Wear - Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands 24

SAND EROSION OF METALS AND PLASTICS: A BRIEF REVIEW*

SUMMARY

Erosion caused by solid particles, such as sand grains, can occur under a variety of service situations but has received littIe disciplined study before about zg6o.

The parameters influencing the extent of the erosion are associated with the impact conditions and the properties of the impacting particles and target surface, Their effects are briefly reviewed and it is concluded that further work is required to elucidate the mechanisms of erosion and relate material properties to erosion resis- tance,

INTRODUCTION

Sand erosion is a form of wear caused by particles irnp~~~ against a target face and removing material. In order to quantify the extent of the damage, E, it is usually expressed as the weight of material removed by unit weight of impacting particles. However, in comparing the erosion of different types of materials, it is more meaningful to use a volumetric loss, E/Q, because the problem is usually manifest- ed by modification of the geometric profile rather than by weight loss.

The term sand erosion is used to describe processes involving solid particles of up to NIOOO~U in size and is often loosely used for industrial materials such as alumina and silicon carbide as well as for quartz or natural sand. Erosion has been reported as a problem in areas as diverse as aero gas turbine@, rocket nozzles2 and transport tube@. For vehicles such as helicopters which are designed to operate over dusty terrains, engine life may be reduced to as little as ~0% of the normal value. Engines in civil transport, used under less erosive conditions but for longer periods 9f time, can also incur erosion damage. A typical example of an eroded compressor blade is shown in Fig. 1.

In the laboratory, the damage is usually simulated by blasting airborne parti- cles against a test piecee-0. Alternative methods involve dropping particles under vacuum on to the face of a stationary specimen? or specimens attached to the ends of a rotating arms. In general, it has been found that a relatively good correlation can be obtained between laboratory data and service damages. However, comparisons with other apparently similar types of wear such as cavitation, rain erosion and abrasive wear, indicate that sand erosion involves rather different mechanisms and materials are rated in different orders of merit.

* Paper presented at the Materials Science Club meeting on “Abrasive wear of materials”, held at the National College of Agricultural Engineering, Silsoe, Bedfordshire, U.K., July 16-17, 1~69.

wear, r4 (1969) 24r-248

242 G. 1’. ‘lT1,I.Y

Fig. I. Typical erosion of a compressor blade.

This paper reviews, very briefly, the current understanding of the processes of sand erosion of different materials under various impact conditions.

THE EROSION PARAMETERS

The impacting particles

Properties of the impacting particles, such as size, density, sharpness and hardness, can markedly influence the severity of erosion. Laboratory testing has often been conducted with synthetic materials such as silicon carbide334 alumina6 and steel spheres4 whereas natural dusts contain a range of geological constituents of which quartz is usually the most common as well as the most erosive present. Studies of a variety of natural sands showed that their erosiveness tends to be directly pro- portional to the percentage of quartz present for tests against an 11% chromium steel at 420 ft./se@. In these experiments, the particles were sieved into the range 1z5-150p

to avoid confusion with effects due to different size distributions. For different types of abrasive, it appears that hardness and sharpness are inter-related so that a soft material is usually more rounded than a hard material. However, there are exceptions to the rule e.g. wind-blown quartz tends to be more rounded than quartz that has been keyed together by vegetation or artificially sized by crushing (see Fig. 2). Investi- gation of the influence of hardness has also involved the use of closely sieved sizes and it has been shown that erosion is strongly dependent upon hardness as measured by diamond pyramid micro-hardness indentations on individual grain@. No work has been reported on the influence of particle density but it is reasonable to consider that hardness and sharpness adequately characterise the abrasiveness.

Particle size is one of the most important engineering criteria for aircraft applications because it determines the liability of particles to become airborne and strongly influences their erosiveness. Particles smaller than N 70 ,U are usually charac-

Wear, 14 (1969) 241-248

243

Fig. 2. 125-150 p sieved fractions showing different sharpness of windblown and crushed quartz. (a) Sample of windblown sand containing 88% quartz; (b) freshly crushed quartz (98% purity).

IO*

3 l- 7;- E 0

s ‘L

P w

0.1 -

/

Epoxy resin

h

/

&

xNX *X~Aluminium alloy

Jh

/ a--a- Aiuminium

A Quartz at 90” and 420 ftjsec

x Ouortz at 90” and 800 ftjsec 1 NGTE test

o Silicon carbide at ZOO and 500 ftisec X (Ref. 121

0.01 b I / 10 100

Particle size (yrn)

I 1000

Fig. 3. Influence of partick size on erosion.

terised as dust and can be raised as high as 75 ft. under the ~sturbing forces of the downwash from helicopters. Studies of the influence of size on erosion indicate that damage increases with particle size 5,*~10. However, this can be a rather complicated situation because small particles (less than zap) can be deflected by the air flow so that they either fail to impact against the target or do so at modified angles and velocitieslf. In addition, different materials exhibit different types of size dependence; engineer: ing alloys and resilient plastics exhibit an initial increase in erosion with particle size till the onset of a saturation plateau where it is independent of size**9 (Fig. 3). The onset of the plateau is itself dependent upon velocity. An essentially brittle material like glass exhibits a power law relation with erosions i.e.

