Printhead with increasing drive pulse to counter heater oxide growth

1. An inkjet printer comprising:

a printhead with an array of ejection devices for ejecting drops of liquid onto a media substrate, each of the ejection devices having a chamber for holding liquid, a nozzle in fluid communication with the chamber and a heater positioned in the chamber for contact with the liquid such that resistive heating of the heater generates a vapor bubble that ejects a drop of the liquid through the nozzle; and

a controller for receiving print data and generating drive pulses to energize the heaters in accordance with the print data, wherein

the controller increases the drive pulse energy during the printhead lifetime such that the drive pulse energy is never less than that of a preceding drive pulse, and

the heater is formed from a TiAlX alloy where Ti contributes more than 40% by weight, Al contributes more than 40% by weight and X contributes less than 5% by weight and comprises W and one or more of Ag, Cr, Mo, Nb, Si, and Ta,

wherein the TiAlX alloy provides surface oxide Al2O3 and TiO2 which directly contact the liquid during use, and

wherein W contributes between 1.7% and 4.5% by weight to enhance Al203 and suppress TiO2 surface oxide.

2. A printer according to claim 1 wherein the printhead is configured to receive a supply of the liquid at ambient temperature, and the drive pulse applied to the heater to generate the vapor bubble has less energy than that required to heat a volume of the liquid equal to the volume of the drop ejected, from a temperature equal to said ambient temperature to the boiling point of the liquid.

3. A printer according to claim 1 wherein the controller is configured to increase the drive pulse energy by increasing the duration of the drive pulse.

4. A printer according to claim 1 wherein the controller is configured to increase the drive pulse energy after a predetermined number of drops are ejected.

5. A printer according to claim 4 wherein the controller monitors a cumulative total of the drops ejected by each of the ejection devices and individually increases the drive pulse energy to any of the ejection devices after it ejects a predetermined number of drops.

6. A printer according to claim 1 wherein the ejection device further comprises a temperature sensor for determining when the heater has a peak temperature less than a predetermined threshold and the controller is configured to increase the drive pulse energy in response to the temperature sensor indicating that the peak temperature is less than the threshold.

7. A printer according to claim 6 wherein the threshold is less than 450 degrees C.

8. A printer according to claim 1 wherein the controller increases the drive pulse duration inversely to a predetermined relationship between actuations of the ejection device and increase in electrical resistance of the heater.

9. A printer according to claim 1 wherein Ti contributes more than 48% by weight, Al contributes more than 48% by weight and X is 0% by weight.

10. A printer according to claim 1 wherein the TiAl component of the heater has a gamma phase structure.

11. A printer according to claim 1 wherein the heater has a microstructure with a grain size less than 100 nanometers.

12. A printer according to claim 1 wherein the TiAlX alloy is deposited as a layer less than 2 microns thick.

13. A printer according to claim 12 wherein the layer is less than 0.5 microns thick.