Surface Engineering of Metals - Principles, Equipment and Technologies Part 12 pot

Fig 5.18 Orientation values of thickness and hardness of some superficial diffusion layers In addition to surface hardness (the measurement of which was introduced to industry as late as the 20th century) it is important to know the hardness of structural elements of the particular zones of the superficial layer, e.g., grains and structural components, especially on cross-sections This last parameter, known as microhardness, came into use only after World War II [28] Fig 5.19 Hardness profile: a) Nitralloy 135M, hardened and tempered to 30±2 HRC; - glow discharge nitrided at 520°C for h; - implanted by nitrogen ions with energy of 100 keV and ion dose of 2·1017 ions of N2+ per cm2; - electron beam hardened with power density of 2230 kW/cm2 and exposure time 0.74·10-4 s; – laser hardned with power density of 1.4 kW/cm2 and exposure time of 0.13 s; b) 18HGT grade steel, gas nitrided at 530°C for 36 h © 1999 by CRC Press LLC Hardness (microhardness) is one of the most basic, universally accepted properties of materials, especially of metals and their alloys, easily measured by various methods, and connected with many other properties of the superficial layer, e.g., wear resistance, strength, residual stresses, plasticity Usually, the higher the stress loading to which the part is subjected, the higher should be the hardness of the surface Unfortunately, a rise in hardness is often connected with a rise in brittleness Hardness depends on the type of material and its structure which, in turn, depends on treatment, especially strain-hardening, heat and thermochemical treatment (Fig 5.19) The hardness of crystalline bodies depends on the limit of elasticity under compressive loading and on the modulus of elasticity The microhardness of superficial layer zones may change during service, especially during wear, as the result of microstructural changes caused by surface tempering, secondary hardening (grinding burns), the breakdown of residual austenite and other factors [21,32] 5.7.3.4 Brittleness Brittleness is a material property, consisting of permanent partition of material under the influence of internal or external forces The partition begins at the tip of the propagating crack and is formed without the presence of any significant plastic deformation Brittleness depends on the type of material, its phase composition, structure, etc and on external factors such as stress distribution, method of loading, temperature, chemical composition of the environment and others Usually, brittleness occurs in solids within certain temperature ranges [26] The majority of materials exhibit brittleness at ambient temperature (so-called cold shortness); others, as e.g., unalloyed open-poured steel, exhibit greater brittleness at elevated temperatures (so-called hot shortness) Metals may exhibit different types of brittleness, e.g., the already mentioned cold shortness and hot shortness, hydrogen embrittlement (caused by excessive diffusion of hydrogen into the metal), pickling embrittlement or embrittlement caused by electroplating of metal objects, temper embrittlement, blue brittleness, etc In the case of superficial layers and coatings, brittleness is an undesirable effect, e.g., brittleness of superficial layers after diffusion, caused by excessive concentration of saturating element, like nitrogen Often, although not always, brittleness is connected with hardness: the higher the hardness, the greater the brittleness of the layer A property opposite to brittleness is ductility - the susceptibility of metals to permanent plastic deformation without the formation of cracks Ductility is one of the basic characteristics of the metallic state Often the term “ductility” is used as a synonym of plasticity but it means a qualitative, non-measurable characteristic, strongly dependent on structure, processes occurring at the atomic level and on the type of slip Usually it is desired that hard but not brittle layers be formed over a ductile core [26, 27] © 1999 by CRC Press LLC 5.7.3.5 Residual stresses Types of residual stresses In all materials subjected to extraneous effects - be they mechanical, thermal, chemical or a combination of any or all of them there occur non-uniform volume changes, both reversible and irreversible, causing the formation of stresses Stresses describe the state of internal forces and moments of forces, brought about by the interaction, in a given locality, of two parts of the material, situated on either side of an apparent crosssection, the forces in question acting on a unit area of the cross-section After the removal of external effects, reversible changes (elastic deformations) undergo atrophy, along with stresses caused by them However, some irreversible changes (plastic deformation) remain in the material, along with stresses caused by them which are referred to as residual stresses [33] Residual stresses, in earlier times referred to as rest or final stresses, are those which are