The effects of load and substrate hardness on the development and maintenance of wear-protective layers during sliding at elevated temperatures

Transitions to low wear rates often occur during sliding between contacting metal surfaces, due to the establishment of high-resistance load-bearing layers. Such layers are developed from compaction of wear debris particles, with adhesion between the particles being an important factor in determining whether the layers are maintained, leading to wear protection, or break down, leading to abrasive wear. They are formed more easily and retained more effectively at higher temperatures, due to increased sintering and adhesion between the debris particles and to enhanced oxidation of these particles. This paper presents the results of a study of the reciprocating sliding wear and friction of dissimilar combinations of pin and disc steel specimens (high-speed steel and high-chrome steel pins and carbon steel discs) at temperatures of 500–600°C, with emphasis on the influence of load and substrate hardness on the development and maintenance of such wear-protective particulate layers. Complex relationships occur between the effects of increased load in producing larger debris particles, in decreasing the critical particle size for establishing the layers and in decreasing the separation between the sliding surfaces, and the effects of hardness of the substrates on the sizes and amounts of wear particles and on the topographies of the wear scars. The relationships are complicated further by oxidation and sintering of debris particles, leading to development of oxide or oxide-containing ‘glaze’ surfaces, and subsequent breakdown of the layers during sliding.