Mechanical Engineers Reference Book 12E Episode 10 pot

Wear and surface treatment 9/81 as tungsten, molybdenum and vanadium. While there is a tendency to think that electrolytic deposits are mainly for corrosion resistance? decorative purposes or electrical/ electronics uses, there are many engineering/tribological applications for electroplates. Hard and soft plates are used, depending on the particular function required. To resist abrasive wear, adhesive pick-up and corrosion, hard chromium plates6' are ideal. Porous or intentionally cracked chromium plates are used for oil retention, as in automotive cylinder liners. The hardness is 8%-900 Ha/ and coatings thickness is typically 20-100 pm. There are several proprietary variations on the plating techniques; some baths giving ultra-hard (1110 HV) deposits and others giving a dense, crack-free, but rather softer (700 13%') coating. Electrolytic coatings tend to build up on outside corners and sharp edges (because of a concentration in the current density) and to give reduced thickness in holes and on inside corners. Soft plates of tin are used to facilitate 'running in', prevent fretting and confer corrosion resistance. Plates of silver, lead, cadmium, tin and antimony are used in heavy-duty sleeve bearings, particularly in aircraft power units. Nickel plate can be deposited from a wide range of solutions and is used to minimize abrasive wear in cases such as sliding contacts on hydraulic rams. Some care should be taken in the selection of electrolytic nickel, particularly with respect to the counterface, because of its tendencies to gall. Nickel is a good undercoat plate for hard chromium. However. because the shock resistance of chromium is poor, it is prudent to make the bulk of the layer nickel, and give a relatively thin top-coat plate of chromium. Electroless nickel plates.68 autocatalytically depositing nickel-phosphorus (Ni-P) or nickel-boron (Ni-B), have many useful tribological applications. In the case of the Ni-P deposits, a hardness of about 500 HV is obtained but can be thermally aged to a hardness in excess of 1000 HV. This is achieved after one hour at 400°C by the precipitation of nickel phosphides. Such hardness is not retained at high temperatures but the Ni-B deposits are superior in this respect. The range of applications of nickel plates can be increased by incorporating fine dispersions of wear-resistant particles in the plating solution (NiC, Sic or A1203). Such coatings are particularly effective in high-temperature wear situations. Electroless nickel is also available with the addition of PTFE. This duplex coating is less hard but has excellent nonstick properties. In general, electroless nickel is not as abrasion resistant as hard chromium plate but, because it is not an electrolytic process, it does perfectly repiicate the component surface without build-up on edges or corners. It is normally applied in thickness between 10 and 50 pm and it has excellent corrosion resistance, particularly in the 'as-plated' condition. Le. straight from the plating bath. Both electroless nickel and hard chromium plate are commonly applied to ferrous materials but they are now also used increasingly for components made in aluminium. Plating baths can usually accommodate components up to a metre or more in size. These coatings are mainly used as a base for paint but they are effective in the presence of a lubricant to ease the deep drawing of steel, and to decrease the wear and fretting of sliding parts, particularly during 'running-in' processes. The identity of the phosphating process is often concealed under a proprietary treatment. In general, they are based on dilute phosphoric acid solutions of iron. manganese and zinc phosphates either separately or in a combination. Accelerators are added to shorten the process time to just a few minutes (in the temperature range 43-72°C). The simplest textiles and filled plastics. It can also be applied to shafts, gears or plain bearings in equipment handling abrasive slur- ries. If there is an element of corrosion then the substrate itself needs t'o be resistant. Alternatively, TiN can be applied over the top of another coating (for instance, electroless nickel). It can be applied to carburized surfaces with only minimal temperling, but if it is to be applied to a nitrided or nitrocarbu- iized material, the compound layer must be removed first. There are now PVD chambers that can coat components up to 3 m long and processing time and costs are being continuously reduced. PYD is a semi-'line-of-sight' process and so holes and re-entrant areas will receive less coating. Typically, at the equivalent of one diameter deep down a hole, the coating thickness will be 30% of that outside the hole. The coatings replicate the underlying surface texture without build-up on edges or comers. Finally, PVD is a high-technology process. It is best suited for high-quality components; bright, clean, free from oxide, burrs and contaminants. Chernic8uE vapour depo~ition~"~~ Chemical vapour deposition (CVD) is, in many ways, a competitor of physical vapour deposition. The process is used to deposit metals and ceramics by the decomposition of a reactive gas at the surface of components placed within the chamber. For instance, titanium carbide is produced by a reaction between titanium tetrachlo- ride and methane, or titanium nitride can be produced by replacing the methane with nitrogen or by using hydrogen and ammonia. Deposition temperatures are very high, usually above 800°C. Thus there are potential problems with softening and distortion of substrates. In fact, the CVD process is most commonly used to coat tungsten carbide tooling, particularly the indexable inserts used for high-speed turning. The cobalt- cemented carbide is an ideal substrate because it has a similar coefficient of thermal expansion to the Tic or TiN coatings. Also, it does not suffer any volume change during cooling and gives go'od support. There: can be problems of decarburization of substrates during deposition and it is common to apply duplex or multiple coatings (for instance, TiN on Tic or A1203 on Tic) to produce graded properties. The coating thickness is typically 1-5 pm. It is possible to apply the process to high- speed or tool steel components but these must be reheat- treated to retrieve the substrate hardness. This will produce distortion and, in many cases, will bring an unacceptable loss in precision. However, for tools with non-critical dimensions, CVD coatings can bring the same benefits to life as those applied by PVD. Equally, because the coatings have high hardness, they can be used to resist low-stress abrasive wear. The syslem is well suited to handling large numbers of small items, the parts being simply jigged on trays in the furnace. CVD coatings are also used to protect against corrosion and corrosivse wear. For instance, chromizing (referred to in Section 9.8.3.2), evein though it is a pack