Erosive wear of coatings and methods to monitor coating wear

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VTT TECHNICAL RESEARCH CENTRE OF FINLAND VTT INDUSTRIAL SYSTEMS

Erosive wear of coatings and methods to monitor coating wear – A literature study

Aino Helle, Peter Andersson, Tiina Ahlroos, Virpi Kupiainen

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Foreword

This literature study forms a part of the work in the PROGNOS- project of prognostics for industrial machinery availability (Official title in Finnish: Teollisuuden käynnissäpidon prognostiikka), particularly in relation to the case dealing with monitoring of erosive wear of screen cylinders. The authors wish to express their gratitude to Tekes - National Technology Agency of Finland, the industrial partners and VTT Technical Research Centre of Finland for funding the project.

Espoo, 29^th September, 2004

Authors

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1 Introduction

In many industrial applications components are subjected to erosive wear caused by fluid flow. Wear resistant coatings may be applied to increase the component life time. When the coating fails the wear rate usually increases rapidly, resulting in a fast deterioration of the performance of the component. In case the coating wear could be monitored such that the coating failure could be detected early enough to prevent wear of the substrate material, recoating of the component could be possible and feasible. In this report modes of erosion, methods for erosion testing, erosive wear of coatings and possible methods for monitoring the coating wear particularly under slurry erosion are reviewed on the basis of literature.

2 Wear modes for erosion

In tribology, erosion is defined as progressive loss of original material from a solid surface due to mechanical interaction between that surface and a fluid, a multicomponent fluid, or impinging liquid or solid particles [ASTM G40-99]. As all kind of wear, erosion causes costly damage in many machine components. In U.K., the Department of Trade and Industry estimated in 2000 that the erosion costs are approximately £20 million per annum [Wood et al. 2004]. In Finland, the losses due friction and wear are about 2.7 billion euros, and by utilizing the new knowledge of tribology, about one billion euros could be saved, which corresponds to about 1 % of Finland’s GNP in 1997 [Tervo 1999].

Many of the erosion modes can be said to be related to each other. Sometimes it is difficult, or even impossible, to define the actual erosion mode. The erosion mechanism depends on the material and is different for brittle and ductile materials, and therefore erosion has been divided into brittle and ductile erosion. Most wear models and equations are inadequate to predict erosion. Meng and Ludema [1995] evaluated a number of wear equations and selected 28 erosion equations for a more detailed study. These contained 33 parameters, with an average of five parameters per equation. The parameters included density and hardness of the particle, moment of inertia, roundness, single mass, size, velocity, rebound velocity, kinetic energy (KE) of particle, density and hardness of the surface material, flow stress, Young's modulus, fracture toughness, critical strain, depth of deformation, incremental strain per impact, thermal conductivity, melting temperature, enthalpy of melting, cutting energy, deformation energy, erosion resistance, heat capacity, grain molecular weight, Weibull flaw parameter, Lamé constant, grain diameter, impact angle, impact angle maximum wear, and KE transfer from particle to target. All the equations also contained one or more constants, either to form a ‘numerical bridge’ between the calculated and measured values, or to represent specific quantitative phenomena not readily measurable. They explored several methods trying to harmonize existing equations:

• The incidence of use of variables: hardness is the most widely used, but it is inadequate as the only material property and not applicable for all types of wear.

• Dimensional analysis: by assuring dimensional compatibility in equations it is expected that some forgotten variables could be found or some candidate variables could be appropriate for consideration.

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• Superposition: adding or multiplying a new variable to the existing ones may work for some variables, but there is too much interdependence between some variables.

• The methods of waveform analysis or system identification: very little value in developing wear equations, because these techniques do not identify the basic mechanisms.

• Expert system methods: these depend on an orderly construct of information and hence cannot impose order into a system.

The result of the evaluation by Meng and Ludema [1995] was that there appears to be no such way that could be used for harmonizing the existing equations without beginning the modelling process in some new way.

In the following, various erosion modes are described according to the erosive medium.

2.1 Liquid impingement erosion and cavitation erosion

According to Halling [1975], liquid impingement erosion occurs when small drops of liquid are striking on the surface of a solid at high speed (1000 m/s) and very high pressures are experienced, exceeding the yield strength of most materials. Thus, plastic deformation or fracture can result from a single impact, and repeated impacts lead to pitting and erosive wear.

Liquids need not to contain particles to produce damage to a solid surface.

ASTM G40 standard defines liquid impingement erosion as “progressive loss of original material from a solid surface due to continued exposure to impacts by liquid drops or jets”.

This definition excludes erosion mechanisms due to:

• The impingement of a continuous jet

• The flow of a single-phase liquid over or against a surface

• A cavitating flow

• A jet or flow containing solid particles.

However, also these mechanisms can cause erosion under some conditions. Liquid impingement erosion and solid particle erosion are quite different mechanisms, and the latter is described in the next chapter. Discrete impacts cause far higher impulsive contact pressures to the surface than continuous jets and the endurance limit and yield strength of the surface can be easily exceeded. Solid material can be removed from a surface even by a single droplet, if the impact velocity is sufficiently high. By comparison of test data it has been estimated that the erosion rate due to droplets can be from one to five orders of magnitude higher than the erosion rate due to a continuous jet with the same quantity of liquid impinging the surface at the same velocity. Liquid impingement erosion can result in a surface composed of sharp peaks and pits [Heymann 1992].

Cavitation is defined as the formation and subsequent collapse of cavities or bubbles within a liquid, and cavitation erosion is the mechanical damage of a solid surface caused by cavities or bubbles collapsing either at or near the surface [ASTM G40-99, Hansson 1992]. Liquid impingement erosion and cavitation erosion damages have many similarities, both being due to small-scale liquid/solid impacts, and sometimes it is difficult to say if the erosion mechanism was impingement or cavitation erosion. Sometimes it is impossible to explain the material loss situation only by mechanical action. The term “Impingement attack” is sometimes used for material loss when the forces of unsteady, turbulent, or bubbly flows are

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believed to remove the protective oxide layers, allowing continuing and accelerating corrosion. In situations of both corrosion and erosion acting simultaneously, the material loss rates can be much higher than the sum of the rates of the individual processes [Heymann 1992].

