TECHNICAL UNIVERSITY OF LIBEREC
FACULTY OF MECHANICAL ENGINEERING
NITROGEN S-PHASE COATINGS FOR FOOD PROCESSING INDUSTRY
DOCTORAL THESIS
2014 VĚRA JAHODOVÁ
ii
TECHNICAL UNIVERSITY OF LIBEREC Faculty of Mechanical Engineering
Department of Material Science
Study Program: 2303V002
Mechanical Engineering Technology Specialization: Material Engineering
Doctoral thesis
NITROGEN S-PHASE COATINGS FOR FOOD PROCESSING INDUSTRY
by
VĚRA JAHODOVÁ
Supervisor: Prof. Ing. Petr Louda, CSc.
iii Prohlášení
Byl(a) jsem seznámen(a) s tím, že na mou dizertační práci se plně vztahuje zákon č. 121/2000 Sb., o právu autorském, zejména § 60 – školní dílo.
Beru na vědomí, že Technická univerzita v Liberci (TUL) nezasahuje do mých autorských práv užitím mé diplomové práce pro vnitřní potřebu TUL.
Užiji-li dizertační práci nebo poskytnu-li licenci k jejímu využití, jsem si vědom povinnosti informovat o této skutečnosti TUL; v tomto případě má TUL právo ode mne požadovat úhradu nákladů, které vynaložila na vytvoření díla, až do jejich skutečné výše.
Dizertační práci jsem vypracoval(a) samostatně s použitím uvedené literatury a na základě konzultací s vedoucím diplomové práce a konzultantem.
Datum: 9/11/2013 Podpis:
Věra Jahodová
iv ANNOTATION
The aim of this doctoral thesis is deposition of nitrogen doped coatings on austenitic stainless steel to enhance the wear resistance in food processing industry. Reactive magnetron sputtering was used to deposit S-phase coatings. Thanks to variable nitrogen concentration in the working atmosphere, coatings with different nitrogen content were created. Structure examination gave evidence of pure S-phase structure without any appearance of other phases (e.g. chromium nitrides). Hardness was increasing along the nitrogen content in S-phase coatings up to three times.
Wear rate loss was significantly decreased (i.e. for over two orders of magnitude) although the values of friction coefficient increased compared to steel substrate.
Resistance to general corrosion was not affected; better behaviour was observed after immersion tests in physiological solution.
Key words: stainless steel, magnetron sputtering, wear, corrosion.
ANOTACE
Cílem dizertační práce bylo vytvoření vrstev dopovaných dusíkem na austenitické korozivzdorné oceli pro zvýšení otěru odolnosti v potravinářském průmyslu. Reaktivní magnetonové naprašování bylo použito k vytvoření vrstev z fáze S. Povlaky s různou koncentrací dusíku byly vytvořeny díky variabilnímu obsahu dusíku v pracovní atmosféře. Zkoumání struktury potvrdilo existenci fáze S bez přítomnosti dalších fází (např. nitridy chromu). Tvrdost se zvyšovala se zvyšujícím se obsahem dusíku až třikrát. Opotřebení bylo výrazně sníženo o dva řády, i když koeficient tření vzrostl oproti ocelovému podkladu. Korozní odolnost nebyla narušena a byla pozorována dokonce lepší odolnost po ponoru ve fyziologickém roztoku.
Klíčová slova: korozivzdorná ocel, magnetronové naprašování, opotřebení, koroze.
v ACKNOWLEDGEMENTS
First, I would like to express sincere gratitude to my advisors, Professor Petr Louda and Dr. Zbigniew Rożek (consultant), for their guidance and great instruction on my research during the PhD study at Technological University of Liberec, Czech Republic.
Second, I would also like to express my sincere thanks to Assoc. Prof. Witold Gulbiński (consultant) and Dr. Tomasz Suszko, for their continuous support, inspiration, and helps in many ways help through my ERASMUS study at Koszalin University of Technology, Poland and financial support of the National Science Centre, Poland, according to the decision DEC-2011/03/B/ST8/06130.
Third, I would like to express my appreciation to Surface Technology Group from Singapore Institute of Manufacturing Technology, Singapore for providing me privileges to use their facilities and for providing great help through my study stay. I would like to thank Dr. Ding Xing Zhao for his brilliant guidance.
Fourth, I would like to thank A*STAR (Agency for Science, Technology and Research, Singapore) for financial support during my stay in Singapore Institute of Manufacturing Technology, Singapore and to the Project OP VaVpI Centre for Nanomaterials, Advanced Technologies and Innovation CZ.1.05/2.1.00/01.0005.
vi TABLE OF CONTENTS
Declaration iii
Annotation / Anotace iv
Acknowledgements v
Table of Content vi
List of Figures vii
List of Tables x
Introduction 1
Thesis Scope 2
Thesis Outline 3
Chapter 1 Literature Review 4
1.1 Stainless Steel 4
1.1.1 Introduction to Stainless Steel 4
1.1.2 Mechanical Properties of Stainless Steel 5
1.1.3 Structure and Surface Finishing of Stainless Steel 7
1.1.4 Chemical Surface Treatment of Stainless Steel 8
1.1.5. Corrosion Resistance of Stainless Steel 9
1.1.6 Coatings Used in Food Industry 11
1.2 S-Phase 13
1.2.1 Introduction to S-phase 13
1.2.2 Structure of S-phase 13
1.2.3 Mechanical Properties of S-phase 20
1.2.4 Magnetic Behaviour of S-phase 23
1.2.5 Corrosion Resistance of S-phase 23
Chapter 2 Materials and Experimental Procedures 25
2.1 Target and Substrates 25
2.2 Testing Solutions 25
2.3 Coatings Production 26
2.4 Thickness Measurement of Coatings 28
2.5 Glow Discharge Optical Emission Spectroscopy 28
2.6 Structure Examination 28
2.7 Hardness Measurement 29
2.8 Surface Examination 29
2.8.1 Scanning Electron Microscopy 29
2.8.2 Atomic Force Microscopy 29
2.9 Tribological Behaviour Testing 30
vii
2.10 Electrochemical Testing 31
2.11 Mercedes Adhesion Test 31
2.12 Immersion Testing 32
Chapter 3 Laboratory Testing 33
3.1 Coating Thickness and Deposition Rate Determination 33
3.2 Chemical Composition Determination 33
3.2.1 GDOES Chemical Composition 33
3.2.2 EDX Chemical Composition 35
3.3 Structure Determination - XRD examination 37
3.4 Mechanical Properties 39
3.4.1 Hardness 39
3.4.2 Tribological Behaviour 40
3.5 Electrochemical Behaviour 42
Chapter 4 Industry Application Testing 46
4.1 Structure Determination - XRD examination 46
4.2 Mercedes Test for Adhesion 47
