ISSN 10683666, Journal of Friction and Wear, 2012, Vol. 33, No. 2, pp. 101–107. © Allerton Press, Inc., 2012. Original Russian Text © A.Ya. Grigoriev, G.V. Vaganov, V.E. Yudin, N.K. Myshkin, I.N. Kovaleva, I.V. Gofman, L.N. Mashlyakovskii, I.V. Tsarenko, 2012, published in Trenie i Iznos, 2012, Vol. 33, No. 2, pp. 133–140.
Friction and Wear of Powder Coatings of Epoxy Composites with Alumosilicate Nanoparticles A. Ya. Grigorieva, *, G. V. Vaganovb, V. E. Yudinc, N. K. Myshkina, I. N. Kovalevaa, I. V. Gofmanc, L. N. Mashlyakovskiib, and I. V. Tsarenkod aV.A.
Belyi MetalPolymer Research Institute, National Academy of Sciences of Belarus, ul. Kirova 32a, Gomel, 246050 Belarus *email:
[email protected] b St. Petersburg Technological Institute (Technical University), Moskovskii pr. 26, St. Petersburg, 190013 Russia cInstitute of Macromolecular Compounds, Russian Academy of Sciences, Bol’shoi proezd 31, St. Petersburg, 199004 Russia d Sukhoi Gomel State Technical University, pr. Oktyabrya 48, Gomel, 246746 Belarus Received December 13, 2011
Abstract—The paper studies the effect of alumosilicate nanofillers of tubular and lamellar shape on the fric tion and wear of epoxy composites. It is shown that the influence of concentration and shape of the fillers on the tribological behavior of the composites is due to variations in their viscoelastic properties and shielding of the contact area of the matrix material with the metallic counterbody by the filler particles. The data evidence that at equal concentrations of alumosilicate fillers in the epoxy matrix, the best tribological characteristics are provided in the case of tubularshaped particles. Keywords: coatings, epoxy composites, alumosilicate nanoparticles, wear, friction. DOI: 10.3103/S1068366612020043
INTRODUCTION Epoxy composite materials are employed as pro tective coatings in various industries owing to their high adhesion to metals, low permeability, and good electroisolating properties. In particular, powder epoxy paint materials are widely used in the oil and gas industry in view of their resistance to harsh environ ments and simple and efficient application technology [1]. Their application substantially prolongs the oper ating and interrepair time of pipelines by providing longterm protection against a large number of unfa vorable effects. Wear resistance and abrasive resistance are intrinsic characteristics of polymer coatings of pipes. Neverthe less, epoxy oligomers are rather susceptible to such surface damage as wear and scratching [2, 3]. Along with peeling, the wear of pipe coatings at the places of fittings and clutches, fastening facilities, and bandages and supports of pipelines, which occurs during mutual movement of these elements as a result of heat expan sion or vibrations, is the main cause of the loss of pro tective abilities. Irrespective of this, the tribological behavior of the epoxy coatings of pipelines remains, in our opinion, not fully understood. Introduction of nanofillers into epoxy coatings is one of the most promising methods to provide required functional characteristics. Natural alumosil icates (galluasite and montmorillonite) with tubular
and laminar particles, respectively, are widely used in epoxy composites. The introduction of up to 5% of nanoparticles into the epoxy matrix substantially modifies its thermal, mechanical, and barrier proper ties [4–6]. The coatings with these two fillers are close not only in physicomechanical characteristics but also in cost. In this connection, it is of interest to compare their tribological characteristics. In addition to the solution of this practical problem, it is of importance to determine the influence on friction and wear mech anisms of the fillers of the same kind (alumosilicates) but of different morphology (tubes and laminas). The aim of the work is to compare the tribological characteristics of the coatings made of powder epoxy composites with alumosilicate nanoparticles of differ ent shapes and establish the influence of the morphol ogy of nanofillers on the mechanisms of coating fric tion and wear. MATERIALS AND METHODS Materials We studied polymer composites based on epoxy resin with alumosilicate particles. Solid epoxy resin Epi cote 1004 with a molecular weight of 1480 and an epoxy group content of 110–1240 mmol/kg (Resolution Per formance Products, USA) were used as the matrix.
