Reaction Mechanism of ZrB2-ZrC Formation in Ni-Zr-B4C System Analyzed by Differential Scanning Calorimetry

The reaction mechanism of ZrB2-ZrC formation in a 30% Ni-Zr-B4C system under argon was revealed by using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results indicated that the reaction mechanism in the Ni-Zr-B4C system was complex. Initially, NixZry and NixBy intermetallics were formed via solid-state diffusion reactions between Ni, B4C and Zr. Then, the eutectic reaction between Ni2B and Ni4B3 lead to the formation of Ni-B liquid. The free C atoms dissolved into the Ni-B liquid to form a Ni-B-C ternary liquid, and then part of the Zr powder dissolved into the surrounding Ni-B-C ternary liquid to form Ni-Zr-B-C quaternary liquid. Finally, ZrB2 and ZrC formed and precipitated out of the saturated liquid. The eutectic liquid plays an important role during the formation of ZrB2-ZrC.

In a previous paper [21], we successfully synthesized ZrC-ZrB 2 /Ni cermet powders using a Ni-Zr-B 4 C system by the SHS method. The SHS-derived feedstock powders were deposited on a magnesium alloy, and atmospheric plasma spraying was used to obtain ZrC-ZrB 2 /Ni cermet coatings. However, the reaction mechanism of ZrB 2 -ZrC formation in the Ni-Zr-B 4 C system needs to be further studied.
In the present work, differential scanning calorimetry (DSC), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to reveal the formation mechanism of ZrB 2 -ZrC in the Ni-Zr-B 4 C system during combustion synthesis. It is expected that these preliminary results will be valuable for promoting the understanding of the reaction mechanism of ZrB 2 -ZrC formation in the Ni-Zr-B 4 C system.

Materials and Methods
The ZrB 2 -ZrC/Ni composites were produced according to the following reaction equation: Commercial powders Ni (~99% in purity, ≤48 µm, ST-nano science and technology Ltd. Co., Shanghai, China), Zr (~99% in purity, ≤38 µm, ST-nano science and technology Ltd. Co., Shanghai, China) and B 4 C (~95% in purity, ≤3.5 µm, Abrasive Ltd. Co., Dunhua, China) were selected as the starting materials. In order to investigate the complex combustion reactions in the Ni-Zr-B 4 C system, DSC experiments were performed on the mixtures of Zr-B 4 C, Ni-B 4 C, Ni-Zr and 30 wt.% Ni-Zr-B 4 C. In 30 wt.% Ni-Zr-B 4 C mixture, Zr and B 4 C powders with a molar ratio of 3:1 were mixed with 30 wt.% Ni. The compositional proportions in the Zr-B 4 C, Ni-B 4 C and Ni-Zr mixtures were in accordance with those in the 30 wt.% Ni-Zr-B 4 C mixture. The weight of powder mixtures subjected to DSC analysis was 15 mg. The reactant mixtures were dry-mixed sufficiently in a container using zirconia balls at a low speed (~50 rpm) for 6 h.
DSC was carried out on a STA 449C Jupiter (Netzsch, Weimar, Germany) apparatus to reveal the reaction mechanism of the Ni-Zr-B 4 C system. The heating process was set to a rate of 10 • C/min in flowing argon gas (99.9% in purity, flow rate: 40 mL/min). Following DSC analysis, the sintered powders were crushed, and the phase composition was analyzed by XRD (D8 Advance, Bruker, Ettlingen, Germany, Cu-Kα radiation, λ = 0.15406 nm) at a scanning speed of 6 • /min and a scanning range of 20-80 • . Microstructures of the reacted samples were characterized by SEM (S-4800, Hitachi, Tokyo, Japan) equipped with an energy-dispersive spectrometer (EDS). Figure 1 displays the DSC curves of the Zr-B 4 C, Ni-B 4 C, Ni-Zr and 30 wt.% Ni-Zr-B 4 C mixtures heated to 1200 • C with a heating rate of 10 • C/min. Moreover, interrupted experiments were performed in order to elucidate the reaction mechanism during the heating process.

