Densification of ZrB₂–SiC nanocomposites prepared using ZrSi₂, B₄C, and C additives

Abstract

Dense ZrB₂–SiC nanocomposites were fabricated at 1450 °C by the high-energy ball milling (HEBM) of ZrB₂ powder with ZrSi₂–B₄C–C additives and reactive spark plasma sintering (R-SPS). The sizes of ZrB₂ and SiC grains were 80–350 nm, and the phases were homogeneously distributed because of the molecular-level homogeneity of the constituents in ZrSi₂ and the homogeneous mixing of the raw powders by HEBM. The deformation of ZrSi₂ or the reactions between the additives during R-SPS did not strongly promote the densification. Fine and homogeneously distributed ZrB₂ and SiC particles formed by HEBM and R-SPS were the major reasons for the low temperature densification. The fine particles which had high surface energy provided the driving force for densification at low temperatures. Also, the fine SiC grains suppressed the growth of ZrB₂ grains during densification. The 4-point bending strength of the composites sintered at 1500 °C was 354 MPa.

I. INTRODUCTION

ZrB₂ has attracted attention in recent years because of its high melting temperature (>3200 °C), excellent oxidation resistance above 1800 °C, relatively low density (6.09 g/cm³), high strength (500–1000 MPa), and moderate fracture toughness (2.5–4.5 MPa m^1/2).1,2 As a consequence, researches have been performed for producing ZrB₂ ceramics with high relative density, fine microstructure, excellent mechanical properties, and high thermal stability.3

However, the material has some problems such as the difficulty of densification. Because of the strong covalent bonding and low self-diffusion coefficients, pressure-assisted sintering such as hot pressing or spark plasma sintering (SPS) above 2000 °C has been required for the densification of monolithic ZrB₂ without sintering additives. As a consequence, strong grain growth deteriorated the mechanical properties of the ultra-high temperature ceramic materials (UHTC).4

To decrease the sintering temperature of ZrB₂–SiC, reactive hot pressing using Zr, Si, B₄C, and C has been performed. The dense ZrB₂ and ZrB₂–SiC composites were prepared without using sintering additives at 1800–1900 °C.5,6 However, the grain sizes of the resultant products were relatively large (1–10 µm) and a careful handling of the reactive Zr metal powder in an inert atmosphere was required for the process.7

Reactive SPS (R-SPS) has been recently introduced to obtain UHTC with high density and a fine microstructure by using good properties such as fast heating/cooling rate, low sintering temperature, and high applicable pressure.8,9 Wu et al. compared the grain size of ZrB₂–SiC-based composites using reactive hot pressing and R-SPS and reported that the composites prepared by R-SPS formed a more homogeneous and fine microstructure because of the fast heating rate and short holding time.10,11

The low temperature densification of ZrB₂ and ZrB₂–SiC has been also achieved without sintering additives by decreasing the particle size down to 9–10 nm using high-energy ball milling (HEBM) and subsequent spark plasma sintering (SPS) at 1425–1650 °C.12–14 The sinterability of UHTC can be enhanced by HEBM due to the increase of surface area by the particle size refinement and the decrease of diffusion distance through the uniform improvement between the components.

Guo et al. densified ZrB₂ by hot pressing at 1450 °C using ZrSi₂ as a sintering additive.15 The low melting temperature of ZrSi₂ (1620 °C) played a key role in the enhanced densification. However, the ZrB₂ densified with ZrSi₂ suffered from a strong decrease of strength above 1400 °C due to the softening of the additive.16

This problem may possibly be solved by adding B₄C and C together with ZrSi₂ and inducing the following reaction:

$$2{\rm{ZrS}}{{\rm{i}}_2} + {{\rm{B}}_4}{\rm{C}} + 3{\rm{C}} \to 2{\rm{Zr}}{{\rm{B}}_2} + 4{\rm{SiC}}\quad .$$

(1)

$$\Delta G = - 547.686 + 0.049078T\quad ,$$

where ΔG is the Gibbs free energy (kJ/mol) and T is the temperature (°C).

During densification, ZrSi₂ transforms into highly refractory ZrB₂ and SiC by reaction (1) which has a minus Gibbs free energy at 20–2000 °C. Consequently, reaction (1) occurs at a low temperature if the reactants have a close contact without having oxide surface layers.

