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Nanomechanical and tribological characterization of silk and silk-titanate composite coatings
Journal Pre-proof Nanomechanical and tribological characterization of silk and silk-titanate composite coatings Joseph A. Arsecularatne, Elena Colusso, Enrico Della Gaspera, Alessandro Martucci, Mark J. Hoffman PII:
S0301-679X(20)30039-6
DOI:
https://doi.org/10.1016/j.triboint.2020.106195
Reference:
JTRI 106195
To appear in:
Tribology International
Received Date: 1 November 2019 Revised Date:
28 December 2019
Accepted Date: 14 January 2020
Please cite this article as: Arsecularatne JA, Colusso E, Della Gaspera E, Martucci A, Hoffman MJ, Nanomechanical and tribological characterization of silk and silk-titanate composite coatings, Tribology International (2020), doi: https://doi.org/10.1016/j.triboint.2020.106195. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Author credit statement Joseph Arsecularatne: Conceptualization, Investigation, Validation, Writing original draft preparation. Elena Colusso: Conceptualization, Investigation, Validation, Writing - original draft preparation. Enrico Della Gaspera: Investigation, Validation, Writing - reviewing and editing. Alessandro Martucci: Conceptualization, Resources, Supervision, Writing - reviewing and editing. Mark Hoffman: Conceptualization, Resources, Supervision, Writing - reviewing and editing.
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Nanomechanical and tribological characterization of silk and silk-titanate composite coatings Joseph A Arsecularatnea, Elena Colussob, Enrico Della Gasperac, Alessandro Martuccib, Mark J Hoffmana1 a
School of Materials Science and Engineering, UNSW Sydney, NSW 2052, Australia Dipartimento di Ingegneria Industriale, Università di Padova, Via Marzolo 9, 35131 Padova, Italy c School of Science, RMIT University, Melbourne (VIC) 3001, Australia b
Abstract This paper investigates the tribological and mechanical properties of silk-based nanocomposite coatings which are finding applications in optics, biomedicine and dentistry, thanks to the exceptional mechanical/optical properties and associated biocompatibility of silk. Three different nanocomposite formulations were synthesized, and thin films were prepared by spin coating at different thicknesses and with different post-deposition annealing processes. Ellipsometry, FTIR spectroscopy, AFM, nanoindentation, scratch testing, continuous/reciprocating wear testing, confocal microscopy and SEM were used to characterize the coatings. The results reveal that their hardness and elastic modulus are in the range 0.56 - 1.30 GPa and 23.6 – 55.4 GPa, respectively, which are much higher than those reported for other silk films in literature. Incorporation of titanate nanosheets also improved coatings’ scratch resistance.
1. Introduction Bionanocomposites are attracting interest in numerous fields, especially in biomedicine, because they have structure and properties that mimic the natural materials. A wide range of synthetic nanomaterials (graphene oxide, carbon nanotubes, metallic nanoparticles etc.) can be integrated in the biopolymer phase for improving their properties such as thermal and electrical conductivity, optical properties and mechanical flexibility. This new class of materials find promising application in the fabrication of flexible sensing devices, lightweight structural component, packaging, implantable devices and drug delivery (Xiong et al., 2018). Because of the similarity with the natural tissues such as enamel, polymer-ceramic bionanocomposites have recently been investigated also for application in dentistry (Lee et al., 2016).
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Corresponding author. School of Materials Science and Engineering, UNSW Sydney, Sydney 2052, Australia. Tel: +61 2 9385 4970; Fax: +61 2 9385 6545; E-mail: [email protected]
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Silk fibroin (SF), a natural protein obtained from Bombyx mori cocoons, is an ideal biopolymer matrix because of its outstanding mechanical properties, biocompatibility and biodegradability, associated with the possibility to be processed in a wide range of forms including fibers, sponges and films (Hu et al., 2012; Koh et al., 2015, Omenetto et al., 2010, Kundu et al., 2014). Albeit the enormous work on films and coatings made with silk proteins (Borkner et al., 2014), there are only a few documented studies of the characterization of the mechanical properties of silk-nanoparticles based coatings (Jiang et al., 2007; Kharlampieva et al., 2010; Hu 2013). Mechanical properties of nanocomposites provide interesting indications about the interaction between the nanoparticles (NPs) and the silk matrix. With the change of the SF/NPs ratio, the interaction between the particles and the fibroin chains can alter the mechanical properties of the material. A similar effect can be obtained by variation in the microstructure of the protein promoted by post-treatments that induce transition in secondary structure of the silk (Ebenstein et al., 2005). These microstructures include a non-crystalline (random-coil) phase, a silk II crystalline phase composed of ordered β-sheets and a metastable less-organized crystalline silk I phase. (Ebenstein et al., 2005; Cebe et al., 2017). In this work, we present the mechanical and tribological characterization of silk-titanate composite coatings. Titanate nanosheets (TNSs) are 2D nanosheets made of a substochiometric titania that have been already integrated in silk matrix to generate nanocomposites materials with tunable optical properties and processability in the micro- and nanoscale (Perotto et al, 2015). Post-deposition processing treatments (soaking in methanol, temperature treatment and ion exchange followed by UV curing) were investigated to correlate the process approach to the mechanical properties of the coatings. Methanol annealing is an efficient way to crystallize fibroin protein in β-sheets, following the random coil to β-form transition, making the silk structure stable in water (Tsukada et al., 1994). The treatment has also an effect on the nanocomposite, by inducing a densification of the TNSs through dehydration and partial deintercalation of the tetramethylammonium ions (Perotto et al., 2015). On the other side, temperature, ion exchange and UV light can be used for TNSs post-processing, increasing the densification of the small clusters (Colusso et al., 2019). We chose to perform a cation exchange with silver ions followed by UV curing because we recently demonstrated that in this way it is possible to synthesize in loco silver nanoparticles (Colusso et al., 2019). This could add further antibacterial properties to the coatings, interesting for application in the biomedical field and dentistry (Fei et al., 2013; Li et al. 2011; Cheng et al, 2012; Zhou et al 2017). Accordingly, with a view to investigate the applications of these coatings in dentistry, the present work also includes comparisons of coatings’ nanomechanical and tribological properties with those of current resin based dental composite (restorative) materials.
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2. Methods Synthesis of Silk-Titanates nanocomposite solution Silk fibroin (SF) solution and TNSs water dispersion were prepared according to the protocol previously reported in literature (Perotto et al, 2015). SF-TNSs nanocomposite solution was synthesized by gently mixing the stock TNSs solution (20% w/v) with the SF solution (5% w/v) in an appropriate ratio. Three different formulations were selected: 100:0 (SF), 80:20 and 50:50 %w/v SF-TNSs. Coating preparation Thin films were prepared by spin coating (SCS G3P-8 spin coater, Cookson Electronics) on alumina slides or silicon wafers, at different speed to tune the final thickness of the samples (1000 or 3000 rpm) for 60 s. Alumina was selected as a substrate material because of its higher elastic modulus and hardness and hence greater resistance to elastic/plastic deformation during nanoindentation tests. Silicon was selected as a substrate because it is ideal for ellipsometry measurements thanks to its reflectivity and low surface roughness. Before coating, substrates were thoroughly cleaned by sonication in acetone, followed by treatment in a hot (60°C) basic piranha solution (H2O:H2O2:NH3 5:3:1 v/v) for 20 minutes, then rinsed with water and finally dried in air. After the deposition, films were dried at 60°C on a hot plate for 4 minutes. Different post-deposition annealing processes were performed on the samples: methanol, temperature, ion exchange and UV light. Methanol annealing (MeOH) was performed by soaking the samples in methanol (HPLC grade 99.9%, SigmaAldrich) for 30 min. Temperature treatment (TT) was effectuated by annealing the sample in an oven at 200° C for 1h in air. Ion exchange was performed by dipping the samples in a 0.1 M silver nitrate (AgNO3, Sigma Aldrich) aqueous solution for 6 min, followed by UV irradiation for 30 min with a lamp (UVL-56 365 nm) to nucleate silver nanoparticles (Colusso et al., 2018). Table 1 reports the different compositions tested in this work. Table1. Nominal composition, spin coating parameters and post deposition treatments of the samples investigated. Sample composition
Spin coating parameters
Post deposition treatments
SF1000/MeOH
Silk Fibroin
1000 rpm x 60 sec
Methanol annealing
SF3000/MeOH
Silk Fibroin
3000 rpm x 60 sec
Methanol annealing
SF1000/Ag
Silk fibroin
1000 rpm x 60 sec
Methanol annealing followed by ion exchange in 1M AgNO3 solution and UV irradiation
SF3000/Ag
Silk fibroin
3000 rpm x 60 sec
Methanol annealing followed by ion exchange in 1M AgNO3 solution and UV irradiation
SF1000/TT
Silk fibroin
1000 rpm x 60 sec
Methanol annealing followed by thermal treatment at 200°C
Acronym
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Silk fibroin
3000 rpm x 60 sec
Methanol annealing followed by thermal treatment at 200°C
SF-20TNSs1000/MeOH
Nanocomposite with nominal concentration 80:20% wt SF/TNS
1000 rpm x 60 sec
Methanol annealing
SF-20TNSs3000/MeOH
Nanocomposite with nominal composition 80:20% wt SF/TNS
3000 rpm x 60 sec
Methanol annealing
SF-20TNSs1000/Ag
Nanocomposite with nominal composition 80:20% wt SF/TNS
1000 rpm x 60 sec
Methanol annealing followed by ion exchange in 1M AgNO3 solution and UV irradiation
SF-20TNSs3000/Ag
Nanocomposite with nominal composition 80:20% wt SF/TNS
3000 rpm x 60 sec
Methanol annealing followed by ion exchange in 1M AgNO3 solution and UV irradiation
SF-20TNSs1000/TT
Nanocomposite with nominal composition 80:20% wt SF/TNS
1000 rpm x 60 sec
Methanol annealing followed by thermal treatment at 200°C
SF-20TNSs3000/TT
Nanocomposite with nominal composition 80:20% wt SF/TNS
3000 rpm x 60 sec
Methanol annealing followed by thermal treatment at 200°C
SF-50TNSs1000/MeOH
Nanocomposite with nominal composition 50:50% wt SF/TNS
1000 rpm x 60 sec
Methanol annealing
SF-50TNSs3000/MeOH
Nanocomposite with nominal composition 50:50% wt SF/TNS
3000 rpm x 60 sec
Methanol annealing
SF-50TNSs1000/Ag
Nanocomposite with nominal composition 50:50% wt SF/TNS
1000 rpm x 60 sec
Methanol annealing followed by ion exchange in 1M AgNO3 solution and UV irradiation
SF-50TNSs3000/Ag
Nanocomposite with nominal composition 50:50% wt SF/TNS
3000 rpm x 60 sec
Methanol annealing followed by ion exchange in 1M AgNO3 solution and UV irradiation
SF-50TNSs1000/TT
Nanocomposite with nominal composition 50:50% wt SF/TNS
1000 rpm x 60 sec
Methanol annealing followed by thermal treatment at 200°C
SF-50TNSs3000/TT
Nanocomposite with nominal composition 50:50% wt SF/TNS
3000 rpm x 60 sec
Methanol annealing followed by thermal treatment at 200°C
SF3000/TT
Characterization of coatings The thickness of the coatings was determined using a VASE ellipsometer (J.A. Woollam Co., NE, USA) at angles of incidence of 65 - 75° on 300–1000 nm wavelength range. Thickness measurements were made on three samples for each type of coating deposited on silicon substrate. The materials were modelled by using a Cauchy dispersion with Urbach absorption. Infrared spectra of the nanocomposite coatings were obtained by FTIR spectroscopy (Jasco 620 FTIR spectrometer). Absorption spectra were determined for thin films deposited on silicon wafers following different annealing on 4000–400 cm−1 range. The spectra given were obtained following subtraction of the spectrum of the silicon substrate.
