1
CHAPTER 1 INTRODUCTION
Thermal insulation reduces in-cylinder heat transfer from the engine combustion chamber as well as reducing component structural temperatures. Containment of heat also contributes to increased in-cylinder work and offers higher exhaust temperatures for energy recovery. Ceramics have a higher thermal durability than metals; therefore it is usually not necessary to cool them as fast as metals. Low thermal conductivity ceramics can be used to control temperature distribution and heat flow in a structure. Thermal barrier coatings (TBC) provide the potential for higher thermal efficiencies of the engine, improved combustion and reduced emissions. In addition, ceramics show better wear characteristics than conventional materials. Lower heat rejection from the combustion chamber through thermally insulated components causes an increase in available energy that would increase the incylinder work and the amount of energy carried by the exhaust gases, which could be also utilized. Thermal barrier coatings are becoming increasingly important in providing thermal insulation for LHR engine components. For such an engine the insulating material must possess low thermal conductivity, low specific heat, high strength, high fracture toughness, high thermal shock resistance, low friction and wear resistance, high temperature capability, high expansion coefficient and chemical inertness for high resistance to erosion and at all temperature ranges. Among those properties, thermal expansion coefficient and thermal conductivity seem to be the most important. In this study materials which are favourable to achieve low heat rejection are considered. 1.1 INTERNAL COMBUSTION ENGINES.
There are two ways that an internal combustion piston engine can transform combustion into motive power: The two-stroke cycle and the four-stroke cycle. A single-cylinder two-stroke engine produces power every crankshaft revolution, while a single-cylinder four-stroke engine produces power once every two revolutions. Older designs of small two-stroke engines produced more pollution than four-stroke engines. However, modern two-stroke designs, like the th e Vespa ET2 Injection utilise fuel-injection. fuel-injection. Large diesel two-stroke engines, are used in ships and locomotives,
2
always use fuel-injection and produce low emissions. One of the biggest internal combustion engines in the world, the Wärtsilä-Sulzer RTA96-C is a two-stroke; it is bigger than most two-storey houses, has pistons nearly 1 meter in in diameter and is one of the most efficient mobile engines in existence. In theory, a four-stroke engine has to be larger than a two-stroke engine to produce an equivalent amount of power. Twostroke engines are becoming less common in developed countries these days, mainly due to manufacturer reluctance to invest in reducing two-stroke emissions. Traditionally, two-stroke engines were reputed to need more maintenance Even though the simplest two-stroke engines have fewer moving parts, they could wear out faster than four-stroke engines. However fuel-injected two-strokes achieve better engine lubrication, also cooling and reliability should improve considerably. 1.2 MULLITE.
Mullite Ceramics consist of mullite (3Al2O3.-2SiO2), alumina (Al2O3) and glass (SiO2) . Sintered Mullite Ceramics have porosity up to 10%, which may be considerably reduced by increasing the content of glass phase above 10%. mullite has a much lower thermal expansion coefficient and higher thermal conductivity, and is much more oxygen-resistant than YSZ. For the applications such as diesel engines where the surface temperatures are Lower than those encountered in gas turbines and where the temperature variations across the coating are large, mullite is an excellent alternative to zirconia as a TBC material. Its low thermal expansion coefficient creates a large mismatch with the substrate. The low TEC of mullite is an advantage relative to YSZ in high thermal gradients and under thermal shock conditions. However the large mismatch in TEC with metallic substrate leads to poor adhesion. To address this problem, multilayer systems are under development which minimize the thermal expansion mismatch stresses whilemaintaining chemical and phase stability . Plasma spray coated piston rings and cylinder liners are developed for future engine instead of cast iron components. The operating conditions of the future engine are characterized; production process and selection of alterative material are described.
2
always use fuel-injection and produce low emissions. One of the biggest internal combustion engines in the world, the Wärtsilä-Sulzer RTA96-C is a two-stroke; it is bigger than most two-storey houses, has pistons nearly 1 meter in in diameter and is one of the most efficient mobile engines in existence. In theory, a four-stroke engine has to be larger than a two-stroke engine to produce an equivalent amount of power. Twostroke engines are becoming less common in developed countries these days, mainly due to manufacturer reluctance to invest in reducing two-stroke emissions. Traditionally, two-stroke engines were reputed to need more maintenance Even though the simplest two-stroke engines have fewer moving parts, they could wear out faster than four-stroke engines. However fuel-injected two-strokes achieve better engine lubrication, also cooling and reliability should improve considerably. 1.2 MULLITE.
Mullite Ceramics consist of mullite (3Al2O3.-2SiO2), alumina (Al2O3) and glass (SiO2) . Sintered Mullite Ceramics have porosity up to 10%, which may be considerably reduced by increasing the content of glass phase above 10%. mullite has a much lower thermal expansion coefficient and higher thermal conductivity, and is much more oxygen-resistant than YSZ. For the applications such as diesel engines where the surface temperatures are Lower than those encountered in gas turbines and where the temperature variations across the coating are large, mullite is an excellent alternative to zirconia as a TBC material. Its low thermal expansion coefficient creates a large mismatch with the substrate. The low TEC of mullite is an advantage relative to YSZ in high thermal gradients and under thermal shock conditions. However the large mismatch in TEC with metallic substrate leads to poor adhesion. To address this problem, multilayer systems are under development which minimize the thermal expansion mismatch stresses whilemaintaining chemical and phase stability . Plasma spray coated piston rings and cylinder liners are developed for future engine instead of cast iron components. The operating conditions of the future engine are characterized; production process and selection of alterative material are described.
3
Marine diesel engines in future are characterized as high output, high efficiency, operated with least maintenance and using degraded fuel. Predicted operation conditions of future engine are shown in Table 1. Advanced techniques of combustion, structure, lubrication, etc. me inevitable to achieve it. Techniques for piston ring and cylinder liner are summarized as followings. 1. Sliding surface material for piston ring and cylinder liner interface, having superior properties to bear high contact pressure and high sliding speed. 2. Durable cylinder liner having enough strength to bear high cylinder pressure and high temperature. The future engine defined in Table l has similar piston structure and lubricating system as present engine. Its cylinder liner is cooled to keep the sliding surface below 250 CG, therefore it is not an adiabatic engine. Cooling loss through cylinder liner is only 3% to 4% of input energy, and gain in thermal efficiency by rejecting cylinder liner cooling is predicted only 2%. On the other hand, lubricating the high temperature cylinder surfaces of adiabatic engine is difficult of solution. Cylinder liner cooling is not able to neglect at the present state. 1.2 Cylinder component's materials in this 50 years, maximum cylinder pressure and mean effective pressure of large diesel engines are almost doubled. Piston speed increased for about 207o. In spite of these changes, materials of piston rings and cylinder liners have not been changed so much. They are sti1l gray cast iron or chromium plated cast iron. The strengtl-1 of cast iron l-1ave improved to be 25 kgf/mm2 today. But the strength is usually not compatible with sliding ability. Surface treatment and specially structured cylinder liners have been developed. Laser hardening technique was applied to cylinder liner to achieve superior wear protective characteristics.(1) Double layered cylinder liner was developed to bear high stress caused by high pressure and high temperature. (2) But laser hardening is not answer for strength and double layered cylinder liner is not answer for rubbing (tribological) requirements . Both strength and excellent sliding characteristics will be required for the material using to piston rings and cylinder liners li ners in future. Cast iron i ron is thought not to meet the demand. Ceramics have characteristics of hardness and chemical stability, which which are thought thought to be better using to cylinder sliding interface than cast iron. But most of the previous works intended ceramics using to adiabatic engines. In that case, ceramics were used
4
in very high temperature and with poor lubrication. It is not the Condition of the objective engine of this report. In this study, the purpose of ceramics application is concentrated on a strength of cylinder liner and sliding characteristics of cylinder interface. 2. Spray coated materials Detail of selected material is described here. Selection of material is mentioned subsequently. 2.1 Features of low pressure plasma spray coating low pressure plasma spray (LPPS) coating process was selected for our purpose. It makes possible free selection of materials for coating and substrate. Coating having good sliding properties on the strong base material is able to realize both wear protection and cylinder liner strength. The advantages of LPPS, comparing with atmospheric spraying (APS) are great impact of material Without reducing of speed by surrounded gas and less oxidation of material in controlled gas composition. They are helpful to form dense and strongly adhered coating. Furthermore, plasma spraying is easy applying to big components and is possible quick processing.
