Tribology International 65 (2013) 295–302

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Tribology International

0301-67

http://d

n Corr

E-m

journal homepage: www.elsevier.com/locate/triboint

Effect of SiO2 and PTFE additives on dry sliding of NiP electroless coating

D. Gutsev a, M. Antonov b,n, I. Hussainova b, A.Ya. Grigoriev a

a Metal-Polymer Research Institute NASB, Gomel, Belarusb Department of Materials Engineering, Tallinn University of Technology, 19086 Tallinn, Ehitajate tee 5, Estonia

a r t i c l e i n f o

Article history:

Received 3 August 2012

Received in revised form

7 December 2012

Accepted 10 December 2012Available online 8 January 2013

Keywords:

NiP self-lubricating coating

SiO2 and PTFE additives

Dry sliding

Coefficient of friction

9X/$ - see front matter & 2013 Elsevier Ltd. A

x.doi.org/10.1016/j.triboint.2012.12.012

esponding author. Tel.: þ372 6203355; fax:

ail addresses: Maksim.Antonov@ttu.ee, mcsim

a b s t r a c t

The aim of the present work is to study the effect of silicon dioxide and polytetrafluoroethylene (PTFE)

additives on NiP coating performance under dry (unlubricated) conditions in unidirectional and

reciprocating sliding modes in Ball-on-Flat configuration. Yttria-stabilized zirconia and chrome steel

balls were used as counterbodies. Microstructural examination of specimens was conducted by a SEM

with EDS. It was found that in contact with steel ball both additives were beneficial, while addition of

SiO2 particles was detrimental for contact against ceramic ball. The highest improvement of tribological

performance was obtained for NiP coated samples having PTFE or mixed additives.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Nickel–based coatings are widely used in mechanical engi-neering, electronics and chemical industry because of their hard-ness, corrosion and wear resistance. Their stable coefficients offriction (COF) in vacuum and air as well as high electricalconductivity make them promising for various applications inmany sectors of industry such as transport (air, sea and land),energy generation, and manufacturing [1–7].

The challenge arises to development of nanostructured coat-ings that benefit from the unique physical and tribologicalproperties of functional layers and nanoparticles incorporatedinto their structures. Significant improvement in tribologicalcharacteristics of NiP coatings can be reached by introducing selflubricating agent. Nowadays, a successful application of fluo-polymer nanofillers such as polytetrafluorethylen (PTFE), perfluor-oalkoxy (PFA), fluorinated ethylene propylene (FEP) and ethylenetetrafluoroethylene (ETFE) for reduction wear losses and increasedurability of the final product is of a great interest for industry.These fillers are found to increase plasticity of Ni-based matrixand prevent crystallisation of the coating at counterbodies con-tact spots where the flash temperature caused by frictionalheating increases to a great extent.

Electroless (chemical) deposition allows obtaining coatings ofuniform thickness of any irregularly shaped surfaces made ofeither metallic or non-metallic materials. A process of deposition

ll rights reserved.

þ372 6203480.

a@mail.ru (M. Antonov).

is based on oxidation–reduction reaction, when reducer is oxi-dised by H2PO4

� and Ni2þ ions are precipitated onto substrate

surface [8,9]. Generally there is a linear relationship between aholding time (in bath) and a final coating thickness [9]. Co-platingof phosphorus (P) results in formation of nickel phosphide (NiPand Ni3P) enabling sufficient increase in hardness of a coating(up to 1000 HV) during heat treatment [10–12].

One of the current trends is a process of chemical co-deposition of nickel added by hard (ceramic) particles such asSiC, B4C, Cr2C3, WC, BN, Si3N4, Al2O3, ZrO2, TiO2, K2Ti6O13, CeO2,SiO2, diamond [5,6,13–18] and carbon nanotubes; or solid lubri-cants such as PTFE, MoS2, WS2, graphite [4,5,19–25], etc. Incor-poration of suitable additives results in increasing hardness andwear resistance as well as decreasing coefficient of friction of thecoatings that allows service in dry conditions without additionallubricant. By co-depositing the PTFE particles, the hydrophobiccoating with excellent anti-sticking performance, and lubricatingability can be obtained. According to authors knowledge the NiPcoatings added by combined PTFE and SiO2 particles have notbeen studied yet. It is supposed that silicon dioxide particlesimpede dislocation movement and act as barriers to retard theplastic deformation of ductile Ni–P matrix and hence increase themicrohardness while PTFE reduces the COF and increase a loadbearing capacity.

