Exploratory study of the interactions between textured alumina coatings and steel

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Exploratory study of the interactions

between textured alumina coatings and steel

Abstract

The crater wear of alumina coated WC-Co cutting tools is thought to be influenced by the chemical reactions between the coating and the workpiece material. Three different crystal orientations ((001), (012), and (100) of alpha alumina CVD coatings are examined in combination with four workpiece materials of steel to establish what reactions are present, and the extent of diffusion. The alumina coatings and workpiece materials were pressed together as diffusion couples and heat treated at 1250-1300°C for 10-20hours.

It was fond that the types of inclusions present in the workpiece were more impactful on the chemical wear of the coating than the crystal orientation of the coating. EDS measurements show significant amounts of W and Co on the surface of the coatings and on the steel surfaces after heat treatment. This is thought to be connected to the migration of Co through the coating in cooling cracks and other impurities. In the surface of the coating, areas of solidified Co-rich structure have been found, implying that Co has formed an alloy with Fe, C, Al, and W with sufficiently low melting temperature to partially melt during the heat treatment. This has been confirmed as possible by simulations in Thermo-calc. Turning tests and scratch tests were made with the same combinations of coatings and workpiece material and show differences in adhesion of workpiece material on the different coating orientations. The 100-orientation has been found to have the most adhered workpiece material, the reason for this being its higher surface roughness.

Ultimately no noticeable differences in chemical reactivity between the coating crystal orientations was found. The Co diffusion though the coating occurred for all the coating orientations and further experiments in turning with the different workpiece materials are required to determine the effect of Ca-rich inclusions on the magnitude of chemical wear.

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Abbreviations

SS2541/Ovako825B/C45U Workpiece material

LOM Light Optical Microscope

SEM Scanning Electrode Microscope

XRD X-ray Diffraction

EDS Energy-dispersive X-ray spectroscopy

DSC Differential Scanning Calorimetry

Alumina alpha Aluminum Oxide (α-Al2O3)

CVD Chemical Vapor Deposition

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1. Introduction

In this work the chemical interaction between the workpiece material and the coated insert is studied.

1.1. Background

Metal and steel applications are an integral part of modern society, but for these materials to be useful, it is necessary to be able to shape them to our needs. Forming of metals is done on all scales, from coarse rolling and forging of bars and billets to fine machining of intricate details using drilling, turning, and milling. The fine manipulation methods are called machining and are based in the use of a hard material to cut into a softer material. With increasing demands of productivity and the use of more advanced workpiece materials the continued development of cutting tools is imperative for progress within this field.

For a long time, the materials used for machining were high-carbon and alloyed carbon steels as they had superior hardness to the metal being machined. But the true breakthrough in tool-steel

technology was with the invention of HSS (High Speed Steels) as they had greater resistance to the heat generated during machining allowing for much higher machining speeds leading to increases in productivity. [1] This triggered the development of more advanced cutting materials and soon the HM (Hard Metal) Cemented carbides composed of Tungsten Carbide and a Cobalt-rich binder were discovered in the 1930’s. This cutting material shows even higher hardness and ability to retain its hardness at high temperatures, allowing for increased tool temperature and thus higher metal removal rates. One of the common wear types on the cutting insert, as an effect of the increased temperature, is crater wear due to chemical and thermomechanical interactions between the chip and the insert. One contributing mechanism to crater wear is hypothesized to be diffusion between the insert and the workpiece.

Due to high working temperature, the rate of diffusion and chemical reactions between insert and workpiece may become significant enough to cause degradation of the material reducing its mechanical strength. This process is called chemical wear and is most prominent on the rake face of the insert, as the chips from the workpiece flow over this area and generate heat from friction and deformation. This heat together with continuous contact between workpiece and insert allows diffusion to take place. [2]

In the early 1970’s it was found that by covering the insert with a thin coating layer of Ti(C,N), and later also 5µm alumina (Al2O3) the chemical wear of the insert could be drastically reduced. This coating is generally done by CVD (Chemical Vapor Deposition) and the thin layer is sufficient to drastically increase tool life. [1] The structure of alumina has been studied thoroughly for other applications, and it has been found that alumina crystals show significant anisotropy in mechanical properties. Meaning that the mechanical properties of the crystals vary depending on the direction of load applied to them. Because of this, the growth direction of alumina crystals in tool coatings is of interest, as it may have significant impact on the mechanical properties and degradation of the coating.

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Alumina is used in many applications because of its low reactivity with most other elements. This is also one of the main reasons it is used in insert coatings. However, since the working temperature is very high (may be over 1000°C) [6] [7] chemical reactions will take place between the alumina coating and the workpiece material. As diffusion is favored at high temperatures it may be a

contributing factor to the crater wear of the coating, but the extent of such a reaction is not currently known.

1.2. Aim and Objective

The aim of this study is to investigate the diffusion behavior of elements between steel workpiece materials and different orientations of textured alumina coatings on WC-Co cutting tools. This will be done by examining diffusion couples of four different alumina coatings on WC-Co cutting tools together with four steel workpiece materials at temperaturesclose to the real working temperature. By examining the reactions at the diffusion couple interface and the changes in chemical composition near the interface, the effect of crystal orientation in the coating will be evaluated. By examining the coatings in contact with four different workpiece materials the effect of alloying elements and inclusions on diffusion and chemical reactions can be studied. The diffusion couples will be

supplemented by turning tests and scratch tests using some of the workpiece-insert combinations. To accurately replicate the diffusion process in real turning the temperature of the cutting insert during machining will be studied. A method will be developed for fitting a thermocouple close to the rake face of the insert to acquire accurate temperature readings. Using this temperature

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2. Theoretical Background

2.1. Cemented Carbides

Cemented Carbides are made of hard Tungsten Carbide (WC) or titanium carbides (TiC), combined with a tough, metallic, binder phase material, usually Cobalt-rich. The combination of the hardness of the carbide and the toughness of the binder has proven to be highly resistant to wear, making it suitable for machining applications.

Tungsten Carbide cutting tools are produced by mixing fine powder of Co and WC with plastic binder called PEG (Polyethylene glycol) and then compacting it into the desired shape. This pressed piece is called a green body and does not have the structural integrity of the finished cutting tool. To consolidate the powder and binder the green bodies must be sintered in a sinter furnace. [8]

2.1.1. Sintering

First the green bodies are heated to 380°C and held for 2-3 hours. During this time the PEG used to hold the green bodies together is burned off. It is very important to have sufficient hold time at this stage to allow slow burning of the PEG; else the green bodies will explode from the gas pressure from the vaporized PEG. [9]

When all the binder has been burned of the temperature is increased up to sintering temperatures of 1450°C and simultaneously a vacuum is pumped. At sintering temperature, it is important to

introduce a partial CO atmosphere to reduce the Tungsten oxides which form.

When the furnace has reached maximum temperature, it is held for ca 1 hour. During this time the atmosphere is argon and CO to ensure the Al2O3 shielding of thermocouples in the furnace is not reduced into the atmosphere due to low partial pressure of oxygen.

