THE VDI-3198 INDENTATION TEST EVALUATION OF A RELIABLE QUALITATIVE CONTROL FOR LAYERED COMPOUNDS

Transcription

1 THE VDI-3198 INDENTATION TEST EVALUATION OF A RELIABLE QUALITATIVE CONTROL FOR LAYERED COMPOUNDS N. Vidakis *, A. Antoniadis *, N. Bilalis** * Technological Educational Institute of Crete, Greece ** Technical University of Crete, Greece ABSTRACT: Mono- or multi-layered coated compounds are powerful alternatives of conventional bulk materials, which normally illustrate a restricted surface performance. Nowadays, advanced and complicated techniques are induced in film production devices, leading in this way to the development of an extended variety of different coating types. Herewith, soft, hard or super-hard films can be produced in single or multilayer textures, serving in this way demanding applications, which require advanced surface attitude. Taking into account the growing industrial and manufacturing demands, a well-organized contest of characterization processes for such coatings is required. This paper describes the methodology and typical results of a fast, reliable and cost effective quality test, which is based on the Rockwell Cone indentation on planar surfaces of coated compounds. This destructive test, may rigorously exhibit two distinctive properties of the coated compound, i.e. the interfacial adhesion as well as the film brittleness and cohesion. The stress strain field, which occurs during the indentation and the relaxation stages respectively, must be thoroughly considered in order to obtain secure conclusions, regarding the quality of the coating substrate system. KEYWORDS: Indentation Tests; Quality Control; Fracture Mechanics; Adhesion; Cohesion; 1. INTRODUCTION The effective improvement of substrate materials with restricted properties, by means of thin coating deposition, has become a well-known and economically satisfactory practice. In this way, advantageous mechanical, physical and chemical properties can be achieved, by means of thin and hard mono- or multi- layer coatings on conventional materials, such as steel and cemented carbides. Nowadays, advanced deposition techniques are being used, exploiting in this way highly sophisticated and evolved depositing devices. These processes ensure the possibility to achieve flexibly excellent surface behaviour. On the other hand they set the quality control of a coated compound an exceptional multi-parametric process, counting the film bulk properties as well as the ones of the compound. Furthermore, the failure type expectations of the coating substrate systems are technological data, which hold great research and industrial merit. The well-known Rockwell C indentation test is prescribed by the VDI 3198 norm, as a destructive quality test for coated compounds [1]. The principle of this method is presented in the upper right part of figure 1. A conical diamond indenter penetrates into the surface of a coated compound, thus inducing massive plastic deformation to the substrate and fracture of the coating. As in every indentation test, the 1/10 th rule must be accomplished, and therefore the overall specimen thickness must be at least ten times greater than the indentation depth. The type and the volume of the coating failure zone, exhibit in a first sight the film adhesion and secondly its brittleness. The coated specimen may be adequately evaluated, by means of conventional optical microscopy. However,

2 the specific quality control method becomes significantly more effective, when Scanning Electron Microscopy and Spectroscopy are utilized. indentation load coating substrate microcracks delamination Figure 1: The principle of the VDI 3198 indentation test. The contact geometry, in combination with the intense load transfer, induces extreme shear stresses at the interface. Well adherent coatings, manage to withstand these shear stresses and prevent extended delamination circumferentially to the imprint. The four different textures of the left part of figure 1, illustrate the imprint shapes that guarantee strong interfacial bonds between the coating and the substrate [1,2]. On the other hand, extended delamination at the vicinity of the imprint indicates a poor interfacial adhesion. Furthermore, radial cracks and poor delamination indicate a strongly adherent coating but also brittle ones. In any case, the explicit and comprehensive description of the stress strain field, which takes place during the indentation process, is of great importance, taking into account that the failure modes of coated compounds are well correlated to specific stress components. Therefore, such parameters are presented in the following paragraph. 2. POTENTIAL FAILURE TYPES OF COATED COMPOUNDS The coating performance under usual or extreme stressing conditions has, as it is already mentioned, two different but evenly fundamental aspects. The first one refers to the mechanical properties of the films as discrete media and corresponds to their bulk properties. In spite of their elementary thickness, coatings illustrate intrinsic mechanical properties, such as their internal cohesion. The related literature indicates the normal components of the stress tensor to be responsible for brittle failures of the coatings [2-7]. Normal stresses greater than the critical ones, i.e. the coating strength, cause coherence release or chipping, as it is shown in the upper left micrograph of figure 2 for TiAlN coating deposited in premium quality 100Cr6 bearing steel. On the other hand, remembering that the film teams up with the ground material, i.e. the substrate, the formation of strong interfacial bonds, the so-called adhesion is a key parameter. The release of the interfacial bonds is correlated with shear stress components of the stress tensor, which cause micro- or macro-delamination. This kind of failure is illustrated in the upper right part of figure 2, for CrN coating deposited on High Speed Steel (HSS). In this dashing case, the film delamination is evident, whereas the ground material is so exposed that even its grinding marks are visible.

