electroplated hard chromium and autocatalytic nickel-phosphorus coatings at elevated temperatures
Jämförelse av prestandan gällande nötning för ytbehandlingarna elektropläterad krom och autokatalytisk nickel vid hög
temperatur
Mats Eriksson
The Faculty of Health, Science and Technology Master Thesis, 30 credit points
Supervisor: Pavel Krakhmalev
Examiner: Jens Bergström
Date: December 23, 2014
Final report - 4th edition
The author would like to express his gratitude toward his supervisor at Karlstads University, Professor Pavel Krakhmalev for his guidance and support throughout this project.
Further personnel at Karlstads University the author would like to send a special thanks are research engineer Christer Burman and lecturer Mikael Åsberg M.Sc., for their help during this project.
Finally, the author also sends his thanks to his supervisors Magnus and Daniel who work for the company which this thesis was written for.
. . . .
Mats Eriksson
Authors Signature
City and Date:
Karlstad, October 2014
regimes it was possible to replace electroplated hard chromium with autocatalytic elec- troless nickel-phosphorus. In this work the dry sliding wear properties of electroplated hard chromium and autocatalytic electroless nickel-phosphorus(10% P) were compared.
All tests and investigations were done by using available equipment at Karlstads Uni- versity. The tests were made to find out how the wear of these coatings behaved at different temperatures, how different substrates influence the wear of these coatings and how the roughness of the substrate surface influence the wear properties of these coatings.
The method used for the wear tests was block-on-ring with a counterformal contact mode. The tests were executed in room temperature, 300◦C and 400◦C; with a normal load of 100N, sliding speed was 150rpm and duration of the tests were 15 minutes.
All tests were done in an argon gas atmosphere.
The coatings was deposited onto the cylinders with a thickness of 30µm. The different substrates used were an austenitic stainless steel(1.4404) and an austenitic-ferritic(duplex) stainless steel(1.4460). Half of the austenitic cylinders had a machined surface and all the others(including duplex cylinders) were machined and grinded to achieve a smoother surface. The blocks used as countersurface were made out of austenitic-ferritic(duplex) stainless steel(1.4460).
Equipment used to investigate the wear tracks were stereo microscopy, profilometer, mi- crohardness tester and scanning electron microscopy(SEM). The coatings were investi- gated in matter such as wear depth, wear mode, wear mechanism, chemical composition, topography, morphology, cross-section and hardness.
The results of this work showed that the nickel coating wear tracks maximal depth were less deep than those of hard chrome, at room temperature. At elevated temperatures the performance varies. The coatings deposited onto cylinders made out of duplex stainless steel performed better than those deposited onto austenitic cylinders. The nickel coat- ing performed better deposited onto the substrates with smooth surface and the chrome
vilka temperaturer det är möjligt att ersätta elektropläterat hårdkrom med autokatalytisk nickel. I detta arbete jämfördes nötningsegenskaper gällande omsörjd glidning av electro- plated hårt krom och autokatalytisk nickel-fosfor(10% P). Alla tester och undersökningar gjordes med hjälp av tillgänglig utrustning vid Karlstads Universitet. Testerna gjordes för att ta reda på hur slitaget av dessa ytbehandlingar skiljer sig vid olika temperaturer, hur olika grundmaterial påverkar slitaget av dessa beläggningar och hur olika ytfinhet av grundmaterialet påverkar slitaget på dessa beläggningar.
Den metod som används för slitagetesterna var ’block-on-ring’. Testerna utfördes i rum- stemperatur, 300◦C och 400◦C; med en last på 100N, glidhastighet var 150 rpm och testerna varade i 15 minuter. Alla tester gjordes i argongas.
Ytbehandlingarna avsattes på cylindrarna med en tjocklek på 30µm. De olika grund- material som användes var ett austenitiskt rostfritt stål (1.4404) och en austenitiskt- ferritiskt(duplext) rostfritt stål(1.4460). Hälften av de austenitiska cylindrarna hade en bearbetad yta och alla andra(inklusive de duplexa cylindrarna) var bearbetade och sli- pade för att uppnå en högre ytfinhet. Blocken som användes som motyta var gjorda av samma austenitisk-ferritiska rostfria stål (1.4460) som vissa cylindrar.
Den utrustning som används för att undersöka nötningsspåren var stereomikroskopi, profilometer, mikrohårdhetstestare och svep elektronmikroskop(SEM). Ytbehanlingarna utvärderades gällande slitagedjup, slitage typ, slitage mekanism, kemisk sammansättning, topografi, gränsyta, tvärsnittsyta och hårdhet.
Resultaten av detta arbete visar att nickelbeläggningens maximala slitagedjup inte var lika djupt som det djup påträffat för hårdkrom, vid rumstemperatur. Beträffande max- imalt djup observerat för testerna vid de högre temperaturerna varierar resultaten. Yt- behandlingarna som var avsatta på cylindrarna tillverkade av duplext rostfritt stål var mindre slitna än de som var avsatta på de cylindrar tillverkade i det austenitiska stålet.
Nickelbeläggningen presterade bättre på substraten med fin ytfinhet, medans krombeläg- gningen presterade bättre på substraten med grov ytfinhet.
