книги / FISMA and the risk management framework the new practice of federal cyber security

of the film. As the adhesion results show that the coating with I hour pre-sputtering time has better adhesion (see Fig. 3), the failure does not occur in figure 10 (a). In addition, failure due to weak adhesion will increase the amount of wear debris, which increases the chance of adhesive wear between the coatings and wear debris.

In this paper, we found that the adhesion behaviour is related to the wear behaviour, as far as the commercial carbon based coating is concerned. However, further work should focus on different coatings and conditions for several reasons: 1) Different coatings have different microstructure and properties. 2) The substrate material used may affect the coating properties. Shima et. al. [18] had found that using different substrate material directly affected the coating wear behaviour because of the substrate elastic modulus, coating adhesion and local asperity deformation.

CONCLUSIONS

The adhesion of the carbon based coatings was found to increase with the pre-sputtering time, while the wear rate will decrease. After combining these two results, the correlation between wear and adhesion was found; it showed that the wear rate will decrease when the adhesion increases. This relationship is better demonstrated over a large range of the critical load and wear rate. To apply this result to other cases, more work should be done, as only carbon based coating were used in this research.

ACKNOWLEDGEMENT

KH Lau acknowledges the support of the research scholarship from the City University of Hong Kong. The authors appreciate the use of the facilities in Advanced Coatings Applied Research Laboratory (ACARL), which is supported by the Innovation Technology Fund of Hong Kong. In addition, a grant from the Research Grant Council of the HKSAR, China (Project No. CityU 1173/03E) also supports this work.

REFERENCES

1.Larsson M., Olsson M., Hedenqvist P., Hogmark S. (2000) Surface Engineering, 16, no. 5, p 436-444.

2.Harry E., Rouzaud A., Julieta P., Pauleau Y. ( 1999) Thin Solid Films, 342, no. 1-2, Mar, p

207-213.

3.Perry A. J. (1983) Thin solidfilms, 107, no. 2, p 167-180.

4.Wang R. (1993) Materials Research Society Symposium Proceedings, 308, Thin Films: Stresses and Mechanical Properties IV, p 227-233.

5.Staia M. H. Castillo EJ, Puchi ES, Lewis B, Hintennann HE. (1996) Surface and Coatings Technology, 86-87, p 598-602.

6.Bienk E. J., Reitz H., Mikkelsen N.J. (1995) Surface & Coatings Technology, 76-77, no. 1-3 pt 2, p 475-480.

7.Schwarzer N., Richter F. ( 1995) Surface & Coatings Technology, 14, no. 1-3 pt 1, p 97-103.

8.Chalker P.R., BullS. J., Rickerby D. S. (1991) Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing, Al40, no. 1-2, p

583-592.

9.Xie Y., Hawthorne H. M. (2001) Surface and Coatings Technology, 141, no. 1, p 15-25.

10.Laugier M. T. (1987) J Vac. Sci. Techno!. AS (1), p 67-69.

11.Farhat Z. N., DingY., Alpas A. T., Northwood D. 0. (1997) Journal ofMaterials Processing Technology, 63, no. 1-3, p 859-864

12.BullS. J. (1999) Wear, 233-235, p 412-423

13.Koski K., Holsa J., Emoult J., Rouzaud A. (1996) Surface & Coatings Technology, 80, no. 1-2, Symposium H on Advanced Deposition Processes and Characterization ofProtective Coatings, p 195-199.

14.Yang S., Camino D., Jones A. H. S., Teer D. G (2000) Surface and Coatings Technology,

124, no. 2, p 110-116.

15.Ronkainen H., Vihersalo J., Vaijus S., Zilliacus R., Ehmsten U., Nenonen P. (1997)

Surface & Coatings Technology, 90, no. 3, p 190-196.

16.Eisenhuttenleute V. D., (editor) (1993) Steel- A handbookfor materials research and engineering, p 35.

17.Shum P. W., Zhou Z. F., Li K. Y. (2004) Wear, 256, no. 3-4, p 362-373.