The means to reduce liquid impingement erosion include modification of impingement conditions, material selection and protective shielding. By modification of the geometry and/or fluid dynamics it is possible to reduce:

• The amount of liquid impacting the surfaces

• The impact velocity of the droplets

• The droplet size

• Time of operation under the most severe conditions.

To assist in the selection of materials, information of the relative erosion resistance of metallic alloys can be obtained. Protection against erosion by shielding or cladding is usually achieved with a harder layer than the base material, or by a composition or microstructure which is more erosion resistant. In some low-intensity environments elastomeric coatings can be used to reduce the impact pressures [Heymann 1992].

2.2 Solid particle erosion

Solid particle erosion is the loss of material that results from repeated impact of small, solid particles in a gaseous or liquid medium [Kosel 1992]. This kind of erosion is to be expected whenever hard particles are entrained in a gas or liquid medium impinging on a solid at any significant velocity (>1 m/s). Sometimes this may be beneficial, for example in sandblasting, but usually it is a serious problem. Slurry erosion (sand particles in a liquid) is usually treated as a different subject. The difference between erosion and abrasion is often unclear. In solid particle erosion, a series of particles strike and rebound from the surface, and cause a force on the material due to their deceleration. In abrasion, abrasive particles slide on the surface under an external force.

Solid particle erosion involves an eroding surface, the particles producing erosion, and the fluid flow bringing the particles into contact with the surface. The factors affecting pure erosion, i.e. erosion in the absence of corrosion, are the following [Finnie 1995]:

• Variables describing the particle flow: particle velocity, angle of incidence, particle rotation, particle concentration in the fluid, nature of the fluid, and fluid temperature

• Particle properties: particle shape, size, hardness, and strength (or resistance to fragmentation)

• Properties of the surface: shape, stress level, residual stress, temperature, stress as a function of strain, strain rate and all material properties of the surface such as hardness, fracture toughness, fatigue, melting point, microstructure etc.

The erosion resistance of ductile and brittle materials differs remarkably from each other when measured as a function of the impact angle: ductile materials have a maximum erosion rate at low angles of incidence while brittle materials have the maximum at or near 90° [Kosel 1992].

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2.3 Slurry erosion

Slurry erosion is defined as the type of wear, or loss of mass, that is experienced by material exposed to a high-velocity stream of slurry, i.e. a mixture of solid particles in a liquid (usually water) [Miller 1992]. Slurry erosion is complex due to many important variables present.

Applicable models to predict it are still lacking. Some of the main variables affecting erosion according to Wood et al. [2004] are:

• Slurry variables:

o Liquid: viscosity, density, surface activity, lubricity, corrosivity, temperature o Particles: brittleness, size, density, relative velocity, shape, relative hardness,

concentration, particle/particle interactions

o Flow field: angle on impingement, particle impact efficiency, boundary layer, particle rebound, degradation, particle drop-out, turbulence intensity

• Component variables:

o Bulk properties: ductility or brittleness, hardness and toughness, melting point, microstructure, shape and roughness

o Surface properties: work hardening, corrosion layers, surface treatments, coating type, coating bond, microstructure

o Service variables: contacting materials, pressure, velocity, temperature, surface finish, lubrication, corrosion, hydraulic design, intermittent slurry flows.

In addition to wear purely caused by the slurry moving past the material or the material moving through the slurry, other wear modes are also encountered when parts or components are subjected to slurries. According to Miller [1992], the most common wear modes are those described in Table 1.

2.4 Erosion/Corrosion

An important aspect in erosion-corrosion interaction is the potential synergy between the two processes. Corrosion may enhance the erosion rate through preferential dissolution or it may also inhibit erosion through the formation of a passive film. To describe the erosion-corrosion interactions in aqueous environments, erosion-corrosion maps have been developed and they identify the regimes of interactions [Stack & Jana 2004]. Erosion-corrosion interaction regimes can be divided into pure erosion, corrosion-affected erosion, erosion-enhanced corrosion, and pure corrosion. Pure erosion occurs when the corrosion rate is negligible as in severe erosion conditions, for example in case of high velocity or angular particles, or in non- corroding conditions. In corrosion-affected erosion there is an increased corrosion component.

The dimensions of the stress field caused by the particles is greater than the scale thickness and the metal loss rate is increased when compared with pure erosion. Erosion-enhanced corrosion is divided into different categories: a steady-state scale thickness, determined by the relative erosion and corrosion rates, and spalling of the scale. In pure corrosion, the ratio of corrosion to erosion rate is very high, resulting in pure corrosion and parabolic scale thickening [Kosel 1992].

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Table 1. The common wear modes under slurry conditions according to [Miller 1992].

Abrasion-corrosion A result of any metal-to metal rubbing in the presence of abrasive solids in a corrosive liquid. When parts of different metals are exposed to slurry, electrolytic effects are included. Apart from high-velocity erosion, this is the most destructive mode when handling slurry.

Scouring wear Caused by abrasive solid particles becoming embedded in the softer material of two materials rubbing against each other, e-.g. elastomer-to- metal rubbing.

Crushing and grinding Takes place in abrasive metal-to- metal contacts.

High-velocity erosion Velocity of slurry greater than 6 to 9 m/s is usually considered as high.

May result in rapid wear.

Low-velocity erosion Usually a low-rate wear mode taking place when there is a flow of slurry at regular low velocities.

Saltation erosion During the transport of a sediment, particles are moved forward in a series of short intermittent bounces from a bottom surface, resulting in erosion.

Cavitation erosion Damage caused by repeated collapse of vapour bubbles in the liquid.

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3 Erosion testing

For acquiring more knowledge about erosive wear on machine components, experimental research has been carried out and several standardised test methods have been developed. The experimental research includes both component tests in actual operational environments and laboratory experiments under conditions simulating some defined operational conditions.

