Tin Films Deposited on Uranium by High Power Pulsed Magnetron Sputtering Under Low Temperature

materials

Article TiN Films Deposited on Uranium by High Power Pulsed Magnetron Sputtering under Low Temperature

Jingjing Ding, Xixi Yin, Liping Fang ID , Xiandong Meng and Anyi Yin *

Institute of Material, China Academy of Engineering Physics, Mianyang 621900, China; [email protected] (J.D.); [email protected] (X.Y.); [email protected] (L.F.); [email protected] (X.M.) * Correspondence: [email protected]; Tel.: +86-0816-3626746

Received: 10 July 2018; Accepted: 6 August 2018; Published: 10 August 2018

Abstract: Depleted uranium (DU) is oxidized readily due to its chemical activities, which limits its applications in nuclear industry. TiN film has been applied widely due to its good mechanical properties and its excellent corrosion resistance. In this work, TiN protection films were deposited on DU by direct current magnetron sputtering (DCMS) and high power pulsed magnetron sputtering (HPPMS), respectively. The surface morphology and microstructures were investigated by atomic force microscope (AFM), scanning electron microscopy (SEM), and grazing incidence X-ray diffraction (GIXRD). The hardness and Young’s modulus were determined by nano-Indenter. The wear behavior and adhesion was analyzed by pin-on-disc tests and scratch adhesion tests and the corrosion resistance was evaluated by electrochemical measurements. The results show that the TiN films that were deposited by HPPMS outperformed TiN film deposited by DCMS, with improvements on surface roughness, mechanical properties, wear behavior, adhesion strength, and corrosion resistance, thanks to its much denser columnar grain growth structure and preferred orientation of (111) plane with the lowest strain energy. Besides, the process of Ti interlayer deposition by HPPMS can enhance the film properties to an extent as compared to DCMS, which is attributed to the enhanced ion bombardment during the HPPMS.

Keywords: DU; TiN ﬁlm; HPPMS

1. Introduction Uranium is widely used in civilian and military applications due to its unique nuclear properties. However, it is chemically active and susceptible to corrosion, especially in salty, humid, and high-temperature environments [1,2]. Surface modification and film techniques have been applied to improve the corrosion resistance of U, including Ni [3], Al [4], Ti-based [5], Cr-based [6] films, and Mo+,C+,N+ [7,8] ion-implantation. These films can increase corrosion resistance to a certain extent. However, it is hard to get a pure surface without oxidation for films deposited on depleted uranium due to its high chemical activity. Inevitably, an undesirable interface adhesion and loose structure is induced. Consequently, the spallation failure of films and corrosion of the substrate easily occur. Thus, there exists an urgent demand for preparing a film with good performance to improve the serve life of depleted uranium (DU). TiN films have been commonly used for protective purposes on different kinds of substrate materials due to its chemical stability and excellent mechanical properties [9–13]. However, the deposition temperature of TiN is above 300 ◦C in most reported literature [14], which is too high for DU for the following two facts: (1) the thick and loose uranium oxide layer that is formed at this

Materials 2018, 11, 1400; doi:10.3390/ma11081400 www.mdpi.com/journal/materials Materials 2018, 11, 1400 2 of 13 temperature would decrease the interfacial adhesion strength between film and substrate intensively; (2) when uranium components are subjected to such high temperatures, they may deform and lose their mechanical properties. Until now, the film deposition under low temperature is still one of the most critical engineering and scientific problems that challenge the film application on uranium. Efforts have been attempted to deposit TiN film on DU under low temperature by various methods including the conventional direct current magnetron sputtering (DCMS) and arc ion plating (AIP) techniques [5]. However, existing techniques encounter such difficulties as: the films deposited by DCMS at low temperature often exhibit a loose structure due to the low ionization rate of less than 1% [15]; AIP has high ionization rate, however, the macroscopic droplets produced in the film deposition process would result in some void defects and decrease the corrosion resistance consequently. High power pulsed magnetron sputtering (HPPMS) is a novel technique in which the power provides the target with pulses of high power densities of a few kW cm−2. This technology enriched in metal ion plasma, could deposit dense structures and offer virtually defect free films at a relatively low temperature [16,17]. In this work, HPPMS technique was utilized to deposit dense TiN films on uranium for the first time. TiN films were deposited on DU by DCMS and HPPMS. The surface roughness, morphology, nano-hardness, adhesion strength, and electrochemical properties of these films were characterized in order to study the differences between conventional DCMS and HPPMS.

