• Sonuç bulunamadı

Mo-n-cu Nanokompozit Kaplamaların Yağlı Ortamda Tribolojik Davranışlarının İncelenmesi

N/A
N/A
Protected

Academic year: 2021

Share "Mo-n-cu Nanokompozit Kaplamaların Yağlı Ortamda Tribolojik Davranışlarının İncelenmesi"

Copied!
113
0
0

Yükleniyor.... (view fulltext now)

Tam metin

(1)

İSTANBUL TECHNICAL UNIVERSITY  INSTITUTE OF SCIENCE AND TECHNOLOGY

M.Sc. Thesis by Sabri ÇAKIR

TRIBOLOGICAL PROPERTIES OF IN LUBRICATED ENVIRONMENT

Mo-N-Cu NANOCOMPOSITE COATINGS

Department : Metallurgical and Materials Engineering Programme : Materials Engineering

(2)
(3)

İSTANBUL TECHNICAL UNIVERSITY  INSTITUTE OF SCIENCE AND TECHNOLOGY

M.Sc. Thesis by Sabri ÇAKIR

(506081417)

Supervisors (Chairmans) : Prof. Dr. Mustafa ÜRGEN (ITU) Asst. Prof. Dr. Özgen AKALIN (ITU) Members of the Examining Committee : Prof. Dr. Mehmet KOZ (MU)

Assoc.Prof Dr. Kürşat KAZMANLI (ITU) TRIBOLOGICAL PROPERTIES OF

IN LUBRICATED ENVIRONMENT

Mo-N-Cu NANOCOMPOSITE COATINGS

JULY 2010

Date of defence examination: 16 July 2010 Date of submission : 16 July 2010

(4)
(5)

İSTANBUL TEKNİK ÜNİVERSİTESİ  FEN BİLİMLERİ ENSTİTÜSÜ

YÜKSEK LİSANS TEZİ Sabri ÇAKIR

(50601417)

Mo-N-Cu NANOKOMPOZİT KAPLAMALARIN İNCELENMESİ

YAĞLI ORTAMDA TRİBOLOJİK DAVRANIŞLARININ

Doç. Dr. Kürşat KAZMANLI (ITU) Prof. Dr. Mehmet KOZ (MU)

Yard. Doç. Dr. Özgen AKALIN (ITU) Tez Danışmanı : Prof. Dr. Mustafa ÜRGEN (ITU) Diğer Jüri Üyeleri :

TEMMUZ 2010

Tezin Enstitüye Verildiği Tarih : 16 Temmuz 2010 Tezin Savunulduğu Tarih : 16 Temmuz 2010

(6)
(7)

v

FOREWORD

I would like to express my special appreciation to my advisors Prof. Dr. Mustafa ÜRGEN and Asst. Prof. Dr. Ozgen AKALIN for their support, sharing experiences and laboratories, advisory and also there have been open doors for sharing ideas. This study was a part of the project “Power Cylinder Design for Improved Lube Oil Consumption” supported by Ford Otosan A.S. Special thanks are given to Mr. Ömer RüĢtü ERGEN and Mr. Göktan KURNAZ of Ford Otosan.

I would like to express my special thanks to Mr.Hayrettin DEMIR and his colleagues in SES Ozalit, 4. Levent for their attentive and rapid performance.

During my thesis work, I learned many things about various subjects from my mentors. They let me share their days and nights, happiness and sorrow. This thesis would not have been possible without them. I would like to acknowledge my mentors who are Assoc. Prof. Dr. KürĢat KAZMANLI, Asst. Prof. Dr. Vefa EZĠRMĠK and Dr. Zafer KAHRAMAN.

I would like to thank Sevgin TÜRKELĠ, Çiğdem KONAK, Hüseyin SEZER and Talat ALPAK for their efforts and recommendations about characterization and imaging. Also I would like to thank research assistants Berk ALKAN, Onur MEYDANOĞLU and Beril AKINCI for their support about preparation of samples and characterization works.

During laboratory works (Preparation of samples, Coating, Cleaning, Testing, Commenting, etc), my friends helped me to cope with all the stuff. I would like to thank Esma ġENEL, Güliz GÜRLÜK, Can TALI, Burak GÜLLAÇ, Semih OTMAN, Alperen SEZGĠN, Oğuz YILDIZ, Serkan OKTAY, Ġkram AHAD, Sebahattin GÜVENDĠK, Özgen AYDOĞAN, Münevver UZUN DOĞDUASLAN, Nagihan SEZGĠN, Duygu ĠġLER, Sinan AKKAYA, Sinem ERASLAN, Erdem ARPAT, Semih ÖNCEL. Also, I would like to thank other friends which shared same classes and ideas with me. I would like to send a special thanks to “Master Vefa and Apaches”.

I would like to acknowledge my BSc. advisor and my current boss Prof.Dr. Nafiye Güneç KIYAK for all of her support and encouraged me to do. Also I would like to thank my friends Sakip ONDER, Tuğba OZTURK, Öznur KAYMAK MUHTAROGLU, Pınar ASLAN, Elif KAYA, Emine GÜNAY, and Cem ERSOZ. Last but not least, maybe one of the greatest acknowledgments goes to my beloved family (Super 9). Thanks to all of you for being with me, support and your patience. May the force be with you!

(8)
(9)

vii TABLE OF CONTENTS Page ABBREVIATIONS ... ix LIST OF TABLES ... x LIST OF FIGURES ... xi SUMMARY ... xv ÖZET ... xvii 1. INTRODUCTION ... 1 1.1 Literature Survey ... 3 1.2 Objectives of Study ... 13 2. GENERAL REMARKS ... 15 2.1 Tribology ... 15 2.1.1 Wear ... 16 2.1.1.1 Abrasive wear……….. ... 16 2.1.1.2 Adhesive wear ... ..17 2.1.1.3 Corrosive wear ... ..18 2.1.1.4 Fatigue wear ... ..18 2.1.2 Lubrication ... 18 2.1.2.1 Liquid lubricants………..…..………. 18 2.1.2.2 Viscosity grades………..………... 19 2.1.2.3 Lubrication mechanisms………..……...… 20 2.1.2.4 Solid lubricants………..……...………….. 22 2.2 Nanocomposite Coatings ... 23

2.2.1 Methods for production of nanocomposite coatings ... 27

2.2.1.1 Cathodic arc ………..…..………... 28

2.2.1.2 Magnetron Sputtering…...………..………... 29

2.2.1.3 Hybrid Coating Methods …...………..………... 31

3. METHOD ... 33

3.1 Sample Preparation ... 33

3.2 Coatings ... 34

3.3 Characterization of Coatings ... 35

3.4 Pin on Disc Tests ... 36

3.5 Ring on Liner Tests ... 38

3.5.1 Ring on liner test system ... 38

3.5.2 Experimental Setup ... 39

3.5.3 Test Matrix ... 41

4. RESULTS AND DISCUSSION ... 43

4.1 Coating Characterization Results ... 43

4.2 Pin On Disc Tests ... 45

4.2.1 Pin coated, CGI naked base mineral oil tests of 07 series coatings ... 45

4.2.2 Pin coated, CGI naked formulated mineral oil tests of 07 series coatings 45 4.2.3 Both samples coated base mineral oil tests ... 48

(10)

viii

4.2.5 Pin coated, cgi naked tests of 09 series coatings ... 52

4.3 Ring on Liner Tests ... 54

4.3.1 MoN-Cu base mineral oil tests ... 55

4.3.2 Mo2N-Cu base mineral oil test ... 57

4.3.3 MoN-Cu formulated mineral oil test ... 59

4.3.4 Mo2N-Cu formulated mineral oil test... 61

4.3.5 Mo2N-Cu synthetic base oil test ... 63

4.3.6 Mo2N-Cu formulated synthetic oil test ... 65

5. CONCLUSION AND FUTURE WORK ... 67

REFERENCES ... 71

APPENDICES ... 75

(11)

ix

ABBREVIATIONS

AES : Auger Electron Spectroscopy ARC : Cathodic Arc Deposition ASM : American Society of Materials

