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Biotribological properties of Ti-6Al-4V alloy treated with self-assembly multi-walled carbon nanotube coating
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Biotribological properties of Ti-6Al-4V alloy treated with self-assembly multi-walled carbon nanotube coating Jun Deng, Songhong Pang, Chenchen Wang , Tianhui Ren ⁎
⁎
School of Chemistry and Chemical Engineering, Key Laboratory of Thin Film and Microfabrication Technology (Ministry of Education), Shanghai Jiao Tong University, 200240 Shanghai, China
ARTICLE INFO
ABSTRACT
Keywords: Ti-6Al-4V CMWNT coating Self-assembly Biotribology
Ti-6Al-4V alloy has been widely applied in biomedical field due to its good biocompatibility, non-toxicity and excellent anti-corrosion. However, the poor tribological performance limits its application. In this paper, in order to enhance the biotribological properties of Ti-6Al-4V alloy, carboxylic multi-walled carbon nanotubes (CMWNT) coatings were grafted on the surface by chemical self-assembly technology. Raman spectroscopy and scanning electron microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDX) were utilized to characterize the microstructures of the coatings. The CMWNT coatings showed lower coefficient of friction and wear rates under dry sliding and simulated body fluid (SBF) lubrication. Furthermore, the biotribological mechanisms of CMWNT coatings under both conditions were discussed. Analyses of worn surfaces indicated that the rolling friction between the microtubes due to the spalling and fracture of CMWNT during the friction process was a major factor for improving biotribological behavior.
1. Introduction Titanium alloys, especially Ti-6Al-4V, have been widely applied in biomedical field due to their biocompatibility, high strength-to-weight ratio, and excellent anti-corrosion [1,2]. However, poor tribological properties of Ti-6Al-4V such as high coefficient of friction and inferior wear resistance, limit its application as bioimplants, such as artificial joints [3]. The wear debris containing Al and V elements are mostly likely to enter in the bloodstream, resulting in aseptic loosening of the prosthesis and inflammation of the surrounding tissues, further leading to the failure of the implants [4,5]. Therefore, several surface treatments have been carried out to enhance the tribological performance of titanium alloy [6]. To date, various modification technologies have been used, these include surface coatings and surface texturing [7–12]. Among them, surface coatings have been considered as a good option to apply on the surface of Ti-6Al-4V plates, due to its directness and effectiveness to improve surface properties. Yetim et al. [13] prepared composite diamond-like carbon (DLC) coating on untreated Ti-6Al-4V alloys. The results reported that the coefficient of friction (COF) and worn rate of Ti-6Al-4V alloys significantly reduced after coating DLC film. Viteri et al. [14] investigated the improved tribological performance of Ti-C-N coating on the Ti-6Al-4V surface. However, these coatings have some drawbacks, such as weak adhesion and instability on the substrate ⁎
during friction process. Thus, suitable lubricated coatings should be considered to be prepared on the surface of titanium alloys. Carbon nanotubes (CNT) have attracted great attention in biomedical application, owing to its high aspect ratio, high mechanical properties and biocompatibility [15–18]. The previous researches have shown that CNT could be utilized as solid lubricants or liquid lubricants, implying its self-lubricating properties [19,20]. Savalani et al. [21] prepared Ti-CNT composite layers reinforced Ti matrices and indicated that the composite layers with CNT exhibited superior wear resistance compared with pure Ti substrates. Wang et al. [22] added CNT to TiO2 coating to evaluate its effect on tribological properties and reported that the COF and wear volume of nanocomposite coatings decreased, suggesting the enhancement of tribological properties. Therefore, CNT is considered to be an ideal coating to enhance tribological properties of titanium alloy. Nevertheless, the preparation methods of CNT on the surface of titanium alloy face some challenges, such as the harsh temperature conditions from heat treatment [20] and chemical vapor deposition (CVD) [23], the weak bonding adhesion and inhomogenous dispersion from electrodeposition [24] and spark plasma sintering [25]. In order to prepare stable CNT coatings on the surface of titanium alloy, chemically attaching CNT by self-assembly without external force is a feasible and simple method, which has attracted much attention [26,27]. According to the previous studies, it is difficult to directly graft CNT onto the surface of materials due to its chemical
Corresponding authors. E-mail addresses: [email protected] (C. Wang), [email protected] (T. Ren).
https://doi.org/10.1016/j.surfcoat.2019.125169 Received 15 September 2019; Received in revised form 10 November 2019; Accepted 15 November 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Jun Deng, et al., Surface & Coatings Technology, https://doi.org/10.1016/j.surfcoat.2019.125169
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inertness [28,29]. Therefore, functionalized CNT with active groups, such as hydroxyl, epoxide and carboxyl groups, is considered for application [30–32]. To improve the bonding strength of the functionalized CNT coatings on the titanium alloy surface, the intermediate layer should be introduced. 3-Aminopropyl triethoxysilane (APS), has been proved to be a suitable choice as the intermediate layer, due to its high activities and strong bonding strength between coatings and substrates [33–35]. Li et al. [36] introduced APS as the intermediate layer to graft graphene oxide on the titanium substrates. However, the results showed that the coating was prone to peel off from the substrates after a certain period [37]. Hence, the second intermediate layer can be considered to reinforce the adhesion. In our previous study, dopamine (DA) has been utilized to improve the bonding strength of graphene oxide with the substrates, owing to its super adhesion, non-toxicity and eco-friendliness [27,37–39]. Furthermore, polydopamine (PDA) self-polymerized by DA monomers can also provide a second reaction plate for coatings. Therefore, DA can be considered to be a fabulous choice for the second intermediate layer. In this study, carboxylic multi-wall carbon nanotubes (CMWNT) were firstly obtained by the oxidization of multi-wall carbon nanotubes. A simple and efficient chemical self-assembly technology was utilized to graft the CMWNT coatings on the surface of titanium alloy. Among that, APS and DA were respectively introduced as the transition layers. The prepared samples were referred to APS-DA-CMWNT. For comparison, CMWNT coated titanium alloy plates using APS as the transition layer were prepared as APS-CMWNT. The results of scratch tests indicated that DA could improve the bonding adhesion between substrate and coating. The biotribological properties of the specimens under dry sliding and simulated body fluid (SBF) lubrication were investigated, respectively. These results illustrated that the CMWNT coatings enhanced the biotribological properties of Ti-6Al-4V alloy. Furthermore, the biotribological properties of the APS-DA-CMWNT samples under both lubrications were superior to that of APS-CMWNT samples. The corresponding biotribological mechanisms were deduced and discussed by surface analysis.
removement of residues from the surface of MWNT during the oxidation and subsequent cleaning process. It can be seen from Fig. 1(c) that the original MWNT exhibited the absorption peak at 1590 cm−1, indicating that the stretching movement of C]C bond. The same characteristic peak at 1572 cm−1 was found in the spectrum of CMWNT sample. However, the new peak of carboxyl group at 1717 cm−1 from –COOH occurred on CMWNT, suggesting the success of the carboxylation reaction. 2.3. Self-assembly of CMWNT coatings on the surface of Ti-6Al-4V alloy The prepared titanium alloy samples were first reacted at 20% HNO3 solution for 30 min at room temperature to remove the oxide films. The surfaces were washed by deionized water and dried by N2. Then, the samples were placed in 5 mol/L KOH solution for 12 h at 40 °C to raise the amount of hydroxyl groups on the surfaces. Afterwards, the hydroxylated titanium alloy plates were placed into a prepared 3% APS aqueous solution (with the adjusted pH of 8.5 by adding 1 mol/L HCl solution) for 1 h at ambient temperature. After washing and drying, the samples were put into 2 g/L DA in Tris solution (pH = 10.5) for 12 h at room temperature during stirring thoroughly, and then washed by deionized water and dried by N2. The prepared samples were abbreviated as APS-DA. After that, the APS-DA samples were put into 2 g/L CMWNT aqueous solution treated in the ultrasound bath for 5 h with 150 W, and then reacted at 60 °C for 24 h. The obtained samples were referred to APS-DA-CMWNT. For comparison, the Ti-6Al-4V plates modified by the intermediate layer of APS were reacted with 2 g/L CMWNT aqueous solution at the same treatment and reaction condition. The prepared samples were abbreviated as APSCMWNT. 2.4. Bonding strength tests The bonding strengths between the Ti-6Al-4V plate and CMWNT coating were measured by scratch tests [40]. The specimens were scraped by coating adhesion automatic scratch tester (WS-2005, China) with the scratch speed of 5 mm/min, the maximum load of 100 N, and the load speed of 100 N/min. APS-DA-CMWNT and APS-CMWNT samples were tested four times, respectively, and the average adhesion strengths were evaluated by the essential load that the coating can withdraw.
