- Email: [email protected]
Design aspects for advanced tribological surface coatings
ELSEVIER
Surface and Coatings Technology lOOG101 (1998) IL6
Design aspects for advanced tribological surface coatings A. Matthews a,,* , A. Leyland a, K. Holmberg
b, H. Ronkainen
b
a Reseurch Centre in SurIfirce Engineering, University of Hull, Hu11, HW 7RX. UK b VTT Manufucturing Technology. PO Box 1704. FIN-02044 VTT, Finland
Abstract A holistic approach to the study of the important tribological contact mechanisms is described, which provides a basis for effective coating design. The mechanisms include macromechanical effects, defining the stress fields, and these are influenced by the hardness. thickness and surface finish levels of coatings and substrates. Micromechanical mechanisms influence cracking. Tribochemical mechanisms can also determine friction and wear performance. Material transfer is another influencing mechanism, and nanomechanical mechanisms at the atomic level influence friction. Examples are given of coatings and treatments that fulfil the needs of these mechanisms in a range of different contact types. 6 1998 Elsevier Science S.A. Keywords:
Tribology;
Coatings: Design: Systems
-
1. Introduction
2. Tribological contact mechanisms
In recent years, there has been a considerable improvement in the level of understanding of the tribology of surface coatings. At the same time, developments in coating technology have provided the possibility of producing surface layers with properties that were previously unachievable. However, these developments in tribology and in coating technology have, in someways, progressed almost independently of each other. In a recent book [ 11, two of the authors attempted to merge these two parallel fields, to describe the properties of the new coatings and to put these in the context of tribological theories. This forms a basis for effective surface design, to provide the exact properties required for a given range of operating conditions. This requires, firstly, an understanding of tribological contacts and the degradation mechanisms that can occur. This paper begins by outlining a holistic basis to the fulfilment of this objective [2,3], followed by an explanation of how the surface requirements necessaryto resist these degradation effects can be met using new coating developments, which often involve hybrid or duplex combinations of treatments or coatings [4,5].
2.1. Mucron~echanical
* Corresponding author. Tel: +44 1482 465073; Fax: +44 1482 466477; e-mail: [email protected] 0257-8972/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PII SO257-8972(97)00578-i
nlechanisrns
These are defined by the stressand strain distributions in the whole contact between two surfaces, the total elastic and plastic deformations that they result in, and the total wear particle formation process and its dynamics. Four main factors influence the macromechanical tribological behaviour: ( 1) the hardness of the coating and substrate, (2) the thickness of the coating, (3) the roughness of the surface, and (4) the size and hardness of any debris in the contact (which may pre-exist or be generated during service). Depending on these factors, a range of phenomena can occur, as shown in Fig. 1. Conflicting demands may be placed on a contact, e.g. the need for low friction (which may necessitate a low shear strength surface) together with a low wear rate (which may require a hard, high shear strength surface). Also a hard, load-supporting layer is often needed to prevent junction growth and bulk plastic deformation. These apparently contradictory requirements may be fulfilled by using a very thin, low shear strength film above a hard coating. This clearly brings into consideration the optimum thickness requirement; e.g. in certain applications a thick, relatively soft film can assist in extending service life by embedding any debris present, and therefore effectively removing this from the contact.
2
z/jTH,CKNESS ,.~._. ::?: OF COATING ..:
Fig. 1, Tribological
phenomena
(a-l ) observed
in sliding
Whilst most macromechanical models consider static or sliding contacts, there is increasing evidence that impact contact conditions are widespread in real applications and these can place even greater demands on the surface and near-surface regions. 2.2.
Micr-ott7eckanical
ntechanistm
These occur at the asperity level and encompass crack generation and propagation, wear particle formation and related phenomena on a micrometre to nanometre scale. In recent years several researchers have identified a multilayered coating approach as a means of reducing surface crack propagation [ 6-91. 2.3.
Tt?hochetnicul
tt~eclwtistns
In many tribological contacts the wear behaviour is controlled by the reaction of the surfaces with the local environment. Also, in certain applications (such as metal cutting) reactions may occur between contacting surfaces due to the heat generated, leading to the production of modified surface chemistries which may promote different (and not necessarily detrimental) behaviour to that of the original surfaces, in the ensuing tribological lifespan. 2.4.
Nunottlechanical
tnechanistns
Recent developments in analysis equipment, such as the atomic force microscope, have provided a means of examining the contact mechanisms that occur at the subnanometre level. In particular. studies using these
contacts.
resulting
from
changes
in coating
parameters
methods have identified the importance vibrations in friction generation.
2.5.
