2023 Volume 64 Issue 6 Pages 1199-1204
The reaction process of ZrC–2ZrB2-based Cu cermets from the (1 − x wt.%)(1B4C–3Zr)–x wt.%Cu system (Zr/B4C = 3 in molar ratio) was explored. Results showed that ZrC and ZrB2 were mostly produced through the dissolution of B4C into a preformed Zr–Cu liquid. With an increase in x, the synthetic ZrC–2ZrB2 and generated heat were reduced. This effect decreased an ability for Zr–Cu liquid to propagate in the reactants, and restrained the dissolution Zr. As a result, the complete synthesis of ZrC–2ZrB2 failed in the systems with a higher Cu content (e.g., 50 wt.%). Furthermore, after the precipitation of ZrC and ZrB2, the liquid surrounding would prevent ZrC and ZrB2 from growing. Increasing Cu content enhanced the amount of Zr–Cu melt. This behavior contributed to a decline in ZrC–2ZrB2 particle sizes, and the production of fine ceramic particles (∼200 nm). It is also revealed that the formation of ZrC–2ZrB2 is a multistep process, which results in the inhomogeneity of ZrC–2ZrB2 particle sizes. A valuable approach was proposed to explore the relationship between reaction process and synthesized products of combustion synthesis-related technique.
Fig. 3 XRD patterns of (a) BZC50 and (b) BZC30 quenched from different temperatures.
ZrC and ZrB2 have good mechanical properties, excellent high-temperature stability, and high electrical conductivity. They are ideal materials for ultrahigh-temperature ceramics, conductor of electricity, and wear-resistant parts.1–4) Among various preparation method, combustion synthesis has the advantages of easy operation, low energy consumption, and high product purity.5,6) Furthermore, this technique has been widely used to combine with other routes to prepare composites containing ZrC and ZrB2 recently.7–11) For instance, fully dense ZrB2–ZrC–SiC ceramic was fabricated by the combustion reaction and subsequent spark plasma sintering.8) ZrB2–ZrC/Fe composite coating was produced by the combination of combustion synthesis and gas tungsten arc welding technology.10) As these ceramic particles are mainly formed by the combustion synthesis reaction, understanding the reaction characteristics is vital to achieve a desired microstructure. Conventionally, the influence rules of kinetic parameters (e.g., the particle size of raw materials) on the synthesized products, were predicted by thermodynamic calculations and/or reaction behaviors (e.g., combustion temperature).12,13) The above methods are valuable, but it may not be completely accurate. As we know, combustion synthesis is a chemical reaction of reactants. In essence, kinetic parameters will influence the reaction processes, which in turn leads to the differences in the synthesized products. Hence, it is required to establish relationships between the reaction process and synthesized products. However, reports about this aspect are limited because of the complicated reaction process.
Copper has excellent electric and heat conductivity. To improve the hardness and wear resistance of copper, ZrC–ZrB2-based Cu cermet was successfully used as the reinforcement in copper matrix composites14) and protective coating on the copper substrate.15) Therefore, ZrC–ZrB2-based Cu cermet was produced by the combustion reaction from the (1 − x wt.%)(1B4C–3Zr)–x wt.%Cu system. The main purpose is to explore the kinetic role of Cu content in the combustion synthesis processes and products, and clarified the relationship between them. It is expected that this work would offer an insight into the combustion reaction, and provide guide to fabricate ZrC–2ZrB2-based cermets by combustion synthesis-related technology.
The starting materials included Zr (98%, <48 µm), B4C (99%, ∼0.5 µm), and Cu (99%, <25 µm) powders. The reactants were constituted of Zr and B4C at a ratio corresponding to ZrC–2ZrB2 with the addition of 10 wt.%–50 wt.% Cu. The detailed compositions are listed in Table 1.
B4C, Zr and Cu powders were ball-milled, and then cold-compacted into tablets (green density: ∼65%). Combustion synthesis experiments were carried out in a glove-box under argon atmosphere, as illustrated in Fig. 1. After the ignition of powder compact using an arc heating source, the power source was turned off.
A schematic diagram of the combustion synthesis experiment employed.
According to Ref. 16), the reaction mechanism of a given system in differential scanning calorimetry (DSC) and combustion synthesis seems to be similar, although the DSC condition is not same as the rapid combustion process. Moreover, regarding to the study on reaction process, DSC analysis can be more visual. Therefore, the influence of Cu content on the reaction process of the (1 − x wt.%)(1B4C–3Zr)–x wt.%Cu system was investigated by DSC (200 F3 Maia, Germany). The heating rate is 30°C/min. The obtained DSC curve was analyzed by a Proteus Thermal Images Software (Netzsch Proteus, Version 6.0.0).
2.4 MeasurementsPhase compositions and microstructures of DSC products and combustion synthesis samples were examined by X-ray diffraction (XRD, D8 Advance, Germany) and field-emission scanning electron microscopy (FESEM, S-4800, Japan) coupled with energy-dispersive X-ray spectroscopy (EDS, Link-ISIS, England), respectively. The hardness of combustion synthesized specimens were tested by the Vickers hardness tester (HV-10, China). The load and loading duration were 2.94 N and 15 s, respectively. The average value of hardness was from five measurements.