1 Introduction 7
1.1 Background . . . . 7
1.2 Hard Chromium . . . . 11
1.3 Electroless Nickel-Phosphorus . . . . 15
1.4 Properties, hard chromium vs electroless nickel . . . . 22
1.5 Tribology . . . . 23
1.6 Wear . . . . 24
1.7 Tribochemical layers . . . . 28
1.8 Surface roughness . . . . 29
1.9 Substrate material . . . . 30
1.10 Definition of problem . . . . 32
2 Method 35 2.1 Execution of tests . . . . 35
2.2 Investigation of wear tracks . . . . 40
3 Results 45 3.1 Unworn coatings . . . . 45
3.2 Wear tracks . . . . 49
3.3 Wear depth . . . . 50
3.4 Wear modes and mechanisms . . . . 51
3.5 Transfer material / Tribolayers . . . . 54
3.6 Hardness tests . . . . 58
4 Discussion 61
5 Conclusions 68
6 References 69
7 Appendix I 74
1 Introduction
T
his thesis was written by request of a Swedish valve manufacturer to investigate whether it is possible to replace hard chromium with electroless nickel as coating material. If a replacement is possible, it would result in reduced costs during the man- ufacturing of this company’s products, contribution to the environment and potentially increasing service life of the valves.1.1 Background
A valve is a device used to control the flow, rate, volume, pressure and direction of the fluid transported in a piping system or similar. Valves are used so the flow of the fluid can be adjusted or isolated. The size and complexity of a valve vary to fit a specific ap- plication like car engine, power plant etcetera. There are many different types of valves like ball, butterfly, check and gate valves; just to mention a few. Each type have different features and functional capacities. Regardless of what valve type they all consist of the same basic parts like body, bonnet, disk, seat and actuator.
The body is the outer casing that contains the internal parts. The bonnet is mounted on top of the body in which the stem runs. The stem connects the actuator and the disk so the valve can be opened or closed. The actuator can be manually operated wheel/lever or operated by a motor. [1]
Surface treatments are used to improve a specific property such as friction, wear or corrosion resistance. The idea of a surface treatment, or coating, is to use a cheap sub- strate material and cover it with a protective surface layer. Today there are several surface treatments available to choose from, all with different advantages and disadvan- tages. Companies always strive to reduce costs by replacing existing coatings with others that are cheaper or increase service life of their products. In 1966 a report made by the initiative of the government in the United Kingdom estimated that 515 million sterling pounds could be saved annually in the UK, due to improved tribological practice. [2]
Today valves are used in various industries, all with different requirements of the valves components. Due to valves are used in many different industries, the media that flows through the valves also varies a lot. These medias can be liquid or gas, some are hot others are cold, they can be alkaline or acidic and some contain particles or grit. All these different environments results in several different problems like corrosion and erosion. To meet these problems valve manufacturers use coatings. For a long time hard chromium has been a common coating due to its high hardness, good corrosion resistance and cheap process [3]. During the last 20 years researchers have tried to find an alternative to hard chromium, due to its hazardous manufacturing process and waste [4].
There are several ways to deposit coatings. Some of the most common techniques are spraying, welding, electroplating and chemical plating. All of these techniques have their own advantages and disadvantages. The most promising alternatives to replace hard chromium is: [5]
• Thermal spray coatings.
• Electro- and electroless plating.
• Vacuum coatings.
• Laser and weld coatings.
Thermal spray coatings
There are many different ways of thermal spray coatings i.e. flame spraying, plasma spraying and high velocity oxygen fuel (HVOF). HVOF coatings are the most common to replace hard chromium among the thermal spray coatings. There are several differ- ent materials that can be deposited by HVOF, meaning the properties of these coatings vary a lot. The most common ones being chrome carbide-nickel chrome(Cr3C2-NiCr) and tungsten carbide-cobalt(WC-Co) [5]. However, these coatings have a micro-cracked and porous structure. Which makes it easy for a corrosive media to reach the substrate [6].
Electro- and electroless plating
Electro- and electroless plating techniques remind of hard chrome plating, since they are both bath processes done with similar technology. The most common material used to replace chrome in both electro- and electroless platings is nickel. Electroless plating is often preferred since this technique can coat complex shapes uniformly [5]. These types of coatings are often used as protection against wear and corrosion. They are considered cheap and are often used to coat large batches of the same substrates. [7]
Since nickel is identified as an EPA 17 hazardous chemical, some argue that replacing hard chromium with coatings made out of nickel isn’t a good idea. Since regulators probably will focus on improving nickel deposition once the problems regarding hard chromium coatings have been solved. [8]
Vacuum coatings
Some of the most common types of vacuum coatings are chemical vapour deposition(CVD) and physical vapour deposition(PVD). The applications of CVD coatings are limited due to high deposition temperatures. CVD coatings can be deposited at lower temperatures, but these coatings are more expensive and the coating materials used are usually haz- ardous [5]. After many years of research scientists created a nano-structured coating made of tungsten-carbide called Hardide. This is a coating deposited by low-temperature CVD and has been commercially available for 10 years. It has proven to be a good alternative to hard chromium for valve applications in several industries. The enhanced corrosion resistance of this coating compared to themal sprayed coatings is due to the low porosity and lack of binding material. [9]
PVD coatings have proven to be as resistant against wear as HVOF coatings. However, due to the high cost of thick PVD coatings, they are seldom thicker than 5µm. Since these coatings are so thin their wear life is rather short. Unlike CVD, PVD coatings suf- fer from line-of-sight problems during deposition which results in a non-uniform coating.
The most common PVD coatings are TiN and CrN. [10]
Laser and weld coatings
Laser cladding is a technique that has become more viable due to better equipment. In this process a solid is fused onto the substrate surface by using a laser beam. Since the coating is welded onto the substrate, the adhesion is excellent. Due to the use of a laser a heat-affected zone is formed at the substrate surface, the depth of this zone can be several hundred microns and can alter the crystal structure of the substrate surface. Meaning the mechanical properties of the substrate will be affected. [8]
Laser cladding coatings have a high quality with low porosity and few imperfections.
Due to the high solidification and fast cooling rate these coatings have a fine solidified microstructure, hence these coatings have good resistance against wear and corrosion.
