With the increasing demand for turbine engine performance, turbine blade structure becomes increasingly complex. The investment of precision casting ceramic core is a serious challenge. The existing solubility is the most important issue restricting the application of alumina-based ceramic cores. Increasing the porosity can enhance the substrate material and the interaction of chemical solutions. The main research direction is to improve the corrosion performance of alumina-based ceramic core. However, the increase of porosity will cause the core strength to plummet, affecting the performance of alumina-based ceramic core. To meet the requirements of investment casting, the organic-inorganic hybrid fiber can improve the porosity and strength of alumna-based ceramic core system. The synergic strengthening mechanism of hybrid fiber modified aluminum ceramic cores pore-system was investigated.

Capacitive corundum powder, fused mullite, and quartz glass powder were used as matrix material, mineralizer, and firing aid, respectively. Meanwhile, organic short-cut fiber as a pore-making agent, inorganic short-cut alumina fiber as reinforcement, coupling agent solution, and hydroxypropyl methylcellulose modified fiber were used to prepare alumina ceramic cores with different hybrid fiber ratios via hot pressing injection. According to aviation industry standard HB 5353.1-3—2004, the key performance parameters of each core sample were obtained. A scanning electron microscope was used to observe the pore characteristics, fracture morphology, and fiber distribution of the core, and analyze the fiber fracture mode. The synergic strengthening mechanism and the pore strength of the hybrid fiber-modified alumina-based ceramic core were investigated.

The introduction of hybrid fibers changes the pore characteristics of the alumina-based ceramic core, and irregularly connected pores and nearly circular closed pores appear in the ceramic core. The fracture mode of the alumina-based ceramic core changes from "transcrystalline fracture" of like-metal to "intergranular fracture" as the proportion of hybrid fibers decreases.

The addition of hybrid fiber reduces the sintering shrinkage rate of the alumina-based ceramic core, the shrinkage rate of the sample NA67 is only 0.2%. For the comparison, the rate is reduced by 77.8% for the sample NA60 without alumina fibers, which may be since an appropriate amount of alumina fiber can effectively increase the center separation of sintered particles, hinder the migration and diffusion of substances, offset the sintering shrinkage stress, and inhibit the sintering shrinkage of the core. The mass-burn loss of alumina-based ceramic core is 19.5% as the proportion of hybrid fibers decreases. As the mass loss mainly comes from the oxidation burn loss of plasticizer and nylon fiber, the alumina fiber has an impact on the migration of crystal water in the sintering process, so the mass-burn loss rate fluctuates slightly.

The number of large pores in the aluminum ceramic core increases, the number of small pores decreases, and the apparent pores in the ceramic core firstly increase and then become stable with the decrease in the proportion of hybrid fibers. Small-aperture pores are mainly composed of closed pores lost by chopped organic nylon fiber (N_sf) fiber, while large-aperture pores are mainly connected pores formed by chopped inorganic alumina fiber (A_sf) crossing closed pores when the hybrid fiber ratio is less than 6∶7. The apparent porosity of the core tends to be stable at 43.31% because the intersected A_sf fibers are clustered among the sintered particles. The effect of A_sf fibers on the increase of the distance between the sintered particles becomes slight, the number and size of connected pores in the ceramic core almost unchange, and the apparent porosity tends to be stable.

The bending strength of the alumina-based ceramic core increases firstly and then decreases as the proportion of hybrid fiber decreases. This is because the appropriate amount of A_sf fiber effectively increases the fiber-matrix interface bonding area. When the core is subjected to external loads, A_sf fiber consumes the more crack propagation energy through the behavior of debonding, fracture, and pull-out, and the bending strength of cores increases significantly. The excessive A_sf fiber is unevenly dispersed in the matrix material, dividing the matrix, reducing the interface bonding area between the agglomerated fiber and the core matrix, weakening the strengthening effect of the fiber on the core, and leading to a slight decline in the bending strength of the core.

The shrinkage rate of the alumina-based ceramic core decreased after firing as the hybrid N_sf/A_sf fiber ratio decreased. The A_sf fibers interspersed in the core matrix effectively hindered the diffusion and migration of sintered particles and reduced the core densification. The shrinkage rate of the sample NA67 after firing was 0.2% when the hybrid fiber ratio was 6∶7, which was 77.8% lower than that of the sample NA60 without hybrid fiber, showing a good dimensional stability.

The increase of apparent porosity of hybrid fiber reinforced alumina-based ceramic core was mainly due to the loss of residual closed porosity by oxidation of N_sf fiber at a high temperature and the formation of connected porosity by A_sf fiber interpenetrating matrix. The number of connected pores, the volume proportion of open pores and the apparent porosity increased, while the volume density decreased as the hybrid fiber proportion decreased.

An appropriate amount of A_sf fiber consumed the crack propagation energy through the mechanism of de-bonding, pulling out, and breaking, and improved the bending strength of alumina-based ceramic core, effectively making up for the strength loss caused by in-situ burning of N_sf. The bending strength of the alumina-based ceramic core reached a maximum value of 20.1 MPa when the hybrid fiber ratio was 6∶7. However, the agglomeration and cross of A_sf fibers could form some penetrating cracks in the core matrix and cut the matrix, affecting the strengthening effect of the fibers when the proportion of hybrid fibers further decreased.