Handbook of Lubrication Episode 2 Part 12 pdf

While surface hardness becomes relatively less significant at high angles of impact, coatings of very hard materials are being used to prevent erosion damage at both high and low angles

FIGURE 5 Effect of impact angle on

ero-sion of ductile metal and brittle solid (From

Wolfe, G F., Lubr Eng., 19, 28, 1963.

With permission.)

FIGURE 6 Erosion as a function of impact

angle (From Eyre, T S., Tribol Int., 11(2),

91, 1978 With permission.)

Erosion

Erosive wear can be caused by either solid participates or liquid droplets striking the surface at high velocities 16,17 In this chapter, emphasis is placed on erosive wear by solid particle impact Typical problem areas include: compressor and turbine blading, helicopter rotor blades, impeller-type pumps, pressure letdown valves, etc

Since the contact stress results from kinetic energy of the particles in the fluid stream, size and density of the particles, their velocity, and angle of impact must all be considered Erosive weight loss is roughly proportional to the square or cube of velocity Relative hardness and shape of the particle, its fracture characteristics, and the ductility of the solid surface also influence the resulting damage

As illustrated in Figure 5, cutting wear predominates at low angles of impingement for ductile metals unless the particles are smooth spheres The harder the surface, the lower the rate of material removal At higher angles of impact, hard, brittle materials show more erosive wear as elastic properties of the surface become much more important Annealed materials often erode less than the same alloy in the hardened state An elastomeric coating may be a very viable solution to erosion at high angles of impingement Figure 6 compares the relative effect of impact angle on wear rates for a metal and a rubber material

While surface hardness becomes relatively less significant at high angles of impact, coatings of very hard materials are being used to prevent erosion damage at both high and low angles of impact For example, Hansen et al.18 evaluated a large variety of metals, alloys, carbides, and ceramics in a sandblast type of test at 20 and 700°C with 90 and 20° angles of impingement As shown in Table 3, some ceramics and carbides with a low metal binder content were more erosion resistant than metals or alloys Hard coatings that were particularly effective included: chemical vapor-deposited SiC, electrodeposited TiB2, bonded

Mo, and bonded WC cermets Generally, coating thicknesses of about 50 to 80 µm (0.002

to 0.003 in.2) were necessary for adequate protection These test results have been partially verified by evaluation of control valve components in coal gasification plants

The above results are of fundamental interest for the following reasons:

1 Contrary to other basic studies (e.g., the results illustrated in Figure 5), some hard, brittle materials can be very effective at high-impingement angles

2 Most metals and alloys, except molybdenum, have essentially the same erosion rates

3 Thin, hard coatings can provide erosion protection for softer metal substrates

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Generally, rougher or more porous surfaces are less prone to lubricant depletion The valleys or pores also serve as reservoir traps for loose debris However, rough surfaces are also more susceptible to fatigue-type wear Sprayed metal coatings such as molybdenum or copper have been used successfully in certain applications These coatings tend to be porous because of oxidation of the metal during the spraying operation Another coating system which should be promising is bonded molybdenum with its surface hardness of about 3000 Knoop The coating could be applied by spraying molybdenum on the substrate, grinding the surface smooth, and then bonding to produce a hard, porous surface Other processes which can produce hard porous surfaces include spark hardening, selective etching, and porous chrome plating

Chemical Wear

Exposure of fresh metal surfaces, coupled with the high pressures and flash temperatures developed at contacting asperities, create ideal conditions for chemical reactions in sliding contacts These reaction films serve to prevent bare metal-to-metal contacts and welding or metal transfer However, under certain conditions, an excessive amount of soft reaction product is produced which then wears away rapidly

This corrosive wear could be attributed to a number of factors These include:

1 Excessively high-operating temperature This promotes lubricant oxidation to form acidic and corrosive products and also increases reaction rates

2 Use of reactive chemical additives (EP agents) Additives containing phosphorous, sulfur, or chlorine are often used in lubricants to form protective inorganic films on heavily loaded bearing surfaces Such compounds corrode certain bearing alloys, and also become overly reactive at high temperatures