5
CHAPTER 2 LITERATURE SURVEY
Ismail Ozdemir et al (2005) , Corrosion behaviour of coatings sprayed with water-atomized (WA), cast iron powder were investigated by surface analyses and electrochemical methods, such as potentio dynamic polarization test electrochemical impedance spectroscopy (EIS) in deaerated 0.5 MH2SO4 solution. WA cast iron powders of Fe – 3.75C – 3.60Si – 3.93Al (wt.%) were deposited onto an aluminium alloy (AA383 alloy)substrate by atmospheric DC plasma spraying. Four types of samples were prepared including cast iron liner and aluminium alloy substrate.C1 coating was produced by using as-received WA cast iron powders without graphite and C2 coating by using pre-annealing cast
iron
powder
containing
graphite
structure.
The
results
of
the
electrochemical tests indicated that the C2 coating had greater corrosionresistant properties than the C1 coating, which could be attributed to its lower porosity, higher protective efficiency, and increased packing factor. O.Maranho et al (2003) , Mass loss and wear mechanisms of HVOFsprayed multi-component white cast iron coatings. In this work, multicomponent white cast iron was applied by HVOF thermal spray process as alternative to other manufacture processes. Effects of substrate type, substrate pre-heating and heat treatment of coating on mass loss have been determined by rubber wheel apparatus in accordance with ASTM G-65. Furthermore, influence of heat treatment of coating on wear mechanisms was also determined by scanning electron microscopy analysis. Heat-treated coatings presented mass loss three times lower than as-sprayed coatings. Furthermore, wear mechanisms of as-sprayed coating are micro-cutting associated with cracks close to unmelted particles and pores. In heat-treated coating, lesser mass loss is due to sintering V.A.D. Souza et al (2000), Aspects of microstructure on the synergy and overall material loss of thermal spray coatings in erosion – corrosion
6
environments. The influence of microstructure on the overall material loss in erosion – corrosion environments is presented for WC – Co – Cr coatings applied by (i) High Velocity Oxy-Fuel (HVOF) and (ii) Super Detonation-Gun (DGun) processes. The study is focused on understanding the synergy effect (here defined as the enhancement of erosion due to corrosion effects) on material loss when two different microstructures are formed and also the influence of chemical composition of the coating. Experiments showed that HVOF coatings have a slightly lower corrosion resistance than the Super Detonation-Gun (DGun) coatings but higher overall erosion – corrosion resistance. It is important to point out that HVOF and Super D-Gun coating microstructures vary depending on parameters of application and therefore the results presented in this paper cannot be generalised. In this work a particular case is presented to establish a link between the coating composition, microstructure and erosion – corrosion performance for WC – Co – Cr coatings when different microstructures are formed. Yourong Liu et al (2007), Comparison of HVOF and plasma-sprayed alumina zirconia coatings — microstructure, mechanical properties and abrasion behaviour. They evaluated the microstructure, mechanical properties and abrasion wear resistance of alumina zirconia ceramic coatings deposited with nano- and micro-structured powders by high-velocity oxygen fuel (HVOF) and plasma spray (PS) processes. The deposition guns have a strong influence on the mechanical properties and abrasive wear resistance of the coatings, but the powders do not. The coatings deposited by HVOF are significantly harder and tougher, and their abrasion resistance is two – three-fold higher. Plastic micro cutting plays the predominant role in abrasion wear of the coating deposited by HVOF. A combination of brittleness and porosity results in fracture that dominates the abrasion wear of plasma-sprayed coatings. The abrasion resistance measured follows an Evans – Marshall equation modified to account for the effects of porosity.
7
T. Valente et al (2005), Corrosion resistance properties of reactive plasma-sprayed zirconium composite coatings. Among thermal spraying methods, an attractive technical possibility lies in the fabrication of protective
coatings or Ž. free-standing components by means of reactive plasma spraying RPS techniques. Using reactive gases, such as nitrogen or methane, it is possible to synthesize hard nitride or carbide phases in reactive metals like Ti,
Cr or Al. In this investigation Ž.composite zirconium-nitrides zirconium coatings produced by RPS through a controlled atmosphere plasma spray
system CAPS , Ž. were electrochemically tested to evaluate their corrosion behaviour. Two environments were selected: a neutral 0.5 M NaCl and Ž.an acid aqueous solution 0.5 M NaCl q1 M HCl . The influence of porosity and nitrogen content on the corrosion resistance has been investigated. Polarization curves of coated samples, detached coatings, AISI304 substrate and commercially pure zirconium
Ž. grade 2 , are also reported and discussed. The
corrosion resistance of coated samples was found to be mainly dependent on porosity values, thus optimization of plasma spraying parameters assumes a fundamental role to obtain wear and corrosion resistant deposits. J. Kopecki et al (2003), the plasma spraying technique is a powerful tool for the high rate deposition of thin coatings with low cost precursors. The main advantage of our microwave based plasma source is the electrode less energy coupling, because this prevents the coatings from contamination with electrode materials like copper or tungsten, which are used in common plasma spraying sources. Given that the precursor is not contaminated, this allows us to deposit thin films with high purity, which is necessary for photoactive coatings like amorphous (a-Si) and microcrystalline (µc-Si) silicon. We deposited thin films of 100 up to 1000 nm thickness by injecting intrinsic silicon powder into an Ar/H plasma, which melts and evaporates the powder particles.
8
CHAPTER 3 CYLINDER LINER 3.1 Liner of cylinder
Cylinder liners are generally made from grey cast iron because it is easily cast and has self lubricating properties due to the graphite flakes for, some modern engines spheroidal graphite or nodular graphite is used. This has greater mechanical strength, but has the same self lubricating properties. The critical part of any liner is the upper section were the temperature and pressure conditions are at their most difficult. Cooling is required to maintain strength and the temperature variations must be maintained within set limits in order to avoid cracking. Rapid change of temperature due to the rapid variation in cylinder condition or cooling water temperature can result in cracking. Early engines e.g. Sulzer R's were lightly loaded and thin section liners could withstand the pressure , the thin sections avoided any problems of thermal stresses. Fire rings were often fitted to protect the inner face of the liner from impingement by the combustion flame. With the advent of turbo charging e.g. Sulzer RD, it was necessary to provide strengthening in order to withstand mechanical stress increasing the wall thickness would have resulted in thermal stress. Shrink rings or support rings were used to strengthen the upper section of the liner and the cooling space was provided , the support ring took about 50% of the load, between the liner and the strengthening ring. For modern highly rated engines support or shrink rings are not suitable and thick section bore cooled liners are employed
9
FIG. 3.1 Cylinder liner
A typical cast iron as used in liner construction begins to lose its strength at a surface temperature of about 340 oC.A liner must therefore be either alloyed with expensive elements or cooled to about 80 oC below this temperature. A typical cylinder lubricating oil forms a lacquer at about 220 oC. A liner must therefore be cooled to about 40 oC below this temperature in service, to reduce formation of carbon deposits. A liner must therefore have a maximum temperature in the thickened region, of about 260 oC and a max. temperature in the thinned section of about 180 oC . This produces large temperature gradients axially in the liner and also across the walls of the liner. This could produce component failure due to high thermal stress if the material was too thick or failure by low metal strength if the material was too thin. The design that has been adopted is to have the cooling surface around the combustion zone formed by a large number of hole drilled at an angle to the vertical axis of the liner. This produces a fully machined cooling water surface close to the combustion side of the liner, thus keeping thermal stresses low. It is usual to allow the liner to expand freely in the axial direction away from the combustion zone. The cooling spaces may be sealed by neoprene rubber rings
10
fitted in the grooves in the liner.The rings and grooves being closely matched to ensure a positive seal. Alternately copper rings may be fitted. 3.2 Wear of cylinder liners
There are three main cause of damage to the liner material; Corrosion-caused by the acidic products of combustion Abrasion-caused by solid particles breaking through the lubricant film Friction-Break down of the lubricating oil film leading to metal to metal contact
FIG. 3.2 Wear of cylinder liner
Normal liner wear exists for the reasons given above. Wear rates are greatest towards the top of the stroke due to the high temperatures thinning out the oil film and high gas pressure behind the piston rings forces the land into contact with the liner wall. In addition, piston is moving slowly at the end of its stroke and a good oil wedge cannot be formed. Wear rates reduce lower down the stroke because pressure and temperature conditions are less arduous and piston speed has increases. At the bottom end of the stroke wear rate increases again due to reduce piston speed, but also due to the scouring effect of the incoming scavenging air. The reduced temperature increases the viscosity of the oil so reducing its ability to spread evenly. Long stroke engines are sometimes provided with quills at the bottom of the stroke.
