Effect of ultrafine powders in lubricants on performance of friction pairs

Effect of ultrafine powders in lubricants on performance of friction pairs

Wear 254 (2003) 645–651 Effect of ultrafine powders in lubricants on performance of friction pairs I.V. Frishberg, N.V. Kishkoparov, L.V. Zolotukhina...

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Wear 254 (2003) 645–651

Effect of ultrafine powders in lubricants on performance of friction pairs I.V. Frishberg, N.V. Kishkoparov, L.V. Zolotukhina∗ , V.V. Kharlamov1 , O.K. Baturina, S.V. Zhidovinova Fine Metal Powers Research & Production GSC, 101 Amundsena Street, 620216 Ekaterinburg, Russia Received 13 May 2002; accepted 20 February 2003

Abstract The effect of ultrafine powders (UFPs) of metals and metal alloys in lubricants on surface properties of steel-to-steel friction pairs was analyzed. It was found that the UFP operation mechanism depended on the origin of metals and their chemical affinity for the material of rubbing surfaces. It was shown that copper alloys had a special significance since they could undergo phase transformations with precipitation of active copper under a load, resulting in appearance of secondary ultrafine composite structures on rubbing surfaces and a dynamic equilibrium between wear and wear-compensation processes. The copper concentration in those structures was determined by the chemical composition of the ultrafine alloys and stability of their crystal lattices. It was proposed to interpret the wear-compensation process by analogy with the principle of an oriented growth of layers. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Friction pair; Wear; Ultrafine powders

1. Introduction Powdered materials based on copper and its alloys find a progressively increasing application as lubricant additives for reduction and compensation of wear of conjugate surfaces [1,2]. Our research team has long been concerned with the development of special ultrafine powders (UFPs) based on metals and alloys. Addition of these modern materials to lubricants improves tribological characteristics of conjugate surfaces [3–5]. Their operation mechanism depends on many factors, the most important of which are the rubbing conditions, type of lubricant, UFP properties, and UFP ability to interact with the rubbing surface [6,7]. The UFPs developed by our team are subdivided into two groups. The first group includes UFPs of metals capable of reacting with the material of the rubbing surface, for example, iron. The second group comprises metastable alloys of metals with a strong interparticle interaction, which are more inert to the rubbing surface material. A typical example of materials of the first group is zinc UFP, which has a relatively low formation energy for some stable intermetal∗ Corresponding author. Tel./fax: +7-3432-288-141. E-mail address: [email protected] (L.V. Zolotukhina). 1 Present address: Institute of Engineering Science, Russian Academy of Science, Urals Branch, 91 Pervomaiskaya Street, 620219 Ekaterinburg, Russia.

lic compounds [8]. The second group comprises UFPs of copper-based alloys, which do not practically interact with iron under equilibrium conditions, but may decompose and release active copper on exposure to thermomechanical effects. Although the mechanism by which copper is released from the alloys and interacts with the rubbing surface is unclear, one may think that the quantity of copper deposited on the rubbing surface depends on the copper alloying technique and the alloy composition. This study deals with the effect of the composition, properties and particular features of the UFPs on conjugate surfaces.

2. Experimental technique The powders used in metal-filled lubricants were prepared by the method of gas-phase metallurgy [9]. The powder particles have a spherical shape. The zinc UFP have the average diameter about 8 ␮m. The UFPs of copper-based alloys have the range of sizes from 0.01 to 3 ␮m, with average diameter about 0.1–0.4 ␮m. The tribological tests were performed on a CMT-1 friction testing machine using steel–steel model pairs by a standard “shaft–bush” procedure. The wear was measured with a profile recorder. The lubrication compounds (LCs) were prepared by introducing (in accordance with UFP properties)

