Christopher DellaCorte

Glenn Research Center, Cleveland, Ohio

Lewis E. Moore, III and Joshua S. Clifton

Marshall Space Flight Center, Huntsville, Alabama

The Effect of Indenter Ball Radius on the Static 

Load Capacity of the Superelastic 60NiTi 

for Rolling Element Bearings 

NASA/TM—2014-216627

April 2014
	
	
	








	



	

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Christopher DellaCorte

Glenn Research Center, Cleveland, Ohio

Lewis E. Moore, III and Joshua S. Clifton

Marshall Space Flight Center, Huntsville, Alabama

The Effect of Indenter Ball Radius on the Static 

Load Capacity of the Superelastic 60NiTi 

for Rolling Element Bearings 

NASA/TM—2014-216627

April 2014


	



	


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NASA/TM—2014-216627 1 

The Effect of Indenter Ball Radius on the Static Load Capacity of 

the Superelastic 60NiTi for Rolling Element Bearings 

Christopher DellaCorte 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Lewis E. Moore, III and Joshua S. Clifton 

National Aeronautics and Space Administration 

Marshall Space Flight Center 

Huntsville, Alabama 35812 

Abstract 

Static load capacity is a critical design parameter for rolling element bearings used in space 

mechanisms because of the potential for Brinell (surface dent) damage due to shock and vibration loading 

events during rocket launch. Brinell damage to bearing raceways can lead to torque variations (noise) and 

reduced bearing life. The growing use of ceramic rolling elements with high stiffness in hybrid bearings 

exacerbates the situation. A new family of hard yet resilient materials based upon nickel-titanium is 

emerging to address such bearing challenges. 60NiTi is a superelastic material that simultaneously 

exhibits high hardness and a relatively low elastic modulus (~100 GPa) and has been shown to endure 

higher indentation loads than conventional and high performance steel. Indentation load capacity has been 

reported for relatively large (12.7 mm diameter) ceramic (Si3N4) indenter balls pressed against flat plates 

of 60NiTi. In order to develop damage load threshold criteria applicable to a wide range of bearing 

designs and sizes, the effects of indenter ball radius and the accuracy of interpolation of the Hertz contact 

stress relations for 60NiTi must be ascertained. In this paper, results of indentation tests involving 

ceramic balls ranging from 6.4 to 12.7 mm in diameter and highly polished 60NiTi flat plates are 

presented. When the resulting dent depth data for all the indenter ball sizes are normalized using the Hertz 

equations, the data (dent depth versus stress) are comparable. Thus when designing bearings made from 

60NiTi, the Hertz stress relations can be applied with relative confidence over a range of rolling element 

sizes and internal geometries. 

Introduction 

Rolling element bearing (REB) design is a mature, well-understood engineering process that differs 

from but is closely linked to bearing selection. Bearing selection is characterized by matching a bearing’s 

known characteristics, like stiffness and load capacity, to the requirements of a specific application 

(Ref. 1). In such instances, a system designer simply looks up the bearing capabilities in a manufacturer’s 

design catalogue and selects the product that meets requirements. In many instances, especially when 

designing space flight hardware, no commercial off-the-shelf (COTS) bearing is suitable. 

Space flight hardware bearing applications often include unusual operating conditions such as 

vacuum compatibility of lubricants and cage materials, extreme low and high temperatures, ultra-high 

precision motion and torque noise (ripple) requirements while maintaining extraordinarily low (drag) 

torque levels (Ref. 2). Among the most challenging bearing requirements is the ability to withstand 

excessive shock loads resulting from launch vibrations and other common space vehicle effects such as 

the discharging of pyrotechnic actuators. When faced with such otherworldly challenges, detailed bearing 

design alterations are often a necessity. 