A4uterid
of atomic lattice
tnrtt~fi’r.tnechritii.sttw
In many contacts the transfer of material from one surface to the other dominates the friction and wear behaviour. Indeed, it can often lead to a like-on-like sliding pairing, rather than the parieing that was originally intended. This will in most cases be disadvantageous, but situations exist [e.g. for some diamond-likecarbon ( DLC ) or MoS, coatings] in which such mechanisms are claimed to be beneficial [ 10,111.
3. Coating design to control tribological
mechanisms
Traditional wear-resistant coatings have been applied by electroplating and thermal spraying techniques. These coatings are. in general, much thicker than the recently developed coatings based on vapour deposition technologies. As a consequence of their thickness the established coatings have been less dependent on the substrate to provide load support, i.e. to ensure that the macromechanical wear mechanisms are resisted. However, these coatings have tended to exhibit limited hardness and certainly provide less control over structural and compositional profiles than the more modern coating methods. In spite of this, a recent study [ 121 has shown that the advanced plasma and vapour deposition technologies account for less than 0.5% of the engineering coatings market in the UK, by value, and even less than this by volume or area treated. There are many reasons
for this, not least the relative expense and the unsuitability of the newer coatings for many low-cost engineering substrate materials. Thus, in order to specify a new coating process to fulfil predefined tribological requirements, the influence of the substrate and coating must be considered together in the overall performance/cost balance. Fortunately, with very recent process developments it is now often possible not only to incorporate the surface benefits of advanced plasma and vapourbased processes, but also to design the coating and substrate together so that they achieve a composite effect which optimizes both tribological performance and cost. In Ref. [l] we cited seven contact conditions which are representative of some typical applications (Fig. 2). These were used to illustrate a structured selection methodology as a series of rules based on empirical and theoretical models. In each of these contact conditions or types one or more of the mechanisms described in Section 2 may operate, and other mechanisms may also
come into play. These contact conditions will be used to illustrate how effective coating design can be used to meet different tribological requirements. The examples are by no means meant to be exhaustive, or even definitive. They are presented merely to highlight how consideration of the requirements of the tribological contact mechanisms can lead to more effective surface design. 3.1. Contact stresses In the majority of tribological contacts, the primary aim will be to ensure that the stresses generated do not lead to adverse macromechanical mechanisms such as surface plastic deformation. For ceramic coatings deposited by vapour deposition methods this requires that there should be adequate load support beneath the coating. This can (for example) be achieved by a plasma diffusion pretreatment of the surface [ 13-151, or by depositing a low-cost supportive interlayer such as electroless nickel [ 16,171. These approaches can ensure that a cheaper, low-alloy steel can be used for the substrate, thus providing a more cost-effective solution. 3.2. Sliding contact
Contact stresses
Sliding
Surface
Fretting
fatigue
impact
Abrasion
This is the most common tribological contact condition and is often combined with high contact stresses. The aim will usually be to reduce both friction and wear, and again the need to prevent plastic deformation will be paramount. In this situation a coating which is not brittle and can to some extent deform with substrate elastic deformations can be beneficial. This applies, for example, to the multilayered DLC/metal carbide coatings that are now available [ 18,191. Fig. 3 illustrates how these can deform under load without failure due to the bending stresses generated. In effect, the more elastic layers allow the brittle layers to slide over each other in the manner of a multileaf book when bent. This provides a coating which combines properties of both hardness and elasticity. For low friction coefficients in sliding, it has been found that DLC and diamond coatings can be particularly effective. This can be explained by the formation of low shear strength microfilms on the hard coating [20%22]. In effect, the contact is considered as a soft coating on a hard substrate, the latter being in this case the underlying hard coating. 3.3. Surfuce jbtigur
Chemical Fig. 2. Typical
dissolution contact
conditions
experienced
by coatings
This condition occurs, for example, in ball and roller bearings and in gears. The ability to tolerate repeated deformation is important and multilayer DLC/metal carbide coatings can again be beneficial in limiting macromechanical mechanisms leading to crack propaga-
multiyr
coating_/ \
‘Ounter face
\
H s H s H 9
\
/
,
Substrate
, m ,.,A U”IU
H = hard (less elastic) layer S = soft (more elastic)
I
7
layer H 9 H ‘I H 8
Substrate
Fig. 3. Simplified without fracture.
schematic representation illustrating how a multilayer structure with alternating elastic properties can allow coating deformation The line through the film shows how the shear occurring in the more elastic layers allows the brittle layers to slide over each other.
tion. Very thin ceramic coatings can also provide benefits [23,24], since they can deflect with the substrate surface under load and therefore avoid macromechanical mechanism-induced failure.
