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Wear 268 (2010) 464–472

Contents lists available at ScienceDirect

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echanical and tribological properties of rubber blends composed of HNBRnd in situ produced polyurethane

. Karger-Kocsisa,b,∗, D. Felhösc,d, D. Xuc

Department of Polymer Technology, Faculty of Engineering and Built Environment, Tshwane University of Technology, Pretoria 0001, South AfricaDepartment of Polymer Engineering, Budapest University of Technology and Economics, H-1111 Budapest, HungaryInstitut für Verbundwerkstoffe GmbH (Institute for Composite Materials), Kaiserslautern University of Technology, D-67663 Kaiserslautern, GermanyDepartment of Polymer Engineering, Faculty of Materials Science Engineering, University of Miskolc, H-3515, Hungary

r t i c l e i n f o

rticle history:eceived 19 December 2008eceived in revised form 14 August 2009ccepted 25 August 2009vailable online 1 September 2009

eywords:

a b s t r a c t

A peroxide curable hydrogenated nitrile rubber (HNBR) was modified by in situ produced polyurethane(PU), added in 50 and 100 parts per hundred rubber (phr). The crosslinked PU was formed from solid PUprecursors, viz. polyol and blocked polyisocyanate, during curing of the HNBR rubber.

Dynamic mechanical thermal analysis (DMTA) revealed that HNBR formed the continuous whereasPU the dispersed phase. Mechanical properties (hardness, tensile modulus, ultimate tensile strength andstrain, tear strength) of the HNBR and HNBR + PU were determined and collated. Tribological properties

liding wearolling wearubberNBRolyurethane

were investigated in pin (steel)-on-plate (rubber) (POP), roller (steel)-on-plate (rubber) (ROP), oscillatingcylinder (steel)-on-rubber plate (Fretting) and rolling ball-(steel)-on-plate (rubber) (RBOP) test configu-rations. Coefficient of friction (COF) and specific wear rate of the HNBR/PU systems were determined. Itwas found that the resistance to wear increased with PU hybridization. On the other hand, the COF didnot change much with the PU content. The friction and wear characteristics strongly depended on the test

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locked polyisocyanate configurations. The worn(SEM) and typical wear m

. Introduction

Polyurethane (PU) is likely the most versatile polymer withespect to preparation methods, properties and application. Amongts properties especially the high resistances to wear and to chem-cals has to be emphasized. Blending is a well established methodo obtain new rubbers with improved properties. Interestingly,nly few works addressed the blending of various rubbers withU. This may be linked with the fact that incorporation of liq-id PU precursors, viz. polyol and polyisocyanate, in rubbers andheir subsequent crosslinking can hardly be achieved. CrosslinkedU processing waste after hygrothermal decomposition proved toe a suitable polymeric extenders for different rubbers, like natu-al (NR) [1], styrene–butadiene (SBR) [2], acrylonitrile–butadieneNBR) [1,3], and polychloroprene (CR) [4] rubbers. Dhamodharant al. [5] combined thermoplastic PU (TPU) with peroxide cur-

ble ethylene–propylene–diene (EPDM) rubber with and withoutaleation. The compatibility between the TPU and EPDM wasarkedly improved by using maleated EPDM. This was associatedith a pronounced improvement in the mechanical properties. TPU

∗ Corresponding author at: Department of Polymer Engineering, Budapest Uni-ersity of Technology and Economics, H-1111 Budapest, Hungary.

E-mail address: karger@pt.bme.hu (J. Karger-Kocsis).

043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2009.08.037

ce of the HNBR systems was inspected in scanning electron microscopenisms concluded.

© 2009 Elsevier B.V. All rights reserved.

has been modified with EPDM (less than 20 wt.%) by Tan et al. [6]who observed a prominent improvement in the mechanical prop-erties and in the abrasion resistance. Wang and Luo [7] followed asimilar strategy. TPU with 8 wt.% EPDM exhibited improved meltprocessability and mechanical properties compared to the neatTPU, especially when maleated polyolefin was added as compatibi-lizer. Wu et al. [8] demonstrated that the morphology of the rubbersystems highly depends on the processing conditions at a fixedTPU/EPDM (major/minor phase) ratio. Sebenik et al. [9] producedan unsaturated TPU to upgrade the properties of NR/polybutadiene(BR) rubber blends via co-crosslinking mechanisms.