process, can be considered as CVD because it occurs by the decomposition of a gas. Finally, it is now possible to combine the PVD and CVD principles in the form of plasma-assisted chemical vapour deposition (PACVD) bringing the flexibility of the chemical process at a much lower temperature (less than 300°C). However. as yet, deposition rates are low and applications are mainly in thin (submicron) coatings for electronics applications. Eectrolytic and electroless coatings Over 30 metals can readily be deposited from aqueous solutiod6 but they do not include alkali or alkali-earth metals and refractory metals such 9/82 Tribology phosphate coating consists of grey to black crystals of Fe3(PO4)* and FeH Pod. Zinc and manganese phosphates produce more complex layers which absorb lubricant more readily, and are effective in reducing adhesive processes such as galling, pick-up and scuffing. In addition to phosphating, there are many chemical conversion coatings which involve dipping components in solutions to develop specific com- pounds. Treatments such as chromating, used on non-ferrous alloys to prevent corrosion, will hold lubricants, and provide a base for bonded lubricant coatings. Sprayed coatings70 In spraying techniques, powders are heated to a semi-molten state and deposited at high velocities onto the component surface. Coating thicknesses vary from about 0.05 to 1.00 mm. The techniques can be divided broadly into flame gun, arc, plasma-arc and detonation gun processes. The merits of any one technique over another need to be assessed with reference to the particular job in hand. Ob- viously the selection must be on a cost-effective basis taking due account of the integrity required for a particular duty. One of the difficulties with these processes is to assess the substrate bond integrity, porosity and general coating qualities on a production basis. Suppliers are well aware of this, and the usual approach is to design their coating methods with care, so that tight control of the process variables is maintained by following the set procedure at every stage. Electric arc spraying is used for metal deposition for wear resistance, corrosion resistance or reclamation. The coating material is fed as two wires and an electric arc is struck between them to cause melting. The molten metal is then propelled onto the substrate by compressed air. In flame spraying, the source of heat is a burning gas, such as acetylene, and the coating material is fed into the gun either as a wire or a powder. It is a relatively cheap process and gives a high deposition rate but, in general, the bond strength is lower and the porosity is higher than that achieved by the electric arc process. The spray-fuse process takes the technique a step further by first spraying and then fusing with a second heat source, such as flame, torch or by induction. This is the basic technique used for the nickel- or cobalt-based Stellite-type alloys and gives excellent resistance to corrosion, erosion, abrasion and fretting. Alloys can have hardnesses up to 900 HV with near-zero porosity. There will, of course, be substrate distortion and the parts will require finish grinding. Coating thicknesses are typically up to 0.5 mm. I- The plasma spray process makes use of an ionized gas (usually argon or nitrogen) to produce much higher temperatures than those created during flame spraying. This allows deposition of higher melting point materials such as metal oxides or metaVceramic mixtures. Such coatings have relatively good substrate adhesion and porosity levels are usually in the range 24% (Figure 9.71). A typical coating thickness would be 0.1 mm and the process finds a wide range of applications in resisting abrasive wear. Substrate heating is minimal. There are several variations on the plasma arc technique. The process can be conducted in a partially evacuated chamber (Low Pressure Plasma Spraying - LPPS), giving reduced porosity, reduced oxidation of the coating material and, because the substrate reaches a higher temperature, better adhesion. The same advantages can be gained by shrouding the arc in a non-oxidizing gas (Inert Atmosphere Plasma Spraying) and both these processes are now used increasingly to deposit the nickel-cobalt-chromium-aluminium-yttrium (MCrA1Y)-type alloys which are used extensively in the aircraft industry to resist high-temperature oxidation erosion and fretting. A third variation is the Transferred Plasma-Arc process in which a secondary electric current is established between the arc and the workpiece. This promotes substrate heating and surface melting and gives more dense and more adherent coatings. Deposition rates can be very high and the technique is typically used to deposit thick (up to 10 mm) abrasion- and erosion-resistant coatings for use in applications such as mining and agriculture. The substrate must be electrically conducting and be able to withstand some thermal distortion. The ultimate spray techniques are those producing the highest particle temperatures and velocities. These are achieved in the proprietary techniques such as Detonation Gun, Jet Coat and Mach Stream. As their names suggest, they are high-velocity techniques based on combustion of high- octane fuels. Porosity is very low (less than half of 1%) and substrate adhesion is excellent. They are used to deposit tough, abrasion- and erosion-resistant coatings such as chromium carbide or tungsten carbidekobalt cermets. Thickness is typically 0.1 mm and substrate heating is minimal. For all spray techniques, a correct substrate preparation is essential. The surface should be clean, free from scale, flash and burrs and should be pre-roughened by a grit-blasting procedure. Attention should be paid to the required coating Plasma- sprayed coating I- Substrate t-i 20 pm Figure 9.71 Plasma-sprayed chromium oxide layer Wear and surface treatment 9/83 compress the apparent differences between coatings and it will be difficult for users to predict the effects in their own applications, where completely different abrasives are Iikely to be present. In addition, a designer or user has to be sure that wear data were obtained under relevant conditions. If high loads are present, it is no use reiying on data produced under Iow-load conditions. Thin coatings or treatments. even though they might be very hard, can be crushed into the substrate and torn away without any benefit. For other wear situations the idea of wear rates is equally doubtful. The designer will rely on data or experience which shows, for instance, that certain coatings are better in fretting situations than others, or that some treatments are effective in corrosive applications. Equally, in situations involving severe wear problems such as seizure, galling and scuffing. wear rate has no meaning. Material is transferred, torn and deformed; surfaces weld and seize. The objective is to eliminate the problem, not merely to reduce it. Wear data, therefore, are most commonly published for relatively simple situations of adhesive or abrasive wear. The range of wear machines is extensive and some, together with examples of wear data, are described below. Adhesive wear data are produced in many test geometries. In most cases, the geometry is chosen because it allows a quick and easy measure of wear, rather than in any attempt to simulate a particular application. The most common wear machine is probably a ‘pin-on-disk’ arrangement, with the wear on the pin being measured from its weight loss or reduction in length and that of the disk from profilometry of the worn groove. Figure 9.72 illustrates the typical mild/severe wear transition that takes place with a normalized medium- carbon steel rubbing dry against itself. If the disk is through- hardened, the severe wear regime is suppressed and a mild distribution so that the spraying procedure can be optimized and, for mass production, automated and computer controlled. Laser dioying and ciaddi~tg~’.’