CHAPTER 2 SURFACE MODIFICATION TECHNIQUES

Transcription

1 14 CHAPTER 2 SURFACE MODIFICATION TECHNIQUES 2.1 INTRODUCTION The selection of technology to engineer the surface is an integral part of an engineering component design. The first step in surface modification technique to determine the surface and substrate engineering requirements which involves one or more of the properties like wear resistance, corrosion and erosion resistance and thermal resistance, fatigue, creep strength, pitting resistance etc.the various surface treatments generally used in engineering practice and presented as under. 2.2 SURFACE MODIFICATION METHODS/ TECHNIQUES A simplified classification of various groupings of non-mechanical surface treatments could be reduced as 1. Thermal treatments 2.Thermo-chemical treatment 3.Plating and coating 4. Implantation The figure 2.1, illustrates different types of surface treatments and typical thickness of engineered surface materials produced by them. The effectiveness depends on particular surface and modification technique. 1. PVD process 2.CVD process 3.Electoless Nickel 4.Composite 5.Thermal spraying 6. Surface welding 7. Ion Implantation 8. Anodising 9. Boronizing 10. Nitriding 11. Carbonitriding 12. Carburizing 13.Nitrocarburising 14. Surface alloying 15. Thermal hardening.

2 15 Numerous processes are used for surface treatments, based on mechanical, chemical, thermal and physical. Their principles and characteristics are obtained as follows: There are two categories of vapor deposition processes: physical vapor deposition (PVD) and chemical vapor deposition (CVD). In PVD processes, the work piece is subjected to plasma bombardment. In CVD processes, thermal energy heats the gases in the coating chamber and drives the deposition reaction. Fig.2.1 Typical thickness of engineered surface layers Physical Vapour Deposition In this process, the work piece or substrate is subjected to high temperature vacuum evaporation or plasma sputter bombardment to deposit thin films by the condensation of a vaporized form of the material onto substrate

3 16 surfaces. This process contains the three major techniques; evaporation, sputtering and ion plating. It produces a dense, hard coating. The primary PVD methods are.ion plating, ion implantation, sputtering and laser surface alloying. Fig.2.2. PVD process using Plasma evaporation Fig.2.3. PVD process using arc sputtering PVD is used in the manufacture of semiconductor wafers, aluminized PET film for snack bags and balloons, cutting tools for metalworking and generally used for extreme thin films like atomic layers and mostly for small substrates. Chemical Vapour Deposition (CVD) In these processes, thermal energy heats the gases in the coating chamber and drives the deposition reaction and then this reactant gas mixture (mixture of gas precursors and coating material also known as a reactive vapour)

4 17 impinges on the substrate. CVD processes can be used to deposit coating materials, form foils, powders, composite materials in the shape of spherical particles, filaments, and whiskers and also in structural applications, optical, chemical, photovoltaic and electronics.. Start-up costs are typically very expensive. CVD includes sputtering, ion plating, plasma-enhanced CVD, lowpressure CVD, laser-enhanced CVD, active-reactive evaporation, ion beam, laser beam evaporation, and many other variations. These variants are distinguished by the manner in which precursor gases are converted into the reactive gas mixtures. It is usually in the form of a metal halide, metal carbonyl, a hydride, or an organ metallic compound. The precursor may be in gas, liquid, or solid form. Gases are delivered to the chamber under normal temperatures and pressures, whereas solids and liquids require high temperatures and/or low pressures in conjunction with a carrier gas. Once in the chamber, energy is applied to the substrate to facilitate the reaction of the precursor material upon impact. The ligand species is liberated from the metal species to be deposited upon the substrate to form the coating. Because most CVD reactions are endothermic, the reaction may be controlled by regulating the amount of energy input. Disadvantages of CVD, the precursor chemicals should not be toxic, and exhaust system should be designed to handle any reacted and unreacted vapors that remain after the coating process is complete. Other waste effluents from the process must be managed appropriately. Retrieval, recycle, and disposal methods are dictated by the nature of the chemical. For example, auxiliary chemical reactions must be performed to render toxic or corrosive materials harmless, condensates must be collected.