Figure 2 displays the DSC curves of BZC50 (50 wt.% Cu) and BZC30 (30 wt.% Cu) systems. Figure 3 illustrates the XRD patterns of the two powder mixtures quenched from different heating temperatures. In BZC50 cooled from 903°C, new Cu51Zr14, Cu10Zr7, CuZr2, ZrC and ZrB2 were identified (Fig. 3(a)). Furthermore, a structure containing Cu and Zr atoms was detected (Fig. 4(a)), and the diffusion phenomenon between B4C and Zr was observed (Figs. 4(b) and 4(c)). Therefore, this exothermic event was caused by the solid-solid reactions of αZr + Cu → Cu51Zr14 + Cu10Zr7 + CuZr2 and αZr + B4C → ZrC + 2ZrB2. In contrast, CuyZrx XRD peaks in BZC30 at 901°C slightly decreased (Fig. 3(b)). As mentioned in Experimental, Zr particle (∼48 µm) is larger than Cu particle (∼25 µm). The reduced Cu content will decrease the contact area between Zr and Cu. This is unfavourable for Zr–Cu diffusion reaction.
DSC curves of (a) BZC50 and (b) BZC30.
XRD patterns of (a) BZC50 and (b) BZC30 quenched from different temperatures.
Microstructures of BZC50 quenched from 903°C (a), (b); corresponding EDS line scan (c).
After BZC50 was heated to 931°C and 968°C, two exothermic peaks emerged (Fig. 2(a)). Since Cu content in BZC50 reached to 50 wt.%, Cu combined with Zr to produce Cu-rich Cu51Zr14. The formation of Cu51Zr14 is exothermic, which then promoted Zr–B4C solid state reaction. So in BZC50 quenched from 931°C and 968°C, Cu51Zr14, ZrC and ZrB2 XRD peaks gradually increased (Fig. 3(a)). By comparison, in BZC30 cooled from 926°C and 965°C, Cu8Zr3 appeared, and Cu51Zr14 peaks remained weak (Fig. 3(b)). This implies that with a reduced Cu content, Zr–Cu reaction tends to prepare CuyZrx compounds with a higher Zr content.
Continuous heating BZC50 introduced an endothermic peak at 1000°C and an exothermic peak at 1029°C. According to the Zr–Cu phase diagram,17) Cu10Zr7 will be melted into Zr–Cu liquid near 900°C (Fig. 5). With the spreading of the melt, Cu, CuZr2 and Zr surrounding dissolved into Zr–Cu liquid, which led to the endothermic event at 1000°C. In this way, more liquid was obtained, as confirmed by a certain amount of solidification structure in Fig. 6(a). As the melt spread over B4C, B4C was surrounded or covered by Zr–Cu liquid, and the latter case resulted in the presence of a gray “interlay” (Fig. 6(b)). Then, B4C dissolved into Zr–Cu liquid and reacted with Zr to yield ZrC and ZrB2, accompanying by the exothermic phenomenon at 1029°C. The assumption was supported by an enlarged image and EDS map. As shown in Figs. 6(c) and 6(d), both the “interlayer” and the transitional region between B4C and Zr–Cu melt were composed of Zr, B, C and Cu atoms, and contained some ceramic particles. Hence, in BZC50 quenched from 1029°C, CuZr2 and Zr almost disappeared; Cu peaks decreased, ZrB2 and ZrC significantly increased (Fig. 3(a)). For BZC30, the first endothermic peak and the fourth exothermic peak are 1024°C and 1073°C, respectively (Fig. 2(b)). Moreover, in BZC30 cooled from 1073°C, Cu was absent; ZrC and ZrB2 peaks strengthened, Zr and CuZr2 peaks weakened (Fig. 3(b)). One could expect that these two peaks correspond to the formation of Zr–Cu liquid and ZrC–ZrB2 as seen in BZC50, but are retarded. The content of Zr in BZC30 is relatively higher. This would restrain the dissolution of Zr-rich phases for the limited solubility of Zr–Cu liquid,18) as supported by the occurrence of less solidification structure in Fig. 7. This behavior is unfavorable for the subsequent dissolution of B4C into Zr–Cu liquid. For this reason, the synthesis of considerable ZrC–ZrB2 was delayed to 1073°C.
Zr–Cu phase diagram.17)
Microstructures of BZC50 mixtures quenched from 1029°C with different magnifications (a) ×500, (b) ×18000, (c) ×35000; EDS of the composite using high magnification (×35000) (d).
Microstructure of BZC30 quenched from 1073°C.
An endothermic peak at 1104°C appeared in the DSC curve of BZC50. This should be the melting of Cu51Zr14, according to Fig. 4. Owing to the melting, the liquid further increased. This effect further promoted the synthesis of ZrC–ZrB2 at 1132°C, and the liberation of free Cu from the melt. Eventually, BZC50 mainly contained ZrC, ZrB2 and Cu. For BZC30, the last exothermic peak, which corresponded to the complete synthesis of ZrC and ZrB2, was delayed to 1158°C. The reason may be the less Zr–Cu liquid formed in this system, as discussed in the fourth exothermic phenomenon.
