Helium Ion Microscopy shows Hydrogen degradation of NBR Elastomer

Acrylonitrile Butadiene Rubber (NBR) elastomers are used in Hydrogen delivery systems for Fuel Cell vehicles. The failure of NBR components in high pressure Hydrogen environments is a safety concern. In this case study, Helium Ion Microscopy (HeIM) reveals Hydrogen Induced phase separation of plasticizer in NBR elastomer samples.

Helium-ion microscopy (HeIM) uses a beam of helium ions generated under high voltage, high vacuum and cryogenic temperatures, from an atom-sized ionization source. Interaction of the helium-ion beam with the sample surface produces ion-induced secondary electrons, which are detected and converted to images. HeIM provides sub-nanometer image resolution levels, significantly superior to standard Scanning Electron Microscopy (SEM). HeIM can also provide images with Rutherford backscattered ions (RBIs), using a separate detector. This capability allows HeIM to give information on elemental compositions that a standard SEM cannot. To leverage its high-resolution imaging and surface sensitivity, HeIM has been utilized in surface morphology studies of a wide range of synthetic and natural materials.

Acrylonitrile Butadiene Rubber (NBR) elastomers are used in sealing components of high-pressure hydrogen delivery systems for Fuel Cell Vehicles. The permeation and dissolution of Hydrogen into filled elastomers has been associated with mechanical surface degradation and phenomena such as swelling, blistering and fracture. Eventually catastrophic failure due to rapid gas decompression (RGD) may occur.

In this case study, samples of two NBR compounds labelled N2 and N5, containing plasticizers and/or fillers, were subjected to high pressure Hydrogen at 27.6–90 MPa and elevated temperature. Both the samples N2 and N5 are vulcanized NBR elastomers, with the difference that N5 contains both plasticizer and fillers while N2 does not contain fillers.  Each sample is a 22.2 mm disk with a thickness of 2.9 mm. A Zeiss ORION PLUS helium-ion microscope providing resolution less than 0.3 nm, was utilized for imaging and analyses of surfaces and fractures. Images of surface morphology of the N2 sample before and after exposure to 28 MPa Hydrogen gas are presented in Figure 1. The images 1 (a) and (b) show cracks about 1 μm wide, prior to Hydrogen exposure. After saturation in a high-pressure H₂ environment for a day, dark spots are visible around the edges of the cracks, as observed from images 1 (c) and (d) The development of spots is indicative of plasticizer agglomeration and Hydrogen induced phase separation.

Reference:

Simmons, K.L., Kuang, W., Burton, S.D., Arey, B.W., Shin, Y., Menon, N.C., and Smith, D.B., H-Mat Hydrogen Compatibility of Polymers and Elastomers, International Journal of Hydrogen Energy, ICHS 2019 Conference, Volume 46, Issue 23, Pages 12300-12310.