Common bearing design materials alterations include the use of vacuum compatible oils and greases 

with low viscosity index (VI), space radiation tolerant (e.g., phenolic or polyimide) cage materials and the 
NASA/TM—2014-216627 2 

use of martensitic stainless steel races and ceramic rolling elements to minimize the potential for 

corrosion during storage before launch. Careful selection of internal clearances, such as those between the 

cage diameters and the bearing race lands, are sometimes required to prevent cage binding at extreme 

temperature conditions. Space bearings often must cope with operating temperatures that can vary by 

hundreds of degrees Fahrenheit. More nuanced and subtle design changes might include altering raceway 

curvature and ball diameters to reduce contact stresses and enhance static load capacity. Despite such 

attention to detail, the extreme dynamic shock loads common in rocket-launched hardware remains a 

vexing problem (Ref. 3). 

System engineering offers approaches to address the static load challenge for space hardware. 

Designers can include vibration isolation methods to reduce load levels to acceptable values. Such 

techniques are often complex and costly and add to overall mass. Another engineering approach is to 

oversize bearing components until the static load capacity exceeds the anticipated load levels. Again, this 

adds weight and also leads to generally higher bearing power consumption for operating machinery once 

in orbit. Recently, new, high carbide content, ultra-hard bearing steels have been developed that provide 

enhanced static load capability and fatigue resistance (Refs. 4 and 5). These steels, like REX 20, offer 

about 30% higher stress capability against Brinell type damage (static indentation loads) compared to 

conventional space bearing steels, like 440C. Corrosion resistance however is a serious problem. To date, 

a need exists for bearing alloys with the corrosion resistance of a stainless steel and load capacity of highcarbide tool steel. The emerging nickel-rich, NiTi alloys offer an avenue to address these challenges. 

Nickel-rich, binary nickel-titanium alloys, such as 60NiTi or alternatively NiTiNOL 60, are intriguing 

candidate materials for use in mechanical components (Ref. 6). 60NiTi (60Ni-40Ti by wt%) exhibits 

attractive properties that impact the design and performance of mechanical components especially rolling 

element bearings (Ref. 7). 60NiTi is highly corrosion resistant and because it contains no iron it is 

immune to the atmospheric rusting behavior experienced by bearing and gear steels, even those deemed 

“stainless” such as 440C. 60NiTi is dimensionally stable yet exhibits extremely high elasticity (so-called 

superelasticity) and can deform up to 5% without incurring permanent deformation. 60NiTi can be 

hardened through simple heat treatment to levels comparable with conventional bearing steels (Rockwell 

C 58-62). Table I contains the most up-to-date properties known for 60NiTi compared to other bearing 

materials. 

TABLE I.—NOMINAL PROPERTIES FOR CONVENTIONAL BEARING ALLOYS AND 60NiTi 

Property 60NiTi 440C Si3N4 M-50 

Density, g/cc 6.7 7.7 3.2 8.0 

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 ~11.210–6/°C 1010–6/°C 2.610–6/°C ~1110–6/°C 

Magnetic Non Magnetic Non Magnetic 

Corrosion resistance Excellent 

(aqueous and acidic) Marginal Excellent Poor 

Tensile/(flexural strength), MPa ~1000 (1500) 1900 (600 to 1200) 2500 

Young’s Modulus, GPa ~95 200 310 210 

Poisson’s ratio ~0.34 0.3 0.27 0.30 

Fracture toughness, =Z
 ~20 22 5 to 7 20 to 23 

Maximum use temperature, °C ~400 ~400 ~1100 ~400 

Electrical resistivity, -m ~1.0410–6 ~0.6010–6 Insulator ~0.1810–6
NASA/TM—2014-216627 3 

One property that differs substantially from that of bearing steel is elastic modulus. 60NiTi has a 

modulus similar to titanium and aluminum and is approximately one-half that of steels and superalloys. This 

unique combination of superelasticity, high hardness and reduced elastic modulus, compared to tool steel, 

has been shown to result in extraordinarily high static load capacity in highly loaded, concentrated contacts 

such as those that occur at ball and roller-raceway contacts in rolling element bearings (Figure 1). 

Figure 2 depicts a generic ball-on-race Hertz contact with an exaggerated deformation to elucidate 

this effect. When loads are transmitted through a bearing, the materials must resist permanent 

deformation. If the loads are applied while the bearing is not rotating (e.g., as would be the case for a 

space instrument during launch), the contact mimics a ball on plate. From a rolling element bearing 

perspective, any damage that reduces fatigue life or causes unacceptable bearing operation (torque noise) 

is to be avoided. An obvious type of damage resulting from an overload condition is denting of bearing 

raceways. 

