lable at ScienceDirect

Vacuum 144 (2017) 27e35

Contents lists avai

Vacuum

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

Surface oxidation of Ni20Cr/Cr3O2 composite processed by ball millingand HVOF thermal spraying

Israel L�opez-B�aez a, *, Enrique Martínez-Franco b, Carlos Agustín Poblano-Salas c,Luís Gerardo Tr�apaga-Martínez d, 1

a Departamento de Ingeniería en Minas, Metalurgia y Geología, Universidad de Guanajuato, Ex Hacienda San Matías s/n., Fraccionamiento San Javier,Guanajuato, Guanajuato C.P. 36025, M�exicob Direcci�on de Ingeniería de Superficies, Centro de Ingeniería y Desarrollo Industrial, Av. Playa Pie de la Cuesta 702, Desarrollo San Pablo, Santiago deQuer�etaro, Quer�etaro C.P. 76125, M�exicoc CIATEQ A.C., Centro de Tecnología Avanzada, Av. Manantiales 23 A, Parque Industrial Bernardo Quintana, El Marqu�es, Quer�etaro C.P. 76246, M�exicod Centro de Investigaci�on y de Estudios Avanzados del Instituto Polit�ecnico Nacional, Libramiento Norponiente # 2000, Fraccionamiento Real de Juriquilla,Quer�etaro, Quer�etaro C.P. 76230, M�exico (on sabbatical leave at CIATEQ A.C.)

a r t i c l e i n f o

Article history:Received 26 March 2017Received in revised form11 July 2017Accepted 11 July 2017Available online 12 July 2017

Keywords:Composite powderOxide dispersionBall millingOxide layerHVOF coating

* Corresponding author.E-mail addresses: israel.baez@ugto.mx (I. L�opez-B�a

edu.mx (E. Martínez-Franco), carlos.poblano@ciatetrapaga@cinvestav.mx (L.G. Tr�apaga-Martínez).

1 (on sabbatical leave at CIATEQ A.C.)

http://dx.doi.org/10.1016/j.vacuum.2017.07.0110042-207X/© 2017 Elsevier Ltd. All rights reserved.

a b s t r a c t

A Ni20Cr/Cr2O3 composite powder was successfully processed by controlling different mechanicalmilling conditions. The powder was heat treated at various temperatures to evaluate the effects of fineCr2O3 particles additions on the microstructure of the Ni20Cr matrix. Thermogravimetric analysis indi-cated an earlier oxidation temperature of the composite powder (250e270 �C) compared to the oxidationtemperature of the Ni20Cr matrix (680e700 �C), the fine Cr2O3 particles may act as nucleation sites topromote matrix oxidation at lower temperatures. Ni20Cr/Cr2O3, commercial, and experimental Ni20Crpowders were used as feedstock to produce coatings by HVOF thermal spraying. Raman studies showedNiCr2O4 and Cr2O3 as main phases in the microstructure of Ni20Cr/Cr2O3 composite coating. A residualNi20Cr phase, processed by ball milling, was preserved in the coatings microstructure; such a phaseimproved oxygen and chromium diffusion, then allowing the formation of a thick oxide protective layerin the coatings during oxidation tests. Oxide scales produced during sliding wear test were stronglyanchored to the Ni20Cr/Cr2O3 coating surface, a wear rate of 20.2 � 10�6 mm3/N$mwas measured whensuch a coating was tested employing a pin-on-disc configuration.

© 2017 Elsevier Ltd. All rights reserved.

1. Introduction

One of the most effective ways of enhancing the performance ofindustrial components is to engineer the surface so as to makethem suitable for tasks that are different from the primary functionof the bulk material [1]. Several processing techniques have beendeveloped to enhance the surface performance of a material, one ofthem is high velocity oxy-fuel (HVOF) thermal spraying. Thistechnique applies consumable powder in the form of finely dividedmolten and semi-molten droplets that collide at high velocity on arough surface to produce a dense coating. Powder characteristics

ez), enrique.martinez@cidesi.q.mx (C.A. Poblano-Salas),

have important effects on coating performance; the chemicalcomposition, particle size distribution, crystallinity, andmorphology of powders can be modified accordingly by the se-lection of different manufacturing routes [2e4]. Mechanical alloy-ing (MA) is a milling technique that was originally developed toproduce oxide dispersion strengthened (ODS) nickel- and iron-basesuperalloys for applications in the aerospace industry [5]. Powderproduction by mechanical milling has been used to produceconsumable powders in thermal spray process when componentsof experimental alloys or composites are individually evaluated[6e8].

