Wear Resistance of Internal WC-CoCr Coatings Produced by High Velocity Oxy-Fuel Spraying

Abstract

Thermal spray coatings, and in particular those applied by HVOF process, are being used in a diverse range of engineering applications to extend component life by retarding wear and corrosion degradation [1]. Coatings performance is strongly dependent on microstructure [2]. The mechanical properties are a complex function of carbide size, shape and distribution, matrix hardness and toughness and the presence of various reaction products. This paper is focused on the wear resistance of WC-based coatings containing 10 mass% Co and 4 mass% Cr as the metallic binder, involving a heat-treated steel rings as substrate, a hot rolled product of structural steel in accordance with DIN EN 10113-2 for steel construction, detaining a tensile strength about 520-680 N/mm2. A feedstock powder of WC-CoCr 86 10 4 agglomerated/sintered was used for coatings production. It has good properties, such as: high resistance against oxidation and corrosion (due to the metallic component), good wear resistance (due to the ceramic component) and with a hardness in range of 1000-1300HV. The coatings have been produced by HVOF spraying of four different WC-CoCr grain size of powder particles, such as:sample 1 (-45+22µm), sample2 (-22+5µm), sample 3 (-10+2µm) and sample 4 (-2+0.5µm), using an ID CoolFlow mono gun developed especially for internal surfaces. This type of gun is a HP HVOF system for high pressure, but cold internal HVOF spraying. The morphology and the structure of the sprayed coatings were investigated by means of scanning electron microscopy (SEM/EDS) and the qualitative and quantitative phase composition of those coatings were determined applying the x-ray diffraction technique (XRD). In order to determine the sliding wear resistance of the coated samples, it was used the pin on ring test method under dry condition, with and without compressed air. This method consists into a stationary ball loaded against a ring sample in relative motion. The sliding wear rate of the tested samples was calculated from the volume of material lost during the test. The investigated samples were examined by means of stereo and light microscopy before and after exposure to wear tests. Based on the obtained results it was demonstrated that the coatings obtained from -10+2µm powder particles exhibit an optimal wear behavior when exposed to severe conditions.