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Silicon nitride granule friction measurements with an atomic force microscope: effect of humidity and binder concentration
Powder Technology 119 Ž2001. 241–249 www.elsevier.comrlocaterpowtec
Silicon nitride granule friction measurements with an atomic force microscope: effect of humidity and binder concentration Anders Meurk a , Joseph Yanez b, Lennart Bergstrom ¨ a,) a
Institute for Surface Chemistry, PO Box 5607, SE-114 86 Stockholm, Sweden b Procera SandÕik AB, SE-126 80 Stockholm, Sweden
Received 1 May 2000; received in revised form 1 December 2000; accepted 18 December 2000
Abstract This study introduces the atomic force microscope ŽAFM. for direct measurement of internal and external friction in ceramic powder pressing. The friction measurements were performed between two single granules, and a granule and a hard metal substrate as a function of granule binder concentration, relative humidity and sliding velocity. We found that the friction coefficient decreased with increasing humidity for a specific binder concentration, the effect being more pronounced for low concentrations. The friction coefficient also decreased with increasing binder concentration. Evaluation of the adhesion force showed a steady increase with both humidity and binder concentration. A substantial difference, more than an order of magnitude, was seen for the highest binder concentration at low and high humidities. We attribute these findings to the hygroscopic nature of the binder, polyŽethylene glycol. ŽPEG.. Softening of the PEG at increasing humidities lowers the friction coefficient but increases the adhesion force. The results are consistent with flowability and angle of friction measurements. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Atomic force microscope; Granule; Friction; Adhesion; Binder; Powder compaction
1. Introduction Industrial compaction of fine ceramic powders requires granulation in order to facilitate handling and feeding prior to die filling w1,2x. Granulation is commonly achieved by spray-drying ceramic slurries that have been optimised with respect to viscosity, solids loading and organic additives, e.g. binder, dispersants and defoamers w3–6x. Controlling processing parameters enables design of granule properties, thereby directly influencing green compact characteristics. During spray-drying, the binder can migrate to the granule surface, particularly if it is water-soluble w7,8x. This results in a hard, binder-rich surface that resists deformation, and is potentially detrimental to the final microstructure. Binders commonly used for powder pressing are polyŽvinyl alcohol. ŽPVA. and polyŽethylene glycol. ŽPEG.. PEG is a low molecular compound made from polymerised ethylene oxide with good lubricating and plasticising properties w9x. The advantages of using PEG,
)
Corresponding author. Tel.: q46-8-790-9900; fax: q46-8-208-998. E-mail address: [email protected] ŽL. Bergstrom ¨ ..
or modified PEG, are its stable pressing properties and its ability to achieve sufficient granule deformation without adding a plasticiser w10x. The plasticising effect of PEG on PVA and the equivalent effect of humidity on the glass transition temperature ŽTg . has been studied in detail w11–18x. Plasticisation lowers the Tg , making the binder more easily deformable. It is desirable to press components at temperatures above the binder Tg , where enhanced densification is realised due to high granule deformability and decreased granule strength. PEG compounds usually have a Tg below 08C, resulting in optimal performance at room temperature. The hygroscopic nature of PEG makes this binder sensitive to changes in relative humidity ŽRH., thereby affecting granule friction and adhesion. A comprehensive study of the effect of seasonal variations in humidity on the final properties of ceramics produced from granules with PVA as a binder appeared recently w19x. It was shown that granules became stiffer during dry winters due to an increased Tg . This resulted in a final microstructure with remaining intergranular porosity. The glass transition temperature of PEG is rather non-sensitive to humidity, but water uptake is substantial w20,21x. Increasing humidity thereby softens the PEG by lowering the shear modulus,
0032-5910r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 Ž 0 1 . 0 0 2 6 0 - 1
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making handling of the granules considerably more difficult due to increased stickiness, and thus a decreased flowability. Both intergranular friction Žinternal friction. and granule–wall friction Žexternal friction. during filling of the die and subsequent pressing is affected by the choice of binder, plasticiser and relative humidity. Granules behave as elastic spheres up to some critical level of stress, where the granules either deform plastically or fracture, depending on the binder concentration w2,22– 24x. The fracture strength of silicon nitride granules must be as low as possible to result in a narrow pore size distribution of the green body w25x. The decrease in granule shear modulus with increasing binder content favours deformation rather than fracture. Deformable, soft granules thus enhance packing by filling intergranular pores, but the high friction forces may yield low-density areas of incomplete granule deformation within the compact w26x. Current techniques of measuring wall friction and internal friction of powders are mainly based on indirect methods. Common techniques of determining the wall friction involve sliding a pressed body against the wall material or examination of the ejection pressure and springback effect w5x. Internal friction measurements of individual granules are scarce. In the Jenike shear cell, and related designs, a shear force is applied transversely to a box containing the powder w27–29x. Both the internal angle of friction and wall coefficient of friction can be determined. The coefficient of internal friction in powder compaction was recently deduced from shear strength experiments, where, however, contributions from die wall friction can affect the result w30,31x. A measure of interparticle friction can be obtained from the ratio of tap density to apparent density Žthe Hausner index. w32x. The angle of repose and flowability measurements are important characteristics but do not describe the sliding behaviour under an applied load. Re-
lating angle of repose data to internal friction is only reasonable for cohesionless materials. In this paper, we present a new method for direct measurement of both internal and external friction and adhesion using the atomic force microscope ŽAFM. w33x. This technique is fast, simple and reliable for sliding friction measurements between a microscopic probe and another surface of arbitrary geometry. Measurements can easily be performed between two single granules or a granule and a flat substrate. The advantage over other techniques is that we measure friction forces on a scale equivalent to the granule interaction forces, without the influence of post-granulation parameters on the results. Also, granule fracture and strength can be followed optically and measured. Once set-up and properly calibrated, the AFM measures friction and adhesion force as a function of sliding velocity, applied load and relative humidity. Standard AFM imaging gives additional information of the granule and substrate roughness. Altogether, these AFM measurements clearly illustrate the importance of humidity and binder concentration on both friction and adhesion, allowing a comparison of the results to flowability measurements, compaction curves, ejection pressure and compact strength.
2. Experimental 2.1. Materials Silicon nitride-based granules were produced by conventional spray-drying with an outlet temperature of 3008C. The aqueous slurry of 30 vol.% solids loading was deflocculated using an anionic polyelectrolyte. A polyŽethylene. glycol binder consisting of a mixture of different molecu-
Fig. 1. Optical image of the spray-dried silicon nitride granules used in this study.
