Plasma nitriding

Plasma nitriding or 'ion' nitriding (sometimes also called plasma ion nitriding) or glow-discharge nitriding, is an industrial surface hardening treatment for metallic materials.

Description

A plasma is the 'fourth' state of matter, the other three being solid, liquid and gaseous states. In the plasma state, matter exists in its excited form, also called the 'ionized' form, meaning the electrons in the outermost orbit have been knocked off. Thus a plasma usually is an ionized gas, and a mixture of neutral as well as charged particles.

Types of plasma

There are hot plasmas typified by plasma jets used for metal cutting, welding, cladding or spraying. There are also cold plasmas, usually generated inside vacuum chambers, at low pressure regimes. Here the high temperature characteristics of the ionized gases are not used, but the electronic properties become more useful. Thus an ionized gas like nitrogen in such a low pressure regime becomes much more reactive. Thus surface treatment using the ionized nitrogen results in hardening of metals by two mechanisms :

:* by diffusion, since the diffiusivity of the excited nitrogen atoms is higher -can be up to ten times faster compared with gas nitriding, forming the Diffusion Zone where precipitation hardening is present, and:* by thermo-chemical reactions which yield a thin layer of very hard iron and alloy nitrides, named Compound Layer or White Layer.

Process

Usually steels, alloy steels etc. are very beneficially treated with plasma nitriding. Plasma nitriding advantage is related to the close control of the nitrided microstructure, allowing nitriding with or without compound layer formation. Not only the performance of metal parts gets enhanced but working lifespan gets boosted. So does the strain limit, and the fatigue strength of the metals being treated.

A plasma nitrided part is usually ready for use. It calls for no machining, or polishing or any other post-nitriding operations. Thus the process is user-friendly, saves energy since it works fastest, and causes little or no distortion. This process was invented by Dr. Bernhardt Berghaus of Germany who later settled in Zurich to escape persecution of his community by the Nazis in 1939. It was only after his death in late 1960s that the process was acquired by Klockner group and popularized world over.

Plasma nitriding is often coupled with PVD physical vapor deposition process and labelled Duplex Treatment, to avail of immensely enhanced benefits. Many users prefer to have a plasma oxidation step combined at the last phase of processing to generate a smooth jetblack layer of oxides which is very resistant to not only wear but corrosion.

Gas and Liquid Bath Nitriding

Gas Nitriding or diffusion nitriding or ammonia (diffusion) nitriding has been around for nearly a century. It is a simple and inexpensive method for surface hardening of 'nitriding alloys' i.e. alloy steels with nitride forming elements and other metals e.g. aluminium, chromium, molybdenum and titanium to name but a few.

Liquid bath nitriding or salt bath nitriding on the other hand is not a classical nitriding technique but rather a carbo-nitriding technique, since it employs rather toxic salts of sodium cyanide. These salts break up at high processing temperatures, as high as 510 degrees Celsius up to 610 degrees Celsius depending upon various factors. This technique produces only a thin 'chemical compound zone' which is hardly 4 to 10 micrometres thick, with no underlying hardened sub-surface. In developed nations, the use of such toxic salts is banned, whilst the process thrives in the Third World without any restrictions, or circumvention of slack laws and regulations.

Gas nitriding consumes a lot of power, apart from using ammonia which is not user-friendly and can lead to physiological problems. The treatment cycles can be as long as 7 to 9 days for achieving deeper cases. Usually only a diffused case of a few hundred micrometres is produced, and the thermo-chemically formed compound layer is machined off. This is called the white layer as it appears whitish under magnification with an optical microscope. Therefore gas nitrided parts are not ready for use once they are nitrided but have to be machined or polished to remove this layer, which is brittle in nature. These limitations were primarily due to use of ammonia NH3 which implies the quantity of hydrogen gas is three times that of nitrogen when ammonia dissociates at high temperature. Though this stoichiometry [ratio of nitrogen to hydrogen] is good for diffusion, it tends to produce a mixture of two phases of iron nitride. A monophase nitride can only be generated when there is reliable manipulation of the gas ratios. This used to be a severe limitation in gas nitriding, which has been rectified by use of sophisticated gas flow controls coupled with automation or computerized controls.

Plasma nitriding can successfully suppress the formation of white layer, or if needed, form a monophase layer which may be epsilon or gamma prime in nature. The former is harder and the latter has good tensile properties. Thus the user has a choice here, either a diffused layer can be generated where no extra-ordinary hard surfaces are called for, or a monophase thin top layer can be made available on top of the diffused layer as needed. The tailor-made nature of the nitrided layer in plasma nitriding is its great strength and versatility.

Though gas nitriding techniques with improved, separate flow channels for nitrogen and hydrogen have been introduced, plasma nitriding by nature is faster due to activation of the main reactive species. It can be up to ten times faster than classical gas nitriding, and produce much lower mechanical distortion due to its ability to use lower processing temperatures.

Automated or computerized controls used with plasma nitriding render it a formidable surface treatment which is basically inexpensive but very effective and easily reproducible. The main uses remain in the automotive sector for treatment of parts such as crankshafts, cams and camshafts, gudgeon pins, connecting rods and levers or actuators. In tools, a wide variety of high speed steel cutting tools, or metal forming tools made from the high carbon high chromium alloy steels, and in toolings, a huge variety of dies and moulds are routinely plasma nitrided to make them last ten times longer.

Lately plasma nitriding has been combined with an additional PVD [physical vapour deposition treatment] to increase its efficiency further. New thin film hard coatings like chromium nitride, aluminium titanium nitride and titanium carbo-nitride are often applied to a plasma nitrided metal article, usually in the same reactor vessel to avoid contamination and eliminate the need for extra cleaning. These are called Duplex Treatments.

Plasma nitriding combined with an oxidation step results into a highly corrosion resistant jetblack glossy layer which is very popular with the automotive industries.

References

*cite book |first=Ramnarayan |last=Chattopadhyay |title=Advanced Thermally Assisted Surface Engineering Processes |publisher=Springer |location=Berlin |year=2004 |pages=90–94 |chapter=Plasma Nitriding |isbn=1-4020-7696-7