2. Materials and Methods

2.1. Sample Preparation The Ti/TiN films were deposited on DU substrate and silicon (100) wafers, the later ones were prepared for phase and morphology analysis. The DU samples with a size of Φ15 mm × 3 mm were grinded using SiC water papers from 500# to 1000# progressively, mechanically polished to minor, and ultrasonically cleaned in acetone and ethanol, respectively, and then put into a vacuum chamber immediately when the substrates dried. The base pressure was lower than 5 × 10−4 Pa and the chamber temperature was fixed to 180 ◦C. Prior to deposition, the DU samples were sputtered by Ar+ ions (2.0 Pa, applied bias voltage −800 V) for 30 min to clean the surface contaminations and partially remove the native oxide layer. After pre-sputtering, the Ti interlayer of approximately 100~300 nm thick was pre-deposited on the DU substrates to reduce the stress at the substrate interface and to enhance the adhesion between the film and the substrate. During deposition, the TiN films were deposited on the substrate with thickness of 4~5 µm. Three deposition modes were designed to compare the structure and properties of the films deposited by HPPMS and DCMS, in order to investigate how the high power pulse introduced into magnetron sputtering process influences performance of the TiN films. Table1 summarizes the deposition parameters for TiN film fabricated by DCMS, HP + DC, and HPPMS modes. In the DCMS mode, both Ti interlayer and TiN film were deposited by DCMS. In the HP + DC mode, the Ti interlayer was prepared by HPPMS, and then TiN layer by DCMS. However, in the HPPMS mode, both Ti interlayer layer and TiN film were deposited by HPPMS. The cathode was operated in a vacuum chamber equipped with a HPPMS power supply by employing the following parameters: frequency of 150 Hz, pulse width of 200 µs, and average power of 2.2 kW. The applied bias voltage of the substrate was −100 V. The deposition gas pressure was 0.3 Pa. Materials 2018, 11, 1400 3 of 13

Table 1. Deposition parameters for TiN ﬁlm fabricated by direct current magnetron sputtering (DCMS), HP + DC, and high power pulsed magnetron sputtering (HPPMS) modes.

Ti Inter-Layer TiN Film Deposition Distance to Ratio of Deposition Rate Mode Targets (mm) Ar/N2 Time Deposition Time Deposition (nm/min) (min) Mode (min) Mode 1 150 160/18 5 DCMS 35 DCMS 62 2 150 160/18 5 HPPMS 35 DCMS 67 3 150 160/18 5 HPPMS 70 HPPMS 28

2.2. Film Characterization The surface and cross-sectional morphology of the samples were observed by scanning electron microscopy (SEM, FEI 200, FEI, Hillsboro, OR, USA). The phase structure of the TiN films was characterized by grazing incidence X-ray diffraction (GIXRD, Philips X’Pert Pro, PANalytical B.V., Almelo, Netherlands) with Cu Kα radiation and the incident angle of 0.5◦, where the scan range was from 30◦ to 80◦. The Hardness and Young’s modulus of the deposited films were measured by a nano-Indenter (Triboindenter, Hysitron 950, Bruker, Billerica, MA, USA) with a Berkovich head. The test was applied with a load of 1 mN and depth of 90 nm. Then, the surface roughness of the TiN films was measured by an atomic force microscope (AFM, Hysitron, Bruker, Billerica, MA, USA) fitted to the indenter. The tribological test of the deposited films was performed on a ball-on-disc tribometer (Tribo-S-D, CSM, Peseux, Switzerland). In this test, SiN balls (diameter of 6 mm) were selected as friction pairs. The balls were under a constant normal load of 1 N, while the discs were circumrotated at a certain speed to generate abrasion of the films (detailed in Table2). The scratch adhesion tests were performed on a Micro Combi Tester (Anton Parr, Graz, Austria), where the loading rate was 18 N/min and the progressive speed was 4 mm/min. Then, the scratch length was 3 mm. The tester was fitted with acoustic emission monitoring equipment, which can detect emission in the vicinity of 100 kHz. Scratches were examined by optical microscopy (OM, VHX-1000C, KEYENCE, Osaka, Japan) correlated with the acoustic emission observations to determine the cracking behavior and the critical loads (Lc).

Table 2. Parameters of the tribological test.

Tribology Pair Load (g) Speed (r/min) Radius of Balls (mm) Φ6 mm SiN 100 160 4

The corrosion behavior of DU and TiN coated DU samples were studied by potentiodynamic polarization techniques on an electrochemical workstation (PARSTAT2263, Ametek, San Diego, CA, USA). All of the electrochemical measurements were conducted using a conventional three-electrode electrochemical cell in aerated neutral NaCl solution with 50 µg/g Cl− at room temperature with the specimen as working electrode, a platinum plate as counter electrode, and a saturated calomel electrode (SCE) as reference. The scan rate of potentiodynamic polarization was 0.2 mV/s.