In addition to the benefit of low temperature densification and the removal of ZrSi₂ having low melting temperatures, the composites prepared by reaction (1) may have very fine and homogeneously intermixed ZrB₂ and SiC grains due to the molecular-level homogeneity of Zr and Si in ZrSi₂. The present authors reported the synthesis of homogeneously intermixed nano ZrB₂–SiC powder (20–80 nm) through reaction (1).17

The SiC particles promote the densification of ZrB₂ and improve the microstructure, mechanical properties, and oxidation resistance of UHTC. The SiC particles with a fine size exhibited a strong grain refining effect of the ZrB₂ matrix and excellent mechanical properties of the resultant ZrB₂–SiC ceramics.18

In this research, the densification of ZrB₂ using ZrSi₂–B₄C–C additives was performed at 1450 °C by using HEBM and R-SPS. The possible mechanisms for the low temperature densification were discussed, and some mechanical properties of the nano ZrB₂–SiC composites were analyzed.

II. EXPERIMENTAL PROCEDURE

ZrB₂ (ZrB₂–F, d₅₀: 2.12 µm, Japan New Metal Co., Osaka, Japan), ZrSi₂ (99.5%, <325 mesh, Alfa Aesar, Ward Hill, MA), B₄C (99%, <10 µm, Alfa Aesar), and carbon black (99.9%, d₅₀: 30 nm, Alfa Aesar) were used as starting materials. The molar ratio between ZrSi₂:B₄C:C was set to 2:1:3, which is the stoichiometric composition to form ZrB₂ and SiC by reaction (1).

The additive contents were 30, 40, and 50 wt% of the total weight (termed ZS30, ZS40, ZS50, respectively), which corresponded to 12.5, 16.1, and 20.8 wt% of SiC after reaction (1). On the other hand, 0, 10, and 20 wt% of nano-SiC powder (T-1, Sumitomo-Osaka Cement Co., Tokyo, Japan, d₅₀: 30 nm, β-SiC, O: 0.45 wt%, C: 3.73 wt%) was mixed with the ZrB₂ powder (termed SC0, SC10, and SC20, respectively) to compare the microstructure and sintering behavior.

The raw materials were mixed using a WC jar and balls for 6 h using a high-energy ball mill (M-8000, Spex Sample Prep., Metuchen, NJ, ball:powder = 10:1, 5 g per batch). After milling, the powder mixtures were granulated through a metallic sieve with a #150 mesh size and were loaded into a graphite mold (inner diameter 10 mm).

The powder mixtures were densified using a spark plasma sintering apparatus (SPS; Dr-Sinter 4000, SPS Syntax, Kamagawa, Japan) at 1400–1500 °C for 30–60 min under 120 MPa pressure in vacuum (heating rate: 50 °C/min). The heating process was controlled using a monochromatic optical pyrometer that was focused on a hole at the side of the graphite mold. The sintering shrinkage of the specimen was analyzed by measuring the displacement of the lower electrode (resolution 0.01 mm) which was connected to a computer to log the shrinkage curves.

Specimens of 2 × 10 × 25 mm size were obtained, which were machined into 1.5 × 2 × 25 mm bars. A 4-point bending test was performed using 5 specimens per condition according to the ASTM standard.19 After grinding and polishing the surface of the sintered specimens, the bulk densities were measured using Archimedes’ method. The theoretical densities of ZrB₂, SiC, ZrSi₂, B₄C, and C to calculate the volume shrinkage by reaction (1) and relative density were 6.09, 3.2, 4.88, 2.52, and 2.27 g/cm³, respectively.20 The phase identification and microstructure of the sintered specimens were analyzed by X-ray diffractometry (XRD; D-Max2200, Rigaku Co., Tokyo, Japan) using Cu K_α1 radiations (wave length: 1.54056 Å) and scanning electron microscopy (SEM; JSM-6700F, Jeol, Tokyo, Japan), respectively. An image analysis program (Nano measurer, China) was used to precisely measure the grain size. At least 200 grains per specimen were measured for the determination of the average grain size.

