稀有金属(英文版) 2017,36(11),858-864

Tribological properties in seawater for Ti/TiCN coatings on Ti6Al4V alloy by arc ion plating with different carbon contents

Jin-Long Li Gang-Yi Cai Hua-Sheng Zhong Yong-Xin Wang Jian-Min Chen

Key Laboratory of Marine Materials and Application Technologies, Zhejiang Key Laboratory of Marine Materials and Protection Technologies, Ningbo Institute of Materials Technologies and Engineering, Chinese Academy of Sciences

School of Mechanical and Electrical Engineering, Henan University of Technology

收稿日期：10 September 2015

基金：financially supported by the National Natural Science Foundation of China (No.51575510);Ningbo International Cooperation Project (No.2013D10005);Zhejiang Provincial Natural Science Foundation of China (No.LY14E010005);

Tribological properties in seawater for Ti/TiCN coatings on Ti6Al4V alloy by arc ion plating with different carbon contents

Jin-Long Li Gang-Yi Cai Hua-Sheng Zhong Yong-Xin Wang Jian-Min Chen

Key Laboratory of Marine Materials and Application Technologies, Zhejiang Key Laboratory of Marine Materials and Protection Technologies, Ningbo Institute of Materials Technologies and Engineering, Chinese Academy of Sciences

School of Mechanical and Electrical Engineering, Henan University of Technology

Abstract：

TiCN coatings incorporated with Ti buffer layer were deposited on Ti6A14V alloy by arc ion plating. The carbon content in TiCN coatings was varied by controlling flow rates of C₂H₂ in reactive gas. The Ti/TiCN coatings have a typical structure of columnar crystal with a total thickness of about 2 μm. The elements of Ti, C and N are present as TiN and TiC in TiCN coatings. A little free carbon appears with carbon content increasing in TiCN coatings.For the TiCN coatings, the hardness, friction coefficient and wear rate decrease with the increase in carbon content. In seawater, both friction coefficient and wear rate have an obvious decrease at lower carbon content compared with those in atmosphere. However, the friction coefficient and wear rate only have a slight decrease, while the carbon content reaches or exceeds 10 at% in Ti/TiCN coatings.

Keyword：

Ti/TiCN coatings; Arc ion plating; Hardness; Seawater; Wear;

Author： Jin-Long Li e-mail:lijl@nimte.ac.cn;

Received： 10 September 2015

1 Introduction

Ti6A14V alloy is widely used in aerospace and marine environment parts due to the excellent corrosion resistance and high specific strength [ 1, 2, 3] .However,the titanium alloy has a poor tribological property,and thus,surface treatment is necessary to improve its wear resistance.For marine equipment,the components are subjected to harsh environment coupling wear and corrosion,and moreover,the lubricate oil also does not employ in water media,and thus,the components are very easy to failure.The protective coatings with excellent lubrication property are a promising candidate to improve wear and corrosion of marine equipment component and extend its service life.

Surface modification and coating techniques were applied and achieved a good effect to improve the wear resistance of titanium alloy [ 4, 5, 6, 7, 8, 9, 10] .By physical vapor deposition,The TiN coating was employed for the protection to titanium alloy in the initial stage [ 11] .The coating has a higher hardness,but the antifriction effect is not good enough.In order to improve antifriction property,carbon was involved into the TiN coatings to form the TiCN coating to obtain a good lubrication effect.Ertuerk et al. [ 12] pointed out that TiCN is the solid solution of TiN and TiC,and thus the coating has a high hardness and excellent lubricating effect in the practical application due to combining the advantages of both TiN and TiC [ 13, 14] .And many reports also have confirmed that TiCN coatings have good protective performance in dry environment [ 15] .However,there has been lack of information on tribological behavior of TiCN coatings in seawater.In seawater,the removal of the protective coatings is from mechanical wear and chemical corrosion.The two mechanisms often are an interaction,i.e.,corrosion can affect wear,and similarly wear will accelerate corrosion.

In this paper,Ti/TiCN coatings with different carbon contents were fabricated on Ti6A14V by arc ion plating.The effect of carbon content on structure and tribological behavior of Ti/TiCN coatings in seawater was studied.