[11]
1.2 Hard Chromium
Hard chromium(HC) is a very common coating, it has been used to coat surfaces since the 1940’s. It is a well known, simple and cheap process with high hardness, good wear and corrosion resistance. Lately hard chromium has received a lot of attention due to the environmental issues with hard chromium plating. During the plating process a lot of hydrogen and oxygen bubbles are produced. When these bubbles rise to the surface of the bath they burst and emit hexavalent chrome(Cr6+) to the surrounding air. This gas has to be removed to protect the workers and the environment. The waste water produced after hard chromium plating process has to be cleansed from hexavalent chrome before disposal [3]. A famous case of contamination with waste water containing hexavalent chromium was when the U.S. company Pacific Gas and Electric Company(PG&E) contaminated the ground water in Hinkley, California. The legal events that followed has been portrayed in the drama film Erin Brockovich starring Julia Roberts [12].
In 2007 hard chrome plating was classified as an environmentally unfriendly process by the U.S. Environmental Protection Agency(EPA) [13]. The same year the European Union directive Restriction of Hazardous Substances(RoHS) decided to ban hexavalent chrome. These classifications have lead to higher restrictions when dealing with chrome plating. It is the higher restrictions and the risen costs due to these restrictions that has stimulated the industry to search for better alternatives. [14] [15]
Hard chromium coatings are used in several fields. Due to the low friction of hard chromium coatings they are used in applications like cylinders, engine vales, shafts and bearings. Other applications are steam turbines and foundry for dies due to the high ther- mal resistance of hard chromium. Hard chromium also possesses high corrosion resistance and is therefore used in applications like chemical, pharmaceutical and food industries.
[16]
1.2.1 Process description
Before the substrate is being electroplated it should be carefully cleaned. Any contamina- tions of the substrate surface will affect the bond quality of the coating. Poor preparation can lead to porosity, low adhesion or a bad uniformity of the coating. The deposition process involves bonding chromium onto the substrate surface by electrolytic means. The substrate is placed in a liquid solution consisting of chromic acid and other various sub- stances (additives). An external power source is needed to provide the current for the plating process. The substrate is connected to the negative pole of the power source and works as a cathode. Another metal part is added to the electrolyte, this part is connected to the positive pole of the power source and acts as the anode. The anode usually consists of tin-lead [17]. When the current is applied, cations in the electrolyte are drawn to the substrate and bond. [18]
Numerous of factors influence the quality of the final plating. The ratio between chromic acid and catalysts is a critical factor. This ratio influences several properties of the final coating like hardness. The temperature of the electrolyte and current density are other factors that also influence the properties of the coating. The conductivity of the electrolyte is reduced as cations build up, this will reduce the current density of the electrolyte which will lead to a decrease in efficiency of the deposition and poor uniformity of the coating [19]. By increasing the bath temperature a lower concentration of the solution is possible, which will reduce costs. But a too high temperature will result in breakdown of the additives in the solution. [20]
1.2.2 Microstructure of hard chromium
The microstructure of the coating is affected by the bath composition. If the solution con- ditions promote formation of fresh nuclei the deposit will form a fine-grained microstruc- ture. A coarse microstructure is formed if the solution promotes growth of existing nuclei.
Deposits with finer grains are harder and less ductile than deposits with coarser grains.
Solution factors that increases the cathode polarization, i.e. current density, temperature and agitation, results in finer grains. [19]
1.2.3 Properties of hard chromium
Hard chromium coatings have a lot of microcracks. How these microcracks arise hasn’t been fully understood yet. One theory is that these microcracks arise due to the relax- ation of internal tensile stresses [21]. The relaxation take place as brittle cracking due to the embrittlement of the coating by absorption of atomic hydrogen. The absorption of hydrogen increase the hardness of hard chrome coatings. When the thickness of the coating increases the intensity of microcracks decreases since the embrittlement of the coating is worst near the substrate surface. Properties such as hardness, corrosion resis- tance and fatigue life are influenced by these mickrocracks. If the deposit is thin enough so these cracks reach the substrate surface, then the corrosion resistance is significantly decreased. Fig. 1.1 illustrates a thick and thin microcracked cross-section. [22]
Figure 1.1: Microcracks of a (A) thick and (B) thin hard chrome deposit. [21]
The high hardness of hard chromium is due to the dissolution of atomic hydrogen in the coating during the plating process. Hence is hard chromium a popular coating for high wear applications. At elevated temperatures the hardness is reduced due to the loss of the atomic hydrogen. Increased porosity can also be a contributing factor since oxygen can diffuse further into the coating.
All types of chromium attracts oxygen and forms an external protecting oxygen layer, Cr2O3. This oxygen layer increase wear resistance and corrosion resistance of hard chrome coating [20]. Fig 1.2 illustrates hardness as a function of coating thickness for electro- plated chromium.
Figure 1.2: Microhardness vs coating thickness of hard chromium. Redrawn [23]
1.2.4 Influence of substrate roughness
The roughness of the substrate surface affects the appearance of the coating. A rough surface will most likely be more uneven due to differences in intensity of the electric field attracting the ions. Whilst a smooth substrate will result in a even coating. Fig. 1.3 illustrates a hard chrome deposition onto two substrate with different roughness. [24]
Figure 1.3: Surface of a hard chrome coating deposited onto a (A) smooth and (B) rough substrate surface. [24]
1.3 Electroless Nickel-Phosphorus
Electroless nickel(EN) plating is a coating technique developed by Brenner and Riddel in the 1940’s. This technique doesn’t use any external power source like an electrolytic deposition does. The current arise from chemical reactions in the electrolyte, this type of reaction is called an autocatalytic reaction. Fig. 1.4 illustrates the differences between an electrolytic and an autocatalytic process.
