How durable is IWC's RC coating?

I bought my son a XVIII Le Petite Prince in August 2017. Unfortunately we had
to return it in December of that year as the AR coating was lifting from the
edges of the crystal. The crystal was replaced and returned. September 2018
the issue returned and the watch was sent back in December. This time it was
assessed as: "No manufacture fault. Watchmaker advised the crystal is rubbing
on something to cause the AR coating to wear away...last replacement was at
cost as a goodwill gesture" Cost for repair $500.

What is stumping us is what could possibly be rubbing it off. surely not the
occasional shirt cuff? The watch is worn in rotation with other watches and
kept in its original box when not in use and he has no watch winder. 80% of
the time he wears shirt sleeves to work.

Is it possible that the durability of the IWC AR coating is that fragile that
a light sleeved shirt or jumper would rub it off so easily? I would seriously
doubt it.

Very disappointed as it was a gift for graduating as an airline pilot

I guess that the top AR could always be removed but he likes the AR coating
and it was one of the selling points of the watch.

Is anyone aware of this as an issue with AR coatings?

I am not aware of this issue with any of my IWC's or those of others.

But, there can be adhesion issues in the sputtering process that applies the
mutl layers of the AR coatings. If something is amiss in the process, the
multi-layer coating can delaminate or flake off the substrate. It can happen,
but a quality supplier will have specs and test for adhesion and flaking as
well as the coating's hardness.

I doubt a shirt sleve will casue the issue.

this is very surprising. Never had any issue with any of my IWC crystal.

What is very surprising is that this has happened twice. Even if first time,
the crystal somehow missed quality cotnrol, the issue for you happened again
with the second crystal.

The IWC coating history. The first watch with coated saphire glass were the
early Ingenieur models 1832 and 3003.Those early coatings were critical and
later models were delivered without coatings, also serviced watches.

That negative experiance could be the reason, why IWC offered watches with
coatings later as several other brands, around 2000.

In fact, even modern coatings habe much less hardness as saphire glass. So,
abrasion or coating scratches are not uncommon, depending of usage intensity.