29-ANALE 2009_Chiculita

THE ANNALS OF UNIVERSITY “DUNĂREA DE JOS“ OF GALAŢI FASCICLE VIII, 2009 (XV), ISSN 1221-4590, Issue 2

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Paper presented at the

International Conference on Diagnosis and Prediction in Mechanical

Engineering Systems (DIPRE’09) 22 - 23 October 2009, Galati, Romania

TRIBOLOGICAL TESTING PROGRAMS FOR UHMWPE MATRIX COMPOSITES

Simona CHICULIŢĂ, Minodora RÎPĂ, Gabriel ANDREI

University “Dunarea de Jos”of Galati, ROMANIA

simona.chiculită@ugal.ro

ABSTRACT This paper aims to review the knowledge on materials and tribology of the ultra

high molecular weight polyethylene (UHMWPE) composites. The various mechanisms proposed for the wear behaviour of UHMWPE are closely associated with its structural characteristics. UHMWPE has a very high wear resistance, good abrasive wear resis-tance, but relatively low processing temperature range. The UHMWPE was improved by adding different reinforcements, fillers or by mixtures materials in order to improve and to enlarge the field of applicability. Test conditions are differing, depending on requirements of the actual application, specialists trying to get an acceptable compro-mise between having close similarity to actual functioning regimes and the need of simple shapes and small dimensions as required by advanced investigation equipments. KEYWORDS: Tribology, testing program, UHMWPE matrix, composite.

1. INTRODUCTION

Polymers play an important role in mechanical and materials engineering, for their easy manufacturing, low unit cost, and also for their excellent tribological performances, high corrosion resistance, low friction and damping of noise and vibrations [21, 30]. A few polymers have valuable tribological properties and many researches are directed towards this relatively limited number of polymers [76].

UHMWPEs have been defined by ASTM D1601 as: “linear polyethylenes which have a relative viscosity of 2.3 or greater, at a solution concentration of 0.05%, at 1350C, in decahydronapthalene” [93]. UHMWPE is a polymer with an unique combination of wear resistance and low-friction surface properties and, being characterized by a good corrosion resistance and impact strength. UHMWPE is a highly wear resistant polymer as compared to other

polymers, especially with polystyrene (PS), polymethil-methacrylate (PMMA), polyetheretherketone (PEEK) [22].

The structure of composite materials is presented in figure 1.

Fig. 1. Systematic illustration of the structural

components of composite materials [20].

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Among the various types of composites, the polymer composite materials (PMCs) are most frequently used nowadays.

Polymer composite materials (PCMs) belong to the group of multiphase materials; they are characterized by a softer matrix (thermoset or thermoplastic matrix) containing harder and stronger particles or fibers (typically glass or carbon). Polymer composites are known for excellent combination of mechanical properties, chemical resistance; damping vibration, coupled with tribological properties such as self-lubrication, low noise emission etc. [28]. The main applications of PCMs are concentrated on tribological components, such as gears, cams, bearings, seals, where the self-lubrication of polymers is of special advantage.

Fillers and reinforcements are used in PCMs in order to improve the mechanical properties, temperature resistance and environmental resistance, and to reduce the cost of the plastic materials [2, 51].

In figure 2 there are presented different ways to overcome the inherent weakness of polymers, various reinforcement materials and internal lubricants have been used to develop polymer composites for the improvement of the coefficient of friction and the wear rate.

Fig. 2. Structural components of polymer composite materials [21].

UHMWPE, pure or reinforced, is often an

excellent alternative to stainless steel, wood and engineering plastics, such as nylon and fluoropolymers, where wear and impact strength are very important. In recent times, there has been a remarkable growth in the large-scale production of reinforcement materials for UHMWPE matrix composites. The reinforcement materials are added to extend the application fields of polymers in tribological loaded systems. Some factors which may influence the wear behaviour of a composite are: fibre type, fibre content, fibre distribution, fibre matrix adhesion and processing temperature and pressure [46, 30].

Reinforcement materials used in polyolefin composites, like UHMWPE composites, include mainly the following: glass fibers, hollow glass bubbles, clay minerals, carbon black, carbonnanotubes, carbon fibers, graphite, wollastonite, magnesium hydroxide, aluminum trihydroxide, attapulgite, titanium dioxide, hydroxyapatite, calcium carbonate, silica, and natural fibers [54, 11]. The main roles of reinforcement materials in polymer composites are: strengthening of the matrix (high load-carrying capacity), improvement in the sub-surface crack arresting capability (better toughness), lubricating effect at the interface by decreasing shear stress and the improvement of the thermal conductivity of the polymer [21].

