Improvement of plasma uniformity and mechanical properties of Cr films deposited on the inner surface of a tube by an auxiliary anode near the tube tail

Houpu WU (吳厚樸), Xiubo TIAN (田修波), Linlin ZHENG (鄭林林),Chunzhi GONG (鞏春志) and Peng LUO (羅朋)

State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin150001,People’s Republic of China

Abstract In order to improve the length of plasma in a whole tube and mechanical properties of Cr films deposited on the inner surface of the tube,a high-power impulse magnetron sputtering coating method with a planar cathode target and auxiliary anode was proposed.The auxiliary anode was placed near the tube tail to attract plasma into the inner part of the tube.Cr films were deposited on the inner wall of a 20#carbon steel tube with a diameter of 40 mm and length of 120 mm.The influence of auxiliary anode voltage on the discharge characteristics of the Cr target, and the structure and mechanical properties of Cr films deposited on the inner surface of the tube were explored.With higher auxiliary anode voltage,an increase in substrate current was observed,especially in the tube tail.The thickness uniformity,compactness,hardness and H/E ratios of the Cr films deposited on the inner surface of the tube increased with the increase in auxiliary anode voltage.The Cr films deposited with auxiliary anode voltage of 60 V exhibited the highest hardness of 9.6 GPa and the lowest friction coefficient of 0.68.

Keywords: high-power impulse magnetron sputtering, auxiliary anode, inner surface, Cr films

1.Introduction

Tubular workpieces are widely used in petroleum and chemical industries, metallurgy, machinery, construction, transportation engineering,electric power engineering,marine engineering and the defense field[1,2].However,in practical application,due to corrosion, friction and wear, the inner surface of tubular workpieces is often seriously damaged, resulting in the shortening of their service life.Therefore, how to improve the quality of the inner surface coating is the focus of tube protection research[3].

For coating methods on the inner surface of a tubular workpiece,the earliest electroplating technology was widely used because of the advantage of low cost and uniformity of films[4].However, films deposited by electroplating usually exhibit the shortcoming of poor adhesion and easy peeling,and especially the fact that electroplating technology seriously pollutes the environment [5].Electroplating has been gradually replaced by chemical vapor deposition and physical vapor deposition (PVD)technology.PVD methods, such as magnetron sputtering and cathodic arcs, have the advantage of a wide range of deposition materials, low deposition temperature, high film quality and outstanding adhesion [6, 7].Therefore, PVD has important application potential in coating methods used on the inner surface of a tubular workpiece.

However,PVD coating methods used for the inner surface of a tube are limited by the shape and size of the tube.It is difficult for the plasma to enter the tube, or it is difficult to ensure the mechanical properties of the coatings in the tube even if the plasma is imported into the tube [8, 9].In addition, poor mechanical properties of the coatings deposited on the inner surface of the tube limit the development of the coating methods on the inner surface of the tubular workpiece [10].

Many coating methods for the inner surface of a tubular workpiece use the cylindrical target placed in the tube as the cathode target.However,these methods have high equipment complexity and are not conducive to depositing coatings in tubes with a small tube diameter[11].Coating methods on the inner surface of a tubular workpiece using a planar cathode target have the advantage of simple hardware and wide applicability.However, there is the problem that the plasma concentration ejected from the cathode target gradually decreases with the increase in tube length in traditional coating methods with a planar cathode target [12].

Therefore,in this work,in order to improve the length of the plasma in the whole tube and mechanical properties of Cr films deposited on the inner surface of the tube,an auxiliary anode near the tube tail was installed, and a novel coating method with a planar cathode target and auxiliary anode for the inner surface of the tubular workpiece was proposed.The auxiliary anode attracted plasma into the deeper part of the tube.Cr films were deposited by high-power impulse magnetron sputtering(HiPIMS)on the inner surface of a 20#carbon steel tube with a diameter of 40 mm and length of 120 mm using this method.The effect of the auxiliary anode voltage on the discharge characteristics of the Cr target, uniformity and mechanical properties of the Cr films on the inner surface of the tube have been studied.

