Nitrogen and helium permeation tests of SiCf/SiC composites

The permeance of inert gases (N2 and He) through coated SiC based ceramic composites both in form of tubes and discs has been measured, in particular SiC–SiCf/SiC functionally graded material tubes made of SiC–SiCf with a SiC polycrystalline sealing coating and chemically vapour infiltrated (CVI) SiCf/SiC composite discs have been tested.The permeation tests have been aimed at verifying the effectiveness of coatings obtained by chemical vapour deposition (CVD) and used for closing the porosity of the composite materials and assuring their gas sealing. The measurements have been carried out at room temperature with operating pressure in the range 130–500 kPa. The SiC–SiCf/SiC functionally graded material has shown very low permeability values. The CVI SiCf/SiC composite tested has shown rather low permeance values, although the material is not gas tight: its reduced permeance was attributed to both the high fibre percentage and the dense SiC coating.