INTRODUCTION AND HISTORY
Two of the main aspects of the mechanical engineering systems are structural integrity and wear integrity. Ensuring structural integrity simply means machine components do not break wear integrity deals with the wear and wear out rates of the machine components. During past years, environmental factors with respect to mechanical system operation have changed drastically. These factors include limited natural resources, limited economic resources, and spiraling manpower costs. In order to resolve conflict due to these factors, the machine design process was forced to address the fact of wear integrity. However, the resolution is not as simple as it appears. Machine wear and friction are a function of assorted variables as lubrication, additives, materials, surfaces, contamination, design etc.
One of the first to verbalize this situation were the British in the Jost Report published in 1966, which summarized the findings of the famous Jost committee. The resulting Jost Report outlined this wear integrity problem and recommended a multidisciplinary approach to this solution. The document labeled the proposed approach “Tribology”, and defined it as “the science and technology of the interacting surfaces in relative motion and of particles related thereto.” This term included the subjects of friction, wear and lubrication. There is a difficulty in that friction is generally characterized as a branch of physics or mechanical engineering, wear is part of the material science of metallurgy, while lubrication is a branch of chemistry. Tribology is thus a complex interdisciplinary subject. e.g.
1 The physics, chemistry, mechanics, and metallurgy of interacting surfaces in relative motion including the phenomena of friction and wear.
2 Fluid film lubrication e.g. hydrostatic, hydrodynamic, aerostatic, aerodynamic.
3 Lubrication other than in fluid film e.g. boundary and solid lubrication.
4 The engineering of bearings and bearing surfaces (e.g. plain and rolling bearings, piston rings, machine slides and gear teeth, etc.) including their design.
The phenomena considered in tribology are between the most fundamental and most common of those encountered by humans in interaction with their largely solid environment. Many manifestations of tribology are beneficial and, indeed, make modern life possible. Many other effects of tribology, however, constitute serious nuisances, and careful design is necessary to overcome the inconvenience arising from excessive friction or wear. On an overall basis, friction uses up, or wastes, a substantial amount of the energy generated by mankind, while a large amount of productive capacity is devoted to replacing objects made useless by wear.
Tribology is the art of applying operational analysis to problems of great economic significance, namely, reliability, maintenance and wear of technical equipment ranging from aircraft to house hold appliances. Surface interaction in a tribological interface are highly complex, and their understanding requires knowledge of various disciplines including physics, chemistry, applied mathematics, solid mechanics, fluid mechanics, thermodynamics, heat transfer, material science, lubrication, machine design, performance and reliability.
The word tribology is derived from the Greek word “tribos” meaning rubbing, thus the literal translation would be “the science of rubbing”. It is only the name tribology that is relatively new, because interest in the consequent part of tribology is older than recorded history. It is known that the drills made during Paleolithic period for drilling holes and producing fire were fitted with bearings made from antlers or bones, and potter’s wheel or stones for grinding cereals, etc., clearly had a requirement for some form of bearing. Records show the use of wheels from 3500 BC, which illustrates the concern in reducing the friction in translationary motion. The transportation of large building blocks and monuments required the expertise of frictional devices and lubricants, such as water-lubricated sleds. It was the renaissance engineer-artist Leonardo da Vinici, who first postulated a scientific approach to friction.
INDUSTRIAL SIGNIFICANCE AND APPLICATION OF TRIBOLOGY
Tribology is crucial to modern machinery, which uses sliding and rolling surfaces. Examples of productive friction are brakes, clutches, driving wheels on trains and automobiles, bolts and nuts. Examples of productive wear are writing with pencil, machining, polishing and shaving. Examples of productive friction and wear are internal combustion and aircraft engines, gear, cams, bearing and seals. According to some estimates the losses, resulting from ignorance of tribology in United States is about 6% of the Gross National Production and approximately about one third of world’s energy resources in present use, appear in the form of friction. Thus, the purpose of research in tribology is understandably the minimization and elimination of losses resulting from friction and wear at all levels of technology where the rubbing of surfaces is involved. Research in Tribology leads to greater plan efficiency, better performance, fewer breakdowns and significant savings.