Results and Discussion
The DSC curve of the Zr-B 4 C mixture is shown in Figure 1a. A broad exothermic peak appears near 1008 • C. The XRD result of DSC product heated to 1200 • C shows that the product mainly consists of a large amount of ZrB 2 , ZrC and a small amount of Zr (see Figure 2). The presence of Zr may have been caused by the incomplete reaction of reactants. Hu et al. [1] studied the mechanism of ZrB 2 and ZrC generation in the Zr-B 4 C system and proposed that the solid-phase synthesis reaction was the main formation mechanism. Zhang et al. [2][3][4] investigated the reaction behavior and formation mechanism in the Cu-Zr-B 4 C system. Effects of heating rate and B 4 C particle size on the reaction process in the Zr-B 4 C system were also explored. Either increasing the particle size of B 4 C or increasing the heating rate may result in a sluggish solid-state reaction between Zr and B 4 C, which leads to the residual of Zr and B 4 C in the DSC products. The diffraction peaks of Zr were also found in the XRD patterns of the above research, but the diffraction peaks of B 4 C were very weak or absent due to the atomic characteristics and crystalline lattice of  The DSC curve of the Zr-B4C mixture is shown in Figure 1a. A broad exothermic peak appears near 1008 °C. The XRD result of DSC product heated to 1200 °C shows that the product mainly consists of a large amount of ZrB2, ZrC and a small amount of Zr (see Figure 2). The presence of Zr may have been caused by the incomplete reaction of reactants. Hu et al. [1] studied the mechanism of ZrB2 and ZrC generation in the Zr-B4C system and proposed that the solid-phase synthesis reaction was the main formation mechanism. Zhang et al. [2][3][4] investigated the reaction behavior and formation mechanism in the Cu-Zr-B4C system. Effects of heating rate and B4C particle size on the reaction process in the Zr-B4C system were also explored. Either increasing the particle size of B4C or increasing the heating rate may result in a sluggish solid-state reaction between Zr and B4C, which leads to the residual of Zr and B4C in the DSC products. The diffraction peaks of Zr were also found in the XRD patterns of the above research, but the diffraction peaks of B4C were very weak or absent due to the atomic characteristics and crystalline lattice of B4C [1][2][3][4].  Figure 1b shows the DSC curve of the Ni-B4C mixture heated to 1200 °C. A small exothermic peak was present at 576 °C, and a large endothermic peak was present at 1026 °C. To better interpret the two peaks, the Ni-B4C mixtures were heated to 900 °C and 1030 °C, respectively, before being cooled down. Figure 3 shows the XRD patterns obtained for  Figure 1b shows the DSC curve of the Ni-B 4 C mixture heated to 1200 • C. A small exothermic peak was present at 576 • C, and a large endothermic peak was present at 1026 • C. To better interpret the two peaks, the Ni-B 4 C mixtures were heated to 900 • C and 1030 • C, respectively, before being cooled down. Figure 3 shows the XRD patterns obtained for DSC products when quenched from 900 • C, 1030 • C and 1200 • C, respectively. When the DSC heating was quenched from 900 • C, the product was mainly composed of Ni 2 B, Ni 3 B and C, indicating that the solid reaction between Ni and B 4 C occurred at this time, corresponding to the exothermic peak appearing at 576 • C on the DSC curve. As shown in Figure Figure 1c shows the DSC curve of the Ni-Zr mixture heated to 1200 °C. As indicated, three exothermic peaks appear at 878 °C, 1030 °C and 1074 °C, respectively. Two endothermic peaks appear at 1146 °C and 1181 °C. In order to determine the reactions occurring near these peaks, the Ni-Zr mixtures were heated