Hardness Measurement of Nano Coatings on Eyeglass Lenses

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Whether used for eye protection or vision correction, plastic eyeglass lenses are preferred over  glass  for  their  considerably  lower weight  and  better  fracture  strength.  In order to provide the required life-long quality of such lenses a specific scratch-resistance is necessary. This scratch-resistance can be determined through hardness measurement.

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Fig.  1:  Scratch-resistant  protective  coatings  are  essential  for  plastic lenses (picture courtesy of Rodenstock)

Current   eyeglass   lenses   made   of   plastic   are commonly  provided  with  an  anti-scratch,  dirt-repellent and  anti-reflective  surface.  They  are  vacuum  coated using a physical vapour deposition (PVD) method with up to 10 protective layers, each only a few nanometres thick,   which   together   ensure   very   high   scratch- resistance.  Hardness  and  scratch-resistance  of  these coatings are directly related: therefore, determining the hardness is a suitable method for quantifying the quality of these protective coatings.

To avoid commingling the hardness results of the coatings with those of the base materials while measur- ing, the test load must be absolutely minimal, as low as

a few micronewtons: The indenter may only penetrate up to one  tenth of  the overall coating depth  in order  to correctly determine its hardness without being influenced by the properties of the substrate (Bückle’s-Rule).

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Fig. 2: Hardness measurement of protective coatings of lenses (Martens hardness). Sample P4 has a significantly lower hardness and was identified as far less scratch-resistant (samples courtesy of Rodenstock)

Another important measuring parameter is the elastic/plastic deformation ratio of the coating material. The coatings must have a very high elastic component to prevent separation from the base material upon deformation. Therefore, multilayer coating systems are used that gradually adjust the modulus of elasticity from the base material to the top coating. These systems also have much higher adhesive bond strengths compared to single-layer coatings.

To secure the functionality of these protective coatings it is important to find the right balance between hardness and elastic behavior.

The PICODENTOR® HM500 is ideal for measuring the hardness and elastic properties of these complex, nano-thin multi-coatings, which requires a measuring system capable of load generation as low as a few micronewtons and highly accurate depth measurement in the picometre range – exactly the designed operating range of the PICODENTOR®. The hardness can then be calculated from the measured load/depth curves. For further info contact FISCHER (860)683-0781 or info@fischer-technology.com