[01] V. I. Dybkov, Department of Physical Chemistry of Inorganic Materials, Institute of Problems of Materials Science, National Academy of Sciences of Ukraine, Kyiv, Ukraine.

Boriding of Fe–Cr alloy (5-30% chromium) and commercial chromium steel (13 and 25% Cr) samples in amorphous boron powder at 850–950ºC and reaction times 3600-43200 s (1-12 h) results in the formation of a surface coating consisting of two boride layers. In the case of Fe–Cr alloys containing 5-15% chromium and a 13% Cr steel, the outer layer bordering the boriding agent consists of the (Fe,Cr)B phase, whereas the inner adjacent to the solid substrate consists of the (Fe,Cr)2B phase. Each layer is a homogeneous phase (microstructure of the first type). With Fe–Cr alloys containing 25 and 30% chromium and a 25% Cr steel, each of two boride layers consists of two phases. The outer layer comprises the (Fe,Cr)B and (Cr,Fe)B phases, while the inner comprises the (Fe,Cr)2B and (Cr,Fe)2B phases. Both layers have a regular network-platelet microstructure of the second type. With Fe–5% Cr and Fe–10% Cr alloys, boriding during 3600 s leads to the formation of a single (Fe,Cr)2B layer. The next (Fe,Cr)B layer occurs after the first-formed (Fe,Cr)2B layer has reached, depending on the temperature of reaction, a thickness of 100-180 µm. With other alloys and steels, a reaction time of 3600 s is sufficient for both boride layers (Fe,Cr)B and (Fe,Cr)2B to occur. The characteristic feature of the layers is a pronounced texture, the strongest reflections being {002} and {020} for the FeB phase and {002} for the Fe2B phase. Diffusional growth kinetics of boride layers are close to parabolic and can more adequately be described by a system of two non-linear differential equations. Values of layer growth-rate constants are provided. Their temperature dependence obeys a relation of the Arrhenius type. Boride layers with the microstructure of the second type exhibit a much higher wear resistance than those with the microstructure of the first type, the difference being more than an order of magnitude.

Fe–Cr Alloys and Chromium Steels, Boride Coatings, Microstructure, Phase Identity, Chemical Composition, Microhardness, Dry Abrasive Wear Resistance

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