2. Experimental procedures 2.1. Materials The medical grade titanium alloy (Ti-6Al-4V) plates were processed into 10 mm × 10 mm × 2 mm (length × width × thickness) and polished to the roughness (Ra) of 0.1 ± 0.02 μm. Before processing, Ti6Al-4V plates were washed with acetone and water for 1 h in the ultrasound bath with 150 W, respectively. DA hydrochloric (98%) and APS (99%) were of analytical grade.
2.5. Biotribological tests The biotribological measurements were carried out on the ball-onplate High-speed Reciprocating Friction and Wear Tester (Yanhua Electronic Technology Co., Ltd., Lanzhou, Gansu, China) to evaluate the biotribological properties of CMWNT coatings on Ti-6Al-4V plates during dry friction and SBF lubrication. Among that, Si3N4 ceramic balls (ф = 6 mm, with the hardness of 1500 HV, the mean roughness (Ra) of 0.02 μm) were used as the stational upper counterpart, and the tested specimens were fixed in a rectangle sink as lower friction pair. All biotribological tests were performed under the vertical load of 10 N, the sliding frequency of 1 Hz and the amplitude of 2 mm for 30 min in dry friction and SBF lubrication, respectively. And the ion concentrations in SBF solution are listed in the Table 1. Samples were tested for at least three times under each condition to guarantee the data repeatability. The worn rates k of Ti-6Al-4V specimens were calculated by Eq. (1) [41]: (mm3/(N·m))
2.2. Preparation of CMWNT Carboxylic multi-wall carbon nanotubes (CMWNT) were prepared by using the following method: 400 mL of prepared mixture acid (V98%H2SO4 : V30%H2O2 = 3 : 1) was transferred to the flask to react with 1 g MWNT, and then heated and refluxed at 100 °C for 12 h during stirring. Subsequently, the mixture was naturally cooled at room temperature for approximately 1 h, and then centrifuged with the speed of 18,000 r/min for 25 min. The upper solution was discarded and the deionized water was added in the stirring state. After washing 7 times, the pH of supernatant was neutral. The lower CMWNT with the concentration of 9.6 g/L was taken out and as a prepared material for subsequent experiments. Fig. 1 showed the SEM images and FTIR spectra of the original MWNT and CMWNT. As exhibited in Fig. 1(a) and (b), the diameters of the MWNT and CMWNT were about 18–30 nm, and the carbon nanotubes were prone to wrap. In addition, the diameters of CMWNT were slightly smaller than that of MWNT, which may be attributed to the
k = V /(X FN ),
(1)
where V is the worn volume of Ti-6Al-4V specimens, X is the sliding distance and FN is the applied normal load. 2
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Fig. 1. SEM images of (a) MWNT and (b) CMWNT and FTIR spectra of two samples (c).
2.6. Characterizations The microscopic structures of the three different Ti-6Al-4V specimens were characterized by Raman spectra (DXR, Thermo Fisher Scientific Co., Ltd., USA). The MWNT and CMWNT powders and surface coatings on prepared Ti-6Al-4V samples were analyzed by ATR-FTIR spectrometer (attenuated total reflection of Fourier transform infrared, Spectrum 100, Perkin Elmer, Inc., USA) with a resolution of 4.0 cm−1 over a scan range of 600–4000 cm−1. The surface coating thicknesses of the samples were measured using the Alpha-step tester (MFT-4000). Scanning electron microscopy (SEM, FEI Nova 450, USA) was employed to characterize the surfaces of the Ti-6Al-4V specimens before and after the biotribological tests as well as morphology of MWNT and CMWNT. And the compositions of chemical element on worn surfaces were analyzed by Raman spectroscopy and Energy Dispersive X-ray Spectroscopy (EDX). White light interferometer (WLI, Bruker, Contour GT-K0) was utilized to observe the three-dimensional wear scar morphology and calculate the worn volume of the specimens. 3. Results
Fig. 2. FTIR spectra of APS-CMWNT and APS-DA-CMWNT samples.
3.1. Surface characterization
showed two new bands of C]O(N) at 1628/1435 cm−1, CeN at 2901 cm−1, indicating that the amidation reaction between DA and CMWNT. The Raman spectra of APS-CMWNT and APS-DA-CMWNT coated titanium alloys were exhibited in Fig. 3(a). As shown in the Fig. 3, compared with the bare titanium alloys (Fig. 3(b)), Raman spectra of APS-CMWNT and APS-DA-CMWNT both displayed two prominent peaks at D band (1333 cm−1) and G band (1588 cm−1), and two weak peaks at 2D and D + G modes, indicating that CMWNT coatings had been successfully grafted. Among those peaks, D-band was related to the transformation of sp2 hybrid carbon to sp3 hybrid carbon, while Gband corresponded to the plane vibration of sp2 hybrid carbon, which
The ATR-FTIR spectra of APS-CMWNT and APS-DA-CMWNT specimens were shown in Fig. 2. It can be found from the spectrum of APSCMWNT that a Si-O-Si band at 904 cm−1 occurred, resulting from the condensation reaction between Si-OH of APS and eOH of CMWNT, suggesting that APS molecules were prone to hydroxylate with CMWNT in aqueous solution. In addition, the occurrence of CeN characteristic peaks at 2922 cm−1 and the weaken epoxy groups at 1100 cm−1 implied that the epoxy groups from CMWNT might react with the amino of APS. Moreover, the appearance of peaks at 1630/1425 cm−1 could infer the structural changes of carboxyl groups owing to the reaction with the amine groups of APS. The spectrum of APS-DA-CMWNT Table 1 Chemical composition of SBF solution. Ion Concentration (mM)
Na+ 142.0
Mg2+ 1.5
K+ 5.0
Cl− 103.3
Ca2+ 2.5
3
HCO32− 10.0
HPO42− 1.0
SO42− 0.5
pH 7.4
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CMWNT coating were significantly lower than that of uncoated plates, and the APS-DA-CMWNT sample exhibited the lowest COF. The results were consistent with the calculated average COF (Fig. 7(c)), implying that CMWNT coating can effectively reduce the COF under dry friction and SBF biolubrication and DA had a positive effect on the reduction of COF. In addition, the worn rates of three titanium alloy plates under both lubrications were calculated in Fig. 7(d). It can be observed that the worn rates of the CMWNT coated samples (APS-CMWNT and APS-DACMWNT) were remarkably lower than that of the uncoated plates under both lubrications, respectively, suggesting that CMWNT can reduce the wear of the titanium alloys. Compared with the wear rate of APSCMWNT, APS-DA-CMWNT showed the superior wear resistance, which was consistent with the results in ACOF (Fig. 7(c)). The results illustrated that the introducing of DA transition layer can remarkably enhance the biotribological properties of CMWNT coatings. By comparing the biotribological properties of the two lubrication conditions, it can be found that the ACOF values of three titanium alloy samples under SBF lubrication were lower than that in dry friction, while the worn rates (Fig. 7(d)) of three samples were higher in SBF solution. This might be ascribed to the corrosive wear by ions in SBF solution on the surface of samples, such as Cl−, SO42− [41].
