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Core drilling in reinforced concrete using polycrystalline diamond (PCD) cutters: Wear and fracture mechanisms
Int. Journal of Refractory Metals & Hard Materials 27 (2009) 394–402
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Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Core drilling in reinforced concrete using polycrystalline diamond (PCD) cutters: Wear and fracture mechanisms S.G. Moseley *, K.-P. Bohn, M. Goedickemeier Hilti Corporation, P.O. Box 333, Feldkircherstrasse 100, LI-9494 Schaan, Liechtenstein
a r t i c l e
i n f o
Article history: Received 11 April 2008 Accepted 25 November 2008
Keywords: Polycrystalline diamond PCD Core drilling Reinforced concrete Wear Fracture
a b s t r a c t The wear and fracture mechanisms of PCD cutters active during the core drilling of reinforced concrete have been documented. All have analogies with well known mechanisms from rotary and rotary-percussive rock drilling and also metal cutting (machining). At rotational velocities up to 5 m/s and normal and cutting forces around 2000 N per 5 mm cutter (equating to principal stresses of around 500 N/mm2 at the cutting edge), gross fracturing of relatively un-worn cutters early in drilling is the predominant failure mechanism. Chipping at various size scales is observed with approximately the same rate of occurrence as cutter wear, which is mainly caused by attrition abrasion in concrete and to a lesser extent by chemical (diffusive) wear in the reinforcement steel. Chipping occurs by brittle fracture (unstable crack growth) at the small- to mid-scale and by fatigue crack growth at the large scale. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The conventional method used to core drill in reinforced concrete uses diamond impregnated segments with saw grit diamonds of typical size 300–600 lm and concentration 5–20 vol.%, held in a metallic matrix. In order to dissipate the large amount of heat generated during the rotary drilling process with segment technologies, water at a flow rate of between 1 and 4 l/min is required for cooling and flushing of the drilling debris. However, in many applications the elimination of this coolant is necessary. A specific example is in the dismantling of decommissioned nuclear power plants. In the European Union alone, by 2015 it is estimated that around 400 major facilities will be at one stage or another of the decommissioning and dismantling (D&D) process [1,2]. During the latter dismantling stage, the requirement to contain and then dispose of large volumes of potentially radioactively contaminated drilling coolant is highly undesirable and therefore ‘‘dry demolition” techniques are the preferred option. New technologies are required to realise this challenge, one of which is the use of polycrystalline diamond (PCD) cutters for core drilling, since it is energetically much more efficient than using conventional segments, only requiring around 100–300 Joules of energy per cm3 of concrete removed in comparison to 750–2000 J/cm3 for an efficient diamond impregnated segment core bit [3,4]. Since PCDtipped core bits can be successfully used with compressed air
* Corresponding author. Tel.: +423 234 3104; fax: +423 234 7104. E-mail address: [email protected] (S.G. Moseley). 0263-4368/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2008.11.014
replacing water as a flushing and cooling medium, coupled with an industrial vacuum cleaner to create a fully closed system with no loss of drilling dust to the atmosphere, this technology is of significant commercial interest. Further aspects of the use of PCDtipped core bits in reinforced concrete can be found in [5]. The data presented in this work has been taken from a number of research and development projects that led to the introduction of the Hilti PCC core bits in 2004. An overview of the observed wear and fracture mechanisms is presented without entering into a detailed analysis of each particular case. It is the intention of this paper to simply illustrate the variety of failure mechanisms observed in drilling reinforced concrete with PCD cutters and therefore it presents only selective results that are representative of observations made in the tests noted. It is an abridged version of the data presented at the ICSHM9 conference in Montego Bay, Jamaica, 10– 14 March 2008. Certain data have been omitted for reasons of commercial secrecy. 2. Experimental The work briefly described in this paper was performed in a series of varied experiments aimed at identifying the main factors influencing the drilling performance and cutter wear behaviour in the dry drilling of reinforced concrete using PCD. Here, the focus is on the wear and fracture mechanisms observed. Some qualitative comparisons are also made between experimental observations and results from fracture mechanics modelling using FRANC-2D ‘‘FRacture ANalysis Code” [6,7] and wear life predictions [8].
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2.1. Single cutter lathe turning tests A number of commercial grades of PCD with grain sizes from 3 to 50 lm and binder metal contents from around 4–20 vol.% were tested. These covered the range of materials typically used for metalworking and rock drilling. Grooves were cut into a 190 mm diameter reinforced concrete cylinder using a single cutter, either brazed or mechanically clamped into a tool holder similar to those used for parting operations, as shown in Figs. 1 and 2. The concrete was a C35 strength limestone-quartz mixed aggregate (4/32 mm size fraction) concrete containing 26 mm diameter reinforcement steel bars, also known as ‘‘rebars” (S235JR, or St37-2). Depth of cut (feed per revolution) was varied from 0.025 to 0.3 mm at 90 to 500 rotations per minute (RPM), equating to surface speeds of 53.7–298.5 m/min at the first cut. Total slot depth was 5 mm (e.g. 50 revolutions at 0.1 mm depth of cut) and up to 120 slots were cut per cutter. The axial rake angle was varied from +10 to 20° and the wedge angle was varied from 70 to 90° in both the up-sharp condition and with various edge preparations. Force and temperature measurements were made during the lathe cutting. 2.2. Drilling rig tests Two different automated drilling rigs were used for actual core bit testing, employing constant power for drilling in concrete and constant feed rate for drilling in rebar. One of these is shown in Fig. 3. Tests were performed both wet and dry. The drilling parameters used were from 1500 to 2700 W power, translated into torques of 12–72 Nm at rotational frequencies of 440–1100 min 1. These simulated the parameters available in commercial drives (drilling tools) that are used in the construction industry. Core bits of nominal diameters between 52 mm and 152 mm with 3–5 PCD cutters per bit were used. Cutters were attached to the core bit by a mechanical clamping system [9]. The reinforced concrete used was the same as that already described in Section 2.1, using either 16 mm or 26 mm rebars. Axial and radial rake angles of the cutters
3 jaw chuck Concrete cylinder
Fig. 3. Photograph of one of the two test rigs used.
were varied at a constant wedge angle. Although a number of grades of PCD were also tested in this stage, the majority of cutter variants tested were based on a coarse grained, low binder content PCD grade (nominal 25 lm diamond grain size with 4 vol.% Co). The PCD was used in the as-received condition and also subjected to various post-processing operations and with different edge preparations (chamfers). Drilling data recorded (at data logging frequencies of between 1 and 10 Hz) included instantaneous drilling speed; power consumption and current drawn; normal force; cutting force; and torque. 2.3. Drilling experiments with commercial diamond drilling tools In the final stages of testing, commercially available Hilti diamond drilling systems were used (DD 250 and DD 200 drives on their respective rigs, such as shown in Fig. 4). These tools deliver approximately 2 kW power to the spindle, supplying between 19 and 92 Nm of torque at 940–215 RPM. The drilling was performed by experienced operators who control the feed force to the core bit by use of a mechanically geared hand wheel to operate at, or close to, the maximum torque for the gear chosen at nearly constant power consumption. The concrete was the same as described in Sections 2.1 and 2.2. Both wet and dry drilling tests were performed on core bits of diameter in the range of 42–202 mm. The typical rate of penetration was of the order of 40 cm/min in con-
FT FN PCD in holder
Fc
Force measurement platform
Fig. 1. Schematic of the single cutter lathe turning tests.