Laboratory experiments are less expensive than field testing, and the risk encountered with is smaller in the laboratory experiments. It is easier to make conclusions on the influence of individual operational parameters on a wear mechanism after a laboratory test than after a field test, while on the other hand the conditions are closer to reality in a field test than in a laboratory experiment. Normally, a compromise for the level of tribological experimental testing has to be made.

Field tests related to erosion studies are typically based on an exchangeable machine component or sample in a realistic process, e.g. a propeller, a pipe line or some other surfaces subjected to a slurry in motion. An example of tests in a large fan has been presented by Bulloch and Callagy [1999]. Results of field tests and laboratory testing related to slurry erosion of stainless steel pipes have been presented by Wood and Jones [2003].

Laboratory experiments have been performed using different configurations, the only feature in common being a fluid sample and a test sample surface in relative motion one to another.

In laboratory tests, either the fluid or the test sample surface is the moving part that provides preconditions for fluid particle collisions on the sample surface.

The common method to evaluate the magnitude of the erosion wear rate is by determining the mass loss of the test sample during the test. In terms of coated surfaces, where the penetration of the coating may lead to a sudden increase in the wear rate, the thickness reduction of the coating is a relevant measure for the erosive wear rate.

Intercomparisons of results from erosion tests with different test equipment, or different procedures with the same equipment, is often unreliable. The reason for this is, in most cases, the variation in the key parameters responsible for the erosion; minor variations in the particle velocity and flow direction, and the particle density, size and shape may strongly influence on the erosive wear. For this reason, the most appropriate way of comparing test results is to use reference materials that are tested at the same time or at identical conditions as the samples for the investigation, and in addition to this the testing conditions should be modelled if more than a single test method and equipment is used in a study.

In the following, some examples of laboratory tests to study erosion are presented.

3.1 Standardised erosion tests

The entire field of standardised tests related to some type of erosion comprises a myriad of standard methods for specific and general purposes. The specific purposes include tests for determining, as examples, the erosion-corrosion characteristics of aluminium pumps with engine coolants [ASTM D2809-94], the dust erosion resistance of optical and infrared transparent materials and coatings [ASTM F1864-98], issues related to spark or arc erosion, the erosion resistance of building materials, the erosion resistance of electric insulation materials and several standards related to actions against earth erosion. In the following,

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standardised test procedures for studies on erosion with relevance for the operational reliability of machines are presented, with a sub-division into gas and liquid erosion.

3.1.1 Standardised gas erosion tests

Two standardised test procedures for studies on gas-borne erosion for various applications within machine building are found among the current DIN and ASTM standards:

• DIN 50 332, which defines "Solid particle erosion test; basic rules". The test is performed as sand-blasting, using compressed air and abrasive particles, against a tilted (0-90°) sample surface, see Fig. 3.1.

• ASTM G76-04, which defines a "Standard test method for conducting erosion tests by solid particle impingement using gas jets". The test is performed as sand-blasting, using compressed air and abrasive particles, at right angle (90±2°) against a sample surface.

Hawthorne et al. [1999] have used the method for scientific laboratory work.

The interaction of solid particles with surfaces under dry conditions is furthermore the issue of the following standard on low-stress abrasive wear testing:

• ASTM G65-00, which defines a "Standard test method for measuring abrasion using the dry sand / rubber wheel apparatus". In this test, a test sample (coated or uncoated) is pressed against a rotating rubber wheel, while quartz sand is flowing into the gap between the wheel and sample, see Fig. 3.2.

Fig. 3.1. The principle for sand-blast and slurry jet erosion tests.

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Fig. 3.2. An ASTM G65 rubber wheel and quartz sand abrasion test in action.

3.1.2 Standardised liquid erosion test

The following standardised test procedures for studies on erosion related to liquids, and with relevance for machine design, is found among the current DIN and ASTM standards:

• ASTM G32-03, which defines a "Standard test method for cavitation erosion using vibratory apparatus". The test is performed by using a high-frequency (20 kHz) ultrasonic actuator, which causes a sample to vibrate, as submerged into a liquid, in the direction perpendicularly to the flat surface of the sample. The vibration gives rise to cavitation at the sample surface, which in turns causes cavitation wear of the sample through surface fatigue. The method has been used for scientific laboratory work [Han et al. 2002].

• ASTM G134-95, which defines a "Standard test method for erosion of solid materials by a cavitating liquid jet". The test is based on a submerged jet of a liquid (often water), which impinges on a test specimen on which cavities collapse and cause erosive wear. The liquid is circulated and filtered in a 40 µm filter, for which reason any effect of solid particles on the wear can be eliminated in practice. The test principle resembles industrial water jet cutting without abrasive particles.

• ASTM G73-98, which defines a "Standard practice for liquid impingement erosion testing". The method is based on the cumulative erosive wear effect of individual liquid drops or minor jets when causing repeated discrete impacts on a material surface.

The interaction of a slurry containing abrasive particles with surfaces under wet conditions is furthermore the issue of the following standard on low-stress slurry abrasion wear testing:

• ASTM G75-01, which defines a "Standard test method for determination of slurry abrasivity (Miller Number) and slurry abrasion response of materials (SAR Number)".

The test employs a crank-driven reciprocating testing machine with a shallow container for the slurry. The test specimen, in the shape of a cylinder with a circular test surface, slides in an oscillating mode, submerged in the abrasive slurry, under pressure against a Neoprene (™DuPont) rubber surface.

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3.1.3 Standardised reporting of results from erosion tests

The following standardised format for result reporting is available:

• ASTM G117-02, which provides a "Standard guide for calculating and reporting measures of precision using data from interlaboratory wear or erosion tests ". The standard has been developed for needs related to the establishing of data banks based on wear and erosion test results from individual laboratories in interlaboratory comparisons.

3.2 Non-standardised erosion tests

3.2.1 Non-standardised gas erosion tests

For laboratory experiments on erosion caused by air-borne particles, a few test methods and types of equipment have been developed. In the following, selected known methods and types of equipment for gas-erosion tests on a laboratory scale are presented:

• Non-standard (performance not conforming with ASTM G76 or similar standard) sand- blast or gas-blast rigs, into which abrasive particles are introduced as dry, into an air or gas stream that has a high velocity. The particles are accelerated in the gas stream and cause impacts on the sample [Hejwowski et al. 2000], see Fig. 3.1. The method has been used both at room-temperature, and at high temperatures for studying components like gas and steam turbine blades and heat transfer pipes [Wood & Wheeler 1998, Nicholls et al.

