Controlling Stress Evolution in Ceramic Thin Films and Coatings Brian W. Sheldon and Janet Rankin, Brown University

Barbara L. Walden, Trinity College

NANOCRYSTALLINE DIAMOND

Bonding at the boundaries between diamond grains is significantly different than bonding inside of

grains

-fringes show atom layers in nanodiamond crystals-bonding studied with spectroscopy (data not shown)

2 nm

• H reacts at grain boundaries and alters bonding induces large tensile stresses.

• Hydrogen plasmas can be used to control both the magnitude of the stress and the stress gradients in the film.

KEY ISSUES:

• Large residual stresses in ceramic coatings and thin films promote failures via fracture and decohesion.

• Stress gradients that arise during film growth can create serious difficulties in small scale structures (e.g., MEMS components that are not flat, etc.).

RESEARCH RESULTS:

• Experiments and modeling have led to new strategies for controlling stress evolution during film growth in carbides, nitrides, and oxides.

• Film growth rate and grain boundary chemistry strongly influence stress evolution in a variety of materials.

• Nanocrystalline films allow better control of stresses (more grain boundary area).

DMR-0305418

GM has provided supplemental financial support for this research

Collaborative Research with Y.T. Cheng at General Motors:

• Use the understanding of stress-control developed in NSF-sponsored research to mitigate the large thermal stresses in NCD coatings.

• Optimize friction and wear properties, while controlling total residual stress.

Steel substrate (cutting tool)

diffusionbarrier

NCD coating

COMPARE WITH STANDARD NCD DEPOSITION AT 800 C: unacceptable

compressive stress of -10 GPa or larger

NEW LOW TEMPERATURE NCD DEPOSITED AT 500 C: processed with hydrogen plasma treatments to obtain

compressive stress of -2 GPa

Proper Control of Film Growth Makes it Possible to Obtain Low Stress, High

Hardness, and Low Friction

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0 1000 2000 3000 4000 5000 6000Cycles

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NCD grown at 500 C has low friction that is similar to 800 C films (materials also have similar hardness).

Motivation:

• Dry-machining of Al alloys requires a hard, low friction coating on the cutting tool. An adequate coating system has not yet been implemented, however, GM has identified nanocrystalline diamond (NCD) as the most promising coating material.

• GM estimates that using NCD coatings in dry machining can potentially save more than $300M annually, while also reducing environmental impact.

• PROBLEM: Diamond coatings on cutting tools fail as a direct result of very high thermal stresses.

Controlling Stress Evolution in Ceramic Thin Films and Coatings Brian W. Sheldon and Janet Rankin, Brown University

Barbara L. Walden, Trinity College DMR-0305418