2.Experimental details

2.1.Deposition parameters

Figure 1 presents the experimental setup of a tube inner surface deposition system with an additional auxiliary anode.Experiments were carried out in a vacuum chamber 400 mm in diameter and 400 mm in height, with a base pressure of 3 × 10-3Pa.A metal planar Cr target (50 mm in diameter,4 mm in thickness, and 99.9% pure) was mounted on a balanced magnetron cathode and powered by a hybrid HiPIMS power supply(coupling DC with pulse power supply in parallel)developed by our group[13].The target discharge current was measured using a digital oscilloscope (Tektronix TDS1012B-SC)via the output current sensor that comes with the hybrid HiPIMS power supply.The working gas was argon(99.999% pure), which was controlled by a standard massflow controller.The argon flow rate was maintained constant at 30 ml min-1.The auxiliary anode was a round 304 stainless-steel plate (50 mm in diameter, 1 mm in thickness),which was located at a height of 200 mm in the vacuum chamber (facing the tail of the tube).The power of the auxiliary anode was supplied by a Zhaoxin DC power supply with a constant voltage mode, where the voltage could be adjusted in the range of 0-60 V.To study the auxiliary anode electron current, a 100 Ω resistor was connected in series at the anode terminal of the power supply powering the auxiliary anode.The voltage across the resistor was recorded through the digital oscilloscope, and the auxiliary anode electron current was calculated.

Figure 1.Experimental setup of a tube inner surface deposition system with an additional auxiliary anode.

The electromagnetic coils were installed outside the chamber wall,and were located at the rear end of the target.The power of the electromagnetic coils was supplied by a Zhaoxin DC power supply, where the current could be adjusted in the range of 0-6 A.The tube was biased by a pulsed power supply,where the voltage could be adjusted in the range of 0-900 V(duty cycle of 75% and frequency of 40 kHz).

The 20# carbon steel tubes (40 mm in diameter,120 mm in length)(wt.%,C 0.17%-0.24%;Si 0.17%-0.37%;Mn 0.35%-0.65%; Cr≤0.25%; Ni≤0.25%; Cu≤0.25%;S≤0.035%;P≤0.035%and Fe-balance.)were utilized as the substrates.The axis of the tube orifice is directly opposite the center of the planar target, and the tube was installed at a distance of 50 mm away from the Cr target surface.

Table 1.Details of deposition parameters for Cr films on the inner surface of the tube using a hybrid HiPIMS power supply with an auxiliary anode.

For the deposition of the Cr coatings on the inner surface of the tube, the (100) silicon wafers and 304 stainless-steel wafers (wt.%, C 0.08%; Si 1.0%; Mn 2.0%; Cr 18.0%-20.0%; Ni 8.0%-10.0%; S 0.03%; P 0.045% and Fe- balance.) were respectively placed at four equally spaced positions in the tube.These wafers were utilized to detect the thickness,structure and mechanical properties of the Cr films at each position in the tube.As shown in figure 2, four different positions are marked as Orifice 1, Middle 2, Middle 3 and Tail 4.Prior to loading into the vacuum chamber, the(100) silicon wafers and 304 stainless-steel wafers were ultrasonically cleaned in ethanol and acetone for 30 min.

Figure 2.Schematic diagram of the sample positions in the tube.

The experiments were performed in three stages: (1)argon plasma etching,(2)Cr plasma etching,(3)deposition of Cr coatings, as presented in table 1.Before the carrying out the experiments,the tube was heated to approximately 150°C for outgassing and then the power was turned off.There was no external heating employed during the deposition of Cr coatings.The temperature of the inner surface of the tube during the coating deposition process was maintained at around 115°C.It was measured by placing thermocouples on the outside surface of the tube.

In order to study the distribution of plasma in the tube,it was evenly cut into four sections, which were fastened and insulated with ceramic sleeves.Then, 100 Ω resistors were connected in series on each section of the tube,and connected in parallel at the negative terminal of the bias voltage power supply.The voltage across the resistors was recorded through the digital oscilloscope, and the substrate current of each section of the tube was calculated.The schematic diagram is shown in figure 3.

Figure 3.Schematic diagram of the substrate current measurement of each section of the tube.

2.2.Coating characterization

Cross-sectional images of the films grown on Si(100)wafers were acquired using a FEI Helios NanoLab 600i Dual Beam System.The nanoscale surface morphology was characterized by atomic force microscopy (AFM, Bruker, AXS Dimension Icon).Structural and phase analyses by x-ray diffraction(XRD) in the Bragg-Brentano mode using a Bruker D8 ADVANCE equipped with a Cu Kα radiation source were conducted on the Cr coatings grown on Si (100) wafers.The grain sizes of the coatings were estimated from the (111)diffraction peak using the Scherrer equation [14, 15]:

where λ is the x-ray wavelength(0.15406 nm for Cu),θ is the Bragg angle of the diffraction peak and B is the full width at half maximum of the peak.