~ These are relatively new processtes. For cladding, the powdered material is biown directly into the laser-generated melt pool. Laser alloying is a similar process, except that the energy is increased to produce more substrate melting and complete surface alloying with the powdered material The reaction area is shielded with an inert gas. A particular advantage of such techniques is that a specific area can be treated, thus minimizing component distortion. Corrosion-resistant surfaces and metallic glasses are being produced but more development is required before the processes have wide industrial application. One of the problems is that of controlling the depth of heating, particularly when thin-layer fusion zones are required. Welding and roll-cladding” These processes involve relatively thick layers, typically up to 2 or 3 cm. Welding can be used to good effect in tribological situations where high-stress abrasive wear is the problem, such as coating digger teeth, tank tracks and on ore-handling equipment. For instance, some of the Stellite-type coatings are applied by weld deposition. Cladcling is usually associated with corrosion or mild wear problems that are encountered in the chemical, food processing or printing industries. The two processes, roll-cladding and weld cladding, are complementary; hard abrasion-resistant materials are difficult to fabricate and are best deposited by welding, while the more corrosion-resistant materials are based on ductile austenites and are amenable to roll-cladding and forming. 9.8.4 ~~ibQ~Qg~~a1 dlata Friction and wear data are available for surface treatments and coatings. Designers can sometimes find them in the open literature or in the advertising information which accompanies the various products. However, before they can make use of such data, they needi to consider how the wear tests were performed. Le. the type of laboratory wear machine that was used, and relate the conditions to those present En their own applications. The object of this sub-section is to acquaint the designer with the wear data that are available and to assist in interpretation and application. As described in Section 9.1, wear is expressed in terms of the rate of material volume removal, usually as a function of applied loading and rubbing distance. For metal-on-metal rubbing under dry conditions, the rate of wear is often proportional to load and distance, and it is just possible that data could be related to a designer’s specific application. However, it is more likely that there will be some further influenoe on the rubbing situation (for instance, a process fluid or lubricant) and that absolute wear data cannot be obtained. The situation in abrasive wear is even more complex. Wear rate is a function of so many factors (for example, hardness, impact velocity and particle shape and size) that there is no prospect of users finding data which have any absolute meaning to their applications. Relative wear data that rank a range of treatments in order of wear resistance can usually be found but, even then, the rainge of wear will depend greatly on the aggressiveness of the abrasive. For instance, two sets of wear tests, one using Sic (2500 HV) as thie abrasive, the other using Si@ (800 HV), may both produce ?he same ranking for a number of coatings or treatments but the relative wear rates will be very different. The harder abrasive will tend to (Severe wear) (Mild wear) I 1000 Sliding speed (cm s-’ ) Figure 9.72 Pin-on-disk tests for normalized 0.4%C steel, showing transition in wear 9/84 Tribology IO-' I r Ll 5 E, Y I r 10-8 - W +- E L m z V Y- a W v) 10-9 lo-" NA rr Sliding speed 154 cm s-l 1 FG I HJ L N Key: A = Fe S deposition B = Fe Sn deposition J = gas nitro- carburizing G = boronizing H = carburizing c = low-temperature K = plasma nitL carbonitriding carburizing Figure 9.73 0.4%C steel Pin-on-disk tests on various surface treatments on wear rate of about 1 X lo-' cm3 cm-' kg-' (volume per distance travelled per unit load) is maintained. Figure 9.73 shows that most of the thermochemical diffusion treatments have the same effect, with only the sulphur-based process failing to give mild wear. This provides an excellent example of the difficulties inherent in wear testing. The sulphur-based treatment is aimed specifically at eliminating scuffing, i.e. under lubricated conditions, and a different test is required to highlight its advantages. Figure 9.74 shows similar wear data for an aluminium alloy, demonstrating the reduction in wear achieved by anodizing (particularly when PTFE is incorporated into the layer) and by the Zinal electrolytic deposition process. In this case, the wear rates are about a factor of 10 higher than those found with medium-carbon steel. Another test geometry used for producing adhesive wear data is 'crossed-cylinders'. Here. the contact stress is high and certainly unsuitable for evaluating thin 10- - - I Y I m c 6 "E 10- u W I m L L m - s E 10- 0 *- v, 10- lo-' I - I m Y I - 5 WE 10- 0 W I m L L m - 5 g 10- 0 c v, 10- Aluminium alloy 0 \/Om n 0 &\ Dural type -I c 0- - \ I I Aluminium alloy 0 Dural type Key 0- - 0 Untreated Dural Anodising Process A \: 0 ProcessB \\ + process^ \ PTFE 0 +0 0 0 0 Pin hardened EN8 /+- in each case 100 1,000 Sliding speed (cm s-' ) Key Aluminium alloy 0% - - - - -0 cu 4.12 Mg 0.62 Sr 0.75 \ Fe 0.63 \\ Mn 0.61 \ \ Zn 0.13 O\' Cr 0.06 o\ \ Dural type \ \ - -0 - \ \' \\ \ \ \ \ L Electrolytic deposition process Pin hardened EN8 in each case 100 1,000 Sliding speed (cm sr1 ) Figure 9.74 Pin-on-disk tests on treated aluminium alloy Wear and surface treatment 9/85 the excellent abrasion resistance of PVD titanium nitride under the light load that was used. However, if high loads or large abrasive particles are employed the results would be completely changed, with only the thicker surface treatments surviving. The lesson on wear data is, therefore, very clear. The designer should treat it with caution, making