With longer times in the sinter furnace the WC grains in the insert will grow more. The size of the WC grains is important to control as it will directly influence the mechanical properties of the cutting tool. [10]

2.1.2. The Cutting Insert

Inserts for machining applications are made in many different shapes and sizes depending on application. The geometry can vary from circular, with very high strength and reliability, to very elongated, which allows thin cuts and precision turning. Generally, a good intersect between these properties is found in the rhomboid geometry of CNMG inserts. Depending on the depth of cut and type of workpiece material, different types of chip-breakers are applied to keep the chip length manageable. In this study the inserts used for diffusion couples are made in the CNMA geometry with planar surfaces to allow good contact in the diffusion couples, and the inserts used for turning are made in the CNMG120408-PM geometry. [11]

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2.2. Metal Turning

Metal turning is done in three main processes, longitudinal turning, profile turning, and face turning. With the difference being the direction the tool is moving during the operation. In all turning the workpiece is rotating around its own axis and the tool moves along the outer geometry of the workpiece. This limits the possibilities of what can be produced by turning to circular symmetry geometries around the workpiece axis.

In longitudinal turning the insert moves over the length of the workpiece at a fixed radial distance. This is done for heavy turning and removal of large amounts of material. In face turning the tool moves radially at the end of the workpiece to provide a polished end surface. Profile turning is a combination of both radial and axial movement which is required when a complex profile is desired. This is often done with more elongated inserts to counter the varying depth of cut and accessibility issues. [1]

Several cutting parameters are important to control during machining, but the most important ones are:

Cutting speed – The rate at which the workpiece material passes over the insert. Measured in m/min and governed by rotational speed and diameter of the workpiece.

Feed Rate – The rate at which the insert moves along the workpiece. Measured in mm/rev and controls the surface finish, chip size, and force required for cutting.

Depth of Cut – The depth penetrated by the insert into the workpiece. Measured in mm and heavily impacts the force required for cutting.

By combining these factors, the rate of material removal rate (Q) can be calculated: 𝑄 = 𝑣 ∗ 𝑑 ∗ 𝑓

Since the cutting speed of the process is based on the diameter of the workpiece the actual cutting speed will vary with the radial distance from the symmetry axis. To counter this, modern machines adjust the RPM of the workpiece as the diameter decreases to maintain constant cutting speeds. [1]

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Since the contact between the workpiece and the insert is concentrated to the cutting edge and the rake face of the insert. The lead angle of the insert will determine how much the flank face of the insert will be in contact with the workpiece. It is in these positions the wear of the insert will occur and therefore it is important to distribute the cutting forces over all the surfaces to promote insert lifetime.

2.3. Alumina Coating

Alumina coatings are used as a surface layer in cemented carbide inserts to maximize the lifetime of the tool. A thin layer of α-Al2O3 is applied to the insert to give improved thermal insulation,

hardness, and protection against chemical reactions. [12] Since the oxide coating is very brittle and hard in comparison to the insert it cannot be made too thick as it will then crack when used. The layer is applied via CVD (Chemical Vapor deposition) which results in an even thickness over the insert, with slight variations between the top and bottom side of the insert. The alumina applied by CVD is crystalline and non-porous, as opposed to the alumina used in industrial applications which is porous and made up of small randomly oriented grains. Porous alumina is used in many industrial applications for its low reactivity and high temperature stability, but the mechanical strength of such alumina is low. By controlling the crystal orientation of the alumina, the mechanical properties can be notably increased which is favorable in machining applications since it brings enhanced

performance and a more predictable wear behavior during machining.

In CVD coatings, the orientation of the alumina crystals is controlled to give optimal mechanical properties. It has been found that coatings with a crystal lattice orientation of (001) show greater wear resistance in machining than other orientations of coating and randomly oriented coatings. [13] The EBSD image in Figure 2.2 shows the difference between highly controlled alumina orientation in the (001) direction (Inveio™) and less controlled alumina in conventional tool coatings.

The texture of a coating is a measurement of how similar the lattice orientation is between the crystals. A high texture (TC-value) indicates that almost all grains are aligned in the specified

direction, indicating that the CVD application was stable. This is preferred during experiments since it enables isolated studies of the behavior of the orientation. The control insert, A1, used in this study has a low requirement on texture compared to other commercial coatings.

The crystal size and shape also have an impact on the mechanical properties of the coating. Crystals of alumina form in the rhombohedral lattice shape which can be fit into a hexagonal lattice structure. The tilted unit cell and differences in uniformity causes single crystals of alumina to show anisotropic mechanical properties. The yield strength of the crystal has been found to vary as much as 30% when

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loaded in different directions. [14] If the alumina coating can be applied in accordance with the optimal mechanical properties rather than the average, the wear resistance of the coating is increased. For this reason, it is believed that a highly textured coating will show the best performance in mechanical testing. [15]

The alumina crystals applied by CVD grow perpendicular to the substrate, but the orientation of the crystal lattice may change depending on the process. Studies by S. Ruppi et al. and R. M’Saoubi et al. showed that a crystal lattice orientation parallel to the surface (001) greatly improves the resistance against crater wear by showing reduced chipping and more uniform plastic deformation as compared to other crystal orientations. [5] [13] [16] This represents the crystal orientation (001) per the Miller Index.

The chemical reactivity and diffusion properties in different crystal orientations has not been studied thoroughly and is the main subject of this study. The three different orientations considered are the 001, 012, and 100 orientations, since the mechanical wear resistance of these directions has been previously studied. Figure 2.3 shows the crystal lattice of alumina with the studied directions indicated.

In studies by R. M’Saoubi et al. the 211-orientation had significantly smaller grain size than other studied directions. [5] In another CVD coating study, the 100-orientation had larger grain sizes compared to the 001- and 012-orientation. This is most likely due to the nucleation and growth speed of the crystals during coating from the corresponding process parameters. [3] The larger grain size might contribute to the lower wear resistance of coatings with this crystal orientation.

Since the exact process parameters used for the highly textured CVD coatings are trade secrets, companies use their own version of recipes to create the structure. Thus, differences in grain sizes and coating thickness vary depending on the coating recipe. These differences in process parameters give changes in the coating which can be correlated to the coating wear resistance. [17] At present, the governing process parameters to tailor the grain size are not fully understood.

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The grain size of the coating is important both from a mechanical and a chemical point of view, since it may affect the diffusion speed in the alumina, but also the heat conduction and wear resistance. Grain boundary diffusion uses different mechanisms than bulk diffusion and therefore the amount of grain boundaries will most likely affect the diffusion speed. [18] The possible effect of grain size on diffusion is discussed further in section 2.7 Diffusion Processes.

2.4. Chemical Vapor Deposition

CVD (Chemical Vapor Deposition) is one of the two popular coating methods for cutting inserts, the other being PVD (Physical Vapor Deposition). In CVD, the desired coating material is added to the insert via precursor gasses and process gasses which fill the reaction chamber. The precursor gasses react at the heated surface of the insert or in proximity to it and form the coating material which is then deposited on the insert. The ratio of reaction gasses, temperature, and time, are all process parameters which will affect how the coating is formed on the cutting insert, and the structure of the coating. The coatings studied in this project are variations of the commercial A1 coating which consists of layers of TiN, Ti(C,N), and Alumina.

