Wear 342-343 (2015) 85–91

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Gamma irradiation effects on thermal, physical and tribologicalproperties of PEEK under water lubricated conditions

Neelima Khare n, P.K. Limaye, N.L. Soni, R.J. PatelRefuelling Technology Division, BARC, Trombay, Mumbai 400085, India

a r t i c l e i n f o

Article history:Received 3 April 2015Received in revised form4 August 2015Accepted 6 August 2015Available online 20 August 2015

Keywords:Sliding wearWear testingHardnessSteelPolymers

x.doi.org/10.1016/j.wear.2015.08.00548/& 2015 Elsevier B.V. All rights reserved.

esponding author.ail address: neelima.khare@gmail.com (N. Kha

a b s t r a c t

Effects of gamma irradiation (upto 3 MGy) on thermal, physical and tribological properties of poly-ether-ether ketone (PEEK) were studied. Several studies have presented data on radiation stability of PEEK.However, none have investigated the effects of gamma irradiation on its tribological properties. Thiswork was carried out for applications of precision mechanisms of fuelling machine of Indian PressurisedHeavy Water Reactors (PHWRs). Hence very slow sliding speed and water lubricated environment wereselected.

Increase in glass transition temperature, melting temperature, hardness and density was observedafter exposure to gamma irradiation. Highest glass transition temperature and melting temperature wereobserved at 0.5 MGy. Tribological properties were also altered after exposure to gamma radiation. Sur-face stresses generated due to crosslinking have resulted into highest coefficient of friction at 0.5 MGydose. Reduced probability of fracture has resulted into lowest wear rate at same dose.

& 2015 Elsevier B.V. All rights reserved.

1. Introduction

Apart from their good tribological properties, some engineeredpolymers have good ductility, formability and light weight. It ismaking them an obvious choice for replacing ceramic and metalliccomponents. Polymeric material for nuclear power plant applicationshould have high radiation resistance and good mechanical/tribolo-gical properties. High energy radiation in the plant can bring majorchanges in the molecular structure and macroscopic properties ofpolymers. Radiation dose required to bring changes in physicalproperties of polymers is considerably less than that is required tocause any significant change in ceramics or metals [1]. The radiationcan bring changes in appearance, chemical/ physical states,mechanical, tribological, electrical and thermal properties. Poly-ether-ether-ketone (PEEK) is one of the most promising polymers, asemi-crystalline thermoplastic with excellent mechanical properties[2]. It displays high glass transition temperature (143 °C) and highmelting point (335 °C) [3]. It can easily be processed in variouscomplex shapes and also it possesses high toughness and excellentwear resistance [4]. It has good stability in both chemically active andradiation environments [5]. Celina et al. [5] studied about differentpolymeric materials components used in acidic-radiation environ-ment. They concluded that PEEK is stable for radiation and thermalbehaviour under acidic environments. Due to these properties, PEEK

re).

and its composites are widely used for various applications e.g.aerospace, nuclear and tribological [6,7]. Lawrence et al. [8] studiedthe effect of gamma radiation on the thermal properties and mor-phology of PEEK and PEEK–alumina composites. They concluded thatincrease in glass transition temperature (Tg) and decrease in meltingtemperature (Tm) take place for both materials. Sasuga et al. [9]studied effect of gamma radiation in terms of change in mechanicalproperties, under oxygen pressure for different aromatic polymers.Under this study they concluded that aromatic polymers which showrelatively high radiation resistance for non-oxidative irradiationdeteriorate vigorously under oxidative irradiation and this is broughtabout mainly by chain scission only. Sharp et al. [10] studied theeffects of radiations on various components of nuclear robot appli-cation. Remotely operated connectors with PEEK insulation havebeen shown to be very resistant, both mechanically and electrically,up to high total doses (10 MGy) and even under thermal stresses(120 °C). Hernandez et al. [11] studied the effect of gamma radiationon water lubricated PEEK components (O-rings, seals etc.) of ITER.They concluded that PEEK shows excellent mechanical behaviourafter irradiation. Sasuga et al. [12] studied effects of ion irradiation onthermal and mechanical properties of crystalline and non-crystallinePEEK. Ash et al. [13] concluded in their work that molecular weightof polymers increases after exposure to gamma irradiation due tocross-linking and results into improvement in tribological/ mechan-ical properties.