1999, Hidalgo et al. 1997]. Re-developments of the air-sand impingement erosion testing have been presented by Wood and Wheeler [1998].

• Centrifugal accelerators for dry particle erosion tests. For tests, usually several samples are attached at the rim of a disc that can be rotated around a vertical axis. Rotation of the disc causes a centrifugal fan action. When particles are dosed onto the disc, the centrifugal fan action ejects the erosive particles towards the surfaces of the samples, causing erosive wear of the samples [Wood & Wheeler 1998, Westergård et al. 2000].

• Whirling arm rigs, in which the samples are attached at the ends of arms pointing out from a centre hub with bearings, see Fig. 3.3. In tests, the sample holders rotate and the samples travel along a circular path. When erosive particles are fed into a test chamber, in which the sample holder is rotating, collisions between the sample surfaces and the particles will lead to erosive wear. The tests are often conducted under vacuum. The method has been used, for instance, for studies on the erosive wear resistance of blades of helicopter rotors and gas turbine compressors [Wood & Wheeler 1998, Maozhong & Jiawen 2002].

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Fig. 3.3. The principle for whirling-arm-rig and slurry pot erosion tests.

3.2.2 Non-standardised liquid erosion tests

For laboratory experiments on erosion caused by liquid-borne particles, several test methods and types of equipment have been developed. In the following, selected methods of relevance and types of equipment for liquid-erosion tests on a laboratory scale are presented:

• Equipment with a high-speed water jet, into which abrasive particles are introduced to form a slurry, see Fig. 3.1. The high velocity of the slurry and the kinetic energy of the particles cause erosive wear of a sample surface. In the test, the components of the slurry are generally not re-cycled [Wood & Wheeler 1998, Wood & Jones 2003]. The test system resembles industrial water jet cutting with abrasive particles.

• Customised equipment with a high-speed clear water jet acting on a sample that is submerged in a liquid containing abrasive particles. The water jet accelerates the particles, which then make collisions with the sample [Zhao et al. 1995, 1999a, 1999b].

• Circuits with re-circulated slurry, which is similar to the high-speed water jet, into which abrasive particles are introduced, but with the difference that the slurry is re-circulated within the equipment (Wood & Wheeler, 1998). The denomination "slurry jet erosion test" has been used for this type of equipment [Wood et al. 1998, Hawthorne et al. 1999].

• A circuit with a re-circulated slurry with abrasive particles, which is pumped into the gap between a rotating disc and a stationary housing with test samples. Acceleration of the slurry by the rotating disc caused impingement and erosive wear of the samples. The test arrangement was designed for simulating the conditions at hydropower stations [Mann 2000, Mann & Arya 2001].

• Slurry pot erosion testers, in which the samples are attached at the ends of arms pointing out from a centre hub with bearings, see Fig. 3.3. In tests, the sample holders rotate and the samples travel along a circular path in the slurry volume, and collisions between the sample surfaces and the slurry particles will lead to erosive wear [Wood & Wheeler 1998, Knuutila et al. 1999, Lathabai et al. 1998].

• The Coriolis slurry erosion tester, in which two sample plates are attached to a rotating disc with a slurry flow from the disc centre, see Fig. 3.4. Due to the centrifugal force, the slurry is flowing outwards from the disc centre, while the erodent particles are directed

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towards the surface of the sample plates by the Coriolis effect. The Coriolis slurry erosion tester offers good reproducibility and good discrimination between materials of different erosive wear resistance [Hawthorne 2002, Hawthorne et al. 2003, Clark et al. 2000, Llewellyn et al. 2004]. The action of the erodent in the Coriolis slurry erosion tester has been studied in detail by Hawthorne et al. [2002] and modelled by Xie et al. [1999].

• A double cylinder test rig, in which a smaller cylinder rotates inside a larger cylinder, with a slurry in between the cylinder walls. The rotational speed can be up to 10.000 rev/min, as an example [Stack & Wang 1999]. Because of the centrifugal action, the method is, however, quite mild in terms of erosive wear of the inner cylinder surface.

The interaction of a slurry containing abrasive particles with surfaces under wet conditions is furthermore the issue of the following non-standard abrasive wear test:

• Micro-abrasive wear tests for the simulation of slurry erosion has been carried out with a wheel-on-flat geometry in a slurry environment [Xie et al. 2001].

Fig. 3.4. The principle for the Coriolis slurry erosion tester; the grey disc with flow channels and samples is rotating with a velocity ω.

4 Slurry erosion of coatings

Erosion of coated surfaces by the action of flowing liquids, clear or containing eroding particles, has been studied by several authors over the years. In most cases, the erodents defined in the literature by different authors are hard, abrasive, mineral or ceramic particles.

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In recent years, several works on the erosion of engineering materials have been presented in the literature on tribology. In the following, a selection of works related to the erosion of coated surfaces is reviewed. The study is limited to metallic, ceramic, diamond and polymeric coatings, with the emphasis on hard coatings.

4.1 Bulk metals

Slurry jet tests by Wood et al. on carbon steel AISI 1020, stainless steel AISI 316 and various metallic, ceramic and polymeric coatings have shown that the erosive wear rate was similar for the two steels, and lower than for a Cu-Ni (90%/10%) alloy. The erosive wear resistance of a 50 µm thick pulsed Cr-coating, deposited by plating, and that of a heat treated composite coating by electroless nickel plating of a Ni/P alloy with SiC particles, was higher than that of the two steel materials. The erosion resistance of the steel materials was superior to that of a plasma sprayed Al2O3-TiC ceramic coating in the comparison [Wood et al. 1998].