3. Results and Discussion

3.1. Surface Morphology and Microstructure The surface morphologies of TiN film by different deposition modes are shown in Figure1. The TiN film that was deposited by DCMS mode shows large micro particles with voids between each particle. While the film fabricated by HP + DC mode appears flattened and it interconnects with no presence of voids. In the case of HPPMS mode, the film presents the most compact and smoothest surface. Materials 2018, 11, x FOR PEER REVIEW 3 of 13

2.2. Film Characterization The surface and cross-sectional morphology of the samples were observed by scanning electron microscopy (SEM, FEI 200, FEI, Hillsboro, OR, USA). The phase structure of the TiN films was characterized by grazing incidence X-ray diffraction (GIXRD, Philips X’Pert Pro, PANalytical B.V., Netherlands) with Cu Kα radiation and the incident angle of 0.5°, where the scan range was from 30° to 80°. The Hardness and Young’s modulus of the deposited films were measured by a nano-Indenter (Triboindenter, Hysitron 950, Bruker, USA) with a Berkovich head. The test was applied with a load of 1 mN and depth of 90 nm. Then. the surface roughness of the TiN films was measured by an atomic force microscope (AFM, Hysitron, Bruker, USA) fitted to the indenter. The tribological test of the deposited films was performed on a ball-on-disc tribometer (Tribo-S-D, CSM, Switzerland). In this test, SiN balls (diameter of 6 mm) were selected as friction pairs. The balls were under a constant normal load of 1 N, while the discs were circumrotated at a certain speed to generate abrasion of the films (detailed in Table 2). The scratch adhesion tests were performed on a Micro Combi Tester (Anton Parr, Austria), where the loading rate was 18 N/min and the progressive speed was 4 mm/min. Then, the scratch length was 3 mm. The tester was fitted with acoustic emission monitoring equipment, which can detect emission in the vicinity of 100 kHz. Scratches were examined by optical microscopy (OM, VHX-1000C, KEYENCE, Japan) correlated with the acoustic emission observations to determine the cracking behavior and the critical loads (Lc).

Table 2. Parameters of the tribological test.

Tribology Pair Load (g) Speed (r/min) Radius of Balls (mm) Ф6 mm SiN 100 160 4

The corrosion behavior of DU and TiN coated DU samples were studied by potentiodynamic polarization techniques on an electrochemical workstation (PARSTAT2263, Ametek, USA). All of the electrochemical measurements were conducted using a conventional three-electrode electrochemical cell in aerated neutral NaCl solution with 50 μg/g Cl− at room temperature with the specimen as working electrode, a platinum plate as counter electrode, and a saturated calomel electrode (SCE) as reference. The scan rate of potentiodynamic polarization was 0.2 mV/s.

3. Results and Discussion

3.1. Surface Morphology and Microstructure The surface morphologies of TiN film by different deposition modes are shown in Figure 1. The TiN film that was deposited by DCMS mode shows large micro particles with voids between each particle. While the film fabricated by HP + DC mode appears flattened and it interconnects with Materialsno presence2018, 11 ,of 1400 voids. In the case of HPPMS mode, the film presents the most compact and smoothest4 of 13 surface.

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Figure 1. The surface morphology of TiN film by different deposition modes: (a) DCMS (b) HP + DC Figure 1. The surface morphology of TiN film by different deposition modes: (a) DCMS (b) HP + DC (c) HPPMS. (c) HPPMS. Further, AFM was used to characterize the surface morphology of the deposited TiN films (FigureFurther, 2), and AFM the averaged was used surface to characterize roughness ( theRa) of surface the samples morphology was calculated of the accordingly deposited (Table TiN films 3). (FigureThe grain2), and size the of averaged TiN decreased surface gradually roughness from ( R aDCMS) of the mode samples (~0.4 was μm) calculated to HPPMS accordingly mode (~0.3 (Table μm).3 ). TheHPPMS grain mode size of will TiN create decreased a high graduallyionization degree from DCMS of target mode material (~0.4s, µwhichm) to causes HPPMS increased mode (~0.3 adatomµm). HPPMSenergymode and mobility will create on the a high substrate ionization surface, degree and ofsubsequently target materials, promotes which the causes small increasedgrain migration adatom energyto the andgrain mobility boundaries on the and substrate grain boundary surface, migratio and subsequentlyn to hence promotesthe refining the grain small sizes. grain The migration TiN film to theby grain DCMS boundaries mode has andthe roughest grain boundary surface with migration Ra of 46.65 to hence nm, thewhile refining the film grain by the sizes. HPPMS The TiN mode film bypresents DCMS modethe smoothest has the surface roughest with surface Ra of 25.89 with nm.Ra of This 46.65 evolution nm, while can be the attributed film by the to the HPPMS enhanced mode presentsion bombardment the smoothest and surface subsequently with R ahigherof 25.89 surface nm. Thisenergy evolution of the growing can be attributedfilms. Besides, to the the enhanced larger ionquantity bombardment of Ar+ ion and bombardment subsequently at higher higher surface bias voltage energy al ofso theresulted growing in enhanced films. Besides, etching the of largerthe quantityasperities of and Ar+ thusion bombardmentsmoothing the surface. at higher bias voltage also resulted in enhanced etching of the asperities and thus smoothing the surface.

