Contact Characteristics of Metallic Materials in Conditions of Heavy Loading by Friction or by Elect by Shirley Wang

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Contact Characteristics of Metallic Materials in Conditions of Heavy Loading by Friction or by Electric Current Marina I.Aleutdinova*1, 2, Viktor V. Fadin1, Aleksandr V. Kolubaev1, 3, Valery A. Aleutdinova4 Institute of Strength Physics and Materials Science of Siberian Branch Russian Academy of Sciences,

Seversk Technological Institute - branch of State Autonomous Educational Institution of Higher Professional Education «National Research Nuclear University «MEPhl» 2

National Research Tomsk Polytechnic University

National Research University Saint Petersburg State Polytechnical University

2/4 pr. Akademicheskii, Tomsk, 634021, Russia *1

aleut@ispms.ru

Abstract Friction of composites having composition TiC+metal was realized at pressure more 100 MPa in lubricant medium. Sliding electric contact of metal materials was carried out at contact current density higher 100 A/cm2 without lubricant. It was shown that these loading parameters cause a friction surface wear which increases at increasing alloying atoms quantity or number of phases in the initial structure of material. Keywords Composite; Initial Conductance

Composite

Structure;

Wear;

Contact

Introduction Contact interaction occurs mostly in contact spots. Microvolumes of contact spots undergo large plastic deformation and transit to other structural state. Processes (plastic deformation, formation of chemical compounds, etc) in contact zone could percolate into depth more than 20 microns below the surface . As a result, a layer of friction induced structures is being formed and these structures define basic contact characteristics – wear resistance and friction coefficient. High wear resistance occurs when contact layer structure becomes stable in friction process (Fedorchenko, 1980). Therefore it is necessary to exclude, in the first turn, the plastic deformation in contact spots. It is often being achieved by raising the

material's yield point, or to be more specific, by increasing the hardness of the initial material structure. As a rule, hardnening leads to decreasing the ductility. Therefore this way may be effective in the absence of structural changes in contact layer during the process of friction. Materials oriented for friction under high pressure must have high structural strength and high hardness of initial structure. Composites based on the titanium carbide could be used as such materials. It is of scientific interest to produce these materials by selfpropagating hightemperature synthesis (SHS), for example, by pressing exothermic powder mixture in combustion wave (Merzhanov, 1995) and then define their performance in friction under pressure above 20 MPa. In addition, it could be interesting to study resources of materials under higher heat flow through contact spots, which might be realized by transmitting electric current through the worn surface. Sintered composites produced by methods of powder metallurgy are used in such friction conditions. Commercial composites can realize satisfactory wear resistance at contact current density lower than 60 A/cm2 during sliding current collection (Braunovic, 2007). Friction at pressure higher 20 MPa or sliding at contact current density above 60 A/cm2 may be assumed to be the heavy work conditions. Initial structure of friction pair materials is one of main

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factors which can provide effective work of friction units in these conditions. The objective of the present research is to get some idea about interconnection of contact characteristics and initial structure of composites obtained by SHS methods and powder metallurgy, under friction loading with rated pressure higher 20 MPa or current loading with contact current density higher 100 A/cm2. Materials and Experimental Metods Sample Preparation The materials of exothermic charge for obtaining model microheterogeneous SHS-composites were powders as follows: titanium, lamp soot, iron (98%Fe+ admixtures), Hadfield steel (HS, 13%Mn). The SHScomposites had the phase compositions presented in Tables 1 and 2. They were fabricated in a closed mold mounted in the working space of a hydraulic press. Combustion was initiated by high-voltage discharge after heating to 773 K. The powder charge was placed under the pressure up to 70 MPa during combustion process. Macroheterogeneous composites (Table 3) were fabricated by granulation of SHS-composite TiC+10%Cu+25%HS in grinder-mill to grain size 0,41,0 mm, then by mixing them with iron and saturating this mixture with bronze (Cu+8%Sn+12%Pb) in proportions specified in Table 3. Testing Metods The hardness HV was measured by the Vickers hardness tester. The bending strength σ was determined on an Instron-1185 testing machine. The lattice parameter a was found using a DRON-3 X-ray diffractometer in cobalt radiation. The thermal

