v99stres

CLUSTER: PROPERTIES AND CHARACTERIZATION Residual Stress - An Integrated Approach Part 1: From Splats To Coatings Jiri Matejicek, Sanjay Sampath - CTSR, SUNY at Stony Brook, NY Paul Brand - NIST, Gaithersburg, MD

SCOPE AND CONNECTIONS

STRESS MEASUREMENT METHODS

Diffraction:
* x-rays: surface measurement
* neutrons: in-depth stress profiles
- non-destructive
- phase distinctive
- moderate limitation on specimen shape + size
- measure stress inside the splats

Material removal:
* hole-drilling, layer removal
- can obtain stress gradient
- destructive
- risk of affecting the stress

Strip curvature:
- non-destructive
- easily applicable to graded coatings
- limited specimen shape + size
- can determine stress profiles in some cases
- allows measurement of in-situ stress evolution
Models:
* numerical, analytical
- concern: material properties
- in conjunction with experiments, can separate
quenching and thermal stress

THROUGH-THICKNESS STRESS PROFILE

XRD measurement:
surface stresses in subsequent specimens
of different thicknesses

Calculation:
stress profile in the entire deposit thickness
calculated from XRD data

EXAMPLE OF STRESS HISTORY Neutron diffraction msmt + calculation

QS = quenching stress TS = thermal stress RS = residual stress

- measured values of residual stress + calculated values of thermal stress
=> determination of quenching stress
- quenching stress is dominant contributor to final stress values
- small difference in CTE => small thermal stress
- different proportion of quenching and thermal stress at different temperatures

STRESS ORIGIN AND SIGNIFICANCE

Origin of residual stress:
- quenching of the impinging splat, restricted contraction
? always tensile
- thermal mismatch between the coating and the substrate
? can be both tensile and compressive

Significance of stress:
- affects deformation behavior, lifetime, ultimately even structural integrity
- can be modified by varying the process conditions

Mechanisms of stress relaxation:
- plastic deformation, creep, annealing, cracking, intersplat sliding
- inelastic behavior makes theoretical predictions difficult
? experimental methods very important

SINGLE SPLATS AND THIN LAYERS Mo - deposition temperature effects

- stress measured by x-ray diffraction and microdiffraction (splats)
- stress changes from tensile at low temperature to compressive at high temperature,
due to increased contribution of thermal mismatch: CTE(Mo) < CTE(Fe)

THROUGH-THICKNESS STRESS PROFILE

- stress measured by neutron diffraction
- APS and wire-arc have both tensile and compressive stress in the deposit, with more
tension near the surface
- HVOF has compressive stress, due to high-velocity-enabled peening action
- deposit stress accommodated by substrate bending

CONCLUSIONS - stress in coatings can be studied by a variety of methods, depending on specimen dimensions and desired information - stresses in single splats, surface layers and through-thickness stress profiles can be determined - experimental methods complemented with calculations can isolate stress contributions at various stages of deposition process (quenching, thermal, residual stress) - the effects of processing parameters on stress and related properties are elucidated, among which the deposition temperature plays prominent role - by changing the temperature,one can achieve tensile or compressive stress in the deposit Outcomes: - better understanding of the process of coating build-up and stress evolution - enables control of deposit properties and performance by process modification

For more information, contact:
Jiri Matejicek - jmatejic@mail.ic.sunysb.edu    Sanjay Sampath - ssampath@ms.cc.sunysb.edu

See also… X. Jiang et al.: Splats - Experiments and Modeling; Temperature Effects on Molybdenum Coatings
                   R. Neiser et al.: Plasma Spraying Process Maps
                   O. Kesler et al.: Residual Stress - Part 2

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VG 4/22/99