Nano oxides reinforced high-entropy alloy coatings synthesized by atmospheric plasma spraying

ABSTRACT The high-entropy alloy coatings reinforced with nano oxides were synthesized by the atmospheric plasma spraying (APS). The crystal structure, microstructure, surface morphology, hardness and wear resistance properties of the CoCrFeNiAl and CoCrFeNiMo high-entropy alloy coatings are investigated. For CoCrFeNiAl high entropy coating (HEC) not only has high hardness (573 ± 19 Hv0.1) but also has good wear resistance [The coefficients of friction (COF) is 0.49 ± 0.04]. The transition of the wear mechanism obviously appears between the two HECs. The influences of the structure, composition and hardness on the tribological behavior of high-entropy alloy coatings were discussed in details. GRAPHICAL ABSTRACT IMPACT STATEMENT Nano oxides enhanced high-entropy coatings (HECs) are successfully fabricated by atmospheric plasma spraying, which exhibit the highest hardness and the best wear resistance in HECs and high-entropy alloys.

HECs can be fabricated by various techniques [13], including magnetron sputtering [17,18], laser cladding [14,19], spraying [15,20], electrodeposition [21,22], etc. However, little attention has been paid to the preparation of HECs by thermal spraying. As one of the most widely-used thermal-spraying technique, atmosphericplasma spraying (APS) is a potential processing route for fabricating HECs due to its simple manufacturing process, wide range of powder selection, and high production efficiency. Therefore, in the present work, two different single-phase solid-solution HEA coatings, i.e. CoCrFeNiAl and CoCrFeNiMo HEAs, doped with nanomulti-component metal oxides were synthesized by the atmospheric plasma spraying (APS). The high hardness and excellent wear resistance performance of the HECs are achieved. The detailed information about the experimental procedure and results are provided in the Supplementary materials. Figure 1(a and b) show the profiles of CoCrFeNiAl and CoCrFeNiMo feedstock powders, respectively. The powders have spherical or near-spherical shapes, and the sizes of powders range from 30 to 50 μm. The size distributions of two feedstock powders are presented in Figure S1, which show near-Gaussian distributions. The XRD spectrum for feedstock powders and coatings are displayed in Figure 1(c). The CoCrFeNiAl HEA powder exhibits a body-centered-cubic (BCC) single-phase structure. The CoCrFeNiMo HEA powder has a face-centered-cubic (FCC) single-phase structure. After spraying, the XRD patterns indicate that the main phase structures do not significantly change, i.e. the BCC structure still occupies the CoCrFeNiAl HEC, and the FCC structure occupies the CoCrFeNiMo HEC. Besides the main phase, a small quantity of unindexed phases is found. The unindexed phases are possibly the complex metallic oxide that are formed during the APS process.
The morphologies and the elemental distributions of the CoCrFeNiAl and CoCrFeNiMo HECs are shown in Figure 2. The surfaces of coatings are rough, which are the inherent characteristics of coatings by APS [23]. Generally, during the APS process, the heat input of the plasma jet on the flying-powder particles is not homogeneous. When the feedstock powders with a wide particle-size range are injected into the plasma jet, different kinetics and thermal energies are obtained by the particles, which can cause the particles to be in various states, including fully-molten, semi-molten, and unmelted particles [24]. The fully-molten particles with an enough velocity can form the well-flattened splats when they strike the substrate, which have been marked in Figure 2(a and g). For the semi-molten particles, they are rebounded on the coatings, which exhibit some rough bulges, as marked in Figure 2(a and g). However, the surface of the CoCrFeNiMo HEC exhibits that some unmelted particles are bonded on the surface of the coating [ Figure 2(g)], which cannot be observed in the CoCrFeNiAl HEC [ Figure 2(a)]. Figure 2(b and h) provide the threedimensional (3D) surface morphologies of the CoCr-FeNiAl and CoCrFeNiMo HECs, which present many bulges appearing on the surface of the CoCrFeNiMo HEC. The surface of the CoCrFeNiAl HEC is flatter than that of the CoCrFeNiMo HEC. Based on Figure 2(b) and (h), the surface-roughness values, R a , of the HECs are measured to be 11.9 ± 0.5 μm for the CoCrFeNiAl HEC, and 14.6 ± 0.7 μm for the CoCrFeNiMo HEC, respectively, which further confirms that the CoCrFeNiAl HEC is smoother than that of the CoCrFeNiMo HEC quantitatively. The thicknesses of the HECs are approximately 310 ± 35 μm for the CoCrFeNiAl HEC and 255 ± 25 μm for the CoCrFeNiMo HEC, respectively [Figure 2(c and i)]. The difference in the thicknesses of two HECs indicates that the deposition efficiency of the CoCrFeNiAl HEC is significantly higher than that of the CoCrFeN-iMo HEC fabricated with the same APS parameters. The aluminum element of the CoCrFeNiAl HEA is replaced by