The many technical aims and functions to be realized through “interacting surfaces in relative motion” lead to a great variety of tribo-technical components and tribo-engineering systems and processes. The technical functions realized through interacting material surfaces can be broadly classified in the following groups:
Transmission of motion
Transmission of work
Generation or reproduction of information
Transportation of materials
Forming of materials
In all engineering systems friction, wear and lubrication process of various process occur. In order to optimize functional technical behavior of these systems and to minimize friction-and –wear induced energy and material losses, tribological knowledge must be applied with respect to all basic engineering activities, namely:
1. design
2. manufacture
3. operation
4. condition monitoring
5. repair
6. recycling
Common tribological components used in industrial application includes sliding-contact and rolling-contact bearings, seals, gears, cams and tappets, piston rings, electrical brushes, cutting and forming tools. Some of the common industrial applications include material processing, internal combustion engine for automotive applications, and gas turbine engines for aerospace applications, railroads and magnetic storage devices.
IMPACT OF TRIBOLOGY
In considering the impact of tribology, three aspects may be emphasized: scientific, economic, and multidisciplinary. From a scientific point of view, tribology is to be considered as being the discipline which tries to explain the most dominant irreversible process in nature and technology. Tribology is attempting to investigate the irreversible process of mechanics in detail and to explain the complex effects of friction-and-wear-induced energy and materials dissipation.
A second important aspect is multidisciplinary nature of tribology. Since Tribology is the science and technology of interacting surfaces in relative motion, it not only includes the work of physicists, chemists and material scientists interested in the surfaces properties of materials, but also the work of engineers who use interacting surfaces for transmission of forces, work, motion etc. Consequently, the attempts for the solution of the tribological problems required the combined effort of these branches.
Wear is an important topic from the economic point of view because it represents one of a very limited number of ways in which material objects loose their usefulness. The causes of loss of usefulness of material objects with a percentage estimate of economic importance of each are given below. Only corrosion is comparable to wear in economic impact.
Loss of usefulness of material
1. Obsolescence (15%)
2. Breakage (15%)
3. Surface deterioration (70%)
a) Wear (55%)
_adhesive wear (25%)
_abrasive wear (20%)
_corrosive wear (2%)
_surface fatigue wear (8%)
b) Corrosion (15%)
TERMINOLOGY
Owing to the multidisciplinary approach of tribology, the International Research Group on Wear of Engineering Materials compiled a “Glossary on Terms and Definitions” for the
field of tribology. The most important terms included in it are:
1) Friction: Friction is defined as the resistance force tangential to common boundary between two bodies when, under the action of external force, one body moves or tends to move relative to the surface of the other. It is therefore a vital factor in the operation of most mechanisms. High friction is needed for the satisfactory functioning of nuts and bolts, paper clips, and tongs, as well as in the familiar processes of walking, gripping objects manually, and building piles of sand or of apples. Low friction, however, is desired in objects that are designed to move continuously, like engines, skis, and the internal mechanism of watches. Constant friction is required in brakes and clutches, as otherwise unpleasant jerky movement would be produced. Frictional forces, such as the traction needed to walk without slipping, may be beneficial; but they also present a great measure of opposition to motion. The major cause of friction between metals appears to be the forces of attraction, known as adhesion, between the contact regions of the surfaces, which are always microscopically irregular. Friction arises from shearing these “welded” junctions and from the action of the irregularities of the one surface plowing across the other surface. In addition to the frictional energy to overcome adhesion developed at real areas of contact between the surfaces, energy is required for micro scale deformation of contacting surfaces during relative motion. The dominant mechanism of energy dissipation in metals and ceramics is plastic deformation. There is little energy lost during elastic deformation of interfaces. Regardless to the type of deformation, breaking of adhesive bonds during the motion requires energy. The total Intrinsic Frictional force ( Fi ) can be expressed as:
Fi = Fa + Fd
Fa Force needed to shear adhered junctions.
Fd Force needed to supply the energy of deformation.
