Erosion of polymer-particle composite coatings by liquid water jets

WEAR ELSEVIER

wear 203-204 ( 1997) LX-97

Erosion of polymer-particle composite coatings by liquid water jets ’Depnmenr

B.J. Briscoe *, M.J. Pickles a*‘,KS. Julian b, M.J. Adams b ofChemical Engineering and Chemical Technology.Imperial College of Science.Technologyazd Medicine,Prince Conwrt Rwd London SW7 2BY. UK b !!nilever Research,Pan Sunlighr Laborarory, Quarry Road Eatr. Bebingron, Wirral L63 3Jw. UK

Abstract

This paper describes a study of the erosion of a polymer-particle coating system by a water jet apparatus. Modified versions of two established wear models were applied to this system; these were based on a simple hardness contmllcd abrasion model and a bulk toughness contm&zd abrasion model. T%emechanical properties of the coatings were modified and their bulk properties were characterized using an indentation hardness technique. The hydrodynamic erosion characteristics of the same coatings wcm examined using a water jet apparatus. The emsion of the coatings was observed qualitatively and was also quantified in terms of a critical water jet pnssun. The modified wear models were then applied to this system, in terms of the bulk mechanical properties of the coatings, and the models were critically evaluated and appraised. 0 1997 Elsevier Science S.A. All righe reserved. Keywords: Erosion:Water&l;Coatings;Hardness:Rame-Lancaster correlation

1. Introduction This paper presents a study of the erosion of a model surface coating system by the action of hydrodynamic flows; this form of surface damage is often classified as a type of erosive wear. In many chemical process plant applications, it is a common procedure to transport fluids which contain a high fraction of solid particles or particle agglomerates. A consequence of this is that the particles, or agglomerates, often impact against the walls of the pipes or the process vessels through which they are passing, and as a result they may become adhered to the walls. Over a period of time, many particles may adhere and a coherent layer may build up; this phenomenon is known as ‘fouling’. These fouling layers may bc detrimental to the performance of the process plant and a common technique for the removal of the deposits is to use water sprays and jets. In other applications, it may bc desirable to maintain the integrity of a dcpositcd surface layer under hydrodynamic flow conditions, for example, in solid and boundary lubrication. The performance of the equipment may he affected detrimentally if tbe lubricantlayershouldbecome eroded. Therefore the study of theremovalof suchsurfacedeposits by the erosive action of liquid flows is of some importance to the optimization of these systems.

’Unsent address: Dcpamnent

of MaterialsScienceandMetallurgy,Univenity of Cambridge.PembrokeSueat,Cambridge,CR2342 UK.

Elsevier Science S.A. All rightsresewed

0043s1648/97/$17.000 1997 PllSOO43-1648(96)07379-6

The flow configuration adopted in this study was a liquid jet, which represents an important practical configuration for the removal of surface deposited layers. In a previous publication, the erosion of a different model coating system was examined using a liquid radial flow shear cell [ 11. ‘I%e major focus of this study has been to measure the bulk mechanical properties of a model deposit system and to examine the influence of these properties on the hydrodynamic induced erosion characteristics of the deposits. To this end, coatings wcrc pnparcd from glass Baliotini spheres bound with various mass fractions of a poly (methylmethacrylate) to produce a model deposit system in which the mechanical properties of the coatings could easi!y be varied. The bulk mechanical properties were characterized using an indentation technique and the coatings were eroded using a waterjet apparatus. Two established wear models were modified and applied to the current configuration.

2. Modified abrasion

models

In principle, two extreme modes of wear may be conceived for the present coating system: the erosion of the bulk of the intcrface. In the case of the data to be describe& the apparent wear process was observed to he erosion of the bulk of the coating. Many models exist for rationalizing the damage caused by erosive wear, although very little literature exists on the spe-

COating itself, or the rupture of the coating-substrate

B.J.13ticaceral. / Wcar203-204(1997)88-97 cific case of hydrodynamic flow induced erosion. At a qualitative level. one can easily appreciate that acoating which is relatively soft and ductile Will be less reSiStaMto efO8iVewear than a coating which is tougher and more durable; this is embodied in many erosion or wear models. This paper expands on these observations and seeks to rationahxe the observed hydrodynamic erosion mechanisms using established wear models. As the erosion patterns observed under hydrodynamic flows resemble those caused by cohesive erosive or abrasive wear, abrasion models may be applied, to a first order, to predict the erosion rates under theseconditions. 2.1. Rabinowicz abrasion model A simple, but nevertheless useful, model of the abrasion process may be derived from a geometric argument. Rabinowicz [2] has derived such a model, in which the material removal rate was predicted to be inversely proportional to the hardness of the abraded surface. The validity of this model, as a practical tool, has been confirmed for materials such as metals, which do not show significant variations in theirelastic elongation prior to rupture [ 31. for a cohesive or abrasive erosion process, the erosion rate is predicted to be inversely proportional to the surface hardness H.

