The notched ball test developed by the materials scientist Dipl.-Ing. Stefan Strobl serves for examining the surface quality of ceramic balls. This method forms the basis of ÖNORM M 6341 on determining the strength of ceramic rolling bearing balls. It permits the comparison and characterization of materials and also ensures the integrity of ceramic surfaces during sample preparation.
Ceramic materials are on the rise. Owing to their excellent properties, such as high hardness, low density, electric insulation and high resistance to corrosion, wear and high temperatures, they are now also used in applications that used to rely on metals in the past.
Major areas in which structural ceramics are used are high-temperature applications. These include, for example, gas turbines operated at high speeds of several thousand revolutions per minute and – because of rising efficiency – more and more combustion engines.
Ceramic materials withstand very high stress without distortion or fatigue and therefore lend themselves for use in bearings and sealings. Highly resistant structural ceramics also are optimally suited for applications in pumps or gears where components are subject to corrosion or wear.
Apart from these undisputable advantages of ceramics, there are also drawbacks, such as higher production costs, brittleness and the phenomenon of sudden material failure. As ceramics are a relatively young material, knowledge is still lacking on its behaviours in all types of environments. Therefore, it is all the more important to control the quality of the material and surface.
Mechanical characteristics of ceramics can only be described statistically and absolutely have to be examined in the actual component itself as parameters such as strength clearly depend on concrete surface quality.
Dipl.-Ing. Stefan Strobl solved this problem by means of an innovative test method. In his PhD thesis prepared at the Institute of Structural and Functional Ceramics of the University of Leoben, the materials scientist developed a new method for testing ceramic balls that are used, for example, in Formula 1 racing cars and large wind turbines.
"The notched ball test can measure the strength of original surfaces of even small components with high accuracy. It easy to perform and also easy to analyze," says Strobl.
In the equatorial plane of the ceramic ball, a notch having a length of approximately 80 percent of the diameter is cut. At the poles, a force is introduced by means of two indenters. Squeezing the notch results in tensile stress at the equator of the ball surface. The fracture load and the geometry of the ball can be used to determine the tensile strength of the material.
As special fixing devices are not needed for preparing and testing the specimens, the test set-up is very simple. In contrast to the standardized 4-point bending test, the new method tests the original surface. As specimens are prepared quickly and flexibly, larger samples can be examined.
The scientific findings of the thesis serve as the basis of the recently published standard ÖNORM M 6341 "Rolling Bearings – Ceramic bearing balls – Determination of the strength – Notched ball test". Its specifications describe the notched ball test for determining the strength of bearing balls made of ceramics, preferably silicon nitride, with diameters from 3 mm to 50 mm.
To facilitate its introduction in an international environment – for example, within the International Organization for Standardization (ISO) – the standard was published in German and English.
For Dr. Gerwin Preisinger who is responsible for the development of hybrid bearings and the ceramic rolling elements required for them at the rolling bearing manufacturer SKF in Steyr (Austria) that supported and funded Strobl's thesis as an industrial partner, the publication of the ÖNORM standard is a highly positive development.
"Rolling elements made of 'technical ceramics' have enormous advantages in comparison with those made of steel: They allow for reduced bearing friction and higher speeds, are less sensitive to soiling and have a longer life. The adoption of a standard dedicated to quality control helps to better position ceramics as a material for rolling bearing applications. On this basis, the notched ball test can be used as a tool for material specifications agreed between suppliers and customers," explains the expert.
In practice, hybrid bearings are used in most cases where the balls are made of ceramics and the rings of steel. Due to negligible abrasion, this combination permits the production of fully encapsulated components with grease lubrication.
Such solutions are required for autonomous systems such as deep-sea pumps without their own supply lines. The attractive characteristics of the material combination of steel and ceramics are in high demand: The hybrid bearing market has recorded two-digit annual growth rates since 2000.
And there are many more potential applications, such as drills that have to accelerate and stop quickly, superconductors, in low-temperature environments and in-vivo applications in medical technology.
That is one more reason for paying utmost attention to material quality. The notched ball test and ÖNORM M 6341 provide the tools required for quality assurance. Their story shows how the work of Austrian researchers can result in innovative, internationally applicable products – with the help of standards.
Author: Herbert Hirner
ÖNORM M 6341 Rolling Bearings – Ceramic bearing balls – Determination of the strength – Notched ball test