to 600 °C, 950 °C, 1030 °C, 1080 °C, 1160 °C and 1200 °C, respectively, and then cooled down. XRD patterns for the DSC products  Figure 1c shows the DSC curve of the Ni-Zr mixture heated to 1200 • C. As indicated, three exothermic peaks appear at 878 • C, 1030 • C and 1074 • C, respectively. Two endothermic peaks appear at 1146 • C and 1181 • C. In order to determine the reactions occurring near these peaks, the Ni-Zr mixtures were heated to 600 • C, 950 • C, 1030 • C, 1080 • C, 1160 • C and 1200 • C, respectively, and then cooled down. XRD patterns for the DSC products of Ni-Zr mixtures quenched at different temperatures are shown in Figure 4. When the Ni-Zr mixture was heated to 600 • C, only the original reactants Ni and Zr were found in the quenched product, and no obvious reaction occurred (see Figure 4). When the Ni-Zr mixture was heated to 950 • C, the diffraction peak intensity of Ni and Zr in the quenched product was obviously weakened. At this time, NiZr, Ni 10 Zr 7 and Ni 5 Zr were generated, which indicated that there was a solid-state reaction between Ni and Zr, resulting in a wide exothermic peak at 878 • C. When the Ni-Zr mixture was heated to 1030 • C, the content of Ni 10 Zr 7 increased significantly, which corresponded to the exothermic peak at 1030 • C (see Figure 4). As the temperature was raised to 1080 • C, the Ni 11 Zr 9 phase appeared, and the content of unreacted Ni and Zr decreased significantly. The production of Ni 11 Zr 9 led to the presence of an exothermic peak at 1074 • C. As the temperature was raised to 1160 • C, Ni 10 Zr 7 disappeared, and there was a large amount of NiZr and a small amount of Ni 11 Zr 9 in the product. Following the Ni-Zr binary phase diagram [23], Ni 10 Zr 7 and Ni will form a eutectic liquid at 1150 • C, which exactly corresponds to the endothermic peak at 1146 • C in the DSC curve. When the Ni-Zr mixture was heated to 1200 • C, the product mainly consisted of NiZr, Ni 11 Zr 9 and a small amount of NiZr 2 , in which the content of Ni 11 Zr 9 phase increased obviously. According to the Ni-Zr binary phase diagram [23], NiZr and Ni will form eutectic liquid phase at 1170 • C. Therefore, it can be deduced that the Ni-Zr eutectic liquid will form after the temperature is gradually raised to 1170 • C, which leads to the endothermic peak at 1181 • C. Subsequently, when the mixture was heated to 1200 • C and then cooled down, Ni 11 Zr 9 and NiZr 2 eventually crystallized from the Ni-Zr eutectic liquid. Figure 1d shows the DSC curve of the 30 wt.% Ni-Zr-B 4 C mixture heated to 1200 • C. As shown, two exothermic peaks were observed at 851 • C and 1088 • C, and two endothermic peaks were observed at 1025 • C and 1159 • C, respectively. In order to make clear the reactions occurring during the heating process, DSC interrupted experiments were carried out for the Ni-Zr-B 4 C mixtures at 900 • C, 1030 • C, 1060 • C, 1100 • C, 1130 • C, 1170 • C and 1200 • C, respectively, and then cooled down. The XRD patterns for the DSC products quenched at different temperatures are shown in Figure 5. When the Ni-Zr-B 4 C mixture was heated to 900 • C, a large amount of Ni 2 B and a small quantity of Ni 4 B 3 , NiZr and Ni 5 Zr were generated in the product, indicating that the wide exothermic peak near 851 • C corresponded to the formation of these Ni x Zr y and Ni x B y phases. This is also consistent with the previous analysis of Ni-Zr and Ni-B mixtures. When the Ni-Zr-B 4 C mixture was heated to 1030 • C, a very small amount of ZrB 2 and ZrC appeared in the product, indicating that a small amount of Zr reacted with B 4 C at this time. As the temperature was raised to 1060 • C, the diffraction peak intensity of Ni 2 B and