Thus,

2.2. Modijied Ratner-L.ancaster model

89

H=ccr, where c is a constant, whose value is approximately 3 for perfectly plastic materials. For PMMA, the value of c is close as 2.6 (81. Thus, applying Rq. (2) and Eq. (3), we may write the modified Ratner-Lancaster correlation as w,=-

k2E

(4)

M

where k is a constant of proportionality. We may then eliiinate the proportionality constants by defining a parameter WY, as follows: W

& (5)

Wr=my’,$

where Y is Poisson’s ratio and the reduced elastic modulus is defined as

F;

The Rabinowicz model has been shown to provide a very reasonable estimate of the abrasion rate for metals and highly plastically deforming materials. However, materials with relatively low elastic moduli and variable strains to mpture. such as organic polymers, normally show a poor fit with this simple mode1 [4]. TheRatner-Lancaster correlation hasbeenshown to provide a rather good indication of the resistance of many polymeric materials to abrasion [ 5,6]. This model assumes that the energy required to remove a unit volume of the material from the surface is inversely proportional to the bulk fracture energy of the material. which is sometimes referred to as the cohesive fracture toughness. This parameter, in the original form of the model, is taken to be proportional to the area under the stress-strain curve for tensile rupture. Thus, if the material is assumed to exhibit a linear stress-strain curve prior to failure (again an assumption adopted in the model), the cohesive toughness is proportional to ( uY+) - ‘, where oY and 4 are the yield stress and strain of the material respectively. Hence, the Ratner-Lancaster correlation may be writtenas [6]

E*=-

(6)

The parameters E* and H were measured for the coatings used in the present study by using the nccmsd indentation technique, described below. lhese paramdas were then applied to the modified Rabinowicz and Ramer-Lmcaskr models and the results of these models were compated with the measured hydrodynamic erosion characteristics of the model coatings. These two models, which am applied to the current data, presume a unit fracture process; that is, material is removed incrementally in each deformation cycle. No account of the proc*Ft of interfacial failure (at Ue coating-substrate interface) is considered. Ai the conclusion of the paper, the significance of the assumptions in these model8 is considered further.

3. Experimentnl 3. I.

where WCis the cohesive wear, or erosion, rate. For coatings, the parameters try and e,. are generally not straightforward to measure. Therefore, a modified form of the Ratner-Lancaster correlation may be derived in terms of the surface hardness and the reduced elastic moddus [ 11. ‘these parameters are

E 1 - v2

techniques

Matt?rials

The system investigated in this study compri8edsoda-lime glass Ballotini spheres, with a nominal size range 4-45 pm, which were bound with a poly(methy!methactylate) (PMMA, average molecular weight 75 000) to produce coherent coatings. The coatings, which wete typicalty 3 mm

B.J. Briscoe et al. / Wear203-204

in thickness,were preparedby mixing the glass spheresin a solution of the polymer in chloroformand allowing the solvent to evaporate.The coatings were subsequentlyannealed at 60 “C for 24 h prior to use. The mechanicalpropertiesof the coatings were modified by varying the bin&r fraction between 2% and 8% by mass. The samples were cast on aluminium substrates,which had been roughenedto an even surfacefinish (R. = 1.6 km) with 400 grade abrasivepaper. The aluminium surfaceswere cleanedin chloroformprior to the deposition.

