Research Papers

Laser Induced Porosity and Crystallinity Modification of a Bioactive Glass Coating on Titanium Substrates

[+] Author and Article Information
Panjawat Kongsuwan

Department of Mechanical Engineering,
Columbia University,
New York, NY 10027
e-mail: pk2261@columbia.edu

Grant Brandal

Department of Mechanical Engineering,
Columbia University,
New York, NY 10027
e-mail: gbb2114@columbia.edu

Y. Lawrence Yao

Advanced Manufacturing Laboratory,
Department of Mechanical Engineering,
Columbia University,
New York, NY 10027
e-mail: yly1@columbia.edu

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received April 22, 2014; final manuscript received December 21, 2014; published online February 16, 2015. Assoc. Editor: Y.B. Guo.

J. Manuf. Sci. Eng 137(3), 031004 (Jun 01, 2015) (12 pages) Paper No: MANU-14-1245; doi: 10.1115/1.4029566 History: Received April 22, 2014; Revised December 21, 2014; Online February 16, 2015

Functionally graded bioactive glass coatings on bioinert metallic substrates were produced by using continuous-wave (CW) laser irradiation. The aim is to achieve strong adhesion on the substrates and high bioactivity on the top surface of a coating material for load-bearing implants in biomedical applications. The morphology and microstructure of the bioactive glass from the laser coating process were investigated as functions of processing parameters. Laser sintering mechanisms were discussed with respect to the resulting morphology and microstructure. It has been shown that double layer laser coating results in a dense bond coat layer and a porous top coat layer with lower degree of crystallinity than an enameling coating sample. The dense bond coat strongly attached to the titanium substrate with a 10 μm wide mixed interfacial layer. A highly bioactive porous structure of the top coat layer is beneficial for early formation of a bone-bonding hydroxycarbonate apatite (HCA) layer. The numerical model developed in this work also allows for prediction of porosity and crystallinity in top coat layers of bioactive glass developed through laser induced sintering and crystallization.

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Fig. 1

CaO·SiO2–Na2O·SiO2 pseudobinary phase diagram of the sodium/calcium silicate system [32]

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Fig. 2

Flow chart of the numerical model. It captures the combined effects of laser heating, densification, pore coalescence, and crystallization to predict degree of porosity and crystallinity.

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Fig. 3

Schematic representation of 45S5 Bioglass laser coating on a titanium substrate. Laser sintering of (a) the dense bond coat layer and (b) the porous top coat layer. The bond coat layer is necessary to achieve strong adhesion on the substrate. The top coat layer with a highly bioactive porous structure is beneficial for early formation of a bone-bonding HCA layer.

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Fig. 4

(a) Top-view optical micrograph and (b) cross-sectional SEM image of deposited glass powder layer by sedimentation; (c) image analysis to characterize porosity. The average porosity of deposited glass powder layer is 39.2%.

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Fig. 5

(a) Top-view optical micrograph and (b) cross-sectional SEM image of the bond coat layer; (c) expanded view at the interface between Ti-substrate and the bond coat. Region at the center has a dense structure.

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Fig. 6

(a) Cross-sectional SEM image of the interface region in Fig. 5(c) denoting a scanning area for (b) EDX composition map and a scanning line for (c) EDX composition profiles across the interface. It indicates titanium diffusion into the glass with a 10 μm wide mixed interfacial layer.

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Fig. 7

Cross-sectional SEM images of (a) direct laser sintering (from the denoted rectangular area in Fig. 5(b)) of a porous glass layer onto a Ti-substrate; (b) a second layer of glass powder deposited on the dense bond coat before another laser irradiation; (c) morphology of resulting dense bond and porous top coat layers by laser coating; (d) the fluorescence micrograph of pores opened to the top surface. Figure 7(a) shows weak attachment of porous glass coating directly on a titanium substrate as compared to the double layer coating with a dense structure having strong adhesion on the substrate and with a highly bioactive porous structure on the top surface in Fig. 7(c).

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Fig. 8

Cross-sectional SEM images of (a) double layer laser coating; (b) expanded view at the porous top coat layer; and (c) the pre-existing interface between two layers. Swelling of the porous top coat layer results from pore coalescence dominating over viscous flow densification. There is no gap between the bond and top coat layers.

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Fig. 9

(a) Optical micrograph of crystallized 45S5 bulk glass heat-treated in the furnace; (b) top-view optical micrograph (expanded view from the denoted rectangular area in Fig. 5(a)); and (c) top-view SEM image of the partially crystallized dense area by laser sintering; (d) EDX composition profiles show that crystals are calcium-rich compared to the amorphous glass

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Fig. 10

XRD spectra of (a) the enameling process sample fired at 750 °C for 15 min in the furnace, (b) laser sintering of the porous top coat layer at 12 W and 2.0 mm/s, and (c) amorphous 45S5 bioactive glass powder deposited by sedimentation. Laser sintering causes less crystallinity than enameling, which is beneficial for bioactivity of implants.

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Fig. 11

(a) Porosity and (b) crystallinity of laser sintering samples versus laser scanning speed. Error bars denote standard deviation. Faster scanning speeds correspond to the formation of higher porosity and lower crystallinity.

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Fig. 12

(a) Porosity and (b) crystallinity of laser sintering samples versus laser power. Error bars denote standard deviation. Crystallinity is more-or-less constant with laser powers.

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Fig. 13

Representative revolution (225 deg) of 2D temperature distribution of a coupled laser-source heat transfer, sintering potential, pore growth, and crystallization model (laser power of 11 W, at 0.35 s of laser irradiation)

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Fig. 14

2D cross-sectional images for (a) crystallinity, (b) sintering potential, and (c) porosity; (d) the morphology of sintered glass in comparison to the numerical porosity (at same laser power of 11 W, numerical irradiation time of 0.35 s, and experimental scanning speed of 2 mm/s)

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Fig. 15

Numerical (dashed lines) and experimental (solid lines) percentages of porosity and crystallinity at different laser powers (at 0.35 s of numerical laser irradiation time and at 2 mm/s of experimental scanning speed). Numerical models predict an increasing trend of porosity and more-or-less constant trend of crystallinity with the laser powers.




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