Fig. 2. Photograph of the reinforced concrete cylinder showing the slots cut.
Fig. 4. Photograph of the DD 200 tool with HD 30 rig used for ‘‘hand tests”.
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crete and 4 cm/min in rebar, equating to a feed per cutter per revolution of the order of 0.25 mm and 0.025 mm, respectively. 3. Results The results documented below refer to the reference grade of PCD, with a nominal mean grain size of 25 lm and a cobalt content of 4 vol.%, as shown in Fig. 5, in most cases using a neutral axial rake angle and T-land chamfer. The active load spectrum (collective) on individual cutters under the conditions described in Sections 2.2 and 2.3 was quantified. For example, the maximum principal stress in the PCD diamond table near the cutting edge is of the order of 400–500 MPa, depending on drilling conditions. This stress is purely compressive when drilling rebar, but is compressive at the rake face and tensile at the flank face when drilling concrete. A cutter will experience tens of thousands of cycles at frequencies of 10–50 Hz while transitioning from concrete to rebar and back to concrete, and the bulk temperature in the PCD is between 300 and 500 °C when dry drilling, depending on drilling parameters and conditions.
3.1. Observed wear and fracture mechanisms As well as the complete loss of the cutter from the bit and catastrophic fracture, five distinct wear and fracture mechanisms were observed, along with the early stages of various modes of cracking associated with these. The mechanisms are schematically illustrated in Fig. 6. Gross fracturing from top and side loading (high normal and radial forces) is the primary failure mechanism, followed by abrasive wear and chipping from front loading (high cutting forces). The relative incidence rate of these for the specific
52mm core bit
40 Percent (%)
396
30 20 10 Outer side
0 Gross fracturing
Inner side Chipping
Abrasionwear
Fig. 7. Relative incidence of the three primary failure mechanisms of the cutters on 52 mm diameter core bits drilled by hand to end-of-life in reinforced concrete. Note that only around half the cutters were able to be categorised, with the remainder subject to secondary damage.
Fig. 5. Microstructure of reference PCD grade primarily used in these experiments.
Fig. 8. Example of gross fracturing caused by high normal forces, evidenced by the geometry of the fracture surface.
Micro cracks Micro chipping on cutting edge Abrasion Chipping due to side impacts Fatigue, delamination Hertzian cracks Chemical wear Fig. 6. Schematic illustration of the wear and fracture mechanisms observed, excluding gross fracture and cutter loss from the core bit. (1) and (6) are the early stages of micro- or meso-scale chipping.
Fig. 9. Example of macro-chipping due to high cutting forces, evidenced by the geometry of the fracture surface.
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All the observed wear and fracture mechanisms are best illustrated using photographs of tested cutters. Examples are shown in Figs. 8–17 without specific reference to the test conditions. The figure captions provide a short description. Mechanisms have been categorised by reference to examples provided in the literature (see Sections 4.1 to 4.3). 3.2. Modelling
Fig. 10. Example of micro-chipping which is primarily on the flank face due to high cutting forces, but also evident on the rake faces due to the normal force.
Fig. 11. Example of chipping due to side impact. Along the intact cutting edge, uniform abrasive wear can be seen on the flank face.
example a of 52 mm diameter core bit with three cutters drilled using a DD 200 tool at 940 RPM and 19 Nm can be seen in Fig. 7.
Actual cutter geometries and measured material property data and load conditions (magnitude and orientation of forces plus bulk temperatures) were used in the fracture mechanics modelling. Stable (fatigue) and unstable (brittle) fracture propagations were proven to be dependent on one another [8], reducing the number of independent Paris constants to one, being the ‘‘m” value in the Paris Law equation, with m 2.5. Wear loss equations with input from measured wear rates in the linear steady-state (stage II wear) regime were also used to predict the wear life. Many cases of different crack initiation sites under different loading conditions (actual measured resultant force magnitudes and vectors plus model cases of purely normal force and cutting force) and cutter geometries were modelled, examples of which are shown in Figs. 18 and 19. An example of the loading history of a PCD cutter when drilling in reinforced concrete with a DD 200 drive is shown in Figs. 20 and 21. In this case, the bit drilled to a depth of over 11 m cutting nearly 50 rebars before end-of-life was achieved at around 5 104 load cycles. This experimental result was in agreement with the modelling, which indicated that under the conditions modelled, wear or micro-chipping would predominate and gross fracture was unlikely except in the new condition, as shown in Fig. 22. Some selected (mainly qualitative) conclusions from the modelling are as follows: Micro-chipping by brittle crack propagation competes with wear. Macro-chipping occurs by fatigue crack growth. Gross fracturing by brittle crack propagation is assisted by unfavourable residual- and thermo-elastic stresses in the cutter but is only predominant in new (un-worn) cutters. Delamination caused by brittle crack formation can only take place when the interface toughness is around 30% or less that of the diamond table itself.
Fig. 12. Examples of three pure wear mechanisms, the nature of which depends on the substrate being drilled.
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Fig. 13. Example of abrasive wear and attritional edge crumbling in a cutter with a standard T-Land (chamfer). Much of the wear on the rake face has been caused by diamonds plucked out of the PCD itself.
Fig. 16. Example of Hertzian cracking, initiated from surface defects on the rake face.
These results are also consistent with a previous analysis [10] of single cutter controlled cyclic compressive loading and unloading experiments (excluding severe shock impact loading). The fracture mode was observed to be brittle shear fracture, and fracture stress is reduced under cyclic loading of the order of 105 cycles. In their experiments, a series of micro-chipping events precedes a regime where ‘‘mini-scale fatigue” occurs at the grain size level, similar to attritional abrasion. The crack continues to grow until conditions are favourable for spontaneous crack growth and massive fracture occurs in a rapid manner. 4. Discussion
Fig. 14. Example of reduced wear in a cutter subject to an alternative edge preparation (drilled under the same conditions as in Fig. 13).
In order to put the findings of this work into context, it is first necessary to briefly review the active wear and fracture mechanisms for hard materials in similar applications. The mechanisms observed in this study can then be directly compared to these other cases. It must be stressed that the active wear mechanisms may differ considerably under different conditions, even in a single field of application. Note that comments in the descriptions placed within brackets have been added by the present authors based on the textual content of the papers referenced. The terms used are in some cases interchanged between authors, and definitions of the scales of wear and fracture (e.g. ‘‘micro-”, ‘‘macro-” and ‘‘gross”) are not necessarily fully consistent with one another. 4.1. Wear mechanisms reported in rock drilling using PCD cutters [10–15]
Fig. 15. Example of both chemical wear (‘‘cratering”) on the rake face and the early stages before delamination of the entire diamond table with a large crack evident close the interface between the PCD and the cemented carbide substrate.