Ni 4 B 3 decreased. According to the analysis of the Ni-B 4 C mixture, Ni 2 B and Ni 4 B 3 can form the Ni-B eutectic liquid at 1018 • C, which corresponds to the endothermic peak at 1025 • C in the DSC curve of Ni-Zr-B 4 C. At the same time, the formation of the Ni-B liquid phase also promotes the contact and reaction between the reactants in the mixture, and the free C atomic can dissolve into the Ni-B liquid phase to form the Ni-B-C ternary liquid phase, which fully contacts with the surrounding Zr powder and B 4 C powder. As the temperature was raised to 1100 • C, a large amount of Ni, ZrB 2 and ZrC were formed in the product, and a large exothermic peak appeared at 1088 • C in the DSC curve. It is speculated that part of the Zr powder directly reacted with B 4 C to form ZrB 2 and ZrC, and part of the Zr powder dissolved into the surrounding Ni-B-C ternary liquid to form Ni-Zr-B-C quaternary liquid. When the concentration of [Zr], [B] and [C] atoms in the Ni-Zr-B-C liquid achieved the thermodynamic condition for the formation of ZrB 2 and ZrC, ZrB 2 and ZrC particles precipitated out of the saturated liquid. It is worth mentioning that a large amount of Ni 10 Zr 7 also appeared in the product at 1100 • C, which was slightly different from the temperature at which Ni 10 Zr 7 appeared in large quantities in the Ni-Zr mixture (1030 • C), which may be due to the influence of the addition of B 4 C in the Ni-Zr-B 4 C mixture. When the Ni-Zr-B 4 C mixture was heated to 1130 • C, the product was mainly composed of a large amount of ZrB 2 , ZrC and a small amount of Ni 10 Zr 7 and Ni 2 B. When the Ni-Zr-B 4 C mixture was heated to 1170 • C, the product consisted of ZrB 2 , ZrC, Ni and a small amount of Ni 2 B. As the temperature was raised to 1200 • C, the product consisted of ZrB 2 , ZrC and Ni, indicating that the reaction of the system had tended to be complete. When the temperature rose from 1130 • C to 1170 • C, the content of Ni 10 Zr 7 decreased rapidly, which was consistent with the results in the previously studied Ni-Zr mixture. When the temperature reached 1150 • C, Ni 10 Zr 7 and Ni could form a Ni-Zr eutectic liquid phase, corresponding to the thermal absorption peak at 1159 • C in the DSC curve of the Ni-Zr-B 4 C mixture. The formation of Ni-Zr liquid phase promotes the contact and reaction between each component, which makes the reaction of the whole system fast and complete. of Ni11Zr9 in the product. Following the Ni-Zr binary phase diagram [23], Ni10Zr7 and Ni will form a eutectic liquid at 1150 °C, which exactly corresponds to the endothermic peak at 1146 °C in the DSC curve. When the Ni-Zr mixture was heated to 1200 °C, the product mainly consisted of NiZr, Ni11Zr9 and a small amount of NiZr2, in which the content of Ni11Zr9 phase increased obviously. According to the Ni-Zr binary phase diagram [23], NiZr and Ni will form eutectic liquid phase at 1170 °C. Therefore, it can be deduced that the Ni-Zr eutectic liquid will form after the temperature is gradually raised to 1170 °C, which leads to the endothermic peak at 1181 °C. Subsequently, when the mixture was heated to 1200 °C and then cooled down, Ni11Zr9 and NiZr2 eventually crystallized from the Ni-Zr eutectic liquid.   In order to better illustrate the above viewpoints, microstructure analysis of DSC quenching products in the 30 wt.