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Tribological characterization of thin, hard coatings
WEAR ELSEVIER
Wear
Tribological
179 (1994) 147-154
characterization
of thin, hard coatings
Sture Hogmark *, Per Hedenqvist Uppsala University, Department
of Technology, Materials Science Division,
Box 534, S-751 21 Uppsala, Sweden
Abstract The use of thin, hard coatings in tribological applications has become more widespread. Thus the need to fully understand the relationships between the intrinsic properties of the coating, the properties of the coating/substrate composite and the tribological performance of the composite in different tribological systems has become more pressing. The present paper contributes to this effort by presenting and discussing the use of a selected number of physical and mechanical tests to obtain a general characterization of the tribological properties of a coated material: its tribological profile. The tribological tests include dry sliding wear, solid particle erosion and microabrasion. Even if the emphasis of the present paper is put on tribological characterization of thin, hard coatings, some of the current techniques frequently employed to characterize some of the more general properties of a coating, e.g. thickness, hardness, adhesion, residual stress state, etc., are treated. Examples from tribology profile determination and general characterization are collected from PVD and CVD coating systems. Keywords:
Hard coatings; Tribological characterization
1. Introduction
Today, thin, hard coatings deposited by physical vapour deposition (PVD), e.g. titanium nitride (TiN), or chemical vapour deposition (CVD), e.g. alumina (A1203), are frequently used to improve tribological performance in many engineering applications. Hence, a thorough understanding of the relationships between the intrinsic properties of the coating, the properties of the coating/substrate composite and the tribological behaviour of the composite in different tribological systems is needed. Further, the ever-present demands for increased productivity, tool life, etc., have, particularly in the case of PVD coatings, triggered the introduction of several new coatings on the market, e.g. chromium nitride (CrN), titanium aluminium nitride (Ti, Al)N) and titanium carbonitride (Ti(C, N)), Other PVD coatings (homogeneous, multi-layered or “superlattices”), e.g. (Ti, Zr)N, (Ti, Nb)N, etc. are currently produced in the research laboratories and can be expected to be introduced on the market in the near future. Reproducible and standardized methods for determination of the tribological properties of coatings are, somewhat surprisingly, scarce. There should thus be a
* Corresponding author.
0043-1648/94/$07.00 0 1994 Elsevier Science SSDI 0043-1648(94)06530-6
S.A. All rights reserved
massive demand for design and implementation of tribological (and, indeed, mechanical) tests specifically aimed at thin, hard coatings and/or coating/substrate composites. Some work along these lines has been performed, but is mostly either out of date or mainly focused on methods intended for adhesion, hardness and thickness determination [l-4]. The tribological performance of a component is governed by its design (geometry), environment (atmosphere, temperature, etc), contact conditions (load, speed, lubrication, etc.) and the materials of which it (and any other parts of the tribosystem) is composed. In order to understand, explain or predict the performance of a given component, one must naturally possess all relevant information about these parameters. Consequently, the number of tests to obtain a general characterization of the coating performance could be infinite. The idea of this work is to select a limited number of tribo tests, each exposing the coated composite to a specified, well-controlled tribological situation. Togetherwith the general parameters of thickness, structure, composition, hardness, adhesion to the substrate, etc., a set of results from the selected tribo tests is suggested to constitute a “tribology profile” of the coated system. Ideally, the tribo tests should be selected in such a way that they generate a tribology profile from which it is possible to predict the performance of any coated system in any given application.
In this paper, three tribology tests, dry sliding, solid particle erosion and microabrasion, have been selected for obtaining the tribology profile of coated systems. Examples are given from PVD TiN, Ti(C, N), (Ti, Al)N, CrN and (Ti, Nb)N (superlattice) on high speed steel (HSS), CVD TiC and Al,O, on cemented carbide (CC) and diamond deposited on CC by the hot flame (CVD) technique.
2. Characterization
of general
coating
Coating Chemical
properties
and methods
Characterization
property composition
used to obtain
them
method(s)
Energy dispersive X-ray spectroscopy, Auger electron spectroscopy, glow discharge optical emission spectroscopy
Microstructure morphology
and
Transmission electron microscopy, scanning electron microscopy, light optical microscopy, X-ray diffraction
Residual
state
X-ray
stress
diffraction,
substrate
Thickness
Ball grinding, cross-section X-ray fluorescence
Hardness
Extrapolation,
Adhesion Fracture
to substrate toughness
Scratch
adhesion
Indentation
2 ._; or
.
Ti x2 ,_,_;*.-_“‘-
: ,._ ‘. .,:., (,‘.,‘:.;‘,.;’ ,:_,..-‘,_ ~. ‘. -.; _-..;,: _ .: . .....Alx! ‘,\ ‘\( ‘.
r
:
, ’ Fe \
theoretical testing
deflection microscopy, models
-r
/ 1 .I -1.
N-
$ -4.
on techniques frequently used for characterization of a few fundamental coating (or coating/substrate composite) properties are given in this section (see Table 1). Energy dispersive X-ray spectroscopy (EDS) is employed to obtain the bulk chemical composition of a specimen, while Auger electron spectroscopy (AES) and/or glow discharge optical emission spectroscopy (GDOES) are utilized to obtain depth profiles, i.e. the relative amounts of the chemical elements as a function of the distance from the coating surface. GDOES, in particular, has proven to quickly yield reliable compositional results (typically < 15 min to obtain a depth profile through a 5-pm thick coating), for reasonable cost and without any time-consuming specimen preparation [5] (see Fig. 1). The limitations are that the technique is truly destructive and only permits analysis of relatively large, flat areas (> 10 n-m?). The techniques utilized for morphology and microstructure studies are light optical microscopy (LOM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) [6-81. The last technique yields the most detailed microstructural information but the specimen preparation is unfortunately rather time-consuming and also relatively expensive. Crystal
coating
.
properties
Some information
Table 1 Some important
-_
0
1 0
I
_,
-
/
I
I
2
Depth
I
I
3
4
-7
1 5
[pm1
Fig. 1. GDOES profile of PVD (Ti, AI)N on HSS. The coating consists of a cyclic repetition of two sublayers with different Ti:AI ratios. From [S].
structure and orientation of a coating is determined using X-ray diffraction (XRD). High coating residual stresses can result in plastic deformation and microcracking, which affects the tribological performance of the coated component. It is thus obviously of great importance to determine the residual stress state. In the literature, the most common technique for residual stress determination is the sin2” method (see e.g. Ref. [9]), which utilizes XRD to obtain the stresses. An alternative method is to measure the deflection of a disc by a surface profilometer both before and after coating deposition [lo]. The coating residual stress parallel1 to the specimen surface, a,,, can then be obtained using the Stoney equation,
(1) where E,/(l - vs) is the biaxial modulus of the substrate and t, and & are the substrate and coating thickness, respectively. R, is the radius of curvature after coating deposition and R, the radius of curvature before. Determination of coating thicknesses can be performed by various means, e.g. ball-grinding [l], X-ray fluorescence [ll] or direct measurements on polished or fractured cross-sections using LOM and/or SEM 16771. The absolute hardness of a thin, hard coating cannot be determined directly by conventional microhardness testing, as the measured hardness is usually influenced by the substrate. Instead, special experimental techniques must be utilized. Several approaches are used; one possibility is to measure the hardness of the coated component at several loads and extrapolate towards an infinitely low load [S]; other approaches involve the application of physical models such as the Jiinsson/ Hogmark or the Burnett/Rickerby models [12,13].