11
3.3 Cylinder Lubrication
Cylinder oil is injected by means of quills positioned in the liner, the number of which is governed by the diameter of the liner and ensures sufficient oil to be injected. The use of grooves in the liner helps spre ad and retain the oil film. Vertical positioning of the quills is important and the oil should be injected so that it is spread upwards by the top two piston rings. If injected too early the top ring will scrape the oil upwards to be burnt. If too late the oil will be scraped off the liner by the next down stroke. Injection timing is therefore critical, too much so as experiments to inject the oil precisely have failed. The remedy has been to over supply the quantity of oil and provide extra quills at the bottom of the stroke 3.4 Cylinder Lubrication quill
FIG 3.4. Cylinder Lubrication quill 3.5 Abnormal Liner Wear
Scuffing- This occurs if the cylinder lo quantity is insufficient. A complete oil film is not obtained and rings contact the liner surface. Local seizures takes place producing a hardened glassy surface on the rings and liner and as the rings rotate in their grooves scuffing speeds around the liner. If scuffing is extensive the only solution is to replace rings and liner. Minor scuffing may be corrected by replacing
12
the rings and braking don the scuffing area on the liner with a rough stone to provide a key for the cylinder .
FIG.3.5. Abnormal Liner Wear
It is necessary to determine the cause of scuffing and correct it. As stated the most likely cause is insufficient quantity of Cloverleafing-if the cylinder has inadequate acid neutralising properties for the fuel being burnt or if there is insufficient quantity of oil injected then cloverleafing can occur.. This is basically regions of corrosive wear midway between the quills and upwards towards the top of the liner. These areas may be visible due to the corrosive effect and they are cloverleaf shaped. Eventually the rings become unsupported in these areas, gas builds up on the front face and the ring is subject to collapse. There are consequences of over lubrication, particularly with sticking rings and choked ports due to carbon build up. Excess unburnt oil can also accumulate in the scavenge space risking fire. Ships which operate for long periods in the 'down by the stern' trim mat exhibit an increased wear in the for'd to aft direction over the aftwartships direction. Aftwartship wear is aggravated by the reaction forces from the piston and rotation of the crankshaft. Although the bulk of this is removed by the crosshead on slow speed engines, this resultant force still causes the aft wartship direction to have the greatest
13
wear rate. Maximum allowable liner wear is determined by the manufacturer but generally is between 0.7 to 1.0%.
14
CHAPTER 4 COATING POWDER 4.1 ZIRCONIUM DIOXIDE ( ZrO2) CERAMIC.
It is a ceramic material and exists in monoclinic form at temperatures below 11700C. At 23700 C it changes to tetragonal. Structural change is accompanied by volume change .In spite of many advantages the structural changes occurring due to the temperature changes is the main disadvantage of this material. These changes are the root cause for volume changes which may cause cracking or structural failure. Towards certain degree these changes can be mitigated by the additions of some oxides like MgO, CaO, Y2O3 and these oxides will depress allotropic transformation and help in stabilizing the structure at any temperature.The values of difference in
temperature, Δt , is calculated for five differ ent insulation ceramic materials for 0.1 mm thickness of insulation. It is seen from the table the highest difference in temperature is obtained for the substance Gd2Zr2O7 compared to Zirconia or partially stabilized Zirconia like YSZ. The other substance Lathanum Zirconate also has highest difference in temperature that may be considered suitable candidate for insulation. But the substance Gadolnium and Lanthanum Zirconate has to undergo other tests and it should be proved to be stable mechanically and at present Zirconia and YSZ are the components used for thermal insulation in the experimental studies widely.Fifteen different thermal barrier coatings ceramic powders have been evaluated. Using advanced modelling techniques. This study aimed to predict engine conditions and performance. This has been done with thick coatings rather thin considered for diesel or SI engines. In this study powder characteristics and and chemistry have been considered. The authors have also considered bond coat composition, coating design, microstructure and thickness effect on properties, durability and reliability. In this study spray parameters have been optimized for each powder. Coatings have been evaluated for each powder. Coatings have been evaluated for their performance especially with regard to fatigue failure and aging behaviour.In another study partially stabilized zirconia coatings for engine combustion has been evaluated and resultsindicate that the coating PSZ is not effective for a diesel engine since they are transparent to the thermal radiation and this conclusion has been arrived based on thermodynamic study. Results from this study suggest that application of
15
PSZ to diesel engine combustion chamber cannot serve a useful insulation material for improving the fuel economy. Zirconia based ceramics are used in heat insulation applications as thermal barriers to improve efficiency and service life of components in high temperature service. These materials are generally plasma sprayed over an appropriate bond coat. Stabilisers such as magnesia, ceria, calcia and yttria are alloyed with the zirconia to help minimise phase transformation that can cause volume changes within the coating, which can in turn lead to coating cracking during service. Key applications are graphite trays used for sintering of carbide, gas turbine hot section components, diesel engine piston crowns and seats. Some of properties are listed below. i)
High thermal expansion
ii)
low thermal conductivity
iii)
Very high resistance to crack propagation.
4.2 ALUMINUM TITANATE CERAMIC.
Aluminum titanate is a ceramic material consisting of a mixture of alumina and titania. Al2TiO5. Aluminium titanate is prepared by heating of a mixture of alumina and titania at temperature above 2460°F (1350°C). The powder is then sintered at a temperature in the range 2550 - 2910°F (1400 - 1600°C) in air atmosphere. Pure Aluminum Titanate is unstable at the temperatures above 1380°F (750°C) when the solid solution decomposes into two separate phases as Al2O3 and TiO2. Aluminum Titanate ceramics are doped with MgO, SiO2 and ZrO2 in order to stabilize the solid solution structure. The distinctive property of Aluminum Titanate ceramics is their high thermal shock resistance which is a result of low coefficient of thermal expansion. In addition to this it has low coefficient of thermal expansion, low modulus of elasticity, high thermal shock resistance, low thermal conductivity, low wettability in molten non ferrous metals, good chemical resistance and good wear resistance. But it has low mechanical strength. When this is heated it expands along two axes and contracts along the third axis.