ASME : American Society of Mechanical Engineers

AW : Anti Wear

Base : Oil with No Additive BCC : Body Centered Cubic CGI : Compacted Graphite Iron

CI : Cast Iron

COF : Coefficient of Friction CVD : Chemical Vapor Deposition EDS : Energy Dispersive Spectroscopy DLC : Diamond Like Carbon

FCC : Face Centered Cubic

FM : Friction Modifier

Formulated : Oil with Additives GCI : Grey Cast Iron

HCP : Hexagonal Closed Packed HSS : High Speed Steel

JCPDS : The International Centre for Diffraction Data OTAM : Otomotiv Teknolojileri Araştırma Merkezi POD : Pin On Disc

PVD : Physical Vapor Deposition

RT : Room Temperature

SAE : Society of Automobile Engineers SEM : Scanning Electron Microscope Tm : Melting Temperature

XPS : X-Ray Photoelectron Spectroscopy XRD : X-Ray Diffraction

At % : Atomic percent Wt % : Weight percent %rh : Relative humidity

(12)

x

LIST OF TABLES

Page

Table 2.1 : Solid materials with self-lubricating capability... 23

Table 3.1 : Chemical Compositions of HSS Discs and 2343 Pins …..…….. 34

Table 3.2 : Coating Parameters ………..……… 35

Table 3.3 : Test Matrix that was applied at the end of the steps………. 42

Table 4.1 : Cu ingredients of coatings……… 44

Table 4.2 : Thickness and hardness values of coatings……….. 45

Table 4.3 : Cumulative results of pin coated, CGI naked pin on disc tests… 53 Table A.1: Surface roughness data of MoN-Cu base mineral oil first test liner………... 86

Table A.2: Surface roughness data of MoN-Cu base mineral oil repeat test liner ………... 87

Table A.3: Surface roughness data of Mo2N-Cu base mineral oil test liner . 88 Table A.4: Surface roughness data of MoN-Cu formulated mineral oil test liner………... 89

Table A.5: Surface roughness data of Mo2N-Cu formulated mineral oil test liner………... 90

Table A.6: Surface roughness data of Mo2N-Cu base synthetic oil (PAO) test liner ………... 91

Table A.7: Surface roughness data of Mo2N-Cu formulated synthetic oil (PAO) test liner ………... 92

(13)

xi

LIST OF FIGURES Page

Figure 1.1 : Molecule schematic of ZDDP. …………...……… 5

Figure 1.2 : COF vs. temperature graph for Cu-Mo coatings and powder mixtures. 8 Figure 1.3 : Temperature dependence of friction coefficient for MoO3 and Ag2MoO4 coatings on alumina substrates………... 9

Figure 1.4 : Coefficient of friction value graphs of MoS2 and MoSe2 respect to a) Relative humidity (at RT) b) Temperature……….… 10

Figure 1.5 : a) Coefficient of friction value graphs of TiN, CrN, MoN b) Coefficient of friction value graphs of TiN-Cu, CrN-Cu, MoN-Cu, c) Wear rates of coatings………...…... 12

Figure 2.1 : Schematic diagram of Hertzian indentation………..………. 16

Figure 2.2 : (a) Ductile material whose abrasive wear is dominated by plastic deformation (b) Fracture propagation is observed in brittle materials………...………... 17

Figure 2.3 : Stribeck Curve………..…... 21

Figure 2.4 : Structures of some solid lubricants a) h-BN, b) Graphite, c)MoS2. 23 Figure 2.5 : Cross sectional image of (a) conventional MoN and (b) nanocomposite Cu/MoN coating……… 25

Figure 2.6 : Arc formation and scattering of atoms from surface in cathodic arc deposition……….. 28

Figure 2.7 : Magnetron sputtering systems and their effective plasma sites. a) Conventional magnetron, b) Unbalanced magnetron…………. 30

Figure 2.8 : Hybrid coating application of Mo-Cu, Ti-Cu and Cr-Cu nanocomposite coatings………... 31

Figure 3.1 : Dimensions of pin samples………... 33

Figure 3.2 : Mechanism of calotest a) Test setup b) Projectile of wear c) Cross section of projectile………... 36

Figure 3.3 : Pin on disc test setup………... 37

Figure 3.4: SEM images of CGI samples with various magnifications a) 200x b) 2000x……….… 37

Figure 3.5 : a) General view of bench test system , b) Load arm………... 39

Figure 4.1 : XRD graph of 07 series………...… 43

Figure 4.2 : XRD graph of 09 series………...… 44

Figure 4.3 : Pin coated, cgi naked with base oil pin on disc test cof vs. distance graph………... 46

Figure 4.4 : 3D profile, 200x and 2000x SEM images of wear scars in pin coated, cgi naked pin on disc test with base oil……….. 46

Figure 4.5 : distance graph………..…... 47

Figure 4.6 : 3D profile, 200x and 2000x SEM images of wear scars in pin coated, cgi naked pin on disc test with formulated oil……… 47 Pin coated, cgi naked with formulated oil pin on disc test cof vs.

(14)

xii

Figure 4.7 : COF vs. distance graph of both samples coated base oil pin on

disc tests……….. 48

Figure 4.8 : 3D profile, 500x and 2000x SEM images of wear scars in both

samples coated pin on disc test with base oil……….. 49 Figure 4.9 : COF vs. distance graph of both samples coated formulated oil

pin on disc tests……….. 50

Figure 4.10 : 3D profile, 500x and 2000x SEM images of wear scars in both

samples coated pin on disc test with formulated oil…………..…. 50 Figure 4.11 : COF vs. distance graphs of 09 series coatings, a) Base Oil,

b) Formulated Oil……… 51

Figure 4.12: SEM images of tested CGI sample in MoN-Cu base oil test…….. 52 Figure 4.13 : COF vs. distance graphs of synthetic oil pin on disc tests……….. 53 Figure 4.14 : 3D surface profiles and microscope images of wear scars in pin

coated, cgi naked pin on disc test with synthetic oils……….. 54

Figure 4.15 : COF vs. time graph of MoN-Cu base mineral oil first test………. 55 Figure 4.16 : COF vs. time graph of MoN-Cu base mineral oil repeat test……. 56 Figure 4.17 : MoN-Cu coating mineral base oil test comparison graph at 80N,

500rpm, and 70°C………... 56

Figure 4.18 : MoN-Cu coating mineral base oil test comparison graph at 160N,

500rpm, and 70°C………... 57

Figure 4.19 : COF vs. time graph of Mo2N-Cu base mineral oil test…………... 58

Figure 4.20 : Mo2N-Cu coating mineral base oil test comparison graph at

320N, 300rpm, 70°C………... 58

Figure 4.21 : Mo2N-Cu coating mineral base oil test comparison graph at

160N, 500rpm, 70°C………... 59

Figure 4.22 : COF vs. time graph of MoN-Cu formulated mineral oil test…….. 60 Figure 4.23 : MoN-Cu coating mineral formulated oil test comparison graph at

