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TRIBOLOGICAL CHARACTERISTICS AND FORMATION OF SUBSURFACE STRUCTURES OF CAST FE-CR-C ALLOYS WITH

DIFFERENT CONTENT OF COPPER, UNDER SLIDING FRICTION

Viktor NOVYTSKYY, Volodymyr LAKHNENKO

Physico-technological Institute of Metals and Alloys of National Academy of Science of Ukraine, UKRAINE v_novytskyy@ukr.net

ABSTRACT Effect of copper contents (0%, 0.4%, and 14%) and modes of heat treatment on

initial structure of Fe-Cr-C alloys (C~1.2%, Cr = 17…19%) and subsurface structure of friction layers and tribological characteristics of these alloys under dry and boundary friction are investigated. The structure of the alloys with 0% and 0.4% of copper consists of a matrix capable to α ↔ γ transformation + eutectic carbides (hard phase). The soft precipitations of copper containing ε-phase (solid lubricant) appear additionally in structure of an alloy with 14% copper. The least wear rate is observed for an alloy with 14% copper, the wear rate of this alloy (after quenching and after quenching and tempering) decreases 35…60 times under dry friction and 2…8 times under boundary friction as compared to other alloys.

KEYWORDS: Fe-Cr-C alloys, alloying by Copper, wear, subsurface structure.

1. INTRODUCTION

The increase in the population’s standard of living and guarantee of its ecological safety demands the continuous development of the new materials [1]. To this category belong the wear resistant materials. Cast alloys can be used as such materials. Varying their chemical composition, modes of heat treatment and using special technological methods, it is possible to synthesize the alloys that satisfy a complex of necessary requirements. We can define three variants of synthesis of cast wear resistant alloys:

1 - creation of alloys with the structure, capable to absorb the external energy and to dissipate it by thermal fields and by reversible phase and structural transformations.

2 - creation of alloys with the relatively thermo-stable structures, capable to dissipation incoming energy by thermal fields, such as eutectic alloys whose durability is determined by the temperature threshold of structural stability.

3 - creation of alloys with heterogeneous structure where in the matrix of the alloy besides a hard phase (eutectic carbides) there is relatively soft phase, capable to serve as solid lubricant and to protect the initial structure against excessive wear.

It is established that optimization of the initial structure of alloys can lead to increase of its wear resistance. The simplest ways of initial structure optimization are alloying and heat treatment.

Copper is used as alloying element in this work. The problem of efficient alloying of alloys by copper for a long time attracted the attention of material engineers, metallurgists, and tribologists. Copper can give alloys necessary combinations of physicomecha-nical characteristics [2]. The data on copper influence on structure, phase transformations, chemical and technological properties of alloys and its operating characteristics [3, 4] allow to use copper alloying for Fe-Cr-C alloys used in energy industries [5, 6]. These alloys should satisfy a complex set of requirements: to be corrosion and erosion resistant, to resist abrasive influence, and to have high wear resistance under sliding friction in different medium.

One of main causes of little longevity of some energy pump seals is adhesion observed during contact of rotating details that result in the increased wear and is a main cause of formation scores and transfer of a material from one detail on another [7].

Use of some Fe-Cr-C alloys [8-12] for details of the pump equipment has confirmed their high operational characteristics, and additional alloying of these alloys with the purpose of optimization of their initial and subsurface friction layer structures has allowed increasing durability of pump parts working in the water environment up to 40,000 hours [13]. High wear resistance of such alloys under boundary friction is reached due to formation in friction surface layers of dissipative structures which are capable of dissipation of friction energy by thermal fields, and by

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convertible phase and structural transformations. The minimal wear rate of such alloys is achieved due to formation of the specific dynamic ratio between phases and its lattice irregularities in subsurface structure of friction layers of alloys [14-16]. Stabilization of this ratio in for as long as possible favours localization of wear process in these layers. It counteracts transition from fatigue wear to adhesion wear, when the underlying layers of a material are involved in the wear process.

To minimize the wear of such alloys we can increase the chromium content in surface friction la-yers which results in reduction of oxidation rate, thus increasing the time between activation and passive-tion processes in these layers [17]. It is also determi-ned that the protective films of high-strength alloys alloyed by oxygen (AAO) are formed on friction surface of Fe-Cr-C alloys. This AAO having liquid-like structure, whose micro-volumes are capable to transform at contact points of friction surfaces into the structurally unstable state, favouring hydrodynamic friction in such points without loss of layer continuity and to hampering the accumulation of lattice defects leading to surface fracture of alloys [18, 19].

The protection of alloy friction surface layers against fracture is also achieved by using different solid lubrications: disulfide molybdenum, PTFE, graphite, gold, silver, indium etc., which deposit on friction surfaces. At the same time, the increase of the power transmitted through the friction parts requires the use of cast alloys with matrix solid lubricant, i.e. lubricant located within the alloy matrix. The combination of several components with different physico-mechanical properties allows creating com-positions with a number of unique and very important properties for engineering: high heat conductivity, self-lubrication, high damping capacity etc. [20, 21].

