Surfaces for Bio-applications
Research Summary
The Nano-scratch Behavior of Biocompatible Hydroxyapatite Reinforced with Aluminum Oxide and Carbon Nanotubes Kantesh Balani, Debrupa Lahiri, Anup K. Keshri, S.R. Bakshi, Jorge E. Tercero, and Arvind Agarwal
Hydroxyapatite (HA) reinforced with sub-micrometer Al2O3 and carbon nanotubes (CNTs) has been synthesized as a coating on the Ti-6Al-4V substrate via plasma spraying. The addition of Al2O3 and CNTs to HA has shown improvement in the hardness and elastic modulus by 65% and 50%, respectively, when compared to HA. Consequently, HA-Al2O3-CNT coatings have been nano-scratched to understand their wear performance. Reinforcement of HA by Al2O3 shows a decrease in the wear volume by more than 13 times, whereas HA-Al2O3-CNT coating demonstrated further wear volume reduction of five times compared to that of HA-Al2O3 coating. INTRODUCTION Biocompatibillity of hydroxyapatite (HA) has been established by various researchers because of its chemical structure similar to that of bone and teeth (Ca/P ratio of 1.66).1,2 Application of HA as a structural material is limited because of its brittleness and low fracture toughness (~0.4 MPa m1/2).1,3 Hence arises the need to reinforce the HA without hampering its biocompatibility. Introduction of ceramics (e.g., ZrO2, Al2O3, mullite, Ni3Al, and SiC,) with better mechanical properties has been applied to enhance the energy absorption by HA matrix.2–6 It becomes a special concern to retain the biocompatibility of the composite while reinforcing secondary materials to improve mechanical properties of HA matrix.2,4 Extensive interfacial reaction between HA and ZrO2 to form tricalcium phosphate (TCP) limits its utilization.4 Toxicity of SiCw restricts its reinforcement Vol. 61 No. 9 • JOM
in HA matrix.7 Alumina (Al2O3), being a bioinert material, has shown much promise.2,8 Reinforcement with carbon nanotubes (CNTs) has shown fracHow would you… …describe the overall significance of this paper? Reinforcement of hydroxyapatite (HA) with Al2O3 and carbon nanotubes (CNTs) have shown improvement in the hardness and elastic modulus by 65% and 50%, respectively, when compared to that of HA alone. In addition, wear volume reduction by more than 65 times is also observed in HA-Al2O3CNT plasma sprayed coatings. …describe this work to a materials science and engineering professional with no experience in your technical specialty? Novel plasma spraying has been utilized to synthesize hydroxyapatite (HA) coatings reinforced with Al2O3 and carbon nanotubes (CNTs) on a real life Ti-6Al-4V body implant substrate. Undeterred biocompatibility with enhanced mechanical properties (modulus by 50%, hardness by 65%, and wear resistance by 68 times) has been observed in HA-Al2O3-CNT coatings in comparison to that of HA alone. …describe this work to a layperson? The rigorous body environment restricts the normal lifetime of body implants in the range of 10–15 years. An early age implant surgery requires repeated surgery operations after the usual lifetime of inserted implant is exhausted. Hence, newly developed HA-Al2O3-CNT biocoating will not only provide enhanced life, but will also provide a friendly surface for adhesion of newly growing bone (bone cells) for strong anchorage with the implant.
