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Various metal types are used by spark plug manufacturers for ground electrodes and center electrodes.
Copper is a great electrical conductor with low resistance.
Almost all spark plugs use a copper cored center electrode to conduct the electricity, jump the gap, and promote heat dissipation. Due to its soft nature and low melting point, using copper as an outer electrode material would result in a plug that lasted only minutes.
"Copper spark plugs" is a term mistakenly used for a standard or traditional spark plug. A standard spark plug typically uses a nickel-alloy outer material fused to a copper core. Some plugs include a coating or pad of precious metals to extend plug life. But as a general rule of thumb, all spark plugs utilize a copper core.
Iridium is 6x harder and 8x stronger than platinum, has a melting point 1,200°(F) higher, and conducts electricity better.
For many years, spark plug manufacturers favored platinum for their long life and performance plugs due to its high melting point. However, increasingly strenuous engine conditions, and the creation of smaller electrodes and longer life plugs required harder and stronger metals. The strength, hardness and high melting point of iridium makes it very well suited for a fine wire plug and ultra long life spark plugs.
Though better than platinum in these respects, it is very expensive, and at higher temperatures it oxidizes, thus rendering pure iridium as an expensively poor choice for spark plug construction. However, when properly blended with other precious metals such as yttria, rhodium or platinum, you can enhance their advantages with the superior strength and hardness of iridium.
The price point of an iridium plug is usually equitable to its iridium content. Some lower priced iridium plugs have just enough iridium content to be marketed as iridium plugs. They likely will not perform or last as long as the plugs with a higher iridium content.
Nickel is a hard, inexpensive metal used in most all spark plug designs. Nickel is not a good conductor and has a high level of resistance, so most center electrodes feature a copper core, covered in a nickel-alloy. This creates a balanced, team effort where the copper core conducts electricity and promotes heat dissipation, and the nickel-alloy outer material prolongs the life and durability of the center electrode.
While nickel is a choice material used in the majority of spark plug designs, it has its limitations. Other materials are harder, stronger, have a lower electrical resistance and a higher melting point, and are therefore used to enhance nickel electrodes and prolong a plugs life in high performance designs.