4.3 Roughness Measurement 49
4.3.1 Roughness – Contact Profilometer 49
4.3.2 Roughness - AFM Measurement 49
4.4 Wear Behaviour in Solutions 50
4.4.1 Wear Testing 50
4.4.2 SEM and EDX Examination of Wear Track 54
4.5 Immersion Tests 60
Chapter 5 Conclusions and Recommendations 66
5.1 Conclusions 66
5.2 Recommendations for Future Work 68
References 69
List of Publications 73
viii LIST OF FIGURES
Fig. 1 Structure of magnetron deposited S-phase - 300 °C [32] 15 Fig. 2 Structure of magnetron deposited S-phase - room temperature [32] 16 Fig. 3 Nitrided layer evaluation at 400 °C after different nitriding time [34] 17 Fig. 4 Influence of different sputtering power density on structure [36] 18 Fig. 5 Influence of different temperature during plasma nitriding [38] 19 Fig. 6 Influence of process time on structure (ion nitriding - 400 °C) [39] 20 Fig. 7 Mechanical properties change along the nitrogen content [42] 21 Fig. 8 Wear track profile after lubricated pin-on-disc wear testing [42] 22 Fig. 9 Cyclic voltammograms of plasma nitrided and untreated 304 SS [49] 24 Fig. 10 Schema of PVD deposition system placed in TU Koszalin, PL 27 Fig. 11 Equipment of ball-on-disc wear test – ASTM G133 30 Fig. 12 Table for adhesion determination of Mercedes test 31
Fig. 13 GDOES analysis of coatings – nitrogen content 34
Fig. 14 GDOES analysis of coatings – aluminum content 34
Fig. 15 SEM and EDX analyze of sample SN01 36
Fig. 16 SEM and EDX analyze of sample SN02 36
Fig. 17 SEM and EDX analyze of sample SN03 36
Fig. 18 Structure of substrate - XRD examination 37
Fig. 19 Selective corrosion attack of substrate after grinding 37
Fig. 20 Structure of coatings – XRD examination 38
Fig. 21 Crystallites size along nitrogen content. 39
Fig. 22 Hardness dependence on nitrogen content 40
Fig. 23 Coefficient of friction of coatings for linear speed 10 cycles per minute 41 Fig. 24 Wear rate of coatings for linear speed 10 cycles per minute in air 42
Fig. 25 Delaminated SN02 surface after wear test 42
Fig. 26 Central part of wear tracks 43
Fig. 27 Results of potentiodynamic studies in 3 w% NaCl solution 44 Fig. 28 Pitting corrosion on substrate surface after electrochemical testing 45 Fig. 29 Structure of deposited coatings - XRD examination 47
Fig. 30 Mercedes test evaluation of coatings 48
Fig. 31 Record of contact profilometer - substrate surface 49
Fig. 32 Record of AFM measurement - substrate surface 50
Fig. 33 Wear scar after wear testing in physiological solution 51 Fig. 34 Wear scar after wear testing in milk solution 52
ix
Fig. 35 COF versus time in physiological solution 53
Fig. 36 COF versus time in milk solution 53
Fig. 37 SEM image of SN1200 after wear in physiological solution 54 Fig. 38 EDX chemical maps of SN1200 after wear in physiological solution 55 Fig. 39 SEM image of SN1200 after wear in milk solution 55 Fig. 40 EDX chemical map of SN1200 after wear in milk solution 56 Fig. 41 SN0903 after wear in physiological solution SEM and EDX examination 56 Fig. 42 SN0903 after wear in milk solution - SEM and EDX examination 57 Fig. 43 SN0804 after wear in physiological solution - SEM and EDX examination 58 Fig. 44 SN0804 after wear in milk solution - SEM and EDX examination 58 Fig. 45 SN0606 after wear in physiological solution - SEM and EDX examination 59 Fig. 46 SN0606 after wear in milk solution - SEM and EDX examination 59
Fig. 47 Beginning of immersion test – 0 hour 60
Fig. 48 Finish of immersion test – 150 hours 61
Fig. 49 SEM image of substrate surface after immersion 62
Fig. 50 AFM image of substrate after immersion 62
Fig. 51 SEM image of SN1200 surface after immersion 63
Fig. 52 SEM image of SN0606 surface after immersion 63
Fig. 53 Record of AFM measurement - substrate surface after immersion 64
x LIST OF TABLES
Table 1 Chemical composition of 304 and 316L steel - in w% [7] 25 Table 2a Working atmosphere composition for laboratory tested samples 27 Table 2b Working atmosphere composition for industrial tested samples 27 Table 3 Calculated deposition rate and coatings thickness 33 Table 4 Nitrogen incorporation into the deposited coatings - GDOES results 35 Table 5 Nitrogen incorporation into the deposited coatings - EDX results 35 Table 6 Average linear roughness values - contact profilometer 49 Table 7 Average roughness values from central line – AFM measurement 50 Table 8 Change in average area (Ra) roughness after immersion
– AFM measurement. 64
Table 9 Change in peak-to-valley area roughness after immersion
– AFM measurement 65
1 Introduction
Nowadays, austenitic stainless steels (ASS) are widely used in chemical, food, construction, and biomedical industries. Main reason for such huge employment of these materials is an excellent resistance to general corrosion and their biocompatibility. However, low mechanical properties foremost low hardness and wear resistance, can shorten the service life of components.
Significant improvement in wear resistance can be reached through construction changes, thermal treatment, working environment adaptation or suitable coating.
There are many wear resistant coatings keeping high hardness and great durability.
Depositing of hard coating should improve wear resistance but it is limited by low hardness of stainless steel. The problems those are to face are poor adhesion and low bearing capacity of substrates material.
Traditional way of increasing the bearing capacity of steel is nitriding (gas and ion nitriding). Conventional nitriding uses high temperature to decrease time of process but it also gives energy to form precipitates of chromium nitrides in steel structure.
Presence of chromium precipitates weakens the corrosion resistance [1].
Low temperature nitriding has found out the way out. During examination of nitrided area of ASS there was new phase explored. Nitrogen stabilised austenite was named as S-phase [2]. This hard and corrosion resistant phase opens new perspectives in steel protection.