&=a&

whilst erosion of resins and composite materials like fibre-glass increases with particle size but does not exhibit the plateau for the range investigated.

Wear, r& (1969) 241-248

244 G. P. TILL?;

The target mate&d

It is usually considered that erosion decreases with increase in hardness of the target surface, and WOOD AND ESPENSCHADE~ give some evidence to support this contention. However, this work was rather limited in the range of materials tested and comparatively hard brittle materials like glass and sintered alumina have been found to exhibit an appallingly high erosion under normal impact. SHELDON AND FINNIE~~ suggest that erosion is proportional to the reciprocal of the plastic flow stress which is itself related to hardness, and give some evidence to support the re- lation in a later paper-is. However, heat-treatment of AISI 1045 steel and a tool steel to obtain a 411 range of hardness produced no significant change in erosion (Fig. 4).

P To

aM0

fool steed *- a---

-L)---

1045 steel

250 pm SillCOn carbide at ZOoand 250 ft /SeC

Diamond pyramid hardness (kg imm*1

Fig. 4. Influence of hardness on erosion’s.

In a study of some of the more common engineering alloys at NGTE, it has been shown that, although softer samples have a poor erosion resistance, there is no general correlation with hardness in the range 200700 kg mma. The figures are given in Table I.

In analysing the erosion of a variety of materials, it was shown that some brittle materials tend to become less resistant at higher hardnesses whereas the

TABLE 1

Material

~-

Nickel-based alloy I P % chromium steel T~ta~urn alloy Cobalt alloys Aluminium alloy Magnesium alloy

Diamond pyrantid havdnass

(k~l~~2j ~.-_--

207 355 340 280-680

55 37

0.6

0.6

1.0

1.3-1.4 2.0

13.0

Wear, rq (1969) 241-248

SAND EROSION OF METALS AND PLASTICS 245

opposite is true of ductile materials 11. However, in comparing different materials, it is essential to define the impact conditions because brittle materials are usually superior under glancing impacts whereas ductile materials are best under normal impact@. Also, consideration of mechanical properties such as hardness involves data evaluated at ambient temperature and relatively low strain rates whereas this is unlikely to be relevant to the erosion process in which very rapid deformation occurs and tempera- tures may be raised by several hundred degrees.

The impact parameters Parameters such as duration, angle, and velocity, of impact can strongly

influence the extent of the resulting damage and have received close attention in a number of investigations. In early work on an SAE 1020 steel FINN1E4 reported that erosion was proportional to a simple power of velocity i.e.

/z=bVn

where 12 was 2.0, However, he subsequently reported work on other materialsi giving a range of values for n between 2.05 and ~4.4. Other worker+* have also reported power law dependence on velocity and there is considerable evidence to show that n is ~2.3 for a very wide range of materials. However, values as high as 6.5 have been reported for tests involving gap steel spheres against glass4 whilst NGTE tests exhibit a value of 2.3 for 125-150~ quartz against glasss.

It has been suggested7 that there is a threshold velocity below which no erosion occurs but calculated values are very low (less than IO ft./set) so that it can usually be neglected. This is in marked contrast with rain erosion for which much higher values have been estimatedId.

0.00

G i+ E Y

ii .1 .c E 2 0.00 9

: ‘5 e w

(

Aluminium (Ref. 3)

30 60 Impact angle (degrees1

Fig. 5. Influence of impact angle on erosion by 300,~ iron pellets at 32.5 ft./set.

The influence of impact angle is dependent upon the type of material; ductile materials exhibit maximum damage for glancing impacts whereas for brittle materials it is at normal irnpacts.4,7,li (Fig. 5).

In early stages of the erosion process, there can be an incubation period when energy is dissipated in roughening the target surface. The mechanism tends to

Wear, 14 (1969) 241-248

240 (i. P. TLI.L\

stabiiise very rapidly to the stage where cumulative weight loss varies linearly with the quantity of abrasive impacted 4,aV11 (Fig. 6). The extent of the incubation period is dependent upon the properties of the materials under test and the impact conditions e.g. it tends to be reduced by increase in velocity or decrease in impact angle for aluminium”. Hard materials exhibit very little incubation whereas soft materials, such as aluminium and resilient plastics, are very susceptible and deposition may occur during the initial stages of tests in which impacts are at angles close to go”. However, the surface quickly becomes saturated and the situation stabilises so that erosion exceeds deposition and a linear rate of loss is established”,11.

0.1 o 1 I

100 200

Weight impacted (g)

Fig. 6. Variation of weight change with weight of dust impacted”.

It is convenient to consider other variables, such as dust concentration, temperature and surface stress, as secondary features because they have a relatively small effect on the damage. Their effects are summarised as follows.

(i) An increase in the concentration of the impacting particles can reduce the extent of the resulting erosion, E, in laboratory tests5 but no significant change was observed in tests on a small gas turbine enginelo.

(ii) An increase in temperature may increase or decrease erosion depending on the material involvedfi-11915. It has been suggested that temperature dependence may be related to ductility as defined by the elongation at fracture, because this is one of the most important parameters determining the amount of energy dissipated in removing material from the target surfacell.

