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A new test method for the intrinsic abrasion resistance of thin coatings
Su,face and Coatings Technology, 50 (1991) 75—84
75
A new test method for the intrinsic abrasion resistance of thin coatings * Asa Kassman, Staffan Jacobson, Lynn Erickson, Per Hedenqvist and Mikael Olsson Uppsala Universily, Institute of Technology, Box 534, S- 751 21 Uppsala (Sweden)
(Received June 25, 1991; accepted July 1, 1991)
Abstract A new method has been developed for checking the mechanical quality of thin coatings by determining their resistance to small-scale abrasion. The method is capable of determining the wear constants for the coating and the substrate individually, even for very thin coatings. The theory for imposed shape wear scars is introduced. Suitable parameters and test procedures are proposed and the repeatability, experimental scatter and general capability of the method are presented. The method has the following advantages: (1) The intrinsic abrasion resistance of the coating and the substrate are measured individually. In contrast to competing wear tests which evaluate the wear resistance of the composite rather than the coating and the substrate individually, these values are independent of the wear scar size. (2) Very thin coatings can be evaluated. In the most favourable situation, where the coating has a considerably higher abrasion resistance than the substrate, the wear constant of coatings of sub-micron thickness can be readily determined. (3) The small volume needed makes the test virtually non-destructive. For example it is fully possible to test the coating of a cutting insert before using it in a tool life test. (4) The craters are produced in a commercially available dimple grinder, which remains useful for its original purposes of preparing transmission electron microscopy samples and coating thickness measurements.
1. Introduction Today, coatings are deposited onto substrates for a large number of purposes. Typical examples are coatings deposited for their desirable optical properties, for decorative purposes or for increasing the wear resistance. In all cases a main problem for the manufacturer and the user is the characterization and evaluation of the mechanical properties of the coatings. Knowing these is useful, e.g. for wear life estimations or ranking
The small dimensions of thin wear-resistant coatings (usually 1—10 j~mthick) make mechanical and tribological evaluations particularly difficult. In most methods [11, 12, 14—16] both coating and substrate will contribute to the overall tribological performance, usually in a complex manner that is hard to predict. Fortunately, some work on the selection and standardization of wear tests for coatings can be found in the literature [9, 10]. In the literature, microhardness measurements and scratch adhesion testing have been proposed as useful
of wear resistant coatings but also as a general check of the overall mechanical quality of the coating, as determined by the porosity, defect density, grain size and so forth. This quality check is also valuable for coatings that are not used primarily for their mechanical properties. Some of the available techniques for mechanical and tribological characterization of coatings are given in Table 1. References to papers, employing or describing
indicators of the quality of thin coatings and coating! substrate composites [10, 17, 18]. However, in these methods it is very hard to separate the influence of the substrate from the intrinsic coating properties. Tests on identical coating materials will ascribe different properties to the coating for different substrates and even for different coating thicknesses. The need for a quick, simple and reproducible method
the techniques are included. *paper presented at the 18th International Conference on Metallurgical Coatings and Thin Films, San Diego, CA, April 22—26, 1991.
0257-8972/91/$3.50
for characterization of the intrinsic mechanical quality of thin coatings is evident. In the following, a first step towards a method of this kind, “the crater grinding method”, is presented.
© 1991
—
Elsevier Sequoia, Lausanne
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Kassman et al. / Test method for intrinsic abrasion resistance
TABLE 1. Characterization techniques for thin coatings Coating property
Characterization method
Reference
Hardness
Microindentation and/or special techniques (e.g. the Jonsson/Hogmark model, the Burnett/Rickerby model) Palmqvist indentation Pull test, indentation test, scratch test 2~’method X-ray sin Ball grinding, cross-section measurements Pin-on-disk tests, erosion tests, etc.
[1—5]
Fracture toughness Adhesion to the substrate Residual stress Thickness Wear resistance
[2] [6, 7] [8] [1, 7] [9—13]
2. Theory and concept of the proposed test
where
2.1. The wear equation formulated for sutface coated materials From classical wear theory for homogenous materials we have
5). L~and L, denote the parts of the load that are carried by the coating and the substrate, respectively. For most sliding wear situations K is also dependent on the nominal contact area. However, in pure abrasion K is often quite independent of the nominal contact area. The distribution of the
.
L
V S
H
(1)
usually called the wear equation and sometimes also referred to as3), Archard’s equation. V= (m), volume S = sliding distance L =ofmaterial load (N) worn off (m and H=hardness (N rn-2), and the wear coefficient K is defined by the equation. The wear properties of the material are included in both K and H and thus it~is often preferred to replace the wear coefficient by 9ie wear constant K, defined as K
H
(2)
K= K(K.~, íç,
L~,L
load will vary during wear while the total load is constant L=L~+L. (5) The total wear rate is naturally the sum of the wear rates of the coating and the substrate dV = d(V~+l’~) = + (6) dS dS dS dS which can be reformulated to a general wear equation ~
for surface coated materials dV dS
(7)
=K~L~+K5L~
is sometimes also called the wear factor or even the wear coefficient just as K.) The wear equation becomes
In the general case, the distribution of the load on the coating and the substrate can not be predicted. There are, however, some exceptions. These cases, of imposed shape wear scars, will be discussed in the
V
following.
(K
(3)
is normally considered to be a characteristic constant of the material, determining the wear rate of a specific wear situation, i.e. if all other triboparameters, such as load, speed, counter material, temperature, environment etc. are constant. However, inhomogeneous materials such as materials with a modified surface or a coating should not be expected to exhibit constant K-values. As soon as both coating and substrate experience wear the K-value becomes a weighted mean of the individual wear constants of the coating and the substrate (K~and K~,respectively). For situations like this, where the wear rate V/S is not constant, the wear equation has to be reformulated as K
.
dV dS
(4) =
2.2. The concept of imposed shape wear scars The distribution of load on a wear scar through a surface coating, as in Fig. 1, can not be predicted in the general case. If, however, one of the interacting bodies exhibit negligible deformation during the formation of the wear scar and in addition the relative motion of the interacting bodies imposes a specific, Load bearing area of the substrate
/
the coatmg area ________ ______
Fig. 1. Load bearing areas of an imposed shape wear scar in a coated substrate.
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Kassman er a!. / Test method for intrinsic abrasion resistance
well defined shape to the scar, the load distribution is predictable. Examples of such geometries are the wear of a ball or a cylinder surface against a non-wearing flat, or the opposite, formation of spherical and cylindrical craters in an initially flat surface. In this investigation grinding of spherical cap shaped craters has been investigated and their geometry will be used in the following derivation. The critical point in the concept of imposed shape wear scars is the understanding of the distribution of load. Intuitively one might imagine that the load distribution should be related to the distribution of load bearing area of the coating and the substrate. Such a load distribution would however lead to unreasonable consequences. Provided the assumption of K~ and K5 being independant is valid, loadload distribution proportional to theconstants distribution of athe bearing area would lead to unbalanced wear rates in the coating and the substrate, i.e. the shape of the wear scar would deviate from the shape of the rigid wearing body, as illustrated in Fig. 2. If the shape of a wear scar is imposed, the individual loads on the coating and the substrate have to adjust to balance the respective wear rates. It is possible to determine the load distribution and the individual Kvalues provided that it is possible to measure the wear scar size as a function of sliding distance, and that the imposed shape is known. In other words; if h =f(S) and V~,V, =f(h, t), (where h is the depth, or some other measure of the wear scar size, and t is the coating thickness) are known or measured functions it is possible to solve the wear equation for K~,K5 and r. In these cases the only unknown is K, which is obvious in the following formulation dV
—
—
dVdh
8
()
—
— ~..
.
-.
~
77
-
~ .
