Influence of NbC additions on microstructure and wear resistance of Ti(C,N)-based cermets bonded by CoCrFeNi high-entropy alloy

Highlights

• Due to the effect of sluggish diffusion of CoCrFeNi HEA binder, bright coreless grains exist in the cermet without NbC. With the increase of NbC addition, the proportion of bright coreless grains and isolated black Ti(C,N) particles increase gradually.

• The hardness of cermets dramatically increases due to the fine microstructure and maximized solid solution strengthening effect of CoCrFeNi HEA binder.

• On the basis of uniform microstructure and superior mechanical properties, the high-temperature wear resistance of cermet with 3 wt% NbC was further improved due to the existence of stable lubricant tribolayers.

Abstract

Ti(C,N)-based cermets bonded by CoCrFeNi high-entropy alloy (HEA) were reinforced via adding NbC. The microstructure, mechanical properties, cutting performance and high-temperature wear resistance of the cermets were investigated in detail. Due to the sluggish diffusion effect of HEA binder, the fine bright-coreless grains exist in cermet without NbC. The increased addition of NbC causes a increase of the proportion of bright-coreless grains resulted from the change of nucleation sites. The higher relative density resulted from the increased wettability of CoCrFeNi HEA binder to ceramic phase. The hardness increased significantly due to the fined grains and maximized solid solution strengthening effect of CoCrFeNi HEA binder. The fracture toughness enhanced because of the bright coreless-complete rim grains with the less lattice strain. A good comprehensive mechanical properties of cermets was obtained at 3 wt% NbC additions with the relative density of 98.13%, the hardness of 1853 HV and fracture toughness of 9.93 MPa m^1/2. Moreover, a small amount of NbC additions may decrease the friction coefficient and improve wear resistance of cermet inserts during machining the GCr15 bearing steel. Additionally, on the basis of uniform microstructure and superior hardness, the high-temperature wear resistance of cermet with 3 wt% NbC was further improved due to the existence of stable lubricant tribolayers. The corresponding average friction coefficient and wear rate are 0.198, 4.14 × 10⁻⁶ mm³/N·m, respectively.

Introduction

In the last decades, Ti(C,N)-based cermets as a hard composite materials are mainly constituted of hard-brittle Ti(C,N) phase and soft-ductile Fe/Co/Ni binder phases [1,2]. Owing to the excellent wear resistance, relatively low fraction coefficient and superior chemical stability, they have been broadly applied in many fields, especially in machining industry and wear resistance parts [[3], [4], [5], [6]]. However, when confronted with the more and more severe service condition such as dry machining or high-temperature, the performance degradation of conventional binders produces a great limitations of application [7,8]. So researchers are dedicated to finding a novel binder to tackle this issue [[9], [10], [11]].

Fortunately, a kind of distinctive multi-component alloys with equiatomic, or nearly equiatomic compositions emerged, which are also defined as high entropy alloys (HEA) [12]. The excellent combination of high temperature hardness and thermodynamic stability makes this type of multi-component alloy a potential candidate to substitute the conventional binder [13]. Recently, a few studies have demonstrated the feasibility of utilizing certain HEAs as alternative binders in cermets or cemented carbides. Zhu et al. successfully fabricated Ti(C,N)-CoCrFeNiAl cermets exhibiting relatively good fracture toughness and remarkable oxidation resistance [14]. Xie et al. prepared the TiO-CoCrFeNiMn composite sustaining superior fracture strength without sacrificing the ductility [15]. Holmström et al. develop WC-CrFeCoNi cemented carbide tools showing excellent resistance of plastic deforming at high-speed cutting [16]. Velo et al. investigated WC-CoCrFeNiMn cemented carbides possessing excellent mechanical behavior, especially for the wear resistance [17]. The advantages of utilizing HEA as a binder can be summarized as fallow: (1) higher hardness, (2) excellent wear resistance, (3) superior high-temperature performance.

However, compared with the conventional WC-Co cemented carbides or Ti(C,N)-Ni/Co cermets, the cermets or cemented carbides using HEA as binder possess relatively low comprehensive mechanical properties resulted from the poor wettability of HEA binder phase on hard phase [18,19]. It has been demonstrated that the poor wettability mainly caused by the higher melting temperature of HEA binder, which is unfavorable for well-wetting of the hard phase due to the shorter solid-liquid transition time [16,20,21]. Additionally, the special sluggish diffusion effects and severe lattice distortion of HEA can restrain solute diffusion during the liquid phase sintering progress, which also inevitably leads to the insufficient wettability of HEA binder to the hard phase [22]. In hence, improving the comprehensive mechanical properties of Ti(C,N)-based cermets bonded by HEA is essential.

Choosing the appropriate alloying elements of HEA binder or adding secondary carbides has been proposed to enhance the wettability between HEA binder phase and hard phase [19]. Meanwhile, adding NbC in conventional Ti(C,N)-Ni/Co cermets was considered as an effective way to lower the sintering temperature, which promote the wettability of Co/Ni binder phase to hard phase [2,[23], [24], [25]], similarly, which also should be suitable for Ti(C,N)-HEA cermets. Additionally, it has been reported that NbC can significantly reinforce the mechanical properties of some hard ceramic-metal composites [[26], [27], [28]]. Moreover, NbC exhibits a high potential for wear and tribological applications [[29], [30], [31]]. In hence, adding NbC in Ti(C,N)-HEA cermets may be a good choice to enhance comprehensive mechanical properties and high-temperature wear resistance.

In this work, the simplest HEA CoCrFeNi was used as a novel binder, and various NbC additions were added to optimize the comprehensive performances of Ti(C,N)-CoCrFeNi-WC-Mo₂C cermets. In addition, the influence of NbC addition on the microstructure, mechanical properties, cutting performance and high-temperature wear resistance of the cermets were also investigated.