In the rock drilling industry, the International Association of Drilling Contractors (IADC) uses a simplified bit grading system with four categories [11] for reporting the condition of the cutters on a bit after it is pulled from the hole. Most drillbit manufacturers have expanded this classification method to reduce ambiguities and to improve consistency in the grading of dull bits (i.e. worn or at end-of-life) within the current IADC structure, an example of which is given in [12]. This grading scheme categorises the condition of the cutters as:
Delamination caused by fatigue crack growth is unlikely as only a small proportion of the interface will be delaminated at the same stage (i.e. number of changing load cycles) as when macro-chipping is likely to occur.
Bond failure / Lost cutter (failure in a joint with the PCD). Chipped cutter (where less than 1/3 of the cutter volume is lost by fracture). Broken cutter (where more than 1/3 of the cutter volume is lost by fracture).
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Fig. 17. Example of chemical wear and binder extrusion at the rake face in cutting rebar. The cutting edge (dotted line) is crumbling by a process of attritional abrasion while the rake face is being smoothly worn by chemical wear. Between the 2nd and 3rd rebar, fracture processes elsewhere caused the bit to ‘‘dull” and the resulting much higher drilling time for the third rebar provoked a significant increase in the cutting edge temperature, which enhanced chemical wear and binder extrusion to the surface (‘‘sweating”).
Smooth wear (abrasive or erosive). Micro-chipping (from high cutting forces, see Fig. 23). Gross-fracturing (from high normal forces, see Fig. 23). Thermally induced wear or fracture (and fatigue).
This categorisation has been yet further expanded upon by [14] where macroscopic wear is described as being due to four components:
Fig. 18. Example of one modelled geometry with a positive axial rake angle (b) of 15° and a wedge angle (a) of 70°.
Delamination (total loss of the diamond table from the carbide substrate). Heat checking (in the supporting cemented carbide substrate). Worn cutter (abrasive wear flatting, ranked according to severity). A more detailed analysis of actual wear and failure mechanisms [13] shows that the ‘‘worn” and ‘‘broken” cases can actually be further broken down into four different active wear and fracture mechanisms. These are described as:
Abrasion (steady-state wear, on the rake face in soft rocks or flank in hard rocks). Impact shock and fatigue (with impact shock caused by high forces interacting with residual stresses in the PCD and impact fatigue being mechanical fatigue). Impact loading (which can cause macro-failure through tensile stresses). Thermal shock / temperature (intensified micro-chipping wear at >350 °C). From both laboratory cutting experiments and full scale bit tests, [15] reported that various modes of wear and failure were observed, being: Joint failure (delamination of PCD-hardmetal and also failure in hardmetal–hardmetal brazed joints). Gross plastic deformation and shear failure (of the hardmetal or the diamond table, respectively). Chipping and spalling at the cutting edge (micro-chipping). Diamond grain micro-chipping, fracture and pullout (attritional abrasion, crushing and detachment of individual diamond grains).
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Fig. 19. Top: Crack initiation sites considered for the modelling of chipping due to the acting cutting force. From left to right are shown micro-chipping (0.07), meso-chipping (0.27) and macro-chipping (0.50), where the numerical values indicate the distance away from the cutting edge as a proportion of the cutter height (i.e. 0.50 = half way up the cutter). Bottom: Crack initiation sites considered for the modelling of gross fracture due to the acting normal force. From left to right are shown at 0.2, 0.5 and 1.0, where the numerical values indicate the distance away from the cutting edge as a proportion of the diamond table thickness (i.e. 0.50 = half way into the diamond layer). Delamination of the entire diamond table is a special case at 1.0, where the crack runs parallel to the interface just inside the diamond table itself.
Fig. 20. Magnitude of the resultant force acting on a single PCD cutter when drilling in reinforced concrete. At the end-of-life, the bit had drilled 11 m and experienced around 45,000 changing load cycles (concrete aggregate-cement-rebar transitions).
Fig. 21. Direction of the resultant force acting on a PCD cutter when drilling in reinforced concrete.
significant, whole diamond grain pullout became prevalent and also both impact wear of the diamond and abrasive wear of the hardmetal were accelerated. Abrasive wear of the hardmetal (when the wear flat width exceeds the diamond table thickness). Heat checking and thermal fatigue of surface layers (in the hardmetal). At low temperatures, failure of individual diamond grains was primarily by crushing or brittle fracture with only occasional fracture at a larger scale under high impact conditions. (As noted previously in [10], shear mode brittle fracture and fatigue crack growth at the grain size level appear to be the underlying mechanisms of these chipping and fracturing events). At high temperatures (cutting edge >750 °C) where thermal effects become
4.2. Wear mechanisms reported in percussive rock drilling using cemented tungsten carbide hardmetal inserts [16–21] Wear of hardmetals in rock drilling was first characterised in the 1970’s with later work providing a greater level of understanding at both a microscopic and macroscopic level. Wear during rock drilling can take place by a number of macroscopic processes: Surface impact spalling (equivalent to micro-cracking). Surface impact-fatigue spalling (from sub-surface hertzian cracking).
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Mechanical fatigue/overload fracture on a larger scale (catastrophic failure or gross fracturing). These processes can be characterised on a microscopic scale as: Preferential abrasion (erosion) of the binder. Extrusion of the binder to the wear surface. Uprooting of WC grains from the binder phase. Cracking/fragmentation of the (large) WC grains. Attritional abrasion of the WC (‘‘crumbling” of grains by fatigue or multiple cracking events). Spalling under point impact (loss on individual grains or small clusters). Chipping/flaking after crack growth by mechanical and/or thermal fatigue.
Fig. 22. Modelling of extent of wear and the crack length in gross fracturing (parameter ‘‘a”) as a function of the number of changing load cycles. Wear should predominate, although in the early stages (new condition) there is some competition between wear and fracture.
4.3. Wear mechanisms reported in metalworking using cemented tungsten carbide inserts [22,23] In the metalworking industry, cutting insert end-of-life or failure is described by well known and understood tool wear and fracture mechanisms, being: Chipping or gross fracture (localised or total failure caused by excessive stress, mechanical shock, dynamic fatigue loading or as a consequence of wear). Flank wear (abrasion of the clearance face). Cratering (diffusive, chemical wear on the rake face by workpiece chips). Built-up edge (adhesion of workpiece chips by localised pressure welding). Notch wear (localised increased wear at the depth-of-cut position). Thermal cracking (due to thermal cycling fatigue). Wear mechanism maps are commonly used to predict the expected primary wear mechanism and intensity when machining a specific substrate material under varying feeds and speeds. The wear regimes are generally then defined as thermally induced (at high cutting speeds) or mechanically induced (at high feeds). 4.4. Comparison of active PCD wear and fracture mechanisms during core drilling in reinforced concrete (present work) with the cases presented in Sections 4.1 to 4.3
Fig. 23. Schematic representations of gross fracturing and micro-chipping [13].
Thermal fatigue. Abrasion.
