Heat Treatment Effects on Mechanical Properties of (alpha+ß) and Lead-tin Brasses

T E C H N I C A L A R T I C L E

Heat Treatment Effects on Mechanical Properties ofAtmospheric Plasma Sprayed FexB Coatings on Al SubstrateO. Culha1, S. Sahin2, I. Ozdemir3, and M. Toparli1

1 Department of Metallurgical and Materials Engineering, Faculty of Engineering, Dokuz Eylul University, Buca, Izmir, Turkey

2 Department of Materials Engineering, Faculty of Engineering, Celal Bayar University, Muradiye, Manisa, Turkey

3 Department of Metallurgical and Materials Engineering, Faculty of Engineering, Bartin University, Bartin, Turkey

KeywordsPlasma Spray; Iron Boride Coating;

Indentation; Mechanical Properties

CorrespondenceM. Toparli,

Department of Metallurgical and Materials

Engineering,

Faculty of Engineering,

Dokuz Eylul University,

Buca, Izmir, 35160,

Turkey

Email: [email protected]

Received: April 1, 2011; accepted:

July 13, 2011

doi:10.1111/j.1747-1567.2011.00781.x

Abstract

In this work, we draw attention to determination of heat-treatment effectson mechanical properties of atmospheric plasma sprayed (APS) FexB coatingson aluminum substrates by micro-indentation technique. With this regard,iron boride powders of Fe-18.8B-0.2C-0.5Si-0.8Al (wt.%) were depositedonto Al substrates by APS in order to improve the mechanical propertiesof Al surface. As-sprayed coatings are composed of mainly FexB and ironmatrix supersaturated with boron owing to the rapid solidification of moltendroplets flattened on a substrate. It was observed that APS coatings exhibitedcharacteristic wavy layered structure having porosity, inclusions, and semi-melted particles. The postfurnace treatment of APS coatings was carried outat temperatures ranged from 450 to 550◦C in an argon atmosphere. Thepost-treatment applied for APS deposits led to increase in hardness of 40%without showing cracks. Furthermore, micro-mechanical properties of FexBcoatings were examined by Shimadzu Dynamic Ultra-Microhardness Tester forestimating Young’s modulus and hardness due to load–unload sensing analysisby applying different loads such as 160, 320, and 640 mN to determine load andindentation depth dependency of APS FexB on Al substrate for each samples,in details.

Introduction

Boriding is a thermomechanical surface-hardeningprocess, in which boron atoms are diffused into thesurface of a workpiece to form borides with the basematerials. Industrial boriding can be applied to mostferrous materials, such as structural steels, cast steels,Armco iron, gray-ductile iron, sintered iron, steel,as well as to nonferrous materials, such as nickel-,cobalt-, titanium-, and molybdenum-based alloys andcemented carbides.1 The major advantage of a mod-ified surface layer by deposition of boride layersis its high-wear resistance.2 It is well known thatconventional boriding process, that is, pack borid-ing, is commonly applied in the temperature range1073–1273 K. Depending on the process tempera-ture, chemical composition of substrate materials,several hours are required in order to obtain desired

hardened surface layer by formation of interstitialboron compounds. The resulting layer may con-sist of either a single-phase boride (FeB or Fe2B)or polyphase boride layer (FeB and Fe2B). How-ever, it is almost impossible to produce hardenedsurface composed of only single phase preferablyFe2B during boriding. The formation of FeB layeris inevitable but undesirable phase because it is verybrittle and has higher thermal coefficient (15 × 10−6

K−1) than that of Fe2B (8 × 10−6 K−1) which leads todevelop thermal stresses during heat cycle. Recently,it has been shown that the aforementioned disad-vantages of conventional boriding process could beeliminated by introducing fast deposition processessuch as atmospheric, vacuum plasma spraying, andplasma transferred arc (PTA) boriding.3 Plasma spray-ing is an attractive coating method because it offers

Experimental Techniques (2011) © 2011, Society for Experimental Mechanics 1

Atmospheric Plasma Sprayed FeB Coatings on Al Substrate O. Culha et al.

Unloading

Loading

P

Pmax

S = dP/dh

hmax hhf

Figure 1 Schematic illustration of a typical P–h response of an

elasto-plastic material to instrumented sharp indentation.

fast and cost-effective solutions for the problems ofwear, corrosion, and thermal stability by depositinga thin layer onto the substrate, thereby satisfying therequired surface specification.

