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Structure, mechanical and tribological properties of TaCx composite films with different graphite powers
Journal Pre-proof Structure, mechanical and tribological properties of TaCx composite films with different graphite powers Jiaojiao Hu, Hang Li, Jianliang Li, Jiewen huang, Jian Kong, Heguo Zhu, Dangsheng Xiong PII:
S0925-8388(20)30132-8
DOI:
https://doi.org/10.1016/j.jallcom.2020.153769
Reference:
JALCOM 153769
To appear in:
Journal of Alloys and Compounds
Received Date: 24 August 2019 Revised Date:
7 January 2020
Accepted Date: 8 January 2020
Please cite this article as: J. Hu, H. Li, J. Li, J. huang, J. Kong, H. Zhu, D. Xiong, Structure, mechanical and tribological properties of TaCx composite films with different graphite powers, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.153769. 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 B.V.
Author Contributions Section Jiaojiao Hu: Conceptualization, Methodology, Investigation, Data curation, Formal analysis, Writing- Original draft preparation. Hang Li: Conceptualization, Methodology, Formal analysis, Supervision, Writing - Review & Editing. Jianliang Li*: Resources, Supervision, Writing - Review & Editing. Jiewen huang: Software, Validation. Jian Kong: Resources, Visualization. Heguo Zhu: Project administration, Funding acquisition. Dangsheng Xiong: Project administration, Funding acquisition.
Structure, mechanical and tribological properties of TaCx composite films with different graphite powers Jiaojiao Hu, Hang Li, Jianliang Li*, Jiewen huang, Jian Kong, Heguo Zhu, Dangsheng Xiong School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing, 210094 Tel.: +86 025 84315325; E-mail address: [email protected] Abstract In this study, the TaCx composite films consisted of amorphous carbon and nanocrystalline TaC were synthesized with varying sputtering powers of graphite by DC magnetron sputtering, and studied their microstructure, mechanical and tribological behaviors. The film exhibits a Ta2C phase and high content a-C composite structure at a graphite power of 30 W, then a new composite structure with a lower a-C content and TaC nanocrystalline was formed, as increasing the graphite power, their hardness and toughness was increased at first and then decreased. The TaC110 film obtained the highest hardness (42.8 GPa), H/E (0.12) and elastic recovery (80.5%), this is due to the hard TaCx grains with high carbon vacancy are encompassed by suitable amorphous carbon phase, that the formed strengthening TaCx phase and nanocomposite structure greatly enhanced the mechanical properties. Nevertheless, the TaC50 film with a relatively low hardness and toughness exhibits the lowest average COF of 0.18 and wear rate of 1.44×10-6 mm3/N·m, which is attributed to the lubrications of partial tantalum oxide oxides and highly graphitized transfer layer on the frictional interface. Key words: Transitional-metal carbides; mechanical properties; friction and wear; magnetron sputtering.
Introduction TaC has great potential value in many applications due to its unique features. For example, it is commonly used for reinforce phase in composites due to its high hardness [1-2]; Predominant electrical conductivity and thermal stability make it suitable as diffusion barriers in semiconductors field [3-4]; It also considered as a promising thermal protection material in aerospace industry due to its excellent thermal stability and ablation resistance [5-6]; Furthermore, it is an alternative material as wear-resistant protective coatings owing to the relatively
low-friction and high hardness [7]. In fact, according to the tribological studies of many binary transition metal carbides film, such as TaC, HfC, TiC [7-9], there had an empirical conclusion that higher carbon content in the film corresponds to superior friction performance. And some studies have demonstrated that the tribological properties of self-lubricating nano-composites were affected by many factors (microstructure, hardness, toughness and internal stress, etc.) [9-10]. For TaC, as a typical ceramic material, the low ductility and high transition temperature greatly limit its further applications [6]. In other words, the coatings with a superior mechanical property are indispensable to achieve functional characteristics for high-speed and heavy-duty cutting tools,
such as hard, tough and simultaneously high resistance to cracking. It is a common method to prepare transitional-metal carbide films by magnetron sputtering, and many different carbon sources could be used, which includes carbide target, graphite target, and hydrocarbons (methane, acetylene and so on). It directly influences the composition and content of carbon atoms of films, thus, the prepared carbide films may have much different structure and properties. Previous researches have suggested that the carbide films by reactive deposition methods with the hydrocarbon as carbon source are prefer to form a high crystallinity, and inevitably introduce hydrogen and formed C-H bonds [10-13]. And the film surface with containing C-H bonds is more easily absorbs H2O and O2 molecules, then the formed C=O bonds by tribo-chemical reaction would lead to an increase in the friction coefficient [14]. Furthermore, some researchers have compared the tribological properties and internal mechanisms of hydrogen-free tetrahedral (ta-C) and hydrogenated (a-C:H) diamond-like carbon coatings, they found that the former is easier to form a graphite transfer layer [15]. The non-reactive sputtering method to deposit carbide films by using carbide target is favor to process control and fewer scattering effect [16-17], and the deposited films usually formed single-phase structure with a nonstoichiometric, such as TaCx, HfCx, WCx [16,18-19]. In this work, TaCx films were synthesized with different sputtering powers of graphite target by DC magnetron sputtering. Our emphasis is analyzing effect of structure and composition on mechanical properties and the friction mechanism of TaCx film, as well as investigating the role and influence of amorphous carbon in binary carbides.