sure that it involves the appropriate wear mechanism and that the test conditions have some relevance to the particular application. coatings. Tests may also be quoted from Falex ‘pin-and-jaw’, or ‘four-ball’ machines. These evaluate scuffing or seizure resistance and a ‘failure-load’ rather than a wear rate is the usual quoted result. A ‘disk-on-disk‘ geometry (sometimes tested with a combination of rolling and sliding) gives a line contact and tests can lead to fatigue pitting as well as adhesive wear. The irange of machines used for performing abrasive wear tests is more limited. Data are produced by rubbing samples against abrasive paper disks (sometimes in a spiral to ensure that fresh abrasive is acting throughout the test). It may also be produced from a grit-blasting machine (erosion) or from a rubber wheel tester. In each case the result is likely to be expressed as a weight loss or penetration depth per unit time; Le. simple relative data. An example of rubber wheel test data for abrasion by Si02 is shown in Table 9.22. It demonstrates Table 9.22 Abrasive wear rates for several coatings and substrates: rubber wheel test Coatinglmaterinl Hardness Worn volume after 100 revs at 130 N load (mm3) ( x~o-~) (HV 100 g) PVD TIN CVD TiN CVD Tic Sprayed WC Sprayed A1203 Sprayed Cr203 High-speed steel Mild st4eel 3500 1800 3200 1500 1200 1300 850 180 0.5 1.0 5.1 9.8 19.3 23.2 5.7 142.0 9.8.5 Selection philosophy As stated earlier, the process of selecting the wear-resistant surface should be started at the design stage. If due considera- tion is given to the environmental factors, and if the user processes are considered at the onset, then preliminary selection will be relatively easy (Figure 9.75). The first stage will be to satisfy the mechanical engineering demands for the component. Ideally, the lowest cost component which will meet their demands is required. This usually means that, when manufacturing a component, it must be made with the cheapest material compatible with its design requirements, and be fabricated with the minimum number of low-cost operations. During this stage, the designer will be considering all the mechanical and environmental requirements; for example, there may be a need for corrosion resistance, good fatigue properties or resistance against creep or impact damage. The next stage is to consider the wear. If the environment or engineering demands dictate that a material, or the condition in which it must be used, are not compatible with the wear processes likely to be encountered in the system, then wear- resistant surface treatments must be employed. It is important that the type of wear is carefully identified; abrasive (two- body, three-body, high or low stress), adhesive, erosive, DECIDE WHETHER Use wear data when SPECIAL TREATMENTS availnbie OR COATINGS ARE REQUl RED t CONSIDER LOCAL FACTORS Figure 9.75 Selection of wear-resistant surface - initial procedure 9/86 Tribology fretting, chemical or fatigue. Also, it is necessary to obtain data on likely rubbing conditions, such as sliding speed, contact pressures, load cycles, hardness and type of any abrasives and the presence of any corrosive medium. If wear data are available and those data are relevant to the load, sliding speed, environment and counterface material, then this will assist in the selection process. However, the designer should look carefully at any wear data found in the trade literature supplied by the surface treatment specialist, or in published papers and journals, and ensure that they were obtained under a relevant wear regime. It is totally inappropriate to assume that wear data obtained under, say, abrasive conditions, would have any meaning to an application involving another wear process such as adhesion, fatigue or corrosion. Obviously, each design and component cannot be subjected to tribological testing, but there are wear and performance characteristics of classes of materials and surface treatments which allow a primary selection. By this stage of the selection process, the engineer should have a reasonably short list of materials and surface treatments that could be used, and should have considered possible manufacturing routes and worked out some detailed requirements; for instance, depth of surface treatment (determined by the loading conditions), hardness, core properties, whether the total surface area of the component needs treating, the required wear life, number of components to be produced, etc. However, a number of options for surface treatment will remain and should be reconsidered when assessing the ‘local’ factors. The final selection must be based on what is practicable, available and economic (Figure 9.76). First. there will be the question of availability. Usually, the user will like to sub-contract any surface treatment fairly locally. The designer will have to ensure that the selected process is available and that the processor has the necessary equipment, working skill and quality control to provide the treatment reliably and reproducibly at the volume of production anticipated. Second, the designer must consider the geometry of the component. Complex geometries, incorporating a number of section thicknesses, are likely to distort when treated at high temperature. This may limit the choice to low-temperature treatments. The heat-treatment history of the component is important. Specific core properties may have been produced by hardening and tempering so that any subsequent surface treatment must be applied at a temperature below that of the final temper. If this is not so, the core may be softened and the coating/substrate combination may then have insufficient load-carrying capacity to withstand the service contact stresses. Depending on the areas to be treated, the geometry may also exclude all ‘line-of-sight’ plating or coating processes. Obvious examples of this are the ceramic and cermet spray processes such as plasma arc and detonation gun. Ion implantation is also a line-of-sight process and physical vapour deposition techniques have only limited ability to penetrate re-entrant surfaces or holes. In some applications. the replication of the surface shape may be important. The electrolytic plating processes tend to deposit thicker coatings on sharp corners and peaks while leaving valleys with little or no coverage. In contrast, electroless coatings tend to replicate the shape perfectly and may be preferred. On a finer scale the designer may need to preserve a particular surface finish, perhaps applied to the component for reasons of controlling friction or to provide specific optical properties, and so may be limited to ion implantation, physical vapour deposition or electroless coatings. In all cases. the designer will be considering the dimensional requirements and making allowances for any final machining or grinding after treatment. Low-temperature processes that AVAILABILITY OF TREATMENT FACl LlTlES COMPONENT GEOMETRY PROCESS TEMPERATURE