Double-cylinder slurry tests by Stack et al., and mapping of the erosion-corrosion severity for mild steel and stainless steel AISI 304 and a selection of thin ceramic coatings, has shown that erosive-corrosive wear can be higher for carbon steel than for stainless steel, and for both steels it can be higher than for thin ceramic coatings [Stack et al. 1999].

4.2 Thin metallic coatings

Slurry jet tests by Wood et al. on various metallic, ceramic and polymeric coatings and steel have shown that the erosive wear rate was lower for a 50 µm thick pulsed Cr-coating, deposited by plating, and for a heat treated composite coating by electroless nickel plating of a Ni/P alloy with SiC particles, than for carbon steel and stainless steel materials in the comparison [Wood et al. 1998]. In a larger overview, Wood [1999] presents electroless Ni plating with SiC particulate as the most wear resistance solution for low impingement angles (30° angle) and low energies, in comparison with a large range of metallic, ceramic and polymeric coatings studied under slurry impingement erosion conditions.

Cavitation erosion tests, performed by Han et al. using a piezoelectric actuator, have shown that a hard chrome plating reduces the weight loss caused by erosion, in comparison with that of an uncoated carbon steel or a multilayer chromium nitride and chromium (CrN/Cr) coating on carbon steel [Han et al. 2002].

4.3 Thermally sprayed thick metallic and metal matrix composite coatings

Slurry pot tests by Lathabai and co-workers on an arc sprayed metal (Cr-Fe, 280 µm thickness after grinding) coating have shown that the erosive wear rate was lower for the metallic coating than for Al2O3 and Cr2O3 coatings in the same study. The erosive wear of the metallic coating was stronger with harder particles (SiC) than with softer (SiO2) particles [Lathabai et al. 1998].

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Re-circulating slurry jet tests performed by Hawthorne et al. have shown that the erosion rate of HVOF-sprayed (high velocity oxy-fuel) metallic coatings (stainless steel 316 L, Ni-W-Cr- Si-Fe-B-C and Co-Cr-Ni-Mo-W-Fe) was higher than for cemented carbide and Cr3C2-based coatings in the comparison, at both 20° and 90° impingement angle [Hawthorne et al. 1999].

Slurry jet impingement tests performed by Wood and co-authors have shown that the erosion resistance of thermally sprayed WC-Co-Cr coatings is similar to that of sintered cemented carbide material grades when the energy impact is low, but that the erosion resistance is weaker for the coating when the energy impact is high. At low impact energy, the impact angle affected the wear rate of the coating, being highest at 90° impingement angle. At high impact energy, under which conditions the erosion was stronger, the impact angle was almost insignificant for the wear rate [Wood et al. 1997]. In a larger overview, Wood [1999] presents thermally sprayed WC-Co-Cr coatings as the most wear resistance solution for high impingement angles (90° angle) and high velocities, in comparison with a large range of metallic, ceramic and polymeric coatings studied under slurry impingement erosion conditions.

Accelerator disc erosion tests with a re-circulating slurry, which have been presented by Mann and Arya, have shown that a thermally sprayed WC10Co5Cr HVOF coating gives less erosive wear than an as-received or plasma nitrided 13Cr-4Ni steel surface. Work by Mann has shown that borided steel T410 has a higher resistance against erosive wear than, for instance, plasma nitrided or WC-coated steel 13Cr-4Ni [Mann 2000, Mann & Arya 2001].

4.4 Thin ceramic coatings

Mapping of the erosion-corrosion severity for a selection of thin ceramic coatings and two steels has been presented by Stack et al. [1999]. Thin ceramic coatings are included for comparison in several works, but in most cases they offer less protection against slurry erosion than the other groups of hard coatings.

4.5 Thermally sprayed thick ceramic coatings

The slurry erosion wear of plasma sprayed oxide ceramics (Al2O3, Cr2O3) has been studied in slurry pot tests by Knuutila and co-authors, who conclude that the erosive effect on the ceramic coatings is stronger with particles of higher hardness, and that a water slurry with a low pH value causes stronger erosion of the oxide ceramics. In comparison with a cast duplex stainless steel reference material, the Cr2O3 coating gave little improvement in the erosion resistance, while the Al2O3 coating gave practically no improvement in a neutral water slurry and an increase in the erosion rate in an acidic water slurry [Knuutila et al. 1999].

Slurry pot tests by Lathabai and co-workers on arc sprayed thick (approx. 240 µm) oxide ceramic coatings (Al2O3, Cr2O3) have shown that the erosive wear rate was higher for the Al2O3 coating than for the Cr2O3 coating, and that the wear of the two ceramic coatings was higher than for the stainless steel AISI 316 and mild steel reference materials. The erosive wear of the ceramic coatings was stronger with harder particles (SiC) than with softer (SiO2) particles [Lathabai et al. 1998].

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Slurry jet tests by Wood et al. on various metallic, ceramic and polymeric coatings and steel have shown that the erosive wear rate was higher for a plasma sprayed Al2O3-TiC ceramic coating of 1060 µm thickness than for carbon steel and stainless steel [Wood et al. 1998].

Similarly, slurry jet-in-slit tests by Zhao and co-authors have shown that the erosion of ceramic coatings (Cr2O3, Al2O3-TiO2, ZrO2) is more severe than for stainless steel when the slurry jet is directed towards the surface, but that the ceramic coatings, particularly Cr2O3, have a higher erosive wear resistance than stainless steel when the jet travels along the surface [Zhao et al. 1995, 1999a, 1999b]. Re-circulating slurry jet tests performed by Hawthorne et al.

have shown that the erosion rate of HVOF-sprayed cemented carbide and Cr3C2-based coatings was lower than that of metallic coatings in the comparison, at both 20° and 90°

impingement angle [Hawthorne et al. 1999].

Taken together the above observations, thermally sprayed Cr2O3 coatings seem to be superior to equally deposited Al2O3 coatings. At impingement angles closer to 90°, the ceramic coatings do not offer any advantages over uncoated steel materials, while at impingement angles closer to 0°, the ceramic coatings seem to offer wear protection. Harder erodent particles cause more wear on ceramic coatings than do softer particles.