coefficient, as this may be detrimental to the fretting behaviour, as can any change which permits or accelerates tribochemical mechanismsand corrosion. 3.5. Abmsion
3.4. Fretting Fretting is a special case of fatigue wear, which is most efficiently reduced by effective macromechanical design measures, e.g. to control the displacements and stresses induced in the contact. However, it is also possible to modify the frictional conditions to reduce fretting, e.g. by means of a solid lubricant coating such as MO& applied by PVD, or a soft electroplated metal. Increasing the surface hardness by coating or thermochemical treatments can also be beneficial, especially if compressive residual stressesare induced, although care may have to be taken not to increase the friction
Contact conditions involving abrasion have long been recognized as being some of the most significant in terms of the amount of material loss. The theoretical understanding of abrasion, and related phenomena such as erosion, has advanced considerably. Nevertheless, it is noticeable that the performance of advanced coatings such as PVD TIN has been less remarkable under abrasive conditions than under sliding conditions. This is partly due to the thickness limitations on TIN, its lack of toughnessand the need for effective load support, especially under three body abrasion with sharp and hard abrasive particles. The load support requirement
A. Matthews
et al. / Surfhce
and Coatings
to prevent adverse macromechanical mechanisms can be fulfilled by utilizing interlayers such as electroless nickel, to improve performance considerably, as demonstrated in, for example, the ASTM rubber wheel test [ 151. Also, it has been found that by multilayering Ti and TiN films it is possible to produce a composite coating which is both hard and tough, thus controlling micromechanical mechanisms and performing well in erosive and abrasive conditions [25]. A coating with similarly effective abrasion resistance can be formed by depositing stainless steel under nitrogen plasma conditions and then post-coat plasma nitriding the coating [4,X26]. 3.6. Impact As in the case of abrasion, surfaces must possess a high toughness in order to absorb impacts that result in severe macromechanical stress fields. They must be sufficiently elastic to be able to accommodate any substrate deformation that may occur under impact. Recently, work has taken place to develop test equipment that will assess coatings under impact conditions 127,281. This has shown that the multilayered coatings that alternate hard (inelastic) and soft (more elastic) layers can provide excellent performance [29], i.e. they prevent the occurrence of micromechanical mechanisms such as crack growth.
Technology
3.8. Combination
(1998)
1-6
5
time during operation. It is then useful to utilize a coating that combines a number of desirable attributes, such as the one in Fig. 4a which represents a functionally graded composition, designed to fulfil a number of operating needs. Fig. 4b illustrates an example of a functionally graded coating from the literature [33].
4. Discussion
and conclusions
Optimal coating design requires careful consideration of the tribological contact mechanisms that operate. These can include the macromechanical mechanisms, as defined by the stress fields in the contact, which can be influenced by coating properties such as hardness, thickness and surface finish, and by the substrate. Micromechanical effects, influencing cracking behaviour, must also be taken into account, as must the tribochemical mechanisms that can influence the chemical stability of the coating/substrate combination in service. Consideration also has to be given to effects (4
T
sllpporting interlayers
3.7. Chemical dissolution Although the figure illustrating this mechanism depicts the workpiece moving over a cutting tool (a typical tribochemical mechanism-dominated application), contacts that involve a chemical reaction can include those subject to aqueous corrosive attack as well as those subjected to high temperature diffusion and oxidation conditions. Again, the early performance of PVD TIN films was not regarded as outstanding in any of these regards. For example, pin-hole defects made such coatings vulnerable to aqueous corrosion. This situation was considerably improved by utilizing interlayer films such as electroless nickel [ 16,171. Another promising coating in this regard is a carbondoped tungsten film which is both dense and hard [ 301. In metal-cutting applications the resistance of TIN films to oxidation was first improved by employing TiAlN compositions [31] and then, more recently, by advanced multiphase ceramics incorporating yttrium, which helps to form a stable oxide on the surface [32].
100-101
diffusion barrier adhesive
I
Steel Substrate
(b)
DLC (a:CH) Matrix // TIC Graphite Ti
contact conditions
As already stated, the above contact conditions may operate individually or in combination. Indeed, the nature of the tribological mechanisms can change with
Fig. 4. (a) Schematic of a functionally graded coating; (b) typical functionally graded composition.
occurring at an atomic level, as there is increasing evidence that these can control the frictional behaviour; this area of study is still in its infancy. Finally, potential material transfer mechanisms must be taken into account, as these can also have a significant effect on the ensuing friction and wear behaviour. Examples have been cited of advanced coatings, developed for specific types of contact conditions which are optimized to take into account each of these mechanisms. Most of these solutions involve combining two or even more coating or treatment methods. They also consider the coating and substrate together, acting to achieve a composite effect of which neither alone is capable, thus achieving what many regard as the primary (but presently under-utilised) philosophy of surface engineering design.
