the effect of hydrogen in lubricated frictional couples

ELSEVIER

WEAR Wear 212 ( 19’J7) 199-205

The effect of hydrogen in lubricated frictional couples

Piotr Kula * , Robert Pietrasik, Bogdan Wendler, Krzysztof Jakubowski Technical Universi&y of L&k lnsriture of Materials Engirterring. ul. Stefuntwskiego l/S, 90-924 L&z. Polmd

Received 22 April 1997; rtcccpted 30 June 1997

The tribological experiments under circumstances of hydrodynamic and elastohydrodynamic lubrication. the immersion test in Iubricating

oil as we11 as the exposure test to the environment have been performed to elucidate the effect of hydrogen on sofi and hardened surface layers. The occurrence of the effect of hydrogen tribosrlrption in lubricated frictional nodes has been confirmed. This effect proceeds at a different rate and in a different way for nitrided surface layers (and homologous layers), on the one hand, and for other soft or hardened

surface layers on the other. The most probable mechanisms of the tribosorption for both the cases mentioned above have been presented and the significance of that phenomenon for the theory as well as for practical operation has been discussed. 8 tW/ Ekvirr Science S.A.

1. Introduction

The need for improvement in design and operational char- acteristics of frictional couples has become a prerequisite for theoretical as well as experimental research on optimisation of associations of hardened superficial layers of machine parts with lubricating fluids [ 1,2] _ That subject matter is also an outcome of recent results concerning counteraction to cata- strophic effects of hydrogen wear by means of appropriate diffusion barriers for hydrogen atoms [ 3,4]. It follows from the relevant papers that hard superficial layers produced with the use of conventional as we11 as of modem nitriding procedures proved to be effective barriers to bulk hydrogenation of machine parts of steei and cast iron [5,6 1. Due to elimi-

nation of hydrogen atoms of the crack nucleation process (including that of fatigue type), the resistance of nitrided layers to hydrogen wear as well as to other types of wear (such as, forexample, that of contact fatigue type) is superior in comparison with superficial layers hardened in tk result of martensitic transformation [ 3 3.

The effect of a considerable amount of hydrogen concentrated in the area of the compound zone of the nitrided layers on the adhesive wear is another important cognitive issue. To date a speculative model exists in which hydrogen atoms desorb easily to a friction contact due to the low energy of hydrogen chemisorption to the surface of the nitrided layer and, as the result, the surface friction drag decreases. The

* Corresponding author. Tel.; +48 42 3 12279; fax: +48 42 366790.

CKM3-1648/97/$17.00 8 1997 Ekevier Science S.A. AlI rights reserved PfZSOO43-1648(97)00155-5

model has been verified in a tribological experiment on dry friction. Independently, a set of examples concerning real friction nodes after performance tests gives support to the model [ 7 j , A similar outcome (of low adhesive wear) has been observed in tribological tests of carbon coatings and explained in terms of hydrogen atoms’ release during dry friction [ 8-101.

In this paper the results of investigations of hydrogen influence on iron and steel surface layers after different heat treatment or cementation procedures are presented. The investigations have been performed under the conditions of static effects of the environment as well as during tribologicaf stand tests with hydrodynamic lubrication or elastohydrodynamic lubrication. The analytic techniques used in the experiments are handled in view of their adequacy to the problem under investigation and the results of the research work are discussed with regard to hitherto state-of-the-art knowledge on the effect of hydrogen on friction nodes.

Analytical techniques of determination of hydrogen concentration in metals and relevant measuring apparatus are mostly employed in metallurgy. As a rule, a metallurgical sample is destroyed during analysis due to its melting or annealing at relatively high temperatures. The amount of hydrogen obtained in that case is being refen-ed to the total volume of the sample under investigation without taking into

200 P. Kdu d al. / Weiw 212 (N97) I9SZOS

consideraticrn the eventual heterogeneity of hydrogen distribution. The apparatus LECO-RH2 is an example of the relevant equipment, in which the hydrogen content in the steel sample is determined owing to hydrogen liberation of the melting sample. Although the method has been used to dif- ferentiate between the hydrogen concentration in the surface layer and in the bulk of the samples submitted previously to a nitriding or carburizing process, the necessary procedure involved was rather arduous and the results deduced with the use of rather simplified models [ 31. In the present work the apparatus LECO-RH2 has been used only as a method for referring to the new one elaborated for hydrogen deterrnina- tion in thin surface layers.

