Effect of heat treatment on the roughness and mechanical properties of dental lithium disilicate glass-ceramics


In dental clinics, it is common to perform small fitting adjustments in dentures using a micro-grinding tool after testing them in the patient's mouth. This procedure increases local roughness and can lead to formation of microcracks on the prosthesis surface. This study aimed to investigate the benefits of a post-finishing heat treatment to surface roughness and crack healing and its effect on the flexural strength of lithium disilicate (LD) dental glass-ceramics. Commercially available lithium metasilicate, Li2SiO3, samples were heat treated at 840 degrees C for 7 min to induce the phase transformation into LD, Li2Si2O5. The LD samples were characterized by X-ray Diffraction, Scanning Electron Microscopy, Vickers hardness, Young's modulus, and fracture toughness. One of the surfaces of the LD samples was sanded aiming to simulate the denture fitting adjustments performed in the dentist's laboratory, generating a rough surface, Group 1. Half of the LD samples had their biaxial flexural strength evaluated by the piston-on-three-ball test (P-3B) and the other half were submitted to a second short-term heat treatment (840 degrees C - 5 min), Group 2, and later assessed by the P-3B. Roughness parameters in both groups were measured by 3D optical profilometry. After the crystallization heat treatment, formation of elongated LD crystals, Li2Si2O5, 35% amorphous phase, and residual Li3PO4 was observed. In addition, the following mechanical property values were obtained: Vickers hardness = 5.8 +/- 0.1 GPa, fracture toughness = 2.2 +/- 0.1 MPa m(1/2), and Young's modulus = 100.3 +/- 0.3 GPa. The samples in Group 1 showed bending strength of 206 +/- 30 MPa and the following roughness parameters: Ra = 0.45 +/- 0.16 mu m, Rz = 22.7 +/- 6.7 mu m, and PV = 27.7 +/- 7.1 mu m. In the samples in Group 2, the Ra, Rz and PV roughness parameters were 0.31 +/- 0.12 mu m, 5.2 +/- 2.5 mu m, and 9.2 +/- 4.7 mu m, respectively. With this decrease in roughness, the bending strength increased by 62%, with a mean value of 331 +/- 59 MPa. In the need for machine finishing of LD-based glass-ceramic dental prostheses, the use of a second short-term heat treatment at 840 degrees C for 5 min generates considerable gains in bending strength, increasing the lifecycle of the prosthesis as a result of reduced surface roughness caused by softening of the remaining amorphous phase in the glass-ceramic. These conditions can be adapted to each chemical and crystallographic composition of the glass-ceramic under study.




Materials Science


Alves, MFRP; Simba, BG; Fernandes, MHFV; Elias, CN; Amarante, JEV; dos Santos, C

nossos autores


The authors would like to thank the LABNANO/CBPF for technical support during the electron microscopy analysis. We are also grateful to Brazilian agencies FAPERJ (grants E-26-201.476/2014 and E-26-202.997/2017) and CNPq (grant 311.119/2017-4) for financial support received. Part of this work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020 & UIDP/50011/2020, financed by national funds through the FCT/MEC and, when appropriate, co-financed by FEDER under the PT2020 Partnership Agreement.

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