Fig. 3. Raman spectra of samples: (a) APS-CMWNT and APS-DA-CMWNT, (b) Ti-6Al-4V.
3.3. Analysis of worn surfaces
was related to the destruction of sp2 structure of graphite or covalent binding of functional groups. In order to more intuitively analyze the grafting of CMWNT on the sample surface, SEM images and compositions analysis of APS-CMWNT and APS-DA-CMWNT were shown in Fig. 4. Fig. 4(a) and (b) exhibited that the coatings of the two samples were relatively uniform. It can be seen from Fig. 4(a) and (a1) that, interwoven CMWNT tubular structure appeared on the surface of Ti-6Al-4V alloys. Furthermore, the corresponding EDX mapping (Fig. 4(a2)) characterized the C elements distribution of surface composition, suggesting the presence of CMWNT, further indicating that CMWNT had been successfully grafted onto the surface of APS-CMWNT specimen. Similarly, Fig. 4(b) and (b1) exhibited a network structure on the surface of titanium alloys, which might be attributed to the introduction of CMWNT. This result can be verified by Fig. 4(b2) with the C elements distribution. Fig. 5 showed the variation of friction force with load measured by scratch tests [40]. As shown in Fig. 5(a), CMWNT coatings on the surface of APS-CMWNT sample were scraped off when the load was more than 77.0 N. After introducing the second adhesive layer DA, the critical load (82.8 N) of APS-DA-CMWNT sample was higher than that in APS-CMWNT, indicating that DA transition layer can enhance the bonding force of CMWNT coatings and the substrate. In order to further analyze the CMWNT coatings on the surfaces of APS-CMWNT and APS-DA-CMWNT samples, the corresponding coating thicknesses were shown in Fig. 6. The thickness of APS-CMWNT coatings was calculated as ~0.44 μm (APS-CMWNT, Fig. 6(a)) and the thickness of APS-DA-CMWNT was ~0.66 μm (APS-DA-CMWNT, Fig. 6(b)) which were greater than the thickness of CMWNT tubes. The results showed that the CMWNT coating was assembled by chemical bonding and physical adsorption. In addition, the thickness of APS-DACMWNT coating was thicker than APS-CMWNT coating, which might be due to the strong adhesion of DA transition layer.
In order to further investigate the wear mechanism of CMWNT coating on the surface of titanium alloy, the wear surfaces of specimens under dry friction and SBF lubrication were characterized by SEM and WLI, respectively. Fig. 8 showed the three-dimensional (3D) microscopic images, SEM micrographs and EDX elemental analysis of the samples in dry condition. The 3D topographic maps of Ti-6Al-4V, APS-CMWNT and APS-DACMWNT were shown in Fig. 8(a–c). It can be seen that the scratch depth of uncoated Ti-6Al-4V sample was approximately 25 μm. While the values of two CMWNT coated samples were lower than that of uncoated Ti-6Al-4V plate, with ~22 μm (APS-CMWNT) and ~19 μm (APS-DACMWNT). The results exhibited that APS-DA-CMWNT had the best wear resistance under dry friction, suggesting that CMWNT coating had remarkably protective effects on Ti-6Al-4V plates. The SEM images under different magnification (200× (Fig. 8(a1–c1)), 2000× (Fig. 8(a2–c2)), 10,000× (Fig. 8(a3–c3))) of the samples under dry friction showed that serious material loss, parallel plough shape and delamination were found on the surface of uncoated titanium alloy plate, resulting from abrasive wear behavior. Compared with the uncoated sample, the two CMWNT coated samples had significant slighter wear scar with swallow scratches, less debris and smaller sized flake-like chips, demonstrating the effective protection. Among the two CMWNT coated samples, APSDA-CMWNT displayed the lightest scratches and the smallest debris. This finding might indicate that DA had further enhanced the wear resistance of CMWNT coating. Moreover, the corresponding EDX analysis (Fig. 8(a4–c4)) showed that the Si element appeared on the worn surface, suggesting the transfer of Si3N4 contents, further demonstrating the adhesive wear mechanism. Compared with that on uncoated Ti-6Al4V plate (1.31 wt%), the Si contents were detected on the worn area of APS-DA-CMWNT (0.81 wt%), next on APS-CMWNT (0.91 wt%), implying the improvement of adhesive wear due to the CMWNT coatings. The above results illustrated that CMWNT coatings can improve the tribological behavior under dry friction. For the Si3N4 balls rubbing against the plates, the SEM images on the worn surface in dry sliding were compared in Fig. 9. It can be observed from Fig. 9(a–c) that the worn scar of uncoated Ti-6Al-4V was the largest, followed by APS-CMWNT, and the lightest appeared on the surface of APS-DA-CMWNT sample. In addition, debris and ploughs can be detected on the magnification of SEM images (Fig. 9(a2–c2)), implying that the materials from the titanium alloy plates transferred to the Si3N4 balls in the wear process, which further demonstrated the
3.2. Biotribological properties On the purpose of researching the protective effects of CMWNT coatings on the titanium alloy substrates, friction tests were respectively carried out under dry friction and SBF lubrication, as exhibited in Fig. 7. Fig. 7(a) and (b) revealed the dynamic COF curves of the samples surface during friction process under both lubrications. It can be observed that the COF tended to be relatively stable after a period of time. Besides, under both lubrications, the COF of the two samples with 4
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Fig. 4. SEM images and Mapping graphs of samples: (a) APS-CMWNT, (b) APS-DA-CMWNT.
Fig. 5. Friction force-load curves for scratch test: (a) APS-CMWNT, (b) APS-DA-CMWNT. 5
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Fig. 6. Thicknesses of samples: (a) APS-CMWNT, (b) APS-DA-CMWNT.
Fig. 7. Biotribological properties of Ti-6Al-4V, APS-CMWNT and APS-DA-CMWNT under dry friction and SBF lubrication: (a) coefficient of friction (COF) curves in dry friction, (b) COF curves under SBF lubrication, (c) average coefficient of friction (ACOF) curves, (d) worn rates. 6
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Fig. 8. 3D micrographs, SEM images and EDX results of wear scars on Ti-6Al-4V samples in dry sliding. (a) Ti-6Al-4V, (b) APS-CMWNT, (c) APS-DA-CMWNT.
mechanism of abrasive wear. Moreover, it is obvious that the slightest ploughs and the least debris were found on the surface of Si3N4 ball against APS-DA-CMWNT sample, next against APS-CMWNT and most severe against the uncoated Ti-6Al-4V plate. The wear spot
observations of the Si3N4 balls were consistent with those of Ti-6Al-4V samples. Therefore, it can be inferred that the mixed wear mechanism might include adhesive wear and abrasive wear. Fig. 10 showed the 3D micrographs (Fig. 10(a–c)), SEM images with 7
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Fig. 9. SEM images of worn scars on the Si3N4 balls in dry friction. (a) Ti-6Al-4V, (b) APS-CMWNT, (c) APS-DA-CMWNT.
