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ADASRI staff develop new technique for testing fracture toughness of zirconia

Researchers use focused ion beam milling to create tiny, sharp starter cracks

August 24, 2020

By Mary Beth Versaci

Researchers from the American Dental Association Science & Research Institute, in collaboration with Argonne National Laboratory in Lemont, Illinois, have developed a new way to create sharp, nanometer-sized notches necessary for measuring the fracture toughness of zirconia materials used in dental restorations.

Their original research report, "Fracture Toughness of Zirconia with a Nanometer Size Notch Fabricated Using Focused Ion Beam Milling," was published in June by the Journal of Biomedical Materials Research.

Photo of Nathaniel Lawson
Dr. Lawson
"A major clinical advantage of using zirconia for dental restorations is its ability to prevent the propagation of cracks introduced through adjusting, polishing or biting on a restoration," said Dr. Nathaniel Lawson, Ph.D., director of the Division of Biomaterials at the University of Alabama at Birmingham School of Dentistry and member of the ADA Clinical Evaluators Panel Oversight Committee of the ADA Council on Scientific Affairs. "Fracture toughness is a test used to measure the ability of a material to resist crack propagation. Fracture toughness testing is notoriously difficult to perform in the laboratory. The work done by the ADA represents an advancement in the methods used to measure this property and to more reliably compare different types of dental ceramics."

Creating a starter crack in nanograin material like dental zirconia is difficult, leading to concerns about the reliability of some of the fracture toughness data in dental literature and marketing. With that in mind, the ADASRI researchers collaborated with the Argonne National Laboratory Center for Nanoscale Materials and the Advanced Photon Source to develop this method.

Photo of Yifeng Liao
Dr. Liao
The ADASRI researchers included Yifeng Liao, Ph.D., manager of research and standards and lead researcher on the study; Max Gruber, engineering research assistant; Henry Lukic, engineering research associate; and Spiro Megremis, Ph.D., research and laboratories director, who were joined by Si Chen with Argonne.

To get accurate fracture toughness results, the researchers needed to create starter cracks that were as sharp as possible and smaller than the grain size of zirconia, which is about 300 nanometers for the zirconia with 3 mol% yttria used in this study.

Using focused ion beam milling, the researchers milled notches that were less than 100 nanometers wide in standard specimens and compared these to standard specimens without the milled notches.

The study found the average fracture toughness for the specimens with milled notches was considerably lower than the specimens without them, showing toughness values may be affected by the testing procedure used.