2) Wear: Wear is the progressive loss of substance from the operating surfaces of the body occurring because of relative motion at the surfaces. It is such a universal phenomenon that rarely do two solid bodies slide over each other or even touch each other without measurable material transfer or material loss. Thus, coins become worn as a result of continued contact with human fingers; pencils become worn after sliding over paper; and rails become worn as a result of the continued rolling of train wheels over them. Only living things (e.g., bone joints) are in general immune to the permanent damage caused by wear because only they have the property of healing through regrowth. The systematic study of wear has been severely hampered by two factors: first, the existence of a number of separate wear processes, which has led to much confusion, especially in terminology; second, the difficulties caused by the small amounts of material involved in wear processes. These difficulties were greatly alleviated when radioactive isotopes of the common engineering metals (iron, copper, chromium, etc.) became available in the 1940s; tracer techniques using these radioisotopes permit measurements of wear, even in small amounts, while it is occurring. This has made it possible to identify types of wear and discover the laws of wear. Wear occurs by mechanical and/or chemical means and is generally accelerated by frictional heating. Wear includes six principal, quite distinct phenomena that only have one thing in common: the removal of solid material from rubbing surfaces. These are Adhesive, Abrasive, Fatigue, Impact by erosion and percussion, Chemical or Corrosive, and Electrical arc induced wear.
Adhesive wear: The most common type, adhesive wear, arises from the strong adhesive forces that are generated at the interface of two solid materials. When solid surfaces are pressed together, intimate contact is made over a number of small patches or junctions. These contacts are sheared by sliding action and the junctions continue to be made and broken, and, if a junction does not break along the original interface, a wear particle is formed. These particles eventually break away. Adhesive wear is undesirable for two reasons: first, the loss of material will eventually lead to deterioration in the performance of the mechanism; and second, the formation of large wear particles in closely fitted sliding members may cause the mechanism to seize at an early stage in its productive life. Adhesive wear is many times greater for unlubricated than for effectively lubricated metal surfaces.
Abrasive wear: Abrasive wear occurs when a hard, rough surface slides over a softer one, and damage the interface by plastic deformation or fracture. Material removal from a surface via plastic deformation can occur by several deformation modes such as plowing, wedge formation, and cutting. Plowing results in a series of grooves as a result of plastic flow of softer material. In plowing process, material is displaced to the sides without the removal of material, however after several plowing material removal can occur. In wedge formation type of wear, an abrasive tip ploughs a groove and develops a wedge on its front. In cutting form of wear, an abrasive tip with large attack angle ploughs a groove and removes the material in discontinuous particles. In case brittle materials with low fracture toughness, wear occur by brittle fracture.
Fatigue wear: Surface-fatigue wear is produced by repeated high stress attendant on a rolling motion, such as that of metal wheels on tracks or a ball bearing rolling in a machine. The stress causes subsurface cracks to form in either the moving or the stationary component. As these cracks grow, large particles separate from the surface and pitting ensues. Surface-fatigue wear is the most common form of wear affecting rolling elements such as bearings or gears.
Impact wear: Repeated impacts result in progressive loss of solid material. The two types of impact wear are erosive and percussive wear. Erosion can occur by jets and streams of solid particles, liquid droplets and implosion of bubbles formed in the fluid. Percussion occurs from repetitive solid impacts.
Chemical wear: Chemical wear occurs when sliding takes place in a corrosive environment. The chemical wear in air is generally called oxidative wear. In absence of sliding, the chemical process of corrosion would form a film typically less than one micrometer thick on surfaces, which would tend to slow down or even arrest the corrosion, but sliding action wears the chemical film, so that chemical attack can continue. Thus, chemical wear requires both rubbing and chemical reaction. It is important in a number of industries such as mining, mineral processing, chemical processing, slurry handling.
Electric-arc-induced wear: When a high potential is present over a thin air film in a sliding process a dielectric breakdown results that leads to arcing. During arcing, a relatively high-power density occurs over a very short period. The heat-affected zone is very shallow. Arcing causes large craters, and any sliding after arc either shears or fractures the lips, leading to three-body abrasion, surface fatigue, corrosion and fretting. Arcing can thus initiate several modes of wear, resulting in catastrophic failures in electrical machinery.
3) Lubrication: The reduction of frictional resistance and wear or other forms of surface deterioration between two load-bearing surfaces by the application of lubricant. In modern lubrication practice, the main concern is to reduce the wear that accompanies sliding and, at the same time, to design lubrication systems that will operate for long periods without inspection or maintenance. Liquid lubricants include natural organics consisting of animal fat, vegetable oils, mineral fractions, synthetic organics and two or more of these materials. Liquid lubricants provide a substantial range of physical and chemical properties. The physical properties are attributed to structure of lubricant. Chemical properties result from the additives used. Some of the important lubricant properties are viscosity, surface tension, thermal properties, volatility, oxidative stability, thermal stability and inflammability.