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(b)
R
R–h a
h
I e 1
2
Fig. 1. Evaluation of coating wear: (a) interference image of friction tracks; (b) diagram of determination of the depth of inden tation h of counterbody into the sample during friction and values of elastic recovery e; 1⎯track profile under loading; 2 ⎯track profile after removing load.
Modified diacynamide (trade mark Casamid 780, Tho mas Swan, Great Britain) served as a hardener. To reg ulate the pouring, BYK 366 or polyacrylate deposited on silica powder (Byk Chemie, Germany) was used. Benzoin (Caffaro, Italy) served as the degasifying agent. Quaternary ammonium saltmodified montmoril lonite Cloisite (MMP15A, Southern Clay Products. Inc., USA) was used as the filler in the form of lami nated nanoparticles (cross dimension 200 nm, the thickness of individual lamina 1 nm). Galluasite (Nat ural Nano Inc., USA) served as the tubeshaped filler (with a diameter of individual nanotubes of 70 nm and a length of 1–3 µm). The nanocomposites were produced by mixing in a melt of epoxy oligomers with a DSM Xplore double screw microextruder (the Netherlands). Mixing was carried out at a rotating speed of the screws of 200 rpm and an extrusion temperature of 100°C for 5 min. The nanocomposites obtained were mixed with all other additives of the lacquering powder composite (hardener, pouring regulator, Benzoin) in the melt and after hardening it was ground and sifted through a sieve with cells 125 µm in size. The powder composite obtained was applied by the electrostatic method on a steel plate or aluminum foil and hardened at 180°C. The thickness of the coatings ranged from 80 to 100 µm. The samples on steel plates were used in the friction tests. The films were used in the mechanical tests. Experimental Methods The mechanical characteristics of the films were determined on a universal Instron1122 tensiletesting machine (IEC, USA). The samples were deformed at an arm velocity of 50 mm/min. In testing, the modulus of elasticity, strength, and breaking deformation were measured. The tribotest results were obtained on a
MTU2K7 reciprocal tribometer (MPRI, NAS of Belarus, Gomel) under normal conditions (GOST 8.395–80) without lubrication. A ShKh15 steel ball (4.7 mm in diameter) was used as the counterbody. The tests were carried out under loads 100, 250, 500, 750, and 1000 mN at a relative sliding velocity of 2.5 mm/s and a friction path of 7.5 m. The coefficient of friction f was found by the regression parameters of the linear model of F = fN kind. The linear wear I of the polymer samples was mea sured on a MII4M Linnik interferometer (LOMO, Russia). An interference picture was obtained under lighting with white light (Fig. 1a). The curvature of interference fringes was measured with an Optizer MII software and hardware system (MPRI, NAS of Belarus, Gomel). The wear was taken as the greatest depth of the friction track profile in cross section in the middle of its length. The indentation h of the spherical indenter into the sample during friction was found by the halfwidth of the track a as h = R − R − a (Fig. 1b). The elastic recovery of the material on the friction track e was evaluated according to the diagram presented in Fig. 1b by the difference between h and I. The morphology of the films and friction tracks was studied by scanning electron microscopy on Supra 55 VP (Carl Zeiss, Germany) and VEGAII LSH (Tescan, Czech Republic). The features of the coating structure were studied by crosswise chipping in liquid nitrogen. 2
2
RESULTS AND DISCUSSION The mechanical characteristics of the studied coat ings in the form of films taken from the aluminum foil are shown in the Table. As seen, only the tensile strength and relative elongation demonstrate a statisti cally significant dependence on the filler concentra tion, the greatest strength being achieved at minimal concentrations of both types of the fillers.
