Determination of anti-icing properties of a gas-thermal coating protective coating with a surface layer of PTFE

. Icing is a negative factor in the operation of various structures and equipment. Ice, due to its physical characteristics, has an aggressive effect on the surfaces of many materials. The use of anti-icing coatings is one of the ways to passively prevent the negative effects of icing. Anti-icing coatings developed by Technological Systems of Protective Coatings LLC (TSZP LLC) are promising coatings intended for operation in the aquatic environment. The authors studied the performance characteristics of the developed anti-icing thermal thermal coatings with a surface layer of PTFE under field testing conditions. The results of the experiments carried out confirmed the anti-icing properties of the coatings under study.


Introduction
Protective coatings are used to prevent the appearance of defects on certain surfaces, as well as to give the surfaces the necessary performance properties.Thermal protective coatings based on aluminum, magnesium and zinc powders provide a high level of corrosion protection due to their very good adhesion.However, for the surfaces of ship hulls, port infrastructure and other hydraulic structures operating in the aquatic environment, resistance to icing is an important factor.
The formation and accumulation of ice on the surfaces of building structures, ships, port infrastructure, power lines, machinery and equipment is a common phenomenon in winter.Icing makes it difficult or even impossible to operate the equipment, increases the load on structural elements, which can lead to their destruction, that is, the occurrence of emergencies.Icing of vessels adversely affects their stability, reduces the buoyancy margin, and sometimes leads to their death [1,2].
The formation of an ice cover on the surface of the coating and its adhesion are determined by the following factors: surface wettability with water, surface roughness, ambient temperature and exposure.
One of the passive ways to prevent the negative impact of icing is the use of anti-icing coatings, which are coatings that, due to their physical and chemical properties, prevent the accumulation of ice on them, which is formed under the conditions of systematic exposure to atmospheric precipitation or water from natural and artificial reservoirs.Such coatings should have hydrophobic properties to minimize the accumulation of water on their surface; on the one hand, on the other hand, they should be characterized by low adhesion of already formed ice for its spontaneous removal under the action of gravity or wind load.
The work [3] presents a detailed review of modern possibilities for creating and imparting hydrophobic properties to a surface.Accordind to the authors [4] the adhesion of ice to hydrophobic surfaces, which were obtained by depositing organosilicon and fluorine compounds on smooth and sandblasted aluminum alloy substrates.It has been established that the value of adhesion decreases linearly with an increase in the contact angle.
The wettability of the surface is determined by the contact angle Ɵ, if the angle is 0° <Ɵ<90°, then the surface is considered hydrophilic, if the angle is 90°<Ɵ<150°, then the surface is considered hydrophobic, and at values of 150°<Ɵ<180°, superhydrophobic.
The hydrophobicity of the surface is characterized not only by a high contact angle, but also by a small drop angle when the surface is tilted.The value of the angle hysteresis (ΔƟ) is defined as the difference between the angles of advancing Ɵa and receding Ɵr.
According to reference [5], the difference between the advancing and receding angles is due to the fact that, in addition to adhesion forces and surface tension forces, a third force similar to friction acts on the three-phase contact line, the main sources of this force are surface roughness and heterogeneity.
Thus, superhydrophobic coatings are characterized by: -high mobility of liquid drops on the surface of the coating, which is determined by the small value of the contact angle hysteresis -no more than 10-15 degrees [6,7]; -low adhesion of ice to the surface due to a small fraction of the wetted surface (˂100 kPa) [8,9].
It should also be noted the effect of temperature on the value of the contact angle.An analysis of the results of the experiments presented in reference [10] clearly shows that as the substrate temperature decreases, the contact angle decreases.