160N, 300rpm, and 70°C……… 60

Figure 4.24 : MoN-Cu coating mineral formulated oil test comparison graph at

160N, 500rpm, and 70°C……… 61

Figure 4.25 : COF vs. time graph of Mo2N-Cu formulated mineral oil test…… 62

Figure 4.26 : MoN-Cu coating mineral formulated oil test comparison graph at

80N, 500rpm, 70………. 62

Figure 4.27 : MoN-Cu coating mineral formulated oil test comparison graph at

160N, 700rpm, and 70°C……… 63

Figure 4.28 : COF vs. time graph of Mo2N-Cu base synthetic oil test…………. 64

Figure 4.29 : MoN-Cu coating base synthetic oil test comparison graph at

320N, 500rpm, and 70°C……… 64

Figure 4.30 : COF vs. time graph of Mo2N-Cu formulated synthetic oil test….. 65

Figure 4.31 : Mo2N-Cu coating formulated synthetic oil test comparison graph

at 80N, 500rpm, and 70°C……….. 66

Figure 4.32 : Mo2N-Cu coating formulated synthetic oil test comparison graph

at 320N, 500rpm, and 70°C……… 66

Figure A.1 : User interface of LabView program………... 76 Figure A.2 : MoN-Cu coating base mineral oil test comparison graphs at 70°C 77 Figure A.3 : Mo2N-Cu coating base mineral oil test comparison graphs at

70°C……… 78

Figure A.4 : MoN-Cu coating formulated mineral oil test comparison graphs

(15)

xiii

Figure A.5 : Mo2N-Cu coating formulated mineral oil test comparison graphs

at 70°C……… 80

Figure A.6 : Mo2N-Cu coating base synthetic oil test comparison graphs at

70°C……… 81

Figure A.7 : Mo2N-Cu coating formulated synthetic oil test comparison

graphs at 70°C ..……….. 82

Figure A.8 : MoN-Cu coating base mineral oil test comparison graphs at 25°C 83 Figure A.9 : Mo2N-Cu coating formulated mineral oil test comparison graphs

at 25°C……… 84

Figure A.10 : Mo2N-Cu coating formulated synthetic oil test comparison

(16)
(17)

xv

TRIBOLOGICAL PROPERTIES OF MO-N-CU NANOCOMPOSITE

COATINGS IN LUBRICATED ENVIRONMENT SUMMARY

Two of the biggest problems in last two decades are air pollution and energy consumption. Governments and companies have been supporting research projects that offer solutions. Car engines (and other combustion engines) place in intersection of these two problems. In European Union, over 200 million vehicles are on roads and numbers are increasing with demand. In all energy consumptions, 30% share belongs to vehicles that use fossil fuels. Besides that, high exhaust emission of vehicles pushed governments into action. Friction in engine and engine parts is 15% of total power loss in a car and biggest source of that loss is piston assembly with 38%. To solve friction problem friction modifiers and anti wear additives are in use. But they do not sufficiently solve the problem and they don’t work well under harsh conditions. Solid lubricant coatings (DLC, MoS2, etc.) also are in use but they are too soft to resist abrasion and can be used only in limited environmental conditions. One of the candidates that can meet increasing tribological requirements is nanocomposite coatings. They both have good mechanical properties and tribological properties.

MoN, MoN-Cu, Mo2N and Mo2N-Cu are very promising nanocomposite coatings because of their good mechanical properties and tribological properties. There are some works in literature about members of that coating family, but tribological properties are not explained well. In this study, tribological properties of these coatings were tried to explain systematically.

In this thesis, first two series of coatings were performed which have different amount of Cu ingredient to observe how Cu content effects the tribological formulated mineral, base and formulated synthetic oils. Tests were applied at boundary lubrication conditions in room environment. Then, actual piston rings were coated with best resultant coatings, and they were used in Ring on liner bench tests. Same oils from pin on disk tests were chosen as lubricant. System worked under various loads and speeds to simulate actual working engine conditions.

(18)
(19)

xvii

Mo-N-Cu NANOKOMPOZİT KAPLAMALARIN YAĞLI ORTAMDA

TRİBOLOJİK DAVRANIŞLARININ İNCELENMESİ ÖZET

Hava kirliliği ve enerji tüketimi son yirmi yılın en büyük sorunlarından ikisidir. Hükümetler ve şirketler bu sorunlara çözüm öneren çalışmaları desteklemektedir. Araba motorları (ve diğer içten yanmalı motorlar) bu iki sorunun kesişiminde yer almaktadır. Avrupa Birliğinde 200 milyonun üzerinde motorlu taşıt kullanılmakta ve bu rakamlar her geçen gün artmaktadır. Tüm enerji tüketimleri içinde, %30luk pay fosil bazlı yakıtlar kullanan araçlara aittir. Enerji tüketimine ek olarak, egzoz gazlarının zararlı etkisi hükümetleri harekete geçirmiştir.İçten yanmalı motorlu araçlarda sürtünme kayıpları aracın toplam güç kaybının %15lik kısmını oluşturmaktadır, bu kaybın %38 ile en büyük kısmı ise piston ailesine aittir. Piston ailesindeki problemlerin çözümü için yağlara sürtünme azaltıcı ve aşınma azaltıcı katkılar ilave edilmektedir. Bu katkılar problemleri yeterli ölçüde çözememekte ve zor koşullar altında etkilerini önemli düzeyde yitirmektedirler. Bir çözüm olarak katı yağlayıcılar kullanılsa da bunlarda zor koşullar altında yüzeyleri terk etmekte veya etkilerini yitirmektedir. Bir diğer çözüm adayı da yüksek mekanik ve tribolojik özelliklere sahip nanokompozit kaplamalardır.

MoN, MoN-Cu, Mo2N ve Mo2N-Cu iyi mekanik ve tribolojik özellikleri ile umut vadeden kaplama ailesidir. Literatürde bu kaplama ailesinin tribolojik özellikleri ile alakalı olarak çalışmalar yer alsa da, düzenli ve detaylı bir araştırmanın eksikliği gözlenmektedir. Bu çalışmada bu kaplama ailesinin tribolojik özellikleri sistematik bir biçimde incelenmiştir.

Tez çalışmaları sırasında öncelikle farklı Cu içeriklerinin tribolojik özelliklere etkisi incelemek amacıyla iki grup kaplama yapılmıştır. Bu kaplamalar sınır yağlama koşulları altında oda koşullarında katkılı ve katkısız mineral yağlar ve yine katkılı ve katkısız sentetik yağlar ile deneylere tabii tutulmuştur. En iyi sonuçları sağlayan kaplamalar gerçek piston segmanlarına uygulanmış ve bu segmanlar gömlek üstü segman deneylerinde kullanılmıştır. Farklı yük ve hızlar altında çalıştırılan sistem ile gerçek motor şartlarına yakın deneyler yapılmıştır.

(20)
(21)

1

1. INTRODUCTION

Mechanical systems consist of many parts which work together in contact. Because of that working manner, energy and wear loses occur. Mechanical energy loses has a 1/3 share on the total energy consumption of the world. The mechanical energy loses in combustion engines are the most common examples. Number of vehicles is increasing day by day with high demand. In European Union (EU), over 200 million passenger cars and half a million coaches and buses are on road [1].

Possible extinction of fossil based fuels in near future and production of harmful exhaust gases, push governments and civil organizations into action. Less emission of exhaust and fuel consumption can be achieved if loses in engines would be overwhelmed. In combustion engines, the main source of energy loss is piston assembly. For decreasing that energy loss, oils have been widely used for many years. In early times, people had used vegetable and animal oils for lubrication and which gave better performance than petrol based lubricants. Through the development of oil refining and new additives, demand shifted to mineral based oils. In recent years, with development of polymer science, synthetic oils are in markets. However, there are limits to combat wear and friction with oil based lubrication Antiwear additive ZDDP and sulfur content of mineral oils causes pollution. In cyclic working systems like piston assembly, lubrication is starving at some points of cycle so surface of counterparts contact [12].

In last two decades, one of the widely used practices for tribological applications is thin film coatings. This practice is based on deposition of materials on substrates with using different deposition methods such as physical vapor deposition (PVD) and chemical vapor deposition (CVD), with different material combinations better tribological properties have been achieved [2]. TiN and CrN (also other nitrides of transition elements) are widely chosen coatings with their wear resistivity for industrial applications.

(22)

2

Under lubrication, these coatings are at very good conditions for combating wear and friction. However, their application is limited. Although they do not wear away, the counter bodies, which are working against them, are severely worn away due to their hard hardness.

