The importance of surface characterization in surface treatment processes

Wear, 81 (1982)

145

145 - 158

THE IMPORTANCE OF SURFACE CHARACTERIZATION SURFACE TREATMENT PROCESSES*

D. M. TURLEY

IN

and E. D. DOYLE

Materials Research Laboratories, Defence bourne, Victoria 3032 (Australia)

Science and Technology

Organization, Mel-

(Received March 24,1982)

Summary Directed energy beam (DEB) processes are being increasingly used to modify surface structure and to improve surfacedependent properties of materials such as wear and fatigue. Before DEB processing, however, the surfaces are usually prepared by machining or abrasive machining processes which themselves may cause considerable alterations to the surface layers. These alterations to the surface layers are discussed and the subsequent effects they may have on DEB processes are described.

1. Introduction The basic aims of directed energy beam (DEB) processes used for surface treatments are to improve surface-sensitive properties such as fatigue, wear and corrosion by plating or by making compositional or structural modifications in the surface layers. It is important to be aware, however, that machining and abrasive finishing processes used to prepare the surfaces before the application of these surface treatments will already have modified the surface layers [ 1 - 31. These prior modifications to the workpiece surface are more significant than generally realized, and it is most likely that they will affect any subsequent surface treatment. It is important therefore for users of DEB processes to be aware of, and to keep in mind, these effects. Characteristic features of a metal surface which may be significantly altered by removal of metal by mechanical means are as follows: (i) topography; (ii) crystal structure, as affected by plastic deformation and/or heating during machining; (iii) residual elastic stresses; (iv) chemical composition, as affected by plastic deformation, reaction with the environment or embedding of abrasives.

*Paper presented at the Conference on Surface Modification of Alloys for Improved Performance, U.S. 6th Army Headquarters, San Francisco, CA, June 1980. 0043-1648/82/0000-00001$02.75

0 Elsevier Sequoia/Printed

in The Netherlands

146

Most attention in the past has been concentrated on the first factor, surface topography, but increasingly it is being realized that surface integrity also encompasses the other three factors. It is now also realized that all four factors may be affected differently and independently by different machining processes, so that something needs to be known about them all before a given surface can be satisfactorify characterized. Further, it must also be realized that expe~men~ly prepared surfaces upon which surface treatments are carried out in laboratory investigations will probably be different from surfaces produced under production conditions. Consequently, the effects of surface treatments in practice may also be different. Finally, a new surface should be fully characterized after a surface treatment has been applied so that meaningful correlations with surface-sensitive properties can be obtained. This means that something also needs to be known about all four factors in respect of each individual surface treatment. In the present paper a short discussion of the above four factors will be given and their possible influence on some of the more commonly recognized DEB processing techniques briefly summarized.

2. Surface finish 2.1. ~uc~ining Surface finish can be predicted solely from geometric considerations. The surface roughness or peak-to-valley height h of a surface produced by machining with a feed rate f and cutting entirely on the nose radius R of the cutting tool is given [ 41 by the following expression: h = f2i8R

(1)

In practice this usually indicates the optimum surface roughness that can be expected; the surface finish actuaIly generated usually departs considerably from this ideal. This is because other factors such as the sharpness of the cutting edge of the tool, the nature of the chip formation process (i.e. continuous or discontinuous), built-up edge (BUE) formation, machine tool vibrations and thermal effects all contribute additional roughness. The theoretical surface roughness is approached in practice when cutting produces a continuous chip without a SUE, the cutting edge is very sharp and vibrations and thermal effects are minimal. An example of such a surface is shown in Fig. 1 which is a scanning electron micrograph of a gold surface produced by machining with a single-point diamond tool under the geometrical conditions expressed in eqn. (1). The deeper grooves in Fig. 1 were caused by pre-existing microdefects at the cutting edge of the diamond tool. The nature of chip formation with and without BUE formation critically affects surface roughness and will now be discussed in more detail. 2.1 .l. Continuous and discontinuous chip formation It is now reasonably well established that even in continuous formation the deformation process is heterogeneous on a microscale

chip and is

147

Fig. 1. Scanning electron micrograph of the surface of a diamond-turned

Fig. 2. Scanning electron micrograph segments remain together.

of a titanium

gold workpiece.