(1997) 88-97

is the imposeddisplacement,and n is an index whose value is greaterthanunity. The Box-Cox curvefitting method [ 101 was adoptedin the currentstudy to fit numericallythe experimentaldata to yield equationsof a similar fotm to the above relation. This technique offers substantially reducederrors compared with other curve fitting techniques [ II]. The experimentalloading and unloading curves were fitted to provide equationsof the form Pi= lm,(h-Ml”’

(7)

and 3.2. Normal indentation

P2= [m,(h-h,)]“*

(8)

wherethe subscriptsI and 2 referto the loading andunloading processes,m and n are constants,hc is the xero offset and h, is the final (recovered) indentationdepth. 3.4. Hardness The hardness H of a material which is indented by an axially symmetrical indenter is given by [?I

where.P,, is the maximum load transmittedto the sample and a is the projectedradius of the indentation in the plane orthogonal to the loading axis. The hardness is usually regarded as a measure of the material resistance to plastic flow. Indentationsin polymeric samples, however, are generally elastoplastic in nature.[ 121 and thereforethe elastic componentof the indentationmust Iirst be extractedin order to calculatethe hardness. If the geometryof the indenter is known, the value of a may be estimatedfrom the penetrationdepth /I by applying a simple geometricargument: a=(&+S)tanB 3.3. Data analysis procedures It is generally found that the load-displacementrelations for both the loading and unloading processesin indentation are of the form Pa h” [ 91, whereP is the transmitted load,h

(IO) where 6 is the half-angle of the cone, hp is the penetration depth correspondingto plastic deformation, and the parameter S is a correction factor which accounts for any deviations in the tip shapefrom perfectgeometry[ 131.The90” stainless steel cone indenterused was examinedusing scanningelectron microscopyimaging and it was found to have a slightly roundedtip; the value of the tip defect 6 was found to be ca. 20 pm. The value of the penetrationdepth corresponding to the plastic component of the deformation hp was obtained from the fitted curves by extrapolating the initial slope of the unloading curve to the zero force axis. This technique is widely usedand has beenshownto provide an accuratemeasureofh, [l4]. 3.X Elastic modulus

Fig. I. Schmmic diagram of the normal indentadon cxperimntal set-u.

The values of the reduced elastic modulus F for the system studied were also calculated From the compliance curves

B.J. Briscor

et al. /Wear

obtained in the indentation experiments. This may be found from a rearrangement of the elastic contact stiffness expression [ 151:

203-204 (1997) 88-97

91

pressure of the water jet on the sample surface Pt was therefore taken as the Bernoulli pressure field: (12)

(11) where the derivative dPs/dh is the slope of the unloadmg curve, evaluated at the point of initial unloading. The values of EC were obtained in the current study from the results of the Box-Cox curve fitting procedure,by evaluatingthe deriv-

ative of the unloadingcurve.Thecomplianceof the apparatus was also taken into accountin the calculations and this technique has been shown to provide values of F which are in very good agreement with those obtained by more conventional methods [ 161. 3.6. Water jet erosion apparatus A conventional water jet erosion apparatus (WJA) was used to study the hydrodynamic erosion characteristics of the samples. The apparatus utilized a high-pressure pump (K-her Ltd., Banbmy, UK) to produce a coherent water jet of ca. 1 mm in diameter.Normal tap water was used as the eroding fluid for all of the experiments. The flow rate of the water was varied by adjusting a bypass control on the pump, which also varied the pressure and the velocity of the water jet obtained. By this means, the pressure of the water jot at tbc nozzk outlet could be varied between 0.3 and 9.0 Mpa, producing jet velocities of up to ca. 40 m s-‘. The samples to be eroded were placed on a platform inside the tank, directly below the outlet of the water jet. A shutter system was incorporated into the apparatus, in order to control accurately the time for which the samples were exposedto

the erosion conditions. In the majority of the data presented in this paper, the samples were exposed to the water jet erosion for a time of 1 min. The apparatus is shown schematically in Fig. 2. The jet velocities in the WJA experiments were not sufficiently high for compressible behaviour in the water jet to occur and the flow was therefore incompressible. The impact

where p is the density of the water. The impinging water jet gives rise to a region of high pressure, known as the stagnation region, at the on the coating surface. The liquid flow then diverges radially outwards, to form a liquid sheet over the surface.‘be radial velocity decreases rapidly, giving rise to a ‘hydraulic jump’, where the thickness of the liquid sheat &eases abruptly. Kaye 171 has studied the removal of relatively weak coatings by the erosive action of liquid jets. His work showed that the area of high shear stress around the stagnation region was responsible for the majority of the erosion and the ‘hydraubc jump’ had little effect on the erosion. Kaye also showed that, for relatively we&coatings. theerosionprocess occurred in two stages. The first stage involved the penetration of the coatings by the water jet and in the second stage the coatings were removed by the hateral fluid Bow.

