the as-achieved CaHPO4 lattice. This is the reason that CaHPO4 shows Ca/P atomic ratio higher than 1. The morphologies of the coatings achieved at the temperature of 343, 353, 363 and 373 K, are shown in Fig.4. All the coatings consist of lots of polygon crystals. These crystals are bonded tightly together to form a dense coating. It can be noted that the coating achieved at 343 K shows lower crystal size and denser morphology than those at other substrate temperatures, which is related to the competition between the epitaxial and parallel growth directions of coatings. At low substrate temperature, the crystal in the coating grows slower in the epitaxial direction of the coating than in the parallel direction, and as a result the coating at 343 K consist of lots of small and dense crystals. With the substrate temperature increasing, the growth velocity of the crystals in coating in the direction parallel to coating surface is faster than that in the epitaxial to coating surface. Thus, the coating presents coarse morphology with some holes, and the hole size increases with increasing the substrate temperature, as shown in Fig.4b~4d. Fig.5 shows the dependence of the measured mass of coatings in this study on the depositing time at different substrate temperatures. Interestingly, the coating mass is found to linearly increase with increasing time, which indicates that the deposition rate keeps unchanged at a specific substrate temperature, and the deposition amount of CaHPO4 monetite coatings is proportional to deposition time. The deposition reaction of CaHPO4 is shown as follows: HPO42- + Ca2+ → CaHPO4 (3) the deposition rate k (in units of g/min) is proportional to d[CaHPO4]/dt under the condition of the fixed solution composition, namely, d[C aH P O 4 ] = kc dt

(4)

where c is a constant. If plotting the data from Fig.5 in the form of k versus 1/T (the absolute temperature), we can obtain the following relation:

In( k ) = 2.530 − 5.629

1 R 2 = 0.992 T

(1)

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a

Fig.4

b

c

d

SEM images of surface morphologies of achieved coatings on HT-C/C at different substrate temperatures: (a) 343 K, (b) 353 K, (c) 363 K and (d) 373 K

Coating Mass/g

0.012

a

373 K 363 K

0.009

353 K

0.006 343 K

0.003 0.000

0

10

20 Time/min

30

40

–12.0

b ln k=2.530-5.629/T R=0.992 Eα=46.7 kJ/mol

In k

–12.5 –13.0 –13.5 –14.0 2.6

Fig.5

2.7

2.8 T-1/×10-3 K -1

2.9

Dependence of the measured mass of CaHPO4 coatings prepared by ultrasonic induction heating deposition on the depositing time at different substrate temperatures (a) and the Arrhenius relationship plot of CaHPO4 coating (b)

where R2 is the determination coefficient. Comparing to Arrhenius equation(2):

k = Ae

−

Eα RT

(2)

A value of 46.7 kJ/mol is then obtained for the apparent activation energy Eα of the deposition process. Thus, we can conclude that the process is kinetically controlled by the surface reaction of Ca and P ions on HT-C/C in our experiment, and the deposition rate is highly elevated by increasing of the substrate temperature. Fig.6a~6d shows the curves of friction force and friction coefficient v.s scratch distance and load. The critical load was determined when the exposed black C/C substrate was

in-situ observed using STM. It can be seen that the critical load of the coating increases with the increasing of substrate temperature, and the bonding strength of the achieved coating could reach its maximum critical load of 37.2 N at 373 K. This adhesive value of monetite coating on HT-C/C is greater than that of HA on HT-C/C, which is high enough for handling before implant and strong enough to survive in the living body[13]. Particularly it is worth noting that for the curves of friction forces v.s loads, they have the similar slope which means that the friction coefficients of all the coatings are almost equal before their failure. The curves of the measured friction coefficients v.s srcatch distances also present this result. As shown in Fig.6, the curves of the friction coefficients show high fluctuation at the initial stage. This is aroused by uneven surface. After that, all the friction coefficients are stable at 0.3 nearby, which is close to the slope of the curve of friction force v.s applied load. All the CaHPO4 coatings show similar friction coefficient, which means that the coatings have the approximate friction force at the same applied load. Thus, we can propose that the crystals in the CaHPO4 have the similar cohesive strength, and the bonding strength between two crystals in the CaHPO4 is almost the same. This indicates that the CaHPO4 coatings have close cohesive strength, which might be related to the linear growth of the coatings in our study. Also, all the curves of friction coefficients show high fluctuations before the failure of as-achieved coatings, and the fluctuation range increases with increasing of substrate temperature. This is aroused by the increasing size of holes in the coatings. These analyses show that the compactness of the coatings does not affect their cohesive strength. From the above analyses, we could suggest that increasing of the C/C substrate temperature favors mainly improving of the adhesive strength of the CaHPO4 coatings to HT-C/C, but does not obviously influence their cohesive strength. Additionally, the scratch morphologies of all the coatings are shown in Fig.7. It is also seen that no obvious fracture or chip is observed at the border or inside the scratch until the coating fails, and the coating materials are squashed along the track. After the coatings fail, they also show no larger pieces peeling off, which indicate the coatings at the

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0

50.0 a

1 mm

0.2 10 0.4 0

0

10

20 Load/N

30

Fig.7

Scratch Distance/mm 2 3 4

1

b

perature

10 0.4

0

10

20 Load / N

30

40

Scratch Distance/mm 2 3 4

1

c

5 0.0

(33.4N)

0.2

20 10

0.4 0

0

10

1

20 Load / N

30

Scratch Distance/mm

2

3

Friction Force/N

4 d

0.2

10

0.4 0

10

3

20 Load/N

30

Arciniegas M, Aparicio C, Manero J M et al . J Euro Ceram Soc[J], 2007,11(27): 3391

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distance and load of as-deposited CaHPO4 coatings at dif-

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ferent temperatures: (a) 343 K, (b) 353 K, (c) 363 K and (d) 373 K

Conclusions

1) CaHPO4 coatings can be prepared on HT-C/C by the induction heating deposition at the substrate temperatures from 343 to 373 K at a frequency of 15 kHz. The CaHPO 4 coatings have a Ca/P ratio of 1.2±0.05 and some carbonate ions are found in their lattice. 2) These coatings show no obvious change in the morphology, but their compactness decreases with increasing of temperature. 3) The adhesive strength of the coatings to HT-C/C increases but their cohesive strength decreases with increasing of substrate temperature. At 373 K, the coating could reach a critical load of 37.2 N. 4) The induction heating deposition process of these CaHPO4 coatings is controlled by surface chemical reaction and the deposition activation energy is 46.7 J/mol.

1

50.0

(37.2N)

20

0

40

as-used substrate temperature possess good adhesion to HT-C/C substrate.

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Fig.6

FrictionCoefficient Coefficienc Friction

Friction Force/N

30

FrictionCoefficient Coefficienc Friction

Friction Force / N

0.2

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Optical images of the scratch test performed on a CaHPO4 coatings on HT-C/C prepared at different substrate tem-

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363 K

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(21.3N)

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Friction Friction Coefficient Coefficienc

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Friction Coefficienc Coefficient Friction

(4.96N)

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