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Effects of Nd:YAG laser treatment on the wettability characteristics of a zirconia-based bioceramic
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Optics and Lasers in Engineering 44 (2006) 803–814
Effects of Nd:YAG laser treatment on the wettability characteristics of a zirconia-based bioceramic L. Haoa,, J. Lawrenceb a
Rapid Manufacturing Research Group, Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Leicestershire LE11 3TU, UK b Manufacturing Engineering Division, School of Mechanical & Production Engineering, Nanyang Technological University (NTU), Nanyang Avenue, Singapore 639798, Singapore Received 18 February 2005; accepted 21 August 2005 Available online 17 October 2005
Abstract By enhancing the wettability characteristics of a zirconia-based bioceramic, magnesia partially stabilised zirconia (MgO–PSZ) using Nd:YAG laser irradiation, beneficial changes in the way biological fluids interact with the material will be achieved. This will consequently improve the bone–implant interface. Contact angle measurements revealed that the Nd:YAG laser-treated MgO–PSZ exhibited a considerable reduction in contact angle, y, implying that laser treatment brought about improved wettability characteristics of this material. The changes in surface properties generated by the laser irradiation and their effects on the wettability characteristics of the MgO–PSZ were analysed. Notably, the complete melting and solidified different microstructure following laser treatment gave rise to the maximum wettability characteristics. It was found that although the increase in surface roughness is the factor influencing the wettability characteristics, it only plays a minor role. Both the enhancement in surface oxygen content and the increase in polar component of surface energy, gpsv , were seen to be influential factors in determining the wettability characteristics of the
Corresponding author. Tel.: +44 (0) 1509 227565; fax: +44 (0) 1509 227549.
E-mail address: [email protected] (L. Hao). 0143-8166/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2005.08.001
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MgO–PSZ. Moreover, the increase in gpsv was found to be the chief mechanism governing the change in wettability characteristics of the MgO–PSZ. r 2005 Elsevier Ltd. All rights reserved. Keywords: Nd:YAG laser; Zirconia; Wettability characteristics; Contact angle; Surface energy
1. Introduction Wettability, which controls the way biological fluids interact with materials, is among the physicochemical characteristics that have been altered with the aim of improving the bone–implant interface. Different approaches are being used in an effort to obtain the desired bone–implant interface. The implant should present a surface conductive to or that will induce osseointegration [1]. The bone cells such as osteoblasts are anchorage cells and have been shown to adhere better on substrate with higher wettability characteristics. The cells attached to carboxylicylic-acidterminated hydrophilic monolayers were about two times more than those attached to methyl-terminated hydrophobic monolayer over 90 min [2]. Radio-frequency glow discharge has been used to increase surface energy and to enhance cell binding [3,4]. Hence, the improved wettability of the zirconia-based bioceramic, which is widely used as orthopaedic implant but is inert, and lack of osseointegration, will be of importance in enhancing its performance. Indeed, Hao et al. have found that the better protein [5,6], human fibroblast [7] and osteoblast [8] cell response to the magnesia partially stabilised zirconia (MgO–PSZ) with higher wettability characteristics. At present, very little work has been published with regard to the use of lasers for altering the surface properties of ceramics in order to improve their wettability characteristics. Notwithstanding this, Kappel [9] showed that the texturing of ceramics (with an excimer laser 248 nm) can improve the adhesion strength by up to 20%. Such an improvement is said to be due to the formation of raised microscopic protrusions over the surface. High-power diode laser (HPDL) treatment of the surfaces of a ceramic tile (SiO2/Al2O3-based ceramic), a clay quarry tile (SiO2/Al2O3/Fe2O3-based ceramic) and Al2O3 and SiO2–TiO2 (crystalline) [10–12] resulted in an improvement in the wettability characteristics of the materials. Recently, Hao and Lawrence [13] improved the wettability characteristics of MgO–PSZ using CO2 laser irradiation and identified the predominant mechanisms governed this enhancement [14]. Yet, no work has been conducted hitherto on the use of Nd:YAG laser process for the wettability modification of bioceramics. This paper describes the utility of a Nd:YAG laser to alter the wettability characteristics of the MgO–PSZ. The contact angle measurement using a set of liquids was used to evaluate the wettability characteristics of the MgO–PSZ before and after Nd:YAG laser treatment. The changes in surface properties generated by the laser irradiation and their effects on the wettability characteristics of the MgO–PSZ were analysed.
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2. Experimental procedures 2.1. Material and laser processing A sheet of 4% MgO–PSZ with dimensions of 50 50 1.5 mm3 (Goodfellow, Ltd.) was used in this work. It was used as-received prior to laser treatment. The defocused beam of a 400 W Nd:YAG was fired back and forth across the surfaces of the MgO–PSZ by traversing the samples beneath the laser beam using the x- and yaxis of the computerised numerical control (CNC) gantry table. The laser energy density (fluence) of the laser beam incident on the surface of the MgO–PSZ could be varied through manipulation of laser power densities and traverse speeds. It was important that the laser energy density had to be within the operating window of the MgO–PSZ for the surface treatment to be effective. A series of experiments were conducted for a wide range of power densities at 0.3 ms pulse width with a 6 mm spot diameter, whilst the traverse speed was set at 10 mm/min with 2 bar pressure oxygen process gas. 2.2. Surface characterisation The surfaces of untreated and Nd:YAG laser-treated MgO–PSZ were characterised by optical microscopy, X-ray photoemission spectroscopy (XPS) and surface tester SV-600. In addition, for analysis in cross-section, the samples were cold mounted in resin before grinding on increasing finer SiC grinding papers followed by a 1 mm finish using diamond paste. Then the polished samples were etched with 10 ml HNO3 and 20 ml HF (48%) for 20 min to affect the ceramic layers. 2.3. Wettability analysis procedure To investigate the effects of laser irradiation on wetting and surface energy characteristics of the MgO–PSZ, a set of sessile drop control experiments were carried out using glycerol, formamide, etheneglycol, polyglycol E-200 and polyglycol 15-200, with known total surface energy (glv ), dispersive (gdlv ) and polar (gplv ) component values. The contact angles, y, of the test liquids on the untreated and CO2 laser-treated MgO–PSZ were determined in atmospheric condition at 25 1C using a sessile drop measure machine (First Ten A˚ngstroms, Inc). In order to estimate the influence of contaminant layers on the measurement results, the specimens of untreated MgO–PSZ were cleaned with acetone in an ultrasonic bath for 2 h, rinsed with distilled water several times and dried in a vacuum oven at 90 1C for 12 h. The test liquids were used to measure y for the cleaned sample. It was observed that the values of y on the cleaned sample are 1.51, 1.21, 1.01, 0.91 and 0.81 for glycerol, formamide, etheneglycol, polyglycol E-200 and polyglycol 15-200, respectively, lower than that of the received sample without the cleaning. It is deemed that the contaminant on the surface of the MgO–PSZ has only a slight influence on the value of y. Since the contaminant is a minor factor active in the wettability characterisation, it is reasonable to preclude of cleaning pre-treatment for
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practical application of CO2 laser treatment. In order to explore the potential of CO2 laser treatment as an industrial and economical processing for altering the wettability of MgO–PSZ, the current research is conducted in an atmospheric environment without pre-cleaning. Each measurement of y lasted for 3 min, with profile photographs of the sessile drop obtained every minute and a mean value being subsequently determined. After the test liquid drops for each liquid attached and rested on the MgO–PSZ surface, the drops consistently reached an equilibrium state in around 6 s. Thereafter, they remained motionless and the magnitude of the y changed little with time.