Fig. 3. The geometry and denotations of the craters.
v= ~h2(3r_h)
(9)
where r is the radius of the sphere and h is the depth of the crater. Initially only the coating is cratered. For crater depths exceeding the coating thickness the part of the wear scar in the substrate becomes 2(3r—h V
S
3h
=
S
\
(10
\ Si
where h 5=h —t
(11)
~ is naturally V— V~.By exchanging L~with L —L5 in eqn. (7) K (L—L )+KL K= (12) S
S
S
only an expression for L5 has to be found. For the case when the coating is not worn through, the situation is uncomplicated and identical to that for a homogeneous material. h
~t
By integrating eqn. (4) with K = K~ and using the volume expression from eqn. (9) we get the following relation between sliding distance and crater depth: (13)
~=
K~L
2.3. Crater depth vs. sliding distance for spherical
h>t
craters
In the following i~, iç and t will be solved from the wear equation for the special case of a spherical cap shaped wear scar, see Fig. 3. The values are determined by fitting experimental crater depth vs. sliding distance values to the relationship theoretically derived below. From fundamental geometry we have ___________________ _________
To find the solution for the more complex situation h > t we use eqn. (8) formulated as =
for cl/i
Ii > t
—
—1
fdI.’5\ =sçLSI —f dS \ cl/i / —
Fig. 2. Example of an imposed shape wear scar (left) and the consequence of unbalanced load distribution. If the load ratio were proportional to the load bearing area ratio an unbalanced situation such as shown to the right would occur.
J
(14) dS ~dh, The vertical wear rate has to be the same whether we study the whole crater or only the bottom part; i.e. dh/dS has to be equal to dh5/dS. Thus, we note that —
-1
(15)
which allows us to extract L5. Now we solve dV5/dh by differentiating eqn. (10):
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78
dV
d / dh ~~~2(3rhs))
Kassinan ci a/./ Test method for intrinsic abrasion resistance
=~[2r(h—t)—(h—t)2]
(16)
By differentiating eqn. (9) and combining eqns. (12) and (14)—(16) we arrive at an expression for the load carried by the substrate /
2rh—h2
LS=LKC/[KS~).(h)(h)s
(17)
_1)+K~]
In the experimental part of this paper, the wear constants of the coating and the substrate have been determined by fitting the experimental S vs. /i values to eqn. (23). It could seem more natural to use h as the independent variable but in that case the derivation of the relationship becomes more messy and the final expression more complex. However, the simplified expression in eqn. (23) is easily rearranged for h=h(S). Unfortunately,
Now, let us expand eqn. (15) by substituting L 5 and
dV5Idh with the expressions derived in eqns. (16) and (17) dhLiç~,çf ~
1
~KS(22ht+t2)+KC[~(ht)(ht)2)])
(18) We will reach the relationship between S and h by integrating eqn. (18): 2+12h+C (2rth—th 1) (19) L \K~ ~
(-~_
2—~(h_t)3+C [r(h—t)
+
this formulation is not well suited to use for curve fitting due to the complicated expressions needed to reformulate ha~tand h>t to the corresponding limits for S.
21)
K5
3. Experimental details
3.1. Test equipment A commercial dimple grinder (Gatan Model 656 Precision Dimple Grinder) is used for grinding craters into the surfaces of coating/substrate composites. Normally, the main applications for this apparatus are prethinning of transmission electron microscopy (TEM) samples, polishing of samples for Auger depth profile analysis and coating thickness measurements.
The wear scar has to match at the interface, i.e. eqn. (19) has to be equal to eqn. (13) when h=t, which determines the values of the constants. Firstly the K~ terms have to match when just passing the interface 2—— (20) 3 C1=—rt
The general construction of the instrument is shown in Fig. 4. A grinding wheel, shaped as a disk cut out from the centre of a sphere, rotates about a horizontal axis. For these tests, the specimen is mounted horizontally in a cup, which is subsequently filled with diamond the wheel grinds axis. the specimen slurry. which During in turn testing, rotates about a vertical
where the
These combined motions result in a crater with a spherical cap shape being ground into the specimen
iç
term should be zero
C 20 This leads to the final expression S= ~.
L
(21)
surface. The normal force between the wheel and the specimen can be varied between 0 and 0.5 N using dead weights. The rotational velocity of the grinding wheel is variable, 0 and velocity 550 r.p.m., while the6 specimen rotates between at a constant of roughly
h~r
r.p.m. Different types of wheels can be used. The most common wheel materials are steel, bronze and delrin
2—h3/3 I I
rh
K~
2rt(h—t/2)—th2±t2h—t3/3
r(h—t)2—(h—r)3/3 h>t
+
(22) Note that for our test set-up r>>h and r>>t (r~101 load ~/////~____~
specimen gnnding wheel
hmm, andh—~t—-~ t can 10-2 be neglected, mm) so that i.e. h3—~t3——th2—~ht2——0. all third-order termsApof plying these approximations, S becomes
‘JTI Sappr =
~
1
h2
h
—
~t
/
mond slurry
K 0
2t(h t/2) —
IC,~
+
(h
—
h>t
(23)
~spec~nsen
(maenetic) platfo
specimen axis
Fig. 4. Schematic view of the dimple grinder.
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Kassrnan et a!. / Test method for intrinsic abrasion resistance
polymer. Any suitable abrasive media can be used, typically either diamond paste or slurry. The shape of the specimen is important. Ideally the test will give a circular crater opening, but rough specimen surfaces get non-circular crater openings. The influence of the surface roughness is most marked for shallow craters. When the craters are shallow compared with the surface roughness, only the protruding parts of the surface are cut off and the crater shape will naturally be very hard to define. The alignment of the point of contact of the grinding wheel with the axis of rotation of the specimen is also a crucial factor. A misfit leads to a smaller crater depth than is indicated by the crater diameter. Utilizing a profIlometer to measure the depth of the worn craters, it is possible to get an indication of the magnitude of the misfit.
TABLE 2. Chemical composition materials investigated
3.2. Test parameters
model [3, 4].
A steel grinding wheel with a diameter of 20 mm was used throughout the test series. The wheel was investigated in the scanning electron microscope (SEM) and measured after the tests and did not exhibit any noticeable wear, Hyprez Liquid Diamond, a commercial diamond slurry of standard concentration, was used as the grinding medium. A diamond size of 2.5 jim was chosen. The slurry was exchanged on every measuring occasion. A standard load of 0.1 N (±0.01 N) was used in all tests, while the rotational velocity of the grinding wheel was varied. Velocities of 112 and 175 r.p.m. (±5% in both cases), corresponding to peripheral velocities of 0.12 and 0.18 m s~, were used.
High speed steel, ASP 30 Cemented carbide, GC 1025
(wt.%) of the substrate
1.30 6.1 85.9 2.6
C, 4.0 Cr, 5.0 Mo, W, 3.1 V, 8.1 Co WC, 3.6 TaC, 2.4 NbC, TiC, 5.5 Co
TABLE 3. Coating thickness, coating hardness and substrate hardness Specimen
Coating thickness (sm)
Coating hardnessa 2) (kg mm
Substrate hardnessa (kg mm2)
TiN/HSS
4.0±0.3 7.0±0.4
2450±200 3300±250
1250±150 1550±100
TiC/cemented carbide
_____________________________________________________
uflardness value determined by the Jonsson—Hogmark hardness
The microstructure of the coatings was characterized by light optical microscopy and SEM of both fractured and polished cross-sections. While the TiN coating displayed a columnar structure the TiC coating was composed of two distinct zones with the inner zone (2.0 ±0.2 ~m thick) consisting of very fine equiaxed grains of sub-micron size. The outer zone consisted of coarser, more columnar grains. In addition, the TiC coating displayed thermal cracks perpendicular to the surface as well as some porosity at the substrate interface.
4. Evaluation of the test 3.3. Recorded parameters The test was interrupted at regular time intervals and the crater diameters were measured using an optical microscope with an accuracy of 0.01 mm. Subsequently, the depth of the craters was calculated from the diameter. 3.4. Materials Monocrystalline Si (100) was tested as a reference material. In wafer form, silicon is an extremely well defined and homogeneous material, Two different surface coating composites, a titanium nitride (TiN) coated powder metallurgy high speed steel and a titanium carbide (TiC) coated cemented carbide have been characterized. In Table 2 the chemical cornpositions of the substrate materials are given. The TiN coating was deposited by reactive electron beam evaporation while the TiC coating was deposited by chemical vapour deposition. In Table 3 the coating thicknesses, coating hardnesses and substrate hardnesses of the two coating composites are given,
4.1. Experimental scatter and reproducibility The experimental scatter and reproducibility of the test were evaluated by grinding craters in uncoated monocrystalline silicon. This material is virtually free from defects and extremely homogeneous. Thus, it can be assumed that all experimental scatter and deviations from the model obtained are due to scatter and errors in the test method. Sources for these deviations and scatter include errors in the measurement of the crater diameter, shifting quality of the diamond slurry, errors in grinding speed and load etc. As is obvious from Fig. 5, the experimental scatter is quite low. The K-values, obtained by fitting eqn. (13) to the experimental h vs. S values for the three tests, differ by less than ±2.5%. Further, it is obvious from studying the V vs. S curves that the volumetric wear rate is constant within the tested crater size interval, i.e. there is no noticeable change in the wear rate due to the increasing nominal contact area during crater growth. It can thus be concluded that the test should
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Kassman
ci a!.