The aim of the present work is to study the effect of silicondioxide and polytetrafluoroethylene (PTFE) additives on NiP coat-ing performance under dry sliding conditions in unidirectionaland reciprocating mode in Ball-on-Flat configuration. The scan-ning method for the evaluation of the critical forces was appliedto improve the differentiation among coatings.

Fig. 2. Schematic representation of NiP coating with SiO2 and PTFE (cross section).

D. Gutsev et al. / Tribology International 65 (2013) 295–302296

2. Materials and methods

2.1. Materials

Composite NiP-based coatings with additions of SiO2 and/orPTFE were deposited using electroless nickel bath containingnanosized (5–20 nm) silicon dioxide (SiO2) particles (GOST14922) at pH 4.670.4 and temperature 90–95 1C during 1 h.Phosphorus content in the NiP coating was 10 wt%. The coatings(Fig. 1) were deposited onto a mild carbon steel substrate(S235J2G3, EN10025). Concentration of SiO2 particles in thesolution was held on the level of 1.5 and 3.0 wt% that resultedin the concentration of silicon dioxide in the coating equal to 4.5and 7.0 vol% respectively that is lower than reported in Ref. [26].The amount of the silicon dioxide was confirmed by EDS analysisof polished samples.

In general, electroless co-deposition processes of second phaseparticles take place at low temperature and the chemical inter-action is not favoured between the particles and the matrix. Theparticles are only physically entrapped into the Ni–P matrix.Therefore heat treatment of these coatings is necessary topromote high hardness and improve wear resistance. Coatingshave been heat treated at 400 1C for 1 h [26,27]. Low-molecularfine-dispersed PTFE particles (size of a single particle was notexceeding 7 mm) were sprayed onto the base coating and melted

Fig. 1. SEM images of surfaces

Table 1Properties of the coatings studied.

Coating Thickness (mm) Microhardn

NiP 6.071.8 8127125

NiP–4.5 vol% SiO2 5.070.7 8497204

NiP–7.0 vol% SiO2 6.670.3 9467345

NiP–PTFE 4.770.8 543763

NiP–4.5 vol% SiO2–PTFE 5.770.1 507148

NiP–7.0 vol% SiO2–PTFE 3.770.1 5937139

Steel substrate (S235J2G3) – 232712

at 340 1C during 1 h. Thickness of the PTFE layer was non-uniform: it was larger at the place of initial position of the PTFEcluster and less between them (Fig. 1c and d). Coatings composi-tions and their thicknesses are given in Table 1. The specimenswere prepared in size of 5 mm�15 mm�30 mm. The coatingswere tested as received without any surface modification. The

of as-deposited coatings.

ess (HV0.05) Roughness parameters (mm)

Ra Rmax Rz

1.570.2 12.671.2 10.571.3

1.570.2 13.072.8 10.771.4

1.270.1 10.971.1 8.871.4

1.470.2 13.473.0 11.171.4

1.470.3 15.775.8 10.373.1

1.670.2 13.275.5 10.672.5

1.270.1 9.871.0 8.371.4

D. Gutsev et al. / Tribology International 65 (2013) 295–302 297

schematic cross-sectional representation of the NiP coating withSiO2 and PTFE is depicted in Fig. 2.

Fig. 3. Determination of the critical force by scanning method (step 2).

2.2. Experimental details

Microstructural examination of specimens was conducted by ascanning electron microscope (SEM) Zeiss EVO MA15 suppliedwith energy dispersive X-ray spectroscopy (EDS) — INCA analyserand Hitachi TM-1000 scanning electron microscopes with EDSmodule. Coating thickness was determined by means of ballcratering method (kaloMAX, BAQ GmbH) with the help of opticalmicroscope Zeiss Axiovert-25 according to EN1071. Ball of 20 mmin diameter was used with 5 mm diamond slurry. Values given inTable 1 are the average of three measurements. Microhardnesswas determined with the help of Buehler Micromet-2001 device.Measurements were done by Vickers diamond indenter usingload of 0.5 N according to ISO 6507-2005. Seven measurements ofeach coating were made, maximum and minimum values wereignored and the average of remaining five values is given inTable 1. Zwick/Roell ZHR 8150 LK Rockwell hardness testingmachine and optical microscope Zeiss Axiovert-25 were appliedfor assessment of coating adhesion to substrate according toCEN/TS 1071-8. Load was 588.6 N (HRA). Roughness of bodieswas measured by Mahr perthometer PGK 120 in contact mode.

