Strengthening of solid solution (Ti,La)(C,N)-based cermets by LaB6 addition

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Highlights

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    Samples of (Ti,La)(C,N)-based solid solution cermets were fabricated by powder metallurgy method.

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    The performance indexes of (Ti,La)(C,N)-based cermets are better than those of Ti(C,N)-La-based cermets with equal La mass fraction With the increase of La content, there is decline of oxygen content in the cermets solid solution.

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    With the increase of La content, there is decline of oxygen content in the cermets solid solution. It is hence deduced that LaB6 functions as deoxidizer for the improvement of cermets quality.

Abstract

(Ti,La)(C,N)-based solid solution cermets are fabricated from self-prepared (Ti,La)(C,N) carbonitrides by powder metallurgy with LaB6 addition in the solid solution. Based on the results of XRD and SEM analyses, it is confirmed that there is complete dissolution of (Ti,La)(C,N) powder. The presence of La can effectively suppress grain overgrowth and thus enhance the homogeneity of porcelain microstructure, consequently increasing the particle size of black phase. On the basis of equal mass fraction of La, the performance of (Ti,La)(C,N)-based cermets is better than that of Ti(C,N)-La-based cermets prepared by simple addition of LaB6 to Ti(C,N)-based cermet powder. The discrepancy can be directly related to the effect of adding LaB6 in the solid solution. The physical and mechanical properties of (Ti,La)(C,N)-based cermets comes to optimum when the content of dissolved La is 0.25 wt%, and there is hardness elevation of 20%. With suitable La addition, there is decrease of oxygen content in the solid solution of cermets. It is hence deduced that LaB6 can improve the performance of cermets as a deoxidizer.

Introduction

Ti(C,N)-based cermets belong to a kind of ceramic-based composites consisting of ceramic particles and metal binders [1,2]. Endowed with the favorable properties of ceramics and metals, the cermets exhibit excellent characteristics such as chemical stability, high specific strength, good polishing performance, and outstanding oxidation and creep resistance [3]. They are widely accepted as potential alternative to traditional WC-Co cemented carbide. Furthermore, owning to low density as well as pre-eminent mechanical performance and chemical stability at elevated temperature, Ti(C,N)-based cermets have been applied in aerospace, petroleum and mining industries.

Nevertheless, Ti(C,N)-based cermets are limited in strength and toughness, and these major deficiencies largely confine the service life and application scope of the composites. According to literatures, such limitations can be circumvented through the doping of a small amount of rare earth (RE) element into the cermets. It was found that the presence of RE element can refine the hard phase and improve the wettability between metallic and ceramic phases, which can subsequently strengthen the binding phase and enhance the mechanical properties of cermets [4]. However, the high oxidation tendency of RE elements and their heterogeneous distribution would inevitably lead to poor stability of cermets, thereby hampering the workability of this design idea [5].

Many researchers investigated the effects of RE elements on the solidification, eutectic transformation and mechanical properties of Ti(C,N)-based cermets, and pointed out that the addition of RE atoms could improve the tensile strength and toughness of the cermets [6]. In the present work, we prepared (Ti,La)(C,N) carbonitrides of different compositions through preliminary solution treatment to investigate the effects of lanthanum solid solution on the fabricated (Ti,La)(C,N)-based cermets.

Section snippets

Preparation of (Ti,La)(C,N) carbonitrides

The composition details for the preparation of (Ti,La)(C,N) carbonitrides are listed in Table 1. Briefly, a mixture of TiO2, LaB6, TiC and C powder was subject to ball milling for 4 h. The as-obtained powder was then sieved using a 200 mesh sieve, followed by carbonitriding at 1800 °C in a ZMT-45-25 high temperature graphite tube furnace under N2 for 3 h.

Preparation of (Ti,La)(C,N) cermets

Compiled in Table 2 is the basic information related to the preparation of (Ti,La)(C,N) cermets from (Ta,Nb)C, WC, Mo2C, Co, Ni and

Characterization of (Ti,La)(C,N) carbonitrides

Fig. 1(a) shows the wide scan XRD patterns of (Ti,La)(C,N) carbonitrides powder (PDF:42-1489). It is deduced that there is complete dissolution of raw materials during the carbonitriding process at 1800 °C, and the microstructures of the samples in different La contents are similar. Fig. 1(b–f) show the peaks in high resolution, and it is observed that with the increase of La content there is a gradual shift of (111), (200), (220), (311) and (222) peaks to higher angles. Referring to

Improvement of mechanical properties by the addition of LaB6

With the addition of LaB6, the density of Lasingle bondS increases from 6.64 to 6.69 g/cm3. The addition of LaB6 is beneficial to the relief of impurity hazard and promotion of interface pore contraction [15]. It is because there is partial decomposition of LaB6 and diffusion of B and La elements into the carbide matrix [16].

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    Influence mechanism of LaB6 in solution sintering.

As depicted in Table 3, the bending strength is the highest when the content of LaB6 is 0.25 wt%. The improvement of mechanical

Conclusion

LaB6 was added to obtain a new hard phase of (Ti,La)(C,N) carbon complex by pre-solution treatment technology. The results of XRD and SEM analyses confirm the complete dissolution of (Ti,La)(C,N) powder during the carbonitriding process. The presence of LaB6 could effectively suppress the overgrowth of grains and hence increase homogeneity of microstructure, causing an increase of particle size in the black phase. On the basis of equal La mass fraction, the performance indexes of

Declaration of Competing Interest

None.

Acknowledgements

The work was financially supported by the Science and Technology Planning Project of Hunan Province (Nos. 2019RS2067, 2020JJ5142), the Natural Science Foundation of Jiangsu province (No. BK20190973), Natural Science Foundation of China (No. 51774127), and Research Foundation of Education Bureau of Hunan Province, China (No. 19C0581). The authors thank Prof. C.T. Au for helpful suggestions.

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