The purpose of this innovation is to develop advanced multilayered coating architectures to protect graphite substrates from hot hydrogen attack. The concept consists of coating the graphite substrate with metallic and non-metallic layers consisting of ZrC; Nb, Mo, and/or Nb-Mo alloy; and/or Mo2C.
The coating architecture is designed with several factors in mind. First, a diffusion barrier coating must be developed to prevent or slow down the diffusion of the hydrogen through the ZrC coating to the graphite substrate during the operational life of the component. Second, the ZrC coating must be strengthened suitably to minimize the creep of the coating. Third, the coefficient of thermal expansion (CTE) mismatch between the ZrC coating and the Gr substrate must be minimized to prevent coating debonding. Fourth, the elements forming the coating should have high melting points and possess low neutron absorption cross-section for nuclear applications. The figure shows a schematic of the proposed coating architecture or similar derivatives.
Several coating layers are introduced between the ZrC coating and the Gr substrate to ensure that the CTE of the coatings is matched with those of the Gr substrate. The choices of the coating compositions were determined by the absolute melting points, Tm, and neutron absorption cross-sections, sa, of the elements. The Mo-Nb binary phase diagram is isomorphous with complete solid solubity betweeen the two elements. An inner coating layer of molybdenum carbide (Mo2C) can be included between the Mo-Nb alloys and the Gr substrate to act as a diffusion barrier.
The ZrC has a high melting point (≈3,523 K), a CTE of 7.6×10–6/K, and is resistant to hydrogen attack. The inner layers of Mo-Nb alloys are designed as a compositionally graded coating designed to match the CTE differences of ZrC and Gr. The Mo-Nb layers are also expected to be compliant so that differences in the thermal expansion of ZrC and other layers can be easily accommodated. Finally, the Mo2C layer is necessary as a diffusion barrier to mitigate the diffusion of carbon into the Mo-Nb layers and that of Mo and Nb into the Gr substrate. Stress analysis of the coating architectures with and without the present concept indicate that the residual stresses will be lower when the coated component is cooled to room temperature from the coating deposition temperature.