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Mechanical properties of epoxy coatings with different fillers Particles
no MMT15A
Galluasite
Concentration c, %
Modulus of elasticity E, HPa
Strength σt, MPa
Deformation to fracture εfr, %
–
1.98 ± 0.17
56 ± 2
5.6 ± 0.4
0.5
1.92 ± 0.15
64 ± 3
5.9 ± 0.4
1
2.10 ± 0.15
55 ± 3
5.6 ± 0.4
3
2.15 ± 0.13
47 ± 3
4.6 ± 0.2
1
1.97 ± 0.12
60 ± 2
5.9 ± 0.5
3
1.99 ± 0.15
51 ± 3
6.5 ± 0.3
5
2.00 ± 0.04
54 ± 4
6.4 ± 0.3
A feature of the friction behavior of polymer mate rials is that in addition to mechanical properties, the tribological characteristics strongly depend on the peculiarities of viscoelastic deformation and stress relaxation, which can be judged by the morphology of chip surfaces. Analysis of the fracture surfaces is most convenient to perform in terms of phenomenological models of fracture of polymer composites based on the features of the structure of crack propagation fronts, the differ ence in the levels of fracture surfaces, and their fine structure [7]. The subsurface area of a chip of unfilled epoxy resin (Fig. 2a) has few fronts of crack propagation and insignificant difference in their surface levels. With fillers, the fracture surface becomes more developed; i.e., the number and length of the crack front lines and the difference in the levels of the crack surfaces increase (Fig. 2b, 2c). Moreover, the correla tion between the maturity of the subsurface chip zone and the filler and its concentration in the composite becomes evident. Also, the ratio of the areas of the characteristic zones varies, which indicates different fracture mechanisms of the materials and kinetics of exposure of the fracture surfaces. The formation of a comb structure is commonly related to a relatively slow propagation of plastic deformations, whereas the socalled mirror zone occurs in the case of the quick development of cracks that are characteristic of brittle fracture. Analysis of the fine structure of the chips substantiates the fact that the introduction of fillers changes the fracture mechanisms of the composites. Thus, the facets on the surfaces of crack development in unfilled resin (Fig. 2d) are of elliptical shape. The small depth and underdevelopment of plastic nuclei at the points of ellipse focus indicate that the rate of propagation of the initial front of fracture is higher than the rate of surface fracture at local areas [7, 8]; i.e., the destruc tion of the unfilled material generally occurs by brittle mechanism. JOURNAL OF FRICTION AND WEAR
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An increase in the MMT15A concentration in the composite by up to 1% is accompanied by decreasing facet dimension, increasing quantity, and more pro nounced manifestation of nuclei of plastic fracture (Fig. 2e). The addition of galluasite results in a total degeneration of the facet structure even at a minimal concentration (Fig. 2f). The observed distinctions can be attributed, in our opinion, to the morphology of the fillers. As seen from Fig. 2g, the fragments of the fine chip structure are limited by flaky MMT15A parti cles, which are apparently more effective in preventing the formation of chip surfaces, diminishing the size of secondary structures (Fig. 2h) and slowing down crack development. The galluasite nanotubes are not funda mentally related to the features of the fine chip struc ture (Fig. 2f); however, the results of the mechanical tests indicate that at a concentration of 1%, the filling yields a statistically meaningful effect on hardness improvement (the Table). The effect is most probably due to the reinforcing ability of the fibers. An increase in the filling up to maximum results in increasing defectiveness of the composite structure. This is due to microcavities whose tops obviously act as stress con centrators, facilitating the exposure of surface cracks and their propagation in depth, thus decreasing the composite strength (Fig. 2i). Therefore, introduction of the filler increases the share of the viscosity component in the viscoelastic deformation of the composites, which leads to a decreasing rate of propagation of the fracture plane. The extreme mode of variation of the mechanical properties with varying concentration is most probably due, on the one hand, to the competitiveness of mate rial reinforcement and inhibition of friction develop ment and, on the other, to the growth of structure defectiveness because of micropores. The tribological properties of unfilled epoxy resins are low and characterized by a significant dependence of the coefficient of friction on load. The data shown in Fig. 3a confirm these regularities for Epicote 1004 resin used as the matrix for the studied composites. 2012
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(a)
10 µm (b)
10 µm (c)
10 µm
(d)
1 µm (e)
1 µm (f)
500 nm
(g)
500 nm (h)
2 µm (i)
2 µm
Fig. 2. SEM images of chip surfaces of epoxy coatings: (a, d) unfilled epoxy resin; (b, e, g, h) filling with 0.5% MMT15A; (i) 3% MMT15A; (c, f) filling with 1% galluasite.