[6] [7] [8] [9] [IO] [I I] [ 121 [I31 [ 141 [I51 [ 161
[ 171 [I81
Acknowledgement Professor Matthews and Dr. Leyland thank their colleagues in the Research Centre in Surface Engineering for their technical input into many of the coatings reported here, and the EPSRC and industrial sources for providing financial support for this work. Professor Holmberg and Ms. Ronkainen thank their colleagues and the VTT for their help and advice, and TEKES and the Swedish Academy of Engineering Sciences in Finland for financial support.
References [I] [2] [3] [4]
[5]
K. Holmberg, 442. K. Holmberg, K. Holmberg, A. Matthews, ings and Thin 1997. A. Matthews,
[ 191 [20]
[?I] [22] [23] [24] [25] [26] [27] [28] [29] [30]
A. Matthews,
Elsevier
Tribology
Series 28 ( 1994) [3l]
A. Matthews, Thin Solid Films 353 ( 1994) 173. Surf. Coat. Technol. 56 (1992) 1. in: Y. Pauleau, P.B. Berna (Eds.), Protective CoatFilms, Kluwer Academic Publishers. Dordrecht.
[32]
[33] A. Leyland.
B. Dorn.
P.R.
Stevenson.
M.
Bin-
Sudin, C. Rebholz, A.A. Voevodin. J. Schneider, J. Vat. Sci. Technol. A I3 (1995) 1303. H. Holleck. V. Schier, Surf. Coat. Technol. 7677 (1995) 328. H. Holleck. M. Lahres, P. Wall. Surf. Coat. Technol. 41 (1990) 179. W.D. Sproul. J. Vat. Sci. Technol. A I2 (1994) 1595. S.A. Barnett, M. Shinn, Annu. Rev. Mater. Sci. 24 (1994) 481. C. Donnet, J.-M. Martin, T. Le Mogne, M. Belin, Proc. Int. Tribology Conf.. Yokohama, 1995. 6. G.B. Hopple. S.H. Loewenthal, Surf. Coat. Technol. 6869 (1994) 398. A. Matthews. R. Artley, P. Holiday. P. Stevenson, The UK Engineering Coatings Industry in 2005, Hull University. Hull (1992). A. Leyland. K.S. Fancey. A. Matthews, Surf. Engng 7 ( 1991) 207. A. Leyland. D.B. Lewis. P.R. Stevenson A. Matthews, Surf. Coat. Technol. 62 ( 1993) 608. A. Matthews, A. Leyland, Surf. Coat. Technol. 71 (1995) 88. A. Leyland. M. Bin-Sudin, A.S. James. M.R. Kalantary. P.B. Wells, A. Matthews, J. Housden. B. Garside. Surf. Coat. Technol. 60 (1993) 474. M. Bin-Sudin. A. Leyland, A.S. James, A. Matthews, J. Housden, B. Car-side, Surf. Coat, Technol. 81 (1996) 215. A. Matthews. S.S. Eskildsen, Diamond Relat. Mater. 3 ( 1994) 901. H. Dimigen. C.P. Klages, Surf. Coat. Technol. 3 ( 1991) 543. M.N. Gardos, in: K.E. Spear, J.P. Dismukes (Eds.), Synthetic Diamond: Emerging CVD Science and Technology, John Wiley & Sons. New York. 1994, 410. A. Erdemir. C. Bindal. J. Pagan. P. Wilbur. Surf. Coat. Technol. 7677 (1995) 559. A. Erdemir. C. Bindal. G.R. Fenske, C. Zuikner, P. Wilbur, Surf. Coat. Technol. 86687 (1996) 692. T.P. Chang. H.S. Cheng, W.D. Sproul, Surf. Coat. Technol. 4344 (1990) 699. T.P. Chang, H.S. Cheng, W.A. Chiou, W.D. Sproul. Tribology Trans. 34 ( I99 I ) 408. A. Leyland. A. Matthews, Surf. Coat. Technol. 70 ( 1994) 19. B. Raehle, MSc Thesis. University of Hull. 1988. R. Bantle. A. Matthews, Surf. Coat. Technol. 7475 (1995) 857. W. Heinke. A. Leyland. A. Matthews. G. Berg, C. Friedrich. E. Brosreit. Thin Solid Films 270 (1995) 431. A.A. Voevodin, R. Bantle. A. Matthews, Wear I85 (1995) 151. C. Rebholz, J.M. Schneider. H. Ziegele, B. Rahle, A. Leyland. A. Matthews. Vacuum (submitted). H. Ronkainen, I. Nieminen, K. Holmberg. A. Leyland, A. Matthews. B. Matthes. E. Broszeit, Surf. Coat. Technol. 49 ( 1991) 468. W.-D. Mtinz, I.J. Smith, L.A. Donahue. A.P. Deeming, P. Halstead, Proceedings of the I st French and German Conference on High Speed Machining, Metz. France, 17-18 June 1997. A.A. Voevodin. J.M. Schneider, C. Rebholz. A. Matthews. Tribology Int. 29 ( 1996) 559.
