INFLUENCE OF CAD/CAM GRINDING IN THE PERFORMANCE OF SINTERED DENTAL ZIRCONIA FRAMEWORK

Carlos Nelson Elias1a, Andréa Mattos Melo1b, Claudinei dos Santos2,3c, 1 Laboratório de Biomateriais, Instituto Militar de Engenharia, Pr. General Tibúrcio, 80,

Pr Vermelha, 22290-270 Rio de Janeiro, RJ - Brazil 2 UniFOA- Centro Universitário de Volta Redonda, Mestrado em Materiais, Volta Redonda, RJ -

Brazil 3 ProtMat Materiais Avançados Ltda, Volta Redonda, RJ – Brazil

a elias@ime.eb.br, ba.matosmelo@gmail.com, cclaudinei@demar.eel.usp.br Keywords: CAD/CAM, zirconia, prosthetic restorations, copings

Abstract. The brittle behavior of ceramics limits the use of these materials under conditions of

cyclic loading, as is the case of fixed partial dentures. To improve toughness and biocompatibility

of ceramics is necessary to employ powders with better purities, adjust the conditions of

compaction and sintering, microstructure control and explore mechanisms for increasing the

toughness. Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) is generally used for dental

ceramic restorations. The zirconia framework is fabricated using the CAD/CAM system. The

ceramic mechanical properties are determined by testing polished sample. Previous work did not

analyze the influence of surface defects induced during dentures grinding. Ceramic restorations

manufacturing are made with coarse grinding high-speed diamond rotatory cutting instruments.

This process induces residual stresses and the high temperature induces surface cracks.

Consequently, a lower strength and reliability of the material is observed. In this work the

mechanical properties of yttria stabilized zirconia were determined with the use of samples of dental

prostheses molded in patients and machined with the use of a dental laboratory CAD/CAM system.

The results showed that the mechanical properties of pre-sintered blocks are different from

prosthetics machined by CAD/CAM and sintered under the same conditions used in the laboratories

of the prosthesis. The defects created during machining reduce the mechanical properties of

zirconia.

Introduction

The literature shows results of zirconia mechanical properties based in pre-sintered block.

Thus, the performance of zirconia prosthesis is not yet quantified. The mechanical properties of the

blocks are different from those sintered and machined prosthesis. It is believed that the machining

process as well as the degradation in aqueous media can influence the surface morphology and

residual stresses in the final product, thus the interpretation of test results with blocks lead to errors

in analysis of the behavior and mechanical properties of the final zirconia prosthesis.

The machining of ceramic green body introduces defects and can induce a compressive

stresses in the prosthesis surface. These defects change flexural strength. The machining process

done after complete sintering of the test sample creates surface defects that act as stress

concentrators [1]. Some internal and external defects coming from the cooling after sintering

process and reduce the Y-TPZ toughness.

Materials and methods

Dental prostheses with three elements were made and submitted to a three bending tests (Fig

1). The partial denture was attached to two dental implants. The dental preparations respected the

threshold setting cervical prepared to shoulder with rounded corners, space for accommodation of

1.0 mm minimum thickness and substructures of 0.5 mm. A total of 8 dental prostheses supported

by implants were machined from blocks of pre-sintered zirconia (ZrO2-3% in a mol Y2O3). The

zirconia blocks were supplied by the company ProtMat Advanced Materials Ltd (Volta Redonda,

RJ Brazil).

Materials Science Forum Vols. 727-728 (2012) pp 1081-1084Online available since 2012/Aug/24 at www.scientific.net© (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.727-728.1081

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 68.181.176.15, University of Southern California, Los Angeles, United States of America-10/04/14,19:02:35)

Figure 1: Dental prostheses model used in the present work.

Results and Discussions

The measures of the connector dimensions (diameter and length) were performed using an

optical microscope (Fig 2). Based on the connector measurements and the applied force the

maximum stresses in the connector of the prosthesis were calculated. For the calculations we

considered the model of a prismatic beam subjected to bending stresses. The stress (σ ) was

calculated by the following expression:

σ = Mf. y / I (1)

where “Mf” is the bending moment, “y” a distance between the neutral line and any point of the

cross section of the beam and “I” is the value of moment of inertia of this section. The bending

moment Mf is calculated by the product of the reaction force in support of a (Q) by the lever arm

(x), which is the distance between the support and the line of force application.

