Microstructure and mechanical properties of Ti(C,N)-based cermets fabricated using Ni-coated mixed powders
Introduction
Titanium carbonitride-based (Ti(C,N)-based) cermet have drawn more attention worldwide from researchers due to their excellent heat and wear resistance, great thermal deformation resistance and low friction coefficient and high hardness [[1], [2], [3], [4]]. Titanium carbonitride-based cermets are composed of a Ti(C,N) (or TiC + TiN) ceramic hard phase and a metallic binder phase in which Ni and Co are always used as thebinder. Specifically, the ceramic hard phase determines the hardness and also contributes to the fracture toughness of cermet when it is combined with a metallic binder phase. Moreover, compared with cemented carbides of WC-Co, the lower density of Titanium carbonitride-based cermets and the less scarce strategic resources needed mean that the cermets have great potential as cemented carbide upgrade materials. These advantages enable cermets to be used as wear parts, molds, drawing dies, and cutting tools [5,6]. However, low toughness and strength of cermets have limited their application in most areas. To increase the toughness of cermets, research has mainly focused on: (1) adjusting composition of hard phase and binder phase [7,8], (2) optimizing sintering conditions [9], and (3) introducing nano-enhancers such as nanoparticles, whiskers, and fibers [[10], [11], [12], [13]].
Meanwhile, cermets that have homogeneous microstructure are also key to increasing their toughness and maintaining high hardness. The conventional method of wet milling is prone to producing an inhomogeneous distribution of binder phase and hard phase. Also, hard phase particles are easy to aggregate and grow. Synthesizing metal-coated ceramic powders to replace the conventional individual raw powders will not only improve the wetting or joining of metal-ceramic interfaces effectively but also lead to a well-controlled and homogeneous microstructure of Titanium carbonitride-based cermets [[14], [15], [16]]. Recently, it has been reported that some methods can be used to synthesize Ni-coated composite powders for preparing Titanium carbonitride-based cermets with improved microstructure. For example, Zhou et al. synthesized Ni-Mo/Ti(C,N)-coated powders by means of Ni(NO3)2·6H2O solution and (NH4)6Mo7O24·4H2O solution via heterogeneous precipitation and thermal reduction [17]. Although cermets fabricated using these coated powders without wet-milling exhibited fine and uniform microstructure without the conventional core-rim structure, the as-sintered cermets via this method tend to form Ni3Ti third phase due to the ultrafine ceramic powders of their high specific surface, thereby decreasing the fracture toughness of cermets. Dios et al. proposed a bottom-up approach via chemical reduction of a nickel precursor onto the surface of Ti(C,N) grains using Ni(NO3)2·6H2O and N2H4·H2O solutions [18]. Cermets produced from core-shell composites powders also had a relatively homogenous and finer distribution of hard phase. But the wide size distribution of hard particles decreased the uniformity of binder phase, and this led to the cermets having poor mechanical properties. Furthermore, mechanical mixing, the sol-gel method, and electroless plating have also been used to produce metal-coated ceramic powders [[19], [20], [21]]. However, current methods for preparing Ni-coated powders are complicated, and the pH of the coating process must be precisely controlled. In the present study, a more convenient method for preparing Ni-coated powders was proposed. In our experiment, ammonium oxalate monohydrate ((NH4)2C2O4·H2O) and nickel chloride hexahydrate (NiCl2·6H2O) were used as precipitants to prepare nickel oxalate (NiC2O4·2H2O), and Ni powders were obtained via reduction of precursor (NiC2O4·2H2O). Reaction processes are as follows:NiCl2 + (NH4)2C2O4 → NiC2O4↓+ 2NH4Cl2NiC2O4 + 3H2→ 2Ni + CO2 + 3CO+ 3H2O
In the present study, chemical co-precipitation coating was used to deposit NiC2O4, which was produced via reaction (1) onto the surface of mixed powders and then via hydrogen reduction (reaction (2)) to obtain Ni-coated mixed powders. Microstructure and the mechanical properties of Ti(C,N)-based cermet prepared using Ni-coated mixed powders were investigated.
Section snippets
Experimental procedures
Preparation of Ni powder was merged with the mixing of Ni and other powders to prepare Ni-coated mixed powders through a chemical method. Analytical grade (NH4)2C2O4·H2O and NiCl2·6H2O were used to obtain Ni powder via chemical co-precipitation. Other raw powders required in the experiment are summarized in Table 1, which lists mean particle sizes and oxygen contents of those ingredients.
Before coating, mixed powders to be coated were prepared. The nominal composition of the mixed powders to be
Phase composition and characterization
Fig. 2 shows XRD patterns of experimental powders under different conditions and of as-sintered cermets.
As is seen in Fig. 2 (a), the XRD pattern of the starting powders exhibited diffraction peaks which corresponding to Ti(C,N), Mo2C, and WC. After the coating experiment, NiC2O4·2H2O diffraction peaks appeared in the XRD pattern of precursor powders (Fig. 2 (b)) that were based on the starting powders. Moreover, NiC2O4·2H2O diffraction peaks disappeared, and Ni peaks appeared with hydrogen
Conclusion
- (1)
Ni-coated mixed powders were successfully synthesized via chemical co-precipitation and hydrogen thermal reduction. Also, titanium carbonitride-based cermets were further sintered by using as-prepared chemical Ni-coated mixed powders through vacuum liquid sintering. XRD patterns of cermets only exhibited diffraction peaks of Ti(C,N) ceramic phase and Ni binder phase.
- (2)
The present cermets that were fabricated using Ni-coated mixed powders exhibited a more homogenous and finer microstructure than
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was financially supported by the National Natural Science Foundation of China (No. 51674148), Key Research and Development Plan of Jiangsu Province (BE2017084) and the Project Funded by Priority Academic Program Development of Jiangsu Higher Education Institutions.
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