The most advantages in terms of loading and self-lubricating are the macroheterogeneous pseudo-alloys [22-24]. Depending on external friction conditions on surface of such materials, a film of the plastic material on hard component parts is formed due to a difference of thermal expansion of antifriction and hard components under heating of friction pair, or as a result of mechanical smearing.

For this purpose, it is possible to use solid lubricant which has soft inclusions distributed in alloy structure. High copper content ε-phase can be used as such inclusion. It appears in an alloy by introduction of copper over the limit of its solubility. During friction copper will emerge on a surface contacting parts, protecting them from direct impact.

In these heterogeneous alloys, an additive action of structural components is realized: a matrix capable to α ↔ γ transformation + hard component (eutectic car-bides) + a soft component (copper containing ε-phase). In this case, the dissipation of friction energy on a macrolevel will be determined by ratio between these

components, and also their cooperative action promo-ting formation of a new structural state. Dissipation of energy on a microlevel will be determined by ability of an alloy to form subsurface layer structures with optimum phase composition and thin structure.

The manufacture of alloys with heterogeneous structure has been investigated in works [25, 26]. It is established that, due to additional alloying, changes of kinetic parameters of crystallization it is possible to obtain in principle new class of alloys. In the structure of these alloys, besides hard precipitations of complex carbides, there is a soft component (copper containing ε-phase), which can serve as solid lubricant. The wear tests of such alloys under dry and boundary friction [27] have shown that these alloys can be used for friction units.

The purpose of this work is to research the tribo-logical characteristics and peculiarities of subsurface friction layers formation of the Fe-Cr-C alloys with the copper content which is dissolved or is higher than a limit of its solubility in a matrix of an alloy.

2. MATERIALS AND METHODS

Alloys of system Fe-Cr-C were chosen as object of research: (1) – (C~1.2%, Cr = 17…19%), the base alloy, (2) – (C~1.2%, Cr = 17…19%) + ~0.4% Cu, developed by as the alloy used in industry [6] and (3) – (C~1.2%, Cr = 17…19%) + ~14% Cu. Alloys were investigated in a cast state, and also after quenching at 1080оС in oil, and after quenching at 1080 С in oil and tempering at 580оС. It has allowed to obtain alloys such as (C ~ 1.2%, Cr = 17…19%) and (C ≈ 1.2%, Cr ~ 17…19%) + ~ 0.4% Cu, with the initial structures composed of a matrix with α + γ-phases or α-phase, and eutectic carbides. For the alloy (C ~1.2%, Cr = 17…19%) + ~14% Cu, the initial structure com-posed of a matrix (α + γ-phases) + a hard phase (eutectic carbides) + soft copper contai-ning ε-phase. The initial structures of studied alloys were examined with optical microscope, and the chemical composition of structural components was determined by scanning electronic microscope with microanalyser REMMA-102. Characteristics of initial and subsurface friction layer structures and phase compositions were studied by X-ray diffraction method. The wear tests were performed on a block-on-ring machine. The ring (counterbody) was made of steel grade 20X13 GOST 5632-72, containing 0.23% C and ~13.0% Cr (similar to steel grade AISI 420) with surface hardness 38…40 HRC. Studied alloys were tested under dry (in air) and boundary (water was supplied to contact zone) friction. The sliding velocity and the specific load were 1 m/s and 5 MPa, respectively. The ratio of the sample area to the counterbody area was 0.08. The wear rate of specimens was determined as ratio of its weight loss to sliding distance.

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3. EXPERIMENTAL RESULTS

3.1. Effect of copper on initial structure, wear rate and redistribution of alloying

elements on friction surfaces

In this experiment, the base Fe-Cr-C alloy (C~1.2%, Cr = 17…19%) was additionally alloyed

with copper in quantity 0%, ~0.4%, and ~14%. Such difference of copper contents in alloys has resulted in essential change of their initial structure in a cast state, and after various modes of heat treatment. In a cast state the matrix of alloys, basically, represents austenite grains around which eutectic carbides of skeletal shape are deposited (fig. 1).

Fig. 1. Micrographs of studied Fe–Cr–C alloys alloyed by 0% Cu (a, b, c), 0.4% Cu (d, e, f), and 14% Cu (g, h, i); a, d, g - cast state, b, e, h - quenching at 1080оС in oil, c, f, I - quenching at 1080оС in oil and tempering at

580оС; 1 - eutectic carbides; 2 - copper containing ε -phase.

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Table 1. Hardness and microhardness of structural component of alloys, containing 0%, 0.4% and 14% copper in a cast state, after quenching at 1080 С in oil, and after quenching at 1080 С in oil and tempering at 580 С.