There is, however, a further option to combine traditional rub-bers with PUs. PU precursors are namely available also in solidform: crystalline polyols and blocked polyisocyanates. Above thedeblocking temperature of polyisocyanate a crosslinked PU mayform if the functionalities of the polyol and polyisocyanate arehigher than two. The deblocking temperature of the polyisocyanatemay be below that of the usual curing temperature of rubbers whichis in favor of this rubber/PU blending strategy. Solid precursors ofin situ crosslinkable PU are marketed mostly as adhesives, glues.

The aim of this work was to check the feasibility of the aboveblending route, i.e. to produce rubber/PU blends using solid insitu crosslinkable PU precursors. In situ preparation of the PUmeans that deblocking of the blocked polyisocyanate along withthe PU formation via polyaddition reactions take place parallel

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ith that of the rubber curing. As rubber component hydrogenatedcrylonitrile–butadiene rubber (HNBR) was selected owing to itsolarity and high temperature resistance. A further aim of this studyas to determine the mechanical and tribological properties (underry sliding and rolling wear conditions) of the related blends. Theesistance to sliding and rolling wear was determined in differentribotest set-ups.

. Experimental

.1. Materials

As PU precursor Collano HCM 555-D (particle size <200 �m;ollano AG, Sempach-Station, Switzerland) was used which is mar-eted as rubbery, one-component hot melt PU adhesive. Accordingo differential scanning calorimetric (DSC) results (cf. Fig. 1) this

aterial shows a melting peak at T ≈ 60 ◦C which is due to the polyolomponent. This endothermic peak is followed by an exothermicne at T ≈ 130 ◦C representing the deblocking of the polyisocyanatend reaction between the –OH and –NCO groups. During coolinghe polyol segments built in the cured PU crystallize at T ≈ 10 ◦C.uring subsequent reheating the DSC trace exhibits the glass tran-

ition temperature (Tg ≈ −50 ◦C) of the PU and the melting of itsolyol soft segments at T ≈ 50 ◦C (cf. Fig. 1).

The composition of the peroxide curable HNBR was theollowing: HNBR (Therban® LT VP/KA 8882 of Lanxess, Lev-rkusen, Germany; acrylonitrile content: 21%, Mooney viscos-ty ML(1 + 4)100 ◦C = 74): 100 part; diphenylamine-based ther-

ostabilizer (Luvomaxx CDPA of Lehmann & Voss, Hamburg,ermany): 1.1 part; zinc-containing mercapto-benzimidazoleompound (Vulcanox® ZMB 2/C5 of Lanxess): 0.4 part; di(t-utylperoxyisopropyl) benzene (Perkadox 14-40 B-PD of Akzo-obel, Düren, Germany; active peroxide content: 40%): 7.5 part;gO: 2 part, triallyl isocyanurate: 1.5 part; ZnO: 2 part. This mixas produced separately and provided by Lanxess. The curing

ime of this base mix to reach 90% crosslinking was ca. 10 min at= 175 ◦C.

This peroxide curable HNBR was mixed with 50 (ca. 30 wt.%)nd 100 (ca. 45 wt.%) parts per hundred rubber (phr; based onhe HNBR) PU precursor on a two-roll mixing mill (LRM-150BE ofabtech, Bangkok, Thailand) at ca. 30 ◦C by setting a friction ratio of.15.

Curing of the HNBR both in the presence and absence of the PUrecursors occurred at T = 175 ◦C for 10 min in a laboratory press.ote that these conditions agree with those needed to reach opti-um cure of the HNBR. For comparison purpose plain PU sheetsere also produced under identical curing conditions from the Col-

ig. 1. DSC traces of the applied PU material during heating, cooling and reheating.

268 (2010) 464–472 465

lano HCM-555D powder. Specimens for the investigations listedbelow were cut/punched from the compression molded sheets ofca. 2 mm thickness.

2.2. Phase structure

To get some insight in the structure of the HNBR/PU the relatedhybrids subjected to dynamic mechanical thermal analysis (DMTA).

DMTA spectra were recorded on rectangular specimens(length × width × thickness = 20 × 10 × ca. 2 mm3) in tension modeas a function of temperature (from −100 ◦C to +100 ◦C) and a fre-quency of 10 Hz using a Q800 device of TA Instruments (New Castle,DE, USA). Tests were run at a static preload 0.01 N with a superim-posed sinusoidal 0.01% strain. Heating of the specimens occurredin a stepwise manner (temperature steps of 5 ◦C were followed byan equilibration period of 3 min at each temperature).