/ Test method for intrinsic abrasion resistance
for the tested composite. In order to give a clearer view of the wear rate modification yielded by the coating,
0.25
60
I:
-
—
0.15
9.
H 0.20 0 10
~th~~me
0.05 H 0 0
50
100
150
20(1
250
Sliding Distance [ml
Fig. 5. Crater size vs. sliding distance for uncoated monocrystalline silicon. Three parallel tests, all with 0.1 N load and 0.18 m s speed. The crater depth and crater volume values as well as fitted curves are indicated.
shown in Fig. 7. The crater depth vs. sliding distance for cemented the volumetric ratecemented atthe different crater depths is Fig. carbide 8. For and this TiCwear composite coated wear carbide rate isis higher shown on in the coated specimen, as is obvious from the wear rate vs. crater depth curves shown in Fig. 9. Also in this case the assumptions of the model seem valid; the individual experimental values are closely approximated by the fitted curves. 4.3. Resulting wear constants and notes on the curve fitting procedure The correlation error for the curve fittings of each test was between 0 and 1.5%, usually less than 1%.
30
are collected in Fig. 10. The scatter between the parallel 2 5 10t4
1:
All2.0 resulting wear constants and experimental scatter t0( 14
~~oat~:
0
500
1000
1500
2000
2500
.~l.5 10 E ~l.O to::
0
Sliding Distance Em]
Fig. 6. Crater depth vs. sliding distance for high-speed steel and TiN coated high-speed steel. Load: 0.1 N, speed: 0.12 and 0.18 m s (two of each speed on each sample). For the coated samples, the individual crater depth readings and the corresponding fitted curves of four parallel tests are shown. For the uncoated samples only the fitted curves of four parallel tests are shown.
be able to detect differences in wear properties larger than about 5%, and further that scatter exceeding the level of that found for silicon is caused by actual inhomogenities in the tested materials.
5(1 lO-’~ 0
~ 0
_____________
1C0atmgT 10
20
30
Crater Depth [gm]
Fig. 7. Volumetric wear rate vs. crater depth. Wear rates for four parallel tests of TiN coated high-speed steel calculated from the respective wear constants from the fitted curves in Fig. 6. Load: 0.1 N, speeds: 0.12 and 0.18 m s~.
TiC coated
4.2. Test evaluation on coated specimen The capability of the test was evaluated on two
tion (PVD) carbide. coating/substrate Forcoated comparison composites; TiN onuncoated high-speed physical substrates steel vapour andwere chemical deposialso vapour deposition coated on cemented tested. Plots showing(CVD) the crater depth TiC vs. sliding distance for the high-speed steel with and without a TiN coating are shown in Fig. 6. As is obvious from Fig. 6, the coating exhibited much higher wear resistance than did the substrate, which means that the coating has a lower wear constant than the substrate in this case. The fact that the individual experimental values fall quite close to the fitted curve indicates that the theoretical model is valid, at least
9~
:
~coated
0
0
50
100
150
200
Sliding Distance [m[
Fig. 8. Crater depth vs. sliding distance for cemented carbide and TiC coated cemented carbide. Load: 0.1 N, speed: 0.12 and 0.18 m s (two of each speed on each sample). For the TiC coated samples, the individual crater depth readings and the corresponding fitted curves of four parallel tests are shown. For the uncoated samples only the fitted curves of four parallel tests are shown.
A. 3.0
Kassman ci a!. / Test method
~
1.5 10t3 2.5 10.13 ~ng
Thickness
1 0 ~ 5(1 l0~
Co -
________________________
0 0
10
20
30
Crater Depth [gm]
Fig. 9. Volumetric wear rate vs. crater depth. Wear rates for four parallel tests of TiC coated cemented carbide calculated from the respective wear constants from the fitted curves in Fig. 6. Load: 0.1 N, speeds: 0.12 and 0.18 m S.
ogenities in the mechanical properties of both the substrate and the coating. These inhomogenities include the “double layer” of two different grain sizes in the TiC coating, possibly a deformation hardened supe~cial 11. Secondly the scatter originates from actual inhomlayer of the coated high speed steel, etc. Provided that the sample is ideal, i.e. that it consists of a substrate and a single coating, both with homogeneous properties, the present method is capable of determining the wear constants of both the coating and the substrate as well as the coating thickness by fitting the experimental values of one single test to the theoretically .derived equation. However, for normal non-ideal specimens like those tested here, the determination of the wear constant of the coating (which normally would be the prime purpose
15
of the test) could show less scatter if predetermined values of the coating thickness and the wear constant
U
.~
•
121
~ ~ ~ ~
~
L
0 (a)
—~
HighSpeedSteet
solo_lu
~
1(5db
damal
Highest value Meanvalue Lowest value
TnantumNitnde
30
-
scm mu 51013
~ —.
_____ _~
• J
Highestvalue Mean value ue lowesival
2(1
15
Ill
of the substrate are set to the equation, as was the case for the TiN coatings in this investigation. The curve is then fitted with only the wear constant of the coating as a free variable. The wear constant of the substrate is readily determined by crater testing either prior to coating deposition or on a sample where the coating has been completely removed by grinding and polishing. The coating thickness can be obtained as a by-product of the crater diameter measurements or by SEM studies of cross-sections.
______________________
~
81
resistance
topography and uneven coating thicknesses, see Fig.
10-t3
2.0 lO~
for intrinsic abrasion
4.4. Material removal mechanisms 4.4.1. TiN coated high-speed steel (HSS) At the periphery of topography the craters (grinding in the TiN/HSS composite, the original marks) was smoothed out, see Fig. 12. Fine grooves and some
~ C.
‘all
5 11
(b)
--
Cemented Carbide
-~
Titaztiummt Carbide
Fig. 10. Average, minimum and maximum wear constants of the tested coatings and substrates. The wear constants of the substrates were determined both on uncoated specimens (by fitting to the equation for uncoated materials, eqn. (13)) and on coated specimens (by fitting to the equation for coated materials, eqn. (23)). The wear constants of the coatings were also detennined in two ways, both by fitting to eqn. (23): substrate only test (left bar); (i) by applying a constant value of K, as determined in the (ii) using both K5 and K, as free variables (right bar). (a) Uncoated and TiN coated high-speed steel. (b) Uncoated and TiC coated cemented carbide.
r
.1
_______________________________________________________
I
tests is higher here (Figs. 6 and 8) compared with the test on silicon (Fig. 5). This is primarily due to the size of the craters in the surface coated composites being less well defined, which is caused by coarse
SOOjmi
Fig. 11. Crater in TiC coated cemented carbide (SEM). The rough topography and uneven coating thickness results in a far from perfectly circular crater. The exact mean diameter is thus hard to determine which causes some scatter to the results.
82
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Icassman ci a!.
lest mel/i Cal f/r I, 1! (Ft Sib abe a sum resi.syari~
20 ~
__~___J
Fig. 12. Pdrt of the era tel (I op su rt aee a rid cross—sect ion) at i a TiN coated high-speed steel sample (SEM). Compare the ap pearance of the original grooved surface with the smooth ground TiN in the crater periphery. Note the cross-section showing the smooth gradual thinning of the coating.
signs of microchipping of TiN were observed at higher magnifications. Wear of the TiN coating can thus be concluded to occur by micro-cutting. No signs of in terfacial spalling were observed and the coatings ap peared to have been very gently thinned, see Fig. 13(a). The exposed HSS substrate displays a somewhat rougher, grooved surface in combination with detach ment of some individual carbides, see Fig. 13(b).