F, mN
(a) 1000 mN
f 0.7
(b)
600 0.6
500
750 mN 0.5
400 500 mN
300
0.4
200 250 mN 100 mN
100 0
1.5
3.0
4.5
6.0
7.5 S, m
0.3 0.2 0
200
400
600
800
N, mN
Fig. 3. Friction force (a) and coefficient of friction (b) of unfilled epoxy resin vs. load. JOURNAL OF FRICTION AND WEAR
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(a)
80 µm (b)
80 µm (c)
105
80 µm
Fig. 4. Optical images of the friction track morphology: (a, b) unfilled epoxy resin after testing under 250 and 1000 mN, respec tively; (c) composite with 1% MMT15A after testing under 1000 mN.
The dependence shown in Fig. 3b is characterized by the presence of two load ranges (100–300 and 500– 1000 mN) over which the coefficient of friction has stable values, although they differ by a factor of two. Such behavior commonly indicates a substantial vari ation in the friction mechanisms, which is confirmed by analysis of the morphology of the wear scars. Comparison of the tribosurface images of unfilled resin shows that under loads >500 mN a net of charac teristic crosswise cracks arises on the tribosurfaces (Fig. 4a, 4b). Such fracture of the polymer surface layer is commonly associated with shear deformations at adhesive seizure of friction pair materials. Since marks of plastic shear are absent on the wear scars, the intensification of adhesive interaction of the unfilled resin with the metallic counterbody is the most proba ble cause of the correlation between the coefficients of friction and load. The introduction of fillers substantially modifies the tribological behavior of materials. Crosswise cracks are not seen even under great loads (Fig. 4c) and the coefficient of friction and wear decline with increasing filler concentration (Fig. 5a, 5b). The distinctions in the tribological properties of the composites depending on the filler can be attributed to the differences in their viscoelastic behavior under loading. This manifests itself in different rates of deformation propagation, which is revealed, in partic ular, via analysis of the fracture surface morphology (Fig. 2e, 2f). A decline in the rate of deformation propagation increases the hysteresis of the elasticity forces, which, in terms of the adhesivedeformation theory of friction [9], leads to energy losses during fric tion contact. Therefore, the observed signs of lower rate of deformation propagation in the MMT15Afilled composites, other conditions being the same, encour age an increase in the sliding resistance forces. Con sidering that the fillers used in the composites differ only in morphology, we may conclude that a tubular shape of the filler (galluasite) is preferred over a flaky JOURNAL OF FRICTION AND WEAR
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one (montmorillonite) for the improvement of the tri bological behavior of epoxy coatings. Analysis of the influence of the filler concentration on the friction of the composites in question should consider two factors. The first one is related to the variations in the mechanical properties of the materi als and the second one, with regard for the effect of adhesion on the friction of unfilled resin, to the shield ing by the filler particles of the contact area of the matrix polymer and metallic counterbody. The symbate mode of the dependence of the tensile strength and the coefficient of friction on the degree of composite filling supports the influence of the mechanical properties (Table). In line with the exist ing theoretical ideas, the coefficient of friction decreases with increasing load at elastic contact [9]. Analysis of elastic recovery of the material in the region of frictional contact (Fig. 5d) indicates an increase in the elastic component of contact interac tion with increasing concentration of nanofillers. Therefore, the introduction of nanofillers encourages the occurrence of elastic deformations over a wide range of loads, which leads to the observed decrease in the coefficient