Fig. 4 represents the true compressive stress–strain curve of the
sintered samples. The yield and compression strengths extracted
from the curves are also summarized in Fig. 5. As can be seen, both
increases by increasing the sintering aid content, due to the
progress in densification and the decrease of retained porosity.
Moreover, an increase in elastic modulus by decreasing the pore
level is observable in the curve, which is in agreement with
previously-reported models [22]. It is also noticeable that, despite
the presence of some porosity in the samples, considerable
compressive strengths were found. This is due to the strengthening
contribution of the interstitially dissolved atoms of carbon and
nitrogen, on the one hand, and the obtained nanometric structures,
on the other hand. Note that the presence of retained porosity in
implants can be advantageous for medical applications. Porosity
decreases the elastic modulus mismatch of the bone and implant,
and subsequently decreases the probability of loosening [23]. Also,
surface porosity can help the mechanical fixation of implants [24].
As well as the better strength of these materials, their higher biocompatibility
of these nickel-free stainless steels is another benefit,
as compared to conventional stainless steels like AISI 316L [8].
The absorbed energy of the samples during the Charpy impact
tests is indicated in Fig. 6, showing an increase in the energy by
increasing the amount of the additive and thereby reducing the
Fig. 4. Compressive true stress–strain curves of the sintered samples.
Fig. 6.