Laser-Plus

EPSRC research grant GR/J/19078

Summary of the final report

Principal Investigators: Professor F R Jones, Dr W M Rainforth, Dr R D Short

The aim of the project was to develop a novel low temperature process suitable for depositing conformal ceramic (or preceramic) coatings, which were of sufficient hardness to provide wear resistance. The specific application was cutting-edges but the emphasis changed during the course of the research. The most wear resistant coating was found to be a carbon coating, with some 20 fold improvement over the reference PVD sputter coated titanium carbonitride coatings. There was no direct relationship between wear rate and hardness. The TiOCN coatings which were prepared by plasma enhanced deposition had very low wear resistance. Plasma polymerisation can be accomplished in an RF or Microwave plasma discharge. Essentially monomeric organo or organometallic molecules fragment in the discharge and during deposition onto a substrate reassemble into a crosslinked conformal film. The degree of fragmentation of the monomeric reactants depends on the frequency and the power of the discharge. With microwave processes supplying the greatest degree of randomisation of monomers. The plasma gas can also be incorporated into the coating. Thus organosilicone and organosilazane compounds are potential sources of low temperature silica and silcon nitride films respectively, especially with O2 and N2 plasma gasses, respectively. There is a major advantage over conventional CVD in that the reactants and effluent are much safer, easier to handle, low cost and without the need to discharge corrosive gasses.
An electron cyclotron resonance (ECR) discharge, reactor manufactured by AsTex (USA) was commissioned and modified for the study. Monomers with potentially high ceramic yield were chosen for study; titanium isopropoxide (TIPT) and propoxide (TPOT), hexamethyldisiloxane (HMDSO), hexamethyldisilazane (HMDZN) and acrylonitrile (AN). Nitrogen and Ammonia were used as ECR gasses. Nitride ceramic-like films were prepared by plasma discharge substitution of oxides by nitrides from TIPT, TPOT and HMDSO. The coatings based on titanium and silicon were found by XPS analysis to contain up to 22% N but 28% O remained in the film. They could also be deposited readily to a thickness of 4 um, which was considered to be a requirement for wear resistance. These TiOCN films had Knoop hardness of >900, which was comparable with the PVD or sputter coated TiCN which was used as a control. Film adhesion was found to be good, however, these coatings had low wear resistance presumably as a result of incomplete conversion into TiCN.
Coatings deposited from HMDZN at a thickness of 1um were characterised to be SiCN and to have a high hardness (approx 16 GPa by nano-indentation). However, they were brittle and poorly adhering. But low thickness coatings of < 0.2 um adhered well. This meant that a thin SiCN coating could be used to prevent carbon diffusion into the steel substrate during deposition of a carbon film. Reports in the literature suggested that C3N4 coatings would be of high hardness and durability. Acrylonitrile was therefore examined as a potential source of a carbonnitride film, which could be deposited onto the SiCN protected steel substrate. These films had a hardness, similar to that of hardened-stainless steel, of up to 6 GPa (by nanoindentation). Their wear resistance was 50 times greater than that of the substrate and 20 times greater than that of the PVD-TiCN reference coating. The high wear resistance has been attributed to a low coefficient of friction caused by a lubricating layer of graphite in the wear attributed to a low coefficient of friction caused by a lubricating layer of graphite in the wear interface. The carbon nitride coatings with silicon carbonnitride barrier layer exhibited good adhesion to the stainless steel substrate. Heating to 700C did not affect the integrity of the films. The chemistry of these coatings showed a strong dependence on the RF bias.
The methodology for depositing multi-layer coatings onto steel substrates to provide wear resistance has been developed. The conditions required for varying the hardness of the coatings has also been established. The potential for depositing multi-layer coatings with specific properties is high and further research is planned.

Contact:

Prof. F.R. Jones,
Department of Engineering Materials,
University of Sheffield,
Mappin Street, Sheffield S1 3JD
Telephone: 0114 222 5477
Fax: 0114 222 5943