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Failure mode analysis of TiN-coated high speed steel: In situ scratch adhesion testing in the scanning electron microscope
Surface and Coatings Technology, 41(1990) 31
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FAILURE MODE ANALYSIS OF TiN-COATED HIGH SPEED STEEL:
IN SITU SCRATCH ADHESION TESTING IN THE SCANNING ELECTRON MICROSCOPE PER HEDENQVIST, MIKAEL OLSSON and STAFFAN JACOBSON Uppsala University, School of Engineering, P.O. Box 534, 5-751 21 Uppsala (Sweden) STAFFAN SODERBERG AB Sandvik Coromant, P.O. Box 42056, S-126 12 Stockholm (Sweden) (Received April 1, 1989)
Summary A scratch test apparatus for in situ testing of coating adhesion in the scanning electron microscope has been designed. Scratch tests were performed on TiN-coated high speed steel substrates with various coating thicknesses and substrate hardnesses. Four groups of coating damage and detachment mechanisms were identified: deformation, crack formation, chip formation and flaking. In all cases, incipient coating failure was associated with a sharp increase in the friction force readings. The maximum normal force that the coating—substrate composite could sustain increased significantly with increasing coating thickness and substrate hardness. An interesting observation was that direct adhesive failure by interfacial fracture only occurred for the combination of a thick (3 pm) coating on a hard (1000 HV) substrate. In all other cases, coating failure was due to a ductile chip formation mechanism. The detailed influence of the coating thickness and substrate hardness on the resulting coating failure modes is illustrated in the paper by the introduction of coating failure maps. The results demonstrate the great potential for in situ studies in order to obtain a better understanding of the basic coating failure mechanisms. In situ scratching allows the dynamics of the process to be studied; the coating damage and detachment mechanisms in front of the tip can be identified and the scratching events can be directly correlated to the corresponding friction force characteristics. Unfortunately, direct extrapolation of results from the in situ test to the conventional scratch test is difficult owing to the smaller tip radius (25 instead of 200 pm) used in the in situ experiments.
1. Introduction Ceramic coatings of high hardness and excellent wear resistance can be produced by chemical vapour deposition (CVD) and physical vapour deposi0257-8972/90/$3.50
© Elsevier Sequoia/Printed in The Netherlands
32
tion (PVD). Currently, such coatings are successfully used for wear protection in various engineering applications. However, if the adhesion of the coating to the substrate is insufficient, premature failure of the coated part may occur owing to coating detachment by interfacial fracture. The evaluation of coating adhesion is a key issue in the surface coating community and several review papers on adhesion testing have been published recently [1 31. Numerous techniques for inducing adhesive failure of a coating! substrate composite have been proposed, e.g. scratching [4 6], indentation [7 91, particle erosion [10, 11] and laser shock-wave exposure [12]. Of these methods, the scratch test is the most widely used and commercial scratch testers are now available [13, 141. In the scratch test, a diamond stylus is drawn over the coated surface. The applied normal force FN is increased, stepwise or continuously, and the critical normal force FN, c at which adhesive failure is first detected is used as a measure of adhesion. It has been suggested that the FN ~ transition is directly associated with the sudden increase in the friction force and acoustic emission readings that is often observed in the scratch test [13, 14]. However, neither of these two methods have proven to be completely reliable and, therefore, a post-test examination of the scratch should also always be performed. Further, great care must be taken in the interpretation of scratch test data since the critical force value cannot be directly related to the strength of the coating—substrate interface. For example, coating thickness, substrate hardness and surface roughness are parameters that are known to strongly affect the FNC readings [13-15]. In addition, the scratching parameters used, such as stylus radius, scratching speed and loading rate, as well as the gradual wear of the diamond stylus have a significant influence On FN, c [3, 161. Hence, it is not surprising that the correlation between scratch test data and actual coating performance in practical applications is generally poor. In spite of these drawbacks, the scratch test is still widely used in coating research laboratories because of its simple and fast operation. In order to obtain a better understanding of how the FN c readings from the scratch test are related to coating adhesion in a practical application, a better theoretical understanding of the scratch process is required. Benjamin and Weaver [4] attempted in their original paper on the scratch test to link the FN, c value to a critical interfacial shear stress using plasticity theory. However, their model was not able to explain the experimental data obtained, and later Weaver [17] suggested that an elastic—plastic approach was required. Recently, Burnett and Rickerby [18, 19] have discussed the mechanics of the scratch test by extending earlier models for elastic—plastic indentation to scratching, an approach which appears to have great potential. The scratch test has also been modelled by Laugier [20, 21] under the assumption that the scratching process can be considered as a fully elastic event. Further development in the theoretical modelling of the scratch test will require a solid understanding of the detailed mechanisms of coating detachment. The current information about coating detachment -
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mechanisms is mainly based on post-test metallographic examination of scratched specimens [15, 18, 22]. In this work, a new test equipment for in situ scratching in a scanning electron microscope is presented. This equipment offers a unique possibility for detailed studies of coating detachment mechanisms during scratching. The potential of the in situ scratch technique is demonstrated by a study of the effects of coating thickness and substrate hardness on the critical force values recorded with TiN-coated high speed steels.
2. Experimental procedure 2.1. Materials ASP 30 high speed steel (chemical composition: 1.30 wt% C, 4.0 wt% Cr, 5.0 wt% Mo, 6.1 wt% W, 3.1 wt% V and 8.1 wt% Co) blanks (25 mm X 10 mm X 3 mm), were used as substrates in all experiments. The ASP 30 blanks were heat treated to three different hardnesses, namely 290 HV, 565 HV and 1000 HV, corresponding to soft-annealed, quenched and overtempered, and quenched and tempered conditions respectively. After the heat treatment, the oxide scale was removed and the blanks were polished with 3 pm diamond paste in the last step. The blanks were subsequently coated with TiN by magnetron sputtering. Coatings of three different thicknesses, 0.4, 1.3 and 3.0 pm, were obtained by variation in the deposition time. 2.2. Test equipment and procedure The scratch test equipment was designed to fit inside a JEOL 25 S scanning electron microscope. This was accomplished by replacing the standard goniometer stage with a modified scratch stage (see Fig. 1). The diamond tip is held stationary at a point corresponding to the centre of the image screen while the specimen is translated manually or by using a motor
(a)
(h)
Fig. 1. The in situ scratch adhesion test stage: (a) overview and (b) detail, showing the diamond stylus (at A) and the mounted specimen (at B).
34
control. For optimum contrast conditions, the specimen stage is tilted 45~ with respect to the incident electron beam. A tip radius of only 25 pm was selected in order to allow the scratching event to be monitored in situ. The normal force (maximum 10 N) is applied by an adjustable spring and both the normal and friction forces are measured using strain gauges. In the present experiments, a scratching speed of 50 pm s was used. Each scratch (length, 15 mm) was made with a constant normal force. The normal force was then increased with 0.1 N for each new scratch until the critical normal force corresponding to coating failure was well exceeded. During scratching, the resulting frictional force was recorded and the various coating damage mechanisms were carefully analysed as they appeared. The scratch specimens were also subjected to a detailed metallographic examination by scanning electron microscopy (SEM) and light optical microscopy (LOM) after the test had been completed.
3. Evaluation of scratch test data 3.1. Critical normal force
The normal force value FN, c, corresponding to incipient coating removal, was estimated using two different techniques. (1) The normal force FN, c~at which a transition from low to high friction force (FF) values is first observed (see Fig. 2(a)). (2) The normal force FN C2 at which the underlying substrate material is first exposed during scratching. In the scanning electron microscope this transition could readily be identified from the atomic number contrast in the backscattered electron mode. A strong correlation between the two different critical force values was observed. Both the FN c~ and FN C2 transitions were found to be well defined experimentally. However, the critical force value displayed irregular
C
(a)
~
1
4
Normal force [N]
(b)
Scratching direction
—~
Fig. 2. (a) Friction force vs applied normal force for a substrate hardness of 560 HV and a coating thickness of 1.3 pm. The two sets of data points corresponds to the two different scratch topographies in (b). (b) Resulting scratch topography when the stylus was seen to jump from coating penetration (at A) to sliding on top of the coating (at B).