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3. Results and discussion

Table 1 Variants of test LCs LC

Parent lubricant

UFP

I [7] II [7] III∗ [3] IV∗ [3] V VI

A A B B B B

Absent Zn Absent Cu–Sn Cu–Sn Cu–Sn–Sb

zinc into plastic lithium grease (A) (the content is 10 mass%) and copper alloys into mineral oil (B) (the content is 2.5 g/l) (Table 1). The powder particles of copper-based alloys are covered with special surfactants, which prevent their precipitation from an oil suspension. The material of the shafts during the tests in LCs V and VI was steel SAE 1044 quenched to HRC 32. The material of the bushes was steel SAE 5015 having HRC 56. In the other cases, the shafts and the bushes were made from steel SAE 1040 (HRC 15) and steel SAE 3316 (HRC 37), respectively. A groove 0.05 mm deep was cut along the shaft circumference at an equal distance from the end faces. Liquid LCs were tested in a leak-tight chamber. Compounds with maximum microstress of the crystal lattice were selected as additives to the mineral oil. The microstress was determined from widening of the lines in X-ray diffraction patterns. The reference was a specially treated sample of bulk copper, which was purified by chemical pickling and vacuum-annealed to remove stresses. The X-ray examination of the test samples and the reference sample was performed using an automated device diffractometer in Cu K␣ radiation under the θ–2θ mode. The angle 2θ was between 40◦ and 150◦ . The X-ray diffraction patterns were processed by the Reschingher method to render the spectrum monochromatic (conversion to K␣1 radiation). The experimental profiles of all the reflections measured in the interval of 2θ angles at hand were analyzed using the Warren–Auerbach harmonic analysis method [10,11]. The experimental width of the reflections was determined by approximating the line shape with the Lorentz function. The true width of reflection (with a correction for the instrumental width) was estimated from nomographic charts using the data due to Vasiliev [11]. The post-test structure of the rubbing surfaces was examined using X-ray phase analysis (XPA), metallographic and electron probe microanalysis (EPMA) methods.

3.1. Zinc UFP The results obtained on the friction testing machine are given in Table 2. The microphotographs in Fig. 1 show the surface of the shafts. The wear after operation in plastic lithium grease was represented by deep marks formed along the whole perimeter of the shaft. No such marks were seen after operation in LC with zinc UFP, while traces of wear were short and shallow. A comparison of the structures of cross microsections of the shafts tested in LCs I and II showed that the grain deformation zone was much larger in the latter case (Fig. 2). The grain deformation zones had different microhardness values. The measured thickness of the grain deformation zone at the base (ferrite–pearlite) interface and the microhardness values of the shaft surfaces (H20 , kg/mm2 ) are given in Table 3. The EPMA data for a shaft cross microsection after operation in the zinc UFP LC showed that the alloying elements were distributed uniformly over the depth of the shaft and corresponded to their standard concentration in the steel SAE 1040. This fact testified to the absence of a reverse transfer, which is typical of like friction pairs. The XPA revealed the presence of zinc on the shaft surface (Fig. 3). Zinc lines (1 0 1) with d = 0.210 nm and (1 0 2) with d = 0.689 nm, and also the ZnO line (1 1 0) with d = 0.623 nm are resolved in the X-ray diffraction patterns in addition to intensive reflections of ␣-Fe. An average thickness of the zinc alloy film was estimated by the X-ray fluorescence method. It was equal to ∼0.04 ␮m. So, the zinc UFP added to the lithium grease formed a thin continuous layer on rubbing surfaces. The depth of the grain deformation zone on the surface of conjugate parts decreased nearly by one order of magnitude. As a result, the wear of the friction pair was reduced more than four times. The scoring load increased nearly by a factor of 6, while the lubricant endurance temperature was 40◦ higher. A result obtained during service tests of this material was also considered to be significant: the zinc layer formed on the surface of rubbing joints protected the steel from corrosion. 3.2. UFPs based on copper alloys Rubbing surfaces had absolutely different structure after they interacted with UFPs of copper alloys. UFPs of a

Table 2 Performance of friction pair in plastic lithium grease and LC with zinc UFP Lubricant

Operation time, τ (min)

Friction length, l (m)

Load, P (N)

Temperature, T (◦ C)

Friction coefficient, η

Wear, ∆ (rel. units)

Plastic lithium grease

60 90

2000 3000

0 500

40 80

– 0.03

– 4.2

LC with zinc UFP

60 90

2000 3000

0 500

40 40

– 0.015

– 1

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Fig. 1. Microphotographs of shaft surfaces after tests in plastic lithium grease (a) and LC containing zinc UFP (b).