2 Experimental 2.1. Films preparation
The TaCx films were co-deposited on Si (1 0 0) wafers with using a tantalum and a graphite target (99.95%, Φ=60 mm) by DC magnetron sputtering (JGP-450). All substrates were ultrasonic bath treatment in alcohol for 15 min. The substrate-to-target distance was fixed at 110 mm. Before deposition, the chamber was evacuated to 3.5×10-3 Pa. Then the flow rate of Ar gas (purity 99.999%) and the working pressure were accurately controlled at 30 sccm and 0.3 Pa during depositions. A pure Cr interlayer with a thickness of 0.1-0.2 µm was deposited as adhesive layer. The tantalum and graphite targets were pre-sputtered for 5 min to remove the impurities of target surface. After that, the sputtering power of Ta target was fixed at 150 W, and graphite power was varied from 30 W to 140 W to obtain TaCx films with different C contents. During deposition, no heating or substrate bias were applied on the substrate. The thickness of films was controlled to be about 2 µm by adjusting the deposition time. And the samples were expressed as TaC30, TaC50, TaC70 and so on, which means the sputtering power of graphite target were 30, 50, 70W. 2.2 Characterization and testing The chemical bond state and elemental composition of the as-deposited films were determined by the X-ray photoelectron spectrum (ESCALAB-250, ThermoFisher Scientific) with monochromatic Al-Kα irradiation. Before analysis, all samples were etched by Ar+ for 5 min to remove surface contaminations. The phases and microstructure were characterized by X-ray diffraction (D8 Advance, Bruker) with Cu Kα radiation (λ = 1.5418 Å) at a scanning speed of 10°/min with a 0.02° step size, the range of 2θ was 30-80o. The topographies of TaCx films were observed by the AFM (AFM-Nanoview 1000, Flyingman Precision). The micro-hardness (H) and elastic modulus (E) of TaCx films were investigated by the nano-indentation tester (NHT-2, Anton Par) with a Berkovich diamond probe tip. All tests were conducted in load-unload mode, and the press-in depth not exceeding 10% of the total film thickness by fixing a maximum load of 10 mN to avoid the effects of substrate and the films surface roughness, at least five indentations were performed at different positions to avoid random errors. The effective fracture toughness was measured from the crack length of the Vickers indentation conducted under a load of 300 gf, which follows the formula: KIC = α(E/H)0.5(P/C1.5), where α is a constant about the Vickers indenter (0.016); E, H and P are elastic modulus, hardness and applied load, respectively; C is the average crack length of four indented corners. The tribological properties of TaCx films were tested by a ball-on-disc tribological tester
(HT-1000, ZhongkeKaihua), A Si3N4 balls (6 mm diameter) was used as the counterface materials All tests were conducted under a normal load of 1.5 N with a velocity of 6.3 cm/s, the friction radius was 3 mm and sustained for 30 min in air atmosphere. The friction coefficient was directly obtained by the attached software, and the wear volume was determined by the laser scanning confocal microscope (SM-1000, Sixian Photoelectric technology). The wear rates were calculated by equation as Kc = V/(FN×d), where V is the wear volume in mm3, d is the sliding distance in m and FN is the normal load in N. After wear test, the morphologies of frictional surface were obtained by field emission scanning electron microscopy (Quanta 250F, FEI). The Raman spectrum of the films surface and wear track were obtained by Raman spectrometer (HORIBA Scientific) with 532 nm laser exciter, by using 10% laser energy to avoid damage of the test area.
3. Results 3.1. Phase and microstructure of TaCx films Fig.1 shows the XRD patterns and the corresponding lattice parameter of TaCx films with various powers of graphite. The sharp peaks between 55°∼70° are derived from Si substrate. The asymmetry broad peak in TaC30 pattern is derived from the overlap of Ta2C (100) at 36.3° and β-Ta (330) at 37.5° sub-peaks. The increased graphite power triggers the phase transition of the Ta2C to tetragonal TaC (Fm3m, #225). The peaks at 34.9°, 40.5°, 58.6°, 70.0°, 73.6° correspond to (111), (200), (220), (311), (222) crystal plane of FCC-TaC (PDF 35-0801), respectively. The (111) preferred orientation of TaC could be related to the minimization of the elastic deformation energy by the closest arrangement of atoms [20]. And the diffraction peak at 34.9° slightly shifts to the lower angle as the power continually increases to 140 W. The calculated lattice constant based on XRD patterns shows in Fig.1 (b), the increased lattice constant means the significant lattice expansion, which is caused by the filling of lattice interstice, residual tensile stress or the replacement of a larger atoms [21-23]. In this work, more carbon atoms enter the octahedral interstice of the Ta lattice, causing the lattice expansion with the carbon power increases from 50 to 140 W. The TaC30 film is not included due to the large calculation error and no comparative value from its two-phase structure. Significantly, the (111) texture become weak with increasing the C power, that may be derived from the random orientation of TaC. Fig.2 shows the Ta 4f and C 1s XPS spectra of the as-deposited TaCx films with different graphite powers. The peak shift of bonding energy in Ta 4f spectra demonstrates that Ta valence
