characterization include B content along the coatings thickness, microstructure,
microhardness, residual stresses evaluation and corrosion resistance before and
after heat-treatment. The B content in the as plated coatings has been
evaluated by Glow Discharge Optical Emission Spectroscopy (GDOES) measurements.
The GDOES analyses were carried out using a JY RF-GD PROFILER HR
instrument, manufactured by Horiba Jobin-Yvon, Longjumeau, France. The
instrument is equipped with a standard 4 mm diameter anode, a polychromathor with 28
acquiring channels and a Quantum XP software. The source conditions were Ar
pressure of 650 Pa and 35W applied power. The selected conditions are necessary
to obtain a flat crater thus increasing the depth resolution. All results have
been obtained with the same source conditions and with the same calibration
method. The calibration was performed with 21 samples selected among Setting-Up
Samples (SUS) and Certified Reference Materials (CRM).
The microstructure of the as plated
and heat treated coatings has been examined by scanning electron microscopy
(Zeiss model EVO 40) (SEM) in cross section after metallographic etching (50%
HNO3 and 50% CH3COOH). To this aim, the
specimens were cut, embedded in resin in cross section and underwent
metallographic preparation to obtain a mirror-like surface.
hardness have been evaluated as mean value of 15 HV0,05
microhardness measures performed in cross section using a Struers Duramin
Residual stress in the coatings was measured by
means of FIB micro-scale ring-core method, according to the procedure that is
already described in previous papers 21-22. The methodology is based on incremental focused ion beam
(FIB) micro-milling, combined with high-resolution in situ SEM imaging, a full
field strain analysis by digital image correlation (DIC)
and analytical/numerical models for residual stress calculation.
Ring-core FIB milling was
incrementally performed at a current of 48 pA at 30 kV. The depth of each
milling step was set to 200 nm, while the inner diameter of the trench was set
to 20 µm. A SEM micrograph is acquired before milling and after each milling
step (1024 × 884, obtained as the integral of 128 different images acquired
with a dwell time of 50ns) (Fig. 1).
In this way, the relaxation strain
vs depth profile is obtained, which allow for the calculation of both the
average residual stress in the coating, as well as the residual stress depth
By assuming that the on-plane
residual stress is equibiaxial for a coating material, the residual stress can
be calculated from the measured relaxation strains by using of the simple
biaxial form of the Hooke’s law, as reported in more detail in previous papers 21-22.
For the evaluation of the corrosion resistance, potentiodynamic
polarization measurements were performed in a 3.5% wt. NaCl solution at room
temperature using an Avesta cell, in order to avoid crevice corrosion
phenomena, in a three electrodes system. The surface of the material specimen is pressed against an
opening in the bottom of the electrochemical cell. A ring of filter paper
tightens the borderline of the specimen. Distilled water is passed through this
filter paper ring into the cell at an extreme low rate. This prevents the
corroding electrolyte to come in contact with this zone, so avoiding aggressive
electrolytes in the artificial crevice. The water flow is controlled by a
peristaltic pump, which delivers 5 ml per hour. The volume of distilled water
diluting the measuring electrolyte is too small to disturb the measurement.
An Ag/AgCl/KCl3M electrode and a Pt wire have been used respectively
as reference and counter electrode. The measurements have been carried out
using an AUTOLAB PGSTAT 30 potentiostat. The scan rate was 1 mV/s and the scan
range was from -100mV vs. OCP till reaching 10-2 A/cm2. Prior
the potentiodynamic polarization the specimens have been immersed in the
electrolyte for 1h in order to reach a steady state.