Low Power Plasma Spraying Research Articles

Effects of spray parameters on the microstructure and property of Al 2O 3 coatings sprayed by a low power plasma torch with a novel hollow cathode

Al 2O 3 coating is deposited using a low power plasma torch with a novel hollow cathode through axial powder injection under a plasma power up to several kilowatts. The effects of the main processing parameters including plasma arc power, operating gas flow and spray distance on particle velocity during spraying, and the microstructure and property of the coating are investigated. The microstructure of the Al 2O 3 coating is examined using optical microscopy and X-ray diffraction analysis. The property of the coating is characterized by dry rubber wheel abrasive wear test. The velocity of in-flight particle is measured using a velocity/temperature measurement system for spray particle based on thermal radiation from the particle. The dependency of the microstructure and property of the coating on spray particle conditions are examined by comparing the particle velocity, and microstructure and abrasive wear weight loss of subsequent coating deposited by low power plasma spray with those of the coating by conventional plasma spray at a power one order higher. X-ray diffraction analysis of the coating revealed that Al 2O 3 particles during low power plasma spraying reach to sufficiently melting state prior to impact on the substrate with a velocity comparable to that in conventional plasma spraying. The experiment results have shown that processing parameters have significant influence on the particle conditions and performance of deposited Al 2O 3 coating. The coating of comparable microstructure and properties to that deposited by conventional plasma spray can be produced under a power one order lower. From the present study, it can be suggested that a comparable coating can be produced despite plasma power level if the comparable particle velocity and molten state are achieved.

High hardness alumina coatings prepared by low power plasma spraying

Alumina coatings were prepared by low power plasma thermal spraying (2.5–4.0 kW). The powders were directly injected into the region between the cathode and the anode intake inside the plasma torch, and the carrier gases were transferred to the plasma flame. The results show that the hardness of Al 2O 3 coatings by low power plasma spray with an internally-fed powder system is higher than that of the coatings by APS with an externally-fed powder. The hardness of the coating increases with increasing plasma energy up to HV=1500 kg/mm 2, when the plasma arc voltage and current are 80 V and 50 A, respectively, and the porosity is estimated lower than 1%. The coatings are mainly composed of γ-Al 2O 3 although the original powders were nearly all α-Al 2O 3. The amount of the γ-Al 2O 3 in the coating increases, and the α-Al 2O 3 decreases with increasing plasma power. The grain size of the coating is 0.1–1.2 μm. The optimum spraying conditions have been established by analyzing the relation of the coating properties with spray parameters.