Tribological and Corrosion Properties of Nickel/TiCBilayered Coatings Produced by Electroless Deposition

and PACVDAli Shanaghi and Paul K. Chu

(Submitted July 29, 2016; in revised form September 3, 2016; published online October 7, 2016)

Ni/TiC bilayered coatings are deposited on hot-working steel (H11) by plasma-assisted chemical vapordeposition and electroless technique. The TiC layer is deposited at 490 �C using a gas mixture of TiCl4,CH4, H2, and Ar, and a dense nanostructured TiC coating with minimum excessive carbon phases and lowchlorine concentration is produced. The effects of the Ni intermediate layer on the microstructure, tri-bology, and corrosion behavior of the nanostructured TiC coating are investigated. The friction coefficientof the Ni/TiC bilayered coating (Ni thickness = 4 lm) at 500 cycles is much smaller than that of the coatingwithout the Ni intermediate layer. The smallest friction coefficient is about 0.2, and the hardness values ofthe Ni/TiC bilayered samples with three different Ni layer thicknesses of 2, 4, and 6 lm are 2534, 3070, and2008 Hv, respectively. The wear mechanism of the Ni/TiC bilayered coatings is abrasive induced by plasticdeformation and fatigue during the sliding process. The smaller groove width on the 4-lm electrolessnickel-Ni3P/TiC bilayered coating correlates with the larger H/E ratio and the 4-lm nickel/TiC bilayeredsample shows the better wear resistance. The polarization resistance of the 6-lm electroless nickel-Ni3P/TiCcoating in 0.05 M NaCl and 0.5 M H2SO4 increases by about 8 and 15 times, respectively. The Ni inter-mediate layer increases the toughness of the coating and adhesion between the hard coating and steelsubstrate thereby enhancing the tribological properties and corrosion resistance.

Keywords corrosion, nanostructure coating, Ni intermediatelayer, PACVD, titanium carbide, wear

1. Introduction

Titanium carbide is a wear-resistant coating on account ofthe high hardness and elastic modulus, good wear resistance,and low friction coefficient (Ref 1, 2). However, wider use ofTiC coatings in the aerospace, automobile and related industryhas been hampered by the substrate mismatch and adhesionproblem caused by the typically low processing temperature inchemical vapor deposition (PACVD) and other commontechniques (Ref 3-6). Plasma-assisted chemical vapor deposi-tion (PACVD) is suitable for wear-resistant coatings ontemperature-sensitive substrates (Ref 7), and plasma CVD hasthe advantage that a coating with a uniform thickness andcomposition can be produced at a low temperature even on asubstrate with a complex shape (Ref 7-10).The success of thedeposition technique depends on the precise control of thecomposition and subsequent stability. For example, a sub-stoichiometric composition with no extra phases such as carbonand oxide phases in the TiC layer can be formed together withthe proper tribological and corrosion behavior respect to other

techniques such as CVD and PVD (Ref 3-6). One way toimprove adhesion of TiC films is by using metallic interlayersbetween the hard coating and soft substrate (Ref 6, 11, 12). Inthis study, the effects of a nickel intermediate layer on thetribological and corrosion properties of nanostructured TiCcoatings prepared by PACVD are investigated.

2. Materials and Methods

The hot-working die steel grade AISI H11 (DIN 1.2343)wasused as the substrate, and the chemical composition of AISI H11

was 0.36% C, 0.56% Si, 0.4% V, 4.6% Cr, 0.52% Mo, 0.37%Mn, and 0.01% P. The AISI H11 substrate (A109 5 mm) wasquenched and tempered to a hardness of 480 HV. Afterpolishing to a roughness of Ra = 2 lm using Al2O3 slurry, thesamples were rinsed with acetone and ultrasonically cleanedwith ethanol.

Electroless nickel deposition was performed in a containingnickel sulfate, sodium hypophosphite, and other chemicals asshown in Table 1 (Ref 13).

Electroless deposition was performed at a temperature of87± 1 �CandpHof5.4 for three different timedurations of10, 20,and 30 min. The pH value of the plating solution was systemat-ically controlled by addition of 50 vol.% H2SO4, and thetemperature of the nickel solution was kept at 87 �C in a waterbath. Prior to deposition, the samples were cleaned ultrasonicallyin acetone, rinsedwith distilledwater, degreasedwith 10%volumeNaOH, rinsing with distilled water again and finally activated anddeoxidized by immersion in 50% HCl for 1 min.

