Substrate-Regulated Growth of Plasma-Polymerized Films on Carbide

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Substrate-regulated Growth of Plasma Polymerized Films on Carbide-forming Metals Behnam Akhavan, Steven Garry Wise, and Marcela M.M. Bilek Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02901 • Publication Date (Web): 27 Sep 2016 Downloaded from http://pubs.acs.org on October 3, 2016

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Substrate-regulated Growth of Plasma Polymerized Films on Carbide-forming Metals Behnam Akhavan,a* Steven G. Wiseb,c, and Marcela M.M. Bileka

a) School of Physics, University of Sydney, NSW 2006, Australia b) The Heart Research Institute, Sydney, NSW 2042, Australia c) Sydney Medical School, University of Sydney, NSW 2006, Australia

*Corresponding Author: Dr Behnam Akhavan School of Physics, University of Sydney New South Wales, 2006 Australia Tel: +61 2 93515961 Fax: +61 2 93517726 Email: [email protected]

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ABSTRACT Although plasma polymerization is traditionally considered as a substrate-independent process, we present evidence that the propensity of a substrate to form carbide bonds regulates the growth mechanisms of plasma polymer (PP) films. The manner by which the first layers of PP films grow determines the adhesion and robustness of the film. Zirconium, titanium, and silicon substrates were used to study the early stages of PP film formation from a mixture of acetylene, nitrogen, and argon precursor gases. The correlation of initial growth mechanisms with the robustness of the films was evaluated through incubation of coated substrates in simulated body fluid (SBF) at 37o for two months. It was demonstrated that the excellent zirconium/titanium-PP film adhesion is linked to the formation of metallic carbide and oxycarbide bonds during the initial stages of film formation, where a 2D-like, layer-bylayer (Frank–van der Merwe) manner of growth was observed. On the contrary, the lower propensity of the silicon surface to form carbides leads to a 3D, island-like (Volmer–Weber) growth mode that creates a sponge-like interphase near the substrate, resulting in inferior adhesion and poor film stability in SBF. Our findings shed light on the growth mechanisms of the first layers of PP films and challenge the property of substrate-independence typically attributed to plasma polymerized coatings. Key words: Plasma polymerization, Growth mechanism, Film stability, Biomaterials, Biocompatible Coatings


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1. INTRODCUTION Plasma polymerization is a simple, one-step, versatile process that can be employed to enable direct covalent immobilization of multiple biologically functional molecules to a surface.1 In this process, a precursor gas is excited into the plasma state under the influence of an electric field and can polymerize on virtually any solid substrate through a process of fragmentation and recombination.2-4 Although this technique has been increasingly applied in recent decades for the development of bio-interfaces,5-6 fabrication of plasma polymer (PP) films with strong interfacial adhesion remains an ongoing challenge. The operational parameters, e.g. plasma energy and system pressure, have typically been the focus of research for the production of PP interfaces that tightly adhere to the bulk biomaterial.7-10 However, the role of substrate chemistry on the properties of PP films has not been widely investigated in the literature since plasma polymerization is traditionally considered as a ‘substrate-independent’ process.11-13 This orthodox view has been questioned lately by a number of studies that showed differences in the properties of PP structures between regions proximate to the substrate and those in the upper regions or bulk of the film.14-17 Furuya et al showed that perfluorocarbon PP films deposited on an aluminium substrate consist of a long chain structure, whereas films deposited on a silver substrate are composed of shorter chains.18 Vasilev and co-workers reported that at early stages of deposition, amine-containing PP films grow faster on a thiol-functionalized gold surface compared to bare gold.14, 16 They have also reported that amine-containing PP films from nheptylamine show an island-like morphology near the substrate, whereas films from allylamine grow smoothly from the earliest stages of deposition.19 The delamination of allylamine and 1-bromopropane PP films from a sodium chloride single crystal substrate enabled Chen et al to conclude that the near-substrate regions of both films contain a higher 3 ACS Paragon Plus Environment

