Study of the wear mechanisms for drill bits used in core drilling

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UPTEC K 18022

Examensarbete 30 hp Juni 2018

Study of the wear mechanisms for drill bits used in core drilling

Emy Guttenkunst

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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

Study of the wear mechanisms for drill bits used in core drilling

Emy Guttenkunst

The thesis work was made in cooperation with the I-EDDA project who evaluates the drill equipment used in core drilling. The aim of this work was to determine how and why the drill bits are worn. The work consisted of two parts; investigate drill bits used in field tests

and develop a lab scale method to be able to change one drill parameter at a time and see how it affects the wear. During the field tests the rotational speed and the pressure on the drill bits were changed between the three boreholes drilled. In the lab test one parameter at a time was changed; the rotational speed, the water flow and the load. The lab test was developed to attempt to replicate the core drilling and was performed by pressing a piece of a drill bit against a rotating stone cylinder. The drill bits from the field

tests and lab test were analysed with the same methods on both macro- and microscale for easier comparison. The results indicate that the lab scale test can be used to evaluate the wear of drill bits. The analyses show rock present on the matrix of all the drill bits, in various amounts. The load has the largest impact on the wear of the drill bits and cause a change in mechanism. A high pressure leads to a higher amount of damaged diamonds and three body abrasive wear on the matrix. Lower pressure leads to polished diamonds and erosive wear on the matrix.

Ämnesgranskare: Erik Lewin Handledare: Urban Wiklund

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Populärvetenskaplig sammanfattning

Nötning av borrkronor som används i kärnborrning

Examensarbetet utfördes i samarbete med I-EDDA, ett projekt där eﬀektivisering och för- bättring av kärnborrningsprocessen är målet. Syftet med examensarbetet var att ta reda på hur och varför själva borrkronorna nöts under borrningen. Det för att kunna minska nötningen på borrkronorna i framtiden. Själva borrprocessen går till så att en borrkrona roteras ner i berget och slipar på stenen. Själva borrkronan är ihålig för att kunna få ut en borrkärna från berget som senare undersöks för att se vilka mineraler som finns i berget. Metoden utnyttjas i gruvor för att kunna följa malmkroppar. Borrkronan består av en metallkropp med många små diamanter inblandat i metallen. När en borrkrona blivit nernött under borrningen behöver hela borrstången tas upp för att kunna byta borrkronan i änden av den, det är ett tidskrävande arbete och kostnaderna för borrningen ökar. Inför examensarbetet borrades tre hål i en berggrund i Skåne, med olika inställningar i varje bor- rhål. Det för att kunna se om parametrarna spelade roll för nötningen av borrkronorna. I ett av hålen användes rekommenderade parametrar från tillverkan, i ett annat användes en lägre rotationshastighet och ett ökat tryck på borrkronan och i det sista borrhålet användes en högre rotationshastighet och ett högre tryck än rekommenderat. För det första hålet krävdes endast en borrkrona för att komma ner till ungefär 130 meter, medan för en lägre hastighet krävdes tre borrkronor för samma djup och för en högre hastighet krävdes två borrkronor, varav den sista fastnade i berget och kunde inte tas upp.

Borrkronorna analyserades i mikroskop, både med liten och stor förstoring, för att kunna förstå vad som hände under borrningen. Arbetet bestod även av en annan del, att utveckla en metod för att härma bergborrningen i ett laboratorium. Bitar från en en borrkrona sågades ut och trycktes sedan, med en bestämd kraft, mot en roterande stencylinder. Sten- cylindern hade en specifik rotationshastighet och vatten spolades mot stenen och borrbiten.

De tre inställningarna varierades för att kunna se hur de påverkade nötningen av borrkronorna. Bitarna analyserades på samma sätt som borrkronorna från bergborrningen för att kunna jämföra resultaten.

Resultaten tyder på att laborationstestet fungerar för att utvärdera nötningen under kärn- borrning. Rotationshastigheten, vattenflödet och lasten hade alla en påverkan på nötningen, men i olika stor grad. Lasten hade den största påverkan då en för hög last krossar diamanter och nöter på matrisen snabbare. En för låg last polerar diamanterna, vilket försämrar slipningen på stenen då diamanten inte längre har några vassa kanter. Även metallen runt diamanterna nöttes olika beroende på lasten. En hög last ledde till att den sten som blivit avverkad slipade på matrisen och gav långa repor på ytan och ett mer polerat utseende. En lägre last gjorde att stenbitarna slungades mot ytan i hög fart, genom vattenflödet mellan berget och borren, vilket gav matrisen en mer prickig och matt yta. På alla borrkronor och testbitar fanns det sten ovanpå eller intryckt i metallen, i olika mängder.

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1 Introduction

The work is a part of the Innovative Exploration Drilling and Data Acquisition project, I-EDDA. A cooperation between Uppsala University, Lund University, Technical Univer- sity Bergakademie Freiberg, Luleå University of Technology, Atlas Copco (now Epiroc), Helmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum, RISE Research Institutes of Sweden and The Geological Survey of Finland. The project aims to develop new technology and methods in exploration rock drilling in order to get as much information as possible from each drilled meter due to the high costs of the drilling. The project is now in the second work phase which consists of evaluation of the drill equipment. This master thesis focused on the investigation and understanding of the wear of the drill bits, which is important to lower the cost of exploration rock drilling. The actual drill bit is not the high cost during the rock drilling, but the replacement of the drill bit takes a lot of time.

A better understanding of the wear of the drill bits and how it is influenced by diﬀerent parameters have the potential to extend the lifetime of the drill bit and therefore lower the costs of the exploration rock drilling.

1.1 Aim and objectives

The aim of the master thesis was to provide fundamental knowledge of the wear of the core drilling bits in exploration rock drilling. The wear of used drill bits were examined in order to understand what mechanisms causes the wear. If the mechanisms are understood the lifetime of the drill bits may be improved, either by changing the settings during drilling or change the manufacturing of the drill bits. An increase in lifetime of the drill bits results in a large decrease in drilling cost. The following objectives were set up:

• Get an understanding of the wear of diamond tools by reading published work.

• Determine the wear mechanisms on both macro and micro scale. Both in drilling and in lab scale tests.

• Determine the wear rate of the drill bits and correlate this to the wear mechanisms.

• Develop a test method in a laboratory in an attempt to replicate the rock drilling.

The work consisted of two parts, both including literature studies. Part one was about analysing a number of drill bits used in exploration drilling, with a variety of parameters used. This to get an understanding of how the drill bits were worn during the drilling, but also to see how diﬀerent parameters impact the wear of the drill bits. The second part was about trying to recreate the the wear of the drill bits during rock drilling in a laboratory, in order to be able to study the impact of individual parameters and to execute simpler and cheaper drill tests in the future.

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1.2 Key questions on drill bit wear during core drilling

During the literature study there were obvious that a number of questions are very important to answer in order to explain the wear of the drill bits and a few of these needed further investigation. Some of the most obvious questions are:

• How are the diamonds worn?

• How is the matrix worn?

• How does the load aﬀect the wear?

• How does the rotational speed aﬀect the wear?

• How does the water amount aﬀect the wear?

• Is there any rock on the surface of the drill bits?

• How does the wear eﬀect the profiles of the drill bit?

