293 Nickel-Titanium Alloys: Corrosion “Proof” Alloys for Space Bearing, Components  and Mechanism Applications 

Christopher DellaCorte *  

Abstract 

An intermetallic nickel-titanium alloy, 60NiTi (60w t%Ni, 40wt%Ti), is shown to be a promising candidate  tribological material for space mechanisms. 60NiTi offers a broad combination of physical properties that  make it unique among bearing materials. 60NiTi is hard , electrically conductive, highly corrosion resistant,  readily machined prior to final heat treatment, and is non-magnetic. Despite its high titanium content,  60NiTi is non-galling even under dry sliding. No other bearing alloy, metallic or ceramic, encompasses all of these attributes. Since 60NiTi contains such a hi gh proportion of titanium and possesses many metallic  properties, it was expected to exhibit poor tribolog ical performance typical of titanium alloys, namely  galling type behavior and rapid lubricant degradation. In  this poster-paper, the oil-lubricated behavior of  60NiTi is studied.   Introduction 

Binary Nickel-Titanium (Ni-Ti) alloys are in widesp read use in the medical and dental industries in  applications where their biocompatibility and unique supe relastic or Shape Memory Effect characteristics  are readily exploited [1]. The Ni-Ti family of alloys tr aces their origins to pioneering work of William J.  Buehler and his colleagues at the Naval Ordnance Laboratory during the late 1950’s [2,3]. The designation NITINOL often used for these alloys is  an abbreviation for Nickel-Titanium Naval Ordnance  Laboratory. Buehler’s early efforts identified both the Nitinol 55 and Nitinol 60 alloys, which contained 55 and 60 weight percent nickel respectively. Nitinol 55 is soft and was found to be easier to mechanically work and form than Nitinol 60 that was prone to excessive work hardening. Several hand tools were fabricated from Nitinol 60 to take advantage of its hi gh hardness, electrical conductivity, non-magnetic  behavior and corrosion resistance [4].   Materials for high performance bearings, gears and other mechanical components require a number of  specific properties and characteri stics. Among these key attributes ar e high strength and hardness, high  thermal conductivity, and the ability to be manufactured to  very high levels of precision with regards to  final dimensions and surface finish. In addition, ex cellent corrosion resistance and good tribological  properties are often of importance especially for applications in extreme environments. Spaceflight hardware destined to operate in the vacuum of spac e, beyond the realm of atmospheric corrosion, often  must be stored for extended periods before launch, and are subject to bearing and gear corrosion  problems. In select applications involving electric machines and sensitive instru mentation, good electrical  conductivity and non-magnetic properties can also  be highly desirable. Unfortunately, no currently  deployed material possesses all of these properties.  Traditional tool steel based bearing materials, such  as M50 and 52100 enjoy widespread application due  to their high hardness, ease of manufacture and good tribological properties. However, these alloys suffer from corrosion attack if not protected and though electr ically conductive they are also highly magnetic. In  addition, when used as bearing rolling elements, their high density leads to high centrifugal forces and limited fatigue life. These considerations have driven  the search and development  of alternate bearing  and mechanical component alloys, namely stainless steels and ceramics.                                                     *NASA Glenn Research Cent er, Cleveland, OH  

Proceedings of the 40th Aerospace Mechanisms Symposium, NASA Kennedy Space Center, May 12-14, 2010   NASA/CP-2010-216272