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concentration of oxygen in comparison with their topsides.15 More recently, we have demonstrated that near-substrate regions of micrometre-thick PP films from both acrylic acid and 1,7-octadiene exhibit a significantly lower degree of crosslinking than the bulk of the films.20 Following these recent advances towards a better understanding of PP film-substrate interactions, herein we report that the carbide-forming nature of a surface plays a key role in determining the robustness of these films through significant influences on both the chemistry and morphology of the initially deposited layers. The aim of this investigation is to highlight the role of early transition metals chemistry on the growth mechanisms of PP films from a mixture of acetylene, nitrogen, and argon gases. These PP films, also known as plasma activated coatings (PAC),21 have recently been shown to provide universal protein-binding platforms that covalently immobilize target biomolecules via surface embedded radicals22 while retaining their bioactive native conformations.23 Zirconium (Zr) and titanium (Ti) were used as model carbide-forming substrates because these early transition metals are widely applied for the fabrication of orthopedic implants.24-28 Silicon wafers were used as control substrates to compare the initial growth mechanisms of PP films deposited on carbide-forming metals (i.e. Zr and Ti) to those of films deposited on surfaces with lower carbide-forming propensity. Growth mechanisms of PP films deposited on different substrates were studied using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), time-of-flight secondary ion mass spectroscopy (ToF-SIMS), and spectroscopic ellipsometry. The correlation of initial growth mechanisms with the robustness of the films was evaluated through incubation of coated substrates in simulated body fluid (SBF). Our findings challenge the substrate-independent nature of plasma polymerization techniques by revealing fundamental aspects of early PP film formation that are critical for enabling the production of robust bioactive interfaces.


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2. EXPERIMENTAL SECTION 2.1. Materials Zirconium and titanium substrates were obtained from Firmetal - China. Double-side polished silicon wafers (boron doped, p-type) with a resistivity of below 50 Ω·cm were purchased from Addison – USA. 100 mm × 30 mm zirconium/titanium and 10 mm × 10 mm silicon substrates were ultrasonicated in acetone and ethanol, rinsed with Milli-Q water, and then dried under a stream of nitrogen gas prior to film deposition. High purity nitrogen, argon, and acetylene gases were obtained from BOC-Australia.

2.2. Plasma polymerization A custom-made plasma polymerization system was utilized for the deposition of PP films onto zirconium, titanium, and silicon surfaces. The plasma polymerization system, schematically illustrated elsewhere,29 was equipped with a radio frequency (RF) electrode and a DC pulsed voltage source connected to the substrate holder. The RF electrode was powered at 50 W (13.56 MHz) using an ENI generator (OEM-6AM-1) and a matching network (RFPP AM-5). The voltage pulses of -500 V, applied to the substrate holder, were generated by a RUP-6 pulse generator (GBS-Electronik GmbH) at a frequency of 3 kHz with a pulse duration of 20 µs, while the chamber wall was grounded. A mixture of acetylene, argon, and nitrogen gases was injected into the chamber through a gas dispenser at the flow rates of 5, 10, and 15 standard cubic centimetre per minutes (sccm), respectively. The gas flow rates were adjusted using Alicat mass flow controllers (Alicat Scientific Inc.) operated by the Flow Vision software (Alicat Scientific, version 1.1.35). Base pressures of below 5.0 ×


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10-2 mTorr were achieved using turbomolecular and scroll pumps. The total pressure of 110 mTor was kept constant, and samples were placed in the same position on the holder throughout the study. Samples were treated by argon plasma (argon flow rate = 40 sccm, RF power = 75 W, bias voltage = -500 V) for 10 minutes prior to plasma polymerization as a final cleaning step. Plasma polymerization was subsequently conducted without exposure to air.

2.3. X-ray photoelectron spectroscopy (XPS) Surface chemistry of PP films deposited onto different substrates was analysed using a SPECS SAGE spectrometer. The system was equipped with a monochromatic Al Kα (hv = 1486.7 eV) radiation source, a hemispherical analyser (PHOIBOS 150), and a MCD9 electron detector. The radiation source was operating at a power of 200 W (10 kV and 20 mA). The electron take-off angle was 90o with respect to the sample surface. All the measurements were conducted at pressures below 5.0 × 10-8 mbar. The survey spectra were collected in an energy range of 0 – 1000 eV at a pass energy of 30 eV and a resolution of 0.5 eV. High resolution (0.1 eV) C1s, Zr3d, and Ti2p spectra were collected at a pass energy of 20 eV. The recorded survey spectra were used for atomic concentration calculations, while high-resolution C1s, Zr3d, and Ti2p spectra were curve fitted to determine the relative concentrations of the various carbon- or metal-containing bonds. Atomic concentration calculations and curve fittings were preformed using the commercial software CasaXPS (version 2.3.1) by applying a linear background, equal full-width at half-maximum (FWHM) peaks with a Gaussian (70%) – Lorentzian (30%) line shape. All spectra were recorded within approximately 24 hours after the deposition allowing a fixed aging time for all samples.


