The electrodeposition of composite coatings based on metal matrix-includedparticle deposits

Historically, the origins of composite (or inclusion) electrodeposition can be traced back to the early 1900s although the majority of modern developm ents can be considered to have taken place over the last 40 years. The incr easing demands of industrial surface engineering have provided a driving fo rce for rapid developments over the last decade. Co-deposition techniques c an be used to produce a wide range of metal matrix-included particle coatin gs. The thickness of the overall coating can range from sub-micron to tens of m icrons 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 fro m hard materials (e.g., SiC, WC, Al2O3, CrC arzd BN), to self-lubricating o nes (e.g, PTFE, C, MoS2 and encapsulated oils) and second metal powder phas es. These deposits combine the advantages of the metal matrix and the included particles. For example, the metal matrix can confer high electrical and the rmal conductivity while the type and degree of inclusion can tailored to th e tribological properties required Important examples include Ni/SiC deposi ts for wear resistance and Ni/PTFE ones for their self-lubricating and anti -stick characteristics. More ambitious composite coatings include those whe re 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 electrodepos ition and highlights the importance of process control in obtaining critica l deposit characteristics for a variety of demanding industrial application s. In order to achieve high quality deposits, it is essential to control th e electrolyte composition and process conditions (e.g., electrolyte flow co nditions, solution pH and organic additive levels). Existing theories canno t 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 profile d, including the emergence of compositionally- and hydrodynamically modulat ed layer coatings, the possibility of slow release coatings for semi-contin uous lubrication and modification of diffusion coatings by heat treatment.