18.Shima M., Okado J., McColl I. R., Waterhouse R. B., Hasegawa T., Kasaya M. (1999)

Wear, 225-229, no. I, p 38-45.

250

CHAPTER13

THE STUDY OF THE ADHESION OF A TiN COATING ON STEEL AND TITANIUM ALLOY SUBSTRATES USING A MULTI-MODE SCRATCH TESTER

Originally published in Tribologv International vol 39, February 2006

J. STALLARD, S. POULAT and D.G. TEER

Teer Coatings Ltd, West Stone House, Berry Hill Industrial Est., Droitwich, Worcs., WR9 9AS, U.K. E-mail:joanne.stallard@teercoatings.co.uk

ABSTRACT

A titanium nitride (TiN) coating was deposited by magnetron sputter ion plating onto steel and titanium alloy polished substrates. The adhesion of the coating on each substrate material was investigated using a newly developed multimode scratch tester. Progressive loading scratch tests, constant load scratch tests, multiple scratch tests in the same track and indentation tests were performed. It was shown that the modified scratch tester can be used to identify not only coating detachment during progressive load scratch tests, but also other failure events such as cracking and cohesive damage to the coatings. By using the additional modes of operation, it was possible to study the fracture mechanisms in more detail i.e. chipping in the scratch track was cohesive for the TiN coated steel and adhesive for the TiN coated Ti alloy.

KEYWORDS

Thin coating, Adhesion, Scratch, TiN, Titanium alloy

INTRODUCTION

Ceramic coatings are the usual choice for increasing the wear lifetime of industrial components, of which TiN is the most widely accepted. The coatings possess different properties for each specific application but their overall performance is very dependent on the adhesion between the coating and the substrate material. Adhesion is measured as the force or the work required to detach a coating from the substrate [1]. Adhesion has been measured using various test methods for which the applications and limitations have been reviewed [2-4]. The Scratch Test is the most popular method for measuring adhesion because it is one of the few tests that can be simply and quickly used to assess relatively well-adhered surface coatings. The coatings studied by scratch testing span a wide range of applications from wear resistant coatings on cutting tools to optical coatings on glass. The scratch tester is now an essential tool for use in industry for quality control purposes or research laboratories for studying the mechanical strength of coatings on machine components.

The conventional scratch test procedure involves drawing a diamond stylus across the coated surface under increasing load until adhesion failure is detected. The critical load (Lc) is defined as the load at which the coated film is removed from the substrate [5]. It is influenced by many factors such as substrate hardness, film thickness, interface bonding, and intrinsic properties of

the deposited film. Many authors [6-8] have reported that the critical load increases linearly with the substrate hardness. This general behaviour has been explained in terms of the increasing load bearing capacity of the substrate as its hardness increases. The various test parameters (scratching velocity, stylus properties, etc.) and coating-substrate composite properties (hardness and surface roughness, etc.) all affect the critical load value [9-12]. Other failure events, such as cracking or cohesive failure are also equally important in determining the behaviour of the coating [13]. The critical load and any failure event can be detected and observed using friction force, acoustic emission (AE) and examination of the scratch track under optical microscopy and Scanning Electron Microscopy (SEM). In recent years, following the continuing improvement of coatings and their properties, numerous efforts have been made to improve available scratch testers. Updated instrumentation and extended operating capabilities have been added. The Teer Coatings ST3001 scratch tester was designed and developed as a computer controlled multi-mode scratch tester to incorporate these new developments. This work was completed as part of the European project entitled "Multimode scratch testing (MMST): Extension of operation modes and update of instrumentation" [14]. To illustrate how these new developments can be used for quality control or research activities a testing programme was implemented: the aim was to identify the failure mechanisms of a PVD TiN coating deposited onto two different substrate materials.

