H S Ubhi1 H Jiang1 M Keeble2, and S Fritze2,

Microstructural Characterisation of an Automotive Spark Plug using EBSD

www.oxford-instruments.com

Conclusions• Successful mechanical polishing was achieved for all the

components and verified by optical metallography.

• SEM/EBSD characterisation was similarly successful in accessing the processes and microstructures of the various components, e.g. coating of the terminal, inter-metallic formation at the interface between the copper and nickel in the central cored electrode, local melting as the erosion mechanism at the platinum electrodes.

IntroductionA pre-requisite for successful metallography is good sample preparation. This study illustrates the methodology for mechanically polishing an automotive spark plug, utilising both bright field and differential interference contrast optical microscopy to assess the surface condition, prior to SEM/EBSD characterisation.

MethodologyAn automotive spark plug that had come out of service was sectioned and polished using the steps given in Table 1. The sample surfaces were examined using optical microscopy and the microstructures of the various components in the spark plug examined using the SEM/EBSD technique. The EBSD apparatus was attached to a FEGSEM and the EBSD data acquired using OI-HKL Nordlys detector combined with CHANNEL5 software.

1Oxford Instruments plc, Halifax Road, High Wycombe, Bucks, HP12 3SE, UK. 2Buehler UK,

Step No. Surface Abrasive Lubricant Force Time Platen Speed Head Speed Relative Rotation

1 Apex DGD 75-15µm Water 6lbs until plane 250 60 Comp.

2 UltraPol 9µm MetaDi Supreme 6lbs 6 min 200 60 Comp.

3 Trident 3µm MetaDi Paste Metadi Fluid 7lbs 5 min 200 60 Comp.

4 Trident 1µm MetaDi Paste Metadi Fluid 7lbs 5 min 200 60 Comp.

5 ChemoMet 0.05µm Alumina / Colloidal Silica 7lbs 5 min 80 60 Contra

6 ChemoMet 5 parts Colloidal Silica+2parts 30%H

2O

2

7lbs 6 min 80 60 Contra

Table 1. Details of polishing steps

Sectioning: Isomet 5000 Mounting: Simplimet 1000 Preparation: EcoMet-AutoMet Microscopy: Nikon MA-200

Al2O3

Insulator - ceramic

The fraction of amorphous phase is 6.6%. Mean size of alumina grains is 3.4µm with a maximum of 16.4µm.

All Euler orientation map

DIC image

EBSP

SEM BEI

Centre Electrode tip

Ni

Pt PtNi

IPF orientation map

20mm

Local misorientation map

20mm

FSD image

10mm20mm

SEM BEI DIC image

IPF orientation mapIPF orientation map

IPF orientation map

SEM BEI

IPF orientation map

SEM SEI

Platinum tip, fine deformed structure, HAZ, local surface melting during service, weld metal – large grains inter-granular cracks.

Pole figures

Platinum electrode, large grains, local surface melting during service.

Pt

Phase map

Pt

Ni

Side electrode

EBSP

200mm 200mm

IPF orientation map

IPF orientation map

SEM SEI SEM BEI

Terminal: Nickel plating with outer oxide film formation

Ferrite

NiAll Euler orientation map

EDS linescan

Terminal

Ni

EBSP

NiO

EDS linescanFerrite

EBSP

NiO

EBSP

DIC image

Phase map

Formation of intermetallic Cr

7C

3

particles in the copper core at the Cu/Ni interface. Nickel Sheath - mean grain size of 10.9µm.

Cr7C3

IPF orientation map of Ni

EDS Ni EDS CrPhase map

Ni

Cu

Cr7C3 particles

EDS Cu

Cored central electrode

500m

m

EBSPDIC image DIC image

Casing/side electrode joint: melting of casing, grain growth in electrode.

Ni

Steel

Pole figures

500mm

CSL boundary map

Pole figures

500mm

IPF orientation map of steel

Casing - thread

Deformation in thread root from thread roll forming process.

IPF orientation map of steel

200mm

DIC image

DIC image

DIC image

IPF orientation map

Local misorientation map

IPF orientation map

Steel

Casing/side electrode joint

Ni

Centre electrode tip

Side electrode

Cored central

electrode

Casing threads

Terminal

Insulator - ceramic

Casing/side electrode

joint

Spark Technical Poster_AW.indd 1 28/6/10 15:41:49