3. Results and discussion 3.1. Morphological observation and surface characterisation With a Nd:YAG laser power lower than 150 W, no visible effects were observed on the MgO–PSZ. In contrast, obvious changes occurred on the material’s surface at power of 200 and 250 W. The typical optical images of the untreated and lasertreated MgO–PSZ are given in Fig. 1. As one can see, there are obvious grooves, which occurred in the manufacturing processing, on the untreated sample as shown in Fig. 1(a). On the other hand, the Nd:YAG laser treatment with 200 W power was most likely to generate partial melting which erased some grooves. In addition, some microcracks appeared on the surface, as shown in Fig. 1(b). The formation of these microcracks is mainly due to laser-induced thermal stress. Since the ceramic has a very high melting point and very low thermal conductivity, a large thermal gradient between the melted spot and substrate exists. This in turn will produce thermal stresses. Since all the grooves disappeared and some cracks on the surface were obvious to be filled and levelled-up, as shown in Fig. 1(c), it is believed that the MgO–PSZ surface were totally melted in the laser processing with 250 W power. It is observed that the filled crack track originated at the grain boundaries of MgO–PSZ, and then the microcracks interconnect with each other and propagate along the boundaries (see Fig. 1(c)). The crack observation by Quyang et al. [15] showed similar phenomena in the characterisation of laser clad yttria partially stabilised ZrO2 ceramic layers on steel 16 MnCr5. Furthermore, a very fine microstructure, which is deemed to be the cellular microstructure, exhibits on the MgO–PSZ (see Fig. 1(c)) and ought to result by the laser-induced rapid solidification. A high heat input from a laser beam facilitates surface-localised melting at a very high efficiency. That is, the major portion of the absorbed energy is used for melting, with only a small fraction going into heating of the solid sub-surface material. This ability to maintain a cold substrate whilst melting a thin surface layer of material results in rapid quenching of the molten layer once the beam is removed. Thermal gradients at the liquid–solid interface layer are very steep. In this case, melt solidification is almost a self-quenching process which allows rapid growth rate. Indeed, the cellular microstructure was also found on the CO2 laser-treated MgO–PSZ [16]. Indeed, the solidified microstructures generated by laser irradiation have been reported by a
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Fig. 1. Typical optical image of the (a) untreated and Nd:YAG-laser-treated sample, (b) at 200 W, and (c) 250 W power.
number of workers conducting research into the laser treatment of various ceramics. Pei et al. [17] noted that both equiaxed and dendritic microstructures were obtained on the laser clad ZrO2 layer. Hao and Jonathan found a difference microstructure on the MgO–PSZ generated at the different CO2 power densities [16]. The crosssectional view of the Nd:YAG-modified MgO–PSZ with 250 W power is given in Fig. 2. As one can see, there is one apparent uppermost layer where no voids, while the subtracted under this layer presented with the grain boundary and many voids. Because the melted material in the laser treatment should fill voids on the uppermost layer, this top layer is considered as the melted layer. As one can see from Fig. 3, the Nd:YAG laser treatment brought about a consistently higher surface roughness on the MgO–PSZ compared with the untreated sample. Nevertheless, Ra does not increase linearly with the power of Nd:YAG laser treatment. The sample treated with 200 W power displayed the highest Ra due to the partial melting and the presence of a few microcracks on the surface. When the power of Nd:YAG laser treatment increased to 250 W, a lower Ra appeared on the surface due to the sample experiencing a complete melting and the subsequently filling of the microcracks; precluding their formation. Furthermore, the Nd:YAG laser treatment resulted in the higher surface oxygen content as shown in Fig. 4. This
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Fig. 2. Typical SEM cross-section view of the MgO–PSZ surface layer following Nd:YAG treatment with 250 W power.
Surface roughness, Ra (µm)
2.0
1.6
1.2
0.8
0.4
0.0 untreated
200 W Nd:YAG Laser Power
250 W
Fig. 3. Relationship between Ra and Nd:YAG laser power density.
is believed to be due to the oxidisation of the MgO–PSZ surface during melting. The surface oxygen content increases as the power of laser treatment increases. 3.2. Wettability characteristics and surface energy analysis As one can see from Table 1, with all the control liquids used, the MgO–PSZ experienced a considerable reduction in y as a result of interaction with the Nd:YAG
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Surface oxygen content, at%
60
40
20
0 untreated
200 W
250 W
Nd:YAG Laser Power Fig. 4. Relationship between surface oxygen content and Nd:YAG laser power density.
Table 1 Mean values of contact angles formed between the selected test liquids and untreated and Nd:YAG lasertreated sample Liquid
Contact angle y (deg) Untreated
Glycerol Formamide Etheneglycol Polyglycol E-200 Polyglycol 15-200
79 73 61 53 35
Nd:YAG laser 200 W
250 W
64 58 49 42 30
55 49 43 37 24
laser beam, indicating higher wettability characteristics after laser treatment. Further, y decreased as the power of the Nd:YAG laser increased. The intermolecular attraction which is responsible for surface energy, g, results from a variety of intermolecular forces whose contribution to the total surface energy is additive [18]. A majority of these forces are functions of the particular chemical nature of a certain material and, as such, the total surface energy comprises gp (polar or non-dispersive interaction) and gd (dispersive component; since van der Waals forces are present in all systems regardless of their chemical nature). Consequently, the surface energy of any system can be described by [19] g ¼ gd þ gp .
