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 Table of Contents  
ORIGINAL ARTICLE
Year : 2018  |  Volume : 5  |  Issue : 1  |  Page : 54-59

Effects of various surface treatments on the biaxial flexural properties of yttria-stabilized zirconia ceramics


1 Department of Substitutive Dental Sciences, College of Dentistry, University of Dammam, KSA
2 Department of Substitutive Dental Sciences, College of Dentistry, Jazan University, KSA
3 Department of Prosthodontics and Crown and Bridge, Maharashtra Institute of Dental Sciences and Research Dental College, Latur, India
4 Department of Prosthodontics and Crown and Bridge, Saraswati-Dhanwantari Dental College and Hospital and Post-Graduate Research Institute, Parbhani, Maharashtra, India
5 Consultant Prosthodontist, Laxmi Dental Clinic, Bangalore, Karnataka, India
6 Department of Pedodontics, Himachal Pradesh Government Dental College and Hospital, Shimla, Himachal Pradesh, India
7 Department of Oral Medicine and Radiology, Saraswati-Dhanwantari Dental College and Hospital and Post-Graduate Research Institute, Parbhani, Maharashtra, India

Date of Web Publication12-Mar-2018

Correspondence Address:
Abhishek Singh Nayyar
44, Behind Singla Nursing Home, New Friends' Colony, Model Town, Panipat - 132 103, Haryana
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/sjos.SJOralSci_30_17

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  Abstract 


Aim: The aim of the present study was to evaluate and compare the influence of different surface treatments and their cumulative effects on the biaxial flexural properties and phase transformation of yttria-stabilized zirconia ceramics.
Materials and Methods: A total of fifty specimens were fabricated by computer-aided design/computer-aided manufacturing machining from Cercon®. The samples were divided into five groups following different surface treatments as control (C), air particle abrasion (Si), mechanical loading (ML), low-temperature degradation (LTD), and cumulative treatment (CT) groups.
Statistical Analysis Used: The results were analyzed by two-way ANOVA and Tukey's honestly significant difference (HSD) test. Two-way ANOVA was used to find significance between the test and the control groups. Tukey's HSD test was carried out to determine any significant difference among the groups.
Results: The highest biaxial flexural strength was observed in the Si group (950.2 ± 126.7 MPa) followed by the LTD group (861.3 ± 166.8 MPa), CT group (851.2 ± 126.5 MPa), and the least with the ML group (820 ± 110 MPa). Significant difference was observed in two-way ANOVA test. Tukey's HSD test showed that there was a significant difference (P ≤ 0.05) between the C and Si groups and C and LTD groups; however, no significant difference was observed (P ≥ 0.05) between the C and ML groups and C and CT groups. X-ray diffraction analysis showed that the control group consisted of 100% tetragonal zirconia while the maximum amount of monoclinic phase was obtained after the LTD treatment.
Conclusions: Air particle abrasion with CoJet Sand, LTD, and CTs had no negative impact on biaxial flexural strength indeed it increased the biaxial flexural strength. Hence, these surface treatments can be done in routine clinical practice to improve the performance of ceramic restorations.

Keywords: Biaxial flexural strength, computer-aided design/computer-aided manufacturing, phase transformation, surface treatments, zirconia


How to cite this article:
Jain T, Porwal A, Bangar BR, Patil SN, Elanangai, Bhandari R, Nayyar AS. Effects of various surface treatments on the biaxial flexural properties of yttria-stabilized zirconia ceramics. Saudi J Oral Sci 2018;5:54-9

How to cite this URL:
Jain T, Porwal A, Bangar BR, Patil SN, Elanangai, Bhandari R, Nayyar AS. Effects of various surface treatments on the biaxial flexural properties of yttria-stabilized zirconia ceramics. Saudi J Oral Sci [serial online] 2018 [cited 2018 Dec 9];5:54-9. Available from: http://www.saudijos.org/text.asp?2018/5/1/54/227129