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An atomic force microscope (Park Systems XE-70) was used to determine the surface roughness of the nanocomposite coatings deposited on silicon. The morphology of the films deposited on alumina after wear test was inspected by Scanning Electron Microscope (FEI Verios 460L SEM). The samples were coated with a 5 nm thick platinum layer before SEM inspection. Nanoindentation and wear testing Nanoindentation was used to measure the mechanical properties, specifically, hardness and elastic modulus of films consisting of different formulations. Their wear behavior was studied using continuous and reciprocating wear tests. The adhesion and scratch resistance of the coating was assessed using scratch testing. All these tests were performed on films deposited on alumina slides. Nanoindentation Force-controlled nanoindentation with a Hysitron TriboIndenter (Hysitron Inc., Minneapolis, USA) was used to measure the indentation hardness (H) and reduced elastic modulus (Er) of silk/silk-titanate coatings. It was fitted with a Berkovich diamond tip with radius of curvature approximately 100 nm. A detailed account of the procedure was given by Oliver and Pharr (Oliver and Pharr, 1992). The required tests were performed under ambient conditions. For each coating, at 100 µN maximum load, two sets of 9 indents at two different locations and separated by roughly 10 mm were carried out. These indents at two different locations were performed under identical conditions to observe any variation of nanomechanical properties over a coating. For some of the selected coatings, indents at 50, 250 and 500 µN maximum loads were also made to observe any variation of nanomechanical properties with indentation load. In order to minimize any possible influence of the mechanical properties of the substrate material on the measured nanomechanical properties of a coating, an attempt was made to control the maximum penetration depth to less than 30% of the thickness of a coating (Dayal et al., 2009). Wear testing: reciprocating Since the present study aims to investigate the applications of SF-TNSs coatings in dentistry, reciprocating wear tests were given priority. However, considering the large number of tests2 possible and to keep the experiments to a manageable number, specimens/conditions for testing were carefully selected to obtain representative results. For the present study, a total of 175 reciprocating wear tests, including 15 preliminary tests (for determining the suitable contact loads, number of cycles, etc) were carried out.
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There are 352 test parameter combinations from: 3 coating compositions; 3 post treatment annealing processes; 2 coating thicknesses; 2 contact loads; 7 lubricated conditions (including the dry condition). For 2 – 3 tests under each parameter combination, the total number of reciprocating wear tests are 704 – 1056.
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The wear tests were performed using a tribometer fitted with a reciprocating module (CSEM Instruments, Peseux, Switzerland) with 6 mm diameter ruby ball antagonist under dry conditions or with a lubricant such as deionized water, artificial saliva (Duffo and Castillo, 2004), or a solution of ethanol and water containing 10% ethanol, 50% ethanol or 100% ethanol. In order to observe the tribological behavior of these coatings in an acidic environment, eight wear tests (four tests on a 450 – 500 nm series sample (SF50TNSs1000/TT) and another four tests on a 150 – 200 nm series sample (SF3000/MeOH)) were carried out using acetic acid solution (pH 3.2) as lubricant. Since preliminary tests revealed wearing out of the coatings under higher loads and higher cycles, loading was restricted to 1 – 2 N and the number of cycles to 100. Maximum sliding speed was 0.67 cm/s and the sliding distance (stroke) 2 mm. Each selected lubricant (at room temperature) was injected to the wear surfaces by a hypodermic needle at flow rate of 90 ml/h with an infusion pump (Imed Gemini PC-1, San Diego, USA). For consistency, the lubricant was not recycled during a test and the coefficient of friction was logged throughout. For each test condition, 2-3 wear tests were carried out. Following wear tests, the width and cross-sectional area of each wear scar were measured at six different locations using a Keyence laser confocal scanning microscope (VK-X200, Keyence corporation, Osaka, Japan) and accompanying software. The wear depth for each location was determined by dividing the cross-sectional area by the corresponding width. Wear testing: continuous (pin-on-disk) These wear tests were performed using a pin-on-disk tribometer (CSEM Instruments, Peseux, Switzerland) with a 6 mm diameter ruby ball antagonist under dry conditions with loads in the range 1 – 2 N and the cycles in the range 250 - 500. The sliding speed was 0.67 cm/s and the diameter of a circular wear track was in the range 4 – 10 mm. Under each test condition, 2 - 3 wear tests were carried out. Following wear tests, the width and cross-sectional area of each wear scar were measured at six different locations as discussed above and the corresponding wear depth calculated. Scratch testing These tests were carried out on a scratch tester (Revetest, CSM Instruments, Peseux, Switzerland) according to the procedure outlined in (Zavgorodniy et al., 2011). The normal load applied to a diamond tip (200 µm radius, Rockwell C) was raised from 1 to 6 N at a rate of 10 N/min. The length of a scratch was 5 mm and the sliding speed was 10 mm/min. Altogether, six different silk/silk-titanate coatings were tested with 2 – 3 scratch tests were made on each coating. Laser scanning confocal microscopy imaging was used to identify any mechanical processes that would occur along a scratch line, e.g., onset of detachment of a coating from the substrate.
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3. Results Characterization of silk-TNSs composite thin films Silk-TNSs composite coatings were fabricated by spin coating starting from a water dispersion of TNSs and SF in water. Both TNSs and SF were processed following the standard protocol reported in literature (Perotto et al., 2015) and detailed in the Methods. For each composition we tested two different spin speeds in order to tune the final thickness of the coatings. As measured by ellipsometry, we obtained two series of samples with different range of thickness, 150-200 nm and 450-500 nm, respectively for 3000 and 1000 rpm. For coatings fabricated at 3000 and 1000 rpm, following MeOH annealing, we observed increases in thickness of approximately 40 and 120 nm respectively, for an increase of TNSs up to 50 %wt. More details are reported in Table S1 in the supporting Information. The measured surface roughness (root-mean-square, rms) of the coatings deposited on silicon after methanol annealing was found to be very low for all the compositions (<1 nm, see Table S1, S.I.). No significant effects of the treatments were observed, except for the samples exposed to ion exchange with Ag followed by UV annealing. For these samples we measured an increase of rms (root-mean-square) roughness, with a maximum of about 9 nm in the case of the SF-50TNSs one. We also characterized the secondary structure of the silk fibroin protein after the postprocessing by observing the variation of the amide I band in the FTIR spectra of the samples (see Figure S2, S.I.). In short, we considered a strong peak in the range 1616-1637 cm-1 a marker of silk II conformation (β-sheets), a broad peak centered at 1640-1647 cm−1 indication of a random-coil structure, while a peak centered at 1650 cm-1 indication of α-helix (Cebe et al., 2017; Jin H.J. et al, 2005; Wilson, D. et al, 2000). After MeOH SF coatings showed a predominant of helix indicated by a peak at 1650 cm-1, and a second peak at 1630 cm-1 representative of a silk I structure. TT does not affect significantly the FTIR spectrum, while Ag+UV induced a complete crystallization in silk II. Considering samples with different TNSs amount, some variations were observed in the case of the MeOH. A broader band around 3500 cm-1 is visible in SF-20TNSs/MeOH and SF50TNSs/MeOH related to OH- vibration, not present in SF. This is likely due to the higher amount of water in the samples, as already verified by thermal gravimetric analysis (Perotto et al., 2015). On the contrary, a sharp peak at 3295 cm−1 (NH stretching vibration) is visible for all samples suggesting strong intermolecular hydrogen bonding. We also observed a decrease of the band at 1632 cm-1 in the case of SF-20TNSs/MeOH compared to the SF/MeOH, that increased again for the SF-50TNSs/MeOH. This means that the presence of 20%wt TNSs reduced the formation of ordered β-sheet structures (silk II). TT and Ag+UV induced an increase of band at 1632 cm-1 for all the SF-TNSs compositions, with a stronger effect in the case of SF-50TNS/Ag that showed a band corresponded to a silk II structure.