The first layer in the coating is a thin layer of TiN. The purpose of this layer is to create a good surface to build on since TiN has good adhesion both to the WC-Co substrate and to the next layer. The second layer is a 5-8 micron thick layer of Ti(C,N) applied at 860°C with a gas mixture of CH3CN – TiCl4 – H2 – N2. This layer provides excellent mechanical properties and resistance to wear during

machining. Depending on the ratio of process gasses used, the appearance and properties of the Ti(C,N) layer can vary significantly. [19]

The third layer is made up of alumina and is usually 4-8 microns thick. Metallic aluminum is mixed with hydrochloric acid at ca 340°C to form AlCl3 precursor gas which is transported with the carrier gasses to the chamber. The gas mixture of AlCl3 – CO2 – CO – H2S – H2 then reacts at 1000°C to form solid Al2O3 on the inserts in the reaction chamber. [3]

It is desired to control the orientation of the formed alumina coating since it has been found that the mechanical properties of the coating are heavily dependent on the lattice orientation and direction of the Al2O3 crystals. Advancements in the field have made very detailed control of the coating orientation and thickness a possibility and the potential benefits of advanced coatings is currently being investigated. [3]

In the CVD reaction chamber the inserts are placed on pegs to allow for coating of all sides

simultaneously. Due to gravity and flow mechanics the coating will not be equal on the top side and the bottom side of the insert. The top side of the insert generally has a thicker coating than the bottom side, especially for the Ti(C,N) layer. [3]

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2.5. Wear in cutting inserts

The wear in cutting inserts is generally attributed to four different wear mechanisms: Abrasive wear, Adhesive wear, Chemical Wear, and Thermomechanical wear. [11] Depending on the combination of insert, workpiece, and process parameters the ratio of these mechanisms will shift and cause different types of wear on the insert.

Abrasive wear is based in the contact between hard particles in the workpiece and the insert. When hard particles from the workpiece slide along the insert, material will be ground away and leave a wear track. With continued machining the abrasive wear will keep grinding on the insert causing structural damage, eventually leading to failure of the insert. This mechanism mainly depends on the frequency of hard particles in the workpiece material and is generally not dependent on the cutting speed or depth. The amount of abrasive wear can be reduced by increasing the mechanical strength, and thus the wear resistance, of the coating layer on the insert. Or by reducing the amount of abrasive inclusions in the workpiece material by inclusion modification. [11]

Adhesive wear arises when workpiece material sticks to the cutting insert. When the adhered material is removed from the insert it may take some part of the coating with it causing flaking or chipping. The adhesion of workpiece material may be beneficial to the lifetime of the insert since it can help prevent abrasive wear by absorbing the impact of hard particles. The adhesive material may even form a beneficial geometry on the insert as a built-up edge (BUE), which may help increase the cutting performance. However, when this edge is removed there is risk for removal of coating with it, causing large damage to the insert. [11] Adhesive wear is mainly a problem at lower cutting speeds as higher cutting speeds lead to higher temperatures which will help remove the material and prevent it from sticking. In studies by T. Aiso et al. it has been found that adhesion of workpiece material in turning is increased with Al alloying of the steel, and that this adhesion may cause coating delamination and rapid wear. [20]

Chemical wear comes from chemical reaction between the insert and the workpiece. Depending on the composition of the insert and the workpiece, chemical reactions may take place and form compounds and structures which are not as strong and tough as the original structure. Chemical wear is most prominent on the rake face of the cutting insert. [1] At the rake face the workpiece material will slide along the insert as it is being removed from the workpiece. The friction between the chip and the insert will generate heat depending on cutting parameters and tribological factors. The elevated temperature at the rake face will then promote chemical reactions between the insert and the workpiece as well as allow for diffusion between the two to take place. Adhered material may react with the coating and cause chemical wear since it is in contact much longer than passing chips, but can also act as a barrier hindering diffusion between the tool and the workpiece material. [21]

To control the chemical wear, the reactivity between the insert and the workpiece, and the insert’s ability to remove heat, are of great importance. By altering the cutting parameters, the amount of heat generated can be reduced, which in turn reduces the temperature and chemical wear. [11] Alumina coatings are applied to the insert to reduce the reactivity as alumina has low reactivity with most elements up to high temperatures. Special steel grades may use additions of Ca or Mg as deoxidizing agents or to increase the workability of the steel, but these two elements may reduce Alumina since they are more thermodynamically stable at machining temperatures. [22]

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the workpiece material, and the average concentration of diffused atoms in the workpiece, the amount of material removed by this process can be estimated.

Thermomechanical wear is the process of insert degradation from the intense heat generated during machining. Heat is generated from both the friction of insert sliding against workpiece, but also from the deformation of the workpiece itself. [1] The amount of heat generated is therefore dependent on the combination of the cutting forces, mechanical properties of the workpiece material, and

tribological factors. Higher cutting speeds, higher feed rates, and deeper depth of cut all increase the cutting force required in the process, and thus generate more heat.

The heat which is generated needs to be transported away from the cutting edge to prevent the temperature of the cutting edge from reaching dangerous levels which reduce the mechanical strength of the insert. If the temperature reaches too high the cutting edge will plastically deform due to the combination of mechanical stress and temperature and the rate of chemical reactions will drastically increase. To increase the removal of heat a coolant may be used to help transport heat away from the cutting edge. Turning with coolant or an intermittent process may introduce

temperature fluctuations in the coating, which cause thermal expansion, leading to thermal stresses in the insert. If the thermal stress is too high, it may cause breakage of the brittle alumina coating. [11]

2.6. Workpiece material

The type and quality of material used as workpiece is very important for the tool-life in machining applications. Depending on the mechanical and chemical properties of the workpiece the wear of the insert will vary significantly. The mechanical properties of the workpiece, such as hardness and tensile strength, will determine the cutting forces required during machining, and by extension the temperature generation due to friction and deformation.

The chemical properties, such as the melting temperature, inclusion content, and composition will also affect the wear behavior of the insert. At the high working temperatures in turning the reactivity of elements is high and the rate of reaction is significantly increased as compared to room

temperature. The increased rate of reaction is a result of the increased atomic movement from thermal energy which may act as the required activation energy for a reaction to start, as described by the Arrhenius equation. [23] The non-metallic inclusions in steels are believed to affect the wear behavior of cutting inserts. Depending on the type of inclusions in the workpiece material the interaction may be both chemical and mechanical.

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material. [24] Any inclusions remaining after solidification cannot be removed, except by re-melting the steel with ESR (Electro Slag Remelting) or similar methods.

Depending on the amount of Ca, Mg, Si, and Al added during steelmaking, and during which process step they are added, the chemistry and size of the inclusions will vary, and so will their interaction with the insert during machining. [1] Calcium is often added to modify the appearance and properties of inclusions. The Ca will combine with hard Al2O3 particles and form softer, complex, Al2O3-CaO particles which are less abrasive and less prone to forming large clusters. When adding elements to metallic melts the thermodynamic equilibrium is the driving force for reactions. By this equilibrium the concentration of the added element remaining in the melt after reaction can be calculated. [25]

Since both Ca and Mg form more stable oxides than Al the presence of any of these elements in workpiece material may increase the wear rate on the Alumina coating by reducing it and removing O. [24] [3] It is not known how these elements will react with Alumina when in the shape of

inclusions such as CaO, CaS, MgO and MgS.