1.3 Setting of the work

The drilling took place in Hörröd, Sweden, and was executed by Lund University. There were three boreholes drilled between August 28th and October 12th, 2017. Figure 1 illustrates the depths of the boreholes and how deep each of the drill bits were used.

Figure 1: An illustrating picture over the three boreholes. To the left of each hole the diﬀerent bits are indicated and to the right the corresponding depths are written.

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The parameters used during drilling is shown in table 1. The parameters were kept constant in the same borehole, while only the rotational speed and the rate of penetration, ROP, diﬀered between the boreholes. Note that in order to control the penetration rate the weight on bit (load) is the parameter changed in reality. A higher penetration rate is achieved with a higher weight on bit. In borehole 1 the recommended parameters from the manufacturer were used. The recommended parameters should give the drill bit a long lifetime. A higher penetration rate which the driller by experience combines with a lower rotational speed, as used in borehole 2, is considered a tougher setting for the drill bit while the lower penetration rate which the driler combines with higher rotational speed, as used in borehole 3, is more gentle. For borehole 1 the drilling was cancelled due to instability in the borehole at a depth of 129,1 m. In borehole two, three drill bits had to be used, as seen in figure 1. The first two drill bits in borehole 2 were changed due to undersize cutting. The drilling in borehole 2 was cancelled while drilling with the third bit due to instability in the borehole, and therefore the last drill bit in borehole 2 was not changed due to undersize cutting. Borehole 3 used two drill bits. The first drill bit was changed because of undersize cutting and the second drill bit got wedged in the borehole and it was not possible to retrieve the drill bit. The drill bits are named after the penetration rate and the order of the drill bits during drilling. For recommended parameters in borehole 1 the drill bit is called R.1, for the lower penetration rate in borehole 3 the drill bits are called L.1 and L.2. For the higher penetration rate in borehole 2 the drill bits are called H.1, H.2 and H.3.

Table 1: The four main parameters used during drilling. The penetration rate and the rotational speed were varied, while the water flow rate was kept constant.

L.1 L.2 R.1 H.1 H.2 H.3 Penetration rate [cm/min] 12 12 18 25 25 25

Rotational speed [rpm] 1200 1200 1000 700 700 700 Water flow rate [l/min] 40 40 40 40 40 40

2 Theory

2.1 Core drilling

The purpose of core drilling is to collect a rock sample in order to examine minerals present or to investigate the quality of the rock and soil. There are diﬀerent kinds of core drilling, used for diﬀerent reasons. Definition drilling, or infill drilling, is used in mines in order to follow the ore bodies and to calculate the concentrations of minerals for production planning. Site investigation is another example of core drilling which can be divided into

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two categories; rock mechanics and soil mechanics. Site investigation is used in order to investigate the rock or soil before constructing for example a tunnel or a building. The last type of core drilling is exploration drilling. It is used to examine what minerals are present in the borehole and if its content is suﬃcient for economic extraction in a mine.

From exploration drilling it is also possible to investigate the planet’s development during millions of years and to identify bacterias down to 2.5 km depth [1].

The core drilling process consists of three parts, the drilling, the retrieving of the core sample and the resuming of the drilling. During the drilling a drill stem, with a drill bit attached at the bottom, is rotated clockwise with an applied load. The applied load (also called weight on bit, WOB) controls the pressure on the drill bit which determines the rate of penetration, ROP. The drill stem consists of a number of drill pipes; each normally with a length of 3 meters, so after 3 meters of drilling another drill pipe is attached to the top of the drill stem to be able to drill deeper into the borehole. Therefore, dissembling and reassembling of the drill stem, which is necessary if the drill bit at the bottom of the stem needs to be changed, takes a lot of time and is not desirable.

The bottom of the drill stem is illustrated in figure 2 and shows the principle of double tube core drilling. The drill stem consist of an outer tube and an inner tube. Between the tubes there is a water flow used to cool the drill bits and to wash out the cuttings from the drilling. A small amount of the water could act as a lubricant when transported between the drill bit and the rock. The water and rock cuttings are then transported to the surface outside the outer tube. Inside the inner tube the core sample is collected. At regular intervals he drilling is paused and roughly meter long core samples are brought up to the surface.

Figure 2: A sketch of the core drilling. The stone sample is collected inside the inner tube.

The water feed is between the inner and the outer tube and is transported to the surface outside the outer tube.

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The diameter of the inner tube is similar to the inner diameter of the drill bit. When the drill bit is worn, however, the diameter of the core sample increases and the pressure in the inner tube increases. This is called undersize cutting. Undersize cutting could also depend on the wear of the outer part of the drill bit. When there is undersize cutting the drilling process has to be stopped and the drill bit replaced. The entire drill stem has to be dissembled and reassembled in order to change the drill bit, which is not cost eﬃcient.

Therefore it is important to chose the most suitable drill bit.

To choose the right drill bit the drillability of the rock needs to be known. The drillability is aﬀected by grain size, rock hardness, weathering and fracturing. A larger grain size makes the rock more abrasive to the bit, as does a larger fracturing. A fine grained, hard rock is less abrasive. Rock strength is reduced by weathering and such rock wears less on the drill bit [2]. A picture of the kind of drill bit used in this work can be seen in figure 3a.

The drill bit consists of eight teeth with waterways between the teeth, where most of the water flushed during drilling goes through rather than in the contact between stone and drill bit. In each tooth there is a hole pointing inwards or outwards, see figure 3b, to further increase the possibility for cuttings to escape from the cutting zone. Figure 3c illustrates one tooth from the drill bit. The upper part of the drill bit, above the dashed line, consists of synthetic diamonds with a size of about 200 µm in a matrix of copper, molybdenum, silver and tungsten. The hardness of the matrix is about 220 HV. Below the dashed line in figure 3c there are no diamonds present and the matrix consists of Ag, Cu and W. The dark grey areas are reamers that grind and smoothens the core samples. There are reamers also on the outside of the drill bit that grind the borehole, in order to broaden the borehole and get a smoother surface for an easier movement of the stem. The reamers are made of cemented carbide consisting of WC-grains in a Co matrix.

(a) (b) (c)

Figure 3: a) An image of the drill bit used [2]. During drilling the upper part is facing downwards and cut the rock. b) An illustrative picture of the drill bit from above. c) An illustrative picture of one tooth from the drill bit. The dashed line indicates a change in material.

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Diamond is the hardest material known to man and as such it ideally should not suffer damage in contact with any kind of rock, which all are much softer. As will be discussed below, this is unfortunately not true. The hardness of the matrix matters when choosing drill bit for the drilling. Softer matrices are used for harder rock material and vice versa [2]. This might seem illogical but the purpose of the matrix is merely to give support to the diamonds which are the ones that should do the actual work with the rock. However, as diamonds wear during drilling, fresh diamonds have to be exposed continuously in order to obtain a sufficient number of cutting points [3]. This is accomplished by wearing off the matrix around the diamonds. A too soft matrix will cause the diamonds to release from the matrix before the end of working life, while a too hard matrix causes the diamonds to wear faster than the matrix and the penetration rate drops due to sliding over the surface [4, 5, 6]. When drilling in hard rock diamonds wear quicker and a softer matrix is used to match the wear rates of the diamonds and the matrix.

