Thick SiC Green Bodies: Degreasing Analysis and Pressureless High Density Sintering

doi:10.15541/jim20230126

Abstract

Abstract:

SiC ceramics with high thickness and high density are highly advantageous for armor protection, but it is difficult to produce bulk SiC ceramics with thicknesses more than 100 mm. This work was concentrated on the problems of easy cracking and non-densification in the thick SiC ceramic sintering, and degreasing products as well as pressure-incomplete degreasing process were investigated. In order to degrease thick ceramics, phenolic resin’s pyrolysis residue was examined using TG-MS. Formaldehyde and other small-molecule byproducts of breakdown were quickly eliminated, but macromolecule byproducts like dimethylphenol were easily retained in the core, leading to the noncompact sintering of thick ceramics. After degreasing optimization, the blank’s surface-core density is almost constant, ranging from 1.81 to 1.84 g/cm³. In contrast, the test group shows no cracking or deformation after sintering at 2150 ℃, and both surface and core of the large thickness ceramics obtain a density up to 3.14 g/cm³ with similar microstructure. The fracture strength of the core is (411±84) MPa, while the fracture strength of the surface is (433±48) MPa. According to the investigation, the ceramic core’s insufficient degreasing is the primary reason for the cracking and poor density.

Key words: large thickness, SiC ceramics, degreasing effect analysis, insufficient degreasing

CLC Number:

TQ174

HUANG Yihua, HUANG Zhengren, SHA Wenhao, ZHOU Yabin, TAN Zhouxi, ZHANG Mingkang. Thick SiC Green Bodies: Degreasing Analysis and Pressureless High Density Sintering[J]. Journal of Inorganic Materials, 2023, 38(10): 1163-1168.

Figures/Tables 9

Fig. 1 Schematic diagram of sampling of large thickness SiC bulk ceramics (Unit: mm)

Table 1 Main pyrolysis products of phenolic resin and their molecular weight

Pyrolysis product	Molecular weight	Result	Pyrolysis product	Molecular weight	Result
Methane	16	Yes	Naphthalene	128	Yes
Acetylene	26	No	Naphthalene trimethylphenol	136	Yes
Carbon monoxide	28	Yes	Methylnaphthalene	142	No
Formaldehyde	30	Yes	Diphenylmethane	168	No
Carbon dioxide	44	Yes	Dibenzofuran	168	No
Benzene	78	Yes	Anthracene	178	No
Toluene	92	Yes	Methyldiphenyl furan	182	No
Phenol	94	Yes	Xanthine	182	No
Xylene	106	Yes	Methylxanthine	196	No
Cresol	108	Yes	Dimethyl xanthine	210	No
Mesitylene	120	Yes	Trimethylxanthine	224	No
Dimethyl phenol	122	Yes

Fig. 2 Ion current intensity of silicon carbide powder degreasing products tested by TG-MS

Fig. 3 Weight loss before and after (incomplete) degreasing of surface (a) and core (b) of large thickness SiC bulk

Fig. 4 Changes of formaldehyde (a) and benzene (b) ion current intensity before and after incomplete degreasing Colorful figures are available on website

Fig. 5 Changes of ion current intensity of dimethyl phenol before and after degreasing and incomplete degreasing Colorful figures are available on website

Table 2 Densities before and after degreasing of different parts of large thickness block bulk

Position	Density after degreasing/(g·cm^-3)	Density before degreasing/(g·cm^-3)
A1	1.84	1.92
A2	1.81	1.86
C3	1.81	1.89
E4	1.83	1.86

Fig. 6 Photo of large thickness bulk SiC ceramics

Fig. 7 Microstructures of surface (a) and core (b) of large thickness SiC ceramics

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