ICMCTF2013 Session E2-3: Mechanical Properties and Adhesion

Thermal exposure of oxidizing metals with concentrated laser irradiation in ambient atmosphere produces metastable dielectric layers with characteristic colors. While most work has focused on continuous wave (CW) laser exposure of metallic substrates, relatively few groups have studied pulsed laser colorization and no studies have addressed mechanical behavior. Oxides on titanium and stainless steel substrates are particularly interesting; the properties of the film and substrate combination control wear and fracture of the oxide with a strong contribution from plastic deformation in the substrate. Films were found to be comprised of multiple crystalline phases with misfit strains leading to through-thickness cracking. Traditional quasistatic and dynamic nanoindentation with multiple tip radii and nanoscratch testing probed mechanical properties, fracture, and wear behavior. Oxides have elastic moduli lower than the metallic substrates, but hardnesses that are much higher, on the order of 10-16GPa. Applying an energy and fracture mechanics analysis, fracture properties and residual stress can be determined from pre-cracked films. Finally, NanoECR results indicated that defect structure and electromechanical response vary with processing conditions. Combining microscopy, diffraction, and indentation techniques provides a unique approach for defining wear behavior of laser-fabricated oxide films in harsh conditions. This work was supported by DTRA Basic Research Award # IACRO 11-4471I, NSF Grant NSF/DMR-0946337, and by Sandia National Laboratories, a Lockheed Martin Company for the USDOE NNSA under contract DE-AC04-94AL85000.

Diamond-like carbon (DLC) coatings are characterized by its high hardness and the outstanding tribological behavior. They are hence used as sliding partners in friction pairs in many industrial applications. However, wear and loose particles trapped between the friction pairs can lead to the apparition of locally applied high pressure on the DLC coating. This effect results in contact damage of the coating and, depending on the penetration depth, influences the substrate. In the case of a soft and compliant substrate, the contact damage leads to cracking and imprinting of the hard and brittle DLC coating into the soft and compliant substrate. Mechanical loading can occur during service life or processing and may lead to the formation of cracks and delaminated areas. The failure mode upon contact damage and mechanical loading of DLC coated titanium substrates has been investigated in this work. DLC coatings with a thickness varying from 50 nm up to 4 μm have been deposited.

The contact damage could be diminished by taking a stiffer substrate. However, this approach is often not suitable since the substrate material is given by the industrial application. In the case of a soft and compliant substrate (e.g. titanium), a suitable interlayer combination which is deposited between substrate and DLC coating can show similar properties as a stiff substrate. The stress field distribution upon indentation has been determined by finite element modeling (FEM) and the contact damage has been simulated by microindentation and then analyzed by scanning electron microcopy (SEM), load-displacement curves and focused ion beam (FIB) cuts. It has been observed that the cracking morphology and the stress field distribution in the substrate/interlayer/DLC system strongly depends on the thickness of the DLC coating and interlayer as well as on the interlayer material.

The mechanical loading was simulated by uniaxial loading and the damage pattern was recorded in situ by optical, scanning electron (SEM) and atomic force microscopy (AFM). The cracking analysis shows that a 50 nm DLC coating exhibits localized areas with a high crack density whereas thicker coatings show regular crack patterns with equidistant cracks. In addition, strain at onset of fragmentation decreases with increasing film thickness and a widening of existing cracks upon further straining has been observed.

Various components need protection against superimposed corrosion and wear (abrasion, erosion) loading, e.g. in off-shore applications. The goal of the research has been to develop PVD multilayer coating by systematically altering the layer architecture in order to protect components against corrosive environments and erosive loadings. A Diamond-Like-Carbon (DLC) toplayer of the multilayer coating system is responsible for the erosion resistance and a multilayer architecture below ensures a good corrosion protection.

One of the primary motivations for development of instrumented indentation was to measure the mechanical properties of thin films. Characterization of thin film properties as a function of temperature is important for both engineering design and scientific considerations. The major challenge in elevated temperature testing of thin films is to obtain clean load-displacement data for shallow indentation depths that correspond directly to the deformation behavior and to avoid artifacts of testing like thermal drift and noise. Thermal drift arises due to thermal expansion/contraction of the indenter load column and gets convoluted with the deformation data being recorded by the instrument. The compliance of the sample mounting can change as a function of temperature making the extraction of accurate modulus values difficult for thin coatings.

Keeping these challenges in view, we have developed a novel high temperature nanoindentation system that can perform accurate nanoscale measurements up to 400 degrees C in vacuum at pressures as low as 10e-6 mBar to prevent oxidation. This system utilizes an active surface referencing technique that measures the differential displacement between the indenter tip during indentation, and a reference sitting on the sample surface. The most important aspect of this active referencing system is the elimination of frame compliance making it suitable for accurate property extraction of thin films as a function of temperature. The challenges associated with elevated temperature nanoindentation testing and recent progress made by us in its application to thin film measurements will be presented. Relevant design modifications and operational refinements that have resulted in minimizing drift rates to less than 7nm/min at 400 degrees C will be discussed. Noise level in measurements was found to be negligible with increase in testing temperature. Case studies on multilayered films will be presented to illustrate the best practices and experimental considerations in nanomechanical testing of thin films at elevated temperatures.

Multilayered thin films exhibit superior mechanical properties compared to their single layered constituents. Metal / metal and metal / ceramic multilayers of Al/W and Al/TiN deposited by magnetron sputtering on silicon substrates were chosen as they form model systems to study the size dependence of mechanical properties of the systems as a function of temperature. W and TiN were used to prevent interlayer diffusion during the deposition and elevated temperature testing of these systems.

In borided low-carbon steels, the morphology displayed by the Fe₂B layer is saw-toothed, and the layer-substrate interface has a columnarity extent. The layer morphology can be explained because the diffusion process is of strongly anisotropic nature. So, the mechanical properties are affected by the anisotropy along the boride layer.

