Application of hardmetals as thermal spray coatings

Thermally sprayed hardmetal coatings have a typical thickness within the range 100–500 μm. Thus, thermal spray enables the functionality of hardmetals to be realized on the surface of large parts, which cannot be produced by powder metallurgy for technical and economical reasons. This article reviews the different types of thermal spray processes, with particular focus on the high velocity HVOF and HVAF deposition techniques which are of most relevance to the application of hardmetal coatings. Feedstock powder preparation technologies are presented. The majority of hardmetal thermal spray coatings are based either on WC or Cr3C2 or hard phases appearing as a result of their interaction. As an alternative, TiC-based compositions are most intensively studied. Thermal spraying generates significant changes in the hardmetal chemical and phase compositions between the feedstock powder to the sprayed coating. Coating formation and microstructures as well as selected properties, such as hardness, the effect of heat treatments and the oxidation in service, as well as corrosion resistance are discussed. As an example for wear protection applications, abrasion wear resistance is shortly discussed. This paper is a partly updated and condensed version of the chapter: “Coatings by thermal spray” in the book “Comprehensive Hard Materials”, V.K. Sarin (Editor-in-Chief) & D. Mari & L. Llanes (Vol. Ed.), Vol. 1 (pp. 471–506), Elsevier, 2014