3.1 Characterization of Ta4HfC5 powder
Fig. 1 shows the XRD patterns of Ta4HfC5 powder calcined from 1000 °C to 1400 °C. The crystalline peaks were mainly Ta2O5 and TaC when the temperature was heated to 1000 °C. Ta2Hf6O17 began to appear between 1000 °C and 1200 °C. After 1200 °C, only hafnium oxides existed in Ta4HfC5 precursor. These hafnium oxides had two different crystal structures: monoclinic HfO2 and orthorhombic HfO2. No other peaks existed as the temperature increased to 1400 °C. The precursor was completely converted to Ta4HfC5 powder. Orthogonal HfO2 completely reacted before 1300 °C, but monoclinic HfO2 needed 1300 °C.
Fig. 1 XRD patterns of powder calcined from 1000 °C to 1400 °C
Ta4HfC5 powder can be prepared at 1400 °C, which below 1600 °C for other works [14,15]. For the Hf–O–C system, the synthesis temperature of HfC is generally higher than 1600 °C [17], which limited the formation of Ta4HfC5 powder at a lower temperature. The precursor is converted to Ta2Hf6O17 at 1100 °C instead of independent Ta2O5 and HfO2[15]. Converting Ta–O–Hf to Ta–Hf–C took lesser energy than converting Ta–O and Hf–O to Ta–Hf–C.
Ta4HfC5 powder was investigated through SEM analysis as shown in Fig. 2. The powder presented good uniformity and dispersion at lower magnification (Fig. 2a). The small amount of agglomeration shown in the figures was the agglomeration of the precursor during drying process. Fig. 2(b) presents that the particle size of Ta4HfC5 powder was nanoscale. Figs. 2(c)–(e) show the homogeneous distribution of the different elements as observed via energy-dispersive spectroscopy, which had over 3000 scan points to ensure the accuracy of the result. Ta and Hf were distributed uniformly. This method solved the problem of the uneven distribution of Ta and Hf when Ta4HfC5 powder was prepared via mechanical ball milling method.
Fig. 2 Morphological characteristics and elemental distribution of Ta4HfC5 powder
Fig. 3 shows the XPS spectrum of Ta4HfC5 powder prepared at 1400 °C. The components at 12.98 eV and 14.68 eV are assigned to Hf–C bonds, and the others at 15.53 eV and 17.18 eV are corresponded to Hf–O bonds. Fig. 3(b) shows the spectrum of Ta4f, which reveals Ta–C bonds at 21.73 eV and 23.63 eV and Ta–O bonds at 24.43 eV and 26.13 eV, respectively. No obvious peaks of tantalum oxide or hafnium oxide were observed in the XRD pattern in Fig. 1, the Hf-O and Ta-O bonds should be derived from contamination during the testing process.
Fig. 3 X-ray photoelectron spectrum of Ta4HfC5 powder
Fig. 4 shows the FTIR spectrum of the Ta4HfC5 precursor. Ta–O–Hf precursor without phenolic resin was prepared through the same process as a reference to analyze the changes in the chemical structure during precursor synthesis. The absorption peak of O–H near 3420 cm−1 showed that associated and free hydroxyl groups existed in the system and indirectly indicated that O–H was connected with multiple metal ions.
The absorption peaks near 2960 cm−1and 2851 cm−1 corresponded to the absorption peak of C–H in –CH2 and –CH3, respectively. After phenolic resin was added, a C=C stretching vibration near 1540 cm−1 appeared, and the vibration absorption peak of C=O stretching vibration corresponding to ketone appeared at about 1578 cm−1. These two peaks were the characteristic peaks of Ta(AcAc)x and Hf(AcAc)x, which corresponded to the C=C and C=O stretching vibration in the enol-type structure.
The absorption peak near 1127 cm−1 weakened and disappeared after phenolic resin was added, but peaks appeared at 1164 cm−1, 1113 cm−1, and 1051 cm−1, which were C–O stretching vibrations. Combined with the spectra of phenolic resin, this peak might be the weak absorption peak caused by Ta–O–C and Hf–O–C.
Fig. 4 FTIR of phenolic resin, Ta–O–Hf precursor, and Ta4HfC5 precursor
3.2 Characterization of Ta4HfC5 ceramics
Table 2 shows the relative density of Ta4HfC5 ceramics. The relative density of the ceramic sintered at 2100 °C/30MPa for 5 min was 81.61%. This result was better than the relative density of Ta4HfC5 ceramics sintered through mechanical ball milling at 2050 °C/32MPa for 20 min, which was 77.9% [10]. The relative density increased to 88.60% when uniaxial pressure increased to 50 MPa. Densification behavior is discussed in detail later in this article.