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2.4. Spectroscopic ellipsometry The thickness and refractive index of PP films deposited on silicon wafers were acquired using a Woollam M2000V spectroscopic ellipsometer. The instrument was equipped with a XLS-100 light source and a control module (EC-400) running by WVASE32 software. Measurements were conducted at 65o, 70o, and 75o angles of incidence in the visible and near-UV spectral regions over a wavelength range of 200 – 1000 nm with 5 nm steps. A Cauchy model was implemented for a wavelength-by-wavelength fit of data obtained. The average values of film thickness and refractive index (at 630 nm) for at least three measurements per sample were reported. The standard deviations of the mean values were less than 10%.

2.5. Atomic force microscopy (AFM) Topographic images and average root-mean-square (RMS) roughness of PP films deposited on polished zirconium, titanium, and silicon surfaces were obtained by Molecular Imaging PicoSPM operating in non-contact AFM mode. Silicon non-contact tips (Nanosensors, PPPNCST-SPL), which had a resonance frequency of 76 – 263 kHz, were utilised. A scanner with a maximum range of 100 µm was used for scanning the samples at a set point of 2 – 3 V and a frequency of 0.5 Hz. The data obtained were processed using PicoScan 5 and WSxM software (Version 5.0, Nanotec Electronica). RMS roughness values were calculated from areas of 1 µm x 1µm.


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2.6. Time of flight secondary ion mass spectroscopy (ToF-SIMS)

ToF-SIMS was conducted using a PHI TRIFT V nanoTOF instrument (Physical Electronics Inc., Chanhassen, MN, USA) to obtain positive ion distribution maps. A 30 eV 79Au+ pulsed liquid metal primary ion source (LMIG) was used to bombard the surface, while the base pressure was below 5 × 10-6 Pa. The dual charge neutralization was achieved by an electron flood gun and Ar+ ions at 10 eV. Positive SIMS counts were recorded over areas of 100 µm × 100 µm, and WincadenceN software (Version 1.8.1 provided by Physical Electronics Inc.) was used to process the ion distribution images.

2.7. Physical stability tests PP-coated zirconium and titanium surfaces were scratched using a custom-built macro scratch unit before incubation, to evaluate their robustness in simulated body fluid even after being physically damaged. Two scratches with a length of approximately 10 mm with an intersection near their middle were made on samples using a sharp high-speed steel tip under a normal load of 3.5 N. PP-coated samples were sterilized on both sides using ultraviolet light for 30 minutes. The samples were incubated along with PP-coated silicon wafers for two months at (37 ± 1)oC in 10 ml of sterile Tyrode’s salt balanced solution (pH = 7.4) with a composition listed in Table 1.30 The Tyrode’s solution is commonly used as a simulated body fluid for biomaterials testing.30-33 After incubation, the Tyrode’s solution was removed, and


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samples were thoroughly rinsed with Milli-Q water, followed by drying under a stream of nitrogen gas. XPS survey spectra of the washed samples were obtained within 24 h.

Table 1. Chemical composition of Tyrode’s salt balanced solution (g/L) NaCl

CaCl2

KCl

NaHCO3

MgCl2

NaH2PO4

8.00

0.20

0.20

1.00

0.10

0.05

2.8. Scanning electron microscopy (SEM) SEM images of PP-coated substrates before and after incubation in Tyrode’s solution were obtained using a FEI Quanta 450 FEG microscope in the secondary electron (SE) mode. The chamber pressure was below 6.5 × 10-6 Tor, while an acceleration voltage of 5 kV was applied at working distances of 5 – 10 mm.