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Nanoindentation Figure 1 depicts representative load (P) displacement (h) curves obtained by indenting to a maximum load of 100 µN for coatings with different percentages of TNSs (450-500 nm series) and post deposition annealing treatments. These P – h curves show considerable variation in the maximum indentation depth with coatings containing 0 or 20% TNSs and MeOH treatment revealing higher depths than other coatings. Conversely, coatings containing 50%TNSs with Ag or TT treatments show lowest indentation depths. Creep (which is revealed as an increase in indentation depth at constant maximum load) is seen to occur for all tested coatings with lower creep noted for the coating with 50% TNSs and TT treatment. A comparison of nanoindentation mechanical properties (average H and Er) obtained under 50 and 100 µN indentation loads revealed greater variability of results for 50 µN load. This is most likely due to the influence of alumina substrates surface roughness on measured H and Er under such low load. Since the measured roughness of the coatings was very low in the case of silicon substrates, the larger variability observed is probably due to the alumina substrate that present a micro-rugosity (see Figure S3(a)). To avoid this issue only nanoindentation mechanical properties obtained under 100 µN load are considered below. The measured nanoindentation hardness of silk-titanate coatings ranged from 0.56 to 1.30 GPa and the elastic modulus from 23.6 to 55.4 GPa (Fig.2a and 2b). In general, measured H and Er show an increase for Ag+UV and TT compared to the MeOH samples. This increase in mechanical properties is more pronounced for coatings that contained a higher fraction of TNS e.g., 50% TNS.
Fig.1. Representative nanoindentation load–displacement (P–h) curves for coatings with different percentage of TNSs (450 – 500 nm series).
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Fig.2. Nanoindentation mechanical properties for coatings with different percentage of TNSs (450-500 nm series) and treatments: methanol aneling (MeOH), ion exchange and UV (Ag) and thermal treatment (TT). (a) hardness, H; (b) reduced elastic modulus, Er
Wear depth The measured wear depth for tests without a lubricant are shown in Fig.3a. The coatings that revealed a higher hardness and elastic modulus during nanoindentation (Fig.1), e.g., those that contained 20 – 50% TNS with TT treatment showed a higher wear depth indicating a lower wear resistance of the coating. This trend, however, was not reflected in the measured coefficient of friction (Fig.3b). Surprisingly, some of the coatings that showed the highest wear depth exhibited lowest coefficient of friction. The possible reasons for this will be considered in the Discussion section of the paper. Additionally, wear depth and coefficient of friction data for specimens containing 50% TNSs with MeOH and Ag treatments are not given in Fig.3 since these tests were not carried out because of the large number of tests required to obtain a complete set of data. It is however possible to estimate these values as will be considered in the Discussion section. During wear tests with a lubricant containing water (including acetic acid at pH 3.2), it was not possible to measure the wear depth for most of the coatings in Fig.3 above due to swelling of the coating adjacent to the wear scar. Water seemed to have seeped into the coating in that region causing its swelling (Fig.4). Occurrence of the swelling and its extent depended on four parameters: coating composition; lubricant composition; coating thickness; postprocessing treatment. Of the 450 – 500 nm series coatings, those containing > 20% TNSs and
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received TT treatment resisted swelling. Although the coating consisting of bare SF and received TT treatment showed some swelling (Fig.4b), it was smaller compared to the coatings that received other secondary treatments (Fig.4c). All thinner coatings (150-200 nm series) which received TT treatment resisted swelling possibly due to greater densification induced by thermal treatment. Moreover, none of the tested coatings swelled when 100% ethanol was used as lubricant. A higher wear resistance was also observed with 100% ethanol compared to all other lubricants tested (Fig.5a). With decrease in wear, coefficient of friction also decreased (Fig.5b). SEM analysis of the wear scar revealed crack formation and partial delamination of the coating which were more pronounced with DI water and artificial saliva lubricants compared to ethanol (Figure S3, S.I.). Additionally, the wear depth measured for the SF-50TNSs1000/TT coating following tests with acetic acid lubricant was in the same range as those measured for other water-based lubricants.
Fig.3. Measured wear depth (a) and coefficient of friction (b) from reciprocating wear tests without lubricant for coatings with different percentage of TNSs (450-500 nm series) and treatments: methanol annealing (MeOH), ion exchange and UV (Ag) and thermal treatment (TT).
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(a) SF3000/TT
(b) SF1000/TT
(c) SF1000/Ag
Fig.4. Typical wear scars (arrowed) on 100% silk specimens wear tested with DI water lubricant and swelling of wear scar sides (arrow heads): no noticeable swelling (a); moderate (b) and large (c) swellings.
Fig.5. Measured wear depth (a) and coefficient of friction (b) for different lubricants using reciprocating wear tests for 100% silk coating (150-200 nm series) A comparison of the specific wear depth for continuous and reciprocating sliding reveals that the wear depth measured in reciprocating sliding is 2 – 4 times greater than that for continuous sliding (Fig.6a). However, the measured coefficient of friction is much higher in continuous sliding compared to reciprocating sliding (Fig.6b). Additionally, coatings with TNSs seem to show lower coefficient of friction (compared to those for coatings without
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TNSs) in continuous/reciprocating sliding. Typical coefficient of friction curves obtained during continuous sliding are shown in Fig.6c. For coatings without TNSs, steady state coefficient of friction is higher than the initial value. However, for the coating that contained TNSs, steady state coefficient of friction is lower than the initial value.
Fig.6. Comparison of measured specific wear depth (a) and coefficient of friction (b) obtained using reciprocating and continuous wear tests without lubricant for different coatings (450-
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500 nm thickness); typical coefficient of friction versus number of cycles curves obtained during continuous wear tests (c).
Scratch testing Laser scanning confocal microscopy analysis of the tested coatings revealed a smoothing of the coating surface due to the scratches (Fig.7a). No indication of the detachment of the coating from substrate, even under highest load, was noted (Fig.7b). Moreover, friction force signal did not show a transition indicating coating detachment (Fig.7c). The scratch width (a measure of scratch resistance) for coatings with MeOH and Ag+UV treatments revealed slightly lower scratch resistance of 100% silk coatings compared to that of composite coatings containing 50% silk and 50% titanates (Fig.8). The scratch resistance of 100% silk coatings with TT treatment is higher and similar to those of the above composite coatings.
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Fig.7. A region of scratch (arrowed) corresponding to the highest applied load and measured forces (SF-50TNSs3000/TT): (a) 3D topography; (b) scratch and surface topography; (c) normal force Fn and tangential force Ft curves
Fig.8. Comparison of measured scratch width for different coatings (450-500 nm thickness)
4. Discussion The present work investigated the nanomechanical and tribological behavior of silk-titanate composite coatings having three different compositions and post deposition annealing processes. The authors’ previous work with these nanocomposite films prepared using a method similar to the one used in the present study has revealed reasonable dispersion of TNSs in SF matrix (Perotto et al, 2015; Magrì et al, 2018). Moreover, for a silk-titanate nanocomposite coating fabricated using the present method, a cross section of coatingsubstrate obtained by cryofracture was imaged using SEM in the authors’ previous work (Colusso et al, 2017) and revealed good coating adhesion to the substrate. According to the nanomechanical data, it appears that the increase of TNSs concentration improves both the stiffness and the hardness of the coating material compared to the bare silk fibroin. This effect is clear in the case of high filling fraction of TNSs (50 %wt), while the SF20TNSs composition did not induce a significant improvement in the mechanical properties.