35
variations across the specimen surface. For a constant normal force, the scratching action of the diamond stylus was sometimes seen to shift between sliding on top of the coating and complete coating penetration. The SEM micrograph shown in Fig. 2(b) illustrates how the appearance of the scratch changes drastically when this occurs. These sudden shifts in the scratching process were always accompanied by a corresponding shift in the friction force readings. Consequently, two sets of friction force values are included in Fig. 2(a) for normal forces above FN. c~.The local variations in the critical force values across the surface may in part be due to experimental scatter. However, local variations in the quality of the interface, i.e. in the FN ~ value, will also give rise to this effect. One implication of this observation is that the results from tests with stepwise increasing normal force should be statistically more significant than those obtained with a continuously increasing normal force. The influence of substrate hardness and coating thickness on the critical normal force readings FN, ci and FN, c2 is presented in Fig. 3. The two graphs clearly illustrate the good agreement between the two critical force parameters. In the following, only the critical force FN Cl will be used in the evaluation of the experimental data. The simplified notation FN, c will be used for this parameter.
—.
Q
________________
0
200
400
600 800
________________
1000 1200
C..)
Substrate Hardness [HV] (a) Fig. 3. Critical normal forces (a) FN
0
200
400
600
800 1000 1200
Substrate Hardness [HV] (b)
ci and (b) FN. c~vs.
substrate hardness.
The dependence of FN, c on substrate hardness and coating thickness depicted in Fig. 2(a) is in good agreement with earlier work using a standard type scratch test [2, 3, 13]. This is illustrated by the data in Fig. 4, which were obtained by Steinmann and Hintermann [13] for CVD TiC coatings on steels of different hardness. The difference in the magnitude of the critical force values is mainly an effect of the different tip radii in the in situ experiments (R = 25 pm) as compared with the standard test (R = 200 pm). Scratch tests performed outside the scanning electron microscope showed that the vacuum ambient in the microscope (about io~Torr) did not influence the scratch process.
36
Z
SO D
:
40
.—T
~~i2
30 20 ~10,
~ 0 0
200
400
600
800
1000
Substrate Hardness [HV] Fig. 4. Critical normal force vs. substrate hardness (from ref. 13). tO
0,4
—
~
0,8
1m~
:~ o C)
A
0,3
06
~
*
Z
u_ V
‘
Co4
.2’
(a)
t—041~x~~
a t=l3
s
0
1
~~~FNC
2
V
3
Normal force [N]
4
(b)
0,2
: ~ 0
200
400
600
800 1000 1200
Substrate Hardness [HV]
Fig. 5. (a) Friction coefficient p vs. normal force for a substrate hardness of 560 HV and a coating thickness of 1.3 pm. The two sets of data points corresponds to the two different scratch topographies in Fig. 2(b). (b) dp/dFN i’s. substrate hardness for all investigated specimens.
3.2. Friction coefficient The experimental curves of FF vs. FN show that the relationship between these two parameters is non-linear even below the critical normal force FN c; see Fig. 2(a). This implies that the friction coefficient p cannot be considered as a constant in the scratch test. If the experimental data are replotted as p vs. FN, an approximately linear relationship is obtained below FN, c, as illustrated by Fig. 5(a). When the different coating—substrate composites were compared, it was found that all lines originated at approximately p = 0.05 but that the slope dp/dFN varied significantly. It is believed that the value 0.05 corresponds to the adhesive component of the friction coefficient (which should be independent of the normal force) while dp/dFN can be considered as a characteristic parameter for the ploughing friction component. Figure 5(b) illustrates that the parameter dp/dFN decreases both with increasing substrate hardness and increasing coating thickness. 4. Coating damage and detachment mechanisms Classification of mechanisms A number of mechanisms for coating damage and detachment were identified, as listed in Table 1. Each mechanism is described by three key 4.1.
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TABLE 1
Identified mechanisms of coating damage and detachment
_______
L
~
L~I
Mechanism (key words)
Acronym
First observed
Mild plastic deformation
MPD
Pronounced plastic deformation
PPD
Stick—slip deformation
SSD
External parallel cracking
EPC
Internal transverse cracking
ITC
External transverse cracking
ETC
Coating debris removal
CDR
Discontinuous chip removal
DCR
~ F~ c
Continuous chip removal
CCR
~ FN, C
Splinter-like parallel flaking
SPF
a FN
Sideward lateral flaking
SLF
~‘
Forward lateral flaking
FLF
~ EN, C
c
C
c
EN, C
words in the table and an acronym is formed using the first letter in each word. The last letter in the acronym refers to the basic damage and detachment process involved, as defined below. D Deformation only (no surface material detached). C Crack formation only (no surface material detached).
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R Remova] of surface material by chip formation. F Flaking (interfacial or cohesive) by brittle fracture. Table 1 also lists when each mechanism is first observed, below or above the critical normal force FN, c. Further, a schematic illustration of each mechanism is included. A detailed description of the different mechanisms is given in the following section. The influence of the properties of the coating—substrate composite on the relative importance of these mechanisms is then discussed. 4.2. Scratch mechanisms below the critical normal force 4.2.1. Deformation For normal force values close to zero, the diamond tip slides on the coating surface without producing any visible surface damage. This corresponds to elastic deformation of the coating—substrate composite only. With increasing normal force, a remaining depression is formed behind the diamond tip by a mild plastic deformation (MPD) mechanism (see Fig. 6(a)). LOM cross-sections (see Fig. 6(b)) reveal that the plastic deformation extends through the coating into the substrate. The fact that surface irregularities on the as-deposited coating surface has become flattened out in the scratch bottom shows that also the nominally brittle coating is plastically deformed.
110 torn
(a)
Scratching direction
—~
(h)
Scratching direction
Fig. 6. (a) The MPD mechanism as observed in the scanning electron microscope; (b) LOM cross-section of the scratch in (a).
The degree of plastic deformation increases with increasing normal force. A transition to a pronounced plastic deformation (PPD) mechanism occurs when significant ridge formation becomes visible along the sides of the scratch (see Fig. 7(a)). The transition from the MPD to the PPD mechanism is not abrupt and is therefore somewhat subjective. However, the onset of PPD can be defined as the normal force at which scratch formation is primarily governed by the plastic deformation of the substrate. With increasing normal force, a wave-like pattern may also appear in the bottom
39
lOjirn II (a)
10 torn .1
(b)
Scratching direction
Scratching direction —a’
Fig. 7. (a) Ridge formation characteristic of the PPD mechanism; (b) wave-like pattern in the bottom of a scratch characteristic of the SSD mechanism.
of the scratch (see Fig. 7(b)). This pattern suggests the onset of a stick—slip deformation (SSD) mechanism and is interpreted as an effect of a cyclic plastic build-up and shearing of material at the leading edge of the tip due to the compressive stresses generated during the stick period. This process appears to be fully plastic since no cracking of the coating is observed. The frequency of the stick—slip motion can be estimated to be 15 Hz from the observed wavelength and the known tip velocity. This frequency is too high to be directly seen in the scanning electron microscope. 4.2.2. Crack formation Transverse (with respect to the scratching direction) surface cracks were often observed during scratching. Tensile stresses are generated behind the moving tip, resulting in internal transverse cracking (ITC) in the scratch bottom. Ideally, these cracks should have a semicircular geometry but, in practice, more complex crack patterns were often observed (see Fig. 8(a)). The number of ITC cracks increases with increasing normal force. At higher normal force values, external transverse cracking (ETC) may also appear
10pm
10pm
II
(a)
Scratching direction
—~
(b)
Scratching direction
Fig. 8. (a) ITC at A and ETC at B; (b) EPC along the sides of a scratch.