2.3. Property determination

For the density determination the Archimedes principle (buoy-ancy method with water) was adopted according to the ISO 1183.The Shore hardness of the rubbers was determined according to ISO868 using a hardness measuring device of Zwick (Ulm, Germany).The Martens (earlier termed to universal) hardness (HM) was deter-mined by the ISO 14577-1 standard using a Shimadzu DUH 202device with a Vickers-type diamond indenter. The maximum forceand its holding time were 5 mN and 2 s (s), respectively, duringthe Martens hardness test. Tensile tests were carried out on 2 mmthick dumbbells (type: S1 according to DIN 53504) on a Zwick 1445(Ulm, Germany) universal testing machine at a deformation rate of500 mm/min. From the related stress–strain curves apart from theultimate properties, the stress values at 100% and 200% elongations(termed M-100 and M-200, respectively) were also read followingthe recommendations of (ISO 378). To determine the tear strengththe recommendation of the ISO 34-1 standard was followed, i.e.angle-type notched specimens were tested at 500 mm/min defor-mation rate.

2.4. Sliding and rolling friction and wear

Sliding friction and wear characteristics were determined inpin (steel)-on-plate (rubber) (POP) configuration using a Wazaudevice (Berlin, Germany), in which a steel pin (100Cr6; arithmeticalroughness, Ra, less than 1 �m) with a hemispherical tip of 10 mmdiameter rotated along a circular path (diameter: 33 mm). The pinwas pushed against the rubber plate with a given load. The fol-lowing parameters were selected for this configuration—normalload was <3 N (varied), sliding speed: 250 mm/s, duration: 1.5 h (h).Measuring both the normal and the friction force components viaa torque load cell the COF values were calculated and monitoredduring the test.

A further sliding test, termed roller (steel)-on-plate (rubber)(ROP), was also used. A rotating steel roller (9SMnPb28k, diam-eter: 10 mm, width: 20 mm, Ra ≈ 0.9 �m) was pressed against arubber strip of 9 mm width in a SOP 3000 device (Dr Tillwich GmbH,Horb-Ahldorf, Germany). The frictional force induced by the torquewas measured online and thus the COF was registered during thetest. The test parameters were—load: <4 N (varied); sliding speed:250 mm/s; duration: max. 1.5 h.

In the fretting test a steel cylinder was oscillating on the surfaceof the fixed rubber specimen. The cylinder was pressed against the

rubber with a given load. The diameter and the contact length ofthe cylinder (Ra ≈ 0.9 �m) were 15 and 12 mm, respectively. Theapplied normal load was <30 N (varied), the frequency of the oscilla-tion was set for 10 or 20 Hz (varied), the stroke selected was <5 mm(varied), and the duration of the measurements was <3 h (varied).

466 J. Karger-Kocsis et al. / Wear 268 (2010) 464–472

F plate (rubber) (POP), roller (steel)-on-plate (rubber) (ROP), oscillating steel cylinder onr

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point of the PU components at +50 ◦C. The more PU is incorporatedin the HNBR rubber the higher the storage modulus beyond the Tg ofthe HNBR is. This means that PU acts like an active filler (reinforce-ment). Important to note, that over +50 ◦C the storage modulus of

ig. 2. Schematic set-up of the tribotesting devices. Designations: pin (steel)-on-ubber plate (Fretting) and rolling ball (steel)-on-plate (rubber) (RBOP).

The rolling friction and wear behaviors of the HNBR/PU blendsere investigated in a home-built machine. On a fixed plate onhich the rubber specimen was glued, a steel ball was rolling alongcircular path (rolling ball (steel)-on-plate (rubber) (RBOP). The

all was driven by a rotating steel plate having a guiding groovehen pressed again the rubber sheet by a given load. The aver-

ge diameters of the circular path and rolling ball (100Cr6 steel;a ≈ 0.9 �m) were 33 and 14 mm, respectively. The applied loadas 90 N, the speed fixed at 252 mm/s, and the duration of the testas 3 h. The evolved torque was measured and the COF determineduring the measurements.

The test configurations used are depicted schematically in Fig. 2.The specific wear rates were determined by Eq. (1):

s = �V

FL(1)

here �V is the volume loss, F is the normal force and L is theverall sliding/rolling distance.