binary Cu–Sn alloy were chosen as materials with strong interparticle interaction. A study of the interaction between copper and tin UFPs at a stepwise increase of the temperature showed [12] that phase transformations in the UFP mixture did not correspond to the phase equilibrium diagram, but involved formation of metastable phases from those most enriched to those least enriched with low-melting tin (Oknov’s rule [13]). To provide for gas-phase deposition of Cu and Sn vapors, we took into account specific features of this system and designed UFPs with particles containing 2–20% tin. By this means, the powder particles included supersaturated ␣solid solution , which released active copper under a thermomechanical effect. One more method of developing a metastable alloy is to obtain UFPs whose crystal lattice has a higher stored energy and, correspondingly, is less stable, releasing active copper more readily under a thermomechanical effect. On the assumption that this situation is possible in a ternary Cu–Sn–Sb alloy, we measured microstresses of crystal lattices in four samples whose compositions are given in Table 4. The X-ray diffraction patterns of those samples and a reference sample, which were obtained for angles of

the reflection from the (1 1 1) plane, are given in Fig. 4. The true widening of the reflections is given in Table 5. As can be seen from Fig. 4, the shape of the line obtained for sample 1 with small concentrations of tin and antimony is similar to that obtained for the reference sample. Powdered Cu–Sn and Cu–Sn–Sb alloys represent solid solutions: copper lines shift towards small angles. This corresponds

Table 3 Metallographic analysis results δ (␮m)

LC I LC II

H20 (kg/mm2 )

Maximum

Minimum

70 20

10 –

Mean 50 <5

402–526 458–659

Fig. 2. Microphotographs (250×) of shaft microsections after operation in plastic lithium grease (a) and LC containing zinc UFP (b).

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Fig. 3. X-ray diffraction patterns of the shaft surfaces after tests in LC containing zinc UFP.

Table 4 Composition of test samples (at.%) Compound

Cu

Sb

Sn

1 2 3 4

>99 95.600 95.000 92.290

0.017 – – 2.730

0.029 4.400 5.000 4.980

Fig. 4. Reflection (1 1 1) obtained for test samples (Table 1) and reference sample.

Table 5 Widening of reflections, β, as corrected for instrumental width Reflection Plane index (h k l)

Compound 1

2

3

4

1 2 3 4 5 6 7 8

0.0096(6) 0.0010 0.0010 0.0154 – – 0.0010 0.0010

0.1556(6) 0.1653 0.2837 0.3816 – – 0.2581 –

0.1391(5) 0.1439 0.1679 0.3063 – – 0.6142 1.1616

0.4050(8) 0.7327 1.3372 – – – – –

(1 1 1) (2 0 0) (2 2 0) (3 1 1) (2 2 2) (4 0 0) (3 1 3) (4 2 0)

Compound

d/d (%)

2 3 4

9.6 7.7 44.2

to an increase in the interplanar spacing when copper is replaced by a metal with a larger ionic radius (atomic radii of copper, tin and antimony are 0.128, 0.161, and 0.158 nm, respectively). Satellite lines, which are pronounced well for samples 2 and 3, are seen on the side of small angles. This observation suggests that the solid solution of tin in copper has an inhomogeneous composition: the tin concentration of fine particles is higher (5–12 mass% Sn) than that of coarse particles (1–1.5% Sn), which are responsible for the intensive reflection. Very wide and relatively symmetric lines were obtained for sample 4. This is an indication that addition of antimony to the same concentration of tin as in samples 2 and 3 causes a more homogeneous distribution of tin and antimony in the copper base. Consequently, fine and coarse particles acquire a more uniform chemical composition. Assuming that widening of β 1 and β i lines (i being the reflection number) (Table 5) at small and large angles, respectively, was caused mainly by microstresses, distortions of the crystal lattice in samples 2, 3, and 4 were estimated from the formula d/d = β/(4 tg θ), where d denotes the interplanar spacing (Table 6). Thus, 4.5–5% of tin in the Cu–Sn alloy led to 8–10% microdistortions of the structure. As was expected, addition of less than 3% Sn caused much greater distortions (up to 40%) of the structure. Compounds 2 and 4 with maximum microstress of the alloy crystal lattice were chosen as addition to lubricants for tests in the friction machine. These compounds correspond to LCs IV, V, and VI in Table 1. A post-test inspection of the shaft surfaces operating in LCs IV, V, and VI showed that the surface were lapped, free of cracks, and had no traces of peeling or pitting. The conjugate surfaces had numerous traces of metal buildup, islet inclusions of copper, and Cu3 Sn, Cu4 Sn and Cu3 Sb intermetallics. The quantity of copper was almost twice as large after operation in LC VI. Microstructure examination for the shaft surface layers showed that the initial structure transforms during rubbing into characteristic secondary structures, namely, a white martensite-like layer down to 20 ␮m deep and an underlying dark-gray pickled layer down to 40 ␮m deep, which is adjacent to the steel base (Fig. 5). Thickness of the newly formed layers is limited by the gap between the shaft and the bush. It does not exceed the gap, but determines the clearance established in the rubbing joint. Both layers have uniform ultrafine structure. The white layer on the shaft operating in LC VI is thinner, while the gray layer is pickled non-uniformly. The newly formed secondary structures fill