Microhardness, МPа matrix #

Copper content in an alloy, weight%

State of an alloy Hardness, HRC

α γ eutectic carbides ε-phase

In cast state 34-38 − 4120 9450 After quenching 59 7650 − 12540

1

0

After quenching and tempering 37 4730 − 8730 −

In cast state 40 − 4530 10900 After quenching 60 6840 − 9270

2

0.4

After quenching and tempering 40 4980 − 6120 −

In cast state 33 − 3620 12540 1190 After quenching 58 5950 4980 9270 1270

3

14.0

After quenching and tempering 48 6620 7110 9270 1890 In the structure of an alloy with 14% copper, the

precipitations of copper containing ε-phase were deposited in the area of eutectic carbides as well as within the austenite grains.

The quenching of alloys results in essential change of alloy structure. The structure of alloys with copper content of 0% and 0.4% represents structureless martensite and eutectic carbides. In the structure of the alloy with 14% copper, there are two times decreasing of austenite quantity is observed, and on boundaries of eutectic carbides α-phase appears, obtained as a result of γ → α transformation by martensite type. However, the arrangement of precipitations of copper contained ε-phase essentially does not change.

The structure of alloys with 0% and 0.4% copper after quenching and tempering represents troosto-sorbite matrix with eutectic carbides. In the alloy with 14% copper, there is decreasing of auste-nite quantity in relation to the quenched state due to additional γ → α transformation by martensite type. The arrangement of precipitation of copper containing ε -phase essentially does not change. Results of deter-mination of hardness and microhardness of alloys after various modes of heat treatment are given in table 1.

The data, given in table 1, show that the microhardness of alloys is significantly changes depending on alloying with copper and modes of heat treatment. In cast state, when the matrix of alloys almost fully consists of equal quantity of austenite, the 0.4% copper content results in the maximal hardening of austenite (γ-phase) matrix. The quen-ching of an alloy with 0.4% copper content does not lead to increase of hardening of martensite (α-phase) in relation to the base alloy, and the microhadness of the α-phase of an alloy with 14% copper is much less than in other alloys. The maximal microhardness of structural components of alloys after tempering is observed already at an alloy with 14% copper due to age hardening, caused by copper content in alloy beyond its solubility in α-and γ-solid solutions [4].

The wear tests of alloys under dry friction at sliding velocity 1 m/s and specific load 5 MPa have shown that the initial structure of alloys influences the wear rate of both the specimens and the counterbodies (fig. 2).

The wear rate of an alloy without copper in a cast state is relatively high, but wear rate of the counterbody is 2 times less. The quenching of this alloy results in increase of wear rate of the specimen by an order of magnitude, and a transfer of specimen material onto the counterbody is observed. The tempering of an alloy results in the greater increase of wear rate of the specimen and the greater transfer of a specimen material onto the counterbody. The friction coefficient is relatively independent from the heat treatment of an alloy, and is in the range 0.4…0.52.

The wear rate of an alloy containing 0.4% copper in cast state has increased 3 times in relation to the base alloy, but the wear rate of the counterbody has decreased four times. The quenching of an alloy and quenching and tempering results in further increase of the specimen wear rate and in increase of specimen material transfer onto the counterbody, but it occurs less intensively than in the base alloy. The friction coefficient is independent of the heat treatment and is in the range 0.4…0.45, which is less than in the base alloy.

The wear rate of an alloy with 14% copper in cast state is slightly less than that of the base alloy, but the wear rate of counterbody decreases 8 times. The wear rates of an alloy after quenching and after quenching and tempering are practically equal, and are, respectively, 60 and 58 times less than the wear rate of the base alloy after the same heat treatment. In this case, the property of copper to plate the contac-ting surface leads to a positive gradient of mechanical properties on depth of subsurface friction layer [28].

The wear tests of alloys in cast state under boundary friction have shown that the wear rate of the specimens and counterbodies significantly decreases as compared to the results obtained under dry friction, i.e. the initial structure of the alloys exerts influence on wear rate of friction pair (fig. 3).

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Fig. 2. Wear rate of studied Fe–Cr–C-alloys alloyed by 0% Cu (a), 0.4% Cu (b), and 14% Cu (c) under dry

friction; 1 - cast state, 2 - quenching at 1080 С in oil, 3 - quenching at 1080 С in oil and tempering at 580 С, □ – specimen; ■ – counterbody.

Fig. 3. Wear rate of studied Fe–Cr–C-alloys alloyed by 0% Cu (a), 0.4% Cu (b), and 14% Cu (c) under boundary

friction; 1 – cast state, 2 - quenching at 1080 С in oil, 3 - quenching at 1080 С in oil and tempering at 580 С, □ - specimen; ■ – counterbody.

The wear rate of an alloy without copper (the

matrix of alloy consists practically of γ-phase) is fairly high. The matrix of alloy after quenching consists already of α-phase (martensite) which results in increased wear rate. The matrix of an alloy after quenching and tempering consists of α-phase as troosto-sorbite. This change of initial structure results in significant decrease of the alloy wear rate and insignificant decrease of the counterbody wear rate. The friction coefficient depends on alloy initial structure and is in the range 0.33…0.85.

