Cryogenics 58 (2013) 1–4

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Cryogenics

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Tribological properties of ultra-high molecular weight polyethyleneat ultra-low temperature

0011-2275/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.cryogenics.2013.05.001

⇑ Corresponding author. Tel.: +86 051683591916.E-mail address: liuht@cumt.edu.cn (H. Liu).

Liu Hongtao ⇑, Ji Hongmin, Wang XuemeiSchool of Materials Science and Engineering, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China

a r t i c l e i n f o

Article history:Received 20 December 2012Received in revised form 22 April 2013Accepted 7 May 2013Available online 17 May 2013

Keywords:Ultra-high molecular weight polyethyleneUltra-low temperatureTribological properties

a b s t r a c t

The hardness, compression properties, creep resistance and tribological properties of ultra-high molecu-lar weight polyethylene at ultra-low temperature were researched in this paper, and the feasibility of itsuse in low temperature components was explored. Studies had shown that the UHMWPE sample at ultra-low temperature had a brittle tendency, and its compression curve was similar to the brittle material, forwhich the brittle fracture occurred in the 20% compression. Besides, the creep resistance of the sample atlow temperature got worse, and its hardness showed an increasing tendency. With the increased exper-imental load, the friction coefficient varied seriously, and during the same load, the friction coefficient atlow temperature was higher than that at room temperature. According to the worn morphology, the sam-ple at low temperature showed a typical feature of fatigue wear and abrasive wear, while at room tem-perature it mainly for abrasive wear.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction 6000.000). Test pressure was 15 MPa, and the test temperature

With the development of science and technology, the applica-tion of cryogenic technology continue expanding, low temperaturematerial is an important guarantee of the reliability of cryogenictechnology [1,2]. Since the material properties at low temperaturewill change drastically, the selection of low temperature materialsmust be strictly [3,4]. At present low temperature materials aremostly austenitic stainless steel, aluminum alloy, titanium alloy,nickel-based alloy and other materials [5–8]. Ultra-high molecularweight polyethylene (UHMWPE) is a kind of linear engineeringthermoplastics and its comprehensive performance is excellent.[9]: low density, only 1/8 of the steel material; high impactstrength; good ductility; good abrasion resistance; excellent chem-ical resistance; good climate resistance and good self-lubricatingproperties [10–15], it can also be used as low temperature mate-rial. This paper studied the compression, friction, creep resistance,ball-indentation hardness properties of UHMWPE at ultra-lowtemperature (liquid nitrogen environment), so as to explore its fea-sibility as ultra-low temperature material.

2. Materials and methods

2.1. Preparation of the materials

The sample was made by thermoforming equipment using ul-tra-high molecular weight polyethylene powder (molecular weight

was 473 K, keeping 1.5 h.

2.2. Experiment

The experiment was done respectively at ultra-low temperatureand room temperature. In this paper, ultra-low temperaturemeaned under liquid nitrogen environment (78 K), and room tem-perature was of 298 ± 2 K.

2.2.1. Ball-indentation hardness testingThe indentation hardness was tested using a hard ball with

small diameter, which was perpendicular pressed in the samplesurface under the experimental load, and was calculated in theaverage pressure of per indentation area (see Fig. 1).

The test instrument was UMT-II microfriction tester, and thereference standard was ISO-2039-1-2001. The experiment wasdone with CoCr 15 ball whose diameter was 4 mm, and the pre-pressing load was 9.8 N, test load was 49 N. The indentation hard-ness conversion formula is:

H ¼ 0:21P0:25pDðh� 0:04Þ

where H is the sample indentation hardness (N/mm2); D is the balldiameter (mm); h is the maximum indentation depth (mm); P is thetest load (N). The experiment takes the average of five repeatedtests as the final result.

Fig. 1. The schematic drawing of the ball-indentation hardness test.

Fig. 2. The variation of indentation depth of UHMWPE at different temperature.

Fig. 3. The stress–strain curve of UHMWPE at room temperature and lowtemperature.

2 H. Liu et al. / Cryogenics 58 (2013) 1–4

2.2.2. Creep performance testingThe creep performance testing was tested on the UMT-II micro-

friction tester, preloading load was 9.8 N, keeping 20 s, and thenapplied constant load 132 N, keeping 30 min. When the loadreached 132 N, setting the indentation depth at this time as 0,recording the depth data.