Tribological performance of polymer is greatly influenced bymany factors and studied by many researchers in past. Jie et al.[14] proved in their work that, the friction coefficient is mainly

Table 1Properties of PEEK, aluminium bronze and 17-4 PH stainless steel.

Property Values

Name PEEK 17-4 PH SS Aluminium bronzeComposition (wt%) No fillers Cr¼17%, Ni¼4%,

Cu¼4%, %Fe¼balance

Al¼8.5–11%, Fe¼2.0–5.0%, Ni¼4.0–6.0%,Mn¼0.0–1.5 %Cu¼balance

Density (gm/cc) 1.31 7.8 7.58Water absorption (%)(immersion after24 h)

0.1 – –

Thermal conductivity(W/m K)

0.252 22.6 39.1

Tensile strength (MPa) 110 1400 690Hardness 105 36 96

N. Khare et al. / Wear 342-343 (2015) 85–9186

influenced by the PV factor (mechanical factor), and the weightloss was mainly influenced by the contact temperature (thermalfactor). Effects of reinforcement on tribological performance wereinvestigated by Davim et al. [15] under dry condition and by Khareet al. [16] under water lubricated conditions. They concluded thatcarbon fibre reinforcement is very effective for improved tribolo-gical performances of PEEK. Tribological performance of PEEK wasnicely evaluated by statistical and other methods by researchers[14,17]. Several studies have presented data on radiation stabilityof PEEK. However, none have investigated the effects of gammairradiation on its tribological properties.

In fuelling machine head of Pressurised Heavy Water Reactors(PHWRs), present design employs components of aluminiumbronze and phosphor bronze rubbing against moving componentsof precipitation hardened stainless steels (e.g. 17-4 PH SS, 13-08MO SS) at slow sliding speeds (0.005–0.05 m/s). Some examples ofthese components are piston-cylinder, ram drives, latch gear-spline shaft etc. These all components are sliding at very low speedand always submerged under water lubrication. To determinetribological performance of these components, it is required tocarry out test on pin-on-disc setup for simulated operated condi-tions. Operating environment is high purity de-mineralised waterat moderate temperature and 10 MPa operating pressure. In theseoperating conditions, wear mechanism of aluminium bronze isadhesive in nature, which produces large size wear debris andcauses significant damage to mating part. These wear debrisaccumulate at other sliding joints and cause further damage to thesurfaces. Hence for long term solution, new materials are neededto study as a replacement for bronze components. In this work, theeffects of gamma radiation on the tribological and thermal beha-viour of natural PEEK under water lubricated condition wereinvestigated. 17-4 PH SS counterface was used because it is com-monly used material for nuclear applications. Comparisonbetween tribological properties of conventional material pair(aluminium bronze and 17-4 PH SS) and selected material pair(PEEK and 17-4 PH SS) was carried out. Accumulated dose for 10years operating life for fuelling machine components is of theorder of 2–2.5 MGy. Thermal, physical, and tribological propertiesevaluation of PEEK was carried out up to 3 MGy.

Tribological performance of a system can be improved byreplacing conventional metallic materials with high performancepolymers. However for nuclear applications careful selection ofmaterial is required due to the presence of radiation. Purpose ofthis work is to establish the effect of gamma irradiation on prop-erties of PEEK and to compare the performance with conventionalmaterial.

2. Experimental

2.1. Materials

PEEK in extruded rod form was purchased from a reputedsupplier. Flat ended cylindrical pins with 4.0 mm diameter atcontact were machined from rods. The 17-4 pH stainless steel discsof 10 mm thickness and 110 mm diameter were machined from aforged rod. Discs were heat treated to achieve hardness value of 36HRC. Discs and pins were ground to achieve surface roughness (Ra)of 0.96 mm. For detailed materials properties refer Table 1.