Hardness (H) and Young’s modulus (E) values of the coatings deposited on the 304 stainless steel were characterized by nanoindentation using a Swiss CSM nanoindentation tester (NHT2) equipped with a Berkovich diamond indenter.The friction coefficients were evaluated on custom-made ballon-disc wear apparatus at a rotation speed of 100 rpm and loading of 20 g with a 6 mm diameter ZrO2ball.A microscratch tester (Huahui MFT-4000) was used to evaluate the adhesion of the coatings using a Rockwell C indent tip(200 μm).The tests were carried out by progressively increasing the normal load from 0.03 to 60 N at a rate of 60 N min-1and with a scratch length of 4 mm.After the test,the scratch tracks were examined using the attached optical scope.

3.Results and discussion

3.1.Discharge character

Figure 4 presents the curves of the target discharge current in HiPIMS discharge, under different voltages of the auxiliary anode.As shown in figure 4,at the beginning of the pulse,the target current of the Cr target powered by the hybrid HiPIMS power supply increased rapidly to a peak, and then slowly decreased to a platform.At the end of the pulse, the target current rapidly decreased to 0 A.The current curves were in accord with the self-sputtering dominant regime in HiPIMS[16, 17].By increasing the auxiliary anode voltage, both the initial peak current and platform current increased.This implied the production of many more charges and higher plasma density near the target [18].As the auxiliary anode voltage increased from 0 to+60 V,the target current reached the peak faster, and the peak value of the target current increased from 69 to 81 A,which was higher than the increase in the target current platform value (from 66 to 70 A).According to Anders et al [16], the initial phase of an HIPIMS discharge pulse is dominated by gas ions, whereas the later phase has a strong contribution from self-sputtering.It can be speculated that the auxiliary anode can enhance the ionization of gas and metal particles, but it is more effective for the ionization of gas.

Figure 4.Target discharge current curves in HiPIMS discharge,under different voltages of the auxiliary anode.

Figure 5 presents the curves of the auxiliary anode electron current in HiPIMS discharge under different voltages of the auxiliary anode.The auxiliary anode electron current is the sum of conductive electron current and ion current.During the off-pulsed period in HiPIMS discharge, one can see that there was a reference value for the auxiliary anode electron current,which was not 0 A.This can be explained by the much higher diffusion coefficient of electrons compared with ions.During the off-pulsed period in HiPIMS discharge,the Cr target still maintained DC discharge.Due to the higher diffusion coefficient of electrons in plasma, more electrons reached the auxiliary anode, hence the auxiliary anode electron current has a value that is not 0 A.At the beginning of the on-pulse, the ions impinging the cathode resulted in secondary electron emission from its surface and could quickly drift to the auxiliary anode, which resulted in the high electron current peak.The discharge was dominated by argon species at the early stage of the discharge, and this was followed by the dominance of metallic species [19].Since the secondary electron emission coefficient for metal ions bombarding the target is much lower than that of gas ions[20],an initial current peak and subsequent lower current peak were observed during the on-pulsed period.As the auxiliary anode voltage increased, the reference value during the off-pulse period and the current peaks during the on-pulse period gradually increased.This observation illustrates that more electrons drifted to the auxiliary anode under the traction of the auxiliary anode with higher voltage.It is worth noting that when the auxiliary positive voltage was above 20 V,after the on-pulsed period,the electron current decreased to a platform value higher than the reference value, then slowly decreased to the lowest value,and finally returned to the reference value.As the auxiliary anode voltage increased,the phenomenon of falling from the platform value to the lowest value was more obvious.This may be explained by taking into account that more electrons drifted to the auxiliary anode under higher auxiliary anode voltage.This resulted in enhanced ambipolar diffusion, hence an improved transport process of ions towards the auxiliary anode [21], which would reduce the electron current.

Figure 5.Auxiliary anode electron current curves in HiPIMS discharge, under different auxiliary anode voltages.

Figure 6.Evolution of substrate current curves with auxiliary anode voltages:(a)0 V,(b)10 V,(c)20 V,(d)30 V,(e)40 V,(f)50 V,(g)60 V.

