The effects of repeated heat-pressing on the mechanical properties and microstructure of IPS e.max Press.

Abstract

We evaluated the effects of repeated heat-pressing on the mechanical properties and microstructure of IPS e.max Press. A total of 20 specimens were fabricated including 10 heat-pressed once and another 10 heat-pressed twice. The density, porosity, and surface roughness (Ra) were evaluated. Three-point flexural strength following the ISO 6872 and Vickers hardness were measured, and fracture toughness (KIC) was calculated. Specimens were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Compared to a single heat-press treatment, the density decreased and porosity increased after two heat-pressing events. A significant difference in density was observed. The flexural strength, Vickers hardness and fracture toughness significantly decreased after two heat-pressing events. The XRD patterns show that the intensity of the crystalline phase better corresponds to lithium disilicate (Li2Si2O5) after two heat-presses than only one. The SEM images detailed the interlocking microstructure of rod-shaped Li2Si2O5 crystals after one heat-press. These became oriented after two heat-presses, and the crystal size became larger. This study showed that repeated heat-pressing was detrimental to the density, porosity, strength, hardness, and toughness of IPS e.max Press.