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Fig. 5. Microphotographs (2000×) of cross microsections of shafts after tests in LC IV (a) and LC VI (b): (1) white martensite-like layer; (2) gray layer.

Table 7 Microhardness of newly formed layers LC

Steel base

Gray layer

White layer

IV V VI

356 412 412

850 630 530

1270 946 795

microdefects, in particular, a mark on the shaft surface. The microhardness values measured on the shaft cross-sections are given in Table 7. The texture of the newly formed layers is improved in the presence of copper alloys. This conclusion is confirmed by a change in the intensity (J) of [1 1 0] and [2 0 0] diffraction lines of ␣-Fe. The ratio J1 1 0 /J2 0 0 = 5.3 for the parent oil and 20 for LC IV. The surface layers of the shafts and the bushes have similar chemical compositions after operation in LCs IV, V, and VI (Table 8). This evidence suggests intensive mass transfer of the material of the friction pairs from one surface to the other. Fig. 6 presents the distribution of copper and alloying elements of the bush over the depth of the shaft surface layer operating in LC IV. The wear curves of the samples also point to the presence of a mass transfer between contacting bodies. These curves are distinguished for a characteristic cycling (Fig. 7), especially in the case of unquenched samples. Negative ∆ values correspond to increase in the size of the samples caused by the mass transfer. Generally, the shafts were worn at a much Table 8 Chemical composition of surface layers (δ < 2 ␮m) of friction pair after operation in LC IV Element

Fe Ni Cr Cu Sn

Concentration (%) Shaft, steel SAE 1040

Bush, steel SAE 3316

93.00 3.20 0.80 0.80 0.01

93.20 2.70 0.70 0.90 0.01

Fig. 6. Distribution of copper and alloying elements of bush over the depth of shaft surface layer after operation in LC IV: (1) Ni; (2) Cu; (3) Cr.

smaller rate. The amount of wear did not exceed thickness of the white layer.

4. Considerations on the operation mechanism of UFPs of metals and alloys It is expedient to add UFPs of metals, which have a chemical affinity for a steel friction surface (Zn), to plastic greases. If the load is small and the temperature is low, they form a continuous antifriction anticorrosion layer on conjugate surfaces. UFPs of copper alloys in the mineral oil reduce and compensate the wear of rubbing joints at high loads and elevated temperatures. Thinking to different chemical potentials and high surface energy, UFPs of copper alloys are readily deposited on steel surfaces, which are accessible in energy terms. If the lubricant contains a binary alloy, part of the ␣solid solution transforms to the Cu4 Sn intermetallic in the rubbing zone on the surface. This process does not require much energy for rearranging the phase transformations, because the ␣solid solution , which has a cubic unit cell, transforms to a cubic lattice of Cu4 Sn. Longtime tribochemical processes

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Fig. 7. Wear curves for shafts after operation in LC III (1), LC IV (2), LC V (3), and LC VI (4).