S. Hogmark,
P. Hedenqvist
The single most important property of a coating/ substrate composite is the adhesion of the coating to the substrate. If this is inadequate, premature failure of the coated part can occur due to coating detachment. Scratch adhesion testing using a diamond stylus which is drawn over the coated surface [14] is by far the most common technique for adhesion testing. Its popularity arises from the ease with which it is performed and the fact that it yields comparatively quick results. It must be emphasised, however, that this test method cannot be used to obtain any quantitative information, it is only capable of determining whether adhesion is “bad” or “sufficient” in any given case. In combination with LOM and/or SEM studies, however, scratch testing can be used to obtain valuable information on coating failure modes [15]. Crack initiation and propagation virtually always precede coating failure. Consequently, the coating fracture toughness is a parameter of great importance. A relatively simple method to obtain a measure of the fracture toughness is to make Vickers’ indentations at high loads (at least 10 kg) and determine the length of the cracks appearing in the corners of the indentation; long cracks indicate a low fracture toughness and vice versa [16]. The need for more sophisticated test methods that are able to actually quantify the coating fracture toughness will, however, be great in the near future; not least since one of the aims of current PVD coating development is to make tougher coatings.
3. Tribological
characterization
3.1. Sliding wear
In machining, which is one of the major applications of hard, wear-resistant coatings, wear occurs on the flank and rake face. Both mechanical and chemical interactions are involved in the wear process, which is governed mainly by cutting forces, cutting speed and the mechanical and chemical properties of the work material. Preferably, tool performance is evaluated by full-scale cutting tests. These tests are, however, expensive due to the extensive consumption of workpiece material and the amount of machine and man hours involved. Also, they are usually not able to distinguish between the influence from continuous wear mechanisms (e.g. adhesive, abrasive and solution wear) and discrete failure mechanisms (e.g. edge chipping, micro fracture etc.) on tool life. Therefore, a laboratory wear test which is capable of reproducing the contact conditions at a tool-workpiece interface in machining has been developed [17]. The test equipment (basic test geometry given in Fig. 2) is based on a conventional lathe with the test pin,
/ Wear 179 (1994) 147-154
Fig. 2. Schematic drawing (test the crossed-cylinders laboratory of coated materials.
149
pin at A, countermaterial at B) of test for evaluation of sliding wear
which consists of the coating/substrate composite to be tested, mounted on the tool holder. A spring loading system is utilized to press the pin against the counter-material which is mounted as the workpiece in the lathe. Strain gauges are used to measure the friction force. Before the start of an experiment, a fine cutting tool is used to produce a new, smooth countermaterial surface. Wear testing with continuous introduction of fresh, unworn countermaterial (a prerequisite for accurate simulation of cutting tool wear) can then be performed by employing the feed function of the lathe. Some representative results obtained by this test procedure are given in Fig. 3a. Conventional pin-on-ring testing can be performed using the same equipment by disengaging the feed function. The wear mechanisms are governed by a combination of the properties of the contacting materials, normal force and sliding speed. In general, it is found that under mild sliding contact conditions against carbon steels PVD coatings are worn (if at all) by a mild abrasive or polishing mechanism with the carbides in the steel countermaterial acting as abrasives. Consequently, wear is mainly governed by the coating hardness, i.e. the test pins with the hardest coating (Ti(C, N) in Fig 3a) display the lowest amount of wear at these conditions. With increasing contact stresses, fatigue induced cracking may result in microchipping of small coating fragments. At severe sliding conditions (high contact pressure and/or high sliding speed), the high tensile stresses generated at the sliding interface result in nucleation and propagation of cracks in the coating perpendicular to the sliding direction. Eventually, intersecting cracks will cause segmentation of the coating and allow coating fragments to be removed by the counter-material (see Fig. 3b). Particularly if the sliding speed is increased, this mechanism is greatly accelerated due to thermal softening of the substrate. The relatively good performance of (Ti, Al)N at high sliding speed (cf. Fig. 3a) is explained by the low oxidation rate of the coating material at elevated temperatures [19]. This, in turn, means that a corresponding lesser degradation of the intrinsic mechanical properties, in particular the coating fracture toughness, occurs.
Pin sliding dir’
a
Sliding
Sliding
Fig. as a of a Tic
speed [minrinl
speed
[m/mini
3. Characteristics of coatings sliding against quenched and tempered steel. (a) Wear of some PVD coatings deposited on HSS test pins function of sliding speed. The hardness of the coatings increases in the order CrN-TiN-(Ti, AQN-Ti(C, N) (From [IS]). (b) Cracking PVD TIN coating and depression of coating fragments into the thermally softened HSS (severe sliding wear regime). (c) Wear of CVD on CC as a function of sliding speed. From [22]. (d) Adhesive wear topography on the TiC coating surface.