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Figure 1. The surface morphology of TiN film by different deposition modes: (a) DCMS (b) HP + DC (c) HPPMS.

Further, AFM was used to characterize the surface morphology of the deposited TiN films (Figure 2), and the averaged surface roughness (Ra) of the samples was calculated accordingly (Table 3). The grain size of TiN decreased gradually from DCMS mode (~0.4 μm) to HPPMS mode (~0.3 μm). HPPMS mode will create a high ionization degree of target materials, which causes increased adatom energy and mobility on the substrate surface, and subsequently promotes the small grain migration to the grain boundaries and grain boundary migration to hence the refining grain sizes. The TiN film by DCMS mode has the roughest surface with Ra of 46.65 nm, while the film by the HPPMS mode presents the smoothest surface with Ra of 25.89 nm. This evolution can be attributed to the enhanced ion bombardment and subsequently higher surface energy of the growing films. Besides, the larger quantity of Ar+ ion bombardment at higher bias voltage also resulted in enhanced etching of the asperitiesMaterials and2018, thus11, 1400 smoothing the surface. 5 of 13

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FigureFigure 2. Atomic 2. Atomic force force microscope microscope (AFM) (AFM) images images of of TiN TiN film ﬁlm by ((aa)) DCMS;DCMS; ( b(b)) HP HP + + DC; DC; and, and, (c) (c) HPPMS mode. HPPMS mode.

Table 3. Surface roughness (Ra), hardness and Young’s modulus of TiN ﬁlm by different Table 3. Surface roughness (Ra), hardness and Young’s modulus of TiN film by different deposition deposition modes. modes.

DepositionDeposition ModeMode DCMS DCMS HP HP + DC+ DC HPPMS HPPMS Ra (nm) 46.65 ± 11.27 37.14 ± 6.65 25.89 ± 5.29 Ra (nm) 46.65 ± 11.27 37.14 ± 6.65 25.89 ± 5.29 Hardness (GPa) 15.75 ± 0.41 20.56 ± 0.76 22.09 ± 0.39 HardnessModulus (GPa)(GPa) 163.6215.75± ± 2.210.41 200.37 20.56± ±4.17 0.76 220.21 22.09± 2.33± 0.39 Modulus (GPa) 163.62 ± 2.21 200.37 ± 4.17 220.21 ± 2.33

Figure 3 presents the cross-sectional morphology of TiN film obtained by different deposition modes. The thickness of the as-deposited film was 4~4.7 μm. The film morphology was evaluated using the well-known structure zone models (SZM) (Figure 4) [18]. The film by DCMS mode presents a porous columnar morphology, which is the typical model of ZONE I. Due to the limited surface diffusion, the atoms that were deposited on the substrate failed to diffuse into the bulk. Hence, the film is consisted of uninterrupted fibrous columns, which exhibits porous and rough morphologies [19]. The HPPMS deposited film exhibits a rather denser columnar morphology identified by model of ZONE T. A high level energy of ion bombardment will enhance the surface diffusion, which gives rise to a different crystallographic orientation of grains, and therefore leads to a competitive growth. The structure mode of HP + DC deposited film is between the modes of ZONE I and T. The abundance of Ti+ ions in the Ti interlayer deposition flux during the HPPMS deposition will improve the transfer to the growth surface to a certain extent. However, in the subsequent DCMS mode where the majority of the ion flux consists of the much lower energy Ar+, the surface diffusion was limited.