2.2. Radiation aging

Sealed samples of PEEK were kept in ice temperature bath andirradiated with gamma rays from a 60Co source in a gammachamber (Model no.: GC-5000, supplied by M/s BRIT India). Theirradiation was carried out by delivering radiation doses of

0.5–3 MGy (50 -300 MRad) at a dose rate of 12.5 kGy/h, as mea-sured by Fricke dosimetry.

2.3. Thermal characterisation

Samples of dimensions approx. 2�2 mm2 were cut from irra-diated PEEK samples. Subsequent thermal characterisation wascarried out using a differential scanning calorimeter (DSC; Model:DSC 131) supplied by M/s. SETARAM Instrumentations, France.Samples were kept in aluminium crucible and heated from 30 to450 °C at 10 °C/min. Subsequent cooling was carried out from meltto 30 °C. Graphs were recorded online and values of glass transi-tion and melting temperatures extracted for various doses.

2.4. Density measurement

The determination of the density was carried out by waterdisplacement, which consists in evaluating the specific gravity of aspecimen. The specific gravity of PEEK samples (unirradiated andirradiated) was determined with a Mettler H35 AR scale, a con-tainer filled with distilled water and attachment for suspendingthe specimens following the procedures laid out in ASTM D792-A.First, the samples of about 10 g weights were conditioned for 48 hat 2371 °C. Samples were weighted first in air, then in water. Fivesamples were measured three times each, which provided for alow standard deviation on the mean value of the density at anygiven dose. Specific gravity was calculated using the followingequation:

aa b

Specific gravity, ρ =−

where a is the apparent mass of specimen in air and b is theapparent mass of specimen completely immersed in water.

2.5. Hardness measurement

Rockwell hardness is a measure of the net increase in depth ofthe indenter. The Rockwell hardness test was conducted at roomtemperature as per ASTMD785. Samples of PEEK having 8 mmdiameter and 10 mm length were prepared. Three samples at eachdose of 0.5, 0.75, 1, 2, 3 MGy were irradiated. Testing was per-formed by first forcing a steel ball indenter (diameter 6.35 mm)into the surface of a sample using a specified minor load (10 kg).The load is then increased to a specified major load (100 kg) andthen decreased back to the original minor load. Rockwell hardnessnumber was obtained from direct reading on M scale. To findaccurate results, at least three readings at different locations ofsamples were determined and average value was reported.

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2.6. Tribological evaluation

The friction and wear tests were conducted on a pin-on-disctest rig. Test rig was specially designed for under water lubricatedtesting. Proper sealing arrangements were provided to prevent anyleakage of water. PEEK pin sample was held by a pin holder againstthe rotating disc mounted on disc holder. Before each test, poly-mer pins and 17-4 PH stainless steel discs were thoroughlycleaned with alcohol/acetone in an ultrasonic bath. Lubricationwas applied on wear track (wear track diameter is 90 mm) using20 drops of de-mineralised water (having 9.5 pH) every minute,keeping it flooded to achieve testing in submerged condition. Thetribological tests were carried out at sliding speeds of 0.005,0.05 m/s and contact pressures 3, 6, 12 MPa. Diameter of pin at thecontact was 4 mm and it was constant for all the tests. Variablecontact pressure was achieved by varying normal load and keep-ing area of contact constant. Ambient temperature around 25 °Cand sliding distance of 1000 m were kept constant for all tests.During the test, friction force was recorded continuouslythroughout the test using a load cell. Plot of variation of coefficientof friction with varying sliding distance was displaced onlineduring the test. After stabilisation of plot average value of COF wasreported. Wear of the pin was evaluated by measuring weight lossafter completing the test using a precision weight balance(AFCOSET ER-182 A).

