Al-Cu-Cr quasicrystalline (QC) coatings were deposited onto mild steel Q235A substrate by low power plasma spraying. The plasma torch was designed such that the powders can be injected axially (parallel to the axis of the plasma arc) into the region between the anode and cathode, and the carrier gases were transferred to the plasma flame, so the heating efficiency of the powders is greatly increased and the velocity of particles is much higher than an externally-fed powder system. The Al-Cu-Cr QC coatings were sprayed via this new technique at different plasma power (3~7kW) by controlling the secondary plasma gases (H 2 and N 2 ) flow or changing the arc current. The results show that, the original powder was mainly composed of a dominant icosahedral quasicrystalline phase I-Al 65 Cu 24 Cr 11 and a minor monoclinic θ-Al 13 Cr 2 . However, besides these two phases, body-centered cubic α- Al 69 Cu 18 Cr 13 , the approximant to quasicrystal I-Al 65 Cu 24 Cr 11 , has been detected in as-sprayed coatings. On the other hand, peak intensity of QC phase I-Al 65 Cu 24 Cr 11 decreased and that of crystalline phases (θ-Al 13 Cr 2 or α-Al 69 Cu 18 Cr 13 ) increased as the plasma power was increased.

The effects of substrate temperature during the low-power plasma spraying (2.5~8.0kW) and subsequent annealing on the phase transformation of Al-Cu-Cr quasicrystalline coatings were investigated. Compared with the original powder mainly composed of icosahedral I-Al 65 Cu 24 Cr 11 quasicrystalline phase and monoclinic θ-Al 13 Cr 4 , the coatings show an icosahedral quasicrystal I-Al 65 Cu 24 Cr 11 and its body-centered cubic approximate phase α-Al 69 Cu 18 Cr 13 . XRD patterns indicated that, for identical spraying condition, a rise in substrate temperature resulted in an increase of the I/α peak intensity ratio in the coatings, and the crystalline phases of AlCu 4 and Al 86 Cr 14 were observed in the coatings after annealing at 1113K for half an hour.