  Introduction Top


The use of ceramics is not new in dentistry. Ceramics are widely used in dentistry, especially, in the field of prosthodontics and restorative dentistry because of their excellent mechanical, physical, and esthetic properties. Due to their excellent mechanical properties, there is even higher use of zirconia in dentistry, especially, yttria-stabilized zirconia (Y-TZP) ceramics.[1] However, certain procedures such as computer-aided design/computer-aided manufacturing (CAD-CAM) machining and various other procedures including sterilization result in development of surface flaws that lead to stress concentration at specific sites.[1],[2] The abrasives regularly used in dental applications include silica-coated alumina particles of different sizes. However, concerns have been raised following treatment with such abrasives. There are studies which have demonstrated the influence of these abrasives on the physical properties of zirconia.[3] Y-TZP ceramics demonstrate superior strength due to their phase transformation phenomenon. During this phenomenon, there is approximately 4% increase in volume due to tetragonal to monoclinic transformation.[4] This transformation may, also, occur during various procedures including air particle abrasion, mechanical loading (ML), and low-temperature degradation (LTD) during autoclaving and cumulative procedures during the fabrication of the prosthesis.[5],[6] When zirconia restorations are subjected to heavy masticatory forces and thermal stresses, it leads to further deterioration of their strength due to crack propagation. Due to this reason, the evaluation of effects of different surface treatment and their cumulative effects becomes the need of the hour. There are a wide variety of CAD-CAM zirconia materials which are available in the market and which have shown excellent physical and mechanical properties when compared to high alumina ceramics.[7] Some such common examples include Lava™, KaVo Everest, and Cercon. However, it is unclear whether or not a different surface treatment along with low-temperature aging and ML together affects the physical properties of CAD-CAM machined Y-TZP ceramics. The aim of the present study was to evaluate and compare the influence of different surface treatments and their cumulative effects on the biaxial flexural properties and phase transformation of Y-TZP ceramics.


  Materials and Methods Top


Fifty disc-shaped specimens of Cercon® base [Figure 1] (Degudent, Hanau, Germany) were prepared as per ISO 6872 1995 standards. The standard described that a test piece should have a minimum thickness of 1.2 (±2) mm and a diameter of 12–16 mm. The specimens were initially milled in large dimensions to compensate for the shrinkage occurring during sintering. In the previous studies, it was noted around 25% for Cercon. The specimens were, then, sintered in sintering oven at 1350°C for about 1.5 h as per the manufacturer recommendations. The materials used in the study are shown in [Table 1]. The specimens were divided into the following groups based on surface treatments:
Figure 1: Disc-shaped specimens

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Table 1: Brands, composition, and manufacturers of materials used

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  • Control group (C) - Consisted of CAD-CAM machined specimens. Not subjected to any treatment after fabrication
  • Air particle abrasion (Si) group - Specimens were sandblasted with 30 μm silica-coated alumina particles (CoJet™ sand) at 0.28 mm pressure. After sand blasting, all the specimens were cleansed in an ultrasonic cleaner for 10 min
  • ML group - A cyclic load of 10,000 cycles was applied centrally to the specimens in 37°C water at 2 Hz using with the load between a minimum and a maximum force from 20 to 250 N. During the loading phase, the maximum force was set to mimic occlusal loading in posterior teeth region which was approximately 25% of the mean biaxial flexural strength
  • LTD group - Specimens were autoclaved at 127°C at 1.5 bar pressure for 12 h which induced degradation
  • Cumulative treatment (CT) group- Specimens were subjected to air particle abrasion, ML, and LTD
  • Density measurements- Density measurements were performed on each sintered specimen using the Archimedes' principle calculated using the following equation:


  • ρ = actual weight/actual suspended × ρw

    Where, ρ = density of the sample (g/cm3) while ρw= density of water (g/cm3)

  • Biaxial flexural strength measurement - BiAxial flexural strength measurement was performed as per ISO 6872 specifications. Instron 8871 Servo hydraulic system (Instron®, US) was used [Figure 2]. A jig was fabricated to hold the specimen. It was designed with a support circle of 11 mm diameter, and three steel balls were positioned at 120° angles. A loading pin was used of length 2 mm and diameter 1.5 mm. Samples were placed on the supporting balls and then loaded with an indenter at a cross head speed of 1 mm/min until fracture occurred. Failure load was recorded using graph data manager software. Biaxial flexural strength (MPa) was calculated using the following formula as per ISO 6872 1995 standards:


  • σ = −0.2387 P (X-Y)/d2

    Where, σ is biaxial flexural strength, P = maximum load, L = length (mm) while d = Specimen's thickness