3. RESULTS AND DISCUSSION

3.1. Formation of metallic carbide/oxycarbides near the substrate To examine the growth of PP films during the initial stages of polymerization, depositions were terminated at times of 15 to 120 s, while all other plasma polymerization conditions were kept constant. The uncoated zirconium surface showed XPS atomic concentrations of 17.6, 75.1, and 7.3% for zirconium, oxygen and adventitious carbon, respectively. The detected oxygen originates from the passive naturally-occurring oxide layer of zirconium as


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described in the literature.34 From Figure 1a, it is observed that by increasing the deposition time, the carbon and nitrogen atomic concentrations increase, whereas that of oxygen and zirconium decrease. These variations in surface chemistry are simply due to the deposition of more carbon- and nitrogen-containing PP fragments at longer deposition times obscuring the signals originating from the substrate. Although the precursor gas mixture contains no oxygen, approximately 9% oxygen was still detected for the film deposited for 120 s. Since no zirconium was observed in the survey spectrum of this sample, it can be concluded that the PP film thickness is greater than the sampling depth of XPS and therefore the detected oxygen could not have originated from the underlying substrate. The observed oxygen is suggested to originate from either plasma phase contamination due to residual oxygen in the plasma chamber or post-deposition oxidation reactions upon exposure to atmosphere. Considering the relatively low base-pressure of the deposition chamber (below 5.0 × 10-5 Torr), the post-deposition oxidation process is expected to play a more significant role in the presence of oxygen in the PP film structure. In this process, reactive radicals generated during the plasma polymerization are initially oxidised into metastable species and subsequently to stable compounds once the samples are in contact with atmospheric oxygen.35 The postoxidation of PP films is a well-known, inevitable mechanism that has been also previously reported for other precursor monomers.36-41 Zirconium signals were only detected in the XPS survey spectra of samples coated for 15, 30, and 60 seconds. At longer deposition times, PP layers thicker than the sensitivity depth of XPS (8 - 10 nm) are deposited onto the surface concealing the PP film-zirconium interface.


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Figure 1. (a) XPS survey elemental composition of uncoated and PP-coated zirconium surfaces as a function of plasma polymerization time. (b) XPS C1s high resolution spectra of PP-coated zirconium surfaces at early stages of growth from films with plasma polymerization times of 15 – 120 s. The C1s spectra were curve-fitted by five components; C1: C-C/C-H, C2: C-O/C-N, C3: C=O/N-C=O, C4: ZrC, and C5: zirconium oxycarbides. (c) XPS Zr3d high resolution spectra of (i) uncoated and (ii) PP-coated zirconium. The Zr3d spectra were curve-fitted by six components; Zr1: (ZrO2)5/2, Zr2: (ZrO2)3/2, Zr3: (ZrOy)5/2, Zr4: (ZrOy)3/2, Zr5: (Zr-Zr/Zr-C)5/2, Zr6: (Zr-Zr/Zr-C)3/2. The plasma polymerization time for the PP-coated sample was 15 s.

XPS C1s high resolution spectra of PP-coated zirconium surfaces at early stages of growth are shown in Figure 1b. The C1s high resolution spectrum of PP film deposited for 120 s consists of C-C/C-H at binding energy (BE) ≅ 284.6 eV, C-O/C-N at BE ≅ 286.5 eV, and 11 ACS Paragon Plus Environment

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C=O/N-C=O at BE ≅ 287.5 eV.38, 42 The C1s spectra of samples coated for shorter times than 120 s (film thicknesses < 10 nm), however, contain information from the PP-substrate interface and exhibit a more complex structure. In addition to organic carbon moieties, two components at binding energies of ≈ 282.5 eV and ≈ 289.8 eV are present that can be assigned to metallic carbide (ZrC) and oxycarbides (ZrOxCy) components, respectively.43-44 Comparison of C1s spectra for PP films deposited on silicon (see Figure S1) and zirconium surfaces highlights the influence of the substrate on the formation of inorganic carbon species at the initial stages of deposition. The concentration of metallic carbide/oxycarbide components for the PP film deposited on the zirconium surface increases from 0% to 6.8% with a decrease in deposition time from 120 to 15 s. On the contrary, no contribution of silicon carbide/oxycarbide compounds can be observed in the C 1s spectra of PP films deposited on the silicon surface. These differences in the chemistry of initially deposited layers on zirconium and silicon surfaces can be rationalized by the markedly higher enthalpy of formation of ZrC (-196.65 kJ.mol-1) compared to that of SiC (-71.55 kJ.mol-1) that also suggests a higher bond strength for ZrC.45 A comparison of Zr3d high resolution spectra of uncoated zirconium (cleaned by argon plasma) and PP-coated zirconium prepared for a deposition time of 15 s, shown in Figure 1c, helps to further elucidate the formation of carbide bonds at very early stages of deposition. The Zr3d is a doublet due to the spin-orbit splitting of Zr3d3/2 and Zr3d5/2 with a separation energy of 2.3 – 2.4 eV.46-47 The high-resolution Zr3d peak can include three groups of components at different oxidation states:46-50 (i) ZrO2-like components at binding energies (BEs) of 183−186 eV, (II) zirconium sub-oxides (ZrOx) with 0 < x < 2 at BEs of 181 – 184 eV, and (iii) zirconium-containing components in the neutral state (Zr-Zr, Zr-C) at BEs of 179−182 eV. For each group of components, two peaks corresponding to 3/2 and 5/2 spins were fitted, giving a total of six peaks. From Figure 1c, it is apparent that the concentration


