Improving the wear and heat resistance of niobium substrate via reactive electrospark treatment using fusible AlCaSiY electrode

Abstract

Protective coatings with a heterophase layered structure were obtained by reactive electrospark treatment (EST) of niobium substrate with a fusible AlCaSiY electrode. The effect of the frequency–energy modes of EST on the phase composition, structure and properties of the coatings was studied. A combination of high discharge current values (400A) and long pulses (100 μs) yields a double-layer coating with the total thickness of 80 μm, which consists of the (Al) matrix strengthened with Nb2CaAl20 particles and a hard (10 GPa) NbAl3 layer. The results of pin-on-plate tribological testing revealed that this structure made it possible to reduce the coefficient of friction from 0.42 to 0.24 and increase wear resistance 3.5-fold compared to those of niobium. Oxidative annealing of Nb plates in air at 700 °C causes their fracture due to the formation of Nb2O5. Indeed, oxidative annealing of the samples subjected to EST leads to in situ formation of a dense Ca12Al10Si4O35-oxide based near surface layer that inhibits oxygen diffusion into the bulk, thus suppressing Nb2O5 formation. Mass gain of the sample having a protective coating after 300-min isothermal exposure was 2.54 mg/cm2, which is 3.6-fold lower than that for the Nb substrate (9.04 mg/cm2).