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graphic image. A topographical granule image is shown in Fig. 2, where the image size constitutes the length scale over which the sliding friction measurements were performed. Prior to friction measurements, the WC substrate was cleaned with ethanol and blown dry with nitrogen. Granules attached to cantilevers were stored at controlled relative humidity. The granules used for flowability measurements were stored in a humidity cabinet for 24 h, then taken out and measured within an hour at ambient conditions equivalent to those prevalent during friction measurements. Angle of repose measurements were done with granules equilibrated 24 h at the ambient humidity of 24%.
2.2. Friction measurements Fig. 2. AFM image of the binder-rich granule surface. The surface RMS value is 63 nm.
lar weights ŽMW s 3000, 15 000 and 40 000. was also added to the slurry. Four different concentrations of PEG were chosen for a comparative study of the adhesion and lubrication behaviour: 0.81, 1.62, 3.25 and 6.5 wt.% PEG. The different granules are denoted A, B, C, and D with increasing binder content. Typical granule size and shape after spray-drying can be seen in Fig. 1. Substrates used for friction measurements were polished tungsten carbide ŽWC. containing a small amount of Co, with a RMS roughness of 3 nm as determined from topographical 4 = 4 mm AFM images. In addition, a single granule glued to a rigid disc was used as substrate for individual granule friction measurements. The granule surface RMS-value was 63 nm, as evaluated from a 4 = 4 mm AFM topo-
An atomic force microscope ŽMultiMode SPM, Nanoscope IIIA, Digital Instruments, USA. was used for sliding friction measurements. In standard topographical imaging mode, a sharp tip is in mechanical contact with the sample surface. Interaction forces deflect the cantilever, which is sensed by a laser reflected from the back of the cantilever onto a position sensitive photo diode ŽPSD.. A computer-controlled feedback loop connects the PSD with a piezoelectric scanner underneath the sample. The piezoelectric scanner adjusts the tip–sample separation distance during scanning by keeping a constant cantilever deflection. Optimising the feedback assures that a constant force is maintained throughout the experiment. Friction measurements are attainable by recording the torsional bending of the cantilever as it slides sidewards a specified distance across the surface. This signal is readily available from the detector and can subsequently be converted into friction force at a particular applied load w34x.
Fig. 3. A typical AFM cantilever with attached granule viewed from below. The granule diameter is approximately 20 mm.
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The relative nature of the AFM necessitates calibration of scanner, detector and cantilever for converting the raw data to meaningful friction values. Cantilever normal spring constants were calibrated by attaching a series of tungsten spheres of varying mass and measuring the corresponding shift in resonance frequency w35x. Spring constants were determined within 10% accuracy, and those used here were typically between 0.5 and 3 Nrm. This is about an order of magnitude higher than those normally used in AFM contact mode, improving the signal to noise ratio during friction measurements. Owing to the fact that reliable torsional spring constants are more difficult to substantiate Žalthough some promising experimental techniques are emerging w36,37x., the cantilevers included in this study were uncoated, tipless, rectangular beams made of silicon. We carefully evaluated dimensions from SEM-images and used materials properties stated by the manufacturer ŽSilicon MDT, Moscow, Russia.. Granules of different PEG-content were glued on to the cantilevers with a two-component epoxy resin by using a micromanipulator consisting of two glass fibres: one for picking up glue and one for moving particles to the cantilever. Fig. 3 shows a granule probe used for friction measurements. All granules used were typically in the 20-mm range. In the case of granule–granule friction measurements, granules spread on a layer of glue and cured for 24 h served as the substrate. All friction measurements were carried out as a function of humidity, sliding velocity, applied load and granule PEG concentration. Several friction–load curves were recorded at different sliding velocities Ž2–120 mmrs. for each granule binder concentration. The applied load was also deliberately increased with humidity in order to test the effect of decreasing binder shear modulus on granule strength. A program specifically developed for AFM friction analysis was used for the evaluation of friction coefficients w38x.
3. Theoretical background 3.1. Friction and adhesion models The friction between rough surfaces in sliding contact is generally well-described by Amonton’s law with a direct proportionality of the friction force to the applied load w39x. Plastic deformation occurs when interlocking asperities are sheared, giving rise to a true area of contact much smaller than the apparent contact area estimated from surface geometry. Friction–load dependence for rough surfaces exhibiting some degree of adhesion is usually described by Ff s F0 q m L
Ž 1.
where Ff is the friction force, F0 the friction at zero load, m the friction coefficient and L the applied load w40x. The
finite friction at zero load has termed this behaviour adhesion-controlled friction, due to the sustained sliding friction at negative loads. Molecularly smooth surfaces, i.e. where a single-asperity contact is achieved, have been shown to conform to contact mechanics theories, such as JKR and DMT w41x. More complex and rough surfaces, such as ceramic granules containing a polymer-rich surface layer, do not generally follow these theories. Softening of the polymer and build-up of contaminants in the contact area do not give a single asperity contact. An attempt to fit the friction–load curves in this study with a numerical contact mechanics model proposed recently w42x resulted in unrealistic fitting parameters. The model of Bowden and Tabor w39x, based on adhesive junctions ruptured by frictional work, is a more realistic approach to describing the frictional behaviour of hard particles. The friction force is described by Ff s t A s CL2r3 q a L
Ž 2.
where C is the contact area multiplied by the intrinsic shear strength Ževaluated at zero applied load. calculated from Hertz theory, and a is a pressure constant w43–45x. This theoretical approach assumes that the elastic deformation is balanced by a variation in shear strength with applied load. Recent experiments on friction of polymer surfaces have been shown to follow Eq. Ž2. closely w46x. The presence of a soft organic binder on the granule surfaces may infer a boundary type lubrication where the interfacial rheology of the polymer will control the friction. This type of frictional behaviour may be described by an equation of the form Ff s k Ž L q La .
n
Ž 3.
where k is the friction factor, La the adhesion force and n the load index. The friction factor is related to components regulating a Hertzian contact, while the load index is to a large extent affected by deviations from a shear strength not being of constant value. The load index can vary between 2r3 and 1; the latter case is equivalent to a multiple asperity contact in accordance with Amonton’s law. It is clear from the above that the coefficient of friction depends on the intrinsic shear strength and the contact pressure. At high loads, the load index approaches unity. For this special case, the friction factor reduces to the pressure constant, thereby corresponding to the coefficient of friction. We have chosen to use the more general Eq. Ž3. for a qualitative evaluation of the influence of binder properties, surface deformation and adhesion of granules in sliding contact. Shear strength variations are related to binder softening, whereas granule deformation is influenced by load, roughness, adhesion and the moduli of elasticity and rigidity.
