"My understanding is that the substrate material of aluminum would influence the hardness of the hardcoat on it. Would this imply that the hardcoat on 2024 alloy is softer than the hardcoat on 6061 alloy? Have you ever tested the HK hardness of hardcoat on 2024 alloy?"
I agree with your assessment of 2024 hardcoat hardness. We know from our experience that it is significantly softer than 6061 but we do not have any test numbers. The general principle is that the higher purity alloys will always yield a denser and harder hardcoat finish. The reason for this is that the alloying elements plate out during the process leaving a porous anodized surface (in anodizing the part is the anode which is sacrificial in electroplating terms). Aluminum, magnesium, and titanium do not plate out but oxidize under the right conditions. Also, if you check MIL-A- 8625 they will note that unsealed hardcoat gives better wear on the Tabor Abrasion test. We have tested sealed and unsealed hardcoat and can confirm that recommendation.