For instance, it is difficult to develop a diffusioniron boride layer on the aluminum surface by con-ventional boriding process. Plasma spraying of ironboride powder successfully performed with a thick-ness of hundreds microns onto aluminum surfaceswhich exhibited good antiwear performance.4 Like-wise, Galvenetto et al.2 modified carbon steel surfacesby depositing iron boride layers with graded com-position by VPS technique to improve their wearresistance.

During the past quarter of a century instrumentedindentation machines have been used increasinglyfor determining Young’s modulus and indentationhardness of bulk solids and thick coatings depositedon solid substrates. A typical experimental run withan instrumented indentation machine consists ofloading normally a Vickers or Berkovich indenteron to the test surface and gradually increasing theload at a predetermined rate to a preselected valueand then unloading the indenter gradually to zeroload. Throughout the indenter loading and unloadingthe indenter load versus indenter displacement withrespect to the original surface of the specimen arerecorded. In the case of an elastic–plastic solid, plas-tic flow will occur around the pointed indenter andwhen the indenter is unloaded and removed from theindented surface, a permanent impression will be leftin the surface of the specimen (Fig. 1).3

Five means are commonly used for coatingYoung’s modulus measurement: (1) nanoindenta-tion, (2) beam bending, (3) vibration, (4) ultrasonicsurface wave emission, and (5) acoustic emission.5–7

Among these methods, indentation is the most pop-ular technique, with which Young’s modulus is oftendetermined through the initial part of the unloadingcurve of a set of indentation data obtained with asharp indenter.8–12 The depth-sensing (or dynamic)micro-indentation method offers great advantagesover the conventional Vickers microhardness test-ing in two aspects. Firstly, apart from microhardness(or microstrength), the method can also providewell-defined mechanical parameters such as elasticmodulus of the interfacial zone. Secondly, as load anddepth of an indentation are continuously monitored,optical observation and measurement of diagonallength of the indent/impression, which can be difficultand subjected to inaccuracy, is no longer required.13,14

In this study, we draw attention to microstructuraland thermomechanical properties of FexB coatings onAl substrates. In this context, FexB-based coatingswere fabricated on Al substrates by using atmosphericplasma spray (APS) technique. The produced coatingswere characterized by scanning electron microscope(SEM). Mechanical properties (Hardness and Young’sModulus) of FexB coatings were obtained frommicro-indentation tests. The as-sprayed layers weresubjected to post-spray heat-treatment process in atemperature ranging from 450 to 550◦C for 1 to2 h in an argon atmosphere in order to improvemechanical properties of sprayed coatings. Thedynamical hardness measurements were examined todetermine modulus and hardness variations of boridelayer under different applied peak loads. With thisregard, it was aimed to show the load and indentationsize dependency of the elasto-plastic properties ofplasma sprayed FexB coating on Al substrate by micro-indentation technique.

2 Experimental Techniques (2011) © 2011, Society for Experimental Mechanics

O. Culha et al. Atmospheric Plasma Sprayed FeB Coatings on Al Substrate

Experimental Study

The FexB powders with a composition of Fe-18.8B-0.2C-0.5Si-0.8Al (wt.%) were sprayed onto gritblasted 30 × 30 × 5 mm aluminum alloy substrate (diecast, Al-11.2Si-2.74Cu; in wt.%) by APS method. Thestandard spray parameters followed for deposition isgiven in elsewhere.15 In order to quantitatively deter-mine the level of porosity, an image analyzer (LUCIA4.21) was used. The Vickers hardness of the coatinglayers was measured with a load of 2.94 N.

The surface morphologies of layers were exam-ined by a Scanning Electron Microscope (JEOL-JSM6060 SEM (JEOL, Tokyo, Japan)). Accelerating volt-age of 20 kV was used for the SEM imaging analyses.Mechanical properties (hardness and young’s modu-lus) of FexB were obtained from load-depth curvesby using Shimadzu Dynamic Ultra-Micro Hardnesstester (Shimadzu DUH-W210S, SHIMADZU, Kyoto,Japan) under 160, 320, and 640 mN applied peakloads. In order to determine the Young’s modulusand hardness, maximum applied load should be suf-ficient to produce a permanent deformation on thecoating surface.

