Microstructure Based Structure-Property Relationships for the Design of Thin Films and Composite Coatings by Multiscale Materials Modeling

Integrated Computational Materials Engineering (ICME) is the next generation methodology for the discovery, development, and deployment of material solutions. ICME refers to tackling materials science and engineering problems by way of merging materials modeling exploiting High-Performance Computing (HPC), experimental and characterization activities, as well as data and its analytics in the solution process and utilizing this toolset to deliver better performing innovative material solutions faster. A crucial element in ICME is the application paradigms of multiscale materials modeling, such as the widely adopted Process-Structure-Properties-Performance (PSPP) approach. The PSPP construct itself is a common frame for experimentalists and modelers alike to convey materials problems from discovery to deployment in terms of the typical progression of how to develop a new material or material solution. To realize PSPP in practice for modeling, workflows are required. These workflows consist of interacting multiscale materials modeling objects, which are interfaced to support the solution of an ICME problem. Commonly, such workflows consist of multiscale models of material manufacturing (processing), digital representation of the following material structure, modeling the properties of the structure in question (for example engineering material properties), and evaluating the performance of the material solution that links the material to its application environment. The PSPP chain as such establishes causal relations from material processing all the way to its application performance. Since optimization-wise this is an imperfect construct, Material Informatics (MI) deals with managing and analyzing the data, ultimately targeting the solution of the coveted inverse problem, where a material is discovered and designed based on its performance requirements and optimized for example with respect to the affiliated costs. The goal is to deploy a material that satisfies the functionality requirements set for the product performance.Applications of ICME began from the need for extreme performance, for example involving material solutions for aerospace applications. Another high end application involves surfaces and coatings for wear resistance and lubrication, engaging material challenges for example in transportation or highly abrasive environments such as mining applications. This is also the domain of the current work, i.e., how to systematically develop better wear resistant surfaces and coatings. The specific challenge is the development, implementation and validation of ICME workflows for microstructure founded design of coatings and thin films for improved wear resistance. Within this scope, the current work develops a multiscale framework for modeling the microstructure and surface topography of complex multiphase coating microstructures and thin films, employing means to model realistic material microstructural morphologies containing also a composite interface character. The behavior of thin solid coatings under sliding abrasive loading is studied, and the possibilities of utilizing Cohesive Zone Modeling (CZM) directly in the modeling of film rupture are established. Next, the focus turns toward the introduction of the microstructural modeling of either thin or composite coating solutions, for which the computational methodologies are developed, implemented and validated. The analyzed cases consider primarily cemented carbide microstructures under abrasive tribological loading conditions. The computational methodology is developed further to increase realism with respect to modeling material interfaces, where Finite Element (FE) based models are interfaced to a Phase Field (PF) based modeling of rapid solidification microstructures as a result of material processing. After establishing a realistic enough description of the composite microstructure, the focus turns to introducing a modeling solution capable of addressing surface roughness and topography, in relation to other coating characteristics. This introduces a methodology enabling modeling of both the coating topography, its microstructure, interfaces with the bulk substrate, and the microstructure of the substrate itself. ICME workflows are discussed, outlined, and set up for the design of wear resistant surfaces utilizing the PSPP principle as a basis, considering especially the microstructure to product linkage in a bottom-up manner. The various use cases support the notion that ICME already provides added value to the solution of tribological problems and material related challenges by adding knowledge for solving problems otherwise difficult and costly to handle. Furthermore, the greatest impact and improved quality of results are obtained when both experimental and modeling approaches are used concurrently and not viewed as alternatives or competitors

Crystal plasticity modeling of transformation plasticity and adiabatic heating effects of metastable austenitic stainless steels

Strain induced phase transformation in metastable 301LN stainless steel generates a heterogeneous multiphase microstructure with a capability to achieve excellent strain hardening. The microstructural deformation mechanisms, prior deformation history and their dependency on strain rate and temperature determine much of the desired dynamically evolving strength of the material. To analyze microscale deformation of the material and obtain suitable computational tools to aid material development, this work formulates a crystal plasticity model involving a phase transformation mechanism together with dislocation slip in parent austenite and child martensite. The model is used to investigate microstructural deformation with computational polycrystalline aggregates. In this context, material's strain hardening and phase transformation characteristics are analyzed in a range of quasi-static and dynamic strain rates. Adiabatic heating effects are accounted for in the model framework to elucidate the role of grain level heating under the assumption of fully adiabatic conditions. The model's temperature dependency is analyzed. The modeling results show good agreement with experimental findings.publishedVersionPeer reviewe

Coupling Molecular Dynamics and Micromechanics for the Assessment of Friction and Damage Accumulation in Diamond-Like Carbon Thin Films Under Lubricated Sliding Contacts

