Characterization of Crn-Based Hard Coating Materials with Addition of Gan

Characterization of CrN-Based Hard Coating Materials with Addition of GaN

Yusei Mizuno+, Tadachika Nakayama, Hisayuki Suematsu and Tsuneo Suzuki

Extreme Energy-Density Research Institute, Nagaoka University of Technology, Nagaoka 940-2188, Japan

Cr1¹xGaxN thin films with various GaN concentrations were prepared on Si(100) substrates by pulsed laser deposition in order to clarify the effects of the GaN content on the material characteristics. The compositions of these films were determined by Rutherford backscattering spectroscopy, while crystal structures were elucidated using Fourier transform infrared spectroscopy and X-ray diffraction, and hardness values were measured by nanoindentation. Analyses determined that x was in the range of 0 to 0.51 and, at x ¯ 0.31, a single B1-(Cr,Ga)N phase was present. In those films for which x ² 0.38, a secondary phase based on B4-GaN also appeared. The hardness increased with increases in x up to 0.31 as the thin films maintained a single B1-(Cr,Ga)N phase, and a maximum hardness of 29.4 GPa was obtained at x = 0.31. [doi:10.2320/matertrans.MAW201806]

(Received May 15, 2018; Accepted July 10, 2018; Published September 25, 2018) Keywords: hard coating, chromium nitride, gallium nitride, solid solution hardening

1. Introduction Therefore, in the present study, Cr1¹xGaxN thin films with various GaN levels (as indicated by x) were prepared on CrN-based hard coating materials have been widely Si(100) substrates by PLD and the impact of varying the applied to cutting tools to improve their useable lifespans amount of GaN was investigated. and cutting performances.14) There have been many reports regarding improvements in the hardness and other character- 2. Experimental istics of these materials following the addition of a third 57) element to CrN. Typical examples include CrSiN and Cr1¹xGaxN thin films were prepared on Si(100) substrates CrAlN810) coatings. It has been determined that CrSiN by PLD, using a Cr50 mol% Ga alloy attached to a Cr plate exhibits hardening as a result of the formation of a as the target. The substrate and the target were positioned nanocomposite structure11) in which nanocrystalline(nc)- 50 mm apart in a chamber that was evacuated to a pressure ¹5 CrN is surrounded by a-SiNx. In contrast, CrAlN forms of less than 1.0 © 10 Pa. A nitrogen plasma was generated (Cr,Al)N as a metastable phase via the partial substitution of by ionizing a 1 sccm flow of N2 gas using a 400 W radio CrN having a B1(NaCl type) structure with AlN having a B1 frequency radical source. During deposition, substrates were structure.12) AlN undergoes a phase transition from a B4 maintained at a temperature of 473 K using an infrared lamp. (würtzite type) structure to a B1 structure at high pressures An ablation plasma was produced by applying a Nd:YAG (approximately 23 GPa).13,14) The B1-AlN dissolves in the laser ( = 355 nm) in the form of intense pulses over brief B1-CrN through a non-equilibrium process, thus improving durations of 7 ns at a repetition rate of 10 Hz to the rotating the hardness by solution hardening.9,10) Our own group has target. The GaN content in the thin films was tuned by investigated new hard coating materials based on solution varying the area of the Cr plate covered by the target (SR) hardening, focusing on Ga added as a third element to CrN. from 0% to 100%. The deposition was carried out for 12 h, Like Al, Ga is a group 13 element and its nitride, GaN, has and the resulting film thicknesses were in the range of 200 to characteristics similar to AlN.15,16) GaN also shows a phase 300 nm. The composition of each film was assessed using transition from a B4 to B1 structure at approximately Rutherford backscattering spectroscopy (RBS). During these 53 GPa.1618) In our previous work, we confirmed that B1- analyses, the film was irradiated with a He2+ ion beam at an GaN can be dissolved to form B1-CrN as a result of the acceleration voltage of 2 MeV. The elemental proportions in epitaxial growth of a CrGaN thin film having a single the films were determined based on simulations performed B1 type phase on a MgO(100) substrate by pulsed laser with the RUMP software package that applied fittings to the deposition (PLD) using a Cr-10 mol% Ga alloy as a target RBS spectra. The bonding states of the films were evaluated material. Consequently, the GaN level in the thin film was by Fourier transform infrared spectroscopy (FT-IR; JASCO only 10 mol%.19) In the case of CrAlN thin films, the AlN FT/IR-4000), acquiring spectra from 375 to 1425 cm¹1 at content has a significant effect on their characteristics.20,21) a resolution of 4 cm¹1. Crystal structures were studied by The hardness of CrAlN thin films has been found to X-ray diffraction (XRD; Rigaku RINT 2500HF+/PC) in the increase with increasing AlN solid solution amounts up to a Bragg-Brentano configuration with Cu K¡ radiation ( = level of approximately 70 mol%. Above this concentration, 0.154 nm). Indentation hardness values were obtained by the films consist of two phases, B1-(Cr,Al)N and B4-AlN, so nanoindentation testing with a Berkovich indenter (Agilent that their hardness decreases drastically. However, in the case Technologies G200), utilizing the continuous stiffness of CrGaN thin films, the effect of the GaN concentration measurement (CSM) technique. Using this method, inden- on the mechanical properties has not been clarified. tation depth profiles of both hardness and Young’s modulus could be obtained with a single indentation cycle.22) For each +Corresponding author, E-mail: [email protected]. Graduate thin film, 15 points were tested to an indentation depth of Student, Nagaoka University of Technology 100 nm. Characterization of CrN-Based Hard Coating Materials with Addition of GaN 1575

