Carbon-Bridge Incorporation in Organosilicate ... - ACS Publications

Dec 23, 2015 - Carbon-Bridge Incorporation in Organosilicate Coatings Using. Oxidative .... Two key processing parameters were found for successful...
1 downloads 0 Views 2MB Size
Download PDF
Research Article www.acsami.org

Carbon-Bridge Incorporation in Organosilicate Coatings Using Oxidative Atmospheric Plasma Deposition Linying Cui,† Geraud Dubois,*,†,‡ and Reinhold H. Dauskardt*,† †

Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States IBM Almaden Research Center, San Jose, California 95120, United States

‡

S Supporting Information *

ABSTRACT: Carbon-bridges were successfully incorporated into the molecular structure of inorganic silicate films deposited onto polymer substrates using an oxidative atmospheric plasma deposition process. Key process parameters that include the precursor chemistry and delivery rate are discussed in the context of a deposition model. The resulting coating exhibited significantly improved adhesion and a 4-fold increase in moisture resistance as determined from the threshold for debonding in humid air compared to dense silica or commercial sol−gel polysiloxane coatings. Other important parameters for obtaining highly adhesive coating deposition on oxidation-sensitive polymer substrates using atmospheric plasma were also investigated to fully activate but not overoxidize the substrate. The resulting carbon molecular bridged adhesive coating showed enhanced moisture resistance, important for functional membrane applications. KEYWORDS: hybrid coating, atmospheric plasma, adhesion, moisture resistance, functional membrane

■

INTRODUCTION Atmospheric plasma chemical vapor deposition is an emerging technology for depositing plasma coatings in ambient air without a vacuum chamber at low cost on large substrates for energy, display, and aerospace applications. However, the abundant oxygen in air poses a significant oxidation challenge for incorporating specific oxygen-sensitive components in the coating. As a result, inorganic oxides, such as silicon dioxide1,2 and zinc oxide,3 have been deposited in air using atmospheric plasma, while oxygen-sensitive coatings were deposited either in an inert environment using atmospheric pressure plasma (e.g., silicon carbide4 and silicon nitride5) or in air with compromised oxidized structures and properties (e.g., TiNx/TiO26). One type of oxygen-sensitive coating is carbon-bridged organic−inorganic hybrid silicates deposited from organosilane precursors containing the Si-CxHy-Si structure. The coating has found applications as low dielectric constant materials,7−12 functional membranes,13−15 and adhesion enhancement layers for protective transparent bilayer coatings on polymer substrates for aerospace and energy applications.16,17 The presence of a carbon-bridge in the inorganic silica network improves the mechanical properties through increased plastic deformation and increased network connectivity,18−21 and also increases the moisture resistance due to the insensitivity of the Si−C bond to moisture attack.13,14,22 Such coatings are commonly synthesized by a sol−gel process21 and recently reported thermal plasma chemical vapor deposition in an inert chamber environment.15 In the latter, a carbon-bridged precursor contained in a vapor was injected into a vacuum © XXXX American Chemical Society

chamber with pure Ar plasma at 0.1 mbar, without any traces of oxygen. In this study, we demonstrated that the carbon-bridge can be incorporated in an inorganic silicate network using oxidative atmospheric plasma deposition. Important processing parameters, including the precursor chemistry and delivery rate, are discussed based upon a deposition model. The resulting organic−inorganic hybrid coatings exhibited significantly improved adhesion (Figure 1) related to increased plastic deformation of the coating. The coating also exhibited a 4-fold increase in moisture resistance as determined from the threshold for debonding in humid air compared to dense silica or commercial sol−gel polysiloxane coatings. Other important processing parameters for obtaining highly adhesive coating deposition on oxidation-sensitive poly(methyl methacrylate) (PMMA) polymer substrates using atmospheric plasma were also investigated to fully activate but not overoxidize the substrate. The resulting carbon molecular bridged adhesive coating showed enhanced moisture resistance important for functional membrane applications.13

■

EXPERIMENTAL METHODS

Coating Deposition. Coatings were deposited by an atmospheric pressure plasma system (Figure 2(a)) [Surfx Technologies LLC, Redondo Beach, CA] integrated with a custom-made high temperature precursor delivery system.1 Glow discharge was generated between Received: October 21, 2015 Accepted: December 23, 2015

A

DOI: 10.1021/acsami.5b09971 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

plasma source into ambient air for energy interchange with air molecules and further reaction, with the residence time up to tens of milliseconds before reaching the substrate. A carbon-bridged organo-silane precursor, 1,2-bis(triethoxysilyl)ethane (BTESE) [Sigma-Aldrich, Saint Louis, MO] with an ethyl bridge connecting two silicon atoms was used to deposit the carbonbridged silica coatings. For comparison, two additional precursors, triethoxy(ethyl)silane (TEES) [Sigma-Aldrich, Saint Louis, MO] and tetraethyl orthosilicate (TEOS) [Sigma-Aldrich, Saint Louis, MO] (Figure 2(b)), were also used. The precursors were first vaporized at an elevated temperature, Tv, with the desired vapor pressure, Pv (Table 1), by passing helium gas through the precursor liquid at the flow rate

Table 1. Vapor Pressure, Pv, of the Precursors BTESE, TEES, and TEOS at the Vaporizing Temperature, Tva

Figure 1. Adhesion energy of the carbon-bridged 1,2-bis(triethoxysilyl)ethane (BTESE) coatings compared to other coatings (TEOS,1 TMCTS,1 and commercial sol−gel polysiloxane protective coatings23).

Precursor

Vaporizing temp, Tv (°C)

Vapor pressure, Pv (Torr)

BTESE

120 131 138 69 100 74 106

1.6 3.2 4.9 26.4 103.4 26.8 103.9

TEES

two parallel perforated electrodes powered at 13.56 MHz. Helium and oxygen of 99.995% purity [Praxair Inc., Santa Clara, CA] were mixed at the volumetric ratio of 99:1 and fed into the capacitive discharge plasma. The plasma power was fixed at 60 W with power density of 11.8 W/cm2, and two plasma gas flow rates of either 20 or 30 L/min were used. The plasma gas flowed out of the plasma zone at a speed of ∼1 m/s, forming the afterglow region where the precursor gas was injected. Then the plasma and precursor gas mixture flowed out of the

TEOS a

Calculated using the Clausius−Clapeyron relation from the product data sheet.52−54.

