impair the mechanical properties of ceramic framework material. Effects of polishing ... according to type of mechanical test (flexural strength and fracture toughness). .... release 8.02/2001 (SAS Institute. Inc.). ..... flaw size and auto-glaze treatment on porcelain strength. J Dent ... Coltene/Whaledent Inc., Cuyahoga Falls, Ohio.
Dental Materials
Impact of surface finishes on the flexural strength and fracture toughness of In-Ceram Zirconia Manal Manawi, DDS, PhD Mutlu Ozcan, DDS, Dr.med.dent., PhD Luiz Felipe Valandro, DDS, MSD, PhD n
Dental restorations made of zirconia are usually selectively adjusted chairside to eliminate occlusal or internal interferences that can impair the mechanical properties of ceramic framework material. Effects of polishing procedures on zirconia after chipping or simply glazing the monolithic zirconia restorations are not known. This study evaluated the effects of different surface treatment procedures—namely, glazing or grinding, finishing, and polishing regimens—on the flexural strength and fracture toughness of a zirconia core material. Forty zirconia specimens were prepared and divided into two main groups (n = 20) according to the type of surface treatment (glazed or ground, finished, and polished). Each group was further divided into two subgroups (n = 10) according to type of mechanical test (flexural strength and fracture toughness). The roughness measurements were performed before mechanical testing. Qualitative evaluation of representative specimens of each subgroup was performed using SEM.
A
ll-ceramic dental materials are commonly used because of their superior biocompatibility and optical properties compared to their metallic counterparts. However, their brittleness, in the form of microfractures, has hampered their wider use and limited their application for extensive fixed dental prostheses (FDPs).1,2 Inherent flaws and defects that can propagate at a microscopic level significantly control the strength and mechanical properties of ceramics.3 One such all-ceramic system, In-Ceram Alumina (Vident), has been developed. It is based on a slip-casting technique to build the framework of the FDPs fired to an open-pore microstructure. The material derives its strength through 138
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Manal Madina, DDS, PhD
Cenk Cura, DDS, PhD
The surface roughness mean (μm; ± standard deviations) recorded for the glazed specimens (0.94 ± 0.2) was significantly lower than that of the finished and polished group (3.01 ± 0.1) ( P < 0.05). The glazed zirconia showed significantly higher flexural strength (385.4 ± 45.4 MPa) and fracture toughness (6.07 ± 1 MPa.m½) values than the ground, finished, polished zirconia (302.4 ± 47.6 MPa and 2.14 ± 0.5 MPa.m½) ( P = 0.002 and P < 0.001 for flexural strength and fracture toughness, respectively). A smooth topographic pattern after glazing could not be obtained after finishing and polishing. Grinding, finishing, and polishing markedly decreased the flexural strength and fracture toughness of zirconia compared to the glazed groups. Received: March 3, 2011 Final revisions: May 16, 2011 Accepted: August 1, 2011
infiltration of the lanthanum glass to the microstructures of the open pores of this ceramic.4 The high flexural strength of glass-infiltrated In-Ceram Alumina (400–605 MPa) has been further improved by adding 33% by weight zirconium oxide.5,6 In-Ceram Zirconia (Vident) demonstrates a flexural strength of 750 MPa and fracture toughness that is two times higher than that of In-Ceram Alumina.5,6 All-ceramic core materials are covered with suitable veneering ceramics, adequately glazed prior to cementation, and not intended to be exposed in the oral environment.7 Also, lately, monolithic zirconia FDPs are available that are not covered by a veneering ceramic, but only glazed or colored.
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Chipping of the veneering ceramics with zirconia core exposure has been commonly reported in clinical studies.8,9 Because adhesion of resinbased materials for direct repair or retrieval of such reinforced ceramics is difficult, one clinical approach to such chipping is to smooth and polish the surface, depending on the size of the defect.8,10 In fact, nonglazed ceramics have been shown to cause undesirable responses to the adjacent periodontal tissues, such as potentially increasing the wear of opposing teeth and increasing susceptibility to plaque accumulation.11 Moreover, surface roughness of allceramics might also influence their mechanical properties.12 Such clinical adjustments could initiate subcritical flaws or larger defects. Under clinical
loading and/or the presence of moisture, nonglazed surfaces might eventually yield to catastrophic failure.13 In addition, the level of surface roughness created during different finishing and polishing procedures can cause stress concentrations and, consequently, decrease the strength. For these reasons, the superficial roughness must be minimized by employing effective polishing techniques. The effect of the processing procedures, polishing, grinding, and glazing on the mechanical properties of glassy matrix ceramics has been studied, but there remains a lack of information for zirconia ceramics.14,15 For this reason, the objective of the present study was to evaluate the effect of different surface treatment procedures, namely glazing or grinding, finishing, and polishing regimens, on the flexural strength and fracture toughness of In-Ceram Zirconia core material. The null hypothesis tested was that the mechanical properties of In-Ceram Zirconia would not be significantly affected by the finishing, polishing, or glazing procedures.
