Jakub Kõo

Determination of Residual Stresses in Thermal and Cold Sprayed Coatings by the Hole-Drilling Method

Residual stresses in most coatings have a significant influence on adhesion, on mechanical properties and on tribological performance. Residual stress can not be measured directly, rather strain is measured and stress is calculated. In this study residual stresses were investigated in thick thermal Ni95Al5 and cold sprayed Ni, Cu, NiZn, AlZn powder coatings using the semi-destructive hole-drilling and layer growing curvature methods. Residual stresses obtained for the thermal spray coating were compressive and with a relatively large deviation of the results indicating the nonuniformity of residual stress. Cold sprayed coatings have more uniform structure than thermal sprayed coatings. Residual stresses were compressive, and results obtained by hole-drilling and curvature method were comparable.

Comparison of Curvature and X-Ray Methods for Measuring of Residual Stresses in Hard PVD Coatings

Physical Vapour Deposition and PVD coatings are designed for several applications, from industrial to biomedical. Residual stresses, arising during coating deposition, have important effect on the coating’s service life as their influence to the mechanical and tribological properties. Our aim was to investigate the residual stresses in five different PVD coatings (TiN, TiCN, TiAlN, TiAlN, nc-(AlTi) N/α -Si3N4) (presence of the Ti as adhesion layer) by the layer growing curvature method and the X-ray diffraction techniques using a plate and a strip as the substrate. Residual stresses were compressive and very large (2.98 - 7.24) GPa in all coatings and comparable in TiN, TiAlN, TiAlN coatings in the case of both methods. The magnitude of residual stresses is influenced by intrinsic strain in the case of layer growth rather than by thermal stress.

On Determination of Residual Stresses in Coatings from the Measured Longitudinal Deformation of a Strip Substrate

This paper presents an algorithm for the calculation of initial stresses from the longitudinal deformation of a straight strip substrate, measured during the simultaneous deposition or dissolution of a coating on the two sides of the substrate. The possibility of prestressing the substrate by an elastic element (spring, semiconductor) is taken into account. An example of application is presented.

A Device for Determination of Residual Stresses in Galvanic Coatings from the Measured Longitudinal Deformation of a Strip Substrate

A method was elaborated for the determination of residual stresses in the galvanic coating by measuring the longitudinal deformation of a straight strip (ribbon) and of a cylindrical (wire) substrate. For this purpose an experimental measuring system was designed. The experiments were carried out using a nickel coating deposited bilaterally from Watts electrolyte on a copper or brass strip substrate, and initial stresses were calculated. The strip substrate was fixed by one end and loaded with a calibrated weight on the other. The longitudinal displacement of the free end of substrate, depending on coating thickness, was determined by measuring the deformation of the elastic element. The effect of the rigidity of an elastic element on initial stresses was investigated. The results are compared with the literature data.

Evaluation of Residual Stresses in PVD Coatings by Means of Strip Substrate Length Variation and Curvature Method of Plate Substrate

The aim of the study was to determine macroscopic residual stresses in Physical Vapor Deposits (PVD) coatings through measurement of the length variation of the strip substrates coated on both sides. The length change of the strip was reduced to the deflection of the middle cross-section of the elastic element and was recorded by four strain gauges. For validating the obtained results, the conventional curvature method was used. As an application, residual stresses in hard AlCrN PVD coatings were investigated. The coatings were nanolayered to achieve better coating toughness for blanking and punching applications. The steel strips and steel plates with two thicknesses were used as the substrate. The values of the compressive residual stresses, determined by both methods for the investigated coatings, were very high (3.3 -3.6 GPa) independent of coating thickness and practically equal within the measurement uncertainty of the method. Good agreement between the experimental results obtained with both methods suggests that the presented method, strip length variation, is applicable for determination of residual stresses in coatings. Compressive stresses in coatings are desirable as they strengthen the coating.

Comparative Analysis of Residual Stresses Determined by Various Methods in Brush-Plated Hard Gold and Silver Coatings

The investigated brush-plated silver and gold coatings are used for refining the surface properties of electric apparatuses. Tensile residual stresses generated in the plated coatings were determined with a thin-walled ring substrate using the curvature and instrumented indentation techniques. These stresses relax over time; the dependence of relaxation time was approximated by a linear-fractional function. The modulus of elasticity and the nanohardness of the coatings were determined by nanoindentation. The surface morphology and structure in cross-section of the coated substrates are presented.

Measurement of Residual Stresses in the Cold-Rolled Fe-Ni-Mn/Invar Thermo-Bimetallic Plate

The paper presents a method for measuring residual stresses in normal thermo-bimetal Fe-Ni-Mn/Invar strips with a thickness of 0.76 mm. For this purpose, a setup was designed which permits to remove layers from a strip substrate by electrochemical etching. Residual stresses in the directions that are longitudinal and transversal to rolling are determined by the curvature method based on the layer growing/removing techniques. As a reference, residual stresses were also determined by hole-drilling technique. Tensile and compressive residual stresses arose both in active and in passive layers and were considerably higher when determined by the hole-drilling technique.

Investigation of Residual Stresses and some Elastic Properties of Brush-Plated Gold and Silver Galvanic Coatings

Nickel-hardened gold and silver coatings were brush-plated from a commercial SIFCO Dalic Solution (Gold Hard Alloy), Code SPS 5370, and Silver Hard Heavy Build, Code SPS 3080, on unclosed thin-walled copper ring substrates. To determine residual stresses, the conventional curvature method (common among the electrodeposition methods) was applied, where the substrate was coated with certain thickness and then the slit increment (bending deflection) of the substrate was measured as an experimental parameter. Residual stresses on gold coatings were also determined by X-ray diffraction (XRD) based on the sin2 method. The values of residual stresses determined by the curvature method and by the XRD technique were comparable. Relaxation of residual stresses was observed. An equation for approximation of the change of residual stresses was applied assuming that the dependence of residual stresses on relaxation time is linear-fractional. The surface morphology and microstructure of the coatings was studied by means of scanning electron microscopy (SEM). The magnitudes of the modulus of elasticity and of the nanohardness of the coatings were obtained by instrumented indentation.

Residual Stress Measurement in Coated Plates Using Layer Growing/ Removing Methods: 100Th Anniversary of the Publication of Stoney’s Paper “the Tension of Metallic Films Deposited by Electrolysis”

In connection with the 100th anniversary of Stoney’s equations, some historical remarks are made with respect to the development of these equations. As an example of the extension of Stoney’s equations, a unique algorithm of the layer growing/removing methods is presented for determination of residual stresses in isotropic inhomogeneous coated plates. Using a computer program based on this algorithm, residual stresses are computed in the galvanic steel coating on the copper plate substrate.

Measurement of Residual Stresses in Electrochemically Metallized Coatings on Thin Cross-Cut Ring Substrate

An experimental method was developed for determination of residual stresses in electrochemically metallized (brush-plated) coatings by measuring the slit increment of the unclosed ring strip substrate after deposition process. The substrate is fixed to a mandrel, which makes free slipping of the edges as well as momentless deformation of the coated substrate possible. The calculation formula is extended Stoney’s formula which takes into consideration the real shape of the substrate, and the difference of the elasticity moduli of the coating and the substrate materials. The difference between the coefficients of thermal expansion of the coating and substrate materials is also taken into account. Residual stresses are determined for four materials (nickel, zinc, copper, silver) from six electrolytes.