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Ash Small

Wed Sept 04 2013, 03:51PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I found this on Wikipedia regarding plasma nitriding:

"Plasma nitriding

Plasma nitriding, also known as ion nitriding, plasma ion nitriding or glow-discharge nitriding, is an industrial surface hardening treatment for metallic materials.

In plasma nitriding, the reactivity of the nitriding media is not due to the temperature but to the gas ionized state. In this technique intense electric fields are used to generate ionized molecules of the gas around the surface to be nitrided. Such highly active gas with ionized molecules is called plasma, naming the technique. The gas used for plasma nitriding is usually pure nitrogen, since no spontaneous decomposition is needed (as is the case of gas nitriding with ammonia). 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.

Usually steels 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. For instance, mechanical properties of austenitic stainless steel like wear can be significantly reduced and the hardness of tool steels can be double on the surface.

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 Nazi persecution. After his death in late 1960s the process was acquired by Klockner group and popularized world over.

Plasma nitriding is often coupled with physical vapor deposition (PVD) process and labeled Duplex Treatment, with enhanced benefits. Many users prefer to have a plasma oxidation step combined at the last phase of processing to produce a smooth jetblack layer of oxides which is resistant to wear and corrosion.

Since nitrogen ions are made available by ionization, differently from gas or salt bath, plasma nitriding efficiency does not depend on the temperature. Plasma nitriding can thus be performed in a broad temperature range, from 260Â°C to more than 600Â°C. For instance, at moderate temperatures (like 420Â°C), stainless steels can be nitrided without the formation of chromium nitride precipitates and hence maintaining their corrosion resistance properties.

In plasma nitriding processes nitrogen gas (N2) is usually the nitrogen carrying gas. Other gasses like hydrogen or Argon are also used. Indeed, Argon and H2 can be used before the nitriding process during the heating of the parts to clean the surfaces to be nitrided. This cleaning procedure effectively removes the oxide layer from surfaces and may remove fine layers of solvents that could remain. This also helps the thermal stability of the plasma plant, since the heat added by the plasma is already present during the warm up and hence once the process temperature is reached the actual nitriding begins with minor heating changes. For the nitriding process H2 gas is also added to keep the surface clear of oxides. This effect can be observed by analysing the surface of the part under nitriding (see for instance )."

I'm not quite sure how I should go about generating the plasma, for the following reasons:

First, if I use microwave energy, the steel workpiece 'may' absorb all the energy, and I'm not sure about whether a few tens of watts would even ionise nitrogen at STP (would I need to reduce the pressure?)

Second, Would a capacitatively coupled plasma work, and if so, how should I go about it?

Third, are there any other methods that I should consider?

Thanks in advance.

Proud Mary

Wed Sept 04 2013, 06:08PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

RF plasma is the stuff for this, so you'd need to make what is effectively a medium power CW HF transmitter to do it. You could get away with two valves, say 6V6 as oscillator, and 807 as power amplifier, with HT 750V/100mA for about 50W RF O/P, but I'm sure other people will have better solutions using modern components!

If you Google nitriding RF plasma you'll find a lot more.

Ash Small

Wed Sept 04 2013, 09:11PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Thanks for the tips and links, PM.

I do have an industrial RF generator (not sure if it works, though). Last time I had a thread on it's use I reached a 'dead end' because I couldn't calculate the load, and therefore couldn't design a 'matching circuit'. Any tips?

Proud Mary

Wed Sept 04 2013, 10:47PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Ash Small wrote ...

Thanks for the tips and links, PM.

I do have an industrial RF generator (not sure if it works, though). Last time I had a thread on it's use I reached a 'dead end' because I couldn't calculate the load, and therefore couldn't design a 'matching circuit'. Any tips?

With a few multi-way ceramic switches to make some tapped inductors, some fixed silver micas to select with another switch, and some variable capacitors, you can create so many permutations of L and C that you can match anything to just about anything. You don't have to calculate anything, because you just tune the output - change or reconfigure L & C by means of the switches - to get the best indication on a meter. I have plenty of this kind of stuff that you can have.