the magnification of 200× (Fig. 10(a1–c1)), the magnification of 2000× (Fig. 10(a2–c2)), the magnification of 10,000× (Fig. 10(a3–c3)), and EDX results of samples during SBF lubrication. It can be seen from the Fig. 10(a–c) that the maximum depths of worn scar on uncoated titanium alloy specimens was up to ~33 μm, while the depths of two samples coated with CMWNT were significantly swallow, ranging from ~24 μm (APS-CMWNT) to ~21 μm (APS-DA-CMWNT). The results indicated that CMWNT coating might plays an active role in the wear resistance of Ti-6Al-4V plates under SBF solution lubrication, and the most apparent effect was reflected on the APS-DA-CMWNT sample. As shown in the SEM images, the wear scar widths of three Ti6Al-4V plates showed the following sequence: Ti-6Al-4V > APSCMWNT > APS-DA-CMWNT. Serious parallel ploughs and stratifications appeared on uncoated Ti-6Al-4V surface, indicating the abrasive wear behavior during SBF solution lubrication. Compared with the uncoated sample, two CMWNT coated specimens had slighter scratches and smaller debris, and APS-DA-CMWNT had the shallowest scratches, ploughs and the least debris, indicating that DA could improve the wear resistance of CMWNT coating. The EDX results (Fig. 10(a4–c4)) showed that Si element on the wear scars of three samples had the following sequence: APS-DA-CMWNT (0.48 wt%) < APS-CMWNT (0.64 wt %) < uncoated Ti-6Al-4V (0.86 wt%), implying that the CMWNT coating could weaken the adhesive wear. The results revealed that CMWNT coating could improve the wear resistance of titanium alloy under SBF lubrication effectively and the DA transition layer had more remarkable effects to protect the titanium alloy substrate, which were consistent with that under dry friction. Fig. 11 showed the SEM micrographs of the worn surface of Si3N4 balls under SBF lubrication. It can be seen from the magnification of
200× (Fig. 11(a–c)) that the size of wear scars of three Si3N4 balls against different plates had the following sequence: APS-DACMWNT < APS-CMWNT < uncoated Ti-6Al-4V plate. Furthermore, it is obvious from the magnifications of SEM images (2000× (Fig. 11(a1–c1)), 10,000× (Fig. 11(a2–c2))) that compared with the uncoated Ti-6Al-4V alloy, the wear scar from Si3N4 balls against APSDA-CMWNT showed slightest ploughs and least debris, next against APS-CMWNT. These findings suggested the occurrence of abrasive wear and the protect effect of CMWNT on the titanium alloy plate under SBF lubrication. In order to further study the transfer of CMWNT coating on Si3N4 balls during the process of friction, the Raman spectra of Si3N4 balls in dry friction and SBF lubrication were displayed in Fig. 12. It is apparent from Fig. 12(a) and (b) that two characteristic peaks of carbon nanotubes at D and G, and two low signals of 2D and D + G, indicating that CMWNT existed on wear scars of Si3N4 balls rubbing with two CMWNT coated samples under both conditions, which further demonstrated the adhesive wear mechanism during both lubrications. 4. Discussion The experimental results and characterization analysis indicated that the CMWNT coating might have an effective protective impact on the Ti-6Al-4V substrates. The APS-DA-CMWNT samples showed superior biotribological properties under dry sliding and SBF lubrications, followed by APS-CMWNT. The corresponding biotribological mechanisms under both lubrications were discussed below, as shown in Figs. 13 and 14. The previous studies had reported that MWNT could enhance tribological properties because carbon nanotubes tended to roll 8
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Fig. 10. 3D micrographs, SEM images and EDX results of wear scars on Ti-6Al-4V samples in SBF lubrication. (a) Ti-6Al-4V, (b) APS-CMWNT, (c) APS-DA-CMWNT.
perpendicularly to the sliding direction due to the lower energy barrier from rolling motion [22,42]. In this work, CMWNT were grafted onto the surface of the Ti-6Al-4V alloy substrates through the DA and APS transition layers. In addition,
the corresponding EDX results indicated that CMWNT appeared on the wear surface of the CMWNT coated specimens in dry friction and SBF lubrication. Furthermore, the Raman spectra of wear scars from the Si3N4 balls verified that CMWNT had adhered to the wear surface, 9
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Fig. 11. SEM images of worn scars on the Si3N4 balls in SBF lubricating conditions. (a) Ti-6Al-4V, (b) APS-CMWNT, (c) APS-DA-CMWNT.
revealing the transfer of CMWNT coating during friction process. Based on the above results and analysis, a schematic diagram of the tribological mechanisms was exhibited in Fig. 13. As shown in Fig. 13, during the process of reciprocating friction, the small broken tubular CMWNT between both friction pairs might be presented due to coiling and
rolling, and subsequently these tubes began to crack and form continuous lubricating layer [43–46]. The relative rolling of the broken tubular CMWNT might result in the transformation of friction from sliding friction to rolling friction, thereby reducing the COF and improving the wear resistance of titanium alloy, which was consistent
Fig. 12. Raman spectrums of wear surfaces of Si3N4 balls against three Ti-6Al-4V samples. (a) In dry friction, (b) under SBF lubrication. 10
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Fig. 13. A schematic diagram of tribological mechanism of CMWNT coating in dry friction.
Fig. 14. A schematic diagram of biotribological mechanism of CMWNT coating under SBF lubrication.
with previous studies [19]. Moreover, the tubular microstructures (Fig. 4) of CMWNT between the friction pairs would reduce actual contact area and the shear strength between the friction pairs, further reducing the COF and adhesive wear on the surface of Ti-6Al-4V in dry sliding [47]. In addition, the biotribological mechanism diagram under SBF lubrication was shown in Fig. 14. It can be observed from Fig. 14 that water molecules might form a lubricating layer between the friction pairs, leading to the decrease of shear strength which might be a critical factor for the reduction of COF compared with that in dry friction. Besides, our previous study revealed that the occurrence of corrosion by ions in SBF solution such as Cl−, SO42− might have a negative effect on the surface of the titanium alloy [41]. This might be the main reason that wear rates of the samples under SBF lubrication were higher than that in dry friction. In addition, the biotribological properties of APS-CMWNT and APSDA-CMWNT samples were compared. The results illustrated that DA transition layer could significantly ameliorate the biotribological properties of CMWNT coatings on the titanium alloy surface. Moreover, scratch tests showed that the adhesive strength of CMWNT coatings on the APS-DA-CMWNT was higher than that of APS-CMWNT, implying that the bonding adhesion of CMWNT coatings on Ti-6Al-4V alloys could be effectively improved by DA transition layer. Compared with the APS-DA-CMWNT sample, the CMWNT coatings of APS-CMWNT were prone to peel off from the surface of titanium alloy in the process of friction, resulting in an increase of abrasive particles, further leading to the aggravation of abrasive wear. In terms of APS-DA-CMWNT samples, the dense PDA transition layer formed by DA self-agglomeration might result in excellent load-carrying capacity on the Ti-6Al4V surface, thereby protecting the titanium alloy plates from severe wear, which was consistent with our previous studies [37]. Therefore, the introduction of DA transition layer could ameliorate the biotribological properties of APS-CMWNT samples.
5. Conclusions The carboxylic multi-wall carbon nanotubes were successfully grafted on Ti-6Al-4V alloy by a simple and efficient chemical self-assembly process. APS and DA self-polymer were introduced as two transition layers to realize the coating preparation. For comparison, the APS-CMWNT samples without DA transition layer were prepared. The results revealed that the biotribological properties of Ti-6Al-4V alloy were significantly improved by CMWNT coatings. The main conclusions were as follows: (1) The biotribological tests results illustrated that the COF and wear rate of CMWNT coatings under dry sliding and SBF lubrication had the following sequence: uncoated Ti-6Al-4V > APSCMWNT > APS-DA-CMWNT. Besides, the COF of these samples in SBF lubrication were lower than those under dry sliding, while the wear rate of samples exhibited opposite variation. (2) The wear surface analysis indicated that the major wear mechanisms during the friction process were adhesive wear and abrasive wear. Due to the relative rolling formed by micro-tubular CMWNT between the friction pairs, the CMWNT coated samples showed excellent biotribological properties. (3) The biotribological properties of APS-DA-CMWNT samples were better than those of APS-CMWNT samples. Due to the existence of DA transition layer, APS-DA-CMWNT had better load-carrying capacity and stronger adhesion on titanium alloy substrates than APSCMWNT. Author contributions J.D. and C.C.W. planned and supervised the study. J.D. and S.H.P. finished the process of preparing samples. J.D. carried out the 11
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biotribological experiments, data analysis, image graphics and initial manuscript writing. C.C.W and T.H.R. assisted to revise and finalize the manuscript. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
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Biotribological properties of Ti-6Al-4V alloy treated with self-assembly multi-walled carbon nanotube coating Jun Deng, Songhong Pang, Chenchen Wang , Tianhui Ren ⁎
⁎
School of Chemistry and Chemical Engineering, Key Laboratory of Thin Film and Microfabrication Technology (Ministry of Education), Shanghai Jiao Tong University, 200240 Shanghai, China
ARTICLE INFO
ABSTRACT
Keywords: Ti-6Al-4V CMWNT coating Self-assembly Biotribology
Ti-6Al-4V alloy has been widely applied in biomedical field due to its good biocompatibility, non-toxicity and excellent anti-corrosion. However, the poor tribological performance limits its application. In this paper, in order to enhance the biotribological properties of Ti-6Al-4V alloy, carboxylic multi-walled carbon nanotubes (CMWNT) coatings were grafted on the surface by chemical self-assembly technology. Raman spectroscopy and scanning electron microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDX) were utilized to characterize the microstructures of the coatings. The CMWNT coatings showed lower coefficient of friction and wear rates under dry sliding and simulated body fluid (SBF) lubrication. Furthermore, the biotribological mechanisms of CMWNT coatings under both conditions were discussed. Analyses of worn surfaces indicated that the rolling friction between the microtubes due to the spalling and fracture of CMWNT during the friction process was a major factor for improving biotribological behavior.