Figure 2. (a) Schematic of the atmospheric plasma deposition source; (b) molecular structures of the precursors, BTESE, TEES, TEOS, and TMCTS. B

DOI: 10.1021/acsami.5b09971 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces of Fhelium measured at room temperature T = 23 °C. The saturated precursor vapor carried by helium was delivered at 135 °C to the immediate plasma afterglow region. The precursor molecule vaporizing rate, rp, can be estimated using the ideal gas law1

rp =

⎞ P ⎛F n = v · ⎜ helium ⎟ ⎝ unit time R T ⎠T = 23oC

with n the mole of vaporized precursor molecules and R the ideal gas constant. The precursor molecular concentration, Nprecursor, in the gas mixture of plasma and precursor gas is1

Nprecurosr

Figure 3. Schematics of the ADCB test specimen.

rvNA = Jplasma + Jprecursor

test system [DTS Delaminator Test System, DTS Company, Menlo Park, CA] under displacement control. The specimens were loaded with a displacement rate of 5 μm/s in tension to produce controlled coating debonding, followed by unloading. The load was measured simultaneously, and the adhesion energy, Gc (J/m2), was calculated from the critical value of the strain energy release rate.23−26 The debond growth rate versus debond driving energy under different humidities was quantified by load relaxation tests conducted in an environmental chamber [Associated Environmental Systems, Ayer, MA] at 23 °C and different humidities. The specimen has the same dimensions as those for the ADCB test. The precracked specimen was first equilibrated in the chamber for 8 h, and then loaded until crack growth initiated. At that point, displacement was fixed. Crack growth continued for several days with monotonically decreasing load. The crack length and debond driving energy were calculated from the load, the fixed displacement, and the specimen dimensions.23−26 The debond growth rate was calculated from the crack length versus time.

with NA Avogadro’s number, and Jplasma and Jprecursor the flow rates of the plasma and precursor gas. Coatings were deposited on silicon wafers and military grade PMMA sheets meeting all requirements of MIL-PRF-25690. The substrate was wiped with ethanol before deposition to remove any surface contamination and dust, and then dried in air for 24 h. Deposition of a uniform coating with controlled thickness was implemented through the use of an X-Y stage that moved the plasma source at 50 mm/s over the substrate in a planar fashion. The relative motion of the plasma source to the substrate resulted in a rectangular array of 0.3 mm line spacing. Characterization Methods. The coating thickness was characterized by ellipsometry [Woollam M2000, J. A. Woollam Inc., Lincoln, NE]. Incident light (45° polarization) at the Brewster angle of the substrate was used to maximize reflection. The polarization versus wavelength (250−1000 nm) of the reflected light was measured and fitted to determine the coating thickness. The elemental composition of the coatings and the fractured surfaces after adhesion testing was analyzed by X-ray photoelectron spectroscopy (XPS) [Physical Electronics Inc., Chanhassen, MN]. An Al Kα (1486 eV) X-ray source was used (spot size ∼1 mm, pass energy 117.4 eV, scan range 0−1000 eV). Before measuring the coating composition, surface contamination was removed by argon ion beam sputtering (sputter rate ∼20 nm/min, sputter time ∼4 min). An elemental depth profile of the fractured surface was also probed with alternating argon ion beam sputtering (1 kV, 0.5 μA, beam size 1 mm × 1 mm, sputter rate ∼35 nm/min for the organosilicate coating, and 0.2 min acquisition time interval). The chemical bonds in the coating were characterized using IR spectroscopy. The spectrum was recorded as dispersions in KBr using a Nexus 670 FT-IR (reflectance mode). Mid-IR in the wavelength range from 650 to 4000 cm−1 was probed at a resolution of 4 cm−1. Coatings on silicon substrates were characterized in transmission mode at the Brewster angle of the silicon substrate. The Young’s modulus was obtained using surface acoustic wave spectroscopy (SAWS). SAWS studies were performed with a laseracoustic thin film analyzer [LaWave, Fraunhofer USA, Boston, MA] in which acoustic waves were generated by a nitrogen pulse laser (wavelength 337 nm, pulse duration 0.5 ns). These were detected using a transducer employing a piezoelectric polymer film sensor. The measured surface wave velocity as a function of frequency was fitted with the theoretical dispersion curve to deduce the Young’s modulus (a value of 0.25 was assigned for Poisson’s ratio, and the input coating density was measured by X-ray reflectivity16). The adhesion energy of the coating on PMMA was quantified using the asymmetric double cantilever beam (ADCB) test.23−26 The asymmetric specimen configuration was specially chosen to confine the debond occurring at the interface of soft polymer substrate and hard coating.23−26 The specimens (Figure 3) were prepared by bonding an uncoated thinner substrate onto a coated thicker substrate using epoxy adhesive [E-20HP Hysol High Strength Epoxy Structural Adhesive, Henkel AG & Company, Düsseldorf, Germany]. The inplane dimensions of the specimen were 9 mm × 70 mm. The thicknesses of the thinner and thicker blank substrates were 3 and 6 mm, respectively. The coating thickness was controlled to be ∼500 nm. The fracture tests were conducted on a micromechanical adhesion

