Synthesis and performance characterization of Hafnium-based multilayer coating applied over carbon/carbon composites with sharp leading edge

Carbon/carbon(C/C)composites have been acknowledged as potential candidates in aerospace vehicles,but their oxygen sensitivity still remains an enormous challenge.In this work,a novel multilayer coating consisted of HfC-2.5 mol.%Hf_(6)Ta_(2)O_(17),HfC-40 mol.%SiC,HfC-2.5 mol.%Hf_(6)Ta_(2)O_(17) and HfC-60 mol.%SiC sublayers from surface to inside was designed and fabricated on the surface of C/C composites with sharp leading edge by plasma spraying.Its ablation resistance was assessed using oxyacetylene torch with a maximum temperature over 2300℃ and compared with monolayered coatings.The multilayer coating revealed preferable ablation retardation capacity evidenced by its integrated profile and less flaw quantity.Such benefits were primarily stemmed from the effective structural design and rational material selection.The former was able to reduce the thermal stress within the ablated scale,the latter contributed to rising the high-temperature resistance and oxygen barrier ability of the coating.

Response mechanism study of alternate ZrC-10vol.%SiC/ZrC-70vol.%SiC coatings with various sublayer thicknesses for cyclic and long-term thermal exposure

To explore the influence of sublayer numbers on the structure evolution and thermal stress level,an alternate coating consisting of ZrC-10 vol.%SiC and ZrC-70 vol.%SiC sublayers was designed in this work.With a basically consistent general thickness,three coatings constituted by 2,4 and 6 sublayers were prepared by plasma spraying,which then were assessed using an oxyacetylene torch under cyclic and long-term exposure.The coating with 6 sublayers was supposed to be the one with the best ablation property,finite element analysis also evidenced its least thermal stress among all these samples.After being ablated for 240 s,the linear and mass ablation rates of the coating with 6 sublayers decreased by 51.27%and 14.41%as compared to that with 4 sublayers.Post-test analysis proved the existence of Si-based products,which help the outmost surface to develop a dense profile.Additionally,the yielded alternate dense/porous scale had a preferable toughness,allowing it to maintain a good integrity.

Oxidation and Mechanical Properties of SiC/SiC-MoSi_2-ZrB_2 Coating for Carbon/Carbon Composites

To improve oxidation resistance of carbon/carbon （C/C） composites, a SiC/SiC-MoSi2-ZrB2 double-layer ceramic coating was prepared on C/C composites by two-step pack cementation. The phase compositions and microstructures of as-prepared multilayer coating were characterized by X-ray diffraction and scanning electron microscopy. The oxidation resistance at 1773 K and the effect of thermal shock between 1773 K and room temperature on mechanical performance of coated specimens were investigated. The results show that the SiC/SiC-MoSi2-ZrB2 coating exhibits dense structure and is composed of SiC, Si, MoSi2 and ZrB2. It can protect C/C composites from oxidation at 1773 K for more than 510 h with weight loss of 0.5%. The excellent anti-oxidation performance of the coating is due to the formation of SiO2-ZrSiO4 complex glassy film. The coating can also endure the thermal shocks between 1773 K and room temperature for 20 times with residual flexural strength of 86.1%.