Properties of HVOF-sprayed Stellite-6 coatings

The wear behavior of the coatings is less influenced by deposition conditions. In ball-on-disk dry sliding tests, all coatings exhibit wear rates of 2-3 × 10−5 mm3/(N·m), higher than those reported for bulk or clad Stellite, because of interlamellar delamination. Strain-induced, “martensitic” phase transformation from the f.c.c. structure to a h.c.p. one is observed over a 1-2 μm depth below the contact surface. Additional tribo-oxidation is onset when frictional heat dissipation has heated the wear debris enough to trigger its reaction with the environment. Correspondingly, a transition to a regime of higher friction occurs (from ≈0.6 to ≈0.8). At 400 °C, lamellar delamination is suppressed but wear rates rise to 5-8 × 10−5 mm3/(N·m) because of abrasive and adhesive wear. At 800 °C, a dense “glaze” tribofilm is formed by sintered debris particles, firmly bonded to a thermally grown oxide scale on the underlying metal surface. The “glaze” protects the coating, lowering the wear rate to ≈1 × 10−5 mm3/(N·m) and the friction coefficient to <0.45. Under high-stress particle abrasion conditions, wear rates of ≈1 × 10−3 mm3/(N·m) are found.