LUO Zihao ¹, XIAO Zhuohao ¹, LI Dong ², GU Zonghui ², GUO Yan ², WEN Fuhua ², FU Zhen ³

(1. Jingdezhen Ceramic University, Jingdezhen 333403, Jiangxi, China; 2. Shandong Yixin Optoelectronics Technology Co., Ltd., Linyi 266107, Shandong, China; 3. Shandong Jingyao Glass Group Co., Ltd., Linyi 276624, Shandong, China)

Extended abstract:

[Significance] Lithium disilicate glass-ceramics (LDGCs) based on the Li₂O-SiO₂ system have emerged as a research hotspot in fields, such as dental restoration and electronic device packaging, owing to their excellent mechanical properties (flexural strength exceeding 400 MPa, fracture toughness of over 3.0 MPa·m^1/2), high chemical stability and biocompatibility. In dental restoration, 70% of the needle-like or rod-like lithium disilicate (LS2) crystals in LDGCs ensure a highly interlocked microstructure, which not only matches the translucency of natural teeth but also meets the mechanical strength for mastication. In electronic packaging, their low dielectric constant, extremely low dielectric loss and ability to withstand high temperatures up to 800 ℃ make them an ideal choice for insulating substrates of high-frequency electronic devices. The effects of chemical composition and heat treatment processes on the microstructure and mechanical properties of LDGCs have been widely studied. However, the influences of additives and heat treatment processes on the microstructure and mechanical properties of LDGCs have rarely summarized. This review is aimed to comprehensively review the progress of Li₂O-SiO₂ system glass-ceramics, focusing on the effects of additives (such as ZrO₂, P₂O₅, Al₂O₃, K₂O, MgO) and heat treatment parameters (heat treatment stages, heat treatment temperature, holding time, heating/cooling rate) on their microstructure and mechanical properties. It is to analyze how different additives and heat treatment processes affect the phase composition, crystal morphology, size and distribution, thereby influencing mechanical properties, such as hardness, flexural strength and fracture toughness. Finally, the existing problems in current research are pointed out, while future development directions are prospected.

[Progress] The effects of two factors, type of additives and heat treatment processes, on properties and performances of LDGCs have been systematically elaborated. ZrO₂ acts as a network former, enhancing the degree of glass network polymerization by forming Si-O-Zr bonds, which synchronously increases the glass transition temperature and crystallization temperature. When the ZrO₂ content ranges from 0.5 to 3 mol%, bulk nucleation dominates and the crystal size increases with increasing content. When the content reaches 5 mol%, it switches to surface nucleation, forming a crystalline layer of about 20 μm only in the near-surface region. The optimal content is 15 wt.%, which induces LS2 crystals to grow in a rod-like shape with a fracture toughness of (3.5±0.3) MPa·m^1/2. Excessive content (30 wt.%) leads to performance degradation, due to the agglomeration and increased porosity. As a typical nucleating agent, the presence of P₂O₅ results in the formation of amorphous Li₃PO₄ as nucleation sites by inducing phase separation. When the addition content is 1.0 mol%, it promotes the formation of dense interlocked structures with high aspect ratio rod-like LS2 crystals, resulting in a flexural strength exceeding 300 MPa. Excessive addition (≥2 mol%) causes oversaturated nucleation, transforming the crystals into a spherical shape and weakening the interlocking effect.Al₂O₃ enters the glass network by substituting [SiO₄] with [AlO₄] tetrahedra. With 0.5–1.0 mol% Al₂O₃, the degree of crystal interlocking is enhanced, with a flexural strength of 300 MPa. As the content of Al₂O₃ reaches 3–6 mol%, glass viscosity is increased, leading to crystal spheroidization and pore formation. Accordingly, the flexural strength is reduced to 135 MPa, but the fracture toughness is increased to 1.74 MPa·m^1/2, due to the increased crystal size. By adding 4.5 mol% K₂O, the glass viscosity is reduced, promoting the crystallization pathway from co-precipitation of LS (lithium metasilicate) and LS2 to the precipitation of LS first and then to LS2, forming long needle-like crystals with a length of about 3 μm. The mixture of multiple alkali metal ions (K⁺/Rb⁺/Cs⁺) can be used to further enhance formation of the interlocked structure, achieving a flexural strength of (271±14) MPa. MgO affects the glass network by regulating the coordination state of Mg²⁺. When the MgO content is ≤1.4 mol%, [MgO₄] enhances network stability. With 2.00–2.74 mol%, [MgO₆] is formed, which promotes phase separation. When the MgO content is 2.74 mol%, a dense interleaved lamellar structure is formed, with a flexural strength of (398±27) MPa. Synergistic doping with Al₂O₃ and ZrO₂ incudes the precipitation of MgAl₂Si₄O₁₂, enabling the flexural strength to exceed 562±107 MPa. Regarding heat treatment processes, synchrotron HT-XRD technology has revised the crystallization mechanism of LDGCs, with the presence of trace LS phases that cannot be identified by traditional XRD, clarifying the co-nucleation growth law of LS and LS2. Two-stage heat treatment (low-temperature nucleation+high-temperature crystallization) is the mainstream process, while three-stage heat treatment is optimal for the crystal aspect ratio through an intermediate holding stage, further increasing the flexural strength. The optimal temperature for crystal growth is in the range of 755–843 ℃, within which LS2 crystals evolve from equiaxed to rod-like shapes. When the temperature exceeds 860 ℃, melting and coarsening occur. An appropriate holding time (90 min of holding in the nucleation stage) ensures the complete transformation of LS to LS2, forming a highly interlocked structure. Excessively long holding time leads to grain coarsening and increase in porosity. Quenching treatment can induce residual compressive stress on the surface, increasing the flexural strength from 287 MPa to 576 MPa. Secondary short-term heat treatment (840 ℃/5 min) is helpful to repair surface defects, increasing the flexural strength to (331±59) MPa.

[Conclusions and prospects] The effects of additives and heat treatment processes on the microstructure and mechanical properties of LDGCs have been reviewed. Although significant progress in LDGCs have been made, they face challenges, such as insufficient information on multi-additive co-doping, lack of in-situ monitoring of high-temperature phase transformation and great difficulty in regulating the directional growth of crystals. Future research should be focused on: (1) systematically exploring the synergistic enhancement effect of multi-additives and clarifying the optimal ratio, (2) introducing artificial intelligence-driven research to change the traditional model, (3) using in-situ characterization technologies such as synchrotron HT-XRD to determine the nucleation saturation point and the optimal temperature for rapid crystal growth, (4) realizing the directional preferential growth of LS2 crystals and constructing needle-like interlocked structures with a larger aspect ratio and (5) combining quenching and chemical ion exchange technologies to build a multi-prestress enhancement mechanism, further expanding the application scenarios of LDGCs in extreme environments.

Key words: Li₂O-SiO₂ system glass-ceramics; oxides; heat treatment processes; microstructure; mechanical properties