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This can be related to the composite structure obtained during the self-assembly of the material. As shown by FTIR spectrum, in the case of SF-20TNSs/MeOH the peak related to a crystalline silk II conformation is lower compared to the SF-50TNSs/MeOH. The maximum H and Er are obtained for samples with 20-50 TNSs and combined treatments, that correspond to compositions with higher crystalline phase (β-sheet) from spectroscopic data. Previous studies have shown that mechanical strength and toughness of silks are controlled by β-sheets (Keten et al., 2010; Koh et al, 2015). In addition, TT and Ag+UV treatments induced a densification of the TNSs by deintercalation of the ions and collapse of the layered structure (Colusso et al., 2019). The plots of nanoindentation hardness (H) and elastic modulus (Er) results against contact depth for all silk and silk-titanate coatings (not shown in the paper) revealed an indentation size effect, that is, H and Er decreased with increasing indentation depth. However, they did not reveal any influence of the substrate in that the curves showed a continuous decrease with increasing depth for 100 µN load and in particular, all hardness values were clearly on single curve with very little scatter. The maximum depth obtained under this load was 63 nm which is ~ 14% of coating thickness. Additional tests carried out at higher indentation loads of 250 and 500 µN (with the corresponding contact depth above 96 nm which is ~ 21% of coating thickness) revealed the influence of substrate in that corresponding H and Er did not follow the aforementioned decreasing trend (with increasing contact depth) but were well above the curves. Additionally, creep occurred for all tested coatings with lower creep noted for the coating with 50% TNSs and TT treatment which is due to increased reinforcement by higher TNS content and greater densification by TT treatment. The increase in indentation depth due to creep can result in a reduction in measured hardness (Chudoba and Richter, 2001). Additionally, the measured nanoindentation H and Er for present silk and silk-titanate coatings (under 100 µN indentation load) were in the range 0.56 - 1.30 GPa and 23.6 – 55.4 GPa, respectively. These H and Er values are comparable to those measured for current dental composites consisting of a resin matrix and micro-/nano-scale glass/silica particle reinforcements (Arsecularatne et al., 2016; El-Safty et al., 2012). A previous experimental study has reported, for a composite graphene-silk film (3 - 4 mm thick) containing 0.5 wt% graphene, relatively higher H (0.12 GPa) and Er (1.9 GPa) compared to the pure silk film (H, 0.053 and Er, 0.33 GPa) (Wang et al., 2017). With further increase in graphene, elastic modulus decreased. Much higher nanomechanical properties measured for the present thin silk and silk-titanate composite coatings appears to be due to greater densification and crystallization associated with the post treatments. During wear tests with lubricants that contained water, swelling of a coating at the sides of the wear scar was observed for some of the tested coatings. It was hypothesised that: (i) smaller water molecules could seep through the microcracks generated during coating wear and caused the swelling; (ii) a liquid containing relatively larger molecules may not seep through these microcracks. To test this hypothesis, wear tests were carried out with lubricants containing varying amounts of ethanol. The wear results obtained with 100% ethanol lubricant supports the above hypothesis since none of the tested coatings swelled. This may
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be attributed to the relatively larger ethanol molecule (compared to the water molecule) which did not seep through the microcracks. However, further studies are required to clarify the interactions between coatings and liquid lubricants during sliding contact. Greater densification and crystallization of the present silk-titanate composite coatings also explain the increased resistance of some of the coatings to swelling during the wear tests with lubricants such as DI water and artificial saliva. Wear tests carried out with such coating revealed lower wear when the lubricant contained more than 50% ethanol and the lowest wear for 100% ethanol lubricant. Measured low wear also corresponded to low friction. Additionally, the wear behavior with acetic acid (at pH 3.2) which was found to be similar to that with other water-based lubricants reveals that the SF-TNSs coatings tested in the present work are resistant to food acids. The wear tests carried out without a lubricant revealed that incorporation of TNS reduced friction during wear. However, a decrease in friction did not necessarily result in decrease in wear particularly, in composite coatings containing 20 – 50% TNSs with TT treatment. These may be attributed to silk-TNSs interface debonding and crack formation during contact sliding (which will increase wear (Kruzic et al., 2018)) and some of the accumulated wear debris acting as miniature rollers (which will decrease friction at the sliding interface (Ma et al., 2013; Zanoria et al., 1995)). To confirm these, further research aimed at revealing the dominant wear mechanism is required. As noted in the Results section of this paper, wear depth and coefficient of friction data for specimens containing 50% TNSs with MeOH and Ag treatments were not given in Fig.3 since these tests were not carried out because of the large number of tests involved. However, from the data in Fig.3, particularly those for 20%TNSs and that for 50%TNSs, it is possible to estimate the average wear depth and the corresponding coefficient of friction values for coatings containing 50% TNSs with MeOH and Ag treatments which are given in Table 2. Table 2. Estimated wear depth and coefficient of friction for coatings containing 50% TNSs with MeOH and Ag treatments Treatment
MeOH
MeOH
Ag
Ag
Contact load (N)
1
2
1
2
Wear depth (µm)
0.40
0.41
0.36
0.39
Coefficient of friction
0.051
0.052
0.053
0.053
It can be seen that the wear depth values in Table 2 are lower than those for the coating with similar TNSs content (with TT treatment). Conversely, they are higher than those for the coatings with similar treatments but lower (20%) TNSs content. However, the coefficient of friction results can be seen to follow the opposite trend in that the estimated values are higher than those for the coating with similar TNSs content (with TT treatment). Conversely, they are lower than those for the coatings with similar treatments but lower (20%) TNSs content.
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Compared to reciprocating wear tests, a higher wear resistance of the silk-titanate coatings in continuous (pin-on-disc) wear tests was observed. The measured high wear during former seems to indicate that the tested coatings are less resistant to mechanical fatigue caused by fluctuating stresses (Kruzic et al., 2018) which occur more frequently during reciprocating sliding than during continuous sliding. In the present study, the average specific wear depth measured during continuous tests is in the range 1.36 – 1.67 µm per N per 1000 cycles. In comparison, the wear depth reported for nine commercial resin based composite materials was in the range 0.016 – 0.07 µm per N per 1000 cycles (Cha et al., 2004). This reveals that the wear resistance of present silk and silktitanate composite coatings is much lower than that of commercial dental composite materials. Present coatings with hardness less than 5 GPa (Fig. 2a) can be considered as soft coatings (Bull, 1991; Bull and Berasetegui, 2006). During scratch testing, with increasing load, the soft coating is progressively plastically deformed and its detachment from the substrate is initiated at a critical load. This coating detachment may be identified from optical/electron microscopy observations of a scratch and/or from a transition in the friction force curve i.e., a sudden rise in friction force. As noted in Section 3, it was not possible to identify the location/load of onset of silk-TNS coating detachment following the current scratch tests. Loads higher than 6 N were not considered for scratch tests because of the relatively thin alumina substrate (0.5 mm thickness) and the possibility of its failure under higher loads. The difficulty in identifying the location/load for onset of coating detachment in the present scratch tests may also indicate strong adhesion of silk and silk-titanate coatings to the substrate. Using the same scratch equipment and test parameters, Zavgorodniy et al (Zavgorodniy et al., 2011) obtained the onset of failure of hydroxyapatite and monetite coatings at contact loads of 1.5 and 5.4 N, respectively. The corresponding critical shearing stress for the coating detachment estimated using the equation of Benjamin and Weaver (Benjamin and Weaver, 1960) was 44 MPa for hydroxyapatite and 84 MPa for monetite. Based on these results it appears that the critical shearing stress for the present silk and silk-titanate coatings is higher than 84 MPa. To confirm this, scratch testing under higher loads is required. The present investigation has revealed that the tested silk and silk-titanate coatings possess nanomechanical properties that are similar to those of resin based dental composites. In addition, tested coatings seem to indicate good adhesion to the ceramic substrate. Moreover, silk/silk-titanate composite coatings that received TT treatment resisted swelling during sliding contact in the presence of a lubricant containing water. However, the wear resistance of the present silk/silk-titanate composite coatings is much lower than that of the current dental composites. Since silk/silk-titanate coatings are biocompatible, further development of these coatings with improved tribological properties will enable their wider application such as in dental implants/protheses and hip/knee joints.
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5. Conclusions Based on the present experimental results and analyses discussed above, following conclusions can be drawn: •
•
•
• •
Measured nanoindentation hardness and elastic modulus of silk and silk-titanate composite coatings were in the range 0.56 - 1.30 GPa and 23.6 – 55.4 GPa, respectively. These H and Er values are much higher than those reported for graphenesilk composite films but comparable to those measured for resin based dental composites. Wear tests on these coatings revealed a 2 – 4 times higher wear depth during reciprocating sliding compared to continuous sliding. This indicates that the coatings possess greater wear resistance during continuous sliding. However, their wear resistance is much lower than that of resin based dental composites. While some of the tested coatings swelled under contact sliding when water was present in the lubricant, others did not. Of the 450 – 500 nm series, coatings containing TNS and received TT treatment resisted swelling. Of the 150 – 200 nm series, all coatings with TT treatment resisted swelling. Silk and silk-TNS coatings revealed good adhesion to the alumina substrate. No coating failure was observed under tested conditions. Scratch resistance of coatings that received TT treatment was higher. Incorporation of TNSs also improved the scratch resistance of a coating.
The present study has revealed high-performance nanocomposite coatings based on inorganic fillers (titanate nanosheets) incorporated within a silk-fibroin matrix. This work highlights the effect of the nanofiller concentration and the control of the secondary structure of the protein on the improvement of the mechanical properties and paves the way for further studies on biomimetic materials based on biopolymers and 2D nanoparticles. Based on the results of the present study, reinforced SF-50TNSs with TT treatment are recommended for applications that require hard and scratch resistant coatings while unreinforced SF coatings with TT treatment are recommended for applications that require wear resistance.
Acknowledgements Alessandro Martucci thanks BIRD 2017 program promoted by the University of Padova, prot. BIRD177407 and RMIT Philanthropic Fund through an International Visiting Fellowship for financial support, Enrico Della Gaspera thanks RMIT University for funding through the Vice Chancellor’s Fellowship scheme.
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Supporting Information
Table S1. Thickness, refractive index and root-mean-square roughness (rms) of nanocomposite coatings deposit on silicon substrates measured by ellipsometry and AFM. Thickness /nm
Refractive index @ 500 nm
SF1000/MeOH
370 ± 5
1.5612 ± 0.0043
SF3000/MeOH
170 ± 12
1.5633 ± 0.0013
SF1000/Ag
384 ± 14
1.5780 ± 0.0041
SF3000/Ag
156 ± 3
1.5914 ± 0.0275
SF1000/TT
382 ± 1
1.5591 ± 0.0002
SF3000/TT
185 ± 30
1.5645 ± 0.0034
SF-20TNSs1000/MeOH
479 ± 39
1.6209 ± 0.0353
SF-20TNSs3000/MeOH
179 ± 30
1.6521 ± 0.0028
SF-20TNSs1000/Ag
444 ± 27
1.6869 ± 0.0693
SF-20TNSs3000/Ag
158 ± 19
1.6581 ± 0.0424
SF-20TNSs1000/TT
440 ± 32
1.6353 ± 0.0609
SF-20TNSs3000/TT
165 ± 6
1.6163 ± 0.001
SF-50TNSs1000/MeOH
494 ± 8
1.7367 ± 0.0424
SF-50TNSs3000/MeOH
208 ± 7
1.6961 ± 0.0058
SF-50TNSs1000/Ag
410 ± 30
1.7874 ± 0.0024
SF-50TNSs3000/Ag
173 ± 7
1.7115 ± 0.0028
SF-50TNSs1000/TT
456 ± 80
1.77275 ± 0.0146
SF-50TNSs3000/TT
150 ± 22
1.7124 ± 0.0868
Sample
rms /nm
0.472
4.35
0.357
0.832
1.705
0.743
0.791
9.854
0.541
24
Figure S2. Representative FTIR spectra of silk-TNSs coating nanocomposite of (a) SF3000, (b) SF-20TNSs3000 and (c) SF-50TNSs3000 samples.
25
Figure S3. SEM micrographs of SF3000/TT as prepared (a) and after the wear test performed with different lubricants: (b) DI water, (c) artificial saliva, (d) 10% ethanol and (e) 100% ethanol.
