We present a comparative study of the elastic stiffness of ultra-thin (5, 20 and 100 nm thick) diamond-like carbon coatings with a sampling depth less than or comparable to the thickness of the coating. The experiments were conducted using atomic force acoustic microscopy, which is a dynamic operation mode of the atomic force microscope that permits the measurement of elastic properties with high spatial resolution. The method is based on the evaluation of the shift of the cantilever resonance frequencies caused by the contact stiffness between the sensor tip and the sample surface. Two sets of measurements are reported: one in which the silicon sensor tips were employed; and another set in which diamond-coated tips were used. The silicon tips showed considerable wear during data acquisition. Using the diamond-coated tips, however, the wear could be avoided and qualitative measurements of high stability could be performed, thereby permitting the comparison of coatings obtained by different deposition techniques. It is also shown how the obtained results can lead to future quantitative measurements on stiff coatings.