Research Papers: Friction and Wear

Experimental and Computational Thermal Modeling of In Vitro Pin-on-Disk Tests of Ultra High Molecular Weight Polyethylene

[+] Author and Article Information
Kathleen A. Lewicki

Thayer School of Engineering
at Dartmouth College,
14 Engineering Drive,
Hanover, NH 03755
e-mail: kathleen.a.lewicki.th@dartmouth.edu

Douglas W. Van Citters

Thayer School of Engineering
at Dartmouth College,
14 Engineering Drive,
Hanover, NH 03755,
e-mail: dvancitters@dartmouth.edu

Contributed by the Tribology Division of ASME for publication in the JOURNAL OF TRIBOLOGY. Manuscript received June 22, 2015; final manuscript received November 9, 2015; published online July 8, 2016. Assoc. Editor: Zhong Min Jin.

J. Tribol 138(4), 041602 (Jul 08, 2016) (7 pages) Paper No: TRIB-15-1220; doi: 10.1115/1.4032819 History: Received June 22, 2015; Revised November 09, 2015

Frictional heating occurring during pin-on-flat tribotesting of ultrahigh molecular weight polyethylene (UHMWPE) pins was measured and modeled. A full factorial experiment was conducted to determine if testing parameters can produce sufficient frictional heat to alter tribological properties of the bovine serum used as lubricant in the system. Temperature of the surrounding bovine serum was monitored during tribotests using varying pin sizes and sliding speeds to determine typical temperature rises due to frictional heating. This work examined two sliding speeds (40 mm/s and 80 mm/s) and two pin diameters (6.35 mm and 9.5 mm) at a single static load. Gravimetric analysis for wear determination and coefficient of friction measurement were performed for each test. Results showed that frictional heating increased the bulk temperature of the surrounding serum and correlated to sliding speed and average coefficient of friction. No correlation was seen at this temperature range between serum temperature rise and wear rate, providing evidence that the tested parameters are acceptable for tribotesting of UHMWPE. A computational model was developed to predict bulk serum temperature increase. This model closely predicted the temperature increase to within 2 °C, which is sufficient accuracy for identifying if bovine serum protein precipitation is likely during tribotesting. This work serves as an initial estimate and prediction for appropriate testing parameters based on lubricant responses to frictional heating.

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Fig. 1

(a) Schematic of thermocouple arrangement and (b) image of actual thermocouple arrangement in tribotester

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Fig. 2

Heat flow diagram for the computational model

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Fig. 3

Experimental temperature results for small bath bulk lubricant temperature shown were sampled at 1 Hz until the temperature reached an asymptote at steady state for all four testing conditions: large diameter pins at 2 Hz, large diameter pins at 1 Hz, small diameter pins at 2 Hz, and small diameter pins at 1 Hz

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Fig. 4

(a) Average and peak coefficient of friction values averaged over five coefficient of friction tests at six pin stations. These tests were performed every 50,000 cycles during the 250,000 cycle test. (b) A typical coefficient of friction trace identifying the peak and average coefficients of friction.

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Fig. 5

Gravimetric analysis was performed at the end of 250,000 cycles and plotted versus temperature. No significant correlation was observed between mass lost and temperature increase.

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Fig. 6

(a) Higher coefficients of friction are statistically associated with greater temperature increases (p < 0.001) and account for 52% of the variance. (b) Coefficient and sliding speed are both significant predictors (p = 0.003) and account for approximately 75% of the variance.

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Fig. 7

Comparison of computational and experimental temperature profile for (a) large pins at 2 Hz, (b) large pins at 1 Hz, (c) small pins at 2 Hz, and (d) small pins at 1 Hz

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Fig. 8

Average increases in temperature to steady state from experimental results are provided as open bars and model results are provided as solid black bars

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Fig. 9

No apparent transfer film or protein deposition occurred in these tests (a), compared with prior longer-term wear tests performed in our laboratory under similarly aggressive conditions ((b) and (c))




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