In earlier studies, indentation experiments were carried out to assess the static load capacity of 

hardened 60NiTi (Ref. 8). Initially, the load limit, defined as the contact stress (or load) that results in a 

permanent dent whose depth exceeds 510-5 of the indenter ball diameter (dp/D), was assessed by 

pressing 12.7 mm diameter silicon nitride balls against highly polished, heat treated 60NiTi prepared by 

powder metallurgy techniques. These experiments showed that the static stress capability for 60NiTi 

exceeded that of 440C by about 25% as shown in Figure 3. 

In terms of stress capability, the tool steel REX 20 has about 30% higher capability than 60NiTi. But, 

as concluded in the earlier research, the reduced modulus of 60NiTi obscures a direct comparison based 

solely upon contact stress. Indeed, with respect to bearing applications that encounter shock loads, a 

comparison based upon load is more relevant. The load capacity data is presented in the following 

Figure 4 and it tells a significantly different story. 

Figure 1.—Hertz concentrated contacts in bearings and gears. 

Figure 2.—Single ball-on-race Hertz concentrated 

contacts in a bearing (exaggerated deformation). 
NASA/TM—2014-216627 4 

Figure 3.—Indent depths as a function of contact stress for 60NiTi, 440C and REX20 

plates loaded 

Figure 4.—Indent depths for 60NiTi plate as a function of load when contacted by a 

12.7mm diameter Si3N4 indenter ball. Data for Stellite 6B, 440C, M50 and REX20 

from References 4 and 5. 

When plotted against contact load, 60NiTi exhibits nearly twice the load capability of tool steel. 

Follow-on experiments using the same general test set-up demonstrated that pre-stressing the contact 

surface prior to indentation could enhance the load capability further (Ref. 9). These results clearly show 

that 60NiTi exhibits excellent resilience and static load capacity when indented with a 12.7 mm diameter 

ceramic indenter ball surface. For bearing design purposes, however, a more generalized load-stress 

relationship must be confirmed. 

In the present study, indentation experiments of hardened 60NiTi are carefully carried out using 

highly polished surfaces loaded against Si3N4 spherical indenters with diameters that range from 6.35 to 

12.7 mm. The resulting data is then examined to determine if the Hertz contact stress equations can be 

accurately used to guide future bearing designs employing 60NiTi for high load applications like space 

mechanisms. 
NASA/TM—2014-216627 5 

Materials 

Vacuum casting is used to produce a 25-mm diameter, 300-mm long 60NiTi rod by melting high 

purity nickel and titanium pellets in a graphite crucible (Figure 5). The 6.4 mm thick, flat plate 

indentation specimens are cut using wire Electrode Discharge Machining (EDM). The specimens are 

hardened by heating treatment (2 hr) at 1000 °C in argon followed by a rapid water quench. The casting 

process yields a uniform, fine-grained microstructure that sometimes contains flaws, such as voids that 

can cause spontaneous fracture during quenching (Ref. 10), especially for large (>50 mm diameter) cross 

section specimens. Such issues are avoided for the relatively small specimens used in the present study 

and the casting process provides an economical path to obtain large quantities of specimens. Previous 

work has shown that 60NiTi prepared both my powder metallurgy and casting exhibit similar hardness 

behavior. Figure 6 shows cross sections of the test material before and after heat treatment that results in 

hardness typically between 59 and 61 on the Rockwell C scale. 

Following heat treatment, one side of the specimen is then metallographically polished with 

progressively finer grades of abrasive until a mirror smooth and flat surface (Rrms~0.03 m) results. 

Such a surface is required to facilitate the measurement of small surface denting. 

Figure 5.—Cast 60NiTi plate and rod ingots. Forward most cast rod used to produce 

indentation specimens tested in the present study. 
NASA/TM—2014-216627 6 

Figure 6.—Microstructure of 60NiTi before and after heat treatment. (a) As-received 60NiTi (lighter phase is Ni3Ti). 

(b) Heat-treated 60NiTi . 