Nitrogen-rich interlayer provides chemical and mechanical similarity and should secure well adhered inter-coating in between stainless steel substrate and hard cover coating. Because the S-phase is possible to be obtained by magnetron sputtering from steel target in argon-nitrogen atmosphere, deposited coating can be also used as top coating not only as interlayer. Because of almost identical structure of S-phase obtained by nitriding and deposition, it is possible to use magnetron sputtering as a reconstruction method. Originally nitrided parts finishing their service life could be missing some top material.
The aim of this study is to deposit S-phase coatings with good adhesion and corrosion resistance for applications where corrosion and friction is combined.
2 Thesis Scope
The aim of study was to deposit metallic coating for enhancing tribological behaviour without affecting resistance to general corrosion what could be used in contact with food.
(1) To determine structure depending on deposition conditions
The main aim is to identify deposition conditions to obtain single phase coating with a good adhesion to steel substrate and based on these results find the process prescription to obtain coating with specified properties.
(2) To study wear behaviour
Wear properties should exhibit lower material wear loss compare to pure (untreated) stainless steel. Wear behaviour will be tested in rotative and linear mode under stable condition and with a presence of solutions.
(3) To evaluate behaviour in corrosive environment
The corrosion resistance should not be affected in comparison to substrate material.
Electrochemical behaviour should confirm not worse resistance to general corrosion.
Immersion test in different solutions will be used to classify the tolerance of these coatings to be applied in food processing industry.
3 Thesis Outline
The dissertation is organized into five chapters. The first chapter of literature review is divided into two parts. The first part summarised knowledge and recent findings on stainless steel and gives the outline on the properties and behaviour of this famous material. The second part represents a short report on S-phase from its discovery to today's application and contains detailed list of useful properties and its comparison in between different ways of S-phase production. The second chapter briefly describes experimental methods used to characterise S-phase coatings, substrate material and coating-substrate system obtained with magnetron sputtering.
The third and fourth chapters discuss the tested behaviour and properties on laboratory (the third chapter) and on industry application (the fourth chapter) level.
The fifth chapter summarizes the results of current study and outlines the possible further research.
4 CHAPTER1LITERATUREREVIEW
1.1 Stainless Steel
1.1.1 Introduction to Stainless Steel
Stainless steel is reasonable title for over 300 materials (alloys). They are divided into different classes or families based on similarities. Stainless steel is a low carbon steel with high chromium content. Steel is possible to be classified by structure to one of four categories; there are austenitic, ferritic, martensitic, and duplex. All steels are due to chromium content corrosion resistant but also high and/or low temperature resistant (retaining the mechanical properties to relatively high/low temperatures).
They are easy to fabricate, clean and recycle. Steel industry is bonded with costs and recycling of materials is lowering the total costs of the product. The use of stainless steel scrap is fundamental to the steelmaking process. Today, stainless steel is made up of approximately 60% recycled material. Batch is composed of 25% reclaimed scrap (includes industrial equipment, tanks) and of 35% industrial scrap (industrial returns or production off-cuts from manufacturing) [3].
Steel types 18/8 (other description 18.8 or 18-8), 18/10 and 18/0 are recommended to the cutlery industry. The prescript compositions are given by two numbers which means concentration of chromium and nickel. The '316' types are often referred to as the 'food' grades [4]. The 304 and 430 types are suitable for food processing and beverage manufacturing, handling, for manufacture, bulk storage and transportation. There is no known official classification, or restriction for stainless steels for food industry applications. Stainless steel 304 or 316 types are generally suitable for storing and handling cold or unheated drinking (town's) water tanks [4].
Material requirements for contact with food are summarized in data sheets from Control of Substances Hazardous to Health (CSOHH). The Advice and recommendation available for stainless steels outlines any risks associated with its handling, fabrication and use [5]. Also cleaning is included. Effective cleaning is essential in maintaining the integrity of the process and in prevention of corrosion.
The choice of cleaning method and the frequency of its application depends on the nature of the process, the food being processed, the deposits formed, hygiene requirements etc.
The following cleaning methods are suitable [5]:
water and steam (hot and cold)
5
mechanical scrubbing - hard and adherent deposits can be removed with stainless steel wool, Scotchbrite sponges.
scouring powder and detergents - any chloride free scouring powders and detergents may be used not to scratch the visible surfaces.
alkaline solutions - solutions containing soda, ammonia or ammonium hydroxide can be used for removing grease and fats.
nitric acid - can also be used to remove iron contamination from stainless steel surfaces, thus preventing “rust staining” effects. Most stainless steel is safe in contact with a wide range of concentrations of nitric acid.
For disinfection of stainless steel equipment [5] there are chemical disinfectants often more corrosive than cleaning detergents, e.g. hypochlorites, chloramine (Sodium hypochlorite or potassium hypochlorites are commercial sterilising agents), tetravalent ammonium salts, iodine compounds. These detergents due to presence of halogen ions, particularly chlorides can cause pitting which will be discussed below.
Vessels and equipment must be rinsed after the use of cleaning agents. While flushing or rinsing with water is usually adequate, it may be necessary to rinse the equipment with a neutralising solution (e.g. soda).
1.1.2 Mechanical Properties of Stainless Steel
Mechanical properties and structure is resulting (given) by chemical composition and processing parameters. Pure iron is soft (even softer than aluminium), but is unobtainable by smelting. Impurities from the smelting process significantly harden and strengthen this material. There are four allotropic forms of iron. Common forms are α and γ, which can exist in atmospheric conditions. Allotropic forms, called as δ and ε exist at very high pressures, even high temperatures.
In text bellow there are listed main elements including their influence on steel [6].
Carbon (always present) content is generally held at low levels with exception of martensitic and precipitation hardening steels. It increases strength of steel.
6 Content of carbon should be low due to chromium carbides precipitation which weakens the corrosion behaviour of steel.
Chromium is a corrosion resistance element. The chromium content should be over 10.5 w% (according to European standard EN10088 [7]) in alloy.
Chromium protects the surface through formation the insoluble, adherent film that prevents the further diffusion of oxygen into the surface and prevents the oxidation of the iron in the matrix. The higher the chromium level the greater the protection. The thickness of protective film is about 1.5 to 2.5 nm [3].
Nickel is the alloying element stabilizing the austenitic structure (e.g. 300 family). The austenite results strength, ductility and toughness, which retains to even cryogenic temperatures. It also makes the material non-magnetic.
Nickel is being added for better corrosion resistance in environment of reducible acids with no direct influence on the development of the passive surface film. These days nickel is being substituted with manganese mainly because of its price.
Manganese is also austenite stabilized element. Manganese is being added to stainless steels to help in de-oxidation, during melting, and to prevent the formation of iron sulphide inclusions which can cause hot cracking.
Manganese also increases toughness in low temperatures. Replacing the nickel, the 200 family is established.
