Smithells Metals Reference Book Part 15 pdf

For moderate sliding speed and load, boundary lubrication can exist with only the oxide film being worn away and replaced at a tolerable rate of wear.. A grease is usually SO-SO% liquid

Wrought PM materials 23-25

Superalloys owe their good strength properties to precipitation hardening by a titanium- aluminium compound, but as the temperature rises, this compound begins to go into solution and loses its effect This puts a ceiling on the working temperture The mechanical alloying process to produce a !he dispersion of a ceramic material-yttria is favoured-that is virtually insoluble in the metal matrix, significantly increasing the temperature at which useful strength is retained A

list of ODS superalloy compositions is given in Table 23.18, and Figure 23.11 shows the improved performance of one of these alloys compared with that of a similar composition made conventionally

23.143 Copper

One of the main users of copper is the electrical industry where conductivity is the primary consideration Increasingly, power plant is required to operate at temperatures well above ambient where pure copper recrystallizes and m e s very soft Increasing the strength and recrystallization temperature by alloying drastically reduces the conductivity The inclusion in pure copper of a small percentage of h e l y dispersed aluminium oxide provides significantly better elevated temperature strength with only a small reduction in conductivity An atomized powder of a dilute aluminium copper alloy is internally oxidized to give a very h e Al,O, particles, the powder being

then processed by compaction, sintering, and working The improved strength at room temperatures enables smaller sections to be used in, for example, miniaturized systems Figure 23.12 gives some

results for a range of alumina contents

23.14.4 Lead

This is another soft metal whose strength can be increaskd by a dispersed oxide phase In this case

lead oxide is used The chief application is to chemical plant especially for handlingsulphuric acid

Table 23.18 NOMINAL COMPOSITIONS (W%) OF MECHANICALLY ALLOYED OXIDE DISPERSION STRENGTHENED SUPERALLOYS

INCOLOY alloy M A 956 Bal 20 4.5 0.5 0.05 0.5

INCONEL alloy MA 954 Bal 1.0 20 0.3 0.5 0.05 0.6

INCONEL alloy M A 758 Bal 1.0 30 0 3 0.5 0.05 0.6

INCONEL alloy M A 6ooo Bal 15 4.5 2.5 0.05 1.1 2.0 4.0 2.0 0.01 0.15

* INCOLOY end INCONEL arc hademarks of the Inco family of companies

Iro=hromhtm afioys: INCOLOY alloy MA 956 sheet, plate, bar, spinnings, rings and forgings have applications in the hot sections of gas turbines and dim1 engines where the resistance of the alloy to creep, oxidation and sulphidation allow higher metal temperatures and longer component life The alloy is being used to replace molybdenum in high-temperature vacuum furnaces for fixtures and heat-treatment trays INCOLOY alloy M A 957

is intended for nuclear power applications, especially fuel cladding in liquid metal cooled reactors Compared with

316 type stainless steel it has higher strength at 700°C and considerable resistance to irradiation damage

Nickel-chromium alloys: INCONEL alloy M A 754 is used for brazed nozzle guide vane and band assemblies in advanced military aero endnes T h e principal advantages of the alloy for these applications are thermal fatigue

resistance, long term creep strength and a high melting point INCONEL alloy M A 758 is highly resistant to attack

by molten glass and is used in spinnercttes for the production of fibre glass The parts are formed by hot spinning plate

G m p'me O D s alloys: The immediate applications for INCONEL alloy MA 6ooo are for first- and second-stage turbine vanes and blades machined from solid bar Forced airfoil components have also been developed The characteristics of INCONEL alloy MA6OOO allow blade cooling to be reduced or eliminated as the metal

temperature CUI be increased by 100 K or more in engines where the stresses are medium or low INCONEL alloy

M A 760 is an industrial gas turbine derivative of INCONEL alloy M A 6M)o having greater resistance to corrosion

and oxidation Initial applications are for machined vanes and blades but forged components are under

development

(Inco Alloys International)

Figure 23.11 Properties of mechanically alloyed Income (Inconel b a

trakmark of Inco Alloys Intemarwnal) compared with a conventional

Section 23.3.1 referred to the mechanical alloying of AI with graphite to produce powder containing

a dispersion of aluminium carbide This powder containing a dispersion of aluminium carbide This powder compacted into billets and extruded gives a product with much improved strength at elevated temperatures (see Figure 23.13) This may have applications in aircraft where weight saving

is of significant value

23.15 Spray Forming

This process is not powder metallurgy in the strict sense of the term in so far as the metal is at no

stage in the form of powder However, for reasons that will be apparent the PM world has adopted

it The process involves gas atomization of a liquid metal, but instead of allowing the droplets to solify as powder, the spray is caused to impinge on a solid surface where the droplets are collected

as a semi-solid layer which solidifies as a layer of dense metal This layer may be built up to any desired thickness, and by suitable choice of design of the original target, the angle of the spray, and other parameters, near-net shapes can be produced For example if the target is a cylinder rotating horizontally and capable of being moved in a controlled manner in the axial direction, a tube of dense deposited metal is formed The deposit has aII the advantages of dense metal produced from powder, is complete absence of macro-segregation and pipe-related defects A further merit

of the process is that by injecting fine refractory powder particles, i.e by entraining them in the atomizing gas stream, ODS material m be deposited

Injection moulding 23-27 Aluminum oxide content, YO)%

is then carefully treated by solvents and/or heat to remove the binder and then at a higher temperature eventually to sinter the metal Shrinkage of the order of 10% linear occurs but this

can be predicted accurately and parts with very close dimensional tolerances and of quite complex shape can be made It is an expensive process but the advantages in eliminating expensive machining operations make it viable for a number of applications

0 ZOO LOO 600

TemDerature loci

Figme 2313 Strengthllempemhae retatiomhip of

iwn with a dispersion of AI,C, produced by m e c h i c d alloyhg compared with mgot-based high strength alloys

D k p d b a tra&tnnrk of Smtermetdlwerk Krebscge

This family of PM materials consists of fine, hard, and usually brittle particles bonded with a relatively soft and tough binder phase which is normally metallic The hard particles are generally between 1 and 5pm, but even finer grades with particles below 1 j are now being made The original hard phase was tungsten monocarbide (WC) and the preferred binder phase was cobalt The name cemented carbide was, and to a large extent still is, used to describe these materials, but

the official name is now hurdmetuL Carbides other than that of tungsten were later added, e.g

those of tantalum, niobium, and titanium, and binder metals other than cobalt have been used,

but the WC/Co-based compositions still have the lion’s share of the market More recently, products

have been developed in which the hard phase is not carbide, but nitride, boride, carbo-nitride,

oxide, or combinations of these Such materials may not strictly be called hardmetals since

hardmetal has been defined in I S 0 standard 3252 as ‘sintered material characterized by high

strength and wear-resistance comprising carbides of refractory metals as the main component together with a metallic binder phase’

Uses

Hardmetals were originally developed as a substitute for diamond as wire drawing dies for tungsten,

and they are stdl used for that purpose, but the largest single use today is as cutting tools It is

for this application that the noncarbide alloys have been developed, but no cost effective substitute has been found for the WC/Co hardmetal where straightforward wear resistance is the primary requirement, and this includes cuffing tools for non-ferrous metals and non-metallic materials Other such cases are wire drawing dies, dies for the compaction of metal powders for the manufacture

of PM parts, rolls for metal rolling miIls, and other large abrasion-resistant parts

Munufacture

The process for the production of hardmetal is a classic example of liquid phase sintering: a mixture

of WC and cobalt powders is pressed and sintered at a temperature above the melting point of cobalt However, to get good results special procedures in the preparation of the powder mix are necessary The i n d e n t s are wet milled together in order to coat each carbide particle with cobalt, and to facilitate this, the cobalt powder must be extremely fine It is well known that very fine powders do not flow readily, if at all, and to overcome this problem the WC/Co mixture is

‘granulated‘, by which is meant the production of agglomerates A favoured method of granulation

Hardmetals and related hard metals 23-29

is the spray drying of a slurry of the powder with a liquid containing also a pressing lubricant

During sintering, the compact shrinks by as much as 50 vol% to become nearly 100% dense The sintering temperature used in practice varies with the composition, being lowest (1400°C) when

the cobalt content is high, and rising to 1600°C) or higher with compositions having high proportions

of the carbides of Ti, Ta, and/or Nb and low cobalt contents In cases where it is not posible to get close to the required shape by direct pressing and sintering, the compact may be presintered

at a lower temperature so as to remove the lubricant and provide sufficient strength for handling

