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Research Papers: Friction and Wear

Effect of Contact Area on the Wear and Friction of UHMWPE in Circular Translation Pin-on-Disk Tests

[+] Author and Article Information
Vesa Saikko

Department of Mechanical Engineering,
Aalto University School of Engineering,
P.O. Box 14300,
Aalto FI-00076, Finland
e-mail: vesa.saikko@aalto.fi

Contributed by the Tribology Division of ASME for publication in the JOURNAL OF TRIBOLOGY. Manuscript received October 7, 2016; final manuscript received March 9, 2017; published online July 10, 2017. Assoc. Editor: Zhong Min Jin.

J. Tribol 139(6), 061606 (Jul 10, 2017) (5 pages) Paper No: TRIB-16-1316; doi: 10.1115/1.4036448 History: Received October 07, 2016; Revised March 09, 2017

Circular translation pin-on-disk (CTPOD) tests were performed for ultrahigh molecular weight polyethylene (UHMWPE) with a view to reproducing wear mechanisms that prevail in total hip prostheses. The contact surface diameter varied from 3.0 mm to 30 mm, while the slide track diameter was fixed, 10 mm. The counterface was polished CoCr, and the lubricant was diluted alpha calf serum. Either the nominal contact pressure (1.1 MPa) or the load (126 N) was kept constant. With a constant contact pressure, the wear factor decreased steeply when the contact diameter exceeded the slide track diameter, apparently because the wear debris was not readily conveyed away from the contact. With constant load, both the wear factor and the coefficient of friction increased linearly with increasing contact area. This trend was in agreement with clinical observations that the wear rate of UHMWPE acetabular cups increased with increasing femoral head size. With nominal contact pressures approaching 10 MPa however, the bearing surface topography markedly differed from clinical observations. This was probably due to overheating of the contact and plastic deformation that resulted in the formation of protuberances not seen clinically. The present study emphasized the importance of test parameters in the pin-on-disk wear screening of prosthetic joint materials. It appeared that the contact surface diameter of the flat-on-flat test should be below the slide track diameter, and that the nominal contact pressure should be of the order of 1 MPa.

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Figures

Grahic Jump Location
Fig. 1

(a) Variation of wear factor k (filled circles) and coefficient of friction μ (open circles) with contact surface area A of nonirradiated UHMWPE against polished CoCr in diluted serum lubricant in CTPOD tests, mean, and standard deviation. Nominal contact pressure (1.1 MPa), slide track diameter (10 mm), and serum lubricant temperature (20 °C) were constant. The dashed line indicates A (78.5 mm2) above which a central circle with increasing diameter on the disk was continually covered by the pin. (b) As in Fig. 1(a), but the load was constant 126 N. Hence, the nominal contact pressure values were 10.0 MPa, 4.5 MPa, 2.5 MPa, 1.6 MPa, and 1.1 MPa.

Grahic Jump Location
Fig. 2

(a) UHMWPE pins with contact surface diameters of 3.0 mm, 4.5 mm, 6.0 mm, 9.0 mm, 12.0 mm, 15.0 mm, 22.5 mm, and 30.0 mm after tests with constant nominal contact pressure 1.1 MPa. Note burnishing. (b) UHMWPE pins with contact surface diameters of 4.0 mm, 6.0 mm, 8.0 mm, 10.0 mm, and 12.0 mm after tests with constant load 126 N. The nominal contact pressure was 10 MPa, 4.5 MPa, 2.5 MPa, 1.6 MPa, and 1.1 MPa, respectively. Note protuberances on center of contact surface, which are a dominating feature especially above 2 MPa contact pressure.

Grahic Jump Location
Fig. 3

(a) Optical micrograph from edge of worn UHMWPE surface with typical burnished appearance. Contact surface diameter 6.0 mm and contact pressure 1.1 MPa. (b) Optical micrograph from center of worn UHMWPE surface showing protuberance formation. Note also criss-cross wear marks. Contact surface diameter 4.0 mm and contact pressure 10 MPa.

Grahic Jump Location
Fig. 4

Schematic showing how the entire disk may be lubricated even when the pin diameter is three times the slide track diameter. Circles drawn with the following line types and their diameters are: thick, slide track, 10 mm; medium, pin at two opposite positions half a cycle apart, 30 mm; thin, extent of contact on disk, 40 mm; and dashed, continually covered by pin, 20 mm. When point (P) is considered to be a location on disk when pin is in its leftmost position, P is on edge of lubricated disk surface. Then, P is considered to be a location on pin when it is in its rightmost position half a cycle later. It can be seen that P can drag lubricant to center of disk (C) because the two marked distances are equal, 10 mm (=slide track diameter).

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