(1)
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Therefore, the contact angle, y, for solid–liquid systems, where both dispersion forces and polar forces are present, can be related to the surface energies of the respective liquid and solid by [19] cos y ¼
2ðgdsv gdlv Þ1=2 þ 2ðgpsv gplv Þ1=2 1, glv
(2)
where gplv and gdlv are the dispersive component and polar component of liquid surface energy, glv , respectively, and, gpsv and gdsv are the dispersive component and polar component of solid surface energy, gsv , respectively. It is possible to adequately . estimate gdsv , for the MgO–PSZ, by plotting the graph of cos y against ðgdlv Þ1=2 glv according to Eq. (2), as shown in Fig. 5. Thus, according to Fowkes [19], the value of gdsv is estimated by the gradient (¼ 2ðgdsv Þ1=2 ) of the line which connects the origin . (cos y ¼ 1) with the intercept point of the straight line (cos y against ðgdlv Þ1=2 glv ) correlating the data point with the abscissa at cos y ¼ 1. Comparing the ordinate intercept points of the untreated and Nd:YAG laser-treated MgO–PSZ liquid systems in Fig. 5, it can been seen clearly that for the untreated MgO–PSZ, the bestfit straight line intercepts the ordinate closer to the origin. This is noteworthy since intercept of the ordinate close to the origin is characteristic of the dominance of dispersion forces acting on the MgO–PSZ material–liquid interfaces of the untreated sample, resulting in poor adhesion [19,20]. On the other hand, the best-fit straight line of samples treated by the laser intercept the ordinate considerably high above the origin. An interception of the ordinate above the origin is indicative of the action of polar forces across the interface, in addition to dispersion forces; hence, improved wettability and adhesion is promoted [19,20]. Furthermore, because none of the bestfit straight lines intercept below the origin, it can be said that the development of an equilibrium film pressure of adsorbed vapour on the MgO–PSZ surface (untreated 1.0 0.8 0.6 0.4 Cos
0.2 0.0 -0.2 -0.4 -0.6 -0.8
Untreated Laser treated 200 W Laser treated 250 W
-1.0 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 (lvd )1/2 / lv . Fig. 5. Plot of cos y against ðgdlv Þ1=2 glv for the untreated and Nd:YAG laser treated MgO–PSZ in contact with the wetting test control liquids.
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and Nd:YAG laser-treated) did not occur [20]. It is not possible to determine the value of gpsv for the MgO–PSZ directly for the plot. Nonetheless, the following can be derived: ðgpsv Þ1=2 ¼
ðgdsv Þ1=2 ða 1Þ . c
(3)
a can be determined from the best-fit line of Wad (work adhesion) against W dad (dispersive component of work adhesion) of the MgO–PSZ, while c can be deduced from the best-fit line of gplv and gdlv of the test liquids. For the MgO–PSZ the value of a is 2.28 (untreated), 2.37 (200 W) and 2.71 (250 W), as c is 2.9 for the set of the test liquids defined previously [7]. It is, therefore, possible to calculate gpsv for the MgO–PSZ using Eq. (3). The determined results of surface energies of the untreated and Nd:YAG laser-treated MgO–PSZ (at various power densities) are given in Fig. 6. As is evident from Fig. 6, Nd:YAG laser treatment increased gsv of the MgO–PSZ by primarily increasing gpsv , since gdsv was similar for all the samples. It is important to note that because of the long range ionic interactions between the MgO–PSZ and the test liquids, it is highly likely that the thermodynamically defined total solid surface energy will be higher than the sum of the gdsv and gpsv components of the surface energy. Indeed, the derivation that leads to Eq. (1) can only be done under the specific assumption that the ionisation potentials are all equal and that dipole–dipole random orientation interactions dominate over dipole-induced–dipole random interactions. Although the increase in (excess) surface free energy will probably be less than the increase in the total lattice energy, on the other hand, an absorbed liquid layer may shield the ionic fields substantially. As such, all the data derived from Eqs. (2)–(3) should be considered as semi-empirical. Notwithstanding
100
Surface energy (mJ/cm2)
90 80
Dispersive component Polar component Total surface energy
70 60 50 40 30 20 10 0 Untreated 200 W 250 W Nd:YAG laser power density
Fig. 6. Relationship between surface energy (gdsv , gpsv and gsv ) of the MgO–PSZ and Nd:YAG laser power.
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this, as the studies by many researchers [11–13,21] found, it is reasonable to conclude from the data obtained from Eqs. (2)–(3) that Nd:YAG laser treatment of the MgO–PSZ surface has caused an increase in gpsv . 3.3. Mechanisms influencing the wettability characteristics A model similar to that for heterogeneous solid surfaces can be developed in order to account for surface irregularities, being given by Wenzel’s equation [22] ra ðgsv gsl Þ ¼ glv cos yw ,
(4)
where ra is the roughness factor defined as the ratio of the real and apparent surface areas and yw is the contact angle for the wetting of a rough surface. It is important to note that Wenzel’s treatment is only effective at the position of the wetting triple line [22]. Eq. (4) shows that if ra is big, that is the solid surface is rough, then cos yw is large, and yw decrease when yw o 901. In this study, the rougher surface generated by the Nd:YAG laser treatment had the lower y than the rough untreated surface. It agrees with Eq. (4) and a previous work [23], that an increase in surface roughness effected a decrease in y and enhanced the wettability. But, the rougher surface generated by laser treatment with 200 W power has a higher y than the sample with a relatively smooth surface brought about by laser treatment with 250 W power. It is postulated that the complete surface melting in the MgO–PSZ surface generated by this power contributes to the decrease in y. In addition, it must be noted that the experimental conditions in place in this study and in the previous work by others are very different. Whereas in the previous work only the surface roughness was altered, in this work the CO2 laser treatments brought about changes in surface energy, surface oxygen and surface roughness, simultaneously. Hence, it implies that the surface roughness might play a minor role in the wettability characteristics of MgO–PSZ. It is well-known that the increase in surface oxygen content results in a higher likelihood of wetting [24–28]. Hence, the observed increase in the wetting performance of the MgO–PSZ would have certainly been influenced by an increase in the surface oxygen content as a result of the laser treatment. In this study, the surface oxygen content increases with the wettability characteristics as the power of laser treatment increases. This suggests that oxygen enrichment in the Nd:YAG laser-treated MgO–PSZ is active in promoting wetting and adhesion. A similar finding was observed by Song et al. [26,28] that the surface oxygen content increased after laser treatment and, in turn, effected a reduction in the y. It is observed from Fig. 7 that there is a clear relationship between the value of cos y and the surface energy, which reveals that an increase in the surface energy brings about a rise in the value cos y and higher wettability. Hence, it is deemed that surface energy is a major factor active in changing the wettability characteristics of the MgO–PSZ. Indeed, it was found by Lawrence [29] that surface energy was the most predominant factor governing the wetting characteristics of the SiO2/Al2O3based ceramic following the irradiation of high-power diode laser. What is more, Hao and Lawrence recently found that the change in surface energy represented the change in the microstructure feature [30] was identified as the main mechanism
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1.0
Wettability (cos , glycerol)
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 40
45
50
55
60
65
70
75
80
85
90
Surface energy, γsv Fig. 7. Relationship between wettability characteristics (cos y, glycerol) and surface energy of the MgO–PSZ.
