Evaluating dental zirconia

Abstract

Objectives: To survey simple contact testing protocols for evaluating the mechanical integrity of zirconia dental ceramics. Specifically, to map vital material property variations and to quantify competing damage modes.

Methods: Exploratory contact tests are conducted on layer structures representative of zirconia crowns on dentin.

Results: Sharp-tip micro- and nano-indentations were used to investigate the roles of weak interfaces and residual stresses in veneered zirconia, and to map property variations in graded structures. Tests with blunt sphere indenters on flat specimens were used to identify and quantify various critical damage modes in simulated occlusal loading in veneered and monolithic zirconia.

Significance: Contact testing is a powerful tool for elucidating the fracture and deformation modes that control the lifetimes of zirconia dental ceramics. The advocated tests are simple, and provide a sound physical basis for analyzing damage resistance of anatomically-correct crowns and other complex dental prostheses.

Keywords: Dental prostheses; Fatigue; Fracture mode; Indentation; Translucency; Zirconia.

1.

Failure modes in ceramic layers on soft substrates. (a) Schematic of crown on dentin, showing fracture and deformation modes at occlusal and cementation surfaces from axial, sliding and edge contacts [12]. (b) Basic experimental setup, indicating damage modes from sphere indenter of radius r at axial load P on flat zirconia layer of thickness d on compliant substrate: C cone crack at contact periphery, Y quasiplastic zone from yield deformation, R radial crack from intaglio surface.

2.

Use of Berkovich nanoindentation to probe elastic modulus variations in dental structures. (a) Across longitudinal section in great ape tooth enamel, showing gradient in values between outer and inner surfaces [58]. (b) At cross section of in-house glass-infiltrated 3Y-TZP (TZ-3Y-E grade, Tosoh, Tokyo, Japan), with value close to that of dental porcelain and tooth enamel at cameo surface increasing to that of core zirconia at depth ~ 150 μm [12].

3.

Corner cracks from Vickers indentations near fused interfaces in bilayers [46]. (a) Porcelain (Lava Ceram, 3M ESPE AG, Seefeld, Germany) on 3Y-TZP (Lava Frame, 3M ESPE AG, Seefeld, Germany), CTE mismatch < 0.5×10⁻⁶ °C⁻¹. Note how cracks either arrest, delaminate, or penetrate interface. (b) Borosilicate glass on same 3Y-TZP, CTE mismatch ~ 5×10⁻⁶ °C⁻¹. Large residual stresses in latter case have caused catastrophic failure of the bilayer.

4.

Critical loads to initiate radial cracks (P_R), cone crack (P_C), and quasiplastic yield deformation (P_Y) in flat 3Y-TZP (Prozyr Y-TZP, Norton, East Granby, CT) on a compliant substrate, as function of zirconia thickness. (a) 3Y-TZP layer of variable d on substrate, P_C and P_Y computed for nominal r = 5 mm [86]. (b) Veneer layer of thickness d₁ on same 3Y-TZP core of thickness d₂, with fixed net thickness d = 1.5 mm [67]. P_R data from tests on polycarbonate substrates scaled for equivalent dentin substrates using eqn. 1.

5.

Critical loads P_R for intaglio radial cracking in monolithic Y-TZP (Prozyr Y-TZP, Norton, East Granby, CT) on a dentin substrate as function of number of contacts with a hard sphere [52]. Data shown for as-polished, sandblasted, and indented (Vickers, 10 N) zirconia undersurfaces. Linear fits to polished and sandblasted cases are in accordance with strength loss from moisture-assisted slow crack growth. Curve through data for indented surfaces is empirical fit, revealing enhanced degradation from mechanical fatigue for flaws within quasiplastic contact zone. Arrows indicate 'runouts', i.e. no failure after 10⁷ cycles.

6.

Inert strength of zirconia after occlusal damage from cyclic contacts with hard sphere of radius r = 3.18 mm, in water. (a) Axial loading of 3Y-TZP bars (Prozyr Y-TZP, Norton, East Granby, CT) with tungsten carbide indenter at two loads shown [55]. (b) Sliding loading on 3Y-TZP discs (Zpex, Heany Dental, Scottsville, NY) with zirconia sphere in mouth-motion machine at relatively low load (data courtesy Marcia Borba and Tomoyuki Okamoto). Data points for specimens on the lower solid lines are failures are from contact sites. Boxes at left axis are strengths of unindented specimens (standard deviation bounds). Abrupt drops in strength after cycling correspond to initiation of cone or other macroscopic cracks from quasiplastic zone.