III. RESULTS AND DISCUSSION

A. Properties of the powders

Figure 1(a) shows the XRD data of the ZrSi₂–B₄C–C powder mixture after HEBM. The decrease of peak intensity and the broadening of the peaks indicated the ultrafine particle size of the ZrSi₂–B₄C–C mixture after high-energy milling. The initial particle sizes of ZrB₂ and ZrSi₂ were 2.12 and 27.5 µm, while the average particle size of the ZrSi₂–B₄C–C mixture after HEBM was calculated to be 8 nm by the Sherrer equation. Carbon black was not detected by XRD because of its amorphous state. The peaks of SiC were not identified after milling, which indicated that reaction (1) did not actively occur during the HEBM process.

FIG. 1

XRD data of sintered ZrB₂–SiC composites. (a) ZrSi₂–B₄C–C powder mixture after HEBM, (b) ZS40, 1200 °C, (c) ZS40, 1400 °C, (d) ZS50, 1450 °C, (e) SC20, 1450 °C, (f) ZS40, 1500 °C, and (g) ZS40, 1650 °C (▼: ZrSi₂, ■: ZrB₂, ●: SiC, □: ZrO₂, ○: graphite).

Figure 2 shows the SEM images of the powders after HEBM and after heating at 1200 °C. Fine and homogeneously intermixed particles were observed after HEBM [Fig. 2(a)]. Although the primary particle size after HEBM was measured to be 8 nm by X-ray analysis, the average particle size analyzed by the image analyzer was 310 nm because the particles were strongly agglomerated by cold welding during the milling process. Cold welding is a general expression in the field of mechanical alloying. During the high-energy milling of powder, the powders form a very hard agglomeration by the mechanical energy even at room temperatures.

FIG. 2

Morpholigy of ZS40 powders (a) after mixing and (b) after heating at 1200 °C.

After heating at 1200 °C, the formation of phases with different sizes and colors was clearly identified [Fig. 2(b)]. The EDS data indicated that the particles larger than 300 nm in size were ZrB₂. Most of the synthesized SiC particles were round in shape and were smaller than 100 nm. Although ZrSi₂ has the molecular-level homogeneity of Zr and Si, the size of ZrB₂ particles was larger than that of SiC particles after reaction (1). The result indicated that the growth rate of ZrB₂ particles is faster than that of SiC particles.17

B. Densification behavior

Table I shows the effect of the nano-SiC powder or ZrSi₂–B₄C–C additive contents on the sintered density. The relative density of the crushed ZrB₂ (SC0) was 95.5% after sintering at 1450 °C under 120 MPa pressure. The enhanced densification of the milled ZrB₂ was caused by the strong decrease of the particle size to about 8 nm and a consequent increase of the rearrangement of the particles and the driving force for densification.12 In addition, the imperfections in the crushed powder promoted the deformation of the particles at relatively low temperatures under high pressure (120 MPa).14

TABLE I Relative density and flexural strength of specimens sintered at various conditions under 120 MPa pressure in vacuum.

The densification of the milled ZrB₂ was clearly suppressed when adding nano-SiC powder (Table I). This result was inconsistent with those reported by Zamora et al. where the addition of SiC promoted the densification of ZrB₂ after HEBM due to a borosilicate glass which was formed during the process.21 (Fig. 5) Figure 4 shows the polished cross-section of SC20 after sintering at 1850 °C. The high sintering temperature was required to obtain dense specimens for the analysis of the microstructure. The nano-SiC particles were strongly aggregated. The aggregated SiC was not fully densified in spite of the high sintering temperature and pressure. Nadeau et al. reported that high temperature and pressure (2100 °C, 2 GPa) were simultaneously required for the densification of high-purity SiC.22

In contrast to the detrimental effect of nano-SiC on densification, the ZrSi₂–B₄C–C system did not suppress the densification of ZrB₂. The relative densities of ZS30–ZS50 were 88.9–94.2% after sintering at 1400 °C for 1 h, which were similar with that of SC0 (91.0%). The densification of ZS40 and ZS50 was mostly completed at 1450 °C (Table I). ZS40 and ZS50 contained different amounts of the additives (40 and 50 wt% of total weight). In all cases, the heating rate was fixed to 50 °C/min.

