1.INTRODUCTION
The High temperature Air Jet Erosion Testing machine is used to test the erosion resistance of solid materials to a stream of gas containing abrasive particulate. The test is performed by propelling a stream of abrasive particulate gas through a small nozzle of known orifice diameter toward the test sample. Material loss, in this case, is achieved via the impingement of small abrasive particles upon the surface of the test sample. Materials such as metals, ceramics, minerals, polymers, composites, abrasives, and coatings can be tested with this instrument. The test specimen, temperature, angle of incidence of the jet stream, abrasive particulate speed and flux density, can be varied to best simulate actual conditions. Special adapters are available to test various geometries and components for user-specified testing applications. The instrument can be configured to test as per ASTM G76 specifications.
1.1 WEAR
Wear is defined as the progressive volume loss of material from a target surface. It may occur due to corrosion, abrasion or erosion. The wear due to corrosion is caused by chemical reactions, which can be prevented by adopting suitable measures, whereas the wear due to abrasion and/or erosion can only be minimized by controlling the affecting parameters.
Wear is one of the most common problems encountered in industries like thermal power plants, hydropower plants, mining industries, food processing industries etc. in which solid liquid mixture is transported through pumps and pipes. Wear is the loss of material from a component due to a mechanical interaction with another object. Many types of solids, liquids, and even high-velocity gases can remove material and change the physical dimensions and functionality of a part. Corrosion and erosion are the main causes of wear. Corrosion is caused by chemical reaction of material with its environment. Erosion wear is due to exposure to moving liquids and gases, which may or may not contain hard particulate.Effect of erosion wear in slurry pumps and pipes is predominantly more as compared to the corrosion. The service life of equipment of slurry transport system is reduced by erosion caused by solid-liquid mixture following through
the slurry transport system. So slurry erosion is important field should be investigated.
1.2 Erosion Wear
Erosion wear is the dominant process which can be defined as the removal of material from a solid surface. It is due to mechanical interaction between the surface and the impinging particles in a liquid stream. In Erosion process there is a transfer of kinetic energy to the surface. With the increase in kinetic energy of the particles impacting at the target surface, it leads to increase in the material loss due to erosion. It depends on the predominant impact angle of particle impingement with the material surface and it will var y from 0 to 90 degrees. Impact angle depend on both fluid particle and particle - particle interaction. This type of wear can be practically found in slurry pumps, angled pipe bends, turbines, pipes and pipe fitting, nozzles, burners etc. The material loss due to erosion increases with the increase in kinetic energy of the particles impacting at the target surface. The volume loss due to erosion is a troublesome problem for slurry transportation systems e.g. mineral transport systems, ash disposal systems. The erosion wear due to the air borne particles in some devices such as jet planes and turbines is also significant due to very high impact velocity.
1.3 Mechanism of Erosion Wear
The commonly accepted erosion mechanisms are classified as:
1.Cutting
2.Ploughing
3.Extrusion and forging
4.Subsurface deformation and cracking
1.Cutting Erosion
If the particles are very sharp it causes cutting erosion and a micromachining action occurs when particles interact with the material surface. Minimal plastic deformation of the surface region occurs in slurry pipeline because of two mechanisms namely, corrosion and erosion. These mechanisms are quite different in various manners
Figure 1.1 Cutting Erosion
2.Ploughing Erosion
Ploughing erosion is a two stage process involving localized plastic deformation of the surface from rounded particle impacts. In first stage process, particle impacts form surface craters with plastic flow of the surface occurring around the particle edges during impact. As a result of the particle collision, an extruded shear lip is formed. The second stage process involves repeated particle impacts causing fatigue of the extruded shear lip regions. The shear lips fail and are broken off.
Figure 1.2 Ploughing Erosion
3.Extrusion and Forging Mechanism
This mechanism is also known as platelet mechanism. The impact of a solid particle spreads the target material over the adjacent crater in the direction of impact. This spread material get further flattened and extended to develop a platelet. A proposed sequence of particle impacts that could cause the material removal due to platelet mechanism is shown.
