Basics of Formation of Iron Borid e Coatings

Abstract

Main features of formation of iron boride layers in surface coatings on iron, its alloys, notably with chromium, and steels are discussed. Those include their microstructure, phase identity, chemical composition, sequence of occurrence, growth kinetics, and dry wear resistance. Two different kinds of layer microstructure (single-phase and two-phase) are shown to exist. Boriding of iron-chromium alloys (5-30% Cr) and chromium steels (13 and 25% Cr) at 850–950oC and reaction times up to 43200 s is found to result 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 compound, while the inner one adjacent to the solid substrate consists of the (Fe,Cr) 2 B compound. Each boride layer is a homogeneous phase. It is a 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 compounds. The outer layer comprises the (Fe,Cr)B and (Cr,Fe)B compounds, while the inner one comprises the (Fe,Cr) 2 B and (Cr,Fe) 2 B compounds. It is a microstructure of the second type that was not observed earlier. With Fe–Cr alloys containing 5% and 10% Cr, boriding during 3600 s leads to the formation of a single (Fe,Cr) 2 B layer. The (Fe,Cr)B layer occurs after the first-formed (Fe,Cr) 2 B layer has attained a thickness of more than 100 µm. With other alloys and steels, a reaction time of 3600 s is sufficient for both boride layers (Fe,Cr)B and (Fe,Cr) 2 B to form. Their distinguishing feature is a {002} texture. 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 exceeds an order of magnitude.