Results and Discussion

Microstructure of the coatings

Coatings sprayed by APS exhibited characteristicwavy, layered structure without showing cracksinside the layers. The average microhardness valueof the iron boride layers is about 760 HV which ismuch lower than borided layers of AISI 1020 steelat 900◦C for 4 h. Typical cross-section microstruc-ture of the coatings sprayed with these parametersis shown in Fig. 2, where the sprayed coatingexhibits a wavy layered structure and consists ofsome unmelted particles and pores, with an averagethickness of approximately 100 μm. X-ray diffractionpatterns of as-sprayed and heat-treated samples areshown in Fig. 3. As seen at higher magnification(Fig. 4(b)), some microcracks are observable insidethe layers. In order to eliminate these microcracksand, preheating substrate before spraying, stoppingcompressed air cooling used for substrate cooling fromthe backside as well as extending spray distance from150 to 170 mm were also tried. However, formationof microcracks was generally not avoided. This isattributed to the cooling rate which leads to gen-eration of residual stresses, which might result incracks in the coating. PTA boriding study conductedby Bourithis et al.16 showed that microcracks wereobserved inside the borided layers with increasingcontent of boron, which lead to the local formation

Figure 2 SEM image of the as-sprayed coatings by APS.

of FeB and, eventually, intergranular cracking. Gal-venetto et al.2 sprayed Fe2B (%100) and FeB+α-Fegraded powder composition by VPS on AISI 1040steel, it was concluded that no cracks were observedinside the iron boride layers. Both studies werefocused on fast formation of boride layers on lowcarbon steel which used commonly for pack boridingprocess.

While, in this study, aluminum surface was mod-ified by depositing boride layers by APS, whichits surface is not possible boriding in conventionalboronizing. The observed cracks inside the coat-ing produced by APS might be attributed to fastcooling during spray process due to the higher ther-mal conductivity of Al compared to low carbonsteel, such cracks might be eliminated by sprayinggraded composition of iron boride powders. Postheattreatment process was applied to treat the sprayedboride layers, enabling the boron diffusion whichdecomposed and formed solid solution during sprayprocess. In order to densify and improve the mechan-ical properties, the coatings were post-treated in afurnace at temperatures of 450–550◦C for 60 to120 min in an atmosphere of argon. Modificationof initial coating microstructure is shown in Fig. 4.Post-treated microstructure became smoother andimproved exhibiting decrease in pore size, that is,coating densification, better adhesion of splat layersas well. As a matter of fact that the hardness ofpredeposited coating increased significantly at therate of 40%. Depending on composition of boridedlayers, measured hardness value changes from 400to 1600 HV. The measured hardness values of as-sprayed and post-treated coatings are much lowercompared with data of borided AISI 1020 steel at900◦C for 4 h, yielding maximum hardness value,



020 30 40 50 60

2 Theta

70 80 90

As-sprayed

H.T. at 450 C

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1:FeB

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nsi

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Figure 3 XRD results of as-sprayed

and heat-treated samples.

(a) (b)

(c) (d)

Figure 4 SEM images of cross-section microstructures of the coatings sprayed by APS (a) as-sprayed and post-treated at (b) 450◦C for 1 h, (c) 500◦C

for 2 h, and (d) 550◦C for 2 h.



1600 HV. On the other hand, several studies showedthat iron-based powder sprayed coatings exhibitedgood antiwear performance.17,18 The improvementat wear performance and reduction in friction coeffi-cient not only depend on coating hardness but also onthe formation of protective oxide films which stronglyaffect the wear mechanisms of the layers. Therefore,coating sprayed with ferroboron powder containingFexB intermetallic phase, boride oxide, and Fe-basedoxide expected to experience with low friction coeffi-cient and improved wear resistance in comparison tothe steel surface which is conventionally boronized.Moreover, contrary to the classical boronizing, ther-mal spraying is a cost-effective promising technique tosolve problems such as wear, corrosion, and thermalstability by producing rapidly solidified thick materialson any substrate with a relatively short cycle time.

