Guide to Surface Engineering

Altering the Chemistry of
the Surface Regions

i) Thermochemical diffusion treatments introduce interstitial elements, such as carbon, nitrogen and boron, or combinations of carbon and nitrogen, into a ferrous metal surface at elevated temperatures. However, the processes are not confined to interstitial diffusion; metallic substitutional elements or metalloids are used in processes such as chromising, aluminizing and siliconising.

Interstitial element diffusion into steels falls into two categories:

1. those carried out at low temperatures, i.e. within the ferritic range, or
2. high temperature treatments in the austenitic range.

Ferritic processes include gas nitriding (typically 525°C), plasma nitriding (400 to 600°C) and nitrocarburising processes (approx. 500°C). For ferritic nitrocarburising processes, many different treatment media may be employed, including salt baths (cyanides or non toxic cyanate mixtures), endothermic ammonia gas mixtures, and methane or propane/ammonia/oxygen mixtures. Typically, such processes produce case-depths of around 250 microns on alloy steels, but they can also be applied to a much wider variety of ferrous alloys. On low carbon mild steel they can produce a thin 'compound layer' (of the order of 10 microns thick) which can improve both wear and corrosion resistance.

The austenitic treatments broadly include carburising employing solid (pack), liquid (salt bath) or gaseous media, carbo-nitriding and boronising. They are performed at temperatures near 900°C and produce much greater case depths (up to several mm) than the ferritic treatments. However, they also produce greater surface growth and distortion. Thermochemical treatments involving diffusion of substitutional elements, chromium (chromising) or aluminium (aluminising), which may be pack, salt bath or vapour processes are often used for elevated temperature service. The substrates are often nickel-based super-alloys or nickel/chromium gas turbine materials.

ii) Electroplating and 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 a diffusion treatment at 400 to 600°C to form Fe/Sn compounds which resist scuffing and confer some corrosion resistance. Bronze coatings may be developed in a similar way to add a bearing surface to a steel substrate.

iii) Oxide coatings on the surface of components can produce significant tribological advantages. When oil is present they prevent scuffing, adhesive wear and metal transfer. On ferrous substrates, chemical conversion layers may be produced by immersion in caustic nitrate solutions. This type of process is applied to needle or roller bearings, gears and piston rings. Similar coatings can be developed by thermal exposure at 300 to 600°C to produce an oxide film. Steam tempering or autoclaving, is applied to high-speed steel drills and zirconium alloy components for this purpose.

iv) Anodising treatments for aluminium alloys produce oxide layers which reduce adhesive wear and are significantly harder than the substrate (up to 500Hv). In this case, the process of hard anodising is carried out in an oxidising acid at around 0°C, so that a layer of oxide up to 500 microns thick is produced. Surface growth is exactly half of that layer thickness. Thinner layers, for decorative or corrosion protection purposes, are produced at room temperature.

Anodising may be followed by treatments to seal the surface and improve the corrosion resistance or by incorporation solid lubricants into the surface to lower friction and reduce wear rates. In this respect, the cellular structure of the layer readily lends itself as a key and a reservoir for low friction polymers.

v) Sulphur treatments incorporate sulphur into the surface of ferrous components. Sulphur, because of its low melting point, and some sulphides because of their crystal structures, have good lubricating properties. These processes are used for anti-scuffing purposes on cylinder liners, gears, CV joints, heavy duty rear axle spiders, textile machinery parts, etc. The processing temperature is generally below 200°C.

vi) Phosphating, The process is based on dilute phosphoric acid solutions of iron, zinc and manganese phosphates. Accelerators are added to shorten the process times to just a few minutes at approx 40 to 70°C. The simplest phosphate coatings consist of grey or black crystals of Fe3(PO₄)2 and some FePO4. Zinc and manganese produce more complex layers which absorb lubricant more readily. They are effective in reducing galling, pick up and scuffing. All phosphate coatings absorb oil and grease, thereby assisting 'running-in' by preventing adhesive wear and fretting.

v) Ion Implantation. In this process, atoms of gaseous or metallic elements are ionised and pass to a high vacuum chamber, where they are accelerated through a mass separator. Selected ions are then further accelerated and implanted into the target component. The implanted species occupy interstitial sites and distort the lattice. It is a low temperature process, typically 150°C for small items and less for larger components. The depth of effect is very shallow, 0.2 microns, but the surface properties such as wear resistance, friction and oxidation/corrosion resistance can be enhanced. This process has been used to improve the performance of forming tools for plastics, press tools and some surgical implants.