Grease is a semisolid lubricant produced by the dispersion of a thickening agent in a liquid lubricant that may contain special ingredients imparting special properties. Greases are used where circulating liquid lubricant cannot be contained because of space and cost and where cooling by oil is not required or liquid lubrication is not feasible.
WEAR OF MATERIALS
The wear process is generally quantified by wear rate. Wear rate is defined as the volume or mass of the material removed per unit time or per unit sliding distance. Wear rate is usually not constant. In general, it is a complex function of time. Wear rate may start low and later rise, or vice-versa, fig 1.
Figure 1 Wear rate After certain duration, the wear rate remains constant for a period and may change if transition from one mechanism to other mechanism occurs during a wear test. The initial period during which the wear rate changes is known as run-in or break-in period. Wear during run-in depends on initial material structure properties and on surface conditions such as surface finish and the nature of films present. Wear rate like friction, of a material is dependent upon the counter face or the mating material, surface preparation and operating conditions.
A self-mated steel pair exhibits high friction and wear. Pairs of dissimilar metals exhibit moderate friction and wear. These are generally used in lubrication applications. Ceramic versus another ceramic or versus itself exhibit moderate friction but extremely low wear. Self-mated ceramic pairs opposed to self-mated metal pairs are desirable as they are not abusive to the mating surface. Since they exhibit very low wear and friction, they are used in lubricated and non-lubricated conditions. Metals pairs are most commonly used because of ease of machinability and low cost. Ceramics are used because they are somewhat inert, strong and can be used at high temperatures.
Wear of Metals and Alloys
Clean metals and alloys in solid contact exhibit high adhesion, consequently high friction and wear. The wear rate of contacting metallic surfaces cleaned in high vacuum can be very high. The slightest contamination mitigates contact or chemical films, which reduce adhesion, resulting in reduction of friction and wear. In case of soft metals, such as Pb and Sn, the contact area is large even at low loads, which result in higher wear rates. In general, wear for alloys tends to be lower than that for pure components.
Figure 2 Wear resistances of different microstructures of steel Steels form the most commonly used family of materials for structural and tribological applications. Based on chemical composition and processing, a variety of microstructures and physical properties of steel can be obtained. The wear resistance of different microstructures is summarized as shown in fig 2.
Effect of temperature (Oxidative wear)
Interface temperature produced at asperity contacts during sliding of metallic pairs under nominally unlubricated conditions result in thermal oxidation, with oxide films several microns thick. A thick oxide film reduces the shear strength of interface, which suppresses the wear because of plastic deformation. Tribological oxidation can also occur under conditions of boundary lubrication when the oil thickness is less than the combined surface roughness of the interface. The oxide film can prevent severe wear.
At low ambient temperatures, oxidation occurs at asperity contacts from frictional heating. At higher ambient temperatures, general oxidation of the entire surface occurs and affects wear. In case of steel, the predominant oxide present in the debris depends on sliding conditions. At low speeds and ambient temperatures, the predominant oxide is α-Fe3O4, at intermediate conditions it is Fe3O4, at high speeds and temperatures it is FeO. Oxygen and other molecules are absorbed on clean metals and ceramic surfaces, and form chemical bonds with them. The slow step inhibiting the continuation of this reaction is the diffusion of the reacting species through the film of the reaction product. Oxidation of iron and many metals follow a parabolic law, with the oxide film thickness increasing with the square root of time,
h = C×√t
h: Oxide film thickness
t: Average growth time
C: Parabolic rate constant at elevated temperatures
Science diffusion is thermally activated, growth rate in oxide film thickness during sliding as a function of temperature similar to thermal oxidation under static conditions, follow an Arrehenius type of relationship,
K =A exp (- Q/ RT)
K: Parabolic rate constant for the growth of oxide film
A: Parabolic Arrehenius constant (kg/m² m²s)
Q: Parabolic activation energy associated with oxide (kJ/mol)
R: Universal gas constant
T: Absolute temperature
Arrehenius constant for sliding is several orders of magnitude larger than that for static conditions, which means that oxidation under sliding condition is much more rapid than that under static oxidation conditions.
