Wear, 34 (1975) 383 - 397 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
EFFECT OF BORONISING METALS*
AND WEAR OF FERROUS
T. S. EYRE Brunel University, &bridge,
Middx. (Ct. Britain)
(Received May 9, 1975)
Summary Boronising is a surface diffusion treatment analogous to carburising, but produces harder surfaces without recourse to quenching. Friction and wear resistance of boronised coatings have been evaluated on steels and grey cast iron under dry sliding conditions. Boronising inhibits adhesive wear during running-in and at loads above the mild/severe transition load. The onset of severe wear is prevented until the boronised layer has been worn away, its life expectancy is extremely long and is inversely related to applied load. Boronising has considerable advantages over other similar surface conversion treatments under adhesive wear conditions and is likely to find application at elevated temperatures and under non-oxidising conditions.
Boronising is a surface diffusion treatment which can be carried out in either a gas, molten salt or pack media at a temperature ‘between 900” 1100 “C depending upon the process and the substrate. Extremely hard surface layers are formed provided they form borides and a number of steels have been successfully treated. Although the process is analogous to carburising the high hardness is attained directly through the formation of borides and does not require quenching. If high mechanical strength is required in the substrate this can be achieved by control of carbon content or by heat treatment after boronising, though care is required to reduce quenching stresses to prevent spalling of the boronised layer. There is a parabolic relationship between the coating thickness and the diffusion time and diffusion depth is greatest in steels with a low alloy content. Although FeB is harder than FesB it should be avoided because of its increased brittleness. It becomes difficult and time-consuming to produce a coating thickness greater than 150 cc (Table 1). From the iron-boron equilibrium diagram (Fig. 1) it can be seen that there are two iron borides, FesB (8.84% B) and FeB (16.25% B). Boron has *Paper presented at the 3rd International Tribology Conference, “Tribology for the Eighties”, Paisley, 22 - 25 September, 1975.
High speed steel
of steel type on thickness
10 - 15
Layer thickness after 6 h (pm)
Layer thickness after 2 h (pm)
Boron diffusion reduced by alloying constituents
Boron diffusion reduced by alloying constituents
z E 3
: E c”
1 3 4
~I~~~‘~ 5 6 7 6 9 1011
Fig. 1. The iron-boron
Fig. 2. Microstructure of boronised steels, (a) ENlA, normalised and boronised; (b) 20 MO Cr 4, hardened and tempered and boronised. (X 150)
a very limited solubility in QI,y and 6 iron and only FezB is stable in the presence of iron regardless of any thermal treatment applied to it. At the low boron potentials usually used in commercial practice Fe2B only is
Boride FezB _ 1390 “C
8.0 x 10-6K-1
Microhardness (H, 100 gr) Coefficient expansion
Thermal conductivity Resistivity
- 20 /._wm
3 of heating
200 400 600 800 1000
(1) Specimens (2) Hardness
of the surface
layer (Hv 30 g)
680 380 330 220 200
1680 1500 1600 1575 1565
heated in a controlled measured after cooling
atmosphere. to room temperature.