Biggest problem of single phased PVD (Physical Vapor Deposition) and CVD (Chemical Vapor Deposition) coatings is their mechanical properties. Hard coatings have low toughness, and coatings with high toughness have low hardness. Both high toughness and hardness properties can be achieved with nanocomposite coatings [52].

Nanocomposite coatings have two or more immiscible phases and generally grain sizes are several atoms to 10 nanometers. At least one phase has nanocrystalline structure and others can be either nanocrystalline or amorphous. Coatings can achieve 100GPa hardness with this process [3]. At production stage many processes can be used alone and together (hybrid). Laser ablation, thermal evaporation, cathodic arc deposition, magnetron sputtering, plasma assisted chemical vapor deposition and others are in use. In this research, cathodic arc deposition and magnetron sputtering are used together as a hybrid system.

Mo based coatings are popular with their good mechanical and tribological properties; there are some publications about their production and applications [5-12]. Molybdenum nitride coatings have high hardness compared to other nitrided transition element coatings and their doped versions with softer elements (such as copper and silver) have improved mechanical properties. Doped coatings have lower friction and wear than TiN both in dry and lubricated conditions [4]. There is not a systematic and detailed publication about tribological applications of Mo based coatings in lubricated environment.

(23)

3

1.1 Literature Survey

Wear between sliding surfaces is usual during start-up and running-in stages of a machine. Oxide films on the metal surfaces and sulfur compounds in the oil; offer limited wear protection. Therefore antiwear additives are presently essential for controlling wear at acceptable levels in modern engines. Zinc dialkyldithiophosphate (ZDDP) has been used as an anti-wear additive for many years in automotive engine oils. ZDDP has been the most effective anti-wear for automotive engine applications and no powerful alternative is available to date.

Ito et al. showed that ZDDP forms a protective tribofilm on the ferrous surface. They worked with an iron oxide layer that contains about 50% of Fe3O4 which was formed by the water-vapor treatment on steel plates, then it were slide against steel cylinders by using a cylinder on plate-type tribometer in an oscillating motion under a mixed lubrication condition in PAO that contains 1% ZDDP. They observed the friction coefficient as below 0.06. They determined that the iron oxide reacts with ZDDP as a result of sliding and forms a multi-layered tribofilm. The thickness of the tribofilm ranges from 30 to 130 nm and at the bottom layer of the tribofilm had 10–30 nm thickness with a gradient composition of Zn, Fe, S, P, and O. The distribution peaks of Zn and S exist in the bottom layer, which is unique to the iron oxide compared with the case of steel. These films serve to separate the two metallic surfaces, and limit the extent the metal/metal adhesion and also, possibly reduce the level of the contact stresses experienced by the metal substrate [13]. ZDDP is preferred with anti-oxidation and anti-wear (AW) properties as easy preparation and low cost. Within this usage, high friction of rubbing surfaces leads to significant increase in fuel consumption, but the anti-wear effects of ZDDP also play an important role on the durability of mechanical systems [13].

Minfray et al. showed that, during the running-in process, iron oxide particles may form in the engine system therefore causes abrasive wear process and eventually damage the steel surfaces. ZDDP forms a thin film on the surface, mainly composed of zinc polyphosphate (or possibly thiophosphate), which typically prevents surface from wear [14].

(24)

4

Fan et al. reported that sulfur is found in the tribofilm, mainly under sulfide forms (zinc sulfide or iron sulfide) but the existence of thiophosphate is still possible. However, sulfur and phosphorus atoms contained in ZDDP molecule cause poisoning of vehicles catalysts (Figure 1.1). Also these Zn containing additives can generate plenty of ashes, leading to operational problems [16]. Therefore development of alternative additives to ZDDP is essential.

Onadera et al. reported that a great number of challenges in the automotive industry are to provide similar or higher wear protection with less amount of sulfur and phosphorus in engine oil formulation, and worked on computer simulations of Zn(PS0.5O2.5)2 and Zn(PO3)2. They found that both is capable of digesting the Fe2O3 wear particle and concluded that Zn (PO3)2 has a better wear prevention performance and it is more environmental friendly [17].

Lin et al. studied the temperature dependence effect of ZDDP in mineral base oil. Flat-on-flat sliding specimens were used in the experiments with a contact pressure up to 88.88 MPa, under different temperatures up to 200°C and under boundary lubrication conditions in an eight-hour tribotest. When the base oil was used, the depth of the wear increases as temperature is rising to 120 °C with a slow linear manner. Moreover, the depth of the wear scar increases sharply when the temperature is above 120 °C. Oil with ZDDP effects wear profile positively and within temperature range, wear profiles stays in a small interval. This study determined that if the contact pressure exceeded 90 MPa, and the surface roughness of a hard surface exceeded 0.3 µm in Ra, then there is no antiwear performance from this ZDDP. Moreover, if the concentration of ZDDP was less than 0.5 wt%, and it was used at over 200 °C, then it loses its antiwear properties after several hours of rubbing. Consequently, the use of ZDDP as an antiwear additive in plain paraffin oil is limited. [18]

There are two main and complementary approaches to reducing friction via engine oil design. One is to employ low viscosity oils, which reduce losses in pumping and under hydrodynamic lubrication conditions. The second is to reduce friction in the boundary lubrication regime by the incorporation of appropriate friction reducing additives. A very important class of friction reducing additives is the oil soluble molybdenum-containing compounds such as molybdenum dialkyldithiocarbamate (MoDTC)

(25)

5

Figure 1.1: Molecule schematic of ZDDP. [15]

Graham et al. worked on molybdenum dialkyldithiocarbamates (MoDTCs) as a wide spectrum and published two review papers [19, 20]. They reported that here are variety of tribological works on MoDTC and in the mixed to boundary lubrication regime, use of the molybdenum dialkyldithiocarbamate (MoDTC) additive results in a very low friction coefficient of around 0.05 making it a very effective friction modifier additive and an essential component of current engine lubricant technology. Its effectiveness in friction reduction comes primarily from forming a MoS2 containing tribofilm on the wear scar. High-resolution transmission electron microscopy studies have indicated the formation of tiny platelets or flakes in wear debris and on wear tracks which appear to be nanocrystals of MoS2, [21]. The reaction sequence by which MoDTC forms MoS2, and the precise conditions under which this occurs are, however, largely unknown. A wide range of friction tests have been carried out on MoDTC. However, MoS2 is insoluble in oils so formation should be supplied within Mo contained compounds thus they have been used for friction reduction.

MoDTC compounds, which contain both molybdenum and sulfur in their structure and are soluble in oil due to their hydrocarbon chain component. These compounds have been found to have better thermal stability than many other organic molybdenum compounds and have also been shown to be very effective friction modifiers.

(26)

6

The depletion rate of MoS2, from the surface is also considered to have an important influence on the friction-reducing ability of the oil solution, and some research has suggested that the addition of antiwear additives to the oil reduces this depletion, thus preserving the friction lowering. Soluble molybdenum additives can produce smoothing of rubbed surfaces. [19].

Graham et al. reported that at below 180 ppm concentration level friction reducing ability lost. One of the most striking characteristics of MoDTC action is the way that friction drops suddenly and very rapidly, often after a prolonged induction period. Interestingly, the Raman surface analysis showed that during this period there was no discernable build up of MoS2, in or around the contact; this only occurred when the friction started to fall. Neither was there any evidence of molybdenum oxides on the surface prior to nor after friction drop. The MoDTC additives are most effective in reducing friction at high concentration and high temperature (up to 0.4 % wt. and 200ºC). Characteristic of MoDTC activity results with formation of tiny nano crystals of MoS2 on the rubbing surfaces. These flake-like crystals form only on the asperity peaks. Formation occurs at solid-solid contact i.e. true boundary conditions. Thus friction modification is lower at rolling than sliding conditions since micro-elasto-hydrodynamic lubricating films prevent solid-solid interaction. The abruptness and rapidity of the appearance of MoS2 and consequent friction reduction suggests that the reaction is autocatalytic [20].

