Damage Tolerance and Microstructure of Dental Ceramics

Objectives: Fabrication of ceramic restorations involves CAD/CAM milling (shaping) and hand-piece grinding (adjusting) with diamond particles. Consequently, microfractures are unavoidable in order to remove the material. This brittle-regime grinding can jeopardize the strength of ceramics. This study aims to elucidate the threshold load below which ceramics will not indulge strength-limiting damages.
Methods: Disc-shaped e.max CAD, Mark-II, and 5Y-PSZ samples were cut, heat treated as per manufacturers’ instructions as needed, and polished to 1-μm finish. Single particle scratch tests were conducted using a nano-indentation machine to reproduce controlled damage under various loads, representing ductile, brittle, and a combination of ductile and brittle material removal regimes. Further, biaxial flexure strength tests were performed to analyze the effect of scratch load and associated microfracture on ceramic strength (n = 6/load). Scanning electron microscopy (SEM) and Focused Ion Beam (FIB) were used to characterize surface and subsurface damages. Statistical analysis was performed using one-way ANOVA and Tukey tests
Results: Different classes of ceramics, namely e.max CAD, Mark II, and 5Y-PSZ, with different mechanical properties and microstructures showed different responses to scratch damages and threshold loads: 70 mN, 130 mN, and 400 mN, respectively (Table 1). Below these threshold loads, the ceramics did not exhibit any strength degradation. The formation of subsurface damage associated with the threshold loads was evaluated by FIB-SEM analysis.
Conclusions: The rank, from high to low, for strength and toughness is 5Y-PSZ, e.max CAD, and Mark II. However, the threshold loads above which the material strength deteriorated showed a different trend, as Mark II withstood higher stresses and deeper subsurface damages relative to e.max CAD. This implies that not only the mechanical properties but also the microstructure play a significant role in the damage tolerance behavior of ceramics. Our findings provide important guidelines for establishing ductile-regime grinding parameters for dental ceramics.