In view of the specific character of the friction phenomenon and forecast reversibility of hydrogen translocation in the zone of the surface layer, the experimental techniques should be non-destructive and enable measurements at as Iow a temperature as possibie. Tbe experimental procedure should also enable stoppage of the friction test, quick transfer of the friction sample to the apparatus for determination of hydrogen in the surface layer and its repeated mounting in the friction tester to continue an interrupted test if the hydrogen’s determination has been non-destructive. In this work two new experimental techniques have been elabomted:

I chromatographic analysis of the anode gas: a destructive method enabling determination of the hydrogen concentration in the surface layer of particular ring-shaped samples for tribological tests ( the hydrogen being accumulated in that layer due to line contact friction at the conditions of elastohydrodynamic lubrication); and low temperature X-ray diffractometry: a non-destructive methodenabling estimation of the degreeof hydrogenation of the surface layers of a particular tribological sample shaped as part of a bearing bush (the hydrogenation being due to conformal frictional contrlct under the circumstances of hydrodynamic lubrication).

Independently, a static test has been performed to estimate the effect of water vapour from the environment as well as the effect of waste engine lubricant on the hydrogen concentration in the surface layers of a steel foil after toughening and different thermo-chemical treatments.

3. StaSic test of the effect of the environment

Samples of Cr-M-AI steel grade, 0.2 mm thick, after three different types of treatment have been used fslr determination of the total hydrogen content in variant I; (A) primary sampIes with sorbitic structure; (B) same primary samples (as A) submitted subsequently to sulfonitriding i 111. due to which the following phase composition has been accomplished: 4 approximately 27.5%: Y’~ approx. I. I %; cw, approx. 7 I .4%0; FeS, traces: (C) same primary sampies (as A) submitted subsequently to a long-time nitriding due to which the foIlowing phase

composition has been accomplished: l , approximately 65.1%; y’. approx. 5.2%; U, approx. 29.7%.

A part of those samples was then vacuum annealed for I h at 823 K ta liberate hydrogen of the samples (variant II). Finally, a part of the latter was exposed to the environment at the condition of charging humidity at 298 K for a period of 24 h i variant III ) or for the same period the samples after the II treatment have been immersed into hot engine lubricant at the temperature 393 K (variant IV). The hydrogen content in the samples has been determined with use of LECO-RH2 apparatus. The results together with theirstatisticalevatuation are given in Table I.

One can see frcm the table that after almost thorough hydrogen removal of samples B and C (due to vacuum annealing), its renewed imbibition to a ievel of concentration like that immediately after corresponding thermo-chemical treatment proceeds rather easily, even in wet air. In the difference, after immersion in waste hot engine lubricant, the level of hydrogenation of the steel containing nitride phases is much greater and depends on the volume fraction of those phases. A similar effect has been reported in an earlier paper [ 31 cuncerning the experiments with cathodic hydrogenation of steel samples. In this connection one shoutd emphasise that hydrogen concentration in the compound zone in samples B and C can be estimated in a similar way, as has been done in Ref. [ 31 taking into account the phase compositions of the samples after sulphonitriding or nitriding as well as the corresponding values of hydrogen contents and making use of the model proposed in that paper - it turns out to have a similar level as that in the relevant paper. At the same time and under the same circumstances, :he changes of the hydrogen content in the primary sampies with an isotropic sorbitic structure are rather insignificant and in all likelihood they do not concern a thin surface layer but rather the total volume of the specimens.

