I was taught in machine work that the harder surface wears when combined with a softer surface when contamination is present. The expression was I think, "The softer material takes the charge and the harder material wears." People want to argue that all the time but think about it. Take a stick of butter out of the fridge, throw some glass shards, sand, woods splinters on it and press them into the butter. Rub it on something harder and see how much it scratches. The butter and contamination embedded in it don't wear much. The aluminum is soft enough that sand and debris is embedding in it. The contamination then wears the crankshaft.