It can be loosely stated that with the exception of built-up edge, notch wear and heat-checking (thermal fatigue) of the cemented tungsten carbide substrate, all the wear and fracture mechanisms observed in similar applications are evident to some extent in the rotary drilling of reinforced concrete using PCD-tipped core bits. Although a further detailed analysis at the microstructural scale would be necessary to quantify the relative contribution of
Table 1 Factors affecting the resistance of a cutter to the main failure mechanisms are indicated with a ‘‘Y”. Note that this is a purely qualitative summary of the results of all testing performed under the range of conditions noted in Sections 2.1 to 2.3.
Gross fracturing Micro-cracking Abrasive and chemical wear Delamination
Cutting edge properties
Material properties & microstructure
Y
Y Y Y
Cutter geometry
Cutter internal construction
Y Y
Residual stresses Y Y
Y
Y
Surface treatments Y Y
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each of the microscopic wear mechanisms, there is clear evidence for cracking, fragmentation and uprooting of the larger diamond grains; attritional abrasion (cracking/crushing) of the diamond grains; and spalling of individual or small clusters of grains. The thermal degradation of the diamond–diamond bonds due to the cobalt content of the PCD also plays a role. The large number of tests performed, where many variables were independently varied to determine how they influence the wear and fracture mechanisms and rates, have resulted in the (simplified) summary provided in Table 1. Due to reasons of commercial secrecy, specific details have been omitted. Naturally, the drilling parameters of the tool (drive) also strongly influence the magnitude of the forces acting and also the temperatures generated at the cutting edge and in the bulk of the cutter.
5. Current status and summary of this work 1. Gross fracturing from top and side loading (high normal and radial forces) is the primary failure mechanism, followed by wear and chipping from front loading (high cutting forces) in roughly equal proportions. 2. The primary parameters (geometry, tool, material, etc) that influence each of the active wear and fracture mechanisms have been identified: The material properties and microstructure, along with the edge and surface preparation of the cutters, influence the resistance to micro-cracking and wear. Delamination can be reduced by altering the internal stress state and metallurgy of the PCD-hardmetal near-interface region. Under the load collectives studied, fracture events from the micro-scale to the macro-scale are primarily influenced by the cutter geometry and the internal stress state, with the PCD microstructure only influencing events at the micro-scale. 3. All the mechanisms observed have analogies with known wear and fracture mechanisms from rock drilling and metal cutting. Wear and micro-chipping compete with each other under typical core drilling conditions. 4. Fracture mechanics modelling has shown good agreement with experimental results. As expected, challenges faced in both rock drilling and metal cutting are all present during the coring drilling of steel reinforced concrete, but are even more accentuated by the limited power and rigidity of conventional construction industry drilling systems which have been designed to work with diamond impregnated segment technologies. Significant improvements in cutter lifetime (10 times higher than the first core bits produced) have been realised following this work to quantify and understand the wear and fracture mechanisms and the necessary actions for even further improvements are known. PCD-tipped core bits for dry coring in reinforced concrete are now commercially available for special applications but at the present time it remains a niche market.
Acknowledgements The authors would like to thank the many Hilti employees who have contributed in the various PCD projects over the years, particularly in Corporate Research and Technology and in the business unit ‘‘Diamond”. Thanks are also due to our external partners for the supply and development of the cutting materials, performing the fracture mechanics modelling and providing technical support and many interesting discussions over a number of years. References [1] European Commission Report. ‘‘Decommissioning of nuclear installations in the European union”. EUR-18860-EN, 1998. [2] OECD Nuclear Energy Agency Report. ‘‘The decommissioning and dismantling of nuclear facilities”. 2002. [3] Hilti Internal Report. ‘‘Basics of demolition - diamond”. 2001. [4] Lammer A. ‘‘Designing tool parameters for optimum drilling performance in reinforced concrete”.in: second international industrial diamond conference – ‘‘diamond at work 2”, 19–20 April 2007, Rome, Italy. [5] Moseley SG, Bohn K-P, Goedickemeier M. ‘‘Core drilling in reinforced concrete using polycrystalline diamond (PCD) cutters: performance modelling”, in: international conference on diamond, cubic boron nitride and their applications – ‘‘intertech 2008”, 5–7 May 2008, Orlando, USA. [6] Ingraffea AR, Wawrzynek PA. ‘‘Discrete modeling of crack propagation: theoretical aspects and implementation issues in two and three dimensions”, Report 91-5, School of Civil and Environmental Engineering, Cornell University; 1991. [7] Wawrzynek PA. ‘‘Interactive finite element analysis of fracture processes: an integrated approach”, M.Sc. Thesis, Cornell University, 1991. [8] Hilti Internal Report. Modeling of failure modes and fracture mechanisms in PCD cutters: mechanical behaviour”. 2003. [9] EP1281463. US6902357, US2003026668, DE10137747, JP2003053613. [10] Dunn KJ, Lee M. ‘‘The fracture and fatigue of sintered diamond compact”. J Mat Sci 1979;14:882–90. [11] Fixed Cutter Bit Dull Grading System. International Association of Drilling Contractors (IADC). 1987. [12] Smith International Incorporated. ‘‘Smith tool dull grading manual”. ST-2067, 2001. [13] Lin T-P, Hood M, Cooper GA, Xiaohong L. Wear and failure mechanisms of polycrystalline diamond compact bits. Wear 1992;156(1):133–50. [14] Ersoy A, Waller MD. Wear characteristics of PDC pin and hybrid core bits in rock drilling. Wear 1995;188:150–65. [15] Ortega A, Glowka DA. Studies of the frictional heating of polycrystalline diamond compact drag tools during rock cutting. Sandia National Laboratories; Report SAND80-2677, 1982. p. 10–5. [16] Osburn HJ. Wear Of rock drilling tools. Powder Metall 1969;12(24):471–502. [17] Larsen-Basse J. Wear of hard-metals in rock drilling: a survey of the literature. Powder Metall 1973;16(31):1–31. [18] Perrott CM. Tool materials for drilling and mining. Ann Rev Mater Sci 1979;9:23–50. [19] Fischmeister HF. Development and present status of the science and technology of hardmetals. in: Proceedings of international conference science hard materials 23–28 Aug, Wyoming, USA: 1981. [20] Almond EA, Lay LA, Gee MG. Comparison of sliding and abrasive wear mechanisms in ceramics and cemented carbides. Proceedings of second international conference science of hard materials, 23–28 September, 1984, Rhodes, Greece. [21] Reshetnyack H, Kuybarsepp J. Mechanical properties of hard metals and their erosive wear resistance. Wear 1994;177:185–93. [22] Trent EM, Wright PK. Metal cutting. 4th ed. Butterworth; 2000. ISBN 9780750670692. [23] European Powder Metallurgy Association (EPMA) Training Course ‘‘Hardmetals: Wear, Fracture and Design” from the ‘‘Design for PM elearning platform”, an EU funded project in the ‘‘Leonardo da Vinci” Education and Culture Programme. Compiled from data provided by Sandvik, Kennametal, Ceratizit, Hilti and the Fraunhofer Institute IKTS. Available from:.