The wear characteristics of CVD Tic-coated CC are also strongly dependent on the sliding speed (Fig. 3~). At low sliding speeds (below 100 m s-l) the coating appears more or less unaffected. At higher speeds, the TiC shows signs of superficial plastic deformation of surface asperities, indicating an adhesive wear mechanism (see Fig. 3d). With increasing sliding distance, this results in a gradual reduction of the coating thickness until the CC is exposed. For both PVD and CVD coatings sliding against plain carbon and quenched and tempered steel, nonmetallic, glassy layers are formed on the coating surface. This is a result of the smearing-out of inclusions from the countermaterial over the tool wear test surface and, depending on the inclusion content of the steel, oxide and/or sulphide layers may form. It has been suggested that due to its high hardness and relatively high chemical stability against steel, the alumina layer formed on TiC/ CC (Fig. 4a) protects the TiC coating from wear [20,21]. Another type of oxide layer, based on silica, can be found on both Tic-coated CC and TiN-coated HSS (see Fig. 4b). These layers are believed to play a minor role in the protection of TIC/CC but might, however, act as a solid lubricant at high sliding speeds, thus reducing the friction in the pin/countermaterial contact zone. In the case of TiN-coated HSS, the effect of the
silica-based layer can be twofold; firstly, the layers may act as a solid lubricant, the same as for TIC/CC, and reduce the temperature in the contact zone and thereby delay the onset of substrate thermal softening (and thus also delay the onset of the crack formation and plucking coating removal mechanism). Secondly, the silica-based layer may act as a binder, holding cracked TiN fragments together. The cohesive properties of coatings as well as the adhesion to the substrate can be evaluated using an austenitic stainless steel countermaterial. Since this material does not contain any hard constituents, the abrasive wear will be suppressed. Austenitic stainless steel generally has a lower thermal conductivity, higher ability to strain harden a thinner surface oxide layer and a greater fracture toughness than carbon steels, properties which all are known to aggravate adhesive wear in sliding contacts. Hence, it is possible to control the proportions of abrasive and adhesive wear through the amount of hard constituents in the countermaterial. It is concluded that a general evaluation of a coated system aimed at metal cutting should be performed by sliding against three types of steel: - soft annealed tool steel (abrasion is enhanced); - stainless steel (adhesion and cohesion is enhanced);
S. Hogmark,
P. Hedenqvist
151
/ Wear 179 (1994) 147-154 Specimen
Raptdly
Fig. 5. Schematic view of the centrifugal erosion tester. The eroding particles are fed into the centre of the rapidly rotating disc. They are then accelerated by the centrifugal force through four radial channels and eventually hit the specimens.
a 400
Fig. 4. Glassy layers. (a) Alumina layer on TiCKC. layer on TiNiHSS.
I ! 3500
I 1
I 1
’
f
I
(b) Silica-based Tii
CrN
(TiAI)N
Ti(C.N)
quenched and tempered carbon steel (a combination of abrasion and adhesion).
__._._._ ._.._.___; /~..~..~ 1:I$
3.2. Erosive wear Solid particle erosion is a fairly common method for wear testing of coatings [23-261. The main advantages of this method are: (i) it is highly reproducible; (ii) it yields relatively quick results; and (iii) it is a statistical method in the sense that a relatively large area is tested, which minimizes the influence from local defects on the mean performance of the coated specimens. The centrifugal erosion tester offers the possibilities to test a relatively large number of samples under identical experimental conditions (particle dose, velocity and angle of impingement) (see Fig. 5). Mass loss measurement is the standard technique to determine erosion rates. In many cases, this technique cannot be used for thin coatings since the mass loss corresponding to complete coating removal is often too small to be accurately measured. An additional problem arises if the particle dose varies over the eroded area, in which case simultaneous erosion of the coating and the substrate occurs as soon as the coating has been worn through. To overcome these problems, several new experimental techniques have been introduced [23,24,26]. These are all based on EDS measurements or image analysis techniques, which makes it possible to restrict the analysis to small surface elements on
c
0:
0
/
:
I I
2
:
1
4 Coating
j. . . .. . .. . .. . .
i
I I
I I
I I
6 8 10 thickness [pm]
1 12
Fig. 6. (a) Critical particle doses (i.e. the particle doses corresponding to complete coating removal on the entire tested area (= 64 mm*); the higher the critical particle dose, the better the erosion resistance) obtained in erosive testing of some different PVD coatings on high speed steel. From 1261. (b) Variation in coating erosion rate as a function of coating thickness for CVD TiC and A1203 on CC. From
v41.
which the particle dose can be considered as being constant. The coating erosion resistance tends to increase with coating (or composite) hardness and thickness (see Fig. 6). Four main erosion mechanisms can be observed for today’s PVD coatings. One is based on the fact that some of the impinging particles will cause lips of coating material to be foned during impact. Subsequent particle impacts can then easily remove the lips. Secondly, depending on the particle impact angle, the impinging
particles might act as single pass cutting edges; each impact removes an infinitely small amount of coating material. Further, a large number of particle impacts on the surface will result in severe deformation and plastic straining of the surface material. In particular, nucleation and propagation of fatigue induced subsurface lateral cracks will occur in the relatively brittle coating. As more and more particles hit the surface, adjacent crack systems will interact and, eventually, coating fragments will be detached (see Fig. 7a). Finally, when the coating thickness has been sufficiently decreased, extensive plastic deformation of the substrate material will cause formation of vertical cracks in the coating. Subsequent particle impacts will then result in severe coating fragmentation and removal. The severity of this mechanism tend to increase with decreasing substrate hardness and coating thickness. Single particle impact studies on polished CVD coated specimens show that the impinging particles cause crack formation, originating from the point of impact, in both TIC and Al,O, coatings. Continued erosion results in overlapping impacts and, in the case of the TiC coatings, plastic straining of the surface material due to the deformation caused by the impinging particles. When the fracture strain of the coating material is exceeded, extensive microchipping occurs and individual crystallites of TiC can be observed on the eroded surface [24], i.e. erosion predominantly occurs by an inter-
crystalline fracture mechanism resulting in detachment of individual grains at impact. The erosion of A&O, coatings is to a large extent governed by chipping by transcrystalline fracture, as proved by the subsurface cracks observed in crosssections (Fig. 7b). The size of the individual chippings is larger than for Tic, which results in a higher erosion rate. For multi-layered Al,O,/TiC coatings on CC, interfacial spalling in the A&O,--TIC interface is the dominant erosion mechanism. 3.3. Microabrasion The abrasive wear resistance of a coated part can be determined by subjecting it to virtually any kind of grinding/polishing treatment or by using the sliding wear test described in Section 3.1 with a carbide-rich workpiece material. However, these types of tests are not straightforward to use for determination of the (intrinsic) abrasive wear resistance of a given coating material, since the substrate material always influences the results, i.e. one obtains the abrasive wear resistance of the coating/substrate composite, which, of course, depends on coating thickness, coating hardness, substrate hardness, etc. Fortunately, a microabrasive wear test that is capable of distinguishing between the contributions from coating and substrate, respectively, has recently been developed [27]. The test utilizes a commercial dimple grinder (Fig. S), normally used for preparation of TEM specimens. At regular intervals, the test is interrupted and the crater diameters are measured using an optical microscope; the measurements are subsequently used for
‘0. Fig. 7. (a) PVD TiN coating failure by initiation and propagation of lateral cracks. (b) Cross-section of an eroded CVD-A1203 coating. Note the crack (at A).
