Posted on Septemeber 24, 2017 at 10:00 PM
Hello friends, Today we will discuss about Thermal Barrier Coatings,which are defined as materials with very high melting temperatures (~3000 degree C), very low thermal conductivity, high specific heat capacity, but with the ability to perform under extreme thermal environments .
Hypersonic vehicles are the future of high speed travel. But there are many challenges associated with hypersonic vehicles, one of which is hot
structures. Hypersonic vehicles are subjected to atmospheric heating due to drag, the leading edges including the nose region, tail edges, and
scramjet inlets are exposed to intense, highly localized heating and severe thermal environment.
The wall temperatures of scramjet rise above 2000 K, at such high temperature strength and toughness of most metals degrades rapidly. This necessitates use of protective mechanisms which shall reduce the heat flux faced by hypersonic vehicle walls.
The following figure illustrates of high temperature of(in red) experienced by re-entry and hypersonic vehicles researched. Images are from Purdue University.
Thermal barrier coatings (TBCs) perform the important function of insulating components, such as gas turbine and aeroengine parts,
operating at elevated temperature. Typical examples are turbine blades, combustor cans, ducting and nozzle guide vanes.
TBCs have made possible the increase in operating temperature of gas turbines.
TBCs are characterised by their very low thermal conductivity, the coating bearing a large temperature gradient when exposed to heat flow. The most commonly applied TBC material is yttria stabilized zirconia (YSZ) which exhibits resistance to thermal shock and thermal fatigue up to 1150°C. YSZ is generally deposited by plasma spraying and electron beam physical vapour deposition (EBPVD) processes.
It can also be deposited by HVOF spraying for applications such as blade tip wear prevention, where the wear resistant properties of this material can also be used. It is common practice to aluminise and pre-coat the substrate material (generally a nickel or cobalt superalloy) with an MCrAlY bond-coat. The bond-coat is necessary to accommodate residual stresses that might otherwise develop in the coating system, caused by the metallic substrate and the ceramic TBC having different coefficients of thermal expansion.
Following image illustrates the TBC.(Source)
High Velocity Oxygen Fuel (HVOF) spraying is a group of thermal spraying processes which spray flames at supersonic speeds.
The detonation gun (D-gun) process was the first type of HVOF process used. In HVOF processes, kinetic energy of particles is more
important than temperature. These processes use high-impact energy to produce dense coatings at relatively low temperatures.
In HVOF spraying, fuel and oxygen are pressed into a combustion chamber in a continuous flow, producing a jet of combustion products at extremely high speed. Powder particles injected into this gas steam are accelerated to a very high velocity. Fusion is obtained by the kinetic impact of the coating particles rather than by their increased temperature: the process is carried out in an ambient atmosphere. Cleaning and surface preparation are critical and require a well-equipped workshop. See more here.
There are different types of HVOF spraying, based on:
1) The fuel used: either hydrogen, natural gas, or kerosene
2) The oxygen source: either pure oxygen or compressed air
Following image illustrates the HVOF.(Source)
1) One of the most common uses of Thermal barrier coatings is in the combustion sections of aircraft turbine engines.
With the demand for fuel economy and increased power, combustion temperatures are approaching the design limits of the metal
alloys from which turbine components are made.
2) IC engines,Scramjet and Ramjet Combustors
3) Rocket nozzles, space re-entry vehicles
Following image illustrates application of TBC in Gas Turbine engines.(Source)
You can refer following resources / web links for more information related to UHTC:
1)Thermal Barrier Coatings
2)Thermal-barrier coatings for more efficient gas-turbine engines-MRS Bulletin
3)Metal-Organic CVD Yttria Stabilized Zirconia (YSZ) FOR Thermal-barrier coatings.. Read more here
4)High-Velocity-Oxygen-Fuel-Spraying-HVOF.. Read more here
5) If you want to go pro and read more indepth then refer.
6) If you want to use of TBC in gas turbines refer.
Hope it helped you learn new thing today...Keep visiting for more such posts :)