Figure 1.4: Difference in set up for an (left) electrolytic deposition; (right) auto-catalytic deposition. [25]
Electroless nickel coatings are used in many engineering applications due to their unique properties like resistance against corrosion, abrasion and wear among others. There are many different materials incorporated into the electroless nickel deposit to improve a certain tribological property. These incorporated materials can be metals, polymers, solid lubricants or ceramics. The most common material to be added is phosphorus or boron, due to there are so many different deposits of electroless nickel it is common to classify these into several groups. These groups are listed below. [25]
• Pure Nickel
• Electroless nickel alloy coatings (i) Alkaline, nickel-phosphorus (ii) Acid, nickel-phosphorus
(a) 3 - 5% P (low phosphorus, LPEN) (b) 6 - 9% P (medium phosphorus, MPEN)
(c) 10 - 14% P (high phosphorus, HPEN) (iii) Alkaline, nickel-boron
(iv) Acid, nickel-boron
(v) Ternary, quaternary alloy
• Electroless nickel composite coatings
(i) Nickel-phosphorus composites (Ni-P-X) (ii) Nickel-boron composites (Ni-B-X)
( X = Al2O3, ZrO2, SiC, C, PTFE etc...)
• Electroless nickel nano-coatings
1.3.1 Process description
The bath used for electroless nickel plating is usually composed of nickel sulfate, sodium hypophosphite and other chemicals. All chemicals used in the bath have different func- tions and is given a certain name. These component names and functions are listed in table 1.1. The bath is normally operated at about 85◦C and at low pH-value. During the plating process the pH-value of the bath is lowered due to formation of hydrogen, this is prevented by adding alkaline salts since the pH-value influences the plating rate. [25]
Table 1.1: Electroless nickel plating bath components and their function. [26]
Component Function
Nickel Ion Metal source
Reducing agent Electron source Complexing agents Stabilizes the bath
Accelerators Activates the reducing agent Buffering agent Long term control of pH pH regulator Short term control of pH
Stabilizer Prevents solution from breakdown Wetting agents Increases wettability of surfaces
Electroless nickel is usually deposited by catalytic reduction of nickel ions with sodium hypophosphite. Due to the slow electrochemical oxidation of hypophosphite ions in aque- ous solutions, there is no reduction of nickel ions at normal bath conditions. Still, nickel ions are reduced at catalytic surfaces and this is the reason for the high ability to produce deposits at surfaces with low current density. When the initial layer of nickel has formed on the substrate, the deposition propagates unassisted due to the nickel layer now acts as a catalyst. It is this property that makes it possible to obtain a uniform coating at internal surfaces and complex geometries. [25]
As mentioned there are many different alloys of electroless nickel coatings. The most common are the nickel-phosphorus coatings. These coatings can be made out of a acid or alkaline solution. The acid solution is most common since the obtained coating has better quality than alkaline coatings and the bath is more stable. For coating substrates made of metals, acid baths are almost always used. The instability of alkaline baths at high temperatures are their main disadvantage. Since alkaline baths can’t handle high temperatures they are frequently used when coating plastics. Stabilizers are frequently used to prevent the baths from decompose too suddenly. The quick decomposition of the baths is a big problem in electroless nickel plating since it leads to higher costs. [25]
1.3.2 Microstructure of electroless nickel
Figure 1.5: Ni-P phase diagram. [27]
The phosphorus content affect the mi- crostructure of electroless nickel de- posits, hence the properties vary with phosphorus content. Low content of phosphorus result in a crystalline mi- crostructure, a medium phosphorus content leads to a semi-crystalline mi- crostructure and the microstructure of high phosphorus coatings are fully amorphous. It’s the degree of crys- tallinity that affects the final proper- ties. Fig. 1.5 illustrates a Nickel- Phosphorus phase diagram, where α and β phases are fully crystalline and the γ phase is fully amorphous. The co- existence of β and γ corresponds to the semi-crystalline phase [27]. The phos- phorus content is controlled by the for- mulation and chemistry of the plating
1.3.3 Heat treatment of electroless nickel
Electroless nickel-phosphorus coatings are often heat treated to enhance the hardness and wear properties. By heat treatment it is possible to achieve a hardness comparable to hard chromium. At 220-260◦C the microstructure of Ni-P coatings starts to change from amorphous to crystalline. For medium phosphorus coatings the metastable phases NiP2 and Ni12P5 may form before the stable Ni3P phase, since this phase forms at tempera- tures above 320◦C. [25]
Usually these coatings are heat treated at 400◦C for 1 hour to achieve maximum hardness.
This is due to the crystallization of nickel and formation of Ni3P precipitations. By heat treating the coating over 400◦C it will result in nickel grain growth and coarsening of the precipitations, which will result in decreased hardness. However, heat treated electroless nickel coatings have less corrosion resistance due to increased crystallinity and crack for- mation has been reported due to shrinkage of the coating [28]. Fig. 1.6 illustrates final hardness by heat treatment at different temperatures for 1 hour. Table 1.2 illustrates potential values of as-plated and heat treated electroless nickel coatings [26].
Figure 1.6: Hardness as function of heat treatment temperature. Redrawn [29]
Table 1.2: Hardness (HV0.1) of electroless nickel coatings. [25]
Phosphorus content As-deposited Heat treated (400◦C / 1h)
2-3% P 650 1200
6-9% P 620 1100
10-12% P 520 1050
1.3.4 Properties of electroless nickel phosphorus coatings
Wear resistance
Electroless nickel phosphorus coatings have very good resistance against wear. Usually are nickel-phosphorus coatings heat treated since higher hardness, theoretically, is corre- lated to lower wear. As mentioned, the hardness of electroless nickel coatings is influenced by the phosphorus content. Illustrated in Fig. 1.7 is the relation between phosphorus content, hardness and wear rate. As the hardness increases the wear rate decreases.
Other factors such as the amount of load applied also influences wear rate. A study has shown that increasing the applied load by 50% led to an increase of wear rate by 10 times of an as deposited nickel-phosphorus coating, whilst the wear rate of a heat treated nickel-phosphorus coating only increased by a factor of 1.3. [26]
Figure 1.7: Effect of phosphorus content on (a) hardness and (b) wear rate of as-deposited electroless nickel phosphorus coatings. [30]
Friction
The frictional properties of electroless nickel phosphorus coatings vary with phosphorus content since phosphorus provides a natural lubricity [25]. Friction is partly responsible for wear, thus is the minimization of friction wanted to increase service life. By heat treating EN coatings generally results in reducing the friction coefficient [26].