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Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Core drilling in reinforced concrete using polycrystalline diamond (PCD) cutters: Wear and fracture mechanisms S.G. Moseley *, K.-P. Bohn, M. Goedickemeier Hilti Corporation, P.O. Box 333, Feldkircherstrasse 100, LI-9494 Schaan, Liechtenstein
a r t i c l e
i n f o
Article history: Received 11 April 2008 Accepted 25 November 2008
Keywords: Polycrystalline diamond PCD Core drilling Reinforced concrete Wear Fracture
a b s t r a c t The wear and fracture mechanisms of PCD cutters active during the core drilling of reinforced concrete have been documented. All have analogies with well known mechanisms from rotary and rotary-percussive rock drilling and also metal cutting (machining). At rotational velocities up to 5 m/s and normal and cutting forces around 2000 N per 5 mm cutter (equating to principal stresses of around 500 N/mm2 at the cutting edge), gross fracturing of relatively un-worn cutters early in drilling is the predominant failure mechanism. Chipping at various size scales is observed with approximately the same rate of occurrence as cutter wear, which is mainly caused by attrition abrasion in concrete and to a lesser extent by chemical (diffusive) wear in the reinforcement steel. Chipping occurs by brittle fracture (unstable crack growth) at the small- to mid-scale and by fatigue crack growth at the large scale. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The conventional method used to core drill in reinforced concrete uses diamond impregnated segments with saw grit diamonds of typical size 300–600 lm and concentration 5–20 vol.%, held in a metallic matrix. In order to dissipate the large amount of heat generated during the rotary drilling process with segment technologies, water at a flow rate of between 1 and 4 l/min is required for cooling and flushing of the drilling debris. However, in many applications the elimination of this coolant is necessary. A specific example is in the dismantling of decommissioned nuclear power plants. In the European Union alone, by 2015 it is estimated that around 400 major facilities will be at one stage or another of the decommissioning and dismantling (D&D) process [1,2]. During the latter dismantling stage, the requirement to contain and then dispose of large volumes of potentially radioactively contaminated drilling coolant is highly undesirable and therefore ‘‘dry demolition” techniques are the preferred option. New technologies are required to realise this challenge, one of which is the use of polycrystalline diamond (PCD) cutters for core drilling, since it is energetically much more efficient than using conventional segments, only requiring around 100–300 Joules of energy per cm3 of concrete removed in comparison to 750–2000 J/cm3 for an efficient diamond impregnated segment core bit [3,4]. Since PCDtipped core bits can be successfully used with compressed air
* Corresponding author. Tel.: +423 234 3104; fax: +423 234 7104. E-mail address: [email protected] (S.G. Moseley). 0263-4368/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2008.11.014
replacing water as a flushing and cooling medium, coupled with an industrial vacuum cleaner to create a fully closed system with no loss of drilling dust to the atmosphere, this technology is of significant commercial interest. Further aspects of the use of PCDtipped core bits in reinforced concrete can be found in [5]. The data presented in this work has been taken from a number of research and development projects that led to the introduction of the Hilti PCC core bits in 2004. An overview of the observed wear and fracture mechanisms is presented without entering into a detailed analysis of each particular case. It is the intention of this paper to simply illustrate the variety of failure mechanisms observed in drilling reinforced concrete with PCD cutters and therefore it presents only selective results that are representative of observations made in the tests noted. It is an abridged version of the data presented at the ICSHM9 conference in Montego Bay, Jamaica, 10– 14 March 2008. Certain data have been omitted for reasons of commercial secrecy. 2. Experimental The work briefly described in this paper was performed in a series of varied experiments aimed at identifying the main factors influencing the drilling performance and cutter wear behaviour in the dry drilling of reinforced concrete using PCD. Here, the focus is on the wear and fracture mechanisms observed. Some qualitative comparisons are also made between experimental observations and results from fracture mechanics modelling using FRANC-2D ‘‘FRacture ANalysis Code” [6,7] and wear life predictions [8].
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2.1. Single cutter lathe turning tests A number of commercial grades of PCD with grain sizes from 3 to 50 lm and binder metal contents from around 4–20 vol.% were tested. These covered the range of materials typically used for metalworking and rock drilling. Grooves were cut into a 190 mm diameter reinforced concrete cylinder using a single cutter, either brazed or mechanically clamped into a tool holder similar to those used for parting operations, as shown in Figs. 1 and 2. The concrete was a C35 strength limestone-quartz mixed aggregate (4/32 mm size fraction) concrete containing 26 mm diameter reinforcement steel bars, also known as ‘‘rebars” (S235JR, or St37-2). Depth of cut (feed per revolution) was varied from 0.025 to 0.3 mm at 90 to 500 rotations per minute (RPM), equating to surface speeds of 53.7–298.5 m/min at the first cut. Total slot depth was 5 mm (e.g. 50 revolutions at 0.1 mm depth of cut) and up to 120 slots were cut per cutter. The axial rake angle was varied from +10 to 20° and the wedge angle was varied from 70 to 90° in both the up-sharp condition and with various edge preparations. Force and temperature measurements were made during the lathe cutting. 2.2. Drilling rig tests Two different automated drilling rigs were used for actual core bit testing, employing constant power for drilling in concrete and constant feed rate for drilling in rebar. One of these is shown in Fig. 3. Tests were performed both wet and dry. The drilling parameters used were from 1500 to 2700 W power, translated into torques of 12–72 Nm at rotational frequencies of 440–1100 min 1. These simulated the parameters available in commercial drives (drilling tools) that are used in the construction industry. Core bits of nominal diameters between 52 mm and 152 mm with 3–5 PCD cutters per bit were used. Cutters were attached to the core bit by a mechanical clamping system [9]. The reinforced concrete used was the same as that already described in Section 2.1, using either 16 mm or 26 mm rebars. Axial and radial rake angles of the cutters
3 jaw chuck Concrete cylinder
Fig. 3. Photograph of one of the two test rigs used.
were varied at a constant wedge angle. Although a number of grades of PCD were also tested in this stage, the majority of cutter variants tested were based on a coarse grained, low binder content PCD grade (nominal 25 lm diamond grain size with 4 vol.% Co). The PCD was used in the as-received condition and also subjected to various post-processing operations and with different edge preparations (chamfers). Drilling data recorded (at data logging frequencies of between 1 and 10 Hz) included instantaneous drilling speed; power consumption and current drawn; normal force; cutting force; and torque. 2.3. Drilling experiments with commercial diamond drilling tools In the final stages of testing, commercially available Hilti diamond drilling systems were used (DD 250 and DD 200 drives on their respective rigs, such as shown in Fig. 4). These tools deliver approximately 2 kW power to the spindle, supplying between 19 and 92 Nm of torque at 940–215 RPM. The drilling was performed by experienced operators who control the feed force to the core bit by use of a mechanically geared hand wheel to operate at, or close to, the maximum torque for the gear chosen at nearly constant power consumption. The concrete was the same as described in Sections 2.1 and 2.2. Both wet and dry drilling tests were performed on core bits of diameter in the range of 42–202 mm. The typical rate of penetration was of the order of 40 cm/min in con-
FT FN PCD in holder
Fc
Force measurement platform
Fig. 1. Schematic of the single cutter lathe turning tests.
