Structural properties and ablation behaviors of homogeneous ZrC multi-layer coated on carbon/carbon composites via a multi-step vacuum plasma spray coating process

Ablation-resistant zirconium carbide (ZrC) layers were coated onto carbon/carbon (C/C) composites using vacuum plasma spraying. The structural and ablative characteristics of ZrC layers applied in single-versus multi-step coating processes were compared. An increase in the number of coating cycles from 4 to 20 in single-step coating increased the ZrC layer's thickness from 32.3 μm to 116.2 μm even though the coating rate per cycle was reduced from 8.1 μm/cycle to 5.8 μm/cycle. By increasing the number of cycles, the substrate surface was continuously heated, enhancing the lateral spread of molten droplets and improving the porosity and hardness of the ZrC layer. On the other hand, multi-step coating, involving repetition of the single-step process, yielded a 20% thickness increase of ZrC layers with a higher coating rate while maintaining comparable mechanical properties despite an identical total number of coating cycles. Furthermore, the multi-step-coated layers showed better thermal oxidation resistance than the single-step-coated layers due to the formation of ZrC/ZrC interfaces having larger grain boundaries and the thicker ZrC_xO_y layers, effectively barring oxygen diffusion. The multi-step coating of identical ZrC layers suggested the better thermal oxidation resistant protection way for C/C composite without the reduction of coated layer's properties.