Qualitative Assessment of Wear Resistance and Surface ...

Qualitative Assessment of Wear Resistance and Surface Hardness of Different Commercially Available Dental Porcelain

The Journal of Contemporary Dental Practice, September 2016;17(9):755-761 755

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ABSTRACT

Introduction: In an attempt to minimize wear damage to the enamel of antagonist teeth, new low and medium fusing ceramic materials have been developed. Manufacturers usually claim that these ceramics are wear-friendly because of their lower hardness, lower concentrations of crystal phase, and smaller crystal sizes. This study aimed to quantitatively analyze the wear strength of various commercially available dental porcelain with tooth enamel as well as the surface hardness of these dental porcelain.

Materials and methods: The basic model was designed as a pin on plate arrangement. The tooth specimens were mounted on the stylus which was centered on the ceramic specimen in a wear testing machine. The dental ceramic specimen was centered in the metal die. A load of 40 N was applied at a rate of 80 cycles/minute for 15 minutes. In the current study, mean wear depth (Ra) value, volumetric loss, and surface hardness were obtained by standard quantification method and were statistically evaluated.

Results: Ceramco-3 was reported to be most abrasive for enamel; however, Duceram love significantly more abraded itself than the other two, Ceramco-3 and Vita Alpha, and generated the lowest loss of enamel. Also, same abrasive type of wear was revealed for all three variants of tested ceramics.

Conclusion: Ceramco-3 was the most abrasive for enamel, while surface roughness (mean wear depth) of Duceram love was maximum and for Ceramco-3 it was minimum. The value of surface roughness for Vita Alpha was in between Duceram love and Ceramco-3. Nonetheless, the mean surface hardness

Qualitative Assessment of Wear Resistance and Surface Hardness of Different Commercially Available Dental Porcelain: An in vitro Study1Abhishek Singh, 2Abhishek Nagpal, 3Salil Pawah, 4Chetan Pathak, 5Gaurav Issar, 6Pankaj Sharma

1,3,4,6Department of Prosthodontics, Sudha Rustagi College of Dental Sciences and Research, Faridabad, Haryana, India2,5Department of Prosthodontics, ITS-CDSR Dental College and Hospital, Ghaziabad, Uttar Pradesh, India

Corresponding Author: Abhishek Singh, Department of Prosthodontics, Sudha Rustagi College of Dental Sciences and Research, Faridabad, Haryana, India, Phone: +919818613206 e-mail: [email protected]

of Duceram love was found to be least and maximum for Vita Alpha.

Clinical significance: In situations of dental wear and wasting tooth disease (Attrition/Abrasion), Duceram can be applied in lieu of Ceramco-3 so as to prevent worsening of existing dentition. However, in younger patients Vita Alpha would offer maximum durability due to its greater surface hardness.

Keywords: Dental ceramics, Profilometer vickers microhardness tester, Wear resistance.

How to cite this article: Singh A, Nagpal A, Pawah S, Pathak C, Issar G, Sharma P. Qualitative Assessment of Wear Resistance and Surface Hardness of Different Commercially Available Dental Porcelain: An in vitro Study. J Contemp Dent Pract 2016; 17(9):755-761.

Source of support: Nil

Conflict of interest: None

INTRODUCTION

The wear of tooth enamel is a natural and unavoidable process. Clinical wear occurs when two materials slide against each other. The tooth enamel may show an acceler-ated wear when opposed by dental ceramics. The pattern of wear may vary according to the ceramic system used and its surface characteristics. Dental esthetics is becom-ing more important, and the emphasis on quality ceramics is consistent with the improving skills of dental techni-cians and the use of layering techniques.1 Developing veneering ceramics with sophisticated optical properties has also contributed to the rise in esthetic standards. By design, the “man-made” external layers of ceramics that are responsible for the unique and individual beauty of the restoration are carried onto the occluding sur- faces and thus influence wear phenomena. Machinable ceramics, even though proven to be wear-friendly, were not included in our study because of the focus on

ORIGINAL RESEARCH10.5005/jp-journals-10024-1925

Abhishek Singh et al

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veneering porcelains porcelain fused to metal (PFM) with high esthetic potential.2

The study of wear is common throughout the bio-materials literature. What is also common is the wide variety of designs, conditions, loads, and measurement techniques and results. This finding highlights the lack of an accepted standard. Difficulty in standardization and the impact of methodologic or operator error can make direct comparisons of studies almost impossible. Harrison reviewed various measurement techniques ranging from commonly available laboratory devices to expensive and rare devices. No single technique was free from operator error, material distortion, or ease in use. Attempts to mimic complicated intraoral motions are admirable, but the resultant rank orders of wear are similar to simpler machines. More complicated wear machines do have the advantage of analyzing the more dynamic and impact or fatigue components of wear and the influence of different variables, but the pin-plate model was chosen for this study because of its ease and simplicity of operation.3 In this study, the authors have genuinely attempted to quantify the wear strength and surface hardness of three commonly used dental porcelain.

