MAE528 Project report Suggala_Bhadoria

MAE-528 Tribological Properties of Cryo-Treated Polymer CompositesProject Report

Project Report

Advanced Composite Manufacturing

MAE 528

Tribological Properties

Of

Cryo-treated Polymer Composites

December15, 2014

By;

Suggala Indraneel (012513187),

Bhadoria Sanchit Singh (012524159)


Table of Content:

1. Introduction

1.1. Tribology

2. Cryogenic Treatment

2.1. Significant Factors in CT2.2. Types of Cryogenic Treatment2.3. Cooling Systems in CT

3. Experimental Set Up

3.1. Assigned values3.2. Composites Examined3.3. Cooling Mediums

4. Results

4.1. Crystallinity4.2. Hardness4.3. Wear4.4. Friction4.5. Polymer composites with varied proportions of fillers

5. Conclusion

6. References


1. Introduction:

Composites has extensively replaced metals in various applications due to their advantageous properties, like non-corrosive, lightweight, good manufacturability. In this report we show how a composite material treated at sub-zero temperatures achieves better tribological properties, with this enhancement the use expands in space rockets, gears, bearings, bullet proofing, transportation of liquid hydrogen, aero planes etc.

1.1 Tribology:

The Science of Tribology concentrates on contact mechanics of moving interfaces that generally involves Energy dissipation. In simple words, “the study of things that rub”

Tribological Properties:

Wear Friction Lubrication Scratch Resistance

The effect on these properties is discussed in detail in the report when composite is treated at low temperatures or at specific cryogenic temperatures.

2. Cryogenic Treatment:

A cryogenic treatment is the process of treating work pieces to sub-zero temperatures (i.e. below −190 °C (−310 °F)) to remove residual stresses and improve wear resistance, friction, hardness, etc. in composites. The effects are at molecular level; usually the low temperature arranges the molecules in an orderly manner and strengthens the bond between them to achieve better properties.

A basic CT consists in a gradual cooling of the component until the defined temperature, holding it for a given time (freezing time) and then progressively leading it back to the room temperature. The aim is to obtain an improvement of mechanical properties, typically hardness and wear resistance, but in recent tests fatigue limit too, and to achieve an optimal ratio between conflicting properties, like hardness and toughness.

2.1. Significant Factors in CT:

• Soaking Temperature: This phase indicates the temperature at which work piece is treated, usually these are sub-zero temperatures (77K, 193K). Authors have given this phase very much importance in contributing towards the enhancement of composite properties.

• Soaking Time: The time for which work pieces are held at the treatment temperatures is called as the Soaking Time, again it varies depending upon the composite being treated and the researcher experimenting. The phase holds importance but less when compared with soaking temperature.


Cooling rate: After a work piece is held at a certain temperature and for dedicated time period, it is brought to the room temperature either gradually or rapidly depending upon the type of cooling system used for the process.

The graph below shows the different phases of above explained processes. Composite is accordingly treated for acquiring the desired properties as per the application.

[Fig 1] DCT temperature profile [4]

2.2. Types of Cryogenic Treatment:

Depending on the minimum temperature reached during the treatment, cycles are categorized as:

• Shallow Cryogenic Treatment (SCT): In this treatment the samples are placed in a freezer (rapid introduction of composite to sub-zero temperature) at soaking temperature, usually 193K; held at this temperature until soaking time and then is exposed to room temperature at instance. This process is not recommended and is thus not preferred by authors as rapid introduction of composite to variable temperatures cause’s damages to the matrix or the fiber, which in stead lowers the tribological properties of it.

Deep Cryogenic Treatment (DCT): In this technique samples are gradually cooled to 77K (or a certain soaking temperature), held for many hours (soaking time) and then again brought to room temperature with moderate increase in temperature. Because this technique does not involve rapid change of temperature, it is more accepted and widely used.


2.3. Cooling Systems in CT:

• Heat Exchanger: A simple working of basic heat exchanger with liquid nitrogen flowing through the pipes, this nitrogen is then outputted as cooled gas, which is introduced in the chamber by a fan. No actual contact between nitrogen and work piece is made.

• Gradual immersion: Work piece is immersed into liquid nitrogen for specific time, then extracted and gradually lead back to room temperature. This method also results in damaging the composite due to sudden variation in temperatures, thus is avoided.

• Direct Nebulization: Most used and preferred method. Liquid nitrogen is nebulized (fine spray) in the chamber. A fan allows homogeneous temperature distribution of the nitrogen in the chamber around the composite to treat it perfectly.

[Fig 2] Sketch of a Direct Nebulization Cryo-System [4]

Above figure shows a commonly used cryo-treatment. A chamber where work piece is placed consists of a sensor which keep knowledge of temperature inside it, this helps to attain perfect temperature inside the chamber and also maintains it throughout the process. This sensor sends data to CU, which also controls the injection of nitrogen through controlling Injectors and flow of nitrogen through Electric Valve coming from the Tank. An electric engine is also placed at the entry of chamber which operates the fan, this fan allows homogeneous distribution of nitrogen through the chamber and around the composite.