1. Introduction Titanium alloys, especially Ti-6Al-4V, have been widely applied in biomedical field due to their biocompatibility, high strength-to-weight ratio, and excellent anti-corrosion [1,2]. However, poor tribological properties of Ti-6Al-4V such as high coefficient of friction and inferior wear resistance, limit its application as bioimplants, such as artificial joints [3]. The wear debris containing Al and V elements are mostly likely to enter in the bloodstream, resulting in aseptic loosening of the prosthesis and inflammation of the surrounding tissues, further leading to the failure of the implants [4,5]. Therefore, several surface treatments have been carried out to enhance the tribological performance of titanium alloy [6]. To date, various modification technologies have been used, these include surface coatings and surface texturing [7–12]. Among them, surface coatings have been considered as a good option to apply on the surface of Ti-6Al-4V plates, due to its directness and effectiveness to improve surface properties. Yetim et al. [13] prepared composite diamond-like carbon (DLC) coating on untreated Ti-6Al-4V alloys. The results reported that the coefficient of friction (COF) and worn rate of Ti-6Al-4V alloys significantly reduced after coating DLC film. Viteri et al. [14] investigated the improved tribological performance of Ti-C-N coating on the Ti-6Al-4V surface. However, these coatings have some drawbacks, such as weak adhesion and instability on the substrate ⁎
during friction process. Thus, suitable lubricated coatings should be considered to be prepared on the surface of titanium alloys. Carbon nanotubes (CNT) have attracted great attention in biomedical application, owing to its high aspect ratio, high mechanical properties and biocompatibility [15–18]. The previous researches have shown that CNT could be utilized as solid lubricants or liquid lubricants, implying its self-lubricating properties [19,20]. Savalani et al. [21] prepared Ti-CNT composite layers reinforced Ti matrices and indicated that the composite layers with CNT exhibited superior wear resistance compared with pure Ti substrates. Wang et al. [22] added CNT to TiO2 coating to evaluate its effect on tribological properties and reported that the COF and wear volume of nanocomposite coatings decreased, suggesting the enhancement of tribological properties. Therefore, CNT is considered to be an ideal coating to enhance tribological properties of titanium alloy. Nevertheless, the preparation methods of CNT on the surface of titanium alloy face some challenges, such as the harsh temperature conditions from heat treatment [20] and chemical vapor deposition (CVD) [23], the weak bonding adhesion and inhomogenous dispersion from electrodeposition [24] and spark plasma sintering [25]. In order to prepare stable CNT coatings on the surface of titanium alloy, chemically attaching CNT by self-assembly without external force is a feasible and simple method, which has attracted much attention [26,27]. According to the previous studies, it is difficult to directly graft CNT onto the surface of materials due to its chemical
Corresponding authors. E-mail addresses: [email protected] (C. Wang), [email protected] (T. Ren).
https://doi.org/10.1016/j.surfcoat.2019.125169 Received 15 September 2019; Received in revised form 10 November 2019; Accepted 15 November 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Jun Deng, et al., Surface & Coatings Technology, https://doi.org/10.1016/j.surfcoat.2019.125169
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inertness [28,29]. Therefore, functionalized CNT with active groups, such as hydroxyl, epoxide and carboxyl groups, is considered for application [30–32]. To improve the bonding strength of the functionalized CNT coatings on the titanium alloy surface, the intermediate layer should be introduced. 3-Aminopropyl triethoxysilane (APS), has been proved to be a suitable choice as the intermediate layer, due to its high activities and strong bonding strength between coatings and substrates [33–35]. Li et al. [36] introduced APS as the intermediate layer to graft graphene oxide on the titanium substrates. However, the results showed that the coating was prone to peel off from the substrates after a certain period [37]. Hence, the second intermediate layer can be considered to reinforce the adhesion. In our previous study, dopamine (DA) has been utilized to improve the bonding strength of graphene oxide with the substrates, owing to its super adhesion, non-toxicity and eco-friendliness [27,37–39]. Furthermore, polydopamine (PDA) self-polymerized by DA monomers can also provide a second reaction plate for coatings. Therefore, DA can be considered to be a fabulous choice for the second intermediate layer. In this study, carboxylic multi-wall carbon nanotubes (CMWNT) were firstly obtained by the oxidization of multi-wall carbon nanotubes. A simple and efficient chemical self-assembly technology was utilized to graft the CMWNT coatings on the surface of titanium alloy. Among that, APS and DA were respectively introduced as the transition layers. The prepared samples were referred to APS-DA-CMWNT. For comparison, CMWNT coated titanium alloy plates using APS as the transition layer were prepared as APS-CMWNT. The results of scratch tests indicated that DA could improve the bonding adhesion between substrate and coating. The biotribological properties of the specimens under dry sliding and simulated body fluid (SBF) lubrication were investigated, respectively. These results illustrated that the CMWNT coatings enhanced the biotribological properties of Ti-6Al-4V alloy. Furthermore, the biotribological properties of the APS-DA-CMWNT samples under both lubrications were superior to that of APS-CMWNT samples. The corresponding biotribological mechanisms were deduced and discussed by surface analysis.
removement of residues from the surface of MWNT during the oxidation and subsequent cleaning process. It can be seen from Fig. 1(c) that the original MWNT exhibited the absorption peak at 1590 cm−1, indicating that the stretching movement of C]C bond. The same characteristic peak at 1572 cm−1 was found in the spectrum of CMWNT sample. However, the new peak of carboxyl group at 1717 cm−1 from –COOH occurred on CMWNT, suggesting the success of the carboxylation reaction. 2.3. Self-assembly of CMWNT coatings on the surface of Ti-6Al-4V alloy The prepared titanium alloy samples were first reacted at 20% HNO3 solution for 30 min at room temperature to remove the oxide films. The surfaces were washed by deionized water and dried by N2. Then, the samples were placed in 5 mol/L KOH solution for 12 h at 40 °C to raise the amount of hydroxyl groups on the surfaces. Afterwards, the hydroxylated titanium alloy plates were placed into a prepared 3% APS aqueous solution (with the adjusted pH of 8.5 by adding 1 mol/L HCl solution) for 1 h at ambient temperature. After washing and drying, the samples were put into 2 g/L DA in Tris solution (pH = 10.5) for 12 h at room temperature during stirring thoroughly, and then washed by deionized water and dried by N2. The prepared samples were abbreviated as APS-DA. After that, the APS-DA samples were put into 2 g/L CMWNT aqueous solution treated in the ultrasound bath for 5 h with 150 W, and then reacted at 60 °C for 24 h. The obtained samples were referred to APS-DA-CMWNT. For comparison, the Ti-6Al-4V plates modified by the intermediate layer of APS were reacted with 2 g/L CMWNT aqueous solution at the same treatment and reaction condition. The prepared samples were abbreviated as APSCMWNT. 2.4. Bonding strength tests The bonding strengths between the Ti-6Al-4V plate and CMWNT coating were measured by scratch tests [40]. The specimens were scraped by coating adhesion automatic scratch tester (WS-2005, China) with the scratch speed of 5 mm/min, the maximum load of 100 N, and the load speed of 100 N/min. APS-DA-CMWNT and APS-CMWNT samples were tested four times, respectively, and the average adhesion strengths were evaluated by the essential load that the coating can withdraw.