Mechanical properties of coatings

Dynamic Ultra-microhardness test is applied to cross-section part of samples for determination of hardnessand Young’s modulus variations of FexB under theloads of 640, 320, and 160 mN applied loads. DynamicHardness results for without heat treatment, 450◦Cfor 1 h, 500◦C for 1 h, and 550◦C for 2 h under 640,320, and 160 mN applied peak loads are shown inFig. 5. The figures were constructed experimentallyusing the data taken from the loading part of depthsetting Dynamic Hardness Values (DHV) measure-ments at various loads. It is seen that DHV numbersdecrease with increasing applied peak load and inden-tation depth. Hardness measurements of FexB showsthat when the heat treatment process temperatureincrease from non-heat treated to 550◦C, DHV ofFexB layer decreases from 1518 DHV to 657 DHV,from 1042 DHV to 475 DHV, from 897 DHV to 653DHV, and finally from 953 DHV to 245 DHV withincreasing applied peak loads from 160 to 640 mN. Ashardness is accepted as an inherent material property,it should not vary with indentation load and size.However, investigations have confirmed that DHVnumber of materials were indentation size dependentespecially at lower peak loads. Increase in hardnesswith decreasing applied peak load cause from differ-ences in indentation depth, therefore this effect iscalled indentation size effect. When the curves areexamined, two different parts of hardness variationcan be clearly seen. The first part of the curve rep-resents the contact region between the indenter andcoatings. So, at this point, indentation depth influ-ences the dynamic hardness value of coatings at nearthe surface region. When the applied load and depthincrease, depth effect systematically decreases. At the

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Figure 5 Hardness-penetration depth curves of the coatings sprayed

by APS as-sprayed and post-treated at 450◦C for 1 h, 500◦C for 1 h,

550◦C for 2 h under (a) 160 mN, (b) 320 mN, and (c) 640 mN applied peak

loads.

second part of the curve, dynamic hardness valueof coatings is approximately constant depending onindentation depth. The first stage of curves shows theload and size dependency of hardness under appliedpeak loads. Figure 5 exhibits this kind of behavior.

In order to determine the Young’s modulus,maximum applied load should be sufficient toproduce a permanent deformation on the coating



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Figure 6 A series of load-displacement plots of the coatings sprayed by APS (a) as-sprayed and post-treated at (b) 450◦C for 1 h, (c) 500◦C for 2 h, and

(d) 550◦C for 2 h under 160, 320, and 640 mN applied peak loads.

surface. So that the values of Young’s modulus,determined from indentations, do not depend onthe value of h and, therefore, on the value of themaximum load, the indentation depth should notexceed 10–20% of the layer thickness, otherwisethe results will be affected by the properties ofthe substrate. In this study, thicknesses of FexBlayers are between 442 and 298 μm depending onheat-treatment conditions and maximum indentationdepth changes among 3.19 and 0.627 μm.

The load–unload (load–displacement) curvesshown in Fig. 6 represent the 640, 320, and 160mN applied load as a function of the displacementof the indenter with respect to the initial positionof the cross-section part of layers. Using the exper-imentally determined Sand hc, the reduced elasticmodulus by micro-indentation was calculated andthe results are shown in Fig. 7 for heat-treated andnon-heat-treated samples. It is clearly seen from thefigures that the extracted reduced elastic modulusalso exhibits a strong peak load dependency as shownin Table 1. According to the result, Young’s modulus

values increase with decreasing applied peak loads.The Young’s modulus analysis of FexB layer showsthat when the heat-treatment temperature increasesfrom non-heat treated to 550◦C, Young’s modulus

678.34

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Pa)


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Figure 7 Young’s modulus variations of as-sprayed and post-treated at

450◦C for 1 h, 500◦C for 2 h, and 550◦C for 2 h under 160, 320, and 640

mN applied peak loads.



Table 1 Dynamic hardness results of samples

Material type Load (mN) Maximum depth (μm) hc (μm) Dynamic hardness value (DHV) Young’s modulus (GPa)

Without heat treatment 160 0.640 0.432 1518 678.34320 1.115 0.776 1011 300.07640 1.948 1.430 657 236.80

450◦C–1 h 160 0.772 0.593 1042 411.64320 1.417 1.058 627 181.26640 2.289 1.791 475 154.84

500◦C–1 h 160 0.839 0.708 897 352.40320 1.216 0.767 836 174.65640 1.944 1.169 653 132.30

550◦C–2 h 160 0.814 0.457 953 202.30320 1.445 0.844 600 110.90640 3.190 2.211 245 52.27

of FexB layer decreases from 678 to 202 GPa, from300 to 110 GPa, and finally from 235 to 52 GPa withincreasing applied peak loads from 160 to 640 mN.

