Effects of yttrium doping on high-temperature oxidation, friction, and wear properties of CrAlN films

This study investigates the deposition of CrAlYN nanomultilayer films with different yttrium (Y) contents on M2 tool steel and single-crystal Si wafer using unbalanced magnetron-sputtering ion-plating technology. Transmission and scanning electron microscopic and scanning morphologies, x-ray diffraction pattern, energy dispersive spectra, nanoindentation, high-temperature oxidation, and high-temperature tribological analyses show that CrAlYN nanomultilayer films have a face-centered cubic (fcc) crystal structure with a modulation period of CrN/YN+AlN/CrN/AlN. CrAlYN films exhibit optimal mechanical performance when Y content is 1.13at%. However, a substantial drop occurs when Y content increases to 2.67at%. As Y content increases, the high-temperature oxidation resistance, friction, and wear of CrAlYN films first increase and then decrease. Notably, films with the Y content of 1.13at% have optimal resistance to high-temperature oxidation, friction, and wear.


Introduction
The remarkable physicochemical properties of rare earth elements have made them 'industrial gold.'Their unique physical and chemical properties have widespread applications in various fields, such as military, electronics, metallurgy, optics, agriculture, chemicals, and medicine [1][2][3][4][5].Recent research has shown that surface treatment can introduce the right amount of reactive yttrium (Y) content into nitride hard films, thereby improving their hardness, adsorption capacity, and resistance to high-temperature oxidation.Additionally, rare earth elements possess a surface purification effect that facilitates the adsorption, deposition, and diffusion of atoms in coatings, reducing deposition times [6][7][8][9].Moser et al [10,11] found that Y doping enhances the hardness and thermal stability of TiAlN films through solid solution strengthening.Zheng Z et al [12] suggested that the outward diffusion of Y can reduce the inward diffusion of oxygen (O); however, Y doping can improve the oxidation resistance of TiN films.Xian G et al [13] conducted a comparative study on the oxidation resistance of ZrAlN and ZrAlYN films and found that at high temperatures, Y precipitates at grain boundaries inhibit the inward diffusion of O and the outward diffusion of metal ions.Additionally, these precipitates slow grain coarsening at high temperatures, thereby enhancing film adhesion.Zhang J et al [14] proposed that the oxidation of TiAlYN films results in the accumulation of Y or its oxide at grain boundaries and interfaces.This accumulation prevents O from diffusing inward to the oxide film/film interface through grain boundaries, reducing the oxidation rate.The aforementioned studies indicate that the addition of an appropriate amount of a rare earth element can substantially improve the hardness and oxidation resistance of nitride films.Y is an ideal doping element for high-temperature cutting coatings [15,16].
CrAlN films are widely used in high-speed cutting processes owing to their resistance to high-temperature oxidation [17][18][19][20][21][22].To further enhance the high-temperature cutting performance of CrAlN coatings, this study investigates the effects of adding trace amounts of Y to CrAlN nanomultilayer films.Moreover, closed-field nonequilibrium magnetron sputtering was used to prepare quaternary CrAlYN nanomultilayer films.The study systematically investigates the impact of rare earth element Y doping on the resistance to high-temperature oxidation, friction, and wear of CrAlYN films.

Methods
The CrAlYN nanomultilayer films were prepared using a Teer-UDP650/4 magnetron-sputtering system.The system has two 99.99% pure Chromium (Cr) targets, one 99.99% pure Aluminum (Al) target, and three AlY alloy targets with 0%, 2.58%, and 4.97% Y contents.Each target measured 380 mm × 175 mm with a thickness of ∼10 mm.The substrates were Φ30 mm × 2 mm M2 tool steel and single-crystal Si films.The M2 tool steel substrates were polished with sandpaper at 2,000 grit and further polished using diamond polishing liquid until Ra was 0.5 μm.Then, the substrates were cleaned using ultrasonics for 15 min with acetone and anhydrous ethanol.The vacuum chamber was evacuated to 5 × 10 −4 Pa before deposition, and the substrate holder rotated at 4 rpm.Argon gas was introduced at a −450 V bias voltage at a flow rate of 40 sccm for 30 min to clean the sample surfaces and remove oxide impurities.The bias voltage was adjusted to −75 V, and the argon gas flow rate was reduced to 25 sccm.For Cr targets, the Cr underlayer deposit current was increased to 4 A. After 5 min, high-purity nitrogen gas (N 2 ) (>99.999%) was introduced as the reaction gas.The nitrogen flow was controlled by the optical emission monitor to help deposit the CrN transition layer.After 10 min, the currents for the AlY and Al targets were gradually increased to 6 A. The CrAlYN nanomultilayer films were deposited for 120 min at this current.
The cross-sectional morphologies of nanomultilayer films were analyzed using a transmission electron microscope (TEM).The main technical specifications are as follows: point resolution: 0.24 nm; information resolution: 0.14 nm; magnification range: 25-1030 K; maximum accelerating voltage: 200 kV; and maximum sample tilt angle: ±40°.Cross-sectional and wear-mark morphologies caused by high-temperature oxidation and wear were analyzed using a ΣIGMA field emission scanning electron microscope (SEM), and the surfaces and cross-sectional compositions of the films were analyzed using an energy dispersive spectrometer (EDS).Isothermal oxidation was performed in a box-type resistance furnace.At 1000 °C, after 2 h of oxidation in static air, the samples were cooled to room temperature.The phase structures of CrAlYN films with different Y contents after being oxidized were analyzed using a Bruker D8 Advance x-ray Diffractometer (XRD).The process specifications are as follows: x-ray source: Cu target Kα ray (λ = 0.154056 nm); current: 40 mA; acceleration voltage: 40 kV; incident angle scan range: 20°-90°.The hardness of the films oxidized at 1000 °C was measured using a G200 Nano-Indenter.To minimize measurement errors, each sample was measured five times and the average was recorded.High-temperature friction and wear tests were performed using an HT-4001 Pin-on-Disk Wear Tester.The temperature was set at 600 °C, and the load was 15 N.The sample was a W-6%Co ball with a diameter of 4 mm, and the rotation speed was set to 637 rpm.The test duration was 1800 s, and a 6-mm wear mark was produced.The tests were performed at 25 °C, and the wear-mark depth was measured using a profilometer.