conductivity λ0 of the composites was determined by comparing it to the thermal conductivity of the aluminium (which was used as a reference) under conditions of stationary heat flow. Contact characteristics of SHS-composites were obtained on a drilling bench involving the “ring butts” loading scheme (Fig.1, a) (Fadin, 2011) at sliding velocity 0,5 m/s, sliding distance 5 km, mutual overlap coefficient equal 1 and with single application of a graphite grease in contact space. Contact characteristics of SHScomposites (Table 2 and Table 3) were as well examined using the “blocks-on-ring” loading condition (fig.1, b) on a Falex friction machine. The sliding velocity was 0.5 m/s. The sliding distance was 660 m for tests on the Falex machine. Stellite 190 served as counterbody in both schemes (Fig.1, a-b). Sintered composites were fabricated by fusing in vacuum at a temperature 1100oC during 2 hours. Contact characteristics of sintered composites at current collection sliding were determined using “block-on-shaft” testing procedure (Fig.1, c) under pressure 0,13 MPa. Sliding speed was 5 m/s at sliding distance 9 km without grease. Steel-45 (50 HRC) served as counterbody. Results and Discussion It is reasonable to start the study of external friction of new model materials from contact characteristics defining the selection of materials, which could be perspective for further study and obtaining general regularities. The friction of composite TiC+50%(Ni,Cr) using the “ring butts” loading scheme shows a sharp rise of friction coefficient f versus the pressure growth. Simultaneously rapid deteorioration of friction surface occurs as a result of intense adhesive interaction with the counterbody.

TABLE 1. PHYSICAL AND MECHANICAL CHARACTERISTICS OF MICROHETEROGENEOUS COMPOSITES BASED ON TIC PRODUCED BY SHS METHOD

λ0, W/m·K 10 22 35 44

Composition, vol % \ Characteristic TiC+50%(Ni,Cr) TiC+30%Cu+20%(Co,Ni,Cr) TiC+50%Cu TiC+30%Cu+20%Fe

HV, MPa 8900 3200 4230 6200

σ, MPa 1200 622 803

аFe, 10-10 m 2,8723

аTiC, 10-10 m 4,3248 4,3212 4,3253 4,3236

аCu, 10-10 m 3,5690 3,6154 3,6154

TABLE 2. HARDNESS, PHYSICAL AND CONTACT CHARACTERISTICS OF MICROHETEROGENEOUS COMPOSITES BASED ON TITANIUM CARBIDE

Composition, vol % \ Characteristic TiC+30%Cu+20%Fe TiC+20%Cu+20%Fe TiC+30%Cu+20%HS TiC+20%Cu+20%HS TiC+10%Cu+25%HS

HV, GPa 6,2 8,8 7,85 10,8 12,1

λ0, W/m·K 44 19 -

aTiC, 10-10m 4,3236 4,3265 4,3290 4,3256 4,3256

aCu, 10-10m 3,6154 3,6154 3,6429 3,6473 3,6577

dW, mm

0,205 0,210 0,176 0,198 0,192

0,910 0,844 0,938 0,869 0,856

р, MPa 130 141 127 137 139

Т, K 425 451 429 443 436

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TABLE 3. CONTACT PARAMETERS OF MACROHETEROGENEOUS SATURATED COMPOSITES, CONTAINING COMPOSITES TIC+10%CU+25%HS AND STELLITE 190 AS GRANULES

f 0,172 0,168

Composition, vol % \ Characteristic [TiC+10%Cu+25%HS]+30%Fe+30%Bronze(Cu+8%Sn+12%Pb) Stellite 190+30%Fe+30%Bronze(Cu+8%Sn+12%Pb) [Commercial САМ-5]

dW, mm 0,613 0,858

р, MPa 194 138

Т, K 438 422

N A

220V 2

FIG. 1. BASIC SCHEMES OF TRIBOLOGICAL LOADING: A) - RING BUTTS, B) - BLOCKS-ON-RING, C) - BLOCK-ON-SHAFT (1 – COMPOSITE SAMPLES, 2 –CONTERBODY, 3 – LUBRICANT)