Figure 6 presents the variation in substrate current curves with the auxiliary anode voltage increasing from 0 to +60 V at different positions in the tube.As shown in figure 6(a),the substrate current curves at Orifice 1 and Middle 2 were similar to the auxiliary anode of 0 V.However, the substrate current obviously decreased at Middle 3 and Tail 4,especially at Tail 4, where the peak value of substrate current was only 0.03 A, far less than that of 0.53 A at Orifice 1 in the tube.Generally, a lower value of substrate current indicates lower ion flux towards the substrate,which contributes to lower ion flux bombardment on the growing coating and fewer particles for the deposition of coatings[22,23].This shows that when there was no auxiliary anode, the length of plasma entering the tube was limited, especially at the tail of the tube, where the plasma density decreased to a low value.This was not beneficial to the uniformity of the films in the tube and the improvement of the quality of the films.When the auxiliary anode was applied near the tail of the tube, the substrate current at each position increased,especially at the end of the tube.With the auxiliary anode voltage increasing to 40 V,the substrate current curves at each position in the tube were similar and the peak value of substrate current at Tail 4 reached 0.80 A.This demonstrates that the auxiliary anode near the tube tail could effectively increase the plasma density in the tube and the length of plasma entering the tube.On the one hand, the auxiliary anode changed the trajectory of electrons by attracting the electrons, thereby reducing the number of electrons directly escaping to the wall of the vacuum chamber.Instead, more electrons move through the tube to the auxiliary anode, which induces more collision ionization with the neutrals in the tube [24], and more electrons moving to the auxiliary anode will lead to the diffusion of the ions towards the auxiliary anode [25], which will further increase the length of plasmas in the tube.On the other hand, as the auxiliary anode voltage increases, the electric potential gradient between the target cathode and the auxiliary anode gradually increases and the probability of collision ionization therefore increases [26].

3.2.Uniformity of the inner film of the tube

Figure 7 presents the thickness variation of the Cr films at different positions in the tube with different auxiliary anode voltages.The data were obtained through three repeated experiments and measurements.The thickness of the film at Orifice 1 gradually decreased from 2.14 to 1.69 μm with the auxiliary anode voltage increasing from 0 to 60 V, while the film thickness at Middle 2 and Middle 3 both increased.It can be speculated that more Cr ions for film deposition were transported deeper into the tube under the action of the auxiliary anode, which would cause the films to be deposited more uniformly.In addition, to further evaluate the uniformity of film thickness with different auxiliary anode voltages, the coefficient of variation (CV) is calculated and shown in figure 8.The CV is the percentage of the ratio of the sample standard deviation to the average film thickness [27].The CV decreased with the increase in auxiliary anode voltage.Therefore,the thickness uniformity of the films increased with the increase in auxiliary anode voltage.

Figure 7.Film thickness of Cr films deposited at different positions in the tube with different auxiliary anode voltages.

Figure 8.CV of film thickness of Cr films deposited at different auxiliary anode voltages.

It is worth noting that, as can be seen from figure 7, the film thicknesses all decreased significantly with the increase in tube length with different auxiliary anode voltages.However, as shown in figure 6, when the auxiliary anode voltage was larger than 40 V,the substrate current at Tail 4 was close to that at Orifice 1.To explain the inconsistency in variation between the substrate current and film thickness in different positions in the tube,as shown in figure 9,it could at first be invoked that the substrate current only represents the flux of ionized species towards the tube,but cannot represent the flux of neutrals, while the species used to deposit the Cr films contained both Cr ions and Cr neutrals [28].However, the auxiliary anode can only affect the distribution of Cr ions,but cannot promote the movement of Cr neutrals towards the tail of the tube.Therefore, the density of Cr neutrals decreases obviously with the increase in tube length,and Cr neutrals are mainly deposited near the orifice of the tube, which leads to the decrease in the film thickness with the increase in tube length.The second point is that the high substrate current at the tube tail is mainly caused by Ar+, but not Cr+.This is because electrons move to the auxiliary anode under traction of the auxiliary anode and collide with neutrals in the process,thus ionizing the neutrals.However, there are almost no sputtered Cr species at the tail of the tube for electron ionization, which leads to the fact that the ionized species at the tail of the tube are mainly gas species.However, ionized gas species cannot be used for film deposition.Therefore,although the substrate current at the tail of the tube is similar to that at the orifice, there is no film deposition.

Figure 9.Schematic diagram of plasma distribution in the tube with and without an auxiliary anode.