2.2 Bedrock

The drilling took place in Hörröd, Sweden. About 30 km south of Kristianstad. An area dominated by igneous rock types, mostly granodioritic granite gneiss composed by the minerals quartz and feldspar, brown area in figure 4 from Sveriges Geologiska Undersökning [7]. The figure shows a surface mapping of the area around Hörröd. The purple are diabase, light blue are quartz arenite, pink are granite and green are amphibolite. The drill site, in the blue circle in the figure, is a brown area indicating there should be granodioritic granite gneiss at the surface, that should dominate during the first part of the drilling. There is also a dark purple line pointing towards the drill site. This means that diorite could be found during drilling deeper into the borehole, because the veins may continue below the surface.

Something that could be expected to be found in the borehole is clay minerals due to chemical weathering of rock. Chemical weathering is described as a slow chemical decomposition of rock and often occurs when water is present in the rock [8]. The probability for weathering is greater at places where the rock is crushed or is cracked. The increased surface area means chemical reactions can more easily occur. The most common reaction for silicates, all the minerals mentioned above are silicates, is by hydrolysis. The water dissociate to H⁺ and OH^– where the hydrogen ion reacts and replaces positive ions in the crystal lattice. That leads to a decomposition of the mineral. Another common substance in the water is CO₂, which forms H₂CO₃ in water. This leads to even faster decomposition of the mineral due to the contribution of more hydrogen ions. An example on a reaction between potassium feldspar, one of the consistuents in granite, water and H₂CO₃ can be seen below. Potassium is replaced with hydrogen, which forms the clay mineral kaolinite and itself ends up in a solution with bicarbonate ions and silica [8].

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Figure 4: Map over the bedrock around the drilling site, indicated by a blue circle. Brown is granodioritic granite gneiss, pink is grainte, green is amphibolite and the blue are quartz arenite. Purple is diabase. Image from Sveriges Geologiska Undersökning [7].

2KAlSi₃O₈

potassium feldspar+ 2(H⁺+ HCO₃)

carbonic acid

+ H₂O

water !

Al₂Si₂O₅(OH)₄

kaolinite + 2K⁺

potassium ion+ 2HCO₃

bicarbonate ion

+ 4SiO₂

silica

From plagioclase the clay mineral illite is formed in a similar way and from pyroxene the clay mineral montmorillonite is formed. The plagioclase and pyroxene in diabase are more sensitive to chemical weathering than the quartz and feldspar in granite and therefore the diabase is more prone to decompose into clay mineralsthan granite is [9].

2.3 Wear of the diamonds

The diamonds used in drill bits are often synthetic diamonds grown to have a octahedral mixed with a cubic shape, which is an advantageous shape when exposed to mechanical stress, see figure 5a. The cubic plane, on the top in the picture, is a {100} plane and the octahedral planes, at the sides of the diamond, are {111} planes. The diamonds can vary in appearance, the diﬀerent planes can diﬀer in size leading to an almost cubic diamond, or an almost octahedral diamond. During drilling, the diamonds are in contact with the rock and are exposed to large pressures and shocks which lead to wear of the diamonds.

Depending on the size of the pressure the mechanisms of diamond wear are diﬀerent. These mechanisms are important for the drillability of the drill bit, sharper cutting edges leads to

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a better drill bit [5]. According to Luo [10] the most desirable diamond is a diamond which is undamaged and has a high protrusion height. The wear of the diamond can be divided into two mechanisms according to Tönshoﬀ [6]; friction wear and fracture of grains. The friction wear occurs att lower loads when very hard rock particles or other released diamonds contact and scratch the diamond surface. Fracture of diamonds occurs predominantly at high loads and is caused by fatigue, or mechanical load or thermal shock. From these mechanisms Luo [10] divides diamonds in used diamond tools into five categories; whole, wear flat, microfractured, macrofractured and pull-outs. Explanations of the categories can be seen below and figure 5 shows images of the diﬀerent diamond categories.

• Whole: Undamaged diamond, figure 5b.

• Wear flat: A polished diamond. The polishing is due to friction wear, i.e. heat assisted chemical wear, at low pressures [3, 11], figure 5c.

• Microfractured: A diamond with microcracks, damages and fragments [10], figure 5d.

Due to fracture of grains at higher pressures [10].

• Macrofractured: Further fracture of the microfractured diamonds [10], figure 5e.

• Pull-out: Not really a diamond but a diamond having been pulled out of the matrix, leaving a hole in the matrix [10], figure 5f.

The ROP has to be large to get a pressure big enough to get microcracks instead of wear flats on the drill bit. Wear flats decreases the number of cutting points, sharp edges, and makes the surface polished, which leads to a decreased ROP [5]. Microfractures leads to new cutting points [12], but also to a decrease in the protrusion height [13]. The protrusion height impacts the wear of the matrix and will be discussed in the next section.

The diamonds can sometimes be cleaved easily, according to Telling [14] the cleavage occurs easiest in the {111} plane due to the lower amount of bonds compared to the other planes.

The lower amount of bonds leads to a low cleavage energy and an impact in the right direction on the diamond leads to a cleavage of the diamond. The cleavage is facilitated by defects along the plane [14] which matches Xuefengs [5] statement, that the microcracks are often initiated at the defects in the diamonds. Therefore it is important to have diamonds with few defects and have a high strength and toughness during core drilling due to the larger resistance to cleavage [4]. In figure 5d cleavage of the {111} plane can be seen. The cubic {100} plane can be seen at the top, and the octahedral {111} planes on the siden of the diamond. The cracks seen at the top of the diamond are parallel to the {111} planes and visualises what Telling [14] talks about.

Almost all of the friction in the borehole is converted to heat. At the very cutting zone, at the diamond/rock interface, the low thermal conductivity in rock the heat go into the drill bits through the diamonds [5]. This heat is soon transported away from the tool, mostly by the water flow, but temporarily high temperatures may be generated. Xuefeng

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(a) (b)

(c) (d)

(e) (f)

Figure 5: a) Ideal form of diamonds used in drill bits. Diﬀerent wear and fractures of the diamonds. b) A whole diamond without any damages. c) Wear flat. d) Microfractured. e) Macrofractured. f) Pull-out.

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[5] calculated in one exemplifying situation the maximum temperature in the diamond/rock interface should be 357°C and would not aﬀect the mechanical properties of the diamonds.

When rock cuttings are gathered around the diamonds a thermal insulator barrier is formed and the temperature could rise to 1477°C. Such high temperatures would cause microburn of the drill bit. Xu [15] calculated the temperature for such a diamond could reach above 900°C. Temperatures above 1000°C leads to wear flats according to Wang [12]. This is due to oxidation of diamonds starting already at 700-800°C and graphitization, which forms a softer material that can be plastic deformed by the rock. Another effect of a high temperature is an increase likelihood of pull-outs. Xu [15] talks about the difference in the coefficients of thermal expansion as a reason to this. Large stresses is induced between the diamond and the matrix when the temperature increases, which detaches the matrix from the diamonds when cooled down. The diamonds can be pulled from the matrix more easily. A diamond pulled out of the matrix before the working life of the diamond has ended will lead to a faster wear of the matrix and decreased working time of the drill bit [10].

2.4 Wear of the matrix

The relation between the diamonds and the matrix is very important during the drilling.