2. REINFORCING MATERIALS FOR UHMWPE COMPOSITES

Many UHMWPE composites have been proposed

by different research teams. UHMWPE can be both matrix and reinforcing material, plain, fiber or powder form.

2.1. Organic Reinforcing Materials for

UHMWPE Composites Overall results indicate that the self-reinforced

UHMWPE composites may be good for biomedical applications. In order to decrease the wear rate of UHMWPE, one possible solution is the mixing of the two components, the UHMWPE powder and UHMWPE fiber. Chemical compatibility of composite components and the adhesion at the interface give excellent mechanical properties, wear volume decrea-sing with the incorporation rate of UHMWPE fibers.

The polymer fibres reinforcing materials are generally used in soft matrices, due of their good strength, modulus and toughness [77]. The properties of UHMWPE fibers are competitive with fiberglass, carbon fibers and aramids, in many aspects [48].

UHMWPE micro-powders are used as reinfor-cing materials to plastics, rubbers and other materials, to enhance wear and scratch resistance, lubricity, chemical resistance and other properties.

UHMWPE is mixed, reinforced especially with polymers and is the most studied combination due to the good results: low volume density, high specific strength, and specific modulus, low volume density and high tensile strength. UHMWPE was mixed with polymers such as: HDPE, nano-epoxy, EPDM, UHMWPE, PMMA, HEMA, besides polymers were added and carbon nanotubes to obtain more efficient materials [69, 92].


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Blending of polymers is a method, widely accepted in the industry, to produce diversified polymeric materials with improved properties. The literature shows that the polymer blends possess an excellent complex of mechanical properties by equal or prevailing concentration of the first component [44, 89].

Other organic reinforcement for UHMWPE is carbon in various forms: carbon black, graphite, carbon nanotubes.

Carbon fibres could be used as an alternative to glass fibres to increase mechanical strength, to improve the sliding and wear characteristics. Graphite incorpo-rated in polymers reduced friction coefficient. Carbon black is often added to UHMWPE to meet antistatic requirements and to improve UV light resistance [8]. Carbon nanotubes (CNT) dispersed in polymer matrix are a relatively new class of nano-materials, improving the tensile behavior, strength, toughness, stiffness, electrical and thermal conductivity [6]. The CNT can be Single-walled CNTs (SWCNTs) have walls consisting of only a single graphite layer and multiwalled CNTs (MWCNTs) have walls consisting of piles of many graphite layers [68]. CNTs have a diameter of a few dozens of nm and a length of some µm. Due to this high-aspect ratio, CNTs provide a good mechanical reinforcement potential, which has been experimentally confirmed by several researches.

The properties of UHMWPE/carbon related to other frequently used engineering polymers could be find in [24] and a comparison of some mechanical properties of various commercial high performance fibers and 5 wt% MWCNT (multiwalled carbon nanotubes)/UHMWPE fibers could be found in [68].

2.2. Inorganic Reinforcing Materials for

UHMWPE Composites The incorporation of inorganic particles or

fibers in the matrix of UHMWPE is an effective method to enhance wear resistance.

Glass fibres are widely used in polymer matrix composites due to their good mechanical properties and relatively low price. Glass fibres are used mainly to increase the mechanical strength, especially tensile strength, and also to improve compression strength and temperature-dependent dimensional stability.

The UHMWPE reinforced with ceramic fibers (Al2O3, BaTiO3, TiO2), has very high temperature resistance, but it is expensive.

Avanzini A. [1] presents the polymeric composites (UHMWPE reinforced with glass or ceramic micro-spheres) tested on INSTRON testing

machine, evaluating the uniaxial tension and com-pression, planar tension and shear. UHMWPE reinfor-cement with ceramic or glass micro-spheres is usually made to improve wear or abrasion resistance. Glass fiber reinforced polymeric composites traditionally show poor wear resistance and high friction due to the brittle nature of the reinforcing fibers. Ranade et. al. [65] have studied the behavior of UHMWPE composites reinforced with glass particle.

Solid lubricants such as MoS2 when added to polymers have proved to be effective in reducing the coefficient of friction and wear rate of composites.

Kaolin is a commonly used filler. Depending on the weight, it is very effective to reduce the friction and wear of UHMWPE in sliding against steel.

The wollastonite fibers are added in polymer matrix to improve mechanical and dielectric properties, heat resistance and dimensional stability, to decrease moisture absorption and impact strength [2], to improve the abrasion resistance [79].