Fig. 2. Photograph of the reinforced concrete cylinder showing the slots cut.
Fig. 4. Photograph of the DD 200 tool with HD 30 rig used for ‘‘hand tests”.
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crete and 4 cm/min in rebar, equating to a feed per cutter per revolution of the order of 0.25 mm and 0.025 mm, respectively. 3. Results The results documented below refer to the reference grade of PCD, with a nominal mean grain size of 25 lm and a cobalt content of 4 vol.%, as shown in Fig. 5, in most cases using a neutral axial rake angle and T-land chamfer. The active load spectrum (collective) on individual cutters under the conditions described in Sections 2.2 and 2.3 was quantified. For example, the maximum principal stress in the PCD diamond table near the cutting edge is of the order of 400–500 MPa, depending on drilling conditions. This stress is purely compressive when drilling rebar, but is compressive at the rake face and tensile at the flank face when drilling concrete. A cutter will experience tens of thousands of cycles at frequencies of 10–50 Hz while transitioning from concrete to rebar and back to concrete, and the bulk temperature in the PCD is between 300 and 500 °C when dry drilling, depending on drilling parameters and conditions.
3.1. Observed wear and fracture mechanisms As well as the complete loss of the cutter from the bit and catastrophic fracture, five distinct wear and fracture mechanisms were observed, along with the early stages of various modes of cracking associated with these. The mechanisms are schematically illustrated in Fig. 6. Gross fracturing from top and side loading (high normal and radial forces) is the primary failure mechanism, followed by abrasive wear and chipping from front loading (high cutting forces). The relative incidence rate of these for the specific
52mm core bit
40 Percent (%)
396
30 20 10 Outer side
0 Gross fracturing
Inner side Chipping
Abrasionwear
Fig. 7. Relative incidence of the three primary failure mechanisms of the cutters on 52 mm diameter core bits drilled by hand to end-of-life in reinforced concrete. Note that only around half the cutters were able to be categorised, with the remainder subject to secondary damage.
Fig. 5. Microstructure of reference PCD grade primarily used in these experiments.
Fig. 8. Example of gross fracturing caused by high normal forces, evidenced by the geometry of the fracture surface.
Micro cracks Micro chipping on cutting edge Abrasion Chipping due to side impacts Fatigue, delamination Hertzian cracks Chemical wear Fig. 6. Schematic illustration of the wear and fracture mechanisms observed, excluding gross fracture and cutter loss from the core bit. (1) and (6) are the early stages of micro- or meso-scale chipping.
Fig. 9. Example of macro-chipping due to high cutting forces, evidenced by the geometry of the fracture surface.
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All the observed wear and fracture mechanisms are best illustrated using photographs of tested cutters. Examples are shown in Figs. 8–17 without specific reference to the test conditions. The figure captions provide a short description. Mechanisms have been categorised by reference to examples provided in the literature (see Sections 4.1 to 4.3). 3.2. Modelling
Fig. 10. Example of micro-chipping which is primarily on the flank face due to high cutting forces, but also evident on the rake faces due to the normal force.
Fig. 11. Example of chipping due to side impact. Along the intact cutting edge, uniform abrasive wear can be seen on the flank face.
example a of 52 mm diameter core bit with three cutters drilled using a DD 200 tool at 940 RPM and 19 Nm can be seen in Fig. 7.
Actual cutter geometries and measured material property data and load conditions (magnitude and orientation of forces plus bulk temperatures) were used in the fracture mechanics modelling. Stable (fatigue) and unstable (brittle) fracture propagations were proven to be dependent on one another [8], reducing the number of independent Paris constants to one, being the ‘‘m” value in the Paris Law equation, with m 2.5. Wear loss equations with input from measured wear rates in the linear steady-state (stage II wear) regime were also used to predict the wear life. Many cases of different crack initiation sites under different loading conditions (actual measured resultant force magnitudes and vectors plus model cases of purely normal force and cutting force) and cutter geometries were modelled, examples of which are shown in Figs. 18 and 19. An example of the loading history of a PCD cutter when drilling in reinforced concrete with a DD 200 drive is shown in Figs. 20 and 21. In this case, the bit drilled to a depth of over 11 m cutting nearly 50 rebars before end-of-life was achieved at around 5 104 load cycles. This experimental result was in agreement with the modelling, which indicated that under the conditions modelled, wear or micro-chipping would predominate and gross fracture was unlikely except in the new condition, as shown in Fig. 22. Some selected (mainly qualitative) conclusions from the modelling are as follows: Micro-chipping by brittle crack propagation competes with wear. Macro-chipping occurs by fatigue crack growth. Gross fracturing by brittle crack propagation is assisted by unfavourable residual- and thermo-elastic stresses in the cutter but is only predominant in new (un-worn) cutters. Delamination caused by brittle crack formation can only take place when the interface toughness is around 30% or less that of the diamond table itself.
Fig. 12. Examples of three pure wear mechanisms, the nature of which depends on the substrate being drilled.
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Fig. 13. Example of abrasive wear and attritional edge crumbling in a cutter with a standard T-Land (chamfer). Much of the wear on the rake face has been caused by diamonds plucked out of the PCD itself.
Fig. 16. Example of Hertzian cracking, initiated from surface defects on the rake face.
These results are also consistent with a previous analysis [10] of single cutter controlled cyclic compressive loading and unloading experiments (excluding severe shock impact loading). The fracture mode was observed to be brittle shear fracture, and fracture stress is reduced under cyclic loading of the order of 105 cycles. In their experiments, a series of micro-chipping events precedes a regime where ‘‘mini-scale fatigue” occurs at the grain size level, similar to attritional abrasion. The crack continues to grow until conditions are favourable for spontaneous crack growth and massive fracture occurs in a rapid manner. 4. Discussion
Fig. 14. Example of reduced wear in a cutter subject to an alternative edge preparation (drilled under the same conditions as in Fig. 13).
In order to put the findings of this work into context, it is first necessary to briefly review the active wear and fracture mechanisms for hard materials in similar applications. The mechanisms observed in this study can then be directly compared to these other cases. It must be stressed that the active wear mechanisms may differ considerably under different conditions, even in a single field of application. Note that comments in the descriptions placed within brackets have been added by the present authors based on the textual content of the papers referenced. The terms used are in some cases interchanged between authors, and definitions of the scales of wear and fracture (e.g. ‘‘micro-”, ‘‘macro-” and ‘‘gross”) are not necessarily fully consistent with one another. 4.1. Wear mechanisms reported in rock drilling using PCD cutters [10–15]
Fig. 15. Example of both chemical wear (‘‘cratering”) on the rake face and the early stages before delamination of the entire diamond table with a large crack evident close the interface between the PCD and the cemented carbide substrate.
