Applications of the scratch test method for coating adhesion assessment

Wear, 115 (1987)

APPLICATIONS OF THE SCRATCH ADHESION ASSESSMENT*

JUHANI

215

215 - 221

TEST METHOD

FOR COATING

VALLI and ULLA M;iKELd

Technical Research Centre of Finland, Metals La&oratory, 02 I50 Espoo ~~~nla~d~

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Summary

The use of hard tribological coatings in metal cutting processes is a common practice today. However, the quality control of these hard coatings is still not satisfactory. One of the most promising test methods is the scratch test for coating adhesion measurement. New important features, which can be achieved by a relatively simple addition to the basic scratch test co~fi~ration, are reported. The introduction of tangential force measurement is an especially valuable extension. The main advantage is that relatively thin (less than 1 pm) coatings can also be studied and we still have the possibility of continuously recording the test results in addition to the optical microscopy studies. An additional advantage is that coatings of the same colour as the substrate can easily be studied. Typical examples are titanium carbide, molybdenum nitride coatings and chromium nitride coatings on a steel substrate. The influence of friction at the coating-indenter interface and surface roughness on the results obtained in the scratch test has been studied,

1. Introduction

The most important property of a coating is its adhesion to the substrate material. A simple and reliable adhesion test method is needed to avoid expensive field tests. Several techniques have been studied for adhesion testing f 1 - 51. Modern physical and chemical vapour deposition methods are used to produce hard tribological coatings on tool materials and they offer previously unattainable adhesion levels which limit the number of applicable adhesion test methods. One of the most promising test methods is the scratch test method for coating adhesion measurement. In the scratch test a stylus is drawn over the sample surface under a stepwise 16, ‘73 or continuously [8] increasing normal force until the coating *Paper Technology,

presented at the Nordic Symposium Lule%, Sweden, June 15 - 18,1986.

on Tribology,

0 Elsevier Sequoia/Prjnted

Lule% University

in The Netherlands

of

216

Fig. 1. The VTT

scratch test equipment.

is detached. Typical scratch test equipment is shown in Fig. 1. The normal force on the indenter causing coating detachment is called the critical normal force and it represents a comparative value of the coating adhesion. The corresponding load of the indenter is called the critical load. In the scratch test the coating detachment can be observed in practice by optical microscopy or using acoustic emission detectors to measure high frequency vibrations caused by coating detachment. Also frictional force monitoring has proved to be a reliable means of coating detachment detection and the sensitivity of the scratch test method in measuring the adhesion of thin (less than 1 pm) hard coatings is enhanced when using friction measurement [ 91. Valli et ~2. [9] have carried out extensive studies into friction monitoring coupled with acoustic emission detection and found that in many cases the former method is more sensitive in detecting failure. In particular, it was shown that on very thin coatings (less than 1 pm) coating failure could be detected only by a change in the frictional force (see Fig. 2) 191. An additional advantage of frictional force monitoring is that coatings with the same colour as the substrate can easily be studied. Typical examples are titanium carbide, molybdenum nitride coatings [ 101 and chromium nitride coatings on a steel substrate.

2. Factors

affecting the scratch test result

The standard deviation of the scratch test results is usually about 10% 20% of the reading. This must be kept in mind when comparing the adhesion of different coatings. The following factors, i.e. substrate hardness, coating thickness, surface roughness, coating hardness, loading rate, indenter tip radius and friction between the coating and the indenter, affect the critical normal force of the scratch test as discussed in a recent review article [ 111”

217

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DISTANCE

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(mm)

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(mm)

(b)

Fig. 2. Chart recorder traces from scratch test (substrate M2 HSS, coating titanium nitride). No meaningful acoustic emission could be detected from the thinner coating in Fig. 2(b) but the tangential frictional force clearly indicates failure.

