Strength Tests on Silicon Nitride Balls

Abstract

For some years ceramic bearing balls based on silicon nitride have been routinely used in technical practice. An important property of bearing balls is their strength, but appropriate testing methods are still missing. In this paper four different methods for strength testing are applied to commercial bearing balls. Each of the tests needs a different type of specimen, their preparation needs a very different effort, and the stress state applied to the specimens is also very different. This causes pros and cons, which are discussed in detail. The conventional 4-point bending test characterises the material in the interior of the balls. The applied stress state is uniaxial. The machining of the bending bars out of the balls is time intensive and costly. The ball on three balls test also characterises interior of the balls. The stress state is biaxial. The machining of the disc shaped specimens out of the balls is less expensive than the production of bending bars, but the finish of the tensile loaded surface needs special care. The data of both types of tests can be converted into each other using Weibull theory. The specimens in the triple ball crush test are as-received bearing balls, which are squeezed together. This causes some kind of contact loading, as will also occur in service. Failure is caused by the creation and growth of contact cracks, followed by a collapse of the compressed and cracked material. A detailed analysis of test results is complicated. It can be speculated that the component’s behaviour is mainly influenced by the toughness of the material and that the flaws in the material or at the component’s surface are of less significance. In the newly developed notched ball test the highest stressed region is a part of the original surface of the balls. Machining of the notch is straightforward. The stress state is almost uniaxial. The strength depends on size of flaws in the surface region. Therefore the notched ball test is a relevant measure to characterize the quality of the bearing balls.

A new test to determine the tensile strength of brittle balls—The notched ball test

“…However, the interpretation of the test results is still a little unclear. Recently, it has been claimed that this test does not determine the strength of the balls but is controlled by toughness and plasticity of the ball material, 8,19 and a similar conclusion was found recently for the contact loading of ceramic tools for metal forming. 20 In this paper, a new strength test -the notched ball test -is proposed.…”

“…For large balls (having a diameter of say 15 mm or larger) it is possible to machine bending specimens out of the balls in order to perform three-or four-point bending tests, 8 but this procedure is very costly. The surface of the specimens has to be prepared very carefully to avoid unrepresentative machining damage.…”

“…High tensile stresses occur in the outer surface region of the ball opposite the notch root (in the ligament). 8 These stresses are used to determine the strength of the ball. Fracture starts from defects which exist in this region.…”

The Rationale for Silicon Nitride Bearings in Orthopaedic Applications

“…Silicon nitride thus prepared has improved strength, albeit at a higher manufacturing cost. 21 A compromise is to combine the two technologies; thus, Si 3 N 4 can be made by post-sintering the reaction-bonded silicon nitride in order to achieve a relatively high strength, at a fraction of the fabrication cost of HIPed Si 3 22 These investigators found increases in specimen bending strength of 56%, 66%, and 35% by the implantation of Ti, Zr, and Cr ions, respectively. 22 Silicon nitride, like other ceramics, is a brittle material; typical material property tests have shown that silicon nitride has a Vickers hardness of 12-13 GPa; Young's modulus of 299 GPa, Poisson's ratio of 0.270, and a typical grain size of 0.59 µm.…”

“…22 Silicon nitride, like other ceramics, is a brittle material; typical material property tests have shown that silicon nitride has a Vickers hardness of 12-13 GPa; Young's modulus of 299 GPa, Poisson's ratio of 0.270, and a typical grain size of 0.59 µm. 23 Composites of silicon nitride with 6 wt% yttrium oxide and 4 wt% alumina were fabricated to measure mechanical strength and related properties, according to ASTM C-1161 standards, using specimens with dimensions of 3 x 4 x 45 mm; results showed a near 100% theoretical material density (3.20 g/cm 3 ), average grain width of 1.5 µm, flexural strength of 923 ± 70 MPa, with a Weibull modulus of 19 and a fracture toughness of 10 ± 1 MPa.m ½ . 24 Both material strength and toughness are at least an order of magnitude higher than typical values reported for alumina, the most common ceramic bearing material in orthopaedic bearings today.…”