Effect of Surface Finishing Methods and Corrosion on Surface Elemental Composition and Roughness of Dental Ceramics

Ozdemir Akkus Niler^1* and Aksoy Gokhan²

^*Correspondence: Ozdemir Akkus Niler, Department of Prosthodontics, Faculty of Dentistry, Okan University, Istanbul, Turkey, Email:

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Introduction

Dental ceramic materials are considered chemically most inert reconstructive materials. Different kinds of ceramics possess different chemical properties. Because of this, inertness of a specific dental ceramic material cannot be generalized. The ceramic structure is based on electric neutrality [1]. With respect to the chemical composition, ceramic materials can be classified as silica based oxide and nonoxide ceramics. Sintered dental ceramics are mainly silica based ceramics, characterized by a continuous glass matrix in which different volume fractions of crystals and particles are interspersed [2]. The glass matrix in conventional ceramics is predominantly based on a potash feldspar or soda feldspar. In addition to feldspar, other oxides are frequently added. To enhance the esthetic appearance of the ceramic material, colorant oxides of Fe, Cu, Co, Mn, and opacifying oxides of Sn, Zn, Al, Zr and Ti are also added. Some of the low or ultra-low fusing dental ceramics demonstrate compositional2 and microstructural differences from traditional porcelain fused to metal ceramics. The composition, microstructure and physical properties of newly launched ceramic materials differ from those of traditional ones, which may affect the inertness and surface roughness of the material that influence plaque accumulation on exposed surfaces [3]. Most commercial dental ceramic materials are partially crystallized feldspathic glasses reinforced with alumina particles which induce increased coarseness in the surface. Modern low-fusing ceramics use fine-grained Lucite crystals or mullite crystals as the reinforcing material. Recently introduced dental glasses or hydrothermal glasses are free from reinforcing alumina particles or crystals are thus smoother and softer than traditional dental ceramics [4].

The surface roughness of ceramic materials may be influenced by many factors, such as the composition and microstructure of the ceramic material, glaze temperature and surface finishing method used.

Materials and Methods

20 ceramic disks (8 mm in diameter and 2 mm in height) were fabricated from A1 shade of Empress II, Zirkonzahn, Noritake Cerabien ZR,Vita VM7 and Finesse veneering ceramics. The ceramic specimen was fabricated in plexiglass mold. Ceramic powder was mixed with a brush on a glass plate using the mixing liquid recommended by producers of each ceramic material. The mold was filled carefully with the mixture and condensed. Excess liquid was removed by applying a piece of adsorbent paper to the surface of the specimen. After condensation the mold was removed leaving the non-sintered test specimen on firing tray. Each specimen was sintered in a calibrated oven according to manufacturer’s instructions by the same experienced dental technician. Test specimens were divided into 2 groups of autoglaze and polishing (n=10). All test surfaces were ground using 1000 grit SiC paper on a rotating disc at 150 revolutions/ min under water cooling. The test specimens were finally ultrasonically cleaned in distilled water. Then, autoglaze was applied according to the manufacturer’s instructions of each material. Mechanical polishing was applied using Shofu All Ceramic Finishing and Polishing wheels using a slow-speed handpiece rotating at approximately 10,000 rpm, as recommended by the manufacturer. Polishing was performed by the same investigator. The adjustment kit consisted of a 4-step process: Two green stones and 2 different polishers were used once for 20 seconds. The specimens were then ultrasonically cleaned with distilled water and dried with a blast of air for 30 seconds before surface analysis. Surface roughness of the specimens was evaluated using a profilometer (Mitutoyo Surf Test 402 Analyzer; Mitutoyo Corp, Kawasaki, Japan). To measure the roughness profile value in μm, the diamond stylus was moved across the surface under a constant load of 3.9 mN and a speed of 0.100 mm/s with a range of 600 μm during testing. This procedure was repeated 10 times at a different location for each specimen to obtain the general surface characteristics of the specimens. The average values of these measurements were considered to be the “Roughness average” (Ra) values. Prior to elemental analysis, the samples were subjected to a standard cleaning procedure involving short exposure in isopropanol under ultrasonic stirring, followed by careful drying in air. This proved to be necessary to ensure that the amount of surface contamination of hydrocarbons that always appear on metallic and ceramic samples of all types was minimized. The analyses were conducted using Energy Dispersive X-Ray Spectrophotometer (EDX). The energy was fixed at 93.9 eV and the analyzed areas were about 0.8 × 2 mm². For each sample, a survey spectrum of the binding energy range of 0-1350 eV was recorded for 44 mins on the horizontally mounted samples. The angle between the sample surface and the analyzer as well as between the incoming X-ray beams was then 450. From the spectrum, the characteristic peaks of elements present in the surface were identified and the apparent elemental surface composition was derived from the peak intensities. Access evaluation software and the standard relative sensitivity given therein by the equipment manufacturer. Before doing this, carbon was excluded from the calculations as this element arises from external contaminants. A survey spectrum was taken from the central area of each ceramic specimen and was repeated for 5 times. The range and mean values (in atom %) were registered and mean average was calculated for each element. After the initial surface analysis, the ceramic specimens were subjected to different levels of corrosion intensity. In the water corrosion experiments, the specimens were placed in the shaking equipment (New Brunswick, Innova, 4080 Incubator Shaker) and exposed to gentle rocking for 18 hours at 37°C in solutions with pH values of 4.5–7 and 9. In the acid corrosion experiments, we modified the conditions of ISO Standard 6872 [5]. According to the standard, all specimens were momentarily exposed to 4% acetic acid at 80°C, and corrosion solution temperature was gradually increased to 80°C for reducing the risk of surface microcrack formation. A single ceramic specimen and 17 mL of corrosive medium were added to each of the polyethylene corrosion bottles, and the bottles were sealed with a screw cap. The bottles were then placed in an oven, and the temperature was increased until reaching the final temperatures of 37°C and 80°C. To avoid a steady-state situation at the surface of the test specimens, we subjected them to gentle rocking during the entire corrosion process in the oven [5]. After cooling to room temperature, the specimens were removed from the bottles, rinsed in distilled water, dried, and a new surface analysis was performed. The data were statistically analyzed by Univariate Analysis of Variance and Bonferroni Test. Multiple comparisons of Ra values between autoglaze and polishing groups for each brand of ceramic were computed by T-Test. For the determination of correlation, between surface elements and roughness average, Spearman correlation test were utilized. Level of significance was chosen as 0.05.

Results

From the spectrum of ceramic surface composition, strong characteristic peaks representing Na, Al, Si were found. The intensity of Si also varied according to different types of ceramic that was determined highest for Noritake CZR and lowest for Zirkonzahn veneering ceramics (Table 1).

Table 1: Surface Si change ratio of glazed and polished groups at different pH values.

Surface composition of polished ceramics was found to be dominated by silicon, for Noritake CZR at all pH values. The highest surface Na change was found for polished Zirkonzahn at high intensity corrosion at pH 2.4 and no surface Na concentration was calculated for Empress (Table 2).