In unlubricated pin-on-disc tests on a 95% alumina, fluctuations were observed in the recorder traces for applied load, displacement, and friction. It was indicated by analysis of the fluctuations, together with consideration of the characteristics of a simple harmonic oscillator, that the frequencies were probably lower than the natural frequency of vibration of the test equipment. It is proposed that the resultant cyclic loading promotes fatigue crack growth in compression and causes wear by degrading the surface.