DFT calculations of metal/ceramic interfaces Collin D. Wick, Shoutian Sun, Abu Mohammad Miraz, and B. Ramu Ramachandran Collaborators: Shuai Shao and Wen J Meng

Thin ceramic coatings offer an important means for providing mechanical protection to machine tools and mechanical components Reducing ceramic coating spallation is important for enhancing life of coated systems Better adhesion between ceramic and metal and stronger resistance to shear are desired Introducing an adhesion layers between metal substrate and ceramic coating is a common industrial practice *The main goal is to find metal/ceramic combinations that maximize strength of interactions and resist spallation Metal “Compressive cracks” in a coating with excessively high residual stresses; taken from P. Ståhle et al. Wear vol. 267 issue. 12, pp. 2142-2156, 2009. Adhesion layer (Cu, Ti, Cr) CrN, TiN, VN Calico Coatings (CrN)

There are two common ways to study systems atomistically Use first principles The VASP software is reasonably user friendly Nearly every atom and atom combination can be examined without any initial fitting Can be confident in most qualitative results extracted from the study Electron density and density of states information can be extracted from the calculations Smaller system sizes, up to around 103 atoms Use molecular models Can study much larger systems (orders of magnitude of that with first principles: 106 atoms) Run simulations on much longer time scales.

We will use First Principles to Predict where Shear failure occurs and Which Metals and Ceramics have the highest Shear Strength Can only investigate the interface between the adhesion layer and the ceramic Limited to fairly small systems and only coherent interfaces Can investigate many different molecular formulas due to the flexibility of first principles calculations Metal CrN, TiN, VN Cu, Ti, Cr

We will determine the strength of adhesion between interfaces and barriers to shear movement for the first few metal layers Work of adhesion shows which layer it is easiest to pull the metal/ceramic system apart. The 2D Generalized Stacking Fault Energy (GSFE) gives the barriers for shear movement Work of Adhesion 2D GSFE

Strongest adhesion at the surface is for Ti, while strongest adhesion overall is for Cr Ti adhesion significantly drops away from the surface and is strongest with the VN ceramic Cu has many different most stable faces with the ceramics. The weakest adhesion is with the CrN surface. For TiN and VN, Cu adhesion is weakest one molecular layer from the ceramic surface Cr adhesion increases away from the ceramic interface, signaling stronger adhesion in its bulk than with the ceramic surface Ti(0001)-XN(111) Cu(001/111)-XN(001/111) Cr(001)-XN(001)

The interface between Ti and TiN have by far the highest barriers for shear The most stable interface is the Ti (0001)-TiN (111) surface The smallest barriers for shear is in the 1st layer equal to 0.12 J/m2 (around three times that further away from the interface). 0th Layer* 1st Layer 2nd Layer 3rd Layer *Notice that the scale is much larger than others

Cr has similar barriers for shear in all layers The most stable interface is the Cr(001)-TiN(001) Cr has clear movement pathways in the x and y directions Barriers are all near 3.8 J/m2, much greater than for Ti-TiN (0.12 J/m2) 0th Layer 1st Layer 2nd Layer 3rd Layer

Cu has smallest barriers at the interface The most stable interface is the Cu(001)-TiN(001). Cu has clear movement pathways in the x and y directions The barrier is higher than Ti, but lower than Cr 1st Layer 0th Layer

For Cr, CrN has the lowest barriers for shear, while TiN has the highest barriers This suggests that TiN will better resist shear displacement than CrN or VN Cr has clear movement pathways for all systems Cr(001)-CrN(001) Cr(001)-VN(001) Cr(001)-TiN(001)

As with Cr, Ti-CrN has the lowest barriers for shear with TiN having the highest The Ti-CrN barriers for shear are significantly lower than for the other systems. TiN appears to be the best overall material for resistance to shear failure Ti(0001)-CrN(111) Ti(0001)-VN(111) Ti(0001)-TiN(111)

Conclusions Ti forms very strong interactions with all ceramics, but are weakened near the interface. Cr has similar if slightly weaker interactions with each ceramic than within its bulk structure. Cu interactions vary significantly with each ceramic TiN has highest barriers for shear displacement for Cr and Ti Future work will examine more metal layers for all systems and….

See Poster on new MEAM potentials for ceramic-metal interactions (preliminary results for Ti-TiN) We fit bulk and surface properties of Ti and TiN using our genetic parameter fitting algorithm Also does a good job of reproducing GSFE and the work of adhesion between Ti and TiN DFT DFT Ref. New Model Wad Ti(0001)-TiN(001) [J/m2] 2.83 4.91 2.13 Wad Ti(0001)-TiN(111) 6.06 9.90 6.95 New Model Reference . Y. Kim and B. Lee, 56, 3481–3489 (2008).