Structural andMechanical Properties of CrN x Coatings Deposited byMedium-FrequencyMagnetron Sputtering with and without Ion Source Assistance

CrNx coatings were deposited on Si (100) and WC-Co substrates by a home-made medium-frequency magnetron sputtering system with and without thermal filament ion source assistance. The structure and composition of the coatings were characterized by X-ray diffraction, atomic force microscopy, scanning electron microscopy, and transmission electron microscopy. The mechanical and tribological properties were assessed by microhardness and pin-on-disc testing. The ion source-assisted system showed a deposition rate of 3.88 μm/h, much higher than the value 2.2 μm/h without ion source assistance. The CrNx coatings prepared with ion source assistance exhibited an increase in microhardness (up to 16.3 GPa) and adecrease in friction coefficient (down to 0.48) at the optimized cathode source-to-substrate distance.


Introduction
Transition metal nitrides, especially chromium nitride (CrN), have been studied extensively due to their unique properties, including high hardness, good wear resistance, as well as excellent corrosion and high-temperature oxidation resistance [1][2][3][4].They are widely applied in industry as protective coatings [5][6][7].Recent studies also revealed magnetic properties in CrN, and it might find applications in the electronic industries [8,9].Many methods have been used for the deposition of CrN films, among which unbalanced magnetron sputtering produces good quality samples at high-deposition rates [2].The pulsed DC reactive magnetron sputtering technique was characterized by improved ionization and a high ion-to-neutral particle ratio during deposition to enhance the quality of coatings [10], but more importantly it increased the kinetic energy of the ions in the plasma, which can enhance the ion bombardment of the substrate and film [11][12][13][14].Therefore, high-quality CrN coatings have been prepared by pulsed dc magnetron sputtering [15,16], and the high-power pulsed magnetron sputtering has also been developed for deposition of CrN coatings [17,18].It is known that low ionization efficiency in the plasma is a hurdle of magnetron sputtering; therefore, plasma or ion sources have been developed to improve ionization efficiency [19,20].In particular, Wei et al. reported dense and thick films of transition metal nitrides, including ZrN and TiN, by means of plasma-enhanced magnetron sputtering, where an electron source of thermal filament type is a key technology [21,22].In order to prepare thick protective coatings with high deposition rate, high microhardness, and ideal surface chemistry, it is necessary to introduce high-density plasma in pulsed dc magnetron sputtering systems.
In this paper, we have prepared CrN x coatings at various magnetron cathode source-substrate distances by medium-frequency (40 kHz) magnetron sputtering with a thermal filament ion source and conducted characterization in comparison with samples prepared without the use of the ion source.We intended to find out the influence of cathode source-to-substrate distances and ion source on the deposition rate, microstructural, mechanical, and

Experiment Details
The CrN x coatings were deposited by using a modified closed field twin unbalanced magnetron sputtering system.The vacuum chamber is φ400 × 500 mm in dimension.The ion source was powered by a supply of 20 A and 24 V and was mounted in the middle-upper area of the chamber.Cr sheets with a purity of 99.99% and an area of 10 × 40 cm 2  were used as a cathode source material.Prior to deposition, a base pressure was less than 5 × 10 −3 Pa.For substrates, p-type Si (100) and mirror-polished WC-Co plates were ultrasonically cleaned in acetone and methanol, rinsed in deionized water, and dried in N 2 before being loaded into the deposition chamber.Then, they were ion etched for 30 min in Ar atmosphere at a pressure of 2.0 Pa and a negative bias voltage of 800 V applied to the substrate holder.N 2 (99.99%) and Ar (99.99%) were used as working gases.First, a layer of pure Cr (about 230 nm) was deposited onto the substrate for 5 min in Ar ambient at 0.25 Pa and −100 V to improve the adhesion.Then, N 2 was let in, and Ar flow rate was tuned to keep an Ar : N 2 ratio of 1 : 1.The total pressure was kept at 0.25 Pa, and the substrate bias was fixed at −100 V.The medium-frequency power used was 7.0 kW, and cathode source-substrate distance was varied between 50 and 140 mm.The substrates temperature was kept at 150 • C. The ion source was a tungsten filament which emitted electrons when heated by a high current and could produce ions of argon gas fed to the outlet of the filament source.The ion source was installed at the upper part of the chamber, similar to the configuration described in the literature [23].
The crystal structure of the deposited CrN x coatings was characterized by X-ray diffraction (XRD, Bruker-Axs D8 advanced which was operated at voltage and current of 40 kV and 40 mA, resp.) with a Cu ka radiation and JEOL JEM 2010 transmission electron microscopy (TEM).The deposition rate was evaluated from the thickness of the films measured with a FTSS2-S4L-3D step profiler.The cross-section micrographs were measured using Sirion FEG scanning electron microscopy (SEM), and the composition of CrN x coatings was determined by using an EDAX genesis 7000 energy dispersive spectroscopy (EDS) system operated at 12 kV.The surface topography was analyzed using an atomic force microscope (AFM) (Shimadzu SPM-9500J3) operated in the tapping mode.The hardness was measured using an HX-1000 microhardness tester with a load of 25 g (the indentation depth was about 250 nm) and taking the average of 5 values.The friction and wear measurements of the CrN x coatings were carried out by using an MS-T3000 ball-on-disk tester which slides in ambient air at 30 • C, at relative humidity of 70%, with a Si 3 N 4 ball of 3 mm in diameter being used as the mating material, on which a 4 N load was applied.The average sliding speed was 0.02 m/s for a fixed sliding time of 30 min, and the friction coefficients were recorded during the test.The energies of the depositing particles were different at different d ss because of the collision of ions (N, Ar, and Cr).The particles bombarded the substrate and heat the substrate.The ratio of ion to neutral particles (Ar, N, and Cr) arriving at the growing film would be different at different d ss .These factors influence the film growth kinetics, which finally determine the orientation and phase structure of the CrN x coatings.The broadening of the diffraction peaks of CrN x coatings is related to the changes in the grain size, thickness, and residual stress in the coating.The different ratio of N/Ar has an influence on the composition of CrN or Cr 2 N [24].Therefore, the formation of CrN or Cr 2 N at different d ss is attributed to the different N/Ar ratio influenced by d ss .