2. Experimental procedures 2.1. Materials The medical grade titanium alloy (Ti-6Al-4V) plates were processed into 10 mm × 10 mm × 2 mm (length × width × thickness) and polished to the roughness (Ra) of 0.1 ± 0.02 μm. Before processing, Ti6Al-4V plates were washed with acetone and water for 1 h in the ultrasound bath with 150 W, respectively. DA hydrochloric (98%) and APS (99%) were of analytical grade.
2.5. Biotribological tests The biotribological measurements were carried out on the ball-onplate High-speed Reciprocating Friction and Wear Tester (Yanhua Electronic Technology Co., Ltd., Lanzhou, Gansu, China) to evaluate the biotribological properties of CMWNT coatings on Ti-6Al-4V plates during dry friction and SBF lubrication. Among that, Si3N4 ceramic balls (ф = 6 mm, with the hardness of 1500 HV, the mean roughness (Ra) of 0.02 μm) were used as the stational upper counterpart, and the tested specimens were fixed in a rectangle sink as lower friction pair. All biotribological tests were performed under the vertical load of 10 N, the sliding frequency of 1 Hz and the amplitude of 2 mm for 30 min in dry friction and SBF lubrication, respectively. And the ion concentrations in SBF solution are listed in the Table 1. Samples were tested for at least three times under each condition to guarantee the data repeatability. The worn rates k of Ti-6Al-4V specimens were calculated by Eq. (1) [41]: (mm3/(N·m))
2.2. Preparation of CMWNT Carboxylic multi-wall carbon nanotubes (CMWNT) were prepared by using the following method: 400 mL of prepared mixture acid (V98%H2SO4 : V30%H2O2 = 3 : 1) was transferred to the flask to react with 1 g MWNT, and then heated and refluxed at 100 °C for 12 h during stirring. Subsequently, the mixture was naturally cooled at room temperature for approximately 1 h, and then centrifuged with the speed of 18,000 r/min for 25 min. The upper solution was discarded and the deionized water was added in the stirring state. After washing 7 times, the pH of supernatant was neutral. The lower CMWNT with the concentration of 9.6 g/L was taken out and as a prepared material for subsequent experiments. Fig. 1 showed the SEM images and FTIR spectra of the original MWNT and CMWNT. As exhibited in Fig. 1(a) and (b), the diameters of the MWNT and CMWNT were about 18–30 nm, and the carbon nanotubes were prone to wrap. In addition, the diameters of CMWNT were slightly smaller than that of MWNT, which may be attributed to the
k = V /(X FN ),
(1)
where V is the worn volume of Ti-6Al-4V specimens, X is the sliding distance and FN is the applied normal load. 2
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Fig. 1. SEM images of (a) MWNT and (b) CMWNT and FTIR spectra of two samples (c).
2.6. Characterizations The microscopic structures of the three different Ti-6Al-4V specimens were characterized by Raman spectra (DXR, Thermo Fisher Scientific Co., Ltd., USA). The MWNT and CMWNT powders and surface coatings on prepared Ti-6Al-4V samples were analyzed by ATR-FTIR spectrometer (attenuated total reflection of Fourier transform infrared, Spectrum 100, Perkin Elmer, Inc., USA) with a resolution of 4.0 cm−1 over a scan range of 600–4000 cm−1. The surface coating thicknesses of the samples were measured using the Alpha-step tester (MFT-4000). Scanning electron microscopy (SEM, FEI Nova 450, USA) was employed to characterize the surfaces of the Ti-6Al-4V specimens before and after the biotribological tests as well as morphology of MWNT and CMWNT. And the compositions of chemical element on worn surfaces were analyzed by Raman spectroscopy and Energy Dispersive X-ray Spectroscopy (EDX). White light interferometer (WLI, Bruker, Contour GT-K0) was utilized to observe the three-dimensional wear scar morphology and calculate the worn volume of the specimens. 3. Results
Fig. 2. FTIR spectra of APS-CMWNT and APS-DA-CMWNT samples.
3.1. Surface characterization
showed two new bands of C]O(N) at 1628/1435 cm−1, CeN at 2901 cm−1, indicating that the amidation reaction between DA and CMWNT. The Raman spectra of APS-CMWNT and APS-DA-CMWNT coated titanium alloys were exhibited in Fig. 3(a). As shown in the Fig. 3, compared with the bare titanium alloys (Fig. 3(b)), Raman spectra of APS-CMWNT and APS-DA-CMWNT both displayed two prominent peaks at D band (1333 cm−1) and G band (1588 cm−1), and two weak peaks at 2D and D + G modes, indicating that CMWNT coatings had been successfully grafted. Among those peaks, D-band was related to the transformation of sp2 hybrid carbon to sp3 hybrid carbon, while Gband corresponded to the plane vibration of sp2 hybrid carbon, which
The ATR-FTIR spectra of APS-CMWNT and APS-DA-CMWNT specimens were shown in Fig. 2. It can be found from the spectrum of APSCMWNT that a Si-O-Si band at 904 cm−1 occurred, resulting from the condensation reaction between Si-OH of APS and eOH of CMWNT, suggesting that APS molecules were prone to hydroxylate with CMWNT in aqueous solution. In addition, the occurrence of CeN characteristic peaks at 2922 cm−1 and the weaken epoxy groups at 1100 cm−1 implied that the epoxy groups from CMWNT might react with the amino of APS. Moreover, the appearance of peaks at 1630/1425 cm−1 could infer the structural changes of carboxyl groups owing to the reaction with the amine groups of APS. The spectrum of APS-DA-CMWNT Table 1 Chemical composition of SBF solution. Ion Concentration (mM)
Na+ 142.0
Mg2+ 1.5
K+ 5.0
Cl− 103.3
Ca2+ 2.5
3
HCO32− 10.0
HPO42− 1.0
SO42− 0.5
pH 7.4
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CMWNT coating were significantly lower than that of uncoated plates, and the APS-DA-CMWNT sample exhibited the lowest COF. The results were consistent with the calculated average COF (Fig. 7(c)), implying that CMWNT coating can effectively reduce the COF under dry friction and SBF biolubrication and DA had a positive effect on the reduction of COF. In addition, the worn rates of three titanium alloy plates under both lubrications were calculated in Fig. 7(d). It can be observed that the worn rates of the CMWNT coated samples (APS-CMWNT and APS-DACMWNT) were remarkably lower than that of the uncoated plates under both lubrications, respectively, suggesting that CMWNT can reduce the wear of the titanium alloys. Compared with the wear rate of APSCMWNT, APS-DA-CMWNT showed the superior wear resistance, which was consistent with the results in ACOF (Fig. 7(c)). The results illustrated that the introducing of DA transition layer can remarkably enhance the biotribological properties of CMWNT coatings. By comparing the biotribological properties of the two lubrication conditions, it can be found that the ACOF values of three titanium alloy samples under SBF lubrication were lower than that in dry friction, while the worn rates (Fig. 7(d)) of three samples were higher in SBF solution. This might be ascribed to the corrosive wear by ions in SBF solution on the surface of samples, such as Cl−, SO42− [41].
