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.
Słowa kluczowe: laser boriding, self-lubricating addition, microstructure, hardness, wear resistance.
Keywords: borowanie laserowe, dodatek samosmarujący, mikrostruktura, twardość, odporność na zużycie.
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, nitrocarburizing or boronizing as well as CVD and PVD methods, were also applied . Diamond like carbon (DLC) coatings  as well as the multicomponent coatings (TiAlN + TiN), produced by PVD methods , 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 considered 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 measured during wear . 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 utilization. Therefore, the solid lubricants seemed to be more acceptable 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 wellknown 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 a [...]
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