■

RESULTS AND DISCUSSION Carbon-bridged highly adhesive coatings were successfully deposited on PMMA using oxygen atmospheric plasma by carefully tuning the precursor chemistry and reaction kinetics. Two key processing parameters were found for successful carbon-bridge incorporation. One was to provide similar or higher concentration of precursor molecules compared to the reactive oxygen species; the other was to have the carbonbridge already present in the precursor. Other factors important for highly adhesive coating deposition were revealed, including the deposition distance, the plasma gas flow rate, and the deposition rate. As a potential application for hydrothermal stable functional membranes, we show that the carbon-bridged coating exhibits superior moisture resistance compared to commercial sol−gel polysiloxane coatings. Carbon-Bridge Incorporation in Oxidative Atmospheric Plasma Deposition. Key Parameter One: High Precursor Concentration in the Gas Phase. In our study, the main reactive species present in the plasma afterglow gas phase was oxygen neutrals which formed in the plasma zone or were excited from ambient air molecules. Other reactive species such as charged species, metastable helium (23S1), and UV photons had negligible concentration because the abundant oxygen in air acts as an efficient quencher. For example, metastable helium (23S1) can be quickly quenched by oxygen with a pseudo-first-order rate constant of ∼107 μs at oxygen molecule concentrations of 1017 cm−3 in ambient air,27 resulting in a lifetime of only 0.1 μs, in contrast to a few milliseconds of residence time (=deposition distance/flow speed) for precursor species in the afterglow. Charged species were quenched even faster than metastable helium. High energy UV photons can also be quickly absorbed,28 and less than 10% of the total power was used to generate them,27 so on the first order, the effect could be neglected. On the other hand, ground-state O atoms C

DOI: 10.1021/acsami.5b09971 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces (3P), metastable molecular O2 (1Δg and 1∑g+), and ozone had concentrations of 1015 cm−3 from the immediate afterglow to tens of millimeters downstream, as determined by simulation and experiments (Figure 5 of ref 27), thus being the dominant reacting species with the precursor in the afterglow reaction. Especially, the ground-state O atoms and ozone possess high energy and can easily overcome the energy barrier to oxidize the precursor. In the gas phase, the oxidation probabilities for the ethoxy terminal group and the carbon-bridge of BTESE are about the same due to similar bond energies for C−C of 88 kcal/mol, C− Si of 92 kcal/mol, and C−O of 98 kcal/mol.1,29,30 The steric hindrance effect is minor, given the small dimension of oxygen radical and ozone compared to the larger BTESE molecule branches. We believe that the key to prevent oxidation of the carbon-bridge was to minimize the reaction time or the oxidants concentration. However, at atmospheric pressure, the collision between molecules is so frequent that it is difficult to limit the residence time of the precursor to be comparable to the time between molecular collisions. We calculated that the number of collisions between one BTESE precursor and the reactive oxygen species throughout the afterglow is about 10,000 (see Supporting Information). Thus, it is more practical to tune the relative concentration of the precursor to reactive oxygen species in order to preserve the carbon-bridge. Since each precursor molecule has six ethoxy groups and one ethane bridge, roughly about 1/7 of oxidation events could occur at the bridge. To preserve the bridge, the precursor concentration should be at least larger than 1/7 of the reactive oxygen species concentration (∼1015 cm−3),27 roughly on the order of 1014 cm−3. This is consistent with previous conclusions that decreasing the energy density per precursor, called the Yasuda parameter,31 results in better preservation of the precursor molecular structure in PECVD coatings. While we did not change the plasma power, increasing the precursor concentration in the gas phase would also lead to a smaller Yasuda parameter. Note that increasing the precursor concentration potentially reduces the effective usage of the precursor and the resulting molecular network connectivity, although a postdeposition process (e.g., thermal or UV cure) could be used to enhance the network connectivity. In our experiments, we observed a BTESE concentration threshold of ∼6 × 1014 cm−3 for carbon-bridge incorporation, on the same order of magnitude as the estimation, under fixed plasma conditions (20 L/min plasma gas flow and 10 mm deposition distance). With a BTESE concentration of 3 × 1014 cm−3 in the afterglow (obtained with a 2.6 mmol/min BTESE delivery rate), there was only ∼0.8 atm % of carbon content in the coating (Figure 4(a)) and no carbon-bridge features in the IR spectrum (Figure 4(b, c)). This is in agreement with previously reported results from our group, showing that hard silica is obtained when sufficient reactive oxygen species were used.1,16 When the BTESE delivery rate was increased to 5.3 mmol/ min with a corresponding BTESE concentration of 6 × 1014 cm−3, the coatings’ carbon content increased markedly to ∼7.3 atm % (Figure 4(a)), together with the carbon-bridge-related IR peaks at the same positions as the sol−gel BTESE coating (synthesized with NaOH as the catalyst and no thermal cure) (Table 2, Figure 4(b, c)): sp3 C−H2 asymmetric and symmetric stretching at ∼2930 and ∼2890 cm−1, respectively,15,32−35 asymmetric bending and wagging of CH2 in the Si-CH2−CH2− Si structure at ∼141015,36 and ∼1273 cm−1, respectively,15,36

Figure 4. (a) Relation between the normalized BTESE delivery rate, the carbon content, and the adhesion to PMMA; (b) comparison of the IR spectra of the BTESE sol−gel coating (no thermal cure) and atmospheric plasma coatings deposited at different BTESE delivery rates, 2.6, 5.3, and 32 mmol/min, with a fixed plasma gas flow rate of 20 L/min and a deposition distance of 10 mm. The plasma coatings with 5.3 and 32 mmol/min precursor delivery rates have all the carbon-bridge-related peaks as the sol−gel coating. (c) Magnified IR spectra (3100−2700 cm−1 and 1800−1250 cm−1) of the BTESE atmospheric plasma coatings.

and the Si−C stretching at ∼780 cm−1.15,35,37 Further increases of the BTESE delivery rate to 32 mmol/min with BTESE concentration of 3.6 × 1015 cm−3 in the afterglow resulted in a higher carbon content of ∼14.4 atm % and more pronounced carbon-bridge-related IR peaks. (As a reference, the carbon content of a fully condensed film, made from BTESE, with no residual ethoxy groups but with the carbon-bridge preserved would be 29 atm.%.) Note that the Si−H peak15 at ∼2200 cm−1 D

DOI: 10.1021/acsami.5b09971 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Table 2. IR Peaks for Carbon-Bridge-Related Bonds Peak position (cm−1)