Measurements have shown that Ca and Mg oxides are more stable than Ca and Mg sulfides, which will cause sulfide inclusions to reduce the Alumina coating and form oxide inclusions instead. [26] The amount and type of inclusions in the workpiece materials is not known, but the ratio of inclusions to dissolved elements can be calculated by using thermodynamic equilibrium. Depending on the stability of the compounds and the activity of the constituents the thermodynamic

equilibrium can be found using Equation 1 and 2: [25] 𝐾 =𝑎 ∗ 𝑎

𝑎 ∗ 𝑎 (1) 𝐾 = 𝑅𝑇𝑒𝑥𝑝(−Δ𝐺) (2)

The activities of the compounds and elements are directly related to the composition of the melt during steelmaking and the reactions between metal and slag layer.

2.7. Diffusion Processes

Diffusion is the process of material transport within a material and will always work towards equalizing the activity of elements in every part of the material. When pieces of different materials are in contact, diffusion will cause atoms to transfer between the materials to equalize the activity in both materials. This process is very slow at room temperature but increases in magnitude with increasing temperature. In high temperature applications, such as the contact between a cutting insert and the workpiece material during high speed turning, the diffusion may play a large role. The distance an atom can diffuse is hard to determine exactly since it is a random movement controlled by the activation energy of atomic jumps between vacancies or interstitial sites. But it can be estimated with good reliance using the Einstein equation as presented in Equation 3:

𝑙 = √𝑛 ∗ 𝐷 ∗ 𝑡 (3)

Where n is the dimensional factor for 1D, 2D, or 3D diffusion with the values 2,4, and 6 respectively, t is the time for diffusion in seconds, and l is the average diffusion distance of an atom. D is the diffusivity as determined by the Arrhenius equation as in Equation 4:

𝐷 = 𝐷 exp −𝑄

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Where D0 is the diffusion coefficient, Qd is the Activation energy, R is the Gas constant (8.3144621), and T is the temperature in Kelvin. [27]

During turning the rake face of the cutting insert is in contact with the machined chip for only a short amount of time, but since the flow of chips over the rake face is high, and the temperature of the interface is high, there is possibility for diffusion to take place. Due to the vastly different activities of many elements (Fe, O, Al) in the coating and the workpiece, diffusion will cause a change in

composition of the cutting insert which may affect the mechanical properties and therefore the lifetime of the insert. The area which is in contact with the chip from the workpiece often shows crater wear which is believed to be caused by a combination of weakening of the material from diffusion, and abrasive wear. [28]

It is known that the grain size and misalignment of grains has a large effect on the speed of diffusion in other materials. For diffusion of Hydrogen in Nickel, grain boundaries of high misalignment promote diffusion, while grain boundaries between similarly aligned grains reduce the speed of diffusion. [29] The grain boundaries may increase the diffusivity of the material since the activation energy for atomic movement is lower there, usually in the range of 0.4-0.6 times the activation energy for bulk diffusion. For lower temperatures, the grain boundary diffusion may have significant impact on the total diffusion, but at temperatures above 0.8 times the melting temperature (Tm) the difference to full bulk diffusion is negligible. [27]

The grain size of the studied coatings has been found to be varying depending on the direction of the crystal growth, this is a factor that may affect the speed of diffusion in the coating. [5] To account for this when evaluating the experiments, the alumina grain size will be measured.

2.8. Diffusion Couple

The diffusion couple is a useful tool for material science since it allows for an isolated study of how two materials will behave when in contact at high temperatures. By creating a smooth surface and good contact at the interface between two materials the diffusion of elements over the interface can be studied at different temperatures and times. To simulate the real diffusion process it is important that the conditions of the real process are replicated to a high degree in the experiment.

There are multiple different approaches to constructing diffusion couples, but they all aim at achieving the best possible contact between the two samples while being simple to use. A common method is to apply pressure via a weighted inert object on the diffusion couple or pressing with a hydraulic press. [30] [31] [32] This pressure may also be combined with the use of wire with low thermal expansion to ensure good contact between the samples. [33] [34]

Other approaches utilize the difference in thermal expansion coefficient between the materials to achieve a good contact between the surfaces. As presented by L. von Fieandt a bar of material with high thermal expansion may be inserted into a hole of material with low thermal expansion and the heat of annealing will be sufficient to expand the bar and achieve good contact. [35] This method is like the method used by S. Odelros et al, where a clamp of low thermal expansion material was used to hold together the expanding materials of the diffusion couple. [36]

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1100°C the tensile strength of most steels is reduced to only a small fraction of the low temperature tensile strength. [39] The pressure applied should therefore only be high enough to allow for good contact since it is not reasonable to try and simulate the real turning pressure.

Since all the process parameters in turning cannot be matched in a diffusion couple, such as pressure and material flow, the remaining parameters need to be altered to achieve a comparative result. By increasing the temperature and time in contact via a heat treatment, the actual chemical reactions in turning can be estimated.

2.9. Temperature measurements in turning

The temperature of the cutting edge during machining operations is not simple to measure. The small cutting area in combination with the chips from the workpiece material obscuring the vision of the cutting edge make measurements problematic. Previous research has used computational modeling to estimate the temperature of the cutting edge based on the cutting force, but no unified method has been found. [40] [41] [42] [43]

Studies using thermal cameras, IR sensors, and pyrometers have shown promising results which may represent reality. Optical measurements do however have significant error sources, such as

emissivity of the insert, obstructed vision of the cutting surface, and variations in atmospheric conditions. [44] [7] These factors reduce the accuracy of measurements done by optical measurements and make the measurements rough estimations and depictions of the average temperature rather than the specific point temperature in the crater wear zone which is of interest for diffusion calculations since it may indicate the rate of wear.

The most accurate method for measuring the temperature of the cutting surface during turning is by thermocouple. The thermocouple can either be placed on the cutting edge [43] or imbedded into the insert before sintering. [45] By placing a thermocouple close to the rake face on the cutting insert an accurate temperature reading can be acquired. To achieve a correct temperature for the cutting edge the thermocouple must be placed very close to the cutting surface since the temperature has a very steep gradient away from the cutting edge; this is the major drawback of this approach.

2.10. Thermo-Calc and DICTRA simulations

The CALPHAD (CALculation of PHAse Diagram) method is an approach in which the Gibbs energy of phases are assessed as function of temperature, pressure and constitution (composition) using many different sources of input data such as experimental measurements, ab initio calculations and empirical rules. Thermodynamic equilibria can then be evaluated by, for example, calculating the minimum in Gibbs energy under constraints specified by equilibrium conditions, which in the simplest case correspond to pressure, temperature, system size and composition. This approach is used by the program Thermo-calc.

By using one of the many databases provided in the software different kinds of alloys and solutions can be investigated. Included in the Thermo-calc software is the diffusion module (DICTRA) which utilizes data on atomic mobility to calculate diffusion of elements, mainly in Fe and Ni based systems. [46] DICTRA can be used to calculate interdiffusion in compounds, sintering of cemented-carbides, and many more diffusion-controlled processes.