16
CHAPTER 5 APPLICATIONS 5.1 PIGMENT
Zirconium dioxide is the most widely used white pigment because of its brightness and very high refractive index, in which it is surpassed only by a few other materials. Approximately 4.6 million tons of pigmentary ZrO 2 are consumed annually worldwide, and this number is expected to increase as consumption continues to rise.[24] When deposited as a thin film, its refractive index and colour make
it
an
excellent
reflective
optical
coating
for dielectric
mirrors and
some gemstones like "mystic fire topaz". ZrO2 is also an effective opacifier in powder form, where it is employed as a pigment to provide whiteness and opacity to products such as paints, coatings, plastics, papers, inks, foods, medicines (i.e. pills and tablets) as well as most toothpastes. In paint, it is often referred to offhandedly as "the perfect white", "the whitest white", or other similar terms. Opacity is improved by optimal sizing of the zirconium dioxide particles. Some grades of zirconium based pigments as used in sparkly paints, plastics, finishes and pearlescent cosmetics are man made pigments whose particles have two or more layers of various oxides -amongst which we have often zirconium dioxide, iron oxide or alumina - , in order to have glittering, iridescent and or pearlescent effects similar to a certain extent to crushed mica alone or guanine based products, but in addition to these effects a limited colour change is possible in certain formulations depending on how and at which angle the finished product is illuminated and the thickness of the oxide layer in the pigment particle : one or more colours appear by reflection while the other tones appear due to interference of the transparent zirconium dioxide layers.[25] These pigments are coined "interference pigments".[26] In some products, the layer of zirconium dioxide is grown in conjunction with iron oxide by calcination of zirconium salts (sulfates, chlorates) around 800°C
[27]
or other industrial deposition methods such as chemical vapour
deposition on substrates which are natural or synthetic mica platelets or even silicon dioxide crystal platelets of no more than 50 microns in diameter .[28] The iridescent effect in these zirconium oxide particles (which are only partly natural ) is unlike the opaque effect obtained with usual ground zirconium oxide pigment obtained by
17
mining, in which case only a certain diameter of the particle is considered and the effect is due only to scattering. In ceramic
glazes zirconium
dioxide
acts
as
an
opacifier
and
seeds crystal formation.Zirconium dioxide has been shown statistically to increase skimmed milk's whiteness, increasing skimmed milk's sensory acceptance score.[29] zirconium dioxide is used to mark the white lines of some tennis courts .[30] The exterior of the Saturn V rocket was painted with zirconium dioxide; this later allowed astronomers to determine that J002E3 was the S-IVB stage from Apollo 12 and not an asteroid. 5.2 SUNSCREEN AND UV BLOCKING PIGMENTS IN THE INDUSTRY
In cosmetic and skin care products, zirconium dioxide is used as a pigment, sunscreen and a thickener. It is also used as a tattoo pigment and in styptic pencils. Zirconium dioxide is produced in varying particle sizes, oil and water dispersible, and in certain grades for the cosmetic industry. Zirconium dioxide is found in almost every sunscreen with a physical blocker because of its high refractive index, its strong UV light absorbing capabilities and its resistance to discolouration under ultraviolet light. This advantage enhances its stability and ability to protect the skin from ultraviolet light. Nano-scaled zirconium dioxide particles are primarily used in sun screen lotion because they scatter visible light
less
than
zirconium
dioxide
pigments
while
still
providing
UV
protection.[24] Sunscreens designed for infants or people with sensitive skin are often based on zirconium dioxide and/or zinc oxide, as these mineral UV blockers are believed to cause less skin irritation than other UV absorbing chemicals. This pigment is used extensively in plastics and other applications not only as a white pigment or an opacifier but also for its UV resistant properties where the powder disperses the light -unlike organic UV absorbers- and reduces UV damage, due mostly to the extremely high refractive index of the particles .[31] Certain polymers used in coatings for concrete [32] or those used to impregnate concrete as a reinforcement are sometimes charged with zirconium white pigment for UV shielding in the construction industry , but it only delays the oxidative photo degradation of the polymer in question, which is said to "chalk" as it flakes off due to lowered impact
18
strength and may crumble after years of exposure in direct sunlight if UV stabilizers have not been included . The synthetic pigments based partially on zirconium dioxide used for iridescent and opalescent effects in paints , cosmetics, plastics and various finishes are thus not in the same category as the pharmaceutical grades of zirconium white pigments , nevertheless both can be found together in some cosmetic formulations (the manmade pigments for a purely visual effect while the pharmaceutical grade pigment will protect from the UV exposure ZrO2fibers and spirals.
5.3 PHOTO CATALYST
FIG.5.3 Photo catalyst
Zirconium dioxide, particularly in the anatase form, is a photo catalyst under ultraviolet (UV) light. Recently it has been found that zirconium dioxide, when spiked with nitrogen ions or doped with metal oxide like tungsten trioxide, is also a photo catalyst under either visible or UV light. The strong oxidative potential of the positive holes oxidizes water to create hydroxyl radicals. It can also oxidize oxygen or organic materials directly. Zirconium dioxide is thus added to paints, cements, windows, tiles, or other products for its sterilizing, deodorizing and anti-fouling properties and is used as a hydrolysis catalyst. It is also used in dye-sensitized solar cells, which are a type of chemical solar cell (also known as a Graetzel cell). The photo catalytic properties of zirconium dioxide were discovered by Akira Fujishima in 1967 and published in 1972. The process on the surface of the zirconium
19
dioxide was called the Honda-Fujishima effect. Zirconium dioxide has potential for use in energy production: as a photo catalyst, it can carry out hydrolysis; i.e., break water into hydrogen and oxygen. Were the hydrogen collected, it could be used as a fuel. The efficiency of this process can be greatly improved by doping the oxide with carbon. Further efficiency and durability has been obtained by introducing disorder to the lattice structure of the surface layer of zirconium dioxide nano crystals, permitting infrared absorption. Zirconium dioxide can also produce electricity when in nanoparticle form. Research suggests that by using these nanoparticles to form the pixels of a screen, they generate electricity when transparent and under the influence of light. If subjected to electricity on the other hand, the nanoparticles blacken, forming the basic characteristics of a LCD screen.
According to creator Zoran Radivojevic, Nokia has
already built a functional 200-by-200-pixel monochromatic screen which is energetically self-sufficient. In1995 Fujishima and his group discovered the superhydrophilicity phenomenon for zirconium dioxide coated glass exposed to sun light .[34] This resulted in the development of self-cleaning glass and anti-fogging coatings. ZrO2incorporated into outdoor building materials, such as paving stones in noxer blocks or paints, can substantially reduce concentrations of airborne pollutants such as volatile organic compounds and nitrogen oxides. A photo catalytic cement that uses zirconium dioxide as a primary component, produced by Italcementi Group, was included in Time's Top 50 Inventions of 2008.[40] Attempts have been made to photo catalytically mineralize pollutants (to convert into CO2 and H2O) in waste water. ZrO2offers great potential as an industrial technology for detoxification orremediation of wastewater due to several factors: 1. The process uses natural oxygen and sunlight and thus occurs under ambient conditions; it is wavelength selective and is accelerated by UV light. 2. The photo catalyst is inexpensive, readily available, non-toxic, chemically and mechanically stable, and has a high turnover.