5 18 Fig.2.4. Schematic diagram of CVD process Electroless nickel plating Electroless nickel (EN) plating is a chemical reduction process that depends upon the catalytic reduction process of nickel ions in solution containing a chemical reducing agent and water and the subsequent deposition of nickel metal without the use of electrical energy. Thus in the EN plating process, the driving force for the reduction of nickel metal ions and their deposition is supplied by a chemical reducing agent in solution. This driving potential is essentially constant at all points of the surface of the component, provided the agitation is sufficient to ensure a uniform concentration of metal ions and reducing agents. The electro less deposits are therefore very uniform in thickness all over the part s shape and size. The process is advantageous when plating complex shape devices, holes, recesses, internal surfaces, valves, threaded parts etc. Electroless (autocatalytic) nickel coating provides a hard, uniform, corrosion, abrasion, and wear-resistant surface to protect machine components in many industrial environments. EN is chemically deposited, making the coating exceptionally uniform in thickness. If carefully process is controlled good surface finish can be produced which eliminates costly machining after plating.

6 19 In a true electroless plating process, reduction of metal ions occurs only on the surface of a catalytic substrate in contact with the plating solution. Once the catalytic substrate is covered by the deposited metal, the plating continues because the deposited metal is also catalytic. High corrosion resistance in the as-deposited condition; maintains better uniform thickness and surface finish; can plate small diameters, deep bores and intricate shapes. Disadvantages: Requires high standards of quality control of surface preparation and plating solution; softer than chrome plating; some metal limitations. Fig.2.5. Electroless Nickel plating process Composite A composite material is a macroscopic, physical combination of two or more materials in which one material usually provides reinforcement. Composites have been developed where no single, quasi-continuous material will

7 20 provide the required properties. In most composites one phase (material) is continuous and is termed the matrix, while the second, usually discontinuous phase, is termed the reinforcement, in some cases filler is applied when the reinforcement is not a quasi-continuous fibre. Matrix-filler nomenclature is one method of categorization. This yields the categories metal matrix (MMC), polymer (plastic) matrix (PMC), and ceramic matrix (CMC) composites the major subdivisions of this section. Other categories are given the shape and configuration of the reinforcing phase. The reinforcement is usually a ceramic and/or glass. If it is similar in all dimensions, it is a particulate reinforced composite; if needle-shaped single crystals, it is whisker-reinforced; if cut continuous filament, chopped fibre-reinforced; and if continuous fibre, fibre composite. For fibre composites configuration gives a further category. If fibres are aligned in one direction, it is a uni-axial fibre composite; if arranged in layers, it is a laminar composite; if a three-dimensional arrangement, it is a 3D weave composite. Laminates and 3D weaves can be further divided by the weave used for the fibre. Ion Implantation In the Ion plating (IP) process, the target material is initially melted while the substrate is bombarded with ions before deposition to raise it to the required temperature. The coating flux ion is attracted to the substrate by biasing the substrate with a negative voltage. Thus sufficient ion energy is available for good inter mixing of coating and substrate at the interfaceion implantation is the introduction of ionized dopant atoms into a substrate with enough energy to penetrate beyond the surface. The most common application is substrate doping. The use of 3 to 500 kev energy for boron, phosphorus or arsenic dopant ions is sufficient to implant the ions from 100 to 10,000A below the silicon surface. The depth of implantation, which is proportional to the ion energy, can be selected to meet a particular application.

8 21 Implantation offers a clear advantage over chemical deposition techniques. The major advantage of ion implantation technology is the capability of precisely controlling the number of implanted dopant atoms. Furthermore, the dopants depth distribution profile can be well-controlled. Disadvantages of Ion Implantation are very deep and very shallow profiles are difficult, not all the damage can be corrected by annealing, typically has higher impurity content than does diffusion. Often uses extremely toxic gas sources such as arsine (AsH3), and phosphine (PH3) and expensive They are generally used in Doping, SIMOX, H and He isolation in GaAs, and Smart cut technologies. Fig. 2.6 Schematic diagram of Ion Implantation used in Doping process Anodizing Anodizing involves the electrolytic oxidation of a surface to produce a tightly adherent oxide scale that is thicker than the naturally occurring film. Anodizing is an electrochemical process during which aluminium is the anode.