in mutual equilibrium within a certain zone of the material and which remain after the removal of external loading Depending on the zone where this equilibration occurs, the following types are distinguished: 1) according to the classification by E Orowan [34], two types of residual stresses include: – macrostresses - formed as the result of any external loading, and balanced out in the entire volume of the body They are regarded as the result of the joint, average interaction of microstresses A definition of this type assumes the material to be homogenous, i.e., having isotropic properties; – microstresses - formed as the result of heterogeneity of the material (blocks of grains, single grains), which usually generate a non-homogenous stress field, often connected with texture and therefore exhibiting preferred orientation (so-called stress-texture) [29]; 2) according to the classification by N.N Davidenkov [34, 35] three types of residual stresses are distinguished (Fig 5.20): – stresses of the I st kind, termed macrostresses (body stresses), caused by the mutual interaction of macroscopic-size zones of the material, balancing out within volumes of the same order of magnitude as the object, within the limits of the entire superficial layer, in zones of dimensions approximating those of the superficial layer or in major zones of the superficial layer (e.g., in a zone with a very big number of grains) They are formed when external effects in the form of, e.g., mechanical loading causes non-uniform plastic deformation or as the result of thermal effects, causing non-uniform expansion of neighboring macrozones For this reason, they were once referred to as thermal stresses The conservation of body continuity requires the formation, between such macrozones, of mutual interaction, tensile or compressive, which we call macrostresses [33] Macrostresses are caused directly by non-uniform plastic deformation, temperature changes, changes in the material structure or a combination © 1999 by CRC Press LLC times referred to as structural stresses Microstresses often constitute the result of the formation of a superficial layer Their chief source is different crystal orientation and the associated anisotropy of elastic and plastic properties of the various crystals Since after treatment (mainly deformation) the microstructure usually exhibits a definite texture, stresses also exhibit a preferred orientation, called stress texture Its final result is the anisotropy of the material’s properties Microstresses may be regarded as the result of total, average interaction of submicrostresses; – microstresses of the IIIrd kind, termed submicrostresses, balancing out within the space of one crystal, thus within zones corresponding to the crystal lattice parameters They are treated as stresses of the material’s crystal lattice, especially in zones with defects In such zones the proper structure is disrupted by the occurrence of own or foreign atoms in improper interstitial and nodal sites or the existence of voids Foreign atoms introduce into the lattice their stress fields, nodal voids cause the absence of stress fields to balance the fields from neighboring atoms Stress fields from foreign atoms in nodal sites also not balance out stress fields from neighboring atoms The energy of the lattice in the vicinity of a defect is in all cases higher than its minimum value corresponding to the state of equilibrium The result of that is the stress field around the defect The range of stress fields is small due to the small range of action of atomic forces and may reach several lattice spacings Stress fields around defects interact with atoms but only with the neighboring ones, upsetting them from their state of equilibrium [33, 35-37] If an atom of gas, e.g., hydrogen, is introduced by diffusion into the crystal lattice of steel, it generates around it compressive residual stresses of the III rd kind Next, as the result of desorption of gas molecules in the internal discontinuities of microstructure, very high pressures are generated in such sites, giving rise to compressive residual stresses of the II nd kind After the saturation of the superficial layer with this element it is usual that a gradient of its concentration will occur (and along with it a gradient of properties) The final result will be that residual stresses of the I st kind will be generated between layers or between the superficial layer and the core [38] In the superficial layer there exist three kinds of residual stresses; they are manifest predominantly as macrostresses Micro and submicrostresses affect the limit of elasticity of the material but have only a small influence on its strength They are added to stresses caused by external effects and for that reason they determine the moment of exceeding of the material’s strength, manifest by the formation of microcracks Submicrostresses may be the cause of high hardness and strength of metal alloys [33] Independently of the kind of stresses, the result of their action is the