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of Zr-Zr/Zr-C compounds (shown as Zr5 and Zr6) significantly increases upon deposition of PP film onto the surface (from 3.5% for the uncoated zirconium to 12.1% for the PP-coated zirconium). The concentration of Zr-Zr bonds in the Zr3d high resolution spectrum is expected to remain unchanged upon plasma polymer deposition. It is, therefore, reasonable to conclude that the increase in the concentration of compounds at BE ≅ 180 eV is solely due to the increase in Zr-C concentration. This information agrees with the C1s high resolution curve-fitting data (Figure 1b) and suggests the formation of metallic carbide bonds in close proximity to the zirconium substrate.

3.2. Morphology of the PP films at early stages of polymerization AFM in tapping mode was used to compare the morphology of the first layers of PP films deposited on the different substrates. Figure 2 shows AFM topographic images of PP films deposited for 15 s onto zirconium and silicon substrates. While early stages of PP film growth on the zirconium substrate show a complete surface coverage (RMS roughness = 0.31 ± 0.04 nm); island-like features, with heights of approximately 6 nm, are observed for the PP-coated silicon surface (RMS roughness = 1.83 ± 0.3 nm). These results indicate two distinctly different types of growth as has also been previously observed for semiconductor materials:5152

a Frank-van der Merwe growth mechanism for the zirconium surfaces resulting in a 2D-

like structure, and a Volmer-Weber mechanism, identified by 3D nucleation and island-like growth, for the silicon surface. The predominance of these mechanisms depends on the strength of interactions within the deposited atoms and between those atoms and the substrate material.53 The formation of strong zirconium carbide/oxycarbide bonds during the initial stages of deposition appears to be an effective driving force to make the zirconium substrate an energetically preferred site for the arriving atoms.51 It can also be hypothesised that carbon atoms arriving to the zirconium substrate are ‘pinned’ on the surface due to the strong affinity 13 ACS Paragon Plus Environment

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of an early transition metal for forming stable carbide bonds. The nucleation density on the zirconium substrate is therefore likely to be significantly higher than that on the silicon surface where ad-atoms would be more mobile. The anchored carbon atoms are expected to act as stable nucleation centres for subsequently deposited species, thus increasing the sticking probability and resulting in a contiguous mode of growth. On the contrary, the island-like growth for the silicon surface may imply that the inter-polymeric bonding is stronger than forces between the deposited atoms and silicon substrate.53-54 Another possible explanation for such an incoherent mode of growth on the silicon surface is post-deposition restructuring.19 The absence of a strong molecular interaction between the coating and the substrate possibly increases the mobility of deposited layers. The post deposition restructuring of PP film is thus more readily achieved on a silicon surface to obtain a conformation with the lowest free energy. A transformation from 3D to 2D growth was observed for silicon surfaces once the substrate became fully masked by the PP film (see Figure S2). As islands grow and coalesce with each other, an interphase rich in voids is likely to form in the close proximity of the surface. This interpretation is consistent with ellipsometry data shown in Figure 3. Significantly higher deposition rates together with lower refractive indices are observed for PP film deposited on silicon surfaces at early deposition times. Such data suggest the presence of a porous, sponge-like interphase in the close proximity of the silicon surface that shows a greater apparent thickness.


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Figure 2. AFM images with locations of Z profiles (shown as white lines) of PP film deposited on (a) zirconium and (b) silicon substrates. Deposition time was 15 s.