3 Excessive moisture in the lubricant This problem is particularly acute in marine applications Tin-base babbitt forms a relatively hard “scab” with seawater contam-ination that can abrade a steel journal Pitting corrosion because of water contamcontam-ination

is a major cause of ball bearing failures on naval aircraft.19

4 Atmospheric corrosion Many industrial components operate, unlubricated, in exposed locations Rust formation of ferrous alloys and subsequent abrasion results in rapid material loss

Changes in bearing alloy composition, electroplating, diffusion treatments, chemical con-version coatings, and organic coatings are all potential solutions to the problem Research

is also being directed toward new techniques such as ion implantation to change the char-acteristics of metal surfaces

Cavitation-Erosion

Cavitation involves gas- or vapor-filled bubbles or pockets in flowing liquids as a result

of the dynamic generation of low pressure Collapse of these bubbles can generate extremely high pressures and velocities in the fluid Adjacent solid surfaces may be rapidly pitted and eroded by this action This type of wear is particularly serious in valves, impeller-type pumps, and propellers Hobbs20 correlated cavitation-erosion with ultimate resilience, ex-pressed as follows:

Plastics and elastomers with high-tensile strength and resilience have been used

success-fully as protective coatings on metal substrates Strong adhesion of the coating is essential Metal overlays or inlays are effective in certain applications when flame- or plasma-sprayed, welded, or electroplated In general, cavitation damage decreases with increasing hardness, particularly among materials of the same general class

SUBSTRATE AND COATING CONSIDERATIONS This section considers the various types of surface treatments shown in Table 1 and the processing variables involved

Coatings Applied on the Surface

Electroplating

This process is applicable to practically any metal surface and, by suitable preparation,

to plastics and many other nonconducting materials Since it is a low-temperature process (<100°C), warpage or dimensional changes are avoided

There are disadvantages Hydrogen embrittlement can occur with certain alloys Quality control and adhesion may be problems Since electroplating is a line-of-sight process, holes, recesses, and complex shapes should be avoided

Despite these problems, a variety of platings are used as wear-resistant coatings These range from soft, conformable coatings such as tin- or lead-base alloys, to hard chrome Thicknesses normally range from 2.54 (0.0001 in.) to 500 µm (0.020 in.), although platings

as thick as 3180 µm (0.125 in.) are possible with some metals Electroplated precious metals such as gold, silver, and rhodium are used for sliding electrical contacts as well as specialized bearing applications For selective plating of worn surfaces in the field, a porous electrode impregnated with proprietary plating solutions can be used to brush-plate limited areas.21 This technique is used to repair scratches or flaws in chrome-plated cylinders and to build

up worn areas on babbitted bearings

Electroplating can increase surface hardness, improve corrosion resistance, provide soft and conformable coatings, or create a nonsoluble material combination with lower adhesion

Electroless Deposition

Certain metals, such as nickel, copper, and cobalt can be deposited by chemical reduction from aqueous solutions at temperatures below 100°C Electroless nickel is most widely used Although more expensive than electroplating, electroless deposits are uniform and protective, and complex shapes including holes and ID surfaces can be coated Most metals, except lead, cadmium, tin, and bismuth can be plated The deposition rate is slow; thicknesses range from 2.54 (0.0001 in.) to 180 µm (0.007 in.)

Electroless nickel plate contains about 8 to 10% phosphorous As deposited, the hardness

is about 500 Vpn (49 Rc), but can be increased to about 1000 Vpn (70 Rc) by heat treating the plated part at 400°C Despite its hardness, practical experience with electroless nickel

as a wear-resistant coating has often been disappointing Lubrication is essential; electroless nickel is not recommended for dry sliding applications Silver plating the opposing surface

is reported to be beneficial When considering electroless nickel for a bearing surface, evaluations should be made under conditions simulating the actual application

Composite platings of very fine hard particles dispersed in an electroless nickel matrix appear to be much more effective than straight electroless nickel for wear resistance The particles are suspended in the plating bath and codeposit with the nickel, Silicon carbide is widely used for the hard particles, but diamond is also commercially available Particle size and shape are critical and the surfaces must be finished so that no sharp peaks project from the surface These platings are used extensively for molds which must resist abrasion from glass fiber-reinforced plastic parts22 and also for guides and rollers subjected to abrasion by textile fibers