Experimental part
To impart anti-icing properties to gas-thermal coatings, a layer of polytetrafluoroethylene (PTFE) was also applied to the surface of the coating using gas-thermal technology.During application, heated to a temperature close to the melting point, PTFE particles at high speed enter the top layer of the thermal coating, which is characterized by heterogeneity, layered structure and high porosity, which ensures good adhesion of PTFE particles to the main coating.
The use of PTFE particles in the surface layer of the protective coating should significantly improve its anti-icing properties, since PTFE is a hydrophobic material characterized by low wettability.
In order to verify this assumption, experiments were carried out to determine the contact angle of wetting, the angles of run-up and rollback of a drop on the surface of the test coating, as well as the adhesion of ice to the coating upon separation.
Studies to determine the contact angle of wetting, in order to determine hydrophobicity, were carried out on the investigated coating of GT AlMg5 + PTFE, as well as on the control coating of GT Al 100% and an uncoated sample of structural carbon steel.
Protective GT AlMg5+PTFE is a layer of a continuous metal coating deposited on the base surface, which includes Al -95 % and Mg -5 %.A coating of PTFE granules with a diameter of 100 µm is applied on top of the HT AlMg5 layer.
Based on the above classification, the surface with HT AlMg5 + PTFE can be considered hydrophobic, a series of measurements gives the value of the wetting angle from 104° to 140°.At the same time, for 100 % Al GTPs, the contact angle value is only 28°, the contact angle on the surface of the control sample made of carbon steel is in the range of 42°-48°, which indicates their hydrophilicity.
The determination of the hysteresis, that is, the measurement of the angles of inflow and outflow of a drop on the coatings, was carried out using an electronic goniometer ZUBR "EXPERT" mini.The analysis of the microrelief of the coatings was carried out using an SJ-411 Surftest 178-580-01D profilometer with a measurement range of the Ra parameter from 0.025 to 1600 µm and an Altami SM 0870 digital stereomicroscope.
A fragment of the profilogram of the surface with GT AlMg5+PTFE coating is shown in Fig. 1.Based on the analysis of the image of the AlMg5+PTFE GT surface (Fig. 2), we can conclude that the profile troughs are not covered with PTFE, i.e. in the depressions, AlMg5 comes to the surface, which has a significantly lower wetting angle compared to PTFE (about 28°) and is a hydrophilic material.Wetting a real rough surface with a liquid can occur in two different modes: 1. Wenzel (Fig. 3  Taking into account the high hydrophilicity of the depression surface and the hydrophobicity of the protrusions, the droplet on the surface of the AlMg5 + PTFE GT, despite the large values of the wetting angle, is held quite firmly, which is confirmed by the angles of the droplet rolling from the inclined surface.Drop rolling occurs at angles of 35°-45°.
Determination of the wetting hysteresis by determining the difference in the angles of advancing Ɵa and receding Ɵr of a drop on the surface of the AlMg5+PTFE GT gave a result of approximately 80°, which clearly does not meet the requirements of hydrophobicity.
Adhesion is a quantitative characteristic that determines the strength of fixing droplets after polycrystallization on the surface.
The adhesion of ice to the surface depends on the chemical structure of its material, the state and morphology of the surface layer, the conditions of ice formation, on which its structure and physical and mechanical properties depend.
To evaluate the anti-icing properties of a coating with a PTFE surface layer, full-scale tests were carried out to determine the adhesion of ice to peel and shear.
It should be noted that at present there is no unified standard procedure for testing the adhesion of ice to surfaces, which significantly complicates the comparison of the obtained experimental data.
For testing, samples were made of shipbuilding steel RSV with a diameter of 20 mm and a height of 20 mm, coated with the test coating from one working end (Fig. 4).The samples have a threaded hole for fixing the screw of the adhesion meter in it.Before testing, an external examination of the samples was carried out and the roughness of the working surface was determined by the parameter Rz.Determination of the roughness of the coating was carried out by the device NOVOTEST-TP1.
Ice adhesion to the coating was measured using an ONIKS-1.AP.005 instrument.