3.3.Microstructure of Cr coatings

Figure 10 presents the cross-section views of the Cr films deposited at Orifice 1 with different auxiliary anode voltages.All the Cr films deposited at Orifice 1 had a columnar structure as revealed by the cross-section micrographs in figures 10(a)-(d).For the Cr film deposited with auxiliary anode voltage of 0 V (figure 10(a)), the columns were separated by large channels of voids.Under higher auxiliary anode voltages, the width of the columns decreased and denser films were deposited.The Cr films deposited with auxiliary anode voltages of 40 and 60 V had tightly packed columns and almost no inter-columnar voids.This is due to the fact that higher plasma density with high auxiliary anode voltage can provide higher ion flux bombardment on the growing film/substrate, thereby improving the compactness of the film and resulting in thinner columns separated by thinner valleys of voids [23, 29].

Furthermore, the tilt angle of the columnar crystal decreased with the increase in auxiliary anode voltage.Higher plasma density with high auxiliary anode voltage will result in more particle bombardment during deposition, and more particle bombardment can make particles scatter more.Particle scattering can inhibit the line-of-sight deposition of shadow effect, leading to a smaller tilt angle of columnar crystal [27].

Figure 10.SEM images of the cross-sections of Cr coatings deposited at Orifice 1 with different auxiliary anode voltages:(a)0 V,(b)20 V,(c)40 V,(d) 60 V.

Figure 11.AFM images of a 2×2 μm2 scan area of the Cr coatings deposited at Orifice 1 with different auxiliary anode voltages:(a)0 V,(b) 20 V, (c) 40 V, (d) 60 V.

Figure 12.Surface roughness of the Cr coatings deposited at Orifice 1 with different auxiliary anode voltages.

Figure 13.XRD patterns for the Cr coatings deposited at Orifice 1 with different auxiliary anode voltages.

As shown in figure 11, AFM analysis was done with a scanning range of 2 × 2 μm2to ascertain the changes in the surface morphology of the coatings deposited at Orifice 1 with different auxiliary anode voltages.As shown in figure 12, the surface roughness (Ra) of coatings deposited with anode voltages of 0, 20, 40 and 60 V was 5.94, 11.7,9.66 and 12.6 nm, respectively.As shown in figure 11, the AFM images of the sample surface micrographs were different between the films deposited with and without an auxiliary anode.The Cr films deposited without an auxiliary anode(voltage of 0 V)presents relatively low-sized spikes in the surface morphology(figure 11(a)).However,the Cr films deposited with an auxiliary anode presented larger and deeper bulges in the surface morphology (figures 11(b)-(d)).This variation in the surface micrographs may be related to the higher ion flux bombardment on the substrate/film with the application of an auxiliary anode [30].

Figure 14.Grain size calculated from the(110)plane of Cr coatings.

Figure 15.(a) Hardness and Young’s modulus, and (b) the H/E ratios of the Cr coatings deposited at Orifice 1 with different auxiliary anode voltages.

Figure 13 shows the XRD patterns of Cr coatings deposited at Orifice 1 with different auxiliary anode voltages.As shown in figure 13, all the Cr films exhibited a strong body-centered cubic Cr (110) peak, and all the (110) peaks were shifted to the left towards lower values, indicating the presence of compressive residual stresses in the films [31].Notably, the angle offset of the diffraction peaks was the smallest with the auxiliary anode voltage of 20 V, indicating the lowest residual stress in the films.This can be explained by the fact that more electrons were constrained in the tube during the film deposition with the application of the auxiliary anode, and the heating effect of the electrons brought about the temperature rise in the substrate [32].The increase in the substrate temperature was conducive to the release of residual stress in the films [33], leading to the reduction in the angle offset.While further increasing the auxiliary anode voltage(e.g.40 and 60 V), the higher ion flux bombardment caused the increase in defects in the films, leading to the increase in residual stress in the films and the angle offset of the diffraction peaks [34].

