vtinventor

Topic: Electrical Resistance of plating metals.

Hello. I manufacture a fishing alarm that is activated when
2 very small rods (each about ½ inch long and the diameter of a small paper
clip) come into contact. The rods are perpendicular to each other and form an
“X” shape. When the rods do touch to complete the circuit, the surface area of
the physical contact is VERY small, being that the rods are round and only
contact at the point at the center of the “X”. The amount of pressure to make
the contact is also VERY small and can be accomplished with a feather. Because
the contact point of the 2 perpendicular rods and pressure is so small I need
to find the best combination of base metal and plating that will allow the lowest
possible electrical resistance, will stand up to light abrasion as the 2 rods
as they rub against each other, and will also not corrode or oxidize easily.
The circuit is powered by a 3 volt coin cell battery and runs at 5ma. Here are
a few questions and any help would be greatly appreciated. Thanks in advance.

1. Does the base metal or plating metal determine the resistance
of the rod? For example if rod is steel and plated with gold would the
resistance of the rod be based on the plating material or the base material or
a combination? If the answer is that the plating metal controls resistance would
using a thicker layer of plating lower resistance even more? Would it matter if
I added a layer of nickel between the steel and gold or multiple layers like
nickel/gold/nickel/gold?

2. When a metal is rubbed against itself, are there other
factors besides “hardness” that would determine how fast it wears? For example
say I have 4 rods, 2 made of steel and 2 of copper and they are exactly the
same size. If I put the 2 steel rods in contact with one another, and the 2
copper rods in contact with one another, and then spin the rods, with both sets
having the exact same amount of pressure on each, will they both wear out
evenly in the same amount of time or will the harder metal wear faster/slower?
Are some metals better to withstand rubbing against itself?

3. I need the surface of my plated rods to be as smooth
(less peaks and valleys) as possible because of the low physical pressure to activate the alarm and
small electrical contact point. Are some base metals or plating metals better at allowing
a smooth surface?

DustinGebhardt

Re: Electrical Resistance of plating metals.

Copper is the most conductive metal, but corrodes easily. Same with silver. Gold would be good. It is a good conductor and doesn't oxidize.


As far as wearability, tin or tin/lead is often used on engine bearings for this reason. I'm not sure how it would perform with regard to corrosion.


Answering your questions:

1: Depending upon the amount of current, thickness of the plated layer, and resistances of the 2 metals, etc, the flow of current through the part can be different. Typically, electrons like to be as far apart as possible, so in a circular part (like your rods), they will typically flow at the surface of the metal. As you add more current (and more electrons) they are forced to move closer together. For AC current, this may not apply. So if you have a steel rod with gold plating, the electricity will typically flow through the gold (it is more conductive) until the current gets high enough to overcome the steel's resistance and begin to flow in that metal. The thickness of the deposit should have an effect on the amount of current you can run before the steel's resistance is overcome.


2. You want to consider the tendency to gall of the plated deposit. Hardness has an affect as well, as a harder part will typically wear more slowly.


3. The initial surface condition of the part will ultimately determine the final condition of the plated part. Some deposits contain brighteners and/or levelers that help smooth out small imperfections, but a smooth substrate is your best option.

-Dustin Gebhardt,
CEF
Plating Engineer
Danaher Tool Group
Gastonia, NC

vtinventor

Re: Electrical Resistance of plating metals.

Thank you Dustin for the excellent information.