The rather light hydrogen imbibition or renewal of its primary content from operational materials (as, for example, lubricating oils) or the environment observed in the work can be, in the authors’ opinion, an important contribution to elu- cidation of the well known phenomenon of low friction in frictiorlal couples in which nitrided or sulphonitrided steel elements occur. It is therefore not very probable that the hydrogen liberation due to ‘thermal spikes’ generated in the frictioncontact is a short-term phenomenon whichtakesplace unless the hydrogen accumulated in the surface layer is exhausted (as has been stipulated in Ref. [ 7 ] ). In contrast to that it is rather probabIe that the hydrogen liberation in Lhe frictional contact with such elements is a long-lasting phenomenon characteristic for appropriate structures of the surface layers of relevant frictional nodes.

4. Tribological test with elastohydroclynamic lubrication

The tests have been performed with use of aFALEX block- on-ring type tribometer with line-shaped friction contact. A

P. Ktriu et ul. / Wear 2 I2 (1 W7) 1 N-205 201

Mean Hydrogen conkIts C, (in ppm) in 0.2 mm thick samples uf the steel grade 38HMJ and carresponding confidence interYak (at confidence level 0.95)

Variam. no. of additioilal treatment

Primary sample stntcturc --

A

0

C

I (samples to refer to

without any additional treatment I _.

I’;, Confidence

interval .-.

2.2: 0.75-3.7 I 19.1 I 1 I m-27.13

27.47 26.27-28.67

II (vtii;mr I with !I! (variant 11 with additional 1 h vacuu~n subsequent exposure for annealing sit 550°C ! 24 h to wet air)

G+ Confidence Cl, Contidericc interval interval

I.11 o.%- 1.67 1.19 1.01-1.36 1.52 O.&l-K!0 13.72 7.50-19.95 0.95 o.o--2.14 X.75 2 I .49-28.0 I

IV (viu-iant II with subsequent immersion for 24hinhotoil)

CH Confidence

intewal

1.42 0.74-2.09 34.32 29.79-38.85

65.m! 5 t .67-78.33

Fig. 1, Scheme of triboneter FALEX used for friction tests.

scheme of kinematics and thatofloading of&he ftictioncoup!e is presented in Fig. I.

Friction tests have been performed throughuzt 2 1: at constant normal luad 15 N and constant rotational speed 150 rpm with the use of warm engine commercial lubricant TEDEX 15W/40 at constant temperature 343 K. Immediately after completion of any friction test the ‘hydrogen concentration in the surface layer of the sample has been determined by means of its electrolytic dissolutrcn and subsequent chromatographic analysis of accumulaied anode gas. A scheme of the electrolyzer used for anodic etching of the samples is presented in Fig. 2. ln Table 2 the results of chromatographic analysis for both annealed and nitrided mild steel without any supplementary treatment, aftercathodic charging in hydrogen (that latter corresponds to the effect of charging with hydrogen in aggressive hydrogen containing media) as we11 asafter friction tests under the circumstances of elastohydrodynamic lubrication.

I. plalinum electrode 2. curhodic area 3. Schatt trjher 3. cone ma&e/ for specimen mounting 5. specimen

6. graduated pipette

7. needIc fir gas sampling 8. shvt-oJrvalve

The hydrogen volume that has been liberated during anodic dissolution and measured by means of the chromatographic method as well as the corresponding mass of hydrogen, has been referred to the mass of the dissolved layer as well as to !$e total mass of the specimen. 1n the case of primary specimens (only after annealing), more reliable results have been obtained referring the volume and the mass of hydrogen to the total mass of the samples. The reason for this is that in an annealed steel there are no important btiers for hydrogen diffusion. Therefore, hydrogen diffuses rather unconstrained in the whoIe sample volume at a temperature near to ambient during cathodic charging and friction tests as well as during anodic dissolution (in that latter case, however, the direction of the diffusion is rather opposite to that in the two former cases).