Common compositions of commercial mechanical alloyednickel-based superalloys typically have a solid solution strength-ened Ni-Cr matrix with small amounts of Al, Ti, W, and Mo [5]. TheNi and Ni20Cr alloy has been extensively studied, specifically whenthe alloy is strengthened with hard particles like Cr3C2, Y2O3, Al2O3,and Cr2O3 [9e12]. In thermal spray coatings, high energy millinghas been employed to produce Ni20Cr/Al2O3 composite feedstock

I. L�opez-B�aez et al. / Vacuum 144 (2017) 27e3528

powders. Pre-milled Al2O3 particles have also been employed toproduce homogeneous dispersion structures with fines oxide par-ticles in the Ni20Cr matrix [11]. It is well known that differentoxides are formed in Ni-Cr alloys depending on the Cr concentra-tion, e.g. the scale formed on alloys with 10e20% of Cr is a mixtureof NiO, Cr2O3, and NiCr2O4, with Cr2O3 particles that start to coa-lesce to form a continuous layer; the presence of grain boundariesenhancing such coalescence [13]. According to Birks [14], when theCr content of the alloy increases, the volume fraction of spinelNiCr2O4 also increases, reducing the total Ni flux through the scalewith a concomitant reduction in the oxidation rate.

In this work, Ni20Cr powder particles were strengthened withfine Cr2O3 particles by ball milling in order to study superficialoxidation of Ni20Cr particles when the dispersion of Cr2O3 particlesis high. In order to evaluate the effect of Cr2O3 particles embeddedin a bulk Ni20Cr matrix, a Ni20Cr/Cr2O3 coating was processed byHVOF thermal spray using the feedstock produced. Thermogravi-metric, x-ray diffraction, and micro-Raman analyses were used tostudy the oxides formed on the Ni20Cr/Cr2O3 coating. Wear andoxidation tests were performed to investigate the formation ofoxide scale in the presence of embedded Cr2O3 particles in theNi20Cr matrix.

2. Materials and methods

2.1. Feedstock processing

An experimental Ni20Cr powder was mixed with Cr2O3 hardparticles to produce a composite powder by ball milling (BM). TheNi20Cr/Cr2O3 composite powder was processed using a horizontalSimoloyer CM01 (Zoz GmbH, Wenden, Germany) attritor. Theexperimental Ni20Cr powder was previously alloyed by high en-ergy ball milling (HEBM) for 30 h under an argon atmosphere andanalyzed [15]. The powder was then mechanically mixed withCr2O3 hard particles (ALDRICH, d50 ¼ 50 mm, 98% purity, angularparticle shape) also employing an argon atmosphere. Dispersion ofCr2O3 particles in the matrix surface was performed by HEBM,whereas the particle size control of the Ni20Cr/Cr2O3 compositepowder was achieved by employing low energy ball milling(LEBM). The weight ratio of Ni20Cr/Cr2O3 was 3.22; also, 5 mmdiameter YSZ ceramic balls and a W01-1/2L milling chamber wereemployed in HEBM and LEBM. Table 1 contains the HEBM andLEBM parameters used to process the composite powder.

The experimental Ni20Cr powder produced by ball milling andan atomized Ni20Cr powder (Tafa, 1262F) were used to produce theNi20Cr coatings by HVOF thermal spraying. The experimentalNi20Cr/Cr2O3 powder produced by ball milling in this work, wasused to produce the composite coating by HVOF. Characteristics ofpowders are in Table 2. The particle size of the experimentalpowders was controlled by mechanical sieving.

2.2. Coating processing

AISI 8620 steel plane substrates (thickness ~ 5 mm) were gritblasted with alumina (ANSI G-20), cleaned with acetone, and driedwith pressurized air before spraying. Powders were dried in anelectric furnace at 120 �C for 1 h before spraying. A Mitutoyo SJ-201/301 roughness tester was used to measure the substrateroughness (Ra, 5.5 ± 0.2 mm). The spraying parameters employed toproduce the coatings with a Sulzer Metco DJ2700H HVOF gun(working with a DJ2702 aircap) using a reducing atmosphere [16]are found in Table 3. The temperature of the substrate-coatinginterface during spraying was approximately 225 �C; it wassensed using a thermocouple placed in a hole made on the sub-strate's surface. The coating was cooled with air directly applied on

the growing coating surface during spraying.

