Biomedical Engineering Reference
In-Depth Information
TABLE 21.8. Micro Hardness Test Conditions
Parameter
Details
Indenter geometry
Triangular pyramid ( α = 136 ° ± 0.2 ° )
Maximum load
50 mN
Loading cycle
100 mN/min loading, 10 s hold, 100 mN/min unloading
Measurement type
3
×
3 matrix with 250
μ
m gap between measurements
TABLE 21.9. Micro Hardness and Elastic Modulus of T i N and DLC Coatings
TiN Coating on Ti6Al4V
Alloy
DLC coating on Cp
Titanium
Parameters
Micro hardness — Vickers (Hv)
3260
±
600
2450
±
480
Elastic modulus of coating (GPa)
346
±
55
147
±
18
21.7.6 Simulated Accelerated Durability Studies
Artifi cial heart valves were identifi ed as one potential application for DLC and
TiN-coated materials. One of the primary requirements of artifi cial heart valves is
durability. This calls for extensive studies on candidate materials for their wear
properties and fatigue resistance. The adhesive and abrasive wear properties of
the DLC/TiN-coated materials have been assessed individually and in combina-
tions using pin-on-wheel and sand slurry tests. The functional durability of the
valves is studied using accelerated durability tests.
To test new designs or material combinations at normal heart rates, whether
in mechanical systems or in animal models for periods over ten years (as required
by the International standards) is unrealistic. Hence, accelerated durability
studies are carried out on the valve models. The test system simulates the actual
working of a valve in a test chamber (Bhuvaneshwar et al., 1989). The unit at the
institute is a hydraulic system capable of testing fi ve valves simultaneously and
the cycling rate can be varied between six and twelve times the normal heart rate
(see Figure 21.7). The hydraulic pressures and fl ow rates through the valves can
be regulated to simulate the physiological conditions in the heart. The rate of
opening and closing of the valve is accelerated to ten times the normal heart rate
to obtain meaningful data in a shorter time.
The system incorporates a fl uid reservoir containing distilled water which is
pumped using a magnetically coupled hydraulic pump through a ten-micron fi lter
to the test chamber. The fi lter is employed to prevent any wear debris or dust
particles reaching the test chamber. A rotating distributor supplies each test valve
with the fl uid in the required pressure range in a sequential manner so that each
valve is opened and closed. The cycling rate is varied by varying the speed of rota-
tion of the distributor, which is driven by a dc motor. The test conditions are listed
in Table 21.10 .
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