Mechanical, tribological, and oxidation properties of Si-containing CrAlN films

Accepted Manuscript Mechanical, tribological, and oxidation properties of Si-containing CrAlN films Y. Kitamika, H. Hasegawa PII:

S0042-207X(18)32095-5

DOI:

https://doi.org/10.1016/j.vacuum.2019.02.031

Reference:

VAC 8558

To appear in:

Vacuum

Received Date: 30 October 2018 Revised Date:

3 February 2019

Accepted Date: 19 February 2019

Please cite this article as: Kitamika Y, Hasegawa H, Mechanical, tribological, and oxidation properties of Si-containing CrAlN films, Vacuum (2019), doi: https://doi.org/10.1016/j.vacuum.2019.02.031. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Mechanical, tribological, and oxidation properties

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of Si-containing CrAlN films

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Y. Kitamika, H. Hasegawa1

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Graduate School of Science & Engineering, Saga University,

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1 Honjyo-machi, Saga 840-8502, Japan

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ABSTRACT

CrXAlYSiZN films were synthesized by radio-frequency magnetron sputtering from alloy

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targets. The metal ratio of Cr to Al in the targets was close to that corresponding to the

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cubic-to-hexagonal phase transition. X-ray diffraction analysis indicated that all prepared

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films had the cubic structure without precipitation of the hexagonal phase. The microhardness

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increased from 30.9 GPa for Cr0.41Al0.59N up to 42.2 GPa for Cr0.43Al0.46Si0.11N. During

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dynamic oxidation, the exothermic peak in the differential thermal analysis curve shifted from

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1286 °C for Cr0.41Al0.59N to 1384 °C for Cr0.43Al0.46Si0.11N. Consistent with the dynamic

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oxidation behavior, the mass gain measured by thermogravimetric analysis for

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Cr0.43Al0.46Si0.11N exhibited the minimum values at isothermal oxidation temperatures of 800–

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1200 °C. In addition, we evaluated the microstructure, microhardness, tribological properties,

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and oxidation behaviors of the films considering the requirements for use as wear-resistant

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coatings.

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Keywords:

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CrAlSiN, microstructure, microhardness, coefficient of friction, oxidation resistance

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1 Corresponding author. E-mail address: [email protected] (H. Hasegawa).

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ACCEPTED MANUSCRIPT Hard protective coatings for cutting tools are designed in order to tailor the mechanical

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strength, tribological behavior, and oxidation resistance to the expected machining conditions,

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such as the workpiece material and cutting speed. CrXAlYN (X+Y=1.0) materials are widely

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used in applications to cutting tools as the favorable surface properties of this material prevent

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tool wear; hence, evaluating the cutting performance of CrXAlYN is important [1–3].

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CrXAlYN materials have been developed as an alternative to TiXAlYN hard coatings by

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incorporating Al atoms into the CrN structure to produce microstructural transitions that result

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in improvement of the physical and chemical properties for protective coating applications.

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Several researchers showed that increasing Al fractions in the range Y = 0.6–0.7 [4–6] led to

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structural changes from the cubic NaCl (B1) to the hexagonal wurtzite (B4) structure. The

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microhardness gradually increases with the incorporation of Al atoms into CrN, where the

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maximum hardness is obtained at an Al content corresponding to the phase boundary between

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the cubic and hexagonal structures [5,6]. Consistent with the hardening of CrN with the

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addition of Al atoms, CrXAlYN exhibits a lower coefficient of friction and better wear

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resistance than CrN [7–9]. Additionally, the oxidation behavior of CrXAlYN has been

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investigated using X-ray diffraction analysis, Fourier transform infrared spectroscopy, and

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thermal analysis. Banakh et al. [10] reported that CrXAlYN (Y = 0.34 or 0.63) resisted

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oxidation at 900 °C without the formation of oxidation products such as Cr2O3 and Al2O3. Zhu

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et al. [11] measured the mass gain of CrXAlYN with Y= 0.18–0.47 under 1000–1100 °C in an

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ambient atmosphere, and showed that the film with Y = 0.47 had the lowest oxidation rate.