Effect of operating conditions (Wear-Regime maps)
The wear regime maps elucidate the role of operating environment on wear mechanisms. No single wear mechanism operates over a wide range of conditions. There are several wear mechanisms, which change in relative importance as the operating conditions are changed. The transitions in dominant wear mechanisms occur as sliding loads and velocities are changed. In some cases, changes also occur as a function of time. The dominant wear mechanisms are based on mechanical strength and interfacial adhesion. Increase in normal load and sliding velocity results in monotonic increase in interface temperature. High interface temperature results in formation of chemical films, mostly formation of oxide films in air. High temperatures result in decrease in mechanical strength and in some cases in structural changes. At high load-velocity (PV) conditions, there may even be localized melting near the surface.
Various regimes of mechanical and chemical wear for a particular sliding material pair are observed on a single wear-regime map (wear-mode map or wear-mechanism map) plotted on axis of normalized pressure and normalized sliding velocity. Normalized pressure is nominal pressure divided by surface hardness ( p/H ) and normalized sliding velocity is sliding velocity divided by velocity of heat flow ( given by the radius of circular nominal contact area divided by thermal diffusivity ). Wear regime map for steel sliding on steel in air at room temperature is shown in fig 3. It can be seen that, in principle, the map can be divided into areas corresponding to different wear regimes, with boundaries of sliding velocities and contact pressure beyond which oxidative wear would be dominant as compared to mechanical wear at low speeds. Prevailing wear mechanisms can give mild or severe wear. Mild wear gives a smooth surface, severe wear produces a surface that is rough and deeply torn, and the wear rate is usually high. The transition between mild and severe wear takes place over a wide range of sliding conditions. These are load- and velocity-dependent. In addition, some are sliding distance-dependent.
Figure 3 Wear map for steel sliding on steel at room temperatureThe mild wear occurs because direct metal-metal contacts are minimized mostly by oxide layer produced because of frictional heating. Mild wear takes place under four distinct sets of conditions.
1. At low contact, pressures and sliding velocities, a thin and ductile oxide film is formed which prevents direct metal-metal contact and is not ruptured at high loads.
2. At higher velocities, a thicker and more brittle film is continuously generated by high interface temperatures. Continuous oxidation replenishes the oxide film.
3. At higher loads, a hard surface layer is formed on carbon steel surfaces because of localized frictional heating followed by rapid quenching as the frictional heat is dissipated. The higher interface temperatures also produce a thicker film of oxide, supported by the hardened substrate.
4. At yet higher sliding velocities, the increased interface temperature produces thick film. Insulating oxide films reduce heat flow from the surface to the underlying conducting substrate resulting in severe oxidation.
Severe wear occurs under conditions where direct metal-metal contact occurs. Severe wear occurs under three distinct sets of conditions.
1. At high contact pressure and low sliding velocities, contact pressure is high enough to rupture the thin oxide layer, which leads to direct metal-metal contact.
2. At moderate contact pressure and sliding velocities, the load is high enough to penetrate thicker but brittle oxide films generated.
3. At high contact pressures and sliding velocities, the sliding conditions are so severe that the local temperature reach melting point of steel, resulting in liquid film in contact, which leads to severe wear.
These wear maps are useful to provide guidance with respect to proper selection of materials and performance envelopes for metals.
CERAMIC COATINGS
Coatings are used for dry and lubricated conditions to meet the conflicting requirements, often to reduce cost. Coatings can be classified as hard wear resistant coatings, moderate friction and soft coatings with low friction and moderate-to-low friction. Hard coatings have found extensive use in highly loaded and/or high temperature (up to 1000°C or even higher) applications to reduce wear. Coatings of ferrous and nonferrous metals, intermetallic alloys and ceramics provide good wear resistance owing to their inherent high hardness. The coatings, varying in thickness from a fraction of a micrometer to several millimeters, can be applied by a variety of deposition techniques, such as electrochemical deposition, thermal spraying, and physical vapor deposition.
Among metals, nickel is most widely used coating after hard chromium. Coatings of ferrous based alloys are used where heavy wear is encountered, under conditions that impose mechanical and thermal shock. Cobalt and nickel based alloy coatings are superior in hardness in elevated temperatures and are corrosion and oxidation resistant.
