It seems as though we at CAP take a fair number of support calls for bad-looking zirconia. Most often, this is related to sintering cycles and/or sintering complexities. Here are the top 5 need-to-know details about sintering zirconia.
- Know Your Heating Elements. You should be aware of the type of heating elements in your furnace, likely, either Molybdenum Silicide (MoSi2) or Silicon Carbide (SCi). Both have pros and cons.
- MoSi2 Heating Elements Require Special Care:
- They form a ceramic quartz outer surface that interacts with green state colorants. Over time the melting point of this quartz becomes lower than the top temperature of the sintering cycle and creates pollution in the furnace chamber. These pollutants can cause greening or darkening of the restorations.
- When sintering in a furnace with MoSi2 heating elements, always use a covered crucible. This will reduce the likelihood of chamber pollution getting into the restoration.
- Quartz Pollutes. The quartz pollution can have an effect on the thermocouple over time (6-12 months). When this occurs the thermocouple will send lowerer temperature readings to the control panel resulting in firing temperatures of up to 100° higher than programed.
- Watch the Temp Throughout. From room temperature to 900°C there is little effect on the restorations. The ramp rate from 900° to top temp, top temp to hold time, consistency of temp through the hold, and then rate of cool back to about 900° are all critical.
- Make Sure Colorants are Dry. Not adequately drying colorants added to green state colored zirconia prior to sintering will shorten MoSi2 heating element and add to the pollution problem in sintering furnaces.
For best results, always follow manufacturers instructions when sintering zirconia.
Learn more about sintering furnaces here.
Thanks for reading,
Bob Cohen, CDT