Each test was repeated three times and their average valueswere presented. The specific wear rate Ws was calculated from thefollowing relationship

⎡⎣ ⎤⎦Wm

F Lmm /Nms

N

3

ρ= Δ

where m∆ is the mass loss, ρ is the density of the irradiated/unirradiated specimen, L is total sliding distance and FN is theapplied load. Aluminium bronze (Pin) and 17-4 pH stainless steel(Disc) material pair was also tested similarly under water lubri-cated condition. Sliding speed of 0.5 m/s and contact pressures 3,6 and 12 MPa were used. The worn surfaces of PEEK and alumi-nium bronze were observed under a scanning electron microscope(SEM) (CT/100 Cam Scan MV2300, UK). Before SEM examinationthe worn surfaces of PEEK were sputtered coated with a thin layerof gold.

3. Results and discussion

3.1. Thermal properties

Fig. 1 shows the variation of glass transition temperature (Tg)and melting temperature (Tm) with absorbed radiation doses.

Fig. 1. Variation of glass transition temperature, Tg, and melting temperature, Tm,with radiation dose.

Glass transition and melting temperatures started from theirlowest value for unirradiated condition and reached to highestvalue for 0.50 MGy irradiated condition. These started decreasingafter 0.50 MGy and reached to some value (still greater than uni-rradiated condition) for 3 MGy irradiated condition. Glass transi-tion temperature at all radiation doses was higher than that forun-irradiated conditions. It indicated radiation stability of PEEKupto 3 MGy. Melting temperature and glass transition temperatureare directly related to the molecular weight of the polymer.Melting point reached to its highest value at 0.50 MGy irradiatedconditions signifies that molecular weight of PEEK is maximum at0.50 MGy condition. Ash et al. [13] concluded in his work thatmolecular weight of polymers increases after exposure to gammairradiation due to cross-linking and results into improvement intribological/mechanical properties. This signifies that highestcrosslinking has occurred at 0.5 MGy irradiation dose among theother selected doses. Shift of Tg to high temperature because ofradiation may arise from restriction of three dimensional mole-cular motions by the newly formed crystallites during cooling runof DSC at given crystallisation temperatures. In terms of degree ofcrosslinking it can be stated that highest and lowest degree ofcrosslinking is indicated at 0.5 MGy and 3 MGy respectively.

3.2. Density measurement

The specific gravity or density of a solid is a property that canbe an indicative of physical changes in a sample. Changes indensity of the material may be due to changes in crystallinity,molecular weight, crosslinking, molecular configuration etc. Themeasured densities as a function of the irradiation dose are pre-sented in Fig. 2. As expected, the densities of the irradiated PEEKgrades are different, a fact explained by the difference in themolecular weight. The density increase is an indicative of mor-phological changes in the polymer structure, consisting mostly incrosslinking occurring. Thermal evaluation displayed the highestcrosslinking at 0.5 MGy dose, however density has not confirmedthe same. Density is almost same at all the irradiation doses.

3.3. Hardness evaluation

Exposure of radiation resulted into increased hardness of PEEKas displayed in Fig. 3. Once increased, hardness is stabilized forfurther increase in irradiation dose upto 3 MGy. Althoughincreased hardness attributed to crosslinking of PEEK however it isnot confirmed that crosslinking is maximum at 0.5 MGy dose.High hardness generally produces wear resistant surfaces howeverother factors also influence this.

Fig. 2. Variation of density with radiation dose.

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3.4. Tribological properties

Friction and wear are serious causes of energy dissipation andmaterial dissipation respectively. Fig. 4 presents typical variationof coefficient of friction (COF) with sliding distance. Once thegraph is stabilized, average value of COF was calculated andreported. Fig. 5(a) and (b) presents the variation of coefficient offriction with change in contact pressure and irradiation doses.General trend is COF reducing to a certain extent then stabilizedwith increase in contact pressure. This is a typical behaviour due toelastic deformation of asperities. Highest COF was depicted bysamples irradiated for 0.50 MGy and 0.75 MGy (only at high slid-ing speed) irradiation dose. The junctions sheared under theapplied tangential force results in the friction force. In general, theinterfacial junctions are influenced by the nature of the mated

Fig. 3. Variation of hardness with radiation dose.

