Laser melting of plasma-sprayed copper–ceramic coatings for improved erosion resistance

Abstract

Copper-based coatings containing a hard ceramic phase can provide an engineering solution to components exposed to particle erosion environments provided the microstructure of these coatings is appropriate. Single-phase ductile copper is damaged by micromachining and ploughing while single-phase brittle materials are thought to be damaged by the generation and propagation of subsurface lateral cracks. Therefore, a multicomponent system composed of a ductile metal such as copper containing hard and brittle ceramic particles is expected to be tough enough to resist particle impacts at 90° and sufficiently hard to deflect eroding particles at low impact angles. Copper–ceramic coatings were manufactured by plasma spraying composite powders. These spray powders were obtained by agglomerating fine copper and boride powders by means of an organic binder in a rotating tumbler. The thickness of as-sprayed coatings reaches about 350 μm. After surface polishing, these coatings were laser melted with parameters ensuring in-depth melting. Polished as-sprayed, surface milled laser-melted coatings and bulk copper specimens were erosion tested at 25° and 300°C at impact angles of 25° and 90° in a laboratory erosion device. This device was designed to impact materials with coarse (32–300 μm) iron ore particles at a speed of 100 m/s. The evaluation of wear damages was done with a laser profilometer and scanning electron microscopy was used to evaluate microstructural changes upon laser treatment as well as surface modifications after erosion tests. Particle erosion resistance of laser-melted plasma-sprayed copper coatings containing 45 vol.% boride phase is improved in comparison with bulk copper. The low angle impact erosion is reduced at both 25°C and 300°C (1.7 times at 25°C and 4.8 times at 300°C) and the normal angle impact erosion resistance is the same as pure copper at both temperatures. As-plasma-sprayed coatings presented the worst erosion resistance. The improvement in erosion resistance of laser-melted coatings is attributed to the formation of large clusters of boride particles upon laser melting plasma-sprayed copper-boride coatings. These rounded boride islands are large enough to deflect the incoming eroding particles and the elastic copper phase dissipates the energy of particles with a high impingement angle.