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A systems rationale for the selection or design of tribological surface coatings
Thin Solid Filrns, 73 (1980) 235-244 © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands
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A SYSTEMS RATIONALE FOR THE SELECTION OR DESIGN OF TRIBOLOGICAL SURFACE COATINGS* ROBERT L. JOHNSON Rensselaer Polytechnic Institute, Troy, New York (U.S.A.) (Received April 2, 1980; accepted April 22, 1980)
Tribological surface coatings have been used to control wear or friction. There are many interacting processes including friction, wear, environment, thermal stresses and mechanical stresses as well as a variety of operating variables that should be part of the design logic. The number, complexity and level of the processes suggest that a systems approach could be helpful. A design logic checklist was prepared at a hierarchy level considered to be useful to both designers and researchers. A review of the processes indicated that composite design technology could be applied and that the coating methods of chemical vapor and vacuum sputter ion deposition were promising for designed coatings as they allow immobilization of the original surface defects and can provide a transitional region of graded interfaces that help to control the system's determinant processes (environment, thermal and mechanical) as well as the friction and wear. 1. INTRODUCTION
Solid surface coatings are used in the mitigation of many tribological problems which range from controlling friction and limiting wear to accommodating hostile environments and sustaining thermal and mechanical stresses. A large number of deposition, conversion and diffusion methods are used to form tribological coatings and each process has areas of preferred utilization. In many situations coatings have been used to resolve tribological problems not anticipated in the original design. Increasingly, however, the option for such coatings is being exercised by the original designer. The selection and design of tribological coatings can be very complex. There are ten or more operating variables and more than 30 interacting structure processes that merit consideration. It is clear that a morphological or systems approach is needed in the development of the logic for design with tribological surface coatings. The method of the morphological box I is suited for the derivation of solutions in such clear-cut cases as the interrelation of various physical phenomena. The introduction of the systems concept to tribology was advanced by G. Soloman and H. Czichost. Czichos 2 has used systems concepts in his treatment of the variables in * Paper presented at the International Conference on Metallurgical Coatings, San Diego, California, U.S.A., April 21-25, 1980. t In meetings of the International Research Group on Wear of Engineering Materials, sponsored by the Organization for Economic Cooperation and Development, Paris.
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the adhesion and abrasion processes in wear-resistant coatings. Adhesion and abrasion wear are among a total of 30 or more processes that are important. In addition, there can be a hierarchy of subprocesses that may require consideration. In this paper we shall discuss the important operating variables for tribological coatings with respect to the coating function processes. It is anticipated that the proposed logic will minimize the failures of coatings to perform complex tribological functions. The logic presented is intended to be on a hierarchy level common to both tribology designers and researchers. Therefore the present objective is to stimulate the use of systems concepts in design to prevent oversight of the complex functions for tribology coatings. In pursuing such objectives a designer should utilize the background developed by Czichos 2. 2.
MORPHOLOGICAL OR SYSTEMS APPROACH
The basic principles advanced by Zwicky I for the morphological approach to problems solution can be paraphrased as follows. (1) A perspective over large fields of material objects and phenomena is visualized. (2) A framework or morphological box of qualitative knowledge is established. (3) A quantitative study and analysis using dimensionless and dimensional morphology for all relevant factors and variables are carried out. (4) Individual relevant solutions to achieve desired processes with operating variables from complex inputs are selected. (5) The relevant solutions from (4) are ordered according to desired processes to provide a systematic realization of achievable performance considering all implications. An ordering or structure of systems has been suggested by Boulding3 using the following rules. (a) The systems are classified by level of complexity. (b) All logic and empiricism valid for less complex systems are also valid for highly complex systems. (c) Highly complex systems have more unknown elements and undiscovered physical laws which are responsible for the working of the system. Czichos 2 has determined the structure of a variety of tribological systems and considered the alternative wear processes from a substantial spectrum of published theory and experiment. For wear-resistant coatings, Czichos (ref. 2, p. 207) suggests two basic aspects in the choice of suitable coatings: (1) the technical functions of the system and the operating conditions of load, velocity and temperature and (2) the type of wear mechanism expected to occur. He suggests the following rules for the design of a tribological wear-resistant coating: (a) a thin surface layer to prevent adhesion; (b) an intermediate layer with ductility to support the hertzian content stresses; (c) a subsurface material offering a transition to the properties of the bulk material. These guidelines for wear-resistant coatings offer good logic that is substantiated by successful practice and research. The total requirements for tribological coatings include several functions other than wear resistance. Friction reduction may be the primary function of most coatings in engineering practice. The operating environment for a tribological
SYSTEMS RATIONALE FOR TRIBOLOGICAL SURFACE COATINGS
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coating may impose special conditions with regard to the interactions, function and stability of the coating. Thermal control and stress interaction problems impact other functions but they have a distinct role in all work processes, especially in energy-intensive machines that can be improved by optimized tribological coatings. Similarly, as indicated by Czichos 2 and as mentioned above, mechanical stress interactions can be optimized by varying the properties of the coating from the surface to the bulk material. Thus it is clear that a morphological or systems approach to the complete design of tribological coatings can indeed require the visualization of perspectives over large fields of material and phenomenological variables and processes, as discussed by Zwicky (ref. 1, p. 285). Fortunately, such concepts are intended for complex systems and the referenced approaches can be extended to provide a series of logic matrices that may prevent errors of omission in the design of tribological coatings. As we go through the hierarchy of determinant processes in the systems structure, increasingly fundamental material concepts can be applied. Some examples will be mentioned. However, hierarchy level in this discussion is only intended to be sufficiently basic to prevent critical omissions in the design of tribological coatings. 3.