Assignment

2980 2930 2890 1410 1273 780

C−H3 asymmetric stretching sp3 C−H2 asymmetric stretching sp3 C−H2 symmetric stretching Asymmetric bending of C−H in Si-CH2CH2−-Si Wagging of C−H2 in Si-CH2CH2-Si Si−C stretching

was absent in all the spectra (Figure 4(b)), suggesting that precursor fragmentation was almost always accompanied by oxidation in atmospheric plasma deposition. Also note that the peak at ∼1273 cm−1 can be assigned to either Si-CH3 or SiCH2−CH2−Si. But BTESE only contains Si-CH2 and Si−O−C groups and pure precursor fragmentation was almost impossible in highly oxidative atmospheric plasma environment, so Si-CH3 groups were unlikely to form. Interestingly, with increasing BTESE delivery rate, the ratio of bridged carbon to ethoxy carbon in the coating also increased. The IR peak at 2890 cm−1 was related to C−H2 stretching in the bridge, and 2980 cm−1 was related to C−H3 stretching in ethoxy (Figure 4(c)). They both increased with increasing BTESE delivery rate; however, the carbon-bridgerelated peak at 2890 cm−1 increased faster. At 5.3 mmol/min BTESE delivery rate, the 2890−2980 cm−1 peak height ratio was 1.6. At 32 mmol/min delivery rate, the ratio increased to 2.1. Because the gas phase oxidation probabilities of the carbonbridge and ethoxy group were similar, the predominant deposition of carbon-bridges suggested that the surface reaction favored the incorporation of bridged over unbridged species at increasing BTESE delivery rate. Such predominance in the surface reaction step can be explained by the precursorsurface interaction mechanism. The incoming precursor species were adsorbed on the substrate surface at probability θa, diffused on the surface for time t, and then either were chemically incorporated at probability θi or desorbed. The total probability for incoming precursor deposition was therefore θa × θi, with the limiting steps varying between precursor adsorption and incorporation determined by precursor availability and reactive oxygen species needed for the incorporation reaction. Under conditions for bridged coating deposition, the reactive oxygen species was deficient in the plasma afterglow region and on the substrate surface, thus becoming a limiting factor for precursor incorporation. Longer surface diffusion time with a higher probability of reaction with the reactive oxygen species would produce more binding chances for chemical incorporation of the precursor species. The diffusion time was longer for precursors with higher molecular weight and lower vapor pressure, which was true for the bridged species. The bridged species also have more (= six) terminal groups for binding than the unbridged species (four terminal groups) and thus a higher probability of being incorporated. This condition was more predominant when the reactive oxygen species were increasingly deficient with increasing BTESE delivery rate, as observed in the preceding IR analysis. Key Parameter Two: The Carbon-Bridged Precursor. Another important parameter for carbon-bridge incorporation in atmospheric plasma deposition is to start with the carbonbridged precursor, e.g. BTESE. We compare BTESE to two unbridged precursors: TEES replaces the ethane bridge with an ethyl terminal group, and TEOS replaces the ethyl terminal

References 15, 15, 15, 15, 15, 15,

32−35 32−35 32−35 32, 36 36 35

group with an ethoxy group (Figure 2(b)). Since TEES and TEOS molecules have a single Si atom compared to two in BTESE, we normalized two TEES or TEOS molecules to one BTESE molecule when comparing their precursor delivery rate. At similar and even higher normalized precursor delivery rates than that for bridged BTESE coating deposition, TEES and TEOS coatings had no carbon-bridge-related peaks in the IR spectra (Figure 5(a, b)): In the TEES coating spectra, the

Figure 5. (a) IR spectra and (b) magnified IR spectra (3100−2700 cm−1 and 1800−1250 cm−1) of BTESE, TEES, and TEOS coatings at high precursor delivery rate. Only the BTESE coatings contain carbonbridges because they have all the carbon-bridge-related IR peaks. (c) The relation between the normalized BTESE delivery rate and deposition rate. E

DOI: 10.1021/acsami.5b09971 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces absence of the peak at ∼1273 cm−1 (wagging of C−H2 in the Si-CH2CH2−Si structure) excluded the possibility of ethyl bridge incorporation, while the peak at ∼1412 cm−1 was due to the remaining Si-CH2CH3 groups.38 This can be explained by a similar deposition model as above. To form a carbon-bridge from two TEES molecules, at least one ethyl terminal group needs to connect to another silicon atom or two Si-CH2 fragments need to combine. However, as noted for BTESE deposition, pure precursor fragmentation was almost impossible in the atmospheric plasma afterglow because the main reactive species were the reactive oxygen species. As a result, all the reacted ethyl groups were in oxidized states in the IR spectrum (Figure 5(a, b)). It is not surprising that TEOS coatings were not bridged either, given that there are only ethoxy groups in the molecule. Interestingly, BTESE also exhibited a 4-fold higher deposition rate compared to the TEES and TEOS precursors (Figure 5(c)) due to its bridged structure. This is due to two main reasons: (1) the preserved carbon-bridge increased the probability for the species to be incorporated when on the substrate surface as discussed above; and (2) if the carbonbridge is oxidized during deposition, the two Si atoms linked through the bridge are both activated to form highly reactive species, in contrast to only one Si atom being activated if oxidation occurred at a terminal group. Such activated species had a higher probability of being incorpoated. Carbon-Bridged Coating PropertyHigh Adhesion. One advantage of the carbon-bridged coating was the significantly improved adhesion which has previously been demonstrated for sol−gel coatings.21 The mechanism for adhesion improvement is that carbon-bridges in an inorganic molecular network (e.g., silicate16,21,39 and silicon carbide40,41) can accommodate plastic deformation of the molecular network,41−43 so that when the coating delaminates, a plastic deformation zone forms near the crack tip which consumes energy and increases adhesion. In contrast, a purely inorganic network is usually brittle, and cannot deform plastically near the crack tip, thus exhibiting lower adhesion. The plasticity contribution to the coatings’ high adhesion was verified by its thickness dependencethe adhesion dropped with decreasing coating thickness below a threshold thickness of ∼100 nm.16 This phenomenon has been well understood by the plastic zone size model: The plastic energy dissipation during coating delamination depends on the plastic deformation zone size, which is an intrinsic coating material property in a thick coating, but constrained to be no larger than the coating thickness in a thin coating.41,42,44,45 Details of the adhesion dependence on coating thickness have been discussed in a separate paper.16 Because the intrinsic plastic zone size was ∼100 nm for the current coatings, we deposited coatings of ∼500 nm thickness for all the adhesion tests, to achieve the maximal plasticity effect. Note that the highly cross-linked epoxy (∼1 μm thick) does not produce an appreciable plasticity contribution to the adhesion in the ADCB test because it is separated from the debonding interface by the coating itself, and the plastic energy contribution from the plastic zone formed in the plastic substrate and the coating itself would dominate the measured adhesion values. The adhesion enhancement mechanism of the carbon-bridge explains the increased adhesion with increasing BTESE delivery rate (Figure 6(a)). For the curve of 20 L/min plasma gas flow rate, the jump of adhesion energy from 14.9 to 18.3 J/m2 corresponded to the transition from brittle inorganic silica

