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Diamond retention in sintered cobalt bonds for stone cutting and drilling
Diamond and Related Materials 8 (1999) 2043–2052 www.elsevier.com/locate/diamond
Diamond retention in sintered cobalt bonds for stone cutting and drilling Steven W. Webb * GE Superabrasives, PO Box 568, 6325 Huntley Road, Worthington, OH 43085, USA Received 10 February 1999; accepted 21 June 1999
Abstract Thin (<1 mm) coatings on diamond do not influence segment density, closed porosity or bulk transverse rupture strength. None of these measures show strong correlation to diamond saw performance. Diamond coatings influence the yield strength of the bond in shear, at the diamond-bond–matrix interface, and more effectively utilize the compressive stress developed in bond fabrication to produce good retention. The improvement in retention, as observed by differential hardness, in 100% cobalt bonds, is Cr-coated>Ti-coated>uncoated. Since retention is only one attribute of a bond, the benefit of a coated crystal in tool performance is system- and condition-specific. Improvement in saw performance will come with reduction in blade angular friction derived from higher and more stable cutting point protrusion. To achieve higher protrusion will require higher retention from nominal or higher bond compressive stress combined with higher diamond–matrix friction. Diamond coatings provide higher friction. Observing improvement in blade performance with a coated crystal requires that the condition/application be performancelimited by retention, not, for example, by acoustic impedance, blade stiffness or crystal crushing. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Diamond; Hardness; Retention; Sawing
1. Introduction Single-crystal diamond cutting points are used in sawing and drilling of hard stone and concrete. The cutting points are bound to a saw blade or drill bit by compression derived from shrinkage upon sintering (contraction) of a metal (glass, plastic) powder forming a ‘bond’. The sintered metal–diamond bond is called a ‘segment’ and is brazed or welded to a blade or drill core and attached to a machine. The forces and geometry of a bound crystal in a segment are shown in Fig. 1. The compressive stress arises from differences in thermal expansion coefficient between the diamond and the matrix augmented by any volume changes due to sintering or crystallization of the matrix. The compressive stress available from the matrix is limited by the yield strength of the matrix. Since yield stress is related to hardness, the hardness of a bond matrix can be indirectly related to retention for fixed diamond–matrix friction/adhesion. * Tel.: +1-614-438-2027. fax: +1-614-438-2235. E-mail address: [email protected] (S.W. Webb)
Scanning electron microscopy (SEM ) micrographs of a bound working crystal cutting point in a saw segment are shown in Fig. 2. Both crystal cutting point and bond are damaged after substantial wear (~5000 impacts) under a predominantly normal load (normal/tangential impact is about six to ten for sawing, two to four for grinding) typical of sawing hard stone. As described [1], most cutting point damage is confined to back-edge ablation due to reflected dynamic tensile strain exacerbated by poor bond contact. Both surface attrition at the leading edge and subsurface conchoidal fractures at the back edge are typical of diamond fatigue. From the top view of the leading edge one can see the effect of debonding of the matrix from the crystal. The tangential impact bend load is deadhering and wearing the matrix–diamond crystal interface. Eventually the bond will no longer hold the crystal and it will ‘popout’ or prematurely fail. For sawing and drilling in hard stone and concrete, where dynamic contact loads per cutting crystal can be >1 GPa, metal bonds derived from sintered metal powders are used. Metal powders and processes are selected
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Fig. 1. Forces on working diamond cutting point in a drill or saw blade; force, velocity relative to machine table.
to create adequate retention for a target application. For lapping and grinding, with lower contact loads per point, softer resin or vitrified bonds suffice. Nevertheless, tools using vitrified and resin bonds usually benefit from a coated diamond cutting point (typically metal-coated, e.g. electroless Ni or Cu) to increase retention. The compressive stress offered with quenched glass or cured resins is small relative to sintered metals. To convert compressive stress in segment fabrication to retention requires diamond–matrix friction. The necessary level of friction and compressive stress depends on the anticipated contact load. Coating the diamond is a good way to control friction. The effective use of ultra-high grade ( UHG) diamond crystals in severe applications requires the highest retention. Higher cutting point quality permits a higher cutting rate, for fixed tool power, or lower power use for a fixed cutting rate, with nominal tool wear rate. This performance is valuable for applications such as road renovation or deep drilling, where cutting speed and tool durability and reliability are paramount. The benefit of UHG crystal cutting points is derived from the higher tolerable impact load between crystal and workpiece, resulting in higher material removal rates with nominal or lower wear rate. Both crystal and bond accommodate a higher contact stress. If the bond fatigues, or fails prematurely, the benefit of UHG cutting points is squandered. This work describes a method for evaluating retention and demonstrates how coatings on diamond can improve retention and tool performance.
2. Background Bond quality can be described by three main attributes: retention, strain transparency or acoustic impedance and abrasion rate.
2.1. Retention Retention R may be quantified as a ratio
R=
(contact area)(matrix compressive stress) (diamond–matrix friction) contact force
,
where contact area refers to that between matrix and diamond (Fig. 1). High protrusion, smooth and coarsemesh crystals reduce contact area. If R>1 the crystal is well retained. The life and power of the tool will be determined by the dynamic solution of the ablation rate of the cutting points and the abrasion rate of the bond. The ablation rate of the cutting points depends on their toughness index (denoted TI ) or compressive fracture strength (denoted CFS ) relative to the dynamic contact stress and spectral content of the strain. TI is a diamondindustry standard accelerated-wear test observing ablation from cycle-controlled, medium strain-rate (relative to sawing), point contact. CFS observes single-crystal crushing failure in low-strain-rate blunt compression. The ratio of contact load to CFS is a ‘stress intensity factor’, which, when plotted versus cycles of contact, can be correlated to fatigue life. For example, a typical diamond cutting point in stone cutting maintains protrusion or ‘sharpness’ for ~30 000 impacts. Failure occurs when accumulated damage, mainly apparent as externalsurface roughness, and internal micro-cracks organize and penetrate the body of the crystal, eventually cleaving it. High stress-intensity and low TI implies rapid fatigue. Force transducers in single-crystal saws put the contact load in sawing Class V (hard) bright-red granite at a rate of 300 cm2/min at ~190 N per crystal, with a strain period of ~300 kHz, oriented 80–85° normal to the stone surface, applied at ~40 Hz intervals (blade rpm). The spectral content of strain is centered on ~v/p, where v is the tool peripheral speed and p is
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(a)
(b) Fig. 2. SEM micrographs of working diamond cutting point in a saw blade: (a) top view, showing back-edge ablation; (b) side view, showing protrusion and back-edge ablation.