cause distortions of the cubic lattice of Cu4 Sn, which transforms to a more stable Cu3 Sn intermetallic having an orthorhombic crystal lattice. Both phase transformations are followed by precipitation of active copper. So, when the lubricant contains UFP of a binary alloy, the formation sequence of phases at the rubbing joint may be presented as follows: ␣solid solution → Cu4 Sn + Cu → Cu3 Sn + Cu. Since diffusion coefficients of the intermetallics are much larger than those of the solid solutions [8], the intermetallics decompose faster and copper is released and deposited on rubbing surfaces more readily. The microstructure of surface layers of conjugate parts is saturated with copper and, consequently, steel particles are transferred from one surface to the other at a faster rate and are retained in the rubbing zone. The surface layers of the steel base acquire an ultrafine state and get strengthened. Copper and alloying elements of the bush penetrate throughout the dispersed layer under the action of a large load. Copper serves as a plastic binder of steel particles. Thus, copper participates in the formation of secondary ultrafine composite structures, which possess a high hardness and good plastic properties. Similar processes take place in friction pairs working in a lubricant with a ternary Cu–Sn–Sb alloy. In this case, phase transformations in the ternary solid solution lead to appearance of two intermetallics by the scheme ␣solid solution → Cu3 Sn + Cu3 Sb + Cu. The crystal lattice of the ternary alloy experiences high stresses and, therefore, the alloy decomposes faster. The presence of two intermetallics and a high mobility of antimony atoms, whose diffusion activation energy is very small [14], cause an increase in the concentration of free copper on the surface of test samples. As a result, plasticity of surface layers of the samples, which worked in the lubricant

with a ternary alloy, is improved and the wear of the friction pair is less. Thus, formation of wear-resistant composite structures on rubbing surfaces is determined by the amount of copper released from the alloy, which, in turn, depends on stability of the alloy crystal lattice. Ultrafine active copper has a special significance for this process, because it favors retention of a large quantity of steel wear particles in the contact zone and formation of secondary ultrafine composite structures. These specific structures have a homogeneous distribution of ultrafine copper in steel and a characteristic (1 1 0) texture. In our opinion, the mechanism of those processes is determined by the long-range effect of copper, which is well known in the theory and practice of crystallization [15]. The essence of this long-range effect is that specific active centers on the surface can retain and orient particles of a crystallizing material at a macrodistance. If these considerations are used in description of processes, which take place during friction with ultrafine copper, then ultrafine active copper inclusions may be viewed as centers of long-range action responsible for appearance of secondary ultrafine composite structures. These centers have an orientation effect on the structure of newly formed layers. In particular, they improve the orientation of steel grains in the (1 1 0) plane. Wear particles, which are formed after a local failure of the surface white layer, are retained and oriented in the contact zone thanks to the same centers. Therefore, wear particles do not leave the friction zone entirely and participate in restoration of failed fragments, reproducing previous structures. Wear and growth of the layer are repeated in characteristic cycles. The information obtained during the research work allows composing new ultrafine materials with optimal tribological properties, which are determined by the chemical composition of alloys and stability of their crystal lattice. For example, our team developed the “Vympel” plastic grease on the basis of a zinc UFP and a family of “Rimet” compounds based on UFPs of copper alloys. Specialists in

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machinery and equipment have recognized these materials. A long-term use of these lubricants showed that they reduce and compensate in-service wear of rubbing joints [16,17].

5. Conclusions 1. Addition of metal and alloy UFPs to lubricants provides a considerable reduction and compensation of wear of rubbing joints. It was found that the effect of the UFPs is determined by the origin of metals and their chemical affinity for the material of rubbing surfaces. 2. It was demonstrated that copper alloys possessing a preset degree of equilibrium and capable of phase transformations with precipitation of active copper under load have a special significance. The quantity of copper released from the alloys depends on stability of the alloy crystal lattice. 3. If copper alloy UFPs are added to the oil, wear and recovery of friction surfaces are found to be in dynamic equilibrium. This relates to formation of secondary ultrafine composite structures on the friction surfaces. The wear-compensation process is suggested to be considered by analogy with the principle of an oriented growth of layers. 4. The obtained results open up new perspectives both in materials science (development of UFPs with a preset composition, structure and properties) and tribology (in-service deposition of wear-resistant coatings on rubbing joints). References [1] D.N. Garkunov, Tribotechnics, M: Mashinostroenye, 1989, 327 pp. [2] S.A. Belyaev, S.Y. Tarasov, A.V. Kolubaev, S.A. Laryonov, Friction and wear in the presence of lubricant with copper-based FMP additive, Friction, Wear, Lubric. 3 (2000) 2. http://www.tribo.ru.

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