Atomic force microscopy (AFM) was used to measure the disjoining pressures of perfluoropolyether lubricant films (0.8-4.3 nm of Fomblin Z03) on both silicon wafers and hard drive disks coated with a diamondlike carbon overcoat. Differences in the disjoining pressure between the two systems were expected to be due to variations in the strength of van der Waals interactions. Lifishitz theory calculations suggest that this substrate switch will lead to relatively small changes in disjoining pressure as compared to the more pronounced effects reported due to changes in lubricant chemistry. We demonstrate the sensitivity of our AFM method by distinguishing between these similar systems.