9 22 The electric current passing through an electrolyte converts the metal surface to a durable aluminium oxide. The difference between plating and anodizing is that the oxide coating is integral with the metal substrate as opposed to being a metallic coating deposition. The oxidized surface is hard and abrasion resistant, and it provides some degree of corrosion resistance. Anodic coatings can be formed in chromic, sulphuric, phosphoric, or oxalic acid solutions. Chromic acid anodizing is widely used with 7000 series alloys to improve corrosion resistance and paint adhesion, and unsealed coatings provide a good base for structural adhesives. However these coatings are often discolored and where cosmetic appearance is important, sulphuric acid anodizing may be preferred. Fig. 2.7 Anodising electrolytic bath Boronising Boronising is also called as boriding. It is a thermo-chemical treatment involving diffusion of boron into the surface of a component from the surrounding environment which results in the formation of a distinct compound

10 23 layer of a metal boride. The reaction takes place between boron and component, therefore it can be generally limited to steels, titanium-based alloys and cobaltbased hard metals. In steels, boronising is carried out in the austenite regime (between C) for several hours, resulting in the formation of layers commonly between 60 and 165 m thick. The surface reaction layer thus formed consists of two separate phases, namely a layer of Fe2B adjacent to the substrate and an outer layer of FeB. The proportions of the two phases are dependent upon the composition of the boronising environment and the alloy content of the steel (higher alloy content favours FeB formation). Care is taken to reduce the proportion of FeB in the boride layer since this always exists in tension; as such, high-alloy and stainless steels are unsuitable for boronising. The hardness of the boronised layer is dependent upon the exact composition of the steel but is commonly in the range MPa (as measured on the Vickers scale). This is significantly higher than many commonly occurring abrasives and, as such, boronising has been employed in situations requiring abrasive wear resistance. A variety of methods are employed to produce the boron-rich environment for the boronising process such as pack boronising, paste boronising, salt bath boronising and gas boronising. In pack boronising (the most commonly employed method), the source of boron is B4C which is mixed with an activator and an inert diluent to make up the pack powder. Nitriding Steels containing nitride-forming elements such as chromium, molybdenum, aluminium, and vanadium can be treated to produce hard surface layers, providing improved wear resistance. Many of the processes employed are proprietary, but typically they involve exposure of cleaned surfaces to anhydrous ammonia at elevated temperatures. The nitrides formed are not only hard but also

11 24 more voluminous than the original steel, and therefore they create compressive residual surface stresses. Therefore, nitrided steels usually exhibit improved fatigue and corrosion fatigue resistance. Similar beneficial effects can be achieved by shot peening. Fig. 2.8 Boronising process Layout Fig. 2.9 Nitriding process in hardening. Laser coating technology is increasingly widespread. Surface alloying is one of many kinds of alteration processes achieved through the use of lasers. It is similar to surface melting, but it promotes alloying by injecting another

12 25 material into the melt pool so that the new material alloys into the melt layer. Laser cladding is one of several surface alloying techniques performed by lasers. The overall goal is to selectively coat a defined area. In laser cladding, a thin layer of metal (or powder metal) is bonded with a base metal by a combination of heat and pressure. Specifically, ceramic or metal powder is fed into a carbon dioxide laser beam above a surface, melts in the beam, and transfers heat to the surface. The beam welds the material directly into the surface region, providing a strong metallurgical bond. Powder feeding is performed by using a carrier gas in a manner similar to that used for thermal spray systems. Large areas are covered by moving the substrate under the beam and overlapping disposition tracks. Shafts and other circular objects are coated by rotating the beam. Depending on the powder and substrate metallurgy, the microstructure of the surface layer can be controlled, using the interaction time and laser parameters. Laser surface treatment can be controlled to achieve alloying, cladding, grain refining or transformation hardening a metal surface without actually affecting the metal itself. A material of poor oxidation can be modified with a surface alloy which can show improved resistance. Laser grain refining eliminates or minimizes surface defects such as inclusions, pores and improves grain structure. Carburizing Carburizing is a heat treatment process in which iron or steel is heated in the presence of carbon material (in the range of 900 to 950 C (1,650 to 1,740 F)). Depending on the amount of time and temperature, the affected area can vary in carbon content. Longer carburizing times and higher temperatures lead to greater carbon diffusion into the part as well as increased depth of carbon diffusion. When the iron or steel is cooled rapidly by quenching, the higher carbon content on the outer surface becomes hard via the transformation from austenite to martensite, while the core remains soft and tough as a ferritic and/or pearlite microstructure.