The initial structure of alloy with 0.4% copper varies similarly to the base alloy after a heat treat-ment, but the wear rate of alloy with 0.4% copper in cast state and after quenching and tempering is lower, due to positive influence of small copper additives on subsurface structure formation of friction layers. The friction coefficient is in the range 0.4…0.77.

Wear rate of alloy with 14% copper decreases most significantly. In this case, the wear rate of alloys after quenching and quenching and tempering decreased 8 and 2.5 times, respectively, as compared

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to the base alloy, but the wear rate of counterbodies is significantly increased. Friction coefficient depends less on the alloy initial structure and is in the range 0.4...0.47.

Such variability of the alloys wear rate, probably results from the initial structure differences due to differences of copper content, modes of heat treatment, and peculiarities of redistribution of alloying elements on friction surfaces of specimens and counterbodies, as well as subsurface structure formation in friction layers. Results of determination of a chemical composition changes in specimen and counterbody surfaces are given in table 2.

Analysing the data in table 2, we can note that the chemical composition of a surface after friction depends not only on the copper content in an alloy and the alloy initial structure (in cast state, after quenching, after quenching and tempering), but on the friction conditions as well.

On the surface of an alloy without copper (in cast state, after quenching, after quenching and tempering) after dry friction, there is increase in Al and Si content and decrease in Cr and Mn content, but after boundary friction increase in Mn content and decrease in Cr and Si content are observed.

On the surface of an alloy with 0.4% copper (in cast state, after quenching, after quenching and tempering) after dry friction, there is an increase in Si content. However, the increase in Mn content in this type of alloy is observed only for an alloy in cast state and the alloy that underwent quenching and tempering. Content of Cu is decreased for alloys in cast state, after quenching, after quenching and tempering, but the content of Mn is decreased in the alloy that underwent quenching and tempering.

The wear tests of this alloy (in cast state, after quenching, after quenching and tempering) under boundary friction show increase in Al, Si, Cr, Cu content and decrease in Mn content on its surface.

On the surface of an alloy with 14% copper (in cast state, after quenching, after quenching and tempering) after dry friction there is an increase in Al, Si, Mn content and a decrease in Cr and Cu content. The wear tests of this alloy under boundary friction show the increase in friction surface Mn, and an additional increase in Si for the alloys in cast state and after quenching, while there is a decrease in Cr, Cu content. The decrease of Si content was observed only after quenching and tempering. The results show an increase of Si content on friction surfaces of the alloys with different copper contents under dry friction, are in accordance to information given in [29]. The copper content in an alloy exerts significant influence on distribution of alloying elements on its surfaces after boundary friction. An increase in Si content is observed only for alloys containing copper as it noted in work [30] and a decrease in Si content for an alloy without copper is observed.

The content of chromium on a friction surface also depends on the copper content in an alloy and its

content is decreased at the copper content 0% and 14%, while an insignificant content of copper (0.4%) in an alloy leads to increase of chromium content on friction surfaces. An increase of chromium content on friction surfaces of specimens was observed by us earlier by alloying a similar alloy with a small amount of vanadium [17]. In this case, we can note that an additional alloying of the base alloy with a small amount of copper or vanadium favours chromium increase on its friction surface.

The content of Mn on friction surfaces is increased for alloys with 0% and 14% copper and is decreased for alloy with 0.4% copper.

The copper content on friction surface has the tendency to increase slightly for an alloy with 0.4% copper and to decrease for an alloy with 14% copper, when there is an appearance of copper on the counterbody surfaces.

The chemical composition of counterbody surfa-ces also changes and it depends on chemical composi-tion of the specimens and friction conditions. The content of Al and Cr increases on the counterbody surfaces that have been tested with the alloys con-taining 0% and 0.4% Cu under dry friction, but if tested with the alloy containing 14% Cu, there is an increase of Al, Cr, and Cu and a decrease of Si and Mn.

The boundary friction tests lead to a decrease of Si on counterbody surfaces tested with the 0% and 14% copper containing alloys and to an increase of Si after test with an alloy containing 0.4% copper. The boundary friction tests of the counterbody and an alloy containing 14% copper lead to an increase of Al, Cr, and Cu on counterbody surface.

The change of copper content in an alloy also results in change of phase composition and thin structure of alloys in both an initial state and after friction. 3.2. The Structure Characteristics of Alloys

in Initial Cast State and after Dry and Boundary Friction

It is shown that a change of copper content in

alloys in cast state does not result in significant change of the austenite (γ-phase) content in its matrixes and constitutes 70…75% (fig. 4), but the additives of 0.4% and 14% of copper result in increase of austenite lattice parameter, which is caused by the difference in size of the nuclear radii copper (0.128 nm) and iron (0.126 nm). There is also an increase in microdistortions of the second-kind

(∆а/а)γ and third kind ( 2u )γ (in γ-phase) by increase of copper content in an alloy and it is most strongly shown for alloy with 14% of copper. Thus, the microdistortions of the second kind (∆а/а)γ and

third kind ( 2u )γ are increased 7 and 1.4 times respectively as compared to the base alloy.

