Microstructures and mechanical properties of CoCrFeNiMn high-entropy alloy coatings by detonation spraying

Highlights

• CoCrFeNiMn high entropy alloy coatings were prepared by denotation spraying.

• A small amount of flocculent oxide was formed in the high entropy coating.

• Microhardness of CoCrFeNiMn coating by detonation spraying was ~471 HV

• The coating exhibited excellent abrasive wear resistance and bonding strength.

Abstract

CoCrFeNiMn high-entropy alloy (HEA) coatings were deposited on 316L stainless steel substrate by detonation spraying technique. The HEA coatings had a dense microstructure and contained a small amount of flocculent metal oxides, showing a main face-centered-cubic (FCC) solid solution structural matrix with amounts of nano-multi-component metal oxides. It was found that the microhardness of CoCrFeNiMn HEA coating by detonation spraying was much higher than that prepared by direct casting or spark plasma sintering due to the multi-component metal oxides reinforcement and fine grain strengthening effect. Furthermore, the HEA coating by detonation spraying exhibited excellent abrasive wear resistance and bond strength. The analysis of coating failure verified that the failure of the HEA coating was mainly due to lamellar structures peeling off. The CoCrFeNiMn HEA coatings with outstanding mechanical properties could be promising for application in future as structural materials.

Introduction

Recently, high-entropy alloys (HEAs) have attracted tremendous attention due to their unique phase structures and excellent physical and chemical properties [[1], [2], [3], [4]]. HEAs are designed in a brand new concept with five to thirteen different metallic elements in equal or approximately equal atomic ratio [5], which are randomly mixed and lead to a high configurational entropy [6], resulting in unique features of the alloys, such as sluggish diffusion, lattice distortion and cocktail effect [7,8]. A series of excellent properties of the HEAs have been gradually discovered, such as excellent oxidation and irradiation resistance [9,10], high corrosion and wear resistance, and high yield strength at elevated temperature [11,12]. It is worth mentioning that a prominent member of the new class of HEAs, CoCrFeNiMn, which forms a simple face-centered-cubic (FCC) solid solution phase structure, has been found to exhibit high strength of ~1 GPa, good ductility of ~60–70%, and fracture toughness values exceeding 200 MPa m^1/2, and it has great potential for future engineering application [13]. Most of the bulk HEAs reported previously are prepared by arc melting, spark plasma sintering [14,15] and direct casting because these methods are relatively high efficiency and low cost. However, these conventional preparation techniques of bulk HEAs are not flexible enough, which impedes the wide application of the bulk HEAs [16]. Consequently, HEA coatings have been explored soon to facilitate their practical application. It should be noted that many methods have already been used to prepare HEA coatings, such as thermal spraying [17], laser cladding [18], plasma transferred arc cladding [19], electrodeposition [20], spark plasma sintering [21,22] and magnetron sputtering [23], among which thermal spraying has an enormous potential to prepare large-area HEA coatings for engineering application due to its advantages of flexible, high deposition efficiency and low costs. However, the HEA coatings prepared by traditional thermal spraying techniques such as atmospheric plasma spraying have many insufficiencies such as layered structure [24], segregation and massive oxidation [25]. Therefore, it is necessary to find a superior deposition method to prepare high quality HEA coatings. Detonation spraying (DS) is a novel thermal spraying technique that has been widely applied in industries to produce coatings with dense microstructures and strong adhesion [26]. As compared with other thermal spraying techniques, the substrate surface is impacted by the detonation wave at a high velocity of 800–1200 ms⁻¹ [27], the detonation products can be heated to 3500–4500 K, and the carrier gas can be heated to 1000–1500 K [28]. The particles with high kinetic energy will deposit a high-density coating with a large compressive stress and it can also improve the bond strength of the coating [29]. The segregation and oxidation of coatings can be suppressed commendably by the DS technique because the heating time for a particle is short and it limits the heat transfer [30]. Therefore, DS is an especially suitable technique to deposit high quality coatings. However, to date there is little work that has been done and reported on HEA coatings by DS.

In this study, CoCrFeNiMn HEA coatings were deposited on 316L stainless steel substrates by detonation spraying technique, and their microstructures, chemical composition and mechanical properties were systematically studied. Their abrasive wear resistance, bond strength and failure mechanism were analyzed and studied based on microscopic images, X-ray diffraction, sandblasting wear tests and tensile tests. The present investigation provides a fundamental basis for the development and engineering application of HEA coatings.