Figure 6. (a) Relation between BTESE delivery rate and the adhesion to PMMA at different plasma gas flow rates of 20 and 30 L/min. (b) XPS depth profile of the fractured surfaces of “specimen *” from Figure 6(a) after the ADCB adhesion test. The sputter rate is ∼35 nm/min for the organosilicate coatings.

(Young’s modulus ∼17.0 GPa) to a ductile carbon-bridged coating (Young’s modulus ∼6.0 GPa). A similar trend was observed for the coatings deposited with 30 L/min plasma gas flow rate (Figure 6(a)). The higher plasma gas flow rate diluted the precursor and reactive oxygen species concentration at the same time, with their relative concentration almost unchanged. So the transition from brittle silica to ductile carbon-bridged coatings stays in the same regime of BTESE delivery rate. Note that delamination at the coating/PMMA interface was verified by XPS depth profiling of the fractured surfaces of the highest adhesion specimen (Figure 6(b)). The fractured surface on both sides had a mixed layer of coating and PMMA composition: The PMMA side had ∼6 atm % Si residues, and the coating side had a carbon-rich surface layer, both of which could be sputtered off within 1 min at a sputter rate of ∼35 nm/min for the organosilicate coatings. Other Factors for Highly Adhesive Coating Deposition. Carbon-bridge incorporation significantly increases energy dissipation during coating delamination. However, to achieve even higher adhesion, the substrate surface properties are also important. The PMMA substrate is very sensitive to oxygen atmospheric plasma exposure,46 so special care is needed to protect the PMMA from overoxidation in the initial stage of deposition before a uniform coating has formed and protects the substrate. The amount of plasma exposure is proportional to the reactive oxygen species concentration at the substrate surface times the exposure time, which can be tuned by deposition distance, plasma gas flow rate, and deposition rate. F

DOI: 10.1021/acsami.5b09971 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Controlling the Oxygen Species Concentration at the Substrate Surface: The Effects of Deposition Distance and Plasma Gas Flow Rate. The concentration of the reactive oxygen species at the substrate surface decreased with increasing deposition distance due to reaction consumption and spontaneous decay over the residence time (= deposition distance/flow speed). Under the study conditions, the residence time was a few milliseconds and on the same order of magnitude of the spontaneous decay lifetime of reactive oxygen species.27 As a result, changing the residence time through the deposition distance or the gas flow speed (via plasma gas flow rate) can significantly affect the oxidation status of the PMMA substrate. By gradually increasing the deposition distance, we observed the overoxidized, activated, and deficiently activated PMMA surface properties (Figure 7). At 5 mm deposition distance,

The adhesion to PMMA increased with increasing deposition rate for both TEES and TEOS coatings deposited at a fixed deposition distance (10 mm) and plasma gas flow rate (20 L/ min) (Figure 8). Since the TEES and TEOS coatings are brittle,

Figure 8. Relation between normalized precursor delivery rate and adhesion of TEES and TEOS coatings.

the adhesion increase was purely due to the improved protection of the PMMA surface, while for the BTESE coatings with carbon-bridges, increased adhesion at higher deposition rate under the above plasma condition was also observed, but with enhanced coating plasticity as another contributor in addition to better substrate protection. We also noticed that the TEES coatings exhibited higher adhesion than the TEOS coatings at similar deposition rate (Figure 8), indicating that the ethyl terminal group in the TEES molecule can also improve the bonding between the coating and PMMA. An Application: Moisture Resistive Coatings on PMMA. Conventional silica coatings are significantly weakened in high humidity environments due to the sensitivity of Si−O bonds to moisture attack.47−49 For example, the threshold debond driving force, measured in terms of the applied strain energy release rate, G = GTH, to cause debonding of SiO2 films dropped by ∼50% when the relative humidity was increased from 5 to 95% at 50 °C (Figure 2 of ref 48). Commercial sol− gel polysiloxane coatings also exhibited very low crack growth thresholds, below 3 J/m2, in the relative humidity range from 15 to 95% (Figure 6 of ref 23). In contrast, the carbon-bridged hybrid membranes were reported to exhibit significantly improved hydrothermal stability up to 150 °C in 5% water− n-butanol solution due to the hydrolytically stable Si−C bond.13,22 The carbon-bridged organosilicate coatings on PMMA by atmospheric plasma deposition (at 5.3 mmol/min BTESE delivery rate, 20 L/min plasma gas flow rate, and 10 mm deposition distance) indeed exhibited up to four times improved moisture resistance with GTH values of ∼4 to ∼16 J/m2 when relative humidity increased from 3 to 80% (Figure 9(a)). Another interesting observation is that the debond growth rate curves shifted to higher values of applied strain energy release rate at a given debond velocity when the environmental humidity increased (Figure 9(a)), opposite to the trend observed in conventional silica coatings.48 The plasticizing effect of moisture on PMMA in humid air plays an important role in this trend. PMMA can absorb up to 2 wt % of water50,51 and exhibits increased plastic deformation in the PMMA plastic zone at the debond tip, dissipating more

Figure 7. Effect of deposition distance and plasma gas flow rate on the BTESE coatings’ adhesion to PMMA. The residence time is estimated from (deposition distance)/(plasma gas flow speed).