In this state, the object can be machined using tools of bonded diamond or other hard material Although it is usual to refer to the binder phase as being cobalt there is some mutual solubility

between it and the carbide, and great care is needed to ensure that the carbon balance is maintained

such that neither the brittle W/Co (eta) phase nor free graphite is formed The toughness is affected also bq the size of the carbide particles; the h e r they are the harder but less shock-resistant is the final product, However, grades with carbide particle size well below 1 pn are reported to combine high hardness with toughness

-Hot Iso-static Pressing

Although the porosity of conventionally produced hardmetal is normally low, porosity can be completely eliminated by hot iso-static pressing (HIPping) Toughness is, thereby considerably increased and the possibility of the rejection of large and expensive components at a late stage of grinding or polishing no longer presents a problem HIPping is now routinely applied to a large number of hardmetal parts including indexable cutting tool tips that are a major product Recently

it has been found possible to combine HIPping with the sintering stage The parts are sintered in vacuum to a density such that the porosity is sealed, and then high pressure gas, usually argon,

is introduced into the furnace The pressure required for full densihtion is lower than that needed for the HIPping of already sintered parts, and the new process, referred to as Sinter-HIP or Pressure Assisted Sintering is rapidly replacing the original two-stage process of vacuum sintering followed

by HIPping in a separate furnace, at least for cutting tool tips

Compositions

Straightforward WC/Co hardmetal appears to be the most cost-effective material for many applications where wear resistance is the primary requirement, including the machining of non-ferrous metals Additions of e.g TaC improve the already good wear resistance, perhaps by acting as grain growth inhibitors, and are especially valuable in applications involving high temperatures When the use is the machining of ferrous materials at high speeds, the situation is different In addition to abrasive wear, reaction between the carbide particles and the steel results

in what is known as crater wear The substitution of more stable carbides such as those of Ta, Nb,

Ti, and Hffor some or all of the WC considerably improves the cratering resistance Wear resistance

is, as would be expected, markedly iniluend by the amount of binder phase as shown in Figure

23.14, but reduction in the cobalt content also makes the alloy more brittle, so a compromise between toughness and wear resistance is necessau

Table 23.19 gives compositions and properties of one manufacturer’s range Table 23.20 lists the IS1 classification of carbides according to use

Alternative Binders

Because of the high price of cobalt and the perceived instability of the countries that produce the bulk of it, continuing efforts have been made to find alternatives Ni, Fe, and Ni/Mo have been used successfully in certain applications, and a recent entry into the field is Ni/Cr which now appears in some commercial grades One grade is based on T i c with MoC and a Ni/Mo binder Good results have been reported also with a superalloy binder which combines conspicuous high-temperature strength with toughness Useful resuls have been reported also with a nickel binder containing ruthenium, but this member of the platinum group is, of course, relatively very costly

Coatings

For tools for the machining of steels the most dramatic improvement has been the development

of surface coatings An ideal tool material fr.om the cutting point of view would be a pure, very

23-30 Sintered materials

Table 23.19 COMPOSITION AND PROPERTIES OF TIZIT GRADES OF HARD METAL

Chemical composlrlon-wdghf %

Average Fmmerse Size strength Densify

16.5

17 83.5

84

a6

77 96.7

81.5

94.2 93.7

6 0.3

0.3 0.6 0.3 0.6

3 1-2

3

3 2-3 2-3

3 3-4

14.9

14.1 14.7 14.6 14.3 14.0 13.5

133 12.8 14.9 14.6 14.2 14.0

**Coatinglthickness liC, Ti(C, N), T i l l L 1 2 p

***Coatinplctbickness Tic, Ti(C, N), TiN 5 p

Hardmetals and refated hard metals

Co@cient

Compressive modulus Coerciw Electrical conductivity expansion Hardness strength of elasticity Poisson's force restiuity (20°C) (20-400"C)

24 19.5 18.70 17.11 15.12 14.72 12.33 10.74 9.55 7.00 5.81 5.25 11.95 7.3 6.5 6.0

5.5 5.5

5

5 5.5

6

5 4.8

WO IS0 CLASSIFICATION CARBIDES ACCORDING TO USE

Steel, steel casting8

Steel, steel castings Steel, steel castings Malleable cast iron with long chips

Steel, steel castings Malleable cast iron with long c h i p

Main groups

of chip remoual

categories

of materio1

Finish turning and boring, high cutting speeds small chip sec

tion, accuracy of dimensions and h e finish, vibration-free

operation

Turning, copying, threading and milling, high cutting speeds,

small or medium chip sections Turning, copying, milling, medium cutting speeds and chi,

sections, plaoing with small chip sections

Turning, milling, planing, medium or low cutting speeds, me- dium or large chip sections, and machining in unfavourable conditions*

strength, with sand inclusion and cavities

I use ami working comiitiom

For operations demanding very tough carbides: turning, plan

ning slotting, low cutting speeds, large chip sections, with the possibility of large cutting angles for machining in unfavour-

able conditions and work on automatic machina

o/

f cut carbide

M

-

K

Steel, steel castings, mangaoese stecl

Grey cast iron, alloy cast iron

Steel, steel casting;, austenitic or manganese

steel, grey cast iron

Steel, steel castingi, austenitic steel, grey

cast iron, high temperature resistant alloys

Mild free cutting steel, low tensile steel Non-ferrous metals and light alloys

Turning, milliog, planing Medium cutting speeds, medium or

Turning, parting off, particularly on automatic machines 7 I

Very hard grey cast iron, chilled castings of over

85 Shore, high silicon aluminium alloys, hardened steel, highly abrasive plastics, hard cardboard,

ceramics Grey cast iron over 220 Brinell, malleable cast

iron with short chips, hardened steez silicon ahm- iuium alloys copper alloys, plastics, glass, hard rubber, hard cardboard, porcelain, stone

rey cast iron up to rine , non-ferrous

cast iron, low tensile steel, 2o :et& copper, b-i%iniul

T,,,

&, &jlin& drilling, b r i n g , broachin&

Turning, milling, planing, boring, broaching, demanding

very tough carbide

51, fondit!ns* i d with k bosPifbility :.irgecut&

urning, m ing, paning, s ttmg, or mac inlog in u avour- P a 3

7 7 I I

angles Turning, milling, planing, slotting, for machining io unfavour- able conditions* and with the possibility of large cutting angles

Rcprodnd from I S 0 recmcndstion 513 by permidon of the British Standards institution, 2 Park Street, London, W I A 2BS

An essential requirement, of course, is that the coating adheres firmly to the substrate and this is facilitated by using multiple coatings only a few micrometres thick Another factor in the equation

is the possibility of reaction between the coating and the substrate and between the coating and the steel workpiece In addition to nitrides, carbides, carbo-nitrides, borides, and oxides especially

alumina-are used, and as many as ten layers are applied, so the permutations are manifold Recent additions to the range of coatings are cubic boron nitride (CBN) and diamond, but the latter is not suitable for the high-speed cutting ofsteel because it reacts with the steel at the high temperatures that are reached The importance of matching the tool to the application should be emphasized

Tool Life

In cutting applications much is made of the improved tool life of modern throw-away inserts achieved by careful design of substrate and coating, but a more important factor from the economic point of view is the improved cutting speed that is made possible The cost of the tool insert is a small part only of the total machining cost, and if the cutting rate can be doubled and the tool life halved in consequence, the overall efficiency may well be much greater than that of prolonging the life of the tool

Acknowledgements

Associazione Industriali Metallurgici Meccanici

American Society for Metals (ASM)

British Powder Metals Federation (BPMF)

BSA Metal Powders

gkm Bound Brook

Hoeganaes Corporation

Hganas AB

Inco Alloys International

Makin Metal Powders

Metal Powder Industries Federation

Table 23.3, and Figure 23.1

Tabla 23.19 (properties ofhardmetals)

24 Lubricants

24.1 Introduction

Lubricants minimize friction and wear in rubbing contacts They may also prevent rusting and with liquid lubricants remove heat Lubricants may be solid, such as graphite, molybdenum disulphide, polytetrduoroethylene and talc; or gaseous, commonly air; but the principal lub-

ricants are liquids such as mineral oil, or the semi-solid greases formed from liquids by the use of

thickening agents

241.1.1 Hydrodynamic lubrication

A viscous fluid interposed between two solid surfaces moving in very close proximity to one another becomes pressurized and holds those surfaces apart, often against considerable loads, provided there is a position of closest approach of the surfaces towards which the net flow of fluid

is directed The effect is called hydrodynamic lubrication

The condition that the net lubricant flow must be towards the position of closest approach of the surfaces is fuElled in journal bearings by the journal automatically assuming an eccentric

position in the bearing It is not fuliilled if the surfaces are absolutely parallel to one another, but if the bearing surface can tilt appropriately as in the Michel tilting pad bearing, considerable loads can be carried The amount of tilt required is almost imperceptible: approximately 1 in 10 OOO If a position of closest approach can be developed by slight elastic or thermal distortion considerable fluid pressures can be developed between apparently parallel surface