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4. Results and discussion 4.1. Granule–die wall friction measurements Friction measurements between a granule and a WCbased hard metal substrate at the relative humidities 38%, 45% and 60% are shown in Figs. Ž4–7.. The majority of the friction–load curves show a non-linear dependence on load, as predicted from theory. The load index, n, is always smaller than 1, which means that the friction coefficient, m , is load dependent; it increases rapidly at low loads but becomes constant at high loads w45x. Hence, a limiting coefficient of friction was therefore evaluated at the highest loads for each measurement, where the friction coefficient equals the value of the friction factor, k, and the pressure constant, a . The adhesion component of the friction force was obtained from fitting experimental friction data to Eq. Ž3.. Parameters obtained from friction–load curves are shown in Table 1, and friction coefficients in Table 2. Fig. 4 shows friction–load curves at 38% RH. A clear trend is observed in which the friction force and the coefficient of friction decrease with increasing binder concentration. The data for the granule with the lowest amount of binder Žgranule A. displays more scatter at high loads than the other concentrations. Considering the relatively low humidity and binder concentration, this granule is more prone to fracture than deformation, resulting in wear at high loads. Granule B shows a non-linear frictional behaviour, implying that the granule deforms elastically. The binder-rich granules ŽC and D. appear to have similar
Fig. 4. Friction–load measurements at 38% RH between a granule and a flat substrate. The solid lines are fits to Eq. Ž3..
Fig. 5. Friction–load measurements at 45% RH between a granule and a flat substrate. The solid lines are fits to Eq. Ž3..
friction–load dependence; both exhibit a relatively large adhesion force and a low friction coefficient. However, the low load index indicates that elastic and geometry-dependent granule material properties contribute considerably to the contact shearing behaviour. Evaluation of granule–wall adhesion at 38% RH reveals an overall low adhesion force, where a clear trend is seen
Fig. 6. Friction–load cycles on a flat substrate showing increasing and decreasing loads for two consecutive measurements. Hollow symbols denote decreasing loads. The solid lines are fits to Eq. Ž1. for granule A and Eq. Ž3. for granule D.
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Table 2 Friction coefficients evaluated from granule friction measurements and angle of repose data Granule PEG ma Žwt.%.
mb
Angle of repose Ž8.
38% RH 45% RH 60% RH 38% RH 24% RH A B C D
0.81 1.62 3.25 6.5 a b
0.12 0.09 0.08 0.07
0.08 0.06 0.06 0.06
0.05 0.06 0.04 0.03
0.07 0.06 0.06 –
42 45 47 59
Granule–substrate measurements. Granule–granule measurements.
taken at increasing and decreasing loads. This so-called adhesion hysteresis clearly asserts that the binder-rich surface changes the contact dynamics. Fig. 6 shows two consecutive measurement cycles for granules A and D. The first and second cycles overlap with minimal hysteresis for granule A. Granule D displays an initial linear increase in friction force with load, but a substantial hysteresis occurs at decreasing loads. The net energy loss during a cycle for molecularly smooth surfaces can be explained by a difference in advancing Žloading. and receding Žunloading. surface energies. In these measurements, the hysteresis most probably occurs due to an irreversible change in shear strength with increasing contact pressure. As the granule is pressed and slid against the substrate, binder softening causes a time-dependent, viscous, surface deformation. The second cycle shows much less adhesion hysteresis and lies between the data for the first cycles. The energy dissipation stemming from hysteresis has been significantly reduced after the first contact, enabling reproducible frictional behaviour. Note, however, that the coefficient of friction is mildly affected by the number of sliding contacts Žmeasurement cycles.. We can relate our friction and adhesion results to observations during pressing. An increased binder content often leads to an improved compaction but may also give rise to more problems with granule sticking to the wall. This is an effect of the reduced friction coefficient and increased adhesion with increased binder content. The large friction-load hysteresis at high binder content can also be one contribution to the large spring-back often observed w47x.
Fig. 7. Friction–load measurements at 60% RH between a granule and a flat substrate. The solid lines are fits to Eq. Ž3..
with respect to binder concentration for all granules except granule B. The restricted ability of the granules to deform, i.e. the brittle character of the binder, leaves the morphology rather than the viscous PEG binder responsible for the low adhesion forces. It is, therefore, believed that granule friction is only partly responsible for the flow and compaction characteristics at low humidities; the surface roughness is also a very important parameter. An interesting observation noted at 45% RH is the shift in relative position between the friction–load curves Žsee Fig. 5.. Granules C and D still group together but the increase in adhesion force has shifted their position above granules A and B. In spite of the higher friction forces, the coefficient of friction is lower at 45% RH compared to 38% RH ŽTable 2.. Granule A still exhibits the highest value, whereas the difference in m between granules B, C and D at 45% RH is almost negligible. The adhesion force increases substantially with RH for all granules. A clear difference between the results for granules C and D at 45% RH compared to those obtained at 38% RH is the increased hysteresis between friction–load curves Table 1 Parameter evaluated from fitting Eq. Ž3. to granule friction measurements Granule
PEG Žwt.%.
Ladh ŽnN. a 38% RH
45% RH
60% RH
338% RH
38% RH
45% RH
60% RH
38% RH
A B C D
0.81 1.62 3.25 6.5
21 6 25 27
60 84 305 187
80 226 390 417
18 2 10 –
0.67 0.67 0.67 0.73
0.72 0.67 0.67 0.67
0.72 0.78 0.67 0.71
0.81 0.98 0.76 –
a b
Granule–substrate measurements. Granule–granule measurements.
Ladh ŽnN. b
na
nb
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Friction measurements performed at 60% RH are shown in Fig. 7. As opposed to the clear trends at lower humidities, there is no particular consistency with respect to the magnitude of the friction force or the relative position of the friction–load curves. However, evaluation of the friction coefficient reveals a continuous decrease compared to lower humidities at all binder concentrations. The adhesion force increases with the binder content. Both granules B and C display a significantly higher adhesion compared to 45% RH, which is most likely a result of the softening of the outer, PEG-rich surface layer. An important consequence for compaction of these easily deformable granules is the low resistance to sliding. This leads to an increased green body density, but also powder handling problems and agglomerate caking because of the relatively strong adhesion force. The inconsistent and deviating sliding behaviour at 60% RH compared to lower humidities brought about an investigation of the effect of sliding velocity on friction. Varying the rate within two orders of magnitude did not significantly affect the friction force. As the load increased, there was a slight tendency of the friction force to decrease at higher velocities. This effect may be explained by binder shear thinning, which is known to occur at high humidities w13x. However, the overall rate dependence was weak and thus of minor importance. Friction coefficients for the above measurements, derived at high loads where the load index approaches unity, are averages of four experiments for each PEG-concentration. The variation in friction coefficient for one particular PEG-concentration did not exceed 10%, whereas the adhesion force was highly dependent on contact history, sometimes varying with a factor of two or three for consecutive measurements. Hence, the distribution of binder at the granule surface, and residual amounts from the spray-drying process, strongly affects the intergranular flow characteristics. Acting as a lubricant in all of the above friction measurements, the effect of binder is more pronounced at high concentrations but probably vanishes above a certain value when the surface binder concentration saturates. Increasing binder concentration and relative humidity allows for improved granule deformation, and consequently improves granule joining during pressing w2,11,12,18,48x.
