Modern Physical Metallurgy and Materials Engineering Part 14 pot

Corrosion and surface engineering 381 is a compound oxide of both the solute and solvent metals. The best-known examples are the spinels with cubic structure (e.g. NiO.Cr 2 O 3 and FeO.Cr 2 O 3 ). It is probable that the spinel formation is temperature- dependent, with Cr 2 O 3 forming at low temperatures and the spinel at higher ones. Stainless steels (ferritic, austenitic or martensitic) are among the best oxidation-resistant alloys and are based on Fe–Cr. When iron is heated above about 570 ° C the oxide scale which forms consists of w ¨ ustite, FeO (a p-type semiconductor) next to the metal, magnetite Fe 3 O 4 (a p-type semiconductor) next and haematite Fe 2 O 3 (an n-type semiconductor) on the outside. When Cr is added at low concentrations the Cr forms a spinel FeO.CrO 3 with the w ¨ ustite and later with the other two oxides. However, a minimum Cr addition of 12% is required before the inner layer is replaced by Cr 2 O 3 below a thin outer layer of Fe 2 O 3 . Heat-resistant steels for service at temperatures above 1000 ° C usually contain 18% Cr or more, and austenitic stainless steels 18% Cr, 8% Ni. The growth of Cr 2 O 3 on austenitic stainless steels containing up to 20% Cr appears to be rate-controlled by chromium diffusion. Kinetic factors determine whether Cr 2 O 3 or a duplex spinel oxide form, the nucleation of Cr 2 O 3 is favoured by higher Cr levels, higher temperatures and by surface treatments (e.g. deformation), which increase the diffusivity. Surface treatments which deplete the surface of Cr promote the formation of spinel oxide. Once Cr 2 O 3 is formed, if this film is removed or disrupted, then spinel oxidation is favoured because of the local lowering of Cr. When chromium-bearing alloys, such as austenitic stainless steels, are exposed to the hot combustion products of fossil fuels, the outer layer of chromium oxide which forms is often associated with an underlying sulphide phase (Figure 12.3a). This duplex structure can be explained by using phase (stability) diagrams and the concept of ‘reaction paths’. Previously, in Section 3.2.8.5, it was indicated that a two-dimensional section could be taken through the full three-dimensional diagram for a metal–sulphur–oxygen system (Figure 3.23). Accordingly, in a similar way, we can extract an isothermal section from the full phase diagram for the Cr–S–O system, as shown in Figure 12.3b. The chemical activities of sulphur and oxygen in the gas phase are functions of their partial pressures (concentration). If the partial pressure of sulphur is relatively low, the composition of the gas phase will lie within the chromium oxide field and the alloy will oxidize (Figure 12.3b). Sulphur and oxygen diffuse through the growing layer of oxide scale but S 2 diffuses faster than O 2 , accordingly, the composition of the gas phase in contact with the alloy follows a ‘reaction path’, as depicted by the dashed line. Figure 12.3c shows the reaction path for gases with a higher initial partial pressure of sulphur. Its slope is such that first chromium oxide forms, and then chromium sulphide (i.e. Cr CS D CrS). Sometimes the oxide scale may crack or form voids. In such cases, the activity of S 2 Figure 12.3 Reaction paths for oxidation and sulphidation of chromium. 382 Modern Physical Metallurgy and Materials Engineering may rise locally within the scale and far exceed that of the main gas phase. Sulphidation of the chromium then becomes likely, despite a low concentration of sulphur in the main gas stream (Figure 12.3d). Relative tendencies of different metallic elements to oxidize and/or sulphidize at a given temperature may be gauged by superimposing their isothermal p S 2 p O 2 diagrams, as in Figure 12.4. For example, with the heat-resistant 80Ni–20Cr alloy (Nichrome), it can be reasoned that (1) Cr 2 O 3 scale and CrS subscale are both stable in the presence of nickel, and (2) Cr 2 O 3 forms in preference to NiO; that is, at much lower partial pressures of oxygen. The physical state of a condensed phase is extremely important because liquid phases favour rapid diffusion and thus promote corrosive reactions. Although nickel has a higher sulphidation threshold than chromium, the Ni–NiS eutec- tic reaction is of particular concern with Ni-containing alloys because it takes place at the relatively low temperature of 645 ° C. 12.2.2 Aqueous corrosion 12.2.2.1 Electrochemistry of corrosion Metals corrode in aqueous environments by an electrochemical mechanism involving the dissolution of the metal as ions (e.g. Fe ! Fe 2C C 2e). The excess electrons generated in the electrolyte either reduce hydrogen ions (particularly in acid solutions) according to 2H C C 2e ! H 2 so that gas is evolved from the metal, or create hydroxyl ions by the reduction of dissolved oxygen according to O 2 C 4e C 2H 2 O ! 