Ind. Eng. Chem. Res. 1993,32, 774-779
774
Realization of the “Zero-Wear” Effect in Composite Coatings Alexander S. Kuzharov’ and Viktor G. Ryadchenko Department of Chemistry, Rostov Agricultural Machine-Building Institute, 1 Gagarin Square, Rostov-on-Don 344070, Russia
This paper demonstrates the potential applications of the latest developments in tribology, specifically, the wearless effect, aimed at enhancing the relability and longevity of friction units incorporating polytetrafluoroethylene fiber-based coating. 1. Introduction
Experience in development and application of bulk polytetrafluoroethylene (PTFE) based friction-resistant materials has demonstrated that introduction of copper or its compounds into the composition appreciably enhances their tribotechnical properties (Bely et al., 1976; Garkunov, 1985; Garkunov and Polyakov, 1974). The potential ways of realizing the “zero-wear” effect under strictly specific limited conditions of lubricant-free friction are so far not clearly understood, though information recently obtained (Garkunov, 1985) gives clear evidence of a noticeable increase in wear resistance and longevity of metal-polymer friction units owing to a complex of physicochemical processes involved in the metal-polymer tribotechnical system. 2. Polytetrafluoroethylene Fiber- and Copper-Based Coatings
The first results in the realization of the “zero-wear” effect in a steel/self-lubricatingPTFE coating friction pair have been obtained on matrices of the type “Pydane” [Werkstatt Betr., 1969 (ref 20)], in which, contrary to recommendations of the proprietary film, copper wire in combination with PTFE fiber has been employed as the working layer. The data thus obtained (Grechko et al., 1982; Grechko, 1982) have shown that after substantial run-in wear due primarily to an imperfect adhesive bonding procedure the “reversed” Pydane practically ceased to wear, retaining friction resistance and load characteristics similar to those of ita counterpart. The latter phenomenon, in the opinion of the authors (Kutkov et al., 19801, is connected with realization of the “zero-wear” effect in a steel/PTFE fiber-based coating friction pair due to tribocoordination (Grechko et al., 1984) of the PTFE tribooxidation products onto copper and subsequent transfer and decomposition of the metal-containing compounds on the conjugated surfaces. The data (Grechko et al., 1982)obtainedduringinvestigations of a composition into which copper was introduced by way of its chemical deposition on the PTFE fiber and during tests of a composition (Kuzharov, 1991) containing copper and complex-forming additives, salicylideneaniline or 8hydroxyquinoline, demonstrate an enhancement in the wear resistance and a reduction in the friction coefficient by a factor of 2. Of particular interest are characteristic inflections of the friction coefficient curve which coincide in time with the formation of a visually detectable copper film on the contacting surfaces. Similar relationships have been observed in the selective-transfer regime during friction of copper alloy/steel pairs in a low-molecular-weightliquid medium (Onishuk et al., 1981; Kuzharov, 1991). This
* Author t o whom correspondence should be addressed.
