Structural changes in eutectic alloys during flame spraying

Abstract

Results of studying microstructural changes in eutectic alloys with different types of spraying are summarized. A distinctive feature of alloys which have eutectic carbides of the type MeC is that they do not change their shape and composition during different methods of spraying. The morphological characteristics of oxides phases are demonstrated. Addition of alloying elements with an affinity for oxygen makes it possible to alter the morphological characteristics.Powder eutectic alloys are used extensively for applying flame sprayed coatings for functional purposes. The comparatively low melting temperature provides good production characteristics for these powders during deposition. The structure of the alloys normally consists of a soft base and harder inclusions which reinforce the matrix in the form of a strengthening carcase. The properties of the strengthening phase (it may contain carbides, silicides, intermetallics, etc.) govern the range of application for these natural composites as wear-and heat-resistant and also high-temperature coatings. The structure of eutectic materials based on nickel, iron, and mixtures which have a relatively low melting temperature undergoes marked changes during deposition caused by high temperature and high heating and cooling rates, and deformation of particles with different temperature, rate, and aggregate states [1].There are a limited number of publications devoted to studying structural and phase transformations of eutectic powder alloys during deposition. The authors of the present work summarize the results of studying structural changes in complexly-alloyed eutectic materials during flame spraying and have analyzed the features of the structure of individual elements of the microstructure.Alloys of the systems Ni-AI, Fe-Ni-Cr-A1-C-Si, Fe-Ni-Zr-AI-Si-C, Fe-Cr-A1-C-Si with intermetallic, carbide, and silicide-carbide strengthening were considered. Coatings were prepared by plasma and detonation deposition. The structure of longitudinal and transverse microsections and also surface fractures was studied by optical and scanning electron microscopy using electron-probe microanalysis. Phase x-ray structural analysis was performed in a DRON-3 unit in CuK~-and FeK~-radiation. Results of the studies are given in Table 1. The main types of structure forming during flame spraying are presented in Fig. 1.Analysis of the microstructure of powder materials with intermetallic strengthening points to a qualitative difference in the phase composition before and after deposition, is somewhat different for the detonation and plasma methods. For coatings of both types there is a typical increase in the content of supersaturated solid solution of aluminum in nickel which is indicated by the diffusion of lines in x-ray diffraction patterns. IntermetaUic phases are impoverished in aluminum, and a marked amount of oxide phases appears reaching with plasma deposition 10% or more. Alongside oxide phases in the form of thin interlayers along grain boundaries and layers compact inclusions f...

Tribotechnical properties of coatings on titanium alloys

“…This favors better structural adaptability of the coating. Furthermore, gas-thermal coatings are characterized by high heterogeneity caused by development of boundaries of different types, namely, intergranular and interphase boundaries and boundaries between deformed particles, layers, and the coating and the substrate, by formation, in the course of deposition, of additional phases, including oxides, that can serve as a solid lubricant, by formation of solid solutions supersaturated with elements that enter into the composition of the coating and ensuring their hardening, and by development of micropores to the extent of 20-30% that are reservoirs for products of friction and for lubricant [ 18]. These processes affect positively the friction characteristics of the coatings and determine their high resistance to seizing and scoring.…”