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The low humidity results in small variations in sliding behaviour. Granule A has a friction coefficient of 0.07, where both the friction and adhesion force increases slightly with increasing number of measurements. Shearing of granule surface asperities leads to wear until a smoother contact is established, yielding a higher adhesion force. The average friction coefficient for the granules B and C is somewhat lower at 0.06. Surprisingly, there is a substantial difference in adhesion force between these systems. The extremely low value of 2 nN for granule B is somewhat unexpected. A possible explanation is a very rough contact between the granules throughout the experiment. Earlier results from intergranular friction measurements for granule C at 30% relative humidity showed a slightly lower friction coefficient Ž0.05. and an even lower adhesion force w34x. Measurements for the highest binder concentration Žgranule D. were very scattered due to difficulties in maintaining a stable contact, and reproducible friction results could not be obtained. The high amount of binder results in surfaces too compliant for a constant force to be maintained by the AFM feedback loop. A comparison between granule–wall and intergranular friction and adhesion ŽFigs. 4 and 8., reveals an overall higher coefficient of friction and adhesion force for granule–wall friction, regardless of PEG concentration. This suggests that wall friction dominates the pressing response at low humidities. 4.3. Flowability and angle of repose measurements The friction and adhesion results have been compared with granule flowability ŽFig. 9. and angle of repose
4.2. Intergranular friction measurements Friction measurements between two single granules at a relative humidity of 38% are shown in Fig. 8 for all granules except the one with the highest PEG-concentration. These results also follow Eq. Ž3., with a high load index. Results for granule B conform to Eq. Ž1., indicating that the friction coefficient can be evaluated regardless of the loading regime. Recalling the high granule surface roughness ŽFig. 2., it is evident that the surface deformation is insufficient for a smooth contact to be maintained, resulting in a multiple-asperity contact.
Fig. 8. Friction-load measurements at 38% RH between two individual granules. Granule D was too soft for a stable contact to be maintained. The solid lines are fits to Eq. Ž3..
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5. Conclusions
Fig. 9. Granule flowability as a function of humidity. Note the switch at high humidities of granules C and D.
measurements ŽTable 2.. Both techniques yielded reproducible results, but data became more scattered with time due to changes in relative humidity. This was especially the case for granule C, where the flowability rate quickly decreased after the third run. At low humidities, the flowability results follow the same trend as the adhesion measurements. Granule B has the best flowability and lowest adhesion, followed by granule A. Granules C and D display a significantly over flowability compared to the granules with lower binder concentrations. The correlation between flowability and adhesion is poorer at the higher humidity of 60%. Granules A and B both exhibit lower adhesion and lower flowability times than the other granules, but the descending order of adhesion and flowability. The flowability of the granules are therefore probably related to both granule morphology and granule friction. A surprising finding was the switch in flowability behaviour of granules C and D at high humidities. It is possible that the binder-rich surface of these granules is so soft at high RH that the granules slowly deform and fuse together with time. Examination of the friction–load curves at 60% RH showed a large adhesion hysteresis, which was not seen as markedly for the other granules. The angle of repose measurements relate to the internal angle of friction for cohesionless solids. For cohesive materials, particle adhesion also play an important role causing the material to pile up and thus the angle of repose to increase. We find that the angle of repose measurements do not correlate well with either the intergranular friction or adhesion data. There is a better correlation with the granule-substrate adhesion values. More intergranular measurements are needed at different RH and controlled surface roughness to clarify this issue.
The atomic force microscope ŽAFM. has successfully been used for direct measurement of friction and adhesion forces between a single spray-dried ceramic granule and a flat, hard metal substrate, and between two individual granules. Using AFM for friction measurements is advantageous, compared to other indirect techniques for directly measuring internal and external friction in powder pressing. The non-linear friction data was evaluated with a simple model that applies to boundary lubrication. This model gives the adhesion force and the limiting coefficient of friction at high loads. We found that the friction is strongly dependent on binder concentration and relative humidity. Our results show that granule–granule friction is lower than granule–wall friction at low relative humidities. We also find that the granule–wall friction coefficient is relatively high, but the adhesion force is low, at low humidities, whereas the situation is reversed at high humidities. The lowest friction coefficients are reported for the highest binder concentration, regardless of humidity. The adhesion force has a strong dependence on humidity, where a markedly stronger force is measured for high binder concentrations and high relative humidities. A good correlation of adhesion with granule flowability is found for all granule compositions at low humidities. We attribute these findings to the hydrophilic character of the binder, where water absorption lowers the shear resistance of the binder-rich granule surface.
Acknowledgements This work has been performed within the Brinell Centre —Inorganic Interfacial Engineering, supported by the Swedish National Board for Industrial and Technical Development ŽNUTEK., and the following industrial partners: Erasteel Kloster, Hoganas, ¨ ¨ Kanthal, Sandvik, Seco Tools and Uniroc.