The addition of molybdenum to the Cr-Fe-Ni matrix increases resistance to localized pitting attack and better resistance to crevice corrosion. Molybdenum is changing the electric behaviour of protective film and enhancing the localized corrosion resistance in chloride environment. The higher the molybdenum content the better the resistance to higher chloride levels.
Nitrogen raises the yield strength and enhances the resistance to pitting corrosion and inter-granular corrosion in austenitic and duplex stainless steels.
Low carbon grades (designated with an “L” since containing less than 0.03%
carbon), are suggested for welding operations, since the lower carbon minimizes the risk of sensitization. High nitrogen steel has been formed into 200 steel family to highlight the importance of this element.
Sulphur is generally kept at very low levels as it can form sulphide inclusions.
It is used to improve machinability (as chip breakers). Sulphur is said to be the initial of pitting formation; it disturbs the electric status of protective film.
7 Mechanical properties of bulk material of ASS [8] published to every final product are tensile strength, yield strength of 0.2% proof and hardness. These properties are possible to be changed by thermal treatment and processing to final shape. These numbers (values of mechanical properties) are important to predict the service lifetime of each part. The lifetime prediction takes into account the perfect status of part surface but any influence of aggressive environment.
1.1.3 Structure and Surface Finishing of Stainless Steel
Austenitic stainless steel has pure austenitic structure. Austenite (also called gamma phase) is high temperature meta-stable phase of iron [9]. Its face-centred cubic (FCC) crystal structure is non-magnetic from nature. Austenite is stabilized by additional elements to retain to room temperature. It is not possible to obtain different phase by conventional thermal treatment. Application of untreated austenitic steel is limited by low hardness and wear resistance what shortens its lifetime. Surface engineering is still developing new surface treatments or coatings to remove this weakness.
Surface as a sharp interface between bulk material and air plays significant role in engineering applications. The importance of surface was highlighted by creation of separate scientific discipline fittingly named Surface engineering. Scientists study interaction between surface and environment (e.g. corrosion) also with combination of external forces (e.g. wear). The results of such studies are technological improvements for higher surface hardness, longer lifetime, to improve fatigue life and corrosion resistance. Fatigue damage is the progressive and localized structural damage that occurs when a material is exposed to cyclic loading. It initiates from the surface imperfections and it is the main reason to take care of surface.
Surface finishing plays crucial role in specific applications [10] . Surface finishing can be distinguished as mechanical or chemical finishing. Requirements of surface finishing are summarized in different normative rules, e.g.:
ASME B46.1-2002 - Surface Roughness, Waviness, and Lay ISO 4287 and 4288 - Geometrical Product Specifications (GPS) DIN ISO 1302, DIN 4768 - Comparison of Roughness Values
8 ASME Y14.36M - Surface Texture Symbols
EN 10088-2 - Technical delivery conditions for sheet/plate and strip of corrosion resisting steels for general purposes - valid in European Union - [7]
The 2B is one of the surface finishing labelling [7]. It means cold rolled, annealed, pickled and light rolled (also called lightly rolled). This is the most common finish for most steel types to ensure good corrosion resistance, smoothness and flatness. Also finishing commonly used for further processing.
Group after cold rolling is specified by change in mechanical properties also called as Surface hardening by cold working. Cold rolled finished are ready to be used for industry applications, e.g. heat-resisting elements.
Various types of polishing operations are commonly used to reduce the surface roughness of metals used in food industry, piping and related components [10]. Electropolishing is an electrolytic process combining electric current and chemicals to remove metal. This electrochemical action produces a smoothing and rounding of the surface profile, resulting in irregularities as small as 0.01 μm. Effect of electropolishing parameters was in details studied by Hryniewicz [11] on steel AISI 430 and AISI 316L.
1.1.4 Chemical Surface Treatment of Stainless Steel
Chemical surface treatment is based on introducing atoms into interstitial positions of material to produce internal stress which results in increased hardness.
This kind of hardening of surface is possible to be reached through solid-solution strengthening mechanisms - substitutional or interstitial solid solution. The increasing concentration of the impurity results in an increase in tensile and yield strengths.
In interstitial solid solutions, solute atoms do not displace solvent atoms. Solvent atoms such as carbon, nitrogen, hydrogen or oxygen enter one of the holes or interstices between the solvent atoms. An excellent example is iron-carbon system.
In this system the carbon (solute atom) atom occupies an interstitial position between iron atoms (solvent atoms). There is a rule of atomic radii ratio for forming the interstitial solid solution [12]. The ratio between solvent and solute atoms should be less than 59 %. The second way of interpretation is based on atomic radii [13]. Atoms
9 which have atomic radii less than one angstrom are likely to form interstitial solid solutions. Examples are atoms of carbon (0.77 A°), nitrogen (0.71 A°), hydrogen (0.46 A°), Oxygen (0.60 A°) etc.
Chemical-heat treatment is a method of introducing the elements (N, C, B, etc.) by diffusion through surface layers with the desired effect on mechanical behaviour of surface and bulk. There is a possibility or need of further heat treatment. For conventional nitriding nitrogen concentration reaches 12 w%. Formation of new phases takes place; there are γ΄ iron nitride (Fe4N) which is stable till 680°C and nitride ε (wide chemical composition and temperature stability range). Hardness can reach 1200 HV. Final treatment is tempering (quenching) [14].
Salt bath for nitrocarburizing [15] consists of alkaline cyanate and alkaline carbonate. Due to these chemical compounds it is very high risk for environment and is almost replaced with more environment friendly methods. Gas nitriding [16] uses the ammonia gas at 500 - 520°C and process times are 40 - 80 hours. For reducing the process time more sophisticated methods - plasma nitriding [17] (carried out in a nitrogen - hydrogen atmosphere at 400 - 600°C and a pressure of approximately 50 - 500 Pa), low-temperature plasma nitriding [18] (below temperature of CrN formation), plasma implantation and glow-discharge nitriding [19] .
Coatings are another solution for enhancing the wear resistance. Nowadays PVD methods are the most used. Magnetron sputtering of non-magnetic steel target can be used to produce structure similar to nitrided layer [20].
1.1.5 Corrosion Resistance of Stainless Steel
Environment influences constantly industrial devices. Chemical attack changes the electric status of surface, what leads to damage. Place affected by chemical attack is weakened and due to combination with loaded stress, the failure becomes more probable. All corrosion is an electrochemical process of oxidation and reduction reactions.
Corrosion hazards in food applications [5], i.e. pitting, crevice, and stress corrosion cracking are highlighted to be avoided. Suitable cleaning and disinfection systems should be used except of hypochlorite or chloride solutions.
