1.1 What are the differences between THC and THP?
Thermal History Coatings (THCs) are ceramic powders applied by air-plasma spraying to produce a robust and durable coating.
Thermal History Paint (THP) is ceramic powder mixed into a water based binder to produce a liquid. This liquid is sprayed onto the surface with an air spray gun.
The THP is lower cost and generally intended for lower temperature, shorter term applications. We would advise which to use depending on the specific application.
1.2 Rotating components?
Yes, both can be used for temperature detection on rotating parts.
1.3 Do you have experience with coating durability on rotating turbine blades?
Yes, we have successfully tested in a range of applications including gas turbine blades. The durability has been proven to be significantly better than standard thermochromic paint. The THC and THP showed little damage after our longest test of 35 hours on a gas turbine.
1.4 Is a calibration procedure required?
Yes, typically samples of the same material as the test component(s) are applied with THC or THP. These samples are individually heat treated by SCS in our lab at specific temperatures for the same duration as the test. The samples are then measured to relate the measurements to the heat treatment temperature.
The details of the calibration depend on the specific application. We will advise how we do it and what we need from you.
1.5 Spot size
The spot size of the measurement is typically 0.5mm. The measurement probe is usually scanned across the surface to build a temperature map of the surface.
The thickness of the Thermal History Paint or Thermal History Coating is typically the same approximately 30 µm.
1.7 Re-use components after?
Typically, yes. THP is non-toxic and the painted component can either be put back with the paint or the paint can be removed by grit blasting the surface.
The THC adheres more strongly (longest test without failure 4500hrs at Didcot power station). The coated component can be reused with the coating on.
1.8 Is it possible to measure the history of temperature rise between turbine start up to the steady state phase offline?
This is only possible with so called ‘on-line’ measurements and not with THC/THP as these are off-line tools.
If you want we can discuss on-line measurements too, as we have the expertise for this.
1.9 How much is the density of THC/THP and how thick it should be applied on a turbine vane or blade?
See also Q10. Typical thickness is between 25µm and 35µm for both THC and THP. We don’t have density measurements. The materials are oxide ceramics.
1.10 How long will the coating survive in an engine? Concerns about interdiffusion at high exposure times?
This depends on the application conditions, e.g. operating temperature and environment. We can discuss the details for your application.
1.11 What is the expected accuracy / precision? Publications state precision of +/-5C up to 900C – is this on a 2sigma (95% confidence) basis?
The precision and accuracy depend on the application conditions (e.g. soot deposits) and the technology variant (THC / THP). The typical precision for THP is approximately ±5°C (1 sigma). For THC it strongly depends on the exposure temperature. The accuracy is more difficult to quantify but in previous case studies on industrial applications our measurements have been in good agreement with thermocouple readings.
1.12 Do you provide the APS gun parameters recommended for a successful coating?
Usually we prefer to use our own coating manufacturer to ensure the successful deposition of the coating. However, we can discuss this.
1.13 How is the painting / coating affecting the valves heat transfer?
We have not investigated the effect of the paint on heat transfer. However, one of our gas turbine OEM customers has investigated this for their turbine blades. Their results indicated that the paint had a negligible effect on the heat transfer. I’m not sure how it would affect the valves but I would expect the effect to be negligible/small.
1.14 What data do you need from us?
We need data from at least one thermocouple located as close as possible to the tested component during operation (e.g. turbine air inlet temperature). We use this data to generate calibration data for the paint specific to your test.
1.15 What is the current temperature rating your “paints” can be used at?
The current temperature range of the paint is 150-900°C however we have recently been successful in measuring up to 1000°C but we have more experience, and hence confidence, in results up to 900°C. The temperature limitation is dictated by the measurement capability rather than the durability of the paint.
1.16 Can you estimate the time it would take to paint parts get them back to us, and then read the data?
We can normally paint and return parts within 2 weeks of receiving them.
1.17 How long does the coating need to run at peak temperature? Minimum/Maximum exposure time? Do you have a recommended operating plan for a THP test?
The details depend on the application. A general guideline for minimum exposure time at peak temperature is 5 minutes. The maximum depends on the expected durability of the THP/THC in the specific application. Our recommended test plan for THP is steady state operation for a defined period of time. We can discuss the duration for your specific application.
1.8 Is the response to temperature peak-latching? Or does the coating respond continuously to time/temperature? Is a thermal history of the part required to determine temperature?
The thermal history of the part is required for temperature measurements. We conduct the calibration using the thermal history of the part to heat treat the calibration samples for the same length of time. The THC/THP responds to the maximum temperature however the temperature measured depends on the calibration. We are investigating the effect of unsteady heating cycles e.g. temperature spikes.
1.19 Do you need to quench the sample at a certain rate after exposure?
We do not usually recommend a particular quench rate but suggest a relatively rapid shutdown after dwell at maximum temperature.