The wear rate of the matrix has to match the wear rate of the diamonds to get a good drillability and long working life. A good drillability is achieved when the height between matrix and diamond tip, protrusion height, is high. To get a high protrusion height the matrix around the diamonds has to be worn oﬀ in order to uncover the diamonds [10]. At this height it is important that the adhesion between matrix and diamond is high [10] in order to avoid pull-outs before the diamond is superannuated. For the same reason the wear rate of the matrix has to be low when the protrusion height is reached. If the matrix is worn too slow the gap between rock and matrix will decrease and the flow of the rock cuttings will be hindered [6]. The slow wear rate would also lead to polishing of the diamonds before reaching protrusion height decreasing the drillability of the drill bit [13].

According to Tönshoﬀ [6] the typical appearance of the matrix wear is a pit in front of the diamond, in the sliding direction, and a ridge behind the diamond. The diamond protects the matrix behind it and therefore the wear rate is lower [6]. The ridge acts like a support for the diamond and prevents it from being pulled out. The wear mechanism leading to that appearance is erosion, the wear when particles bombards a surface [16].

Xuefeng [5] talks about another wear mechanism for the matrix is delamination wear. At higher temperature rock melts onto the matrix, due to the high adhesion between rock and matrix matrix is worn oﬀ when the rock is worn oﬀ. Delamination wear occurs when repeated contact leads to plastic deformation of the matrix where the shear stress is greatest, a bit below the surface of the matrix. Microcracks starts to form in that area which grows together parallel to the surface. The cracks eventually goes up to the surface and a patch

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loosens from the surface [16].

Xuefeng [5] examined diamond drill bits with a matrix made of WC grains and a softer binder phase. At a low ROP the softer binder phase eroded from the matrix, leading to a loss of WC grains. At a higher ROP the dominant wear mechanism was three body abrasion due to the high amount of rock cuttings in the contact between rock and matrix. The rock cuttings was the third body in the system. The large frictional forces led to abrasion and deattachment of large patches of the matrix [5].

The diamonds play an important role for the wear of the matrix and vice versa. With larger diamonds the protrusion height can be magnified and the rock cuttings can flow more easily between the surfaces, reducing the wear of the matrix. The concentration of diamonds on the surface aﬀects the wear of the matrix, a higher concentration of diamonds on the surface protects the matrix more and leads to a longer working life for the drill bit [10].

3 Method

3.1 Analysis

Drill bits wear was examined on both at macro- and microscale. The overall amount of wear was measured with a caliper. The volume was then divided by the drill depth, to get a measure of wear from a drillers point of view, or by the eﬀective sliding length of the bit, to get a measure of wear from a tribological point of view.

One strategy for understanding the wear mechanisms of the core drills is to examine the behaviour of the diamonds on the worn surface. The diamonds were categorised in three diﬀerent groups, similar to what Luo proposed [10]; whole, fractured and pull-out. Two teeth from each drill were studied in a stereo microscope and with the help of pictures of the surfaces taken with a Light Optical Microscope, LOM, each diamond could be color coded.

No sample preparation was needed when taking the pictures in the LOM. The pictures were taken on the half of a tooth, see blue box in figure 6, giving a total of four pictures per drill bit. From the color coding on the pictures, the distribution between the three diamond categories could be determined.

Figure 6: The pictures in LOM was taken inside the blue area on the drill bit.

The matrix was examined by a number of diﬀerent methods. The pictures from the LOM was used to study the surfaces on a macroscale. To measure the surface roughness an optical

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profiler with Vertical Scanning Interferometry, VSI, was used. The instrument used was a WYKO NT1100. The VSI was used also to measure the ridges present behind diamonds to determining the maximum support the matrix could provide. It is assumed that once a diamond is crushed or pulled out, the corresponding ridge will rapidly be worn away. Also wear of test bits in replicating lab tests and surface finish were measured with VSI.

To study the diamonds and the matrixes at a higher magnification a Scanning Electron Microscope, SEM, was used. The models used were Zeiss Merlin, Leo 1550 and Leo 1530.

Energy-dispersive X-ray spectroscopy, EDS, was used together with the SEM in order to make element analysis. The EDS detector used was Oxford instruments X-Max. The characteristic X-ray lines of Si and W have an overlap and therefore the EDS system reports Si and W in the matrix when it actually is only W.

The cross sections were made with a Focused Ion Beam, FIB, model FEI Strata DB235 for both SEM and Transmission Electron Microscopy, TEM, samples. The specific area was coated with Pt before making the cross section in order to protect it from the ion beam.

The way of coating with the Pt leads to a diﬀerence in contrast between the layers. Some of the Ga sputtered onto the surface becomes implanted into the matrix, which could be detected during EDS analysis. The FIB is used to make cross sections but also for sample preparation for the TEM analysis. With the TEM it was possible to look at the samples at extremely high magnification, with a resolution of a few nanometers. The instrument was FEI Titan Themis 200 XFEG. A SuperX EDS detector was used to visualise the plastic deformation of the surface layer of the matrix and if there was rock at the surface.

Table 2 compares the methods used for the field tests and the replicating lab test bits. The same analyses have been made, except for the cross section and the TEM studies that has not been made on the replicating lab test bits.

Table 2: Analyses made on the field tests and on the replicating tests.

Wear rate Diamonds VSI SEM Cross section TEM

Field tests x x x x x x

Replicating tests x x x x

3.2 Replicating lab scale test

The purpose was to attempt to replicate the wear mechanisms in core drilling in a laboratory, in order to decrease the costs when investigating the behaviour of drill bits and allow for studies of influence of specific parameters. Figure 7 compares core drilling with the setup in the laboratory. The principle for core drilling can be seen in figure 7a. A force is applied from above while the drill is rotated against the rock. The water flow is per- pendicular to the rotational direction of the drill bit. In the lab test, a stone cylinder was

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rotated instead of the drill bit. The test bit was pressed horizontally with a specific load against the stone cylinder and water was applied from the top, making the flow parallell to the rotational direction of the stone cylinder. During the replicating tests the samples were moved along the stone cylinder in order to wear on new rock and avoid making a deep track at one specific position in the rock, which could lead to undesired wear on the edges of the drill bit. The method used was developed from methods in previous work [17, 18, 19]

performed with cemented carbide drill bit inserts used in rotary drilling.

(a) (b)

Figure 7: Sketch of the a) core drilling and b) the setup from the replicating test. Instead of rotating the drill as in core drilling a stone cylinder was rotated in the replicating test.

The stone cylinder was made out of granite from eastern Skåne and was placed in a lathe.

The setup consisted of; a lathe with a stone cylinder mounted in it, a water pump to obtain a constant water flow and a sample holder where the samples were placed and then pressed against the stone cylinder. The setup can be seen in appendix A and an image of the sample holder in appendix B. The sample holder was attached to a strain gauge arrangement for load measurement which measured and collected the applied load.

The samples were prepared by cutting pieces from an unused drill bit with a hacksaw.

A tooth was cut from the drill bit, and then split in half to get two parallel sides. The bottom was removed to not include the reamers or the softer material without diamonds.

To imitate the core drilling, the flat outer surface of the piece was used, rather than the grooved top surface. The flat surface at the outside of the drill bit is more similar to the flat surfaces of the drill bits after the core drilling, compared to the grooved top of the unused drill bit. Therefore, the outside was pressed against the stone cylinder. Figure 8 illustrates where on the drill bit the samples were cut out from, the arrow indicates which side was worn against the stone cylinder.

