In this project we will focus on hard metals and oxide materials. For the hard metals we focus on interface related properties as mechanical strength. In the case of the oxide materials our primary interest is on ionic and electronic transport properties. We emphasis the atomic-scale approach and electronic-structure calculations are a key component. The project is divided into three subprojects.
1. Plastic deformation of cemented carbides
Due to the success of various coatings in cemented carbides in recent decades, abrasive wear has become less of an issue, and instead, high temperature plastic deformation of the bulk material is often limiting tool life. There is evidence to support that grain boundary sliding occurs at these temperatures and that it is facilitated by binder phase grain boundary infiltration.
In this project we will use both DFT based calculations and MD using interatomic potentials. We will study the effects of temperature, deformation rate, and Co binder phase concentration in the boundary on sliding and decohesion. Special emphasis will be directed towards properties of other type of binder phases as Ni and Fe. Temperature dependent effects will also be investigated using free energy integration methods. Using DFT these type of calculations become computationally expensive.
Stresses will be obtained as function of interface sliding and separation distance. We will investigate grain boundary sliding behavior of WC bicrystals and this will be used in collaboration with the Applied Mechanics, Chalmers to calibrate cohesive zone laws for use in continuum models. These models will enable direct comparisons with experimental stress-strain curves.
2. Ionic and electronic transport in solid state ionic materials
The aim with this subproject is to characterize different mechanisms that affect the overall conductivity, ionic and electronic. Special emphasis is directed to protonic conductors. We will do this by using DFT to study the energetics of point defect and defect complexes. In order for this to be done properly it is important to have an accurately described band gap, position of valence and conduction bands. We will derive that by using the more computationally demanding hybrid xc-functional and the GW method. For the elementary proton transfer process the computationally demanding RPA technique will be used. We will investigate possible polaronic localization of electron-hole defetcs and the mobility barriers for those kind of defects.
3. Grain boundary barriers in solid state proton conductors
Limitations in proton conductivity have up to now prevented successful implementation of ceramic materials in solid oxide proton conductors. It has become clear that in one of the more important candidate materials the perovskite oxide barium zirconate (BZO), the boundaries between grains in the material are the prime source of inhibited proton conductivity. DFT will be used for detailed energetic calculations and evaluation of electronic properties, while computationally less demanding model potential calculations allow us to study larger systems. We will also consider the entropy of formation of grainboundaries es well as the segregation entropy. This will involve full phonon computations for quite large systems.