An investigation of diamond thin film deposition on steel substrates

Abstract

The motivation behind this work was the exploration of the possibility of diamond deposition on steel substrates for low friction and low wear applications. Materials such as tungsten carbide are commercially available as diamond coated tools, where the diamond coating greatly extends the tool lifetime and performance. The diamond deposition on steel differs in terms of limitations to the diamond deposition on tungsten carbide. The main limitations of steel are its sensitivity to elevated temperatures which are commonly used for diamond deposition and a large difference in the thermal expansion coefficients of steel and diamond. Overcoming those challenging limitations would result in an introduction of competitive products for many applications. This project was a pioneering work in diamond deposition on steel substrates at Aston University in co-operation with Teer Coatings Ltd (Miba goup). The main focus was on the use of an interlayer as a facilitator of enhanced diamond growth and its adhesion towards the steel substrate. Particular attention was given to amorphous carbon coating being a buffer layer for subsequent diamond growth, followed by the investigation of diamond film growth on tungsten coated steel substrates. Interlayers were deposited using the magnetron sputtering technique at Teer Coatings. Diamond thin films were deposited at Aston University using microwave plasma chemical vapour deposition (CVD) with methane and hydrogen as a deposition gas mixture. Investigation of diamond growth from amorphous carbon films coated on steel substrates was found despite the initial promising results to provide low diamond nucleation coverage resulting samples with a sparse population of diamond crystals. The focus of the study changed into an investigation of diamond growth on steel substrates coated with metallic interlayers. As an enhancement for diamond nucleation a pre-treatment of seeding the substrates with nanocrystalline diamond particles, transferred onto the substrates by immersion into a diamond suspension, was developed and used further in this work. Tungsten coating was chosen as the main interlayer material for its diffusion barrier properties, carbide formation, specific thermal expansion coefficient and no inclination to hydrogen embrittlement. The direct tungsten deposition onto a substrate was found problematic and was initially solved by the development of a structured CrW interlayer (1 μm thick) on which an optimization of diamond CVD deposition conditions was performed. The need for a reliable temperature measurement resulted in creation of a setup with thermocouple mounted at the bottom of a substrate holder and a suitable calibration of the setup to be able to calculate the temperature of the substrate surface. CrW was found to have poor adhesion properties and a new MoW interlayer (1 μm thick) possessing excellent adhesion characteristics was developed. The diamond films deposited using previously optimised diamond deposition conditions was found to be at 785 °C. The ≈250 nm thick diamond films showed a good adhesion strength while the MoW interlayer was proved to be an effective diffusion barrier. The previously optimised diamond deposition conditions were found to deteriorate the steel substrate’s properties and further low temperature diamond deposition conditions were optimised for diamond growth at 650 °C. The resulting ≈250 nm thick films showed poor diamond adhesion characteristics due to weaker bonding between diamond and the substrate. The steel substrate did not undergo any softening during the diamond deposition. The effect of different diamond deposition temperatures, as well as the different thickness of the MoW interlayer on stress within diamond film, was studied. Lowest amount of compressive stress of 1.6 GPa was found for a sample coated with the thickest MoW (8.3 μm) and diamond deposition conditions at 650 °C. The sample showed superior adhesion upon Rockwell C indentation, while poor adhesion was observed by means of scratch testing using WC ball as an indenter.