O P E R A T I N G VARIABLES FOR COATINGS
In Table I a series of operating variables for tribological surface coatings is listed. The functional level of the listed variables is hopefully such as to prevent omission in design considerations but not to cover all that is important at a more detailed level. For several of the variables listed, there are considerations that can increase the number of factors listed to the point of being intractable for a general guide. TABLE I OPERATING VARIABLES
Total load (force) Unit load (stressor pressure) Slidingor rollingvelocities Slidingdistanceor cycles Temperatureof bulk substrate Temperatureof surface Hardness(yieldstrengthin compression)of bulk Hardnessof surface Configuration(geometry,topography,textures) Chemistry(thermodynamicconditions) Examplesof parametricfactors Pressure-velocityP V Wear coefficientK
= WHI_ Wearvolumexhardness I Ld Wotalload x slidingdistancel
Total load and unit load are listed separately because they are examples of parametric design factors. Pressure-velocity (ref. 4, p. 96) and wear coefficient (ref. 5, pp. 267-335) are used effectively in engineering practice. The primary usefulness of
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the wear coefficient is in the region of adhesive wear and the pressure-velocity factor is indicative of a rate of energy release that establishes a limit for the "mild" adhesive wear anticipated for mechanical components including seals 6. Sliding or rolling velocities are important to the limitations induced by thermal control as well as to rheological phenomena at the interface. A variety of observations suggest that rheology (viscoelasticity in particular) may play an important role in the tribological behavior of solids as well as in nominally liquid lubricants 7. Sliding distance and stress cycles are typical parameters that may dictate the design life of a mechanical component. Although equally relevant to other wear mechanisms, the parametric wear coefficient for adhesive wear includes sliding distance. Because the strength, rheology, thermal control, hardness and chemistry are temperature dependent, the temperatures of both the surface and the bulk materials are important in tribology. Many failures can be attributed to the thermal mismatch of the surface and bulk materials. A comprehensive treatise on surface mechanics including thermal transients has been provided by Ling 8 and is relevant to tribological coatings. Hardness is, of course, a mechanical strength (compressive yield strength) although many "hardness" measuring methods do not properly provide such data. Etastoplastic indentation occurs in concentrated contacts and the stress transfer from a coating to the substrate is critical to the mechanical integrity of the coating 9. There is no question that methods which provide a coating in which there is a gradual transition in both hardness and other physical properties from bulk to surface can be effective in tribology. As Czichos (ref. 2, p. 209) suggests, however, there can be advantages to a very thin outer layer on an intermediate substrate layer with the desired mechanical and other properties graduating from the bulk material to the substrate layer. The variable of configuration includes a number of geometric considerations. The first is the shape of the tribological mechanical components at the contact; the mating surfaces may be counterformed (as with two spheres or a sphere on a fiat in loaded contact), conformed (as with a sphere or ball in a conforming socket) or planar (two nominally fiat surfaces in contact). The second is the topographic macrofeatures such as waviness which occur by either design or accident in the manufacturing process or result from thermal and mechanical distortions in the operating system. For plane surfaces, such as the primary elements in a face seal, topographical features ~an determine performance. The third is the microtopography or roughness of the surfaces which results from the manufacturing method of finishing. The lay of the microroughness features and other factors of geometric texture such as a "matte" finish can be especially important for coatings. To carry the hierarchy concept further, it is clear that metallurgical texture (crystal orientation) is important and that the fine details of tribology involve atomistic mechanics of materials (ref. 5, pp. 181-265). A further variable is the change in configuration that results from the operating processes, e.g. changes in form and topography from wear processes. The chemistry of the system is a variable that is widely relevant to tribological processes. Again, many hierarchy levels can be developed. The dominant chemical
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reactions at the surface are the formation of oxide films on metals or the reaction of lubricating films with the operating environment. In this it is axiomatic that the oxygen in air is the most powerful contributor or addition agent in lubricating processes. Chemical thermodynamics and kinetics are powerful tools in determining the useful environments and methods of application for tribological coatings 7. The mechanical activation of chemical processes has been demonstrated in metal cutting tribology and chemical wear can be a dominant process, as has been documented for mechanical carbons 4. Advances towards more energy-efficient machines suggest a general trend towards higher operating temperatures, higher stress loading and higher speeds for tribological components; thus the chemical variable will be of increasing importance. 4.
SYSTEM PROCESSES
Friction control is subject to the processes listed in Table II and is a primary objective in the use of surface coatings. There is a whole industry which provides the service of applying solid film lubricants. Films have a finite life but need no maintenance and do provide controlled (usually low) friction. For example, 1000 parts on a modern automobile may have coatings with molybdenum disulfide or graphite as lubricating pigments. Friction has a direct impact on energy conservation in all levels of our society and its productivity. TABLE II
TABLE III
DESIGN FOR FRICTION CONTROL
DESIGN FOR WEAR CONTROL
Rheology(viscoelasticand shear) Adhesion at surface(atomistic) Adhesion of coating (mechanical, chemical,metallurgical) Film thickness Area of shear Junction growth Surficialdeformation Substrate deformation Surfaceinteractions Topography
Adhesion (wearcoefficient) Abrasion (particulates) Corrosion(chemical) Impact Erosion Fatigue Plasticity Film thickness(coatingadhesion,wear life) Discontinuities(stresscracks, roughnessetc.) Incipient weardominatesmitigation,as in "fretting" which starts with adhesion and proceeds through abrasion, corrosion to fatigue. The final failure is often by fatigue.
