METHODS FOR THE VALIDATION OF ADVANCED THIN HARD PROTECTIVE COATINGS - AN EUROPEAN
PROGRAM
METODA ZA VALIDACIJO NAPREDNIH TANKIH ZAŠČITNIH PLASTI - EVROPSKI PROGRAM
PETER MAYR, H. VETTERS, A. SCHULZ
Stiftung Institut fur VVerkstofftechnik, Bremen, Badgasteinerstr. 3 D, 28359 Bremen
Prejem rokopisa - received: 1997-10-01; sprejem za objavo - accepted for publication: 1997-10-21
Specific surface engineering technologies are widely used to improve decisively product characteristics vvhereas thin hard coatings are increasingly adopted in a broad range of industries. The quality assurance infrastructure for coated products is essential to determine their functional properties. For the reliable assessment of the intrinsic properties (coating thickness, hardness, chemical compositon, adhesion) as well as the tribological performance standardised test methods are required! International groups from research institutes and industries are working on this actual topic to develop comprehensive test methodologies including instrument calibration and reference samples. These will then be proposed to committees for standardisation for adoption as European standards.
Key words: thin hard coatings, characterisation methods, properties, standards
Specifične površinske inženirske tehnologije se široko uporabljajo za povečanje karakterističnih proizvodov in tanke trde prevleke se uporabljajo v širokem spektru te industrije v naraščajočem obsegu. Zagotovitev kakovsti infrastrukture za prekrite proizvode je bistvena za določitev njihovih funkcionalnih lastnosti. Za zanesljivo opredelitev specifičnih lastnosti (debelina, trdota, kemijska sestava in adhezija prekritja) in triboloških lastnosti so zahtevane standardne testne metode. Mednarodne skupine iz raziskovalnih institutov in iz industrije delajo na tej aktualni temi s ciljem, da razvijejo razumljive metodologije za testiranje vključno s kalibracijo instrumentov in referenčnimi etaloni. Te bodo kasneje predložene komitetom za sprejem kot evropski standardi.
Ključne besede: tanke trde prevleke, metoda karakterizacije, lastnosti, standardi
1	SURFACE ENGINEERING GAINING ADVANCED THIN HARD PROTECTIVE COATINGS
Specific surface engineering technologies are widely used to improve decisively product characteristics. Thin hard coatings are increasingly adopted in a broad range of industrial sectors as a means of achieving advanced products. The mechanical properties of these advanced coatings will determine the service life because elastic and plastic behaviour control fracture and adhesion of the coating. The use of wear resistant coatings can lead the retention of better tolerances through the lifetime of tools or machine parts. In ali these cases mechanical re-liability requires optimum matching of the different com-ponents to be "assembled", that can only be obtained by model calculations requiring precise knovvledge of the characteristics and functional properties1. It is estab-lished that surface coatings and treatments can lead to considerable improvements in the performance of a vvide range of components. Modern PVD and CVD hard coating techniques often allow for alternative, environmen-tally friendly solution to wear protection2 4.
2	QUALITY ASSURANCE - A KEY OBJECTIVE
As reported5-6, a broad selection of materials is being offered for ceramic coatings. When selecting a coating
for certain application, the substrate, the interface and possible interlayer and the environment influence strongly the on service condition of the coated part. A wide variety of coating procedures leads to a vvide range in functional properties, vvhich can be classified by methods of test7. In order to allovv industry to take fuli advantage of this rapidly developing area it is essential to establish a quality assurance infrastructure for coated products by standardised test methods. In vievv of foster-ing international collaboration betvveen research organi-sations, standard bodies and industry on pre-standar-disation research in advanced materials the "Versailles Project on Advanced Materials and Standards" (VAMAS) is fulfilling a unique role. Within several tech-nical vvorking areas (TWA) formal links vvith international standards organisations and increasing industrial involvement have been formed. In fact one activity on developing methods of test of thin hard coatings is novv established9. The European Committee for Standardisation (CEN) has also established a technical committee on ceramic materials vvith a vvorking group "Methods of test of ceramic coatings" (CEN TC 184AVG 5).
3 CHARACTERISATION OF PROPERTIES
Many methods exist for the measurement of properties of coatings (table 1), but for thin hard coatings is a great paucity of standards. Improved test methods, there-
fore need to be developed or the existing procedure should be formalised and draft standards prepared. The test routine should lead to a subsequent process control during the production routine which includes prelimi-nary quality control as well as quality control after the coating procedure. Their properties are defined as: hardness residual strain, composition, morphology, microstructure, thickness of coating, elasticity, electrical con-ductivity, magnetic susceptibility, thermal conductivity, etc and on service conditions are indicated by wear and corrosion resistance, fatigue and life time oriented behaviour, thermoshock stability, and other charactenstics " 10
Table 1: Characterisation of reference samples
composition bulk materials_
spectroscopy, wet chemical analysis
composition of coatings
X-ray fluorescence, EPMA, AES, GDOS, SIMS, SNMS
thickness of coat-ings
roughness___
microstructure
grain size and tex-
ture__
coating internal stress__
cap grinding, cross-section ing, X-ray diffraction, step height measurement. NDT-test methods^ 3D profilometry
optical microscopy, SEM, TEM, XRD phase-analysis
XRD, quantitative metallography
X-ray diffraction, flexural strain, Ravleigh wave analysis
density
hardness, hardness gradient
vveighting and volumina evaluation indentation method
^iguioin-------
adhesion of coatings scratch t™tin«>. indentation method ultrasonics, acoustic microscopy
modulus of elastic-
iH.