2.3. Metallographic preparation

In order to find the distribution of Cr2O3 fine particles within thecomposite powder; the latter was mounted in a conductivephenolic resin and prepared by conventional grinding and polish-ing procedures. For the case of coatings, these were mounted in ahard-phenolic resin in order to prevent deformation of the coat-ings' edge during grinding and polishing. Grinding was performedby employing silicon carbide emery paper, from P240 to P1200 gritsize, whereas polishing was carried out by employing 3 and 1 mmdiamond paste.

2.4. Characterization techniques

Morphological analyses of powders and coatings were per-formedwith a Philips/XL30ESEMmicroscope (high vacuum, 30 kV),whereas local semi-quantitative analyses were performed usingenergy dispersive X-ray spectroscopy (EDS). Conventional ZAFcorrections were performed by the equipment software. Thestructural analyses were carried out by using a Rigaku/Dmax2100diffractometer employing Co-Ka1 radiation. Powder oxidation wasstudied by thermogravimetric analysis performed at 10 �C/min in aTGA/SDTA851 Mettler Toledo apparatus, 30 mg of each powderwere used, approximately. Identification of oxides were performedon a polished sample employing a micro-Raman Spectrometer/Dilor-Labram II, the measurements were made at standard condi-tions, 20 mW, excitation wavelength of 632.8 nm (HeeNe laser),and a spot size ~ 2 mm.

2.5. Sliding wear test

Wear tests were conducted using a CSM Instruments Pin-on-Disc (POD) Tribometer at standard temperature and pressure con-ditions. The coatings' surface was polished on 1000 grit SiC pol-ishing paper before POD testing. The wear test was performedemploying fully stabilized 5.4 mm diameter ZrO2 balls as counter-parts, a 2 N load, a linear speed of 0.2 m/s, and a sliding distance of600 m. After each test, a circular wear track was formed on thespecimen surface and a wear scar on the ZrO2 pin, respectively.Weight loss was evaluated by calculating the volume of wear trackafter the test. The average cross-sectional area of thewear trackwasevaluated at two zones along the track length. The diameter of thewear track was ~14 mm. The pin weight loss was evaluated bycalculating the volume of the wear scar based on the circumferenceformed after the test.

3. Results and discussion

3.1. Composite powder

Mixing of a ductile metal with fine ceramic particles by me-chanical milling allows the insertion of fine hard particles on thesurface of larger ductile particles. On the other hand, smaller ductileparticles tend to agglomerate and enclose fine hard particles in theearly stages of milling processing [5]. In this sense, a chemicalcomposition gradient is expected between the powder surface andits core of larger experimental Ni20Cr particles. Fig. 1a and b showsthe Ni20Cr particles before and after mixing with Cr2O3 particles byball milling, respectively. SEM images clearly show that Ni20Cr/Cr2O3 particles are less flat, have a smaller size, and have a moresaturated surface with fine particles, compared with the initialNi20Cr particles. During milling of Ni20Cr and Cr2O3 particles, theflake shape of initial Ni20Cr particles tend to disappear while

Table 1Milling parameters and conditions employed for processing the Ni20Cr/Cr2O3 composite powder.

Energy condition Angular velocity, rpm Ball/Powder weight ratio Batches, weight, g Milling time, h

HEBM 1800 5.5 100 6LEBM 1200 2.75 200 1

Table 2Conventional and experimental feedstock powders.

Powder Condition Aspect ratio Meshsize (mm)

Particle size, d50 (mm)

1262F (Ni20Cr) Conventional �53 þ 20 25.8Ni20Cr Experimental 1.42 ± 0.23 �67 þ 30 49.4Ni20Cr/Cr2O3 Experimental 1.43 ± 0.25 �70 þ 33 47.4

Table 3HVOF spraying parameters employed to produce the coatings.

Parameter Parameter value

Spraying distance, m 0.3Gun traversal speed, m/s 2Horizontal plume pass, mm 5Powder feed rate, g/min 38C3H8 flow, LPM 56Air flow, LPM 264Oxygen flow, LPM 164Nitrogen, LPM 12.5g (fuel/oxygen ratio) 0.26

Fig. 1. SEM images of experimental powders. a) Ni20Cr and b) Ni20Cr/Cr2O3 particlesprocessed by HEBM.

I. L�opez-B�aez et al. / Vacuum 144 (2017) 27e35 29

increasing their equiaxial shape. The inclusion of fine hard particleson the surface of Ni20Cr particles results in the promotion of a finemicrostructure in such particles and a loss of workability.