circumstance combined with the absence of wear of the sample and the counterbody under the conditions of steady-state operation and extreme loading support the effect of “zero wear” during friction of a copper-fluoroplastic composite over steel with no lubricant available. Investigation of the mechanism of lubricating action (Kutkov et al., 1980)by scanning electron microscopy and electron diffractometry has made it possible to determine that observable areas of copper film start forming on the abrasive streaks, i.e., in the region of surface defects, ensuring fairly high concentration of the active surface centers, and on the level of the Servovite film exhibits nonuniformity and discontinuity in thickness, which varies within 100-300 nm. The electron diffractometry data give conclusive evidence of the existence of a nonoxidized copper layer on the surface. Simulation of processes involved in the zone of friction contact by differential thermogravimetric analysis (DTGA) has shown that a t temperatures exceeding the PTFE decomposition temperature a sharp change in the mass loss occurs, and additional peaks appear on the DTG and differential thermal analysis (DTA) curve,which are absent on the thermograms of the noninteracting components. Yet, the availabletechniques prove inadequate for isolating and identifying the product formed in this case. On the diffractogram, peaks characteristic of the oxides and fluorides of copper have been detected. If it is remembered that chelates of perfluorinated diketones of heavy metals, of copper in particular, are highly volatile, the following pattern of their formation in the process of thermaloxidation reaction may be presupposed:
OH
I
(-CF2)m-CF-(CF2)k-CF=O
cu
T > 600 K T c 693 K
,CF=O (CF2)k ‘CF-0
\
fCUlP
I
(CF2)m
Proceeding from the results of the investigation (Kutkov et al., 19801, the authors have suggested the following structure of frictional contact resulting from the friction of a copper-fluoroplastic composite over steel (Figure 1). The illustrated structure of the boundary layer comprises a layer of Servovite copper film linked with the surface of the steel crystal lattice and a metal-polymer layer oriented in the direction of friction, which is anchored on the surface of the Servovite film due to complexing. However, use of pure copper in the form of fibers complicates the textile framework manufacturing procedure, and introduction of copper into the matrix or chemical (electrochemical) deposition of copper onto the framework of the coating degrades the physical and mechanical properties and increases the wear rate of the
0888-588519312632-0774$04.00/0 0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32,No. 5, 1993 775 Scheme I
NH
I c=o Figure 1. Tribocoordination scheme.
&OH
I.L 0.16 0.12
0.08 0.04
O %K
W3
%L
0
Figure 2. Effect of low-molecular-weightcomplex in combination with ligand on wear resistance of CACs.
3
2
1 XI.%O
t
% x2s
0
1
Figure 3. Geometrical interpretation of canonical transformation of response surface level.
composite material, which is ruled out altogether in using the copper-containing components. Thus, though introduction of copper into fluoroplastic compositions substantially enhances their wear reeistance due to realization of the “wearlnessness” effect, it seems that a more promising way is the introduction of copper into compositions in the form of chemical compounds.
3. Coatings Based on Polytetrafluoroethylene Fibers and Complex Compounds of Copper In studies by Kuzharov et al. (19911,a positive effect of copper on the tribotechnical properties of PTFE fiberbased CACs has been noted and the feasibility in principle has been disclosed of improving their tribotechnical properties through realization of the “zero-wear” effect thanks to formation in the zone of friction contact of coordination compounds of copper with the PTFE decay products. On the basis of this fact, an assumption was made that if the processes under review exist under the real conditions of friction interaction, introduction of prefabricated complex compounds into the compositions must bring about an improvement in their tribotechnical properties, also assume their approximately equal level in
those instances when boundary layer structures closely approximating each other in composition and makeup are being formed on the friction surface. In this connection, an attempt was made to improve the tribotechnical properties of coatings by introducing copper coordination compoundsinto the compositionsbeing studied in a variety of ways, i.e., by addition into the matrix a low-molecularweight complex in combination with a ligand, by addition of a complex anchored on the surface of a PTFE powder, or by introduction of a polymer complex in the form of fibers into the framework of the coating. 3.1. Optimization of Makeup and Tribotechnical Properties of Composition Filled with Low-Molecular-Weight Copper(I1) Complex in Combination with Ligand. The composition consisted of the following: the framework, which was made from commercial knitted fabric based on PTFE fibers and poly(ethy1ene terephthalate), the matrix from phenol-formaldehyde resin (varnish), and acrylonitrile rubber. By comparative tests of a number of experimental coatings additives, the most suitable for these conditions have been selected: salicylidene-p-methoxyanilinateof bivalent copper (K) and salicylidene-p-paramethoxyaniline(L).