4OH  The corrosion rate is therefore associated with the flow of electrons or an electrical current. The two reactions involving oxidation (in which the metal ion- izes) and reduction occur at anodic and cathodic sites, respectively, on the metal surface. Generally, the metal Figure 12.4 Superimposition of isothermal sections from Cr–S–O and Ni–S–O systems. surface consists of both anodic and cathodic sites, depending on segregation, microstructure, stress, etc., but if the metal is partially-immersed there is often a distinct separation of the anodic and cathodic areas with the latter near the waterline where oxygen is read- ily dissolved (differential aeration). Figure 12.5 illus- trates the formation of such a differential aeration cell; Fe 2C ions pass into solution from the anode and OH  ions from the cathode, and where they meet they form ferrous hydroxide Fe(OH) 2 . However, depending on the aeration, this may oxidize to Fe(OH) 3 , red-rust Fe 2 O 3 .H 2 O, or black magnetite Fe 3 O 4 . Such a process is important when water, particularly seawater, collects in crevices formed by service, manufacture or design. In this form of corrosion the rate-controlling process is usually the supply of oxygen to the cathodic areas and, if the cathodic area is large, can often lead to intense local attack of small anode areas, such as pits, scratches, crevices, etc. In the absence of differential aeration, the formation of anodic and cathodic areas depends on the ability to ionize. Some metals ionize easily, others with diffi- culty and consequently anodic and cathodic areas may be produced, for example, by segregation, or the join- ing of dissimilar metals. When any metal is immersed in an aqueous solution containing its own ions, positive ions go into solution until the resulting electromotive force (emf) is sufficient to prevent any further solution; this emf is the electrode potential or half-cell potential. To measure this emf it is necessary to use a second reference electrode in the solution, usually a standard hydrogen electrode. With no current flow- ing, the applied potential cancels out the extra potential developed by the spontaneous ionization at the metal electrode over and above that at the standard hydrogen electrode. With different metal electrodes a table of potentials E 0  can be produced for the half-cell reactions M ! M nC C ne 12.8 where E 0 is positive. The usual convention is to write the half-cell reaction in the reverse direction so that the sign of E 0 is also reversed, i.e. E 0 is negative; E 0 is referred to as the standard electrode potential. It is common practice to express the tendency of a metal to ionize in terms of this voltage, or potential, E 0 , rather than free energy, where G DnFE 0 for the half-cell reaction with nF coulomb of electrical Figure 12.5 Corrosion of iron by differential aeration. Corrosion and surface engineering 383 Table 12.1 Electrochemical Series Electrode reaction Standard electrode potential E 0 (V) Cs D Cs C C e 3.02 Li D Li C C e 3.02 K D K C C e 2.92 Na D Na C C e 2.71 Ca D Ca 2C C 2e 2.50 Mg D Mg 2C C 2e 2.34 Al D Al 3C C 3e 1.07 Ti D Ti 2C C 2e 1.67 Zn D Zn 2C C 2e 0.76 Cr D Cr 3C C 3e 0.50 Fe D Fe 2C C 2e 0.44 Cd D Cd 2C C 2e 0.40 Ni D Ni 2C C 2e 0.25 Sn D Sn 2C C 2e Reactive metals 0.136 Pb D Pb 2C C 2e 0.126 H D 2H C C 2e 0.00 Cu D Cu 2C C 2e C0.34 Hg D Hg 2C C 2e C0.80 Ag D Ag C C e C0.80 Pt D Pt 2C C 2e C1.20 Au D Au C C e Noble metals C1.68 charge transported per mole. The half-cell potentials are given in Table 12.1 for various metals, and refer to the potential developed in a standard ion concentration of one mole of ions per litre (i.e. unit activity), relative to a standard hydrogen electrode at 25 ° Cwhichis assigned a zero voltage. The voltage developed in any galvanic couple (i.e. two half cells) is given by the difference of the electrode potentials. If the activity of the solution is increased then the potential increases according to the Nernst equation E D E 0 C RT/nF lna. The easily ionizable ‘reactive’ metals have large negative potentials and dissolve even in concentrated solutions of their own ions, whereas the noble metals have positive potentials and are deposited from solution. These differences show that the valency electrons are strongly bound to the positive core in the noble metals because of the short distance of interaction, i.e. d atomic ' d ionic . A metal will therefore displace from solution the ions of a metal more noble than itself in the Series. When two dissimilar metals are connected in neutral solution to form a cell, the more metallic metal becomes the anode and the metal with the lower tendency to ionize becomes the cathode. The Electro- chemical Series indicates which metal will corrode in the cell but gives no information on the rate of reactions. When an anode M corrodes, its ions enter into the solution initially low in M C ions, but as current flows the concentration of ions increases. This leads to a change in electrode potential known as polarization, as shown in Figure 12.6a, and corresponds to a reduced tendency to ionize. The current density in the cell is a maximum when the anode and cathode potential curves intersect. Such a condition would exist if the two metals were joined together or anode and cathode regions existed on the same metal, i.e. differential aeration. This potential is referred to as the corrosion potential and the current, the corrosion current. In many reactions, particularly in acid solutions, hydrogen gas is given off at the cathode rather than the anode metal deposited. In practice, the evolution of hydrogen gas at the cathode requires a smaller additional overvoltage, the magnitude of which varies considerably from one cathode metal to another, and is high for Pb, Sn and Zn and low for Ag, Cu, Fe and Ni; this overvoltage is clearly of importance in electrodeposition of metals. In corrosion, the overvoltage arising from the activation energy opposing the electrode reaction decreases the potential of the cell, Figure 12.6 Schematic representation of (a) cathode and anode polarization curves and (b) influence of oxygen concentration on cathode polarization. 