UPTEC K21 004
Examensarbete 30 hp Februari 2021
Method development for a tribological diffusion couple of rock and cemented carbide
Alma Fjällström
Teknisk- naturvetenskaplig fakultet UTH-enheten
Besöksadress:
Ångströmlaboratoriet Lägerhyddsvägen 1 Hus 4, Plan 0
Postadress:
Box 536 751 21 Uppsala
Telefon:
018 – 471 30 03
Telefax:
018 – 471 30 00
Hemsida:
http://www.teknat.uu.se/student
Abstract
Method development for a tribological diffusion couple of rock and cemented carbide
Alma Fjällström
In a diffusion couple, the intimacy of the contact between the two parts is of high importance for the results. In a tribological contact, matter can transfer from one part to another and a very intimate contact is formed. A new method for investigating a tribological diffusion couple created in this way and consisting of rock and a cemented carbide (CC) drill bit button, is developed in this thesis.
This is done as further studies of this couple can contribute to the understanding of drill bit wear in rock drilling. A complete
experimental route, including sample preparation, tribological contact, heat treatment and analysis of samples, is presented. Heat treatment of samples was conducted both in an atmosphere of flowing argon and inside an evacuated and sealed quartz vacuum ampule. Heat treatment in flowing argon was rejected as an oxide formed on the sample surface. Samples in quartz ampules were heat treated at either 1000 °C for 2 h or 21 h, or at 1100 °C for 2 h. Samples were repeatedly imaged with Scanning Electron Microscopy (SEM) and analysed with Energy Dispersive X-ray Spectroscopy (EDS) during the process. As Si and W have characteristic X-ray peaks in close proximity, the need for a detection method other than EDS to detect diffused Si in CC arose. Wavelength Dispersive X-ray Spectroscopy (WDS) performed well in that respect. Diffused Si could be found in the superficial Co pockets of the CC structure, by analysis with WDS.
Metodutveckling för ett tribologiskt diffusionspar av berg och hårdmetall
Popular science summary in Swedish
Inom materialvetenskapen används diffusionspar ofta för att reda ut vad som sker i en kontakt mellan två material. Det klassiska tillvägagångssättet är att polerade ytor av de två material som ska studeras trycks ihop och hålls ihop med en kraft medan de värms upp.
Vilket material, om något, som diffunderar in i det andra och vilka eventuella nya faser som bildas är exempel på relevant information för den som vill förstå ett materialsystem.
I detta examensarbete tas en metod fram för att framställa ett diffusionspar på ett annat sätt än det klassiska, nämligen genom att använda ytor som har gnidits mot varandra så att material har överförts.
Två ytor som gnids mot varandra interagerar ofta med varandra. Vetenskapsområdet för just ytor i kontakt med varandra som rör sig relativt varandra heter tribologi och inom tribologi studeras ofta friktion, nötning och smörjning av olika ytor. Ett forskningsom- råde inom tribologi omfattar den kontakt som uppstår mellan berg och borrkrona vid bergborrning. Det finns olika typer av bergborrning, en vanligt förekommande är top- phammarborrning där borrkronan slås in mot berget upprepade gånger samtidigt som den roterar. Detta leder till att det relativt spröda berget krossas och på så sätt avverkas.
De delar av en bergborrkrona som främst är i kontakt med berget kallas för stift och är oftast gjorda av hårdmetall.
Hårdmetall är ett material bestående av två faser som är blandade med varandra. Den ena fasen är keramen wolframkarbid (WC) som är hård och spröd och den andra fasen är metallisk kobolt (Co) som är mjukare, vilket gör att den tål mer deformation än WC utan att spricka. Vid den tuffa kontakten mellan berg och hårdmetallstift i bergborrning tros höga temperaturer kunna uppstå. Dessutom nöts hårdmetallen och sten överförs från berget till hårdmetallstiftets yta.
Den överförda stenen påverkar sannolikt prestandan för hårdmetallstiftet och det finns en möjlighet att de förhöjda temperaturerna som kan uppstå vid bergborrning leder till att den överförda stenen diffunderar in i hårdmetallen. Som ett sätt att utreda detta utvecklas i detta examensarbete en metod för framställande och analys av ett tribologiskt diffusionspar bestående av hårdmetall och berg. Två olika bergarter används, sandsten och granit (Bohusgranit).
Hårdmetallstift polerades och utsattes för en tribologisk kontakt med antingen sandsten eller Bohusgranit. I kontakten uppstod ett nötningsmärke på hårdmetallstiftet, och för att utnyttja varje polerat hårdmetallstift maximalt så placerades fyra sådana nötningsmärken ut på stiftet. Det kapades sedan i fyra delar så att varje del fick ett nötningsmärke.
Efter det värmebehandlades proverna. Värmebehandling i flödande argon testades men visades leda till att en oxid växte på proverna. Denna oxid växte bland annat i gränsytan mellan de två material som var tänkta att studera. Därför testades värmebehandling i vakuumampuller gjorda i kvarts, vilket inte ledde till någon oxidbildning och därför bedömdes fungera bra. Proverna i ampullerna värmebehandlades vid 1100 ◦C i 2 h eller vid 1000 ◦C i antingen 2 h eller 21 h.
Proverna undersöktes med svepelektronmikroskopi (SEM) och energidispersiv röntgen- spektroskopi (EDS) i flera steg under processens gång för att se hur de påverkades av
de olika stegen, såsom kapning och värmning. För att studera diffusionen av bergma- terial (främst kisel) in i hårdmetallstiftet gjordes tvärsnitt av proverna. Dessa tvärsnitt gjöts in i plast och polerades så att en blank yta uppstod, vilken kunde användas för analys i våglängdsdispersiv rötgenanalys (WDS). Tvärsnitten studerades även med EDS och avbildades med SEM.
De olika metoderna fungerade olika bra för att hjälpa till att utreda om diffusion av kisel hade skett eftersom de har olika energiupplösning och analyserar olika stor volym av provet. EDS kunde inte separera kisel och volfram, vilket gjorde den till en otillräcklig metod för att studera diffusion i just detta diffusionspar. WDS presterade bättre i det avseendet.
Det visade sig att värmebehandlingen av detta tribologiska diffusionspar måste ske syre- fritt och temperaturer kring 1000-1100 ◦C är tillräckliga för att påverka diffusionen så att kisel kan hittas i hårdmetallen. Det skulle vara intressant att använda Augerelektron- spektroskopi (AES) för att karaktärisera proverna eftersom denna metod skulle kunna prestera bra då djupet som kiselatomer har vandrat till är ganska grunt. Dessutom skulle det vara intressant att värmebehandla fler prover längre tid än 21 h, för att se ifall diffu- sionsdjupet ökar. Metoden borde, med vissa anpassningar, fungera även för att undersöka andra tribologiska diffusionspar.