Table 1 Results of compositional analysis by RBS. x = 0.51

Si sub. FWHM: 1.65 deg. Si sub. Si

x = 0.38 FWHM: 1.52 deg. Si sub. Si sub.

x = 0.31 Intensity (a.u.) FWHM: 1.40 deg.

Ga-N Cr-N sub. Si

x = 0.51 x = 0 FWHM: 0.52 deg. Si sub. x = 0.38 B1-CrN ICDD #11-0065 (111) (200) B4-GaN ICDD #76-0703 (002) (100) (101) x = 0.31 Absorbance (a.u.) 30 35 40 45 50 55 60 65 Diffraction angle 2θ (deg.) x = 0.10 Fig. 2 XRD patterns of Cr1¹xGaxN thin ﬁlms having various x values. FWHM indicates full width at half maximum of the (200) peaks.

x = 0

25 1400 1200 1000 800 600 400 Wavenumber, k /cm-1 20 Fig. 1 FT-IR spectra of Cr1¹xGaxN thin ﬁlms having various x values. /nm

t 15 , size 3. Results and Discussion 10

3.1 Compositional analysis and phase identification Crystallite 5 Table 1 summarizes the compositions of the Cr1¹xGaxN thin films as determined by RBS. Each film contained approximately 50 mol% N, in good agreement with the 0 stoichiometric expectation for CrN and GaN. The x values 0 0.1 0.2 0.3 0.4 0.5 0.6 (representing the Ga/(Cr + Ga) ratio) ranged from 0 to 0.51 Ga / (Cr+Ga), x in conjunction with SR values from 0% to 100%. Figure 1 Fig. 3 Crystallite size in the B1 phase (as calculated using Scherrer’s shows the FT-IR spectra obtained from the Cr1¹xGaxN equation) as a function of the relative proportion of Ga. thin films. It has been reported that B1-CrN and B4-GaN generate broad peaks at approximately 400 cm¹1 (due to CrN bonding) and 570 cm¹1 (due to GaN bonding), films with x ¯ 0.31 produced peaks attributable to B1-CrN respectively.2325) The spectra of the films for which with orientation along the (200). In contrast, those films x ¯ 0.31 exhibit a broad peak at around 400 cm¹1 due to with x ² 0.38 also generated peaks at 2ª values of 34.3° and the presence of B1-CrN. In the case of the film having 36.3° due to the appearance of a secondary phase. Although x = 0.31, this peak is shifted to a lower wavenumber as these peak positions do not exactly coincide with the B4- compared to the film with a lower x. This would be expected GaN (002) and (101) peaks, it was concluded they can still based on changes in the bond length with dissolution of be ascribed to the phase based on B4-GaN, considering the GaN in the CrN lattice. In contrast, the Cr1¹xGaxN thin films FT-IR results. The appearance of the secondary phase was for which x ² 0.38 generated an additional peak at 570 cm¹1 caused by GaN, which could not dissolve in the B1 phase, as a result of the appearance of B4-GaN, indicating that adopting the B4 phase as the stabilized phase for GaN. the GaN was not fully dissolved in the CrN. Figure 2 The (200) peaks also broadened and shifted to lower 2ª provides the XRD patterns of films with various x values, values with increasing x. Figure 3 plots the crystallite size along with the patterns for B1-CrN and B4-GaN from the in the B1 phase in the thin film (as calculated from the International Centre for Diffraction Data (ICDD). The thin (200) peaks using Scherrer’s equation) as a function of x. 1576 Y. Mizuno, T. Nakayama, H. Suematsu and T. Suzuki