bonds have different states. The peaks near 21.9 and 23.8 eV, 22.5 and 24.3 eV for 4f7/2 and 4f5/2 are attributable to Ta-Ta bonds and Ta-C bonds, respectively [24]. According to Ta 4f spectra of TaC30 film and the XRD result, it can be deduced that Ta bonding states are composed of less Ta2C and more metallic Ta, due to the fact that the equal amounts of Ta-Ta and Ta-C bonds in Ta2C [25]. The peak position of the Ta-C bonds is consistently shifts to the left as the graphite power increases, except for the TaC140 film, which indicates that x value (the atomic ratio C/Ta in TaCx phase) increases and Ta-Ta bonds in metallic Ta decreases [7, 26]. The broad shoulder peak near 25.9 eV is identified as the Ta-O bonds of the Ta2O5 phase, which may be related to residual oxygen in the vacuum chamber and target impurities [24]. In the C 1s spectra (Fig.2b), the sharp peak 282.9 eV and a broad peak 284.8 eV can be attributed to C-Ta bonds and C-C bonds, respectively. Based on the sub-peak area ratio of the fitted XPS spectra, table 1 lists the calculated carbon and tantalum content and the corresponding deposition parameter. As graphite power increases from 30 to 50 W, the total carbon fraction decreases from 60.99 to 38.01 at.%, then linearly increases to 49.07 at.% at 110 W and followed by a decrease to 40.26 at.% at 140 W. A certain amount of amorphous carbon (a-C) in these films is present and ranges from 10 to 36.91 at.%, which decreases firstly and then increases with increasing graphite power. It is normally recognized that the deposition rate is proportional to power of graphite. However, the low-energy carbon atoms under low power show poor migration ability, which incompletely react with Ta atoms and leads to high a-C percentage. On the contrast, energetic particles at higher sputtering power of 140 W might bombard the surface of pre-deposited film, which results in resputtering of lightweight carbon atoms and enrichment of amorphous carbon [27]. The x in TaCx decreases firstly from 0.61 to 0.36, then increases to 0.77 as the graphite power raising. The higher x value means the metal site is decreased. In other words, the electron charge increases in the vicinity of carbon ions, while it decreases in the vicinity of metal ions for TaCx. Additionally, the trend of x also can be seen from the degree of the valley between 4f 7/2 and 4f 5/2, that the deeper valley means a higher x [26]. In this study, the trends of bonding energy and the spectra shape are good consistent with the C/Ta values. The calculated average grain size based on XRD full spectrum fitting is 7.5~15.7 nm. In the light of XPS and XRD observation, we can conclude that TaCx nanocomposite film consists of a certain amount of amorphous carbon and TaC nanocrystallines
with high vacancy concentration, including a few inevitable tantalum oxides (Ta2O5). The surface topographies of TaCx films are obtained by atomic force microscope (AFM) and presented in Fig.3. For the TaC30 film, the relatively smooth surface topography demonstrates the typical feature of amorphous, which combined with lower crystallinity and higher a-C content from the XRD and XPS results. The root mean square roughness (Rq) increases from 2.61 to 3.39 nm, and then decreases to 1.53 nm as graphite power increases, accompanied with the needle-like surface for TaC50 film gradually evolves to dome-shaped for TaC110 film. It might relate to inverse the sputtering ratio of a-C and TaC, and bombardment effect of the enhanced ions energy on removing the deposition atoms, which flatten the island-like growing film [8, 27]. 3.2. Mechanical properties The hardness (H) and elastic modulus (E) are shown in Fig.4 (a), and monotonically increase from 25.4 GPa, 264.1 GPa for TaC30 to the maximum value of 42.8 GPa and 356.5 GPa for TaC110 respectively, while the hardness is 26% higher than the pure TaC film (H of 34.0 GPa and E of 350 GPa), but similar in E [7]. Fig.4 (b) shows the calculated H/E and H3/E2 values. It is noticeable that the H/E of TaC50 film reaches 0.11, while the TaC30 is 0.96. The film would show optimized wear resistance when the H/E value exceeds 0.11, due to the enhanced elastic and plastic deformation resistance from the increased H/E and H3/E2, which also can be used to evaluate the toughness of the film [28-31]. The load-unload curve and elastic recovery are also presented in Fig.4 (c, d), the elastic recovery achieves the maximum of 80.5% and all films exceed 60%, which defined as the ratio of We (elastic deformation work) and We + Wp (plastic deformation work). This highly elastic film with low plastic deformation has high resistance of crack initiation and propagation [32-33]. Fig.5 shows the Vickers indentation morphology of TaC50, TaC90, TaC140 films and the effective fracture toughness KIC values as a function of graphite power. As graphite power increases, it can be seen from the increased KIC value that the fracture toughness gradually enhances. The indentation morphology evolves from long radial cracks to edge microcracks. For TaC50 film, obvious radical cracks propagation along the indenter corners can be observed, which indicates poor toughness compared to the TaC140 film. The edge microcracks at the indentation of TaC140 film are caused by different plastic deformation response of amorphous carbon and hard particle of TaCx [34-36].
3.3 Tribological performance The effect of compositions on the tribological behaviors was investigated and the relationships between the tribological, mechanical properties and compositions were also discussed. As shown in Fig.6 (a), the friction coefficient curves violently fluctuate and very rough except for TaC50 and TaC70 films, which are attributed to the welded junctions with high shear strength instantly formed and detached between the asperities of sliding surface during wear test [28]. The average friction coefficient decreases from 0.44 to 0.18 as C power increases from 30 to 50 W, as shows in Fig.6 (b). Further raising graphite power lead the COF monotonically increases to 0.64. The wear rate is varied in a similar tendency with that of average friction coefficient, as Fig.6 (c) shows, the TaC50 film achieves the lowest value (1.44×10-6 mm3/N·m). The SEM images of TaCx films worn surface with various graphite powers are presented in Fig.7. It is obvious that TaC30 film exhibits catastrophic failure and numerous debris cover on the wear track due to its low hardness and poor toughness, while the TaC50 film is relatively smooth. Significantly, the wear track widths and wear debris are reduced for the films at higher graphite sputtering power due to the enhanced loading capacity from the increased hardness [34]. The TaC70 and TaC90 film surface undergoes shear fracture and the formed debris is flattened by the grinding ball during friction (Fig.7 d-f). The TaC110 and TaC140 film show smoother wear track and narrow widths, and only little tiny wear debris exist in Fig.7 (h) with the higher magnification, which is due to their high hardness (42.8 GPa) and toughness (H/E > 0.11). But obvious scratches and plowing groves are also can be observed in Fig.7 (g, i), which means there is no lubricating layer on the wear track.