Materials and methods Ceramic specimen preparation
In-Ceram Zirconia specimens were made according to the manufacturer’s specifications. A metal template was created (25 mm x 3.0 mm x 4.0 mm), and the impression of the mold was made using an additional silicon impression material (Swiss Tec, Coltene/Whaledent Inc.) according to the two-step impression technique. The metal template was placed over the impression material and held under finger pressure until setting was complete. It then was removed and poured with In-Ceram special plaster (Vita In-Ceram, Vident). Once the plaster set, it was painted with a special isolating medium (Vita isolating gel, Vident).
In-Ceram zirconia powder was mixed with the mixing liquid in a glass beaker according to the manufacturer’s instructions. The powder slip was added and mixed with a spatula into the liquid in small portions under vibration in an ultrasonic unit for seven minutes to ensure a homogeneous mixture. The mixture was then placed in a furnace (Vita Vaccumat 200, Vident) for 30 minutes at a temperature ranging from 130–190°C until shrinkage occurred. The plaster was separated from the ceramic specimens and placed in the furnace for a 40-minute holding time at 1,120°C. After glass infiltration of the specimens, they were placed in the furnace for a 40-minute holding time and then subjected to another cycle for five minutes at 1,100°C for controlled glass infiltration. Finally, the specimens were air-abraded in a sandblasting unit (Rocatector Junior, 3M ESPE) using 50 μm Al2O3 (Korox, BEGO USA) at a maximum pressure of 3 bars and cleaned in an ultrasonic bath (Quantrex 90, L&R Manufacturing Company) with ethanol for 15 minutes. Specimens obtained for flexural strength testing (20 mm length, 1.5 mm thickness, and 4 mm width; n = 20) and fracture toughness (20 mm length, 3 mm thickness, and 4 mm width; n = 20) were randomly divided into two groups of 10 according to type of surface treatment.16,17 Two additional specimens for each surface treatment group were manufactured for SEM analysis. Experimental groups
Group 1 specimens were glazed at a temperature of 900°C according to the manufacturer’s instructions, with a holding time of one minute. Group 2 specimens were submitted to wet grinding and finishing with a diamond bur (No. 4138, www.agd.org
KG Sorensen), followed by wet polishing using silicon rubber tips (No. 9232PM, KG Sorensen) and 6-μm diamond paste (KG Sorensen; batch No. 7023). Final polishing was performed along the long axis of the specimen. One operator, blinded to the objectives of the study, performed all of these procedures for 10 seconds. All specimens were then cleaned in an ultrasonic bath with acetone at room temperature for 15 minutes. Surface roughness measurement
Surface roughness of the specimens was determined using a profilometer (Perthometer equipped with RHT 6-250, Mahr Federal Inc.). Five measurements of R a values were made at a distance of 3.5 μm from one another, and mean values were calculated. Flexural strength
A type 500 universal testing machine (Lloyd Instruments Ltd.) was used for the flexural strength test. The specimens were fixed centrally, and the load was applied perpendicular to the long axis of the specimen (2 mm/min) at room temperature (20°C) in inert conditions (moisture-free). The flexural strength (M) of the specimens was calculated (MPa) according to this equation: M = 3Wl/2bd2 where W was the applied load (N), l was the test span (mm), b was the width of the specimen (mm), and d was the thickness of the specimen (mm).18 Fracture toughness
Fracture toughness was measured using the single-edge notch beam technique (Fig. 1).18 The width, thickness, and crack length of the
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Dental Materials Impact of surface finishes on the flexural strength and fracture toughness of In-Ceram Zirconia
grinding, finishing, and polishing were examined qualitatively on the surface of the specimens at a magnification of 1,000X.
Table 1. Means (and standard deviations) of flexural strength and fracture toughness for In-Ceram Zirconia after glazing or grinding, finishing, and polishing. Flexural strength (MPa)
Fracture toughness (MPa.m½)
Glazing
385.4 ± 45.4
6.07 ± 1
Grinding, finishing, and polishing
302.4 ± 47.6b
2.14 ± 0.5B
0.002