Table 2Sliding test conditions with constant load.

Parameter Confi

Ball-O

Mode recip

Ball diameter (m) 3�1

Ball material ZrO2

Load, N (g) 0.98

Amplitude (m) 5�1

Radius (circumference) of track (m) –

Frequency (Hz) 5

Linear velocity, (m s�1) 0.050

Duration (min) 5 (30

Wear path length (m) 15 (9

Number of repetitive sliding events (motions) other the same

point on the track, times

3000

Atmosphere Air, 2

n Test with duration of 300 min was performed for NiP–PTFE coating to

Table 3Scanning test conditions to determine the critical load.

Feature/parameter

Configuration

Mode

Ball diameter (m)

Ball material

Load, N (g)

Amplitude (m)

Length of scan (plate movement) (m)

Frequency (Hz)

Feed i.e. sample movement rate, cycles mm-1 (duration of test, min)

Atmosphere

n For determination of the COF used for tracing the critical force (F

Tribological sliding tests were done using CETR UMT-2 tribo-meter. Main conditions of the testing are given in Tables 2 and 3.High purity yttria-stabilized zirconia (95% ZrO2, 5% Y2O3) ceramicballs of hardness HV10 – 1250 and roughness Ra – 0.08 mm as wellas steel balls (EN 100Cr6/ AISI 52100 bearing chrome steel) ofhardness HV 848 (converted from HRC) and roughness Ra – 0.07 mm

guration

n-Plate Ball-On-Disk

rocating unidirectional

0�3 3�10�3

and EN 100Cr6 (AISI 52100) ZrO2 and EN 100Cr6 (AISI 52100)

(100) 0.98 (100)

0�3 –

5 (31.4)�10�3

–

0.033

0n) 60

00n) 120

(180,000n) 3822

372 1C, relative humidity 4575%

estimate the resistance and to monitor wear mechanism.

Description

Ball-On-Plate, plate is coated

Ball is reciprocating, plate is moving, force is increased

3�10�3

EN 100Cr6 (AISI 52100)

0.10–8.92 (10–910) for coatings without PTFE

0.98 - 98.98 (100 - 10100) for PTFE containing coatings

0.49 (50)n

1�10�3

10�10�3

5

50 (1.7)

200 (6.7)

800 (26.7)

3200 (106.7)

12,800 (426.7)

Not moving

Air, 2372 1C, relative humidity 4575%

ig. 3).

Fig. 4. Evolution of coatings COF under constant load in reciprocative and unidirectional regimes with ZrO2 and 100Cr6 balls.

Fig. 5. Effect of lower specimen feeding rate on critical force for NiP–7 vol%SiO2–

PTFE coating determined by scanning method.

D. Gutsev et al. / Tribology International 65 (2013) 295–302298

were used. Prolonged tests were carried out to monitor the wearmechanism.

NiP coatings, especially with PTFE additives are having excel-lent wear resistance and low COF and the tribological test toobtain reliable material loss data would be extremely timeconsuming. The substrate and coating (NiP coating usually repli-cate the substrate roughness) were prepared to have roughness(Table 1) being high comparing to that in real applications(Ra¼0.1–2.0 mm). Surfaces with high roughness are used muchoften since they are easier to produce and they are economicallyfeasible. Coatings have generally better adhesion if they aredeposited onto rough surface [4]. However, high roughness leadsto generation of extreme stress concentration in the asperitiesunder loading and may result in high wear and low load carryingcapacity. In addition to high roughness, coating has a multilayerstructure with phases (PTFE, NiP, SiO2; Fig. 2) having different

density, COF and wear rate. PTFE is present only on the top of thecoating and it is not reasonable to measure the mass or volumeloss of such multilayer coating. It is also noted by other research-ers that NiP deposits are too thin for reliable wear rate measure-ments [4] even for much thicker coatings (35 mm [27], 25–30 mm[26]). The scanning test (Table 3, Fig. 3) similar to those used withanother tribological devices [28,29] was applied instead to deter-mine the critical load that may be carried by coating. The idea ofthe method is to move the coated specimen in two directions.Specimen is moving back and forward as during usual reciprocat-ing test (along X axis, amplitude 1 mm, Fig. 3) while at the sametime it is continuously and unidirectionally being moved tosupply fresh coating into contact (along Y axis, 10 mm). The feedrate is the number of reciprocative cycles done by specimen in X