the cutting point protrusion, typically 10–40% of the crystal diameter, depending on workpiece or the ‘bluntness’ of the contact. In interrupted cuts, where the tool engages/disengages the workpiece, the spectral content broadens. Higher frequencies are more absorbed by the cutting point and workpiece. At nominal load, high frequency will lead to rapid ablation ‘polishing’ of the cutting points. High wear and polishing ‘blunt’ the blade, increasing the angular friction and tool power. The intense strain wave passing into the stone is surface shear (Raleigh) with bulk, reflected compression/ tension. Using 18 km/s as the mean velocity, the strain
wavelength is ~60 mm. This wavelength will disperse only at flat surfaces (curvature >60 mm), such as cracks and external crystal facets. The dynamic contact stress supported by the bond can be ~1 GPa. This dynamic stress is above the strength of dense steel, but below the compressive strength of UHG diamond (4–5 GPa). Based on static crushing, the stress intensity factor sawing granite at 300 cm2/min is only 1/4=25% for the cutting point, consistent with the ~30 000 cycles-to-failure (typical wear rate ~10 mm/min). The stress intensity factor for bonds is >300%, implying rapid fatigue of the bond–
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crystal interface. However, this analysis neglects important strain absorption by crystal ablation and transmission to the tool frame and motors. To address retention problems without changing powder, powder processing or restricting blade use, crystal coatings are used. The capability of a coating to improve retention depends on the nature of the coating (e.g. completeness, thickness, stress, adhesion, density, bonding, solid-state wetting, etc.) and requires understanding of how coatings effect retention. 2.2. Strain transparency (acoustic impedance) Cutting point ablation is important to limit bond yield. Facile transmission and period lengthening of ~300 kHz contact strain from the cutting point edge to the tool shaft, motor and frame is also essential to performance. The diamond cutting point acts as a ‘lens’ for strain, much like glass as a lens for light. The idea is to pass energy of the widest bandwidth with the least absorption, dispersion, scatter, or reflection. An inefficient cutting point absorbs strain and ablates like an inefficient lens absorbs light and gets hot. The more strain energy passed into the tool, the less ablation the cutting points must endure. The energy passed into the tool (or optical system) is defocused, the period lengthened and absorbed there with less cost (i.e. heat in motor, vibration in the frame). Transmitting the strain is better than using the cutting points as an ablative. However, at the contact edge the strain amplitude is the most intense, and the strain period is shortest. Absorption/ablation must be managed by designing a capability for strain-energy absorption into the cutting point. This is accomplished by controlling defects as strain and density gradients within, and on, the cutting point. With proper control of strain gradients the diamond will act as an ablative. Cracks formed at the contact surface are arrested, absorbed or diverted back to the external surface, where they remove strain energy as debris or heat. With poor control over strain gradients cracks do not return to the surface. Ablation removes less strain energy per unit of wear. Neglecting system effects, the strain energy absorption per micrometer of wear defines the cutting point quality [2,3]. Although single-crystal diamond, as a distinct phase, is not unique, the defect and strain gradient structures in different grades and types of diamond crystal are unique and define utility. The strain–optical properties of the varied defects are critical to the ablative and transmissive properties of the cutting point. Defects typically include inclusions, microcracks, roughness, substituents such as nitrogen, and dislocation/twin structures. Their type, number density and spatial correlation are important. The transmission of dynamic tensile/compressive and
shear strain is a problem of acoustics. Acoustic impedance is a propagation velocity v gradient dv d(앀E/r) impedance¬ # , dx dx discounting the effects of curvature, wavelength-sensitivity and dispersion along a surface. Impedance is sensitive to density gradients in the strain path. The most problematic gradients are sharp-cracks. A crack in, or on, the cutting point, or at the interface due a debonded, partially failed crystal or void, poses severe impedance and catalyzes cutting point fatigue. Indeed, the absorption/reflection of strain energy by cracks, with downshift of transmitted energy, is used for crack detection and diagnosis [4]. The presence of an adherent, graded-stiffness or -density phase at the interface can improve transparency. 2.3. Abrasion rate The bond matrix is worn by contact with stone debris dragged by cutting fluid at a relative speed of ~20– 60 m/s in a gap of ~50–100 mm (protrusion). This shear stress produces a hydrodynamic force ‘floating’ the blade or bit (acting as a lubricant/coolant) and a drag force eroding the blade and helping pump debris out of the cut. In order to maintain a stable number of cutting points and protrusion, the dynamic solution to −
dp dt
#crystal wear rate−bond wear rate,
(2)
where p, the protrusion normalized by the mean mesh size of the cutting point, must be bounded and stable. The unbounded, unstable solutions, p=0 and p>mesh size, describe overwhelmed (crushed crystals, popouts) blades. Bond wear rate (micrometers/minute) must approach the crystal wear rate to achieve a stable solution. The crystal wear rate depends on the stress intensity and protrusion. A higher protrusion means a higher total and normal load with a higher wear rate for fixed diamond quality. The bond wear rate depends on the cutting fluid viscosity, the stone debris type and the concentration, depth-of-cut, bond hardness, exposed bond area (i.e. concentration of cutting points) and protrusion. A higher protrusion keeps the soft bond away from the hard stone, thus lowering the bond wear rate. The most common solution to Eq. (2) is a multiperiod, short-duration oscillation from observable dulling and sharpening cycles [1]. For a badly operated blade the periods will be few and long; dressing or other intervention is required. For overwhelmed blades, monotonic dulling to stall may occur. Depth-of-cut may be used to adjust the bond wear
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rate (at a fixed cutting rate) to stabilize protrusion if bond retention or machine torque are limiting performance or to set protrusion for a desired surface finish or to mitigate thermal damage. Clearly, proper diagnosis of the state of the blade is required to know what adjustments to retention, transmission or abrasion rate are required. The cutting point acts as an important, but not self-sufficient, part of a system of strain energy transfer. 2.4. Hardness A proposed method of evaluating retention is called ‘differential hardness’. This is a similar concept to fiber push or pull testing [5]. The hardness of the matrix is compared with that of the bond under a bound diamond. A large and negative difference in hardness implies poor retention. With this technique it is possible to evaluate the retention of cutting points at edges versus the middle of the segment (e.g. die wall effects) and the effect of clustering of crystals.
3. Experimental Diamond–cobalt fired PM powder segments were machined to ~20 mm flatness using a 170/200 RVG diamond wheel. A flat surface (relative to impresser curvature) is necessary in order to avoid bending indentors and altering deformation. A blunted 120° diamond indentor, with 60 kg load, Rockwell C scale was used to evaluate hardness on and off the diamonds. Since the indentor makes a hole of about the same diameter as the exposed and bound diamonds (~0.1–0.3 mm) it is not necessary to align the indentor on the bound diamond. Provided the impression is of a similar scale to the diamond, and the diamond crystal either does not fail or fails completely, it is the metal under and around the diamond that provides resistance to indentation, not the diamond. The presence of the bound diamond disturbs the strain field under the indentor, but, on average, the relative quality of the bond matrix is probed. The depth of penetration at 60 kg load is ~1/3 of the diamond-diameter, so the effect of machining damage is minimal. The diamond crystals are severely damaged by the machining. The damage is helpful in two ways: (1) the diamonds will completely yield to the indentor, and (2) there will be pre-existing damage to the diamond–matrix interface, similar to that in a real tool application. Any detachment, deadhesion or porosity at the interface, originally present or created by machining, will be easier to observe. All bonds were prepared using the same cobalt metal powder ( Euro-Tungsten grade UF denoted ‘CoUF’), blending, firing (2 h in H gas at 500–600°C ) and hot2 pressing (Fritsch press) procedures. The diamond types
Fig. 3. SEM of surface of indented machined diamond-containing segment: uncoated crystal, 225×; center of indentation is to left-center of image.
were MBS1960 diamond, 45/50 mesh (~0.3 mm diameter) uncoated, titanium ‘Ti’-coated and chromium ‘Cr’coated. The coatings were uniformly thick and complete, and applied using proprietary GE technology. All segments were 1.5 wt% diamond. The segments were 2×4×10 mm3 machined on all sides. The method was to locate a diamond, visually align the indentor on that diamond, indent, then move the indentor over the matrix, indent, alternately, monitoring the matrix hardness to diagnose indentor changes and differencing only alternating hardness values. As a baseline, non-diamond-containing segments were examined as well. Saw blade tests were also done to confirm the hardness measures; details of that are described within this text.
4. Results 4.1. SEM of machined surfaces Figs. 3–5 show images of the machined, indented diamond crystals, with no coating, Ti and Cr coatings after acid etching to clean debris and metal post-machining. In all cases the diamond crystal is shattered, pushed down and detached from the metal bond. The planar, non-porous, cleanly detached walls of the bond after impression suggest no direct chemical or physical bonding of crystal, or coating, to the metal matrix. Any intermediate phases (e.g. alloys, intermetallics, carbides) of yield strength greater than retention, as tested by indentation, are well bonded to the diamond, not the cobalt. Almost every indentation produced audible ‘cracking’. This noise was not indentor cracking, as the 1 Trademark of General Electric Company, USA.
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Fig. 4. SEM of surface of indented machined diamond-containing segment: Ti-coated crystal, 225×.
