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Lithium disilicate glass-ceramics show great potential in the fabrication of anisotropic structures due to their high-strength needle-form crystalline phase subjective to processing-induced alignment. Here we explore this strategy by focusing on the mechanistic aspects of fracture. Through different variations of the fracture toughness test we demonstrate the effect of bulk crystal orientation on fracture energy, toughening mechanisms, crystal size and aspect ratio. Using the anisotropic Poisson's ratio of the Li2Si2O5 crystal phase obtained by Resonant Ultrasound Spectroscopy, we apply the new geometry factor developed by Strobl et al. to provide a more accurate insight into the anisotropic crack propagation behavior in LS2 glass-ceramics. Raman spectra and X-Ray Diffraction patterns are resolved for the Li2Si2O5 phase aligned at plane parallel, anti-plane and random orientations. We show that Li2Si2O5 crystallites oriented perpendicular to the crack growth plane tended to induce large scale crack deflection unless the crack was forced into a straight path, thereby promoting crystallite fracture, increased KIc-values and accentuated R-curve behavior. Crystal aspect ratio and residual stresses in the glass have been identified as important influencing factors on crack growth behavior and toughening mechanisms.

Processing-induced alignment of Li2Si2O5 crystals in biomedical lithium disilicate glass-ceramics have been shown to increase the monotonic resistance to fracture and better simulate the mechanical behavior of natural dental tissues. The (mis)orientation of the [001] direction in the Li2Si2O5 crystal phase relative to the crack growth plane was induced by injection-molding of a high aspect ratio composition in order to determine the effect of crystal alignment. The role of particulate geometry was assessed by comparing two compositions having randomly oriented Li2Si2O5 crystals of similar phase fraction but dissimilar crystal aspect ratios. Static and cyclic fatigue experiments in varying frequencies and R-ratios were conducted to clarify the degradation of crack bridging mechanisms induced by the crystal phase during repetitive opening and closure of the crack faces. Both crystal alignment and aspect ratio showed to play important roles in affecting the subcritical crack growth resistance the lifetime of lithium disilicate biomedical components.