What is The Through-Diamond-Technique Method?

While both methods – UCI- and Rebound – are successfully used in the field and solve many on-site hardness testing applications, there are limitations concerning the kind of material under test and its size and weight, respectively. Furthermore, because of the influence of Youngs-Modulus, most conventional testing methods do not allow to measure different materials without firstly calibrating or adjusting the instrument.

What are the advantages of the TDT method?

With Transpyramidal Indenter Viewing, or also known as Through Diamond Technique, we overcome this “handicap”: practically all kinds of material from steel to rubber and from aluminum to plastics can be tested without the necessity of instrument calibration. With this instrument we developed an optical mobile hardness tester, allowing real Vickers hardness measurement under load without having to face the disturbing influence of the elastic properties of the test material, i.e. Youngs-Modulus.

The system consists of the portable base instrument including a graphical LCD display and the TDT probe (see Figure 3).

By applying a certain test load (e.g. 50 N) the diamond penetrates into the material. The indentation size of the Vickers diamond, i. e. the lengths of the diagonals, is automatically measured under load by viewing through the diamond with an optical system having a CCD camera. Data evaluation is then made in the instrument.

As Vickers hardness is simply defined as the ratio between test load and indentation size (the diagonals of the indentation), the TDT-measurement of the diagonal length immediately produces a Vickers hardness value for the applied test load. The live picture of the indentation displayed on the instrument’s LCD also allows immediate characterization of measurement reliability, i.e. the quality of the Vickers diamond indentation.

The physical method of TDT hardness testing – traced back to Vickers hardness – allows mobile testing of different materials without the necessity of calibrating the instrument. By viewing through the diamond under load, TDT opens up mobile hardness testing not only to new applications like coils, thin layers and coatings but also to different materials like plastics, glass and high-tech materials such as ceramics or intermetallics.

While testing under load by viewing through the diamond, the TDT instruments even allow to measure the hardness of elastic or soft materials. Other types of tests, such as Brinell, Vickers or Knoop tests, have their difficulties. The problem with trying to apply some of the “older types” of tests, is that the indentations themselves can at times almost completely recover, and there is no permanent impression left, thus making measurements impossible.

The TDT method eliminates that problem. It involves pressing a diamond punch of known geometry into the surface of a material. The indentation size will be monitored under load during the test.

In some industries, aluminum or soft metal alloys such as solder would be considered “soft” materials. But as the testing of rubbers, plastics, and polymers becomes more commonplace, even the softest metals will seem comparatively hard. It is a relative term. Applications for testing soft materials are nonetheless widespread.

The automotive industry tests the hardness of paints and tires. The microelectronics and photonics industries test low-dielectric constant films, chemical and mechanical polishing pads, bond pads, solders, and electronic packaging materials. The biomaterials industry tests polymer joint-implant materials, nail polish and drug particles. The medical field even tests biological samples such as liver, cartilage, and arterial tissues. Determining meaningful hardness values for soft materials has always been challenging, and despite recent advances in methods and instruments, continues to be so.

The Through Diamond Technique (TDT)

The innovation with this technique is the evaluation of the Vickers diamond indentation, which takes place by viewing through the Vickers diamond using a CCD camera. For that purpose it is necessary to light up the inner surfaces of the diamond using light-emitting diodes (LED) geometrically arranged.

In order to obtain the highest resolution of the indentation picture it is necessary to match the wavelength of the LED light and the spectral sensitivity characteristics of the CCD chip.

A special lens system was developed and adjusted to the LED to ensure maximum resolution. Computer assisted evaluation of the indentation and determination of the diagonal’s length occurs in three steps.

A first step locates the approximate position of the indentation. After that the exact course of the indentation’s border is determined in local vicinities (so-called Areas of Interest) by applying suitable “transition filters” for determination of any grey scale transition.

Finally the indentation surface and the diagonals are determined using the intersections of the calculated borders and the edges of the Vickers diamond. According to the definition of Vickers, the HV value is calculated for the applied test load.