molybdenum, which significantly improves the melting point of the CoCrFeNiMo HEA powders. In this case, some of the CoCrFeNiMo HEA powders are not fully melted, which leads to the low deposition efficiency. After polishing the cross sections of HECs, the SEM images show some dark and bright regions [ Figure 2(d and j)]. Further energy-dispersive spectrometry (EDS) analyzing the elemental distributions in the dark and bright regions indicates that the dark regions are the oxide banded structure because the oxygen content in the dark region approaches about 50 at. %, and the oxide banded  structure is mainly composed of a variety of complex metal oxides ( Table 1). The elemental distribution in the bright region matches well with the HEA solid solution (5 ∼ 35 at. %) ( Table 1). Figure 2(e and k) exhibit the polished surfaces of the CoCrFeNiAl and CoCrFeNiMo HECs, which are composed of a great number of metal oxides and HEA matrix with some cavities embedded into the surface of coatings. The cavity formation is due to the loose adhesion between the spraying layers, and gas escaping from coatings [23,24]. It is obvious that a large number of layered wrapping structures exist on the polished surface of the two coatings. The layered wrapping structure consists of dark banded region of multi-component metal oxides and bright region of HEA matrix. The local regions covered by blue rectangles in Figure 2(e and k) are enlarged, as shown in Figure 2(f and l) that are further characterized by the electron probe micro analysis (EPMA). It is obvious that the compositions in the HEA solid solutions are homogeneously distributed, and an enrichment of metal oxides is in the oxide of layered wrapping structure. The EPMA result of the area in Figure 2(f) and Table 1 indicate that the aluminum element is mainly enriched in the dark region, and the average content is 37.7 at. % in the CoCrFeNiAl HEC ( Table 1). The EPMA result of the area in Figure 2(l) and Table 1 indicate that the chromium element is mainly enriched in the dark region, and its atomic percent is 34.4% in the CoCrFeNiMo HEC (Table 1) Figure S2). Accordingly, both HECs are composed of the HEA matrix and multi-component metal oxides.
The TEM analysis can further characterize the microstructures of the HEA matrix and multi-component metal oxides in the HECs. Figure 3(a and d) present the bright-field images of the HEA matrix and nano-multi-component metal oxides for the CoCrFe-NiAl and CoCrFeNiMo HECs, respectively. Figure 3(b and e) show the morphologies of the HEA matrix for the CoCrFeNiAl and CoCrFeNiMo HECs, respectively. It can be clearly seen that the phase of the matrix for the CoCrFeNiAl HEC is determined to be a solidsolution with a BCC structure, and the grain size is about 200 ∼ 500 nm [ Figure 3(b)]. For the CoCrFeNiMo HEC, the phase structure of the matrix is FCC solidsolution, and the gain size is almost the same as the value in the CoCrFeNiAl HEC. The bright-field image and the selected-area electron diffraction CoCrFeNiAl HEC) and 49 ± 12 nm (for the CoCrFeNiMo HEC) are clearly observed in Figure 3(c and f). The polycrystalline diffraction rings are observed in the (SAED) patterns of multi-component metal oxides for the CoCrFeNiAl and CoCrFeNiMo HECs are shown in Figure 3(c and f), respectively. From this, numerous nano-grains with the average grain diameter of 38 ± 8 nm (for the CoCrFe-NiAl and CoCrFeNiMo HECs, which point to the phases of nano-multi-component metal oxides, as exhibited in Figure 3(c and f), respectively. Figure 4(a and b) present the indentations of the Vickers hardness for the CoCrFeNiAl and CoCrFeNiMo HEC, respectively. To exclude the experimental error, the hardness tests were repeated ten times for each HEC. The average Vickers hardness (an HV diamond Indenter with a rectangular pyramid) values of two HECs are shown in Figure 4(c), which are 573 ± 19 and 319 ± 10 Hv 0.1 for the CoCrFeNiAl and CoCrFeNiMo HEC, respectively. For a comparison, the hardness values of other HEAs, HECs, steels and titanium alloy with different compositions are also listed in Figure 4(c), in which the hardness of the CoCrFeNiAl HEC is significantly higher than those of other coatings and alloys, even higher than the value of the CoCrFeNiMo HEC. Figure 5(a) shows the coefficients of friction (COF) for the CoCrFeNiAl and CoCrFeNiMo HECs as the functions of the sliding distance. A small COF means a high wear resistance because it requires more energy to remove the same volume of the material [30]. The COFs of the CoCrFeNiAl and CoCrFeNiMo HECs are 0.49 ± 0.04 and 0.75 ± 0.02, respectively [ Figure 5(a)]. For a comparison, the COF of the FeCoCrNiMo0.2, AlCoCrFeNiTi, AlCoCrFeNiSi, CoCr2FeNiTi HECs, and Cr3C2-50NiCrMoNb coating are also marked by dash lines in Figure 5(a). It can be clearly seen that the CoCrFe-NiAl HEC exhibits the best wear resistance. Figure 5(b-g) show the SEM images of the wear traces of the CoCr-FeNiAl and CoCrFeNiMo HECs. From Figure 5 Table 2.
