Typically, in bubble jet print heads a plurality of electrically resistive heater elements are deposited on a support substrate, that is formed e.g. of metal or ceramic material and has a heat control coating e.g. SiO.sub.2. Metal electrodes are formed to selectively apply voltage across the heater elements and a protective coating is provided over the heater elements and electrodes. Printing ink is supplied between the heater elements and orifices of the print head and heater elements are selectively energized to a temperature that converts the adjacent ink to steam rapidly, so that a shock wave causes ejection of ink from the related orifice.
This ink jet printing approach is becoming increasingly useful; however, a major problem still exists in providing print heads wherein the heater elements are capable of a long operative life, particularly when used in high speed printing modes. Primarily, this is because protecting the drop ejectors from physical and chemical damage still presents a major technical problem.
Various protective cover constructions have been developed to isolate the print ink from the heating resistor and electrode elements of the print head and to protect those elements from physical and electrolytic damage. However, it would be desirable to further extend the life of the print heads and/or enable higher operating print speeds with those devices.
We have found that significant failure modes occur because of cracking of the heat resistor films and/or crazing of their protective cover layer(s), which can allow the commencement of electrolytic attack on the composite structures.