governing the wettability characteristics of the MgO–PSZ following CO2 laser irradiation [14]. Since a significant increase in the total surface energy was governed by the marked enhancement in gpsv , while gdsv only changed slightly after laser treatment, as shown in Fig. 6, the increase in gpsv , in particular, has a positive effect upon the action of wetting and adhesion [31]. The changes in gsv are thought to be due to the fact that Nd:YAG laser treatment of the MgO-PSZ caused the surface melting of the surface; a transition that is known to effect an increase in gpsv [32] and, thus, an improvement in the wettability and an increase in the adhesion at the interface in contact with the control liquids. Similarly, Lawrence [29] observed a sharp reduction in y at the point of melting for an Al2O3/SiO2-based oxide compound after high diode power laser (HPDL) treatment. Especially, the surface energy of the MgO–PSZ with the laser-solidified microstructure is 75.9 mJ/cm2 and is much higher than the untreated sample surface energy, 52.8 mJ/cm2. This solidified microstructure generated by laser surface melting might be attributed to the changes in surface energy and, thereof, in the wettability characteristics. Indeed, work conducted by Zhang et al. [33] found that considerable improvement in the bond strength of a Si3N4 ceramic could be realised only when excimer laser treatment of a structural alloy steel (SAE 4340) resulted in surface melting.
4. Conclusions Owning to the interaction with Nd:YAG laser beam, the MgO–PSZ displaced a considerable reduction in y with all the control liquids used, implying that laser treatment effects higher wettability characteristics. Notably, the sample exhibiting the complete melting and solidified microstructure following laser treatment
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displayed the maximum wettability characteristics. It was found that although the increase in Ra is the factor influencing the wettability, it plays only a minor role. Both the enhancement of surface oxygen content and surface energy were observed to be influential in promoting the enhanced wettability characteristics. The surface energy of the MgO–PSZ with the laser-solidified different microstructure is 75.9 mJ/cm2 and is much higher than the untreated sample surface energy, 52.8 mJ/cm2. This solidified microstructure generated by laser surface melting might be attributed to the changes in surface energy and, thereof, in wettability characteristics. Particularly, the increase in gpsv , which is caused by the surface melting and resolidification into a different microstructure following laser treatment, has a clear relationship with the wettability characteristics and is thought to be the chief mechanism governing the change in wettability characteristics of the MgO–PSZ. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
Puleo DA, Nanci A. Biomaterials 1999;20:2311–21. Scotchford CA, Cooper E, Leggett GJ, Downes S. J Biomed Mater Res 1998;41:431–42. Michaels CM, Keller JC, Stanford CM. J Oral Implantol 1991;17:132–9. Kawahara D, Ong JL, Raikar GN, Lucas LC, Lemons JE, Nakamura M. Int J Oral Maxillofacial Implant 1996;11:435–42. Hao L, Lawrence J. Colloid Surf B: Biointerface 2004;34:87–94. Hao L, Lawrence J. J Biomed Mater Res 2004;69:748–56. Hao L. Lawrence J Mater Sci Eng: C 2003;23:627–39. Hao L, Lawrence J, Chian KS. J Biomater Appl 2004;19:81–105. Kapplel. Excimers help ceramic stick together. Opt Laser Eur 1998;34. Lawrence J, Li L, Spencer JT. Surf Coat Technol 1999;115:273–81. Lawrence J, Li L. J Phys D 1999;32:1075–82. Lawrence J, Li L, Spencer JT. Appl Surf Sci 1999;138–139:388–93. Hao L, Lawrence J. J Phys D 2003;36:1292–9. Hao L, Lawrence J. J Laser Appl 2004;16:252–7. Ouyang JH, Nowotny S, Richter A, Beyer E. Surf Coat Technol 2001;137:12–20. Hao L, Lawrence J. Proceedings of the IMechE part B. J Eng Manuf 2004;218:59–76. Pei YT, Ouyang JH, Lei TC. Surf Coat Technol 1996;81:131–5. Adam NK, Elliott GEP. J Chem Soc 1958;18:2206–15. Fowkes FM. Ind Eng Chem 1964;56:40–52. Dann JR. J Colloid Interface Sci 1970;32:302–20. Agathopoulos S, Nikolopoulos P. J Biomed Mater Res 1995;29:421–9. Wenzel RN. Ind Eng Chem 1936;28:988–94. Uelzen T, Muller J. Thin Solid Film 2003;434:311–5. Ueki M, Naka M, Okamoto I. J Mater Sci Lett 1986;5:1261–2. Li J. Xiyou Jinshu/Rare Met 1993;12:84–96. Song Q, Netravali AN. J Adhes Sci Technol 1998;12:957–82. Lawrence J, Li L. Laser Modification of the Wettability Characteristics of Engineering materials. London: Professional Engineering; 2001. Song Q, Netravali AN. J Adhes Sci Technol 1998;9:983–7. Lawrence J. Proc R Soc London, Ser A 2002;458:2445–63. Hao L, Lawrence J. Mater Sci Eng A 2003;364:171–81. Neumann AW. Adv Colloid Interface Sci 1974;4:106–91. Chattoraj DK, Birdi KS. Adsorption and the Gibbs Surface Excess. New York: Plenum Press; 1984. Zhang XM, Yue TM, Man HC. Mater Lett 1997;30:327–32.
