ICMCTF2010 Session E2-2: Mechanical Properties and Adhesion

Multilayered coatings of TiN/CrN, TiN/NbN and NbN/CrN of different thicknesses of alternating layers have been deposited by direct current (dc) magnetron sputtering onto Si<100>. For comparison, single layer reference coatings of TiN, CrN and NbN of ~700 nm thickness have been synthesized. The mechanical characterization was conducted using a combination of micro – and nano - indentation. In-situ SEM micro-/nano- indentation was used to observe the deformation process including any pile-up, sink-in, and fracture mechanisms specific to each coating. These information have been supplemented with the microstructural deformation mechanisms characterized by TEM studies on the cross-sections containing indentation imprints. Subsequently, the tribological properties have been characterized at both nanoscale (ex-situ nanoscratch and nanowear) and microscale (in-situ SEM microscratch). This gave an insight into the tribological resistance of the thin films independent of the substrate as well as in the film-substrate system collaboration.

In a result it was found out that the mechanical response strongly depends on the scale of analysis and the type of multilayer. The deformation mechanisms of the constituent layers in the multilayer stack combine in each multilayer film differently giving new characteristic deformation mechanism. From the nanotribological studies, it was revealed that the scratch deformation cannot be used for wear estimation. But, the microscratch results are in agreement with the micromechancial studies.

Finally, the finite element model of the multilayer deformation has been developed to elaborate the deformation and fracture response of the multilayers. The visualization of stress and strain distribution under the load and in the residual state supplements the information obtained in the experimental process. It also suggests that the interfaces play a critical role for the shear stress distribution.

The main conclusions from the point of view of particular applications will be presented.

In this work, three-dimensional finite element analyses were conducted to understand the stress distribution during scratch tests on coated systems with orthotropic material properties. Different from analyses commonly found in the literature, the mechanical behavior of both film and substrate was considered elastic-perfectly plastic. Besides, it is known from previous works that orthotropic materials may alter the contact stresses in the film surface, modifying the location and probability of nucleation and propagation of cohesive cracks. To study this material aspect, the orthotropic properties were analyzed for the substrate. Those material properties were the Young's modulus and the yield stress. Results were analyzed based on stress distribution in the film and substrate, which are mainly linked to the failure of this system. Results have indicated that systems with orthotropic properties are less prone to film failure commonly seen in experimental scratch tests.

Metallic platinum is important for industrial applications and the demand has increased considerably in the past decade, particularly in applications such as jewellery and catalytic converters which together account for over 80% of platinum consumption. The platinum coated systems are used in many applications where the surface properties such as hardness, scratch resistance or electrochemical activity play an important role. Our research study was focused onto platinum-vanadium single and multilayered systems and we have been particularly interested to compare the scratch resistance of as deposited and annealed coatings. A single platinum layer and multilayer systems of platinum and vanadium (double and triple layers) deposited on vanadium substrates were used in this research. They have been annealed at 700⁰C, 800⁰C and 900⁰C under vacuum conditions for 45 minutes and afterwards characterised using several complementary techniques.

The phase formation and coating morphology have been determined by x-ray diffraction and scanning electron microscopy respectively while the scratch resistance was tested using a nano-scratch tester. The phase analysis showed that several phases, PtV, Pt₂V₃ and Pt₂V were formed as a consequence of annealing. The coating morphology and roughness were also affected by elevated temperatures. More importantly, it was found that the scratch resistance of annealed coatings is significantly better in comparison to scratch resistance of as deposited coatings. It indicates that the platinum-vanadium phases formed during annealing process has significant role in increasing the scratch resistance and improving the adhesion properties of platinum-vanadium coated systems.

Indentation experiments were conducted inside a scanning electron microscope to measure adhesive strength of individual alumina splats plasma-sprayed atop a steel substrate. In-situ type of experimental evaluations made characterization of interfacial crack propagation possible by direct observation. Strain energy increase of brittle alumina splats originated from indentation deformation was correlated to the strain energy release rate through the characterization of the interfacial crack propagation. An analytical model previously reported and evaluated in the studies of adhesive strength of thin-films was employed. The average calculated strain energy release rate of 80 J/m² was determined for single splats. This high value suggests that splat adhesion can possess a significant contribution to the adhesion of thermal sprayed coatings.

The most important functional requirements for continuity and performance of hard wear-resistant coating is adhesion. Scratch test used to control and to measure adhesion of thin hard coatings. In this work, thin films of Ti-Ta-B-N with different nitrogen contents were deposited on Toolox and D2 substrates by closed-field unbanced magnetron sputtering (CFUBMS). Microstructure and phase composition of thin films were studied by X-Ray Diffractometer (XRD) and Scanning Electron Microscopy (SEM). To measure adhesion of Ti-Ta-B-N thin films have been scratch tested at the a standart mode using progressive load operation. The effect of different nitrogen contents and substrates (on adhesion) were investigated.