ENlA Carburised ENlA Boronised EN8 Boronised Grey C.I. Boronised
770827 1500 - 1600 1500 - 1600 1200 - 1300
(H, 100 g)
formed. With increasing boron potential FeB forms in the saturated FesB layer. The boronised layer can either have a jagged, saw-toothed interface with the substrate or a relatively straight interface; the former occurs on plain carbon steels and the latter on low alloy steels (Fig. 2). The geometry of the interface has a critical effect on the adhesion of the layer. Experience
has shown that the straight interface obtained on alloyed steels often leads to spalling, particularly at sharp comers and under impact conditions. When boride layers are applied to steel substrates the mismatch in the coefficient of expansion may give rise to residual stresses in the material. On cooling there will be a residual compressive strain in the FezB layer and tension in the steel substrate, but because of the ductility of the steel the residual strains at the interface are probably low. The saw-tooth type interface assists considerably in the dispersion of the residual strain over a greater surface area and increases adhesion by mechanical keying. Some physical and mechanical properties of borides are given in Table 2. Thermo-chemical diffusion treatments are widely used for improving the friction and wear characteristics of steels and cast irons under sliding conditions. These surface treatments derive their properties from: (1) increase in surface hardness which will undoubtedly improve its resistance to abrasive and fatigue damage. Under sliding conditions increased hardness and yield strength will, for a given load, reduce the contact area and thus increase the contact stress. There is some evidence that increased hardness by production of a tempered martensitic struct:lrc: brings no over.. all improvement in wear resistance and some evidence that a decrease in wear resistance is produced; (2) a change in surface chemical composition may so change the chemical compatibility between the friction couple that the tendency to adhesion between them decreases and wear resistance is thus improved. Some improvements brought about by tufftriding and sulphinuzing are thought to be attributed to this reason; (3) friction developed between sliding surfaces develops heat and this may seriously affect the structure of the surface by softening, so a measure of thermal stability is of advantage and undoubtedly this is so in the case of nitrided surfaces. Carburised and boronised ENlA were heated for thirty minutes in a controlled atmosphere at temperatures up to 1000 “C. As expected, the carburised surface tempered to a lower hardness and at 800” and 1000 “C a normalised structure was produced. The boronised surface retained the same structure and hardness up to 1000 “C (Table 3) and this appears to be unique for surface diffusion treatments. Boronising looks extremely good in all these respects; the surface hardness is considerably in excess of that obtained in either carburising or tufftriding (Table 4). The change in composition and structure brought about by boronising has already been discussed and these changes should therefore be favourable in sliding wear resistance. Wear of metals under sliding conditions is characterised by a number of important features. In the early stages of wear when two comparatively clean metal surfaces come together, adhesive forces cause metal transfer from one surface to the other with the production of metallic wear debris and a high wear rate. If the transferred layers are rough even higher wear rates of the other surface may be encouraged. If running-in is successfully completed oxide layers develop which prevent further adhesion and the wear rates diminish. Wear is also
(e) Fig. 3. Microstructure of metals investigated, (a) ENlA steel, normalised and boronised, (b) EN8 steel, normalised and boronised, (c) Grey cast iron, boronised, (d) Grey cast iron, tufftrided, (e) ENIA steel, carburised. (X 60)
characterised by transitional behaviour when a low wear rate (oxidative wear) changes to a much higher wear rate (metallic wear). These two stages of wear are often qualitatively referred to as mild and severe wear respectively .
For the purpose of this investigation it was decided to examine the effect of boronising of two plain carbon steels and a grey cast iron, the composition and hardness of which are given in Table 5. The two steels are representative of those that are widely used in the carburised condition (ENlA) and in the normalised condition (EN8) for wear applications. ENlA steel was also carburised to the same depth as the boronised layer to establish a comparative basis. Grey cast iron is very widely used in the as cast condition for its excellent sliding wear characteristics and is also sometimes used under more arduous conditions in the tufftrided condition. Boronised and tufftrided cast irons have therefore been included in this investigation. The microstructures of these materials and surface treatments are shown in Fig. 3. It will be observed that the carburised and borunised layers are approximately 0.005 in. thick and the tufftrided layer approximately 0.0005 in. The boronised layers on all three materials have a saw-toothed diffusion front and the surfaces are relatively rough and porous in comparison with the carburised layer on ANlA steel. The boronised grey cast iron is particularly porous. Graphitisation of the pearlite in the grey cast iron has also occurred to produce more free ferrite than was originally present in the untreated iron. A well established pin-on-disc technique was used, both friction force and wear being measured continuously throughout the duration of the test. Most tests were terminated after equilibrium conditions were established, usually within two hours. Some tests were continued for considerably longer periods to wear through the boronised layers. Sliding speed was held constant at 300 cm/s and the normal load on the wear pin was varied between 3.1 and 28 kg. The material under test was used in the form of a wear pin 7.5 cm long, 6.0 mm diam. with a flat end. A steel disc was used, the composition and hardness of which is given in Table 5. A new wear pin and TABLE 5 Composition of materials C
MO -. _-
0.15 0.35 0.45
1.20 0.60 1.00
t wear disc materials
a new disc wear track were used for each test. All experiments were carried out dry in a normal laboratory environment of 20 “C and a relative humidity of 65%. Results For each material a family of wear curves was obtained and these are shown for EN8 steel in Fig. 4. From these curves the mild (oxidative) to severe (metallic) transition diagrams were plotted and these are shown for all three materials in Fig. 5, Transition diagrams were plotted from equilib-
Fig, 4. Family of wear curves for EN8 steel. 10-4r
Fig, 5. Transition wear diagram for ENlA steel, EN8 steel and Grey Cast Iron.