Compositions and microstructures of CrAlYN nanomultilayer films
The compositions and thicknesses of the three CrAlYN        Figure 5 shows the wear-mark morphologies and depths of the CrAlYN films with different Y contents after high-temperature friction and wear tests at 600 °C.As shown in figure 5(a), the wear-mark width of the CrAlN film without Y was ∼58 μm, with the deepest point at 0.83 μm.Moreover, the wear-mark width of the CrAlYN film with an Y content of 1.13at% (figure 5(b)) was 42 μm, with the deepest point at 0.45 μm.Furthermore, at an Y content of 2.67at%, the wear-mark width of the CrAlYN film was 60 μm (figure 5(c)), and the deepest point of wear reached 4.6 μm.The film was worn out and failed because the wear-mark depth exceeded its thickness.

Discussion
The experimental results indicate that the doping ratio of Y substantially affects the high-temperature performance of the CrAlYN film, and the film doped with 1.13at% Y demonstrated optimal performance in high-temperature friction and wear experiments.XRD and EDS results reveal that during the oxidation of the CrAlN nanomultilayer film without Y, the main oxide layers formed are Cr 2 O 3 and Al 2 O 3 .After 1.13at% Y doping, the rate of inward diffusion of oxygen during the oxidation process of the CrAlN film decreased considerably.This is mainly because during the oxidation process of the CrAlYN nanomultilayer film doped with a small amount of Y, the reactive element Y in the YN+AlN modulation layer, driven by the oxygen potential gradient, diffuses outward to the oxide layer/gas interface and accumulates at the grain boundaries of the oxide layer.As the ionic radius of Y is greater than those of Cr and Al, the accumulation of Y at the grain boundaries serves as pinning points for dislocations at the interface, which can prevent the outward diffusion of Cr and Al ions, thus reducing the growth rate of the oxide layer.This changes the growth pattern from the outward diffusion of metal ions to the inward diffusion of oxygen.
As Y content exceeded 1.13at%, the oxide layer of the CrAlYN nanomultilayer film formed many porous structures.This substantially decreased the oxidation resistance of the film.To better understand this process, an x-ray photoelectron spectroscopy analysis was performed on the composition of the oxidized CrAlYN film with an Y content of 2.67at%, as shown in figure 6.The fitting results for O1s show that the oxide layer is primarily composed of Cr 2 O 3 and Al 2 O 3 oxides.The fitting results for Y3d indicate that an excessive amount of Y led to a second-phase Y-Al oxide in the oxide layer during oxidation.The second-phase Y-Al oxide introduces defects into the oxide layer that is primarily composed of Cr 2 O 3 and Al 2 O 3 and destroys its dense structure [23].Additionally, the porous structures provide pathways for rapid outward diffusion of Cr and Al ions and inward diffusion of O, accelerating the formation of an oxide layer.This considerably decreases the hardness of the film and the strength of the film/substrate interface, thereby causing a rapid decline in the high-temperature friction and wear of the film.

