Ind. Eng. Chem. Res. 1993,32, 763-773
763
REVIEWS Polytetrafluoroethylene Fiber-Based Composite Antifriction Coatings Alexander S. Kuzharov Department of Chemistry, Rostov Agricultural Machine-Building Institute, 1 Gagarin Square, Rostov-on-Don 344070, Russia
This review paper presents the principal advances in the development of composite antifriction coatings based on polytetrafluoroethylene fibres. Information is given about their chemical composition, their properties, and the effect of the service conditions on their tribotechnical characteristics. The paper discusses the appropriate fields of use and the most frequently encountered designs of friction units incorporating such coatings.
1. Introduction Composite antifriction coatings (CACs) are coatings consisting of two or more constituents (separate fibers or other reinforcing components) and a matrix, the tribotechnical properties of which are different from the sum of the properties of their component parts. Development of such CACs is aimed at producing within a coating a combination of constituents possessing different properties in order to obtain tribotechnical characteristics other than those of the individual constituents. The most important characteristics, as applied to such coatings, are wear resistance and friction coefficient, which predetermine both the applicability of coatings in the movable joints of machines and mechanisms and their service life. The unique physicochemical and tribotechnical properties of polytetrafluoroethylene (PTFE),i.e., its chemical inertness (Kataev et al., 1975)and its friction coefficient (Chichinadse, 1988)in contact with metal alloys, which is the lowest of the known polymers, have enabled its wide application as the main antifriction component in many polymeric antifriction materials (Semenovand Savinsky, 1976),coatings (Evans, 1970),and lubricants (Papok and Ragozin, 1975). However, the incurable defect of the bulk polymer, Le., its cold flow under load (Evans, 1970),limits the fields of application of PTFE, in the tribotechnical sense, to low loads and sliding rates. The preparation of PTFE fibers (Sigal and Koziorova, 1972;Maslennikov, 1973)has made it possible to overcome the above disadvantage and has opened a way for development of new materials and coatings with enhanced tribotechnical and strength characteristics. PTFE fibers have found the most extensive tribotechnical application in the preparation of CACs. Use of various matrices, for example,polymer [Des. Eng., 1973 (ref 22);Bull. Atl., 1975 (ref 28); Werkstatt Betr., 1969 (ref 46); Technica, 1973 (ref 56)1, metallic (Gnusov et al., 1972;Teplyakov et al., 19801,and metalpolymeric (Kuzharov et at., 1985);combination of PTFE fibers with other types of fibers [ Werkstatt Betr. 1969 (ref 46);Bull. Atl., 1975 (ref 28);Prod. Eng., 1976a,b(refs 60,79)1 and with various fillers (Hodes, 1973;Tschacher and Gubitz, 1969);preparation with natural [Prod.Eng., 1976a,b,(refs 60,79)1and artificial [ WerkstattBetr., 1969 (ref 46);Bull. Atl., 1975 (ref 28);Prod. Eng., 1976a (refs 60,79)1fibers or with metal wire [Des.Eng., 1973 (ref 22); Technica,1973 (ref 5611;modification of fibres [Prod.Eng., 1976a,b (refs 60,7913;etc. have made it possible to obtain a wide range of CACs possessing a great variety of specific
properties. These properties enable them to be used in the friction units of automobiles [Des.Eng., 1973 (ref 22)1, tractors (Reynolds, 1974),aircraft [Evans, 1970;Reynolds, 1974; Chironis, 1970;Ind. Lubr. Tribol., 1975 (ref 1); Reinsch, 19621, spacecraft (Evans, 1970; Graig, 1962; Sottosaanti and Baker, 1978),cargo-handling machines (Reynolds, 1974; Production, 1971 (ref 3111 and the equipment of steel-rolling mills, etc. (Reinsch, 1962). Despite the wide renown and successful use of such coatings in turns out that there are no papers summarizing the composition, properties, methods of preparation, practical applications,and mechanism of lubricating action of such coatings. In this connection, the authors have undertaken to discuss the information available in publications and the results of their own investigations with aview to evaluate primarily the pros and cons of the PTFE fiber-based CACs for substantiating their practical use. Furthermore, it seemed appropriate for the authors to s u m up the advancesin this field of modern tribotechnical materials and to discuss the possible routes to further progress. The references cited in this paper have been published during the past 30years; however, in this review primary attention is paid to the publications of the last decade. 2. Polytetrafluoroethylene Fibers
Notwithstanding limited fields of application (Evans, 1970),PTFE fibers make it possible to obtain materials and coatings which are superior in their qualities to filled composites. The expenditures involved in the production of PTFE fibers are required when bearings incorporating coatings based on PTFE-fiber textiles are put to practical use. 2.1. Preparationof PolytetrafluoroethyleneFibers. To date, the most widespread method of commercial preparation of PTFE fibers (Sigal and Koziorova, 1972) is based on their fabrication from aqueous dispersions of PTFE with the use of a thickening agent, which depends upon formation of fibers from an auxiliary polymer filled with PTFE and followed by thermal treatment. As aresult, the auxiliary polymer breaks and the particles of polytetrafluoroethylene are sintered,transforming into fibers. The spinning composition for the preparation of PTFE consists of an aqueous colloidal dispersion of PTFE, a thickening polymer, and a surface-active substance. Forming of fibers from spinningcompositions is effected by the wet-spin method. This being the case, poly(viny1
0888-588519312632-0163$O4.OOIO 0 1993 American Chemical Society
