MICROSTRUCTURE, GROWTH KINETICS, AND ABRASIVE WEAR RESISTANCE OF BORIDE LAYERS ON Fe–30% Cr ALLOY

Abstract

Two boride layers are found to form at the interface between reacting phases in the course of boriding a Fe–30% Cr alloy in boron powder with KBF4 (activator) in the temperature range of 850–950 °C and reaction times 3600–43200 sec (1–12 h). Each of these layers is single-phase structurally (crystallographically) and two-phase compositionally (chemically). The outer boride layer bordering boron consists of the crystals of the (Fe,Cr)B and (Cr,Fe)B compounds, while the inner layer adjacent to the alloy base comprises the crystals of the (Fe,Cr)₂B and (Cr,Fe)₂B compounds. The characteristic feature of both layers is a profound texture. Diffusional layer-growth kinetics are close to parabolic and can alternatively be described by a system of two non-linear differential equations dx/dt = (k_B/x) – (rgk_Fe/py), dy/dt = (k_Fe/y) – (qk_B/sgx), where x is the outer FeB layer thickness (m), y is the inner Fe₂B layer thickness (m), k_B is the FeB layer growth-rate constant (m²· sec^-1), k_Fe is the Fe₂B layer growth-rate constant (m²· sec^-1), g is the ratio of the FeB and Fe₂B molar volumes, p = q = r = 1, and s = 2 (factors from the chemical formulae of FeB and Fe₂B). The temperature dependence of the layer growth-rate constants obeys a relation of the Arrhenius type K = Aexp (–E/RT), where K stands for any constant, A is the frequency factor, E is the activation energy, R is the gas constant, and T is the absolute temperature. Application of the least-squares fit method yielded the following equations: k_B = 3.42 · 10^-8 x exp(–175.4 kJ · mol^-1/RT) m²· sec^-1, k_Fe = 7.45 · 10^-9 exp(–144.6 kJ · mol^-1/RT) m²· sec^-1. Microhardness values are 18.1 GPa for the outer boride layer, 15.2 GPa for the inner layer, and 1.75 GPa for the alloy base. The dry abrasive wear resistance of the outer boride layer, found from mass loss measurements, is more than 300 times greater than that of the Fe–30% Cr alloy base. Such a huge increase in wear resistance is due to the microstructure of boride layers having a peculiar regular arrangement of enhanced rigidity.