—~
40
(see Fig. 8(a)). The ETC cracks are significantly fewer than the ITC cracks but generally larger in size. The ridge formation that occurs along the sides of the scratch as a result of the PPD mechanism will generate high tensile stresses in the coating in a direction perpendicular to the scratch. This is due to the higher hardness and higher modulus of elasticity of TiN than that of the substrate. This bending stress in the coating leads to the formation of cracks that run along the sides of the scratch (see Fig. 8(b)). The corresponding damage mechanism is referred to as external parallel cracking (EPC). 4.2.3. Coating debris removal The plastic deformation of the coating associated with the PPD mechanism may eventually cause the fracture strain of the TiN coating to be exceeded. This results in cohesive fracture of the TiN along the sides and in front of the tip. Figure 9 shows that this type of cohesive fracture results in detachment of minute coating debris by a coating debris removal (CDR) mechanism. Since the fracture process is restricted to the coating itself the substrate is not exposed, i.e. FN
10pm II Scratching direction
——“>
Fig. 9. CDR in front of the moving stylus.
4.3. Scratch mechanisms above the critical normal force 4.3.1. Removal by chip formation Figure 10(a) shows how the substrate material is being exposed in the scratch bottom by a discontinuous chip removal (DCR) mechanism. This mechanism can be regarded as the natural extension of the CDR mechanism to higher normal forces. The transition from CDR to DCR is experimentally well defined. The detached debris is significantly larger under DCR conditions, and the scratch bottom exhibits a considerably rougher surface with adhesive scale formation (see Fig. 10(b)). In addition, the onset of DCR is always accompanied by a marked increase in frictional force. Therefore,
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10pm II (a)
Scratching direction
10pm II (b)
—~
Scratching direction
—~
Fig. 10. (a) The DCR mechanism; (b) resulting scratch bottom.
DCR is directly associated with normal forces above the critical force transition. However, the onset of DCR does not directly imply the occurrence of interfacial fracture. Examination of the detached chips shows that many consist of heavily deformed coating—substrate composite material. Apparently, the interface is sometimes able to accommodate large plastic strains without interfacial fracture. Substrate exposure caused by a continuous chip removal (CCR) mechanism was also observed for some coating—substrate combinations. Figure 11(a) illustrates the surprisingly ductile behaviour of the coating— substrate composite at CCR conditions. No signs whatsoever of interfacial fracture are seen in this case. CCR may be initiated directly from the PPD mechanism or via CDR—DCR. A built-up edge composed of deformed substrate material frequently forms at the tip (see Fig. 11(b)). This may significantly change the scratching geometry. 4.3.2. Flaking Three mechanisms of flaking were observed in the in situ experiments. In common for the different flaking mechanisms is that material detachment
L4
___
i~
,~.
2Opm (a)
Scratching direction
Fig. 11. (a) CCR stylus.
—~
10pm I—’ (b)
Scratching direction
—~
in front of the diamond tip; (b) built-up-edge formation in front of the
42
(a)
Scratching direction
~
(b)
Scratching direction
Fig. 12. (a) Example of the SPF mechanism as observed in the scanning electron microscope; (b) SPF resulting in almost complete removal of the coating on both sides of the scratch.
Scratching direction
—>
Fig. 13. Flakings, characteristic of the SLF mechanism.
predominantly occurs outside the scratch. The onset of flaking always results in a transition to high friction force values. Figure 12(a) shows an example of the splinter-like parallel flaking (SPF) mechanism. This type of flaking is caused by fracture between two neighbouring EPC cracks. Figure 12(b) demonstrates that the SPF mechanism can result in almost complete removal of the coating along the sides of the scratch while the scratch bottom is still covered by coating material. Fracture may occur along the interface or within the coating itself (cohesive failure). A second flaking mechanism, sideward lateral flaking (SLF), is shown in Fig. 13. This type of flaking is very similar to the lateral spalling damage that is observed for scratching of bulk ceramics. In bulk materials, lateral crack formation and spalling is associated with the stresses generated at the elastic—plastic zone boundary upon unloading, i.e. when the diamond tip has passed over a surface element. It is believed that the SLF mechanism has the same origin although the existence of EPC and ETC cracks probably
43
5pm II (a)
Scratching direction
~
25pm II
(b)
Scratching direction
—~
Fig. 14. (a) FLF in front of the moving diamond tip; (b) resulting scratch topography at FLF conditions.
accelerates the process. The SLF mechanism can lead to both cohesive and interfacial fracture depending on the properties of the coating/substrate composite. The SPF and SLF flaking mechanisms are both characterized by sideward flaking in relation to the diamond tip. Figure 14(a) shows a different situation in which flaking is taking place in front of the tip by a forward lateral flaking (FLF) mechanism. This mechanism is believed to be caused by the build-up of compressive stresses in front of the tip during scratching. This may cause the coating to buckle, resulting in the removal of semicircular flakes. In the present experiments, this mechanism was always associated with interfacial spalling. From Fig. 14(b), which shows the resulting scratch at FLF conditions, it can be seen that the SLF and FLF mechanisms may be difficult to distinguish if the scratch process is not monitored in situ.
5. The influence of coating—substrate properties on the scratching process The coating thickness and substrate hardness not only influence the critical normal force value, but also have a strong effect on the occurrence and relative importance of the various coating damage mechanisms. In all, twelve different mechanisms of coating damage and detachment were identified in the in situ experiments. Of these mechanisms, only three were found to be directly associated with the critical force transition, namely CCR, DCR and FLF. In addition to the three critical force transition mechanisms, SPF and SLF were often observed at and above the critical force transition. However, these two mechanisms appeared erratically and were therefore considered as secondary failure mechanisms. In Fig. 15(a) a coating failure (CF) map based on the observed primary failure mechanism for each tested coating/substrate combination is
44 4
‘—‘4
E
a
:1(“3
~3
\
\
\
.2
‘\
\
2
\
SLF
\
.9
2
I ladhesive &
DCR
SPF
_
1
IcoCesee)
No Ftaking
00 0
200
400
600
SPF& SLF I
Icoheeve) — — —
.~
CCR
1
I
w
FLF
Oo (a)
b
800 1000 1200
0
(b)
Substrate Hardness [HV]
200
400
600
800 1000 1200
Substrate Hardness [HV]
Fig. 15. Schematic CF maps indicating the observed (a) primary and (b) secondary failure mechanisms for each coating—substrate combination tested.
