412 Poisson’s Ratio ~.34 ~.34 .30 .27 .30  Fracture  Toughness TBD TBD 22 MPa/ √m 5-7 MPa/ √m 20-23  MPa/√m  Max. Use Temp ~400°C ~400°C ~400°C ~1100°C ~400°C  Elect. Resistivity ~80x10-6 Ωcm ~80x10-6 Ωcm ~36x10-6 Ωcm Insulator ~60x10-6-6  Ω-cm  TBD means “to be determined”   Shock loading is a significant design challenge for space mechanisms. On orbit bearing loads, in the absence of gravity tend to be very low but the severe launch vibration environment can lead to bearing damage through the Brinell effect in which hard rolling elements dent more vulnerable races [4]. For applications that demand long life and ultra smooth operation, such Brinell damage can be catastophic. In such cases, great care is taken using vibration isolation features and tie-down systems to avoid the problem. These add weight and complexity. At times, load capacity design margins are increased for bearings and this leads to increased mass and power consumption. Clearly, the development of more resilient bearings is an advantage for aerospace bearings and mechanisms.  Recent investigations into the hardness and Brinell damage sensitivity of 60NiTi show a potential pathway to engineer ball bearings that are highly resistant to indentation damage [1]. In preliminary indentation tests either Si 3N4 or 60NiTi balls (12.5mm diameter) were pressed into flat plates made from 60NiTi and  the bearing alloys 440C, M50 and Stellite 6B. Initial experiments were carried out at light loads that were increased on subsequent trials. In this manner, both the classic Brinell Hardness Number (BHN) and the threshold load to achieve the first observable dent were obtained. The data, tabulated in Table II, shows that 60NiTi provides significantly more static load capacity (higher threshold load) than the other materials tested.   

Table II. Hertz contact stresses and contact diameter at the threshold load for various plate  and indenter material combinations. 

Plate   Indenter Threshold load,  kgf (lbs) Peak Stress,  GPa (ksi) Contact Dia.,  mm (in) Avg. Stress,  GPa (ksi)  Stellite 6B Si 3N4 10 (22) 2.06 (299) 0.30 (0.012) 1.37 (199)  440 C Si 3N4 51 (112) 3.48 (504) 0.52 (0.021) 2.32 (336)  M50 Si 3N4 150 (331) 5.09 (738) 0.74 (0.029) 3.39 (491)  60NiTi Si 3N4 552 (1214)* 5.56 (806) 1.36 (0.054) 3.71 (537)  Stellite 6B 60NiTi 15 (33) 1.56 (226) 0.42 (0.017) 1.04 (151)  440 C 60NiTi 150 (331) 3.33 (483) 0.92 (0.036) 2.22 (322)  M50 60NiTi 501 (1102) 5.02 (728) 1.37 (0.054) 3.35 (486)  60NiTi 60NiTi 1512 (3327) 5.90 (856) 2.19 (0.086) 3.94 (571)  *Using 1.2 m (50in) dent depth fatigue criterion.  Threshold load is ~360kgf (~800 lb) using more stringent 0.5 m  (20in) dent depth criterion for quiet running bearing.   The reasons for this behavior are complex and discussed in detail in Reference 1 but briefly, the large elastic deformation range of 60NiTi combined with its high hardness and relatively low elastic modulus result in an increased contact area, reduced peak and average stresses and enhanced static load capacity. Given these somewhat unexpected results and capabilities, the next logical steps are to design and fabricate bearings from 60NiTi for evaluation.  Bearing design and manufacturing is a field that is mature and well developed for steels and, in the case of rolling elements (balls and rollers), silicon nitride ceramics. 60NiTi is a superelastic material in the class of intermetallics (neither a metal nor ceramic with respect to atomic bonding) and has key physical properties and attributes that differ from the traditional bearing materials. Among the relevant differences are a relatively low elastic modulus, limited tensile ductility (a tendency for brittle failure in tension), and a