4. Discussion Above all, the TaCx films with small x value contain higher a-C content, and show poor mechanical properties and lower resistance to crack expansion. While for the composite films with high x and low a-C content exhibit high hardness, enhanced toughness and resistance to cracking. These changes in mechanical properties could be illustrated by the phase transition, amorphous carbon content and grain size. For TaC30 film, the concomitant of metallic Ta and Ta2C phases, as well as the higher a-C is favorable for the reduction of resistance for dislocation moving, grain rotation and grain boundary slip, eventually results in low hardness and toughness [7-9, 19, 34]. For TaC50 film, the increased fraction of Ta-C covalent bond and decreased a-C content are
responsible for the improved hardness and elastic modulus. As the sputtering power of graphite increases from 70 to 110 W, the films exhibit extreme hard and tough, in particular for TaC110 film is regarded as super-hard, simultaneously tough and flexible advanced film (H>40 GPa, H/E>0.12, elastic recovery of 80.5%) [33]. The enhancement could be explained that the stronger covalent bond was formed, which determined by the changed pseudo gap between the bonding state and the anti-bonding state through adjusting the Fermi level by carbon vacancies [16，37-38]. Other than that, the TaCx grains wrapped in a-C network, results in the formation of nanocomposite structure, which will effectively restrain dislocation, grain boundary sliding, and serve as “strong glue” for pinning nanocrystallites. The two-phase interface can consume crack energy and prevent crack propagation and amorphous carbon at grain boundaries can effectively release stress concentrations near the crack tip, finally hardness and toughness enhanced [7, 34]. Fine-grain strengthening is one of the important factors of TaCx films that cannot be ignored. The TaC30 and TaC50 film with higher a-C content exhibit relatively poor mechanical properties, but have lower friction coefficient and wear rate. In order to further explore the reasons for this phenomenon, the wear track and corresponding sliding ball are characterized by Raman spectroscopy. As shown in Fig.8, no characteristic peaks appear on the unworn surface (B region in the inserted optical picture), in spite of 15.49 at.% amorphous carbon is present in as-deposited TaC50 film. By contrast, the dominant D peak around 1340 cm-1 and G peak near 1590~1600 cm-1 are presented for the wear track (A) of film and counterpart. Meanwhile, the formed Ta2O5 on the wear track is confirmed through the peaks appeared around 660 cm-1 and 815 cm-1 [39], and obvious transfer layer covers on the sliding ball surface. Normally, D peak represents A1g symmetric breathing mode of disordered sp2 hybrid carbon atoms, while G peak corresponds to the stretching mode of sp2 hybrid carbon ring or chain. According to the three-stage model proposed by Ferrari and Robertson [40], the transfer layer in this work belongs to nanocrystalline graphite. The increased ID/IG indicates the raise of the six-membered ring carbon cluster and aggregation of the sp2 hybrid phase, which is conducive to the formation of graphitized transfer layer, consequently, enhanced the lubrication effect [27, 40-42]. A probable illustration about the formation mechanism of the tribological-layer is that the lattice distortion under high shear force induces carbon release and segregate to the grain boundary, and then they are gradually brought to friction interface by the sliding of counterpart to form a
carbon transfer layer [41-42]. According to the results of Raman spectrum, the tribological layer containing Ta2O5 and graphite clusters is mainly caused by tribochemical reaction and the transition from disordered or amorphous carbon to graphitic material induced by the shear during sliding. In general, the films with higher a-C content have lower hardness and toughness, and easily produce cracks. Therefore, those films are more likely to form partial oxides and highly graphitized wear debris due to the relatively poor mechanical properties, which can act as lubricant to improve anti-friction and wear resistance. Regarding to the films with low a-C content, the obtained excellent crack propagation and wear resistance are derived from their high hardness and toughness, but high friction coefficient is appeared, which is the consequence of without wear products lubrication of at the friction interface.
5. Conclusions The TaCx composite films consisted of amorphous carbon and TaCx grains with high vacancy concentration were deposited at different powers of graphite target via DC magnetron sputtering. (1) As the power of graphite increases, the phase of TaCx films transforms from mixed phase of Ta2C and Ta to FCC-TaC phase, and the a-C fraction varies from 10% to 36.9%, which depends strongly the energy of the particles. (2) The TaC110 film obtains super-hard (42.8 GPa), tough (H/E>0.11) and high resistance to cracking (elastic recovery of 80.5%). Those excellent mechanical properties are attributed to the composited structure consisted of hard TaCx grains and soft a-C phase enriched near grain boundaries. (3) A highly graphitized and partial oxidized tribological layer is responsible for the enhanced wear resistance for TaC50 film. And wear resistance is not proportional to mechanical properties.
Acknowledgements This work is financially supported by Natural Science Foundation of China (No. 51101087) and China Postdoctoral Science Foundation (No.2013M540450, 2014T70520), Fundamental Research Funds for the Central Universities (No. 30917014106).
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Highlights 1. Composite structure with vacancies and a-C shows excellent mechanical properties. 2. The TaC100 film obtained high hardness (42.8 GPa) and elastic recovery (80.5%). 3. The lubricating effect of tribological layer on TaCx composite films is clarified. 4. The TaC50 film achieved low COF (0.18) and wear rate (1.44× ×10-6 mm3/N·m).