764 Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 Table I. Physicomechanical and Tribotechnical Properties of Bulk-Polymerized, Film, and Fibrous P T F E polytetrafluoroethylene properties bulk-polymerized film density, g/cm3 failure compressivestress, MPa elongation, % modulus of compression, MPa, at T = 293 K wear intensity, z h friction coeff as a function of the oper conditions
2.2
14 250-500 (100-200) 400 at P = 1.25 MPa, 3.8 X V = 0.32 m d 0.2-0.05
2.2 40 40-120 800 1.1X
lo4 at P = 20 MPa,
v = 0.1 ms-1
0.2-0.05
fiber
2.3 420 13 3000 0.8 X lo4 at P = 50 MPa, V = 0.1 m d 0.12-0.02
Table 11. Properties of P T F E Fibers country Russia USA Japan
elongation, YJ 29-30 13 40
strength, d t e x 12-15 15.3 18
permissible static load, MPa 300-500 420 420
alcohol) or viscose is used as the thickening polymer. The setting bath is composed of aqueous solutions of some metal sulfates or acetone; in the case of poly(viny1alcohol) a mixture of aqueous solutions of a inorganic salt and a inorganic acid is the setting bath and in the case of viscose the setting bath is acetone. After the fibers are formed they are subjected to finishing and heat-treatment (sintering) operations. Sintering of the polytetrafluoroethylene fibers (the Russian name is “polyfen”) is effected on machines equipped with a system of heated rollers, a t a temperature of 618-653 K. The process involves formation of continuous polymer structure, compaction of the fiber, and up to 20% shrinkage. The PTFE fiber, prestrengthened during the sintering, is subjected to stretching during which recrystallization of the polymer takes place due to orientation of molecules and supramolecular formations along the axis of the fiber, resulting in a substantial improvement in the strength of the fiber and its reduced elongation at rupture. 2.2. Properties of Fibers and Their Advantages over Bulk Polymers and Thin Films. The properties of PTFE are definbd by its structure and chemical composition (-CF2-),, (Flory, 1974). The energies of C-F (124 kcal/mol) and C-C (77 kcal/mol) bonds ensure sufficiently high strength of the PTFE molecules (Sigal and Koziorova, 1972). According to Flory (1971), low solubility of PTFE, high melting point of PTFE (600-613 K), and viscosity of the melt, which are considered to be the index of hardness of the PTFE chain, are explained by small twisting of the polymer chain, by polarity of C-F bonds, and by Coulombic interactions of the far- and nearorder dipoles. The physicomechanical properties of PTFE are largely dependent on the degree of orientation of the supramolecular structures in the polymer sample. In this connection, any increase of the degree of order of the supramolecular formations leads to an improvement in the performance characteristics of the polymer, including its tribotechnical properties. In practice, increase of the degree of order is achieved by the preparation of thin films of fibers from PTFE (Evans and Senior, 1982). In doing so, such a transition from a bulk polymer to films and fibers leads to increases in the most important performance characteritics. Comparative data on the physicomechanical and tribotechnical characteristics of bulk-polymerized, film, and fiber PTFE are illustrated in Table I [Kataev et al., 1975; Ind. Lubr. Tribol., 1975 (ref 113. As can be seen, ordering of the PTFE structure has a particular effect on the tribotechnical properties, this effect being most evident in going from the thin film to the fibers.
friction coeff on steel 0.06 0.05-0.06 0.06
oper temp range, K 113-548 113-548 113-548
ref 42 1
78
In particular, transition from bulk polymer to film and further to fiber enhances the basic physicomechanical properties of PTFE [Chironis, 1970; Ind. Lubr. Tribol., 1975 (ref 1); Arkles et al., 19741; for example, the compression strength is increased 3&40 times, resulting in asignificant increase in the carrying capacity of bearings, and the wear resistance is increased 500-fold (Tsallagova, 1967). Comparison of the physicochemical properties of fibers produced by Russia, the USA, and Japan (Table 11) shows that the characteristics of the fibers of different makes are practically identical. In consideration of the latter fact, thin-film coatings and PTFE fibre-based CACs have a number of advantages over bulk polymers (Evans and Senior, 1982),due to their reduced creep: greater carrying capacity; increased working rates thanks to improved heat abstraction; lesser working clearance resulting from smaller thermal expansion; reduced cost, since the coatings and CACs are characterized by low specific consumption of materials as compared with the bulk-polymer products. 3. Principles of Reinforcement and Main Types of Coatings with Polytetrafluoroethylene Fibers
Similar to other fibrous materials [Mater. Des. Eng., 1963 (ref Sl)], the PTFE fiber-based CACs have, in the majority of cases, two principal phases-the matrix and the fibrous framework. They differ from the structural composites,for one thing, by the presence in the framework of PTFE fibers with low adhesion to the binder and, for another, by the necessity to bring the PTFE fibers to the surface of the working layer (Turner, 1974;Bondar, 1975). This being the case, the following factors must be considered in designing the PTFE fibre-based CACs: (1) compatibility of the CAC constituents (Lubin, 1988); (2) material of the matrix, which must ensure high elasticity and reduced shrinkage as compared with similar characteristics of the fibrous framework, and by virtue of this fact exclude internal strains in the fibrous framework [Mater.Des. Eng., 1963(ref 8l)l and formation of cracks; (3) the environment, service conditions, and operational cost-effectiveness of the CACs; (4) selection of a method of enhancing the adhesion of the PTFE fibers; (5) choice of a structure of the reinforcing framework. The adhesion of the PTFE fibers is enhanced either by use of complex filaments containing PTFE fibers and reinforcement fibers (Matt, 1973) or through activation of the polytetrafluoroethylene fibers surface in alkali metal melts (Hodes, 1973, 1982). The CACs discussed below may be arbitrarily subdivided into five classes in terms of arrangement of their fibers