Rheology is the study of the deformation or flow of matter in terms of stress, strain, temperature and time 1°. In achieving low friction the rheology sought is low shear strength which responds to anisotropic factors in crystal structure internal bonding forces as in molybdenum disulfide 11. Contaminants, melting points, heat of fusion, hardness and other properties have been shown to relate to the shear properties of solid film materials 7. Clearly an extensive systems hierarchy for achieving the desired (high or low) shear strength can be developed. Adhesion at the surface is common to all tribological contacts. Material transfer from one surface to another should be expected with the lower shear strength material being predominantly transferred. Thus, in real systems, shearing
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occurs between or adjacent to the transfer film and its parent body. If shearing occurs at the interface, wear and other forms of degradation will be minimized. Adhesion of the coating to the base material is critical to its function. Mechanical, chemical and metallurgical factors may contribute to such adhesion. For a film to be retained and to perform its function, its adhesion to the base material must tolerate mechanical stresses and elastoplastic distortions, thermal stress and environment or process fluid displacement. For minimum friction, lubricating films must be very thin (i.e. 10 -4 cm indium on steel) (ref. 12, p. 115) to minimize film-to-base adhesion problems and to achieve minimum real shear area as determined by the deformation of the base material. The area of shear can be controlled by the base material properties if the film is very thin 12. If the film deformation fills the nominal voids or interstices between loadsupporting asperities, elastic and plastic deformation allows growth of the shearing function. Transferred film material and strain processes can promote junction growth that increases friction. For soft metals and their films especially, the terms "prow formation" and "frontal bulge" have been applied to the build-up of the real shearing area 13. If the adhering prow formation is work hardened or otherwise transformed in a manner to allow its penetration into the mating surface, a two-body abrasion mechanism with plowing action may increase friction. Similarly, if the topographies of the mating surfaces can interact, Coulomb or roughness friction becomes important. The surface interactions may be physical (i.e. roughness or other geometry), chemical or metallurgical. The primary determinant processes for friction control are associated with the rheology of the coating surface. As discussed and listed in Table II, there are many interrelated processes that also merit consideration. Wear control is subject to the process mechanisms and properties listed in Table III. Wear is a primary determinant in life capabilities and maintenance requirements for production, transport and service machines. Thus it influences the productivity and cost effectiveness of machines as well as energy conservation. Adhesion wear mechanisms are relevant to adhesion friction under nominal or mild wear conditions. Shear occurs at or adjacent to the interface and the Archard wear coefficient 5 and other design parameters such as pressure-velocity numbers 6 can be used. Abrasive wear occurs with hard particles plowing and plastically displacing or shearing grooves in mating surfaces. Free non-deformable particles arising from wear, from chemical reaction or from contamination serve as cutting edges. Corrosion or chemical wear is caused by the reactions responsible for the removal of surface material and is a function of the reactivity of the surface with the reactant (air, lubricant additives, process fluid or contaminants), the local stresses and the temperatures. The corrosive wear product formed in air may be volatile (e.g. CO 2 for mechanical carbon) or abrasive (e.g. Fe203 for steel). Thus the interaction of various wear processes occurs. Impact wear and erosion can be similar processes in tribological coatings. Impacts by a mating surface or by a free particle (erosion) cause elastoplastic deformation. Impact craters which may be formed by plastic flow at the edge produce a bulge next to the indentation 9. In effect, the indentation and the bulge will
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have a greater surface area than the original surface. If slip occurs at the coatingbase material interface, the bond may be ruptured and the coating loosened. Fatigue wear theories usually involve discontinuities in the structure. Accumulation of inclusions or cracks gives rise to fatigue pitting or the fracture of tribological components. Coating procedures may serve to remove or immobilize defects and also to prevent dislocation concentrations (ref. 2, p. 207). Ion-plated coatings increase bending fatigue 14 endurance and also are believed to have good potential for tribological fatigue problems. The environment control for tribological coatings will determine the functional performance. Each environment listed in Table IV provides special considerations. Air is the usual environment and its effects are compensated for if not understood. Vacuum is unique for most tribological mechanical components in that the usual oxide surface films and oxygen interaction will not occur 15. In addition, some coating materials may be volatile. Other materials such as graphite that need interlaminar adsorbates to function as lubricants cannot be used to reduce friction. If convective heat is not removed from the vacuum, this dramatically compounds the thermal control problem in mechanical components. Moisture is a typical adsorbate that makes graphite lubricate 15; moisture also contributes to many other aspects of lubrication chemistry. Increasingly, tribological mechanical components use process fluids for lubrication although seldom do they perform real lubrication functions. Contamination is usually abrasive dirt but may be water or other reactive materials. Perhaps thin soft coatings capable of holding abrasives should be avoided or the coatings must be used in such a manner as to immobilize the abrasive action of particulates. The thermal control in mechanical components requires design interactions8 to minimize the problems and to enhance the tribological properties of materials (Table V). Some coating materials have their best performance in an elevated temperature range 16. Transient temperatures are a part of tribology technology with asperity contacts giving extreme flash temperatures and thermal elastic nodes establishing shifting wear areas 17. Concentrated frictional energy release in tribology develops thermal stresses in surface coatings and, in combination with mechanical stresses, causes extensive surface cracking that may initiate spalling and other failure modes. For some coating metals and polymers, even modest limits for operating variables result in achieving the melting points at the surface; thus liquid viscous shear may give low friction but the melting may often lead to mechanical failures. Solid transformations, including recrystallization and texturing, occur readily in sliding contacts ~5. It is not usual to design coatings to accommodate the complex thermal interactions. Parametric use of thermal conductivity, thermal TABLE IV
TABLE V
DESIGN FOR ENVIRONMENT INTERACTION
DESIGN FOR THERMAL INTERACTIONS
Vacuum Air Moisture Process fluid Liquid lubricant Contamination
Extreme temperatures Transient temperatures Melting points Transformations Conductivity Expansion
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expansion and deformation processes with reference to the base material can improve thermal control. In the selection of mechanical interactions of coatings most designers make use of hardness values to the exclusion of the other similarly important material properties listed in Table VI. Coating failures are often complex stress fractures and fracture mechanics is being applied to wear processes. Composite systems are necessary to satisfy the complex interactions in tribological coatings; by the most simple considerations, however, the use of a coating on a bulk solid is a composite. There are many options in the mechanics of composite materials~ 8 that are available to coatings designers for the control of mechanical and thermal interactions. The mechanical requirements vary for different tribological mechanical components (e.g. rolling element bearing and face seals) where coatings are important. Failure mechanisms are commonly associated with discontinuities (such as surface cracks imposed by material forming and finishing), stress raisers (such as hard particles and weakening voids) and surface changes by tribological processes (stress cracks, work hardening, recrystallization etc.). Similarly, damping can influence transient stressrelated failure mechanisms and is worthy of consideration. TABLE VI DESIGN FOR MECHANICALINTERACTION Compressive yield stress Modulus of elasticity Poisson's ratio Fatigue Creep Impact Discontinuities Damping
5. DESIGN LOGIC
It has been indicated that one determinant function usually leads to the selection of tribological coatings. The preceding discussion indicates the multiplicity and complexity of interacting processes and variables that are inherent in most coating applications. The systems approach advocated here is intended to make designers of coatings consider in a more comprehensive manner the processes that may be important. From this discussion and consideration of the cited references a checklist has been assembled and is presented as Table VII. In general terms, very thin easily sheared films that may be a metal, a metal compound or a polymer can provide low friction. Such films can be part of a surface composite system that allows a transition of thermal, mechanical and chemical properties from the surface to the base material; sputter-ion-plated 19 and chemically vapor-deposited coatings 2° are promising. The base material may require modification by etching21, diffusion or ion implanting to accommodate essential thermal, mechanical and chemical processes. Chemical conversion treatments provide property transitions and surface geometry that aids adhesion of the outer coating material. It is in providing the transition structure of the coating that some of the less prominent processes (environment, thermal and mechanical)
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TABLE VII DESIGN LOGIC FOR TRIBOLOGICAL COATINGS: CHECKLIST
nucTi~
m~olo~ Adhesic~of coatin~ Film th/ckness Area of shear
Jtmcticn ~rowth Cn surface substrate Surface interactions ~lhesicn Abrasion Corrosion
~t Erosion
~ati~ue Plasticit~ Film Thickness Discontinuities ~/IRC~MENr
Vacuum Air Moisture Process fluid Liquid lubricant Contamination
TH~RV~L
Ta~oerature extremes Temperature transients m e l t ~ points Transformations
Conductivity Expansi~ ~CHANICA~
CU~. yield stress Elastic modulus Poisscn, s ratio
~a_ti~e iimi~ cr~ 9 ~ct Disc~tinuities m,ging Variables may have strong (s), medium (m) or weak (w) influence on the determinant processes.