Morina et. al, reported that MoDTC is more effective in friction reduction when used with ZDDP. [22]. Increase of MoDTC concentration in the ZDDP containing lubricant from 50 to 250 ppm Mo level caused an increase in MoS2 formation in the wear scar, thus further reduction of friction. In the case of only ZDDP containing lubricant, increase in temperature causes an increase in phosphate glass tribofilm formation in the wear scar. Higher phosphate chain length at the top layers of the tribofilm is shown to be related with friction increase but lowers the wear.

Johnson, et al. reported that MoDTC undergoes partial ligand exchange with ZDDP and it has been proposed that this may influence the reactivity of the MoDTC additive [23].

(27)

7

Neville et al. made a review about tribological effects of additives and coatings on piston family [24]. They reported that components of the piston assembly are considered to be the most complicated tribological components to analyze because they experience large variations in load, speed, temperature and lubrication regime (from boundary to hydrodynamic lubrication regime). Traditionally, low cost and readily available cast irons (CI) namely grey cast iron and nodular CI were used as piston ring materials. However, because of high strength and good fatigue properties, recently nitrided stainless-steel and tool steel are widely being used as piston ring materials. The compression ring experiences extremely high temperatures resulting in severe wear. Arc-ion plating of Cr–N or Cr on steel or cast iron compression rings has been reported to give low friction loss [25]. However, Cr–N plating gives much lower friction compared to Cr plating resulting in 90% reduction in ring wear and 15% reduction in bore wear [26]. This is because the electrodeposited Cr coatings are very densely packed and the low porosity compared to Cr–N and give poor oil entrapment and therefore, it is very difficult to maintain oil film at the sliding interface. Piston rings made of cast iron or steel can also be treated by gas or ion nitriding. Recently, ceramic coatings, plasma sprayed molybdenum, and diamond like carbon (DLC) coatings are also becoming popular as piston ring coatings. The DLC coating on piston ring significantly improves the friction properties, engine reliability and work life, however, high internal stress of DLC which limits the thickness of DLC coatings further reduces the longer working life [25]. DLC is considered as a promising non-ferrous coating because of its excellent tribological properties. Since the temperature and environmental parameters greatly affect the stability of friction and wear of DLC coatings, the use of lubricating oil will isolate the coating from the surrounding hostile environment and act as a coolant to keep the temperature in the allowable limit. In addition, the lubricant additives may interact with the DLC coating and produce low friction and wear resistant tribofilm. But at high temperatures, DLC transform to graphite. Thus it cannot be used well under boundary conditions where lubricant film thickness is below surface roughness [26]. Most of the lubricants developed so far are customized to form the tribofilm that will adhere to ferrous materials and until now no lubricants have been designed for the non-ferrous coatings [26].

(28)

8

To design new lubricants for the DLC coating, it is essentially important to understand how this coating interacts with the existing lubricant additives. Knowledge of how DLC interacts with existing lubricant additives is contradictory. Some groups showed that no stable tribofilm was found on the DLC surface when additive containing oils were used [27, 28], while other research groups observed tribofilm formation on DLC at boundary lubrication condition [29-31]. The physical properties of DLC coatings will significantly vary with deposition techniques, type of dopant used and the hydrogen content in the DLC matrix [31].

For automotive engine applications, solid lubricant films such as DLC, CrN, Mo and other ceramic films became popular in last decade. Therefore many groups began to study ceramic coatings which have self lubricant property.

Wahl et al. firstly use CuMoO4 and CuO oxide powders, then they coated Cu and Mo and oxidize them in high temperatures for tribology tests. They used Cu-Mo coated (ion beam deposition) alumina specimens in tests which are applied in 25 to 650°C temperatures. They observed formation of CuMoO4 and MoO3 within increasing temperatures and decrement of COF as 0.5 to 0.2. Authors defined this decrement with formation of crystalline oxide phases [32]. Also this work showed that powders and coatings have near COF values (Figure 1.2).

Figure 1.2: COF vs. temperature graph for Cu-Mo coatings and powder mixtures [32] Softening of oxides over brittle-ductile transition temperatures is known. This temperature is in 0.4-0.7 Tm interval (where Tm is melting temperature of material in Kelvin unit).

(29)

9

Tm is 800°C for CuMoO4 and MoO3 which was formed with high temperature oxidation of Cu and Mo coatings. Therefore softening is predicted in 200-550°C intervals. Results of experiment are compatible with that prediction and friction decreased in 200-500°C intervals [32].

Gulbinski et al. studied tribological properties of MoO3-Ag, V2O5-Cu, V2O5-Ag coatings, which are coated with reactive magnetron sputtering, in 100-700°C temperature intervals (Figure 1.3).

Figure 1.3: Temperature dependence of friction coefficient for MoO3 andAg2MoO4 coatings on alumina substrates [33]

In Ag-MoO3 binary system Tm for MoO3 is 800°C but at %16mol Ag2O it decreased to 530°C. For more Ag ingredient, decrement of eutectic temperature to 496°C is reported [33].

Zabinski et al. investigated tribological behaviors of micro structural oxides. Authors declared that solid lubricants can be used for friction problems in high-low temperature, radiation, and high vacuum conditions. Graphite and MoS2 are widely used in industry, but graphite does not service in vacuum and MoS2 has a dramatically shortened life under high humidity conditions and its friction coefficient increases an order of magnitude [34]. Both lubricant oxidize over 350°C and lose their lubrication property. Recently, WS2 is preferred instead of MoS2 because it increases operation temperature about 100°C [34].

Kubart et al. investigated the comparison of tribological properties of MoS2 and MoSe2 coatings at elevated temperatures and different humidity [35].

(30)

10

MoSe2 has same crystal structure with MoS2 but information about lubrication property of this alloy is very limited. Both coatings are produced with DC magnetic sputtering in Ar gas environment.

Ball-on-disc tests were performed at various temperatures and humidity. COF of MoSe2 does not change much with humidity, but this is not true for MoS2. In MoS2 section, COF values increase till 40%rh, then decreases some amount (Figure 2.7a)

a) b)

Figure 1.4: Coefficient of friction value graphs of MoS2 and MoSe2 respect to a) Relative humidity (at RT) b) Temperature [35]

Both coatings wear rates are close and MoS2 has lower rates until 200°C, but over it, the wear rate is considerably lower than that of MoSe2. Authors stated that higher coefficient of friction in the case of MoSe2 could be caused by stronger forces between layers. For both coatings working temperature is limited to 300ºC because of rapid increase on both friction and wear rate.

Donnet and Erdemir stated that various solid lubricants can be used with oxide solid lubricants, new forms through that usage also can be used as solid lubricants and lubrication can continue with them [36]. For example, while MoS2 and PbO are mixed, PbMoO2 forms [37] and it can be used as solid lubricant at high temperatures. And also they states; there is no such coating that can work under any type of application conditions. To overcome such variations in performance and durability, researchers have been exploring novel coating architectures having multi-layers, micro surface texturing, nano-structures and or composites.

New types of coatings appear to work better than their ancestors in both the machining and sliding contact applications.

(31)

11

Some of the nano-composite coatings are superhard and depending on their compositions, some of them are able to provide better performance and durability over broad application conditions [36].

There are limited amount of work about MoN and Cu doped variations of MoN nanocomposite coatings in literature. Oxides of these coatings have solid lubricant property. Suzsko et al. interested about that topic and also our research group has been working with that coating family and their tribological applications for two decades [4-12,38,39].

