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Materials Letters 60 (20

Tribological properties of oxidised boride coatings grown on AISI 4140 steel

Saduman Sen a,⁎, Ugur Sen a, Cuma Bindal b,1

a Sakarya University, Technical Education Faculty, Department of Metal Education, 54187 Esentepe Campus, Sakarya—Turkeyb Sakarya University, Engineering Faculty, Department of Materials and Metallurgical Engineering, 54187 Esentepe Campus, Sakarya—Turkey

Received 28 March 2005; accepted 13 March 2006Available online 4 April 2006

Abstract

In this study, we investigated the wear behaviour of borided and borided + short-duration oxidized AISI 4140 steel. Boronizing was carried outin a slurry salt bath consisting of borax, boric acid and ferro silicon. Also, short-duration oxidizing treatment was applied to borided steel toproduce glass-like boron oxide layer. The short-duration oxidizing was performed at 750 °C for 3 min. Optical and scanning electron microscope(SEM) cross-sectional examinations of borided layer revealed a needle-shaped morphology. The presence of non-oxide boride type ceramics FeBand Fe2B formed on the surface of steel substrate was confirmed by classical metallographic technique and X-ray diffraction (XRD) analysis. Thehardness of borides formed on the surface of steel substrate and unborided steel substrate were 1446–1690 HV0.1 and 280 HV0.1, respectively. Thewear behaviour of borided steel were characterised by using a pin-on-disc technique. The borided and short-duration oxidized steels, in the form ofpins were allowed to slide against a hard AISI 440C stainless steel disc (63 HRc). The sliding velocity of 1 m s−1 for borided and short-durationoxidized steel and the nominal load on the pin was 20 N. The highest wear rates were observed on disc slide against the base steel, whilst thelowest wear rates occurred during sliding against the borided and short-duration oxidized steel surfaces. It was observed that the frictioncoefficient of unborided (hardened + tempered) and borided steels ranged from 0.50 to 0.60, but after short-duration oxidizing, the frictioncoefficient of borided steel was dropped to 0.12.© 2006 Elsevier B.V. All rights reserved.

Keywords: Boronizing; Hardness; Wear; Friction coefficient; Short-duration oxidizing; Steel

1. Introduction

Boronizing transforms the surface of work-piece into ametallic boride layer. The boride layer is formed by diffusionof boron atoms in the base metal at high temperature [1–6].Diffusion of boron into the surface of various metals andalloys results in the formation of metallic borides whichprovide extremely hard, wear and erosion resistant surfaces[1,7]. This treatment is a thermo-chemical treatment in whicha material is kept in a boron-giving environment from 850 to1000 °C for 2–10 h [8]. The surface of material should becleaned well previously and the boron can be given either assolid, paste or gas. The application temperature varies with

⁎ Corresponding author.E-mail address: sdmnsen@sakarya.edu.tr (S. Sen).

1 Tel.: +90 264 346 00 90 334; fax: +90 264 346 02 62.

0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2006.03.036

the type of material and the depth needed. If boronizing isapplied to a material surface the resultant boride layerincreases the wear resistance considerably. Furthermore, itdecreases the friction coefficient [8–10]. Boronizing is veryeffective, especially on low alloy steel with chromium[11,12].

The oxidation of boronized steels produces B2O3 phases onboride layer. The reactions that took place on the surface ofboride layer at open atmosphere are as follows [13];

8FeB þ 3O2 ¼ 4Fe2B þ 2B2O3 ð1Þ

4Fe2B þ 3O2 ¼ 8ðα−FeÞ þ 2B2O3 ð2Þ

Formation of boric acid on the oxidized layer during thesliding test was firstly reported by Takeuchi et al. [13]. Bindaland Erdemir [14] and Erdemir et al. [15,16] showed that thefriction coefficient of the surfaces including boron oxide is

Fig. 1. A schematic drawing of a pin-on-disc machine.

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decreased by the formation of boric acid on the worn surface bythe humidity of open air when reaction took place on B2O3

phases as follows;

B2O3 þ 3H2O ¼ 2H3BO3 ð3Þ

The main objective of this study was to investigate theeffect of wear behaviour of boronized and boronized + short-duration oxidized AISI 4140 low alloy steel, beside frictioncharacteristics. All samples were studied by performingmicrohardness measurements, sliding wear tests, opticalmicroscopy, scanning electron microscopy (SEM) and X-raydiffraction analysis.

