M Bawazir
| M Bawazir |
M Bawazir, Sonaj Vardhaman, Chek Hai Lim, Yu Zhang
Preventive and Restorative Sciences, University of Pennsylvania, School of Dental Medicine
Introduction
Dental ceramics are brittle and prone to damage during CAD/CAM milling and chairside grinding. The crack propagation from these flaws under continuous loading in the presence of saliva are responsible for their strength degradation, resulting in premature failure. Thus, improvements in survivability of dental ceramic restorations lie in the elimination of milling/grinding induced microcracks. In the manufacturing industry, there exists a transition region between milling/grinding and polishing where material can be effectively removed predominantly by plastic flow without introducing strength degrading cracks, a regime known as ‘ductile grinding’. Here, we aim to define the critical parameters of ductile–brittle transition region in various classes of dental ceramics by determining the threshold load and depth of cut below which the material removal is dominated by plastic flow.
Methods
Specimens of 3Y-TZP (Zpex), 5Y-PSZ (Zpex Smile), and e.max (IPS e.max CAD) were prepared to 1 mm thickness. Specimens were polished down to 1 μm finish, using a polishing machine. A progressive load single scratch test was performed using Nanovea mechanical tester (n = 10, P = 10–360 mN) with 5 µm conical-diamond-indenter under varying speeds (0.3, 3, and 30 mm/min). The data obtained from the scratch tests was used to determine the ductile–brittle transition region, establish the threshold load and depth of cut for ductile grinding. The scratch morphology was analyzed using scanning electron microscope. Data was analyzed using one-way ANOVA and Tukey’s tests.
Results
: 3Y-TZP, 5Y-PSZ, and e.max underwent a transition from brittle to ductile regime grinding as a function of load. The threshold loads at the boundaries of transition region were 81±2.6 to 121±12 mN, 87±2 to 130±1 mN, 48±4 to 78±4 mN, respectively, under speed of 0.3 mm/min. Regardless of the different threshold loads identified, the critical depth of cut was ≤1 µm for all ceramics. Further, with increasing the speed to 30 mm/min, the transition zone was delayed significantly (p < 0.05) and the threshold loads were 143±9.7, 142±2.5, and 91±4 mN, respectively.
Conclusion
All tested ceramics underwent a transition from brittle to ductile regime grinding at different threshold loads which may attribute to the different microstructures. The transition zone for all tested ceramics delayed with increasing scratch speed.