MATERIALS AND METHODS

A stainless steel die was fabricated by computer-aided design/computer-aided manufacturing (CAD–CAM) milling of dimension 165 inch diameter × 8 inch thickness. The circular stainless steel die was having eight counter bores equidistant from each other at an angle of 45° to precisely fit into the slots in the wear testing machine. A circular slot of dimension 20 mm in diameter × 1.5 mm in thickness in the center of the metal die was made. Two equidistant holes, of dimension 2 mm in diameter × 3 mm in depth, were made in the stainless steel die as locking device for the ceramic specimen.

Fabrication of Ceramic Disk Specimen and Metal Substructure

Wax patterns for the metal substructure were fabricated directly from the stainless steel die of dimension 20 mm in diameter × 3 mm in depth. The thickness of metal struc-ture was 1.5 mm in dimension and 1.5 mm was kept for the ceramic application. The patterns were invested using phosphate-bonded investment, employing a powder liquid ratio of 150 gm: 35 mL according to the manufac-turer’s recommendation. One hour was allowed for the investment to harden. Wax elimination or burn out at a temperature 750ºC was carried out by placing the casting ring in a thermostatically controlled furnace. The Ni–Cr alloy was melted and casted with the use of an induction

casting machine. The rings were allowed to bench cool and the castings were divested.

Conditioning of the Metal Substructure

The sprues were severed with the help of carbide disks in a high-speed grinder. The thickness of the metal specimen was measured at three different locations using Vernier caliper and the mean thickness was calcu-lated. The recommended thickness for the present study is 1.5 mm for the metal substructure. The total number of metal specimen prepared was 30. The castings were finally cleaned in an ultrasonic unit and transferred to the furnace on a firing tray for oxidation at a temperature of 950°C, to ensure a chemical bond between the metal and the porcelain.

Application of Opaque Layer and Porcelain Buildup

The opaque was mixed to creamy consistency with the help of a glass spatula. The metal sample was held with an artery forceps and a small bead of opaquer was picked up with the ceramic brush. The coated alloy samples were transferred to the firing tray and placed near the muffle to allow moisture to evaporate. The furnace used for the firing was calibrated periodically using the recommended firing cycle to assure accuracy of firing temperature of 975°C. Following fusion, a uniform layer of shade opaque of the corresponding shade was applied using the ceramic brush and the samples were fired in the furnace at a temperature of 970°C (Fig. 1). Porcelain

Fig. 1: The metal ceramic samples being placed in the furnace for the porcelain firing



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machine was used for the analysis of the wear. The system consisted of a styli which was opposed by a rotating disk on which the ceramic specimen via the stainless steel die was mounted. The ceramic disk was initially placed on the circular slot present in the stainless die and then the die was fixed on the wear testing machine.

Loading Protocols and Sample Testing

A 40 N force was recommended to be applied by the tooth specimen on the surface of ceramic disk in sliding range of 10 mm. The sliding range of 10 mm was achieved in circular path by adjusting the track diameter to 2 mm on the wear testing machine. Each tooth specimen was attached to the styli of the wear testing machine which was centered onto the ceramic specimen. The ceramic specimen was centered on the metal die such that the tooth enamel tip contacts the surface of the ceramic specimen in pin on plate arrangement. The surface pro-filometery measuring system and surface profile projec-tor (Figs 2 and 3) were used to measure the amount of surface wear of ceramic disk during the sliding process at Department of Mechanical Engineering, Krishna Institute of Engineering and Technology, Ghaziabad (India). The amount of wear on the enamel cusp was measured by measuring the vertical height loss of tooth structure by digital Vernier caliper at National Physics Laboratory, New Delhi.

The surface hardness was measured by Vickers microhardness tester at National Physics Laboratory, New Delhi (Fig. 4). The surface topography of the tested

powder was mixed with modeling liquid with a glass spatula to prevent metal contamination. Condensation was achieved by mild vibration with the serrated Lacron carver and blotted to semi-dry state with the help of white tissue paper. Each specimen was air dried for at least 10 minutes and transferred to the firing tray, and after a preheating of 5 minutes was fired at a temperature of 960°C.