Optics and Lasers in Engineering 44 (2006) 803–814
Effects of Nd:YAG laser treatment on the wettability characteristics of a zirconia-based bioceramic L. Haoa,, J. Lawrenceb a
Rapid Manufacturing Research Group, Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Leicestershire LE11 3TU, UK b Manufacturing Engineering Division, School of Mechanical & Production Engineering, Nanyang Technological University (NTU), Nanyang Avenue, Singapore 639798, Singapore Received 18 February 2005; accepted 21 August 2005 Available online 17 October 2005
Abstract By enhancing the wettability characteristics of a zirconia-based bioceramic, magnesia partially stabilised zirconia (MgO–PSZ) using Nd:YAG laser irradiation, beneficial changes in the way biological fluids interact with the material will be achieved. This will consequently improve the bone–implant interface. Contact angle measurements revealed that the Nd:YAG laser-treated MgO–PSZ exhibited a considerable reduction in contact angle, y, implying that laser treatment brought about improved wettability characteristics of this material. The changes in surface properties generated by the laser irradiation and their effects on the wettability characteristics of the MgO–PSZ were analysed. Notably, the complete melting and solidified different microstructure following laser treatment gave rise to the maximum wettability characteristics. It was found that although the increase in surface roughness is the factor influencing the wettability characteristics, it only plays a minor role. Both the enhancement in surface oxygen content and the increase in polar component of surface energy, gpsv , were seen to be influential factors in determining the wettability characteristics of the
Corresponding author. Tel.: +44 (0) 1509 227565; fax: +44 (0) 1509 227549.
E-mail address: [email protected] (L. Hao). 0143-8166/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2005.08.001
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MgO–PSZ. Moreover, the increase in gpsv was found to be the chief mechanism governing the change in wettability characteristics of the MgO–PSZ. r 2005 Elsevier Ltd. All rights reserved. Keywords: Nd:YAG laser; Zirconia; Wettability characteristics; Contact angle; Surface energy
1. Introduction Wettability, which controls the way biological fluids interact with materials, is among the physicochemical characteristics that have been altered with the aim of improving the bone–implant interface. Different approaches are being used in an effort to obtain the desired bone–implant interface. The implant should present a surface conductive to or that will induce osseointegration [1]. The bone cells such as osteoblasts are anchorage cells and have been shown to adhere better on substrate with higher wettability characteristics. The cells attached to carboxylicylic-acidterminated hydrophilic monolayers were about two times more than those attached to methyl-terminated hydrophobic monolayer over 90 min [2]. Radio-frequency glow discharge has been used to increase surface energy and to enhance cell binding [3,4]. Hence, the improved wettability of the zirconia-based bioceramic, which is widely used as orthopaedic implant but is inert, and lack of osseointegration, will be of importance in enhancing its performance. Indeed, Hao et al. have found that the better protein [5,6], human fibroblast [7] and osteoblast [8] cell response to the magnesia partially stabilised zirconia (MgO–PSZ) with higher wettability characteristics. At present, very little work has been published with regard to the use of lasers for altering the surface properties of ceramics in order to improve their wettability characteristics. Notwithstanding this, Kappel [9] showed that the texturing of ceramics (with an excimer laser 248 nm) can improve the adhesion strength by up to 20%. Such an improvement is said to be due to the formation of raised microscopic protrusions over the surface. High-power diode laser (HPDL) treatment of the surfaces of a ceramic tile (SiO2/Al2O3-based ceramic), a clay quarry tile (SiO2/Al2O3/Fe2O3-based ceramic) and Al2O3 and SiO2–TiO2 (crystalline) [10–12] resulted in an improvement in the wettability characteristics of the materials. Recently, Hao and Lawrence [13] improved the wettability characteristics of MgO–PSZ using CO2 laser irradiation and identified the predominant mechanisms governed this enhancement [14]. Yet, no work has been conducted hitherto on the use of Nd:YAG laser process for the wettability modification of bioceramics. This paper describes the utility of a Nd:YAG laser to alter the wettability characteristics of the MgO–PSZ. The contact angle measurement using a set of liquids was used to evaluate the wettability characteristics of the MgO–PSZ before and after Nd:YAG laser treatment. The changes in surface properties generated by the laser irradiation and their effects on the wettability characteristics of the MgO–PSZ were analysed.
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2. Experimental procedures 2.1. Material and laser processing A sheet of 4% MgO–PSZ with dimensions of 50 50 1.5 mm3 (Goodfellow, Ltd.) was used in this work. It was used as-received prior to laser treatment. The defocused beam of a 400 W Nd:YAG was fired back and forth across the surfaces of the MgO–PSZ by traversing the samples beneath the laser beam using the x- and yaxis of the computerised numerical control (CNC) gantry table. The laser energy density (fluence) of the laser beam incident on the surface of the MgO–PSZ could be varied through manipulation of laser power densities and traverse speeds. It was important that the laser energy density had to be within the operating window of the MgO–PSZ for the surface treatment to be effective. A series of experiments were conducted for a wide range of power densities at 0.3 ms pulse width with a 6 mm spot diameter, whilst the traverse speed was set at 10 mm/min with 2 bar pressure oxygen process gas. 2.2. Surface characterisation The surfaces of untreated and Nd:YAG laser-treated MgO–PSZ were characterised by optical microscopy, X-ray photoemission spectroscopy (XPS) and surface tester SV-600. In addition, for analysis in cross-section, the samples were cold mounted in resin before grinding on increasing finer SiC grinding papers followed by a 1 mm finish using diamond paste. Then the polished samples were etched with 10 ml HNO3 and 20 ml HF (48%) for 20 min to affect the ceramic layers. 2.3. Wettability analysis procedure To investigate the effects of laser irradiation on wetting and surface energy characteristics of the MgO–PSZ, a set of sessile drop control experiments were carried out using glycerol, formamide, etheneglycol, polyglycol E-200 and polyglycol 15-200, with known total surface energy (glv ), dispersive (gdlv ) and polar (gplv ) component values. The contact angles, y, of the test liquids on the untreated and CO2 laser-treated MgO–PSZ were determined in atmospheric condition at 25 1C using a sessile drop measure machine (First Ten A˚ngstroms, Inc). In order to estimate the influence of contaminant layers on the measurement results, the specimens of untreated MgO–PSZ were cleaned with acetone in an ultrasonic bath for 2 h, rinsed with distilled water several times and dried in a vacuum oven at 90 1C for 12 h. The test liquids were used to measure y for the cleaned sample. It was observed that the values of y on the cleaned sample are 1.51, 1.21, 1.01, 0.91 and 0.81 for glycerol, formamide, etheneglycol, polyglycol E-200 and polyglycol 15-200, respectively, lower than that of the received sample without the cleaning. It is deemed that the contaminant on the surface of the MgO–PSZ has only a slight influence on the value of y. Since the contaminant is a minor factor active in the wettability characterisation, it is reasonable to preclude of cleaning pre-treatment for
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practical application of CO2 laser treatment. In order to explore the potential of CO2 laser treatment as an industrial and economical processing for altering the wettability of MgO–PSZ, the current research is conducted in an atmospheric environment without pre-cleaning. Each measurement of y lasted for 3 min, with profile photographs of the sessile drop obtained every minute and a mean value being subsequently determined. After the test liquid drops for each liquid attached and rested on the MgO–PSZ surface, the drops consistently reached an equilibrium state in around 6 s. Thereafter, they remained motionless and the magnitude of the y changed little with time.