Conclusions
This study investigated the effects of Y contents on the resistance of CrAlN nanomultilayer films to hightemperature oxidation and friction.The CrAlYN films have an alternate nanomultilayer structure, and Y doping does not change the original fcc crystal structure.As Y content increases, the hardness and elasticity moduli of CrAlYN films initially increase and then decrease.When Y content is 1.13at%, the hardness of CrAlYN films reaches its maximum value of 29.4 GPa.The resistance of CrAlYN films to high-temperature oxidation is closely related to their Y content.When Y content is 1.13at%, the films show optimal resistance to high-temperature oxidation.However, when Y content increases to 2.67at%, the oxidation resistance of CrAlYN films considerably drops.As Y content increases, the resistance of CrAlN nanomultilayer films to high-temperature friction and wear increases and then drops.Specifically, CrAlYN films with an Y content of 1.13at% show optimal resistance to high-temperature.

Figure 3 (
b) shows the cross-sectional morphology of the CrAlYN film with an Y content of 1.13at% oxidized at 1000 °C for 2 h.EDS analysis shown in figure3(e) indicates that the depth of the oxidation layer in the CrAlYN film was ∼0.4 μm, which was less than that in the CrAlN film.This shows that trace Y doping improves the oxidation resistance of the CrAlN film.Additionally, the N content in the oxidation layer was low, and the Cr, Al, and Y contents were mostly unchanged.As Y content continued to increase, the depth of the oxidation layer in the CrAlYN film with an Y content of 2.67at% (figure3(c)) increased to ∼1 μm, and a more porous oxidation layer structure was formed.The EDS analysis, as shown in figure 3(f), indicates that the oxygen-rich region reached a depth of 1.2 μm; the N content in the oxidation layer was low; and Cr diffused toward the surface.After 1.2-μm deep, the elements in the CrAlYN film stabilized.The oxidation resistance of the CrAlYN film considerably decreased when Y content reached 2.67at%, which is consistent with the surface EDS and XRD results.

Figure 2 .
Figure 2. X-ray diffraction patterns of the CrAlYN film with different Y contents after being oxidized at 1000 °C for 2 h.

3. 3 .
Resistance of CrAlYN films to high-temperature friction and wear The friction coefficient curves of the CrAlYN films with different Y contents at 600 °C are shown in figure 4. The friction coefficient curves of the CrAlN films fluctuated and averaged 0.374, as shown in figure4.After the addition of 1.13at% Y, the friction curves of the CrAlYN films slightly fluctuated, and the average friction coefficient was found to be 0.213, thereby depicting a significant improvement in high-temperature friction performance.However, as Y content increased to 2.67at%, the friction coefficient curves fluctuated more, and the average friction coefficient rose from 0.213 to 0.634, decreasing the high-temperature friction performance of the films.

Figure 4 .
Figure 4. Friction coefficient curves of the CrAlYN films with different Y contents at 600 °C.

Figure 5 .
Figure 5. Wear-mark morphologies and depths of the CrAlYN films with different Y contents after high-temperature friction and wear tests at 600 °C.(a) 0.00 at% Y.(b) 1.13at% Y. (c) 2.67at% Y.

Table 1 .
nanomultilayer films with different Y contents are shown in table 1.As Y content increases in the AlY alloy target, Y content in the CrAlYN film gradually increases from 0 to 2.67at%; the Cr content slightly decreases; the Al content slightly increases; and the N content remains relatively constant (table 1).Composition, thickness and hardness of CrAlN and CrAlYN films.
The cross-sectional TEM morphologies of the CrAlYN nanomultilayer films with different Y contents are shown in figure 1.The CrAlN, CrAlYN (1.13% Y), and CrAlYN (2.67% Y) films exhibited similar multilayer structures with alternating lighter and darker regions and roughly consistent growth directions.Energy spectrum analysis of the CrAlYN (1.13% Y) and CrAlYN (2.67% Y) films identified the lighter region as the CrN layer with a thickness of 4.7 nm and the darker region as the alternating AlN and YN+AlN layers with a thickness of ∼5.0 nm.The periodic structure is CrN/YN+AlN/CrN/AlN, which is a typical nanomultilayer structure.Y was primarily doped into the modulation layer of AlN in the multilayer film.The lower-right corner of figure 1(c) shows a selected area electron diffraction pattern of the CrAlYN (2.67% Y) nanomultilayer film.The experimental results showed that the CrAlYN film had an fcc polycrystalline structure and doping with Y did not alter it.3.2.Resistance of CrAlYN nanomultilayer films to high-temperature oxidationTable2presents the compositions and hardness of the CrAlYN films with different Y contents oxidized at 1000 °C for 2 h.The CrAlN films oxidized at 1000 °C contained 14.81at% O on their surfaces, as shown in table 2. After Y was added, the O content on the surface of the CrAlYN film with the Y content of 1.13at% dropped to 11.42at%.As Y content further increased to 2.67at%, the O content on the film surface increased to 19.86at%, whereas the nitrogen (N) content rapidly dropped, indicating a considerable increase in the oxidation film.The indentation hardness and modulus of the CrAlYN films oxidized at 1000 °C with different Y contents increased and then decreased.

Table 2 .
Composition, hardness, and modulus of CrAlYN films after being oxidized at 1000 °C for 2 h.