Ind. Eng. Chem. Res., Vol. 32,No. 5, 1993 766 and layers: with random, unidirectional, two-directional, and triaxial orientation of fibers, as well as in combination of laminated coatings. 3.1. CACs with Random-Oriented Fibers. Reinforcement of CACs with orientation of fibers is effected by use of chopped fibers. The physicomechanical properties of such CACs are largely a function of the length of fiber, the fiber length-to-diameter ratio, the surface treatment of the fibers, their mixing in the composite mixture, and the degree of filling of the latter. Thorough mixing of the fibers in the binder encourages homogenization of the mixture and preparation of macroisotropic properties to CACs (Lubin, 1988). The length of fibers in such coatings varies within 1-100 mm (Runton, 1961;Couchand Geller, 1966;Sirenko, 1985), and its optimum values constitute 6-12 mm, with a fiber length-to-diameter ratio of 50-100. In so doing, distribution of the reinforcing and the antifriction phases may be both uniform in thickness and variable, depending on the thickness, in the range of 1:4 to 4:l due to settling of the fibers in the process of polymerization of the matrix [Maschinenmarkt,1962(ref 71)l. As to composition, they are practically the same as the materials of other type, except that monocrystals, whiskers, etc. may be added into the matrix for improvement of the strength characteristics, as well as various fillers for enhancement of the antifriction properties (Hodes, 1973; Tschachder and Gubitz, 1969). Thicknesses of such composite-material coatings are equal to 0.1-0.75 mm (Runton, 1961). The random-oriented-fiber CACs are relatively inexpensive, with the use of high technology, but their production requires a large expenditure of energy in upgrading of PTFE fiber adhesion. 3.2. CACs with Unidirectional Orientation of Fibers. CACs with unidirectional orientation of fibers are reinforced by a framework of orthotropic or cylindrical construction (Freund, 1969). Orthotropic orientation of fibers may be obtained by their parallel placement (Freund, 1969). This method has the advantages of simplicity and practicability and may be especially advantageous in piecework or small-lot production. Cylindrical orientation of fibers is obtained by the use of the parallel-winding method which makes it possible to orient the fibers in any direction. Use of such construction enables the complex filaments to be impregnated with a binder in the process of preparation and to automate the process. The recommended thickness of coatings lies within the range of 0.20.5 mm. CACs of this class are resorted to rarely because of their limitations, such as lack of cross-linkingbetween the fibers and distribution of the fibers over the entire thickness of the coating, which result respectively in great shear strains and stresses in the composite material and in substantial rise in its production cost. CACs with random and unidirectional orientation of fibers have a disadvantage which is common to them: their inadequate carrying (load-bearing) capacity. 3.3. CACs with Two-Directional Orientation of Fibers. CACs with two-directional orientational of fibers are essentially systems reinforced by a framework with longitudinal/lateral (woven) (Bateman, 1976) and cylindrical orientation of fibers obtained by a method of spiral two-row winding (Banks and Benion, 1973). Most commonly employed in practice are composites with a woven-base framework containing PTFE fibers, which differ from the other types of textile both in composition and in structural design (Lancaster, 1973). Since there is a need to limit mobility of the PTFE fibers
in the working layer, reinforced fiber-free fabrics are rarely used. Reinforcement of the textiles is effected through interweaving of the PTFE fibers with more rigid highmodulus fibers of other nature [Prod.Eng., 1976a,b(refs 60,7913.An important factor ensuring reliable operation of the CACs under conditions of high specific loads is the density of the weave structure, which is responsible for the percentage and the strength of the binder in the interfiber spaces (Kuzharovet al., 1985;Butzow and Harris, 1974). The recommended thickness of such coatings lies within the range of 0.1-0.6 mm. I t should be noted that bearings with fabric-based CACs [Bull. Atl., 1975 (ref 28); Technica, 1973 (ref 56); Prod. Eng. 1976a,b(refs 60,791;Banks and Benion, 1973;Butzov and Harris, 1974;Lancaster, 1973;Newell et al., 19681. It has been found (Campbell, 1980), that frictional heating in CAC-faced bearings is the main limiting factor in selection of the rotation rate and that performance characteristics are largely a function of the thermal stability of the reinforcement fibers and the binder (Neale, 1967; Campbell, 1978,1980). It has been demonstrated that serviceability of the CACs is influenced by four factors: surface roughness of the counterbody, strain characteristics of the coatings, clearances in the joint, and bearing geometry. The optimum wear resistance of bearings with such coatings correponded to the counterbody roughness R = 10 gm (Graig, 1962). Among the main advantages of the CACs of this class are their high wear rsistance, load-carrying capacity, and damping power; freedom from seizure of the friction surfaces; and the possibility of automation of the CACfaced bearings production process, which makes them the only choice in a number of units of machines and mechanisms. However, disadvantages of the CAC-based products such as high cost, stringent requirements on the counterbody surface roughness, and narrow range of the operating rates must not be overlooked. 