(a)
(b)
Fig. 5. SEM of surface of indented machined diamond-containing segment: Cr-coated crystal, 439×; note clean slipping and failure of crystal–bond interface as well as interrupted fracture of crystal.
reference hardness of the matrix was unchanged. Every bound, machined diamond crystal failed under indentation loading. A 3 mm diameter spherical WC ball indentor at 60 kg was tried, but it gave poor discrimination. The indentations were too shallow to sample the region under the bound diamond crystals. Crystals were also crushed by the relatively soft WC indentor, confirming extensive machining damage. Figs. 6 and 7 show EDAX Cr and Ti maps for coated crystals prior to indentation. There is a distinct interface between sintered Co (Cr or Ti<500 ppm) and diamond (Cr or Ti<20 ppm) that is rich in Cr or Ti. This interface is due to the coating; ~1.2 mm for Ti and 1.75 mm for Cr. Although smearing and relief may have occurred in machining, there is no evidence for bulk diffusion of the coating into the bond or diamond during sintering. The coating is a distinct 1–2 mm phase between diamond and matrix cobalt, bound to the diamond. The action
Fig. 6. SEM image (a) and EDAX Cr map (b) of the surface of a machined diamond-containing segment: Cr-coated crystal.
of the coating is local to the crystal, influencing impedance or adhesion/friction at the diamond–bond interface. The SEM contrast observed from the interface suggests that the coating is a distinct phase. This phase has been verified by X-ray diffraction to be a carbide [6 ]. To act as an acoustic transmissive layer, chemical barrier, or friction surface the coating must be dense and integral. If the coating debonds or cracks its effect is worse than if it were not present. Use of coatings can be risky if coating quality is not closely observed. Minimal stress in the coating with good solid-state wetting is required. These are easier to achieve with thin coatings if uniform thickness and integrity can be achieved. Finally, temperature and chemistries that may degrade the diamond, and/or coating, restrain available coating processes. 4.2. Differential hardness Table 1 shows the results of the differential hardness test. The bond under the crystal is weaker than the
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matrix. The hardness of the diamond-free segments was the same as the matrix in all diamond-containing segments. The presence of diamond at 1.5 wt% does not inhibit bulk sintering; this may not be true at higher diamond volume fractions. The variation in hardness under the diamond is higher than the matrix owing to variable indentation geometry. The uncoated segments show the highest difference hardness, −10%, indicative of weak retention. For this cobalt powder, the Cr coating provides the highest retention, +1.6%. The Cr coating reinforces the segment under the diamond. Ti coating is not as good as Cr, −3%, but better than uncoated diamond. The Cr coating was so strong that the indentor tips frequently cracked (tips failed before the bond ). Tip cracking never occurred with uncoated or Ti coated segments. The strength of the Cr coating must derive from a new, higher-strength surface at the diamond–matrix interface.
(a)
4.3. Differential density
(b) Fig. 7. SEM image (a) and EDAX Ti map (b) of the surface of a machined diamond-containing segment: Ti-coated crystal.
To test if density was controlling the hardness of a segment, the water and gas (He pycnometry) density was measured ( Table 2). The difference between gas and water density is an indicator of microporosity (water does not wet metal relative to gas). The presence of diamond retards densification by increasing macropores by ~2×. There is no coating effect on density or porosity. The sensitivity of differential hardness to coating is not due to densification, e.g. improved pressure transmission or inhibited gas formation (metal oxide+carbonCO) in sintering. The improvement may be due to improved friction/adhesion from the coating.
Table 1 Hardness (Rockwell C scale) of diamond and matrix sintered metal Uncoated
Average hardness Standard deviation Difference No. of samples
Cr-coated
Ti-coated
Diamond-free segment
Diamond
Matrix
Diamond
Matrix
Diamond
Matrix
62.4333 4.820
68.639 1.551 6.206 18
71.667 4.933
70.500 0.707 −1.167 2
65.440 4.239
67.552 2.004 2.112 23
18
3
15
67.6 0.699 10
Table 2 Density of uncoated, coated-diamond and diamond-free segments: theoretical density of cobalt 8.92 g/cm3; ET CoUF cobalt powder; theoretical density of 15conc segments 8.719 g/cm3 Difference in % density
Density (%)
Uncoated Cr Ti CoUF
Water
Gas
Standard deviation
Water
Gas
Gas−water
95.3 94.9 94.6 97.8
95.9 95.7 95.9 100.2
0.2 0.9 1.0
2.5 2.9 3.2
4.1 4.3 4.1
0.6 0.8 1.3 2.2
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Table 3 TRS of segments with coated crystals; three-point bend to fail
Uncoated Cr-coated Ti-coated CoUF (diamond-free)
Mean (GPa)
Standard deviation (GPa)
1.03 1.07 1.01 1.41
0.13 0.09 0.08 0.03
4.4. Transverse rupture strength (TRS) To investigate the tensile strength of the coated interface the TRS (three-point bend to fail ) of the segments was evaluated ( Table 3). Adding diamond weakens the segments in tension, not by effecting density, but by adding weakly bonded surfaces. The coated interface did not improve the bulk tensile strength. Improvement in retention with Cr coating lies with an increase in shear strength, not tensile strength, due to friction at the diamond–matrix interface. 4.5. Powder processing Retention is derived from the product of compressive stress in the matrix and diamond–matrix friction producing a support force countering the contact load (Fig. 1) and thus minimizing interface yield and fatigue. A highquality coating improves friction. However, if compressive stress is reduced owing to processing, performance of the coating is wasted. To investigate this, several powder processes were tested using test diamond blades. Fired, unfired, granulated/non-granulated, nominal (Co6101) and ultrafine (CoUF ) cobalt powders (all Eurotungsten ‘ET’) were saw tested with the same crystal concentration, uncoated UHG crystals, blades, depthof-cut, traverse rate, tip speed, machine, stone and cutting rate. Segments were prepared as described above. The cutting rate was set high (relative to bond hardness) to create retention as the performance-limiting effect. We used a small 7◊ diameter blade attached to a grinder, rather than a conventional diamond saw, to minimize machine/blade deformation. The key saw performance observables [1] include machine power (tangential blade force for fixed
speed) w, the inverse blade angular friction coefficient (tangential/normal blade force, 1/tan w), a measure of cutting efficiency and b, the radial location of the average blade force against the stone, constrained by depth-ofcut and blade diameter. Large b and w imply a blade cutting well, with high protrusion, low wear and low power. The observation that b and w are correlated [1] implies that high protrusion produces low blade friction. High protrusion directs the contact force into the blade, thereby reducing torque and machine power. It also helps keep the soft bond/blade off the hard workpiece and abrasive debris, thus improving blade life. Since b and w are dimensionless, they are insensitive to the number of cutting points and contact area. Table 4 shows that saw performance is not related to segment density. As shown earlier, density is not well related to retention either. Although density is necessary to make high-quality segments, it is not sufficient to predict tool performance. Granulating the cobalt powder prior to making segments produced poor saw performance. Granulation is typically used (with compaction) to reduce density gradients in green bodies to limit shrinkage and distortion in sintering. A coarser cobalt powder (ET Co6101) also produced lower saw performance. Coarse particles and granulation apparently reduce compressive stress, thus impairing retention. Firing the cobalt powder (in hydrogen) produced marginally better segments, presumably by reducing surface oxides that reduce contact area at the diamond–matrix interface. 4.6. Saw testing 4.6.1. Calibration If saw performance is limited by machine/blade stiffness, crystal crushing (i.e. non-ablative wear) or severe abrasion rate mismatch, a coating will have no effect. If retention or impedance limits performance, a coating may help. Performance limits are sensitive to condition, e.g. stone type, depth-of-cut, tool speed and available torque, spindle stiffness and cutting rate, etc. In a multidimensional problem, such as sawing hard stone, it is very easy to miss the operating window in which any single effect will be controlling. Having selected the best powder process and coating