(K)
(L)
Qualitative choice of these components involved a great body of experimental data on the development and operational use of such composites, with due consideration of the peculiar operating conditions of heavy-loaded friction units (Ryadchenko,1988). Further work covered optimization of the quantitative proportion of the components. Use of the screening experiment method made it possible a t the very first stage to exclude the content of varnish and of acrylonitrile rubber from the variable factors, assuming their values t o be constant and equal respectively to 13% and 77% by mass. The gradient motion (steep ascent method) enabled the determination of the wear resistance optimum, with the content of salicylidenep-methoxyaniline amounting to 0-3 % and that of copper(11)salicylidene-p-paramethoxyanilinateto 0-3 %. Since substantial curvature of the response surface (Figure 2) prevents its description within the narrow area of the factorial space in terms of a linear equation, it seems
776 Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 a
0.08
0.06 0.04
1
I
I
1
0
100
50
150
P, MPa 0
I,0.10
1
200
100
300
P, MPa
b
I, mm 0.08
0.06 0.04
0.02 0
50
150
100
0
P,MPa Figure 4. Effect of copper complex anchored on PTFE powder surface on tribotechnical properties of CACs. (a) Antifriction properties; (b) wear resistance. I,' base; V, base + 5% K.
expedient to describe the process of wear by a secondorder polynomial (Tschacher and Gubitz, 1969; Turner, 1974): 2
2
2
9 = bo + C b i X i + Z b i j X i X j+ C b i i X ? + c i=l
i#j
(1)
i=l
where bo, bi, bij, and bii are the parameters of the model and e is the random error. In the region of the extremum, the second-order design has been implemented. After processing of the results by computer, the following wear relationship has been obtained:
I 0.135 + O.OIXl - 0.152X2- o.ooo4x,xz+ 0.0012Xl2+ o.058x22 (2) For determining the optimum conditions of the wear processes, a canonical analysis of the second-order model has been performed (Evdokimov et al., 1980).
Z = 0.0016kls2+ 0.0571x22
XlS = -4.15;
with a new center
(3)
a,, = 1.32
Geometrical interpretation of the canonical transformation of the response surface equation is illustrated in Figure 3. The numbers in the figure represent the calculated values of the amount of wear. The new axes and the slope angle value have been obtained by computation. Parallelism of axes XI and XIS, XZ and 22s suggests tht the pairwise interaction coefficient is insignificant.
160 240 P, MPa Figure 5. Effect of fibrous polymeric complex on tribotechnical properties of CACs. (a) Antifriction properties; (b) wear resistance. 0,base + heterocyclic polyamides; A, base + Cu + Vinol + 8-hydroxyquinoline.
80
The data given in Figure 3 demonstrate that wear resistance of a composition, into which a low-molecularweight complex in combination with ligand (Kuzharov et al., 1985) has been introduced, is 2-3 times that of the nonfilled one, and its friction coefficient is somewhat higher, amounting to 0.07-0.09. I t has been found (Kuzharov and Ryadchenko, 1988) that the principal factor affecting the wear resistance is XI (complex compound of bivalent copper) though, for instance, in metal-plating lubricants (Onishuk et al., 1981), the effect of X2 ligand is also tangible. The latter effect is due to the availability in the metal-plating lubricants of a liquid carrier by means of which these compounds are being conveyed into the zone of friction contact. In the case under consideration, transition of metal ions into the solution is impossible, because the mobility of both the metal ions themselves and the added ligand is restricted. This leads obviouslyto a direct interaction of the products of tribodestruction of the matrix and the fluoroplastic with the ions of metal formed in the process of tribodecomposition of the complex compound. Considering the fact that the triboreaction proceeds within the microvolumes of the friction contact and that concentration of the added ligand in the nearest environment of the metal (in this particular case, copper) is substantially inferior to concentration of the matrix and the PTFE tribooxidation products, the probability of interaction between the metal and these products is significantly higher than that with the ligand. It is precisely this fact that explains the
Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 777
rl.