corresponding to 5.1 ms residence time, the PMMA surface received too much plasma exposure and was overoxidized to form a low molecular-weight weak layer46 that significantly reduced the adhesion. When the deposition distance was increased to 10−15 mm, corresponding to 10.2−15.3 ms residence time, the reactive oxygen species concentration was lower, so that the PMMA surface was not over oxidized but still activated with polar groups,46 which improved the adhesion with the coating. When the deposition distance was further increased to 20 mm, corresponding to 20.4 ms residence time, the activation was deficient and the adhesion began to decrease. A similar trend was observed when the plasma gas flow rate was decreased to 20 L/min (Figure 7). Because the gas flow speed was lower, the adhesion trend shifted to shorter deposition distance. Note that although lower plasma gas flow rate resulted in a higher concentration of reactive oxygen species in the plasma source, this linear effect was minor compared to the exponential decay of reactive oxygen species over residence time in the afterglow region, so the deficient surface activation stage occurred at a similar residence time of ∼23.0 ms. Controlling the Exposure Time: The Effect of Deposition Rate. Exposure time is another important parameter for controlling the plasma exposure to PMMA in the initial stage of deposition before a uniform coating has covered the substrate. The exposure time is inversely proportional to the deposition rate. So increasing the deposition rate can effectively reduce the overoxidation risk for the highly oxygen-sensitive PMMA substrate, thus improving adhesion. G

DOI: 10.1021/acsami.5b09971 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 9. (a) Moisture resistivity of the carbon-bridged organosilicate coatings on PMMA, investigated by the shift of the debond driving energy versus the debond growth rate at different relative humidity at 23 °C. (b) Atomic force microscopy image of the PMMA substrate surface after coating delamination at different humidities. (c) The XPS atomic percentage analysis of the fractured surface on the coating and PMMA substrate sides after the debond growth rate tests. (d) The XPS depth profile of the carbon percentage on the coating side of the fractured surface (sputter rate ∼35 nm/min for the organosilicate coatings).

■

CONCLUSIONS Carbon-bridged organic−inorganic hybrid coatings were successfully deposited on PMMA, using oxidative atmospheric plasma, from the carbon-bridged precursor, BTESE. The coatings exhibited more than double the adhesion compared to plasma silica coatings without carbon-bridges and commercial sol−gel polysiloxane coatings. Under fixed plasma conditions, the two most important deposition parameters for incorporating the carbon-bridges were using a carbon-bridged precursor and maintaining a high precursor delivery rate. Other deposition parameters, such as the deposition distance, the plasma gas flow rate, and the deposition rate to achieve good substrate surface properties for high adhesion were also studied. As an application, the carbon-bridged, highly adhesive coating on PMMA was shown to exhibit superior moisture resistance than commercial sol−gel polysiloxane coatings in humid air.

energy and increasing the adhesion. Increased adhesion of the coating and PMMA substrate enhanced this plasticity contribution, favoring bond rupture and plastic deformation in the PMMA. This was evidenced by a significant increase in the debond surface roughness on the PMMA side from ∼10 to ∼75 nm when the relative humidity increased from 3 to 80% (Figure 9(b)). The carbon percentage of the fractured surface on both the coating and PMMA sides also increased with increasing humidity (Figure 9(c)), another evidence for the increased plastic deformation in PMMA. Because the substrate side had a noticeable amount of Si (∼1−6 atomic %) for all the test specimens (Figure 9(c)) and the coating side had a highcarbon-content surface layer which can be sputtered off within 0.8 min (sputter rate ∼35 nm/min for organosilicate) (Figure 9(d)), the crack path can be determined at the coating/PMMA interface for all the tests, more toward the PMMA side at higher humidity. Incorporating moisture insensitive carbon-bridges and improving the adhesion significantly enhanced the coatings’ resistance to moisture. These initial results are quite promising for hydrothermally stable functional membrane applications. Nevertheless, additional studies, such as long-term storage in humid environment and tests at low temperature where plasticity is limited to some degree, are needed to fully assess the coating’s hydrothermal stability.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b09971. Details of the calculation (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail (Geraud Dubois): [email protected]. H

DOI: 10.1021/acsami.5b09971 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces *E-mail (Reinhold Dauskardt): [email protected].