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S0925-8388(20)30132-8
DOI:
https://doi.org/10.1016/j.jallcom.2020.153769
Reference:
JALCOM 153769
To appear in:
Journal of Alloys and Compounds
Received Date: 24 August 2019 Revised Date:
7 January 2020
Accepted Date: 8 January 2020
Please cite this article as: J. Hu, H. Li, J. Li, J. huang, J. Kong, H. Zhu, D. Xiong, Structure, mechanical and tribological properties of TaCx composite films with different graphite powers, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.153769. 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 B.V.
Author Contributions Section Jiaojiao Hu: Conceptualization, Methodology, Investigation, Data curation, Formal analysis, Writing- Original draft preparation. Hang Li: Conceptualization, Methodology, Formal analysis, Supervision, Writing - Review & Editing. Jianliang Li*: Resources, Supervision, Writing - Review & Editing. Jiewen huang: Software, Validation. Jian Kong: Resources, Visualization. Heguo Zhu: Project administration, Funding acquisition. Dangsheng Xiong: Project administration, Funding acquisition.
Structure, mechanical and tribological properties of TaCx composite films with different graphite powers Jiaojiao Hu, Hang Li, Jianliang Li*, Jiewen huang, Jian Kong, Heguo Zhu, Dangsheng Xiong School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing, 210094 Tel.: +86 025 84315325; E-mail address: [email protected] Abstract In this study, the TaCx composite films consisted of amorphous carbon and nanocrystalline TaC were synthesized with varying sputtering powers of graphite by DC magnetron sputtering, and studied their microstructure, mechanical and tribological behaviors. The film exhibits a Ta2C phase and high content a-C composite structure at a graphite power of 30 W, then a new composite structure with a lower a-C content and TaC nanocrystalline was formed, as increasing the graphite power, their hardness and toughness was increased at first and then decreased. The TaC110 film obtained the highest hardness (42.8 GPa), H/E (0.12) and elastic recovery (80.5%), this is due to the hard TaCx grains with high carbon vacancy are encompassed by suitable amorphous carbon phase, that the formed strengthening TaCx phase and nanocomposite structure greatly enhanced the mechanical properties. Nevertheless, the TaC50 film with a relatively low hardness and toughness exhibits the lowest average COF of 0.18 and wear rate of 1.44×10-6 mm3/N·m, which is attributed to the lubrications of partial tantalum oxide oxides and highly graphitized transfer layer on the frictional interface. Key words: Transitional-metal carbides; mechanical properties; friction and wear; magnetron sputtering.
Introduction TaC has great potential value in many applications due to its unique features. For example, it is commonly used for reinforce phase in composites due to its high hardness [1-2]; Predominant electrical conductivity and thermal stability make it suitable as diffusion barriers in semiconductors field [3-4]; It also considered as a promising thermal protection material in aerospace industry due to its excellent thermal stability and ablation resistance [5-6]; Furthermore, it is an alternative material as wear-resistant protective coatings owing to the relatively
low-friction and high hardness [7]. In fact, according to the tribological studies of many binary transition metal carbides film, such as TaC, HfC, TiC [7-9], there had an empirical conclusion that higher carbon content in the film corresponds to superior friction performance. And some studies have demonstrated that the tribological properties of self-lubricating nano-composites were affected by many factors (microstructure, hardness, toughness and internal stress, etc.) [9-10]. For TaC, as a typical ceramic material, the low ductility and high transition temperature greatly limit its further applications [6]. In other words, the coatings with a superior mechanical property are indispensable to achieve functional characteristics for high-speed and heavy-duty cutting tools,
such as hard, tough and simultaneously high resistance to cracking. It is a common method to prepare transitional-metal carbide films by magnetron sputtering, and many different carbon sources could be used, which includes carbide target, graphite target, and hydrocarbons (methane, acetylene and so on). It directly influences the composition and content of carbon atoms of films, thus, the prepared carbide films may have much different structure and properties. Previous researches have suggested that the carbide films by reactive deposition methods with the hydrocarbon as carbon source are prefer to form a high crystallinity, and inevitably introduce hydrogen and formed C-H bonds [10-13]. And the film surface with containing C-H bonds is more easily absorbs H2O and O2 molecules, then the formed C=O bonds by tribo-chemical reaction would lead to an increase in the friction coefficient [14]. Furthermore, some researchers have compared the tribological properties and internal mechanisms of hydrogen-free tetrahedral (ta-C) and hydrogenated (a-C:H) diamond-like carbon coatings, they found that the former is easier to form a graphite transfer layer [15]. The non-reactive sputtering method to deposit carbide films by using carbide target is favor to process control and fewer scattering effect [16-17], and the deposited films usually formed single-phase structure with a nonstoichiometric, such as TaCx, HfCx, WCx [16,18-19]. In this work, TaCx films were synthesized with different sputtering powers of graphite target by DC magnetron sputtering. Our emphasis is analyzing effect of structure and composition on mechanical properties and the friction mechanism of TaCx film, as well as investigating the role and influence of amorphous carbon in binary carbides.