Mf = Q.x (2)

Figure 2: Connector part of the dental prosthesis.

Considering that the connector has a circular section, the moment of inertia of its cross

section can be obtained by the equation using a round beam and maximum stress ( σmax) is

maximum when y = d / 2.

Substituting the values of “y” and “R” in the equation (1) and assuming for purposes of

calculating that the prosthesis is symmetric with respect to the pontic, the reactions at the supports

are equal, corresponding half the applied force. There is the expression of maximum tension stress.

σmax = Κ 16 Mf/πd3

Applying the bending moment specific to the different measures of L/2 and the forces

applied to the fracture will have the value of the applied tension needed to fracture the prosthesis.

The values of maximum stress and diameter of the connector found are shown in Table 1.

1082 Advanced Powder Technology VIII

Table 1: Data from the bending test at three points.

Load [kgf] Diameter[mm] L[mm] Maximum stress [MPa]

63,79 4,29 9,45 388,0

58,24 3,14 9,78 143,7

75,1 4,34 8,69 434,9

71,09 4,19 10,03 427,6

59,34 4,4 9,82 404,6

111,12 4,3 11,8 849,9

106,84 4,41 10,05 750,8

71,76 4,23 9,76 432,1

Results of the literature [2,3] show that the mechanical properties of the used zircônia blocks

are from 900 to 1200 MPa. The mechanical properties are determined with polishing ceramic

samples. Table 1 shows that the machined sample tested in the present work has lower mechanical

properties than the good finishing one.

Based on the results, we conclude that the maximum fracture stress is inversely proportional

to the area of the connector. This result may be associated with increased probability of finding a

critical defect due to an increase in the area of work, as well as the worst surface finish and possibly

to increased wear of cutting tools during machining operations. Although the maximum strength

increases with the volume, the tension decreased. Figure 3 shows the Weibull distribution of the

results.

Figure 3: The Weibull distribution to the stress values (a) machined samples, (b) polished samples.

The parameter “m” is the Weibull module. Given the values calculated in this study was

obtained for polished and machined prosthesis respectively m=8.3, m=5.3. It is concluded that there

was a large spread in the values of fracture toughness of the samples after machining. This variation

in results may be explained by the different defects introduced during machining. To check the

accurance of fit of the experimental results of the equation we use the correlation coefficient (R2).

The values were close to 1, indicating that the results are adequately described by the equation.

Despite the small number of samples may be associated with the polishing phase

transformation and consequent reduction of the bending resistance of some polished samples. The

lack of standardization of the finishing and polishing step can be associated with the spread of

results in the samples polished.

Conclusions

1. The process of machining CAD / CAM introduces defects on the surface of the prosthesis.

2. The dental prostheses bending strength reduces after machining of zirconia.

3. The maximum fracture stress is inversely proportional to the area of the connector.

4. The relationship between compressive strength and thickness of the prosthesis is directly

proportional.

Materials Science Forum Vols. 727-728 1083

Acknowledgments

The authors would like to thank the ProtMat Advanced Materials Co for providing the zirconia

blocks. This research was supported by a grant from CNPq under the 302251/2009-6 project and

FAPERJ E_26/102.714/2008 project.

References

[1] T. Osmac, C. Oblack and P Jevnikar: Dental Materials Vol. 15 (1999), p. 426.

[2] C. dos Santos and C.N. Elias: Key Engineering Materials Vol. 396 (2009), p. 603.

[3] C. dos santos, M. Koizumi, J. Daguano, C.N. Elias and A Ramos: Materials Science &

Engineering. A Vol. 502 (2009) p.6.

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Advanced Powder Technology VIII 10.4028/www.scientific.net/MSF.727-728 Influence of CAD/CAM Grinding in the Performance of Sintered Dental Zirconia Framework 10.4028/www.scientific.net/MSF.727-728.1081

DOI References

[1] T. Osmac, C. Oblack and P Jevnikar: Dental Materials Vol. 15 (1999), p.426.

http://dx.doi.org/10.1016/S0109-5641(99)00070-6 [2] C. dos Santos and C.N. Elias: Key Engineering Materials Vol. 396 (2009), p.603.

http://dx.doi.org/10.4028/www.scientific.net/KEM.396-398.603