Effect of protecting coatings on mechanical properties of metal surfaces

. The effects of epoxy-polyester based powder coatings on the mechanical properties of plates made of rolled steel sheets were investigated in this work. The effect of coatings of the order of 100 µm thickness on the mechanical properties of specimens of 0.7-1.5 mm thick was considered. Tests carried out at room and elevated temperatures showed that thin coatings had an insignificant effect on the mechanical properties of the plates in bend and tensile tests. In particular, the tensile and bending stiffness of the plates is virtually unchanged in the presence of coatings, despite the fact that the modulus of elasticity determined in tests of steel plates with coatings is always somewhat lower than the Young's modulus of steel, due to the increased thickness of specimens determined in the presence of coatings. In compression strength tests, on the contrary, the influence of coatings is significant. In the overcritical mode of deformation, the load carrying capacity of coated plates is significantly reduced and their critical load of stability loss becomes 1.2-2.3 times lower (depending on substrate thickness) compared to uncoated samples. This effect can be explained by the influence of residual temperature stresses that arise in the samples after coating.


Experimental research methods
The tests were carried out on slab samples with a length of 120 mm, a width of 12 mm and a thickness of 0.7 mm, 0.9 mm, 1.2 mm and 1.5 mm (Fig.1). The material of the plates was rolled sheet steel (unalloyed structural steel with a low carbon content). The surface of the plates was electrostatically powder coated with an epoxy-polyester coating. Prior to coating, the steel surface was degreased and phosphatised. Coating was carried out in a painting booth by Gema (Switzerland). Drying was carried out at a temperature of 120 oC for not more than 5 minutes. Polymerization of the sprayed layer was carried out in a thermal chamber at 150°C for 30 minutes. Samples were cooled in air for several hours. The thickness of the coatings on the samples was about 100 µm with a variation of ±30 µm. The thickness of the coatings was measured using a micrometer. Batches of coated and uncoated specimens with different substrate thicknesses were produced for bending, tensile and stability tests at room and elevated temperature. Each batch for each type of test consisted of five samples. a b Mechanical testing was carried out on an Instron 5969 (UK) with Bluehill 3 software. In the three-point bend tests, the distance between supports was 100mm. The test speed was 1mm/min. To accurately measure the flexural movement of the plate, a 50mm gauge contact extensometer was used on a deflectometer mounted centrally under the bend specimen. Bend tests were carried out to a maximum strain of 0.1% on the stretched fibres. As a result of the bend test, the modulus of elasticity was determined. The software automatically selected the most representative interval in the stress-strain diagram to determine the modulus of elasticity of the specimens.
In tensile tests, the working length of the specimens was 80 mm. Tests were performed at a speed of 0.5 mm/min during the linear stage of deformation and at a speed of 3 mm/min during the plastic stage of deformation and fracture. A video extensometer with a measurement baseline of 50 mm was used in the tests, so the stress-strain diagram was plotted with high accuracy up to the fracture of the samples.
When tested for loss of stability in compression, the length of the working part of the specimen (the distance between the grips) was 86 mm. The test speed was 1 mm/minute. As a result of the test, a diagram of relation of displacement to applied compressive load in the linear range of deformation of the specimen and in the mode of supercritical deformations after the loss of stability was plotted. The moment of instability loss was established visually.
Tests at elevated temperature were carried out using an environmental chamber. The test temperature was 700C. Exposure of the specimens at the elevated temperature after mounting in the grips and before the start of the test was 5 minutes. A contact extensometer with a 25 mm base was used in the elevated temperature tensile tests.

Research results
A comparison of the characteristic load-displacement diagrams obtained for different thicknesses of coated and uncoated specimens in flexure tests is shown in Figure 2. Each diagram in this figure is the result of averaging experimental data for five specimens of the same type. It can be seen that the stiffness of the plates (slope of the diagrams) changes insignificantly in the presence of coatings. However, if we recalculate the modulus of elasticity of the plates from the results of these tests, it will always be lower for the coated specimens than for the uncoated ones, due to their increased thickness determined by taking into account the presence of coatings (see Table 1). Provided the plate deflections and loads are measured with high accuracy, this type of test can be used to identify the elastic modulus of coatings [6][7][8][9][10][11][12]. In particular, a Young's modulus value of about 3 GPa was found for the investigated coatings. However, from the point of view of calculating the stress-strain state of structures, the established insignificant influence of coatings in most cases can be neglected.  The results of the tensile test are shown in Fig. 3. Each of the diagrams shown here is obtained by averaging the experimental data for five specimens of the same type. It should be noted that standard values for the tensile strength of 08PS sheets can be between 275 and  Table 1), and the greater the thickness of the sample, the higher its ultimate strength. The influence of heating up to 70 oC appears to be insignificant (see Table 1), however, in the results obtained for coated plates it can be noticed that heating leads to the increase of their ultimate strength by  MPa which may indirectly testify to the presence of appropriate level of residual stresses in steel substrates at room temperature which relax during heating. a b c d

Conclusions
The present study shows that the critical load of stability loss for coated plates with an elastic modulus two orders of magnitude lower than that of the substrate can be reduced by more than a factor of two. This phenomenon can be explained by the influence of residual thermal stresses acting in the coating and in the substrate. Heating, polymerisation, and cooling of coatings applied to steel plates result in sufficiently considerable residual stresses. Determination of the level of these stresses and modelling of their effects on the plate's supercritical behaviour will be carried out in subsequent work by the authors.