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ture toughness enhancement by up to three times, HA crystallinity increase by 27%, bending strength of up to 180 MPa, without deterioration of the coating’s biocompatibility.1,7 In addition, apatite precipitation has been observed on the CNT surface endorsing non-toxicity of human fiber osteoblasts in the CNT reinforced HA coating.1 Consequently, macro-scale pin-on-disk wear resistance improvement of up to 1.5 times was also observed in CNT reinforced HA matrix.3 The current work focuses on the synergistic effect of secondary reinforcements such as sub-micrometer Al2O3 (150 nm particle size) and CNTs in attaining enhanced fracture toughness and wear resistance of HA matrix. HA coatings reinforced with Al2O3 and CNTs are synthesized using plasma spraying, and wear resistance of HAAl2O3-CNT biocomposites is quantified using nano-scratching by evaluation of its wear volume. Since our previous work insinuates biocompatibility of HA-CNT composite,1 the addition of bio-inert Al2O3 as reinforcement is expected to further enhance the fracture toughness and wear resistance of coatings without affecting its biocompatibility. Though the effect of Al2O3 and CNT reinforcements on the biocompatibility of HA composite is not the focus of the current work, it must be mentioned that research confirming biocompatibility of these composites is published elsewhere.9 The two-fold importance of (i) enhancing toughness by Al2O3 (bio-inert) and rendering CNT re-bars (bio-active), while (ii) retaining biocompatibility (bioinertness of Al2O3 and apatite precipitation on CNTs) 63
Table I. Plasma-Sprayed HA-Al2O3-CNT Coatings Coating % th. Thickness ρth Ρexpt Sample (μm) (g/cc) (g/cc) Densification HA HA-A HA-A16C
300 400 450
3.16 3.30 3.27
2.96 3.04 3.01
94 92 92
makes this research a novel approach. See the sidebar for a description of materials and methods. RESULTS AND DISCUSSION Plasma-sprayed HA-Al2O3-CNT coatings were thick (>300 Mm) with
a density greater than 92% theoretical density as shown in Table I. Explosion of spray dried Al2O3 agglomerate (~15–45 Mm diameter) into individual sub-micrometer size Al2O3 particles is observed in HA-A coating (Figure 1a). The motive of spray drying is to retain the sub-micrometer Al2O3 particles embedded in HA matrix via solid state sintering, and induce toughening by serving as secondary reinforcements. With CNT reinforcement (in HA-A16C coatings), CNT pullouts are visible in the fractured surface (Figure 1b). It must be stated that the cushioning effect rendered by spray dried agglomerate can strongly retain CNTs in
MATERIALS AND METHODS Plasma Spraying of HA Composite Coatings
Table A. Representative Plasma Spray Parameters for HA-Al2O3-CNT Coatings
Three powder feedstock have been Plasma Spray Optimized prepared: (i) spray dried HA powder Parameters Values (particle size 15–55 μm), (ii) HA blended 500–700 A with 20 wt.% spray dried10 Al2O3 (denoted Gun Current 22–23 kW as HA-A). Spray dried Al2O3 has 99.9%+ Power Argon (30.2 slm) purity, bimodal size of 40-60 μm, with an Primary Gas Helium (28.3 slm) agglomerate size of 15-50 μm. (iii) HA Secondary Gas 100 mm blended with 20 wt.% of composite spray Standoff Distance dried Al2O3-CNT (corresponding to HA18.6 wt.% Al2O3-1.6 wt.% CNT (denoted as HA-A16C). CNTs are multiwalled, 95%+ pure with outer diameter of 40–70 nm, and 0.5–2 mm long. Composite spray drying has been performed to render dispersion of CNTs in Al2O3 powder agglomerate. Powder feedstock has been blended for one hour in a jar mill. Consequently powder feedstock are plasma sprayed to synthesize coatings on the Ti-6Al-4V substrate (50 mm × 50 mm × 2 mm) using Praxair’s SG-100 gun with parameters shown in Table A. Indentation toughness of HA-Al2O3-CNT coatings was calculated using Vickers indentation (load of 100 g, and dwell time of 15 s), using the formula:1
E K IC 0.016 H
the matrix.12 Consequently, retention of CNTs in the HA matrix as reinforcements becomes viable. Thereby, energy utilized in fracturing CNTs is expected to further enhance the fracture toughness of HA-A16C coatings. Synergistic contribution of exploded sub-micrometer-sized Al2O3 particles and CNT pullouts has been shown to provide exceptional contribution toward enhancing the fracture toughness of HA-A16C coatings, as explained later, without deterring the biocompatibility of these coatings.9 Interestingly, current work elucidates the effect of sub-micrometer Al2O3 and CNT reinforcement in the HA matrix in terms of nano-scratching of these coatings. Typical load-displacement curves of HA-Al2O3-CNT coatings upon nanoindentation are shown in Figure 2. Hardness and elastic modulus of HA coating are observed to be 2.52 GPa and 68.1 GPa, respectively (Table II). Enhancement of hardness and Young’s modulus to 3.52 GPa (by 39.7%) and 94.1 GPa (by 38.2%), respectively, are achieved in the HA-A coatings. In conjunction, a significant increase of 158% in the fracture toughness of HA-A coating (to 1.83 MPa m1/2, compared to 0.71 MPa m1/2 for HA) is attributed to the distri-
0.5
P a 1.5
(1)
where E is the Young’s modulus (assuming the Young’s modulus of HA to be 100 GPa, that of HA-A as 150 GPa, and of HA-A16C as 160 GPa), H is the hardness, P is the applied load, and a is the crack length from the center of indent.