can be accommodated. Using vacuum processes such'as sputter etching 22 and the various coating methods 14' 19, it may also be .possible to remove, or immobilize surface discontinuities and defects that can influence wear and mechanical failures. 6.
CONCLUDING REMARKS
A systems approach is suggested for the design of surface coatings. There are many interacting processes, including friction, wear, environment, thermal stresses and mechanical stresses, to be considered. The operating variables are numerous and influence the determinant processes. A design logic checklist is provided at a systems hierarchy level that can be used by a designer and further developed by researchers.
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R.L. JOHNSON
General consideration of the logic suggests that composite materials t e c h n o l o g y s h o u l d be a c o a t i n g d e s i g n e r ' s tool. F u r t h e r m o r e , c h e m i c a l v a p o r a n d v a c u u m d e p o s i t i o n m e t h o d s h a v e special p o t e n t i a l for surface c o a t i n g s ; t h e y c a n r e d u c e o r i g i n a l surface defects a n d a l l o w a t r a n s i t i o n in c o m p o s i t i o n t h a t s h o u l d m i n i m i z e failures by p r o c e s s e s n o t c o n s i d e r e d in the o r i g i n a l design. ACKNOWLEDGMENT T h i s d e s i g n r a t i o n a l e was d e v e l o p e d w i t h the s u p p o r t of the U.S. A r m y R e s e a r c h Office in their p r o g r a m of F u n d a m e n t a l R e s e a r c h in T r i b o l o g y at R e n s s e l a e r P o l y t e c h n i c Institute. REFERENCES
1 F. Zwicky, Morphological Astronomy, Springer, Berlin, 1957. 2 H. Czichos, Tribology: A Systems Approach to the Science and Technology of Friction, Lubrication and Wear, Elsevier, New York, 1978. 3 K.E. Boulding, General systems theory the skeleton of science, Manage. Sci., 2 (1956) 197. 4 R.R. Paxton, Manufactured Carbon: A Self Lubricating Material for Mechanical Devices, CRC Press, Cleveland, Ohio, 1979. 5 P.M. Ku (ed.), Interdisciplinary approach to friction and wear, NASA Spec. Publ. SP-181, 1968. 6 K. Schoenherr and R. L. Johnson, Seal Wear, Wear ControlHandbook, Vol. II, American Society of Mechanical Engineers, New York, 1980, to be published. 7 R.L. Johnson and H. E. Sliney, Fundamental considerations for future solid lubricants, NASA Tech. Memo. TMX-52659, 1969 ; Proc. Air Force Materials Laboratory-Midwest Research Institute Conf. on Solid Lubricants, in Tech. Rep. AFML-TR-70-127. 1970, p. 40, Air Force Materials Laboratory. 8 F.F. Ling, Surface Mechanics, Wiley-Interscience, New York, 1973. 9 F.E. Kennedy and F. F. Ling, Elasto-plastic indentation of a layered medium, Trans. A S M E (April 1974)97 103. 10 G.W. Rowe et al., Friction, Wear, and Lubrication-- Tribology Glossary of Terms and Definitions, International Research Group on Wear of Engineering Materials, Organization for Economic Cooperation and Development, Paris, 1969. 11 E.R. Braithwaite, Lubrication and Lubricants, Elsevier, Amsterdam, 1967. 12 F.P. Bowden and D. Tabor, The Friction and Lubrication of Solids, Oxford University Press, London, 1958. 13 M. Antler, The wear of electrodeposited gold, A S L E Trans., 11 (3) (1968) 248-260. 14 T. Spalvins, Survey of ion plating sources, Trans. 26th Natl. Vacuum Symp., American Vacuum Society, New York, October 2-5, 1979; NASA Tech. Memo. TM-79269. 15 D.H. Buckley, Friction, wear, and lubrication in vacuum, NASA Spec. Publ. SP-277, 1971. | 6 H.E. Sliney, Plasma-sprayed metal-glass and metal-glass fluoride coatings for lubrication to 900 °C, A SLE Trans., 17 (3) 182-189. H. E. Sliney and J. W. Graham, Tribological properties of self-lubricating fluorid~metal composites to 900 °C a review and some new developments, A S L E Paper 74-LUB-2, 1974. 17 J.P. Netzel, Observations of thermoelastic instability in mechanical face seals, Wear, 59 (1980) 135-148. 18 R.M. Christensen, Mechanics of Composite Materials, Wiley-Interscience, New York, 1979. 19 T. Spalvins, Coatings for wear and lubrication, Thin Solid Films, 53 (1978) 285-300. 20 H.E. Hintermann, Chemical vapor deposition applied in tribology, Wear, 47 (1978) 407415. 21 B. Bhushan, Surface pretreatment of thin Inconel X-750 foils for improved coating adherence, Thin Solid Films, 53 (1978) 99-107. 22 R.S. Berg and G. J. Kominiak, Surface texturing by sputter etching, J. Vac. Sci. Technol., 13 (1) (1976) 403405.