The commonly used materials are oxides, carbides, nitrides, borides and silicides of the refractory transition materials, nonmetallic oxides and nonmetallic nonoxides such as aluminum oxide, chromium oxide, zirconium oxide, silicon dioxide, tungsten carbide, molybdenum disilicide etc. The Vickers hardness of these coatings varies from about 600 to 3500 kg/mm² or higher. Ceramic bonded and plasma sprayed coatings of CaF2-BaF2, CaF2 and PbO-SiO2 exhibit flow characteristics above their softening point and are thus used for solid lubrication at high temperature.
Ceramic coatings find following tribological applications:
1. Bearings and seals operating in extreme environments.
2. Slurry pump seals, acid pump seals, valves, knife sharpeners.
3. Cutting tools, metal working tools, gas turbine blades of aircraft engines.
4. Gun steels, magnetic heads, tape-path components of computer tape devices.
Coating deposition technique by PLASMA SPRAYING PROCESS
Figure 4 Plasma spray processA schematic of plasma spray process is shown in fig4. In this process, a DC arc is struck from a high frequency arc starter between a center tungsten electrode in the torch and a water-cooled copper nozzle, which forms an anode, while a stream of inert gases is passed through the high temperature plasma stream from the gun nozzle. The spraying material is generally in the powder form and fed into the plasma flame by carrier gas, where it melts and gains high velocity due to high plasma enthalpies and is propelled to the substrate.
COMPARISION OF WEAR BEHAVIOUR OF PLASMA-SPRAYED Al2O3 COATINGS WITH AND WITHOUT LASER TREATMENT
Introduction
Plasma sprayed ceramic coatings are used in many engineering applications to improve wear and corrosion resistance. However, their protective properties are strongly limited by micro structural defects at the surface. To improve such properties, surface laser treatments are widely used. This topic focuses on an experimental study of the wear behavior and tribological characteristics of plasma-sprayed Al2O3 coatings, before and after CO2 laser treatment. Its main objectives are:
(1) To determine the effects of laser treatment on adherence, porosity and micro
hardness of the ceramic coatings.
(2) To compare the wear behavior of laser-treated ceramic coatings with that of
non-laser-treated ceramic coatings.
(3) To determine the wear mechanism.
Experimental procedure
Figure 5 Geometry of test specimensTo know how the laser treatment affects wear, mechanical and tribological tests were carried out before and after the treatment and the results were compared. The geometry of the two specimens used, block and ring, was defined according to the ASTM G77 requirements. The ring was made of SAE 4620 steel, and the block was formed by an AISI 1043 metallic substratum on which a bond layer and the ceramic coating were sprayed.
Table 1 Coating characteristics
CERAMIC COATING
Powder size
-90 + 15μm
Fusion temperature
1982°C
Composition
98.5% Al2O3, 1% SiO2, 0.5% other oxides
Thickness
0.30mm
BOND LAYER
Composition
89.5% Ni, 5.5% Al, 0.5% Mo
Thickness
0.05mm
Fig 5 shows the geometry of the two types of test specimens. The Al2O3 coatings were deposited by plasma spray onto the AISI 1043 steel
Table 2 Surface laser treatment parameters
POWER
2000 W
SPEED
3500 mm/min
DENSITY
22.2W/mm²
ENERGY
5.7 J/mm²
BEAM
15×6 mm INTEGRATED
PREHEATING
700°C
PROTECTIVE GAS
Argon
substrates. These substrates had previously been shot blasted with synthetic corundum to provide suitable roughness to ensure adequate adherence between coating and substrate. Prior to the spraying the Al2O3 coating, the bond layer was deposited by plasma spray to improve bond strength. The features of the coating and the bond layer are shown in Table 1.
The fusion of surface was carried out by means of a 5 kW CO2 laser (Spectra Physics) operating in continuous mode, and using an integrator beam to provide homogenous power distribution. The spot was 12mm long and 6mm wide. To decrease the thermal effects of treatment on the coating, the specimens were preheated by putting them in a furnace for 4 minute at 700°C. The laser-treatment parameters were optimized to obtain the following results
· Thickness of molten layer of about 0.1mm.
· Minimum cracking and vertical cracks only.
· Maximum reduction of porosity in molten zone.
· Good adherence between coating and substrate.
The best results were obtained by using the parameters in the table 2.
