Material property analysis and hardness measurement provide a broad spectrum of data to a wide variety of markets. Traditionally hardness has been defined as the resistance of a material to permanent penetration by another harder material. Hardness results performing Rockwell, Vickers and Brinell tests are determined after the test force has been removed. Any effect of elastic deformation under the indenter is therefore ignored.
Instruments testing hardness under load by continuously monitoring a complete cycle of increase and removal of test force obtain results that are equivalent to traditional hardness values. In addition, these instruments offer the capability to record additional material properties such as the indentation modulus and elastic and plastic hardness properties. All materials parameters determined from the force/indentation depth set are described and defined in the international standard, DIN EN ISO 14577-1-
Instrumented indentation test for hardness and materials parameters. All of these values can be obtained in one automated measurement by evaluating hardness vs. load, hardness vs. indentation depth and indentation depth vs. load curves. The method also provides information regarding creep and elastic recovery properties of the specimen. This convenient way of extracting material properties values eliminates optical inspection of the indentation that can lead to subjective errors.
The resulting hardness values are designated as Martens hardness HM. Martens hardness is determined from values given by the force/indentation depth curve during the increasing of the test force. Martens hardness includes the plastic and elastic deformation, therefore the hardness values can be calculated for all materials. Martens hardness is measured under applied test force; therefore the method also can be described as dynamic hardness testing under load.
Martens hardness is defined as the test force F divided by As(h) the surface area of the indenter penetration beyond the zero-point contact with the material; it is expressed in N/mm2. It is defined for both Vickers and Berkovich indenters:
The equation shows that it is sufficient to determine the load F and the indentation depth h of the indenter to calculate the corresponding hardness of a material.
These automated instrumented indentation tests garner more advantages when measurements are required on very thin and on very soft coatings. In order to reliably determine the hardness of the layer, the penetration depth must not exceed 1⁄10 of the layer thickness. The instruments allow for the selection of very small load ranges down to a few micronewtons. With traditional hardness test methods, one would not be able to optically determine the size of an indentation at a few micronewtons on a hard substrate, which is essentially invisible to the human eye. In addition, the large test loads employed will penetrate further than 1⁄10 of the layer thickness resulting in hardness results that are influenced by the substrate material. In a worst-case scenario, the coating layer will simply be penetrated through to the substrate. In these instances, instruments capable of measurements in the micro and nano range are required.
Industrial applications
General applications for microhardness measurement include paint, metal, hard and soft glass coatings, hard and soft plastics, and rubber material.
Microhardness measurement is becoming an important requirement in a variety of markets. Paint and lacquer coatings in the automotive segment must be sufficiently thick and must also have specific mechanical properties. Hardness measurements provide information about the degree of polymerisation, changes in hardness due to temperature influences, brittleness arising due to UV radiation, the alteration of viscoelastic properties due to weather influences, scratch resistance and other characteristics of coatings.
• Electroplated coatings. Electroplating applications also require microhardness measurement for determining the functional effectiveness of electroplating layers. For example, connectors have contact surfaces selectively plated with gold-the HM is between 1,200 and 6,000 N/mm2, depending on the alloy-for cost reasons; thin layers of only up to 0.8 micrometer are used. In this case, microhardness measurements allow one to draw conclusions about the abrasion resistance and the bonding capability.
• Rubber articles. The reliability of engineered rubber articles may be undermined through aging, brittleness, exposures to chemicals UV radiation or in other ways. The microhardness of the materials, which may lie between 0.2 N/mm2 and 500 N/mm2, depending on the composition and condition of the material, can indicate the level of damage caused. The elastic depth resistance may be up to 99% in the case of rubber.
• Paint and lacquer coatings. The automotive market has a particular interest in microhardness measurement. The elasticity of paint or its ability to bend or stretch with the metal it covers while maintaining the desired appearance characteristics is important. Nano- and microhardness testing where the load is incrementally increased over a specified range, then subsequently released under controlled conditions, provides valuable information on elasticity. The very light loads on the indenter associated with this method make it practically nondestructive. An additional benefit includes the possibility to test the painted product again in the future.
• Hard material coatings. Hard material coating layers improve the performance of deformation and metal removing tools. The wear resistance arises because of the high level of hardness in the layers that are applied, for example, 20 to 25 kN/mm2 for TiN. Coatings with thicknesses of only a few micrometers or even only a few tens or hundreds of nanometers are gaining in importance because of their excellent properties. Hard material coatings of TiN, TiC or diamond-like carbon with thicknesses of 1 to 4 micrometers are already common for tools and engine components. Highly complex coating systems in the nanometer range have been developed during the past years to achieve scratch-resistant, soil-resistant, antistatic, reflecting or
storage-capable surfaces.
Quality assurance for these applications requires reliable measurement of microhardness. Conventional hardness testing devices are only partially suitable for this purpose because they use excessively high testing forces. Here, the testing bodies pass through the layers and measure a mixed hardness consisting of the protective layer and the substrate.
• Ultra-thin protective coatings. Microelectronics as well as the miniaturization of structures and components necessitates instrumentation technology that adapts to these circumstances. For example, in the electronics industry the printed conductors and their coating thickness are becoming ever finer and thinner. Because the indentation depth of the indenter for hardness tests should only be at most 10% of the coating thickness, the test load must be reduced accordingly to a minimum. Very thin, transparent nanometer coatings are applied to hard discs or CDs and DVDs to increase the wear protection and abrasion resistance.
• Coated eye glass lenses. As a rule, eyeglasses are used as visual aids and for occupational safety. Protective eyeglasses, for example, must exhibit a certain hardness to offer protection to the impact of chips of any material. When eyeglasses are used as visual aids, synthetic glasses are popular because of their significantly lower weight and better breaking resistance in comparison to normal glass. Today's synthetic glasses receive several coatings of different thicknesses in the nanometer range to obtain a scratch resistant, soil resistant and anti-reflective surface. Testing the mechanical properties of such thin coatings requires a measurement system with a high-precision distance measurement in the picometer range and a load generation down to a few micronewtons.
Advances in microhardness measurement technology enable a broad range of markets to enhance the quality of the products they are producing. Q
Paul Lomax is marketing director at Fischer Technology Inc. (Windsor, CT). For more information, call (800) 243-8417 or e-mail [email protected].
Tech tips
• Instruments testing hardness under load by continuously monitoring a complete cycle of increase and removal of test force obtain results that are equivalent to traditional hardness values.
• Martens hardness is determined from values given by the force/indentation depth curve during the increasing of the test force.
• General applications for microhardness measurement include paint, metal, hard and soft glass coatings, hard and soft plastics, and rubber material.