Internal stress
The amount of internal stresses in electroless nickel coatings differ due to the composi- tion of the coating, illustrated in Fig. 1.8. High phosphorus content results in neutral or compressive stresses while low phosphorus content results in tensile stresses. Heat treating electroless nickel will increase the internal stresses even further due to the micro structural changes starting at 220◦C. The internal stresses can also increase due to minor changes of the electrolyte. [28]
Figure 1.8: Internal stress as function of phosphorus content. Redrawn [28]
1.4 Properties, hard chromium vs electroless nickel
A number of different properties of hard chromium and electroless nickel are displayed in table 1.3.
Table 1.3: Comparison of properties. [31]
Hard Chromium Electroless Ni-P*
Tensile Strength [MPa] <200 >700
Hardness as plated [HV0.1] 800-1000 480-500
Hardness after heat treatment [HV0.1] -** 1100
Coefficient of thermal expansion [10−6 / ◦C] 6 12
Elongation [%] <<0,1 1-1,5
Melting point [◦C] +1610 +890
Thermal conductivity [W/m K] 67 8
Taber wear resistance [mg/1000 cycles] 2-3 15-20***
*P content = 10%
**Heat treatment decreases hardness of hard chrome.
***2-9 mg/1000 cycles, when coating is heat treated.
1.5 Tribology
T
he word tribology is Greek for rubbing. The science of tribology is defined as, "the science and technology of interacting surfaces in relative motion" and it comprises the study of friction, wear and lubrication. Friction is a force that arises when two surfaces in contact move over each other and the loss of material that occurs due to this force is called wear. In most cases wear is unwanted due to it leads to increased movement between two components(vibrations) and loss of precision. Lubrication is a common way to reduce friction and thus wear. Even if a lubricant isn’t added to the tribological system, atmospheric components like oxygen have a similar effect. [2]1.5.1 Sliding friction
Sliding friction is a force that arise from the sum of two forces, an adhesion force(Fadh) and a deformation force(Fdef). The adhesive force depends on the area of true contact (asperity contact points). The size of this force depends on the amount of asperity contacts and the shear strength of the asperities. Assume all the asperities have the same shear strength(s) and denote all the asperity contact points as (A). Then the adhesive force of friction will be Fadh= A∗ s. [2]
The deformation force is the amount of force needed to plough the asperities of the harder surface through the softer counter-surface. The size and shape of the asperities affect this force, as well does the hardness of the softer surface. This force can be calcu- lated Fdef = H ∗ a ∗ x, where H is the hardness of the softer surface material; a and x describe the geometry of the asperities. [2]
This means the total friction force, Ftot = Fadh + Fdef = A∗ s + H ∗ a ∗ x.
1.6 Wear
Loss of material due to wear can occur by different modes and mechanisms. A wear mode is classified as what process the material is removed. Some typical modes for wear are adhesive, abrasive, corrosive and fatigue wear. The wear mechanism describes the process of deformation. A wear mechanism can be due to a mechanical, chemical or thermal nature. Examples of common mechanical mechanisms are ductile fracture, brittle fracture and plastic deformation. It’s important to understand that several mechanisms can occur simultaneously but one is more dominant than the others. [35]
Fig. 1.9 illustrates the interrelations between some descriptive keywords of wear. [36]
Figure 1.9: Illustration of descriptive keywords in wear and their interrelations. [36]
1.6.1 Adhesive wear
Adhesive wear is a common wear mode during sliding wear. Adhesive wear occurs when the asperities of two surfaces come in contact. The asperities adhere to each other and form asperity junctions. Due to the tangential motion of the surfaces the weaker ma- terial’s asperity separates from the bulk and material is removed. Fig. 1.10 illustrates adhesive wear. [35]
Figure 1.10: Basic mechanism of adhesive wear. F is tangential force and A is contact area and τx is shear strength. [35]
There are properties and factors that influence the severity of adhesive wear. A simple theoretical analysis of the amount of material loss due to adhesive wear is the Archard wear equation. Where the rate of wear, Q, is calculated by equation 1.1. [2]
Q = K∗ W
H (1.1)
W = normal load; H = Hardness of the softer surface; K = wear coefficient.
Equation 1.1 states that the wear rate is inversely proportional to the hardness of a material. Meaning, the harder a material is the lower the wear rate will be.
Another way to lower wear rate is by lowering the coefficient K. This value is a measure- ment of the probability that a junction will wear due to adhesion and is always less than unity. An acceptable value of K, in an application, is less than 10−5. The factors that
influence the value of K is the surface energy between the two surfaces. This energy can be altered by the material pair compatibility, use of lubrication and load distributions.
[37]
The validity of equation 1.1 has been discussed since it doesn’t take any physical or chemical properties into account. A new equation that describes adhesive wear has been proposed where these properties have been taken into account. [38]
During sliding contact heat is generated at the interface of the surfaces. How the rise of temperature at the interface affect wear rate depends on how fast the heat is generated.
At slow sliding speeds the heat is quickly conducted away and the temperature remains the same. When the sliding speed is increased the heat is generated faster, resulting in higher temperature at the interface and the sliding can be considered adiabatic. For metals the high interface temperature can lead to softening and even melting if they are extreme. Chemical reactions such as oxidation are promoted by high temperatures at the interface. How normal load and sliding velocity influence the wear rate and temperature are illustrated in Fig. 1.11. [2]
1.6.2 Abrasive wear
Abrasive wear is when surface material is removed(or deformed) by a particle, it can also be due to a protuberance in the counter-surface. If the damage is caused by a free particle it is denoted as three-body abrasive wear, if the damage is done by a fixed protuberance it is denoted as two-body abrasive wear. Fig. 1.12 illustrates these different types of wear.