10 Corrosion is the gradual degradation of material by interaction with its environment. It is possible to categorize signs of corrosion as uniform attack, crevice corrosion, pitting, intergranular corrosion, selective leaching, erosion corrosion, stress corrosion, and hydrogen damage [21]. Combination of types also occurs. Localized corrosion is more dangerous in high rates of metal penetration at specific sites but the remainder of surface goes largely unaffected. The attack can be hard to detect because the damage may have only a small opening visible to the eye at surface.
Stainless steel is sensitive to these forms of corrosion [5], [9] :
Crevice corrosion - develops from the compositional difference between inside and outside solutions in the tight gap and is resulted from the basis of aeration, metal concentration, pH, chloride ion concentration or inhibitor concentration. Crevice corrosion occurs in the edges (contacts) of parts – with different or similar material. Non-metal-to-metal or metal-to-metal contacts are present in all piping. In joins, the crevices are hard or almost impossible to detect because of very small critical gab size.
Intergranular corrosion – also called sensitisation and typically occurs during welding or temperatures between about 450-850°C. Chromium can combine with carbon to form chromium carbides along the grain boundaries. Depleting of chromium from bulk affects the passive layer and causes the reduction of corrosion resistance.
Stress corrosion cracking (SCC) - a relatively rare form which occurs in a very specific combination of tensile stress, temperature and corrosive species (e.g.
chloride ions). Hot water tanks and swimming pools are typical environment for SCC occurrence.
Pitting corrosion – the most dangerous and most common local break in the passive layer caused in aqueous environments rich in chloride and/or sulphides. The unanticipated occurrence and unpredictable propagation rate make it difficult to take it into consideration in practical engineering designs.
Because the pitting corrosion is a serious danger, Pitting Resistance Equivalent (PRE) was established as a tool to compare the risk of pitting attack. It is base of chemical composition of steel in weight percent [4]:
11 PREN = % Cr + 3.3 (% Mo) + K* (% N)
K has a value 30 for austenitic grades of steel [22].
(earlier there was no nitrogen taken into consideration and only PRE was calculated)
Stainless steel 304 has a PREN value from 17.5 to 20 and stainless steel 316L from 23.1 to 28.5 [4]. Because the coefficient K is 30 even small nitrogen content has certain influence.
The chemical composition is not the only thing which influences the corrosion resistance of ASS. Thermal treatment (e.g. annealing) do not need to change chemical composition but is an energy source for precipitation formation. Treatment temperature should not cross the value 400 to 500 °C. This temperature depends on chemical composition of steel. The most dangerous are chromium precipitates (e.g.
nitrides). Such formation depletes unbounded chromium from the solution and the corrosion resistant is reduced [1].
Following the PREN calculation, nitrogen became very important. High-nitrogen stainless steels are important class of engineering materials. Steel is being considered "high-nitrogen" if it contains more nitrogen than can be retained in the material by processing at atmospheric pressure. For most austenitic materials, this limit is about 0.4 wt%. Nitrogen additions have a beneficial effect on pitting and crevice corrosion resistance. It appears that nitrogen in solid solution increases creep and fatigue resistance but still there exists the temperature limit for use; it is 500 °C [23].
Stainless steel 316L contains molybdenum which is added to further enhance of corrosion resistance [24]; the corrosion resistance to general corrosion is much higher than to SS 304 and also to pitting corrosion. Austenitic SSs are sensitive to crevice corrosion which occurs at the edges, corners or joints. This corrosion is hard to avoid the only solution is to adjust the corrosion environment or make changes in construction.
1.1.6 Coatings Used in Food Industry
Aggressive environment increase the requirements on resistance not to cut the service life of mechanical parts. If it is not possible to influence the surroundings, surface should be modified.
12 Coatings can possess special properties why to use them, they are for example:
abrasion resistant, anti-stick, corrosion resistant, wear resistant, galling resistant, noise reducing etc. All these properties are not utilized in food processing industry.
the most common desired property is a friendly behaviour to living creatures.
In the text below there are some examples of commonly used coating proved to contact with food.
Porcelain enamel - Industrial porcelain enamel (also known as vitreous enamel) is a thin layer of ceramic or glass applied to a metal substrate and is used to protect surfaces from chemical attack and physical damage, modify the structural characteristics of the substrate, and improve the appearance of the product.
Porcelain enamel is used most often in the manufacture of products that will be expected to come under regular chemical attack or high heat such as cookware, burners, and laboratory equipment. There is an indicated future trend of coating all outdoor mild steel products in a weather-resistant porcelain enamel.
Porcelain enamel is also used in individual artistic production for a very long time, but still very popular even to these days [25].
Ceramic Coatings – by TRIPLEX Pro™ - thermal sprayed ceramic coatings for wear and abrasion resistance on crankshafts, pumps, piston rods, textile parts, seals and valve seats, they can consist from Al2O3 (alumina), chromium oxide, TiO2
(titania) and combination. High refractive index, surpassed by few other materials, allows titanium dioxide to be used at relatively low levels to achieve its technical effect and change in colour - pigment [26].
Teflon® - is a commercial name for polytetrafluoroethylene - PTFE. It is a fluorocarbon solid, as it is a high-molecular-weight compound consisting wholly of carbon and fluorine. PTFE is hydrophobic: neither water nor water-containing substances wet this material. PTFE has one of the lowest coefficients of friction against any solid. The best known brand name of PTFE is Teflon by DuPont Co [27].
The food processing industry is still developing industrial branch possesses wide importance. There many coating under testing procedure to become certificated instrument for the food contact applications. Coatings already tested for food industry are for example diamond-like carbon (DLC), DLC–Si–O, Ni–P–PTFE, silica coatings (SiOx), [28] etc.
13 1.2 S-Phase
1.2.1 Introduction to S-phase
To solve the problem of poor tribological behaviour of stainless steel, low temperature plasma nitriding process was invented in the middle 1980’s. There were some different observation and explanation of nitrided layer in austenite. Together with the process carried out at 400°C, there was also new phase discovered and described. Supersaturated austenite was designed as S-phase already in 1980’s [2].
The crystallographic study of S-phase was developed to enrich the ASTM (American Society for Testing and Materials) index. S-phase was separated from the ordinary austenite with a higher hardness but comparable corrosion resistance. In a parallel investigation, second group of scientists Zhang and Bell measured the hardness at level of 700 HV 0.05. All these studies were using based material labelled as 304 and 316L stainless steel.
