Abrasive wear behaviour of steels and advanced HVOF-sprayed WC-M coatings

This thesis concerns a study of the three-body abrasive wear behaviour of two groups of materials with different abrasive particles using the Dry Sand Rubber Wheel (DSRW) test method. This investigation can be divided into three sections: In the first section, the abrasion of a range of steels with an ash from a biomass power station was compared with that observed for abrasion with a conventional silica abrasive. It was seen that the wear rate of the steels when abraded with silica increased in proportion to the applied load and decreased with the hardness of the steel. However, the bottom-ash was more friable than the silica abrasive, and as such, significantly more abrasive crushing was observed during the tests with the bottom-ash abrasive. It is proposed that the wear is dominated by abrasion by the larger particles in the distribution, and that damage is limited by the maximum load which the particles can sustain before failing. In the second section, the motion of particles in the DSRW test with silica abrasive against a range of steels, as a function of applied load and the hardness of the steels was studied. The results showed that particle rolling through the contact is favoured by low applied loads and low testpiece hardness whereas particle sliding through the contact is favoured by high applied loads and high testpiece hardness. A model was proposed to provide an analysis of the motion of particles in the DSRW test. The effect of hardness on particle rotation is well predicted by the model, but the effect of the applied load on particle motion observed experimentally is opposite to that which is predicted by the model. The shortcomings of the model are discussed, and the model has been qualitatively modified to account for this discrepancy. In the third section, five different WC-metal cermet powders were deposited as coatings by HVOF thermal spraying. These were a WC-nickel alloy, a WC-iron alloy and three types of WC-Co powders with different carbide grain sizes. Characterisation of the coatings showed decomposition of WC during spray process for all the coatings. The results show different solubilities of W and C in the binders and different precipitation characteristics. DSRW tests were performed to assess the wear resistance of the coatings with silica and alumina abrasives. It was found that the coatings had different wear rates and mechanisms when abraded with silica compared with alumina. The differences in the wear behaviour of the coatings are due to the differences in powder characteristics, the extent of reaction and decarburisation during spraying, and the subsequent development of the microstructure in the coating during splat solidification at high cooling rates