same - they always induce defects and elastic deformations of the crystal lattice Further on in this book the term “residual stresses” should be understood as residual stresses of the I st kind Each surface treatment in which the limit of elasticity is exceeded by any element of the superficial layer or core structure leaves behind a trail in the © 1999 by CRC Press LLC form of residual stresses, especially those of the I st kind In the majority of finished machine parts and structures there exist residual stresses left behind by treatment or assembly operations Residual stresses are characterized by their sign (“-” compressive and “+” tensile), their value, distribution, gradient and depth of penetration Factors causing the formation of residual stresses Such factors can usually be classified as being of three kinds: – mechanical, stemming from non-uniform plastic deformation of superficial layers during mechanical cold work They are accompanied by non-uniformly distributed and interconnected processes of force action, reorientation, refinement, expansion or contraction of structural components Macrodeformations give rise to reorientation of structural components in layers situated closer to the real surface relative to deeper situated zones Microdeformations, on the other hand, reveal themselves within the volumes of separate components, due to their refinement into fragments and blocks and to mutual elastic-plastic interaction of neighboring grains Resulting from that is local increase or decrease in material density, enhanced by the movement of dislocations, their distribution and kind [37] Plastic deformation due to cold work causes changes in material density (a rise in volume of approximately 0.3 to 0.8 [21]), conducive to the rise of compressive stresses Plastic stretching of the superficial layer by forces of friction and by machining chips also causes the formation of compressive stresses Residual stresses caused by mechanical factors are sometimes termed mechanical residual stresses; – thermal, caused by thermal expansion of the material and stemming from non-uniform heating or cooling of various layers of the material (macrodeformations) or of its particular fragments (microdeformations) During heating, especially if it is non-uniform, there occurs non-uniform thermal expansion causing plastic deformation which prevails all the way up to melting point In the liquid state, the volume of all metals (with the exception of bismuth and antimony) is smaller than in the solid state Fig 5.21 shows a diagram of the formation of residual stresses using water quenching of 100 mm dia heated steel bar as an example [39, 40] Upon heating, surface temperature is usually slightly lower than that of the core With progress of cooling time, the difference between surface temperature (curve S) and core temperature (curve C), in other words - the temperature gradient - rises The material of the superficial layer and of layers situated deeper diminishes in volume with the progress of the cooling process, shrinking (linear changes of approximately 0.5%), causing the formation of tensile stresses (curve 1) At the same time compression of the still hot core, gives rise to compressive stresses there (curve 3) The temperature gradient between surface and core rises until it reaches point M The maximum temperature difference (approximately 600 K) corresponds to maximum tensile stresses at the surface and maximum compressive © 1999 by CRC Press LLC panied by a simultaneous process of stress formation The stresses are compressive if the specific volume is increased and tensile if decreased In turn, all volumetric changes within the volume of a given component are accompanied by changes in neighboring zones [37] Greatest residual stresses are formed during hardening, caused by the transformation of austenite to martensite which proceeds at a very high linear rate (in ferrous alloys the rate of growth of martensite nuclei is approximately 33% that of the speed of sound in a crystal) Martensitic transformation in the heated material occurs as the result of quenching at a known rate of heat extraction, highest at the surface, causing a volumetric increase in the superficial layer When the carbon content in martensite is 1%, volume increase of martensite relative to austenite is approximately 4% In the slower cooled core, martensitic transformation is retarded The core is subjected to stretching, causing compressive stresses at the surface Next, the onset of martensitic transformation in the core causes the stretching of the outer layers which were hardened earlier and, in consequence, the compression of the core Changes in specific volume which are due to structural transformations are greater than those brought about by thermal expansion Stresses caused by these factors are termed structural residual stresses Other examples of external forces causing the formation of residual stresses with varied value and range of action may be, besides pressure (mechanical stresses) and temperature (thermal