Effect of High Temperature Heat Treatment on Phase Composition and Microstructure of SiBCN/HfC Ceramic Composites

doi:10.15541/jim20190595

Abstract

Abstract:

Amorphous SiBCN powders were prepared by high-energy ball milling-mechanical alloying method. The SiBCN/HfC ceramic composites were consolidated by spark plasma sintering (SPS). The influence of high temperature heat treatment on the microstructural evolution and phase composition was investigated. The results showed that the oxygen was introduced during the mechanical alloying process, leading to the oxidation of BN to form B₂O_3. HfO₂ was formed in the sintering process result from HfC oxydation and reduced to HfB₂ by carbothermal reduction reaction after heat-treatment at 1600 ℃ for 1 h. Both HfO₂ and HfC were reduced to HfB₂ during the heat-treatment at 1650 ℃ and 1800 ℃ by reaction of HfC + C + B₂O₃ → HfB₂ + CO. Introduction of oxygen causes phase transformation of the SiBCN/HfC ceramic composites after heat treatment at high temperatures, during which ceramic matrix becomes loose and porous due to volatilization of the gaseous byproducts. Therefore, control of oxygen content is the key to the real applications of SiBCN/HfC ceramic composites at high temperatures.

Key words: SiBCN, HfC, HfB₂, high temperature heat treatment

CLC Number:

TQ174

WEI Yuquan,YANG Yong,LIU Meng,LI Qile,HUANG Zhengren. Effect of High Temperature Heat Treatment on Phase Composition and Microstructure of SiBCN/HfC Ceramic Composites[J]. Journal of Inorganic Materials, 2020, 35(8): 931-938.

Figures/Tables 10

Fig. 1 XRD patterns of raw powders and milled powders

Fig. 2 Microstructure and EDS element mapping of the amorphous SiBCN powders (a) TEM image with inset showing selected area electron diffraction; (b) HRTEM image; (c) EDS spot analysis of (a); (d) SEM image and (e) EDS elemental maps of (d)

Fig. 3 XPS spectra of amorphous SiBCN powders

Fig. 4 EFTEM images (a) and EELS spectra (b) of the amorphous SiBCN powders

Fig. 5 XRD patterns of the SiBCN/HfC ceramic composites before and after heat-treatement for 1 h at different temperatures

Fig. 6 Microstructure (a) and EDS element overlap mapping (b) of the as-sintered SiBCN/HfC ceramic composites, and its corresponding element distribution displayed in lower two rows

Fig. 8 Change of standard Gibbs free energies of reactions (1)-(9) as a function of temperature

Table 1 Change of standard Gibbs free energies of reactions (6-9) at different heat treatment temperature

Reaction	ΔG_{1300 ℃}/(kJ·mol^-1)	ΔG_{1400 ℃}/(kJ·mol^-1)	ΔG_{1500 ℃}/(kJ·mol^-1)	ΔG_{1600 ℃}/(kJ·mol^-1)	ΔG_{1650 ℃}/(kJ·mol^-1)	ΔG_{1800 ℃}/(kJ·mol^-1)
HfO₂ + B₂O₃ + 5C = HfB₂ + 5CO(g)	169.8	91.0	12.6	-65.3	-104.1	-210.0
HfO₂ + 2BN + 2C = HfB₂ + 2CO(g) + N₂(g)	246.5	196.6	146.9	97.4	72.8	-0.7
HfO₂ + 3C = HfC + 2CO(g)	115.6	82.1	48.8	15.7	-0.8	-50.1
HfC + 2C + B₂O₃ = HfB₂ + 3CO(g)	54.1	8.8	-36.2	-81.0	-103.3	-169.9

Fig. 9 Surface morphologies and corresponding EDS results of the SiBCN/HfC ceramic composites before and after heat treatment at different temperatures (a) As sintered; (b)1400 ℃; (c)1600 ℃; (d)1800 ℃

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