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Holy shnikies.  You guys need an education in chemistry.  Nickel > Copper (in cost), Nickel < Copper (thermal conductivity).  Nickel is in place because of its very stable pure nature (does not corrode - oxidize - easily or quickly).
 

HORRIBLE blanket statement.  Some alloys conduct heat better than pure metals.  Do not make comments like this with zero knowledge of the subject matter.
 
Edit:
Kyle, BS Chemical Engineering, University of Cincinnati

Obviously this manufacturer does not make quality products! I do not see why we have to work so hard to get it to work after paying good money for it!?
 
I'd rather buy something ready to go.

Results differ when lapping. There are even people that have 0C difference between lapped and unlapped. This happens when the CPU is concave and the HS is convex... meaning that they meat together quite well. I'd say this isn't too common though, as almost everyone sees some improvement after a lapping.

In fact, I’m a little late with my findings about lapping my Ultra.
 
I went down 5c, yes FIVE; and all because the concave in my ultra (the QA in thermalright really s8cks!)
 
It was a pain to do the lapping, i knew it was going to take some hours.
I did it slowly and nice, but soon i find out i dont have the patience for this!
I still finished it slowly and nice, but I didn’t gave it a nice shinning ending. I focused in leave it plain.
 
At the end it did gave more than I expected.
 
I have a couple of pictures, but they are shameless

After lapping the knickle off the heatsink..exposing the copper..could applying the AS5(or your brand) seal off the copper so that it does not oxidize? Just a common sense thought...

This is a feature of the heatsink to help the thermal compound spread more evenly.  Contrary to the common belief, a mirror surface is unnecessary in the presence of properly-applied thermal compound.

Actually, your picture shows the wear starting from the middle and moving outward as time progressed.  This indicates a convex surface, not concave.  Like the grooves, this is a feature of the heatsink.  The heatspreaders on most modern CPUs are actually slightly concave.  Thermalright knows this so they designed their heatsink with a slight convex shape to provide a better mate with the heat spreader.

As I've heard from a poster concerning a similar thread on another site...
 
Apparently Thermalright prepares their heatsinks with a convex bottom because majority of Intel processors maintain a concave surface.  It's much easier to take away than add to.
 
Being that we're enthusiast I'm sure they took into account that we tend to go the extra mile to make sure our components work BEST.  So their quality control has an emphasis on joe stupid and what "might" happen.
 
I know plenty of those people who would be upset to read their thermalright directions and find out they'd have to lap their CPU before applying the heatsink to generate the best numbers.

Just to dig up this zombie of a thread to illuminate the question of why the nickel?  The one thing mentioned is the need to seal away the copper from the affects of oxidization.  That is indeed one benefit to coating the base with nickel since it is nearly corrosion proof.  The other more important reason though is that copper is a very soft metal, easily scratched.  Nickel on the other hand is not.  There in lies the most important reason, to reduce the chance of scratching the contact surface.  In nearly all cases the machining marks are quite noticeable in ThermalRights products.  Of course the affects of these small grooves on thermal efficiency are is emolliated by the use of TIM.  Most people will agree that lapping is not to remove the grooving left behind by the machining though it is a natural byproduct.  Lapping is primarily to obtain a flat surface not a smooth surface.  That said deep scratches would increase the chance of having air pockets in between the heatspreader and the heatsink.  This of course would have a noticeable affect on thermal efficiency, thus the primary reason the base is plated with nickel.

Oxidisation (turning green) should not really be a problem given the lack of moisture inside the case and the fact that no air can get to the contact areas.
 
I guess its more of an issue if the CPU / HS is sitting in retail packaging on a shelf somehwere.

well, i have read through this, and i have to say, im pretty darn confused. im set on buying this cooler(as it seems to be the best for air cooling). however, i dont know weather to "lap" this thing or not. hell, i dont even know how to lap anything. let alone a 60$ computer part which is pretty damn important to keeping your 350$ processor running right. so will someone plese direct me on what the wise thing to do with this heatsink for a q9450 (yes, i know their not out yet, but thats what i want to buy) or possibly a q6600.

?can you give an example of an impure metal? that's not an alloy.

