Tribotechnical properties of coatings on titanium alloys

Abstract

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...

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