The nanostructured TiC coatings were deposited on sampleswith Ni intermediate coatings with 3 different thicknesses in a

Ali Shanaghi, Materials Engineering Department, Faculty ofEngineering, Malayer University, P.O. Box: 95863-65719, Malayer,Iran; and Paul K. Chu, Department of Physics and Materials Science,CityUniversity ofHongKong, Tat CheeAvenue, Kowloon,HongKong,China.Contact e-mails: alishanaghi@gmail.com,paul.chu@cityu.edu.hk,and a.shanaghi@malayeru.ac.ir.

JMEPEG (2016) 25:4796–4804 �ASM InternationalDOI: 10.1007/s11665-016-2378-8 1059-9495/$19.00

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PACVD reactor using a mixture of TiCl4, CH4, H2, and Ar. Theplasma was triggered by a pulsed DC power supply. Thesubstrates were placed on cylindrical cathodes and kept at anegative potential at a pressure of 5 mbar. Table 2 lists theimportant deposition conditions.

The crystalline structure of the coating was determined bygrazing incidence x-ray diffraction (GIXRD) (Philips PW-1730diffractometer) using the continuous scanning mode using Cu

Ka radiation (k = 0.154056 nm). The film composition wasdetermined by x-ray photoelectron spectroscopy (XPS, PHI5802) using monochromatic Al Ka radiation. An argon ionbeam was used to sputter off about 40 nm of the surface toremove contaminants before acquisition of the Ti2p and C1sspectra. The surface morphology, film uniformity, and homo-geneity were studied by scanning electron microscopy (SEM,Philips XL-30), and the thickness of the coating was deter-mined by scanning electron microscopy (SEM).

The Vickers microhardness was determined with a load of20 g and loading time of 15 s. The hardness values reportedhere represent an average of five measurements conducted atdifferent locations of the specimens. The adhesion of the TiCcoatings on hot-working steel H11 was evaluated using amicroscratch tester with a diamond tip (radius of 10 mm). Thesliding speed was 15 lm s�1. The critical load (Lc) wasdefined as the smallest load at which the coatings started todelaminate from the hot-working steel H11. Delamination ofthe TiC coatings was confirmed by optical microscopy. Themechanical properties such as Young�s modulus (E) andhardness (H) were determined by nanoindentation (A HysitronInc. Tribo Scope) using acno-spherical (radius R = 1.0 lm)diamond indenter (supplied by Dr. U.D. Hangen, SURFACE,Huckelhoven). The wear tests were conducted on a pin-on-disk(POD) apparatus (ASTM G99-90) under dry sliding conditions.A WC-Co ball 6 mm in diameter was employed. The load was5 N, and sliding speed was 0.1 m/s. The tests were performedfor 1220 cycles (radius R = 2 cm), and the temperature andrelative humidity were 28 �C and 36%, respectively. Prior tothe tests, the samples were cleaned with ethanol and dried inhot air. The friction and wear data were mean values of twotests.

The electrochemical measurement was carried out on theEG&G potentiostat/galvanostat (Prinston, model 273A) in0.5 M H2SO4 and 0.05 M NaCl. The standard electrochemical

Table 1 Ingredients in the solution used for electrolessdeposition of nickel

Component Concentration

Nickel sulfate (g/l) 24Sodium citrate (g/l) 15Sodium lactate (ml/l) 10Sodium acetate (g/l) 5Sodium hypophosphite (g/l) 15

Table 2 Important processing parameters in PACVD ofnanostructured TiC coatings

Parameters Value

Pulsed voltage (V) 590Pressure (mbar) 4-8Duty cycle (%) 40Process time (h) 2H2 (Nl/min) 1.6Ar (Nl/min) 0.05CH4 (Nl/min) 0.1-0.5TiCl4 (Nl/min) 0.05Temperature (�C) 490

Fig. 1 GIXRD diffraction patterns: (a) Ni intermediate layer, (b) TiC nanostructure coating, and (c) Ni/TiC bilayered coatings

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cell consisted of three electrodes including the coated sample asthe working electrode, saturated calomel as the referenceelectrode, and platinum net as the auxiliary electrode. Thecorrosion potential (Ecorr) and corrosion current density (icorr)were calculated from the intersection of the cathodic and anodicTafel curves using the Tafel extrapolation method. Thepolarization resistance (Rp) was determined by the Stren-Gearyequation (Ref 14).

