Out-gassing of Tungsten Disulfide and Molybdenum Disulfide

Out-gassing is a property of a material that expresses its weight loss and tendency to condense onto a colder surface when tested at an elevated temperature [i]. The test required for out-gassing for tungsten disulfide listed in the AMS 2530 specification is ASTM E595.  There is not an out-gassing test required for molybdenum disulfide under AMS 2526 but, we tested it for comparison to tungsten disulfide.​

From the ASTM E595 specification paragraph 1.5 – The criteria used for the acceptance and rejection of materials shall be determined by the user and based upon specific component and system requirements. Historically, a total mass loss (TML) of 1.00% and collected volatile condensable material (CVCM) of .10% have been used as screening levels for rejection of spacecraft materials.

For this test we send out a 2-gram sample of material to an ISO/IEC 17025:2017 accredited lab. The test is performed in a vacuum environment of less than 5 x 10-5 Torr for a duration of 25hrs at 125 degrees C. Here is an example of our previous WS2 & MoS2 out-gassing test results:

Table showing outgas test results for tungsten disulfide and molybdenum disulfide, showing percentage results for TML, CVCM, and WVR tests.

Our total mass loss changes slightly from lot to lot but, never exceeds the 1.00% limit. This test is not viewed as a requirement by most of our competitors for the AMS 2530 specification so, they do not test for it.  We perform this test on every lot number to ensure a quality product to our customers.  

Here is some out-gassing test results from NASA. The testing was from 1969, around the time tungsten disulfide was starting its use as a space lubricant. This testing was prior to NASA standardizing on out gas testing to the ASTM E595 specification. In figure 4 below are the test results by NASA [ii]:   

Graph showing weight loss rates of MoS2 and WS2 with increasing temperature. MoS2 data indicates a sharp increase in weight loss rate around 2000°F, with sulfur peaks observed on a mass spectrometer at that point. WS2 data shows very low weight loss up to approximately 1600°F, then increases sharply, with sulfur peaks also observed near the higher temperatures.

No weight loss was observed for MoS2 up to 1700' F (930' C). Beyond 1700' F (930' C), the weight loss rate increased with temperature. At 2000° F (1090' C), sulfur peaks were observed, indicating the liberation of sulfur because of thermal dissociation [ii].  Similar results were found from another experiment when heating MoS2 powder in vacuum to 1832' F (1000 C) and reported a constant weight loss rate at 1742' F (950' C) [iii].

WS2 began to have a detectable loss rate at 1600' F (870' C), and sulfur peaks were observed at 1900' F (1040' C). The rate-temperature slope for WS2 is less than that of MoS2.

References:

[i]  VEST, C. E. (1993). LUBRICATION OF SPACECRAFT MECHANISMS. Johns Hopkins APL.

[ii] Brainard, W. A. (1969). THE THERMAL STABILITY AND THE FRICTION OF THE DISULFIDES, DISELINIDES, AND DITELLURIDES OF MOLYBDENUM AND TUNGSTEN IN VACUUM. NASA. Cleveland: Lewis Research Center.

[iii] Johnston, R. R. M.; and Moore, A. J. W.: Water Adsorption on Molybdenum Disulfide Containing Surface Contaminants. J. Phys. Chem., vol. 68, no. 11, Nov. 1964, pp. 3399-3405

​[iv] Vazirisereshk MR, Martini A, Strubbe DA, Baykara MZ. Solid Lubrication with MoS2: A Review. Lubricants. 2019; 7(7):57.