D ISTO RTl ON COMPONENT MASS COMPONENT SIZE HANDLING FACILITIES FOR LARGE NUMBERS (JIGGING ETC) PRACTICAL COATING THICKNESS I LABOUR SUITABILITY TOXICITY ETC ECONOMICS OF THE PROCESS Figure 9.76 Selection of wear-resistant surfaces - local factors require no post-treatment finishing operations and retain dimensions after treatment are particularly attractive. Third, the question of component mass, size and numbers must be considered. These are obvious points to look for but it is surprising how easy it is to overlook such facts as: the length of the component you want to nitride is a few inches longer than any salt bath available in the country, a component requiring a coating applied by physical vapour deposition turns out to be too heavy to be handled inside a vacuum chamber, or a particular heat treatment service does an excellent job on 10 components but is totally impracticable to deal with 100 000, because the logistics and cost of jigging would be prohibitive. The designer should always seek advice from the contractor before finalizing on the surface treatment. A fourth point to consider is the practical coating or treatment thickness. Although the wear performance of a particular treatment may be excellent it is advisable to check whether the particular process is capable of giving the desired depth of treatment to withstand the design pressure throughout its anticipated service life. Some of the processes, particularly ion plate deposits, have excellent hardness, low friction, good corrosion and anti-galling characteristics but it is imprac- tical to deposit more than a few microns onto a substrate. Similarly, some thermochemical diffusion treatments, electrolytic/electroless plates and sprayed surfaces cannot be Wear 2nd suriace t~eatment 9/87 and, with good documentation, faults can be traced back through the process and quickly rectified. developed to adequate depths for highly stressed surfaces. For this reason, the design engineer must consider not only the surface but also sub-surface stressing on a particular component. If the coating or surface treatment technique is to be brought ‘in-house’. i.e. out of the hands of sub-contractors, it is important to consider all the implications. When introducing new skills. new processes, high technology, processes requiring the use of chemicals, salt baths, toxic materials or radia- tion, all the technical and practical aspects must be appre- ciated. It is equally important to consider how they will be received at shopfloor level. Finally, it is importan: to consider the economics. Surface treatments can provide tribological and environment- compatible surfaces on relatively cheap substrates, and unless the designer has a knowledge of the range of treatments available. it will be difficult to select the most cost-effective solution. 9.8.6 Quality control A designer or engineer who is proposing the use of a surface coating or treatment on a critical component requires confidence, not only that he or she has chosen the correct solution to the problem, but confidence in the integrity and reliability of the treatment process. Certification of a product should. of course, be the duty of the supplier, but it is up to both the user and the surface treater to agree what surface properties are important and how to specify them. This should lead to a formal quality-control procedure with full documentation. The surface properties to be specified depend on the specific application. However, jt is likely that, for modification techniques such as case hardening, nitriding, induction hardening, etc the two key properties will be hardness and case depth. For coatings, the thickness and hardness will again be important but the bond strength to the substrate will be critical. No matter how hard or thick it is, if the coating falls off, it cannot do its jobs. For sprayed coatings, porosity may be an issue (for instance, allowing a corrosive medium access to the substrate) and, in :some applications, surface finish may need to he specified. A further question to be answered is whether the quality- control tests should be carried out on the actual component (in which case they may need to be nom-destructive) or whether they should Re performed on a test coupon ( a small, flat plate sample) that is treated at the same time. The danger in using a coupon is that it may not experience the same treatment conditions as the component. For instance, it is nearly always easier to (effectively treat a flat plate than to deal with a sample with complex geometry. The ideal solution is to include with the batch some components (or representative sections) that are expendable and can be used for destructive quality-control tests. The number of samples and the frequency of checks would, again, be a matter of agreement between user and supplier. It should be decided on the hasis of batch sizes and on the likely consequences to the end product of faulty surface treatment being undetected. The detection of mistakes is, of course, the very purpose behind quality-control )tests and some ideas for test procedures are given below. However, the real key to control is in quality assurance, i.e. the close definition and monitoring of the treatment procedure itself. The key treatment parameters should be identified and the user should demand full documentation of each treatment cycle. This way, the product is reliable and there is less need for costly and time-consuming monitoring of the final items. There should always he some quality-control testing, the unknown can sometimes occur 9.8.6.1 Hardness and case-depth sf treated surfaces The only reliable way of determining the hardness and depth of hardening is to prepare a polished cross-section through the treated sample and perform a series of hardness tests from the surface into the hulk. Ohviousiy, this is destructive and must be performed either on a tab sample or on sacrificed components. For case-hardened components the depth of hardening is defined as the distance from the surface to a plane at which the hardness is 550 HV. The measurements must be made using a load of 1 kg. The same method is applied to induction and flame-hardened surfaces but this time the convention is to define the limiting hardness at 400 HV. This definition also applies to nitrided surfaces, but only for conventional nitriding steels. For other nitrided steels (for instance, a high-alloy stainless steel) the specification would have to he agreed between the parties concerned. All these conventions apply to steels with a case depth of more than 0.3 mm and are covered by IS0 Standards 2639,3754 (1976) and 4970 (1979). For cases less than that value it will be necessary for parties to draw up their specification. In particular, for very shallow treatments (for instance, the surface ‘compound‘ layers produced by nitrocarhurizing processes) it may be necessary to use very light hardness loads, perhaps as low as 0.1 kg. This demands expert metallographic preparation and careful measurements, taking account of the wide statistical spread in low-load hardness values. For ultra-thin diffusion layers, such as those produced by ion-implantation, detection of the hardened surface is possible only by an ultra micro-hardness tech- niq~e.’