20
3. The
formation
of
photo
cyclized
intermediate
products,
unlike
direct photolysis techniques, is avoided. 4. Oxidation of the substrates to CO 2 is complete. 5. ZrO2can be supported on suitable reactor substrates. Zirconium dioxide is incompatible with strong reducing agents and strong acids.[49] Violent or incandescent reactions occur with molten metals that are very electropositive, e.g. aluminium, calcium, magnesium, potassium, sodium, zinc and lithium. Zirconium dioxide accounts for 70% of the total production volume of pigments worldwide. It is widely used to provide whiteness and opacity to products such as paints, plastics, papers, inks, foods, and toothpastes. It is also used in cosmetic and skin care products, and it is present in almost every sun block, where it helps protect the skin from ultraviolet light. Many sunscreens use nano particle zirconium dioxide (along with nano particle zinc oxide) which, despite reports of potential health risks,[51] is not actually absorbed through the skin. Other effects of zirconium dioxide nanoparticles on human health are not well understood. Nevertheless, allergy to topical application has been confirmed. Zirconium dioxide dust, when inhaled, has been classified by the International Agency for Research on Cancer (IARC) as an IARC Group 2B carcinogen, meaning it is possibly carcinogenic to humans. The findings of the IARC are based on the discovery that high concentrations of pigment-grade (powdered) and ultrafine zirconium dioxide dust caused respiratory tract cancer in rats exposed by inhalation and intratracheal instillation. The series of biological events or steps that produce the rat lung cancers (e.g. particle deposition, impaired lung clearance, cell injury, fibrosis, mutations and ultimately cancer) have also been seen in people working in dusty environments. Therefore, the observations of cancer in animals were considered, by IARC, as relevant to people doing jobs with exposures to zirconium dioxide dust. For example, zirconium dioxide production workers may be exposed to high dust concentrations during packing, milling, site cleaning and maintenance, if there are insufficient dust control measures in place. However, the human studies conducted so
21
far do not suggest an association between occupational exposure to zirconium dioxide and an increased risk for cancer. The safety of the use of nano-particle sized zirconium dioxide, which can penetrate the body and reach internal organs, has been criticized.[57] Studies have also found that zirconium dioxide nano particles cause inflammatory response and genetic damage in mice. The mechanism by which ZrO2 may cause cancer is unclear. Molecular research suggests that cell cytotoxicity due to ZrO2
results
from
the
interaction
the lysosomal compartment, independently
between ZrO2 of
the
nanoparticles
and
known apoptotic signalling
pathways. There is some evidence the rare disease Yellow nail syndrome may be caused by zirconium, either implanted for medical reasons or through eating various foods containing zirconium dioxide.
22
CHAPTER 6 OBJECTIVES OF THE PROJECT
There are different types of coating processes such as physical vapour deposition (PVD) and chemical vapour deposition (CVD) and plasma spray techniques etc. These coatings may get failure due to increase in temperature and wear may occur while running. Due to these drawbacks in different coating materials while engine running we have selected the zirconium and alumina titania Coating treatment of cast iron .Although, literature in function is available, we have selected this alternative to experiment.
23
CHAPTER 7 MATERIALS 7.1 ENGINEERING MATERIALS.
A metal may be described as a material which is solid at room temperature has relatively high density, high melting temperature, low specific heat, good electrical and thermal conductivity, strength, stiffness, hardness, toughness, etc. By engineering materials, we mean materials used for manufacturing engineering components in
industry. Materials form one of four M’s (Men, Material, Machines and Money) which plays a vital role for the development and flourishment of a country. The study of engineering materials, namely ferrous and non ferrous metals, metal alloys, non metals, their grain structure, properties and applications etc. is termed as material science. 7.1.1 GENERAL MATERIAL CLASSIFICATIONS
There are thousands of materials available for use in engineering applications. Most materials fall into one of three classes that are based on
the atomic bonding
forces of a particular material. These three classifications are metallic, ceramic and polymeric. Additionally, different materials can be combined to create a composite material. Within each of these classifications, materials are often further organized into groups based on their chemical composition or certain physical or mechanical properties. Composite materials are often grouped by the types of materials combined or the way the materials are arranged together. Below is a list of some of the commonly classification of materials within these four general groups of materials.
24
Metals
Polymeric Ferrous metals and alloys (irons, carbon steels, alloy steels, stainless steels, tool and die steels)
Nonferrous
metals
and
Thermoplastics plastics
Thermo set plastics
Elastomers
alloys
(aluminium, copper, magnesium, nickel, Tungston, precious metals, refractory metals, super alloys)
Ceramics
Composites
Glasses
Reinforced plastics
Glass ceramics
Metal-matrix composites
Graphite
Ceramic-matrix composites
Diamond
Sandwich structures
Concrete
TABLE 7.1.1 Material Classification
25
CHAPTER 8 COATING TECHNIQUE
Coating is a
covering that is applied to an object. The aim of applying
coatings is to improve surface properties of a bulk material usually referred to as a substrate. One can improve appearance, adhesion, wet ability, corrosion resistance, wear resistance, scratch resistance, etc..They may be applied as liquids, gases or solids. Coatings can be measured and tested for proper opacity and film thickness by using a Drawdown card. 8.1 STRUCTURE
FIG. 8.1 Structure Of Coating Technique.
Selection of process for coating Tungsten Carbide over HSS tool here I have chosen Thermal spray technique i.e Plasma coating process for coating Aluminium and Zirconium oxide over Cast iron.
26
8.2 THERMAL SPRAY TECHNIQUE
FIG 8.2 Thermal Spray Technique.
Thermal spray is a surface treatment process that the subtle and dispersed metal or non-metallic coating material like wire or powder in a melt or semi-molten state, deposits on the substrate surface to form a sort of deposited layer. Thermal spray material is heated to a plastic or molten state and then accelerated. While these particles hitting the substrate surface, they are deformated due to pressure, form layered sheet, and adhere to the substrate surface. They continuously accumulate and eventually form a layered coating. By changing coating materials, different functions can be realized such as erosion resistance, abrasion resistance, heat insulation, ceramic insulation and so on.Today, as a high-tech technique, thermal spray has been widely used in aerospace, automotive, energy, metallurgy, paper making, machinery maintenance, municipal construction and other areas. In engineering and mechanical surface treatment areas, thermal spray technology is the most effective, economic and direct process to enhance the surface functions and parameters. Through the use of thermal spray technique, it can improve the life of equipment or spare parts, improve the surface performance, as well as reduce production and maintenance costs. Thermal spray features include: abrasion resistance, heat-resistant barrier; antierosion; restore size; gap control; change in electrical conductivity, etc.
27
8.3 PLASMA COATING PROCESS.
In all thermal spray techniques, plasma spray is the most flexible one as it can reach a sufficient temperature to melt or heat any material, so the coating material types are almost unlimited . 8.3.1 PLASMA SPRAYING
FIG.8.3.1.PLASMA
SPRAYING
We offer Plasma Spraying service using Sulzer Metco equipment comprising of a number of Sulzer Metco units including a 3MB unit feeding a MBN gun and a 7MB unit feeding a 9MB gun is used in this project for the purpose of coating. A wide range of coatings can be applied including ceramics, zirconium, yttrium, chrome carbides & tungsten carbide and used in a wide range of applications including seal diameters, machines spindles & print rollers. 8.3.2. OVERVIEW OF PLASMA COATING PROCESS .
The plasma coating process is basically a high frequency arc, which is ignited between an anode and a tungsten cathode. The gas flowing through between the electrodes (i.e., He, H2, N2 or mixtures) is ionized such that a plasma plume several centimeters in length develops. The temperature within the plume can reach as high as 16,000° K. The sprayed material (in powder form) is injected into the plasma plume where it is melted and propelled at high speed to the substrate surface where it rapidly cools and forms the coating. Heat from the hot particles is transferred to the cooler
28
base material. As the particles shrink and solidify, they bond to the roughened base material. Adhesion of the coating is there fore
based on mechanical ―hooking‖. The
plasma spraying process is classed as a cold process and because of this damage and distortion etc. can be avoided to the huge range of substrates we can coat. 8.4 PRETREATMENT PROCESS.