Figure 3. Deposition rate and refractive index (at λ = 630 nm) of PP film deposited on silicon surfaces as a function of plasma polymerization time. ToF-SIMS is a highly sensitive technique for surface characterization with a sampling depth of only 1 - 2 nm.55 When used in imaging mode, this technique can probe the homogeneity of the ultrathin PP films deposited on zirconium and silicon surfaces. The distribution of Zr+ and Si+ counts as a function of plasma polymerization time are presented as image tiles in Figure 4. By increasing the plasma polymerization time from 15 to 120 s, the number of Zr+ and Si+


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counts decrease as the underlying substrate is concealed by PP film fragments. The ion distribution maps of samples deposited for 120 s do not show any Si+ and Zr+ signals, as both substrates are completely masked by sufficiently thick layers of PP films. Comparison of Zr+ and Si+ ion distribution maps of samples polymerized for 30 s provide valuable information on the uniformity of the coatings. As observed, no Zr+ counts are detected for the coating deposited on zirconium surface, whereas significant signals from the substrate (Si+) are still recorded for the coating deposited onto the silicon substrate. The absence of Zr+ signals for the zirconium substrate implies that a PP layer with a thickness greater than the sampling depth of ToF-SIMS is uniformly deposited on the surface. The detection of Si+ signals is not, however, expected for a conformal PP film as the film thickness, as measured by ellipsometry (14.8 ± 1.3 nm), is well above the sensitivity depth of ToF-SIMS (1 – 2 nm). It can be therefore argued that the Si+ signals originate from areas that are not fully concealed by the PP film. These data are consistent with AFM observations and provide further evidence of a contiguous and an island-like mode of growth for PP films deposited on zirconium and silicon substrates, respectively.

Figure 4. ToF-SIMS distribution maps of Si+ and Zr+ originating from the underlying substrates as a function of plasma polymerization time.


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3.3. Early stages of PP film growth on a titanium surface In order to ensure that the results observed on the zirconium substrate are not specific to that particular case, a titanium substrate was also used to study the early stages of the PP film growth. Similar to zirconium, titanium is a carbide-forming metal that is widely used for medical applications such as dental implants and total hip/knee replacements.27-28, 56 The PP film was deposited on the titanium surface for a polymerization time of 15 s providing a surface with an XPS chemistry of 62.6, 10.7, 21.2, and 5.5% for carbon, nitrogen, oxygen, and titanium, respectively. The contribution of ~ 5% titanium to the surface chemistry ensures that the XPS information is originating from the PP film-titanium interface. The XPS Ti2p high resolution spectra of uncoated and PP-coated titanium are shown in Figure 5a and 5b, respectively. Ti2p3/2 peaks for TiO2, titanium sub-oxides (TiOx with 0 < x < 2 ), and titanium in a neutral environment (Ti-Ti, Ti-C) were fitted at BEs of 458.8 – 459.2, 456.4 – 457.0, and 453.8 – 454.6 eV, respectively.57-59 Corresponding Ti2p1/2 peaks with a separation energy of 5.7 eV [∆ = ( 2 / ) − ( 2/ )] were also fitted in the spectra.57-58 It is observed that similar to the zirconium substrate, the concentration of Ti-Ti/Ti-C compounds substantially increases from 3.8% for the uncoated titanium to 13.6% upon deposition of PP film onto the surface. The C 1s high resolution spectra of PP-coated titanium (Figure 5c) also shows the presence of titanium carbide and oxycarbide components at BEs of 282.5 eV and 289.8 eV respectively; implying the formation of metallic carbon-containing species in close proximity of the titanium surface at initial stages of polymerization. The presence of such carbon-containing compounds suggests that carbon atoms arriving to a carbide-forming metal surface may substitute for both metal (M) and oxygen (O) atoms,60-61 forming a proposed structure schematically depicted in Figure 5d. The AFM topography of PP-coated titanium, shown in Figure 5e, also agrees with the AFM observations on the zirconium surface 17 ACS Paragon Plus Environment

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(Figure 2a) and shows a 2D-like manner of growth for early layers of PP (RMS roughness = 0.23 ± 0.06 nm). These results once again confirm that the carbide-forming nature of a substrate is critically important in determining the growth mechanism of PP films and may suggest that the conclusions of this study are likely to be more generally applicable to other carbide-forming materials.

