A comparison of the dry sliding wear behavior of NiCoCr medium entropy alloy with 316 stainless steel

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Abstract

The aim of this work was to determine the wear behavior of the f.c.c. medium entropy alloy NiCoCr, which has been shown by others to possess both a very good strength-ductility balance and an especially high fracture toughness. Pins of NiCoCr were tested in dry sliding wear against disks of yttria-stabilized zirconia at a sliding velocity of 0.1 m/s both in air and in argon. Identical wear tests on 316 stainless steel were performed for comparison. It was found that the wear rates of the NiCoCr pins were lower than those of the 316 stainless steel in air, but that the wear rates of the two materials were almost identical in dry argon. Scanning electron microscopy analysis of both the worn pins' surfaces and cross sections indicated that the worn surface of NiCoCr pins were flatter and showed less fracture due to the better toughness compared to the 316 stainless steel pins. Wear tests in air resulted in the formation of oxides. X-ray photoelectron spectroscopy of the NiCoCr pin showed that the oxides in the debris were Cr2O3 and Co2O3. The oxides play an important role in the development of a mechanically mixed layer, which formed on the NiCoCr pin's tip to protect the contacting material during wear tests in air.

Introduction

The equi-atomic NiCoCr medium entropy alloy exhibits an extremely high room temperature fracture toughness above 200 MPa m1/2, as demonstrated by Gludovatz et al. [1]. The alloy, which is a single-phase f.c.c. solid solution, shows even better strength and ductility with decreasing temperature. Indeed, the alloy shows a good balance of strength and ductility over the temperature range from −196 °C to 400 °C [[1], [2], [3]], while the results of fracture toughness tests showed that the NiCoCr medium-entropy alloy represents one of the toughest materials in any materials class ever reported [1].

There have been several studies of the room temperature wear behavior of both bulk high entropy alloys (HEAs) and HEA coatings [[4], [5], [6], [7]], which have generally been of the sliding pin-on-disk type conducted at room temperature in air. Tsai and Yeh [8] discussed the wear properties of HEAs and noted that the wear resistance of f.c.c. single-phase HEAs increased with increasing hardness, a well-known feature referred to as the Archard law [9,10], i.e.W=KFL/Hwhere W is the volume loss of materials through wear, K is the wear coefficient, F is the normal load, L is the sliding distance, and H is the hardness of the material. Tsai and Yeh [8] found that HEAs containing an ordered second phase had improved wear resistance, especially if the second phase was hard and was the majority phase. Thus, compared to the wear behavior of 52100 (SUJ2) bearing steel, the wear resistance of the HEA Al0.2Co1.5CrFeNi1.5Ti is 3.6 times better, even though these two materials have similar hardness values [4]. Similarly, the HEA Co1.5CrFeNi1.5Ti has a wear resistance comparable to that of H13 tool steel even though the hardness of H13 steel is greater [4]. Very recent wear testing of NiCoCr against an Inconel 718 counterpart at elevated temperatures found much lower wear rates at higher sliding temperatures, and concluded that the wear mode changes from abrasive wear at room temperature to oxidative and adhesive wear at 200 °C. However, the authors of [11] didn't investigate the important effect of environment on the wear rate of the materials and didn't consider the contribution of frictional heating to the contact temperature during sliding wear tests.

Sliding contact produces substantial heating during the wear tests as energy is transformed from the friction force, which results in a temperature increase of the contact area of the pins [12]. The temperature increase can influence the pin's hardness and make plastic deformation in the pin/disk contact area much easier. The wear rate also changes with hardness according to Eq. (1). This temperature increase can also lead to greater oxidation of the pin tip. In iron-based metals, it is thought that an oxide film forms on the surface that could increase the wear resistance [13]. Although an oxide could work as protection of the metallic surface during wear, it may break off to produce wear particles, i.e. wear debris [14]. When wear tests are performed at a slow speed and limited load with the contact temperature lower than 400 °C, the oxides in iron-based alloys are usually soft and part of the oxide film spalls to produce mild oxidative wear [14]. If the wear debris is much harder than the metal layer, it could lead to severe wear, which has been found in NiAl-based alloys [15].

This work is a study of the wear behavior of equi-atomic NiCoCr using pin-on-disk sliding wear tests against an yttria-stabilized zirconia (YSZ) disk in both air and argon. These results are compared with those from similar tests performed on 316 stainless steel, which is a single-phase austenitic alloy that exhibits good oxidation and corrosion resistance at relatively low cost and is used in numerous applications, despite its suboptimal wear resistance. It is of interest to study if the higher yield strength of NiCoCr compared to 316 stainless steel will produce better wear resistance.

Section snippets

Materials and methods

An ingot of equi-atomic NiCoCr alloy was prepared by arc-melting elemental Ni, Co and Cr (purities >99.9%) under argon. The ingot was melted three times and flipped to another face before the next melting to ensure homogeneity. The alloy was then placed in a Bridgeman furnace and a rod 71 mm long and 12 mm diameter was produced. Both the NiCoCr rod and a commercial 10 mm diameter 316 stainless steel rod were machined into pins 19 mm long, 9.5 mm diameter with hemispherical tips. All pins were

The wear behavior of NiCoCr alloys and 316 stainless steel pins

The results from wear tests of NiCoCr alloys and 316 stainless steel pins performed against YSZ disks at room temperature in air and argon are shown in Fig. 1. Three tests were conducted to obtain the average wear rates of each alloy under the different atmospheres and to calculate the error bars. The two materials both have much greater mass loss after wear tests in air compared to tests under argon. The NiCoCr pins had an average mass loss of 31 mg in air and 4.9 mg in argon, while the 316

Discussion

The results in Fig. 1 indicated that the mass loss of the NiCoCr pins during wear tests against YSZ was less than that for 316 stainless steel pins, especially in air. This may be partially attributed to the higher hardness of the NiCoCr material, as predicted by the Archard law. The material's toughness can also influence its wear performance, particularly through its influence on abrasion. The failure strain of NiCoCr alloys during tensile tests at room temperature is 70% [1], while this

Summary

Dry sliding pin-on-disk wear tests of NiCoCr alloy against yttria-stabilized zirconia were conducted in air and argon and the results compared to tests on 316 stainless steel. The results and comparison indicate that:

1. The wear rate of the NiCoCr pins in air at 0.1 m/s velocity was much lower than the wear rate of the 316 stainless steel: the wear rates of the NiCoCr and the 316 stainless steel under argon were similar and much lower than the wear rates in air.

2. The worn surfaces of NiCoCr

Declaration of competing interest

There is no conflict of interest.

Acknowledgements

This work was supported by National Science Foundation grant number 1758924.

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