235
A SYSTEMS RATIONALE FOR THE SELECTION OR DESIGN OF TRIBOLOGICAL SURFACE COATINGS* ROBERT L. JOHNSON Rensselaer Polytechnic Institute, Troy, New York (U.S.A.) (Received April 2, 1980; accepted April 22, 1980)
Tribological surface coatings have been used to control wear or friction. There are many interacting processes including friction, wear, environment, thermal stresses and mechanical stresses as well as a variety of operating variables that should be part of the design logic. The number, complexity and level of the processes suggest that a systems approach could be helpful. A design logic checklist was prepared at a hierarchy level considered to be useful to both designers and researchers. A review of the processes indicated that composite design technology could be applied and that the coating methods of chemical vapor and vacuum sputter ion deposition were promising for designed coatings as they allow immobilization of the original surface defects and can provide a transitional region of graded interfaces that help to control the system's determinant processes (environment, thermal and mechanical) as well as the friction and wear. 1. INTRODUCTION
Solid surface coatings are used in the mitigation of many tribological problems which range from controlling friction and limiting wear to accommodating hostile environments and sustaining thermal and mechanical stresses. A large number of deposition, conversion and diffusion methods are used to form tribological coatings and each process has areas of preferred utilization. In many situations coatings have been used to resolve tribological problems not anticipated in the original design. Increasingly, however, the option for such coatings is being exercised by the original designer. The selection and design of tribological coatings can be very complex. There are ten or more operating variables and more than 30 interacting structure processes that merit consideration. It is clear that a morphological or systems approach is needed in the development of the logic for design with tribological surface coatings. The method of the morphological box I is suited for the derivation of solutions in such clear-cut cases as the interrelation of various physical phenomena. The introduction of the systems concept to tribology was advanced by G. Soloman and H. Czichost. Czichos 2 has used systems concepts in his treatment of the variables in * Paper presented at the International Conference on Metallurgical Coatings, San Diego, California, U.S.A., April 21-25, 1980. t In meetings of the International Research Group on Wear of Engineering Materials, sponsored by the Organization for Economic Cooperation and Development, Paris.
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R . L . JOHNSON
the adhesion and abrasion processes in wear-resistant coatings. Adhesion and abrasion wear are among a total of 30 or more processes that are important. In addition, there can be a hierarchy of subprocesses that may require consideration. In this paper we shall discuss the important operating variables for tribological coatings with respect to the coating function processes. It is anticipated that the proposed logic will minimize the failures of coatings to perform complex tribological functions. The logic presented is intended to be on a hierarchy level common to both tribology designers and researchers. Therefore the present objective is to stimulate the use of systems concepts in design to prevent oversight of the complex functions for tribology coatings. In pursuing such objectives a designer should utilize the background developed by Czichos 2. 2.
MORPHOLOGICAL OR SYSTEMS APPROACH
The basic principles advanced by Zwicky I for the morphological approach to problems solution can be paraphrased as follows. (1) A perspective over large fields of material objects and phenomena is visualized. (2) A framework or morphological box of qualitative knowledge is established. (3) A quantitative study and analysis using dimensionless and dimensional morphology for all relevant factors and variables are carried out. (4) Individual relevant solutions to achieve desired processes with operating variables from complex inputs are selected. (5) The relevant solutions from (4) are ordered according to desired processes to provide a systematic realization of achievable performance considering all implications. An ordering or structure of systems has been suggested by Boulding3 using the following rules. (a) The systems are classified by level of complexity. (b) All logic and empiricism valid for less complex systems are also valid for highly complex systems. (c) Highly complex systems have more unknown elements and undiscovered physical laws which are responsible for the working of the system. Czichos 2 has determined the structure of a variety of tribological systems and considered the alternative wear processes from a substantial spectrum of published theory and experiment. For wear-resistant coatings, Czichos (ref. 2, p. 207) suggests two basic aspects in the choice of suitable coatings: (1) the technical functions of the system and the operating conditions of load, velocity and temperature and (2) the type of wear mechanism expected to occur. He suggests the following rules for the design of a tribological wear-resistant coating: (a) a thin surface layer to prevent adhesion; (b) an intermediate layer with ductility to support the hertzian content stresses; (c) a subsurface material offering a transition to the properties of the bulk material. These guidelines for wear-resistant coatings offer good logic that is substantiated by successful practice and research. The total requirements for tribological coatings include several functions other than wear resistance. Friction reduction may be the primary function of most coatings in engineering practice. The operating environment for a tribological
SYSTEMS RATIONALE FOR TRIBOLOGICAL SURFACE COATINGS
237
coating may impose special conditions with regard to the interactions, function and stability of the coating. Thermal control and stress interaction problems impact other functions but they have a distinct role in all work processes, especially in energy-intensive machines that can be improved by optimized tribological coatings. Similarly, as indicated by Czichos 2 and as mentioned above, mechanical stress interactions can be optimized by varying the properties of the coating from the surface to the bulk material. Thus it is clear that a morphological or systems approach to the complete design of tribological coatings can indeed require the visualization of perspectives over large fields of material and phenomenological variables and processes, as discussed by Zwicky (ref. 1, p. 285). Fortunately, such concepts are intended for complex systems and the referenced approaches can be extended to provide a series of logic matrices that may prevent errors of omission in the design of tribological coatings. As we go through the hierarchy of determinant processes in the systems structure, increasingly fundamental material concepts can be applied. Some examples will be mentioned. However, hierarchy level in this discussion is only intended to be sufficiently basic to prevent critical omissions in the design of tribological coatings. 3.