Fig. 5 Fractured surfaces of Ta4HfC5 ceramics at different uniaxial pressures:
(a) 30 MPa, (b) 40 MPa and (c) 50 MPa
Fig. 5 shows the SEM images of the fractured surfaces of all Ta4HfC5 ceramics. The fracture morphology at 30 MPa was basically granular agglomeration, sintering phenomenon was hardly observed. At 40 MPa, grain size increased, porosity decreased, but agglomeration could still be observed. Sintered neck was observed when uniaxial pressure increased to 50 MPa. The agglomeration of powder basically disappeared.
The grain sizes of the ceramic still had good uniformity and did not increase too much compared with the initial Ta4HfC5 powders. This result was obviously superior to Ta4HfC5 ceramics sintered via mechanical ball milling.
Table 1 shows the mechanical properties of Ta4HfC5 ceramics sintered under different uniaxial pressures. The increase in relative density could obviously improve the hardness and flexural strength, the hardness increased from 5.85 ± 0.43 GPa to 10.07 ± 0.61 GPa at the same holding time. Flexural strength increased from 5.85 ± 0.43 GPa to 10.07 ± 0.61 GPa, but it slightly affected fracture toughness. The fracture toughness for all compositions were similar in the range of 2.36–2.56 MPa m1/2.
Table 1 Relative density and mechanical properties of Ta4HfC5 ceramics
Sintering Conditions
℃/MPa/min
|
Relative density
(%)
|
Mechanical properties
|
Hv
(GPa)
|
KIC
(MPa·m1/2)
|
σ
(MPa)
|
2100/30/5
|
81.61
|
5.85 ± 0.43
|
2.36 ± 0.06
|
119.94 ± 5.21
|
2100/40/5
|
83.27
|
6.64 ± 0.48
|
2.40 ± 0.14
|
151.76 ± 11.3
|
2100/50/5
|
88.60
|
10.07 ± 0.61
|
2.56 ± 0.12
|
244.21 ± 10.4
|
3.3 Densification behavior of Ta4HfC5 ceramics
Fig. 6 shows the sintering curves of Ta4HfC5 ceramics sintered via spark plasma sintering under different uniaxial pressures. All the densification rates showed negative growth before 500 °C. The part of gas between particles would expand as temperature increased and would discharge slowly from the gap between mold and punches. Hence, a slower heating rate was applied before 500 °C because a rapid heating rate at the beginning would cause the gas instantly expand and generate local high pressure, which would quickly overflow powder from the gap, even directly destroy the mold.
All the powders started shrinking from 600 °C to 2100 °C because of the partial sintering of ceramics and the exhaustion of gas at a higher temperature. The gas in the pores produced spark discharge, generated ionization, and released a large amount of plasma, thereby promoting sintering densification. The maximum shrinking rates as the uniaxial pressure increased were 2.13×10−3, 4.45×10−3, and 2.35×10−2 mm/s at 30, 40, and 50 MPa, respectively.
Fig. 6 Sintering curves of Ta4HfC5 ceramics under different uniaxial pressures:
(a) 30, (b) 40, and (c) 50 MPa
The sintering displacement curves of the three ceramics still increased slowly at 2100 °C when sintering temperature was constant. This finding indicated that relative density would continue to increase with the extension of the constant sintering time. If the sintering time was long enough, nearly full-dense Ta4HfC5 ceramics might be theoretically obtained.
In addition, the high-speed moving plasma hit the surface of the particles, which would eliminate the oxides on surface and make it easier to purify and activate. We added additional phenolic resin before to make it possible to remove the oxides, which was beneficial to further increase the densification of ceramics.
For the densification behavior of Ta4HfC5 ceramics, fully dense Ta4HfC5 ceramic was difficult to sinter at approximately 2000 °C and would need a higher uniaxial pressure because of the numerous covalent bonds and high melting points of the Ta–Hf–C system. Some pores were inevitably left during the shorter sintering time because of low self-diffusion coefficients and small liquid phase filling. Nanoscale particles had a higher sintering activity; however, their relative density was still higher than that sintered through mechanical ball milling under similar conditions.
Although spark plasma sintering had the advantages of the faster heating rate and shorter sintering time, no obvious advantages were observed when sintering Ta–Hf–C ceramics at around 2000 °C. In some works, TaC/TaC-HfC ceramics are sintered without sintering additives, and the relative density is over 90%. However, most works have used hot pressing, which involves longer time, higher temperature, or higher pressure [18,20,21]. However, the ultrafine powder is still difficult to obtain full densification ceramic via hot pressing [22]. Ta–Hf–C ceramics sintered through spark plasma sintering can also achieve a higher densification in a shorter time but will require a higher sintering temperature than hot pressing [10].