Figure 6 Alpha LWF-1 TribometerWear tests of the block-on-ring type were carried out using an Alpha LWF-1 tribometer ( as shown in fig 6), with linear contact between a steel with Al2O3 coating and an SAE 4620 steel ring, with various loads an speeds, under dry, abrasive and lubricated conditions. Half of the tests were performed with laser-treated coatings and rest with non-treated coatings.
Table 3 Range of variables of wear test
CONDITION
SPEED (rpm)
LOAD (N)
DRY
50
45
100
90
150
136
200
408
ABRASIVE
50
45
100
90
150
136
200
408
LUBRICATED
200
1768
The abrasive material used was alumina powder (90+15 μm size) with water in ratio of 1:5. During the tests, the abrasive was removed three times every minute. The lubricant was oil 0.822g/cm³ density and 107 and 11.3 CSE viscosity at 38° and 100°C respectively. The tests were carried out in environmental conditions. The control variables are given in table 3. Adherence tests were carried out which determines the degree of adhesion of a coating to a substrate, or the cohesive strength of coating at tension normal to the surface.
The test consists of coating one face of substrate fixture, bonding this coating to the face of a loading fixture and subjecting this assembly to a tensile load normal to the plane of coating. This test is performed at ambient temperature.
Results
The effects of laser on plasma-sprayed coatings regarding the adherence, porosity and microhardness were analyzed to achieve a better understanding of wear after laser-treatment.
Coating control results
Adherence in non laser-treated coatings ranged from 48 to 64 MPa. In this case, the bond layer failed, so the failure can referred as adherence failure. On the other hand, for laser treated coatings, adherence ranged from 56 to 65 MPa, i.e. the mean value increased. Failure occurred sometimes within the coating itself, so such failure can be referred as semi cohesive.
Figure 7 Cross sectional Optical Micrograph of laser treated Al2O3 ceramic coating Before treatment, porosity was about 5 to 6%. After treatment, at a surface depth between 0.05 and 0.1 mm it almost became zero, and coating contained a few vertical cracks. Then comes a 0.1 mm thick transition zone where porosity increases gradually to values it had before treatment. Fig 7 shows this effect.
In non-treated zone, the microhardness was an average of 1161 HV, where as the mean microhardness in the treated zone was an average of 1441 HV, which implies an increase of almost 25%. This accounts for better behavior of treated coatings when some of the wear mechanisms such as abrasion or adhesion are applied.
Wear results
The wear results were obtained for pairs indicated in table 3, at different loads and speeds, under dry abrasive and lubricated conditions.
Comparative wear study: ceramic steel
Wear behavior of ceramic coating, regardless of whether it is treated or non-treated, is good compared with that of steel, under any conditions Figs 8, 9 and 10 shows this behavior. In spite of the quantitative difference, qualitatively wear behavior in both steel and the ceramic coating is very similar, verifying the existence of three stages in the development of wear: the first stage of running in, the second stage of steady wear with a linear relationship between wear and time and third stage of failure with sudden deterioration.
Figure 9 Wear comparisons between treated ceramic coating and steel under dry and abrasive conditions
Figure 8 Wear comparisons between non-treated ceramic coating and steel under dry and abrasive conditions
Figure 10 Wear comparison between ceramic coating and steel under lubricated conditions
Wear of ceramic coating An increase in load results in higher and that for lighter loads results in higher and that for light loads, there is a linear relation between load and wear. The wear rate decrease as speed increases, although under some conditions, especially for laser treated coatings, there are speeds at which wear are minimum.
Figure 11 Wear comparisons between treated and non-treated coatings sliding against SAE-4620 steel under dry conditions
Figure 12 Wear comparisons between treated and non-treated coatings sliding against SAE-4620 steel under abrasive conditions
Figure 13 Wear comparisons between treated and non-treated coatings sliding against SAE-4620 steel under lubricated conditions
Under dry conditions, as shown in fig 11, laser treated show less wear than non-treated ones, and this improves even further load is increased. The increase in hardness from laser treatment is responsible for better behavior.
Fig 12 shows that, under abrasion conditions, the treated coating has a remarkably better behavior than non-treated coatings. The abrasion condition implies a certain amount of lubrication, since the abrasive material consists of certain amount of water diluted alumina particles.
Under lubricated conditions, as shown in fig 13 despite good wear behavior shown by both the treated and non-treated coatings, the wear of laser-treated ceramic is worse than non- treated ceramic.
