Descriptions of crack growth behaviors in glass-ZrO₂ bilayers under thermal residual stresses

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• 作者：Renan Belli^a ; ^{rbelli@dent.uni-erlangen.de" class="auth_mail" title="E-mail the corresponding author} ; Michael Wendler^a ; ^b ; José ; I. Zorzin^a ; Anselm Petschelt^a ; Carina B. Tanaka^c ; Josete Meira^c ; Ulrich Lohbauer^a
• 关键词：Thermal incompatibility ; Residual stresses ; Slow crack growth ; Crack shielding ; Dental zirconia ; Cooling rate ; Birefringence
• 刊名：Dental Materials
• 出版年：2016
• 出版时间：September 2016
• 年：2016
• 卷：32
• 期：9
• 页码：1165-1176
• 全文大小：5748 K

文摘

This study was intended to separate residual stresses arising from the mismatch in coefficients of thermal expansion between glass and zirconia (ZrO₂) from those stresses arising solely from the cooling process. Slow crack growth experimentes were undertaken to demonstrate how cracks grow in different residual stress fields.

Methods

Aluminosilicate glass discs were sintered onto ZrO₂ to form glass–ZrO₂ bilayers. Glass discs were allowed to bond to the ZrO₂ substrate during sintering or prevented from bonding by means of coating the ZrO₂ with a thin boron nitrade coating. Residual stress gradients on “bonded” and “unbonded” bilayers were assessed using birefringence measurements. Unbonded glass discs were further tested under biaxial flexure in dynamic fatigue conditions in order to evaluate the effect of residual stress on the slow crack growth behavior.

Results

When fast-ccoling was induced, residual tensile stresses on the glass increased significantly on the side toward the ZrO₂ substrate. By allowing the bond between glass and ZrO₂, those tensile stresses observed in unbonded specimens are overwhelmed by the contraction mismatch stresses between the ZrO₂ substrate and the glassy overlayer. Specimens containing residual tensile stresses on the bending surface showed a time-dependent strength increase in relation to stress-free annealed samples in the dynamic biaxial bending test, with this effect being dependent on the magnitude of the residual tensile stress. The phenomenon observed is explained here on the basis of the water toughening effect, in which water diffuses into the glass promoting local swelling. An additional residual tensile stress at the crack tip adds an applied-stress-independent (K_res) term to the total tip stress intensity factor (K_tip), increasing the stress-enhanced diffusion and the shielding of the crack tip through swelling of the crack faces.

Significance

Residual stresses in the glass influence the crack growth behavior of veneered-ZrO₂ bilayered dental prostheses. The role of water in crack growth might be of higher complexity when residual stresses are present in the glass layer.