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Table 2. The chemical composition of specimen surfaces of Fe-Cr-C alloys containing 0%, 0.4%, and 14% copper and surface of counterbodies made of steel 20X13 in an initial state and after sliding friction.

Chemical composition*,**, weight% Copper content in an alloy, weight%

State of an alloy

Surface state of specimen and counterbody Al Si Mn Cr Cu Fe

specimen 0.37 0.80 0.85 17.3 − rest Initial counterbody − 0.80 0.80 13.0 − rest

specimen 0.56 1.20 0.70 15.2 − rest Dry friction counterbody 0.20 0.30 0.60 13.7 − rest

specimen 0.46 0.78 1.00 16.5 − rest In cast state Boundary

friction counterbody − 0.50 0.36 13.0 − rest specimen 0.62 1.00 0.57 16.0 − rest Dry

friction counterbody − 0.20 0.74 13.5 − rest specimen 0.27 0.55 1.38 16.6 − rest

After quenching Boundary

friction counterbody − 0.60 0.42 13.2 − rest specimen 0.40 0.78 0.80 15.8 − rest Dry

friction counterbody 0.28 0.32 0.78 14.3 − rest specimen 0.28 0.65 1.10 16.0 − rest

0

After quenching

and tempering

Boundary friction counterbody 0.20 0.50 1.20 13.7 − rest

specimen 0.16 0.70 0.70 16.6 0.38 rest Initial counterbody − 0.80 0.80 13.0 − rest specimen 0.20 0.80 0.72 18.1 0.34 rest Dry

friction counterbody 0.25 0.70 0.67 14.4 − rest specimen 0.18 0.80 0.50 18.2 0.42 rest

In cast state Boundary

friction counterbody 0.40 0.90 0.40 14.0 − rest specimen 0.10 0.80 0.76 16.4 0.20 rest Dry

friction counterbody 0.10 0.70 0.46 14.9 − rest specimen 0.40 0.80 0.65 18.6 0.48 rest

After quenching Boundary

friction counterbody 0.30 0.95 0.50 13.5 − rest specimen 0.30 0.74 0.32 16.4 0.26 rest Dry

friction counterbody − 0.66 0.65 15.1 − rest specimen 0.30 1.30 0.50 17.6 0.40 rest

0.4

After quenching

and tempering

Boundary friction counterbody 0.25 1.00 0.60 14.2 − rest

specimen 0.24 0.46 0.30 19.3 13.8 rest Initial counterbody − 0.80 0.80 13.0 − rest

specimen 0.37 1.20 0.50 18.8 10.1 rest Dry friction counterbody − 0.46 0.60 14.4 6.00 rest

specimen 0.40 1.10 0.70 15.6 8.60 rest In cast state Boundary

friction counterbody − 0.17 0.68 12.8 5.00 rest specimen 0.40 0.50 0.40 16.5 9.00 rest Dry

friction counterbody 0.20 0.60 0.60 15.1 3.80 rest specimen 0.10 0.70 0.40 15.4 8.60 rest

After quenching Boundary

friction counterbody 0.40 0.45 1.30 14.0 3.40 rest specimen 0.30 0.65 0.35 18.0 10.0 rest Dry

friction counterbody − 0.60 0.30 14.4 7.60 rest specimen − 0.55 0.20 17.4 8.70 rest

14.0

After quenching

and tempering

Boundary friction counterbody 0.50 0.74 0.64 13.5 4.80 rest

* - The carbon content in steel 20X13 is 0.23%, in an alloys (C ~1.2%, Cr = 17…19%) alloyed with 0%, 0.4%, and 15% copper – 1.19%, 1.21%, and 1.21%, respectively. The microanalyser characteristics have not allowed determining the content of carbon on specimen and counterbody surfaces after friction. ** - Taking into account the definition locality of a chemical composition by means of a microprobe, the definition of a chemical composition has been made from surface equal to contact surface of a specimen with a counterbody.

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The change in value of the microdistortions of second

kind (∆а/а) and third kind ( u 2 ) enables to analyse the structure-stress state of subsurface layers of a material [31-35].

The size of coherent particles (Dγ) of austenite in matrix decreases significantly by increasing of copper content of alloy. The density of dislocations (ργ) has the minimal value at 14% copper content in an alloy.

The obtained data for α-phase allow to analyse only the changes of the dislocations density (ρα), whose quantity has a minimum for an alloy with 14% of copper.