Figure 3 shows the sintering shrinkage behavior. The shrinkage became more distinct when the additive content increased because of the volume shrinkage of 11.2% by reaction (1). In all cases, the shrinkage stopped below 1650 °C, indicating that the densification was mostly completed during heating. The shrinkage curves could be divided into three stages.

FIG. 3

Shrinkage behavior during sintering at 1650 °C for 5 min at a pressure of 120 MPa. (a) SC0, (b) ZS30, (c) ZS40 and (d) ZS50.

The displacement data in Fig. 3 did not precisely reflect the sintering behavior of the powder because the data were obtained from the displacement of the lower electrode of the SPS (resolution 0.01 mm). The expansion below 1000 °C was the summation of the thermal expansion of the powder, graphite punches, and graphite spacers.

The expansion stopped when the first sintering stage started because the thermal expansion of the punches was compensated by the shrinkage of the powder. The onset temperatures of the first sintering stage were 1100, 1192, 1173, 1165 °C for SC0 and ZS30–ZS50, respectively. According to the results from the XRD analysis (Fig. 1), reaction (1) was mostly completed at this stage. The melting temperature of ZrSi₂ is 1620 °C, and the material can be densified above 1100 °C by plastic deformation.15,23 However, the deformation of ZrSi₂ did not have a strong effect on the densification of the ZS systems because the silicide mostly transformed into ZrB₂ and SiC below the temperature of the first stage. The reaction was reported to occur rapidly once started.17 The oxides which formed on the surface of ZrSi₂ prevent the reaction to occur at low temperatures. Once the oxide became unstable during heating, the reaction was reported to be mostly completed within a few seconds. In addition, the first stage was observed in SC0 which did not contain ZrSi₂. Also, the onset temperature of the first sintering stage is too low to induce the densification of ZrB₂ and SiC by thermal diffusion.

During SPS, the pressure at the contact point between the particles is amplified by the applied pressure until the local pressure exceeds the yield strength at the processing temperature.24 In addition, at the initial stage of SPS, the small contact point between the particles concentrates the current, which can induce the local increase of temperature and electromigration.25 Electromigration usually occurs at current densities over 1000 A/cm² and when the powder has a relatively low electrical resistivity (resistivity of HfC: 60 × 10⁶ Ω cm at 300 K).26 The local increase of pressure and temperature as well as electromigration at the contact point promoted the deformation and rearrangement of the particles at the first stage. The rearrangement was reported to be promoted by the ultrasmall particle size (∼10 nm) produced by the high-energy milling.14

The onset temperatures of the second stage for SC0 and ZS30–ZS50 were 1214, 1310, 1315, and 1290 °C, respectively, where the shrinkage began to occur. The results indicated that the densification of the second stage was initiated by that of the ZrB₂ pre-treated by HEBM because the onset shrinkage temperature of SC0 was the lowest among the tested specimens. The ZrSi₂–B₄C–C additives increased the onset shrinkage temperature of the finely crushed ZrB₂ powder. However, the onset temperature of shrinkage of the ZS systems decreased slightly when the additive content increased. The second stage is a sintering regime where the accelerated densification occurred and the pore distribution changes from open porosity to closed porosity.

Particle size has an important effect on the solid-state sintering in terms of temperature. The sintering temperature was reported to decreases with particle size, especially when the size was below 30 nm.27 The sintering of tungsten was reported to start at 825 °C for 31 nm particles, while the behavior occurred at 625 °C when using 9 nm powder.27 Consequently, the formation of ultrafine particles (8 nm) by HEBM is one of the major reasons for the low temperature sintering of the ZrB₂–ZrSi₂–B₄C–C powder.

At the beginning of the significant shrinkage, the densification of ZS was effectively promoted by the fine particles formed by reaction (1). The crystal size refinement and homogeneous mixing of the starting powders by HEBM increased the reaction area and thus promoted reaction (1). During the reaction, ultrafine ZrB₂ and SiC phases nucleated at the interface of ZrSi₂ and C.17 Growth of the new phases was suppressed by SPS because of the fast heating/cooling rate and the low densification temperature.