-Fc-Fc-Fe-
I l l
PTFE-g-PSAA
'Cu
.--3
PTFE-p-PSSAA
Figure 6. Boundary structure formation mechanism.
practically observed independence of the wear resistance of the composition from the concentration of ligand. Thus, the complex compounds of copper intensify on the friction contact the tribochemical processes promoting improvement of the performance characteristics. 3.2. Makeup and Properties of CompositionFilled with Copper Complex Anchored on the Surface of Polytetrafluoroethylene Powder. Coatings (Chironis, 1970;Turner, 1974)filled with fine-grained PTFE powder are well-known. However, practically they do not differ in their makeup and properties from the conventional compositions based on PTFE fibers. The basic composition, similarly to the above-cited example, is filled with PTFE powder featuring a surface-grafted binuclear copper(I1)complexwith acrylsalicylamide,obtained according to Scheme I (Pomogailo et al., 1986). Introduction into the composition of a binuclear copper complex with acrylsalicylamide anchored on the surface of the PTFE powder brings about a 3-4-fold increase in the wear resistance (Kovalev et al., 1987; Kuzharov et al., 1989; Uflyand et al., 19901, as compared to the base composition. Constant values of the wear rate (Figure 4) at specific loads of 100-150 MPa support the notion of the "zero-wear" effect. Low friction coefficients of this composition in contrast to the previous one may be explained, in the main, by the cohesive break of the PTFE particles possessing low values of the intermolecular shear resistance in the system of binuclear copper complexwith acrylsalicylamideanchored on the PTFE surface. 3.3. Makeup and Properties of Composition with Framework Containing Heterocyclic Polyamide Fiber Complex. This composition radically differs from the preceding ones by the method of introduction of the complex compound. Through investigation of the tribotechnical properties of a composition based on PTFE fibers and phenolic-elastomer binder, into which a complex in the form of fibers of heterocyclic polyamide [meta or para isomers of poly[oxyphenylbenzoxazolylterephthalamides]] (Kolot et al., 1978)was introduced, it has
L
CulP-0
J"
'Cuod5
-cu-cu-cu
-
-Fc
I- l
Fc-
I
PTFE-g-PSAA
-
l Fc -
I
*Cuodr
+
Cu'
been found (Figure 5) that the character of the friction coefficient and the character of wear curves of this composition are similar to the relationship observed in the "zero-wear" regime for the pair comprising copper alloy and steel in the low-molecular-weight liquid medium. Its wear resistance is 2-2.2 times higher than that of the composition based on PTFE fibers, Vinol, copper, and 8-hydroxyquinoline (Pomogailo et al., 1986). Equal wear-rate values in the range of specific loads from 100 to 200 MPa testify in favor of the "zero-wear" effect. Yet, attention should be focused on the fact that the friction coefficients at specific loads of 10-100 MPa are high and amount to 0.2-0.1, which is due to large energy expenditures required to break the bonds within the polymer volume, i.e., to bring about its cohesive failure. It is well-known (Nikolis and Prigozhin, 1979; Baramboim, 1978) that part of the frictional interaction energy (the "activation" energy) characterized by the P V factor is expended on the initiation of the "zero-wear" regime i.e., on the progress of a complex of physicochemical processes bringing about a reduction in friction and formation of wear self-compensation systems. By way of example, a composition filled with complex in combination with ligand requires less friction interaction energy than a composition based on binuclear copper(I1) complex with acrylsalicylamide anchored on PTFE powder, whereas composition containing fiber-type complex involves the maximum friction interaction energy. Thus, selection of proper complex compounds for each PVloading condition may bring about a realization of the "zero-wear" effect since the "activation" energy is a function of the complex bond energy in the composition. Thus, the upgrading of wear resistance observed in all the above cases is associated with intensification of tribochemical processes in the zone of friction contact and is indicative of the adequate structure of the boundary layers, which indirectly confirms the occurrence of the "zerowear" effect in the composite systems. 4. Mechanism of Lubricating Action in Composites Filled with Complex Copper Compounds The results of investigation of transfer films on conjugated faces by present-day physicochemical methods made it possible to suggest the following model of the mechanism of lubricating action of coatings based on PTFE and complex compounds of bivalent copper (Figures 6 and 7). Figure 6 demonstrates a model of the boundary structure formation mechanism for a composition filled with low-molecular-weight complex in combination with
778 Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993
stage I
stage 2
Figure 7. Boundary layer formation scheme: 1,layer of complexes; 2,Servovite film; 3,steel; 4,polymeric layer filled with complex compounds; 5,layer of polymers.
fibers and modification of existing fibers to be used in manufacture of the reinforcement framework, or to the development of new compositions for fabrication of CACs matrices, it seems promising to use as a conceptual basis the effects of wearless, abnormally low friction, and hydrogen wear discovered by Soviet researchers. Much consideration should be given to the processes of selforganization involved in the frictional behavior of such CACs.
