Formation of antifriction surface-pereiodic nanostructures under the action of ultrashort laser pulses

In this paper, we studied the method of forming of periodic surface structures resulting by the action of femtosecond laser radiation. The dependence of the formed structure pulsations period on the orientation angle of the sample was established. Studies of the formed periodic surface structures resulting by the action of femtosecond laser radiation were made on the basis of the obtained images from a scanning electron microscope. A study of the coefficient of friction of the sample before and after processing on the tribometer was carried out. It was concluded that LIPSS created on the surface of molybdenum reduced the friction coefficient by half.


Introduction
Laser-induced periodic surface structures (LIPSS) are a unique phenomenon that can be observed on almost any material after irradiating it with linearly polarized laser beams, especially when using femtosecond laser radiation [1][2][3]. Over the past few years, research activities in the field of LIPSS have increased significantly, since their generation in a one-step process provides a simple method of nanostructuring and surface functionalization for controlling optical, mechanical or chemical properties. After exposure to metal by femtosecond radiation, the sample surface acquires a lower coefficient of friction and wear, which underlines the huge potential of LIPSS in tribological applications. The coefficient of friction is reduced by more than two times [4].
The frictional qualities depend on the microstructure of the surface, namely on a certain degree of roughness or porosity, in which the oil is retained in the cavities and pores. The appearance of antifriction properties under dry friction conditions is facilitated by the presence of such components in the material, which themselves have a lubricating effect and are present on the friction surface and provide low friction [5,6]. A widely used approach is to reduce the coefficient of friction through the use of lubricants. Due to the grooves and roughness available on the surface, the lubricant remains in the grooves. This effect allows the lubricant to stay longer between the pipes.

Experimental work
The femtosecond laser system TETA-10 was used as a radiation source in this work. The parameters of the laser radiation were the following: the duration of the radiation pulse τ = 300 fs, the energy per pulse ε = 150 μJ. The sample was a round molybdenum plate. The sample prepared for the experiment was subjected to laser processing. The dependence of the period of the formed structure of the pulsations on  [7]. The power value was 1.2 W, and the sample displacement speed was 1 mm / s. Using the motorized rotary translator Standa 8MR151, the sample was set to: Ɵ = 0; 5; 15; 25; 35; and 45 degrees, then laser radiation passes through the focusing lens, and then strokes begin to form. The scheme of this experiment is shown in Figure 1. The principal trajectory of the surface scan is presented in the implemented scheme in Figure 2. The speed of movement of the sample is υ1 = 1 мм/с; υ2 = 2 мм/с; υ3 = 3 мм/с.

Research
The studies of the formed periodic surface structures resulting from the action of femtosecond laser radiation were made on the basis of the obtained images of the scanning electron microscope Quanta 200 3D. The criterion for determining the best treatment regimes was the formation of periodic structures without any pronounced defects in the form of exfoliating melt ablation or a phase explosion.   There are differences in the dynamics of the growth of the PSS period in comparison with the data obtained as a result of calculation by the formula and from experimental data [8]. The growth trends of the PSS period with a change in the orientation angle of the sample to laser radiation described in  [9]. The difference in growth dynamics can be explained by several factors: 1) the effect of a laser-induced plasma torch formed on the surface of the sample during processing; 2) some inaccuracy when setting the position of the sample relative to the focal plane of the focusing system; 3) factors not specified in the literature that have a certain influence on the result.
The study of the coefficient of friction on the surface of molybdenum before and after treatment was carried out on the CSM tribometer. The specimen was mounted on a movable table, a movable rod was adjusted, and a steel ball was fixed as a counter-body. The force with which the counter-body acted on the sample was 10 N. Between the counter-body and the sample was a lubricant -industrial oil [10]. In the work program, the parameters were set for the speed and distance that the counterbody would travel by moving along the model. The speed was equal to 10 cm / s, and the distance was 40 m. After making all the parameters, tests were carried out, during which graphs of the friction coefficient were plotted in the working window of the program (Fig. 5). In Figure 5A, the friction of the counter body against the surface of untreated molybdenum using oil occurred. The friction coefficient here was about 0.11. After molybdenum was treated with laser radiation, the coefficient of friction of the counter body on the surface of the sample under study decreased by a factor of 2 and became equal to 0.055. This graph can be seen in Figure 5B. It can be concluded that LIPSS formed on the surface of molybdenum helps to reduce the friction coefficient by 50%. This is due to the fact that LIPSS are alternating grooves in which oil is retained in the grooves, which in turn provides low friction, and this effect can be achieved by reducing the area of the contacting surface due to the formation of some roughness.

Conclusion
The laser-induced periodic surface structures created on the surface of molybdenum reduced the friction coefficient by half. After exposure to the metal by femtosecond laser radiation, the sample surface acquired a lower coefficient of friction and wear, which underlines the enormous potential of periodic surface structures caused by a laser in tribological applications. The developed surface micro-relief, in addition to the lubrication retention functions, helps to reduce the contact area on contact of mated friction pairs. The considered method for the formation of periodic surface structures can be used in industry in the treatment of friction pairs to increase the antifriction properties.