Figure 14 Wear of SAE 4620 steel against laser treated and non-treated coatingsSteel wear The steel that is in contact with treated ceramic coating shows less wear than non-treated coatings, as shown in fig14. The reason for this change is the microstructural change undergone by the coating owing to the laser treatment. Under lubricated conditions, the result as shown in fig 10 are different and, although overall wear is small, it is higher for ring that contacts the treated coatings. These facts are relevant from the industrial point of view, because the laser treatment cannot only improve the performance of ceramic coatings in some circumstances, but can also reduce the wear of their paired steel. Under abrasion condition, the values obtained are low for treated coatings than for non-treated coatings. Under lubrication, the friction of treated coating is slightly higher.
Wear map Three-dimensional wear maps, have been obtained relating wear sliding distance
Figure 16 Wear map of Al2O3 coating at different loads under dry conditions (speed=150 rpm)
Figure 15 Wear map of Al2O3 coating at different speeds under dry conditions (load=45N)
and speed or load. Fig 15 and fig 16 illustrates sliding speed at which wear is minimum and effect of load on wear of laser-treated ceramic coatings.
Microstructure and fractography
By the use of optical microscope and SEM, the wear mechanism and the microstructural changes were found. Fig 19, corresponding to the plasma sprayed Al2O3 coating before treatment, shows the alumina particles, which are molten and plastically distorted because they contact the substrate at a velocity of 300 m/s. the rapid quenching causes high porosity as shown. The X-ray diffraction pattern of plasma sprayed coatings, as shown in fig17, indicate that alumina is mainly gAl2O3.
The rapid cooling caused by sudden impact of molten alumina particles and the cold surface leads to a predominantly g phase.
Fig 20 indicates the plasma sprayed coatings after laser treatment, showing the needle shapes that characterize αphase Al2O3.
Figure 17 X-ray diffraction pattern of Al2O3 without laser treatment
When the laser treatment is used to melt at room temperature a thin layer of surface, g phase dominates. If the test specimen is heated before treatment the predominant phase is α. The X-ray diffraction pattern as shown in fig 18 proves this fact.
Figure 18 X-ray diffraction pattern of Al2O3 with laser treatment
After wear tests, in both coatings, treated and non-treated, there exist flattened zones with plastically distorted particles, as well as zones where particles have been torn and whose edges suggest brittle fracture. This is shown in fig 21 and fig 22.
Considering all the above facts, the main wear mechanism of Al2O3 coatings with and without laser-treatment is: first, plastic deformation of particles due to high temperature and pressure; second, cracking due to fatigue in plastically deformed zones. For laser treated coatings, cracking also occurs due to treatment. Finally, the coating itself undergoes cracking and loosening of particles with wear debris. The same sequence is repeated when the ceramic coating comes in contact with new particles.
Figure 19 Surface of plasma sprayed Al2O3 before laser treatment
Figure 20 Surface of plasma sprayed Al2O3 after laser treatment
Figure 22 Plastically deformed zone on worn surface of laser treated Al2O3 coating (load=136N, speed=150 rpm)
Figure 21 Plastically deformed zone on worn surface of laser treated Al2O3 coating (load=45N, speed=150 rpm)
Conclusions
1. Surface laser treatment is a method that allows improvement, in some circumstances, of the tribological behavior of the plasma sprayed coatings, modifying their microstructure from g phase to predominantly α phase Al2O3.
2. The surface treatment of plasma sprayed ceramic coating by laser allows molten layer thickness of 0.1mm. Whenever preheating of coating is carried out, the layers present minimum amount of cracking.
3. The increase in hardness generated from the treatment causes the behavior under dry and abrasive wear conditions to be better for the treated than for the non-treated coatings.
4. Under lubricated conditions despite good behavior shown by both treated and non-treated coatings, it is worse for treated coatings.
5. Wear of steel is lower when the steel contacts the laser treated rather than the non-treated coatings.
6. Friction and Wear are necessary evil.
BIBILOGRAPHY
1) Journal: Tribology International (volume-29, No-6, 1996)
2) Books: 1. Tribology Technology (volume-1)
Peter B Senholozi
2. Friction and Wear of Materials
Earnest Rabinowicz
3. Principles and application of Tribology
Bharat Bhushan
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