4.6 Diamond coatings

Water-sand slurry experiments with a variety of ceramics, ceramic coatings and diamond coatings, presented by Wheeler and Wood [1999], have shown that SiC coated with diamond by chemical vapour deposition (CVD) was more wear resistant than uncoated SiC, and more wear resistant than WC. A minimum coating thickness of 15-20 µm was required to give the CVD diamond coating a high resistance against erosive wear. Diamond coating detachment (and poor fracture toughness of the SiC substrate material) was seen as the main limitation of the wear resistance.

In a larger overview, Wood [1999] presents the results of a larger comparison with slurry impingement erosion tests with a large range of metallic, ceramic, diamond and polymeric coatings. Particularly CVD diamond coatings, but also diamond-like carbon (DLC) coatings by physical vapour deposition (PVD), showed a high resistance against volumetric erosive wear in slurry jet impingement tests at 90° angle, offering significant improvement in wear resistance when compared with steel. Coating detachment turned out to be a limiting factor for the erosion resistance of these coatings at 90° impingement angle.

5 Methods to monitor coating wear

The most common methods to measure wear involve direct determination of the amount of removed material as mass loss or volume loss. The determination of mass loss requires weighing the part or specimen before and after a wear process. If the absolute or relative wear loss is very small, the mass change may become too small for any weighing method. The weighing method also includes the risk for having extraneous matter present due to improper cleaning. Often it is more reliable and appropriate to determine wear volumes or dimensional changes instead of mass loss, particularly when materials with different densities are compared. By measuring dimensional changes information about the wear distribution or the

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extent of localised wear can also be obtained. The dimensional changes can be measured e.g.

by using profilometers (stylus or optical, also for three-dimensional profiling) or optical microscope determination of wear scar dimensions. Small micro-hardness indentations or scratches of different controlled depths can also be used to determine the extent of wear by determining how the size of the indents or scratches reduces during the wear exposure [Rabinowicz 1995, Ruff 1997].

In the case of coatings, the amount of wear could also be determined by measuring the coating thickness before and after the wear exposure. A variety of methods, both standardised and non-standardised, exist for coating thickness measurements based on e.g. optical methods (optical microscopy or scanning electron microscopy of cross sections), material removal (ball cratering, coulometric method by anodic dissolution, step height profilometry), electromagnetic methods (eddy current, magnetic flux, capacity), as well as scattering or excitation (beta back scattering, ultrasonic, x-ray fluorescence) [Holmberg & Matthews 1994, ISO 3882].

The above methods can be used for monitoring coating wear mainly in situations where the test or process can be stopped periodically and the samples removed for measurement, for example during laboratory tests. The use of replica techniques would allow dimensional changes to be measured also in situ, by periodically cleaning the worn surface and taking replica samples from it. When on-line monitoring of wear is required, direct measurement of weight loss or dimensional changes is more difficult or even impossible, and often indirect methods are more feasible. The present literature search on wear monitoring of coatings gave fairly little results in other areas except for tool wear monitoring during machining, which is a subject of intensive research. For tool wear monitoring, a number of indirect methods such as e.g. measuring forces, vibration, acoustic emission and sound, are used [Dimla 2000, Jantunen 2002]. Methods for direct wear measurement based on machine vision have also been developed [Kurada & Bradley 1997, Lanzetta 2001].

Some corrosion monitoring methods should also be applicable for monitoring erosive wear of coated components under wet conditions, e.g. during slurry erosion. Even if the measurements would indicate a corrosion rate which is lower than that of the actual wear rate due to erosion, an indication of the break through of the coating should be possible to be obtained by some corrosion monitoring methods. Erosion-corrosion processes of coated samples have been studied by using electrochemical methods which have also been able to respond to coating breakdown [Wood et al. 2002].

In the following, methods which could be suitable for on-line monitoring of coating wear are reviewed with focus on methods which are applicable for metallic or other inorganic coatings under slurry erosion.

5.1 Electrochemical measurements

Electrochemical corrosion monitoring methods are used to assess the electrochemical activity associated with corrosion, yielding results which can be used to estimate the corrosion rate or to identify situations that are likely to promote corrosion [Cowan & Winer 2001].

In linear polarization resistance (LPR) measurement the current needed to maintain a specific voltage shift, typically of the order of 10 mV, is measured while a small voltage (or

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polarization potential) is applied to the electrodes in a solution. The measured current is directly related to the corrosion on the surface of the electrode. The ratio of the applied potential and the resulting current is the polarization resistance. The measurement of corrosion rate by LPR method is rapid but the method has the disadvantage that it can only be successfully applied in fluids that are conductive, such as aqueous solutions. It will not work in environments where the electrodes may become contaminated by, for instance, oil [Cowan

& Winer 2001, MSC 2004, Caproco 2004].

In galvanic/potential monitoring no polarization current is needed since a natural voltage or electrochemical potential difference exists between electrodes of dissimilar metals when they are immersed in a solution. The potential difference generates a current which is related to the corrosion rate [Cowan & Winer 2001, MSC 2004].

Electrochemical noise (EN) arises from the random fluctuations in potential and current during an electrochemical process. The electrochemical potential is related to the driving force of the reaction whereas the current is related to the corrosion rate (kinetics of the reaction). One of the advantages of the use of EN measurements is the fact that localized corrosion processes, which may be difficult to monitor with other techniques, tend to give particularly strong EN signals, and the method can be used to predict the type and severity of corrosion that is taking place [Holcomb et al. 2004, Cottis & Llewellyn 1996].

Electrochemical current noise can be measured between a pair of identical electrodes, or on a single electrode under potentiostatic control. When using two similar working electrodes, both current and potential noise can be measured simultaneously since the potential noise of the working electrode pair, both electrodes being at the same potential, can be measured with respect to a third working electrode or a reference electrode [Holcomb et al. 2004, Cottis &

Llewellyn 1996, Wood et al. 2002]. EN measurements have been successfully applied for e.g.

the investigation of corrosion-wear of stainless steel under oscillating sliding conditions [Wu

& Celis 2004] and of erosion-corrosion processes of stainless steel with a thermally sprayed coating, showing also clear response on coating breakdown [Wood et al. 2002]. Detection of localised corrosion or corrosion type by EN method has been studied by Cottis et al., for instance in the references [Cottis et al. 2001, Al-Mazeedi & Cottis 2004].

