I know most of this site is dedicated to older 911s so this may not apply to the vast majority of users, but the information found on a multitude of different subjects on this Off Topic forum never ceases to amaze me, so I've decided to share the following information I've put together:
Checking for wear on carbon ceramic brake (CCB) discs:
This will depend on whether or not the rotor hub/hats have Proceq markings (three of them) or not. If the hub of the brake disc does in fact have the Proceq markings on it, then it’s very simple procedure to determine brake disc wear, assuming that you have access to the Carboteq measuring tool from Proceq (known as the VAS6813 in the Porsche world). Since Proceq started to be commercially available in 2014, odds are that carbon ceramic discs manufactured before 2014 will not have the Proceq markings on the hubs. This video is self-explanatory on what the markings look like and how to take the measurements:
https://youtu.be/asnGXOmQsYU
If you own a disc that does not have the Proceq markings on the rotor hubs, then determining carbon ceramic brake (CCB) disc wear follows a separate procedure on estimating what percentage of wear has taken place. The following steps yield a much more imprecise result, but it’s better than taking a leap of faith regarding purchasing a vehicle with used carbon ceramic discs. First you will need to understand the information stamped on the brake disc hub (assuming it’s different from the one shown in the video posted above):
Step 1- visually inspect the discs to find the three wear indicators (on each side of the disc). These round indicators (approximately 2 cm. or 3/4”) are spaced 120° from each other. When the discs have relatively little use, these can be at times hard to see unless you shine a bright light on them. If these wear indicators have a different color and can be easily seen, this means your disc is approximately 50% used. If these wear indicators can be seen, then you need to proceed to Step 2.
Step 2- measure thickness of the brake disc. You will need a very accurate micrometer / digital caliper for this. The minimum thickness will be stamped on the brake disc hub (each disc could have a different amount printed on its hub). If the obtained result equals minimum thickness plus 0.2mm measurement stamped on the hub, then you need to proceed to Step 3.
Step 3- weigh the brake disc. The original weight of the disc will be stamped on the brake disc (it could vary per disc on the same vehicle). The scale used to measure the disc must have the ability to measure ± 1 gram and the range that it needs to be able to weigh must be between 0 and 12 kilograms. Each manufacturer will have the maximum allowable weight loss for each disc indicated in their service manual.
Other miscellaneous information: it is normal to have what appear to be micro cracks/fractures all over the disc, even when new, even on the cooling duct cross-members. What is not normal is to have cracks from the hub to the braking surface of the disc (requires immediate replacement). Also not normal is that the braking surface presents multiple areas where the surface seems to have chipped off. It is also not normal, and requires immediate replacement, are chips on the edge border of the disc. To say that carbon discs are a “lifetime part” is simply not true. Based on use, the “lifetime” will vary greatly from one owner to another (assuming same vehicle). If to this we add that each vehicle will have different wear characteristics, then determining expected usage left becomes even more complicated, but by following the steps listed above, one can have a better guess. To say that discs have X more time left or Y more miles left in them is a completely absurd statement to make as there is no way to determine this as there are too many variables to consider.
I am not a mechanic nor pretend to be one. I am currently going through a purchase process for a 2011 Cayenne Turbo equipped with PCCB brakes and knowing that brake disc replacement can easily cost $28K, I wanted to dig deeper into this. All of the information provided above is the summary of days of research among various well known automotive related web boards such as this one, but truthfully said, if it were not for the clarifications and technical documentation (my source for images used) obtained from Maja Bašelj from ReDisc, I would have not been completely sure of what I’ve posted above. My hope is that this information will be of help to others in similar situation in the future and they can find a place where they can find all of the information they need to make an educated decision.