Taking into considersltion the results for nitrided mild steel, the biocking effect to hydrogen diffusion into the butk substrate due to the nitrided surface layer has been confirmed explicitly as well as hydrogen agglomeration in a greater amount in the area of the nitrided layer. In that case, more reliable values of concentrations have been obtained when referring the mass of hydrogen liberated from the samples under investigation to the mass of the dissolved layers. Ihe hydrogen concentrations determined in that way correspond well with other results obtained in other work [ 3j as well as with the results of calculations presented in Section 3.

For both groups of samples of the mild steel under investigation, an intense hydrogen imbibition into the steel during

I

2

t

Fig. 2. Scheme of the electrolytic unit wed for anodic etching of surface layers of the samples used in tribological tests.

202 F. Kulu em al. /Wear 212 (19!J7) I9!L205

Table 2 The results of the chromatographic analysis of rhe and gas lilxvuted during ekctmlytic dissolution of the sampler of mild steel submitted to different treaUnents

Volume of nitrogen Volume of hydrogen Mass of hydrogen Hydrogen concentration Hydrogen concenrratIon gas contained in the gas contained in the liberated to anode in the surfutx layer in the surface layer anode gas anode gas area referred to the total mass referred to decrement of

(ml) (ml) !cLI of specimen the specimen’s mass

Wpm) @pm)

Steel immediately after 1.630 4.089 365.09 16.59 332.49 annealing ( prinxuy -1 Annealed steel after 2.800 7.055 629.9 I 28.63 488.68 cathodic charging with

hymen hakd steet after 0.869 7.07 1 631.31 28.69 469.37 friction test Steel immediately after 0.488 0.272 24.29 I.10 2Q.W nitriding Nibickd s-1 after 4,330 2.840 253.57 t I.52 138.34 cathodic cha.@g with

hytin Nitrikcl aeel after 12.738 I.283 114.55 5.21 165.73 frktio?? test

friction under circumstances of elastohydrodynamic lubrication has been ascertained due to substantial contact stress concentration. The hydrogen concentration in the samples for friction tests (or, eventually, in their surface layers) has incrtased after two hours of friction testing to a level corresponding to that after cathodic charging with hydrogen for the same material. That result is a full corroboration to the role and place of the theory of hydrogen wear in modern tribology as well as in the theory of machine operation.

5. Tribobg&d experiment pedomed under ds of hydrod~ hIbricatloIl

The tests have been performed using the same tribometer FALEX (Fig. I ) and particular specimtzns to it (Fig. 3) in the form of a piece of a bearing bush mounted on the driving shaft of the friction tester. That particular shape afforded the possibility of performing friction tests under the circumstances of hydrodynamic lubrication as well as subsequent low-temperature X-ray diffractometric analysis of the specimen’s structure. The friction tests have been performed each time in the space of 4 h under the circumstances of hydrodynamic lubrication using engine lubricant TEDEX

I= 12 .

Fig. 3. Shape and dimensions of specimens for friction tests.

l§W/40 at a temperature stabilised at 343 K. Sampfes of ingot iron as-delivered as well as after long term vacuum nitriding have been submitted to testing [5,6J _

X-ray diffractometric investigations have been performed with the use of Siemens D-500 diffractometer unit and of an Anton Paar low temperature ?TK camera equipped with a micropmessor temperature controller combined with a Pt 100 temperature sensitive detector, electric microheater and programmer of liquid nitrogen flow through the sample holder. Monochromatic Co Ka radiation has been selected owing to a Siemens graphite monochromator. After prlmary analysis in a rather wide angle range, some of the diffraction lines have been selected for their subsequent precise step- scanning with steps A (2@) = 0.02” and constant counting time 60 s for each step. When X-ray analysing, the sample temperature was maintained at a constant value of 173 K with precision *O. 1 K. Owing to use of the low temperature X- ray camera it became possible to cool the sample enough immediately after having finished friction tests and to main- tain the hydrogen concentration in its surface layer during the time necessary for performing the X-ray diffractometric measurements. To avoid water, steam and carbon dioxide forming into drops on the specimen’s surface the camera has been evacuated to a value of residual pressure below IO-” hPa.

