Wear Improvement in Ti-6Al-4V by Ion Implantation
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WEAR IMPROVEMENT IN Ti-6A1-4V BY ION IMPLANTATION
R.G. VARDIMAN Naval Research Laboratory Washington, D.C. 20375
ABSTRACT The friction and wear of Ti-6Al-4V are found to be sharply reduced by carbon implantation followed by heat treatment. Optimum wear resistance is developed at 400 0 C, at which the microstructure of the implanted layer shows a dense array of TiC precipitates up to 60 nm in size. The implanted layer in this case is worn through in a few thousand cycles of the ball on disc test, but by implanting at two energies to achieve a deeper carbide layer, negligible wear was found even after 20,000 cycles. No wear improvement was found for nitrogen implantation.
INTRODUCTION Titanium alloys have always presented a particular problem with regard to wear, yet many applications involve fretting or wear situations. Most standard coating methods do not work well on titanium usually because of adhesion problems [1]. It is not always possible to lubricate the surfaces, and under unlubricated conditions rapid material transfer and severe wear can occur with even moderate loads [2]. Ion implantation creates a surface layer with no adhesion problems, since the affected layer is continuous with the underlying material. Wear improvement has been achieved in a variety of materials by implantation, usually with nitrogen [31. Two recent studies [4,5] have reported substantial wear improvement in Ti-6A1-4V with nitrogen implantation. The present work uses carbon implantation with post-implantation heat treatment to produce a low friction, wear resisting surface. Variations in the implanted layer microstructure are seen to correlate with wear behavior. A dual energy implant, increasing the depth of the affected layer, produces a larger than expected improvement in wear rate. A nitrogen implanted specimen tested for comparison did not show any wear improvement. EXPERIMENTAL METHODS The starting material was m-B processed (mill annealed) Ti-6A1-4V. This microstructure is characterized by equiaxed a grains and smaller, elongated S grains [6]. Wear discs were cut approximately 2.6 cm square and 0.45 cm thick, and the surface polished to 3 pm diamond. For transmission electron microscopy (TEI), 3 mm discs were mechanically polished to 100-150 Pm thickness, then electropolished on one side before implantation. Final thinning was from the opposite side, with the implanted surface masked. For implantation, wear specimens were clamped to an aluminum plate, while TEM specimens were attached with silver paint. Heat sinking was good in both cases, and maximum temperature during implantation is not expected 0 First, 2x1017 to exceed 150 C. Two types of carbon implantation were used. 2 at./cm was implanted at 75 keV. All TEM specimens received this treatment and one set of wear discs. One wear disc was first implanted to 3x1011 2 at./cm at 175 keV, followed by the above treatment, in order to give a much Mat. MR. sot. sp. Proc. Vol. 27 (1984)
Elsevier Science Publishing Co., Inc.
700
deeper layer of carbon. Nitrogen was keV, follo
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