NR 3/2017 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 143

Laser boriding of 100CrMnSi6–4 steel using BaF2 self-lubricating addition

Adam Piasecki*, Mateusz Kotkowiak, Michał KulkaInstitute of Materials Science and Engineering, Poznan University of Technology, Poznan, Poland, *adam.piasecki@put.poznan.pl

100CrMnSi6–4 steel, being a high carbon chromium steel with increased content of manganese and silicon, is commonly used in the bearing industry as a standard material. This material is predominantly applied to elements of rolling bearings taking into consideration its good wearability as well as good resistance to contact fatigue. The diffusion boronizing was a thermochemical treatment which improved tribological properties of this steel. In this study, instead of the diffusion process, the laser boriding was used in order to produce a boride layer on this material. The two-step process was used during laser alloying. First, the surface of the specimen was coated by a paste with alloying material. The alloying material consisted of the mixture of amorphous boron and BaF2 as a self-lubricating addition. Next, the surface was remelted by a laser beam using TRUMPF TLF 2600 Turbo CO2 laser. The microstructure of the layer consisted of the remelted zone with eutectic mixture of iron borides, borocementite and martensite as well as the heat-affected zone with martensite, bainite and retained austenite. The continuous laser-borided layer was obtained at the surface. It was uniform in respect of the thickness because of the high overlapping used during the laser treatment (86%). The hardness decreasing was observed in remelted zone compared to the laser-alloyed layer with boron only. However, the significant increase in wear resistance of laser-borided layer was caused by BaF2 self-lubricating addition. The formation of tribofilm on the worn surface was the reason for improved tribological properties of the self-lubricating layer.

Key words: laser boriding, self-lubricating addition, microstructure, hardness, wear resistance.

Inżynieria Materiałowa 3 (217) (2017) 143÷148DOI 10.15199/28.2017.3.6© Copyright SIGMA-NOT MATERIALS ENGINEERING

1. INTRODUCTION

Nowadays, there was a lot of literature data which described various techniques of improving tribological properties of the bearing steel [1÷5]. Some of these methods consisted in a special heat treatment [1, 2]. The surface treatments, such as diffusion nitriding, nitrocar-burizing or boronizing as well as CVD and PVD methods, were also applied [3]. Diamond like carbon (DLC) coatings [4] as well as the multicomponent coatings (TiAlN + TiN), produced by PVD methods [5], often increased the tribological properties of bearing steel. The wear resistance of the materials was often improved by increasing the hardness. The surface layers of higher hardness were usually characterized by the better tribological properties. In a con-sidered friction pair, the material with lower hardness usually wore more intensively. However, the wear mechanism also influenced the resistance to friction wear. If the oxidation was confirmed as the main wear mechanism, the oxides ensured the lubrication of the parts, e.g. the V2O5 oxides appeared on the surface of vanadized layers and because of that the lower friction coefficient was meas-ured during wear [6].

The coefficient of friction, characteristic of the mating parts, could be reduced by oils, used as lubricants. But oils proved to be very dangerous in the use as well as during production and utili-zation. Therefore, the solid lubricants seemed to be more accept-able for lubrication. Many different solid lubricants were applied for improving the wear resistance, e.g.: metals, fluorides, sulfides, sulfur and tungstates. Fluorides, such as CaF2 or BaF2, were well-known as solid lubricants which could work at elevated temperature (even above 500°C), ensuring reduction of friction coefficient [7, 8]. Barium fluoride was characterized by a low hardness and very good lubrication properties [9÷11]. This lubricant could be added as a component of composite materials, produced by hot-pressing and sintering [9], spark plasma sintering (SPS) [10] or pulse electric current sintering (PECS) [11]. The mixture of BaF2 and CaF2 fluo-rides was very often used as a lubricating material [12÷15]. Such a mixture was applied during the formation of composite materials by SPS [12], hot pressing and sintering [14, 15] as well as for pro-ducing the plasma-sprayed composite coatings [13]. Laser alloying

was also the surface treatment which enabled to produce the self-lubricating layers. Such a treatment was successfully implemented regarding the 100CrMnSi6–4 bearing steel which was alloyed with boron and calcium fluoride [16÷18]. It resulted in improving the tribological properties.

