Microstructural evolution in Fe-Cr-B based alloy powder and thermally sprayed coatings and their wear performance

Chokethawai, Komsanti (2010) Microstructural evolution in Fe-Cr-B based alloy powder and thermally sprayed coatings and their wear performance. PhD thesis, University of Nottingham.

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Abstract

To date, the development of an amorphous alloy has concentrated entirely on a multi-component Fe-based amorphous alloy (Fe-Cr-B based alloy systems) containing boron and/or carbon. The major driving force behind these developments lies in the desire to produce an amorphous coating which exhibits both excellent thermal stability and wear performance, whilst achieving significant cost reductions over existing materials used in wear applications.

Three commercially available gas atomised Fe-Cr-B based alloy powders, namely Armacor M, Armacor C and Nanosteel SHS7170 and a locally designed powder (KC1) were employed in this study. High velocity oxy-fuel (HVOF) thermal spraying was used to deposit coatings of these multi-component Fe-based amorphous alloys, approximately 300 micrometers thick on a mild steel substrate. The microstructures of the feedstock powders and the coatings before and after heat treatment were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC).

The as-sprayed Armacor M, Armacor C and Nanosteel SHS7170 coatings are a mixture of amorphous and crystalline phases, whereas the as-sprayed KC1 coating is composed of only an amorphous phase. The fraction of amorphous phase decreases in the order KC1 >> Nanosteel SHS7170 > Armacor C > Armacor M.

The amorphous phase in these coatings crystallises on annealing at about 600-700 degree Celsius and the crystallisation temperature depends on the content of solute elements (i.e. boron and carbon). Thermal stability of the amorphous phase containing boron and carbon is higher than those phases containing boron.

After heat treatment of the coatings above the crystallization temperature (~700 degree Celsius) the amorphous phase in the coatings devitrified into a multiphase structure. In the KC1 coating, formation of a very fine mixed structure of α-Fe, carbide, borocarbide and/or boride by decomposition of the amorphous phase can be observed, bringing about a hardness of (~12.4-13.2 GPa). In contrast, the hardness of the Armacor M and Armacor C coatings, containing boron decreased when annealed at temperatures of 750 degree Celsius or higher.

The wear performance of the sprayed SHS7170, Armacor C and Armacor M coatings was evaluated by sliding wear (ball-on-disc) testing under unlubricated conditions. The testing was performed with ceramic and steel counterfaces as a function of load and sliding distance. The results show that the SHS7170 coating, containing larger proportion of amorphous phase exhibits higher sliding wear resistance, compared with that of the Armacor M and Armacor C coatings. Wear mechanisms were investigated and related to properties (i.e. fracture toughness) and microstructures of the coating.

Item Type:Thesis (PhD)
Supervisors:McCartney, D.G.
Shipway, P.H.
Faculties/Schools:UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering
ID Code:1771
Deposited By:Dr Komsanti Chokethawai
Deposited On:02 Dec 2011 16:11
Last Modified:02 Dec 2011 16:11

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