shows micrographs of AISI H13 and M2 steels
treated in a borax bath containing dissolved ferroniobium
and aluminum, at 1000 -C for 4 h. In both cases the layers
were homogenous, with high thickness regularity. It can be
clearly seen that the layer formed on AISI M2 steel is
considerably thicker. This one measured 9.0T0.3 Am, while
the other one measured 6.2T0.2 Am. This is due to the fact
that AISI M2 steel has a higher carbon content, making it
possible for more carbon atoms to diffuse and form a thicker
niobium carbide layer [1,4]. The X-ray diffractograms of the
coated AISI H13 and AISI M2 steels are shown in Fig. 3. In
both cases it was verified that the layers consisted of
niobium carbide (NbC). The type of layer formed in the
borax bath can be explained considering the free energies of
carbide and oxide formation relative to the dissolved
element. The niobium carbide layers were formed on H13
and M2 steels because the niobium element presents a
relatively small free energy of carbide formation and a
relatively high free energy of oxide formation, higher than
that of B2O3 [3,6]. The carbide layers were formed by the
direct combination of carbon in the parts with dissolved
niobium in the bath. Its composition and hardness are
almost independent of the substrate [1], not being influenced
by the existing alloying elements of the steels. The
hardnesses of the layers on AISI H13 and AISI M2 steels
were 2333.3T56.4 and 2344.9 T69.6 HV0.050, respectively.
These levels of hardness are expected for NbC layers
obtained by TRD treatments [5].