Furthermore, elasto-plastic properties of coatings,such as C constant, yield strength, indentation hard-ness, and strain hardening exponent variation ofFexB coatings depending on applied loads and with–without heat treatment was calculated by indenta-tion algorithms, as represented Fig. 8. According tothe results, C constants, yield strengths, indentationhardness’s, and strain hardening exponents of FeBcoatings were decreased from 325 to 76 GPa, from4.62 to 1.15 GPa, from 27.95 to 4.41, and finallyfrom 0.2621 to 0.2600 by increasing applied loadfrom 160 to 640 mN and heat-treatment temperaturefrom 450 to 550◦C, respectively. Indentation size andheat-treatment effect on elasto-plastic properties ofFexB coatings were clearly seen in Fig. 7. Accordingto the several studies,19,20 load depended elastic mod-ulus (125–624 GPa) and hardness (17–33 GPa) wereobtained at 80, 160, 320, and 640 mN applied peakloads depending on boriding process time. In addition,finite element modeling was applied to simulate themechanical response of FexB layer on low alloy steelsubstrate. When the results of mechanical propertieswere compared from current studies, it can be con-sidered that indentation hardness, Young’s modulus,and elasto-plastic properties of coatings show sim-ilarities. When the diffusion-controlled productionregime and atmospheric plasma spraying comparedeach other, FexB layer on different substrate showssame indentation properties.20,21

As known, indentation size effect played an impor-tant role while increasing indentation loads anddepths. In addition to this effect, heat treatmentat 450–550◦C at inert atmosphere influenced theindentation algorithm results. Concurrently, compre-hensive theoretical and computational studies have

emerged to elucidate the deformation mechanismin order to systematically extract material propertiesfrom P versus h curves obtained from instrumentedindentation. Analysis algorithms for determinationof elasto-plastic properties were established basedon the identified dimensionless functions.21 Thesealgorithms allow for the calculation of the indenta-tion response for a given set of properties, and alsofor extraction of yield strengths and strain harden-ing exponent from a given set of indentation data.The main aim of heat treatment was to obtain theimprovement of mechanical properties of FexB coat-ings on Al substrate by APS. Young’s modulus, yieldstrength, indentation hardness, and strain hardeningexponents of coatings systematically decreased andmechanically diverged from brittle behavior. In addi-tion, residual stresses and mechanical mismatch effectof Al substrate-FeB coating was also decreased due tothe heat treatments.

Conclusion

In this paper, the authors were deposited iron boridepowders of Fe-18.8B-0.2C-0.5Si-0.8Al (wt.%) ontoan Al substrate by APS. The microstructure andmechanical impact phenomenon of the coating wasreached as below:

• Iron boride layers deposited by APS showed char-acteristic wavy, layered structure without showingcracks. Iron boride coating is mainly is consisted ofFexB phase according to the stoichiometric powdermixing.

• Smoother and improved microstructure, that is,decrease in pore size, coating densification wereobtained after post-treatment of APS coating.The measured hardness of post-treated coatingincreased remarkably at the rate of 40%.



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Figure 8 Indentation hardness and strain hardening exponent variations of coatings.

• Hardness measurements of FexB coatings showsthat when the heat-treatment process tempera-ture increase from non-heat treated to 550◦C, DHVof FexB decreases from 1518 DHV to 657 DHV,from 1042 DHV to 475 DHV, from 897 DHV to653 DHV, and finally from 953 DHV to 245 DHVwith increasing applied peak loads from 160 to640 mN.

• The Young’s modulus analysis of FexB layershows that when the heat-treatment temperatureincreases from non-heat treated to 550◦C, Young’smodulus of FexB layer decreases from 678 to202 GPa, from 300 to 110 GPa, and finally from235 to 52 GPa with increasing applied peak loadsfrom 160 to 640 mN.

• Indentation properties of FexB coatings such as Cconstants, yield strengths, indentation hardness’s,and strain hardening exponents of FexB coatings

were decreased from 325 to 76 GPa, from 4.62to 1.15 GPa, from 27.95 to 4.41, and finally from0.2621 to 0.2600 by increasing applied loadfrom 160 to 640 mN and heat-treatment tempera-ture from 450 to 550◦C, respectively.

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Heat Treatment Effects on Mechanical Properties of (alpha+ß) and Lead-tin Brasses

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