The inner wall of liquid-fuel rocket combustor consists of NiCrAlY as a bonding layer and ZrO2 as a top layer on Cu-0.7%Cr substrate by plasma spray coating process. This process has advantage so short working time and excellent thermal barrier characteristics. But it has inherent possibility of cracking due to large difference in thermal expansion coefficient among bonding layer, top layer and substrate. It is also difficult to obtain uniform thickness of coating layer. Another serious problem is oxidation of substrate, which results in low durability. Electroplating processes have ensuring an uniform layer on the inner wall of the liquid-fuel rocket combustor, cheaper cost than spray coating. It could be replaced to Ni-P plating as a bonding layer having an excellent adhesion and Cr plating as top layer having excellent oxidation resistance and hardness at high temperature.
In this paper, Ni-P/Cr plating process has been developed for applying to inner wall of liquid-fuel rocket combustor to observe the high temperature properties of bonding layer Ni-P with P contents and top layer Cr plating.
The Ni-P and Cr electroplating was obtained from each the Watts bath containing low(2.12%), medium(6.97%) and high phosphorous(10.57%) acid and Sargent bath. In order to observe the change of microstructure of the interface, Ni-P/Cr electrodeposited specimen was heat treated at 750℃ for 0, 1, 2, 4, 8,16 and 32 hours. The surfaces and cross section of Ni-P and Cr electroplating layer were analyzed with scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction(XRD) and transmission electron microscope(TEM). In result, as electroplating, the low-P(2.12wt.%P) and medium-P(6.97wt.%P) alloys are presented as microcrystalline nickel or a crystalline phase. The high-P(10.57wt.%P) alloys showed an amorphous phase. Cr plating had cracked morphologies, but the distribution and density of crack was almost similar with respect to current density and plating time. The Ni?P electroplating layer became completely crystallized through heat treatment. And heated Ni-P layer phases would appear Ni3P and Ni5P2. The XRD patterns had just change from no heating to 1 hour at 750 °C and no significant changes from 2 hours to 32 hours because crystallization of the Ni-P has been completed during 1 hour of heat treatmet. The surface of Cr electroplating layer has significant morphology change but no effect of surface. And XRD pattern had increasing peak of Cr Oxide and Cr with heat treatment time. From the SEM cross section images, a band layer was formed by the inter-diffusion between the Ni-P and Cr layer due to the heat treatment. Moreover, it extended with increasing heat treatment time. We expected that band layer will improve the adhesion between Cr and Ni-P layer because of interdiffusion. TEM analysis showed that the band layer was consist of a solid solution of Ni and Cr and no P was found in band layer.