All studied alloys in a cast state had severe wear rate under dry friction (fig. 2), which was the least for the 14% copper containing alloy, but, in this case, the wear rate of a counterbody decreased 8 times as compared to a counterbody tested with the base alloy. In this case, the cladding action of copper (6%) on a counterbody surface is to protect the contacting surfaces from direct contact. Therefore, there was no significant change in quantity of α- and γ-phases and their lattice parameters in subsurface structure of friction layer as compared with this alloy in the initial state. At the same time, there is an insignificant increase in microdistortions of the second kind

(∆а/а)γ and the third kind ( 2u )γ; the size of coherent particles (Dγ) does not change, but the density of dislocations in γ-phase is increased twice in relation to the ones in the initial state. There is a significant increase in microdistortions of second kind (∆а/а)α, but an insignificant decrease in

microdistortion of third kind ( 2u )α in α-phase. The size of coherent particles (Dα) is increased, but the density of dislocations ρα is decreased. The tendency towards an equilibration of dislocation density and total concentration of stacking faults (1.5α+β) in α-and γ-phases is observed.

The wear rate of the studied alloys is decreased under boundary friction (fig. 3), due to a heat absorp-tion by the present water, which changes the condi-tions of the subsurface structures formation. The least wear rate is observed for an alloy with 0.4% copper content which is probably caused by optimization of subsurface structure under friction. The quantity of γ-phase in subsurface friction surfaces of the tested alloys does not change significantly after boundary friction in relation to the alloys’ initial state and makes up 70…80% (fig. 4). The lattice parameter of γ-phase is decreased for all alloys in relation to their initial state, but the magnitude of change is similar to an initial state, i.e., if the copper content in an alloy is increased, the lattice parameter of γ-phase is increased. The microdistortions of the second kind

(∆а/а)γ and the third kind ( u 2 )γ are increased in alloys with 0% and 0.4% copper content, but for an alloy with 14% copper, the inverse process is

observed. The size of coherent particles (Dγ) for alloys with 0.4% and 14% copper content is increased slightly after friction test in relation to its initial state and in the alloy with 0.4% copper the size of (Dγ) achieves the greatest value. The density of dislocations in γ-phase is increased for all alloys after friction and their value is almost identical and does not depend on the copper content in an alloy, but the density of dislocations in α-phase is decreased in an alloy without copper, almost does not change for an alloy with 0.4% copper, and is increased in an alloy with 14% copper. Thus, the insignificant decrease of wear rate for an alloy with 0.4% copper is the result of lager value of microdistortions of the second kind

(∆а/а)γ and the third kind ( u 2 )γ, the size of coherent particles (Dγ), and also density of dislocations in α-phase.

3.3. Structure Characteristics of Alloys in Initial State after Quenching and after Dry

and Boundary Friction

The quenching of studied alloys has resulted in significant change of their initial structure. Thus, the matrix of alloys with 0% and 0.4% copper content consists of α-phase representing martensite (fig. 1), and the austenite content in an alloy with 14% copper has decreased from 75% to 46% as compared to this alloy in a cast state (fig. 4). Thus, the lattice parameter of γ-phase has not changed as compared to an alloy in cast state and is equal 0.36218 nm, and the lattice parameter of α-phase has increased from 0.28767 to 0.28782 nm. The microdistortions of the second kind (∆а/а)γ and density of dislocations ργ have decreased 2 times as compared to the alloys in cast state, but the size of coherent particles (Dγ) has increased 3 times as compared to the cast state. It is significant, that after quenching there is the tendency to increase the lattice parameter of α-phase that parallels the increase of the copper content in an alloy. These results concur with the data obtained for Fe-Cu systems, when alloying of iron by copper results in significant increase of the lattice parameter of α-phase [36, 37].

The dry friction tests (fig. 2) of alloys after quen-ching have shown that the wear rate of the alloys with the 0% and 0.4% copper content (with martensite matrix) is increased 13 and 2.5 times, respectively, as compared to similar alloys in a cast state, and the seizure is observed. The wear rate of an alloy with 14% copper (where α and γ-phases are still present in the matrix), decreases 4 times and the counterbody wear rate doesn’t change. In this case, fatigue wear is observed. The difference of initial structures of alloys with 0%, 0.4%, and 14% copper has caused manifes-tation of various kinds of wear mode that has affected formation of subsurface structure of friction layers. In the friction layers of alloys with 0% and 0.4% copper

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Fig. 4. Effect of Cu on the subsurface structure characteristics of Fe–Cr–C alloys under conditions: in initial state (a), after dry (b), and boundary (c) friction; 1 - cast state, 2 - quenching at 1080 С in oil, 3 - quenching at 1080 С

in oil and tempering at 580 С

I - quantity of γ-phase,%; II - lattice parameter of γ-phase (аγ), nm; III - lattice parameter of α-phase (аα), nm;

IV - microdistortions of the II-kind (∆а/а); V - microdistortions of the third kind ( 2u ), nm; VI - size of coherent particles (D), nm; VII - dislocations density (ρ), cm–2; VIII - total concentration of stacking faults (1.5α+β);

□ - the value for α-phase; ■ - the value for γ-phase; Х, XX - the data were not obtained because of degraded line (220) and correspondingly (311).