Contents
1 Introduction 1
1.1 Aim and limitations . . . 1
1.2 Diffusion and diffusion couples . . . 1
1.3 Rock types . . . 3
1.4 Cemented carbides . . . 4
1.5 Rock drilling . . . 5
1.5.1 Wear of cemented carbides in rock drilling . . . 6
1.6 Materials analysis . . . 7
2 Method 9 2.1 Sample preparation . . . 9
2.2 Tribological contact . . . 11
2.3 Heat treatment . . . 13
2.4 Analysis . . . 15
3 Results and discussion 15 3.1 Tribological contact . . . 15
3.2 Heat treatment . . . 19
3.3 Analysis . . . 24
4 Conclusions 30
5 Future Work 31
6 Acknowledgements 31
Appendix A Oxidized samples 34
Appendix B WDS analysis results 36
1 Introduction
Diffusion couples are very useful in the field of materials science as they allow the study of material interaction. They help increase the understanding of phases formed in the interface of two materials and can be helpful in for example the creation of phase diagrams.
Diffusion couples are usually heated to increase the movement of atoms, and thereby the diffusion. An intimate contact between the two parts of the couple is important, and it is possible that the transferred material from a tribological contact can provide such an intimate contact.
An interesting tribological contact is the one between rock and a drill bit in rock drilling.
As the drill bit hits the rock surface, rock is transferred to the drill bit surface. Transferred rock can be seen both on drill bits worn in the field and on drill bits worn in laboratory tests. It is proposed in current research in the area, that the transferred rock, now in intimate contact with the drill bit, diffuses into the drill bit as elevated temperatures can arise in rock drilling. To facilitate further investigations of this, the development of a tribological diffusion test method for cemented carbide and rock is investigated in this thesis.
1.1 Aim and limitations
The aim of this thesis is to find a complete method for studying diffusion of elements in a tribological contact of cemented carbide and rock. The complete method needs to contain a sample preparation technique, a method to get a controlled tribological contact, a heat treatment procedure and adequate analysis methods. Deciding actual diffusion depths, chemical compositions and identifying any new phases formed, are outside the scope of this thesis.
This is a degree project in the Chemical engineering program at Uppsala University and is therefore constrained to 30 hp and one semester.
1.2 Diffusion and diffusion couples
Diffusion is the random movement of particles in a medium. For materials in a crystalline solid state, diffusion means that atoms move from one lattice site to another at random.
Atoms in a lattice are constantly oscillating around their equilibrium positions. Occa- sionally, these oscillations become so large that the atom moves to another equilibrium site. Not all sites are equally preferred, so there are a few different types of solid-state diffusion. The structure of the lattice and the size of the diffusing atom are the main factors in deciding which diffusion mechanism takes place.[1]
Before going into further detail about some specific diffusion mechanisms, a more general example will be presented. Imagine a crystal rod with a high number of impurities in the left half due to a manufacturing error. There is no driving force for the impurity atoms to move in any specific direction, so they take steps both to the right and to the
the sample.[1] As different lattice sites have different properties, diffusion needs to be discussed in a more specific way.
In any lattice structure there are interstitial sites. They are the sites located in between lattice atoms and are usually smaller than the sites for the lattice atoms. Atoms placed in these sites are called interstitial atoms. If an interstitial atom moves from its intersti- tial site to another close by interstitial site, without displacing any of the lattice atoms permanently, it is said to have diffused with an interstitial mechanism. However, the sur- rounding atoms will have to move apart so that the interstitial atom can move through.
This temporary distortion causes an energy barrier that needs to be overcome for inter- stitial diffusion to occur. Atoms diffusing in this way are usually small alloy elements, that normally are positioned in the interstitial sites. So-called self-interstitials can also diffuse in this way. Self-interstitials are formed when lattice atoms are knocked from their original sites and end up in an interstitial site.[1]
For larger solute atoms in the interstitial position, moving through the interstitial mecha- nism is not very likely. This is due to the fact that a large temporary distortion is needed to let the atom through to the next interstitial site. An alternative way this diffusion can happen is through the interstitialcy mechanism, where the relatively large intersti- tial atom pushes a nearby lattice atom into an interstitial position and takes the now vacant place in the lattice itself. This causes only a comparatively small distortion of the lattice.[1]
Vacancy, or substitutional, diffusion is another diffusion mechanism in solids. Here, the fact that some lattice sites are naturally unoccupied, is used. These unoccupied sites are called vacancies. Any atom adjacent to a vacancy can move into the empty site and is then considered to have moved with a vacancy mechanism. This is thought to be the main diffusion mechanism for all pure metals and practically all substitutional alloys.[1]
The most common way to investigate diffusion in solid state materials is by using a diffusion couple. The setup for a diffusion couple can vary, but it commonly includes two materials clamped together by an applied force as in Figure 1. The surfaces in contact are flat and well-polished to have as intimate contact as possible between the two parts. The intimacy of the surfaces is of very high importance for the success of the test, and several methods have been developed to ensure a more intimate contact than in the classic setup.
One approach is to let a bulk material be one part while the other part is a layer deposited on the surface. Various electrolytic or electroless plating techniques can be used to form such a layer on some metals. Chemical vapour deposition (CVD), plasma spraying and Physical Vapor deposition (PVD) are other examples of techniques that can be used to create the thin part of this kind of diffusion couple.[2] For diffusion couples where the materials have different thermal expansion coefficients, another way to ensure intimate contact has been reported. By inserting a cylinder of one material into a cylindrical hole of the same dimension in another material, a diffusion couple that increases the contact during heating is created.[3]
Figure 1: Schematic image of a classic diffusion couple. Materials A and B are clamped together by an external force, F. The interface between materials A and B is where diffusion
usually is studied.
After the couple is created, it is generally heat treated at a temperature relevant for the specific system to increase diffusion rates. After the heat treatment, the couple is usually quenched to prevent phase transformation during cooling. This preserves the system in a high temperature equilibrium.[2] Samples are then commonly cut and a cross section of the contact is usually polished before analysis. An array of analysis methods is available, and the choice of method is based on the nature of the diffusion couple and if the relevant result is expected to be either a phase change, a change in composition or both.
1.3 Rock types
Rock types are commonly divided into three classes according to how they are formed.
The main classes are igneous rock, sedimentary rock and metamorphic rock. Igneous rocks are formed by solidification of melts from rock material. This process can occur at the surface of the earth or below ground. Sedimentary rocks are formed of fragments of eroded rocks and minerals and organisms that have died and sedimented to the bottom of the ocean. This always happens at the earth’s surface and in a medium, such as water or air, that collects the fragments. These loose fragments are then cemented in a process that neither requires high temperatures nor high pressures. Metamorphic rock is formed by the metamorphosis of any type of rock to another. This transformation often requires that a certain temperature and pressure condition is fulfilled, and it therefore takes place under the earth’s surface. The initial state can be an igneous rock, a sedimentary rock or even another metamorphic rock. The transformation causes changes in structure, texture and in some cases even in mineralogical composition through recrystallisation. Sometimes an exchange of elements with surrounding rock occurs during the transformation.[4]
All rock types consist of at least one, but more commonly multiple, minerals. Minerals are inorganic, crystalline and naturally occurring substances that can consist of pure elements, chemical compounds or can be composed of an- and cations. Over 4500 different minerals are known, but only a few of them form rock. Among the most common rock forming minerals are feldspar, mica and quarts.[5, 6] Feldspars have the general composition of XZ4O8 with X as any of Ca, K, Na, NH4, Ba or S, or as a combination of these. Z can be either Al, Si or B, or a combination of these.[7] Mica has two common forms: the dark bi- otite (K(Mg,Fe) AlSi O (OH,F) ) and the light muscovite (KAl (Si ,Al)O (OH,F) ).[8]
as the composition of the rock type varies.[9] Sandstone is generally considered to be a soft but abrasive rock.[10] For this thesis, a sandstone that consists almost exclusively of quartz is used to have a more simple chemistry.