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Figure3 presents the cross-sectional morphology of TiN film obtained by different deposition modes. The thickness of the as-deposited film was 4~4.7 µm. The film morphology was evaluated using the well-known structure zone models (SZM) (Figure4)[ 18]. The film by DCMS mode presents a porous columnar morphology, which is the typical model of ZONE I. Due to the limited surface diffusion, the atoms that were deposited on the substrate failed to diffuse into the bulk. Hence, the film is consisted of uninterrupted fibrous columns, which exhibits porous and rough morphologies [19]. The HPPMS deposited film exhibits a rather denser columnar morphology identified by model of ZONE T. A high level energy of ion bombardment will enhance the surface diffusion, which gives rise to a different crystallographic orientation of grains, and therefore leads to a competitive growth. The structure mode of HP + DC deposited film is between the modes of ZONE I and T. The abundance of Ti+ ions in the Ti interlayer deposition flux during the HPPMS deposition will improve the transfer to the growth surface to a certain extent. However, in the subsequent DCMS mode where the majority of the ion flux consists of the much lower energy Ar+, the surface diffusion was limited. Materials 2018, 11, x FOR PEER REVIEW 6 of 13

Figure 3. Cross-section morphology of TiN film grown by: (a) DCMS (4.7 μm); (b) HP + DC (4.7 μm); Figure 3. Cross-section morphology of TiN ﬁlm grown by: (a) DCMS (4.7 µm); (b) HP + DC (4.7 µm); and, (c) HPPMS (4 μm). and, (c) HPPMS (4 µm).

Figure 4. The structure zone model (SZM) [18].

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Figure 3. Cross-section morphology of TiN film grown by: (a) DCMS (4.7 μm); (b) HP + DC (4.7 μm); Materials 2018, 11, 1400 7 of 13 and, (c) HPPMS (4 μm).

Figure 4. TheThe structure structure zone model (SZM) [18]. [18]. Materials 2018, 11, x FOR PEER REVIEW 7 of 13

GIXRD measurements were performed on TiN filmsfilms and the corresponding XRD patterns are shown inin FigureFigure5 .5. Figure Figure5a 5a exhibits exhibits a strong a strong (200) (200) orientation, orientation, weak weak (220) (220) orientation, orientation, and scarce and scarce (111) orientation.(111) orientation. Figure Figure5b presents 5b presents a mixture a mixture of more of pronounced more pronounced (200) orientation (200) orientation with less (111)with andless (220)(111) orientation.and (220) orientation. Figure5c exhibits Figure (111) 5c preferredexhibits orientation.(111) preferred It is proposedorientation. that It the is competition proposed betweenthat the surfacecompetition energy between and strain surface energy energy during and filmstrain growth energy might during contribute film growth to the might changes contribute in preferred to the orientationchanges in preferred [20]. The orientation preferred orientation[20]. The preferred of film hasorientation an important of film effect has an on important its performance effect on [21 its]. Asperformance reported, [21]. the preferred As reported, orientation the preferred of (111) orientat planeion is of the (111) most plane closely is the packed most closely and it exhibitspacked and the lowestit exhibits strain the energy,lowest whilestrain (200)energy, exhibits while the(200) lowest exhibits surface the lowest free energy surface [22 free,23]. energy In the [22,23]. DCMS mode,In the theDCMS applied mode, substrate the applied temperature substrate is so lowtemperature (Ts = 453 K)is thatso low the surface(Ts = 453 energy K) that controlled the surface the growth energy of thecontrolled TiN film the and growth the (200) of preferredthe TiN film orientation and the favors (200) preferred [22]. In the orientation case of HPPMS favors mode, [22]. theIn abundancethe case of + ofHPPMS Ti ion mode, bombardment the abundance increased of Ti+ the ion adatom bombardment mobility increased and diffusivity, the adatom thereby mobility improving and diffusivity, the strain energy,thereby whichimproving facilitated the strain the preferred energy, which orientation facilitated of (111) the [preferred23]. orientation of (111) [23].

Figure 5. Grazing incidence X-ray diffraction (GIXRD) of TiN film deposited by three modes: Figure 5. Grazing incidence X-ray diffraction (GIXRD) of TiN ﬁlm deposited by three modes: (a) DCMS (ba) DCMS HP + DC (b) ( cHP) HPPMS. + DC (c) HPPMS.

3.2. Hardness (H) and Young’s Modulus (E) 3.2. Hardness (H) and Young’s Modulus (E) Hardness and Young’s modulus values of the film deposited by different modes are listed in Hardness and Young’s modulus values of the film deposited by different modes are listed Table 3. The hardness of HPPMS film (H = 20.56 GPa) was higher as compared to that of DCMS film in Table3. The hardness of HPPMS film ( H = 20.56 GPa) was higher as compared to that of (H = 15.75 GPa), and the hardness of HP + DC film (H = 22.09 GPa) was between them. The Young’s DCMS film (H = 15.75 GPa), and the hardness of HP + DC film (H = 22.09 GPa) was between them. modulus exhibits a similar tendency. The enhanced hardness was attributed to both a dense The Young’s modulus exhibits a similar tendency. The enhanced hardness was attributed to both a microstructure without inter-columnar voids that were prepared during film growth in HPPMS dense microstructure without inter-columnar voids that were prepared during film growth in HPPMS discharge [24] and the fact that (111) is the hardest orientation in TiN [25]. discharge [24] and the fact that (111) is the hardest orientation in TiN [25]. 3.3. Wear Behavior The wear behavior of the TiN films was evaluated by pin-on-disc tests. Figure 6 plots the film friction coefficient against sliding time, and then Table 4 shows the corresponding average tribology coefficient (μ) of TiN film. It can be identified that the DCMS TiN has the highest wear coefficient (μ = 0.56). The film by HP + DC mode exhibits a reduced friction coefficient (μ = 0.42). While, the HPPMS deposited one has a further improved wear resistant (μ = 0.34) when compared to the two modes described above.