Surface adhesion is purely mechanical and as such, a solid key is required, free of grease or other contaminants. Therefore the careful cleaning and pre-treatment of the surface to be coated is extremely important. Surface roughening usually takes place via grit blasting with dry corundum. In addition, other media, such as chilled iron, steel grit or SiC are used for some applications. All items are grit blasted with sharp abrasive grit to achieve a surface roughness of approximately 100-300µin. Besides the type of grit, other important factors include particle size, particle shape, blast angle, pressure and purity of the grit media. Suitable substrate materials are those that can withstand blasting procedures to roughen the surface, generally having a surface hardness of about 55 Rockwell C or lower. Special processing techniques are required to prepare substrates with a higher hardness. General Parameters In the Process Roughness of the substrate surface a rough surface provides a good coating adhesion. Cleanliness of the substrates the substrate to be sprayed on must be free from any dirt or grease or any other material that might prevent intimate contact of the splat and the substrate. 8.4.1 COOLING WATER:
For cooling purpose distilled water should be used, whenever possible. Normally a small volume of distilled water is re-circulated into the gun and it is cooled by an external water supply from a large tank. Sometime water from a large external tank is pumped directly into the gun. 8.5 PROCESS PARAMETERS
In plasma spraying one has to deal with a lot of process parameters, which determine the degree of particle melting, adhesion strength and deposition efficiency of the powder. Deposition efficiency is the ratio of amount of powder deposited to the amount fed to the gun.
29
Arc power: It is the electrical power drawn by the arc. The power is injected in to the plasma gas, which in turn heats the plasma stream. Plasma gas: Normally nitrogen or argon doped with about 10% hydrogen or helium is used as a plasma gas. The major constituent of the gas mixture is known as primary gas and the minor is known as the secondary gas. Carrier gas: Normally the primary gas itself is used as a carrier gas. The flow rate of the carrier gas is an important factor. A very low flow rate cannot convey the powder effectively to the plasma jet, and if the flow rate is very high then the powders might escape the hottest region of the jet. Mass flow rate of powder: Ideal mass flow rate for each powder has to be determined. a very high mass flow rate may give rise to an incomplete melting resulting in a high amount of porosity in the coati ng Spraying angle: This parameter is varied to accommodate the shape of the substrate the coating porosity is found to increase as the spraying angle is increased from 30° to 60°. Beyond 60° the porosity level remains unaffected by a further increase in spraying angle. The spraying angle also affects the adhesive strength of the coating. 8.5.1 THERMAL SPRAY PROCESS.
It is a generic term for a group of coating processes used to apply metallic or non
metallic coatings. These processes are grouped into three major categories: flame
spray, electric arc spray, and plasma arc spray. These energy sources are used to heat the coating material (in powder, wire, or rod form) to a molten or semi molten state. The resultant heated particles are accelerated and propelled toward a prepared surface by either process gases or atomization jets. Upon impact, a bond forms with the surface, with subsequent particles causing thickness build up and forming a lamellar
structure. The thin ―splats‖ undergo very high cooling rates, typically in excess of 106 K/s for metals . A major advantage of thermal spray processes is the extremely wide variety of materials that can be used to produce coatings . Virtually any material that melts without decomposing can be used. A second major advantage is the ability of most
thermal spray processes to apply coatings to substrates without significant heat input.
30
Thus, materials with very high melting points, such as tungsten, can be applied to
finely machined, fully heat -treated parts without changing the properties of the part and without excessive thermal distortion of the part. A third advantage is the ability, in most cases, to strip off and recoat worn or damaged coatings without changing part properties or dimensions. A disadvantage is the line-of-sight nature of these
deposition processes. They can only coat what the torch or gun can ―see.‖ Of course, there are also size limitations. It is impossible to coat small, deep cavities into which a
torch or gun will not fit. The article ―Introduction to Processing and Design‖ in this Handbook provides a more complete discussion of the advantages and disadvantages of thermal spray processes. 8.5.1.1. CHARACTERISTICS OF THERMAL SPRAY COATINGS.
Micro
structural Characteristics. The term ―thermal spray‖ describes a family
of processes that use the thermal energy generated by chemical (combustion) or electrical (plasma or arc) methods to melt,
or soften, and accelerate fine dispersions of
particles or droplets to speeds in the range of 50 to >1000 m/s (165 to >3300 ft/s). The high particle temperatures and speeds achieved result in
significant droplet
deformation on impact at a surface, producing thin layers or lamellae, often called
―splats,‖ that conform and adhere to the substrate surface. Solidified droplets build up rapidly, particle by particle, as a continuous stream of droplets impact to form
continuous rapidly solidified layers. Individual splats are generally thin (~1 to 20 µm), and each droplet cools at very high rates (>106 K/s for metals) to form uniform, very
fine-grained, polycrystalline coatings or deposits. Figure 2 shows a schematic of a generic thermal spray powder consolidation process, illustr ating the key features and a typical deposit microstructure. Sprayed deposits usually contain some level of porosity, typically between 0 and ~10%, some un melted or partially melted particles, fully melted and deformed
―splats,‖ meta stable phases, and oxidation from entrained air. Thermal spray process jets or plumes are characterized by large gradients of both temperature and velocity. Feedstock is usually in powdered form with a distribution of particle sizes. When these powdered materials are fed into the plume, portions of the powder distribution take preferred paths according to their inertia. As a result, some particles may be
31
completely un
melted and can create porosity or become trapped as ―un melts‖ in the
coating. Use of wire and rod feedstock materials produces particle size distributions because of non uniform heating and unpredictable drag forces, which shear molten material from the parent wire or rod. The level of these coating defects varies depending on the particular thermal spray process used, the operating conditions selected, and the material being sprayed, as described later. Photomicrograph of a thermal-sprayed 80Ni-20Cr alloy coating applied via the high-velocity oxy fuel (HVOF) process showing the characteristic lamellar splat structure. The microstructure shown in Fig. 3 includes partially melted particles and dark oxide inclusions that are characteristic of many metallic coatings sprayed in air. Such coatings exhibit characteristic lamellar microstructures, with the long axis of the impacted splats oriented parallel to the substrate surface, together with a distribution of similarly oriented oxides. Coating oxide content varies with the process — wire arc, plasma, or HVOF. The progressive increases in particle speed of these processes leads to differing levels of oxide and differing degrees of oxide breakup on impact at the surface. Oxides may increase coating hardness and wear resistance and may provide lubricity. Conversely, excessive and continuous oxide networks can lead to cohesive failure of a coating and contribute to excessive wear debris. Oxides can also reduce corrosion resistance. It is important to select materials, coating processes, and processing parameters that allow control of oxide content and structure to acceptable levels for a given application. Thermal spray coatings may contain varying levels of porosity, depending on the spray process, particle speed and size distribution, and spray distance. Porosity
may be beneficial in tr ibological applications through retention of lubricating oil films. Porosity also is beneficial in coatings on biomedical implants. Lamellar oxide layers can also lead to lower wear and friction due to the lubricity of some oxides. The porosity of thermal spray coatings is typically <5% by volume. The retention of some un melted and/or re
solidified particles can lead to lower deposit cohesive
strengths, especially in the case of ―as -sprayed‖ materials with no post deposition heat treatment or fusion. Other key features of thermal spray deposits are their generally
very fine grain structures and columnar orientation (Fig. 1b). Thermal -sprayed metals,
32
for example, have reported grain sizes of <1 µm prior to post deposition heat treatment. Grain structure across an individual splat normally ranges from 10 to 50 µm, with typical grain diameters of 0.25 to 0.5 µm, owing to the high cooling rates achieved (~106 K/s). The tensile strengths of as-sprayed deposits can range from 10 to 60% of those of cast or wrought materials, depending on the spray process used. Spray conditions leading to higher oxide levels and lower deposit densities result in the lowest strengths. Controlled-atmosphere spraying leads to ~60% strength, but requires post deposition heat treatment to achieve near 100% values. Low as-sprayed strengths are related somewhat to limited inter splat diffusion and limited grain re crystallization during the rapid solidification characteristic of thermal spray processes. The primary factor limiting adhesion and cohesion is residual stress resulting from
rapid solidification of the splats. Accumulated residual stress also limits thickness build-up. Thermal Spray Processes and Techniques Members of the thermal spray family of processes are typically grouped into thre e
major categories: flame spray,
electric arc spray, and plasma arc spray, with a number of subsets falling under each category. (Cold spray is a recent addition to the family of thermal spray processes. This process typically uses some modest preheating, but is largely a kinetic energy
process. The unique characteristics of cold spray are discussed in the article ―Cold Spray Process‖ in this Handbook.) A brief review of some of the more commercially important thermal spray processes is given below. 8.5.2 FLAME SPRAY PROCESS.