impact point

[

3.7. Quantification

of rhe observed erosion

The erosion characttristics of the sample coatings were quantified by measuring the mass loss from each sampkafter erosion, using an analytical bahmce. The erosion was additionallyquantifiedbyacriticaljetpreJsunP~forthefailun of the coatings. Above the critical pressure, thecoatings were found to be eroded rapidly by a near catastrophii failure mechanism. Each of the samples was also photographedafter the erosion, to provide a visual record of the damage produced. 3.8. Determination of the surfocc topography Laser prolilometty was used to quantify the erosion in terms of the volume loss for some of the samples and to determine accurately the surface topography of these stmtpies. This device was capable of recording the smfe profik to an accuracy of jrO.2 pm, within the operating range of & 300 )rm. The laser sensor (Rodenstock LM600, Optische Werke G. Rodenstock, Germany) was motmted on a linear stage (Heidenhain, Germany) and the samples wen placed on an X-Y stage below the laser sensor. Ihe aatttples were scanned underneath the. laser sensor in a raster fashion and the surface height data were mcorded atttcnnatically during the experiments at the appmpriate stage positions, to build up a three-dimensional profile of the surface. The appmatus is shown schematically in Fig. 3.

4. Results W

4. I. Bulk mccham’cal properties

to Fig. 2. Schematic diagram of the water jet erosion mpsrsuu.

6.1. Briscoe et al. /Wear

Fig. 3. Sck&c

203-204

(1997) 88-97

diagramof the lasersurfaceprotilottte3er

30

3” 3 B10 a

0.2

0.1

0.0

Ftg. 4. Compli~

0.3

*

6plr curves&tsined fmin the iwhtdon

of catings of

varyingcompositions.

of the coatings were modified. Fig. 4 shows several experimental compliance curves obtained with varying proportions of the PMMA binder phase. It is clear that, as the mass fraction of the PMMA in dte coatings was increased,the indenterdisplacementcorresponding to the common maximum applied load of ca. 20 N was substantiallyreduced. The values of the hardnessIf and the reducedelastic modulus EC were computedfrom the experhttentalcompliance curves, using the Box-Cox technique.Fig. 5 showsthe variation in the values of H and E* for these coatings as the PMMAmass fraction was altered.Both Hand F werefound to increase considerably, in an approximatelyexponential manner,as the mass fraction of the PMMA in the coatings was increasedand thus the coatings became much tougher and more durable as the binder fractionwas increased.

as the mechanical properties

4.2. Hydrodynamic erosion characteristics All of the sample coatings were observedto show similar responseswhen subjected to the water jet erosion and they were all found to be eroded by apparentlythe same mechanism, which involved a cohesive wearprocess.No evidence of interfacialfailure was observed.It wasfound that a critical

jet pressurePti, existed, which was specific to each composition, for the initiation of signigcant erosion. Below this critical pressure, the coatings showed no visible signs of erosion. even at& 10 mitt or more imposed exposuretime. Atjetpressureswhichwerequalto,orlargerthan,thecritical pressure,the erosion of the samples took place by a catastrophic failure mechanism. which followed an initiation period of ca. l&l5 s. This failure mechanismwas a result of the hydrostaticand shearloading action imposedon the coatings owing to the waterjet. During the initiation period. no erosion of thespecimenswas observed,confirmingthatthe range of jet velocities used was insufficient to cause compressible behaviour in the waterjet. Continued exposureto the waterjet atter the rapid erosion appeamdto producelittle increasein the observederosion. Similarly, increasingthe jet pressureabove the critical pressureproducedno significant increasein the level of the measurederosion comparedwith that which was observedat the criticalpressure.Fig. 6shows typical photographsof the erosion patterns observed above and below the critical pressure,for coatings of varrouscompositions. The erosion patterns were generally found to be erosion was found to occur in the centre of the samples, directly below the jet axis. Themasserosionforeachofthespecimenswasdetermined using an riectronic balanceand Fig. 7 showsthe variation in the percentagemass erosion for a coating containing 3% PMMA.as thejet pressurewas vatied. It is evident from this figure that at pressuresbelow the critical pressureP,,, there was virtually no erosion of the coatings, whilst at pressuns greater than Pnit, a high level of mass erosion occurred.It was found that once Pti, wes exceeded, the level of mass erosion for all of the specimenswas similar in magnitudeand dte percentageerosion therefore did not provide a useful indicationof the durabilityof the coatings underthe waterjet erosion.Therefore,the parameterP$ was chosenas a measure of the susceptibilityof the coatings to the erosionand this parameterwas used in order to apply the Rabinowicz and Ratner-Lancastercorrelations.