S-phase is also possible to be found in different materials from steel, e.g. Inconel 690 (mainly nickel-chromium) alloy [29].
The temperature limit for forming S-phase is between 400 – 450 °C and it depends on steel composition. If the temperature is crossed the chromium nitrides precipitation takes place. Chromium is highly attracted by nitrogen atoms more than any others what explains the primary precipitation of chromium nitrides.
1.2.2 Structure of S-phase
The new phase discovered after low temperature nitriding is nitrogen supersaturated phase with austenitic expanded cell structure, higher hardness and magnetic behaviour. This phase was named as S-phase or expanded austenite.
Nitrogen atoms are placed in interstitial positions in cells and form solid solution. It is also possible to obtain austenite expanded (stabilized) by carbon [30].
S-phase structure is derived from Cr-Ni-Fe austenitic structure. XRD peak positions are not constant. They are resulted by the nitrogen concentration.
Compared to the primary austenite all peak positions are shifted to lower values.
Peaks are wide at the bottom and not always symmetrical. The shape, position, and intensity of peak were firstly described by Ichii in 1985 [2].
14 Structure of nitrided layer in temperature of only 400 °C was fitted with labels of S1 to S5. All these peaks appear between 30 to 100 degree (Cu Kα radiation) diffraction patterns. For example, S1 peak is derived from γ Fe (111).
After nitriding, a compound layer and an underlying diffusion zone are formed near the surface of the austenitic steel. The compound layer (known as the white layer) consists predominantly of ε - Fe2-3(C, N) and γ΄ - Fe4N phases as a mixture (there are two sub-layers in pure ferrite) [31]. The hardened diffusion zone is composed of interstitial solid solution of nitrogen dissolved in the ferrite lattice and nitride and/or carbo-nitride precipitation for the alloy steels containing the nitrides forming elements [16].
Magnetron deposited S-phase is the phase with stable chemical composition.
Study of Kappaganthu and Sun [32] exhibited the differences between S-phase obtained in process of low temperature nitriding and reactive magnetron deposition.
Not all peaks of S-phase were detected in the deposited structure. Nitrogen is building in the (200) direction and forms the pure phase. Increasing nitrogen content shifts the peak position to the lower angles; see Fig. 1. Very interesting result is the metallic deposition of steel target in pure argon. Metallic deposition builds the BCC and/or ferritic structure (α(110) is used in Fig. 1) characterized with the ferritic peak position, but it is not possible to obtain BCC structure with such chemical composition using conventional methods.
15 Fig 1: Structure of magnetron deposited S-phase - 300 °C [32].
Along the increasing nitrogen gas composition in the sputtering gas mixture, the crystalline structure of the stainless steel films changed from BCC ferrite, to expanded FCC austenite and finally to the MN type cubic nitride. This phase was identified as (Fe0.69Cr0.2Ni0.1Mo0.01)N phase. It is also the evidence of wide range of nitrogen concentration in S-phase. Here should be highlighted that the substrate temperature is important process parameter. There is amorphous layer formed in case of no heated substrates [32].
16 Fig 2: Structure of magnetron deposited S-phase - room temperature [32].
After comparison of two XRD structure examination it is possible to find peaks at similar positions but the shape is different - more broader. Also some peaks were not built. In case of presented plots (Fig. 1 and 2) the higher angle peaks are not presented for 10 % N2 but for 25 % N2 the peaks are merged to one. No chromium nitrides under both deposition temperatures were observed. Adhesion to substrate was not studied [31].
The broadening of the peaks for the nitrided samples is a consequence of the nitrogen concentration gradient in the analysed volume and an anticipated high density of microstructural defects in the γN layer, i.e. stacking faults, dislocations etc.
[31]. Asymmetry of the peaks obtained for expanded austenite is described to the presence of gradients in stress, composition and stacking fault probability. In addition, also texture gradients could contribute to the asymmetry. Such texture gradients originate from grain rotation caused by plastic deformation in the expanded austenite case during growth [33].
The evolution of S-phase [34] during nitriding at 400 °C was studied in time sequence (see Fig. 3). The shifts of peak positions are displayed with different shape and broadness. Vertical dashed lines denote the positions of untreated austenitic steel 316L and they are the evidence of shifting to lower 2θ degree due to nitrogen presence.
17 Fig 3: Nitrided layer evaluation at 400 °C after different nitriding time [34].
For reactive magnetron deposition, the dependence of nitrogen concentration in coatings on nitrogen content in atmosphere is experimentally given in the work of Kappaganthu [35]. It also works as the instruction to choose the final chemical composition. This kind of dependence is typical for every specific deposition equipment and should be found.
It is also necessary to find processing setting of deposition equipment to obtain optimal and stable deposition. One example regarding the obtained structure on different power density was described [36] connected to reactive magnetron sputtering in 25 vol% of nitrogen in working atmosphere (Fig. 4). Along increasing power density the structure is changing form amorphous through ferritic to S-phase.
It is important to hold the other parameters unchanged to identify the main influence of every single parameter.
18 Fig 4: Influence of different sputtering power density on structure [36].
After examination of homogeneous powder of stress-free S-phase with different nitrogen contents obtained by gaseous nitriding it was proved that the S-phase has a face centred cubic structure (FCC) and that stacking faults contribute to systematic deviations of the X-ray line profiles from their fault free positions [37].
After crossing the chromium nitride formation temperature, changes in structure are detected. The used method of obtaining S-phase does not play any role, the one of the main influences is the temperature but connected to chemical composition of steel; SS 304 [38] after plasma nitriding (duration of process four hours) possesses changes in structure. The temperature difference of 40 °C caused the CrN formation, see Fig. 5.
19 Fig 5: Influence of different temperature during plasma nitriding [38].
The second main parameter of process is treatment (or deposition) duration/period [39]. In a plot (see Fig. 6) there are these nitriding periods displayed:
A1 for thirty minutes; B1 for sixty minutes; C1 for six hours; D1 for twelve hours and base material is untreated SS 316L. Processing temperature was 400 °C. The formation of chromium nitrides started without crossing the critical temperature. The process is studied to details in study of Mientus and Elmer [40] and it requires deep knowledge of plasma physics.
20 Fig 6: Influence of process time on structure (ion nitriding - 400 °C) [39].
1.2.3 Mechanical Properties of S-phase
Mechanical properties especially the hardness is the main highlighted property of S-phase differing from austenite. Higher hardness is followed by better wear resistance.
