Glazed MoSi2 Elements
If the MoSi2 (sometimes called Super) elements for your dental sintering furnace look like the above picture, don’t panic – what you are seeing is a normal build-up of natural Silica oxide on the surface of the element.
That said, as with most things in life, there are two sides to every story. In this case, too much oxide (glazing) can affect both element life as well as the quality of your Zirconia product. Let’s take a deep dive on both and get you up to speed.
Glaze, or Oxide?
If you have been reading our blog, or have read our articles in the Journal of Dental Technology, you at least have a passing familiarity with the oxide forming properties of MoSi2 heating elements. If this article is your first visit to our blog, that is okay – thank you for reading, and I will get you caught-up.
The technical term for the “glazing” is oxide. Molybdenum, the base material of MoSi2 elements, has a high melting temperature (~2600C). Because of this, it is used – in its pure form – as a heating element material for very high temperature applications … so long as the furnace atmosphere is not air.
The problem with air and Molybdenum is that above 760C, Molybdenum rapidly oxidizes. In layman’s terms, that means it goes poof. Because of this, Molybdenum is used mainly in vacuum applications where air is not a concern.
This poof-factor is a problem when it comes to dental sintering, because, well, dental sintering is not done in a vacuum or other process atmosphere. This is where the Si2, or Silica Oxide, comes into play.
As MoSi2 heating elements get hot, the Silica oxide migrates to the surface of the element, coating the Molybdenum and insulating it from the furnace atmosphere (in our case, air). This process is called oxidization, which is why the resulting layer/ material is called oxide.
The reason for the milky appearance is due to Silica’s natural color – which is a milky white. This shouldn’t be a surprise as Silica is most commonly found in nature as quartz.
What’s the problem?
So far, this all sounds good, right? The Silica oxide protects the Molybdenum base material, creating a heating element that works in an air atmosphere and successfully sinters implants, crowns, and other dental components.
As with most things in life, too much of a good thing is, well, no longer good. Dental sintering applications are tough on standard MoSi2 elements. Each high-power cycle from room temperature to 1600C creates a condition where the elements naturally produce large amounts of Silica oxide, resulting in the milky glazed appearance.
The problem with building too much oxide is twofold.
For one, as the oxide layer gets thicker, it starts to act as an insulator (like wrapping the heating element in a blanket). This is a problem as – at temperature – the Silica oxide “floats” on the base Molybdenum material. Most dental furnaces have vertical element configurations, so the Silica oxide – thanks to gravity – migrates down the element and starts to pool around the bend at the bottom of the element.
Consequently, the resulting build-up of oxide causes the bend to run hotter than the rest of the element. When that section runs hotter, it gets weaker. If one area is weaker than another, mechanical stress is created, and being that MoSi2 elements are similar in mechanical strength to glass, any type of mechanical stress is not a good thing and will result in breakage.
Also, as the oxide becomes thicker – and since it expands and contracts at a rate different from that of the base material – the oxide will “flake” or spall from the element, causing contamination in the furnace, and ultimately, a Zirconia product that is not true to the color originally intended.
How can I fix the problem?
Generally, for MoSi2 elements in a dental sintering application, we consider a lifespan of 12-18 months to be acceptable. There are many variables that influence life – cycle rates, furnace usage, furnace construction, to name a few – so, as the saying goes, your mileage may vary.
That said, if your elements are lasting less than 8 months, there is something wrong worth investigating.
If your heating elements show signs of heavy glazing, and are breaking either near the bend or near to the terminals, it may be worth exploring a different style of element that is better suited to dental sintering.
Our Super HT heating elements are formulated so that they form a thinner oxide layer under highly cyclic applications. While a thinner oxide layer may seem like it would reduce element life, the opposite is true in dental furnace applications. Since the oxide layer is thinner, there is less oxide buildup, less hot spots, and as a result, less stress on the element and less breakage.
A side benefit of the thinner oxide layer is that there is far less flaking/ spallation of the oxide layer, resulting in less contamination in the furnace, and a purer, more color-true Zirconia product.
Both of which are most definitely good things.