(a) (b)
To carry out the pull-off tests, a special equipment was designed and manufactured, consisting of a base (Fig. 5) and an installation plate.The base is attached to the mounting plate with screws (Fig. 6).The tests were carried out in natural conditions in the following order: -at a steady temperature below 0 °C, water was poured into the base to a level 2 mm below the inner ledge, when freezing, the volume of water increases by about 10 % and the ice surface, in this case, is at the level of the inner ledge; -samples are also cooled to temperature; -after freezing of water in the form of a base, a film of water about 1 mm thick was applied to the surface of the ice cover using a syringe, on which a sample with the test coating was placed with the working side down, and a special screw was screwed into the threaded hole on the opposite side; -after the water freezes in the form and the exposition established by the experiment plan, the screw head is placed in the grip of the pulling device of the adhesion meter, which is installed on the fixture plate, after which the detachment is made.As a result of the tests, it was found that the adhesion of ice to the tested surface is 0.07 MPa, for comparison, adhesion to the surface of structural steel is 2.417 MPa.
Special tooling was also designed and manufactured to carry out ice shear adhesion tests.The rig is a mounting plate on which a cylindrical mold with an inner diameter somewhat smaller than the outer diameter of the sample is rigidly fixed (Fig. 7).The form is rotated relative to the plane of the base at an angle of 90°.Shear adhesion tests were carried out in the following order: -when preparing the tests, the equipment: the plate, together with the mold, was installed in a vertical position in a multi-place device; -water was poured into the mold to a level 2 mm below the edge of the mold; -molds with water and samples were kept until the water in the molds freezes at a temperature corresponding to the experiment in natural conditions; -after freezing of water in the mold, a film of water was applied to the surface of the ice with a syringe, and a sample was placed on top of the mold, with a screw screwed into the threaded hole; -the form with the established sample was kept for the time stipulated by the experiment; -at the end of the exposure time set by the experiment plan, the base together with the mold was placed in a horizontal position, the screw head is placed in the grip of the pulling device of the adhesion meter, which is installed on the mounting plate of the fixture, to prevent the occurrence of a bending moment from the non-working end surface, the sample was pressed with a special stop, onto the surface which a layer of grease was applied, after which shearing is performed.

Conclusion
In the course of carrying out studies to determine the properties of the AlMg5+PTFE GT anti-icing coating, the mechanisms of ice cover formation on surfaces were considered, including taking into account their morphology.The contact angle of wetting of the anti-icing coating of GT AlMg5+PTFE was determined.
Studies have been carried out to determine the hysteresis of the angles of advancing and receding drops on the surface of the AlMg5 + PTFE GT coating.
The theoretical mechanism of wetting of the AlMg5+PTFE HT surface is considered with allowance for its morphology.
The values of adhesion of ice to the coating of GT AlMg5 + PTFE under natural conditions for separation and shear are experimentally determined.
The adhesion value for the AlMg5+PTFE coating at shear is 3.3 times greater than at peel adhesion at a temperature of -4°C and 1.7 times greater than at a temperature of -9°C, which is associated with the surface morphology of this coating.
In the process of conducting comprehensive studies of the properties of the AlMg5 + PTFE GT protective coating, it was found that, in terms of the combination of properties, these coatings cannot be considered hydrophobic, since they do not fully meet the requirements for hydrophobic coatings.
The AlMg5+PTFE GT coating can be considered to have anti-icing properties, since the adhesion of ice to this coating is 35 times less than that of structural carbon steel.
The results obtained confirm that one of the most significant factors affecting the adhesion value is the chemical structure of the material.Thus, the height of the microroughness of the surface of the control steel sample is 10 µm in Rz, while the surface roughness of the AlMg5+PTFE GT is 217 µm in Rz (22 times more), while the value of ice adhesion to the AlMg5+PTFE GT is 35 times less than to become.

Fig. 3 .
Fig. 3. Wetting a real rough surface with a liquid.

Fig. 4 .
Fig. 4. View and drawing of a sample for testing to determine the adhesion of ice to the separation.