PROPIEDADES MECÁNICAS DE ACEROS BORURADOS AISI 1018 EVALUADAS POR INDENTACIÓN A ESCALA MICRO Y NANOMÉTRICA

Abstract

The present study evaluated the indentation size effect on the Fe2B layer at micrometric scale, using the Vickers technique and on the Fe2B/substrate interface, at nanometric scale by mean of the Berkovich nanoindentation technique. First, the Fe2B layers were obtained at the surface of AISI 1018 borided steels by the powder-pack boriding method. The thermochemical treatment was carried out at temperatures of 1193, 1243 and 1273 K with 4, 6 and 8 h of exposure times for each temperature. The boriding of AISI 1018 steel resulted in the formation of saw-toothed Fe2B surface layers. The formation of a jagged boride coating interface can be attributed to the enhanced growth at the tips of the coating fingers, due to locally high stress fields and lattice distortions. For that reason, the evaluation of the mechanical properties at the tips of the boride layer is of great importance in order to understand the behavior of borided steel. In the case of the microindentation, the applied loads were set in the range of 10 to 1000 g, and were applied at 50 μm from the surface, in order to evaluate the hardness of the Fe2B layer. On the other hand, loads in the range of 10 to 500 mN, were employed to characterize the hardness in the tips of the Fe2B/substrate interface at nanometric level, for the different conditions of the boriding process. The results achieved in both cases, showed that the measured hardness depended critically on the applied load, which indicated the influence of the indentation size effect (ISE). The load dependence of hardness was analyzed by mean of five empirical models, the classical power law approach, the Hays/Kendall approach, the Elastic Recovery model, the Proportional Specimen Resistance (PSR) model, and the Modified Proportional Specimen Resistance (MPSR) model. The true hardness in the tips of the Fe2B/substrate interface and in the pure zone of the Fe2B layer was obtained for each model, and the hardness value depended on the boriding parameters and the model adopted. Finally, the nanoindentation technique was used to estimate the state of residual stresses in this critical zone of the Fe2B/substrate interface.