Table 4 Saw testing of varied cobalt bond processes: ET CoUF cobalt; density under water; 7.5 mm depth-of-cut, 300 cm2/min, bright-red granite (Class V ), cutting rate/wear rate ratio ~2.28 m2/mm; average performance over ~200×0.6 m stone passes; 30 m/s speed Powder process
Density (%)
b (deg)
w (deg)
Power (kW )
Fired Unfired Coarse-fired Granulated-fired Granulated-unfired
96.6 95.4 97.2 96.3 95.4
76.6 76.4 75.9 75.3 74.6
80.7 80.5 80.1 78.3 78.2
1.2 1.2 1.3 1.3 1.1
Standard deviation
1.6
6.0
0.5
0.3
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Table 5 Saw test results of lower-grade uncoated diamond crystals using same conditions and bond: Barre granite, 35/40, 25conc; three replicates×two blades; minimum 200×0.6 m passes; ET CoUF cobalt, depth-of-cut=10 mm, cutting rate =300 cm2/min Crystal
b (deg)
w (deg)
Power (kW )
Wear performance (m2/mm)
MBS940 diamond
75.3 74.8 74.7 74.9 0.3
81.6 82.8 81.9 82.1 0.6
1.3 1.4 1.2 1.3 0.1
6.06 7.34
74.2 74.8 74.8 74.6 0.3
81.7 81.0 81.9 81.7 0.5
1.2 1.2 1.3 1.3 0.1
5.86
Average Standard deviation MBS930 diamond
Average Standard deviation
it remains to verify the effect of coating in a saw test. To do so requires calibration of the test platform and evaluation of test sensitivity. To demonstrate contrast of the test system, lower grade crystal products, MBS940 diamond and MBS930 diamond, were tested with the same cutting rate and depth-of-cut, replicate blades, ET CoUF cobalt in soft granite (Barre). The choice of soft stone and lower-grade crystals was to ensure good retention and machine stiffness, thus isolating the effect of crystal grade. ET CoUF cobalt retains
7.13 0.27
6.17 6.01 0.21
1. a high contact force condition ( low crystal concentration and high cutting rate) or, 2. an abrasive condition leading to lower bond area ( Fig. 1). A severe application will test system effects such as machine stiffness and crystal crush strength. With higher crystal wear rate a closer match of bond wear rate will be required, else the blade will develop long dulling cycles and render mean performance discrimination difficult to ascertain. 4.6.2. Evaluation of coating in hard application Saw tests with 45/50 Cr-coated UHG crystals were performed in hard bright-red granite at a relatively high cutting rate of 300 cm2/min. MBS950 diamond is used as a reference uncoated HG product to verify the test. Several depths-of-cut were tested to find the operating window for best discrimination. The results shown are averages. The time-variations in forces and angles were collected to evaluate blade dynamic stability (dull–sharp cycles) but are not shown. A blade with dull–sharp period of >6 m2 of stone was considered unstable. The results are shown in Table 6. 300 cm2/min in soft Barre granite with an HG diamond product results in ~82° of angular friction ( Table 5). The same cutting rate in harder red-granite with UHG diamond produces the same 82° of friction. At a fixed grade, increasing the
Table 6 Saw test results: average ~200×0.6 m stone passes; bright-red granite; 30 m/s speed Crystal type
Depth-of-cut (mm)
Cutting rate (cm2/min)
b (deg)
w (deg)
Power (kW )
UHG Cr
10 7.5 7.5 15 7.5 10 7.5 10 7.5 5
300 150 150 450 300 300 150 200 150 100
74.5 76.9 76.5 69.5 75.9 75.1 76.9 71.6 73.3 76.4
82.0 85.1 85.0 74.8 80.4 82.2 85.1 79.8 83.8 84.1
1.8 1.6 1.5 2.2 1.4 1.8 1.6 1.6 1.8 1.2
UHG uncoated
MBS950 diamond
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cutting rate reduces the angular blade friction. The power increases with grade (and protrusion), but not as consistently as blade friction. The blade friction is a more sensitive performance indicator. Indeed, the power samples only ~1/3 of the total force on the blade. The out-of-plane force measures deflection and deviation and is also a performance variable [1], but it is a much smaller effect. Replicate blades are within 0.1° and 0.1 kW. The benefit of increasing depth-of-cut is shown. At 300 cm2/min ( UHG crystals), moving from 7.5 mm to 10 mm increases the average normal component of the load, increasing total blade force, but improving efficiency. This is accomplished by diverting more force into the blade spindle owing to more favorable kinematics and higher, stable and steady protrusion. 450 cm2/min showed a precipitous increase in angular friction, indicative of large loss in protrusion due to crushing. The contact load, for uncoated or coated UHG crystals, at 300–400 cm2/min is too severe. Crushing, not retention is limiting the performance. The effect of Cr coating is not going to be observable in this condition range. 4.6.3. Saw testing — evaluation of coating in abrasive application Another method to create retention-limiting performance is to increase substantially the bond wear rate at a lower contact load (no crushing or machine deflections). By increasing bond wear rate, protrusion will tend to increase, minimizing the compressive ‘grip’ on the cutting points. One method to increase bond wear rate is to go to a high-cutting rate, soft, but abrasive workpiece, such as granite-aggregate concrete. This test was performed with uncoated and Ti-coated MBS960 diamond crystals. For concrete cutting, constant power machines are common, thus cutting rate, not power, is a variable. Table 7 shows that the cutting rate with the coated diamond was lower than uncoated from the higher total forces derived from higher, stable protrusion. However, the 300% improvement in wear resistance far surpasses the 20% decrease in cutting rate. Observation of the worn cutting points confirmed that crushing was a not Table 7 Concrete cutting test: Tysaman machine; 46 m/s speed; depth-of-cut 63 mm; power 20/50 hp available; 55 crystals/cm2 visible
Cutting rate (cm2/min) Wear performance (m2/mm) Wear rate (mm/min) Crystal count (% of visible) wholes/working Crushed popouts
a limitation and that the average number of working cutting points was higher with coated diamond.
5. Conclusions Thin coatings on diamond do not influence segment density, closed porosity or bulk TRS. None of these measures shows a strong correlation with diamond saw performance. Diamond coatings influence the yield strength of the bond in shear, at the diamond–bond matrix interface, and more effectively utilize the compressive stress developed in bond fabrication to produce good retention. The improvement in crystal cutting point retention, as observed by differential hardness, in 100% cobalt PM bonds, is Cr-coated>Ti-coated> uncoated. Since retention is only one attribute of a bond, the benefit of a coated crystal in tool performance is system- and condition-specific. Improvement in saw performance will come with reduction in blade angular friction derived from higher and stable cutting point protrusion. To achieve higher protrusion will require higher retention from nominal or higher bond compressive stress combined with higher diamond–matrix friction. Diamond coatings provide higher friction. Observing improvement in blade performance with a coated crystal requires that the condition/application be performance-limited by retention, not, for example, by acoustic impedance, blade or machine stiffness or crystal crushing. The differential hardness test requires an indentor of radius less than the bound and exposed diamond crystal, a relatively soft bond or weak bond–crystal interface. Careful attention to the indentor tip is required and must be compensated for in the differencing.
Acknowledgements The assistance of technical experts within GE Superabrasives is gratefully acknowledged including M. Wright, T. Whittington, D. Turner, D. Dawley, T. Wilson, J. Adjunta, M. Loh, G. Braeuniger, S. Hayden and S. Snyder. The author thanks GE and GE Superabrasives for permission to publish this work.
MBS960 diamond
MBS960 diamond — Ti coated
References
3780 16.6 22.7
3058 39.1 7.8
50 21 29
59 24 17
[1] S.W. Webb, W.E. Jackson, J. Manuf. Eng. Sci. 109 (1998) 94. [2] J.J. Mecholsky, R. Linhart, B.D. Kwitkin, J. Mater. Res. 13 (1998) 3153. [3] W.E. Jackson, S.W. Webb, J. Mater. Res. 12 (1997) 1646. [4] J.L. Rose, R.M. German, K.F. Hens, S.M. Menon, Proc. Am. Control Conf. 10 (1995) 1732. [5] N.P. Bansal, J.I. Eldridge, J. Mater. Res. 13 (1998) 1530. [6 ] E.S.K. Menon, I. Dutta, J. Mater. Res. 14 (1999) 565.
