Enhancing the tribological properties of fluoroplastics operating in a hydraulic fluid environment

. The issues of an increasing the wear resistance of fluoroplastics under conditions of dry friction against steel and in the environment of hydraulic fluids are discussed in this article. The statement about the prospects of radiation treatment for strengthening fluoroplastics is substantiated. Comparative tribological tests were carried out on a laboratory friction machine according to the friction scheme of the end of a cylindrical sample made of steel 40X13 on a fluoroplastic plate. It has been established that radiation-modified fluoroplastic has increased wear resistance. The antifriction properties of strengthened fluoroplastic during friction in a hydraulic fluid environment are investigated. The influence of load and speed on the friction coefficient is analyzed. Based on the study, a radiation modification of fluoroplastic rod seals of hydraulic cylinders and hydraulic piston machines is proposed, with a possible increase in wear resistance up to 4 times.


Introduction
Piston devices are widely used in hydraulic systems of machines.The operation of hydraulic cylinders, piston pumps, valve gear, valve mechanisms and distributors depends on the effective operation of seals.Among the sealing materials, fluoroplastic materials and composites with a fluoroplastic matrix stand out.The use of various fillers affects the volumetric properties of the material.The antifriction properties of composites are determined mainly by the fluoroplastic matrix.Loss of seal performance is mainly due to wear of friction surfaces, so much attention is paid to the study of seal performance.In [1], the reasons for the leakage of the rod seal in an aircraft actuator are considered.Actuators in modern aircraft are made in the form of hydraulic cylinders.The research carried out showed that the main reason for failure of the combined seal is wear.A universal failure analysis method is proposed that can be used for various types of seals.Work [2] is devoted to the study of rectangular polymer seals for rectangular hydraulic rods in reciprocating motion.The effectiveness of the seal was assessed by the criterion of mass leakage per friction cycle.A method for constructing sealing performance maps based on an operating parameter is proposed.[3] Was tested the reliability of an aircraft hydraulic swivel coupling and the failure mechanism of the swivel coupling seals, which was analyzed using the finite element method.It has been established that during the operation of an aircraft hydraulic swivel coupling, metal parts and seals wear out.The failure mechanism was analyzed using finite element analysis.The influence of the clearance fit on the reliability of aircraft hydraulic swivel coupling was analyzed.In [4], the process of destruction of a hydraulic reciprocating seal is analyzed.It has been established that seal failure largely depends on the actual operating parameters of the hydraulic cylinder.A model for predicting seal reliability has been created.In [5], the surface morphology of fluoroplastic was studied as a result of changes in mechanical processing parameters.The proposed model provides a basis for optimizing the production of fluoroplastics.In [6], fluoropolymer and polymers with self-lubricating properties used in bearings where the use of liquid lubricant is not possible were studied.Tribological solutions were investigated for extreme temperatures ranging from -196 to 300 °C, which simulate the extreme operating conditions of bearings in space.The tests were carried out according to the disk-on-finger scheme at a pressure of 1 MPa.As a result, positive solutions were obtained that ensure a low friction coefficient.In [7], the friction forces between the seals and the rod in a hydraulic cylinder were studied.The influence of speed, external load, seal diameter and seal profiles on the steady-state friction force of hydraulic cylinders was studied.It has been established that a hydraulic drive with a combined piston seal is less sensitive to load changes than a hydraulic drive with an o-ring.In [8], experiments were conducted to study the durability of the seal on a new unit for reciprocating shaft movement with a pneumatic drive by changing the pressure from 100 to 400 bar at a shaft rotation speed of 0.15 m/s.It was found that surface temperature generated by frictional heat in the seal area was responsible for seal failure.In [9], the friction characteristics and performance of three polymer composite materials for seals in contact with steel were studied.It was found that sealing composites based on fluoroplastic had lower friction coefficients and less leakage during testing.In [10], the tribological properties of fluoroplastic material under cryogenic conditions were studied.The suitability of the material for use in valves at low temperatures has been established.In [11], the possibility of creating a bank of experimental data on friction coefficients for predicting the tribological properties of polymers based on processing noise signals was studied.The friction coefficients of pairs of four polymers and seven metals are determined by five temperatures.In [12], the influence of basalt fiber on the tribological properties and structure of polytetrafluoroethylene was studied in relation to use in arctic climate conditions.It has been established that the characteristics of basalt fluorocarbon composites are on par with those of composites with carbon fibers.In [13], the properties of compositions based on fluoroplastic-4 with hightemperature glassy polyarylate and phenylone after explosive pressing of powder composite mixtures were studied.An effective technology for radiation strengthening of fluoroplastic has been developed [14].The mechanical properties of radiation-strengthened fluoroplastic were studied using the kinetic microindentation method [15].However, the tribological properties of fluoroplastic under operating conditions in hydraulic fluids experimentally have not been sufficiently studied.
The purpose of the work is to determine the tribological properties of fluoroplastic when sliding on steel in a hydraulic fluid environment and to determine the effectiveness of radiation hardening treatment to increase the wear resistance of the friction coupling.