3.4. CACs with Triaxial Orientation of Fibers. CACs with triaxial orientation of fibers are reinforced by frameworks with a three-dimensional weave, i.e., pile, bulky, or knitted fabrics (Runton, 1961). Of particular merit are bearings for which the coating framework is made of pile fabric incorporating the base fibers and the PTFE fibers. The base is essentially cotton and Dacron fibers, which readily stick to the metal surface, or metal (copper or stainless-steel) fibers, which may be soldered to this surface. Anchored on the fabric by means of the base fibers are bundles of PTFE fibers which, when trimmed, form pile having a height of several millimeters. The pile is then filled with thermosetting resin or with ita solution. Phenolic resins are the most commonly used. The resin is partially cured at a temperature of 383-403 K during 1-15 min (to Shore hardness number up to 98). The prepreg thus obtained is adhesively bonded or soldered to a metal support, and the product is then subjected to thermal treatment. The prepared coatings possess greater wear resistance compared to that of above the discussed CACs, because application of the normal loads is along the axis of the fibers (Sirenko, 1985). The working layer of the CACs reinforced with threedimensional or knitted fabrics is prepared from the PTFE fibers, and the carrying layer involves a combination of cotton filaments and fibers of Dacron, Orlon, nylon, etc. The thicknesses of the CACs reinforced with triaxial textiles by far exceed the thicknesses of the coatings discussed above. At present such CACs are regarded as the most
766 Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 Table 111. Brief Characteristics of Materials Used in Quantity Production of Bearings oper conditions and tribotechnical char design features of coating composn trade name of material type of fiber in fabric type of binder country P8JPdyn,MPa V, mwl T, K cc 0.05-0.16 175/350 0.1 214-436 Duralon 20% Dacron; 80% Teflon thermosetting USA 0.02-0.10 1051175 0.75 193-448 thermosetting, modified Nomexteflon Teflon-polyamide, cotton USA 1411560 1 193-448 0.026-0.055 Teflon-polyester thermosetting Fiberglide USA 2101430 0.2 23 (1231-453 0.03-0.08 thermosetting USA, BRD Fiberglide-6 Teflon-glass fiber 1751310 203-493 0.03-0.08 Teflon-copper (stainless steel) wire phenolic, polyimide Pydane France 2101420 0.2 123-453 0.03-0.16 Fiberslip Teflon-carbon (glass) fiber phenolic USA 0.05-0.16 2001500 153-473 Polyfen-Vinol phenolic Naftlen Russia
sophisticated and expensive, but they hold much promise due to the feasibility of discarding the metal support. 3.6. Combination Laminated CACs. The combination laminated CACs constitute multilayer systems (Matt and Roland, 1972) whose tribotechnical properties are varied through the use of different types of weaves and through bringing the required quantity of PTFE fibers into the working layer (Spokes, 1967). These CACs differ from the ones considered earlier in that bronze, babbitt, lead, and other antifriction additives are added into their working layer in order to enhance the performance characteristics, the composition therewith being of variable thickness (Gnusov et al., 1972; Teplyakov et al., 1980). Among the most popular are combination two-layer CACs with random orientation of fibers in the working layer and two-directional orientation of fibers (fabric) in the carrying layer [Tech. Rundsch., 1977 (ref 70); Ind. Lubr. Tribol., 1975 (ref 1); Roawland and Wyles, 1973; Barret, 19753,or with two-directional orientation of fibers (fabric-fabric) in both layers (Harrison, 1973;White, 1964; Board, 1976). Structurally, the CACs featuring random orientation of PTFE fibers in the working layer and two-directional orientation of the reinforcing fibers in the carrying layer [Tech. Rundsch., 1977 (ref 70); Ind. Lubr. Tribol., 1975 (ref 1);Roawland and Wyles, 1973;Barret, 19751constitute sparase-weave fabrics, most often of high-modulus (glass or carbon) fibers, the spaces between which are filled with randomly arranged PTFE fibers [Ind.Lubr. Tribol., 1975 (ref l)]. The recommended working layer thicknesses of the coatings are 0.25-0.60 mm. Brief characteristics of the materials used in bearing production are given in Table 111. Use of the combination laminated CACs makes it possible to automate the production process and to set up a large-scale production of relatively inexpensive bearings. The pros and cons of the combination CACs discussed above are similar to those of the CACs with longitudinal/ lateral orientation of fibers. 4. Matrices
The choice of a matrix is governed primarily by the service conditions and is realized depending on ita compatibility with other constituents of the CAC. Of particular importance in practical operation are mechanical strength in a designated temperature range, resilience (elasticity) adequate to sustain mechanical stresses within designated limits, zero or minimum shrinkage during curing, freedom from tendency to age, or change of properties in the course of operation. Also, due consideration must be given to the physicochemical properties, such as chemical stability and resistance to attacks resulting from moisture, atmospheric conditions, deep vacuum, oils, and solvents (Kardashov, 1976). 4.1. Polymeric Binders. The principal characteristics of the polymer binders are their adhesive and cohesive properties. As is evidenced by literary publications, in bearings for which framework is based on PTFE fibers,
Table IV. Principal Characteristics of Binders ultimate strenath at 293 K at at t eof atshear, nonuniform uniform no. agesive MPa tear,MN.m-l tear,MPa Phenolic-Elastomer Adhesives 1 VK-3 20 650 19 2 VK-32-200 21 320 18 3 VK-13 20 450 16 4 VK-13M 23 450 18 5 6 7 8
KLN-I VK9 L4 BOV-3
213-473 213-473 213-573 213-473
25 20.5 14.5
213-333 213-353 213-323 213-443
10.5 40 50
213-333 213-333 213-333
Hot-Setting Epoxy-Silicone Adhesives 21-36
2 13-573
(re? uoroplastic)
9 K-153 10 Epoxide-I1 11 VK-32-M 12 K-350-61
Epoxy Adhesives 120-220 240
15 15 6
oF:iE,?