5 I lO‘?cm)
Fig. 6. Effect of boronising on wear of EN8 steel, at 8.5 kg and 300 cm/s. Fig. 7. Wear of EN8 steel normalised and boronised at 8.5 kg and after 1800 m of
(a) Fig. 8. Appearance of disc counterface, (a) EN8 steel, normalised, transferred metal particle adhering to the steel disc, (b) EN8 steel, boronised, smooth disc surface with no adhesion.
rium wear rates after all, running-in wear had been completed. However, it was observed that at loads just below the mild-severe transition, considerable running-in occurred with the production of metallic wear tracks and metallic debris. Boronising completely inhibited running-in wear (Fig. 6) and tufftriding had a similar effect. Even after these surface layers had worn away low wear rates persisted and it was observed that this condition was associated with the presence of a black layer on the disc surface. Once this layer was worn off a higher wear rate was then obtained. A sample of the transferred layer from a boronised experiment was removed and subjected to analysis which showed it to be rich in boron. It was also observed that running-in of both ENlA and ENS steels in the load range 6.5 - 8.5 kg was characterised by metallic transferred particles building up on the wear track of the steel disc counterface, although after the completion of running-in and oxidative mild wear a smooth black wear track was obtained. Two
lo- 8 _i
Fig. 9. Wear characteristics
Fig. 10. Wear curves
further tests were therefore carried out on EN8 steel, one not boronised, the other boronised and wear tested at 8.5 kg, the wear test being terminated after 1,800 m, partly through the running-in period. The wear curves are shown in Fig. 7 and it will be observed that as expected considerable wear of the untreated EN8 steel and negligible wear of the boron&d EN8 steel occurred. The disc wear tracks for these experiments are shown in Fig. 8 and it will be seen that the wear mechanisms are quite different. Above the transition load the boronised coatings inhibited severe wear (Fig. 9) and a considerable sliding distance was necessary before the coating was worn away. Because of the complex nature and extensive time duration of some of these experiments the effect of boronising is only shown schematically (Fig. 10). From these results it was possible to obtain the life/ load relationship for the coating which for EN8 steel is shown in Fig. 11.
40 of sliding
Fig. 11. Life us. load of the boronised
on EN8 steel.