-_______
________________
________
5. Concluding remarks and discussion of the proposed test 5.1. Applicable to very thin coatings One of the main advantages of the proposed method is that it is capable of making quality checks of very
________
-______
-_______ -_____________
_______
______
________________ -___________
_______
_______
______
______
.
______
______
4.4.2. TiC coated cemented carbide The original, as-deposited topography of the TiC coating is rapidly flattened out at the periphery of the crater and a fairly smooth surface is produced where fine grooves as well as detachment of individual TiC grains can be detected, see Fig. 14(a). This demonstrates that wear predominantly occurs by two mechanisms; micro-cutting and intercrystalline fracture. The inner fine-grained TiC zone displayed a much smoother ground surface than the outer coarse-grained zone. The tendency to intercrystalline fracture appeared very small in the inner zone. Grinding proceeds through both the outer and inner TiC zones and into the substrate without any evidence of interfacial spalling or subsurface crack formation, The exposed cemented carbide substrate had almost the same appearance as the outer TiC zone, i.e. fine grooves in combination with detachment of individual WC grains or parts of grains, see Fig. 14(b).
______________
______
I
1O~ifl
-~
.
Fig. I ~. Crater in TiN coated high-speed steel (SEM). (a) Detail of the obliquely ground interface. The thin TiN streaks in the elsewhere bare HSS zone cover the bottom of the deepest grooves of the original uncoated specimen. (b) Bare HSS in the bottom
of the crater. Note that the carbides are usually nicely cut through and that in one case a carbide has been pulled out.
thin coatings. As in most available tests, the coating may rapidly be worn through and during the remainder of the test both the coating and the substrate will influence the wear rate. Quite uniquely though, in the proposed crater grinding test the properties of the coating and the substrate are still separable. The resuiting wear constant values are independent of the wear scar size and the coating thickness. It is not necessary to make any crater size reading before the coating is worn through. This was demonstrated by the fact that identical Kr-values (for TiN and TiC) were obtained when fitting all the depth readings to the curve as when only fitting the depths larger than 10 jim. This means that all data needed to determine the coating properties can be collected when it is completely worn through. (Note that these
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Kassman ci a!. / Test method for intrinsic abrasion resistance
83
be used to characterize coatings of metal cutting inserts prior to cutting tests in order to correlate the tool life with the intrinsic wear properties of the coating. 5.3. Thickness-measurement in the same operation The well defined geometry of the crater makes it possible to directly measure the thickness of the coating, exactly at the spot where the wear test is performed,
I
5 ~
I
by measuring the radius of the whole crater and the radius of the part of the crater that extends down into the substrate, and then apply simple geometry. This method is often referred to as ball grinding thickness measurement.
(a)
5.4. Properties of the diamond slurry
I
~J
(It) Fig. 14. Crater in TiC coated cemented carbide (SEM). (a) Very fine grooves and detachment of individual TiC grains at the periphery of the crater. (b) Bare cemented carbide in the bottom of the crater. Note the fine abrasive grooves and detachment of wc grains.
coatings, for which the wear constant was successfully determined, were too thin (4 and 7 jim) for ordinary microhardness measurements.) Provided the coating has significantly higher wear resistance than the substrate, coatings thinner than 1 jim should be possible to evaluate, 5.2. Small samples One of the main advantages of the proposed method is that it uses simple, flat specimens. Further, it only requires about 4 mm2 per test and it is so mild that it does not significantly affect adjacent material and allows very thin samples. Thus, it is possible to make a number of parallel tests closely spaced on a small specimen and in addition it could be used as a virtually non-destructive test if the crater is not ground in the active region of the investigated specimen. It could e.g.
The main disadvantage is the practical problem of providing a standardized, homogeneous diamond slurry with a satisfactory long time stability. So far it has only been observed but not fully investigated. One way of getting around the problem would be to check the properties of the slurry against a standard specimen prior to each test. If the slurry properties change between tests the achieved wear constants could be normalized with respect to the value achieved for the standard specimen. One suitable such specimen could be monocrystalline silicon in the form of a wafer intended for fabrication of electronic components. Silicon wafers are easily accessible, the material is very homogeneous, well defined and has been proven to show very little scatter in the initial tests.
5.5. Suitable for standardization The proposed test appears promising for standardization. It results in a single wear constant parameter, comparable to a hardness test value, As in hardness measurements the value is dependent on the load, which thus has to be specified. Also in analogy with hardness measurements, the geometry of the abrasives (for crater grinding) or the indenter (for hardness measurement) has an influence on the result, and thus has to be specified. In the grinding test the properties of the slurry would have to be strictly systematized to make the test reproducible. In addition the grinding wheel speed, shape and material would have to be strictly specified in a standardized test. Acknowledgments The authors are indebted to Dr. Leif Westin of Kloster Speedsteel AB and Dr. Staffan Soderberg of AB Sandvik Coromant for supplying the specimens. The financial support from the National Swedish Board
84
A.
Kassman et a!. / Test method for intrinsic abrasion resistance
for Technical Development for this work is greatly appreciated.
References
1 M. Olsson, Ph.D. Thesis, Acta Univ. Upsali., 200 (1989). 2 P. K. Mehrothra and D. T. Quinto, Techniques for evaluating mechanical properties of hard coatings, J. Vac. Sci Technol. A, 3 (1985) 2401. 3 B. Jönsson and S. Hogmark, Hardness measurements of thin films, Thin Solid Films, 114 (1984) 257. 4 0. Vingsbo, S. Hogmark, B. Jönsson and A. Ingemarsson, in P. J. Blau and B. R. Lawn, (eds.), Microindentation Techniques in Materials Science and Engineering ASTM special technical publication, 889, Philadelphia, PA, 1986. 5 P. J. Burnett and D. S. Rickerby, Assessment of coating hardness, Surf Eng., 3 (1987) 69. 6 D. S. Rickerby, A review of the methods for the measurement of coating—substrate adhesion, Surf Coat. Technol., 36 (1988) 541. 7 H. Ronkainen, S. Varjus, K. Holmberg, K. S. Fancey, A. R. Pace, A. Matthews, B. Matthes and E. Broszeit, Coating evaluation methods: A round robin study, D. Dowson, C. M. Taylor and M. Godet, (eds.), Proc. 16th Leeds—Lyon Symp. on Tribo!ogy, Lyon, France, 5—8 Sept., 1989, p. 453.
8 A. J. Perry, Aspects of the characterization of hard coatings, T. S. Sudarshan and D. G. Bhat (eds.), Surface Modification Technologies, The Metallurgical Soc., 1988, p. 121. 9 J. E. Kelley, J. J. Stiglich Jr and G. L. Sheldon, Methods of characterization of tribological properties of coatings, T. S. Sudarshan and D. G. Bhat (eds.), Surface Modification Technologies, The Metallurgical Soc., 1988, p. 169. 10 R. F. Bunshah, Selection and Use of Wear Tests for Coatings, ASTM Special Technical Publication 769, 1982, p. 3. ii M. Olsson, P. Hedenqvist, B. Stridh and S. Soderberg, Solid particle erosion of hard chemically vapour deposited coatings, Surf Coat. Technol., 37 (1989) 321. 12 P. Hedenqvist, Comparative solid particle erosion of Ti(C, N) and TiN coatings on high speed steels, J. Jacobsen, M. Klarskov and M. Eis (eds.), Proc. 4th Nordic Symp. on Tribology, Lubrication, Friction and Wear, Hirtshals, Denmark~10—13 June, 1990, p. 485. 13 K.-H. Habig, Wear behaviour of surface coatings on steels, Tribol. mt., 22(2) (1989) 65. 14 T. F. Page and J. C. Knight, Factors affecting the tribological behaviour of thin hard TiN and TiC coatings, Surf Coat. Technol., 39/40 (1989) 339. 15 D. S. Rickerby and P. J. Burnett, The wear and erosion resistance of hard PVD coatings, Surf Coat. Technol., 33 (1987) 191. 16 S. 1. Bull, D. S. Rickerby and A. Jam, The sliding wear of titanium nitride coatings, Surf Coat. Technol., 41 (1990) 269. 17 A. Thomas, Microhardness measurement as a quality control technique for thin hard coatings, Surf Eng., 3 (1987) 117. 18 1. C. Knight and T. F. Page, Scratch induced subsurface deformation behaviour of ceramic coatings, Surf Eng., 6 (1990) 263.