4 THE VALIDATION OF METHODS OF TEST
In practice, many of the qualitative tests are used and
recommended, because they are suitable for several coat-
ing materials, simply to handle and commonly based on
visual examination13. Because of limited applicability of
the standard puli test a modified shear test is recom-
mended for the test of adhesion of ceramic coatings12. In
practice often accepted is the Rockvvell indentation test1-
for the qualitative examination of thin hard coatings,
where the remaining plastic deformation after indenta-
tion causes a "spalling off" of the layer, so an area of
failure remains. But in spite of reference samples pro-
ducing calibration data under reproducible measurement
conditions this method of test is not valuable until now
to be adapted as a standard.
4.1 Reference specimens for calibration
Calibration requires the production of an ensemble of
vvell characterised specimens vvith hard coatings of per-
fectly controlled chemical composition, microstructure,
and thickness uniformity, likely to become certified ultimate^ as reference specimens. The follovving methods are used to assess these properties.
4.2	Measurement of the coating thickness
For the measurement of the coating thickness the methods of stylus profilometry14 and cap grinding15 are employed. A further development of the cap grinding method allovvs the in-situ detection of the coating relief during abrasive extraction16. This method can also be modified for the determination of the coating resistance against abrasive vvear under defined friction control.
4.3	Measurement of the chemical composition
Chemical analysis23 vvas carried out from various lay-ers of defined (N/Ti)-composition vvhich has been real-ised by reactive magnetron sputtering vvith nitrogen vvhere the N-concentration has been varied by trimming the gas flovv18. The spectrochemical analysis of the layer led to a constancy of the composition of ±1% atomic%, vvhich has been certified by reference labs18.
4.4	Measurement of the coating hardness
It vvas found by round robin comparison that existing standards methods of hardness tests on hard coatings are rarely valuable for certification. Hardness measurement by load-indentation1819 on TiN coated test plates of tool steel shovved under certain conditions, regardtng the surface quality, chemical homogeneity, smooth interface morphology, hard substrate material, reproducible hardness values of 20 GPa by means of a test force betvveen 250 mN (HU 0.25) and 1 N (HU 1) vvithin an error vvidth of less than ±2,5% of the value as determined above. These platelets, described in18, could act as calibration samples for hardness measurement.
The surface morphology, respectively the surface roughness, plays an important role on accuracy and re-producibility. Doubtless, the calibration of the Instruments, their stability and constancy during the test pen-ods give a guarantee for exact results. Several procedures, especially for the nanohardness scale, have been developed and controlled through various round robin tests19.
4.5 Adhesion of the coating
In the same way the influences on scratch test sensi-tivity and reproducibility of results are carried out. Obeying the vvhole system, stylus geometry and adhesive debris, surface quality, the existence of lubricants, hu-midity and temperature the function of influencing parameters is studied22. Then, using various coating/sub-strate composites of defined combination, scratch test failure modes are analyzed and various features cata-logued. Improving the NDT techniques for in-situ control during process routines, ultrasonic test procedures are validated20. For thin hard films it vvill be realised by surface acoustic measurement techniques. The input ot one specified research vvork is to enhance and cahbrate the surface acoustic vvave velocity measurement tech-nique vis a vis reference coated substrate standards vvith
the aim of developing a comprehensive test methodology for coating thickness. This measurement methodology will then be proposed to CEN for adoption as an Euro-pean standard.
5	assessment of tribological performance
Engineering components often suffer from wear and friction resulting at best in energy inefficiencies and losses in performance, and in the worst cases cata-strophic failure. There has been considerable recent in-terest in improving the tribological performance of components by the application of wear resistant coatings to the surface of the components22.
For reliable assessment of the tribological performance of potential coating systems, robust and effective testing procedures are required. Testing of the cavitation erosion by ultrasonic affects in water can be estimated as a valuable method for standardisation18.
Current friction and wear testing procedures are not sufficiently well defined to allow data comparisons, and evaluation of the significance of reported data is difficult. The vvork to be carried out in one prenormative research22 involves the development of improved procedures for uniaxial sliding wear testing; reciprocating sliding wear testing and abrasive testing.
6	assessment of life time of coated parts
Whatever the main technological function may be, in practice mechanical properties of the coating substrate compound vvill determine the Iife time vvith on service conditions because the interactive elastic and plastic behaviour of coating and substrate controls the fatigue en-durance, fracturing conditions and the adhesion of the coating. At first, the aims of the research vvork conducted in one current program22 are to develop and validate the methods of test to determine the elastic properties of the surface layer. In comparison to the tribological test procedures quantitative assessments based on scientific in-vestigations due to structure and composition of reference samples are provided. These items vvill involve the development of reference materials and the conduction of round-robin intercomparisons.
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11	European Envisaged Standard ENV 1071-5, Methods of Test of Ceramic Coatings: Determination of the Porosity, 1995
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"H. Jehn, G. Reiners: Charakterisierung diinner Schichten, DIN Fach-bericht 39, Beuth Verlag GmbH., Berlin, Wien, Ziirich, 1993, ISSN 0179-275X
14 European Envisaged Standard CEN ENV 1071-1, Determination of the Coating Thickness: Step height measurement by means of a contact profilometer, 1992
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the Coating Thickness; Cap grinding method, 1992 16H. Kltimper-VVestkamp: Prakt. Metallographie (German DGM, Spec. Vol. 26, 1994), Mikrotribometer zur Qualitatssicherung von Rand-schichten (Pat. DE-P 4434661.1) 433-442
17	European Envisaged Standard CEN ENV 1071- 3: Characterization of adhesion: Scratch adhesion test, 1992
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23	European Envisaged Standard CEN ENV 1071-4; Determination of the chemical composition, 1995
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