|M.Sc Student||Fischer Horowitz Miri|
|Subject||Influence of deposition temperature and microstructure|
of Cr-N interlayer on the adhesion of diamond
coatings on WC-Co substrates
|Department||Department of Chemistry||Supervisor||Professor Alon Hoffman|
|Full Thesis text|
The most renowned property of diamond is its exceptional hardness. By depositing diamond films on tungsten carbide substrate (WC-Co), the hardness of diamond can be combined with the toughness of WC-Co, resulting in an excellent wear resistance material for tribological applications. However, poor adhesion of diamond coating on WC-Co substrates leads to a lesser lifetime than expected for the diamond coated tools. The prime reasons for the lack of proper adhesion are the preferential formation of graphitic layer at the interface and the residual stresses at the interface. Previous studies in our group showed that the adhesion can be improved by the use of a Cr-N interlayer, which (i) prevents the diffusion of Co from the substrate surface and (ii) provides an intermediate CTE, when compared to diamond and WC-Co, which helps to minimize the residual stresses to some extent.
In the first section, we compare the wear performance of diamond coated WC-Co inserts with a Cr-N interlayer, deposited using a commercial reactor and a custom-built reactor. For comparison, a commercially available diamond coated WC-Co tool without an interlayer was also included. Wear performance of the diamond coated tools was tested by turning and impact tests. Machining experiments of diamond coated WC-6%Co tools with an Al-Si alloy work piece showed excellent wear performance, however their performance upon the impact was not superior. On the other hand, impact test of diamond/Cr-N/WC-10%Co tools showed improved coating-substrate interface fatigue strength. Thus, this combined study demonstrated that diamond coatings on WC-10%Co tools with a Cr-N interlayer are better candidates for machining hard composite materials. In addition, it is found that the tool lifetime is strongly influenced by the deposition temperature.
In the second section, we investigated the effect of the deposition temperature, in the range of 500-750 °C, on the adhesion property of diamond films on WC-10%Co substrates. Adhesion was determined using Vickers hardness test by examining the failure occurred at the weaker interface, which is found to be the diamond/Cr-N interlayer. An in-depth interface analysis of the weaker interface is carried out to determine the fundamental reasons for the failure, using secondary ion mass spectrometry, x-ray photoelectron spectroscopy and x-ray diffraction techniques. Our analysis showed that improved adhesion was obtained for the films deposited at 650 °C. In addition, the formation of the Cr-C layer is strongly influenced by the deposition temperature, which in turn influences the adhesion strength. The optimum deposition temperature (650 °C) is an optimization of two main factors of the adhesion, compressive stress and chemical bonding, which have opposite temperature dependences.
In the third section, the influence of Cr-N microstructure on the adhesion is investigated by depositing diamond films on polished and unpolished Cr-N interlayer. Our results showed that diamond films deposited on the unpolished sample possess better adhesion due to (i) enhancement in the diamond seed density, (ii) improvement in ‘mechanical interlocking’ between the interlayer and the diamond film and (iii) an increase in surface energy of the interlayer surface.