Highlights • Presents a nanomechanical and tribological characterization of nanocomposite coatings made of silk fibroin and titanates nanosheets
• Results are presented for three different formulations of nanocomposite coatings with different post-deposition annealing processes
• It is revealed that the measured nanomechanical properties are much higher than those •
reported for graphene-silk composite films in the literature Incorporation of titanates nanosheets improved the scratch resistance of a composite coating
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. They also declare that they have approved the final article for submission. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0301-679X(20)30039-6
DOI:
https://doi.org/10.1016/j.triboint.2020.106195
Reference:
JTRI 106195
To appear in:
Tribology International
Received Date: 1 November 2019 Revised Date:
28 December 2019
Accepted Date: 14 January 2020
Please cite this article as: Arsecularatne JA, Colusso E, Della Gaspera E, Martucci A, Hoffman MJ, Nanomechanical and tribological characterization of silk and silk-titanate composite coatings, Tribology International (2020), doi: https://doi.org/10.1016/j.triboint.2020.106195. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Author credit statement Joseph Arsecularatne: Conceptualization, Investigation, Validation, Writing original draft preparation. Elena Colusso: Conceptualization, Investigation, Validation, Writing - original draft preparation. Enrico Della Gaspera: Investigation, Validation, Writing - reviewing and editing. Alessandro Martucci: Conceptualization, Resources, Supervision, Writing - reviewing and editing. Mark Hoffman: Conceptualization, Resources, Supervision, Writing - reviewing and editing.
1
Nanomechanical and tribological characterization of silk and silk-titanate composite coatings Joseph A Arsecularatnea, Elena Colussob, Enrico Della Gasperac, Alessandro Martuccib, Mark J Hoffmana1 a
School of Materials Science and Engineering, UNSW Sydney, NSW 2052, Australia Dipartimento di Ingegneria Industriale, Università di Padova, Via Marzolo 9, 35131 Padova, Italy c School of Science, RMIT University, Melbourne (VIC) 3001, Australia b
Abstract This paper investigates the tribological and mechanical properties of silk-based nanocomposite coatings which are finding applications in optics, biomedicine and dentistry, thanks to the exceptional mechanical/optical properties and associated biocompatibility of silk. Three different nanocomposite formulations were synthesized, and thin films were prepared by spin coating at different thicknesses and with different post-deposition annealing processes. Ellipsometry, FTIR spectroscopy, AFM, nanoindentation, scratch testing, continuous/reciprocating wear testing, confocal microscopy and SEM were used to characterize the coatings. The results reveal that their hardness and elastic modulus are in the range 0.56 - 1.30 GPa and 23.6 – 55.4 GPa, respectively, which are much higher than those reported for other silk films in literature. Incorporation of titanate nanosheets also improved coatings’ scratch resistance.
1. Introduction Bionanocomposites are attracting interest in numerous fields, especially in biomedicine, because they have structure and properties that mimic the natural materials. A wide range of synthetic nanomaterials (graphene oxide, carbon nanotubes, metallic nanoparticles etc.) can be integrated in the biopolymer phase for improving their properties such as thermal and electrical conductivity, optical properties and mechanical flexibility. This new class of materials find promising application in the fabrication of flexible sensing devices, lightweight structural component, packaging, implantable devices and drug delivery (Xiong et al., 2018). Because of the similarity with the natural tissues such as enamel, polymer-ceramic bionanocomposites have recently been investigated also for application in dentistry (Lee et al., 2016).
1
Corresponding author. School of Materials Science and Engineering, UNSW Sydney, Sydney 2052, Australia. Tel: +61 2 9385 4970; Fax: +61 2 9385 6545; E-mail: [email protected]
2
Silk fibroin (SF), a natural protein obtained from Bombyx mori cocoons, is an ideal biopolymer matrix because of its outstanding mechanical properties, biocompatibility and biodegradability, associated with the possibility to be processed in a wide range of forms including fibers, sponges and films (Hu et al., 2012; Koh et al., 2015, Omenetto et al., 2010, Kundu et al., 2014). Albeit the enormous work on films and coatings made with silk proteins (Borkner et al., 2014), there are only a few documented studies of the characterization of the mechanical properties of silk-nanoparticles based coatings (Jiang et al., 2007; Kharlampieva et al., 2010; Hu 2013). Mechanical properties of nanocomposites provide interesting indications about the interaction between the nanoparticles (NPs) and the silk matrix. With the change of the SF/NPs ratio, the interaction between the particles and the fibroin chains can alter the mechanical properties of the material. A similar effect can be obtained by variation in the microstructure of the protein promoted by post-treatments that induce transition in secondary structure of the silk (Ebenstein et al., 2005). These microstructures include a non-crystalline (random-coil) phase, a silk II crystalline phase composed of ordered β-sheets and a metastable less-organized crystalline silk I phase. (Ebenstein et al., 2005; Cebe et al., 2017). In this work, we present the mechanical and tribological characterization of silk-titanate composite coatings. Titanate nanosheets (TNSs) are 2D nanosheets made of a substochiometric titania that have been already integrated in silk matrix to generate nanocomposites materials with tunable optical properties and processability in the micro- and nanoscale (Perotto et al, 2015). Post-deposition processing treatments (soaking in methanol, temperature treatment and ion exchange followed by UV curing) were investigated to correlate the process approach to the mechanical properties of the coatings. Methanol annealing is an efficient way to crystallize fibroin protein in β-sheets, following the random coil to β-form transition, making the silk structure stable in water (Tsukada et al., 1994). The treatment has also an effect on the nanocomposite, by inducing a densification of the TNSs through dehydration and partial deintercalation of the tetramethylammonium ions (Perotto et al., 2015). On the other side, temperature, ion exchange and UV light can be used for TNSs post-processing, increasing the densification of the small clusters (Colusso et al., 2019). We chose to perform a cation exchange with silver ions followed by UV curing because we recently demonstrated that in this way it is possible to synthesize in loco silver nanoparticles (Colusso et al., 2019). This could add further antibacterial properties to the coatings, interesting for application in the biomedical field and dentistry (Fei et al., 2013; Li et al. 2011; Cheng et al, 2012; Zhou et al 2017). Accordingly, with a view to investigate the applications of these coatings in dentistry, the present work also includes comparisons of coatings’ nanomechanical and tribological properties with those of current resin based dental composite (restorative) materials.
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2. Methods Synthesis of Silk-Titanates nanocomposite solution Silk fibroin (SF) solution and TNSs water dispersion were prepared according to the protocol previously reported in literature (Perotto et al, 2015). SF-TNSs nanocomposite solution was synthesized by gently mixing the stock TNSs solution (20% w/v) with the SF solution (5% w/v) in an appropriate ratio. Three different formulations were selected: 100:0 (SF), 80:20 and 50:50 %w/v SF-TNSs. Coating preparation Thin films were prepared by spin coating (SCS G3P-8 spin coater, Cookson Electronics) on alumina slides or silicon wafers, at different speed to tune the final thickness of the samples (1000 or 3000 rpm) for 60 s. Alumina was selected as a substrate material because of its higher elastic modulus and hardness and hence greater resistance to elastic/plastic deformation during nanoindentation tests. Silicon was selected as a substrate because it is ideal for ellipsometry measurements thanks to its reflectivity and low surface roughness. Before coating, substrates were thoroughly cleaned by sonication in acetone, followed by treatment in a hot (60°C) basic piranha solution (H2O:H2O2:NH3 5:3:1 v/v) for 20 minutes, then rinsed with water and finally dried in air. After the deposition, films were dried at 60°C on a hot plate for 4 minutes. Different post-deposition annealing processes were performed on the samples: methanol, temperature, ion exchange and UV light. Methanol annealing (MeOH) was performed by soaking the samples in methanol (HPLC grade 99.9%, SigmaAldrich) for 30 min. Temperature treatment (TT) was effectuated by annealing the sample in an oven at 200° C for 1h in air. Ion exchange was performed by dipping the samples in a 0.1 M silver nitrate (AgNO3, Sigma Aldrich) aqueous solution for 6 min, followed by UV irradiation for 30 min with a lamp (UVL-56 365 nm) to nucleate silver nanoparticles (Colusso et al., 2018). Table 1 reports the different compositions tested in this work. Table1. Nominal composition, spin coating parameters and post deposition treatments of the samples investigated. Sample composition
Spin coating parameters
Post deposition treatments
SF1000/MeOH
Silk Fibroin
1000 rpm x 60 sec
Methanol annealing
SF3000/MeOH
Silk Fibroin
3000 rpm x 60 sec
Methanol annealing
SF1000/Ag
Silk fibroin
1000 rpm x 60 sec
Methanol annealing followed by ion exchange in 1M AgNO3 solution and UV irradiation
SF3000/Ag
Silk fibroin
3000 rpm x 60 sec
Methanol annealing followed by ion exchange in 1M AgNO3 solution and UV irradiation
SF1000/TT
Silk fibroin
1000 rpm x 60 sec
Methanol annealing followed by thermal treatment at 200°C
Acronym
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Silk fibroin
3000 rpm x 60 sec
Methanol annealing followed by thermal treatment at 200°C
SF-20TNSs1000/MeOH
Nanocomposite with nominal concentration 80:20% wt SF/TNS
1000 rpm x 60 sec
Methanol annealing
SF-20TNSs3000/MeOH
Nanocomposite with nominal composition 80:20% wt SF/TNS
3000 rpm x 60 sec
Methanol annealing
SF-20TNSs1000/Ag
Nanocomposite with nominal composition 80:20% wt SF/TNS
1000 rpm x 60 sec
Methanol annealing followed by ion exchange in 1M AgNO3 solution and UV irradiation
SF-20TNSs3000/Ag
Nanocomposite with nominal composition 80:20% wt SF/TNS
3000 rpm x 60 sec
Methanol annealing followed by ion exchange in 1M AgNO3 solution and UV irradiation
SF-20TNSs1000/TT
Nanocomposite with nominal composition 80:20% wt SF/TNS
1000 rpm x 60 sec
Methanol annealing followed by thermal treatment at 200°C
SF-20TNSs3000/TT
Nanocomposite with nominal composition 80:20% wt SF/TNS
3000 rpm x 60 sec
Methanol annealing followed by thermal treatment at 200°C
SF-50TNSs1000/MeOH
Nanocomposite with nominal composition 50:50% wt SF/TNS
1000 rpm x 60 sec
Methanol annealing
SF-50TNSs3000/MeOH
Nanocomposite with nominal composition 50:50% wt SF/TNS
3000 rpm x 60 sec
Methanol annealing
SF-50TNSs1000/Ag
Nanocomposite with nominal composition 50:50% wt SF/TNS
1000 rpm x 60 sec
Methanol annealing followed by ion exchange in 1M AgNO3 solution and UV irradiation
SF-50TNSs3000/Ag
Nanocomposite with nominal composition 50:50% wt SF/TNS
3000 rpm x 60 sec
Methanol annealing followed by ion exchange in 1M AgNO3 solution and UV irradiation
SF-50TNSs1000/TT
Nanocomposite with nominal composition 50:50% wt SF/TNS
1000 rpm x 60 sec
Methanol annealing followed by thermal treatment at 200°C
SF-50TNSs3000/TT
Nanocomposite with nominal composition 50:50% wt SF/TNS
3000 rpm x 60 sec
Methanol annealing followed by thermal treatment at 200°C
SF3000/TT
Characterization of coatings The thickness of the coatings was determined using a VASE ellipsometer (J.A. Woollam Co., NE, USA) at angles of incidence of 65 - 75° on 300–1000 nm wavelength range. Thickness measurements were made on three samples for each type of coating deposited on silicon substrate. The materials were modelled by using a Cauchy dispersion with Urbach absorption. Infrared spectra of the nanocomposite coatings were obtained by FTIR spectroscopy (Jasco 620 FTIR spectrometer). Absorption spectra were determined for thin films deposited on silicon wafers following different annealing on 4000–400 cm−1 range. The spectra given were obtained following subtraction of the spectrum of the silicon substrate.