384 Modern Physical Metallurgy and Materials Engineering i.e. hydrogen atoms effectively shield or polarize the cathode. The degree of polarization is a function of current density and the potential E to drive the reaction decreases because of the increased rate of H 2 evolution, as shown in Figure 12.7 for the corrosion of zinc and iron in acid solutions. Corrosion can develop up to a rate given by the current when the potential difference required to drive the reaction is zero; for zinc this is i Zn and for iron i Fe . Because of its large overvoltage zinc is corroded more slowly than iron, even though there is a larger difference between zinc and hydrogen than iron and hydrogen in the Electrochemical Series. The presence of Pt in the acid solution, because of its low overvoltage, increases the corrosion rate as it plates out on the cathode metal surface. In neutral or alkaline solutions, depolarization is brought about by supplying oxygen to the cathode area which reacts with the hydrogen ions as shown in Figure 12.6b. In the absence of oxygen both anodic and cathodic reactions experience polarization and corrosion finally stops; it is well-known that iron does not rust in oxygen-free water. It is apparent that the cell potential depends on the electrode material, the ion concentration of the electrolyte, passivity and polarization effects. Thus it is not always possible to predict the precise electrochemical behaviour merely from the Electrochemical Series (i.e. which metal will be anode or cathode) and the magnitude of the cell voltage. Therefore it is necessary to determine the specific behaviour of different metals in solutions of different acidity. The results are displayed usually in Pourbaix diagrams as shown in Figure 12.8. With stainless steel, for example, the anodic polarization curve is not straightforward as discussed previously, but takes the form shown in Figure 12.9, where the low-current region corresponds to the condition of passivity. The corrosion rate depends on the position at which the cathode polarization curve for hydrogen evolution crosses this anode curve, and can be quite high if it crosses outside the passive region. Pourbaix diagrams map out the regions of passivity for solutions of different acidity. Figure 12.8 shows that the passive region is restricted to certain conditions of Figure 12.7 Corrosion of zinc and iron and the effect of polarization. pH; for Ti this is quite extensive but Ni is passive only in very acid solutions and Al in neutral solutions. Interestingly, these diagrams indicate that for Ti and Ni in contact with each other in corrosive conditions then Ni would corrode, and that passivity has changed their order in the Electrochemical Series. In general, passivity is maintained by conditions of high oxygen concentration but is destroyed by the presence of certain ions such as chlorides. The corrosion behaviour of metals and alloys can therefore be predicted with certainty only by obtaining experimental data under simulated service conditions. For practical purposes, the cell potentials of many materials have been obtained in a single environment, the most common being sea water. Such data in tabular form are called a Galvanic Series, as illustrated in Table 12.2. If a pair of metals from this Series were connected together in sea water, the metal which is higher in the Series would be the anode and corrode, and the further they are apart, the greater the corrosion tendency. Similar data exist for other environments. 12.2.2.2 Protection against corrosion The principles of corrosion outlined above indicate several possible methods of controlling corrosion. Since current must pass for corrosion to proceed, any factor, such as cathodic polarization which reduces the current, will reduce the corrosion rate. Metals having a high overvoltage should be utilized where possible. In neutral and alkaline solution de-aeration of the electrolyte to remove oxygen is beneficial in reducing corrosion (e.g. heating the solution or holding under a reduced pressure preferably of an inert gas). It is sometimes possible to reduce both cathode and anode reactions by ‘artificial’ polarization (for example, by adding inhibitors which stifle the electrode reaction). Calcium bicarbonate, naturally present in hard water, deposits calcium carbonate on metal cathodes and sti- fles the reaction. Soluble salts of magnesium and zinc act similarly by precipitating hydroxide in neutral solutions. Anodic inhibitors for ferrous materials include pota- ssium chromate and sodium phosphate, which convert the Fe 2C ions to insoluble precipitates stifling the anodic reaction. This form of protection has no effect on the cathodic reaction and hence if the inhibitor fails to seal off the anode completely, intensive local attack occurs, leading to pitting. Moreover, the small current density at the cathode leads to a low rate of polarization and the attack is maintained. Sodium benzoate is often used as an anodic inhibitor in water radiators because of its good sealing qualities, with