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w31x K. Shinohara, M. Oida, B. Golman, Powder Technol. 107 Ž2000. 131–136. w32x L.C.Y. Chan, N.W. Page, Powder Technol. 90 Ž1997. 259–266. w33x G. Binnig, C.F. Quate, C. Gerber, Phys. Rev. Lett. 56 Ž1986. 930–933. w34x A. Meurk, Royal Institute of Technology, Stockholm, 1999. w35x J.P. Cleveland, S. Manne, D. Bocek, P.K. Hansma, Rev. Sci. Instrum. 64 Ž1993. 403–405. w36x G. Bogdanovic, A. Meurk, M.W. Rutland, Colloids Surf., B Ž1999. accepted for publication. w37x A. Feiler, P. Attard, I. Larson, Rev. Sci. Instrum. Ž2000. submitted for publication. ¨ w38x J. Odlund, Uppsala University, Uppsala, 2000. w39x F.P. Bowden, D. Tabor, The Friction and Lubrication of Solids, Oxford Univ. Press, London, 1950. w40x A. Berman, J. Israelachvili, in: B. Bhushan ŽEd.., MicrorNanotribology and its Applications, Kluwer Academic Publishing, Amsterdam, 1997, pp. 317–329. w41x E. Meyer, R.M Overney, K. Dransfeld, T. Gyalog, Nanoscience: Friction and Rheology on the Nanometer Scale, World Scientific Publishing, Singapore, 1998. w42x R.W. Carpick, D.F. Ogletree, M. Salmeron, J. Colloid Interface Sci. 211 Ž1999. 395–400. w43x B.J. Briscoe, D. Tabor, in: D.T. Clark, W.J. Feast ŽEds.., Polymer Surfaces, Wiley, Chichester, 1978. w44x M.J. Adams, B.J. Briscoe, S.L. Kremnitzer, in: K.L. Mittal ŽEd.., Physicochemical Aspects of Polymer Surfaces, Plenum, New York, 1983. w45x B.J. Briscoe, M.J. Adams, Tribology in Particulate Technology, Adam Hilger, London, 1987. w46x D.H. Gracias, G.A. Somorjai, Macromolecules 31 Ž1998. 1269– 1276. w47x C.W. Nies, G.L. Messing, in: J.A. Mangels ŽEd.., Advances in Ceramics, 1984, pp. 58–66. w48x R.A. Youshaw, J.W. Halloran, Am. Ceram. Soc. Bull. 61 Ž1982. 227–230.
Silicon nitride granule friction measurements with an atomic force microscope: effect of humidity and binder concentration Anders Meurk a , Joseph Yanez b, Lennart Bergstrom ¨ a,) a
Institute for Surface Chemistry, PO Box 5607, SE-114 86 Stockholm, Sweden b Procera SandÕik AB, SE-126 80 Stockholm, Sweden
Received 1 May 2000; received in revised form 1 December 2000; accepted 18 December 2000
Abstract This study introduces the atomic force microscope ŽAFM. for direct measurement of internal and external friction in ceramic powder pressing. The friction measurements were performed between two single granules, and a granule and a hard metal substrate as a function of granule binder concentration, relative humidity and sliding velocity. We found that the friction coefficient decreased with increasing humidity for a specific binder concentration, the effect being more pronounced for low concentrations. The friction coefficient also decreased with increasing binder concentration. Evaluation of the adhesion force showed a steady increase with both humidity and binder concentration. A substantial difference, more than an order of magnitude, was seen for the highest binder concentration at low and high humidities. We attribute these findings to the hygroscopic nature of the binder, polyŽethylene glycol. ŽPEG.. Softening of the PEG at increasing humidities lowers the friction coefficient but increases the adhesion force. The results are consistent with flowability and angle of friction measurements. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Atomic force microscope; Granule; Friction; Adhesion; Binder; Powder compaction
1. Introduction Industrial compaction of fine ceramic powders requires granulation in order to facilitate handling and feeding prior to die filling w1,2x. Granulation is commonly achieved by spray-drying ceramic slurries that have been optimised with respect to viscosity, solids loading and organic additives, e.g. binder, dispersants and defoamers w3–6x. Controlling processing parameters enables design of granule properties, thereby directly influencing green compact characteristics. During spray-drying, the binder can migrate to the granule surface, particularly if it is water-soluble w7,8x. This results in a hard, binder-rich surface that resists deformation, and is potentially detrimental to the final microstructure. Binders commonly used for powder pressing are polyŽvinyl alcohol. ŽPVA. and polyŽethylene glycol. ŽPEG.. PEG is a low molecular compound made from polymerised ethylene oxide with good lubricating and plasticising properties w9x. The advantages of using PEG,
)
Corresponding author. Tel.: q46-8-790-9900; fax: q46-8-208-998. E-mail address: [email protected] ŽL. Bergstrom ¨ ..
or modified PEG, are its stable pressing properties and its ability to achieve sufficient granule deformation without adding a plasticiser w10x. The plasticising effect of PEG on PVA and the equivalent effect of humidity on the glass transition temperature ŽTg . has been studied in detail w11–18x. Plasticisation lowers the Tg , making the binder more easily deformable. It is desirable to press components at temperatures above the binder Tg , where enhanced densification is realised due to high granule deformability and decreased granule strength. PEG compounds usually have a Tg below 08C, resulting in optimal performance at room temperature. The hygroscopic nature of PEG makes this binder sensitive to changes in relative humidity ŽRH., thereby affecting granule friction and adhesion. A comprehensive study of the effect of seasonal variations in humidity on the final properties of ceramics produced from granules with PVA as a binder appeared recently w19x. It was shown that granules became stiffer during dry winters due to an increased Tg . This resulted in a final microstructure with remaining intergranular porosity. The glass transition temperature of PEG is rather non-sensitive to humidity, but water uptake is substantial w20,21x. Increasing humidity thereby softens the PEG by lowering the shear modulus,
0032-5910r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 Ž 0 1 . 0 0 2 6 0 - 1
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making handling of the granules considerably more difficult due to increased stickiness, and thus a decreased flowability. Both intergranular friction Žinternal friction. and granule–wall friction Žexternal friction. during filling of the die and subsequent pressing is affected by the choice of binder, plasticiser and relative humidity. Granules behave as elastic spheres up to some critical level of stress, where the granules either deform plastically or fracture, depending on the binder concentration w2,22– 24x. The fracture strength of silicon nitride granules must be as low as possible to result in a narrow pore size distribution of the green body w25x. The decrease in granule shear modulus with increasing binder content favours deformation rather than fracture. Deformable, soft granules thus enhance packing by filling intergranular pores, but the high friction forces may yield low-density areas of incomplete granule deformation within the compact w26x. Current techniques of measuring wall friction and internal friction of powders are mainly based on indirect methods. Common techniques of determining the wall friction involve sliding a pressed body against the wall material or examination of the ejection pressure and springback effect w5x. Internal friction measurements of individual granules are scarce. In the Jenike shear cell, and related designs, a shear force is applied transversely to a box containing the powder w27–29x. Both the internal angle of friction and wall coefficient of friction can be determined. The coefficient of internal friction in powder compaction was recently deduced from shear strength experiments, where, however, contributions from die wall friction can affect the result w30,31x. A measure of interparticle friction can be obtained from the ratio of tap density to apparent density Žthe Hausner index. w32x. The angle of repose and flowability measurements are important characteristics but do not describe the sliding behaviour under an applied load. Re-
lating angle of repose data to internal friction is only reasonable for cohesionless materials. In this paper, we present a new method for direct measurement of both internal and external friction and adhesion using the atomic force microscope ŽAFM. w33x. This technique is fast, simple and reliable for sliding friction measurements between a microscopic probe and another surface of arbitrary geometry. Measurements can easily be performed between two single granules or a granule and a flat substrate. The advantage over other techniques is that we measure friction forces on a scale equivalent to the granule interaction forces, without the influence of post-granulation parameters on the results. Also, granule fracture and strength can be followed optically and measured. Once set-up and properly calibrated, the AFM measures friction and adhesion force as a function of sliding velocity, applied load and relative humidity. Standard AFM imaging gives additional information of the granule and substrate roughness. Altogether, these AFM measurements clearly illustrate the importance of humidity and binder concentration on both friction and adhesion, allowing a comparison of the results to flowability measurements, compaction curves, ejection pressure and compact strength.