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An atomic force microscope (Park Systems XE-70) was used to determine the surface roughness of the nanocomposite coatings deposited on silicon. The morphology of the films deposited on alumina after wear test was inspected by Scanning Electron Microscope (FEI Verios 460L SEM). The samples were coated with a 5 nm thick platinum layer before SEM inspection. Nanoindentation and wear testing Nanoindentation was used to measure the mechanical properties, specifically, hardness and elastic modulus of films consisting of different formulations. Their wear behavior was studied using continuous and reciprocating wear tests. The adhesion and scratch resistance of the coating was assessed using scratch testing. All these tests were performed on films deposited on alumina slides. Nanoindentation Force-controlled nanoindentation with a Hysitron TriboIndenter (Hysitron Inc., Minneapolis, USA) was used to measure the indentation hardness (H) and reduced elastic modulus (Er) of silk/silk-titanate coatings. It was fitted with a Berkovich diamond tip with radius of curvature approximately 100 nm. A detailed account of the procedure was given by Oliver and Pharr (Oliver and Pharr, 1992). The required tests were performed under ambient conditions. For each coating, at 100 µN maximum load, two sets of 9 indents at two different locations and separated by roughly 10 mm were carried out. These indents at two different locations were performed under identical conditions to observe any variation of nanomechanical properties over a coating. For some of the selected coatings, indents at 50, 250 and 500 µN maximum loads were also made to observe any variation of nanomechanical properties with indentation load. In order to minimize any possible influence of the mechanical properties of the substrate material on the measured nanomechanical properties of a coating, an attempt was made to control the maximum penetration depth to less than 30% of the thickness of a coating (Dayal et al., 2009). Wear testing: reciprocating Since the present study aims to investigate the applications of SF-TNSs coatings in dentistry, reciprocating wear tests were given priority. However, considering the large number of tests2 possible and to keep the experiments to a manageable number, specimens/conditions for testing were carefully selected to obtain representative results. For the present study, a total of 175 reciprocating wear tests, including 15 preliminary tests (for determining the suitable contact loads, number of cycles, etc) were carried out.
2
There are 352 test parameter combinations from: 3 coating compositions; 3 post treatment annealing processes; 2 coating thicknesses; 2 contact loads; 7 lubricated conditions (including the dry condition). For 2 – 3 tests under each parameter combination, the total number of reciprocating wear tests are 704 – 1056.
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The wear tests were performed using a tribometer fitted with a reciprocating module (CSEM Instruments, Peseux, Switzerland) with 6 mm diameter ruby ball antagonist under dry conditions or with a lubricant such as deionized water, artificial saliva (Duffo and Castillo, 2004), or a solution of ethanol and water containing 10% ethanol, 50% ethanol or 100% ethanol. In order to observe the tribological behavior of these coatings in an acidic environment, eight wear tests (four tests on a 450 – 500 nm series sample (SF50TNSs1000/TT) and another four tests on a 150 – 200 nm series sample (SF3000/MeOH)) were carried out using acetic acid solution (pH 3.2) as lubricant. Since preliminary tests revealed wearing out of the coatings under higher loads and higher cycles, loading was restricted to 1 – 2 N and the number of cycles to 100. Maximum sliding speed was 0.67 cm/s and the sliding distance (stroke) 2 mm. Each selected lubricant (at room temperature) was injected to the wear surfaces by a hypodermic needle at flow rate of 90 ml/h with an infusion pump (Imed Gemini PC-1, San Diego, USA). For consistency, the lubricant was not recycled during a test and the coefficient of friction was logged throughout. For each test condition, 2-3 wear tests were carried out. Following wear tests, the width and cross-sectional area of each wear scar were measured at six different locations using a Keyence laser confocal scanning microscope (VK-X200, Keyence corporation, Osaka, Japan) and accompanying software. The wear depth for each location was determined by dividing the cross-sectional area by the corresponding width. Wear testing: continuous (pin-on-disk) These wear tests were performed using a pin-on-disk tribometer (CSEM Instruments, Peseux, Switzerland) with a 6 mm diameter ruby ball antagonist under dry conditions with loads in the range 1 – 2 N and the cycles in the range 250 - 500. The sliding speed was 0.67 cm/s and the diameter of a circular wear track was in the range 4 – 10 mm. Under each test condition, 2 - 3 wear tests were carried out. Following wear tests, the width and cross-sectional area of each wear scar were measured at six different locations as discussed above and the corresponding wear depth calculated. Scratch testing These tests were carried out on a scratch tester (Revetest, CSM Instruments, Peseux, Switzerland) according to the procedure outlined in (Zavgorodniy et al., 2011). The normal load applied to a diamond tip (200 µm radius, Rockwell C) was raised from 1 to 6 N at a rate of 10 N/min. The length of a scratch was 5 mm and the sliding speed was 10 mm/min. Altogether, six different silk/silk-titanate coatings were tested with 2 – 3 scratch tests were made on each coating. Laser scanning confocal microscopy imaging was used to identify any mechanical processes that would occur along a scratch line, e.g., onset of detachment of a coating from the substrate.
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3. Results Characterization of silk-TNSs composite thin films Silk-TNSs composite coatings were fabricated by spin coating starting from a water dispersion of TNSs and SF in water. Both TNSs and SF were processed following the standard protocol reported in literature (Perotto et al., 2015) and detailed in the Methods. For each composition we tested two different spin speeds in order to tune the final thickness of the coatings. As measured by ellipsometry, we obtained two series of samples with different range of thickness, 150-200 nm and 450-500 nm, respectively for 3000 and 1000 rpm. For coatings fabricated at 3000 and 1000 rpm, following MeOH annealing, we observed increases in thickness of approximately 40 and 120 nm respectively, for an increase of TNSs up to 50 %wt. More details are reported in Table S1 in the supporting Information. The measured surface roughness (root-mean-square, rms) of the coatings deposited on silicon after methanol annealing was found to be very low for all the compositions (<1 nm, see Table S1, S.I.). No significant effects of the treatments were observed, except for the samples exposed to ion exchange with Ag followed by UV annealing. For these samples we measured an increase of rms (root-mean-square) roughness, with a maximum of about 9 nm in the case of the SF-50TNSs one. We also characterized the secondary structure of the silk fibroin protein after the postprocessing by observing the variation of the amide I band in the FTIR spectra of the samples (see Figure S2, S.I.). In short, we considered a strong peak in the range 1616-1637 cm-1 a marker of silk II conformation (β-sheets), a broad peak centered at 1640-1647 cm−1 indication of a random-coil structure, while a peak centered at 1650 cm-1 indication of α-helix (Cebe et al., 2017; Jin H.J. et al, 2005; Wilson, D. et al, 2000). After MeOH SF coatings showed a predominant of helix indicated by a peak at 1650 cm-1, and a second peak at 1630 cm-1 representative of a silk I structure. TT does not affect significantly the FTIR spectrum, while Ag+UV induced a complete crystallization in silk II. Considering samples with different TNSs amount, some variations were observed in the case of the MeOH. A broader band around 3500 cm-1 is visible in SF-20TNSs/MeOH and SF50TNSs/MeOH related to OH- vibration, not present in SF. This is likely due to the higher amount of water in the samples, as already verified by thermal gravimetric analysis (Perotto et al., 2015). On the contrary, a sharp peak at 3295 cm−1 (NH stretching vibration) is visible for all samples suggesting strong intermolecular hydrogen bonding. We also observed a decrease of the band at 1632 cm-1 in the case of SF-20TNSs/MeOH compared to the SF/MeOH, that increased again for the SF-50TNSs/MeOH. This means that the presence of 20%wt TNSs reduced the formation of ordered β-sheet structures (silk II). TT and Ag+UV induced an increase of band at 1632 cm-1 for all the SF-TNSs compositions, with a stronger effect in the case of SF-50TNS/Ag that showed a band corresponded to a silk II structure.