% Ni-Zr-B4C mixture at different temperatures was carried out. The SEM images are shown in Figure 6. It can be seen from Figure 6a that, at room temperature, the raw material mixed powder presented a loose and uniform microstructure. When the temperature was 900 °C, the Ni powder no longer presented a flower shape, but it became denser and bound more closely with the surrounding Zr powder and B4C powder, as shown in Figure 6b. Some NixBy compounds formed around it by energy spectrum analysis. When the temperature rose to 1060 °C, the formation of a liquid phase was observed (see Figure 6c). Combining the EDS-point scanning spectrum (see Figure  6g) with the SEM image, point 1 was rich in Ni and B and thus mainly contained the Ni-B liquid phase. When the temperature rose to 1100 °C, the EDS-point scanning spectrum (see Figure 6h) of point 2 in Figure 6d contained Zr, Ni, B and C and, thus, possibly mainly contained the Ni-Zr-B-C liquid phases. When the temperature was further increased to 1170 °C, a large amount of liquid phase was formed, and a small number of ceramic particles were precipitated out of the liquid phase (see Figure 6e). When the temperature was increased to 1200 °C, a large number of ceramic particles formed in the product, as shown in Figure 6f. These results indicate that the microstructure evolution of DSC-quenched products is consistent with the previously inferred reaction mechanism analysis. In order to better illustrate the above viewpoints, microstructure analysis of DSC quenching products in the 30 wt.% Ni-Zr-B 4 C mixture at different temperatures was carried out. The SEM images are shown in Figure 6. It can be seen from Figure 6a that, at room temperature, the raw material mixed powder presented a loose and uniform microstructure. When the temperature was 900 • C, the Ni powder no longer presented a flower shape, but it became denser and bound more closely with the surrounding Zr powder and B 4 C powder, as shown in Figure 6b. Some Ni x B y compounds formed around it by energy spectrum analysis. When the temperature rose to 1060 • C, the formation of a liquid phase was observed (see Figure 6c). Combining the EDS-point scanning spectrum (see Figure 6g) with the SEM image, point 1 was rich in Ni and B and thus mainly contained the Ni-B liquid phase. When the temperature rose to 1100 • C, the EDS-point scanning spectrum (see Figure 6h) of point 2 in Figure 6d contained Zr, Ni, B and C and, thus, possibly mainly contained the Ni-Zr-B-C liquid phases. When the temperature was further increased to 1170 • C, a large amount of liquid phase was formed, and a small number of ceramic particles were precipitated out of the liquid phase (see Figure 6e). When the temperature was increased to 1200 • C, a large number of ceramic particles formed in the product, as shown in Figure 6f. These results indicate that the microstructure evolution of DSC-quenched products is consistent with the previously inferred reaction mechanism analysis.

Conclusions
Based on DSC and XRD analysis of Zr-B4C, Ni-B4C, Ni-Zr and 30 wt.% Ni-Zr-B4C mixtures, the reaction mechanism in 30 wt.% Ni-Zr-B4C mixture under DSC conditions is proposed as follows: (i) Firstly, some intermetallic NixBy (mainly Ni2B and Ni4B3) and NixZry (mainly NiZr and Ni5Zr) formed via solid-state diffusion reactions of Ni, B4C and Zr at about 851 °C. (ii) Then, Ni2B and Ni4B3 formed a Ni-B eutectic liquid at about 1025 °C, and the free C atoms dissolved into the Ni-B liquid to form a Ni-B-C ternary liquid. When the mixture was heated to about 1088 °C, part of the Zr powder directly reacted with B4C through a solid-state diffusion reaction, and part of the Zr powder dissolved into the surrounding Ni-B-C ternary liquid to form Ni-Zr-B-C quaternary liquid. (iii) Finally,

Conclusions
Based on DSC and XRD analysis of Zr-B 4 C, Ni-B 4 C, Ni-Zr and 30 wt.% Ni-Zr-B 4 C mixtures, the reaction mechanism in 30 wt.% Ni-Zr-B 4 C mixture under DSC conditions is proposed as follows: (i) Firstly, some intermetallic Ni x B y (mainly Ni 2 B and Ni 4 B 3 ) and