13 TEF-9 14 VK-8
15 VK-10 16 SP-6K 17 Cyacrin-ED 18 Cyacrin-ED
13.5 21
15-25
100 150
213-523
14
High-Heat-Resistant Adhesives 15.5 130 21.5 13 130 5
Polyimide Adhesives 100
Cyanoacrylate Adhesives 18 100 13 100
up to 1273 up to 1473 393-573
20 20
213-18 373-13
either a thermosetting or thermoplastic polymer base is employed, as may be required by the operating conditions (Kataev et al., 1975; Kovalev, 1977; Movsisyan, 1980; Petrova, 1977; Popilov, 1972). Yet, in selection of a polymeric binder in addition to the above-mentioned properties due consideration should be given to its technology, availability, or shortage of supply, and relative cheapness. The most popular binders are phenol-formaldehyde, epoxy, silicone, polyimide, cyanoacrylate, polyamide, and polyester. The principal characteristics of the binders of these groups are listed in Table IV. For enhancing elasticity and heat resistance phenolformaldehyde resins modified by various thermoplastic materials and elastomers [polyamides, poly(viny1acetals), synthetic rubbers, etc.] are used. Such matrices demonstrate good performance in load-bearing structures. In particular, phenolieelastomer adhesives successfullycombine the positive qualities of the phenol-formaldehyde resins with those of rubbers, i.e., high heat resistance of the former and enhanced elasticity of the latter, possessing therewith good adhesivity to metals and plastics. CACs based on these adhesives are resistant to thermal aging and cyclic exposures to temperatures variable from 213 to 353 K, are water- and oil-resisant, and exhibit high vibratory-load strength (Popilov, 1972). As compared to the above-cited adhesives, binders modified by poly(viny1 acetals) feature a number of disadvantages such as
Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 767 brittleness, insufficient strength at nonuniform tear, low impact elasticity, and inadequate water resistance. The epoxy nonmodified binders are characterized by impermeability, but they possess low resistance to thermal destruction as compared to the phenol-formaldehyde binders, their operating temperature range is from 213 to 323 K, and they are not used in load-bearing constructions. CACs produced by hot-setting of epoxy adhesives have high strength characteristics within the temperature range of 213-373 K. Better technologicalcharacteristics, enhanced elasticity, and heat resistance, versus those af the hot- and coldsetting epoxy binders, are exhibited by modified epoxyresin binders, which makes it possible for some of them to be used in CAC load-bearing constructions. Among obvious advantages of these binders are their resistance to attacks by water, oil, and gasoline (Kardashov, 1976). The epoxy-siliconebinders form an extremely promising group because they can be used in a wide range of temperatures from 213 to 673 K, although they display reduced elasticity, as compared with the phenolic-elastomer binders. The latter feature prevents their use in bearings operating under conditions of alternating motion (Petrova, 1977). The high-heat-resistant silicone binders exhibit relatively low plasticity and strength at normal temperatures, but they retain their strength at very high temperatures (Popilov, 1972). The polyimide binders cure at temperatures of 533-577 K and retain their initial strength performance during long-time operation at 533 K (20-30 OOO h). Some of them display high resistance to attacks from water and artificial tropical climate (Petrova, 1977). The cyanoacrylate adhesives feature good adhesivity to various materials, contain no solvent, and rapidly cure in air. They are moisture-, gasoline-, and oil-resistant and are serviceable within the operating temperature range from 213 to 373 K, and they possess good physicomechanical characteristics (Kataev et al., 1975). Matrices based on polyamide resins are formed by the use of their melta. 4.2. Metal-Polymer Matrices. Upgrading of the physicomechanical and tribotechnical properties of coatings is also achieved by the use of metal-polymer matrices. A wide varity of methods are available for improvement of the physicomechanical properties of the polymer matrices through addition of the following into their composition. 1. Various inorganic fillers may be added: MoS2, MoSez, MoTe2, WS2, WSe2, WTez, PbO (Hodes, 1973;Tschacher and Gubitz, 1969); metal powders (Kutkov et al., 1980); alloys (Gnusov et al., 1971; Hodes, 1973). 2. Organometallic and complex compounds may be added (Kutkov et al., 1980; Grechko et al., 1984; Kovalev et al., 1987; Kuzharov et al., 1985,1986; Pomogailo et al., 1972). 4.3. Metal Matrices. Because performance characteristics (serviceability, rate) of sliding-contact bearings with PTFE fiber-based coatings depend in large measure on effective heat abstraction from the zone of friction contact, CACs with a metal matrix have been developed (Gnusov et al., 1972; Hodes, 1973; Hein, 1974; Sysoev et al., 1974). 5. Tribotechnical Properties of Coatings Based on Polytetrafluoroethylene Fibers and Mechanism of Their Lubricating Action The performance characteristics of CACs are governed by the composition of the matrix and the reinforcement
rnT 1
-
0.032 0.030 0.028
, I
0 50 100 150 P,MPa Figure 1. Effect of reinforcementfiber type on antifriction properties of CACs: 1, polyfen-Arimide-PM, 2, polyfen-Lavsan; 3, polyfenVinol; 4, polyfen-cotton. Table V. Wear Resistance of Compositions Based on Phenolic-Elastomer Matrix and Linen-Weave Fabric Containing 60% Polyfen and 40% Reinforcement Fibers during Oscillatory Motion (V,,, = 0.025 ms-I) sp load, MPa
25 50 70 100 125 175
wear intensity r h x 1Olo Arimide-PM Lavsan Vinol 1.06 3.28 17.8 75 208 1150
1.07 3.75 23.6 90 252 1580
1.24 3.64 20.7 97 283 1320
Cotton 1.22 3.86 22.8 93 276 1460
phase, by various additives, and by the design features of the framework. 5.1. Effect of Reinforcement Fiber and Matrix Materials on Tribotechnical and Physicomechanical Properties of Coatings. An essential precondition in determining the physicomechanical and tribotechnical potentialities of PTFE fiber-based CACs is the impartial assessment of the technological capabilities of fibers and adhesive binders. Investigation of the effect of reinforcing fibers (Barchan et al., 1978,1987;Ryadchenko et al., 1984;Cordiano et al., 1956)on tribotechnical properties of the CACs was carried out with the use of CACs with a twill-weave fabric framework involving the PTFE fibers and the following reinforcing fibers: poly(viny1 alcohol) (Vinol), polyester (Lavsan), cotton, and polyimide (Arimide-PM). The phenolic-elastomer binders were used as the matrix. The results of tribotechnical testa illustrated in Figure 1 and in Table V show that the materials of the studied reinforcing fibers do not affect the friction coefficient. Reduction in the friction coefficient value in response to pressure increase from 25 to 50 MPa is explained by the fact that an ever-increasing number of low friction coefficient PTFE fibers come into contact, while ita rise at loads ranging from 50 to 175 MPa is connected with upgrading of the PTFE viscoelastic properties. Stabilization of the friction coefficient value at loads exceeding 175 MPa is explained by an increase in the actual area of contact and, consequently, by a decrease in the actual load [Prod. Eng., 1976a,b (refs 60, 7911. Similar friction coefficient values (0.02-0.06) have been obtained for CACs in which the framework contained highmodulus and high-strength carbon fibers. At the same time, investigations (Barchan et al., 1978, 1987; Ryadchenko et al., 1984) have shown that wear resistance is a function of the type of reinforcing fibers. The somewhat better wear resistance of the coatings containing Arimide-PM may be attributed to their high mechanical characteristics (Maslennikov, 19731,while use of high-modulus and high-strength carbon fibers as reinforcing fibers, E = (3.85-4.55) X lo5 MPa, 8~ = (14-