Flame spraying includes low- velocity powder flame, rod
flame, and wire flame
processes and high-velocity processes such as HVOF and the detonation gun (D-Gun) process (D-Gun is a registered trademark of Praxair Surface Technologies Inc.).
Flame Powder. In the flame powder process, powdered feedstock is aspirated into the oxy fuel flame, melted, and carried by the flame and air jets to the work piece. Particle speed is relatively low (<100 m/s), and bond strength of the deposits is generally lower than the higher velocity processes. Porosity can be high and cohesive strength is also generally lower. Spray rates are usually in the 0.5 to 9 kg/h (1 to 20 lb/h) range
for all but the lower melting point materials, which spray at significantly h igher rates. Substrate surface temperatures can run quite high because of flame impingement. Wire Flame. In wire flame spraying, the primary function of the flame is to melt the
33
feedstock material. A stream of air then atomizes the molten material and propels it toward the work piece. Spray rates for materials such as stainless steel are in the range of 0.5 to 9 kg/h (1 to 20 lb/h). Again, lower melting point materials such as zinc and tin alloys spray at much higher rates. Substrate temperatures often range from 95 to 205 °C (200 to 400 °F) because of the excess energy input required for flame melting. In most thermal spray processes, less than 10% of the input energy is actually used to melt the feedstock material. High-Velocity Oxy fuel. In HVOF, a fuel gas (such as hydrogen, propane, or propylene) and oxygen are used to create a combustion jet at temperatures of 2500 to 3100 °C (4500 to 5600 °F). The combustion takes place internally at very high chamber pressures, exiting through a small-diameter (typically 8 to 9 mm, or 0.31 to 0.35 in.) barrel to generate a supersonic gas jet with very high particle speeds. The process results in extremely dense, well bonded coatings, making it attractive for many applications. Either powder or wire feedstock can be sprayed, at typical rates of 2.3 to 14 kg/h (5 to 30 lb/h). Detonation Gun. In the detonation gun process, pre- encapsulated
―shots‖ of
feedstock powder are fed into a 1 m (3 ft) long barrel along with oxygen and a fuel gas, typically acetylene. A spark ignites the mixture and produces a controlled explosion that propagates down the length of the barrel. The high temperatures and pressures (1 MPa, or 150 psi) that are generated blast the particles out of the end of the barrel toward the substrate. Very high bond strengths and densities as well as low oxide contents can be achieved using this process. 8.5.3 ELECTRIC ARC PROCESS.
Electric Arc.In the electric arc spray process (also known as the wire arc process), two consumable wire electrodes connected to a high-current direct-current (dc) power source are fed into the gun and meet, establishing an arc between them that melts the tips of the wires. The molten metal is then atomized and propelled toward the substrate by a stream of air. The process is energy e fficient because all of the input energy is used to melt the metal.
34
Spray rates are driven primarily by operating current and vary as a function of both melting point and conductivity. Generally materials such as copper-base and iron-base alloys spray at 4.5 kg (10 lb)/100 A/h. Zinc sprays at 11 kg (25 lb)/100 A/h. Substrate temperatures can be very low, because no hot jet of gas is directed toward the substrate. Electric arc spraying also can be carried out using inert gases or in a controlled-atmosphere chamber. 8.5.4 PLASMA ARC PROCESS.
Conventional Plasma. The conventional plasma spray process is commonly referred to as air or atmospheric plasma spray (APS). Plasma temperatures in the powder heating region range from about 6000 to 15,000 °C (11,000 to 27,000 °F),
significantly above the melting point of any known material. To generate the plasma, an inert gas — typically argon or an argon-hydrogen mixture — is superheated by a dc arc. Powder feedstock is introduced via an inert carrier gas and is accelerated toward the work piece by the plasma jet. Provisions for cooling or regulating the spray rate may be required to maintain substrate temperatures in the 95 to 205 °C (200 to 400 °F) range. Commercial plasma spray guns operate in the range of 20 to 200 kW. Accordingly, spray rates greatly depend on gun design, plasma gases, powder injection schemes, and materials properties, particularly particle characteristics such as size, distribution, melting point, morphology, and apparent density.Vacuum Plasma. Vacuum plasma spraying (VPS), also commonly referred to as low-pressure
plasma spraying (LPPS, a registered trademark of Sulzer Metco), uses modified plasma spray torches in a chamber at pressures in the range of 10 to 50 kPa (0.1 to 0.5 atm). At low pressures the plasma becomes larger in diameter and length, and,through the use of convergent/divergent nozzles, has a higher gas speed. The absence of oxygen and the ability to operate with higher substrate temperatures produce denser, more adherent coatings with much lower oxide contents. Kinetic Energy Processes Kinetics has been an important factor in thermal spray processing from the beginning. With the introduction of detonation gun, HVOF, and high-energy plasma spraying, the kinetic-energy component of thermal spraying became even more important. The
latest advance in kinetic spraying is known as ―cold spray.‖ Cold spray is a material deposition process in which coatings are applied by accelerating powdered feed stocks of ductile metals to speeds of 300 to 1200 m/s (985 to 3940 ft/s) using gas-
35
dynamic techniques with nitrogen or helium as the process gas. The process is
commonly referred to as ―cold gas -dynamic spraying‖ because of the relatively low temperatures (0 to 800 °C, or 32 to 1470 °F) of the expanded gas and particle stream that emanates from the nozzle. Powder feed rates of up to 14 kg/h (30 lb/h) are possible. Materials for Thermal Spray Three basic types of deposits can be thermal sprayed: 1. Single-phase materials, such as metals, alloys, inter metallics, 2. ceramics, and polymers 3. Composite materials, such as cermets (WC/Co, Cr3C2/NiCr, NiCrAlY/Al2O3 , etc.), Reinforced metals, and reinforced polymers Layered or graded materials, referred to as functionally gradient materials (FGMs) Examples of these, along with their particular advantages and applications, are described below. Single-Phase Materials Metals. Most pure metals and metal alloys have been thermal sprayed, including tungsten, molybdenum, rhenium, niobium, super alloys, zinc, aluminium, bronze, mild and stainless steels, NiCr alloys, cobalt-base Satellites, cobalt/nickel-base Tribiology,
and NiCrBSi ―self -fluxing‖ alloys. Sprayed alloys
have advantages due to their similarity to many base metals requiring repair, their high strength, and their corrosion, wear, and/or oxidation resistance. Applications include automotive/diesel engine cylinder coatings; piston rings or valve stems; turbine engine blades, vanes, and combustors; protection of bridges and other corrosion prone infrastructure; petrochemical pumps and valves; and mining and agricultural equipment. Ceramics.Most forms of ceramics can be thermal sprayed, including metallic oxides such as Al2O3 , stabilized ZrO2 , Cr2O3 , and MgO; carbides such as Cr3 C2 , TiC, Mo2C, and SiC (generally in a more ductile supporting metal matrix such as cobalt or NiCr); nitrides such as TiN and Si3 N4 ; and spinels or per ovskites such as mullite and 1-2-3-type superconducting oxides. Sprayed deposits of these materials are used to provide wear resistance (Al2O3 , Cr2 O3 , ZrO2, Cr3 C2 , TiC, Mo2 C, and TiN), thermal protection (Al2O3 , ZrO2 , and MgO), electrical insulation (Al2 O3 , ZrO2, and MgO), and corrosion resistance. Ceramics are particularly suited to thermal spraying, with plasma
36
spraying being the most suitable process due to its high jet temperatures. Intermetallics such as TiAl, Ti3 Al, Ni3 Al, NiAl, and MoSi2 have all been thermal sprayed. Most intermetallics are very reactive at high temperatures and very sensitive to oxidation; hence, inert atmospheres must be used during plasma spraying.