In this work, the laser alloying of 100CrMnSi6–4 steel with boron and addition of BaF2 was investigated. The microstructure, phase composition, and some mechanical properties of such self-lubricating layer were studied.

2. EXPERIMENTAL PROCEDURE

The chemical composition of the investigated 100CrMnSi6–4 bear-ing steel was shown in Table 1. The external diameter 20 mm, in-ternal diameter 12 mm and height 12 mm were characteristic of the ring-shaped specimens which were used for the study.

The two step laser-alloying process was shown in Figure 1. At the beginning, the surface of the sample was covered with the paste which was made from amorphous boron and BaF2 powders. The thickness of the coating was about 100 µm. Boron and barium fluoride were blended with a mass ratio of 10:2. The next step of the treatment consisted in the laser remelting. The surface of the sample, coated with alloying material, was irradiated by the laser beam. TRUMPF TLF 2600 Turbo CO2 laser was used during la-ser treatment. Its nominal power of the laser was equal to 2.6 kW. The parameters of the laser heat treatment were as follows: laser beam power P = 1.43 kW, scanning rate vl = 2.88 m/min and the la-ser beam diameter d = 2 mm. The scanning rate (vl) resulted from rotational speed (n) 45.85 min–1 and feed rate (vf) 0.28 mm per revolution. The multiple laser tracks ovelapped with the distance f = 0.28 mm

Next, the polished and etched cross section of the sample was Table 1. Chemical composition of 100CrMnSi6–4 steel, wt %Tabela 1. Skład chemiczny stali 100CrMnSi6–4

C Cr Mn Si Cu P S Fe

1.03 1.52 1.08 0.59 0.11 0.022 0.012 balance

144 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVIII

used for microstructure analysis. To reveal the microstructure the sample was polished by the abrasive paper with different granular-ity, and finally with Al2O3 abrasive slurry. After that, the specimen was etched by the solution consisting of 5% nital. For microstruc-ture observations, an light microscope (LM) and scanning electron microscope (SEM) Tescan Vega 5135 were applied. The phase analysis was performed by the X-ray diffraction using PANalytical EMPYREAN diffractometer with Cu Kα radiation. Microhardness measurements were carried out by the Buehler Micromet II appara-tus, at the load of 50 G (0.49 N). For this investigation the polished cross-section of the sample was used.

Wear resistance test was carried out using Amsler type MBT-01 device. The frictional pair consisted of the laser-alloyed specimen and plate-shaped counterspecimen made of sintered carbide S20S. (Fig. 2). The chemical composition of sintered carbide was shown in the Table 2. This material was characterized by the density

10.7 g/cm3 and hardness 1430 HV. The parameters of the wear resistance test were as follows: load P = 49 N and the specimen speed 0.26 m/s, resulting from the rotational speed of the specimen n = 250 min–1 and from its external diameter (20 mm). The wear test lasted 1 hour. The wear resistance was evaluated by the relative mass loss of the components of the frictional pair (sample and countersample), according to the equation::

Δmm

m mmi

i f

i=

−

(1)

where: Δm – loss of mass, mg, mi – initial mass of the specimen or counterspecimen, mg, mf – final mass of the specimen or counter-specimen, mg.

3. RESULTS AND DISCUSSION

The microstructure of laser-borided layer obtained on 100CrMn-Si6–4 steel with BaF2 self-lubricating addition was shown in Fig-ure 3. A high quality was characteristic of laser-alloyed layer. The layer was continuous, compact and uniform in respect of its thick-ness because of the high overlapping of the laser tracks (86%).

In the areological system, consisting of a layer and a substrate, the three zones were observed: laser-remelted zone – MZ (1), heat-affected zone – HAZ (2) and the substrate without visible effects of heat treatment (3). The average thickness of the remelted zone was equal to about 410 µm and was higher in the axis of the track and lower at the contact of adjacent tracks. The heat-affected zone was characterized by the thickness in the range of 160÷190 µm. Based on XRD patterns (Fig. 4), the MZ consisted of eutectic mixture of borides (FeB, Fe2B, Fe3B) with martensite (identified as Fe-α). Ad-ditionally, borocementite Fe3(C, B) and barium fluoride (BaF2) were detected. The particles of solid lubricant were visible mainly close to the surface. Such a distribution of barium fluoride was advisable tak-ing into consideration wear behaviour of the laser-alloyed layer. It fa-