Electrical resistance (ER) monitoring is another method which can be used for on-line measurement of corrosion, and it is also suitable for situations where the metal loss is due to mechanical processes such as erosion. Metal loss from the surface of a measuring element results in a decrease of the cross-sectional area of the element, with a corresponding increase in its electrical resistance. The rate of change of the resistance is a measure of the rate of metal loss due to corrosion, erosion or some other form of wear. ER measurements can be made either periodically or continuously. Since actual metal loss must occur before a response is obtained, this method may be somewhat slower than electrochemical measurements [Cowan & Winer 2001, MSC 2004, Caproco 2004].

5.2 Radionuclide technique

Radionuclide methods for wear monitoring are based on activation of the monitored sample or component to produce gamma emission. As the sample or component wears, gamma emitting particles are removed from the surface. Two approaches can be used for measuring the wear, i.e. the tracer approach or the marker approach. In the tracer approach, a gamma detector is used to measure the increase in gamma radiation intensity of the wear debris collected in a

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filter or trap or in the fluid line. In the marker approach, wear can be measured directly by placing a detector to indicate the activity remaining in the activated part [Blatchley 1992].

Activation of the monitored sample or component can be carried out by neutrons in a nuclear reactor, which usually results in the part becoming thoroughly radioactive. Alternatively, if activation is performed by heavy charged particles such as protons, deuterons or alfa-particles using a cyclotrone, only a thin surface layer of the component will be activated. The advantage of thin layer activation is that the total activity of the components remains low, hence reducing the need for special protective measures [Scherge 2003]. The activated thin surface layer acts as a marker, allowing wear measurement from the remaining activity of the component. When the marker is very shallow, a precision in the order of 1% of the activated depth can be achieved in wear measurements. Ultra-shallow activated layers can be produced by recoil implantation, which is a process where a target foil is bombarded instead of the actual part. Reaction products in the foil recoil with enough energy to escape and be implanted downstream to the surface of the part, which is being activated. This method is very suitable to materials that are easily degraded due to radiation damage. With surface layer activation, both marker and tracer approaches can be used whereas the use of neutron activation is mostly restricted to the tracer method unless the material to be activated is already in a thin coating [Blatchley 1992].

The advantages of the surface layer activation include the high precision possible, especially for markers, and the fact that both marker and tracer readings can be taken in-situ without interfering with the experimental system or process. Critical properties of gamma-emitting radionuclides that determine their suitability for wear monitoring, are their half-life and gamma-ray energy. The rate of decay must be slow enough to allow a reliable signal to persist throughout the planned measurement interval. For quick tests, half-lives of a few days are sufficient. For long-term monitoring, reliable measurements can typically be made for up to six half-lives. If a component being monitored is contained within some material around it, the gamma radiation must be sufficiently energetic to penetrate the surrounding material to allow detection from the outside [Blatchley 1992].

The radionuclide technique has been successfully applied to many engine components and even to large industrial systems [Blatchley 1992, Scherge 2003, Schneider et al. 2003]. A short introduction to the surface layer activation technique by Blatchley can be found from reference [Blatchley 2004].

5.3 Fiber optic sensors

The first experiments on the use of optical fibres as sensing elements date back to the early 1970s. Research and development work has resulted in a wide range of sensors, e.g. for temperature, pressure, strain, acceleration, displacement, liquid level, flow, rotation, acoustic, and chemical and biomedical sensing. Fiber optics allow the production of sensors which are small and lightweight, easily multiplexable on a single fiber network, and immune to electromagnetic interference (EMI). A number of reviews on the developments in fiber optic sensor technology have been published. This chapter is based on the reviews by Kersey [1996], and Grattan & Sun [2000].

Optical sensors can be divided into intrinsic and extrinsic devices. In the former, the interaction occurs within an element of the optical fiber itself, whereas in the latter the optical

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fiber is used to couple light, usually to and from the region where the light beam is influenced by the measurand. In many sensor systems the parameter of interest is measured at a specific location. This ‘point’ measurement capability is the most common sensor operation mode, used for example when monitoring temperature, pressure, acceleration, or many chemical parameters. This is also how many fiber optic sensors operate: the sensor tip may be sensitized via a chemical indicator to respond to pH, the sensor may have a prism tip for liquid level monitoring, or resonant structures may be mounted at the end of the fiber for pressure or acceleration measurement, to mention a few examples. Fiber optic sensing techniques offer also possibilities for distributed sensing, as a result of the ability to spatially discriminate the measurand at different locations along the length of the fiber itself. Most distributed fiber optic methods utilise non-linear optical effects in the fiber material, since these exhibit varied and distinctive responses to external measurands. Distributed sensing has been employed for example in some types of strain sensing as well as for temperature measurements. If the fiber is not sensitive along its entire length, but is locally sensitized at particular predetermined points along the length of the fiber network, a ‘quasi-distributed’

sensor system is obtained. This technique has been applied to pressure and chemical sensing, e.g. using different fiber types.

The common feature of fiber optic sensors is that they contain an optical fiber, at least one of a range of optical sources and a modulation scheme by which the parameter that is being measured introduces a change in the optical signal, which can be sensed at the detector and employed through the signal processing scheme. For example, the operation of interferometric sensors is based on the measurand-induced phase shift proportional to the influence of the measurand on the optical system (e.g. strain or temperature induced change in fibre length or optical path). Fibre Bragg gratings (FBG) are simple intrinsic devices photo-inscribed into a fiber, and they form one of the most intresting development areas in fiber optic sensors, particularly in the area of embedded sensing in materials for creating smart structures. They are based on the photosensitivity of silica fiber doped with germanium when illuminated with UV light. Absorption of UV light causes a change in the absorption characteristics of the glass resulting in a shift in the index of the glass at certain wavelengths. Illuminating the glass with a broadband source of light, a narrowband component is reflected at the Bragg wavelength.