2. Experimental 2.1. Materials Silicon nitride-based granules were produced by conventional spray-drying with an outlet temperature of 3008C. The aqueous slurry of 30 vol.% solids loading was deflocculated using an anionic polyelectrolyte. A polyŽethylene. glycol binder consisting of a mixture of different molecu-
Fig. 1. Optical image of the spray-dried silicon nitride granules used in this study.
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graphic image. A topographical granule image is shown in Fig. 2, where the image size constitutes the length scale over which the sliding friction measurements were performed. Prior to friction measurements, the WC substrate was cleaned with ethanol and blown dry with nitrogen. Granules attached to cantilevers were stored at controlled relative humidity. The granules used for flowability measurements were stored in a humidity cabinet for 24 h, then taken out and measured within an hour at ambient conditions equivalent to those prevalent during friction measurements. Angle of repose measurements were done with granules equilibrated 24 h at the ambient humidity of 24%.
2.2. Friction measurements Fig. 2. AFM image of the binder-rich granule surface. The surface RMS value is 63 nm.
lar weights ŽMW s 3000, 15 000 and 40 000. was also added to the slurry. Four different concentrations of PEG were chosen for a comparative study of the adhesion and lubrication behaviour: 0.81, 1.62, 3.25 and 6.5 wt.% PEG. The different granules are denoted A, B, C, and D with increasing binder content. Typical granule size and shape after spray-drying can be seen in Fig. 1. Substrates used for friction measurements were polished tungsten carbide ŽWC. containing a small amount of Co, with a RMS roughness of 3 nm as determined from topographical 4 = 4 mm AFM images. In addition, a single granule glued to a rigid disc was used as substrate for individual granule friction measurements. The granule surface RMS-value was 63 nm, as evaluated from a 4 = 4 mm AFM topo-
An atomic force microscope ŽMultiMode SPM, Nanoscope IIIA, Digital Instruments, USA. was used for sliding friction measurements. In standard topographical imaging mode, a sharp tip is in mechanical contact with the sample surface. Interaction forces deflect the cantilever, which is sensed by a laser reflected from the back of the cantilever onto a position sensitive photo diode ŽPSD.. A computer-controlled feedback loop connects the PSD with a piezoelectric scanner underneath the sample. The piezoelectric scanner adjusts the tip–sample separation distance during scanning by keeping a constant cantilever deflection. Optimising the feedback assures that a constant force is maintained throughout the experiment. Friction measurements are attainable by recording the torsional bending of the cantilever as it slides sidewards a specified distance across the surface. This signal is readily available from the detector and can subsequently be converted into friction force at a particular applied load w34x.
Fig. 3. A typical AFM cantilever with attached granule viewed from below. The granule diameter is approximately 20 mm.
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The relative nature of the AFM necessitates calibration of scanner, detector and cantilever for converting the raw data to meaningful friction values. Cantilever normal spring constants were calibrated by attaching a series of tungsten spheres of varying mass and measuring the corresponding shift in resonance frequency w35x. Spring constants were determined within 10% accuracy, and those used here were typically between 0.5 and 3 Nrm. This is about an order of magnitude higher than those normally used in AFM contact mode, improving the signal to noise ratio during friction measurements. Owing to the fact that reliable torsional spring constants are more difficult to substantiate Žalthough some promising experimental techniques are emerging w36,37x., the cantilevers included in this study were uncoated, tipless, rectangular beams made of silicon. We carefully evaluated dimensions from SEM-images and used materials properties stated by the manufacturer ŽSilicon MDT, Moscow, Russia.. Granules of different PEG-content were glued on to the cantilevers with a two-component epoxy resin by using a micromanipulator consisting of two glass fibres: one for picking up glue and one for moving particles to the cantilever. Fig. 3 shows a granule probe used for friction measurements. All granules used were typically in the 20-mm range. In the case of granule–granule friction measurements, granules spread on a layer of glue and cured for 24 h served as the substrate. All friction measurements were carried out as a function of humidity, sliding velocity, applied load and granule PEG concentration. Several friction–load curves were recorded at different sliding velocities Ž2–120 mmrs. for each granule binder concentration. The applied load was also deliberately increased with humidity in order to test the effect of decreasing binder shear modulus on granule strength. A program specifically developed for AFM friction analysis was used for the evaluation of friction coefficients w38x.
3. Theoretical background 3.1. Friction and adhesion models The friction between rough surfaces in sliding contact is generally well-described by Amonton’s law with a direct proportionality of the friction force to the applied load w39x. Plastic deformation occurs when interlocking asperities are sheared, giving rise to a true area of contact much smaller than the apparent contact area estimated from surface geometry. Friction–load dependence for rough surfaces exhibiting some degree of adhesion is usually described by Ff s F0 q m L
Ž 1.
where Ff is the friction force, F0 the friction at zero load, m the friction coefficient and L the applied load w40x. The
finite friction at zero load has termed this behaviour adhesion-controlled friction, due to the sustained sliding friction at negative loads. Molecularly smooth surfaces, i.e. where a single-asperity contact is achieved, have been shown to conform to contact mechanics theories, such as JKR and DMT w41x. More complex and rough surfaces, such as ceramic granules containing a polymer-rich surface layer, do not generally follow these theories. Softening of the polymer and build-up of contaminants in the contact area do not give a single asperity contact. An attempt to fit the friction–load curves in this study with a numerical contact mechanics model proposed recently w42x resulted in unrealistic fitting parameters. The model of Bowden and Tabor w39x, based on adhesive junctions ruptured by frictional work, is a more realistic approach to describing the frictional behaviour of hard particles. The friction force is described by Ff s t A s CL2r3 q a L
Ž 2.
where C is the contact area multiplied by the intrinsic shear strength Ževaluated at zero applied load. calculated from Hertz theory, and a is a pressure constant w43–45x. This theoretical approach assumes that the elastic deformation is balanced by a variation in shear strength with applied load. Recent experiments on friction of polymer surfaces have been shown to follow Eq. Ž2. closely w46x. The presence of a soft organic binder on the granule surfaces may infer a boundary type lubrication where the interfacial rheology of the polymer will control the friction. This type of frictional behaviour may be described by an equation of the form Ff s k Ž L q La .