75
A new test method for the intrinsic abrasion resistance of thin coatings * Asa Kassman, Staffan Jacobson, Lynn Erickson, Per Hedenqvist and Mikael Olsson Uppsala Universily, Institute of Technology, Box 534, S- 751 21 Uppsala (Sweden)
(Received June 25, 1991; accepted July 1, 1991)
Abstract A new method has been developed for checking the mechanical quality of thin coatings by determining their resistance to small-scale abrasion. The method is capable of determining the wear constants for the coating and the substrate individually, even for very thin coatings. The theory for imposed shape wear scars is introduced. Suitable parameters and test procedures are proposed and the repeatability, experimental scatter and general capability of the method are presented. The method has the following advantages: (1) The intrinsic abrasion resistance of the coating and the substrate are measured individually. In contrast to competing wear tests which evaluate the wear resistance of the composite rather than the coating and the substrate individually, these values are independent of the wear scar size. (2) Very thin coatings can be evaluated. In the most favourable situation, where the coating has a considerably higher abrasion resistance than the substrate, the wear constant of coatings of sub-micron thickness can be readily determined. (3) The small volume needed makes the test virtually non-destructive. For example it is fully possible to test the coating of a cutting insert before using it in a tool life test. (4) The craters are produced in a commercially available dimple grinder, which remains useful for its original purposes of preparing transmission electron microscopy samples and coating thickness measurements.
1. Introduction Today, coatings are deposited onto substrates for a large number of purposes. Typical examples are coatings deposited for their desirable optical properties, for decorative purposes or for increasing the wear resistance. In all cases a main problem for the manufacturer and the user is the characterization and evaluation of the mechanical properties of the coatings. Knowing these is useful, e.g. for wear life estimations or ranking
The small dimensions of thin wear-resistant coatings (usually 1—10 j~mthick) make mechanical and tribological evaluations particularly difficult. In most methods [11, 12, 14—16] both coating and substrate will contribute to the overall tribological performance, usually in a complex manner that is hard to predict. Fortunately, some work on the selection and standardization of wear tests for coatings can be found in the literature [9, 10]. In the literature, microhardness measurements and scratch adhesion testing have been proposed as useful
of wear resistant coatings but also as a general check of the overall mechanical quality of the coating, as determined by the porosity, defect density, grain size and so forth. This quality check is also valuable for coatings that are not used primarily for their mechanical properties. Some of the available techniques for mechanical and tribological characterization of coatings are given in Table 1. References to papers, employing or describing
indicators of the quality of thin coatings and coating! substrate composites [10, 17, 18]. However, in these methods it is very hard to separate the influence of the substrate from the intrinsic coating properties. Tests on identical coating materials will ascribe different properties to the coating for different substrates and even for different coating thicknesses. The need for a quick, simple and reproducible method
the techniques are included. *paper presented at the 18th International Conference on Metallurgical Coatings and Thin Films, San Diego, CA, April 22—26, 1991.
0257-8972/91/$3.50
for characterization of the intrinsic mechanical quality of thin coatings is evident. In the following, a first step towards a method of this kind, “the crater grinding method”, is presented.
© 1991
—
Elsevier Sequoia, Lausanne
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76
Kassman et al. / Test method for intrinsic abrasion resistance
TABLE 1. Characterization techniques for thin coatings Coating property
Characterization method
Reference
Hardness
Microindentation and/or special techniques (e.g. the Jonsson/Hogmark model, the Burnett/Rickerby model) Palmqvist indentation Pull test, indentation test, scratch test 2~’method X-ray sin Ball grinding, cross-section measurements Pin-on-disk tests, erosion tests, etc.
[1—5]
Fracture toughness Adhesion to the substrate Residual stress Thickness Wear resistance
[2] [6, 7] [8] [1, 7] [9—13]
2. Theory and concept of the proposed test
where
2.1. The wear equation formulated for sutface coated materials From classical wear theory for homogenous materials we have
5). L~and L, denote the parts of the load that are carried by the coating and the substrate, respectively. For most sliding wear situations K is also dependent on the nominal contact area. However, in pure abrasion K is often quite independent of the nominal contact area. The distribution of the
.
L
V S
H
(1)
usually called the wear equation and sometimes also referred to as3), Archard’s equation. V= (m), volume S = sliding distance L =ofmaterial load (N) worn off (m and H=hardness (N rn-2), and the wear coefficient K is defined by the equation. The wear properties of the material are included in both K and H and thus it~is often preferred to replace the wear coefficient by 9ie wear constant K, defined as K
H
(2)
K= K(K.~, íç,
L~,L
load will vary during wear while the total load is constant L=L~+L. (5) The total wear rate is naturally the sum of the wear rates of the coating and the substrate dV = d(V~+l’~) = + (6) dS dS dS dS which can be reformulated to a general wear equation ~
for surface coated materials dV dS
(7)
=K~L~+K5L~
is sometimes also called the wear factor or even the wear coefficient just as K.) The wear equation becomes
In the general case, the distribution of the load on the coating and the substrate can not be predicted. There are, however, some exceptions. These cases, of imposed shape wear scars, will be discussed in the
V
following.
(K
(3)
is normally considered to be a characteristic constant of the material, determining the wear rate of a specific wear situation, i.e. if all other triboparameters, such as load, speed, counter material, temperature, environment etc. are constant. However, inhomogeneous materials such as materials with a modified surface or a coating should not be expected to exhibit constant K-values. As soon as both coating and substrate experience wear the K-value becomes a weighted mean of the individual wear constants of the coating and the substrate (K~and K~,respectively). For situations like this, where the wear rate V/S is not constant, the wear equation has to be reformulated as K
.
dV dS
(4) =
2.2. The concept of imposed shape wear scars The distribution of load on a wear scar through a surface coating, as in Fig. 1, can not be predicted in the general case. If, however, one of the interacting bodies exhibit negligible deformation during the formation of the wear scar and in addition the relative motion of the interacting bodies imposes a specific, Load bearing area of the substrate
/
the coatmg area ________ ______
Fig. 1. Load bearing areas of an imposed shape wear scar in a coated substrate.
A.
Kassman er a!. / Test method for intrinsic abrasion resistance
well defined shape to the scar, the load distribution is predictable. Examples of such geometries are the wear of a ball or a cylinder surface against a non-wearing flat, or the opposite, formation of spherical and cylindrical craters in an initially flat surface. In this investigation grinding of spherical cap shaped craters has been investigated and their geometry will be used in the following derivation. The critical point in the concept of imposed shape wear scars is the understanding of the distribution of load. Intuitively one might imagine that the load distribution should be related to the distribution of load bearing area of the coating and the substrate. Such a load distribution would however lead to unreasonable consequences. Provided the assumption of K~ and K5 being independant is valid, loadload distribution proportional to theconstants distribution of athe bearing area would lead to unbalanced wear rates in the coating and the substrate, i.e. the shape of the wear scar would deviate from the shape of the rigid wearing body, as illustrated in Fig. 2. If the shape of a wear scar is imposed, the individual loads on the coating and the substrate have to adjust to balance the respective wear rates. It is possible to determine the load distribution and the individual Kvalues provided that it is possible to measure the wear scar size as a function of sliding distance, and that the imposed shape is known. In other words; if h =f(S) and V~,V, =f(h, t), (where h is the depth, or some other measure of the wear scar size, and t is the coating thickness) are known or measured functions it is possible to solve the wear equation for K~,K5 and r. In these cases the only unknown is K, which is obvious in the following formulation dV
—
—
dVdh
8
()
—
— ~..
.
-.
~
77
-
~ .