13 26 Fig Pack Carburising process Generally it is used for low-carbon workpiece to increase their toughness and ductility; and it produces case hardness depths of up to 0.25 inches (6.4 mm). Carburization of steel involves a heat treatment of the metallic surface using a source of carbon. Early carburization used a direct application of charcoal packed onto the metal (initially referred to as case hardening), but modern techniques apply carbon-bearing gases or plasmas (such as carbon dioxide or methane). The process depends primarily upon ambient gas composition and furnace temperature, which must be carefully controlled, as the heat may also impact the microstructure of the rest of the material. For applications where great control over gas composition is desired, carburization may take place under very low pressures in a vacuum chamber. Plasma carburization is increasingly used in major industrial regimes to improve the surface characteristics (such as wear and corrosion resistance, hardness and load-bearing capacity, in addition to quality-based variables) of various metals, notably stainless steels. The process is used as it is environmentally friendly (in comparison to gaseous or solid carburizing). It also provides an even treatment of components with complex geometry (the plasma

14 can penetrate into holes and tight gaps), making it very flexible in terms of component treatment. 27 The process of carburization works via the implantation of carbon atoms in to the surface layers of a metal. Gas carburizing is normally carried out at a temperature within the range of C to 950 C. In oxy-acetylene welding, a carburizing flame is one with little oxygen, which produces a sooty, lowertemperature flame. It is often used to anneal metal, making it more malleable and flexible during the welding process. A main goal when producing carbonized work pieces is to insure maximum contact between the workpiece surface and the carbon-rich elements. In gas and liquid carburizing, the work pieces are often supported in mesh baskets or suspended by wire. In pack carburizing, the workpiece and carbon are enclosed in a container to ensure that contact is maintained over as much surface area as possible. Pack carburizing containers are usually made of carbon steel coated with aluminium or heat-resisting nickel-chromium alloy and sealed at all openings with fire clay. It's possible to carburize only a portion of a part, either by protecting the rest by a process such as copper plating, or by applying a carburizing medium to only a section of the part. The carbon can come from a solid, liquid or gaseous source; if it comes from a solid source the process is called pack carburizing. Packing low carbon steel parts with a carbonaceous material and heating for some time diffuses carbon into the outer layers. A heating period of a few hours might form a high-carbon layer about one millimetre thick.

15 28 Liquid carburizing involves placing parts in a bath of a molten carboncontaining material, often metal cyanide; gas carburizing involves placing the parts in a furnace maintained with a methane-rich interior. Cyaniding Cyaniding is a case hardening process that is fast and efficient; it is mainly used on low carbon steels. The part is heated to C ( F) in a bath of sodium cyanide and then is quenched and rinsed, in water or oil, to remove any residual cyanide. 2NaCN + 2NaCNO 2NaCNO 2NaCNO + O 2 NaCO 3 +CO + 2N 2CO CO 2 + C This process produces a thin, hard shell (between mm [0.010 and inches]) that is harder than the one produced by carburizing, and can be completed in 20 to 30 minutes compared to several hours so the parts have less opportunity to become distorted. It is typically used on small parts such as bolts, nuts, screws and small gears. The major drawback of cyaniding is that cyanide salts are poisonous. Fig Hardening using Cyaniding process.