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Vapor Deposited Coatings

Although the principles of vapor deposition have been known for over 80 years, industrial applications have been very limited Two application techniques are being used: chemical vapor deposition (CVD) and physical vapor deposition (PVD).23 In CVD, the coating is formed either from gaseous chemical reactants at the substrate surface, or by thermal de-composition of volatile compounds such as the carbonyls In PVD, the coating is evaporated

or sputtered from the source to the substrate Recent interest centers around the use of thin, hard coatings on cutting tools In CVD of titanium carbide on cemented tungsten carbide tool bits, as an example, titanium tetrachloride is vaporized, mixed with hydrogen and methane, and fed into a reaction chamber containing the tool bits These parts are heated

to 800 to 1000°C and the following reaction takes place at the surfaces;

Strong bonding takes place because of some diffusion Tool life is reportedly improved by factors of 4 to 10 Certain carbides, nitrides, borides, and oxides of metals such as titanium, silicon, tungsten, and chromium can be deposited

CVD is also used commercially to apply hard, wear-resistant coatings of silicon carbide

on carbon-graphite seal faces Test results have shown that this material runs best against itself in displaying outstanding resistance to wear by abrasives in the fluids.24

This process has limitations It is most economical when a large number of parts are treated simultaneously, but part size is limited by the size of the reaction chamber Process temperatures are so high that many substrate alloys would be annealed Reduction of the process temperature will retard diffusion and reduce adherence of the coating Vapor dep-osition should be useful for creating hard surfaces on small parts made from stainless steels and nickel- or cobalt-base superalloys

The lower processing temperature with PVD permits coating of high-speed steel tools without excessively softening the substrate Adherent coatings have been obtained at tem-peratures below 500°C

Sprayed Coatings

Any material that can be melted without decomposition can be sprayed as a surface coating.25 Plasma or detonation gun coatings of ceramics, carbides, and refractory metals (Mo and W) are of particular value for upgrading wear resistance A major disadvantage is that this is a line-of-sight process The densest and most adherent coatings are those sprayed perpendicular to the surface As the impact angle decreases, coating quality drops and spraying angles below 45° are definitely not recommended The amount of heat that must

be dissipated limits the ability of even specialized “mini-gun” equipment to coat bores less than about 75 mm (3 in.) in diameter and longer than 100 mm (4 in.) Flat surfaces and outside diameters are no problem

Practically any metallic substrate can be spray coated Size is no impediment With reasonable care, bulk temperature of the substrate can be kept below 175°C (350°F) While steel grit blasting is normal practice, steel particles embedded in the substrate can rust and cause blisters in the coating In critical applications, grit blasting with sharp, fresh Al2O3 abrasive avoids this problem All coating should be done as soon as possible after abrasive blasting Since it is difficult to roughen hard metal surfaces such as hardened steel by abrasive blasting, a thin-sprayed undercoat of metal such as nickel-chrome or nickel alu-minide should be applied first to provide a rough base for the final coating

For ceramic and carbide coatings, thicknesses range from about 100 (0.004 in.) to 1000

µm (0.04 in.) Heavier coatings of metals can be deposited, and plasma spraying is widely used for salvage and repair work If the coatings are to be finish-ground for bearing or shaft

surfaces, the as-sprayed coating should be at least twice as thick as the finished coating This will ensure minimum porosity and optimum cohesion and adhesion The finished coating thickness should be as thin as possible to minimize problems Coating vendors should be consulted in selecting sliding combinations Mating a sprayed oxide coating against a metal

is particularly risky since the metal tends to transfer to the ceramic as islands of work-hardened material which can then severely abrade the opposing metal surface Such com-binations should be carefully evaluated before specifying them for practical applications Where substrate corrosion is a problem, corrosion-resistant metal undercoatings, e.g., nickel aluminide, can help Soft nickel plate has also been used, but must be grit blasted before the final coating is applied Care is needed to avoid exposing the substrate

Other variations of these coatings involve spraying and fusing The fusing step requires very high temperatures which could affect the metallurgy of the substrate

