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Measurements to international standards

Within our laboratory measurements are taken according to international standards to ensure compliance with the technical and quality specifications agreed with the customer:

1 Thickness measurement (Calotest procedure)

At the customer's request a certificate under the UNI-EN-10204: 2004 is issued which attests to the coating thickness achieved.
With the Calotest test, in reference to the ISO 26423 standard, it is possible to very accurately control the coating thickness. The piece to be tested is placed in contact with a normed size stainless steel sphere, interposing a fine diamond paste between the parts in contact. The sphere, rotating at a predetermined speed, generates through friction an impression on the coated test specimen, which is then analysed by optical microscope to determine the thickness with an error margin to the tenth of a micron.

Schematisation of a Calotest impression on a coated specimen: the abrasive material (crystalline diamond dust) is harder than the protective coating and removes it, making the substrate visible.

2Measuring the coating’s adhesion (Scratch test procedure and Mercedes test)

The adhesion of the coating is estimated by evaluating the interfacial forces between the substrate material and the coating on top of it. The measurement can be carried out with:

  • Scratch test method (standards ASTM C1624, ISO 20502, EN1071-3)
  • Mercedes test method (standard VDI 3198), with the Rockwell C indentation method.

With the scratch test method, a Rockwell indenter, placed perpendicular to the substrate, applies a normal force of increasing intensity to it; the indenter simultaneously runs across the surface at a predefined speed. The coating material undergoes a series of ruptures, up to delamination, of which the critical loads are studied and used to define the qualitative/quantitative resistance to scratching.

With the Rockwell C test on the other hand, an HRC impression is made on the coated surfaces and then controlled by an optical microscope, with interest in circumferential geometry. The applied force generates first an elastic and then a plastic deformation until the coating breaks. The analysis of the type of break provides information on the coating’s adhesion and on its fragility.

3 Colour measurement (colorimetric spectrum procedure)

Colour measurement is carried out with a special electronic instrument called a spectrocolorimeter, suitable for quick and easy control of the colour of any material in compliance with standard DIN 5033.

The instrument captures the light radiation reflected by the sample and analyses it by placing it in a lab colour space (or CIELAB or CIE 1976), which is a virtual environment defined by the intensity and combination of light frequencies in which the colour is going to place itself thanks to the coordinates.

The figure below represents the colour solid for the lab space.

The aesthetic accuracy is evaluated through a comparison between the colour coordinates of a reference sample and those of the coating’s colour.

4 Measurement of the coating’s instrumental hardness (nano indentation procedure)

Hardness is a mechanical property of all solid materials, which indicates the surface scratch resistance (brittle materials) and permanent deformation (ductile materials).
The measurement of hardness, together with that of the elastic modulus (or Young's modulus), is essential in the characterisation of thin coatings.
The instrumental test that determines these parameters is the nano-indentation, performed in compliance with ASTM E2546-07, ISO 14577, DIN 50359 standards.

The operating principle is as follows: a fine diamond tip (indenter) penetrates the coating perpendicularly to the surface, applying an increasing load up to a predefined value. The load is then reduced, allowing the relaxation of the material. A software reconstructs a load-unload curve/depth of process, analysed with algorithms to calculate the instrumental hardness (expressed in Vickers or MPa) and the elastic modulus (expressed in MPa).

5Roughness measurement (procedure with roughness tester)

Each surface has its own roughness that can alter frictional properties to a certain extent. This roughness can be random (for example depending on the surface forming process) or have preferential trends (for example in pieces produced with machine tools).
The roughness profile is analysed by means of mathematical models, which estimate how much the surface patterns deviate from an ideally perfect surface and the parallel sections. The roughness is usually around a few microns.
The measurement of roughness is done with a device called a roughness tester, which inspects the surface with a probe and records irregularities, carrying them forward onto a diagram with a tolerance of one thousandth of a micron.

6 Measurement of the friction coefficient (dynamic test procedure)

The measurement of the friction coefficient is in part indicative of the substrate’s condition of wear, dependent also on the possible use of lubricants and by the contact mechanisms between the parts.
The dynamic friction coefficient depends on several factors: the chemistry and surface morphology of contact materials, the geometry of the test, the applied loads and the chemical test environment (dry air, humid, inert gas, vacuum).
Among the different test geometries are: Pin-On-Disk, Ball-On-Disk, Block-On-Ring. In each of them, the sample is brought into dynamic contact with a counterpart, whose material and shape depend on the tests. By means of a load cell the perpendicular force to the sliding plane is measured continuously, which is then correlated with the dynamic friction coefficient.
An assessment of the state of wear can also be made through microscopic inspection of the trace left by the indenter or by differential weighing before and after the test.

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