Figure 5. XPS Ti2p high resolution spectra of (a) uncoated titanium cleaned by argon plasma and (b) PP-coated titanium. The Ti2p spectra were curve-fitted by six components; Ti1: (TiO2)3/2, Ti2: (TiO2)1/2, Ti3: (TiOx)3/2, Ti4: (TiOx)1/2, Ti5: (Ti-Ti/Ti-C)3/2, Ti6: (Ti-Ti/Ti-C)1/2 (c) XPS C1s high resolution spectrum of PP-coated titanium. The C1s spectrum is curvefitted by five components; C1: C-C/C-H, C2: C-O/C-N, C3: C=O/N-C=O, C4: TiC, and C5: 18 ACS Paragon Plus Environment

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titanium oxycarbides. (d) Proposed structure of carbon (C), oxygen (O) and carbide-forming metal (M) elements on a PP-coated early transition metal substrate at the first stages of plasma polymerization (e) AFM image of a PP film deposited on a titanium substrate with the location of the Z profile indicated by the white line. The plasma polymerization time for the PP-coated sample was 15 s.

3.4. The physical stability of PP films in simulated body fluid Orthopedic implants which are surface engineered using thin polymeric coatings are exposed to scratching during abrasive insertion into the body. It is, therefore, crucially important that the coatings resist delamination upon contact with biological media even when scratched. To simulate such conditions, PP-coated zirconium and titanium surfaces were scratched and incubated along with PP-coated silicon substrates in Tyrode’s simulated body fluid (SBF) for 2 months at 37 oC. The physical robustness of the coatings was evaluated using SEM and XPS analyses. SEM images of PP-coated zirconium, titanium, and silicon surfaces before and after incubation in SBF are shown in Figure 6. As observed, no evidence of film cracking, buckling, or delamination is witnessed for the PP-coated zirconium or titanium after incubation; whereas the coating deposited under the same conditions on the silicon surface shows either a complete delamination or partial failure in the form of buckling or wrinkling. The failure of PP film on silicon surfaces highlights the adverse role of the porous interphase in film failure and suggests that the initial stages of PP films growth are strongly substratedependent. The variations of zirconium and titanium atomic concentrations measured by XPS as a function of incubation time are shown in Figure 7. The limited changes of zirconium and titanium signals, originating from the underlying substrate, indicate that the thickness of PP film surface coverage is not being reduced with incubation time. This excellent resistance to failure is believed to be underpinned by the formation of metallic carbide/oxycarbide bonds


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at the interface and a coherent 2D-like manner of growth during the initial stages of deposition.

Figure 6. PP-coated zirconium, titanium, and silicon surfaces, prepared under the same plasma polymerization conditions, before and after incubation in Tyrode’s SBF for two months at 37 oC. Scale bar = 200 µm.


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Figure 7. (a) zirconium and (b) titanium atomic concentrations from XPS survey spectra of PP-coated surfaces before and after incubation in Tyrode’s SBF for different durations.

4. CONCLUSIONS We have presented evidence that the carbide-forming propensity of a substrate plays a key role in regulating the growth mechanisms of plasma polymer (PP) films through influencing both the chemistry and morphology of the initially deposited layers. It was demonstrated that PP films from a mixture of acetylene, nitrogen, and argon grow in a 2D-like, layer-by-layer (Frank–van der Merwe) manner on zirconium and titanium surfaces during the initial stages of deposition, in contrast to a 3D island-like (Volmer–Weber) form of growth on silicon surfaces. The initial layer-by-layer growth on zirconium and titanium substrates is proposed 21 ACS Paragon Plus Environment

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to be due to the formation of metallic carbide/oxycarbide bonds at the interface, whereas the island-like growth for the silicon surface is likely to be as a result of the inter-polymeric bonding within the plasma polymer being stronger than the forces between the deposited atoms and the silicon substrate. Our film stability studies showed excellent physical stability for PP films deposited either on zirconium or titanium surfaces, whilst the coating plasma polymerized on the silicon surface exhibited either partial failure or complete delamination. Such different behaviours highlight the role of the substrate in film stability and suggest that the initial stages of PP growth are strongly substrate-dependent. Substrate-regulated plasma polymerization holds great promise for the fabrication of robust bioactive surfaces on other carbide-forming early transition metals such as chromium and niobium.

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the Australian Research Council (ARC). The authors acknowledge the facilities, and technical assistance of the Australian Microscopy & Microanalysis Research Facility. We are thankful to Dr. John Denman for undertaking ToF-SIMS measurements. ASSOCIATED CONTENT Supporting information is available free of charge via the Internet at http://pubs.acs.org/.

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Figure 1 140x117mm (300 x 300 DPI)

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Substrate-Regulated Growth of Plasma-Polymerized Films on Carbide

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