Fig. 2. Optical microscope and SEM cross-sectional views of the borided AISI 414

2. Experimental details

2.1. Test material

The test material used in this study was AISI 4140 steelessentially containing, 0.41 wt.% C, 1.01 wt.% Cr, 0.85 wt.%Mn, 0.24 wt.% Mo, 0.12 wt.% Ni, 0.2 wt.% Si, 0.024 wt.% P,and 0.031 wt.% S. The unborided and borided materials undertest were used in the form of a wear pin 50 mm long and 6 mmdiameter with a flat end. An AISI 440C stainless steel (63 Rc)disc which is ground and polished (Ra=0.24 μm) was used asthe counter-face material in the wear tests. The samples wereultrasonically cleaned in acetone and ethyl alcohol, dried andweighed in an electronic balance having a resolution of0.01 mg.

2.2. Boronizing and short-duration oxidizing treatments

Boronizing was performed in a slurry salt bath consisting ofborax, boric acid (boron supplier) and ferro-silicon (activator).The temperatures of the bath were 850 and 950 °C, and theboronizing procedure was conducted at atmospheric pressurefor 2–6 h. After boronizing, short-duration oxidizing treatmentof steels was performed in a furnace at 750 °C for 3 min in openair.

The boride layer morphology and types of borides formed onthe surface of AISI 4140 steel substrate were confirmed bymeans of optical microscopy, scanning electron microscopy(SEM) and X-ray diffraction analysis. An Olympus BO71optical microscope and a Jeol JSM-5410 SEM instrument werealso utilized in this study. X-ray diffraction (XRD) analysis wasperformed on the borided test material, utilizing a Rigaku high-

0 steel (a) at 850 °C and (b) 950 °C for 4 h showing morphology of borides.

Fig. 3. X-ray diffraction (XRD) pattern of borided AISI 4140 steel at (a) 850 °C and (b) 950 °C for 6 h.

Table 1The layer thickness and hardness of borides formed on the surface of AISI 4140steel

Properties ofborided AISI4140 steel

Boronizing temperature and time

850 °C 950 °C

2 h 4 h 6 h 2 h 4 h 6 h

Boride layerthickness (μm)

38,4±3.58

58,8±4.78

73,6±6.33

127,1±10.67

173,8±11.74

202,7±19.53

Distance fromsurface (μm)

Hardness HV0.1

5 1446 1539 1539 1632 1583 169020 1446 1539 1539 1632 1583 169050 565 1364 1446 1632 1583 153975 485 544 549 – – –100 485 442 481 1632 1583 1539140 485 434 449 807 1292 1539180 485 434 420 427 970 1191220 – 434 420 427 442 583260 – – – 427 431 431300 – – – 427 431 420

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resolution diffractometer with CoKα radiation of 0.17902 nmwavelength for X-ray diffraction (XRD) analysis of boridesformed on AISI 4140 steel surfaces.

The thickness of the boride layer was measured by means ofan optical microscope attached to an optical micrometerinstrument. The hardness of unborided steel and borides formedon the surface of AISI 4140 steel were determined from surfaceto interior by means of a Leitz microhardness tester with a loadof 100 g.

A pin-on-disc technique was used, both friction force andwear being measured continuously through the duration of thetest. A schematic drawing of a pin-on-disc machine used in thewear test is shown in Fig. 1. Before wear testing, the roughnessof unborided steel and borided steel surfaces were measured byPerthometer S8P instrument. The sliding velocity was heldconstant at 1 m s−1 and the normal loads on the wear pin were20 N. At least five tests were performed under the same con-dition for reproducibility of data. All experiments were carriedout versus AISI 440C stainless steel (63 HRc) at roomtemperature (about 20 °C) and relative humidity was about60±6.0%.

3. Results

Optical and SEM examinations of the cross-sections of boridedAISI 4140 steel revealed a needle-shaped compact and porosity-freestructure to a depth of 202 μm and good bonding of the film tosubstrate. The microstructural cross-sectional view of the borided steelsurfaces has three distinct regions at higher magnifications; (i) boridelayer formed on borided steel which includes FeB and Fe2B layers [17],(ii) transition zone under the boride layer and, (iii) matrix. Optical and

SEMmicrographs showing morphology of FeB (darker region near thesurface) and Fe2B phases as shown in Fig. 2. The prominent phasesseen in the boride layer are FeB, Fe2B and CrB (see Fig. 3). Thethickness of borides depends strongly on boronizing time and processtemperature. The microhardness values of unborided and boridesformed on the steel substrate were about 280 HV0.1 and 1446–1690 HV0.1, respectively. As a result, the hardness of borides is muchhigher than that of the unborided substrate. These can be attributed tothe presence of FeB, Fe2B and CrB. Boride layer thickness and