presented. A second CF map is given in Fig. 15(b) which in a similar way shows the dominant secondary failure mechanisms. In the latter case, a distinction is made between flaking by predominantly interfacial or cohesive fracture. The primary CF map in Fig. 15(a) demonstrates that coating failure occurs by a chip formation mechanism in all but one case in the present experiments. This result is surprising since chip formation in general was not found to be associated with interfacial fracture. Apparently, a thin, brittle coating on a steel substrate can exhibit a quite ductile behaviour during scratching. The ductility of the coating—substrate composite increases with decreasing coating thickness and substrate hardness. The only material combination for which the critical force transition was found to be directly related to adhesive failure of the coating/substrate interface was in the case of the thickest coating on the hardest substrate. This was also the material combination that exhibited the highest critical force value in the present tests. Flaking by one of the two secondary mechanisms, SPF and SLF, was observed for all coating—substrate composites except those with the thinnest coating (see Fig. 15(b)). However, interfacial flaking was only obtained for the coatings on the hardest substrate. In the other cases, flaking was caused by cohesive failure within the coating itself. The frequency of the SPF and SLF mechanisms was found to increase with increasing coating thickness and decreasing substrate hardness. The coating thickness and substrate hardness also have a strong influence on the relative importance of the coating damage mechanisms below the critical force transition, i.e. plastic deformation and crack formation. However, the relationships appear to be fairly complex. At low normal force values, all coating—substrate composites exhibit the mild plastic deformation which, with increasing normal force, changes to the more pronounced plastic deformation mechanism with extensive ridge formation. At higher normal forces extensive cracking of the coating is observed during the scratching event. It was found that cracking generally appears in the order EPC ITC ETC. The only significant deviation from this -+
—~
45
pattern was observed for the thickest coating on the hardest substrate, i.e. for the hardest composite. In this case, the ITC cracks were formed before the EPC cracks. While ITC cracking was observed for all coating—substrate composites, the tendency for EPC and ETC cracking was found to increase with increasing coating thickness and decreasing substrate hardness and these two crack formation mechanisms were not observed for the specimens with the thinnest coating. This behaviour is similar to what was observed for the SPF and SLF flaking mechanisms which indicates that EPC and ETC cracks are precursors for SPF and SLF flaking respectively. The relationships between and the relative importance of the identified coating damage mechanisms appear to be fairly complex. However, three main routes (see Fig. 16) corresponding to low, medium and high composite hardness can be distinguished. Each route can be directly related to one of the final primary coating failure mechanisms. In the figure the various mechanisms are schematically shown in the order that they generally appear with increasing normal force.
~ —
—
V
-
ai~.
—
—-‘ -
____I, 1~
V
—
V
,_
,7
__,
~
b~.
17
7
~ Fig. 16. Schematic representation of the three main coating damage mechanism routes: (a) low, (b) medium and (c) high composite hardness.
6. Discussion
In situ observation of scratch formation in the scanning electron microscope offers a unique opportunity to study the dynamics of the various coating damage and detachment mechanisms that occur during scratching. In principle, the information obtained from this type of experiment should not be presented as individual SEM micrographs but as video recordings of
(a)
Scratching direction
—~.
(b)
Scratching direction
Sc at hing direction
‘—~
(d)
Scratching direction
—>
~rn20~ (c)
~
Fig. 17. Image sequence, illustrating the unique opportunity to observe the actual, dynamic scratch process when using the iii situ scratch equipment (CCR conditions).
the different scratch mechanisms. The image sequence in Fig. 17 is an attempt to illustrate the dynamics of the scratching process, in this case during continuous chip removal. Two additional advantages of the in situ test compared with conventional scratch testing are that the material deformation and detachment mechanisms in front of the moving tip can be studied and that the observed events on the image screen can be correlated to any recorded variations in friction force during the test. Studies of coating detachment mechanisms by post-test examination techniques are of necessity restricted to analysis of the areas outside the actual scratch since the moving tip will change the surface characteristics drastically as it passes over a surface element. 1-lowever, our in situ experiments show that the processes that occur in front of the tip are generally very different from the processes that take place along the sides of the scratch. This may be the reason why chip formation processes are rarely discussed in conjunction with the scratch test. However, the plastic behaviour of the nominally brittle coating is not so surprising considering the high hydrostatic stress state in front of the moving tip and that plastic behaviour of brittle materials is well known in tribological applications. From studies of machining and abrasion
47
it is known that chip formation occurs by cyclic shearing of material in the primary shear zone which extends from the cutting tip to the surface just in front of the chip. Each shearing event normally starts at the cutting tip and propagates outwards toward the surface. For a coating—substrate composite, it is of particular interest to see whether the shear front changes direction when it reaches the interface. If the shear front continues to propagate along the interface, it will soon result in interfacial spalling of the coating in front of the chip. However, our experiments show that this only occurs to a limited extent and only during DCR conditions. The fact that chip formation does not necessarily lead to interfacial fracture may suggest that this failure mode is of little interest with respect to adhesion testing. However, the various coating damage mechanisms are frequently interrelated. For example, chip formation also leads to ridge formation along the sides of the scratch. This may, in turn, produce parallel cracks and interfacial fracture by the SPF mechanism. Another example of the importance of in situ studies of scratch formation is that these allow us to directly distinguish between SLF and FLF mechanisms. This would be difficult by post-test examination only since these two mechanisms give rise to very similar damage patterns. In the present experiments, the relationships between the friction force readings and the events taking place on the image screen were not systematically investigated. This technique appears to have great potential and the incorporation of an acoustic emission detector in the in situ test is expected to give additional valuable information. One criticism that may be raised against the in situ test is that the small tip (R = 25 pm) used will give a more plastic response of the coating— substrate composite and will therefore give rise to different coating detachment mechanisms from those for the conventional (R 200 pm) test. It may be that the very ductile behaviour with continuous chip formation that was observed in the present experiments will not occur with a larger tip radius. For example, the more extreme cases of continuous chip formation were always associated with significant built-up-edge formation, a phenomenon that is less likely to occur with a larger tip radius. Hence, the effect of tip radius on the various scratch mechanisms is an area that should be investigated in more detail in future work. The existence of chip formation is an important issue since it will drastically affect the stress and deformation pattern around the tip, and it therefore needs to be incorporated in the theoretical models for scratching. Another result of the present work which is~of importance for theoretical work is that the friction coefficient p is not a constant in scratching but increases with increasing normal force. The variation in p becomes more pronounced using a smaller tip radius but the effect should be significant also for a 200 pm tip radius. (It should be noted that this effect refers to forces below FN, c~and is not related to the existence of chip formation above FN C.) For a given normal force, p decreases with both increasing substrate hardness and coating thickness, i.e. with increasing composite hardness. Lower p values mean lower shear stresses at the interface and,
48
consequently, the relationship depicted in Fig. 3 between FN and coating thickness and substrate hardness respectively, could partly be explained as a strictly frictional effect. However, we found no systematic variation in the critical friction force or its corresponding friction coefficient with coating thickness or substrate hardness. Another result of the present work that should be explored further is the significance of the observed local variations in FN, c values over the specimen surface. In principle, the critical normal force should not be represented by a single number but by a statistical distribution function. This becomes especially important for components in which only parts of the coated surface are subjected to high stresses, such as a cutting tool insert. In this case, the probability of coating failure could possibly be analysed using Weibull statistics as utilized in failure prediction of brittle bulk material. From the experimental work and the discussion above, the complexity of the scratch process should have become evident. It is believed that further insight on the coating damage and detachment mechanisms can be obtained from systematic work on different coating—substrate composites. In particular, the importance of plastic deformation processes as precursors to adhesive failure should be investigated, with special emphasis on the effect of tip radius on the scratching process. ~,
Acknowledgment The financial support of the National Swedish Board for Technical Development (STUF) is greatly acknowledged.
References 1 2 3 4 5
6 7 8
9 10 11 12 13 14
J. Valli, J. Vac. Sci. Technol. A, 4 (1986) 3007. J. Valli, U. Mäkelä and A. Matthews, Surf. Eng., 2 (1986) 49. D. S. Rickerby, Surf. Coat. Technol., 36 (1988) 541. P. Benjamin and C. Weaver, Proc. R. Soc. London, Ser. A, 254 (1960), 163. 0. Berendsohn, J. Testing Evaluation, 1, (1973), 139. A. J. Perry, Surf. Eng., 2 (1986), 183. T. Sumomagi, K. Kuwahara and H. Fujiyama, Thin Solid Films, 79 (1981) 91. S. S. Chiang, D. B. Marshall and A.-G. Evans, in J. Pask and A. Evans (eds.), Surfaces and Interfaces in Ceramic and Ceramic—Metal Systems, Materials Science Research, Vol. 14, Plenum, New York, 1981, p. 603. P. C. Jindat, D. T. Quinto and G. J. Wolfe, Thin Solid Films, 154 (1987) 361. B. Jonsson, L. Akre, S. Johansson and S. Hogmark, Thin Solid Films, 137 (1986) 65. D. S. Rickerby and P. J. Burnett, Surf Coat. Technol., 33 (1987) 191. J. L. Vossen, in K. L. Mittal (ed), Adhesion Measurement of Thin Films, Thick Films and Bulk Coatings, ASTM Spec. Tech. Publ. 640, 1978, p. 122. P. A. Steinmann and H.-E. Hintermann, J. Vac. Sci. Technol. A, 3 (1985) 2394. J. Valli and U. Mãkelã, Wear, 115 (1987) 215.