direction while it travels 1 mm in Y direction. Frequency ofreciprocation in X direction is same for all tests. High value offeed rate means that the velocity in Y direction is low and thetime required to finish test is longer. The test comprises twosteps. The first step is required to obtain COF of the coating inmild conditions with constant force of 0.49 N (feed rate is800 cycles mm�1) to ensure that the coating is not damaged.The second step is done to determine the maximum critical forcethat the coating is able to sustain. The only difference betweenthe steps is that during the second step the force is continuouslyincreased from low to high level. The analysis of the second stepresults are done after the test. When the force is low enough theCOF is similar to that observed during the first step. The forcecorresponding to the COF that is 50% higher than the COF of thecoating determined during the first step is assumed as a criticalone. Fresh surface of coating was used for both steps. At least3 tests were performed and the average value is calculated.Scanning test was done only with the steel ball that is mostlyused as counterbody for NiP coatings.

Fig. 6. Coefficient of friction and critical forces of coatings tested by scanning method.

D. Gutsev et al. / Tribology International 65 (2013) 295–302 299

3. Results

It is evident from Table 1 that there is an increase in thehardness values obtained due to the SiO2 particle reinforcement

in the Ni–P matrix. It is known that fine precipitations create anadditional barrier for the movement of dislocation to propagate.Addition of PTFE droplets results in the lower hardness values ascompared to untreated coatings (Table 1). Lower hardness values

Fig. 7. SEM image of NiP–PTFE coating after 300 min reciprocative test (extended duration). PTFE layers are appeared as the dim grey coloured areas (indicated by arrows).

D. Gutsev et al. / Tribology International 65 (2013) 295–302300

of the samples with Ni–PTFE and NiP–7.0 vol% SiO2–PTFE coatingscan also be attributed to lower thickness of these coatings andrelated increased influence of softer substrate during hardnessmeasurement.

Microstructure of NiP-based coatings is composed of charac-teristic globular shaped grains (Figs. 1, 2). It was found (by EDS)that grain boundaries have higher phosphorus content than graininteriors. Coating with high content of SiO2 (NiP–7.0 vol% SiO2)has slightly smaller size of grains, lower roughness and low levelof porosity and other defects (outgrowths protruding above thesurface, agglomerates having low adhesion with coating, etc.) ascompared to base NiP coatings (Fig. 1; Table 1). PTFE clusters ofNiP–SiO2––PTFE coating are shown in Fig. 1(d). Coating with SiO2

has more uniform distribution of PTFE clusters (compare Fig. 1cand d). There is significant dispersion of thickness measurementresults that is associated with high roughness of initial substratesand deposited coatings (Table 1). Roughness of the deposit addedby SiO2 is decreased while roughness of the deposit coated byPTFE is slightly increased. Single radial cracks without adhesivedelamination were characteristic for all coatings studied thatcorresponds to the Class 1 according to CEN/TS 1071-8 indicatinggood adhesion of the coatings to the substrate.

Behaviour of the coatings in both modes (reciprocating andunidirectional sliding) is similar (Fig. 4). COF of NiP–PTFE andNiP–SiO2–PTFE coatings was usually improved by additives com-paring to the basic NiP coating. Addition of SiO2 (NiP- 7 vol%SiO2

coating) is favourable only when steel ball (100Cr6) is used. Whenceramic ball (ZrO2) with hardness exceeding that of the NiP–SiO2

coating is used, the SiO2 additives do not prevent the coating fromthe excessive wear. Wear mechanism in this case could becharacterised by growing role of a brittle mode of fracture.

Behaviour of coatings during scanning test was similar (com-paring to conventional test results), generally resulting in highervalue of critical force when sufficient supply of fresh coatingmaterial was enabled (low value of feed rate), Fig. 5. The lowestcritical force was observed during tests with slow movement ofthe sample in Y direction (high feed rate) when the contactbetween the side wall of wear crater and steel ball took place.In conditions when wear debris are accumulated and are servingas solid lubricant (sample is not moving in Y direction) thesufficiently high critical force was measured.

