The electrodeposition of composite coatings based on metal matrix-included particle deposits

Historically, the origins of composite (or inclusion) electrodeposition can be traced back to the early 1900s although the majority of modern developments can be considered to have taken place over the last 40 years. The increasing demands of industrial surface engineering have provided a driving force for rapid developments over the last decade. Co-deposition techniques can be used to produce a wide range of metal matrix-included particle coatings. The thickness of the overall coating can range from sub-micron to tens of microns while the included particles typically have sizes in the range 0.05-50 microns. The metal matrices (which include Ni, Co, Cu, Pb and Cr) can be deposited by electroplating or by electroless plating; particles range from hard materials (e.g., SiC, WC, Al2O3, CrC arzd BN), to self-lubricating ones (e.g, PTFE, C, MoS2 and encapsulated oils) and second metal powder phases. These deposits combine the advantages of the metal matrix and the included particles. For example, the metal matrix can confer high electrical and thermal conductivity while the type and degree of inclusion can tailored to the tribological properties required Important examples include Ni/SiC deposits for wear resistance and Ni/PTFE ones for their self-lubricating and anti-stick characteristics. More ambitious composite coatings include those where a semi-continuous release of the included particles occurs under service conditions, e.g., slow release of oil particles or PTFE fragments. This paper provides a concise review of the field of composite electrodeposition and highlights the importance of process control in obtaining critical deposit characteristics for a variety of demanding industrial applications. In order to achieve high quality deposits, it is essential to control the electrolyte composition and process conditions (e.g., electrolyte flow conditions, solution pH and organic additive levels). Existing theories cannot adequately predict the deposit composition and properties from knowledge of bath composition and process conditions and further work in this area is essential. Recent developments in composite coating technology are profiled, including the emergence of compositionally- and hydrodynamically modulated layer coatings, the possibility of slow release coatings for semi-continuous lubrication and modification of diffusion coatings by heat treatment.