The volume loss was recorded by surface scans of the wearracks, using a white light profilometer (FRT Fries Research Tech-ology, Bergisch Gladbach, Germany). The width and the depth ofhe wear tracks were determined from the surface scans. The shapef the cross-section of the wear tracks was assumed to be an idealalf ellipse when calculating the worn volume. (The depth of theear track is the half of the minor axis, while the width of the wear

rack is the major axis of the ellipse.)Worn surfaces of the specimens were inspected in a scanning

lectron microscope (SEM; JSM5400 of Jeol, Tokyo, Japan). Prioro SEM investigation the specimens were sputtered with an Au/Pdlloy using a device of Balzers (Lichtenstein).

. Results and discussion

.1. Phase structure

Figs. 3 and 4 display the DMTA traces (storage modulus, E′, andoss factor, tan ı, as a function of temperature) for the HNBR, PUnd their blends. PU has a glass transition temperature (Tg) at ca.50 ◦C, and a sharp transition related to the melting of the soft

egments at T = 50 ◦C. Note, that this was well reflected in the DSC

urves of the PU, as well (cf. Fig. 1). By contrast, the HNBR has a Tg ofa. −25 ◦C. The DMTA traces of the HNBR/PU exhibit characteristicsf incompatible blends. Interestingly, the melting of the soft phasef the PU was markedly reduced in the HNBR/PU formations. Thisay be an effect of dipole interactions between –CN(HNBR) and

Fig. 3. Storage modulus versus temperature traces for the HNBR, PU and HNBR + PUhybrids.

–OH(PU) disturbing the crystallization of the polyol component inthe PU.

The storage moduli below the Tg of PU are in the same range forthe investigated materials. The storage moduli versus temperaturecurves of the HNBR/PU hybrids reflect both Tgs of the HNBR and ofthe PU (−25 ◦C and at −50 ◦C, respectively), as well as the melting

Fig. 4. Loss factor versus temperature traces for the HNBR, PU and HNBR + PUhybrids.

Wear 268 (2010) 464–472 467

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he HNBR + PU hybrids is smaller than that of the neat HNBR. Theeason may be that the melted PU segments have no load bearingapacity and thus “weaken” the HNBR matrix. At this transition theobility of the PU segments increases, resulting in a decrease in

he storage modulus (and increase in the loss factor as it is to see inig. 4). The DMTA response suggests that HNBR is the continuoushase (matrix) even in the hybrid with 100 phr PU content.

In Fig. 4 the loss factor versus temperature traces from the DMTAests are collated for the materials studied. As expected the highesteak is to observe at the Tg (−25 ◦C) of the neat HNBR rubber. Theecrease of the loss factor peak at the Tg of HNBR with increasing PUontent is in accord with the expectations. In the loss factor versusemperature curve of the neat PU one can observe two humps athe two already mentioned transitions of the PU (cf. Fig. 3 at −50 ◦Cnd at +50 ◦C).

In order to characterize the crosslinked structure one can definehe mean molecular mass between the crosslinks (Mc). Accordingo the rubber elasticity theory the inverse of the plateau modulus1/Epl) at temperatures above the Tg correlates with the Mc:

c = 3�RT

Epl(2)

here Epl is the plateau modulus, � is the density, R is the universalas constant (8.314 J/(K mol)), and T is the absolute temperature. Inhe present context Epl data are reported as read at T = 293 K.

Alternatively, the apparent crosslink density (�c) can be consid-red:

c = �

Mc(3)

It has to be emphasized that both Mc and �c are apparent values.hey reflect not only the chemical crosslinking (which is likely con-tant for the HNBR recipes) but also different interactions between

he components of the hybrid system. The network-related data areummarized in Table 1. One can see that with increasing PU contenthe apparent Mc decreases and �c increases. It is noteworthy thatbove T = 50 ◦C different ranking would appear with respect to therosslink density parameters.

able 1echanical and tribological data of the HNBR rubbers with and without PU.