little tendency for pitting. Some metals are naturally protected by their adher- ent oxide films; metal oxides are poor electrical con- ductors and so insulate the metal from solution. For the reaction to proceed, metal atoms have to diffuse through the oxide to the metal–liquid interface and electrons back through the high-resistance oxide. The Corrosion and surface engineering 385 Figure 12.8 Pourbaix diagrams for (a) Ti, (b) Fe, (c) Ni, (d) Al. The clear regions are passive, the heavily-shaded regions corroding and the lightly-shaded regions immune. The sloping lines represent the upper and lower boundary conditions in service. Figure 12.9 Anode polarization curve for stainless steel. Table 12.2 Galvanic Series in sea water Anodic or most reactive Mg and its alloys Cu Zn Ni (active) Galvanized steel Inconel (active) Al Ag Mild steel Ni (passive) Cast iron Inconel (passive) Stainless steel (active) Monel Pb Ti Sn Stainless steel (passive) Increasing reactivity Brass Decreasing reactivity Cathodic or most noble corrosion current is very much reduced by the formation of such protective or passive oxide films. Al is cathodic to zinc in sea water even though the Electro- chemical Series shows it to be more active. Materials which are passivated in this way are chromium, stainless steels, Inconel and nickel in oxidizing conditions. Reducing environments (e.g. stainless steels in HCl) 386 Modern Physical Metallurgy and Materials Engineering destroy the passive film and render the materials active to corrosion attack. Certain materials may be artifi- cially passivated by painting. The main pigments used are red lead, zinc oxide and chromate, usually sus- pended in linseed oil and thinned with white spirit. Slightly soluble chromates in the paint passivate the underlying metal when water is present. Red lead reacts with the linseed oil to form lead salts of various fatty acids which are good anodic inhibitors. Sacrificial or cathodic protection is widely used. A typical example is galvanized steel sheet when the steel is protected by sacrificial corrosion of the zinc coating. Any regions of steel exposed by small flaws in the coating polarize rapidly since they are cathodic and small in area; corrosion products also tend to plug the holes in the Zn layer. Cathodic protection is also used for ships and steel pipelines buried underground. Auxiliary sacrificial anodes are placed at frequent intervals in the corrosive medium in contact with the ship’s hull or pipe. Protection may also be achieved by impressing a d.c. voltage to make it a cathode, with the negative terminal of the d.c. source connected to a sacrificial anode. 12.2.2.3 Corrosion failures In service, there are many types of corrosive attack which lead to rapid failure of components. A familiar example is intergranular corrosion and is associated with the tendency for grain boundaries to undergo localized anodic attack. Some materials are, however, particularly sensitive. The common example of this sensitization occurs in 18Cr–8Ni stainless steel, which is normally protected by a passivating Cr 2 O 3 film after heating to 500–800 ° C and slowly cooling. During cooling, chromium near the grain boundaries precipitates as chromium carbide. As a consequence, these regions are depleted in Cr to levels below 12% and are no longer protected by the passive oxide film. They become anodic relative to the interior of the grain and, being narrow, are strongly attacked by the corrosion current generated by the cathode reactions elsewhere. Sensitization may be avoided by rapid cooling, but in large structures that is not possible, particularly after welding, when the phenomenon (called weld decay) is common. The effect is then overcome by stabilizing the stainless steel by the addition of a small amount (0.5%) of a strong carbide-former such as Nb or Ti which associates with the carbon in preference to the Cr. Other forms of corrosion failure require the component to be stressed, either directly or by residual stress. Common examples include stress-corrosion cracking (SCC) and corrosion-fatigue. Hydrogen embrittlement is sometimes included in this category but this type of failure has somewhat different characteristics and has been considered previously. These failures have certain features in common. SCC occurs in chemically active environments; susceptible alloys develop deep fissures along active slip planes, particularly alloys with low stacking-fault energy with wide dislocations and planar stacking faults, or along grain boundaries. For such selective chemical action the free energy of reaction can provide almost all the surface energy for fracture, which may then spread under extremely low stresses. Stress corrosion cracking was first observed in ˛-brass cartridge cases stored in ammoniacal environments. The phenomenon, called season-cracking since it occurred more frequently during the mon- soon season in the tropics, was prevented by giving the cold-worked brass cases a mild annealing treatment to relieve the residual stresses of cold forming. The phenomenon has since extended to many alloys in different environments (e.g. Al–Cu, Al–Mg, Ti–Al), magnesium alloys, stainless steels in the presence of chloride ions, mild steels with hydroxyl ions (caustic embrittlement) and copper alloys with ammonia ions. Stress corrosion cracking can be either