3.2 Influence of Cu content on the reaction productsFigure 8 is the XRD results of combustion synthesized specimens. With an increased Cu content, Cu peaks gradually enhanced, as indicated by the green dotted rectangle. Accordingly, ZrC and ZrB2 peaks reduced. Clearly, increasing Cu content will decrease the contents of Zr and B4C in the reactants, and thereby the gainable ZrC–2ZrB2 lessened. In addition, when Cu content ranged from 10 wt.% to 20 wt.%, the products mainly contained ZrC, ZrB2 and Cu (Figs. 8(a)–8(b)). One reason may be the preformed Zr–Cu liquid, as the diffusion of atoms in melt is much faster than that in solid. After B4C dissolved into Zr–Cu liquid, it is easier for B and C atoms to combine with Zr, contributing to the complete reaction. However, besides ZrC, ZrB2 and Cu, residual Zr was detected in BZC30, BZC40 and BZC50, as indicated by the purple dotted ellips (Figs. 8(c)–8(e)). In addition, the residual Zr increased with increasing Cu content. The phenomenon can be explained via the DSC analysis. For instance, the DSC curve of BZC50 showed five exothermic peaks. The first three events were the formation of CuyZrx and ZrC–2ZrB2. The total peak area, which was calculated by the Proteus Thermal Images Software, was 8578 J/g. In contrast, the last two corresponded to ZrC–2ZrB2-forming reaction. The total area was 54076 J/g. This implies that the generated heat of combustion reaction mainly originated from the fabrication of ZrC–2ZrB2. With an increased Cu content, the obtained ZrC–2ZrB2 reduced. Accordingly, the released heat should decreased. This effect would decrease an ability for Zr–Cu liquid to propagate in the reactants, and the solubility Zr in Zr–Cu liquid. As a consequence, the complete reaction failed. We noted that minor amount of ZrO2 was identified in DSC products and combustion synthesis samples, as indicated by the red dotted circles in Fig. 3(a) and Fig. 8, respectively. In some experiments, the pump-down time may be relatively short. Consequently, the residual O2 in the heating chamber reacted with Zr to prepare ZrO2.
XRD spectra of the combustion synthesized products from (a) BZC10, (b) BZC20, (c) BZC30, (d) BZC40, and (e) BZC50 systems.
Figure 9 is the fractured surface morphology of these specimens listed in Fig. 8. Both ZrC and ZrB2 grains presented with irregular morphologies. With increasing Cu content, the particle sizes of ZrC and ZrB2 decreased from about 2 µm to 200 nm (Figs. 9(a)–9(e)). In general, the particle size becomes larger at a higher temperature. However, an increased Cu content decreased the combustion heat, as discussed in XRD results of combustion synthesized specimens. Moreover, ZrC–ZrB2 was mainly produced by the dissolution-reaction-precipitation method. After the precipitation, the liquid surrounding will restrain the conglomeration and growth of ZrC–ZrB2. With increasing Cu content, the amount of formed liquid increased, as mentioned before. This contributed to a decline in ZrC–ZrB2 particle sizes. It is worth to mention that the size of ceramic particles is inhomogeneous. This phenomenon can be understood by the DSC analysis. For instance, ZrC–ZrB2-forming reaction in BZC30 occurred at 926°C, 965°C, 1073°C and 1158°C. The initially formed ZrC and ZrB2 at 926°C coarsened during the subsequent temperature increase. In contrast, for the ZrC and ZrB2 produced at 1158°C, there were insufficient Zr and B4C sources and a much shortened time for the particle growth.
Microstructures of the combustion synthesized products from (a) BZC10, (b) BZC20, (c) BZC30, (d) BZC40, and (e) BZC50 systems.
Figure 10 shows the hardness of combustion synthesized samples. The maximum value is about 362 HV. As we know, the combustion reaction will release an amount of heat, which in turn lead to the occurrence of considerable pores and low hardness.9,19,20) To improve the mechanical properties and extend the application of ZrC–ZrB2-based Cu cemert (e.g., metal matrix composites, wear-resistant part and coating), a further work is recommended, such as hot isostatic pressing sintering,8) casting.14) From Fig. 10, it can also see that with increasing Cu content, the hardness first enhances and then reduces. Generally, an increased metal additive can decrease the porosity of resultant samples.21,22) It may be for this reason that the hardness enhances. However, with a further increased Cu content, the hardness gradually reduces. A similar phenomenon was found in the combustion synthesis experiments of TiB2–TiN/Ni–Al composites.23) This can be explained by the decreased ZrC and ZrB2 in the resultant products, since they have a higher hardness in comparison with Cu.
The hardness of combustion synthesized ZrC–ZrB2-based Cu cermet.
This work was supported by the National Natural Science Foundation of China (U20A20293) and Qing Lan Project of JiangSu Province.