Three parameters were changed during the tests to see how these aﬀected the drill bits.

The parameters were the load, the rotational speed and the water flow. There are more variations in lab scale due to the simplicity in changing parameters, which can be seen in

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Figure 8: Illustrative picture over the size and geometry of the samples. The arrow indicates the surface facing the stone cylinder during the test.

table 3. Moreover, this facilitates tests where only one parameter is changed. In drilling several parameters often change, here both the ROP and rotational speed. The samples are named after the parameter which has been changed, except for the initial set, called median, which is included in all the comparative tests.

Table 3: The eight parameter sets used in the lab tests. The first row is the initial set, for the following six one parameter has been changed. For the last set two parameters have been changed.

Load [N] Rotational speed [rpm] Flow rate [l/min] Time [s]

Median 200 395 0.33 45

LL 100 395 0.33 40

HL 400 395 0.33 40

LS 200 270 0.33 65

HS 200 470 0.33 35

LF 200 395 0.025 40

HF 200 395 2 40

HLHF 600 395 9 35

To calculate the load to be used inte the replicating test the parameters from the core drilling were used together with the geometry of the drill bit. The nominal contact area, A, was calculated for the drill bit, as four teeth with the hole outwards and four teeth with the hole inwards. The grooves were assumed to be flat. This gave a contact area of 1610 mm². With the load, F, used in the core drilling a contact pressure, p, could be calculated with equation 1. The pressure should be the same in the replicating test as in the core drilling to obtain similar conditions. By calculating an approximate contact area for the samples a suitable load could be determined by using the same equation. The pressure calculated for the core drilling, with a WOB 40 kN, was 25 MPa corresponding to a load of 2.5 kN in the replicating test with a contact area of 100 mm² of the sample. This was however too high for the equipment used so three much lower loads had to be used.

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p = F

A (1)

The stone rotational speeds to be used were calculated from the parameters used in the core drilling and from the diameters of the drill bit and the stone cylinder. To calculate the sliding velocity between the drill bit and the rock, equation 2 was used. Odrill is the circumference of the drill bit, 194 mm, taken as the average between that of from the outer and inner diameter, 76 mm and 48 mm respectively. !drill is the rotational speed of the drill bit. Aiming for the same sliding velocity using the stone cylinder, equation 3 could be used to calculate the rotational speed for the stone cylinder, !cylinder, where Ocylinder

is the circumference of the stone cylinder, 455 mm. For the recommended parameters, 1000 rpm in the core drilling, a stone rotational speed of 420 rpm should be used in the replicating test. The slower drilling, 700 rpm, were calculated to correspond to 300 rpm in the replicating test and the faster drilling, 1200 rpm, to a stone rotational speed of 510 rpm.

The lathe had limitations in the rotational speed, the calculated rotational speeds could not be used. Instead, the available rotational speeds closest to the calculated rotational speeds were used.

v = O_drill⇤ !drill (2)

!_cylinder = v

O_cylinder (3)

A fix lateral feed of 0.42 m/m was applied to ensure that no wear would occur on the sides of the sample. The run time diﬀered between the diﬀerent rotational speeds; higher rotational speed gave a shorter run time due to the faster lateral movement. It was not possible to calculate the water flow corresponding to the water flow used in the drilling process due to the waterways in the drill bit where most of the water flows. Four arbitrary water flows were chosen. The length of the tests were limited by the length of the stone cylinder.

Before the test started the test bit was weighed and the length, width and thickness was measured with a calipers. The sample was then placed in the sample holder. The water flow was put on an the data collection started. The sample was pressed against the stone cylinder until the desired load was applied and then the lathe was turned on. After the test finished the lathe, the water flow and the data collection were stopped. The sample was taken out of the sample holder, cleaned in running water and dried with a Kimtech wipe tissue. The area of the wear was measured with a calipers and the sample was weighed.

The worn surface was studied in a stereo microscope in order to get a perception of how the surface had changed. The sample was then mounted back into the sample holder and the

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test was repeated to a total of five runs per parameter set. When remounting the sample it was important to mount it the same way as for the previous runs, so the wear would occur at the same place.

3.3 Separation of a single diamond

To understand the wear of the diamonds it is necessary to know how the diamonds looked before the drilling. Ideally they should have a morphology like in figure 5a. However, damages from manufacturing are not easily observed as the diamonds are embedded in the matrix. The damages on the diamonds could be both from the manufacturing and the core drilling. However, damages from manufacturing are not easily observed as the diamonds are embedded in the matrix. Therefore, a single diamond from a unused drill bit was carefully, mechanically and chemically separated from the matrix. The metals in the matrix around the diamonds were Ag, Cu, Mo and W. Tungsten is hard to dissolve, one way to do it is by using a mixture of 20 ml HF, 10 ml H₂O and 5 ml HNO₃ [20]. Copper and silver can be dissolved in HNO₃ and small amounts of molybdenum can be dissolved in HNO₃ [20].

A small piece of an unused drill bit was placed in a beaker and 20 ml of concentrated HNO₃ was poured into the beaker. After a while the solution was heated to 60°C in order to increase the reaction rate. The solution was stirred periodically. After a hour of heating the experiment was cancelled, even though not all of the matrix had been dissolved. The solution was filtered through a Büchner funnel and rinsed with water. The filter paper was examined in a stereo microscope in search for diamonds. Also, the matrix had become porous and diamonds could be pulled from the matrix and put on a carbon tab for further examination in SEM.

4 Results - Field tests

4.1 Rock material

The other thesis worker in the I-EDDA project [21] made a mapping of the rock samples during the final stage of this master thesis. The main rock material found in the boreholes are granite and diabase. Clay were found in the boreholes, which indicates chemical weathering of the rock. There were also veins of quartz. The short distance between the three holes makes it reasonable that all three holes have approximately the same layers, albeit at slightly diﬀerent depths. For a more complete picture of exactly which rock is present in the three holes the reader is referred to the other master thesis which is to be finalised later this year. The rock material that each drill bit drilled in right before being changed can be seen in table 4, and also what condition that rock material was in.

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Table 4: The rock material the diﬀerent drill bits drilled in right before being changed.

Drill bit Rock material Condition Reason for change

L.1 Granite Crushed Undersize cutting

R.1 Diabase Crushed Instability of the borehole

H.1 Granite Crushed Undersize cutting

H.2 Diabase Whole Undersize cutting

H.3 Diabase and Granite Crushed Instability of the borehole

4.2 Wear rate

The wear of the drill bits depends on the rock material drilled through. The minerals in the rock aﬀects the wear rate, but also the amount of crushed rock in the bore hole. The depths drilled varies between all the drill bits, figure 1, as do the wear. The profiles of the drill bits can be seen in figure 9 and gives a perception of the amount of material lost. Drill bit L.1 has the most material left after the drilling while drill bit H.1 has the least amount of material left. Drill bits R.1, H.1 and H.2 has a similar geometry after drilling, for drill bit H.1 the amount of materials lost is higher.

(a) L.1 (b) L.1 (c) R.1 (d) H.1 (e) H.2 (f) H.3

Figure 9: The profiles of the teeth on the diﬀerent drill bits, the side the arrow points at in figure a). The wavy profiles from before the drilling is indicated in grey and after the drilling in black.