Property [unit] Composition

HNBR

Mc [g/mol] 1931�c [mol/m3] 5.47E+02tan ı at Tg = −25 ◦C [–] 1.36Density [g/cm3] 1.057Shore A [◦] 42HM [MPa] 1.45M-0.01 DMTA [MPa] 4.0M-100 [MPa] 1.3M-200 [MPa] 2.7Tensile strength [MPa] 4.4Tensile strain [%] 280Tear strength [kN/m] 4.2

POP COF steady-state [–] 1.00E+00Specific wear rate [mm3/N m] 1.13E−01

ROP COF steady-state [–] 2.95E+00Specific wear rate [mm3/N m] 3.79E−02

Fretting COF steady-state [–] 9.00E−01Specific wear rate [mm3/N m] 1.50E−03

RBOP COF steady-state [–] 3.38E−02Specific wear rate [mm3/N m] 3.26E−04

a The Shore D hardness value; M-0.01 represents the stiffness at T = 293 K read from th

Fig. 5. Tensile stress versus strain curves for the investigated rubbers.

3.2. Hardness and mechanical properties

The hardness values (Martens, Shore) increased with increas-ing PU contents—cf. Table 1. A similar tendency was observed forthe moduli, derived from the DMTA and static tensile tests, respec-tively.

The stress–strain behavior of HNBR was also improved byadding PU (cf. Fig. 5). The prominent strain to failure of the neatPU is obvious based on the related stress versus strain curves (cf.Fig. 5). At low tensile strain (<∼40%) the PU behaves like an elas-tic material. After reaching a threshold stress value the materialundergoes a yielding process (∼40% < strain > ∼200%). Afterwardsa strain hardening is to observe whilst the stress and strain valuesreach ∼36 MPa and ∼1300%, respectively. This hardening processis connected with the orientation and break-up of supermolecu-lar entities (crystallites) and with the orientation of the polymermolecules during stretching. By contrast, the neat HNBR showsa hyperelastic tensile behavior which is characteristics for elas-tomers. The maximal stress and strain values at break of HNBR,

are ∼4.4 MPa and ∼280%, respectively. Incorporation of 50 phr PUin the HNBR yields an increase in tensile strength (∼5.7 MPa) and inthe strain to failure (∼302%), as well. Increasing PU content of theHNBR/PU hybrids (up to 100 phr) is associated with further increase

HNBR + 50 phr PU HNBR + 100 phr PU PU

684 388 301.55E+03 2.74E+03 3.85E+040.84 0.66 0.11.058 1.062 1.136

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302 375 130010.6 12.9 74.3

1.15E+00 1.07E+00 4.2E−014.53E−02 3.07E−02 2.1E−04

3.23E+00 2.52E+00 9.3 E−012.97E−03 4.68E−04 8.3E−05

1.75E+00 1.52E+00 7.00E−011.37E−03 2.38E−03 1.5E−04

3.42E−02 3.34E−02 4.43E−021.34E−04 1.96E−04 3.47E−04

e DMTA spectra recorded at 0.01% strain.

468 J. Karger-Kocsis et al. / Wear 268 (2010) 464–472

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ig. 6. Force versus displacement curves for the investigated rubbers during tearests.

n their stiffness (M data), tensile strength (∼9 MPa), and strain toailure (∼375%). The related results are summarized in Table 1.

Similar large difference is to observe in the results of the tearests. The tear strength of neat PU (74.3 kN/m) proved to be almostbout one magnitude higher than that of the HNBR and HNBR/PUybrids (cf. Fig. 6). As expected, the neat HNBR shows the small-st tear strength (4.2 kN/m). Incorporating 50 and 100 phr PUn the HNBR the tear strength increases to 10.6 and 12.9 kN/m,espectively—cf. Table 1.

Observing the force versus displacement curves of the tear testcf. Fig. 6) one can conclude that incorporation of PU in the HNBRncreases the maximal force as well as the related displacementuring tearing.

.3. Coefficient of friction (COF)

In Fig. 7 the measured dynamic COF are summarized for theaterials studied in different type wear tests. Note that in Fig. 7

he COF results are depicted in logarithmic scale. In POP, ROP andretting tests the smallest COF was measured for the neat PU. This isot surprising because the PU is a relatively hard rubber comparedo the softer HNBR (cf. Table 1). Incorporation of 50 phr PU into theNBR causes an increase in the COF data for all dry sliding tests.urther increase of the PU content (up to 100 phr) reduces the COFompared to that of HNBR + 50 phr PU. Nevertheless, the COF is stilligher than that of the neat HNBR. This is not the case for the ROPests, where the COF of the HNBR + 100 phr PU is smaller than that

f the neat HNBR. For the sliding tests the average COF values arehe highest for the ROP and the smallest for the POP measurements.