Thick transfer layer forms on the counterbody surface. This fact indicates to low heat resistance of surface layer and instability of its structure. Assuming that adhesion could be reduced by temperature decreasing in friction zone, copper was injected into the composite for more efficient friction heat removal. Besides, cobalt was added to the exothermal mixture to improve the heat stability of the surface layer (SL). In this way the composite TiC+30%Cu+20%(Co,Ni,Cr) of improved heat conductivity, was manufactured and tested for friction (Table 1). As a result friction coefficient decreased (Fig. 2) but a thick transfer layer on counterbody formed due to adhesion and thus wear resistance of composite decreased. Assuming that heat stability of the SL could be increased more considerably, SHS-composite TiC+25%Co+25%Cr was fabricated. Structural state of its SL has not changed during the friction under contact pressure up to 50 MPa. However, the SL temperature was too high so that grease burned out, and quasiperiodic cracks appeared on friction surface as a result of probable excitation of friction autooscilations. These results indicate on the insufficient improvement of heat resistance. It should be mentioned that the TiC lattice parameters in these composites are almost the same values (Table 1). Hence TiC properties in different composites are the same, and difference in friction behavior is determined by the composition of metallic matrix. It is known that large plastic deformation of pure metals does not lead usually to nanostructural and amorphous state due to easy relaxation process. Intermetallic compounds could be transferred to the amorphous state due to hindrance in occurring the

relaxation processes at high strains. Thus the relaxation in a zone of stress appearance is a basic factor to define the type of structure (Glezer, 2010). This indicates on the fact that the presence of elements solution or several phases in contact spot inhibits the effective relaxation of stress generated during the moment of contact.

FIG.2. FRICTION COEFFICIENT VERSUS PRESSURE ON THE WORN SURFACE OF COMPOSITES: 1 – TiC+50%(Ni-Cr), 2 – TiC+30%Cu+20%(Co,Ni-Cr), 3 – TiC+50%Cu, 4 – TiC+30%Cu+20%Fe, 5 – CAM-5

Therefore large plastic deformation at such contact spot will lead to its rapid destruction. Complex composition of contact layer or of microvolumes of contact spots could be caused by complex assortment of chemical elements or phases in initial material structure. Besides, complex composition of metallic matrix leads to low heat conductivity of a composite. This allows assuming that composites having a complex structure, complex set of chemical elements or phases in its structure are not capable of easy stress relaxation in SL and would collapse under pressure loading faster than composites with simpler structure.

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Therefore it is necessary to carry out friction tests on a composite TiC+50%Cu since it has rather high thermal conductivity (Table 1) and plain composition. Tests showed that the contact of this composite takes place with lower friction coefficient as compared to that of composites containing Ni or Cr (Fig. 2). The durability of composite TiC+50%Cu is higher than the that of composites containing nickel due to lower SL temperature. However copper does not wet the titanium carbide. Therefore the interphase boundaries TiC/Cu have low mechanical strength and large thermal resistance. This results in brittle fracture of SL under pressure of 38 MPa, therefore further increasing the pressure is unreasonable. The increase of heat conductivity and the improvement of mechanical properties are gained by SHS-composite TiC+30%Cu+20%Fe (Table 1) creation. The structures of microheterogeneous SHS-composites are unifomely as a rule and are similar to the structure of composite TiC+30%Cu+20%Fe (Fig.3). Wear process of this composite occurs with low friction coefficient (Fig. 2), and there are no structural changes in material of SL. It has higher wear resistance as compared to other composites described above. Presented here data lead us to conclusion that the absence of solution and phase diversity in initial structure corresponds to rather high stability of SL structure and realization of satisfactory sliding contact characteristics.

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conductivity and hardness are determined by composition and content of a metal binder. Since iron is a filler without the impurity atoms it serves to improve the heat conductivity and simultaneously reduce the hardness of the composite. The situation is opposite for composites having the Hadfiel steel as a fillerand iron/manganese solid solution binder. Moreover, the solution of manganese in copper is being formed as follows from the growth of copper lattice parameter (Table 3). Decrease of metal binder content leads to the increase of hardness and, agreeably, to the decrease of wear track width dW. That means that wear resistance increases with hardness. SL of composites TiC+Cu+HS bear evidences of adhesive wear as distinct from SL of TiC+Cu+Fe composites. Therefore, the wear resistance of TiC+Cu+HS composites is slightly lower than that of TiC+Cu+Fе ones. Friction coefficients could be compared reasonably only if dW are equal but it is impossible judging by the data of Table 2. The low temperature Тd of the diesel oil in a bath is an indicator of low friction force under boundary lubrication. Effective method of gaining the high working capacity of tribological materials is a production of macroheterogeneous metal matrix composite (MMC). In this composite, the hard granules of SHS-composite TiC+10%Cu+25%HS are inserted in a soft composite matrix which is an pseudoalloy of composition Fe+Bronze(Cu+8%Sn+12%Pb) as shown on Fig.4. Such