chip: cracking is incomplete

and the

Fig. 3. Scanning electron micrograph of a machined titanium surface (corresponding to Fig. 2). The rows of fracture separated by reIatively smooth regions should be noted. The arrow indicates the cutting direction.

dominated by a series of shear instabilities on a relatively thin shear front [ 5, 6 ] . Doyle et al. [ 71 suggest that discontinuous chip formation is a further extension of this process, the distinction being that the lamella spacing is much greater and the chip is divided into regular and discrete segments which are relatively undeformed. Cracking is associated with the discontinuous chips caused by shear instabilities. The cracking may be complete so that each segment is separate from the others or it may be partial so that the segments remain together (Fig. 2). This type of chip fo~ation has a very deleterious effect on surface finish as illustrated in

148

Fig. 3, where each row of fractures with each shear instability.

is the remnant

of the crack associated

2.1.2. Chip formation with built-up edge It is common in machining for a BUE to be present at the cutting edge of the tool (as shown in Fig. 4) particularly when machining at lower cutting speeds in steel workpieces. The BUE is in effect a stagnation zone of workpiece material. The BUE is generally unstable in that it completely or partially breaks away from the cutting edge and then re-forms; the periodicity of this cyclic process can vary from many seconds to a fraction of a second. When the BUE breaks away it is usually carried off as part of the chip and/or left on the generated workpiece surface. It is because the BUE may be left on the newly generated workpiece surface that BUE formation is undesirable when optimum surface finish is required (Fig. 5).

Fig. 4. Optical micrograph of a section at the root of a quick-stop chip showing a BUE formed during the machining of a plain carbon steel. Tears on the rake face side of the chip and the newly generated workpiece surface are indicated by arrows. Fig. 5. Scanning electron micrograph of an interrupted cut in plain carbon steel. Tears on the workpiece surface shouId be noted (the BUE is arrowed).

2.2. Grinding It has generally been agreed that the surface finish produced by grinding is a result of the complex interaction of many abrasive grits with the workpiece surface. However, a recent study [8,9] has shown that an important feature of the grinding process affecting surface finish is the continual pickup of workpiece material onto the wheel and the subsequent redeposition of this adhered metal back onto the ground surface; this phenomenon was termed redeposition. The general appearance of a dry-ground surface of 70-30 brass where the redeposition process was operating is shown in Fig. 6(a). This surface is very rough and there are discrete pieces of attached metal and smeared layers; it should be compared with Fig. 6(b) which shows a much smoother surface produced under conditions which

(a)

(b)

Fig. 6. Scanning electron micrographs of 70-30 brass surfaces: (a) ground dry showing the presence of discrete pieces of metal and smeared surface layers; (b) ground using a cutting oil as a lubricant.

Fig. 7. Schematic illustration of the redeposition process: A, start of redeposition; A-B, smearing of redeposited metal; B-C, cutting zone.

prevented redeposition. A schematic illustration of the redeposition process is shown in Fig. 7, Workpiece material picked up by the grinding grits between B and C begins to be redeposited at A, and increasing plastic deformation and smearing occurs between A and B as the interaction between adhered metal on the grit tops and the workpiece surface progressively increases. Taper sections through these surfaces have confirmed that multiple redeposition occurs; many layers of heavily deformed redeposited metal contour the grinding grooves (Fig. 8(a)) and comp~atively large pieces of metal protrude above the general macroscopic level of the ground surface (Fig. 8(b)) corresponding to the discrete pieces of metal in Fig. 6(a). Lubricants have a marked effect on the redeposition process. Grinding with a cutting oil or with wheels packed with soap virtually eliminated the redeposition process (Fig. 6(b)). The surface finish was then much smoother in comparison with dry grinding (Fig. 6(a)), and corresponded closely to that generated by the interaction of the grinding grits in the cutting zone. The

(a)

(h)

Fig. 8. Optical micrographs of transverse taper sections through dry ground surfaces of 70-30 brass showing (a) many layers of heavily deformed metal contouring the grinding grooves (the boundaries between the layers are arrowed) and (b) large pieces of metal protruding above the general macroscopic level of the surface (arrowed).