The effect of PU content on the performance of the hybrids inBOP tests is similar as described before. Adding 50 phr PU to the

ig. 7. COF results of the tested materials under POP, ROP, Fretting and RBOP con-itions.

Fig. 8. Specific wear rate results of the tested materials in POP, ROP, Fretting andRBOP conditions.

HNBR increases the measurable COF compared to the neat HNBR.However, with further increase of the PU content of the hybrids theCOF decreases again. Surprisingly the highest COF was measured forthe neat PU, the reason of which is not clear at the moment.

3.4. Specific wear rate, wear mechanisms

The specific wear rates of the different materials and test meth-ods are summarized in Fig. 8. The specific wear rate (Ws) wascalculated by adapting Eq. (1).

For the better understanding the results of the sliding and rollingwear tests will be discussed separately.

3.4.1. Pin-on-plate (POP)In Fig. 8 the wear results show that PU incorporation into the

HNBR matrix enhances the wear resistance of the resulting hybrids.The differences between the specific wear rates of the materialsare quite large for POP tests (note that in the diagram logarith-mic scale is applied). An extremely large difference (three orders ofmagnitudes) was observed between the neat HNBR and neat PU.

In Fig. 9 the left column (pictures a, d, g, j) shows the SEMpictures of the worn surfaces after POP test for the four differentmaterials. It is clearly to see how the wear mechanisms changewith the incorporation of PU into the HNBR. The neat HNBR showscrater-like surface damage which is linked with its “weakness”(poor mechanical performance in absence of fillers). Incorporationof PU triggers the appearance of Schallamach waviness [10] in thewear track. The related wear mechanisms are depicted in Fig. 10a–c.Wear particles can be found on the worn surface of the HNBR + PUhybrids, too. On the other hand, the crater-like feature of the weartracks of the HNBR + PU hybrids, being similar to that of the neatHNBR, may suggest that HNBR forms the continuous phase in thehybrids. This would be in line with the DMTA results, as discussedbefore. The worn surface of the neat PU shows extrusion-type fail-ure [11]. The plastic deformation of PU creates “flakes” which areoriented in the sliding direction (cf. Fig. 11a).

3.4.2. Roller-on-plate (ROP)The specific wear results of the ROP tests show similar tenden-

cies like the POP results (cf. Fig. 8). The difference between the neatHNBR and neat PU covers again three orders of magnitude.

Under ROP conditions the wear mechanisms for the neatHNBR and HNBR + 50 phr PU hybrid show similarities with thePOP results (cf. Fig. 9b and e). The neat HNBR rubber experi-

ences a crater-like damage during ROP tests (cf. Fig. 9b) similarto POP conditions. Schallamach-type waviness appears for theHNBR + 50 phr PU hybrid when tested under ROP conditions (cf.Fig. 9e), like to the POP tests (cf. Fig. 9d). The wear situationchanges with increasing PU content (cf. Fig. 9h). At 100 phr PU con-

J. Karger-Kocsis et al. / Wear 268 (2010) 464–472 469

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ig. 9. SEM pictures taken from the worn surface after POP, ROP and Fretting tesscillation was vertical.

ent flake-like wear mechanism appears, which suggests increasedeformability (toughness) for this hybrid. For the neat PU mate-ial (cf. Fig. 9k) minor damage can be established after the ROPests. This is in accord with the specific wear results (cf. Fig. 8).he neat PU shows neither Schallamach-type wave formation norurface cracking when sliding against a relatively smooth counterody.

.4.3. FrettingThe neat HNBR shows the same crater-like surface structure as

efore for POP and ROP tests (cf. Fig. 9c). The specific wear resultsor the fretting configuration did not result in enhanced wear resis-

ance of the hybrids with their increasing PU content. The reasonor this finding may be linked with the reciprocating movement ofhe counterpart due to which the wear debris will not be removedrom the contact area. By this way a wear protecting layer may formn the surface. On the other hand, significant fatigue type damage

te: the sliding direction for POP and ROP tests is downwards, in fretting tests the

(cf. Fig. 11b) can be established when PU is incorporated into theHNBR rubber (cf. Fig. 9f and i). However, SEM investigation of theworn surface of the neat PU reveals notable plastic deformationafter the fretting test (cf. Fig. 9l).

3.4.4. Rolling ball-on-plate (RBOP)The specific wear rate results in RBOP test are on the one hand

smaller as under dry sliding conditions; on the other hand theirchange with the PU content is unexpected. The neat HNBR andneat PU show identical wear behavior (cf. Fig. 8). Because the mate-rial characteristics of HNBR and PU are very different (cf. Table 1),different wear mechanisms are expected during their RBOP tests.

