Diffusion -hardened surfaces

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. 

In contrast, in thick samples oxygen diffusion restrictions may limit the initial oxidation to the surface of the polymer. The effect of surface cross-linking may be to further restrict diffusion of oxygen into the bulk, giving a hardened surface layer which protects the bulk polymer from further oxi-... 

Property PVD coated with 3 pm TiN layer Oxygen diffusion hardened (ODH) (30 pm hardened surface)... 

Choose material couples that are resistant to interaction in sliding (metal-to-metal wear resistance). Hardfacing alloys such as cobalt-base and nickel-chromium-boron alloys have been used for many years for applications involving metal-to-metal wear. Other surfaceengineering options include through-hardened tool steels, diffusion (case)-hardened surfaces, selective surface-hardened alloy steels, and some platings. 

Nitriding The formation of a dispersion-hardened surface region by diffusion of nitrogen into a metal-alloy surface containing a material that will form a metal-nitride dispersed phase. 

The same atom-centered polyhedra can be used to describe interstitial diffusion in all the many metal structures derived from both face-centered cubic and hexagonal closest packing of atoms. In these cases the polyhedra are centered upon a metal atom and all the tetrahedral and octahedral interstitial sites are empty. The hardening of metals by incorporation of nitrogen or carbon into the surface layers of the material via interstitial diffusion will use these pathways. 

Surface curvature, color coding for, 10 340 Surface defect densities, 9 731 Surface deformation, case hardening by, 16 207-208 Surface diffusion... 

Carburizing One of several thermochemical treatments involving, by heating, carbon absorption from suitable material in contact with the surface layer, and diffusion into solid (ferrous) alloys. Surface hardening processes may then be... 

Cementation process. This process, now little used, consists of heating wrought iron or low-carbon steel in powdered charcoal or leather dust for 6 to 11 days in a closed boxlike furnace at 650 to 700°C. At these temperatures, carbon diffuses slowly into the surface of the steel, thus producing a thin coat of high-carbon steel over a core of low-carbon steel. This is essentially a case-hardening procedure, and steel produced in this manner is used largely in the manufacture of tools. 

Three primary mechanisms have been suggested for enhanced adhesion via silane coupling agents.5 The classical explanation is that the functional group on the silane molecule reacts with the adhesive resin. Another possibility is that the polysiloxane surface layer has an open porous structure. The liquid adhesive penetrates the porosity and then hardens to form an interpenetrating interphase region. The third mechanism applies only to polymeric adherends. It is possible that the solvent used to dilute and apply the silane adhesion promoter opens the molecular structure on the substrate surface, allowing the silane to penetrate and diffuse into the adherend. 

Consider the absorption of oxygen from air in the aeration of a lake or the sohd surface diffusion in the hardening of mild steel in a carburizing atmosphere. Both these processes involve diffusion in a semi-infinite medium. Assume that a semi-infinite medium has a uniform initial concentration of CAo and is subjected to a constant surface concentration of CAs. Derive the equation for the concentration profiles for a preheated piece of mild steel with an initial concentration of 0.02 wt% carbon. This mild steel is subjected to a carburizing atmosphere for 2 h, and the surface concentration of carbon is 0.7%. If the diffusivity of carbon through the steel is 1 X 10 11 m2/s at the process temperature and pressure, estimate the carbon composition at 0.05 cm below the surface. 

The surface hardening of a mild steel component by the diffusion of carbon molecules is a transient mass diffusion process. 

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