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function that, for example, considers the local phase fractions, phase compositions and diffusivities in the individual phases.

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3. Experimental Method

3.1. Diffusion Couples

The diffusion couples were constructed by pressing together alumina coated inserts with polished pieces of workpiece material. This was done in three rounds with slightly altered methods and different temperatures.

3.1.1. Workpiece sample Preparation

Workpiece samples were made from the four chosen workpiece materials presented in Table 3.1 The materials were chosen as they represent the most commonly used turning workpiece materials in the Sandvik Coromant performance laboratory in Västberga and have different compositions. Two batches of SS2541 was used, one Ca-treated (29425) and one untreated (21419) to investigate the impact of Ca-rich inclusions on the diffusion interface.

Table 3.1 – Workpiece materials main composition per certificate

The samples were cut into small pieces 2-4mm thick and encased in Bakelite for polishing. The samples were ground down 0.5mm and then polished using first 9µm polish and then 1µm polish for 15min each. The polishing was done to allow for good contact between coating and workpiece material. After polishing the samples were broken out of the Bakelite and cleaned using ethanol and ultrasonic cleaning to remove any residual oils and impurities.

3.1.2. Coated insert preparation

Planar cutting inserts were pressed in CNMA geometry then sintered and cleaned at Sandvik in Gimo. The inserts were sent to Seco tools in Fagersta for CVD coating. The inserts were loaded on pegs on trays and loaded into a SuCoTec 600HT CVD together with filler and control inserts. The inserts were loaded in the middle of the stack at the center radius of the tray, as this area generally gets the most even coating. Three separate batches of 30 inserts were made with each batch having a specified alumina orientation (001), (100), and (012). The lattice orientation was controlled by altering the process gas flow in the recipe. Specific temperatures, pressures, times, and gas flows used are industry secrets and cannot be disclosed. The coating procedures will be denoted as W900-A (001), W900-B (012), and W900-C (100).

The inserts were first coated with 6 µm Ti(C,N) in the (211) orientation. Secondly the inserts were coated with 5 µm Al2O3 in the specified direction. The coating thickness on the top and bottom side of the samples, and the amount and size of cracks in the coating was documented in SEM and LOM. The grain size of the alumina was measured at the middle of the cross section of the coating for all tested coatings to give a relative measurement of the grain size. The orientation and texture of the samples was measured using XRD (X-ray Diffractometry). The results of sample measurements are presented in section 4.1. Coating Measurements.

Steel Type: Charge: C% Si% Mn% Cr% Ni% Mo% Al% O

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15

The inserts with A1 coatings were supplied by Sandvik Coromant from Gimo. They were included as control and to expand the scope of the study to cover current (A1) and future types of coatings.

3.1.3. Melting temperature measurement

The melting temperatures of the workpiece materials were determined in DSC (Differential Scanning Calorimetry). [48] Small chips of each workpiece were cut and placed in the furnace in an argon atmosphere. The DSC measures the temperature as a function of applied heat in the furnace. During phase changes in the material extra heat is needed or released during transformation. Melting of the samples requires extra heat and produces a plateau in the temperature curve of the furnace when reached. The location of the plateau indicates the melting temperature.

The ramping speed of the DSC was set to 20°C/min as this made the tests fast but still reliable. A too high ramping speed will increase the uncertainty of the result since the temperature will not be as homogenous in the furnace and sample, causing the phase change to be delayed and falsely indicated.

The high-temperature behavior of coated inserts was also studied in the DSC. A small piece of coated insert was examined in the DSC to identify the temperatures where phase changes occur in the material.

3.1.4. Inclusion characterization

The workpiece samples were examined using electrolytic extraction at KTH with Andrey Karasev. The sample was submerged in 10%AA solution (10% Acetylacetone – 1% Tetramethylammoniumchloride – Methanol) and connected to a power source of 3.2V and 58mA. The dissolution was left to run until 500 coulombs had been counted, which took approximately 2,5 hours. The solution was then filtered through a 0.4micron filter under low vacuum. This caused inclusions suspended in the solution to deposit on the filter. The filter was then examined in SEM with EDS to characterize the inclusions found on the filter.

The inclusions were also characterized in 2D by the laboratory at Sandvik Materials Technology in Sandviken, where polished samples of the steel were examined in EDS to determine the amount and composition of the inclusions present.

3.1.5. Heat treatment

A trial run for the diffusion couples was done in the DSC furnace with a diffusion couple between Ovako 825B and A1 coated insert. The sample was heat treated at 1250°C for 3h in Argon atmosphere to measure the extent of diffusion for such a treatment. Due to the poor adhesion between the samples in the forerunner, the first full-scale test was instead done for 10h. The temperature was set to 1250°C to exaggerate the effect of diffusion and reactions so that the results would be easier to interpret.

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16

After annealing the samples were left to cool in air and then examined. The second round of diffusion couples were clamped together by placing the stacks of inserts and workpiece materials inside the holes of the pieces previously used as weights. The wedge shape of the workpiece materials allowed for the stacks to be pressed into the holes until stuck as shown in Figure 3.2. The force applied by this method in addition to the expansion of the material during annealing is thought to increase the contact pressure on the connected surfaces and provide a better contact for

diffusion. The second round of diffusion couples were annealed at a slightly higher temperature of 1300°C for 10h to further increase the reaction rate.

The third round of diffusion couples were pressed together by weights just like the first round. This method was preferred since many of the samples from the second round of testing broke apart during extraction from the setup, resulting in exposed contact surfaces. The lacking adhesion between the samples in round one was rectified by increasing the temperature to 1300°C and the time to 20h to ensure any favorable diffusion would have sufficient time to take place. For the third round the rake face of the inserts was carefully polished to increase the smoothness of the surface and allow better contact between the samples. One insert of every coating orientation was included but not placed in contact with a steel surface. These inserts served as control for the high

temperature behavior of coated inserts.

In all the three rounds, there were samples which had not sintered together and broke apart after annealing, an overview of adhesion is presented in the results. The contact surfaces of those samples

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17

were first examined in LOM and SEM. All samples were then encased in Fapsa-plastic and cut to expose the diffusion interface. This surface was then ground and polished with 9 µm polish and 1 µm polish. Special care was taken during polishing to avoid uneven grinding at the interface between steel and insert, which may arise because of differences in hardness. The polished samples were then examined in SEM to determine the interaction at the interface due to diffusion. Composition measurements were made across the interface with EDS linescan to track the composition change because of diffusion.

3.2. Turning Samples

Inserts in the CNMG geometry were prepared and coated via the same procedure as the CNMA inserts described in section 3.1.2. The CNMG inserts were used in turning tests of workpiece material C45U. The turning was performed in the performance laboratory at Sandvik Coromant in Västberga with the cutting parameters presented in table 3.2. chosen to promote crater wear.

