Thesis
PROPIEDADES MECÁNICAS DE ACEROS BORURADOS AISI 1018 EVALUADAS POR INDENTACIÓN A ESCALA MICRO Y NANOMÉTRICA
Autor
Hernández Sánchez, Enrique
Institución
Resumen
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.