| The application of plasma sprayed alumina coatings have been limited due to their high porosity and poor toughness. This study presents the preparation of the nanostructured alumina based composite coatings for strengthening and toughening of the alumina ceramic coatings. In this thesis, the agglomerated nanopowders were spray-dried using the commercial available nanoparticles of Al2O3, TiO2, CeO2 and ZrO2 as raw materials. Subsequently, the nanopowders were sintered at high temperature to form the sprayable nanocrystalline feedstocks. The Al2O3 based nanocoatings were fabricated by plasma spray technology. The nanostructured powders and coatings were characterized by XRD, SEM and TEM to investigate the growth of the grains and the evolution during the whole plasma spray process. The “growth model” of the nanograins as well as the various stages of the coating formation model were established to reveal the formation mechanism, structural features, performance characteristics and friction and wear properties of the plasma sprayed Al2O3 based nanocoatings. A comparative study was made with the conventional Al2O3-13%TiO2 coating created from micron-sized feedstock and the results showed that:According to the free-holding density and flowability of the powder, the optimum spray-granulation-process parameters of the reconstituted Al2O3-13%TiO2 agglomerated nanopowders were set as 3% binder content, 30% solid content and 300°C drying temperature. In order to avoid the excessive growth of the nanoparticles, a suitable sintering temperature was determined as 1000℃-1100°C. A Low-power plasma jet enhancement treatment for the nanopowders can further improve the feeding spray-ability. As shown from a spray online monitor, the velocity and surface temperature of the flighting feedstocks in the plasma jet increase with the spraying power. The typical grain growth characteristics of the nano-feedstock obtained by quenching experiments showed that, the grains of surface-layer were fully melted to form columnar crystal, the grains beneath surface-layer were partially melted to form liquid phase sintering microstructures, and the grains in the center did not melt and maintained the original solid phase sintering microstructures.Plasma sprayed alumina-based nanocoatings exhibited a bimodal microstructure consisting of partially melted zone(PM) and fully melted zone(FM). Nanoparticles in FM zone were mainly columnar grain growth consolidation, and there were nanoscale grains in FM zone. PM zone was formed containing unmelted nanoparticles(retained from the starting powder) with liquid or solid phase sintering-structures. FM zone was dominated by γ-Al2O3, however PM zone was dominated by α-Al2O3. The ratio of FM to PM zone and thus the ratio of the γ-Al2O3 to α-Al2O3 reflects the melting degree of the coating. With the increase of the spraying power, the ratio of γ-Al2O3 increased, and the grain size of the coatings increased as well. The melting degree and average grain size of the coatings can be calculated quantitatively based on the XRD analysis results, and can be controlled by adjusting the spraying power, the two match produces the best comprehensive properties when the spraying power is 35 kW. The structure-heredity existed in the nanostructured Al2O3-13%TiO2 feedstocks and coatings.The hardness, strength and toughness of the nanocoatings are superior to conventional coatings. Compared with the conventional coatings, the hardness, the bonding strength and the toughness of nanostructured Al2O3-13%TiO2 coatings can be increased up to 24%, 32% and 1 time, respectively. The addition of CeO2 and ZrO2 improves the sintering behavior of the nano-feedstocks and the microstructure of the coatings, which help to enhance the mechanical properties. Weibull statistical analysis showed that the nanocoating exhibited a distinct bimodal distribution: PM zone with low hardness and high toughness, however, FM zone with high hardness and brittleness. Based on the crack tip shielding toughening, toughening mechanism model of the plasma sprayed nanostructured Al2O3-13%TiO2 coatings was established, and the toughening effect of the nanostructure were analyzed.The friction and wear properties of the alumina-based nanocoatings are better than the conventional coatings. The friction coefficient of the nanostructured alumina-based coating(sprayed at the power of 35 kW) is about 1/2 of the conventional coatings, and its wear weight loss is only about 1/4 ~ 1/3 of the conventional coatings, when it suffered the friction under the load of 500 N. CeO2 and ZrO2 can be added to improve wear resistance of the nanocoatings. The different wear properties of the coatings are attributed to the different wear mechanisms: The wear mechanism of nanostructured Al2O3-13%TiO2 coating under low load is abrasive wear mainly associated with nanoparticles rolling and self-repairing function, high load of "pseudo adhesive wear" for primary and fatigue wear relatively minor. Whereas, the wear mechanism of conventional coating is dominated by abrasive wear at low load and accompanied by obvious fatigue wear at high load.Based upon these experimental observations, a conclusion can be developed to explain the two mechanism that alumina-based nanocoating has excellent mechanical and tribological properties, One is “Performance complementary” resulted from the bimodal distribution of FM zone and PM zone, the other is “indirect strengthening and toughening” caused by the nanocrystals and micro-defects in nanocoagtings. In addition, the special three-dimensional network structure in the nanocoatings also played an important role. |
PDF Full Text Request