Critical forces for coatings determined with feed rate of 800,12,800 cycles mm�1 and when the sample was not moving inY direction are given in Fig. 6. Addition of PTFE enables to reducethe COF of coatings several times during conventional tests(Fig. 4) and down to 37% when determined by scanning method(Fig. 6). Addition of silicon dioxide was beneficial in reduction ofthe COF only in case of NiP–4.5 vol% SiO2–PTFE coating. Criticalforces of PTFE-containing coatings are order of magnitude higher

than that of plain NiP or NiP with silicon dioxide additives. PlainNiP has higher critical force when the sample is not moving ormoving very slowly (high value of feed rate). Addition of SiO2 toNiP is beneficial only when supply of fresh coating is sufficient(feed 800 cycles mm�1). Addition of silicon dioxide to the NiP–PTFE coating cannot significantly improve the load bearingcapacity of the tribosystem.

SEM images after testing of coatings by conventional (Fig. 7)and scanning method (Fig. 8) with results of EDS analysis(Table 4) are showing that the highest peaks of NiP and later thatof steel substrate material are first removed from the surface. Ironis present on the surface even after testing with low force (a1–a3,d1, d2, g1, g2; Table 4) that indicates that highest peaks arealready worn. In the case of PTFE containing coatings the load iscarried by PTFE clusters providing almost no wear for substrateand NiP (Fig. 7). Particles of wear debris (NiP and steel) werefound to be embedded into the PTFE clusters (Fig. 7b). Mechanicalmixing of components was observed (b4, e3, h2; Table 4).

Main mechanism of coatings degradation is adhesive andoxidative wear off at the contact points where high flash tem-perature is generated. Wear debris are more oxidised than initialcoating. Fine wear debris (b4, c2, c3, f5; Table 4) have highercontent of oxygen than coarse ones (a3, b3, g5; Table 4). PTFEcontent (fluorine, Table 4) in all locations of PTFE containingcoatings was sufficiently high indication that it is covering thewhole surface. PTFE is transferred even to damaged surface ofcoating (e4, Table 4) Rapid reduction of PTFE was observed afterfailure of coatings (f1–f5, j1–j3; Table 4). Scuffing was observedonly after failure of the coating when force was higher thancritical for given coating.

4. Discussion

PTFE addition is beneficial if reduced COF and increasedcritical force are required. This can be explained by formation ofthe PTFE-rich mechanically mixed layer responsible for good anti-friction properties (Figs. 4–8) [4,30].

SiO2 may be required for long runs in real applications toprovide a load-bearing capacity for protection of NiP–PTFE con-taining tribo-layer from shearing. Indeed, silicon dioxide addi-tions were beneficial to increase the hardness of the NiP and NiP–PTFE coatings. However they are able to reduce the COF for NiPcoating only with steel counterbody when tested under low loads(0.98 N, Fig. 4). Critical loads of silicon dioxide containing coatingsmay be improved only when supply of fresh coating surface issufficient (Figs. 5, 6). High stresses generated during the contactbetween ceramic counterbody and silicon dioxide leads to therupture of the nickel oxide and PTFE film, direct and fatigue

Fig. 8. SEM images of NiP, NiP–PTFE and NiP–7.0 vol% SiO2–PTFE coatings after testing by scanning method with indication of force and locations where EDS analysis was

performed (see also Table 4).

D. Gutsev et al. / Tribology International 65 (2013) 295–302 301

crashing and removal of the silicon dioxide that starts acting asan abrasive that in its turn leads to higher wear rates. Similarsignificant reduction of wear resistance was also observed in thecase of NiP–PTFE coating sliding in air and in nitrogen atmosphere[31] when the formation of nickel oxide layer was impeded.

5. Conclusions

1.

PTFE addition reduces coefficient of friction and increases loadcarrying capacity of NiP coating when rubbed against ceramicor steel counterbody.

Table 4Results of EDS analysis of worn surfaces of coatings. Locations are depicted

in Fig. 8.