results for grinding with a water-soluble oil were intermediate between the redeposition clearly is a two extremes (of Figs. 6(a) and 6(b)); consequently, process that can occur during normal production grinding. Redeposition has also been found to occur when grinding steels. In comparison with 7CP30 brass, one major difference was that the redeposited steel often appeared to be in an oxidized state. When examined visually, areas with oxidized redeposited metal had the appearance of areas of grinding burn. 2.3. Measurement of surface finish Surface finish is usually measured on a stylus-type instrument such as a Talysurf and from the resultant surface profile trace an assessment of surface finish is made. This is usually expressed in terms of some parameter such as a centre-line average (c.1.a.) value which is taken as a measure of the surface finish. A c.1.a. value, however, does not necessarily give an accurate assessment of surface finish as it may fail to record fine detail. Also, much information regarding the nature of the surface may not be obtained if only a Talysurf measurement is made. In this regard, it is unlikely that a proper assessment of the nature of the surfaces shown in Figs. 3, 5 and 6(a) could be obtained just from a Talysurf reading.

3. Crystal structure The general pattern of the plastic deformation in the surface layers of machined and abraded surfaces is best shown by optical metallography

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e~~~a~on of taper sections of specimens of 70-30 brass [1 - 31 f This material is a convenient model because evidence of plastic defo~ation can be revealed witb very high sensitivity and detail by using various etching techniques. The taper sections of the ground surface shown in Fig. 8 clearly demonstrate that two distinct layers are present beneath the surface. The outermost layer adjacent to the surface is dark etching, and it is so deformed that no details of its structure can be resolved by optieaf microscopy, This outpost surface layer has been termed the fr~en~d layer. Below the fragmented layer shown in Fig. 8 is a second layer which has been termed the deformed layer. Within the deformed layer strain markings are revealed by etching (Fig. 8), and from the nature of these markings it cau be ascertained that the amount of plastic deformation decreases in this layer with increasing depth beneath the surface. In fact etchants can be used to show different strain ievels within the deformed layer [l] ; this is illustrated di~~ammatic~ly in Fig. 9 which shows the effect of abrading on P1200 grade silicon carbide abrasive paper on the subsurface defo~ation. For abr~ion and machining processes the depths of the fragmented and deformed layers are affected by various factors, e,g. they increase with increasing depth of cut and decreasing rake angle [2] , Of the two layers the fragmentid layer is the more important as its nature and properties will have a large effect on the surfacedependent properties of the metal. For this reason, the fragmented layer will now be discussed in more detail. Surface Fragmented Layer 596 Compression ho-strain Line

[

v

Elastic-plastic Boundary

75

Fig. 9. ~~a~rnatic i~lus~ation of the damaged Iayer beneath an abraded surface, drawn as a transverse taper section. (From Sam&s [ 1 J .)

Previous work [Z] ‘has shown that the fragmented layer has its origins in the primary shear zone, which is a region of intense shear generated immediately ahead of the cutting tool. The deformation processes occurring in the shear zone are associated with the formation of shear bands f73 which is a high strain deformation phenomenon [lo] , As the chip separates from the workpiece, part of the materiat in the shear zone is left on the

152

Fig. 10. Transmission electron micrograph (longitudinal section) showing subgrains near a machined surface of 70-30 brass. Fig. 11. Transmission electron micrograph (transverse section) showing small subgrains beneath an abraded copper surface (- - -, boundary between surface and electrodeposit).

newly generated workpiece surface where it forms the fragmented layer while the rest goes into the chip. Since the structure of the fragmented layer cannot be resolved by optical microscopy, detailed studies [ 11, 121 have been made with the transmission electron microscope. The dislocation substructure observed at the machined surface of 70-30 brass is shown in Fig. 10. Slab-shaped subgrains which lie parallel to the machined surface are present; they are approximately 0.5 pm wide, 0.03 - 0.10 pm deep and up to 1.5 pm long. The dislocation substructure at an abraded copper surface is shown in Fig. 11. In this case the slab-shaped subgrain structure has recrystallized at the surface to give a fine subgrain structure. The presence of subgrains in Figs, 10 and 11 indicates that the amount of deformation at the surface is very high. 3.2. Strain distributions beneath machined surfaces The sequence of deformation structures observed with increasing strain in cold rolling is similar to that observed beneath machined surfaces as one progresses from the elastic-plastic boundary to the surface (see Fig. 9). Thus, by using this correlation, it is possible to make estimates of the strain distribution beneath machined surfaces [II] and such a dist~bution is shown in Fig. 12. The strain at the surface is very high, greater than 99% by cold rolling (natural strain, 6 - 7) and it decreases linearly with depth within the fragmented layer to a strain equivalent of 90% reduction (natural strain, 2.3). In the deformed layer the strain decreases exponentially until the elastic-plastic boundary is reached. The maximum strain at the surface was

