Graphite-loading-dependent porosity and mechanical performance of LDM-printed kaolin ceramics

Controlling processing-induced defects while maintaining mechanical integrity remains a central challenge in extrusion-based ceramic additive manufacturing. This study examined the effect of graphite addition on the mechanical and microstructural response of kaolin ceramics fabricated by Liquid Deposition Modeling (LDM). Seven formulations, designated K0 and KG1-KG10 and corresponding to graphite fractions of 0-33.33 wt.% on a dry-solid basis, were printed, dried, sintered at 1200 C, and characterized by compression and three-point bending tests, microscopy, SEM/EDS, FTIR, XRD, and simultaneous TGA/DTG/DTA. KG3 produced the highest mean compressive strength (60.69 MPa) and flexural strength (2.39 MPa). Its compressive strength was 128.8% higher than that of K0. Games-Howell comparisons placed KG1, KG2, KG3, and KG4 in a single highest-strength group, indicating that the compressive benefit is sustained across a composition window rather than confined to a single formulation. In bending, the 8.2% increase for KG3 relative to K0 was not statistically significant. At higher graphite additions, both properties decreased, accompanied by graphite depletion and increasingly pronounced pore-related defects after sintering. Porosity determined from the ratio of measured to theoretical density increased from 11.02% for KG3 to 21.06% for KG10, over the same interval in which the compressive strength fell by 67.4%. XRD and FTIR indicated the transformation of the kaolin matrix toward mullite-containing aluminosilicate phases. The results show that limited graphite addition can improve compressive performance, whereas excessive addition promotes defect-controlled degradation. The measured flexural response limits these ceramics to non-load-bearing porous or thermally stable components unless further improvements in defect control and bending resistance are achieved.