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already there is γ-phase in quantity at 20% and 50% and with high level of microdistortion of the second

kind (∆а/а)γ and the third kind ( u 2 )γ for an alloy with 0.4% copper (fig. 4). The lattice parameter of α-phase is decreased which indicates the separation of alloying elements under friction. The microdistortions of the second kind (∆а/а)α decrease, and the

microdistortions of third kind ( u 2 )α don’t change significantly as compared to its value in initial state.

For the alloy with 14% copper, when the fatigue wear was observed, the quantity of austenite practically has not changed, while the density of dislocations has increased 6 times. Because of degraded lines (311), it is impossible to determine the changes of other parameters of thin structure in γ-phase. There is a decrease of the lattice parameter of α-phase, while the microdistortions of the second kind (∆а/а)α and the microdistortions of third kind

( u 2 )α don’t change, and the density of dislocations ρα has decreased 2 times.

Thus, the least wear rate is observed for an alloy with 14% copper under dry friction, which is 60 and 35 times less than the wear rate of alloys with 0% and 0.4% copper. In this case, the phase composition and thin structure of subsurface friction layers of an alloy undergo the least changes, which can be explained by formation of plating copper film on the counterbody surface (3.8% copper), that significantly changes subsurface layer structure formation condi-tions under dry friction. The obtained results are in accordance with the data given in work [38], when the presence of a plating film deposited on one or both friction surfaces exerts influence on friction energy dissipation and leads to minimizing of the probability of thermomechanical wear commencement.

The boundary friction tests of alloys after quenching have shown (fig. 3) that the wear rate of alloys with 0%, 0.4%, and 14% copper content decreases 1.5, 7.5, and 2 times respectively, as compared to the results obtained under dry friction. The minimal wear rate is observed in the alloy with 14% copper, which is 7...8 times less, than for alloys with 0% and 0.4% copper content. However, it is necessary to underscore that the wear rate of counter-body is increased more than 6 times. After friction in subsurface layers of alloys with 0% and 0.4% copper content appears 65% and 45% of austenite, respect-tively, and in an alloy with 14% copper the quantity of austenite is increased from 35% to 70% as compa-red to initial state (fig. 4). Taking into account prac-tically identical wear rate of alloys with 0% and 0.4% copper, it is possible to assume that the changes of structure characteristics of their subsurfaces after fric-tion will be identical. It is established that the crystal lattice parameters of α- and γ-phases are larger for an alloy with the content 0.4% copper, while the micro-distortions of the second kind (∆а/а)α, the

microdistortions of third kind ( u 2 )α, the size of coherent particles (Dα), density of dislocations ρα,

the microdistortios of third kind ( u 2 )γ, the size of coherent particles (Dγ) for alloys with 0% and 0.4% copper are equal. In this case we can note that if the wear rates of the alloys are equal, so there are certain structure characteristics of their subsurfaces.

The lattice parameters of α and γ-phases in friction layers of alloy with 15% copper are decreased relative to its initial state. The microdistortions of the second kind (∆а/а)γ, (∆а/а)α, the microdostortions of

the third kind ( u 2 )γ, density of dislocations ρα and ργ have the minimal values in comparison with those in the alloys containing 0% and 0.4% copper. In this case, the plating copper film formed on a counterbody surface (3.4% copper) provides positive influence, dividing contacting surfaces and promoting more equal distribution of loading in a contact zone, thus decreasing structure stress in a surface layer of an alloy.

Thus, the least wear rate is observed for an alloy with 14% copper under boundary friction. In this case, the microdistortions of the second kind (∆а/а) in α- and γ-phases, the microdistortions of the third kind

( u 2 )γ, densities of dislocations ρα and ργ in subsurfase friction layers have the minimal values in comparison with the alloys containing 0% and 0.4% copper.

3.4. Structure Characteristics of Alloys in Initial State after Quenching and

Tempering and after Dry and Boundary Friction

The quenching and tempering of alloys has

resulted in significant change in their structure. The structure for alloys with 0% and 0.4% copper content is changed from martensite structure to troosto-sorbite (fig. 1). In an alloy with 14% copper, besides α-phase in matrix, there is a significant quantity of residual austenite, though his quantity is decreased slightly from 46% (after quenching) to 30% (after tempering). The quenching and tempering of alloys with 0% and 0.4% copper leads to decrease of their lattice parameter of α-phase, the microdistortions of the second kind (∆а/а)α and the microdistortions of the

third kind ( u 2 )α, and density of dislocations ρα, while the size of coherent particles (Dα) does not change and a total concentration of stacking faults (1.5α+β) for α-phase is increased (fig. 4).

The quenching and tempering of an alloy with 14% copper leads to decrease of the lattice parame-ters of α- and γ-phases, which indicate intensive allocation of alloying elements. There is a decrease of the microdistortion of the second and third kind,

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density of dislocations in α-phase. The difference of structure characteristics of alloys in initial state has predetermined a character of subsurface layer structure formation under friction and wear rate of these alloys.