For very high pressures over tiny contact areas films as thin as lOnm have been measured

24.2

In hydrodynamic lubrication friction is purely viscous and is directly dependent on the area of the

film, the rate of shear and the viscosity of the lubricant Coefficients of friction are as low as 0.001- 0.003 and wear is negligible When the speed of sliding and the oil viscosity are insufficient for the

viscous film to carry the load, contact occurs between asperities on the opposing surfaces Friction then rises and wear ensues Where viscous effects are absent or negligible, lubrication is independent of the nominal k e a of contact and is said to be of boundary type Where boundary and viscous effects occur together, the conditions are said to be those of ‘mixed‘ lubrication

When sliding speeds are very low indeed a ‘stick-slip’ or jerky motion arises due in part to the elastic response of the drive and in part to the coefficient of static friction exceeding the coefficient

of dynamic friction This undesirable effect can be suppressed by the use of special lubricants

containilrg fatty acids, acid phosphates, or similar materials able to react with the metal surfaces to produce soft, easily-sheared layers of soap which considerably reduce the coefficient of static friction

For moderate sliding speed and load, boundary lubrication can exist with only the oxide film being worn away and replaced at a tolerable rate of wear Under these conditions lubricants which produce low friction do not necessarily produce low wear For most machine bearings, boundary

lubrication is inadequate, since the oxide film wears away very rapidly and direct metal to metal

contact o a r s The surface asperities weld together momentarily and are broken apart again to produce wear debris largely composed of metallic particles This is called adhesive wear, or Scufhg

Friction, wear and boundary lubrication

24-1

24-2 Lubricants

Boundary lubrication is the most important mode in the chipless-forming type of metal working

operation since the local pressures have to be high enough to exceed the yield strength of the metal In such operations the surface area of the workpiece is being enlarged and new, easily- weldable areas are being created For severe operations, therefore, solid lubricants are used which can resist high pressures and are capable of extension to cover the new areas Examples of such lubricants are waxes, graphite, molybdenum disulphide, talc, whiting and pastes of dry soap and

fats For more severe operations such as tube and bar drawing thick layers of lubricants may be

built up before the operation by, for example, phosphating, baking on a coating lime, treating with special inorganic salts, or by coating with soft metals such as lead

24.3.1 Viosity

Viscosity is probably the most important property of a lubricating oil or grease For most fluids in laminar flow there is a linear relationship between the shear stress and the rate of shear The constant of proportionality is the viscosity or, more specifically, the dynamic viscosity to distinguish it from the kinematic viscosity which is given by the ratio of viscosity to density

Dynamic viscosity may be dehed as the shear stress necessary to move a flat surface at unit speed over a parallel surface unit distance apart when the intervening space is filled with the fluid

In SI units, dynamic viscosity is expressed in N s m-2, but the centipoise or nN s m-2 is at present commonly used Direct measurements are relatively few Usually dynamic viscosities are derived from measurement of density and kinematic viscosity, which is easily measured in calibrated glass capillary tubes using gravity flow The units of kinenytic viscosity are m2s-I but centistokes (1 x 10-6m2 s-’) are most commonly used Obsolescent units such as Redwood and Saybblt seconds and Engler degrees derived originally from short elllux tube instruments are still to be found, but nowadays they are derived from measurement of kinematic viscosity with which they have almost linear relationships

With increase of temperature, the viscosity of gases rises while that of liquids falls A moderately viscous mineral oil falls from l N s m - ’ at 5°C to O.OINsm-f at 100°C This property is commonly expressed as the Kinematic Viscosity Index (KVI) measured over the range 37.8-

98.9”C (100-210°F) and expressed in values usually ranging between 0 and 100 However, a more rational Dynamic Viscosity Index, which may be measured over any convenient range, has been

proposed Oils with a KVI of 35 and below are known as Low Viscosity Index (LVI) oils, those

between 35 and 80 are Medium Viscosity Index (MVI) oils, those between 80 and 110 are High Viscosity Index (HVI) oils and those over about 110 are Very High Viscosity Index (VHVI) oils

With minerals oils and oils of similar molecular weight, viscosity is independent of the rate of shear, exept possibly under very high pressures Shear rates in elastohydrodynamic lubrication (or EHL) are said to be ‘Newtonian’ Oils of very high molecular weight, such as silicones, exhibit a reduction in viscosity at quite moderate rates of shear and are said to be ‘non-Newtonian’ While such loss in viscosity may be only temporary, there may also be permanent loss due to mechanical breakdown of very high molecular weight polymer molecules

The lubricant viscosity usable in practice depends on the type of machine involved Low viscasity oils are used for high speeds and high viscosities for low speeds Very approximately 3Nsm-* is the maximum viscosityat which machines can be started up, while 0.002 is the minimum for maintaining hydrodynamic lubrication undsr running conditions

243.2 Bormdsry lubricaticm properties

This property of a lubricant is essentially its ability to produce on one or both of the rubbing surfaces a layer of adequate thickness of coherent and adherent, low shear strength material which

will minimize metal-to-metal contact and reduce friction Lubricants exemplifying this property

under very mild, slow speed conditions are the fatty acids which produce soap layers on metals

with active oxide surfaces, such as copper

Under such mild conditions the property is often known as ’lubricity’ Under more s p x e conditions of load, speed and temperature, the property is known as ‘Extreme Pressure’ or ‘EP’ As

or part by thermal decomposition, oxidation or by hydrolysis Thermal decomposition followed

by polymerization results in the formation of materials of very high molecular weight, especially

insoluble coke-like substances, as well as low molecular weight materials including gases The

viscosity and !lash point of the lubricant therefore generally drops Similar materials are also formed during oxidation, but in addition highly oxygenated species including lacquers and organic acids are formed and the viscosity generally increases This viscosity increase, the amount of insoluble material and the increase in organic acidity, are conveniently used as expressions of the degree of oxidation Lubricants based on esters and lubricants containing esters or salts as

components, e.g as additives, are subject to hydrolysis and here again acidity, perhaps with a

corrosion test for a sensitive metal such as lead, are used as criteria of stability

Water-containing lubricants require particularly clean working conditions as contamination may lead to bacterial attack and thus to unpleasant odour, corrosion, and reduced effectiveness Systems should be prepared and regularly cleaned by flushing with 5% solutions of caustic soda, detergent solutions or both Biostats or biocides may also be helpful

24.3.4 physical properties

The thermal capacity of an oil is particularly important as in many cases the flow of oil is used to remove heat Thermal capacity varies from around 2000 Jkg-’K for mineral oils to around 1500 for silicones and triaryl phosphate esters, which compare with 4200Jkg-’K for water

In general, lubricants must not evaporate rapidly at the highest temperatures of usage At well above their maximum usable temperatures, mineral oils and similar flammable oils reach their flash points at which, in particular apparatus, the evaporation is sufficient to reach the lower explosive limit with air High volatility is, however, occasionally desirable In some metalworking operations, working is followed by an annealing operation and any lubricant remaining must evaporate away cleanly

At low temperatures paraffinic mineral oils reach a lower limit of usage at the pour point At about -7°C a separate wax phase comes out of solution from the rest of the oil and at about

- 10°C sufficient needle-like crystals form to block the flow

24.4 Mineral oils

The most important type of oil, the mineral oils, are made from petroleum They are abundant, are comparatively low in cost and are available in a wide range of viscosities The fatty oils are inferior

in all respects except boundary friction properties which, where required, can easily be provided by

the incorporation of a low concentration of fatty material The various synthetic oils are of

growing importance and find &ective use in extreme applications where some particular property

or properties justifies their high price but, in general, their scale of use is very Limited

There are, broadly speaking two types of mineral oil: paraffinic, having comparatively long alkyl side chains in the molecule and consequently high KVI, together with a ‘wax’ pour point; and naphthenic, having comparatively short side chains, low to medium VI and a ‘Viscosity’ pour point With both types a wide range of viscosity grades can be made Low viscosity mineral oils

are frequently known as ‘spindle oils’ from their early use in textile machinery while the more

viscous oils are known as ‘cylinder oils’ from their use in steam endnes Grades between these two extremes are called ‘light machine’ and ‘heavy machine’ oils These basic grades are produced by refining and intermediate grades are produced by blending

The wide range of viscosities available is indicated in Figure 24.1 for paraffinic oils of 95 KVI to

BS 4231, in which each grade is designated by its mid-point viscosity at 373°C (100°F) expressed

in centistokes (mm’s- ’) Figure 24.1 also indicates in its margins the important classification system of the Society of Automotive Engineers (SAE), the ‘ W or winter grades of which are classified in dynamic units, the remainder in Saybolt seconds