Surface and Coatings Technology lOOG101 (1998) IL6
Design aspects for advanced tribological surface coatings A. Matthews a,,* , A. Leyland a, K. Holmberg
b, H. Ronkainen
b
a Reseurch Centre in SurIfirce Engineering, University of Hull, Hu11, HW 7RX. UK b VTT Manufucturing Technology. PO Box 1704. FIN-02044 VTT, Finland
Abstract A holistic approach to the study of the important tribological contact mechanisms is described, which provides a basis for effective coating design. The mechanisms include macromechanical effects, defining the stress fields, and these are influenced by the hardness. thickness and surface finish levels of coatings and substrates. Micromechanical mechanisms influence cracking. Tribochemical mechanisms can also determine friction and wear performance. Material transfer is another influencing mechanism, and nanomechanical mechanisms at the atomic level influence friction. Examples are given of coatings and treatments that fulfil the needs of these mechanisms in a range of different contact types. 6 1998 Elsevier Science S.A. Keywords:
Tribology;
Coatings: Design: Systems
-
1. Introduction
2. Tribological contact mechanisms
In recent years, there has been a considerable improvement in the level of understanding of the tribology of surface coatings. At the same time, developments in coating technology have provided the possibility of producing surface layers with properties that were previously unachievable. However, these developments in tribology and in coating technology have, in someways, progressed almost independently of each other. In a recent book [ 11, two of the authors attempted to merge these two parallel fields, to describe the properties of the new coatings and to put these in the context of tribological theories. This forms a basis for effective surface design, to provide the exact properties required for a given range of operating conditions. This requires, firstly, an understanding of tribological contacts and the degradation mechanisms that can occur. This paper begins by outlining a holistic basis to the fulfilment of this objective [2,3], followed by an explanation of how the surface requirements necessaryto resist these degradation effects can be met using new coating developments, which often involve hybrid or duplex combinations of treatments or coatings [4,5].
2.1. Mucron~echanical
* Corresponding author. Tel: +44 1482 465073; Fax: +44 1482 466477; e-mail: [email protected] 0257-8972/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PII SO257-8972(97)00578-i
nlechanisrns
These are defined by the stressand strain distributions in the whole contact between two surfaces, the total elastic and plastic deformations that they result in, and the total wear particle formation process and its dynamics. Four main factors influence the macromechanical tribological behaviour: ( 1) the hardness of the coating and substrate, (2) the thickness of the coating, (3) the roughness of the surface, and (4) the size and hardness of any debris in the contact (which may pre-exist or be generated during service). Depending on these factors, a range of phenomena can occur, as shown in Fig. 1. Conflicting demands may be placed on a contact, e.g. the need for low friction (which may necessitate a low shear strength surface) together with a low wear rate (which may require a hard, high shear strength surface). Also a hard, load-supporting layer is often needed to prevent junction growth and bulk plastic deformation. These apparently contradictory requirements may be fulfilled by using a very thin, low shear strength film above a hard coating. This clearly brings into consideration the optimum thickness requirement; e.g. in certain applications a thick, relatively soft film can assist in extending service life by embedding any debris present, and therefore effectively removing this from the contact.
2
z/jTH,CKNESS ,.~._. ::?: OF COATING ..:
Fig. 1, Tribological
phenomena
(a-l ) observed
in sliding
Whilst most macromechanical models consider static or sliding contacts, there is increasing evidence that impact contact conditions are widespread in real applications and these can place even greater demands on the surface and near-surface regions. 2.2.
Micr-ott7eckanical
ntechanistm
These occur at the asperity level and encompass crack generation and propagation, wear particle formation and related phenomena on a micrometre to nanometre scale. In recent years several researchers have identified a multilayered coating approach as a means of reducing surface crack propagation [ 6-91. 2.3.
Tt?hochetnicul
tt~eclwtistns
In many tribological contacts the wear behaviour is controlled by the reaction of the surfaces with the local environment. Also, in certain applications (such as metal cutting) reactions may occur between contacting surfaces due to the heat generated, leading to the production of modified surface chemistries which may promote different (and not necessarily detrimental) behaviour to that of the original surfaces, in the ensuing tribological lifespan. 2.4.
Nunottlechanical
tnechanistns
Recent developments in analysis equipment, such as the atomic force microscope, have provided a means of examining the contact mechanisms that occur at the subnanometre level. In particular. studies using these
contacts.
resulting
from
changes
in coating
parameters
methods have identified the importance vibrations in friction generation.
2.5.
A4uterid
of atomic lattice
tnrtt~fi’r.tnechritii.sttw
In many contacts the transfer of material from one surface to the other dominates the friction and wear behaviour. Indeed, it can often lead to a like-on-like sliding pairing, rather than the parieing that was originally intended. This will in most cases be disadvantageous, but situations exist [e.g. for some diamond-likecarbon ( DLC ) or MoS, coatings] in which such mechanisms are claimed to be beneficial [ 10,111.