2 Experimental

As the substrate,the commercial Ti6A14V sheets were employed with a size of 15 mm×15 mm×5 mm.Before deposition,the titanium sheets were mechanically ground and polished to a mirror finish and ultrasonically cleaned in acetone medium for three times.The multi-arc ion plating equipment (Hauzer Flexicoat 850) was used to prepare the coatings.Prior to deposition,the vacuum chamber was pumped down to a base pressure below 1×10^-3 Pa.The substrate temperature was fixed at 450℃during deposition.First,Ti buffer layer was deposited on Ti6A14V substrate to improve adhesion.Second,TiCN coatings were prepared by sputtering a titanium target (purity>99.5 wt%) in N₂(99.99%) and C₂H₂ (99.99%) mixed atmosphere.And the flow rate of N₂ was fixed at 500 ml·min^-1,and C₂H₂ flow rates are 20,40 and 80 ml-min^-1,respectively.

The morphologies of the coatings and wear track were observed by field emission scanning electron microscope(FESEM,S4800) equipped with energy-dispersive spectroscopy (EDS,OXFORD X-Max).The chemical states of the elements were characterized by X-ray photoelectron spectroscopy (XPS,AXIS UTLTRADLD,Japan) using Al Kαsource with 12 kV and 10 mA.High-resolution scans for the elements were performed with pass energy of20 eV.The phase structure was investigated by X-ray diffraction (XRD,Bruker D8 X-ray facility) using Cu Kαradiation (λ=0.154 nm) at 40 kV and 40 mA.And the scanning speed was 4 (°)·min^-1 with step size of 0.02°.

Nanoindentation test was carried out using a MTS Nano indenter@G200 system with a continuous stiffness measurement option.The eight indentations in each sample configured on different areas were performed to have reliable statistics.Wear tests were performed on a UMT-3 MT tribometer (CETR,USA).The tests with a ball-ondisk reciprocating mode were sliding against WC balls with a diameter of 6 mm.The tests were conducted in artificial seawater which was prepared according to Standard ASTM D 1141-98.Based on cross-profile of the wear track by Alpha-Step IQ profile-meter,the wear losses(V) of coating can be obtained after the sliding tests.The wear rate (W) can be calculated by the following equation:

where S is the sliding distance and L is the normal load applied.

3 Results and discussion

3.1 Structure

Figure 1 shows SEM top-view and cross-section images of Ti/TiCN coatings with different carbon contents.From the top-view images,all Ti/TiCN coatings are composed of big particles covered a lot of small round particles.Ti/TiCN coatings become dense with the increase in carbon content.The cross-section images show that the coatings have a feature of clear columnar crystals.The thickness of TiCN coating is 1600 nm and that of Ti buffer layer is 400 nm from cross-section image and EDS composition results.

Table 1 shows contents of Ti,C and N in TiCN coatings by XPS analysis.The carbon content increases from8.8 at%to 16 at%with the increase in C₂H₂ flow rate in reactive gas.The chemical states of elements are shown in Fig.2.By deconvolution,Ti presents as predominant TiN,TiC and TiO.N is identified from TiN.C 1s spectra reveal that TiC is predominant in TiCN coatings,and free carbon appears and its amount increases with the increase in carbon content.A lot of contaminated carbon is also found on the surface of TiCN coating.

Figure 3 shows XRD patterns of Ti/TiCN coatings with different carbon contents.TiCN diffraction peak (200) is detected for all coatings,and the peaks are particular broad,implying that its crystals are very small.This also can be proved by small particles and dense structure observed from top-view and cross-section SEM images of Ti/TiCN coatings shown in Fig.1.The TiCN diffraction peak becomes strong with the increase in carbon content.By arc ion plating,as-deposited coatings have an obvious preferred orientation for the different parameters.For the low carbon content,there is an obvious TiN (012) preferred orientation.However,the coatings have an obvious TiN(015) preferred orientation with the increase in carbon content.As shown in Fig.3,the strong Ti diffraction peaks(100),(101) and (002) also are found for all coatings.The reason is that the TiCN coatings are not too thick,and thus,the signals are detected from the Ti buffer layer and Ti6A14V substrate.