768 Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 Table VI. Strength Performance of CACs with Framework of Linen-Weave Fabric Containing 60% Polyfen and 40% Reinforcement Fibers ult shear ult tear residual def stren h, after removal of tvue of matrix T . &a % a$,:: 200-MPa SD load, um phenolic-elastomer 7.0 6.9 42 phenolic-elastomer 6.9 6.7 36 modified epoxy 3.5 7.2 42 cold-setting epoxy 3.5 6.9 48 epoxy-silicone organic 4.7 7.9 46 Table VII. Tribotechnical Properties of Sunken-Loop Knitted Fabric Framework (60% Polyfen, 40% Reinforcement Fibers) versus Type of Matrix in Rotary Motion (P= 50 MPa, V = 0.025 m d ) Darameters twe of matrix IL I h x io7 phenol-formaldehyde 0.06 1.6 phenol-formaldehyde 1.8 0.04-0.05 0.06 4.8 epoxy-silicone organic 0.05 3.4 epoxy 0.03-0.04 5.4 epoxy 2.4 0.04 epoxy-silicone organic 1.2 0.03-0.04 phenol-formaldehyde modified by poly(viny1 acetal) phenolic-elastomer 0.03-0.04 1.8
21) X lo2MPaand E = (2.45-3.15) X lo5MPa, 6~ = (24.531.5) X lo2 MPa, brings about a 3-4-fold increase in the wear resistance. It has been shown (Ryadchenko et al., 1988)that bringing more than 60-80 % of reinforcing fibres into the friction contact zone reduces the wear resistance of coatings with a woven-type framework and increases their friction coefficient. Such degrading of tribotechnical properties is associated with the fact that a large number of high friction coefficient fibers become involved in the process of frictional interaction and that formation of the PTFE transfer films therewith becomes hindered. Accordingly, the best wear resistance is attained at the PTFE fiber to carbon fiber ratio of 3:1, and low friction coefficient values are ensured whenever at least 30 % of the bearing surface is occupied by the PTFE fibers. Thus,the nature of thereinforcement fibers has no effect on the friction coefficient but defines the wear of the coatings, while bringing into the friction contact zone more than 70% of the reinforcement phase degrades the tribotechnical properties of the coatings. Selection of a matrix is primarily a function of the conditions under which it will be operating and is realized depending on the physical and physicochemicalproperties of the polymers. It has been demonstrated (Ryadchenko, 1988) that under alternating load the best strength and wearresistance performance is exhibited by compositions incorporating a phenol-formaldehyde or phenolic-elastomer-based matrix (cf. Tables VI and VII) and, in the case of a rotary motion, phenol-silicone matrices. This being the case, the composition of the matrix is responsible for the wear-resistance factor of the coating, while the antifriction and strain-response characteristics are independent of the type of the investigated matrices. 5.2. Effect of Reinforcement Framework Design on Properties of Coatings. The performance characteristics of CACs of identical chemical composition are primarily a function of their design features in terms of the weave structure and orientation of fibers. One of the ways of controlling the wear resistance of such coatings is the choice of the optimum types of weave of antifriction and reinforcing fibers and of the mode of the orientation
a b C Figure 2. Types of weaves/loops in CAC framework (a) 11/2-layer twill fabric; (b) sunken-loop tricked knit-fed fabric; (c) warpknitted fabric.
relative to the direction of sliding motion (Bely, 1982). Most suitable for CAC frameworks woven or loop (knitted) textures, whose great diversity makes it possible to vary the tribotechnical properties of the resulting composites over wide limits. The overwhelming majority of CACs are manufactured on the basis of woven textures [ Werkstatt Betr., 1969 (ref 46); Bull. Atl., 1975 (ref 28); Des. Eng., 1973;Prod. Eng., 1976a,b (refs 60, 79); Ind. Lubr. Tribol., 1975 (ref l)], whereas loop structures find practically no application even through from the technological point of view they exhibit a number of advantages over woven ones. Among these are the multiformity of diverse weaves; 5-10 times higher productivity in processing into knitted as compared with woven fabrics, with 3-3.5 times less basic material consumption and overall labour expenditures (Guseva, 1981); and the possibility of producing seamless hoses, thus obviating joints. Besides, the loop structures are pliable in various directions which is of fundamental importance in constructing bearings of complex shape. For coatings based on woven or loop structures (cf. Figures 2-4) which enable to obtain structures with approximately similar distributions of antifriction fibers and reinforcing fibers and to make comparisons of tribotechnical properties of coatings without substantial change in their chemical compositions, investigations (Kurzharov et al., 1986) have shown that the type of the weave or loop structure has practically no effect on the antifriction properties but is a decisive factor in defining the wear rate. It has been found (Kuzharovet al., 1986)that orientation of fibers with respect to the rate vector (Figure 5 ) as exemplified by warp-knitted fabric with practically unidirectional orientation of fibers in the working layer affects the wear resistance of the coating; in doing so, orientation of fibers normal to the rate vector is preferable. This regularity may be used in designing anisotropic-structure coatings. Thus, the type of weave structure and orientation of fibers in the coatings of radial bearings practically does not affect the antifriction properties but is a decisive factor in defining the rate of wear of the composition, and loopstructure materials are at least equal to the woven-structure ones as far as the tribotechnical properties are concerned. 5.3. Mechanism of Lubricating Action of Polytetrafluoroethylene Fiber-Based Composite Materials. I t is universally acknowledged (Tanaka et al., 1973; Makinson and Tabor, 1964; Uetz and Breckel, 1967) that the lubricating action mechanism both of pure PTFE and of PTFE-based composite materials is connected to the low values of energy of intermolecular interaction in polymers (Uetz and Breckel, 19671,the structure of which represents highly oriented crystal planes or plates, 10-40 nm thick, separated by nonoriented areas of polytetraflu-
Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 769
a
b
C
Figure 3. Friction coefficient verus specific load and sliding rate friction of CACs on steel-45. HRC = 60-65; phenol-formaldehyde matrix; framework a, b, and c correspond to Figure 2.