153

iyJ

04

t&2

a.3

0.4

0.5

t

,

1

,

1

FRAGMENTED REGIONS

I

ZONE

OF HIGHER

DEFORMATION DEFORMED

(6)

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w

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-

INCREASING PROPORTION OF REGIONS OF LOWER DEFORMATION

NATURAL STRAIN

0.6 mm

(A)

99 x REDUCTION

3.0 INCREASE IN DEFAMATION

2.0

0

0405 DEPTH

SIZE OF TWINS

0.010 BENEATH

@OS MACHINE0

Fig. 12. Strain distribution beneath a machined 0.0050 in; rake angle, 0”; substructures present

found to be approx~a~ly tool rake angles.

const~t

0~020

0.025 in

SURFACE

surface of 70-30 brass (depth are also indicated).

of cut,

for varying depths of cut and cutting

3.3. Phase transformations Most of the energy that goes into deforming the surface layers of the workpiece is subsequently transformed into heat. Thus, under some machining conditions appreciable temperatures can be developed in the surface layers. Factors producing high temperatures are (1) large feeds and speeds, (2) blunt or worn tools (large wear land) and (3) inadequate lubrication or coolant. High temperatures may cause the following structural changes in the surface layers: (1) subgrain growth; (2) recrystallization; (3) overtempering; (4) aging or overaging; (5) phase transformations, the most notable of which is the re-austenitization of the surface layers of steel and the subsequent tr~sfo~a~on of these layers to martensite. In conclusion, the crystal structure of a surface is greatly affected by a finishing process which involves metal removal by mechanical means, and consequently the surface properties and behaviour can be expected to behave very differently from the bulk solid.

4. Residual elastic stress It is now clear from the above that in all machining and metal finishing operations the surface layers of the workpiece are heavily plastically deformed, so much so that there are steep plastic strain gradients within these layers. Contained within the surface layers there is a high concentration of defects such as dislocations which have associated with #em elastic stresses, and it is the sum of these stresses which primarily give rise to the overall residual stress. Since the process of plastic deformation is by its very nature

154

heterogeneous then the distribution of elastic stresses will be non-uniform, but the overall direction of the stresses will be either tensile or compressive, In addition to the effects of plastic deformation there are other sources of residual stress in the surface layers. These are associated with temperature effects and are (i) the result of non-uniform thermal expansion and contraction and (ii} the precipitation of other phases, phase tr~sformations etc. Generally, low speed machining processes produce compressive stresses in the surface layers whereas high speed machining processes, in which appreciable temperatures are generated, produce high tensile stresses. For grinding, low wheel speeds, low feed rates and softer wheels tend to produce lower tensile stresses in the surface layers [ 131. It is usually not sufficient to know just the nature and magnitude of the residual stress at the surface. The variation in residual stress with depth beneath the surface is also important, as often the peak stress is not at the surface and the residual stress may also change sign just below the surface,

5. Chemical composition

The chemical composition of the surface is affected by (1) plastic deformation, (2) reaction with the environment and (3) embedding of a brasives. 5.1, Plastic deformation Plastic deformation in the fragmented layer is so intense that it can result in chemical changes in the surface layers. For example it is not uncommon for graphite flakes in cast iron to be squeezed out of their containing cavity, resulting in a carbon-depleted layer at the surface [ 13. Manners et al. [ 141 have studied the effects of various surface-finishing processes on the surface composition of leaded copper alloys. They found that generally lead was squeezed out of the surface layers and the lead content at the surface was considerably less than in the bulk. Conversely, brittle constituents such as primary silicon in Al-Si alloys may be selectively removed from the surface because they fracture rather than deform.

5.2. Reaction with the en~~r~nrnent Surfaces can alter their surface chemistry by reacting with the environment. An oxide layer is formed on the freshly generated metal surface and if the surface heating during machining is appreciable then relatively thick oxide layers can form. The visual appearance of grinding bum is an example of this. As indicated previously, grinding bum has been associated with redeposited metal that has been oxidized. Chemical reactions can also occur between the surface and lubricants [ 151.