~ However, this is really an academic tool and not suitable for quality control. 9.8.6.2 Hardness of coatings In the case of coatings, the whole layer usually has a constant hardness, so the idea of a ‘case-depth’ is inappropriate. For dense coatings such as electrolytic chromium plate or electroless nickel the measurement of hardness on a polished section is straightforward, usually being performed at a load of 1 kg. For sprayed coatings the results may be affected by porosity, the material collapsing into sub-surface voids and giving low hardness values. In this case, hardness would be quoted alongside porosity (discussed below). With very thick coatings, the hardness can be measured directly on the surface, provided that the surface finish is good and the load is not so high that the coating collapses into the substrate. For very thin coatings, only 1 or 2 pm thick (typical of coatings applied by physical vapour deposition), no direct method is practicable, The hardness can be predicted by performing a series of tests into the surface at different loads and using the analysis described by Thomas.74 9.8.6.3 Coating thickness Obviously, a micrograph prepared at a known magnification from a polished section provides a positive record of coating thickness. However, this is a measure at one single point and it is often important to map the coating thickness over the contours of the components. For coatings in the thickness range up to 30 pm, X-ray fluo- res~ence~~ gives excellent results (provided there is atomic number contrast between the coating and substrate). It now supersedes the traditional method using electrons - Beta- backscattering. It is particularly effective for measuring the 9/88 Tribology thickness of coatings such as titanium nitride or zirconium nitride applied by either physical or chemical vapour deposition. For thicker coatings, other techniques are available. These can be based on the use of eddy currents, ultrasonics, thermal waves. etc. 9.8.6.4 Coating porosity Porosity is important in most sprayed coatings and the best method of measurement is to prepare micrographs of polished sections. Porosity can then be assessed by a visual comparison with ‘standard’ photographs. The major coating companies have established such standards, usually covering a range from 0.25% to 10% porosity. An absolute measure of porosity can be made from a micrograph by making an area of line tracing and determining either the area or length of the voids as a ratio of the total (for a homogeneous structure. volume, area and line porosity are equal). It is important that the polished section is prepared with care, so that the polishing procedure itself does not pluck out material and create ‘false’ porosity. This is a particular problem with the harder plasma-sprayed ceramics and causes frequent disputes between users and coating contractors. The polishing procedure should have been worked out previously and it should then be included in the overall quality-control specification. A second possibility is to measure porosity by vacuum impregnating the coatings with a low-viscosity fluid and measuring the weight taken up. This relies on accurate knowledge of the volume of coating (Le. the area and the average thickness). on the pores being interconnected and on all the pores being filled. (For very fine pores, capillary forces may prohibit complete filling.) These are significant points but, if the process can be perfected, it does have the merit of determining a volume porosity. In contrast, a polished section gives only a single point value. 9.8.6.5 Coating adhesion The adhesion is the key property and the most difficult to quantify. In fact, measurements are usually qualitative rather than quantitative. To produce an absolute measure of bond strength (in units of forcehnit area required to detach the coating) it is possible to use a tensile-type test. This relies on gluing a peg to the coating with a high-strength epoxy and measuring the tensile force required to pull off the coating. In practice, the glue usually has a lower bond strength than the coatinghnterface or the coating fails cohesively. Additionally, the difficulties in setting up and pulling the pins accurately at right angles to the surface mean that a statistical approach, with multiple tests, is required. For thin coatings, such as those applied by physical or chemical vapour deposition, a scratch test technique” is available. This uses a spherical diamond which is dragged across the surface at a steadily increasing load. The point of coating detachment is detected by measuring the friction or by monitoring the acoustic emission, and a ‘critical’ load is assigned as a measure of adhesion. Such a test usually requires a flat plate tab sample. For thicker coatings there are a number of alternatives. Samples can be bent in a closely defined way and the bent area inspected for flaking. Alternatively, they can be thermally shocked, thermally cycled, impacted or subjected to high-load indentations (for instance, using a Vickers hardness tester with a pyramidal diamond indentor). In such tests, it is usual for coatings to crack, but without flaking from the substrate. The procedure should be worked out between the user and contractor and designed so that an unsatisfactory coating is highlighted. It may actually be necessary for the coater to deliberately produce coatings with a poor bond so that a relevant procedure can be developed. It is also preferable for the test to bear some relation to the actual application so that, for instance, components subjected to bending in service would be given a bend-type adhesion test. 9.8.6.6 Surface finish If there is a need to preserve a particular surface finish, the quality control is a matter of a ‘before’ and ‘after’ texture measurement using a stylus profilometer. In most cases, it will be sufficient to determine the most common texture parameter, R,, the ‘centre-line-average’. However, some coatings, even though they may generally replicate the underlying surface, have a subtle texture of their own. This is true of some of the evaporated physical vapour deposition coatings and also some electrolytic coatings. In that case, it may also be necessary to monitor other surface finish parameters, particularly those relating to the ‘shape’ of the surface (skewness) and the sharpness of peaks and valleys (rms slope or average wavelength). 