Fig. 3. The geometry and denotations of the craters.
v= ~h2(3r_h)
(9)
where r is the radius of the sphere and h is the depth of the crater. Initially only the coating is cratered. For crater depths exceeding the coating thickness the part of the wear scar in the substrate becomes 2(3r—h V
S
3h
=
S
\
(10
\ Si
where h 5=h —t
(11)
~ is naturally V— V~.By exchanging L~with L —L5 in eqn. (7) K (L—L )+KL K= (12) S
S
S
only an expression for L5 has to be found. For the case when the coating is not worn through, the situation is uncomplicated and identical to that for a homogeneous material. h
~t
By integrating eqn. (4) with K = K~ and using the volume expression from eqn. (9) we get the following relation between sliding distance and crater depth: (13)
~=
K~L
2.3. Crater depth vs. sliding distance for spherical
h>t
craters
In the following i~, iç and t will be solved from the wear equation for the special case of a spherical cap shaped wear scar, see Fig. 3. The values are determined by fitting experimental crater depth vs. sliding distance values to the relationship theoretically derived below. From fundamental geometry we have ___________________ _________
To find the solution for the more complex situation h > t we use eqn. (8) formulated as =
for cl/i
Ii > t
—
—1
fdI.’5\ =sçLSI —f dS \ cl/i / —
Fig. 2. Example of an imposed shape wear scar (left) and the consequence of unbalanced load distribution. If the load ratio were proportional to the load bearing area ratio an unbalanced situation such as shown to the right would occur.
J
(14) dS ~dh, The vertical wear rate has to be the same whether we study the whole crater or only the bottom part; i.e. dh/dS has to be equal to dh5/dS. Thus, we note that —
-1
(15)
which allows us to extract L5. Now we solve dV5/dh by differentiating eqn. (10):
A.
78
dV
d / dh ~~~2(3rhs))
Kassinan ci a/./ Test method for intrinsic abrasion resistance
=~[2r(h—t)—(h—t)2]
(16)
By differentiating eqn. (9) and combining eqns. (12) and (14)—(16) we arrive at an expression for the load carried by the substrate /
2rh—h2
LS=LKC/[KS~).(h)(h)s
(17)
_1)+K~]
In the experimental part of this paper, the wear constants of the coating and the substrate have been determined by fitting the experimental S vs. /i values to eqn. (23). It could seem more natural to use h as the independent variable but in that case the derivation of the relationship becomes more messy and the final expression more complex. However, the simplified expression in eqn. (23) is easily rearranged for h=h(S). Unfortunately,
Now, let us expand eqn. (15) by substituting L 5 and
dV5Idh with the expressions derived in eqns. (16) and (17) dhLiç~,çf ~
1
~KS(22ht+t2)+KC[~(ht)(ht)2)])
(18) We will reach the relationship between S and h by integrating eqn. (18): 2+12h+C (2rth—th 1) (19) L \K~ ~
(-~_
2—~(h_t)3+C [r(h—t)
+
this formulation is not well suited to use for curve fitting due to the complicated expressions needed to reformulate ha~tand h>t to the corresponding limits for S.
21)
K5
3. Experimental details
3.1. Test equipment A commercial dimple grinder (Gatan Model 656 Precision Dimple Grinder) is used for grinding craters into the surfaces of coating/substrate composites. Normally, the main applications for this apparatus are prethinning of transmission electron microscopy (TEM) samples, polishing of samples for Auger depth profile analysis and coating thickness measurements.
The wear scar has to match at the interface, i.e. eqn. (19) has to be equal to eqn. (13) when h=t, which determines the values of the constants. Firstly the K~ terms have to match when just passing the interface 2—— (20) 3 C1=—rt
The general construction of the instrument is shown in Fig. 4. A grinding wheel, shaped as a disk cut out from the centre of a sphere, rotates about a horizontal axis. For these tests, the specimen is mounted horizontally in a cup, which is subsequently filled with diamond the wheel grinds axis. the specimen slurry. which During in turn testing, rotates about a vertical
where the
These combined motions result in a crater with a spherical cap shape being ground into the specimen
iç
term should be zero
C 20 This leads to the final expression S= ~.
L
(21)
surface. The normal force between the wheel and the specimen can be varied between 0 and 0.5 N using dead weights. The rotational velocity of the grinding wheel is variable, 0 and velocity 550 r.p.m., while the6 specimen rotates between at a constant of roughly
h~r
r.p.m. Different types of wheels can be used. The most common wheel materials are steel, bronze and delrin
2—h3/3 I I
rh
K~
2rt(h—t/2)—th2±t2h—t3/3
r(h—t)2—(h—r)3/3 h>t
+
(22) Note that for our test set-up r>>h and r>>t (r~101 load ~/////~____~
specimen gnnding wheel
hmm, andh—~t—-~ t can 10-2 be neglected, mm) so that i.e. h3—~t3——th2—~ht2——0. all third-order termsApof plying these approximations, S becomes
‘JTI Sappr =
~
1
h2
h
—
~t
/
mond slurry
K 0
2t(h t/2) —
IC,~
+
(h
—
h>t
(23)
~spec~nsen
(maenetic) platfo
specimen axis
Fig. 4. Schematic view of the dimple grinder.
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79
Kassrnan et a!. / Test method for intrinsic abrasion resistance
polymer. Any suitable abrasive media can be used, typically either diamond paste or slurry. The shape of the specimen is important. Ideally the test will give a circular crater opening, but rough specimen surfaces get non-circular crater openings. The influence of the surface roughness is most marked for shallow craters. When the craters are shallow compared with the surface roughness, only the protruding parts of the surface are cut off and the crater shape will naturally be very hard to define. The alignment of the point of contact of the grinding wheel with the axis of rotation of the specimen is also a crucial factor. A misfit leads to a smaller crater depth than is indicated by the crater diameter. Utilizing a profIlometer to measure the depth of the worn craters, it is possible to get an indication of the magnitude of the misfit.
TABLE 2. Chemical composition materials investigated
3.2. Test parameters
model [3, 4].
A steel grinding wheel with a diameter of 20 mm was used throughout the test series. The wheel was investigated in the scanning electron microscope (SEM) and measured after the tests and did not exhibit any noticeable wear, Hyprez Liquid Diamond, a commercial diamond slurry of standard concentration, was used as the grinding medium. A diamond size of 2.5 jim was chosen. The slurry was exchanged on every measuring occasion. A standard load of 0.1 N (±0.01 N) was used in all tests, while the rotational velocity of the grinding wheel was varied. Velocities of 112 and 175 r.p.m. (±5% in both cases), corresponding to peripheral velocities of 0.12 and 0.18 m s~, were used.
High speed steel, ASP 30 Cemented carbide, GC 1025
(wt.%) of the substrate
1.30 6.1 85.9 2.6
C, 4.0 Cr, 5.0 Mo, W, 3.1 V, 8.1 Co WC, 3.6 TaC, 2.4 NbC, TiC, 5.5 Co
TABLE 3. Coating thickness, coating hardness and substrate hardness Specimen
Coating thickness (sm)
Coating hardnessa 2) (kg mm
Substrate hardnessa (kg mm2)
TiN/HSS
4.0±0.3 7.0±0.4
2450±200 3300±250
1250±150 1550±100
TiC/cemented carbide
_____________________________________________________
uflardness value determined by the Jonsson—Hogmark hardness
The microstructure of the coatings was characterized by light optical microscopy and SEM of both fractured and polished cross-sections. While the TiN coating displayed a columnar structure the TiC coating was composed of two distinct zones with the inner zone (2.0 ±0.2 ~m thick) consisting of very fine equiaxed grains of sub-micron size. The outer zone consisted of coarser, more columnar grains. In addition, the TiC coating displayed thermal cracks perpendicular to the surface as well as some porosity at the substrate interface.
4. Evaluation of the test 3.3. Recorded parameters The test was interrupted at regular time intervals and the crater diameters were measured using an optical microscope with an accuracy of 0.01 mm. Subsequently, the depth of the craters was calculated from the diameter. 3.4. Materials Monocrystalline Si (100) was tested as a reference material. In wafer form, silicon is an extremely well defined and homogeneous material, Two different surface coating composites, a titanium nitride (TiN) coated powder metallurgy high speed steel and a titanium carbide (TiC) coated cemented carbide have been characterized. In Table 2 the chemical cornpositions of the substrate materials are given. The TiN coating was deposited by reactive electron beam evaporation while the TiC coating was deposited by chemical vapour deposition. In Table 3 the coating thicknesses, coating hardnesses and substrate hardnesses of the two coating composites are given,
4.1. Experimental scatter and reproducibility The experimental scatter and reproducibility of the test were evaluated by grinding craters in uncoated monocrystalline silicon. This material is virtually free from defects and extremely homogeneous. Thus, it can be assumed that all experimental scatter and deviations from the model obtained are due to scatter and errors in the test method. Sources for these deviations and scatter include errors in the measurement of the crater diameter, shifting quality of the diamond slurry, errors in grinding speed and load etc. As is obvious from Fig. 5, the experimental scatter is quite low. The K-values, obtained by fitting eqn. (13) to the experimental h vs. S values for the three tests, differ by less than ±2.5%. Further, it is obvious from studying the V vs. S curves that the volumetric wear rate is constant within the tested crater size interval, i.e. there is no noticeable change in the wear rate due to the increasing nominal contact area during crater growth. It can thus be concluded that the test should
A.