Oxidation stability may be significantly increased by the use of antioxidants as shown in

Typical physical properties for mineral oils are given in Table 24.1

-18-16 -io -5 a 5 io 15 zo 30 40 50 m m 80 90 3 -

mm far

5w.ulw.iuw

Figme 24.1 BS 4231 rangesfor 95 KVI oils with SAE limits

extrapolated to permit comparison with the SAE grades

N B Such oils normally have a pour point of approximately - 10°F, but the lines have been

Mineral oils 24-5

Life h

Figure 24.2 Approximate life of well-refined mineral oils

TaMe 24.1 TYPICAL PHYSICAL PROPERTIES OF HIGHLY REFINED MINERAL OILS

Naphthenic oils Par&nic oils Light Heavy Light Heavy Bright Spindle machine machine machine machine stock

Density at 25'C

Viscosity (mNsm-') at 30°C

60°C 100°C

Dynamic viscosity index

92

- 43

2.0 1.6 1.3

35

163

0.880 45.0

120 3.9

68

-40

2 8

2 0 1.6

3.4

28 2.2

125

300

24-6 Lubricants

24.5 Emulsions

For many applications, particularly in metalworking and for the provision of fireresistant

lubricant$ emulsions of water and lubricating oils are used Because their thermal capacities are close to that of water, emulsions of 1-lWA oil in water are used where heat dissipation is more

important than lubrication, as for instance in high speed metal cutting, in grinding and in rolling Emulsions are also used for fireresistant hydraulic fluids where a low cost fluid is required The

water should be clean and free from acids with hardness preferably between 15 and 50p.p.m CaCOJ equivalent Very soft water may cause foaming while very hard water may reduce stability and corrosion protection Oil/water emulsions, however, cannot be used as a direct replacement

fer oil in conventional hydraulic systems

Water in oil emulsions with about 40% deionized water are used as cylinder lubricmts for

reciprocating compressors handling oil-soluble gases and as fire-resistant lubricants to replace oil,

however, the presence of water accelerates the fatigue failure of heavily loaded rolling bearings

Emulsions are non-Newtonian At high rates of shear their viscosities are close to that of the base oil, which is usually a spindle oil They are not broken down by shear Their useful life is largely limited by emulsion stability To limit loss of water and loss of emulsion stability the maximum continuous operating temperature is about 65 "C

24.6 Water-based lubricants

Aqueous solutions, either true or colloidal, of various substances are widely used as coolants and lubricants The water-glycol type of fire-resistant lubricants are solutions of 5045% polyglycols, sometimes including ethylene glycol Two or three grades between 0.03 and 0.07 N s m-2 at 38°C (100°F) are usually available Their VIS are very high and pour points gery low, e.g -40"C.They are

completely shear stable but operating temperatures are usually limited to 65°C in order to avoid excessive loss of water

Solutions of corrosion inhibitors and load carrying additives are used for high speed cutting and grinding operations

24.7 Synthetic oils

A large number of synthetic lubricants have been described but only a few are of commercial

importance

24.7.1 Diesters

These were developed principally for aviation gas turbines because of their very high VI, and low

pour point gives them a wide range of usage while the susceptibility to antioxidants allows them to work continuously at 120450°C with bearing temperatures up to 250°C

The load carrying capacity of the diesters in spur gear rigs is about twice as high as that of equiviscous mineral oil; under boundary conditions they appear to be equal to mineral oil while in heavily loaded ball and roller bearings the diesters appear to provide somewhat superior protection against Furface fatigue

24.7.2 Neoptyl plyol esters

This group, based on neopentyl alcohols and mixed aliphatic acids, has significantly better thermal

stability and the possibility of rather higher viscosity (0.002-0.008 Nsm-') than the diesters

These esters are suitable for continuous operation at 200°C with hot spots of 275°C but at these

high operating temperatures it is necessary to avoid the use of cadmium, magnesium and silver because of corrosion and to use silicone and fluorinated types of elastomers

The load-carrying properties of the neopentyl esters are better than those of the diesters,

because of their higher viscosity They apparently do not promote surface fatigue pitting in heavily loaded ball and roller bean'ngs

Table 24.2

Mixed Mixed

hexyl) di(iso- erythritol erythritol phosphate (1oooCS Medium Chloro- Base oil sebacate octy9 azelate ester ester ester Fluorocarbon Polyglycol at 25°C) phenyl phenyl

60 "C

100°C Temperature for vapour pressure of 0.001 mmH& '

Flash point, open, "C

Approximate cost relative to mineral oil

0.911 0.016

0.006 5 0.002 87

145 -60 1.40 1.28 1.05

-

1960

2 100 0.154 0.149 0.142

117

230

5

0.911 0.016 5

0.006 77 0.00301

141

< -65

-

1.38 1.18

-

1 960

2 100 0.151 0.148 0.144

144 -60

-

260

10

1.02 0.087 0.025 0.083

164 -25 1.76 1.43 1.22

1 870

1910

2 loo 0.150 0.148 0.146

-

277

5

0.97 0.140 0.083 0.045

200 -55 1.81 1.81 1.94

-

1550

-

0.162 0.159 0.155

24-8 Lubricmts

24.73 Trisryl phosphate esters

These esters find their greatest use as fire-resistant hydraulic fluids in diecasting machines, and like machines, as well as in the governor systems of large steam turbines Viscosities range from 0.0036

to 0.008 N S ~ - ~ at 100°C Their oxidation stability is good, but thermal degracfation is catalysed

by steel This limits their maximum operating temperature to about 120°C The fire-resistance is

on a par with that of the water-in-oil emulsions, and is generally adequate for industrial purposes The hydrolytic stability of these esters is rather poor and appears to be catalysed by acid impurities or similar substances developed by initial hydrolysis If the acidity can be kept low by filtration through fullers earth the degradation can be very greatly retarded Hydrolysis may result

in some corrosion of aluminium and steel but the phosphate esters are not corrosive to cadmium, zinc or other common metals

Triaryl phosphate esters tend to damage conventional rubber elastomers and therefore butyl, silicone or fluorinated types are preferred Ordinary paints are also affected and those based on epoxy resins should be used

These esters are good lubricants under both boundary and hydrodynamic (including efasto- hydrodynamic)conditions but they promote fatigue pitting of rolling element bearings and, for a life equivalent to that when using mineral oils, a 2077 reduction in load may be required

Toxicity is largely related to the amount of ortho-tolyl isomer in the oil which is accordingly kept to a low value Particular attention should be paid to personal cleanliness and good ventilation wherever people come into contact with these oils

247.4 Fluorocarbons

Usually these are polymers of trifluorovinyl chloride, the terminal groups being fluorine The range

of oils with pour points below 20°C is only 0.002-0.004 Nsm-' Densities and volatilities are

unusually high They are exceptionally stable to strong oxidizing agents such as fuming nitric acid, hydrogen peroxide, etc Thermally, they are completely stable below 300°C and the degradation at high temperatures is depolymerization so that carbonaceous deposits are not formed

Fluorocarbons are non-corrosive to metals Load carrying capacity under boundary conditions

is rather better than that of equiviscous mineral oil, but they may have a lower protection against fatigue failure of heavily loaded rolling bearings

24.75 Polyglycds

These oils are also known as polyalkylene glycols, polyoxyalkylenes, glycols and polyethers The water soluble types are mainly polyethylene oxides and have high pour points and very high viscosity indices, while the mainly polypropylene oxides are water insoluble with low pour points and somewhat lower viscosity indices A wide viscosity range is covered: from 0.008 to 19.5Nsm-'at 38°C

Polyglycols are very responsive to oxidation inhibitors and when inhibited are much more stable to oxidation than mineral oils At about 250°C polyglycols exhibit rapid thermal decom- position, but as the products of decomposition are volatile they do not form deposits Polyglycols

are not corrosive to the usual metals, but since even the water-insoluble grades are slightly

hygroscopic rust inhibited grades are preferred wherever moisture may enter the oil

The water-soluble types have important uses as components of water-glycol type fire-resistant lubricants and automotive brake fluids and are very good lubricants under hydrodynamic and elastohydrodynamic conditions Under boundary conditions they are not very good but may be provided with suitable properties by the addition of small amounts of long-chain fatty acids Typical physical properties of these synthetic lubricating oils are given in Table 24.2

24.8 Greases

2A&1 Composition

The standard definition of a lubricating grease is 'A solid to semi-fluid product of dispersion of a thickening agent in a liquid lubricant Other ingredients imparting special properties may be included.' The most common types of thickener are calcium and lithium metal soaps Bentonite is