Fig. 1. The scheme of laser alloying [17]Rys. 1. Schemat laserowego stopowania [17]

Fig. 2. Scheme of wear resistance test. Load F = 49 N, rotational speed n = 250 min–1

Rys. 2. Schemat badania odporności na zużycie przez tarcie. Obciążenie P = 49 N, prędkość obrotowa n = 250 min–1

Table 2. Chemical composition of sintered carbide S20S, wt %Tabela 2. Skład chemiczny węglika spiekanego S20S, % mas.

WC TiC + TaC + NbC Co

58 31.5 10.5

Fig. 3. Microstructure of laser-alloyed 100CrMnSi6–4 steel with boron and BaF2; 1 – remelted zone (MZ), 2 – heat-affected zone (HAZ), 3 – substrate; LMRys. 3. Mikrostruktura laserowo stopowanej stali 100CrMnSi6–4; borem i BaF2; 1 – strefa przetopiona (SP), 2 – strefa wpływu ciepła (SWC), 3 – podłoże; mikroskop świetlny

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cilitated delivering the lubricant into the contact area of mating parts and the formation of tribofilm between them. The microstructure of the heat affected zone probably consisted of a mixture of martensite, bainite and retained austenite. In the places, in which laser tracks overlapped, the tempered martensite could be observed in HAZ be-cause of the probable tempering of the previously produced tracks.

BaF2 should be applied as a lubricant working in dry conditions, i.e. in the environment free from water or damp. Particles of this lubricant could be rinsed out during preparing the metallographic specimen. Such a situation caused the decreased amount of BaF2 observed on microsection. Therefore, the fracture of the surface layer was prepared. SEM image of this fracture revealed the pres-ence of light BaF2 particles in this fracture (Fig. 5). The presence of BaF2 was also confirmed by qualitative X-ray microanalysis by EDS method (Fig. 6). Figure 6a showed the SE image of barium fluoride particle (occurring directly close to the surface) together with a lin-ear profile of barium content. The increase in Ba concentration in the light BaF2 particle was clearly visible. EDS map of Ba distribu-tion in considered area was presented in Figure 6b. It also confirm the relatively high barium concentration in this area (Fig. 6b). Some

BaF2 particles, visible in the fracture (Fig. 5), revealed the same shape which was characteristic of the powder, used as alloying mate-rial. Probably, during laser alloying, there were BaF2 particles which were remelted, partially remelted or even vapourized. However, some of them weren’t subject to these transformations and stayed in the unchanged shape. Such particles were not visible on microsec-tion because they were smeared during grinding or polishing.

The microstructure analysis by LM or by SEM as well as phase analysis by XRD confirmed that laser processing parameters were adequate in order to avoid substantial thermal decomposition and vaporization of the BaF2 powder during the laser alloying process. A significant amount of the lubricant was observed in the laser-borided layer, produced with BaF2 self-lubricating addition. A simi-lar effect was obtained if the bearing steel was laser-alloyed with boron and calcium fluoride (CaF2) [16÷18].

The hardness profiles of laser-alloyed layers were shown in Figure 7. The measurements were performed perpendicular to the laser-alloyed surface along the axis of a track (Fig. 7a) as well as along the contact of tracks (Fig. 7b). The results were compared to the laser-alloyed 100CrMnSi6–4 steel with boron only [17]. The occurrence of iron borides and martensite was the reason for hard-ness increase at the surface and in the entire remelted zone. For the laser-alloyed layer with boron and BaF2, microhardness of MZ varied between 690 and 857 HV in the axis of a track or between 750 and 946 HV at the contact of tracks. The laser-alloyed layer with boron only, was characterized by significantly higher hard-ness of MZ (875÷1450 HV) [17]. The addition of soft BaF2 phase

Fig. 4. XRD patterns of laser-alloyed 100CrMnSi6–4 steel with boron and BaF2Rys. 4. Dyfraktogram rentgenowski laserowo stopowanej stali 100CrMn-Si6–4 borem i BaF2