2 Experimental 2.1. Films preparation
The TaCx films were co-deposited on Si (1 0 0) wafers with using a tantalum and a graphite target (99.95%, Φ=60 mm) by DC magnetron sputtering (JGP-450). All substrates were ultrasonic bath treatment in alcohol for 15 min. The substrate-to-target distance was fixed at 110 mm. Before deposition, the chamber was evacuated to 3.5×10-3 Pa. Then the flow rate of Ar gas (purity 99.999%) and the working pressure were accurately controlled at 30 sccm and 0.3 Pa during depositions. A pure Cr interlayer with a thickness of 0.1-0.2 µm was deposited as adhesive layer. The tantalum and graphite targets were pre-sputtered for 5 min to remove the impurities of target surface. After that, the sputtering power of Ta target was fixed at 150 W, and graphite power was varied from 30 W to 140 W to obtain TaCx films with different C contents. During deposition, no heating or substrate bias were applied on the substrate. The thickness of films was controlled to be about 2 µm by adjusting the deposition time. And the samples were expressed as TaC30, TaC50, TaC70 and so on, which means the sputtering power of graphite target were 30, 50, 70W. 2.2 Characterization and testing The chemical bond state and elemental composition of the as-deposited films were determined by the X-ray photoelectron spectrum (ESCALAB-250, ThermoFisher Scientific) with monochromatic Al-Kα irradiation. Before analysis, all samples were etched by Ar+ for 5 min to remove surface contaminations. The phases and microstructure were characterized by X-ray diffraction (D8 Advance, Bruker) with Cu Kα radiation (λ = 1.5418 Å) at a scanning speed of 10°/min with a 0.02° step size, the range of 2θ was 30-80o. The topographies of TaCx films were observed by the AFM (AFM-Nanoview 1000, Flyingman Precision). The micro-hardness (H) and elastic modulus (E) of TaCx films were investigated by the nano-indentation tester (NHT-2, Anton Par) with a Berkovich diamond probe tip. All tests were conducted in load-unload mode, and the press-in depth not exceeding 10% of the total film thickness by fixing a maximum load of 10 mN to avoid the effects of substrate and the films surface roughness, at least five indentations were performed at different positions to avoid random errors. The effective fracture toughness was measured from the crack length of the Vickers indentation conducted under a load of 300 gf, which follows the formula: KIC = α(E/H)0.5(P/C1.5), where α is a constant about the Vickers indenter (0.016); E, H and P are elastic modulus, hardness and applied load, respectively; C is the average crack length of four indented corners. The tribological properties of TaCx films were tested by a ball-on-disc tribological tester
(HT-1000, ZhongkeKaihua), A Si3N4 balls (6 mm diameter) was used as the counterface materials All tests were conducted under a normal load of 1.5 N with a velocity of 6.3 cm/s, the friction radius was 3 mm and sustained for 30 min in air atmosphere. The friction coefficient was directly obtained by the attached software, and the wear volume was determined by the laser scanning confocal microscope (SM-1000, Sixian Photoelectric technology). The wear rates were calculated by equation as Kc = V/(FN×d), where V is the wear volume in mm3, d is the sliding distance in m and FN is the normal load in N. After wear test, the morphologies of frictional surface were obtained by field emission scanning electron microscopy (Quanta 250F, FEI). The Raman spectrum of the films surface and wear track were obtained by Raman spectrometer (HORIBA Scientific) with 532 nm laser exciter, by using 10% laser energy to avoid damage of the test area.
3. Results 3.1. Phase and microstructure of TaCx films Fig.1 shows the XRD patterns and the corresponding lattice parameter of TaCx films with various powers of graphite. The sharp peaks between 55°∼70° are derived from Si substrate. The asymmetry broad peak in TaC30 pattern is derived from the overlap of Ta2C (100) at 36.3° and β-Ta (330) at 37.5° sub-peaks. The increased graphite power triggers the phase transition of the Ta2C to tetragonal TaC (Fm3m, #225). The peaks at 34.9°, 40.5°, 58.6°, 70.0°, 73.6° correspond to (111), (200), (220), (311), (222) crystal plane of FCC-TaC (PDF 35-0801), respectively. The (111) preferred orientation of TaC could be related to the minimization of the elastic deformation energy by the closest arrangement of atoms [20]. And the diffraction peak at 34.9° slightly shifts to the lower angle as the power continually increases to 140 W. The calculated lattice constant based on XRD patterns shows in Fig.1 (b), the increased lattice constant means the significant lattice expansion, which is caused by the filling of lattice interstice, residual tensile stress or the replacement of a larger atoms [21-23]. In this work, more carbon atoms enter the octahedral interstice of the Ta lattice, causing the lattice expansion with the carbon power increases from 50 to 140 W. The TaC30 film is not included due to the large calculation error and no comparative value from its two-phase structure. Significantly, the (111) texture become weak with increasing the C power, that may be derived from the random orientation of TaC. Fig.2 shows the Ta 4f and C 1s XPS spectra of the as-deposited TaCx films with different graphite powers. The peak shift of bonding energy in Ta 4f spectra demonstrates that Ta valence