Granite is a very common igneous rock type that constitute a large part of Sweden’s bedrock. It consists mainly of feldspar, quartz and biotite. It is considered to be a hard and abrasive rock type.[10] As for the sandstone, the composition of minerals in granite vary greatly, and there commonly is a geographical variation of appearances.[11] The type of granite used for this thesis is "Bohusgranit" which has a pink looking colour stemming from potassium feldspar.
1.4 Cemented carbides
The structure of Cemented Carbide (CC) combines the high strength of grains of ceramic WC and high toughness from a binder matrix, usually consisting of metallic Co. The resulting properties for the composite are relatively high strength and wear resistance while still being ductile. This makes the material excel in machining of hard-to-process materials, such as carbon steel or titanium alloys. Its properties also makes CC the material most commonly used to make drill bit buttons for rock drilling (See section 1.5). For rock drilling, the CC is generally uncoated [12], in contrast to the CC tools used in metal machining that usually have a ceramic coating.[13] An important property of the CC is the content of binder phase and hard phase, as this ratio determines the hardness/ductility balance of the material. For rock drilling, CC with 7-10 wt% binder phase is commonly used.[10]
The manufacturing of this material has historically been made by a powder route rather than by a casting route due to the high melting temperature of WC. The melting tem- perature for pure WC is 2870 ◦C, while pure Co (hcp structure) has a melting point of 1495 ◦C.[14] To produce cemented carbides with the powder route, a powder mixture of WC, Co and possible additives are ground and subsequently pressed and sintered.[15] For the cemented carbide grade that is most common in commercial applications, a sintering temperature of 1400 ◦C is used, which is above the temperature where an eutectic liquid is formed in the material (1320 ◦C).[16]
When using Co as a binder in CC, small amounts of W and C are usually added. The added W and C affect the structure of the Co as it most likely is the reason for Co having an fcc structure after the sintering of a cemented carbide. For pure Co, hcp is otherwise the most stable conformation at low temperatures. In the finished button, WC is in a simple hexagonal structure.[16, 17]
The carbon content in the binder phase (commonly referred to as carbon activity) during sintering is highly important for the properties of the CC button. If the carbon activity is too low there is risk of forming the undesired, brittle eta phase (M6C). On the other hand, if the carbon activity is a bit too high, the relatively soft and undesired graphite will be formed.[18] It is also known that the WC grains coarsen during liquid phase sintering and it is believed that the rate of coarsening depends on the carbon activity in the Co. A higher carbon activity is reported to give a higher average grain size of the WC grains.[18]
The size of the WC grains is important for the macroscopic properties of the button, such as hardness.
Today, extensive research to find new binder phase materials other than Co is performed.
This is done as it is possible that the European Union will restrict the usage of Co due to studies showing a carcinogenic effect in mice and rats.[19] There is also a hope that a change of binder phase material will reduce material cost.[19] Recent developments in the field of nano science have also spurred investigations on CC with nanocrystalline grains.[15]
1.5 Rock drilling
Rock is generally a hard but brittle material, which allows for little to no plastic deforma- tion. This makes brittle fracturing the main process for material removal in rock drilling.
One method for rock drilling is rotary percussive drilling, which is well suited for hard and tough rock types but can be used on all rock types. Rotary percussive drilling is done with a drill bit, also known as drill crown, mounted on an electro-hydraulic machine.
Drill crowns for rotary percussive drilling usually consist of steel cylinders with studs of cemented carbide (see Section 1.4) protruding from the face of the cylinder. These studs are commonly called inserts or buttons and can vary in size and shape depending on the type of rock that the drill is intended to be used in. The face geometry of the steel cylinder and the positioning of buttons vary and an appropriate drill crown is selected for anticipated drilling conditions.[10] Two examples of drill crown geometries and button shapes are presented in Figure 2.
Figure 2: Two drill crowns with coated steel cylinders and buttons made of cemented carbide.
Image courtesy of Sandvik.
A rotary percussive drill breaks rock by repeated impacts of the protruding buttons on the drill crown into the rock, while the drill crown rotates so that the buttons come into contact with a new position on each impact. This crushes the rock and the resulting rock mass, the cuttings, is transported away from the impact site by a medium flushed through the holes seen in the drill crowns in Figure 2. This medium is either pressurized water or pressurized air, mainly depending on if the drill site is above or below ground. As the contact environment between drill crown and rock is a harsh one, the air or water also fulfils the purpose of cooling the contact.[10]
experiences is also hard to measure but it is commonly estimated to about 2 kN, which is 200 N per button on a drill crown equipped with 10 buttons.[10]
1.5.1 Wear of cemented carbides in rock drilling
The CC buttons used in rock drilling gets worn in the harsh contact with the rock.
Reshaping of the worn buttons is usually made to make the button regain its original geometry. This can only be done a few times before the button has lost so much material that it is no longer possible to reshape. When this happens, the whole drill crown needs to be replaced. Both the reshaping process and the change of drill crown leads to loss of drilling time as the crown needs to be taken out of the drill hole.[12] Buttons worn in the field are exposed to uncontrolled conditions, so test rigs have been developed to mimic the conditions in rock drilling. Two such test rigs, an ASTM B611 steel wheel test and a modified ASTM G65, have been presented and improved by Gant et al. in [20].
Several papers concerning the wear of CC in rock drilling have been published, among others [21–26]. All sources report that rock is transferred to the CC button in the contact, either forming a cover of rock on the surface or by intermixing with the Co. The great variation in drilled rock type, drilling conditions and sometimes even the change of rock composition within a single drill hole makes comparisons difficult. However, attempts have been made to give a comprehensive presentation of the wear behaviour of CC in rock drilling, among others in [12] and [27]. Studies of the transferred rock, using Transmission Electron Microscopy (TEM), have confirmed that rock adhered to the surface of a worn drill bit is atomically tight and perfectly bounded.[12] Rock also penetrates the CC button in deeper channels between the carbide grains. Embrittlement and degradation of the binder phase is also reported, as well as oxidation and corrosion of superficial WC grains.