The refinement of ZrB₂ grains after densification can also be explained by Zener effect.28 The ultrafine secondary phase, which is placed at the grain boundaries of the matrix phase, exerts a drag force against the boundary movement and causes the inhibition of grain growth. Liu et al. reported that nano-SiC particles suppressed the growth of ZrB₂ grains from 7.5 to 2 µm.29 ZS40 and ZS50 became highly dense after the isothermal heating at 1450 °C for 60 min under 120 MPa pressure. The average grain size of ZrB₂ in the ZS decreased with increasing the additive content, which will be explained in the following paragraphs. The densification of ZS was improved when the additive content increased. The promotion of Zener effect and consequent suppression of grain growth is believed to be the main reason because the deformation of ZrSi₂ did not strongly induce the densification of the ZS system.

The shrinkage rate decreased rapidly at the third stage, indicating that the densification was mostly completed during heating. The third stage started at 1520, 1640, 1592, and 1572 °C for SC0 and ZS30–ZS50, respectively. The result indicated that the densification of the powder mixture was mainly performed by that of the crushed ZrB₂. Consequently, the densification mechanism of the ZrB₂–ZrSi₂–B₄C–C system is considered to be solid-state sintering like the case of high-energy milled ZrB₂.12,14

Based on the above discussion, the densification mechanism of the ZrB₂–SiC composites prepared using ZrSi₂, B₄C, and C powder mixtures could be proposed as follows. The deformation of ZrSi₂ and the reaction between ZrSi₂, B₄C, and C did not result in the significant densification. The densification became distinct after the reaction was completed. Ultrafine and homogeneously distributed ZrB₂ and SiC particles which were introduced by HEBM and R-SPS effectively promoted the densification.

Table I summarized the 4-point bending strength of the composites. The relative high porosity of ZS50 sintered at 1400 °C induced the low strength of the specimens compared to those sintered at 1500 °C. A maximum bending strength of 354 MPa was obtained.

C. Phase compositions and microstructures after densification

Figures 1(b)–1(g) show the XRD data of the ZS and SC systems after heating at different temperatures. In ZS, the peaks of ZrSi₂ (major peak: 38°) and B₄C mostly disappeared and the formation of SiC by reaction (1) was identified after heating at 1200 °C [Fig. 1(b)].

The phase formation did not strongly change up to 1450 °C [Figs. 1(c)–1(e)]. The results indicated that the additive system was mostly consumed by reaction (1) below the onset temperature of shrinkage (≥1165 °C) because the Gibbs free energy of reaction (1) is negative at and above room temperature. Reaction (1) was reported to occur at and above 860 °C when ZrSi₂–B₄C–C powders were strongly mixed by HEBM.17 The reaction between ZrSi₂–B₄C–C was reported to occur rapidly once started.17 The oxides formed on the surface of ZrSi₂ prevent the reaction from occuring at low temperatures. Once the oxide became unstable at high temperatures, the reaction was reported to be completed within a few seconds.

The ratio of the main peak intensity of ZrB₂ and SiC was similar between ZS50 and SC20 which contained similar amounts of SiC [20.8 versus 20 wt%, Figs. 1(d) and 1(e)]. The result also indicated that ZrSi₂ was mostly consumed to form ZrB₂ and SiC during densification.

The intensity of the ZrB₂ peaks increased and the width decreased when the sintering temperature was at and above 1500 °C [Figs. 1(f) and 1(g)] because the grain coarsening became distinct at 1500 °C.

In both the ZS and SC systems, the presence of ZrO₂, which was formed during the storage and crushing of ZrB₂ powder, was evident in the sintered specimens. The amorphous zirconium hydroxides on the surface of the raw powders were crystallized during densification. B₄C and C in the ZS systems can effectively remove ZrO₂ by carbo–boro thermal reduction;

$$2{\rm{Zr}}{{\rm{O}}_{2({\rm{s}})}} + {{\rm{B}}_4}{{\rm{C}}_{({\rm{s}})}} + 3{{\rm{C}}_{({\rm{s}})}} \to 2{\rm{Zr}}{{\rm{B}}_{2({\rm{s}})}} + 4{\rm{C}}{{\rm{O}}_{({\rm{g}})}}\quad ,$$