Diamond retention in sintered cobalt bonds for stone cutting and drilling Steven W. Webb * GE Superabrasives, PO Box 568, 6325 Huntley Road, Worthington, OH 43085, USA Received 10 February 1999; accepted 21 June 1999
Abstract Thin (<1 mm) coatings on diamond do not influence segment density, closed porosity or bulk transverse rupture strength. None of these measures show strong correlation to diamond saw performance. Diamond coatings influence the yield strength of the bond in shear, at the diamond-bond–matrix interface, and more effectively utilize the compressive stress developed in bond fabrication to produce good retention. The improvement in retention, as observed by differential hardness, in 100% cobalt bonds, is Cr-coated>Ti-coated>uncoated. Since retention is only one attribute of a bond, the benefit of a coated crystal in tool performance is system- and condition-specific. Improvement in saw performance will come with reduction in blade angular friction derived from higher and more stable cutting point protrusion. To achieve higher protrusion will require higher retention from nominal or higher bond compressive stress combined with higher diamond–matrix friction. Diamond coatings provide higher friction. Observing improvement in blade performance with a coated crystal requires that the condition/application be performancelimited by retention, not, for example, by acoustic impedance, blade stiffness or crystal crushing. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Diamond; Hardness; Retention; Sawing
1. Introduction Single-crystal diamond cutting points are used in sawing and drilling of hard stone and concrete. The cutting points are bound to a saw blade or drill bit by compression derived from shrinkage upon sintering (contraction) of a metal (glass, plastic) powder forming a ‘bond’. The sintered metal–diamond bond is called a ‘segment’ and is brazed or welded to a blade or drill core and attached to a machine. The forces and geometry of a bound crystal in a segment are shown in Fig. 1. The compressive stress arises from differences in thermal expansion coefficient between the diamond and the matrix augmented by any volume changes due to sintering or crystallization of the matrix. The compressive stress available from the matrix is limited by the yield strength of the matrix. Since yield stress is related to hardness, the hardness of a bond matrix can be indirectly related to retention for fixed diamond–matrix friction/adhesion. * Tel.: +1-614-438-2027. fax: +1-614-438-2235. E-mail address: [email protected] (S.W. Webb)
Scanning electron microscopy (SEM ) micrographs of a bound working crystal cutting point in a saw segment are shown in Fig. 2. Both crystal cutting point and bond are damaged after substantial wear (~5000 impacts) under a predominantly normal load (normal/tangential impact is about six to ten for sawing, two to four for grinding) typical of sawing hard stone. As described [1], most cutting point damage is confined to back-edge ablation due to reflected dynamic tensile strain exacerbated by poor bond contact. Both surface attrition at the leading edge and subsurface conchoidal fractures at the back edge are typical of diamond fatigue. From the top view of the leading edge one can see the effect of debonding of the matrix from the crystal. The tangential impact bend load is deadhering and wearing the matrix–diamond crystal interface. Eventually the bond will no longer hold the crystal and it will ‘popout’ or prematurely fail. For sawing and drilling in hard stone and concrete, where dynamic contact loads per cutting crystal can be >1 GPa, metal bonds derived from sintered metal powders are used. Metal powders and processes are selected
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 25 - 9 63 5 ( 9 9 ) 00 1 6 7- 3
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Fig. 1. Forces on working diamond cutting point in a drill or saw blade; force, velocity relative to machine table.
to create adequate retention for a target application. For lapping and grinding, with lower contact loads per point, softer resin or vitrified bonds suffice. Nevertheless, tools using vitrified and resin bonds usually benefit from a coated diamond cutting point (typically metal-coated, e.g. electroless Ni or Cu) to increase retention. The compressive stress offered with quenched glass or cured resins is small relative to sintered metals. To convert compressive stress in segment fabrication to retention requires diamond–matrix friction. The necessary level of friction and compressive stress depends on the anticipated contact load. Coating the diamond is a good way to control friction. The effective use of ultra-high grade ( UHG) diamond crystals in severe applications requires the highest retention. Higher cutting point quality permits a higher cutting rate, for fixed tool power, or lower power use for a fixed cutting rate, with nominal tool wear rate. This performance is valuable for applications such as road renovation or deep drilling, where cutting speed and tool durability and reliability are paramount. The benefit of UHG crystal cutting points is derived from the higher tolerable impact load between crystal and workpiece, resulting in higher material removal rates with nominal or lower wear rate. Both crystal and bond accommodate a higher contact stress. If the bond fatigues, or fails prematurely, the benefit of UHG cutting points is squandered. This work describes a method for evaluating retention and demonstrates how coatings on diamond can improve retention and tool performance.
2. Background Bond quality can be described by three main attributes: retention, strain transparency or acoustic impedance and abrasion rate.
2.1. Retention Retention R may be quantified as a ratio
R=
(contact area)(matrix compressive stress) (diamond–matrix friction) contact force
,
where contact area refers to that between matrix and diamond (Fig. 1). High protrusion, smooth and coarsemesh crystals reduce contact area. If R>1 the crystal is well retained. The life and power of the tool will be determined by the dynamic solution of the ablation rate of the cutting points and the abrasion rate of the bond. The ablation rate of the cutting points depends on their toughness index (denoted TI ) or compressive fracture strength (denoted CFS ) relative to the dynamic contact stress and spectral content of the strain. TI is a diamondindustry standard accelerated-wear test observing ablation from cycle-controlled, medium strain-rate (relative to sawing), point contact. CFS observes single-crystal crushing failure in low-strain-rate blunt compression. The ratio of contact load to CFS is a ‘stress intensity factor’, which, when plotted versus cycles of contact, can be correlated to fatigue life. For example, a typical diamond cutting point in stone cutting maintains protrusion or ‘sharpness’ for ~30 000 impacts. Failure occurs when accumulated damage, mainly apparent as externalsurface roughness, and internal micro-cracks organize and penetrate the body of the crystal, eventually cleaving it. High stress-intensity and low TI implies rapid fatigue. Force transducers in single-crystal saws put the contact load in sawing Class V (hard) bright-red granite at a rate of 300 cm2/min at ~190 N per crystal, with a strain period of ~300 kHz, oriented 80–85° normal to the stone surface, applied at ~40 Hz intervals (blade rpm). The spectral content of strain is centered on ~v/p, where v is the tool peripheral speed and p is
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(a)
(b) Fig. 2. SEM micrographs of working diamond cutting point in a saw blade: (a) top view, showing back-edge ablation; (b) side view, showing protrusion and back-edge ablation.