0.426 45 450 : E 40 : HIT 400 0.424 35 350 /GPa nm / IT 30 300 /GPa a H 0.422 E 25 250

0.420 20 200 15 150

Lattice constant, 0.418 10 100 Young's modulus,

Indentation hardness, 5 50 0.416 0 0.1 0.2 0.3 0.4 0.5 0.6 0 0 Ga / (Cr+Ga), x 020406080100 Indentation depth, h /nm Fig. 4 Lattice constant of the B1 phase, as calculated from the (200) peak position in each XRD pattern, as a function of the relative proportion Fig. 5 Typical depth proﬁles for the indentation hardness and Young’s of Ga. modulus values obtained from Cr1¹xGaxN thin ﬁlm with x = 0.31.

The crystallite growth of the B1 phase was prevented by the 50 350 increasing of GaN content in the thin films. This was : E 45 : HIT 300 probably caused by forcibly dissolving of B1-GaN, which GPa / is a high-pressure phase, in CrN. The crystallite size for the /GPa 40 250 E IT thin films with x ² 0.31 decreased drastically. Since the thin H 35 200 film with x = 0.38 has the secondary phase based on B4- GaN, x = 0.31 was close to maximum solubility of B1-GaN 30 150 into CrN. Therefore, it was considered that the crystallites 25 100 of the B1 phase were hardly to grow for thin films with modulus, Young's x ² 0.31. The lattice constant for the B1 phase, as calculated Indentation hardness, 20 50 from the (200) peak positions in XRD patterns, is plotted 15 0 as a function of x in Fig. 4. This constant increased by 0 0.1 0.2 0.3 0.4 0.5 0.6 approximately 1.4% as x increased from 0 to 0.31, as a Composition ratio of Ga / (Cr+Ga), x result of the dissolution of GaN in the CrN to form a B1- Fig. 6 Indentation hardness and Young’s modulus values of Cr1¹xGaxN (Cr,Ga)N phase. As x was increased to 0.31 or more, the thin films as functions of the relative proportion of Ga. lattice constant plateaued. We believe that those films for which x ² 0.38 consisted of B1-(Cr,Ga)N supersaturated with GaN and the secondary phase based on B4-GaN. of B4 type phase in the thin films, the hardness and Young’ There is a possibility that Cr dissolved to the secondary modulus both decreased. There were some reports that phase, forming B4-(Ga,Cr)N. Vickers and indentation hardness value of the B4-GaN showed approximately 12 GPa26) and 20 GPa,27) respectively. 3.2 Indentation hardness These values were lower than that of CrN fabricated in this Typical indentation hardness and Young’s modulus depth study. Similar to CrAlN thin films, the presence of a B4 profiles obtained during a single indentation test with the type phase in the thin films prevented hardening. CSM technique are shown in Fig. 5. At depths ranging from 0 to 40 nm, the hardness initially increased rapidly and then 4. Conclusion became constant, after which it decreased slightly with increasing depth beyond 40 nm. This decrease in the hardness Cr1¹xGaxN thin films for which x ¯ 0.51 were successfully could have resulted from the Si substrates because the film prepared on Si(100) substrates by PLD. The films with thickness were all from 200 to 300 nm. Herein, the intrinsic x ¯ 0.31 had a single phase with a B1 structure, and the hardness and Young’s modulus values were calculated by lattice constant of these films was found to increase with averaging over indentation depths from 30 to 40 nm. Figure 6 increasing x. From these results, it was determined that GaN plots the intrinsic hardness and Young’ modulus values was dissolved in the CrN lattice, forming a B1-(Cr,Ga)N obtained by averaging data from 15 indentations for each phase. Conversely, films for which x = 0.38 or 0.51 consisted sample. Both hardness and modulus increased with increases of two phases: B1-(Cr,Ga)N supersaturated with GaN and in x up to 0.31 as the thin films maintained a single B1- a secondary phase based on B4-GaN. The nanoindentation (Cr,Ga)N phase. At x = 0.31, the maximum hardness of hardness and Young’s modulus values of the Cr1¹xGaxN thin 29.4 GPa was obtained. This trend could have been caused by films increased with increases in x up to 0.31, giving a solution hardening of B1-GaN to give B1-CrN and by a maximum hardness of 29 GPa. As the B4-GaN appeared in reduction in the crystallite size, as evident in Fig. 3. In the thin films, the indentation hardness decreased. Based on addition, increases in the Young’s modulus could also con- the above results, GaN addition up to approximately 30 mol% tribute to the hardening. Simultaneous with the appearance can contribute to the hardening of CrN. Characterization of CrN-Based Hard Coating Materials with Addition of GaN 1577

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