Transition metal nitrides have been used routinely as coatings for cutting tools and their mechanical properties have often been measured by nanoindentation at room temperature. However, these materials are used at temperatures up to 1000˚C. In this work we have measured the high temperature properties of a range of transition metal nitrides, including TiN in bulk and thin film form and TiAlN with the aim of understanding how the materials behave at these elevated temperatures. It will be shown how simple nitrides soften considerably at temperatures far below their operating temperature and also how microstructural changes that might have little effect on the room temperature properties can become quite dominant at elevated temperatures. In this work the mechanical properties have been measured by high temperature nanoindentation and micropillar experiments while observation of the deformation has been carried out by TEM.

Residual stresses existing in most polycrystalline hard coatings have significant influence on the adhesion, mechanical properties and tribological performance. The aim of the research is to optimize the XRD-sin²ψ technique so that surface residual stresses can be determined with high precision and good reliability. In the X-ray diffraction (XRD) sin²ψ stress measurement, the value of residual stress is determined through a linear regression between two parameters derived from experimentally measured diffraction angle (2θ), whereas the linear regression precision factor R² reflects the accuracy of the stress determination. Thus, the accuracy of the obtained residual stress value depends strongly on how precise the 2θ values are measured out of a series of very broad and sometimes irregularly-shaped diffraction peaks. In practice, uncertainty arises when different diffraction peak positioning methods are applied, mainly including the maximum intensity (I_max) method, the middle point of half maximum (MPHM) intensity method, the area/gravity centre method, and the parabolic approaching method. However, up to date little comparative experimental research has been done on the precision of residual stresses determined by using the different 2θ measurements.

In this paper, glancing angle XRD experiments have been conducted to determine the surface residual stress of a large number of samples, including an electron beam evaporated ZrO₂ based thermal barrier coating and several magnetron sputtered transition metal nitride coatings, as well as a few shot-peened superalloy gas turbine components. On the XRD scans of each tested sample, the 2θ values were determined using the above mentioned peak-positioning methods, and then the obtained residual stress and the R² values were compared with each other.

The experiments reveal that, the residual stresses obtained range from -0.08 Gpa to -6.67 Gpa depending on the tested materials. The relative precision R² values vary between 0.25 and 0.99 depending on both the quality of the obtained diffraction curves and the methods of the 2θ value determination. The parabolic approaching method resulted in the highest precision of linear regression, with R² = 0.93 ± 0.07 out of the obtained results, which showed low deviation of the determined residual stress value in all cases; both the MPHM (R² = 0.86 ± 0.16) and gravity centre (R² = 0.91 ± 0.11) methods could also give good results in most cases; and the I_max method (R² = 0.71 ± 0.27) exhibited substantial uncertainty depending on the nature of individual XRD scans. Based on the experiments, an optimized surface residual stress measurement method is recommended.

For hot forming tools classical wear protection coatings mostly are not sufficient for an industrial use. Due to high process temperatures the tribological properties are much different compared to room temperature conditions and the use of lubricants is limited. Additionally thermal shock conditions and the increase of sticking workpiece material are leading to severe wear.

Recent developed Ti-B-N coatings are thermally stable, antiadhesive against workpiece material and wear resistant. Examples for these ternary boron based coatings obtained through plasma-enhanced chemical vapor deposition will be presented.

Coatings of the ternary Ti-B-N system could be adapted to the requirements of hot forming applications by variation of the boron and nitrogen content.

The deposition of gradient coatings was also developed to adapt the different material properties to the coating. The coatings exhibit a nanostructured crystal morphology in means of a nanocomposite assembly where amorphous regions are containing nanocrystalline components. This can be used in different layer designs for further enhancements of the mechanical load capabilities of tool surfaces. In the presentation the properties of different multilayer systems with altering material contents from pure TiB2 to nitrogen containing regions will be shown. Boron containing systems are also known as thermally stable against degradation and chemically stable against molten aluminium.

An additional pretreatment by plasma nitriding in so called 'single-duplex processes' additionally enhances the mechanical load ability. Generally there is some need to optimize the nitriding parameters which have a significant influence on the nitriding depth, the maximum hardness near the surface and the gradient of the hardness decrease. Different nitriding processes were developed and studied concerning the influence on the crack formation behaviour of tool surfaces.

Current developments showed in different fields of forming applications a very high potential of these coatings. In aluminium die casting applications the sticking of casted material could be significantly reduced resulting in better product surfaces and an increased lifetime of dies. In precision forging processes an overall stabilization of the planned tool life could be achieved and enabled a continuous production without needs of reworking the tools. Finally an increase in process reliability paired with a longer tool life has been possible.