Sputtering

In this process, atoms of material from a negatively charged target of the coating material are vaporized by bombardment with positive ions of an inert gas such as argon These atoms are then transported, in the vapor phase, through the plasma of ionized gas and deposited

on the surface to be coated.26,27Coatings of metals, alloys, solid lubricants, and hard materials such as oxides and carbides can be applied Substrate heating is negligible These coatings are characteristically uniform and very thin, ranging in thickness from 50 (500 Å) to 1000

nm (10,000 Å) The process is ideal for applying wear-resistant coatings on precision components such as gas bearings or rolling contact bearings because no subsequent finishing

is required

Reverse sputtering before coating removes any contamination from the surface and enables outstanding adherence Even very hard coatings such as TiB2 or Cr2O3 can be flexed, brinelled, or bent over a small radius without cracking Wear life of a sputtered coating appears to compare favorably with similar coatings 1000 times thicker deposited by other processes

Although the process requires a vacuum, it can be automated to some extent Graded coatings can be applied, without breaking vacuum, if the equipment has provision for multiple targets

In ion implantation, the evaporated material is ionized and accelerated to the workpiece

by electrical fields The ions actually penetrate the surface Both wear and corrosion resistance can be affected.28

Hard Facings

By welding, or spraying and subsequent fusion, various wear-resistant alloys can be deposited on metal substrates.29,30 This technique is widely used for heavy-duty industrial and construction equipment which is subject to severe wear by abrasion or impact-abrasion Hard facings are used on new equipment and also find wide application in building up and salvaging worn parts The process has many advantages: wide ranges of coating materials are available, heavy deposits are feasible, repairs can be made in the field, metallurgical bonding is obtained, some coatings are corrosion resistant, and expensive materials are conserved by applying the coatings on low-cost substrates For abrasion resistance, hard materials, e.g., metal-bonded tungsten carbide, cobalt alloys, and nickel-chrome-boron are used For maximum impact resistance, high manganese work-hardening steels perform best Chrome steels and low alloy or carbon steels are in many cases comparable or better in abrasion resistance than the more expensive cobalt-base alloys

Aside from processing temperature and cost, the major drawback to hard facings is the possibility of cracking in some applications, particularly with thick deposits The very high-surface temperatures involved may also affect the substrate

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Organic Coatings

Many plastic and elastomeric coatings have been used as wear-resistant surfaces on metal substrates Typical examples include:

Teflon ® Acrylics Nylon Polyurethanes (rigid) Vinyls Polyamide - imides Epoxies Polyphenylene sulfide Phenolics Aromatic polyesters

In many cases, these plastics also provide corrosion protection

By compounding powdered solid lubricants, such as MoS2, graphite, or Teflon into a suitable resin matrix, a variety of wear-resistant solid lubricant films with outstanding fric-tional characteristics have been obtained These coatings are covered in the chapter on Lubricant Types and Their Properties (Volume II)

Elastomeric coatings effectively prevent erosive or abrasive wear under certain conditions

As long as the velocity of an eroding particle is not too high, the elastomer surface deforms and recovers elastically with no damage In abrasive-blasting booths, durable rubber gloves protect the operator’s hands Hard, tough polyurethane elastomers are used to coat steel tires

on industrial equipment such as forklifts and carts which operate on rough, hard surfaces Some new abrasion-resistant plastic coatings have provided unique durability on the hulls

of icebreakers.31 A large number of candidates were screened and a nonsolvented poly-urethane and a nonsolvented epoxy were selected for trials After four years of service, both coatings have remained essentially intact Similar coatings have shown promise for pre-venting cavitation damage on ship propellers

These plastic-and elastomeric coatings can be applied by spraying, brushing, dipping, and fluidized bed Surface preparation is a major consideration and abrasive grit blasting has been found to be very suitable Durable coatings usually require heat curing at temper-atures up to 175°C (350°F), although some newer materials require tempertemper-atures as high as 350°C (660°F) This can be a problem with certain substrate materials, especially age-hardened aluminum alloys

