Dynamic (impact) burnishing
The utilization of kinetic energy of steel, ceramic or glass particles in the form of shot or pellets, hurled by centrifugal force, stream of compressed air or energy of detonation of explosive gases,
or the kinetic energy of a smooth tool (hammering) hitting the surface of the treated metal object in order to cold harden the superficial layer. The layer thickness is usually less than in static burnishing, while the applications are similar. In some cases, dynamic burnishing is also applied as a means of introduction of compressive stresses in order to deform thin-walled objects.
Surface Engineering of Metals Principles, Equipment, Technologies
Tadeusz Burakowski and Tadeusz Wierzchon (PG: 21)
Pad welding
A modification of surfacing, accomplished with the use of welding torches, for overlaying of the metal substrate with a layer of alloy material in order to obtain a coating with properties either:
– similar to those of the substrate to replenish worn material (repair)
– different to those of the substrate to enhance service life.
Pad welding causes some insignificant melting of the substrate material, allowing a metallurgical bond between the substrate and the coating. Pad welding is carried out with the utilization of welding techniques, mainly arc and flame (oxy-acetylene) heating. Materials used for pad welding and for the generation of coatings with special properties are: carbon and low alloy
steels, austenitic high manganese and chromium-nickel steels, chromium and chromium-tungsten steels, high speed steels, high chromium cast irons, alloys such as Co-Cr-W, Ni-Cr-B, Ni-Mo and sintered carbides. The thickness of pad welded layers usually reaches several millimeters. In the past. Pad welding was considered a modification of plating.
Surface Engineering of Metals Principles, Equipment, Technologies
Tadeusz Burakowski and Tadeusz Wierzchon (PG: 25)
Etching
Removal of layers of scale, rust, oxides or alkaline salts from the
surface of metals and alloys, carried out before final pickling and deposition of
electroplated coatings. It can be carried out by:
– chemical means (electroless) - by immersion in acidic solutions, reacting
with metal oxides,
– electrolysis - in an electrolytic process, where the metal may be pickled by
the anode or the cathode.
Surface Engineering of Metals Principles, Equipment, Technologies
Tadeusz Burakowski and Tadeusz Wierzchon (PG: 30)
Plasma emission guns
The advent of the plasma emission gun came in the 1960s. It works at temperatures lower than those used in thermal emission guns and in conditions of softer vacuum. It is more resistant to the effect of atmosphere of the technological process and is characterized by long life (up to 5500 h), reliability and repeatability of beam parameters. The emitter of electrons is, directly or indirectly, plasma, generated by glow discharge.
In plasma cathode guns, the direct emitter is plasma generated by glow discharge in nitrogen, argon, helium, hydrogen or methane. Electrons exit the plasma zone as the result of thermal movements . Extraction of electrons is facilitated by the emission diode. Next, the electrons are formed into a beam, in a manner similar to that in thermal emission guns. Because of the absence of a potential barrier at the plasma boundary, the scatter of initial velocities of extracted electrons is significantly higher than in the case of thermal emission. This is conducive to errors in representation by the focusing system. The current beam is controlled by varying plasma parameters (discharge current and voltage).
The working pressure in the discharge zone of the plasma cathode is 10-3 to 10-1 Pa, the accelerating voltage is max. 60 kV, the power reaches 10 kW and the convergence angle of the beam is 10-2 to 10-1 rad. Guns with a plasma cathode are used in applications not requiring high treatment precision which is achievable when thermal emission guns are used. Current density from a plasma cathode may be higher by an order of magnitude than that obtained from a thermal emission cathode. Power density on the load may reach 10^7 kW/cm2.
Surface Engineering of Metals Principles, Equipment, Technologies
Tadeusz Burakowski and Tadeusz Wierzchon (PG:67)
Tribological wear
The result of interaction of the rubbing elements is tribological wear, understood as the process of destruction and removal of material from the surface of solids, due to friction, and manifest by a continuous change of dimensions and shapes of the rubbing elements. The causes of wear are in most cases of a mechanical character, less often mechanical, combined with the chemical interaction of the surrounding medium. The basic causes of wear are;
– Elastic and plastic deformation of peaks of asperities and their workhardening,
– The formation at the rubbing surface of oxide layers, preventing, on the one hand, galling and deep detachment of particles, but, at the same time, layers which are brittle, flaky and easy detachable; exposed surfaces mayundergo secondary oxidation, etc.,
– Building-in of fragments of the superficial layer of one rubbing material into the surface of the second material. During sliding wear this causes scratching of the surface and, with extended time (multiple formation of new asperities), destruction of surface,
– Adhesive bonds between contacting elements of the surface, conducive to transportation of metal from one superficial layer to the other which accelerates wear,
– Accumulation of hydrogen in the superficial layer of steel and cast iron elements which, depending on service conditions, may accelerate wear even by a factor of 10.
Surface Engineering of Metals Principles, Equipment, Technologies
Tadeusz Burakowski and Tadeusz Wierzchon (PG: 396)
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