C - Chamber

S - Sensor

CU - Control Unit

EV – Electric Valve

E – Engine

I – Injector

F – Fan

T - Tank


3. Experimental set-up:

A series of polymer composites are selected for cryogenic treatment along with filler added PEI composites. A set of base matrices were selected on the basis of crystallinities and proven tribo-potential either in adhesive or abrasive wear modes. PTFE and its composites which exhibit excellent wear performance in adhesive wear modes, PI which was known as a very good abrasive wear resistant material etc., were selected as the experimental material.

3.1. Pin-on-disc apparatus was used, hardened and polished carbon steel disc (German standard Cr6)

• Radius = 16.5mm• Sliding Speed, v = 0.2 m/s• Contact load, Fn = 5 N• Sliding Distance, s= 1800m• Counter body = AISI 304 steel

3.2. Composites examined• PTFE (Polytetrafluoroethylene), • PA6 (Polyamide6 (nylon))• PI (Polyimide)

3.3. Experimental medium• Room temperature • LH2

• LHe (4.2K)• LN2 (77K)

4. Results and discussion:

4.1 Crystallinity

Crystallinity refers to the degree of structural order in a solid. In a crystal, the atoms or molecules are arranged in a regular, periodic manner. The degree of crystallinity has a big influence on hardness, density, transparency and diffusion.Perhaps no fundamental property affects the physical properties of a polymer in so general a way as the degree of crystallinity.


[Fig 3] DSC curve of Untreated and Cryo-Treated PEI and PTFE [1]

Differential scanning calorimetry (DSC) provides a rapid method for determining polymer crystallinity based on the heat required to melt the polymer. DSC is a technique that measures heat flow into or out of a material as a function of time or temperature. Polymer crystallinity can be determined with DSC by quantifying the heat associated with melting (fusion) of the polymer. This heat is reported as Percent Crystallinity by normalizing the observed heat of fusion to that of a 100 % crystalline sample of the same polymer.

Crystallinity measurement is done on the selected PEI, PTFE and comparison is performed with the DSC results. The Curves indicate that glass transition temperature (Tg) increases from 215.30oC to 252.91oC in case of PEI and from 335.93oC to 341.35oC in PTFE, The glass transition temperature (Tg) of a plastics is the point at which a reversible transition of amorphous phases from a hard brittle condition to a visco-elastic or rubber-elastic condition occurs. Due to the sub-zero temperature what we are inputting, the activation energy is exceeded which intern allows molecular chains to slide past each other hence increasing Tg which shows the increase in crystallinity of the polymer.

4.2 Hardness: Hardness is the resistance of a material to localized deformation. In most polymers, the deformation is carried is plastic deformation of the surface. For some polymers, the hardness is defined at the resistance to elastic deformation of the surface. When the composite polymer is treated under cryogenic environment, Crystallinity increases with the increased Tg.

Hardness measurement is performed on PTFE using Rockwell treatment and comparison is done with help of scanning electron microscope. (For untreated and cryo-treated PTFE)


[Fig 4] SEM of PTFE (a) untreated (b) magnified view (c) cryo-treated (d) magnified view [1]

If we consider in a polymer composite, at low temperatures the movement of the macromolecules is hindered, bonds are frozen in. Therefore, temperature differential contraction takes place between fiber and matrix leading to fiber-matrix interfacial bonding exhibiting increased hardness.In the case of PTFE which is already 99% crystalline, at sub-zero temperatures as Tg is increased, the isolated single molecules or particles assembled together (Agglomeration) and a dense network of micro fibrils originated for each particle (Fibrillation) in the composite. This typical behavior of agglomeration and excessive fibrillation leads to increase of hardness. This increased hardness and crystallinity lead to enhancement of wear and friction properties.

4.3 Wear

Wear is erosion or loss of dimension from plastic deformation if the composite is originated at the interface between two sliding surfaces. To enhance the wear properties, cryogenic treatment is done on a series of thermoplastic engineering polymers such as PA6 (polyamide6 (nylon)), PTFE (poly tetra floro ethylene), PI (polyimide) at different temperature mediums with Liquid Nitrogen, Liquid Hydrogen, Liquid Helium. Investigation using pin-on-disc apparatus is done on the untreated and cryogenic treated composite polymers to evaluate the cryogenic treatment effect up on the polymers.


Graph analysis says PA6, PTFE performed enhanced results under cryogenic temperature in all different temperature mediums. During the solidification stage of manufacturing, some of the molecules get caught in a random pattern. But molecules move at subzero and deep cryogenic temperatures, albeit slowly. The molecules do move and form into a tighter denser, realigned pattern. When ramp up to room temperature, the molecules stay in the new relationship, producing less random and more even spacing, which in turn reduces the open areas between the grid matrixes to one another. The resulting product possesses an improved bonding of molecules to each other and a better wear pattern.

In the case of PI, a negative impact is seen in liquid hydrogen and liquid helium medium. With the fillers added PI, Positive and negative effects are observed. From here it is clear that the material composition plays a key role in impacting the properties of the polymer composites.

4.4 Friction

Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. Along with the Wear analysis, friction analysis is also done up on the same polymer composites.


From the graph, the results show that after the cryogenic treatment polymers showed desirable output with decreased coefficient of friction in almost all the three different temperature mediums. Except in the liquid helium environment, significant properties are achieved.

At low temperature bonds are frozen in and movement of macro molecules are hindered resulting,

(1) The increase of stiffness of the polymer composites at low temperatures. This effect can reduce the deformation of the polymer composite pins when being in contact with the metal counterpart under a certain load, which, in turn, can reduce the real contact area of the two counterparts(2) The liquid lubricating effect of the cryogenic media (either liquid nitrogen or liquid hydrogen) between the two contact partners. This effect can probably form a thin liquid (or even partly gaseous) lubricating film between the two counterparts, thus contributing to a reduction of the shear stresses during sliding. Besides these interpretations, the thermoplastics, PEI, and PA6, 6, generally perform better than the thermosetting resin, in spite of similar filler types and amounts. The reason for this is probably due to the fact that thermoplastics still maintain a better toughness in comparison to thermosetting resins, even though it is generally reduced at lower temperatures.

4.5 Polymer composites with varied proportions of fillers

A considerable enhancement of the properties in the polymer composites were seen under cryogenic treatment. So, a study is done on the experimenting polymer composite by adding different fillers with varied proportions. This study is performed to understand the microstructural change in the material due to cryogenic treatment.

When composites are subjected to LN2 temperature, differential contraction takes place between fiber and matrix leading to fiber matrix de-bonding. This effect become more pronounced as fiber concentration increases. Large internal stresses do get generated because of the thermal shock and residual stresses are generated. When stress relaxation does not occur, it leads to micro structure cracking.

[Fig 10] a) Untreated and b) Cryo-Treated PTFE Composite at different Loads [1]


Fig [11] a) Untreated and b) Cryo-Treated PEI Composite at different Loads [1]

Fig [12] a) Untreated and b) Cryo-Treated PI Composite at Different Loads [1]

From the above graphs with the varied fiber concentrations, following are the salient observation. Cryo-treatment has certainly indicated a potential to enhance the abrasive wear performance of the material. However the extent of improvement dependent on the material and in the case of composites, it depended on the types of fillers and fibers also. In the case of neat polymers, abrasive wear performance improved for all the polymers due to cryo-treatment. Enhancement was maximum for PEI (=~30%) and PTFE (=~60%). For PI and PEI copolymer it was, however, small (=~2%). Increase in hardness due to cryogenic treatment was found to be prominent factors responsible for enhancement in wear performance of polymers and composites. Cryo-treatment proved effective in the case of composite containing PTFE and graphite. Cryo-treatment worked synergistically for a combination of filler viz, PTFE and graphite.


5. Conclusions

• Friction and wear characteristic of composite system depends upon the type of fabric material• Optimized Cryo-Treatment can improve the Tribological properties of the composites [3]• Excessive treatment may negatively affect the polymer [3]• Excessive fibre and fillers may give a negative impact• Gradual immersion is not advisable and affect micro structure of polymer composite• Care must be taken while manufacturing a Hybrid Polymer composite• Special attention and temperature measurement will require further investigation, due to complex

contact conditions [5]• Investigations on the effectiveness of Cryo-Treatment on the performance of polymer composites

have been reported and has proved to be an effective technique enhancing hardness, crystallinity, abrasive wear and frictional coefficient.


6. References:

[1] J. Indumathi, J. Bijwe, A.K. GhoshWear, M. Fahim, N. Krishnaraj.

Wear of Cryo Treated Engineering Polymers and Composites.

[2] W. Hubner, T. Gradt, T. Schneider, H. Borner.

Tribological Behavior of Materials at Cryogenic Temperature.

[3] Zhao-Zhu Zhang, Hui-juan Zhang, Fang Guo, Kun Wang, Wei Jiang, 2009.

Enhanced Wear Resistance of Hybrid PTFE,Kevlar Fabric,Phenolic Composite by Cryogenic Treatment.

[4] P. Baldissera and C. Delprete, 2008.

Deep Cryogenic Treatment A Bibliographic Review.

[5] G. Theiler, W. Hubner, T. Gradt, P. Klein, K. Friedrich, 2002.

Friction and Wear of PTFE Composites at Cryogenic Temperatures.

[6] Susheel Kalia, 2009.

Cryogenic Processing: A Study of Materials at Low Temperatures.

[7] Geraldine Theiler, Thomas Gradt, 2006.

Influence of Hydrogen Environment on the Tribological Performance of Polymer Composites.

[8] B. Suresha and G. Chandramohan, 2006.

Friction and Wear Characteristics of Carbon-Epoxy and Glass Epoxy Woven Roving Fiber Composites.

[9] Zhong Zhang, Patrick Klein, Geraldine Theiler and Wolfgang Hubner.

Friction of Polymer Composites in Liquid Hydrogen Media.

[10] K. S. Mahesh Lohith, V. B. Sondur, V. V. Sondur, 2013.

Influence of Cryogenic Treatment on the Friction Coefficient of Nylon 6 and Caprolactam - Graphite Composite

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MAE528 Project report Suggala_Bhadoria