In the rock drilling industry, the International Association of Drilling Contractors (IADC) uses a simplified bit grading system with four categories [11] for reporting the condition of the cutters on a bit after it is pulled from the hole. Most drillbit manufacturers have expanded this classification method to reduce ambiguities and to improve consistency in the grading of dull bits (i.e. worn or at end-of-life) within the current IADC structure, an example of which is given in [12]. This grading scheme categorises the condition of the cutters as:
Delamination caused by fatigue crack growth is unlikely as only a small proportion of the interface will be delaminated at the same stage (i.e. number of changing load cycles) as when macro-chipping is likely to occur.
Bond failure / Lost cutter (failure in a joint with the PCD). Chipped cutter (where less than 1/3 of the cutter volume is lost by fracture). Broken cutter (where more than 1/3 of the cutter volume is lost by fracture).
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Fig. 17. Example of chemical wear and binder extrusion at the rake face in cutting rebar. The cutting edge (dotted line) is crumbling by a process of attritional abrasion while the rake face is being smoothly worn by chemical wear. Between the 2nd and 3rd rebar, fracture processes elsewhere caused the bit to ‘‘dull” and the resulting much higher drilling time for the third rebar provoked a significant increase in the cutting edge temperature, which enhanced chemical wear and binder extrusion to the surface (‘‘sweating”).
Smooth wear (abrasive or erosive). Micro-chipping (from high cutting forces, see Fig. 23). Gross-fracturing (from high normal forces, see Fig. 23). Thermally induced wear or fracture (and fatigue).
This categorisation has been yet further expanded upon by [14] where macroscopic wear is described as being due to four components:
Fig. 18. Example of one modelled geometry with a positive axial rake angle (b) of 15° and a wedge angle (a) of 70°.
Delamination (total loss of the diamond table from the carbide substrate). Heat checking (in the supporting cemented carbide substrate). Worn cutter (abrasive wear flatting, ranked according to severity). A more detailed analysis of actual wear and failure mechanisms [13] shows that the ‘‘worn” and ‘‘broken” cases can actually be further broken down into four different active wear and fracture mechanisms. These are described as:
Abrasion (steady-state wear, on the rake face in soft rocks or flank in hard rocks). Impact shock and fatigue (with impact shock caused by high forces interacting with residual stresses in the PCD and impact fatigue being mechanical fatigue). Impact loading (which can cause macro-failure through tensile stresses). Thermal shock / temperature (intensified micro-chipping wear at >350 °C). From both laboratory cutting experiments and full scale bit tests, [15] reported that various modes of wear and failure were observed, being: Joint failure (delamination of PCD-hardmetal and also failure in hardmetal–hardmetal brazed joints). Gross plastic deformation and shear failure (of the hardmetal or the diamond table, respectively). Chipping and spalling at the cutting edge (micro-chipping). Diamond grain micro-chipping, fracture and pullout (attritional abrasion, crushing and detachment of individual diamond grains).
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Fig. 19. Top: Crack initiation sites considered for the modelling of chipping due to the acting cutting force. From left to right are shown micro-chipping (0.07), meso-chipping (0.27) and macro-chipping (0.50), where the numerical values indicate the distance away from the cutting edge as a proportion of the cutter height (i.e. 0.50 = half way up the cutter). Bottom: Crack initiation sites considered for the modelling of gross fracture due to the acting normal force. From left to right are shown at 0.2, 0.5 and 1.0, where the numerical values indicate the distance away from the cutting edge as a proportion of the diamond table thickness (i.e. 0.50 = half way into the diamond layer). Delamination of the entire diamond table is a special case at 1.0, where the crack runs parallel to the interface just inside the diamond table itself.
Fig. 20. Magnitude of the resultant force acting on a single PCD cutter when drilling in reinforced concrete. At the end-of-life, the bit had drilled 11 m and experienced around 45,000 changing load cycles (concrete aggregate-cement-rebar transitions).
Fig. 21. Direction of the resultant force acting on a PCD cutter when drilling in reinforced concrete.
significant, whole diamond grain pullout became prevalent and also both impact wear of the diamond and abrasive wear of the hardmetal were accelerated. Abrasive wear of the hardmetal (when the wear flat width exceeds the diamond table thickness). Heat checking and thermal fatigue of surface layers (in the hardmetal). At low temperatures, failure of individual diamond grains was primarily by crushing or brittle fracture with only occasional fracture at a larger scale under high impact conditions. (As noted previously in [10], shear mode brittle fracture and fatigue crack growth at the grain size level appear to be the underlying mechanisms of these chipping and fracturing events). At high temperatures (cutting edge >750 °C) where thermal effects become
4.2. Wear mechanisms reported in percussive rock drilling using cemented tungsten carbide hardmetal inserts [16–21] Wear of hardmetals in rock drilling was first characterised in the 1970’s with later work providing a greater level of understanding at both a microscopic and macroscopic level. Wear during rock drilling can take place by a number of macroscopic processes: Surface impact spalling (equivalent to micro-cracking). Surface impact-fatigue spalling (from sub-surface hertzian cracking).
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Mechanical fatigue/overload fracture on a larger scale (catastrophic failure or gross fracturing). These processes can be characterised on a microscopic scale as: Preferential abrasion (erosion) of the binder. Extrusion of the binder to the wear surface. Uprooting of WC grains from the binder phase. Cracking/fragmentation of the (large) WC grains. Attritional abrasion of the WC (‘‘crumbling” of grains by fatigue or multiple cracking events). Spalling under point impact (loss on individual grains or small clusters). Chipping/flaking after crack growth by mechanical and/or thermal fatigue.
Fig. 22. Modelling of extent of wear and the crack length in gross fracturing (parameter ‘‘a”) as a function of the number of changing load cycles. Wear should predominate, although in the early stages (new condition) there is some competition between wear and fracture.
4.3. Wear mechanisms reported in metalworking using cemented tungsten carbide inserts [22,23] In the metalworking industry, cutting insert end-of-life or failure is described by well known and understood tool wear and fracture mechanisms, being: Chipping or gross fracture (localised or total failure caused by excessive stress, mechanical shock, dynamic fatigue loading or as a consequence of wear). Flank wear (abrasion of the clearance face). Cratering (diffusive, chemical wear on the rake face by workpiece chips). Built-up edge (adhesion of workpiece chips by localised pressure welding). Notch wear (localised increased wear at the depth-of-cut position). Thermal cracking (due to thermal cycling fatigue). Wear mechanism maps are commonly used to predict the expected primary wear mechanism and intensity when machining a specific substrate material under varying feeds and speeds. The wear regimes are generally then defined as thermally induced (at high cutting speeds) or mechanically induced (at high feeds). 4.4. Comparison of active PCD wear and fracture mechanisms during core drilling in reinforced concrete (present work) with the cases presented in Sections 4.1 to 4.3
Fig. 23. Schematic representations of gross fracturing and micro-chipping [13].
Thermal fatigue. Abrasion.