Also, crack formation both on the composite scale and the WC grain scale is reported.[12]
Angseryd et al. have published results from a test rig showing that more rock got adhered to the CC surface in a dry contact, compared to a contact situation wet by water.[28]
Material is removed from the surface of the CC button as it gets worn and there are a few material removal mechanisms reported. The removal of fragments from crushed WC grains as well as the detachment of whole or parts of WC grains have been reported.[12]
Extrusion of Co as the WC skeleton gets deformed has been previously reported, [29], but Beste et al. suggested in [12] that the crushing of binder intermixed with rock is a more important material removal mechanism for the binder phase. Their published theory is that binder extrusion only is of high importance in the beginning of the wear, before a rock/binder composite is formed and becomes the surface in contact with the rock.[12] The interdiffusion of rock into the CC button via the binder phase is important to investigate to get a better understanding of the wear of CC in rock drilling. It has been suggested that the diffusion of Si in the binder phase can form cobalt silicides, which could make the button less wear resistant.[21]
As previously mentioned, the wear behaviour of CC varies between rock types. Sandstone and granite, the two rock types used in this investigation, have slightly different wear results. In a master thesis presented by Oskarsson [30], it was shown that sandstone caused a higher wear of CC than granite.
1.6 Materials analysis
As materials analysis is an important part of this thesis, this section will briefly describe some selected analysis methods to give the foundation on which the discussion lies. The techniques described are Electron Probe Microanalyzer (EPMA), Scanning Electron Mi- croscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), Wavelength Dispersive X-ray Spectroscopy (WDS) and Auger Electron Spectroscopy (AES).
An electron microscope and an electron microprobe have very similar setup, their main difference is their purpose. While the electron microscope tends to be more frequently used for imaging, the microprobe is generally used for materials analysis. Both instruments are setup in a vacuum chamber, in which the pressure varies, depending on equipment requirements. Above the chamber, there is an electron source inside what commonly is referred to as the column. The purpose of the electron source is to emit electrons with a small variation in energy and to provide a high and reliable current of electrons. In high resolution microscopes and microprobes, a Field Emission Gun (FEG) is commonly needed. A FEG produces electrons by using an electric field to extract them from the tip of a tungsten rod. Compared to other electron generating techniques, FEG gives a higher brightness (higher electron current) and a more narrow beam. It does however have higher demands on the surrounding vacuum, which leads to a higher cost. Electromagnetic lenses inside the column are used to focus the beam onto a sample.[31]
The electrons in the focused beam are called primary electrons. When they hit the sample, they can collide with the matter in it. From this collision, the primary electrons can scatter either elastically or inelastically. When an elastic collision occurs, the primary electron does not lose any energy, but only change direction. Some of these collisions lead to primary electrons leaving the sample almost the same way they entered. These electrons are called backscattered electrons (BSE).[31]
Primary electrons that instead collide inelastically with the material, loses energy. One inelastic event is the ionization of an atom in the material, as the primary electron knocks one valence electron away from the atom. The knocked valence electron is now referred to as a secondary electron (SE2), generally having comparatively low energy. Due to their low energy, they can not escape the sample from as large depths as the backscattered electrons, as shown schematically in Figure 3. If a primary electron instead collides with an electron tightly bound to the core of an atom and knocks it away, the atom becomes excited. An electron from an outer shell falls into the newly created vacancy and as it does so, excess energy needs to be released to relax the atom. This is done by the release of either a characteristic X-ray or an Auger electron. An Auger electron is an electron released from the outer shells of the relaxing atom and it typically has very low energy.
The Auger electrons can therefore only escape the sample when generated very close to the surface. X-rays can escape the sample from greater depths.[32]
Figure 3: Schematic image of the activated volume in a sample interacting with an electron beam. All signals are generated in the whole activated volume, but green areas mark where the
labelled signals can escape the sample from. A signal is able to escape from areas of lighter green as well. The depth of the activated volume varies but is usually in the sub-micrometre to
micrometre range.[32] Image source [33].
Due to how Auger electrons and characteristic X-rays are generated, their energy cor- responds to the energy difference between two atomic shells in the atom from which it was released. EDS and WDS are both techniques that analyse the characteristic X-rays emitted from a sample. While EDS measures the energy of the X-rays, WDS measures the wavelength of the X-ray. Comparing the two techniques, EDS is faster and less sen- sitive to variations in sample topography. On the other hand, WDS has a better energy resolution and a better noise-to-signal ratio, both properties making it a better choice for qualitative analysis.[32] AES analyses the Auger electrons, and as only the ones generated close to the surface can escape the sample, it is a very surface sensitive analysis method with good lateral resolution.[32]
As can be seen in Figure 3, the volume from which the electrons and X-rays can escape is wider than the diameter of the electron beam. This means that whichever technique that is used to analyse emitted electrons and X-rays, the diameter of the analysed volume is larger than the beam diameter. How much larger depends on which type of signal that is analysed. By increasing the diameter of the beam, the analysed volume becomes even bigger. When analysing characteristic X-rays using EDS and WDS, the analysed volume is larger than the analysed volume for any type of electron.[32]
How the primary electrons spread in the sample and what depths they reach, depends
on factors such as the average atomic number of the sample, the energy of the primary electrons, the angle with which the beam hits the surface and the diameter of the beam.
Samples with a higher average atomic number increase the probability of an elastic scatter event for the primary electrons. This makes it more likely for the electrons to backscatter and escape the sample.[32]
To get an understanding of how the electrons scatter within a specific material, compu- tations can be used. For the inelastically scattered electrons, a fairly straight forward mathematical description is made by the Bethe range of the electrons, using the fact that the electrons lose energy in each collision.[31] For the elastically scattered electrons there is no energy loss. They scatter in a random angle after each collision, so computer simu- lations are used to estimate the movement of the electrons. A commonly used simulation technique for this is Monte Carlo simulation.[31]
2 Method
2.1 Sample preparation
The samples used for this thesis were CC buttons with ∼ 20 vol% (11.6 wt%) binder phase, which is a higher binder phase content than in buttons commonly used in rock drilling. They were chosen as a higher content of binder phase most probably facilitate diffusion and hence exaggerate the effect to be studied in this thesis. The binder phase consisted mainly of Co with small amounts of W and C and the hard phase consisted solely of WC. The buttons were 10 mm in diameter.
The mantle surfaces of the buttons were polished to remove the uneven layer of irregular composition covering the buttons after sintering (sinter skin), and to give a defined surface.
The polishing was made with the button fastened with the calotte end in the chuck of a lathe, see picture of setup in Figure 4. As the sample rotated with a speed of 30 rpm in the lathe, a Dremel was used to polish it with a slurry containing 45 μm diamond particles on a polishing wheel. DP-Lubricant Blue (Struers) was used as a lubricant and to prevent hazardous dust of Co to be emitted. Polishing was performed until the sintering skin was removed, which was confirmed by investigations in SEM. The button was removed from the lathe and cleaned in an ultra-sonic acetone bath for 3 minutes to remove the diamond particles. A new polishing wheel was then covered in a slurry containing 9 μm diamond particles and the Dremel was again used to polish the surface of the button.