3. Result and Discussion

Figure 1 shows the GIXRD diffraction patterns, and thereference peaks of TiC can be indexed to the ICCC-JCPDSvalues (titanium carbide No. 31-1400). The diffraction peak ofthe electroless nickel coating at 44.5� suggests that it amor-phous or microcrystalline. The chemical composition of the Ni-P deposit is 91.2% Ni and 8.8% P. Structural changes from theamorphous state to crystalline state have been observedfrom Ni-P electroless coatings at over 250 �C (Ref 15-17).Figure 1(a) shows that the Ni-P is amorphous under the as-plated condition. After depositing the TiC nanostructure

coating at 490 �C, the Ni-P samples crystallize and producenickel and nickel phosphide (Ni3P) (Fig. 1c). The TiC diffrac-tion peak of (200) at 43.9� confirms that the coating is TiC andthe (200) plane implies that the TiC coating is deposited underthermodynamically stable conditions (Ref 18, 19). After theTiC coating is deposited on the electroless nickel intermediatelayer, the TiC (111) peak is reduced. The deposition process ofa perfect TiC coating without a carbon phase is not a simpleprocess because stable carbon phases are easily formed in thefilm from excess carbon components under activated gaseousconditions. Hence, selection of the proper processing param-eters requires great care in order to produce perfect TiC withoutexcess carbon phases (3).

The TiC composition determined by XPS is shown in Fig. 2.According to Fig. 2(a), different Ti2p, C1s, and O1sXPS spectraare observed after sputtering the surface. Ion bombardmentreduces the O1s intensity suggesting that oxygen is a surfacecontaminant. Figure 2(b and c) displays the C1s and Ti2p spectraof the coatings. The Ti2pXPS signal is composed of Ti2p3/2 andTi2p1/2 doublets with binding energies of 455.2 and 460.8 eV,respectively. Actually, Ti has two oxidation states, and therelative intensity at 458.4 and 464 eV related to titanium oxide(TiO2) (Ref 20-22) is observed to decrease after surface

Fig. 2 XPS spectra: (a) Surface of TiC nanostructure coatings before and after argon sputtering to a depth of 40 nm, (b) C1s spectra of TiC,and (c) Ti2p spectra

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sputtering and also for the relative intensity of binding energy atabout 455.25 and 460.86 eV, related to titanium carbide (TiC)(Ref 20-22). The C1s spectra exhibit two components at 281.6and 284.8 eV. The former is related to C-Ti observed to increaseafter surface sputtering, and the latter is related to C-O whichdecreases after surface sputtering (Ref 20-22), suggesting thatthese are surface species. TheTi2p andC1s peaks indicate that thecoating is TiC with a C/Ti ratio of 1.1.

Figure 3 presents the SEM micrographs, and the filmthicknesses are summarized in Table 3. The nickel intermediatelayer increases the thickness of the TiC hard coating by more

than 0.2 lm compared to the TiC hard coating without anintermediate layer. It is related to the decrease in the activationenergy of the Ti-C reaction on the Ni surface and decreases thesize of the TiC particles. To limit the effects of defects, reducethe reaction between the steel substrate and plasma, andenhance the reaction between Ti and C without changing theplasma conditions such as the duty cycle and operatingtemperature, a protective interlayer (e.g., electroless nickel) isdeposited between the TiC nanostructured coating and steelsubstrate. The electroless nickel provides the catalytic activityto deposit pure TiC by PECVD, and electroless nickel provides

Fig. 3 SEM micrographs: (a) Electroless nickel intermediate layer (b) Single nanostructured TiC coating, and Ni/TiC bilayered coatings withthree different thicknesses of (c) 2 lm, (d) 4 lm, and (e) 6 lm

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a uniform thickness (Ref 23). The combination of theelectroless nickel interlayer and plasma CVD hard coating isexpected to improve the mechanical and corrosion properties ofthe duplex coating (Ref 13). A relatively smooth surface isobserved from the Ni-Ni3P/TiC bilayered coating, and a denseNi-Ni3P/TiC bilayered coating is obtained. The Ni layerimproves the growth rate of the TiC coating by enhancingnucleation.

As shown in Table 3, the microhardness of the electrolessnickel(as-plated condition) and Ni-P after applying a coating at490 �C are about 510 and 800 Hv, respectively, and thehardness values of the Ni/TiC bilayered with three different Nilayer thicknesses of 2, 4, and 6 lm are 2534, 3070, and2008 Hv, respectively. In comparison, the hardness of thenanostructured TiC coating is 2290 Hv. The hardness of the Ni/TiC coating decreases when the thickness of electroless nickelis increased from 4 to 6 lm. The hardness of the 4 lmelectroless nickel/TiC bilayer is the highest, followed by 2 and6 lm.

The critical loads LC determined by the scratch adhesiontest are listed in Table 3. Compared to the coated samples, the4-lm Ni/TiC bilayered coating has the largest critical load. Thehigh hardness of the 4-lm Ni/TiC bilayered coating is related tothe interfaces being an effective obstacle against latticedislocation slip that is the dominant deformation mechanismin nanostructured coatings (Ref 24, 25). Moreover, theinterfaces may depend on the thermo-dynamical equilibriumconditions and deposition temperature (Ref 26). Therefore, thethickness of the Ni electroless increases from 4 to 6 lm. Itenhances the ductile part of the belayed coating (Ni electroless)rather than hardening part (TiC coating) decreasing adhesion ofthe TiC coating. As a result, the 4-lm Ni/TiC bilayered coatinghas the largest critical load. The interfacial integrity andadhesion of the Ni electroless deposited coatings are importantproperties and can be enhanced by the presence of theappropriate thickness interlayer. Further studies indicate thatthe properties of titanium carbide are related to the chemicalcomposition and texture and also affect the adhesion andresidual stress in the thin films (Ref 8, 24, 27). However, theexact nature of the chemical bonding between ductile interme-diate layer and hard top coating requires further research, andmore work is being conducted in our laboratories.

Figure 4 shows the variations in the friction coefficients onthe coated samples. The friction coefficient decreases when a 4-lm electroless nickel intermediate layer is used and the smallestfriction coefficient is about 0.2. The large friction coefficient ofthe TiC coating without the intermediate layer is probably dueto the brittle nature of TiC and poor adhesion. The tribologicalproperties of ceramic coatings depend on properties such ascoating thickness, surface roughness, coefficient of friction,

hardness, stiffness, as well as elastic modulus ratios of thecoatings to the substrate. The ratio of hardness (H) to elasticmodulus (E), H/E, is a more appropriate factor to determine thetribological properties of hard coatings (Ref 28). According toTable 3, a large H/E ratio leads to better wear resistance for the4-lm nickel/TiC bilayered coating. This is achieved byreducing the elastic modulus (E) and increasing the hardness(H). The latter can be as low as and very close to that of thesubstrate so that coating delamination can be avoided.

The GIXRD diffraction patterns of the coated samples(Fig. 1) show that during TiC deposition, amorphous Ni isconverted into crystalline Ni and Ni3P phases in the electrolessnickel intermediate layer at 490 �C. A mixture of crystallineand amorphous structures enhances the mechanical adhesionbetween the intermediate layer and steel substrate. Theelectroless nickel-Ni3P intermediate modifies the stress discon-tinuity between the TiC nanostructure coating and steelsubstrate. To reduce the influence of the physical properties(source of stress) between the hard coating and soft substrate, aproper interlayer can improve coating adhesion. However, thestress in the Ni-Ni3P/TiC bilayered coating has a gradeddistribution and so the ability to support loading and resistplastic deformation is improved. In addition, the electrolessnickel layer not only works as an intermediate layer, but alsoreacts with the CH4 and TiCl4 precursors to enhance coatingbonding. Nonetheless, the exact nature of the chemical bondingrequires further research, and more work is being conducted inour laboratories.