Results and Discussion
Also shown in Figure 1 is the influence of the hot filament ion source; CrN x coatings deposited with ion source assistance have different diffraction peaks at all values of d ss .
In particular, at d ss of 90 mm, the intensity of (200) peak is enhanced.The slightly higher intensity of the XRD peaks in the CrN coatings deposited with the ion source assistance can be related to the enhanced ion bombardment from the plasma, which may lead to a higher substrate temperature as well as a higher ion-to-neutral particle ratio.Figure 2(a) shows bright-field TEM images of CrN x coatings deposited without ion source assistance, which reveals nonuniform CrN x grains.The corresponding selected area diffraction pattern reveals obvious CrN and blurry Cr 2 N phases.On the contrary, with the use of ion source Figure 2(b), uniform CrN x grains are observed and selected area diffraction shows obvious diffraction rings of CrN and Cr 2 N, revealing the polycrystalline nature of the film, with diffraction points attributed to the Si (100) substrate.The uniform CrN x grains with distinct grain boundaries suggest higher microhardness of the corresponding coatings.
Figure 3 shows the deposition rate of the CrN x coatings as a function of d ss .As a general tendency, the deposition rate of the coatings decreases with increasing d ss .The deposition rate is further increased at d ss values of 90 mm and 140 mm with the assistance of the ion source.At d ss = 50 mm, however, the ion source assistance tends to decrease the deposition rate.
At larger source-substrate spacing, the sputter-produced particles arriving at the substrate decrease in number due to the collision of Cr and N atoms with the plasma of N 2 , Ar, N + , Ar + , and secondary electrons.However, with the use of thermal filament ion source, more ions, especially Ar ions, were generated, which bombarded the Cr target and produced more Cr particles [3,25].As a  result, the deposition rate is higher than that without ion source assistance.At close source-substrate spacing (d ss = 50 mm), with thermal filament ion source assistance, the increased amount of Ar and other ions causes severe re sputtering of the film surface.Therefore, the deposition rate becomes lower than that without ion source assistance.With the increase of d ss , the energies of deposition particles decreased because of the collision in the plasma, especially at d ss much larger than molecular mean-freepath.Hence, the deposition rate at larger d ss with the ion source assistance becomes larger than without ion source assistance.
Figure 4 shows typical three-dimensional AFM morphologies taken from the CrN x coatings deposited at d ss of 50, 90, and 140 mm without and with thermal filament ion source assistance.The topographies shown in Figure 4(a) suggest that the CrN x coatings deposited at 50 mm are composed of columns with irregular tops.When the sourcesubstrate spacing increases to 90 mm, the size of the extrusive tops was significantly reduced (Figure 4(b)).At even larger d ss , the extrusive tops become more regular and an even smoother surface is observed (Figure 4(c)).Figure 5 shows the root-mean-square (RMS) roughness calculated from the AFM images of the CrN x coatings deposited at various d ss without and with thermal filament ion source assistance.Corresponding to the AFM observations in Figure 4, the RMS roughness of the CrN x coatings deposited at 50 mm was relatively large at shorter source-substrate distance.The use of thermal filament ion source gives rise to the reduction of RMS roughness from 10.3 to 8.6 nm at d ss = 50 mm.At larger source-substrate distance, the difference becomes negligible.
When the source-substrate spacing is small, the deposition rate is high (Figure 3) and the growth is columnar, which gives a large roughness.More energetic ions bombard the substrate, which affected the nucleation kinetics [26], resulting in rapid growth of the grain, thereby exhibiting larger     particle sizes.With increasing d ss , frequent collision reduces the kinetic energies of particles reaching the surface, and the deposition is slowed down, leading to uniform surfaces with extrusions of smaller particle size and higher density.Figure 6 shows cross-section SEM images of the CrN x coatings deposited without and with ion source assistance at  d ss of 90 mm.One sees a columnar growth throughout the whole film thickness without the ion source assistance.In the process of using ion source, columnar growth is not obvious and the coating becomes denser, apparently resulting from the energetic bombardment by ions produced with the ion source assistance.
Figure 7 shows the microhardness of the CrN x coatings as a function of source-substrate distance, which exhibits the same trend, but the values are higher when ion source assistance is applied during deposition.At d ss = 90 mm, the highest hardness is observed.This is accounted for by better crystallization and higher N concentration (measured by EDS) measured from the samples.The CrN x coating deposited with the thermal filament ion source had higher N concentration (as shown in Figure 8), and the films were denser, with much finer grains (as shown Figure 2).It is believed that the grain size rather than the existence of Cr 2 N phase influences the hardness values [27], which explains the further improvement of the microhardness at d ss = 90 mm.
Figure 9 shows the friction coefficients of CrN x coatings.The average friction coefficient of the CrN x films prepared without ion source is 0.53 and is decreased to 0.48 with ion source assistance.This is consistent with the enhanced microhardness and the reduction in the surface roughness of the CrN x coatings.