transgranular or intergranular. There appears to be no unique mechanism of transgranular stress corrosion cracking, since no single factor is common to all susceptible alloys. In general, however, all susceptible alloys are unstable in the environment concerned but are largely protected by a surface film that is locally destroyed in some way. The variations on the basic mechanism arise from the different ways in which local activity is generated. Breakdown in passivity may occur as a result of the emergence of dislocation pile-ups, stacking faults, micro-cracks, precipitates (such as hydrides in Ti alloys) at the surface of the specimen, so that highly localized anodic attack then takes place. The gradual opening of the resultant crack occurs by plastic yielding at the tip and as the liquid is sucked in also prevents any tendency to polarize. Many alloys exhibit coarse slip and have similar dislocation substructures (e.g. co-planar arrays of dislocations or wide planar stacking faults) but are not equally susceptible to stress-corrosion. The observation has been attributed to the time necessary to repassivate an active area. Additions of Cr and Si to susceptible austenitic steels, for example, do not significantly alter the dislocation distribution but are found to decrease the susceptibility to cracking, probably by lowering the repassivation time. The susceptibility to transgranular stress corrosion of austenitic steels, ˛-brasses, titanium alloys, etc. which exhibit co-planar arrays of dislocations and stacking faults may be reduced by raising the stacking- fault energy by altering the alloy composition. Cross- slip is then made easier and deformation gives rise to fine slip, so that the narrower, fresh surfaces created have a less severe effect. The addition of elements to promote passivation or, more importantly, the speed of repassivation should also prove beneficial. Intergranular cracking appears to be associated with a narrow soft zone near the grain boundaries. In ˛- brass this zone may be produced by local dezincifica- tion. In high-strength Al-alloys there is no doubt that it is associated with the grain boundary precipitate-free zones (i.e. PFZs). In such areas the strain-rate may be so rapid, because the strain is localized, that repassivation cannot occur. Cracking then proceeds even though Corrosion and surface engineering 387 the slip steps developed are narrow, the crack dis- solving anodically as discussed for sensitized stainless steel. In practice there are many examples of intergranular cracking, including cases (1) that depend strongly on stress (e.g. Al-alloys), (2) where stress has a com- paratively minor role (e.g. steel cracking in nitrate solutions) and (3) which occur in the absence of stress (e.g. sensitized 18Cr–8Ni steels); the last case is the extreme example of failure to repassivate for purely electrochemical reasons. In some materials the crack propagates, as in ductile failure, by internal necking between inclusions which occurs by a combination of stress and dissolution processes. The stress sensitivity depends on the particle distribution and is quite high for fine-scale and low for coarse-scale distributions. The change in precipitate distribution in grain boundaries produced, for example, by duplex ageing can thus change the stress-dependence of intergranular failure. In conditions where the environment plays a role, the crack growth rate varies with stress intensity K in the manner shown in Figure 12.10. In region I the crack velocity shows a marked dependence on stress, in region II the velocity is independent of the stress intensity and in region III the rate becomes very fast as K IC is approached. K ISC is extensively quoted as the threshold stress intensity below which the crack growth rate is negligible (e.g. 10 10 ms 1 ) but, like the endurance limit in fatigue, does not exist for all materials. In region I the rate of crack growth is controlled by the rate at which the metal dissolves and the time for which the metal surface is exposed. While anodic dissolution takes place on the exposed metal at the crack tip, cathodic reactions occur at the oxide film on the crack sides leading to the evolution of hydrogen which diffuses to the region of triaxial tensile stress and hydrogen-induced cracking. At higher stress intensities (region II) the strain-rate is higher, and then other processes become rate-controlling, such as Figure 12.10 Variation of crack growth rate with stress intensity during corrosion. diffusion of new reactants into the crack tip region. In hydrogen embrittlement this is probably the rate of hydrogen diffusion. The influence of a corrosive environment, even mildly oxidizing, in reducing the fatigue life has been briefly mentioned in Chapter 7. The S–N curve shows no tendency to level out, but falls to low S-values. The damage ratio (i.e. corrosion fatigue strength divided by the normal fatigue strength) in salt water environments is only about 0.5 for stainless steels and 0.2 for mild steel. The formation of intrustions and extrusions gives rise to fresh surface steps which form very active anodic sites in aqueous environments, analogous to the situation at the tip of a stress corrosion crack. This form of fatigue is influenced by those factors affecting normal fatigue but, in addition, involves electro-chemical factors. It is normally