Laser processing allows changing the defective substructure of materials in order to control their properties. Critical values of temperature and pressure in the laser-processing zone create unique opportunities for obtaining various structures on the surface and in the depth of materials. Simplicity, safety and ease of automation contribute to the fact that laser processing finds more and more applications in various fields of science and technology. There are many different ways to determine defects on the surface of materials after laser treatment. Some of them are connected with the use of special devices, and some with the use of special algorithms, models and software products for automatic identification and calculation of created defects. Each of the means has its advantages and disadvantages. In this work, we propose the model for the analysis of SEM images with the aim of not only qualitative but also quantitative analysis of defects in materials. This model makes possible to obtain the values of defect areas in relation to the base material. This is important for a more accurate and high-quality selection of laser processing parameters in order to create materials with predetermined structure and properties. In addition, such model can facilitate the calculation of surface defects for scientists, engineers and technologists.
Laser processing is a modern physical and technical method of changing the structure and properties of materials. Thanks to high temperatures and pressures, all materials can be laser processed. In addition, laser technologies have a number of advantages over other traditional processing methods: manufacturability, accuracy, safety, ease of automation. The change in the properties of materials after laser treatment is associated with a change in the defective substructure of the materials. A defect should be understood as any change in the structure of the material after processing, which differs from the structure of the original (base) material. At the same time, defects can both improve and worsen the properties of materials.
Very often, during research, there is a need to quickly, qualitatively, and quantitatively evaluate
defective changes in the material after laser treatment. It should be noted that there are many methods
for surface analysis [
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For laser processing of materials, we used a system based on Nd:glass laser (Figure 1). The pulse duration is 50 ns, the radiation flux density is 2×108 - 5×109 W/cm2.
Experiments conducted on laser shock wave treatment of ZnO nanopowder using a thermal protective copper screen showed that a large number of dimples are formed on the reverse side of the copper foil (Figure 2).
There are two possible mechanisms for the formation of these dimples: the first is caused by the pressing of ZnO conglomerates into the surface of copper (stamping effect), since the nanopowder has a higher hardness than copper, the second is related to the chipping pulse due to the passage of a shock wave through the material.
With help of a laser possible to accelerate small particles of materials to high speeds (≤10 km/s). In this way, it is possible to simulate the impact of micrometeoroids on the surface of various spacecraft. This can reduces the risks of damage to spacecraft after collision with space debris, and it can also help to protect life and health of astronauts.
a b Figure 2. Scheme of laser shock wave processing of ZnO nanopowder (a) and SEM image of the reverse side of copper screen after processing (b): 1 - transparent condensed medium; 2 - thermal protective copper screen; 3 – ZnO nanopowder, 4 - steel substrate
Based on the device for laser shock-plasma acceleration of fine-dispersed materials (Patent 86399 Ukraine, IPC C23C 24/00) was developed a new method for modeling the impact of micrometeoroids on the surface of spacecraft materials (Figure 3).
As can be seen from the figure, when aluminum microparticles collide with a target made of the same aluminum, defects (dimples or craters) form on the surface. The shape and size of the craters depend on the shape and size of the microparticles, as well as on the parameters of the laser pulse.
Also, with the help of lasers, it is possible to change the structure of the material at depths significantly exceeding the depth of the thermal action of the laser pulse. Thus, after irradiation of titanium through a transparent condensed medium without a thermal protective coating, its strength increased, and plasticity decreased. The study of fractography of titanium samples after static stretching showed a decrease in the number of pores in the material after laser treatment due to the passage of the shock wave, which can be the reason for the change in mechanical properties. (Figure 4).
The algorithm for SEM images analyzing is based on a matrix model [
Then we conducted an empirical elemental analysis and found that the defects in the SEM images have a shade from 0 to 50, and the base material has a shade from 51 to 255. In this way, we can convert a gray image to binary (black and white), where all shades up to 50 will be equal to 0, and shades from 51 to 255 will be equal to 1.
Then we can calculate the number of black pixels in the received image, this number will be equal to the number of defects on the surface. After that, we can compare the number of black pixels to the total number of pixels in the image, that is, look for the ratio of the area of defects to the area of the entire surface of the material. Or we can compare the areas of defects before and after laser treatment on two different images.
Image analysis of the copper screen reverse side with the help of the proposed model using the MATLAB (Figure 5) showed that the ratio of the area of dimples to the area of the entire material is 18.28%.
a a b b
Surface analysis of the aluminum target after aluminum microparticles impact with help of the proposed model (Figure 6) showed that the ratio of the area of the formed craters to the area of the entire material is 0.95%.
Fractography analysis of titanium before (Figure 7) and after (Figure 8) laser shock wave treatment using the proposed algorithm showed that the ratio of the pore areas before and after irradiation to the area of the entire material decreased from 4.19% to 1.95%, or approximately 2.15 times. a b
There are many different ways to determine defects on the surface of materials after laser treatment. Some of them are connected with the use of special devices, and some with the use of special algorithms, models and software products for automatic identification and calculation of created defects. Each of the means has its advantages and disadvantages.
The work presents various types of defects after laser processing, which was used in our previous experiments. The proposed model allows to quantitatively estimate the number of defects after laser processing. The model for analyzing SEM images makes it possible to obtain the values of defect areas in relation to the base material. The analysis algorithm is based on a matrix model implemented in MATLAB. This is important for a more accurate and high-quality selection of laser processing parameters in order to create materials with predetermined structure and properties. In addition, such model can facilitate the calculation of surface defects for scientists, engineers and technologists.