Lightweight materials such as aluminum alloys and aluminum-based composites are the most preferred materials in the automotive and aerospace industries due to their high strength to weight ratio, wear resistance, thermal conductivity, and corrosion resistance. However, the usage of these materials is based on the realization of the required geometrical and dimensional tolerances of the finished component with the least environmental impact. These requirements necessitate the need for dry machining of these materials to avoid the environmental impact of coolant and for easy reuse of chips collected during the machining process. Machining of these materials in a dry cutting condition is a highly challenging task. The difficulties in the machining of aluminum alloys are due to the aggravated formation of a built-up edge on the rake surface of the cutting tool. This leads to deterioration of machined surfaces, higher cutting forces and tool wear. On the other hand, for Al/SiC composites, the presence of hard SiC reinforcing material gives rise to rapid tool wear. To address this machining challenge, a selection of cutting tool with high thermal conductivity, high hardness, and the low friction coefficient is required. In this regard, diamond-based tools such as polycrystalline diamond (PCD) and hot-filament chemical vapor deposition (HFCVD) based diamond coated carbide tools are the potential tools for processing of these materials. The commercially available PCD tools possess low thermal conductivity, low chemical stability than CVD diamond due to the presence of cobalt binder. Also, it is very difficult to fabricate a complex geometry of the cutting tools using PCD and also it is not economical for high volume production. In this view, the uniform diamond coating on the complex geometry of the WC-Co cutting tool can be possible through the CVD method. However, to achieve a highly adhesive diamond coating on WC-Co is a challenge due to the outward diffusion of cobalt and reactivity with carbon at a higher deposition temperature during diamond growth. This catalytic effect of cobalt induces coating delamination during machining and it limits the survival limit of the diamond coated cutting tool. The outward diffusion of cobalt from WC-Co was hindered by doping boron in diamond lattice during diamond deposition. Besides, boron doped graded layer diamond coating is a prime novel in this present research work. The architecture was consisting of three-layer such as bottom layer boron-doped diamond (BDD) followed by a transition layer and eventually ended with a nanocrystalline diamond coating (NCD) on the top layer. This was designated as BMTN (BDD/transition layer/NCD). The bottom layer BDD arrests the diffusion of cobalt during diamond deposition and the presence of top layer NCD reduces the friction during machining due to its lower friction coefficient. The interface stresses between BDD and NCD could be possible due to its different morphological variation in the coating. The presence of the transition layer relieves the interface stress by gradually changing the deposition conditions from BDD to NCD. To compare the BMTN coating performance, conventional diamond coatings such as microcrystalline diamond coating (MCD) and nanocrystalline diamond coatings were considered for this present work. The quality of the diamond film, residual stresses and the presence of different phases in the diamond coatings were analyzed through Raman spectroscopy method. The compressive residual stress is higher for NCD coating due to its more intrinsic stresses and sp2 phases, whereas in BMTN coated tool, lower compressive residual stress was achieved due to reduced interface stresses. The results of the scratch test show that BMTN coated tool has higher adhesion strength with a critical load (Lc) of 67.71 N. The tribological results show that the NCD coated tool exhibited a lower friction coefficient and it was slightly lower than BMTN coated tool. However, the usage of NCD coated tool in a machining operation was limited due to its more amounts of sp2 phases at the coating interface. Besides, the fracture toughness of diamond coatings was examined through the Vicker’s indentation test method. The results show that BMTN coated tool attains higher fracture toughness (KIC =12.6 MPam1/2) than other tools.
Similarly, the performance of the BMTN coated tool was compared with MCD, NCD, BDD, and BMTN coated tools while machining against Al-Cu alloy/25% SiC MMC material in dry cutting condition. In addition, the uncoated tool, TiAlN coated tool and PCD tool was considered for this MMC machining. The tool wear results show that BMTN coated tool exhibits low flank wear (VB = 0.25 mm) than other tools. The surface finish of the component machined through BMTN coated tool generates lower Ra value of 331.53 nm than other diamond-based tools.
Thus, this study indicated the better performance of boron doped graded layer diamond coated tool in the machining of aluminum 6061 alloy and aluminum-based composites and this could be considered as a potential cutting tool for high performance manufacturing applications