Figure 1.3 Extrusion and Forging Mechanism
4.Subsurface deformation and Cracking
A blunt particle striking the target surface at high velocity causes localized plastic deformation at the point of contact, which develops cracks leading to wear by brittle fracture. This type of wear mechanism is known as subsurface deformation or cracking. As the particle moves over the surface, small increment in the plastic strain takes place and the residual stresses develop in the deformed layer. Since the ratio of particle size to contact asperity size is 100:1, the material experiences cyclic loading and unloading as it moves over the surface. This leads the anisotropy in the surface layer and subsequent crack generation. When cracks generated from various parts of the solid link together through crack propagation, loose eroded material is formed. The ploughing process also causes subsurface plastic deformation and may also contribute to the generation and propagation of subsurface cracks. When subsurface cracks propagate, the target material loses out as small wear particles causing loss of material from the surface. Thus the solid particle striking at large impact angles causes brittle fracture and the material is removed from the surface by the formation and intersection of cracks as shown in Figure 1.8. The fracturegenerates the new surface in following two steps:
1.Cracks and voids can be nucleated at or below the surface
2.Cracks will propagate from these nucleated or pre-existing flaws to develop long cracks.
Thus the subsurface damage processes have dominant effect on erosion behavior of materials particularly brittle type.
Figure 1.4 Subsurface deformation and Cracking
1.4 Types of Erosion wear
Erosion can be classified into various types depending upon interaction taking place between the target surface and the impacting substance.
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1.5 Solid Particle Erosion (SPE)
It is the loss of material that results from repeated impact of small, solid particles entrained in air or gas. In some cases SPE is a useful phenomenon, as in sandblasting and high-speed abrasive cutting, but it is a serious problem in many engineering systems, including steam and jet turbines, pipelines and valves carrying particulate matter. Solid particle erosion is to be expected whenever hard particles are entrained in a gas medium impinging on a solid at any significant velocity (greater than 1 m/s).
1.6 Cavitation Erosion
It is defined as the repeated nucleation, growth, and violent collapse of cavities, or bubbles, in a liquid. In pr actice, all liquids contain gaseous, liquid, and solid impurities, which act as nucleation sites for the cavities. When the liquid that contains cavities is subsequently subjected to compressive stresses, that is, to higher hydrostatic pressure, these cavities will collapse. This collapse is directly responsible for the erosion process.
1.7 Liquid Impingement Erosion
It has been defined as "progressive loss of original material from a solid surface due to continued exposure to impacts by liquid drops or jets". Liquid impingement erosion is related to repeated impacts or collisions between the surface being eroded and small discrete liquid bodies. The significance of the discrete impacts is that they generate impulsive contact pressures on the solid target, far higher than those produced by steady flows thus, the endurance limit and even the yield strength of the target material can easily be exceeded, thereby causing damage by purely mechanical interaction.
1.8 Slurry Erosion
It is defined as that type of wear, or loss of mass, that is experienced by a material exposed to a stream of slurry. This erosion occurs either when the material moves at a certain velocity through the slurry or when the slurry moves past the material at a certain velocity. Slurries erode by the action of abrasive particles in the liquid which results in the failure of the surface of material in one or the other mode depending upon the conditions to which the system is exposed. Slurry erosion is a serious problem for the industries, which deals with the liquids having solid particles entrained in them. When such a mixture of liquid and solid particles termed as slurry come in contact with the machine element, the removal of material takes place from the surface making the component redundant from the surface.
Chapter 2
Literature Review
2. LITERATURE REVIEW
The purpose of this literature review is to provide background information on the issues to be considered in this project and to emphasize the relevance of the present study. This treatise embraces various aspects of ceramic coatings with a special reference to their erosion wear characteristics.