One important mechanical parameter in design is the fracture toughness value. In recent years, several attempts to determine the fracture toughness of different borided steels have been carried out using the Vickers microindentation test.

The purpose of this work is to estimate fracture toughness along the Fe₂B layer using the Berkovich nanoindentation technique. First, the boriding of AISI 1018 steel was developed by the powder-pack method at a temperature of 1273 K with an exposure time of 8 h. The boride layer thickness was estimated in 210 microns. Berkovich nanoindentations tests were performed on the “pure zone” of the Fe₂B layer at distances of 25, 50 and 75 microns from the surface of the borided steel, where the indentation loads were varied between 10 to 500 mN at each distance. The behavior of the hardness as a function of the indentation load showed the presence of the indentation size effect (ISE) at the different distances from the surface, in which the apparent hardness values of the Fe₂B layer were estimated by the model of geometrically necessary dislocations. In addition, the measurements of the cracks emanated from the corners of the indentation marks were evaluated for the different applied loads.

Finally, the mechanisms of various crack patterns and existing models used to estimate the fracture toughness such as stress-analysis-based model and energy-based models were discussed as a function of the anisotropic nature of the boride layer.

Extraction of fundamental materials properties, such as the critical resolved shear stress (CRSS) required to move dislocations and anisotropic single crystal elastic constants requires isolation and testing of individual microstructural units. We have performed these tests on single samples within polycrystal samples using nanoindentation [1], micro-pillar compression and micro-cantilever testing [2]. The study was performed on 99.98% pure zirconium.

Nanoindentation was performed to extract indentation hardness and modulus as a function of crystal orientation (measured with EBSD). Evaluation of the local flow fields around two of these indents was performed using HR-EBSD and reveals evidence of a complex stress state, indicating the inherent difficulty in interpreting nanoindentation results to quantitatively assess key mechanical properties (similar to a prior study in Ti [2]).

To simplify the testing geometry and therefore the applied stress state, we have fabricated small scale mechanical test specimens using FIB (similar to previous work in titanium [3]) and observed both differences in the activation of each slip system as well as a mechanical size effect.

Micro-pillars offer a simple geometry for uniaxial testing but in practice understanding the strain state is difficult. In our experiments mechanical instabilities and difficulties in making a ‘perfect’ test specimen at the small scale result in non-uniform deformation states and difficulties interpreting the onset of plastic strain. This makes rational extraction of key parameters not trivial.

Micro-cantilevers with an equilateral cross section provide a slightly more complex strain state (i.e. bending) but their stable deformation process is well suited to continuum based modeling approaches to extract mechanical properties. Each cantilever was milled to contain a volume of material at a particular orientation with respect to the beam design in order to apply the maximum shear stress to an individual slip system (i.e. single slip) and they were carefully measurement and then tested with a nanoindenter. The load-displacement response was recorded and compared to the deformation of a similar 3D finite element crystal plasticity model [4]. The CRSS in the model until the experiment and simulation load-displacement curves matched well.

[1] Oliver, W.C., and G.M. Pharr. JMR (1992)

[2] Britton, T.B., H. Liang, Dunne, F.P.E, and Wilkinson, A.J., Proc. Roy. Soc. A (2010)

[3] Gong, J., and Wilkinson A.J., Acta Mat., (2009)

[4] Dunne, F. P. E., Rugg, D, and Walker, A., IJP (2007)

TiFeN, TiN and TiFeMoN nanocomposite films with a wide range of mechanical properties have been deposited on Si using a dual ion beam system to investigate the correlation between mechanical properties and performance. Mechanical properties were determined by nanoindentation, tribological behaviour assessed by nano-scratch testing and their dynamic toughness by nano-impact testing.

Failure behaviour of the films was strongly correlated with the ratio of hardness to modulus (H/E) in the film. In the nano-scratch test nanocomposite thin films of TiFeN with very high H/E ratios failed dramatically at low critical load, with failure leading to large-area delamination. Films with slightly lower H/E were found to possess a more optimum combination of hardness and toughness for applications where they could be exposed to high shearing forces and do not show the same failure behaviour. In the nano-impact test films with high resistance to plastic deformation (H^3/E^2) showed improved performance at low impact forces but not at higher forces.

Their suitability for high temperature applications has been investigated using a recently developed modification to a commercial nanoindentation instrument (NanoTest) enabling nano-scale friction measurement at 750C.

Microstructural and basic mechanical properties characterization of WC - based coatings obtained by standard HVOF method was showed in this article. Three different feedstock powders of WC-Co 83-17 type was used to deposition of coating o steel substrate. First of them was the standard powder of Amperit 526.074 type, second one it was powder by Inframat from category of Infralloy™ S7400superfine powders. And the last it was the standard Amperit 526.074 modified by nanoparticles of carbides. The aim of investigation was related to comparison of microstructure and some mechanical properties of coatings depending of used types of powders and characterization of nanocarbides influence on basic mechanical properties of coatings. The range of investigations included short characterization of feedstock powders by SEM, EDS, XRD and EBSD method and their technological properties as well. In second step the characterization of deposited coatings were made, especially evaluation of theirs overall quality, porosity, micro-hardness distribution, adhesion of coatings to substrate alloys and theirs tendency to cracks. To characterization of coatings microstructure the same methods were used. Adhesion to substrate alloy and tendency to crack of coatings were characterized by bend test and Brinell hardness measurement on polished top surface of carbide coatings.

Financial support of Structural Funds in the Operational Program - Innovative Economy (IE OP ) financed fr om the European Regional Development Fund - Project No POIG.0101.02-00-015/09 is gratefully acknowledged.