80 80
Kassman
ci a!.
/ Test method for intrinsic abrasion resistance
for the tested composite. In order to give a clearer view of the wear rate modification yielded by the coating,
0.25
60
I:
-
—
0.15
9.
H 0.20 0 10
~th~~me
0.05 H 0 0
50
100
150
20(1
250
Sliding Distance [ml
Fig. 5. Crater size vs. sliding distance for uncoated monocrystalline silicon. Three parallel tests, all with 0.1 N load and 0.18 m s speed. The crater depth and crater volume values as well as fitted curves are indicated.
shown in Fig. 7. The crater depth vs. sliding distance for cemented the volumetric ratecemented atthe different crater depths is Fig. carbide 8. For and this TiCwear composite coated wear carbide rate isis higher shown on in the coated specimen, as is obvious from the wear rate vs. crater depth curves shown in Fig. 9. Also in this case the assumptions of the model seem valid; the individual experimental values are closely approximated by the fitted curves. 4.3. Resulting wear constants and notes on the curve fitting procedure The correlation error for the curve fittings of each test was between 0 and 1.5%, usually less than 1%.
30
are collected in Fig. 10. The scatter between the parallel 2 5 10t4
1:
All2.0 resulting wear constants and experimental scatter t0( 14
~~oat~:
0
500
1000
1500
2000
2500
.~l.5 10 E ~l.O to::
0
Sliding Distance Em]
Fig. 6. Crater depth vs. sliding distance for high-speed steel and TiN coated high-speed steel. Load: 0.1 N, speed: 0.12 and 0.18 m s (two of each speed on each sample). For the coated samples, the individual crater depth readings and the corresponding fitted curves of four parallel tests are shown. For the uncoated samples only the fitted curves of four parallel tests are shown.
be able to detect differences in wear properties larger than about 5%, and further that scatter exceeding the level of that found for silicon is caused by actual inhomogenities in the tested materials.
5(1 lO-’~ 0
~ 0
_____________
1C0atmgT 10
20
30
Crater Depth [gm]
Fig. 7. Volumetric wear rate vs. crater depth. Wear rates for four parallel tests of TiN coated high-speed steel calculated from the respective wear constants from the fitted curves in Fig. 6. Load: 0.1 N, speeds: 0.12 and 0.18 m s~.
TiC coated
4.2. Test evaluation on coated specimen The capability of the test was evaluated on two
tion (PVD) carbide. coating/substrate Forcoated comparison composites; TiN onuncoated high-speed physical substrates steel vapour andwere chemical deposialso vapour deposition coated on cemented tested. Plots showing(CVD) the crater depth TiC vs. sliding distance for the high-speed steel with and without a TiN coating are shown in Fig. 6. As is obvious from Fig. 6, the coating exhibited much higher wear resistance than did the substrate, which means that the coating has a lower wear constant than the substrate in this case. The fact that the individual experimental values fall quite close to the fitted curve indicates that the theoretical model is valid, at least
9~
:
~coated
0
0
50
100
150
200
Sliding Distance [m[
Fig. 8. Crater depth vs. sliding distance for cemented carbide and TiC coated cemented carbide. Load: 0.1 N, speed: 0.12 and 0.18 m s (two of each speed on each sample). For the TiC coated samples, the individual crater depth readings and the corresponding fitted curves of four parallel tests are shown. For the uncoated samples only the fitted curves of four parallel tests are shown.
A. 3.0
Kassman ci a!. / Test method
~
1.5 10t3 2.5 10.13 ~ng
Thickness
1 0 ~ 5(1 l0~
Co -
________________________
0 0
10
20
30
Crater Depth [gm]
Fig. 9. Volumetric wear rate vs. crater depth. Wear rates for four parallel tests of TiC coated cemented carbide calculated from the respective wear constants from the fitted curves in Fig. 6. Load: 0.1 N, speeds: 0.12 and 0.18 m S.
ogenities in the mechanical properties of both the substrate and the coating. These inhomogenities include the “double layer” of two different grain sizes in the TiC coating, possibly a deformation hardened supe~cial 11. Secondly the scatter originates from actual inhomlayer of the coated high speed steel, etc. Provided that the sample is ideal, i.e. that it consists of a substrate and a single coating, both with homogeneous properties, the present method is capable of determining the wear constants of both the coating and the substrate as well as the coating thickness by fitting the experimental values of one single test to the theoretically .derived equation. However, for normal non-ideal specimens like those tested here, the determination of the wear constant of the coating (which normally would be the prime purpose
15
of the test) could show less scatter if predetermined values of the coating thickness and the wear constant
U
.~
•
121
~ ~ ~ ~
~
L
0 (a)
—~
HighSpeedSteet
solo_lu
~
1(5db
damal
Highest value Meanvalue Lowest value
TnantumNitnde
30
-
scm mu 51013
~ —.
_____ _~
• J
Highestvalue Mean value ue lowesival
2(1
15
Ill
of the substrate are set to the equation, as was the case for the TiN coatings in this investigation. The curve is then fitted with only the wear constant of the coating as a free variable. The wear constant of the substrate is readily determined by crater testing either prior to coating deposition or on a sample where the coating has been completely removed by grinding and polishing. The coating thickness can be obtained as a by-product of the crater diameter measurements or by SEM studies of cross-sections.
______________________
~
81
resistance
topography and uneven coating thicknesses, see Fig.
10-t3
2.0 lO~
for intrinsic abrasion
4.4. Material removal mechanisms 4.4.1. TiN coated high-speed steel (HSS) At the periphery of topography the craters (grinding in the TiN/HSS composite, the original marks) was smoothed out, see Fig. 12. Fine grooves and some
~ C.
‘all
5 11
(b)
--
Cemented Carbide
-~
Titaztiummt Carbide
Fig. 10. Average, minimum and maximum wear constants of the tested coatings and substrates. The wear constants of the substrates were determined both on uncoated specimens (by fitting to the equation for uncoated materials, eqn. (13)) and on coated specimens (by fitting to the equation for coated materials, eqn. (23)). The wear constants of the coatings were also detennined in two ways, both by fitting to eqn. (23): substrate only test (left bar); (i) by applying a constant value of K, as determined in the (ii) using both K5 and K, as free variables (right bar). (a) Uncoated and TiN coated high-speed steel. (b) Uncoated and TiC coated cemented carbide.
r
.1
_______________________________________________________
I
tests is higher here (Figs. 6 and 8) compared with the test on silicon (Fig. 5). This is primarily due to the size of the craters in the surface coated composites being less well defined, which is caused by coarse
SOOjmi
Fig. 11. Crater in TiC coated cemented carbide (SEM). The rough topography and uneven coating thickness results in a far from perfectly circular crater. The exact mean diameter is thus hard to determine which causes some scatter to the results.
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lest mel/i Cal f/r I, 1! (Ft Sib abe a sum resi.syari~
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Fig. 12. Pdrt of the era tel (I op su rt aee a rid cross—sect ion) at i a TiN coated high-speed steel sample (SEM). Compare the ap pearance of the original grooved surface with the smooth ground TiN in the crater periphery. Note the cross-section showing the smooth gradual thinning of the coating.
signs of microchipping of TiN were observed at higher magnifications. Wear of the TiN coating can thus be concluded to occur by micro-cutting. No signs of in terfacial spalling were observed and the coatings ap peared to have been very gently thinned, see Fig. 13(a). The exposed HSS substrate displays a somewhat rougher, grooved surface in combination with detach ment of some individual carbides, see Fig. 13(b).
