Concurrent impression and uniaxial compression creep studies were performed on three Yb-SiAlON materials. Stress exponents were approximately 1 in compression and 2 in impression. The higher stress exponents were due to the complex stress field in the impression creep test, which caused microstructural dilation. The dilated multi-grain junctions also became filled with additional intergranular glassy phase. Focused ion beam milling and in situ lift-out specimen preparation combined with transmission electron microscopy was successful in identifying microstructural changes after creep testing. These observations have important implications in the design of creep-resistant materials in complex stress fields.