of friction with increasing filler con centration. The effect of shielding of the polymer matrix mate rial with nanofiller particles has been reported many times in the literature. In particular, in work [10], dis persion and increasing nanofiller concentration on the friction surface accompanied by decreasing roughness were observed. The authors of [11] found an increase in the microhardness of the friction surfaces of the composites with silicate particles. The fillers of galluasite and montmorillonite, hav ing similar densities (≈2.5 g/cm3), occupy approxi mately equal volumes. It is possible to show that in this case, they must have similar shielding effects regard less of the particle shape since in any arbitrary section of the sample they provide equal ratios of matrix and filler areas. Considering that the used fillers also have similar hardness (1–2 units on the Mohs hardness 2012
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GRIGORIEV et al. I, µm
(a)
f
MMT15A galluasite
0.75
(b) Epicote 1004 0.5% galluasite 1.0% galluasite 3.0% galluasite
1.2 1.0
0.70 0.8 0.65 0.6 0.60 –0.4 0.55 –0.2 0.50 0 I, µm 1.2 1.0
1
2
Epicote 1004 + 1% galluasite + 3% galluasite + 5% galluasite
3
4 c, wt %
0 e, µm
(c)
0.4
0.6
0.2
400
600
800 N, mN
0.5% MMT15A (d) 1.0% MMT15A 3.0% MMT15A
0.6
0.8
200
0.4 0
0.2
–0.2 0
200
400
600
800 N, mN
0
200
400
600
800 N, mN
Fig. 5. Tribological characteristics of epoxy composites with alumosilicate nanoparticles of different kinds: (a) coefficient of fric tion vs. nanoparticle concentration; (b, c) load dependences of wear; (d) load dependence of elastic recovery of wear scars.
scale), their effects on the tribological behavior of the epoxy resin must obviously be similar at equal concen trations. This is supported by the approximately equal (a difference of 15–25%) wear of the epoxy compos ites at equal concentrations of galluasite and montmo rillonite. A decrease in the adhesive interaction in the fric tion pair is the most probable mechanism of the effect of the fillers’ shielding on wear. The coefficient of fric tion is apparently subjected to the effect of the differ ence in the mechanical properties of the composites. CONCLUSIONS The influence of the concentration and morphol ogy of alumosilicate nanofillers on the friction and wear of powder epoxy composites is related both to variations in the viscoelastic properties of the epoxy composites and shielding by filler particles of the con tact area of the matrix material with the metallic coun terbody. The different coefficients of friction of the compos ites with equal concentrations of the nanofillers can be due to differences in their morphology. The laminar montmorillonite particles more efficiently inhibit deformation propagation in the composite during fric
tion, thereby increasing the hysteresis of the elasticity forces, which results in greater sliding resistance forces compared to galluasitecontaining composites. The reduction in the coefficient of friction with increasing nanofiller concentrations is attributed to stronger effects of matrix material shielding and, hence, decreasing adhesive component of the friction force. Moreover, filling encourages the occurrence of elastic deformations over a larger load range, which, according to theoretical findings, also leads to decreasing friction forces. Therefore, the best tribological characteristics are achieved via introducing tubular alumosilicate parti cles at equal concentrations of the fillers. The work was supported by the Belarusian and Russian Foundations for Basic Research BRFBR no. T10P051 and RFBR no. 00390005Bel). DESIGNATIONS a⎯halfwidth of friction track; c⎯filler concen tration; e⎯elastic recovery of friction trace; F⎯fric tion force; f⎯coefficient of friction; I⎯linear wear; h⎯depth of counterbody indentation in the sample during friction; N⎯normal load to sample.
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