Hardness and elastic modulus was in depth profile studied in plasma-nitrided 316L steel by nanoindentation topography [41]. The idea was to determine the influence of nitrogen concentration on the local mechanical properties in different depths through the nitrided layer. At 5 μm below the surface, the (111) oriented grains are the softest and the (001) oriented grains are the hardest. This anisotropy is completely reversed at a depth of 19 μm, which corresponds to the non-nitrided region. The evolution of the elastic modulus Ehkl exhibits more or less similar trends but appears more sensitive to the neighbouring grains, partly due to lateral diffusion.
Dependence of hardness on single grain orientation was not fully uncovered and explained.
21 Change in mechanical properties is also possible to be studied along the nitrogen content. The highest measured hardness of S-phase was published as 20 GPa [42].
The nitrogen content in sputtered S-phase was described as 30 and 40 N/100 Me (number of nitrogen atoms per metal atoms), see Fig. 7. Further increase in nitrogen concentration led to decrease in hardness because of new phases forming out.
Dahm [42] has explained the increased hardness by presence of interstitial- substitution clusters, which explained the change in behaviour of chromium bonding.
Fig 7: Mechanical properties change along the nitrogen content [42].
During the ball-on-disc sliding on 304 steel treated with glow discharge in nitrogen, the wear mechanisms changed from adhesive to a three-body abrasive [43]. During abrasive wear deep scars in the wear track are grooved by wear particles (debris).
Dominated oxidative mechanism of wear was confirmed for S-phase (obtained by sputtering) with the overall decrease in wear. The plateaus of oxide materials were observed in the wear track [42]. The change in wear track profile is also evidence of wear behaviour improvement. Lower material loss along increasing nitrogen content
22 can be estimated. The improvement (or wear loss lowering) is valid for both dry and lubricated wear, see Fig. 8.
Fig 8: Wear track profile after lubricated pin-on-disc wear testing [42].
COF is possible to be decreased by nitriding at low (475°C) and also high (570°C). COF and wear rate of low-temperature nitrided steel is lower compare to high-temperature nitrided and untreated SS 304 [44]. Here is also mentioned that the friction is influenced by the amorphous layer on the top of nitrided material, but the total wear rate is lower by almost three orders of magnitude compare to polished surface. The specific wear rate seems to decrease with increasing nitrogen content but the COF has not clear dependence.
Mechanical properties were also studied according to thermal stability. Austenite is unstable phase of iron. Experiments by X.Y. Li at all [45] were carried out in different temperatures and for different time periods. Before the annealing process, the S-phase was pure and free from any CrN precipitates possessing high hardness.
The surface hardness depended on annealing time and has slightly decreased. This change is the evidence of a thermal phase transformation. Final phases after 20
23 hours annealing in 600 °C were CrN, ferrite and austenite with a very fine lamellar structure.
Thermal stability of S-phase is highly dependent on annealing period; these results are describing the use limitation of S-phase coating.
1.2.4 Magnetic Behaviour of S-phase
Magnetic behaviour was named as the most important property of S-phase to be pronounced as new phase. Mössbauer phase analysis shows that the nitrogen expanded austenite is predominantly ferromagnetic while the carbon expanded austenite remains paramagnetic [30]. It has been also confirmed by magnetic force microscopy that two different sub-layers are formed. The layer close to the surface shows magnetic domains and the underlying one is paramagnetic. It can be concluded that the nitrogen concentration at the ferromagnetic/paramagnetic sub- interface is approximately 5 w% (17.5 at%). In other study [46] the level of nitrogen concentration for ferromagnetic change is 10 at%.
The formation of magnetic domains takes place already after few minutes of gas nitriding and also after ion sputtering in pure nitrogen as a tool to destroy the chromium oxide protective layer [47]. The penetration of nitrogen seems to start locally on microscopic level.
1.2.5 Corrosion Resistance of S-phase
Intensively discussed behaviour of S-phase is corrosion resistance. Corrosion resistance of stainless steel should not be cut by surface treatment because high corrosion resistance to almost each environment is significant requirement for application.
Temperature during nitriding plays a significantly crucial role. The nitriding process performed at 450 °C caused the CrN precipitation [48] which decreased the available chromium concentration for forming of chromium oxide protective layer. In contrast to high temperature nitriding, low temperature nitriding could increase the corrosion resistance behaviour in chloride containing solutions. Resistance to pitting corrosion was tested in 3.5 w% NaCl pH neutral solutions by cyclic voltammograms [49]. The reverse scan builds the loop, see Fig. 9. The large area is surrounded the bigger susceptibility to pitting corrosion occurs. After the repassivation, a compact passive layer is re-established and the pit damage stops its propagation.
24 Fig 9: Cyclic voltammograms of plasma nitrided and untreated 304 SS [49].
Comparison in corrosion resistance between S-phase and CrN coatings were studied earlier in 1990's [50]. Sputtered CrN and S-phase were tested in NaCl and FeCl3 solutions. S-phase possesses better electrochemical behaviour than CrN and untreated steel. But S-phase is not better than CrN coatings in corrosion-wear resistance. The final behaviour is influenced by surface defects.
25 CHAPTER 2 MATERIALS AND EXPERIMENTAL PROCEDURES
2.1 Target and Substrates
Austenitic stainless steel for substrates and the target was supplied by Nova Trading S.A., Poland following the standard EN 10088-2 in chemical composition (see Tab. 1) and surface finishing. Each coating was deposited from AISI 316L ASS (target).
a) Samples Preparation for Laboratory Testing
The first set of substrates was made of commercially available stainless steel 316L. Substrates were cut from bar Ø 28 mm to discs with 3 mm thickness. Surface finishing was done manually with the use of grinding papers, final polishing with the alumina suspension (5 µm) in water. Samples were ultra-soundly cleaned in ethanol for 15 minutes just before introducing into the deposition chamber.
b) Samples Preparation for Industrial Testing
The second set of samples was laser cut from the 3 mm thick plate of AISI 304 with surface finishing 2B. The 2B labelling is achieved by cold rolling, heat treating and pickling, final light rolling using highly polished rolls gives the surface a smoothness and reflectivity. The only manual preparation was removing the laser cut residues from the back side because of placing to testing equipment. Samples were ultra-soundly cleaned in ethanol for 15 minutes just before introducing into the deposition chamber.
Tab. 1: Chemical composition of 304 and 316L steel - in w% [7].
C Mn Si P S Cr Mo Ni N
304 0.08 2.0 0.75 0.045 0.03 18-20 - 8-10.5 0.1
316L 0.03 2.0 0.75 0.045 0.03 16-18 2-3 10-14 0.1
2.2 Testing Solutions
Liquids used in this testing are commercially available products.
a) milk solution was prepared from milk powder (Full cream milk powder (26%)) - a product of Lactos company, Prague, Czech Republic. Milk powder contains minimally 26 % of milk fat, 26 to 27 % of proteins and lactose 35 to 39 %. Renewed milk was prepared by dissolving 65 g of milk powder in 450 ml of cold water.