Fig. 3. Raman spectra of samples: (a) APS-CMWNT and APS-DA-CMWNT, (b) Ti-6Al-4V.
3.3. Analysis of worn surfaces
was related to the destruction of sp2 structure of graphite or covalent binding of functional groups. In order to more intuitively analyze the grafting of CMWNT on the sample surface, SEM images and compositions analysis of APS-CMWNT and APS-DA-CMWNT were shown in Fig. 4. Fig. 4(a) and (b) exhibited that the coatings of the two samples were relatively uniform. It can be seen from Fig. 4(a) and (a1) that, interwoven CMWNT tubular structure appeared on the surface of Ti-6Al-4V alloys. Furthermore, the corresponding EDX mapping (Fig. 4(a2)) characterized the C elements distribution of surface composition, suggesting the presence of CMWNT, further indicating that CMWNT had been successfully grafted onto the surface of APS-CMWNT specimen. Similarly, Fig. 4(b) and (b1) exhibited a network structure on the surface of titanium alloys, which might be attributed to the introduction of CMWNT. This result can be verified by Fig. 4(b2) with the C elements distribution. Fig. 5 showed the variation of friction force with load measured by scratch tests [40]. As shown in Fig. 5(a), CMWNT coatings on the surface of APS-CMWNT sample were scraped off when the load was more than 77.0 N. After introducing the second adhesive layer DA, the critical load (82.8 N) of APS-DA-CMWNT sample was higher than that in APS-CMWNT, indicating that DA transition layer can enhance the bonding force of CMWNT coatings and the substrate. In order to further analyze the CMWNT coatings on the surfaces of APS-CMWNT and APS-DA-CMWNT samples, the corresponding coating thicknesses were shown in Fig. 6. The thickness of APS-CMWNT coatings was calculated as ~0.44 μm (APS-CMWNT, Fig. 6(a)) and the thickness of APS-DA-CMWNT was ~0.66 μm (APS-DA-CMWNT, Fig. 6(b)) which were greater than the thickness of CMWNT tubes. The results showed that the CMWNT coating was assembled by chemical bonding and physical adsorption. In addition, the thickness of APS-DACMWNT coating was thicker than APS-CMWNT coating, which might be due to the strong adhesion of DA transition layer.
In order to further investigate the wear mechanism of CMWNT coating on the surface of titanium alloy, the wear surfaces of specimens under dry friction and SBF lubrication were characterized by SEM and WLI, respectively. Fig. 8 showed the three-dimensional (3D) microscopic images, SEM micrographs and EDX elemental analysis of the samples in dry condition. The 3D topographic maps of Ti-6Al-4V, APS-CMWNT and APS-DACMWNT were shown in Fig. 8(a–c). It can be seen that the scratch depth of uncoated Ti-6Al-4V sample was approximately 25 μm. While the values of two CMWNT coated samples were lower than that of uncoated Ti-6Al-4V plate, with ~22 μm (APS-CMWNT) and ~19 μm (APS-DACMWNT). The results exhibited that APS-DA-CMWNT had the best wear resistance under dry friction, suggesting that CMWNT coating had remarkably protective effects on Ti-6Al-4V plates. The SEM images under different magnification (200× (Fig. 8(a1–c1)), 2000× (Fig. 8(a2–c2)), 10,000× (Fig. 8(a3–c3))) of the samples under dry friction showed that serious material loss, parallel plough shape and delamination were found on the surface of uncoated titanium alloy plate, resulting from abrasive wear behavior. Compared with the uncoated sample, the two CMWNT coated samples had significant slighter wear scar with swallow scratches, less debris and smaller sized flake-like chips, demonstrating the effective protection. Among the two CMWNT coated samples, APSDA-CMWNT displayed the lightest scratches and the smallest debris. This finding might indicate that DA had further enhanced the wear resistance of CMWNT coating. Moreover, the corresponding EDX analysis (Fig. 8(a4–c4)) showed that the Si element appeared on the worn surface, suggesting the transfer of Si3N4 contents, further demonstrating the adhesive wear mechanism. Compared with that on uncoated Ti-6Al4V plate (1.31 wt%), the Si contents were detected on the worn area of APS-DA-CMWNT (0.81 wt%), next on APS-CMWNT (0.91 wt%), implying the improvement of adhesive wear due to the CMWNT coatings. The above results illustrated that CMWNT coatings can improve the tribological behavior under dry friction. For the Si3N4 balls rubbing against the plates, the SEM images on the worn surface in dry sliding were compared in Fig. 9. It can be observed from Fig. 9(a–c) that the worn scar of uncoated Ti-6Al-4V was the largest, followed by APS-CMWNT, and the lightest appeared on the surface of APS-DA-CMWNT sample. In addition, debris and ploughs can be detected on the magnification of SEM images (Fig. 9(a2–c2)), implying that the materials from the titanium alloy plates transferred to the Si3N4 balls in the wear process, which further demonstrated the
3.2. Biotribological properties On the purpose of researching the protective effects of CMWNT coatings on the titanium alloy substrates, friction tests were respectively carried out under dry friction and SBF lubrication, as exhibited in Fig. 7. Fig. 7(a) and (b) revealed the dynamic COF curves of the samples surface during friction process under both lubrications. It can be observed that the COF tended to be relatively stable after a period of time. Besides, under both lubrications, the COF of the two samples with 4
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Fig. 4. SEM images and Mapping graphs of samples: (a) APS-CMWNT, (b) APS-DA-CMWNT.
Fig. 5. Friction force-load curves for scratch test: (a) APS-CMWNT, (b) APS-DA-CMWNT. 5
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Fig. 6. Thicknesses of samples: (a) APS-CMWNT, (b) APS-DA-CMWNT.
Fig. 7. Biotribological properties of Ti-6Al-4V, APS-CMWNT and APS-DA-CMWNT under dry friction and SBF lubrication: (a) coefficient of friction (COF) curves in dry friction, (b) COF curves under SBF lubrication, (c) average coefficient of friction (ACOF) curves, (d) worn rates. 6
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Fig. 8. 3D micrographs, SEM images and EDX results of wear scars on Ti-6Al-4V samples in dry sliding. (a) Ti-6Al-4V, (b) APS-CMWNT, (c) APS-DA-CMWNT.
mechanism of abrasive wear. Moreover, it is obvious that the slightest ploughs and the least debris were found on the surface of Si3N4 ball against APS-DA-CMWNT sample, next against APS-CMWNT and most severe against the uncoated Ti-6Al-4V plate. The wear spot
observations of the Si3N4 balls were consistent with those of Ti-6Al-4V samples. Therefore, it can be inferred that the mixed wear mechanism might include adhesive wear and abrasive wear. Fig. 10 showed the 3D micrographs (Fig. 10(a–c)), SEM images with 7
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Fig. 9. SEM images of worn scars on the Si3N4 balls in dry friction. (a) Ti-6Al-4V, (b) APS-CMWNT, (c) APS-DA-CMWNT.