Literature Cited *2
Figure 8. System self-organization scheme.
ligand and for a composition for which the framework incorporates complex compounds in the form of heterocyclic polyamide fibers. Figure 7 illustrates such a model for a composition filled with PTFE powder with binuclear copper complex grafted on its surface. The first stage (at the start of the friction interaction process) involves adsorption and chemical fixation of copper complex compounds on the surface of steel, to all probability due to d-d interaction of the central atom of the complex with the Fe atoms of the surface of friction. The subsequent acts of friction interaction lead to a tribochemical reducing decomposition or a tribochemical exchange with formation on the conjugated surface of a Servovite film and an organic coat chemically bonded with it, which ensures low friction coefficient values and high wear resistance. The boundary structure formation mechanism is illustrated in Figure 6, and the boundary layer formation scheme is illustrated in Figure 7. Decomposition of the thus obtained structure results in increasing energy intensity in the defect area of the friction surface and in intensification of the processes aimed at reduction of this area, and the system self-organizes. The “zero-wear”regime sets up (Kovalev et al., 1987; Grechko and Ryadchenko, 1987)the scheme which is illustrated in Figure 8. 5. Conclusion
Use of the most up-to-date developments in tribology, for example the “zero-wear” effect, permits substantially widening of the fields of application of the PTFE fiberbased CACs. The latter fact permits the hope that other approaches to investigation of the processes of friction and wear will prove effective in terms of enhancement of quality of such coatings. In this connection, understanding that any further perfection of properties of the PTFE fiberbased CACs may be directed either to the search of new
(1) Baramboim, N. K. Mechanochemistry of high molecular weight compounds; Khimiya: Moscow, USSR, 1978. (2) Bely, V. A.; Sviridenok, A. I.; Petrakovets, M. I. Friction and wear of materials based on polymers; Nauka i Tekhnika: Minsk, USSR, 1976. (3).Chironis, N. Woven Teflon Bearings Run Dry for a Lifetime. Prod. Eng. 1970,41,136-138. (4) Evans, R. K. PTFE fibres add strength and flow resistance to resins. Des. Eng. 1970,June, 70-74. (5) Evdokimov, Yu. A.; Kolesnikov, V. I.; Tetrin, A. I. Planning and analysis of experiments for solution of problems of friction and wear; Nauka: Moscow, USSR, 1980. (6) Garkunov, D. N.; Polyakov, A. A. Increasing wearstability of aircraft construction components; Engineering Industry: Moscow, USSR, 1974. (7) Garkunov, D. N. Tribotechnics; Engineering Industry: Moscow, USSR, 1985. (8) Grechko, V. 0.;Danushina, G. A.; Kuzharov, A. S.; Vysochina, V. N. Antifriction composition. Authors Sert. USSR 971,670,1982. (9) Grechko, V. 0.Development of copper-containingantifriction coatings based on polytetrafluorethylene fibres. Dissertation, Novocherkassk, USSR, 1982. (10) Grechko, V. 0.; Kuzharov, A. S.; Vysochina, V. N.; Popov, Yu. I.; Stenko, L. V. Layer antifriction material. USSR 1,047,059, 1984. (11) Grechko, V. 0.; Ryadchenko, V. G. Effect of bivalent copper coordination compounds on tribotechnical properties of metallopolymeric systems. Abstracts of Papers, All Union Conference on chemistry of unaqueous solutions of inorganic and complex compounds; Rostov-on-Don, USSR; Nauka: Moscow, 1987;p 332. (12) Kolot, V. N.; Kudryavtaev, G. I.; Litovchenko, G. D.; Kravchenko, V. G.; Kvitko, I. Ya. Synthesis and properties of chelating heterocyclic polyamides. Polyamidobenzoxazoles. Vysokomol. Soedin. 1978,22,546-550. (13) Kovalev, V. N.; Grechko, V. 0.;Rydchenko, V. G. Mechanism of lubricating action, development and investigation of tribotechnical properties of materials based on polytetrafluorethylene fibres. Antifriction materials of special purpose; Novocherkassk, USSR, 1987;pp 110-117. (14) Kuzharov, A. S. Coordination tribochemistry of selective transfer. D.Sc. Thesis, Rostov-on-Don, 1991. (15) Kuzharov, A. S.;Rydchenko, V. G. Effect of copper complex compounds on tribotechnical properties of coatings based on PTFE fibres. Modernproblems of tribology. Abstracts ofPapers,AllUnion Conference, Nikolaev, USSR N K I Nikolaev, 1988;320-321. (16)Kuzharov, A. S.; Grechko, V. 0.; Ryadchenko, V. G.; Pomogailo, A. D.; Uflyand, I. E. Antifriction self-lubricating polymeric composition. Authors Sert. USSR 1,482,168,1989.
