Structure and Mechanical Properties of Coatings Produced by Dynamic Ion Mixing

Ion implantation is now applied on a wide scale for Si doping in the semiconductor industry but its application to other materials is more recent. The ion beam mixing of predeposited surface layers with substrate materials has expanded the potential uses of ion beams to modify material properties. However, the shallow implanted or mixed layers (about 0·1 μm) are often insufficient for many technological applications and a new method combining ion beam mixing with simultaneous deposition, dynamic ion mixing (DIM), removes the thickness limitation imposed by finite ion ranges and allows for the buildup of thicker coatings on engineering components. The characteristic of the DIM technique is the use of a high energy (100-400 keV) heavy ion beam (Ar ⁺ or Xe ⁺ ) to produce localised energetic deposition of the target atoms. One of the most important features of this surface alloying process is to produce atomic mixing at the film/substrate interface which is important in the first stage of coating. This deposition results in an intermixed layer of graded composition which improves markedly the adhesion performance of the film. Moreover, the high atomic displacement rate achieved during heavy ion bombardment can also be considered to be a modification of the process itself which is used to control the microstructural state of the film. For instance, crystallisation of amorphous coatings can be obtained by DIM at temperatures much lower than those required in conventional thermal treatment. Some recent examples of coatings produced by this method on stainless steel and tool steel are described, including intermetallics (CuNi, FeAl) and ceramics (TiC, SiC, TiB ₂ ). The microstructural characteristics of the coatings were studied by TEM observations on crosssectional samples and composition-depth profiles were carried out by secondary ion mass spectroscopy. In all cases it was demonstrated that thick and adherent coatings with homogeneous and controlled structures could be produced. The adhesion performance of the coatings has been studied by means of scratch tests and the friction and wear behaviour assessed on a standard ball on disc tribometer. The hardness of the coatings was determined from nanoindentation experiments.