  • Weibull analysis – Weibull analysis was carried-out to determine the variability of flexural strength values. The formula used was:


  • P (σ) =1-exp (−[σ/σ0]m)

    Where, P = Probability of failure, r = strength at a given P, σ0= characteristic parameter while m = Weibull modulus

  • X-ray diffraction analysis (XRD analysis): XRD analysis was carried out to determine the crystalline phase. Five specimens were selected from each group for the analysis. XRD data were obtained with a θ-20 diffractometer (Models: Rigaku Ultima IV and JEOL JD × 3530) using cu-Kα radiation. Garvie and Nicholson's method was used to determine monoclinic phase in the samples. It was expressed in terms of percentage of the tetragonal phase that was transformed to monoclinic phase.


  • Xm = (Im1+ Im2)/(Im1+ Im2+ It)

    Where, I = intensity at angular position 20°.
Figure 2: Biaxial flexural strength testing using universal testing machine

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Statistical analysis used

Two-way ANOVA was used to find significance between the test and the control groups. Tukey's honestly significant difference (HSD) test was carried out to determine any significant difference among the groups.


  Results Top


[Table 2] depicts the mean biaxial flexural strength plus respective standard errors of mean of Cercon specimens. There was increase in the biaxial flexural strength of cercon air particle abrasion (Si) group, LTD group, and CT group except in the ML group where biaxial flexural strength was actually decreased as compared to the control group (C). Highly significant difference (P = 0.000) was found between the control and test groups in two-way ANOVA which was used to find significance between the test and the control groups [Table 3]. Tukey's HSD test was carried out, further, to determine any significant difference among the groups wherein a statistically significant difference (P< 0.05) was observed between C and Si and C and LTD group specimens. On the contrary, there was no significant difference (P > 0.05) was found between C and ML and C and CT group specimens [Table 4]. Weibull analysis was carried out to determine the variability of flexural strength values which showed that there was no monoclinic (m) phase present in the control group; however, other groups showed variable amounts of m phase. The variation was observed from 0% to 27%. Si and ML groups showed 8% and 6.2% m phase, respectively while LTD and CT groups showed variation in m phase from 26.43% to 12.58% [Table 5].
Table 2: Comparison of biaxial flexural strength and Weibull Statistics of Cercon® Group specimens

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Table 3: Two-way ANOVA results

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Table 4: Tukey's honest significant difference test results

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Table 5: Relative amount of monoclinic zirconia (%) of the tested groups