Coating Position Elements (wt%)

Ni P F Si O Fe

NiP a1 83.1 12.7 – – 2.0 2.2

a2 85.2 11.5 – – 1.4 2.0

a3 77.3 11.2 – – 8.4 3.1

b1 85.6 10.4 – – 1.3 2.7

b2 84.9 11.8 – – 0.9 2.4

b3 72.8 10.6 – – 5.5 11.1

b4 52.5 8.7 – – 31.3 7.4

c1 81.0 11.0 – – 3.1 4.9

c2 51.2 8.0 – – 24.2 16.7

c3 52.7 8.3 – – 26.2 12.8

c4 2.9 0.4 – – 2.7 94.0

NiP–PTFE d1 82.2 12.1 2.9 – 1.3 1.6

d2 29.5 2.8 95.5 – 0.8 1.3

e1 82.0 12.7 1.9 – 0.5 2.9

e2 81.8 12.0 2.2 – 2.1 2.0

e3 49.8 5.5 40.8 – 1.1 2.4

e4 55.2 10.3 15.9 – 2.1 16.5

e5 69.4 10.7 2.0 – 2.5 15.5

f1 0.1 0.1 0.7 – 1.1 98.0

f2 2.5 0.4 0.0 – 22.1 75.0

f3 0.5 0.2 0.8 – 8.5 90.1

f4 3.0 0.5 0.0 – 16.6 80.0

f5 1.3 0.2 0.0 – 30.4 68.1

NiP–7.0 vol% SiO2–PTFE g1 78.7 10.8 2.8 0.8 3.4 3.6

g2 80.7 12.5 1.8 0.7 1.5 2.9

g3 64.5 3.3 27.7 0.3 1.3 2.8

g4 80.4 11.5 2.1 0.5 1.8 3.9

g5 68.0 9.6 11.2 1.9 6.0 3.4

h1 72.2 9.8 3.1 0.7 6.0 8.3

h2 39.5 6.2 15.1 1.2 19.4 18.6

i1 30.2 2.6 2.2 0.2 5.7 59.2

i2 77.5 10.3 2.3 0.5 4.1 5.3

i3 62.9 8.5 3.0 0.6 4.3 20.8

j1 15.2 2.2 1.3 0.1 4.9 76.4

j2 4.5 0.5 0.8 0.1 7.5 86.7

j3 3.6 0.6 0.8 0.0 4.6 90.3

D. Gutsev et al. / Tribology International 65 (2013) 295–302302

2.

Addition of silicon dioxide to NiP is beneficial only in the caseof steel counterbody under low loads or with sufficient supplyof fresh coating material into contact zone.

3.

Nano-sized SiO2 addition to NiP coating slightly reduces thesize of grains and roughness.

4.

Method for determination of the load carrying capacity (cri-tical force) of the thin coatings in Ball-On-Plate configurationis proposed.

Acknowledgements

The authors would like to acknowledge DoRa programme(personal grant for D.Gutsev) as well as Estonian Ministry ofEducation and Research (Targeted finance Project no. SF0140062s08)and Estonian Science Foundation (grants ETF 8850 and 8211) for thefinancial support of the study.

References

[1] Reidel W. Electroless nickel plating. Melksham, Wiltshire: Redwood Press:1991.

[2] Bozzini B, Cavallotti PL, Buratti A, Krueger F. Controllo del processo dideposizione chimica autocatalitica di riporti funzionali di Ni–P. AIFM Galva-notecnica 1991;2:53–66.

[3] Belen’kijj MA, Ivanov AF. Electrodeposition of metallic coatings. Moscow:Metallurgija; 1985.

[4] Sahoo P, Das SK. Tribology of electroless nickel coatings — a review. Materialsand Design 2011;32:1760–75.

[5] Balaraju JN, Sankara Narayanan TSN, Seshadri SK. Electroless Ni–P. compositecoatings 2003;33:807–16Journal of Applied Chemistry 2003;33:807–16.

[6] Agarwala RC, Agarwala V. Electroless alloy/composite coatings: a review.Sadhana 2003;28:475–93.

[7] Myshkin NK, Grigoriev AY, Gutsev DM, Ignat M, Chainet E, Grandvallett V,et al. Tribological behavior of thin electroplated and chemically depositedNi–P coatings on copper substrates. Journal of Friction and Wear 2010;31:413–8.

[8] Mallory GO, Hajdu JB. Electroless plating — fundamentals and applications.Noyes: William Andrew Publishing; 1990.

[9] Barker D. Electroless deposition of metals. Transactions of the Institute ofMetal Finishing 1993;71:121–60.