Fig. 2a shows the morphology and microstructure of Ni20Cr/Cr2O3 particles, most of them have a solid core (see particle Z1).However, some particles have an evident lamellar microstructurewith embedded Cr2O3 particles (see particle Z2). Fig. 2b shows across section of a Ni20Cr/Cr2O3 particle including polished andunpolished areas. Localized semi-quantitative analyses by EDSwere performed to compare the oxygen concentration associatedwith fine Cr2O3 particles; Table 4 summarizes these analyses per-formed on P1, P2, and P3 zones, indicated in Fig. 2b. The Ni/Cr ratiois expected to be 4 for the Ni20Cr matrix (P1 zone), analyses indi-cate that the composite particle core was composed mainly by Niand Cr. However, contamination by Fe is evident as a result of wearof steel grinding tools (balls, vessel, and blades). P2 zone, has a Ni/Cr ratio around 3.0 as the concentration of fine Cr2O3 particlesbegins to increase at that zone. P3 zone, localized on an unpolishedarea of the powder, has a Ni/Cr ratio close to 1.2 because of fineCr2O3 particles are incrusted on surface particle, which increasedthe Cr and O concentration. Powder contamination with YSZ wasdetected on P2 and P3 zones, severe wear of ceramic balls used asgrinding media caused this effect.

The Ni20Cr and Cr2O3 powder particles were successfully mixedby HEBM according to the XRD pattern I included in Fig. 3, whichshows characteristic phases of both components. In order to eval-uate chemical interactions or phase transformations at relativelyhigh temperatures, powder composite samples were heat-treatedin air at 400 �C and 800 �C for 3 h, respectively. Analysis of thefull width at half-maximum (FWHM) of peaks, included in Table 5,indicates the presence of a moderate crystallization when thecomposite was heat-treated at 400 �C (both components coexists atthis temperature, pattern II), and an evident crystallization, whenthe heat-treating temperature was 800 �C. XRD pattern III shows astrong presence of NiO and NiCr2O4; both phases are expected to

form due to the known oxidation mechanism of Ni20Cr alloy [14].The strong presence of both phases is thought to be formed byexcessive oxidation of the Ni20Cr/Cr2O3 composite powder; that is,embedded Cr2O3 particles could accelerate Ni20Cr oxidation.

Table 5 presents a comparison of angular positions (2 thetadiffraction peaks) of Ni(Cr) solid solution at the different heat-treatment temperatures. The angular positions of the Ni(Cr)phase increased due to the mobility of Cr to grain boundaries toform oxides [17]. Fig. 4 shows the thermogravimetric analysis, theresult corroborates the early oxidation of composite powder, i.e.,the composite powder suffered a significant oxidation around

Fig. 2. Cross section of polished Ni20Cr/Cr2O3 powder. a) Polished and unpolishedparticles and b) localized points for semi-quantitative analyses.

Table 4Semi-quantitative analysis of the Ni20Cr/Cr2O3 particle shown in Fig. 2b.

Evaluated zone Chemical elements (wt. %) Ni/Cr

Ni Cr O Fe Zr Y

P1 / inner 77.5 20.0 0.9 1.6 e e 3.9P2 / middle 67.7 22.4 6.0 2.2 1.6 0.1 3.0P3 / outer 42.7 34.3 14.1 2.1 5.2 1.6 1.2

Fig. 3. XRD patterns of composite powder Ni20Cr/Cr2O3. Pattern I at room tempera-ture, pattern II at 400 �C, and pattern II at 800 �C.

I. L�opez-B�aez et al. / Vacuum 144 (2017) 27e3530

250e270 �C, while for the Ni20Cr powder a moderate oxidationstarted around 680e700 �C. The zoomed area (a) shows the initialoxidation temperature of the composite powder; which may beattributed to the strong influence of small Cr2O3 particles. Ac-cording to XRD and TGA results, significant formation of the Cr2O3phase is favored because Cr2O3 fine particles are embedded in theNi20Cr matrix; these particles acted as nucleation sites to form athermodynamically stable oxide [14,18].

Fig. 4. TGA curves of experimental (Ni20Cr) and composite powder (Ni20Cr/Cr2O3).