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5. Concluding remarks and discussion of the proposed test 5.1. Applicable to very thin coatings One of the main advantages of the proposed method is that it is capable of making quality checks of very
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4.4.2. TiC coated cemented carbide The original, as-deposited topography of the TiC coating is rapidly flattened out at the periphery of the crater and a fairly smooth surface is produced where fine grooves as well as detachment of individual TiC grains can be detected, see Fig. 14(a). This demonstrates that wear predominantly occurs by two mechanisms; micro-cutting and intercrystalline fracture. The inner fine-grained TiC zone displayed a much smoother ground surface than the outer coarse-grained zone. The tendency to intercrystalline fracture appeared very small in the inner zone. Grinding proceeds through both the outer and inner TiC zones and into the substrate without any evidence of interfacial spalling or subsurface crack formation, The exposed cemented carbide substrate had almost the same appearance as the outer TiC zone, i.e. fine grooves in combination with detachment of individual WC grains or parts of grains, see Fig. 14(b).
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1O~ifl
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Fig. I ~. Crater in TiN coated high-speed steel (SEM). (a) Detail of the obliquely ground interface. The thin TiN streaks in the elsewhere bare HSS zone cover the bottom of the deepest grooves of the original uncoated specimen. (b) Bare HSS in the bottom
of the crater. Note that the carbides are usually nicely cut through and that in one case a carbide has been pulled out.
thin coatings. As in most available tests, the coating may rapidly be worn through and during the remainder of the test both the coating and the substrate will influence the wear rate. Quite uniquely though, in the proposed crater grinding test the properties of the coating and the substrate are still separable. The resuiting wear constant values are independent of the wear scar size and the coating thickness. It is not necessary to make any crater size reading before the coating is worn through. This was demonstrated by the fact that identical Kr-values (for TiN and TiC) were obtained when fitting all the depth readings to the curve as when only fitting the depths larger than 10 jim. This means that all data needed to determine the coating properties can be collected when it is completely worn through. (Note that these
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Kassman ci a!. / Test method for intrinsic abrasion resistance
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be used to characterize coatings of metal cutting inserts prior to cutting tests in order to correlate the tool life with the intrinsic wear properties of the coating. 5.3. Thickness-measurement in the same operation The well defined geometry of the crater makes it possible to directly measure the thickness of the coating, exactly at the spot where the wear test is performed,
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by measuring the radius of the whole crater and the radius of the part of the crater that extends down into the substrate, and then apply simple geometry. This method is often referred to as ball grinding thickness measurement.
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5.4. Properties of the diamond slurry
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(It) Fig. 14. Crater in TiC coated cemented carbide (SEM). (a) Very fine grooves and detachment of individual TiC grains at the periphery of the crater. (b) Bare cemented carbide in the bottom of the crater. Note the fine abrasive grooves and detachment of wc grains.
coatings, for which the wear constant was successfully determined, were too thin (4 and 7 jim) for ordinary microhardness measurements.) Provided the coating has significantly higher wear resistance than the substrate, coatings thinner than 1 jim should be possible to evaluate, 5.2. Small samples One of the main advantages of the proposed method is that it uses simple, flat specimens. Further, it only requires about 4 mm2 per test and it is so mild that it does not significantly affect adjacent material and allows very thin samples. Thus, it is possible to make a number of parallel tests closely spaced on a small specimen and in addition it could be used as a virtually non-destructive test if the crater is not ground in the active region of the investigated specimen. It could e.g.
The main disadvantage is the practical problem of providing a standardized, homogeneous diamond slurry with a satisfactory long time stability. So far it has only been observed but not fully investigated. One way of getting around the problem would be to check the properties of the slurry against a standard specimen prior to each test. If the slurry properties change between tests the achieved wear constants could be normalized with respect to the value achieved for the standard specimen. One suitable such specimen could be monocrystalline silicon in the form of a wafer intended for fabrication of electronic components. Silicon wafers are easily accessible, the material is very homogeneous, well defined and has been proven to show very little scatter in the initial tests.
5.5. Suitable for standardization The proposed test appears promising for standardization. It results in a single wear constant parameter, comparable to a hardness test value, As in hardness measurements the value is dependent on the load, which thus has to be specified. Also in analogy with hardness measurements, the geometry of the abrasives (for crater grinding) or the indenter (for hardness measurement) has an influence on the result, and thus has to be specified. In the grinding test the properties of the slurry would have to be strictly systematized to make the test reproducible. In addition the grinding wheel speed, shape and material would have to be strictly specified in a standardized test. Acknowledgments The authors are indebted to Dr. Leif Westin of Kloster Speedsteel AB and Dr. Staffan Soderberg of AB Sandvik Coromant for supplying the specimens. The financial support from the National Swedish Board
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for Technical Development for this work is greatly appreciated.
References
1 M. Olsson, Ph.D. Thesis, Acta Univ. Upsali., 200 (1989). 2 P. K. Mehrothra and D. T. Quinto, Techniques for evaluating mechanical properties of hard coatings, J. Vac. Sci Technol. A, 3 (1985) 2401. 3 B. Jönsson and S. Hogmark, Hardness measurements of thin films, Thin Solid Films, 114 (1984) 257. 4 0. Vingsbo, S. Hogmark, B. Jönsson and A. Ingemarsson, in P. J. Blau and B. R. Lawn, (eds.), Microindentation Techniques in Materials Science and Engineering ASTM special technical publication, 889, Philadelphia, PA, 1986. 5 P. J. Burnett and D. S. Rickerby, Assessment of coating hardness, Surf Eng., 3 (1987) 69. 6 D. S. Rickerby, A review of the methods for the measurement of coating—substrate adhesion, Surf Coat. Technol., 36 (1988) 541. 7 H. Ronkainen, S. Varjus, K. Holmberg, K. S. Fancey, A. R. Pace, A. Matthews, B. Matthes and E. Broszeit, Coating evaluation methods: A round robin study, D. Dowson, C. M. Taylor and M. Godet, (eds.), Proc. 16th Leeds—Lyon Symp. on Tribo!ogy, Lyon, France, 5—8 Sept., 1989, p. 453.
8 A. J. Perry, Aspects of the characterization of hard coatings, T. S. Sudarshan and D. G. Bhat (eds.), Surface Modification Technologies, The Metallurgical Soc., 1988, p. 121. 9 J. E. Kelley, J. J. Stiglich Jr and G. L. Sheldon, Methods of characterization of tribological properties of coatings, T. S. Sudarshan and D. G. Bhat (eds.), Surface Modification Technologies, The Metallurgical Soc., 1988, p. 169. 10 R. F. Bunshah, Selection and Use of Wear Tests for Coatings, ASTM Special Technical Publication 769, 1982, p. 3. ii M. Olsson, P. Hedenqvist, B. Stridh and S. Soderberg, Solid particle erosion of hard chemically vapour deposited coatings, Surf Coat. Technol., 37 (1989) 321. 12 P. Hedenqvist, Comparative solid particle erosion of Ti(C, N) and TiN coatings on high speed steels, J. Jacobsen, M. Klarskov and M. Eis (eds.), Proc. 4th Nordic Symp. on Tribology, Lubrication, Friction and Wear, Hirtshals, Denmark~10—13 June, 1990, p. 485. 13 K.-H. Habig, Wear behaviour of surface coatings on steels, Tribol. mt., 22(2) (1989) 65. 14 T. F. Page and J. C. Knight, Factors affecting the tribological behaviour of thin hard TiN and TiC coatings, Surf Coat. Technol., 39/40 (1989) 339. 15 D. S. Rickerby and P. J. Burnett, The wear and erosion resistance of hard PVD coatings, Surf Coat. Technol., 33 (1987) 191. 16 S. 1. Bull, D. S. Rickerby and A. Jam, The sliding wear of titanium nitride coatings, Surf Coat. Technol., 41 (1990) 269. 17 A. Thomas, Microhardness measurement as a quality control technique for thin hard coatings, Surf Eng., 3 (1987) 117. 18 1. C. Knight and T. F. Page, Scratch induced subsurface deformation behaviour of ceramic coatings, Surf Eng., 6 (1990) 263.