STUDY OF TRIBOLOGICAL PROPERTIES OF PLASMA-MODIFIED UHMWPE

全文:

Introduction. Interest in the new available nanocomposites (NCs) is due to the possibility of controlling their physicomechanical and tribological characteristics under the operating conditions of friction units in various machines and mechanisms. This progressively developing direction is considered to be almost the only method for producing functional materials, which makes it possible to form articles with improved performance characteristics [1-5]. Nanocomposites are currently widely used as structural and tribological materials, which replace traditional ferrous metals and alloys; they play an important role in reducing material consumption in the manufacture of machines and mechanisms, as well as in enhancing their reliability and durability. A low cost and the availability of thermoplastic polymers and powdered nanofillers, as well as the possibility of using traditional methods and equipment to produce NCs, ensure high efficiency of their manufacture and the introduction of these materials into machine building [6; 7]. Designing promising NCs is based on an analysis of the operating conditions of friction units, such as bearings and seals of various machines. For friction units that do not undergo impact loads, composite materials can be modified by spherical strengthening particles without using fibrous materials [8; 9]. This makes it possible to use plasma-chemical methods most efficiently to modify and synthesize the original materials for NCs. In the course of designing NCs, the problem of selecting a polymer matrix and a nanofiller (particles of a strengthening phase) is solved. The polymer matrix should meet the requirements, such as a high or sufficient mechanical strength, resistance to aggressive media, satisfactory tribological (primarily, antifriction) charac-teristics, as well as sufficient thermal and wear resistance. In order to improve the mechanical strength of a polymer matrix, a nanofiller should have higher hardness and strength than the material of the matrix, high or satisfactory antifriction characteristics, a high thermal conductivity, and a satisfactory adhesion to the material of the matrix or a chemical bond with it. In this work, results of studying the vacuum plasma-arc modification of UHMWPE on the mechanical and tribological characteristics of polymer composites are presented. Experimental. Ticona Ultra-high-molecular-weight polyethylene (UHMWPE) (Germany) with a molecular weight of 2-7 mln carbon units and composite materials on its basis were studied. Specimens were produced by hot compaction at a pressure of 10 MPa, a temperature of 190 °C, and a rate of subsequent cooling of 3-4 K/min. The vacuum plasma-arc modification of UHMWPE was carried out using the method described in [10-12]. This method makes it possible to preliminarily treat original UHMWPE powder by ions of an inert gas in glow discharge, as well as to simultaneously CuO nanopowder and to deposit in onto surfaces of UHMWPE particles. Thus, powdered composite mixtures of the polymer matrix and filler with various mass ratios were prepared; both untreated mixtures and mixtures preliminarily treated in glow discharge were used. The wear resistance and the coefficient of sliding friction were determined using a Tribometer instrument (CSM Instruments, Switzerland) with the pin-on-disc arrangement. The counterbody was a polished ball made from the 100Sgb steel, which was an analog of the ShKh15 steel with a hardness of 1550 HV. Young’s modulus of 220 GPa, and a density of 3.0-3.2 g/sm3. The test conditions were as follows: the counterbody was a polished ball3 mm in diameter, the counterbody material was 100Sgb steel, the normal load was 1 N, the radius of the wear ring was 9-11 mm, the linear velocity was 20 cm/s, and the specified duration of the test was 5000 rev. After the tests, the diameter of a wear spot on the counterbody (the stationary ball) and the width of a wear groove on the rotating specimen were visually determined using MBS-10 and AXIOVERTCA25 (Karl Zeiss, Germany) microscopes. The depth of the wear groove was measured using a WYKONT 1100 optical profilometer (VEECO, United States). The depth and width of the grooves were measured in four to six diametrically and orthogonally opposite regions and then averaged. The test results were used to determine the wear rate I (the ratio of the thickness of the worn layer to the friction path), the coefficient of friction f, and the microhardness of the friction surface H. The mechanical characteristics of the polymer composites (PCs) with various concentrations of the filler were determined according to GOST 11262-80. During the tensile tests, the modulus of elasticity Ep, the ultimate strength σpp, and the relative elongation εpp were measured. Results and discussion. Fig. 1-3 show the dependences of the physicochemical characteristics of the PCs in tension on the concentration of CuO in these composites. The effect of the method for preliminary plasma-chemical treatment on the characteristics of the PCs was studied. An analysis of the dependences of σpp and εpp has shown that they have fairly pronounced extreme points. For the ultimate tensile strength, these extreme points correspond to a concentration of CuO of approximately 2-3 %. For both untreated NCs and NCs preliminarily treated by glow discharge, the curves of the dependences of the ultimate strength on the concentration of CuO are almost identical. With an increase in the concentration of CuO powder, the modulus of elasticity Ep rises from ~ 500 MPa to 3500 MPa or higher. Fig. 1. Ultimate strength of NCs based on UHMWPE and CuO: 1 - after preliminary ion treatment; 2 - untreated Fig. 2. Ultimate relative elongation of NCs based on UHMWPE and CuO: 1 - after preliminary ion treatment; 2 - untreated Fig. 3. Modulus of elasticity in tension of NCs based on UHMWPE and CuO: 1 - after preliminary ion treatment; 2 - untreated The nonmonotonous dependence of the modulus of elasticity on the concentration of CuO is also confirmed by the fact that, at very high concentrations of CuO, single values of the modulus of elasticity of 6000 and even 8000 MPa can be recorded. The observed effects are apparently due