Introduction Yttrium aluminum silicate (YAS) glass ceramics, known for their high-temperature resistance, excellent hardness,and mechanical strength, attract wide attention as coating materials for bonding or protecting silicon carbide ceramics. The precipitation of the y-Y2Si2O7 crystalline phase in YAS glass, particularly in comparison with SiC, is advantageous due to the matched coefficients of thermal expansion between y-Y2Si2O7 and SiC. This often leads to its use as a sintering aid or in anti-oxidation coatings for SiC. A glass composition with reported proportions of 22Y2O3-19Al2O3-59SiO2 (in mole) has been designed to selectively precipitate only the y-Y2Si2O7 crystalline phase through process engineering, achieving a softening temperature exceeding 1 400 ℃. However, this glass composition initially undergoes a droplet-type phase separation, resulting in the precipitation of y-Y2Si2O7 crystals in regions enriched with Y2O3 and SiO2. The large gap between these two phase-separated regions leads to non-uniform distribution of y-Y2Si2O7 grain sizes. The cross-sectional morphology exhibits a fibrous structure, which may affect the mechanical properties of the glass ceramics. Compared to fibrous structures, spherical grains have a higher stress distribution, providing the glass ceramics with greater strength. Previous studies have shown that TiO2 and CaF2 are unsuitable as nucleating agents for achieving uniform and fine grains in YAS glass. In this study, a base glass has a composition of 22Y2O3·19Al2O3·59SiO2., The addition of ZnO into the base glass was found to inhibit large-sized phase separation in YAS glass, resulting in a significant refinement of precipitated grain sizes. This led to a substantial improvement in the fracture toughness of the glass ceramics. By observing the crack propagation patterns in the glass ceramics, the toughening mechanism has been elucidated.
Methods On the basis of the glass composition 22Y2O3·19Al2O3·59SiO2, a series of glasses with varying compositions,2xZnO·22Y2O3·(19–x)Al2O3·59SiO2 (2x=0, 3, 6, 12, 18, in mole), were obtained by gradually replacing Al2O3 with ZnO at a ZnO:Al2O3 ratio of 2:1. The glasses were subjected to a nucleation temperature (Tg+30 ℃) heat treatment for 3 h, followed by ramping to the crystallization peak temperature (Tc) and holding for 2 h to obtain glass ceramics for testing. Characteristic temperatures of the glasses were determined using DSC at a heating rate of 10 K/min. The glass ceramics, after fine polishing, were etched for 60 s in a 4% HF solution (volume fraction) and then subjected to SEM testing. Vickers hardness of the samples was measured under a 2.94 N load. Fracture toughness (KIc) of the samples was determined under the ASTM C-1421-01b standard
Results and discussion As the amount of ZnO gradually increases, the glass transition temperature (Tg) and Tc of the glass decrease progressively. The addition of ZnO reduces both Tg and Tc of the YAS glasses, which is advantageous for lowering the sealing temperature when used in conjunction with SiC ceramics. XRD patterns, SEM images, and EDS patterns confirm that the mechanism for the precipitation of y-Y2Si2O7 in 22Y2O3·19Al2O3·59SiO2 glass belongs to crystallization through phase separation. After the addition of ZnO to YAS glass, nearly spherical grains are precipitated, and the distribution of grains becomes more uniform. With increasing ZnO content, the grain size gradually decreases. The addition of ZnO has a minor impact on the hardness and density of both YAS glass and glass-ceramics. However, a significant improvement is observed in the fracture toughness of the glass ceramics,showing a 65% increase for the glass ceramic containing 18% ZnO compared to the YAS glass ceramic. In glass ceramics formed through phase separation and grain growth, both the size and distribution of grains are uneven. Cracks propagate in a straight-line manner, resulting in longer crack lengths under the same load. In contrast, when cracks propagate in glass ceramics with similar and evenly distributed grain sizes, numerous circumferential cracks around grains occur, leading to increased energy consumption during the crack propagation process and resulting in relatively shorter crack propagation distances.
Conclusions As ZnO gradually replaces Al2O3, both the Tg and Tc of YAS glasses decrease. This leads to reduce the sealing temperature when used as sealing materials. With the addition of ZnO, the crystal phases deposited from the YAS glasses transitions from only the y-Y2Si2O7 grains to two phases, i.e., ZnAl2O4 and y-Y2Si2O7. The content of precipitated ZnAl2O4 phase increases with the gradual introduction of ZnO. Although no significant increase in hardness was observed following the compositional change from 0 to 18% of ZnO (in mole), a distinct enhancement of about 1 GPa in hardness after crystallization occurs for the glass with ZnO.Despite a decrease in KIc of YAS glass from 0.82 MPa·m0.5 to 0.71 MPa·m0.5 with the addition of ZnO, the prepared glass ceramics with 18% ZnO (in mole) shows a significant enhancement of KIc, reaching a maximum value of 1.92 MPa·m0.5. The microscopic mechanisms of this enhancement was then revealed by further investigation using SEM.