155

5.3. Embedding of abrasives There is noi considerable evidence to show that, during abrading, polishing or grinding, particles of abrasive can become embedded in the surface. In abrasion and polishing, this phenomenon is mostly associated with soft metals, while for grinding, em~dding of abrasive can occur quite readily with hard materials such as in quenched and tempered low alloy steel of hardness 500 HV and even in tool steel with a hardness of about 1000 HV. Abrasive embedded in the surface can be difficult to detect by optical microscopy but is readily detected by electron probe microanalysis and by scanning electron microscopy, particularly in the latter case if the surface is given a light etch before examination. Abrasion studies have shown [ 161 that when abrading annealed aluminium on sihcon carbide papers coated with a thin layer of wax the surfaces were virtually free from embedded abrasive, whereas surfaces abraded on uncoated abrasive papers exhibited large amounts of embedded abrasive. It has been found that commonly used cutting fluids such as kerosene, alcohols and extreme pressure oils are superior in reducing embedded abrasive to water, which in turn is an improvement on dry abrasion. Carbon tetrachloride, an excellent fluid for the low speed cutting of aluminium, reduces pick-up to lower levels, However, none of the liquid lubricants restricted pick-up to the levels achieved by the soft solid coatings soap and wax. The soft solid lubricants are reported to be most effective because primarily they mechanically restrict the movement of free abrasive particles across the w orkpiece surface. For grinding [ 171, most embedding of abrasive occurred when the wheel was freshly dressed. Dressing produces much loose and broken grit in

(a)

fb)

Fig. 13. X-ray area scans for aluminium on surfaces of steel (500 HV) ground with (a) a freshly dressed wheel and (b) a wheel not freshly dressed packed with soap. The large white areas in (a) indicate embedded abrasive whereas the small spots in (b) are only background.

156

the surface of the wheel, and this abrasive grit is very readily embedded in the workpiece surface during grinding even if the workpiece is very hard. As for abrasion, packing the wheel with wax or soap virtually eliminated embedding of abrasive when grinding. Figure 13(a) is an X-ray area scan for aluminium of a surface of steel (500 HV) ground with a freshly dressed alumina wheel and should be compared with Fig. 13(b) which is an area scan obtained when the wheel was not freshly dressed and packed with soap.

6. Conclusions It is apparent from the preceding discussions that the effects of metal removal processes on the nature and structure of surfaces can be varied and complex. The area of most obvious consideration is where ion implantation is used to produce surfaces for wear or corrosion resistance under engineering conditions. The process of ion implantation is known [18] to produce a dense network of dislocations formed as a result of the aggregation of a number of point defects resulting in some degree of hardening, The process has been likened to a shot-peening process on a microscale. It is difficult to conceive, however, that the ion implantation process could further increase the dislocation density of the surface layers of a machined component which already have been plastically deformed to a very high degree. Indeed, the implantation process may cause relaxation effects and subgrain growth. Perhaps a more important consideration is the influence of changes in crystal structure at the surface of machined components on the penetration of implanted ions. Deamaley [IS] points out that initially ion implantation as a means of improving the surface performance of engineering components was regarded with some scepticism because estimates of the range of penetration of even light atoms such as nitrogen in steel was only up to 100 nm. This estimate was based on the a~umption that the dominant factor controlling the depth and concentration of the injected atoms is the kinetic energy of the ions and the transfer of this energy to the atoms of the component. This estimate overlooks, however, the inward migration of atoms both during implantation and during service life. This inward migration can be facilitated by rises in temperature and by the production of a high concentration of vacancies by the ion bomb~dment process itself. More importantly, however, the fragmented layer and the upper regions of the deformed layer can be at least an order of magnitude deeper than the 100 nm quoted above, and they have a very high concentration of subgrain boundaries and dislocations which would greatly facilitate the diffusion of ion-implanted species to many times their normal depth. Thus variations in the depths of the fragmented layer produced by different surface finishing processes could be expected to have a Iarge effect on ion implantation. With regard to service life where ion-implanted surfaces are being used for wear resistance, the wear process itself will generate heat and produce further deformation substruc-