49 15 A. J. Perry, Thin Solid Films, 107 (1983) 167. 16 P. A. Steinmann, Y. Tardy and H.-E. Hintermann, Thin Solid Films, 154 (1987) 333. 17 C. Weaver, J. Vac. Sci. Technol., 12 (1975) 18. 18 P. J. Burnett and D. S. Rickerby, Thin Solid Films, 154 (1987) 403. 19 P. J. Burnett and D. S. Rickerby, Thin Solid Films, 157 (1988) 233. 20 M. Laugier, Thin Solid Films, 76 (1981) 289. 21 M. T. Laugier, Thin Solid Films, 117 (1984) 243. 22 J. H. Je, E. Gyarmati and A. Naoumidis, Thin Solid Films, 136 (1986) 57.
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49
31
FAILURE MODE ANALYSIS OF TiN-COATED HIGH SPEED STEEL:
IN SITU SCRATCH ADHESION TESTING IN THE SCANNING ELECTRON MICROSCOPE PER HEDENQVIST, MIKAEL OLSSON and STAFFAN JACOBSON Uppsala University, School of Engineering, P.O. Box 534, 5-751 21 Uppsala (Sweden) STAFFAN SODERBERG AB Sandvik Coromant, P.O. Box 42056, S-126 12 Stockholm (Sweden) (Received April 1, 1989)
Summary A scratch test apparatus for in situ testing of coating adhesion in the scanning electron microscope has been designed. Scratch tests were performed on TiN-coated high speed steel substrates with various coating thicknesses and substrate hardnesses. Four groups of coating damage and detachment mechanisms were identified: deformation, crack formation, chip formation and flaking. In all cases, incipient coating failure was associated with a sharp increase in the friction force readings. The maximum normal force that the coating—substrate composite could sustain increased significantly with increasing coating thickness and substrate hardness. An interesting observation was that direct adhesive failure by interfacial fracture only occurred for the combination of a thick (3 pm) coating on a hard (1000 HV) substrate. In all other cases, coating failure was due to a ductile chip formation mechanism. The detailed influence of the coating thickness and substrate hardness on the resulting coating failure modes is illustrated in the paper by the introduction of coating failure maps. The results demonstrate the great potential for in situ studies in order to obtain a better understanding of the basic coating failure mechanisms. In situ scratching allows the dynamics of the process to be studied; the coating damage and detachment mechanisms in front of the tip can be identified and the scratching events can be directly correlated to the corresponding friction force characteristics. Unfortunately, direct extrapolation of results from the in situ test to the conventional scratch test is difficult owing to the smaller tip radius (25 instead of 200 pm) used in the in situ experiments.
1. Introduction Ceramic coatings of high hardness and excellent wear resistance can be produced by chemical vapour deposition (CVD) and physical vapour deposi0257-8972/90/$3.50
© Elsevier Sequoia/Printed in The Netherlands
32
tion (PVD). Currently, such coatings are successfully used for wear protection in various engineering applications. However, if the adhesion of the coating to the substrate is insufficient, premature failure of the coated part may occur owing to coating detachment by interfacial fracture. The evaluation of coating adhesion is a key issue in the surface coating community and several review papers on adhesion testing have been published recently [1 31. Numerous techniques for inducing adhesive failure of a coating! substrate composite have been proposed, e.g. scratching [4 6], indentation [7 91, particle erosion [10, 11] and laser shock-wave exposure [12]. Of these methods, the scratch test is the most widely used and commercial scratch testers are now available [13, 141. In the scratch test, a diamond stylus is drawn over the coated surface. The applied normal force FN is increased, stepwise or continuously, and the critical normal force FN, c at which adhesive failure is first detected is used as a measure of adhesion. It has been suggested that the FN ~ transition is directly associated with the sudden increase in the friction force and acoustic emission readings that is often observed in the scratch test [13, 14]. However, neither of these two methods have proven to be completely reliable and, therefore, a post-test examination of the scratch should also always be performed. Further, great care must be taken in the interpretation of scratch test data since the critical force value cannot be directly related to the strength of the coating—substrate interface. For example, coating thickness, substrate hardness and surface roughness are parameters that are known to strongly affect the FNC readings [13-15]. In addition, the scratching parameters used, such as stylus radius, scratching speed and loading rate, as well as the gradual wear of the diamond stylus have a significant influence On FN, c [3, 161. Hence, it is not surprising that the correlation between scratch test data and actual coating performance in practical applications is generally poor. In spite of these drawbacks, the scratch test is still widely used in coating research laboratories because of its simple and fast operation. In order to obtain a better understanding of how the FN c readings from the scratch test are related to coating adhesion in a practical application, a better theoretical understanding of the scratch process is required. Benjamin and Weaver [4] attempted in their original paper on the scratch test to link the FN, c value to a critical interfacial shear stress using plasticity theory. However, their model was not able to explain the experimental data obtained, and later Weaver [17] suggested that an elastic—plastic approach was required. Recently, Burnett and Rickerby [18, 19] have discussed the mechanics of the scratch test by extending earlier models for elastic—plastic indentation to scratching, an approach which appears to have great potential. The scratch test has also been modelled by Laugier [20, 21] under the assumption that the scratching process can be considered as a fully elastic event. Further development in the theoretical modelling of the scratch test will require a solid understanding of the detailed mechanisms of coating detachment. The current information about coating detachment -
-
-
33
mechanisms is mainly based on post-test metallographic examination of scratched specimens [15, 18, 22]. In this work, a new test equipment for in situ scratching in a scanning electron microscope is presented. This equipment offers a unique possibility for detailed studies of coating detachment mechanisms during scratching. The potential of the in situ scratch technique is demonstrated by a study of the effects of coating thickness and substrate hardness on the critical force values recorded with TiN-coated high speed steels.
2. Experimental procedure 2.1. Materials ASP 30 high speed steel (chemical composition: 1.30 wt% C, 4.0 wt% Cr, 5.0 wt% Mo, 6.1 wt% W, 3.1 wt% V and 8.1 wt% Co) blanks (25 mm X 10 mm X 3 mm), were used as substrates in all experiments. The ASP 30 blanks were heat treated to three different hardnesses, namely 290 HV, 565 HV and 1000 HV, corresponding to soft-annealed, quenched and overtempered, and quenched and tempered conditions respectively. After the heat treatment, the oxide scale was removed and the blanks were polished with 3 pm diamond paste in the last step. The blanks were subsequently coated with TiN by magnetron sputtering. Coatings of three different thicknesses, 0.4, 1.3 and 3.0 pm, were obtained by variation in the deposition time. 2.2. Test equipment and procedure The scratch test equipment was designed to fit inside a JEOL 25 S scanning electron microscope. This was accomplished by replacing the standard goniometer stage with a modified scratch stage (see Fig. 1). The diamond tip is held stationary at a point corresponding to the centre of the image screen while the specimen is translated manually or by using a motor
(a)
(h)
Fig. 1. The in situ scratch adhesion test stage: (a) overview and (b) detail, showing the diamond stylus (at A) and the mounted specimen (at B).