26 b) physiological solution - 0.9% Sodium Chloride In Water For Injection Fresenius - is the branch name for a solution of 0.90% w/v of NaCl, about 300 mOsm/L or 9.0 g per liter.
2.3 Coatings Production
Coatings can be produced by many manifold methods. S-phase was invented in the nitrided layer but it was proved that it is possible to be obtained by magnetron sputtering [51]. Magnetron sputtering is a vacuum technique friendly to environment.
Reactive magnetron sputtering – is the one of the physical methods of thin films fabrication. This method is based on sputtering of solid target by ions generated in plasma discharge. Inelastic collisions between argon atoms and surface atoms of the target result in sputtering of target atoms. Deposited film is composed of atoms originating from the target with addition of reactive gas atoms present in a processing atmosphere.
The vacuum chamber was pumped down to the ultimate pressure of 5.10-3 Pa before heating up to avoid high temperature oxidation. Samples were heated up to 300 ºC by irradiation from resistance wire placed 10 mm from sample holder.
Deposition temperature was measured with thermo couple type J placed on back side surface of the sample. Samples were finally cleaned in argon glow-discharge for 10 minutes.
Metallic coatings were deposited by pulsed, reactive magnetron sputtering in Ar/N2 atmosphere. Magnetron sputtering source was driven by pulsed power supply (1 kHz with 100 kHz modulation) operated in an unbalanced mode. Sample bias was held at UB = - 70 V during deposition.
Settings of all equipment were chosen based on experience with the use of equipment. The deposition chamber is laboratory designed and the schema is presented in Fig. 10. The distance between substrate and target was 70 mm.
For deposition two principles were followed. For laboratory testing the coatings were deposited in the conditions of increasing partial of nitrogen with stable argon pressure; detailed working atmosphere composition is summarized in Tab. 2a.
For industrial testing the ratio between flows was to be followed. The flows of argon and also of nitrogen were changing from process to process. The flows were set by flowmeters in Standard Cubic Centimeter per Minute (sccm). The value of total
27 process pressure was not taken into account. The working atmosphere is given in Tab. 2b.
Fig 10: Schema of PVD deposition system placed in TU Koszalin, PL.
Tab 2a: Working atmosphere composition for laboratory tested samples.
Sample Partial pressure of Ar [10-1 Pa]
Partial pressure of N2 [10-1 Pa]
SN00 4 0
SN01 4 1
SN02 4 2
SN03 4 3
Tab 2b: Working atmosphere composition for industrial tested samples.
Sample Argon flow [sccm] Nitrogen flow [sccm]
SN1200 12 0
SN0903 9 3
SN0804 8 4
SN0606 6 6
28 After deposition process samples were left in vacuum chamber to cool down to the room temperature to avoid high temperature oxidation of surface. Samples before all testing were sealed in plastic bags to minimize the damage of surface.
2.4 Thickness Measurement of Coatings
Contact profilometer (Taylor-Hobson Form Talysurf Series 2, placed in SIMTech, SG and Hommel-Etamic T8000, placed in TU Koszalin,PL) was used to measure the thickness of deposited coatings. Certain area of sample was tightly covered by silicon wafer before deposition. The needle was touching the surfaces (coated and uncoated) in one single movement along the sample. The height of obtained step is equal to the coating thickness.
2.5 Glow Discharge Optical Emission Spectroscopy
Glow Discharge Optical Emission Spectroscopy (GDOES) is a method for determining chemical composition as a function of depth for coating thicknesses from a few hundred nm to maximum 50 µm. Argon RF plasma above the surface of the sample is ignited and material is gradually removed at a relatively high rate. The removed atoms are excited and therefore emit light in the visible range and then measured and analysed. HORIBA Jobin Yvon GD-Profiler HR™ (placed in SIMTech, SG) was used for the chemical composition determination.
2.6 Structure Examination
X-ray diffraction is an instrumental technique that is usually used to study crystalline materials. When a focused monochromatic X-ray beam interacts with planes of atoms, part of the beam is diffracted; the distances between the planes of the atoms can be measured and calculated by applying Bragg's Law.
Bragg's Law is
where the integer n is the order of the diffracted beam, λ is the wavelength of the incident X-ray beam (1.78897 nm for cobalt), d is the distance between adjacent planes of atoms (the D-spacings), and θ is the angle of incidence of the X-ray beam.
The results (diffractograms) are finally compared with databases to find the individual material. Diffractogram gives the information about the dimension of the elementary
29 cell (peak position), content of the elementary cell (peak intensity) and strain and/or crystalline size (peak broadening). D8 DISCOVER X-ray Diffractometer from BRUKER was used and is placed in SIMTech, SG.
2.7 Hardness Measurement
The nanohardness tester from Micro Materials Ltd (placed in SIMTech, SG) was chosen to provide quantitative data about the hardness and Young’s modulus of coatings, using an indentation method where a tip of known geometry is driven into the sample surface. The force is applied by an electromagnetic actuator on the indenter, whilst the displacement is measured via a capacitive system, giving a force resolution of 10 μN and displacement resolution <1 nm.
For the measurement of the surface hardness, Berchovich diamond indenter was used. The nanohardness measurement was performed by the load not reaching 5 mN and the indentation depth was only 1/10 of the total coating thickness to avoid the substrate influence. The linear loading and unloading speed was set on 1000 nm per minute. The resulted value was averaged from at least twelve indentations.
2.8 Surface Examination
2.8.1 Scanning Electron Microscopy
Scanning electron microscope (SEM) Zeiss ULTRA Plus (placed in TU Liberec, CZ) is essentially a high magnification microscope, which uses a focussed electron beam to produce images of sample by generally scanning in a raster scan pattern, and the beam's position is combined with the detected signal to produce an image of surface. The secondary electrons come from a small layer on the surface and yield the best resolution.
The principle of energy-dispersive X-ray microanalysis (EDX), complementary to SEM, is that the electron beam generates X-rays within the specimen. By measuring the energy of the X-rays it is possible to determine which elements are present in the specimen. Point, linear, and area (composition map) analyses are possible to obtain.
2.8.2 Atomic Force Microscopy
Atomic force microscopy (AFM) JPK NanoWizard III (placed in TU Liberec, CZ) is used to measure surface topography of specimen on a scale from Å to 100 µm. The tip with a radius of 20 nm is held several nanometers above the surface using a