2.2.3. Compression performance testingThe stress of wear-resisting material in actual use was com-

monly compressive stress. The reference standard of compressionexperiment was DIN EN ISO 604-2003. Sample size was10 mm � 10 mm � 5 mm, the test instrument was CSS-400 elec-tronic universal testing machine, and the test speed was 2 mm/min. And the stress and strain were calculated using the followingequations:

r ¼ P=Ao

where r was the stress, P was the experimental load, and Ao was thesectional area.

e ¼ ðLo � LÞ=Lo

where e was the strain, L was the length of the sample after defor-mation, Lo was the original length of the sample.

2.2.4. Friction and wear testingThe friction and wear performance test was done on UMT-II

friction and wear testing machine, adopting reciprocating slidingfriction test. The friction pair were CoCr 15 ball (U = 4 mm) andthe block UHMWPE (20 mm � 10 mm � 5 mm). The test were doneat room and low temperature in the 68.6 N, 78.4 N, 88.2 N, 98 N,107.8 N, 117.6 N, 127.4 N seven different applied load, sliding dis-tance of 5 mm, and the speed of 1 mm/s, each friction time was120 min. Finally the sample surface morphology was analyzed byscanning electron microscopy.

3. Experimental results and discussion

3.1. The effects of low temperature on the creep performance

Fig. 2 shows the variation of the indentation depth of UHMWPEat different temperature. As was shown in Fig. 2, at low tempera-ture, the indentation depth of UHMWPE reached to the maximumin a very short time, after which the curve tended to moderate andslighted ups and downs. This may due to the shortcomings of thesample microstructure, the microscopic lattice would be brokenat ultra-low temperature. While at room temperature, the indenta-tion depth of UHMWPE increased rapidly, but the increasing rate

was slower than that of low temperature, then the indentationdepth increased more slowly and the curve turned to flat, whichshowed the sample performance was more stable at room temper-ature. The indentation depth of UHMWPE at low temperature was190 lm, while at room temperature it was 160 lm, it can be seenthat the creep resistance of UHMWPE at room temperature wasbetter than that at low temperature.

3.2. The compression performance of UHMWPE at ultra-lowtemperature

According to the stress-strain curve (see Fig. 3), the sample atultra-low temperature reflected the compression characteristicsof brittle material, the compressive stress was 90.56 MPa. The sam-ple at low temperature occurred breakage during the small defor-mation, the cross-section and axis was about 45�, and themaximum shear stress existed on this cross section. While thecompressive stress was 23.24 MPa at room temperature, the sam-ple occurred significantly deformation, the height got shorter, andthe middle part presented cydariform. With the stress gradually in-creased, the sample kept on deformation, finally the load increasedrapidly while the compressive strain varied little. The sample of

Fig. 5. The friction coefficient of UHMWPE under different load at room temper-ature and low temperature.

H. Liu et al. / Cryogenics 58 (2013) 1–4 3

UHMWPE did not failure after compressive yield. Therefore thecompressive stress of UHMWPE at ultra-low temperature washigher than that at room temperature, and its compressive proper-ties can meet the use of low temperature.

3.3. The ball indentation hardness of UHMWPE at ultra-lowtemperature

According to Fig. 4, the ball indentation hardness of UHMWPEat room temperature was 24.98 MPa, while at low temperature itwas 28.89 MPa. The reason can be concluded as follows: the micro-structure of the materials at low temperature appeared more clo-sely, which made the movement of the molecular chain becomemore difficult. The elasticity of the sample was lost gradually,while its stiffness increased. When the temperature dropped to acertain low temperature, the movement of molecular chain wascompletely frozen, the ductility of the material was lost, and theslip of the molecular chain suffered a greater obstacle. However,the ball indentation hardness of the sample at low temperaturehad a little change, similarly to that at room temperature, whichalso showed good performance.

3.4. The tribological properties of UHMWPE at ultra-low temperature

3.4.1. Friction coefficientThe friction coefficient of the sample under different load at low

temperature and room temperature were tested. The experimentalpressure was 68.6 N, 78.4 N, 88.2 N, 98 N, 107.8 N, 117.6 N, 127.4 Nrespectively. Fig. 5 showed the comparison of the friction coeffi-cient under different load at low temperature and roomtemperature.