ENlA steel gave very similar results to EN8 steel whilst grey cast iron exhibited rather more scatter, and tended towards a shorter life. However, the scatter was insignificant compared to the magnitude of the improvements obtained. Similar effects were also observed for the tufftrided grey cast iron although the degree of improvement was considerably less than that for boronised grey cast iron. No improvements were observed for the carburised ENlA steel, the transition diagram is almost identical to that for the normalised material (Fig. 9). EN8 steel is superior to carburised ENlA steel, even though the latter is considerably harder. It was observed that carburised ENlA steel exhibited the adhesion and metal transfer type wear characterised by normalised ENlA and EN8 steels. Metallurgical examination Two basic wear mechanisms have been observed during the course of these experiments and these are illustrated in Fig. 8. During running-in and at loads above the mild/severe transition a metallic wear mechanism operates which is characterised by strong adhesion between pin and disc. Plastic deformation, adhesion and fracture of metallic particles from the pin surface result in transfer to the disc counterface and the production of metallic debris. The wear rates obtained during running-in are similar to those obtained during severe wear and are proportional to the applied load. The second wear mechanism is characterised by the production of a nonmetallic wear surface which is either oxide or transferred material from the boronised or tufftrided layers. A very fine black wear debris is produced. This mechanism of wear operates under the following conditions: (1) At loads of 3.1 kg and below oxidative wear occurs with no evidence of prior metallic adhesive wear.
(a) Fig. 12. Effect of boronising on surface metallurgy (a) plastic deformation, phase transformation and oxide on normalised ENS steel, (b) no plastic deformation, phase transformation or oxide on the boronised EN8 steel. The pearlite grain size is larger than in (a) because of grain growth during boronising. (X 60)
Fig. 13. SEM view of mild and severe wear surfaces (a) mild (oxidative) wear of grey cast iron (3.1 kg), (b)severe (metallic) wear of grey cast iron (15 kg). (X 60)
(2) At loads between 6.5 - 10 kg oxidative wear occurs after some prior metallic adhesive wear. (3) Above the mild/severe transition load an oxidative wear mechanism occurred which was promoted by the presence of boronised and tufftrided layers. This wear region was extended by the presence of transferred layers of the coatings investigated and once these were completely worn away conditions reverted to metallic adhesive wear. The presence of boronised layers completely inhibited the plastic deformation and metal transfer characteristics of ENlA and EN8 steels and these features are shown in Fig. 12. The wear surfaces examined in the Scanning Electron Microscope (SEM) confirm the general features of oxidative and metallic wear already discussed (Fig. 13). At 3.1 kg there is no significant difference in the surface topography of untreated, boron&d and tufftrided grey cast irons, but at 15 kg which is above the mild/severe
(cl Fig. 14. SEM views of worn boronised steel surfaces (a) (X 300), (b) (X 1500), appearance of boronised layer, (c) taper section viewed in SEM. (X 300)
transition there is considerable evidence of plastic deformation and production of metallic wear debris on the surface. As a result of most of the experiments described the surface layers are completely worn away and it was necessary to repeat one of the tests carried out on the boronised EN8 steel at 6.5 kg which was interrupted before the boronising was completely worn away. Figure 14 shows the surface topography to have a granular pitted appearance with some areas which are relatively smooth projecting above the remainder of the surface. These smooth areas of roughly polygonal shape appear to be borides and this was confirmed by preparing a taper/SEM microsection to enable the microstructure and wear surface topography to be viewed simultaneously (Fig. 14, ref. 1). Discussion Under dry sliding conditions for all the metals and surface treatments investigated two wear mechanisms predominate. An adhesive wear process takes place during running-in and above the mild/severe transition load which