Specimen platform (magnetic)
Fig. 8. Schematic view of the dimple grinder. The grinding shaped as a disc cut out from the centre of a sphere, rotates a horizontal axis. The specimen is mounted horizontally in subsequently filled with abrasive slurry. During testing, the grinds (at a load of 20 g) the specimen which in turn rotates a vertical axis. The combined motions resufts in a crater spherical cap-shape being ground into the specimen surface.
wheel, about a cup wheel about with a
S. Hogmark,
P. Hedenqvist
a
/ Wear 179 (1994) 147-154
153
5. The tribology profile The three types of tribo tests described in this paper, sliding wear, particle erosion and microabrasion, each generate characteristic data of the tribological properties of coated materials. Together with the coating thickness, residual stress state, structure and chemical composition, these data can be regarded to constitute a tribology profile. Given the tribology profile of a number of coating/substrate combinations, it is possible to select and to predict the performance of the best combination for any given application. In spite of the above statements, the general recommendation to the design engineer who wants to use a coated component is to perform a field test or a test which simulates the desired application as closely as possible before making the final choice. 6. Tomorrow’s coatings
$ 100
3
0
Fig. 9. (a) Representative V, versus (SL - VJK~) plot obtained from microabrasion tests of TiN on HSS. (From [S]). (b) Abrasive wear constants for some PVD coatings on HSS, superlattice (Ti, Nb)N/ HSS, Tic/CC and hot flame deposited diamond on CC. From [11,28, 291.
calculation of the coating and substrate wear volumes. These parameters can then, in turn, be inserted in the expression (derived from the original model in [27]): f&r=
3 + !L
(2) \ I
KC
4
where S is the sliding distance, L is the applied load and V, and V, are the wear volumes of coating and substrate, respectively. & and 4 are the wear constants of coating and substrate. The wear constant of a material is a measure of its wear rate, i.e. a high wear rate corresponds to a high wear constant. Assuming that K~ is known (it can be determined in advance on uncoated specimens), the wear constant of the coating is easily obtained by plotting V, versus (SL -VJr$) (see Fig. 9a). The intrinsic microabrasive resistance of coatings usually increases with coating hardness (Fig. 9b). The microabrasion test not only supplies information on the coating resistance to abrasive wear, it also yields much more information of the investigated composite; any possible multilayered structure as well as pores and/ or inclusions (secondary phases) in the coating is easily detected. Insufficient coating adhesion and cohesion will be detected and the coating thickness can be determined from the experimental results as a “bonus” (cf. ball grinding).
The number of different PVD and CVD coatings commercially available is high and rapidly growing. As already indicated, we can expect e.g. multilayered or superlattice coatings or coatings of new materials like C,N,, to appear on the market soon. We will also have to face the fact that the matter of choosing the optimum coating system for a given application will be increasingly difficult since each coated system has its own tribology profile. Thus, it will be necessary to obtain tribology profiles along the guidelines given in this paper to meet the needs of design engineers. Today, it is possible to manufacture coatings with hardness sufficient for almost any application and adhesion is usually not a problem. Instead, the main limiting factor is the coating ductility. A good test for ductility measurements of coated materials is still needed. The scratch test and/or indentations will give qualitative information on the ductility of coating with relatively good adherence, but the scatter is too high. The next generation of coatings will probably have a more active tribological role than just passive reduction of friction and increase of wear resistance. More emphasis will certainly be put on the “third body layer” which develops by a combination of mechanical and chemical reactions between surfaces in sliding contact. A further improvement of the tribological properties of coatings can possibly be offered by the introduction of constituents which control the formation of a beneficial tribo film. Very hard coatings on relatively ductile materials, i.e. diamond coatings on CC or TiC coatings on HSS, require a gradual reduction in coating hardness (and corresponding increase in ductility) towards the interface in order not to fail by brittle fracture. The development of coatings with ductility gradients but maintained heat resistance is another future challenge in coating development.
154
S. Hogmark,
P Hedenqvist
7. Conclusions The main conclusion from this survey of methods used for mechanical and tribological characterization of thin, hard coatings are: - As each application emphasises a particular set of merits, the tribological performance of a coating cannot be predicted by one single parameter. - A general tribological characterization of a coated component should encompass several reproducible lab tests, e.g. sliding wear, erosion, microabrasion, in order to obtain a “tribology profile”. - Given the tribology profile of a number of coating/ substrate combinations, it is possible to select and to predict the performance of the best combination for any given application. - The ideal coating should be: (a) well adhering; (b) sufficiently hard to resist abrasion; (c) sufficiently ductile to avoid flaking; (d) chemically resistant; (e) active in tribo film formation; (f) adjusted gradually to the substrate.
Acknowledgements Dr Erich Bermann (Balzers AG), Dr Leif Westin (Erasteel Kloster AB) and Mr Stig Pettersson (Uddeholm Tooling AB) are recognized for providing a neverceasing flow of coatings and substrates. The authors gratefully acknowledge the financial support from the National Swedish Board for Technical and Industrial Development (NUTEK).
References VI
J.E. Kelly, J.J. Stiglich Jr. and G.L. Sheldon, Methods of characterization of tribological properties of coatings, in T.S. Sudarshan and D.G. Bhat (eds.), Surface Modification Technologies, The Metallurgical Society, 1988, p. 169. 121 R.F. Bunshah, Selection and use of wear tests for coatings, ASTM Special Technical Publication 769, 1982, p. 3. PI H. Ronkainen, S. Vajus, K. Holmberg, K.S. Fancey, A.R. Pace, A. Matthews, B. Matthes and E. Broszeit, in D. Dowson and M. Godet (eds.), Proc. 16th Leeds-Lyon Symp. on Tribology, Lyon, France, 1989, p. 453.