Suzsko et al. made a research about tribological properties of MoN coatings in 20-400°C interval and reported that above 250°C; oxide softening leads the decrease in friction [38]. At room conditions, COF is observed as 0.42. Tribo oxidation produces a tribofilm which includes mixture of absorbed water from environment and molybdenum oxide. Over 100°C and till 250°C that mechanism fails because of vaporizing and friction increases due to leave of brittle oxides. Over 250°C, oxides become soft and at 400°C friction is about 0.55 however, there is a high wear.

Suszko et al. worked about tribological properties of Mo2N-Cu in various conditions and Cu ingredients. Doping with Cu, increases hardness till 3% level but above that level, hardness of coatings begin to soften and reaches under hardness of raw Mo2N. Highest COF value was reached at 100°C and about 400°C COF values reached the minimum. Researchers explained the reason of the decrement as formation of MoO3 and CuMoO4. Wear rate increases with Cu ingredient at 300°C and 400°C. At 100°C, wear rate increases till 6% Cu and wear becomes negligible for higher content of Cu. Raman spectroscopy test results let authors to determine the strong adhesion between nitride phases and oxide structure. They propose that this adhesion decreases the wear rate. Over 300 °C, the beginning of thermal oxidation results with a high wear. Because soft and constantly growing molybdenum oxide/copper molybdate layer, which is solid lubricant and forms at that condition, easily worn out [39].

Öztürk et al. investigated the comparative tribological results of transition elements nitride coatings [4]. They coated TiN, CrN, MoN and Cu contained versions of them in hybrid coating system on HSS substrates. Coated samples were tested in dry ball on disc and reciprocating tests under room conditions. In sliding tests Cu doping does not affected the COF well in TiN and CrN but in MoN side, effect is neutral.

(32)

12

However there is no good benefit for COF, Cu addition affected wear positively. In both test series minimum wear and COF values belong MoN-Cu coatings with a great difference. Raman studies showed oxide formation of transition elements on wear debris. Therefore low friction behavior was explained with oxide formation and crystal chemical approach [40]. Mo oxides have highest ionic potential thus it has the best results.

Figure 1.5: a) Coefficient of friction value graphs of TiN, CrN, MoN b) Coefficient of friction value graphs of TiN-Cu, CrN-Cu, MoN-Cu, c) Wear rates of coatings [4]

b

(33)

13

1.2 Objectives of Study

Literature survey showed that additives of motor oils can not perform well to solve friction loses and environmental problems. ZDDP which is used in most of the IC engine lubricants produces pollution and increases friction while preventing wear. Also MoDTC which is leader friction modifier and it is preffered because of its Mo2S production capability and actually Mo2S and Mo-oxides are lubricious. Also it can not perform well without ZDDP. Also near 200°C, effect of both additives decreases and generally diesel engines can reach over that temperatures within their high load and low speed cycles where most of the friction loss and wear take place. Also studies showed that surface roughness has effect on performance of both additives and within usage of vehicles; surface roughness of liners decreases thus effect of additives so. By that manner engine part producers offer coated (electrodeposited, spray coated and recently nanocomposite coated) rings since beginning of 90’s. This coatings decreases wear and friction for a limited amount with using formulated oils. DLC is a popular material family and it is applied to engine parts with its low friction property but it causes wear to counter element. Also at high temperatures it transforms to graphite and under boundary lubrication conditions its usage is significantly limited.

PVD coated MoN causes less friction than metallic Mo coating, which have been used on piston rings, and also film durability is longer when compared. However MoN has less friction, its wear rate is higher (but even less than Cr coating family) [4]. MoN-Cu nanocomposite family has known with its self lubricant effect and its mechanical and tribological properties has supremacy to other most of nanocomposite coatings which are in used on parts for machining applications. Also this coating family has Mo ingredient thus possible Mo2S formation can be predicted if they would be used with mineral based oils which contains S. Even tribological properties of MoN-Cu was studied, Mo2N-Cu is not well studied and comparative study of that family is a need.

Therefore in this study, tribological effects of nanocomposite MoxN-Cu coating family was investigated. First coatings were applied to pins and pin on disc tests were performed as pin coated, cgi naked and both parts coated series.

(34)

14

Then lowest friction and wear owner coatings were applied to rings and they were used in ring on liner bench tests. In following chapters, results will be debated.

(35)

15

2. GENERAL REMARKS

In this chapter, theoretical background of study is introduced. This thesis was built on two main topics which are tribology and nanocomposite coatings. In following parts, brief information is given to understand the applications and for detailed information references are recommended.

2.1. Tribology

Tribology is a Latin word and name for the science of friction, wear and lubrication. If elements slide over one another causes friction. If friction is over threshold, then some particles are removed from softer surfaces and wear occurs (then that particles can initiate removing particles from harder surfaces). If there is/are another body that makes motion easier, that effect is called as lubrication.

Specimens are tried to be smooth or have defined roughness for tribological tests. Recent polishing systems can achieve 100Å~1000Å surface roughness values for macrotribology test applications. Therefore, contact occurs in very small area as molecular level. If specimens have bumps, that their hardness values are higher, they should be removed with caution. Otherwise true contact can not be achieved and wear topologies will be as parallel valleys. In this work, this effect is observed because of droplets on the surface of coated elements, so some part of experiment matrix is repeated.

In an ideal contact of two surfaces, where roughness of surfaces considered as spherical, they will initially touch at either a point or along a line. With the application of the smallest load, by the effect of elastic deformation, line contact enlarges to area contact across which the loads are distributed as pressures. This type of contact is analyzed with Hertzian indentation. In figure 2.1, spherical bearing is considered as harder.

(36)

16

Figure 2.1: Schematic diagram of Hertzian indentation [41]

When load P is applied to softer surface, that surface deforms elastically. Radius of contact area can be calculated with the following formula [12]

3 1 2 1

1

1

2

1

.

1





E

E

PxR

a

(2.1)

Where R is radius of spherical specimen, E1and E2 are elastic modulus of hard and soft specimens respectively Then contact area is A (A=πa2).

2.1.1 Wear

Wear has been recognized as material removal from a surface due to interaction of sliding surfaces. Almost all machines lose their durability; reliability and lifetime of machines shorten due to wear. Therefore wear causes serious economical loss. To prevent wear, surface modifications such as hardening and ceramic thin film coatings are widely used.

Wear types are described with many terms. Generally wear can be investigated under four main topics. They are abrasive, adhesive, corrosive and fatigue wear. Wear mechanism can be either one of them or complex.

2.1.1.1 Abrasive wear

If the contact interface between two surfaces has interlocking of an inclined or curved contact, a certain volume of surface material is removed and a channel shaped wear formed on the softer surface. This type of wear is called abrasive wear.

(37)

17

Abrasive wear particles generally have sharp edges and stays on track; therefore they cause further wear with microcutting. If wearing material has a ductile property, a cord-like, long wear particle is removed. If material is brittle, then crack propagation occurs (Figure 2.2). Abrasive wear generally causes fast failure of elements.

Figure 2.2: (a) Ductile material whose abrasive wear is dominated by plastic deformation (b) Fracture propagation is observed in brittle materials

2.1.1.2 Adhesive wear

Elements which slide relatively can stick each other because of chemical reactions or heating which caused by friction. Therefore friction force increases instantly. There is three possible case occur;

Stick and slip: Specimens holds and releases each other for small intervals. There is not particle removal, but this is very effective for crack propagation.

Scuffing: If specimens locally weld together (without melting) and tear apart since high degree of relative sliding and poor lubrication conditions, then case is called as scuffing. If melting of surfaces occurs during welding, wear amount increases and this is called galling [53].

(38)

18

Seizure: The stopping of relative motion as the result of interfacial friction Seizure may be accompanied by gross surface welding [53].The removed particles can block contact of surfaces and also they can change wear mechanism to abrasive wear.

2.1.1.3 Corrosive Wear

Corrosive wear is material loss with degradation because of liquids, liquid solutions that contain solid particles or corrosive gases, when sliding takes place. Corrosion accompanies with the wear process in all environments, except in a vacuum and inert atmospheres. Corrosion wear is main reason for cracking, and cracking decreases lifetime in an important manner.