EXPERIMENTAL DEIAILS

Substrate preparation

ASP23 powder metallurgy steel and TA 46 titanium alloy (032 mm and height 6mm), ground and polished to a surface roughness of R. = 0.02 Jlm were used as substrate materials for the coating. The substrates were placed in an ultrasonic bath in acetone for 15 minutes and then dried in hot air to remove the residual solvent. The substrates were then placed on a precleaned sample holder for coating. A TiN coating was deposited in an industrial closed field unbalanced magnetron sputter ion-plating (CFUBMSIP) system with titanium targets. The substrates were Ar plasma ion cleaned using pulsed DC bias prior to deposition and a thin, approximately 0.1 Jlm, adhesion promoting Ti layer was first deposited by DC magnetron sputtering, again with a pulsed DC bias on the substrates. Nitrogen, under controlled flow conditions, was introduced into the chamber after deposition of the initial Ti interlayer to produce the stoichiometric TiN coating, the reactive deposition conditions being maintained via an automatic feedback control optical emission monitoring system.

Characterisation ofthe films

A standard hardness tester (Wilson I Rockwell B503-R) using a !50 kgf load was also used to assess the adhesion of the coatings, using the HFl to HF6 scale [15]. The microhardness was measured using a Fischerscope HlOO ultra-microhardness tester with a load of 50 mN. For indentation depths of more than 10% of the coating thickness a composite hardness value was obtained [16, 17]. Coating thickness was assessed using the ball crater taper-section technique [18,19]. Optical microscopy was used to examine and measure the coatings.

Progressive load scratch test procedure

Adhesion was measured using a Teer Coatings ST3001 scratch tester with a 0.2 mm tip radius Rockwell diamond indenter. The diamond tip was drawn across the coatings with a loading rate of 100 N min-1 and a sliding speed of 10 mm min· 1• The increasing load scratch test method used has been accepted as a European standard [8]. A start load of 5 N was used in order to identify the start of the scratch track and the test was stopped after a dramatic increase in friction occurred, which corresponds to the substrate being exposed. Using optical microscopy

During each test a computer recorded the normal load, friction force and AE signals.

Constant load scratch test procedure

To assess the homogeneity of the coating along the coating surface and to test the repeatability, constant load scratch tests were completed. A 0.2 mm tip radius Rockwell diamond indenter was loaded to the critical load Lc2 determined during the progressive load scratch test and was

1

then drawn across the coating surface with a sliding speed of 10 mm min· • The number of failure events along the length of the scratch track was then analysed. During each test a computer recorded the normal load, friction force and AE signals.

Multiple scratch test procedure

To assess the wear resistance of the coating, uni-directional multiple scratch tests in the same track were performed. A 0.2 mm tip radius Rockwell diamond indenter was loaded to the critical load Lc2 value determined during the progressive load scratch test and was then drawn across the coating surface with a sliding speed of 10 mm min· 1• During each test and for each pass a computer recorded the normal load, friction force and AE signals.

Indentation procedure

To assess the fracture mode of the coatings, some medium load indentations were completed. A 0.2 mm tip radius Rockwell diamond indenter was loaded from 5N to a final load value, which was chosen following the analysis of the progressive load scratch test data. The loading rate was 100 N min·1• The final load was then maintained for a selected duration and, finally, it was

1

removed with an unloading rate of l 00 N min- • Different final loads were selected. During each test, for the three stages, loading, pause and unloading, a computer recorded the AE signal.

RESULTS

The results of the thickness, hardness and Rockwell indents are shown in Table 1. Due to the substrate's influence on the hardness result the TiN coating on the Ti alloy substrate had a composite hardness lower than for the coating on the steel substrate. An example of a crater taper section used to measure the thickness of the TiN coating on the steel sample is shown in Fig. 1 (a). A comparison of Rockwell indents on the coated steel and Ti alloy is shown in Fig. l

(b) and (c). No coating failure was observed for the Rockwell indent on the coated steel sample (HF1) but cracking and small edge chips were observed for the coating on the Ti alloy (HF3), this was expected as the Ti alloy was a softer substrate and would produce more deformation under load.

Table 1: Thickness, hardness and Rockwell adhesion ofthe coatings