B.J. Briscoe .?Ial. /Wear 203-204 (1997) 88-97

Fig. 6. photognphsofmCtypiulerosioaprnanrobmvedinmCpMMA/glruBlllocinieatingruDdawr*rjet~. (b) 8% PMMA.etwkd atS.OMPII; (c) 5% PMhWemded aI l.OMPa (bclowcriticpI prrm~e);

(8)2%PMMA,wded~ (d) 5% Pkd&fA.mduJ~4.0MP&

l.OMh

6-

0

I

4 3 jct&m Fig. 7. Variation in the percentage mass erosion with the waterjcl prcssun, for a txlaling containing 3% PhMA and for an exposum time of I min. The value of the critical pressure was identified for each coating composition and these data are shown in Fig. 8. A linear increase in the critical pressure with the mass fraction

OT.U........ 0 I

1

3 4 PMMh-l+

5

I... 6

7

8

of the PMMA binder in the coatings is seen, and the entrapolated line intersects the zero pressure axis at ca 1% PMMA content. This would suggest that, at PMMA mass tiactions

B.J. BrircocN al. /Wear 203-204(I 997) 88-97

94

below this value, the samples would not have sufficient cohesive strength to remain as coherent coatings and would therefore have no resistance to the water jet erosion. It is also notable that the values of the critical jet pressures for the coatings studied are considerably lowerthantheirmeasured

central eroded area is clearly visible. The initial surface profile was recorded before the erosion and this was subtracted from the profile of the eroded sample to provide an estimate of the volume eroded from the sample. This value was calculated as being 17.0 nun), or 4.6% of the total sample volume, which

hardnesses.This would suggestthatthe mechanismof the failurefor these samples may have involved an element of

comparesverywell withthegfavimetrically measuredmass erosionof 4.79%forthesamesample. Severalsurfaceprofiles were also recorded for the samples

rapid fatigue at pressures below the yield stresses of the coatings. Stress concentration would occur at existing flaws in the coatings, leading to failures at relatively low jet pressures. As these coatings were porous in nature, there may also have been a component of the failure which was associated with the internal hydrostatic stresses, caused by the penetration of the water into the coating structures. 4.3. Laser projlotnetry

results

The laser profilometer was used to scanspecificareasof thesamplecoatings,in orderto assessaccurately thesurface topography and the volume eroded from these samples. Fig. 9 shows a typical topographical map and a surface plot of the eroded area of a typical sample containing 6% PMMA. lhis scan was performed over a relatively large area (14 X 12 mm*) in relatively coarse steps ( 100 pm). The shape of the

over a much smaller area, with a greatly reduced step size, to assess more accurately the surface topography of the samples. Scanning electron microscopy was also used for this purpose, to show the structure present within the coatings. Fig. 10(a) shows a typical topographic map of a coating containing 6% PMMA, scanned over an area of 100 X 100 fim*. with a step size of 0.5 Frn. The structure of the coating is shown and the glass Ballotini spheres are visible in this figure. The electron micrograph given in Fig. lO( b) shows the structure of the sample coatings more clearly. The Ballotini spheres appear to be coated with the polymer binder and the surface of the coating is also completely covered with a thin layer of the binder.

Rg. 10. (a) Atopogmphic~p,md (b) a&ngekc@onmiaographof the surface structureof a PhIMA/glsss Bslletiai costing containing 6% PMMA.

B.J. 8,iscoe er al. /Wear 203-204 (I 9971 M-97

The modified abrasion models, which were described ear-

lier, were applied to the coating systems using the measured values of the parametersHand E*. The erosion predictions were then correlated with the measured erosion characteristics of the coatings, which were obtained from the water jet studies. The reciprocal of the critical jet pressure P&l was used here as a measure of the susceptibility of the coatings to the water jet erosion. The variation in the values of the parameter H-' with the PMMAmass fraction in the coatings is shown in Fig. 11. The data show that the value of H- 'wasdecreased as the PfvlMA binder content was increased. Thus, as the coatings became tougher, owing to the increased polymer binder content, the Rabinowicz erosion model would suggest that the cohesive erosion rate should become reduced. Fig. 12 shows the values of H-' plotted against the reciprocal of the critical jet pressures P;: of the coatings. The data show that a good correlation exists between the values of H-' and P&: for these coatings, indicating that the simple, hardness based, abrasion model provides a reasonable estimate of the erosion durability for these coatings. The correlation is observed to be linear in