reduced by plating, cladding and painting but difficulties may arise in localizing the attack to a small number of sites, since the surface is continually being deformed. Anodic inhibitors may also reduce the corrosion fatigue but their use is more limited than in the absence of fatigue because of the probability of incomplete inhibition leading to increased corrosion. Fretting corrosion, caused by two surfaces rubbing together, is associated with fatigue failure. The oxidation and corrosion product is continually removed, so that the problem must be tackled by improving the mechanical linkage of moving parts and by the effective use of lubricants. With corrosion fatigue, the fracture mechanics threshold K th is reduced and the rate of crack propagation is usually increased by a factor of two or so. Much larger increases in crack growth rate are produced, however, in low-frequency cycling when stress-corrosion fatigue effects become important. 12.3 Surface engineering 12.3.1 The coating and modification of surfaces The action of the new methods for coating or modifying material surfaces, such as vapour deposition and beam bombardment, can be highly specific and energy- efficient. They allow great flexibility in controlling the chemical composition and physical structure of surfaces and many materials which resisted conventional treatments can now be processed. Grain size and the degree of crystalline perfection can be varied over a wide range and beneficial changes in properties produced. The new techniques often eliminate the need for the random diffusion of atoms so that temperatures can be relatively low and processing times short. Scientifically, they are intriguing because their nature makes it possible to bypass thermodynamic restrictions on alloying and to form unorthodox solid solutions and new types of metastable phase. 388 Modern Physical Metallurgy and Materials Engineering Table 12.3 Methods of coating and modifying surfaces (after R. F. Bunshah, 1984; by permission of Marcel Dekker) Atomistic deposition Particulate deposition Bulk coatings Surface modification Electrolytic environment Electroplating Electroless plating Fused salt electrolysis Chemical displacement Vacuum environment Vacuum evaporation Ion beam deposition Molecular beam epitaxy Plasma environment Sputter deposition Activated reactive evaporation Plasma polymerization Ion plating Chemical vapour environment Chemical vapour deposition Reduction Decomposition Plasma enhanced Spray pyrolysis Thermal spraying Plasma-spraying Detonation-gun Flame-spraying Fusion coatings Thick film ink Enamelling Electrophoretic Impact plating Wetting processes Painting Dip coating Electrostatic spraying Printing Spin coating Cladding Explosive Roll bonding Overlaying Weld coating Liquid phase epitaxy Chemical conversion Electrolytic Anodizing (oxide) Fused salts Chemical-liquid Chemical-vapour Thermal Plasma Leaching Mechanical Shot-peening Thermal Surface enrichment Diffusion from bulk Sputtering Ion implantation Laser processing The number and diversity of methods for coating or modifying surfaces makes general classification diffi- cult. For instance, the energies required by the various processes extend over some five orders of magnitude. Illustrating this point, sputtered atoms have a low thermal energy (<1 eV) whereas the energy of an ion beam can be >100 keV. A useful introductory classification of methods for coating and modifying material surfaces appears in Table 12.3, which takes some account of the different forms of mass trans- fer. The first column refers to coatings formed from atoms and ions (e.g. vapour deposition). The second column refers to coatings formed from liquid droplets or small particles. A third category refers to the direct application of coating material in quantity (e.g. paint). Finally, there are methods for the near-surface modification of materials by chemical, mechanical and thermal means and by bombardment (e.g. ion implantation, laser processing). Some of the methods that utilize deposition from a vapour phase or direct bombardment with particles, ions or radiation will be outlined: it will be apparent that each of the processes discussed has three stages: (1) a source provides the coating or modifying specie, (2) this specie is transported from source to substrate and (3) the specie penetrates and modifies the substrate or forms an overlay. Each stage is, to a great extent, independent of the other two stages, tending to give each process an individual versatility. 12.3.2 Surface coating by vapour deposition 12.3.2.1 Chemical vapour deposition In the chemical vapour deposition (CVD) process a coating of metal, alloy or refractory compound is produced by chemical reaction between vapour and a carrier gas at or near the heated surface of a substrate (Figures 12.11a and 12.11b). CVD is not a ‘line-of- sight’ process and can coat complex shapes uniformly, having good ‘throwing power’. 