34
control. For optimum contrast conditions, the specimen stage is tilted 45~ with respect to the incident electron beam. A tip radius of only 25 pm was selected in order to allow the scratching event to be monitored in situ. The normal force (maximum 10 N) is applied by an adjustable spring and both the normal and friction forces are measured using strain gauges. In the present experiments, a scratching speed of 50 pm s was used. Each scratch (length, 15 mm) was made with a constant normal force. The normal force was then increased with 0.1 N for each new scratch until the critical normal force corresponding to coating failure was well exceeded. During scratching, the resulting frictional force was recorded and the various coating damage mechanisms were carefully analysed as they appeared. The scratch specimens were also subjected to a detailed metallographic examination by scanning electron microscopy (SEM) and light optical microscopy (LOM) after the test had been completed.
3. Evaluation of scratch test data 3.1. Critical normal force
The normal force value FN, c, corresponding to incipient coating removal, was estimated using two different techniques. (1) The normal force FN, c~at which a transition from low to high friction force (FF) values is first observed (see Fig. 2(a)). (2) The normal force FN C2 at which the underlying substrate material is first exposed during scratching. In the scanning electron microscope this transition could readily be identified from the atomic number contrast in the backscattered electron mode. A strong correlation between the two different critical force values was observed. Both the FN c~ and FN C2 transitions were found to be well defined experimentally. However, the critical force value displayed irregular
C
(a)
~
1
4
Normal force [N]
(b)
Scratching direction
—~
Fig. 2. (a) Friction force vs applied normal force for a substrate hardness of 560 HV and a coating thickness of 1.3 pm. The two sets of data points corresponds to the two different scratch topographies in (b). (b) Resulting scratch topography when the stylus was seen to jump from coating penetration (at A) to sliding on top of the coating (at B).
35
variations across the specimen surface. For a constant normal force, the scratching action of the diamond stylus was sometimes seen to shift between sliding on top of the coating and complete coating penetration. The SEM micrograph shown in Fig. 2(b) illustrates how the appearance of the scratch changes drastically when this occurs. These sudden shifts in the scratching process were always accompanied by a corresponding shift in the friction force readings. Consequently, two sets of friction force values are included in Fig. 2(a) for normal forces above FN. c~.The local variations in the critical force values across the surface may in part be due to experimental scatter. However, local variations in the quality of the interface, i.e. in the FN ~ value, will also give rise to this effect. One implication of this observation is that the results from tests with stepwise increasing normal force should be statistically more significant than those obtained with a continuously increasing normal force. The influence of substrate hardness and coating thickness on the critical normal force readings FN, ci and FN, c2 is presented in Fig. 3. The two graphs clearly illustrate the good agreement between the two critical force parameters. In the following, only the critical force FN Cl will be used in the evaluation of the experimental data. The simplified notation FN, c will be used for this parameter.
—.
Q
________________
0
200
400
600 800
________________
1000 1200
C..)
Substrate Hardness [HV] (a) Fig. 3. Critical normal forces (a) FN
0
200
400
600
800 1000 1200
Substrate Hardness [HV] (b)
ci and (b) FN. c~vs.
substrate hardness.
The dependence of FN, c on substrate hardness and coating thickness depicted in Fig. 2(a) is in good agreement with earlier work using a standard type scratch test [2, 3, 13]. This is illustrated by the data in Fig. 4, which were obtained by Steinmann and Hintermann [13] for CVD TiC coatings on steels of different hardness. The difference in the magnitude of the critical force values is mainly an effect of the different tip radii in the in situ experiments (R = 25 pm) as compared with the standard test (R = 200 pm). Scratch tests performed outside the scanning electron microscope showed that the vacuum ambient in the microscope (about io~Torr) did not influence the scratch process.
36
Z
SO D
:
40
.—T
~~i2
30 20 ~10,
~ 0 0
200
400
600
800
1000
Substrate Hardness [HV] Fig. 4. Critical normal force vs. substrate hardness (from ref. 13). tO
0,4
—
~
0,8
1m~
:~ o C)
A
0,3
06
~
*
Z
u_ V
‘
Co4
.2’
(a)
t—041~x~~
a t=l3
s
0
1
~~~FNC
2
V
3
Normal force [N]
4
(b)
0,2
: ~ 0
200
400
600
800 1000 1200
Substrate Hardness [HV]
Fig. 5. (a) Friction coefficient p vs. normal force for a substrate hardness of 560 HV and a coating thickness of 1.3 pm. The two sets of data points corresponds to the two different scratch topographies in Fig. 2(b). (b) dp/dFN i’s. substrate hardness for all investigated specimens.
3.2. Friction coefficient The experimental curves of FF vs. FN show that the relationship between these two parameters is non-linear even below the critical normal force FN c; see Fig. 2(a). This implies that the friction coefficient p cannot be considered as a constant in the scratch test. If the experimental data are replotted as p vs. FN, an approximately linear relationship is obtained below FN, c, as illustrated by Fig. 5(a). When the different coating—substrate composites were compared, it was found that all lines originated at approximately p = 0.05 but that the slope dp/dFN varied significantly. It is believed that the value 0.05 corresponds to the adhesive component of the friction coefficient (which should be independent of the normal force) while dp/dFN can be considered as a characteristic parameter for the ploughing friction component. Figure 5(b) illustrates that the parameter dp/dFN decreases both with increasing substrate hardness and increasing coating thickness. 4. Coating damage and detachment mechanisms Classification of mechanisms A number of mechanisms for coating damage and detachment were identified, as listed in Table 1. Each mechanism is described by three key 4.1.
37
TABLE 1
Identified mechanisms of coating damage and detachment
_______
L
~
L~I
Mechanism (key words)
Acronym
First observed
Mild plastic deformation
MPD
Pronounced plastic deformation
PPD
Stick—slip deformation
SSD
External parallel cracking
EPC
Internal transverse cracking
ITC
External transverse cracking
ETC
Coating debris removal
CDR
Discontinuous chip removal
DCR
~ F~ c
Continuous chip removal
CCR
~ FN, C
Splinter-like parallel flaking
SPF
a FN
Sideward lateral flaking
SLF
~‘
Forward lateral flaking
FLF
~ EN, C
c
C
c
EN, C
words in the table and an acronym is formed using the first letter in each word. The last letter in the acronym refers to the basic damage and detachment process involved, as defined below. D Deformation only (no surface material detached). C Crack formation only (no surface material detached).
38
R Remova] of surface material by chip formation. F Flaking (interfacial or cohesive) by brittle fracture. Table 1 also lists when each mechanism is first observed, below or above the critical normal force FN, c. Further, a schematic illustration of each mechanism is included. A detailed description of the different mechanisms is given in the following section. The influence of the properties of the coating—substrate composite on the relative importance of these mechanisms is then discussed. 4.2. Scratch mechanisms below the critical normal force 4.2.1. Deformation For normal force values close to zero, the diamond tip slides on the coating surface without producing any visible surface damage. This corresponds to elastic deformation of the coating—substrate composite only. With increasing normal force, a remaining depression is formed behind the diamond tip by a mild plastic deformation (MPD) mechanism (see Fig. 6(a)). LOM cross-sections (see Fig. 6(b)) reveal that the plastic deformation extends through the coating into the substrate. The fact that surface irregularities on the as-deposited coating surface has become flattened out in the scratch bottom shows that also the nominally brittle coating is plastically deformed.
110 torn
(a)
Scratching direction
—~
(h)
Scratching direction
Fig. 6. (a) The MPD mechanism as observed in the scanning electron microscope; (b) LOM cross-section of the scratch in (a).