9.8.7 Closure Surface engineering is the obvious solution to many of today’s engineering and wear problems. It allows optimization of the surface and the substrate in a cost-effective way. To be most effective, the principle of surface engineering must be adressed at the design stage of a component, with the designer making full use of available data on the coating or treatment properties, including friction and wear. Then, having made a selection, the designer should be entitled to a good quality and reliable treatment service and it is the duty of the coating contractor to provide a certified product. The science of surface engineering is expanding quickly. There are increasing possibilities of combining treatments and coatings to produce even more specialized properties. Coat- ings might be ion-implanted, either after or during their deposition. They might be laser-glazed to further increase hardness or diffusion. One coating might be applied to another to produce a combination of properties (e.g. abrasion resistance and corrosion protection). The time may come when expensive alloying elements can be added exclusively to the surface of a cheap component (perhaps by ion-plating) so that it can then be effectively nitrided. Designers and engineers should be kept abreast of developments and be continuously aware of possibilities for improving component life, reliability and economics. Acknowledgements The authors would like to thank Mr M. Farrow of the Surface Science Division, Northern Research Laboratories, UKAEA, for his invaluable technical assistance in assembling this information. 9.9 Fretting 9.9.1 Introduction Tomlin~on’~ first investigated the phenomenon of €retting in 1927 after observations of red rusting of the grips of fatigue- testing machines. He coined the term ‘fretting corrosion’, by which name it is commonly known, and carried out the first Fretting 9189 quantitative st~dy.’~ Ne considered that damage of the surfaces by fretting was initiated by mechanical wear produced by sliding of one surface on another and that the corrosion observed with base metals in air was a consequence of the wear. He stated that ‘although the presence of oxidation products shows that chemical action accompanies fretting. the process is nevertheless certainly not one of corrosion as ordinarily understood’. The distinction between fretting wear and ordinary wear is that fretting generally occurs at contact surfaces that are intended to be fixed in relation to one another but which actually undergo minute alternating relative movement. A classical #example is the damage to the races of wheel bearings of automobiles during shipment by rail or ship, initially ascribed to brinelling caused by impact as a result of vibration but now known to be caused by slip between ball and race (and called ‘false brinelling’). When fretting occurs on base metal in air, the wear debris always consists of oxides of the metal so that the most common symptom of fretting is the red-brown mud (comprising essen- tialiy red iron oxide mixed with oil or grease) shown as a patchwork over contacting steel surfaces. Frettin.g damage occurs when two loaded surfaces in contact undergo relative oscillatory tangential movement (known as ‘slip’) as a result of vibration or stressing. Amplitudes of relative moment are small and often difficult to measure or even to predict by analysis. It is a deterioration process often ignored or not understood by designers. As a consequence, many fretting problems often come to light only late in the product development or, even worse, when the product is out in service. In such cases it is often too late to allow anything but minor redesign, and the only option then available is to employ some type of palliative (which is rarely a Bong-lasting solution), when fundamental changes in design concept can be the only successful solution. Possible situations in which relative movement and hence fretting can occur, either intentionally or otherwise, are legion. More common ones are flexible couplings, press fits of bearing raceways and hubs on axles, riveted aircraft structures, screws in surgical implants, steel ropes, heat exchangers. business machines and electrical contacts. Reports of the occurrence of fretting have increased over the years, in part due to the increased demands placed on materials by virtue of higher power densities and stresses in modern machinery. Fretting and fretting fatigue have become increasing problems in the aerospace industry, particularly with the use of exotic alloys based on titanium and aluminium. A number of excellent review papers are available in the literature which rovide summaries of the state of knowledge at various times!^' Although fretting damage by itself is seldom sufficient to cause failure directly, the irregularities produced in the surface cause loss of dimensional accuracy and wear products can cause excessive friction and seizure of closely fitting parts. A number of different terms have been used in the literature, including fretting, fretting wear, fretting corrosion, fretting fatigue, false brinelling, rubbing fretting, impact fretting and impact-slide fretting. It is worth providing some defini- tions to distinguish between these various terms. Although fretting was initially described as fretting corrosion, it is better to use the term ‘fretting’ as a general title to cover a number of aspects of the phenomenon. In the first instance, fretting was observed with materials such as low-carbon steels, which at room temperature and in the presence of oxygen and water vapour, produce copious amounts of finely divided red oxide identified as haematite (Fe2Q3) and fretting was understood to be closely associated with corrosion on the material surface. The oxidized particles are extremely hard and become em- bedded in the contact surfaces producing a ied staining which even mild abrasive cleaning will not remove. This staining can often be used to diagnose the occurrence of fretting. It has often been stated that the oxidized debris was responsible for abrasive wear of the surfaces or even that it alleviated the damage by acting as a miniature ball bearing. However, fretting is known to occur in non-oxidizing environments such as high-purity inert gases where there is no oxidized debris and corrosion effects are minimal. It is generally accepted that fretting processes are caused by high-frequency relative movement of contacting surfaces even with slip amplitudes of less than 1 pm. The upper transition between fretting and oscillatory wear is not well defined, but somewhere between 100 and 200 pm is the upper limit at which the wear process assumes the characteristics of unidirectional or reciprocating sliding wear. Fretting wear is best used to describe processes in which ultimate failure of the component occurs by loss of the material surface leading to fracture, loss of function or pressure bound- ary rupture (applies to heat exchanger tubes, pressure vessels). In such instances, the component geometry is such as to allow escape of the wear debris with continuing penetration of the component thickness. This is to be contrasted with the more common fretting processes in