Table 2Friction coefficients and wear rates of unborided, borided and short-durationoxidized steel against 440C steel disc

Treatment conditions Friction Wear rates

Coefficients (μ) (× 10−6 mm3/N m)

Normalised steel 0.55 28.94Quenched + tempered steel 0.62 23.82Borided steel for 2 h 0.53 3.23Borided steel for 4 h 0.54 4.98Borided steel for 6 h 0.50 7.54Borided for 4 h and short-duration oxidized steel

0.12 1.62

Fig. 5. The variation in the wear rate with boronizing time during sliding againstthe 440C steel disc.

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hardness values as a function of boronizing time and processtemperature are given in Table 1. Detailed information was given inthe Ref. [18] about the boronizing characteristics of the boronized AISI4140 steel. In addition to the study, the effect of wear behaviour ofboronized and boronized + short-duration oxidized AISI 4140 lowalloy steel, beside friction characteristics was studied in this study.

Before wear testing, the borided steel, unborided steel and discsurfaces roughness were measured as 0.18–0.32, 0.28 and 0.24 μm,respectively. It was observed that the friction coefficient of unboridedand borided AISI 4140 steels were ranged from 0.50 to 0.60. But, aftershort-duration oxidizing, the friction coefficient of borided steel wasdropped to 0.12. The friction coefficient of borided or base steel against440C steel is initially low, but increase substantially with boronizingtime and reach values of about 0.50–0.60. The lowest frictioncoefficient, i.e. 0.12, was observed on those borided steel surfacessubjected to short-duration oxidizing. The range of friction coefficientsand wear rates of unborided, borided + short-duration oxidized steel aregiven in Table 2 and Figs. 4 and 5.

The highest wear rates were observed on base steel during slidingagainst 440C steel disc, whilst the lowest wear rates occurred onborided and short-duration oxidized steel surfaces. The variation inthe wear rate with boronizing time was indicated in Fig. 6 for boridedsteel at 850 °C. It is clear from the Fig. 5 that at nominal load, theborided steel exhibits a considerably lower specific wear rate. Thewear rate values of borided steel surfaces were ranged from 3.23 to7.54×10−6 mm3/N m whereas wear rate value of unborided steel was28.9×10−6 mm3/Nm andwear ratewasmeasured as 1.62×10−6 mm3/Nm

Fig. 4. Friction coefficients of unborided, borided at 850 °C

for borided + short-duration oxidized steel surface. It is possible that thewear rate of borided steel increases faster with boronizing time than thatof the unborided steel.

The scanning electron micrographs of the worn surfaces ofunborided and borided steels are illustrated in Fig. 5. The wornsurfaces are essentially composed of grooves running along the slidingdirection. The grooves are much finer in the borided steel and, as aresult, the worn surfaces appear smoother in the case of borided steel ascompared with the unborided steel.

4. Discussion

The boride layer formed on steel by boronizing consists of atop FeB layer followed by a Fe2B layer. The optical and SEMexaminations demonstrated that the borides formed on thesurface of AISI 4140 steel have needle-shaped morphology.Thus, it is important to know which boride phases are res-ponsible for the observed tribological behaviour [7]. And also tocharacterise the coatings the following properties were deter-mined: phase composition, boride layer thickness, hardness andsurface roughness [19]. The thickness of borides depends

for 4 h and short-duration oxidized steel at 750 C for 3.

Fig. 6. Optical and SEM micrographs of the worn surfaces of (a) unborided steel, (b) borided steel at 850 °C for 4 h and (c) borided and short-duration oxidized steel.

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strongly on boronizing time and temperature. Depending on theboronizing time and temperature, the thickness of boride layersranged from 38 to 203 μm. The microhardness values ofunborided and borides formed on the steel substrate were about280 and 1446–1690 HV0.1 respectively. As a result the hardnessof borides is much higher than that of unborided substrate.These are the consequences of the presence of hard FeB andFe2B phases in the boride layer. The roughness of the boridedsteel surface was low i.e. 0.18–0.32 μm similar to the roughnessof polished unborided steel surface. The roughness value ofunborided steel was measured as 0.28.