Incorporation of 50 and 100 phr PU into the HNBR matrix resultsin a decrease of the specific wear rate. The smallest wear rate wasestablished for the HNBR + 50 phr PU hybrid.

SEM investigation of the worn surfaces after RBOP tests is morecomplicated than for sliding ones. The wear groove in this test can

470 J. Karger-Kocsis et al. / Wear 268 (2010) 464–472

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ig. 10. Schematic sketch about the formation of the crater-like (c), and Schallamaormation of Schallamach-type waves [10] (a), triggered by surface cracking (b). Then the material behavior.

e separated into three parts as it is shown in Fig. 12a. This is dueo the fact, that during orbital rolling of the ball, a spinning motions superimposed to the forward rolling [12]. This causes a relativeliding at the outer and at the inner radius of the wear groove.

t the outer side this relative sliding shows in the rolling direc-

ion, while at the inner radius in the opposite direction. The middleart of the wear groove is exposed to drilling motion. SEM picturesvidence for all investigated materials that the relative sliding at

ig. 11. Schematic sketch of extrusion- [11] (a), and fatigue (gauging)-type (b) surface dary sliding, while the latter phenomenon is related to repeated and/or reciprocated wear

vy (d) surface structures. The onset of both surfaces structures is preceded by theg over an already cracked surface results in different wear mechanisms depending

the inner and outer radii produces differently oriented wear pat-terns (compare the SEM pictures in the left and right columns inFig. 12). For the neat HNBR and for HNBR + PU hybrids some kindof Schallamach waviness is to see on the surface at the inner and

the outer radii of the wear track. At the same time in the middlepart of the wear tracks less surface damage but prominent depo-sition of wear debris can be observed (cf. Fig. 12c, f, i, and l). Forthe neat PU similar wear phenomena can be resolved at the inner

mage developments. The former is typical for thermoplastic materials during theiring motions.

J. Karger-Kocsis et al. / Wear 268 (2010) 464–472 471

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ig. 12. SEM pictures taken from the worn surface after RBOP test. Note: picture (arrows.

nd at the outer radii of the wear track. But in this case, insteadf a typical surface waviness, an extrusion-type “flake”-formationan be resolved (cf. Fig. 12k and m). This type of surface damages more typical for thermoplastic materials. In case of PU, the wearebris are deposited again in the middle part of the wear trackcf. Fig. 12l).

. Conclusion

Aim of this work was to produce novel hybrid rubbers by blend-ng peroxide curable hydrogenated nitrile rubber (HNBR) with solidolyurethane precursors (polyol and blocked polyisocynate) whichndergo in situ curing during the crosslinking of HNBR, and study

s the overview of a segment of the wear track. The sliding direction is indicated by

their basic mechanical, dry sliding and rolling wear properties. Theresults can be summarized as follow:

- The PU, when present up to 100 phr content in the HNBR, formeda disperse reinforcing phase. This was concluded from DMTAresults. PU incorporation enhanced the tensile properties (mod-ulus, strength, and strain) and hardness.

- The COF and specific wear results depended on the type of thetribotests. The specific wear rate was more sensitive to the PUhybridization than the COF. The specific wear rate was promi-nently reduced under POP and ROP testing with increasing PUcontent. By contrast, PU hybridization was less effective in fret-

4 / Wear

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A

pS5S

R

72 J. Karger-Kocsis et al.

ting tests. PU “reinforcement” changed the failure mode fromcratering (neat HNBR) to Schallamach waviness with roll forma-tion (HNBR + PU), while the neat PU failed by plastic deformationunder dry sliding.In rolling tribotests the COF was not markedly influenced by thehybridization. On the other hand, reduction in the specific wearrate was achieved when PU was incorporated in the HNBR matrix.The characteristic failure modes under dry rolling were: crater-ing for HNBR, Schallamach waviness for HNBR + PU, and surfaceplastic deformation for the neat PU.

cknowledgements

This work was performed in the framework of the followingrojects: intergovernmental cooperation between Hungary andlovenia and Kristal of the EU (Contract No.: NMP3-CT-2005-15837). The PU sampling by Collano (nolax) AG (Sempach-Station,witzerland) is gratefully acknowledged.

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