Table 4. The average tribology coefficient of three deposition modes.

Deposition mode 1 2 3 Average tribology coefficient 0.56 0.42 0.34

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3.3. Wear Behavior The wear behavior of the TiN films was evaluated by pin-on-disc tests. Figure6 plots the film friction coefficient against sliding time, and then Table4 shows the corresponding average tribology coefficient (µ) of TiN film. It can be identified that the DCMS TiN has the highest wear coefficient (µ = 0.56). The film by HP + DC mode exhibits a reduced friction coefficient (µ = 0.42). While, the HPPMS deposited one has a further improved wear resistant (µ = 0.34) when compared to the two modes described above.

Table 4. The average tribology coefﬁcient of three deposition modes.

Deposition mode 1 2 3 Average tribology coefﬁcient 0.56 0.42 0.34 Materials 2018, 11, x FOR PEER REVIEW 8 of 13 Article TiN Films Deposited on Uranium by High Power Pulsed Magnetron Sputtering under Low Temperature

wear. Figure 7b presents a relatively light and narrow grooves with tiny debris, suggesting that the wear mechanism is dominated by abrasive wear in HP + DC mode. Meanwhile, the film by HPPMS mode exhibits a smooth surface with the lightest grooves (Figure 7c), identified by abrasive wear. The optical images are in good agreement with the friction coefficient results, which proves that the film deposited by HPPMS mode possesses excellent tribological characteristics.

FigureFigure 6. 6.The The ﬁlm film friction friction coefﬁcient coefficient against sliding sliding time time by by three three modes. modes.

The wear tracks of TiN films by three modes were observed by optical microscopy (OM). Deep The wear tracks of TiN films by three modes were observed by optical microscopy (OM). Deep and and wide grooves with some obvious debris were formed on the film by DCMS mode (Figure 7a), widewhich grooves indicates with that some abrasive obvious wear debris occurred were accompanied formed on by the adhesion film bywear. DCMS Figure mode 7b presents (Figure a7 a), whichrelatively indicates light that and abrasive narrow wear grooves occurred with tiny accompanied debris, suggesting by adhesion that wear.the wear Figure mechanism7b presents is a relativelydominated light andby abrasive narrow wear grooves in HP with + tinyDC mode. debris, Meanwhile, suggesting the that film the by wear HPPMS mechanism mode exhibits is dominated a by abrasivesmooth surface wear in with HP the + DC lightest mode. grooves Meanwhile, (Figure the7c), filmidentified by HPPMS by abrasive mode wear. exhibits The optical a smooth images surface withare the in lightestgood agreement grooves with (Figure the 7frictionc), identified coefficient by results, abrasive which wear. proves The that optical the imagesfilm deposited are in by good agreementHPPMS with mode the possesses friction excellent coefficient tribological results, whichcharacteristics. proves that the film deposited by HPPMS mode possesses excellent tribological characteristics.

Figure 7. Cont.