i Wear 179 (1994) 147-154 141 A.J. Perly, J. Valli and P.A. Steinmann, Slrlj: C‘ncrting~s~~chnn/ 36 (1988) 559. M. Olsson and S. [51 M. Bromark, M. Larsson, P. Hedenqvist, Hogmark, Su$ Eng., submitted. [61 M. Olsson, PhD Thesis, Acta Universatis Upsaliensis, Vol. 200, 1989. [71 P. Hedenqvist, PhD Thesir, Acta Universatis Upsaliensis, Vol 360, 1991. PI A. Schulz, H.-R. Stock, H. Vetters and P. Mayr, Pratt. Met., 27 (1990) 269. and D.G. Bhat (eds.), Su$ace [91 A.J. Perry, in T.S. Sudarshan Modification Techniques, The Metallurgical Society, 1988, p. 121. and S. Hogmark, Surf Eng., sub[lOI M. Larsson, P. Hedenqvist mitted. IllI M. Wargelius, MSc Thesis, internal report UPTEC 94 032E, Uppsala University, Department of Technology, 1994. in WI 0. Vingsbo, S. Hogmark, B. J(insson and A. Ingemarsson, P.J. Blau and B.R. Lawn (eds.), Microindentation Techniques in Materials Science and Engineering, ASTM Spec. Techn. Publ., 889 ASTM, Philadelphia, PA, 1986, p. 257. P31 P.J. Burnett and D.S. Rickerby, Thin Solid Films, 157 (1988) 233. P41 A.J. Perry, Surf: Eng., 2 (1986) 183. [I51 P. Hedenqvist, M. Oisson, S. Jacobson and S. Sbderberg, Su$ Coatings Technol., 41 (1990) 31. WI P.K. Mehrotra and D.T. Quinto, J. Vat. Sci. Technol., A3(6) (1985) 2401. 1171 M. Olsson, B. Stridh, U. Jansson and S. Sijderberg, Wear, 124 (1988) 195. M. Olsson and S. WI M. Larsson, M. Bromark, P. Hedenqvist, Hogmark, in M. Kozma (ed.), Proc. 6th Znt. Congress on Triborogy, Budapest, Hungary, Vol. 1, 1993, p. 216. 1191 W.-D. Miinz, J. Vat. Sci. TechnoL, A4(6) (1986) 2717. in K. Holmberg PO1 M. Olsson, L. Rosengren and P. Hedenqvist, and I. Nieminen (eds.), Proc. 5th Znt. Congress on Triborogy, Helsinki, Finland, Vol. 2, 1989, p. 134. B.M. Kramer and N.P. Suh, J Eng. Ind., 102 (1980) 303. PI P. Hedenqvist and M. Olsson, Tribal. Znt., 24(3) (1991) 143. PI (231 B. JGnsson, L. Akre, S. Johansson and S. Hogmark, Thin Solid Films, 137 (1986) 65. B. Stridh and S. SGderberg, Surj v41 M. Olsson, P. Hedenqvist, Coatings Technol., 37 (1989) 321. ~251 P.J.. Burnett and D.S. Rickerby, Su$ Coatings Technol., 33 (1987) 191. 1261 M. Bromark, M. Larsson, P. Hedenqvist, M. Olsson and S. Hogmark, in M. Kozma (ed.), Pmt. 6th Znt. Conpss on TriboZogv, Budapest, Hungary, Vol. 1, 1993, p. 204. t271 A. Kassman, L. Erickson, M. Olsson, P. Hedenqvist, S. Jacobson and S. Hogmark, Surf: Coatings Techno/., 50 (1991) 75. M. Olsson and S. 1281 M. Larsson, M. Bromark, P. Hedenqvist, Hogmark, in M. Kozma (ed.), Pnx. 6th Znt. Congress on Tribology, Budapest, Hungary, Vol. 1, 1993, p. 210. v91 A. Alahelisten, M. Olsson and S. Hogmark, in M. Kozma (ed.), Proc. 6th Znt. Congress on Tribology, Budapest, Hungaly, Vol. 3, 1993, p. 280.
Wear
Tribological
179 (1994) 147-154
characterization
of thin, hard coatings
Sture Hogmark *, Per Hedenqvist Uppsala University, Department
of Technology, Materials Science Division,
Box 534, S-751 21 Uppsala, Sweden
Abstract The use of thin, hard coatings in tribological applications has become more widespread. Thus the need to fully understand the relationships between the intrinsic properties of the coating, the properties of the coating/substrate composite and the tribological performance of the composite in different tribological systems has become more pressing. The present paper contributes to this effort by presenting and discussing the use of a selected number of physical and mechanical tests to obtain a general characterization of the tribological properties of a coated material: its tribological profile. The tribological tests include dry sliding wear, solid particle erosion and microabrasion. Even if the emphasis of the present paper is put on tribological characterization of thin, hard coatings, some of the current techniques frequently employed to characterize some of the more general properties of a coating, e.g. thickness, hardness, adhesion, residual stress state, etc., are treated. Examples from tribology profile determination and general characterization are collected from PVD and CVD coating systems. Keywords:
Hard coatings; Tribological characterization
1. Introduction
Today, thin, hard coatings deposited by physical vapour deposition (PVD), e.g. titanium nitride (TiN), or chemical vapour deposition (CVD), e.g. alumina (A1203), are frequently used to improve tribological performance in many engineering applications. Hence, a thorough understanding of the relationships between the intrinsic properties of the coating, the properties of the coating/substrate composite and the tribological behaviour of the composite in different tribological systems is needed. Further, the ever-present demands for increased productivity, tool life, etc., have, particularly in the case of PVD coatings, triggered the introduction of several new coatings on the market, e.g. chromium nitride (CrN), titanium aluminium nitride (Ti, Al)N) and titanium carbonitride (Ti(C, N)), Other PVD coatings (homogeneous, multi-layered or “superlattices”), e.g. (Ti, Zr)N, (Ti, Nb)N, etc. are currently produced in the research laboratories and can be expected to be introduced on the market in the near future. Reproducible and standardized methods for determination of the tribological properties of coatings are, somewhat surprisingly, scarce. There should thus be a
* Corresponding author.
0043-1648/94/$07.00 0 1994 Elsevier Science SSDI 0043-1648(94)06530-6
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massive demand for design and implementation of tribological (and, indeed, mechanical) tests specifically aimed at thin, hard coatings and/or coating/substrate composites. Some work along these lines has been performed, but is mostly either out of date or mainly focused on methods intended for adhesion, hardness and thickness determination [l-4]. The tribological performance of a component is governed by its design (geometry), environment (atmosphere, temperature, etc), contact conditions (load, speed, lubrication, etc.) and the materials of which it (and any other parts of the tribosystem) is composed. In order to understand, explain or predict the performance of a given component, one must naturally possess all relevant information about these parameters. Consequently, the number of tests to obtain a general characterization of the coating performance could be infinite. The idea of this work is to select a limited number of tribo tests, each exposing the coated composite to a specified, well-controlled tribological situation. Togetherwith the general parameters of thickness, structure, composition, hardness, adhesion to the substrate, etc., a set of results from the selected tribo tests is suggested to constitute a “tribology profile” of the coated system. Ideally, the tribo tests should be selected in such a way that they generate a tribology profile from which it is possible to predict the performance of any coated system in any given application.
In this paper, three tribology tests, dry sliding, solid particle erosion and microabrasion, have been selected for obtaining the tribology profile of coated systems. Examples are given from PVD TiN, Ti(C, N), (Ti, Al)N, CrN and (Ti, Nb)N (superlattice) on high speed steel (HSS), CVD TiC and Al,O, on cemented carbide (CC) and diamond deposited on CC by the hot flame (CVD) technique.
2. Characterization
of general
coating
Coating Chemical
properties
and methods
Characterization
property composition
used to obtain
them
method(s)
Energy dispersive X-ray spectroscopy, Auger electron spectroscopy, glow discharge optical emission spectroscopy
Microstructure morphology
and
Transmission electron microscopy, scanning electron microscopy, light optical microscopy, X-ray diffraction
Residual
state
X-ray
stress
diffraction,
substrate
Thickness
Ball grinding, cross-section X-ray fluorescence
Hardness
Extrapolation,
Adhesion Fracture
to substrate toughness
Scratch
adhesion
Indentation
2 ._; or
.
