Comparison of Tribological Performance of Ultra High Molecular Weight Polyethylene and Poly Ether Ether Ketone.
Polymers are used in many areas related to tribology due to their superior mechanical properties such as high strength, self-lubrication, low friction coefficient, easy fabrication. The parameters affecting the friction and wear behavior of polymers and their relations with structural properties of polymers were investigated in tribological studies of polymers [1,2]. However, there is much to be researched in this area because polymers are often forced by applications involving contact with abrasive particles where roughness is an important parameter to be considered.
UHMWPE is one of the most common material used in orthopaedic implants for the last three decades . UHMWPE wear debris, which is the main reason of aseptic loosening in total joint replacements, causes osteolysis [4,5]. Therefore, reducing the wear debris of UHMWPE has long been a technical challenge.
PEEK is considered as an alternative to UHMWPE. Clinical  and experimental [7,8] studies in the total hip joints demonstrated that PEEK asetabular cups have superior wear performance over UHMWPE. Scholes and Unsworth (2009) , have also been measured low wear rates in knee replacements. However, in high contact stress conditions, using PEEK tibial implant instead of UHMWPE tibial implant results in unconformity and misalignment of implants.
The investigation and comparison of the friction and wear behaviour of UHMWPE and PEEK pins against stainless steel discs are presented in this study. The experiments are conducted on a self-made pin on disk tribometer, and the sample surfaces are prepared with a defined low level Ra of 0.02-0.03 [micro]m and tested against a commercial stainless steel disc. The motivation of the current study is to compare the friction and wear behaviour of UHMWPE and PEEK under bovine serum lubrication.
Materials and Methods
A 12 mm diameter UHMWPE and PEEK rods are supplied from the market and machined in 8 mm diameter as shown in figure 1. The densities of UHMWPE and PEEK, quoted by the manufacturer, are 0.93 gr.[cm.sup.-3] and 1.31 gr.[cm.sup.-3], respectively. The counter surface is AISI 304 stainless steel. The discs, 3 mm thick, are first polished on abrasive paper and then on diamond paste. The resulted surface roughness value is [R.sub.a]=0.02-0.03 [micro]m.
The experiments are performed on a self-made pin-on-disc tribometer (figure 2) by using UHMWPE and PEEK pins against stainless steel discs. The wear and friction coefficients of UHMWPE and PEEK against stainless steel discs are measured and compared under bovine serum lubricated conditions. Tests are carried out at room temperature (20-25[degrees]C) and the discs are rotated at a constant speed of 60 rpm, which resulted in a sliding speed of 0.2 [m.s.sup.-1]. The loading weights of 10 N is applied through the pin, which resulted in a contact pressure of 2 MPa between pin and disc. The computer read the frictional force data then friction coefficient was calculated as dividing this data by normal load.
The total sliding distance for wear tests is determined as 100 km with equal intervals (20 km) according to previous studies [10,11], and the wear is measured by the weight loss. A control pin is immersed in the bovine serum bath during the tests to monitor and compensate for water absorption. The tested pins together with control pin are cleaned and dried, and weighed using a precision scales (Radwag AS 220), accurate to 0,1 mg, after after every interval (20 km). The water absorption amount in the control pin is added to weight loss of the test pin to calculate the net weight loss of the pin. The wear factor ([mm.sup.3]*[(N.m).sup.-1]), k is calculated as,
k = V/(P.X) (1)
where V is the volume loss ([mm.sup.3]) of pins; P is the applied load (N) through the pins, and X is the sliding distance (m) on discs .
The experiments were carried out using UHMWPE and PEEK pins on stainless steel discs in bovine serum. The tribological test results allowed for clear comparison of the materials' tribological performance. The average friction coefficient values measured along the total sliding distance of 100 km are shown in figure 3. The measured friction coefficient of UHMWPE pin on stainless steel disc under bovine serum is approximately 0.09, while the friction coefficient of PEEK pin on stainless steel disc is higher and less stable, reaching an average of 0.14. These results have a good agreement with literature [12,13]. Petrica et al demonstrated that the friction coefficient of PEEK higher than for UHMWPE . Golchin et al. provided friction coefficients of UHMWPE and PEEK in water lubrication as 0.13 and 0.21, respectively . However, friction coefficient is lower under bovine serum lubrication than it is under water lubrication [14,15]. Fisher et al. found that the friction coefficient of UHMWPE pins on stainless steel discs under bovine serum lubrication is 0.07-0.2 when the surface roughness Ra is 0.014-0.078 [micro]m , which is in a good agreement with current results in this study.
The wear of UHMWPE and PEEK is calculated by measuring the weight loss of the test pins after every 20 km of sliding distance. The measured water absorption amount of test pin is also considered to calculate the total weight loss. Figure 4 shows the wear curves of UHMWPE and PEEK pins, in terms of weight loss (mg) versus sliding distance (km) and volume loss ([mm.sup.3]) versus sliding distance (km). A comparison of wear losses show that the wear amount of PEEK pins were significantly higher than of UHMWPE pins against stainless steel counterfaces. The wear loss in PEEK after 100 km of sliding distance is predicted as 4.20 mg (3.23 [mm.sup.3]) while it is 0.32 mg (0.34 [mm.sup.3]) for UHMWPE discs. Golchin et al are investigated the tribological behaviour of polymers against stainless steel in water lubrication and concluded that the UHMWPE provided the lowest wear of all polymers including PEEK .
In figure 5, the specific wear factors are plotted for UHMWPE and PEEK pins against stainless steel discs under bovine serum lubrication. After 100 km of sliding distance, the predicted wear factor of UHMWPE (0.34*[10.sup.-7] [mm.sup.-3]*[Nm.sup.-1]) is almost 10 fold lower than of PEEK (3.3*[10.sup.-7] [mm.sup.-3].[Nm.sup.-1]), consisted with the literature [11,13]. Figure 5 is also pointed out that the wear factor of UHMWPE was decreased with sliding distance while it was increased for PEEK. Similarly, Petrica et al have demonstrated that the wear factor was increased from low wear regime to high wear regime for PEEK, while it was decreased for UHMWPE .
The UHMWPE and PEEK pins against stainless steel discs are tested on a self-made pin-on-disc tribometer under bovine serum lubrication, and a comparison of their tribological properties are presented in this study. The friction coefficient, wear loss and wear factor of two polymers are compared. Results from the tribometer tests demonstrate that UHMWPE has a lower friction coefficient and wear factor than PEEK. From the results of current study, it is concluded that more tribological studies should be carried out regarding the use of PEEK, especially in artificial implants.
Received 1 April 2019
Accepted 7 July 2019
Published online 31 March 2020
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Ahmet Cagatay Cilingir
Department of Mechanical Engineering, Sakarya University, Serdivan, Sakarya, Turkey
* Coresponding author
E-mail address: email@example.com (Professor, Department of Mechanical Engineering, Sakarya University, Serdivan, Sakarya, Turkey)
Caption: Figure 1: Prepared disc and pin samples
Caption: Figure 2: A self-made pin-on-disc tribometer
Caption: Figure 3: Friction Coefficients: a) UHMWPE/Stainless steel and b) PEEK/Stainless steel
Caption: Figure 4: Comparison of wear curves of UHMWPE and PEEK
Caption: Figure 5: Comparison of wear factors of UHMWPE and PEEK after 20 km and 100 km sliding distances
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|Title Annotation:||Original Article|
|Author:||Cagatay Cilingir, Ahmet|
|Publication:||Trends in Biomaterials and Artificial Organs|
|Date:||Apr 1, 2020|
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