Tensile Adhesion Strength of Atmospheric Plasma Sprayed MgAl₂O₄, Al₂O₃ Coatings

This study analyses the distribution of stress during the testing of glued cylindrical specimens with thermally sprayed MgAl2O4, Al2O3 oxide coatings in order to evaluate the tensile adhesion strength. The set of studies that make up this work were conducted in order to evaluate the influence of the geometric parameters of cylindrical test specimens, 25 mm in diameter by 16–38.1 mm in height, on the measured tensile adhesion strength of the specimens. The stress and strain states inside the coating and at the coating-substrate interface were determined using the finite element modelling method. The debonding mechanisms, failure mode and influence of the coating microstructure on bond strength are also discussed. The finite element stress analysis shows a significant level of non-uniform stress distribution in the test specimens. The analysis of the results of the modelling stresses and strains using the finite element method for six types of cylindrical specimens, as well as the values obtained for the adhesion testing of MgAl2O4, Al2O3 coatings, show a need to increase the height of the standard cylindrical specimen (according to ASTM C633-13 (2021), GOST 9.304-87). The height should be increased by no less than 1.5–2.0 times to reduce the level of a non-uniform stress distribution in the separation area

Evaluation of the influence geometric parameters of a cylindrical specimen for tensile adhesion testing of thermally sprayed coating

The problem of calculating the required adhesion strength and the accuracy of measuring the adhesive/cohesive strength of thermally sprayed coatings (thermal barrier, electrically insulating, irradiation) on a cylindrical test specimen is of practical interest from the point of view of improving the reliability of highly loaded parts of internal combustion engines of agricultural machines, various parts of the aerospace and nuclear industries. There have been carried out a set of studies on the application in practice of standard test method for assessing adhesion or cohesion strength of electrically insulation plasma sprayed coating Al2O3 on standard and elongated specimens with a diameter of 25 mm in accordance with standards ASTM C633-13, GOST 9.304-87. In order to assess the influence of witness specimen’s geometry at common coating adhesion, there were selected 6 types of cylindrical specimens. A finite-element stress analysis was used to evaluate stress-strain state of the two bonded cylindrical specimens of the same geometry with coating by tensile testing. The uniformity of stress distribution at the interface coating/substrate was found for three elongated specimens. Compared to standard tensile testing of a cylindrical specimen, the applied significance of the conducted studies that it is possibility to obtain more accurate experimental data of bond strength. There is also needed further research in the field of determining the most optimal geometry of the witness specimen for evaluating the tensile adhesion strength of thermal spray coating