NiAl-Al 2 O 3 intermetallics based composite coatings were prepared by cold spraying of Ni/Al-Al 2 O 3 composite powders followed by post-spraying annealing treatment. The phase transformation mechanism from Ni/Al mechanical alloy to intermetallics was explored to aim at controlling the microstructure of the composite coating. Results showed that, with the porous Ni/Al and Ni/Al-Al 2 O 3 green compacts, self-propagating high-temperature synthesis (SHS) reaction was ignited at a temperature of 500-600°C. However, SHS reaction was not able to be ignited for the cold-sprayed dense Ni/Al alloy coating with or without substrate. SHS reaction was even not ignited for the Ni/Al-40vol.%Al 2 O 3 composite coating, although the thermal conductivity of the coating was significantly decreased by the addition of Al 2 O 3 ceramic particles. The phase transformation from Ni/Al mechanical alloy to NiAl intermetallics during post-spraying annealing can be evidently attributed to diffusion mechanism.

Thermal spraying may be a promising approach for the deposition of NiAl-Al 2 O 3 composite coatings. To understand the intrinsic mechanical property of NiAl-Al 2 O 3 composite, dense NiAl-Al 2 O 3 composites were prepared by plasma activated sintering using the ball milled Ni/Al- Al 2 O 3 composite powders contain both micro-sized Al 2 O 3 strengthening particles and submicron-sized Al 2 O 3 dispersoids homogeneously distributed in the NiAl matrix phase. The angular morphology of the Al 2 O 3 particles in composite powder was changed to spherical or near spherical morphology after plasma activated sintering. Compared to the NiAl-Al 2 O 3 composites with low Al 2 O 3 content, NiAl- 60vol.%Al 2 O 3 composite consisted of less fine submicron-sized Al 2 O 3 dispersoids. Melting induced mechanism was proposed to explain the spheroidization and the change of Al 2 O 3 particle size distribution. The hardness of the NiAl-Al 2 O 3 composites with both micro-sized Al 2 O 3 strengthening particles and submicron-sized Al 2 O 3 dispersoids increased with the increase of Al 2 O 3 content. The relation was employed to theoretically estimate the hardness using volume fractions of components and corresponding hardness. The estimated hardness was compared with the observed ones. It was found that bimodal sized particles reinforcement results in a higher hardness than the theoretical value, which could be attributed to the dispersion hardening effect.

FeAl intermetallic compound coating was prepared by cold spraying a mechanically alloyed Fe(Al) alloy powder followed by post-spray annealing at 950°C. The high-temperature abrasive wear test was carried out for the annealed cold-sprayed FeAl at a temperature range from room temperature to 800°C. The high temperature abrasive wear of a heat-resistant stainless steel 2520 was performed for comparison. The results showed that the annealing treatment of the as-sprayed Fe(Al) alloy coating at a temperature of 950°C results in the formation of dense FeAl intermetallic compound coating with no particle boundaries. It was found that with the increase of the test temperature the wear rate of the stainless steel increased at the temperature higher than 400°C, while the wear rate of cold sprayed FeAl coating tended to decrease at the temperature higher than 400°C. The high temperature abrasive wear resistance of the cold-sprayed FeAl intermetallic compound coating increased with the increase of the abrasive wear temperature in a temperature range from 400°C to 600°C and changed little in the temperature range from 600°C up to 800°C. The wear resistance of cold-sprayed FeAl coating was higher than that of heat-resistant 2520 stainless steel under 800°C by a factor of 3.