Greases 24-9 used for high temperatures, above about 140°C Esterified silica, vat dyestuffs and urea compounds

are used for the most specialised applications

The fatty acids of the metal soaps also influence the properties of the grease Mixed acids from tallow, stearic acid and hydroxy stearic acid are probably the most widely used Complex soaps formed by the co-crystallization of two compounds permit operation at high temperatures

A grease is usually SO-SO% liquid lubricant, commonly low and medium viscosity mineral oil but high viscosity residual oils are used for high temperatures and low speeds For special purposes, synthetic oils are used

Additives are commonly used in greases for particular purposes as follows:

To prevent rapid oxidation during storage and in use

To reduce catalytic oxidation of the grease by cuprous-metals, e.g in the cages of rolling element bearings To prevent rusting, particularly of rolling element bearings, during storage and use

To prevent scuffing and wear under boundary lubrication conditions, particularly those arising temporarily from shock loads

Extreme pressure

24.82 Properties

The essential property of a grease is that it possesses a yield stress up to which it only deforms elastically and above which it flow plastically When flow commences the ratio shear stress/rate of shear decreases smoothly until at shear rates in the region of lo6 s-‘ it closely approaches that ratio for the liquid phase of the grease, i.e its viscosity Above the yield stress greases are non- Newtonian liquids, and at any point the ratio shear stress/rate of shear is called its ‘apparent

viscosity’, which is, in effect, the viscosity a Newtonian fluid would have if it exhibited the same

shear stress at the same shear rate

The significance of these properties, in relation to plain bearings is that under stationary conditions grease tends to remain in place in clearance spaces and at the ends d bearings Thus lubricant is available immediately the machine starts up ag+n, and grease clinging to the ends of bearings acts as a seal to exclude dirt

Semi-fluid greases of negligible yield stress reduce leakage from gearboxes by virtue of their very high apparent viscosity at low rates of shear They also permit feeding through loug narrow bore piping, particularly at subzero temperatures

The yield stress of a grease is not easily measured and for production quality control and other

ordinary purposes the worked Penetration (IP 50/69, ASTM D217C8) i.e the depth in mm of the penetration in the grease of a special metal cone under its own weight, is used The National Lubricating Grease Institute has classified greases according to their consistency after a specified amount of mechanical working as follows:

T a b 24.3 NLGI GREASE CLASSIFICATION

445-415

W 3 0 355-385 310-340

265-295 220-250 175-205

130-160 85-115

No 2 grade is popular since it combines satisfactory yield properties with easy pumpability, but

where there are extreme vibration and shock loads a No 3 grade is preferred Grades more fluid

than No 0 or stiffer than No 3 are not normally used for roller bearings

Important properties of the thickener structure are temperature stability, resistance to water and

mechanical stability Table 24.4 lists the various types of thickeners and indicates the extent to which they have these properties

Tsble 24.4

Thickener type Temperature stabiky Wafer resistance Mechanical Stability

COMPARISON OF GREASE THICKENERS

Synthetic oils are used in place of mineral oils for the liquid phase where their special advantages outweigh their greater m t Diesters are particularly useful where low volatility and good performance at low temperature are needed, e.g in aircraft bearings Polyglycols are used

where good oxidation stability and good lubrication between steel and bronze are required, also

for special cases where the liquid phase is required to evaporate at high temperature without

passing through deposit-forming decomposition stages Silicones are used where good stability is

required at high temperatures without the conditions of load and speed being at all severe

Fluorocarbons are, however, preferred in spite of their very high cost where maximum resistance

to oxidation is required, e.g from contact with liquid oxygen or ozone

24.9 Oil additives

Plain mineral oils are used in many units and systems for the lubrication of bearings, gears and other mechanisms where their oxidation stability, operating temperature range, ability to prevent wear, etc are adequate The addition of fatty oils improves boundary lubrication properties at the expense of oxidation stability and demulsibility, but over the last 30 years oil-soluble chemical Compounds called ‘additives’ have been developed which improve or confer a wide range of

properties The functions required of these ‘additives’ gives them their common names as indicated

in Table 24.5

Tabk 245 TYPES OF ADDITIVE

Viscosity index improvers

Neutralise contaminating strong acids formed for example by com- bustion of high sulphur fuels or, less often, by decomposition of active

EP additives Reduce surface foam Reduce oxidation Various types are: oxidation inhibitors, retarders; anti-catalyst metal deactivators, metal passivators

Reduces rusting of fmous surfaces swept by oil Reduce wear and prevent scutfing of rubbing surfaces under steady load operating conditions, nature of film uncertain

Type (1) Reduces corrosion of lead Type (2) Reducff corrosion of cuprous metah

Reduce or prevent deposits formed at high temperatures, e.g in i.c

engines

Prevent deposition of sludge by dispersing a finely divided suspension of insoluble material formed at low temperature

Form emulsions either water in oil or oil in water according to type

Prevent scuffing of rubbing surfaces under shock load operating con-

ditions mainly by formation of inorganic surface films

Reduce friction under boundary lubrication condition, increase load carrying capacity especially where limited by frictional temperature rise,

by formation of organic surface film Examples are fatty acids and their esters

Reduce pour point of paraffinic oils Reduces loss of oil by gravity, eg from vertical sliding surfaces, or centrifugal forces

Reduce the decrease in viscosity due to increase of temperature

Oil additives 24-11 249.1 Machinery lubricants

As shown in Tables 24.6 and 24.7 below, additives and oils are combined in various ways to

provide the performance required It must be emphasized, however, that indiscriminate mixing can

produce undesirable interactions Indeed some additives may be included in a blend simply to

overcome problems caused by other additives

24.93 cuttingoils

Factors entering into the selection of cutting oils are: the material of the workpiece; the speed and nature of the operation; whether cooling is more important than lubrication; and the compatibility

of the cutting oil with the machine tool

Table 24.7 gives a very general system of lubricant selection

Table 24.6

Type of machinery usual base oil type Usual additwes Speeiol reqt&ements

TYPES OF OIL REQUIRED FOR VARIOUS TYPES OF MACHINERY

Food processing Medicinal white oil

Plain roll-neck bearings HYI

distillates

Gears (st&/steel) HVI or MVIN

Gears (steel/bronze) HVI

Hermetically sealed MVIN

refrigerators

None None Antioxidant Anti-rust Anti-wear

Pour point deprasant

VI Improver Anti-foam Antioxidant

h t i - N S t None or fatty oil

Antioxidant, anti-rust Anti-wear EP antioxidant Anti-foam Pour point depressant Oiliness, tackiness None

Detergent Dispersant Antioxidant Acid-neutralizer Anti-foam Anti-wear Corrosion inhibitor

Safety in case of ingestion Best demulsibility Minimum viscosity change with temperature Minimum wear of steel/

steel

Ready separation from

water, good oxidation stability

Maintenance of oil film on

hot surfaces, resistance

to washing away by wet Steam

Low deposit formation tendency

Protection against wear and scuffing

Maintains smooth sliding

at very low speeds Keeps film on vertical stirfaces

Good thermal stability, miscibility with refrigerant, low floc

point

Vary with type of engine

thus affecting additive combination

Table 24.7 CHIP-FORMING METALWORKING LUBRICANTS

Soluble oil (oil-in- LVI oil Emulsifiers With 20-50 parts water

cutting operation where cooling and absence of fuming important

24-12 Lubricants

Table 24.1

Type of lubricmrt Rase lubricant Additive R m r h

Aqueous cutting solution Water Rust inhibitora As for soluble oil

Inactive EP cutting oil HVI Mild EP but no free For cutting yellow metal

alloys where good

lubrication without staining required Active EP cutting oil HVI Mild EP and free For heavy cuts on tough

24.9.3 Lubricants for chipless-forming

Lubricants for chipless-forming probably present a greater range of diversity than any other branch

of lubrication Table 24.8 gives the types of lubricants used in drawing, stamping and pressing, and Table 24.9 gives lubricants used in rolling

TaMe 24.8

PRESSING

TYPES OF LUBRICANTS FOR DRAWING, STAMPING AND

Metul Lubricant in order of severity of operation

Steel Mineral oils of medium to heavy viscosities

Fatty oiymineral oil blends Soap solutions

Soap/fat pastes Baked-on-lime coatings Soft metak e.g lead Dilute soap solutions Light mineral oil Soap/fat pastes with solid lubricants

Dried on soap Colloidal graphite in low volatile mineral oils Graphite in volatile solvents

Mineral oils, viscosity increasing with seventy Mineral oil with l0-15% fatty oil

Brass and copper

Table 24.9 ROLLING OILS

M e a l

Lubriemrt in order of severity of operation

Mineral oil/fatty oil blends and with lubricity and

EP additives Palm oil Mineral oil Mineral oil with lubricity additive Mineral oils of viscosity from 40 mN s m-’ at 20°C to