It can be loosely stated that with the exception of built-up edge, notch wear and heat-checking (thermal fatigue) of the cemented tungsten carbide substrate, all the wear and fracture mechanisms observed in similar applications are evident to some extent in the rotary drilling of reinforced concrete using PCD-tipped core bits. Although a further detailed analysis at the microstructural scale would be necessary to quantify the relative contribution of
Table 1 Factors affecting the resistance of a cutter to the main failure mechanisms are indicated with a ‘‘Y”. Note that this is a purely qualitative summary of the results of all testing performed under the range of conditions noted in Sections 2.1 to 2.3.
Gross fracturing Micro-cracking Abrasive and chemical wear Delamination
Cutting edge properties
Material properties & microstructure
Y
Y Y Y
Cutter geometry
Cutter internal construction
Y Y
Residual stresses Y Y
Y
Y
Surface treatments Y Y
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each of the microscopic wear mechanisms, there is clear evidence for cracking, fragmentation and uprooting of the larger diamond grains; attritional abrasion (cracking/crushing) of the diamond grains; and spalling of individual or small clusters of grains. The thermal degradation of the diamond–diamond bonds due to the cobalt content of the PCD also plays a role. The large number of tests performed, where many variables were independently varied to determine how they influence the wear and fracture mechanisms and rates, have resulted in the (simplified) summary provided in Table 1. Due to reasons of commercial secrecy, specific details have been omitted. Naturally, the drilling parameters of the tool (drive) also strongly influence the magnitude of the forces acting and also the temperatures generated at the cutting edge and in the bulk of the cutter.
5. Current status and summary of this work 1. Gross fracturing from top and side loading (high normal and radial forces) is the primary failure mechanism, followed by wear and chipping from front loading (high cutting forces) in roughly equal proportions. 2. The primary parameters (geometry, tool, material, etc) that influence each of the active wear and fracture mechanisms have been identified: The material properties and microstructure, along with the edge and surface preparation of the cutters, influence the resistance to micro-cracking and wear. Delamination can be reduced by altering the internal stress state and metallurgy of the PCD-hardmetal near-interface region. Under the load collectives studied, fracture events from the micro-scale to the macro-scale are primarily influenced by the cutter geometry and the internal stress state, with the PCD microstructure only influencing events at the micro-scale. 3. All the mechanisms observed have analogies with known wear and fracture mechanisms from rock drilling and metal cutting. Wear and micro-chipping compete with each other under typical core drilling conditions. 4. Fracture mechanics modelling has shown good agreement with experimental results. As expected, challenges faced in both rock drilling and metal cutting are all present during the coring drilling of steel reinforced concrete, but are even more accentuated by the limited power and rigidity of conventional construction industry drilling systems which have been designed to work with diamond impregnated segment technologies. Significant improvements in cutter lifetime (10 times higher than the first core bits produced) have been realised following this work to quantify and understand the wear and fracture mechanisms and the necessary actions for even further improvements are known. PCD-tipped core bits for dry coring in reinforced concrete are now commercially available for special applications but at the present time it remains a niche market.
Acknowledgements The authors would like to thank the many Hilti employees who have contributed in the various PCD projects over the years, particularly in Corporate Research and Technology and in the business unit ‘‘Diamond”. Thanks are also due to our external partners for the supply and development of the cutting materials, performing the fracture mechanics modelling and providing technical support and many interesting discussions over a number of years. References [1] European Commission Report. ‘‘Decommissioning of nuclear installations in the European union”. EUR-18860-EN, 1998. [2] OECD Nuclear Energy Agency Report. ‘‘The decommissioning and dismantling of nuclear facilities”. 2002. [3] Hilti Internal Report. ‘‘Basics of demolition - diamond”. 2001. [4] Lammer A. ‘‘Designing tool parameters for optimum drilling performance in reinforced concrete”.in: second international industrial diamond conference – ‘‘diamond at work 2”, 19–20 April 2007, Rome, Italy. [5] Moseley SG, Bohn K-P, Goedickemeier M. ‘‘Core drilling in reinforced concrete using polycrystalline diamond (PCD) cutters: performance modelling”, in: international conference on diamond, cubic boron nitride and their applications – ‘‘intertech 2008”, 5–7 May 2008, Orlando, USA. [6] Ingraffea AR, Wawrzynek PA. ‘‘Discrete modeling of crack propagation: theoretical aspects and implementation issues in two and three dimensions”, Report 91-5, School of Civil and Environmental Engineering, Cornell University; 1991. [7] Wawrzynek PA. ‘‘Interactive finite element analysis of fracture processes: an integrated approach”, M.Sc. Thesis, Cornell University, 1991. [8] Hilti Internal Report. Modeling of failure modes and fracture mechanisms in PCD cutters: mechanical behaviour”. 2003. [9] EP1281463. US6902357, US2003026668, DE10137747, JP2003053613. [10] Dunn KJ, Lee M. ‘‘The fracture and fatigue of sintered diamond compact”. J Mat Sci 1979;14:882–90. [11] Fixed Cutter Bit Dull Grading System. International Association of Drilling Contractors (IADC). 1987. [12] Smith International Incorporated. ‘‘Smith tool dull grading manual”. ST-2067, 2001. [13] Lin T-P, Hood M, Cooper GA, Xiaohong L. Wear and failure mechanisms of polycrystalline diamond compact bits. Wear 1992;156(1):133–50. [14] Ersoy A, Waller MD. Wear characteristics of PDC pin and hybrid core bits in rock drilling. Wear 1995;188:150–65. [15] Ortega A, Glowka DA. Studies of the frictional heating of polycrystalline diamond compact drag tools during rock cutting. Sandia National Laboratories; Report SAND80-2677, 1982. p. 10–5. [16] Osburn HJ. Wear Of rock drilling tools. Powder Metall 1969;12(24):471–502. [17] Larsen-Basse J. Wear of hard-metals in rock drilling: a survey of the literature. Powder Metall 1973;16(31):1–31. [18] Perrott CM. Tool materials for drilling and mining. Ann Rev Mater Sci 1979;9:23–50. [19] Fischmeister HF. Development and present status of the science and technology of hardmetals. in: Proceedings of international conference science hard materials 23–28 Aug, Wyoming, USA: 1981. [20] Almond EA, Lay LA, Gee MG. Comparison of sliding and abrasive wear mechanisms in ceramics and cemented carbides. Proceedings of second international conference science of hard materials, 23–28 September, 1984, Rhodes, Greece. [21] Reshetnyack H, Kuybarsepp J. Mechanical properties of hard metals and their erosive wear resistance. Wear 1994;177:185–93. [22] Trent EM, Wright PK. Metal cutting. 4th ed. Butterworth; 2000. ISBN 9780750670692. [23] European Powder Metallurgy Association (EPMA) Training Course ‘‘Hardmetals: Wear, Fracture and Design” from the ‘‘Design for PM elearning platform”, an EU funded project in the ‘‘Leonardo da Vinci” Education and Culture Programme. Compiled from data provided by Sandvik, Kennametal, Ceratizit, Hilti and the Fraunhofer Institute IKTS. Available from: