MECHANICAL TESTING WITH MODELLING OF STRESS DISTRIBUTION IN NICKEL THIN COATINGS ON DIFFERENT SUBSTRATES AS A METHOD FOR ASSESSING ADHESION QUALITY

Abstract

Covering materials with thin coatings is often necessary for protecting them from wear and
corrosion, ensuring electrical conductivity, and in planar technologies for forming the structural
elements of MEMS components too. For the coating and substrate composite system to be of intact
integrity, it is necessary to know and coordinate their mechanical properties. The most important
property that evaluates the integrity of the composite systems is adhesion strength between the
substrate and coatings. Poor adhesion can lead to the deterioration of the protected material or the
failure of a component. Still, it can also lead to more serious accidents resulting in significant
property damage, serious injuries, or loss of life. Therefore, the analysis of material adhesion
problems and causes of their failure is of great importance for forensic engineers.

Thin foils of brass and stainless steel were chosen as the substrate materials. Nickel coatings with
different thicknesses were deposited from sulphamate electrolyte using DC-electrochemical
deposition with optimal current density. Two methods were selected for the mechanical testing of
materials adhesion: the bidirectional bending test and the cross-hatching adhesion test. A special
structure was made for the bidirectional bending test. The critical number of bending cycles until
material failure, i.e. adhesion loosening, is determined. In addition to the experimental testing of
bi-directional bending, numerical stress modeling was performed in ANSYS, using the actual
dimensions and mechanical properties of the nickel-coated samples. By applying the finite element
method (FEA), a detailed stress distribution analysis was carried out, which enabled the
identification of critical zones. These results contributed to a better understanding of the
mechanical behavior of the system during bending and assisted in the interpretation of the
experimental data, particularly in areas with high stress concentrations that may cause damage.
After performing the crosshatch test, the area of the remaining coating on the substrate was
evaluated, and the adhesion quality was assessed according to a standardized category. The
measurement results of the methods are compared and systematized. A relation was made between
adhesion quality criteria.

When we talk about the quality of adhesion in a material system, many factors that affect the
strength of adhesion must be considered. The crystallographic and mechanical properties of both
the substrate and the coating must be known, the deposition process parameters must be carefully
selected, and the thickness of the coating must be taken into account. Thinner coatings and
matching substrate and coating crystallographic properties enable better adhesion. Mathematical
modeling and simulation of mechanical tests facilitate monitoring of the testing process and
facilitate easier selection of optimal materials and parameters.

In scientific and engineering practice, many tests for adhesion evaluation indicate that it is a severe
problem and task. Connecting the results of adhesion testing for systems that are different in their
properties with the analysis of stress distribution in the systems will contribute to a better
understanding of the phenomena that occur in complex composite systems. Based on the
measurements made by different methods, a quantitative relationship was established between the
adhesion criteria defined by the methods.

Keywords: coating adhesion, nickel electrodeposition, bidirectional bending test, the critical cycle
number, crosshatch adhesion testing, stress distribution analysis