Perturbation of the grating, e.g. as a result of temperature or strain, results in a shift in the Bragg wavelength of the device. This shift can be detected in either the reflected or transmitted spectrum.

One advantage of fiber optic sensors is the relative ease of multiplexing of several fibres into an array that uses a common source and detection system. The three major multiplexing arrangements that may be used are wavelength division (WDM), frequency division (FDM), and time division (TDM). The number of sensors on a single network can be extended by using various combinations of the multiplexing techniques.

More detailed descriptions of the various fibre optic sensors and their possibilities can be found in the before mentioned review articles [Kersey 1996, Grattan & Sun 2000] as well as in the numerous references in them. Even if it is not possible to utilise fibre optic sensors to direct wear monitoring of coatings they should provide a possibility at least for the detection of coating break through by embedding the fibres between the coating and the substrate. The distributed sensor system could in some applications possibly even allow the determination of the location of the coating damage.

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5.4 Smart wear sensors

Continuous wear monitoring is possible by using smart wear sensors based on capacitive, resistive or conductive principles. Yaxley and Knight [1999] have reported on the development of smart sacrificial wear sensors for monitoring the condition and performance of cone crushers. The sensors were embedded in key areas of the liners of the crusher. The signals from the sensors were relational to the length of the sensors. As the sensors were worn away at the same rate as the liners, they gave a good representation of the liner wear. The sensors studied were of four different types: single resistive strip sensors, multiple surface mount resistive sensors, multiple surface mount capacitive elements, and discrete level wear sensors based on conductivity. The single resistive sensor was made by coating a ruthenium oxide resistive element onto the top surface of a ceramic substrate, with nickel and solder plated wraparound terminals running along the length of the sensor. The multiple surface mount sensors were made in a similar manner but instead of a longer strip they consisted of an array of resistors or capacitors stacked together. Problems encountered included short circuits and irregular spikes in the signal, brittle nature of the resistive element and profound effect of temperature on the signal from the capacitive sensor. The discrete level wear sensor was constructed from conductive wire loops positioned in a metal tube filled with epoxide pottant. The wire loops extended different distances along the sensor body. As the sensor was worn, the wire loops broke at the respective levels. This kind of sensor can be seen advantageous in certain situations although it is more likely that a single wire loop would be used for obtaining an indication for final system shutdown or early warning.

The wear range measured by Yaxley and Knight [1999] was around 50 mm and the accuracy they achieved was in the range of 1 mm. In many applications there is, however, a need to monitor wear in much smaller dimensions. In order to be able to monitor the wear of a sliding bearing surface while in contact with a shaft or flat counter face, Kreider and Ruff have studied sensors consisting of electrically conductive films separated from the bearing with insulating films [Kreider & Ruff 1996, Ruff & Kreider 1997]. Wear was quantified by changes in the electrical resistance of the sensor due to the reduction of the thickness of the conducting film. A constant current was fed to the conducting film and the resistance was recorded throughout the test. Most of the increase in the resistance of a single layer was measured during the last 20% of its life. Wear-through of the conducting film was indicated by a high resistance value. The performance and resolution of the sensor can be enhanced by using multilayer-laminated thin-films. As the initial thickness of each layer in the laminate is known, the progression of wear can be followed when each conducting layer wears through.

The sensor must not change the performance of the shaft or bearing which is measured. This can be achieved by both miniaturization of the sensor such that its area is an insignificant fraction of the wear surface and by selecting the sensor materials such that their wear resistance is comparable to that of the substrate material. Commercial electrical conductor thin-films can be produced with line widths of about 0.1 mm using metal shadow masks, and much smaller line widths can be achieved by photolithography. The thickness of the sensor can be designed to be the same as that of the wear depth to be measured.

The smart sensors described above can be used for measuring local wear on the location where they are placed. Hence the use of this type of sensors is mainly restricted to applications where the critical wear location is known or the wear is uniform.

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6 Conclusions

The parameters affecting erosion are diverse and include material properties, environmental factors and process parameters. The erosion modes were reviewed according to the media causing erosion, i.e. liquid impingement erosion, cavitation erosion, solid particle erosion, slurry erosion and the combination of erosion and corrosion.

Comparison of the results of different erosion tests is often unreliable, due to the wear being strongly influenced by the variation of key parameters responsible for it. Even minor variations in e.g. particle velocity and flow direction may have a strong influence on erosion test results. The use of reference materials that are tested at the same time or at identical conditions as the test samples is advisable for enabling comparison of results. A number of both standardised and non-standardised erosion tests exist. Test procedures for erosion studies with relevance for the operational reliability of machines are presented in the report, with a sub-division into gas and liquid erosion.

Erosion of coated surfaces has been studied by a number of authors showing that hard metallic coatings such as hard chrome plating and ceramic coatings can be successfully used to increase the erosion resistance of steels. However, in many cases thin ceramic coatings offer less protection against slurry erosion than the other groups of hard coatings. Thick ceramic coatings seem to offer wear protection at low impingement angles but not at angles close to 90^o. Diamond and diamond-like coatings can be used to increase erosion wear resistance, the coating detachment being the main limitation to their ability for wear protection.

The most common methods to measure wear involve direct determination of the amount of removed material as mass loss or volume loss. Due to differences in density, comparison of wear between different materials is, however, easier by dimensional changes or volume loss than by mass loss. Dimensional changes can be used for determining wear distribution or localised wear, and for coatings the reduction of coating thickness is a useful indication of wear. When on-line monitoring of wear is required, indirect methods are often more feasible.

On-line corrosion monitoring methods could also be applicable for monitoring wear under slurry erosion. Even if the measurements would indicate a corrosion rate which is lower than that of the actual wear rate due to erosion, an indication of the break through of the coating should be possible by some corrosion monitoring methods. Other methods possibly suitable for being adapted to on-line monitoring of coating wear under slurry erosion include radionuclide technique, fibre optic sensors and smart sensors based on capacitive, resistive or conductive principles.

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Erosive wear of coatings and methods to monitor coating wear