bonds have different states. The peaks near 21.9 and 23.8 eV, 22.5 and 24.3 eV for 4f7/2 and 4f5/2 are attributable to Ta-Ta bonds and Ta-C bonds, respectively [24]. According to Ta 4f spectra of TaC30 film and the XRD result, it can be deduced that Ta bonding states are composed of less Ta2C and more metallic Ta, due to the fact that the equal amounts of Ta-Ta and Ta-C bonds in Ta2C [25]. The peak position of the Ta-C bonds is consistently shifts to the left as the graphite power increases, except for the TaC140 film, which indicates that x value (the atomic ratio C/Ta in TaCx phase) increases and Ta-Ta bonds in metallic Ta decreases [7, 26]. The broad shoulder peak near 25.9 eV is identified as the Ta-O bonds of the Ta2O5 phase, which may be related to residual oxygen in the vacuum chamber and target impurities [24]. In the C 1s spectra (Fig.2b), the sharp peak 282.9 eV and a broad peak 284.8 eV can be attributed to C-Ta bonds and C-C bonds, respectively. Based on the sub-peak area ratio of the fitted XPS spectra, table 1 lists the calculated carbon and tantalum content and the corresponding deposition parameter. As graphite power increases from 30 to 50 W, the total carbon fraction decreases from 60.99 to 38.01 at.%, then linearly increases to 49.07 at.% at 110 W and followed by a decrease to 40.26 at.% at 140 W. A certain amount of amorphous carbon (a-C) in these films is present and ranges from 10 to 36.91 at.%, which decreases firstly and then increases with increasing graphite power. It is normally recognized that the deposition rate is proportional to power of graphite. However, the low-energy carbon atoms under low power show poor migration ability, which incompletely react with Ta atoms and leads to high a-C percentage. On the contrast, energetic particles at higher sputtering power of 140 W might bombard the surface of pre-deposited film, which results in resputtering of lightweight carbon atoms and enrichment of amorphous carbon [27]. The x in TaCx decreases firstly from 0.61 to 0.36, then increases to 0.77 as the graphite power raising. The higher x value means the metal site is decreased. In other words, the electron charge increases in the vicinity of carbon ions, while it decreases in the vicinity of metal ions for TaCx. Additionally, the trend of x also can be seen from the degree of the valley between 4f 7/2 and 4f 5/2, that the deeper valley means a higher x [26]. In this study, the trends of bonding energy and the spectra shape are good consistent with the C/Ta values. The calculated average grain size based on XRD full spectrum fitting is 7.5~15.7 nm. In the light of XPS and XRD observation, we can conclude that TaCx nanocomposite film consists of a certain amount of amorphous carbon and TaC nanocrystallines
with high vacancy concentration, including a few inevitable tantalum oxides (Ta2O5). The surface topographies of TaCx films are obtained by atomic force microscope (AFM) and presented in Fig.3. For the TaC30 film, the relatively smooth surface topography demonstrates the typical feature of amorphous, which combined with lower crystallinity and higher a-C content from the XRD and XPS results. The root mean square roughness (Rq) increases from 2.61 to 3.39 nm, and then decreases to 1.53 nm as graphite power increases, accompanied with the needle-like surface for TaC50 film gradually evolves to dome-shaped for TaC110 film. It might relate to inverse the sputtering ratio of a-C and TaC, and bombardment effect of the enhanced ions energy on removing the deposition atoms, which flatten the island-like growing film [8, 27]. 3.2. Mechanical properties The hardness (H) and elastic modulus (E) are shown in Fig.4 (a), and monotonically increase from 25.4 GPa, 264.1 GPa for TaC30 to the maximum value of 42.8 GPa and 356.5 GPa for TaC110 respectively, while the hardness is 26% higher than the pure TaC film (H of 34.0 GPa and E of 350 GPa), but similar in E [7]. Fig.4 (b) shows the calculated H/E and H3/E2 values. It is noticeable that the H/E of TaC50 film reaches 0.11, while the TaC30 is 0.96. The film would show optimized wear resistance when the H/E value exceeds 0.11, due to the enhanced elastic and plastic deformation resistance from the increased H/E and H3/E2, which also can be used to evaluate the toughness of the film [28-31]. The load-unload curve and elastic recovery are also presented in Fig.4 (c, d), the elastic recovery achieves the maximum of 80.5% and all films exceed 60%, which defined as the ratio of We (elastic deformation work) and We + Wp (plastic deformation work). This highly elastic film with low plastic deformation has high resistance of crack initiation and propagation [32-33]. Fig.5 shows the Vickers indentation morphology of TaC50, TaC90, TaC140 films and the effective fracture toughness KIC values as a function of graphite power. As graphite power increases, it can be seen from the increased KIC value that the fracture toughness gradually enhances. The indentation morphology evolves from long radial cracks to edge microcracks. For TaC50 film, obvious radical cracks propagation along the indenter corners can be observed, which indicates poor toughness compared to the TaC140 film. The edge microcracks at the indentation of TaC140 film are caused by different plastic deformation response of amorphous carbon and hard particle of TaCx [34-36].
3.3 Tribological performance The effect of compositions on the tribological behaviors was investigated and the relationships between the tribological, mechanical properties and compositions were also discussed. As shown in Fig.6 (a), the friction coefficient curves violently fluctuate and very rough except for TaC50 and TaC70 films, which are attributed to the welded junctions with high shear strength instantly formed and detached between the asperities of sliding surface during wear test [28]. The average friction coefficient decreases from 0.44 to 0.18 as C power increases from 30 to 50 W, as shows in Fig.6 (b). Further raising graphite power lead the COF monotonically increases to 0.64. The wear rate is varied in a similar tendency with that of average friction coefficient, as Fig.6 (c) shows, the TaC50 film achieves the lowest value (1.44×10-6 mm3/N·m). The SEM images of TaCx films worn surface with various graphite powers are presented in Fig.7. It is obvious that TaC30 film exhibits catastrophic failure and numerous debris cover on the wear track due to its low hardness and poor toughness, while the TaC50 film is relatively smooth. Significantly, the wear track widths and wear debris are reduced for the films at higher graphite sputtering power due to the enhanced loading capacity from the increased hardness [34]. The TaC70 and TaC90 film surface undergoes shear fracture and the formed debris is flattened by the grinding ball during friction (Fig.7 d-f). The TaC110 and TaC140 film show smoother wear track and narrow widths, and only little tiny wear debris exist in Fig.7 (h) with the higher magnification, which is due to their high hardness (42.8 GPa) and toughness (H/E > 0.11). But obvious scratches and plowing groves are also can be observed in Fig.7 (g, i), which means there is no lubricating layer on the wear track.
