FENG Bocong, GAO Yuzong, TANG Cong, ZHAO Zhe, ZENG Zibin, LYU Donglin
(School of Electromechanical Engineering, Guangdong University of Technology, Guangzhou 510006, Guangdong, China)
Extended abstract:[Background and purposes] Si3N4 ceramics are widely regarded as one of the most promising advanced engineering ceramic materials, due to their excellent high-temperature mechanical properties, outstanding thermal stability and remarkable thermal shock resistance, with extensive applications in various fields, including electronic power devices, ceramic cutting tools, biomedical implants and aerospace components. However, conventional machining methods often suffer from low efficiency, long production cycles, high costs and difficulties in ensuring precision, when fabricating complex-shaped Si3N4 components, leading to the emergence of additive manufacturing technologies as a viable alternative. Among these, digital light processing (DLP) has attracted widespread attention for its high forming efficiency, superior dimensional accuracy and excellent surface quality, yet the inherent gray color of Si3N4 powder, coupled with its high light absorption and significant refractive index, severely restricts the curing accuracy and penetration depth during photopolymerization, a challenge that has prompted various surface modification strategies, which nevertheless remain relatively scarce in terms of systematically and simultaneously improving the rheological behavior and photopolymerization characteristics of Si3N4 slurries, along with the mechanical performance of sintered bodies. Therefore, this study was motivated to employ low-melting-point glass powder as a coating material for Si3N4 particles, combined with ball milling and sintering processes for surface modification, while utilizing DLP as the shaping technique to systematically investigate the comprehensive effects of this coating on the rheological properties, curing behavior, microstructure and mechanical and thermal properties of the resulting ceramics.[Methods] Si3N4 powder was used as the raw material coated with low-melting-point glass powder, employing MgO and Y2O3 as sintering aids, while the photosensitive resin system consisted of OPPEA and HDDA with Irgacure 819 as the photoinitiator, KMT-3331 and BYK-110 as dispersants and DOP as the plasticizer. The Si3N4 powder was blended with 0–5 wt.% glass powder and sintering aids, then ball-milled, dried, sieved and heat-treated at 700 ℃ to form a uniform coating. The coated powder was mixed with resin and additives through vacuum homogenization to prepare photosensitive slurry, which was layer-wise cured via DLP at 30 μm per layer to obtain green bodies. The samples were subsequently vacuum-debinded at 500 ℃ for 3 h and air-debinded for 4 h, followed by pressureless sintering at 1850 ℃ for 8 h. The slurry's rheology was characterized with a rotational rheometer (MCR301), while curing parameters (Ew, Sw, Ed, Sd) were determined by using single-layer curing tests based on quasi-Beer-Lambert and Beer-Lambert semi-logarithmic models. Phase identification was performed by using XRD, microstructure was examined by using SEM and bending strength of the sintered ceramics was evaluated via three-point bending, along with measurement of Vickers hardness (10 kg, 10 s) and Archimedes density.[Results] As the content of low-melting-point glass powder for coating was increased from 0 wt.% to 5 wt.%, the viscosity of the Si3N4 slurry decreased from 0.594 Pa·s to 0.477 Pa·s, indicating effective improvement in rheological properties, while the curing behavior showed systematic changes with increasing depth sensitivity coefficient (Sd) but decreasing critical energy density (Ed), along with reduced width sensitivity coefficient (Sw) yet enhanced critical energy density (Ew) and curing width. XRD analysis results confirmed the negligible influence of the glass powder on phase composition of the final ceramics, due to decomposition at 1850 ℃ during sintering and low Al2O3 content (27.87 wt.%). SEM results revealed high densification and reduction in average grain size from 2.61 μm to 2.29 μm (12.26%), consistent with liquid-phase sintering. The flexural strength increased from (679.5±46.6) MPa to (776.5±54.7) MPa (14.3% improvement), due to grain refinement, while Vickers hardness decreased from (15.90±0.85) GPa to (15.300±0.265) GPa and fracture toughness displayed an initial rise followed by a decline, reflecting the critical effect of glass phase content.[Conclusions] To address the challenge of conventional surface modification methods in synergistically optimizing both the Si3N4 slurry properties and the mechanical performance of the sintered bodies, surface coating modification of Si3N4 powder was implemented by precisely controlling the content of the low-melting-point glass powder, combined with ball milling and sintering processes. This treatment significantly reduced the viscosity of the Si3N4 slurry and improved its curing performance. During sintering, an appropriate amount of glass powder partially volatilized without adversely affecting composition or densification of the ceramics. Furthermore, the glass phase formed during sintering effectively refined the grains, resulting in a maximum increase of 14.3% in flexural strength and 31.1% in fracture toughness for the Si3N4 ceramics, thereby achieving synergistic optimization of printing processability and mechanical properties.
Key words: Si3N4 powder; surface coating modification; digital light processing (DLP); low-melting-point glass powder