This was done for 30 minutes to remove any scratches caused by the coarser diamond particles. Buttons were then cleaned in acetone and ethanol for 3 minutes respectively in an ultra-sonic bath.
Figure 4: Setup for polishing of samples. Handheld Dremel with polishing wheel covered in diamond slurry was held against button fastened in lathe.
When comparing buttons before and after polishing, see Figure 5, there is a distinct difference in the appearance of the lower part of the mantle surface. The lower part of the polished button is now shiny, but on the calotte and the upper part of the polished button the sample is unaffected, as those parts were fastened in the lathe. In SEM images of the polished and unpolished sample, see Figure 6, the appearance of the sinter skin can be seen, where Co (darker) cover large parts of the surface and WC grains (lighter) protrude from the surface. In the polished sample, the surface is level, and a more representative distribution of the phases in the CC is visible.
Figure 5: Button before (left) and after (right) polishing. Diameter of button is 10 mm.
Figure 6: SEM images of mantle surface of button before (left) and after (right) polishing.
Lighter areas are WC-grains and darker are Co. Acceleration voltage 3 kV. Note the different scales.
2.2 Tribological contact
A rock cylinder of either sandstone or granite, see Figure 7, was mounted in a lathe. The polished CC button was placed in a sample holder, which was mounted in a test rig (see Figure 8). The test rig has previously been described by Heinrichs et al. in [34] and is reported to give similar results to rock drilling [22, 34, 35]. The test rig enables a dry sliding contact between the rotating rock cylinder and the mantle surface of the button with an applied normal force (in this case 50 N). In one rotation of the rock cylinder, the sample was fed 0.84 mm towards the chuck, so that the button would mostly come into contact with unaffected rock. Two sliding lengths were tested; 5 m and 10 m. Test conditions are presented for each mark in Table 1.
Figure 7: Rock cylinders, roughly 20 cm in diameter, that were used to make wear marks, with a close-up of their surfaces. Sandstone to the left and "Bohusgranit" to the right. The close-up
of granite is of wet rock to enhance the difference in colour of the grains.
Figure 8: Setup for the rock/CC contact where the mantle surface of a CC button was fed along a rotating rock cylinder.
Table 1: Conditions for the creation of wear marks on the CC buttons. The rock cylinder was rotating with a speed of 30 rpm, and the CC buttons were all applied with a force of 50 N.
Samples with "FS" in the name were used to test furnace settings while "S" and "L" marks samples with the shorter respectively longer hold time in heat treatment (see Section 2.3).
Sample Rock Sliding velocity [m/s] Sliding length [m]
Sa-FS-01 Sandstone 0.40 10
Sa-FS-02 Sandstone 0.20 5
Sa-FS-03 Sandstone 0.20 5
Sa-1000-S Sandstone 0.20 10
Sa-1000-L Sandstone 0.20 10
Sa-1100 Sandstone 0.20 10
Gr-1000-S Granite 0.23 10
Gr-1000-L Granite 0.23 10
Gr-1100 Granite 0.23 10
After the tribological test, a wear mark had become visible on the mantle surface of the CC button, see Figure 9. The sample was then loosened and rotated a quarter of a turn and fastened again. Another wear mark was created in this new position of the button and in this way each button got four wear marks around the mantle. The samples were blown with compressed air to remove any loose particles before imaging in SEM and elemental analysis in EDS.
Figure 9: Sa-FS-02 with a wear mark after 5 m sliding contact with sandstone.
After the wear marks had been studied, each button was cut into quarters with a Diamond Cut-off Wheel (BOD15, Struers). One CC button became four samples with wear marks.
In the cutting process, water was used as a coolant and an anti-corrosive agent (Corrozip, Struers) was added to it to protect the machine. After the cutting, samples were thor- oughly rinsed in cold water. They were then submerged in acetone in an ultra-sonic bath for 3 minutes and subsequently transferred to ethanol and put in the ultra-sonic bath for another 3 minutes. The wear mark was imaged in SEM and an elemental analysis of the adhered rock was performed using EDS.
2.3 Heat treatment
Three samples were used to test the furnace settings and the heat treatment conditions for these samples are presented in Table 2. Sa-FS-01 and Sa-FS-02 were both heat treated in a zirconia crucible in a tube furnace under flowing argon atmosphere. Sample Sa-FS-01 was subsequently quenched in 5◦C water. Sample Sa-FS-02 cooled down inside the closed furnace under flowing argon.
Sample Sa-FS-03 was sealed into a vacuum quarts ampule. The quartz ampule was created from a quartz tube that had previously been sealed in one end. The sample was introduced, and the open end of the tube was connected to a vacuum pump via a rubber tube. The pump was turned on and pumped until a pressure of 2*10-2 bar was reached inside the quartz tube, to remove all oxygen. The quartz tube, still connected to the pump, was subsequently heated in the middle using a welding flame. Inside the flame, the softened quartz was shaped to an end, to create a sealed vacuum ampule. Both welding procedure and finished ampule are presented in Figure 10. After the ampule was finished,
Figure 10: Welding flame used to seal a vacuum quarts ampule, while connected to a vacuum pump via the orange tube. To the right is the finished ampule with sample inside. The
diameter of the ampule is 10 mm.
Table 2: Heat treatments for the samples used to test furnace settings.
Sample Atmosphere Ramping Hold temperature Hold time Quenched
Sa-FS-01 Flowing Argon 4 h 1000 ◦C 2 h Yes
Sa-FS-02 Flowing Argon 4 h 1000 ◦C 1 h No
Sa-FS-03 Vacuum Ampule 3 h 1000 ◦C 1 h Yes
As the heat treatment in flowing argon was not satisfactory, as will be further discussed in section 3.2), the rest of the samples were all encapsulated in ampules as described above.
Times and temperatures for the heat treatments are presented in Table 3. They were all quenched after heat treatment. After quenching, samples were taken out of the ampule by wrapping the ampule in paper and crushing one end with a hammer. The respective sample was made sure to be in the other end to avoid damage. The samples were blown with compressed air to remove loose particles and shards of quartz before studies in SEM and EDS.
Table 3: Heat treatments. All samples were heat treated in a crucible furnace, inside a vacuum ampule. The ramping time was 3 hours and heat treatment was ended by quenching the
ampule in cold water.
Sample Hold temperature Hold time
Sa-1000-S 1000 ◦C 2 h
Sa-1000-L 1000 ◦C 21 h
Sa-1100 1100 ◦C 2 h
Gr-1000-S 1000 ◦C 2 h
Gr-1000-L 1000 ◦C 21 h
Gr-1100 1100 ◦C 2 h
2.4 Analysis
Imaging with SEM and element analysis with EDS was performed in the wear marks on the mantle surfaces in several stages of the sample preparation process. After the analysis of the heat treated mantle surfaces, samples were cast in conductive plastic and a cross section of the wear mark was cut and polished. This was then analysed with SEM, EDS, and for some samples also WDS.