50 mN s m-’ at 40°C with lubricity additives Brass and copper Oil-in-water emulsion

Aluminium

Oil additives 24-13 BIBLIOGRAPHY

F P Bowden and D Tabor, ‘The Friction and Lubrication of Solids’, Parts I and 11, Oxfore, 1954, 1964

E L H Bastian, ‘Metalworking Lubricants’, McGraw-Hill, New York, 1951

‘Lubrication and Wear: Fundamentals and Application to Design’, Proc Inst m c h Engrs, 1967-68, 182, part 3A

A Glossary of Petroleum Terms’, Inst Petroleum, London, 1961

25 Friction and wear

The friction and wear characteristics of materials are not intrinsic properties but, rather, depend

on a large number of variables including the physical, chemical and mechanical properties of the

material and surfaces and the environment

required to initiate or maintain motion If Wis the n o d reaction of one body on the other, the

coefficient of friction p is defined as ,p = F/ W

STATIC A N D KINETIC FRICTION

If the force to initiate motion of one of the bodies is F, and the force to maintain its motion at a given speed is F k , there is a corresponding coefficient of static friction ps=Fs/Wand a coefficient of

kinetic friction pk=Fk/W! In some cases these c&cients are approximately equal, in most cases

& > p k and there is a tendency for intermittent or ‘stick-slip’ motion to occur

BASIC LAWS OF FRICTION

The two basic laws of friction, which are valid over a wide range of experimental conditions, state that:

1 The frictional force F between solid bodies is proportional to the normal force between the

surfaces, i.e p is independent of W

2 The frictional force F is independent of the apparent area of contact

25.1.2 Friction of dubricated materials

When clean metal surfaces are placed in contact they do not touch over the whole of their apparent area of contact The load is supported by surface irregularities (asperities) which deform

plastically as the load is applied The area of real contact is approximately proportional to the

load and almost independent of the size and geometry of the surfaces.’ This is also the case when asperity contact is primarily elastic: which may occur with well run-in surfaces, particularly in the

presence of a lubricant or surface oxide films The limiting values to the true area of contact3 for a wide range of practical situations are W / p and lOW/p, where Wis normal load and p is plastic

flow pressure of the asperities, of the same order as the indentation hardness of the material

For very clean surfaces strong adhesion occurs at regions of real contact, a part of which may be

5 1

25-2 Friction and wear

due to cold-welding, and these junctions must be sheared if sliding is to take place Thus, it is almost impossible to slide such surfaces in a vacuum and complete seizure often occurs as shown

in Table 25.1 However, if the surfaces are contaminated the adhesion is much weaker because the

formation of strong junctions is inhibited For example, hydrogen or nitFogen atmospheres have

little effect on in-vacuo coefficients of friction, but the smallest trace of oxygen or water vapour

produces a profound reduction in friction (Table 25.1) A further reduction in the coefficient

of friction often occurs at high sliding speeds, p-aticularly at speeds sufficient to produce local hot-spots and surface melting: e.g ice at 0.1 m s or steel at 500 m s-l for which p may be less

than 0.1

Table 25.1 STATIC FRICTION OF METALS (SPECTROSCOPICALLY PURE) IN VACUUM

(OUTGASSED) AND IN AIR (UNLUBRICATED)

Condirions Ag A1 Co Cr Cu Fe In Mg Mo Ni Pb Pt

p,metal onitselfin vacuo S S 0.6 1.5 S 1.5 S 0.8 1.1 2 4 S 4

ps metal 011 itself in air 1.4 1.3 0.3 0.4 1.3 1.0 2 0.5 0.9 0.7 1.5 1.3

S signilies gross seizure (@=lo)

Friction values for metal couples in air depend on a number of factors Principal ones are the

tendency for formation of oxide fdms, the degree of deformation in sliding, the ability of oxide films to survive sliding contact and the tendency for transfer of material from one surface to the other Table 25.2 shows the relative hardnesses of some common metals and their oxides and the load

(forasphericalslider onpolishedsurfaces)at whichappreciilblemetalliccontact occurs.Thus,theoxide

on copper is not easily penetrated, whereas the very hard aluminium oxide on the soft aluminium

substrate is readily shattered during sliding Thick oxide films, such as produced by anodizing aluminium, may be more protective because sliding deformation can be restricted entirely to the oxide Similarly, with very hard metal substrates, such as chromium, the surface deformation may

be so small that the oxide is never ruptured

Table 25.2 BREAKDOWN OF OXIDE FILMS PRODUCED DURING SLIDING

Vickers hardness (kg mm-') Load (g) at which

The static coefficients of friction of a number of metals and alloys on steel are shown in Table 25.3 Of particular note are the values for indium and lead, which are the same as those for sliding

on themselves (see Table 25.1) Pick-up occurs on the steel surface such that the sliding couple

becomes the metal on itself

Static friction of ferrous materials is shown in Table 25.4 The data illustrate the

effect of increasing hardness on reducing friction through greater support'of the surface oxide,

the effect of second phases such as carbides and graphite in d u c i n g adhesion of junctions, and the effect of the very thin oxide coating on austenitic stainless steel which is easily ruptured in slid-

ing leading to a high coefficient of friction

Friction 25-3

Table 25.3 STATIC FRICTION OF UNLUBRICATED METALS AND ALLOYS (PREPARED GREASE FREE)

The results quoted are for sliders of pure metals and alloys sliding over 0.13% C, 3.42% Ni, normalized steel

The results on mild steel are essentially the same

Steel (0.13 C, 3.42 Ni) Tin (pure)

White metal (tin-base):

Cast iron (pearlitic) 200 0.3-0.4

Pure iron (cold-welded) 150 1-1.2

Ps

0.5 0.5 0.35 0.5 0.8

0.9

0.8

0.5 0.7

-

VPN

(Hoffman) Tool steel (C 0.8, 900 0.3-0.4

containing carbides)

(bard bright)

Table 25.4 STATIC FRICTION OF UNLUBRICATED FERROUS MATERIALS ON THEMSELVES

Tabfe 25.5 FRICTION OF VERY HARD SOLIDS

(a) Bonded tungsten carbide (cobalt binder)

0.2 0.9 Rises rapidly above 1800°C

2!W Frictionandwear

Very hard solids often have low wfficients when sliding on themselves or other materials

because of the limited surface deformation that occurs during sliding (Table 25.5)

Similarly, very low coefficients of friction may be obtained by plating hard metal substrates with thin soft metal f h s (Table 25.6) The substrate supports the load while slidmg occurs within the soft film Typical film thicknesses are 1 to 10 pn

Table 25.6 FRICTION OF THIN MmALLIC FILMS

(Sliding on a 6 m m diameter steel sphere)

Co&cient of static friction p s

Lood Indiumfilm Indiumfilm Leadfilm Copperfilm

B on steel on silwr on capper on steel

The friction of many materials is little alFected by high or low temperatures (see Table 25.7)

Exceptions are when the plastic flow pressure changes significantly or when oxide 6lms become very much thicker

Table 25.7 FRICTION OF MATERIALS SLIDING ON THEMSELVES AT LOW AND HIGH

1.52 0.26 0.81 0.97 1.06

of true contact may increase with time because of creep and the starting friction may be correspondingly larger Thirdly, the friction may show changes with speed which reflect the visco-

elastic properties of the polymer but the most marked changes occur as a result of frictional heating Even at speeds of only a few m s-l the friction of unlubricated polymers can rise to very high values On the other hand at extremely high speeds the friction may fall again because of the formation of a molten lubricating a m

The main effect of speed of sliding is the generation of high I dtemperatures produced by frictional heating at the regions of real contact Local hot-spots may produce phase changes or alloy formation at or near the sliding interface, they may produce local melting and they may

greatly change the rate of surface oxidation At speeds of a few m s-l these effects are not BS

marked as at very high speeds (see Table 25.10) but they may still be significant In general the kinetic friction at moderate speeds is of the same order as the static friction (compare previous

tables) but is usually somewhat smaller Results in Table 25.9 are for stationary sliders rubbing on

a mild-steel disc rotating at a few m s-' The materials are grouped in descending order of friction

At very high sliding speeds the friction generally falls off because of the formation of a very thin molten surface layer which acts as a lubricant Although this is, broadly speaking, the main

trend other factors may considerably change the behaviour For example, with steel sliding on

diamond the friction first diminishes and then increases, because at higher speeds the steel is

Friction ?,5-5

transferred to the diamond so that the sliding resembles that of steel on steel In some cases the metals may fragment at these very high speeds particularly if they are of limited ductility Again, if appreciable melting occurs the friction may rise at high speeds because of the viscous resistance of the liquid interface: this occurs with bismuth

Table 25.8

LOW SLIDING SPEEDS'.'.'

