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Hardness of Complex Coating Systems for Optical Components
The demands placed on the performance of optical components have skyrocketed and, in response, highly complex coating systems have been developed to produce surfaces that are scratch-resistant, dirt-repellent, anti-static and reflective. Various curing processes are integral to the production of optical coatings, making it difficult but important to find the decisive balance between coating hardness and elasticity.
Quality control therefore requires correspondingly powerful measurement methods and systems. For the standard-compliant determination of such material parameters as hardness and elastic modulus the instrumented indentation test can be used, even thin coatings of less than 100 nanometres in thickness can be measured accurately.
With the load/indentation depth method according to DIN EN ISO 14577 and ASTM E 2546, the indenter, typically a Vickers or Berkovich pyramid is pressed with continuously increasing test load into the material and then reduced in the same manner while simultaneously measuring the respective indentation depths. Important technological characteristics can be calculated from the resultant load/unload cycle, for example the Martens hardness. The elastic modulus of indentation can be determined when the test load is reduced.
Fig. 1: Depth-dependent profile of the Martens hardness (HM) of two differently composed optical coatings. Marked in blue is the area where already an influence from the base material is given according to Bückle’s rule.
The figure 1 presents the measurement of Martens hardness and the associated standard deviation on two plastic lenses, samples courtesy of Rodenstock GmbH, Munich. The samples were produced under the same process conditions but exhibit differences in the composition of the coating system. As result a significant change of the hardness from one coating to the other can be seen.
At a certain indentation depth, the substrate material starts to become detectible. In order to avoid that influence while measuring the coating, the indentation depth must be limited to no more than 1/10 of the coating thickness (Bückle's Rule). The coefficients of variation for the two samples, 1.73 and 1.60 percent, respectively, as achieved using the FISCHER PICODENTOR HM500, demonstrates the potential for accuracy.
Fig. 2: The principle of instrumented indentation test: a designates the load increase, b the load decrease.
Although only the Martens hardness can be measured depth-dependent using standard methods, additional mechanical properties such as the Vickers hardness or the elastic modulus of indentation can be determined via the ESP (Enhanced Stiffness Procedure) method, which employs partial loading and unloading.
Conclusion: If the right balance between coating hardness and elasticity for coatings on optical components has to be determined the FISCHER PICODENTOR® HM500 is the suitable instrument to evaluate these parameters. For further consultancy please contact your local FISCHER representative.
Measuring the micro-hardness of tooth enamel in dentistry
The enamel – the outermost layer – of the tooth is made of a very hard, abrasion- and acid-resistant calcium-phosphate mineral compound. However, the consumption of chemically aggressive foods, imperfect dental care and mechanical wear and tear all combine to gradually soften or even remove the enamel: once this protective covering is damaged, bacteria can pass through to the core, resulting in tooth decay. Supposedly, dental rinses and toothpastes can protect the enamel and make it more impermeable. But is this effect actually measurable and thereby provable?
Human nutrition has changed greatly over the past few decades. The food industry now offers an enormous variety of tempting food products containing large amounts of sugars and acids, and the average per-person consumption of these soft drinks, snacks and convenience foods has skyrocketed.
At the same time, awareness of daily preventive dental care has also increased significantly. The toothbrush, whether conventional or electric, is now a normal part of nearly everyone’s basic personal hygiene regimen. Manufacturers of toothpastes and mouthwashes have developed products to protect the tooth enamel by making it more resistant to the “repeated acid attacks” associated with eating. Various products can prevent, slow down, or even reverse the degradation of already softened surfaces. However, systematic optimisation of these products is only possible if their effectiveness can be tested through accurate measurements.
The Department of Dentistry at the University of Bern was engaged by the Swiss Dental Association (SSO) to investigate the micro-hardness of the enamel, its modulus of elasticity and the relations between the various surface hardnesses. In a controlled experiment, human teeth were subjected to the caustic effects of such beverages as sugary and acidic soft drinks, orange juice and (only seemingly harmless) rose hip tea. The specimens, fixed in an embedding compound, were then measured with the FISCHERSCOPE® HM2000 at a test load of 50 mN. The results showed a significant decrease in surface hardness and elastic modulus compared with the "untreated" enamel. The consequences are obvious: prolonged exposure to acidic liquid can cause tooth decay, because it attacks and softens the tooth enamel.
