Wear resistant surface layers

Figure 4.19. The metal aluminium forms a hard and wear-resisting surface layer of aluminium oxide AI2O3 corundum when exposed to the oxygen of the air. This oxide layer is crucial for the corrosion resistance of aluminium.

Electroplating. Chromium is electroplated onto various substrates in order to realize a more decorative and corrosion- or wear-resistant surface (24—32). About 80% of the chromium employed in metal treatment is used for chromium plating over 50% is for decorative chromium plating (see Metal surface treatments). Hard chromium plating differs from decorative plating mostiy in terms of thickness. Hard chromium plate may be 10 to several 100 p.m thick, whereas the chromium layers in a decorative plate may be as thin as 0.25 p.m, which corresponds to about two grams Cr per square meter of surface. 

Problem 4.8. The aim of carburization is to increase the carbon concentration in the surface layers of steel in order to achieve a harder wear-resistant surface. This is usually done by holding the steel in a gas mixture containing CH4 and/or CO at a temperature where austenite is present. A thick plate of 0.3 wt % C steel is carburized at 930 °C. The carburizing gas used in the treatment holds the surface concentration at 1.0 wt % C. Assume the plate is so thick that the diffusing carbon does not reach the opposite side of the plate over the course of the treatment. Given that the diffusivity of carbon in iron at this temperature is > = 1.1 x 10 m /s, determine how long it will take for the carbon concentration to reach 0.6 wt % C at a depth of 0.3 mm ... 

Hard (engineering) chromium layers are also deposited directly on a variety of metals. The purpose in applying these layers is to obtain wear resistant surfaces with a high hardness or to restore original dimensions to a workpiece. In addition, the excellent corrosion resistance resulting from these layers make them suitable for outdoor applications. 

It is nowadays widely accepted that hard, wear and corrosion resistant surface layers can be produced on Austenitic stainless steel by means low temperature nitriding and/or carburizing in a number of different media (salt bath, gas or plasma), each medium having its own strengths and weaknesses (Bell, 2002). In order to retain the corrosion resistance of austenitic stainless steel, these processes are typacally conducted at temperatures below 450 °C and 500 °C, for nitriding and carburizing respectively. The result is a layer of precipitation free austenite, supersaturated with nitrogen and/or carbon, which is usually referred to as S-phase or expanded austenite (Sun et al, 1999 Li, 2001 Li, et al., 2002 Christiansen, 2006). 

To produce wear-resistant or hardened surfaces, thin layers of borides can be prepared on metal surfaces by reaction and diffusion (see Metal SURFACE treatments). Boride powders can be formed iato monolithic shapes by cold pressing and sintering, or by hot pressiag. 

Hard plating is noted for its excellent hardness, wear resistance, and low coefficient of friction. Decorative plating retains its brilliance because air exposure immediately forms a thin, invisible protective oxide film. The chromium is not appHed directiy to the surface of the base metal but rather over a nickel (see Nickel and nickel alloys) plate, which in turn is laid over a copper (qv) plate. Because the chromium plate is not free of cracks, pores, and similar imperfections, the intermediate nickel layer must provide the basic protection. Indeed, optimum performance is obtained when a controlled but high density (40—80 microcrack intersections per linear millimeter) of microcracks is achieved in the chromium lea ding to reduced local galvanic current density at the imperfections and increased cathode polarization. A duplex nickel layer containing small amounts of sulfur is generally used. In addition to... 

The second approach, that of surface coating, is more difficult, and that means more expensive. But it is often worth it. Hard, corrosion resistant layers of alloys rich in tungsten, cobalt, chromium or nickel can be sprayed onto surfaces, but a refinishing process is almost always necessary to restore the dimensional tolerances. Hard ceramic coatings such as AbO, Cr203, TiC, or TiN can be deposited by plasma methods and these not only give wear resistance but resistance to oxidation and... 

Often it is the properties of a surface which are critical in an engineering application. Examples are components which must withstand wear or exhibit low friction or resist oxidation or corrosion. Then the desired properties can often be achieved by creating a thin surface layer with good (but expensive) properties on a section of poorer (but cheaper) metal, offering great economies of production. 

Other diffusion coatings include manganising to produce austenitic or martensitic surface layers on steel. Mixed Mn/Cr diffusion coatings are readily produced by pack techniques. Carbide coatings based on Ti, V and Cr, singly or in combination, are mainly produced for their wear-resisting properties There are now several commercial boronising processes Con-... 

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