JayS wrote:I just noticed that there was a tender floated by GTRE in Jan Feb 2019 period for Machining, Brazing and TBC for SCB. Most likely for Kaveri. Given the inflow of a number of tenders in last frlew months on various modules, its safe to conclude they are building new prototypes. Are they intending to use SCB now in Kaveri..? Could be. But I am not sure 100%.
Also, another thing I noticed just now, DMRL person during AI2017 Seminar mentioned DMRal has done Preliminary development for 7YSZ ceramic TBC coating using EB-PVD, in collaboration with ARCI.
And then a couple of months ago, there was a tender floated by DMRL for setting up a EBPVD facility for TBC. What for..? Industrialization of the Tech..? ARCI was the one perhaps who did the TBC for HTSE1200 SCBs. Looks like a lot is happening in the background.
JayS, wrt the highlighted and underlined parts, pls note that the 7-8 YSZ TBC (over a metallic bond coat) providing decrease of the underlying metal-substrate temperature up to 150 deg C etc, is old hand (typically 4+ decade old).
GE has been using 7-8 YSZ TBC etc from mid-1990s (on static parts like combustor linings) and late 1990s on stator (vane)/rotors (blade) of HPT.
So, I'm quite sure/confident that Kabini HPTs would also have some form of YSZ based TBC.
However, the TBC application (and thus its effectiveness) technology though has changed from various type of plasma spray tech to, and nowadays it's almost universally via, physical vapor deposition (PVD) tech.
One of the
major reason or switching to PVD tech (other than other-major advantages like life and surface finish) is
to address the cooling-hole closure issues.
A PVD deposited TBC typically has particle size of 1.4-1.5 micro-m (compared to >5 micro-m of plasma spray tech) - and that compares very well to typical 0.25-0.50 mm dia of the cooling holes. So, the earlier issues of cooling-hole closure or the cooling flow-reduction is almost eliminated by the PVD technology.
People did try a lot of other approaches to circumvent this problem like, drilling holes after coating, filling holes with wires during spraying, oversizing holes etc - but none really worked well.
The issue with 7-8YSZ based top-coat TBC applications is that
YSZ allows for maximum surface temperature capability of about 1200 deg C - beyond that degradation of the coating (in form of reduced strain tolerance and a decrease in thermal fatigue life of the coating) takes place due to changes in microstructure.
So, in most turbofan HPT applications the
make-or-break technological aspect is film/convectional cooling effectiveness
i.e. for a Kabini/Kaveri example, the blade-surface film cooling technology, has to be effective enough to bring down the blade-TBC surface temperature from TET levels of 1455deg C to ~1200deg C. So that
TBC can further ensure the substrate-metal-blades are operating at about 1050-1100 deg C levels.
This 1050-1100 deg C temp levels are firmly within DS casted blade regime - the SC casted blades can add another 100-150deg C to it, but the applied TBC simply wouldn't support any more temp creep (due to 1200 deg C max phase stability boundary limits).
My hypothesis is, and I'll be the happiest person to be proven wrong, that we haven't seen much of SC casted HPT applications for Kabini/Kaveri, because of this limitation.
So, to summarise,
1) we have
reached the max level of TeT that DS casted blades would allow via Kaveri/Kabini - this after implementing a 200-250 deg C temp advantage via film-cooling and TBC.
2) to increase TeT levels by another 100-150 deg C, SC is the way to go.
But the
limitation there is film-cooling tech available with us, because it still needs to negate this 100-150deg C temp increase, so that the TBC itself doesn't start spalding and cracking etc. IoW, SC material tech temp advantage will get nullified without further advancement in film-cooling temp technology (a severely closely-guarded tech, which nobody, repeat nobody, will ever part with).
However, recently we have seen reports of Indian Rare Earths Limited
developing bi-layer TBC technology of Lanthanum Zirconate (LZ) over Yttrium Stabilized Zirconia (YSZ).
Now referring to my previous post (on this thread) pls note,
1) Lanthanum Zirconate (LZ), has
much higher thermal and phase stability - close to 2000deg C. It also has lower thermal conductivity and sintering tendency compared to YSZ.
(Thermal Conductivity - 2 W/m/K of YSZ vs 1.56 W/m/K of LZ)
2) LZ is also less oxygen transparent than YSZ, providing better bond coat oxidation resistance and minimises the growth of TGO (Thermally Grown Oxide layer) - Wiki has good details about TGO and it's impact on TBC.
But the
disadvantage of course is LZ has lower coefficient of thermal expansion compared to YSZ - so i
t cannot be applied directly on the NiCrAlY bond coat.
So, the solution is therefore to have
LZ applied as a top coat material over YSZ forming a bilayer TBC.
Now suddenly, with this technology, the issue of 1200deg C limit of TBC is gone - so we don't really need the blade-surface film-cooling tech to be able to bring down the ambient/operating temp to 1200 deg C etc. As this bilayer (LZ-YSZ) TBC will happily work upto a much higher temp (theoretically upto 2000 deg C).
So now suddenly, as a chain reaction, the SC casting tech is attractive again - now we can get those additional 100-150deg C TeT levels without worrying about TBC spalding and other issues. Yes, the temp for the substrate-blades are going to be higher, but that's exactly why SC casting is being attempted for.
Pls further note, DRDO has
already assembled and validated the bi-layer YSZ-LZ coated flaps in an aero-engine for test cases involving rapid thermal transients, supersonic flow of combustion products, vibratory loads of about 4 ‘g’, sustained 1,000 h equivalent of engine operation and more than 30,000 nozzle actuations.
And I'm sure testing on rotor blades of the HPT also is going on and news will surface in due course.
So, net-net ...
Kaveri is already delinked from LCA - so French can shove their costly-and-oh-so-advanced M88-core to their sun-never-shine areas.
We are
on the threshold of arriving at modern turbo-fan tech etc.
If only GoI et all have the foresight to actually fund the last-mile flight-certification of Kaveri quickly, so that the only other unknown area (of any turbofan tech) of rotor-dynamics and CFD can be certified/baselined - but that's too much to ask from animal-husbandry-degree-yielding-baboons to comprehend let alone plan and execute.
And for some posters here in BRF, as is evident from some of the previous posts, lesser said the better it is - and silly me, here I was thinking I'm in a forum called Bharat-Rakshak.
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PS: I should have posted this AFTER I'd completed the 4 part series write-up - but no b/w, so is getting shelved. Anyway I have posted most of the stuff I'd have posted there anyway.