The structural effects of hydrogen accumulation in the surface layers of specimens during fluid friction have been compared to those on X-ray diffractograms of specimens submitted to cathodic charging with hydrogen. First. during primary analysis, the diffraction lines were selected forwhich the changes linked to hydrogen translocation in the area of the friction contact when performing mm-destructive friction tests were the best-marked [ t 21. It turned out that for

203

(4 afict dldic charging

with hydrogen --

k j(b)

after fluid frition test

1 d?

161 162 163 i64 165 161 162 163 164 16S

28 Fig. 4. The effects of cathodic charging with hydrogen (a) and of fluid friction (II) on the shays ad position of the diffraction doublet (310),,.,L.

annealed iron and mild steel the most convenient were the high-angle diffraction lines of the cr-iron whereas for the nitrided layers the diffraction lines of the hexagonal +-nitride were more convenient. In Fig. 4(a) the shape and position of the diffraction doublet (31Q)q,+ recorded from the surface of the specimens of the ingot iron as delivered are compared with those for same doublet recorded from the specimen after cathodic charging with hydrogen. A rather distinct change of shape and position of the doublet due to the effect of phase work-hardening in the result of hydrogen atom displacement into interstices is apparent. That comparison gives a rather good diffractometric standard of the effect of cathodic charging with hydrogen. Taking this into accaunt one can say that the effect of hydrogen imbibition into ingot iron when performing friction tests under circumstances of hydrodynamic lubrication is negligible (Fig. 4(b) 1. Further- more, inalterability of the shape of diffraction lines of a-iron after fiuid friction tests is an indication of due separation of both surfaces influencing no contact with each other in the friction contact across a thin oil film. In the result of that contactless interaction there was in all likelihood no plastic deformation of any surface irregularities.

The results of similar analysis performed for nittided ingot iron are presented in Fig. 5, where the ( 101) diffraction line of the +-phase in the samples cooled down to 173 K is under investigation. The changes of shape and position of that line after fluid friction test (Fig. 5(b) ) are appreciable, however not as distinct as those in Fig. 5(a) obtained for the sample after cathodic charging with hydrogen used as the reference standard. That remark corroborates the stipulated phenomenon of physico-chemical interaction between &on nitride and the lubricated film in the case under consideration where the surface irregularities of the specimen and those of the counter-specimen in the friction node are not in dire&contact. It has been found in Ref. [ I2 1, after some precise diffractometric measurements, that the shape changes of diffraction lines of the e-phase, either due to cathodic char&g in bydro- gen or to tribochemical hydrogenation of surfxe layers, are rather systematised and indicative of the creation of a new phase of hexagonal structure, in all likelihood a hydride phase, Making use of computer analysis of the shape of diffraction lines, the separation of lines has been calculated and the parameters of elementary ccllsof the separate phases have been determined. At the temperature 173 K the values of

Fig. 5. Compaison of the effect of the cathodic charging with hydrogen (a) and that of the fluid friction test (b) on the shape and position of the l101 j

diffraction line of the E-phase.

P. K14h et al. / Wear 212 (1997) 19% 205

those parameters were as follows: u = 0.27 I6 nm, c = 0.4424 nm for &on nitride and a = 0.273 1 nm, c = 0.4424 nm for the new phase which is being created as the result of charging with hydrogen.

6. Di!5cUs&.mottheresults

The results of different investigations presented in the work point out the complicated character of lubricating fluid’s influence on the surface layers of specimens in the friction node. A distinguishing mark of that complexity is the phenomenon of hydrogen atom transfer from hydrocarbon mol- ecules in lubricating fluids to surface layers of lubricated machine parts which fallows a different course for different structures (solid solutions or interstitial phases) in hardened surface layers as well as in tough materials.