The particles in three-body abrasive wear can be contaminants such as airborne grit, they can also be created locally in form of wear debris. Two-body abrasive wear can be caused by differences in hardness and roughness between the two surfaces in contact. [2]
Figure 1.12: Illustration of (a) two-body and (b) three-body abrasive wear. [2]
The properties of the particles and protuberances influence the rate of wear. If the particles are much harder than the counter surface the more severe the wear becomes.
The shape of the particles also influences the wear rate, a sharp particle causes greater wear than rounded ones and larger particles causes more severe wear than smaller ones.
This is due to that the flow stress of a small volume is higher than a large volume.
Common wear mechanisms for abrasive wear are plastic deformation and brittle fracture.
[2]
1.7 Tribochemical layers
A tribochemical layer, or tribolayer, is a layer formed on either surface interface during wear. The layer is formed due to chemical reactions with the ambient medium or a lubricant. The chemical reactions forming the layer take place because of the increased temperature due to friction. Other factors that influence the formation of tribolayers are chemistry of the mating materials, sliding speed, load and formation of clean reactive surface. Tribolayers also influences the wear process. An oxide layer, one of the most common tribolayers, can work as an lubricant and hence reduce friction and wear, but some tribolayers can increase wear. This is due to brittle chipping of the tribolayer causing wear debris, resulting in increasing wear. This type of wear if often called tribochemical wear and is illustrated in Fig. 1.13. [39]
Figure 1.13: Illustration of tribochemical wear. [39]
1.8 Surface roughness
The contact conditions influence the wear mode and wear mechanisms during sliding wear.
Surface roughness is such a condition that influences wear and friction. For most engi- neering materials friction isn’t affected by surface roughness due to plastic deformation of the asperities. Wear, however, is affected by stresses at the contact area. A numeri- cal analysis has shown that rough interfaces show significantly higher contact pressures compared to smooth interfaces, hence more plastic deformation occurs. If the coating in contact has very high hardness, a study has shown that increasing surface roughness leads to increasing wear and friction. [40]
1.8.1 Covering power
The roughness of the substrate also influences how well the coating covers the substrate.
Covering power is a term used in electroplating, it describes how well the coating is deposited onto the substrate with uniform thickness. There are several factors influencing the covering power like what kind of substrate is used, current density and the electrolyte composition. There are three different types of results possible after different plating techniques, these are illustrated in Fig. 1.14. [41]
Figure 1.14: The different types of covering power: (a) negative throwing, (b) geometric levelling and (c) true levelling. [41]
1.9 Substrate material
Coatings behave differently depending on what substrate they are deposited onto. This is due to the adhesive forces between the substrate and coating differs. Good adhesion between the coating and substrate is crucial to its function. Adhesion is defined by the ASTM(American Society for Testing and Materials) as the "condition in which two surfaces are held together by either valence forces or by mechanical anchoring or by both together". The adhesion performance is influenced by factors of the interfacial region such as its atomic bonding, thickness and fracture toughness. Failure of adhesion is often related to fracture mechanisms instead of bonding. Absence of fracture modes, strong bonding and low stress gradients at the interface all result in good adhesion. [42]
An important factor is the coating/substrate-system ability to withstand loads. This ability to withstand normal and tangential forces is known as the load-carrying capacity and it is influenced by the mechanical properties of coating as well as the substrate, but also external factors such as contact geometry. A study was conducted to find out how parameters, such as coating thickness, coating type, substrate hardness and elasticity influenced the load-carrying capacity. This study concluded that the most influential factors, regarding the substrate, of the load-carrying capacity were young’s modulus and hardness [43].
Another important property is the coefficient of thermal expansion(CTE). If the CTE of the substrate differs significantly from the coating this can lead to failure due to poor adhesion. It can also result in cracks of the coating(due to relaxation of residual thermal stresses) initiated at the coating-substrate interface, when the coating is exposed to high temperatures. [44]
1.9.1 Stainless steel
Stainless steels can be divided into three main groups, these are austenitic, ferritic and martensitic. Austenitic stainless steels have a primary phase of austenite with a FCC crystal structure. Ferritic stainless steels have a primary phase consisting of ferrite phase and BCC crystal structure. Duplex stainless steels consist of a mixture of austenite and ferrite. The influence of ferrite is why duplex steels derives its higher strength than austenitic steels, since BCC structure requires more energy to initiate slip. [45]
A comparison of mechanical properties between austenitic and duplex stainless steel are illustrated in table 1.4. The chemical composition of these steels are listed in table 1.5.
Grades are AISI 316L and AISI 329.
Table 1.4: Comparison of mechanical properties of austenitic and duplex stainless steels.
Tensile strength [MPa]
Young’s Modulus [MPa]
Hardness, Brinell [HB]
Thermal Expansion [µm/m◦C]
Austenitic SS [46] 485 193 217 16
Duplex SS [47] 550 200 230 16
Table 1.5: Chemical composition of the austenitic and duplex stainless steels.
Fe [%]
C [%]
Cr [%]
Ni [%]
Mo [%]
Mn [%]
Si [%]
P [%]
S [%]
Austenitic SS [46] - <0,03 16-18,5 10-14 2-3 <2 <1 <0,045 <0,03 Duplex SS [47] - 0.10 28 4,5 1,5 2 1 0,04 0,03
1.10 Definition of problem
The purpose of this thesis is to examine the differences in high temperature wear prop- erties for two different coatings deposited onto duplex and austenitic stainless steel. The influence of the substrates surface roughness was also examined.
The coatings to be evaluated are electroplated hard chromium(reference) and medium phosphorus electroless nickel(10% P). The electroless nickel coating was chosen by the company who this thesis was written for.
