In Situ Synthesis of ZrB₂–SiC Composite Powders by Carbothermal Reduction Method

Abstract

ZrB₂–SiC composite powders were synthesized in situ via carbothermal reduction using ZrO₂, HBO₂ and carbon black as the starting materials. The influences of HBO₂ contents (40.6–45.5 wt%), boron sources (HBO₂, H₃BO₃ and B₂O₃) and carbon sources (black carbon, activated carbon, graphite and petroleum coke) on phase composition and morphology of ZrB₂–SiC composite powders were analyzed. The obtained samples were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). Results showed that when the HBO₂ content was 43.2 wt%, impurity-free ZrB₂–SiC composite powders could be successfully produced at 1600 °C for 90 min in argon atmosphere. Meanwhile, columnar ZrB₂ and granular SiC particles were combined interactively. Compared with control groups, products fabricated via black carbon revealed the better crystallinity and finer particle size.

Introduction

Carbon/carbon (C/C) composites have incomparable advantages over other materials in aerospace and other high-tech fields due to their excellent properties, such as low density, low coefficient of thermal expansion, high specific strength, and high specific modulus. Because of the internal lattice defects, C/C composites are easily oxidized at high temperature in oxidizing atmosphere, which strongly restricted their applications [1, 2]. At present, ZrB₂–SiC ultra-high temperature ceramic coatings are actively used to protect C/C composites owing to the good oxidation resistance [3,4,5], and spraying is widely applied to prepare the coatings for its flexible operation [6, 7]. However, due to the different densities between ZrB₂ and SiC, the components of coatings are easy to demix during spraying, if the two powders are mixed mechanically. Therefore, the synthesis of ZrB₂–SiC composite powders has become a hot issue.

There are various methods to prepare ZrB₂–SiC composite powders, including self-propagation high-temperature synthesis method, sol–gel method, carbon thermal reduction method and so on [8,9,10,11]. Among different synthesis routes, carbon thermal reduction method has the advantages of simple process, short product cycle and low production costs. In this study, ZrB₂–SiC composite powders were synthesized in situ by carbothermal reduction at 1600 °C for 90 min in Ar, using ZrO₂, HBO₂, black carbon and SiC as the starting materials. The in situ reaction could make the solidification mass transfer more fully and impede delamination effectively in spraying. The effects of HBO₂ content, different boron and carbon sources on phase composition and morphology of ZrB₂–SiC composite powders were discussed.

Experiment

Zirconia powders (ZrO₂, ≤10 µm), boric acid (H₃BO₃, 96%, ≤10 µm), boron oxide (B₂O₃, 98%, ≤20 µm), carbon black (C, ≤10 µm), activated carbon (C, ≤20 µm), graphite (C, ≤20 µm), petroleum coke (C, ≤20 µm) and silicon carbide (SiC, ≤0.5 µm) were provided as the raw materials. Since H₃BO₃ could volatilize easily which would do harm to the furnace owing to acid corrosion, H₃BO₃ was put into a vacuum drying oven at 120 °C for 12 h to get HBO₂, according to Eq. 1 [12]. In theory, ZrB₂–SiC composite powders can be produced by Eq. 2. Because of the volatilization of HBO₂ during calcination, the amount of HBO₂ was adjusted to obtain the highly pure powders.

$$ {\text{H}}_{ 3} {\text{BO}}_{3} \left( {\text{s}} \right) \to {\text{HBO}}_{2} \left( {\text{s}} \right) + {\text{H}}_{2} {\text{O}}\left( {\text{g}} \right). $$

(1)

$$ {\text{ZrO}}_{2} \left( {\text{s}} \right) + 2{\text{HBO}}_{2} \left( {\text{s}} \right) + 5{\text{C}}\left( {\text{s}} \right) + {\text{xSiC}}\left( {\text{s}} \right) \to {\text{ZrB}}_{2} \left( {\text{s}} \right) + 5{\text{CO}}\left( {\text{g}} \right) + {\text{H}}_{2} {\text{O}}\left( {\text{g}} \right) + {\text{xSiC}}\left( {\text{s}} \right). $$

(2)

Three groups of composite powders were prepared according to the ratio of Eq. (2), referred to as ZS01, ZS02, ZS03, in which the mass fraction of SiC was 20 wt%. In order to study the impact of HBO₂ content, extra HBO₂ of 80, 100, 120 wt% were added to ZS01, ZS02, ZS03 respectively. Therefore, the content of ZrO₂, HBO₂, carbon black and SiC was shown as Table 1.

Table 1 Content of the ingredients of different groups

The three groups of powders were mixed by ball mill at 300 rpm for 20 min with ZrO₂ balls as the grinding media. Then, the mixed powders were put into different graphite crucibles and sealed in a graphite resistance furnace. The samples were heated to 1600 °C at a rate of 10 °C/min in argon atmosphere and then held for 90 min. After cooling to room temperature, gray powders were obtained. In addition, different boron sources were added to composite powders to compare the phase composition and morphology of the products while the mass ratio of Zr and B was constant. Further, the black carbon was replaced by activated carbon, graphite and petroleum coke to study the effect of different carbon sources. XRD (D/max-RA, Japan) was used to analyze the phase composition of ZrB₂–SiC composite powders. The morphology of the products was characterized by SEM (D/max-RA, Germany).