and structural stresses), chemical interaction (e.g., formation of chemical compounds by atoms introduced through diffusion and substrate atoms) and physico-chemical (e.g., implantation with the formation of chemical compounds) Through the change of chemical composition, such interaction causes changes in the specific volume of the material or in the coefficient of thermal expansion As an example, the saturation of iron and its alloys with nitrogen increases volume and decreases the thermal expansion coefficient of the saturated layer relative to that of the core which causes compressive stresses to be set up in the layer and tensile stresses in the core Usually, residual stresses are formed as the result of joint interaction of several forces (causes) and their separation is usually difficult For example, during hardening, when the effects of thermal and structural stress formation overlap, structural stresses tend to either raise or diminish thermal stresses, depending on the size and shape of the element’s crosssection plane, rate of heat extraction and steel hardenability Tying in the above to point U in Fig 5.21 [40] the following can be stated: – structural stresses raise thermal stresses if they are formed in the core before and in the superficial layer after reaching point U and vice versa; – structural stresses across the entire cross-section or after passing through point U counteract thermal stresses; – greatest compressive stresses in the superficial layer and tensile in the core are formed when transformation in the core occurs before and in the superficial layer after passing through point U © 1999 by CRC Press LLC When, after removing the external forces, residual stresses prove to be only slightly less than the material’s strength, the material may deform, warp, suffer delamination or exfoliation If they prove to be greater, the material will crack Residual stresses are superimposed on operating stresses, induced by external forces (see Fig 5.44) – They can be added to them, resulting in the material being destroyed already under operating stresses, lower than material strength, sometimes under quite small loads Residual stresses can also cause the material to crack spontaneously [37]; it is said that residual stresses reduce material strength In the superficial layer, these are usually tensile stresses – They may be subtracted from operating stresses, resulting in destruction of the material only when operating stresses exceed the material’s strength; it is then said that residual stresses raise material strength In the superficial layer these are usually compressive stresses Residual stresses are formed in the superficial layer and in the core Usually, the value of residual stresses is greatest in the superficial layer and, the greatest stress gradients are located there, especially at the interface between the superficial layer and core (Fig 5.22) Residual stresses in the superficial layer usually occur in zones of texture, plastic deformation, and elastic deformation, but it is in the textured zone that they assume their highest values Their distribution and value depend on the type of material and its three-dimensional and metallographic structure, on strength and thermal characteristics, on external factors (e.g., rate of heat extraction) and on the associated strain-hardening of the superficial layer, as well as on wear resistance General functional expression of residual stresses In the broadest sense, residual stresses σ w may be expressed by an implicit function of the most important, mutually interacting parameters in the form below: σw = f (m, t, k, o) (5.16) where: m = f1 (c, w, f, ch, s) - is the function of the primary material (core, superficial layer, coating), described mainly by its properties: c - thermal (especially: thermal conductivity, thermal expansion, specific heat); w - mechanical (especially strength: Young’s modulus, Poisson ratio); f - physical (e.g., ion implantation); ch - chemical (especially: chemical composition, formation of chemical compounds of diffusing atoms with substrate atoms); s - structural (especially: roughness and valley bottom radius) and metallographic (especially grain type, size and orientation, defects); t - technology of formation of superficial layer or coating (type, number, sequence and parameters of treatment operations; temperature, temperature variation rate, temperature gradient, pressure, loading, feed rate, energy, element concentration, etc.); k - shape and size of component in which residual stresses are measured; o - interaction of core with superficial layer or coating © 1999 by CRC Press LLC Fig 5.23 Distribution of residual stresses, resulting from: a) diffusion chromizing of D2 grade steel; b) TiC coating of D2 steel; c - boriding of 1045 steel; designations: B boriding; Cr - chromizing; Ti - TiC treatment; H - hardening; T - tempering (From Janowski, S [41] With permission.) In the absolute sense, a given value of residual stresses when all other parameters are equal depends heavily