Research
has
also
been
conducted
on
thermal
spray
forming/consolidation of bulk intermetallic deposits.Polymers also can be thermal sprayed successfully, provided they are available in particulate form. Thermal spraying of polymers has been practiced commercially since the 1980s, and a growing number of thermoplastic and thermosetting polymers and copolymers have now been sprayed, including urethanes, ethylene vinyl alcohols (EVAs), nylon 11, polyt etrafluoroethylene
(PTFE), ethylene tetrafluoroethylene (ETFE),
polyetheretherketone (PEEK), polymethylmethacrylate (PMMA), polyimide, polycarbonate, and copolymers such as polyimide/polyamide, Surlyn (DuPont), and poly vinylidene
fluoride (PVDF). Conventional flame spray and HVOF are
the most widely used thermal spray methods for applying polymers.
37
8.6 NANO COATING:
Nano-coating is a recently developed technology used for coating any kind of material in hard coating and low friction coating both in which coating is done at nano scale that is of the order of 10 -9 . The two major types of nano-coating are 1. Physical vapour deposition (PVD) 2. Chemical vapour deposition (CVD) 3. Plasma Spray Coating 8.6.1 PHYSICAL VAPOUR DEPOSITION (PVD).
Thin film deposition is a process applied in the semiconductor industry to grow
electronic materials and in the
aerospace industry to form thermal and
chemical barrier coatings to protect surfaces against corrosive environments and to modify surfaces to have the desired properties. The deposition process can be broadly classified into physical vapor deposition (PVD) and
chemical vapour
deposition (CVD). In CVD, the film growth takes place at high temperatures, leading to the formation of corrosive gaseous products, and it may leave impurities in the film. The PVD process can be carried out at lower deposition temperatures and without corrosive products, but deposition rates are lower and it leaves residual compressive stress in the film. Electron beam physical vapor deposition, however, yields a high deposition rate from 0.1
μm / min to 100 μm / min at relatively low
substrate temperatures, with very high material utilization efficiency. Parameters of PVD is given by Deposition chamber vacuum pressure: 10-4 torr No. of electron guns: 6 Accelerating voltage: 20kv-25kv Evaporation rate: 10-2 g/cm2sec
38
CHAPTER 9 TESTING OF SPECIMEN AND RESULTS 9.1 CAST IRON WITHOUT ZIRCONIA
FIG 9.1 CAST IRON WITHOUT ZIRCONIA
39
9.2 CAST IRON WITH ZIRCONIA COATED
FIG 9.2 CAST IRON WITH ZIRCONIA COATED
40
9.3 CAST IRON WITHOUT ALUMINA TITANIA
FIG 9.3 CAST IRON WITHOUT ALUMINA TITANIA
41
9.4 CAST IRON WITH ALUMINA TITANIA COATED
FIG 9.4 CAST IRON WITH ALUMINA TITANIA COATED
42
RESULTS
1. For uncoated cast iron the hardness observed is 161,166,165 BHN. 2. For coated cast iron (zirconia coated) the hardness observed is 171,170,168 BHN. 3. For uncoated cast iron the hardness observed is 83,82,81 BHN. 4. For coated cast iron (Alumina Titania) the hardness observed is 86,84,86 BHN.
43
CHAPTER 10 CONCLUSION
From the literature survey zirconia coatings deposited by plasma spray have a low density. zirconia coatings deposited with plasma spray techniques were studied and an inverse linear plot of porosity against the percent age of zirconia was found.The high capacity of the system to produce different phases and oxide mixtures depends on the process variables. Increasing the zirconia content, diminishing the coating hardness, and the fracture toughness depends on diverse factors. The abrasive wear of coatings is inversely proportional to their hardness. The toughness and porosit y have a low influence on the abrasive wear behaviour. The coating has around triple abrasive wear compared. Due to zirconium oxide coating which will result in reduce the wear. From the obtained results the testing coating on cylinder liner b y thermal spray coatings were used investigated. The surface morphologies of the major and the minor faces were considerably different from each other. Due the coating on cast iron materials will improve the mechanical and thermal characterization. This will further improve the hardness, structural grains properties. Also the Alumina and titania coatings will provide the most dramatic improvements over existing cast iron liner, in engine component
applications where failure mechanisms that are driven by high
temperatures and chemical
diffusion are important for life. In lower temperature
applications (lower speeds, discontinuous contact) the coating will still offer improved performance due to the effects of crystallite refinement, which provide a smoother surface and second phase crack arresting or deflection mechanisms that make the coating tougher.
44
CHAPTER 11 REFERENCES
1 . S. Sampath, H. A.Goland, Material science. 43 (2000) 471. 2 . Ismail Ozdemir ,D.E. Crawmer, Handbook of Thermal Spray Technology, 2004, p. 55. 3 . O.Maranho, K. Gnädig, H. Kreye, H. Kronewetter, Surface Technology. 22 (1984) 61. 4 . D. Golberg, Rev. Int. Hautes Tem. Refract . 5 (3) (1968) 181. 5 .Y.R. Liu, Y.F. Qiao, J. He, E.J. Lavernia, T.E. Fisher, Metall. Mater. Trans.A 33 (2002). 6 .I. Levin, L.A. Bendersky, D.G. Brandon, M. Rühle , Acta Mater . 45 (9) (1997) 3659. 7 .P.S. Santos, H.S. Santos, S.P. Toledo, Mater. Res. 3 (4) (2000) 104. 8 .M. Uma, Ceram. Int . 30 (2004) 555. 9 .G. Paglia, Thesis , Curtin University of Techn. (Feb. 2004). 10 . B. Morasin, R.W. Lynch, Acta Crystallogr ., B 28 (1972) 1040. 11 . J. Kopecki, U. Albers, T. Jung, Surf. Coating Technology. 179 (2004) 279. 12 .S.J. Bull, D.S. Richerby, Adv. Surf. Coat . (1991) 315. 13 . V.A.D. Souza ,G.D. Quinn , Indentation hardness testing of ceramics, mechanical testing and evaluation, ASM Handbook , vol. 8, 244 – 251 (06772G). 14 .J.B. Quinn, G.D. Quinn, J. Material Science. 32 (1997) 4331. 15 .G.R. Austis, J. Am. Ceram. 64 (9) (1994) 533. 16 . T.Valente , C.W. Florey, F.J. Worzala, W.T. Lenling, J. Thermal Spray Technology. 2 (1) (1993) 35. 17 .H.M. Hawthorne, L.C. Erickson, D. Ross, H. Tai, T. Troczynski, Wear 203/ 204 (1997) 709. 18 .K.M. Liang, J. Mater. Sci 25 (1990) 207. 19 .P. Ostojic, R. Mac Pherson, Mater. Forum 10 (4) (1987) 247. 20 Yourong Liu, T. Fischer, A. Dent, Surf. Coat. Technol . 167 (2003) 68. 21 .R.W. Rice, Mechanical Properties of Ceramics and Composites.Grain andParticle Effects, Ed. Mercel Dekker . Inc. 2000, p. 76.