3 There is also the probability to handle failure mechanisms, the so-called mixed failure modes, which are not easy to be classified. In such cases it is difficult to conclude whether the fracture is a result of de-cohesion or delamination. Normally, this kind of failure is caused by a combination of normal and shear stresses, according to the scenarios illustrated in the bottom part of the same figure [2-7]. These scenarios may include the effect of shearing, bending and buckling. Single failure modes coating Mixed failure modes delamination zone substrate Tensile stress concentration decohesion (microcracks) Figure 2: The potential failure types of coated compounds owing overstressing. The ensemble of the aforementioned failure modes can appear during the evaluation of Rockwell C indentations into coated compounds. The reasons have to be gone after the breadth of parameters that define the overall performance of each specific compound, such as the bulk mechanical properties of the component materials and their mismatch, the film thickness, the deposition temperature, the chemical affinity between the coating and the substrate and many others. However the evaluation of the test results easily depicts primarily the interfacial adhesion and secondly the film brittleness. Obviously, the aforementioned data are qualitatively determined, bearing in mind that they are results of optical observations. The specific stress field, which occurs during the indentation tests, is well approached analytically by the plasticity theory for non-layered materials. However, elastoplastic indentations on layered compounds cannot be examined using analytical methods, considering that this kind of problems is associated with three non-linear sub-problems, i.e. the plastic deformation of the substrate, the anisotropic behaviour of the compound and the contact problem itself. Nevertheless, the VDI test has been examined by means of arithmetical techniques, such as the Finite Elements Method, especially using the capabilities of contact elements and other non-linear modules of various FEM codes. Despite the fact that such methods illustrate great research interest, such non-linear FEM analyses are complicated up to time exhaustive, and up today they cannot practically be exploited by the coating industry.

4 3. TYPICAL EXPERIMENTAL RESULTS CASE STUDIES The optical or SEM observations of examined specimens readily yield practicable and applicable information, especially when they are applied in comparative analyses. A first typical application refers to the performance of the same coating on different substrates. It is well known and explained that the interfacial adhesion is a compound property, so that it is very regular that a certain film, deposited under identical conditions and specifications, exhibits excellent adhesion to a particular substrate but a very poor to another one. Figure 3, illustrates such a case. In the left micrograph, CrN is deposited onto premium quality bearing steel 100Cr6, and evidently exhibits ideal adhesion. The interfacial bonds are so strong that even at the region where the substrate piles up, there is not any indication of delamination. On the other hand, the same coating on High Speed Steel (see the right micrograph of figure 3) indicates very poor adhesion and a remarkable number of radial micro-cracks. Both micrographs have been taken with the aid of optical microscopy. To increase the contrast of the pictures, a graphite layer were spread onto the specimens before the indentation, which is a very common practice when conventional microscopes are used. pile - up delamination cracks Adequate interfacial adhesion Poor interfacial adhesion Figure 3: Typical experimental cases of the same coating, well and poor adherent on different substrates under identical conditions. A second but still typical request of coating producers is the failure resistance of coated compounds, after the completion of a marginal thermal circle. The conservation of the bulk properties and the interfacial performance of coatings is usually a key demand also for coating users. This parameter becomes more composite, considering that coatings experience chemical transformations at elevated temperatures, besides any impact to their mechanical properties. The flexibility of the Rockwell C indentation method, when a fast decision has to be taken, is presented with the aid of the second experimental assignment. A contest between the two Titanium and Aluminum derivative films, i.e. of TiAlN and TINALOX on cemented carbide substrates was carried out with the aid of the Rockwell-indentation test, according to the VDI indications [8]. This test was performed for both coatings, in as deposited status, as well as after their oxidation at high temperatures (750 o C) and recovery at room conditions. Figures 4 and 5 illustrate the corresponding results. The left SEM micrographs of both figures state that at room temperatures the adhesion of both coatings, being at their as deposited status is comparable, yet illustrating a more ductile behaviour for the TINALOX coating and a more brittle for the TiAlN one. This behaviour is common when cemented carbides are used as substrate materials. At room temperature, the TiAlN coating exhibits an increased brittleness, despite its lower hardness than the TINALOX coating and its lower