2.1 On Solid Particle Erosion Wear of Materials
Solid Particle Erosion (SPE) is a typical erosive wear mode where particles strike against surfaces and promote material loss. During flight a particle carries momentum and kinetic energy, which can be dissipated during impact, due to its interaction with a target surface. It is to be noted that solid particle erosion is different from the other forms of erosion like liquid impact erosion, slurry erosion, cavitation erosion etc. Material removal due to solid particle erosion is a consequence of a series of essentially independent but similar impact events. Thus, the contact between the hard particles and the component surface is of a very short duration. From this point of view, erosion is completely different from the other closely related processes like sliding wear, abrasion, grinding and machining wherein the contact between the tool/abrasive and the work-piece/target is continuous.
In some cases SPE is a useful phenomenon, as in sand-blasting and high-speed abrasive water jet cutting, shot peening of rotating components, cutting of hard and brittle materials by abrasive jets and rock drilling [4-6], but it is a serious problem in many engineering systems, including steam and jet turbines, pipelines and valves used in slurry transportation of matter and fluidized bed combustion systems. Gas and steam turbines operate in environments where the ingestion of solid particles is inevitable. In industrial applications and power generation, such as coal-burning boilers, fluidized beds and gas turbines, solid particles are produced during the combustion of heavy oils, synthetic fuels, pulverized coal etc. and causes erosion [13, 14] leading to the damage of compressor gas path components, such as stator vanes or rotor blades, leading to gradual changes in their surface finish and geometry [15, 16]. Similarly, a helicopter operating in a sandy or dusty field will generate a dust cloud that will be ingested by the compressor resulting in a progressive metal loss from both the leading and trailing edges of the airfoils [17]. Erosion is thus expected whenever hard particles are entrained in a gas or liquid medium impinging on a solid at any significant velocity. In both cases, particles can be accelerated or decelerated and their directions of motion can be changed by the fluid.
Degradation of materials due to solid particle erosion, either at room temperature or at elevated temperature, is encountered in a large variety of engineering industries as illustrated in table 2.1.
Systems
Components
Chemical plant
Transport tubes carrying abrasive materials in an air
stream[17]
Hydraulic mining
Pumps and valves[18]
machinery
Propellant system
Rocket motors trail nozzle, gun barrel[12]
Combustion system
Burner nozzle, reheater, super heater and economizer
tube banks[13, 14]
Fluidized bed combustion
Boiler heat exchanger tubes in bed tubes, tube banks
and expander turbine[17, 18-19]
Coal gasification
Turbine, lock hopper valves[17]
Coal liquefaction
Valve to throttle the flow of product steam[17]
Aircraft engine
Compressor and turbine blades[18]
Helicopter engine
Rotor and gas turbine blades[20]
Table 2.1 Examples of components subjected to solid particle erosion
In order to minimize damage caused by erosive wear, many authors propose the use of surface coatings. Fluidized bed combustion boilers, turbines and engines are normally exposed to erosive environments and the erosion leads to many accidents [19-22].
The ceramic coatings are considered as powerful barriers against deterioration of machine parts exposed to particulate flow at high temperatures [7]. Ceramic coatings have great potential for many applications due to their good thermal protectiveness, high hardness and wear resistance among others. For example, the wear resistant coatings are widely used in textile industry to improve the life time of different thread guiding elements, guiding and distribution rollers, ridge thread brakes, distribution plates, driving and driven rollers, gullets, tension rollers and thread brake caps [8].
Applications of ceramic coatings produced by different deposition methods are increasingly used to extend the service life of mechanical components. This is because the coatings themselves have high hardness and chemical inertness and have excellent wear resistance, which makes it possible to protect the surface from erosive environments. Friction and wear are surface phenomena and are of high concern especially in industrial components resulting in huge economic losses and sometimes lead to catastrophic failure. Hence, it is of utmost importance to minimize their ill effects. Use of coatings would enhance the wear resistance as well as anti-friction resistance of the materials. In addition, coatings enable use of relatively cheaper materials for machine components. Recently, cermet coatings are used to further increase the erosion resistance through increasing the toughness of the coatings [9-13]. The erosion resistance of the coatings is influenced not only by the impact angle, particle velocity and environment temperature, but also is strongly dependent on the coating process. For the application of these materials to components, different techniques in the field of surface engineering can be considered. Some researchers made use of processes such as thermal spraying, sputtering, physical vapour deposition, chemical vapour deposition, detonation spraying and electro-spark detonation to obtain protective coatings against erosive wear. Out of all these surface modification techniques, however, the most widely reported one is thermal spraying.