Finishing of the Samples and Fabrication of Tooth Specimen

The samples were finished by light grinding under medium pressure using sintered diamond bur placed in the micrometer straight handpiece. The thickness was frequently checked with the help of a Vernier caliper to avoid any over or under adjustments and to achieve the desired thickness. Each sample was ultrasonically cleaned for 5 minutes and glazed. Freshly extracted 25 premo-lars were sectioned buccolingually. The sectioned tooth was mounted on acrylic block. To behave as antagonist, 30 mounted teeth samples were fabricated for each ceramic sample. Furthermore, all ceramic samples were divided into following groups: Group 1: 10 samples of ceramic disk vita alpha.Group 2: 10 samples of ceramic disk duceram love.Group 3: 10 samples of ceramic disk ceramco-3.

Wear Analysis

The wear testing was done at National Physics Laboratory, New Delhi. The computer-controlled wear testing

Fig. 2: Surface profilometer and surface profile projector for measuring the mean wear depth

Fig. 3: Surface profile projector


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samples was studied under scanning electron micro-scope (SEM) at Indian Institute of Technology, New Delhi. The ceramic samples were first mounted on the metal circular disk, then the metal circular disk was mounted on the SEM and the images were taken at a magnification of 500× individually for all three ceramic samples. The results obtained were statistically analyzed using one-way analysis of variance and Tukey’s honest significant difference (HSD) test.

RESULTS

The wear analysis of tooth ceramic samples using the computer-controlled machines was conducted for a total of three groups of metal ceramic samples against tooth specimen of each group consisting 10 samples as tooth ceramic antagonists. The wear analysis of tooth specimen was done by measuring the volume loss of the tooth specimen by digital micrometer and mean wear depth analysis of ceramic disk specimen was done by surface profilometer at three points and their mean was taken. Table 1 shows the mean, standard deviation, and standard error values of mean wear (Ra) of three brands of teeth. The respective mean values of groups I to III were 0.853667, 1.033, and 0.419333 with a standard deviation of 0.171795, 0.139411, and 0.124011 respectively. Highly significant difference was observed between mean wear rate (Ra) of three brands of teeth, where f-value of 139.3 was found to be significant at 0.00 level (Table 1). The surface hardness analysis for ceramic samples was done by Vickers microhardness tester for a total of three

Fig. 4: Mounting of ceramic specimen on Vickers microhardness tester

Figs 5A to C: The SEM photomicrograph shows wear pattern of: (A) Vita Alpha; (B) duceram love; and (C) ceramco-3 at magnification 500× on the central region of the porcelain disks

groups. The SEM photomicrograph was also taken for qualitative analysis (Figs 5A to C). Table 2 shows mean, standard deviation, standard error values of surface hard-ness of three commercially available dental ceramics. The respective mean values of groups I to III were 453.687, 412.317, and 435.097 with a standard deviation of 26.0891, 12.5462, and 15.2809 respectively. Highly significant dif-ference was observed between mean surface hardness (VHN) of two commercially available dental ceramics (Table 2). Table 3 shows mean, standard deviation values, and volume loss of three commercially available dental ceramics. The respective mean values of groups I, II, and III were 1.41928, 1.37532, and 1.56058 with a standard

A

B

C



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Table 1: Mean wear rate (Ra) of three commercially available dental ceramics

Ceramic materialMean wear depth (Ra)

f-value p-valuen Mean SD* Max MinVita alpha 10 0.853667 0.171795 1.2 0.61 139.3 0.001Duceram love 10 1.033 0.139411 1.29 0.81Ceramco-3 10 0.419333 0.124011 0.63 0.2Total 30 0.768666667 0.296789 1.29 0.2

*SD: Standard deviation

Table 2: Surface hardness of three commercially available dental ceramics

Ceramic materialMean surface hardness

f-value p-valuen Mean SD* Max MinVita alpha 10 453.687 26.0891 497.3 412.6 36.06 0.001Duceram love 10 412.317 12.5462 445.8 397.4Ceramco-3 10 435.097 15.2809 465.9 411.9Total 30 433.7 25.2705 497.3 397.4


Table 3: Volume loss (mm3) of three commercially available dental ceramics (Vita alpha, duceram love and ceramco-3)

Ceramic material

Volume loss (mm3)p-valuen Mean SD* Max Min

Vita alpha 10 1.41928 0.54864 2.1666 0.628 0.001Duceram love 10 1.37532 0.31878 1.8526 0.942Ceramco-3 10 1.56058 0.46339 2.1666 0.9106Total 30 1.45173 0.44503 2.1666 0.628


deviation of 0.54864, 0.31878, and 0.46339 respectively. A highly significant difference was observed between mean volume losses (mm3) of three commercially avail-able dental ceramics (Table 3).