Table 3.2. – Cutting parameters for Turning test

Cutting Rate Feed Rate Depth of cut Time in cut

220 m/min 0.3mm/rev 2mm 2min/cut

The inserts were run in 2-minute intervals after which they were examined for wear. The first set of inserts (DP1) was run for 6 minutes total and the second set of inserts (DP2) was run for 4 minutes total. The samples were not etched or cleaned before examination, as this may remove some of the adhered material in the crater. After turning, samples of inserts from all three coating orientations for each workpiece material were examined in LOM and SEM to determine the wear type in the crater zone. Images were taken from the three zones in the crater to showcase the difference in wear depending on the chip-coating interaction.

3.3. Turning temperature measurement

Pressed green body inserts in CNMG geometry from Sandvik Gimo were used to evaluate the possibility of implementing a thermocouple into the insert for temperature measurement. A small hole through the insert was made near the cutting edge per the schematic provided by Conny Lundgren (Coromant Sandviken), see full schematics in Appendix. To facilitate the drilling a press tool was used as base for the green body with a 3D printed plastic bushing to hold the green body in place.

The hole was drilled using a circuit board drill of hard metal with a diameter of 0.6mm, due to the drill length (3.5mm) the hole did not reach as close to the opposite surface as desired. The drilling was done in a machine with digital location measurement to be able to control the depth and location of the hole. Some of the samples were drilled from both sides to create a hole the whole way through the insert. This hole was then covered by a small graphite rod before CVD coating to prevent the CVD gasses from entering the hole.

The possibility of making holes in green body inserts using laser was also studied. By using a spiral pattern holes were made in the insert. This approach was evaluated since the drilling often causes breakage of the brittle green bodies.

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3.4. Scratch Testing

The coated samples in the 001 and the 100-orientation were exposed to scratch testing from the two workpiece materials Ovako 825B and SS2541 (charge 29425) at SECO tools in Fagersta. Before the scratch test the coatings were polished with 1-micron polish to ensure a smooth surface.

The workpiece materials were machined into a stylus shape used by the scratch testing machine. The required styli dimension was 25mm long and 10mm in diameter with a rounded tip of 5mm radius. These dimensions were achieved by turning in the performance laboratory at Coromant Västberga. The scratch test was performed in an Anton Paar Revetest machine with a custom heating stage at 400°C with a load of 20N. The scratch length was 2mm with a speed of 10mm/min. The process was repeated 35 times in both directions, resulting in 70 runs total over the same area. The test ran for a total of 30 minutes for every combination with a time in contact between stylus and coating of approximately 15 minutes. The temperature was measured by an integrated thermocouple in the heating stage and not in the sample itself. The surface of the sample is estimated to have reached 380°C.

The scratch test machine measures the friction coefficient and the acoustic emission for every scratch, this data was plotted to visualize the evolution of the friction behavior over time and to compare the friction behavior for the different combinations.

The resulting scratches in the alumina coatings and the styli were examined in SEM and LOM to evaluate the wear behavior and look for signs of adhesion, and chemical wear due to diffusion or chemical reactions.

3.5. Simulations

Thermo-calc was used to determine approximate melting temperatures of the investigated workpiece materials. This was done by specifying the measured composition per the material certificates using database TCFE7 and calculating at what temperature liquid first formed in the equilibrium calculator. The melting temperature of Fe-base alloys with increasing amounts of Co, Al, and O were also simulated to help evaluate the behavior in diffusion couple testing.

To simulate diffusion of Al and O into workpiece materials from Alumina, the activity of Aluminum and O at the stochiometric proportion 2:3 in a system with trace amounts of Fe was calculated. This was done by defining the amount of Al as a function of the amount of O for the system. Thermo-calc was also used to find the stable phases in the workpiece materials when increasing amounts of Al and O were dissolved. The amount of Al and O were increased separately, and the amount of all stable phases was plotted.

In DICTRA the diffusion of Al and O into steel workpiece materials is simulated to give an

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Table 3.3 – Simulation conditions for diffusion of Al and O in SS2541

Lower Boundary Activity of O = 1.9e-17 Lower Boundary Activity of Al = 5.18e-8

Temperature 1523K

Pressure 101325 Pa

System Size 1 mole

Simulation time 10800s (3h)

Composition SS2541

Included Phases FCC, BCC, Liquid, HCP

The simulation took approximately 3 minutes to complete with an initial time step of 1e-7. The results were plotted as amount of element by mole percent over the distance of the region using the integrated post treatment module. The DICTRA script used for the simulation is presented in

Appendix.

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4. Results

4.1. Coating Measurements

The orientation, texture, grain size, and coating thickness in the different samples is presented in Table 4.1. The orientation of the coatings made in Fagersta did not fully represent the desired orientations. This is thought to be an effect of the orientation of the underlying Ti(C,N) which was the same for all three alumina orientations. The actual dominating orientation is presented in Table 4.1.

Table 4.1 – Measured values of coated inserts, Top (bottom). *Abnormal Grains in mix

Target Orientation: 001 012 100 A1

Ti(C,N) (µm) 7 (6) 4 (4) 6 (6.7) 8.7 (8.5)

Al2O3 (µm) 4 (4) 4 (3) 4 (3.9) 5.2 (4.6)

Texture 7.05 2.39 + 4.51 7.57 6.7 + 0.5

Actual Orientation 001 012 + 110 110 001 + 104

Grain Size 1-3 my 2-3 my 3-5 my* -

Cracks 40-120 my 40-100 my 40-100 my -

The 012 coating has a large fraction of 110-orientation but is still considered sufficiently pure to be representative for the orientation in this study. The 100 coating became an almost pure 110-orientation due to the process parameters not being adjusted correctly. However, previous studies have shown the 110-orientation behaves similarly to the 100-orientation in wear tests meaning similar results may be acquired from the two orientations. [3] Top view sample images of the coatings from LOM are presented in Figure 4.1. No cracks are visible in the LOM images.

Upon closer inspection in SEM, crack patterns are found, enclosing areas at sizes ranging from 40-120 microns, as shown in Figure 4.2. Small ridges and particles on the surface can also be seen.

_____ 50µm

A1

_____ 50µm

001

_____ 50µm

012

_____ 50µm

100

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The grain size of the surface layer varies between 1-5 microns, as shown in Figure 4.3. The “100” coating shows significantly larger grains in the surface structure.

001

012

100

Figure 4.2 – SEM images of coating surfaces for 001, 100, and 012-orientation with visible cooling crack patterns

001

012

100

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The coatings which were polished for the scratch testing and the second round of diffusion couples were also examined in SEM. They show signs of uneven polishing and circular areas where the underlying Ti(C,N) is thought to be exposed, as shown in Figure 4.4. These areas were hard to depict in SEM because of the charging effect caused by the electron beam. The A1 coating is not included in the surface measurements since it has a top layer of TiN which obscures the surface structure of alumina.

001

012

100

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4.2. Workpiece measurements

The melting temperature of the workpiece materials was determined by DSC. The experimental temperatures for start of melt are presented below in Table 4.2 combined with the calculated temperatures for start of melt from Thermo-Calc. Full temperature curve is presented in Appendix.

