ICMCTF2011 Session E2-1: Mechanical Properties and Adhesion

The stress and strain relationships expressed in terms of nanoindentation force and depth are applied to identify the phase transitions due to stress-induced crystallizations at different indentation depths. The further provision of the function of electrical contact resistance (ECR) in the present indentations system plus the aids of the diagrams of Gibbs free energy and the Raman spectra allow to be more clear and complete to track the paths of phase transition. The predictions of the critical stress and strain corresponding to some phase transitions by the present model are proved to have good consistence with the experimental results appearing a noticeable turning. The borders for different phase transitions arising in the loading/unloading process can be determined in the stress-strain diagram. The loading/unloading rate is of importance to the phase species created in the nanoindentation process and the critical stress and strain forming in phase transition.

Within sufficiently long time scales and depending on the temperature all solid materials show to a certain extent time dependent mechanical behavior. There are many different models to describe this effect, but none of which is sufficiently and generally applicable to the problem of analyzing mechanical surface tests like indentation and scratch in a proper and physical manner. The way out of this dilemma usually is to perform the tests fast enough in order to reduce the influence of the time dependency of certain mechanical parameters like Young's modulus and yield strength in order to apply the classical none-time dependent analyzing techniques.

However, this bypass has its limits

a) with respect to the materials it could be properly applied to

b) because it explicitly excludes the time dependency of the material properties, which is a great disadvantage if exactly this dependency shall be discussed

Thus, a new method is required providing the means for taking time into account generally and in an applicable manner. In principle, such a method is already at hand and its extension to time dependency is rather straight forward. It is called "the concept of the effectively shaped indenter" [1].

Within the talk it will be demonstrated how the concept has to be adapted not only to take time, even load rate into account, but also to become applicable for layered materials and multiaxial indentation tests.

[1] A. Bolshakov, W.C. Oliver, and G.M. Pharr, MRS Symp. Proc 356, p 675 (1995)

The adhesion strength of industrial Diamond-like Carbon (DLC) coatings is a crucial factor for the performance of the coated components in high load automotive applications. On an industrial scale metallic adhesion layers are frequently used to achieve a certain interfacial strength between the DLC coating and the steel substrates. However, details concerning the correlation between the microstructure, the chemical gradients and the load bearing capacity in terms of interfacial strength of the adhesion layers are not fully understood. In this work Cr-based adhesion layers with different interfacial strength ranging from excellent to poor were investigated in terms of the microstructure, the chemical gradients and their resistance against interfacial failure. From Energy Filtered TEM analysis clear differences in the microstructure and the chemical gradients are found for the two systems. The local structure nicely correlates with the local hardness and Young's modulus, determined on low angle cross sections of the coating systems using nanoindentation. In addition, SEM in-situ bending tests were performed on FIB milled DLC bending beams. By varying the dimensions of the beams, focused investigations on the interfaces of the DLC coating systems with different adhesion strengths can be performed, allowing a quantification of the interfacial strength. Herewith, basic design microstructural based design principles for well adherent Cr-based adhesion layer for DLC coating systems can be obtained.

Thin films and coatings are widely used for their functional properties that strongly depend on their mechanical behaviour and stability. During the elaboration process by physical vapor deposition methods, high internal compression stresses can develop and may then result to failure by delamination and buckling. A plethora of buckling patterns has been observed such as straight-sided wrinkles, circular blisters or telephone cord structures. During the past decade, the buckling phenomenon has been extensively studied in the framework of the Föppl-Von Karman theory of thin plates assuming no pressure mismatch between the lower (inside) and the upper (outside) parts of the buckled films.

It is the purpose of this work to investigate the pressure level inside the buckles. In this context, straight-sided wrinkles have been first induced on a 400 nm nickel thin film by a uni-axial compression of the polycarbonate substrate. To investigate the pressure effect, some closed and airtight wrinkles have been selected and analyzed by atomic force microscopy (AFM). Wrinkles have been cut by a focus ion beam (FIB) technique and then re-analyzed by AFM. An increase of 20 nm of the maximal deflection has been observed after the FIB opening process. This experimental result suggests the presence of a low vacuum environment inside the buckle, between the film and the substrate. This behaviour is discussed in the framework of the Föppl-Von Karman theory of thin plates.

Diamond like Carbon – coatings are used in the automotive industry since more than 15 years now. This is due to their extraordinary wear and friction reducing properties which can mainly be attributed to their high hardness, low affinity to metals and very low coefficients of friction. In order to choose a coating system which is adapted to a tribological application, one has to consider the whole tribological system comprising the coated substrate, the counter body and eventually present lubricants. After that, the mechanical coating properties, for example coating hardness, yield strength, adhesion, Young’s modulus and intrinsic stress, have to be adjusted to the tribological system.

In this work, multiaxial nanoindentation and it's modelling has been used in order to determine the mechanical properties of DLC-coatings and to predict the behaviour of different DLC-coating systems in various mixed-load applications.

In a first step, vertical and lateral nanoindentation has been carried out during coating development. This allowed us to determine the mechanical properties of different coatings as well as to calculate the coating behaviour for different loading cases in order to identify critical areas in the coating and potential for improvement.