FRICTION OF STEEL ON POLYMERS ROOM TEMPERATURE,

Low density polythene (no plasticizer)

Low density polythene (with plasticizer)

High density polythene (no plasticizer)

Soft wood

Lignum vitae

PTFE (low speeds)

FTFE (high speeds)

Filled PTFE (15% glass fibre)

Filled PTFE (15% graphite)

Filled PTFE (W/, bronze)

Rubber (polyurethane)

Rubber (isoprene)

Rubber (isoprene)

Dry Wet Dry Dry Dry Dry or wet Dry or wet Dry or wet Natural Natural Dry or wet Dry or wet Dry Dry

B Y Dry Dry

Wet (water-alcohol solution)

0.4 0.15 0.5 0.5 0.5 0.4 0.1 0.15 0.25 0.1 0.06 0.3 0.12 0.09 0.09 1.6 3-10 2-4

Table 25.9 KINETIC FRICllON OF UNLUBRICATED MATERIALS

SLIDING ON MILD STEEL AT SPEEDS OF A FEW m S - '

Nickel, mild-steel

Aluminium, brass (7030) cadmium, magnesium

Chromium (hard plate), steel (hard)

Copper, copper+admium alloy

0.46

0.34 0.34 0.18 0.4 0.4

0.2

0.13 0.08

Table 25.10

600 m S-') SLIDING ON A SPHERE OF BALL-BEARING STEEL

W T I C FRICTION OF UNLUBRICATED METALS AT VERY HIGH SLIDING SPEEDS (UP TO

Duration of Coefficient offiiction p t

25-6 Friction and wear

25.13 Friction of lubricated snrfaces

Length of

chain K

Lubricant

DEFINITIONS

When moving surfaces are separated by a relatively thick 6lm of lubricant the resistance to motion

is due entirely to the viscosity of the interposed layer The friction is extremely low (p=O.oOl~.OOOl) and there is no wear of the solid surfaces These are the conditions of hydrodynamic lubrication under which bearings operate in the ideal case If the pressures are too high or the sliding speeds too low the hydrodynamic film becomes so thin that it may be less than the height of the surface irregularities The asperities then rub on one another and are separated

by h sonly one or two molecular layers thick The friction under these conditions ( p z O o 0 5 to 0.15) is much higher than for ideal hydrodynamic lubrication and some wear of the surfaces occurs This type of lubricated sliding is called 'boundary' lubrication.'" The friction does not depend on the viscosity of the lubricant, but on a more elusive property sometimes called 'oiliness' Under boundary conditions as for unlubricated surfaces the frictional resistance is proportional to the load and independent of the size of the surfaces

In certain circumstances a further type of lubrication, known as elastohydrodynamic lubri- cation, may obtain It arises in the following ~ a y " * ' ~ ~ ' ~ , ' ~ U nder conditions of severe Ioading the moving surfaces may undergo appreciable elastic deformation: this not only changes the

geometry of the surfaces, it also implies that very high pressures are exerted on the oil film The main effect of this is to produce a prodigious increase in the viscosity of the oil For example at

contact pressures of 30,60,1M) kgmm-2 (such as may occur between gear teeth of hardened steel) the viscosity of a simple mineral oil is increased by 200,40 OOO and 1 OOO 000 fold respectively Thus the harder the surfaces are pressed together the more difficult it is to extrude the lubricant Consequently effective lubrication may obtain under conditions where it would normally be expected to break down

In general, elastohydrodynamic lubrication becomes effective when the oil film thickness is of the order of lO-'-lpm This is very much thicker than the boundary film (1-1Onm)

but it is very small in engineering terms Consequently for practical exploitation of elastohydro- dynamic lubrication the surfaces must be very smooth and carefully aligned

Most boundary lubricants are used as additives, dissolved as a few per cent in a mineral oil:

be beneficial but later they lead to polymerization, gumming and the formation of other deleterious products

Table 25.12 STATIC FRICTION OF VARJOUS METALS

(SPECTROSCOPICALLY PURE) LUBRICATED WITH 1%

SOLUTION OF LAURIC ACID (M.P UT) IN PARAFFIN

OIL AT ROOM TEMPERATURE

0.4

1.4 1.0 0.5

0.7

1.3 1.4

0.3 0.05 0.34 0.10 0.15 0.10

0.3

0.25 0.55

0.10 0.095 0.095 0.085

0.1 05 0.105 0.105 0.08

0.10 0.095 0.095 0.085

Mineral oils

Light machine Thick gear Solvent r e 6 n d

Heavy motor

BP parafEn Extreme pressure Graphited oil Oleic acid Trichlorethylene Alcohol Benzene

0.16 0.125 0.15 0.195 0.18 0.09-0.1 0.13 0.08 0.33

0.43 0.48

0.2

0.19 0.15 0.2 0.205 0.22 0.09-0.1 0.15 0.08

Rape oil Castor oil Mineral oil fatty acids

25-8 Frietion and wear

251.5 Extreme prerrsme (EP) lubhut&

Even the best boundary lubricants (e.g long-chain acids or soaps) cease to provide any lubrication above about 200°C Since localized hot-spots of very much higher temperature an often reached

in running mechanisms it is necessary to use surface films that have a high melting point and

which, as far as possible, possess a low shear strength One obvious method is to coat the metal

with a thin film of a, softer metal These fdms are effective up to their melting point but are gradually worn away with repeated sliding Other materials which are very efFective are listed in Table 25.15

Molybdenum disulphide 0.07-0.1 -8OO'C

Another approach is to form a protective film in situ by chemical attack, a small quantity of a suitable reactive compound being added to the lubricating oil The most common materials are additives containing sulphur or chlorine or both Phosphates are also used The additive must not

be too reactive, otherwise excessive corrosion will occur The results in Table 25.16 are based on

laboratory experiments in which metal surfaces were exposed to H,S or HCI vapour and the frictional properties of the surface examined The results show that the films formed by H,S give a higher friction than those formed by HCI: however in the latter case the films decompose in the p"sence of water to liberate HCI and for this reason chlorine additives are less commonly used

than sulphur additives

The detailed behaviour of commercial additives depends not only on the mctivity of the metal

and the chemical nature of the additive but also on the type of carrier fluid used (e.g aromatic, naphthenic, paratfinic) Further the chemical reactions which occur are far more complicated than

originally supposed With sulphurized additives oxide formation appears to be at least as

important as sulphide formation With phosphates the surface reaction is still the subject of dispute

T a b U16 EFFECT OF SULPHIDE AND CHLORIDE FILMS ON FRICTION OF METALS

Metal

chloride film

Sulphide film

C o d with Cowred with,

Clean Dry lubric&hgofl Dry lubricutingoil

The differences in friction are not very marked showing that the friction is a very poor criterion of

the effectiveness of an EP lubricant Marked ditferences in seizure-preventing properties are often

Wear 25-9

Table 25.17 KINETIC FRICTION, INITIAL SEIZURE LOADS AND WELD LOADS OF BALLBEARING

STEEL SURFACES LUBRICATED WITH TYPICAL EP ADDITIVES.’6 FOUR BALL MACHINE

FRICTION MEASUREMENTS AT 10 kg LOAD

Lubricant Coefficient Initial seizure Weld

offriction load load

Mineral oil Zinc di-secbutyl thio-phosphate 0.09

Mineral oil Sulphurized (10%wt) sperm oil (5% wt) 0.095

Chlorinated additive (1% wt) 0.085

Mineral oil Tributyl phosphate (1% wt) -

Parallin oil Tributyl phosphate (lxwt) -

Mineral oil Tricresyl phosphate (1% wt) -

Paratfin oil Tricresyl phosphate (1% wt) -

accompanied by almost indistinguishable coefficients of friction The last four lines of the table also show that EP effectiveness depends to some extent on the nature of the base oil

25.2 Wear

DEFINITIONS

Wear is the progressive loss of substance from the operating surface of a body occurring as a result

of relative motion at the surface Wear is usually detrimental, but in mild form may be beneficial, e.g during the running-in of engineering surfam The major types of wear are abrasive wear, adhesive wear, erosive wear and fretting Abrasive wear is wear by displacement of material caused

by hard protuberances or particles AdKesive wear is, strictly, wear by transference of material from one surface to another due to the process of solid-phase welding Adhesive wear is often used, loosely, to describe other metal-to-metal wear mechanisms, including the removal of particles detached by fatigue arising from cyclic contact stresses and in which no adhesion occurs Erosive

wear is loss of material from a solid surface due to relative motion in contact with a fluid which

contains solid particles or collapsing vapour bubbles Fretting is a wear phenomenon occurring between two surfaces having oscillatory motion of small amplitude and is used, frequently, to include fretting corrosion, in which a chemical reaction predominates