Fig. 5. Fracture of laser-alloyed layer with boron and BaF2Rys. 5. Przełom laserowo stopowanej warstwy borem i BaF2

Fig. 6. Qualitative X-ray microanalysis using EDS method: a) SE im-age with linear profile of barium content, b) EDS map of barium dis-tributionRys. 6. Jakościowa mikroanaliza rentgenowska metodą EDS: a) obraz SE z liniowym profilem stężenia baru, b) mapa rozmieszczenia baru

146 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVIII

caused the decrease in hardness of the remelted zone. Additionally, the lower laser beam power was applied during the formation of this layer (1.17 kW). It resulted in diminished depths of MZ and HAZ (Fig. 7). The hardness of the heat affected zone changed from about 700 HV directly below MZ to the hardness characteristic of the sub-strate. A gradual decrease of hardness was observed between the MZ and the substrate, contrary to the laser-alloyed layer with boron only. Such a situation could cause the stress distribution between laser-alloyed layer and substrate to be more advantageous.

The results of the wear tests were presented in Figure 8 and Table 3. In spite of the diminished hardness, the relative mass loss of the laser-borided layer, produced using BaF2 as a self-lubricating addi-tion, was smaller than that-measured for the laser-alloyed layer with boron only [16]. It confirmed the advantageous influence of BaF2 solid lubricant on the wear resistance.

The advantageous tribological behaviour of the specimen, laser-borided with the addition of BaF2, was caused by the tribofilm which was formed on the worn surface. The scheme of tribofilm producing was presented in Figure 9. During the first stage of the

Fig. 7. The hardness profiles of laser-alloyed layers formed on 100CrMnSi6–4 steel: a) along the axis of track, b) along the contact of tracksRys. 7. Profile twardości warstw stopowanych laserowo wytworzonych na stali 100CrMnSi6–4: a) wzdłuż osi ścieżki, b) wzdłuż styku ścieżek

Table 3. The relative mass loss of specimen and counterspecimenTablica 3. Względny ubytek masy próbki i przeciwpróbki

MaterialRelative mass loss Dm/mi

Specimen Counterspecimen

Laser-borided 100CrMnSi6–4 steel with BaF2

0.000248003 0.0000100013

Laser-borided 100CrMnSi6–4 steel [16] 0.000470434 0.0000367108

Fig. 8. The results of wear resistance testsRys. 8. Wyniki prób odporności na zużycie

Fig. 9. Scheme of tribofilm producing: a) initial stage consisting in lapping, b) smearing the lubricant on the surface of the specimen, c) forma-tion of tribofilm of diversified thicknessRys. 9. Schemat powstawania tribofilmu: a) wstepny etap docierania, b) rozmazywanie lubrykantu na powierzchni próbki, c) powstawanie tribofilmu o zróżnicowanej grubości

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wear, the mating parts were lapped (Fig. 9a). Then, BaF2 particles appeared on the surface and were smeared on it (Fig. 9b). Finally, the tribofilm of diversified thickness was observed on the surface of a laser-alloyed specimen and, as a consequence, the mass wear diminished. In the regions, in which the lubricant particles occurred directly on the surface after the process of laser alloying, lapping stage could be not observed. In that case, the barium fluoride was smeared at once, creating the tribofilm on the surface.

The presence of the BaF2 on the worn surface was confirmed by the X-ray microanalysis. The increased concentration of barium was distinctly visible in the EDS surface element distribution - map from the worn surface (Fig. 10).

4. CONCLUSIONS

Laser boriding with the addition of barium fluoride (BaF2) was ap-plied to produce self-lubricating layer on 100CrMnSi6–4 bearing steel. The microstructure was characterized by the three zones: laser remelted zone (MZ), heat affected zone (HAZ) and the substrate without visible changes in microstructure. The high value of over-lapping (86%) caused that the layer was compact and uniform in respect of their thickness. The cracks as well as gas pores were not observed in the microstructure.

The MZ consisted mainly of eutectic mixture of iron borides, borocementite and martensite. This zone was also characterized by the presence of BaF2 solid lubricant. It influenced the microhard-ness profile obtained. The soft BaF2 phase caused the decrease in hardness of the remelted zone compared to the laser-alloyed layer with boron only.