the cutting point protrusion, typically 10–40% of the crystal diameter, depending on workpiece or the ‘bluntness’ of the contact. In interrupted cuts, where the tool engages/disengages the workpiece, the spectral content broadens. Higher frequencies are more absorbed by the cutting point and workpiece. At nominal load, high frequency will lead to rapid ablation ‘polishing’ of the cutting points. High wear and polishing ‘blunt’ the blade, increasing the angular friction and tool power. The intense strain wave passing into the stone is surface shear (Raleigh) with bulk, reflected compression/ tension. Using 18 km/s as the mean velocity, the strain
wavelength is ~60 mm. This wavelength will disperse only at flat surfaces (curvature >60 mm), such as cracks and external crystal facets. The dynamic contact stress supported by the bond can be ~1 GPa. This dynamic stress is above the strength of dense steel, but below the compressive strength of UHG diamond (4–5 GPa). Based on static crushing, the stress intensity factor sawing granite at 300 cm2/min is only 1/4=25% for the cutting point, consistent with the ~30 000 cycles-to-failure (typical wear rate ~10 mm/min). The stress intensity factor for bonds is >300%, implying rapid fatigue of the bond–
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crystal interface. However, this analysis neglects important strain absorption by crystal ablation and transmission to the tool frame and motors. To address retention problems without changing powder, powder processing or restricting blade use, crystal coatings are used. The capability of a coating to improve retention depends on the nature of the coating (e.g. completeness, thickness, stress, adhesion, density, bonding, solid-state wetting, etc.) and requires understanding of how coatings effect retention. 2.2. Strain transparency (acoustic impedance) Cutting point ablation is important to limit bond yield. Facile transmission and period lengthening of ~300 kHz contact strain from the cutting point edge to the tool shaft, motor and frame is also essential to performance. The diamond cutting point acts as a ‘lens’ for strain, much like glass as a lens for light. The idea is to pass energy of the widest bandwidth with the least absorption, dispersion, scatter, or reflection. An inefficient cutting point absorbs strain and ablates like an inefficient lens absorbs light and gets hot. The more strain energy passed into the tool, the less ablation the cutting points must endure. The energy passed into the tool (or optical system) is defocused, the period lengthened and absorbed there with less cost (i.e. heat in motor, vibration in the frame). Transmitting the strain is better than using the cutting points as an ablative. However, at the contact edge the strain amplitude is the most intense, and the strain period is shortest. Absorption/ablation must be managed by designing a capability for strain-energy absorption into the cutting point. This is accomplished by controlling defects as strain and density gradients within, and on, the cutting point. With proper control of strain gradients the diamond will act as an ablative. Cracks formed at the contact surface are arrested, absorbed or diverted back to the external surface, where they remove strain energy as debris or heat. With poor control over strain gradients cracks do not return to the surface. Ablation removes less strain energy per unit of wear. Neglecting system effects, the strain energy absorption per micrometer of wear defines the cutting point quality [2,3]. Although single-crystal diamond, as a distinct phase, is not unique, the defect and strain gradient structures in different grades and types of diamond crystal are unique and define utility. The strain–optical properties of the varied defects are critical to the ablative and transmissive properties of the cutting point. Defects typically include inclusions, microcracks, roughness, substituents such as nitrogen, and dislocation/twin structures. Their type, number density and spatial correlation are important. The transmission of dynamic tensile/compressive and
shear strain is a problem of acoustics. Acoustic impedance is a propagation velocity v gradient dv d(앀E/r) impedance¬ # , dx dx discounting the effects of curvature, wavelength-sensitivity and dispersion along a surface. Impedance is sensitive to density gradients in the strain path. The most problematic gradients are sharp-cracks. A crack in, or on, the cutting point, or at the interface due a debonded, partially failed crystal or void, poses severe impedance and catalyzes cutting point fatigue. Indeed, the absorption/reflection of strain energy by cracks, with downshift of transmitted energy, is used for crack detection and diagnosis [4]. The presence of an adherent, graded-stiffness or -density phase at the interface can improve transparency. 2.3. Abrasion rate The bond matrix is worn by contact with stone debris dragged by cutting fluid at a relative speed of ~20– 60 m/s in a gap of ~50–100 mm (protrusion). This shear stress produces a hydrodynamic force ‘floating’ the blade or bit (acting as a lubricant/coolant) and a drag force eroding the blade and helping pump debris out of the cut. In order to maintain a stable number of cutting points and protrusion, the dynamic solution to −
dp dt
#crystal wear rate−bond wear rate,
(2)
where p, the protrusion normalized by the mean mesh size of the cutting point, must be bounded and stable. The unbounded, unstable solutions, p=0 and p>mesh size, describe overwhelmed (crushed crystals, popouts) blades. Bond wear rate (micrometers/minute) must approach the crystal wear rate to achieve a stable solution. The crystal wear rate depends on the stress intensity and protrusion. A higher protrusion means a higher total and normal load with a higher wear rate for fixed diamond quality. The bond wear rate depends on the cutting fluid viscosity, the stone debris type and the concentration, depth-of-cut, bond hardness, exposed bond area (i.e. concentration of cutting points) and protrusion. A higher protrusion keeps the soft bond away from the hard stone, thus lowering the bond wear rate. The most common solution to Eq. (2) is a multiperiod, short-duration oscillation from observable dulling and sharpening cycles [1]. For a badly operated blade the periods will be few and long; dressing or other intervention is required. For overwhelmed blades, monotonic dulling to stall may occur. Depth-of-cut may be used to adjust the bond wear
S.W. Webb / Diamond and Related Materials 8 (1999) 2043–2052
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rate (at a fixed cutting rate) to stabilize protrusion if bond retention or machine torque are limiting performance or to set protrusion for a desired surface finish or to mitigate thermal damage. Clearly, proper diagnosis of the state of the blade is required to know what adjustments to retention, transmission or abrasion rate are required. The cutting point acts as an important, but not self-sufficient, part of a system of strain energy transfer. 2.4. Hardness A proposed method of evaluating retention is called ‘differential hardness’. This is a similar concept to fiber push or pull testing [5]. The hardness of the matrix is compared with that of the bond under a bound diamond. A large and negative difference in hardness implies poor retention. With this technique it is possible to evaluate the retention of cutting points at edges versus the middle of the segment (e.g. die wall effects) and the effect of clustering of crystals.
3. Experimental Diamond–cobalt fired PM powder segments were machined to ~20 mm flatness using a 170/200 RVG diamond wheel. A flat surface (relative to impresser curvature) is necessary in order to avoid bending indentors and altering deformation. A blunted 120° diamond indentor, with 60 kg load, Rockwell C scale was used to evaluate hardness on and off the diamonds. Since the indentor makes a hole of about the same diameter as the exposed and bound diamonds (~0.1–0.3 mm) it is not necessary to align the indentor on the bound diamond. Provided the impression is of a similar scale to the diamond, and the diamond crystal either does not fail or fails completely, it is the metal under and around the diamond that provides resistance to indentation, not the diamond. The presence of the bound diamond disturbs the strain field under the indentor, but, on average, the relative quality of the bond matrix is probed. The depth of penetration at 60 kg load is ~1/3 of the diamond-diameter, so the effect of machining damage is minimal. The diamond crystals are severely damaged by the machining. The damage is helpful in two ways: (1) the diamonds will completely yield to the indentor, and (2) there will be pre-existing damage to the diamond–matrix interface, similar to that in a real tool application. Any detachment, deadhesion or porosity at the interface, originally present or created by machining, will be easier to observe. All bonds were prepared using the same cobalt metal powder ( Euro-Tungsten grade UF denoted ‘CoUF’), blending, firing (2 h in H gas at 500–600°C ) and hot2 pressing (Fritsch press) procedures. The diamond types
Fig. 3. SEM of surface of indented machined diamond-containing segment: uncoated crystal, 225×; center of indentation is to left-center of image.
were MBS1960 diamond, 45/50 mesh (~0.3 mm diameter) uncoated, titanium ‘Ti’-coated and chromium ‘Cr’coated. The coatings were uniformly thick and complete, and applied using proprietary GE technology. All segments were 1.5 wt% diamond. The segments were 2×4×10 mm3 machined on all sides. The method was to locate a diamond, visually align the indentor on that diamond, indent, then move the indentor over the matrix, indent, alternately, monitoring the matrix hardness to diagnose indentor changes and differencing only alternating hardness values. As a baseline, non-diamond-containing segments were examined as well. Saw blade tests were also done to confirm the hardness measures; details of that are described within this text.
4. Results 4.1. SEM of machined surfaces Figs. 3–5 show images of the machined, indented diamond crystals, with no coating, Ti and Cr coatings after acid etching to clean debris and metal post-machining. In all cases the diamond crystal is shattered, pushed down and detached from the metal bond. The planar, non-porous, cleanly detached walls of the bond after impression suggest no direct chemical or physical bonding of crystal, or coating, to the metal matrix. Any intermediate phases (e.g. alloys, intermetallics, carbides) of yield strength greater than retention, as tested by indentation, are well bonded to the diamond, not the cobalt. Almost every indentation produced audible ‘cracking’. This noise was not indentor cracking, as the 1 Trademark of General Electric Company, USA.