In a second step, vertical and lateral nanoindentation and it's modelling was carried out on a DLC-coated component from the automotive industry which showed sometimes good and sometimes poor performance in a real industrial tribological application. In this case and for the first time we were able to physically quantify the failure of a coating system by a real multiaxial, 3-dimensional nanoindentation test and it’s modelling.

Compositionally graded and nanostructured thin films not only provide new technological opportunities, but are also interesting from a scientific perspective. Deliberate design of such advanced thin films has revealed superior and unique combinations of mechanical properties, such as high hardness, toughness, wear resistance etc.

Instrumented nanoindentation is now a commonly used tool in assessing the properties of materials for a wide range of applications. Analysis of nanoindentation data is typically carried out using the methods popularised by Oliver and Pharr, with Young’s modulus and hardness determined by a power law fit to the unloading data of the load displacement plot. One of the key limitations of this analysis technique is that it does not allow consideration of time dependent deformation mechanisms such as creep or visco-elastic behavior exhibited by some materials. Hence if testing such materials, experimentalists find Young’s modulus values dependent on the experimental unloading rate.

Conventional nanoindentation techniques use a fast unloading rate in order to minimise the influence of time dependent deformation on calculated Young’s modulus. Such techniques have been used on two representative samples - a visco-elastic polymer and gold, tested at room temperature and higher (creep) temperatures respectively. Oliver and Pharr analysis of this data returns acceptable Young’s moduli but the results still exhibit unloading rate dependence. Additionally, failure to account for time dependent depth change can lead to physically unrealistic fitting constants in the power law (m>2), calling into question the validity of the fit.

Results using a new analysis method which allows determination of the time dependent contributions to the Nanoindentation depth will be presented. This enables calculation of a Young’s modulus independent of the unloading rate and with physically meaningful fitting constants in the power law. The technique also provides estimates for the stress field in the indented region during unloading.

The boriding process enhances mechanical and chemical properties at the surface of steels through the formation a Fe₂B coating or FeB/Fe₂B coatings. The thickness of the coating formed (known as the case depth), which affects the mechanical and chemical behavior of borided steels, depends on the boriding temperature, the treatment time and the boron potential that surrounds the surface sample. Based on the formal theory of boride growth in different steels, the formation of the FeB borided phase results from the transformation of Fe₂B crystals at the outermost part of the sample due to the high boron potential at the surface. When treatment times and temperatures are increased, FeB regions grow from compact and oriented crystals of Fe₂B and become much deeper. The layers grow preferentially in the (002) plane, thereby increasing the mechanical stresses at the FeB/Fe₂B interface due to lattice distortions in this zone.

For this reason, the present study estimated the adhesion of the FeB/Fe₂B coating formed at the surface of AISI 316 borided steel by means of interfacial indentation test. This technique is used to create and propagate a crack in the FeB/Fe₂B interface, and defining the apparent fracture toughness, which can represent the adhesion and the mechanical support of the aforementioned interface. First, the boriding process was carried out at the surface of AISI 316 steels by developing the powder-pack method, at temperatures of 1123, 1173, 1223 and 1273 K with 2,4,6,8 and 10 h of exposure. Mechanical properties like Young modulus and hardness were obtained by depth-sensing nanoindentation technique for each surface layer. Also, Vickers microindentation fracture technique was used to generate microcracks at the FeB/Fe₂B interface with different indentation loads. The applied loads, crack lengths generated from the corners of the indentations, the Young modulus, and hardness values were set as the experimental parameters for determining the apparent fracture toughness of the FeB/Fe₂B interface.

Considering the set of experimental parameters and the depth of the FeB layer, the apparent fracture toughness for the FeB/Fe₂B interface is in the range of 3.69 to 4.31 MPa m ^½.

This study evaluated the indentation size effect on Fe₂B/substrate interface applying the Berkovich nanoindentation technique. First, the Fe₂B layers were obtained at the surface of AISI 1018 borided steels by developing the powder-pack boriding method. The treatment was carried out at temperatures of 1193, 1243 and 1273 K with 4, 6 and 8 h of exposure times for each temperature. The boriding of AISI 1018 steel results in the formation of saw-toothed Fe₂B surface layers. The formation of a jagged boride coating interface can be attributed to the enhanced growth at the tips of the coating fingers, due to locally high stress fields and lattice distortions. Thus, the mechanical properties achieved at the tips of the boride layer are of great importance in the behavior of borided steel.

Applied loads in the range of 10 to 500 mN were employed to characterize the hardness in the tips of the Fe₂B/substrate interface for the different conditions of boriding process. The results showed that the measured hardness depends crucially on the applied load, which indicated the influence of the indentation size effect (ISE). The load dependence of hardness was analyzed by using the classical power law approach, a proportional specimen resistance (PSR) model, a modified proportional specimen resistance (MPSR) model, and elastic recovery model. The true hardness in the tips of the Fe₂B/substrate interface was obtained for each model for the set of experimental parameters whose values are similar between the empirical equations adopted in these models. Finally, the nanoindentation technique was used to estimate the state of residual stresses in the critical zone of the Fe₂B/substrate interface.