The significant increase in wear resistance was obtained as a consequence of an addition of BaF2 as a self-lubricant. The rela-tive mass loss of the laser-borided layer, produced using CaF2, was almost twice smaller than that-measured for the laser-alloyed layer with boron only. EDS pattern from the worn surface indicated that the thin tribofilm, consisting of BaF2, was produced during the war. The formation of this tribofilm consisted of the three stages: lap-ping, smearing the lubricant on the surface of the specimen and, finally, appearing of the tribofilm of diversified thickness.

REFERENCES

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[2] Polat S., Türedi E., Atapek Ş. H., Köseoğlu M.: Wear behavior of heat treated DIN 100Cr6 steels. International Iron and Steel Symposium Karabük, Türkiye, 2÷4 April (2012) 181÷196.

[3] Zenker R., Spies H.-J., Buchwalder A., Sacher G.: Combination of thermal surface treatment by high energy beams with thermochemical treatment and hard protective coating — state of the art. Proceedings of the 15th IFH-TSE International Federation for Heat Treatment and Surface Engineering Congress (2006) 214÷219.

[4] Podgornik B., Vižintin J., Borovšak U., Megušar F.: Tribological proper-ties of DLC coatings in helium. Tribol. Lett. 47 (2) (2012) 223÷230.

[5] Rasmussen I. L., Guibert M., Belin M., Martin J. M., Mikkelsen N. J., Ped-ersen H. C., Schou J.: Wear monitoring of protective nitride coatings using image processing. Surf. Coat. Technol. 204 (2010) 1970÷1972.

[6] Młynarczak A.: Modyfikowanie budowy i właściwości jedno- i wieloskładnikowych dyfuzyjnych warstw węglików chromu, wanadu i tytanu wytwarzanych na stalach metodą proszkową. Wyd. Politechniki Poznańskiej (2005).

[7] Yang J.-F., Jiang Y., Hardell J., Prakash B., Fang Q.-F.: Influence of ser-vice temperature on tribological characteristics of self-lubricant coatings: A review Front. Mater. Sci. 7 (1) (2013) 28÷39.

[8] Allam I. M.: Solid lubricants for applications at elevated temperatures: A review. Journal of Materials Science 26 (15) (1991) 3977÷3984.

[9] Cui G., Lu L., Wu J., Liu Y., Gao G.: Microstructure and tribological prop-erties of Fe–Cr matrix self-lubricating composites against Si3N4 at high temperature. Journal of Alloys and Compounds 611 (2014) 235÷242.

Fig. 10. Worn surface of laser-alloyed layer with boron and BaF2: a) SE image of the surface, b) EDS map of Ba distributionRys. 7. Zużyta powierzchnia warstwy stopowanej laserowo borem i BaF2: a) obraz SE powierzchni, b) mapa rozmieszczenia Ba otrzymana metodą EDS

[10] Ouyang J. H., Li Y. F., Wang Y. M., Zhou Y., Murakami T., Sasaki S.: Mi-crostructure and tribological properties of ZrO2(Y2O3) matrix composites doped with different solid lubricants from room temperature to 800°C. Wear 267 (2009) 1353÷1360.

[11] Kim S.-H., Lee S.-W.: Wear and friction behavior of self-lubricating alu-mina–zirconia–fluoride composites fabricated by the PECS technique. Ceramics International 40 (2014) 779÷790.

[12] Shi X., Yao J, Xu Z., Zhai W., Song S., Wang M., Zhang Q.: Tribologi-cal performance of TiAl matrix self-lubricating composites containing Ag, Ti3SiC2 and BaF2/CaF2 tested from room temperature to 600°C. Materials and Design 53 (2014) 620÷633.

[13] Huang C., Du L., Zhang W.: Friction and wear characteristics of plasma-sprayed self-lubrication coating with clad powder at elevated temperatures up to 800°C. Journal of Thermal Spray Technology 23 (2014) 463÷469.

[14] Liying Y., Zuomin L.: Microstructure and mechanical properties of Ti–48Al–2Nb–2Cr matrix high temperature self-lubricating composites by addition of 38% CaF2–62% BaF2 eutectic solid lubricants. Journal of Composite Materials 41 (2007) 3079.