Imaging of the samples was made with a SE2 detector in a Field emission gun SEM (FEG- SEM) equipped with a Gemini column. Three different instruments were used with the same settings (Zeiss 1530, Zeiss 1550 and Zeiss Merlin) and they were considered to have given equivalent results. Analysis of composition with EDS was also used in all three instruments. The EDS detectors were all 80 mm2 silicon drift detectors and software used for analysis was Oxford AZtec. All EDS analyses were performed with an acceleration voltage of 7 kV.
Cross sections of all samples from Table 3 were investigated using a JEOL JXA-8530F field emission electron probe microanalyzer (FEG-EPMA) that was equipped with a WDS detector utilizing a Thallium Acid Phthalate (TAP) crystal. Analyses were made with an acceleration voltage of 10 kV and a probe current of 5.3 nA.
3 Results and discussion
3.1 Tribological contact
After the contact between a rock cylinder and CC buttons, wear marks were visible on the buttons. The surface after contact, see Figure 11, is no longer smooth and polished as it was before the contact, see Figure 6. Investigations using SEM and EDS confirmed the presence of adhered rock on the samples. The rock has been partially embedded into the binder phase and in many cases taken its place. Grains of WC have been deformed and, in some cases, cracked. Comparing with the descriptions of drill bits used in rock drilling, as reported by Beste et al. [12], there are a lot of similarities. The surface is not
Figure 11: Mantle surface of a CC button after sliding contact with sandstone for 10 m.
Darkest areas are rock. Arrow in top left corner indicates sliding direction of rock. Image taken using 3 kV acceleration voltage.
Two sliding lengths were tested for sandstone: 5 m and 10 m. After comparing the results, it was decided to do all the following tests with 10 m as the surface appeared to have more adhered rock, see Figure 12. Sliding lengths were not tested for granite, but 10 m gave a well-defined wear mark with visible rock adhered to it.
Figure 12: Mantle surface of two different CC buttons after sliding contact with sandstone for 5 m (left) and 10 m (right). Arrow in top left corner indicates sliding direction of rock. Image
taken using 3 kV acceleration voltage.
It is noteworthy that the axial feed of 0.84 mm per rotation of the rock is smaller than the width of any of the wear marks. This means that once the mark has reached a width larger than 0.84 mm, the trailing edge of the wear mark will slide against rock that already has been passed. A comparison of trailing and leading edge was made, and no distinct difference could be seen consistently. However, areas imaged and studied are always chosen in the middle of the mark to avoid any possible effects from this, such as rock adhering differently in different parts of the wear mark.
As the aim with the tribological contact is to create a surface where rock is transferred to the CC button and where the two materials are in intimate contact, loss of stone during the cutting process was studied. Images of the wear mark from sandstone on CC, before and after the cutting process, are shown in Figure 13. It appears as if some sandstone was removed from the wear mark during the cutting process. The adhered rock looks smoother after cutting, as if loosely attached rock particles have fallen off in some areas.
However, large amounts of sandstone remain adhered to the surface. For the granite wear marks, the cutting process affected the amount of rock on the surface to a higher degree than for the sandstone wear marks. It can be seen in Figure 14 that the amount of granite is reduced when comparing the wear mark before and after the cutting process.
There is still rock adhered, but the patches of rock are smaller. It is likely that the stone remaining on the surface of the button after the cutting process, which includes time in an ultra-sonic bath, is in intimate contact with the CC.
Figure 13: Sandstone sample before and after the cutting process. The areas depicted are not the same and samples have different orientation, which can be seen from the arrow indicating
sliding direction of rock. Images taken with 3 kV acceleration voltage.
Figure 14: The exact same area in a wear mark by granite on a CC button, before and after the cutting process. Arrow indicates sliding direction of rock, images taken with 3 kV
acceleration voltage.
A closer look of the granite samples is provided in Figure 15. Looking at sample Gr- 1000-S and comparing images after tribological contact and after the subsequent cutting process, the loss of rock is clear. The large rock coverage in the upper right corner has disappeared and WC grains previously covered by this rock is now visible. A small amount of the transferred rock remains in close proximity to one of these WC grains, and appears to be in intimate contact with Co. A similar behaviour is seen for Gr-1000-L and Gr-1100.
Although large patches of rock have been removed, small patches remain in the Co. So, the loss of rock is perhaps not as extensive as indicated by Figure 14, and the rock that has fallen off was perhaps not in the desired intimate contact anyway.
As rock is a natural material, a variation in composition is expected. The sandstone used was chosen as it was supposed to only contain quartz (SiO2). However, when analysing the sandstone transferred to the CC buttons using EDS, small amounts of Al, Na and Mg were detected. This was not the case for all studied transferred sandstone, probably as a result of the variation in composition of the rock. For granite, which consists of multiple minerals, the variation in composition can be seen macroscopically on the rock cylinder as it has areas of different colour (see Figure 7). The variations could also be seen in EDS examinations of the transferred granite. The minerals have different properties which probably affect their tendency to adhere, and to remain adhered, to the CC.
Figure 15: Three different stages of the sample preparation process for the granite samples. All three pictures for each sample are taken in the exact same area. From top to bottom:
Gr-1000-S, Gr-1000-L, Gr-1100. From left to right: after the tribological contact, after the cutting process, heat treated. Large arrow in top left corner indicate sliding direction of rock for all three samples and scale is true for all samples. Small arrows indicate step formation in
the heat treated samples.
3.2 Heat treatment
Sample Sa-FS-01 was heavily oxidized after heat treatment. This could be seen macro- scopically as the sample had turned black and was confirmed in SEM with the use of EDS.
To investigate if this was a result from the heat treatment or the quenching process, sam-
There is a high uncertainty in the carbon content in these EDS analyses, as a carbon contamination formed a layer on top of the analysis sites. The carbon contamination was visible when the site was imaged in SEM with a lower acceleration voltage than 6 kV, which is the acceleration voltage used for the image in Figure 16. The contamination partially covered site 1 and possibly the other sites too. EDS analysis of the area marked by cross 1 shows high content of W and O but also Co and C, see Figure 17. It is possible that this is a combined oxide of Co and W. The slightly lighter grey area, distinguishable at some positions close to WC grains, e.g. at the right arrow, has the same contrast as the area marked with cross 2. This site has a high W content and O content and some C, see Figure 17. This is possibly a tungsten oxide. EDS analysis of the black area at cross 3 (not included in this report) gave a very low signal, as was the case for many similar black areas. They are concluded to be pores in the structure.
At the left arrow in Figure 16, rock is visible in intimate contact with the oxide. Hence, the rock has lost contact with the CC, making the diffusion couple a different one than intended. The need for an oxygen free heat treatment became clear. When studying the mantle surface of the wear mark (see Appendix A), it is visible that the oxide also grew over the rock. This is not visible in the cross section. The oxide outside the rock perhaps disappeared in the polishing of the cross section.
Figure 16: Polished cross section of sample Sa-FS-02 with EDS analysis points marked by white crosses. White arrows point to areas of interest. Acceleration voltage 6 kV.
