EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

MoSi2 as oxidation protective sintering aid in addition to reinforcing SiC fibers to increase fracture toughness are well known strategies to improve ZrB2-based UHTCs. However, the combination of MoSi2 with SiC fibers results in a detrimental reaction above a sintering temperature of 1700°C, jeopardizing the beneficial effect of the fibers. A buffer layer able to obstruct Mo-diffusion is hence mandatory. Three functionally graded ZrB2-based UHT-ceramics have been developed upon variation of the buffer layer composition, in order to enable the combination of MoSi2 with SiC fibers in oxidation resistant and failure tolerant ZrB2-based composite.
In this study, the intermediate buffer layers consisted of ZrB2 with either Si3N4, or polymer derived ceramics such as SiCN or Hf/SiBCN, separating the SiC fiber reinforced ZrB2-ZrSi2 bulk from the outermost ZrB2-MoSi2 oxidation resistant layer. The efficacy of each buffer in preventing SiC fibers from MoSi2 attack was tested upon hot pressing at 1700°C and oxidation in air up to 1650°C.
Microstructural investigations based on scanning- and transmission electron microscopy (SEM/TEM) in combination with energy dispersive X-ray spectroscopy (EDS) were employed to analyze the phase distribution and evolution. The absence of Mo in- and below all buffer layers demonstrates the effectiveness of the intermediate diffusion barrier upon high temperature processes. Several secondary and residual glassy phases were determined, based on which the influence on the MoSi2 diffusion is discussed.