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Nanoindentation Figure 1 depicts representative load (P) displacement (h) curves obtained by indenting to a maximum load of 100 µN for coatings with different percentages of TNSs (450-500 nm series) and post deposition annealing treatments. These P – h curves show considerable variation in the maximum indentation depth with coatings containing 0 or 20% TNSs and MeOH treatment revealing higher depths than other coatings. Conversely, coatings containing 50%TNSs with Ag or TT treatments show lowest indentation depths. Creep (which is revealed as an increase in indentation depth at constant maximum load) is seen to occur for all tested coatings with lower creep noted for the coating with 50% TNSs and TT treatment. A comparison of nanoindentation mechanical properties (average H and Er) obtained under 50 and 100 µN indentation loads revealed greater variability of results for 50 µN load. This is most likely due to the influence of alumina substrates surface roughness on measured H and Er under such low load. Since the measured roughness of the coatings was very low in the case of silicon substrates, the larger variability observed is probably due to the alumina substrate that present a micro-rugosity (see Figure S3(a)). To avoid this issue only nanoindentation mechanical properties obtained under 100 µN load are considered below. The measured nanoindentation hardness of silk-titanate coatings ranged from 0.56 to 1.30 GPa and the elastic modulus from 23.6 to 55.4 GPa (Fig.2a and 2b). In general, measured H and Er show an increase for Ag+UV and TT compared to the MeOH samples. This increase in mechanical properties is more pronounced for coatings that contained a higher fraction of TNS e.g., 50% TNS.
Fig.1. Representative nanoindentation load–displacement (P–h) curves for coatings with different percentage of TNSs (450 – 500 nm series).
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Fig.2. Nanoindentation mechanical properties for coatings with different percentage of TNSs (450-500 nm series) and treatments: methanol aneling (MeOH), ion exchange and UV (Ag) and thermal treatment (TT). (a) hardness, H; (b) reduced elastic modulus, Er
Wear depth The measured wear depth for tests without a lubricant are shown in Fig.3a. The coatings that revealed a higher hardness and elastic modulus during nanoindentation (Fig.1), e.g., those that contained 20 – 50% TNS with TT treatment showed a higher wear depth indicating a lower wear resistance of the coating. This trend, however, was not reflected in the measured coefficient of friction (Fig.3b). Surprisingly, some of the coatings that showed the highest wear depth exhibited lowest coefficient of friction. The possible reasons for this will be considered in the Discussion section of the paper. Additionally, wear depth and coefficient of friction data for specimens containing 50% TNSs with MeOH and Ag treatments are not given in Fig.3 since these tests were not carried out because of the large number of tests required to obtain a complete set of data. It is however possible to estimate these values as will be considered in the Discussion section. During wear tests with a lubricant containing water (including acetic acid at pH 3.2), it was not possible to measure the wear depth for most of the coatings in Fig.3 above due to swelling of the coating adjacent to the wear scar. Water seemed to have seeped into the coating in that region causing its swelling (Fig.4). Occurrence of the swelling and its extent depended on four parameters: coating composition; lubricant composition; coating thickness; postprocessing treatment. Of the 450 – 500 nm series coatings, those containing > 20% TNSs and
10
received TT treatment resisted swelling. Although the coating consisting of bare SF and received TT treatment showed some swelling (Fig.4b), it was smaller compared to the coatings that received other secondary treatments (Fig.4c). All thinner coatings (150-200 nm series) which received TT treatment resisted swelling possibly due to greater densification induced by thermal treatment. Moreover, none of the tested coatings swelled when 100% ethanol was used as lubricant. A higher wear resistance was also observed with 100% ethanol compared to all other lubricants tested (Fig.5a). With decrease in wear, coefficient of friction also decreased (Fig.5b). SEM analysis of the wear scar revealed crack formation and partial delamination of the coating which were more pronounced with DI water and artificial saliva lubricants compared to ethanol (Figure S3, S.I.). Additionally, the wear depth measured for the SF-50TNSs1000/TT coating following tests with acetic acid lubricant was in the same range as those measured for other water-based lubricants.
Fig.3. Measured wear depth (a) and coefficient of friction (b) from reciprocating wear tests without lubricant for coatings with different percentage of TNSs (450-500 nm series) and treatments: methanol annealing (MeOH), ion exchange and UV (Ag) and thermal treatment (TT).
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(a) SF3000/TT
(b) SF1000/TT
(c) SF1000/Ag
Fig.4. Typical wear scars (arrowed) on 100% silk specimens wear tested with DI water lubricant and swelling of wear scar sides (arrow heads): no noticeable swelling (a); moderate (b) and large (c) swellings.
Fig.5. Measured wear depth (a) and coefficient of friction (b) for different lubricants using reciprocating wear tests for 100% silk coating (150-200 nm series) A comparison of the specific wear depth for continuous and reciprocating sliding reveals that the wear depth measured in reciprocating sliding is 2 – 4 times greater than that for continuous sliding (Fig.6a). However, the measured coefficient of friction is much higher in continuous sliding compared to reciprocating sliding (Fig.6b). Additionally, coatings with TNSs seem to show lower coefficient of friction (compared to those for coatings without
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TNSs) in continuous/reciprocating sliding. Typical coefficient of friction curves obtained during continuous sliding are shown in Fig.6c. For coatings without TNSs, steady state coefficient of friction is higher than the initial value. However, for the coating that contained TNSs, steady state coefficient of friction is lower than the initial value.
Fig.6. Comparison of measured specific wear depth (a) and coefficient of friction (b) obtained using reciprocating and continuous wear tests without lubricant for different coatings (450-
13
500 nm thickness); typical coefficient of friction versus number of cycles curves obtained during continuous wear tests (c).
Scratch testing Laser scanning confocal microscopy analysis of the tested coatings revealed a smoothing of the coating surface due to the scratches (Fig.7a). No indication of the detachment of the coating from substrate, even under highest load, was noted (Fig.7b). Moreover, friction force signal did not show a transition indicating coating detachment (Fig.7c). The scratch width (a measure of scratch resistance) for coatings with MeOH and Ag+UV treatments revealed slightly lower scratch resistance of 100% silk coatings compared to that of composite coatings containing 50% silk and 50% titanates (Fig.8). The scratch resistance of 100% silk coatings with TT treatment is higher and similar to those of the above composite coatings.
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15
Fig.7. A region of scratch (arrowed) corresponding to the highest applied load and measured forces (SF-50TNSs3000/TT): (a) 3D topography; (b) scratch and surface topography; (c) normal force Fn and tangential force Ft curves
Fig.8. Comparison of measured scratch width for different coatings (450-500 nm thickness)
4. Discussion The present work investigated the nanomechanical and tribological behavior of silk-titanate composite coatings having three different compositions and post deposition annealing processes. The authors’ previous work with these nanocomposite films prepared using a method similar to the one used in the present study has revealed reasonable dispersion of TNSs in SF matrix (Perotto et al, 2015; Magrì et al, 2018). Moreover, for a silk-titanate nanocomposite coating fabricated using the present method, a cross section of coatingsubstrate obtained by cryofracture was imaged using SEM in the authors’ previous work (Colusso et al, 2017) and revealed good coating adhesion to the substrate. According to the nanomechanical data, it appears that the increase of TNSs concentration improves both the stiffness and the hardness of the coating material compared to the bare silk fibroin. This effect is clear in the case of high filling fraction of TNSs (50 %wt), while the SF20TNSs composition did not induce a significant improvement in the mechanical properties.
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This can be related to the composite structure obtained during the self-assembly of the material. As shown by FTIR spectrum, in the case of SF-20TNSs/MeOH the peak related to a crystalline silk II conformation is lower compared to the SF-50TNSs/MeOH. The maximum H and Er are obtained for samples with 20-50 TNSs and combined treatments, that correspond to compositions with higher crystalline phase (β-sheet) from spectroscopic data. Previous studies have shown that mechanical strength and toughness of silks are controlled by β-sheets (Keten et al., 2010; Koh et al, 2015). In addition, TT and Ag+UV treatments induced a densification of the TNSs by deintercalation of the ions and collapse of the layered structure (Colusso et al., 2019). The plots of nanoindentation hardness (H) and elastic modulus (Er) results against contact depth for all silk and silk-titanate coatings (not shown in the paper) revealed an indentation size effect, that is, H and Er decreased with increasing indentation depth. However, they did not reveal any influence of the substrate in that the curves showed a continuous decrease with increasing depth for 100 µN load and in particular, all hardness values were clearly on single curve with very little scatter. The maximum depth obtained under this load was 63 nm which is ~ 14% of coating thickness. Additional tests carried out at higher indentation loads of 250 and 500 µN (with the corresponding contact depth above 96 nm which is ~ 21% of coating thickness) revealed the influence of substrate in that corresponding H and Er did not follow the aforementioned decreasing trend (with increasing contact depth) but were well above the curves. Additionally, creep occurred for all tested coatings with lower creep noted for the coating with 50% TNSs and TT treatment which is due to increased reinforcement by higher TNS content and greater densification by TT treatment. The increase in indentation depth due to creep can result in a reduction in measured hardness (Chudoba and Richter, 2001). Additionally, the measured nanoindentation H and Er for present silk and silk-titanate coatings (under 100 µN indentation load) were in the range 0.56 - 1.30 GPa and 23.6 – 55.4 GPa, respectively. These H and Er values are comparable to those measured for current dental composites consisting of a resin matrix and micro-/nano-scale glass/silica particle reinforcements (Arsecularatne et al., 2016; El-Safty et al., 2012). A previous experimental study has reported, for a composite graphene-silk film (3 - 4 mm thick) containing 0.5 wt% graphene, relatively higher H (0.12 GPa) and Er (1.9 GPa) compared to the pure silk film (H, 0.053 and Er, 0.33 GPa) (Wang et al., 2017). With further increase in graphene, elastic modulus decreased. Much higher nanomechanical properties measured for the present thin silk and silk-titanate composite coatings appears to be due to greater densification and crystallization associated with the post treatments. During wear tests with lubricants that contained water, swelling of a coating at the sides of the wear scar was observed for some of the tested coatings. It was hypothesised that: (i) smaller water molecules could seep through the microcracks generated during coating wear and caused the swelling; (ii) a liquid containing relatively larger molecules may not seep through these microcracks. To test this hypothesis, wear tests were carried out with lubricants containing varying amounts of ethanol. The wear results obtained with 100% ethanol lubricant supports the above hypothesis since none of the tested coatings swelled. This may
17
be attributed to the relatively larger ethanol molecule (compared to the water molecule) which did not seep through the microcracks. However, further studies are required to clarify the interactions between coatings and liquid lubricants during sliding contact. Greater densification and crystallization of the present silk-titanate composite coatings also explain the increased resistance of some of the coatings to swelling during the wear tests with lubricants such as DI water and artificial saliva. Wear tests carried out with such coating revealed lower wear when the lubricant contained more than 50% ethanol and the lowest wear for 100% ethanol lubricant. Measured low wear also corresponded to low friction. Additionally, the wear behavior with acetic acid (at pH 3.2) which was found to be similar to that with other water-based lubricants reveals that the SF-TNSs coatings tested in the present work are resistant to food acids. The wear tests carried out without a lubricant revealed that incorporation of TNS reduced friction during wear. However, a decrease in friction did not necessarily result in decrease in wear particularly, in composite coatings containing 20 – 50% TNSs with TT treatment. These may be attributed to silk-TNSs interface debonding and crack formation during contact sliding (which will increase wear (Kruzic et al., 2018)) and some of the accumulated wear debris acting as miniature rollers (which will decrease friction at the sliding interface (Ma et al., 2013; Zanoria et al., 1995)). To confirm these, further research aimed at revealing the dominant wear mechanism is required. As noted in the Results section of this paper, wear depth and coefficient of friction data for specimens containing 50% TNSs with MeOH and Ag treatments were not given in Fig.3 since these tests were not carried out because of the large number of tests involved. However, from the data in Fig.3, particularly those for 20%TNSs and that for 50%TNSs, it is possible to estimate the average wear depth and the corresponding coefficient of friction values for coatings containing 50% TNSs with MeOH and Ag treatments which are given in Table 2. Table 2. Estimated wear depth and coefficient of friction for coatings containing 50% TNSs with MeOH and Ag treatments Treatment
MeOH
MeOH
Ag
Ag
Contact load (N)
1
2
1
2
Wear depth (µm)
0.40
0.41
0.36
0.39
Coefficient of friction
0.051
0.052
0.053
0.053
It can be seen that the wear depth values in Table 2 are lower than those for the coating with similar TNSs content (with TT treatment). Conversely, they are higher than those for the coatings with similar treatments but lower (20%) TNSs content. However, the coefficient of friction results can be seen to follow the opposite trend in that the estimated values are higher than those for the coating with similar TNSs content (with TT treatment). Conversely, they are lower than those for the coatings with similar treatments but lower (20%) TNSs content.