O P E R A T I N G VARIABLES FOR COATINGS
In Table I a series of operating variables for tribological surface coatings is listed. The functional level of the listed variables is hopefully such as to prevent omission in design considerations but not to cover all that is important at a more detailed level. For several of the variables listed, there are considerations that can increase the number of factors listed to the point of being intractable for a general guide. TABLE I OPERATING VARIABLES
Total load (force) Unit load (stressor pressure) Slidingor rollingvelocities Slidingdistanceor cycles Temperatureof bulk substrate Temperatureof surface Hardness(yieldstrengthin compression)of bulk Hardnessof surface Configuration(geometry,topography,textures) Chemistry(thermodynamicconditions) Examplesof parametricfactors Pressure-velocityP V Wear coefficientK
= WHI_ Wearvolumexhardness I Ld Wotalload x slidingdistancel
Total load and unit load are listed separately because they are examples of parametric design factors. Pressure-velocity (ref. 4, p. 96) and wear coefficient (ref. 5, pp. 267-335) are used effectively in engineering practice. The primary usefulness of
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the wear coefficient is in the region of adhesive wear and the pressure-velocity factor is indicative of a rate of energy release that establishes a limit for the "mild" adhesive wear anticipated for mechanical components including seals 6. Sliding or rolling velocities are important to the limitations induced by thermal control as well as to rheological phenomena at the interface. A variety of observations suggest that rheology (viscoelasticity in particular) may play an important role in the tribological behavior of solids as well as in nominally liquid lubricants 7. Sliding distance and stress cycles are typical parameters that may dictate the design life of a mechanical component. Although equally relevant to other wear mechanisms, the parametric wear coefficient for adhesive wear includes sliding distance. Because the strength, rheology, thermal control, hardness and chemistry are temperature dependent, the temperatures of both the surface and the bulk materials are important in tribology. Many failures can be attributed to the thermal mismatch of the surface and bulk materials. A comprehensive treatise on surface mechanics including thermal transients has been provided by Ling 8 and is relevant to tribological coatings. Hardness is, of course, a mechanical strength (compressive yield strength) although many "hardness" measuring methods do not properly provide such data. Etastoplastic indentation occurs in concentrated contacts and the stress transfer from a coating to the substrate is critical to the mechanical integrity of the coating 9. There is no question that methods which provide a coating in which there is a gradual transition in both hardness and other physical properties from bulk to surface can be effective in tribology. As Czichos (ref. 2, p. 209) suggests, however, there can be advantages to a very thin outer layer on an intermediate substrate layer with the desired mechanical and other properties graduating from the bulk material to the substrate layer. The variable of configuration includes a number of geometric considerations. The first is the shape of the tribological mechanical components at the contact; the mating surfaces may be counterformed (as with two spheres or a sphere on a fiat in loaded contact), conformed (as with a sphere or ball in a conforming socket) or planar (two nominally fiat surfaces in contact). The second is the topographic macrofeatures such as waviness which occur by either design or accident in the manufacturing process or result from thermal and mechanical distortions in the operating system. For plane surfaces, such as the primary elements in a face seal, topographical features ~an determine performance. The third is the microtopography or roughness of the surfaces which results from the manufacturing method of finishing. The lay of the microroughness features and other factors of geometric texture such as a "matte" finish can be especially important for coatings. To carry the hierarchy concept further, it is clear that metallurgical texture (crystal orientation) is important and that the fine details of tribology involve atomistic mechanics of materials (ref. 5, pp. 181-265). A further variable is the change in configuration that results from the operating processes, e.g. changes in form and topography from wear processes. The chemistry of the system is a variable that is widely relevant to tribological processes. Again, many hierarchy levels can be developed. The dominant chemical
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reactions at the surface are the formation of oxide films on metals or the reaction of lubricating films with the operating environment. In this it is axiomatic that the oxygen in air is the most powerful contributor or addition agent in lubricating processes. Chemical thermodynamics and kinetics are powerful tools in determining the useful environments and methods of application for tribological coatings 7. The mechanical activation of chemical processes has been demonstrated in metal cutting tribology and chemical wear can be a dominant process, as has been documented for mechanical carbons 4. Advances towards more energy-efficient machines suggest a general trend towards higher operating temperatures, higher stress loading and higher speeds for tribological components; thus the chemical variable will be of increasing importance. 4.
SYSTEM PROCESSES
Friction control is subject to the processes listed in Table II and is a primary objective in the use of surface coatings. There is a whole industry which provides the service of applying solid film lubricants. Films have a finite life but need no maintenance and do provide controlled (usually low) friction. For example, 1000 parts on a modern automobile may have coatings with molybdenum disulfide or graphite as lubricating pigments. Friction has a direct impact on energy conservation in all levels of our society and its productivity. TABLE II
TABLE III
DESIGN FOR FRICTION CONTROL
DESIGN FOR WEAR CONTROL
Rheology(viscoelasticand shear) Adhesion at surface(atomistic) Adhesion of coating (mechanical, chemical,metallurgical) Film thickness Area of shear Junction growth Surficialdeformation Substrate deformation Surfaceinteractions Topography
Adhesion (wearcoefficient) Abrasion (particulates) Corrosion(chemical) Impact Erosion Fatigue Plasticity Film thickness(coatingadhesion,wear life) Discontinuities(stresscracks, roughnessetc.) Incipient weardominatesmitigation,as in "fretting" which starts with adhesion and proceeds through abrasion, corrosion to fatigue. The final failure is often by fatigue.
