The presence of the extremely hard ceramic particles in metal matrix composites (MMCs) reduces drastically the Lifetime of ordinary tools, a serious drawback at earlier stages of the development of MMCs [1
]. Very hard materials, such as cubic boron nitride (CBN), and particularly those based on diamond, are the right choice for the tools, due to their extreme hardness, high wear resistance and thermal conductivity. Cobalt bonded diamond (PCD) early became the choice material due to its increased wear resistance, a prominent conclusion of the works of Durante et al. [2
] and El-Gallab and Sklad [3
]. Sub-surface damage is also minimized, particularly in finishing operations, as found by Hung et al. [1
]. PCD tools, albeit their widespread use, also have shortcomings due to the presence of the binder phase. This is much less wear resistant than diamond, softens at high contact temperatures and may also induce graphitization of the diamond grains, thus limiting the maximum usable cutting speed [4
]. Alternative hard tools are brazed thick CVD diamond plates or CVD direct coatings. The latter have the advantages of multiple tips per tool, allow the use of chip breakers [5
], are easier to prepare and are much less geometry dependent, a critical issue for small drilling and milling tools. The performance of brazed CVD diamond plates matches or surpasses that of PCD tools, due to their binder-free pure diamond construction, that reduces the tendency for diamond grain pull out by the hard particles [6
Regarding tools directly coated with a thin CVD diamond film, they may fail by delamination of the diamond coating after a few seconds [6
] due to a combination of small thickness and most likely inadequate substrate and edge geometry. The thickness of a coating is by itself no guaranty that the tool life increases, as was found by D’Errico et al. [6
], where turning inserts with 20 µm thick coatings had a ten times longer tool life than those with a 50 µm coating. This is due most likely to the increase of the cutting edge radius and subsequent increase of cutting forces, as was found for other workpiece materials [8
]. Other authors [9
] indicate that this dominant tool wear mechanism, coating failure, depends on the cutting speed and feed rate, and is due to the high interfacial stresses at the bonding surface. These arise from the high temperatures at the cutting tip and are due to different thermal expansions between the coating and substrate. Skordaris and co-authors [10
] thoroughly studied the temperature-dependent residual stresses in the diamond film structure which significantly affect the interfacial fatigue strength of the diamond coatings during interrupted cutting operations. These effects may also arise in the turning of composites due to the existence of hard particles. Delamination of the diamond coating can be delayed if less aggressive machining conditions, such as smaller feeds, are used as was found by Chou and Liu [9
], although at the cost of productivity. If adhesion is not an issue, then CVD diamond coatings generally wear out along the cutting edge due to the effects of localised forces and thermal load [13
]. One of the first works where the adhesion of the diamond to hard metal substrate tools was studied and modified [14
] showed that surface roughness and chemistry are of paramount importance. A finer microstructure of the substrate, etching of the tungsten carbide grains and removal of surface Co resulted in visible tool life improvements due to enhanced adhesion levels, when the right deposition parameters are used. Using hard metal tools as substrates, Kremer and co-workers addressed the effects of surface roughness [15
], in 6 µm thick coatings, including the use of a bi-layered (micro-/nanocrystalline diamond) tool. The rougher single layer microcrystalline (MCD) coating had the best wear behaviour, with tool-life being limited by abrasive flank wear, while the bi-layer coating peeled easily due to the stresses imposed by adhesive wear and cutting edge attrition. Also using low surface roughness diamond coatings, Qin et al. obtained an opposite result when turning an Al–20%SiC composite [16
]. The smoother nanocrystalline diamond (NCD) evidenced greater wear delamination resistance than MCD when used as 25–30 µm thick coatings on WC-Co indexable tool inserts. The same trend was observed by Shen et al. in a multi-layered architecture of micro/nano crystalline diamond coating of tungsten carbide (WC-Co) cutting inserts [17
Successive nano- and micro-structured layers may absorb a part of the residual stresses and decelerate the crack propagation, contributing to a tool life augmentation when compared to nanocomposite coatings [12
]. A similar multilayer configuration developed by the authors of the present paper more than tripled the critical load for delamination in pin-on-disk tribological sliding [18
] and showed extended life in abrasive erosion tests [19
]. It was shown that the use of an N
= 4 coating system, starting with microcrystalline diamond and ending with a smoother NCD top coating yields much better results than lower order alternatives with N
= 2 or N
= 1 (either micro- or nano-crystalline), under laboratory conditions regarding tribological behavior and adhesion. Following these results, the N
= 10 system here tested follows the trend observed in our previous work and is thus optimized regarding processing conditions.
In the present work, indexable triangular turning inserts were produced from a silicon nitride ceramic and coated with diamond in a multilayer approach that combines the very good adhesion of MCD with the top smoothness of NCD. These tools were tested for the first time in the machining of Al-matrix composites containing Al2O3 particles. Tool performance, cutting force, wear and tool life are analysed for two different thicknesses. Despite the research done over the years on the machining of Al base composites containing hard particles with diamond tools, there have been no reports so far that compare the merits of tools with cutting surfaces as different as a direct CVD coating and a brazed CVD or PCD tip can be. In this work, the tool life of such tools in the turning of SiC–Al and Al2O3–Al composites, gathered from several sources are graphically presented and compared with the results of the present work.
Silicon nitride ceramic triangular inserts were produced and coated with ~12 and ~24 µm thick, ten-fold multilayer micro-/nanocrystalline (MCD/NCD) diamond films by hot filament CVD, starting with MCD for maximizing adhesion and ending with NCD on top to ensure smoothness.
The tool life of triangular ceramic inserts coated with multilayered MCD/NCD films were tested in the dry turning of an Al based metallic matrix composite containing 15 vol % Al2O3 particles.
The thicker coatings (24 µm thick) perform well within the following limits: DOC values below 1.5 mm for feeds of 0.4 mm·rev−1; maximum speeds of 750 m·min−1; and feeds up to 0.4 mm·rev−1.
Within the limits of viable working machining parameters, the multilayer diamond coated inserts behave better than most reported CVD diamond systems and as well as most reports using PCD tools, especially when considering the existence of three tips per tool, as opposed to one for PCD or brazed CVD thick films.
For delamination-free turning conditions, the wear of the flank and corner occurs progressively, with the worn surfaces having a smooth appearance, typical of a micro-scale abrasive wear mechanism.
Crater wear is also present, due to the negative tool normal rake, and for the longer machining tests, some surface cracks may appear. Crater wear occurs by successive wear of the layers, delaying total delamination of the diamond coating from the substrate, unlike what would happen with monolayer coatings.
Optical profilometry was used to measure the depth of the wear craters, showing that the substrate was not reached even after 2000 m of machining length.