Quantitative characterization of defects in stereolithographic additive manufactured ceramic using X-ray computed tomography

doi:10.1016/j.jmst.2021.11.060

Abstract

Abstract:

Ceramics have been widely fabricated by additive manufacturing (AM). Compared to conventional technologies, the strength of additive manufactured ceramic is relatively low owing to the formation of defects during manufacturing process. These defects have significant effects on the microstructure and mechanical properties of additive manufactured ceramics. However, systematic research on defects, including defect geometrical features, quantitative statistics, and formation mechanism, as well as the intrinsic relationship with mechanical properties, need to be studied in depth. In this work, Al₂O₃ ceramics were prepared from photosensitive slurries with different solid loadings by using stereolithographic (SL) additive manufacturing. The defects, including their sizes and distributions, in both green and sintered bodies were investigated by using scanning electron microscopy (SEM) and X-ray computed tomography (X-CT). Geometrical features and quantitative statistics of the defects were evaluated and discussed to reveal their formation mechanism. Moreover, the intrinsic relationship between defects and mechanical properties of the additive manufactured ceramic was revealed. This study can give some fundamental understanding of the defects in additive manufactured ceramics.

Key words: Ceramic, Stereolithographic additive manufacturing, Defects, X-CT, Strength

Keqiang Zhang, Qiaoyu Meng, Xueqin Zhang, Zhaoliang Qu, Rujie He. Quantitative characterization of defects in stereolithographic additive manufactured ceramic using X-ray computed tomography[J]. J. Mater. Sci. Technol., 2022, 118: 144-157.

Figures/Tables 22

Fig. 1. (a) Main components of X-CT, (b) schematic drawing of X-CT and 3D reconstruction.

Table 1. X-CT scanner parameter settings.

Scanner parameter settings	Acceleration voltage (kV)	Target powder (W)	Exposure time (s)	Number of projectors	Voxel size (μm³)
Value	60	5	1	2401	2.4 × 2.4 × 2.4

Fig. 2. SEM images of the fractured cross-sections of Al2O3 green cylinders: (a) A45, (b) A50, (c) A55, (d) A60, (e) A65.

Fig. 3. SEM images of the fractured cross-sections of Al2O3 sintered cylinders: (a) A45, (b) A50, (c) A55, (d) A60, (e) A65.

Fig. 4. Exemplar X-CT images of Al2O3 green cylinders with artifacts and noise: (a1) A45, (b1) A50, (c1) A55, (d1) A60, (e1) A65. High contrast voids and noisy voxels are segmented: (a2) A45, (b2) A50, (c2) A55, (d2) A60, (e2) A65.

Fig. 5. (a) 3D model. X-CT reconstructed images of Al2O3 green cylinders: (b) A45, (c) A50, (d) A55, (e) A60, (f) A65.

Fig. 6. Exemplar X-CT images of Al2O3 sintered cylinders with cupping artifacts and noise obscuring small voids: (a1) A45, (b1) A50, (c1) A55, (d1) A60, (e1) A65. High contrast voids and noisy voxels are segmented: (a2) A45, (b2) A50, (c2) A55, (d2) A60, (e2) A65.

Fig. 7. 3D X-CT reconstructed images of Al2O3 sintered cylinders: (a) A45, (b) A50, (c) A55, (d) A60, (e) A65.

Fig. 8. Correlation between experimental flexural strength and X-CT derived strength.

Fig. 9. 3D X-CT reconstructions showing defect distributions in Al2O3 green cylinders: (a) A45, (b) A50, (c) A55, (d) A60, (e) A65.

Fig. 10. 3D X-CT reconstructions showing defect distributions in Al2O3 sintered cylinders: (a) A45; (b) A50; (c) A55; (d) A60; (e) A65 (surface colors represent the defects volume).

Table 2. Summary of major defect types observed in both SL additive manufactured Al2O3 green and sintered bodies.

Defect types	Features		Causes
Defect types	Green body	Sintered body	Green body	Sintered body
Pores	Entrapped gas, voids	Entrapped gas	Incomplete degassing, high viscosity	Insufficient energy, volatilization of organic matter
Delaminations (or poor inter-layer bonding)	Not found	Improper bonding between successive tracks or layers	Not found	Residual stresses, over-cured
Surface roughness	Layer-by-layer microstructure, ridge-like, staircase	Layer-by-layer microstructure on surface, ridge-like, staircase	Layer by layer deposition, light scattering, orientation of the surface, etc.	Layer by layer manufacturing strategy, volatilization of organic matter, orientation of the surface, etc.

Fig. 11. (a) Pore volume rate of Al2O3 green cylinders vs. solid loading. Pore size distributions of Al2O3 green cylinders: (b) A45, (c) A50, (d) A55, (e) A60, (f) A65.

Fig. 12. Calculated sphericity coefficient of pore distribution for Al2O3 green bodies: (a) A45, (b) A50, (c) A55, (d) A60, (e) A65.

Fig. 13. Pore size vs. calculated sphericity coefficient of pores in Al2O3 green bodies: (a) A45, (b) A50, (c) A55, (d) A60, (e) A65 (Zooms showing typical 3D visualization of pore shape).

Fig. 14. (a1, a2) Defect volume rate of Al2O3 sintered cylinders. Pore size distributions of Al2O3 sintered cylinders: (b1) A45; (b2) A50; (b3) A55. Delamination size distributions of Al2O3 sintered cylinders: (c1) A45; (c2) A50; (c3) A55.

Fig. 15. Calculated sphericity coefficient of pore distributions of Al2O3 sintered samples: (a) A45, (b) A50, (c) A55.

Fig. 16. Pore size vs. calculated sphericity coefficient of pores in Al2O3 sintered cylinders: (a) A45, (b) A50, (c) A55.

Fig. 17. (a) Volume rate of defects in green and sintered bodies, (b) zoom-in view of the volume rate.

Table 3. Statistical data from quantification of pores detected in Al2O3 green and sintered bodies.

Solid loading (vol.%)	Number
	10³-10⁴ (μm³)		10⁴-10⁵ (μm³)
	Green body	Sintered body	Green body	Sintered body
45	84	662	0	21
50	49	88	0	9
55	24	5742	6	4

Fig. 18. Illustration of pore evaluation in debinding and sintering.

Fig. 19. X-CT reconstructions showing delaminations and typical 3D visualization of delamination shapes in Al2O3 sintered cylinders: (a) A45; (b) A50; (c) A55.

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