Ti x2 ,_,_;*.-_“‘-
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r
:
, ’ Fe \
theoretical testing
deflection microscopy, models
-r
/ 1 .I -1.
N-
$ -4.
on techniques frequently used for characterization of a few fundamental coating (or coating/substrate composite) properties are given in this section (see Table 1). Energy dispersive X-ray spectroscopy (EDS) is employed to obtain the bulk chemical composition of a specimen, while Auger electron spectroscopy (AES) and/or glow discharge optical emission spectroscopy (GDOES) are utilized to obtain depth profiles, i.e. the relative amounts of the chemical elements as a function of the distance from the coating surface. GDOES, in particular, has proven to quickly yield reliable compositional results (typically < 15 min to obtain a depth profile through a 5-pm thick coating), for reasonable cost and without any time-consuming specimen preparation [5] (see Fig. 1). The limitations are that the technique is truly destructive and only permits analysis of relatively large, flat areas (> 10 n-m?). The techniques utilized for morphology and microstructure studies are light optical microscopy (LOM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) [6-81. The last technique yields the most detailed microstructural information but the specimen preparation is unfortunately rather time-consuming and also relatively expensive. Crystal
coating
.
properties
Some information
Table 1 Some important
-_
0
1 0
I
_,
-
/
I
I
2
Depth
I
I
3
4
-7
1 5
[pm1
Fig. 1. GDOES profile of PVD (Ti, AI)N on HSS. The coating consists of a cyclic repetition of two sublayers with different Ti:AI ratios. From [S].
structure and orientation of a coating is determined using X-ray diffraction (XRD). High coating residual stresses can result in plastic deformation and microcracking, which affects the tribological performance of the coated component. It is thus obviously of great importance to determine the residual stress state. In the literature, the most common technique for residual stress determination is the sin2” method (see e.g. Ref. [9]), which utilizes XRD to obtain the stresses. An alternative method is to measure the deflection of a disc by a surface profilometer both before and after coating deposition [lo]. The coating residual stress parallel1 to the specimen surface, a,,, can then be obtained using the Stoney equation,
(1) where E,/(l - vs) is the biaxial modulus of the substrate and t, and & are the substrate and coating thickness, respectively. R, is the radius of curvature after coating deposition and R, the radius of curvature before. Determination of coating thicknesses can be performed by various means, e.g. ball-grinding [l], X-ray fluorescence [ll] or direct measurements on polished or fractured cross-sections using LOM and/or SEM 16771. The absolute hardness of a thin, hard coating cannot be determined directly by conventional microhardness testing, as the measured hardness is usually influenced by the substrate. Instead, special experimental techniques must be utilized. Several approaches are used; one possibility is to measure the hardness of the coated component at several loads and extrapolate towards an infinitely low load [S]; other approaches involve the application of physical models such as the Jiinsson/ Hogmark or the Burnett/Rickerby models [12,13].
S. Hogmark,
P. Hedenqvist
The single most important property of a coating/ substrate composite is the adhesion of the coating to the substrate. If this is inadequate, premature failure of the coated part can occur due to coating detachment. Scratch adhesion testing using a diamond stylus which is drawn over the coated surface [14] is by far the most common technique for adhesion testing. Its popularity arises from the ease with which it is performed and the fact that it yields comparatively quick results. It must be emphasised, however, that this test method cannot be used to obtain any quantitative information, it is only capable of determining whether adhesion is “bad” or “sufficient” in any given case. In combination with LOM and/or SEM studies, however, scratch testing can be used to obtain valuable information on coating failure modes [15]. Crack initiation and propagation virtually always precede coating failure. Consequently, the coating fracture toughness is a parameter of great importance. A relatively simple method to obtain a measure of the fracture toughness is to make Vickers’ indentations at high loads (at least 10 kg) and determine the length of the cracks appearing in the corners of the indentation; long cracks indicate a low fracture toughness and vice versa [16]. The need for more sophisticated test methods that are able to actually quantify the coating fracture toughness will, however, be great in the near future; not least since one of the aims of current PVD coating development is to make tougher coatings.
3. Tribological
characterization
3.1. Sliding wear
In machining, which is one of the major applications of hard, wear-resistant coatings, wear occurs on the flank and rake face. Both mechanical and chemical interactions are involved in the wear process, which is governed mainly by cutting forces, cutting speed and the mechanical and chemical properties of the work material. Preferably, tool performance is evaluated by full-scale cutting tests. These tests are, however, expensive due to the extensive consumption of workpiece material and the amount of machine and man hours involved. Also, they are usually not able to distinguish between the influence from continuous wear mechanisms (e.g. adhesive, abrasive and solution wear) and discrete failure mechanisms (e.g. edge chipping, micro fracture etc.) on tool life. Therefore, a laboratory wear test which is capable of reproducing the contact conditions at a tool-workpiece interface in machining has been developed [17]. The test equipment (basic test geometry given in Fig. 2) is based on a conventional lathe with the test pin,
/ Wear 179 (1994) 147-154
Fig. 2. Schematic drawing (test the crossed-cylinders laboratory of coated materials.
149
pin at A, countermaterial at B) of test for evaluation of sliding wear
which consists of the coating/substrate composite to be tested, mounted on the tool holder. A spring loading system is utilized to press the pin against the counter-material which is mounted as the workpiece in the lathe. Strain gauges are used to measure the friction force. Before the start of an experiment, a fine cutting tool is used to produce a new, smooth countermaterial surface. Wear testing with continuous introduction of fresh, unworn countermaterial (a prerequisite for accurate simulation of cutting tool wear) can then be performed by employing the feed function of the lathe. Some representative results obtained by this test procedure are given in Fig. 3a. Conventional pin-on-ring testing can be performed using the same equipment by disengaging the feed function. The wear mechanisms are governed by a combination of the properties of the contacting materials, normal force and sliding speed. In general, it is found that under mild sliding contact conditions against carbon steels PVD coatings are worn (if at all) by a mild abrasive or polishing mechanism with the carbides in the steel countermaterial acting as abrasives. Consequently, wear is mainly governed by the coating hardness, i.e. the test pins with the hardest coating (Ti(C, N) in Fig 3a) display the lowest amount of wear at these conditions. With increasing contact stresses, fatigue induced cracking may result in microchipping of small coating fragments. At severe sliding conditions (high contact pressure and/or high sliding speed), the high tensile stresses generated at the sliding interface result in nucleation and propagation of cracks in the coating perpendicular to the sliding direction. Eventually, intersecting cracks will cause segmentation of the coating and allow coating fragments to be removed by the counter-material (see Fig. 3b). Particularly if the sliding speed is increased, this mechanism is greatly accelerated due to thermal softening of the substrate. The relatively good performance of (Ti, Al)N at high sliding speed (cf. Fig. 3a) is explained by the low oxidation rate of the coating material at elevated temperatures [19]. This, in turn, means that a corresponding lesser degradation of the intrinsic mechanical properties, in particular the coating fracture toughness, occurs.