Figure 17: EDS spectra for locations 1 (top) and 2 (bottom) in Figure 16.
Sample Sa-FS-03 was used to test if a vacuum quartz ampule was a viable solution for heat treatment of this kind of sample in an atmosphere without oxygen. As the ampules were made of quartz, the main constituent of sandstone and granite, there is a risk of contamination. To minimize this risk, samples were placed so that contact between sample and ampule only occurred in a few points and that these contact points were as far away from the wear mark as possible. This is further discussed in Section 3.3.
Judged by the time it took for the sample to stop glowing after it was put in water, cooling takes longer time in the ampules, as compared to quenching directly in cold water. This is due to the fact that the ampules contained vacuum, which is insulating. For this thesis, the need to exclude oxygen is deemed to be more important than the need to cool fast, so possible effects of a prolonged cooling time have not been investigated.
Shards and dust of quartz were generated as the sample was removed from the ampule.
Even though compressed air was used on the sample before analysis, some residues re-
Figure 18: Area outside the wear mark of Sa-FS-03 after heat treatment. A large quartz particle is visible in the bottom right corner. Two small quartz particles are visible in the left
part of the image. Image taken using 3 kV acceleration voltage.
Studying the images in Figure 15, and now focusing on comparing the cut samples with the heat treated ones, a reconfiguration of Co is visible for all three samples. Where once a smooth surface, clear steps have now appeared in the Co of all three samples. One area with steps is indicated for each sample with a small arrow. The steps are formed as the now more mobile Co rearrange in preferred crystallographic directions. The steps indicate that even the shortest time and lowest temperature is enough to affect the samples by increasing the mobility of Co. Sample Gr-1000-S has the least distinct steps, while sample Gr-1000-L and Gr-1100 both have more pronounced transformation. It can also be seen in these pictures that in all three samples, Co has moved and cover parts of WC grains after the heat treatment. Images of the wear marks from sliding against sandstone after heat treatment are presented in Figures 19 to 21. Steps in the Co are observed in these samples as well, but not as clearly visible as in Figure 15. It is possible that the formation of steps in the Co is less pronounced in the sandstone samples, but it is also possible that the steps are less visible due to rock covering a larger part of the sandstone wear mark relatively to the granite wear mark.
The appearance of the transferred granite is fairly unchanged for the samples heat treated at 1000◦C, but not for sample Gr-1100. When comparing the rock in the top right corner of the pictures for this sample (Figure 15 bottom row), the rock looks more porous, almost like a sponge, after heat treatment. The area covered by rock also appears to be reduced in size. For the sandstone, pores appear in the rock for all three samples, as visible in Figures 19 to 21. In Sa-1100, that was heat treated at 1100 ◦C, the pores are larger than for the two samples heat treated at 1000 ◦C.
Figure 19: Sa-1000-S after heat treatment. Large arrow indicates sliding direction of rock and small arrow points to step formation in the Co. Image taken using 3 kV acceleration voltage.
Figure 20: Sa-1000-L after heat treatment. Large arrow indicates sliding direction of rock and small arrow points to step formation in the Co. Image taken using 3 kV acceleration voltage.
Figure 21: Sa-1100 after heat treatment. Large arrow indicates sliding direction of rock and small arrow points to step formation in the Co. Image taken using 3 kV acceleration voltage.
There is a possibility that the heat treatment does not only affect the diffusion. The temperatures used for heat treatment lies far below the melting point of WC, but, as evident by the formation of steps in the Co, close enough to the melting point of Co to give the Co a higher mobility. This makes it possible for the heat treatment to affect the structure of the cemented carbide itself. As the mobility of Co has been high enough for it to move to more preferred directions it is not unlikely that the samples have been affected in other ways. In Section 1.4, the concept of coarsening of WC grains was introduced. The published articles treating this matter only mention it at the sintering temperature, which generally takes place at higher temperatures than the heat treatment in these experiments.
As no cross sections of samples before heating have been studied, no comparison can be made, and it is not in the scope of this thesis.
3.3 Analysis
When analysing the cross sections, it is important to remember that CC is a 3D material and that pockets of binder phase that are not connected in the studied cross section, might be connected in another cross section. Mainly, Si was used to study the diffusion as it has been suggested that Si form compounds that are potentially degrading CC [21]. A search for the other main constituent in quartz, oxygen, would set much higher demands on sample preparation and sample handling since samples easily gets contaminated by oxygen.
Studying the cross sections of the heat treated samples, there are regions with an intimate contact between rock and cemented carbide. Such an example is presented for sample Gr- 1000-L in Figure 22. Here it can be seen that the transferred rock perfectly follows the topography of the CC. Such areas were found in the cross sections of all samples heat treated at 1000 ◦C.
Figure 22: Cross section of sample Gr-1000-L with small arrows pointing at the areas containing rock. The middle arrow points to a place where the rock is adhered to a WC grain, and the other two where rock is adhered to Co. The dark layer above the rock is the conductive
plastic the sample was moulded in. Image taken using 3 kV acceleration voltage.
In Figure 23 an area in sample Sa-1000-S is imaged. Rock is visible between the CC and the plastic, and has WC grain fragments intermixed. It appears as if the rock is not closely adhered to the surface of the CC. The appearance is vastly different to the rock in Figure 22, where the contact between CC and rock was intimate. It is worth mentioning that the two samples have been tested against different types of rock, but as sites with the intimate contact seen between rock and CC in Figure 22 have been found also in samples tested against sandstone, no conclusion is to be drawn from this. As the rock in Figure 23 remains close to the CC, with no apparent attachment to it in this cross section, it is assumed that what is visible here is a flake of rock that is attached to the CC somewhere else, most likely behind the depicted cross section.
Figure 23: Cross section of sample Sa-1000-S. Cross marks the spot for the EDS and WDS analyses, results in Figure 24. Image taken using 3 kV acceleration voltage.
Analysis with EDS could confirm that adhered layers, such as the ones visible in Figures 22 and 23, were rock. But as the aim of this thesis is to find a method for a tribological diffusion couple, diffused atoms needed to be found. This was proven difficult with EDS as the characteristic X-ray peak for W Mα (1.775 keV) and for Si Kα (1.740 keV) are very close and overlap in EDS. As this is the only peak for Si, the need for a technique that can separate these two peaks arose. However, when there was a high amount of Si in the analysed area, as was the case in the transferred rock layers, a peak shift towards lower energies, compared to the W peak, could be seen. But smaller amounts of Si would not affect the position or the shape of the peak as much, as there always will be a dominating W signal in the binder phase of the CC.
WDS in EPMA proved to be a good technique for separating W Mα and Si Kα, as can be seen from the comparison between the EDS spectrum and the WDS spectrum recorded in the same area, in Figure 24. It is not possible to separate the Si peak from the W peak in the EDS spectrum, but in the WDS spectrum the Si peak is well separated from the W peak. Values for the WDS analysis have been converted from c/s per mm to arbitrary unit per keV to ease the comparison between EDS and WDS. This was done upon export from the JEOL software and has possibly affected the data numerically. However, the shape and separation of peaks appear the same as before the data export.