4. Discussion Above all, the TaCx films with small x value contain higher a-C content, and show poor mechanical properties and lower resistance to crack expansion. While for the composite films with high x and low a-C content exhibit high hardness, enhanced toughness and resistance to cracking. These changes in mechanical properties could be illustrated by the phase transition, amorphous carbon content and grain size. For TaC30 film, the concomitant of metallic Ta and Ta2C phases, as well as the higher a-C is favorable for the reduction of resistance for dislocation moving, grain rotation and grain boundary slip, eventually results in low hardness and toughness [7-9, 19, 34]. For TaC50 film, the increased fraction of Ta-C covalent bond and decreased a-C content are
responsible for the improved hardness and elastic modulus. As the sputtering power of graphite increases from 70 to 110 W, the films exhibit extreme hard and tough, in particular for TaC110 film is regarded as super-hard, simultaneously tough and flexible advanced film (H>40 GPa, H/E>0.12, elastic recovery of 80.5%) [33]. The enhancement could be explained that the stronger covalent bond was formed, which determined by the changed pseudo gap between the bonding state and the anti-bonding state through adjusting the Fermi level by carbon vacancies [16，37-38]. Other than that, the TaCx grains wrapped in a-C network, results in the formation of nanocomposite structure, which will effectively restrain dislocation, grain boundary sliding, and serve as “strong glue” for pinning nanocrystallites. The two-phase interface can consume crack energy and prevent crack propagation and amorphous carbon at grain boundaries can effectively release stress concentrations near the crack tip, finally hardness and toughness enhanced [7, 34]. Fine-grain strengthening is one of the important factors of TaCx films that cannot be ignored. The TaC30 and TaC50 film with higher a-C content exhibit relatively poor mechanical properties, but have lower friction coefficient and wear rate. In order to further explore the reasons for this phenomenon, the wear track and corresponding sliding ball are characterized by Raman spectroscopy. As shown in Fig.8, no characteristic peaks appear on the unworn surface (B region in the inserted optical picture), in spite of 15.49 at.% amorphous carbon is present in as-deposited TaC50 film. By contrast, the dominant D peak around 1340 cm-1 and G peak near 1590~1600 cm-1 are presented for the wear track (A) of film and counterpart. Meanwhile, the formed Ta2O5 on the wear track is confirmed through the peaks appeared around 660 cm-1 and 815 cm-1 [39], and obvious transfer layer covers on the sliding ball surface. Normally, D peak represents A1g symmetric breathing mode of disordered sp2 hybrid carbon atoms, while G peak corresponds to the stretching mode of sp2 hybrid carbon ring or chain. According to the three-stage model proposed by Ferrari and Robertson [40], the transfer layer in this work belongs to nanocrystalline graphite. The increased ID/IG indicates the raise of the six-membered ring carbon cluster and aggregation of the sp2 hybrid phase, which is conducive to the formation of graphitized transfer layer, consequently, enhanced the lubrication effect [27, 40-42]. A probable illustration about the formation mechanism of the tribological-layer is that the lattice distortion under high shear force induces carbon release and segregate to the grain boundary, and then they are gradually brought to friction interface by the sliding of counterpart to form a
carbon transfer layer [41-42]. According to the results of Raman spectrum, the tribological layer containing Ta2O5 and graphite clusters is mainly caused by tribochemical reaction and the transition from disordered or amorphous carbon to graphitic material induced by the shear during sliding. In general, the films with higher a-C content have lower hardness and toughness, and easily produce cracks. Therefore, those films are more likely to form partial oxides and highly graphitized wear debris due to the relatively poor mechanical properties, which can act as lubricant to improve anti-friction and wear resistance. Regarding to the films with low a-C content, the obtained excellent crack propagation and wear resistance are derived from their high hardness and toughness, but high friction coefficient is appeared, which is the consequence of without wear products lubrication of at the friction interface.
5. Conclusions The TaCx composite films consisted of amorphous carbon and TaCx grains with high vacancy concentration were deposited at different powers of graphite target via DC magnetron sputtering. (1) As the power of graphite increases, the phase of TaCx films transforms from mixed phase of Ta2C and Ta to FCC-TaC phase, and the a-C fraction varies from 10% to 36.9%, which depends strongly the energy of the particles. (2) The TaC110 film obtains super-hard (42.8 GPa), tough (H/E>0.11) and high resistance to cracking (elastic recovery of 80.5%). Those excellent mechanical properties are attributed to the composited structure consisted of hard TaCx grains and soft a-C phase enriched near grain boundaries. (3) A highly graphitized and partial oxidized tribological layer is responsible for the enhanced wear resistance for TaC50 film. And wear resistance is not proportional to mechanical properties.
Acknowledgements This work is financially supported by Natural Science Foundation of China (No. 51101087) and China Postdoctoral Science Foundation (No.2013M540450, 2014T70520), Fundamental Research Funds for the Central Universities (No. 30917014106).
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Highlights 1. Composite structure with vacancies and a-C shows excellent mechanical properties. 2. The TaC100 film obtained high hardness (42.8 GPa) and elastic recovery (80.5%). 3. The lubricating effect of tribological layer on TaCx composite films is clarified. 4. The TaC50 film achieved low COF (0.18) and wear rate (1.44× ×10-6 mm3/N·m).
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