DISCUSSION

The current study was undertaken to determine the wear resistance and surface hardness of three commercially available veneering dental ceramics. The model for this present study was designed as a pin on plate arrange-ment. The design for this model was a variation in the design given by Metzler et al.2 In this model, tooth speci-mens were mounted on the stylus which was centered on the ceramic specimen in a wear testing machine. The dental ceramic specimen was centered onto the metal die. A load of 40 N was applied at a rate of 80 cycles/minute for 15 minutes. The wear losses were evaluated after 15 minutes of testing. Previous studies4-6 have indicated volumetric wear changes, linearly with time which has been co-related with clinical data. When comparing with veneering ceramics (PFM), our results have shown good co-relation with previous studies.7,8 Simple height loss can also be misleading if specimens are not of a uniform diameter because as wear progresses, the surface area of the contact increases or decreases. This results in a

nonlinear rate of wear and becomes increasingly diffi-cult to compare different samples.9 In the current study, volume loss measured was in accordance with previous studies.6,10 It was found that the mean volume loss of tooth specimen against Ceramco 3 (1.56058 mm3) was maximum followed by Vita Alpha (1.41928 mm3) and least was with Duceram love (1.37532 mm3), as shown in Table 3. In order to provide uniformity between the tooth specimens, the cuspal morphology was flattened using diamond burs. The flattened surface provided an even contact over the ceramic samples. This study was also aimed to measure the surface roughness (mean wear depth) using surface profilometer. It was found that the mean surface roughness (mean wear depth) of Duceram love was maximum (0.33 µm) and for Ceramco 3 it was minimum (0.41933 µm). The value for Vita Alpha was in between Duceram love and Ceramco-3. It was hypoth-esized that the wear characteristics of enamel and the dif-ferent veneering ceramics are similar, as shown in Table 1.

Moreover, it is a common perception that hardness of a material is directly proportional to its abrasiveness. Hardness alone cannot explain abrasive wear.11 In fact, several studies have found no correlation between hard-ness and wear.8,12 Wear appears to be more related to roughness and fracture resistance than strict hardness values; therefore, the properties of the studied porcelains need to be examined. Proprietary formulations have not been revealed, but generalizations can be drawn.13-17 It was found that the mean surface hardness of Duceram love was least (412.31) and maximum for Vita Alpha (453.68), as shown in Table 2. Therefore, Duceram love enamel wear mechanism was expected to be “soft abra-sive” as defined by Richardson, resulting in lower rates of enamel when compared with other ceramic–enamel couples. The results differed from the study done by


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Derand and Vereby in 1999.1 The difference could be in the variation of composition between dental ceramic available in the past and the currently available dental ceramic.18-22

At lesser amount of load ranging from 10 to 20 N, amount of saliva does play an important role in reduc-ing the wear but at higher load, the amount of wear was not affected by the presence of third body medium like saliva. Since in our study the load applied was 40 N, there-fore third body medium was not considered. The SEM photomicrography showed changes in the wear pattern on the individual dental ceramic disk. The surface char-acteristics of Duceram love was found to be coarser than the ceramic disk of Ceramco-3 and Vita Alpha.23 The wear pattern revealed that Duceram love underwent more wear than the other two ceramic disk. So, the qualitative analysis was also well co-related with the quantita-tive results obtained. This was also well supported by Richardson, in his study, where soft abrasive undergoes more wear than the hard abrasive.13 So, in this study, Duceram love was a soft abrasive compared to Vita Alpha and Ceramco-3 against tooth enamel.

As the model for present study was designed as a pin on plate arrangement, the tooth specimens were mounted on the stylus which was centered on the ceramic specimen in a wear testing machine. The dental ceramic specimen was centered in the metal die. Here mean wear depth (Ra) value, volumetric loss, and surface hardness were obtained by standard quantification method and were statistically evaluated. It was found that the mean surface roughness (mean wear depth) of Duceram love was maximum (0.33 µm) and for Ceramco 3 it was minimum (0.41933 µm). The value for Vita Alpha was in between Duceram love and Ceramco-3. The mean surface hardness of Duceram love was least (412.31) and maximum for Vita Alpha (453.68). At lesser amount of load ranging from 10 to 20 N, amount of saliva does play an important role in reducing the wear but at higher load, the amount of wear was not affected by the presence of third body medium like saliva. Since in our study the load applied was 40 N, therefore third body medium was not considered. Although a number of in vitro studies have accomplished, but to determine the wear loss of tooth opposing metal ceramic restoration it is desirable that clinical in vivo studies should be undertaken to have more meaningful clinical implications.

CONCLUSION

Ceramco-3 was the most abrasive for enamel and gener-ated, along with Vita Alpha, the highest volumetric loss of enamel. Duceram love was significantly more abraded itself than the other two and generated the lowest loss of

enamel. For the three materials tested, surface hardness was greatest for Vita Alpha followed by Ceramco-3 and was least for Duceram love. The same abrasive type of wear was revealed for all three ceramics.

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