Based upon these flat plate results, one can conclude that the Hertz stress model is useful in predicting  denting behavior of NiTi. Further, the behavior of NiTi to static loads is the same whether the indentation is from a small ball or a large ball. In order to complete the assessment and establish a stress capability limit that can be used with confidence for bearing design, however, the dent capability of a real bearing  race, with dual curvature must be measured. In the present study, a series of experiments are carried out in which the inner race of a 50-mm ball  bearing is loaded by a single 8.74-mm (0.344-in) diameter silicon nitride ball under loads that range from  2200 to 11,000 N (1000 to 5000 lb). These loads correspond to mean Hertz contact stress from 2.1 to 3.5 GPa. The resulting dents are characterized for form, shape and depth. This data is then used to firmly  establish load capability of 60NiTi bearing races. In addition, the newly established stress capability is then applied to the design of ball bearings that support a simple notional space mechanism, a reaction  wheel. By analysis, the wheel assembly is then put through estimated space vehicle launch loads to  determine its ability to survive without damage. Materials and Procedures The test specimens consist of a ceramic indenter ball and a 60NiTi bearing race. The indenter is a  standard grade 5 Si 3N4bearing ball (8.74-mm dia.) and the inner race of a 50-mm bore ball bearing  made from 60NiTi. The general material properties of these materials are shown alongside more conventional tool steel bearing materials in Table 1. Table 1. Nominal Properties for 60NiTi and Si 3N4 Test Specimens and Conventional Bearing  Steels Property 60NiTi 440C Si 3N4 M-50 Density 6.7 g/cc 7.7 g/cc 3.2 g/cc 8.0 g/cc Hardness 56 to 62 HRC 58 to 62 HRC 1300 to 1500 Hv 60 to 65 HRC Thermal conductivity W/m-°K~9 to 14 24 33 ~36 Thermal expansion ~11.2 /g11710–6/°C 10 /g11710–6/°C 2.6 /g11710–6/°C ~11 /g11710–6/°C Magnetic Non Magnetic Non Magnetic Corrosion resistance Excellent (Aqueous and  acidic)Marginal Excellent Poor Tensile/(Flexural  strength)~1000(1500) MPa 1900 MPa (600 to 1200) MPa 2500 MPa Young’s Modulus ~95 GPa 200 GPa 310 GPa 210 GPa Poisson’s ratio ~0.34 0.3 0.27 0.30 Fracture toughness ~20 /g48/g51/g68/g18/g165/g80 /g21/g21/g3/g48/g51/g68/g18/g165/g80 5 to /g26/g3/g48/g51/g68/g18/g165/g80 20 to /g21/g22/g3/g48/g51/g68/g18/g165/g80 Maximum use temp ~400 °C ~400 °C ~1100 °C ~400 °C Electrical resistivity ~1.04 /g11710–6/g159-m ~0.60 /g11710–6/g159-m Insulator ~0.18 /g11710–6/g159-m Silicon Nitride is a hard ceramic material that is increasingly being used for balls in rolling element  bearings because of its excellent corrosion resistance, low density, and fatigue life capability. Under the  poorly lubricated conditions, ceramic-steel hybrid bearing configurations are typically more wear resistant  than all-steel bearings.  The bearing race is made from powder metallurgy processed 60NiTi manufactured according to the steps  detailed previously in the literature (Ref. 9). Briefly, pre-alloyed powder is made by atomization using 60 wt% nickel and 40 wt% titanium feedstock. The powder is placed inside an evacuated mild steel can  that is then sealed by welding. The canned powders are consolidated under high pressure and  temperature resulting in a fully dense 60NiTi ingot. The ingot is cut using a combination of electric discharge machining, turning and grinding operations to produce slightly oversized bearing races. The  77