Microstructures of Stainless Steels Exhibiting Reduced Friction and Wear After Implantation with Ti and C
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MICROSTRUCTURES OF STAINLESS STEELS EXHIBITING REDUCED FRICTION AND WEAR AFTER IMPLANTATION WITH Ti AND C*
D. M. FOLLSTAEDT, F. G. YOST, Sandia National Laboratories,
AND L.E. POPE Albuquerque, NM 87185
ABSTRACT Implantation of Ti and C into stainless steel discs of Types 304, 15-5 PH, Nitronic 60 and 440C has previously been reported to reduce wear depths by up to - 85% and friction by - 50% in unlubricated pin-on-disc tests. Our earlier studies relating microstructure to friction and wear results in Type 304 are first summarized; these indicate that the improvements in the surface mechanical properties are due to an amorphous surface layer, similar to the amorphous layer observed in pure Fe implanted with Ti and C. We have now examined the other three implanted steels and found similar amorphous layers. These results strongly suggest that the amorphous surface alloy is responsible for reduced friction and wear in all the steels.
INTRODUCTION Ion implantation has been studied for about a decade as a method to reduce friction and wear on metal surfaces. Many studies have examined the effects of N implantation, but recently we and others have investigated steels implanted with Ti or Ti and C. This treatment has produced improvements in a wide variety of tests, including unlubricated pin- or ball-on-disc tests [1-4], lubricated tests [3,4], and tests of abrasive wear by diamond particles [5]. Here we examine implanted discs from our unlubricated tests, which gave reductions in maximum wear depth of up to - 85% and reductions in friction of - 50% [1]. Based on our observations in pure Fe implanted with Ti and C [6], we expected an amorphous surface alloy. Below we show that the amorphous phase forms in all the steels tested, and we propose that it is responsible for the reduced friction and wear in types 440C, Nitronic 60, and 15-5, as previously shown for 304 [7,8]. The details of our pin-on-disc tests are given [i]. Mechani17 elsewhere 16[l 2 22 cally polished discs were implanted with 2x10 Ti/cm (5xl0 Ti/cm at 180, 17 2 150, 120 and 90 keV) and 2x10 C/cm (30 keV). Samples for transmission electron microscopy (TEM) were obtained from the wear test specimens by jet electropolishing from the unimplanted side to produce electron-transparent areas which included the surface alloy. Energy dispersive (x-ray) spectroscopy (EDS) was used with TEM analysis at 100 and 120 kV and with scanning (secondary) electron microscopy (SEM) examination of wear tracks at 20 keV.
SUMMARY OF TYPE 304 RESULTS Our microstructure observations on 304 are discussed in detail elsewhere [7]. Unimplanted areas of the disc consisted of mostly fcc material of 20-30 ýIm grain size with a high density of defects. The TEM sample from implanted but unworn areas showed a free-standing amorphous surface layer which could be examined isolated from the crystalline substrate, just as in pure Fe [6]. EDS indicated the depth-averaged composition of the amorphous layer *This work performed at Sandia National Laboratories supported by the U. S. Department o
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