which the geometry traps the debris, restricts access of the environment and produces little loss of section. Seizure of closely fitting parts such as bearing raceways, press fits, machine tool slideways and gear couplings is a characteristic mode of failure for fretting wear. Fretting fatigue is a consequence of fretting damage to components subjected to cyclic stressing in which the fretting scar acts as a fatigue crack initiator. It usually occurs where fretting wear is minimal since either the production of copious amounts of wear debris interfere with the fretting process or the wear front progresses at such a rate so as to obliterate the propagating crack. The elimination of excessive wear of a component can often lead to fretting fatigue problems, much to the consternation of the machine designer, developer or operator. A number of papers and publications provide re- views of this extensive topic.85,86 False brinelling is a specific term used to describe the damage to rolling element bearings particularly where the bearing has been subjected to vibration from neighbouring machinery during a period of inoperation. The damage does resemble a brinelling indentation and can lead to rough running and premature fatigue failure during subsequent operation of the bearing.87 Although the classical forms of fretting involve loaded contacting surfaces, it is now recognized that fretting can occur between surfaces that undergo separation and repeated contact. Thus, the term rubbing fretting should be used to differentiate between loaded non-separating surfaces and those which experience periodic impact and slide, for which the terms impact fretting or impact-slide fretting are used. It should be noted that oblique collision of elastic bodies yields small-scale sliding under the Hertzian contact forces and milliseconds contact duration which can cause significant fretting wear damage. It is largely due to such occurrences of impact fretting problems in nuclear power plant that this topic has received much attention in recent 9.9.2 Source of relative movement 9.9.2.1 Forcelstress exciied Since it can be stated that wear cannot arise from surfaces which are not subject to relative slip and that the most successful method of overcoming fretting is to eliminate the fretting movement, it is worth examining how the relative [...]... steel) 10- 15 L I 1 o3 I I I , , , I , 104 I I I I I l l 1 I 105 I 1 1 1 , 1 I 1 106 Test duration (cycles) Figure 9.79 Effectof test duration on specific wear rate for mild steel under rubbing fretting conditions I I , , , , , 107 10- 4 c c 0 10- 5 Y z 8 L a m l 3 P s : R’I 0 3 7 -+-+ 10- 6 71 Lewis (1978) X Feng and Uhlil + Ohmae and Tsukizoe 0 Halliday 0 Halliday and Hurst I 10- 17 7 I I l l 10 100 IO... amplitude on specific wear rate for mild steel under rubbing fretting conditions: comparison with other workers +/+ +w \+ + X Feng and Uhlig f Wright 10- 15 i 100 I 9 Hz 67 800 cycles 90 urn 25 prn 50000 cycles (hardened steel) 4 100 0 Load ( N ) I 1 I , 1 1 1 1 10 000 Figure 9.81 Effect of load on specific wear rate for mild steel under rubbing fretting conditions: results of Feng and Uhlig and Wright 9/94... the initial stage are comparable with adhesive wear, in most cases the wear rate falls significantly as steady-state conditions are established For further discussions on fretting theories, see references 100 -103 9.9.3.4 Concept of specific wear rate Archard'" has expressed the wear behaviour of materials ucder unidirectional or reciprocating sliding with the term Volume of material removed Load X sliding... large changes in electrode potential for the baser metals Fretting can therefore show its effects more by continuous disruption of the passivating film Experiments in which the potential of the fretting surfaces is kept constant show a linear relationship between corrosion current and slip amplitude Calculations show that the bulk of the material removed is the result of mechanical action rather than... than chemical d i ~ s o l u t i o n ~ ~ Corrosion reactions can be controlled by imposing a cathodic potential on the system (or on metals which display passivation, an anodic potential) It has been observed that cathodic protection can @ve a significant improvement in fretting fatigue behaviour 0 100 200 300 400 500 600 Temperature ("C) Figure 9.83 Rubbing fretting of mild steel in air (after Hurricks)... Fretting 9 /101 Table 9.24 Rubbing fretting wear of steel at 125 pm slip amplitude various materials versus mild Counterface Specific wear rate (m3N-'m-*) ( X Mild steel Nylon 6.6 Carbon-graphite filled PTFE Amorphous carbon-graphite Woven PTFE fibre/glass fibre + resin PTFE flockiresinlNomex cloth 300-600 80-160 10- 20 9 6 8 4-5 For additional information on materials and manufacturers see reference. .. 1%increase in power is required to maintain ship speed for every 10 pm increase of a roughness parameter called the mean apparent amplitude (MAA).ls5 It is also stated that typical hull deterioration rates are between 10 pm and 40 pm MAA per annum, with new ships having a hull roughness of approximately 130 p m MAA while that of old ships may exceed 100 0 pm MAA The efficiency of aircraft is also strongly influenced... plate on AI plate Parcolubrited steel on Tin on steel AI on AI Zn plated steel on AI Fe plated steel on AI steel Feng and Uhlig 67800cycles 457800cycles 0 Sop 67800cycles 0 lop 67800cycles X 230p f 1 9Op 10 100 Frequency (Hz) Figure 9.82 Effect of frequency on specific wear rate for mild steel under rubbing fretting conditions: results of Feng and Uhlig Fretting 9/95 surface is more susceptible to fretting... Fe304) formed This behaviour has also been found by Lewis'" with mild steel in air u~pto 500"C, in which the specific wear rate for high-amplitude (100 p n ) fretting shows a considerable reduction with increasing temperature with a lesser reduction for low slip (10 pm) I: would appear that the formation of a compacted oxide layer on mild steel increasingly reduces the fretting wear rate by providing a protective... amplitude of 100 pm the volume of material removed is directly proportional to the slip amplitude In other words, the wear volume is directly proportional to the load and the sliding distance and specific wear rates correspond typically to those of sliding wear At low slip amplitudes, the evidence suggests that damage is much lower Some investigators claim that there is no measurable damage below 100 pm . thin 10- - - I Y I m c 6 "E 10- u W I m L L m - s E 10- 0 *- v, 10- lo-' I - I m Y I - 5 WE 10- 0 W I m L L m - 5 g 10- 0 c v, 10- . Halliday 0.73 R.’ I 0 Halliday and I 7 10 100 Hurst 10- 17 I Ill Slip amplitude (pm) 10- 4 c c 0 10- 5 z 8 Y- L m al 3 P .s : 10- 6 IO Figure 9.80 Effect of slip. and Rightmire 45 p 4360 N (hardened steel) 10- 15 L I I I I,,,, I I I I I Ill1 I I 111,11 I I I,,,,, 1 o3 104 105 106 107 Test duration (cycles) Figure 9.79 Effect

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