The wear behaviour of borided steels depends on themicrostructure, microhardness, friction characteristics of thecoating materials and environmental conditions [20]. Table 2and Fig. 4 denote the variation of the friction coefficients of thesamples as a function of sliding time. The friction coefficient ofborided or base steel against 440C steel are initially low, butincrease substantially with boronizing time and reach values ofabout 0.50–0.55. Friction coefficient of unborided steel sample(hardened + tempered) is 0.60 at 20 N load and 1 m s−1 slidingspeed. The friction coefficient of boronized steels is nearly

11.7% lower than that of the unborided steel sample, and thecoefficient of friction value of a normalized steel sample isequal to the boronized steel, see Table 2. The lowest frictioncoefficient, i.e. 0.12, was observed on those borided steelsurfaces subjected to short-duration oxidizing. The coefficientof friction of short-duration oxidized sample is 80% lower thanthat of the unborided steel and 77% lower than that of boridedsteel samples. Previous tribological studies by Erdemir [21,22]and Bindal [14] showed that the lowest friction coefficient wasobserved on borided steel surfaces subjected to short-durationoxidizing.

Table 2 and Fig. 5 present the variation of specific wear rateof the worn samples. The specific wear rates of borided steelsare considerably lower than those of the unborided steels. Thehighest specific wear rates were observed on base steel duringsliding against 440C steel disc, whilst the specific lowest wearrates occurred on borided and short-duration oxidized steelsurfaces. The fact that, the hardness of the borided steel is about6 times higher than that of the unborided steel which is oneobvious reason for its low wear rate [7]. The specific wear ratevalues of borided steel surfaces were ranged from 3.23 to

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7.54×10−6 mm3/N m whereas specific wear rate value ofunborided steel was 28.9 10−6 mm3/N m and specific wear ratewas measured as 1.62×10−6 mm3/N m for borided and short-duration oxidized steel surface. Fig. 4 showed that, the longer theboronizing time, the higher the boronizing temperature, thehigher the specific wear rate became. Subrahmanyam andGopinath [23] explained that the formation of FeB phase besidesthe Fe2B phase resulted in the decrease of wear resistance ofboronized steels. It is explained in the earlier studies that, FeBphases were highly prone to cracking and scaling as a result ofthe large differences in their inherent stress levels, (Fe2-B=compressive strain /FeB=tensile strain) [9,24,25]. It wasshown in the earlier study that, the higher the boronizing time,the longer the treatment temperature, the thicker the FeB borideson the borided layer became [24]. As it can be seen in Table 2,specific wear rate of quenched + tempered steel is 17.7% higherthan that of the normalized steel. Specific wear rates of boridedsteel samples are nearly 89% lower than that of the normalizedsteel. In addition, borided and short-duration oxidized steelsample has a lowest specific wear rates which is nearly 94.4%,93.2% and average of 68.6% lower than that of the normalized,quenched + tempered and borided steels, respectively.

Boronizing of steel surfaces can result in improved wearresistance, but the friction coefficients of such surfaces can stillbe high. To achieve low friction, such surfaces must besubjected to short-duration oxidizing during which some boronatoms diffuse to the surface and react spontaneously withoxygen in the environment to form a thin boron oxide layer.During cooling, the boron oxide reacts with the moisture in airto form a layer of boric acid (Eq. (1). The low friction is a directconsequence of this boric acid film, mainly because of the film'slayered triclinic crystal structure. Under nominal load, thecrystalline layers can align themselves parallel to the slidingdirection and, once so aligned, can slide over one another toprovide low friction coefficient [21]. The low frictioncoefficient and wear rate can be attributed to the formation ofB2O3 during short-duration oxidizing and the formation ofH3BO3 in the duration of wear process performed.

2Bþ 3=2O2YB2O3 DH1023 ¼ −1218 kj:mol−1 ð4Þ

1=2 B2O3 þ 3=2H2OYH3BO3 DH1023 ¼ −45:1 kj:mol−1

(5)

The SEM micrographs demonstrated that the worn surfacesare essentially composed of grooves running along the slidingdirection. And wear debris is formed on the unborided steel (seeFig. 6). The results are similar to the previous studies realized byHabig [19] showed that the grooves are much finer in theborided steel and, as a result, the worn surface appears smootherin the case of borided steel as compared with unborided steel.The worn surfaces of boronized + short-duration oxidized steelsamples are too smother than that of the worn surfaces ofunborided and borided steels.