Rheology is the study of the deformation or flow of matter in terms of stress, strain, temperature and time 1°. In achieving low friction the rheology sought is low shear strength which responds to anisotropic factors in crystal structure internal bonding forces as in molybdenum disulfide 11. Contaminants, melting points, heat of fusion, hardness and other properties have been shown to relate to the shear properties of solid film materials 7. Clearly an extensive systems hierarchy for achieving the desired (high or low) shear strength can be developed. Adhesion at the surface is common to all tribological contacts. Material transfer from one surface to another should be expected with the lower shear strength material being predominantly transferred. Thus, in real systems, shearing
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occurs between or adjacent to the transfer film and its parent body. If shearing occurs at the interface, wear and other forms of degradation will be minimized. Adhesion of the coating to the base material is critical to its function. Mechanical, chemical and metallurgical factors may contribute to such adhesion. For a film to be retained and to perform its function, its adhesion to the base material must tolerate mechanical stresses and elastoplastic distortions, thermal stress and environment or process fluid displacement. For minimum friction, lubricating films must be very thin (i.e. 10 -4 cm indium on steel) (ref. 12, p. 115) to minimize film-to-base adhesion problems and to achieve minimum real shear area as determined by the deformation of the base material. The area of shear can be controlled by the base material properties if the film is very thin 12. If the film deformation fills the nominal voids or interstices between loadsupporting asperities, elastic and plastic deformation allows growth of the shearing function. Transferred film material and strain processes can promote junction growth that increases friction. For soft metals and their films especially, the terms "prow formation" and "frontal bulge" have been applied to the build-up of the real shearing area 13. If the adhering prow formation is work hardened or otherwise transformed in a manner to allow its penetration into the mating surface, a two-body abrasion mechanism with plowing action may increase friction. Similarly, if the topographies of the mating surfaces can interact, Coulomb or roughness friction becomes important. The surface interactions may be physical (i.e. roughness or other geometry), chemical or metallurgical. The primary determinant processes for friction control are associated with the rheology of the coating surface. As discussed and listed in Table II, there are many interrelated processes that also merit consideration. Wear control is subject to the process mechanisms and properties listed in Table III. Wear is a primary determinant in life capabilities and maintenance requirements for production, transport and service machines. Thus it influences the productivity and cost effectiveness of machines as well as energy conservation. Adhesion wear mechanisms are relevant to adhesion friction under nominal or mild wear conditions. Shear occurs at or adjacent to the interface and the Archard wear coefficient 5 and other design parameters such as pressure-velocity numbers 6 can be used. Abrasive wear occurs with hard particles plowing and plastically displacing or shearing grooves in mating surfaces. Free non-deformable particles arising from wear, from chemical reaction or from contamination serve as cutting edges. Corrosion or chemical wear is caused by the reactions responsible for the removal of surface material and is a function of the reactivity of the surface with the reactant (air, lubricant additives, process fluid or contaminants), the local stresses and the temperatures. The corrosive wear product formed in air may be volatile (e.g. CO 2 for mechanical carbon) or abrasive (e.g. Fe203 for steel). Thus the interaction of various wear processes occurs. Impact wear and erosion can be similar processes in tribological coatings. Impacts by a mating surface or by a free particle (erosion) cause elastoplastic deformation. Impact craters which may be formed by plastic flow at the edge produce a bulge next to the indentation 9. In effect, the indentation and the bulge will
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241
have a greater surface area than the original surface. If slip occurs at the coatingbase material interface, the bond may be ruptured and the coating loosened. Fatigue wear theories usually involve discontinuities in the structure. Accumulation of inclusions or cracks gives rise to fatigue pitting or the fracture of tribological components. Coating procedures may serve to remove or immobilize defects and also to prevent dislocation concentrations (ref. 2, p. 207). Ion-plated coatings increase bending fatigue 14 endurance and also are believed to have good potential for tribological fatigue problems. The environment control for tribological coatings will determine the functional performance. Each environment listed in Table IV provides special considerations. Air is the usual environment and its effects are compensated for if not understood. Vacuum is unique for most tribological mechanical components in that the usual oxide surface films and oxygen interaction will not occur 15. In addition, some coating materials may be volatile. Other materials such as graphite that need interlaminar adsorbates to function as lubricants cannot be used to reduce friction. If convective heat is not removed from the vacuum, this dramatically compounds the thermal control problem in mechanical components. Moisture is a typical adsorbate that makes graphite lubricate 15; moisture also contributes to many other aspects of lubrication chemistry. Increasingly, tribological mechanical components use process fluids for lubrication although seldom do they perform real lubrication functions. Contamination is usually abrasive dirt but may be water or other reactive materials. Perhaps thin soft coatings capable of holding abrasives should be avoided or the coatings must be used in such a manner as to immobilize the abrasive action of particulates. The thermal control in mechanical components requires design interactions8 to minimize the problems and to enhance the tribological properties of materials (Table V). Some coating materials have their best performance in an elevated temperature range 16. Transient temperatures are a part of tribology technology with asperity contacts giving extreme flash temperatures and thermal elastic nodes establishing shifting wear areas 17. Concentrated frictional energy release in tribology develops thermal stresses in surface coatings and, in combination with mechanical stresses, causes extensive surface cracking that may initiate spalling and other failure modes. For some coating metals and polymers, even modest limits for operating variables result in achieving the melting points at the surface; thus liquid viscous shear may give low friction but the melting may often lead to mechanical failures. Solid transformations, including recrystallization and texturing, occur readily in sliding contacts ~5. It is not usual to design coatings to accommodate the complex thermal interactions. Parametric use of thermal conductivity, thermal TABLE IV
TABLE V
DESIGN FOR ENVIRONMENT INTERACTION
DESIGN FOR THERMAL INTERACTIONS
Vacuum Air Moisture Process fluid Liquid lubricant Contamination
Extreme temperatures Transient temperatures Melting points Transformations Conductivity Expansion
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expansion and deformation processes with reference to the base material can improve thermal control. In the selection of mechanical interactions of coatings most designers make use of hardness values to the exclusion of the other similarly important material properties listed in Table VI. Coating failures are often complex stress fractures and fracture mechanics is being applied to wear processes. Composite systems are necessary to satisfy the complex interactions in tribological coatings; by the most simple considerations, however, the use of a coating on a bulk solid is a composite. There are many options in the mechanics of composite materials~ 8 that are available to coatings designers for the control of mechanical and thermal interactions. The mechanical requirements vary for different tribological mechanical components (e.g. rolling element bearing and face seals) where coatings are important. Failure mechanisms are commonly associated with discontinuities (such as surface cracks imposed by material forming and finishing), stress raisers (such as hard particles and weakening voids) and surface changes by tribological processes (stress cracks, work hardening, recrystallization etc.). Similarly, damping can influence transient stressrelated failure mechanisms and is worthy of consideration. TABLE VI DESIGN FOR MECHANICALINTERACTION Compressive yield stress Modulus of elasticity Poisson's ratio Fatigue Creep Impact Discontinuities Damping
5. DESIGN LOGIC
It has been indicated that one determinant function usually leads to the selection of tribological coatings. The preceding discussion indicates the multiplicity and complexity of interacting processes and variables that are inherent in most coating applications. The systems approach advocated here is intended to make designers of coatings consider in a more comprehensive manner the processes that may be important. From this discussion and consideration of the cited references a checklist has been assembled and is presented as Table VII. In general terms, very thin easily sheared films that may be a metal, a metal compound or a polymer can provide low friction. Such films can be part of a surface composite system that allows a transition of thermal, mechanical and chemical properties from the surface to the base material; sputter-ion-plated 19 and chemically vapor-deposited coatings 2° are promising. The base material may require modification by etching21, diffusion or ion implanting to accommodate essential thermal, mechanical and chemical processes. Chemical conversion treatments provide property transitions and surface geometry that aids adhesion of the outer coating material. It is in providing the transition structure of the coating that some of the less prominent processes (environment, thermal and mechanical)
SYSTEMS RATIONALE FOR TRIBOLOGICAL SURFACE COATINGS
243
TABLE VII DESIGN LOGIC FOR TRIBOLOGICAL COATINGS: CHECKLIST
nucTi~
m~olo~ Adhesic~of coatin~ Film th/ckness Area of shear
Jtmcticn ~rowth Cn surface substrate Surface interactions ~lhesicn Abrasion Corrosion
~t Erosion
~ati~ue Plasticit~ Film Thickness Discontinuities ~/IRC~MENr
Vacuum Air Moisture Process fluid Liquid lubricant Contamination
TH~RV~L
Ta~oerature extremes Temperature transients m e l t ~ points Transformations
Conductivity Expansi~ ~CHANICA~
CU~. yield stress Elastic modulus Poisscn, s ratio
~a_ti~e iimi~ cr~ 9 ~ct Disc~tinuities m,ging Variables may have strong (s), medium (m) or weak (w) influence on the determinant processes.
