TY - THES

T1 - Comparison of Technological and Scientific Methods for Fracture Toughness Determination of Silicon Nitride

AU - Munz, Maximilian

N1 - embargoed until 11-09-2028

PY - 2023

Y1 - 2023

N2 - Ceramic balls, made of silicon nitride, can be machined to have accurate dimensions, a smooth surface and have a high resistance against corrosion and abrasion. These features are paramount in bearing applications, which makes ceramic balls great fits for this kind of use. In service, the lifetime of such balls is limited by surface spalling due to contact crack initiation and growth. Hence, it is important to know the fracture resistance of these materials. A simple method to determine this property is the standardized indentation fracture (IF) method. Such experiments are conducted by placing a Vickers hardness indentation on flat, polished cross sections of the bearing balls. The investigated location is thus below the surface or in the center of a ball. The microstructure and the residual stress states of the bearing balls may vary in radial direction. Measurements on material from the ball interior may thus not reflect the properties of the surface, which is relevant for in-service performance. Using the ball surface directly as test surface would also simplify the test execution dramatically. This work aimed to establish fracture resistance testing on spherical surfaces of bearing balls with various diameters from 5.55 mm to 47.6 mm using different indentation loads. These results reflect the local properties from the part of the bearing ball, which is relevant for the application. Additionally, insight on the properties of different parts of the balls were gained by comparing these results with values obtained at other radial locations of the balls (i.e. different ¿reference states¿): the sub-surface 1 mm below the surface, the center of the balls and the surface of separately fabricated standard bend bars. Additionally, the fracture toughness was measured with various standardized methods. The measurements of the indentation fracture resistance indicate that sufficiently large indentation loads must be used to obtain meaningful results. Further, empirical corrections were developed to convert the results obtained on curved surfaces to hypothetical results obtained on various flat ¿reference¿ surfaces. These results indicate deviations between the reference locations and the ball surface but also different material behaviour between different ball sizes. All fracture resistance values were higher than the fracture toughness measurements.

AB - Ceramic balls, made of silicon nitride, can be machined to have accurate dimensions, a smooth surface and have a high resistance against corrosion and abrasion. These features are paramount in bearing applications, which makes ceramic balls great fits for this kind of use. In service, the lifetime of such balls is limited by surface spalling due to contact crack initiation and growth. Hence, it is important to know the fracture resistance of these materials. A simple method to determine this property is the standardized indentation fracture (IF) method. Such experiments are conducted by placing a Vickers hardness indentation on flat, polished cross sections of the bearing balls. The investigated location is thus below the surface or in the center of a ball. The microstructure and the residual stress states of the bearing balls may vary in radial direction. Measurements on material from the ball interior may thus not reflect the properties of the surface, which is relevant for in-service performance. Using the ball surface directly as test surface would also simplify the test execution dramatically. This work aimed to establish fracture resistance testing on spherical surfaces of bearing balls with various diameters from 5.55 mm to 47.6 mm using different indentation loads. These results reflect the local properties from the part of the bearing ball, which is relevant for the application. Additionally, insight on the properties of different parts of the balls were gained by comparing these results with values obtained at other radial locations of the balls (i.e. different ¿reference states¿): the sub-surface 1 mm below the surface, the center of the balls and the surface of separately fabricated standard bend bars. Additionally, the fracture toughness was measured with various standardized methods. The measurements of the indentation fracture resistance indicate that sufficiently large indentation loads must be used to obtain meaningful results. Further, empirical corrections were developed to convert the results obtained on curved surfaces to hypothetical results obtained on various flat ¿reference¿ surfaces. These results indicate deviations between the reference locations and the ball surface but also different material behaviour between different ball sizes. All fracture resistance values were higher than the fracture toughness measurements.

KW - Silicon Nitride

KW - Bearing Balls

KW - Indentation Fracture Resistance

KW - Fracture Toughness

KW - Hardness

KW - Siliziumnitrid

KW - Kugellagerkugeln

KW - Eindruckrisszähigkeit

KW - Bruchzähigkeit

KW - Härte

M3 - Master's Thesis

ER -