There are a lot of different tribological properties that can be evaluated to find out in what types of applications electroless nickel-phosphorus outrank hard chromium. In this study these questions will be addressed:
1. In what temperature regime does the electroless nickel-phosphorus coating show less amount of wear than hard chromium?
2. How does the substrate material influence the wear properties of these coatings?
3. How does the substrate surface roughness influence the wear properties of these two coatings?
Earlier work
Several studies have been made about electroless nickel-phosphorus coatings. These stud- ies have investigated how compounds in the deposition process, phosphor content and heat treatments affect the properties and performance of these coatings. There are a lot of studies comparing wear properties of electroless nickel-phosphorus coatings and electro- plated chromium, most of these studies are quiet old(1970-80’s) and are not accessible through the internet. The few articles found were these coatings have been compared regarding wear properties, electroplated chromium has shown better resistance to wear.
Some studies regarding hard chrome have reported large amounts of material loss of the counter-surface, but only minor material loss of the hard chromium coating. In these studies the subjected load during the wear tests was high. [29]
A company who tested to replace hard chromium with electroless nickel in their products is Caterpillar Inc.. They began investigating the possibilities to replace hard chromium with electroless nickel coatings. Once they figured out the ideal bath and process pa- rameters their products coated with electroless nickel showed improved quality over hard chrome. The cost of the nickel coating itself was more expensive than hard chrome. But when they included costs like materials, manpower, energy and time in to the equation they found out that the electroless nickel process was cheaper, due to elimination of ex- pensive post- and pre-plating steps like masking and grinding. [32]
A study set out to investigate the tribological characteristics of electroless nickel coat- ings(Taheri et al. [33]), found that the roughness of the substrate were increased after depositing electroless nickel. However, this only applies to smooth substrate surfaces, for rough substrate surfaces the opposite effect was observed. The difference in thickness of the applied coating didn’t influence this effect at the certain roughness’s investigated.
Vernhes et al. [34] did a study were hard chromium was compared to two other coating regarding wear. The three coatings were deposited onto the same austenitic stainless steel used in this thesis work and the countersurface was coated with Stellite 6 using the same austenitic substrate. What they found was that the hard chromium coating was sub-
jected to the most severe wear and the mating surface was heavily worn as well. Vernhes also observed quite large amounts of transferred material from the countersurface onto the hard chromium, with the chrome coating intact.
2 Method
H
ow the specimen were prepared, tested and evaluated are explained below. The coated part of the valve that these test are meant to evaluate is the red shaft in Fig. 2.1.Figure 2.1: Cross section of a butterfly valve. [48]
2.1 Execution of tests
2.1.1 Wear Test
Dry sliding wear tests were performed in a wear test rig designed by Karlstads University illustrated in Fig. 2.2. The technique used was counterformal "block on ring" illustrated in Fig. 2.3a. The counterformal contact was chosen since the equipment was designed for this type of test. Before the cylinders and blocks were tested in the test rig, they were all cleaned in alcohol.
Figure 2.2: Equipment used to run the wear tests, furnace and shaft.
Figure 2.3: Block on ring, (a) counterformal and (b) conformal contact. [49]
2.1.2 Cylinders and blocks
The blocks used as counter surface all had the same dimensions and were made out of duplex stainless steel 1.4460(AISI 329). The cylinders were coated with either hard chrome or electroless nickel phosphorus and the substrate material was either austenitic stainless steel 1.4404(AISI 316L) or duplex stainless steel 1.4460. The hard chromium and electroless nickel phosphorus coatings were supplied by the Swedish company Brink AB Förnicklingsfabriken. All the supplied coatings had the same thickness 30±10µm.
The cylinders coated with electroless nickel were post heat treated for 1 hour at 300◦C.
The roughness and outer diameter of the cylinders also varied. The different types of cylinders used are listed below in table 2.1.
Table 2.1: Properties of the different cylinders tested.
Substrate material Coating Surface roughness Outer diameter
Austenitic SS HC Rough 100mm
Austenitic SS MPEN Rough 100mm
Austenitic SS HC Smooth 90mm
Austenitic SS MPEN Smooth 90mm
Duplex SS HC Smooth 90mm
Duplex SS MPEN Smooth 90mm
Drawings of the cylinder and block are found in Appendix I.
2.1.3 Test conditions
All the wear tests were performed in an argon gas atmosphere to avoid oxidation, the gas used was Instarg containing 2% H2 and 98% Ar. All the tests were done with the same sliding speed, duration and normal load at various temperatures. The same sliding speed was used to get the same amount of contact points onto the cylinder, since the smooth cylinders had different diameters than the rough ones. Two tests were done for each temperature, in total six tests per cylinder were done. All six tests were done on a single cylinder. The parameters used were:
• Sliding speed = 150 revolutions per minute
• Duration = 15 minutes
• Load = 100 N (F in Figure 2.3)
• Temperatures = Room temperature (RT), 300 and 400◦C
To get the tests to be accurate to a real application, the load applied to the block during the tests was calculated by using the graphs illustrated in Fig. 2.4. The pressure between the coated shaft and bushing during normal operation of the valve are 110-200 MPa, according to the client. The same load were used for all the tests, since the differences in pressure between the Ø100mm and Ø90mm cylinders were negligible.
The pressure values plotted in the graphs seen in Fig. 2.4 were obtained from the line con- tact - contact pressure and dimensions tool available at the website http://www.tribology- abc.com/sub1.htm. The values used in this tool are listed in table 2.2.
Table 2.2: Values used in the Hertzian line contact calculator tool.
Young’s modulus for HC [MPa]
Young’s modulus for MPEN [MPa]
Young’s modulus for SS [MPa]
Poisson’s ratio
Radius [mm]
(a)
(b)
Figure 2.4: Pressure vs Load graph for (a) ∅100mm and (b) ∅90mm cylinder.