The degree of plastic deformation increases with increasing normal force. A transition to a pronounced plastic deformation (PPD) mechanism occurs when significant ridge formation becomes visible along the sides of the scratch (see Fig. 7(a)). The transition from the MPD to the PPD mechanism is not abrupt and is therefore somewhat subjective. However, the onset of PPD can be defined as the normal force at which scratch formation is primarily governed by the plastic deformation of the substrate. With increasing normal force, a wave-like pattern may also appear in the bottom
39
lOjirn II (a)
10 torn .1
(b)
Scratching direction
Scratching direction —a’
Fig. 7. (a) Ridge formation characteristic of the PPD mechanism; (b) wave-like pattern in the bottom of a scratch characteristic of the SSD mechanism.
of the scratch (see Fig. 7(b)). This pattern suggests the onset of a stick—slip deformation (SSD) mechanism and is interpreted as an effect of a cyclic plastic build-up and shearing of material at the leading edge of the tip due to the compressive stresses generated during the stick period. This process appears to be fully plastic since no cracking of the coating is observed. The frequency of the stick—slip motion can be estimated to be 15 Hz from the observed wavelength and the known tip velocity. This frequency is too high to be directly seen in the scanning electron microscope. 4.2.2. Crack formation Transverse (with respect to the scratching direction) surface cracks were often observed during scratching. Tensile stresses are generated behind the moving tip, resulting in internal transverse cracking (ITC) in the scratch bottom. Ideally, these cracks should have a semicircular geometry but, in practice, more complex crack patterns were often observed (see Fig. 8(a)). The number of ITC cracks increases with increasing normal force. At higher normal force values, external transverse cracking (ETC) may also appear
10pm
10pm
II
(a)
Scratching direction
—~
(b)
Scratching direction
Fig. 8. (a) ITC at A and ETC at B; (b) EPC along the sides of a scratch.
—~
40
(see Fig. 8(a)). The ETC cracks are significantly fewer than the ITC cracks but generally larger in size. The ridge formation that occurs along the sides of the scratch as a result of the PPD mechanism will generate high tensile stresses in the coating in a direction perpendicular to the scratch. This is due to the higher hardness and higher modulus of elasticity of TiN than that of the substrate. This bending stress in the coating leads to the formation of cracks that run along the sides of the scratch (see Fig. 8(b)). The corresponding damage mechanism is referred to as external parallel cracking (EPC). 4.2.3. Coating debris removal The plastic deformation of the coating associated with the PPD mechanism may eventually cause the fracture strain of the TiN coating to be exceeded. This results in cohesive fracture of the TiN along the sides and in front of the tip. Figure 9 shows that this type of cohesive fracture results in detachment of minute coating debris by a coating debris removal (CDR) mechanism. Since the fracture process is restricted to the coating itself the substrate is not exposed, i.e. FN
10pm II Scratching direction
——“>
Fig. 9. CDR in front of the moving stylus.
4.3. Scratch mechanisms above the critical normal force 4.3.1. Removal by chip formation Figure 10(a) shows how the substrate material is being exposed in the scratch bottom by a discontinuous chip removal (DCR) mechanism. This mechanism can be regarded as the natural extension of the CDR mechanism to higher normal forces. The transition from CDR to DCR is experimentally well defined. The detached debris is significantly larger under DCR conditions, and the scratch bottom exhibits a considerably rougher surface with adhesive scale formation (see Fig. 10(b)). In addition, the onset of DCR is always accompanied by a marked increase in frictional force. Therefore,
41
10pm II (a)
Scratching direction
10pm II (b)
—~
Scratching direction
—~
Fig. 10. (a) The DCR mechanism; (b) resulting scratch bottom.
DCR is directly associated with normal forces above the critical force transition. However, the onset of DCR does not directly imply the occurrence of interfacial fracture. Examination of the detached chips shows that many consist of heavily deformed coating—substrate composite material. Apparently, the interface is sometimes able to accommodate large plastic strains without interfacial fracture. Substrate exposure caused by a continuous chip removal (CCR) mechanism was also observed for some coating—substrate combinations. Figure 11(a) illustrates the surprisingly ductile behaviour of the coating— substrate composite at CCR conditions. No signs whatsoever of interfacial fracture are seen in this case. CCR may be initiated directly from the PPD mechanism or via CDR—DCR. A built-up edge composed of deformed substrate material frequently forms at the tip (see Fig. 11(b)). This may significantly change the scratching geometry. 4.3.2. Flaking Three mechanisms of flaking were observed in the in situ experiments. In common for the different flaking mechanisms is that material detachment
L4
___
i~
,~.
2Opm (a)
Scratching direction
Fig. 11. (a) CCR stylus.
—~
10pm I—’ (b)
Scratching direction
—~
in front of the diamond tip; (b) built-up-edge formation in front of the
42
(a)
Scratching direction
~
(b)
Scratching direction
Fig. 12. (a) Example of the SPF mechanism as observed in the scanning electron microscope; (b) SPF resulting in almost complete removal of the coating on both sides of the scratch.
Scratching direction
—>
Fig. 13. Flakings, characteristic of the SLF mechanism.
predominantly occurs outside the scratch. The onset of flaking always results in a transition to high friction force values. Figure 12(a) shows an example of the splinter-like parallel flaking (SPF) mechanism. This type of flaking is caused by fracture between two neighbouring EPC cracks. Figure 12(b) demonstrates that the SPF mechanism can result in almost complete removal of the coating along the sides of the scratch while the scratch bottom is still covered by coating material. Fracture may occur along the interface or within the coating itself (cohesive failure). A second flaking mechanism, sideward lateral flaking (SLF), is shown in Fig. 13. This type of flaking is very similar to the lateral spalling damage that is observed for scratching of bulk ceramics. In bulk materials, lateral crack formation and spalling is associated with the stresses generated at the elastic—plastic zone boundary upon unloading, i.e. when the diamond tip has passed over a surface element. It is believed that the SLF mechanism has the same origin although the existence of EPC and ETC cracks probably
43
5pm II (a)
Scratching direction
~
25pm II
(b)
Scratching direction
—~
Fig. 14. (a) FLF in front of the moving diamond tip; (b) resulting scratch topography at FLF conditions.
accelerates the process. The SLF mechanism can lead to both cohesive and interfacial fracture depending on the properties of the coating/substrate composite. The SPF and SLF flaking mechanisms are both characterized by sideward flaking in relation to the diamond tip. Figure 14(a) shows a different situation in which flaking is taking place in front of the tip by a forward lateral flaking (FLF) mechanism. This mechanism is believed to be caused by the build-up of compressive stresses in front of the tip during scratching. This may cause the coating to buckle, resulting in the removal of semicircular flakes. In the present experiments, this mechanism was always associated with interfacial spalling. From Fig. 14(b), which shows the resulting scratch at FLF conditions, it can be seen that the SLF and FLF mechanisms may be difficult to distinguish if the scratch process is not monitored in situ.
5. The influence of coating—substrate properties on the scratching process The coating thickness and substrate hardness not only influence the critical normal force value, but also have a strong effect on the occurrence and relative importance of the various coating damage mechanisms. In all, twelve different mechanisms of coating damage and detachment were identified in the in situ experiments. Of these mechanisms, only three were found to be directly associated with the critical force transition, namely CCR, DCR and FLF. In addition to the three critical force transition mechanisms, SPF and SLF were often observed at and above the critical force transition. However, these two mechanisms appeared erratically and were therefore considered as secondary failure mechanisms. In Fig. 15(a) a coating failure (CF) map based on the observed primary failure mechanism for each tested coating/substrate combination is
44 4
‘—‘4
E
a
:1(“3
~3
\
\
\
.2
‘\
\
2
\
SLF
\
.9
2
I ladhesive &
DCR
SPF
_
1
IcoCesee)
No Ftaking
00 0
200
400
600
SPF& SLF I
Icoheeve) — — —
.~
CCR
1
I
w
FLF
Oo (a)
b
800 1000 1200
0
(b)
Substrate Hardness [HV]
200
400
600
800 1000 1200
Substrate Hardness [HV]
Fig. 15. Schematic CF maps indicating the observed (a) primary and (b) secondary failure mechanisms for each coating—substrate combination tested.