the magnification of 200× (Fig. 10(a1–c1)), the magnification of 2000× (Fig. 10(a2–c2)), the magnification of 10,000× (Fig. 10(a3–c3)), and EDX results of samples during SBF lubrication. It can be seen from the Fig. 10(a–c) that the maximum depths of worn scar on uncoated titanium alloy specimens was up to ~33 μm, while the depths of two samples coated with CMWNT were significantly swallow, ranging from ~24 μm (APS-CMWNT) to ~21 μm (APS-DA-CMWNT). The results indicated that CMWNT coating might plays an active role in the wear resistance of Ti-6Al-4V plates under SBF solution lubrication, and the most apparent effect was reflected on the APS-DA-CMWNT sample. As shown in the SEM images, the wear scar widths of three Ti6Al-4V plates showed the following sequence: Ti-6Al-4V > APSCMWNT > APS-DA-CMWNT. Serious parallel ploughs and stratifications appeared on uncoated Ti-6Al-4V surface, indicating the abrasive wear behavior during SBF solution lubrication. Compared with the uncoated sample, two CMWNT coated specimens had slighter scratches and smaller debris, and APS-DA-CMWNT had the shallowest scratches, ploughs and the least debris, indicating that DA could improve the wear resistance of CMWNT coating. The EDX results (Fig. 10(a4–c4)) showed that Si element on the wear scars of three samples had the following sequence: APS-DA-CMWNT (0.48 wt%) < APS-CMWNT (0.64 wt %) < uncoated Ti-6Al-4V (0.86 wt%), implying that the CMWNT coating could weaken the adhesive wear. The results revealed that CMWNT coating could improve the wear resistance of titanium alloy under SBF lubrication effectively and the DA transition layer had more remarkable effects to protect the titanium alloy substrate, which were consistent with that under dry friction. Fig. 11 showed the SEM micrographs of the worn surface of Si3N4 balls under SBF lubrication. It can be seen from the magnification of
200× (Fig. 11(a–c)) that the size of wear scars of three Si3N4 balls against different plates had the following sequence: APS-DACMWNT < APS-CMWNT < uncoated Ti-6Al-4V plate. Furthermore, it is obvious from the magnifications of SEM images (2000× (Fig. 11(a1–c1)), 10,000× (Fig. 11(a2–c2))) that compared with the uncoated Ti-6Al-4V alloy, the wear scar from Si3N4 balls against APSDA-CMWNT showed slightest ploughs and least debris, next against APS-CMWNT. These findings suggested the occurrence of abrasive wear and the protect effect of CMWNT on the titanium alloy plate under SBF lubrication. In order to further study the transfer of CMWNT coating on Si3N4 balls during the process of friction, the Raman spectra of Si3N4 balls in dry friction and SBF lubrication were displayed in Fig. 12. It is apparent from Fig. 12(a) and (b) that two characteristic peaks of carbon nanotubes at D and G, and two low signals of 2D and D + G, indicating that CMWNT existed on wear scars of Si3N4 balls rubbing with two CMWNT coated samples under both conditions, which further demonstrated the adhesive wear mechanism during both lubrications. 4. Discussion The experimental results and characterization analysis indicated that the CMWNT coating might have an effective protective impact on the Ti-6Al-4V substrates. The APS-DA-CMWNT samples showed superior biotribological properties under dry sliding and SBF lubrications, followed by APS-CMWNT. The corresponding biotribological mechanisms under both lubrications were discussed below, as shown in Figs. 13 and 14. The previous studies had reported that MWNT could enhance tribological properties because carbon nanotubes tended to roll 8
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Fig. 10. 3D micrographs, SEM images and EDX results of wear scars on Ti-6Al-4V samples in SBF lubrication. (a) Ti-6Al-4V, (b) APS-CMWNT, (c) APS-DA-CMWNT.
perpendicularly to the sliding direction due to the lower energy barrier from rolling motion [22,42]. In this work, CMWNT were grafted onto the surface of the Ti-6Al-4V alloy substrates through the DA and APS transition layers. In addition,
the corresponding EDX results indicated that CMWNT appeared on the wear surface of the CMWNT coated specimens in dry friction and SBF lubrication. Furthermore, the Raman spectra of wear scars from the Si3N4 balls verified that CMWNT had adhered to the wear surface, 9
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Fig. 11. SEM images of worn scars on the Si3N4 balls in SBF lubricating conditions. (a) Ti-6Al-4V, (b) APS-CMWNT, (c) APS-DA-CMWNT.
revealing the transfer of CMWNT coating during friction process. Based on the above results and analysis, a schematic diagram of the tribological mechanisms was exhibited in Fig. 13. As shown in Fig. 13, during the process of reciprocating friction, the small broken tubular CMWNT between both friction pairs might be presented due to coiling and
rolling, and subsequently these tubes began to crack and form continuous lubricating layer [43–46]. The relative rolling of the broken tubular CMWNT might result in the transformation of friction from sliding friction to rolling friction, thereby reducing the COF and improving the wear resistance of titanium alloy, which was consistent
Fig. 12. Raman spectrums of wear surfaces of Si3N4 balls against three Ti-6Al-4V samples. (a) In dry friction, (b) under SBF lubrication. 10
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Fig. 13. A schematic diagram of tribological mechanism of CMWNT coating in dry friction.
Fig. 14. A schematic diagram of biotribological mechanism of CMWNT coating under SBF lubrication.
with previous studies [19]. Moreover, the tubular microstructures (Fig. 4) of CMWNT between the friction pairs would reduce actual contact area and the shear strength between the friction pairs, further reducing the COF and adhesive wear on the surface of Ti-6Al-4V in dry sliding [47]. In addition, the biotribological mechanism diagram under SBF lubrication was shown in Fig. 14. It can be observed from Fig. 14 that water molecules might form a lubricating layer between the friction pairs, leading to the decrease of shear strength which might be a critical factor for the reduction of COF compared with that in dry friction. Besides, our previous study revealed that the occurrence of corrosion by ions in SBF solution such as Cl−, SO42− might have a negative effect on the surface of the titanium alloy [41]. This might be the main reason that wear rates of the samples under SBF lubrication were higher than that in dry friction. In addition, the biotribological properties of APS-CMWNT and APSDA-CMWNT samples were compared. The results illustrated that DA transition layer could significantly ameliorate the biotribological properties of CMWNT coatings on the titanium alloy surface. Moreover, scratch tests showed that the adhesive strength of CMWNT coatings on the APS-DA-CMWNT was higher than that of APS-CMWNT, implying that the bonding adhesion of CMWNT coatings on Ti-6Al-4V alloys could be effectively improved by DA transition layer. Compared with the APS-DA-CMWNT sample, the CMWNT coatings of APS-CMWNT were prone to peel off from the surface of titanium alloy in the process of friction, resulting in an increase of abrasive particles, further leading to the aggravation of abrasive wear. In terms of APS-DA-CMWNT samples, the dense PDA transition layer formed by DA self-agglomeration might result in excellent load-carrying capacity on the Ti-6Al4V surface, thereby protecting the titanium alloy plates from severe wear, which was consistent with our previous studies [37]. Therefore, the introduction of DA transition layer could ameliorate the biotribological properties of APS-CMWNT samples.
5. Conclusions The carboxylic multi-wall carbon nanotubes were successfully grafted on Ti-6Al-4V alloy by a simple and efficient chemical self-assembly process. APS and DA self-polymer were introduced as two transition layers to realize the coating preparation. For comparison, the APS-CMWNT samples without DA transition layer were prepared. The results revealed that the biotribological properties of Ti-6Al-4V alloy were significantly improved by CMWNT coatings. The main conclusions were as follows: (1) The biotribological tests results illustrated that the COF and wear rate of CMWNT coatings under dry sliding and SBF lubrication had the following sequence: uncoated Ti-6Al-4V > APSCMWNT > APS-DA-CMWNT. Besides, the COF of these samples in SBF lubrication were lower than those under dry sliding, while the wear rate of samples exhibited opposite variation. (2) The wear surface analysis indicated that the major wear mechanisms during the friction process were adhesive wear and abrasive wear. Due to the relative rolling formed by micro-tubular CMWNT between the friction pairs, the CMWNT coated samples showed excellent biotribological properties. (3) The biotribological properties of APS-DA-CMWNT samples were better than those of APS-CMWNT samples. Due to the existence of DA transition layer, APS-DA-CMWNT had better load-carrying capacity and stronger adhesion on titanium alloy substrates than APSCMWNT. Author contributions J.D. and C.C.W. planned and supervised the study. J.D. and S.H.P. finished the process of preparing samples. J.D. carried out the 11
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biotribological experiments, data analysis, image graphics and initial manuscript writing. C.C.W and T.H.R. assisted to revise and finalize the manuscript. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
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