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Fig. 4. SEM of surface of indented machined diamond-containing segment: Ti-coated crystal, 225×.
(a)
(b)
Fig. 5. SEM of surface of indented machined diamond-containing segment: Cr-coated crystal, 439×; note clean slipping and failure of crystal–bond interface as well as interrupted fracture of crystal.
reference hardness of the matrix was unchanged. Every bound, machined diamond crystal failed under indentation loading. A 3 mm diameter spherical WC ball indentor at 60 kg was tried, but it gave poor discrimination. The indentations were too shallow to sample the region under the bound diamond crystals. Crystals were also crushed by the relatively soft WC indentor, confirming extensive machining damage. Figs. 6 and 7 show EDAX Cr and Ti maps for coated crystals prior to indentation. There is a distinct interface between sintered Co (Cr or Ti<500 ppm) and diamond (Cr or Ti<20 ppm) that is rich in Cr or Ti. This interface is due to the coating; ~1.2 mm for Ti and 1.75 mm for Cr. Although smearing and relief may have occurred in machining, there is no evidence for bulk diffusion of the coating into the bond or diamond during sintering. The coating is a distinct 1–2 mm phase between diamond and matrix cobalt, bound to the diamond. The action
Fig. 6. SEM image (a) and EDAX Cr map (b) of the surface of a machined diamond-containing segment: Cr-coated crystal.
of the coating is local to the crystal, influencing impedance or adhesion/friction at the diamond–bond interface. The SEM contrast observed from the interface suggests that the coating is a distinct phase. This phase has been verified by X-ray diffraction to be a carbide [6 ]. To act as an acoustic transmissive layer, chemical barrier, or friction surface the coating must be dense and integral. If the coating debonds or cracks its effect is worse than if it were not present. Use of coatings can be risky if coating quality is not closely observed. Minimal stress in the coating with good solid-state wetting is required. These are easier to achieve with thin coatings if uniform thickness and integrity can be achieved. Finally, temperature and chemistries that may degrade the diamond, and/or coating, restrain available coating processes. 4.2. Differential hardness Table 1 shows the results of the differential hardness test. The bond under the crystal is weaker than the
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matrix. The hardness of the diamond-free segments was the same as the matrix in all diamond-containing segments. The presence of diamond at 1.5 wt% does not inhibit bulk sintering; this may not be true at higher diamond volume fractions. The variation in hardness under the diamond is higher than the matrix owing to variable indentation geometry. The uncoated segments show the highest difference hardness, −10%, indicative of weak retention. For this cobalt powder, the Cr coating provides the highest retention, +1.6%. The Cr coating reinforces the segment under the diamond. Ti coating is not as good as Cr, −3%, but better than uncoated diamond. The Cr coating was so strong that the indentor tips frequently cracked (tips failed before the bond ). Tip cracking never occurred with uncoated or Ti coated segments. The strength of the Cr coating must derive from a new, higher-strength surface at the diamond–matrix interface.
(a)
4.3. Differential density
(b) Fig. 7. SEM image (a) and EDAX Ti map (b) of the surface of a machined diamond-containing segment: Ti-coated crystal.
To test if density was controlling the hardness of a segment, the water and gas (He pycnometry) density was measured ( Table 2). The difference between gas and water density is an indicator of microporosity (water does not wet metal relative to gas). The presence of diamond retards densification by increasing macropores by ~2×. There is no coating effect on density or porosity. The sensitivity of differential hardness to coating is not due to densification, e.g. improved pressure transmission or inhibited gas formation (metal oxide+carbonCO) in sintering. The improvement may be due to improved friction/adhesion from the coating.
Table 1 Hardness (Rockwell C scale) of diamond and matrix sintered metal Uncoated
Average hardness Standard deviation Difference No. of samples
Cr-coated
Ti-coated
Diamond-free segment
Diamond
Matrix
Diamond
Matrix
Diamond
Matrix
62.4333 4.820
68.639 1.551 6.206 18
71.667 4.933
70.500 0.707 −1.167 2
65.440 4.239
67.552 2.004 2.112 23
18
3
15
67.6 0.699 10
Table 2 Density of uncoated, coated-diamond and diamond-free segments: theoretical density of cobalt 8.92 g/cm3; ET CoUF cobalt powder; theoretical density of 15conc segments 8.719 g/cm3 Difference in % density
Density (%)
Uncoated Cr Ti CoUF
Water
Gas
Standard deviation
Water
Gas
Gas−water
95.3 94.9 94.6 97.8
95.9 95.7 95.9 100.2
0.2 0.9 1.0
2.5 2.9 3.2
4.1 4.3 4.1
0.6 0.8 1.3 2.2
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Table 3 TRS of segments with coated crystals; three-point bend to fail
Uncoated Cr-coated Ti-coated CoUF (diamond-free)
Mean (GPa)
Standard deviation (GPa)
1.03 1.07 1.01 1.41
0.13 0.09 0.08 0.03
4.4. Transverse rupture strength (TRS) To investigate the tensile strength of the coated interface the TRS (three-point bend to fail ) of the segments was evaluated ( Table 3). Adding diamond weakens the segments in tension, not by effecting density, but by adding weakly bonded surfaces. The coated interface did not improve the bulk tensile strength. Improvement in retention with Cr coating lies with an increase in shear strength, not tensile strength, due to friction at the diamond–matrix interface. 4.5. Powder processing Retention is derived from the product of compressive stress in the matrix and diamond–matrix friction producing a support force countering the contact load (Fig. 1) and thus minimizing interface yield and fatigue. A highquality coating improves friction. However, if compressive stress is reduced owing to processing, performance of the coating is wasted. To investigate this, several powder processes were tested using test diamond blades. Fired, unfired, granulated/non-granulated, nominal (Co6101) and ultrafine (CoUF ) cobalt powders (all Eurotungsten ‘ET’) were saw tested with the same crystal concentration, uncoated UHG crystals, blades, depthof-cut, traverse rate, tip speed, machine, stone and cutting rate. Segments were prepared as described above. The cutting rate was set high (relative to bond hardness) to create retention as the performance-limiting effect. We used a small 7◊ diameter blade attached to a grinder, rather than a conventional diamond saw, to minimize machine/blade deformation. The key saw performance observables [1] include machine power (tangential blade force for fixed
speed) w, the inverse blade angular friction coefficient (tangential/normal blade force, 1/tan w), a measure of cutting efficiency and b, the radial location of the average blade force against the stone, constrained by depth-ofcut and blade diameter. Large b and w imply a blade cutting well, with high protrusion, low wear and low power. The observation that b and w are correlated [1] implies that high protrusion produces low blade friction. High protrusion directs the contact force into the blade, thereby reducing torque and machine power. It also helps keep the soft bond/blade off the hard workpiece and abrasive debris, thus improving blade life. Since b and w are dimensionless, they are insensitive to the number of cutting points and contact area. Table 4 shows that saw performance is not related to segment density. As shown earlier, density is not well related to retention either. Although density is necessary to make high-quality segments, it is not sufficient to predict tool performance. Granulating the cobalt powder prior to making segments produced poor saw performance. Granulation is typically used (with compaction) to reduce density gradients in green bodies to limit shrinkage and distortion in sintering. A coarser cobalt powder (ET Co6101) also produced lower saw performance. Coarse particles and granulation apparently reduce compressive stress, thus impairing retention. Firing the cobalt powder (in hydrogen) produced marginally better segments, presumably by reducing surface oxides that reduce contact area at the diamond–matrix interface. 4.6. Saw testing 4.6.1. Calibration If saw performance is limited by machine/blade stiffness, crystal crushing (i.e. non-ablative wear) or severe abrasion rate mismatch, a coating will have no effect. If retention or impedance limits performance, a coating may help. Performance limits are sensitive to condition, e.g. stone type, depth-of-cut, tool speed and available torque, spindle stiffness and cutting rate, etc. In a multidimensional problem, such as sawing hard stone, it is very easy to miss the operating window in which any single effect will be controlling. Having selected the best powder process and coating