3.2. Coatings evaluation

The SEM images of the cross sections of the conventionalNi20Cr, experimental Ni20Cr, and Ni20Cr/Cr2O3 coatings obtainedusing the HVOF thermal spraying process on steel substrates areshown in Fig. 5. In coating C1 (conventional Ni20Cr powder) thelamellar structure was not evident like that observed in coating C2(experimental Ni20Cr powder), see Fig. 5a and b. Coating C3(experimental Ni20Cr/Cr2O3 composite powder) showed a fine

lamellar structure on localized zones, where the splat preserved theoriginal lamellar structure of composite powder, as observed inFig. 2a. On the other hand, the microstructure of composite coatingC3 showed zones exclusively formed by Ni20Cr. The thickness ofcoatings C1, C2, and C3 were ~282, 387, and 364 mm, respectively.

3.2.1. Oxides analysisRaman spectra of selected oxygen-rich zones are shown in

Fig. 6a, Raman band positions of NiO, NiCr2O4, Cr2O3, and g-Fe2O3employed as references are drawn as vertical lines [19e21]. Fig. 6bshows the cross-sectional morphology of an as-sprayed Ni20Cr/Cr2O3 coating, oxygen-rich zones I, II, and III were specificallyselected to performed the analyses. The Cr2O3 phase is identified inthe three zones; however, zone III exclusively shows the presenceof this phase. The extensive formation of Cr2O3 can be related tonucleation points from embedded Cr2O3 particles in the Ni20Crmatrix when an oxidizing atmosphere is present. NiCr2O4 and NiOare not well defined in zone II because the characteristic bands of g-Fe2O3 (oxidation of Fe, see Table 4) are broad, especially at

Table 5Diffraction peak positions of Ni(Cr) solid solution in the Ni20Cr/Cr2O3 powder at various heat treatment temperatures.

Heat treatment temperature (�C) Pattern 2q (Deg) FWHM (Deg)

(111) (200) (111) and (200)

25 I 51.7 60.3 0.9400 II 51.7 60.4 0.8800 III 51.9 60.7 0.5

Fig. 5. Backscattered electron images of cross section of as-sprayed coatings producedusing the various feedstock powders. a) Conventional Ni20Cr powder, b) experimentalNi20Cr powder, and c) experimental Ni20Cr/Cr2O3 powder.

Fig. 6. Raman spectra acquired from localized zones of Ni20Cr/Cr2O3 coating. a)Reference oxides and b) morphology of oxygen rich zones I, II, and III.

I. L�opez-B�aez et al. / Vacuum 144 (2017) 27e35 31

700 cm�1 [20,22]. The NiCr2O4 phase is well defined in zone I and isthe most abundant phase observed in the cross section of coating.This characteristic phase is also observed in the micrographshowed in Fig. 5c, where predominant gray dark zones correspondto the NiCr2O4 phase. XRD pattern of as-sprayed Ni20Cr/Cr2O3coating corroborates the formation of NiO, NiCr2O4, and Cr2O3phases in the coating (see Fig. 7).

3.2.2. Oxide coating layersAs sprayed C1, C2, and C3 coatings were heat-treated at 800 �C

for 30 h in air to form a protective oxide layer. However, coatingspallation was evident on sample C1-HT (Fig. 8a shows the scalespallation) which occurred by delamination. The average thicknessof the fine oxide layer formed was 0.8 ± 0.2 mm. The oxide layer oncoating C2-HT had a thickness of 2.0 ± 0.8 mm; this value was larger

Fig. 7. XRD pattern of as-sprayed Ni20Cr/Cr2O3 coating.

I. L�opez-B�aez et al. / Vacuum 144 (2017) 27e3532

than the reported for C1-HT, it is important to mention that bothcoatings were produced employing powders with the same nom-inal chemical composition but processed by different routes. It iswell known that powder alloys processed by HEBM possess ananocrystalline structure that increases atomic transport, i.e.,crystallite interfaces providing paths for high oxygen diffusivitywhen an oxidation process is present [23]. However, coatingsproduced by HVOF lose the initial crystallinity of powders, whichenhances the surface oxidation at high temperatures [24]. In thecase of coatings processed using nanocrystalline feedstock, a finalmicrostructure of coating with a fine grain structure characterizedby a “multimodal structure”, inwhich both nano-crystalline regionsand submicron grain regions, was observed [25,26]. In this sense, amultimodal structure is expected in sample C2-HT (experimental

Fig. 8. Protective oxide layer of a) coating C1-HT, commercial powder 1262F; b) coating C2-Hd) semi-quantitative analysis on P1, P2, and P3.

feedstock powder processed by HEBM), which showed an oxidelayer with a larger thickness than coating C1-HT (commercialfeedstock powder); for sample C2-HT no evident spallation wasobserved.

Coating C3-HT that had fine Cr2O3 particles embedded in theNi20Cr matrix, showed the greatest oxide layer thickness of the set,2.5 ± 0.6 mm. Coating C3-HT showed localized zones where theoxide layer was thicker, as shown in Fig. 8c; the longest of theselocalized zones was around 6.5 mm in length (see the enclosedoxide layer in the white box). Semi-quantitative analyses of coatingC3-HT were performed on P1, P2, and P3 zones, see Fig. 8ced. Ahigh Ni atomic concentration on P1 could be found due to theformation of NiO and/or NiCr2O4 (these oxides were previouslyidentified by XRD and Raman analyses). Similar Ni and Cr atomicconcentrations were evident on P2, which suggested that NiCr2O4and/or Cr2O3 were also present in this zone. Semi-quantitativeanalyses on P3 and backscattered electron images analyses,mainly indicated the presence of the Ni20Cr metal matrix andCr2O3. The oxides were related to P1, P2, and P3 according to oxidesthat commonly integrate the oxide layers in Ni20Cr alloys [17,18].For coatings C1-HT, C2-HT, and C3-HT, the thicknesses and micro-structure of oxide layers were strongly influenced by the originalphases present in the coatings and powder processing routes.

3.2.3. Sliding wear behavior of coatingsTable 6 shows wear data of coatings C1, C2, C3, and Zr2O pins.

The wear volume loss and wear rate of coating C2 were the highestof the set (coating processed using Ni20Cr experimental powder asfeedstock), whereas coating C3 showed the lowest values. On theother hand, when comparing thewear volume loss andwear rate ofZr2O pins, the pin that acted as a counterpart with coating C2showed the lowest values, in contrast with the pin employed withcoating C3, which showed the largest values. These results suggestthat fine Cr2O3 particles embedded in the Ni20Cr matrix may havepromoted the formation of hard phases during the POD wear test;such hard phases acted as an abrasive medium, which significantlyincreased wear of the Zr2O pins. TGA result of pulverized Ni20Cr/

T, experimental powder Ni20Cr; c) coating C3-HT, experimental powder Ni20Cr/Cr2O3;

Table 6Wear results of coatings against ZrO2 pins.

Coatings and pins* Wear volume loss (mm3) Wear rate (x 10�5) (mm3/N*m)

C1 0.044 3.71C2 0.072 6.01C3 0.024 2.02Zr2O* for C1 0.004 0.31Zr2O* for C2 0.003 0.27Zr2O* for C3 0.008 0.66

Zr2O* for C1, Zr2O* for C2 and Zr2O* for C3 corrrespond to pins.C1, C2 and C3 correspond to coatings.

Fig. 9. TGA curve of pulverized Ni20Cr/Cr2O3 coating.

Fig. 10. Wear tracks using a backscattered electron detector, a) coating C1 and b)coating C2.

I. L�opez-B�aez et al. / Vacuum 144 (2017) 27e35 33

Cr2O3 coating corroborates that significant hard phases (oxides)continued forming to relative low temperatures (~270 �C), seeFig. 9.

Fig. 10 shows wear tracks of coatings C1 and C2. Formation ofoxygen-rich zones were observed as characteristic for both finalsliding tracks. A high concentration of fine free oxygen-rich parti-cles (debris) and less consolidated oxide scales were observed on awear track of coating C2 (see Fig. 10b), suggesting that the mildoxidative wear regime was operating for this coating [27]. On theother hand, a low amount of free oxygen-rich particles (see Fig. 10a)with moderately consolidated oxide scales were observed on weartrack of coating C1, suggesting a high sliding wear strength of thatcoating (see Table 6). Coating C2 that was processed using experi-mental Ni20Cr powder showed a larger amount of debris arisingfrom unconsolidated oxide scales resulting in the highest wear rateof set.

Fig. 11aeb shows a section of a wear track generated on coatingC3, backscattered and secondary radiation was used to contrast thesliding wear products. The analyses performed on the wear trackindicated that sliding wear produced a deep and discontinuousoxide layer on the surface, which provided good protection againstfurther wear. Results from EDS semi-quantitative analyses per-formed on an oxygen-rich zone of the wear track (see Fig. 11ced)revealed high atomic concentration of O, Ni, and Cr, in that order.Fig. 11c shows some striations at the consolidated oxide scales inthe Ni20Cr/Cr2O3 coating; such striations are thought to havestrongly abraded the ZrO2 pin surface causing a high wear rate(6.6 � 10�6 mm3/N*m). As previously discussed, the averagethickness of the oxide formed on coating C3 was high (see Fig. 8c),this suggests that consolidated oxide scales produced duringsliding wear were strongly anchored to the coating, resulting in alower wear rate of 20.2 � 10�6 mm3/N*m.