2.1. The influence of lubrication Friction is a dominant factor affecting the results of the scratch test as can be seen in Fig. 3, in which results of lubricated scratch tests are shown. The use of a thin ion-plated silver overlay increases the critical normal force remarkably. Pure oil does not affect the critical normal force owing to the high stress levels encountered [ll]. The use of oil can even enhance the repeatability of testing, especially in the case of hard coatings on high strength substrates. When the same test equipment was used to study the adhesion of optical coatings on glass substrates the ambient relative humidity affected the critical normal force. 2.2. The influence of surface roughness Poor surface quality affects the general performance of a coating. The surface roughness R, value should not exceed 0.25 pm if reproducible results are required. Figure 4 shows the effect of surface roughness on the critical normal force of the scratch test [12]. The titanium nitride coating thickness was about 2 pm and the R, value of the high speed steel substrate varied between 0.06 and 0.51 pm. When the surface was smooth (R, = 0.06 pm) the critical

218

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Fig. 3. Typical tangential force and acoustic emission traces as a function of normal force in the scratch test. The diamond tip radius was 0.2 mm and the titanium nitride coating thickness on the high speed steel substrate was 2.3 pm. The silver coating thickness was 0.5 pm. The mean value L, and the standard deviation of ten individual scrat.ch tests are also shown.

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Fig. 4. The effect of surface roughness on the critical normal force of the scratch test. Diamond tip radius 0.2 mm. High speed steel substrate (hardness 68 HRC). Titanium nitride coating thickness about 2 pm.

219

Critical normal force

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The normal

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micrograph of the scratch force is shown. in Fig. 5. A more detailed force in this case. Constant

ACOUSTIC EMISSION (mV)

A 2

15

1

(N)

of a scratch

FORCE (N)

I

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normal force was about 13 N. A typical optical channel is shown in Fig. 5, where also the normal The continuous mode of loading was used analysis was used to confirm the critical normal NORMAL FORCE (N)

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Fig. 6. Typical acoustic emission and tangential tance when using four different constant normal

force curves forces.

as a function

of sliding dis-

220

normal forces of 10, 12, 13 and 15 N were used as shown in Fig. 6. When the normal force was 10 and 12 N only some local spots caused acoustic emission, showing coating failure. The corresponding tangential force curve also indicates these spots. A remarkable increase in the acoustic emission was observed between 12 and 13 N. When the normal force was further increased to 15 N the acoustic emission was continuously above the zero level.

3. Conclusions The scratch test has proved to be a reliable method of coating detachment detection. There are several variables affecting the scratch test result but most of them are well understood empirically. For example, a poor surface finish decreases the critical normal force reading, showing worse adhesion compared with a polished surface. This does not necessarily mean a real difference in coating adhesion. Pure oil contaminant does not affect the scratch test but soft well-adhering solid lubricants on hard tribological coatings do.

Acknowledgments Mr. E. Niemi and Mr. J. Rannisto are gratefully acknowledged for their help in the surface roughness experiments. The authors wish to acknowledge the financial support of this work by the Technology Development Centre in Finland and Nordisk Industrifond.

References 1 K. L. Mittal, Electrocomponent Sci. Technol., 3 (1976) 21. 2 A. J. Perry, P. Laeng and H. E. Hintermann, Proc. 8th Int. Conf. on Chemlcul Vapor Deposition, Paris, September 15 - 18, 1981, Electrochemical Society, Pennington, NJ, 1981, p. 475. 3 D. S. Campbell, in L. I. Maissel and R. Glang (eds.), Handbook of Thin Film Technology, McGraw-Hill, New York, 1970, Chapter 12.3. 4 P. A. Steinmann, P. Laeng and H. E. Hintermann, Oberfliiche-Surf., 23 (4) (1982) 108. 5 J. Valli, U. MlkelP, A. Matthews, Assessment of coating adhesion, Surf. Eng.. 2 (1986) 49 - 53. 6 0. S. Heavens, J. Phys. Radium, 11 (1950) 355. 7 P. Benjamin and C. Weaver, Proc. R. Sot. London Ser. A, 254 (1960) 163. 8 H. E. Hintermann and P. Laeng, Haftung als Basis fiir Stoffuerbunde, Deutsche Gesellschaft fiir Metallkunde, Bremen, 1983, p. 87. 9 J. Valli, U. MZkelii, A. Matthews and V. Murawa, J. Vat. Sci. Technoi. A, 3 (1985) 2411.

221 10 J. Valli, U. MSkeli and H. T. Cr. Hentzell, Proc. 13th Int. Conf MetaZJurgical San Diego, CA, 1986, in J. Vat. Sci. Technol. A, 4 (1986) 2854. 11 J. Valli, Proc. 13th. Int. Conf. Metallurgical Coatings, San Diego, CA, 1986, Sci. Technol. A, 4 (1986) 3011. 12 J. Valli, E. Niemi and J. Rannisto, to be published in Finn. J. Tribal.

Coatings,

in J. Vat.