Fig. 4. Variation of coefficient of friction with sliding distance at 0.75 MGy, 12 MPa,0.05 m/s.

Fig. 5. Variation of coefficient of friction with contact press

surfaces as well as by the surface chemistry and the stresses in thesurface layers. Irisawa et al. [18,19] showed in their work thatcoefficient of friction can be represented by following equation:

gkH

μ σ=

where H is the microhardness of softer material, s is the stressrequired to slide the abrasive particle, g is the shape factor of theabrasive particle (or asperity), and k is a constant. In current work,difference in hardness of both the surfaces is very high. Asperitiesof hard metallic counterface are sliding on soft polymer material,hence abrasion is talking place. Here g is constant because samecounterface is being used for all the PEEK surfaces; however H isdifferent for irradiated and unirradiated material. Coefficient offriction decreases with either increase in hardness or decrease insurface stresses. Results showed that hardness of irradiatedmaterial at all the selected doses is approximately the same. Thisindicated that surface stress generated due to crosslinking ofpolymer is responsible for change in COF. Highly cross linkedmaterial at 0.5 MGy and 0.75 MGy is indicating maximum COF inmost of the conditions. These results also indicate that withincrease in contact pressure effect of surface stresses is reduced.

Due to much difference in the hardness of PEEK and 17-4 PH SS,negligible wear of 17-4 PH SS was observed. Wear rate of PEEK wascalculated and reported here. Fig. 6(a) and (b) presents variation ofwear rate of PEEK with contact pressure and sliding speed fordifferent radiation doses. Wear rate is different at different irra-diation doses and lowest for samples irradiated at 0.5 and0.75 MGy. However order of wear is same for all the conditions.Sample irradiated at 0.5 MGy indicated sharp decrease in wearrate with increase in contact pressure. This phenomenon wasobserved only at high speed. Contact pressure and sliding speedhave little effect on wear rate of irradiated/unirradiated samples.

Wear and friction are surface phenomena, modification ofsurfaces leads to change in wear and friction mechanisms. Wearrate may change with either change in hardness or in surfaceconditions. In general increased hardness is responsible forincreased wear resistance of a material. In current work, hardnessof irradiated material at all the selected doses is approximatelysame however wear rate is different. Reason behind this may bethat, as degree of crosslinking is changing, surface condition is alsochanging. Formation of three dimensional networks due tocrosslinking and resulting surfaces stresses caused improved sur-face conditions. Highest degree crosslinking at 0.5 MGy indicatedlowest wear rate.

Coefficient of friction (COF) and wear rates of unirradiated andirradiated PEEK were compared with aluminium bronze. Fig. 7(a)and (b) shows COF and wear rate respectively for irradiated/uni-rradiated PEEK and aluminium bronze, under water lubricatedenvironment. For most of the conditions, all materials exhibited

ure and radiation dose (a) 0.005 m/s and (b) 0.05 m/s.

Fig. 6. Variation of wear rate with contact pressure and radiation dose (a) 0.005 m/s and (b) 0.05 m/s.

Fig. 7. The relationship with contact pressure for PEEK materials and aluminium bronze at 0.05 m/s (a) coefficient of friction and (b) wear rate.

N. Khare et al. / Wear 342-343 (2015) 85–91 89

lower COF than that of aluminium bronze. However, at 3 MPacontact pressure, 0.5 and 0.75 MGy condition showed higher COFthan aluminium bronze. COF for metallic material pairs comprisestwo components namely adhesion and ploughing. At higher con-tact pressure, adhesion component of friction might be dominat-ing for aluminium bronze and 17-4 PH SS material pair, whichresults into high COF. All materials exhibit lower wear rate thanaluminium bronze except at 3 MPa contact pressure. Increasedcontact pressure resulted into high wear rate of aluminium bronzemay be due to change in wear mechanism from abrasive toadhesive.

Replacement of aluminium bronze with PEEK may result intoreduction in wear rate. However it all depends on value of irra-diation dose.