Pin sliding dir’
a
Sliding
Sliding
Fig. as a of a Tic
speed [minrinl
speed
[m/mini
3. Characteristics of coatings sliding against quenched and tempered steel. (a) Wear of some PVD coatings deposited on HSS test pins function of sliding speed. The hardness of the coatings increases in the order CrN-TiN-(Ti, AQN-Ti(C, N) (From [IS]). (b) Cracking PVD TIN coating and depression of coating fragments into the thermally softened HSS (severe sliding wear regime). (c) Wear of CVD on CC as a function of sliding speed. From [22]. (d) Adhesive wear topography on the TiC coating surface.
The wear characteristics of CVD Tic-coated CC are also strongly dependent on the sliding speed (Fig. 3~). At low sliding speeds (below 100 m s-l) the coating appears more or less unaffected. At higher speeds, the TiC shows signs of superficial plastic deformation of surface asperities, indicating an adhesive wear mechanism (see Fig. 3d). With increasing sliding distance, this results in a gradual reduction of the coating thickness until the CC is exposed. For both PVD and CVD coatings sliding against plain carbon and quenched and tempered steel, nonmetallic, glassy layers are formed on the coating surface. This is a result of the smearing-out of inclusions from the countermaterial over the tool wear test surface and, depending on the inclusion content of the steel, oxide and/or sulphide layers may form. It has been suggested that due to its high hardness and relatively high chemical stability against steel, the alumina layer formed on TiC/ CC (Fig. 4a) protects the TiC coating from wear [20,21]. Another type of oxide layer, based on silica, can be found on both Tic-coated CC and TiN-coated HSS (see Fig. 4b). These layers are believed to play a minor role in the protection of TIC/CC but might, however, act as a solid lubricant at high sliding speeds, thus reducing the friction in the pin/countermaterial contact zone. In the case of TiN-coated HSS, the effect of the
silica-based layer can be twofold; firstly, the layers may act as a solid lubricant, the same as for TIC/CC, and reduce the temperature in the contact zone and thereby delay the onset of substrate thermal softening (and thus also delay the onset of the crack formation and plucking coating removal mechanism). Secondly, the silica-based layer may act as a binder, holding cracked TiN fragments together. The cohesive properties of coatings as well as the adhesion to the substrate can be evaluated using an austenitic stainless steel countermaterial. Since this material does not contain any hard constituents, the abrasive wear will be suppressed. Austenitic stainless steel generally has a lower thermal conductivity, higher ability to strain harden a thinner surface oxide layer and a greater fracture toughness than carbon steels, properties which all are known to aggravate adhesive wear in sliding contacts. Hence, it is possible to control the proportions of abrasive and adhesive wear through the amount of hard constituents in the countermaterial. It is concluded that a general evaluation of a coated system aimed at metal cutting should be performed by sliding against three types of steel: - soft annealed tool steel (abrasion is enhanced); - stainless steel (adhesion and cohesion is enhanced);
S. Hogmark,
P. Hedenqvist
151
/ Wear 179 (1994) 147-154 Specimen
Raptdly
Fig. 5. Schematic view of the centrifugal erosion tester. The eroding particles are fed into the centre of the rapidly rotating disc. They are then accelerated by the centrifugal force through four radial channels and eventually hit the specimens.
a 400
Fig. 4. Glassy layers. (a) Alumina layer on TiCKC. layer on TiNiHSS.
I ! 3500
I 1
I 1
’
f
I
(b) Silica-based Tii
CrN
(TiAI)N
Ti(C.N)
quenched and tempered carbon steel (a combination of abrasion and adhesion).
__._._._ ._.._.___; /~..~..~ 1:I$
3.2. Erosive wear Solid particle erosion is a fairly common method for wear testing of coatings [23-261. The main advantages of this method are: (i) it is highly reproducible; (ii) it yields relatively quick results; and (iii) it is a statistical method in the sense that a relatively large area is tested, which minimizes the influence from local defects on the mean performance of the coated specimens. The centrifugal erosion tester offers the possibilities to test a relatively large number of samples under identical experimental conditions (particle dose, velocity and angle of impingement) (see Fig. 5). Mass loss measurement is the standard technique to determine erosion rates. In many cases, this technique cannot be used for thin coatings since the mass loss corresponding to complete coating removal is often too small to be accurately measured. An additional problem arises if the particle dose varies over the eroded area, in which case simultaneous erosion of the coating and the substrate occurs as soon as the coating has been worn through. To overcome these problems, several new experimental techniques have been introduced [23,24,26]. These are all based on EDS measurements or image analysis techniques, which makes it possible to restrict the analysis to small surface elements on
c
0:
0
/
:
I I
2
:
1
4 Coating
j. . . .. . .. . .. . .
i
I I
I I
I I
6 8 10 thickness [pm]
1 12
Fig. 6. (a) Critical particle doses (i.e. the particle doses corresponding to complete coating removal on the entire tested area (= 64 mm*); the higher the critical particle dose, the better the erosion resistance) obtained in erosive testing of some different PVD coatings on high speed steel. From 1261. (b) Variation in coating erosion rate as a function of coating thickness for CVD TiC and A1203 on CC. From
v41.