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Lin et al. [12] showed that the oxidation of CrXAlYN with Y = 0.23 and 0.60 began at 650–

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700 °C and was complete at 1180–1190 °C. It is well known that severe cuttings (e.g., dry or semi-dry machining and processing of

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high-strength materials) damage cutting tools and reduce their lifetime. In order to satisfy the

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requirements of these challenging machining conditions, Si atoms were added to CrXAlYN in

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an attempt to enhance the surface properties. Several authors prepared CrXAlYSiZN using a

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ternary alloy target, or a binary alloy target with a second metallic target, and reported the

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microstructure [13,14], mechanical properties [15,16], tribological properties [17,18],

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oxidation resistance [19,20], and thermal stability [21]. In this study, CrXAlYSiZN materials

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were deposited by radio-frequency (RF) magnetron sputtering in mixed N2–Ar discharges

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from CrX–AlY–SiZ targets. The metal ratio of Cr (X) and Al(Y) was close to that corresponding

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to the phase boundary between the cubic and hexagonal structures. After deposition, we

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comprehensively evaluated several properties relevant for their application as wear-resistant

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coatings using X-ray diffraction, hardness and friction tests, and thermal analyses.

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CrXAlYSiZN films were synthesized on polished (111)-oriented Si wafers and stainless steel

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substrates by RF magnetron sputtering, using Cr0.40–Al0.60, Cr0.40–Al0.55–Si0.05, and Cr0.40–

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Al0.50–Si0.10 alloy targets. The Si wafer substrates were used for identifying the chemical

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composition, crystal structure, microhardness, coefficients of friction, and wear depths,

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whereas the stainless steel substrates were used for thermal analyses. These films were

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synthesized under a mixed gas atmosphere of pure N2 and pure Ar at a total pressure of 1.0 Pa -3-

ACCEPTED MANUSCRIPT and a N2 partial pressure of 0.35 Pa. The power density of the targets was 10.6 W/cm2, and the

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deposition temperature was 250 °C. A negative bias voltage of -50 V was applied to the

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substrate and the distance between the target and substrate was 45 mm for all deposition

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processes. The film thickness was adjusted to 3.0–3.5 µm for all samples.

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The chemical composition of the films was measured using energy-dispersive X-ray

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analysis (EDS; Oxford Instruments X-Max80). The crystal structure of the films on the Si

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wafers were evaluated using X-ray diffraction (XRD; Shimadzu XRD-7000) using Cu-Kα

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radiation at 40 kV and 30 mA in θ–2θ scan mode. The peaks of the XRD patterns were

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identified by comparison with Joint Committee on Powder Diffraction Standards (JCPDS)

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reference spectra.

The microhardness of the samples was measured from load–displacement curves obtained

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using a dynamic ultra-micro hardness tester (Shimadzu DUH-211S) under maximum

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depth-control mode. The maximum loads were ~20 × 10−3 N, and the maximum indentation

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depth was <10% of the film thickness. The friction tests were performed with a ball-on-disc

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tribometer (Rhesca FPR-2100) under dry conditions at 25 °C with a relative humidity of 42%.

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A SiC ball with diameter of 4.8 mm was used as a counterpart material, and the mode for

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rotational motion at a radius of 2.0 mm and speed of 500 rpm was used with test loads of 2.94

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and 4.90 N. The surface roughness of the as-deposited sample, and the wear depth after

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friction tests were determined using a surface-roughness tester (Mitutoyo SJ-301).

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ACCEPTED MANUSCRIPT Dynamic oxidation and isothermal oxidation tests were conducted using differential

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thermal analysis (DTA; Shimadzu DTA-50) and thermogravimetry analysis (TGA; Shimadzu

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TGA-51H), respectively. To eliminate the effect of the substrate material on the DTA and

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TGA measurements, the films were chemically removed from the stainless steel substrates

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using a dilute solution of hydrochloric acid. After filtering and cleaning, the films were

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crushed into powders. For the dynamic oxidation test, the powder samples were placed in a

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platinum crucible and heated from room temperature to 1400 °C in air at a heating rate of

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15 °C/min. Al2O3 blocks were used as an inert standard sample in the DTA measurements. In

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the isothermal oxidation tests, the mass gain of each powder was recorded as a function of

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time during isothermal oxidation at 800–1200 °C.