FIG.3. MICROSTRUCTURE OF COMPOSITE TIC+30%CU+20%FE

This regularity has been found out using the “ring butts” loading scheme but it should be tested in other cases of contact interaction, for example, under pressure p>100 MPa. Such friction conditions could be obtained in an oil bath using the “block-on-rings” loading scheme (Fig.1, b). Compositions of SHScomposites fabricated for this research are given in Table 2. Evidently, the crystalline lattice parameter is approximately the same in all composites that is the properties of TiC are the same too. Thus heat

FIG.4. MICROSTRUCTURE OF MACROHETEROGENEOUS COMPOSITE: 1 – FILLER (SHS-COMPOSITE), 2 – PSEUDOALLOY FE+BRONZE(CU+8%SN+12%PB)

an initial structure of material allows to realize rather small wear track width dW under relatively high pressure sliding on the worn surface under the boundary lubrication (Table 3). For comparison the same sliding contact characteristics of composition antifrictional material (САМ-5) containing Stellite 190

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as a heat resistant component are given. Composite САМ-5 is applied in commercial production of rotary drill bits for oil well. It follows from the comparison that model metal matrix-filled composite displays higher contact characteristics over the composite

CAM-5 in laboratory friction conditions under constant pressure and sliding velocity. This is explained by higher hardness of TiC+10%Cu+25%HS granules in comparison with hardness of Stellite 190 granules.

TABLE 4. OPERATIONAL CAPABILITY PARAMETERS OF DRILL BITS HAVING SUPPORT SLIDER BEARINGS MATERIALS CONTAINING DIFFERENT HARD GRANULES (MANAGEMENT OF DRILL WORKS, STREGEVOY-SITY)

Composition of filler, vol %

advancing, m

durability, hour

[TiC+20%Cu+20%Fe]+30%Fe +30%Bronze(Cu+8%Sn+12%Pb)

217-255

37-46

[TiC+20%Cu+20%HS]+30%Fe +30%Bronze(Cu+8%Sn+12%Pb)

89-112

15-17

Stellite 190+30%Fe +30%Bronze(Cu+8%Sn+12%Pb)

221-323 189-256 53-247

31-46 35-43 15-33

It is reasonable to justify this regularity on other types of materials with different properties under effect of external forces of other nature. Sliding contact of sintered metal composites and metals under electric current could be realized for this purpose. The main SL destruction factor is a contact current density in

Maximum of contact electric conductivity rs-1 corresponds to an inflection point of current-voltage characteristics as well as to sharp growth of wear intensity Ih (fig.5, b) that indicates on the beginning of catastrophic wear process (Fadin, 2009). Contact of composite Cu+10%Graphite+70%Fe shows the highest rs-1 and the lowest Ih, i.e. the highest wear resistance among other composites. One can see also that catastrophic wear process of a Fe-based composite starts at higher current density. Catastrophic wear process of Cu+10%Graphite+70%BBS composite based on ball bearing steel (BBS) starts under lower j and lower rs-1 as compared to characteristics of the Cu+10%Gr+70%Fe composite. The Hadfield steel (HS) based composite Cu+10%Gr+70%HS has the lowest contact characteristics. The decrease in both rs-1 and wear resistance corresponds to the increase in the alloy additives amount in the composites. Hence we can draw a conclusion: the catastrophic wear of composites that have more complicated composition is due to the increase of concentration of alloy additives [BBS (Fe+1.5%Cr) and HS (Fe+13%Mn)] starts both at lower current density jc and lower wear resistance (Fig.5, b).

this case. Contact characteristics such as specific surface conductance rs-1=j/U and wear intensity Ih could be considered as indicators of surface destruction. Contact current density j=i/Aa depends on the contact electrical resistance r, which is being determined by the structural state of SL (i is the current that flows through nominal (geometric) contact area Аа). Linear wear intensity Ih=h/L is a main indicator of SL destruction (h – loss of of the block’s height on sliding distance L). Special surface conductivity rs-1 of sintered composites' contact increases within some interval of contact current density j and contact voltage drop U (Fig. 5a).

Wear resistance of the material could be estimated explicitly only in operating conditions. The drilling bits have been fabricated for this estimation. Their journal bearings had initial structure of macroheterogeneous MMC. The ratio and compositions of components are presented in Table 4. Friction conditions in a boreholeare characterized by approximate sliding velocity 0,5 m/c, pressure about 120 MPa, existence of vibration and occasional impact loading. As follows from Table 4, the operability parameters of bits made with granules TiC+Cu+HS are appreciably lower than the same parameters of other bits. This result is in agreement with the above suggestion that too complex chemical and phase composition of initial structure leads to accelerated deterioration of the contact layer during sliding in heavy duty conditions.