In the CoCrFeNiAl HEC, the average composition distribution in the grooves is almost same as that of the CoCrFeNiAl HEA matrix with no oxygen. The other two areas show obviously oxidation, in which the atomic ratios of metallic elements are still near equal ( Table  2). The oxygen contents in the three regions for the CoCrFeNiMo HEC are all very high (13.12 ∼ 20.27   [38][39][40][41]; The SEM micrographs of the wear traces of (b)-CoCrFeNiAl; (c, d)-higher magnification for CoCrFeNiAl HECs; (e)-CoCrFeNiMo; (f, g)-higher magnification for CoCrFeNiMo HECs. at. %). Some oxides, such as iron oxides, appear in the three regions, which indicates that the oxidative wear is one of its wear mechanisms in the CoCrFeNiMo HEC. Accordingly, the wear mechanisms must include the mild oxidative wear. At the end of friction, abrasive grooves were clearly observed in the wear trace of the CoCrFeN-iMo HEC. For the CoCrFeNiAl HEC, a smooth wear trace with a few of shallowly-abrasive grooves is observed [ Figure 5(b)]. The wear debris in the CoCrFeNiAl HEC is a particle-like shape and finer (around 5 ∼ 20 μm) than that of the CoCrFeNiMo HEC [ Figure 5(c and f)].
For the CoCrFeNiMo HEC, the flakes are a plateletlike shape. The large debris accumulates on the worn surface with the increasing of the COF. These results illustrate that more adhesive and abrasive wear features took place in the CoCrFeNiMo HEC, as compared with that in the CoCrFeNiAl HEC. According to the theory of the adhesive wear, the adhesive material can slide between the surfaces of the friction pairs during the subsequent sliding process. Thus, if the friction continues, the partially-transferred materials will fall off the surface due to work hardening, oxidation, or other reasons, resulting in the loss material from the surface [31]. The wear rate of the CoCrFeNiAl HEC was less than those of the CoCrFeNiMo HEC and the substrate [ Figure  S3]. The CoCrFeNiMo HEC suffered the severe adhesive wear, which is evidenced by the intensively-plastic flow and plowing grooves with a high stress [ Figure 5(f and g)] [30,[32][33][34][35][36]. The materials loss in the CoCrFeN-iMo HEC seems to occur primarily by ploughing with the wedge formation due to the intensively-plastic deformation induced by the hard rubbing-pair, in particular by the oxide particles generated during the wear process itself [37].
The results of the present study demonstrate that both HECs are composed of the single-phase solid-solution HEA matrix and nano-multi-component metal oxides. Usually, the hardness of the BCC HEA is obviously higher than that of the FCC HEA [30]. High hardness is one of the prerequisites for the HEA with excellent wear resistance. Besides the properties of the HEA matrix, the layered wrapping structure in the coating is of importance for its wear performance [42]. On the wear trace of the CoCrFeNiAl HEC, the atomic ratios of the metallic elements in the wear debris are still near equal. During the wear process, the nano-multi-component metal oxides flake off, and freely distribute on the surface of the coating, which thus effectively stop the occurrence of the severe adhesion wear. The friction-reducing effect due to a denudation of nano-multi-component metal oxides, and the formation of the micron-size particlelike debris on the surface of the coatings can effectively lubricate the friction process between the CoCrFeNiAl HEC surface and the rubbing-pair (Si 3 N 4 ) [16,[42][43][44]. Therefore, the CoCrFeNiAl HEC containing nano-multicomponent metal oxides exhibits high hardness and good wear resistance.