on the method of measurement Numerical values of residual stresses, obtained by different measurement methods, may differ by several to several tens percent In certain cases © 1999 by CRC Press LLC differences exceeding 100% and even results with opposite signs may be obtained [41, 42] Residual stresses in a superficial layer directly affect the layer’s cohesion but their action may also be of an indirect nature - by forcing the migration of atoms with small diameters (e.g., hydrogen, carbon, nitrogen, boron) through the crystal lattice of the host material The force exerted by stress gradient on an atom in an interstitial position is, admittedly, not big in comparison with the force exerted by a concentration or temperature gradient However, local stresses may cause migrations of interstitial atoms to sites preferred by geometry or thermodynamics (vacancy clusters, dislocation lines, grain boundaries and stacking faults) causing significant local stresses, favoring the initiation of cracks [38] When knowingly shaping the properties of the superficial layer, it is endeavored to obtain, as the final result, compressive residual stresses in the superficial layer, while in the core - tensile residual stresses with a small gradient Compressive stresses in the superficial layer may even attain a value equal to approximately 50% of the material’s ultimate strength [37] The value of compressive residual stresses obtained as the result of surface diffusion treatments may even reach 2400 MPa (Fig 5.23) [41] As an example, the value of compressive stresses in nitrided layers on low alloy nitriding steels and on high alloy structural steels may reach 900 MPa [38] In the case of mechanical strain hardening, the depth of penetration of stresses is usually greater than the depth of hardening even by several tens percent With a rise of stress value at the surface, the depth of their penetration diminishes [37] The value of residual stresses rises when mechanical strain hardening is coupled with heat treatment of thermo-chemical treatment (Fig 5.24) Generally, with a rise in the strength of the mechanically strain-hardened material and in the strain-hardening parameters (mainly, the loading force), residual stresses in the superficial layer increase Their value, depth of penetration and character of distribution may all be controlled by treatment operation parameters In almost all cases the formation of compressive stresses in the superficial layer causes a rise of fatigue strength (with tensile stresses the effect is opposite) and hardness, wear resistance and corrosion resistance A greater degree of plastic deformation causes an increase in residual stresses and in fatigue strength Regardless of the root cause of formation of residual stresses, their value and distribution affect strength properties, especially fatigue strength, resistance to dynamic loading and to brittle cracking (see Section 5.8.1), as well as tribological properties, especially contact fatigue (see Section 5.8.2) [42] A particularly significant effect of residual stresses on mechanical properties, especially fatigue, is revealed in the case of superficial layers containing technological or structural flaws, surrounded by stress concentrations © 1999 by CRC Press LLC In surface shaping treatment processes the following types of technological residual stresses are formed: – quenching stresses, caused by volumetric changes due to predominantly phase transformations but also to heating and cooling, – casting stresses, caused by solidification and cooling, – welding stresses, caused by phase transformations and thermal expansion In all superficial layer shaping treatment operations, the character and value of technological residual stresses change during the technological process (see Fig 5.10) and from process to process [13] in the following manner: – at first, the superficial layer contains only primary (initial) residual stresses, created during the previous treatment operation (in the steelmaking process, forging, casting, cold forming or heat treatment) and being the net result of a superimposition of effects which had occurred prior to the considered operation; – under the influence of the treatment operation considered, technological residual stresses are created which, when added to initial stresses, become resultant stresses; – resultant stresses of the considered treatment operation constitute, at the same time, the initial stresses for subsequent treatment operation Technological residual stresses not constitute a value which is constant in time or for any location Under the influence of external forces occurring during storage or service, technological stresses become service stresses and their value and distribution change, due to processes of relaxation and redistribution (Fig 5.25) Fig 5.25 Redistribution and relaxation of residual stresses during service: a) in 1045 steel, induction hardened and subjected to fatigue testing (From Janowski S [42] With permission.); and b) structural steel, subjected to wear testing (From Svecev, V.D [43] With permission.); - before test; - after test © 1999 by CRC Press LLC Fig 5.26 Adsorption at solid/gas; solid line - profile of substance concentration (i) vs distance from physically pure solid surface; dashed line - profile of substance concentration vs distance from solid surface in reference system; surface concentration excess ni is represented by the shaded area (From Oœcik, J [45] With permission.) surface forces for a certain time, dependent on the character of the adsorbate and adsorbent, on temperature and pressure, and finally leaves that surface or is desorbed Commensurate with the saturation of the surface, the rate of adsorption decreases while the rate of desorption increases When both rates are equal, desorption equilibrium is set Molecules of the adsorbate at the surface of the adsorbent form adsorption layers We distinguish positive adsorption when the concentration of the substance is greater in the superficial layer than in the deeper phase, and negative adsorption when the concentration in the superficial layer is less than in the deeper phase In most cases, positive adsorption of gases, vapors and dissolved substances occurs at solid surfaces The molecules of a very volatile phase (adsorbate) are then subjected to spontaneous densification in the thin layer at the surface of the very condensed phase (adsorbent) Fig 5.26 shows the profile of gas concentration at the interface with a solid, vs distance z from the physically pure surface The area covered between points BC and E expresses the surface excess (in concentration) of the adsorbed gas substance, relative to the reference concentration of the gas phase © 1999 by CRC Press LLC The surface excess n i of the adsorbed gas substance i (or volumetric excess), which is the surface (or volumetric) concentration, expresses the excess in the number of moles of that substance in comparison with the number of moles which would be present in a reference system without adsorption, given the same equilibrium pressure (5.17) adsorption space surface adsorbent layer where nia - number of moles of substance i in field FBDH; nig - number of moles of substance i in field FEDH; nip - number of moles of substance in field ABF; Cia - local concentration of substance i in the adsorption space; Cig - local concentration of substance i in the gas phase; Cip - local concentration of substance i in the superficial layer of the adsorbent; V - local volume of adsorption space; V - local volume of superficial layer of adsorbent Due to the very small depth of permeation of the adsorbate into the adsorbent, the quantity n ip (or C ip ) is sufficiently small to be neglected in expression (5.17) With this assumption, the quantity n i corresponds to the total amount of substance i (adsorbate) remaining within the field of adsorbent forces Fig 5.27 Types of adsorption isotherms of gases and vapors, according to Brunauer; ni - total amount of adsorbed substance i; p - pressure; po - pressure of saturated gas Type I - typical curve for chemical adsorption, less frequent for physical adsorption; types II to V - various curves for physical adsorption; the most frequent is type II, least frequent - type V The amount of a substance adsorbed by the superficial layer depends on its pressure and on temperature For a gas mixture, the partial pressure © 1999 by CRC Press LLC of the given substance is taken into consideration At fixed pressure (p = const), the amount of adsorbed substance (gas, vapours) is only a function of temperature and usually decreases with its rise For constant temperature (T = const) the amount of adsorbed substance, expressed by the so-called adsorption isotherms, depends only on pressure and increases with its rise (Fig 5.27) [45, 46, 48] Naturally, the amount of adsorbed substance depends on the material of the superficial layer (adsorbent) and the type (structure) of the adsorbate, as well as on conditions of adsorption (p, T), increasing with an increase of the adsorbent surface The higher the molecular mass of the adsorbate and the higher the condensation temperature, the easier it is adsorbed Usually, gases and vapors are adsorbed in amounts which grow with the temperature of the boiling point For example, the volume of ammonia (113.4 cm 2/g) adsorbed by the surface of charcoal at room temperature is close to 40 times greater than that of hydrogen Adsorbed to an even greater degree than gases are vapours of substances which are in the liquid state at room temperature, e.g., gasoline, ether, alcohol, etc When the surface is reached by molecules of a substance which is adsorbed stronger than the considered molecules, the adsorption of the latter is reduced The process of surface binding of the adsorbate may be divided into three groups, mainly from the point of view of forces acting between the adsorbent and the adsorbate Physical adsorption (also termed: molecular, sur face, specific or physisorption) consists of densification of a substance at the surface of the adsorbent under the influence of intermolecular forces of attraction, socalled Van der Waals forces The character of these forces is the same as in intermolecular interaction in gases, liquids and in solids These are forces induced by resonant vibrations of electrons in molecules coming into close proximity (so-called electro-kinetic or dispersion forces) and electrostatic forces associated with the presence of electrical dipoles in molecules of the adsorbate (so-called polar molecules), quadrupoles or, generally, multipoles, caused by a non-uniform distribution of electron density in molecules In the case of an apolar adsorbent, it is mainly the action of forces of dispersive attraction; in the case of a polar adsorbent, the multipoles of the adsorbate molecules are additionally attracted by an electrostatic field which enhances the adsorption of these molecules This is especially true if the surface contains ions of the same sign or dipoles of same orientation Adsorbed molecules cause a reduction in surface energy, as a result of which a certain amount of energy, called heat of adsorption, is exchanged with the environment It assumes a value of the order of heat of evaporation of the adsorbate and usually is contained within the limits of 40 kJ/mole Physical adsorption is thus an exothermic process Physical adsorption usually occurs instantaneously if not hampered by side effects (e.g., slow diffusion of the adsorbate to the surface or its slow © 1999 by CRC Press LLC permeation into pores within the adsorbent) It is a dynamic and reversible process which means that molecules of the adsorbate are not permanently connected to the surface of the adsorbent but are in a state of constant exchange with molecules of the gas phase During adsorption equilibrium, the number of molecules settling down on the surface is equal to the number of molecules passing to the gas phase in the same time As a result, the number of molecules at the surface remains constant Adsorbed molecules of the adsorbate maintain their individual characteristics The energy of the superficial layer plays a significant role in the phenomenon of adsorption Good adsorption properties will be featured by an adsorbent with high surface energy (e.g., resulting from an induced state of stress), as well as with a high surface to mass ratio It is therefore obvious that with a rise of the surface, e.g., due to refinement of molecules forming it, the active surface also rises and so does the intensity of adsorption [9] The effectiveness of physical adsorption increases with the lowering of temperature approaching the temperature of condensation of the adsorbed gas On the other hand, a rise of temperature causes a decrease in the intensity of adsorption, unless this temperature rise causes effects of chemical activation and the associated presence of stronger chemical bonds Further, with a rise of pressure, the amount of adsorbed substance rises out of proportion to the former and the higher the former, the slower the latter (see Fig 5.27, curves II to V) Starting from a certain boundary pressure, sometimes difficult to determine, further rise in pressure does not affect the amount of the adsorbed substance The adsorbent appears as if it were saturated (see Fig 5.27, curve I) This case occurs seldom in physical adsorption but takes place mainly in chemical adsorption The final mass of the adsorbed adsorbate is less in the case of chemical than in physical adsorption In all cases (curves I to V) the effect of pressure on the amount of adsorbed substance is particularly big in the zone of low temperatures and pressures Physical adsorption - as was noted - is a reversible process and the adsorbate may be removed, e.g., by lowering the pressure It is reclaimed in a condition that is chemically unchanged Taking into account the very small value of the activation energy, of the order of kJ/mole, physical adsorption is a process that is very fast even at very low temperatures [39], in particular on smooth surfaces The thickness of physically adsorbed layers corresponds to several molecule diameters of the adsorbate [40] Condensation adsorption (also called capillary) consists of such a high densification of gases and vapors of the adsorbent that after covering the surface with a monomolecular layer they undergo condensation to the liquid state This type of adsorption takes a somewhat longer time than the physical, it is partially reversible, i.e., the desorption curve differs from that of adsorption (this is the so-called sorption hysteresis), it may be treated as a version of physical adsorption Its course is plotted by isotherms of the type depicted by curves IV and V in Fig 5.27 Maxi- © 1999 by CRC Press LLC mum adsorption occurs when pressure p is lower than the pressure of saturated vapor p o Chemical adsorption (chemisorption) is also often called activated adsorption because it calls for a much higher activation energy than physical adsorption and is of the order of 20 to 80 kJ/mole Forces binding molecules of the adsorbate with surface molecules of the adsorbent are significantly greater but with a shorter range of effectiveness These are forces of chemical bonds For that reason the value of heat of chemical adsorption is significantly higher than the heat of physical adsorption It is of the order of 30 to several hundred kJ/mole, thus of the same order as the heat of chemical reaction It is usually an irreversible process Gas, once chemically adsorbed, is very difficult to remove If it undergoes desorption it usually changes its chemical state For example, oxygen adsorbed on the surface of charcoal at room temperature is so strongly bound that it is released in the form of carbon dioxide Chemical adsorption proceeds slowly, especially at low temperatures, and its rate rises with temperature, similarly to the rate of chemical reactions Kinetics indicates the presence of energy of thermal activation Chemical adsorption is limited to a monomolecular superficial layer Additional amounts of gases or vapours may be adsorbed physically in the second and subsequent layers over the monomolecular, chemisorbed first layer There is no sharp dividing line between physical and chemical adsorption, although extreme case may be unequivocally distinguished This constitutes proof that usually chemical adsorption is the next phase of physical adsorption which cannot take place in the presence of additional energy, enabling a closer approach of atoms (molecules) of gases and vapours to those of the surface Thus, considering the phenomenon of adsorption of nitrogen in iron it has been determined that at temperatures up to 200ºC nitrogen is adsorbed physically and above 200ºC chemically [39] New bonds created as the result of chemical adsorption at the surface of a metal are always to some degree polarized, due to the difference in electronegativeness between atoms forming them This causes an insignificant increase or decrease in the concentration of conducting electrons in the metal which may be detected by a measurement of changes in electrical conductivity Physical adsorption does not bring about such electrical effects [40] Chemical adsorption, to a degree greater than physical, depends on surface condition, i.e., on its structure and method of preparation It should be remembered that the entire surface is not homogenous as regards energy For this reason, the concept of active centers has been introduced in which adsorption takes place (see Section 5.7.3.11) The role of active centers - characterized by higher surface energy - is taken by areas with high free energy, particularly all defects of the crystal structure, atoms situated at edges and nodes of crystals They exhibit highest adsorption energy During chemical adsorption, when additional energy appears, enabling an even closer approach of gas atoms to those of the surface, a chemical © 1999 by CRC Press LLC reaction may take place where a surface atom that joins with an atom (molecule) of the gas is “extracted” from the substrate structure and creates a new chemical compound [45] In those cases, surface chemical bonds of the adsorbate with the adsorbent are created A chemisorbed molecule at the surface may undergo deformation, chemical bonds may be relaxed or even totally severed, with the formation of free atoms and radicals which takes place in the process of gas nitriding in an ammonia atmosphere: 2NH3 ♦ 2N + 3H2 [9] When a molecule of gaseous adsorbate undergoes dissociation into component atoms or radicals which, in turn, undergo adsorption, a process of this type is called dissociation chemisorption [46] Dissociation chemisorption of gases on transition metals is a non-activated process and, consequently, it is determined by thermodynamics and not by its kinetics There are, however, exceptions, e.g., a small amount of activation energy is necessary in the case of chemisorption of nitrogen on the surface of steel Transition metals are particularly active in chemisorption [41] A chemisorbed molecule is more chemically active than the nonadsorbed molecule For example, the nascent nitrogen released during the dissociation of NH3, whose lifespan is to 1.5 s, undergoes chemisorption at the metal surface and later diffusion during the nitriding process [44] The heat of binding of atomic nitrogen is close to twice that of molecular nitrogen At the surface of tungsten its value is 646.4 kJ/mole Chemical adsorption may be treated as a chemical reaction between molecules of the adsorbate with atoms of the superficial layer of the metal [32] Energy of chemisorption bonding has a value close to that of the energy of chemical bonding in free molecules For example, the heat of chemisorption of carbon monoxide on the surface of transition metals is 170 to 350 kJ/mole [9] Fig 5.28 Potential energy vs distance of adsorbate molecule from metal surface © 1999 by CRC Press LLC Fig 5.29 Potential energy curve in plane perpendicular to ideal metal surface; Ea,m activation energy of migration of adsorbed particle from site A to unoccupied adjacent site B; Em