a
1 Mm
b
1 Mm
Nanoindentation and Nano-scratching of HA-Al2O3-CNT Coatings Nanoindentation of HA-Al2O3-CNT coatings is carried out using Hysitron’s® 100 nm tip radius Berkovich tip at a load of 1,500 MN (ramp rate of 150 MN/s), and dwell time of 3 s. Hardness and elastic modulus of HA-Al2O3-CNT coatings is evaluated from after tip-area calibration on a standard quartz sample. Nano-scratch testing is performed at a ramp load (of 33.33 MN/s) reaching the maximum load of 1,000 MN. Consequently, wear volume is calculated using the equation:11
Wv
1 cos(70.3)d 2n l 2
(2)
where 70.3o is the included half-angle of Berkovich tip, dn is the depth of the wear track, and l is the length of the wear track. Depth of wear track is calculated by subtracting the scratched depth from the unscratched depth obtained from the sectional traverse of Berkovich tip.
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Figure 1. Fractured surface of (a) HA-A coating showing exploded sub-micrometer Al2O3 particles, and (b) HA-A16C coating showing CNT pullouts.
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▲
800
Sink-in
600 400
HA
200 0
0
20
40 60 Indentation Depth (nm)
bution of sub-micrometer Al2O3 in the HA matrix. A feeble sink-in behavior of HA-A coating is observed in Figure 2, insinuating accommodation of particle sliding during indentation. Further, CNT reinforcement has shown enhancement in hardness by 65.5% (to 4.17 GPa) and that of Young’s modulus by 50.4% (102.4 GPa) compared to that of HA (Table II). Enhancement of mechanical properties is attributed to CNTs which acts as re-bars (Figure 2b) allowing efficient load transfer between matrix/reinforcement, pile-up of material along the edges of indent by pyramidal diamond Berkovich tip in different coatings is observed in Figure 3a–c. Despite increasing hardness and elastic modulus, increased pile-up behavior depicted due to the addition of sub-micrometer Al2O3 (Figure 3b) and CNTs (Figure 3c) indicate enhanced localized plasticity (Table II). Correspondingly, higher resistance of material to indentation cracking is confirmed herewith. Successively, nano-scratching of the coatings is performed to assess their tribological properties. Nano-scratches were performed on the polished surfaces of plasma sprayed coatings. The higher average surface roughness of HA-A and HAA16C coatings (Ra of 26 and 35 nm, respectively) when compared to that of HA (Ra of 5 nm) corresponds to the
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Figure 2. Load-displacement behavior of plasma sprayed HAAl2O3-CNT coatings.