[15] Zhen J., Li F., Zhu S., Ma J., Qiaoa Z., Liu W., Yang J.: Friction and wear behavior of nickel-alloy-based high temperature self-lubricating compos-ites against Si3N4 and Inconel 718. Tribology International 75 (2014) 1÷9.

[16] Piasecki A., Kotkowiak M., Kulka M., Dziarski P.: Laser boriding of 100CrMnSi6–4 steel using CaF2 self-lubricating addition. Inżynieria Materiałowa 6 (2015) 459÷463.

[17] Piasecki A., Kulka M., Kotkowiak M.: Wear resistance improvement of 100CrMnSi6–4 bearing steel by laser boriding using CaF2 self-lubricating addition. Tribology International 97 (2016) 173÷191.

[18] Kotkowiak M., Piasecki A., Kulka M.: Laser alloying of bearing steel with boron and self-lubricating addition. Archives of Mechanical Technology and Materials 36 (2016) 7÷11.

148 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVIII

Laserowe borowanie stali 100CrMnSi6–4 z zastosowaniem dodatku samosmarującego BaF2

Adam Piasecki*, Mateusz Kotkowiak, Michał KulkaInstytut Inżynierii Materiałowej,  Politechnika Poznańska, adam.piasecki@put.poznan.pl

Inżynieria Materiałowa 3 (217) (2017) 143÷148DOI 10.15199/28.2017.3.6© Copyright SIGMA-NOT MATERIALS ENGINEERING

Słowa kluczowe: borowanie laserowe, dodatek samosmarujący, mikrostruktura, twardość, odporność na zużycie.

1. CEL PRACY

Celem pracy było wytworzenie warstw stopowanych borem i flu-orkiem baru charakteryzujących się zwiększoną odpornością na zużycie przez tarcie. Wśród wielu metod zmniejszenia współczyn-nika tarcia w parze trącej istotne znaczenie ma stosowanie olejów. Jednak ze względu na problem z nimi związany, na każdym etapie użytkowania, od produkcji po utylizację, duże znaczenie zyskują lubrykanty stałe. Substancje te można podzielić m.in. na: metale, tlenki, fluorki, siarczki, siarczany, wolframiany. Wśród tych związ-ków na szczególną uwagę zasługują fluorki CaF2 i BaF2, które mogą pracować w wysokiej temperaturze, zapewniając zmniejszenie współczynnika tarcia. Fluorek baru charakteryzuje się małą twar-dością i bardzo dobrymi właściwościami smarnymi. W niniejszej pracy wytworzono warstwę stopowaną laserowo borem i fluorkiem baru, w wyniku czego spodziewano się uzyskać warstwę o więk-szej odporności na zużycie przez tarcie, nawet przy zmniejszonej twardości w porównaniu z warstwą stopowaną laserowo wyłącznie borem. Warstwy powierzchniowe wytworzono na stali łożyskowej 100CrMnSi6–4. Stal ta charakteryzuje się zwiększoną hartowno-ścią oraz należy do stosunkowo tanich materiałów.

2. MATERIAŁ I METODYKA BADAŃ

Skład chemiczny stali łożyskowej 100CrMnSi6–4 zastosowanej do badań przedstawiono w tabeli 1. Proces laserowego stopowania borem i BaF2 przeprowadzono dwuetapowo (rys. 1). Na począt-ku powierzchnia próbki została pokryta pastą zawierającą proszki amorficznego boru i fluorku baru (BaF2). Następnie przeprowadzo-no proces stopowania laserem CO2 TRUMPF TLF Turbo 2600. Do obserwacji metalograficznych stosowano mikroskop świetlny oraz skaningowy mikroskop elektronowy Tescan Vega 5135. Analizę rentgenowską przeprowadzono za pomocą dyfraktometru rentge-nowskiego PANalytical EMPYREAN. Pomiary mikrotwardości przeprowadzono na urządzeniu Buehler Micromet II przy obcią-żeniu 50 G (ok. 0,49 N). Badanie odporności na zużycie przepro-wadzono na na maszynie badawczej typu Amsler MBT–01. Układ pary trącej przedstawiono na rysunku 2. Jako przeciwpróbkę zasto-sowano węglik spiekany S20 (tab. 2).