Test Setup and Procedures 

Four different size (6.35, 7.94, 9.52 and 12.7 mm diameter) indenter balls made from the ceramic 

Si3N4 are used to dent the 60NiTi surfaces in these experiments. The general configuration, shown in 

Figure 7, is essentially a Brinell test and it closely mimics the approach used by Park and his colleagues 

to establish the static load capacity of advanced, high carbide content steels (Refs. 4 and 5). 

In our tests, the ball is loaded against the flat plate surface using a computer controlled load frame. 

The indent ball is held in a hemispherical recess machined into a standard steel push rod. A small volume 

of silicone vacuum grease is applied to the surface of the recess prior to inserting the ball to help secure 

the ball against gravity and to ease subsequent ball removal. Each 25 mm diameter indent plate specimen 

is marked using an ink pen in 12 locations that mimic the face of a clock. These markings aid accurate 

identification of the dent area, which can be invisible to the unaided eye. At least three repeat experiments 

are conducted at each test load. Because of the wide range of indenter ball diameters, not all test loads are 

used for every indenter ball. Rather, loads are selected to result in dent depths below the static load 

capacity and above, based upon earlier work. The tested loads are clearly delineated in the data table. 

To conduct a test, the plate specimen is placed directly beneath the ball and aligned by eye such that 

the indent occurs at the desired position. The push rod-ball assembly is then manually lowered by 

pressing the “lower head” control button on the load frame controller. When the ball is within 1mm of the 

surface the control button is released and the operator then engages an automated computer controlled 

indent program that slowly lowers the ball until a load cell detects contact. The load is then increased 

linearly at approximately 100 kgf per minute until the desired static load is reached at which time the load 

is held steady (within ~5%) by the control system. After a dwell period (typically 1 min) the push rod-ball 

assembly is raised to remove the load and the test concludes. 

Following the indent experiments, the flat plate specimens are measured using a non-contacting 

optical profilometer. With this technique, first a fairly broad area (square mm) in the vicinity of the dent 

area is scanned to develop a low-resolution surface topography image. This area image guides 

subsequent, higher resolution depth measurements to be made on the dent itself, which may not be 

precisely positioned near the ink markings due to operator error. By stitching multiple measurements 

together, a top view, topographical plot of the dent and the surrounding undisturbed surface is then made. 

An example of such a dent measurement plot is shown in Figure 8. 
NASA/TM—2014-216627 7 

Figure 7.—Brinell test configuration used for indentation experiments to measure static load capacity. 

(a) Load frame. (b) Ball on disk set-up. (c) Dent map, top view 

Figure 8.—Topographical contour plot of dented 60NiTi surface as measured by optical (noncontact) 

surface profilometry. 

Results and Discussions 

The hardened and polished 60NiTi specimen surfaces were loaded with the Si3N4 indenter balls and 

the resulting dents were examined as described in the previous section. The indentation data is 

summarized in Table II using a nominal constant value of 100 GPa for the modulus of 60NiTi and 

300GPa for the modulus of Si3N4. The data table presents the average dent depth values. Dent depth data 

scatter was low with repeatability typically within 10% of the average. As mentioned previously, not all 

indenter ball sizes were run at all the test loads. The majority of the data was collected at stress levels 

between 2.0 and 4.0 GPa based upon earlier testing. 
NASA/TM—2014-216627 8 

TABLE II.—STATIC LOAD INDENTATION DATA SUMMARY—Si3N4 BALL VERSUS 60NiTi FLAT PLATE 

[Cast 60NiTi material, hardened to Rockwell C60 and polished to 0.03 m rms surface roughness.] 