2.1.1.4 Fatigue Wear

This type of wear occurs when a surface is stressed in a cyclic manner. Fatigue wear can be observed on parts which are subjected to rolling such as ball bearings and sliding motion parts such as piston rings. Surface finish, residual stress, hardness, and microstructure effect the fatigue wear. Surface treatments such as nitriding, carburizing, and shot peening can be applied to prevent fatigue wear.

2.1.2. Lubrication

Lubricants are introduced between two sliding solids by adding as solid, liquid or gaseous form. Lubrication is used at the sliding interfaces in order to reduce friction and wear. Also they can deport heat and debris which are generated during the sliding process.

2.1.2.1 Liquid lubricants

Main usage of liquid lubricants is to control friction, wear, and surface damage, therefore increase service life of parts. Wear and surface damage generally occur under boundary lubrication conditions; they are not observed in full hydrodynamic conditions. Liquid lubricants can be classified under three main sections.

Liquid organic Lubricants: Animal and vegetable based oils are called organic lubricants. However their lubrication characteristics are good, their service life is short. Nowadays, their service area is very limited compared to mineral based and synthetic oils.

(39)

19

Mineral based liquid lubricants: They are most common lubricants for mechanical systems. They are obtained from distillation of fossil based fuels. Following distillation; sulphur-nitrogen, hydrocarbons which has high density and waxes are refined with rafination process. Waxes increase solidification and aromatic hydrocarbons decrease viscosity rapidly during heating. Sulphur and nitrogen based alloys cause corrosion. Chemical additives such as friction modifiers (MoS2, WS2, Mo-DTC etc.) and anti-wear (ZDDP, halocarbons, etc.) are widely used for recent commercial engine oils. At high temperatures, mineral oils have poor oxidation properties and also viscosity of oil decreases. At low temperatures, solidification is their main disadvantage [12].

Synthetic oils: Synthetic oils are invented in last two decades, and being popular in recent years. Their main structure is polymerized hydrocarbons from crude oils. Firstly, they were very expensive compared to mineral based oils, but in cycle cost analyses, their effects on parts often superior to mineral based oils because equipments need fewer repairs. Also their viscosity–temperature range and low-temperature flow can be improved and controlled with synthetics. The flash points of synthetic oils are higher than a mineral oil for a certain viscosity. Therefore they can be used for higher temperatures for engine applications.

2.1.2.2 Viscosity Grades

Viscosity is a fluid property which is degree of resistance to shear stress and tensile stress. The Society of Automotive Engineers (SAE) has established a numerical code system for grading motor oils according to their viscosity characteristics. SAE grades include, low to high, 0, 5, 10, 15, 20, 25, 30, 40, 50 and 60. If a grade includes W letter, this means viscosity value is for winter or cold start at low temperature. SAE J300 document defines the viscosity values related to these grades.

Kinematic viscosity is performing with measuring the time for a standard amount of oil to flow through a standard orifice, at defined temperatures. More viscous oils flow in longer time and thus graded with higher SAE code.

There are two types of grading; they are single grade and multi grade. For single grade, there are eleven grades which are 0W, 5W, 10W, 15W, 20W, 25W, 20, 30, 40, 50, and 60. Winter coded grades are defined with measuring at different cold temperatures.

(40)

20

Lower grade means, that oil can pass at lower the temperature. For example, if an oil passes at the specifications for 10W and 5W, but fails for 0W, then that oil must be labeled as an SAE 5W. Therefore that oil cannot be labeled as either 0W or 10W [18]. For single non-winter grade oils, the kinematic viscosity test is applied at 100 C and higher viscosity values are graded with higher numbers.

Most vehicle engines work in wide temperature interval as winter to summer. Oils have low viscosity for high temperature and high viscosity for low temperatures. The difference of viscosities is very high for single grade oils. Therefore, polymer based viscosity improvers are added to oil to make it multi grade and more stable for temperature ranges. Multi grade oils are numbered with consecutive two symbols; first symbol is for cold and second one is for high temperatures. The viscosity of multi grade oil changes logarithmically respect to temperatures, but change is lessened compared to single grade oils. 15W40 graded oils are used for this research.

2.1.2.3 Lubrication mechanisms

While sliding motion takes place in a lubricated environment, three different lubrication regimes can be observed. They are boundary, mixed and hydrodynamic regimes (Figure 2.3). Piston ring motion includes three regimes together. While piston is moving in cylinder, it moves in cyclic manner (like sinusoidal motion). In one way motion, it accelerates to midpoint and reaches maximum velocity, then decelerates to end and stops there. Therefore velocity of lubricant changes with distance, and friction occurs between layers of lubricant and depends on shear stresses. Shear stresses in a viscous fluid can be described with following equation:

dy

dv

. (2.2)

Where η is viscosity of fluid, dv/dy is change in velocity respect to distance. Then friction force due to shear stresses is

A

dy

dv

(41)

21

Where A is contact area of moving part on fluid. Therefore coefficient of friction (COF) can be written as

F

F

s

(2.4)

Where μ is COF, Fs is friction force and F is load.

Boundary lubrication is the regime that lubricant film thickness is less than surface roughness. In this regime, lubrication is nearly ineffective and surfaces can contact, therefore the highest wear rate and friction loss occurs. Small offsets and cavities on surfaces interact and without cooling effect of lubrication; stick-slip, scuffing and seizure can be observed. Especially engines which work in high velocities and high loads would have boundary condition and starving. Therefore hardly repairable and even catastrophic failures can occur [19].

Figure 2.3: Stribeck Curve [54]

In mixed lubrication, oil film thickness is equal to surface roughness and even some higher. Therefore surfaces does not contact and slip over lubricant film. Least wear and friction are observed in hydrodynamic lubrication. Recently interests shift to this regime.

(42)

22

In hydrodynamic lubrication, lubricant film thickness is above the surface roughness. It is observed when lubricant is overflow or collects on a region. In this case, surfaces move in the lubricant; by that manner extensive friction force and tensile stress (so surface fatigue) on surfaces is produced by viscosity. These effects are proportional with amount of fluid.

2.1.2.4 Solid Lubricants

Solid lubricants are in use where liquid lubricant usage is limited (such as high temperature and vacuum applications). Solid lubricious materials and alloys are used on surfaces for reducing wear and friction. Strong surface adhesions cause high friction coefficients and high wear rates. Materials which have low tensile strength can be easily deformed even under low loads and low friction coefficients can be reached. By that manner, solid lubricants are generally mechanically anisotropic and they have low tensile strength.

Generally, lamellar crystal structured materials, soft metals and polymer materials are used as solid lubricants; classification can be seen on Table 2.1. In lamellar crystal structured materials, atoms on same layer are closer and have stronger bonds compared to atoms on different layers which are far and bonded with weak van der Waals bonds. Therefore, layers can slip relatively and that leads low friction. Soft metals have multiple shear systems and recrystallization property which prevents deformation hardening, so they can use as lubricant for a long time. Polymers can give low friction with motion of fluorides in their structure that leads easy shape change. Most common polymer lubricant is polytetrafluoroethylene (PTFE, Teflon). Liquid and solid lubricants are generally used in many applications to decrease friction and wear rate. In high and low temperatures, vacuum, radiation and high load applications, solid lubricant are obligatory because liquid lubrication is limited and even useless for these type conditions.

Solid lubricants generally form a tribofilm on the sliding surfaces where they are used. Solid can be used as single or as an additive in liquid lubricants. Most common solid lubricants are Molybdenum disulphide (MoS2), PTFE and graphite [20]. Least coefficient of friction values are reached with lamellar structured solid lubricants. Structures of some solid lubricants are shown in Figure 2.4.