n
Ž 3.
where k is the friction factor, La the adhesion force and n the load index. The friction factor is related to components regulating a Hertzian contact, while the load index is to a large extent affected by deviations from a shear strength not being of constant value. The load index can vary between 2r3 and 1; the latter case is equivalent to a multiple asperity contact in accordance with Amonton’s law. It is clear from the above that the coefficient of friction depends on the intrinsic shear strength and the contact pressure. At high loads, the load index approaches unity. For this special case, the friction factor reduces to the pressure constant, thereby corresponding to the coefficient of friction. We have chosen to use the more general Eq. Ž3. for a qualitative evaluation of the influence of binder properties, surface deformation and adhesion of granules in sliding contact. Shear strength variations are related to binder softening, whereas granule deformation is influenced by load, roughness, adhesion and the moduli of elasticity and rigidity.
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4. Results and discussion 4.1. Granule–die wall friction measurements Friction measurements between a granule and a WCbased hard metal substrate at the relative humidities 38%, 45% and 60% are shown in Figs. Ž4–7.. The majority of the friction–load curves show a non-linear dependence on load, as predicted from theory. The load index, n, is always smaller than 1, which means that the friction coefficient, m , is load dependent; it increases rapidly at low loads but becomes constant at high loads w45x. Hence, a limiting coefficient of friction was therefore evaluated at the highest loads for each measurement, where the friction coefficient equals the value of the friction factor, k, and the pressure constant, a . The adhesion component of the friction force was obtained from fitting experimental friction data to Eq. Ž3.. Parameters obtained from friction–load curves are shown in Table 1, and friction coefficients in Table 2. Fig. 4 shows friction–load curves at 38% RH. A clear trend is observed in which the friction force and the coefficient of friction decrease with increasing binder concentration. The data for the granule with the lowest amount of binder Žgranule A. displays more scatter at high loads than the other concentrations. Considering the relatively low humidity and binder concentration, this granule is more prone to fracture than deformation, resulting in wear at high loads. Granule B shows a non-linear frictional behaviour, implying that the granule deforms elastically. The binder-rich granules ŽC and D. appear to have similar
Fig. 4. Friction–load measurements at 38% RH between a granule and a flat substrate. The solid lines are fits to Eq. Ž3..
Fig. 5. Friction–load measurements at 45% RH between a granule and a flat substrate. The solid lines are fits to Eq. Ž3..
friction–load dependence; both exhibit a relatively large adhesion force and a low friction coefficient. However, the low load index indicates that elastic and geometry-dependent granule material properties contribute considerably to the contact shearing behaviour. Evaluation of granule–wall adhesion at 38% RH reveals an overall low adhesion force, where a clear trend is seen
Fig. 6. Friction–load cycles on a flat substrate showing increasing and decreasing loads for two consecutive measurements. Hollow symbols denote decreasing loads. The solid lines are fits to Eq. Ž1. for granule A and Eq. Ž3. for granule D.
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Table 2 Friction coefficients evaluated from granule friction measurements and angle of repose data Granule PEG ma Žwt.%.
mb
Angle of repose Ž8.
38% RH 45% RH 60% RH 38% RH 24% RH A B C D
0.81 1.62 3.25 6.5 a b
0.12 0.09 0.08 0.07
0.08 0.06 0.06 0.06
0.05 0.06 0.04 0.03
0.07 0.06 0.06 –
42 45 47 59
Granule–substrate measurements. Granule–granule measurements.
taken at increasing and decreasing loads. This so-called adhesion hysteresis clearly asserts that the binder-rich surface changes the contact dynamics. Fig. 6 shows two consecutive measurement cycles for granules A and D. The first and second cycles overlap with minimal hysteresis for granule A. Granule D displays an initial linear increase in friction force with load, but a substantial hysteresis occurs at decreasing loads. The net energy loss during a cycle for molecularly smooth surfaces can be explained by a difference in advancing Žloading. and receding Žunloading. surface energies. In these measurements, the hysteresis most probably occurs due to an irreversible change in shear strength with increasing contact pressure. As the granule is pressed and slid against the substrate, binder softening causes a time-dependent, viscous, surface deformation. The second cycle shows much less adhesion hysteresis and lies between the data for the first cycles. The energy dissipation stemming from hysteresis has been significantly reduced after the first contact, enabling reproducible frictional behaviour. Note, however, that the coefficient of friction is mildly affected by the number of sliding contacts Žmeasurement cycles.. We can relate our friction and adhesion results to observations during pressing. An increased binder content often leads to an improved compaction but may also give rise to more problems with granule sticking to the wall. This is an effect of the reduced friction coefficient and increased adhesion with increased binder content. The large friction-load hysteresis at high binder content can also be one contribution to the large spring-back often observed w47x.
Fig. 7. Friction–load measurements at 60% RH between a granule and a flat substrate. The solid lines are fits to Eq. Ž3..
with respect to binder concentration for all granules except granule B. The restricted ability of the granules to deform, i.e. the brittle character of the binder, leaves the morphology rather than the viscous PEG binder responsible for the low adhesion forces. It is, therefore, believed that granule friction is only partly responsible for the flow and compaction characteristics at low humidities; the surface roughness is also a very important parameter. An interesting observation noted at 45% RH is the shift in relative position between the friction–load curves Žsee Fig. 5.. Granules C and D still group together but the increase in adhesion force has shifted their position above granules A and B. In spite of the higher friction forces, the coefficient of friction is lower at 45% RH compared to 38% RH ŽTable 2.. Granule A still exhibits the highest value, whereas the difference in m between granules B, C and D at 45% RH is almost negligible. The adhesion force increases substantially with RH for all granules. A clear difference between the results for granules C and D at 45% RH compared to those obtained at 38% RH is the increased hysteresis between friction–load curves Table 1 Parameter evaluated from fitting Eq. Ž3. to granule friction measurements Granule
PEG Žwt.%.
Ladh ŽnN. a 38% RH
45% RH
60% RH
338% RH
38% RH
45% RH
60% RH
38% RH
A B C D
0.81 1.62 3.25 6.5
21 6 25 27
60 84 305 187
80 226 390 417
18 2 10 –
0.67 0.67 0.67 0.73
0.72 0.67 0.67 0.67
0.72 0.78 0.67 0.71
0.81 0.98 0.76 –
a b
Granule–substrate measurements. Granule–granule measurements.