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  Discussion Top


The aim of the present study was to evaluate and compare the influence of different surface treatments and their cumulative effects on the biaxial flexural properties and phase transformation of Y-TZP ceramics. The performance of brittle materials such as ceramics can be determined by evaluating strength which is described as ultimate strength required to fracture or lead to plastic deformation of the physical structure.[8] There are different methods discussed in the literature to measure flexural strength as 3-point test, 4-point test, or biaxial flexural test. Among these, biaxial flexural strength test is widely recognized as the maximum tensile stress occurs within the central loading areas.[9],[10],[11] As observed in the previous studies, air borne particle abrasion during sandblasting as well as polishing procedures creates internal flaws leading to decreased strength. The results of the present study, however, on the contrary, indicated that after air borne particle abrasion with 30 μm silica-coated alumina particles, there was an improvement in the biaxial flexural strength in the tested specimens. This can be explained by the fact that tetragonal to monoclinic phase transformations within the physical structure creates a layer of compressive stress that counteracted the degradation of strength by surface flaws. However, the surface flaws created by sandblasting have not exceeded the compressive layer thickness which could have resulted in decrease in strength rather than an increase.[12] In the present study, air particle abrasion resulted in approximately 8% monoclinic to tetragonal phase transformations. The studies in the past have shown similar results with a conclusion that the improvement in strength was because of an increase in monoclinic phase percentages.[13],[14],[15],[16] A study conducted by Zhang et al. concluded that increase in strength of CoJet sand-blasted specimens was attributed to their smaller size as well as their soft and round configuration.[17] Curtis et al. reported similar behavior with 25 μm Al2O3 particles.[18] The results in the present study, also, showed that mechanical cyclic loading at 10,000 cycles in water using a force of 250 N did not significantly (P > 0.05) affect the biaxial flexural strength in the tested specimens. Similarly, a recent study investigated the flexural strength of In-Ceram Zirconia after fatigue and used a force of 50 N for 20,000 cycles. The results of the said study concluded no significant difference between before and after cycling flexural strength.[19] Furthermore, the materials used in the study conducted by Sobrinho et al. showed higher fatigue resistance than in the present study which might be explained because of the differences in the constitution of the said ceramics since In-Ceram Zirconia contains 35% zirconium oxide while Procera is a polycrystalline ceramic which would have higher fatigue resistance.[20] Curtis et al. evaluated the effect of biaxial flexural strength of zirconia after subjecting the specimens under 500, 700, and 800 N force for 2000 cycles and found that the strength of specimens was not deteriorated. Although the samples were tested to 100,000 cycles using an 80 N force, the biaxial flexural strength was still no different.[21] In the present study, LTD did not show any significant reduction in the biaxial flexural strength in the tested specimens. Numerous studies conducted in the past have shown that autoclaving at 134°C for around 1 h has a similar effect as 3–4 years of ageing.[22],[23] Therefore, accelerated ageing test was performed with autoclaving at 134°C for around 10 h under 0.2 MPa pressure which induced degradation in zirconia.[23] Similar findings have been reported by Pröbster and Diehl.[24] A yet another study reported no statically significant difference in flexural strength of zirconia aged at 37°C for 1 year.[25] Shimizu et al. carried out an experiment to determine the effect of temperature on the specimen flexural strength after placing them in saline solution for 3 years and distilled water at 121°C for 2000 h. Their investigation confirmed that there was no significant change in the flexural strength in ceramic specimens even after such a long LTD treatment.[26] An interesting finding of the present study was that the biaxial flexural strength of the CT group increased as compared to the control (C) and ML groups, however, was lesser as compared to the air particle abrasion (Si) and LTD groups. This can be explained by the fact that compressive force generated by tetragonal to monoclinic transformation overcomes the deteriorating effects of different surface treatments. Similar observation was reported by Kosmac et al. and Guazzato et al. in their studies.[12],[27] The m values observed in various studies have shown varying results from the normal values quoted for dental ceramics to values which were found to be considerably higher.[3],[14],[28],[29],[30] Few groups demonstrated less Weibull modulus as compared to the control group signifying that surface treatment might have affected the reliability of the clinical performance of ceramics. However, larger Weibull values represent that there are fewer critical flaws and indicate a smaller error in the judgment of clinical strength of the said ceramics.[14] The characteristic physical and mechanical properties of zirconia are attributed to its tetragonal to monoclinic phase conversions. The observations of the present study were in agreement with the findings of the previous studies where the control (C) group consisted of 100% tetragonal zirconia.[3],[12],[31] Y-TPZ zirconia remains stable in tetragonal state between 1145°C to below room temperature. Different surface treatments lead to phase transformations imparting characteristic physical and mechanical properties to the said materials.[3],[12],[14],[32],[33] This could be explained by the fact that when they are exposed to stress, change in crystal cell structure occurs and this results in phase transformation including tetragonal to monoclinic phase transformation.[34],[35] In the present study, the greatest amount of monoclinic phase was detected following LTD treatment. Similar results were found in previous studies conducted by Kosmac et al.[3] and de Kler et al.[36] Furthermore, few studies have shown a complete absence of monoclinic content in the control (C) group.


  Conclusions Top


  • The conclusions from the present study were as follows:


    1. The highest biaxial flexural strength was observed in air particle abrasion (Si) group followed by the LTD group, CT group and least with the ML group
    2. A 100% tetragonal zirconia was observed in the control group while the greatest amount of monoclinic percentage was observed after LTD treatment.


    Limitations of study

    One of the major shortcomings of the present study was that it did not mimic clinical conditions exactly which might produce different results due to the presence of saliva and pH changes wherein the restoration is actually under a set of biological conditions of moisture contamination and pH balance changes inside the oral cavity.

    Acknowledgment

    The authors would like to thank all the patients who contributed in the study without whom this study would not have been feasible.

    Financial support and sponsorship

    Nil.

    Conflicts of interest

    There are no conflicts of interest.



     
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        Figures

      [Figure 1], [Figure 2]
     
     
        Tables

      [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]



     

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