Load, 

kg 

6.35 mm (1/4 in.) ball 7.94 mm (5/16 in.) ball 9.52 mm (3/8 in.) ball 12.7 mm (1/2 in.) ball 

Mean 

stress, 

GPa 

Dent depth 

(dP), 

m 

dp/D10-3 Mean 

stress, 

GPa 

Dent depth 

(dP), 

m 

dp/D10-3 Mean 

stress, 

GPa 

Dent depth 

(dP), 

m 

dp/D10-3 Mean 

stress, 

GPa 

Dent depth 

(dP), 

m 

dp/D10-3 

50 2.65 0.11 0.017 2.3 0.06 0.008 2.0 ~0 ~0 ---- ---- ---- 

100 3.35 0.56 0.088 2.9 0.19 0.024 2.6 0.15 0.016 ---- ---- ---- 

150 3.83 1.50 0.236 3.3 0.59 0.074 2.9 0.18 0.019 ---- ---- ---- 

200 4.20 2.30 0.365 3.6 1.0 0.126 3.2 0.57 0.06 2.7 0.20 0.016 

250 4.54 3.60 0.564 3.9 1.7 0.214 3.5 1.0 0.105 2.9 0.36 0.028 

300 4.83 5.30 0.84 4.15 3.0 0.378 3.7 1.6 0.17 3.0 0.56 0.044 

350 ---- ---- ---- ---- ---- ---- ---- ---- ---- 3.2 0.66 0.052 

400 ---- ---- ---- ---- ---- ---- ---- ---- ---- 3.4 0.83 0.065 

450 ---- ---- ---- ---- ---- ---- ---- ---- ---- 3.5 1.36 0.107 

500 ---- ---- ---- ---- ---- ---- ---- ---- ---- 3.6 1.68 0.132 

550 ---- ---- ---- ---- ---- ---- ---- ---- ---- 3.7 1.99 0.157 

Figure 9.—Normalized dent depth (dp/D) versus mean contact 

stress for polished 60NiTi plate indented with Si3N4 balls 

ranging in size from 6.35 to 12.7 mm diameter. 

The data is plotted as the ratio of dent depth to indenter diameter as a function of mean contact stress 

in Figure 9. The Hertz equations are used to calculate the contact stress and utilizing the materials 

properties found in Table I for both the ceramic indenter and the 60NiTi. 

Examination of Figure 9 reveals that when the Hertz stress relations are employed for all the data a 

rather smooth curve results. Despite the wide range of indenter diameters, all the data falls along the same 

line crossing the dent depth threshold at a stress level of about 3.0 GPa. This stress limit threshold is, as 

pointed out previously in the introduction section of this paper, above the stress limit for M50 and 440C 

steels but well below the limit for the high-carbide tool steel REX 20. Nonetheless, when the low elastic 

modulus and large elastic deformation range is taken into account, extremely high static loads can be 

endured when employing 60NiTi in concentrated contacts. Further, the Hertz equations, which can be 

generalized to both point and line contacts, can be considered appropriate for use in bearing design. 
NASA/TM—2014-216627 9 

Referring back to the illustration in Figure 1, a bearing designer can readily incorporate the raceway 

curvature and contact radii between a ball and race to calculate stresses when considering 60NiTi for 

raceways, balls or both surfaces. By using 3.0 GPa as the static stress capability, internal bearing 

geometry can be tailored to accommodate design loads in the same manner as traditional steel bearings. 

The significant difference, however, is that the load capability will be higher than for steels as depicted in 

Figure 4. This is a direct result of the low elastic modulus that reduces stress through higher elastic 

deformation coupled by the high hardness and large elastic range of 60NiTi. 

This design approach described in the preceding paragraph will, however, be conservative. Recent 

preliminary stress modeling work for rolling element bearings made using low modulus components, like 

60NiTi, has shown that bearing load capacity is further enhanced compared to steel bearings because of 

compliance effects (Ref. 11). Bearings are more complex than the single ball-on-plate geometry 

considered in the present study. When loaded radially, multiple ball-raceway contacts are engaged to 

support the load. The lower stiffness of the superelastic materials (60NiTi) enhances load sharing 

amongst adjacent ball-race contacts effectively lowering peak stress and increasing the bearing static load 

capacity. This effect and the detailed quantification of its magnitude are still under investigation, but 

represent yet another potential of the use of reduced stiffness construction materials in bearings. 

As always, care must be taken when extrapolating the results presented here to real applications. 