2.2 Investigation of wear tracks
2.2.1 Stereo microscopy
Initially after the tests the wear tracks were investigated in a stereo microscope, model Olympus SZH10, seen in Fig. 2.5. This was done to be able to compare the wear tracks on a larger scale. The pictures collected from these investigations were taken by using the software Leica QWin Pro.
Figure 2.5: The stereo microscope used to photograph the wear tracks.
2.2.2 Sample Preparation
To be able to evaluate the wear tracks left on the cylinders in the SEM and profilometer the cylinders had to be cut into smaller pieces, a Struers Unitom-50 saw seen in Fig.
2.6 was used. The part of the wear track that was considered as most severe worn by ocular examination was chosen to be investigated. The cross-section parts investigated in scanning electron microscopy were cut multiple times and moulded into Polyfast, then they were grinded and polished according to Metalog Method D found on Struers home- page(http://www.struers.com).
Figure 2.6: The saw used to cut the cylinder for evaluation.
2.2.3 Hardness Test
Hardness measurements were performed by a microhardness tester made by Future-Tech Corp. seen in Fig. 2.7. These tests were made to examine the hardness of the coatings and substrates. The load used for the indentations were 25 grams. The distance between every measured point were 50µm. In total ten points were measured of each series the, first in the coating and the remaining in the substrate. In total was six series measured for each cylinder. The series where done on cylinders that had been tested but also unused cylinders were tested.
Figure 2.7: The equipment used to measure hardness. Future-Tech Corp. modell FM-7.
2.2.4 Evaluation of wear depth
To evaluate the topography of both unworn and worn parts of the cylinders an Interfer- ometric optical profiler, WYKO NT3300 with software Vision32 version 2.303, was used as seen in Fig. 2.8. To avoid vibrations during these measurement the samples had to be fixed in a small vice similar to the one illustrated in Fig. 2.9. The size of the investigated areas was adapted to fit each individual wear track. The same width was always used due to curvature of the cylinders.
Figure 2.8: The profiler used to measure wear depth and topography.
Figure 2.9: The profiler used to measure wear depth and topography. [51]
2.2.5 Scanning Electron Microscopy
A LEO Gemini 1530 Scanning Electron Microscope(SEM) was used to investigate the wear tracks, seen in Fig. 2.10. The wear tracks were investigated by cross-section and also perpendicular from the surface. The perpendicular investigations were done to establish wear mode and wear mechanism. The cross-section investigations were done to confirm coating thickness, depth of eventual interfacial cracks of the coatings and also to see if there were any differences of the coatings regarding cross-section wear damages.
The detectors used during these investigations were a Backscatter Electron detector(BsE) and a Secondary Electron detector(SE). The SE detector was used to investigate the morphology of the coatings and to conclude wear modes and mechanisms. The pictures taken using the BsE detector was used to distinguish transfer material, substrate material and coating. To conclude the chemical composition of the different areas of the pictures taken by the BsE was done by Energy Dispersive Spectroscopy(EDS).
Figure 2.10: The Scanning electron microscopy used to investigate the wear tracks.
3 Results
T
he results from the different tests and evaluations are presented in this section.3.1 Unworn coatings
The topography of the unworn coatings was investigated in the profilometer. The results are illustrated in Fig. 3.1. Notable is the difference in roughness of the smooth and rough cylinders for the two coatings. The cylinders coated with electroless nickel have a larger difference between valley and peak than chrome, these values are listed in table 3.1.
The differences in peak-to-valley values were also found during the perpendicular SEM investigations illustrated in Fig. 3.2 & 3.3. Notable in these pictures is also the cracked interface of hard chromium.
Table 3.1: Difference between peaks and valleys for the smooth and rough cylinders.
Coating - roughness Peak-to-valley [µm]
Chrome Rough 3,0
Nickel Rough 6,0
Chrome Smooth 0,80
Nickel Smooth 1,60
Figure 3.1: Coating profile of the unworn cylinders, (A) smooth substrate coated with nickel, (B) smooth substrate coated with chrome, (C) rough substrate coated with nickel and (D) rough substrate coated with chrome.
Figure 3.2: Coating surface of an unworn area coated with chromium onto (A) a rough substrate surface and (B) a smooth substrate surface.
Figure 3.3: Coating surface of an unworn area coated with nickel onto (A) a rough substrate surface and (B) a smooth substrate surface.
The cross-section results of the unworn cylinders illustrated in Fig. 3.4A show how deep the cracks found on the chrome cylinder interfaces are. As seen these cracks are distributed throughout the entire coating. The height of both the unworn nickel and chrome coating were measured in the SEM to 30µm.
An interesting and unexpected result was that the unworn interface of the duplex cylin- ders coated with electroless nickel had some cracks. Fig. 3.5A illustrates this cracked interface. The cross-section investigations illustrating how deep these cracks were is shown in Fig 3.5B, this figure shows that these cracks reach the substrate.
Figure 3.4: Cross-section of the unworn (A) hard chromium and (B) electroless nickel coating.
Figure 3.5: Unworn cracked surface of a duplex cylinder coated with electroless nickel, (A) perpendicular view and (B) cross-section view. Red arrow indicates that these cracks are perpendicular to sliding direction.
3.2 Wear tracks
3.2.1 Stereo microscopy
The pictures of the wear tracks taken by using the stereo microscope are illustrated in Fig. 3.6 and 3.7. Typical patterns observed for the different cylinders. Notable in Fig.
3.7B are the small white pores. These pores were also found on unworn parts of these cylinders. The maximal depth of these pores were found to be 20µm by profilometer investigations. (Red arrow indicates sliding direction).
Figure 3.6: Wear track of the cylinders coated with chrome with (A) austenitic and (B) duplex substrate. Magnification 1.5.
Figure 3.7: Typical wear track left (A) austenitic and (B) duplex cylinders coated with nickel. Magnification 2.