3. Results and discussion 3.1. Morphological observation and surface characterisation With a Nd:YAG laser power lower than 150 W, no visible effects were observed on the MgO–PSZ. In contrast, obvious changes occurred on the material’s surface at power of 200 and 250 W. The typical optical images of the untreated and lasertreated MgO–PSZ are given in Fig. 1. As one can see, there are obvious grooves, which occurred in the manufacturing processing, on the untreated sample as shown in Fig. 1(a). On the other hand, the Nd:YAG laser treatment with 200 W power was most likely to generate partial melting which erased some grooves. In addition, some microcracks appeared on the surface, as shown in Fig. 1(b). The formation of these microcracks is mainly due to laser-induced thermal stress. Since the ceramic has a very high melting point and very low thermal conductivity, a large thermal gradient between the melted spot and substrate exists. This in turn will produce thermal stresses. Since all the grooves disappeared and some cracks on the surface were obvious to be filled and levelled-up, as shown in Fig. 1(c), it is believed that the MgO–PSZ surface were totally melted in the laser processing with 250 W power. It is observed that the filled crack track originated at the grain boundaries of MgO–PSZ, and then the microcracks interconnect with each other and propagate along the boundaries (see Fig. 1(c)). The crack observation by Quyang et al. [15] showed similar phenomena in the characterisation of laser clad yttria partially stabilised ZrO2 ceramic layers on steel 16 MnCr5. Furthermore, a very fine microstructure, which is deemed to be the cellular microstructure, exhibits on the MgO–PSZ (see Fig. 1(c)) and ought to result by the laser-induced rapid solidification. A high heat input from a laser beam facilitates surface-localised melting at a very high efficiency. That is, the major portion of the absorbed energy is used for melting, with only a small fraction going into heating of the solid sub-surface material. This ability to maintain a cold substrate whilst melting a thin surface layer of material results in rapid quenching of the molten layer once the beam is removed. Thermal gradients at the liquid–solid interface layer are very steep. In this case, melt solidification is almost a self-quenching process which allows rapid growth rate. Indeed, the cellular microstructure was also found on the CO2 laser-treated MgO–PSZ [16]. Indeed, the solidified microstructures generated by laser irradiation have been reported by a
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Fig. 1. Typical optical image of the (a) untreated and Nd:YAG-laser-treated sample, (b) at 200 W, and (c) 250 W power.
number of workers conducting research into the laser treatment of various ceramics. Pei et al. [17] noted that both equiaxed and dendritic microstructures were obtained on the laser clad ZrO2 layer. Hao and Jonathan found a difference microstructure on the MgO–PSZ generated at the different CO2 power densities [16]. The crosssectional view of the Nd:YAG-modified MgO–PSZ with 250 W power is given in Fig. 2. As one can see, there is one apparent uppermost layer where no voids, while the subtracted under this layer presented with the grain boundary and many voids. Because the melted material in the laser treatment should fill voids on the uppermost layer, this top layer is considered as the melted layer. As one can see from Fig. 3, the Nd:YAG laser treatment brought about a consistently higher surface roughness on the MgO–PSZ compared with the untreated sample. Nevertheless, Ra does not increase linearly with the power of Nd:YAG laser treatment. The sample treated with 200 W power displayed the highest Ra due to the partial melting and the presence of a few microcracks on the surface. When the power of Nd:YAG laser treatment increased to 250 W, a lower Ra appeared on the surface due to the sample experiencing a complete melting and the subsequently filling of the microcracks; precluding their formation. Furthermore, the Nd:YAG laser treatment resulted in the higher surface oxygen content as shown in Fig. 4. This
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Fig. 2. Typical SEM cross-section view of the MgO–PSZ surface layer following Nd:YAG treatment with 250 W power.
Surface roughness, Ra (µm)
2.0
1.6
1.2
0.8
0.4
0.0 untreated
200 W Nd:YAG Laser Power
250 W
Fig. 3. Relationship between Ra and Nd:YAG laser power density.
is believed to be due to the oxidisation of the MgO–PSZ surface during melting. The surface oxygen content increases as the power of laser treatment increases. 3.2. Wettability characteristics and surface energy analysis As one can see from Table 1, with all the control liquids used, the MgO–PSZ experienced a considerable reduction in y as a result of interaction with the Nd:YAG
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Surface oxygen content, at%
60
40
20
0 untreated
200 W
250 W
Nd:YAG Laser Power Fig. 4. Relationship between surface oxygen content and Nd:YAG laser power density.
Table 1 Mean values of contact angles formed between the selected test liquids and untreated and Nd:YAG lasertreated sample Liquid
Contact angle y (deg) Untreated
Glycerol Formamide Etheneglycol Polyglycol E-200 Polyglycol 15-200
79 73 61 53 35
Nd:YAG laser 200 W
250 W
64 58 49 42 30
55 49 43 37 24
laser beam, indicating higher wettability characteristics after laser treatment. Further, y decreased as the power of the Nd:YAG laser increased. The intermolecular attraction which is responsible for surface energy, g, results from a variety of intermolecular forces whose contribution to the total surface energy is additive [18]. A majority of these forces are functions of the particular chemical nature of a certain material and, as such, the total surface energy comprises gp (polar or non-dispersive interaction) and gd (dispersive component; since van der Waals forces are present in all systems regardless of their chemical nature). Consequently, the surface energy of any system can be described by [19] g ¼ gd þ gp .