3. Coating design to control tribological
mechanisms
Traditional wear-resistant coatings have been applied by electroplating and thermal spraying techniques. These coatings are. in general, much thicker than the recently developed coatings based on vapour deposition technologies. As a consequence of their thickness the established coatings have been less dependent on the substrate to provide load support, i.e. to ensure that the macromechanical wear mechanisms are resisted. However, these coatings have tended to exhibit limited hardness and certainly provide less control over structural and compositional profiles than the more modern coating methods. In spite of this, a recent study [ 121 has shown that the advanced plasma and vapour deposition technologies account for less than 0.5% of the engineering coatings market in the UK, by value, and even less than this by volume or area treated. There are many reasons
for this, not least the relative expense and the unsuitability of the newer coatings for many low-cost engineering substrate materials. Thus, in order to specify a new coating process to fulfil predefined tribological requirements, the influence of the substrate and coating must be considered together in the overall performance/cost balance. Fortunately, with very recent process developments it is now often possible not only to incorporate the surface benefits of advanced plasma and vapourbased processes, but also to design the coating and substrate together so that they achieve a composite effect which optimizes both tribological performance and cost. In Ref. [l] we cited seven contact conditions which are representative of some typical applications (Fig. 2). These were used to illustrate a structured selection methodology as a series of rules based on empirical and theoretical models. In each of these contact conditions or types one or more of the mechanisms described in Section 2 may operate, and other mechanisms may also
come into play. These contact conditions will be used to illustrate how effective coating design can be used to meet different tribological requirements. The examples are by no means meant to be exhaustive, or even definitive. They are presented merely to highlight how consideration of the requirements of the tribological contact mechanisms can lead to more effective surface design. 3.1. Contact stresses In the majority of tribological contacts, the primary aim will be to ensure that the stresses generated do not lead to adverse macromechanical mechanisms such as surface plastic deformation. For ceramic coatings deposited by vapour deposition methods this requires that there should be adequate load support beneath the coating. This can (for example) be achieved by a plasma diffusion pretreatment of the surface [ 13-151, or by depositing a low-cost supportive interlayer such as electroless nickel [ 16,171. These approaches can ensure that a cheaper, low-alloy steel can be used for the substrate, thus providing a more cost-effective solution. 3.2. Sliding contact
Contact stresses
Sliding
Surface
Fretting
fatigue
impact
Abrasion
This is the most common tribological contact condition and is often combined with high contact stresses. The aim will usually be to reduce both friction and wear, and again the need to prevent plastic deformation will be paramount. In this situation a coating which is not brittle and can to some extent deform with substrate elastic deformations can be beneficial. This applies, for example, to the multilayered DLC/metal carbide coatings that are now available [ 18,191. Fig. 3 illustrates how these can deform under load without failure due to the bending stresses generated. In effect, the more elastic layers allow the brittle layers to slide over each other in the manner of a multileaf book when bent. This provides a coating which combines properties of both hardness and elasticity. For low friction coefficients in sliding, it has been found that DLC and diamond coatings can be particularly effective. This can be explained by the formation of low shear strength microfilms on the hard coating [20%22]. In effect, the contact is considered as a soft coating on a hard substrate, the latter being in this case the underlying hard coating. 3.3. Surfuce jbtigur
Chemical Fig. 2. Typical
dissolution contact
conditions
experienced
by coatings
This condition occurs, for example, in ball and roller bearings and in gears. The ability to tolerate repeated deformation is important and multilayer DLC/metal carbide coatings can again be beneficial in limiting macromechanical mechanisms leading to crack propaga-
multiyr
coating_/ \
‘Ounter face
\
H s H s H 9
\
/
,
Substrate
, m ,.,A U”IU
H = hard (less elastic) layer S = soft (more elastic)
I
7
layer H 9 H ‘I H 8
Substrate
Fig. 3. Simplified without fracture.
schematic representation illustrating how a multilayer structure with alternating elastic properties can allow coating deformation The line through the film shows how the shear occurring in the more elastic layers allows the brittle layers to slide over each other.
tion. Very thin ceramic coatings can also provide benefits [23,24], since they can deflect with the substrate surface under load and therefore avoid macromechanical mechanism-induced failure.