Ahmed Elkholy et al. [20]studied the erosion wear of aluminium and cast iron having Brinell
hardness 121 and 230 respectively by using the equation, W = KVn. W is wear in term of mass
loss of specimen per gram of sand. K is constant corresponding to other parameter like
concentration of slurry, particle size and impact angle. The value of n determined by value of
different velocities fitting in above equation. Then Belzona molecular ceramic steel (BMS) was
test under identical condition as for Al and Cast Iron. The observation show that wear increase
with increase in impact velocity. Impact Velocity affects more on BMS than Al and Cast Iron.
He founded that the amount of wear varied with particle size with an exponent of 0.616. Study
was conducted with 30° impact angle on cast iron. He assume that impact angle α is independent
of velocity, an equation for wear of cast iron (hard and brittle) was established as
W = 1.061Ã- 10 -8 [1+ sin ( α− α1Ã- 180 -90)]V2.3875 ................................................... Eq. (2.1)
90− α1
ß™R1 is the angle at which wear develops and it is taken as zero for simplicity. Observations are taken at ß™= 60° and 90° with different velocities. The 90° impact angle showed higher wear for any arbitrary velocity value.
Andrews and Horsfield [21]performed the test with a jet of gas and solids particles to study the mechanics of an eroding surface. They stated that increasing the particle concentration causes a decrease in the erosion rate due to the interference of the particles themselves.
Shook et al. [22] measured the particle size distribution in a flow of sand and water in a pipe.The distribution was not homogeneous and higher particle concentrations occur in the bottom of the pipe. That is, for a fixed mean of solids concentration, as the slurr y velocity increases the particle distribution becomes more uniform, which results in less wear at the bottom of pipe and more near the top and sides of a pipe.
Singh et al. [23]found that both 304 and 316 stainless steels have the same rate of wear when impinged with an air jet containing SiC particles that were 160 microns in diameter and angular shapes. In both metals wear rate fastest when the impingement angle was at 30° and it was the slowest at 90°.This information is very useful when designing a test because it indicates where attention must be directed to evaluate the maximum wear locations. Wear measurement must not be concentrated only at a section of a flow loop where the flow makes an abrupt 90° change.
Lynn et al. [24] have studied particles size effect on slurry erosion using a pot tester at a constant velocity of 18.7 m/s and using a relatively a dilute suspension 1.2% by weight of silica carbide in oil for different equisized diameter ranging from 20µ to 500µ. Tests were performed on steel specimens over a maximum period of 6o minutes. They conclude that for particles sizes greater than about 100µ the erosion rate was proportional to the Kinetic energy dissipated by particles during impact but for particles size less than 100µ other metal removal mechanism become increasingly significant. Both collision efficiency and impact velocity of particles decreased with decreasing particles size.
Miller and Miller[25] have shown that erosion rate increases rapidly as the slurry concentration increases to 10% by weight, but by increasing concentration more than 20% by weight the erosion rate dependence is relatively unaffected by further increases in concentration.
Gupta et al. [26] studied the effect of velocity, concentr ation and particle size on erosion wear. The experiment was performed by pot tester for two pipe materials, namely brass and mild steel. They evaluated that for a given concentration, erosion wear increases with increase in velocity and for a given velocity, erosion wear also increases with increase in concentration but this increase is comparatively much smaller. They also concluded that erosion wear decreases with decrease in erodent particle size.
Gandhi et al. [27]measured the erosion rate of steel plating by a jet of sand and water. The particles ranged from 200 to 900 microns and the solids concentration from 20 to 40 % by weight. Using velocities from 3 to 8 m/s they evaluated that the erosion rate = Æ’ (velocity).
When changing the material of the eroding surface the exponent of particle velocity also changes.
2.6