Chemical Conversion Coatings

Various processes are used to form in situ inorganic coatings on metals.1,32,33 Unlike bonded solid lubricant films, these conversion coatings do not necessarily provide low friction

or long life Their primary function is to prevent bare metal-to-metal contacts and promote surface smoothing during the early stages of run-in when surface imperfections can penetrate through the lubricant film The most common types are phosphates, sulfides, and oxides Phosphating to produce a complex inorganic phosphate surface is the most widely used process For application, parts are immersed in aqueous solutions of phosphates at a tem-perature of about 93°C (200°F).32A manganese phosphate coating is generally best for wear resistance because it is relatively soft and tends to “smear” over the contact area Zinc phosphate produces a harder coating and is used primarily as a substrate pretreatment for improving the adherence of protective polymer coatings Coating thicknesses range from 2.54 to 38 µm (0.0001 to 0.0015 in.), depending on the application temperature and bath composition Such coatings can be applied to cast iron, steel, zinc, and cadmium, but not

to stainless or other corrosion-resistant alloys As a rule of thumb, 50% of the coating thickness penetrates the surface and 50% appears as dimensional growth The coatings are porous (more so in thicker layers) and this helps to retain the lubricant Phosphating is particularly useful for applications such as gears or piston rings where initial conformity may not be ideal Besides their beneficial effects on sliding, these coatings also provide corrosion protection

Sulfide coatings are generally applied from molten salt baths at temperatures ranging from 190°C to about 550°C.1,33 Both electrolytic and chemical processes are used Coatings are characteristically Jess than 10-µm (0.0004-in.) thick Because the salt baths contain cyanides

or cyanates, the coating actually consists of iron nitrides as well as sulfides Unlike the phosphates which are easily friable, sulfide coatings are very wear resistant They can be applied to a wide variety of ferrous alloys, even low-carbon steels which normally would not respond well to nitriding

Oxide films of significant thickness can also be produced on metal surfaces.1,33Anodizing aluminum to produce a hard A12O3 surface is widely used to improve wear resistance Magnesium, titanium, and beryllium can also be anodized These anodized coatings are hard and brittle surface layers, supported on relatively soft substrates; brinnelling loads can crack the coatings, resulting in high wear A similar result would be obtained if a hard particle were trapped between two anodized surfaces

Since anodizing is done at temperatures below 100°C, metallurgical changes are no prob-lem Films as thick as 100 µm (0.004 in.) are used By first creating a porous, hard-anodized coating and then impregnating it with Teflon® or other solid lubricant, improved sliding performance can be obtained These anodized films have a relatively short wear life in dry sliding; however, a thin wiped film of oil or grease increases the life dramatically

Ferrous alloys can be oxidized by various processes.1,33Proprietary salt baths of caustic/ nitrate solutions or molten nitrate/nitrite baths are often used Heating steel in steam at 260

to 400°C can also produce an adherent oxide coating The latter process, part of the “Ferrox” treatment, is often used for treating piston rings Like the phosphate treatments, these oxide coatings minimize bare metal contacts and prevent scuffing in lubricated applications such

as gears, needle bearings, and piston rings

Thermal Treatments

Steels and cast irons which contain enough carbon to be through-hardened in thin sections can be case hardened by localized surface heating to produce a hard, wear-resistant martensitic structure with a tough, ductile core Two production techniques are being widely used: induction heating and flame hardening.34In addition, electron beam and laser hardening are becoming more commonplace.35,36 Flame hardening does not lend itself to close control, but it is particularly suitable for large parts The other three processes can be closely controlled

by varying the energy input Advantages of these thermal treatments are reduced energy consumption, ability to selectively harden surfaces which require wear resistance, high production rates, and ease of automation Gears, cams, and shafts are among the many machinery components that can be surface hardened by these methods

Chill casting is also used to harden critical surfaces on cast iron parts which contain about 3% carbon Instead of allowing slow cooling with formation of graphite flakes, chills are used to cool the cast iron rapidly and cementite (Fe3C) is formed.37 Other carbide-forming elements such as chromium and vanadium are also added to promote surface hardness Cast cam shafts and cam followers can be hardened by this method

Diffusion Treatments

A variety of commercial diffusion treatments increase the wear resistance of metals, particularly steel and iron parts Case carburizing and nitriding are the most prominent, and many modifications are available to achieve specific changes in surface chemistry and metallurgical structure Extensive information is available in the literature1,37,38 and from suppliers In many cases, the difference in chemistry has significant effects on lubrication and sliding behavior

Diffusion treatments uniquely permit selection of a steel with optimum core strength Wear resistance is then provided by the surface diffusion process Gears, splines and many

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other components subject to bending stresses particularly benefit from this approach As an added bonus, case carburizing or nitriding improves the fatigue properties of steels, Nitriding carbon or low alloy steels also upgrades their corrosion resistance, but lowers that of stainless steel

