Boro-nitriding coating on pure iron by powder-pack boriding and nitriding processes

doi:10.1016/j.matlet.2016.04.135

Materials Letters

Volume 176, 1 August 2016, Pages 261-264

https://doi.org/10.1016/j.matlet.2016.04.135 Get rights and content

Highlights

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Boro-nitriding was successfully carried out by using the power pack technique.
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The iron nitride layer on the boride layer was flat, compact and well adhered.
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Microhardness gradient of the produced layers favoured the tribological performance.
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The multicomponent exhibited superior wear resistance compared to a boriding process.

Abstract

To alleviate spallation and crack difficulties exhibited by a borided metallic surface when it is subjected to a normal, heavy and sliding load under dry conditions, a boron nitride coating was produced on pure iron in two stages: boriding the iron surface at 950 °C for 6 h and then nitriding the pre-borided iron at 550 °C for 6 h. The powder-pack technique was used in both stages. XRD measurements confirmed that the grown layers were nitrides and duplex borides. The produced diffusion of the layers reached 240 µm in depth as measured by SEM images. The measured microhardness across the case favoured the interphase cohesion between the iron nitrides and iron borides layers. Consequently, the multicomponent coating exhibited superior wear resistance to an applied normal load under dry sliding contact conditions in comparison to borided iron.

Introduction

Hard coatings with carbides, borides and nitrides have been successfully utilised for engineering applications where specific properties at particular locations are required without compromising the bulk material strengths [1], [2], [3], [4], [5]. In particular, resistant layers of borides are produced in ferrous and non-ferrous materials through the well-developed process of boriding. In ferrous materials, this thermochemical diffusion treatment generally possesses superior hardening features than those found in conventional processes like carburizing, nitriding or chromising, due to the formation of single (Fe₂B) or duplex (FeB+Fe₂B) hard phases [3], [6]. However, while the single Fe₂B layer of ferrous materials promotes a surface with high compressive stresses, the FeB phase is very brittle and develops a surface subjected to high tensile stresses [2], [7], [8], [9]. At the end of the boriding process when the temperature decreases to ambient, and if the duplex phase is produced in the boride layer, stresses from such phases can lead to crack formation at the FeB/Fe₂B interface. This latter, due to those phases exhibit different coefficients of thermal expansion, it can cause spallation leading to the separation of the duplex layer, or else crack formation can appear under mechanical strain or thermal and mechanical shocks [10], [11]. On the other hand, the thermochemical process of nitriding is also used to improve the wear and corrosion resistance of engineering components, producing a hard case and a soft and tough core. Nevertheless, significant variation has been identified in the hardness gradient resulting in ablated tribological performances [12], [13]. To mitigate the brittleness and those variations in microhardness, two multicomponent surface treatments such as boro-nitriding are being investigated. Nonetheless, very little work has been devoted to assess both the microstructural and mechanical characteristics of boron nitride coatings on ferrous materials [14], [15], [16]. In this study, boride-nitride layers on pure iron have been investigated. Aspects of the film formation and the characteristics of their mechanical response to static and dynamic loads (namely hardness and friction behaviour) under dry conditions were the focus of the study.

Section snippets

Experimental

Cylindrical specimens (25.4 mm in diameter×7 mm thickness) were cut from an ARMCO iron bar with composition: Mn, 800 ppm; C and P, 200 ppm; and S, 150 ppm [17]. Due to the fact that the formation kinetics and structure of diffusion layers formed in boriding is influenced by the chemical composition of the steels [14], the substrate pure iron used in this work was selected to curb the effect of alloying elements in order to solely analyse the characteristic boride and nitride layers and some of their

Results and discussion

The morphology of a borided iron (first stage of the boro-nitriding) depicted the typical saw-toothed structure with the FeB/Fe₂B and Fe₂B/substrate interfaces. The iron boride FeB developed from the surface to the interior. The existence of the FeB and Fe₂B was verified by X-ray diffraction.

The cross-section of the boro-nitrided iron (second stage) with the coating layer comprising three different regions is shown in Fig. 1. (i) An iron nitride layer formed on the surface of the coated iron,

Conclusions

Pure iron was successfully boro-nitrided comprising a flat outermost nitride layer with ~15 µm thickness and a polyphase boride, constituted by an inner layer of Fe₂B and an outer layer of FeB with typical saw-toothed structure with a diffusion depth ranging 120–150 µm, without accounting the columnar depth. The formed nitride layer exhibited a narrow transition zone between the nitride phase and the high hardness boride layer, which could be considered to favour the cohesion of the interface.

Acknowledgments

The authors would like to thank the Secretary of Public Education of Mexico/TecNM for the financial support (Project no: 5642.15-P). The CEM-ITESM, I.P.N-ESIME (Dr. I. Campos, leader of the Surface Engineering Group) and the University of Leeds, U.K., for offering facilities to implement the project is also gratefully acknowledged.

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View full text

Boro-nitriding coating on pure iron by powder-pack boriding and nitriding processes

Highlights

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgments

Appl. Surf. Sci.

Mater. Sci. Eng. A

Mater. Eng.

Surf. Coat. Technol.

Tribol. Int.

Surf. Coat. Technol.

Surf. Coat. Technol.

J. Mater. Process. Technol.

Tribol. Int.

Wear