Deposited Hybrid Silica Membranes with a Controlled Retention of Organic Bridges. J. Mater. Chem. A 2013, 1 (18), 5567−5576. (16) Cui, L.; Lionti, K.; Ranade, A.; Larson-Smith, K.; Dubois, G. J.m.; Dauskardt, R. H. Highly Transparent Multifunctional Bilayer Coatings on Polymers Using Low-Temperature Atmospheric Plasma Deposition. ACS Nano 2014, 8 (7), 7186−7191. (17) Dong, S.; Zhao, Z.; Dauskardt, R. H. Dual Precursor Atmospheric Plasma Deposition of Transparent Bilayer Protective Coatings on Plastics. ACS Appl. Mater. Interfaces 2015, 7, 17929. (18) Rathore, J. S.; Interrante, L. V.; Dubois, G. Ultra Low-k Films Derived from Hyperbranched Polycarbosilanes (HBPCS). Adv. Funct. Mater. 2008, 18 (24), 4022−4028. (19) Oliver, M. S.; Dubois, G.; Sherwood, M.; Gage, D. M.; Dauskardt, R. H. Molecular Origins of the Mechanical Behavior of Hybrid Glasses. Adv. Funct. Mater. 2010, 20 (17), 2884−2892. (20) Dubois, G.; Volksen, W.; Magbitang, T.; Sherwood, M.; Miller, R.; Gage, D.; Dauskardt, R. Superior Mechanical Properties of Dense and Porous Organic/Inorganic Hybrid Thin Films. J. Sol-Gel Sci. Technol. 2008, 48 (1−2), 187−193. (21) Dubois, G.; Volksen, W.; Magbitang, T.; Miller, R. D.; Gage, D. M.; Dauskardt, R. H. Molecular Network Reinforcement of Sol−Gel Glasses. Adv. Mater. (Weinheim, Ger.) 2007, 19 (22), 3989−3994. (22) Matsuda, Y.; Rathore, J. S.; Interrante, L. V.; Dauskardt, R. H.; Dubois, G. Moisture-Insensitive Polycarbosilane Films with Superior Mechanical Properties. ACS Appl. Mater. Interfaces 2012, 4 (5), 2659− 2663. (23) Kamer, A.; Larson-Smith, K.; Pingree, L. S. C.; Dauskardt, R. H. Adhesion and Degradation of Hard Coatings on Poly (methyl methacrylate) Substrates. Thin Solid Films 2011, 519 (6), 1907−1913. (24) Kanninen, M. F. Augmented Double Cantilever Beam Model for Studying Crack-Propagation and Arrest. Int. J. Fract. 1973, 9 (1), 83−92. (25) Suo, Z. G.; Hutchinson, J. W. Sandwich Test Specimens for Measuring Interface Crack Toughness. Mater. Sci. Eng., A 1989, 107, 135−143. (26) Anderson, T. L. Fracture Mechanics: Fundamentals and Applications; CRC Press: Boca Raton, FL, 2005. (27) Jeong, J. Y.; Park, J.; Henins, I.; Babayan, S. E.; Tu, V. J.; Selwyn, G. S.; Ding, G.; Hicks, R. F. Reaction Chemistry in the Afterglow of an Oxygen-Helium, Atmospheric-Pressure Plasma. J. Phys. Chem. A 2000, 104 (34), 8027−8032. (28) Li, G.; Li, H.-P.; Wang, L.-Y.; Wang, S.; Zhao, H.-X.; Sun, W.-T.; Xing, X.-H.; Bao, C.-Y. Genetic Effects of Radio-Frequency, Atmospheric-Pressure Glow Discharges with Helium. Appl. Phys. Lett. 2008, 92 (22), 221504. (29) Walsh, R. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; John Wiley and Sons Ltd: 1989; pp 371−391. (30) Walsh, R. Bond Dissociation Energy Values in SiliconContaining Compounds and Some of Their Implications. Acc. Chem. Res. 1981, 14 (8), 246−252. (31) Yasuda, H. K. Some Important Aspects of Plasma Polymerization. Plasma Processes Polym. 2005, 2 (4), 293−304. (32) Grill, A.; Neumayer, D. A. Structure of Low Dielectric Constant to Extreme Low Dielectric Constant SiCOH Films: Fourier Transform Infrared Spectroscopy Characterization. J. Appl. Phys. (Melville, NY, U. S.) 2003, 94 (10), 6697−6707. (33) Smith, A. L. Infrared Spectra-Atructure Correlations for Organosilicon Compounds. Spectrochim. Acta 1960, 16 (1−2), 87− 105. (34) Qi, H.; Han, J.; Xu, N. Effect of Calcination Temperature on Carbon Dioxide Separation Properties of a Novel Microporous Hybrid Silica Membrane. J. Membr. Sci. 2011, 382 (1−2), 231−237. (35) Wu, H. J.; Interrante, L. V. Preparation of a Polymeric Precursor to Silicon Carbide via Ring-Opening Polymerization: Synthesis of Poly[(methylchlorosilylene)methylene] and Poly(silapropylene). Chem. Mater. 1989, 1 (5), 564−568. (36) Lionti, K.; Volksen, W.; Magbitang, T.; Darnon, M.; Dubois, G. Toward Successful Integration of Porous Low-k Materials: Strategies

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported in part by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Department of Energy, under Contract No. DE-FG02-07ER46391.

■

REFERENCES

(1) Cui, L.; Ranade, A. N.; Matos, M. A.; Pingree, L. S.; Frot, T. J.; Dubois, G.; Dauskardt, R. H. Atmospheric Plasma Deposited Dense Silica Coatings on Plastics. ACS Appl. Mater. Interfaces 2012, 4 (12), 6587−6598. (2) Nowling, G. R.; Yajima, M.; Babayan, S. E.; Moravej, M.; Yang, X.; Hoffman, W.; Hicks, R. F. Chamberless Plasma Deposition of Glass Coatings on Plastic. Plasma Sources Sci. Technol. 2005, 14 (3), 477−484. (3) Watanabe, M.; Cui, L.; Dauskardt, R. H. Atmospheric Plasma Deposition of Transparent Semiconducting ZnO Films on Plastics in Ambient Air. Org. Electron. 2014, 15 (3), 775−784. (4) Guruvenket, S.; Andrie, S.; Simon, M.; Johnson, K. W.; Sailer, R. A. Atmospheric Pressure Plasma CVD of Amorphous Hydrogenated Silicon Carbonitride (a-SiCN:H) Films Using Triethylsilane and Nitrogen. Plasma Processes Polym. 2011, 8 (12), 1126−1136. (5) Nowling, G. R.; Babayan, S. E.; Jankovic, V.; Hicks, R. F. Remote Plasma-Enhanced Chemical Vapour Deposition of Silicon Nitride at Atmospheric Pressure. Plasma Sources Sci. Technol. 2002, 11 (1), 97− 103. (6) Dong, S.; Watanabe, M.; Dauskardt, R. H. Conductive Transparent TiNx/TiO2 Hybrid Films Deposited on Plastics in Air Using Atmospheric Plasma Processing. Adv. Funct. Mater. 2014, 24 (20), 3075−3081. (7) Dams, R.; Vangeneugden, D.; Vanderzande, D. Plasma Deposition of Thiophene Derivatives Under Atmospheric Pressure. Chem. Vap. Deposition 2006, 12 (12), 719−727. (8) Gates, S. M.; Dubois, G.; Ryan, E. T.; Grill, A.; Liu, M.; Gidley, D. Adjusting the Skeleton and Pore Structure of Porous SiCOH Dielectrics. J. Electrochem. Soc. 2009, 156 (10), G156−G162. (9) Volksen, W.; Miller, R. D.; Dubois, G. Geraud Dubois, Low Dielectric Constant Materials. Chem. Rev. 2010, 110, 56−110. (10) Volksen, W.; Magbitang, T. P.; Miller, R. D.; Purushothaman, S.; Cohen, S. A.; Nakagawa, H.; Nobe, Y.; Kokubo, T.; Dubois, G. J. M. A Manufacturing Grade, Porous Oxycarbosilane Spin-On Dielectric Candidate with K ⩽ 2.0. J. Electrochem. Soc. 2011, 158 (7), G155− G161. (11) Volksen, W.; Purushothaman, S.; Darnon, M.; Lofaro, M. F.; Cohen, S. A.; Doyle, J. P.; Fuller, N.; Magbitang, T. P.; Rice, P. M.; Krupp, L. E.; Nakagawa, H.; Nobe, Y.; Kokubo, T.; Dubois, G. J. M. Integration of a Manufacturing Grade, k = 2.0 Spin-On Material in a Single Damascene Structure. ECS J. Solid State Sci. Technol. 2012, 1 (5), N85−N90. (12) Volksen, W.; Lionti, K.; Magbitang, T.; Dubois, G. Hybrid Low Dielectric Constant Thin Films for Microelectronics. Scr. Mater. 2014, 74 (0), 19−24. (13) Castricum, H. L.; Sah, A.; Kreiter, R.; Blank, D. H. A.; Vente, J. F.; ten Elshof, J. E. Hybrid Ceramic Nanosieves: Stabilizing Nanopores with Organic Links. Chem. Commun. (Cambridge, U. K.) 2008, 9, 1103−1105. (14) Castricum, H.; Sah, A.; Geenevasen, J. J.; Kreiter, R.; Blank, D. A.; Vente, J.; ten Elshof, J. Structure of Hybrid Organic−Inorganic Sols for the Preparation of Hydrothermally Stable Membranes. J. Sol-Gel Sci. Technol. 2008, 48 (1−2), 11−17. (15) Ngamou, P. H. T.; Overbeek, J. P.; Kreiter, R.; van Veen, H. M.; Vente, J. F.; Wienk, I. M.; Cuperus, P. F.; Creatore, M. PlasmaI

