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Research Papers

Surface Hardening of Titanium Articles With Titanium Boride Layers and its Effects on Substrate Shape and Surface Texture

[+] Author and Article Information
Anthony P. Sanders

 Ortho Development Corporation, 12187 South Business Park Drive, Draper, UT 84020tsanders@orthodevelopment.com

Nishant Tikekar, Curtis Lee

Department of Metallurgical Engineering, University of Utah, 135 South 1460 East, Room 412, Salt Lake City, UT 84112

K. S. Ravi Chandran

Department of Metallurgical Engineering, University of Utah, 135 South 1460 East, Room 412, Salt Lake City, UT 84112ravi.chandran@utah.edu

J. Manuf. Sci. Eng 131(3), 031001 (Apr 15, 2009) (8 pages) doi:10.1115/1.3106032 History: Received May 13, 2008; Revised December 22, 2008; Published April 15, 2009

There is widespread interest in engineering improved properties into the surface layer of manufactured articles. One method for doing so involves a novel boriding process that creates hardened surface layers by the growth of a dual layer TiB2+TiB coating on titanium articles. The objective of the present work was to demonstrate the fundamental feasibility of this process by producing uniform thick boride coating layers on titanium articles and to polish them to a very fine surface texture suitable for biomedical implant bearing surfaces. A powder pack diffusion boriding process was used to grow dual layer TiB2+TiB coatings on simple shapes. Lapping processes were used to polish the borided articles. Evaluation was carried out using measurements of surface texture, geometric form, and hardness, and by metallurgical analysis. Boriding on as-received titanium articles resulted in shape distortion that hampered the subsequent polishing efforts. Hence, further articles were treated with stress-relief annealing prior to boriding, at temperatures below and above the β-transus of the substrate article. Annealing itself caused some form distortion, which was eliminated by lapping. Then, after boriding the annealed articles, varying surface textures and shape distortions were observed. Articles annealed above the β-transus had surface textures with significant peak-to-valley roughness (1455μm), and the texture appeared to be patterned upon the substrate microstructure. However, form distortion seemed to be alleviated. For articles annealed below the β-transus, form distortion was not alleviated, and the articles exhibited wavy surface textures with a high peak-to-valley roughness (up to 50μm). Whether combined or independent, the surface texture changes and shape distortion that occurred during boriding thwarted the polishing processes; the articles could not be uniformly polished to a roughness less than 0.05μm within the coating thickness. To achieve uniformly polished dual layer TiB2+TiB surfaces on titanium articles using the pack boriding technique, it appears that the substrate raw materials should be free of residual stresses and consist of a fine microstructure.

Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic—powder pack diffusion boriding process

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Figure 2

Morphological variants of the coating produced by the boriding process. Sections of coatings made at (a) 950°C and (b) 1050°C.

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Figure 4

Borided ∅6.35 mm ball after polishing

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Figure 5

Metallographic sections through a borided, polished ball: (a) region of polished TiB2, (b) region denuded of TiB2 but retaining the TiB whiskers

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Figure 6

Lapped surface of ∅25.4 mm borided Ti–6Al–4V disk. Lighter areas in the figure were mirrorlike; darker areas were dull.

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Figure 7

Micrographs of sections in (a) mirrorlike and (b) dull surface areas of disk in Fig. 6

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Figure 8

Post-anneal appearance of (a) Group B1 and (b) Group B2 balls

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Figure 9

Example roundness measurement traces from (a) Group B1 and (b) Group B2 balls. Dimensions are maximum (+) and minimum (−) diametral deviations relative to the mean diameter (microns).

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Figure 10

Postboride appearance of ball surfaces: (a) Group B1, (b) Group B2.

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Figure 11

Typical postboride roundness measurement traces: (a) Group B1 and (b) Group B2. Dimensions are maximum (+) and minimum (−) diametral deviations relative to the mean diameter (microns).

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Figure 12

Micrographs of balls that were annealed prior to boriding: (a) Group B1, and (b) Group B2

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Figure 13

Group B2 ball after boriding and polishing: (a) Low magnification, and (b) higher magnification, focusing on smoothly polished area (original magnification 180×).

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Figure 14

Postboride surface texture of pre-annealed disks: (a) Disk 1 and (b) Disk 2

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Figure 16

Polishing results after boriding the pre-annealed disks: (a) Disk 1 and (b) Disk 2. The geometric pattern is the reflection of a playing card positioned close to the surface. Dull areas were unpolished low-lying areas.

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Figure 3

SEM micrographs of (a) as-received ball surface and (b) as-borided ball surface. Images are not from identical locations.

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Figure 15

8×6 mm SWLI profilometer scans of postboride disk surfaces: (a) Disk 1 and (b) Disk 2. In each, the upper figure is the 8×6 mm area scan, and the lower figure is a plot of the roughness for the line drawn on the area scan.

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