294 Stainless steels such as 440C are widely used in  the bearing and gear industry where corrosion  resistance and high hardness are required. These martensitic stainless steels are reasonably low cost,  easy to machine prior to heat treatment and are di mensionally stable. When prepared through vacuum  melting processes, they achieve very uniform, fully dense microstructures which lead to fine surface finishes and good fatigue behavior. Despite being referred to as “stainless”, however, the 400 series martensitic steels are prone to corrosion and are more  accurately referred to as “corrosion-resistant”  alloys rather than stainless. They are also highly magnetic.  Silicon Nitride ceramics, on the other hand, are essent ially corrosion proof. They can be polished to very  fine surface roughness and are quite wear resistant. S ilicon nitride’s low density compared to steels also  makes it ideal for ultra high-speed applications becaus e lower centrifugal stresses result. These attributes  make silicon nitride the material of choice for high stiffness, high load, and high-speed bearings and for  applications that include corrosive conditions and aggressive sliding environments. Such applications  include bearings for gas turbine hot sections, cryog enic oxidizer turbopumps and components for diesel  engine fuel injection systems. Though non-magnetic, silicon nitride is an electrical insulator. It is also  more expensive to manufacture than steels owing to the complexity and cost of  the high temperature,  high pressure powder metallurgy pr ocessing required. Anot her shortcoming of s ilicon nitride is the  difficulty of fabricating it into bearing raceways and other complex geometries.  Historically, metallic alloys with high concentrations of titanium are poor tribologica l materials in that they  do not respond well to lubrication by organic fluids [5]. For instance, alloys such as Ti-6Al-4V exhibit  galling behavior in dynamic contacts even under condit ions well lubricated by oils and greases. During  contact, titanium readily transfers to the counter-face  leading to rough surfaces, high friction and wear. In  addition, titanium alloys are recognized as being chem ically aggressive causing degradation of many  lubricants [6]. When titanium alloys must be used due to other attributes like high specific strength or  corrosion resistance, tribological contact is avoided through the use of thick barrier coatings and claddings.    Based upon a wealth of negative experience with titanium alloys in tribological contacts, 60NiTi would appear an unlikely candidate as a bearing material. On the other hand, ceramic materials with high concentrations of titanium can exhibit desirable tribological properties. Titanium carbide (TiC) and  Titanium dioxide (TiO 2) are good examples. TiC coatings are often used to improve the surface finish and  performance of stainl ess steel rolling element s in bearings and TiO 2, in the form of rutile, has been put  forth as a potential solid lubricant under certain cond itions [6, 7]. These ceramic materials, however, are  brittle and cannot be used as structural elements. In  the present paper, we present evidence that  hardened NiTi alloys can be produced that are devoid of shape memory or superelastic behavior and that such alloys display excellent tribological properties under oil-lubricated contact conditions.  

Materials and Procedures 

The 60NiTi balls evaluated in this work was manuf actured via a high temperature proprietary powder  metallurgy process roughly similar to that described in  the literature [8]. Pre-alloyed 60Ni-Ti powder was  HIPed into rough, spherical ball blanks that were then ground, polished, and lapped to produce high  quality (Grade 5) bearing balls 0.5 inch (12.5 mm) in diameter. 60NiTi plates were cut from cast and hot  rolled plate stock. A multi-step t hermal process (heat treatment) was used to enable rough machining of  the plates and grinding of the bearing balls in a softe ned state followed by lapping to a very fine surface  finish in a final hardened condition. The finished 60NiTi ball specimens, shown in the photograph in Figure 1, are bright and shiny in appearance a nd resemble conventional polished steel balls.    The elemental composition of the bearing material, as measured by atomic emission spectroscopy and  energy dispersive, semi-quantitative X-ray analysis, were consistent and showed the specimens are  nominally 55 atomic percent Nickel with the balance ti tanium. This translates to 60 weight percent Nickel,  40-weight percent titanium, hence the historical designation of NITINOL 60 or 60NiTi. Density was  NASA/CP-2010-216272