No they are not foam pads.They are rubber.Specifically , they are
Sarcon TIM more than likely.
And it's not to prevent chipping.It really is thermal interface material.
You're probably correct about it being the cheapest compound out there , but too little of it isnt an issue
It's hard to see , but if yah look at the pic of the heatsinks , you can still see the thermal goop on the northy and southy heatsinks.It's probably 1 millimeter thick ! there's probably enough of it to mount 25 cpu's.
For shame Asus !!
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On a side note , the day I got my maximus , I tried to redo the heatsinks.It looks like they have done a better job . the northy heatsink is actually epoxied on with thermal epoxy.