I updated my Abrams turret composite armor illustrations based on the armor of the M1150 ABV and other information I got from some experts.
It turns out the only times the physical thickness of the turret armor was increased was on the M1A1 in 1985 and on the M1A2 SEPv3 in 2020.
The downgraded composite armor of export Abrams models likely only reverted back to the original BRL-1 NERA arrays which used rubber.
However, the M1A1s sold to the Australians has the same third generation composite armor used by American Abrams tanks, just without the DU armor
The original BRL-1 composite armor of the M1 Abrams was a rubber based Non-Explosive Reactive Armor developed by the Aberdeen Ballistics Research Laboratory.
The Stillbrew armor of the Chieftain tank was actually very effective against APFSDS rounds
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Testing of the armor prototypes in 1985 showed that it can easily defeat the new L23 Tungsten alloy 120mm APFSDS round of the Challenger 1 even at a distance of just 1,000 meters.
At that distance, the L23 APFSDS can penetrate 455mm of rolled homogeneous steel armor.
They also tested the Stillbrew against the 3BK-14 125mm HEAT-FS shell which can penetrate 450mm and the armor was also able to defeat it.
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Stillbrew Armor:
~460mm vs APFSDS
~500mm vs HEAT
This means that Chieftain tanks equipped with the Stillbrew had superior KE protection over the new Challenger 1 tanks with the Chobham armor.
However, the Stillbrew was only added on the turret of the Chieftain and the hull remained to be very vulnerable to more modern threats.
The hull composite armor of the T-14 Armata is actually only ~650mm thick and mainly consisted of three steel armor plates sloped at 40°
However, it also uses special Silicon Carbide nanoceramic armor inserts which can be equal to almost 200% of steel armor against APFSDS rounds
Nanoceramic armor achieves this higher effectiveness by having more compact nano scale grains which makes them up to 70% harder than conventional ceramics.
Its been public knowledge that the T-14 Armata used nanoceramic armor ever since it was revealed back in 2015 but the actual thickness of the armor and how they were arranged only became known very recently.
The T-90 series of main battle tanks actually had an improved turret composite armor that was twice as effective as the original reflector plate arrays used by the T-72B.
The new design used a 50mm thick RHA plates with two layers of rubber sheets and high hardness steel plates.
This improved double reflector plate NERA design was developed for the Object 188 (T-90 prototype) in the late 1980s and would have also been used by other future Soviet tanks like the Object 187.
The Israeli Merkava III & IV main battle tanks actually use NxRA and SLERA armor arrays.
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Non-eXplosive Reactive Armor and Self-Limiting Explosive Reactive Armor are two types of reactive armor that bridge the gap between NERA which uses rubber and ERA which uses explosives.
Non-eXplosive Reactive Armor (NERA supposedly stands for Non-Energetic Reactive Armor) uses reactive polymers combined with gas generating chemical compounds to provide a highly energetic reaction that would destabilize the shaped charge jet.
While this allows NxRA to be significantly more effective than NERA, it is still very much less effective than Explosive Reactive Armor.
It also doesn't have the same multi-hit capability as NERA as the highly energetic reaction of the expanding gases effectively destroys it.
In 2003, the US military acquired four T-84 main battle tanks from Ukraine for testing and evaluation.
They were shocked to discover that the M829A2 120mm DU APFSDS of the M1 Abrams couldn't penetrate the hull armor of the T-84 when its covered with the Kontakt-5 ERA.
"but wasn't the M829A2 already designed to defeat the Kontakt-5?"
During the development of the M829A2 in the early 1990s, the US was only able to test the new round using T-72M1, T-72A and T-72B tanks equipped with the Kontakt-5 as well as a single T-80U mod. 1989 from the UK.
Due to their limited samples of Soviet armor, the US military had no idea if the M829A2 would also be effective against older Russian main battle tanks when they're equipped with the Kontakt-5.