Testing workpiece overcoat
View Patent ↗A method and apparatus for testing workpiece overcoats is described.
1. A method of evaluating an overcoat on a workpiece, the workpiece having the overcoat prior to the method of evaluating the overcoat being performed, the method of evaluating the overcoat comprising:
submerging the workpiece having the overcoat in an electrochemical cell comprising a corrosive fluid;
establishing a circuit path in the electrochemical cell using the workpiece as a working electrode and a reference electrode disposed in the corrosive fluid;
applying an excitation signal at one or more frequencies to the circuit path;
measuring a modulus of impedance value for one or more frequencies within a range of 1 Hz to 10 Hz at one or more time points; and
dividing the measured modulus of impedance value by a high frequency modulus of impedance value measured at a frequency above 10 Hz to generate a normalized modulus of impedance value.
2. The method of claim 1 , further comprising displaying the normalized modulus of impedance value.
3. The method of claim 2 , wherein the normalized modulus of impedance value is measured using a frequency response analyzer coupled to the electrochemical cell and displayed on a display of a computer operative coupled to the frequency response analyzer.
4. The method of claim 1 , wherein the normalized modulus of impedance value (log|Z|) is at a fixed frequency of 1 Hertz.
5. The method of claim 1 , wherein the high frequency modulus of impedance value is measured at a frequency of 10 4 Hz.
6. The method of claim 1 , further comprising:
setting a reference value; and
determining if the normalized modulus of impedance (log|Z|) value is above the reference value.
7. The method of claim 6 , wherein the reference value is an average log|Z| of a reference workpiece, without the overcoat, when submerged in the corrosive fluid and evaluated using the excitation signal.
8. The method of claim 1 , further comprising determining an effectiveness of the overcoat to prevent corrosion of the workpiece based on the normalized modulus of impedance value.
9. The method of claim 1 , wherein the overcoat comprises carbon.
10. The method of claim 9 , wherein the workpiece is a magnetic recording head wafer that comprises an aluminum titanium carbide (AlTiC) substrate.
11. The method of claim 9 , wherein the overcoat has a thickness in a range of 10 Angstroms to 30 Angstroms.
12. The method of claim 1 , wherein the corrosive fluid is an electrolyte solution.
13. The method of claim 1 , wherein the corrosive fluid is a solution that includes H 2 SO 4 .
14. The method of claim 1 , wherein the circuit path has an open circuit potential after allowing the workpiece to equilibrate in the corrosive fluid for a time period.
15. The method of claim 14 , wherein the excitation signal has an amplitude of 5 mV and the normalized modulus of impedance value is measured at ambient temperature and pressure.
16. An electrochemical impedance spectroscopy measurement system, comprising:
an electrochemical cell containing a corrosive fluid and a submerged workpiece having an overcoat;
a circuit path in the electrochemical cell that includes the workpiece as a working electrode and a reference electrode disposed in the corrosive fluid;
a frequency response analyzer that applies an excitation signal at one or more frequencies to the circuit path;
the frequency response analyzer being electrically coupled to the circuit path and measuring a modulus of impedance value for one or more frequencies within a range of 1 Hz to 10 Hz at one or more time points; and
a computer that is coupled to the frequency response analyzer and configured to normalize the measured modulus of impedance value by dividing the measured modulus of impedance value by a high frequency modulus of impedance value measured at a frequency above 10 Hz.
17. The electromechanical impedance spectroscopy measurement system of claim 16 , further comprising a display of the normalized modulus of impedance value.
18. The electromechanical impedance spectroscopy measurement system of claim 16 , wherein the high frequency modulus of impedance value is measured at a frequency of 10 4 Hz.
19. The electromechanical impedance spectroscopy measurement system of claim 16 , wherein the overcoat is a carbon overcoat.