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FIG.5. CURRENT DEPENDENCE OF SURFACE ELECTRIC CONDUCTANCE (a) AND WEAR INTENSITY (b) OF CONTACT OF COMPOSITES:: 1 - Cu+10% GRAPHITE +70%Fe; 2 - Cu+10% GRAPHITE +70%BBS; 3 - Cu+10% GRAPHITE +70%HS – 3.

FIG.6. CURRENT DEPENDENCE OF SURFACE ELECTRIC CONDUCTANCE (a) AND WEAR INTENSITY (b) OF CONTACT OF STEELS: 1 - STEEL-3; 2 – BBS; 3 - HSS; 4 - HS

One can assume that the use of simple (noncompositional) material structure would lead to the increase of rs-1, jc and decrease of Ih. To verify this assumption we realized a sliding electric contact of steel-3 (НV=1360 MPa), ball bearing steel (BBS, 63 HRC), high speed steel (HSS, 64 HRC) and Hadfield steel (HS, НV=2430 МПа). As illustrated in Fig. 6 the maximal conductivity of a steel contact corresponds to the rapid growth of wear intensity Ih at current density jc. Figure 6 also shows that high speed steel (HSS), which is composed of several phases could not fit in this regularity owing to high wear at low current density. The conductivity rs-1 of steel-3 contact is higher than rs-1 of other steels (Fig.6, а). Also the contact current density jc is higher at sliding of steel-3 than jc of other steels. This caused by lower special electric resistance ρ and higher heat conductivity λ of steel-3. Wear intensity Ih of steel-3 is lower than Ih of other steels (Fig.1, b). The reason for such a friction behavior of steels could be found from taking in account their chemical composition as follows: steel-3 (>98%Fe), BBS (Fe+1.5%Cr+2% admixture), HSS (Fe+6%W+5%Mo) and HS (Fe+13%Mn). One can see that the increase in concentration of alloying elements and admixture atoms in initial structure leads to realization of catastrophic wear under lower jc.

Composites TiC+Cu+HS can realize satisfactory contact characteristics at friction in oil. It may be marked that satisfactory contact characteristics were observed as well at dry friction of composite WC+HS (Kulkov, 2009). This indicates that the Hadfield steel may be used as a metallic binder of hard alloy in some cases. However such results were obtained at stationary conditions work when sharp changes of pressure and temperature were absent in SL. This leads to stable distribution of elements and phases in the SL structure. The relaxation of temperature and mechanical stresses are occurred at constant speed in this case. But the tracks of adhesion interactions are observed at friction of composites containing Hadfield steel. Deformation of contact spot and SL microvolumes leads to distribution of surface atoms and phases, which differs from the distribution of elements in the initial structure. Hence it may be occasionally formed a structure state of SL which is approximately similar to the SL structure in materials of different initial structure during electric contact. In this case contact characteristics would be roughly equal. Thus contact characteristics of composite Cu+Graphite+Fe are close to those of steel-3 especially in the beginning of catastrophic wear (Fig.5 and Fig.6). It could be suggested, that such a structural state of SL determines the certain limit of contact characteristics for all metallic materials. This limit could be expressed as rs-1<350 S/cm2 и jc <300 A/cm2. Conclusions

Heavy-duty loading of metallic materials by pressure or electric current during sliding leads to plastic deformation and destruction of contact surfaces that could be expressed as Ih>0 μm/km. The surface layer deterioration occurs easier when there exists the increase either in the alloying elements content or

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phases in the initial structure of metallic materials. Therefore, the catastrophic wear begins at lower numerical values of pressure or contact current density under conditions of friction or sliding current collection. Catastrophic wear of surface layer of metallic materials which slide without lubrication under the current occurs at certain limiting contact current density jc <300 A/cm2.

carbide

produced

the

process

combustion

method.“ Journal of Friction and Wear 32, 6 (2011): 462466. Fadin, Viktor V. and Aleutdinova, Marina I. “Effect of the phase composition of steel-based composites on the electrical resistance of the friction zone under conditions of current collection “Russian Physics 52 ,6 (2009): 607611.

ACKNOWLEDGMENT

Study was carried out according to program of Basic researches of Siberian Branch RAS (program III.20.2, project III.20.2.4), as well as supported by Russian Foundation for Basic Researches (projects No. 13-0800076 and No. 13-08-98098).

Fedorchenko, Ivan M., Pugina, Ludmila I. Composition

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