The demands on the surface properties of optical components have increased significantly in recent years. Highly complex coating systems have been developed in order to produce scratch resistant, stain-resistant, antistatic or reflective surfaces. Quality control for these coatings requires correspondingly high-performance measurement procedures and systems.

Both in the development phase and during the subsequent quality control phase of the manufacturing process, these coatings can be optimally characterized with the PICODENTOR® HM500. Material parameters such as the Martens hardness, Vickers hardness or penetration modulus can be determined in conformance with standards. Even thin coatings of less than 1 micrometer can be measured accurately with instrumented indentation.

In the following account, the scratch resistance of coated optical lenses was determined on four samples from different production batches by means of instrumented indentation. The lenses were measured in 10 places with a maximum test load of 15 mN and a total time of 30 seconds. The usual hardness for these protective layers is approximately 50 N/mm2 , which corresponds to a penetration depth of about 4 µm for a 15 mN test load. The coefficient of variation for the Martens hardness results ranged between 0.2% and 0.8%. This attests to both the homogeneity of the coatings and the excellent repeatability of the measurement system. Minimal differences in properties can be easily verified by means of instrumented indentation testing. The production process can be adjusted as necessary.

Various hardening processes play an important role in the production of optical coatings. The decisive factor is a good balance between hardness and the elastic properties of the coating. Two plastic lenses with an identical coating but different lengths of hardening time with UV light were also subjected to a test of the Martens hardness. In this case as well, a coefficient of variation of less than 1.7% also demonstrates the accuracy of the measurement. Whereas the measurement of the Martens hardness with the standard measurement procedure is already depth dependent, the depth-dependent determination of additional properties such as the Vickers hardness or the indentation modulus can be carried out by using the Enhanced Stiffness Procedure (ESP) with partial loading and unloading. Beyond a certain penetration depth, one starts to see the influence of the substrate. In order to measure the coating independently, the depth of penetration must not exceed 1/10 of the coating thickness. By generating forces down to a few micronewtons and performing high-precision distance measurements in the picometer range, the PICODENTOR® HM500 can measure the hardness of even very thin coatings. The extremely sensitive placement of the indenter enables precise zero point determination and avoids damaging the sample surface before testing.

In particular, due to the specific structure of the PICODENTOR® HM500, the ease of sample preparation and the speed of measurement, the system is suitable not only for the laboratory but also for quality and process control during production.