Most carburizing and carbonitriding processes are done above the transformation tem-perature of the steels Quenching or subsequent heat treatment to achieve desired properties may induce dimensional changes or warpage Since nitriding is done below the transformation temperature, dimensional changes are minimized In many cases, no subsequent finishing

is required

Proprietary molten salt bath processes are used to apply very thin (10 to 100 µm), relatively soft nitride coatings on steel These are often times very effective Application temperatures are about 570°C (1050°F)

Table 4 compares some commercial diffusion processes Practical limits on case depths are established by diffusion rates of the elements The heavier the case, the higher the cost and the greater the dimensional changes Case depth wear resistance is generally ranked as follows:

If brinelling results because of indentation-type loading, either the case depth must be increased or the substrate strength upgraded

Other diffusion processes have been developed to upgrade wear resistance of both ferrous and nonferrous alloys Siliconizing39and bonding40are examples

Miscellaneous Treatments

This category includes cold-working, spark-hardening, sintered porous surface coatings impregnated with solid lubricants, and laser alloying A variety of mechanical reduction and burnishing processes can also upgrade surface hardness to some extent, as well as provide improved surface texture and fatigue resistance

Spark hardening is used routinely to apply thin, wear-resistant coatings such as tungsten carbide on tools, chucks, dies, etc A positively charged electrode of the coating material

is vibrated against a negatively charged substrate Each time contact is made, current dis-charges from a condenser and material is deposited on the surface The resulting surface is normally rough, but proprietary processes are available for better finishes Wolfe41 found that sparked silver coatings were particularly promising, possibly because the pores acted

as lubricant reservoirs This suggests that a sparked layer of silver might inhibit fretting wear

Impregnating a porous surface layer with a solid lubricant has been the subject of a number

of investigations Best known material is probably the DU supplied by Glacier Metals, Ltd Spherical bronze particles are sintered on a steel or bronze backing and the porous layer is then impregnated with Teflon®and lead The material is produced as flat-strip stock which can then be machined, punched, or rolled to form washers, sleeve bearing inserts, etc

In laser alloying, the laser creates a thin, molten layer on the metal surface Alloying elements are then introduced into the molten skin This technique can form a coating whose chemistry and corrosion or wear resistance is markedly different than the substrate.36

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Table 5 SOME PRACTICAL APPLICATIONS FOR WEAR-RESISTANT COATINGS

TYPICAL APPLICATIONS FOR WEAR-RESISTANT COATINGS Most surface treatments listed in Table 1 are particularly applicable to ferrous alloys.

Ultimate choice depends on factors such as: cost, effect of process temperature on the substrate, and the dominant modes of wear Hard, diffusion coatings are particularly valuable for gears, cams, crankshafts, etc., where through hardening would result in a brittle material

Stainless steels can be hardened to thin-case depths by nitriding or bonding For thicker

coatings, spraying or hard facing would be best

Aluminum, titanium, and magnesium alloys can be anodized to improve wear resistance.

However, where concentrated loading is encountered, substrate deformation and subsequent cracking of the coating is likely Diffusion treatments for these metals and alloys are limited

to a few proprietary processes which involve electroplating followed by thermal diffusion.33 With the exceptions of chemical vapor deposition and hard facing, all processes listed in

Table 1 for applying coatings on the surface (e.g., spraying, plating, sputtering, etc.) can

be used with these alloys Thin layers of tin alloys are often used to upgrade the performance

of aluminum bearings

Plasma-sprayed oxide coatings are particularly effective for hard surfacing aluminum and titanium alloys as long as solid particle erosion or impact loading is not a problem Metal-bonded carbides would be more suitable for erosion resistance Mismatches in thermal expansion coefficients, as on aluminum, are rarely a problem as long as the coatings are reasonably thin, on the order of 50 to 150 µm (0.002 to 0.006 in.) Like anodizing, the real problem is substrate deformation under load

When coatings are required on copper alloys, electroplating or spraying techniques are

applicable

Superalloys are frequently plasma-sprayed with ceramics or carbides for improved wear

resistance Aluminum oxide or nickel-chrome bonded chrome carbide are very effective at high temperatures (to 1000°C) Bonding also produces very hard surface coatings

Table 5 categorizes coating processes used to resolve wear problems One obvious