Scanning electron microscopy (SEM) images were acquired using a Zeiss Gemini 500 scanning electron microscope equipped with an Inlens detector for energy filtering and detection of low voltage back scattered electrons. The microscope was operated at an accelerating voltage of 5 kV in order to prevent significant e-beam induced damage during high resolution imaging (although some damage was still observed). Average apparent grain sizes (dg) were estimated from measuring the area of ~300-400 grains using ImageJ 8 on both ceramic surface and crosssection images (see examples in Figure of main text). dg values were estimated from the measured areas by assuming grains are roughly circular and dg is the mean of all measured dg values.