3.5. Wear mechanisms of irradiated and unirradiated PEEK

Surface topographies of worn surfaces clearly supported wearperformance trends of unirradiated and irradiated PEEK. The wornsurface of unirradiated PEEK as shown in Fig. 8(a) exhibited wearmarks in the form of continuous narrow cutting grooves. Groovesare formed due to ploughing of soft material by hard asperities ofcounterface material. In addition, some chip like large size weardebris formed during the sliding process was also visible. Weardebris might have originated due to detachment of soften plasti-cized surface from bulk material. Fig. 8(b) indicates more entrap-ment of wear debris on the surface which might have resulted intodecreasing coefficient of friction and wear rate with increase incontact pressure. Main wear mechanism of unirradiated PEEK isabrasive wear caused by hard asperities of the counterface. Fig. 8(c) indicates worn surface of PEEK irradiated at 0.50 MGy. Itshowed comparatively smooth topography, sliding marks were notmuch visible. Entrapped wear debris was clearly visible on the

surface. Main wear mechanisms for this condition can be stated asvery mild wear. Fig. 8(d) indicates worn surface of PEEK irradiatedat 0.75 MGy. It also showed smooth topography however slidingmarks are much visible than sample irradiated at 0.50 MGy. Backtransferred PEEK and embedded wear debris are clearly visible onthe surface. Fig. 8(e) and (f) indicates worn surfaces of samplesirradiated at 3 MGy conditions, as wear performances indicatedthat 3 MGy exhibited very high wear rate. Micrographs areshowing back transferred, entrapped wear debris. Presence ofwear particles supports its wear performance.

Fig. 8(g) and (h) shows increased surface distress and delami-nation due to increasing contact pressure from 3 MPa to 6 MPa.This has resulted into sudden transition in wear rate of aluminiumbronze. Fig. 8(i) shows large extent of adhesive wear and severeplastic deformation of aluminium bronze in sliding direction,resulting surface delamination. Surface was covered with plasti-cally deformed wear debris and surface pits due to material pullout at many locations. This tendency of plastic deformation andadhesion increased the probability of formation of asperity junc-tions, resulting in much higher friction coefficient.

4. Conclusions

Natural PEEK samples were irradiated up to 3 MGy for gammaradiation dose. Thermal, physical and tribological properties wereevaluated at various radiation doses. Following conclusions weredrawn:

� Exposure to gamma irradiation caused crosslinking of PEEK andresulted into increase in glass transition temperature, meltingtemperature, density and hardness.

Fig. 8. Worn surfaces of PEEK pins (a) unirradiated (V¼0.005 m/s, P¼6 MPa), (b) unirradiated (V¼0.05 m/s, P¼12 MPa), (c) irradiated at 0.50 MGy (V¼0.005 m/s,P¼12 MPa), (d) irradiated at 0.75 MGy (V¼0.05 m/s, P¼12 MPa), (e) irradiated at 3 MGy (V¼0.005 m/s, P¼12 MPa), (f) irradiated at 3 MGy (V¼0. 05 m/s, P¼12 MPa),(g) aluminium bronze (V¼0.05 m/s, P¼3 MPa), (h) aluminium bronze (V¼0.05 m/s, P¼6 MPa) and (i) aluminium bronze (V¼0.05 m/s, P¼12 MPa).

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� Thermal evaluation indicated highest degree of crosslinking at0.5 MGy radiation dose, although it was not supported byhardness and density evaluation.

� Degree of crosslinking was correlated with the tribologicalproperties. Highest COF was observed at 0.5 MGy conditionsdue to increase in surface stresses. Lowest wear was observed at0.5 MGy conditions due to improved surface conditions by for-mation of three dimensional networks.

� Abrasion associated with the ploughing marks and the presenceof wear particles were dominant wear phenomenon for irra-diated/unirradiated PEEK.

� Adhesive wear, severe plastic deformation and delaminationwere the main wear mechanisms of aluminium bronze.

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