The coatingscharacterization include B content along the coatings thickness, microstructure,microhardness, residual stresses evaluation and corrosion resistance before andafter heat-treatment.
The B content in the as plated coatings has beenevaluated by Glow Discharge Optical Emission Spectroscopy (GDOES) measurements.The GDOES analyses were carried out using a JY RF-GD PROFILER HRinstrument, manufactured by Horiba Jobin-Yvon, Longjumeau, France. Theinstrument is equipped with a standard 4 mm diameter anode, a polychromathor with 28acquiring channels and a Quantum XP software. The source conditions were Arpressure of 650 Pa and 35W applied power. The selected conditions are necessaryto obtain a flat crater thus increasing the depth resolution.
All results havebeen obtained with the same source conditions and with the same calibrationmethod. The calibration was performed with 21 samples selected among Setting-UpSamples (SUS) and Certified Reference Materials (CRM).The microstructure of the as platedand 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, thespecimens were cut, embedded in resin in cross section and underwentmetallographic preparation to obtain a mirror-like surface. The coatinghardness have been evaluated as mean value of 15 HV0,05microhardness measures performed in cross section using a Struers DuraminVickers micro-indenter.Residual stress in the coatings was measured bymeans of FIB micro-scale ring-core method, according to the procedure that isalready 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 fullfield strain analysis by digital image correlation (DIC)and analytical/numerical models for residual stress calculation. Ring-core FIB milling wasincrementally performed at a current of 48 pA at 30 kV.
The depth of eachmilling step was set to 200 nm, while the inner diameter of the trench was setto 20 µm. A SEM micrograph is acquired before milling and after each millingstep (1024 × 884, obtained as the integral of 128 different images acquiredwith a dwell time of 50ns) (Fig. 1).
In this way, the relaxation strainvs depth profile is obtained, which allow for the calculation of both theaverage residual stress in the coating, as well as the residual stress depthprofile. By assuming that the on-planeresidual stress is equibiaxial for a coating material, the residual stress canbe calculated from the measured relaxation strains by using of the simplebiaxial form of the Hooke’s law, as reported in more detail in previous papers 21-22.For the evaluation of the corrosion resistance, potentiodynamicpolarization measurements were performed in a 3.5% wt. NaCl solution at roomtemperature using an Avesta cell, in order to avoid crevice corrosionphenomena, in a three electrodes system. The surface of the material specimen is pressed against anopening in the bottom of the electrochemical cell. A ring of filter papertightens the borderline of the specimen. Distilled water is passed through thisfilter paper ring into the cell at an extreme low rate.
This prevents thecorroding electrolyte to come in contact with this zone, so avoiding aggressiveelectrolytes in the artificial crevice. The water flow is controlled by aperistaltic pump, which delivers 5 ml per hour. The volume of distilled waterdiluting the measuring electrolyte is too small to disturb the measurement.An Ag/AgCl/KCl3M electrode and a Pt wire have been used respectivelyas reference and counter electrode. The measurements have been carried outusing an AUTOLAB PGSTAT 30 potentiostat. The scan rate was 1 mV/s and the scanrange was from -100mV vs. OCP till reaching 10-2 A/cm2.
Priorthe potentiodynamic polarization the specimens have been immersed in theelectrolyte for 1h in order to reach a steady state.