But, in cooperation with industry, dentistry has found a way to help prevent dental caries and to re-mineralise the tooth through the use of low-dose fluoride. In a second step, the affected teeth were briefly soaked in a mouthwash. New hardness measurements on the same sample now showed a demonstrable hardening of the tooth surface. The advertised effect could actually be detected: toothpastes and dental rinses used in daily oral hygiene indeed offer effective protection of tooth enamel against the damaging influences of acidic foods.
With the FISCHERSCOPE® HM2000, mechanical properties such as micro-hardness and the modulus of indentation can be determined on tooth enamel in order to draw conclusions on the effectiveness of dentifrices and rinses. For further details please contact your FISCHER representative.
Micro-hardness testing of dental composites
In the last several years, great efforts have been made to replace amalgam fillings – which are fraught with disadvantages and therefore no longer used – with inlays made of modern composite resins. Because of the substantial challenges set by intensive daily use, these materials must be extremely durable and able to retain their shape. In order to verify a given composite’s suitability to this purpose, it is therefore necessary to determine precisely key mechanical properties such as micro-hardness and elasticity. Only then is it possible to guarantee optimal long-term results for the patient.
Enticing snacks and beverages with high sugar and/or acid content are a constant temptation in today’s vast offering of fast and prepared foods. Even the best oral hygiene has trouble keeping up; often, the side effects of unhealthy eating habits are cavities. As a result, the removal of caries and the placement of fillings have become common practice. However, the age of ugly black fillings is coming to an end, as tooth-coloured inlays and onlays of composite resin are replacing their unsightly amalgam precursors.
Composite fillings are made of microscopic ceramic and glass particles set in a light-activated resin. Because the colour can be matched to existing natural teeth, they represent a big cosmetic improvement over old amalgam fillings; nonetheless, they must still be able to retain their shape and withstand the wear and stresses to which teeth are normally exposed. These are just the minimum suitability requirements for their use in dental repair work.
The demands placed on dental composites are therefore quite high: they must be able to guarantee a certain hardness, a certain elasticity, long-term stability in colour and form, as well as flawless adhesion to the healthy tooth material.
The micro-hardness and elasticity parameters (Young’s modulus) of the various, often nearly identical makeups of dental composites can be determined precisely and accurately using the FISCHERSCOPE® HM2000. Without extensive preparatory effort, samples can be placed into the HM2000 and measured; a few minutes later, the process is done. Using its high-precision sample stage, automated test sequences can be programmed to, for example, detect inhomogeneities in the composite material. Comparing “untreated” composites to samples exposed to e.g. acids can provide the basis for deducing the filling’s ultimate durability in daily use. The HM2000’s high repeatability precision and its extreme accuracy make it possible to see even the slightest of differences.
Fig.1: FISCHERSCOPE® HM2000 Family
With the FISCHERSCOPE® HM2000, mechanical properties such as micro-hardness and elasticity can be determined precisely for dental composite resins, allowing conclusions to be drawn about their imperviousness to the impacts of aggressive food substances. For further details please contact your FISCHER representative.
Nanoindentation on intermediate layers in thin foils
Measuring the true mechanical properties of micron-scale intermediate layers in thin multilayer foils – without influence from the surrounding layers – is a challenge that very few instruments can meet. It takes highly responsive nanoindentation technology and very precise positioning of the indenter.
Determining the hardness and elastic characteristics on the intermediate layers of multiplex foil systems presents several challenges. A normal “top-down” style indenta-tion will yield composite properties for the entire sample, but not those of the individual layers, which requires measurements to be performed within each layer on a cross-sectioned sample. Due to the thinness of these layers, it is crucial to control the measurements precisely and to use extremely small indentations. Fortunately, nanoindentation technology, employing indents on the micron and nano scale, now allows hardness and elastic modulus measurements even on thin interior layers. Coupled with a high magnification microscope and a very precise positioning stage, nanoindentation is ideally suited for testing micron-scale components and films.