(1)
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Therefore, the contact angle, y, for solid–liquid systems, where both dispersion forces and polar forces are present, can be related to the surface energies of the respective liquid and solid by [19] cos y ¼
2ðgdsv gdlv Þ1=2 þ 2ðgpsv gplv Þ1=2 1, glv
(2)
where gplv and gdlv are the dispersive component and polar component of liquid surface energy, glv , respectively, and, gpsv and gdsv are the dispersive component and polar component of solid surface energy, gsv , respectively. It is possible to adequately . estimate gdsv , for the MgO–PSZ, by plotting the graph of cos y against ðgdlv Þ1=2 glv according to Eq. (2), as shown in Fig. 5. Thus, according to Fowkes [19], the value of gdsv is estimated by the gradient (¼ 2ðgdsv Þ1=2 ) of the line which connects the origin . (cos y ¼ 1) with the intercept point of the straight line (cos y against ðgdlv Þ1=2 glv ) correlating the data point with the abscissa at cos y ¼ 1. Comparing the ordinate intercept points of the untreated and Nd:YAG laser-treated MgO–PSZ liquid systems in Fig. 5, it can been seen clearly that for the untreated MgO–PSZ, the bestfit straight line intercepts the ordinate closer to the origin. This is noteworthy since intercept of the ordinate close to the origin is characteristic of the dominance of dispersion forces acting on the MgO–PSZ material–liquid interfaces of the untreated sample, resulting in poor adhesion [19,20]. On the other hand, the best-fit straight line of samples treated by the laser intercept the ordinate considerably high above the origin. An interception of the ordinate above the origin is indicative of the action of polar forces across the interface, in addition to dispersion forces; hence, improved wettability and adhesion is promoted [19,20]. Furthermore, because none of the bestfit straight lines intercept below the origin, it can be said that the development of an equilibrium film pressure of adsorbed vapour on the MgO–PSZ surface (untreated 1.0 0.8 0.6 0.4 Cos
0.2 0.0 -0.2 -0.4 -0.6 -0.8
Untreated Laser treated 200 W Laser treated 250 W
-1.0 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 (lvd )1/2 / lv . Fig. 5. Plot of cos y against ðgdlv Þ1=2 glv for the untreated and Nd:YAG laser treated MgO–PSZ in contact with the wetting test control liquids.
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and Nd:YAG laser-treated) did not occur [20]. It is not possible to determine the value of gpsv for the MgO–PSZ directly for the plot. Nonetheless, the following can be derived: ðgpsv Þ1=2 ¼
ðgdsv Þ1=2 ða 1Þ . c
(3)
a can be determined from the best-fit line of Wad (work adhesion) against W dad (dispersive component of work adhesion) of the MgO–PSZ, while c can be deduced from the best-fit line of gplv and gdlv of the test liquids. For the MgO–PSZ the value of a is 2.28 (untreated), 2.37 (200 W) and 2.71 (250 W), as c is 2.9 for the set of the test liquids defined previously [7]. It is, therefore, possible to calculate gpsv for the MgO–PSZ using Eq. (3). The determined results of surface energies of the untreated and Nd:YAG laser-treated MgO–PSZ (at various power densities) are given in Fig. 6. As is evident from Fig. 6, Nd:YAG laser treatment increased gsv of the MgO–PSZ by primarily increasing gpsv , since gdsv was similar for all the samples. It is important to note that because of the long range ionic interactions between the MgO–PSZ and the test liquids, it is highly likely that the thermodynamically defined total solid surface energy will be higher than the sum of the gdsv and gpsv components of the surface energy. Indeed, the derivation that leads to Eq. (1) can only be done under the specific assumption that the ionisation potentials are all equal and that dipole–dipole random orientation interactions dominate over dipole-induced–dipole random interactions. Although the increase in (excess) surface free energy will probably be less than the increase in the total lattice energy, on the other hand, an absorbed liquid layer may shield the ionic fields substantially. As such, all the data derived from Eqs. (2)–(3) should be considered as semi-empirical. Notwithstanding
100
Surface energy (mJ/cm2)
90 80
Dispersive component Polar component Total surface energy
70 60 50 40 30 20 10 0 Untreated 200 W 250 W Nd:YAG laser power density
Fig. 6. Relationship between surface energy (gdsv , gpsv and gsv ) of the MgO–PSZ and Nd:YAG laser power.
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this, as the studies by many researchers [11–13,21] found, it is reasonable to conclude from the data obtained from Eqs. (2)–(3) that Nd:YAG laser treatment of the MgO–PSZ surface has caused an increase in gpsv . 3.3. Mechanisms influencing the wettability characteristics A model similar to that for heterogeneous solid surfaces can be developed in order to account for surface irregularities, being given by Wenzel’s equation [22] ra ðgsv gsl Þ ¼ glv cos yw ,
(4)
where ra is the roughness factor defined as the ratio of the real and apparent surface areas and yw is the contact angle for the wetting of a rough surface. It is important to note that Wenzel’s treatment is only effective at the position of the wetting triple line [22]. Eq. (4) shows that if ra is big, that is the solid surface is rough, then cos yw is large, and yw decrease when yw o 901. In this study, the rougher surface generated by the Nd:YAG laser treatment had the lower y than the rough untreated surface. It agrees with Eq. (4) and a previous work [23], that an increase in surface roughness effected a decrease in y and enhanced the wettability. But, the rougher surface generated by laser treatment with 200 W power has a higher y than the sample with a relatively smooth surface brought about by laser treatment with 250 W power. It is postulated that the complete surface melting in the MgO–PSZ surface generated by this power contributes to the decrease in y. In addition, it must be noted that the experimental conditions in place in this study and in the previous work by others are very different. Whereas in the previous work only the surface roughness was altered, in this work the CO2 laser treatments brought about changes in surface energy, surface oxygen and surface roughness, simultaneously. Hence, it implies that the surface roughness might play a minor role in the wettability characteristics of MgO–PSZ. It is well-known that the increase in surface oxygen content results in a higher likelihood of wetting [24–28]. Hence, the observed increase in the wetting performance of the MgO–PSZ would have certainly been influenced by an increase in the surface oxygen content as a result of the laser treatment. In this study, the surface oxygen content increases with the wettability characteristics as the power of laser treatment increases. This suggests that oxygen enrichment in the Nd:YAG laser-treated MgO–PSZ is active in promoting wetting and adhesion. A similar finding was observed by Song et al. [26,28] that the surface oxygen content increased after laser treatment and, in turn, effected a reduction in the y. It is observed from Fig. 7 that there is a clear relationship between the value of cos y and the surface energy, which reveals that an increase in the surface energy brings about a rise in the value cos y and higher wettability. Hence, it is deemed that surface energy is a major factor active in changing the wettability characteristics of the MgO–PSZ. Indeed, it was found by Lawrence [29] that surface energy was the most predominant factor governing the wetting characteristics of the SiO2/Al2O3based ceramic following the irradiation of high-power diode laser. What is more, Hao and Lawrence recently found that the change in surface energy represented the change in the microstructure feature [30] was identified as the main mechanism
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1.0
Wettability (cos , glycerol)
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 40
45
50
55
60
65
70
75
80
85
90
Surface energy, γsv Fig. 7. Relationship between wettability characteristics (cos y, glycerol) and surface energy of the MgO–PSZ.