Results and Discussion

Effect of HBO ₂ Content. Figure 1 showed the XRD patterns of ZrB₂–SiC composite powders prepared with different contents of HBO₂. It was obvious that impurity-free ZrB₂–SiC powders were synthesized. When the amount of HBO₂ decreased from 43.2 to 40.6 wt%, the narrowed peaks and reduced intensities of ZrB₂ implied the increment of grain size of the products. With extra 120 wt% HBO₂, diffraction peak of ZrB₂ decreased while the HBO₂ phase was not detected in the Fig. 1c. The reason was that HBO₂ was converted to B₂O₃ during heating and a part of B₂O₃ was involved in the synthesis of ZrB₂ grains while the excess was present in the furnace as B₂O₃ vapor. The partial pressure of B₂O₃ vapor led to the change in Gibbs free energy (ΔG), which resulted in a slight decrease in the intensity of ZrB₂ phase. Meanwhile, the strongest peak of SiC appeared with 43.2 wt% HBO₂. In summary, the amount of HBO₂ had a certain influence on the crystallinity of ZrB₂–SiC composite powders.

Fig. 1

XRD patterns of final products prepared with different contents of HBO₂. a ZS01 b ZS02 c ZS03

Effect of Different Boron Sources. XRD patterns of ZrB₂–SiC composite powders prepared with different boron sources were given in Fig. 2. As Fig. 2a, b showed, powders with good crystallinity were successfully synthesized when HBO₂ or H₃BO₃ were chosen as the boron source. However, the intensity of ZrB₂ peak decreased when B₂O₃ was used instead of HBO₂ or H₃BO₃.

Fig. 2

XRD patterns of final products prepared with different boron source. a HBO₂ b H₃BO₃ c B₂O₃

The resultant samples were also characterized by SEM to investigate the morphology, shown in Fig. 3. From Fig. 3a, powders synthesized by HBO₂ demonstrated uniform size of 1.5 μm. And columnar ZrB₂ and granular SiC particles were combined interactively. Grains in Fig. 3b grew larger in comparison to those in Fig. 3a, implying size increase took place with H₃BO₃. When the boron source was replaced by B₂O₃, ZrB₂ grains revealed poor crystallinity and irregular shapes.

Fig. 3

SEM images of final products prepared with different boron source. a HBO₂ b H₃BO₃ c B₂O₃

Effect of Different Carbon Sources. XRD patterns of ZrB₂–SiC composite powders produced with different carbon sources were presented in Fig. 4. As Fig. 4a, b showed, impurity-free ZrB₂–SiC composite powders could be obtained with carbon black or petroleum coke. However, the faint peaks of carbon and zirconia were detected when the carbon source was replaced by activated carbon, which was probably because that activated carbon with a small specific surface area could not play a sufficient reduction effect in the reaction. Furthermore, compared with Fig. 4a, b, the intensity of ZrB₂ and SiC was reduced. From Fig. 4c, the peak of carbon was still existed when graphite was used instead of activated carbon, which could also introduce impurities.

Fig. 4

XRD patterns of final products produced with different carbon source. a Carbon black b Petroleum coke c Graphite d Activated carbon

Figure 5 showed the SEM images of ZrB₂–SiC composite powders synthesized with different carbon sources. From Fig. 5a, columnar ZrB₂ and granular SiC particles were combined interactively and the particle size of ZrB₂ was estimated to 1.5 μm. When the carbon source was replaced by petroleum coke or graphite, ZrB₂ grains which were distributed on the surface of SiC increased to about 2 μm in size. Furthermore, the grain boundaries of ZrB₂ began to melt and the morphology became irregular, shown in Fig. 5b, c. As Fig. 5d showed, ZrB₂ particles with the size of 4 µm could be produced by activated carbon. The different specific surface area of carbon materials might be the reason for the influences of carbon source on the fabrication of ZrB₂. Based on above results, it could be concluded that the grains of powders prepared by carbon black possess relatively finer particle size and better crystallinity compared with control groups.

Fig. 5

SEM images of final products synthesized with different carbon source. a Carbon black. b Petroleum coke. c Graphite. d Activated carbon

Conclusion

Impurity-free ZrB₂–SiC composite powders were successfully produced from ZrO₂, HBO₂ SiC and carbon black by in situ carbothermal reduction method. Effects of HBO₂ content, various carbon and boron sources on the phase composition and morphology of ZrB₂–SiC composite powders were investigated. When the HBO₂ content was 43.2 wt%, the highly pure composite powders with regular shapes could be synthesized at 1600 °C for 90 min in Ar and ZrB₂ was estimated to 1.5 μm in particle size. Compared with samples prepared by petroleum coke, graphite or activated carbon, the composite powders fabricated by black carbon possessed relatively better crystallinity and finer particle size.