Diamond-like carbon (DLC) coatings have proven to be an excellent thin film solution for reducing friction of tribological systems as well as providing resistance to wear. These characteristics yield greater efficiency and longer lifetimes of tribological contacts with respect to surface solutions targeting for example automotive applications. However, the route from discovery to deployment of DLC films has taken its time and still the design of these solutions is largely done on a trial-and-error basis. This results in challenges both in designing and optimizing DLC films for specific applications and limits the understanding, and subsequently exploitation, of many of the underlying physical mechanisms responsible for its favorable frictional response and high resistance to various types of wear. In current work multiscale modeling is utilized to study the friction and wear response of DLC thin films in dry and lubricated contacts. Atomic scale mechanisms responsible for friction due to interactions between the sliding surfaces and shearing of the amorphous carbon surface are utilized to establish frictional response for microstructure scale modeling of DLC to DLC surface contacts under dry and graphene lubricated conditions. Then at the coarser microstructural scale both structure of the multilayer, substrate and surface topography of the DLC coating are incorporated in studying of the behavior of the tribosystem. A fracture model is included to evaluate the nucleation and growth of wear damage leading either to loss of adhesion or failure of one of the film constituents. The results demonstrate the dependency of atomistic scale friction on film characteristics, particularly hybridization of bonding and tribochemistry. The microstructure scale modeling signifies the behavior of the film as a tribosystem, the various material properties and the surface topography interact to produce the explicitly modeled failure response. Ultimately, the work contributes towards establishing multiscale modeling capabilities to better understand and design novel DLC material solutions for various tribological applications

Crystal Plasticity Modeling of Grey Cast Irons under Tension, Compression and Fatigue Loadings

The study of the micromechanical performance of materials is important in explaining their macrostructural behavior, such as fracture and fatigue. This paper is aimed, among other things, at reducing the deficiency of microstructural models of grey cast irons in the literature. For this purpose, a numerical modeling approach based on the crystal plasticity (CP) theory is used. Both synthetic models and models based on scanning electron microscope (SEM) electron backscatter diffraction (EBSD) imaging finite element are utilized. For the metal phase, a CP model for body-centered cubic (BCC) crystals is adopted. A cleavage damage model is introduced as a strain-like variable; it accounts for crack closure in a smeared manner as the load reverses, which is especially important for fatigue modeling. A temperature dependence is included in some material parameters. The graphite phase is modeled using the CP model for hexagonal close-packed (HCP) crystal and has a significant difference in tensile and compressive behavior, which determines a similar macro-level behavior for cast iron. The numerical simulation results are compared with experimental tensile and compression tests at different temperatures, as well as with fatigue experiments. The comparison revealed a good performance of the modeling approach

Crystal plasticity with micromorphic regularization in assessing scale dependent deformation of polycrystalline doped copper alloys

It is planned that doped copper overpacks will be utilized in the spent nuclear fuel repositories in Finland and in Sweden. The assessment of long-term integrity of the material is a matter of importance. Grain structure variations, segregation and any possible manufacturing defects in microstructure are relevant in terms of susceptibility to creep and damage from the loading evolution imposed by its operating environment. This work focuses on studying the microstructure level length-scale dependent deformation behavior of the material, of particular significance with respect to accumulation of plasticity over the extensive operational period of the overpacks. The reduced micromorphic crystal plasticity model, which is similar to strain gradient models, is used in this investigation. Firstly, the model’s size dependent plasticity effects are evaluated. Secondly, different microstructural aggregates presenting overpack sections are analyzed. Grain size dependent hardening responses, i.e., Hall-Petch like behavior, can be achieved with the enhanced hardening associated with the micromorphic model at polycrystalline level. It was found that the nominally large grain size in the base material of the overpack shows lower strain hardening potential than the fine grained region of the welded microstructure with stronger strain gradient related hardening effects. Size dependent regularization of strain localization networks is indicated as a desired characteristic of the model. The findings can be utilized to provide an improved basis for modeling the viscoplastic deformation behavior of the studied copper alloy and to assess the microstructural origins of any integrity concerns explicitly by way of full field modeling

Computer modelling and simulation approach to developing wear resistant materials

VTT researchers have been pioneers in international science with their computer modelling and simulation techniques for the development of coated surfaces with superior wear resistance and low friction properties. They have introduced a novel PPSP (Performance-Properties-Structure-Processing) multi-scale concept that is based on linking wear and friction performance by micro-FEM computer models to mechanical surface properties, surface microstructure and coating processing parameters. The modelling methods have been applied on 1–5 μm thick hard coatings, such as TiN, DLC and MoS2, on steel as well as on about 200 μm thick thermally sprayed WC-CoCr coatings developed through a Process Mapping concept. The novel approach offers completely new possibilities of systematic and focused material development of wear resistant and low friction coated surfaces with the aim to control and prolong the lifetime of machine components and industrial tools