Crystallization toughening of a model glass-ceramic

Lithium disilicates are commercially important and among the most widely studied and well-characterized glass-ceramics, but their toughening mechanism - and those of most glass-ceramics - is still unknown. In this work, stoichiometric lithium disilicate glasses were used as a model system and crystallized using carefully designed and controlled two-stage heat treatments to give different crystallized volume fractions while maintaining a constant grain size of approximately 12 μm. This original procedure allowed us to separately control the effects of these two microstructural parameters on the mechanical properties of the glass-ceramic. The hardness, elastic modulus, four-point bending strength, indentation fracture toughness and double-torsion fracture toughness were measured for samples with different crystallized volume fractions, ranging from the parent glass to fully crystallized samples. We found that the bending strength increases rapidly with crystallization at crystal volume fractions below 10% and reaches a value 2.5 times greater than that of the original glass. For a fully crystallized sample, the fracture toughness increases almost fivefold, from 0.75 to 3.5 MPa.m1/2. Laser confocal microscopy was used to reveal the topography of the fracture surfaces. Three mechanisms that contribute to toughening - crack deflection, crack bowing and trapping, and crack bridging - were evaluated. A model incorporating the elastic modulus, crystal fracture toughness and crystallized volume fraction was proposed and successfully tested to explain the increased fracture toughness with crystallized volume fraction for the full range of crystallization in LS2 glass-ceramics.