coefficient, as this may be detrimental to the fretting behaviour, as can any change which permits or accelerates tribochemical mechanismsand corrosion. 3.5. Abmsion
3.4. Fretting Fretting is a special case of fatigue wear, which is most efficiently reduced by effective macromechanical design measures, e.g. to control the displacements and stresses induced in the contact. However, it is also possible to modify the frictional conditions to reduce fretting, e.g. by means of a solid lubricant coating such as MO& applied by PVD, or a soft electroplated metal. Increasing the surface hardness by coating or thermochemical treatments can also be beneficial, especially if compressive residual stressesare induced, although care may have to be taken not to increase the friction
Contact conditions involving abrasion have long been recognized as being some of the most significant in terms of the amount of material loss. The theoretical understanding of abrasion, and related phenomena such as erosion, has advanced considerably. Nevertheless, it is noticeable that the performance of advanced coatings such as PVD TIN has been less remarkable under abrasive conditions than under sliding conditions. This is partly due to the thickness limitations on TIN, its lack of toughnessand the need for effective load support, especially under three body abrasion with sharp and hard abrasive particles. The load support requirement
A. Matthews
et al. / Surfhce
and Coatings
to prevent adverse macromechanical mechanisms can be fulfilled by utilizing interlayers such as electroless nickel, to improve performance considerably, as demonstrated in, for example, the ASTM rubber wheel test [ 151. Also, it has been found that by multilayering Ti and TiN films it is possible to produce a composite coating which is both hard and tough, thus controlling micromechanical mechanisms and performing well in erosive and abrasive conditions [25]. A coating with similarly effective abrasion resistance can be formed by depositing stainless steel under nitrogen plasma conditions and then post-coat plasma nitriding the coating [4,X26]. 3.6. Impact As in the case of abrasion, surfaces must possess a high toughness in order to absorb impacts that result in severe macromechanical stress fields. They must be sufficiently elastic to be able to accommodate any substrate deformation that may occur under impact. Recently, work has taken place to develop test equipment that will assess coatings under impact conditions 127,281. This has shown that the multilayered coatings that alternate hard (inelastic) and soft (more elastic) layers can provide excellent performance [29], i.e. they prevent the occurrence of micromechanical mechanisms such as crack growth.
Technology
3.8. Combination
(1998)
1-6
5
time during operation. It is then useful to utilize a coating that combines a number of desirable attributes, such as the one in Fig. 4a which represents a functionally graded composition, designed to fulfil a number of operating needs. Fig. 4b illustrates an example of a functionally graded coating from the literature [33].
4. Discussion
and conclusions
Optimal coating design requires careful consideration of the tribological contact mechanisms that operate. These can include the macromechanical mechanisms, as defined by the stress fields in the contact, which can be influenced by coating properties such as hardness, thickness and surface finish, and by the substrate. Micromechanical effects, influencing cracking behaviour, must also be taken into account, as must the tribochemical mechanisms that can influence the chemical stability of the coating/substrate combination in service. Consideration also has to be given to effects (4
T
sllpporting interlayers
3.7. Chemical dissolution Although the figure illustrating this mechanism depicts the workpiece moving over a cutting tool (a typical tribochemical mechanism-dominated application), contacts that involve a chemical reaction can include those subject to aqueous corrosive attack as well as those subjected to high temperature diffusion and oxidation conditions. Again, the early performance of PVD TIN films was not regarded as outstanding in any of these regards. For example, pin-hole defects made such coatings vulnerable to aqueous corrosion. This situation was considerably improved by utilizing interlayer films such as electroless nickel [ 16,171. Another promising coating in this regard is a carbondoped tungsten film which is both dense and hard [ 301. In metal-cutting applications the resistance of TIN films to oxidation was first improved by employing TiAlN compositions [31] and then, more recently, by advanced multiphase ceramics incorporating yttrium, which helps to form a stable oxide on the surface [32].
100-101
diffusion barrier adhesive
I
Steel Substrate
(b)
DLC (a:CH) Matrix // TIC Graphite Ti
contact conditions
As already stated, the above contact conditions may operate individually or in combination. Indeed, the nature of the tribological mechanisms can change with
Fig. 4. (a) Schematic of a functionally graded coating; (b) typical functionally graded composition.
occurring at an atomic level, as there is increasing evidence that these can control the frictional behaviour; this area of study is still in its infancy. Finally, potential material transfer mechanisms must be taken into account, as these can also have a significant effect on the ensuing friction and wear behaviour. Examples have been cited of advanced coatings, developed for specific types of contact conditions which are optimized to take into account each of these mechanisms. Most of these solutions involve combining two or even more coating or treatment methods. They also consider the coating and substrate together, acting to achieve a composite effect of which neither alone is capable, thus achieving what many regard as the primary (but presently under-utilised) philosophy of surface engineering design.
[6] [7] [8] [9] [IO] [I I] [ 121 [I31 [ 141 [I51 [ 161
[ 171 [I81
Acknowledgement Professor Matthews and Dr. Leyland thank their colleagues in the Research Centre in Surface Engineering for their technical input into many of the coatings reported here, and the EPSRC and industrial sources for providing financial support for this work. Professor Holmberg and Ms. Ronkainen thank their colleagues and the VTT for their help and advice, and TEKES and the Swedish Academy of Engineering Sciences in Finland for financial support.
