Microstructure and properties of DC magnetron sputtered NiAl–Hf coatings

NiAl–0.1 at.% Hf coatings were successfully deposited via direct current magnetron sputtering from an alloy target onto a variety of substrates, including: glass; (1 1 1) oriented Si wafers and CMSX-4® (registered trademark of the Cannon-Muskegon Corporation, Muskegon, MI), a second generation Ni-base superalloy. The coatings were characterized using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The as-deposited coatings were found to be crystalline, consisting of the B2 structured NiAl phase, and to exhibit dense columnar Zone T microstructures. Many of the coatings were textured and the degree of texturing was observed to change with the deposition power used and substrates that were used. NiAl coatings deposited on Si at low powers (⩽50 W) tended to develop random orientations, whereas coatings deposited at higher powers developed strong growth textures. Coatings deposited on Si(1 1 1) exhibited (1 1 1) textures, while coatings deposited on glass exhibited (2 1 1) textures. Thicker coatings (10-μm thick) deposited on CMSX-4® superalloy substrates at 400 W exhibited random textures, which could be attributed to the use of higher Ar pressure during deposition. The hardness and the modulus of the different coatings, as measured via nanoindentation testing, varied with crystallographic orientation. Coatings with (1 1 1) and (2 1 1) textures exhibited higher reduced moduli and lower hardness values than randomly oriented coatings, which is in agreement with prior reports on the mechanical behavior of bulk NiAl alloys. Heat treatment of the coatings in argon gas at 500 and 1000 °C for 1 h resulted in decreases in coating hardness, but in no change in the coating modulus. The hardness change is attributed to a combination of grain growth and the annihilation of defects.