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Surface Engineering in a Nutshell

Introduction

Surface engineering may well be thought of as a very specialised subject deep within engineering or metallurgy. Take a look around the area where you are now. Do you see any items that are surface engineered? In fact, to find a manufactured item without some form of surface engineering applied to it may be far more difficult. Consider that piece of cake with the special "surface engineered" coating of pink icing you are about to eat from a plate that has a surface coated with a pretty ceramic glaze.

Also don't let us forget nature's own complex biological surface engineering. Surfaces that are self-repairing, heat regulating, loaded with sensory systems to name a few.

Definition

To make changes to the surface of a material.

Purpose

To gain or improve upon the desired surface properties of a material. To improve a components; performance, service lifetime, aesthetics or economics.

Surface Engineering Processes

There are many processes for modifying surface properties. These can be grouped into three categories:

1. Surface modification without changing the material chemically

Changes made by thermal or mechanical means, altering metallurgy or surface texture.

2. Surface modification by altering surface chemistry

These processes involve diffusion of new elements into the surface of the material. The original substrate material constituents play an active part in the modified surface.

3. Surface modification by adding new material onto the surface (coating).

These processes essentially add new material to the surface as a coating and do not involve the substrate material constituents at the surface.

Some processes may actually involve all of these categories during stages of producing the surface engineering system. In these cases the category will be judged by the final surface compared to the original substrate surface. Example: A steel substrate is cleaned and grit blasted (cat. 1), then it is thermally sprayed with aluminium (cat. 3), finally it is heat treated to aluminise the surface producing aluminides with the substrate constituents (cat. 2). So many category 3 type processes are used to pre-place material for a category 2 process. Another example: A steel is grit blasted (cat. 1) then thermally sprayed with a nickel chromium based self fluxing alloy (cat. 3), then heated to fuse the coating. Some degree of substrate/coating melting, diffusion and alloying takes place, but not enough to involve substrate constituents in the bulk or surface of the coating, so in this case this would be judge as category 3. Many welding type coating processes are similar.

Surface Modification without Changing the Material Chemically

a. Thermal Processes

Some materials respond well to surface heat treatment, particularly those that undergo phase transformations like the martensitic reaction hardening of carbon steels, low alloy steels and cast irons. Rapid heating processes like flame, induction, laser or electron beam are good techniques for localised surface heat treatment. One advantage of localised heat treatment is that only the surface is changed, so the bulk properties like toughness remain the same and component distortion reduced.

b. Mechanical Processes

Some materials respond well to cold working. Working the surface by peening, shot blasting, explosive hardening or other specialised machining processes induce compressive stresses, increasing hardness and fatigue resistance.

Changing surface texture using machining and blasting.

c. Other Processes

Modification of surfaces by chemical/electro-etching, laser engraving, various chemical, solvent and ultrasonic cleaning processes could also be included here. (laser engraved anilox rolls)

Many of the above processes will also be necessary pre-treatments for many other surface engineering systems.

Surface Modification by Changing Surface Chemistry

a. Thermochemical Diffusion Processes

Interstitial element diffusion (diffusion mechanism in which diffusant moves in between host atoms in the lattice; low activation energy process; interstitial diffusants like carbon, nitrogen and boron in steel feature high diffusion coefficient.

Common Processes:

  • Carburising (carburizing)
  • carbonitriding
  • nitriding
  • nitrocarburising (nitrocarburizing)
  • boronising (boronizing)
  • Chemical vapour deposition CVD, usually refers to "high-tech" thin film coating, but really most of the above processes are equally CVD process, as the diffusant elements are transported in a gaseous precursor compound to the substrate surface, where chemical reactions release the diffusant into the surface.

    Substitutional element diffusion, aluminium, chromium, silicon (diffusion mechanism in which diffusant substitutes for the host atoms by displacing them from their lattice sites; high activation energy process; substitutional diffusants like aluminium, chromium, silicon in steel feature low diffusion coefficient.

    Common Processes:

  • aluminising (aluminizing, calorising, alonising)
  • chromising (chromizing)
  • siliconising (siliconizing)
  • Toyota Diffusion process in which vanadium and niobium are diffused into steel.
  • Coating followed by thermal diffusion treatments, when used in combination, are included in this category. One process involves the electrolytic deposition of tin on to ferrous materials. This is followed by diffusion heat treatment to form Fe/Sn compounds that resist scuffing and add some corrosion resistance. Aluminium coatings deposited by thermal spray are also diffusion treated to form aluminides with the substrate to enhance wear and corrosion/oxidation resistance at high temperatures.

    b. Electrochemical Processes

    Conversion coatings aided by an electrolytic process.

    Common Processes:

  • Anodising (anodizing) of aluminium, titanium
  • c. Chemical Conversion Coatings

    Common Processes:

  • Phosphating
  • chemical blacking
  • chromating
  • d. Ion Implantation Processes

    Ion implantation is a process in which ions are injected into the near-surface region of a substrate. High-energy ions are produced in an accelerator and directed as a beam onto the surface of a substrate. The ions impinge on the substrate with kinetic energies greater than the binding energy of the solid substrate and form an alloy with the surface upon impact. Virtually any element can be injected into the near-surface region of any solid substrate. Commonly implanted substrates include metals, ceramics, and polymers. The average ion penetration depth is usually around 10 nanometres and 1 micrometre.

    Surface Modification by Adding New Material onto the Surface (Coating)

    Common Processes:

  • Welding Type Processes
  • Thermal Spray Process
  • Electroplating
  • Electroless plating
  • Mechanical plating
  • Galvanising, molten bath -tin aluminium (Al not aluminide) babbit
  • PVD Physical Vapour Deposition
  • CVD Chemical Vapour Deposition
  • Painting
  • Spin Coating
  • Powder Coating
  • Lubricants
  • Tiling
  • Cladding
  • Author: Gordon England
    Date: 1 Jan 2007
    Copright: © Copyright Gordon England

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