(2)

$$7{\rm{Zr}}{{\rm{O}}_{2({\rm{s}})}} + 5{{\rm{B}}_4}{{\rm{C}}_{({\rm{s}})}} \to 7{\rm{Zr}}{{\rm{B}}_{2({\rm{s}})}} + 3{{\rm{B}}_2}{{\rm{O}}_{3({\rm{l}},{\rm{g}})}} + 5{\rm{C}}{{\rm{O}}_{({\rm{s}})}}\quad ,$$

(3)

$${\rm{Zr}}{{\rm{O}}_{2({\rm{s}})}} + {{\rm{B}}_2}{{\rm{O}}_{3({\rm{l}})}} + 5{{\rm{C}}_{({\rm{s}})}} \to {\rm{Zr}}{{\rm{B}}_{2({\rm{s}})}} + 5{\rm{C}}{{\rm{O}}_{({\rm{g}})}}\quad .$$

(4)

However, the reduction reactions did not actively occur because reactions (2)–(4) have negative Gibbs free energy above 1424, 1218, and 1509 °C at a standard condition while the value of reaction (1) is negative at room temperature.16 Consequently, B₄C and C were preferentially consumed by reaction (1) before reactions (2)–(4) can actively occur.

Figure 4 displays the microstructure of the ZS systems. The amount of SiC, depicted as dark grains, in ZS50 was larger than that of ZS30 [Figs. 4(a) and 4(b)]. The average grain size of ZrB₂ and SiC in ZS30 was 352 and 81 nm after sintering at 1400 °C for 60 min, whereas the values changed to 213 and 90 nm in ZS50. By the application of the HEBM and ZrSi₂ additive which has molecular-level homogeneity between Zr and Si, dense ZrB₂–SiC composites with fine and homogeneously distributed ZrB₂ and SiC grains were successfully fabricated. The small grain size and homogeneous distribution of the ZrB₂ and SiC phases are related with the strong mixing and efficient size refinement of raw powders by HEBM and low sintering temperature by R-SPS under high pressure. (Fig. 5).

FIG. 4

Polished cross-section of the specimens after sintering for 1 h under 120 MPa pressure in vacuum. (a) ZS30, 1400 °C, (b) ZS50, 1400 °C, and (c) ZS50, 1500 °C.

FIG. 5

Polished cross-section of SC20 after sintering at 1850 °C for 1 h under 120 MPa pressure in vacuum.

Two reasons are considered to explain the difference in the grain size between ZrB₂ and SiC after densification. First, the growth of ZrB₂ grains was faster than that of SiC, which was also observed during the synthesis of ZrB₂–SiC powder mixture by the reaction of ZrSi₂–B₄C–C.17 Second, a part of the coarse ZrB₂ powder did not get crushed below 200 nm or intermixed with the additives after HEBM.

The agglomeration and growth of SiC grains occurred when the sintering temperature increased to 1500 °C [Fig. 4(c)], which result was in accordance with that of the XRD [Figs. 1(f) and 1(g)].

IV. CONCLUSIONS

The ZrB₂–SiC nanocomposites were successfully fabricated by the R-SPS at 1450 °C. The ultrafine particle size (∼10 nm) caused by HEBM of ZrB₂ promoted the rearrangement of particles and the densification at the first and second stage of sintering. The formation of ZrB₂ and SiC by the reaction of ZrSi₂–B₄C–C was mostly completed at 1200 °C. The resultant nano-SiC did not inhibit the densification of ZrB₂. The sizes of SiC grains were 100–300 nm and the phases were homogeneously distributed because of the molecular-level homogeneity of Zr and Si in ZrSi₂ and the high-energy milling of the raw powders. By contrast, the nano-SiC powder which was added to the crushed ZrB₂ powder formed strong aggregation and suppressed the densification of ZrB₂–SiC composites. The deformation of ZrSi₂ and the reaction between ZrSi₂, B₄C, and C did not strongly promote the densification. Instead, ultrafine and homogeneously distributed ZrB₂ and SiC particles prepared by HEBM and R-SPS effectively promoted the densification. A maximum bending strength of 354 MPa was obtained after sintering the ZrB₂–SiC composites at 1500 °C.