Taking into account the different depths drilled (for the moment neglecting the different rock types) and a varying rotational speed, a wear rate was calculated. Figure 10a shows the wear rate per meter sinking depth of the borehole and figure 10b the wear rate per sliding distance of the drill bit. From the figures it can be seen that drill bits H.1 and H.3 has a much higher wear rate than the other drills, both per sinking depth and per sliding distance. Drill bit H.2 has a lower wear rate than the other drills in the same borehole, but a larger wear rate than the drill bits L.1 and R.1. Although the difference is small, R.1 has a smaller wear rate than L.1 per sinking depth, while it is greater per sliding distance.

This shift is because of the diﬀerent rotational speeds for the two drills. L.1 has a higher rotational speed and will, therefore, have a higher sliding distance at the same depth for

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the two drills. R.1 has had a larger wear than L.1 and that leads to a larger wear rate per sliding distance. On the other hand R.1 had a greater sinking depth than L.1 and due to this diﬀerence in depth R.1 has a smaller wear rate per sinking depth than L.1.

(a) (b)

Figure 10: Wear rate for the different drill bits per a) sinking depth [mm²/m] and b) sliding distance [µm²/m]. The different rotational speeds impacts the wear rate and explains why the ratio between the different boreholes varies depending on wether it is calculated per sinking depth or sliding distance.

4.3 Diamonds

Diamonds from a unused drill bit were examined to see if defects were present before the drilling started. There are defects present in the diamonds before drilling, see figure 11.

Figure 11a shows a diamond which seems to be defect free, figure 11b shows a hole in another diamond, about 70 µm wide. A crack is present in one diamond, see figure 11c.

Examining the diamonds after drilling and dividing them into three groups; whole, fractured and pull-out, gives the results in figure 12. It can be seen that R.1 diﬀers from the other three drills. Drill bits L.1 and H.1-H.3 have very similar distributions, around one third of each category. Drill bit R.1 has a higher proportion of whole diamonds and almost half the amount of fractured diamonds compared to the other drills.

During the studies in a stereo microscope and SEM further observations were made on the wear of the diamonds. Drill bit L.1 had a higher amount of wear flats, while R.1 had more microfractured diamonds. Drill bits H.1, H.2 and H.3 had a lot of macrofractured diamonds.

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(a) (b) (c)

Figure 11: Diamonds from an unused drill bit. a) A diamond with visually no defects, b) a diamond with a hole, about 70 µm wide, c) a crack in a third diamond.

Figure 12: Distribution of diamond wear for the drill bits. R.1 diﬀers from the other drills with a higher amount of whole diamonds and a lower amount of fractured diamonds.

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4.4 Matrix

In the previous section the diamonds on the surface were investigated in order to try to explain the wear mechanisms. In this section the matrix is examined to be able to see if there is any differences. By a visual inspection of the drill bits surfaces, figure 14, one difference can be seen. The surface of R.1, figure 14b, is rough with some distinct ridges that has formed behind the diamonds, in the sliding direction. The white spots on the drill is rock attached to the surface. The surface of L.1, figure 14a, looks smooth, disregarding the ridges and he ridges are very distinct and are present behind most of the diamonds, which is different from the other drill bits. During SEM studies the pull-outs in L.1 appeared to be deeper than the pull-outs at the other drill bits. For drill bit H.1, figure 14c, the surface visually looks even smoother and in the bottom of the picture the surface looks polished.

H.1 has ridges, but not as distinct as for drill bit R.1. By visually inspection of drill bit H.2, figure 14d, the drill bit seems to have a rougher surface than H.1, but smoother than R.1 and not as distinct ridges as drill bit R.1.

The ridges were measured in an optical profilometer using VSI, in order to see if the height of the ridges diﬀers between the drill bits. Based on the images in figure 14, the five seemingly largest ridges were chosen from one tooth on each drill bit. The smallest large ridges were located on drill bits H.1 and H.2, while the largest ridges were measured on H.3. The ridges of R.1 were larger than the ridges on L.1.

Figure 13: The height of the ridges behind the diamonds for the drill bits.

4.4.1 Low ROP and high rotational speed

The matrix for L.1 diﬀered between the outer, middle and inner part of the drill bit, figure 15. A magnification of the matrix is located below the overview for easier comparison, e.g.

figure 15d is a magnification of figure 15a. At the outer part of the drill bit, figure 15a and 15d, the matrix has scratches and a low amount of impregnated rock. At the middle of the

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(a) L.1 (b) R.1

(c) H.1 (d) H.2 (e) H.3

Figure 14: The surface of the drill bits. The sliding direction for the drills is from left to right. The diﬀerence in color is due to diﬀerent light when taking the pictures.

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drill bit, figure 15b and 15e, there is a lot of impregnated rock into the matrix. The rock appears to be smooth, which could indicate that it has been melted. At the inner part of the drill bit, figure 15c and 15f, there is also a lot of rock on the surface, but instead of being impregnated into the matrix the rock visually looks to had adhered to the surface of the matrix. The rock has a smooth surface, as in the middle, and could be melted.

(a) Outer part (b) Middle part (c) Inner part

(d) Outer part (e) Middle part (f) Inner part

Figure 15: The matrix on drill bit L.1. The matrix diﬀered between a) outer part, b) middle part and c) inner part.

4.4.2 Recommended parameters

For the drill bit R.1 the matrix diﬀered between the outer part and the inner part of the drill bit. In figure 16 the matrix at the outer part of the drill bit can be seen. Figure 16a shows an overview of the matrix. There is a lot of rock impregnated to the surface, which can be seen clearer in figure 16b. It shows a close up of the ridge behind the diamond to the bottom right in figure 16a. Figure 16c is a close up och figure 16b and shows the impregnated rock. The rock at the top, which is darker, visually looks to be smoother than the rock on the sides, which could indicate melted rock at the top.

At the inner part of the drill bit the amount of impregnated rock is lower than at the outer part. In figure 17a a lot of rock is placed in clusters above the matrix. When increasing the magnification, figure 17b, the matrix looks smoother compared to the matrix at the outer

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(a) (b) (c)

Figure 16: The surface for the outer part of drill bit R.1. a) An overview of the surface, there is a lot of impregnated rock. b) Magnification of the matrix at the ridge behind the diamond at the bottom right in a). c) Further magnification of the ridge, showing impregnated rock.

part of the drill bit and not as much impregnated rock. Increasing the magnification further, figure 17c, scratches can be seen in the matrix and rock cuttings above the matrix.

(a) (b) (c)

Figure 17: The surface of the inner part of drill bit R.1. a) A lot of rock on the surface, b) magnification of the matrix, c) further magnification of the matrix. There are scratches on the matrix, and the rock is not impregnated as at the outer part of the drill bit.

4.4.3 High ROP and low rotational speed

The appearance of the matrix for drill bits H.1-H.3 is similar between the three drill bits.

The matrix had two distinct appearances, polished, figure 18a, and with some rock impregnated into the surface, figure 18b. On some places, mainly on drill bit H.3, there was a lot of rock cuttings above a polished surface, figure 18c. The polished surfaces arises at the outer parts of the drill bit, while the areas with impregnated rock is more common on the inside of the drill bit.