Metallic fillers offer the highest electrical and thermal conductivity of the composites material. One of the earliest fillers used to make plastics conductive was black carbon. The matrix UHMWPE composites may include metallic fillers such as: copper, bronze, titanium, zirconium and others. Metallic fillers increase thermal conductivity, acting as conductive path to conduct heat and electric in composites materials. The metal powders offer well defined morphology and a higher intrinsic conductivity than carbon black.

One could cite the results of researches on metal-polymer composites as Ftoroplast F-4, PE, kaprone (as matrix) and bronze, copper, nickel, iron, titanium and aluminum oxide (as metal fillers) [23]. Other researches show that the UHMWPE with added dispersing copper powder had great improvement in tribological characteristics, mechanical behaviours (anti-creep behavior) and heat conductivity [41]. The inclusion of zirconium particles into a UHMWPE matrix can effectively reduce the wear rate of the component without sacrificing the impact toughness [63]. The electrical resistivity and tensile properties of composites formed by the incorporation of metal powders such as aluminum, copper, and iron in a HDPE matrix are governed by the shape of the filler and the amount of filler content, degree of crystallinity and the adhesion between metal fillers and polymer [29].

In table 1 is presented a collection of published data about UHMWPE composites.


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Table 1. Collection of published data about UHMWPE composite. Matrix and

reinforcement Wt % Reinforced

Composites References

UHMWPE/carbon, CB UHMWPE with 1, 8wt% CB [14] Reinforced carbon [14, 16, 40, 50, 70] UHMWPE/CNTs UHMWPE nanocomposites containing up to

10wt% of nanofibres [22] Reinforced carbon [5, 13, 18, 22, 62, 80,

91] UHMWPE/MWNT UHMWPE fibers with 1, 5wt% MWCNT [82] Reinforced carbon [6, 68, 82] UHMWPE/SWNT UHMWPE with 2, 5, 10 wt% SWNT [24] Reinforced carbon [24, 74] UHMWPE/UHMWPE, PP, PET, nano-epoxy, HDPE, MMWPE, PP, etc.

UHMWPE/PP ratio (80:20) [44] blends of 70wt% poly(vi nylidene fluoride) (PVDF), 30% of UHMWPE [8]

Reinforced polymer [3, 8, 27, 28, 30, 31, 32, 42, 43, 44, 49, 52, 55, 66, 69, 72, 84, 86, 88, 89, 90, 92]

UHMWPE/CB UHMWPE/MoS2 UHMWPE/PFPE UHMWPE/CB+PFPE

UHMWPE composites filled with solid lubricants, MoS2 (3.3, 5wt%), and CB (3.3, 5 wt%), and one liquid lubricant, PFPE (1, 2, 5wt%), and also UHMWPE/2.5% CB+2% PFPE [64]

Reinforced lubricants

[38, 72, 64, 71]

UHMWPE/glass, ceramic microspheres

UHMWPE reinforced with 0.15, 0.25wt% glass microspheres [4]

Reinforced ceramic [4, 65, 77, 78]

UHMWPE/wollastonite UHMWPE composites filled with wollastonite fibers of aspect ratios 10:1, 15:1 and 20:1 have been prepared. [79]

Reinforced ceramic [79, 81, 131]

UHMWPE/HA The composite PE10/HA20 contains 10% UHMWPE, 20% HA, and 70% NaCl and the composite PE20/HA20 contains 20% UHMWPE, 20% HA, and 60% NaCl. [58]

Reinforced ceramic [45, 58]

UHMWPE/Al2O3 UHMWPE filled with 15-50wt% Al2O3 [67] Reinforced ceramic [67, 73] UHMWPE/BaTiO3 UHMWPE were filled with 25 wt% BaTiO3

[10] Reinforced ceramic [10, 59]

Nano-TiO2/UHMWPE UHMWPE were filled with 1, 3, 6, 10 wt% nano-TiO2 powder [87]

Reinforced ceramic [87]

UHMWPE/nano-MMT UHMWPE were filled with 5, 10, 15 wt.% nano-MMT [83]

Reinforced organoclay

[35, 60, 83]

UHMWPE/kaolin UHMWPE composites filled with about 17–20wt.% kaolin [25]

Reinforced organoclay

[25]

UHMWPE/copper, bronze, titanium, Zirconium, silver

UHMWPE were filled with 10, 20wt.% Zr [53] 50%UHMWPE+30%bronze+20%Quasicrystal[36]

Reinforced metal [19, 23, 26, 29, 33, 36, 41, 53, 63]

UHMWPE/α-tocopherol UHMWPE resin powder with α-tocopherol at 0.1 and 0.3wt.% and consolidated these blends [56]

Reinforced antioxidant

[56, 61, 85]

3. TRIBOLOGY OF UHMWPE

COMPOSITES

The wear and friction of non-metallic solids have some fundamental similarities with metals, there are also significant differences in the wear mechanisms involved and the level of friction or wear which occurs. The tribological properties of solids are significantly affected by environmental parameters such as lubri-cants, atmosphere, temperature, humidity and so on. Furthermore, the conditions of contact and frictional movement between the solids are very effective aspects in tribological tests, from the mechanical point of view.

Wear testing could be divided into groups related to the geometry of the contact type: point contact; line

contact; and plane contact (fig. 3). Testing could be subdivided into two categories: rolling contact type; and sliding contact type, according to the movement of the contacting components [11]. Of the direction movement point of view, sliding contact type can be classified: unidirectional friction; and reciprocating friction.

Fig. 3. Sliding contact type [94].


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A classification of the wear mechanism of polymer is described in figure 4. The explanation of the wear mechanism of polymers can be most efficiently given if we follow one of the three systems of classification. Depending on the classification, wear of a polymer sliding against a hard counterface may be termed as abrasive, adhesive, chemical wear etc. [21].

Fig. 4. Simplified approach to the classification of the

wear of polymers [21].

A compilation of the different wear mechanisms leading to material loss is shown in figure 5 [17]. Besides, there is always an overlap of wear mechanisms in any particular wear process and the combination of different mechanisms may represent the actual phenomena.

Fig. 5. Basic tribological interactions leading to wear particle generation [17].

The wear behavior of the polymer materials can

be investigated in standard or specific tests the laboratory. The pin-on-disc and block-on-ring are two

commonly applied configurations for sliding wear tests according to the Standardisation Organisations.

4. TRIBOLOGICAL TESTING PROGRAMS FOR UHMWPE MATRIX

COMPOSITES ON PIN-ON-DISC CONTACT TYPE

Tribological properties of polymer materials are

traditionally tested on small-scale test rigs because of cost-efficiency and flexibility [75].

As a representative example of the unidirectional friction type testing rig, pin-on-disc testing machines can be introduced. Pin-on-disc tests allow the determination of the characteristics of friction and wear of surfaces in sliding contact, lubricated or dry. Ussualy, the disc is rotating against a pin, the stationary sample. Rotational speed of the disc, normal load and wear trace diameter can be adjusted in accordance with the standard test (ASTM G99).

A typical curve for the variation of the friction coefficient on pin-on-disc testing machine under mild conditions is shown in figure 6.

Preliminary tribological tests were performed on the pin-on-disc module of the tribometer UMT-2 (CETR®, SUA) at the Tribology Laboratory of the University “Dunarea de Jos” of Galati, in order to characterize the pure and reinforced UHMWPE.

Fig. 6. Tipical variation of COF in a pin-on-disc test with a flat ended cylindrical pin.

The tribometer UMT-2 is a micro tribometer with

single-platform, multiple-configuration, fully-computerized unique acclaimed modular design for practically all common tribological tests on micro scales. With the module pin-on-disc on UMT-2 tribometer, friction and wear characteristics of sliding surfaces in contact under dry conditions were determined (fig. 7).


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Fig. 7. Module pin-on-disc of UMT-2: 1 - double sensor for friction and load, 2 - suspension system, 3 - support pin, 4 - pin, 5 - disc mass, 6 – disc.

Research results allow the preliminary tribological characterization of the material couple UHMWPE - 316L steel. Quality of surface manufac-turing is a key factor in improving the wear behavior, the rough surfaces with Gaussian distribution gives conditions for the decrease of the wear intensity, reducing the danger of posting steel wear particles and their incorporation into the mass of PCM.

The objective of wear evaluation is to determine the wear rate and its dependence on the test conditions (e.g. load, range of motion, lubricant and temperature).

Table 2 presents a collection of published data about testing UHMWPE and UHMWPE composites materials on pin-on-disc tribometer.

Table 2. Collection of published data about testing

UHMWPE and UHMWPE composites materials on pin-on-disc tribometer. Ref. Contact type Couple of materials Tribometer Test parameters

[105] pin-on-disc under dry friction conditions

Disc sample-Aluminum Pin sample-UHMWPE

pin-on-disc tribometer Normal pre-loads: 8-46.5 N Rotor pin speed: 50 rpm Duration of experiment: 120 min

[15]

pin-on-disc lubricants testing

Disc sample-UHMWPE Pin sample-Co–28Cr–6Mo alloy, 316L, 316L coated with TiN and TiCN

pin-on-disc tribometer, Wazau TRM1000

Normal load: 67.5 N Pressure: 0.88 MPa Tangential sliding velocity: 46 mm/s Sliding distances: 1000 m