If you can determine the frequency, and describe the output stage as best you can, and put up some pictures, we should be able to work something out.

An old electric fire bar - the type with nichrome wire on a ceramic former - can come in handy as a dummy load.

Billybobjoe

Thu Sept 05 2013, 01:31AM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

What's wrong with just using DC?

I've modified my vacuum setup for nitriding, and performed a test using only argon (didn't have pure N2 available). IIRC a few hundred volts established a nice plasma which closely covered the surface of some stainless nuts and bolts I put in the chamber. This was around 1 torr.

See attached.
] electrical_discharges_in_gases_and_principles_of_i on_nitriding.pdf[/file]

Ash Small

Thu Sept 05 2013, 07:19PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Thanks for the links.

It looks like I'll need a capacitatively coupled DC bias of at least several hundred volts either way.

Initial reading of the above links suggests that using RF as well could speed up the process considerably, as well as avoiding some of the possible problems encountered when using a purely DC system.

The anode doesn't appear to be shown in the diagram below. I'm wondering where it would be located, and what form it would take?

Proud Mary

Thu Sept 05 2013, 08:25PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Here's a video of a nitrogen RF plasma generator, Mr Ash, but it looks jolly expensive, when you estimate the cost of all the bits and pieces:

Ash Small

Fri Sept 06 2013, 12:08AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Proud Mary wrote ...

Here's a video of a nitrogen RF plasma generator, Mr Ash, but it looks jolly expensive, when you estimate the cost of all the bits and pieces:

I can only see foreline pumps in the video, no turbo or diff pumps. I have two......I have an industrial 600W RF generator.....I was the first stainless steel worker to be accepted by the Guild of Master Craftsmen, back in the day, in the early nineties, before a lot of members of this forum were born.....My last job before I retired was working for Oxford Instruments Plasma Technology building machines that do this (and a lot more besides), ending up in quality control.......I think I can build it.

johnf

Fri Sept 06 2013, 09:42AM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

Ash
I do this sort of thing for my day job
any form of plasma source is sufficient--but heres the catch, the source has to be at around 50 to 100kv more positive than the piece being nitrided.
if the ion source is earth potential bound then you will need to bias the subject minus by the above --but this has problems as well as the potential tends to focus on the subject.

so the best is to run the subject piece at earth potential or slightly negative and jack the ion source up by most of the accelerating voltage.
At the levels of voltage i suggest the nitriding will be around a micron deep.

Also the ion source should be relatively energetic to disassociate the N2 molecule otherwise the penetration depth will halve --mass energy balance

I'm waiting on my new power supply 150kV 40mA to do bigger items in a reasonable time ie around an hour

Ash Small

Fri Sept 06 2013, 11:23AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

johnf wrote ...

Ash
I do this sort of thing for my day job
any form of plasma source is sufficient--but heres the catch, the source has to be at around 50 to 100kv more positive than the piece being nitrided.
if the ion source is earth potential bound then you will need to bias the subject minus by the above --but this has problems as well as the potential tends to focus on the subject.

so the best is to run the subject piece at earth potential or slightly negative and jack the ion source up by most of the accelerating voltage.
At the levels of voltage i suggest the nitriding will be around a micron deep.

Also the ion source should be relatively energetic to disassociate the N2 molecule otherwise the penetration depth will halve --mass energy balance

I'm waiting on my new power supply 150kV 40mA to do bigger items in a reasonable time ie around an hour

Hi John, thanks for the input. I appreciate that you are extremely knowledgable in these subjects.

The link relating to RF above mentions a DC bias of 400V. The link relating to DC plasma only mentions a few thousand volts at most.

You say I'm better off using 50-100kV and a separate ion source to obtain complete dissociation.

Would something like the ion source with ECR that I've outlined here be suitable?

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