governing the wettability characteristics of the MgO–PSZ following CO2 laser irradiation [14]. Since a significant increase in the total surface energy was governed by the marked enhancement in gpsv , while gdsv only changed slightly after laser treatment, as shown in Fig. 6, the increase in gpsv , in particular, has a positive effect upon the action of wetting and adhesion [31]. The changes in gsv are thought to be due to the fact that Nd:YAG laser treatment of the MgO-PSZ caused the surface melting of the surface; a transition that is known to effect an increase in gpsv [32] and, thus, an improvement in the wettability and an increase in the adhesion at the interface in contact with the control liquids. Similarly, Lawrence [29] observed a sharp reduction in y at the point of melting for an Al2O3/SiO2-based oxide compound after high diode power laser (HPDL) treatment. Especially, the surface energy of the MgO–PSZ with the laser-solidified microstructure is 75.9 mJ/cm2 and is much higher than the untreated sample surface energy, 52.8 mJ/cm2. This solidified microstructure generated by laser surface melting might be attributed to the changes in surface energy and, thereof, in the wettability characteristics. Indeed, work conducted by Zhang et al. [33] found that considerable improvement in the bond strength of a Si3N4 ceramic could be realised only when excimer laser treatment of a structural alloy steel (SAE 4340) resulted in surface melting.
4. Conclusions Owning to the interaction with Nd:YAG laser beam, the MgO–PSZ displaced a considerable reduction in y with all the control liquids used, implying that laser treatment effects higher wettability characteristics. Notably, the sample exhibiting the complete melting and solidified microstructure following laser treatment
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displayed the maximum wettability characteristics. It was found that although the increase in Ra is the factor influencing the wettability, it plays only a minor role. Both the enhancement of surface oxygen content and surface energy were observed to be influential in promoting the enhanced wettability characteristics. The surface energy of the MgO–PSZ with the laser-solidified different microstructure is 75.9 mJ/cm2 and is much higher than the untreated sample surface energy, 52.8 mJ/cm2. This solidified microstructure generated by laser surface melting might be attributed to the changes in surface energy and, thereof, in wettability characteristics. Particularly, the increase in gpsv , which is caused by the surface melting and resolidification into a different microstructure following laser treatment, has a clear relationship with the wettability characteristics and is thought to be the chief mechanism governing the change in wettability characteristics of the MgO–PSZ. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
Puleo DA, Nanci A. Biomaterials 1999;20:2311–21. Scotchford CA, Cooper E, Leggett GJ, Downes S. J Biomed Mater Res 1998;41:431–42. Michaels CM, Keller JC, Stanford CM. J Oral Implantol 1991;17:132–9. Kawahara D, Ong JL, Raikar GN, Lucas LC, Lemons JE, Nakamura M. Int J Oral Maxillofacial Implant 1996;11:435–42. Hao L, Lawrence J. Colloid Surf B: Biointerface 2004;34:87–94. Hao L, Lawrence J. J Biomed Mater Res 2004;69:748–56. Hao L. Lawrence J Mater Sci Eng: C 2003;23:627–39. Hao L, Lawrence J, Chian KS. J Biomater Appl 2004;19:81–105. Kapplel. Excimers help ceramic stick together. Opt Laser Eur 1998;34. Lawrence J, Li L, Spencer JT. Surf Coat Technol 1999;115:273–81. Lawrence J, Li L. J Phys D 1999;32:1075–82. Lawrence J, Li L, Spencer JT. Appl Surf Sci 1999;138–139:388–93. Hao L, Lawrence J. J Phys D 2003;36:1292–9. Hao L, Lawrence J. J Laser Appl 2004;16:252–7. Ouyang JH, Nowotny S, Richter A, Beyer E. Surf Coat Technol 2001;137:12–20. Hao L, Lawrence J. Proceedings of the IMechE part B. J Eng Manuf 2004;218:59–76. Pei YT, Ouyang JH, Lei TC. Surf Coat Technol 1996;81:131–5. Adam NK, Elliott GEP. J Chem Soc 1958;18:2206–15. Fowkes FM. Ind Eng Chem 1964;56:40–52. Dann JR. J Colloid Interface Sci 1970;32:302–20. Agathopoulos S, Nikolopoulos P. J Biomed Mater Res 1995;29:421–9. Wenzel RN. Ind Eng Chem 1936;28:988–94. Uelzen T, Muller J. Thin Solid Film 2003;434:311–5. Ueki M, Naka M, Okamoto I. J Mater Sci Lett 1986;5:1261–2. Li J. Xiyou Jinshu/Rare Met 1993;12:84–96. Song Q, Netravali AN. J Adhes Sci Technol 1998;12:957–82. Lawrence J, Li L. Laser Modification of the Wettability Characteristics of Engineering materials. London: Professional Engineering; 2001. Song Q, Netravali AN. J Adhes Sci Technol 1998;9:983–7. Lawrence J. Proc R Soc London, Ser A 2002;458:2445–63. Hao L, Lawrence J. Mater Sci Eng A 2003;364:171–81. Neumann AW. Adv Colloid Interface Sci 1974;4:106–91. Chattoraj DK, Birdi KS. Adsorption and the Gibbs Surface Excess. New York: Plenum Press; 1984. Zhang XM, Yue TM, Man HC. Mater Lett 1997;30:327–32.