can be accommodated. Using vacuum processes such'as sputter etching 22 and the various coating methods 14' 19, it may also be .possible to remove, or immobilize surface discontinuities and defects that can influence wear and mechanical failures. 6.
CONCLUDING REMARKS
A systems approach is suggested for the design of surface coatings. There are many interacting processes, including friction, wear, environment, thermal stresses and mechanical stresses, to be considered. The operating variables are numerous and influence the determinant processes. A design logic checklist is provided at a systems hierarchy level that can be used by a designer and further developed by researchers.
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General consideration of the logic suggests that composite materials t e c h n o l o g y s h o u l d be a c o a t i n g d e s i g n e r ' s tool. F u r t h e r m o r e , c h e m i c a l v a p o r a n d v a c u u m d e p o s i t i o n m e t h o d s h a v e special p o t e n t i a l for surface c o a t i n g s ; t h e y c a n r e d u c e o r i g i n a l surface defects a n d a l l o w a t r a n s i t i o n in c o m p o s i t i o n t h a t s h o u l d m i n i m i z e failures by p r o c e s s e s n o t c o n s i d e r e d in the o r i g i n a l design. ACKNOWLEDGMENT T h i s d e s i g n r a t i o n a l e was d e v e l o p e d w i t h the s u p p o r t of the U.S. A r m y R e s e a r c h Office in their p r o g r a m of F u n d a m e n t a l R e s e a r c h in T r i b o l o g y at R e n s s e l a e r P o l y t e c h n i c Institute. REFERENCES
1 F. Zwicky, Morphological Astronomy, Springer, Berlin, 1957. 2 H. Czichos, Tribology: A Systems Approach to the Science and Technology of Friction, Lubrication and Wear, Elsevier, New York, 1978. 3 K.E. Boulding, General systems theory the skeleton of science, Manage. Sci., 2 (1956) 197. 4 R.R. Paxton, Manufactured Carbon: A Self Lubricating Material for Mechanical Devices, CRC Press, Cleveland, Ohio, 1979. 5 P.M. Ku (ed.), Interdisciplinary approach to friction and wear, NASA Spec. Publ. SP-181, 1968. 6 K. Schoenherr and R. L. Johnson, Seal Wear, Wear ControlHandbook, Vol. II, American Society of Mechanical Engineers, New York, 1980, to be published. 7 R.L. Johnson and H. E. Sliney, Fundamental considerations for future solid lubricants, NASA Tech. Memo. TMX-52659, 1969 ; Proc. Air Force Materials Laboratory-Midwest Research Institute Conf. on Solid Lubricants, in Tech. Rep. AFML-TR-70-127. 1970, p. 40, Air Force Materials Laboratory. 8 F.F. Ling, Surface Mechanics, Wiley-Interscience, New York, 1973. 9 F.E. Kennedy and F. F. Ling, Elasto-plastic indentation of a layered medium, Trans. A S M E (April 1974)97 103. 10 G.W. Rowe et al., Friction, Wear, and Lubrication-- Tribology Glossary of Terms and Definitions, International Research Group on Wear of Engineering Materials, Organization for Economic Cooperation and Development, Paris, 1969. 11 E.R. Braithwaite, Lubrication and Lubricants, Elsevier, Amsterdam, 1967. 12 F.P. Bowden and D. Tabor, The Friction and Lubrication of Solids, Oxford University Press, London, 1958. 13 M. Antler, The wear of electrodeposited gold, A S L E Trans., 11 (3) (1968) 248-260. 14 T. Spalvins, Survey of ion plating sources, Trans. 26th Natl. Vacuum Symp., American Vacuum Society, New York, October 2-5, 1979; NASA Tech. Memo. TM-79269. 15 D.H. Buckley, Friction, wear, and lubrication in vacuum, NASA Spec. Publ. SP-277, 1971. | 6 H.E. Sliney, Plasma-sprayed metal-glass and metal-glass fluoride coatings for lubrication to 900 °C, A SLE Trans., 17 (3) 182-189. H. E. Sliney and J. W. Graham, Tribological properties of self-lubricating fluorid~metal composites to 900 °C a review and some new developments, A S L E Paper 74-LUB-2, 1974. 17 J.P. Netzel, Observations of thermoelastic instability in mechanical face seals, Wear, 59 (1980) 135-148. 18 R.M. Christensen, Mechanics of Composite Materials, Wiley-Interscience, New York, 1979. 19 T. Spalvins, Coatings for wear and lubrication, Thin Solid Films, 53 (1978) 285-300. 20 H.E. Hintermann, Chemical vapor deposition applied in tribology, Wear, 47 (1978) 407415. 21 B. Bhushan, Surface pretreatment of thin Inconel X-750 foils for improved coating adherence, Thin Solid Films, 53 (1978) 99-107. 22 R.S. Berg and G. J. Kominiak, Surface texturing by sputter etching, J. Vac. Sci. Technol., 13 (1) (1976) 403405.