A new study from the University of Arizona compared the strength of microwave versus conventional sintering of Zirconia, and found that while all zirconia is marginally harder (5-20%) as a result of microwave sintering, the most dramatic results occur over time when traditionally sintered zirconia deteriorates in the mouth.  This study is the most comprehensive independent analysis of zirconia sintering methods to date, and is available to our clients as a PDF via our online request form.

The study, entitiled "Flexure Strength and Hydrothermal Degradation of Yttria-Stabilized Zirconia: Microwave vs. Conventional Sintering," written by Professor Pedro Peralta and Kirk Wheeler of the Department of Mechanical and Aerospace Engineering at Arizona State University came to the following conclusions:

1. "Samples sintered in the microwave oven had an increased flexure strength compared to samples sintered in a conventional furnace. This was attributed to a reduced grain size and a sintering process that encourages rapid densification due to enhanced sintering kinetics produced by the microwaves."
   
   
2. "Samples were tested in a steam environment at ~125°C and ~200kPa pressure for 75hrs and a reduction of flexure strength by ~43% was found for conventional samples and only ~14% for microwave samples."  In other words, after 10 years of simulated testing in the mouth, microwave sintered zirconia does not deteriorate significantly and is nearly 50% stronger than traditionally sintered zirconia.

The chart to the left illustrates some of the most dramatic results from the study.  By applying pressure and temperture for a short period of time, the Hydrotherm test is designed to simulate conditions in the mounth over a longer period of time, in this case a 75 hour high-pressure high-temperature liquid environment which is designed to simulate 10 years.

High Strength formula Crystal brand dental zirconia was used in the test, milled down to ADA 69 / ISO 6872 standard sized bars which are approximately the size of an average dental bridge.  With conventional firing, Crystal achieved aaverage bending strength of just less then 1400 MPa, which makes it a VERY strong material. With microwave sintering of the same sample lots, nearly 1500 MPa was achieved, the highest number that we have EVER seen for Zirconia.

More importantly, when a random selection of samples from the same lot were placed in hydrotherm environment, the conventional samples fatiqued quickly, and their bending strength dropped by more than 40%.  This property of dental ceramics losing strength over time when surrounded by liquid and body temperatures has been well known, and because 800+ MPa still represents a very strong dental material, this degredataion has never been considered to be of serious consequence.  In most people's opinions, dental restorations are not designed to last a lifetime.

Dental Laboratory Milling Supplies, who markets the Crystal brand of zirconia, however, offers a lifetime warranty on their materials, so "lasting a lifetime" represents an entirely different type of objective to them -- for Crystal, it is a business imperitive.  Therefore, DLMS was delighted to see that after 10 years simulated time in the mouth, their microwave sintered high strength materials were still at nearly 1300 MPa bending strength, which is 50% stronger than the conventionally sinterered samples.

Other brands of zirconia were also tested in the study, including 3M LAVA and the Crystal High Translucency product which conventionally sinters at less than 1000 MPa.  For these other comparable and competitive products, which are doped with other materials give the translucency or other properties, the results CAN be even more dramatic, with microwave sintering producing as much as 20% additional hardness over conventional sintering.