3.2 Hardness and modulus

Figure 4 shows the variation of hardness with the indentation depth of Ti/TiCN coatings with different carbon contents.The hardness is identified at the peaks in the hardness curves,and the depth is near 1/10 total thickness of the coatings.The hardness values of the coatings are23.54,21.77 and 19.06 GPa for C₂H₂ flow rates of 20,40and 80 ml·min^-1,respectively.With carbon content of the coatings increasing,the hardness decreases,and moreover,the hardness values of all coatings are far higher than that of Ti6A14V substrate.Elastic modulus (E) and hardness(H) are two key parameters of coating materials,and the relationship between them is of interest to material researchers.The ratio of hardness to modulus (H³/E²) can predict the ability of a coating to resist mechanical failure,and the higher ratio implies that the coating has a higher elastic recovery and excellent mechanical failure resistance in local region of the coating.By indentation technique,it is possible to conveniently evaluate the ratio of hardness to modulus to measure load-depth data.For Ti/TiCN coatings,the values of H³/E² were calculated and they are 0.04,0.19 and 0.11 GPa for C₂H₂ flow rates of 20,40 and80 ml·min^-1,respectively.The H³/E² ratio first increases and then decreases with the increase in carbon content.This reveals that the incorporation of carbon into the coatings can improve the toughness of the coating,but too more carbon does not contribute to the increase in toughness.

Fig.1 SEM images of Ti/TiCN coatings with different carbon contents:a,b 20 ml·min^-1,c,d 40 ml·min^-1 and e,f 80 ml·min^-1;EDS spectra of marked g Area 1 and h Area 2 in f

  下载原图

Table 1 Contents of Ti,C and N from TiCN coatings by XPS (at%)

Fig.2 Series XPS spectra of a Ti 2p,b C 1s and c N 1s of Ti/TiCN coatings with different carbon contents

Fig.3 XRD patterns of Ti/TiCN coatings with different carbon contents:a Ti6A14V,b 20 ml·min^-1,c 40 ml·min^-1,and d 80 ml·min^-1

Fig.4 Variation of hardness with indentation depth of Ti/TiCN coatings with different carbon contents

3.3 Wear resistance

Figure 5 shows the variation of friction coefficient with sliding time of Ti/TiCN coatings deposited with different carbon contents against Si₃N₄ ceramic balls in atmosphere or seawater.In atmosphere,the friction coefficient decreases with the increase in carbon content,from 0.8 to0.4,and then to 0.3.Compared with that in atmosphere,the friction coefficient in seawater decreases from 0.8 to 0.7and from 0.4 to 0.3 for the Ti/TiCN coatings with C₂H₂flow rates of 20 and 40 ml·min^-1,respectively.However,the friction coefficient almost does not change for Ti/TiCN coatings with C₂H₂ flow rate of 80 ml·min^-1 in seawater compared with that in atmosphere.Figure 6 shows the wear rates for Ti/TiCN coatings in atmosphere and seawater.In atmosphere,the Ti/TiCN coatings with C₂H₂ flow rates of 40 and 80 ml·min^-1 have a similar wear rate which is far lower than that of the Ti/TiCN coating with C₂H₂flow rate of 20 ml·min^-1.In seawater,the wear rates of the Ti/TiCN coatings decrease with the increase in carbon content.For C₂H₂ flow rate of 20 ml·min^-1,the wear rate obviously decreases from 1.64×10^-3 mm³·Nm^-1 in atmosphere to 2.44×10^-4 mm³.Nm^-1 in seawater.However,the wear rate has a slightly decrease for Ti/TiCN coatings with C₂H₂ flow rates of 40 and 80 ml·min^-1 in seawater compared with that in atmosphere.

Tribological results reveal that friction coefficient greatly decreases with the increase in carbon content for Ti/TiCN coatings.The reason can be explained as follows.First,Ti/TiCN coatings become dense and smooth with the increase in carbon content.Second,free carbon appears and increases with the increase in carbon content in Ti/TiCN coatings,and such free carbon plays lubrication and antifriction role.The presence of free carbon can be seen from XPS deconvolution analysis of C 1s.Though the free carbon induces a slight decrease in hardness,the wear rate has a great decrease from1.64×10^-3 to 8.32×10^-5 mm³·Nm^-1 for Ti/TiCN coatings with C₂H₂ flow rate of 20 ml·min^-1 compared with that of 40 ml·min^-1.Figure 7 shows the morphologies of the track in atmosphere for Ti/TiCN coatings with different carbon contents.There are many cutting furrows,which is typical abrasive wear characteristic,on track surface of Ti/TiCN coating with C₂H₂ flow rate of 20 ml·min^-1.However,the Ti/TiCN coatings with C₂H₂ flow rates of 40 and80 ml·min^-1 have a smooth track surface.This can be contributed to a lubrication role of free carbon,and thus,a small wear rate can be obtained.