(43)

23

In atmospheric environment and over 500°C temperature, lamellar solid lubricants lose their lubricant property (such as degradation of MoN over 350°C [22]). In this conditions oxides, fluorides and sulfides are used.

Main disadvantage of oxide lubricants is their brittle property. Therefore they can be broken and they can leave the surface easily. Also many oxide alloys can not be used under room temperature as a lubricant

Figure 2.4: Structures of some solid lubricants a) h-BN, b) Graphite, c) MoS2 [21] Table 2.1: Solid materials with self-lubricating capability [16]

Classification Key Examples Typical Range of COF

Lamellar solids MoS2, WS2, HBN, Graphite 0.002-0.7

Soft Metals Ag, Pb, Au, Sn, In 0.2-0.35

Mixed oxides

CuO-Re2O7, CuO-MoO3,

PbO-B2O3 0.1-0.3

Single oxides B2O3, Re2O7, ZnO, MoO3 0.1-0.6

Halides and sulfates of alkaline earth metals

CaF2, BaF2, SrF2, CaSO4,

BaSO4 0.15-0.4

Carbon-based solids

Diamond, DLC, Glassy carbon,

Fullerenes 0.003–1

Organic materials/polymers

Zinc stearite, Waxes, Soaps,

PTFE 0.04-0.4

Bulk or thick-film (>50 µm) composites

Metal-, polymer-, and ceramic-matrix composites consisting of graphite, WS2, MoS2, Ag, CaF2, BaF2, etc.

0.05-0.4

Thin-film (<50 µm) composites

Electroplated Ni and Cr films consisting of PTFE, graphite etc., particles as lubricants

Nanocomposite or multilayer coatings consisting of MoS2, Ti,

DLC, etc.

(44)

24

2.2. Nanocomposite Coatings

Nanocomposite coatings were studied by many researchers and companies in last two decades. This type of coating includes more than one type of materials and generally their grain size is less than 10nm.

There is a great interest about lower fuel and material consumption in numerous advanced tribological systems and also parts that build up the systems are desired to be more compact, more reliable and longer lifetime. Nanocomposite coatings serve that needs with their superior mechanical properties (superior hardness and toughness, greater resistance to contact deformation) and high chemical inertness, these coatings can significantly lower friction and wear losses and at the same time increase resistance to fatigue, erosion and corrosion, which have increasingly become the lifetime limiting factors for mechanical components in many industrial applications. [44, 45]

Nanocomposite coatings can be formulated to increase loading capacity of sliding surfaces and thus improve their resistance to scuffing. They can be designed as superlattice and multilayer films, whose properties are specifically tailored to meet the harsh application conditions of advanced tribosystems [3, 44, and 46]. It is also possible to design nanocomposite coatings that can form replenishing and self-lubricating tribofilms on sliding surfaces and therefore it helps to increase the lubricity [4, 36]

In recent years, with the selection and proper use of metallic and ceramic phases, excellent mechanical properties can be achieved. Various hardness properties can be achieved as less than 10GPa to 100 GPa [47, 48]. Recent experimental studies have confirmed that most nanostructured and composite coatings possess relatively high fracture toughness [49]. It is proposed that their nano-size grain morphology can provide an ideal condition for resisting crack initiation. Even if a crack has initiated within a grain, strong grain boundaries can deflect it back to the grain and so prevent crack growth.

Most conventional coatings have anisotropic and columnar micro-structure. The column boundaries may be relatively weak and so very susceptible to deformation or fracture. On the other hand, nanostructured coatings, the grains are extremely small and the morphology is neither anisotropic nor columnar.

(45)

25

Figure 2.5 shows the cross-sectional morphology of a conventional MoN film with columnar morphology and a superhard Cu/MoN film with undistinguished morphology.

The average grain size of nanocomposite film is in a range that is not well suited for crack initiation and propagation. In other words, Griffith’s criteria for crack growth are very difficult to meet in nanostructured and composite coatings [44,50].Therefore one of the main reasons why nanocomposite coatings exhibit superior wear properties is that they are better able to prevent crack initiation and growth.

Figure 2.5: Cross sectional image of (a) conventional MoN and (b) nanocomposite Cu/MoN coating [29]

Briefly, nanocomposite coatings can be designed to provide tunable mechanical properties. Beside high hardness and toughness, strong adhesion between film and substrate materials is another important aspect for most mechanical and tribological applications [44]. If adhesion of coating is poor, then coatings can rapidly leave the surface under the effect of high normal and shear forces in most tribological applications. Poor adhesion may often be the result of high residual stress or stress build-up at the film–substrate interface; the main reason is inadequate physical and chemical intermixing, or poor mechanical interlocking at the interface between coating and substrate. In most tribological applications, adhesion determines lifetime of coatings, load-bearing capacity and, therefore the effectiveness in such applications.

(46)

26

Recent researches guided to achieve very strong bonding between coatings and their substrates. Arc-PVD with bias and high-power pulse magnetron sputtering can provide an atomically clean surface and promote high levels of inter-diffusion or mixing between coating and substrate atoms at the interface and thus ensure strong bonding [4].

In sliding application, temperatures can reach very high levels and some of coatings at the sliding surfaces may oxidize. By the increasing desires for dry machining and high temperature metal-forming, chemical and thermal stability of coatings become much more important than before.

Liquid lubricants have existed for centuries and have been refined with additives over the years to provide better lubrication. The additive packages of these lubricants have reached their limits and further optimization does not seem feasible. Especially in vehicles, performance demands of customers increase day by day. Thus loads operating temperatures increase, and liquid lubricants can not satisfy this need. But nanocomposite coatings can supply that demand and they may further be optimized to provide low friction and wear even in the near absence of ZDDP- and Mo-DTC-type additives in liquid lubricants. Maybe in future, such coatings can replace liquid lubricants.

The increased flexibility of PVD and CVD techniques gave opportunity to produce novel coatings with complex microstructure architectures and chemical compositions. In industry cathodic arc and magnetron sputtering systems are widely used. In recent years, both systems are used in hybrid manner. This type of usage is most ideal to produce superlattice and multilayer coatings. In these processes, the physical shape or architecture of conventional deposition systems is not greatly changed, but additional targets and appropriate power sources are added to enable coating deposition by one of the methods alone, two or three of them in sequence, or altogether [51].

In recent years, multilayered coatings have become very popular. Because of their layered structure, these coatings provide much better fracture toughness and higher surface hardness and also better tribological properties for coated parts and components. Elemental or chemical ingredients used in each layer may themselves become lubricious, thus enabling tools to operate without external lubrication.

Referanslar

Benzer Belgeler

“ İletişimsizlik­ lerin giderek arttığı günümüzde, unutul­ muş veya unutulmaya yüz tutan insana tekrar merhaba diyebilmek için büyütül­ müş, dev insan

(2007) Nepal'de, yaptıkları bir çalışmada çeşit, çevre ve çeşitxçevre interaksiyonunun önemli olduğunu ve bazı çeşitlerin tüm çevre koşullarında stabil bir

Average yield of cultivars was analyzed in a 2-waY,no interaction model to see differences in cultivars and obtain genetic co.ponents for.. Tarla Bitkileri Merkez

The features are extracted from the normalized segmented iris region using Gabor wavelet transform.The feature extraction algorithm is in given in Algorithm 1.. From the

When all data were merged, participants had an accuracy level that is significantly higher than 50% in detecting agreeableness (male and female), conscientiousness (male

Esas olarak, cevher parajenezinin benzer olduğu bu evrelerden birincisi kuvars diyorit, tonalit ve granodi- yoritlerden oluşan granitoidin yerleşmesi ile birlikte, anakayaç içinde

questionnaire form can be summarized under eleven modules, which are: General Firm Information, Market Properties and Competition Structure, Firms’ Strategies,

The autonomy of the female self in late 19 th century and freedom from marriage are some of the themes that will be discussed in class in relation to the story.. Students will