Let us take into consideration the former case concerning the influence of the lubricated fluids (and that of the environment) on the hardened surface layers created by means of different technological modes of nitriding (or other processes similar to it). It has been proved in fotmer papers that, due to the specific structure of the surface layers created as a result of the processes, the layers turn out to be effective barriers to hydrogen atom diffusion in depth from the specimen’s surface and, therefore, a considerable amount of hydrogen atoms are being accumulated in a rather thin surface layercontaining nitride phases [ 4,121. The new experimental results obtained in the present work not only corroborate those particular features of the nit&led layers but also supplement their scope as well as tbe structural description of hydrogen’s influence on iron nitrides.

In the result of low-temperature X-ray investigations of nittided surface layers on iron it has been pointed out that a greater concentration of hydrogen atoms is not uniform in those layers and that it concerns only that part of the E-iron nitride sublayer where a phase transformation took place due to which a new phase has been created (in aH likelihood of hydride type) with slightly greater hexagonal ceil parameters than those of the primary qhase. Though that statement is in agreement with the hitherto existing theory of hydrogen’s distribution in the nitrided layers [ 121, it indicates however that a discussion should be undertaken concerning real depth of effectively hydrogenized zone as well as the value of hydrogen concentration in the zone. It should be supposed that the real hydrogen concentration in a part of the nitrided layer closer to the layer’s surface is several times greater than that estimated in the hitherto theoretical papers as well as experimental ones [ 121.

The hexagonal hydride phase in the surface layer of nitrided, sulphonitridcd or cyanide treated machine parts is in all likelihood thermodynamically stable under circumstances of working conditions. It also occurs as the direct result of a technological process and very easily as an effect of a secondary hydrogenation from lubricating oils or the environment (only if it has dissociated earlier due to vacuum

mnealing) , Restoration of the hydride phase in the compound zone has been observed in all the experiments described. It occurs irrespectively (or not) of the frictional contact arising and, therefore, it points out the entirely physico-chemical nature of the phenomenon.

The processes of decomposition and restoration of the hydride phase following in cyclic succession in dry or lubricated friction nodes give a contribution to the explanation of low friction obsemed fortbe &on nitride sublayerin nitrided surface layers of machine parts making use of the effect of ‘self-lubrication’ by hydrogen presented prcviausly [ 3,7]. The occurrence of the hydride phase on the surface and its physico-chemical interaction with hydrocarbons explains, at least in part, the high stability of the oil boundary layer at the nitrided or sulphonitrided surfaces in difference to that of the boundary layers at other substrates. That difference has been revealed during tribological tests in which the length of time of electrical microdisconnection as well as the frictional contact resistance have been measured [ 13 ] +

For iron and annealed or toughened steel the effect of hydrogen in the lubricated frictional nodes has been confirmed only in the case of etastohydrodynamic lubrication. Under the circumstances of a strenuous contact hydrogen tribosorption occurs across the surface layer, which has been confirmed making use of the results of the chromatographic analysis of the anode gas.

The mechanism of the hydrogen’s tribosorption across the iron and soft steel under the circumstances of cathiic polar- isation in acidic solutions has been presented earlier in a series of papers C. 8,141. It is assumed in these works that the hydrogen atom transport takes place along dislocations which are being set in motion in microregions of the surface layer submitted to a plastic deformation under circumstances of considerable supply of hydrogen atoms and ions into the surface

layer. If the mechanism of hydrogen transport in the depth of the

substrate in lubricated frictional nodes is rathtir similar to the one mentioned above, the explanation of the manner of the hydrogen atoms’ gain from hydrocarbons is more difficult. Very likely a weakening of hydrogen bonding in hydrocarbons takes place due to their absorption on the frictional surface and the thermal spikes occurring in the result of plastic work and shearing of the asperities in the zone of the elastohydrodynamic contact promote the hydrocarbon’s dissociation. A similar phenomenon has been named *dissociation by chemisorption’ in the literature [ 141. Therefore the mechanism of hydrogenation of soft steel is in all likelihood a superposition of a number of phenomena in the field of micromechanics and physicochemistry of the frictional contact. In design, selection of materials as well as optimisation of the service conditions of the frictional nodes working under circumstances of the elastohydrodynamic lubrication, one should take into consideration the possibiiity of hydrogen tribosorption in order to avoid brittleness of the surface layer as well as nucleation and propagation of fatigue cracks.