to changes in molecular mobility (the flexibility of chains) that result from the effect of various factors, primarily the degree of crystallinity and the pattern of supramolecular structures [13]. In addition, the pattern of supramolecular structures in filled crystalline polymers also determines the mechanism of the fracture of a polymer, such as the propagation of cracks, as well as adhesive or cohesive fracture, which is also governed by the concentration of a filler. In connection with this, reactions of the polymer with the filler, which are usually considered as the necessary condition for the reinforcing effect of the filler to occur, can lead to either an increase or decrease in the strength characteristics depending on the degree of change in the molecular mobility of chains in surface layers. Fig. 4 shows the dependences of the coefficient of friction on the applied load. It can be seen in these dependences that the optimum concentration of the filler at which the minimum values of the coefficient of friction are observed is 2 and 3 wt % for the NCs preliminarily treated by glow discharge and the untreated NCs, respectively. For unfilled UHMWPE treated by glow discharge, the coefficient of friction exceeds by 1.6-2 times the minimum values of the coefficient of friction obtained for the filled materials; for the specimen produced by compacting original (not subjected to any treatment) UHMWPE, the values off are substantially higher than for the preliminarily treated UHMWPE specimen. Fig. 5 shows the concentration dependences off I, and H obtained at a maximum load of 19 N. With an increase in the concentration of the filler, the wear rate decreases and the microhardness of the surface rises. The specimens preliminarily treated in glow discharge have substantially lower values of the wear rate, the microhardness, and the coefficient of friction. As follows from an analysis of the results obtained, the regularities of the contact and friction of the PCs are in line with the molecular-mechanical theory of friction and wear. Friction results in the wear of the NCs, which is the mode of surface fracture typical of a composite. The wear of the materials under study occurs apparently due to both adhesion and deformation. For the majority of the specimens tested, elastic deformation can be considered as the dominant mode of the friction and rupture of friction junctions. During the running-in of the preliminarily ion-treated specimens under the maximum load, plastic deformation accompanied by fatigue wear occurred. When the tests continued, this mode of contact changed into the elastic deformation of the contact of the specimen with the rigid surface of the disc. b а Fig. 4. Dependences of coefficient of friction on applied load at various concentrations of CuO, wt %: a - after preliminary treatment in glow discharge: 1 (1); 2 (2); 4 (3); 0 (4); and original UHMWPE (5); b - untreated: 1 (1); 2 (2); 3 (3) b a c Fig. 5. Dependences of: a - coefficient of friction; b - wear rate; c - microhardness on concentration of CuO under a load of 19 N: untreated (1); after preliminary treatment in glow discharge (2); calculated wear rate (3) The calculation estimate of the wear rate of a nanocomposite material is based on considering numerous factors, such as the physicomechanical characteristics of the material of a surface being worn out, the parameters of the roughness of the rigid surface of the disc, the parameters that characterize the loading of the contact, and the frictional fatigue of the material. An analysis of the results of this study has allowed us to assume that the parameters that govern the wear of the specimens of the polymer composite material are the contact pressure p, the coefficient of friction f, the hardness of the surface H, the average values of the maximum peak-to-valley height R, and the curvature radius of peaks r, as well as the parameters b and of the power approximation of the initial portion of the bearing ratio curve. The wear rate I of the specimens of the PCs that were synthesized without preliminary ion treatment and contained 1-4 wt % CuO is satisfactorily described by the following analytical dependence (fig. 5, curve 3) [14; 15]: where the criterion was obtained from the results of studying the parameters of the roughness of the disk friction surface using profilometry. It can be seen from the data in fig. 5 that the calculated values of I nearly coincide with the experimental values. Conclusion. The results of studying the effect of the vacuum plasma-arc modification of UHMWPE on the mechanical and tribological characteristics of the polymer composites are presented. It has been found that the deformation and strength characteristics of the NCs depend on the concentration of CuO; at concentrations of nanopowder of 1-2 %, a substantial influence of preliminary treatment in glow discharge is observed. It has been established that, with an increase in the concentration of the filler, the wear rate decreases; at a concentration of CuO of up to 1 %, the wear rate drops sharply. The microhardness of the surface simultaneously increases. The wear rate, the microhardness, and the coefficient of friction of the specimens preliminarily treated in glow discharge are substantially lower. The results obtained can form a basis for developing compositions of polymers with nanofillers in order to produce polymer composite materials with the UHMWPE matrix and required characteristics.

作者简介

I. Karpov

Reshetnev Siberian State Aerospace University; Siberian Federal University

31, Krasnoyarsky Rabochy Av., Krasnoyarsk, 660037, Russian Federation; 79, Svobodny Av., Krasnoyarsk, 660041, Russian Federation

A. Ushakov

Reshetnev Siberian State Aerospace University; Siberian Federal University

31, Krasnoyarsky Rabochy Av., Krasnoyarsk, 660037, Russian Federation; 79, Svobodny Av., Krasnoyarsk, 660041, Russian Federation

A. Lepeshev

Krasnoyarsk Scientific Center, SB RAS

50, Akademgorodok, Krasnoyarsk, 660036, Russian Federation

L. Fedorov

Siberian Federal University

Email: sfu-unesco@mail.ru
79, Svobodny Av., Krasnoyarsk, 660041, Russian Federation

A. Shaihadinov

Siberian Federal University

79, Svobodny Av., Krasnoyarsk, 660041, Russian Federation

参考

补充文件