Electrically conductive stainless steel at 700º?

[spudbeaux]

I'm seeking some advice since this is completely out of my realm of experience. Here's a quick background and then I'll get right to the problem. Working on a test fixture for measuring materials from RT to 700ºC. The fixture inside a small furnace is made from 316L and alumina, the alumina to act as an electrical insulator and the 316L since it's fairly inexpensive, easy to machine, easy to weld, and resistant to high temperature. Any wiring inside of the furnace chamber is also 316L with alumina fish spine beads. The contacts we're using are an unknown type of stainless steel, just labeled as "stainless steel". Initially, everything was working well but after a dozen or so tests the contacts have developed some scale and ceased to function as a decent electrical conductor. The contacts are bars (~100mm x 10mm x 1mm) which pivot at one end and have a fine tip (almost like a stylus on a turntable) on the other. I'm looking for a material that can be used which wouldn't develop a similar problem under the test conditions, cycling from room temp to 700º several times a day. I'd rather avoid having to cap the tip of contact with platinum or something similar and I'm hopeful some type of stainless steel would be suitable for this application. I appreciate any advice or information you can provide. Thank you in advance.

 

[Compositepro]

Stainless steels rely on a non-conducive chromium oxide skin on the surface to provide corrosion protection. Not a good choice.

 

[spudbeaux]

Is there type of fairly inexpensive and durable material that could be used that would hold up these conditions? As a last resort the tip of the contact could be ground down and capped w/ Pt. Thanks again.

 

[Compositepro]

It is common practice to gold plate important electrical contacts. You can just go buy an "electrical contact" from a catalog.

 

[racookpe1978]

Non-corrosive surface finish certainly restores itself after scratching or exposure. But non-electrical conductive? Perhaps less so then a bare copper or polished silver solid wire, but it is conductive.

 

[btrueblood]

Why not just clean the contacts after each cycle, to restore the tips to a shiny bright condition? Or do they oxidize enough in one cycle to affect measurements?

What are the contacts making contact with? How much current do you want the contacts to pass?

Without evactuating the chamber, or purging with inert gas, there is no way to prevent the build up of scale on typical stainless steels at that temperature. Nichrome might oxidize more slowly.

What might work, is to use fine-grain graphite contacts, though they will slowly oxidize in air at that temperature (but the oxide is CO2, which is a gas, and won't stick around on the surface to impede electrical conduction). You would get longer life from them with a nitrogen or argon purged atmosphere.

Gold plating on steel or stainless will eventually peel off, as the oxygen in air will still get past the plating film to oxidize the metal underneath.

 

[spudbeaux]

Cleaning the contacts after each cycle would be ideal but given the location in the chamber it would be very difficult and may also lead to someone breaking some of the smaller pieces of alumina. This test is also intended to be used by a broad group of people as a QC check and, given some of the people that work here, I have little faith that things wouldn't be broken almost immediately.

The contacts will generally be in contact with Ag or Pt pads. The current will be quite low but it will vary depending on the material under test. It's characterizing various dielectric insulators by measuring resistance across a temperature range. Voltage would be anywhere from 0.1 to 100V, current generally down in the nA or µA range. Resistance measurements can be around 10^11 ohms down to 10^2 ohm (meter has low limit of 200 ohm) depending on the temperature.

Unfortunately, the furnace will need to be operated without nitrogen or argon. We have plenty of both but it would invalidate the data we're after.

Right now I'm trying a few different formulations with Au/glass and Pt/glass to see what sticks to the random stainless probes. I'll look into the fine-grain graphite contacts as I'm completely unfamiliar with it.

Thank you for the help, I really appreciate it.

 

[btrueblood]

You probably would be better off ponying up for gold, platinum or rhodium tips (i.e. a fairly thick chunk, not a thin plating layer), and soldering/brazing them onto the ends of your stainless probes. The varying resistance of graphite with temperature will probably screw up any resistance measurements you make, so it is likely to be a non-starter.

 

[IRstuff]

A poor man's integrated circuit probe starts with ~20-mil tungsten welding rods. Repeated dipping in electrolytic sodium hydroxide solution results in a very nice convex, but sharp, point that stands up well to repeated probing. It's been 40 years since I made any, so the details are a bit sketchy, but I vaguely recall using about 1 amp current. Don't remember much about the molarity, though. We got tips with ~1-micron radius and we were able to probe 5-6 micron lines on a chip. Tungsten's melting point is in excess of 3000C, and is quite hard and cheap. I bought the rods at a regular hardware store, but you'd probably have to get them at a specialty shop these days. Some tungsten rods are pre-ground for welding, and may be sharp enough as is.

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[EdStainless]

But W does not like high temp in air, it will oxidize.
But coating the W with Au would be a good option.

The problem with your test stand is that you are at the mercy of the contact resistance, and unless the metals involved are soft, oxidation resistant, and good conductors you can easily have more resistance is just the contact than in the entire rest of the circuit.

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[spudbeaux]

It's funny you mention that. I've used some TIG electrodes as contacts before for a different test, that was in a 400º tube furnace and under nitrogen. As it stands right now I'm looking for a quick and inexpensive solution. I'm certain I could use Au or Pt wire along the probe and make sufficient contact but I have very little in house. I tested a Au/glass coating which had very poor adhesion after firing, this material was developed for SiN and Al2O3 substrate, not stainless. I'm trying a high purity Ag coating right now, just waiting for it to finish firing. I think (and I could be very wrong) the 700º testing conditions would keep the contact highly conductive by breaking down any Ag2S that may develop at lower temperature. Meh, it at least sounds sciency to me. I'll see how big of a disaster it turns out to be in about an hour, I'll report back. Thanks again, we'll MacGyver this sucker yet. Did I mention I'm powering a set of 4 SPDT relays with an old cellphone charger? The initial start on the 5V coil pulls about 230 mA which is about 30 mA higher than the I/O controller connected to the computer can handle.

 

[btrueblood]

Why do you need the whole wire to be gold/Pt? Just the point making contact to your test pads should be enough, or is the junction resistance from the ss probe to the gold tip to the test pad enough to skew your numbers?

 

[spudbeaux]

The Ag coating looks promising but time will tell. I used an extra probe and sanded back the scale. Then I applied some Ag paste (which I'll likely need to thin out with additional solvent) to two points. After firing there is no significant resistance between the two Ag coated areas but I am seeing a high resistance between two points directly on the steel. I'll sand back the scale once again on the steel and begin cycling to 700C (firing temp of Ag paste is higher) and keep measuring resistance until the bare steel increases to an unacceptable level and then compare that to the Ag points. The adhesion seems great and should be durable enough for this application. I'm hopeful the elevated temperature will keep the silver from tarnishing but I'll be holding off coating the contact points in the furnace until I've cycled the extra probe at least a dozen times.

 

[spudbeaux]

Sorry, just saw your post. The contact point to the part under test is part of a longer, pivoting bar of stainless steel. It's all one piece from the fulcrum to the tip, connected to the other end is some 316L 22AWG wire. Where the "bar" or probe connects to the wire and the contact point to the part under test is where the increased resistance is screwing up the testing. I would just replace the 316L wire with Ag or Pt and run it along side the probe to the tip eliminating the two spots where changes in conductivity is critical. This is a small muffle furnace, about the size of a toaster oven. I'd probably only need about 2' of wire but all that I've got laying around is about 6" of super fine Ag wire.