Influence of Low-Temperature Environmental Exposure on the Mechanical Properties and Structural Stability of Dental Zirconia Tariq F. Alghazzawi, BDS, MS, MSMtE, PhD, 1 Jack Lemons, PhD, 2 Perng-Ru Liu, DDS, MS, DMD, 3 Milton E. Essig, DMD, 4 Alfred A. Bartolucci, PhD, 5 & Gregg M. Janowski, PhD 6 1 Assistant Professor of Prosthodontics and Materials Engineer, Director of Laboratories, Department of Prosthetic Dental Sciences, College of Dentistry, Taibah University Madina, Saudi Arabia 2 Professor, Department of Prosthodontics, The University of Alabama at Birmingham, Birmingham, AL 3 Professor and Chair, Department of Prosthodontics, The University of Alabama at Birmingham, Birmingham, AL 4 Professor Emeritus, Department of General Dental Sciences, The University of Alabama at Birmingham, Birmingham, AL 5 Professor, Department of Biostatistics, The University of Alabama at Birmingham, Birmingham, AL 6 Professor and Assistant Dean for Assessment and Accreditation, Department of Materials Science and Engineering, The University of Alabama at Birmingham, Birmingham, AL Keywords Low-temperature degradation; zirconia; transformation. Correspondence Tariq F. Alghazzawi, BEC 254, 1530 3rd Ave S., Birmingham, Al 35294-4461. E-mail: tariq@uab.edu This project was supported by the American College of Prosthodontists Education Foundation (ACPEF) Research Fellowship Award on November 28, 2008. The authors deny any conflicts of interest. Accepted October 5, 2011 doi: 10.1111/j.1532-849X.2011.00838.x Abstract Purpose: The effect of dental fabrication procedures of zirconia monolithic restora- tions and changes in properties during low-temperature exposure in the oral environ- ment is not completely understood. The purpose of this study was to investigate the effect of procedures for fabrication of dental restorations by low-temperature simu- lation and relative changes of flexural strength, nanoindentation hardness, Young’s modulus, surface roughness, and structural stability of yttria-stabilized zirconia. Materials and Methods: A total of 64 zirconia specimens were prepared to simulate dental practice. The specimens were divided into the control group and the accelerated aging group. The simulated group followed the same procedure as the control group except for the aging treatment. Atomic force microscopy was used to measure surface roughness. The degree of tetragonal-to-monoclinic transformation was determined using X-ray diffraction. Nanoindentation hardness and modulus measurements were carried out on the surface of the zirconia specimens using a nanoindenter XP/G200 system. The yttria levels for nonaged and aged specimens were measured using energy dispersive spectroscopy. Flexural strength was determined using the piston-on-three- ball test. The t-test was used to determine statistical significance. Results: Means and standard deviations were calculated using all observations for each condition and evaluated using a group t-test (p < 0.05). The LTD treatment resulted in increased surface roughness (from 12.23 nm to 21.56 nm for Ra and 15.06 nm to 27.45 nm for RMS) and monoclinic phase fractions (from 2% to 21%), with a con- comitant decrease in hardness (from 16.56 GPa to 15.14 GPa) and modulus (from 275.68 GPa to 256.56 GPa). Yttria content (from 4.43% to 4.46%) and flexural strength (from 586 MPa to 578 MPa) were not significantly altered, supporting longer term in vivo function without biomechanical fracture. Conclusion: The LTD treatment induced the tetragonal-to-monoclinic transformation with surface roughening in zirconia prepared using dental procedures. Zirconia is a polymorphic material that exists in three crys- tal structures: monoclinic, tetragonal, and cubic. 1 Pure zirconia is monoclinic from room temperature to 1170 ◦ C. 2 Above that temperature, it transforms into the tetragonal phase. At a temperature of 2370 ◦ C, zirconia transforms into a cubic phase. The tetragonal phase may be stabilized by adding small amounts of metallic oxides, such as Y 2 O 3 , MgO, CeO, or CaO, but it is, in fact, metastable at room temperature. Processes such as grinding and sandblasting can trigger the tetragonal- to-monoclinic phase transformation. 3,4 This transformation is accompanied by a 3% to 4% volume expansion that induces compressive stresses, thereby closing the crack tip and pre- venting further propagation. 5 This characteristic, known as transformation toughening, leads to the increased fracture Journal of Prosthodontics 00 (2012) 1–7 c  2012 by the American College of Prosthodontists 1