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Figure 7. The wear tracks for the films after ball on disc wear tests against SiN ball: (a) DCMS Figure 7. The wear tracks for the films after ball on disc wear tests against SiN ball: (a) DCMS (b) HP + (b) HP + DC (c) HPPMS. DC (c) HPPMS. The HP + DC mode provides high energy ion bombardment during the Ti interlayer growth, whichThe HP shows + DC a certain mode improvement provides high in surface energy roughness, ion bombardment porosity, hardness, during and the adhesion Ti interlayer strength, growth, whichas compared shows a certain to DCMS improvement mode. Further, in surface HPPMS-depo roughness,sited TiN porosity, films hardness,show superior and performance adhesion strength, in as comparedstructure and to DCMSmechanical mode. performance, Further, HPPMS-deposited therefore resulting in TiN enhanced filmsshow tribology superior resistance. performance in structure and mechanical performance, therefore resulting in enhanced tribology resistance. 3.4. Adhesion 3.4. AdhesionTo observe and quantify the adhesion strength of TiN film by different deposition modes, progressive micro-scratch tests were performed under the same condition (applied normal load To observe and quantify the adhesion strength of TiN film by different deposition modes, increased linearly from 0 to 10 N). The optical micrographs of scratches TiN film by different modes progressiveafter scratch micro-scratch test are shown tests in wereFigureperformed 8 and the ge undernerated the acoustic same emission condition signals (applied during normal scratch load increasedtest are linearly shown fromin Figure 0 to 109. N).It suggests The optical that micrographsTiN films that of were scratches deposited TiN filmwith by mode different 1 and modes 2 afterexperienced scratch test similar are shown failure in Figuremodes8 [26]. and In the DCMS generated TiN, acousticthe angular emission cracks signals were clearly during observed scratch test are showninitiating in from Figure the9. edge It suggests of scratch that at TiN a very films low that load were of 4 depositedN. Then, some with new mode angular 1 and cracks 2 experienced were similarconstantly failure being modes produced [26]. In along DCMS the TiN, edge theof scratc angularh as cracksthe load were progressed. clearly The observed angle between initiating the from the edgeangular of crack scratch and at the a very forward low direction load of 4of N. the Then, diamond some scribing new angular head was cracks almost were maintained constantly at 45°. being producedAt the alongload of the 5.5 edgeN, the of signs scratch of chipping as the load appeared progressed. at the Thearea anglebetween between the angular the angular crack and crack the and the forwardedge of scratch direction from of film-substrate the diamond interface. scribing The head results was show almost that maintainedthe film-substrate at 45 interface◦. At the close load of 5.5 N,to the edge signs of of scratch chipping was damaged appeared with at the the area appearance between of theangular angular crack crack and failed and theat a edgecritical of load scratch fromof film-substrate5.5 N. While, the interface. pre-deposition The results treatment show of that Ti interlayer the film-substrate using HP interfacehas a positive close effect to the on edge adhesion strength of the TiN film by improving the critical load to almost 8 N. The TiN film that was of scratch was damaged with the appearance of angular crack and failed at a critical load of 5.5 N. produced by HPPMS mode shows completely different failure modes under the same conditions. While, the pre-deposition treatment of Ti interlayer using HP has a positive effect on adhesion strength There were no angular cracks observed in Figure 8c. The semi-circular brittle cracks formed at the of therear TiN of the film indenter by improving (Figure 8c) the in criticalresponse load to the to tensile almost stresses 8 N. The that TiN are generated film that wasduring produced sliding, by HPPMS mode shows completely different failure modes under the same conditions. There were no angular cracks observed in Figure8c. The semi-circular brittle cracks formed at the rear of the indenter Materials 2018, 11, 1400 10 of 13

(Figure8c) in response to the tensile stresses that are generated during sliding, which played as typical failure throughout the remaining process until spalling occurs at the load of 16 N. Obviously, the HP deposition mode improved the scratch resistance of TiN ﬁlm signiﬁcantly, which may also attribute to the enhanced ion bombardment during the HPPMS mode, which may make metal ions implanted and incorporatedMaterials 2018 in, the11, x substrate,FOR PEER REVIEW and hence has an enhancement on interface. 10 of 13

FigureFigure 8. The 8. opticalThe optical micrographs micrographs of of scratches scratches TiN f ﬁlmilm by by different different modes modes after after scratch scratch test: ( test:a) DCMS (a) DCMS (b) HP(b +) HP DC + ( DCc) HPPMS. (c) HPPMS.

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Figure 9. The acousticacoustic emissionemissionsignals signals generated generated by by TiN TiN ﬁlm film with with different different modes modes during during scratch scratch test: test:(a) DCMS (a) DCMS (b) HP (b) + HP DC + (DCc) HPPMS. (c) HPPMS.

