Residual stresses can severely limit performance of thin films / coatings (see Broader Impact).

1. Grain Boundary Chemistry and Intrinsic Growth Stresses in Nanocrystalline Diamond (NCD) FilmsBrian W. Sheldon, Brown University, DMR 0805172 • Residual stresses can severely limit performance of thin films / coatings (see Broader Impact). • Our experiments show very large stresses due to grain boundary formation during NCD growth - these become more compressive at lower temperatures (the opposite trend is observed in most other materials). Growth also leads to substantial H incorporation at boundaries. • Using density functional theory (DFT) to inform stress models predicts that trapped H limits tensile stress due to g.b. closure (in agreement with the experimental trend). This H trapping can also lead to additional compressive stress via additional carbon insertion at g.b.’s. • Residual stress control via grain boundary chemistry should be broadly applicable. Multiscale Model – Correct Trend with No Adjustable Parameters: • Stress evolution described by cohesive zone / finite element model • Plasma CVD model of surface chemistry • DFT describes cohesive zone forces during grain bdry formation (with surface H effects) film substrate Intrinsic Stress (GPa) Compressive offset: surface H • consistent with insertion of methyl radicals at grain boundaries (possible because trapped H holds boundaries open) • also described with DFT data at different growth temperatures Publications: Xiao et al, APL (2008); Qi et al, PRL (2009)

2. Grain Boundary Chemistry, Stress, and Friction in NCD Coatings: Implementation for Dry Machining of Al AlloysBrian W. Sheldon, Brown University, DMR 0805172 MOTIVATION: The high hardness, chemical stability, and low contact friction of NCD make these coatings very promising for dry machining of Al alloys (potential annual savings exceeding $100M).  GENERAL MOTORS COLLABORATION: This NSF award is primarily focused on grain boundary induced residual stresses, which can be varied widely in NCD (see previous slide). In addition to controlling residual stress, low friction is also critical for GM’s dry machining applications, and recent measurements at GM demonstrate that some of the corresponding changes in grain boundary chemistry also have a substantial impact on friction. The comparison below shows that films with more transpolyacetylene (TPA) bonding at grain boundaries do not show the desired low friction. High TPA As-grown NCD Surface Friction remains high in films with high TPA Low TPA Normalized Intensity (Arb. Units) Low Friction with low TPA Raman Shift (cm-1) 1600 1800 1200 1400 1000