1 Typical CVD reactions for depositing boron nitride and titanium carbide, respectively, are: BCl 3g C NH 3g 500–1500 ° C ! BN s C 3HCl g TiCl 4g C CH 4g 800–1000 ° C ! TiC s C 4HCl g It will be noted that the substrate temperatures, which control the rate of deposition, are relatively high. Accordingly, although CVD is suitable for coating a refractory compound, like cobalt-bonded tungsten carbide, it will soften a hardened and tempered high- speed tool steel, making it necessary to repeat the high-temperature heat-treatment. In one variant of the process (PACVD) deposition is plasma-assisted by a plate located above the substrate which is charged with a radio-frequency bias voltage. The resulting plasma zone influences the structure of the coating. PACVD is used to produce ceramic coatings (SiC, Si 3 N 4 ) but the substrate temperature of 650 ° C (minimum) is still too high for heat-treated alloy steels. The maximum coating thickness produced economically by CVD and PACVD is about 100 µm. 12.3.2.2 Physical vapour deposition Although there are numerous versions of the physical vapour deposition (PVD) process, their basic design is 1 The term ‘throwing power’ conventionally refers to the ability of an electroplating solution to deposit metal uniformly on a cathode of irregular shape. Corrosion and surface engineering 389 Figure 12.11 Experimental CVD reactors (from Bunshah, 1984; by permission of Marcel Dekker). Figure 12.12 (a) Evaporation-dependent and (b) sputter-dependent PVD (from Barrell and Rickerby, Aug 1989, pp. 468–73; by permission of the Institute of Materials). either evaporation- or sputter-dependent. In the former case, the source material is heated by high-energy beam (electron, ion, laser), resistance, induction, etc. in a vacuum chamber (Figure 12.12a). The rate of evaporation depends upon the vapour pressure of the source and the chamber pressure. Metals vaporize at a reasonable rate if their vapour pressure exceeds 1Nm 2 and the chamber pressure is below 10 3 N m 2 . The evaporant atoms travel towards the substrate (component), essentially following lines-of-sight. When sputtering is used in PVD (Figure 12.12b), a cathode source operates under an applied potential of up to 5 kV (direct-current or radio-frequency) in an atmosphere of inert gas (Ar). The vacuum is ‘softer’, with a chamber pressure of 1–10 2 Nm 2 .As positive argon ions bombard the target, momentum is 390 Modern Physical Metallurgy and Materials Engineering transferred and the ejected target atoms form a coating on the substrate. The ‘throwing power’ of sputter- dependent PVD is good and coating thicknesses are uniform. The process benefits from the fact that the sputtering yield (Y) values for metals are fairly similar. (Y is the average number of target atoms ejected from the surface per incident ion, as determined exper- imentally.) In contrast, with an evaporation source, for a given temperature, the rates of vaporization can differ by several orders of magnitude. As in CVD, the temperature of the substrate is of special significance. In PVD, this temperature can be as low as 200–400 ° C, making it possible to apply the method to cutting and metal-forming tools of hardened steel. A titanium nitride (TiN) coating, <5 µm thick, can enhance tool life considerably (e.g. twist drills). TiN is extremely hard (2400 HV), has a low coefficient of friction and a very smooth surface tex- ture. TiN coatings can also be applied to non-ferrous alloys and cobalt-bonded tungsten carbide. Experience with the design of a TiN-coated steel has demonstrated that the coating/substrate system must be considered as a working whole. A sound overlay of wear-resistant material on a tough material may fail prematurely if working stresses cause plastic deformation of the supporting substrate. For this reason, and in accor- dance with the newly-emerging principles of surface engineering, it has been recommended that steel surfaces should be strengthened by nitriding before a TiN coating is applied by PVD. Two important modifications of the PVD process are plasma-assisted physical vapour deposition (PAPVD) and magnetron sputtering. In PAPVD, also known as ‘ion plating’, deposition in a ‘soft’ vacuum is assisted by bombardment with ions. This effect is produced by applying a negative potential of 2–5 kV to the substrate. PAPVD is a hybrid of the evaporation- and sputter-dependent forms of PVD. Strong bonding of the PAPVD coating to the substrate requires the latter to be free from contamination. Accordingly, in a critical preliminary stage, the substrate is cleansed by bombardment with positive ions. The source is then energized and metal vapour is allowed into the chamber. In the basic magnetron-assisted version of sputter- dependent PVD, a magnetic field is used to form a dense plasma close to the target. The magnetron, an array of permanent magnets or electromagnets, is attached to the rear of the target (water-cooled) with its north and south poles arranged to produce a magnetic field at right angles to the electric field between the target and substrate (Figure 12.13a). This magnetic field confines electrons close to the target surface, increases the rate of ionization and produces a much denser plasma. The improved ionization efficiency allows a lower chamber pressure to be used; sputtered target atoms then become less likely to be scattered by gas molecules. The net effect is to improve the rate of deposition at the substrate. Normally the region of dense plasma only extends up to about 6 cm from the target surface. Development of unbalanced magnetron systems (Figure 12.13b) has enabled the depth of the dense plasma zone to be extended so that the substrate itself is subjected to ion bombardment. These energetic ions modify the chemical and physical properties of the deposit. (In one of the various unbalanced magnetron configurations, a ring of strong rare-earth magnetic poles surrounds a weak central magnetic pole.) This larger plasma zone can accommodate large complex workpieces and rapidly forms dense, non-columnar coatings of metals or alloys. Target/substrate separation distances up to 20 cm have been achieved with unbalanced magnetron systems. Figure 12.13 Comparison of plasma confinement in conventional and unbalanced magnetrons (PVD) (from Kelly, Arnell and Ahmed, March 1993, pp. 161–5; by permission of the Institute of Materials). [...]... gap Materials World, May, p 259, Institute of Materials Helson, J E F A and J¨ rgan Brime, H (eds) (1998) Metu als as Biomaterials John Wiley and Sons Ltd Ratner, B (ed.) (1996) Biomaterials Science Academic Press, New York, USA Vincent, J (1990) Materials technology from nature Metals and Materials, June, p 395, Institute of Materials Williams, D F (1991) Materials for surgical implants Metals and Materials, ... of its biocompatibility and ease of manipulation and fixing Polymers, particularly silicones and polyurethanes, may be used to replace flexible tissues of the nose, cheek and ear regions of the face Polysiloxanes have been used for onlays in the area of the molar bone in 402 Modern Physical Metallurgy and Materials Engineering the lateral side of the mandible or over the forehead to smooth it Reinforced... by the use of wood, and tennis boomed In the 1970s, ‘large-head’ rackets of different shape with a larger area of 105 in2 came into vogue The greater stiffness and strength enabled string tensions to be raised and the effective playing area, the so-called ‘sweet spot’, was considerably increased The ‘sweet spot’ is the central portion 408 Modern Physical Metallurgy and Materials Engineering of the stringed... 10 11 12 13 14 15 16 17 18 19 20 Time in seconds Figure 14. 3 Shaft bending in typical swing of golf club (after Horwood, 1994) −1 × 10 410 Modern Physical Metallurgy and Materials Engineering standard testing procedures for shafts, club heads, etc to be established internationally 14. 4.2 Golf club shafts The principal design parameters for a shaft are weight, bending stiffness, bend point and torsional... techniques Industrial Materials Science and Engineering, Chapter 12 (L.E Murr, (ed.)) Marcel Dekker, New York Picraux, S T (1984) Surface modification of materials — ions, lasers and electron beams Industrial Materials Science and Engineering, Chapter 11 (L.E Murr, (ed.)) Marcel Dekker, New York Shreir, L L (1976) Corrosion, Vol 1 and 2, 2nd edn Newnes-Butterworth, London Trethewey, K R and Chamberlain,... 400 Modern Physical Metallurgy and Materials Engineering 100 THR Success TKR Success 90 Success of hip and knee replacements 80 70 60 50 40 30 20 10 0 0 5 10 15 20 25 Time in years Figure 13.6 The failure rate for a total hip replacement (THR) and total knee replacement (TKR) (courtesy R Grimer, Royal Orthopaedic Hospital, Birmingham) the movement of a metal ‘hinge’ but had a problem of fatigue and/ or... during the design and development stage As a preliminary, we have taken this opportunity to describe the structure and properties of wood Wood provides a benchmark for alternative materials in sport and many mimic its cellular structure In the more general context of engineering materials, wood remains extremely important in tonnage and particularly volumetric terms Wood has always been, and remains, the... students of science and engineering Longman, Harlow Chapter 13 Biomaterials 13.1 Introduction Biomaterials are materials used in medicine and dentistry that are intended to come in contact with living tissue The familiar tooth filling is where most humans first encounter biomaterials but increasingly many people now rely on more critical implants such as joint replacements, particularly hips, and cardiovascular... not, of course, particularly strong It is made up of layers including an outer epidermis and an inner dermis, a dense network of nerve and blood vessels It is therefore virtually impossible to make an artificial skin from biomaterials to match this complexity Nevertheless 404 Modern Physical Metallurgy and Materials Engineering skin replacements have been made from polymers with an open structure which... beam current above 5 mA so that process times can be shortened Currently, implantation requires several hours The ions 392 Modern Physical Metallurgy and Materials Engineering Figure 12.15 Coating by plasma-spray torch (from Weatherill and Gill, 1988; by permission of the Institute of Materials) may be derived from any element in the Periodic Table: they may be light (most frequently nitrogen) or heavy, . S 2 Figure 12.3 Reaction paths for oxidation and sulphidation of chromium. 382 Modern Physical Metallurgy and Materials Engineering may rise locally within the scale and far exceed that of the main gas. phase. 388 Modern Physical Metallurgy and Materials Engineering Table 12.3 Methods of coating and modifying surfaces (after R. F. Bunshah, 1984; by permission of Marcel Dekker) Atomistic deposition Particulate. is 390 Modern Physical Metallurgy and Materials Engineering transferred and the ejected target atoms form a coating on the substrate. The ‘throwing power’ of sputter- dependent PVD is good and coating

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