205

Conclusions References 7,

I.

2

3.

4.

5.

6.

The effect of hydrogen on lubricated friction;al couples proceeds in a different way in the case of nitrided surface layers (and homologous to rhose) on rhe one hand. rind in the case of other soft or hardened surface layers on the other. Hydrogenation of nitrided elements is confined rather to a thin e-iron nitride subiayer in which a particular hydride phase of hexagona structure is being formed. The hexagonal phase rich in hydrogen is in al1 likelihood thermodynamically stable under the circumstances of the service conditions and undergoes everlasting restoration under circumstances of lubricating oi!s or environment even without the necessity of direct frictional conract, The cyclic succession of decomposition and restoration processes of the hydride phase in dry or lubricated fric- tiona: nodes well explains the phenomenon of low friction occurring for the e-iron nitride sublayer in nitrided machine elements if one takes into consideration the effect of ‘self-lubtication’ by hydrogen reported in the earlier

paper I71 - The tribosorption of hydrogen from oils to soft or toughened surface layers on steeis proceeds enrirely in a zone of a concentrated contact under circumstances nf elastohydrodynamic lubrication. Hydrogenation of soft or toughened surface layers on steels results rather from a superposition of a number of phenomena in the tield of micromechanics and physicochemistry of frictional contact. One that should be pointed out is in all likelihood the hydrogen atoms’ imbibition from hydrocarbons resulting from their dissociation by chemisorption under circumstances of repeating thermal spikes being generated in the zone of asperities’ contact.

I t 1 V.A, Belyi. D,N, Garkunov. 1.V. Kragielski, A.A. Polyakav, Proc. Int. Conf. on Wear of Materials, St. J_.ouis. Ml. April 1977.

131 G.P. Szpierikow. Physicffhemistry of Friction. Miiisk. Uniwcrsitctskojc. 1991.

[ 3 J P, Kula. Wear 178 ( 1994) Il7-121.

! 31 T. Zukroczymski. N. Luknmski. J. Flis. J. Electrochem. Sot.. 140

( 1993) 12.

[ 5 I 2. Ha<. Europcan patent No, 34761. [ 6 1 2. Ha<. P. Kuta. Proc. 11 Internarioaal Conference on Grbutizing and

Nitridina with Atmospheres. Cleveland, OH. &8 December 1995 pp.

227-23 I * I 7 ] F’. Kula. Wcu 20 I ( 1996) 155-162.

18 J S.J. Pyun. J.S. Kim, Steel research. Materials Technology 62 ( 1991)

314319.

I91 C.H. Kim, S.I. Pyun. Journal of Mater. Sci. ( I991 1,

I IO] S.I. Pyun. J.S. Hyun,J.S. Kim, Mater, Sci. Eng. ( I*1 ).

1 I I 1 Z. Hz& P. Kula, III inrmationalTribology CongressEUROTRIB’ll I. Warszawa. Vol. IV/A. pp. 125-135.

I 121 I? Kuln. Z. Gawroriski, K. Jakubowski. R. Pietrsik. iniynieria

Matcrialowa. in press, ( 1997 I ( in Polish1 [abstract in English 1.

I 13 1 P. Kula. R. Pietrasik, Mechanical Engineering 85 ( 1996) 85-92 (in

Polish I i abstrxt in EngIishl.

I 14 I E. Lnarska. Noyzs Publications. Park Ridge, NJ. 1985. pp. 32 l-329.

Biographies

Piotr Kula is associate professor in the lnsrituteof Materials Engineering and Chipless Technology in the Technical Uni- versity of E&ii, Poland. He is a specialist on surface engineering as well as tribology and also he is coauthor of successful modern technological processes for making of hardened sutidce !ayers.

Dr Bogdan Wendler and engineers Krtysztof Jakubowski and Robert Pietrasik zlre scientific workers in the Institute as well.

the effect of hydrogen in lubricated frictional couples

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