Molybdenum disilicide heating elements

[Frank5]

I am interested to know failure analysis of MoSi2 heating elements including microstructures, reasons for the failures. Please post some relevant information. Thanks in advance.

 

[JagdishRawat]

I knew it that the concentrate, molybdenum disulphide (MoS2), we produce is also used as heating element after further processing.

But i never knew (or tried to find out) that it is molybdenum disilicide.

Interesting.

What is the price of heating element along with its weight?

Thanks

Met1CO

 

[blacksmith37]

Moly disulfide is an (EP) lubricant, moly disilicide is the heating element.

 

[Frank5]

Hi,Met1CO (Materials:

A typical ‘U’ shape MoSi2 heating element with dimensions (Le= 10.5”, Lu = 6.5”, and A= 1”) will weigh 0.72 pounds. Approximate cost of this will be $300 in this country. MoSi2 elements sustain high power densities, 14 W/cm2. But they are brittle at room temperature. Hope this answers your question.
I have a question why some of the purchased MoSi2 elements have short heating life?. Any metallurgical materials science analysis will be appreciated. Thanks.

 

[cloa]

Its ceramic- you want the ceramics forum.

 

[Metalhead97]

MoSi2 heating element life-cycles depend heavily on the environment in which the heat treating is done. Switching between reducing environments and oxidizing environments can lower the life time of the heating element because the reducing environment removes the protective silica layer that forms on the surface. This layer protects the heating element from internal oxidation. Before use MoSi2 heating elements are intentionally oxidized to produce the silica layer.

 

[Frank5]

Thanks to Metalhead97!

What if you produce initially too thick a silica layer and expect longer service life?
Specially, when I run non-stop continuous cycle for 21 days, I have problems with these elements. The silica layer gets depleted completely from the surface.
I had measured ~2 to 3 µm silica layer in the as purchased elements. Is this optimal thickness? Did someone had measured and have information?
Does any one have a microstructure of the element to share?. Thanks!

 

[MTbadger]

- What temperature are you running the elements at? (1800-1850C is possible, but this will result in a shorter life than if you are running at 1600C.)
- How well is your furnace insulated? (this determines how much power you are pumping into the elements to maintain your desired temp- more power because of losses means higher element temps = shorter life)
- What is the mode of failure? (is the element thinning gradually or failing while still close to original thickness)
- What is atmosphere? (I assume air...)

My experience is that you can run at 1600-1650C for hundreds to several thousand hrs of hours in air with somewhat standard mosi2 elements. At 1850C, you might typically last only a few hundred hours in a well insulated lab size furace.

MoSi2 elements typically fail by gradual thinning due to oxidation losses. When they drop to a certain thickness, the power density capability is exceeded and they heat up hotter in the thin area than a thick area and burn out. The thinning is also exacerbated by grain growth of the MoSi2, as the grains grow due to the high temps you can sometimes see an orange peel look on the surface.

Manufacturers typically add small amounts of other elements to MoSi2 to help prevent grain growth. Some elements are better than others in this respect and manufacturers have different grades for different conditions. MoSi2 elements also do not prefer to be turned on and off frequently, using them at lower temperatures can actually speed up oxidation thinning (pesting behavior)

So to prolong MoSi2 element life: make sure furnace is insulated well, you are using the right grade and good elements, and avoid high ramp up/down rates.

see Kanthal site for more info:

http://www.kanthal.com/products/fur...ments/molybdenum-disilicide-heating-elements/

 

[Frank5]

Thanks for the response MTbadger (materials). Response opens up few more and other questions.

• The furnace temperature is 1534C for 512 h (elements must be working at 1650C). Cycle duration is 512 h at 1534C.
Question: Scenario like this how many hours of life can be expected from the elements?

• Furnace is well insulated. Furnace is operating at 70% of the rated power steady state conditions.
Question: What maximum temperature the terminals might attain can be considered as normal? Or, what temperature of the terminals is considered as ‘over-heated terminals’?. Did some one measure temperature of the terminals how much was noted?.

Question: Why corrosion product is seen on the terminals?

• 75% elements are thinning before they fail, and 25% of the elements failing still close to the original diameter.
Question: What does this tell us?

• Air is the atmosphere.

• Question: Why oxidation losses? Do you mean to say fresh air is not available to oxidize?

• Question: Can you comment on the microstructure of MoSi2? E.g., what is the matrix, what phases are present? What is desirable and what is not?

• Question: Do you have a microstructure to share as an example? Or can you direct me to a resource in open domain?