Surface Failure of Alumina Balls Due to Repeated Stresses Applied in Rolling Contact at Temperatures to 2000 degree F
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION CLEVELAND OH LEWIS RESEARCH CENTER
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The five-ball fatigue tester was used to study the behavior of alumina balls under repeated stresses applied in rolling contact. Hot-pressed and cold-pressed-and-sintered 12-inch alumina balls were tested at 800 and 7000 F, maximum Hertz stresses of 250,000 to 850,000 psi, a contact angle of 200, and a shaft speed of 950 rpm with a mineral oil lubricant. Failure appearance in alumina was unlike fatigue pits found in bearing steels and a crystallized-glass ceramic. A typical failure was a shallow eroded area approximately 1 mil deep progressing slowly from a very small pit to one spanning the track width. Failure appearance and rate of progression were similar for hot-pressed and cold-pressed-and- sintered alumina. Tests at 800 F with mineral oil lubrication over a range of stresses show that life varies inversely with stress raised to a power that ranges from 9.4 to 10.8 for the hot-pressed alumina and from 6.0 to 8.1 for cold-pressed-and- sintered alumina. The load capacity of hot-pressed alumina at 800 F was one-fifteenth that of a typical bearing steel, seven times that of cold-pressed- and-sintered alumina, and about 15 percent greater than a crystallized-glass ceramic.
- Laminates and Composite Materials
- Metallurgy and Metallography