October AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 1 9 4 4 running performance (noise and vibration) in a precision ball bearing. Indentation results are shown in Fig. 3 comparing Nitinol 60 to conventional bearing steel (440C stainless steel) and a high-carbide (M62) tool steel used in highly loaded applications. Stress limits for the NiTi alloy fall about midway between 440C and M62, further confirming the suitability of NiTi as a bearing material. However, stress limits only tell a part of the story. Applied load, not stress, is the key design factor when engineering a bearing for a specific application. Because NiTi alloys have a much lower elastic modulus than steel, the load limits aremuch higher. This is additionally amplified by the su- perelastic nature of NiTi alloys, which generally exhibit elastic elongation limits of several percent (compared with less than 1% for a typical steel). Figure 4 displays replotted data from Fig. 3 based on load instead of stress showing the significant improvement inbearing static load capacity impartedby using Nitinol 60. The combination of relatively low elastic modulus, high hardness, and extensive elastic deformation range result in three to five times higher indentation load capacity. For space launch and other high shock-load applications, such enhanced levels of load capacity can dramatically change ma- chine designs. When other beneficial properties like electrical conductivity, nonmagnetic behavior, and corrosion resistance are considered, the use of NiTi alloys for bearings makes a compelling case comparedwith traditional and even high-per- formance steels. In fact, no current class of bearing structural alloys provides such high levels of load capacity or such a com- bination of enabling properties. In this sense, dimensionally stable, hardened NiTi intermetallic structural alloys are a new class of materials for use in demanding mechanical engineer- ing applications. In the course of producing and certifying bearings for space flight use, NASA evaluated a wide variety of material properties and identified several unexpected, yet welcome, findings that make NiTi alloys a significant addition to the me- chanical component community. RECENT REVELATIONS Table 1 summarizes select material properties for Nitinol 60, which are highly relevant for bearing and other structural applications. As previously mentioned, hardness levels are comparable to conventional bearing alloys like AISI 440C as is tensile strength and toughness. Corrosion resistance is far su- perior to the martensitic steels like AISI 52100 and grade M50 tool steel and much better than 440C. However, unlike other hard steels, NiTi alloys like Nitinol 60 are nonmagnetic and in con- trast to silicon-nitride ceramic, they are electrical conductors. One notable property that differs significantly from the bearing steels is the elastic modulus. The modulus of Nitinol 60 is only one-half that of steel (e.g., ~95 GPA compared to 200 GPa). Lower modulus has a direct effect on ball bearing behavior in that for a given load, the contact area between a ball and its raceways will be enlarged accompanied by a com- mensurate decrease in stress. The stress reduction results in performance advantageswith respect to load capacity in a ball bearing, especially those exposed to heavy static loads (e.g., shock loads during rocket launch). To assess the effects of modulus on ball bearing con- tacts, a series of indentation experiments were undertaken in which 0.125 to 0.5-in. diameter silicon-nitride indenter balls were loaded against highly polished flat plates made of hardened Nitinol 60. Loads varied from a few pounds to sev- eral thousand pounds. The goal was to determine the stress level that produced a permanent dent that would cause poor TABLE 1 − NOMINAL PROPERTIES OF 60NiTi, SILICON CARBIDE, AND CONVENTIONAL BEARING STEELS Property Material 60NiTi AISI 440C Si 3 N 4 M 50 tool steel Density, g/cm 3 6.7 7.7 3.2 8.0 Hardness 56-62 HRC 58-62 HRC 1300-1500 HV 60-65 HRC Thermal conductivity, W/m ⋅ K 9-14 24 33 ∼ 36 Thermal expansion, × 10 − 6 / ° C ∼ 11.2 10 2.6 ∼ 11 Magnetism Nonmagnetic Magnetic Nonmagnetic Magnetic Corrosion resistance Excellent (aqueous and acidic) Marginal Excellent Poor Tensile (flexural) strength, MPa ∼ 1000 (1500) 1900 600-1200 2500 Young’s modulus, GPa ∼ 95 200 310 210 Poisson’s ratio ∼ 0.34 0.3 0.27 0.3 Fracture toughness, MPa √ m ∼ 15 22 5-7 20-23 Max. use temperature, ° C ∼ 400 ∼ 400 ∼ 1100 ∼ 400 Electrical resistivity, × 10 − 6 Ω ⋅ m ∼ 1.04 ∼ 0.60 Insulator ∼ 0.18 6 FEATURE