As can be seen from Fig. 5, the friction coefficient of UHMWPE atroom temperature had something in common with that at lowtemperature. It showed an increasing tendency when the loadchanged from 68.6 N to 78.4 N, and it appeared a decline whilethe load was over 78.4 N. According to the two curves, it can beseen that under the same load, the friction coefficient at low tem-perature was higher than that at room temperature, mainly be-cause of the hardness of the sample asperities at ultra-lowtemperature was higher, and the mechanical gearing effect wasbetter. When the load was light, the asperities of the friction pairmutual embedded little, and the friction coefficient was low; withthe load gradually increased, the asperities embedded more andthe mechanical gearing effect was enhanced, which caused theincreasing of the friction coefficient. When the load increased toa certain extent, the friction pair running-in rapidly, the asperities

Fig. 4. The ball indentation hardness of UHMWPE at room temperature and lowtemperature.

occurred large yield and finally be polished, meanwhile the frictioncoefficient appeared to decline. Compared to room temperature,the wear phenomenon of UHMWPE at low temperature appearedmore seriously, but the UHMWPE can still be used as a cryogenicmaterial.

3.4.2. The wear lossFigs. 6 and 7 showed the morphology of the worn sample at low

temperature and room temperature with a magnification of 100times, and the experimental load was 68.6 N. The worn sample

Fig. 6. The morphology of the worn sample at low temperature.

Fig. 7. The morphology of the worn sample at room temperature.

Table 1The average wear loss of the samples under different pressure and temperature.

Wear loss Load

68.6 N 78.4 N 88.2 N 98 N 107.8 N 117.6 N 127.4 N

Room temperature (mg) 2.63 2.67 2.82 2.91 3.11 3.19 3.25Low temperature (mg) 3.11 3.18 3.33 3.41 3.54 3.57 3.58

4 H. Liu et al. / Cryogenics 58 (2013) 1–4

in Fig. 7 showed more serious tearing effect, and the worn surfacealong with the moving direction appeared obvious deformation,this mainly due to the good plastic deformation capacity ofUHMWPE at room temperature. It can be seen that the wear mech-anism of the sample at room temperature mostly were abrasivewear. While the sample at low temperature showed higher hard-ness and the feature of fragile material, the mechanical gearing ef-fect of the sample asperities were better during the friction effect,showing a typical feature of fatigue wear and abrasive wear.

According to the grinding crack image and the amplificationproportion of the wear morphology, the width of the grindingcrack was observed and the wear loss was calculated as shownin Table 1. Table 1 showed that the wear loss increased with theexperimental pressure, and under the same pressure the wear lossof low temperature was bigger than that of room temperature. Thiscan also explain the friction coefficient of low temperature werebigger than that of room temperature.

4. Conclusions

The following conclusions can be drawn from this study:

(1) The sample at ultra-low temperature would have a largerindentation depth within 3 min, which can reach to190 lm, and the curve of the indentation depth had aslightly fluctuating. This can be due to the pores and defectsexisting on the microstructure of the sample. While theindentation depth of the room temperature only can reach175 lm, 92% of that at low temperature. Thus, the creepresistance of UHMWPE at low temperature was almost thesame with that at room temperature.

(2) The compressive strength of the sample at ultra-low tem-perature was 90.56 MPa, compared to 23.24 MPa at roomtemperature. Therefore the compressive strength ofUHMWPE at ultra-low temperature was higher than thatat room temperature, and its compressive properties canmeet the use of low temperature.

(3) The ball indentation hardness of UHMWPE at room temper-ature was 24.98 MPa, while at low temperature it was28.89 MPa. It indicated that the UHMWPE at low tempera-ture also showed a good performance.

(4) The trend of the friction coefficient at room temperature hadsomething in common with that at low temperature. During

the same load, the friction coefficient at low temperaturewas higher than that at room temperature. According tothe worn morphology, the sample at low temperatureshowed a typical feature of fatigue wear and abrasive wear,while at room temperature it mainly for abrasive wear.

Acknowledgement

This work was supported by the Fundamental Research Fundsfor the Central Universities (NO. 2012LWA01).

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