is characterised by metallic transfer from the pin to the disc counterface and the production of a metallic debris. The transferred particles adhere strongly to the disc surface and under the influence of plastic deformation become elongated into a roughly ellipsoid shape. Both the appearance and shape of the transferred particles and wear debris are typical of an adhesive wear mechanism. There is less evidence of adhesion of the grey cast iron/steel disc couple as indicated by a lower frictional resistance and less tendency to adhesion of grey iron particles on the steel counterface. Considerable evidence of plastic deformation and phase transformation exists on wear pin surfaces and wear debris from the untreated steels and cast iron. These surfaces and debris are considerably hardened during the adhesive friction process. Grooving of the wear pin surfaces has been caused by contact with the hard transferred particles on the steel disc counterface. It is, therefore, apparent how an adhesive wear process can become even more catastrophic due to the presence of hard debris particles causing abrasive wear. The wear rates during running-in and above the mild/severe transition load fall on a straight line and are proportional to the applied load, and are some two orders of magnitude higher than wear produced by oxidative wear. At loads of 3.1 kg and below and after running-in is completed at higher loads an oxide develops on the hardened surface of both the pin and disc and the wear rate is considerably reduced. This oxidative wear process does not have any of the characteristic features of adhesive wear. Wear takes place by fracture of oxide which produces a plate like debris. A mild/severe wear transition is exhibited for all three untreated metals the main characteristic of which is a higher bulk wear pin temperature and a greater tendency to plastic deformation of the pin to produce a “skirt” of deformed metal around the edges of the wear face. ENlA steel has the lowest and grey cast iron the highest transition load. Increased carbon content in the pearlitic matrix and availability of free graphite both increase the transition load. Increasing the hardness of ENlA steel by carburising does not improve the wear characteristics of ENlA steel and these results confirm the findings of other investigations [ 21. It would therefore appear that for a given carbon content and hardness the best result is achieved with a pearlitic microstructure. The effect of bdronising is to prevent adhesive wear under all conditions, at least until the boronised layer itself has been removed from both friction surfaces. In this respect tufftriding has a similar effect. Of the three surface treatments investigated the tufftrided surface is softest (700 HV), the boronised surface is hardest (1500 HV) whilst the carburised surface is somewhere between (800 HV). The carburised steel (SOOHV) exhibited adhesive wear and was no better than the normalised ENlA steel. This result, therefore, disposes of any meaningful relationship between hardness and adhesive wear resistance. Both the hardest and softest surfaces inhibited adhesive wear and this behaviour can be attributed to chemical incompatibility between the pin and disc surfaces. The life of both boronised and tufftrided layers was inversely proportional to load with the boronised sur-
face being greatly superior to the tufftrided surfaces. It must however be pointed out that the boronised surfaces were much thicker, it would however be difficult to produce much thicker tufftrided layers. A further remarkable feature of the boronised microstructure is its ability to retain its hardness even after being subjected to temperatures up to 1000 “C. Although this was not a feature that played any significant part in this investigation it could be a valuable property in other high temperature wear applications. The affect of both boronising and tufftriding would appear to be worth further consideration in wear applications where the absence of oxygen might otherwise promote adhesive wear. Boronising has a higher load bearing capacity than the tufftrided layer which is probably related to both hardness and coating thickness and this imparts greater resistance to wear above the mild/severe transition load. At 21 kg for example, the boronised cast iron has a low wear rate for a considerable sliding distance until it is worn away whilst the tufftrided cast iron has a high wear rate immediately rubbing commences. Boronising considerably improves the wear characteristics of the materials investigated and has only one major disadvantage which is associated with the inherent brittleness of the coating. In this respect coatings on plain carbon steels are superior to those on alloy steels. Great care, therefore, should be taken in avoiding applications involving impact loads or treating components with sharp edges. Boronising is superior to other coatings evaluated and its friction and wear resistance are likely to be extended to higher temperatures than would be expected of other treatments, particularly under non-oxidising atmospheres. Conclusions (1) grey cast (2) life time (3) load. (4)
Boronising prevents adhesive wear of ENlA steel, EN8 steel and iron. Wear above the mild/severe transition load is inhibited during the of the boronised layer. The life of the boronised layer is inversely related to the applied Boronising
to the other diffusion
Acknowledgements The author would like to thank Borax Consolidated Ltd., and Danite Hard Metals Ltd., for their financial assistance and the provision of boronised samples. J. Perry and K. Dutta gave considerable assistance with the experimental work. References 1 T. S. Eyre and K. Dutta, ASLE/ASME 2 J. C. Gregory, Tribology, 3 (2) (1970)
Lubric. Conf., Paper 74LC-lB-1 ‘7’3 - 83.