Hot isostatic pressing of plasma sprayed Ni-base alloys

This article reports the effects of hot isostatic pressing (HIPing) on the microstructure and properties of plasma sprayed Ni-based alloy coatings. Hot isostatic pressing was used as a post- spray treatment on plasma sprayed Ni-5Al, Ni-20Al, and NiCrAl coatings. The aim was to densify the coatings and modify physical properties such as strength, amount of porosity, and hardness. The coatings were HIPed at 750 to 950 ‡C at pressures of 50 to 200 MPa and held for 1 h. The treated coatings were examined by optical microscopy and scanning electron microscopy (SEM). Coating porosity was determined using a combination of an image analyzer and SEM. Near- zero porosity levels could be obtained, and HIP treatment at increasing temperatures and pressures changed the microstructure and increased the microhardness of the coatings. Mechanical testing of the coatings was performed on a Dynamic Mechanical Analyzer (DMA) from ambient to — 1000 ‡C. The results showed that the elastic modulus of HIPed coatings was greater than as-sprayed coatings up to — 750 ‡C. These changes can be related to plastic flow, interlamellar diffusion, and creep that occur at increased temperatures and pressures.