In this example, a 30 µm thick metallic foil sandwiched between a polymeric sheet and a rubberized top coat is analysed. Because the free-standing sample was not structurally rigid, to minimise sample compliance the part was mounted in epoxy and polished to a mirror finish to expose the metallic inner layer as cross-section.
The PICODENTOR® HM500 was chosen for this test due to its sensitive load resolution (≤100nN) and precise positioning capability (≤0.5µm). The values for indentation hardness (HIT), Vickers hardness (HV) and indentation modulus (EIT) were recorded for the metallic layer. The Martens hardness (HM) was measured as well and plotted as a function of indentation depth; variation in HM is an indicator of potential influence from surrounding layers. The cut edge of the metallic layer was identified under the integrated microscope (with magnification up to 1000x) and a series of indentations were made – located precisely in the centre of the 30 µm thick target layer.
Fig.1: Indents performed precisely at the centre of cross-sectioned layer
Fig 2: The graph shows reproducible load-displacement curves from each indent in Fig. 2.
Tab.1: Mean value X, standard deviation s and coefficient of variation V of mechanical properties measured from the indents in Fig. 2
With its ultra-sensitive measuring head, high-resolution microscope and precise stage, the PICODENTOR® HM500 makes it easy to accurately determine the hardness and elastic properties of micron-scale features, like cross-sectioned foils. For further details please contact your FISCHER representative.
Detection of hairline cracks in bearings for medical devices
The components of medical devices are subject to very stringent safety and quality requirements. An example is the slide bearings used in up-market X-ray equipment: The flawless functioning of these bearings depends on, among other things, the quality of their surfaces. All defects in those surfaces, even the finest hairline cracks, must be ruled out – a major challenge for the inspection metrology employed.
Bearings, such as those used for holding and smoothly moving the X-ray tubes in medical devices, are actually gliding on a thin film of liquid. If the surfaces have any damage, such as the thin fractures that can occur during the machining of the components, the sliding properties cannot be ensured for the device’s entire service lifetime. Technically, such cracks near the surface are difficult to detect, as optical inspection methods very quickly reach their limits with fine hairline cracks. However, measuring the electrical conductivity of the material provides a simple and quick way to detect even tiny fissures, as this method is very sensitive to structural inconsistencies.
Fig.1: Modern medical equipment makes high demands of the quality of the components used; seen here is an X-ray and injection device
FISCHER’s mobile handheld instrument SIGMASCOPE® SMP350, used together with the probe FS40, non-destructively measures the electrical conductivity of non-ferrous metals extremely quickly and precisely using the phase-sensitive eddy current method, which produces exactly the right conditions required for crack detection.
To check a surface for cracks, the probe FS40 is passed over the surface of the sample in free-running mode. Eddy currents are formed in the material, which are in turn recorded by the probe and converted to a signal in the instrument. In the case of the bearings we examined as a test, the conductivity measured was in the range of 18.3 MS/m, with very low variation. Any hairline cracks in the material will hinder the spread of the eddy currents – even the very finest, optically invisible faults. In practice, when the tested bearings showed a value of e.g. 14 MS/m, that would clearly indicate that a crack had been found by means of electrical conductivity.
The probe FS40 has a range of measurement frequencies (60-480 kHz), making it suitable for different material thicknesses. Using a lower measurement frequency results in the eddy current field penetrating deeper into the sample material. The measurements on the bearings described above were carried out using the probe’s highest frequency of 480 kHz, thus looking specifically for damage in the material near the surface. For use on tiny parts, the optional probe FS24, which has a smaller measuring head than the FS40, is recommended.
For the detection of fine hairline cracks in slide bearings produced for use in medical X-ray equipment, measuring the electrical conductivity – as implemented in the SIGMASCOPE® SMP350 from FISCHER – is an excellent solution. And between the two probes FS40 and FS24, a wide variety of part geometries can be accommodated flexibly, completing the ideal system for this purpose. For more information, please contact your local FISCHER representative.