and formation of a 10-40-nm-thick transfer film on the surface (Uetz and Breckel, 1967).
r,,407 18
6. Applications of PTFE Fiber-Based CACs
12
6
t
1
I
12
6
0
30
60
90
120
150
q,deg
Figure 5. Effect of angle of orientation of fibers in coating working layer on tribotechnical properties. Matrix from epoxy resin PTFE fibers orients by hand laying.
oroethylene of uniform thickness up to 1nm, which can readily slip under the action of minor shearing forces (Makinson and Tabor, 1964). The phenomenon of the friction coefficient increase with an increase in the sliding rate can be explained by the possibility of tribochemical interaction between the macromolecules of PTFE and the surfice of the metal. At low sliding rate the reaction is directed chiefly to the surface of the metals, and as a result, a transfer film from PTFE is formed on this surface. With an increase in the rate, i.e., higher energy levels, the reaction develops within the volume of the boundary layer, causing a more pronounced interaction of macromolecules with each other and an increase in the friction coefficient value. It should be noted that under difficult conditions transfer of large particles, several fractions of a micron in size, occurs, while under light-duty conditions there prevails slippage of thin layere, several tenths of a nanometer thick,
The fields of application of the PTFE fibre-based CACs are defined mainly by their properties which afford a number of advantages over the conventional bearing materials. Operation of bearings with PTFE fibre-based CACs require, as a rule, no lubrication of the friction units. Also, this arrangement ensures high load-carrying capacity and low friction coefficients, freedom from seizure on the friction surfaces, high vibration and chemical strength, a wide operating temperature range, high wear resistance and chemical strength, a wide operating temperature range, hgh wear resistance and damping properties, unique dimensional stability of bearings, and high insulation characteristics. In addition, such bearings require no maintenance, which makes it possible to design friction units practically for the entire life span of the machines. The above-mentioned advantages of the CAC bearings along with their ability to operate under vacuum have predetermined their application in missiles and spacecraft (Evans, 1970; Graig, 19621, for example, in the design of a rotary rocket nozzle (Sottosaanti and Baker, 1978)or in the suspension of carrier vehicles for moving on the lunar surface (Hein, 1974). Use of CACs in various types of aircraft constructions, in the B-1, F14-A,F-111,Mirage-6, Boeing-707,etc. [Evans, 1970; Reynolds, 1974; Chironis, 1970; Ind. Lubr. Tribol. 1975 (ref 1);Reinsch, 1962;Graig, 1962;Production, 1971 (ref 31); Tech. Rundsch., 1977(ref 70); Berneyand Chaivre, 1966; Bruch, 1970; Sukharev, 1977; Burns, 1972; Aerosp. Saf., 1970 (ref 15); Ascani et al., 19801, have provided a reduction in the weight and size of the friction units and enabled an overall enhancement of reliability of conventional and vertical-lift aircraft. The most effective is application of CACs in wing high-lift systems, in elevator hinges, in spline-and-hinge joints, etc. The large scale of application of CAC bearings is evidenced by the fact that more than 1000 such bearings of different standard sizes are incorporated in the Boeing-727 airliner construction. Another field of extensive application of friction units incorporating PTFE fiber-based CACs is the automotive industry [ Werkstatt Betr., 1969 (ref 46); Tech. Rundsch., 1977 (ref 70); Muschinenmarkt, 1962 (ref 71); Reynolds et al., 1974;Production, 1971(ref 31);Berneyand Chaivre, 1966; Cooper, 1977;Booser, 1964;White, 1963,1965). Use of CACs in, for example, the steering and suspension
770 Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993
b
a
d
e
f
three types of CACs: (1) Fibriloid, universal CACs representing a fabric (twill 1/11 of PTFE fibers and polyamide, impregnated with a polymeric binder; (2) Fabroid, CACs comprising two layers of fabric woven from PTFE fibers and glass fibers and designed for aircraft application; (3) Fiberglide, a CAC similar to Fabroid but featuring Dacron fibers instead of glass fibers, which enables it to be operated under conditions of high loads and vibrations. Similar CACs with different PTFE to reinforcement fiber ratios, different types of polymer, and metal-polymer binders are also used by other firms. The main tribotechnical, physicomechanical,and chemical properties of bearings with PTFE fiber-based CACs are summarized in Table IX. Of fundamental importance in practical application of bearings with PTFE fiber-based CACs is the prediction of their tribotechnical properties depending on the operating conditions. Although it is an obvious fact that the real performance statistics can be furnished only by service tests, analytical methods are available which provide a means for making correct forecasts, corresponding in the majority of cases to the actual behavior of CACs in friction units. Thus, according to the findings of Ina Elges [Masskatalog K227D, 1980 (ref 39)1, the antifriction properties, specifically the friction coefficient, may be represented as a product of three factors: P
Figure 6. Different types of mobile joints.