Figure 24: Analysis results from EDS (left) and WDS (right) performed on sample Sa-1000-S in position marked in Figure 23. The orange spectrum in the left image is the measured EDS
spectrum while the blue and red lines are software placements of the Si and W peak, respectively.
WDS analyses for Sample Sa-1000-S, Sa-1000-L, Gr-1000-S and Gr-1000-L are located in Appendix B. The analyses of samples Sa-1000-S, Sa-1000-L and Gr-1000-L all show a Si signal about 0.5 μm down from the surface, while Gr-1000-S showed no sign of Si. None of the samples yields a clear signal for diffusion of Si deeper than this, but the noise in the WDS results makes it hard to determine if there is a Si signal or not in some cases.
It is possible that rock is transferred from the adhered rock layer and into the CC in the polishing procedure, but is deemed to be unlikely for the presented results. Si should have been detected in multiple places of the CC cross section if this was the case, but Si signals have only been found in close proximity of the rock/CC interface.
In two samples (Sa-1100 and Gr-1100) no rock could be seen in the cross sections using SEM with an acceleration voltage of 3 kV. Even after repeated re-polishing, to reach new cross sections, no rock could be seen. Before cross sectioning, it had been confirmed by EDS that there were transferred rock in the wear marks on both samples. This indicates that something happened either in the plastic casting or in the polishing process that made the rock detach from the CC. As both samples with missing rock were heat treated at the higher temperature, 1100 ◦C, it is possible that the rock was more brittle compared to the lower temperature samples and therefore was more easily removed during polishing.
It is also possible that the lack of rock in these samples is due to problems with the polishing machine. Both of these samples were polished simultaneously, while all other cross sections were polished on other occasions. The lack of duplicates makes it impossible to say with any certainty why no rock could be found in these cross sections. Results from WDS analysis on the two samples are shown in Figures 25 to 28. It can be seen in these figures that even though they seemingly do not to have rock adhered to the surface, there still is Si in the Co close to the surface. This indicates that the Si diffused into the
Figure 25: Cross section of sample Sa-1100 with crosses marking where WDS analysis was performed. WDS results are presented in Figure 26. Acceleration voltage 3 kV.
Figure 26: WDS analysis results for sample Sa-1100. Left image corresponds to marking 1 in Figure 25, and right image to marking 2 in the same Figure.
Figure 27: Cross section of sample Gr-1100 with a cross marking the location of WDS analysis.
WDS results are presented in Figure 28. Image taken using 3 kV acceleration voltage.
Increasing the hold time for the heat treatments will probably increase the diffusion depth and could also increase the strength of the Si signal in analysis. As Si does not appear to have diffused great lengths from the wear surfaces in a detectable amount, the risk of contamination from the ampule can be considered zero in these tests.
It is important in any analysis to be aware of the analysed volume. There is, for example, a risk of activating adhered rock when analysing a Co-area that is close to the surface.
This could yield a Si signal which could carelessly be interpreted as a Si content further away from the surface than what is correct. For this thesis, diffusion depths are impor- tant, and a good knowledge of the beam size and the activated volume would make the interpretation of results easier to translate to diffusion distance from surface. Analysis volume is commonly estimated with the help of Monte Carlo simulations as mentioned in Section 1.6, but would be a poor method for this specific case as CC is a two phase mate- rial with vastly different average atomic number in its two phases. A complex simulation would have to be made for each individual analysis site when the locations of the WC grains had been decided. This makes it hard to estimate the analysed volume for EDS and WDS analyses in this case. As an alternative, another analysis method could be used instead. AES could be an interesting method to use, as it has an activated volume which is several orders of magnitude smaller, and has a better lateral resolution, than both EDS and WDS.
One of the ways CC gets affected in this type of contact with rock is that superficial Co is extruded or otherwise removed, making the material slightly depleted of Co just below the wear mark. This probably decreases the possibility of diffusion, which is problematic in a diffusion test. However, it resembles the actual application to a higher degree and can therefore be helpful in giving useful understanding of diffusion and wear in rock drilling.
4 Conclusions
A method including sample preparation, tribological contact, heat treatment and analysis of a tribological diffusion couple has been developed. The procedure in this thesis has been proven successful for the tribological contact of CC/granite and CC/sandstone, as diffused Si was detected in the Co-pockets close to the worn surfaces of the samples. The diffusion depths of Si were small, probably as a result of short holding times at elevated temperature. An oxygen free heat treatment process is needed for a rock/CC tribological diffusion couple, mainly to ensure contact between desired species.
The energy resolution of EDS is not sufficient to study diffusion of Si in a material where W is present in large quantities. WDS has proven to be a better alternative in that type of sample.
The method for a tribological diffusion couple developed in this thesis could also be used for other tribological diffusion couples, if material is transferred from one part to the other, but adaptation of the method would be needed. Heat treatment temperatures and hold times need to be chosen specifically for each specific couple and application. When analysing other diffusion couples, EDS might be a sufficiently good method if there are no overlapping energies.
5 Future Work
To get a better understanding of the Co depletion in CC after a tribological contact it would be interesting to compare the composition of CC in the wear mark compared to an unaffected area. One possible way to do so would be through image analysis of SEM images of polished cross sections.
An evaluation of possible phases formed as rock constituents diffuse into the CC would be interesting, but as Si does not diffuse to great depths with the selected times and temperatures, such an analysis might be difficult. Further investigations of heat treatment temperatures and hold times could result in greater diffusion depths, which would enable investigations of formed phases. As the rock got affected by the heat treatments used in this thesis, 1050◦C could be a good temperature to test for longer hold times. Moreover, to get statistically confirmed results, more samples need to be tested for each heat treatment setting.
For a better understanding of the wear process in rock drilling, it could be interesting to study the diffusion in buttons with the commercially used amount of binder phase. In any future work with a rock/CC tribological diffusion couple, EDS is recommended only to be used on the mantle surfaces of the wear marks, to get an increased understanding of the minerals transferred from each rock type. For analysis of diffusion depths of Si in CC WDS, or possibly AES, is recommended. If using WDS, measures to reduce noise could be important to investigate, as this could ease the detection of Si.
6 Acknowledgements
I am grateful to the people who have helped me in the writing of this thesis. My warmest thanks to the Ångström Tribomaterials Group and the knowledgeable, helpful and kind persons in it. I am especially grateful to my supervisor Jannica Heinrichs for always taking the time to listen to my confused wonderings and for all the fun we have had during the work with this thesis.
A special thanks to Jaroslaw Majka and Iwona Klonowska for the help with WDS analyses on the beautiful Yoko. A special thanks also to Susanne Norgren at Sandvik for providing samples and knowledge about Cemented Carbides. I am grateful for all the discussions regarding heat treatment methods with Pedro Berastegui and for the help with handling of a rock cylinder I received from Mattias Sjödin.
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