Ind. Eng. Chem. Res., Vol. 32,No.5, 1993 779 (17) Kuzharov, A. S.; Grechko, V. 0.; Ryadchenko, V. G.; Ponomarev,V. I.; Bobukh, I. A. Regulationoftribotechnicalproperties of multicomponent polymeric system by coordination compounds. Abstracts of Papers, All Union Conferenceon chemistry of unaqueous solutions of inorganic and complex compounds, Rostov-on-Don; Nauka: Moscow, 1985;p 314. (18) Kuzharov, A. S.;Grechko, V. 0.;Ryadchenko, V. G.; Shadrin, V. I.; Bobukh, I. A,; Gribinnik, V. G. Antifriction self-lubricated composition. Authors Sert. USSR 1,347,420,1985. (19) Kutkov, A. A.; Grechko, V. 0.;Kuzharov, A. S.; Suckhov, V. V.; Vlasenko, L. A. Study on mechanism of friction of copperfluoroplastic composite. Frict. Wear 1980,1123-1128. (20)Lager mit Teflon Gletschicht. Werkstatt Betr. 1969, 102, 224. (21)Nikolis, G.; Prigozhin, I. Self-organizationinunequilibrium systems; Mir: Moscow, USSR, 1979; p 512. (22) Onishuk, N.Yu.; Kuzharov, A. S.; Kutkov, A. A.; Popov, Yu. I.; Poltavets, Yu. G.; Stenko, D. A. Improvement of tribotechnical properties of metal-plaquer lubricants by complex-forming compounds. Frict. Wear 1981,2,625-629.
(23) Pomogailo, A. D.; Uflyand, I. E.; Golubeva, N. D. Study on immobilizedcatalysts. XVI. Investigation of structure features and catalytic properties of mono- and binuclear cobalt complexes. Kinet. CQtQl.1986,26,1104-1110. (24) Ryadchenko, V. G. Structure and properties of wearstable coating of heavy loading bearings baaed on polytetrafluorethylene fibres and copper complex compounds. Dissertation, Novocherkassk, USSR, 1988. (25)Tschacher, M.; Gubitz, F. Trockenauflageraufder Grundlage von der PTFE. Schweiz. Bauztg. 1969,87,408-412. (26) Turner,P. H.Filament wound bearing. U.S. Patent 3,802,756, 1974. (27) Uflyand, I. E.; Kokoreva, I. V.; Sheinker, V. N., Kuzharov, A. S. Antifriction self-lubricating coating on the base of fixed metal complexes. Surface Engineen'ngPlastics: FundamentaLs,Processes and Applications in Corrosion and Wear; Strafford, K. W . ,Datta, P. K., Gray, J. S., Eds.; Horwood: Sussex, Great Britain, 1990;pp 686-690.
Received for review August 24, 1992 Accepted January 11,1993
