SiC Heating Elements – a Primer
Earlier this year, I wrote an article for The Journal of Dental Technology titled Making Friends with Your Sintering Oven Heating Elements. If you read that article – and stayed awake until the end – you will know that it was a quick primer on the care and feeding of MoSi2 elements – the often U-shaped elements found in many dental Zirconia sintering furnaces (maybe even the one sitting in your lab). These elements are generally the modern choice when it comes to dental sintering furnace design, but there certainly are other elements being used in the market by various manufacturers.
One of these element types is Silicon Carbide, commonly referred to as SiC. Although they achieve the same ends as MoSi2 elements, they are an entirely different beast requiring their own specialized maintenance and care. So, without further ado, let’s talk about SiC.
A Quick Primer on SiC Elements
SiC is a bit of a wonder material – not only is it mechanically very hard, but it has interesting electrical properties as well (it is classified as a semiconductor). And, it goes without mention that SiC has a very high tolerance to heat – in terms of heating elements, SiC has a maximum temperature of 2927F. This unique mix of qualities makes SiC a critical – if not widely known – material for everyday life. Without it, we would not have high-performance brakes for airliners, ceramic high-velocity munition protection plates for body armor, LEDs, high-power electronic devices, cutting discs for Dremel (and other) brand tools, foundry crucibles … the list goes on.
From a heating element standpoint, SiC has been used in this capacity from nearly the time it was commercialized in the early 20th century. As mentioned in my MoSi2 article, low temperature applications have it easy when it comes to element choice. For applications in the 500F-2300F temperature range, metallic wire of varying alloys may be used to get the job done. If the application calls for heat hotter than 2300F, there really are only two choices available – SiC and MoSi2.
It is worth mentioning that SiC is by no means outdated. If you have a SiC-fired furnace in your lab, outfitting it with a new set of elements will ensure that this asset is a solid workhorse that will serve for years to come.
How SiC Elements Work, and How They Fail
SiC elements, like MoSi2 heating elements, are merely electrical resistors. With respect to Messrs. Ohm and Watt, when a high current is applied to a conductor with a low resistance, the result is heat energy. Normally, when I am working with electronics, this results in loss of the oh-so-rare and highly precious “Magic Smoke”, but when you are sintering dental Zirconia in the lab, your results are far more profitable.
Just like MoSi2 heating elements, SiC elements are unique when it comes to electrical properties. Bear with me while I go down a bit of an electrical rabbit hole – it will be worth it, I promise.
With metallic heating elements, the resistance of the wire is the same no matter the temperature – given a length of metallic wire, its resistance will be the same regardless of whether it is as room temperature or at 1000F.
MoSi2 elements act funny in that they have virtually no resistance until a certain temperature – cold resistance is at least an order of magnitude less than hot resistance. Once MoSi2 elements are at operating temperature, however, their resistance will stay consistent for the rest of their life.
With SiC, the measured resistance at room temperature is also different from the resistance measured when hot, but the material still does have a relatively high resistance when it is cold – so, no special control considerations like there are with MoSi2.
For SiC, however, resistance does change with age.
Those of you with SiC furnaces understand that it is time to change elements when the furnace no longer reaches set point temperature. This is because SiC elements age – towards the end of their life, their resistance can be up to 300 times their original resistance. Going back to Mr. Ohm’s law, if resistance increases, either more voltage or more current will have to be added to maintain the same power output (furnace temperature).
When this happens, the elements have most likely been used to full life expectancy, which, incidentally, in the case of dental furnaces, is typically 14-18 months.
The other common mode of failure when it comes to dental furnaces is element overpowering.
If a SiC element looks like the picture above, it is being overpowered. The bubbly/ glazed appearance is due to the Silica “cooking” out of the base material. Just like with MoSi2 elements where the Silica forms a protective oxide layer for the Molybdenum, enabling it to be used in air atmospheres, SiC also forms a protective Silica oxide layer that also allows the base material – Carbon – to be used in an air atmosphere.
As you can guess, a reduction in Silica content is not going to mean good things for heating element life.
Unfortunately, there is not much that you – the furnace consumer – can do about element overpowering. This is something for the furnace manufacturer to solve. That said, fixing the problem is not hard – if you are adventurous, and would like to try and eek every bit of performance and life out of your existing equipment, give me a call and I will be glad to give you some ideas.
Best Practices for Extending the life of SiC Heating Elements
Theory and understanding are great, but when it comes down to it, you are dental laboratory technicians and owners, not heating element specialists. Heating elements are an expensive consumable, and really, all you want to do is sinter more and buy elements less.
To that end, here are the five most common mistakes (and how to avoid them), that I see with SiC elements:
1 – Change all Heating Elements at the Same Time
Remember the saying about water? It follows the path of least resistance, right? Because the resistance for SiC elements changes over time, it is important to change all of the heating elements at the same time because, like water, electricity also follows the path of least resistance. If there are four SiC elements in the furnace, and only one is changed out, that new, shiny element is going to have the least resistance as compared to the other three. When power is applied, significantly more current is going to flow through the new element overpowering it, prematurely aging it to the level of the existing elements.
2 – Buy New Terminal Straps and Clips with Your Elements
The next time you replace your SiC elements, look at the braided Aluminum straps that supply power to your heating elements. Do they appear to be white and fuzzy? If they are, it is time to replace the straps. That white, fuzzy looking stuff is Aluminum oxide, and as it falls off (spalls), small gaps are created between the strap and the element. Just like the spark plug in your car, a high voltage potential will cause an arc between a gap. In the element world, we call these micro-arcs, and they will steadily eat away at your element terminals, causing failure.
Similarly, you may want to change your element spring clips as well. Over time, these clips lose their springiness, loosening the connection and causing the same issue.
3 – Buy from a Reputable Source
Unlike MoSi2 elements, there aren’t as many grades when it comes to SiC – especially in the spiral type heating elements that are mostly used in dental sintering ovens. For straight, non-spiraled elements, there is an option to go to a higher density material which will increase life, but for the spiral cut elements, they are already made of the highest density material available.
In short, be careful about who you give your money. Many companies make SiC elements – especially low-cost operations out of Asia – but the science of manufacturing long-life elements that won’t contaminate your Zirconia product is best left to the reputable handful of companies that have been manufacturing SiC since the early 1900s.
4 – Reduce Thermal Shock
SiC elements are a ceramic product – and, just like untampered glass, they don’t like to see sudden temperature differentials. Do yourself a favor, and if your oven doesn’t have a controlled ramp-down programmed into your sintering cycle, after removing the Zirconia casts, close the oven door to allow it to naturally cool down at a conservative rate. Your elements will last much longer and your pocketbook will surely thank you.
5 – Consult an Expert!
SiC heating elements are not a commodity! They may not be something you think about every day, but choosing elements that perform for an acceptable life without contaminating your product is not a task left for chance. I would love to help you with your furnace problems – feel free to drop me a line at firstname.lastname@example.org if you would like to chat.