which the particle dose can be considered as being constant. The coating erosion resistance tends to increase with coating (or composite) hardness and thickness (see Fig. 6). Four main erosion mechanisms can be observed for today’s PVD coatings. One is based on the fact that some of the impinging particles will cause lips of coating material to be foned during impact. Subsequent particle impacts can then easily remove the lips. Secondly, depending on the particle impact angle, the impinging
particles might act as single pass cutting edges; each impact removes an infinitely small amount of coating material. Further, a large number of particle impacts on the surface will result in severe deformation and plastic straining of the surface material. In particular, nucleation and propagation of fatigue induced subsurface lateral cracks will occur in the relatively brittle coating. As more and more particles hit the surface, adjacent crack systems will interact and, eventually, coating fragments will be detached (see Fig. 7a). Finally, when the coating thickness has been sufficiently decreased, extensive plastic deformation of the substrate material will cause formation of vertical cracks in the coating. Subsequent particle impacts will then result in severe coating fragmentation and removal. The severity of this mechanism tend to increase with decreasing substrate hardness and coating thickness. Single particle impact studies on polished CVD coated specimens show that the impinging particles cause crack formation, originating from the point of impact, in both TIC and Al,O, coatings. Continued erosion results in overlapping impacts and, in the case of the TiC coatings, plastic straining of the surface material due to the deformation caused by the impinging particles. When the fracture strain of the coating material is exceeded, extensive microchipping occurs and individual crystallites of TiC can be observed on the eroded surface [24], i.e. erosion predominantly occurs by an inter-
crystalline fracture mechanism resulting in detachment of individual grains at impact. The erosion of A&O, coatings is to a large extent governed by chipping by transcrystalline fracture, as proved by the subsurface cracks observed in crosssections (Fig. 7b). The size of the individual chippings is larger than for Tic, which results in a higher erosion rate. For multi-layered Al,O,/TiC coatings on CC, interfacial spalling in the A&O,--TIC interface is the dominant erosion mechanism. 3.3. Microabrasion The abrasive wear resistance of a coated part can be determined by subjecting it to virtually any kind of grinding/polishing treatment or by using the sliding wear test described in Section 3.1 with a carbide-rich workpiece material. However, these types of tests are not straightforward to use for determination of the (intrinsic) abrasive wear resistance of a given coating material, since the substrate material always influences the results, i.e. one obtains the abrasive wear resistance of the coating/substrate composite, which, of course, depends on coating thickness, coating hardness, substrate hardness, etc. Fortunately, a microabrasive wear test that is capable of distinguishing between the contributions from coating and substrate, respectively, has recently been developed [27]. The test utilizes a commercial dimple grinder (Fig. S), normally used for preparation of TEM specimens. At regular intervals, the test is interrupted and the crater diameters are measured using an optical microscope; the measurements are subsequently used for
‘0. Fig. 7. (a) PVD TiN coating failure by initiation and propagation of lateral cracks. (b) Cross-section of an eroded CVD-A1203 coating. Note the crack (at A).
Specimen platform (magnetic)
Fig. 8. Schematic view of the dimple grinder. The grinding shaped as a disc cut out from the centre of a sphere, rotates a horizontal axis. The specimen is mounted horizontally in subsequently filled with abrasive slurry. During testing, the grinds (at a load of 20 g) the specimen which in turn rotates a vertical axis. The combined motions resufts in a crater spherical cap-shape being ground into the specimen surface.
wheel, about a cup wheel about with a
S. Hogmark,
P. Hedenqvist
a
/ Wear 179 (1994) 147-154
153
5. The tribology profile The three types of tribo tests described in this paper, sliding wear, particle erosion and microabrasion, each generate characteristic data of the tribological properties of coated materials. Together with the coating thickness, residual stress state, structure and chemical composition, these data can be regarded to constitute a tribology profile. Given the tribology profile of a number of coating/substrate combinations, it is possible to select and to predict the performance of the best combination for any given application. In spite of the above statements, the general recommendation to the design engineer who wants to use a coated component is to perform a field test or a test which simulates the desired application as closely as possible before making the final choice. 6. Tomorrow’s coatings
$ 100
3
0
Fig. 9. (a) Representative V, versus (SL - VJK~) plot obtained from microabrasion tests of TiN on HSS. (From [S]). (b) Abrasive wear constants for some PVD coatings on HSS, superlattice (Ti, Nb)N/ HSS, Tic/CC and hot flame deposited diamond on CC. From [11,28, 291.
calculation of the coating and substrate wear volumes. These parameters can then, in turn, be inserted in the expression (derived from the original model in [27]): f&r=
3 + !L
(2) \ I
KC
4
where S is the sliding distance, L is the applied load and V, and V, are the wear volumes of coating and substrate, respectively. & and 4 are the wear constants of coating and substrate. The wear constant of a material is a measure of its wear rate, i.e. a high wear rate corresponds to a high wear constant. Assuming that K~ is known (it can be determined in advance on uncoated specimens), the wear constant of the coating is easily obtained by plotting V, versus (SL -VJr$) (see Fig. 9a). The intrinsic microabrasive resistance of coatings usually increases with coating hardness (Fig. 9b). The microabrasion test not only supplies information on the coating resistance to abrasive wear, it also yields much more information of the investigated composite; any possible multilayered structure as well as pores and/ or inclusions (secondary phases) in the coating is easily detected. Insufficient coating adhesion and cohesion will be detected and the coating thickness can be determined from the experimental results as a “bonus” (cf. ball grinding).
The number of different PVD and CVD coatings commercially available is high and rapidly growing. As already indicated, we can expect e.g. multilayered or superlattice coatings or coatings of new materials like C,N,, to appear on the market soon. We will also have to face the fact that the matter of choosing the optimum coating system for a given application will be increasingly difficult since each coated system has its own tribology profile. Thus, it will be necessary to obtain tribology profiles along the guidelines given in this paper to meet the needs of design engineers. Today, it is possible to manufacture coatings with hardness sufficient for almost any application and adhesion is usually not a problem. Instead, the main limiting factor is the coating ductility. A good test for ductility measurements of coated materials is still needed. The scratch test and/or indentations will give qualitative information on the ductility of coating with relatively good adherence, but the scatter is too high. The next generation of coatings will probably have a more active tribological role than just passive reduction of friction and increase of wear resistance. More emphasis will certainly be put on the “third body layer” which develops by a combination of mechanical and chemical reactions between surfaces in sliding contact. A further improvement of the tribological properties of coatings can possibly be offered by the introduction of constituents which control the formation of a beneficial tribo film. Very hard coatings on relatively ductile materials, i.e. diamond coatings on CC or TiC coatings on HSS, require a gradual reduction in coating hardness (and corresponding increase in ductility) towards the interface in order not to fail by brittle fracture. The development of coatings with ductility gradients but maintained heat resistance is another future challenge in coating development.
154
S. Hogmark,
P Hedenqvist
7. Conclusions The main conclusion from this survey of methods used for mechanical and tribological characterization of thin, hard coatings are: - As each application emphasises a particular set of merits, the tribological performance of a coating cannot be predicted by one single parameter. - A general tribological characterization of a coated component should encompass several reproducible lab tests, e.g. sliding wear, erosion, microabrasion, in order to obtain a “tribology profile”. - Given the tribology profile of a number of coating/ substrate combinations, it is possible to select and to predict the performance of the best combination for any given application. - The ideal coating should be: (a) well adhering; (b) sufficiently hard to resist abrasion; (c) sufficiently ductile to avoid flaking; (d) chemically resistant; (e) active in tribo film formation; (f) adjusted gradually to the substrate.
Acknowledgements Dr Erich Bermann (Balzers AG), Dr Leif Westin (Erasteel Kloster AB) and Mr Stig Pettersson (Uddeholm Tooling AB) are recognized for providing a neverceasing flow of coatings and substrates. The authors gratefully acknowledge the financial support from the National Swedish Board for Technical and Industrial Development (NUTEK).
References VI
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