Biomedical Engineering Reference
In-Depth Information
The microhardness of the inal bulk material was studied
using Vickers method and the results are presented in Table 6.5.
Compared with widely used medicine 316L microcrystalline
stainless steel (250 ± 10 HV0.2), microhardness of sintered
nanocrystalline austenitic nickel-free nitrogen containing stainless
steel Fe
54
Cr
24
Mn
21
Mo
1
N obtained by mechanical alloying is
signiicantly higher (520 ± 10 HV0.2). The result is two times
greater than in austenitic steel obtained by conventional methods.
This effect is directly connected with structure reinement and
obtaining of nanostructure as well as introduction of nitrogen.
Nitrogen dissolved in austenitic stainless steel increases its strength,
which is caused by large amount of solution hardening. The grain
size hardening in N-alloyed austenitic stainless steels is based on
the grain size dependence of the yield strength as described by the
Hall-Petch equation. The effect of N content on grain boundary
hardening increases proportionally as the N content of the steel
increases. Grain boundary hardening increases therefore with
increasing N content of the steel and is related to the strong afinity
between Cr, Mo, and N atoms.
Table 6.5
Results of microhardness and corrosion tests in Ringer's
solution [31]
Corrosion
rate
[mm/year]
I
corr
[A/cm
2
]
E
corr
[mV]
R
p
[Ohm/cm
2
]
Sample
HV0.2
250 9.0 × 10
-5
-220 0.300
Fe
65
Cr
18
Ni
12
Mo
2
Mn
2
(316L)
450
Fe
54
Cr
24
Mn
21
Mo
1
N 520 6.9 × 10
-6
-263 0.073
3747
510 2.8 × 10
-6
-414 0.030
Fe
54
Cr
24
Mn
21
Mo
1
N + 5% HA
14385
580 9.2 × 10
-7
-322 0.009
Fe
54
Cr
24
Mn
21
Mo
1
N + 10% HA
28143
170 1.3 × 10
-4
-1021 1.420
Fe
54
Cr
24
Mn
21
Mo
1
+ 5% HA
22
Addition of 5% of HA slightly decreases microhardness
(510 ± 10 HV0.2) while 10% of HA results in growth of microhard-
ness to 580 ± 10 HV0.2. It is worth to mention that microhardness of
the same material before nitrogen absorption treatment is three