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Compared to reciprocating wear tests, a higher wear resistance of the silk-titanate coatings in continuous (pin-on-disc) wear tests was observed. The measured high wear during former seems to indicate that the tested coatings are less resistant to mechanical fatigue caused by fluctuating stresses (Kruzic et al., 2018) which occur more frequently during reciprocating sliding than during continuous sliding. In the present study, the average specific wear depth measured during continuous tests is in the range 1.36 – 1.67 µm per N per 1000 cycles. In comparison, the wear depth reported for nine commercial resin based composite materials was in the range 0.016 – 0.07 µm per N per 1000 cycles (Cha et al., 2004). This reveals that the wear resistance of present silk and silktitanate composite coatings is much lower than that of commercial dental composite materials. Present coatings with hardness less than 5 GPa (Fig. 2a) can be considered as soft coatings (Bull, 1991; Bull and Berasetegui, 2006). During scratch testing, with increasing load, the soft coating is progressively plastically deformed and its detachment from the substrate is initiated at a critical load. This coating detachment may be identified from optical/electron microscopy observations of a scratch and/or from a transition in the friction force curve i.e., a sudden rise in friction force. As noted in Section 3, it was not possible to identify the location/load of onset of silk-TNS coating detachment following the current scratch tests. Loads higher than 6 N were not considered for scratch tests because of the relatively thin alumina substrate (0.5 mm thickness) and the possibility of its failure under higher loads. The difficulty in identifying the location/load for onset of coating detachment in the present scratch tests may also indicate strong adhesion of silk and silk-titanate coatings to the substrate. Using the same scratch equipment and test parameters, Zavgorodniy et al (Zavgorodniy et al., 2011) obtained the onset of failure of hydroxyapatite and monetite coatings at contact loads of 1.5 and 5.4 N, respectively. The corresponding critical shearing stress for the coating detachment estimated using the equation of Benjamin and Weaver (Benjamin and Weaver, 1960) was 44 MPa for hydroxyapatite and 84 MPa for monetite. Based on these results it appears that the critical shearing stress for the present silk and silk-titanate coatings is higher than 84 MPa. To confirm this, scratch testing under higher loads is required. The present investigation has revealed that the tested silk and silk-titanate coatings possess nanomechanical properties that are similar to those of resin based dental composites. In addition, tested coatings seem to indicate good adhesion to the ceramic substrate. Moreover, silk/silk-titanate composite coatings that received TT treatment resisted swelling during sliding contact in the presence of a lubricant containing water. However, the wear resistance of the present silk/silk-titanate composite coatings is much lower than that of the current dental composites. Since silk/silk-titanate coatings are biocompatible, further development of these coatings with improved tribological properties will enable their wider application such as in dental implants/protheses and hip/knee joints.
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5. Conclusions Based on the present experimental results and analyses discussed above, following conclusions can be drawn: •
•
•
• •
Measured nanoindentation hardness and elastic modulus of silk and silk-titanate composite coatings were in the range 0.56 - 1.30 GPa and 23.6 – 55.4 GPa, respectively. These H and Er values are much higher than those reported for graphenesilk composite films but comparable to those measured for resin based dental composites. Wear tests on these coatings revealed a 2 – 4 times higher wear depth during reciprocating sliding compared to continuous sliding. This indicates that the coatings possess greater wear resistance during continuous sliding. However, their wear resistance is much lower than that of resin based dental composites. While some of the tested coatings swelled under contact sliding when water was present in the lubricant, others did not. Of the 450 – 500 nm series, coatings containing TNS and received TT treatment resisted swelling. Of the 150 – 200 nm series, all coatings with TT treatment resisted swelling. Silk and silk-TNS coatings revealed good adhesion to the alumina substrate. No coating failure was observed under tested conditions. Scratch resistance of coatings that received TT treatment was higher. Incorporation of TNSs also improved the scratch resistance of a coating.
The present study has revealed high-performance nanocomposite coatings based on inorganic fillers (titanate nanosheets) incorporated within a silk-fibroin matrix. This work highlights the effect of the nanofiller concentration and the control of the secondary structure of the protein on the improvement of the mechanical properties and paves the way for further studies on biomimetic materials based on biopolymers and 2D nanoparticles. Based on the results of the present study, reinforced SF-50TNSs with TT treatment are recommended for applications that require hard and scratch resistant coatings while unreinforced SF coatings with TT treatment are recommended for applications that require wear resistance.
Acknowledgements Alessandro Martucci thanks BIRD 2017 program promoted by the University of Padova, prot. BIRD177407 and RMIT Philanthropic Fund through an International Visiting Fellowship for financial support, Enrico Della Gaspera thanks RMIT University for funding through the Vice Chancellor’s Fellowship scheme.
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Supporting Information
Table S1. Thickness, refractive index and root-mean-square roughness (rms) of nanocomposite coatings deposit on silicon substrates measured by ellipsometry and AFM. Thickness /nm
Refractive index @ 500 nm
SF1000/MeOH
370 ± 5
1.5612 ± 0.0043
SF3000/MeOH
170 ± 12
1.5633 ± 0.0013
SF1000/Ag
384 ± 14
1.5780 ± 0.0041
SF3000/Ag
156 ± 3
1.5914 ± 0.0275
SF1000/TT
382 ± 1
1.5591 ± 0.0002
SF3000/TT
185 ± 30
1.5645 ± 0.0034
SF-20TNSs1000/MeOH
479 ± 39
1.6209 ± 0.0353
SF-20TNSs3000/MeOH
179 ± 30
1.6521 ± 0.0028
SF-20TNSs1000/Ag
444 ± 27
1.6869 ± 0.0693
SF-20TNSs3000/Ag
158 ± 19
1.6581 ± 0.0424
SF-20TNSs1000/TT
440 ± 32
1.6353 ± 0.0609
SF-20TNSs3000/TT
165 ± 6
1.6163 ± 0.001
SF-50TNSs1000/MeOH
494 ± 8
1.7367 ± 0.0424
SF-50TNSs3000/MeOH
208 ± 7
1.6961 ± 0.0058
SF-50TNSs1000/Ag
410 ± 30
1.7874 ± 0.0024
SF-50TNSs3000/Ag
173 ± 7
1.7115 ± 0.0028
SF-50TNSs1000/TT
456 ± 80
1.77275 ± 0.0146
SF-50TNSs3000/TT
150 ± 22
1.7124 ± 0.0868
Sample
rms /nm
0.472
4.35
0.357
0.832
1.705
0.743
0.791
9.854
0.541
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Figure S2. Representative FTIR spectra of silk-TNSs coating nanocomposite of (a) SF3000, (b) SF-20TNSs3000 and (c) SF-50TNSs3000 samples.
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Figure S3. SEM micrographs of SF3000/TT as prepared (a) and after the wear test performed with different lubricants: (b) DI water, (c) artificial saliva, (d) 10% ethanol and (e) 100% ethanol.
Highlights • Presents a nanomechanical and tribological characterization of nanocomposite coatings made of silk fibroin and titanates nanosheets
• Results are presented for three different formulations of nanocomposite coatings with different post-deposition annealing processes
• It is revealed that the measured nanomechanical properties are much higher than those •
reported for graphene-silk composite films in the literature Incorporation of titanates nanosheets improved the scratch resistance of a composite coating
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. They also declare that they have approved the final article for submission. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: