Development of new multiscale polymer composites for water lubricated tribological contacts

dissertation

Hari Shankar Vadivel (Author), Mitjan Kalin (Mentor), Nazanin Emami (Co-mentor)

Abstract

Moving towards a Green Economy, there is a growing demand to use environmentally friendly tribological systems that has resulted in industries turning towards new materials and water-based lubrication to satisfy their needs. Considering the low viscosity of water, tribological contacts lubricated with it are likely to operate in boundary/mixed lubrication regime for relatively long periods. Naturally, the most critical attributes of contact materials for water lubricated tribological systems are that they should have low friction and high wear resistance under these boundary lubricating conditions, which will inevitably be met during start-up, running, and shut down of a tribological operation. High performing thermoplastics that possess excellent mechanical properties, recyclability, low friction, high resistance to wear, corrosion, and chemical solutions are suitable candidates for demanding tribological applications. In research carried out at the Luleå and Ljubljana on numerous polymers, Ultra High Molecular Weight Polyethylene (UHMWPE) has been observed to perform well under water-lubricated conditions. However, if these polymers, including UHMWPE, are used in their pure/unflled state as tribological material in water-lubricated applications, mixed wear and friction performance with unsatisfactory service life has been obtained. One way to improve the properties and performance of a polymer is by adding reinforcements/fillers. The comxiii bined addition of micro and nano reinforcement materials to create novel multiscale polymer-based composites has shown great potential in this regard. In this thesis, UHMWPE based multiscale polymer composites for water lubricated tribological contacts are developed and evaluated for their mechanical, thermal and tribological properties. The research starts with evaluating the infuence o f p article size, molecular weight, and processing of various UHMWPE grades on their thermomechanical properties and tribological performance. It is found that all the diferent UHMWPE materials display similar thermomechanical properties and tribological performance. Based on the information gathered and after selecting one UHMWPE grade, various composites containing carbon-based reinforcements such as Nanodiamonds (ND), Graphene Oxide (GO) and Short Carbon Fibres (SCF) in diferent q uantities ( wt%) are manufactured. The Multiscale composite containing all the reinforcement materials, i.e. UHMWPE (89wt%) + GO (0.5wt%) + ND (0.5wt%) + SCF (10wt%), shows the best tribological performance. The oxidation and degradation temperatures are signifcantly delayed, indicating an improvement in service life. To gain a better insight into their service life, the developed composites are subjected to accelerated hygrothermal ageing. It is found that even after ageing at elevated temperature and humidity for a signifcant duration, the Multiscale composite’s integrity, structure and tribological performance are not afected n e gatively. F or c o ntinued r e search a n d d e velopment t o wards utilising such composites in practical applications, their time-dependent properties are evaluated. Viscoelasticity (VE) and viscoplasticity (VP) are analysed in short-term creep tests. In addition, supporting loading/unloading tests are conducted to evaluate stifness degradation. In general, the addition of reinforcements is observed to improve the time-dependent behaviour. More specifcally, t he M ultiscale c omposite d isplays t he h ighest r esistance to creep and stifness degradation. Furthermore, for better understanding of the performance of such composites in hydropower applications and to get them closer to real-world use, it is essential to verify their tribological behaviour under the relevant tribological conditions. This includes higher contact pressure and diferent lubrication c onditions, i ncluding starved ( dry), seawater and Environmentally Acceptable Lubricant (EAL). In tribological tests conducted with this premise, the performance of the Multiscale composite is found to be dependent on the type of lubrication used. As the fnal study in this thesis, the developed Multiscale composite is compared with other developed and commercial materials. It is observed that its tribological performance under demanding conditions is on par with the rest of the materials studied. To summarise the findings from all the studies; The particle size, molecular weight or processing of UHMWPE is found not to affect its thermomechanical p roperties and tribological performance. A synergistic efect is o btained i n t he M ultiscale c omposite by the successful inclusion of all the fllers. It e xhibits a 2 1% l ess c oefcient of fri ction value and 15% lower specific wear rate compared to unfilled UHMWPE under DI water lubrixiv cation. The extended service life of the Multiscale composite is evident from its delayed oxidation and degradation temperatures and ability to retain tribological performance even after undergoing hygrothermal ageing. A maximum of 77% and 70% improvement in modulus and stress at yield, respectively, is witnessed. The parameters for the viscoplastic strain model for UHMWPE composites are extracted, and the behaviour of multiscale composites for long-term performance is predicted. Under seawater lubrication, a maximum reduction of 77% in friction coefcient and 88% in specifc wear rate is obtained for the multiscale composite, compared to neat UHMWPE. Wear is reduced by 75% for the same under EAL lubrication. All these results and outcomes contribute towards the development of novel UHMWPE-based multiscale composites for water lubricated applications.