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(a) (b) (c)

Figure 18: Drill bits H.1-H.3. Matrix a) which appears to be polished, b) with rock impregnated and c) which looks polished with rock on top.

Figure 19 shows magnifications of the matrix. Figure 19a shows a polished surface. The grains of the matrix can be distinguished on the surface for drill bits H.1-H.3. Figure 19b shows matrix about to flake oﬀ and figure 19c shows a loosened flake from the matrix.

(a) (b) (c)

Figure 19: a) Polished surface at H.1, b) matrix about to flake oﬀ at H.1, c) matrix has flaked oﬀ at H.2.

4.5 Cross section

Two cross sections were made by Martina Grandin, Uppsala university, on one drill bit from each borehole. All the images in this section is taken by her. The cross section were taken at a place with little rock and one with a lot of rock. The surface of the drill bit for all the figures is at the top, the grey lines at the top are Pt from the sample preparation. The darker areas in the images are rock and the lighter areas are matrix.

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4.5.1 Low ROP and high rotational speed

The cross section at the place with a small amount of rock at the surface for L.1 can be seen in figure 20. Rock impregnated in the matrix can be seen in figure 20a, at a depth of about 1 µm. The matrix has not been deformed at a great depth, as can be seen at the round grains. In figure 20b a deformation of the matrix of about 700 nm can be seen.

Magnification of the figures can be seen in appendix C.1.1.

(a) (b)

Figure 20: a) An overview of the cross section from drill bit L.1 at the place with a lower amount of rock at the surface. b) A close up of the matrix and the rock to the right in a).

For the cross section at the place with a higher amount of rock it is obvious in figure 21 that rock has been melted and fused into the matrix and then been deformed with the matrix. An overview of the cross section can be seen in figure 21a. The rock is present below the matrix and is deformed with it. The black holes in figure 21b visually looks like gas bubbles, that indicates that the rock has been melted and could have been injected or mixed into the matrix. The matrix has been deformed at a depth of almost 3 µm. Larger images of the figures can be seen in appendix C.1.2

(a) (b)

Figure 21: a) An overview of the cross section from drill bit L.1 at the place with a higher amount of rock at the surface. b) A close up of the rock to the right in a).

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4.5.2 Recommended rotational speed

For the recommended rotational speed the cross section with a small amount of rock, see figure 22, it was not possible to observe any rock at a low magnification, figure 22a. With increased magnification, figure 22b, small fragments of rock could be seen at the surface.

The matrix has been heavily deformed to a depth of about 3 µm, visible as elongated and smaller grains.

(a) (b)

Figure 22: The cross section for R.1 at the place with a low amount of rock at the surface.

a) An overview of the cross section, b) a close up of the rock to the right in a).

The cross section at the place with a higher amount of rock can be seen in figure 23. Figure 23a is an overview of the cross section. A lot of rock can be seen. To the left a solid piece has been pressed into the matrix. To the right, magnification in figure 23b, the rock looks solid to the left and porous to the right. This could be an indication that the rock has been pressed into the matrix and then been melted at the surface. Figures with increased size for both cross sections can be seen in appendix C.2.

(a) (b)

Figure 23: The cross section for R.1 at the place with a lot of rock at the surface. a) An overview of the cross section, b) a close up of the rock to the right in a).

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4.5.3 High ROP and low rotational speed

The cross section with a small amount of rock was similar to the one for L.1, figure 24.

The overview of the cross section, figure 24a shows round grains, with a small deformation.

When using a high magnification, figure 24b a few nanometers of rock could be seen at the surface. The deformation of the matrix was about 400 µm deep,

(a) (b)

Figure 24: The cross section for H.1 where there was no to little rock on the surface. a) Overview of the cross section. b) At higher magnification, a black line at the surface appears which could be rock. The matrix is deformed to about 400 nm depth.

The cross section at the place with more rock at the surface can be seen in figure 25. In the overview of the cross section, figure 25a, it can be seen that rock is present further down in the matrix, at a depth of 5 µm at some places. Figure 25b is a close up of figure 25a. The larger rock piece in figure 25a visually seems to have been cracked and would therefore have been pressed down into the matrix while the rock in figure 25b could have been cracked or melted into the matrix. Larger sizes of the figures in this section can be seen in appendix C.3.

(a) (b)

Figure 25: The cross section for H.1 at the place with a lot of rock at the surface. a) Overview of the cross section. b) A close up of a piece of rock in the matrix.

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4.6 Transmission Electron Microscope studies

Transmission Electron Microscopy, TEM, studies were made on drill bit R.1 by Lisa Toller, Uppsala University. All the images in this section are taken by Lisa Toller. She examined the cross sections on two diﬀerent places at the R.1 drill bit. The two positions were chosen by the amount of rock on the drill bit surface, in order to see how rock aﬀects the matrix.

By visual inspection position 1 seemed to have little to no rock on the surface while position two had more rock on the surface, see figure 26.

Figure 26: The two positions that were examined in the TEM. Position one had little to no rock on the matrix while position two visually had more rock on the matrix. The two bottom images are taken with an imaging angle of 52°to increase the topographic features.

Figure 27 shows the samples for the TEM studies, taken with High-Angle Annular Dark- Field imaging (HAADF). In both samples deformation of the matrix can be seen, as elongation of the grains. Especially the heavy Mo and W grains are visually striking in these images. The elongation in these images should be compared with figure 24 where the orig- inal spherical shape of these grains is evident. For position 1, figure 27a, the depth of he deformation is about 3 µm. For position 2, figure 27b, the depth of the deformation is about 8 µm. In the following subsections the samples have been examined at higher magnifications, in order to see if there were any rock fragments present and how the matrix had been deformed.

4.6.1 Position one

Figure 28a shows a higher magnification of the sample from position 1. The surface of the drill bit is at the same level as the blue arrow. The dark grey and light grey lines on the surface is platinum from the sample preparation. At this scale it is evident that the matrix has been heavily sheared and deformed and the grains are flat and thin and almost parallell to the surface, which can be seen clearer in figure 28b taken with bright-field TEM (BF TEM). The diﬀerent contrasts indicates diﬀerent elements and the contrast within the

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(a) (b)

Figure 27: An overview of the TEM samples for a) position 1 and b) position 2. The deformation depth for position 1 is about 3 µm while the deformation depth at position 2 is approximately 8 µm. The surfaces are just below the blue lines and above is the protective platinum.

grains are mainly due to dislocations. In none of the pictures in figure 28, rock can be seen.

(a) (b)

Figure 28: a) A closer picture of the matrix. b) The matrix has been deformed and the grains are almost parallell to the surface. The surface is at the same level as the blue arrow and the dark grey and light grey layers at the top are platina from the sample preparation.

Another position on the same cross section is imaged in figure 29. An overview can be seen in figure 29a. The light grey and dark grey layers are platinum from the sample preparation.

Close to the surface deformed matrix can be seen that again is almost parallell to the surface. The white line inside the blue box, between the surface and the platina, could be rock. Figure 29b is a close up of figure 29a where the white area is unusually thick.