contribution of exploded sub-micrometer Al2O3 in rendering rough surface topography, Figure 4 and Table III. Highest wear volume loss is observed in the HA coatings (177.1 ± 1.1 × 10–21 m3). Enhanced roughness of HA-A coatings induces jerky movement of indenter tip arising from the existing protrusions and cavities. Thereby a wide scatter is observed in the coefficient of friction for HA-A coating (standard deviation of 0.11 with an average value of 0.18), Figure 5 and Table III. Existence of exploded sub-micrometer Al2O3 particles also render enhanced wear-resistance. This is confirmed from the higher coefficient of friction13 shown by the HA-A coating (0.18) when compared to that of HA (0.12). High coefficient of friction corresponds to higher value of lateral force experienced by Berkovich tip during nano-scratching. The lateral force arises from the resistance of material in letting the Berkovich tip traverse. Hence wear volume reduction of 13.5 times is observed in HA-A coatings in comparison to that of HA coating. With the addition of CNTs in the HA-A16C coating, a reduction of coefficient of friction from 0.18 (for HA-A) to 0.13 is observed. But, the effect of higher initial surface roughness (Ra ~ 35 nm) has dropped to lower standard deviation in the coefficient of friction (of 0.02 for an average value of 0.13). Hence, the
Table II. Mechanical Properties of Plasma-Sprayed HA-Al2O3-CNT Coatings
Coatings
Hardness (GPa)
Fracture Toughness (MPa m1/2)
HA HA-A HA-A16C
2.52 ± 0.40 3.52 ± 0.33 4.17 ± 0.40
0.71 ± 0.19 1.83 ± 0.11 2.92 ± 0.23
Vol. 61 No. 9 • JOM
Incorporation of sub-micrometer Al2O3 in the HA matrix has increased hardness, elastic modulus and fracture
5,600 5,500 5,400 5,300 5,200 5,100
20 0 −20
5,000 500 600 700 800 9001,000 1,100 X(nm) a
2,200 2,100 2,000 1,900 1,800
1,700
20 10 0
1,800 1,900 2,000 2,100 2,200 2,300 2,400 X(nm) b
1,700 1,600 1,500 1,400 1,300
Pile-up
1,200
Elastic Modulus (GPa)
Indentation Depth (nm)
Plasticity Index
68.1 ± 6.3 94.1 ± 2.1 102.4 ± 7.4
86.7 ± 6.8 70.8 ± 2.4 65.2 ± 1.9
0.49 0.51 0.54
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CONCLUSIONS
Z(nm)
HA-A16C
Z(nm)
1,000
Z(nm)
HA-A
presence of CNTs is introducing homogeneity in the plasma sprayed HAA16C coating. Consequently, reduced scratch depth (of 67 nm, from 102 nm in HA-A, Table III) in HA-A16C coating corresponds to the contribution of graphitic planes of CNTs serving as lubricants. This synergistic effect of enhanced wear resistance by exploded Al2O3 particles dispersed in HA matrix, reduced penetration depth because of enhanced hardness and modulus of HA-A16C coating, and ease of sliding of the Berkovich tip on the HA-A16C coating surface by CNT lubrication result in significant improvement in the wear resistance (68.3 times) of HAA16C coatings.
Y(nm)
1,200
HA-A
Y(nm)
1,400 Indentation Load (MN)
HA-A16C
HA
Y(nm)
1,600
0 −20
−1,000 −900 −800 −700 −600 −500 −400 −300 X(nm) c Figure 3. Pile-up observed in (a) HA, (b) HA-A, and (c) HA-A16C coatings during nanoindenation at a load of 1500 MN.
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HA
1
Z(nm)
Y(Mm)
0 −100 −200 −300
10 4
2
0 −2 X(Mm)
Coefficent of Friction
HA-A
−6 −4 −2 0 2 4 6 8
0.8
HA
0.6 0.4
Figure 5. Variation of coefficient of friction for HA, HA-A, and HA-A16C coatings.
0.2
−4
0 0
Z(Mm)
Y(Mm)
10 8 6 4 2 0 −2 −4 −6 −8
0.5 0.0 −0.5 0 2 X(Mm)
4
6
b
Z(Mm)
Y(Mm)
10 8 5 3 0 −3 −5 −8
0.3 0 −0.3 −0.5
−10 −6 −4 −2
0 2 X(Mm)
4
HA-A
HA-A16C
a
−6 −4 −2
HA-A16C
6
c Figure 4. Scratched surface topography of (a) HA, (b) HA-A, and (c) HA-A16C coatings. Note the enhanced surface roughness of HA-A and HA-A16C coatings.