The chemical compositions measured using EDS were Cr0.41Al0.59N, Cr0.38Al0.57Si0.05N, and

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Cr0.43Al0.46Si0.11N with nitrogen contents of ~55 at.%; these films were regarded as

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stoichiometric nitrides. Fig. 1 shows XRD patterns of as-deposited CrXAlYSiZN samples with

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the expected peak positions for CrN (JCPDS No. 11-0065). Here, the prefix c- refers to the

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cubic structure of the metastable nitrides. The XRD spectrum of Cr0.41Al0.59N indicates the

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presence of a cubic (NaCl-type) polycrystalline microstructure with (200) preferred

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orientation. The maximum solubility of Al atoms in CrN while maintaining the cubic structure

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has been explored using numerical and experimental methods. A previous study used the band

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parameters and structural maps to predict that the cubic-to-hexagonal phase transformation in

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CrXAlYN occurs at Y=0.77 [5], while XRD studies showed that the cubic structure of CrXAlYN

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ACCEPTED MANUSCRIPT remained unchanged up to Y=0.68–0.71 [6,13]. Here, Cr0.41Al0.59N, as well as the

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Si-containing compositions, showed the cubic phase and a relatively strong (111) peak and

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broad (200) peak were identified in the XRD patterns. Previous structural analyses of

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CrXAlYSiZN [13–15,18,21] showed that the composition-dependent phase could be

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categorized as pure cubic phase or a mixture of cubic and hexagonal AlN (JCPDS No.

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25-1133). Hexagonal AlN was not identified in our XRD data, which indicates the formation

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of a solid-solution phase by the incorporation of Si atoms into CrXAlYN. Additionally, the

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previous studies by Endrio et al. [13], Park et al. [17], and Liu [21] et al. suggest that an

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amorphous phase (e.g., Si–N and Si3N4) accumulate at the grain boundaries of CrXAlYSiZN.

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Fig. 2 shows the coefficient of friction (µ) for CrXAlYSiZN versus sliding distance (l) at test

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loads of 2.94 and 4.90 N. The average surface roughness of the as-deposited films was below

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0.03 µm. The µ values increased gradually between the initial state (l = 0 m) and the transient

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state (l = ca. 20 m) and plateaued at a steady state (l = beyond 20 m). At a test load of 2.94 N,

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the range of µ values at the steady state was 0.38–0.78 for Cr0.41Al0.59N, 0.28–0.59 for

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Cr0.38Al0.57Si0.05N, and 0.33–0.67 for Cr0.43Al0.46Si0.11N (Fig. 2(a)). At the higher test load of

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4.90 N, the range of µ values at the steady state for all the films decreased to µ = 0.31–0.56

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(Fig. 2(b)). This was attributed to reduced adhesion between the SiC ball and the films with

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the higher test load.

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Fig. 3 shows the wear depth after friction tests at loads of 2.94 and 4.90 N. At 2.94 N, the

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wear track on all films had a depth of <0.41 µm. The measured wear depths at 4.90 N ranged -6-

ACCEPTED MANUSCRIPT from 0.75 µm for Cr0.41Al0.59N to 0.35 µm for Cr0.43Al0.46Si0.11N. Consistent with the different

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wear depths, the microhardness increased from 30.9 GPa for Cr0.41Al0.59N to 39.7 GPa for

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Cr0.38Al0.57Si0.05N and 42.2 GPa for Cr0.43Al0.46Si0.11N. Similar hardening effects resulting

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from adding of Si atoms into CrXAlYN were confirmed by several researcher. They suggest

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that hardening phenomena can be attributed to solid-solution hardening, crystal-grain

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refinement, and high cohesive grain boundary energy [17, 21].

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Fig. 4 shows DTA curves for the CrXAlYSiZN samples during dynamic oxidation from room

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temperature to 1400 °C. The first small DTA peak was observed at ~350–600 °C, the

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temperature at which the exothermic reactions occurred via the recovery process and

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recrystallization [12,22]. In general, the recovery process is strongly correlated with the

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relaxation of deposition-induced lattice defects and residual stress at elevated temperature.

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The intensity of the DTA signal for Cr0.41Al0.59N increased at temperatures above 600 °C, and

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exothermic peaks were observed at 1175 and 1286 °C. According to the oxidation behavior

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reported in several previous studies [10–12], the precipitation of hexagonal AlN and the

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formation of oxidative products, such as Cr2O3, Al2O3, and (Cr,Al)2O3, were confirmed.