25.2.1 Abrasive wear

Abrasive wear rates and relative wear resistance (defined as wear of a reference material divided by wear of a test material) vary considerably for abrasives of different hardness, size and shape Wear rates increase approximately linearly with increasing applied load per unit area up to loads at

which extensive failure of the abrasive occurs Figure 25.1 shows the major effect of relative

hardness of the worn surface and abrasive on volume wear rate Thus, relative wear rates in practice may vary over a wide range, as shown in Tables 25.18 to 25.22 Bulk properties of

materials are very approximate guides only to abrasive wear resistance, but wear resistance generally increases as the material bulk hardness increases, except when material is hardened by prior plastic deformation

25.2.2 Adhesive wear

Metal-to-metal wear involves the contact and interaction of asperities on two surfaces Local stresses at asperities may be high even when applied loads are low Adhesive wear is promoted by two major factors:

1 The tendency for different materials to form solid solutions or intermetallic compounds with one another Thus, material combinations of different crystal structure and chemical proper-

ties tend to have lower wear rates and friction Figure 25.2 illustrates the tendency of metal

couples to adhere together

2 The cleanliness of the surface Cleaner surfaces are more likely to bond together Surfaces having a thick oxide &n have low wear Stainless steels and nickel alloys, that do not form thick oxides, have poor adhesive wear resistance

25-10 Friction and weur

1

Hardness of worn surface/hardness of abrasive

Figme 25.1 Efect of abrasim hardness on wear rate of merallic materid and

cerMdcs worn on 80-400 pin commercial bonded abrasiws undm an applied stress of

I MNm-z.z'*22~23 (Reproduced from The Fulmer Materials Opiimizer by permission

of Fulmer Research Instizute Ltd.)

In metal-to-metal wear, two forms of wear debris are often observed; at very low and very high loads the debris is mainly oxide, but at intermediate loads it is metallic The transition from oxidative to metallic wear is accompanied by a rapid increase in wear rate The transition load

varies for different materials, microstructures, sliding speed and environment Thus, wear rates of

materials vary by several orders of magnitude (Table 25.23)

Surface treatments are often beneficial in metal-to-metal wear, through a change in surface chemistry, an increase in surface hardness, a change in surface structure or a change in surface topography Certain coatings are beneficial during running-in, e.g phosphating and sulphidized

coatings, causing metal asperity separation and adherence of lubricant films Tables 25.24 and 25.25 show the performance of a number of coated and uncoated metal pairs

25.2.3 Erosive wear

Erosive wear due to the impact of a stream of solid particles is dependent on the size, hardness,

velocity and angle of impact of the particles Wear rate generally increases rapidly with increasing particle size and hardness and impact velocity For strong and tough materials the maximum wear

rate occurs at an impact angle of about 30°, but for hard and brittle materials it occurs at an

impact angle of about 90" and for tough and elastic materials at an impact angle close to 0"

(Figure 25.3) Thus, material ranking order changes occur for different erosive wear environments

25-12 Friction and wear

Tabk 25.1% COMMONLY USED MATERIAL

Wear rates relative to 0.4:4 C low alloy steel quenched and tempered to about 500 V i e k s hardness

Wear in Wear on WeaF by Wear of Wear by laboratory commercia[ blast f m a c e ball mill flint stone jaw bonded

Type of' Typical commercially wear grinding agricul- siliceous flint material auailuble materials by coke sliding impact quartz ores turd soil ores abrasive

Mn/NI ferrous alloys, manual

3.5% B, Ni.alloy

The Sinter wag produced from foreign ore with ASTM $strength index of about 47

Reproduced by courtesy of Fulmer Research Institute Ltd

5 1.0

0.8 0.8

- 1.0 1.0

- 1.0

0.9 -1.0 1.2

0.3 1.5

0.3 0.4

1 .o 0.5

0.25-0.4

- 0.3 0.2-0.5 -0.3

1 O 0.55

-

- 0.55 0.8

-

0.250.7 0.35 0.45-0.8 0.85

1Jsually convenient with good

design to facilitate replacement

Usually convenient with good

design to facilitate replacement

Replacement can be difficult if

applied in situ These materials

are often chosen because hard

weld may he built up and

worn away several times to its

total depth under severe wear

situations

Cast irons are very suitable materials to resist medium to high stress abrasive wear due to their good wear resistance and reasonable cost At very severe levels of impact abrasion, however, inadequate toughness can be a pro- blem and only materials of the work- hardening type should be nsed Also cheaper materials may be preferred due to the ex- cessively high wear rates involved Due to the very large quantity production in- volved, steels tend to be comparatively cheap

Thus steels with low wear rates become a competitive materials choice

Their main application lies in hardened steels

to resist medium stress abrasion as very low wear rates can be obtained

Austenitic manganese steels can be used in more severe situations due to their work- hardening capability

For medium and high stress abrasion hard- facings give low wear rates generally, and SO

are used in many situations to resist abrasive wear, e.g excavator teeth and other earth moving applications

These materials have the merit that a combination of strength i.e toughness and hardness, may be readily obtained by varying the alloying method of manufacture, and treatment; thus giving suitable combinations of these properties lo

suit a particular application and wear situation Various techniques

of surface hardening can also be employed to improve resistance to abrasive types of wear Other pro- ducts are sintered metals and metal coatings, e.g Cr plate and sprayed coatings

Table 25.18b

quenched and tempered to about 500 vickerb

Sliding by blast by blast 384 pm

a

c

d

!

Type of Typical commercially available wear furnace furnace Pint Ease and convenience of

As above with 2% by volume

25 x 0.4 mm diam, wire fibres

Concrete tile - 6mm wear

resistant surface

Wear resistant rubbers, 55"-

70" shore hardness

65" shore hardness rubber

with saw tooth surface profile

Polyurethane

High density polyethylene

Epoxy resin based PTFE

Calcined bauxite filled epoxy

4.5 3.5

40

11

- 0.2 -0.2 6.9 0.9

Convenient if ceramic is bolted

in place Less convenient if ce- ramic is fixed by adhesive or cement as long curing times may lead to unacceptably long down-times

U s 4 in sheet form where transparency is required Long curing times can lead to unacceptably long down-times

Can be messy and difficult un- der dirty conditions

Banded and bolted Sticking with adhesive can be difficult under dirty conditions

Usually used in sheet form

Difficult to bond plastic to component Solid moulded components are superior but are limited to small sizes

Possible to achieve very high hardness but brittleness tends to be a problem Most suitable to resist low stress ab- rasion by low density materials and powders

Glass is brittle and so it is only used

at the lowest levels of abrasive wear Useful to resist wear of irregularly shaped components and when ab- rasion is of low to medium stress

Very useful to resist impact abrasion - most wear resistant at 90" impact angles Softer types of rubber are used for low stress impact abrasion

Resilient rubber for more severe impact

Low coefficient of friction, good anti- sticking properties Best for low stress abrasion by fine particles Resin bon- ded aggregates are trowellable and so are useful to resist wear or irregularly shaped components

Also useful in large flat areas, especially when cur- ing time is no real pro- blem, e.g aircraft hanger flooring, etc

Easily castable Bonding of rubber to com- ponent is a very large pro- blem in high stress ab- rasive wear Good anti- sticking properties and low density

Composite plastics are only as tough as their bonding matrix and there- fore find more applications where low stress abrasion

by powders or small par- ticles takes place

-

Figure 25.3 Eflkct of impact angle on erosion wear of materials impacted with dry 0.2-1.5mn

(Reproduced from The Fulmer Materials Optimizer by permission of Firlmer Research Institute Ltd.)

The performance of materials in erosion by sandy water and in pneumatic conveying are given in Table 25.26

In cavitation, vapour bubbles formed at low pressure collapse in high pressure regions Cavitation erosion is wear resulting from localized high impact stresses when bubbles collapse

at or close to a surface The cavitation erosion resistance of a range of materials is given in Table 25.27

25.2.4 Fretting wear

Fretting wear occurs when two contacting surfaces are subject to very small oscillatory slip (of no

more than I50 pm) Damage occurs when oxide films are disrupted locally, and may proceed by continuous formation and removal of the oxide, by the abrasive action of the oxide or by localized formation and failure of metal-to-metal adhesive bonds .The rate of fretting wear is normally very low-about 0.1 mg per lo6 cycles, per MN m-’ normal load, per pm amplitude of slip for mild steei However, localized cyclic stresses may enhance fatigue crack initiation causing up to SO%

reduction in fatigue strength

Fretting damage is reduced by eliminating slip (by increasing the contact pressure or separating the surfaces) by lubrication (to separate surfaces and wash away debris) and by surface treatments such as electrodeposits of soft metals or chemical conversion coatings of phosphate and suiphi- dized coatings on steels and anodized coatings on aluminium alloys