95

the composition range studied and the slope of the line is approximately 0.01. Therefore, if we assume that the value of the constraint factor c in Eq. (3) is 3, the critical jet pressure for the failure of the coatings is seen to be ca. 3% of the tensile yield stress. Thus, the failure of the coatings occurred at pressures which were much smaller than their yield

increased, the values of Wy were decreaM. Thus, &modified Ratner-Lancaster model oredicted that the erosion rates of these coatings would show a decrease. as the PMMA content was increased. Fig. 14 shows a plot of the reciprocal of the critical jet pressures for the sample coatings against the values of W: calculated from the theoretical m&L The agreement between the modified Ratner-Lancaster model and the experimental erosion data is good, although the correlation is no better than that obtained from tha simple Rabinowicz model. The correlation was again seen tobe linear and the slope was approximately 0.05. The value of this slope

0.010-

0.002 d.ocQl 0

. . m . m 4 6 8 10 knmAeoatrml% Fig. II. Variation in lf-’ with the PMMA mass fraction in the .~atings.

am 0.0

Fig.

.

. 2

.

.

0.1

0.2

I

.

-

.

1

0.3 0.4 0.3 0.6 0.7 I/PC&AlPa12. Correlation between the recipmcal of Pti, and the pwameler H’

0.0 0.1 0.2 0.3 0.4 ImliI MPa~l Fig. 14. Conelatmn buwecn the rsiproalofP,and

0.5

0.6

thep&mmw~.

03

%

8.1. Briscoeet al. /Wear 203-204 (1997) 88-97

is equal to the reciprocal (k2( I- vr)) in JZq. (5).

of the proportionaJity

constant

Discussion

6.

TJte correlations for both the Rabinowicz and RatnerLancaster analyses are remarkably similar, apart from the differences in the slopes, which are due to the different proportionality constants. These similarities are probably due to tlte fact that the strain to failure e,. does not vary by a large amount over the range of coating compositions studied. This behaviour is more akin to that of metals, rather than most polymers where + often varies considerably. However, for a given polymer class. er may remain tamer constant and given the relatively narrow composition range for the coatings in the present study, the variation in er was probably not large enough to be significant. Therefore, tlte results for both tJte RabinowiczandRatner-LancasbanaJysesareratJtersimiJar. It is notable that tJtc correlations for this system are linear within the composition range studied, whilst in the abrasion of certain polymeric systems these plots are found to be nonlinear. In two-body abrasion, the wear rate is often found to be proportional to (urer) - “, where the value of n is significantly less than unity [ 181. Tltis commonly observed lack of linearity in the Ratner-Lancaster correlation is often attributed to debris inclusion in the contact, which reduces the effective topography of the abrading member. Nosucheffects will occur in the present hydrodynamic flow induced erosion experiments. Both of the models adopted here imply a unit rupture process; they suppose that the erosion is produced by a series of unit deformation events which provide the accttmuJated erosion rate. The relevant material properties are those for a disruption process which occurs on previously undeformed material. The evidence from the current WJA studies is that an element of fatigue may be involved. That is, the material is progressively disrupted at stresses and strains lower than that required for unit cohesive failure. Thus, the processes of erosion occur by tJte accumulation of fatigue induced damage. In abrasion studies, it is supposed that parameters, such as H, v and so on, probably correlate with the fatigue life or fatigue tougJmess of the material. Thus, whilst the correlation with unit fracture criteria is proven, this cannot be regarded as providing mechanistic evidence that this is the origin of the erosion process.

correlated with the values of P;;i: obtained for tire coatings, by applying modified forms of two abrasion models. The correlations between tJte mechanical properties and the erosion characteristics of the coatings were seen to be good, within Ute composition range studied. However, neither of these theoretical models was able to predict the actual observed erosion mechanisms and therefore the use of such models should be approached with some caution. These models are useful as correlations and it is interesting to note that even tJte simple hardness controlled model provides a reasonable fit to the experimental data.

Acknowledgements The authors wish to thank Unilever Research for sponsoring this work.