3. WYNIKI I ICH DYSKUSJA

Mikrostrukturę wytworzonej na stali 100CrMnSi6–4 laserowo bo-rowanej warstwy z dodatkiem BaF2 przedstawiono na rysunku 3. Otrzymana warstwa charakteryzowała się wysoką jakością. Była zwarta i jednorodna na jej grubości, co wynikało z dużego stop-nia nakładania się ścieżek laserowych (86%). W areologicznym systemie składającym się z warstwy i podłoża można było wyróż-nić trzy strefy: strefę przetopioną MZ (1), strefę wpływu ciepła HAZ (2) i podłoże bez widocznych efektów obróbki cieplnej (3).

Na podstawie badań XRD stwierdzono, że strefa MZ składała się z mieszaniny eutektycznej FeB, Fe2B, Fe3B, Fe3(C, B) z marten-zytem oraz cząstek BaF2 (rys. 4). W strefie wpływu ciepła wystę-powała prawdopodobnie mieszanina martenzytu, bainitu i austeni-tu szczątkowego. W miejscu nakładania się ścieżek następowało odpuszczanie martenzytu, o czym mogą świadczyć zamieszczone profile twardości. Średnia grubość strefy przetopionej wynosiła ok. 410 mm, a grubość strefy wpływu ciepła mieściła się w zakresie 160÷190 mm.

Stwierdzono, że cząstki lubrykanta stałego znajdowały się w strefie przypowierzchniowej warstwy, w tym bezpośrednio przy powierzchni, co było korzystne ze względu na cel jego stosowa-nia i szybsze wytworzenie filmu złożonego z BaF2 na powierzchni próbki. Na przełomie warstwy stopowanej zaobserwowano jasne cząstki tej fazy (rys. 5). Obecność cząstek BaF2 potwierdzono za pomocą mikroanalizy rentgenowskiej EDS (rys. 6).

Analiza mikrostruktury oraz składu fazowego (XRD) potwier-dziły, że parametry procesu stopowania laserowego zostały dobrane prawidłowo. Uniknięto bowiem znacznego rozkładu i odparowania fluorku baru podczas procesu.

Warstwa laserowo stopowana borem i BaF2 charakteryzo-wała się znacznie mniejszą twardością w strefie przetopionej (690÷946 HV) w porównaniu z warstwą stopowaną wyłącznie borem (875÷1450 HV). Wpływ na taki stan rzeczy miało prawdo-podobnie zastosowanie BaF2 w materiale stopującym oraz więk-sza moc stosowanej wiązki laserowej. Pomiędzy strefą przetopioną i podłożem zaobserwowano mniejszy gradient twardości, co może powodować korzystny rozkład naprężeń.

Wyniki prób odporności na zużycie przez tarcie przedstawiono na rysunku 8 i w tabeli 3. Pomimo mniejszej twardości warstwa z dodat-kiem BaF2 charakteryzowała się dwukrotnie mniejszym względnym ubytkiem masy w porównaniu z warstwą stopowaną wyłącznie bo-rem. Prawdopodobną przyczyną zmniejszenia zużycia próbki podczas próby tarcia jest powstanie na jej powierzchni tribofilmu zawierają-cego BaF2 zmniejszającego współczynnik tarcia (rys. 9). Obecność BaF2 na powierzchni próbek po badaniach zużycia potwierdzono za pomocą mikroanalizy rentgenowskiej metodą EDS (rys. 10).

4. PODSUMOWANIE

W wyniku laserowego stopowania stali 100CrMnSi6–4 borem i BaF2 otrzymano warstwę składającą się ze strefy przetopionej oraz strefy wpływu ciepła. W strefie przetopionej przy powierzchni wy-stępowały cząstki BaF2. Wytworzona warstwa charakteryzowała się dwukrotnie większą odpornością na zużycie przez tarcie w porów-naniu z warstwą laserową stopowaną tylko borem, pomimo znacznie mniejszej twardości. Zaobserwowane mniejsze zużycie było spowo-dowane powstawaniem na powierzchni próbki tribofilmu złożonego z fluorku baru BaF2, który zmniejszał w znacznym stopniu współ-czynnik tarcia pomiędzy współpracującymi elementami (para trąca).