Appropriate selection and application of a bearing material cannot be made using one parameter, such as 

static indent load capacity, alone. Future investigations to quantify the static load capacity of full-scale 

bearings made with 60NiTi races are needed to verify the true load capability of this emerging class of 

superelastic bearing materials. Recent experiments using 50 mm bore bearings have shown that durable 

hybrid (60NiTi races with Si3N4 balls) ball bearings can be produced. In time, sufficient quantities of 

production bearings will be available to more fully explore and exploit this unique class of materials. 

Summary Remarks 

The results presented in this paper demonstrate that the superelastic 60NiTi exhibits a high tolerance 

to static indentation loads and has a mean contact stress limit above 3.0 GPa. Because of its low elastic 

modulus and large elastic deformation range, 60NiTi can withstand static loads that exceed the 

capabilities of traditional bearing materials. Further, the observation that the dent depth data, when 

normalized using the Hertz stress relations, falls on the same smooth curves indicates that the generalized 

Hertz equations can be used to guide bearing design. These results, combined with 60NiTi’s intrinsic 

corrosion resistance and low density provide further reasons to consider 60NiTi as a viable candidate for 

selected bearing applications. 

References 

1. Brandlein, J., Eschmann, P., Hasbargen, L., and Weigand, K.: Ball and Roller Bearings: Theory, 

Design and Applications, 3rd edition, John Wiley & Sons, West Sussex, England, 1999. 

2. Bisson, E.E., and Anderson, W.J.: Advanced Bearing Technology, National Aeronautics and Space 

Administration, NASA SP-38, 1964, Chapter 9, pages 259-288. 

3. A.R. Leveille and J.J. Murphy, “Determination of the Influence of Static Loads on the Output Torque 

of Instrument Ball Bearings,” in Proceedings of the Charles Stark Draper Lab. International Ball 

Bearings Symposium, (1973). 

4. W. Park, M.R. Hilton, A. R. Leveille, P.C. Ward, D.E. McClintock, and D.W. Smith, “Rolling 

Contact Fatigue and Load Capacity Tests of M62 Bearing Steel,” in Proceedings of 32nd Aerospace 

Mechanisms Symposium, NASA/CP—1998-207191, pp. 237-251, (1998). 

5. W. Park, M.R. Hilton, P.C. Ward, G.W. Henderson, A. R. Leveille, D.E. McClintock, and D.W. 

Smith, “Microstructure, Fatigue Life and Load Capacity of PM Tool Steel REX20 for bearing 

Applications,” Lubrication Engineering, volume 55, number 6, pp. 20-30, 1999. 
NASA/TM—2014-216627 10 

6. DellaCorte, C., Pepper, S.V., Noebe, R., Hull, D.R., and Glennon, G., 2009, “Intermetallic NickelTitanium Alloys for Oil-Lubricated Bearing Applications,” NASA/TM—2009-215646. 

7. DellaCorte, C., Noebe, R., Stanford, M.K., and Padula, S.A., “ Resilient and Corrosion-Proof Rolling 

Element Bearings Made From Superelastic Ni-Ti Alloys for Aerospace Mechanism Applications,” 

Proceedings of the 2011 ASTM Rolling Element Bearings Conference, Anaheim, California, April 

13-15th, 2011 and NASA/TM—2011-217105. 

8. DellaCorte, C., Moore, L.E.III, and Clifton, J.C.: “Static Indentation Load Capacity of the 

Superelastic NiTi for Rolling Element Bearings,” NASA/TM—2012-216016, May 2012. 

9. DellaCorte, C., Moore, L.E.III, and Clifton, J.C.: “The Effects of Pre-Stress on the Static Indentation 

Load Capacity of the Superelastic NiTi,” NASA/TM—2012-216479, January 2013. 

10. Stanford, M.K., Thomas, F., and DellaCorte, C.: “Processing Issues for Preliminary Melts of the 

Intermetallic Compound 60NITINOL, NASA/TM—2012-216044, November 2012. 

11. Krantz, T.L.: “On Calculation Methods and Results for Straight Cylindrical Roller Bearing 

Deflection, Stiffness and Stress,” Proceedings of the ASME 2011 International Design Engineering 

Technical Conferences & Computers and Information in Engineering Conference IDETC/CIE 2011, 

August 29-31, 2011, Washington, D.C., paper number DET2011-47930.