Figure 5 shows the optical micrographs of the grooves afterthe wear tests conducted on a pin-on-disk (POD) apparatus(ASTM G99-90) under dry sliding conditions. The smallergroove width on the 4-lm electroless nickel-Ni3P/TiC bilay-ered coating with respect to those observed from otherbilayered coatings and TiC coatings correlates with the highermagnitude of H/E ratio. This results in a smaller contact area,and the smaller number of junctions requires less energy toshear during sliding (Ref 12).

Cracks are observed from the side of the groove on thesingle TiC coating indicative of the brittle nature of TiC.Delamination is observed from the Ni-Ni3P/TiC bilayeredcoatings albeit drastically decreased at the groove on the 4-lmelectroless nickel/TiC bilayered coating. The nature of weardebris detected from the grooves of the Ni/TiC and single TiCcoatings is distinctly different. The debris from the Ni/TiCbilayered coatings is more ductile than that on the single TiCcoating.

Figure 6 shows the result of wear mass loss of the diskmade of hot-working steel substrate, single TiC coating, andNi/TiC bilayered coatings with three different thicknesses ofNi as well as pins made of W-Co for different number of

Table 3 Thickness and mechanical properties of the coated samples

Sample Thickness (lm) Microhardness (Hv) Critical load (mN) H/E Friction coefficient

Ni/TiC (2 lm) 4.75 2534 37 0.068 0.25Ni/TiC (4 lm) 6.72 3070 43.5 0.092 0.2Ni/TiC (6 lm) 8.73 2008 24 0.033 0.3TiC 2.5 2290 32 0.064 0.25Electroless nickel (after applying coating) 4 800 12.9 0.045 0.5Electroless nickel (as-plated condition) 4 510 9 0.036 0.59Uncoated steel 450 0.68

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cycles. The wear mass loss increases with cycles. Comparedto the TiC coating, the Ni-Ni3P/TiC bilayered coatings showsmaller mass losses, especially the 4-lm electroless nickel-

Ni3P/TiC bilayered coating. In addition, the mass loss fromthe W-Co pins increases with cycles and the W-Co pin as thecounter body of the Ni-Ni3P/TiC bilayered coatings shows

Fig. 5 Optical micrographs of the grooves generated by the wear tests: (a) Single TiC coating and Ni/TiC bilayered coatings with three differ-ent thickness of Ni: (b) 2 lm, (c) 4 lm, and (d) 6 lm

Fig. 4 Friction coefficients vs. number of cycles at a load of 5 N at room temperature for the hot-working steel substrate and coated samples

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less wear mass loss compared to the TiC coating and hot-working steel substrate.

The wear of the single TiC coating obeys the adhesivemechanism, and the wear mechanism on the electroless nickel-Ni3P/TiC bilayered coating is adhesive, abrasive, or a combi-nation of the two. The 4-lm electroless nickel-Ni3P/TiC

bilayered coating is harder, and there is no gross adhesionbetween the bilayered coating and counter materials. This canexplain the occurrence of delamination. The wear of the 2- and4-lm Ni/TiC bilayered coatings is governed by the abrasivemechanism but that observed from the 6-lm coating is acombination of cutting and plowing. A sudden reduction in the

Fig. 6 Wear mass loss for (a) the coated sample and (b) pins, at three different numbers of cycles such as 200, 600, and 1200

Fig. 7 Potentiodynamic results obtained from the uncoated steel and coated samples in (a) 0.05 M NaCl and (b) 0.5 M H2SO4

Table 4 Polarization parameters of the uncoated steel and coated samples in 0.05 M NaCl and 0.5 M H2SO4

Solution Thickness of Ni, lm ba, mV/dec bc, mV/dec Ecorr, mV icorr, lA/Cm22 Rp, X/Cm22

0.05 M NaClUncoated steel 114 147 �602 59.2 471.5Ni-Ni3P/TiC coating 2 131 158 �435 0.2 155,694.3

4 147 178 �399 0.12 291,705.76 153 195 �402 0.09 414,167.9

TiC coating 129 161 �481 0.61 51,045.80. 5 M H2SO4

Uncoated steel 54 91 �609 981 15Ni-Ni3P/TiC coating 2 69 119 �524 51 372.3

4 128 143 �529 32 917.76 132 139 �465 9 3270.8

TiC coating 71 103 �534 83 220.1

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friction coefficient curve is also observed from the coatings,especially the 2-lm one. It is due to buildup of oxide debris atthe interface of the coating and counterpart. This oxide also actsas a lubricant giving rise to smaller friction coefficients on theNi-Ni3P/TiC bilayered coatings (Ref 29). However, investiga-tion of the exact wear mechanism of the Ni-Ni3P/TiC bilayeredcoatings requires further research, for example, phase andelemental analyses of the wear tacks by SEM, EDS, and Ramanscattering. The exact nature of chemical bonding and crosssection of the wear track requires more study as well.

Figure 7 presents the potentiodynamic results obtained fromthe coated samples in 0.05 M NaCl and 0.5 M H2SO4 at roomtemperature (28 �C), and the results are shown in Table 4. Theopen circuit potentials of the uncoated steel in 0.05 NaCl and0.5 H2SO4 are �602 and �609 mV versus SCE, respectively.Compared to the corrosion potential of hot-working steel, thecorrosion potentials of the TiC coating and Ni-Ni3P/TiCbilayered coatings increase positively. Both the anodic andcathodic branches of the coated samples shift to smallercurrents, i.e., smaller metal corrosion rates. Table 4 shows thatthe polarization resistance of the 6-lm electroless nickel-Ni3P/TiC coating in 0.05 M NaCl and 0.5 M H2SO4 increases byabout 8 and 15 times, respectively. The data reveal improvedcorrosion resistance and absence of substrate corrosion. Thecorrosion damage on the bare steel and TiC coatings in 0.5 MH2SO4 is more than that in 0.05 M NaCl, and the corrosionresistance of the 6-lm electroless nickel/TiC coating is the best.The 4-lm electroless nickel/TiC coating is second and the 2 lmone-third and worst. The trend in the corrosion resistance canbe explained by the formation of a barrier film on the bilayeredcoating and so the corrosion resistance increases with thethickness of the layer thickness. It is known (Ref 30) thatcolumnar, island and brittle structure, and high hardness asinherent properties of TiC coatings cause the presence of cracksand pinholes in coatings. Penetration of moisture and corrosivefactors from defects led to corrosion reactions at the interface ofthe coating and substrate. This undermines the abrasionbehavior by decreasing the adhesion of the TiC coating. As aresult, the Ni intermediate coating as an amorphous layerimproves the corrosion resistance at the interface between thecoating and substrate and consequently the wear behavior aswell.

4. Conclusion

Homogenous, smooth, low friction nanostructured TiCcoatings are deposited on hot-work steel substrate by plasmaCVD after an amorphous electroless nickel intermediate layerhas been deposited. The Ni/TiC bilayered coatings have asmother surface and are more uniform and compact than asingle TiC coating. The nickel-Ni3P/TiC bilayered coating witha 4-lm Ni intermediate layer is smoother and more stable (interms of friction) than the other Ni-Ni3P/TiC bilayered andsingle TiC coatings. Microscopic investigation of the weartracks shows that the TiC coating surface without electrolessnickel has extensive grooves after the wear test in contrast toNi-Ni3P/TiC bilayered coating. The hardness and adhesion ofthe TiC coatings are enhanced by using the optimal thickness ofelectroless nickel intermediate layer. The resistance to wear isalso improved, especially during long wear tests. The dominantabrasive wear mechanism of the Ni-Ni3P/TiC bilayered coat-

ings is plastic deformation and fatigue during the slidingprocess. The 6-lm nickel-Ni3P/TiC bilayered coating exhibitsbetter corrosion resistance than the other Ni-Ni3P/TiC andsingle TiC coatings in aggressive media such as NaCl andH2SO4. The corrosion resistance of the Ni-Ni3P/TiC bilayeredcoating increases with the Ni interlayer thickness in bothsolutions. This study shows that by selecting the optimalthickness of the electroless nickel intermediate layer, betterwear resistance and improved corrosion behavior can beaccomplished.

Acknowledgments

The authors would like to express their thanks to IranianNanotechnology Initiative Council. The work was financiallysupported by Malayer University Research Grant and Hong KongResearch Grants Council (RGC) General Research Funds (GRF)No. CityU 11301215 and City University of Hong Kong AppliedResearch Grant 9667122.

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4804—Volume 25(11) November 2016 Journal of Materials Engineering and Performance