(iii) Increase in tensile stress in the target material as a result of externally applied loads tends to decrease erosion ll. This is important because of the relevance to the centrifugal stresses in rotor blading of compressors.

THE MECHANISMS OF EROSION

It is generally accepted that the mechanisms of erosion are different for ductile

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SAND EROSION OF METALS AND PLASTICS 247

and brittle materials, but the precise details of the processes are still far from under- stood. In one of the earliest studies, MARTLEW 16 drew an analogy with the cutting action of a machine tool operation to generate equations relating erosion to impact angle. For normal impacts, damage was considered to be caused by repeated plastic deformation of the surface resulting in work-hardening and propagation of small cracks, so that fragments are eventually detached from the surface. FINNIE* used a very similar model to explain the erosion under glancing impacts but deveIoped equations ignoring the damage that can occur under normal impact. Considering the erosion of brittle materials, he suggested that if perfectly elastic behaviour occurs, cracking is caused by Hertzian stresses set up on impact and subsequently material is removed as the cracks interact, The theories of FINNIE and MARTLEW both predict that erosion is dependent upon the square of velocity in contrast to the slightly higher exponent usually prevalent.

In a very comprehensive study of the problem, BITTERS,? recognised that it is necessary to exceed threshold conditions in order to cause damage. The dependence on impact angle was explained by arguments similar to those of MARTLEW; glancing impacts were considered to cause what was termed “cutting” erosion whilst “de- formation” erosion predominated at angles close to the normal. NEILSON AND GILCHRIST~ subsequently developed a more descriptive approach which effectively simplified BITTER’S analysis but also recognised the likelihood of deposition occurring.

Studies at NGTE, aimed more specifically at elucidating the influence of particle size and velocity, have led to the suggestion that the mechanical properties of the particle may be as important as those of the target in determining the degree of erosion. There appears to be ample evidence to accept FINNIE’S model for brittle materials but it seems unlikely that single impacts can remove chips from the surface in the manner postulated. Examination of the impact topography indicates that an impacting particle initially causes pitting with an extruded lip around the edge but subsequently bursts so that fragments scour the surface causing secondary damage. Erosion is caused by subsequent impacts removing the rather vulnerable extruded lips. For glancing impacts, damage may be due predominantly to the primary indentation whilst for normal impacts, secondary scouring may be responsible for most of the damage. Measurements of the degree of fragmentation show that it is influenced by velocity and initial size and it has been suggested that the secondary damage is a very important part of the process*.

COWCLUSIONS

From this very brief review of sand erosion, it is clear that most of the para- meters have been investigated in some detail but the basic mechanisms are not yet fully elucidated. This is particularly important in connection with the erosion rating of different materials and the selection and development of suitable erosion-resistant alloys and coatings.

REFERENCES

I W. i\. HIBBERT, Helicopter trials over sand and sea, J. Roy. Aerun. Sue., 69 (659) (1965) 746. 2 J. H. NEILSON AND A. GILCHRIST, An experimental investigation into aspects of erosion in

rocket motor tail nozzles, Wear, rr (1969) 123.

Wear, I# (1969) 241~-248

248 G. P. TILLY

3 J. H. BITTER, A study of erosion phenomena Part II, Weav. 6 (1963) 169. 4 I. FINNIE, Erosion of surfaces by solid particles, Wear, .T (1960) 87. 5 C. D. WOOD AND P. W. ESPE~SCHAD~, Mechanisms of ‘dust’erbsion, SAE Sumnwr Meeting

Preprint 880A, Sot. Automative Engrs., New York, 1964. 6 J. H. NEILSON AND A. GILCHRIST, Erosion by a stream of solid particles, Wear, II (1968) I I 1. 7 J. H. BITTER, A study of erosion phenomena Part I, Wear, 6 (1~63) 5. 8 J. E. GOODWIN, W. SAGE AND G. P. TILLY, A study of erosion by solid particles, Proc. Inst.

Mech. Engrs. (London), 184 (1969). g W. SAGE AND G. P. TILLY, The significance of particle size in sand erosion of small gas turbines,

Awon. J., 73 (1969) 427. IO J. E. MONTGOMERY AND J. M. CLARK, Dust erosion parameters for a gas turbine, SAE

Summer Meeting Preprint 5384, Sot. Automotive Engrs., New York, 1962. I I G. P. TILLY, Erosion caused by airborne particles, Wear, 14 (1969) 63-79. 12 G. L. SHELDON AND I. FINNIE, On the ductile behaviour of nominally brittle materials during

erosive cutting, J. Eng. I&., 88 (1966) 387. 13 1. FINNIE, J. WOLAK AND Y. KABIL, Erosion of metals by solid particles, I. Muter. 2 (3) (1967)

682. 14 A. A. FYALL AND R. N. C. STRAIN, Rain erosion aspects of aircraft and guided missiles, J.

Izoy. Aeron. Sot., 66 (1962) 447. 15 H. C. DUFFIN, unpublished NGTE Rept., 1960. 16 D. L. MARTLEW, unpublished NGTE Rept., 1958.

Wear, 14 (1969) 241-248