 
 
I would check out your temps first and figure out if you are running high or not.  Are you going to OC your CPU or not?  If you are doing good temp wise and are not confident in your lapping skills...then why do it?  If you are going to OC your CPU to the max and you are temp limited then go for it.  I am probably going to do it just for a fun experiment with my next heatsink but only if I get really really bored one day.  
 
Regarding your health, make sure to do the lubricant part.  It will help prevent breathing in metal dust.  Try to avoid breathing in metal dust at all possible...not good for the health.  Metal can be pretty toxic.  

 
 
Any metal that you can think of has a purity factor.  Copper, Gold, Silver, Aluminum...you name it.  You can have a chunk of copper that is 98.0% pure.  Pure isn't exactly 100% either.  Usually the highest purity of anything you can get is something like 99.9998%
 
An alloy is an intentional or calculated mixture of metals where the impurities that I am talking about will be components in an elemental metal that serve no purpose or may affect the function of the metal such as it's conductive properties.    
 

However, they were clearly making ASSUMED statements, not educated assurtions.  But yes, pure copper does have a higher CP (0.385 J/(g•K)) and thermal conductivity (401 W/(m•K)) than brass (0.380 & 109, respectively), as an example.  But to say any pure metal has a high thermal conductivity than any other alloy is just plain wrong.  Correct me if I'm wrong, but aren't certain alloys better conductors of heat and electricity because of the better latice structure than their pure forms (in the same way that pure steel is weaker than say, Inconel)?  Thank you for the correction and I will try not to flame in the future.  Please share some of your knowledge on the subject for us all to be more enlightened.
 
Don't flame back with your judgement of my profession and education.

BOYS, Don't make me come over there.  If I have to get up, ain't nobody gonna be happy.  Y'all woke me up outta a good nap after watchin the superbowl and I have to go to work in the morning.  So hush up, get back in yer beds and go to sleep.  Night now.

 
Well don't throw stones if you live in a glass house and these sort of conflicts will be avoided.  I will never attack a person for a particular statement or for lacking knowledge about something unless it is in defense to someone doing it to me...I like the eye for an eye style basically.  In the future, please don't insult other people's intelligence and I will try to stop flaming in defense  
 
Where did you find the statement that any pure metal will conduct better than any other alloy?  If I ever said that, I was mistaken because that is too vague of a statement to hold any ground.  Indeed it is possible for an alloy to have a higher thermal conductivity than a pure metal but that is all relative to which alloy and metal you are comparing and under what conditions you hold them to.  
 
My entire purpose in posting on this thread, and I think that you will agree with me, is that it seems odd that there would be a layer of Nickel between the CPU and the Copper/Aluminum sink seeing that nickel has a lower thermal conductivity than Copper and Aluminum.  You should be better with thermodynamics than I am so I am seriously asking this.  Wouldn't the Nickel technically limit the flux of heat away from the CPU?  Heat can't be transferred at any higher rate than what the Nickel would allow and Copper would do so at a faster rate.  So in an extreme case, it would be like putting a rubber insulator between your CPU and Copper heatsink and expect the heat transfer to remain the same as if you had direct contact with the Copper.

I never said you did.  Someone else in the thread eluded to that, which prompted my first post.

I totally agree with you about Nickel's "insulating" (relative) property.  Yes, you are correct about your first assursion, the total heat flux can only be as great as the least conductive substance/interface in the system.  My personal opinion (and I do not work in the thermal solutions industry, so I would have no way of verifying it personally), is that the nickel is there to keep the metal from oxidizing BEFORE being put into use.  In the same way Intel and AMD package silica (to absorb water) with the heatsinks that come with the processors.  The vendors of thermal solutions have no control over the storage condition after it leaves there shipping docks, and therefore cover their butts by coating the entire heatsink in a very stable metal.
 
And example of how easily copper oxidizes (even without much contact with oils or water) is at Lowes or Home Depot in the plumbing section.  Just look at the copper water pipes, they look tarnished.  And I know, supply companies don't ship them with a brilliant finish, but they definitely look dull.  If you bought a brand new $60 heatsink that looked like that you'd be very mad, and forced to lap, or polish the interface (either CPU-to-HS or HS-to-air) in order to obtain acceptable results (because cupric and cuprous oxide has much lower thermal conductivities than copper metal).