Improper Use Of Multilayer Actuators
Piezoelectric multilayer actuators have demonstrated continuous operation in harsh conditions for billions of cycles. However it is important to use them properly, as improper use can cause immediate and irreversible damage. Here are the typical failures for multilayer actuators.
Too high inertial force, insufficient preload
Actuator stacks tolerate very little tensile stress. Static pulling forces will lead to a mechanical failure, but it is also important to consider inertial forces, which can be generated by the actuator decelerating its own mass and any added load. If insufficient preload is applied to the stack, then there might be a loss of preload during operating, e.g. if temperature changes or if frequency is high. In such case, there may be high tensile stress in the stacks causing delamination and failure. It is recommended for actuators operated under dynamic conditions, that a minimum of 10 MPa preload is available during all operating conditions. Similarly, suddenly discharging a charged piezo stack, i.e. through a short-circuit, may lead to permanent damage.
Too high operating frequency causing self-heating and thermal breakdown
Piezoelectric elements dissipate a portion of the incoming electrical energy, leading to self-heating. The resulting temperature rise depends on the geometry of the piezo and its surroundings: conduction, convection, etc. Furthermore, as performance and capacitance increase with temperature, so does the dissipation. It is also important to keep in mind that thermal conductivity of the ceramic is relatively low, so in the case of self-heating, the core of the actuator is warmer than its surface. The maximum conditions of voltage amplitude and frequency should be determined for the application and the temperature monitored to make sure that it remains below the specified maximum.
Voltage or current spikes
The actuator will react to any spike in the signal, generating inertial forces that can be substantial. If the preload is not sufficient, these spikes can lead to a delamination of the ceramic structure and failure of the actuator. Drive electronics should include suitable protection so that the actuator does not see such spikes. A simple resistor in series with the actuator will form a low pass filter and effectively limit the frequency content and maximum current.
Uneven loading of stacks
Piezoelectric multilayer stacks are designed to generate and sustain axial loads. Torsion and bending will generate shear or tensile stress in the material and may lead to cracking. Also, a localized load on the ceramic will generate tensile stress at a distance from the contact point, leading to crack initiation. Once a crack appears in the ceramic material, it may propagate to the active material and lead to a loss of dielectric strength. Always make sure that the loading of the stack is as even as possible during all operating conditions. Use de-coupling mechanisms to avoid unwanted stress and if bending cannot be avoided, take it into account when determining the preload, so that tensile stress is avoided.
Environment and contamination
Piezoelectric actuators are electrical devices and as such, subject to electro-chemical degradation when a voltage is applied. The main factors affecting the degradation are temperature, DC voltage, humidity or any other chemical contamination. The electrochemical degradation leads to migration within the material, which manifests as an abnormal current draw and ultimately results in short-circuit. For continued operation in severe environment, it may be necessary to hermetically seal the actuator.
Loss of polarization
The piezoelectric property is given to the component during the poling process. This is done by applying an electrical field at elevated temperature. The poling can be weakened or even lost if a too high negative electrical field is applied, the operating temperature is increased either due to increased temperature of the surroundings or self-heating, if too high compressive forces are applied or a combination of these factors.