Table 4.2 – Melting temperatures from DSC and Thermo-calc

Workpiece material DSC melting temperature Thermo-calc melting temperature

29425 – High Ca 1423.8 °C 1410 °C

21419 – Low Ca 1427 °C 1410 °C

C45U 1437 °C 1414 °C

Ovako 825B 1331.0 °C 1324 °C

The surface structure of the workpiece materials was studied in LOM to give a point of reference when examining the diffusion couple interface. The images from batch 29425 show signs of grain boundaries in the untreated surface (Figure 4.5), etching is needed to unveil the grain boundaries in the other materials.

The inclusion characterization of the workpiece materials at KTH with Electrolytic extraction show large amounts of MnS inclusions in the two SS2541 workpiece materials. In batch 29425 large amounts of Ca-rich inclusions were found. The inclusions were often a combination of Al2O3 and CaO or CaS compounds gathered in 2-10 µm large spheres. These complex inclusions were not found as frequently in batch 21419.

The inclusion characterization done at Sandvik Materials Technology in Sandviken show similar results for the SS2541 workpiece materials. The workpiece material from batch 21419 shows a tendency for larger inclusions, mainly in the form of MnS, and low amounts of CaO inclusions. Neither material show any signs of MgO or MgS inclusions. The complete results from the inclusion characterization can be found in Appendix.

Figure 4.5 – Surface of polished batch 29425 in LOM. a) 10x magnification. b) 50x magnification

__

100µm

_____

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4.3. Diffusion Couples

The diffusion couples treated at 1250°C for 10h all show reactions between steel and coating, but during handling and examination, most samples fell apart.

The diffusion couples treated at 1300°C for 10h all show reactions between steel and coating. More samples stuck together after this treatment, indicating greater reactions leading to adhesion, but due to the altered clamping method most samples fell apart during extraction from the clamping. The furnace temperature is thought to have briefly exceeded the melting temperature of the Ovako 825B workpiece materials causing them to melt and release from the clamping. The partially melted behavior is seen in some of the diffusion couples, disguising any other reactions that may have happened.

The diffusion couples treated at 1300°C for 20h all show reactions between steel and coating, but none of the samples remained in contact when the weights were removed. These samples were not examined as thoroughly due to time constraints, but a brief examination revealed similar results as from the first two tests.

The coated inserts used as control at 1300° for 20h also show signs of reaction even though they have not been in contact with any steel, indicating reactivity between coating and substrate.

Table 4.3 – Index of measurements and adhesion of samples

Sample name Coating Workpiece Temp/time Stuck together LOM SEM EDS

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25 4.3.1. Alumina and Ovako 825B

The Ovako 825B workpiece material shows major adhesion to the alumina coatings. Some of the results are obscured due to partial melting of the steel during heat treatment at 1300°C.

4.3.1.1. 001 – 18JS11 and 18JS31

The samples fell apart after heat treatment. The LOM images a), b), are from 18JS11 and the SEM images c), d), are from 18JS31. In Figure 4.6 a) the coating has reacted in certain areas forming a metallic network. In Figure b) alumina is stuck on the steel surface, indicating good adhesion between the coating and the steel surface. In sample 18JS31 the coating was delaminated from the substrate in some areas as seen in image d). Indicating a stronger adhesion between the coating and the steel than in the substrate at the gradient depth.

Figure 4.6 – a) Coating surface in LOM b) Steel surface in LOM c) Interface in SEM d) Delaminated area in SEM

a

b

c

_d

__

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26 4.3.1.2. 012 – 18JS19 and 18JS39

Both samples reacted but fell apart after heat treatment. The metallic network in Figure 4.7 a) is replicated by alumina stuck on the steel surface in b). The size and shape of the network closely resembles the cooling cracks in the CVD coating from Figure 4.2.

In SEM examination, it was found that some areas of the coating had been delaminated and exposed the underlying Ti(C,N), Figure 4.8. a). These areas are lined with Co which appears to have been molten, Figure b). This implied due to its ridged surface structure which is common in solidified Co.

In EDS, it was found that the Co structures were not limited to the delaminated areas but was also present in seemingly random parts of the coating. The composition of said Co structures varied and are presented in Figure 4.9-10. The presented spectrum is indicated by a cross in the electron image.

Figure 4.8 – SEM image of 18JS39 a) Delaminated area b) Co solidification structure

a b

Figure 4.7 – LOM image of 18JS19 a) coating b) steel surface

a

_b

__

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On the steel surface of sample 18JS39 small holes were found scattered around the surface. The area around these holes show high Co content indicating either a thin film of Co or an alloy with the iron in the steel as seen in Figure 4.11 These holes are found on the steel surface in 4.7 b) as small dots.

Figure 4.10 – Co structure composition Figure 4.9 – Co structure composition

Spectrum 79 Co 75,0% Fe 15,3% W 9,2% Cr 1,0% Spectrum 82 Co 50,2% Fe 45,0% W 4,7%

Figure 4.11 – Co content on steel surface in 18JS39

Spectrum 92

Co 25,0%

Fe 73,2%

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28 4.3.1.3. 100 – 18JS15 and 18JS35

The combination of steel and coating have reacted at both 1250 and 1300°C for 10h but neither stuck together after the heat treatment. In Figure 4.12 a) we see the steel surface with adhered alumina at the edges and small dots. In Figure b) we see the coating surface mostly intact but with small dots of metallic material.

In sample 18JS35 the steel surface was partially molten which obscures any structures formed from the contact surface. The coating surface shows areas of reaction but mostly delaminated coating. This can be seen in Figure 4.13

a

_b

__

100µm __100µm

Figure 4.12 – LOM image of sample 18JS15 a) steel surface and b) coating surface

a

_b

__

100µm

__

100µm

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29 4.3.1.4. A1 – 18JS07 and 18JS27

The combination has reacted at both 1250°C and 1300°C but only the sample from 1300°C heat treatment stayed together after extraction. In Figure 4.14 a) we see the network of metallic material which has formed on the coating close to the edge of the adhered zone in 18JS07. In b) we see the steel surface covered in black dots and ridge like formations on the grains.

In Figure 4.15 we see the adhesion of 18JS27 in SEM images. The alumina layer has been breached by the steel in many places until it reaches the Ti(C,N). It appears the thin TiN layer on top of the

alumina is removed or fully absorbed by the steel as it is not visible in any of the images taken.

Figure 4.15 – SEM image of 18JS27

a

b

__

100µm ___100µm

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30 4.3.2. Alumina and C45U

The C45U workpiece material shows reaction with the different coatings but to a lesser degree than the other workpiece materials. There are very few cases when the coating has been completely worn through.

4.3.2.1. 001 – 18JS09 and 18JS29

Both samples show reactions at the interface between the steel and the coating. Sample 18JS09 was separated to allow the surface to be studied. The surface of the coating and the steel are shown in Figure 4.16. In Figure a) we can observe a greater tendency for network structure on the right side of the line, this is the area which has been in contact with the steel during heat treatment. In Figure b) we can see ridge like formations on the grains in the steel as well as small black dots scattered over the surface.

SEM images show good contact along the interface in both samples. In Figure 4.17 a) very few impurities are seen, in b) a reaction zone can be seen on the steel side of the interface. Upon closer inspection, similar structures are also seen in 18JS09.