16 29 Carbo-nitriding Carbo-nitriding is similar to cyaniding except a gaseous atmosphere of ammonia and hydrocarbons is used instead of sodium cyanide. If the part is to be quenched then the part is heated to C ( F); if not then the part is heated to C ( F). Ferritic Nitro Carburizing Ferritic nitro-carburizing diffuses mostly nitrogen and some carbon into the case of a workpiece below the critical temperature, approximately 650 C (1,202 F). Under the critical temperature the work piece s microstructure does not convert to an austenitic phase, but stays in the ferritic phase, which is why it is called ferritic nitro-carburization. Fig.2.12 Carbonitriding Process Applications

17 30 Parts that are subject to high pressures and sharp impacts are commonly case hardened, e.g. firing pins and rifle bolt faces, or engine camshafts. Cladding It is the bonding together of dissimilar metals. It is distinct from welding or gluing as a method to fasten the metals together. Cladding is often achieved by extruding two metals through a die as well as pressing or rolling sheets together under high pressure. Laser Surface Treatment (LST) It can be controlled to achieve alloying, cladding, grain refining or transformation hardening a metal surface without actually affecting the bulk of the metal itself. LST can be categorized into three main sections and its various effects on a substrate can be shown as in table 1.1 [Gnanamuthu D.S., 1979]. A laser beam can enhance surface properties to a controlled, confined extent depending on the power, dwell rime of the beam and the thermal characteristics, i.e., heating and cooling of the surface treated. Surface treatment prospects by lasers were observed with pulsed lasers at first. Being inertialess, it has high processing speeds with very rapid stop and start facility. A material of poor oxidation or corrosion or wear resistance but low cost can be modified with a surface alloy which can show improved resistance. Table 2.1 Effects of application of Laser beam on materials Heating Melting Shocking Annealing Transformation hardening Alloying Cladding Glazing Grain Refining Shock hardening

18 Laser Grain refining eliminates or minimizes surface defects such as inclusions, pores and improves the grain structure. 31 Laser Cladding Laser cladding is a method of depositing material by which a powdered or wire feedstock material is melted and consolidated by use of a laser in order to coat part of a substrate or fabricate a near-net shape part (additive manufacturing technology).it is used to improve mechanical properties or increase corrosion resistance, repair worn out parts, and fabricate metal matrix composites to improve mechanical properties or increase corrosion resistance, repair worn out parts, and fabricate metal matrix composites. Fig Laser cladding method of depositing material Other Surface Treatment Processes Numerous processes are used for surface treatments, based on mechanical, chemical, thermal and physical. Their principles and characteristics

19 32 are obtained as follows: Short Peening, Water-Jet Peening and Laser Peening. In short peening the surface of the work piece is hit repeatedly with large number of cast-steel, glass or ceramic shot (size of 0.125mm to 5mm diameter), making overlapping indentation on the surface; this action causes plastic deformation of the surfaces. Thus improving the fatigue life of the component. Extensively used on shafts, gears, springs, oil-well drilling equipment, and jet engine parts. In water-jet peening, a water jet at pressure as high as 400 MPa impinges on the surface of the work piece, inducing compressive residual stresses. This have been successfully used on steels and aluminum alloys. In laser peening, the surface is subjected to laser shocks from high powered laser up to 1KW and at energy levels of 100 J/pulse. This method has been used on jet engine fan blades with compressive residual stresses deeper than 1mm. Roller Burnishing (Surface Rolling) The surface of the component is cold worked by hard and highly polished roller or rollers; this process is used on various flat, cylindrical or conical surfaces. Roller burnishing improves surface finish by removing scratches, tool marks and pits. Explosive Hardening The surface is subjected to high transient pressures by placing a layer of explosive sheet directly on the work piece surface and detonating it. Large increase in surface hardness can be obtained by this method. Railroad rail surfaces can be hardened by this method.

20 33 Mechanical Plating Fine particles of metal are compacted over the work piece surfaces by impacting them with spherical glass, ceramic, or porcelain beads. The beads are propelled by rotary means. This process typically is used for hardened-steel parts for automobiles. Case Hardening The formation of martensite in case hardening of steels causes residual stresses on surfaces. Such stresses are desirable, because they improve the fatigue life of components by delaying the initiation of fatigue cracks. Hard Facing A relatively thick layer, edge or point of wear-resistant hard metal is deposited on the surface by any of the welding techniques. Hard coatings of tungsten carbide, chromium and molybdenum carbide are also deposited by this method. Typical applications for hard facing include valve seats, oil-well drilling tools and dies for hot metal working. Surface Texturing Manufactured surfaces can be further modified by secondary operations for technical, functional, optical or aesthetic reasons. These additional processes generally consist of etching, electric arcs, laser pulses, atomic oxygen with reacts with surfaces and produces fine, cone like surface textures. Diffusion Coating In this process, an alloying element is diffused into the surface of the substrate, thus altering its properties. Such elements can be supplied in solid, liquid or gaseous state. This process is given different names, depending on the