References [I] [2] [3] [4]
[5]
K. Holmberg, 442. K. Holmberg, K. Holmberg, A. Matthews, ings and Thin 1997. A. Matthews,
[ 191 [20]
[?I] [22] [23] [24] [25] [26] [27] [28] [29] [30]
A. Matthews,
Elsevier
Tribology
Series 28 ( 1994) [3l]
A. Matthews, Thin Solid Films 353 ( 1994) 173. Surf. Coat. Technol. 56 (1992) 1. in: Y. Pauleau, P.B. Berna (Eds.), Protective CoatFilms, Kluwer Academic Publishers. Dordrecht.
[32]
[33] A. Leyland.
B. Dorn.
P.R.
Stevenson.
M.
Bin-
Sudin, C. Rebholz, A.A. Voevodin. J. Schneider, J. Vat. Sci. Technol. A I3 (1995) 1303. H. Holleck. V. Schier, Surf. Coat. Technol. 7677 (1995) 328. H. Holleck. M. Lahres, P. Wall. Surf. Coat. Technol. 41 (1990) 179. W.D. Sproul. J. Vat. Sci. Technol. A I2 (1994) 1595. S.A. Barnett, M. Shinn, Annu. Rev. Mater. Sci. 24 (1994) 481. C. Donnet, J.-M. Martin, T. Le Mogne, M. Belin, Proc. Int. Tribology Conf.. Yokohama, 1995. 6. G.B. Hopple. S.H. Loewenthal, Surf. Coat. Technol. 6869 (1994) 398. A. Matthews. R. Artley, P. Holiday. P. Stevenson, The UK Engineering Coatings Industry in 2005, Hull University. Hull (1992). A. Leyland. K.S. Fancey. A. Matthews, Surf. Engng 7 ( 1991) 207. A. Leyland. D.B. Lewis. P.R. Stevenson A. Matthews, Surf. Coat. Technol. 62 ( 1993) 608. A. Matthews, A. Leyland, Surf. Coat. Technol. 71 (1995) 88. A. Leyland. M. Bin-Sudin, A.S. James. M.R. Kalantary. P.B. Wells, A. Matthews, J. Housden. B. Garside. Surf. Coat. Technol. 60 (1993) 474. M. Bin-Sudin. A. Leyland, A.S. James, A. Matthews, J. Housden, B. Car-side, Surf. Coat, Technol. 81 (1996) 215. A. Matthews. S.S. Eskildsen, Diamond Relat. Mater. 3 ( 1994) 901. H. Dimigen. C.P. Klages, Surf. Coat. Technol. 3 ( 1991) 543. M.N. Gardos, in: K.E. Spear, J.P. Dismukes (Eds.), Synthetic Diamond: Emerging CVD Science and Technology, John Wiley & Sons. New York. 1994, 410. A. Erdemir. C. Bindal. J. Pagan. P. Wilbur. Surf. Coat. Technol. 7677 (1995) 559. A. Erdemir. C. Bindal. G.R. Fenske, C. Zuikner, P. Wilbur, Surf. Coat. Technol. 86687 (1996) 692. T.P. Chang. H.S. Cheng, W.D. Sproul, Surf. Coat. Technol. 4344 (1990) 699. T.P. Chang, H.S. Cheng, W.A. Chiou, W.D. Sproul. Tribology Trans. 34 ( I99 I ) 408. A. Leyland. A. Matthews, Surf. Coat. Technol. 70 ( 1994) 19. B. Raehle, MSc Thesis. University of Hull. 1988. R. Bantle. A. Matthews, Surf. Coat. Technol. 7475 (1995) 857. W. Heinke. A. Leyland. A. Matthews. G. Berg, C. Friedrich. E. Brosreit. Thin Solid Films 270 (1995) 431. A.A. Voevodin, R. Bantle. A. Matthews, Wear I85 (1995) 151. C. Rebholz, J.M. Schneider. H. Ziegele, B. Rahle, A. Leyland. A. Matthews. Vacuum (submitted). H. Ronkainen, I. Nieminen, K. Holmberg. A. Leyland, A. Matthews. B. Matthes. E. Broszeit, Surf. Coat. Technol. 49 ( 1991) 468. W.-D. Mtinz, I.J. Smith, L.A. Donahue. A.P. Deeming, P. Halstead, Proceedings of the I st French and German Conference on High Speed Machining, Metz. France, 17-18 June 1997. A.A. Voevodin. J.M. Schneider, C. Rebholz. A. Matthews. Tribology Int. 29 ( 1996) 559.