vapilot2004,
What is surface recession? One of our engineers thought it was a fancy way of saying wear? Is this correct?
If you think of ceramics in the same way as metallics, oxidation or corrosion are mechanisms, particularly when combined with heat, which do attack either of the materials. In turbine combustor, CMC applications, one of the environmental attacks comes from water. So in order to protect the base CMC material, a protective layer material is added to prevent either oxidation or corrosion leading to base material component failure. So, it becomes important to know the rate of surface recession or the resistance to environmental attack provided by the protective layer. It isn't so much wear as it is oxidation or corrosion of the component material. Hope this answers your question.