MoSi2 Elements - A Primer
We have all had that moment – where the essential tool in the shop or office suddenly needs a replacement something-or-other that costs a surprising amount of money. As someone with a hobby habit (to put it lightly),my personal essentials range from the relatively esoteric – cutting tools/blades (looking at you, planing knives) to one of the usual suspects – desktop printer ink.
“That” Type of Consumable
As dental laboratory folks, perhaps a more apropos analogy is already sitting on your desk or bench – your sintering oven. Although it is a common, everyday item for use in the lab, the MoSi2 heating elements inside that give it its high-temperature qualities are anything but ordinary. They are highly engineered marvels of material science – definitely “that” type of consumable.
That said, you don’t have to be a heating systems expert to get the most out of your heating elements – just like me and my woodworking, a bit of information goes a long way towards the improvement of the final product. Let’s just hope your dental castings turn out better than my furniture.
A Quick Primer on MoSi2 Elements
In general, resistive heating elements scale in complexity and cost according to their temperature. For basic heating applications – such as a toaster – simple metallic wire that has favorable properties in the 500F-800F range is typically chosen. Similarly, these types of wires are also commonly used for lower temperature industrial heating applications (below 800F).
Broadly categorized as “Ni-Chrome” wire or NiCr, this resistance wire is a mixture of Nickel and Chromium; the latter forms a protective oxide layer on the surface of the Nickel, allowing it to last for a long time under (relatively) high temperatures.
Up to about 2230F (1200C), metallic elements of varying grades and compositions are a viable option. Beyond 1200C, things start to get a bit more interesting. In the temperature ranges required for Zirconia or Cobalt Chromium sintering, Silicon Carbide (the subject of a forthcoming article) and MoSi2 are the only materials from which elements can be made and still have acceptable longevity and performance characteristics such as low spallation/flaking (useful when considering the cosmetic appearance of the sintered item – more on this later).
MoSi2 elements are made out of Molybdenum and Silicon; the latter, similar to the NiCr example above, forms a protective oxide layer on the Molybdenum. In an air atmosphere, Molybdenum vaporizes when heated above a certain point. The utility of the oxide layer goes without saying.
MoSi2 elements are typically graded by their maximum element temperature in Celsius – 1700, 1800, and 1900 (and HT). The latter grade is specific to Sandvik/ Kanthal elements, and is designed specifically for high temperature/highly cyclic applications.
A common error when specifying MoSi2 elements – though this applies to all heating elements as well – is specifying an element grade based on the process temperature only, not taking into consideration element temperature. In general, furnace elements must run approximately 50C hotter than the process temperature in order to maintain a given set point. In the world of heating elements, surface loading – measured as Watts per square inch – directly influences element life. The relationship between element life and surface loading is non-linear. Once an element is powered past approximately 80% of its maximum surface loading, life is dramatically reduced.
So, even a temperature delta as low as 50C can have big implications. Depending on your furnace, if you are sintering at 1600, 1700 grade elements may be fine. If you are sintering at 1650, you will want to use 1800 grade elements to achieve maximum element lifespan.
A Note on Zirconia Discoloration
One of the more common issues I hear from dental sintering folks who use MoSi2 elements is discoloration of their sintered Zirconia product.
Broadly, the discoloration is caused by contaminants bonding with the Zirconia. More specifically, the discoloration is caused primarily by Iron and Molybdenum leaching out of the elements and into the furnace environment.
Preventing Iron leaching is a relatively straight forward process – don’t buy cheap MoSi2 elements! Sarcasm aside, this is a good guideline to follow. All MoSi2 elements will have trace amounts of Iron present as a contaminant, but the best and most pure elements will relegate Iron to trace amounts rather than problematic amounts.
As the saying goes – you get what you pay for. Although it is tempting to look on Alibaba or other Chinese sourcing sites for MoSi2 heating elements … caveat emptor. I am sure that the Chinese and other second-source markets will catch-up at some point, but the fact of the matter is that high-purity MoSi2 elements are difficult – and relatively expensive – to manufacture. There are only a handful of reputable manufacturers in the world for these types of elements, and it has taken them decades to perfect the manufacturing of high-purity MoSi2 heating elements.
As far as Molybdenum contamination goes, this one is a bit trickier. As previously mentioned, MoSi2 heating elements form a protective Silicon oxide layer. These two materials work well together, but there is a bit of an inherent challenge in highly cyclic applications such as dental sintering, since Silicon heats and cools at a different rate than Molybdenum (they have different Coefficients of Thermal Expansion). Over time, the Silicon builds up on the surface of the Molybdenum, and as it grows thicker, it begins to crack and flake off of the element and into the furnace environment.
As the Silicon flakes, trace amounts of Molybdenum come along with it, causing contamination. There are solutions in the marketplace – super-high purity elements designed specifically to form thinner oxide layers in highly cyclic applications – that can be explored to combat this issue. Alternatively, using elements that are rated several temperature grades higher than the process temperature may also help.
Best Practices for Extending the life of MoSi2 Heating Elements
In a perfect world, MoSi2 elements in continuous applications (no cycling to ambient temperature and back to process temperature) can expect 2-3 years of life. In the dental furnace world, things are a bit worse as cycling exacerbates the oxide growth and subsequent flaking issue. That said, dental laboratories should be expecting 12-18 months of life out of their elements, maybe shorter, maybe longer, depending on how much the furnace is utilized, and what grade of element is being used.
In general, here are 5 things that any dental laboratory sintering operation can do to help extend element life:
1 – Order terminal straps and clips with each new element.
The straps that come with your elements are braided Aluminum. Over time, these straps oxidize (a tell-tale sign of this is the formation of what looks like frosting on the straps), and as the oxide layer spalls/flakes off the straps, the formerly snug connection between the element terminals and the straps becomes progressively looser. The resulting gap becomes the perfect spot for arcs to form between the terminal and the terminal strap which, over time, eats away at the terminals and causes premature element failure.
2 – Ensure that the terminal shanks are insulated, but able to move independently.
Like anything that is heated, MoSi2 elements need some freedom to move as they naturally expand and contract during heating and cooling cycles. Although the elements are able to produce a tremendous amount of power at high temperatures, they are mechanically fragile. In many respects, handling them is akin to handling glass. A common error is to use “two shank” element holders – they hold both element terminals together in a rigid block, leaving no room for movement. Our recommendation is to use single shank holders.
Also, make sure to use the correct terminal strap length. Straps that are too long – or too short – can place stresses on the terminals, also causing breakage.
3 – Double check your element temperature rating, and consider moving up a grade.
Back to the element surface loading discussed earlier in this article: moving up a temperature grade (from 1700 to 1800, for example), provides greater operating headroom as the element temperature is a lower percentage of the element maximum temperature. In turn, this could significantly lengthen element lifespan. For common sintering applications at 1600C, making the jump from 1700 to 1800 grade elements is a small enough price delta that the improved element lifespan typically provides a quick enough ROI to justify the switch.
4 – If you are getting less than one year of life, or are looking for maximum life, consider trying elements made specifically for dental sintering.
There is no way around it, specialized elements are expensive. But, they do make a difference. High tech elements such as Kanthal Super 1900 HT – that are engineered to manage oxide growth – have a proven track record in the market.
5 – Consult an expert!
High temperature heating elements are not a commodity. MoSi2 and SiC elements are an integral part of the engineered system that is your sintering furnace. Contact an expert to make sure that you are getting the best life and the best results from this highly precise system. We live and breathe heating elements – drop us a line anytime.