References

[II

B.J. Briscoe, M.J. Pickles, KS. Julian and M.J. Adams. WearsIBI183 ( 199.5)Is-14s. I21 E. Rabhmwin Fricfion and Wear of Meteriots. Wiley, New York. 1965. 131 MM. Khnschm and N.A. Babickev. Dokl. Akd Nauk. 88 (1953) 445-449 (English translation. National Science Foundplion. NSF-uIS). [4] J.K. Lancau~r. in 8.1. Brisca and MJ. Adams (e&x), Tribologyin Pmiculofc Tdmdqv. Adam Hi&r, Bristol. 1987. [SJSB.Rnma:I.I.FubaovLO.V.Rdyulrcvich~dE.G.L~.inD.I. Janus (cd.). Abrasionof Rubber.hlacL.mn. Lmdm. 1967. [6] J.K. Lancaster, Wear, 14 (1969) 223-239. [7J D. T&r, 771.~Hardness ofMelds.Clamdon. Oxford. 1951. [El KB.Putliek.L.S.A.SmilhandL.E.Mi1kr.I.Phys.D. 10(1977)617. 191 I.N. SlKddmn. fnt. 1. ,&?. SCi.. 3 (1%5) 47-37. [IO] G.B.P. BoxmdD.R. Cox. 1. R. Sm. Sot.. 26 (1964) 211-243. B.J. Brixoc and K.S. Sebastian. Pmt. R. Sot. Londm Ser. A. 452 (1996) 439-457. 121 D. T&or, Rev.Phys.Technol..( 1970) 145-179. [ 131 C.W. Shih. M. Yang and J.C.M. Li. J. Mater. Res.. 6 (12) (1991) 2623-2628. 141 M.F. Doemcr and W.D. Nix.J. Mater. Res., I ( 1986) 601-6G9. [IS] GM. Phm. W.C. Oliver and F.R. Brown. J. Mater. Res.. 7 (1992) 613-617. I61 K.S. Sebastian. PhD Thesis.Impuial College.London. 1994. [ 171 P.L. Kaye. PbD Thesis.University of cambridge. 1995. 1181 B.J. BriscceandP.D. Evans. Wear. 133 (1989) 47-64.

[II] [ [ [

Biographies 7.

Conclusions The hydrodynamic

erosion characteristics of a polymerparticle composite coating system have been examined using a water jet apparatus. The samples were found to be eroded by a bulk cohesive mechanism, which led to rapid failures once the critical jet pressure P,, was exceeded. The bulk mechanical

properties of the coatings were measured and

Brian Btiscoe

is Professor of Interface Engineering, in the Particle Technology Group, Department of Chemical Engineering, Imperial College, London, UK. He was previously an Ernst Qppenheimer Research Fellow in Surface Science at the Cavendish Laboratory, Cambridge, UK. His awards include the Beilby Medal (RSC) 1983, RSC Interdisciplinary Medal 1988.LMech.E. Silver Medal for Tribology 1990, and the SC1 Sir Eric Rideal, Founder’s Medal 1991. He is the

.203-204 (1997) 88-97

Editor-in-Chief of the journal Tribology Intemutional. Professor Briscoe has a long-standing interest in the tribology of organic materials and ceramics and in interfacial phenomena in the processing of solids and solid suspensions.

Matthew Pickles attained his MEng and subsequent PhD in the Department of Chemical Engineering, Imperial College, London, UK. his PhD thesis was entitled ‘Hydrodynamic erosion of coatings’ and the work described in this paper formed part of his PhD study. He is currently employed as a post-doctoral researcher in the Tribology Group of the Department of Materials Science and Metallurgy, University of Cambridge, UK. His current research interests are in the characterization of the durability of multilayered coating systems.

97

Kate JulianisabiochemicalengineerwithUnileverResearch. Port Sunlight, UK specializing in hygienic manufacturing for home and personal care products. She is a Corporate Member of the Institution of Chemical Engineers and aChartered Engineer. Mike Adams is a Senior Scientist with Unilever Research, Port Sunlight, UK and is involved with the application of materials science and engineering to product development and processing. He is also a Visiting Professor in the Department of Chemical Engineering at Imperial College, London, UK and Chairman of the Institute of Physics Tribology Group. His interests in the field of tribology ‘include applications to polymers, fibres. powders and soft solids. His publications in his field include two coedited book% ZXboIogy in Particulate Technology ( 1987) and Solid-Solidlnteractions ( 1996).