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Therefore, the observed DTA peaks at 1175 and 1286 °C were presumably correlated to the

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precipitation and complete oxidation, respectively. The respective exothermic DTA peaks of

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Cr0.38Al0.57Si0.05N and Cr0.43Al0.46Si0.11N shifted toward higher temperatures up to 1358 and

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1384 °C, respectively, suggesting that the incorporations of Si atoms into CrXAlYN delayed

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the complete dynamic oxidation observed for Cr0.41Al0.59N. The oxidation behavior of

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CrXAlYSiZN studied by Tritremmel et al. [23] indicates that the formations of (Cr,Al)2O3 and

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SiO2 proceed with nitrogen release and oxygen absorption, and these oxides acts as a

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diffusion barrier. Fig. 5 presents the TGA results of the CrXAlYSiZN samples during isothermal oxidation at

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800–1200 °C. The TGA curves showed parabolic oxidation, where the oxidation behavior

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changed drastically with increasing temperature. The final measured mass gain (∆m) of

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Cr0.41Al0.59N increased from 4% at 800 °C up to 17% at 1200 °C (Fig. 5(a)), whereas

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Cr0.38Al0.57Si0.05N exhibited a slightly lower mass gain (Fig. 5(b)) than Cr0.41Al0.59N. The

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smallest ∆m values at all temperatures were observed for Cr0.43Al0.46Si0.11N (Fig. 5(c)). The

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parabolic rate constants (kp) were calculated on the assumption that the oxidation process of

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CrXAlYSiZN accords to a parabolic law, and the logarithm of kp was plotted versus the

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reciprocal of an absolute temperature (T) (Fig.5 (d)). The activation energy obtained from

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Arrhenius equations of Cr0.41Al0.59N and Cr0.38Al0.57Si0.05N were ~42 kJ/mol, and that of

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Cr0.43Al0.46Si0.11N slightly increased up to ~51 kJ/mol.

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Our results showed that Cr0.43Al0.46Si0.11N exhibited the highest hardness and oxidation

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resistance of all composition tested. Previous structural analyses indicated that the addition of

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Si into CrXAlYN resulted in the formation of an amorphous phase around the crystalline phase

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[13,17,20]. The amorphous phase can harden CrXAlYN by preventing plastic deformation and

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propagation of dislocations in addition to the solid-solution hardening from the incorporation

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of Si atoms into the CrXAlYN structure. Furthermore, as amorphous phases have no grain -8-

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boundaries, oxidation resistance is enhanced by suppression of grain boundary diffusion of

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oxygen. In conclusion, we produced CrXAlYSiZN films by RF magnetron sputtering and analyzed

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their microstructure, microhardness, coefficient of friction, wear depth, and oxidation

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behavior. All films were solid solutions with a cubic structure, and a maximum hardness of

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42.2 GPa was obtained for Cr0.43Al0.46Si0.11N. The average frictional coefficient was 0.31–0.56,

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and the minimum wear depth was 0.35 µm with a test load of 4.90 N. With the incorporation

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of Si atoms into CrXAlYN, the exothermic DTA peaks during dynamic oxidation appeared at

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higher temperatures up to 1384 °C. The minimum mass gain at the respective isothermal

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oxidation temperature over the range of 800–1200 °C was obtained for Cr0.43Al0.46Si0.11N.

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Acknowledgments

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This work was supported by Japan Keirin Autorace (JKA) and its promotional funds from

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autoracing. The authors acknowledge the Analytical Research Center for Experimental

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Sciences, Saga University for assistance with the XRD analyses. We also thank Editage

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(https://www.editage.jp) for English language editing.

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Figure captions

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Fig. 1. XRD patterns of as-deposited CrXAlXSiZN on Si wafers. The prefix “c-” and the

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symbol “*” refer to the cubic structure and Si-substrate, respectively.

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Fig. 2. Coefficient of friction for CrXAlXSiZN as a function of sliding distance at test loads of

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(a) 2.94 N and (b) 4.90 N.

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Fig. 3. Wear depth after friction testing and microhardness of CrXAlYSiZN. The error bars represent the standard error.

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Fig. 4. DTA curves of CrXAlYSiZN during dynamic oxidation from room temperature to 1400

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ºC.

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Fig. 5. TGA curves during isothermal oxidation at 800–1200 ºC for (a) Cr0.41Al0.59N, (b)

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Cr0.38Al0.57Si0.05N, (c) Cr043Al0.46Si0.11N, and (d) Arrhenius plots of CrXAlYSiZN.

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CrXAlYSiZN films prepared on the basis of the phase transformation behavior of CrXAlYN Cr:Al ratio tailored to maximize the hardness while maintaining the cubic phase

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Maximum hardness of 42.2 GPa measured for Cr0.43Al0.46Si0.11N composition

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The Cr0.43Al0.46Si0.11N composition showed the best oxidation resistance

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