3.5. Corrosion Corrosion Resistance Figure 10 presents the potentiodynamic polarizati polarizationon curves of DU and and the the DU DU coated coated with with TiN TiN by different modes in aerated 50 ug/gug/g NaCl NaCl solution solution and and Table Table 55 summarizessummarizes thethe correspondingcorresponding electrochemical corrosioncorrosion parameters.parameters. A A highly highly active active dissolution dissolution behavior behavior with with regard regard tothe to the bare bare DU DUwas was observed observed in curve in curve a. Above a. Above the corrosion the corrosion potential potential (Ecorr), the(Ecorr), anodic the current anodic density current increased density −6 2 increasedsharply and sharply it finally and it reachedfinally reached the limiting the limiting current current density density of approximately of approximately3.4 3.4× 10× 10−6−6A/cm A/cm22.. TiN films films could could prevent prevent DU DU from from rapid rapid corrosion, corrosion, as de asmonstrated demonstrated by curve by curve b, c, b,d. Especially, c, d. Especially, all of theall ofTiN the films TiN were films characterized were characterized by a passive by a region. passive However, region.However, the current the density current (icorr) density of HPPMS (icorr) −8 2 −7 2 TiNof HPPMS (2.6 × 10 TiN−8 A/cm (2.62) ×was10 lower−8 A/cm by an2) wasorder lower of magnitude by an order than ofthat magnitude of HP + DC than TiN that (2.7 × of 10 HP−7 A/cm + DC2) −7 2 andTiN two (2.7 orders× 10−7 ofA/cm magnitude2) and twothan ordersthat of ofDCMS magnitude TiN (5.1 than × 10 that−7 A/cm of DCMS2), respectively. TiN (5.1 As× 10reported,−7 A/cm the2), electrochemicalrespectively. As reported,resistance the of electrochemicalfilm is strongly resistancerelated to of the film microstructure is strongly related and surface to the microstructure morphology [11,27].and surface The relatively morphology lower [11 ,corrosion27]. The relativelyresistance lower of the corrosion DCMS TiN resistance film is mainly of the DCMS due to TiNits porous film is columnarmainly due structure to its porous so that columnar the solution structure can easily so that reach the the solution DU substrate can easily via reach the pinholes. the DU substrate The denser via structurethe pinholes. in HP The + DC denser TiN structure films contributes in HP + DC to the TiN enhanced films contributes electrochemical to the enhanced resistance electrochemical as compared toresistance DCMS TiN. as compared In addition, to DCMS HPPMS TiN. TiN In addition, provides HPPMS a competitive TiN provides growth a competitive structure with growth few structure defects andwith the few lowest defects porosity, and the which lowest accounts porosity, for which the best accounts corrosion for the resistance. best corrosion resistance.

Figure 10. PotentiodynamicPotentiodynamic polarization polarization curves curves of of DU DU substrate substrate and and TiN TiN films ﬁlms by by different different modes: modes: (a) DCMS, ( b) HP + DC, DC, ( (cc)) HPPMS, HPPMS, and and ( (d)) depleted depleted uranium uranium (DU).

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Table 5. The electrochemical corrosion parameters of DU and DU coated with TiN by different modes: (a) DCMS, (b) HP + DC, and (c) HPPMS in aerated 50 ug/g NaCl solution.

Deposition Mode Ecorr (mV) Icorr (A/cm2) DU −773 ± 97 3.4 ± 0.7 × 10−6 DCMS −230 ± 21 5.1 ± 3.8 × 10−7 HP + DC −113 ± 14 2.7 ± 1.4 × 10−7 HPPMS −87 ± 19 2.6 ± 1.1 × 10−8

4. Conclusions In this study, TiN film has been deposited on DU under low temperature by direct current magnetron sputtering (DCMS) and high power pulsed magnetron sputtering (HPPMS), respectively. The experimental results shown in this work demonstrate that the HPPMS deposited TiN film exhibited a compact and smooth surface as compared to DCMS that was deposited TiN. The DCMS deposited TiN films exhibited porous columnar structure, which corresponds to zone I in the well-known structure zone model (SZM); while, the HPPMS counterpart shows competitive texture growth, which corresponds to zone T in SZM. The preferred orientation of DCMS produced TiN films changed from (200) to (111) with HPPMS. The HPPMS fabricated TiN film reveals a high hardness of 22.09 GPa, Young’s modulus of 220.21, low friction coefficient of 0.34, high adhesion of Lc 16 N, and an improved corrosion resistance. Besides, the process of Ti interlayer deposition by HPPMS can enhance the film properties as compared to DCMS, which is attributed to the enhanced ion bombardment during the HPPMS. In summary, the results that are shown in this work prove that the TiN film deposited by HPPMS on DU could significantly improves its mechanical, tribological and corrosion performance and therefore serves as a promising surface protective coating for DU.

Author Contributions: J.D. analyzed the electrochemical data and wrote the manuscript, X.Y. and X.M. performed the measurements on the properties, L.F. contributed to the supervision of research, checked and polished the manuscript, A.Y. performed the experiments on the sample preparation and co-wrote the manuscript. Funding: This work is support by the National Science Foundation of China (Grant Nos. 11702261 and 61604138), the Discipline Development Foundation of China Academy of Engineering and Physics (Grant Nos. 2015B0301065 and Nos. 2015B0302061) and the Strategic High-Tech Key Project (Grant No. BGX2016010304). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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