2
4 6 8 Lateral Displacement (Mm)
toughness of HA-A coating by 39.7%, 38.2%, and 158%, respectively, when compared to that of HA alone. Thereby, wear resistance has improved by 13.5 times determined via nano-scratching. Hardness increase of 65.5%, elastic modulus increase of 50.4%, and fracture toughness increase of more than three times is observed with the CNT and sub-micrometer Al2O3 reinforcement in comparison to that of HA coating. Consequently, incorporation of CNTs rendered decreased penetration depth, enhanced plastic pile-up and reduced coefficient of friction. Wear resistance of HA-A16C coating is improved by 68.3 times in comparison to that of HA coating. Superior mechanical properties of HA-A16C coatings without deterring biocompatibility9 portray them as potential body-implant material. ACKNOWLEDGEMENTS KB acknowledges INIMETIITK20080236 (IITK), and BT/PR11224/ MED/32/57/2008 (DBT). AA acknowl-
Table III. Tribological Properties of HA-Al2O3-CNT Coatings
Coatings
Surface Roughness (Ra, nm)
Coefficient of Friction
Maximum Scratch Depth (nm)
Wear Volume (10–21 m3)
Wear Resistance Improvement
HA HA-A HA-A16C
5 nm 26 nm 35 nm
0.12 ± 0.01 0.18 ± 0.11 0.13 ± 0.02
644.2 101.9 67.3
177.7 ± 1.1 13.1 ± 0.2 2.6 ± 0.02
1 13.5 times 68.3 times
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10
edges support from the National Science Foundation CAREER Award (NSF-DMI-0547178) and the Office of Naval Research (N00014-08-10494). References 1. K. Balani et al., Biomaterials, 28 (2007), pp. 618– 624. 2. B. Viswanath and N. Ravishankar, Scripta Mater., 55 (2006), pp. 863–866. 3. K. Balani et al., Acta Biomater., 3 (2007), pp. 944– 951. 4. I. Mobasherpour et al., Ceramics International (2008), doi:10.1016/j.ceramint.2008.08.017. 5. Z. Evis and R.H. Doremus, Mater. Res. Bull., 43 (2008), pp. 2643–2651. 6. S. Nath, K. Biswas, and B. Basu, Scripta Mater., 58 (2008), pp. 1054–1057. 7. Z. Xihua et al., Ceramics International (2009), doi:10.1016/j.ceramint.2008.10.027. 8. E. Champion, S. Gautier, and D.B. Assollant, J. Mater. Sci. Mater. Med., 7 (1996), p. 125. 9. J. Tercero et al., Mat. Sci. Eng. C (2009), doi:10.1016/ j.msec.2009.05.001. 10. K. Balani et al., J. Nanosci. Nanotech., 7 (2007), pp. 3553–3562. 11. K. Balani et al., Acta Materialia, 56 (2008), pp. 5984–5994. 12. K. Balani and A. Agarwal, Surf. Coat. Tech., 202 (2008), pp. 4270–4277. 13. S.R. Bakshi et al., JOM, 59 (7) (2007), pp. 50–53. Kantesh Balani, assistant professor, is with the Department of Materials and Metallurgical Engineering, Indian Institute of Technology Kanpur, Kanpur-208 016, India; +91-512-259-6194; fax +91512-259-7505; email:
[email protected]; Debrupa Lahiri, graduate student, Anup K. Keshri, graduate student, S.R. Bakshi, post-doctoral researcher, and Arvind Agarwal, associate professor, are with the Department of Mechanical and Materials Engineering, Florida International University, Miami, Florida; and Jorge E. Tercero is with Separation Technologies, LLC, Coral Gables, Florida.
JOM • September 2009