157

tures beneath the worn surface so that when the deformed surface layers produced by surface finishing are worn away further inward migration of ions will continue and provide resistance to wear. Another factor is the effect of ion implantation on residual stress. It is known [lS] that ion implantation can produce compressive residual stresses in the surface layers and this is considered an extra benefit of the process. Grinding, however, may introduce high residual tensile stresses [ 131 in the surface layers so that the situation arises where a compressive residual stress due to impl~~tion is supe~mpo~d onto a high tensile residual stress due to grinding and consequently the resultant residual stresses will be complex and not necessarily compressive at the surface. Ion implantation is not expected to have any effect on embedded abrasive, so that any embedded abrasive present in the surface layers would still have a detrimental effect on wear behaviour. Laser glazing, ~volv~g as it does the rapid chilling of thin surface layers from the molten state, is unlikely to be greatly affected by surface integrity considerations. Of course, if the component has a very high quality surface finish it may “harden” to the laser beam, i.e. the greater part of the laser beam is reflected rather than absorbed. Conversely very poor surface finishes with surface cracks and fissures could act as crack initiators in hard martensitic layers. Also, compositions changes that occur in the surface layers because of machining will influence the final composition of the rapidly chilled alloy surface. This may be important when attempts are made to carry out any aging treatments of met&able structures. Similar comments apply to laser-hardening treatments. Finally it must be stated that, apart from the need to characterize surfaces before DEB processing, post~h~ac~~zation of DEB-proce~ed surfaces needs to be carried out before meaningful correlations can be obtained with surface-sensitive properties.

References 1 L. E. Samuels, Metallographic Polishing by Mechanical Methods, Pitman, London, 2nd edn., 1972. 2 D. M. Turley, J. Znet. Met., 96 (1968) 82. 3 D. M. Turley, E. D. Doyle and L. E. Samuels, hoc. Znt. Conf. on Production En&neering, Tokyo, 1974, Part II, Japan Society of Precision Engineering, Tokyo, 1974, p. 142. 4 E. J. A. Armarego and R. Ii. Brown, The Machining of Metals, Prentice-Hall, Englewood Cliffs, NJ, 1969, p. 172. 5 J. T. Black, J. Eng. Znd., 94 (1972) 307. 6 S. Ramalingam and J. T. Black, Metall. Trans., 4 (1973) 1103. 7 E. D. Doyle, D. M. Turley and S. Ramaiingam, hoc. 4th Tewksbury Symp. on Fracture at Work, Melbourne, February i979, University of Melbourne, Melbourne, 1979, Paper 16. 8 D. M. Turley and E. D. Doyle, Muter. Sci. Eng., 21 (1975) 261.

158 9 E. D. Doyle and D. M. Turkey, Proc. 4th North American ~etu~~ork~~g Research Conf., Colu~&us, OH, 1976, Society of ~anufaeturing Engineers, Dearborn, MI, 1976, p. 346. 10 M. Hatheriy and A. S. Mahn, ilfet. Technol., 6 (19’79) 308. 11 D. M. Turley, J. Inst. Met., 99 (1971) 271. 12 D. M. Turley and L. E. Samuels, J. Aust. Inst. Met., I7 (1972) 114. 13 M. Field, W. P. Koster, J. B. Kahls, R. E. Snider and J. Maranchik, Jr., Machining of high strength steels with emphasis on surface integrity, AF MDC 70-1, 1970 (MetCut Research Associates Inc., Cincinnati, OH). 14 V. J. Manners, J. V. Craig and F. H. Scott, J. Inst. Met., 95 (1967) 173. 15 J. G. Horne, E. D. Doyle and D. Tabor, Proc. 1st Int. Conf. on Lu&~ica~ion challenges in Me~alworkj~g and Processing, Chicago, ZL, 1978, Illinois Institute of Technology Research Institute, Chicago, IL, 1978. 16 R. W. Johnson, Wear, 16 (1970) 351. 17 D. M. Turley and P. A. Ewing, Proc. Conf. on Lubrication, Friction and Wear in Engineering, Melbourne, 1980, Institution of Engineers, Canberra, 1980, p. 181. 18 G. Dearnaley, Proc. Znt. Conf. on Advances in Surface Coating Technology, London, 1978, Vol. 1, Welding Institute, Cambridge, 1978, p. 111.