Competition of fracture mechanisms in monolithic dental ceramics: flat model systems

Abstract

Monolithic (single layer) glass-ceramic restorations often fail from chipping and fracture. Using blunt indentation of a model flat porcelain-like brittle layer bonded onto a dentin-like polymer support system, a variety of fatigue fracture modes has been identified and analyzed: outer cone, inner cone, and median cracks developing in the near-contact region at the occlusal surface; radial cracks developing at the internal cementation surface along the loading axis. Our findings indicate that monolithic glass-ceramic layers are vulnerable to both occlusal surface damage and cementation internal surface fracture. Clinical issues in the longevity of ceramic restorations are discussed in relation to biting force, physical properties of ceramic crowns and luting cement, and thicknesses of ceramic and cement layers.

Figure 1

Schematic showing evolution from (a) model ball on flat layer tests to (b) actual dental crown morphology.

Figure 2

Schematic illustration of damage modes in flat ceramic coatings on compliant substrates with sphere of radius r at load P in water for (a) single-cycle loading and (b) multi-cycle loading. Note: O, outer cone cracks; Y, yield; I, inner cone cracks; M, median cracks; R, radial cracks.

Figure 3

Critical loads and standard deviations for onset of contact damage under single-cycle loading in various dental ceramics, including: Dicor, glass, porcelain (Mark II), zirconia (Y-TZP), glass infiltrated alumina (Inceram alumina, A-Infi), Empress I (EI), Empress II (EII), glass infiltrated zirconia (Inceram zirconia, Z-Infi), and alumina. The composition, manufacturer, and material properties of these dental ceramics and glass are summarized in Table 1. Note: O, outer cone cracks; Y, yield.

Figure 4

Examples of near-contact induced occlusal surface fracture morphologies in thick (d ≈ 6 mm) monolith glass bars (not subject to significant flexure upon loading), showing differences in critical loads for the onset of various damage modes for single-cycle and cyclic loading. Note: O, outer cone cracks; I, inner cone cracks; M, median cracks.

Figure 5

Examples of failure of brittle layers on compliant polycarbonate substrate by various damage modes: (a) O, outer cone crack, (b) I, inner cone crack, (c) M, median crack, and (d) R, radial crack. Note: photos are showing the glass coating layers only. The thicknesses of the glass layers are 1 mm for (a), (b), and (d) and 2.2 mm for (c). The top surface of the glass layers are abraded with 600 grit SiC to introduce a controlled flaw population, while the bottom surface of the glass coatings are etched with 9.5% hydrofluoric (HF) acid in (a), (b), and (c), and sandblasted with 50 μm alumina particles in (d) (mimicking the sandblast treatment in dentistry).

Figure 6

Number of cycles to failure n_F as function of maximum contact load P_m for soda-lime glass/polycarbonate bilayers. Glass thickness (a) d = 1 mm, (b) d = 2.2 mm, and (c) d = 1 mm. In (a) and (b), the top, occlusal surface of glass layer is lightly abraded with 600 grit SiC particles, while the bottom, internal surface of glass is etched with HF. Note failure from different crack modes at different P_m. In (c), the top surface of glass is abraded, while the bottom surface is sandblasted with 50 μm alumina particles. Note the dominance of cementation internal surface radial (R) fracture in (c). Note: O, outer cone cracks; I, inner cone cracks; M, median cracks; R, radial cracks. Lines represent 95% confidence bounds to the radial crack initiation data.

Figure 7

Projection of critical loads P_R for the onset of cementation internal surface radial (R) cracks in soda-lime glass plates cemented to tooth dentin. Glass thickness (a) d = 1 mm and (b) d = 2.2 mm. The top surface of glass plates is lightly abraded with 600 grit SiC particles, whereas the bottom surface is sandblasted with 50 μm alumina particles. Note: O, outer cone cracks; I, inner cone cracks; M, median cracks; R, radial cracks. Lines represent 95% confidence bounds to the radial crack initiation data.

Figure 8

Prediction of the dependence of critical load P_R on the cement modulus and thickness for a glass-ceramic (Empress I, Ivoclar-Vivadent, Schaan, Liechtenstein) layer (d = 1.5 mm) cemented to tooth dentin. Calculations are based on Eqs. 4 - 6 (Appendix), assuming the strength of Empress I is 160 MPa and the elastic modulus of tooth dentin is 18 GPa. Two distinctive regions separated by an arrow “A”, which represents the elastic modulus value of tooth dentin E_s ≈ 18 GPa, of cement modulus can be identified.