References

[1] U. Sen, S. Sen, F. Yilmaz, Structural characterization of boride layer onboronized ductile irons, Surf. Coat. Technol. 176 (2000) 222–228.

[2] A.K. Sinha, Boriding (boronizing), ASM Handbook, OH, USA, J. HeatTreat. 4 (1991) 437.

[3] K.H. Habig, R. Chatterjee-Fischer, Wear behaviour of boride layers onalloyed steels, Tribol. Int. (August 1981) 209–215.

[4] M. Carbucicchio, M. Bardini, G. Palamborini, Mössbauer and metallo-graphic analysis of borided surface layers on armco iron, J. Mater. Sci. 15(1980) 711–719.

[5] M. Carbucicchio, G. Palamborini, Effect of alloying elements on thegrowth of iron boride coatings, J. Mater. Sci. Lett. 6 (1987) 1147–1149.

[6] U. Sen, S. Sen, The fracture toughness of borides formed on boronizedcold work tool steels, Mater. Charact. 50 (4/5) (2003) 261–267.

[7] B. Venkataraman, G. Sundararajan, High speed sliding wear behavior ofboronized medium carbon steel, Surf. Coat. Technol. 73 (1995) 177–184.

[8] Enver Atik, Umut Yunker, Cevdet Meriç, The effects of conventional heattreatment and boronizing on abrasive wear and corrosion of SAE 1010,SAE 1040, D2 and 304 steels, Tribol. Int. 36 (2003) 155–161.

[9] A.G. Mathuschka, Boronising, Carl Hanser Verlag, München, 1987,pp. 1–87.

[10] B. Selcuk, R. Ipek, M.B. Karamis, V. Kuzucu, An investigation on surfaceproperties of treated low carbon and alloyed steels (boriding andcarburizing), J. Mater. Process. Technol. 103 (2000) 310–317.

[11] B.S. Mann, Boronizing of cast martensitic chromium nickel stainless steeland its abrasion and cavitation–erosion behaviour, Wear 208 (1997)125–131.

[12] R.H. Biddulph, Boronising for erosion resistance, Thin Solid Films 45(1977) 341–347.

[13] E. Takeuchi, K. Fujii, T. Katagiri, Sliding wear characteristic of gasboronized steel, Wear 55 (1979) 121–130.

[14] C. Bindal, A. Erdemir, Ultralow friction behavior of borided steel surfacesafter flash annealing, Appl. Phys. Lett. 68 (7) (1996) 923–925.

[15] A. Erdemir, G.R. Fenske, R.A. Erck, F.A. Nichols, D.E. Busch,Tribological properties of boric acid forming surfaces. Part II: Mechanismsof formation and self-lubrication of boric acid films on boron and boricacid containing surfaces, J. Soc. Tribol. Lubr. Eng. 47 (3) (1991) 178–184.

[16] A. Erdemir, G.R. Fenske, R.A. Erck, A study of the formation and self-lubrication mechanisms of boric acid films on boric oxide coatings, Surf.Coat. Technol. 43/44 (1990) 588–596.

[17] A.G. Matushka, Boronizing, Carl Hanser Verlag, München, 1987, p. 100.[18] S. Sen, U. Sen, C. Bindal, The growth kinetics of borides formed on

boronized AISI 4140 steel, Vacuum 77 (2) (2005) 195–202.[19] K.H. Habig, Wear behaviour of surface coatings of steels, Tribol. Int. 27

(2) (1989) 65–73.[20] H. Dogan, F. Findik, O. Morgul, Friction and wear behaviour of implanted

AISI 316L SS and comparison with a substrate, Mater. Des. 23 (7) (2002)605–610.

[21] A. Erdemir, C. Bindal, Formation and self-lubrication mechanism of boricacid on borided steel surfaces, Surf. Coat. Technol. 76/77 (1995) 443–449.

[22] A. Erdemir, Tribological properties of boric acid and boric acid formingsurfaces. Part 1: Crystal chemistry mechanism of self-lubrication of boricacid, J. Soc. Tribol. Lub. Eng. 47 (3) (1991) 168–172.

[23] J. Subrahmanyam, K. Gopinath, Wear studies on boronized mild steel,Wear 95 (1984) 287–292.

[24] S. Sen, I. Ozbek, U. Sen, C. Bindal, Mechanical behavior of boridesformed on borided cold work tool steel, Surf. Coat. Technol. 135 (2001)173–177.

[25] W. Fichtl, Boronizing and its practical applications, Mater. Eng. 2 (1981)276–281.