The grain sizes of Cr coatings calculated from the Cr(110) diffraction peak using the Scherrer formula are plotted in figure 14.The grain size of the coating deposited without an auxiliary anode was 12 nm, while applying an auxiliary anode voltage of 20 V resulted in an abrupt increase in the grain size to 71 nm.This was probably a consequence of the increased adatom mobility due to the higher ion flux during the deposition with the auxiliary anode.Higher adatom mobility promoted atomic migration to the grain boundaries and therefore caused an increase in the grain size [35].In addition,when the auxiliary anode was applied, the electrons were constrained in the tube, and the heating effect of the electrons brought about a temperature rise in the substrate,so as to promote grain growth, which might be another reason for the abrupt grain growth.Further increase in the auxiliary anode to 60 V brought about a decrease in grain size to 14.9 nm.The prevention of migration of grain boundaries by the defects was likely the main factor to decrease the grain size due to the enhanced ion flux bombardment [34].

Figure 15 shows the hardness, Young’s modulus and H/E ratios of the Cr coatings deposited at Orifice 1 with different auxiliary anode voltages.The hardness gradually increased from 7.1 to 9.6 GPa with the auxiliary anode voltage increasing from 0 to 60 V.As the auxiliary anode voltage increased from 0 to 60 V,the H/E ratio increased from 0.032 to 0.044.It has been shown that the ability of a coating to resist mechanical degradation and failure is improved by a high H/E ratio (resistance against elastic strain to failure),which implies a longer ‘elastic strain to failure’ for the material to allow the redistribution of the applied load over a large area,delaying failure of the film[36,37].The improved hardness and H/E ratios can be mainly attributed to its denser microstructure with the increase in auxiliary anode voltage [34].

The adhesion of the Cr coatings deposited at Orifice 1 with different auxiliary anode voltages was evaluated using the microscratch test.The scratch track images are presented in figure 16.As shown in figure 16(a), for Cr coating deposited with an auxiliary anode voltage of 0 V, the critical load for the first adhesive chipping at the edge (LC2) was found to be as low as 5.1 N.Afterwards, delamination along the side of the track was observed as the load was increased.As the auxiliary anode voltage increased from 0 to 40 V, the LC2increased to 10.1 N, which showed an improvement in coating adhesion.With further increase in the auxiliary anode voltage, the LC2of the coating decreased to 8.6 N.This variation in coating adhesion may be related to the variation in ion flux bombardment on the substrate/film with different auxiliary anode voltages.The ion flux bombardment was enhanced as the auxiliary anode voltage was increased to 40 V.A more intensive ion bombardment produced densification of the microstructure and improved interfacial engineering.This led to an increase in coating adhesion [31].However, with the further increase in the auxiliary anode voltage(e.g.60 V),the larger residual stress might result in a lower adhesion strength.It should be noted that the relatively low critical loads for all coatings may be related to the soft stainless-steel substrate used in the current study[34],and the sputtered particles or ions arrived at the tube wall with a glancing angle.

Figure 16.Optical micrographs of the scratch tracks of Cr coatings deposited at Orifice 1 with different auxiliary anode voltages: (a) 0 V,(b) 20 V, (c) 40 V, (d) 60 V.

Figure 17 presents the coefficient of friction of the Cr coatings deposited at Orifice 1 with different auxiliary anode voltages.As shown in figure 17, first, the friction coefficient of the coatings increased and then decreased with the increase in the auxiliary anode voltage.The highest friction coefficient of the coating deposited with the auxiliary anode voltage of 20 V was related to the relatively high surface roughness of the coating.By further increasing the auxiliary anode voltage,the friction coefficient gradually decreased, which might be due to the denser microstructure and higher hardness of coatings [38].

Figure 17.Coefficient of friction of the Cr coatings deposited at Orifice 1 with different auxiliary anode voltages.

4.Conclusion

In this paper, a novel coating method with a planar cathode target and auxiliary anode for the inner surface of a tubular workpiece was proposed to increase the length of plasma in the whole tube and mechanical properties of Cr films deposited on the inner surface of the tube.With higher auxiliary anode voltage, an obvious increase in substrate current at each position in the tube was obtained,especially in Tail 4.The thickness uniformity, compactness, hardness and H/E ratios of the Cr films deposited on the inner surface of the tube increased with the increase in auxiliary anode voltage.As the auxiliary anode voltage increased from 0 to 40 V, the LC2increased from 5.1 to 10.1 N.With further increase in the auxiliary anode voltage, the LC2of the coating decreased to 8.6 N.The Cr films deposited with auxiliary anode voltage of 60 V exhibited the highest hardness of 9.6 GPa and the lowest friction coefficient of 0.68.

Acknowledgments

The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (Nos.12075071 and 11875119) and Heilongjiang Touyan Innovation Team Program (HITTY-20190013).