DOI: 10.1021/acsami.5b09971 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Addressing Plasma Damage. ECS J. Solid State Sci. Technol. 2015, 4 (1), N3071−N3083. (37) Fonseca, J. L. C.; Apperley, D. C.; Badyal, J. P. S. Plasma Polymerization of Tetramethylsilane. Chem. Mater. 1993, 5 (11), 1676−1682. (38) Al-Oweini, R.; El-Rassy, H. Synthesis and Characterization by FTIR Spectroscopy of Silica Aerogels Prepared Using Several Si(OR)4 and R″Si(OR′)3 Precursors. J. Mol. Struct. 2009, 919 (1−3), 140−145. (39) Maidenberg, D. A.; Volksen, W.; Miller, R. D.; Dauskardt, R. H. Toughening of Nanoporous Glasses Using Porogen Residuals. Nat. Mater. 2004, 3 (7), 464−469. (40) Matsuda, Y.; Kim, N.; King, S. W.; Bielefeld, J.; Stebbins, J. F.; Dauskardt, R. H. Tunable Plasticity in Amorphous Silicon Carbide Films. ACS Appl. Mater. Interfaces 2013, 5 (16), 7950−7955. (41) Matsuda, Y.; Ryu, I.; King, S. W.; Bielefeld, J.; Dauskardt, R. H. Toughening Thin-Film Structures with Ceramic-Like Amorphous Silicon Carbide Films. Small 2014, 10 (2), 253−257. (42) Litteken, C.; Dauskardt, R. Adhesion of Polymer Thin-Films and Patterned Lines. Int. J. Fract. 2003, 120 (1−2), 475−485. (43) Rowland, H. D.; King, W. P.; Pethica, J. B.; Cross, G. L. W. Molecular Confinement Accelerates Deformation of Entangled Polymers During Squeeze Flow. Science 2008, 322 (5902), 720−724. (44) Dauskardt, R. H.; Lane, M.; Ma, Q.; Krishna, N. Adhesion and Debonding of Multi-Layer Thin Film Structures. Eng. Fract. Mech. 1998, 61 (1), 141−162. (45) Volinsky, A. A.; Moody, N. R.; Gerberich, W. W. Interfacial Toughness Measurements for Thin Films on Substrates. Acta Mater. 2002, 50 (3), 441−466. (46) Cui, L.; Ranade, A. N.; Matos, M. A.; Dubois, G.; Dauskardt, R. H. Improved Adhesion of Dense Silica Coatings on Polymers by Atmospheric Plasma Pretreatment. ACS Appl. Mater. Interfaces 2013, 5 (17), 8495−8504. (47) Michalske, T. A.; Freiman, S. W. A molecular Interpretation of Stress Corrosion in Silica. Nature 1982, 295, 511−512. (48) Lane, M. W.; Liu, X. H.; Shaw, T. M. Environmental Effects on Cracking and Delamination of Dielectric Films. IEEE Trans. Device Mater. Reliab. 2004, 4 (2), 142−147. (49) Sharratt, B. M.; Wang, L. C.; Dauskardt, R. H. Anomalous Debonding Behavior of a Polymer/Inorganic Interface. Acta Mater. 2007, 55 (10), 3601−3609. (50) Goodelle, J. P.; Pearson, R. A.; Santore, M. M. Water-Uptake Kinetics in Poly(methyl methacrylate) Films with a Fluorescent Rotor Probe. J. Appl. Polym. Sci. 2002, 86 (10), 2463−2471. (51) Kusy, R. P.; Whitley, J. Q.; Kalachandra, S. Mechanical Properties and Interrelationships of Poly(methyl methacrylate) Following Hydration over Saturated Salts. Polymer 2001, 42 (6), 2585−2595. (52) Sigma-Aldrich. 2014 triethoxy(ethyl)silane (TEES) MSDS. (53) Sigma-Aldrich. 2014 tetraethyl orthosilicate (TEOS) MSDS.. (54) Gelest Inc. 1,2-bis(triethoxysilyl)ethane(BTESE) MSDS.

J

DOI: 10.1021/acsami.5b09971 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX