Surface Engineering of Metals - Principles, Equipment and Technologies Part 13 pptx

Aboundary case of adhesion is the occurrence of chemisorption bonding atthe interface with the formation of a superficial layer, constituting a chemi-cal compound [58].. The wetting angl

From expression (5.22) it follows that the path of diffusion a = 2(τD) [40].

Phenomenological diffusion equations Complex phenomena of

diffu-sion may be considered with the help of thermodynamic descriptions, rived from the action of forces connected with heterogeneity of concentration

de-of system components The most general form de-of the thermodynamic tion for diffusion of the first component is the following [49]:

(5.24)

where: J 1 - diffusion flux of first component; m 1 , m 2 , m n - chemical potential

of first, second, n-th component; M 11 , M 12 , M 1n - kinetic constants

describ-ing, in sequence, the diffusion of first, second and n-th component, M 1T ,

M 1p , M 1 Φ - kinetic constants, describing, in sequence, the diffusion of the first component in the presence of a gradient of temperature T, pressure p

or other potential Φ (e.g., concentration)

Phenomenological equation (5.24) may be written in an analogousmanner for each component of the thermodynamic system

If diffusion occurs in isothermal (T = const), isobaric (p = const) or

isopotential (Φ = const) conditions, the corresponding components ofthe phenomenological equations (5.24) will equal 0 and the flux of diffu-sion will depend only on the gradients of chemical potentials [49]

It follows from these equations that for multi-component systems,the driving force of diffusion is not concentration gradients but chemi-cal potential gradients In multi-component systems, the componentsmutually interact to affect chemical potentials It may, therefore, hap-

pen that the concentration gradient dc/dx = 0 but the gradient of chemical

potential dm/dx  0 The phenomenon of uphill diffusion will then

take place, i.e., from places of lower to places of higher concentration.This occurs, e.g., during diffusion chromizing of alloyed tool steels,containing tungsten and vanadium, resulting in a higher concentra-tion of these elements in the superficial layer than in the core This, inturn, has a benign effect on the properties of the core and of the diffu-sion layer [47]

A second conclusion from the phenomenological equation is the bility of diffusion, drivenonly by a temperature gradient, i.e., thermodif-

possi-fusion (also known under the name of Soret’s effect) Besides, what has

been experimentally proven, atoms may diffuse with the temperaturegradient (e.g., carbon, nitrogen or zinc in iron) or against it (e.g., hydro-gen in iron) The result of thermodiffusion is self-diffusion of vacancies,with the temperature gradient (in platinum) or against it (in gold) Thephenomenon of thermodiffusion may be of major significance in cases ofthese surface treatments where big temperature gradients occur, with thetime of their duration defined, as in e.g., thermal spraying or pad weld-ing When the time of existence of a temperature gradient is very short,

1= − 1 1 µ1− 1 2 µ2− −K 1 µ − 1 − 1 − 1Φ Φ

thermodiffusion is negligible (e.g., in pulse laser or electron beam ing) [26, 49, 52, 55, 57].

heat-A third conclusion from the phenomenological equation (5.24) is the sibility of diffusion, driven by a pressure gradient or - to put in other words

pos under the influence of a stress gradient The direction in which atoms difpos fuse will cause a lessening of residual stresses: interstitial atoms will diffusefrom compressed zones to those subjected to tensile stresses, while vacancieswill be displaced in the opposite direction This effect is the cause of internalfriction In the case of a solution composed of atoms of different sizes, thebigger atoms will migrate to the zone under tensile stresses, while the smalleratoms to the zone with compression These effects may occur during burnish-ing, thermal (and explosive) spray, ion implantation and induction harden-ing [49, 52]

dif-Finally, the phenomenological equation gives rise to the conclusionthat there is a possibility of diffusion, forced by a variable magnetic field,

in other words, by a gradient of the magnetic potential, or by the flow of astrong electrical current (electrical potential gradient) In the first case,the magnetostrictive effect causes volumetric changes of the material andthese, in turn, force the movement of interstitial atoms In the secondcase, during self-diffusion, atoms have a tendency to migrate in the direc-tion of the anode, while vacancies migrate toward the cathode Interstitialatoms in the majority of metals migrate in the direction of the cathode Theseprocesses have been observed in induction hardening, without, however, anysignificant effect on hardening results [49, 52]

Diffusion plays the following roles:

– primary, in the case of medium and high temperature treatments oflong duration (temperatures above 500ºC; times, several hours and longer).Examples: thermo-chemical treatments, CVD, dip metallizing

– secondary, in the case of short processes at low temperatures (e.g., PVD)

or at high temperatures (e.g., thermal spraying, pad welding);

– ternary, or almost none, in the case of treatments carried out atambient temperature and slightly above (e.g., burnishing, electroplating)

In all cases, a rise of temperature significantly intensifies diffusion cesses and extension of time causes an increase in the amount of thediffused element Intensification of diffusion is also aided by the existence

pro-of residual stresses and, to a lesser extent, by electric and magnetic fields Inall cases, of capital importance are element concentration and chemical po-tential

In surface engineering, the most important role is that played by thediffusion of particles of gases and metals or non-metals into metal alloys,resulting in the formation of chromized, borided, silicized, sulfurized andother layers (including combinations)

5.7.3.10 Adhesion

The concept of adhesion Adhesion (from Latin: adhesio - cling together)

is a phenomenon of permanent and strong joining of superficial layers of

two different (solid or liquid) bodies (phases) brought into mutual

con-tact A specific case of adhesion is cohesion which occurs when the

bod-ies in contact are of the same material Adhesion may be caused by sorption

ad-Adhesion is caused by the presence of attraction forces (e.g Van derWaals, ion and metallic bonds) between particles of touching bodies Aboundary case of adhesion is the occurrence of chemisorption bonding atthe interface with the formation of a superficial layer, constituting a chemi-cal compound [58]

Fig 5.33 Diagrams showing: a) adhesion of two different bodies; b) cohesion of two

identical bodies; γ - surface stresses at interfaces.

The strength of adhesion is described by the value of force (or amount

of work) necessary to separate adhering bodies, applied to a unit contact

surface [8] The work of adhesion (adhesive separation) W a for a reversible

and isothermal process is characterized by a decrement of free energy f a

per unit surface of adhesive contact, equal to the difference in surfacetensions of the two surfaces (Fig 5.33) In the case of adhesion of liquid(1) to liquid (2) we have

W a = — f a = γl1l2 — (γl1g + γl2g ) < 0, (5.25)where: γl1l2 - surface tension between liquids (1) and (2); γl1g - surface ten-sion between liquid (1) and gas; γl2g - surface tension between liquid (2)and gas

Wettability In the case of adhesion of liquid (1) to a solid, equation

(5.25) cannot be used to calculate the work of adhesion W a because surfaceenergies between solid and liquid γsl1 and between solid and gas γsg cannot

be measured directly The difference in surface energies of an unwettedand a wetted surface, i.e., the so-called wetting tension β = γsg – γsl1 is ex-

pressed by the cosine of the boundary angle Θ (also known as the wetting angle) corresponding to the state of equilibrium:

de-If γsg > γsl the interface forms a concave meniscus (Fig 5.34a) and the

surface tension force g sl is tangent to the curved surface of the liquid The

Fig 5.34 Schematic representation of interfaces forming a concave or convex

menis-cus: a) wetting liquid (concave meniscus); b) non-wetting liquid (convex meniscus).

vertical component of that force is equal to γl1g cos Θ and the state of

equilib-rium is reached when the following condition is met:

γsg = γsl + γlgcosΘ (5.27)

The above equation is known as the wetting equation.

If γsg < γsl the interface will have a convex meniscus because the liquidwill drop at the side wall (Fig 5.34b) The condition of equilibrium re-mains the same since in both cases the boundary (wetting) angle is [9, 59]

(5.27a)

If the wetting angle Θ is equal to zero, full wetting occurs If Θ = 180º, thecase of absolute non-wettability occurs [9, 58] In accordance with the corre-lation expressed by (5.27a), full wettability of a solid by a liquid takes placewhen γsg – γsl ⊕ g lg (for an angle equal to zero).

The wetting angle is an angle formed between the wetted surface of the solidand the tangent to the curvature of the meniscus of the wetting liquid, at thepoint of contact between the liquid and the solid (Fig 5.35) A knowledge of thevalue of the wetting angle is of significant importance in flotation processes,lubrication and in the production of laundry agents [58].

From an equation equivalent to (5.25) it follows that W a = γsg (1 ++ cosΘ) for the full range of values: 0 ≤ Θ ≤ 180º When total wettabilityoccurs (Θ = 0), Wa = 2γsg = W l , which means that the work of adhesion W a equals the work of decohesion W d When the liquid totally wets the surface ofthe solid, the boundary angle Θ = 0 and Wa becomes greater than 2γsg (when

of adhesion between solids and, hence, the necessity of reverting to mental methods [58].

experi-When adhesive joining of solids is effected it is justifiable to aim for highvalues of surface energy However, high energy surfaces easily absorb va-pors, gases and contaminations, conducive to a drop in free energy In order

to obtain a good adhesive connection it is, therefore, necessary to remove theadsorbed layers by e.g., creating a vacuum or elevating the temperature Theprocess of adhesion of solids is also intensified by other forms of surfaceactivation, e.g., by the introduction of energy in the form of ultrasonic vibra-tions, radiation (microwave and corpuscular), by defecting the superficial layer(e.g., deformation, treatment at cyclically varied temperatures, oxidation andimmediate reduction, pulsed pressure) [59]

When two different bodies adhere, the value of adhesion force is ticularly big in the case of full contact across the entire surface of the twobodies:

par-– at the interface of two liquid phases (e.g., water - mercury);

– upon significant plastic deformation of contacting bodies (e.g., welding of metals), conducive to total contact and strengthening of thestructure of the adhesive connection (seam);

cold-– upon the introduction of a liquid on to the surface of a solid inconditions of total wettability (e.g., glueing, welding, painting), conducive to(after solidification) obtaining of exceptionally durable adhesive connection(seam);

– upon the formation on a solid surface of a second solid as a newphase, due to the creation and growth of two-dimensional crystallizationnuclei(e.g., electroplating of metals, vacuum deposition of solid particles onmetal surfaces by PVD methods);

– with dry friction, occurring particularly intensively in the case ofmetals with same or similar chemical composition, conducive to the cre-ation of adhesive spot welding and causing adhesive wear [16, 72]

In the case of strong adhesion of solids, with time and due to diffusion,there comes about a progressively stronger bond This so-called “inter-growth” or “in-growth” may lead to an atrophy of the interface as theresult of an unlimited solubility in the solid state, i.e., the transformation oftwo joined phases into a single phase This process occurs mainly in theadhesion of same materials

Adhesion occurs often in everyday life Dust particles are attached towalls by adhesion, chalk adheres to the classroom board, glue joins the gluedmaterial, etc

5.7.3.11 Catalysis

Concept and types of catalysis Catalysis (from Greek: katalisis -

decom-position) is a term introduced in 1836 by J.J Berzelius and used to scribe a phenomenon consisting of acceleration and deceleration of cer-

de-tain chemical reactions by substances called catalysts (In stricter terms,

the effect consists of variations in the rate at which a chemical reaction

achieves the state of equilibrium.) Catalysts participate in the chemical tion (but do not participate in the stoichiometric equation), themselves nei-ther being used up nor appearing among the reaction products Their amountand chemical composition do not undergo any changes during the reaction).Usually, to effect a significant change in the rate of a reaction, only a smallamount of catalyst is sufficient, relative to the amount of reacting substances.The following types of catalyses are distinguished:

reac-– positive catalysis - when the (positive) catalyst accelerates the rate

of a reaction; this is the most frequently used type of catalysis;

– negative catalysis - when the (negative) catalyst, in this case called inhibitor, decelerates the rate of reaction (as well as the stability and

selectivity of the catalyst); this is the type of catalysis used less often, mainly

to slow down corrosion processes by the application of various corrosioninhibitors;

– autocatalysis - when the product of the reaction (or one of the

in-termediate products) exerts a catalytic effect, which is usually positive Insuch a case, the reaction rate rises with the accumulation of that product.All catalytic reactions are, from the point of view of thermodynamics,spontaneous, i.e., they are accompanied by a drop in free energy A catalystdoes not change the state of chemical equilibrium but only the time of achiev-ing that state The same catalyst usually changes the rate of a reaction bothfrom left to right, as well as from right to left Catalysts act selectively, chang-ing the rate not of every reaction but only of one of those thermodynamicallypossible within a given system

Catalysts may have the form of solids (such catalysts are called tacts), liquids and gases Examples of good solid catalysts are platinum, pal-ladium and oxides of certain metals Catalysts of numerous biochemical pro-cesses (digestion, oxidation of sugars in the bloodstream, fermentation) arecalled enzymes

con-To date, there is no satisfactory theory which would explain the action ofcatalysts The mechanism of catalysis may be interpreted as the formation

by the catalyst with one or more substrates (in this case - initial substances)

of a non-stable intermediate bond This bond suffers immediate dissolutionwhich indirectly or directly leads to the formation of the final products ofthe reaction and to a regeneration of the catalyst (its return to the initialform) A reaction with the formation of the intermediate bond is faster thanwithout it, i.e without the catalyst From the point of view of kinetics, thecatalytic reaction is one with a lower activation energy

Depending on the physical state of the catalyst, we distinguish thefollowing types of catalyses [60, 62]:

– homogenous - in which the catalyst occurs in the same phase (solid,

liquid or gaseous) as the reacting substances;

– heterogeneous - in which the catalyst occurs in a different state of

aggregation than reacting substances; most often, the catalyst is a solidand comes in contact with reacting substances only through its surface

Heterogeneous catalysis In the case of the system occurring most

fre-quently in surface engineering: i.e alloy - gas, in which the catalyst is rated from the substrates by an interface, heterogeneous catalysis takes place[62] It is connected by an unbreakable bond with the formation of an adsorp-tion layer, its structure and with the character of interaction between surfaceand metal In the system: solid - gas, the chemical reaction occurs at theinterface The phase containing the substrates, i.e., the gas phase, is simply a

sepa-“reservoir” of particles which are subject to transformation, as well as thosecreated by the reaction Classical heterogeneous catalysis is based on reac-tions caused by the action of the solid’s field of forces on substrate particles.The range of action of the forces is limited to distances comparable to anatomic or particle diameter, thus, of the order of tenths of a nanometer [8].The mechanism of heterogeneous catalysis is complex It is, however, anindisputable fact that in this case, a significant role is played by the adsorp-tion of substrate particles at the surface of the catalyst, by chemisorption, touse a stricter term

The reaction of heterogeneous catalysis comprises five stages: diffusion ofsubstrates to the catalyst, adsorption, chemical transformations at the sur-face, desorption, and diffusion of reaction products from the catalyst surface[60, 62]

Due to diffusion, substrate particles approach the catalyst surface andbecome adsorbed by it However, not every process of adsorption is con-ducive to catalysis but only such which is accompanied by the creation of

a chemical bond between the substrate and the surface, in other words, bychemisorption This process is accompanied by the coming close of par-ticles of reacting substances and the simultaneous rise of their chemicalactivity, under the influence of forces exerted by surface atoms of thecatalyst In the next stage, the newly created products break away fromthe catalyst and finally, by diffusion, permeate into the core of the otherphase Thus, compounds created at the catalyst surface in the case ofheterophase catalytic reactions are intermediate compounds [9]

In order for catalysis to occur, a condition must be met This condition

is that the binding energy of the adsorption compound be contained withincertain limits, i.e., that it be neither too small nor too great, because the forma-tion of an excessively stable bond between substrate or product with thesurface renders further reaction difficult The rate of completion of a catalyticprocess depends on its conditions and is determined by the rate of the slow-est of the above-mentioned stages of the process

The change of energy in the second, third and fourth stage is trated by Fig 5.36 Adsorption of substrates is connected to activation

illus-energy, corresponding to an increment in enthalpy along the length 1 - a’ The stage of adsorption ends at the point marked 3 Next, an active com- plex a is formed which is adsorbed at the surface of the catalyst (stage:

3 - a”) In the next phase (a” - 4) the products of reaction are adsorbed and these finally break away from the catalyst surface The interval 4 - a’’’

which the particle later goes into a state of permanent bonding to thesurface The intermediate phase enables the drop of activation energyfor adsorption to a comfortably low value This value, however, remains

high in the case of metals not containing unpaired electrons d For that reason, metals which have valence electrons only on the s or p shell

belong to those, characterized by weak chemisorption Among such als, also, are alloy components of steel, such as Fe, Cr, Mo, Ni, Ti, Co Inthis way, the steel surface affects heterogeneous reactions occurring dur-ing thermo-chemical treatment [9]

met-Fig 5.37 Dependence of catalytic activation k of some elements on the atomic number

Z of the element, for the reaction of ammonia dissociation at 800ºC, under a pressure

of 0.1 MPa (From Karapetjanc, M.Ch [61] With permission.)

Fig 5.37 shows examples of the correlation between the atomic

num-ber Z of an element and the catalytic activity of certain metals for the

reaction of ammonia decomposition at a temperature of 800ºC and pressure

of 0.1 MPa Catalytic activity was determined by investigating the rate ofdecomposition occurring during the contact of substrates with a known mass

of catalyst in given conditions of pressure and temperature, in other words,

as a certain empirical measure, enabling a comparison of catalysts As can bededuced from Fig 5.37, the highest catalytic activity was exhibited by Fe, Ruand Os A practical conclusion follows that iron contained in steel catalyzesthe decomposition of ammonia during the process of gas nitriding in anatmosphere of NH3; on the other hand, it is difficult to nitride e.g., nickel andits alloys in this way [9]

Catalytic interaction of alloying elements of the steel matrix also takesplace in the process of formation of titanium carbide layers in an atmo-sphere of TiCl4 + H2 + CH4 during the initial stages of layer formation onhigh chromium tool steels (the effect of chromium)[63], as well as in theprocess of formation of duplex titanium nitride layers on top of nitrided

layers (the effect of the nitrided surface) [64, 65] The condition for a catalyticreaction of particles at the metal surface is their prior chemisorption Whentwo particles react, at least one of them, but most often both, are chemisorbed.Thus, chemisorption constitutes a basic stage, preparing the particle for reac-tion.

Generally, metals may be categorized into various groups, regardless ofthe number of gases which may be chemisorbed by them This division isshown in Table 5.1 [62] It should be emphasized that the categorization isonly qualitative It follows unequivocally from the table that properties ofchemisorption are exhibited by transition metals Assuming the premisethat the condition of a catalytic reaction between two particles is theirprior chemisorption, it can easily be predicted which metals will catalyzethe synthesis of ammonia (Class A) or the reaction of hydrogen with oxy-gen (Classes A, B1, B2)

Table 5.1

Metals according to their tendency to chemisorption

It should be noted that one of the better known and most undesirableproperties of heterogeneous catalysts is their tendency to become deacti-vated or poisoned by so-called toxins These may enter the substrates ascontaminations and act in a momentary or permanent manner, depending

on whether their action stops or not after their expulsion from the system

A toxin may also be a by-product; an example of that is the formation ofhydrogen chloride and its chemisorption during the formation of titaniumcarbide in a CVD process, carried out in an atmosphere of vapours ofTiCl4 + H2 + CH4 [66] Metallic catalysts are particularly sensitive to tox-ins, especially to compounds of sulfur and nitrogen, containing free pairs

of electrons which form strong coordinate bonds with the metal surface Atoxin is, therefore, a substance which is adsorbed more than the sub-strates and in that way renders their access to the reactive surface impos-sible [62]

Ti, Zr, Hf, V, Nb, Ta, Cr, Mo,

In surface engineering, catalysis facilitates the carrying out of certaindiffusion processes, as well as the application of some PVD and CVDtechnologies Its most significant role, however, is in thermal spraying(catalytic action of exhaust gases) Of great ecological significance arecatalytic coatings, sprayed onto surfaces of fuel-fired heating equipment.Exhaust gases or metal and ceramic catalysts, introduced into the gasstream, flow around them, including exhaust gases from vehicles withcombustion engines, in order to reduce the content of nitrous oxides [67]and carbon monoxide [68] which are harmful to the environment Forthose applications catalysts are made of e.g oxides of manganese, vana-dium, titanium, and aluminum oxides or their mixtures, with stronglydeveloped surfaces (porous and rough coatings).

5.8 Practically usable properties of

the superficial layer

The superficial layer is always produced with a clearly defined aim; it isalways designated to be exposed to appropriate external hazards, bothchemical and physical Practically usable properties of the superficial layer,beneficial to the service of the part in conditions of one type of hazard,may prove to be less beneficial in conditions of a different type of hazard.For example, anti-corrosion superficial layers usually impair fatiguestrength [69]

Practically usable properties of the superficial layer are, therefore, the sult of matching its potential with external hazards (Fig 5.38) Service prop-erties change in the process of service, with time of use of the product In onlyvery few cases can potential properties be equivalent to usable properties ofthe superficial layer

reFig 5.38 Usable properties of the superficial layer (p unit pressure; v velocity; T

-temperature; a - atmosphere; l - lubricant; r - radiation; e - electromagnetic field).

Appropriate combinations of parameters (characteristics) of the perficial layer may be best for appropriate combinations of external haz-ards The latter may be mechanical stresses (including variable), fric-tion, chemical interaction with the environment (oxidizing, reducing,inert), physical, by electric current or magnetic field or a combination ofany of them

su-Fig 5.39 Most important usable properties of the superficial layer.

Among the most important usable properties of the superficial layerare the following (Fig 5.39): strength, tribological, anti-corrosion, decora-tive and some others

5.8.1 Strength properties

5.8.1.1 General characteristics

In a general sense, strength is the resistance to destructive action of chanical factors, such as various types of loading In a strict sense it meansthe value of resistance put up by the forces of cohesion of a solid to exter-nal loading or the ability to withstand this loading and determines theboundary value of stresses which, when exceeded, cause fracturing of thesolid (machine part or any structural component) The strength of a mate-rial is usually described as a load per unit area of cross-section Strengthdepends on the method of loading and on the type of material

me-We distinguish tensile, compressive, torque and crushing Each of these

types of strength may be further divided into fixed strength (including long-term strength, so-called creep strength) and periodically variable

(including fatigue strength) [70-73]

By changing the properties of the material of the superficial layer, erage properties of the material in its entire volume are changed Thedegree to which a change of superficial layer properties affects averagebulk properties of the material is proportional to the ratio of the cross-section of the superficial layer to that of the core

av-The greatest influence of the superficial layer is not on static strength but on dynamic strength, especially in conditions of multiple periodically

variable loading This causes the creation of a mutually interacting set ofeffects, and successively developing properties which significantly reduce

material strength and often lead to its failure, described as material tigue (first time by J.V Poncelet in 1939).