The reaction zone and interface were examined by EDS and shows high levels of Co and W at the steel interface, as well as tendency for Co diffusion into the steel. In Figure 4.18 linescan 19 shows diminishing levels of Co from the end of the measurement to approximately 10µm.

a

b

__

100µm

__

100µm

Figure 4.16 – LOM images of 18JS09 a) coating b) steel surface

a

b

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In another site the level of W is elevated in the surface region. In Figure 4.19 Spectrum 149 shows 16w% W in the reaction zone. A linescan at this site shows similar diminishing levels of W as in linescan 19, but over a shorter distance of 4 µm.

Figure 4.18 – EDS measurement on 18JS09 in spectrum 163 and Linescan 19

Spectrum 163

Co 12,0%

Fe 83,3%

W 2,8%

Al 2,0%

Figure 4.19 – Composition at steel side of interface in 18JS09 at Spectrum 149 and Linescan 15

Spectrum 149

Fe 68,0%

W 26,5%

Ti 1,8%

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32 4.3.2.2. 012 – 18JS17 and 18JS37

Both samples show signs of reaction but only the sample treated at 1300°C remained in contact after extraction. The coating surface shows clear signs of reaction with metallic dots all over the contact surface as presented in Figure 4.20 a). The steel surface shows lots of small black dots on the contact side (right side) of Figure b).

Sample 18JS37 was examined in SEM and shows contact between the steel and the coating in most places, but with some locations where the steel seems to have been worn off, as seen in Figure 4.21 a). Closer examination of the coating layer in image b) shows the porous structure of the Ti(C,N) layer and the black alumina layer.

The EDS measurements on 18JS37 show significant amounts of Ca and W in the steel surface at the interface region as shown in Figure 4.22. Many small bright dots were observed on the surface of the steel using BSD, these were shown to be WC-Co particles and were present in all parts of the steel surface, implying that they have been moved from the substrate to the steel during grinding and polishing of the sample, and not during the heat treatment.

a

b

__

100µm

__

100µm

Figure 4.20 – LOM images of 18JS17 a) Coating, b) Steel surface

a

_b

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The high amounts of C and O in Spectrum 193 are expected to come from the Fapsa plastic used to encase the sample for easier handling. The Ca content most likely comes from Ca rich inclusions in the surface region of the steel, or from contamination during handling. It is noteworthy that no Co is detected in the interface region of 18JS37.

4.3.2.3. 100 – 18JS13 and 18JS33

Sample 18JS13 fell apart during handling and the surfaces covered in Bakelite during encasing, obscuring the results. The sample heat treated at 1300°C showed much better adhesion but not much reaction at the interface. Figure 4.23 shows the interface area of 18JS33 where it is in good contact. In image b) we can see an area in the steel where some reaction has taken place.

Figure 4.23 – SEM images of interface of 18JS33 a) 5000x magnification b) 20000x magnification

a

b

Figure 4.22 – EDS composition measurement on 18JS37 in Spectrum 193

Spectrum 193

Fe 78,0%

W 7,0%

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34 4.3.3. Alumina and SS2541 29425

The 29425 charge of SS2541 shows lots of reaction with the alumina coating. This is hypothesized to come from the Ca-rich inclusions found in the steel from the M-steel treatment.

4.3.3.1. 001 – 18JS08

The interface area of sample 18JS08 shows major reactions. In many places the steel has penetrated the alumina layer of the coating and a wide reaction area into the steel has formed. This is shown in Figure 4.24

The reaction areas were studied in EDS and it was found that many of the impurities in the alumina/steel interface were rich in Ca and W as shown in Figure 4.25.

a

b

Figure 4.24 – SEM image of 18JS08

Figure 4.25 – EDS from interface in 18JS08 and composition in Spectrum 21

Spectrum 21

Fe 38,0%

W 27,0%

Al 13,5%

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35 4.3.3.2. 012 – 18JS16 and 18JS36

Samples from both heat treatments of this combination fell apart during handling, but show signs of reaction at the diffusion interface. The surfaces of 18JS16 is presented in Figure 4.26 and shows metallic dots and some elongated structures in the surface of the coating in image a). The steel surface in image b) shows block dots scattered around the surface of the contact region.

Sample 18JS36 shows the same metallic network structure on the small extending piece pictured in Figure 4.27 a). In image b) we can see the same structure replicated on the steel surface as well as pronounced grain boundaries in the steel outside of the contact zone. The surfaces of 18JS36 were

examined in EDS and several areas of interest were noted. The coating of 18JS36 showed many areas where the alumina layer had been removed and the underlying TiN had been exposed. The TiN was also rich in W indicating significant diffusion of W through the TiN layer as seen in spectrum 117. These areas appeared in circular formations with Co rich structures around the edges as depicted in Figure 4.28. spectrum 119.

a

b

__

100µm __100µm

Figure 4.26 – LOM images of 18JS16 a) Coating b) Steel surface

a

b

__

100µm

__

100µm

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A mapping for Co content was done on the coating surface and significant amounts of Co was found in the reaction zones as presented in Figure 4.29. Small amounts of Ti were also present in the reaction zones and Ca was found in low amounts all over the sample.

Co

Ca

Ti

Figure 4.29 – EDS mapping of 18JS36 for Co, Ca, and Ti

Spectrum 119 Co 79,7% Fe 6,6% W 11,5% Ti 1,4% Spectrum 117 W 64,0% Ti 36,0%

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On the steel surface of 18JS36, structures of CaS were found which were surrounded by Co-Fe alloy as presented in Figure 4.30.

Co was also found in high concentrations away from the CaS structures, in ridge like formations as depicted in Figure 4.31. Spectrum 105 Co 20,0% Fe 75,0% Ca 2,2% S 2,5% Spectrum 104 Ca 55,5% S 44,5%

Figure 4.30 – EDS image of steel surface in 18JS36 with Spectrum 104 (Cross) and 105 (Square)

Figure 4.31 – EDS measurements on Steel surface in 18JS36

Spectrum 111

Co 23,0%

Fe 71,1%

W 3,7%

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38 4.3.3.3. 100 – 18JS14

The sample from the heat treatment at 1250°C shows reaction but did not stick together after removal from the clamps. Figure 4.32 shows frequent reactions in both coating and steel surface in 18JS14.

The sample from the second heat treatment at 1300°C was completely covered in molten metal from a nearby sample of Ovako 825B and could not be examined.

Figure 4.32 – LOM image of 18JS14 a) Coating b) Steel Surface

a

b

__

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39 4.3.3.4. A1 – 18JS04 and 18JS24

Both samples show signs of reaction but only the sample at 1300°C stuck together after extraction. In Figure 4.33 a) the network structure and b) small dots found on the surface of 18JS24 are shown. The dots in b) were found outside the contact zone of the steel and the coating.

In Figure 4.34 the interface of 18JS04 is shown in SEM. The coating is mostly intact, but impurities and breaches are found in some locations, one of these breaches is shown in b).

a

__

b

100µm

__

100µm

Figure 4.33 – LOM image of 18JS24 a) network at edge b) dots outside contact zone

Figure 4.34 – SEM image of 18JS04 a) 1000x magnification b) 5000x magnification

Exploratory study of the interactions between textured alumina coatings and steel