Materials
Samples of sealing material are made of fluoroplastic (Ф-4) (GOST 10007-80) and radiationmodified fluoroplastic Ф-4РМ, obtained by thermoradiation treatment of fluoroplastic with ionizing gamma radiation.Coupled element steel 40Х13.The fluoroplastic samples are made in the form of disks, the steel ones are made in the form of a quill cylinder (Fig. 1).Hydraulic fluid АМг-10 was selected as the working fluid.

Equipment and technologies
Tribological tests were carried out on a modernized friction machine МАСТ-1 (Fig. 2).The drive power of the friction machine has been increased and a new friction unit, which makes it possible to implement a friction scheme for sliding the end of a ring sample along a plane has been developed.The steel coupled sample 1 is pressed with its end surface against a fluoroplastic plate placed in the rotating cup 2, which transmits the friction moment to the force sensor 4 through a lever 3. The electric motor 5 rotates the coupled sample 1 and the force sensor records the friction moment between the sample and the coupled sample.When testing in a liquid environment, the cup is filled with hydraulic fluid.The tests were carried out according to the method [16].

Results
Friction coupling in hydraulic cylinders and piston pumps operate under variable speed conditions.At dead points, the speed is 0, which corresponds to the boundary friction regime and dry friction, in the limit, when the wear rate of the contacting surfaces is greatest.To compare the wear resistance of fluoroplastic in the original state and the modified state, tests were carried out in dry friction mode at a speed of 2.5 m/s, typical for average speed of the rods and pistons of hydraulic machines.In Fig. Figure 3 shows the effect of load on the friction coefficient of fluoroplastic samples on steel under dry friction conditions.The experimental results showed that the antifriction properties of fluoroplastic in the asreceived condition are significantly better than those of the modified one.The friction coefficient of the modified fluoroplastic is 1.5 times greater than that of the non-strengthened one.The friction surface of the steel sample is very smooth (Ra), so the nature of the friction of the samples is to overcome the adhesive interaction forces.The surface energy of fluoroplastic is very low.Taking into account the assumption that the force of adhesion interaction is directly proportional to the value of surface energy, then the friction coefficient should be proportional to the surface energy.As shown in [15] work, the hardness of fluoroplastic after radiation hardening is almost 2 times greater than the initial one.According to [1], surface energy [15] σ is nonlinearly proportional to hardness σ -Н 1/2 .This explains the increase in the friction coefficient after radiation treatment.Wear tests were also carried out in dry friction mode at a speed of 2.5 m/s.The amount of wear was determined by weight with subsequent conversion to linear wear rate (Ф4=1.07E-07,Ф4М=2.78E-08).
Figure 4 shows a comparison of the wear rates of fluoroplastic samples.To study the effect of hydraulic fluid on the antifriction properties of radiation-hardened fluoroplastic during friction against steel, experiments at extended load and speed ranges were carried out.Fig. 5 shows the test results.An increase in load under conditions of constant sliding speed leads to an increase in the friction coefficient in all test modes.An increase in sliding speed under constant load conditions leads to an increase in the friction coefficient.This behavior of a friction coupling is typical for hydrodynamic friction.The increase in the friction coefficient is explained by the fact that as the speed increases, the front resistance on the movement of the sample in the liquid increases.

Conclusions
Radiation modification of fluoroplastic increases the wear resistance of hydraulic cylinder rod seals and plungers of piston hydraulic pumps.Replacing fluoroplastic materials with radiation-modified ones is promising for increasing the service life of hydraulic cylinders and piston hydraulic pumps.The expected increase in resource is approximately 4 times.

Fig. 3 .
Fig. 3. Dependence of the friction coefficient on load in dry friction conditions at a speed of 2.5 m/s: 1 -Ф4, 2 -Ф4М

Fig. 4 .
Fig. 4. Wear rate of fluoroplastic materials: 1 -Ф4, 2 -Ф4М It has been established that under test conditions, radiation-strengthened fluoroplastic has greater wear resistance by 3.8 times than non-strengthened one.To study the effect of hydraulic fluid on the antifriction properties of radiation-hardened fluoroplastic during friction against steel, experiments at extended load and speed ranges were carried out.Fig.5shows the test results.