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Registered Member #230
Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
Ash The original article is really PVD not nitriding per se. The reactive gases in this case could be ammonia and titanium tetrachloride giving TiN as a coating and HCL as a waste gas. typically the partial pressure used is 10^-3 10^-4 millibar during the coating process. The bias of 400 volts is to give the TiN a few eV so that a chemical bond results on the surface ligands (surface dangling bonds) So this is a coating process where films of up to a micron can be grown. Nitriding is different so you need to implant nitrogen in the interstitial areas and then heat treat to make the nitrogen become part of the crystaline matrix. The temperature required to do this is much lower than the diffusion nitriding process that drives ammonia or cyanide into the crystaline matrix. This process usually causes part distortion due to the high temperatures involved so post process grinding is required to make the part true again
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
John,
That makes a lot of sense. Eliminating the need for post-nitriding machining is one of my objectives.
I came across some PECVD machines at OIPT, some of which apparently used a pulsed DC bias, so I'm reasonably familiar with that process.
I have a couple of question regarding heating:
Do you heat treat the piece as a separate process afterwards, or do yopu use the 'beam current' (for want of a better term) to heat the part while nitriding?
How do you monitor the temperature of the part whilst nitriding? Thermocouples? and, if so, what type, and does the nitriding process adversely affect the accuracy of the thermocouples themselves?
Registered Member #396
Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176
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.
Is 50-10kv really necessary for nitriding? What does a higher voltage physically do to the N2?
I haven't seen many specific voltages quoted in my research, but this article quotes 700v and a process time of 10-50 hours. Many other articles suggest that over 2000v the risk of arcing is high.
Registered Member #230
Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
Ash / billybobjoe Implant depth is proportional to acceleration. A surface layer is just that and if hard will require an underlying hard layer to support it. Nitriding gives a gradiated hardness so that the surface layer does not crack when the underlying layer "gives" the cyanide process / ammonia gives very deep nitriding many micron. Ion implantation usually is to around 1 micron with 100kV acceleration. heat treatment depnds on the target matrix but is usually post implantaion and is somewhere between 400 -600 degrees Celsius. diffusion implantation usually is much higher 700 -900 Celsius. post implantation heat treatment also cause the ions to diffuse albeit to a lower extent
the pressure in the chamber where the work pieces are around 10^6 millibar or better. The ion source internally could be three orders of magnitude higher in pressure. the difference being taken up by differential pumping
As for time a 40mA beam is good for around 1sq metre of surface area per hour
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Well, the 5kW, 50kV Xformer described elsewhere on this forum (which, incidentally, John Futter inspired me to design and build after a thread I had a few years ago on 'another forum'), fitted with a doubler should provide ~40mA @ ~100kV should be up to the job.
My main concern is pumping ~4-5kW into a workpiece of ~1kg in a vacuum (it's going to get pretty hot), so I'll need to monitor the temperature of the workpiece in order to prevent distortion, and the need for post-nitriding machining.
I suppose the first thing to do is to run some tests with thermocouples, and, maybe some test pieces. (After I finish the Xformer. I'm now thinking of doubling the amount of ferrite used in the design outlined in the thread on it, in order to further reduce capacitance, etc.
Registered Member #396
Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176
johnf wrote ...
Ash / billybobjoe Ion implantation usually is to around 1 micron with 100kV acceleration. heat treatment depnds on the target matrix but is usually post implantaion and is somewhere between 400 -600 degrees Celsius. diffusion implantation usually is much higher 700 -900 Celsius. post implantation heat treatment also cause the ions to diffuse albeit to a lower extent
the pressure in the chamber where the work pieces are around 10^6 millibar or better. The ion source internally could be three orders of magnitude higher in pressure. the difference being taken up by differential pumping
As for time a 40mA beam is good for around 1sq metre of surface area per hour
Wow, I had severely underestimated the effort required to successfully do this. I imagine obtaining a feedthrough for 100kv at 10^-6 millibar would not be easy/cheap (I am assuming you meant 10^-6 instead of 10^6).
How are industrial setups doing this successfully at 1kv and 1 torr as I've read numerous places? Simply very long process times?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Billybobjoe wrote ...
Wow, I had severely underestimated the effort required to successfully do this. I imagine obtaining a feedthrough for 100kv at 10^-6 millibar would not be easy/cheap (I am assuming you meant 10^-6 instead of 10^6).
How are industrial setups doing this successfully at 1kv and 1 torr as I've read numerous places? Simply very long process times?
Unsuppressed auto spark plugs, when modified and welded to a flange, can be used for HV feedthroughs. You have the 'long' end inside the chamber, and place the plug at the top of the chamber, with the outer connection under oil. I also have some ceramic stuff that can be used to make your own feedthroughs. You can buy purpose bulit feedthroughs, but you shouldn't need one rated for 100kV, unless you are putting lots of amps through it.
Another 'trick' I'm planning to use on another project is to put a doubler/multiplier 'inside' the chamber. This way the voltage at the feedthrough is 'managable'. You would need glass diodes (VMI do 'military spec' HV glass diodes), and capacitors that don't outgas (maybe capacitor plates aka vacuum capacitors, or 'maybe' ceramic caps would work, after the plastic coating is removed (Plazmatron had a thread on a related subject ).
EDIT: The lower voltage systems presumably won't get the same penetration, unless diffusion at high temp is carried out afterwards, which could lead to distortion, and the need for post nitriding machining, I guess.
N+ ions at 100keV will penetrate a lot more than N+ ions at 1keV.
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Plasma nitriding questions