5 hardness mismatch to the substrate. This behaviour is highlighted by the radial micro cracks in the vicinity of the imprint. Even after the oxidation and thermal recovery to the room conditions, which are presented in the right micrographs of the same figure, the failure zone around the formed imprint is greater for the TiAlN coating. The decisive remark of this experimental branch is that the TINALOX coating is better adherent to the same ground material, keeping satisfactorily this property after oxidation. 100µm 100µm Figure 4: Qualitative explanation of the heating and oxidation effect on the adhesion of TiAlN Coating on Cemented Carbide Substrate. 100µm 100µm Figure 5: Qualitative explanation of the heating and oxidation effect on the adhesion of TINALOX Coating on Cemented Carbide Substrate.

6 Each of the four-presented micrographs incorporates a detailed aspect of the coating surface, which indicate the effect of the chemical phenomena, i.e. oxidation, on the structure and consequently on the expected film performance. 4. CONCLUSIONS This paper presents a very applicative quality control method, which is be used to characterize the adhesion of thin coatings on various substrates, as well as to outline their cohesion expectations. This manly qualitative test can be exploited for research as well as for industrial purposes. This method can be easily applied using common equipment, such as a typical sclerometer and a conventional optical microscope. However, a Scanning Electron Microscope and its Energy Dispersive X-Ray (EDX) analyzer, offer more extensive information, yet qualitative. The Rockwell C test method onto coated specimens is ideal for comparative and parametric analyses, and usually saves evolution cost and time. On the other hand the specific test is supplemental to other qualitative methods, such as the scratch test. The research and development capabilities of the test were exhibited in the present research work in two different but ordinary quality control cases, using the capabilities of optical and scanning electron microscopy respectively. REFERENCES 1. Verein Deutscher Ingenieure Normen, VDI 3198, (1991), VDI verlag. 2. Vidakis N., (1997), Determination of the fatigue strength of thin hard coatings for the prediction of their life time on hybrid bearings steel races, used in high speed spindles of machine tools, PhD Thesis, Aristoteles Univ. of Thessaloniki, Greece. 3. Djablella H., Arnell R., (1992), Finite elements analysis of the contact stresses in an elastic solid coating on an elastic substrate, Thin Solid Films, Vol 213, pp Komvopoulos K., (1988), Finite elements analysis of a layered elastic solid in normal contact with a rigid surface, ASME Journal of Tribology, Vol 110, pp Ihara T., et. al., (1986), A finite elements analysis of contact stress and strain in an elastic film on a rigid substrate, ASME Journal of Tribology, Vol 108, pp Tian H., Saka N., (1991), Finite element analysis of an elastic-plastic two layer half space: normal contact, Wear, Vol 148, pp Lin W., et. al., (1991), Analysis of a transversely isotropic half space under normal and tangential loadings, ASME Journal of Tribology, Vol 113, pp Bouzakis, K.-D. et. al., (1999), Quantification of Properties Modification and Cutting Performance of (Ti x Al 1-x )N coatings at Elevated Temperatures, Int. Journal Surface and Coating Technologies, vol , pp