We compare the antifriction properties of different types of coatings on titanium alloys under conditions of boundary friction as applied to parts of the hydraulic cylinders of an aircraft. We show that gasthermal titanium carbide coatings cladded with nickel and with both copper and nickel have better antifriction characteristics than ones obtained by chrome electroplating, nickel chemoplating, thermooxidation, anodization, etc.Titanium alloys are known to have low antifriction properties and to interact very actively with air at temperatures above 500 ~ C, which causes seizing and losses in serviceability of parts in real friction assemblies. In this case, the coefficients of dry friction of titanium on titanium and other metals are as high as 0.48-0.68.The low wear resistance of titanium can be explained by features of its hexagonal close-packed crystalline lattice, which assures twinning along several planes at once in the case of sliding friction. This causes accumulation of vacancy-type defects and pronounced activation of surface layers, i.e., an increase in internal energy [1]. A decrease in the accumulated energy (passivation) reveals itself in seizing. In this case, one observes interdiffusion of atoms from contacting metals according to the vacancy mechanism. As a result, common lattices are formed at the sites of contact.Greases slightly improve the antifriction properties of titanium alloys. Surface layers interacting with grease and gaseous media are modified due to saturation with oxygen, nitrogen, hydrogen, and other elements. An increase in the carbon content in the surface layer of a titanium alloy [2] results from decomposition of molecules of grease that penetrates into microcracks of the friction surface. This process causes an adsorptive decrease in strength that initiates and intensifies a dispersion process associated with an increase in the density of blocked dislocations to values exceeding the critical one. Intense destruction of tribo-elements of a titanium alloy in a grease medium is also connected with penetration of other elements into surface layers [3].For hardening of surfaces of titanium and its alloys, one uses various coatings that can be divided into three groups:--diffusion coatings obtained by saturation of the surface layer with various elements, e.g., oxygen, nitrogen, boron, carbon, silicon, etc.;--electroplating coatings, chemical coatings, and coatings obtained by deposition from vapors, melts, etc.; --gas-thermal coatings and coatings obtained by various methods of vacuum physical deposition, e.g., the ion-plasma method, the magnetron method, electron-beam spraying, etc.To the last group, we assign coatings obtained by hard-facing, which have found no application in practice due to oxidation of the substrate material, formation of brittle intermetaUides, and low purity of the hard-faced surface.The most extensively used and promising methods of hardening titanium alloys in the aircraft industry are: chrome electroplating, thermooxidation, nickel chemoplating, and anod...
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...
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