Table 4 Saw testing of varied cobalt bond processes: ET CoUF cobalt; density under water; 7.5 mm depth-of-cut, 300 cm2/min, bright-red granite (Class V ), cutting rate/wear rate ratio ~2.28 m2/mm; average performance over ~200×0.6 m stone passes; 30 m/s speed Powder process
Density (%)
b (deg)
w (deg)
Power (kW )
Fired Unfired Coarse-fired Granulated-fired Granulated-unfired
96.6 95.4 97.2 96.3 95.4
76.6 76.4 75.9 75.3 74.6
80.7 80.5 80.1 78.3 78.2
1.2 1.2 1.3 1.3 1.1
Standard deviation
1.6
6.0
0.5
0.3
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Table 5 Saw test results of lower-grade uncoated diamond crystals using same conditions and bond: Barre granite, 35/40, 25conc; three replicates×two blades; minimum 200×0.6 m passes; ET CoUF cobalt, depth-of-cut=10 mm, cutting rate =300 cm2/min Crystal
b (deg)
w (deg)
Power (kW )
Wear performance (m2/mm)
MBS940 diamond
75.3 74.8 74.7 74.9 0.3
81.6 82.8 81.9 82.1 0.6
1.3 1.4 1.2 1.3 0.1
6.06 7.34
74.2 74.8 74.8 74.6 0.3
81.7 81.0 81.9 81.7 0.5
1.2 1.2 1.3 1.3 0.1
5.86
Average Standard deviation MBS930 diamond
Average Standard deviation
it remains to verify the effect of coating in a saw test. To do so requires calibration of the test platform and evaluation of test sensitivity. To demonstrate contrast of the test system, lower grade crystal products, MBS940 diamond and MBS930 diamond, were tested with the same cutting rate and depth-of-cut, replicate blades, ET CoUF cobalt in soft granite (Barre). The choice of soft stone and lower-grade crystals was to ensure good retention and machine stiffness, thus isolating the effect of crystal grade. ET CoUF cobalt retains
7.13 0.27
6.17 6.01 0.21
1. a high contact force condition ( low crystal concentration and high cutting rate) or, 2. an abrasive condition leading to lower bond area ( Fig. 1). A severe application will test system effects such as machine stiffness and crystal crush strength. With higher crystal wear rate a closer match of bond wear rate will be required, else the blade will develop long dulling cycles and render mean performance discrimination difficult to ascertain. 4.6.2. Evaluation of coating in hard application Saw tests with 45/50 Cr-coated UHG crystals were performed in hard bright-red granite at a relatively high cutting rate of 300 cm2/min. MBS950 diamond is used as a reference uncoated HG product to verify the test. Several depths-of-cut were tested to find the operating window for best discrimination. The results shown are averages. The time-variations in forces and angles were collected to evaluate blade dynamic stability (dull–sharp cycles) but are not shown. A blade with dull–sharp period of >6 m2 of stone was considered unstable. The results are shown in Table 6. 300 cm2/min in soft Barre granite with an HG diamond product results in ~82° of angular friction ( Table 5). The same cutting rate in harder red-granite with UHG diamond produces the same 82° of friction. At a fixed grade, increasing the
Table 6 Saw test results: average ~200×0.6 m stone passes; bright-red granite; 30 m/s speed Crystal type
Depth-of-cut (mm)
Cutting rate (cm2/min)
b (deg)
w (deg)
Power (kW )
UHG Cr
10 7.5 7.5 15 7.5 10 7.5 10 7.5 5
300 150 150 450 300 300 150 200 150 100
74.5 76.9 76.5 69.5 75.9 75.1 76.9 71.6 73.3 76.4
82.0 85.1 85.0 74.8 80.4 82.2 85.1 79.8 83.8 84.1
1.8 1.6 1.5 2.2 1.4 1.8 1.6 1.6 1.8 1.2
UHG uncoated
MBS950 diamond
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cutting rate reduces the angular blade friction. The power increases with grade (and protrusion), but not as consistently as blade friction. The blade friction is a more sensitive performance indicator. Indeed, the power samples only ~1/3 of the total force on the blade. The out-of-plane force measures deflection and deviation and is also a performance variable [1], but it is a much smaller effect. Replicate blades are within 0.1° and 0.1 kW. The benefit of increasing depth-of-cut is shown. At 300 cm2/min ( UHG crystals), moving from 7.5 mm to 10 mm increases the average normal component of the load, increasing total blade force, but improving efficiency. This is accomplished by diverting more force into the blade spindle owing to more favorable kinematics and higher, stable and steady protrusion. 450 cm2/min showed a precipitous increase in angular friction, indicative of large loss in protrusion due to crushing. The contact load, for uncoated or coated UHG crystals, at 300–400 cm2/min is too severe. Crushing, not retention is limiting the performance. The effect of Cr coating is not going to be observable in this condition range. 4.6.3. Saw testing — evaluation of coating in abrasive application Another method to create retention-limiting performance is to increase substantially the bond wear rate at a lower contact load (no crushing or machine deflections). By increasing bond wear rate, protrusion will tend to increase, minimizing the compressive ‘grip’ on the cutting points. One method to increase bond wear rate is to go to a high-cutting rate, soft, but abrasive workpiece, such as granite-aggregate concrete. This test was performed with uncoated and Ti-coated MBS960 diamond crystals. For concrete cutting, constant power machines are common, thus cutting rate, not power, is a variable. Table 7 shows that the cutting rate with the coated diamond was lower than uncoated from the higher total forces derived from higher, stable protrusion. However, the 300% improvement in wear resistance far surpasses the 20% decrease in cutting rate. Observation of the worn cutting points confirmed that crushing was a not Table 7 Concrete cutting test: Tysaman machine; 46 m/s speed; depth-of-cut 63 mm; power 20/50 hp available; 55 crystals/cm2 visible
Cutting rate (cm2/min) Wear performance (m2/mm) Wear rate (mm/min) Crystal count (% of visible) wholes/working Crushed popouts
a limitation and that the average number of working cutting points was higher with coated diamond.
5. Conclusions Thin coatings on diamond do not influence segment density, closed porosity or bulk TRS. None of these measures shows a strong correlation with diamond saw performance. Diamond coatings influence the yield strength of the bond in shear, at the diamond–bond matrix interface, and more effectively utilize the compressive stress developed in bond fabrication to produce good retention. The improvement in crystal cutting point retention, as observed by differential hardness, in 100% cobalt PM bonds, is Cr-coated>Ti-coated> uncoated. Since retention is only one attribute of a bond, the benefit of a coated crystal in tool performance is system- and condition-specific. Improvement in saw performance will come with reduction in blade angular friction derived from higher and stable cutting point protrusion. To achieve higher protrusion will require higher retention from nominal or higher bond compressive stress combined with higher diamond–matrix friction. Diamond coatings provide higher friction. Observing improvement in blade performance with a coated crystal requires that the condition/application be performance-limited by retention, not, for example, by acoustic impedance, blade or machine stiffness or crystal crushing. The differential hardness test requires an indentor of radius less than the bound and exposed diamond crystal, a relatively soft bond or weak bond–crystal interface. Careful attention to the indentor tip is required and must be compensated for in the differencing.
Acknowledgements The assistance of technical experts within GE Superabrasives is gratefully acknowledged including M. Wright, T. Whittington, D. Turner, D. Dawley, T. Wilson, J. Adjunta, M. Loh, G. Braeuniger, S. Hayden and S. Snyder. The author thanks GE and GE Superabrasives for permission to publish this work.
MBS960 diamond
MBS960 diamond — Ti coated
References
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50 21 29
59 24 17
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