The friction tests were carried out to know the effect of oxide

formation rate during sliding wear. As previously mentioned,coating C3 presented the lowest wear rate of the set and is relatedto the early formation, and anchoring strength, of oxides. Results offrictional test showed that the coefficient of friction (m) of coatingC3 was greater that coatings C2 and C1 (see Fig. 12); this frictionalbehavior could be a consequence of oxide striation, which can be aresult of strongly anchoring of consolidated oxide scales on thecoating during POD test. In the early stages of POD test, the m valuesof coating C3 did not have a significant variation compared to thoseof coatings C2 and C1. This implies that localized temperature onwear track during POD test higher than 270 �C should have beenpresent (see TGA results shown in Fig. 9), enabling the formation ofconsolidated oxide scales on coating C3.

Fig. 11. Morphology of the wear track and semi-quantitative analysis of a consolidate oxide scale formed on coating C3. a) Backscattered electron image, b) secondary electronimage, c) oxygen rich zone with striations, and d) EDS analysis.

Fig. 12. Friction coefficients evolution of coatings C1, C2, and C3.

I. L�opez-B�aez et al. / Vacuum 144 (2017) 27e3534

4. Conclusions

According to TGA results, the oxidation temperature of Ni20Crpowder is reduced between 250 and 270 �C when fine Cr2O3 par-ticles are embedded in their surface, forming a new Cr2O3 phaseduring the oxidation process. Fine Cr2O3 particles acted as nucle-ation sites to form a thermodynamically stable oxide in the Ni(Cr)matrix at relatively low temperatures.

The thickness of oxide layers formed on Ni20Cr coatingsincreased from 0.8± 0.2 to 2.0 ± 0.8 mmwhen the feedstock powderwas processed by HEBM. The fine microstructure achieved by se-vere plastic deformation of ball milling increased the atomictransport of oxygen through the surface of coating.

Sliding wear rate of composite coating was low compared tothat of the Ni20Cr coating. Consolidated oxide scales on wear trackformed on the composite coating were strongly anchored to thesurface coating, as oxygen penetrated deep into the coating duringtesting. It is believed that fine Cr2O3 particles dispersed in the

coating act as nucleation sites to promote a quick oxidation of theNi20Cr matrix.

Acknowledgments

The authors wish to thank the Directorate for Research Supportand Postgraduate Programs at the University of Guanajuato fortheir support in the final reviewing of English-language version ofthis article. Also, the authors acknowledge the technical assistanceof J.E. Urbina-�Alvarez, F. Rodríguez-Melgarejo, CINVESTAV - Quer-�etaro, CIATEQ A.C., and LICAMM UG. This work was supported byCONACYT - M�exico (Project 45246 and scholarship 226900).

References

[1] R. Manish, Surface Engineering for Enhanced Performance against Wear,Springer-Verlag, 2013.

[2] A.D. Salman, M. Ghadiri, M.J. Hounslow, Handbook of Powder Technology:Particle Breakage, Elsevier, AmstCerdam, 2007.

[3] J.R. Davis, Handbook of Thermal Spray Technology, ASM International, 2004.[4] P. Fauchais, G. Montavon, G. Bertrand, From powders to thermally sprayed

coatings, J. Therm. Spray. Technol. 19 (2010) 56e80.[5] C. Suryanarayana, Mechanical Alloying and Milling, Marcel Dekker, New York,

2004.[6] Z. Qiang, L. Chang-Jiu, W. Xiu-Ru, R. Zhi-Liang, L. Cheng-Xin, Y. Guan-Jun,

Formation of NiAl intermetallic compound by cold spraying of ball-milled Ni/Al alloy powder through postannealing treatment, J. Therm. Spray. Technol. 17(2008) 715e720.

[7] S.M. Hashemi, M.H. Enayati, M.H. Fathi, Plasma spray coatings of Ni-Al-SiCcomposite, J. Therm. Spray. Technol. 18 (2009) 284e291.

[8] N. Eigen, F. G€artner, T. Klassen, E. Aus, R. Bormann, H. Kreye, Microstructuresand properties of nanostructured thermal sprayed coatings using high-energymilled cermet powders, Surf. Coat. Technol. 195 (2005) 344e357.

[9] D. Srinivasan, P.R. Subramanian, Differential role of nanoscaled oxide dis-persoids (Y2O3vs Al2O3) in the high-temperature structural stability of NiCralloys, Metall. Mater. Trans. A 37 (2006) 3455e3468.

[10] �S Houdkov�a, M. Ka�sparov�a, Experimental study of indentation fracturetoughness in HVOF sprayed hardmetal coatings, Eng. Fract. Mech. 110 (2013)468e476.

[11] L. Zhao, J. Zwick, E. Lugscheider, The influence of milling parameters on theproperties of the milled powders and the resultant coatings, Surf. Coat.Technol. 168 (2003) 179e185.

[12] L. Liu, J. Xu, A study of the erosion�corrosion behavior of nano-Cr2O3 particlesreinforced Ni-based composite alloying layer in aqueous slurry environment,Vacuum 85 (2011) 687e700.

I. L�opez-B�aez et al. / Vacuum 144 (2017) 27e35 35

[13] H.V. Atkinson, A review of the role of short-circuit diffusion in the oxidation ofnickel, chromium, and nickel-chromium alloys, Oxid. Met. 24 (1985)177e197.

[14] N. Birks, G.H. Meier, F.S. Pettit, Introduction to the High-temperature Oxida-tion of Metals, Cambridge University Press, New York, 2006.

[15] I. L�opez-B�aez, E. Martínez-Franco, H. Zoz, L.G. Tr�apaga-Martínez, Structuralevolution of Nie20Cr alloy during ball milling of elemental powders, Rev.Mex. Fís. 57 (2011) 176e183.

[16] W. Rusch, Comparison of Operating Characteristics for Gas and Liquid FuelHVOF Torches, Thermal Spray 2007: Global Coating Solutions, ASM Interna-tional, 2007, pp. 572e576. May 14-16, 2007 (Beijing, China), ASMInternational.

[17] G. Calvarin, R. Molins, A.M. Huntz, Oxidation mechanism of Ni-20Cr foils andits relation to the oxide-scale microstructure, Oxid. Met. 53 (2000) 25e48.

[18] I.G. Wright, B.A. Wilcox, R.I. Jaffee, The high-temperature oxidation of Ni-20%Cr alloys containing various oxide dispersions, Oxid. Met. 9 (1975) 275e305.

[19] J.H. Kim, I.S. Hwang, Development of an in situ Raman spectroscopic systemfor surface oxide films on metals and alloys in high temperature water, Nucl.Eng. Des. 235 (2005) 1029e1040.

[20] D.L.A. de Faria, S. Venancio-Silva, M.T. de Oliveira, Raman microspectroscopyof some iron oxides and oxyhydroxides, J. Raman Spectrosc. 28 (1997)

873e878.[21] B.D. Hosterman, J.W. Farley, A.L. Johnson, Spectroscopic study of the vibra-

tional modes of magnesium nickel chromite, MgxNi1-xCr2O4, J. Phys. Chem.Solids 74 (2013) 985e990.

[22] Y.S. Li, J.S. Church, A.L. Woodhead, Infrared and Raman spectroscopic studieson iron oxide magnetic nano-particles and their surface modifications,J. Magn. Magn. Mater. 324 (2012) 1543e1550.

[23] R.K. Gupta, N. Birbilis, J. Zhang, Oxidation Resistance of Nanocrystalline Alloys,INTECH Open access publisher, 2012.

[24] D.S. Doolabi, M.R. Rahimipour, M. Alizadeh, S. Pouladi, S.M.M. Hadavi, Effect ofhigh vacuum heat treatment on microstructure and cyclic oxidation resis-tance of HVOF-CoNiCrAlY coatings, Vacuum 135 (2017) 22e33.

[25] L. Ajdelsztajn, F. Tang, G.E. Kim, V. Provenzano, J.M. Schoenung, Synthesis andoxidation behavior of nanocrystalline MCrAlY bond coatings, J. Therm. Spray.Technol. 14 (2005) 23e30.

[26] M. Roy, A. Pauschitz, J. Bernardi, T. Koch, F. Franek, Microstructure and me-chanical properties of HVOF sprayed nanocrystalline Cr3C2-25(Ni20Cr)coating, J. Therm. Spray. Technol. 14 (2006) 372e381.

[27] F.H. Stott, The role of oxidation in the wear of alloys, Tribol. Int. 31 (1998)61e71.