[10] Kunst H. Verlschleiss metallischer werkstoffe und seine vermin-derungdurch oberflaechenschichten Grafenau: Expert Verlag; 1982.

[11] Gawne DT, Ma U. Structure and wear of electroless nickel coatings. MaterialsScience and Technology 1987;3:228–38.

[12] Lu GX, Li GF, Yu FC. The effect of phosphorus content and heat treatment onthe wear resistance of electroless deposited Ni–P alloys. Wear1989;103:269–78.

[13] Jiaqiang G, Lei L, Yating W, Bin S, Wenbin H. Electroless Ni–P–SiC compositecoatings with superfine particles. Surface and Coatings Technology2006;200:5836–42.

[14] Balaraju JN, Kalavati, Rajam KS. Influence of particle size on the microstruc-ture, hardness and corrosion resistance of electroless Ni–P–Al2O3 compositecoatings. Surface and Coatings Technology 2006;200:3933–41.

[15] Alirezaei Sh, Monirvaghefi SM, Salehi M, Saatchi A. Effect of alumina contenton surface morphology and hardness of EN-Al2O3 electroless compositecoatings. Surface and Coatings Technology 2004;184:170–5.

[16] Sheela G, Pushpavanam M. Diamond-dispersed electroless nickel coatings.Metal Finishing 2002;100:45–7.

[17] Low CTJ, Wills RGA, Walsh FC. Electrodeposition of composite coatingscontaining nanoparticles in a metal deposit. Surface and Coatings Technology2006;201:371–83.

[18] Jin Y, Hua L. Preparation and tribological properties of Ni–P electrolesscomposite coating containing potassium titanate whisker. Journal of Materi-als Science and Technology 2007;23:387–91.

[19] Ramalho A, Miranda JC. Friction and wear of electroless NiP and NiP PTFEcoatings. Wear 2005;259:828–34.

[20] Wu Y, Liu L, Shen L, Hu W. Study of self-lubricant Ni–P–PTFE–SiC compositecoating. Journal of Materials Science 2005;40:5057–9.

[21] Ramalho A, Miranda JC. Tribological characterization of electroless NiPcoatings lubricated with biolubricants. Wear 2007;263:592–7.

[22] Zhao Q, Liu Y. Modification of stainless steel surfaces by electroless EN andsmall amount of PTFE to minimize bacterial adhesion. Journal of FoodEngineering 2006;72:266–72.

[23] Zou TZ, Tu JP, Zhang SC, Chen LM, Wang Q, Zhang LL, et al. friction and wearproperties of electroless EN-MoS2 composite coatings in humid air andvacuum. Materials Science and Engineering: A 2006;426:162–8.

[24] Moonir-Vaghefi SM, Saatchi A. Deposition and properties of electrolessnickel-phosphorus-molybdenum disulfide composites. Metal Finishing1997;95:46–52.

[25] Koshy RA, Graham ME, Marks LD. temperature activated self-lubrication inCrN/M2N nanolayer coatings. Surface and Coatings Technology 2010;204:1359–65.

[26] Hazan Y, Zimmermann D, Z’Graggen M, Roos S, Aneziris C, Bollier H, et al.Homogeneous electroless Ni–P/SiO2 nanocomposite coatings with improvedwear resistance and modified wear behaviour. Surface and Coatings Tech-nology 2010;204:3464–70.

[27] Dong D, Chen XH, Xiao WT, Yang GB, Zhang PY. Preparation and properties ofelectroless Ni–P–SiO2 composite coatings. Applied Surface Science 2009;255:7051–5.

[28] Vlad M, Szczerek MM, Michalczewski R, Kajdas C, Tomastik C, Osuch-S"omka E.The influence of antiwear additive concentration on the tribological behaviourof a-C:H:W/steel tribosystems. Proceedings of the IMechE, Part J: Journal ofEngineering Tribology 2010;224:1079–89.

[29] Podgornik B, Jacobson S, Hogmark S. DLC coating of boundary lubricatedcomponents-advantages of coating one of the contact surfaces rather thanboth or none. Tribology International 2003;36:843–9.

[30] Antonov M, Hussainova I. Cermets surface transformation under erosive andabrasive wear. Tribology International 2010;43:1566–75.

[31] Rosset EA, Mischler S, Landolt D. Dissipative processes in tribology surfacechemistry effects on friction of Ni–P/PTFE composite coatings. TribologySeries 1994;27:329–36.