The Cooling Rate and Its Impact on Zirconia Strength

One of many famous songs from Tom Petty includes the line “The waiting is the hardest part.” Zirconia was, no doubt, at the top of his mind while he was writing those words. At least, a dentist could be forgiven for thinking so when considering the cooling time of a zirconia restoration.

Zirconia requires significant heat energy to convert a restoration milled from a block or a puck to a sintered version of the definitive restoration. One analogy used to describe sintering is the process of making a snowball. The stage between a large, loosely packed larger version and the smaller, more compact version of the snowball gives us a visual for what happens during the sintering process. Zirconia restorations are over-milled and, by design, decrease in volume by 22-24% while sintering in the oven.

Due to the high temperatures required for sintering, it is reasonable to expect that a restoration will require some time to cool down to room temperature. With all the advances in technology, isn’t there a way to speed up the process of cooling down a zirconia restoration? More importantly, with the materials available today, if we were to decrease the time required to cool a restoration from the sintering temperature to room temperature, what influence would that have on the strength of the definitive restoration?

One reason to select zirconia is for the strength of the definitive restoration. Flexural strength is the material property that helps to compare a variety of options available for zirconia and is helpful in comparison to other materials, such as lithium disilicate. The idea is that the higher the flexural strength, the decreased likelihood of a fractured restoration.

Impact of Cooling Rate

Uasuwan (see reference) and colleagues evaluated the impact of altering the cooling rate following the sintering process for 5Y monolithic zirconia and 5Y multi-layered zirconia. The zirconia used for the study was Cercon® xt and Cercon® xt ML, available from Dentsply Sirona. Flexural strength was determined using a 4-point “bending device” using standardized material samples. Inherently, a 4-point bending test will provide clinically relevant and, more importantly, lower flexural strength values when compared to a 3-point bending test.

The investigators found that the “fast” cooling technique resulted in a 16.5% decrease in flexural strength in comparison to “slow" and “normal” cooling periods based on manufacturers' recommendations. The reason was thought to be related to an increased amount of monoclinic phase zirconia. Differences between monolithic and multi-layered zirconia were insignificant.

The recommendation: use a normal cooling rate for zirconia to optimize the material properties.

What Else Impacts Zirconia ’s Strength?

Guidelines for tooth preparation are developed to achieve an optimal thickness of the finished zirconia restoration, as the thickness impacts the strength of the definitive outcome. From a practical point of view, my interest as a restorative dentist focuses on fracture strength when the zirconia is at or near the recommended limit. A second molar may likely represent the most common areas of the mouth where a full-coverage zirconia restoration would be the material of choice.

The selection of zirconia for a second molar is influenced by the advertised material properties in terms of “strength” in managing the significant occlusal forces expected. In contrast, another area where there might be challenges to the recommended minimal thickness of the zirconia would be a lower incisor, specifically the area near the finish line of the margin. The occlusal forces could be lower relative to a second molar.

One of the challenges when comparing materials that fit within the zirconia category is the variety between the blocks or the pucks. In general terms, 0.6mm is considered the recommended limit for thickness of zirconia. Working with Figure 2 as a general characterization of how the material thickness influences the fracture strength, 0.6mm thick zirconia has one-third of the strength expected from a 1.5mm thick sample.

There are likely several causes that can be attributed to material failure. For this article, let’s consider a full-coverage zirconia restoration with a restoration thickness at or near the limit (0.6mm thickness for a second molar or a lower incisor). Now add in a second confounding variable where the restoration was not able to cool down to room temperature at the rate or speed recommended by the manufacturer. The challenge is that there is no way to know just by looking at the finished zirconia restoration.

What is the Best Way to Evaluate Zirconia?

Flexural strength is a bending test that has been a common way to evaluate dental materials in various categories. Zirconia as a material is very strong in compressive loading scenarios. Still, it is challenged in that it is not elastic and does not bend and return to its original shape.

Fracture toughness has been championed as a more practical representation of an inelastic material in function, as it represents the resistance to fracture of a cracked material. When working with monolithic zirconia at or near its limit, fracture toughness provides clinically relevant information on how the material might react in function over time. Fracture toughness might help to assess the risk or probabilities of success — or perhaps failure — of a monolithic restoration.

Consider the example of a little crack in the windshield of your car or truck. A windshield with a low fracture toughness would transition from a small crack to a large crack extending across your windshield quickly. Obviously, a large crack in the windshield becomes a safety hazard requiring immediate attention by replacing the windshield.

So, while it may be that waiting is the hardest part when it comes to cooling, it remains, for now at least, the best option to preserve zirconia strength.

Douglas G. Benting, D.D.S., M.S., F.A.C.P. is a member of Spear Resident Faculty.

Reference

Uasuwan, P., Juntavee, N., & Juntavee, A. (2023). Flexural Strength of High Yttrium Oxide-Doped Monochrome and Multilayered Fully Stabilized Zirconia upon Various Sintered Cooling Rates. Journal of Prosthodontics, 32(6), 118-128.