Keywords

dissertations;UHMWPE;composites;tribology;viscoelasticity;thermomechanical;

Data

Language:	English
Year of publishing:	2022
Typology:	2.08 - Doctoral Dissertation
Organization:	UL FS - Faculty of Mechanical Engineering
Publisher:	[H. S. Vadivel]
UDC:	678.7:539.92:621.89(043.3)
COBISS:	102801411
Views:	426
Downloads:	76
Average score:	0 (0 votes)
Metadata:

Other data

Secondary language:	Slovenian
Secondary title:	Razvoj novih polimerih kompozitov različnih velikostnih skal za vodno mazane tribološke kontakte
Secondary abstract:	V časih, ko stremimo v smeri zelenega gospodarstva, vedno bolj narašča povpraševanje po uporabi okolju prijaznih triboloških sistemov. To je povzročilo, da se je industrija usmerila k novim materialom in mazanju z mazivi na vodni osnovi. Glede na nizko viskoznost vode morajo zato vodno mazani tribološki kontakti delovati v režimu mejnega ali mešanega mazanja, in to dolgotrajno, tekom celotne življenjske dobe. Seveda je pri tem najbolj kritično, da morajo zagotavljati nizko trenje in visoko odpornost proti obrabi pod temi zahtevnimi pogoji tako med zagonom, kot delovanjem in zaustavitvijo delovanja. Za take zahtevne tribološke aplikacije so zato med polimeri najprimernejši visoko zmogljivi termoplasti, ki imajo odlične mehanske lastnosti, možnost recikliranja, nizko trenje, visoko odpornost proti obrabi, koroziji in različne kemične značilnosti. V raziskavah, opravljenih v Luleå-i in Ljubljani s številnimi polimeri je bilo ugotovljeno, da se polietilen z ultra visoko molekulsko maso (UHMWPE) dobro obnese v zmernih pogojih mazanja z vodo. Vendar pa podobni polimeri, in tudi UHMWPE, brez uporabe ojačitev in dodatkov v zahtevnih aplikacijah mejnega mazanja z vodo, ne dosegajo zadovoljivih rezultatov glede obrabe, trenja ter življenjske dobe. Eden od načinov za izboljšanje lastnosti in učinkovitosti polimerov je torej dodajanje ojačitev in dodatkov. Kombinirano xvii dodajanje mikro in nano ojačitvenih materialov za ustvarjanje novih večskalnih kompozitov je v tem pogledu pokazalo velik potencial. V tem doktorskem delu smo razviti in ovrednotili večskalne polimerne kompozite na osnovi UHMWPE za tribološke kontakte, mazane z vodo. Raziskava se začne z oceno vpliva velikosti delcev, molekulske mase in postopkov izdelave različnih vrst UHMWPE na njihove termomehanske lastnosti in tribološko delovanje. Ugotovljeno je bilo, da imajo vsi različni materiali UHMWPE podobne termomehanske lastnosti in tribološke lastnosti. Na podlagi zbranih informacij in po izbiri ene vrste UHMWPE smo izdelali različne kompozite, ki vsebujejo ojačitve na osnovi ogljika, kot so nanodiamant (ND), grafen oksid (GO) in kratka ogljikova vlakna (SCF) v različnih količinah (wt%). Kompozit z ojačitvami na več skalah, ki vsebuje vse ojačitvene materiale v razmerju: UHMWPE (89wt%) + GO (0,5wt%) + ND (0,5wt%) + SCF (10wt%), je izkazoval najboljše tribološke lastnosti. Pri tem kompozitu se je tudi temperatura oksidacije in razgradnje zvišala, kar kaže na izboljšanje življenjske dobe večskalnega kompozita. Da bi še bolje ocenili njihovo življenjsko dobo smo razvite kompozite izpostavili pospešenemu higrotermnemu staranju. Ugotovljeno je bilo, da se celo po dolgotrajnem staranju pri povišani temperaturi in vlažnosti, struktura in tribološko delovanje večskalnega kompozita ni poslabšalo. Nadalje smo z namenom uporabe takšnih kompozitov v praktičnih aplikacijah raziskave in razvoj usmerili tudi v njihove časovno odvisne lastnosti. Analizirali smo viskoelastičnost (VE) in viskoplastičnost (VP) v kratkotrajnih testih lezenja. Poleg tega smo izvedli še podporne preskuse cikličnega obremenjevanja, da smo ocenili vplive na togost kompozita. Na splošno opazimo, da dodajanje ojačitev izboljša časovno odvisno vedenje. Natančneje, večskalni kompozit kaže najvišjo odpornost na lezenje in zmanjševanje togosti. Za boljše razumevanje delovanja večskalnih kompozitov v hidroenergetskih aplikacijah in za njihovo približevanje uporabi v resničnem svetu smo dodatno preveriti njihovo tribološko obnašanje v ustreznih triboloških pogojih. To vključuje višji kontaktni tlak in različne pogoje mazanja, vključno s pomanjkljivim mazanjem (suhim), morsko vodo in klasičnim okolju prilagojenem mazivu (EAL). V triboloških testih, opravljenih s temi cilji, je bilo ugotovljeno, da je zmogljivost večskalnega kompozita odvisna od vrste mazanja. Kot zaključno študijo v tem doktorskem delu pa smo razviti večskalni kompozit primerjali z nekaterimi drugimi razvitimi in komercialnimi materiali. Ugotovljeno je, da je njegova tribološka zmogljivost v zahtevnih pogojih podobna ali enaka ostalim materialom. Če torej povzamemo ugotovitve vseh izvedenih študij v okviru doktorskega dela, lahko na kratko zapišemo; ugotovljeno je bilo, da velikost delcev, molekulska masa ali postopek izdelave UHMWPE ne vplivajo na njegove termomehanske lastnosti in tribološko delovanje. Z uspešnim vključevanjem vseh predvidenih polnil in dodatkov, je bil v večskalnem kompozitu dosežen sinergijski učinek. Ta kompozit izkazuje 21% nižji koeficient trenja in 15% nižjo specifično stopnjo obrabe v primerjavi z osnovnim UHMWPE xviii pri mazanju z vodo. Podaljšana življenjska doba večskalnega kompozita je razvidna iz njegove višje temperature oksidacije in razgradnje ter sposobnosti, da ohrani dobro tribološko delovanje tudi po higrotermičnem staranju. Opazili smo do 77% oziroma 70% izboljšanje modula in napetosti tečenja. Določeni so bili parametri za model viskoplastične deformacije za kompozite UHMWPE in napovedano je bilo obnašanje večskalnega kompozita za dolgotrajno delovanje. Pri mazanju z morsko vodo se z večskalnim kompozitom doseže največje zmanjšanje koefcienta trenja, kar za 77%, in specifčne stopnje obrabe zmanjšane celo za 88%, v primerjavi z osnovnim UHMWPE. Pri enakih razmerah se pri mazanju z mazivom EAL obraba zmanjša za 75%. Dobljeni izvirni rezultati bodo prispevali k nadalnjemu razvoju novih večskalnih kompozitov na osnovi UHMWPE za aplikacije z vodnim mazanjem.
Secondary keywords:	kompoziti;UHMWPE;tribološki kontakti;polimerizati;polietilen z ultra visoko molekulsko maso;Tribologija;Disertacije;
Type (COBISS):	Doctoral dissertation
Study programme:	0
Embargo end date (OpenAIRE):	1970-01-01
Thesis comment:	Univ. v Ljubljani, Fak. za strojništvo
Pages:	XXIV, 211 str.
ID:	14834751