An Energy-dispersive X-ray spectroscopy (EDS) analysis was made on the area inside the blue square in figure 29b. That area can be seen in figure 29c. The black-ish region in the bottom is the matrix of the drill bit and the grainy area at the top is the platina. The EDS

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analysis showed that the white area in between contained Si, O, Ca and some Al and Fe which indicate that there is rock on the surface. The two darker circles contained Ag and Cu. The maps from the EDS analysis can be seen in Appendix D.1. The thickness of the rock layer is approximately 50 nm.

(a) (b) (c)

Figure 29: a) Overview of another place in position 1. The white area inside the blue square could be rock and is examined at a higher magnification in b). Figure c) is a close up of figure b). An EDS analysis was made on the area in figure c) indicating that the bright area is rock.

4.6.2 Position two

For position 2 an overview can be seen in figure 30a taken with bright-field scanning transmission electron microscope, BF STEM. The white and grey areas at the top of the picture are background and platinum, respectively. The matrix is deformed to a larger depth than the matrix at position 1. Figure 30b is a close up of figure 30a. The matrix very close to the surface does not have the elongation of the grains as at position 1. Instead the matrix seems to have been recrystallised, which can be seen clearer in figure 30c. An EDS analysis was made on the matrix in order to see if it had been recrystallised. Instead of clear grains as for position 1 Ag and Cu are spread even over the grey area. The dark grains are mostly W, and the brighter grains are Mo. Mo is also distributed over the grey area, which could indicate a recrystallisation of the matrix. The maps from the EDS analysis can be seen in appendix D.2. Observe that Si and W have overlaps in the characteristic X-ray lines, so the amount of Si is diﬃcult to determine.

Position 2 had visually more rock on the surface than position 1 when choosing the position.

In figure 31, focusing on a diﬀerent area in the same cross section, it is possible to see rock on the surface as a light grey area between the dark matrix at the bottom and the dark grey platinum above. The picture is taken with BF STEM. The thickness of the rock is approximately 100-200 nm, depending on the measuring points. Figure 31b is taken with

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(a) (b) (c)

Figure 30: a) An overview of position 2. b) A magnification of the blue square in a). The matrix does not have the same elongation of the grains as as position 1. A closer look of the blue square in b) is shown in c). An EDS analysis was made on the area in c) indicating that a recrystallisation of the matrix could have happened.

Scanning Transmission Electron Microscopy (STEM) which invert some of the contrasts, and shows rock on the surface of the drill bit. An EDS analysis was made which verified it was rock. The maps of the EDS analysis can be seen in appendix D.3. The grey area at the top is platinum, the matrix is the white area at the bottom where there is Mo, W, Ag and Cu. The dark grey areas are mainly Si, Al, O and Ca, indicating that there is rock on the surface. The central part of the image, which contain rock-element even proved to be crystalline, indicating it is a fragment pressed into the surface. The light grey area between the rock and the matrix could be Cu from the matrix.

(a) (b)

Figure 31: Rock on the surface of the drill bit at position 2. a) Rock with a thickness of 100-200 nm. b) Rock with a thickness of 200 nm. An EDS analysis was made at figure b) and showed that the dark grey area was rock.

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5 Results - Replicating lab test

5.1 Wear rate

The wear rates of the drill bits differs between the five runs of a specific test bit. During the first run the covering top of most matrix on the surface is rapidly worn off, exposing the diamonds, which leads to a large material loss of the sample during that run and a large wear rate, see figure 32. Orange dots are the size of the worn area and the blue dots are the material losses after each run. In the first five runs, figure 32a, the material losses and areas differs a bit, but when increasing to 20 runs, figure 32b, the wear rate stabilises and the drill bit reaches a steady state. The large increase in worn area after about 115 kNm occurs due to remounting and wear at a new area on the test bit. The wear rates for all the replicating lab tests can be seen in Appendix E.

(a) (b)

Figure 32: The wear rates for the Median during a) five runs and b) 20 runs. The blue dots shows the change in weight and the orange dots the worn area after each run.

5.2 Diamonds

The diamonds on the surface of the experimental bits was investigated and divided into four groups; whole, microfractured, macrofractured and pull-out. The diamonds on the surface were color coded according to the diﬀerent wear mechanisms. The figures in appendix F shows how these images could look like.

In figure 33 the amount of whole, microfractured, macrofractured and pull-out diamonds of the experimental samples have been illustrated. In figure 33a it is possible to see a trend in the behaviour of the diamonds for LL, median and HL. A lower load leads to a higher amount of whole diamonds. When increasing the load, the amount of whole diamonds decreases while the amount of fractured diamonds increases. The proportion of microfractured

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diamonds is similar between the loads, while the proportion of macrofractured diamods increases with higher load. When increasing both the load and the water flow, the amount of whole diamonds increases and the amount of microfractured decreases compared to the situation when only the load was increased. The amount of macrofractured diamonds is about the same as for HL. Comparing this to the sets with a variation in water flow indicates that the amount of water is important for the behaviour of the diamonds.

Figure 33b shows the amount of whole, fractured and pull-out diamonds for different rotational speeds. The difference in behaviour of the diamonds is small. The amount of whole, microfractured and pull-out diamonds is similar for the three speeds, while the amount of macrofractured diamonds at 395 rpm is more than 2 times that at 270 and 470 rpm. For the different water flows in figure 33c the amount of whole diamonds is similar for a low water flow and an average water flow while it increases with a high water flow. The proportion of microfractured diamonds decreases with increasing water flow, while the proportion of macrofractured diamonds is the highest at an average water flow. In total, the amount of fractured diamonds is the same between a low water flow and an average water flow, while for a high water flow the total amount of fractured diamonds decreases.

Figure 33d shows the development of the diamonds when increasing the test length form 5 runs to 20 runs. A longer sliding distance decreases the amount of whole diamonds and increases the amount of micro- and macrofractured diamonds. The length of the test was still too short in order to get a change in the amount of pull-outs.

5.3 Matrix

To compare if the same mechanisms are dominant in the replicating lab tests as for the field tests the height of the ridges were measured. Five ridges on each sample were measured one time in the optical profilometer. The ridge sizes for the samples can be seen in figure 34.

The ridge sizes in figure 34a increases with an increase of the applied load. LL gave a ridge size of 77 µm and HL a ridge size of 116 µm. When increasing the load further and applying a higher water flow the increasing trend of the ridge sizes decreases, which corresponds to a lower ridge size for a increased water flow, figure 34c. For the diﬀerent rotational speeds, see figure 34b, there is a slightly decreasing ridge size with increasing rotational speed. The ridge sizes decreases from 100 µm to 89 µm from a low to a high rotational speed. For an increase in water flow the size of the ridges decreases, figure 34c. For a low water flow the average ridge size is 100 µm, while for a high water flow the average ridge size is 85 µm.

Comparing the change in ridge size for the longer run of the median after 5, 10, 15 and 20 runs, see figure 34d, shows that there is only a small diﬀerence. The ridge sizes increases with about 5 µm between 5 and 10 runs, and then decreases with about 1 µm between every five runs.

Study of the wear mechanisms for drill bits used in core drilling

Examensarbete 30 hp Juni 2018

Study of the wear mechanisms for drill bits used in core drilling

Emy Guttenkunst

Abstract

Study of the wear mechanisms for drill bits used in core drilling

Populärvetenskaplig sammanfattning

Contents

1 Introduction

2 Theory

3 Method

4 Results - Field tests

5 Results - Replicating lab test