Conclusion
We have prepared CrN x coatings by medium-frequency magnetron sputtering and demonstrated the improvement of the structural and mechanical properties of coatings by introducing thermal filament ion source during deposition.The CrN x coatings deposited with ion source assistance exhibited an increase in microhardness from 13.25-16.3GPa at a source-substrate distance of 50 mm and from   A deposition rate of 3.88 μm/h was achieved, and the roughness was 6.0 nm for the coatings deposited at d ss of 90 mm.The results show that the use of simple ion source assistance may be promising for high-rate deposition of CrN x coatings.

Figure 1 :
Figure 1: XRD patterns of CrN x coatings deposited at various d ss .

Figure 1
Figure 1 shows the XRD spectra of CrN x coatings deposited under various source-substrate distances, d ss .The CrN x coatings contain two phases of fcc CrN and hexangular Cr 2 N, their corresponding PDF numbers being 65-2899 and 79-2159, respectively.Only the CrN (200) peak is observed for the coating deposited at d ss = 90 mm.With increasing d ss , the XRD data show the structure of the CrN x coatings to be changed from CrN to a mixture of Cr 2 N + CrN.At the lower extreme, d ss = 50 mm, the planes of Cr 2 N (002), together with CrN (200) and (111), can be seen whereas at larger distance of 140 mm, the films deposited exhibit inconspicuous overlap of Cr 2 N (002) and CrN (200) orientations.The energies of the depositing particles were different at different d ss because of the collision of ions (N, Ar, and Cr).The particles bombarded the substrate and heat the substrate.The ratio of ion to neutral particles (Ar, N, and Cr) arriving at the growing film would be different at different d ss .These factors influence the film growth kinetics, which finally determine the orientation and phase structure of the CrN x coatings.The broadening of the diffraction peaks of CrN x coatings is related to the changes in the grain size, thickness, and residual stress in the coating.The different ratio of N/Ar has an influence on the composition of CrN or Cr 2 N[24].Therefore, the formation of CrN or Cr 2 N at different d ss is attributed to the different N/Ar ratio influenced by d ss .Also shown in Figure1is the influence of the hot filament ion source; CrN x coatings deposited with ion source assistance have different diffraction peaks at all values of d ss .In particular, at d ss of 90 mm, the intensity of (200) peak is enhanced.The slightly higher intensity of the XRD peaks in the CrN coatings deposited with the ion source assistance can be related to the enhanced ion bombardment from the plasma, which may lead to a higher substrate temperature as well as a higher ion-to-neutral particle ratio.

Figure 2 :
Figure 2: Bright-field TEM images and selected area diffraction of CrN x coatings deposited at d ss = 90 mm.The view directions are normal to the coating surface.(a) Samples prepared without ion source assistance, (b) with ion source assistance.

Figure 3 :
Figure 3: Deposition rate as a function of the d ss of the CrN x coatings.

Figure 4 :
Figure 4: AFM morphologies of CrN x coatings deposited at d ss of 50 mm (a), 90 mm (b), and 140 mm (c).The images on the lefthand side are of samples prepared without ion source, and those on the right-hand side are of samples prepared with ion source assistance.

Figure 5 :
Figure 5: Variation in rms roughness measured from AFM images of the CrN x coatings as a function of d ss .

Figure 6 :
Figure 6: Cross-sectional SEM image of CrN x coatings deposited at d ss = 90 mm (a) without and (b) with ion source assistance.

Figure 7 :
Figure 7: Variation of microhardness of CrN x coatings as a function of d ss .

Figure 9 :
Figure 9: Variation of friction coefficient with sliding time of CrN x coatings at d ss = 90 mm.
16.0-17.0GPa at the optimized d ss of 90 mm.The friction coefficient was decreased typically from 0.53-0.48.