systems of various types of automobiles results in improved stability of the car on the road, simplified maintenance, and easier steering. In addition to the above-mentioned and already traditional spheres of application, bearings with PTFE fiberbased CACs are used in shipbuilding (Popilov, 1972), namely, in the construction of hydrofoil ships, in cargohandling equipment [Reynolds, 1974;Chironis, 1970;Ind. Lubr. Tribol., 1975 (ref 1); Reinsch, 1962; Graig, 1962; Sottosaanti and Baker, 1978; Production, 1971 (ref 31)1, in the agricultural machinery and power engineering industries, in metallurgy, in the machine-tool industry, in the chemical engineering industry, in the food industry, etc. [Des.Eng., 1973 (ref 22); Prod. Eng., 1976a,b (refs 60, 79); Tech.Rundsch., 1977 (ref 70);Maschinenmarkt, 1962 (ref 71);Reynolds, 1974;Chironis, 1970;Production, 1971; Reinert, 1964; Graig and Remorenko, 1966). A major step forward in the expansion of the field of uses for the PTFE fiber-based CACs is the setting up of a commercial production of standard friction units of various standard sizes and functional applications. The most important and quantity produced are radial rocker bearings (Figure 6a), articulated heads (Figure 6b), lugs (Figure 6c), and special-purpose bearings (Figure 6d-h). The weight of such friction units varies, depending on their loading and size, from 60 g to 240 kg and more. A wide range of friction units with PTFE fiber-based CACs is brought to the market by a number of companies, such as ADR, SKF, Lear Siegler, Hunger SFE, Ina Elges, and the Saratov State Bearing Plant. [Lear Siegler, 1976 (ref 86); Masskatalog K227D, 1980 (ref 39); Aeronautical catalogue, Ivry-sur-Seine, 1980 (ref 4811 and by other manufacturers. CACs employed in standardized friction units produced by different companies substantially differ in both design and composition. Specifically,Lear Siegler [Lear Siegler, 1976 (ref 86)l relies mainly on the following
= PPfTf"
each of which is a function only of the applied load (Figure 71, temperature (Figure 8), and rate (Figure 9). Although relationships between these factors and the load, temperature, and rate are different for different CACs, the character of these relationships remains invariable. The servicelife of bearings with PTFE fiber-based CACs can also be expressed as afunction of an operation factors. Thus, Hunger DFE suggests the following empirical relationships for determining the service life of PTFE fiberbased CAC articulated bearings:
or
G,=-Cl0OOK
io5
PKOf TKn,x% where G, is the service life, in cycles; Gh is the service life in hours; C is the dynamic coefficient of serviceability depending on the type and size of the articulated bearing, in kilonewtons; P is the equivalent load of the bearing, in kilonewtons; fl is the angle of turn, in degrees; f is the frequency of turn, in minutes; KL is the load direction coefficient equal to 1.0 at constant direction and 0.13 at reversible direction; KT is the coefficient defining the temperature effect, varying from 1.0 to 0.3 withira the temperature range from 313 (KT = 1.0) to 373 K (KT= 0.3) (Table X); and KW is the CAC wear-resistance coefficient determined by the serviceability characteristics (Figure 10). As is evident from the information cited above, the tribotechnical properties of friction units with PTFE fiberbased CACs are governed by the operating conditions, primarily by the load, the relative sliding rate, and the temperature. It is precisely these three factors which define the spheres and conditions of application of the PTFE fiber-based CAC bearing units.
Ind. Eng. Chem. Res., Vol. 32,No. 5,1993 771 Table VIII. Effect of Binder Type on Tribotechnical Properties of Coating with Various Forms of Motion ( V = 0.026 m.s-l)m tribotechnical char
0.035 0.04
Ih 7.5 x 10-9 1.2x 10-8
phenolic-elastomer phenol-formaldehyde
0.03 0.035
9.0x 1.6X
10-9
Arimide-PM
phenolic-elastomer phenol-formaldehyde
0.04-0.045 0.045-0.05
2.1 X 8.3 X
le7
Lavsan
phenolic-elastomer phenol-formaldehyde
0.03-0.035 0.03-0.04
1.8 X 1.2 x 10-7
reinforcing framework linen-weavefabric
reinforcing fibers (40%) Arimide-PM
type of matrix phenolic-elastomer phenol-formaldehyde
sunken-loop, knitted fabric
Lavsan
linen-weavefabric sunken-loop knitted fabric
form of motion
l.4
Adhesive VK-32-200was used as the phenolic-elastomer matrix, and adhesive VS-350modified polyvinylacetylenewas used as the phenolicformaldehyde matrix.
Table IX. Tribotechnical. Phvsicomechanical. and Chemical Prowrties of CAC Bearings property specification friction coeff on steel low: 0.02-0.08 at P = 20-210 MPa; increases with the circumferential rate, at temperatures below 233 K and specific loads below 20 MPa freedom from seizure at elevated loads, low sliding rates, and intermittent antifriction properties operation varies as a function of the binder; high in various environments chemical resistance high; depends on the conditions of loading and the integrity of coating wear resistance for the fabric in the coating, maximum 3 shrinkage, % varies from 73 to 523 K depending on the type of binder thermal resistance 0 . 5 ms-1 (ref 22) (up to 1 ma-') (ref 8) sliding rate excellent damping properties vibration and impact resistance compression strength dynamic, MPa 210 static, MPa 420 cold flow practically zero 0.2-0.65 thickness of coating, mm tolerances ISO,JT7,JT8 even during the running-in period lubricant-free operation at temperatures exceeding 607 K starts decomposing with ecology liberation of gaseous products ~~
~~
~
ZEEB
CLP
0.06
0 2
0.04 0.02
0,001
0.01
0.1
1
V, rn0s-l 1.67
Figure 9. Rate coefficient curve. 2
0
4
6
8
10
P-lO'.MF%
Figure 7. Load coefficient curve. fT
1.5 1.o
0.5
0 223 273 320 T,K Figure 8. Temperature coefficient curve.
The operating temperature conditions impose special requirements on such friction units, since the operating temperature range must be narrower than the temperature limits within which each of the CAC components is capable of realizing its functional properties. The most vulnerable element in this context is the polymeric binder of the CAC, which in the majority of cases predetermines the operating
Table X
T,K KT
T