21 diffused element, that describe diffusion process, such as carburizing, nitriding, and boronizing. 34 Electroplating The work-piece (cathode) is plated with a different metal (anode) while both are suspended in a bath containing a water-base electrolyte solution. The metal ions from the anode are discharged under the potential from the external source of electricity, combine with the ions in the solution, and are deposited on the cathode. All metals can be electroplated, with thickness ranging from a few atomic layers to a maximum of about 0,05mm.Typical application include copper plating aluminum wire and phenolic boards for printed circuits, chrome plating hardware, tin plating copper electrical terminals for ease of soldering and plating various components for good appearance and resistance to wear and corrosion. Electroforming A variation of electroplating, electroforming is actually a metalfabrication process. Metal is electrodeposited on a mandrel, which is then removed; thus, the coating itself becomes the product. Simple and complex shapes can be produced by electroforming, with wall thickness as small as 0.025mm. Conversion Coating In this process, also called chemical-reaction priming, a coating forms on metal surfaces as a result of chemical or electro chemical reactions. Various metals, particularly steel, aluminum, and zinc, can be conversion coated. Oxides that naturally form on their surfaces are a form of conversion coating; phosphates, chromates, and oxalates are used to produce conversion coatings.

22 These coatings are used for purposes such as pre-painting, decorative finishes, and protection against corrosion. 35 Hot Dipping In this process, the work piece, usually steel or iron, is dipped into a bath of molten metal, such as zinc, tin, aluminum and terne (lead alloyed with 10% to 20% tin). Hot dipped coatings on discrete or sheet metal provide galvanized pipe, plumbing supplies, and many other products with long time resistance to corrosion. Various pre-coated steel sheets are used extensively in automobile bodies. Porcelain Enameling Metals may be coated with a variety of glassy coatings to provide corrosion and electrical resistance and for service at elevated temperatures. These coatings are usually classified as porcelain enamels and generally include enamels and ceramics. Enameling involves fusing the coating material on the substrate by heating them both to 425 C to 1000 C to liquefy the oxides. Typical application include household appliances, plumbing fixtures, chemical processing equipment, signs, cook ware and jewelry; they are also used as protective coatings on jet-engine components. Organic Coatings Metal surfaces maybe coated or pre-coated with a variety organic coatings, films and laminates to improve appearance and corrosion resistance. Coatings are applied to the coil stock on continuous lines, with thickness generally of mm to 0.2mm. Such coatings have a wide range of properties: flexibility, durability, hardness, resistance to abrasion and chemicals, color, texture and gloss. Application of organic coatings are coatings for naval aircraft that subjected to high humidity, rain, sea water, pollutants, aviation fluids,

23 Deicing fluids and battery acids that are also impacted by particles such as dust, gravel, stones and deicing salts. 36 Painting Because of its decorative and functional properties, paint is widely used as a surface coating. Paints are basically classified as enamels, lacquers and water base paints, with a wide range of characteristics and applications. They are applied by brushing, dipping, spraying, or electro statically. Diamond Coating Important advances have been made in diamond coating of metals, glass, ceramics and plastics, using various chemical and plasma assisted vapor deposition process and ion beam enhanced deposition techniques. Development of these techniques, combined with important properties of diamonds, such as hardness, wear resistance, high thermal conductivity and transparency to ultra violet light and microwave frequencies has enabled the production of various aerospace and electronic parts and components. Diamond Like Carbon (DLC) By using a low temperature, ion beam assisted deposition process, this relatively recently developed materials is applied as a coating of a few nanometers in thickness. Less expensive than diamond films, but with cylinder properties as diamond, DLC has applications in such areas as tools and dies, gears, bearing, micro electro-mechanical systems, and micro scale probes.