presented. A second CF map is given in Fig. 15(b) which in a similar way shows the dominant secondary failure mechanisms. In the latter case, a distinction is made between flaking by predominantly interfacial or cohesive fracture. The primary CF map in Fig. 15(a) demonstrates that coating failure occurs by a chip formation mechanism in all but one case in the present experiments. This result is surprising since chip formation in general was not found to be associated with interfacial fracture. Apparently, a thin, brittle coating on a steel substrate can exhibit a quite ductile behaviour during scratching. The ductility of the coating—substrate composite increases with decreasing coating thickness and substrate hardness. The only material combination for which the critical force transition was found to be directly related to adhesive failure of the coating/substrate interface was in the case of the thickest coating on the hardest substrate. This was also the material combination that exhibited the highest critical force value in the present tests. Flaking by one of the two secondary mechanisms, SPF and SLF, was observed for all coating—substrate composites except those with the thinnest coating (see Fig. 15(b)). However, interfacial flaking was only obtained for the coatings on the hardest substrate. In the other cases, flaking was caused by cohesive failure within the coating itself. The frequency of the SPF and SLF mechanisms was found to increase with increasing coating thickness and decreasing substrate hardness. The coating thickness and substrate hardness also have a strong influence on the relative importance of the coating damage mechanisms below the critical force transition, i.e. plastic deformation and crack formation. However, the relationships appear to be fairly complex. At low normal force values, all coating—substrate composites exhibit the mild plastic deformation which, with increasing normal force, changes to the more pronounced plastic deformation mechanism with extensive ridge formation. At higher normal forces extensive cracking of the coating is observed during the scratching event. It was found that cracking generally appears in the order EPC ITC ETC. The only significant deviation from this -+
—~
45
pattern was observed for the thickest coating on the hardest substrate, i.e. for the hardest composite. In this case, the ITC cracks were formed before the EPC cracks. While ITC cracking was observed for all coating—substrate composites, the tendency for EPC and ETC cracking was found to increase with increasing coating thickness and decreasing substrate hardness and these two crack formation mechanisms were not observed for the specimens with the thinnest coating. This behaviour is similar to what was observed for the SPF and SLF flaking mechanisms which indicates that EPC and ETC cracks are precursors for SPF and SLF flaking respectively. The relationships between and the relative importance of the identified coating damage mechanisms appear to be fairly complex. However, three main routes (see Fig. 16) corresponding to low, medium and high composite hardness can be distinguished. Each route can be directly related to one of the final primary coating failure mechanisms. In the figure the various mechanisms are schematically shown in the order that they generally appear with increasing normal force.
~ —
—
V
-
ai~.
—
—-‘ -
____I, 1~
V
—
V
,_
,7
__,
~
b~.
17
7
~ Fig. 16. Schematic representation of the three main coating damage mechanism routes: (a) low, (b) medium and (c) high composite hardness.
6. Discussion
In situ observation of scratch formation in the scanning electron microscope offers a unique opportunity to study the dynamics of the various coating damage and detachment mechanisms that occur during scratching. In principle, the information obtained from this type of experiment should not be presented as individual SEM micrographs but as video recordings of
(a)
Scratching direction
—~.
(b)
Scratching direction
Sc at hing direction
‘—~
(d)
Scratching direction
—>
~rn20~ (c)
~
Fig. 17. Image sequence, illustrating the unique opportunity to observe the actual, dynamic scratch process when using the iii situ scratch equipment (CCR conditions).
the different scratch mechanisms. The image sequence in Fig. 17 is an attempt to illustrate the dynamics of the scratching process, in this case during continuous chip removal. Two additional advantages of the in situ test compared with conventional scratch testing are that the material deformation and detachment mechanisms in front of the moving tip can be studied and that the observed events on the image screen can be correlated to any recorded variations in friction force during the test. Studies of coating detachment mechanisms by post-test examination techniques are of necessity restricted to analysis of the areas outside the actual scratch since the moving tip will change the surface characteristics drastically as it passes over a surface element. 1-lowever, our in situ experiments show that the processes that occur in front of the tip are generally very different from the processes that take place along the sides of the scratch. This may be the reason why chip formation processes are rarely discussed in conjunction with the scratch test. However, the plastic behaviour of the nominally brittle coating is not so surprising considering the high hydrostatic stress state in front of the moving tip and that plastic behaviour of brittle materials is well known in tribological applications. From studies of machining and abrasion
47
it is known that chip formation occurs by cyclic shearing of material in the primary shear zone which extends from the cutting tip to the surface just in front of the chip. Each shearing event normally starts at the cutting tip and propagates outwards toward the surface. For a coating—substrate composite, it is of particular interest to see whether the shear front changes direction when it reaches the interface. If the shear front continues to propagate along the interface, it will soon result in interfacial spalling of the coating in front of the chip. However, our experiments show that this only occurs to a limited extent and only during DCR conditions. The fact that chip formation does not necessarily lead to interfacial fracture may suggest that this failure mode is of little interest with respect to adhesion testing. However, the various coating damage mechanisms are frequently interrelated. For example, chip formation also leads to ridge formation along the sides of the scratch. This may, in turn, produce parallel cracks and interfacial fracture by the SPF mechanism. Another example of the importance of in situ studies of scratch formation is that these allow us to directly distinguish between SLF and FLF mechanisms. This would be difficult by post-test examination only since these two mechanisms give rise to very similar damage patterns. In the present experiments, the relationships between the friction force readings and the events taking place on the image screen were not systematically investigated. This technique appears to have great potential and the incorporation of an acoustic emission detector in the in situ test is expected to give additional valuable information. One criticism that may be raised against the in situ test is that the small tip (R = 25 pm) used will give a more plastic response of the coating— substrate composite and will therefore give rise to different coating detachment mechanisms from those for the conventional (R 200 pm) test. It may be that the very ductile behaviour with continuous chip formation that was observed in the present experiments will not occur with a larger tip radius. For example, the more extreme cases of continuous chip formation were always associated with significant built-up-edge formation, a phenomenon that is less likely to occur with a larger tip radius. Hence, the effect of tip radius on the various scratch mechanisms is an area that should be investigated in more detail in future work. The existence of chip formation is an important issue since it will drastically affect the stress and deformation pattern around the tip, and it therefore needs to be incorporated in the theoretical models for scratching. Another result of the present work which is~of importance for theoretical work is that the friction coefficient p is not a constant in scratching but increases with increasing normal force. The variation in p becomes more pronounced using a smaller tip radius but the effect should be significant also for a 200 pm tip radius. (It should be noted that this effect refers to forces below FN, c~and is not related to the existence of chip formation above FN C.) For a given normal force, p decreases with both increasing substrate hardness and coating thickness, i.e. with increasing composite hardness. Lower p values mean lower shear stresses at the interface and,
48
consequently, the relationship depicted in Fig. 3 between FN and coating thickness and substrate hardness respectively, could partly be explained as a strictly frictional effect. However, we found no systematic variation in the critical friction force or its corresponding friction coefficient with coating thickness or substrate hardness. Another result of the present work that should be explored further is the significance of the observed local variations in FN, c values over the specimen surface. In principle, the critical normal force should not be represented by a single number but by a statistical distribution function. This becomes especially important for components in which only parts of the coated surface are subjected to high stresses, such as a cutting tool insert. In this case, the probability of coating failure could possibly be analysed using Weibull statistics as utilized in failure prediction of brittle bulk material. From the experimental work and the discussion above, the complexity of the scratch process should have become evident. It is believed that further insight on the coating damage and detachment mechanisms can be obtained from systematic work on different coating—substrate composites. In particular, the importance of plastic deformation processes as precursors to adhesive failure should be investigated, with special emphasis on the effect of tip radius on the scratching process. ~,
Acknowledgment The financial support of the National Swedish Board for Technical Development (STUF) is greatly acknowledged.
References 1 2 3 4 5
6 7 8
9 10 11 12 13 14
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