Microstructure and amorphization induced by frictional work in Fe–Cr–B alloy thermal spray coatings
Introduction
Some amorphous metallic alloys exhibit unique mechanical and chemical properties such as a high tensile strength and corrosion resistance due to the lack of long-range order in their atomic arrangement [1]. However, industrial application of these amorphous alloys has been restricted owing to the difficulties encountered in the production of bulk quantities and the limitations in thickness caused by rapid quenching methods such as melt spinning. Thus, amorphous metallic alloys have usually been used in the form of coatings in the field of combating wear and corrosion. Several deposition methods for amorphous alloy coatings have been reported, such as electrochemical deposition, electric-spark alloying, laser- and electron-beam technologies, ion-plasma and thermal spraying. Of these, thermal spray coating has become an advantageous method due to the high deposition rates and low operation costs involved 2, 3, 4.
One of the major issues in amorphous alloy coatings has been to acquire a high fraction of amorphous phases, since most metallic materials have been reported to exhibit a higher tensile strength and wear resistance in the amorphous state than in the common crystalline state 5, 6. Recently, a new possibility of obtaining amorphous surface layer via solid-state amorphization was proposed 7, 8. That is, certain alloy powders can be sprayed in their crystalline form in the coating and transformed into an amorphous state on the surface layer by grinding or by wear in service. The use of these coatings in combating wear can have great advantages, since this alloy coating has been reported to exhibit excellent wear resistance and corrosion resistance 9, 10, 11. Furthermore, it is possible to avoid considerable efforts and constraints in the coating process and/or alloy composition which have previously been required to obtain a high fraction of the amorphous phase directly from the melt.
The formation of an amorphous phase induced by sliding wear or mechanical grinding was reported recently [12]. However, there has been little investigation of the formation of amorphous surface layers induced by frictional work in the Fe–Cr–B alloy system. Furthermore, no report or discussion has been presented on the effect of microstructure and/or phase composition on friction-induced amorphization.
The present paper reports on the microstructural evolution of Fe–Cr–B alloy coatings under several different thermal spraying process conditions. Investigations were also made of the friction-induced amorphization reaction in dependence on the microstructure of the coatings.
Section snippets
Experimental
Fe–Cr–B alloy powder with a particle size of 5–45 μm was used in the present work. The chemical composition of the powder, analyzed by inductively coupled Ar plasma emission spectroscopy, is shown in Table 1.
Spray coating was performed on a carbon steel substrate by detonation-gun thermal spraying. The substrate was grit-blasted with abrasive SiC particles and degreased before thermal spraying. In the present investigation, spray coating was carried out with various fuel gas contents using
Microstructure characterization
Fig. 1 shows the microstructure of the initial Fe–Cr–B alloy powder. The Fe–Cr–B alloy powder exhibits a spherical shape, which is characteristic of gas-atomized powder. The microstructure of the alloy powder can be seen better from observations using backscattered electron imaging and X-ray diffraction, as shown in Fig. 1(b) and (c). The alloy powder was composed of Cr-rich boride phases, such as Cr1.65Fe0.35B0.96 (JCPDS 35-1180) and Cr2B (JCPDS 38–1399), embedded in an Fe–Cr solid solution
Conclusions
The major conclusions resulting from the work presented in this paper can be listed as follows. The initial Fe–Cr–B alloy powder was composed of Cr-rich boride phase (such as Cr1.65Fe0.35B0.96 and Cr2B) embedded in an Fe-rich Fe–Cr solid solution matrix phase. Thermal dissolution of the boride phase occurred during the thermal spraying process, resulting in a supersaturation of boron in the Fe–Cr solid solution phase. The Fe–Cr solid solution phase was crystalline in the as-deposited state and
Acknowledgements
The authors would like to express thanks for financial assistance provided by the School of Environmental Engineering and the Center for Advanced Aerospace Materials, POSTECH.
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