The dry friction tests of alloys after quenching and tempering have shown that wear rate of the alloys with 0% and 0.4% copper content (with troosto-sorbite matrix) is increased 1.1 and 1.2 times, respect-tively, relatively to the similar alloys after quenching when they have martensite structure, and the increase of transfer of the specimen material on the counterbody noted. The wear rate of an alloy with 14% copper does not change significantly in relation to the one after quenching, but the wear rate of counterbody is increased 5 times. It is significant that the wear rate of an alloy with 14% copper is 55 and 36 times less than those of the alloys containing 0% and 0.4% copper. In case of an alloy with 14% copper fatigue wear is observed, but for the alloys with 0% and 0.4% copper adhesive wear takes place. The differences of the wear process of alloys have prede-termined a character of subsurface layer structure formation under friction. In subsurface friction layers of alloys with 0% and 0.4% copper there is 30...50% of γ-phase, while for an alloy with content 14% copper, the quantity of γ-phase is only insignificantly increased from 30 to 35%, but a lattice parameter of γ-phase is significantly increased relatively to its in initial state and has the maximal value as compared to the other alloys.

The microdistortions of the second kind (∆а/а)γ and the size of coherent particles (Dγ) have the maximal value in subsurface friction layer of an alloy with 0.4% copper, but minimal value for an alloy with 14% copper, while the microdistortions of third kind

( u 2 )γ and a total concentration of stacking faults (1.5α+β) for γ-phase is maximal. The density of dislocations in γ-phase has the tendency to decrease with an increase of copper content in an alloy.

The lattice parameter of α-phase in subsurface friction layers of alloys decreases with the increase of copper content in an alloy, and its minimal value, as well as minimal density of dislocations (ρα) is observed in alloy with 14% copper, but at the same time there are the maximal values of both the size of coherent particles (Dα) and a total concentration of stacking faults (1.5α+β) for α-phase.

In that way, the minimal wear rate of an alloy with 14% copper after quenching and tempering is characterized by 35% γ-phase with the maximal lattice parameter. At the same time, the microdistortions of the second kind (∆а/а)γ, the size of coherent particles (Dγ), and the density of dislocations in γ-phase have the minimal value, but

the micro-distortions of third kind ( u 2 )γ and a total concentra-tion of stacking faults (1.5α+β) for γ-phase is the maximal.

The α-phase at the same time has the minimal value of both lattice parameter and density of dislocations ρα, but the maximal value of the size of coherent particles (Dα) and a total concentration of stacking faults (1.5α+β) for α-phase.

The boundary friction tests of alloys after quenching and tempering have shown that wear rate of alloys with 0%, 0.4%, and 14% copper content decreases 72, 117, and 6 times respectively, relatively to wear rate of similar alloys under dry friction and thus for all alloys the fatigue wear is observed. The minimal wear rate process for the given mode of thermal processing is noted for an alloy with 14% copper, which is 5 and 2 times less than for alloys with 0% and 0.4% copper, respectively. It is worth mentioning that, if the quantity of γ-phase in subsurface friction layers is increased in alloys with 0% and 0.4% copper after boundary friction as compared to the ones after dry friction, the quantity of γ-phase in subsurface friction layer for an alloy with 14% copper has not changed with friction conditions and has the minimal value among the tested alloys. It is necessary to note that an alloy with 0.4% copper and counterbody have identical wear rate in spite of the friction asymmetry determined by scale factor. In this case the fixed ratio between α- and γ-phases is observed in friction layer of an alloy with almost equal values of microdistortions of third kind, the sizes of coherent particles, density of dislocations, and a total concentration of stacking faults (1.5α+β). The minimal wear rate of an alloy with 14% copper is characterized by an increase of lattice parameter of α and γ-phases, and by the minimal value of microdistortions of second kind and density of dislocations in subsurface friction layer. In this case, the surface layer of conterbody contains 4.8% copper which protects contacting surfaces from severe wear.

4. CONCLUSIONS

1. The initial structure of cast Fe-Cr-C (1.2% C,

17...19% Cr) alloys with the various content of copper (0%, 0.4%, 14%) influences on the structure of near-surface friction layers and tribological characteristics of these alloys.

2. During the friction process there is a change of chemical composition of friction surfaces of cast alloys which depends on initial chemical composition of alloys and test conditions.

3. The alloys with 15% copper (in cast state, after quenching, after quenching and tempering) have minimal wear rate under dry friction. In this case, there is a least change of quantity of γ-phase in near-surface friction layers relative to their initial state and optimization of thin structure parameters in these layers.

4. The alloys with 15% copper (after quenching, after quenching and tempering) have minimal wear rate under boundary friction, but significant wear rate of counterbodies is marked.

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5. The alloy with 0.4% copper is used in the industry for manufacturing energy pumps. The alloy with 14% copper is a basis for heterogeneous alloys creation with matrix solid lubricant, i.e. lubricant located within the alloy matrix (cast composites).

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