Ladh ŽnN. b
na
nb
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Friction measurements performed at 60% RH are shown in Fig. 7. As opposed to the clear trends at lower humidities, there is no particular consistency with respect to the magnitude of the friction force or the relative position of the friction–load curves. However, evaluation of the friction coefficient reveals a continuous decrease compared to lower humidities at all binder concentrations. The adhesion force increases with the binder content. Both granules B and C display a significantly higher adhesion compared to 45% RH, which is most likely a result of the softening of the outer, PEG-rich surface layer. An important consequence for compaction of these easily deformable granules is the low resistance to sliding. This leads to an increased green body density, but also powder handling problems and agglomerate caking because of the relatively strong adhesion force. The inconsistent and deviating sliding behaviour at 60% RH compared to lower humidities brought about an investigation of the effect of sliding velocity on friction. Varying the rate within two orders of magnitude did not significantly affect the friction force. As the load increased, there was a slight tendency of the friction force to decrease at higher velocities. This effect may be explained by binder shear thinning, which is known to occur at high humidities w13x. However, the overall rate dependence was weak and thus of minor importance. Friction coefficients for the above measurements, derived at high loads where the load index approaches unity, are averages of four experiments for each PEG-concentration. The variation in friction coefficient for one particular PEG-concentration did not exceed 10%, whereas the adhesion force was highly dependent on contact history, sometimes varying with a factor of two or three for consecutive measurements. Hence, the distribution of binder at the granule surface, and residual amounts from the spray-drying process, strongly affects the intergranular flow characteristics. Acting as a lubricant in all of the above friction measurements, the effect of binder is more pronounced at high concentrations but probably vanishes above a certain value when the surface binder concentration saturates. Increasing binder concentration and relative humidity allows for improved granule deformation, and consequently improves granule joining during pressing w2,11,12,18,48x.
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The low humidity results in small variations in sliding behaviour. Granule A has a friction coefficient of 0.07, where both the friction and adhesion force increases slightly with increasing number of measurements. Shearing of granule surface asperities leads to wear until a smoother contact is established, yielding a higher adhesion force. The average friction coefficient for the granules B and C is somewhat lower at 0.06. Surprisingly, there is a substantial difference in adhesion force between these systems. The extremely low value of 2 nN for granule B is somewhat unexpected. A possible explanation is a very rough contact between the granules throughout the experiment. Earlier results from intergranular friction measurements for granule C at 30% relative humidity showed a slightly lower friction coefficient Ž0.05. and an even lower adhesion force w34x. Measurements for the highest binder concentration Žgranule D. were very scattered due to difficulties in maintaining a stable contact, and reproducible friction results could not be obtained. The high amount of binder results in surfaces too compliant for a constant force to be maintained by the AFM feedback loop. A comparison between granule–wall and intergranular friction and adhesion ŽFigs. 4 and 8., reveals an overall higher coefficient of friction and adhesion force for granule–wall friction, regardless of PEG concentration. This suggests that wall friction dominates the pressing response at low humidities. 4.3. Flowability and angle of repose measurements The friction and adhesion results have been compared with granule flowability ŽFig. 9. and angle of repose
4.2. Intergranular friction measurements Friction measurements between two single granules at a relative humidity of 38% are shown in Fig. 8 for all granules except the one with the highest PEG-concentration. These results also follow Eq. Ž3., with a high load index. Results for granule B conform to Eq. Ž1., indicating that the friction coefficient can be evaluated regardless of the loading regime. Recalling the high granule surface roughness ŽFig. 2., it is evident that the surface deformation is insufficient for a smooth contact to be maintained, resulting in a multiple-asperity contact.
Fig. 8. Friction-load measurements at 38% RH between two individual granules. Granule D was too soft for a stable contact to be maintained. The solid lines are fits to Eq. Ž3..
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5. Conclusions
Fig. 9. Granule flowability as a function of humidity. Note the switch at high humidities of granules C and D.
measurements ŽTable 2.. Both techniques yielded reproducible results, but data became more scattered with time due to changes in relative humidity. This was especially the case for granule C, where the flowability rate quickly decreased after the third run. At low humidities, the flowability results follow the same trend as the adhesion measurements. Granule B has the best flowability and lowest adhesion, followed by granule A. Granules C and D display a significantly over flowability compared to the granules with lower binder concentrations. The correlation between flowability and adhesion is poorer at the higher humidity of 60%. Granules A and B both exhibit lower adhesion and lower flowability times than the other granules, but the descending order of adhesion and flowability. The flowability of the granules are therefore probably related to both granule morphology and granule friction. A surprising finding was the switch in flowability behaviour of granules C and D at high humidities. It is possible that the binder-rich surface of these granules is so soft at high RH that the granules slowly deform and fuse together with time. Examination of the friction–load curves at 60% RH showed a large adhesion hysteresis, which was not seen as markedly for the other granules. The angle of repose measurements relate to the internal angle of friction for cohesionless solids. For cohesive materials, particle adhesion also play an important role causing the material to pile up and thus the angle of repose to increase. We find that the angle of repose measurements do not correlate well with either the intergranular friction or adhesion data. There is a better correlation with the granule-substrate adhesion values. More intergranular measurements are needed at different RH and controlled surface roughness to clarify this issue.
The atomic force microscope ŽAFM. has successfully been used for direct measurement of friction and adhesion forces between a single spray-dried ceramic granule and a flat, hard metal substrate, and between two individual granules. Using AFM for friction measurements is advantageous, compared to other indirect techniques for directly measuring internal and external friction in powder pressing. The non-linear friction data was evaluated with a simple model that applies to boundary lubrication. This model gives the adhesion force and the limiting coefficient of friction at high loads. We found that the friction is strongly dependent on binder concentration and relative humidity. Our results show that granule–granule friction is lower than granule–wall friction at low relative humidities. We also find that the granule–wall friction coefficient is relatively high, but the adhesion force is low, at low humidities, whereas the situation is reversed at high humidities. The lowest friction coefficients are reported for the highest binder concentration, regardless of humidity. The adhesion force has a strong dependence on humidity, where a markedly stronger force is measured for high binder concentrations and high relative humidities. A good correlation of adhesion with granule flowability is found for all granule compositions at low humidities. We attribute these findings to the hydrophilic character of the binder, where water absorption lowers the shear resistance of the binder-rich granule surface.
Acknowledgements This work has been performed within the Brinell Centre —Inorganic Interfacial Engineering, supported by the Swedish National Board for Industrial and Technical Development ŽNUTEK., and the following industrial partners: Erasteel Kloster, Hoganas, ¨ ¨ Kanthal, Sandvik, Seco Tools and Uniroc.
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