[134] pin-on-disc under dry conditions

Disc sample- stainless steel Pin sample-UHMWPE

pin-on-disc tribometer Normal load: 30 N Sliding distance: 60…360 m

[57] pin-on-plate lubricants testing

Disc sample- stainless steel Pin sample-UHMWPE

Reciprocating testing machine for tribotesting of biomaterials

Normal load: 225 N Stroke: 25.4 mm Frequency: 1 Hz

[12] tri-pin-on-disc under dry conditions, lubricants testing

Disc sample- 316L Pin sample-UHMWPE

a custom built tri-pin-on-disc tribometer

Normal load: 75N Sliding 36.17 km/600 rpm, Distance/speed: 18.085 km/300 rpm

[40] pin-on-disc lubricants testing

Disc sample–316L Pin sample-UHMWPE

pin-on-disc wear testing system

Normal load: 315N Speed disc rotates: 120 rpm Sliding velocity of 125mm/s Sliding distance of 62.5km

[37]

pin-on-disc lubricants testing

Disc sample-UHMWPE (GUR1050) Pin sample-Co-Cr-Mo alloy

pin-on-disc wear testing machine, JT-Tohshi Inc., Japan

Contact stress: 5MPa Sliding velocity: 60mm/s Test durations 138.9h Sliding distance: 30 km.

[102] pin-on-disc under water lubricated sliding conditions

Disc sample-Ti6AI4V Pin sample-UHMWPE (GUR 4150 HP)

a conventional pin-on-disc tribometer equipped with a computer

Normal contacts stress: 5 MPa Sliding speed: 0.25 m/s Sliding distance: 105 km

[7] pin-on-disc under dry conditions, lubricants testing

Disc sample– UHMWPE Pin sample-UHMWPE, Torlon, CoCr alloy, PH17–4

pin-on-disc tribometer Normal load: 20 N Linear velocity: 0.18 m/min Angular velocity: 100 cycles/min Test duration: 10.000 cycles=1 km


Disc sample-UHMWPE Pin sample-Ti–6Al–4V

pin-on-disc tribometer Normal loads: 20N Pressure: 4 MPa Distance traveled: 40km Speed of pin: 134 rpm


Disc sample-stainless steel Pin sample-UHMWPE

pin-on-plate wear testing machine

Reciprocating speed: 1Hz Rotational speed: 1Hz Normal loads: 10, 40N


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Table 2. Collection of published data about testing UHMWPE and UHMWPE composites materials on pin-on-disc tribometer (continued).

Ref. Contact type Couple of materials Tribometer Test parameters

[71] block-on-disc under dry conditions

Disc sample- stainless steel, epoxy resin Pin sample-UHMWPE/carbon

pin-on-disc tribometer Contact pressure: 3…5 MPa Sliding velocity: 0.33 m/s Sliding distance: 284 m Relative humidity: 30 %


Disc sample-UHMWPE, UHMWPE+5-10wt% CNF Pin sample- stainless steel

pin-on-disc tribometer, unidirectional sliding tests

Contact pressure: 3.5 MPa Linear sliding distance: 13.2 km Frequency: 180 rpm, 150,000 cycles


Disc sample-stainless steel Pin sample-UHMWPE (3.87-14.19 vol.%) in cotton polyester composites

pin-on-disc sliding wear machine, model TR-201 CL-M2 Ducom, India

Normal load: 19.6, 58.8 N Pressure: 597, 1194 kPa Sliding speed: 2.22 m/s Sliding distance: 4 km

[9] pin-on-disc under dry friction

Disc sample-stainless steel, C-120 steel alloy Pin sample-polyamide 6.6 reinforced with UHMWPE

pin-on-disc tribometer Normal load: 20-100N Pressure: 0.22-1.09 MPa Sliding distance: 25…40 km Sliding velocity: 0.33…0.40 ms-1


Disc sample-stainless steel Pin sample-UHMWPE/epoxy composites

pin-on-disc tribometer model TR-201 CL-M2 Ducom, India

Normal load: 40 N Pressure: 0.78MPa Sliding velocity: 2.33 m/s Sliding distance: 8.4 km

5. CONCLUSIONS

In conclusion, new technologies require materials

showing properties improved performance. In this context, composites and especially nanocomposites are suitable materials to meet the emerging demands arising from technologic advances.

Polymers and polymer-based composites are widely used because of the combination of good mechanical and tribological properties, especially in dry friction conditions where lubricants cannot be used.

The results of this study indicate that UHMWPE and UHMWPE composites are extensively studied and improved after being tested in very different test conditions.

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