Fig.5 Friction coefficients with sliding time of Ti/TiCN coatings with different carbon contents in a atmosphere and b artificial seawater.Conditions:room temperature,load of 1 N,frequency of 5 Hz and diameter of Si₃N₄ balls of 3 mm

Fig.6 Wear rates of Ti/TiCN coatings in different environments

In seawater,all Ti/TiCN coatings have a decreasing trend of friction coefficient with that different degrees,compared with that in atmosphere.For Ti/TiCN coatings with C₂H₂ flow rates of 20 and 40 ml·min^-1,the friction coefficient decreases by 0.1,and there is near no change for the coatings with C₂H₂ flow rate of 80 ml·min^-1.For wear rate,Ti/TiCN coatings with C₂H₂ flow rate of 20 ml·min^-1has a great decreases by an order of magnitude,but other TiCN coatings have a slight decrease in wear rate,compared with in atmosphere.Here,the lubrication of seawater plays a crucial role in decreasing the friction coefficient and wear rate.

Figure 8 shows the morphologies of the wear tracks of Ti/TiCN coatings in seawater.It is found the surface of the wear track is smoother compared with that in atmosphere.In particular,the Ti/TiCN coating with C₂H₂ flow rate of20 ml·min^-1 has a more obvious change of the surface of the wear track.For Ti/TiCN coatings incorporated with higher carbon content,the free carbon already plays a role of lubrication and antifriction,and thus,the friction coefficient and wear rate have not an obvious decrease in seawater compared with that in atmosphere.It is necessary to analyze the corrosion products on wear track in seawater.The composition on wear track was detected by EDS,and the results are shown in Fig.8g.The EDS spectrum reveals that no obvious corrosion products are found.The reason can be explained as follows:(1) TiCN coating has an excellent corrosion resistance;(2) it is difficult to form obvious corrosion products due to the short wear time of30 min;and (3) even though a small amount of corrosion products are formed,the products are easy to be removed by the tribological pair during wear test.

The reason for such a great decrease in wear rate of Ti/TiCN coatings with C₂H₂ flow rate of 20 ml·min^-1 in seawater compared with that in atmosphere is analyzed as following.From the images of the track,the wear mechanism is completely different.For Ti/TiCN coatings,there is serious abrasive wear on wear track in atmosphere,but no abrasive characteristic is observed in seawater.By EDS analysis,the elemental composition reveals that there are only Ti,Al and C on the track in atmosphere,but a lot of Ti,N,C and O exist on the track in seawater.The cross sections of the wear tracks of the coatings are shown in Fig.8h.In atmosphere,the depth of the wear track reaches4μm,which is larger than the thickness of the coatings.However,the depth of the wear track is only 1μm in seawater,and the depth is shallower than the thickness of the coatings.Above results imply that the coating has been already removed in atmosphere.However,TiCN coating still remains on the track in seawater.The reason is that little carbon content does not play the lubrication role,and seawater has lubrication and antifriction effect during tribological test.For higher carbon content,Ti/TiCN coatings have a similar tribological performance in seawater compared with those in atmosphere due to lubrication role from free carbon.

Fig.7 SEM images and corresponding enlarged images of wear tracks of Ti/TiCN coatings with different carbon contents in atmosphere:a,b 20 ml·min^-1,c 40 ml·min^-1 and d 80 ml·min^-1

Fig.8 SEM images and corresponding enlarged images of wear tracks of Ti/TiCN coatings with different carbon contents in artificial seawater:a,b 20 ml·min^-1,c,d 40 ml.min^-1 and e,f 80 ml·min^-1,g EDS spectrum of marked area in f,h depth profiles of wear track from Figs.7a and8a

4 Conclusion

Ti/TiCN coatings were deposited on Ti6A14V alloy by arc ion plating with different carbon contents by controlling reactive gas ratios of C₂H₂ to N₂.The Ti/TiCN coatings have a typical structure of columnar crystal with a total thickness of about 2μm.The coating becomes dense,and roughness decreases with the increase in carbon content.Ti,C and N are present as TiN and TiC in TiCN coatings.A little free carbon appears with carbon content increasing.Higher carbon content contributes to an obvious decrease in the friction coefficient and wear rate.But the friction coefficient and wear rate have not obvious change,while the carbon content reaches or exceeds 10 at%for Ti/TiCN coatings in seawater or atmosphere.

Acknowledgments This study was financially supported by the National Natural Science Foundation of China (No.51575510),Ningbo International Cooperation Project (No.2013D10005) and Zhejiang Provincial Natural Science Foundation of China (No.LY14E010005).

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